Faculteit der Natuurwetenschappen, Wiskunde en Informatica
Master Analytical Chemistry
Research on the surface interaction of metastable helium and ions for the
removal of carbon layers on optical devices
Marij Stoevenbelt
Research on the surface interaction of metastable helium and ions for the
removal of carbon layers on optical devices
Master research thesis by
M. Stoevenbelt
Analytical chemistry
Supervisor
Dr. W. Th. Kok
Daily supervisor
Ing. N. Koster
This report reflects the work of my master research at TNO in the department of semiconductor
equipment in Delft, The Netherlands.
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Table of content
Preface ........................................................................................................................................ 1
Abstract ...................................................................................................................................... 2
Samenvatting .............................................................................................................................. 3
1. Introduction ........................................................................................................................ 4
1.1 Scientific question ....................................................................................................... 4
1.2 Plasma .......................................................................................................................... 4
1.3 Plasma deposition ........................................................................................................ 5
1.3.1 Chemical vapour deposition ................................................................................. 5
1.3.2 Physical vapour deposition................................................................................... 5
1.4 Plasma cleaning ........................................................................................................... 5
1.5 Plasma Analysis and Test Setup (PATS) .................................................................... 6
1.6 Metastable helium........................................................................................................ 7
2. Materials and methods ..................................................................................................... 10
2.1 Introduction ............................................................................................................... 10
2.2 Langmuir probe ......................................................................................................... 10
2.3 Optical spectrometer .................................................................................................. 11
2.4 Metastable helium sensor .......................................................................................... 11
2.5 Used substances and samples .................................................................................... 12
2.6 Operating conditions.................................................................................................. 12
2.6.1 Langmuir probe .................................................................................................. 12
2.6.2 Spectrometer....................................................................................................... 14
3. Experimental part ............................................................................................................. 15
3.1 Literature search ........................................................................................................ 15
3.2 Setting up the system ................................................................................................. 15
3.2.1 Set up .................................................................................................................. 15
3.2.2 Results ................................................................................................................ 15
3.2.2.1
Langmuir probe ........................................................................................... 15
3.2.2.2
The spectrometer ......................................................................................... 17
3.3 Getting to know the equipment ................................................................................. 17
3.3.1 Set up .................................................................................................................. 17
3.3.2 Results ................................................................................................................ 17
3.4 Plasma dependence on power and pressure parameters ............................................ 19
3.4.1 Set up .................................................................................................................. 19
3.4.2 Results ................................................................................................................ 20
3.4.2.1
Power dependence ...................................................................................... 20
3.4.2.2
Pressure dependence ................................................................................... 21
3.5 Differences at different positions .............................................................................. 23
3.5.1 Set up .................................................................................................................. 23
3.5.2 Results ................................................................................................................ 24
3.6 Difference in cleaning rate ........................................................................................ 25
3.6.1 Set up .................................................................................................................. 25
3.6.2 Results ................................................................................................................ 26
3.7 Sensor for helium metastables ................................................................................... 27
3.7.1 Set up .................................................................................................................. 27
3.7.2 Results ................................................................................................................ 27
3.8 Metastable helium or not ........................................................................................... 30
3.8.1 Set up .................................................................................................................. 30
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
3.8.2 Results ................................................................................................................ 32
4. Conclusions ...................................................................................................................... 33
5. Recommendations ............................................................................................................ 34
6. Acknowledgements .......................................................................................................... 35
7. References ........................................................................................................................ 36
Appendices .................................................................................................................................. i
Appendix 1 .............................................................................................................................. i
Appendix 2 ............................................................................................................................ iv
Appendix 3 ........................................................................................................................... vii
Appendix 4 ............................................................................................................................ xi
Appendix 5 .......................................................................................................................... xiv
Research on the surface interaction of metastable helium plasma
List of abbreviations
Ar
CVD
CW
EM
EMC
H
He
He*
I-V
N.A.
Ne
Ni+
O2
O2/Ar
PATS
PVD
RF
SMIRP
Te
Vf
Vp
argon
chemical vapour deposition
continuous wave
electromagnetic
electromagnetic compatibility
hydrogen
helium
metastable helium
current-voltage
not applicable
electron density
ion density
oxygen
oxygen/argon
plasma analysis test setup
physical vapour deposition
radio frequency
shielded microwave induced remote plasma
electron temperature
floating plasma potential
plasma potential
Marij Stoevenbelt
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Preface
Plasma surface cleaning is becoming more and more popular in the field of physics and
industry (such as solar and semicon). In some industries it is very important to have materials
with a clean surface and a high degree of cleanlyness. The materials that have to be clean are
often very sensitive to heat load and electromagnetic fields as well. Plasma is an interesting
option because of the possibility to create plasma at moderate temperatures and atmospheric
pressure. For this purpose TNO developed the SMIRP source, the shielded microwave
induced remote plasma. This microwave source is capable of creating plasma at a relatively
low temperature.
The research question asked in this thesis is about plasma cleaning using helium gas. It is
found experimentally that helium cleans just as well or better than for instance hydrogen. Not
understood is how this works and why it works. The explanation seems to be that the cleaning
is done by metastable helium, but this is never proven without doubt. Sputtering is proven to
work but a more complex mechanism is assumed. The purpose of this research study is to
investigate whether the cleaning is really done by metastable helium or if it is done by helium
ions.
In order to investigate this, a small literature research is done first to get an overview of what
is happening in this field of expertise. After the theoretical part it is needed to get to know the
system and all the apparatus attached to it, including installing some of the equipment. Some
introduction experiments will be done, to know how the equipment reacts as well as to see
how different types of plasma behave inside the chamber.
To conclude the practical time in this internship, more complex experiments will be done as
to determine whether the cleaning is done by metastable helium or by helium ions. Besides
this, the optimal conditions such as the distance of the sample to the source are investigated.
In chapter 1 an introduction is given to the chemistry behind plasma cleaning, the plasma
cleaner and the approach to the scientific question. After this an explanation of the equipment
installed on the vacuum chamber is given in chapter 2. In chapter 3 the experimental setup
and results are given. The last two chapters (4 and 5) contain some conclusions and remarks
regarding future research.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Abstract
Being able to understand why mild helium plasma cleaning works as well as hydrogen plasma
cleaning is important to TNO since it can help them to advice clients about the right cleaning
tool and process for their equipment. Helium can play an important role in this since it is an
inert and relatively safe gas, preventing a number of problems observed with other cleaning
methods. The plasma analysis test setup (PATS) is equipped with a Langmuir probe and a
spectrometer for the purpose of investigation. During the small literature search starting this
internship it is found that many researchers observe the effects of mild helium cleaning, but
they do not have an explanation for it.
At the end of the research period the conclusion had to be drawn that there was no answer yet
on the original research question as to whether the plasma cleaning is done by metastable
helium or by helium ions. However, some very interesting aspects of helium cleaning came
up.
During the experiments with the spectrometer it became clear that there are definitely
differences between the different types of plasma. In helium plasma the different transitions to
the two metastable states showed differences as well for different circumstances, indicating
that they are a good tool to tune the plasma in a way that one species dominates. This can be
important in future cleaning rate experiments.
The cleaning rate experiments showed that using continuous wave plasma it is possible to
clean samples even at a further distance from the source. They also showed that the sample
facing away from the source is cleaned as well, this proves no line of site is necessary for
plasma cleaning when using helium plasma. This is an indication that there are long living
species present, which might be metastable helium atoms. Although the sensor as constructed
to measure the metastables was not working properly, it showed potential for the future.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Samenvatting
Voor TNO is het belangrijk om te begrijpen waarom helium plasma net zo goed lijkt te
reinigen als bijvoorbeeld waterstof plasma, zodat ze hun klanten kunnen adviseren over de
methode die gebruikt moet worden. Helium plasma kan hier een belangrijke rol in spelen
aangezien helium van zichzelf zachter is, waardoor minder schade aan het oppervlak van het
te reinigen voorwerp ontstaat. De Plasma Analysis Test Setup (PATS) is uitgerust met een
Langmuir probe en een spectrometer om plasma’s te kunnen observeren. Tijdens de kleine
literatuurstudie die in de opdracht was verwerkt, is gebleken dat veel onderzoekers zien dat
helium plasma inderdaad goed cleaned, maar niemand kan uitleggen waarom het zo is.
Uiteindelijk is aan het eind van de stage de conclusie getrokken dat er nog geen antwoord is
gevonden op de oorspronkelijke onderzoeksvraag of helium reiniging nu met behulp van
metastabielen of met ionen wordt gedaan. Er zijn echter wel een aantal interessante aspecten
van helium reiniging ontdekt.
Tijdens spectrometer experimenten werd duidelijk dat er wel degelijk verschillen bestaan
tussen de verschillende typen plasma. Bij helium plasma lieten de ratio’s voor de
verschillende transities naar de twee metastabiele staten, verschillen zien bij veranderende
omstandigheden. Dit impliceert dat er een goede manier is om plasma te optimaliseren zodat
een van de twee metastabiele toestanden overheerst. Voor toekomstige experimenten kan dit
van groot belang zijn.
De experimenten met betrekking tot de cleaning rates laten zien dat het bij gebruik van
continuous wave mogelijk is om oppervlakken te reinigen die verder van de bron af staan.
Ook hebben ze laten zien dat het mogelijk is om te reinigen zonder dat er een zichtlijn met de
bron is. Dit is een indicatie dat er reactieve atomen of moleculen aanwezig zijn met een lange
levensduur. De sensor, gebouwd om de hoeveelheid metastabielen te meten, werkte helaas
niet zoals gewenst, maar liet wel mogelijkheden voor de toekomst zien.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
1. Introduction
An overview of the theoretical background is provided on the principals and techniques used
in this research project. The first part of this chapter covers the general information about
plasma and the possibilities when using it. The second half of the chapter contains some
information about the specific equipment used during the research study.
1.1 Scientific question
Plasma cleaning is usually done by reactive gases such as hydrogen, oxygen or by sputtering
using inert gases such as argon. An experiment was done using helium plasma and although
not expected it was found that it had a high cleaning rate for samples coated with carbon.
Investigation revealed that helium is capable of cleaning surfaces as well as hydrogen, but
with less harmful side effects. Sputtering is not expected since the plasma has a low energy,
but, then what is causing the cleaning of the surface? The theory is that neutral helium atoms
with an high internal energy (so-called metastables) are capable of forming short lasting
volatile species with the contaminants on the surface or breaking the bonds connecting it to
the surface. The advantage of soft cleaning is that there is less damage to the surface. The
mechanism behind the helium cleaning is not yet fully understood and needs more research.
The search for answers to this question is described in this report.
1.2 Plasma
Plasma is considered the fourth state of matter (see figure 1). Plasma is a partially ionised gas,
in which gas interactions are dominated by charged species like ions, anions and electrons.
Like gas, plasma does not have a definite shape or a definite volume. The presence of a nonnegligible number of charged particles makes the plasma electrically conducting so it
responds strongly to electromagnetic fields. Depending on the type of atoms, the proportion
between the number of atoms ionised and neutral and the energy of the particles, there are
many different types of plasma; each having their own properties.1 Plasma can be created in
many different ways, as long as there is enough energy added to the gas.
H2O (s)
H2O (l)
H2O (g)
Solid
Liquid
Gas
H,H2,H+,e-,H2H2O O,O ,O ,O-,O 2
3
2
Plasma
Add energy
Figure 1 States of matter
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Figure 1 shows how states of matter are reached by adding enough energy, finally resulting in
plasma. When it reaches plasma state it is transformed into a wide variety of species like
molecules, atoms, ions or anions having high internal energies. Plasma can be used for
cleaning as well as for deposition. In section 1.3 and 1.4 both processes will be addressed.
1.3 Plasma deposition
Plasma deposition involves the formation of a thin coating at the substrate surface. Plasma
deposition can be done in two different processes; chemical vapour deposition (CVD),
involving a reactive gas binding molecules to the surface, and sputtering/physical vapour
deposition (PVD), which involves the sputtering of the target material and condensing it on a
surface.
1.3.1 Chemical vapour deposition
In CVD the chemical that is to be deposited is added to the gas used to create the plasma.
During plasma activity radicals and ions of the chemical are created and are directed towards
the target surface. At the surface the radicals and ions bind to the chemicals present in the
surface layer.2
1.3.2 Physical vapour deposition
The other plasma deposition method is PVD that mainly consists of deposition technologies
in which material is released from a source and transferred to the substrate. The two most
important technologies are evaporation and sputtering.3
Using evaporation, the sample is placed inside a vacuum chamber after which the temperature
is increased until the point is reached where it starts to boil. The molecules which are released
condense on all other surfaces inside the chamber.
In case of sputtering, the material is released from the source at much lower temperature than
evaporation. The substrate is placed in a vacuum chamber with the source material, named a
target, and plasma is activated. The ions created are accelerated towards the target, causing
atoms of the source material to be ejected and condense on all surfaces including the
substrate.
1.4 Plasma cleaning
Plasma cleaning involves the removal of impurities and contaminants from surfaces through
the use of energetic plasma created from gaseous species. Plasma cleaning can be divided in
two processes, chemical cleaning and physical cleaning. In case of a reactive gas, chemical
reactions are possible at the surface. When using an inert gas to create plasma, the excited
ions can collide with a surface and remove a small amount of material, this is mechanical
cleaning .4 Different types of plasma cleaning are pictured in figure 2 using hydrogen (H) as
the plasma gas.
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Research on the surface interaction of metastable helium plasma
Volatilisation
Soft sputtering
Marij Stoevenbelt
Sputtering
Figure 2 Different types of plasma cleaning processes in H2 plasma environment
There are different types of processes to perform plasma cleaning:
 Volatilisation; A radical having little kinetic energy can arrive at the surface. The
radical can bond to the element present at the surface. This can lead to the formation
of a new compound such as methane in figure 2. The new compound is then released
from the surface. This is a very mild method of removing contamination.
 Soft (chemical) sputtering; A radical or ion can have enough energy to break bonds of
the contaminant with the surface. When the radical arrives at the surface, the
transferred energy is used to remove the bond of a contaminant and the surface. This
is still a mild method of plasma cleaning.
 Sputtering; An ion having a high kinetic energy can knock other atoms off the
surface, including atoms from the substrate. This is a very aggressive cleaning method
which can lead to damage of the surface.
Helium is an inert gas, so it should have no abilities to bond to other atoms. The cleaning
processed are therefore expected to be soft sputtering or sputtering. In earlier experiments
however, it is noted that helium is also capable of soft cleaning, which could indicate
volatilisation. Volatilisation would mean that helium is capable of making a short living bond
to another atom. This could be possible with metastable helium.
1.5 Plasma Analysis and Test Setup (PATS)
The Plasma Analysis and Test Setup (PATS) system (see figure 3) is a dedicated plasma
cleaner for research on plasma processes and cleaning recipes or other applications involving
low pressure plasmas. The main focus is aimed at cleaning delicate surfaces like mirrors,
electronics and thin layers. For this a special type of plasma called Shielded Microwave
Induced Remote Plasma (SMIRP) is developed at TNO. SMIRP has the great advantage that
is relatively cold and has no electromagnetic (EM) fields outside the plasma formation region;
hence it is possible to clean electromagnetic compatibility (EMC) sensitive components. The
cleaning is performed by volatilisation and soft sputtering and not, in contrast with other types
of cleaning, by sputtering. Depending on the contaminant to be cleaned an appropriate gas can
be chosen.
At this moment 4 microwave (SMIRP) sources and a radio frequency (RF) source are
mounted onto the PATS. The microwave sources can be operated in continuous wave as well
as pulsed mode. There are cages put over the microwave sources (as a type of Faraday cage)
to keep the plasma creation at one place where it can be observed and manipulated. During
continuous mode the sources are continuously coupling power into the gas. When pulsed
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
mode is used the effective power introduced is lower, depending on the duty cycle
(Tpulse/period) with which the microwave power is coupled into the gas. During the
experiments only one source was used to limit the amount of variables.
Figure 3 The PATS system.
The PATS is equipped with analytical equipment like a Langmuir probe, UV/VIS
spectrometer, mass spectrometer and an ellipsometer. These are used to study the plasma
processes and surface interactions. Besides cleaning, the PATS can also be used for surface
modification of polymers and studies involving sterilization and disinfection of surfaces for
medical, food and pharmaceutical applications. The latter possibilities are beyond the scope of
this thesis and will not be explained here.
1.6 Metastable helium
Since helium is an inert mono-atomic gas, collisions between atoms and electrons are
perfectly elastic. When helium is partially ionised, some electrons are not bound to the
nucleus, resulting in a plasma in which the free charge carriers (electrons and ions) create a
good electrically conductive environment. If electrons are then accelerated under a suitable
potential, the velocity (and thus, the energy) of the electrons continually increase. If an atom
received enough energy to become excited it is possible that this atom relaxes to a state from
where it is not possible to relax to the ground state directly, a metastable radical is formed .
The metastables can have energies of 19.8 or 20.6 electron volts.5 These energies depend on
the spin of the two electrons in the excited state. When they are oriented antiparallel (singlet
or para state) the energy is 20.6 eV and when the electrons are oriented parallel (triplet or
ortho state) the energy is 19.8 eV.
The theoretical lifetime for metastable helium is quite different from the normal lifetime of
excited species in a plasma. Normally an excited species has a lifetime in the order of 10-9
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
seconds, the triplet state has a lifetime of 2*10-2 seconds and the singlet state has a lifetime of
4200 seconds6. Taking into account this long lifetime and the long mean free path (the
average distance covered by a particle between successive impacts) of helium in a vacuum the
reactivity of helium plasma might be explained. When the helium atoms cannot notice each
other within the chamber they are likely to encounter something else first. This means there is
a relatively large amount of metastables present.
Helium can be considered to be metastable when either the spin of the electrons prevents a
relaxation to the groundstate or when the dipole is not capable of relaxing to the ground state.
Metastable excited helium states have a long lifetime since the transition to the ground state is
a quantum mechanically forbidden transition by dipole selection rules. Metastable helium has
to be excited to a higher energy level, or have enough internal energy to reverse a spin orbit,
to be able to relax to the ground state.7 Different helium states with their energy levels are
given in figure 4. The red circles represent the metastable states, since from there only
indirect, in the case of para helium (left side), or no relaxation, in case of the ortho helium
(right side), to the ground state is possible. The most important emission lines for this project
are some of the singlet and triplet lines. The following transitions are the ones of the most
interest;
Singlets;
3p → 2s 502 nm
3d → 2p 668 nm
3s → 2p 728 nm
Triplets;
3p → 2s 389 nm
3d → 2p 588 nm
32 → 2p 707 nm
The 3p → 2s transitions are the most interesting since it involves a direct relaxation to the
metastable states. The measurement of these emission lines gives information about the
quantity of metastable helium present.
Also the ratios of the same singlet and triplet transition are important since they can give a
measure for the amount of singlets present versus the amount of triplets present. In the end the
metastables wanted are those who express the largest cleaning rates and the lowest heat load
or damage to the surface. The ratios between singlet and triplet transitions can help to
determine the optimal cleaning settings.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Figure 4 helium energy levels and transitions8
The states with a red circle are the metastable states. From here no direct relaxation to the
ground state is possible. The transitions from 2p → 2s are not measurable with the
spectrometer used during this research although they could give extra information about
singlet life time.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
2. Materials and methods
2.1 Introduction
As described earlier, a Langmuir probe and an optical spectrometer are used during the
research. Added to this standard analytical equipment is a sensor to detect the amount of
metastable helium. All equipment is introduced first, after which a short part follows about
the gases, substances and samples used. The last part of the chapter contains the primary
settings for all equipment.
2.2 Langmuir probe
The Langmuir probe (figure 5) can be used to determine a number of characteristics of a
certain plasma. The tip of the probe is inserted into the plasma and a varying potential is
applied to this tip. Electrons and ions will be either repelled or attracted to the tip, resulting in
a typical I-V (current-voltage) characteristic. Using the obtained I-V characteristic, the ion
density, electron density, electron temperature, the ion flux, floating potential and the plasma
potential can be calculated. Langmuir probes are routinely used to determine the plasma
parameters in areas as diverse as: low pressure plasmas for materials processing, the design of
ion sources and new plasma chambers, and edge plasmas in fusion devices.
Figure 5 View of a Langmuir probe.
In the I-V curve three different regions can be distinguished, as can be seen from figure 6. At
a high negative voltage, all electrons are effectively repelled and the probe attracts only ions,
so this is called the ion saturation region. At a high positive voltage, ions are repelled
effectively and only electrons are attracted by the probe, so this is called the electron
saturation region. The region in between is called the electron saturation region.9
In the Plasma Diagnostics Introduction to Langmuir Probes, Technical Information Sheet
5318 formulas are given which the software program uses to calculate the characteristics of
the plasma.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Figure 6 Typical I-V current
2.3 Optical spectrometer
With optical spectroscopy the wavelengths of emitted light by the transitions of atoms,
molecules or ions from an excited state to a lower energy state can be examined. Each
element emits and absorbs a characteristic set of discrete wavelengths according to changes in
its electronic structure, by observing these wavelengths the elemental composition of the gas
or plasma can be determined.
Using an optical spectrometer enables observation of the emission lines which are specific for
helium ions or (metastable) helium atoms. Using these results, an estimation can be made as
to which states are dominant in the plasma.
By coupling energy into a gas, some of the atoms or molecules become excited. Excited states
have a certain lifetime, after which the atom will relax to a lower energy state. When they do,
they emit a photon at a unique wavelength, corresponding to the energy differences, ΔE = hν
= hc/λ, h=Planck constant, ν= frequency, c= speed of light and λ= wavelength. The brightness
of the emission lines can give a lot of information about the abundance of the amount of
atoms or ions in a certain state.
2.4 Metastable helium sensor
There are several possibilities to measure the amount of metastable helium in a plasma but
according to Miura and Hopwood10 the most practical one is to build a metastable helium
(He*) sensor (see figure 7). The measurable current released by the plasma is composed of
several components, the electron current (Ie), the ion current (Ii), the photon current (Iᵞ) and
the metastable current (IHe*). In order for the sensor to work, a shielding mechanism has to be
applied to shield the sensor from all the currents except for the metastable current which leads
to the following Itot= IHe*. This means only neutrals are captured on the surface of the sensor,
composed of both atoms and metastable helium atoms. When the sensor is not shielded, the
signal is dominated by ions and electrons hitting the surface of the sensor. In figure 7 the
mechanism of the sensor is depicted. It is fabricated by creating an outer probe of 1x1 cm
aluminium sheet with a hole in the centre. The hole is 3 mm in diameter. The backside of the
outer probe is covered with kapton tape which does not conduct electricity. The inner probe is
made of stainless steel and is only exposed to the environment through the hole in the outer
probe, the backside is again covered with kapton tape to prevent the conduction of electricity.
Potentials can be applied to shield the probe from electrons and ions. A negative voltage is put
on the outer probe, shielding the sensor from positive ions. A positive voltage is applied to the
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Research on the surface interaction of metastable helium plasma
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inner probe, thus shielding the sensor from electrons. He* has enough internal energy to free
an electron from the surface, so by measuring the resulting current on the inner probe the
amount of He* can be measured.
Figure 7 Schematic overview of the helium metastable sensor10
2.5 Used substances and samples
For plasma analysis the need for chemicals is very low. Cleaning using a ultrasonic bath is
needed because everything mounted into the chamber has to be very clean. The parts that
need cleaning are submerged in ethanol and cleaned. Also alcoholic cleaning tissues (70 %
isopropyl alcohol) are used for this purpose.
The gases used to create the plasma (hydrogen, oxygen, argon and helium) are 5.0 (99.999%)
pure. The gas flows are kept constant by the equipment controlling the vacuum chamber.
The samples used during the course of these experiments are round quartz glasses of 1 inch
diameter. The samples are coated on one side with a carbon layer, either 20 nm or 125 nm
thick.
2.6 Operating conditions
2.6.1 Langmuir probe
The Langmuir probe used is a commercially available probe purchased from Hiden
Analytical. The default settings for the Langmuir probe are mainly the probe settings. The
default settings are depicted in figure 8 and 9. followed by an explanation.
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Research on the surface interaction of metastable helium plasma
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Figure 8 Langmuir sweep settings
The settings in figure 8 can be explained as follows;
Potential ramp; the voltage is swept from -50V to +50V, using a 0.1V interval between each
step. Range; the gain is set to 100 mA in order to prevent a shutdown when a strong peak is
observed. The probe impedance is set on 4.9 Ohms, which limits the chance of electrical
break through. The tip is cleaned before each new scan by means of ion bombarding by
applying 100V to it for 200 ms. Since we use continuous wave mode, no trigger signal is used
and gate timing is therefore disabled.
Figure 9 Probe positioning of the Langmuir probe
The settings in figure 9 are relatively simple, the standard probe position (park position) is 35
cm from the starting position. This means the probe is inserted in the chamber for about 20
cm.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
2.6.2 Spectrometer
The initial spectrometer used during the research period is a commercially available Andor
Mechelle (Lot Oriel) spectrometer with an ICCD detector attached to it, type ME5000- iStar
DH734. This detector has a spectral range of 200 to 975 nm with a resolution of 4 pixels at
full width half max. The Andor Mechelle spectrograph is based on a grating principal and
patented optical design, which gives extremely low cross-talk, equally spaced order separation
and maximum resolution. The spectrograph offers a simultaneously recorded wavelength
range from UV to NIR with very high resolution and no overlapping wavelengths, thus
enabling for high resolution real time measurements. The accompanying software is capable
of detecting which elements are present is the spectrum.11 The spectrometer camera can be
cooled.
A second type of spectrometer used during the experiments is a commercially available CCD
minispectrometer from Hamamatsu, model TM-UV/VIS C10082CAH. This type has a
spectral range of 200 to 800 nm with a resolution of 1 nm.
The spectrometer is capable of scanning in dark and in reference mode, so the spectra can be
corrected for background noise or used to compare different spectra.
The spectrometer used is a very simple model only capable of taking a scan and saving it in
excel extension. This file can then be opened in excel to be studied further.12
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Research on the surface interaction of metastable helium plasma
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3. Experimental part
3.1 Literature search
Before the practical part started a small literature search was done. The overall conclusion
derived from the found literature is described in this section. The full result of the literature
study is described in appendix 1.
The overall conclusion that can be drawn from the papers found is that a lot of researchers
observed the effect of helium cleaning, but nobody has determined the exact nature of the
mechanism involved. However there are some other useful conclusions to be drawn from the
articles, which are given here.
One article suggested that the cleaning by helium is done by metastable helium radicals
reacting with the surface, thereby reducing the surface impurities.
When helium plasma is ignited in a fusion reactor (which is a high energetic plasma) the
following is observed; Hydrogen rapidly appears when the discharge is started and slowly
falls off, disappearing completely when the discharge is stopped. The main source for this
hydrogen suggested by the author is adsorbed water but according to some other authors this
could also be released from the vessel wall. The following is also observed; Hydrogen and
helium are both present during plasma cleaning, even if they are not present in the gas used to
produce the plasma.
Accumulation of helium by the vessel wall is influenced by the wall material itself. Graphite
holds less helium than stainless steel, resulting in a lower release during plasma treatment.
No correlation is found between helium pressure within a stainless steel vessel and the gas
species of the main plasma. During experiments a net loss of helium particles is observed and
the main source of helium desorption is the stainless steel wall.
Non-oxygen plasma leads to higher oxygen and nitrogen levels on the surface being cleaned.
Storage has distinct effects on the surface composition, which is depending on the plasma
type used.
3.2 Setting up the system
3.2.1 Set up
Before the plasma analysis test setup (PATS) could be used to observe plasma and its
characteristics, the machine and all the equipment attached to it had to be functioning
correctly. Since the system was new, most of the equipment needed to be installed and
configured for the first time. The optimal settings had to be determined during the
experiments.
3.2.2 Results
3.2.2.1 Langmuir probe
While setting up the system, some difficulties arose. First of all the Langmuir probe was not
working properly, since the probe could not be moved into the vacuum chamber. After
contact with the supplier, the problem was solved. However, the results of the first
measurements were not as expected. Instead of a curve looking like the curve in figure 6, a
deviating curve as shown in figure 10 was observed.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
pulsed microwave scan (350 W, 0,5 mbar)
1,00E-03
current (nA)
8,00E-04
6,00E-04
4,00E-04
2,00E-04
0,00E+00
-2,00E-04
-60
-40
-20
0
20
40
60
voltage (V)
Figure 10 The first Langmuir results
Since this is not the best curve to perform calculations on, improvements and/or
optimalisations were needed. Cleaning of the tip before each measurement helped to improve
the signal by a factor 2. This was not enough however, since the noise scaled along with the
signal. After doing a fourier transformation (appendix 2) it became clear that there was
another signal interfering with the measurement. There were a few other possible sources for
the problems encountered. These problems were addressed one at a time to determine the
cause of the bad signal. The possible causes were the power supply, the plasma source and the
type of plasma creation.
The power supply did not seem to be the problem since getting the power from another source
only led to a worse signal (appendix 2).
Using an RF source instead of a shielded microwave source did not help either, which we
expected, since the RF source is not shielded.
When trying to create plasma using the continuous wave mode of the microwave source
instead of the pulsed mode, a definite improvement of the signal was observed. The problem
occurred during pulsed mode of the microwave source. Measuring the plasma using the
continuous wave mode resulted in a scan looking like a typical curve (see figure 11). The
figures of all the experiments are described in appendix 2.
current (nA)
scan continuous wave (350 W, 0,5 mbar)
0,004
0,0035
0,003
0,0025
0,002
0,0015
0,001
0,0005
0
-0,0005
-60
-40
-20
0
20
40
60
voltage (V)
Figure 11 I/V curve using the continuous wave mode
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
3.2.2.2 The spectrometer
The spectrometer needed to be attached to the vacuum chamber as well. The apparatus had to
be calibrated first. The Andor spectrometer showed no response at all when measurements
were done. After some testing, the suspicion arose that the image intensifier was not working
properly and the spectrometer had to be sent to the manufacturer. A spectrometer was needed,
so another spectrometer had to be arranged. A simple Hamamatsu spectrometer from another
department was borrowed for the duration of the repairs.
3.3 Getting to know the equipment
3.3.1 Set up
The first experiment was also meant to get a better understanding of the behaviour of different
types of plasma. The plasma was activated first, to pre-clean the vacuum chamber. After the
conditioning, the plasma was ignited again and scans were made using the Langmuir probe
and the spectrometer. The Langmuir probe provided information about the electron
temperature, the plasma voltage etcetera and the spectrometer provided information about the
gas species present in the chamber by means of the emission lines.
This experiment was performed using helium, hydrogen and oxygen/argon (50/50) plasma.
Afterwards the results were compared for the different types of plasma.
3.3.2 Results
Using the Langmuir probe and the spectrometer, measurements were done with helium,
hydrogen and oxygen/ argon plasmas. The measurements were performed with the standard
operating mode. This meant an input power of 350 W and a pressure inside the chamber of
0,5 mbar. A Langmuir probe graph taken for helium plasma is shown in figure 12, and a
spectrometer graph for helium plasma is shown in figure 13. The full results including
hydrogen and oxygen/argon plasma are described in appendix 3.
Helium plasma (350 W, 0,5 mbar)
0,0012
current (nA)
0,001
0,0008
0,0006
0,0004
0,0002
0
-0,0002
-50
-30
-10
10
30
50
voltage (V)
Figure 12 Langmuir probe signal helium plasma
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Research on the surface interaction of metastable helium plasma
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He plasma (350 W, 0,5 mbar)
intensity (counts)
60000
50000
40000
30000
20000
10000
0
250
350
450
550
650
750
850
wavelength (nm)
Figure 13 Spectrometer results from the helium plasma
In figure 13, among others, the peaks resulting from direct relaxation to metastable helium
states are visible. These lines are at 502 nm for the singlets and 389 nm for the triplets. The
other transitions for the singlets are visible at 668 nm and 728 nm, and the triplet transitions
are visible at 588 nm and 707 nm (see figure 4).
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Research on the surface interaction of metastable helium plasma
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3.4 Plasma dependence on power and pressure parameters
3.4.1 Set up
The pressure inside the chamber and the power input into the system could affect the electron
density inside the chamber. To test this, the plasma parameters and the emission lines were
monitored using the Langmuir probe and the spectrometer at different input powers and
pressures. This experiment was done using helium, hydrogen and oxygen/argon plasma. The
input powers used were 350, 500 and 750 W at continuous wave. The pressures used were
0,2, 0,5 and 1 mbar. The standard settings were 350W input power at a pressure of 0,5 mbar.
When the input power was changed, the pressure was kept constant and when the pressure
was changed, the input power was kept constant. The electron density for the different types
of plasma was measured using the Langmuir probe. The Langmuir probe was positioned
about 20 cm inside the chamber above the plasma source used for the experiments. The fibre
from the spectrometer was placed in front of the antenna of the source, thereby observing the
plasma through the middle of the plasma cloud.
Using the Langmuir probe not only the electron density was determined but also the electron
temperature, the plasma potential and the ion density. It is also of importance to see if there
are differences between the characteristics of the plasma when either the input power or the
pressure is changed.
Using the spectrometer any possible changes in emission lines were observed for the different
types of plasma at different pressures and input powers. The figure regarding He plasma at
standard settings is depicted in figure 14, but for means of comparison only tables with peak
heights at each setting are described in this chapter.
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Research on the surface interaction of metastable helium plasma
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3.4.2 Results
3.4.2.1 Power dependence
The most important characteristics are summarised in this section, the rest is described in
appendix 3. The results for the plasma potential (Vp), electron temperature (Te), ion density
(Ni+) and the electron density (Ne) are depicted in figure 14 a, b, c and d respectively for the
different input powers.
b) Te in Kelvin (0,5 mbar)
50
45
40
35
30
25
20
15
10
5
0
O2_Ar
He
H
350 W
500 W
Te (1000K)
Vp (V)
a) Vp in volts (0,5 mbar)
4,50
4,00
3,50
3,00
2,50
2,00
1,50
1,00
0,50
0,00
O2_Ar
He
H
350 W
700 W
W input power
9,00
8,00
7,00
6,00
5,00
4,00
3,00
2,00
1,00
0,00
O2_Ar
He
350 W
500 W
W input power
700 W
700 W
d) Ne per m3 (0,5 mbar)
Ne (/m3)
Ni+ (10^15/m3)
c) Ni+ per m3 (0,5 mbar)
500 W
W input power
1,00E+16
1,00E+14
1,00E+12
1,00E+10
1,00E+08
1,00E+06
1,00E+04
1,00E+02
1,00E+00
O2_Ar
He
H
350 W
500 W
W input power
700 W
Figure 14 a) plasma potential, b) electron temperature, c) ion density, d)electron density.
As can be seen from figure 14 the trends are not the same for each type of plasma. The most
interesting result is the trend for the plasma potential (Vp), since the trend for hydrogen (green
trace) and oxygen/argon (blue trace) is an increase while helium (red trace) stays more or less
the same as shown in figure 14a. This could indicate that helium plasma is saturated with
excited species at a lower power input than hydrogen and oxygen/argon plasma. The electron
temperature (Te in figure 14b) for oxygen/argon plasma follows a decreasing trend while
helium plasma follows the same trend as for the plasma potential. This indicates a difference
in behaviour under different circumstances.
Regarding the ion density (Ni+) and electron density (Ne) shown in figure 14c and d
respectively, the same contradiction can be noted. Where the trend for oxygen/argon
contradict each other, the values for helium stay more or less the same and seem to be highly
similar.
When the I-V characteristics could not be determined from the curve a value is missing.
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Research on the surface interaction of metastable helium plasma
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When zoomed in more peaks are visible in the spectrum obtained by the spectrometer, but
they are not significant for the reviewing of the results so they are not named in this report.
The peak heights of the most intense peaks are given in appendix 3. The most important
results however, are the singlet and triplet metastable lines, and from those the 502 nm and
389 nm are the most interesting ones. The results for the singlet and triplet transitions are
depicted in table 1, including the ratio of the singlet and triplet line set associated with the
same transition. These ratios might give an insight as to the metastables present in the plasma
at different input powers.
Table 1 Spectrometer results helium plasma triplet (t) vs singlet (s) emission, results in counts
Emission line
350 W
500 W
700 W
502 nm (s)
8528
9840
11081
389 nm (t)
24273
26684
29410
Ratio
2,85
2,71
2,65
668 nm (s)
15198
18057
20580
588 nm (t)
50466
58643
64668
Ratio
3,32
3,25
3,14
728 nm (s)
2155
2367
2627
707 nm (t)
10965
11913
13154
Ratio
5,09
5,03
5,01
The ratios are calculated by dividing the peak height of the triplet by the peak height of the
singlet. When comparing the ratios for the direct relaxation to the metastable state (502 and
389 nm, 3p → 2s transition), a decrease is visible for increasing power input. This decrease is
also visible for the 3d → 2p transition (668 and 588 nm), but is not so obvious for the 3s →
2p transition (728 and 707 nm). This means there is a difference in equilibrium between
singlet and triplet state at different input powers, resulting in an ability to tune the wanted
plasma conditions.
Overall there is an increase in absolute signal when increasing the power input. This is also
visible for hydrogen and oxygen/argon plasma. The increase in signal is similar for all
plasmas.
Combining figure 15 and table 1 results in the following observations. The plasma potential
for helium remains the same as well as the ion density, but the electron density is decreasing.
The singlet emission lines increase faster than the triplet emission lines with increasing power
input. For a higher amount of the preferred metastables a higher input power is required.
3.4.2.2 Pressure dependence
The most important characteristics are summarised in this section, the rest is described in
appendix 3. The results for the plasma potential (Vp), electron temperature (Te), ion density
(Ni+) and the electron density (Ne) are depicted in figure 15 a, b, c and d respectively for the
different pressures.
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Research on the surface interaction of metastable helium plasma
a) Vp in volts (350 W)
b) Te in Kelvin (350 W)
45
40
35
O2_Ar
25
He
20
H
15
10
Te (1000K)
Vp (V)
30
5
0
0,2
0,5
Marij Stoevenbelt
4,50
4,00
3,50
3,00
2,50
2,00
1,50
1,00
0,50
0,00
O2_Ar
He
H
0,2
1
mbar pressure
c) Ni+ per m3 (350 W)
0,5
mbar pressure
1
d) Ne per m3 (350 W)
1,00E+17
1,00E+15
O2_Ar
1,00E+14
He
1,00E+13
H
1,00E+12
0,2
0,5
mbar pressure
1
Ne (/m3)
Ni+ (/m3)
1,00E+16
1,00E+16
1,00E+14
1,00E+12
1,00E+10
1,00E+08
1,00E+06
1,00E+04
1,00E+02
1,00E+00
O2_Ar
He
H
0,2
0,5
mbar pressure
1
Figure 15 a) plasma potential, b) electron temperature, c) ion density, d) electron density
As can be seen from figure 15 the trends are not the same for each type of plasma. The most
interesting result are the trends for helium plasma (red trace), since the trend is decreasing for
all parameters with increasing pressure (including the ones depicted in appendix 3). This
indicates that the characteristics of helium plasma change when changing the pressure inside
the vacuum chamber. This trend is not visible for the other types of plasma. This indicates
that the behaviour observed with helium plasma is unique for helium. This also implies an
ability to create a plasma with certain characteristics.
When the I-V characteristics could not be determined from the curve a value is missing.
When zoomed in more peaks are visible in the spectrum obtained with the spectrometer, but
they are not significant for the reviewing of the results so they are not named in this report.
The peak heights of the most intense peaks are given in appendix 3. The most important
results however, are the singlet and triplet metastable lines. These results are given in table 2,
including the ratio of the complementing singlet and triplet line. These ratios might give an
insight as to the metastables present in the plasma at different pressures.
Table 2 Spectrometer results helium plasma triplet (t) vs singlet (s) emission, results in counts
Emission line
0,2 mbar
0,5 mbar
1 mbar
502 nm (s)
7187
8528
8752
389 nm (t)
19281
24273
25943
Ratio
2,68
2,85
2,96
668 nm (s)
8510
15198
17879
588 nm (t)
30295
50465
61947
Ratio
3,56
3,32
3,46
728 nm (s)
1990
2155
2292
707 nm (t)
8243
10965
12776
Ratio
4,14
5,09
5,57
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
The ratios are calculated by dividing the peak height of the triplet by the peak height of the
singlet. For the transition directly to the metastable state (502 and 389 nm) an increase in ratio
is visible, indicating a faster increase of the triplet lines comparing to the singlet transition.
This implies a higher amount of relatively short living metastables at a higher pressure.
For helium plasma there is an overall increase in absolute signal when increasing the pressure.
For hydrogen and oxygen/argon plasma however, there is a decrease in signal with increasing
pressure. This is another indicator of different behaviour of different types of plasma.
Combining figure 16 and table 2 results in the following observation. The electron density as
well as the relative amount of singlets decrease with increasing pressure. For better metastable
results a lower pressure is required.
3.5 Differences at different positions
3.5.1 Set up
There can be a difference in electron density, temperature and ion density and therefore in
activity at different positions inside the Faraday cage and the whole chamber. To check this,
the Langmuir probe was inserted into the Faraday cage or placed above it as depicted in figure
16, between brackets is the distance to the microwave source. At each position, Langmuir
measurements were done to evaluate differences. Using the spectrometer, at the same
positions measurements were done to observe (if any) notable changes in the emissions at the
respective positions.
1. at half cage height above the cage (14 cm)
2. just above the cage (10,5 cm)
3. in the upper part of the cage (8 cm)
4. at half height of the cage (5 cm)
Figure 16 The measurement positions around the Faraday cage
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Research on the surface interaction of metastable helium plasma
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3.5.2 Results
The line of sight for the spectrometer is blocked on the other side of the cage, this to ensure an
ending line of sight. The results in this paragraph only contain the results for helium plasma.
Full results for all plasmas are given in appendix 4. With the Langmuir probe only the first
three positions are measured, since the last two positions are outside the plasma creation area
and are expected to give similar results. Using the spectrometer the 3rd position is not
measured. Instead a measurement is done through the antenna of the microwave source,
giving a line of sight through the quartz glass separating the vacuum from the atmospheric
pressure. In figure 17 and table 3 the results from the measurements using the spectrometer
and the Langmuir probe are depicted. The results for the spectrometer are noted as the ratios
between singlet and triplet lines to observe any changes in composition of the plasma.
a) Vp (350W, 0,5 mbar)
b) Te in K (350W, 0,5 mbar)
40
35
25
O2_Ar
20
He
15
H
10
Te (1000K)
Vp (V)
30
5
0
middle
upper
9,0
8,0
7,0
6,0
5,0
4,0
3,0
2,0
1,0
0,0
O2_Ar
He
H
middle
above
position towards the cage
d) Ne per m3 (350W, 0,5 mbar)
O2_Ar
He
H
middle
upper
position towards the cage
above
Ne (/m3)
Ni+ (/m3)
c) Ni+ per m3 (350W, 0,5 mbar)
1,E+18
1,E+16
1,E+14
1,E+12
1,E+10
1,E+08
1,E+06
1,E+04
1,E+02
1,E+00
upper
above
position towards the cage
1,E+18
1,E+16
1,E+14
1,E+12
1,E+10
1,E+08
1,E+06
1,E+04
1,E+02
1,E+00
O2_Ar
He
H
middle
upper
above
position towards the cage
Figure 17 a) plasma potential, b) electron temperature, c) ion density, d) electron density.
As can be seen from figure 18 the trends are not the same for each type of plasma. The most
interesting result is the trend helium plasma (red trace), since the trend for all parameters
(including those in appendix 4) is decreasing. This implies a change in plasma characteristics
when moving away from the plasma creation area. These characteristics change in a different
manner for different types of plasma.
The ion density decreases for all plasma types with increasing distance from the plasma
creation area.
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Research on the surface interaction of metastable helium plasma
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Table 3 Spectrometer results helium plasma triplet (t) vs singlet (s) emission, results in counts
Emission
Through the
middle of the
Just outside
Half height
line
antenna
cage
the cage
above the cage
502 nm (s)
64543
21900
13189
9656
389 nm (t)
64433
47838
15542
10948
Ratio
1,00
2,18
1,18
1,13
668 nm (s)
64418
45977
19918
14922
588 nm (t)
64300
64503
47290
34913
Ratio
1,00
1,40
2,37
2,34
728 nm (s)
31100
10852
7328
6362
707 nm (t)
64436
53771
20891
16063
Ratio
2,07
4,95
2,85
2,52
The results from the measurement through the antenna are not compared to the other
positions, since these measurements are from a different line of sight and therefore not
directly comparable to the other positions.
The ratio is decreasing for the transition directly to the metastable state when moving away
from the plasma creation area. This means that when plasma cannot be created anymore, the
triplet state decay faster than the singlet states. This leads to the observation that the lifetime
of the singlet metastable state is longer than for the triplet metastable state.
The ratios for the indirect relaxations to the metastable state contradict each other so no
conclusive observations can be made from these data. Inside the cage the ratio for the 3s →
2p transition (728 and 707 nm) is higher than the ratio for the 3d → 2p transition.
These results indicate a difference in characteristics of the plasma at different positions inside
the chamber.
There seems to be a step in ratios when measuring inside and outside the cage. Outside the
cage the ratios seem to remain more or less the same, while all absolute intensities decrease.
The intensities for the emission lines of hydrogen and oxygen/argon plasma (appendix 4)
decrease outside the cage as well, this decrease is much steeper than for helium plasma
however. This indicates a difference in decay within the vacuum chamber for different types
of plasma, which might be related to the metastables present in helium plasma.
3.6 Difference in cleaning rate
3.6.1 Set up
There have been cleaning rate experiments performed on another device, but no cleaning rate
experiments have been done in the PATS yet. Cleaning rates are a good indicator on how
efficient the cleaning process is. To observe the cleaning rates and also the differences in
cleaning related to the orientation of the sample, two samples were placed inside the chamber
at the same time. The samples were on one side covered with a carbonlayer and the other side
was plane quartz.
One sample was placed facing towards the source and one sample was placed facing away
from the source. There were a few reasons for choosing these orientations. A first reason was
the long lifetime of metastable helium, which allows the metastables to fly through the
chamber long enough to reach the backside of the sample. A second reason was that
metastable helium can only relax to the ground state by colliding with another atom, that is
not helium. So, although the mean free path (the average distance covered by a particle
between successive impacts) is long, most of the collisions metastable helium (He*) has will
not result in a high loss of He* and will only deflect metastables in its path. This means that a
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
sample facing away from the source can be reached by the metastables through collisions with
other helium atoms. It was expected that the sample facing towards the source is cleaning
faster than the sample facing away from the source.
3.6.2 Results
The first cleaning rate experiment on the PATS is conducted using two quartz plates where
one side is coated with 125 nm carbon. The microwave source is operated at 350W input
power in continuous wave (CW) mode, at 0,5 mbar pressure. The samples, which have a
round shape, are positioned about 1 cm above the cage to which the plasma creation area is
limited. In figure 18 the placement of the samples can be observed.
Figure 18 The placement of the carbon samples relative to the source
The result was a cleaning rate of 0,66 nm/hour for the sample facing the source, for the
sample facing away from the source a rate of 1,34 nm/hour was observed (see table 3). The
result is not as expected so the experiment is repeated with samples containing 20 nm of
carbon using two different settings and at two different distances from the cage. First to repeat
the initial experiment and another setting to clean as is normally done by hydrogen cleaning:
1000 W input power at a 10% duty cycle, which means the plasma source is operational 10%
of the time and the average power input is 100 W. The results are given in table 4.
Table 4 Cleaning rate results in nm/hour
settings
Carbon layer
Distance to
(nm)
the cage (cm)
CW, 350 W
125
1
20
1
20
10
pulsed, 1000
20
1
W, 10% duty
20
10
facing towards
the source
0,66
5,21
2,65
1,01
<LDL
facing away
from the source
1,34
1,89
2,32
6,96
<LDL
The first repeating (with the same settings but with a 20 nm sample) experiment using
continuous wave (CW) shows higher cleaning rates when facing towards from the source,
which is in contradiction to the first experiment. The second experiment at a greater distance
shows comparable cleaning on both sides.
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Research on the surface interaction of metastable helium plasma
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The samples exposed to the pulsed plasma also show differences. The 1 cm, pulsed plasma
exposure again showed considerable higher cleaning rates for the sample facing away from
the source, while for the sample further away no cleaning rate was detected. This result is
puzzling, however it showed that reactive species can reach the backside of these samples.
The reason why the surface facing the source is cleaned with a lower cleaning rate is still not
understood.
At a distance of 10 cm a significantly lower cleaning rate is observed for pulsed plasma
compared to continuous wave plasma. The question that comes from this result is the
following; How far do ions and metastables travel with a certain type of plasma? These results
might indicate that the travelling distance in a pulsed plasma is much lower than in a
continuous wave plasma.
3.7 Sensor for helium metastables
3.7.1 Set up
The sensor as described in section 2.4 has been constructed. The distance of the sensor
relative to the cage containing the plasma creation area can be varied from 0,5 cm to 45 cm
away from the cage. The maximum distance is limited by the wall of the chamber. The
plasma type and pressure were different than those in the article of Miura and Hopwood10, so
this experiment was used to search for the optimal settings by which Ii=0, Ie=0, Iᵞ=0 and Itot=
IHe*. The potentials put on the probes were +25V on the outer probe and -35V on the inner
probe in an attempt to tune the photon current (Iᵞ). At a certain distance from the source the
signal should not change anymore, since there only the photon current should be detected.
The plasma creation area is then so far away that no ions etc. should be able to reach the
probe surface. The position of the sensor relative to the source was also varied from looking
directly at the source to completely looking away from it and facing the side to reduce the
amount of photons reaching the sensor surface. According to Miura and Hopwood the signal
should become a flat line when moved far enough away from the source.
The second part of the experiment was to change the voltages on the inner and outer probe in
order to sweep the settings and observe the differences. According to Miura and Hopwood10,
there should be a limit as to where the values change significantly. The potential on the outer
probe was changed from 0 V until +50V and the potential on the inner probe is changed from
-50 V to +15V. According to Miura and Hopwood there will be a flat part when the inner
potential is below -10V in combination with a positive potential on the outer probe since in
that region all ions and electrons are repelled effectively.
3.7.2 Results
During this research it is not yet accomplished to make Itot equal to IHe*. The results that are
accomplished are described in this section. The measurements for the first part of the
experiment are done with the sensor facing the source, facing away from the source and
facing sideways relative to the source. The positions are depicted in figure 19.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
1. facing towards and away from the source
2. facing sideways
Figure 19 The position of the sensor relative to the cage
When the sensor is facing away from the source or sideways there is no direct line of sight,
thus preventing any direct light on the sensor. The distance to the source is also a little larger
when facing sideways, but the measurements are taken at the same relative point for every
situation. The graphical results for the measurements are given in figure 20, the full results are
given in appendix 5.
current (nA)
He* sensor results at moving distance
(350 W, 0,5 mbar)
2500
facing towards source
2000
facing away from
source
1500
sideways to the source
1000
500
0
0
10
20
30
40
distance from source (cm)
50
Figure 20 He* sensor results at moving distance from the source
When zooming in on the results further away from the source (see appendix 5) it is observed
that the expected flattening of the signal is not present, the signal keeps decreasing. At a 5 to
15 cm distance from the source the curve shows a change in behaviour. This could indicate a
difference is gas-composition at different positions inside the chamber. This behaviour is not
an anomaly since the change in behaviour is also observed when the distance is scanned in
reverse. It can also be noted that the signal starts at a lower point when the sensor is facing
away from the source or sideways. This can be explained by the fact that the sensor in reality
is further away from the cage when facing sideways than the other two positions (see figure
21) and this difference in distance becomes less important when moving further away. This
does not explain why the signal is lower when the sensor is facing away from the source.
Another explanation is therefore that the sensor was cleaner when these experiments were
started.
This lead to the thought that the stabilisation of the sensor could be of influence on the results
obtained. Every time the vacuum chamber is opened air which holds water and carbon is
introduced into the chamber. To check whether it takes time to get a stable signal, the
chamber is opened for a while and then closed again. After the plasma is ignited and the
28
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
potentials are applied to the sensor, the signal is followed for a while. The results are depicted
in figure 21.
stabilising of the sensor (350 W, 0,5 mbar)
1400
current (nA)
1200
1000
800
600
400
200
0
0
1000
2000
3000
4000
5000
6000
7000
8000
time (sec)
Figure 21 Stabilisation of the He* sensor
The signal is not stable within a few minutes as can be noted from figure 22. An explanation
for the curve depicted is that in the first 6 minutes the water at the surface of the sensor is
being removed and after this the cleaning of the surface slowly starts. The signal is decreasing
in such a slow manner that after about 1000 seconds an accurate measurement can be
performed.
The results of the ion and electron shielding experiment are depicted in figure 22 and 23. In
figure 22 the results of the measurement at 1 cm distance from the cage and facing the source
are depicted and in figure 23 the results of the measurement at 1 cm distance from the cage
and facing away from the source are depicted. The results from the measurements at a
distance of 6 cm from the cage are given in appendix 5.
Changing the voltage of the sensor at 1 cm facing the source
(350 W, 0,5 mbar)
30
0V outer
25
+5V outer
20
+10V outer
current (nA)
15
+15V outer
10
+20V outer
5
+25V outer
0
+30V outer
+35V outer
-5
+40V outer
-10
+45V outer
-15
+50V outer
-20
-60
-50
-40
-30
-20
-10
0
10
20
inner probe (V)
Figure 22 voltage sweep facing the source at 1 cm distance
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Changing the voltage of the sensor at 1 cm facing away from
the source (350 W, 0,5 mbar)
8
current (nA)
6
4
0V outer
2
+5V outer
+10V outer
0
+15V outer
-2
+20V outer
-4
+25V outer
-6
+30V outer
-8
+35V outer
-10
+40V outer
-12
-14
-50
-40
-30
-20
-10
0
inner probe (V)
Figure 23 voltage sweep facing away from the source at 1 cm distance
The most important observation from the figures above is the fact that although there is not a
flat line below -10V on the inner probe, it is observed that at a positive inner probe voltage
and an increasing positive outer probe voltage the measured current becomes lower. When a
negative inner probe voltage is applied and an increasing outer probe voltage the measured
current becomes higher. This behaviour is noted for both distances.
This means that either the sensor is too big for the environment inside this vacuum chamber
or the potentials applied to the probes are not large enough.
When this experiment is to be repeated the following result (figure 24) will be the wished
result.
Figure 24. The wished result for the ion and electron shielding experiment
3.8 Metastable helium or not
3.8.1 Set up
The most important question to be answered was whether there are metastable helium atoms
present and if they are responsible for (part of) the cleaning that is observed. First of all, to
determine if there are metastables present a set up must be made that excludes everything
else. To exclude electrons and ions, the same principle as the He* sensor could be used. When
all electrons and ions are repelled only neutrals can reach the centre. To test this, a box was
created. This box has two integrated grids which can be kept at a certain potential (see figure
25). Appropriately chosen potentials should shield the sample from any electrons and ions.
30
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
The surface of the sample was grounded to prevent charging. In figure 26 the position of the
box is depicted.
The cleaning rate results from section 3.6.2 suggested that cleaning species might be able to
reach the bottom of the box and cause carbon removal (cleaning) of the sample.
Figure 25 Schematic view of the box
sample position
box
Figure 26 the box to check for metastable helium
31
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
3.8.2 Results
A sample is put on the bottom of the box (see figure 27), the carbon surface of the sample was
grounded to prevent charging and the box is mounted into the chamber as in figure 26. In
table 5 the results of the conducted experiments are given.
Figure 27 the alignment of the sample in the box
Table 5 cleaning results from the box
sample
Pressure
Remark
4562
0,1 mbar
4563
4564-1
0,1 mbar
0,5 mbar
4564-2
0,5 mbar
4564-3
0,1 mbar
Pump down
over night
N.A.
N.A.
Pump down
over night
cage removed
plasma mode
cleaning rate
(nm/hour)
pulsed, 1000 W, 10% duty
<LDL
continuous wave, 1000 W
continuous wave, 1000 W
<LDL
<LDL
continuous wave, 1000 W
<LDL
continuous wave, 1000 W
<LDL
The conditions for sample 4564-3 were different than for the other samples. When the cage is
removed from the plasma source the whole vacuum chamber becomes a plasma creation area.
At first the grids were connected to earth to measure base line cleaning. However, no
significant baseline cleaning has been observed at varying settings. The method was
abandoned due to lack of time.
32
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
4. Conclusions
The overall conclusion is that a definitive answer about the cleaning mechanisms of
metastable helium cannot be given at this point. There are some conclusions possible though
and they are shown here.
There are definitely differences between different types of plasma, which include the electron
density and electron temperature. The reactions to a change in settings or position within the
vacuum chamber are different for each type of plasma. For metastable helium the singlet and
the triplet states behave different as well when the settings are changed. This leads to the
conclusion that the plasma characteristics can be tuned and the spectrometer is an appropriate
way of monitoring them.
Spectrometer results in the power input and pressure dependency experiments also proved
that the preferred singlet metastable is most abundant at higher input powers and a low
pressure. These ratios are a good way of monitoring the processes inside the vacuum
chamber.
The emission lines of helium decrease much slower when moving the spectrometer away
from the plasma source than for hydrogen and oxygen/argon. This indicates a slow decay of
excited helium gas, therefore excited species can travel further away from the source than
other plasma types.
The cleaning experiments proved that cleaning is possible for samples without a direct line of
sight. This leads to the conclusion that there are species with a lifetime long enough to reach
the backside of something placed inside the chamber.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
5. Recommendations
It will remain difficult to distinguish between metastable cleaning and ion cleaning. There are
some recommendations for future research on this topic.
The cleaning properties of the system should be investigated and optimised. For this purpose
different gases such as helium, hydrogen and argon can be used. This ensures the use of both
a reactive gas (hydrogen) and an inert gas (argon) to compare the results of helium with.
Different settings have to be investigated as well, like the pressure inside the chamber, the
power input and the way of coupling power into the source, the location of the samples and
the orientation of the samples.
The possibilities for a metastable helium sensor (as in section 3.7) should be reinvestigated at
typical PATS settings, in order to design, construct and calibrate a new sensor. When this is
successful the experiments in section 3.7 can be repeated to observe the metastable helium
currents. The sensor can help to create an understanding of the behaviour of metastables at
different positions in the chamber and relative to the plasma source.
The Andor spectrometer should be attached to the PATS because of its large wavelength
range and sensitivity. Then it might be possible to see the metastables as well as the ions and
if they can be measured the ratios can be compared for different plasma conditions. This can
also be coupled to the transition probabilities to accurately determine the amounts present.
When a good and proven method to measure the amount of ions and metastables (singlet and
triplet separately) is created successfully, an attempt can be made to couple the cleaning rate
and heat load to these amounts. The cleaning efficiency can then be observed per species and
they can be tuned.
Finally an attempt can be made to understand the question as to why the cleaning rate is
sometimes larger for the sample facing away from the source than for the sample facing the
source.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
6. Acknowledgements
I learned a lot from the things encountered during this project. It made me aware of some of
the principles of vacuum technologies and plasma techniques, since they are completely
different from those normally encountered by me. I have also learned about equipment
problem solving, which was also new to me, since before all the equipment I worked with had
a service contract and no manual troubleshooting was required. The most important thing I
have learned though has to do with critically looking at experiments and results. This is
however a process that is not finished and I have some things to learn about this. After some
difficulties in the beginning, the project came to a proper end. A lot of the insights I gained
are a result of the guidance at TNO by René Koops, Norbert Koster, Edwin te Sligte, Peter
Bussink and also Timo Huijser, who always took the time to answer my questions. There are
a few other persons to thank at TNO since they made it possible for me to go to TNO cheerful
every day. First of all my roommate Elfi van Zeijl for all her fun and interesting stories and
Anshella Ramdin for her friendship. Besides this Roland van Vliet deserves a thank you as
well since he made it possible to do something about my dental problem during the time I was
part of his department. I would like to thank the rest of the department for the kind and warm
welcome as well as the friendly time I experienced during my time at TNO.
Besides the guidance at TNO, I would like to thank Wim Kok from the University of
Amsterdam for the guidance, support and opportunity to pursue the internship I wished to do.
35
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
7. References
1
2
3
4
5
6
7
8
9
10
11
12
http://en.wikipedia.org/wiki/Plasma_(physics), visited November 14th 2009.
A.C. Jones & M.L. Hitchman, Chemical Vapour Deposition; Precursors, Processes
and Applications, The Royal Society of Chemistry, Cambridge, 2009.
D.M. Mattox, Handbook of physical vapor deposition (PVD) processing, Film
formation, adhesion, Surface preparation and contamination control, Noyes
publications, New York, 1998.
http://www.yieldengineering.com/default.asp?page=264, visited November 13th 2009.
http://www.chromatography-online.org/topics/helium.html, visited November 13th
2009.
W. Sesselmann, B, Woratschek, J. Kuppers, G. Ertl, H. Haberland, Interaction of
metastable noble gas atoms with transition-metal surfaces: Resonance ionization and
auger neutralization, Physical review B, 1987, 35(4), 1547-1559.
J. H. Hetherington, The Spectrum of Helium and Calcium, E. Lansing MI, 2000, 1, 16.
J.J. Brehm, W.J. Mullin, Introduction to the structure of matter, a course in modern
physics, John Wiley & Sons, New York, 1989.
Hiden analytical, Plasma Diagnostics Introduction to Langmuir Probes, Technical
Information Sheet 531.
Naoto Miura & Jeffrey Hopwood, Metastable helium density probe for remote
plasmas, Review of scientific instruments, 2009, 80, 113502-1 until 113502-5.
Andor Technology, Mechelle manual, users guide, 2008.
Hamamatsu Photonics, HARDWARE INSTRUCTION MANUAL, K29-B60419B,
version 1.2.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Appendices
Appendix 1
Observations obtained from literature:
Overall conclusions:
The overall conclusion that can be drawn from the papers found is that a lot of researchers
observed the effect of helium cleaning, but nobody has determined the exact nature of the
mechanism involved. However there are some other useful conclusions to be drawn from the
articles, which are given here.
One article suggested that the cleaning by helium is done by metastable helium radicals
reacting with the surface, thereby reducing the surface impurities.
When helium plasma is ignited in a fusion reactor (which is a high energetic plasma) the
following is observed; Hydrogen rapidly appears when the discharge is started and slowly
falls off, disappearing completely when the discharge is stopped. The main source for this
hydrogen suggested by the author is adsorbed water but according to some other authors this
could also be released from the vessel wall. The following is also observed; Hydrogen and
helium are both present during plasma cleaning, even if they are not present in the gas used to
produce the plasma.
Accumulation of helium by the vessel wall is influenced by the wall material itself. Graphite
holds less helium than stainless steel, resulting in a lower release during plasma treatment.
No correlation is found between helium pressure within a stainless steel vessel and the gas
species of the main plasma. During experiments a net loss of helium particles is observed and
the main source of helium desorption is the stainless steel wall.
Non-oxygen plasma leads to higher oxygen and nitrogen levels on the surface being cleaned.
Storage has distinct effects on the surface composition, which is depending on the plasma
type used.
Relevant quotes from articles including reference;
1. Shortening the pump-down time can be done by discharge cleaning. When applied just
after the start of the pump-down, the pumping time can be reduced by one fourth. The
final pressure is not changed by this procedure.
2. When activating helium plasma a strong emission line of neutral hydrogen is observed,
although no hydrogen is introduced into the vacuum. When the plasma is fully operational
the intensities of the emission lines of helium are increased as well. In the final stage of
the discharge the emission lines of helium are rather strong.
The same can be seen for the other situation, when hydrogen plasma is used, emission
lines for helium are visible.
3. A SEM image after helium sputtering of the surface of a tungsten sample shows no
blistering or bubbles. From EDX analysis it was found that the deposition layer on the
surface contains trace amounts of iron (1.3 at.%). Oxygen was not detected. Using a
quadrupole the release of helium was followed. It was revealed that some amounts of
helium were incorporated into the deposition layer though helium is an inert gas.
The quadrupole was used to investigate the presence of hydrogen during the helium
plasma discharge. The measurements revealed the following;
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
-
4.
5.
6.
7.
When the plasma discharge begins, hydrogen rapidly appears and then slowly falls off.
When the discharge was turned off, hydrogen disappears immediately.
The main source of the hydrogen is guessed to be adsorbed water.
The trapping mechanism of helium and hydrogen in tungsten might be the same when
looking at respectively helium and hydrogen plasma. The release curves look exactly
the same. The hydrogen curve in helium plasma however looks different. This is
believed to be due to the differences in concentration distribution in the layer.
- The H/W ratio in hydrogen plasma is estimated around 0.15. The He/W and H/W ratio
in helium plasma is estimated around 0.08 and 0.075 respectively. (He+H)/W ratio is
0.155, which is very close to the value of H/W in hydrogen plasma. When hydrogen
emission during helium plasma discharge can be repressed, the values might be equal.
- Hydrogen isotope and helium retention in the layer is considered to depend on ion
flux, tungsten atom flux, incident energy and temperature. Therefore, there is a
possibility that some amounts of the layers formed in the reactor contain a large
amount of hydrogen isotopes and helium.
The erosion rate of tungsten by helium plasma sputtering was 1.8 times larger than that by
hydrogen plasma sputtering. This is due to the difference in weight between He+ and H2+.
When comparing accumulation in stainless steel and in graphite, differences can be
observed for helium as well as for hydrogen. When increasing the temperature using a
stainless steel wall no change in accumulation is visible, but by changing to graphite the
accumulation of hydrogen is cut in half and helium is even reduced to 1/3. This results at
high temperatures in a reduction of helium release to 1/6.
The results indicate that stainless steel accumulates helium gas several times more
compared with graphite by plasma irradiation under helium glow discharge cleaning
(GDC), which suggests that the helium behaviour during plasma confinement experiment
in LHD originates from the stainless steel used as the first wall of the vacuum vessel.
By using helium gas, radicals are introduced on the surface which react and thereby
reduce the impurities on the surface.
Nitrogen and oxygen tend to associate to form amide groups on polymer surfaces.
The effect of storage on the oxygen and nitrogen concentration using different plasmas;
- Polystyrene, helium plasma; A slight increase in nitrogen (2,5 instead of 2%) and a
marked increase in oxygen concentration (14 instead of 10%) is seen after storage.
- Polystyrene, 33% nitrogen/ 67% hydrogen plasma; The nitrogen concentration
decreased slightly over time (9 instead of 15%), while the oxygen concentration
increased (15 instead of 9%). The oxygen is present in single, double and multi bonds.
- Polystyrene, 10% oxygen/ 90% helium; There seems to be an apparent loss of oxygen
(13 instead of 16%) and a slight increase in nitrogen (1 instead of 0,5%).
- Polystyrene, 10% oxygen/ 90% helium followed by 33% nitrogen/ 67% hydrogen; A
loss of nitrogen (8 instead of 15%) and an increase in oxygen concentration is seen (13
instead of 8%).
- Polystyrene, 33% nitrogen/ 67% hydrogen followed by 10% oxygen/ 90% helium;
The oxygen concentration is initial a little higher than for the previous experiment
(14%), but is stable during storage, the nitrogen concentration appears to decrease (6
instead of 12%).
- Polyethyleneterephthalate, 10% oxygen/ 90% helium followed by 33% nitrogen/ 67%
hydrogen; The oxygen concentration is stable but high at 25%, the nitrogen
concentration seems to decrease after storage (7 instead of 15%).
- Polyethyleneterephthalate, 33% nitrogen/ 67% hydrogen followed by 10% oxygen/
90% helium; The oxygen concentration decreases a little bit to 25%, the nitrogen
concentration decreases as well (9 instead of 11%).
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
For polystyrene, oxygen being present in the plasma leads to higher initial concentration,
but the levels decrease over time. When there is no oxygen present the opposite happens.
The concentration nitrogen seems to behave the same but to a lesser extent.
8. At the prevailing plasma densities, the effects of metastable states of helium on the line
intensities would be overestimated if the metastable states are assumed to be in a
quasisteady state (i.e. the population determined entirely by the ground-state populations).
9. Since the minimum peak-to-peak RF voltages lie around 200 V it is easiest in helium gas
to achieve breakdown. As the voltage and the power are increased, an increase of the light
emission intensity is observed, followed by an expansion of the plasma volume.
Generally, mixtures of gases require a higher voltage to ignite and sustain the plasma, the
same goes for air and mixtures including air.
Pure helium is characterised by the lowest excitation temperature at a given power input.
Slight temperature decrease is observed at higher power levels. Regarding the electron
temperature the same behaviour is observed in low pressure discharges.
10. No correlation is found between helium pressure and the gas species of the main plasma
experiment. During the main experiments, a net loss of helium particles is observed. There
is a difference between the amount of atoms calculated and measured, which is mostly
explained by implantation in the wall. In helium plasma experiments, about half of the
inlet helium is implanted into the wall, even though the wall is saturated with helium
atoms by the He GDC. The main source of helium gas desorption in the LHD is also the
stainless steel wall, since the area of stainless steel is much larger than that of the graphite.
Literature references:
1.
K. Akaishi et al. 1997, production of ultrahigh vacuum by helium glow discharge
cleaning in an unbaked vacuum chamber, Vacuum, 48, no. 7-9, 767-770.
2.
M. Goto et al. 2003, Determination of the hydrogen and helium ion densities in the
initial and final stages of a plasma in the Large Helical Device by optical spectroscopy, ,
Physics of Plasmas, vol. 10, no. 5, 1402-1410.
3.
K. Katayama et al. 2007, Helium and hydrogen trapping in tungsten deposition layers
formed by helium plasma sputtering, Fusion engineering and design, 82, 1645-1650.
4.
Y. Kubota et al. 2003, Investigation of the trapped helium and hydrogen ions in plasma
facing materials for LHD using thermal desorption spectrometer and alternating glow
discharge cleanings, Journal of nuclear materials, 313-316, 239-244.
5.
S. Marais et al. 2005, Unsaturated polyester composites reinforced with flax fibers:
effect of cold plasma and autoclave treatments on mechanical and permeation
properties, Composites Part A: Applied science and Manufacturing, 36 issue 7, 975-986.
6.
R.W. Paynter 1998, XPS studies of the modification of polystyrene and
polyethyleneterephthalate surfaces by oxygen and nitrogen plasmas, surface and
interface analysis, 26, 674-681.
7.
R. W. Paynter 2000, XPS studies of the ageing of plasma treated polymer surfaces,
Surface and interface analysis 29, 56-64.
8.
R. Prakash et al. 2005, Characterization of helium discharge cleaning plasmas in
ADITYA tokamak using collisional-radiative model code, Journal of Applied Physics,
97, 043301-1-043301-7.
9.
E. Stoffels et al. 2002, Plasma needle: a non-destructive atmospheric plasma source for
fine surface treatment of (bio) materials, Plasma Sources Science and Technology, 11,
383-388.
10. H. Suzuki et al. 2003, Behavior of helium gas in the LHD vacuum chamber, Journal of
nuclear materials, 313-316, 297-301.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Appendix 2
Setting up the system
Fourier transformation of a scan using the microwave source;
A moving average is put over the data first to exclude random noise as much as possible.
After this a Fourier transformation (standard tool in the data analysis tool) is performed on
each scan separately. The scans are then put in a plot and compared. 10 scans are taken but
putting all scans in one plot is confusing, so 5 scans are put in a plot at the same time (figure
28 and 29).
after averaging scan 1-5
0,0014
amplitude
0,0012
scan 1
0,001
scan 2
0,0008
scan 3
0,0006
scan 4
0,0004
scan 5
0,0002
0
0
0,5
1
1,5
2
1/Voltage
Fig. 28 The Fourier signal from scan 1 to 5
after averaging scan 6-10
0,0014
amplitude
0,0012
scan 6
0,001
scan 7
0,0008
scan 8
0,0006
scan 9
0,0004
scan 10
0,0002
0
0
0,5
1
1,5
2
1/Voltage
Fig. 29 The Fourier signal from scan 6 to 10
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Research on the surface interaction of metastable helium plasma
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From these results it is clear that there is some kind of interference. The distance between the
peaks is the same and the signal decreases by the same amount with each peak. Since the
sampling speed of the Langmuir probe is not exactly known, the frequency cannot be
calculated but the voltage change is known so the frequency is set as 1/V.
Then the power supply was tried. In the manual it is mentioned that the power for the
computer as well as the probe have to be drawn from the same phase. However, the signal is
not very good so another power supply for only the computer as well as all equipment is tried.
In figure 30 the normal situation is given and in figure 31 the result from the different power
supply is depicted. There is only one trace in figure 31 since the result for both attempts gave
the same result.
current
normal power supply
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
-0,002
-60
-40
-20
0
V
20
40
60
40
60
Fig. 30 Langmuir results using the normal power supply
current
different power supply
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
-0,002
-60
-40
-20
0
V
20
Fig. 31 Langmuir result using another power supply
The signal only gets worse and a repeating wave becomes visible. It is therefore not a good
idea to switch to another power supply. Everything is connected to the original power supply
again.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Until now the microwave source was used in a pulsed mode, but it is also possible to use the
microwave source in continuous wave mode or to use an RF source to create plasma. Using
all modes a scan is made and they are plotted in figure 32 to see the possible differences.
plasma modes compared
Rf source
current (nA)
pulsed mode
0,004
0,0035
0,003
0,0025
0,002
0,0015
0,001
0,0005
0
-0,0005
continuous wave
-50
-30
-10
10
30
50
voltage (V)
Fig. 32 the different plasma modes compared
The continuous wave mode gives the best results since there are no interferences and the
power put into the system is handled the most efficient. The signal is also much higher, which
makes calculations easier for the software.
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Research on the surface interaction of metastable helium plasma
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Appendix 3
plasma dependence on power and pressure parameters
Power dependence
The other parameters determined by the Langmuir probe are the floating plasma potential
(Vf), the electron temperature (Te) in eV and the ion flux. These results are given in figure 33.
a) Vf in volts (0,5 mbar)
b) Te in eV (0,5 mbar)
40
4,0
3,5
30
3,0
2,5
O2_Ar
10
He
H
0
Te (eV)
Vf (V)
20
O2_Ar
2,0
He
1,5
H
1,0
350 W
500 W
700 W
0,5
-10
0,0
350 W
-20
W input power
500 W
W input power
700 W
c) ion flux (0,5 mbar)
ion flux
1,00E+20
1,00E+19
O2_Ar
He
1,00E+18
1,00E+17
350 W
500 W
700 W
W input power
Figure 33 a) floating plasma potential, b) electron temperature (eV), c) ion flux at different
input powers
The most interesting result depicted in figure 33 is the fact that the floating potential for
hydrogen (green trace) is positive, while the potential for helium (red trace) and oxygen/argon
(blue trace) is predominately negative.
When the I-V characteristics could not be determined from the curve a value is missing.
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The results for the spectrometer measurements are given in table 6, 7 and 8 for respectively
helium, hydrogen and oxygen/ argon plasma. From these tables ratios can be calculated as
well as absolute differences can be seen. Using this data an interpretation can be made as to
the changes resulting from a change in environmental characteristics.
Table 6 Peak heights helium plasma
Peak (nm)
350 W
500 W
319
3065
3444
389
24273
26684
447
11576
13459
492
3311
3835
502
8528
9840
588
50466
58643
668
14420
18057
707
10965
11913
728
2155
2367
700 W
3824
29410
15113
4295
11081
64668
20580
13154
2627
Table 7 Peak heights hydrogen plasma
Peak (nm)
350 W
500 W
434
5086
5737
486
21118
23653
656
64664
64666
700 W
7037
30496
64666
Table 8 Peak heights oxygen/ argon plasma
Peak (nm)
350 W
500 W
697
4412
5852
707
6123
7186
737
9042
12248
750
25977
32861
763
25923
34982
772
8161
10946
777
20025
30265
795
7590
10166
801
8762
11787
811
38245
52315
826
4583
6026
840
10183
13700
842
15496
20952
700 W
5950
7305
12481
33194
35700
11149
31747
10392
12023
53475
6148
13916
21372
For all types of plasma an increase in signal can be observed with increasing power input.
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Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Pressure dependence
The other parameters determined by the Langmuir probe are the floating plasma potential
(Vf), the electron temperature (Te) in eV and the ion flux. These results are given in figure 34.
a) Vf in volts (350 W)
b) Te in eV (350 W)
50
4,0
40
3,5
3,0
30
He
10
H
Te (eV)
Vf (V)
2,5
O2_Ar
20
O2_Ar
2,0
He
1,5
H
1,0
0
0,2
0,5
1
0,5
-10
0,0
0,2
-20
mbar pressure
0,5
mbar pressure
1
c) ion flux (350 W)
3,00E+19
ion flux
2,50E+19
2,00E+19
O2_Ar
1,50E+19
He
1,00E+19
5,00E+18
0,00E+00
0,2
0,5
mbar pressure
1
Figure 34 a) floating plasma potential, b) electron temperature (eV), c) ion flux at different
pressures
The most interesting result depicted in figure 34 is the fact that for helium (red trace) all
characteristics show a decrease with increasing pressure.
When the I-V characteristics could not be determined from the curve a value is missing.
The results for the spectrometer measurements are given in table 9, 10 and 11 for respectively
helium, hydrogen and oxygen/ argon plasma. From these tables ratios can be calculated as
well as absolute differences can be seen. Using this data an interpretation can be made as to
the changes resulting from a change in environmental characteristics.
Table 9 Peak heights helium plasma
Peak (nm) 0,2 mbar
0,5 mbar
319
2949
3065
389
19281
24273
447
9085
11576
492
2982
3311
502
7187
8528
588
30295
50465
668
8510
15198
707
8243
10965
728
1990
2155
1 mbar
3127
25943
12345
3376
8752
61947
17879
12776
2292
- ix -
Research on the surface interaction of metastable helium plasma
Table 10 Peak heights hydrogen plasma
Peak (nm) 0,2 mbar
0,5 mbar
434
5292
5086
486
19351
21118
656
53708
64664
Marij Stoevenbelt
1 mbar
3479
12315
40805
Table 11 Peak heights oxygen/ argon plasma
Peak (nm) 0,2 mbar
0,5 mbar
1 mbar
697
5090
4412
913
707
6261
6123
566
737
10528
9042
1336
750
23896
25977
4466
763
30639
25923
3071
772
9588
8161
1260
777
26489
20025
1254
795
8886
7590
1189
801
10647
8762
1074
811
49771
38245
3582
826
5248
4583
929
840
11508
10183
1424
842
18418
15496
1909
From table 9 to 11 it can be derived that for helium the intensity increases with increasing
pressure, while the opposite goes for oxygen/argon and hydrogen. This means that the gases
react different on a change in pressure inside the chamber.
-x-
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Appendix 4
Differences at different positions
The other parameters determined by the Langmuir probe are the floating plasma potential
(Vf), the electron temperature (Te) in eV and the ion flux. These results are given in figure 35.
a) Vf (350W, 0,5 mbar)
b) Te in eV (350W, 0,5 mbar)
30
7,0
25
6,0
5,0
O2_Ar
15
He
H
10
Te (eV)
Vf (V)
20
O2_Ar
4,0
He
3,0
H
2,0
5
1,0
0,0
0
middle
upper
above
middle
position towards the cage
upper
above
position towards the cage
c) ion flux (350W, 0,5 mbar)
1,00E+22
ion flux
1,00E+21
O2_Ar
1,00E+20
He
1,00E+19
H
1,00E+18
1,00E+17
middle
upper
above
position towards the cage
Figure 35 a) floating plasma potential, b) electron temperature (eV), c) ion flux
The biggest observation to be made from these results is the fact that the floating potential
decreases a little bit when moving away from the source although still inside the cage, but the
electron temperature decreases rapidly within the same step.
- xi -
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
In table 12 to 14 the results are given for the peak heights, determined at different positions of
the spectrometer relative to the cage containing the plasma. When observing the plasma
through the antenna, the fiber was placed on the outside of the vacuum chamber.
Table 12 Peak heights helium plasma
Peak
Through the
Middle of the
(nm)
antenna
cage
319
27734
5742
389
64433
47838
447
64607
25093
492
33277
8321
502
64543
21900
588
64300
64503
668
64418
45977
707
64436
53771
728
31100
10852
Just outside
the cage
3907
15542
13234
6727
13189
47290
19918
20891
7328
Half height
above the cage
3749
10948
10241
5860
9656
34913
14922
16063
6362
Table 13 Peak heights hydrogen plasma
Peak
Through the
Middle of the
(nm)
antenna
cage
434
2436
6818
486
7014
9555
656
20188
23407
Just outside
the cage
3563
3508
3297
Half height
above the cage
3836
3544
3299
Table 14 Peak heights oxygen/ argon plasma
Peak
Through the
Middle of the
(nm)
antenna
cage
697
7528
12344
707
9175
14173
737
15339
23392
750
43193
49063
763
43859
59760
772
13030
21775
777
34837
41003
795
12266
21374
801
14090
24237
811
60611
64570
826
6998
13994
840
16129
25886
842
24686
37773
Just outside
the cage
4193
4039
4711
7100
7609
4717
7550
4638
4654
9048
4266
3764
4632
Half height
above the cage
4058
3961
4354
6111
6578
4403
6407
4344
4306
7630
4116
3373
4017
The most important observation is the fact that the emission intensities decrease slower for
helium than for hydrogen and oxygen/argon plasma.
Helium plasma has the most intense lines directly looking through the antenna into the
plasma. This measurement cannot be compared directly to the other results since these
measurements are taken perpendicular to the other positions. The differences between these
measurements and the other positions are interesting to observe.
- xii -
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Hydrogen and oxygen/ argon plasma have the most intense emission lines just above the
plasma, but still inside the cage. The decrease observed when measuring the spectral lines
outside the cage instead of inside is drastically. For the most intense lines the signal decreases
by a factor 8 instantly. Once outside the cage the decrease is minimal, indicating some sort of
steady state. This leaves room for more than one conclusion, since it can mean that the
effectiveness of the plasma is minimal, but also that the plasma is still effective with no
difference between different positions in the chamber.
- xiii -
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Appendix 5
Sensor for helium metastables
In table 15 the full results are shown for the measurements done using the sensor. These data
are processed in figure 21 in section 3.7.2.
Table 15 full results of the moving He* sensor
distance from
sensor facing
sensor facing away
the source
towards the source
from the source
(cm)
(nA)
(nA)
0,5
1930,5
709,5
1
1955,5
689
1,5
1839,5
645
2
1716,5
605
2,5
1585,5
569
3
1431,5
536
4
1187,5
482
5
1039,5
432
6
947
388,5
7
880
346,5
8
822
306
9
749,5
270,5
10
676,5
235
11
599,5
202
12
520,5
173
13
445
144,5
14
376,5
118,5
15
311
95,4
16
254,5
74,95
17
204,5
57,75
18
161
42,55
19
125
31,5
20
96,05
22,65
21
72,2
16,1
22
55,6
11,6
23
42,05
8,865
24
33,15
7,265
25
27,2
6,395
26
23,9
5,75
27
21,55
5,175
28
19,7
4,685
29
17,9
4,26
30
16,5
3,875
31
15,2
3,54
32
14
3,235
33
13
2,96
sensor facing
sideways
(nA)
967
974,5
987
1000
1024,5
*
*
1055,5
*
*
*
*
843,5
*
*
*
*
472,5
404,5
350
292,5
246,5
202,5
168
136,5
110
90,05
71,05
54,8
42,85
33,4
26,8
21,7
19,05
17,25
15,7
- xiv -
Research on the surface interaction of metastable helium plasma
distance from
the source
(cm)
34
35
36
37
38
39
40
41
42
43
44
45
sensor facing
towards the source
(nA)
11,95
11,15
10,4
9,705
9,025
8,415
7,875
7,37
6,88
6,41
5,955
5,495
sensor facing away
from the source
(nA)
2,715
2,495
2,28
2,08
1,9
1,715
1,56
1,39
1,23
1,095
0,93
0,785
Marij Stoevenbelt
sensor facing
sideways
(nA)
14,6
13,5
*
*
*
*
9,395
*
*
*
*
6,94
(*) not measured
As said in section 3.7.2 when observing the part furthest away from the source it can be noted
that the decrease of the signal continues instead of flattening off. This is shown in figure 36.
current (nA)
zoomed in furthest away from the source
80
facing the source
70
facing away
60
facing sideways
50
40
30
20
10
0
20
25
30
35
40
45
50
distance from the source (cm)
Figure 36 zoomed at the part furthest away from the source
In figure 37 the results of the measurement at 6 cm distance from the cage and facing the
source are depicted and in figure 38 the results of the measurement at 6 cm distance from the
cage and facing away from the source are depicted.
- xv -
Research on the surface interaction of metastable helium plasma
Marij Stoevenbelt
Changing the voltage of the sensor at 6 cm facing the source
(350 W, 0,5 mbar)
2
0
0V outer
current (nA)
+5V outer
-2
+10V outer
+15V outer
-4
+20V outer
+25V outer
-6
+30V outer
+35V outer
+40V outer
-8
-10
-50
-40
-30
-20
-10
0
inner probe (V)
Figure 37 voltage sweep facing the source at 6 cm distance
Changing the voltage of the sensor at 6 cm facing away from
the source (350 W, 0,5 mbar)
2
1
oV outer
0
+5V outer
current (nA)
-1
+10V outer
-2
+15V outer
-3
+20V outer
-4
+25V outer
+30V outer
-5
+35V outer
-6
+40V outer
-7
-8
-50
-40
-30
-20
-10
0
inner probe (V)
Figure 38 voltage sweep facing away from the source at 6 cm distance
At 6 cm distance the same trend is seen as for a distance of 1 cm. This implies that the
shielding is not sufficient.
- xvi -