Characterization of a slot antenna microwave plasma source for hydrogen
plasma cleaning
D. Korzec, F. Werner, A. Brockhaus, and J. Engemann
Microstructure Research Center, University of Wuppertal, 42287 Wuppertal, Germany
T. P. Schneidera) and R. J. Nemanich
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202
~Received 31 August 1994; accepted 18 February 1995!
A new large volume microwave plasma source has been used for the production of a hydrogen
plasma. The source consists of an annular waveguide cavity with axial slots on the inner side which
acts as a field applicator to sustain a plasma at 2.45 GHz. The plasma is contained in a fused silica
bell jar of 16 cm in diameter and 20 cm in height. The distance between the slots corresponds to a
waveguide wavelength. The source is able to generate a highly dissociated ~up to 90%! hydrogen
plasma for cleaning purposes. Stable operation of the plasma source is shown for a pressure range
of 0.1–1.3 mbar and a power range of 600–2000 W. The plasma can be ignited over the entire
examined pressure range, and the power needed for discharge ignition is below 1.7 kW. The
minimum ignition power is 1050 W for a pressure of 0.7 mbar. A double Langmuir probe and optical
emission spectroscopy were used to characterize the hydrogen plasma as a function of microwave
power, pressure, and position. The results indicated a typical ion density of 1.531011 cm23 which
is an order of magnitude less than that obtained for argon under similar conditions. The typical
electron temperature is 2.5 eV for microwave power of 2 kW and pressure of 0.7 mbar. © 1995
American Vacuum Society.
I. INTRODUCTION
Semiconductor surface cleaning which does not introduce
damage and stress is crucial for continued advancements in
ultra-large-scale integration ~ULSI! processing.1 The standard methods for semiconductor surface cleaning are different wet2 or ultraviolet ~UV!-ozone based3 procedures. New
technologies based on clustered approaches will require in
situ surface preparation processes. Furthermore, the integration of entire integrated circuit processing in an ultrahigh
vacuum facility ~sample preparation, focused ion beam implantation, maskless etching, molecular beam epitaxy, surface analysis! may be possible.4 These approaches will require vacuum compatible cleaning or surface preparation
processes. In this study we explore the use of a new plasma
source which can be scaled for large area single wafer systems.
For ultrahigh vacuum ~UHV! surface characterization and
molecular-beam epitaxy ~MBE!, surface preparation techniques often employ high temperature annealing. One
method of Si surface preparation under UHV conditions is an
UV flash of the substrate at a temperature greater than
1000 °C.5 Alternatively, the combination of a wet chemical
treatment and UHV annealing to temperatures of 850 °C can
be used to obtain atomically clean surfaces.2 The diffusion of
impurities and the degradation of existing oxide structures is
a limitation of these techniques. Attempts were made to
clean Si and GaAs surfaces at much lower temperatures with
Ar or Xe low energy ion bombardment.6 For large wafer
sizes, the generation of such ion beams requires sophisticated
equipment. Similar cleaning effects may be achieved with
much simpler plasma sputtering systems.7 The problem with
a!
Present address: SEMATECH, 2706 Montopolis, Austin, TX.
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J. Vac. Sci. Technol. A 13(4), Jul/Aug 1995
all techniques involving ion bombardment is the induced
crystal structure disorder and the ion incorporation. While
high temperature annealing may remove some aspects of the
disorder and the impurities, defects usually remain.
The suppression of some of these effects can be expected
when using hydrogen ions instead of noble gas ions. Previous studies of surface preparation have explored hydrogen
ion bombardment from an ion source,8 from an electron cyclotron resonance ~ECR! plasma polarized source,9 or from a
plasma beam extracted by magnetic fields.10 The low pressure hydrogen ion beam process exhibits low removal rates,
i.e., 1.5 nm/min for SiO2 at 300 eV and 500 °C substrate
temperature. In comparison, for argon ions under similar
conditions a removal rate of 15 nm/min is obtained.11,12 In
spite of the reduced energy transfer, surface damage resulting
from hydrogen ion bombardment has been reported13 for a
hydrogen ion beam generated by a capacitively coupled rf
ion source.14 A plasma process with a reduced ion energy and
enhanced chemical component could potentially reduce or
eliminate this problem.
Good quality low damage GaAs surface cleaning was
achieved using atomic hydrogen generated by thermal dissociation of hydrogen molecules at the mbar pressure range.15
One limitation of the method employed is the use of a hot
filament in front of the sample to produce atomic hydrogen.
The hot filament can result in excessive surface radiative
heating, leading to thermal desorption of As,16 and impurity
incorporation due to evaporation from the filament. Application of the technique to silicon could also result in dopingimpurity diffusion. The results, however, demonstrate the
need for a surface preparation process with high chemical
activity, controllable thermal load, low ion bombardment,
and operation in the mbar pressure range.
0734-2101/95/13(4)/2074/12/$6.00
©1995 American Vacuum Society
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Korzec et al.: Characterization of a SLAN plasma source
There have been numerous applications of hydrogen
plasma based surface preparation and cleaning processes for
different semiconductor applications. Low temperature epitaxial silicon growth can be obtained by MBE or chemical
vapor deposition ~CVD! on hydrogen terminated silicon
surfaces.17–19 Remote hydrogen plasma cleaning of silicon
surfaces prior to low temperature silicon dioxide deposition
can be used to achieve low defect density silicon–silicon
dioxide interfaces.20–23 Similar improvement can be
achieved prior to nitridation.24 High quality metalorganic
chemical vapor deposition ~MOCVD! growth of GaAs on Si
has been demonstrated after hydrogen plasma cleaning of the
Si.25 Passivation of Zn acceptors26 and other surface changes
in InGaAs27 was examined. Neutralization of shallow donor
levels in GaAs from Ge, Sn, S, Se, and Te28 and changes of
the work function, electron affinity, and band bending of decapped GaAs29 has been reported. Remote hydrogen plasma
exposure was successfully used for cleaning of the Ge
surface.30,31 Recent results have demonstrated that H-plasma
surface preparation of diamond can result in a negative electron affinity.32,33
Some common methods of generating a hydrogen plasma
employ filaments of some type. For example, a large family
of arc discharges and other sources were developed for
atomic hydrogen generation.34 –39 The major drawback of all
of these methods is the hot filaments used for electron emission. The filaments require frequent replacement, and filament evaporation can cause contamination of the process.
More useful for semiconductor applications are the filamentless methods for plasma generation. Plasma excitation
with different frequencies can be used for this purpose.
Chang et al. have used 30 MHz inductively coupled excitation for the generation of a hydrogen plasma in a quartz
chamber for etching of semiconductors and their oxides.40
Gao used 13.56 MHz excitation enhanced with an axial magnetic field for the production of an atomic hydrogen beam.41
Hodgson used the frequency of 200 MHz for atomic hydrogen generation.42 The microwave frequency of 2.45 GHz and
the rf frequency of 13.56 MHz have been broadly used for
cleaning purposes.30,43
Many different architectures of the cleaning discharge can
be employed. Goodyear and von Engel44 have used an inductively coupled discharge for generation of ionized and dissociated hydrogen. Remote plasma processing can be used to
reduce possible cross-contamination and the contamination
due to chemical erosion of the discharge chamber walls.45
Remote plasma excited hydrogen46,47 also can achieve a reduction of plasma damage or other deleterious effects on the
surface morphology. Hershcovitch used a radio frequency
~rf! excited hollow cathode discharge for H production.48 Another very efficient method of hydrogen plasma generation is
a multipolar microwave discharge.49 The drawback of this
technique is that a plasma chamber made of stainless steel
was used, causing high wall recombination losses, and hence
a low dissociation coefficient.50 The wall recombination can
be reduced with a quartz plasma chamber. A very efficient
microwave discharge can be produced from a system which
employs a unique multipole plasma excitation with a chamber that is constructed of quartz. The system is termed the
JVST A - Vacuum, Surfaces, and Films
2075
FIG. 1. Schematic diagram of the slot antenna plasma source: ~a! axial cross
section, ~b! radial cross section.
slot antenna ~SLAN! plasma source.51 The characterization
of this plasma source for semiconductor processing is the
subject of this work.
II. SETUP
A. Vacuum system
The plasma system is mounted directly to a stainless steel
chamber. This vacuum chamber is cylindrical in shape ~40
cm in diameter and 60 cm in height!, and is pumped with a
Rootsblower with a pumping speed of 500 m3/h and a rotary
backing pump with a pumping speed of 60 m3/h. The gas
supply is controlled by mass flow controllers ~MKS!. Pressure measurements are made with a gas independent pressure
transducer ~baratron! from 10 mbar to 1021 mbar and with an
ionization gauge for pressures below 1021 mbar. The plasma
source is attached to the vacuum chamber via a 250 ISO–K
flange.
One of the main components of the plasma source is the
quartz bell jar @Fig. 1~a!# used as the plasma chamber. The
dimensions are: diameter of 160 mm and height of 200 mm.
The quartz chamber is vacuum sealed to the stainless steel
base flange with a Viton O-ring. To avoid microwave leakage
from the plasma chamber and to allow for efficient microwave power coupling, the quartz chamber is enclosed in a
metal casing consisting of a cooling ring, the inside wall of
the cavity ring, the walls of the fan assembly, and fan assembly grid.
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Korzec et al.: Characterization of a SLAN plasma source
In this study, the SLAN has been operated with hydrogen
in the range 1022 –1 mbar without external magnetic fields.
An ECR version with multipolar permanent magnet arrangement for pressure ranging up to 1024 mbar will be a subject
of a separate publication.
B. Microwave coupling
The SLAN microwave applicator consists of an annular
waveguide ~ring cavity! with slot antennas on the inner side
@Fig. 1~b!#. The diameter of the ring cavity was chosen to
develop a standing electromagnetic TE10 wave. The circumference of the middle line of the annular waveguide is five
waveguide wavelengths l g @see Fig. 1~b!#. At the inner side
of the ring cavity there are five resonant coupling slots, onehalf of a free-space wavelength long ~6.12 cm!, equally
spaced azimuthally, and axially directed, i.e., perpendicular
to the direction of wave propagation inside the waveguide.
The distance of the capacitive coupler to the nearest slot is
l g /4 @see Fig. 1~b!#. The standing wave nodes ~E-field
minima! correspond to positions of the coupling slots. Hence
the slots cut the lines of maximum current flow and radiate in
equal phase since they are spaced at a distance of l g . Each
of the slot antennas couples part of the microwave energy
into the plasma. Aspects of the electromagnetic field in the
source are described in more detail elsewhere.51
The microwaves are coupled into the ring cavity with an
adjustable coupling probe ~copper rod with diameter of 8
mm! which is positioned transversally through the input
waveguide into the annular waveguide. In order to match the
impedance of the plasma, the immersion depth of the adjustable coupling probe into the ring cavity can be varied @see
Fig. 1~b!#. Furthermore, the end position of the input waveguide is adjustable by means of a movable plunger. For all
the experiments described in this article, the reflected power
was less than 2% of the applied power.
The SLAN is powered by a 2000 W, 2.45 GHz microwave
generator MW-GIR 2M 130-2K-01 ~Muegge Electronics
GmbH!. The microwave power is supplied to the SLAN via
a circulator and a rectangular waveguide WR-340. Between
the generator and the plasma load, a three port circulator
directs the reflected power from the plasma into a watercooled dummy load to protect the magnetron from possible
damage.
C. Langmuir probe measurements
The plasma parameters are measured in this work with a
double Langmuir probe ~DLP!. The two probe tips are identical and are formed from tungsten wire ~0.5 mm in diameter
and 3.5 mm in exposed length!. The probe can be moved
axially and radially @see the coordinates of the system as
shown in Fig. 1~a!#. The measurement system consists of a
power supply which both generates the probe voltage and
measures the probe current. The system is controlled by a
DLP plasma diagnostics system developed by Brockhaus
et al.,52 and the system is based on an iterative method for
calculation of ion density and electron temperature as described by Peterson and Talbot.53
J. Vac. Sci. Technol. A, Vol. 13, No. 4, Jul/Aug 1995
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For calculation of the electron temperature and ion concentration the average ion mass is needed. The dissociation
energy of H2 ~8.5 eV! is much less than the ionization energy
of hydrogen molecules ~15.4 eV!. The collisional cross sections for dissociation are an order of magnitude higher than
collisional cross sections for ionization. Furthermore, the gas
phase recombination of hydrogen atoms to form hydrogen
molecules requires a three body process. Therefore, for the
pressures used in this study and under the assumption of the
absence of heavier molecules ~i.e., water vapor or oxygen,
etc.!, the gas phase recombination can be neglected.54 The
surface recombination probability of hydrogen atoms at the
quartz surface is very low ~below 1023 for room
temperature!.55 No measurements of hydrogen fragmentation
were made, but dissociation coefficients @n~H!/n~H1H2!# in
similar microwave discharges in the same pressure range
vary from 40% for about 1 mbar to 90% for about 0.2 mbar
pressure.56 The hydrogen molecules and atoms are the start1
ing species for the production of H1
2 and H ions, respectively. The threshold for hydrogen atom ionization, 13.6 eV,
is lower than that for hydrogen molecules, and the collisional
cross sections for ionization of molecules is only slightly
1
higher than for atoms, so similar ratios of H1
2 to H and the
respective neutral species can be expected. Consequently, for
the calculations of the electron temperature and ion concentration from the DLP measurements, the mass of atomic hydrogen was assumed, which would result in less than a 5%
error for a pressure of 0.2 mbar and less than 20% for a
pressure of 1 mbar. If the plasma consists mostly of H1
2 , the
maximum error is 40%.
D. Optical emission measurements
Optical emission spectroscopy is a recognized technique
for the monitoring of plasma cleaning processes.57 The central part of the optical emission spectroscopy system employed in these studies is a scanning monochromator ~Verity
Instruments, Inc.! with focal length of 200 mm, spectral
resolution of 0.2 nm, and spectral sensitivity range from 135
to 900 nm. The light is collected by a light guide coupler
~EP200 Mmboc! which is fixed to the monochromator and
positioned as described below. The light emission spectrum
was measured as a function of pressure for three positions
along the length of the system. At the first position, in the
center of the discharge the light was collected through a hole
in the ring cavity and through one of the slots used for coupling the microwave power. In this position the microwave
coupling slot reduces the collection angle to 0.04 Sr. At the
second position 11 cm downstream from the source center,
the light was collected through a hole in the air cooling ring,
and again the collection angle is reduced by the hole, resulting in a collection angle of 0.09 sr. At the third position 32
cm downstream, the spectrum was analyzed through a quartz
viewport in the process chamber wall. The collection angle at
this position is determined by the lightguide coupler, and the
value of this angle is 0.15 sr. Two wavelengths, corresponding to atomic transitions Ha and Hb ~656.3 and 486.1 nm,
respectively!, were used to analyze the plasma emission for
the presence of atomic H.
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Korzec et al.: Characterization of a SLAN plasma source
2077
FIG. 2. The dependence of the microwave power for plasma ignition ~j!
and extinction ~d! as a function of pressure in the vacuum chamber for pure
hydrogen.
III. RESULTS AND DISCUSSION
A. Plasma operation range
The power and pressure operation range of the system is
important for the characterization of a plasma source and for
determining the limitations of the process for which the
source can be used.
The upper limit of the microwave power that can be
coupled into the plasma source is largely determined by the
thermal properties of the chamber materials. The thermally
most fragile component of the source is the vacuum seal,
which for the system used here can withstand 260 °C. As
noted, the system is fitted with an air cooling system. The air
cooling system had a maximum air flow of 350 m3 h21 and a
maximum inlet–outlet pressure difference of 150 Pa, and
with these parameters allowed for application of the full microwave power available in these experiments ~up to 2 kW!.
The minimum power needed to maintain the plasma discharge is termed the extinction power of the discharge. The
measurements of the extinction power as a function of pressure for hydrogen are shown in Fig. 2. For each measurement, stable operation was obtained, and the power was
manually reduced until the discharge was extinguished. For
powers close to the extinction limit, a distortion of the discharge ring pattern appears @Fig. 3~c!#. In the investigated
pressure range the extinction power was found to decrease
with increasing pressure. This indicates that less power is
required to sustain the discharge at higher pressures. The
main reason for this effect is that the plasma volume is found
to decrease with increasing pressure. The details of the
plasma volume will be discussed in the following sections.
We note that the extinction power is different for different
gases. For similar measurements with Ar gas, it was found
that the argon discharge can be sustained with 200 W, which
is the minimum power supplied by the 2 kW magnetron.58
The higher power requirements of the H plasma with respect
to the Ar plasma also correlates with the observation of ion
concentrations which are an order of magnitude higher in the
JVST A - Vacuum, Surfaces, and Films
FIG. 3. Photographs of the discharge with a view parallel to the source axis.
The photographs were obtained at microwave power and pressure in the
process chamber of ~a! 1600 W and 0.2 mbar; ~b! 1600 W and 0.8 mbar; ~c!
850 W and 0.8 mbar, respectively. The grid pattern recognizable in all three
photographs is the fan assembly grid.
argon discharge than in the hydrogen discharge.51 We note
that for efficient generation of a plasma, ionization must occur. The ionization energy of hydrogen molecules ~15.4 eV!
is close to that of argon ~15.6 eV!. Furthermore, the ionization energy of atomic hydrogen is 13.6 eV. Thus, it appears
that the differences in the ionization energy cannot account
for the observed increased power consumption for H-plasma
operation. The most probable reason for the higher power
consumption in the hydrogen discharge is that molecular dis-
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Korzec et al.: Characterization of a SLAN plasma source
sociation, vibrational and rotational excitation can also occur.
Because of the low dissociation threshold of 8.5 eV and a
high collision cross section,59 the electrons effectively couple
more energy into dissociation rather than ionization.
In general, a much higher power is needed for ignition of
the plasma than for sustaining the plasma. The minimum
power for discharge ignition over the pressure range of interest is plotted in Fig. 2. The values were obtained by adjusting the coupling probe and plunger to determine whether
plasma ignition could be obtained at a specified power setting. The positions of the adjustable coupling probe and of
the movable plunger are generally different for the plasma
ignition ~maximum electric field at the plasma chamber wall!
and for the minimum of reflected power of the stabilized
discharge ~optimal coupling of the surface wave60!. As indicated in the figure, a minimum microwave power of 1050 W
was needed for the ignition of the discharge for a pressure of
about 0.7 mbar. When the pressure is increased or decreased
from this optimum value, the ignition power increases but
does not exceed a value of 1.7 kW over the entire pressure
range examined. The plasma ignition is related to the breakdown electric field. A similar behavior was obtained for the
dependence of the breakdown electric field as a function of
pressure at an excitation frequency of 3.2 GHz in hydrogen
gas.61
B. Radial distribution of plasma parameters
At low pressure ~less than 0.3 mbar! the plasma is nearly
uniform over the whole diameter of the chamber @Fig. 3~a!#.
At increased pressures ~0.3–0.8 mbar! the plasma intensity is
increased towards the chamber walls, and ten lobes of increased light emission can be observed in the vicinity of the
cylindrical quartz wall @Fig. 3~b!#. The positions with minimum light intensity at the quartz wall correspond to the positions where the slot antennas in the ring cavity are located,
because superposition of the radiated fields from each slot
results in maximum E-field intensities beside the slots.51 According to the description of Margot-Chaker et al., this light
pattern can be identified as a coaxial mode TE51 with the
plasma forming the inner conductor or a surface wave mode
with m55.62 At high pressures ~over 0.8 mbar! and insufficient microwave power ~below 900 W!, a nonsymmetrical
plasma is observed @Fig. 3~c!#.
In the following we correlate the change in the plasma
radial distribution with the plasma parameters. In Fig. 4~a!
the radial distribution of the ion density versus position is
plotted for three different pressures. The measurements were
obtained from the DLP which was positioned as noted in the
plots. The data were taken by sweeping the probe across the
central plane of the SLAN ~z50! with eccentric center of
rotation. The ion concentration distributions change from
fairly homogeneous ~deviations from the mean value of less
than 2%! at a pressure of 0.2 mbar to strongly inhomogeneous ion concentrations for pressures of 0.45 and 0.7 mbar.
Thus, the observed nonuniformities of the ion densities is
similar to the visual observations of the plasma.
We consider three effects in an attempt to explain the
observed pressure dependence of the ion density: ~1! screening of the electromagnetic waves, ~2! ambipolar diffusion of
J. Vac. Sci. Technol. A, Vol. 13, No. 4, Jul/Aug 1995
2078
the ions and electrons, and ~3! ion recombination. The high
density plasma near the chamber walls could screen the microwaves and result in a nonuniform plasma with greatest
ion density near the walls. The microwaves will penetrate
through a skin depth which can be estimated. The skin depth
d for angular frequencies v higher than the electron collision
frequency n is given by63
d'
S
D
c
3n2 v2
11 2 1
,
vp
8v 2vp
~1!
where v pe is the electron plasma angular frequency given as
v pe 5
Ae
e 2n e
.
0m e
~2!
The electron collision frequency n is for hydrogen nearly
independent of the electron energy and can be approximated
by64 n'5.83109 Hz3p ~Torr!.
We assume that the ion and electron densities are similar,
thus using the typical electron concentration at the plasma
chamber wall of 1011 cm23, the electron plasma angular frequency of 1.7831010 rad s21 is obtained. Using this value,
the angular excitation frequency of 1.5431010 rad s21 and a
collision frequency of 1 GHz ~for 0.2 mbar!, then Eq. ~1!
gives a skin depth of 2.3 cm. This suggests that, for the 16
cm diameter chamber employed here, the greatest part of the
microwave power will be absorbed in the relatively thin
plasma layer at the plasma chamber wall. The preferential
microwave heating of this region is also displayed in the
electron temperature radial distributions @Fig. 4~b!#. For all
pressures the electron temperature is a factor of 2–3 times
higher at the plasma chamber wall than in the discharge center. The higher electron temperature correlates with higher
ion densities, because the production rates of ions increase
strongly with increasing electron temperature.66
Unfortunately, the screening effect does not explain the
observed pressure dependence. As shown in Fig. 4~a!, the
concentration n e decreases with increasing pressure. This indicates that the electron concentration will also decrease with
increasing pressure. From Eqs. ~1! and ~2!, we would find
that the higher pressure should result in an increased skin
depth. This is inconsistent with our observations. Hence the
changes in microwave coupling depth cannot explain the increased inhomogeneity of the radial ion density distribution
with increasing pressure.
We consider now diffusion processes. Here the assumption is that the ions and electrons are generated in the skin
depth and then diffusing towards the chamber center. The
coefficient of ambipolar diffusion has been given as65
S D
D a 5D i 11
Te
,
Ti
~3!
where T e and T i are the electron and ion temperatures, respectively. D i is the coefficient of thermal diffusion for positive ions, which can be assumed as constant over the pressure range of this study ~about 431025 m2 mbar s21 for
atomic ions!.61 For the plasma excitation we assume that the
ion temperature is 500 K and the electron temperatures are
shown in Fig. 4~b!. Thus, the ambipolar diffusion will de-
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Korzec et al.: Characterization of a SLAN plasma source
2079
FIG. 5. Ion concentration ~a! and electron temperature ~b! as a function of
axial position at the center of the SLAN for microwave power of 1400 W
and for three different pressures.
FIG. 4. Ion concentration ~a! and electron temperature ~b! as a function of
radial position for a microwave power of 1400 W and for three different
pressures.
crease significantly as the pressure is increased. This will
lead to an increased confinement of the plasma near the
chamber walls.
The ion concentration gradient occurs only when losses of
ions in the discharge volume or along the axis of the volume
are present. Strong axial ion loss from the generation volume
plays an important role in determining the radial profile of
ion concentration. A large flux of ions lost axially will lead to
a hollow plasma density.
We consider now the ion recombination processes for H1
2
and H1 since these are the relevant ions and the concentrations are pressure dependent. The most probable ion recombination mechanism for hydrogen is the so-called dissocia67
tive recombination ~e1H1
2 →H1H!. This mechanism does
not require triple collision, since the third body is produced
in the course of recombination. The typical values of the
dissociative recombination coefficient at T e ,1 eV are in the
range of a 5(10 28 – 10 210 )/T e ~a is given in cm3 s21 and
T e in eV!. In contrast, the main mechanism of proton recombination is due to three-body recombination. This process
can take place when minimum two electrons are sufficient
close to the positive ion. First, one electron recombines with
JVST A - Vacuum, Surfaces, and Films
ion; the second electron takes the recombination energy of
the first electron. This process scheme allows conservation of
energy and momentum. The coefficient of three-body recombination increases strongly with increasing electron concentration and with decreasing electron temperature. The typical
value is 10212 cm3 s21 for n e 510 10 cm23, T e 51 eV ~pressure of 0.7 mbar! and hydrogen as the discharge gas. This
value is two to four orders of magnitude lower than the coefficient of dissociative recombination.
Results56 have shown that the atomic H dissociation coefficient of hydrogen increases from 40% to 90% as the pressure decreases from 1 to 0.2 mbar. The H1 and H1
2 ion concentrations should have a ratio similar to that of neutral
species. Thus the high pressure plasmas would be expected
to have a higher concentration of H1
2 and a higher ion recombination rate. The higher recombination rate would lead
to a larger concentration gradient extending from the ion
source.
Thus it appears that all three mechanisms contribute to
cause a narrow plasma region near the chamber walls.
C. Axial distribution of plasma parameters
The axial dependence of the ion densities deduced from
the axial DLP measurements is shown in Fig. 5~a!. The ion
concentration decreases by a factor of 10 at a distance of 8.5
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Korzec et al.: Characterization of a SLAN plasma source
cm and by a factor of 30 at a distance of 12 cm for all three
pressures investigated. The ion concentration decreases
slowly near the ring coupler but more rapidly with increasing
axial distance from the edge of the ring coupler. The region
with nearly constant ion density is longer for higher pressure
~6 cm for 0.7 mbar! than for low pressure ~4 cm for 0.2
mbar!. The nearly constant density region is an indication of
the length of the primary discharge at the plasma chamber
wall. The results indicate that with increasing pressure the
layer of plasma with a high ion concentration gets thinner. To
maintain a constant microwave power deposition, the primary discharge gets longer.
The axial dependence of the electron temperature is
shown in Fig. 5~b!. The electron temperature is nearly constant as a function of axial position for all three pressures.
This statement can be made, because the error bars shown in
the figures mean the standard deviations of the measured
values. For a given pressure the electrons are ‘‘hotter’’ in the
regions of maximum microwave power coupling, i.e., near
the plasma chamber wall @Fig. 4~b!#. The almost constant
axial electron temperature distributions show that there is no
microwave power coupling at any position on the source
axis.
For the radial distribution, three different effects were
considered. For the axial distribution, because of the constant
electron temperature, the recombination mechanisms will be
the most important. Unlike the results of the radial distributions, the axial dependence of ion concentration for different
pressures exhibits a larger change for each pressure. An obvious additional mechanism to be considered is wall recombination in the process chamber. This effect results in a substantial decrease in ion density. The effect could prove a
limitation for ion-assisted processes but it could also prove a
substantial benefit for remote processes which rely largely on
neutral radicals for the chemical processes.
D. Plasma parameters for different process
conditions
The DLP measurements with the probe positioned in the
center of the plasma chamber ~z50! were performed to determine the ion concentration and electron temperature versus microwave power for several different hydrogen pressures. Prior to each measurement, the plasma coupling was
optimized for minimum reflected power. The ion density and
electron temperature were deduced from the measurements,
and the results are plotted in Fig. 6.
The ion concentration in the center of the plasma chamber
increases rapidly as a function of microwave power only
between 800 and 1000 W @see the curve for 0.2 mbar in Fig.
6~a!#. The maximum ion concentration of 1.531011 cm23 is
obtained for the lowest processing pressure ~0.2 mbar!. For
increasing microwave power from 1 to 2 kW, only a slight
increase of the ion concentration ~less than 50%! was observed. This behavior can be explained by the field distribution in the SLAN plasma system. The increase of power in
the low range causes a commensurate increase of the plasma
density at the positions where microwaves are coupled into
the plasma: directly at the surface of the ring cavity. For
higher power the plasma density at this position reaches a
J. Vac. Sci. Technol. A, Vol. 13, No. 4, Jul/Aug 1995
2080
FIG. 6. Ion concentration ~a! and electron temperature ~b! as a function of
microwave power measured in the center of the SLAN ~point 0 in Fig. 1! for
four different pressures.
critical value and does not allow the microwaves to penetrate
deeper into the plasma, due to the skin effect @see Eq. ~1!#.
The surplus power is then consumed mainly by the surface
wave propagating at the plasma-fused silica interface.60 As a
result, the discharge expands in the z direction. A slight increase of the maximum ion concentration at the center of the
cavity can be a consequence of the increased area due to the
expansion along the z direction. This is because the ions and
electrons diffuse from the high density wall regions to the
central point of the plasma chamber.
The explanation of the dependence of the ion concentration versus power also correlates with the microwave matching properties of the plasma. For pressure variations, it was
necessary to change the position of the movable plunger and
coupling probe for each pressure to maintain the minimum
reflected power. In contrast, for power variations over the
range of 1–2 kW, the power was readily swept without substantial position change of the adjustable coupling probe and
movable plunger positions. This implies that the impedance,
which is ‘‘seen’’ from the ring cavity through the slot antennas, does not change substantially with microwave power.
The plasma impedance per unit area decreases with increasing power because of the increasing plasma concentration
and hence conductivity, but the plasma area increases such
that the effective total impedance remains unchanged.
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Korzec et al.: Characterization of a SLAN plasma source
2081
sured from the DLP are very low ~in the nA range!, and it
was impossible to analyze in order to obtain a meaningful
electron temperature. At the discharge center, for a pressure
increase from 0.1 to 1 mbar, the electron temperature is reduced from 3.2 to 0.5 eV. This is attributed to an increased
frequency of both elastic and inelastic collisions of the electrons at increasing pressure. For low pressure measurements,
the electron temperature at the 24 cm downstream position is
only slightly lower than at the discharge center ~2.7 eV in
comparison with 3.2 eV!. The similarity of the two curves is
also a manifestation of the weak change of electron temperature versus axial position as shown in Fig. 5~b!.
E. Optical emission spectroscopy
FIG. 7. Ion concentration ~a! and electron temperature ~b! as a function of
pressure for a microwave power of 1400 W measured at three positions on
the symmetry axis ~z! of the SLAN: in the center of the SLAN ~j!, 24 cm
downstream ~d!, and 36 cm downstream ~m!.
Displayed in Fig. 7 is the ion concentration and electron
temperature versus pressure for three different axial positions: ~1! at the center of the discharge, ~2! at 24 cm downstream from the discharge center, and ~3! at 36 cm downstream from the discharge center. When the pressure is
increased by a factor of 4, from 0.15 to 0.6 mbar, the ion
concentrations decrease by factors of 33, 32 and 2 for the
three positions, respectively. It is worth noting that ion concentration versus pressure at the 36 cm downstream position
exhibits a very weak dependence in comparison to that at
two other positions. This dependence can be explained by the
diffusion based transport mechanism described in Sec. III B.
For the two positions closer to the discharge center, the increase of pressure results in a strong enhancement of ion
losses due to H1
2 dissociative recombination and consequently in a strong decrease of ion concentration. At the 36
cm downstream position, the ion and electron concentrations
are very low ~less than 108 cm23! and volume recombination
is negligible. Therefore, at this position, the chamber wall
losses limit the ion flux and ion concentration, and a weak
dependence on pressure is observed.
The electron temperature versus pressure for the discharge
center and at the 24 cm downstream position is shown in Fig.
7~b!. At the 36 cm downstream position, the currents meaJVST A - Vacuum, Surfaces, and Films
Optical emission spectra were obtained versus pressure
for two different microwave powers at all three of the different axial positions. The emission intensity versus pressure
for the measurements is plotted in Fig. 8.
Consider first the dependence on microwave power. For
emission at the center of the plasma region, the emission
intensity increases by approximately 50% for both emission
lines as the power increases from 1.4 to 2.0 kW. For the
second position ~z511 cm! the emission intensity increases,
respectively, by approximately 100% @Fig. 8~b!#. This effect
can be explained once more with an understanding of the
behavior of a surface wave excited plasma, i.e., the length of
the discharge increases with increasing power. This results in
the extension of the intense discharge to a position of at least
11 cm downstream. In the center of the discharge the intense
plasma is present for all microwave power greater then 1 kW.
Furthermore, as described above, the microwaves are
screened, resulting in a smaller change of emission intensity.
Consider now the pressure dependence. When the pressure changes from 0.15 to 1 mbar at 1.4 kW, the ion concentration in the middle of the discharge changes by a factor of
approximately 100 @Fig. 7~a!# while the light intensity
changes only by a factor about 4. The electron energy thresholds for excitation of the Ha ~1.89 eV! or Hb ~2.55 eV!
transitions are much lower than the ionization threshold
~13.6 eV! of atomic hydrogen.68 Taking into account the
change of the electron temperature from 3 eV for 0.15 mbar
to 0.5 eV for 1 mbar @Fig. 7~b!# and assuming the Maxwell–
Boltzmann electron energy distribution one obtains a large
increase of electrons with energy higher than the ionization
threshold and an even larger increase of electrons with energy higher than the Ha transition. But all electrons with
enough energy for Ha excitation do not actually cause the
transition. There are a large number of other atomic and molecular processes which can consume the energy of such
electrons.
Since the plasma density decreases with axial displacement, it is anticipated that a similar decrease of the light
emission intensity would be observed with increasing axial
position. All three measurement series are made with the
same settings of the monochromator. As noted previously,
the collection angle for the three positions is 0.04, 0.09, and
0.15 sr, respectively. Furthermore, the measured volumes and
multiplier voltages are different. When corrected for these
effects but neglecting reflected light and plasma inhomoge-
2082
Korzec et al.: Characterization of a SLAN plasma source
2082
FIG. 9. The temperature of a substrate holder made of Al2O3 measured as a
function of time for two axial positions: 12 cm and 24 cm from center of
SLAN. The plasma was sustained with a pressure of 0.45 mbar, hydrogen
flow of 50 sccm, and microwave power of 1.4 kW.
FIG. 8. Emission intensity of spectral lines of transitions Ha and Hb for
microwave powers of 1400 and 2000 W measured across a diameter of the
plasma volume as a function of pressure for three positions: ~a! in the center
of the SLAN, ~b! 11 cm downstream from the center of the SLAN, and ~c!
42 cm downstream from the center of the SLAN. ~The axis at the right is
corrected for the collection efficiency at each different position.!
neity, for positions 1, 2, and 3, the corrections are, respectively, 1.0, 0.44, and 0.001. With this correction, the light
emission intensity per unit volume 32 cm downstream is
approximately a factor of 1000 lower than in the center of
the discharge. This is comparable to the changes of the ion
concentration which decreases by three orders of magnitude
@compare the ion concentrations in the center of the discharge and 36 cm downstream in Fig. 7~a!#.
F. Substrate temperature effects
In previous studies of low density hydrogen plasma cleaning of Si it was found that elevated substrate temperature
J. Vac. Sci. Technol. A, Vol. 13, No. 4, Jul/Aug 1995
from 150 to 400 °C is required for effective removal of residual oxides from semiconductor surfaces in a hydrogen
plasma.69 The high density plasma employed in this study
can result in substantial heating. To characterize this effect,
Si wafers were positioned in the system and the temperature
determined. A substrate holder block with dimensions 50
310034 mm made of Al2O3 was used to support the silicon
substrates. The temperature of the substrate holder was measured as a function of time for two distances from the source
center: 12 and 24 cm. The temperature of the substrate
holder versus time is plotted in Fig. 9. The measurements
were made with a hydrogen pressure of 0.45 mbar, microwave power of 1.4 kW, and a hydrogen flow of 50 sccm. The
stabilized temperatures for the two distances were 620 and
270 °C. Typical temperatures for hydrogen plasma cleaning
range from 150 to 450 °C. For temperatures higher than
480 °C, the hydrogen desorbes from the surface resulting in
an unpassivated surface.70 The results indicate that a minimum distance of about 24 cm is required such that the
plasma induced heating is reduced to allow control of the Si
surface temperature.
Each surface placed in a plasma is heated by ion bombardment with a power density ~per unit area! P ion and by the
energy of chemical reactions, which in this case will be
mostly heat of hydrogen atom recombination into hydrogen
molecules P rec . The energy input is balanced by heat losses
caused by heat radiation Q rad , thermal conduction through
the substrate holder Q con , thermal conduction via gas cooling
Q gas , and thermal heating of the substrate holder, resulting in
an increase of the temperature. This energy balance can be
expressed by the following equation:
A ~ P ion1 P rec! 5
dQ rad dQ con dQ gas
dT
1
1
1c p m
,
dt
dt
dt
dt
~4!
where A is the area of the substrate holder exposed to the
plasma, c p is the specific heat of the substrate holder material, and m is the mass. When the plasma potential V f with
2083
Korzec et al.: Characterization of a SLAN plasma source
respect to the substrate holder, and ion saturation current J sat
are known, the power density of ion bombardment can be
roughly estimated as
P ion5J satV f 5qn i
A
S
kT e kT e
1
2 1ln
mi q
2
A D
kT e
,
mi
~5!
where n i is the ion concentration in the bulk plasma, m i is
the average ion mass, and T e is the electron temperature.
This equation gives an overestimated value of power deposited on the surface due to the ions because it does not take
into account the energy dissipation in the plasma sheath.
With this limitation in mind, we have used the maximal measured value of the ion concentration of 231011 cm23 and an
electron temperature of 2.5 eV, which yield a power dissipation of 1.13 W cm22.
The power dissipated per area unit due to recombination
of hydrogen atoms to hydrogen molecules can also be calculated from the number of hydrogen atoms arriving at the
heated surface per unit time and the surface area nA . The
following formula describes the dissipated power density:
P rec5 g E recn A 5 hg E rec
p
A2 p mkT
,
~6!
where E rec is the recombination energy, which is equivalent
to the dissociation energy and is equal 8.5 eV per molecule,56
h is the dissociation ratio defined as the proportion of the
atomic hydrogen concentration to the sum of atomic and
molecular hydrogen concentration in the neighborhood of the
heated surface, and g is the probability of recombination
when an hydrogen atom collides with a wall surface. Assuming the neutral particle temperature T of 500 K, Eq. ~6! gives
hg3719 W cm22. For a typical dissociation rate of 80% and
a wall recombination probability of 331023,55 the power dissipated due to recombination is 1.7 W cm22. Therefore, at
the center of the source, the power density due chemical
recombination is greater than that due to ion bombardment.
In the downstream region of the hydrogen beam, the ion
concentration decreases by several orders of magnitude. At
this position, and for the plasma parameters considered here,
the power due to ion bombardment can be neglected. This
result is consistent with a previous experimental result,71
where it was found that atomic hydrogen recombination to
molecular hydrogen accounts for 90% of the energy reaching
the surface of the substrate holder.
When the temperature of the substrate holder is similar to
the temperature of the fixing clamps and to the gas temperature, the whole deposited power is used to heat the substrate
holder @see Eq. ~4!#. The rate of temperature increase measured directly after switching on the plasma process for the
two distances from the source center was 1.7 and 0.6 K s21,
respectively. The specific heat c p of the Al2O3 is approximately 1.0 kJ kg21 K21 ~it varies as a function of temperature from 776 J kg21 K21 at 300 K to 1238 J kg21 K21 for
1100 K72!. Using an average value of the specific heat, the
geometrical dimensions of the substrate holder and the density of Al2O3 ~3.965 g cm23!, the heat capacity of the substrate holder is found to be 80 J K21. At the beginning of the
heating process the heat loss through thermal radiation can
JVST A - Vacuum, Surfaces, and Films
2083
be neglected because the temperature of the substrate holder
is low ~below 500 K!. Also, the heat loss due to thermal
conduction can be neglected because it is proportional to the
temperature difference between substrate holder and the surrounding gas. Taking these assumptions into account, the
power deposited into the substrate holder can be determined
from the initial heating rate. With this approach, the values
136 and 48 W are obtained for the power deposited for the
two positions, respectively. Assuming that this effect is
largely due to differences in the atomic H dissociation, the
results indicate that the dissociation coefficient in the downstream position is a factor of 2.8 lower than in the primary
discharge. This analysis has neglected the heat due to ion
bombardment, the possible differences of the surface recombination probability, and the local pressure variations at the
center of the source and at the remote position.
Taking into account the total area of the substrate holder
of 112 cm2, assuming a surface recombination factor for
Al2O3 of 331023 the total power dissipation due to recombination is 193 W. With this value dissociation fractions for
the two positions are 70% and 25%, respectively. These values are in agreement with calculations made with an analytical model.44 According to measurements of Trainor et al.73
for the pressure range considered in our work the volume
recombination of hydrogen atoms can be neglected. The decrease of the recombination fraction with increasing distance
from the hydrogen atom source is then ascribed to wall
losses of atomic hydrogen.
IV. CONCLUSIONS
This study has characterized a slot antenna system for the
generation of a H plasma. The results are summarized below.
~1! The slot antenna plasma excitation principle can be applied for efficient production of atomic hydrogen.
~2! The minimum ignition power of 1050 W is observed for
0.7 mbar.
~3! The extinction power of hydrogen ~600–1000 W! is a
factor of 5 higher than for argon. This increase is attributed to power consumed by dissociation processes.
~4! Electron temperatures from 0.5 to 3 eV ~increasing with
increasing pressure! and maximum ion concentrations of
1.531011 cm23 ~decreasing with increasing pressure! are
measured.
~5! For pressures greater than 0.5 mbar, both ion density and
the light emission intensity are much higher at the
plasma chamber wall than in the discharge center. The
observation is attributed to effects arising from the skin
depth of the plasma, and the diffusion and recombination
of the charged species.
~6! For pressures below 0.5 mbar a nearly homogeneous distribution of ion density and light emission intensity is
observed, and this is attributed to enhanced ambipolar
diffusion.
~7! The heating due to the plasma was measured for two
positions: in the primary plasma discharge and in the
remote discharge. In the primary discharge region both
the ion bombardment and recombination contribute to
the heating effects. For remote positions, recombination
2084
Korzec et al.: Characterization of a SLAN plasma source
is the dominant effect depositing thermal energy to the
substrate. The proper choice of distance can be used for
preheating the substrates to an optimal substrate temperature during the cleaning process.
~8! Except in the region of the primary plasma discharge, the
ion bombardment can be neglected for calculation of the
heat dissipated to the sample and holder.
~9! The dissociation fraction decreases from 70% to 25%
when the discharge from the discharge center increases
from 12 to 24 cm. This is attributed to recombination at
the chamber walls.
The application of the hydrogen plasma produced as described in this article is not limited to semiconductor surface
preparation and cleaning. The remote hydrogen plasma can
be used to assist numerous deposition processes. As examples: the growth of GaAs by molecular beam epitaxy,74 by
plasma enhanced chemical vapor deposition from metalorganic precursors,75 or by use of Ga evaporated from a Knudsen cell and AsH3 diluted in hydrogen.76
ACKNOWLEDGMENTS
The authors wish to thank R. Dahlhaus for technical assistance. This work is supported in part by the Ministry of
Science and Research/Northrhine, Westfalia, Germany, and
by the Board of Science and Technology of the State of
North Carolina.
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