UNIVERSITÀ DEGLI STUDI DI CAMERINO
FACOLTÁ DI SCIENZE E TECNOLOGIE
Corso di Laurea Magistrale in Fisica - Classe LM-17
Dipartimento di Fisica
Characterization of carbon based resistive
patterns on synthetic diamond plates for
microdevice design
TESI DI LAUREA SPERIMENTALE
IN STRUTTURA DELLA MATERIA
Laureando:
Kamili Yimamu
Relatore:
Prof. Roberto Gunnella
Correlatori:
Dr. Augusto Marcelli
Dr. Daniele Di Gioacchino
Anno Accademico 2011/2012
Contents
Abstract……………………………………………………………....…1
Abstract……………………………………………………………...….2
Introduction………………………………………………………….…3
Chapter 1………………………………………………………………..5
1.1
Introduction to carbon based materials …………………………….…..6
1.1.1 CVD diamond and diamond like carbon (DLC)……………….….9
1.2
Sample description and manufacturing techniques…………………....11
1.2.1 Laser irradiation…………………………………………………..11
1.2.2 The Focused Ion Beam (FIB)…………………………………….12
1.2.3 FIB images of laser irradiated line on the selected CVD diamond
plate…………………………………………………………..........14
Chapter 2………………………………………………………………16
2.1
Introduction to Raman/IR spectroscopy………………………………..17
2.1.1 Raman modes……………………………………………………….19
2.2
Analysis of Raman data…………………………………………….…..20
Chapter 3…………………………….……………………………...…33
3.1
Electrical property test……………………………………………........34
3.1.1
Electrical setup and instruments………………………………....36
3.1.2
Resistivity measurement procedures…….…….………………...38
3.1.3
R-T curve analysis……………………………………….……....39
3.1.4
Sensitivity curve analysis………………………………………..43
3.1.5
Heater behaviour test…………………………………………….44
Chapter 4……………………………………………………………....49
4.1
4.2
Temperature sensing technology…………………………………….....50
4.1.1
Primary and Secondary thermometers…………………………...51
4.1.2
Resistive temperature sensor (RTDs)…………………………….51
Possible designing of a practical temperature sensor and a single photon
detector
using
synthesized
diamond
plates……………………………………………………….…………...53
4.2.1
4.2.2
Design of a resistive temperature sensor………………………....53
Possible layout of a superconducting single photon detector…….55
Conclusion…………………………………………………………….58
i
References………………………………………………………..…....60
Acknowledgments………………………………………………….....62
ii
Abstract
The study presented in this work is dedicated to the characterization of a
surface modified CVD diamond plate. Although diamond is probably the most
resistant material on the Earth, when a laser beam or ion beam illuminates diamond
film surface, significant changes may occur to the composition, structure and
morphology of diamond according to the size and intensity of the beam used.
Compositional change points out the occurrence of phase transition among
carbon species while structural change refers to the alterations of tetrahedrally
oriented sp3 bonded crystalline structures and formation of sp2 like graphitic clusters.
A high resolution FIB set up has been used to monitor morphological change whereas
to understand structural and compositional changes, a 532 nm visible light excitation
has been carried out using Raman spectroscopy
The modification on the structure and composition may lead to fundamental
changes of the physical properties of diamond. Electrical transport measurements
showed that after laser irradiation the exposed area became a poor electrical
conductor, and patterned lines on CVD diamond exhibit thermometric properties
similar to usual Resistive Temperature Detector (RTDs). A detailed characterization
of these properties allows designing specific applications for such material.
To complete this work two different applications have been considered: a
resistive temperature detector and a superconducting single photon detector. Two
simple layouts for such devices of interest for the Physics Department of the
University of Camerino have been outlined.
1
Abstract
Lo studio presentato in questo lavoro è dedicato alla caratterizzazione di una
versione modificata di una lamina di diamante cresciuta con deposizione da CVD.
Sebbene il diamante èil materiale piùresistente sulla Terra, quando un fascio laser o
fascio di ioni illumina la superficie del film di diamante, possono verificarsi
cambiamenti significativi di composizione, struttura e morfologia a seconda delle
dimensioni e l'intensitàdel fascio utilizzato.
Cambiamenti di composizione evidenziano la presenza di transizioni di fase tra
le varie specie di carbonio, mentre il cambiamento strutturale indica alterazioni del
tetraedro orientato della struttura sp3 e la formazione di strati grafitici. Un FIB ad alta
rsoluzione è stato utilizzato per monitorare il cambiamento morfologico e per
comprendere i cambiamenti chimico-strutturali, mentre un laser di 532 nm è stata
utilizzata come sorgente per la spettroscopia Raman.
La modifica della struttura e composizione può portare a cambiamenti
fondamentali delle proprietàfisiche del diamante. Misure di trasporto elettrico hanno
mostrato che dopo l'irradiazione laser l'area esposta è diventata un pessimo
conduttore elettrico, e le linee tracciate su diamante CVD hanno proprieta' simili a
quelle dei sensori di temperatura resistivi (RTD). Una dettagliata caratterizzazione
di queste proprietàpermette future applicazioni specifiche per tale materiale.
Per completare questo lavoro sono state considerate due diverse applicazioni di
possibile utilizzo nel laboratorio di basse temperature del Dipartimento di Fisica: un
rilevatore di temperatura resistivo ed un rivelatore superconduttore a singolo fotone.
2
Introduction
Carbon has one of the most important and fascinating element and, accordingly,
also carbon based materials trigger a lot interests in science and technology because
of their outstanding chemico-physical and mechanical properties. Carbon mainly
occurs in the form of diamond as sp3 hybridized bonds or in the form of graphite as
sp2 hybridized structures. Some amorphous like carbon species, e.g., glassy carbon,
soot and diamond like carbon, are complex network of sp2 and sp3 coordinated bonds.
Diamond exhibits numerous outstanding properties such as highest hardness,
highest thermal conductivity, optical transparency, chemical inertness, wide band gap
and low wear resistance etc. which made diamond an extraordinary material in many
cutting edge researches and industrial applications such as cutting and grinding tools,
heat exchangers and windows.
Although both diamond and graphite are made by carbon atoms, their
properties are very different. While diamond can be obviously used as an abrasive
material because of its great hardness, graphite is soft and it is a perfect lubricant in
many applications [Hu, 2011]. Such large differences in the properties are usually
derived by differences in the crystal structures. The crystal structure of diamond is a
tetrahedrally oriented face centred cubic (FCC) structure with sp 3 hybridized bonds
and an electron configuration: 1s22s12p3. Graphite has a hexagonal ring like structure
with sp2 hybridization and an electronic configuration: 1s22s22p2. The sp3
hybridization of diamond is characterized by four valence electrons and C-atoms
form strong σ bonds with adjacent atoms. The extreme physical properties of
diamond are due to these σ bonds.
Early researches on diamond based materials are strongly limited by the high
cost of the natural diamond. However in 1950s the first artificial bulk diamonds
synthesized under high pressure and high temperature conditions. Since 1980s, the
chemical vapour deposition method, an efficient alternative to obtain even thick
diamond films from a gas mixture has been subjected of intense worldwide
researches [Hu, 2011].
Being the hardest material on the Earth, while on one hand its high hardness
enables diamond to be exceptional in some applications, on the other hand it makes
extremely difficult to machine it with conventional mechanical grinding and
polishing methods. Many attempts have made to achieve patterning or machining of
diamond, over recent years laser beam ablation and ion beam irradiation are found to
be useful and these techniques highly extends the application areas of various CVD
diamond plates.
3
In this thesis we present a study of compositions, structures, physical properties
as well as applications of a surface patterned CVD diamond plates. When the surface
of a CVD diamond plate is exposed to a laser or an ion beam, the crystalline structure
of the irradiated region changes and induces irreversible modifications of its physical
properties such as electrical and optical property. These alterations represent a
damage of the original materials but the resulting material has its own property
important to characterize.
In this work, a laser irradiated 0.7×0.7 cm2 thin CVD synthetic diamond plate
was accurately studied. A line of width of ~30 um was patterned with a high power
laser beam inducing damages on the diamond substrate. We then probed the
irradiated and non-irradiated line with a high resolution FIB/SEM microscopy. A nondiamond phase was detected all along the irradiated area and down to several tens of
microns inside. To better characterize the non-diamond phases present in the material
we investigated the molecular structure of C atoms in the irradiated region with the
Raman spectroscopy. A complete mapping of the irradiated region showed the
presence of three carbon species i.e., diamond, glassy carbon and amorphous like
carbon.
Raman measurements across the carbon resistive line allow the interpretations
of the different phases in terms of G and D peaks i.e. evaluation of the G peak width,
G peak position and intensity ratio of D and G peaks.
The Focused Ion Beam has been also used to set on the line, a four-contact and
four connecting wires have been planted by FIB depositing technique using pure gold
to allow electrical transport experiments.
Electrical transport experiments were performed to obtain some important
information on the thermometric property of these carbon-based materials that could
be useful to design unique devices such as resistive temperature sensor.
4
Chapter 1
The materials we investigated in this work are irradiated Chemical Vapour
Deposition (CVD) diamond substrates. If the irradiation process is performed using a
laser or an ion beam the original diamond is locally modified and the properties
change, therefore it is necessary to characterize the region where material has been
exposed. In this section we show how we recognize, after the irradiation, the atomic
and molecular structural and electronic changes following the irradiation.
Original CVD diamonds have been irradiated by a strong laser source or a
Focused ion beam (FIB). In this chapter we will describe how these methods may
pattern these materials to design a device. Advanced patterns can be obtained by a
FIB microscope. An important characteristic of a FIB microscope is indeed its spatial
resolution: it may guarantee operations or measurements in the nanometer range.
Using advanced ion imaging techniques, minimizing the potential damage of the ion
beam, we applied the FIB microscope also to measure the dimensions of large laser
irradiated lines obtained on our CVD diamond plates.
5
1.1 Introduction to carbon based materials
Carbon is a chemical element with symbol C and atomic number 6, as a
member of group 14 on the periodic table; it is non-metallic and tetravalent–making
four valence electrons which are available to form covalent chemical bonds. The
electron configuration of carbon is 1s22s22p2 and a simple electronic layout is given
in Fig.1.1.
Fig.1.1 Electronic configuration of a carbon atom
A single atom of carbon is a very short-lived species and, therefore, it is
stabilized in various multi-atomic structures with different allotropes. The three best
known allotropes are graphite, diamond, and not ordered amorphous phases of
carbon. The amorphous phase is an assortment of carbon atoms in a non-crystalline,
irregular or glassy state while graphite has a highly ordered bidimensional crystalline
structure. In graphite a C atom is bonded trigonally to three others in a plane
composed of fused hexagonal rings, just like those in aromatic hydrocarbons. In the
case of diamond the carbon atoms are arranged in a variation of the face-centred
cubic crystal structure called diamond lattice and is a transparent crystal of
tetrahedrally bonded carbon atoms and that crystallizes into the diamond lattice.
Depending on impurities diamond can be coloured and even black diamond may be
found. Diamond is characterized by extreme physical properties different from its
two carbon allotropes. Diamond is typically highly transparent, while graphite is
always opaque and black. Diamond is the hardest material known on the Earth, while
graphite is soft enough to be used to write on a paper foil. Diamond has a very low
electrical conductivity, while graphite is a very good conductor, however, under
normal conditions; diamond has among the highest thermal conductivity of all known
6
materials.
Carbon forms a great variety of crystalline and disordered structures because it
is able to exist in three different hybridisations, sp 3, sp2 and sp1. In the sp3
configuration, as in diamond, each of carbon atom‟s four valence electrons is
assigned to a tetrahedrally directed sp3 orbital, which makes a strong σ bond to an
adjacent atom. σ bonds are nearest-neighbour, 2-center, short-range bonds which fix
the C-C skeleton of the lattice, while π bonds are multi-centre conjugated bonds
giving rise to long range interactions [Ferrari et al., 2000].
Fig.1.2 Sp3 hybridisation in the diamond lattice and the σ bond layout.
In the three-fold coordinated sp2 configuration of graphite, three of the four
valence electrons form trigonally directed sp2 orbitals, i.e., form σ bonds within a
plane. The fourth electron of the sp2 hybridized atom lies in a π orbital, normal to the
σ bonding plane [Rybachuk, 2007]. Because it is near the Fermi level, this π orbital
forms a weaker bond with another π orbital of one or more neighbouring atoms. π
orbitals geometrically lie normal to a σ bond plane. The three σ bonds and the π bond
usually compose a ring plane of sp2 clusters.
7
2
Fig.1.3 Sp hybridisation of graphite and the π bond (red)
In the sp1 configuration, two of the four valence electrons form σ orbitals, each
one forming a σ bond directed along the ±x-axis, while the other two electrons
contribute to π orbitals in the y and z directions [Robertson, 2002] (Fig.1.4).
Fig.1.4 Sp1 hybridisation and the σ bond
Actually, the properties of any carbon material depend on the ratio between sp 2
and sp3 bonds. In addition to sp3 content, clustering of sp2 bond also plays an
important role in the different properties of carbon based materials, especially, for
optical, electrical and mechanical properties.
8
1.1.1 CVD diamond and diamond like carbon (DLCs)
The extreme physical properties of diamond derive from its strong, directional
σ bond. Diamond has a wide 5.5 eV band gap, the largest bulk modulus of any solid,
the highest atom density, the largest room temperature thermal conductivity, smallest
thermal expansion coefficient, and largest limiting electron and hole velocities of any
semiconductor [Robertson, 2002].
Due to its extraordinary properties, diamond offers many possible applications
in microelectronics or wear resistive coatings, extremely high power density
materials. It may sustain high mechanical loads or survive to extreme conditions,
although its high cost, both of natural and synthetic materials significantly limits its
range of practical applications. However, recent progresses in the chemical vapour
deposition technology made it possible to the production of synthetic diamond
materials at commercial scale and reasonable cost. At present synthetic CVD
diamonds are often the only material able to met demanding requirements suitable
only to natural diamond. These CVD diamonds show mechanical, tribological, and
even electronic properties almost equivalent to natural diamond.
Contamination, structural defects and grain size may negatively affect the
behaviour of diamond, especially electrical performance. It is then important to grow
diamond as films or substrates of single-crystal quality. Many published results have
been performed on polycrystalline diamonds and free-standing CVD single-crystal
diamonds [Majdi, 2010].
In this work the material we investigated is a laser irradiated polycrystalline
CVD diamond shown in Fig.1.6.
Chemical vapour deposition (CVD), as its name implies, involves a gas-phase
chemical reaction occurring above a solid surface, which causes deposition onto that
surface. All CVD techniques for producing diamond films require a means of
activating gas-phase carbon-containing precursor molecules. This generally involves
thermal (e.g. hot filament) or plasma activation (D.C, R.F, or microwave), or use of a
combustion flame (oxyacetylene or plasma torches). The structural forms of the films
deposited by the CVD method can be amorphous, polycrystalline and
monocrystalline. The reactant gas species of intrinsic diamond films include methane
(CH4), hydrogen (H2) and argon (Ar) [Hu, 2011].
Usually, growth of diamond requires that the substrate be maintained at a
temperature in the range 1000-1400 K, and the precursor gas be diluted in an excess
of hydrogen and the resulting films are polycrystalline, with a morphology that is
sensitive to the precise growth conditions. Growth rates for the various deposition
processes vary considerably, and it is usually found that higher growth rates can be
9
achieved only at the expense of a corresponding loss of film quality. 'Quality' here is
a subjective concept that, as an example, may refer to factors such as the ratio
between sp3 (diamond) to sp2-bonded (graphite) carbon present in the sample, the
composition (e.g. C-C versus C-H bond content) and/or the degree of crystallinity
[May, 1995]. The growth conditions of diamond deposition also determines film
thickness, by varying growth conditions the thickness of the film can be varied from
0.01mm to 2mm.
Another interesting carbon based material with similar physical properties to
diamond is what has been called Diamond-like carbon (DLC): a metastable form of
amorphous carbon containing a significant fraction of sp3 bonds. DLC‟s with a high
sp3 content are called tetrahedral amorphous carbon (ta-C) and its hydrogenated
analogue (ta-C:H) [Ferrari et al., 1999]. In general, the amorphous carbon may
contain sp3 (diamond-like), sp2 (graphite-like) and even sp1 sites. It can be
characterized by a high mechanical hardness, chemical inertness, optical
transparency, and it is a wide band gap semiconductor. Due to its attractive properties
DLCs have attracted a considerable interest as a coating material. It has been used as
coating for antireflection, wear resistant, corrosion resistant and biomedical
applications. The group of materials known as DLC contains not only the amorphous
carbons (a-C) but also hydrogen alloys, (a-C:H). Depending on the hydrogen content
and the ratio among sp2/sp3 bonds, DLCs can be divided in different groups outlined
in Fig.1.5 [Robertson, 2002].
Fig.1.5 the ternary phase diagram of sp2, sp3 bonds vs. hydrogen content with the different
diamond like carbon phases.
10
1.2 Sample description and characterization method
In the present work we characterized carbon based patterns obtained via laser
irradiation on CVD diamond plates. The four samples of CVD synthetic diamond are
shown in Fig.1.6. As we can see from the photographs, four different shapes have
been patterned. Characterizing compositions and electrical properties of these
patterns is important and interesting to design other patterns with improved
properties.
Fig.1.6 Images of four CVD diamond platelets. On each plate a line was patterned by laser.
The sizes of the plates are approximately 0.7×0.7 cm2.
1.2.1 Laser irradiation
Recent advances in chemical vapour deposition (CVD) processes have
increased the availability of high-purity polycrystalline diamonds for optical and
electronic applications. For some applications diamond needs to be patterned with
micron or submicron size features. However, high hardness value and extreme
brittleness made diamond a very difficult material to machine. Laser irradiation may
overcome this difficulty. Milling and patterning of diamond may also be done but
they are much slower than laser ablation, an important parameter to consider if a
significant thickness reduction or large aspect ratios have to be obtained [Smedley et
al., 2009].
Previous studies pointed out that the exposure to laser beam heats a diamond
film and the energy induces formation of different non-diamond phases, such as
mixed amorphous and graphitic phases. The formation of a graphitic layer during the
laser irradiation of a diamond surface is due to the phase transformation from
diamond to graphite due by the energy released by the laser pulses [Chao et al., 2005]
11
In the present thesis, we described the analysis performed on the sample,
labelled as „1‟ in Fig 1.6, and characterized the composition of its non-diamond
phases produced by laser irradiation.
Similar patterns on CVD diamonds can be in principle achieved using a
Focused Ion Beam microscope, which may allow simultaneously two different
functions i.e., micromachining and imaging.
.
1.2.2 The Focused Ion Beam (FIB)
The Focused Ion Beam (FIB) microscope has gained widespread use in
fundamental materials‟ studies and technology applications over the last decade
because it offers both high-resolution imaging and flexible micromachining in a
single platform [Volkert et al., 2007]. A FIB setup is a scientific instrument that
resembles a scanning electron microscopy (SEM). However, while the SEM uses a
focused beam of electrons to image the sample in a vacuum chamber, a FIB setup
uses a focused ion beam of ions instead. Secondary electrons are generated by the
interaction of the ion beam with the sample surface and can be used to obtain highspatial-resolution images. In most commercially available systems, Ga+ ions are used,
and their sputtering action enables precise machining of samples.
Imaging and sputtering with an ion beam are the two basic functions of the FIB
and require a highly focused beam. The smaller the effective source size, the more
current can be focused to a point. Unlike the broad ion beams generated from plasma
sources, high-resolution ion beams are defined by the use of a field ionization source
with a small effective source size on the order of 5 nm, therefore enabling the beam
to be tightly focused. Most widespread are instruments using liquid-metal ion sources
(LMIS), especially gallium ion sources. Ion sources based on elemental gold and
iridium are also available. Another one important function of FIB is called microconstructing that it can locally deposit materials with an assist of gas precursors.
Source ions are then generally accelerated to energy of 1-50 keV, and focused
onto the sample by electrostatic lenses. LMIS produce high current density ion beams
with very small energy spread. A modern FIB can deliver tens of nanoamperes of
current to a sample, or can image the sample with a spot size of the order of a few
nanometres.
12
Fig.1.7 Schematic layouts of functions of FIB, as the primary ion beam, emitted from a
metal tip, hits the materials’ surface; (a) it sputters material, (b) produces secondary
electrons and ions which are collected to form an image of the material surface. (c) When a
gas is introduced by a nozzle in the vicinity of the impact point the gas is adsorbed by the
surface of the material and the secondary electrons produced by FIB break the chemical
bonds of the adsorbed gas and form a deposited layer of material.
As showed in Fig 1.7, a gallium (Ga+) primary ion beam hits the sample surface
and sputters a small amount of material, which leaves the surface as either secondary
ions or neutral atoms. The primary beam also produces secondary electrons. As the
primary beam hits the sample surface, the signal from sputtered ions or secondary
electrons is collected to form an image. At low primary beam currents, a very small
amount of material is sputtered and modern FIB systems can easily achieve 5 nm
imaging resolution. At high primary currents, a great deal of materials can be
removed by sputtering, allowing precision milling of the specimen at nano scale.
Using FIB a deposition occurs if a gas source is introduced near the impact point. The
gas is introduced by a nozzle which is positioned a few hundreds of microns above
the area of interest. The gas is then adsorbed on the surface of the material. When the
FIB hits the surface, secondary electrons with energy ranging from a few eV to a few
hundreds of eV are generated. These secondary electrons will break chemical bounds
of the adsorbed gas molecules which will separate into different components: some of
which remains volatile, others will contribute to the deposited layer on the surface.
13
1.2.3 FIB images of laser irradiated line on the selected CVD diamond plate
A set of high-resolution FIB images has been collected at the LIME laboratory
of the University of Rome III (Rome) showing details of the patterned lines on CVD
samples. These images (Fig. 1.8) show details and dimensions of the carbon-based
resistive lines. We used the same instruments also to produce specific patterns on
other CVD diamond plates in order to compare the effect of an ion beam with the
laser irradiation procedure.
In the panels (a), (c), (d) of Fig.1.8 we may recognize two lines, one internal
and one external on the selected CVD diamond plate. Their widths are ~30 um (left
line) and ~28 um (right line) with a separation of ~74 um. In the next we will discuss
only parameters of the external line. In panel (b) we show a highly enhanced SEM
image of the carbon line where deformations and damages induced by the laser
irradiation can be recognized. The right and left corners of the two carbon lines, with
a longitudinal FIB milling, on the selected sample are shown in (c) and (d) panels.
14
Fig.1.8 FIB images showing the carbon lines on the surface of the selected CVD diamond
plate (a) width and inter distance of the internal and external carbon lines, (b) highly
enhanced SEM images of the carbon line,(c),(d) left and right corners of the two carbon lines
with a longitudinal FIB milling, (e) a transverse milling of the carbon line by FIB,(f) a further
milling to bigger depth, (g) a longitudinal milling by FIB, (g) highly enhanced seem of the
longitudinally milled part and a columnar structure of the microcrystalline diamond phases.
In the panel (e) we show a vertical profile of the carbon line produced by FIB
milling on the selected sample. We clearly recognize an amorphous region (the dark
area in the panel (e)) of ~ 20 micron wide and ~36 micron deep. The removal of
diamond by FIB milling is much slower than the removal of materials from the
irradiated channel pointing out a much lower density. Further milling is shown in
panel (f). Here the depth reaches ~83 micron and the diamond phase is recognized in
the lower layers. We can see clearly the boundary at around several ten micron depth
between the amorphous and the diamond phase. In the panels (g) and (h), a
longitudinal milling is shown. We can recognize columnar ordered structures in panel
(h), which may be assigned to a micro-crystal phase of diamond, i.e., a
recrystallization of the material following the laser irradiation.
Summarizing, a FIB microscope can provide accurate measurements of
patterned materials on diamond and useful information about the composition of the
probed area.
15
Chapter 2
As discussed in chapter 1, CVD diamond plates patterned by laser show large
morphological changes associated to the presence of different carbon-based phases.
To characterize properties of carbon-like materials originating at the diamond surface
we used different spectroscopic techniques.
To understand laser irradiation effects on a CVD diamond it is also useful to
understand the electrical properties of the different phases in order to use these “new
materials” for specific applications.
Starting from synthesized CVD diamond, i.e., a sp 3 hybridized tetrahedral
carbon phase, after exposure to laser the originally C bonds rearrange and with
different amounts and distributions new C-based bonds appear. In this chapter we will
introduce vibrational spectroscopies and, in particular Raman spectroscopy, and the
spectroscopic characterization of carbon-based phases at high-spatial resolution.
16
2.1 Introduction to IR/Raman spectroscopy
IR spectroscopy is one of the most widely used and versatile analytical
methods. Infrared light was discovered in the early 1800, but only at the end of the
century the first IR spectra were published. Later, it was pointed out the advantage to
combine an IR spectrometer with a microscope to extract accurate molecular
information from small areas of a sample. However, only in 1990‟s the first
commercial IR microscopes become available [Marcelli et al., 2012]. Infrared (IR)
spectroscopy is a non-destructive tool capable to analyze materials in any state: solid,
liquid or gas, providing a valuable knowledge about chemical forces between atoms,
and vibration frequencies. This tool allows identifying chemical and/or molecular
species present at the surface or inside the material under investigation. Infrared
spectroscopy is based on absorption or reflection processes of the radiation.
Raman spectroscopy is another vibrational technique that is based on light scattering
(see next section).
IR photons are absorbed when a dipolar excitation occurs. Dynamic dipoles are
oscillations in the density of electrons or electron charges due to atomic vibrations. A
dynamic dipole absorbs a photon when the electric field of the radiation is parallel to
the charge oscillation, and when the frequencies of the light and of the oscillation are
similar (resonance case). Dynamic dipoles occur in both molecules and extended
systems, and their natural frequencies are related to the masses of the displaced
atoms. Vibrational modes are also referred to as normal modes. Due to energy and
momentum conservations, only photons with energy and momentum matching that of
the available excitation may be absorbed; light of higher or lower frequencies cannot
be absorbed. An absorption spectrum is then a plot showing how well different
frequencies of light couple the possible excitations of the sample under investigation
The infrared spectrum of a sample is recorded by illuminating the sample with
a beam of infrared light. When the frequency of the IR is the same as the vibrational
frequency of a bond, a photon absorption process occurs. Analysis of the transmitted
light reveals how much energy is absorbed at each frequency (or wavelength). This
can be achieved by scanning the wavelength range using a monochromator.
Alternatively, the whole wavelength range is measured at once using a Fourier
transform instrument and then a transmittance or absorbance spectrum is generated
using a dedicated procedure. Analysis of the position, shape and intensity of peaks in
the spectrum reveals details about the molecular composition of the investigated
sample.
17
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique based on inelastic scattering
of monochromatic light, usually generated by a laser source. It is a popular technique
substantially non-destructive, since laser may heat the sample and living systems
cannot properly investigated by Raman.
Fig.2.1 Schematic descriptions of (a) the Raman scattering process (b) an energy diagram of
the infrared absorption process, the Rayleigh scattering (elastic scattering), the Stokes and
anti-Stokes scattering processes.
When a light quantum hv0 heats a surface, an elastic scattering process occurs.
It is the Rayleigh scattering of photons with energy hv0. This process has the highest
probability. However, also inelastic processes in which the vibrational energy is
modified by hvs may occur. The inelastic process is called Raman scattering and
quanta of energy hv0 ±hvs are emitted. Because at ambient temperature, according to
Boltzmann‟s law, vibrations of atoms in the excited state are much less than that of
the ground state atoms, it is more efficient to excite ground-state atoms to a
vibrationally excited state than to observe the decay energy from atoms vibrating in
the excited state. Hence, the emitted quanta having energy hv0-hvs are larger than the
emitted quanta with energy of hv0+hvs. The Raman lines corresponding to the quanta
18
hv0-hvs are the Stokes lines whereas those at high energy (hv0+hvs) are called antiStokes lines. As the intensities of anti-Stokes lines are lower, only Stokes lines are
usually observed in a Raman spectrum [Chu et al., 2006].
Spectral resolution of the Raman spectroscopy is an important parameter that
measures the ability to resolve spectral features and bands as separate components.
The spectral resolution required depends by each experiment. Routine analysis for
basic sample identification typically requires low/medium resolution. In contrast,
characterisation of polymorphs and crystalline systems often requires high resolution,
since these systems can be recognized among them by subtle changes in their Raman
spectra, typically hidden in a low resolution spectrum. If the spectral resolution is too
low, information is lost, and a correct identification and characterisation of a sample
is difficult.
2.1.1 Raman modes
Raman spectroscopy is a very effective way to investigate the detailed bonding
structure of a carbon film. The method is widely used to distinguish bonding type,
domain size, and sensitivity to internal stress in amorphous and nanocrystalline
carbon films [Chalker et al., 1991]. Raman spectra are usually discussed in the
context of short distance ordered sp3 and sp2 bonds.
The great versatility of carbon materials arises from their strong dependence of
the physical properties by the ratio of sp2 (graphite-like) to sp3 (diamond-like) bonds.
Indeed carbon materials mainly consist of a complex mixture of these two bonding
types. There are many forms of sp2 bonded carbons with various degrees of graphite
ordering, ranging from micro-crystalline graphite to glassy carbon. In general, an
amorphous carbon is a mixture of sp3, sp2 (see Fig.2.2 (C)) and even sp1 bands, with
an additional presence of hydrogen and nitrogen atoms [Ferrari et al., 1999]. Pure
diamond has a single Raman active mode at 1332 cm-1, which is a zone centre mode
with a T2g symmetry, a symmetry characteristic of triply degenerated d-orbitals. A
second carbon form is the sp2, or graphite trigonal form, in which each carbon is
bonded to three adjacent carbon atoms in the same plane, at 120°, giving rise to a
graphitic hexagonal structure. Highly ordered, single crystalline sp2 graphite gives
rise to a single sharp doubly degenerate E 2g Raman active band at 1580 cm-1,
typically referred as G band [Ferrari et al., 1999]. However, the most common source
of pure graphite occurs in natural crystals. Under normal deposition conditions, this
pure phase is not common and an amorphous phase is often detected. As film
becomes more amorphous, the 1580 cm-1 band slightly shifts in energy and broadens
significantly.
19
Distortions in the crystalline structure of the amorphous graphite are associated
to small domains of sp3 carbon. The disorder induces a second Raman active mode
(A1g mode) at 1350 cm-1, i.e., a visible excitation typically referred as the D band.
Fig.2.2 (a) Bond stretching of a pair of sp2 sites, it appears when C sp2 sites are arranged as
olefinic chains or aromatic rings; (b) the A1g breathing mode of 6-fold aromatic rings
activated by disorder; (c) a mixture of sp2 and sp3 sites
The G and D peaks are due to sp2 sites only. The G peak in particular, is due to
bond stretching of all pairs of sp2 sites. The D peak is due to breathing modes of the
aromatic ring as described in Fig.2.2. Parameters of Raman spectra such as positions,
widths and intensities of the D and G peak are closely related to the density, size, and
structure of the sp2 clusters. The properties of sp2 clusters are in turn closely related
to the sp3 content of DLC, enabling us to measure the sp3 content from the parameters
of Raman spectra [Cui et al., 2010].
2.3 Analysis of Raman data
The original material of our samples is a CVD diamond plate. Usually, the
CVD process involves methane, hydrogen, and also inert gases and oxygen. Films
produced by CVD deposition techniques can yield crystalline and amorphous
structures. In the case of carbon films an additional question regards the atomic
bonds. Are bonds in the films a three-fold coordinated sp2 as in graphite of a fourfold
coordinated sp3 as in diamond? Films may have four different microstructures:
amorphous or crystalline diamond, and amorphous or crystalline graphitic structures
20
[Nemanich et al., 1988]. In our case, as mentioned above, the laser irradiation alters
the original bonds or microstructures and, in the irradiated region new forms of
carbon structures are formed.
In this work we used the Raman spectroscopy (50x objective, 5 sec. acquisition
times) to characterize the formed carbon phases or microstructures in the laser
irradiated area on the synthesized CVD diamond plate. Data collected with Raman on
the carbon line give information related to ~1000 points along the resistive carbon
line in both the longitudinal and the transversal directions. Checking all Raman
spectra of the ~1000 points separately, we recognized in our sample three types of
carbon phases i.e., the diamond, the glassy carbon and the amorphous states, which
are the main phases detected on the carbon line. Some additional not assigned peaks
detected in a few spectra, have been assigned to dusts or contaminations of the
sample. Representative Raman spectra of the carbon phases are shown in Fig.2.3.
Fig.2.3 Raman spectra characteristic of three different carbon phases observed in the
irradiated region, (a) diamond (b) glassy carbon, (c) amorphous carbon
To further investigate the degree of disorder or amorphization of the carbon
21
phases detected on the patterned lines we made an additional set of measurement
using a Bruker Senterra Raman microscope (532 nm laser line and 100x objective)
available at the Porto Conte Ricerche laboratory (SS) The laser power was 50 mW
and the acquisition time was two seconds and the spectra were recorded in the range
70-4500 cm-1. In this measurements Raman data were recorded along a direction
from the CVD diamond area to the laser irradiated area as indicated in Fig.2.4. Total
of 16 points were recorded with a fixed step of 400 nm within a 6 μm long path.
Figure.2.4 Highly enhanced image of the laser irradiated carbon resistive line. The red arrow
shows the direction along data was recorded by the Raman microscope.
Looking at data from the first set of Raman spectroscopy measurements, the
laser irradiation caused amorphization of original CVD diamond and glassy carbon
phases appear. From the second set of Raman data we obtained the typical curves
showed in Fig.2.5.
22
Fig.2.5 Raman peaks obtained from experiments performed across the irradiated region (a)
a high intensity diamond peak, (b), (c) variation of height and position of D and G peaks in
the amorphous carbon phases. (d) Collection of Raman spectra of 16 different points. The
red arrows points out spectral changes along the direction indicated in Fig.2.4.
Fig 2.5 (a) shows a single high intensity peak at 1332 cm-1, introduced
previously and corresponding to pure diamond. From curves in panels (b), (d) we
find that in addition to the sharp peak at 1332 cm-1 a second rather broad shoulder at
1420 cm-1 occurs in the lowest three spectra. In the fourth spectrum, the peak at 1420
cm-1 shifts to 1580 cm-1. It reflects the zone centre E2g mode of pure graphite and it is
usually designated as “G” peak [Ferrari et al., 1999]. A small shift of the diamond
peak at 1332 cm-1 is observed and its final position occurs at 1343 cm-1 Furthermore
the intensity of the peak decreases. It may be referred to a zone-edge A1g mode due to
the disorder. It is usually designated as the “D” peak. The observed double peaks at
1343 cm-1 (D band) and at 1580 cm-1 (G band) must be assigned to a glassy carbon
phase and the D band has a higher intensity than the G band. The extremely weak
shoulder observed at 1100 cm-1 in Fig 2.5 (d) may be attributed to nano-crystalline
diamonds.
23
The three stage model
Visible Raman spectroscopy has a limited use to characterize DLC/HDLC
films, especially to estimate the content of sp3 and sp2 fractions in these materials.
Recently, based on atomic and electronic structures of disordered carbon, Ferrari and
Robertson [Ferrari et al., 2000] introduced a three stages model showing also that
disordered, amorphous and diamond like carbon phases in amorphous C-H films can
be directly characterized by measuring position and width of the G-peak and the
intensity ratio of G and D peaks obtained from Raman spectra, rather than by a direct
measure of their intensities. The three stages model is shortly described below.
In the Stage 1 the average G peak position moves from 1581 cm-1 to 1600 cm-1. The
D peak appears and increases in intensity following the Tuinstra & Kneonig (TK)
relation:
I ( D) / I (G)  1/ La
(2.1)
where La is the in-plane correlation length or grain size of graphite. This means that
the ratio is proportional to the number of rings at the edge of the grain.
The Stage 2 is the introduction of the topological disorder into the graphite
layer. The bonding is still mainly sp2, but a weaker bond softens the vibrational
modes and the G peak shifts to 1510 cm-1. Schwan et al. [Shwan et al., 1995] have
found that the maximum values of I(D)/I(G) ratio is four for a-C films therefore TK
relation is no longer valid in this stage because for DLCs La is always less than 1 nm
[Robertson. 2002]. Recent data on high temperature depositions of ta-C suggests that
for La below 2 nm, the ratio is modified to as equation (2.2) [Ferrari et al., 2000].
I ( D) / I (G)  La 2
(2.2)
The G peak is due to sp2 sites, but the D peak is only due to six-fold rings. Low sp2
contents in the films are expressed by a low I(D)/I(G) ratio, whereas a higher ratio
points to a clustering of sp2 sites [Kahn et al., 2008], so that the ratio is proportional
to the number of rings per cluster. So, I(D)/I(G) falls as the number of rings reduces
and the fraction of chain groups rises An important factor for DLCs is that La is
always less than 1 nm, so that the TK relationship is never valid for them, while the
Equation (2.2) still holds [Robertson, 2002]. A good example of the stage 2 is the
amorphization of the glassy carbon by irradiation [McCulloch et al., 1995].
The Stage 3 is the conversion of sp2 sites to sp3 sites. In this case, due to the
confinement of π electrons in shorter chains, the G peak shifts up to 1570 cm-1 and
I(D)/I(G)=0 due to the lack of rings .
Actually, in this framework the development of a D peak points out an increase
24
of disorder in the graphite, but also an ordering of the a-C phase.
The Raman spectra of the carbon-based patterns have been analysed using the
three-stage model proposed by Ferrari and Robertson.
Fig.2.6 The three stage model proposed by Ferrari and Robertson. The upper panel shows
3
the shift of the G peak with respect to the sp content. The lower panel indicates the variation
3
of the I(D)/I(G) ratio with respect to the sp content along the three stages.
amorphization trajectory
ordering trajectory
Fig.2.7. Change of the sp2 configuration in the different carbon amorphous stages. From left
to right: graphite, nanocrystalline graphite, a-C, ta-C.
The three stages model can simply summarized as the following three
independent processes:
25
(1) Graphite to nano-crystalline graphite (nc-G)
(2) Nano-crystalline graphite to sp2 a-C
(3) a-C to ta-C [Ferrari., 2001]
The structural change in (1) is known to be a uniform crystallinity of graphite
and this change may due to the decrease of graphitic clustering (La). In process (2),
bond angle and bond bending disorder occurs to the nano crystalline graphitic layers,
this causes reduction of number of ordered six-fold rings and increase of chains, or in
other words, the stage 2 imply the disordering of graphitic layers and ordering of a-C.
The process (3) is a passing from a-C to ta-C, the sp3 content dominates and the sp2
sites changes gradually from rings to chains.
ID/IG ratio and sp3 content
As previously mentioned, the parameters of the Raman spectra such as
positions, widths, intensities and peak intensity ratio I(D)/I(G) of D and G peaks are
closely related to density, size and structure of sp 2 clusters. The properties of sp2
clusters are in turn closely related to the sp3 content of DLC. In this way we may
estimate the sp3 content from parameters of the Raman spectra [Cui et al., 2010].
The I(D)/I(G) ratio (intensity of the D peak to that of the G peak) is different in DLC
films synthesized by different methods and sometimes even for films synthesized
with the same method and also for a single sample in different locations [Kagi et
al.,1994]. Here we define the intensity I(D) and I(G), separately, as the peak area of
D and G peaks.
The intensity ratio of D and G bands can be obtained by fitting the curve
corresponding to bands of a pair of Gaussian peaks after background removal and a
pair of Lorentzian or a Breit-Wigner-Fano (BWF) function are also widely used
[Casiraghi, 2005].
Since the spectral resolution of our experiments is 10 cm-1 both D and G
peaks appear in all spectra much wider than the spectral resolution and a Lorentzian
curve is more suitable to fit curves. An example of D and G peak fitting with a
Lorenzian curve after normalisation is shown in Fig.1.8.
In this work, we are mainly interested in the evaluation of the carbon phases.
We then just considered two peaks centred around 1343 cm-1 and 1582 cm-1 assigned
to the D and G bands, respectively and the peak positions slightly shift in different
coordinates as we show in Table 1. We may able to find a peak with very low
intensity near the D peak at around 1100 cm-1. In the present work we neglect this
contribution because the integrated area of that peak is less than 5% of the G peak.
26
Fig.2.7 An example of peak fitting with a Lorentzian for both D and G peaks, a linear
background removal in the wave number region 1000-2000 cm-1 has been considered
We have obtained peak position, peak width and peak area of both D and G
peaks by Lorentzian fit. Usually the peak intensity of D and G peaks are defined
either as peak height or peak area.
27
Table.1 peak area, peak height, FWHM and ID/IG ratio of D and G peaks of the Raman
spectra in 10 selected points on the sample surface
Data Type of peak Peak Area Peak
step
Heights
points
1
2
3
4
5
6
7
8
9
10
Peak
positions
FWHM
ID/IG
Sp3cont Total peak
ent
area
1.44
0.196 182335.61
1.32
0.190 177248.62
1.23
0.204 167618.0
1.20
0.239 157345.12
1.17
0.239
1.18
0.227 158907.34
1.13
0.229 185048.81
1.09
0.239 140990.64
0.98
0.257 147658.31
1.08
0.240 131263.98
D peak
34770.16 925.35
1333.88
48.63
G peak
24131.79 309.85
1588.99
49.64
D peak
50952.81
707.9
1344.23
60.84
G peak
38722.28
298.7
1590.20
62.99
D peak
45902.47 410.02
1344.45
71.26
G peak
37455.82 346.90
1587.33
68.73
D peak
44519.59 328.03
1342.30
86.39
G peak
37116.73 313.18
1581.55
75.44
D peak
43414.07
333.0
1343.19
82.99
G peak
36928.36 322.57
1581.85
72.88
D peak
44878.31 349.79
1344.07
81.67
G peak
37956.05 329.94
1582.56
73.23
D peak
54176.30 435.72
1344.18
79.15
G peak
48136.69 411.99
1582.21
74.38
D peak
38845.44 344.86
1343.63
71.70
G peak
35409.07 318.04
1580.06
70.87
D peak
35859.27 338.12
1342.60
67.51
G peak
36526.48 327.21
1576.46
71.06
D peak
36079.17 360.64
1343.41
63.68
G peak
33384.54 309.85
1579.96
68.59
152700
From Table.1, we can clearly see that the I(D)/I(G) ratio decreases from point 1
to point 10, i.e., the I(D)/I(G) ratio decreases along the line which goes from the non
irradiated region, i.e., the diamond area near the carbon resistive line, to the laser
irradiated region or the carbon resistive line on the CVD diamond plate. The total
length of the measured line by Raman is ~6 um. In this region 16 points are set at
~400 nm each. We show here a plot of the I(D)/I(G) ratio using the Raman data spot
along a 6 micron long path. In this way, we may underline variations of the I(D)/I(G)
ratio from the laser irradiated diamond area on CVD diamonds.
28
Fig.2.8 variation of the I(D)/I(G) ratio along the data acquisition path
The I(D)/I(G) ratio decreases from 1.44 to 0.98 along the acquisition path from
the diamond area to the laser irradiated area (see Table.1) and the variation range of
the I(D)/I(G) ratio well matches the second stage of the three stages model. As we
pointed out previously, the D peak is due to six-fold rings, so that the decrease of the
I(D)/I(G) ratio implies a reduction of the number of rings in favour of the increase of
olefinic chains in the detected area.
Fig.2.9 shift of the G peak along the Raman data acquisition path
The position and width of D and G peaks have been widely used as a reference
to determine the deposition parameter, film properties as well as structures. We
concentrated on the G peak because in our case it is always present and it is the best
defined peak and essential to extract the maximum information from this peak. From
Table.1 we can see a decrease of the G peak, i.e., from the maximum value of 1590
29
cm-1 to the minimum at 1579 cm-1. The variation of the G peak position as a function
of data acquisition coordinates is shown in Fig.2.9. The decrease of the G peak
position may also refer to the increase of not ordered phases along the Raman data
acquisition path.
Even recently, the presence of sp3 sites in amorphous carbon films is not
detected by Raman. This is because most Raman experiments are performed using
visible laser excitation and at these wavelengths sp2 sites have a much larger Raman
cross section, weakening the contribution of sp3 sites. Therefore, DLCs‟ visible
Raman spectra are dominated by a broad G peak, even when the sp3 fraction exceeds
80% [Adamopoulos et al., 1999]. Extract information from the G peak is at present
the only way to improve the evaluation of sp3 sites. Although there is no relationship
between the G peak position and sp3 content, summarizing experimental data from
different sources Ferrari and Robertson were able to evaluate that the sp3 content in
HDLC films is related to the G-peak position (ωG). Accordingly, Singha et al. have
taken average of the experimental data presented in the reference [Tamor et al., 1994;
Ferrari et al., 2000] and obtained an empirical equation:
Sp3content  0.24  48.9(G  0.1580)
(2.3)
where ωG denotes the G peak position should be in terms of inverse of the
micrometer unit. The Eq. (2.3) implies that for ωG at 1580 cm-1, the sp3 fraction in the
films is ~0.24 [Singha et al. 2006]. According to the Eq. (2.3) we evaluated the sp3
content associated to the 10 selected points reported in Table.1. The variation of the
sp3 content along the path is shown in Fig.2.10.
3
Fig.2.10 trend of the variation (a) of the sp content along the path, (b) the I(D)/I(G) with
3
respect to the sp content
30
From Table.1 and Fig.2.10 (a) we see that the sp3 content increases from 0.196
to 0.257 and the variation of the I(D)/I(G) ratio is correlated with the sp3 content (see
Fig.2.10) (b). Increasing the sp3, the I(D)/I(G) decreases and we may find that the
values of the sp3 content for the first two points are <0.2, i.e., the maximum sp3 value
of the second stage, and other eight points are > 0.2. According to the three stages
model, first two points should belong to the end of the stage 2 and the others for the
beginning of the stage 3. The end of stage 2 may correspond to a completely
disordered, almost fully sp2-bonded a-C consisting of mainly six-fold rings and other
ring like configurations (consists of five-, six-, seven- and eightfold disordered rings)
and few sp3 contents according to Ferrari et al. [Ferrari et al., 2000]. The presence of
sp3 contents which exceeded 0.2 (in Fig.2.10 (b)) imply also that different crystalline
phases (e.g., ta-C) may occur at the corresponding data spots.
Another important Raman parameter is the FWHM of the G peak. According to
Casiraghi et al. [Casiraghi et al., 2005] the FWHM of the G peak probes bond lengths
and bond angle disorder in sp2 clusters, all parameters with a close relations with the
stress induced on clusters. Increasing the sp3 content, sp2 clusters within the sp3
network become smaller and more strained, causing the increase of both the bond
length and the bond angle disorder. Therefore, the FWHM of the G peak increases vs.
the sp3 content as shown in Fig.2.11.
Fig.2.11.the variation of FWHM as a function of sp3 content
It was proposed that the G peak width is partially determined by the graphitic
cluster size, [Schwan et al., 1996] and the plot of the I(D)/I(G) ratio with respect to
the G peak width should have a linear behaviour.
31
Fig.2.12. variation of the I(D)/I(G) as a function of the FWHM of the G peak.
From Fig.2.12, it can be seen that the first five points are distributed along a
straight line while others are strongly scattered. The behaviour suggests that the first
five points are strongly affected by the presence of aromatic rings while for other data
the influence of graphitic clustering is less relevant or negligible. This result further
supports previous data, obtained by considering parameters such as the G peak
position, the I(D)/I(G) ratio and the sp3 contents.
Structural information of the carbon phases on the carbon resistive line are
derived from the visible Raman spectroscopy at 532 nm. Along with the Raman
acquisition data, the number of the graphitic aromatic rings decreased while the
number of olefinic chains increased and, as the bond length also the bond angle
disorder may cause the amorphization of carbon-based phases.
32
Chapter 3
We hypothesized that laser annealed non-diamond line on a CVD diamond has
similar thermometric property of RTDs, i.e., their resistance change with temperature
in a measurable way, so that they may eventually monitor temperature changes in a
range and with an accuracy that depend on how fast this material responds to a
temperature variation.
In this chapter we will introduce specifically the material‟s R-T measurement
techniques, the experimental set-up and procedures used. Results will also present
with a preliminary analysis limited only by the number of experiments performed.
The electrical measurement includes also tests of material‟s heating response
associated to the power dissipation according to the Ohm‟s law due to the current
flowing through it. Because in these materials, the resistance changes with
temperature, they are not ideal heating systems, although a micro-heater working in a
limited temperature range could be designed using properly designed carbon-based
patterns. In fact, in a micro-heater device, the small amount of dissipated power does
not produce a significant change of the resistance. According to their thermoelectric
properties we will discuss in this chapter the possible use of our materials as microheaters.
33
3.1 Electrical property test
A key issue of the analysis presented in this thesis is the characterization of the
electrical properties of the sample, i.e., the check of resistance change in a wide
temperature range from room temperature to the liquid helium temperature (4.2 K).
Fig 3.1(a) shows the sample investigated. In the picture it is easy to identify the
two lines on the transparent plate: a-polycrystalline synthetic diamond manufactured
via the Chemical Vapour Deposition (CVD) technique.
Fig 3.1 (a) The laser irradiated diamond plate, (b) the Cu sample holder where the diamond
plate is glued for the transport experiments.
The two lines on the CVD diamond plate were obtained via a laser technique
suitable to realize micro-carbon resistive patterns on diamond plates. This type of
manufacture of relatively low cost has been realized by the Diamond Materials
GmbH, in Germany. The widths of these two lines were measured by the FIB
microscope of the LIME laboratory (University of Roma Tre, Italy) and as depicted
in Fig 2.7 are approximately 30 micron. The purpose of the measurements described
in this section is to obtain the R-T plots of these structures and investigate the type of
response vs. temperature. Actually, we are considering their thermometric behaviour
34
to manufacture a suitable micro-sized thermometer. Before discussing the
measurement, we will introduce the electrical circuit and the setup used for the
experiments.
3.1.1 Electrical setup and instruments.
To measure the resistivity of the sample vs. temperature, it is necessary to
explain the electric set-up as shown in Fig 3.4. The instruments used for the circuit
are following:
1. HP 8116A Function Generator;
2. Power Supply;
3. Oscilloscope;
4. Resistance box (62.3 Ω);
5. Agilent 34970A Data Acquisition Unit (multimeter);
6. Cryostat;
7. Cryogenic Resistive Insert;
8. Signal Recovery 7265 Lock-In Amplifier;
9. Lake Shore218 Temperature Monitor;
10. DT470 Diode Thermometer;
10. Heater Supply.
In the next we will also give a brief introduction to the characteristic of the Cryostat
and of the Lock-In amplifier because of their importance in our experimental setup.
Fig.3.2 the Helium-4 immersion cryostat used for the experiments
35
The cryostat is the device used to maintain the sample at cryogenic
temperatures. Such low temperatures may be obtained using various refrigeration
methods, most commonly using a cryogenic fluid bath such as liquid helium.
Actually, it is a stainless still vessel, similar in construction to a vacuum flask or a
dewar. Cryostats have numerous applications in science, engineering, and medicine.
In the present measurements we have applied the Helium-4 immersion cryostat
shown in Fig.3.2. It can lower the temperature to a maximum of 4.2 K by filling it
with liquid He.
A lock-in amplifier (also known as a phase-sensitive detector) is a type of
amplifier that can extract a signal with a known carrier wave from an extremely noisy
environment. It is essentially a homodyne with an extremely low pass filter (making
it very narrow band). Lock-in amplifiers use mixing, through a frequency mixer, to
convert the phase and amplitude signals to a DC, actually a time-varying lowfrequency voltage signal.
We have applied a signal recovery 7265 lock-in amplifier working in the
frequency range from 1MHz to 250 kHz, full scale voltage sensitivity down to 2 nV
and a current sensitivities up to 2 fA. Cryogenic resistance thermometry requires the
use of the four-contact technique. Using this technique a sensor with a resistance of a
few ohms can be measured accurately through leads with a resistance of several
hundred ohms. Two wires are used to supply the excitation current. The other two
wires are used to measure the voltage across the sensor. Since the low current flowing
in these wires the voltage drop along them is negligible, and their resistance can also
be neglected.
Fig.3.2 shows the four-wire contact used in present work. The double „U‟
shaped lines are separately the external and internal carbon resistive lines on the
CVD diamond platelets. In the experiment described in this work only the external
resistive line has been considered.
Fig.3.3 A schematic picture of the carbon resistive lines on diamond. In this setup the four
contacts and the four connecting lines were manufactured by FIB using gold.
36
The four contacts indicated as (V+, I+) and (V-, I-) were connected to the
external resistive lines as showed in Fig.3.3 by gold materials. They connect the
carbon resistive lines to the power source (AC power supply, 107 Hz). In this way a
complete and flexible circuit can be obtained. The small sample holder (a semicircle
copper plate shown in Fig.3.1 (b)) used to hold the sample, has been fixed on the
cylindrical copper sample holder shown in Fig.3.3.
Fig.3.4 The cylindrical Cu sample holder (on the left without the plastic cover and on the right
with the cover shielding the contacts).
As can be seen from Fig.3.4, the carbon resistive line is connected to the
external power supply using a 4-contacts setup described in Fig.3.3 and a Dt-470
diode mounted near the sample that measures the temperature of the copper plate.
The cylindrical sample holder is mounted on the insert metal bar (called cryogenic
resistive insert) long enough to allow the sample to reach the bottom of the cryostat.
Fig.3.5 (a) The electric set-up and (b) the rack with the signal recovery 7265 Lock-in
Amplifier, the Lakeshore 218 temperature monitor, the Oscilloscope and the Heater supply
37
Fig.3.5 shows the whole electric set-up for the measurement. A resistance box,
which has a constant value of 62.3 Ω is connected in series to the sample and the
voltage on the resistance box is measured by a multimeter. In this setup the current
flowing through the carbon line is the same flowing through the resistive box. It can
be calculated as
Iline  V / Rbox
(3.1)
where V is the voltage applied by the source, Rbox is the value of the resistance box
and Iline is the current flowing through the carbon line. A lock-in amplifier is used to
measure the voltage across the carbon line. The resistance of the carbon line can be
evaluated as
Rline  Vline / Iline
(3.2)
where Vline refers to the voltage measured by the multimeter and Rline refers to the
resistance of the carbon line.
An adiabatic vacuum immersion cryostat has been used to measure the R-T
characteristic of the carbon line from room temperature to the liquid helium
temperature (4.2 K). To reach such a low temperature we first filled the cryostat with
LN2 (Liquid Nitrogen). A continuous refilling with LN2 decreases the temperature
inside the cryostat down to 77 K and after several times refilling when the system is
cool enough the liquid helium starts to be poured into the cryostat until the cryostat is
full.
3.1.2 Resistivity measurement procedures
Measurements of the resistance of the carbon resistive line in the temperature
range 4.2-300 K were performed by three different methods as discussed below, after
a check that the whole circuit was properly connected.
 Fast cooling. In this procedure the temperature of the sample is decreased by
quickly lowering the insert down to the bottom of the cryostat. The cooling rate
of this procedure is very high.


Slow heating. In this case the insert is slowly lifted up with different steps.
Slow cooling. In this procedure a 2 mm thick Teflon tape is inserted between the
semicircle copper base plate on which the sample is fixed, and the bigger copper
sample holder to reduce the thermal exchange. The cooling process is achieved
by lowering down slowly the insert until it reaches the bottom of the cryostat.
38
During the three procedures, we have used the DT-470 diode sensor and a
Lakeshore 218 temperature monitor to measure the temperature. The voltage across
the carbon line, provided by an AC power supply at the constant frequency of 107
Hz, is measured by the signal recovery 7265 lock-in amplifier while a Lab-View
program controlled the whole circuit and collects data.
3.1.3 R-T curve analysis
We collected three separate sets of data, which correspond to the three
procedures mentioned above. Datasets contain time, temperature and voltage applied.
Using Eq. (4.1) and (4.2) we calculated the corresponding values of the current
flowing through the carbon line and the nominal resistance of the carbon line
obtaining the corresponding R-T curves of the non-diamond resistive line that shown
similar characteristics (seeFig.3.6).
Fig.3.6 R-T curves of the carbon resistive line on the diamond plate for the three different
procedures. (a) Fast cooling, (b) Slow cooling, (c) Slow heating, (d) Comparison of R /R
(300K) values for 10 Ohm Speer, 51 Ohm Speer, 68 Ω Allen Bradley and carbon line in three
different R-T measurements
39
In Fig.3.6 (a), (b), (c) the R-T curves show an almost linear behaviour in the
temperature range from approximately 50 K to room while a small exponential
decrease of the resistance occurs in the region below 50 K. (d) shows a comparison of
R/R (300K) character of 10 Ω Speer, 51 Ω Speer, 68 Ω Allen Bradley resistors and of
carbon line in fast cooling, slow cooling and slow heating processes. Here, R (300K)
refers to the nominal resistance value of the resistors at 300 K. In our case the R
(300K) value for the carbon line is 63.2 Ω and the R/R (300K) values vary from 1.0
to 1.2 in the range of room temperature to the liquid helium temperature (4.2 K)
while other sensors show very large ranges, for example in 68 Ω Allen Bradley
resistor the R/R (300K) values varies from 1.0 to ~1000.
The important features of the R-T relationship are the form and smoothness of
the curve and the temperature coefficients. The term smoothness here is used to point
out not only a lack of abrupt changes but also that the first and second derivatives do
not contain maxima or minima. If a curve is not smooth, it is very difficult to fit it
with an analytic expression. An accurate interpolation of temperatures measured by a
device with this drawback is a cumbersome process [Kopp et al., 1972].
For the R-T curves of Carbon Resistor Thermometers (CRTs), none of the
existing interpolation equations are suitable to allow a measurement better than 10-3 T
over a wide range including temperatures above 20 K. Many of the equations that are
used relate the lnR to 1/T in some non-linear fashion, with a number of coefficients to
be determined with a proper calibration ranging from two to five, depending by the
temperature range, the accuracy required, and the type of CRT. An original empirical
equation still widely used is
ln R  C / ln R  A  B / T
(3.3)
where A, B and C are arbitrary constants that have to be experimentally determined.
Using this equation Clement and Quinnell (1952) found an accuracy of ±0.5% in the
range 2 to 20K for a group of Allen-Bradley resistors [Bedford et al., 1997].
Interpolation or fitting calibration of data can be a problem to achieve a high
accuracy: there are no simple physical expressions for the R value of a resistance
thermometer vs. T. So, polynomial expressions with plenty of mathematical
flexibility but no physical justification are often used
ln R   M n (ln T )n
（3.4）
It is up to an experimenter to decide how many terms have to be used in the
polynomial. Low order polynomials are unable to fit data over a wide range, while
trying to fit noise, high order ones develop wiggles while outside the selected range
40
they diverge immediately [Muirhead, 1958].
To interpolate the R-T curves in Fig.3.6 with the Eq. (3.4) the logarithmic scale
was applied to plot lnR-lnT as shown in Fig.3.7.
41
Fig.3.7 The lnR-lnT interpolation curves for the: (a) slow cooling, (b) slow heating, (c) fast
cooling procedures
Fig 3.7 shows the lnR-lnT characteristics for the three procedures, i.e. (a) slow
cooling and (b) slow heating. Curves in both (a) and (b) are fitted with polynomials
of the 6th order, while the curve (c), labelled as fast cooling, is fitted with of the 7th
order.
Teble.2: Coefficients used to make linearize the curves in Fig.3.6
Coefficients
Fast cooling
Slow cooling
Slow heating
Temperature (K)
300K to 4.1K
7th order
300 to 4.1K
6th order
4.1K to 300K
6th order
M0
3.1611
4.184
4.2451
M1
2.8955
0.37934
0.24355
M2
-2.8798
-0.33743
-0.22
M3
1.5226
0.15329
0.10215
M4
-0.46354
-0.038191
-0.02697
M5
0.081264
0.0049044
0.003495
M6
-0.0076104
-0.00025662
-0.00018979
M7
0.00029394
The nonlinear relationship between temperature and resistance is described by a
higher order polynomial:
ln R  M 0  M1 ln T  M 2 (ln T )2 
(3.5)
where the coefficients M0, M1 and M2 etc. depend on the conducting material and
define the temperature-resistance relationship. The number of higher order
42
polynomial terms considered is a function of the required accuracy of the
measurement.
3.1.4 Sensitivity curve analysis
The main characteristics to select a thermometer are accuracy, reproducibility
on thermal cycling, long-term drift, magnetic errors, sensor size, and cost. Accuracy
is usually the main parameter. It is limited by the sensitivity and resolution of the
thermometer, which depend on the temperature. Sensitivity is defined as a
dimensionless quantity that gives the relative change in the sensor‟s output, e.g., the
resistance that corresponds to a given relative change in the temperature:
(dR / R) / (dT / T ) . If the sensor exhibits high sensitivity to achieve a given level of
accuracy a reduced precision is required in the measuring system. High sensitivity
sensors usually have a narrow working temperature range [Leigh, 1988].
43
Fig.3.8 Sensitivity vs. temperature of (a) fast cooling (b) slow cooling (c) slow heating (d)
comparison of (a), (b), (c) procedures with various commercial cryogenic thermometers
The relative sensitivity is also dimensionless and it is defined as
(dR / R) / (dT / T ) or equivalently | (d ln R) / (d ln T ) | . We applied the latter
expression and in Fig.3.8 we show the comparison of the relative sensitivity of
different commercial temperature sensors with our sample for three different
procedures. The sensitivity of our sample increases with the temperature and shows a
rather low relative sensitivity in all procedures compared to commercial
thermometers. The variation of the relative resistivity is rather smooth in the
temperature range from 4.2 to 293K with relative changes approximately from a
minimum of 0.005 up to 0.2.
These values point out that a carbon resistive line is a suitable cryogenic
temperature sensor. Indeed, because the relative sensitivity is |d(lnR)/d(lnT)| , lower is
the value of the relative sensitivity and faster is the change of the resistance with the
temperature.
3.1.5 Heater behaviour test
It is worth studying the heating behaviour of the carbon resistive line, when the
electric current flows through it. According to the Joule law
P  I 2R
(3.5)
a heating occurs due to the electrical transport. This critical issue has then to be
considered. For high accuracy applications the carbon resistive line cannot be suitable
to use as a heating device, because its resistance decreases with temperature (its
thermometric property). When temperature increases then generates less heat than
expected inducing a large error. However, being its nominal sensitivity quite small it
is still can be used as a microheater, in particular when due its small size, it generates
44
a negligible amount of heat, not sufficient to modify significantly its resistance.
In this work we have performed three sets of measurement to characterise the heating
behaviour of a selected sample pattern at the temperatures of 4.2, 77 and 293K.
The method applied for the three sets of measurements is the following: change
the voltage and measure the current through the carbon resistive line. Recording the
variation of the temperature and then calculate the corresponding dissipated power
according to the Eq. (3.5). At the end, check the increment of the temperature due to
the dissipated power.
During these measurements we used the same electrical set-up and instruments
applied for the R-T measurements shown in Fig. 3.4. We have collected three sets of
data corresponding to measurements at 4.2, 77 and 293 K, respectively. Data
collected contains the values of voltage applied, measured current, measured
resistance and time. To see the variation of temperature with respect to different
voltages applied during the measured time we provided temperature vs. time curves
corresponding to the sets of measurement introduced above.
Fig.3.8 shows the change of the temperature in response to the change of current vs.
time at 4.2, 77 and 293 K, respectively.
45
Fig.3.8 Time-temperature characteristics in response to different voltages (heating
behaviour) at the temperature of (a) 293 K, (b) 77 K, and (c) 4.2 K.
Fig.3.8 (a) shows the power dissipation on the carbon resistive line at the
temperature of 4.2K reached by immersion of the sample into the liquid helium bath.
Because of the self-heating contribution, the temperature of the near region of the
carbon line increased and the temperature change was detected by a DT-470 diode
thermometer. The resistance of the carbon line is calculated by
I V / R
(3.6)
At each voltage loop, e.g., 100 mV to 1 V, 2 V etc. changes the current I flowing
through the carbon line according to the Ohm‟s law. Apparently, the dissipated power
also changes correspondingly.
From Table.3 we can see that the increasing power causes changes for the
resistance. According to the thermometric property of the sample the resistance
should decrease with increasing temperature. Here, we can find that the resistance
increases initially for the small increase of dissipated power and it starts to decrease
46
with the increasing power and the variation is approximately of the order of 10-2. As
we have assumed previously, the decrease of resistance can be explained by its
thermometric property, i.e., the increase of the dissipated power generates more heat
and the latter further increases the temperature of the sample and of the near region.
The increased temperature induces a lowering of the resistance.
Table.3 the measured resistance, voltage applied, electric current and calculated power for
the case described in Fig.3.8 (a).
Carbon resistance (Ω)
Voltage (V)
Current (A)
Power (W)
77.64
77.80
100mV
500mV
628.3uA
3.126mA
30.63uW
761.0uW
77.83
77.72
1.0V
1.5V
6.341mA
9.495mA
3.13mW
7.004mW
77.62
2.0V
12.66mA
12.44mW
To investigate the effect induced by the temperature we have performed similar
measurements at 77 and 293 K. Before testing the heater behaviour at 293 K, we
inserted a 2 mm thick Teflon layer between the sample holder as in Fig.3.1 (a) and
the main copper sample holder in Fig.3.2. The Teflon layer is a thermal insulator and
highly reduces the thermal exchange between the sample and the sample holder,
affecting the change of temperature due to the power dissipation.
Table.4 displays the same characteristic data corresponding to the case in
Fig.3.8 (b). This measurement was carried out at 293 K, a much higher temperature
of the previous measurements. From Table.4 we find that the change of resistance
with the increasing dissipated power is negligible and this can be due to the relative
sensitivity of the sample. We have seen from Fig.3.7 (d) that the relative sensitivity of
the sample during the three cooling procedures increases with the temperature. From
the definition of the relative sensitivity, lower is the value of the relative sensitivity,
faster will be the change of resistance with temperature. For this reason the variation
of the resistance due to the dissipated power at 293K is negligible.
A similar behaviour is shown in Fig.3.8 (c). For this procedure the sample is
fixed to the sample holder insert and immersed into a vessel with a liquid nitrogen
(LN2) reaching an intermediate temperature but using the same electrical set-up of the
previous measurements. From Table.5 we show that the change of the resistance with
the increasing dissipated power is small, i.e., of the order of 10-2Ω. We also point out
that the noise in the plot in Fig.3.8 (c) is due to acoustic noises and vibrations present
in the laboratory during the experiments.
47
Table.4. the measured resistance, applied voltage, electric current and calculated power for
the case described in Fig.3.8 (b)
Carbon resistance (Ω)
Voltage (V)
Current (A)
Power (W)
64.81
100mV
626.9uA
25.42uW
64.81
1V
6.34mA
2.60mW
64.81
1.5V
9.49mA
5.83mW
64.81
2V
12.66mA
10.39mW
64.81
2.5V
15.82mA
16.21mW
Table.5. the measured resistance, applied voltage, electric current and calculated power for
the case described in Fig.3.8 (c)
Carbon resistance (Ω)
Voltage (V)
Current (A)
Power (W)
73.27
100mV
627.1uA
28.82uW
73.31
1V
6.35mA
2.95mW
73.29
2V
12.66mA
11.77mW
The heating efficiency is an important parameter to characterize a micro heater
device. It can be defined as the rate of change of temperature of the object due to the
power dissipation in the material. For our sample we compared the temperature
change at 4.2, 77 and 293 K for the currents of 6.35 mA and 12.67 mA.
Table.6. Comparison of the values of temperature changes (∆T) at 4.2, 77 and 293 K for
electric currents of 6.35 mA and 12.67 mA.
Voltage (V)
Current (A)
1V
6.35mA
2V
12.67mA
∆T (K)
(4.2K/77K/293K)
0.07K/0.02K/0.02K
∆T (K)/T(K)
(4.2K/77K/293K)
1.7×10-2/2.6×10-4/6.8×10-5
Dissipated power (W)
3.13mW /2.60mW /2.95mW
0.15K/0.12K/0.13K
3.6×10-2/1.6×10-3/4.4×10-4
12.44mW /16.21mW /11.77mW
From Table.6 we see that values of ∆T for each characteristic voltage are in the same
order of magnitude and according to this we may claim that the heater exhibits a
similar heating efficiency for all temperatures.
48
Chapter 4
In this chapter we will give an introduction to the temperature sensing
technology and thermometers particularly to the Resistive Temperature Detector
(RTD), a kind of temperature sensing device with similar thermometric property of a
laser irradiated CVD diamond.
According to the investigated thermometric property of a selected sample we
propose a simple design of a resistive temperature sensor. The extreme properties of
diamond may trigger other applications such as single photon detector devices. We
predicted that a CVD diamond plate must be a good substrate material for a
superconducting single photon detector; more interestingly the laser ablation and FIB
technique possibly be available to fabricate such device.
49
4.1 Temperature sensing technologies
Among the sensing technologies, temperature sensing is the most common.
Temperature is usually measured in terms of physical properties such as the pressure
of a gas, the equilibrium vapour pressure of a liquid, the electrical resistance, the
magnetic susceptibility, and the junction voltage of a diode [Kar et al., 2007]. Based
on the measurement of physical properties several temperature sensing techniques are
currently in use. The most common of these are RTDs, thermocouples, thermistors,
and integrated Silicon Based Sensor [Baker, 1998]. The best device option for the
application depends on the required temperature range, linearity, accuracy, cost,
features, and easiness of design of the necessary support circuitry.
Each of these sensor technologies is suitable for specific temperature ranges
and environmental conditions. The sensor‟s temperature range, ruggedness, and
sensitivity are just a few characteristics that are used to determine whether or not the
device will satisfy the requirements of the application. However, none temperature
sensor is suitable for all applications [Baker, 1998].
It is then important choose a suitable temperature sensor for a specific
application. The sensor has to be chosen based on the required resolution, precision,
and reproducibility. Further, the sensor has to withstand the effect of thermal cycling.
The selection of a sensor for a specific application then requires the identification of
the most important parameters of the application among the many such as: operating
temperature range, type of excitation, sensitivity, package size, thermal and electrical
response times, power dissipation, and environmental compatibility [Kar et al.,
2007].
Some important factors mentioned above can be defined as following:
Resolution is the smallest increment that a measuring device can measure;
Precision or accuracy is the degree of closeness of the measurement to the
quantity‟s actual value.
Reproducibility (repetition accuracy) is the maximum deviation of the
measured value taken from a series of measurements under the same operating
conditions.
50
4.1.1 Primary and secondary thermometers
Thermometers can be divided into two separate groups according to the level of
knowledge about the physical basis of thermodynamic laws and quantities: primary
and secondary thermometers.
Primary thermometers are measuring instruments that enable the temperature to
be determined without any previous calibration with other thermometers. In all cases,
physical variables linked with temperature by physical relationships are measured.
For example, gas thermometers, noise thermometers or the measurement of black
body radiation. These measurements often involve considerable effort and expense.
Industrial applications primarily involve the use of secondary thermometers,
i.e., sensors that have to be calibrated. In practical case, resistance thermometers or
thermocouples are frequently used.
When positioning the measuring devices in the experimental setup one simple
but fundamental principle must be observed: the sensor has to be installed as close as
possible to the point of measurement and should be shielded as much as possible
from the environmental source of heat. It is also important to understand that the
sensor primarily measures it own temperature.
Larger temperature differences between the ambient temperatures of the
medium may result in systematic errors. If the sensor is packed in a thick protective
box, this means that it is not in a good thermal contact with the actual measuring
point. Temperature changes will slowly be detected or with a not negligible error
[Ebinger, 2000].
4.1.2 Resistive temperature detectors (RTD’s)
The electrical conductivity of a metal depends on the motion of electrons
through its crystal lattice. Due to thermal excitation, the electrical resistance of a
conductor varies according to its temperature and this forms the basic principles of
resistance thermometry.
Resistance thermometers are called RTD‟s (resistance temperature detectors)
while thermistors are temperature sensors that change electrical resistance with
temperature. The superior sensitivity and stability of these devices in comparison to
thermocouples, give them important advantages in low and intermediate temperature
ranges. Resistive thermometers exhibit the most linear signal with respect to
temperature of any sensing device and able to sense temperatures with extreme
accuracy, have consistent and repeatable performance and a low drift error. Small
deviations from the straight-line response, however, require the use of interpolating
51
polynomials to calculate resistance values among defined temperature points.
There are three main categories of RTD sensors; thin film, wire-wound, and coiled
elements. While these types are those most widely used in the industry there are other
applications were more exotic systems have to be used, e.g., carbon resistors are used
at ultra low temperatures
Carbon resistors have been used as cryogenic thermometers during the past 20
years because of their high sensitivity at low temperatures from 1 K to 100 K. AllenBradleys were introduced as temperature sensors by Clement and Quinnell (1952).
Although CRT these are less reproducible than metallic resistance thermometers, they
are very popular because of their small size and low cost. The resistance of a typical
CRT unit is roughly 1 kΩ at 1 K [Kar et al., 2007].
52
4.2 Possible designing of a practical temperature sensor and single photon
detector using synthesized diamond plates
In previous chapters we presented and discussed the characterization of laser
irradiated CVD diamonds. The original material, i.e., CVD diamond triggered a great
attention because of its prominent physical properties suitable for many applications
in science and industry. In this work, we studied not only the material‟s physical
properties but also investigated applications and proposed a design of two new
devices, completely different for the applications but similar in the manufacturing
technique: a resistive temperature sensor, and a superconducting single photon
detector.
4.2.1 Design of a resistive temperature sensor
A thermometric property, namely the resistance change vs. temperature, is
detected by measuring the resistance in the range from 4.2 to 293 K. Like many
carbon resistive thermometers (CRTs), our sample shown a smooth exponential
increase in the resistance with decreasing temperature, as seen in Fig.3.6. Our sample
shows a nominal resistance of 64.7 Ω at 293 K that increases up to 77.6 Ω at 4.2 K.
Since the sample performed a significant thermometric property and a reasonable
sensitivity in a cryogenic temperature region, it can be applied as a practical
temperature sensor.
As shown in Fig.3.1 (a), a shape similar to the letter „U‟ patterned on the CVD
diamond plate with a length of around 1.5 cm and width of 30 um exhibits such a
thermometric property. The measured resistance of this line is ~ 65 Ω at room
temperature.
A rough sketch of a temperature sensor is given in Fig.4.1 according to the
layout of common thin film RTDs. Unlike a simple thin film RTD, e.g., a platinum
RTD, the sensing element in our device is not metal but a continuous irradiated
carbon resistive line. In the case of PRTDs the Pt metal should be evaporated on
substrate and a patterning technique must be applied to form Pt conducting wires.
53
Fig.4.1. A four point contact temperature sensor made by laser irradiated of a synthetic CVD
diamond plate. Black lines are the carbon resistive lines while the yellow lines are conducting
wires made by gold using the FIB technique.
According to R-T results discussed in chapter 3 (see Fig.3.6), we estimated that
to have the ~1kΩ resistance of a typical platinum RTD, a continuous line of
approximately 23 cm has to be patterned. A 1.2×1.2 cm2 CVD diamond plate has
been considered in our layout and a carbon resistive line 30 um wide and 23 cm long
has to be patterned on the surface. The shape of this patterned line is shown
inFig.4.1: one vertical line and 28 horizontal lines ~0.8 cm length for each are
required.
The black line shown in the picture is the carbon resistive line produced by
laser irradiation and it has approximately 1 KΩ resistance. To eliminate the effect of
the external lead wire resistance we considered a four-point contact method. Two
yellow lines are for the current flowing through the carbon resistive line while the
other two yellow lines are for the applied voltage. The four-point contact may fix
differences in lead resistance.
In our layout, we considered that the four-wires together with the four end
contact points can be manufactured by FIB technology. In principle, other techniques,
such as the metal evaporation with chemically etching or laser ablation could also be
considered. The four lead wires (blue) are spot welded to the external contact points
and the junction is then covered with a drop epoxy to help holding the external leads
firmly on the welding points.
In the case of platinum thin film RTDs, a very thin glass layer is usually coated
on the surface of the platinum metal layer to protect it from harmful chemicals and
gases. For our device, producing a coating layer is an option because the carbon
54
resistive lines are relatively more stable resistant to effects induced by chemicals or
gases than platinum.
As previously introduced, RTDs can read the resistance by measuring a
variation of the voltage and the resistance is converted into the temperature by a
proper device. Also in our case, the temperature of the measured object is obtained
using an external temperature monitor connected to the device.
4.2.2 Possible layout of a superconducting single photon detector
The information contained in the previous chapters and the expertise associated
with the patterning and characterization methods of carbon based materials such as
laser ablation and FIB, could be used also to design unique photonics devices of
submicrometer dimensions. Among the many, a possible example of feasible device
is a superconducting single photon detector made by a CVD diamond plate as the
substrate.
Superconducting single photon detectors (SSPDs) may offer single-photon
sensitivity from visible light to mid-infrared wavelengths. The operation principle of
such devices is based on the formation of a normal state after absorption of a single
photon. The first detectors of this type have been developed a decade ago by
Gol‟tsman and colleagues [Gol‟tsman et al., 2001]. Experimental tests showed that
the absorption of a photon in a nanowire induces a supercurrent and a resistive hot
spot is formed, leading to a reasonable voltage pulse if the cross section of the
nanowire is small enough. The early detectors were based on a NbN superconducting
wire about 10 nm thick and 200 nm wide. These detectors operate at cryogenic
temperature well below the critical temperature (T c) of the device.
In our layout, we started from a classical design: a SSPD similar to the device
introduced by Gregory et al., based on a different material and different patterning
methods. The fabrication procedures we propose:
1) polish a surface of 1.5×1.5 mm2 of thin CVD diamond plate;
2) evaporate an interface material (Cr, ~5 nm thick) on the surface of the
CVD diamond substrate.
3) deposit a 100 nm thick Ti superconducting metal layer (Tc=0.39K)
4) pattern the superconducting metal layer (e.g., by laser ablation) and form a
8 um wide continuous superconducting wire.
5) built a four-wire contact with gold using FIB milling technique.
55
Fig.4.2. Schematic layout of a superconducting single photon detector, (Top panel) the
surface of the simulated SSPD; (lower panel is a side-view of the device. The light blue refers
to the CVD diamond substrate, the green lines are the Ti metal wire, the dark blue lines are
the external lead wires, the black lines are the laser ablated area, the yellow lines are the
four-contact wires and the orange is the Cr interface layer.
Our SSPD layout consists of three layers of material i.e., a synthetic CVD
diamond substrate (1.5×1.5 mm2), a Cr interface layer (5 nm thick) and a Ti layer.
The Cr interface should allow the growth of a homogeneous Ti metal layer ~100 nm
thick. Actually, an isotropic Ti metal layer can be deposited with this technique but,
after that, the thin Ti layer has to be patterned to obtain a continuous wire. In the
literature, the electron-beam lithography is widely used for SSPD manufacturing
while our design allows faster and cheaper procedure based on the the laser ablation.
In Fig.4.2 we show a schematic layout for a proposed SSPD. The 1×1 mm2
diamond area is represented by black and green colours. The black line (~40 micron
wide) refers to the laser ablated area on the Ti layer while the green stripes (~8
micron wide) is a continuous Ti wire formed after the removal of the Ti (indicated in
black).
The size of the meandering (a curved of bended shape) Ti wire is ~22 mm long
and ~8 micron wide. The filling factor (ratio of the occupied area by the
superconducting meander to the nominal area) of the device is ~0.17. A device of this
56
size is assumed to be suitable for as IR detectors because these dimensions are
comparable to the IR wavelengths
To enhance performances, when the length of the superconducting wire is large
compared to both width and thickness, it is important to have a uniform cross section
all along the wire. In our case it is possible to obtain a flat and homogenous thickness
in the superconducting layer while a control of the width of the wire is much more
critical. Indeed, patterning of the superconducting layer with a laser beam hardly
produces a uniform width of the removed pattern (black stripes in the figure).
Moreover, also the convex edges of the black stripes result in a not uniform shape of
the superconducting wire. Errors due to non-uniformity of the stripes can be roughly
estimated by FIB/SEM images of the carbon resistive line (see Fig.1.8). The
maximum observed convexity is ~2 micron to compare with a 30 micron nominal
width, i.e., with a maximum error of ~7%. Since non-uniformity in the cross section
of the superconducting wire induces differences in the critical current along the wire,
this results in a degradation of the accuracy in the measured photon energy.
Much more uniform superconducting wires could be produced by FIB milling,
another possible technical method in principle capable to manufacture such
structures. However, with the accuracy required the FIB procedure is a very time
consuming process and to obtain a superconducting stripe on a 1×1mm2 area it may
require tens of hours. As a consequence, the FIB milling technique could be suitable
only to produce much smaller size SSPDs, e.g., a 100×100 um2 device, with
nanometer size superconducting stripes characterized by extremely uniform cross
sections. The FIB patterning can then extend the detection range of these devices to
the visible light region with high accuracy capability.
Other important parameters like detection efficiency, dead time, timing jitter
and dark counts, should be considered to fully characterize a SSPD. However, a
discussion of the performances of a real device starting with its parameters is possible
only with a final layout. This chapter only focus the possible use of new carbon-based
materials, new manufacturing techniques and geometries of SSPD devices.
57
Conclusion
Due to its many extreme physical and mechanical properties diamond plays an
important role in many challenging scientific areas and technologies, although its
high cost still prevent a wider use in many standard applications that would have a
great benefit from the possible use of diamond based materials. The recent advances
in the CVD technology made it possible to the synthesis of large polycrystalline
diamond films at reasonable cost and quality allowing new applications and
researches. In this thesis we will describe the characterization of laser irradiated
synthetic polycrystalline CVD diamond plates. The main aim of this work was to
improve the understanding of morphological and physical properties of these
materials and in particular to investigate the effects of laser irradiation or ion beam
exposure on high quality CVD diamond plates to explore new potential applications
of this exceptional material.
Starting from the knowledge of the diamond atomic structure and the electronic
configurations of a carbon atom and the carbon allotropes we attempted to
characterize carbon based materials obtained by laser irradiation. A high spatial
resolution analysis of the irradiated area has been performed using the FIB/SEM
microscopy. These images at nm resolution clearly showed precise carbon-based
pattern ~30 um wide, alterations and damages of the original diamond substrate.
After the morphological analysis a spectroscopic characterization of this
material has been performed using Raman spectroscopy at 1 micron spatial resolution
and 10 cm-1 spectral resolution. This approach allowed us to present and discuss a full
compositional analysis of the irradiated materials. Pure diamond, glassy carbon and
amorphous carbon phases have been identified and mapped in the irradiated region.
The analysis show that the irradiated region is mainly formed by sp 2 bonded aromatic
rings with additional olefinic chains and from the edge to the centre of the carbon line
the number of aromatic rings decreases while the number of chains increases.
Because the carbon based patterns have different electronic properties from diamond,
an electrical characterization of the material has been performed.
Diamond is an insulator with unique heat transport properties and we realized on one
carbon based pattern, using the FIB technique, four contacts and connecting wires
with pure gold material. This simple layout has been used in the LAMPS laboratory
at the Laboratori Nazionali of Frascati to perform electrical transport experiments.
The data collected showed that the carbon resistive line exhibits thermometric
properties with a high sensitivity at low temperatures. Moreover, the carbon resistive
line is characterized by a heating behaviour that may dissipate heat energy according
to Joule‟s law.
58
Summarizing, the main results of this work are:
a) several ten micron size patterns can easily be obtained by laser irradiation with a
minimum of submicron sized convexes at the edge. The quality of these patterns can
be highly improved by a focused ion beam; and a large surface area could still be
patterned on diamond by a beam raster scan, although this is a very time consuming
process for both laser and FIB;
b) the newly formed carbon phases obtained in the irradiated region are due to a
phase transformation induced by the high power of the laser. The phase
transformation can be explained in the framework of a graphitic clustering. More sp 2
bonded olefinic chains with a few sp3 contents are found in the central region of the
carbon line while more sp2 bonded aromatic rings occur near the edge. As a
consequence a more intense laser beam induces a greater amorphization;
c) the measured R-T characteristics of the carbon resistive line points out the
occurrence of a thermometric character similar to RTDs. The curves of sensitivity vs.
temperature suggest also that the carbon line is a suitable cryogenic temperature
sensor;
d) The heater behaviour obtained from the curves of the electrical power induced
temperature change as a function of time. The initially investigated thermometric
property of the carbon line (R-T behaviour) confined its application only as a microheater.
The occurrence of a thermometric property of the laser irradiated resistive line
suggested the application in the temperature sensing technology and we present in
Chapter 4 a simple layout of a resistive temperature sensor. The advancements in
CVD technology that made possible to manufacture large polycrystalline and also
single crystalline diamond thin films, coupled with the unique physical properties of
these synthetic diamonds such as high optical transparency and extreme hardness,
and the technical availability of patterning and polishing make possible new
applications also in the photonics area. In the chapter 4 we present a simple layout of
a superconducting single photon detector.
59
References
Q. Hu, Diamond-Based Material: Synthesis, Characterization and Applications,
(2011)
A. C. Ferrari, J. Robertson, Physc. Rev. B61, 14095, (2000)
M. Rydbachuk, Fabrication and properties of diamond-like carbon films in discharge
plasmas, Doctoral thesis, Queensland University of Technology, (2007)
J. Robertson, Diamond-like amorphous carbon, Material Science and Engineering,
37, 129-281, (2002)
S. Majdi, Electronic Characterization of CVD Diamond, (2010)
Paul W. may, CVD Diamond – a New Technology for the Future? Endeavour
Magazine, 19, 3, (1995)
J. Robertson, Advances in Diamond-like Carbon, Material science and Engineering,
37, 129, (2002)
J. Smedley, C Jye, J Bohon, T. Rao, D. A. Fischer, Laser patterning of diamond. Part
II. Surface nondiamond carbon formation and its removal, Journal of Applied Physics
105, 123108 (2009)
C.L. Chao, W. C. Chou, K. J. Ma, T. T. Chen, Y. M. Liu, S. W. Huang and H. Y. Lin,
Machining of CVD diamond film by RIE, Laser ablation and Thermo-chemical
Polishing, (2005)
C.A. Volkert and A.M. Minor, Focused Ion Beam Microscopy and Micromachining,
MRS BULLETIN, 32, (2007)
A. Marcelli, A. Cricenti, W. M. Kwiatek and C. Petibois, Biological applications of
synchrotron radiation infrared spectromicroscopy, Biotechnology advance, (2012)
P. K. Chu and L. H. Li, Characterization of amorphous and nanocrystalline carbon
films, Material Chemistry and Physics, 96, (2006)
P. R. Chalker, Characterization of diamond and diamond-like films, in: R.E. Clausing
(Ed), Diamond and diamond like films and Coatings, Plenum Press, New York, 127150, (1991)
A. C. Ferrari, J Robertson, Phys Rev B, (2000)
M. Kahn, M. Cekada, R. Berghauser, W. Waldhauser, C. Bauer, C. Mitterer, E.
Brandstatter, Accurate Raman spectroscopy of diamond-like carbon films deposited
by an anode layer source, Diamond & Related materials, 17, 1647-1651, (2008)
J. Robertson, properties and prospects for non-crystalline carbons, Journal or NonCrystalline Solids, 299-302, (2002)
A. C. Ferrari, Mat. Res. Soc. Symp. Proc., 675, Material Research Society, (2002)
J. Robertson, Diamond-like amorphous carbon, Material science and Engineering, 37,
129-281, (2002)
60
D.G. McCulloch, S. Prawer, J.appl. Phys, 78, 3040, (1995)
A. C. Ferrari, A model to interpret the Raman spectra of disordered, amorphous and
nanostructured carbons, Material research society, 675, (2001)
W.G. Cui, Q.B. Lai, L. Zhang, F.M. Wang, Quantitative measurements of sp 3 content
in DLC with Raman spectroscopy, Surface & Coatings Technology, 205, (2010)
H. Kagi, K. Takahashi and A. Masuda, Raman Frequency of Graphitic Carbon in
Antarctic Urelites, Proc. NIPR Symp. Antarc. Meteorites, 7, 252-261, (1994)
C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev.B, 72, (2005)
G. Adamopoulos, K .W .R. Glikes, J. Robertson, N. M. J. Conway, B. Y. Kleinsorge,
A. Buckley, D. N. Batchelder, Ultraviolet Raman characterization of diamond-like
carbon films, Diamond and Related Materials, 8, 541-544, (1999)
M. A. Tamor, W. C. Vassell, K. R. Carduner, Appl. Phys. Lett., 58, 592 (1991)
A. Singha, A. Ghosh, N. R. Ray, A. Roy, Quantitative Analysis of Hydrogenated DLC
Films by Visible Raman Spectroscopy, cond-mat.mtrl-sci, 2, (2006)
J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S. R. P. Silva, Raman spectroscopy on
amorphous carbon films, J. Appl. Phys. 80 (1), 1, (1996)
F. Kopp, Carbon Resistors as low Temperature Thermometers, Review of Scientific
Instruments, 43, 327, (1972)
R.E. Bedford, T. J. Quinn, Techniques for approximating the international
temperature scale of 1990, (1997)
C. Murinhead, Temperature measurement and control, (1958)
Thermometry, Oxford instruments
B. Baker, Temperature Sensing Technology, Microchip Technology, (1998)
J. R. Leigh, Temperature measurement and control, (1988)
S. Kar, R. Sharma, Cryogenic Temperature Sensors, Defence Science Journal, 57, 3,
195-208, (2007)
Klaus Ebinger, Basic Information on Temperature Sensors, Turck industrial
Automation, (2000)
G.Gol‟tsman, O. Okunev, G. Ghulkova, A. Lipatov, A. Semenov, K.Smirnov, B.
Voronov, A. Dzadanov, C. Williams, and R. Sobolewski, Picosecond superconducting
single-photon optical detector, Applied Physics Letter, 79, 705-707, 6, (2001)
61
Acknowledgments
I would like to express my greatest gratitude to those who have helped and supported
me throughout the work of the thesis. I am deeply grateful to my supervisor
Professor. Roberto Gunnella, whose kindly guidance and encouragement enabled me
to work at the Laboratori Nazionali di Frascati of the INFN (National Institute for
Nuclear Physics) for the experimental part of the thesis.
It is with immense gratitude that I acknowledge the support and help of my Cosupervisor Dr. Augusto Marcelli who provided me the chance of work in an advanced
and comfortable laboratory environment and his continuous support and guidance
made it possible to complete this work.
This thesis would not have been possible without the support and experimental
leadership of Dr. Daniele Di Gioacchino and Dr. Alessandro Puri who helped during
the experiments and also in the analysis.
I would like to thank Dr. E. Woerner at the Diamond Materials GmbH who fabricated
the CVD diamond plates and at the same time I want to acknowledge Prof. E. Pace
and the Florence section of INFN for their collaboration to realize the pattern on
CVD diamond plates.
I owe my deepest gratitude to Dr. M. Piccinini and Dr. A. Notargiacomo who
provided measurements and data for the structural analysis of the material studied in
this thesis.
I wish to thank Dr. Riccardo Natali, who suggested me the idea to design
micordevices and helped me to work on the layouts described in this thesis.
A special thank of my mine goes to my parents for their undivided support and
interest who inspired me and encouraged me to go my own way, without whom I
would be unable to complete my project. At last but not the least I want to thank my
friends and colleagues who appreciated me for my work and motivated me.
62

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