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Plasma-assisted deposition techniques for hard coatings
Vacuum/volume 41/numbers 7-9/pages 2190 to 2195/1990
0042-207X/9053.00 + .00
Printed in Great Britain
© 1990 Pergamon Press plc
Plasma-assisted deposition techniques for hard coatings R F B u n s h a h and C D e s h p a n d e y , Department of Materials Science and Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA 90034, USA
This paper reviews the most commonly used deposition techniques for hard coatings, namely Plasma-Assisted Chemical Vapor Deposition (PACVD), Reactive Sputtering (RS) and Activated Reactive Sputtering (ARE). The role of plasma parameters and process parameters as well as their interdependency is discussed in terms of the three steps in deposition of films, i.e. generation of the depositing species, transport from source to substrate and film growth on the substrate.
1. Introduction The use of plasma-assisted vapor deposition (PAVD) processes for deposition of compounds (oxides, carbides, nitrides, sulfides, etc.) has spread into various types of industrial applications. These include dielectric films for microelectronics, optical and magnetic applications, hard carbide and nitride films for cutting and forming tools, sulfides for solid-state lubrication and solid electrolytes, etc. In fact, PAVD methods can be said to have opened up a new area in materials synthesis. Many compound films, which were hitherto difficult to deposit, are now routinely synthesized. The role of the plasma and its effect on growth and properties of the film in any plasma-assisted deposition technique is, however, very complex. To accomplish this, our understanding of the detailed physics and chemistry of these glow discharges must be improved. In this paper, we will briefly discuss the role of the plasma in film growth by plasma-assisted processes to highlight the plasma parameters that are significant in controlling the growth and properties of the films. 2. Model of film growth by vapor deposition techniques Film growth by any vapor deposition technique can be seen in terms of three basic steps: (1) generation of the depositing species; (2) transport from source to substrate; (3) film growth on the substrate. Additionally, if a plasma is employed in the deposition process, one has to understand the effect of the plasma on each of the three general steps given above ~. Thus, there are two sets of interactive parameters in all plasmaassisted processes. They are the plasma parameters (electron density, electron energy and electron energy distribution function) and the process parameters (evaporation/sputtering rate, reactive/inert gas pressure, flow rate, substrate temperature, substrate bias, etc.). The model of film growth by PAVD process can therefore be schematically represented as shown in Figure I. One might therefore picture in a plasma-assisted deposition process that the depositing species undergo various types of reactions in the 2190
KINETIC S
PLASMA
RATEOF GENERATION ] OF VAPORSPECIES NATURE OF GASESUSED, I FLOW RATES,PRESSURE i ~ INLET LOCATION, ETC. I
]
~[ GENERATION(EXCITATION)
CONFIGURATION,EXCITATION v~ MODERF, D.C.,MICROWAVE, LASER, ETC. POWER/CURENTSUPFK)RTING
I
SUBSTRATE POSITION 0NRT PLASMA) POTENTIAL TEMPERATURE TYPE - INSULATINGOR CONDUCTING
Figure 1. Schematic showing the effect of plasma and deposition variables on the three steps of film growth by vapor deposition processes.
plasma leading to the formation of excited neutral species, ions, free radicals, etc. which may react to form a precursor species that in turn deposit on the substrate, migrate on the surface, react and form the film. The reactions forming the above species, i.e. the plasma volume chemistry, in turn are controlled by both the process as well as the plasma parameters, as discussed below. 3. Plasma parameters relevant to film formation The most important plasma parameters relevant to film growth and properties in any plasma-assisted process are electron density, electron energy and electron energy distribution function. Electrons play a primary role in sustaining the plasma by ionization. The ionization efficiency of the electron is energy dependent. It increases with energy and passes through a maximum for electrons in the energy range of 50-100 V. It is therefore desirable to have low energy electrons (50-100 eV)
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
for optimum ionization. It is in this respect that a process like activated reactive evaporation has some advantages, as will be discussed in a later section. The rate of chemical reactions occuring in plasma is given by 2 (1)
R A = neV A
where ne is the electron number density and vA is the collision frequency for a given reaction A. The collision frequency VA can be written as:
VA = N
f:
(E/2me) l/2aA(E)f(E)dE
(2)
where N is the number density of colliding species, a(E) is the collision cross-section and fiE) is electron energy distribution function. Substituting equation (2) into (1), the rate of reaction can be written as:
R A = NeN
(E/2me)~/2aA(E)f(E)dE.
In order to achieve better control of film properties, it is desirable to independently control the above parameters. However, it is not always possible in all the deposition processes to achieve the process parameter flexibility conferred by the ability to vary them independently of each other. The nature and degree of intercoupling of the variables controlling the above parameters determine the advantages and limitations of a given deposition process. The presence of a plasma introduces additional contraints as some of the variables controlling the process parameters also affect the plasma parameters. To understand and optimize plasma assisted deposition processes, it is necessary to evaluate this inter relationship between the process parameters and plasma parameters. 5. Plasma-assisted deposition teclmiques in current usage
(3)
If the electrons are assumed to have a Maxwellian velocity distribution at a temperature Te and if the cross-section for a given reaction is approximated by a step function of magnitude a 0 and threshold energy E 0 then the reaction rate is given by E
RA = neNtroVe(l + ~ 3 ) .
over, ion bombardment of the growing film can also lead to reduction of absorbed impurities and trapped gases in the films.
(4)
Equation (4) reveals that the rate of any chemical reaction in a plasma is primarily dependent on electron density, electron energy and distribution function fiE). Thus it is advantageous to have independent control over these parameters to adequately control the reaction occurring in the plasma, i.e. the plasma volume chemistry and hence the structure and properties of the deposited films. 4. Deposition parameters relevant to film growth Apart from the plasma parameters, the deposition parameters also influence the growth and properties of the films, produced by any vapor deposition process. The most important deposition parameters are: (I) Rate of generation of vapor species which determine the deposition rate and stoichiometry of the films. (2) Partial pressure of all species in the gas phase which determines the mean free path of these species and hence affect the growth rate. In reactive deposition processes, parial pressures also determine the probability of the collisional reactions between various atomic and molecular species during transit from source to substrate and hence influence the formation of precursor molecules which in turn affect the growth and properties of the film. (3) Gas flow rate is an important process parameter, particularly in reactive deposition processes, since along with the metal species in the vapor phase, it controls the stoichiometry of the films. (4) Substrate temperature controls the composition, structure and morphology of the films by affecting the adatom mobility on the substrate as well as the rate of any chemical reaction occurring on the substrate. (5) Substrate bias, together with substrate temperature, also influences the structure and morphology since it controls the intensity of the ion bombardment of the growing film. More-
The most commonly used plasma assisted techniques for the deposition of compounds can be classified under the following two categories: (I) Plasma-Assisted Chemical Vapor Deposition (PACVD) processes. (2) Plasma Assisted Physical Deposition (PAPVD) Processes such as: (i) Reactive Sputtering, (RS) using dc, rf, magnetron geometries and ion beams. (ii) Activated Reactive Evaporation (ARE). They are briefly discussed below. 5.1. Plasma-assisted chemical vapor deposition. Plasma-assisted chemical vapor deposition involves forming solid deposits by initiating chemical reactions in a gaseous discharge 3. The discharge can be excited by using either if, microwave or photonic excitation. It produces a wide variety of chemical species in ionized and excited states, free radicals as well as ions and electrons. The nature, type, concentration and energy of these species determine the growth and properties of the films. The important parameters controlling film growth by PACVD are as follows: (i) Reactant partial pressure and flow rate. (ii) rf power. (iii) Substrate temperature and substrate bias. The above variables affect process parameters such as deposition rate on the one hand and plasma parameters such as electron density, electron energy and distribution function on the other. For example, the partial pressure of the reactant gas together with rf power determines the rate of dissociation of the reactive gas and hence the deposition rate. These same process variables also determine the electron energy and electron density. As the substrate floating potential depends on average electron energy, pressure and rf power (which in turn controls the substrate bombardment), the substrate bombardment of the growing film and deposition rate are both dependent on the same set of process variables, i.e. pressure and rf power. This interdependence of process and plasma parameters makes it difficult to obtain high deposition rates by PACVD processes. A variety of reactor designs have been used for carrying out PACVD in the laboratory. However, only parallel plate reac2191
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
tors have been used for production applications. Detailed information on theory and practice of PACVD processes can be found in excellent reviews Reinberg4, Hollahan and Rosier 5, Rand 6, Yasuda 7 and Hollahan and Bells. 5.2. Sputter deposition. The sputter deposition process involves a target and a plasma of neutral working gas such as argon. The target material is transferred to the vapor phase by positive ion bombardment from the plasma via momentum transfer from the ions to the target atoms. The most important parameters controlling the growth and properties of the films by sputter deposition processes are:
(a) [ SUBSTRATE(S)] E
L
E
C
T
R
(1) Diode geometry using dc or rf excitation. (2) Magnetron geometries with dc or rf excitation. (3) Reactive sputtering using diode and/or magnetron geometries with dc or rf excitation. In this process, a working gas (argon) is used in combination with a reactive gas. For a detailed review of the physics and applications of sputter deposition processes, refer to review articles by Bossen and Cuomo 1° and Thornton~l. 5.3. Activated reactive evaporation (ARE). The activated reactive evaporation (ARE) process developed by Bunshah ~2 involves evaporation of metal in the presence of a plasma of the reactive gas alone. There is no working gas in the ARE process. The two basic variants of the A R E process are: (1) the Activated Reactive Evaporation process with an electron beam evaporation source, and (2) the A R E process with a resistance heated source. Both of these processes are illustrated in Figure 2.
2192
~
REACTIVE . GAS ~///,( ~ ' ~ - ~ / / ' ~
"
pIJ7
INJECTION~ FLUX
4POWER SUPPLY
C O A T I N ~ - -
-4-
BAR~IER ELECTRON BEAM EVAPORATOR
(1) Target voltage and current. (2) Reactant partial pressure and flow rate. (3) Substrate temperature and substrate bias. Similarly to PACVD processes, these variables affect both process parameters as well as plasma parameters. For example, in conventional diode sputtering using either dc or rf, the deposition rate is dependent on target voltage and current as well as on pressure. However, these same parameters also determine the average energy of the secondary electrons as discussed below. The target voltage determines the energy of the secondary electrons ejected at the target. These are accelerated across the cathode sheath by a potential equal to target potential. The partial pressure on the other hand determines the mean free path and hence the collision frequency (No of collisions per unit length) of the electrons. As electrons lose energy in each collision, the average electron energy functionally depends on pressure. Thus the target voltage in conjunction with the operating pressure determines the average electron energy. Due to the relatively high voltage levels involved in diode sputtering, the energy of the secondary electrons is very high. Bombardment of the substrate by such high energy electrons leads to substrate heating and radiation damage and is thus a limiting factor in conventional dc and rf sputtering processes using the diode geometry. The target voltage/current and reactive gas flow rate exhibit a complex relationship in reactive sputter deposition processes due to target poisoning effects. This issue has been discussed in detail by Deshpandey and Bunshah 9. There are many variants of the sputter deposition processes, such as:
]
O
I
VACUUM PUMPS
VACUUM CHAMBER
MO AG NE C ILS ~TCRYSTALx-x[ ~ E / R]/ n
PYREXGLASS
SH,ELOS
I
u
tJ
~
EVAPORATION
I'~
EVAPORATION SOURCE TRANSFORMER
I PROPORTIONAL CONTROLLER I
I DEPOSITION CONTROLLER
SIGNAL FROM QUARTZCRYSTAL
=i
I
OSCILLATOR I
Figure 2. Schematic of the activated reactive evaporation system: (a) using an electron beam evaporation source, (b) using a resistance heated evaporation source. In A R E using an electron beam (eb) source, the metal is evaporated by the beam in presence of a reactive gas. The plasma is generated by accelerating the secondary electrons from the plasma sheath above the molten pool towards a probe biased to a low ac or positive dc potential. Nath and Bunshah t3 modified the A R E process for use with resistance heated sources. The metal is evaporated from a resistance heated source in the presence of the reactive gas. The plasma is generated by accelerating thermionically emitted electrons from a heated filament towards a positively biased anode. A transverse magnetic field is applied to cause the electrons to travel in spiral paths thereby increasing the probability of ionization.
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
Apart from the above two basic geometries, many other variants of the ARE process have been developed. For further details, the reader is referred to a review of Bunshah. The important process parameters controlling the growth and properties of films by the ARE process are: (1) Evaporation rate. (ii) Plasma parameters such as electron density, electron energy and distribution function. (iii) Substrate temperature and bias. Unlike PACVD and sputter deposition processes, the above variables can be controlled independently. For example, one can control the deposition rate via the evaporation rate by controlling the eb current or the heating current passing through the boat source. This does not significantly affect the plasma parameters, which are controlled through an auxiliary anode potential. This ability to control plasma and deposition parameters relatively independently offers the ARE process much greater flexibility to deposit films with varying stoichiometry, structure and properties at high rates, as compared to PACVD and RS processes.
6. Limitations of current plasma-assisted techniques As discussed earlier, the presence of the plasma in the sourcesubstrate space significantly affects the processes occurring at each of the three steps in film deposition, which are: (l) generation of species, (2) transport from source to substrate, (3) film growth on the substrate. Moreover, the effect of the 'plasma' on the above three steps differs significantly between various processes. Such differences are manifest in terms of the
types and concentration of the metastable species, ionized species, and energetic neutrals which in turn influence the reaction paths or steps involved in the overall reaction for film formation and the physical location of these reaction sites. Deshpandey and Bunshah 14 have discussed in detail the role of plasma in Plasma-Assisted Deposition Processes. They have shown that the advantages and limitations of various plasma assisted deposition techniques can be addressed in terms of the differences in plasma interactions at the source, during transport and at the substrate in the respective processes. Comparisons between the three currently used plasma-assisted deposition techniques viz. Reactive Sputtering (RS), Activated Reactive Evaporation (ARE) and Plasma-Assisted Chemical Vapor Deposition (PACVD) in terms of plasma/sourceplasma/volume and plasma-substrate interactions is shown in Table 1. Also indicated in this table are the limitations/advantages inherent to each process. As can be seen from this table, each of the above processes suffers from limitations in terms of one or more of the following: (1) Control over the supply of the source material in vapor form. (2) Control of the number density and energy distribution of electrons and hence the associated plasma volume chemistry. Most of the above limitations are due to the interdependency of the 3 reactions, i.e. plasma-source, plasma-volume and plasma-substrate reactions. For an ideal plasma assisted process, one should be able to control each of the above reactions independently of each other.
Table 1. Comparison of plasma-assisted deposition processes Reactive sputtering evaporation
Activated reactive vapor deposition process
Plasma-assisted chemical
Source reactions
target poisoning; hence difficult to to achieve high rate in case of compounds especially oxides
no target poisoning due to stirring action in liquid pool; hence high rates can be achieved as compared to reactive sputtering
not applicable
Transport reactions (plasma-volume reactions)
probability of electron-atom/ molecule reaction is small (i) because of high electron energy, (ii) because in magnetron sputtering most electrons are confined to region near the target surface
low energy electrons; hence increase in probability of electron and gasatom/molecule reactions
depends on gas flow, pressure and rf power. These parameters also control deposition rate and substrate bombardment*
Plasma surface reactions
plasma-surface reactions are due to substrate bombardment by energetic ions and electrons, which depend on electron energy
substrate bombardment can be controlled independently of the source power and pressure, as substrates can be located outside the plasma
electron energy cannot be controlled independently of rf power, gas pressures. Substrate bombardment therefore cannot be controlled independently
electron energy cannot be controlled independently of target voltage, pressure. Substrate bombardment, therefore, cannot be controlled independently in diode and rf sputtering *For example, if the flow rate is increased keeping rf power constant, more collisions occur in plasma volume, increasing the possibility of polymerization. On the other hand, if rf power is increased, keeping the flow rate constant, more dissociation takes place, but also more bombardment of film because of higher electron temperature (kTe) and higher floating potential V~p~ 3 kTo). This causes changes in composition, structure, stresses in the film. This interdependence may also cause other competing reactions to occur (e.g. gas phase nucleation) which can make it difficult to obtain high deposition rates by PACVD processes. 2193
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
In view of the above, attempts have been made to develop hybrid processes by combining various features of the PlasmaAssisted Deposition techniques to extend the processing capabilities and to overcome the limitations of the individual techniques. Many modifications of the PAVD techniques have been developed and have been discussed by Deshpandey and Bunshah 14. 7. Survey of materials synthesized using plasma-assisted
techniques It is difficult to present all the results on material and coatings synthesized using plasma-assisted techniques in a short review. We have therefore presented a list of representative materials prepared by PACVD, RS and ARE techniques in Table 2. Also, in view of the considerable interest in materials such as cubic boron nitride and diamond, recent research in the synthesis of these materials is briefly discussed.
7.1. Cabin boron nitride (CBN) films. Cubic boron nitride and diamond films have aroused a great deal of interest in recent years for their potential application in high power microwave devices as well as in optoelectronic devices such as uv lasers and detectors. PACVD as well as PAPVD techniques have been explored. For example Weissmantel et a115 have reported CBN films by electron beam evaporation of boron in a NH3 plasma. Satou and Fujimoto ~6 have used 30 keV N~- ion beam bombardment to produce CBN films whereas Shanfield and Wolfs o n 17 have produced CBN films from borozine in a rf-excited NH3 plasma. Recentely Seidel et a P s have prepared CBN films by reactive diode sputtering. A novel process based on the activated reactive evaporation has been developed by Bunshah et a119, for synthesizing CBN films. Unlike most processes hitherto employed for the synthesis of CBN films which require higher deposition temperatures or the use of toxic chemicals such as borozine, diborane, etc.
this process uses boric acid which is a non-toxic, chemically inert starting material. This process, called the Activated Dissociation Reduction Reaction Process (ADRRP), involves evaporation of boric acid in a NH3 plasma. Cubic boron nitride films have been deposited using the above techniques at substrate temperatures as low as 400°C 2°. Deposition rates up to 1500 A min-t have been achieved. These films show the characteristic absorption at 6.8 and 12.5/~m corresponding to B - N and N - B - N bending vibrations. Transmission electron diffraction confirms the existence of the simple cubic phase. No hexagonal phase is detected in the films. The films transmit in the uv region down to 220/~m and the band gap calculated from absorption co-efficient vs energy plot is found around 5.5 + 3 eV. The refractive index of the films obtained by ellipsometry is around 2.36. 7.2. Diamond films. PACVD techniques have also been used successfully to deposit crystalline diamond films. Most of these techniques involve plasma decomposition of hydrocarbon + H2 gas mixtures. Various excitation geometries using filament anodC 1, rf22 as well as microwave23 have been used. Usually a very low concentration of CH4 and high substrate temperatures are required to nucleate diamond phase. Often the substrate has to be abraided with diamond particles prior to deposition, in order to nucleate the diamond phase. Moreover, films produced by PACVD techniques show a large grain-faceted microstructure and a rough surface morphology, which limits their potential applications in optics and electronics. Recently, a plasma-assisted process based on ARE has been suecesfully used to synthesize non-faceted, smooth, polycrystalline diamond films at low substrate temperature 24'25. The process involves evaporation of graphite in H2 plasma. By appropriately controlling the H2 partial pressure, evaporation rate and plasma parameters, optically transparent, smooth diamond films can be obtained at temperature as low as 350°C.
Table 2. Compounds and synthesized deposition rates obtained with commonly used deposition processes Compounds
Activated reactive evaporation (A min- 1)
Reactive sputtering (~ min- l)
Plasma-assisted chemical vapor deposition (A rain-1)
Carbides, e.g. TiC, HC, ZrC, VC Nitrides, e.g. TiN, HfN, ZrN Oxides, e.g. TiO2, ZrO 2, AI203, SiO 2 Sulphides, e.g. TiS 2, MoS 2, MoS 3
2000-3000
400-500
150-400
2000-3000
300-400
60-150
1000-2000
200-800
200-300
1500-2000
50-200
50- lO0
500- 1000 1000-1500 1-2 #m h- J "
50-75
50- I00
Novel Materials Superconducting materials, e.g. Nb3Ge, CuMo6Ss Photovoltaic materials, e.g. a-Sill, CulnS2 Optoelectronic materials, e.g. indium tin oxide, zinc oxide Cubic boron nitride Diamond i-C, etc.
2194
1000-2000
1000-1500
300
I000 A h -l 200
R F Bunshah and C Deshpandey: Plasma-assisteddeposition techniques
8. Conclusions (I) Plasma-Assisted Vapor Deposition processes are very powerful techniques for deposition of compound films and novel metastable materials such as fl-SiC, cubic boron nitride, i-C and crystalline diamond films. (2) It is very important to develop process modifications where each of the process steps can be independently controlled. This is very difficult in many PAVD processes since the presence of a plasma introduces more interdependent process variables than the available knobs to vary the process parameters. (3) Processes such as ARE which use low energy electrons to generate the plasma offer greater diversity of plasma volume chemistry for material synthesis. (4) Separation of plasma from the substrate is also desirable for improving the film purity, lowering the deposition temperature and bombardment-induced damage. (5) Plasma diagnostic studies are essential to determine the specific species present in the plasma and evaluate probable reaction paths leading to the desired deposition. (6) The use of sources such as inorganic materials (such as boric acid for CBN synthesis) and tailor-made organo-metallic precursor molecules can lead to even greater strides in the deposition of films of novel materials (such as crystalline diamond). (7) The potential of PAVD processes for the deposition of complex modulated structures in films is bright.
Acknowledgements We are currently engaged in a variety of projects related to material synthesis and thin films. Most of the work in our laboratory is supported by grants from federal agencies such as NSF, NASA, DOE, DOD, etc. and contracts from various industries. We gratefully acknowledge the support and interest of all our sponsors which is crucial for ongoing and future work. We are especially thankful to NYSIAD Corporation,
New York and DIASYN Corporation, Canada for their support of a research project on diamond coatings.
References i R F Bunshah and C V Deshpandey, J Vac Sci Technol, A(3), 553 (1985). 2 j A Thornton, In Deposition Technologies for Films and Coatings (Edited by R F Bunshah), p. 26. Noyes, Park Ridge, NJ (1982). 3 T D Bonifield, In Deposition Technologies for Films and Coatings (Edited by R F Bunshah), p 365. Noyes, Park Ridge, NJ (1982). 4A R Reinberg, Ann Rev Mater Sch 9, 341 (1979). 5j R Hollahan and R S Rosier, In Thin Film Processes (Edited by J L Vossen and W Kern), p. 335. Academic Press, New York (1978). 6M J Rand, J Vac Sci Technol, 16, 420 (1979). 7 H Yasuda, In Thin Film Processes (Edited by J L Vossen and W Kern), p. 361. Academic Press, New York (1978). s j R Hollahan and A T Bell (Eds), Techniques and Application of Plasma Chemistry. Wiley, New York (1974). 9 C Deshpandey and R F Bunshah, Surface Coat Technol, 27, 1 (1986). lo j L Bossen and J J Cuomo, In Thin Film Processes (Edited by J Vossen and W Kern), p I1. Academic Press, New York (1978). 11j A Thornton, ibid, p 75. 12R F Bunshah, US Patent 3,791,852 (Feb 1974). 13p Nath and R F Bunshah, US Patent 4,336,277 (June 1982). 14C V Deshpandey and R F Bunshah, Thin Solid Films, 163, 131 (1988). 15C Weissmantel,K Bowilogrea, D Dietrich, H J Erler, H J Hinnerberg, S Klose, W Norwich and G Reisse, Thin Solid Films, 72, 9 (1980). 16M Satou and F Fujimoto, Japan J Appl Phys, 22, LI71 (1983). 17S Shanfield and R Wolfson, J Vac Sci Technol, AI, 323 (1983). is K H Seidel, K Reichelt, W Schaal and H Dimigen, Thin Solid Films, 151, 243 (1987). J9 R F Bunshah, K L Chopra, C Deshpandey and V D Vankar, US Patent, 4,714,625 (1987). 2op Lin, C Deshpandey, H J Doerr, R F Bunshah, K L Chopra and V D Vankar, Thin Solid Films, 153, 487 (1987). 21 A Sawbe and T Inuzulu, Thin Solid Films, 137, 89 (1986). 22 S Matsumoto, In Proc 7th lnt Symp on Plasma Chemistry, Eindhoven, July (1985) (Edited by C J Timmermann), p 79. UPAC, Oxford. 23N Fujimoto, T Imei and A Doi, Vacuum, 36, 99 (1986). 24C Deshpandey, R F Bunshah and H J Doerr, US Patent, 4,816,291 (March 1989). 25 C Deshpandey, M C Radhakrishna, H J Doerr and R F Bunshah, Paper Presented at SDRO/RST-ONR Diamond Technol Initiative Syrup, Crystal City, VA, 11-13 July (1989).
21 95
0042-207X/9053.00 + .00
Printed in Great Britain
© 1990 Pergamon Press plc
Plasma-assisted deposition techniques for hard coatings R F B u n s h a h and C D e s h p a n d e y , Department of Materials Science and Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA 90034, USA
This paper reviews the most commonly used deposition techniques for hard coatings, namely Plasma-Assisted Chemical Vapor Deposition (PACVD), Reactive Sputtering (RS) and Activated Reactive Sputtering (ARE). The role of plasma parameters and process parameters as well as their interdependency is discussed in terms of the three steps in deposition of films, i.e. generation of the depositing species, transport from source to substrate and film growth on the substrate.
1. Introduction The use of plasma-assisted vapor deposition (PAVD) processes for deposition of compounds (oxides, carbides, nitrides, sulfides, etc.) has spread into various types of industrial applications. These include dielectric films for microelectronics, optical and magnetic applications, hard carbide and nitride films for cutting and forming tools, sulfides for solid-state lubrication and solid electrolytes, etc. In fact, PAVD methods can be said to have opened up a new area in materials synthesis. Many compound films, which were hitherto difficult to deposit, are now routinely synthesized. The role of the plasma and its effect on growth and properties of the film in any plasma-assisted deposition technique is, however, very complex. To accomplish this, our understanding of the detailed physics and chemistry of these glow discharges must be improved. In this paper, we will briefly discuss the role of the plasma in film growth by plasma-assisted processes to highlight the plasma parameters that are significant in controlling the growth and properties of the films. 2. Model of film growth by vapor deposition techniques Film growth by any vapor deposition technique can be seen in terms of three basic steps: (1) generation of the depositing species; (2) transport from source to substrate; (3) film growth on the substrate. Additionally, if a plasma is employed in the deposition process, one has to understand the effect of the plasma on each of the three general steps given above ~. Thus, there are two sets of interactive parameters in all plasmaassisted processes. They are the plasma parameters (electron density, electron energy and electron energy distribution function) and the process parameters (evaporation/sputtering rate, reactive/inert gas pressure, flow rate, substrate temperature, substrate bias, etc.). The model of film growth by PAVD process can therefore be schematically represented as shown in Figure I. One might therefore picture in a plasma-assisted deposition process that the depositing species undergo various types of reactions in the 2190
KINETIC S
PLASMA
RATEOF GENERATION ] OF VAPORSPECIES NATURE OF GASESUSED, I FLOW RATES,PRESSURE i ~ INLET LOCATION, ETC. I
]
~[ GENERATION(EXCITATION)
CONFIGURATION,EXCITATION v~ MODERF, D.C.,MICROWAVE, LASER, ETC. POWER/CURENTSUPFK)RTING
I
SUBSTRATE POSITION 0NRT PLASMA) POTENTIAL TEMPERATURE TYPE - INSULATINGOR CONDUCTING
Figure 1. Schematic showing the effect of plasma and deposition variables on the three steps of film growth by vapor deposition processes.
plasma leading to the formation of excited neutral species, ions, free radicals, etc. which may react to form a precursor species that in turn deposit on the substrate, migrate on the surface, react and form the film. The reactions forming the above species, i.e. the plasma volume chemistry, in turn are controlled by both the process as well as the plasma parameters, as discussed below. 3. Plasma parameters relevant to film formation The most important plasma parameters relevant to film growth and properties in any plasma-assisted process are electron density, electron energy and electron energy distribution function. Electrons play a primary role in sustaining the plasma by ionization. The ionization efficiency of the electron is energy dependent. It increases with energy and passes through a maximum for electrons in the energy range of 50-100 V. It is therefore desirable to have low energy electrons (50-100 eV)
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
for optimum ionization. It is in this respect that a process like activated reactive evaporation has some advantages, as will be discussed in a later section. The rate of chemical reactions occuring in plasma is given by 2 (1)
R A = neV A
where ne is the electron number density and vA is the collision frequency for a given reaction A. The collision frequency VA can be written as:
VA = N
f:
(E/2me) l/2aA(E)f(E)dE
(2)
where N is the number density of colliding species, a(E) is the collision cross-section and fiE) is electron energy distribution function. Substituting equation (2) into (1), the rate of reaction can be written as:
R A = NeN
(E/2me)~/2aA(E)f(E)dE.
In order to achieve better control of film properties, it is desirable to independently control the above parameters. However, it is not always possible in all the deposition processes to achieve the process parameter flexibility conferred by the ability to vary them independently of each other. The nature and degree of intercoupling of the variables controlling the above parameters determine the advantages and limitations of a given deposition process. The presence of a plasma introduces additional contraints as some of the variables controlling the process parameters also affect the plasma parameters. To understand and optimize plasma assisted deposition processes, it is necessary to evaluate this inter relationship between the process parameters and plasma parameters. 5. Plasma-assisted deposition teclmiques in current usage
(3)
If the electrons are assumed to have a Maxwellian velocity distribution at a temperature Te and if the cross-section for a given reaction is approximated by a step function of magnitude a 0 and threshold energy E 0 then the reaction rate is given by E
RA = neNtroVe(l + ~ 3 ) .
over, ion bombardment of the growing film can also lead to reduction of absorbed impurities and trapped gases in the films.
(4)
Equation (4) reveals that the rate of any chemical reaction in a plasma is primarily dependent on electron density, electron energy and distribution function fiE). Thus it is advantageous to have independent control over these parameters to adequately control the reaction occurring in the plasma, i.e. the plasma volume chemistry and hence the structure and properties of the deposited films. 4. Deposition parameters relevant to film growth Apart from the plasma parameters, the deposition parameters also influence the growth and properties of the films, produced by any vapor deposition process. The most important deposition parameters are: (I) Rate of generation of vapor species which determine the deposition rate and stoichiometry of the films. (2) Partial pressure of all species in the gas phase which determines the mean free path of these species and hence affect the growth rate. In reactive deposition processes, parial pressures also determine the probability of the collisional reactions between various atomic and molecular species during transit from source to substrate and hence influence the formation of precursor molecules which in turn affect the growth and properties of the film. (3) Gas flow rate is an important process parameter, particularly in reactive deposition processes, since along with the metal species in the vapor phase, it controls the stoichiometry of the films. (4) Substrate temperature controls the composition, structure and morphology of the films by affecting the adatom mobility on the substrate as well as the rate of any chemical reaction occurring on the substrate. (5) Substrate bias, together with substrate temperature, also influences the structure and morphology since it controls the intensity of the ion bombardment of the growing film. More-
The most commonly used plasma assisted techniques for the deposition of compounds can be classified under the following two categories: (I) Plasma-Assisted Chemical Vapor Deposition (PACVD) processes. (2) Plasma Assisted Physical Deposition (PAPVD) Processes such as: (i) Reactive Sputtering, (RS) using dc, rf, magnetron geometries and ion beams. (ii) Activated Reactive Evaporation (ARE). They are briefly discussed below. 5.1. Plasma-assisted chemical vapor deposition. Plasma-assisted chemical vapor deposition involves forming solid deposits by initiating chemical reactions in a gaseous discharge 3. The discharge can be excited by using either if, microwave or photonic excitation. It produces a wide variety of chemical species in ionized and excited states, free radicals as well as ions and electrons. The nature, type, concentration and energy of these species determine the growth and properties of the films. The important parameters controlling film growth by PACVD are as follows: (i) Reactant partial pressure and flow rate. (ii) rf power. (iii) Substrate temperature and substrate bias. The above variables affect process parameters such as deposition rate on the one hand and plasma parameters such as electron density, electron energy and distribution function on the other. For example, the partial pressure of the reactant gas together with rf power determines the rate of dissociation of the reactive gas and hence the deposition rate. These same process variables also determine the electron energy and electron density. As the substrate floating potential depends on average electron energy, pressure and rf power (which in turn controls the substrate bombardment), the substrate bombardment of the growing film and deposition rate are both dependent on the same set of process variables, i.e. pressure and rf power. This interdependence of process and plasma parameters makes it difficult to obtain high deposition rates by PACVD processes. A variety of reactor designs have been used for carrying out PACVD in the laboratory. However, only parallel plate reac2191
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
tors have been used for production applications. Detailed information on theory and practice of PACVD processes can be found in excellent reviews Reinberg4, Hollahan and Rosier 5, Rand 6, Yasuda 7 and Hollahan and Bells. 5.2. Sputter deposition. The sputter deposition process involves a target and a plasma of neutral working gas such as argon. The target material is transferred to the vapor phase by positive ion bombardment from the plasma via momentum transfer from the ions to the target atoms. The most important parameters controlling the growth and properties of the films by sputter deposition processes are:
(a) [ SUBSTRATE(S)] E
L
E
C
T
R
(1) Diode geometry using dc or rf excitation. (2) Magnetron geometries with dc or rf excitation. (3) Reactive sputtering using diode and/or magnetron geometries with dc or rf excitation. In this process, a working gas (argon) is used in combination with a reactive gas. For a detailed review of the physics and applications of sputter deposition processes, refer to review articles by Bossen and Cuomo 1° and Thornton~l. 5.3. Activated reactive evaporation (ARE). The activated reactive evaporation (ARE) process developed by Bunshah ~2 involves evaporation of metal in the presence of a plasma of the reactive gas alone. There is no working gas in the ARE process. The two basic variants of the A R E process are: (1) the Activated Reactive Evaporation process with an electron beam evaporation source, and (2) the A R E process with a resistance heated source. Both of these processes are illustrated in Figure 2.
2192
~
REACTIVE . GAS ~///,( ~ ' ~ - ~ / / ' ~
"
pIJ7
INJECTION~ FLUX
4POWER SUPPLY
C O A T I N ~ - -
-4-
BAR~IER ELECTRON BEAM EVAPORATOR
(1) Target voltage and current. (2) Reactant partial pressure and flow rate. (3) Substrate temperature and substrate bias. Similarly to PACVD processes, these variables affect both process parameters as well as plasma parameters. For example, in conventional diode sputtering using either dc or rf, the deposition rate is dependent on target voltage and current as well as on pressure. However, these same parameters also determine the average energy of the secondary electrons as discussed below. The target voltage determines the energy of the secondary electrons ejected at the target. These are accelerated across the cathode sheath by a potential equal to target potential. The partial pressure on the other hand determines the mean free path and hence the collision frequency (No of collisions per unit length) of the electrons. As electrons lose energy in each collision, the average electron energy functionally depends on pressure. Thus the target voltage in conjunction with the operating pressure determines the average electron energy. Due to the relatively high voltage levels involved in diode sputtering, the energy of the secondary electrons is very high. Bombardment of the substrate by such high energy electrons leads to substrate heating and radiation damage and is thus a limiting factor in conventional dc and rf sputtering processes using the diode geometry. The target voltage/current and reactive gas flow rate exhibit a complex relationship in reactive sputter deposition processes due to target poisoning effects. This issue has been discussed in detail by Deshpandey and Bunshah 9. There are many variants of the sputter deposition processes, such as:
]
O
I
VACUUM PUMPS
VACUUM CHAMBER
MO AG NE C ILS ~TCRYSTALx-x[ ~ E / R]/ n
PYREXGLASS
SH,ELOS
I
u
tJ
~
EVAPORATION
I'~
EVAPORATION SOURCE TRANSFORMER
I PROPORTIONAL CONTROLLER I
I DEPOSITION CONTROLLER
SIGNAL FROM QUARTZCRYSTAL
=i
I
OSCILLATOR I
Figure 2. Schematic of the activated reactive evaporation system: (a) using an electron beam evaporation source, (b) using a resistance heated evaporation source. In A R E using an electron beam (eb) source, the metal is evaporated by the beam in presence of a reactive gas. The plasma is generated by accelerating the secondary electrons from the plasma sheath above the molten pool towards a probe biased to a low ac or positive dc potential. Nath and Bunshah t3 modified the A R E process for use with resistance heated sources. The metal is evaporated from a resistance heated source in the presence of the reactive gas. The plasma is generated by accelerating thermionically emitted electrons from a heated filament towards a positively biased anode. A transverse magnetic field is applied to cause the electrons to travel in spiral paths thereby increasing the probability of ionization.
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
Apart from the above two basic geometries, many other variants of the ARE process have been developed. For further details, the reader is referred to a review of Bunshah. The important process parameters controlling the growth and properties of films by the ARE process are: (1) Evaporation rate. (ii) Plasma parameters such as electron density, electron energy and distribution function. (iii) Substrate temperature and bias. Unlike PACVD and sputter deposition processes, the above variables can be controlled independently. For example, one can control the deposition rate via the evaporation rate by controlling the eb current or the heating current passing through the boat source. This does not significantly affect the plasma parameters, which are controlled through an auxiliary anode potential. This ability to control plasma and deposition parameters relatively independently offers the ARE process much greater flexibility to deposit films with varying stoichiometry, structure and properties at high rates, as compared to PACVD and RS processes.
6. Limitations of current plasma-assisted techniques As discussed earlier, the presence of the plasma in the sourcesubstrate space significantly affects the processes occurring at each of the three steps in film deposition, which are: (l) generation of species, (2) transport from source to substrate, (3) film growth on the substrate. Moreover, the effect of the 'plasma' on the above three steps differs significantly between various processes. Such differences are manifest in terms of the
types and concentration of the metastable species, ionized species, and energetic neutrals which in turn influence the reaction paths or steps involved in the overall reaction for film formation and the physical location of these reaction sites. Deshpandey and Bunshah 14 have discussed in detail the role of plasma in Plasma-Assisted Deposition Processes. They have shown that the advantages and limitations of various plasma assisted deposition techniques can be addressed in terms of the differences in plasma interactions at the source, during transport and at the substrate in the respective processes. Comparisons between the three currently used plasma-assisted deposition techniques viz. Reactive Sputtering (RS), Activated Reactive Evaporation (ARE) and Plasma-Assisted Chemical Vapor Deposition (PACVD) in terms of plasma/sourceplasma/volume and plasma-substrate interactions is shown in Table 1. Also indicated in this table are the limitations/advantages inherent to each process. As can be seen from this table, each of the above processes suffers from limitations in terms of one or more of the following: (1) Control over the supply of the source material in vapor form. (2) Control of the number density and energy distribution of electrons and hence the associated plasma volume chemistry. Most of the above limitations are due to the interdependency of the 3 reactions, i.e. plasma-source, plasma-volume and plasma-substrate reactions. For an ideal plasma assisted process, one should be able to control each of the above reactions independently of each other.
Table 1. Comparison of plasma-assisted deposition processes Reactive sputtering evaporation
Activated reactive vapor deposition process
Plasma-assisted chemical
Source reactions
target poisoning; hence difficult to to achieve high rate in case of compounds especially oxides
no target poisoning due to stirring action in liquid pool; hence high rates can be achieved as compared to reactive sputtering
not applicable
Transport reactions (plasma-volume reactions)
probability of electron-atom/ molecule reaction is small (i) because of high electron energy, (ii) because in magnetron sputtering most electrons are confined to region near the target surface
low energy electrons; hence increase in probability of electron and gasatom/molecule reactions
depends on gas flow, pressure and rf power. These parameters also control deposition rate and substrate bombardment*
Plasma surface reactions
plasma-surface reactions are due to substrate bombardment by energetic ions and electrons, which depend on electron energy
substrate bombardment can be controlled independently of the source power and pressure, as substrates can be located outside the plasma
electron energy cannot be controlled independently of rf power, gas pressures. Substrate bombardment therefore cannot be controlled independently
electron energy cannot be controlled independently of target voltage, pressure. Substrate bombardment, therefore, cannot be controlled independently in diode and rf sputtering *For example, if the flow rate is increased keeping rf power constant, more collisions occur in plasma volume, increasing the possibility of polymerization. On the other hand, if rf power is increased, keeping the flow rate constant, more dissociation takes place, but also more bombardment of film because of higher electron temperature (kTe) and higher floating potential V~p~ 3 kTo). This causes changes in composition, structure, stresses in the film. This interdependence may also cause other competing reactions to occur (e.g. gas phase nucleation) which can make it difficult to obtain high deposition rates by PACVD processes. 2193
R F Bunshah and C Deshpandey: Plasma-assisted deposition techniques
In view of the above, attempts have been made to develop hybrid processes by combining various features of the PlasmaAssisted Deposition techniques to extend the processing capabilities and to overcome the limitations of the individual techniques. Many modifications of the PAVD techniques have been developed and have been discussed by Deshpandey and Bunshah 14. 7. Survey of materials synthesized using plasma-assisted
techniques It is difficult to present all the results on material and coatings synthesized using plasma-assisted techniques in a short review. We have therefore presented a list of representative materials prepared by PACVD, RS and ARE techniques in Table 2. Also, in view of the considerable interest in materials such as cubic boron nitride and diamond, recent research in the synthesis of these materials is briefly discussed.
7.1. Cabin boron nitride (CBN) films. Cubic boron nitride and diamond films have aroused a great deal of interest in recent years for their potential application in high power microwave devices as well as in optoelectronic devices such as uv lasers and detectors. PACVD as well as PAPVD techniques have been explored. For example Weissmantel et a115 have reported CBN films by electron beam evaporation of boron in a NH3 plasma. Satou and Fujimoto ~6 have used 30 keV N~- ion beam bombardment to produce CBN films whereas Shanfield and Wolfs o n 17 have produced CBN films from borozine in a rf-excited NH3 plasma. Recentely Seidel et a P s have prepared CBN films by reactive diode sputtering. A novel process based on the activated reactive evaporation has been developed by Bunshah et a119, for synthesizing CBN films. Unlike most processes hitherto employed for the synthesis of CBN films which require higher deposition temperatures or the use of toxic chemicals such as borozine, diborane, etc.
this process uses boric acid which is a non-toxic, chemically inert starting material. This process, called the Activated Dissociation Reduction Reaction Process (ADRRP), involves evaporation of boric acid in a NH3 plasma. Cubic boron nitride films have been deposited using the above techniques at substrate temperatures as low as 400°C 2°. Deposition rates up to 1500 A min-t have been achieved. These films show the characteristic absorption at 6.8 and 12.5/~m corresponding to B - N and N - B - N bending vibrations. Transmission electron diffraction confirms the existence of the simple cubic phase. No hexagonal phase is detected in the films. The films transmit in the uv region down to 220/~m and the band gap calculated from absorption co-efficient vs energy plot is found around 5.5 + 3 eV. The refractive index of the films obtained by ellipsometry is around 2.36. 7.2. Diamond films. PACVD techniques have also been used successfully to deposit crystalline diamond films. Most of these techniques involve plasma decomposition of hydrocarbon + H2 gas mixtures. Various excitation geometries using filament anodC 1, rf22 as well as microwave23 have been used. Usually a very low concentration of CH4 and high substrate temperatures are required to nucleate diamond phase. Often the substrate has to be abraided with diamond particles prior to deposition, in order to nucleate the diamond phase. Moreover, films produced by PACVD techniques show a large grain-faceted microstructure and a rough surface morphology, which limits their potential applications in optics and electronics. Recently, a plasma-assisted process based on ARE has been suecesfully used to synthesize non-faceted, smooth, polycrystalline diamond films at low substrate temperature 24'25. The process involves evaporation of graphite in H2 plasma. By appropriately controlling the H2 partial pressure, evaporation rate and plasma parameters, optically transparent, smooth diamond films can be obtained at temperature as low as 350°C.
Table 2. Compounds and synthesized deposition rates obtained with commonly used deposition processes Compounds
Activated reactive evaporation (A min- 1)
Reactive sputtering (~ min- l)
Plasma-assisted chemical vapor deposition (A rain-1)
Carbides, e.g. TiC, HC, ZrC, VC Nitrides, e.g. TiN, HfN, ZrN Oxides, e.g. TiO2, ZrO 2, AI203, SiO 2 Sulphides, e.g. TiS 2, MoS 2, MoS 3
2000-3000
400-500
150-400
2000-3000
300-400
60-150
1000-2000
200-800
200-300
1500-2000
50-200
50- lO0
500- 1000 1000-1500 1-2 #m h- J "
50-75
50- I00
Novel Materials Superconducting materials, e.g. Nb3Ge, CuMo6Ss Photovoltaic materials, e.g. a-Sill, CulnS2 Optoelectronic materials, e.g. indium tin oxide, zinc oxide Cubic boron nitride Diamond i-C, etc.
2194
1000-2000
1000-1500
300
I000 A h -l 200
R F Bunshah and C Deshpandey: Plasma-assisteddeposition techniques
8. Conclusions (I) Plasma-Assisted Vapor Deposition processes are very powerful techniques for deposition of compound films and novel metastable materials such as fl-SiC, cubic boron nitride, i-C and crystalline diamond films. (2) It is very important to develop process modifications where each of the process steps can be independently controlled. This is very difficult in many PAVD processes since the presence of a plasma introduces more interdependent process variables than the available knobs to vary the process parameters. (3) Processes such as ARE which use low energy electrons to generate the plasma offer greater diversity of plasma volume chemistry for material synthesis. (4) Separation of plasma from the substrate is also desirable for improving the film purity, lowering the deposition temperature and bombardment-induced damage. (5) Plasma diagnostic studies are essential to determine the specific species present in the plasma and evaluate probable reaction paths leading to the desired deposition. (6) The use of sources such as inorganic materials (such as boric acid for CBN synthesis) and tailor-made organo-metallic precursor molecules can lead to even greater strides in the deposition of films of novel materials (such as crystalline diamond). (7) The potential of PAVD processes for the deposition of complex modulated structures in films is bright.
Acknowledgements We are currently engaged in a variety of projects related to material synthesis and thin films. Most of the work in our laboratory is supported by grants from federal agencies such as NSF, NASA, DOE, DOD, etc. and contracts from various industries. We gratefully acknowledge the support and interest of all our sponsors which is crucial for ongoing and future work. We are especially thankful to NYSIAD Corporation,
New York and DIASYN Corporation, Canada for their support of a research project on diamond coatings.
References i R F Bunshah and C V Deshpandey, J Vac Sci Technol, A(3), 553 (1985). 2 j A Thornton, In Deposition Technologies for Films and Coatings (Edited by R F Bunshah), p. 26. Noyes, Park Ridge, NJ (1982). 3 T D Bonifield, In Deposition Technologies for Films and Coatings (Edited by R F Bunshah), p 365. Noyes, Park Ridge, NJ (1982). 4A R Reinberg, Ann Rev Mater Sch 9, 341 (1979). 5j R Hollahan and R S Rosier, In Thin Film Processes (Edited by J L Vossen and W Kern), p. 335. Academic Press, New York (1978). 6M J Rand, J Vac Sci Technol, 16, 420 (1979). 7 H Yasuda, In Thin Film Processes (Edited by J L Vossen and W Kern), p. 361. Academic Press, New York (1978). s j R Hollahan and A T Bell (Eds), Techniques and Application of Plasma Chemistry. Wiley, New York (1974). 9 C Deshpandey and R F Bunshah, Surface Coat Technol, 27, 1 (1986). lo j L Bossen and J J Cuomo, In Thin Film Processes (Edited by J Vossen and W Kern), p I1. Academic Press, New York (1978). 11j A Thornton, ibid, p 75. 12R F Bunshah, US Patent 3,791,852 (Feb 1974). 13p Nath and R F Bunshah, US Patent 4,336,277 (June 1982). 14C V Deshpandey and R F Bunshah, Thin Solid Films, 163, 131 (1988). 15C Weissmantel,K Bowilogrea, D Dietrich, H J Erler, H J Hinnerberg, S Klose, W Norwich and G Reisse, Thin Solid Films, 72, 9 (1980). 16M Satou and F Fujimoto, Japan J Appl Phys, 22, LI71 (1983). 17S Shanfield and R Wolfson, J Vac Sci Technol, AI, 323 (1983). is K H Seidel, K Reichelt, W Schaal and H Dimigen, Thin Solid Films, 151, 243 (1987). J9 R F Bunshah, K L Chopra, C Deshpandey and V D Vankar, US Patent, 4,714,625 (1987). 2op Lin, C Deshpandey, H J Doerr, R F Bunshah, K L Chopra and V D Vankar, Thin Solid Films, 153, 487 (1987). 21 A Sawbe and T Inuzulu, Thin Solid Films, 137, 89 (1986). 22 S Matsumoto, In Proc 7th lnt Symp on Plasma Chemistry, Eindhoven, July (1985) (Edited by C J Timmermann), p 79. UPAC, Oxford. 23N Fujimoto, T Imei and A Doi, Vacuum, 36, 99 (1986). 24C Deshpandey, R F Bunshah and H J Doerr, US Patent, 4,816,291 (March 1989). 25 C Deshpandey, M C Radhakrishna, H J Doerr and R F Bunshah, Paper Presented at SDRO/RST-ONR Diamond Technol Initiative Syrup, Crystal City, VA, 11-13 July (1989).
21 95