INTRODUCTION Over the last two decades, there has

been an amazing surge of development in surface modification technologies­methods that leave a material's bulk properties inviolate while imparting unique performance characteristics to its surface. Increasingly, these tech­niques are making use of plasmas, lasers, ion and electron beams, and chemical and physical vapor deposi­tion processes. Surface-modified ma­terials are used in a variety of fields, including electronic and photonic de­vices, thermal insulation, tribology and decorative coatings. The substances used to modify a particular material's surface by coating, cladding or implan­tation are gaseous species, metals, alloys, refractory compounds, intermet­allics and polymers. Depending on per­formance requirements, the thickness of implanted or coated surface may vary from a few angstroms to several mils.

Based upon the processing tool, surface modification techniques may be classified under two broad cate­gories- plasma and laser. Plasma-as-

Electroplating Ion Plating

Plasma Techniques for the Surface Modification and Synthesis of Novel Materials K. Upadhya and T.e. Tiearney

-"""'!~I~'~~ ,,'~"'::''''Al- GE Medical Systems-Milwaukee, Wisconsin

500 ~m

sisted techniques are, essentially, variants of chemical and physical vapor deposition processes, which rely on the vapor transport of materials to con­struct new surfaces. The mean energy (E ) of a gas atom with respect to tem­perature is given by

E = 3/2 kT

where k is the Boltzman constant and T is the absolute temperature.

Since it is frequently undesirable to significantly raise a substrate's tem­perature, the high temperatures as­sociated with vapor transport make it unsuitable for a number of applica­tions. From this perspective, plasma­assisted ionization of the vapor atoms of the coating material is a suitable al­ternative method for applying coat­ings. With the ionized vapor species having great reactivity and the capac­ity to be accelerated in the electric field for transport, the resulting coating may possess superior physical and mechani­cal properties.

When ionized species are used for surface modification, two different methods can be adopted for coating the

substrate material. In the first option, ion implantation, a small ion flux with a high mean energy per ion is used; in the second option, plasma-assisted coating, a high ion flux is used, with sufficient mean energy per ion to en­sure a good adhesion of the coating to the substrate. Plasma-assisted coat­ing processes include ion plating, plasma-assisted chemical vapor depo­sition and ion beam mixing. It is worth noting that in both instances, there is an upper limit to the energy flux which can be deposited on the surface to be modified if the substrate properties are to remain unaltered.

J Source Formation Chemical Reduction

Transport of the

Ion implantation is the process whereby one or more ionic species are introduced into the surface of the material using a low-flux, high-energy, ion beam. This results in modified physical and chemical properties of a very thin near-surface region. Since ion implantation is a non-equilibrium process, the thermodynamic solubility limits of the system may be exceeded with or without subsequent precipita­tion. Theoretically, it is possible to implant any kind of ion into a sub­strate material. Ion plating, on the other hand, is a deposition process in which the substrate surface is coated with a film deposited from a high ion flux with sufficient energy of the beam obtained from an activated source and a pre-induced ionized plasma. Table I lists various surface modification proc­esses, and Table II identifies the char­acteristics of various deposition tech­niques for the compound films.

of Coating Material

f Thermal Energy Source

~ of

Reactant Species

Step 1

Vapor Phase • Reactant Plasma Reaction Species Gaseous Anodization

" to the

Thermal Substrate Plasma Reduction

Step 2 Step 3

Figure 1. Schematic representation of the three steps involved in coating fabrication.

6

The parameters which govern the performance of the coatings fall into two groups-volume properties, which relate to the contacting bodies as a whole, and the surface properties, which determine the contacting inter­face of these bodies. In the first group, the important properties are yield strength and hardness, followed by

JOM • June 1989

With the growing availability of reliable plasma­generating devices, the application of plasma in thin-film deposition technology is increasing rapidly. The surge in the use of plasma-assisted deposition techniques is a direct result of research activities aimed at processing or synthesizing thin films of superconducting materials, diamond or diamond-like carbon, cubic boron nitride and ~-SiC.

Young's modulus, shear modulus and stored elastic energy. Other properties of interest may be the ratio offracture stress in tension to yield stress in compression and thermal properties. The second group includes the chemi­cal reactivity and adsorptive proper­ties of the substrate.

THEORETICAL BACKGROUND

The uniqueness of the plasma-as­sisted deposition processes lies in their ability to synthesize films at relatively low substrate temperatures (300-500°C). These processes offer the pos­sibility of varying film properties over a wide range by suitably controlling the plasma parameters (Le., electron density, energy and distribution func­tion). Because of this versatility, plasma-assisted deposition processes have undergone phenomenal growth in industrial applications, depositing films of dielectrics, metallics, semicon­ductors, micro-electronics, optics and optoelectronics, hard carbides and nitrides, and sulfide films for solid lubricants, especially in space applica­tions and solid-state batteries.

Essentially, there are three steps in the formation of a coating film on the substrate (Figure 1): synthesis or cre­ation of the depositing species, trans­port of these species from the source to the substrate, and nucleation and growth of the film onto the substrate. These steps can operate independently or interdependently, depending on the particular process. It is preferable to have a process where the steps operate independently, thereby allowing great­er flexibility and control.

THE ROLE OF PLASMA

Increasingly, plasmas have been used as an important tool in materials processing during the last two dec­ades. The main attraction of plasma utilization is its ability to provide an

1989 June • JOM

Table I. Methods of Coating Deposition

Atomistic Environment

Electrolytic Electroplating Electroless Plating Fused-Salt

Electrolysis Chern. Displace.

Vacuum Vacuum Evap. Ion Beam Dep. Molecular Beam Epitaxy

Plasma Sputter Coating ARE Polymerization Ion Plating

Chemical Vapor

CVD Reduction Decomposition Plasma Enhanced

Spray Pyrolysis

Liquid Phase Epitaxy

Particulate

Thermal Spraying Plasma Spraying Detonation Gun

Flame Spraying

Fusion Coating Thick-Film Ink Enameling

Electrophoretic

Impact Plating

Explosive Bonding Roll Bonding

Overlaying

Weld Overlay

in-situ source of ions, activated atoms and energized molecules at a relatively low temperature, which enhances the various physical and chemical proc­esses and thus produces a deposited film with more desirable properties. In film deposition, the plasma generates the vapor species (for sputtering depo­sition), activates and enhances the reactions for deposition of metal or compound films, and modifies the struc­ture/morphology ofthe films. As men­tioned earlier, plasma has advan­tageous, as well as deleterious effects on the process parameters, depending upon the deposition techniques; for example, in addition to enhancing the reaction kinetics in reactive sputter­ing deposition, it may also produce target poisoning (i.e., over a critical flow rate ofthe gas, the deposition rate decreases by a factor of three or more). The critical flow rate is a function of

Surface Mod. Bulk Coatings

Wetting Process Painting Dip Coating

Electrostatic Spraying Printing

Spin Coating

Cladding

Shot Peening Thermal

Techniques

Chemical Conversion Electrolytic Anodization (Oxide)

Fused-Salt Technique

Chemical Liquid CVD

Thermal Plasma

Mechanical Treatments

Surface Enrichment

Diffusion from Bulk

Sputtering

Ion Implantation

power to the target, pressure into the chamber, and target size.

The relationship between the film properties and gas injection rate is non-linear, and it is believed to result from a complex phenomenon involving dependence of the sticking coefficient on the growth rate, composition struc­ture, and temperature of the growing film.l Hence, the selection of a plasma­assisted process for a particular appli­cation is not a straightforward deci­sion. Some of the important factors which should be included for consid­eration are the materials to be depos­ited, the deposition rate required, the maximum substrate temperature tol­erable, adhesion of the film to the substrate, the throwing power of the process, and the facility for substrate bombardment for the modification of the structure/morphology of the de­positing film.

Table II. Typical Characteristics of Deposition Processes

Thermal Characteristic CVD Electro-Deposition Spraying

Mechanism of Species Production Chemical reaction Deposition from From flames

solution or plasma Deposition Rate Moderate Low to high Very high

(200-2,500 Nmin.) Species Deposited Atoms Ions Droplets Throwing Power

Complex-Shaped Objects Good Good No Into Small, Blind Holes Limited Limited Very limited

Metal Deposition Yes Yes, limited Yes Alloy Deposition Yes Quite limited Yes Refractory Deposition Yes Limited Yes Energy of Species Deposited Can be high with Can be high Can be high

plasma-assisted CVD

Bombardment of Substrate! Deposit by Inert Gas Ions Possible No Yes

Growth Interface (by External Means) Yes (by rubbing) Yes No

7

Table III. A Comparison of Physical Vapor Deposition Processes

Characteristics Evaporation Ion Plating Sputtering

Mechanism of Species Production Thermal energy Thermal energy Momentum transfer

Deposition Rate Can be very high Can be high Low except for (up to 75 11m/min.) (up to 25 11m/min.) pure metals

(Cu: 111m/min.) Species Deposited Atoms and ions Atoms and ions Atoms and ions Throwing Power

Complex-Shaped Objects Poor: line-of-sight Good, but Good, but

coverage except for non-uniform non-uniform gas scattering

Into Small, Blind Holes Poor

Metal Deposition Yes Alloy Deposition Yes Refractory Deposition Yes Energy of Deposited

Species Low Bombardment of

Substrate/Deposit by Inert Gas Ions Not normally

on geometry Growth Interface

Perturbation Not normally Substrate Heating

(by External Means) Yes, normally

The presence of the plasma in the source-substrate vicinity greatly affects the process parameters at each of the three steps of film deposition. How­ever, the effect of the plasma on each of these three steps will be different in terms of the types and concentration of the ionized species, activated atoms, energized molecules and radicals, which, in turn, will affect the reaction paths and the distributions and physi­cal locations of active reaction sites on the surface of the substrate. Also, as noted by Deshpandey et al.,2 the ioni­zation probability of atoms is maxi­mum if the electron energy is in the range of 50-60 eV and decreases with further increases in energy. Therefore, it is preferable to have low-energy elec­trons, as is the case in activated reac­tive evaporation-type processes. Bun­shah3 has shown that the advantages and limitations of various plasma-as­sisted deposition techniques can be addressed in terms of the differences in the plasma interactions at the source, during transport, and at the substrate during film deposition in the various processes. Table III compares the three

C S

Cathode .. (Substrate)

thickness thickness

Poor Poor Yes Yes Yes Yes Yes Yes

Can be high Can be high

Yes Yes or no depending

Yes Yes

Yes or no Not generally

most commonly used plasma-assisted deposition techniques-reactive sput­tering, activated reactive evaporation (ARE) and plasma-assisted chemical vapor deposition in terms of plasmal source-plasmalvolume and plasmal substrate interactions. Also, the table catalogs the advantages and limita­tions inherent in each of these proc­esses.

Plasma/Source Interactions

In sputtering deposition, vapor spe­cies are generated by momentum trans­fer from positive ions bombarding the target and thus knocking out atoms from the target material. Therefore, the rate of vapor species generation will be dependent on the power input from the plasma to the target (i.e., the cathode voltage and current in the case of d.c. and radio frequency sputtering). Thus, sputtering rate, in this case, is totally plasma -dependent. In ion beam deposition techniques, on the other hand, the sputtering rate is indepen­dent of the plasma. In the ARE process (developed and perfected by Bunshah et al. 4.5), metal atoms are produced by

G F

.. Anode

Figure 2. Schematic illustration of various zones associated with a plasma. Zone C is Crook's dark space; zone S is the glow boundary; zone G is the boundary of a visible glow; and F is Faraday's dark space.

8

evaporation from the source, which may be heated by a thermionic elec­tron beam, a plasma electron beam, resistance heating or arc heating. In this process, plasma is created in the source-substrate space by injecting low­energy electrons (20-200 eV). The source of the low-energy electrons can be a thermionically heated cathode or plasma sheath above the molten metal pool with an appropriately spaced anode biased to a low positive poten­tial. Radio-frequency heating can also be used to form plasma. Thus, in the ARE process, the vapor species are generated by thermal energy imparted to the source material. The rate of generation of vapor species varies di­rectly with the vapor pressure of the source material, which, in turn, is dependent on the surface temperature of the source material. The plasma has little effect on the rate of evaporation of source material. Also, numerous studies have been reported on specific metal/reactive gas combinations to elucidate the role of physical and chemi­cal processes occurring during reac­tive sputtering.6-25 The general con­sensus among these studies is that plasmaltarget reactions may play an important role in the overall reactive sputtering reaction mechanism(s).

Role of Plasma During Material Transport

During the plasma-vapor species interactions, there are three impor­tant reactions which occur simulta­neously: electron impact ionization, excitation of atoms and molecules, and dissociation.

These can be represented as:

e- + A = A+ + e-(excitation) (1)

e- + A = A + + 2e-(electron impact-ionization) (2)

e- + A = B+ + C + 2e-(dissociation) (3)

The rates of these reactions can be represented26 as:

(4)

where Ris the rate of reaction, Ne is the electron concentration, Ki is a rate constant, and (A) is the concentration of A.

Further, the rate constant for these reactions has been shown to be27

Ki =(~J!J Ef(E)si(E)dE (5)

where me is the electron mass, f(E) is the electron distribution function, and olE) is the collision cross section for a particular reaction.

Thus, from Equations 4 and 5, one can easily estimate the rate of forma­tion of a particular species in the

JOM • June 1989

plasma. Thornton,28 in an excellent publication, has described in detail the analytical model and illustrated the principle mechanism of radical forma­tion in a plasma. Numerous types of radical metastable species as well as excited and ionized species may be generated in the plasma (Table IV).

Plasma/Substrate Interactions

The substrates, when exposed to a plasma, are bombarded by ions, elec­trons and energized atoms. As dis­cussed by Holland,29 the nature and energy of these bombarding species is dependent on the process parameters and geometrical aspects of the sub­strate within or outside the plasma zone. As a result of substrate bombard­ment, numerous changes may take place, such as substrate heating, sub­strate surface cleaning, re-emission or sputtering of deposited material, gas incorporation in the depositing film, and modification in the film structure/ morphology. Thus, plasma/substrate interactions may have a significant effect on the final properties of the growing film.

The main reason for the substrate bombardment in a plasma or glow discharge is the potential developed on the surface of the substrate with re­spect to the plasma. Due to the dif­ference in the mobility of electrons and ions, a space charge region forms (C in Figure 2) adjacent to the surface in contact with the plasma from which electrons are largely excluded.

The nature of this region C (sheath) depends mainly on the current density passing across it. The main role ofthis sheath is to produce a potential barrier so that electrons are electrostatically deflected away from the substrate. The magnitude of this potential barrier will adjust itselfin a manner to balance the electron flow to the substrate to be equal to that drawn out in the external circuit. Thus, it is true that any surface coming in contact with plasma will de­velop a potential which will be some­what negative with respect to the plasma. Thus, an electrically isolated surface in contact with the plasma develops a negative potential with respect to the plasma so that the elec­tron flux equals to the ion flux. Gener­ally, this potential is referred to as the floating potential. The significance of this analysis is that the substrate bombardment by species is dependent on the floating potential, which, itself, is dependent on the electron energy and distribution functions. To exert control over the substrate bombard­ment and, therefore, on the resulting various substrate reactions that mod­ifY the nucleation and growth phenome­non of the film, the electron energy and distribution function can be varied

1989 June • JOM

Table IV. Products and Activated Species Expected in a Plasma Environment at 8,OOOK

Plasma Gas Elements Source

Products

Argon Carbon Compound

C2, C3, etc.

C+, C2+, etc. S+, S2' S

FeO, Fe, Fe> °2,0,0',0-Ti02, Ti20 3

, Ti30., Ti, Ti+, Ti2+, etc.

Table V. Synthesis Techniques for Novel Materials

Novel Materials

Superconducting Materials (e.g., Nbpe, CuM0

6S

8)

Photovoltaic Materials (e.g., A-SiH, CulnS2)

Optoelectronic Materials (e.g., Indium Tin

ARE (Nmin.)

1,000-1,500

1,500-2,000

Processing Technique

RS (Nmin.) PACVD Others

50-200

60-150 Oxide, Zinc Oxide) Cubic Boron Nitride

500-1,000 1,000-1,500 1,500 Nmin. *

Diamond or Diamond­Like Coating

I-C

*ADRRP

tMPACVD

300 1,000 Mr. 200 Nmin.

10,000 Mr. t 2 )lIn/hr.

SYNTHESIS OF NOVEL MATERIALS

Materials possessing unique combinations of phys­ical, mechanical, electrical and optical properties are classified as novel materials. A few examples of such materials include: diamond, which is electrically insu­lating but has extremely high thermal conductivity; materials with high hardness values and high ductility, such as microlaminate composites; and materials with a metastable structure,n such as ~-SiC, cubic BN and synthetic diamond and similar materials. Some novel materials synthesized by plasma-assisted techniques are listed in Table V.

The unique combination of optical, physical and electronic properties of diamond has been the main driving force for the development of new and less ex­pensive methods for diamond synthesis. Potential ap­plications for diamond include reliable infrared window coatings, high-temperature semiconductors, high­energy microwave amplifiers, powerful pulsed lasers, wear coatings, and as a heat sink material. Diamond film is hard, stiff, and has a very low coefficient of friction. I! can transmit light from the far infrared through ultraviolet. I! is also one of the few known materials that is both an excellent thermal conductor and an electrical insulator. Aerospace manufacturers may be the first to use diamond thin film as a coating material on optical windows and domes. Diamond film will protect fragile sensors and lasers from hostile environments. These devices are used in heat seeking missiles, satellites and in night-vision systems. Most of these devices operate in the far-infrared wave range (Le., 7 11m and upwards) which permits them to see through clouds and distinguish cooler objects from others.

The optical properties of window materials are destroyed under stress. For example, germanium dis­torts when heated by supersonic night or high·powered lasers. Zinc selenide flakes and spalls off on impact. Zinc sulfide has better mechanical properties but a narrower far-infrared bandwidth. Diamond solves most of these problems. I! is almost transparent to far­infrared waves with 7 11m and greater and is hard enough to shed rain and ice at hypersonic speed. Also, its high refractive index of 2.4 makes it a suitable window material with integral lenses.

There are several methods, such as PACVD,3~39 ion beam deposition.'o .. sputtering'2 and hot filament techniques," which have been employed for diamond film fabrication. Different types of diamond for different applications can only be synthesized by different tech­niques of diamond crystallization. For example, dia-

mond for high-temperature semiconductor applications can be prepared by creating an alternating layered structure of dielectric and semiconducting single crys· tal diamond films, with the thickness probably in the range of one microinch. When lower temperature, lower pressure diamond synthesis is used, there is an equal possibility of nucleation for both diamond and graphite crystal. Therefore, in PACVD for the synthesis of diamond, the presence of a super-equilibrium con­centration of atomic hydrogen, obtained by the disso­ciation of H2 into the plasma, is critical. When the concentration of atomic hydrogen is at least one order of magnitude higher than that for thermal equilibrium, nucleation of graphite is then suppressed. A plasma rich in atomic hydrogen can be obtained by various types of electrical discharges. The temperature and pressure range for diamond synthesis are 900-1 ,OOO°C and 5-10 torr. The morphology of diamond single crystals and diamond film is very sensitive to CH, concentrations in Hz, pressure, temperature and physi­cal location of substrate into the plasma. A diamond crystal grown by PACVD is shown in Figure 3.

Similar to diamond, there has been a surge in re­search interest in synthesizing cubic BN film because of its unique combination of high hardness, physical, mechanical and optical properties. Salon et al." have employed 30 keY N

j' ion beam bombardment to syn­

thesize cubic BN fi m. Shamfield et al .. s and many others'· " have reported producing cubic BN films by radio frequency·excited NH, plasma using borozine, by electron beam evaporation of boron in an NH, plasma and by reactive diode sputtering techniques. In a combined study, Bunshah and Chopra'l.9 have reo ported a unique process for syntheSizing cubic BN films which might be of significant importance, because of its potential applications in micro- and optoelectronics as well as in solid lubrications and hard coatings. They claimed to have used boric acid, a non·toxic material, as opposed to the toxic materials (e.g., borozine or diborane) used by others for the preparation of cubic BN film. Bunshah et al. 'I.50.S' have termed their process the activated dissociation reduction reaction process (ADRRP), and it involves evaporation of boric acid in an NH3 plasma. They have further claimed to deposit cubic BN at 400°C with the deposition rate up to 1,500 Nmin. Finally, they could not detect any hexagonal phase in cubic BN film which shows characteristic absorption at 6.8 and 121.5 11m corresponding to B-N and N·B-N bending vibrations.

9

a

Figure 3. Micrographs of PACVD diamond. (a) Single crystal diamonds. (b) Diamond film.

independently of other process pa­rameters.

Plasma has been applied in both major types of deposition techniques [i.e., chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes]. In plasma-assisted CVD (PACVD), radio frequency, microwave and photon (ultraviolet and laser) excitation has been used to generate the plasma. However, in PAPVD, d.c., radio frequency, triode, magnetron geometries and reactive sputtering methods have been used. Also, Bun­shah et a1.30 ,31 have developed a very useful deposition PAPVD technique,

namely ARE, in which metal atoms are produced from an evaporation source and may be heated by a thermionic electron beam, a plasma electron beam, resistance heating or arc heating. The gas employed is only reactive at a pres­sure less than 103 torr. The plasma is generated in the source-substrate space by simply injecting low-energy elec­trons (20-200 eV).

OUTLOOK

Plasma has proven to be a very important tool in thin-film deposition technologies. As a result, established processes for surface modification, such as the hot-dip molten method, diffu­sion, vacuum evaporation and electro­plating, are being replaced by rela­tively new plasma-assisted processes. However, there is a need to develop plasma-assisted deposition processes where most of the process parameters can be controlled independently. Al­though plasma-substrate interactions result in desirable substrate chemis­try changes (i.e., heating, surface clean­ing and sputtering of loosely bonded species), there is a need to separate plasma from the substrate to obtain film purity, lower the deposition tem­perature and minimize bombardment­induced film damage.

Editor's Note: The synthesis and application of diamond film will be discussed by Dr. Upadhya in a short course he is preparing with Jeffrey T. Glass and Andrze R Bedzian for presentation at the 1989 TMS Fall Meeting in Indian· apolis, Indiana. For more information, refer to the ad which appears on page 56 of this issue.

References 1. A.J. Thornton, Deposition Technologies for Films and Coatings, ed. RF. Bunshah (New Jersey: Noyes, 1982), p. 232. 2. C. Deshpandey and RF. Bunshah, invited paper pre· sented in 1988 MRS Meeting, Boston, Massachusetts. 3. RF. Bunshah, Thin Solid Films 21 (107), (1983). 4. RF. Bunshah, op. cit. 1, p. 5. 5. L. Holland, Vacuum Deposition of Thin Films (London: Chapman and Hall, 1956), p. 62. 6. J.L. Vossen and J.J. Cuomo, Thin Film Processes, ed. J. Vossen and W. Kern (New York: Academic, 1978). 7. C. Deshpandey, doctoral thesis, University of Sussex, Falmer, Brighton, United Kingdom (1981). 8. J.A Thornton, op. cit. 1, p. 232. 9. W. Westwood, Prog. Surf Sci., 7 (1976), p. 71. 10. J. Heller, Thin Solid Films, 17 (1973), p. 163. 11. KG. Geraghty and L.F. Donaghey, J. Electrochem.

s:

Soc., 123 (1976), p. 1201. 12. L.F. Donaghey and KG. Geraghty, Thin Solid Films, 38 (1976), p. 271. 13. S. Maniv and W.D. Westwood, J. Appl. Phys., 51 (1980), p. 718. 14. S. Maniv and W.D. Westwood, J. Vac. Sci. Techno!., 3 (1980), p. 743. 15. AM. Stirlig and W.D. Westwood, J. Appl. Phys., 41 (1970), p. 742. 16. AJ. Stirlig and W.D. Westwood, Thin Solid Films, 7 (1971), p. 1. 17. T. Abe and T. Yamashina, Thin Solid Films, 30 (1974), p.19. 18. F. Shinoki andA Itoh,J.Appl. Phys., 46 (1975), p. 381. 19. B.R. Natarajan, AH. Etoukhy, J.E. Green and T.L. Barr, Thin Solid Films, 69 (1980), p. 201. 20. B.R Natarajan, A.H. Ethoukhy, J.E. Green and T.L. Barr, Thin Solid Films, 69 (1980), p. 217. 21. C. Deshpandey and L. Holland, Thin Solid Films, 97 (1982), p. 265. 22. C. Deshpandey and L. Holland, International Con· ference on Metal Coatings (Tokyo: Iron and Steel Institute of Japan, 1982), p. 276. 23. AR Nyiesh and L. Holland, J. Vac. Sci. Technol., 20 (1982), p. 1389. 24.AR Nyiesh andL. Holland, Vacuum, 31(1981),p. 371. 25. L. Holland, Thin Solid Films, 86 (1981), p. 227. 26. AT. Bell, Techniques and Applications of Plasma Chemistry, ed. J.R Hollahan and AT. Bell (New York: Wiley and Sons, 1974), p. 31. 27. F.J. Kampas, Semiconductors and Semi-Metals, ed. J.I. Pankov, vol. 21 (New York: Academic, 1984), p. 159. 28. J.A. Thornton, Thin Solid Films, 107 (1983), p. 3. 29. L. Holland, Surface Technologies, 11, (1980) p. 145. 30. RF. Bunshah and AC. Raghuram, J. Vac. Sci. Tech., 9 (1972), p. 1385. 31. Ibid., p. 1389. 32. RC. DeVries, Am. Rev. Mat. Sci., 17 (1987), p. 161. 33. D.S. Whitmell and RF. Williamson, Thin Solid Films, 35 (1976), p. 255. 34. L. Holland and S.M. Ojha, Thin Solid Films, 38 (1976), p. L17. 35. D.A Anderson,Phil. Mag., 35 (1977), p. 17. 36. S. Berg and L.P. Anderson, Thin Solid Films, 58 (1979), p. 117. 37. AR Badzian and RC. Devries, Mat. Res. Bull., 23 (1988), p. 385. 38. AR Badzian, T. Badzian, R Roy, R Messier and KE. Spear, Mat. Res. Bull., 23 (1988), p. 531. 39. AR Badzian et al., SPIE, 683 (1986), p. 127. 40. S. Aisenberg and R Chabot, J. Appl. Phys., 39 (1968), p.2915. 41. E.G. Spencer, H.P. Schmidt, D.C. Joy and F.J. Sawsadone, Appl. Phys. Lett., 29 (1976), p. 118. 42. G. Gantherin and C. Weismental, Thin Solid Films, 50 (1978), p. 135. 43. S. Matsumoto, Y. Sato, M. Tsutsumi and N. Setaka, J. Mat. Sci., 17 (1982), p. 3106. 44. M. Salon, and F. Fujimoto, Japan J. Appl. Plug., part 2, 22 (1983), p. 171. 45. S. Shamfield and R Wolfson, J. Vac. Sci. Tech., (A) 1 (1983), p. 323. 46. C. Weissmantel, J. Vac. Sci. Tech., 18 (1981), p. 19. 47. P. Lin et aI., Thin Solid Films, (1987). 48. KL. Chopra et aI., Thin Solid Films, 126 (1985),p. 307. 49. RF. Bunshah, KL. Chopra, C.V. Deshpandey and V.D. Vankar, U.S. patent no. 4,714,625 (1987). 50. P. Lin et aI., Thin Solid Films, 153 (1987), p. 487. 51. RF. Bunshah, op. cit. 1, p. 14.

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JOM • June 1989