Ion Plating: a personal account of the early years

Author: Donald M. Mattox
donmattox@mpinm.com

It all began with vacuum cadmium plating of high strength steel compressed gas bottles.

After serving as a meteorologist and Air Weather Officer in the USAF during and after the Korean War, I obtained a M.S. degree in solid state physics from the University of Kentucky in 1961 (see more in Endnote A).

I reported to Sandia Corporation in April of 1961 as a Technical Staff Member and began work as a solid-state physicist in the metallurgical engineering group. The Sandia “metallurgy group” was composed of a mix of material science people with specialties ranging from metallurgy, organic adhesives, surface cleaning, and ceramic fabrication, to process “troubleshooters” and technology transfer. In order to learn more about material science I began teaching a noon-hour course on material science for interested laboratory personnel – I was teaching the course when President John F. Kennedy was assassinated on Nov. 22, 1963.

The metallurgy department had an old CVC vacuum system that wasn’t being used. It had a glass bell jar with a cable hoist, a manually cranked high vacuum gate valve, and an oil diffusion pump. I had done some vacuum work at University of Kentucky so I decided to get it working.

My “bible” was Leslie Holland’s book Vacuum Deposition of Thin Films (1956) (Chapman Hall). I got the system into operation with a filament evaporator then added a DC sputtering capability. I controlled the pressure for sputtering by manually “cracking” the gate valve. My mass flow controller was a manual needle valve. I had a CEC LC-031 DC high voltage power supply that used mercury thyratron rectifier tubes. It could get to 5000 volts DC though it tended to arc internally at the high voltages. When the sputtering cathode arced the voltage would go down, the current would go up, the tubes would light up and the plates in the high voltage transformer would buzz. The only way to turn off a “hard arc” was to manually turn off the power supply – there was no arc suppression/quenching circuit in that power supply!

One of my first troubleshooting trips was to a contractor’s processing line for applying vacuum deposited cadmium (“VacCad”) plating to high-pressure gas bottles. They were having problems with the adhesion of the cadmium coating to the steel. The problem was simple – they were blowing off the part with “house air” and the air contained oil from the compressor. That got me thinking about the adhesion of vacuum deposited coatings [1]. When I discussed this with the metallurgists they maintained that to get good adhesion you needed solid solubility. Being a solid state physicist I thought just getting good atom-to-atom contact should give you good adhesion.

In the library I found that the field of high vacuum surface science studies on atomically clean surfaces really began in the mid-1950s when H. Farnsworth produced atomically clean surfaces by repeated sputtering and annealing in high vacuum [2,3]. I thought “What if you started depositing using thermal evaporation while you are sputtering the substrate surface. Might that not give a “clean” interface and good atom-to-atom contact?” So in one afternoon I tried vacuum evaporation while the sputter cleaning was still going on. It worked great! When I evaporated copper into the plasma there was a nice green glow indicating excitation/de-excitation of the copper atoms and probably ionization.

I decided that the definitive experiments should involve immiscible materials so I chose silver and iron, which are immiscible even in the molten state. I coated a series of iron tensile bars with silver and elongated them to test the adhesion. My “ion plating” technique gave adhesion greater than that detectable by the test.

I presented the work at a Gordon Research Conference on Adhesion in the summer of 1963. My paper (“Deposition by Exploding Wire and Ion Plating”) was scheduled for presentation on Thursday evening. On Wednesday I had yet to received permission to present the paper because a patent application was being made. On Thursday I received permission to present the paper. After the Thursday evening all-you-can-eat lobster dinner I was completely full of lobster and gave the paper. The paper and the subsequent discussion went on for 3 hours! The concept of ion plating was first published in the technical literature in 1964 [4] and the patent was issued in 1967 [5]. Of course in hindsight, there was very little ionization of the evaporated material in the plasma that I used – bombardment was mainly by the inert gas ions used for sputtering.

The 1967 (priority 1964) ion plating patent covered the use of both thermal evaporation and chemical vapor precursors as sources for the depositing material [6]. In 1969 (priority 1964) H.F. Sterling and R.C.G. Swann patented the use of chemical vapor precursors in an RF plasma with no substrate bias as a means of depositing coatings by what was later called plasma enhanced chemical vapor deposition (PECVD) [7,8].

Bob Culbertson of EIMAC/Varian expressed an interest in using the ion plating process for metallized ceramics at a low temperature and we metallized ceramics using a WCl4 precursor [9]. Bob later patented ion plating for depositing low-electron-emission coatings (C and TiC) on electron tube grids [10]. One of the claims in his 1971 patent (filed 1968) used both a filament evaporator and a chemical vapor precursor to deposit coatings such as TiC. This was the first use of ion plating as a hybrid deposition process of PVD/PECVD. His patent was probably the first report on what came to be known as “reactive ion plating” [11]. Y. Murayama was the first to use the term “Reactive Ion Plating” in the peer-reviewed literature in 1975 [12]. The Japanese seemed to pick up the use of the term “ion plating” much more quickly than did others.

The term “ion plating” was used in the 1967 paper published in Electrochemical Technology and in the patent. The term came under some criticism and was discussed in the Journal of the Electrochemical Society in 1968 [13]. Over the years others have used different terminology for much the same process. These include: Ion Vapor Deposition (IVD), Ionized Physical Vapor Deposition (iPVD), Ion Assisted Deposition (IAD), Ion Beam Assisted Deposition (IBAD), Biased Activated Reactive Deposition (BARE), and Energetic Condensation.

The Sandia reactor group was developing nuclear reactors for providing pulses of neutrons; the SPR series of pulsed neutron reactors [14]. The SPR II reactor contained 105 kg of U-Mo alloy of which 93% of the uranium was 235U (more than the 64 kg that was in the gun-type “Little Boy” atomic bomb of WW II). When pulsed by bringing the components together, the uranium parts attained very high temperatures very quickly. In the SPR I reactor the uranium was protected by electroplated nickel and the nickel had to be periodically replaced. The old nickel had to be chemically stripped and the surface “pickled” all of which was undesirable since they lost some 235U in the process and the surface was roughened. They wanted a substitute for the electroplated nickel coating.

We ion plated samples of depleted uranium with aluminum and subjected them to corrosion tests, which it passed with flying colors [15]. We then coated some mock-ups of the reactor components. When it came to coating the real parts we had to strip the nickel and then pump down overnight – all the while with an armed guard by our side. The coated parts worked very well [16].

In analyzing the coating substrate-coating interface we found that there was an appreciable layer of UAl2 formed at the interface due to the clean interface, the heating from the sputter cleaning, and thermal diffusion during the deposition.

While we were working on the uranium project we were also developing other fixtures for ion plating, several of which were described in the patent. These included a “cold finger” fixture for keeping the back-side of the sample cooled to liquid nitrogen temperature during deposition. We used that technique to deposit metallization electrodes on semiconductor materials to avoid “pipe diffusion”.

We were also using a rotating cage fixture (patterned after the barrel plating technique used in electroplating) for coating small parts [17]. Ion plating using the cage fixture was used to vacuum coat ultra-high vacuum components with gold to reduce hydrogen outgassing.

The ion plating patent also covered the use of a grid in front of an insulator or irregular surface to provide bombardment. We used this arrangement to coat ceramics and polymers [18,19]. Ion plating was also used to coat metals with chromium for glass-to-metal sealing [20].

The use of ion plating for aluminum coating spread quickly through the AEC nuclear weapons complex for coating uranium and plutonium as well as silver on beryllium for diffusion bonding [21]. R.T. Bell and J.C. Thompson at the Oak Ridge Y-12 Plant used ion plating as a “strike” for electroplating in 1973 [22] as did others [23].

In January 1965 Time Magazine ran a short news item called “Plating with a permanence” about the ion plating process and the article caught the attention of McDonnell-Douglas. They sent a group to look at what we were doing. They saw the rotating cage fixture and they took the idea back and developed their “IVADIZING” machine for coating fasteners for the aerospace industry [24-26]. This became the basis for the IVD (“ion vapor deposition”) process, which is widely specified for military and aerospace applications [27].

T. Spalvins and Donald Buckley of NASA (Cleveland) picked up the ion plating process for their studies of lubrication in space using low-shear metal films [28]. Based on the NASA work General Electric began using ion plating for coating their X-ray tube bearings with low shear metals for lubrication in vacuum. The dissemination of the ion plating process was aided by the fact that the patent was in the public domain.

In the sixties, RF sputtering began to be used for sputtering dielectric materials. I converted an old diathermy machine for RF sputtering. We began to sputter dielectric materials [29]. Several months after we began rf sputtering FCC inspectors showed up! We had been broadcasting all over the radio spectrum and my laboratory was right by the runway at Kirkland AFB. We were interfering with the aircraft-to-ground communication. Since we were only doing it sporadically they had a hard time finding us. I was shut down – but the “powers that be” at Sandia then purchased an RF system for my group with proper shielding.

The mid-sixties was also the time that scanning electron microscopy (SEM) was coming onto the scene as a commercially available analytical tool [30] (1965 – Cambridge Scientific Instruments – “Stereoscan”). I began to look at the fracture cross-section morphology of thick ion plated coatings. It was immediately obvious that ion bombardment during deposition was disrupting the columnar growth and densifying the deposit as well as affecting the surface morphology [31,32]. For a period of time the densification by bombardment was called “atomic peening” [33] though this descriptive terminology has fallen out of favor.

At about the same time as I was doing the ion plating work using thermal evaporation, several others were doing similar work using “bias sputtering” [34-37]. However, they did not seem to do much analytical work on the films other than that related to the function they were studying. Some authors later called this process “sputter ion plating [38].

In the early work on ion plating it was found that ultrafine particles were deposited on the interior walls of the chamber but not on the negatively biased substrate [39]. The particles that formed in the plasma attained a negative charge and were repelled by the negative potential on the substrate. Later the same phenomenon was observed in high rate sputtering [40,41].

In the case of reactive materials such as titanium, the deposited ultrafine particles were stable when exposed to air, apparently because of a very thin surface oxide layer. When disturbed the fine particles would “burn.” To quote from Handbook of Physical Vapor Deposition Processing, 2nd edition (2010) (p. 322, section 9.9.2), Donald M. Mattox, Elsevier.

“In the early work on ion plating, the particles formed in the plasma and deposited on the walls were called ‘black sooty crap’ (BSC) and could be very combustible. One game was to ask an observer to wipe the particles off a window with a paper towel. When the window was wiped, the towel caught fire and a flame front moved over the interior surface of the chamber, which was covered with BSC.”

In hindsight I wish that I had done more studies on the charged ultrafine particles since 20 years later ultrafine (nano-) particles became an important field of study.

By the mid 1970s several papers had been published on how ion plating could be used to tailor the properties of thick deposits using both continuous and periodic bombardment during deposition. It was well established that forming the interface by beginning the deposition during sputtering cleaning gave good adhesion, bombardment with reactive ions allowed reactive deposition of dense compound materials, and bombardment during deposition could be used to control intrinsic stress, morphology, and crystallographic orientation of the deposited coating materials.

In the book chapter “Ion Plating Technology” in the first edition of Deposition Technologies for Films and Coatings, Development and Applications edited by R.F. Bunshah et al (1982) a rather extensive bibliography of papers on ion plating and related subjects was provided for the period 1963 to 1980 [42].

With the advent of arc vaporization in the early 1980s [43], and HIPIMS in the early 2000s [44] where a large percentage of the vaporized material is ionized to condensable “self-ions,” bombardment effects on the properties of coatings entered a new realm [45].

Some time in the early 1970s someone called attention to the fact that Bernard Berghaus, of plasma nitriding fame, had patented a process similar to “ion plating” in Europe in the mid-1930s [46]. His work in this area seems to have been an extension of his earlier work on ion nitriding and sputter deposition [47-49]. Berghaus apparently did not pursue any application of the process nor did he publish anything in the technical literature about the process that I could find. Leslie Holland didn’t discuss Berghaus’ coating work in his book Vacuum Deposition of Thin Films so it must not have been well known at that time (1956).

By the mid-70s my group became involved in solar energy (depositing selective solar thermal absorbers), fusion reactor technology (coating TOKAMAK limiters) and in developing a lithium source for the Sandia fusion PBFA fusion reactor experiments. Ion plating just became another “tool.”

Using ion plating for tailoring the coating stress was carried over into stress control for the deposition of thick Mo coatings for the BOLVAPS lithium ion source for Sandia fusion energy experiments. At low pressure sputtering the high energy reflected neutrals from the sputtering target may bombard the deposit and affect the stress in the deposit [50]. The technique of “pressure pulsing” for stress control in the sputter deposition of thick deposits was developed to tailor the stress in thick sputter deposited coatings [51]. Pressure pulsing relies on bombardment by high-energy reflected neutrals from the sputtering target at a low pressure environment rather than accelerating ions from a plasma.

In a 2013 publication Professor Allan Mathews, University of Sheffield, Sheffield, UK, credited Ion Plating with being the “First Wave” of “Plasma Assisted PVD Processing” [45].

Conclusion:

The quest for better adhesion by having a clean interface led to a serendipitous way to modify film/coating properties (optical, metallurgical, electrical, chemical, etc.) by energetic particle bombardment during deposition of PVD and PECVD films/coatings.

Acknowledgements:

From the mid-1960s and through the following years I had a number of technicians and several staff members who worked with me on ion plating projects. Notable among the technicians were: Frank Rebarchik, Ray Bland (who later became a Laboratory {shop} Supervisor), George Kominiak (who later became a Technical Staff Member), and Chuck Peeples. Notable among the technical staff members were: Bob Cuthrell, Ray Berg, Janda Panitz, Don Sharp, Art Mullendore, Jack Houston, and John Whitley. Special thanks to my supervisors who gave me the leeway to pursue ion plating projects. These include, in the early years: John McDonald, Charlie Bild, Lou Berry, Bill O’Neal and Dick Schwoebel. Thanks also to Lew Jones who took me under his wing when I first arrived at Sandia and taught me some of the nuances of troubleshooting at production facilities.

Endnote A: I graduated from Eastern Kentucky State University during the Korean War. I joined the USAF for four years to become an officer and be trained as a meteorologist. I attended two semesters of meteorology at Massachusetts Institute of Technology (MIT) (1953-54) before being deployed to Eglin AFB, FL as a weather forecaster.

After a year I was assigned to the 58th Weather Recon Sq. at Eielson AFB, Alaska as an Air Weather Officer. My job was as an in-flight weather observer on synoptic weather fights that also included air sampling for detecting Russian nuclear tests. The flight routes were over the Bering Sea to Attu at the western end of the Aleutian Island chain and north to near the North Pole. I flew about 1500 hours in B-29 and then B-50 aircraft.

After leaving the USAF I went to graduate school at the University of Kentucky (Lexington, KY) under the GI Bill. I was in the Physics Department and was a graduate assistant in the Solid State Physics laboratory. My advisor was Professor Lee Gildart. In 1960 Dr. Gildart left unexpectedly and I was offered a position in the High Energy Physics lab counting cloud chamber tracks. No way was I interested in doing that! That sent me looking for a job.

I wanted to move to the Southwest so I traveled around CO, AZ, NM interviewing. I had noted that Sandia Corporation, a US Atomic Energy Commission (AEC) laboratory run by AT&T in Albuquerque, NM used bicycles to get around their Technical Area but I didn’t know much about what they did (about 6 months after I went to work for Sandia they quit using bicycles in the Tech Area). At their Personnel Office I said that I wanted to interview for a staff member position. It turned out that this was pretty unusual in that Sandia usually went to universities to recruit and had very few “walk-ins” for a technical staff member position.

Personnel had several people from the Metallurgy Group come over for an interview. Up to that time the metallurgy group had been “handbook engineers” but they wanted to expand into R&D. They asked “Can you do research?” Of course I said yes since I had co-authored several peer- reviewed papers while at the U of K. They offered me $750 per month salary – a magnificent sum compared to what I was making at U of K!

I had the option of going to work immediately outside the technical (“tech”) area in the “leper colony” and wait for my security clearance or return in about 6 months after I had a clearance. I chose to wait since I wanted to finish my Masters Degree [A-1]. This was the beginning of my work and sojourn at the Sandia National Laboratories until I retired in 1989 to become active as a consultant and course instructor in Management Plus, Inc. (MPI) with my wife Vivienne. MPI is a consulting and education training organization in the field of vacuum coating.

A-1. D.M. Mattox and L. Gildart, “Energy gaps in bismuth trioxide,” J. Phys. Chem. Solids 18(2-3) 215 (1961)

References:

  1. D.M. Mattox and J.E. McDonald, “Interface formation during thin film deposition,” J. Appl. Physics 34, 2493 (1963)
  2. H.E. Farnsworth, R.E. Schlier, T.H. George and R.M. Burger, “Ion bombardment cleaning of germanium and titanium as determined by low-energy electron diffraction,” J. Appl. Phys, 26, 252 (1955)
  3. H.E. Farnsworth, “Preparation, structural characterization, and properties of atomically clean surfaces,” (AVS Welch Award presentation) J. Vac. Sci. Technol., 20, 271 (1982): doi: 10.1116/1.571282
  4. D.M. Mattox, ”Film deposition using accelerated ions,” Electrochem. Technol. 2, 295 (1964); Sandia Corp. Development Report SC-DR-281-63 (1963)
  5. Donald M. Mattox, “Apparatus for coating a cathodically biased substrate from plasma of ionized coating material,” USP 3329601 (priority date, Sept. 5, 1964; filed, Sept. 30, 1966; publication, July 4, 1967) (assigned USAEC)
  6. D.M. Mattox, “Tungsten CVD in a gas discharge,” Sandia Corp. Development report, SC-DC-67-2026A (1967); presented at the “Conf. on Chemical Vapor Deposition of refractory metals, alloys, and compounds,” Gatlinburg, TN (Sept. 12-14, 1967)
  7. Henry Frank Sterling and Richard Charles George Swann, “Method of forming silicon nitride coating,” USP 3485666 (priority date, 8 May 1964; filed, 3 May 1965; published, 23 Dec. 1969) (assigned International Standard Electric Corp.); British pat. 1104935; French pat. 1442502
  8. R.C.G. Swann: https://ethw.org/w/index.php?title=Special:Search&search=The+Birth+of+Glow+Discharge+Chemistry+%28aka+PECVD%29+-+extensive+references&searchToken=chbbl6060oeonkahx2lto80hu – extensive references
  9. R. Culbertson and D.M. Mattox, “High strength ceramic-metal seals metallized at room temperature,” 8th Conf. Tube Technol. IEEE Conf. Record TK 6563, 3, U52, p. 101 (1966)
  10. Robert D. Culbertson, Russell C. McRae, and Harold P. Meyn, “Nonemissive electrode structure utilizing ion-plated nonemissive coatings,” USP 3604970 (filed 14 Oct. 1968; published 14 Sept. 1971) (assigned Varian Assoc.)
  11. Donald M. Mattox, “Short history of reactive evaporation,” pp. 50-51, SVC Bulletin, Society of Vacuum Coaters (Spring 2014)
  12. Y. Murayama, “Thin film formation of InO2, TiN and TaN by RF reactive ion plating,” J. Vac. Sci. Technol. 12, 818 (1975)
  13. Discussion on “Adherence and porosity of ion plated gold,” C.F. Schroeder and J.E. McDonald, J. Electrochem. Soc., 115(12) 1255 (1968) – Reply by Donald M. Mattox
  14. T.R. Schmidt and J.A. Reuscher, “Overview of Sandia National Laboratories pulse nuclear reactors”, SAND-94-2466C (1994); also CONF-941102-28 (Conference: Winter meeting of the American Nuclear Society (ANS), Washington, DC, 13-18 Nov 1994); OSTI ID: 10190030
  15. R.D. Bland, J.E. McDonald, and D.M. Mattox, “Ion plated coatings for the corrosion protection of uranium,” Sandia Development Report SC-DR-65-519 (1965)
  16. D.M. Mattox and R.D. Bland, “Aluminum coating of uranium reactor parts for corrosion protection,” J. Nucl. Mater., 21, 349 (1967)
  17. D.M. Mattox and F.N. Rebarchik, “Sputter cleaning and plating small parts,” Electrochem. Technol., 6, 74 (1968)
  18. D.M. Mattox, “Metallizing ceramics in a gas discharge,” J. Am. Ceramic Soc. 48, 385 (1965)
  19. “High strength ceramic-metal seals metallized at room temperature,” R. Culbertson and D.M. Mattox, 8th Conf. Tube Technol., IEEE Conf. Record TK 6563, 3, U52, p. 101 (1966)
  20. D. M. Mattox, “Chromizing molybdenum for glass sealing,” Rev. Sci. Instrum. 37, 1609 (1966)
  21. G. Mah, P.S. Mcleod, and D.G. Williams, “Characterization of silver coatings deposited from a hollow cathode source,” J. Vac. Sci. Technol., 14, 152 (1977)
  22. R.T. Bell and J.C. Thompson, “Applications of ion plating in metals fabrication,” Oak Ridge Y-12 Plant, Y-DA-5011 (1973)
  23. J.W. Dini, “Ion plating can improve coating adhesion,” Metal Finishing, 91(9) 15-20 (Sept. 1993)
  24. K.E. Steube and L.E. McCrary, “Thick ion vapor deposited aluminum coatings for irregular shaped aircraft and spacecraft parts,” J. Vac. Sci. Technol., 11, 362 (1974)
  25.  J. Carpenter, G. Kesler, A. Klein, L. McCrary, and K. Steube, USP “Glow discharge coating apparatus,” 3750623 (filed 11 Feb 1972; published 7 Aug. 1973) (assigned to McDonnell Douglas Corp.)
  26. Kenneth E. Steube, “Glow discharge-tumbling vapor deposition apparatus,” USP 2926147 (filed 15 Nov 1974; published 16 Dec. 1975) (assigned to McDonnell Douglas Corp.)
  27. MIL-DTL-83488D; “Coating; High purity aluminum,” (01 April 1999); superseding MIL-C-83488C, “Coating; Aluminum ion vapor deposited,” (01 Dec. 1978)
  28. T. Spalvins, J.S. Przbyszewski, and D.H. Buckley, ”Deposition of thin films by ion plating on surfaces having various configurations,” NASA Lewis TN-D3707 (1966)
  29. G.J. Kominiak and D.M. Mattox, ”Physical properties of thick sputter deposited glass films,” J. Electrochem. Soc. 120, 1535 (1973)
  30. C.W. Oatley, W.C. Nixon and R.F.W Pease, “Scanning electron microscopy,” Adv. Electron Phys., 21, 181 (1965)
  31. D.M. Mattox and G.J. Kominiak, ”Structure modification by ion bombardment during deposition,” J. Vac. Sci. Technol. 9, 528 (1972)
  32. R.D. Bland, G.J. Kominiak, and D.M. Mattox, ”Effects of ion bombardment during deposition on thick metal and ceramic deposits,” J. Vac. Sci. Technol., 11, 671 (1974)
  33. A.G. Blackman, Metall. Trans. 2, 699 (1971)
  34. G.K. Wehner, ”Growth of solid layers on substrates which are kept under ion bombardment before and during deposition,” USP 3,021,271 (filed April 27, 1959, publication, Feb, 1962)
  35. R. Frerichs,”Superconductive films by protected sputtering of tantalum or niobium,” J. Appl. Phys., 33, 1898 (1962)
  36. L.I. Maissel and P.M.Schaible, “Thin films formed by bias sputtering,” J. Appl. Phys., 36, 237 (1965)
  37. Orla Christensen, “Characteristics and applications of bias sputtering,” Solid State Technology, p. 39 (Dec. 1970)
  38. N.M. Renevier, V.C. Fox, D.G. Teer, and J. Hampshire, “Coating characteristics and tribological properties of sputter-deposited MoS2/metal composite coatings deposited by closed-field unbalanced magnetron sputter ion plating,” Surf. Coat. Technol., 127(1) 24 (2000)
  39. “A short history of ultrafine (nano-) particles formed in vacuum,” Donald M. Mattox. pp. 54-56, SVC Bulletin, Society of Vacuum Coaters (Summer 2014)
  40. G. Selwyn, J. McKillop, K. Haller and J. Wu, J. Vac. Sci. Technol., A8, 1726 (1990)
  41. G. Jellum and D. Graves J. Appl Phys., 67, 6490 (1990)
  42. “Ion Plating Technology,” Donald M. Mattox, Ch. 6, pp. 244 – 268, in Deposition Technologies for Films and Coatings: Development and Applications, edited by R. F. Bunshah et al, Noyes Publications (1982)
  43. I.I. Aksenov, V.A. Belous, and V.E. Strel’nitskij, “Vacuum-arc surface modification and coating deposition methods in KIPT, Ukraine (Historical Review)” Proceedings 55th Annual Technical Conference, pp. 3-8 Society of Vacuum Coaters (2012); also pp. 48-52, SVC Bulletin, Society of Vacuum Coaters (Summer 2012)
  44. A.P. Ehiasarian, R. New, W-D. Munz, L. Hultman, U. Helmersson, V. Kouznetsov, “Influence of high power densities on the composition of pulsed magnetron plasmas,” Vacuum 65 (2) 147–154 (2002) doi:10.1016/S0042-207X(01)00475-4
  45. Allan Matthews, “Plasma Assisted PVD: The Past and Present,” p. 24-27, SVC Bulletin, Society of Vacuum Coaters, (Fall 2013); also Keynote paper ISSP 2013 (12th International Symposium on Sputtering and Plasma Processes) July, 2013, Kyoto, Japan
  46. “Improvements in and Relating to the Coating of Articles by Means of Thermally Evaporated Materials,” B. Berghaus, UK Patent# 510, 993 (priority date Aug. 26, 1937, filed Aug. 23, 1938, issued Aug.11, 1939)
  47. B. Berghaus, “Improvements in and relating to the coating of articles by means of thermally vaporized material,” UK patent #510,993 (filed; publication 1938); also “An improved process for metallizing metallic articles by means of cathode disintegration in vacuum,” UK patent #520592-A (priority date, Nov. 19, 1937, publication date, Oct. 26, 1938); also http://patent.ipexl.com/assignee/Bernhard_Berghaus_1.html
  48. Wilhelm Burkhardt and Rudolf Reinecke, “Method of Coating Articles by Vaporized Coating Material,” USP# 2,157,478 (priority date, June 17; 1936; filing date, June 15; 1937, publication May 9, 1939) (assigned to Bernard Berghaus) – thermal evaporation source
  49. Wehnelt Eilhart, Hermann Ramert, Wilhelm Burkhart, “Coating of articles by means of cathode disintegration,” USP 2,239,642 (priority date , May 27, 1936; filing date, May 21, 1937; publication date, April 22, 194) (assigned to Bernard Berghaus) – sputtering source
  50. J.A. Thornton and D.W. Hoffman, “Internal stresses in titanium, nickel, molybdenum and tantalum films deposited by cylindrical magnetron sputtering,” J. Vac. Sci. Technol. 14, 165 (1977)
  51. D.M. Mattox, R.E. Cuthrell, C.R. Peeples and P.L. Dreike, “Preparation of thick stress-free Mo films for a resistively heated ion source,” Surf. Coat. Technol., 36, 117-124, 1988