US 3799862 A
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March 26, 1974 R. c. KRUTENAT 3,799,862
APPARATUS FOR SPUTTERING Original Filed May 13. 1970 United States Patent O i 3,799,862 APPARATUS FOR SPUTTERING Richard C. Krutenat, Middletown, Conn., assignor to United Aircraft Corporation, East Hartford, Conn. Original application May 13, 1970, Ser. No. 36,798, now
abandoned. Divided and this application Nov. 19, 1971,
Ser. No. 200,566
Int. Cl. C23c 15/00 U.S. Cl. 204--298 1 Claim ABSTRACT OF THE DISCLOSURE Apparatus generating very high power densities in the vicinity of a sputtering target and liquefaction thereof is utilized to provide very rapid deposition of high temperature alloys with minimal thermal fractionation.
CROSS-REFERENCE TO RELATED APPLICATION The application is a division of copending application Ser. No. 36,798 filed by Richard C. Krutenat on May 13, 1970 for Metal Deposition by Liquid Phase Sputtering, now abandoned.
BACKGROUND OF THE INVENTION The present invention relates in general to apparatus for sputtering of metals and alloys and, more particularly, to apparatus for the formation or deposition of alloys or coatings in a high deposition rate sputtering process from a liquid surface.
The generation of metal deposits and apparatus therefor are recognized in the art as, for example, in the patents to Moseson 3,305,473 and 3,393,142 and that to Rausch 3,428,426. Sputtering is generally considered to involve the solid state sublimation of a target material caused by ion impact on its surface. The sublimed material forms a unique high energy atomic beam `which is ideally suited to the preparation of metal or alloy films and coatings which are characterized by excellent adherence to the coating substrate and an exceptionally iine grain structure.
The major drawback to the conventional sputtering process is its low deposition rate, nominally about 500 angstroms per minute, in a diode configuration. These low rates are not consonant with the generation of coatings in the thicknesses usually required, as for example on gas turbine engine parts for oxidation resistance, or in the production of alloy deposits in bulk product dimensions.
Recently new coatings have been developed for providing dramatic improvements in the operating lifetimes of components exposed to dynamic oxidizing environments at very high temperatures. They are currently applied by thermal evaporation techniques. However, these new coatings alloys have chemistries of relative complexity and thermal vaporization, in all but a few special cases, causes fractionation of an alloy into its constituent parts, the more volatile constituents being vaporized lirst, thus rendering the deposits from the vapor heterogeneous in composition. To obtain a deposit of homogeneous character over an extended time period requires that the vaporing liquid composition be adjusted and continually modified by a feed of appropriate solid to correct for the fractionation tendency.
The sputtering techniques substantially reduce the fractionation tendency of alloys because atoms removed by sputtering escape without relation to their natural vaporization tendency and, therefore, the sputtered vapor composition is primarily dependent upon the proportion of alloying elements at the sputtered surface and the fraction of sputtered atoms in the total vapor at any given time.
3,799,862 Patented Mar. 26, 1974 ICC SUMMARY oP THE INVENTION This invention pertains to improved sputtering apparatus adapted to generate power densities sufficient to liquefy a high temperature alloy target having a melting point in excess of about 2000 F. providing deposition by a combination of both thermal vaporization and sputtering at a very high rate with minimization of alloy fractionation during deposition.
BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic of the preferred apparatus of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred system for effecting deposition by condensation of vapor sputtered from a liquid metal target is diagrammed schematically in the drawing. A vacuum chamber 1 with suitable valves and pumps and insulated feed-throughs is exhausted through port 2 against a controlled argon leak admitted through opening 3 to maintain a dynamic pressure of 1-5 X10-3 torr. A water cooled copper hearth 4 containing the alloy target charge 5 and electrically insulated from the chamber by dielectric sleeve 6, is connected to a controlled power supply 7 and negatively biased. A shield 8 is positioned symmetrically around the hearth on the inside of the cham'ber so that only the liquid target surface is allowed to sputter. An electrically heated thermionic emission filament 9, insulated from the chamber, is placed about one centimeter above the liquid level in the hearth and is powered by an alternating current power supply 10. A bias of 60L volts is imposed on the filament by a regulated direct current power source 11, causing a glow discharge to form between the filament 9 and all objects at ground potential, including the chamber, provided that the iilament is at its operating temperature of about 2500 K. A discharge current of about 2-6 amperes per square inch of hearth surface is required, which area in the described apparatus was 6.25 inches.
An adjustable magnetic field of 1-200 gauss, aligned to provide its iield axis normal to the liquid target surface, is produced by a solenoid 12 powered by a direct current power supply 13. The magnetic field increases the degree of ionization in the discharge, and as the negative bias of the hearth is increased the charge is melted by an intense ion bombardment. As the alloy target passes into the liquid phase a substantial and unexpected current increase from power supply 7 occurs lwhich causes rapid sputtering of the liquid surface and allows the thermionic filament temperature to be reduced or, in some cases, allows the filament current to =be shut off. Thermionic and other unknown electron emission mechanisms provide the electrons necessary to support the glow discharge formerly supplied by the tiilament. Thus, in a regime of the diode type, sputtering occurs when the target is molten and at a thermionic emission temperature.
The process was investigated in the case of elemental tin which has a conveniently low melting point and a very low Vapor pressure such that virtually no thermally induced vapor would be obtained from the liquid target. A comparison of the respective sputtering rates of liquid and solid tin targets at specific bombarding energies revealed that the yield from the liquid target was up to 15 percent higher than that for the solid target.
In another case, an alloy having a nominal composition, by weight, of 25 percent chromium, 6 percent aluminimum, 0.1 percent yttrium, balance iron, was liquiiied and deposited by condensation on a substrate. The deposit analyzed at, by weight, 31 percent chromium, 4 percent aluminum, 0.1 percent yttrium, balance iron. In
this particular case a power density of 130 watts per square centimeter of liquid target surface was used at a hearth potential of 2600 volts. The total power input was 9.9 lkilowatts for 180 minutes during which time 120 grams of a total charge of 250 grams was removed from the target. A coating deposition rate of mils per hour was experienced at a distance of 3.5 inches.
In the case of an iron-30 percent vanadium alloy, which would yield a theoretical deposit with a calculated analysis of 0.02 percent vanadium by thermal evaporation, revealed a deposit of 10.3 percent vanadium, balance iron, or better than 500 times the predicted value for vanadium by thermal evaporation alone. The iron-vanadium target was sputtered at 105 watts per square centimeter of melt surface with a total system power of 6.3 kilowatts for a period of 225 minutes, in which time 93 grams of a 315 gram target were removed. The deposition rate was 2.7 mils per hour at a substrate separation of 3.5 inches.
There is, of course, inherent in this process a combination of two effects, sputtering of the liquid surface and thermal evaporation. It has been established, however, that sputtering can exert a significant beneficial effect on the composition of the -vapor and, in systems where constituent vapor pressures are low, predominates as the method for vapor production. The method has also been demonstrated to provide exceptionally high sputtering rates and the capability for retaining to a large degree the chemical integrity of the target composition in the deposited material.
The very high sputtering rates achievable from liquid phase targets are thought to occur for the following reasons:
(l) Atoms are released more readily from the surface of a target as the temperature is increased.
(2) Electron emission from the ion bombardment surface increases on liquefaction and contributes to a higher ion density in the gas phase over the liquid metal helping to sustain the plasma. This is probably the primary reason why the use of the thermionic filament becomes unnecessary when a high temperature liquid is sputtered.
(3) The higher metal atom content in the vapor over the liquid itself provides a source of ions for self-sputtering by the atoms of the target material itself. Self-sputtering is an eicient process because the equal mass of the bombarding ion and the surface atom of the sputtering material is the most ideal case for momentum transfer. And it has been shown that, after startup, the system pressure can be reduced and the sputtering mechanism can be retained even in an ultrahigh vacuum regime.
Certain other features and advantages of the present method are provided for coating objects, such as turbine hardware, with complex alloys. The intense electron emission from the liquid pool undergoing ion bombardment is a useful source of heat for parts being coated, preheat of such parts often improving coating adherence and integrity. In fact the ballistic electrons resultant from this intense electron emission are utilized not only for a preheating function in the usual sense but as means for controlling the substrate temperature during the entire coating sequence. Above an ion bombardment threshold density of about 60 milliamperes per square centimeter, the process itself develops, in terms of deposition current, a substantial insensitivity to the target potential. In view of this insensitivity and since the heating effect of the ballistic electrons is dependent upon the target potential, it has been found possible by simple variation of the target potential, generally within the range of about 1000-1700 volts, to precisely regulate the substrate temperature.
It should be mentioned that these ballistic electrons also provide another unique but desirable feature in terms of the resultant coating itself although they play no part in the deposition per se. Their energy is such that they provide actual working of the coating, or electron peening, during the entire deposition phase, or during growth rather than subsequent to it. 'I'.he result is a coating of unique morphology as compared to coatings, even of the same basic chemistry, achieved by other coating methods.
The pressure range over which sputtering is conducted requires only mechanical pumps for the attainment of the desired vacuum conditions. The ion bombardment density is uniform over any desired surface area so that complex hearth shapes, conforming generally to the shape of the article to be coated, can be utilized for conservation of the target material. Thus, collection efficiencies of 25-50 percent have been obtained on substrates similar in size to the 2.5 x 2.5 inch hearth utilized in the program. Because the size of the liquid pool can be large in relation to the size of the part being coated, shadowing effects on geometrically complex parts such as turbine blades are minimized.
It is evident that in appropriate circumstances other suitable means of liquefaction of the target, such as induction heating or electron beam melting, can be utilized and that sputtering from the liquid phase can occur as long as the biasing requirements are met. Also, any suitable gas such as argon or combinations of gases providing the required discharge density in the vicinity of the liquid pool can be utilized for sputtering. It is also evident that, if sufficient power density is provided at the liquid surface, ionization of the vapor atoms in the process of sputtering can provide the necessary ion bombardment for sustaining the deposition operation which would preclude the necessity for the use of a discharge gas altogether.
In this apparatus, the bias in the substrate may in actual practice be either positive or negative With'respect to the plasma as long as the substrate is biased positive relative to the target or, in other words, as long as the substrate is more positive than the target. Typically, at the beginning of the coating cycle, the substrate is biased negative with respect to the plasma to provide cleaning of its surface by sputtering. The substrate bias is then made more positive, generally at about to +100 volts for a target potential of -1000 to -1700 volts, so that the effect of deposition overcomes the effect resultant from the sputtering loss, if any.
Emitter current for a 4 inch diameter pool is normally set at about 100 amperes at l2 volts.
Thus, in the deposition process, the substrate sees sputtered atoms and ions, thermally derived atoms and ions and ballistic electrons, all of which contribute to the unique character of the coating. For this to occur, sputtering from a liquid surface is required. This is not to imply that the entire species to be deposited need be the resultant of both sputtered and thermally derived atoms. For example, it may be desirable to` concurrently sputter from both liquid and solid surfaces, particularly where one species is either of very high melting point or is incompatible for other reasons with the melt. Thus, actual species like carbon, alumina or perhaps some of the refractory metals may be evaporated from the solid state. However, the principal species comprising the major portion of the coating buildup will still necessarily be sputtered from a liquid surface.
Thus, the invention in its broader aspects is not limited to the specific steps, parameters, compositions, combinations and improvements described, but departures may be made therefrom within the scope of the appended claims without departure from the principles of the invention and without sacrifice of its chief advantages.
1. Apparatus for liquefying and sputtering alloys having melting points in excess of about 2000 F. Which comprises:
a vacuum chamber having access means for evacuation of the chamber and admission of a controlled gas leak;
a hearth in the chamber for mounting an alloy target to be sputtered with its sputtering surface oriented horizontally, the hearth being configured at its upper end to contain a liquid in electrical isolation from the chamber;
means for negatively biasing an alloy positioned in the hearth at high voltage;
an emission shield surrounding the hearth, substantially shielding the hearth from ion bombardment, the shield having a vertically directed opening in its upper end defining an alloy surface area to be sputtercd;
an electrically heated alternating current thermionic emission filament, insulated from the chamber and positioned immediately adjacent and horizontally across the upper end of the hearth for melting the alloy target, the lament allowing the passage of sputtered ions thereby;
means for applying a direct current voltage to said ilament;
means for producing a magnetic eld aligned vertically and substantially coincident with the upper end of the hearth;
means for mounting a deposition substrate vertically above the upper end of the hearth;
and means for biasing the deposition substrate positive relative to the target.
References Cited UNITED STATES PATENTS 2,148,045 2/1939 Burkhardt et al 204-192 3,404,084 10/1968 Hamilton 204-298 et al. 204/29'8 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner U.S. Cl. X.R.