US 3294669 A
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Dec. 27, 1966 c. THEUERER 7 3,294,669
APPARATUS FOR SPUTTERING IN A HIGHLY PURIFIED GAS ATMOSPHERE Filed July 22, 1963 2 sheets sheet 1 f 1m DIRECT I.5 HH|||HI" I CURRENT l9 3 hhlln.
HI- l D/RE C T CURRENT lNl EN TOR H. C. THEUERER A T TOR/VEV Dec. 27, 1966 H. c. THEUERER 3,294,669
APPARATUS FOR SPUTTERING IN A HIGHLY PURIFIED GAS ATMOSPHERE Filed July 22, 1963 2 Sheets-Sheet 2 5 DIRECT I5 40 CURRENT DIRECT CURRENT practice of the present invention.
United States Patent APPARATUS FOR SPUTTERING IN A HIGHLY PURIFIED GAS ATMOSPHERE Henry C. Theuerer, New York, N.Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed July 22, 1963, Ser. No. 296,550
2 Claims. (Cl. 204298) The present invention relates to a technique for cathodic sputtering of thin films and to the apparatus employed therefor.
In recent years, considerable interest has been generated in thin films and the preparation of such films by cathodic sputtering techniques. Unfortunately, considerable difliculty has been encountered in the preparation of certain of these films since the materials being sputtered,
typically composed of one or more of the chemically active elements, evidence a strong atfinity for active gases such as oxygen and nitrogen. Accordingly, satisfactory deposition can only be attained when the reactive gases in the sputtering ambient are maintained at partial pressures less than 1() torr, so necessitating the use of elaborate high vacuum techniques.
In accordance with the present invention, a cathodic sputtering technique suitable for the deposition of superconductive and other reactive materials is described wherein the partial pressure of active gases is maintained at 10 torr and higher. The inventive technique involves cathodic sputtering in an apparatus having a configuration such that active gases entering the sputtering chamher as contaminants in the inert gas react with sputtering material at their point of entry and are deposited upon the walls of an anode member substantially surrounding .the'cathode, so resulting in the purification of the inert gas prior to its reaching the area of the system in which coating of the substrate occurs.
FIG. 1 is a front elevational view of an apparatus suitable for the practice of the present invention;
FIG. 2 is a front elevational view of an alternative apparatus suitable for the practice of the present invention; FIG. 3 is a front elevational view of an apparatus suitable for the practice of the present invention wherein two cathodes in combination with an enveloping anode are employed; and
FIG. 4 is a front elevational view of an apparatus suitable for the practice of the present invention wherein two cathodes are connected to a D.-C. power supply through a pair of switches and resistors.
With reference now more particularly to FIG. 1, there is shown anode member 11 which defines the limits of sputtering chamber 12. Anode member 11 is provided with an outlet 13 for connection to a vacuum pump (not shown) and an inlet 14 for the introduction of an inert gas into chamber 12. Shown disposed within sputtering chamber 12 is a cathode member 15 which is supported by anode 11 and electrically insulated therefrom by coverglass seal 16. Cathode 15 is comprised of the material which is required to be deposited upon a plane workpiece or substrate 17. Cathode 15 is connected to the negative pole 18 of a DC. high tension supply, the positive pole of which is connected, as at 19, to anode 11 and to ground.
FIG. 2 shows an alternative apparatus for use in the Shown in the figure is vacuum chamber 21 provided with an outlet 22 for connection to a vacuum pump (not shown). Shown disposed within chamber 21 is an anode member 23 which defines a space gradient in an area substantially surrounding cathode 24 which is supported thereby and electri- 'cally insulated therefrom by insulator 25. Cathode 24 is comprised of the material which is required to be deposited upon a plane workpiece or substrate 26 supported on work table 27, workpiece 26 being situated at a point opposite cathode 24 within the noted space gradient. Inlet 28 is provided for introducing an inert gas into the volume defined by the intercept of the plane of said cathode and said space gradient. Cathode 24 is connected to the negative pole 29 of D.-C. high tension supply, the positive pole of which is connected, as at 30, to anode 23 and to ground.
FIG. 3 shows still another apparatus suitable for the practice of the present invention. Shown in this figure is vacuum chamber 31 provided with an outlet 32 for connection to a vacuum pump (not shown) and an inlet 33 for the introduction of an inert gas. Shown disposed within chamber 31 is an anode member 34 having aflixed thereto tightly fitting lid 35, anode 34 completely enveloping cathodes 36 and 37, respectively, which are electrically insulated therefrom by means of insulators 38 and 39. In practice, insulators 38 and 39 are so fitted as to provide sufficient clearance to permit inert gases introduced into vacuum chamber 31 to flow into the sputtering chamber, i.e., the interior of anode 34. Cathodes 36 and 37 are comprised of materials which are required to be deposited upon a plane workpiece or substrate40 supported on work table 41 positioned between cathodes 36 and 37. Work table 41 is an electrical resistance element which may be heated by means'of electrodes 42 and 43. A movable shutter 44 for controlling coating is shown disposed between cathode 36 and substrate 40. Cathodes 36 and 37 are connected to the negative pole 45 of a D.-C. high tension supply, the positive pole of which is connected, as at 46 to anode 34 and to ground.
Still another alternative apparatus is shown in FIG. 4 wherein the apparatus of FIG. 3 is modified by removing the substrate support 41 and electrodes 42 and 43 which are connected thereto, in addition to removing shutter 44 and connecting cathodes 36 and 37 to a D.-C. high tension supply through shorting switches 51 and 52, respectively, and resistors 53 and 54, respectively.
The present invention may conveniently be described in detail by reference to a first illustrative example in Which tantalum is employed as the cathode in an apparatus similar to that shown in FIG. 1.
The conditions employed in cathodic sputtering are known (see Vacuum Deposition of Thin Films, L. Holland, J. Wiley & Sons, Inc., New York, 1956). In this process, the vacuum chamber is first evacuated. A partial pressure of a nonoxidizing or nonreactive gas as, for example, hydrogen or any of the members of the rare gas family such as helium, argon or neon, is then introduced to the chamber. The system may be operated statically by maintaining a fixed pressure of gas in the system.
It may also be operated dynamically such that a fixed gas pressure is maintained with a constant flow of gas through the system. The extent of the vacuum used in either case is dependent on consideration of several facmaximum pressure may not exceed the value at which a stable glow discharge may be obtained, but in practice is usually dictated by power supply limitations since increasing the pressure also increases the current flow between anode 11 and cathode 15. A practical upper limit in this latter respect is 200 microns of Hg for a sputtering voltage of the order of 3000 volts. The ultimate maximum pressure is that at which the sputtering can be reasonably controlled within the prescribed tolerances. It follows, from the discussion above, that the minimum pressure is determined by the lowest deposition rate which can be economically tolerated.
After the requisite pressure is attained, cathode 15, which may be composed of tantalum or alternatively may be covered with tantalum, for example, in the form of a foil, is made electrically negative with respect to anode 11.
The minimum voltage necessary to produce sputtering is dependent upon the particular material employed as the cathode. For example, a D.-C. potential of about 1500 volts is required to produce a sputtered layer of tantalum suitable for the purposes of this invention. The minimum voltage for other metals, as well as compounds and alloys, is known in the art. Increasing the potential difference between anode 11 and cathode 15 has the same effect as increasing the pressure, that of increasing both the current flow and the rate of deposition. Accordingly, the maximum voltage is dictated by consideration of the same factors controlling the maximum pressure.
The spacing between anode and cathode is not critical. However, the minimum separation is that required to produce a glow discharge which must be present for sputtering to occur. Many dark striations occur in the glow discharge produced during sputtering. Some of these striations are well known and have been given names as, for example, Crookes Dark Space. For the best efficiency during the sputtering step substrate 16 should be positioned immediately without Crookes Dark Space. Location of substrate 16 closer to cathode 14 limits the rate of deposition. Locating substrate 16 further away from cathode 14 results in the impingement on the substrate by a smaller fraction of the total metal sputtered, thereby increasing the time necessary to produce a deposit of a given thickness. The anode should not penetrate the cathode dark space, and for this purpose the minimum distance from any point on the cathode surface and the anode should be one-half inch.
It must be noted that the location of Crookes Dark Space changes with variations in the pressure, it moving closer to the cathode with increasing pressure. As the substrate is moved closer to the cathode it tends to act as an obstacle in the path of gas ions, which are bombarding the cathode.
Accordingly, the pressure should be maintained sufficiently low so that Crookes Dar-k Space is located beyond the point at which a substrate would cause shielding of the cathode.
The balancing of these various factors of voltage, pressure, and relative positions of the cathode, anode and substrate to obtain a high quality deposit is well known in the sputtering art. In addition to the effect of the parameters noted above, it is to be appreciated that the actual sputtering rate is also dependent on the particular material being sputtered. For the purposes of this invention sputtering is allowed to proceed in all directions from the cathode. The sputtering rate must be sufiiciently high to ensure capture of reactive gases before they can reach the coating area. Sputtering rates of the order of 50 A./mm. are satisfactory for this purpose.
With reference now more particularly to the example under discussion, by employing a proper voltage, pressure and spacing of the various elements within the vacuum chamber, a layer of tantalum is deposited upon substrate 17. The sputtering is conducted for a period of time calculated to produce the desired thickness.
During the course of the sputtering reaction, argon entering vacuum chamber 12 via conduit 14 is ionized in the region surrounding anode 11, the resultant charged particles then bombarding cathode 15, so causing cathode particles to dislodge from the crystal lattice in all directions toward anode 11. Thus, a cloud of sputtering material is produced and as the argon, containing traces of reactive bases, enters vacuum chamber 11 via conduit 14, the sputtering material reacts with or getters the reactive gases, so forming compounds which are deposited upon the interior surfaces of anode 11 and permitting the nowpurified inert argon to reach that part of the system in which coating of the substrate occurs.
To obtain the best quality of deposit, it is preferred that the substrate he possessed of a smooth surface which is completely free from sharp changes in contour. The substrate should also be able to withstand temperatures within the range of 300400 C. since it may be heated to temperatures in this range during the deposition. In the cathodic sputtering process the substrate surface is heated by bombardment with gas ions in addition to acting as a sink for the heat released by the deposition of the sputtered layer. However, in certain cases it has been found advantageous to heat the substrate surface to temperatures within the range of 6501000 C. during the sputtering process. This technique has proven satisfactory for obtaining thin films of elemental, alloy and compound superconducting materials which manifest the properties of the bulk material.
All types of refractory materials such as glass, ceramics and high melting metals are suitable for use as substrate materials. In those cases wherein additional heating is required during sputtering, magnesium oxide has proven particularly satisfactory.
Several examples of the present invention are described in detail below. These examples are included merely to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention. It will be appreciated by those skilled in the art that the main impact of the present invention lies in the use of partial pressures of 10* torr and higher in order to obtain satisfactory deposition, conventional sputtering parameters otherwise being employed.
Example I A cathodic sputtering apparatus similar to that shown in FIG. 3 was employed wherein two V Si rods one inch in diameter and inch in length, prepared by alloying the constituents were used as cathodes. The anode consisted of a nickel can and lid made from 0.012 inch stock and was 4 inches high and had an inner diameter of 2% inches. The substrate was a sheet of crystalline mag nesium oxide obtained from the Norton Company. In the apparatus actually employed, the anode was grounded, the potential difference being obtained by making the cathodes negative with respect to ground.
The system was initially evacuated by pumping to a pressure of approximately 5 X 10 mm. of Hg. In order to condition the system, titanium purified argon maintained at a pressure of 150 microns was introduced into the system and sputtering initiated by applying 1500 volts at 4.3 milliamperes to each electrode, a temperature of 700 C. being maintained. Sputtering was conducted for minutes to bring the target to steady state. This is required since sputtering rates for silicon and vanadium are different. Following the substrate was introduced and the system evacuated to 5X10 mm. of Hg. Next, the substrate support was outgassed at 900 C. for 30 minutes after which argon was admitted at a pressure of microns. Finally, the substrate support was heated to 700 C. and sputtering begun by applying 1800 volts at 5.0 milliamperes to each electrode.
Sputtering was conducted for 50 minutes yielding a layer of V Si approximately 3470 A. thick. Coating was terminated by replacing the tantalum shield, turning OFF the power used to heat the substrate surface, and after 50 minutes turning OFF the sputtering voltage. The resultant film was found to be superconducting in liquid helium at 13.9 K.
Example II The procedure of Example I was repeated with the exception that the substrate was maintained at 900 C. during sputtering at a current of 4.0 milliamperes in an argon atmosphere maintained at 125 microns of Hg. Sputtering was continued for 50 minutes yielding a coating of 2300 A. The resultant film was superconducting in liquid helium at 13.6 K.
Example III The procedure of Example I was repeated employing V Ge cathodes, a substrate temperature of 1000 C., a supporting voltage of 1900 volts D.C. and an argon pressure of 100 microns. Sputtering was continued for one hour yielding a coating of 5690 A. The resultant film was superconducting in liquid helium at 68 K.
Example IV The procedure of Example I was repeated employing V Ga cathodes, a sputtering voltage of 2250 volts D.-C. and an argon pressure of 100 microns, the substrate being heated to 1000 C. during sputtering. Sputtering was continued for 30 minutes yielding a film of 2560 A. The resultant film was superconducting in liquid helium at 12.35 K.
Example V The procedure of Example I was repeated employing Ta cathodes, a sputtering voltage of 1500 volts DC. with the substrate heated to 300 C. and an argon pressure of 85 microns. Sputtering was conducted for minutes yielding a coating of 2030 A. The resultant film was superconducting in liquid helium at 3.9 K.
Example VI The procedure of Example I was employed with the exception that an apparatus similar to that shown in FIG. 3 was employed with niobium cathodes.
Stabilization of the system was attained by first biasing the upper cathode with 94,000 ohms with respect to the lower cathode and applying a potential of 1500 volts D.-C. for minutes under an argon pressure of 100 microns, the substrate being positioned atop the lower cathode. Following, the electrical bias was reversed and substrate coating initiated at a current of 5.5 mill-iamperes in an atmosphere of argon maintained at 100 microns of Hg. Sputtering was continued for 50 minutes yielding a coating of 7660 A. in thickness. The resultant film was superconducting in liquid helium at 9.6" K.
While the invention has been described in detail in the foregoing specification and the drawing similarly illustrates the same, the aforesaid is by way of illustration only and is not restrictive in character. It will be readily appreciated by those skilled in the art that although the invention is described in terms of superconducting materials, the described technique may suitably be employed in any sputtering technique wherein reactive gases are deleterious.
What is claimed is:
1. A cathodic sputtering apparatus including a first vacuum chamber, means for evacuating said first vacuum chamber, means for introducing an inert gas into said first vacuum chamber, an anode member having a tightly fitted conductive lid defining a second vacuum chamber in which are disposed in succession (a) a first non-shielded cathode member, (b) a substrate support, and (c) a nonshielded second cathode member comprising the same material as said first cathode, at least one .of said cathode members being disposed significantly nearer to said means for introducing an inert gas than said substrate support, together with means for applying a potential gradient between said anode and said cathode members in all areas within said second. vacuum chamber including the area surrounding said second cathode member at the furthest points from said substrate support, to effect a flow of electrons from said cathode to said anode member.
2. Apparatus in accordance with claim 1 wherein means is provided for biasing one of said cathode members more negatively than the other.
References Cited by the Examiner UNITED STATES PATENTS 2,200,909 5/ 1940 Berghaus et a1 204-298 2,305,758 12/1942 Berghaus et al. 204-192 2,998,376 8/1961 Smith 204-298 3,073,770 1/ 1963 Sinclair et al. 204-192 3,210,263 10/1965 Jones 204-298 OTHER REFERENCES Ser. No. 283,312, Berghaus et al. (A.P.C.), published May 1943.
JOHN H. MACK, Primary Examiner.
R. K. MIHALEK, Assistant Examiner.