High Rate Deposition Of Carbides By Activated Reactive Evaporation

Abstract

Process and apparatus for the production of carbide films at high rates by physical vapor deposition. The metal is evaporated in a vacuum chamber by an electron beam, the hydrocarbon gas is introduced into the chamber, and the metal vapor atoms and gas atoms are activated by electrons deflected from the electron beam to the reaction zone by a low voltage electrode at the reaction zone. The reaction takes place primarily in the vapor phase in the reaction zone, rather than on the substrate. A high reaction efficiency is obtained with the activated atoms and a deposition rate in the range of 1 to 12 micrometers per minute and higher is achieved.

Description

The invention herein described was made in the course of or under a grant with the United States Department of the Army.

BACKGROUND OF THE INVENTION

This invention relates to process and apparatus for the formation of carbide films on substrates, such as cutting tools and wear and abrasion resistant surfaces, and for use as superconductors and for the synthesis of carbide powders. The films are produced by physical vapor deposition of the carbide compound by reactive evaporation in a zone away from the substrate. Carbide films have been prepared by chemical vapor deposition in the production of tool bits and the like; however, the rate of deposition has been relatively slow. Oxide and nitride films have been produced by reactive evaporation, normally at a relatively low rate of deposition. It is an object of the present invention to provide process and apparatus for the production of carbide films by reactive evaporation, and in particular, to produce the carbide film at a high rate of deposition, typically in the range of 1 to 12 micrometers per minute.

A number of processes are available for depositing compounds from the vapor phase onto a substrate: chemical vapor deposition; sputtering of the compound from a target of the same composition; direct evaporation of the compound from an evaporation source of the same composition; and reactive evaporation in which metal vapor atoms from an evaporation source containing the appropriate metal react with reactive gas atoms present in the vapor phase to form compounds. A typical example of reactive evaporation formation of compounds is 2Al (vapor) + 3/2 O.sub.2 (gas) .fwdarw.Al.sub.2 O.sub.3 (solid).

The choice of process depends on the ability to deposit compounds of desired composition and structure, and the rate of deposition.

The sputtering process suffers from a low deposition rate, typically in the order of 0.1 micrometers per minute, and sometimes the breakup of the compound into fractions which may not recombine to yield the desired composition in the deposited film. Direct evaporation of compounds is often possible but some compounds of particular interest have very high melting points which require the use of a high intensity heat source to produce appreciable evaporation rate, and also the possible breakup of the compound into fractions during evaporation. In the reactive evaporation process, the evaporant is a metal whose melting point is lower than the metal compound, and higher metal atom densities in the vapor phase are easier to produce and high deposition rates are possible.

In reactive evaporation, metal vapor atoms react with gas atoms to form compounds. In the example set out above, vaporized aluminum metal atoms combine with oxygen to form aluminum oxide. Similarly, vaporized titanium metal atoms should react with a hydrocarbon gas such as acetylene to form titanium carbide, but titanium carbide has not been produced by the prior art reactive evaporation process.

A variety of reactive evaporation processes are reported in the literature with the reaction usually taking place on the substrate and in the presence of a large excess of the gas atoms. Also, the prior art processes have produced only thin films at very low deposition rates, such as films in the order of a few thousand Angstroms thick at a rate in the order of 0.2 micrometers per minute. None of the prior art discloses a process or apparatus which produces a carbide film by a reaction in the vapor phase away from the substrate, with the reaction independent of substrate temperature and with deposition rates in the order of 1 to 12 micrometers per minute, nor does the prior art consider varying the stoichiometry of the resultant compound. It is an object of the present invention to provide such a process and apparatus.

The deposition rate in a reactive evaporation process depends upon the supply of metal vapor atoms, the supply of gas atoms, collision between metal and gas atoms, and the probability of a subsequent reaction to form a compound when such collisions occur. The overall yield of the reaction would be determined by all of the above mentioned factors, any one of which can be the rate limiting step.

There is an inverse relationship between the partial pressure of the metal vapor or gas and its mean free path, i.e., the average distance between collisions. At low partial pressures, the mean free path exceeds the source to substrate distance and collision between the reactants can occur only on the substrate. As the partial pressures of the reactants are increased, the mean free path decreases, eventually becoming smaller than the source to substrate distance, so that collisions between the reactants now occur in the vapor phase. For high deposition rates, the supply of vaporized metal atoms and gas atoms, i.e., their partial pressures, must be large and hence collision between the reactants takes place primarily in the vapor phase.

For high deposition rates, an adequate supply of metal atoms in the vapor phase can be obtained by using a high rate evaporation source and an adequate supply of gas atoms is provided by having a sufficiently high partial pressure of gas atoms in the gas phase, resulting in a high collision rate between the metal and gas atoms in the vapor phase. These conditions have been utilized in reactive evaporation processes in the past in producing oxide and nitride films; however the carbide films and the high deposition rates of the present invention have not been obtained.

One of the rate limiting steps mentioned above affecting the overall yield of the reaction is the probability of a reaction when the two reacting species collide. This reaction probability can be increased by activating one or both of the reactants.

Certain improvements have been made in the reactive evaporation process, which broadly are referred to as activation, providing an activated reactive evaporation process. In U.S. Pat. No. 2,920,002 to Auwater entitled "Process for Manufacture of Thin Films" (reissued as RE26,857), thin film oxides of silicon, zirconium, titanium, aluminum, zinc and tin are produced by reactive evaporation. The reaction is stimulated ionizing the oxygen by passing it through an ionization chamber having two electrodes with an electric power supply providing several thousand volts across the electrodes. The ionized oxygen passes from the ionization chamber into the vacuum chamber. In an alternative configuration, the oxygen is ionized by passage through a magnetic field on its way into the vacuum chamber.

M. T. Wank and D. K. Winslow in Appl. Phys. Letters, 13, 286, (1968) describe the deposition of films of aluminum nitride by evaporating aluminum from an RF heated crucible and reacting the aluminum deposited on the substrate with nitrogen gas which has been disassociated by a 60 Hertz discharge at the end of the gas feed tube. Deposition rates of 0.1 to 0.2 micrometers per minute were obtained.

B. B. Kosicki and D. Khang in Journal Vac. Sci. Tech. 6, 592 and (1969) and U.S. Pat. No. 3,551,312 describe the production of gallium nitride thin films by depositing pure gallium from a resistance heated source onto a substrate in the presence of activated nitrogen gas. The nitrogen gas was made chemically active by partial disassociation in a microwave discharge located in the line between the gas source and vacuum chamber. They also suggest the use of a high energy D.C. discharge between electrodes capable of producing an intense visible glow. Deposition rates of 0.2 to 0.3 micrometers per minute were obtained.

None of the prior art known to applicant discloses the production of carbide films by reactive evaporation in the vapor phase, i.e., not on the substrate, nor the production of such film at high deposition rates at least about 0.8 micrometers per minute and up to 12 micrometers per minute and higher, with thicknesses typically in the range of 25 to 100 micrometers.

SUMMARY OF THE INVENTION

The invention provides for physical vapor deposition of carbide films by reactive evaporation with activated metal vapor and hydrocarbon gas atoms in the vapor phase in a reaction zone, i.e., in the space between the substrate and the metal source. The source metal is heated and vaporized in a vacuum chamber by an electron beam to provide the metal vapor atoms in the reaction zone. The hydrocarbon gas is introduced into the reaction zone and the metal vapor and gas atoms are activated by some of the electrons of the metal heating electron beam which are deflected into the reaction zone by a low voltage field produced by a deflection electrode positioned at the reaction zone. The activation of the reactants increases the probability of a reaction between them, and the metal vaporhydrocarbon gas reaction is achieved with a high efficiency producing the desired metal carbide film on the substrate. With this process, deposition rates substantially higher than those previously obtained are achieved and substrate temperature is not a limiting factor as in chemical vapor deposition. The acts of compound synthesis and film growth are separated. The microstructure of the deposit and hence its physical and mechanical properties, are dependent on the substrate temperature which can be varied at will. Hence the properties of the deposit can be controlled, which is an important advantage. The stoichiometry of the compound (i.e. the ratio of cations to anions) can be varied by changing the ratio of the reactant species supplied. This is an advantage since properties are often controlled by stoichiometry. Deposition rates of 1 to 12 micrometers per minute and higher are obtained.

DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a schematic vertical sectional view of a vacuum chamber and associated equipment suitable for performing the process of the invention and incorporating the presently preferred embodiment of the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus includes a vacuum chamber which may comprise a conventional cover or dome 10 resting on a base 11 with a sealing gasket 12 at the lower rim of the cover 10. A support and feed unit 13 for a source metal rod 14 may be mounted in the base 11. The unit 13 includes a mechanism (not shown) for moving the metal rod 14 upward at a controlled rate. Cooling coils 15 may be mounted in the unit 13 and supplied with cooling water from a cooling water source 16. An electron gun 20 is mounted in unit 13 and provides an electron beam along the path 21 to the upper surface of the metal rod 14, with the electron gun being energized from a power supply 22.

A substrate 24 on which the carbide film is to be deposited, is supported in a frame 25 on a rod 26 projecting upward from the base 11. The substrate 24 may be heated by an electric resistance heater 27 supported on a bracket 28. Energy for the heater 27 is provided from a power supply 29 via a cable 30. The temperature of the substrate 24 is maintained at a desired value by means of a thermocouple 32 in contact with the upper surface of the substrate 24, with the thermocouple connected to a controller 33 by line 34, with the controller output signal regulating the power from the supply 29 to the heater 27.

The desired low pressure is maintained within the vacuum chamber by a vacuum pump 36 connected to the interior of the chamber via a line 37. Gas from a gas supply 39 is introduced into the zone between the metal rod 14 and substrate 24 via a line 40 and nozzle 41. A shutter 43 is mounted on a rod 44 which is manually rotatable to move the shutter into and out of position between the metal rod 14 and substrate 24.

A deflection electrode, typically a tungsten rod 46, is supported from the base 11 in the reaction zone between the metal rod 14 and substrate 24. An electric potential is provided for the rod 46 from a voltage supply 47 via line 48. An electric insulating sleeve 49, typically of glass, is provided for the rod 46 within the vacuum chamber, with the metal surface of the rod exposed only in the zone between the source and substrate. When a potential is connected to this electrode, some of the electrons from the beam 21 are deflected to the reaction zone, as indicated by the dashed line 50. The gun 20 is the preferred source of electrons for the electrode 46, but a separate electron gun could be added if desired.

Various components utilized in the apparatus described above may be conventional. The evaporation chamber may be a 24 inch diameter and 36 inch high water cooled stainless steel bell jar. The vacuum pump may be a 10 inch diameter fractionating diffusion pump, with an anti migration type liquid nitrogen trap. The source metal unit 13 may be a 1 inch diameter rod fed electron beam gun, self-accelerated 270.degree. deflection type, such as Airco Temescal Model RIH-270. The power supply 22 may be an Airco Temescal Model CV30 30 kw unit which may be operated at a constant voltage such as 10 kilovolts, with a variable emission current.

Various sizes and shapes of substrates can be utilized. A typical substrate is a 6 inch by 6 inch metal sheet in the order of 5 mils thick. Various metals have been used including stainless steel, copper, titanium and zirconium. In one embodiment the substrate is based about eight inches above the surface of the metal source 14. The heater 27 may be a 4 kilowatt tungsten resistance heater providing for heating the substrate to 700.degree. C. and higher. The reaction takes place primarily in the vapor phase in the reaction zone away from the substrate, and the reaction is independent of substrate temperature. As discussed below, the density of the deposit is a function of the substrate temperature and when a carbide powder is desired, the substrate may be left at ambient temperature or heated to a relatively low temperature, resulting in a powdery deposit.

The source metal may be a solid rod or billet and for the feed unit mentioned above, the rod is 0.975 inches diameter and 6 inches in length. Titanium, zirconium, hafnium, vanadium, niobium and tantalum have been utilized in making carbide films. Alloys and mixtures of metals may be used to produce mixed carbides and alloy carbides.

A hydrocarbon gas which readily disassociates is desired for the process. The hydrocarbon gas for the reaction is introduced into the vacuum chamber through a series of needle valves and the preferred range for gas pressure is 2 .times. 10.sup.-.sup.4 torr to 8 .times. 10.sup.-.sup.4 torr. Acetylene is the preferred gas for the synthesis of carbides and ethylene may also be utilized.

The supply 47 provides a low voltage to the deflection electrode 46 and preferably is a D.C. supply with a variable output voltage. The usual potential for the deflection electrode is in the range of about 50 to about 200 volts, and preferably in the range of 60 to 100 volts. Higher voltages may be used if desired. A Lambda Model 71 continuously variable voltage D.C. power supply has been utilized. A.C. potential also has been used on the deflection electrode, with voltages in the same range.

By way of example, titanium carbide was formed by activated reactive evaporation deposition utilizing titanium metal and acetylene gas, with the following reaction:

2Ti (vapor) .times. C.sub.2 H.sub.2 (gas) .fwdarw. 2TiC (solid) .times. H.sub.2 (gas).

The vacuum chamber was initially pumped down to 10.sup.-.sup.6 torr pressure and was then purged with the gas to 10.sup.-.sup.4 torr for a few minutes. The chamber was again pumped down to 10.sup.-.sup.6 torr. This procedure was used to minimize the presence of extraneous gases.

When the pressure in the chamber was again down to 10.sup.-.sup.6 torr, the electron gun was turned on and a molten pool of metal was formed by the electron beam at the upper end of the rod 14. The shutter 43 was in position blocking the substrate 24. The reaction gas was then introduced into the vacuum chamber at a controlled rate to obtain the desired chamber pressure. The power supply for the deflection electrode 46 was turned on and the potential increased until the reaction began, as indicated by a substantial increase in current in the electrode 46. When steady state conditions were obtained the shutter 43 was moved to one side and the carbide film was deposited on the substrate. The process was continued until the desired thickness of film was obtained, after which the shutter was moved to the blocking position and the various supplies were turned off.

The gas partial pressure within the chamber, the deflection electrode potential, and the electron beam current required to produce the carbide film are somewhat interrelated and may be varied over a substantial range. With higher electron beam currents, the deflection electrode potential required for initiating the reaction decreases. Similarly, with higher gas partial pressures, the required electrode potential decreases. For the example given, successful formation of carbide films was achieved with electron gun power in the range of one kilowatt to three kilowatts, with gas partial pressure in the range of 1 .times. 10.sup.-.sup.4 to 3 .times. 10.sup.-.sup.4 torr, and electrode potential in the range of 60 to 90 volts. The following are additional examples of materials used in the production of carbide films by the process of the invention:

2Zr + C.sub.2 H.sub.2 .fwdarw. 2ZrC + H.sub.2

2Hf + C.sub.2 H.sub.2 .fwdarw. 2HfC + H.sub.2

2V + C.sub.2 H.sub.2 .fwdarw. 2VC + H.sub.2

2Nb + C.sub.2 H.sub.2 .fwdarw. 2NbC + H.sub.2

2Ta + C.sub.2 H.sub.2 .fwdarw. 2TaC + H.sub.2 2Ti + C.sub.2 H.sub.4 .fwdarw. 2TiC + H.sub.4

2(Hf.sub.x - Zr.sub.y) + C.sub.2 H.sub.2 .fwdarw. 2(Hf.sub.x - Zr.sub.y)C + H.sub.2

where x = 97, y = 3.

Acetylene (C.sub.2 H.sub.2) is a preferred hydrocarbon gas for the process because it is a highly unsaturated hydrocarbon. Methane (CH.sub.4) is not a suitable gas due to the saturated carbon-hydrogen bond. Ethylene (C.sub.2 H.sub.4) is an unsaturated hydrocarbon gas which may be used in some instances.

The acts of compound formation and deposit growth are separate steps in this process. The character of the deposit changes with substrate temperature. In the range from 0.degree.C to about 0.3 Tm (Tm being the melting point of the compound in degrees Kelvin) the deposit is of less than full density, the density increasing with deposition temperature. The morphology exhibited by the deposit is of tapered crystallites with porosity in between the crystallites. Such deposits show microhardness values of about 3,000 kg/mm.sup.2 at a 50 gram load for TiC and unit stoichiometry and these microhardness values are comparable to those reported for TiC synthesized by other methods. At substrate temperatures greater than 0.3 Tm approximately, the deposit becomes fully dense and its morphology now shows columnar grains across the thickness of the deposit. Such a structure contains fewer imperfections (i.e. porosity, cracks etc.) and hence exhibits much higher microhardness values, 4,000 to 5,000 kg/mm.sup.2 at 50 gram load. This is to be expected since the properties of brittle materials such as ceramics are primarily governed by the imperfection content.

It has been found that the stoichiometry of the carbide deposit (i.e. the carbon to metal ratio) can be controlled by changing the relative amounts of the reactants. For example, the carbon to metal ratio is increased by increasing the partial pressure of the carbon containing gas at a constant evaporation rate of titanium. As an example, with an evaporation rate of 0.66 grams per minute of titanium and an acetylene pressure of 5.10.sup.-.sup.4 torr, a titanium carbide deposit of unit stoichiometry (TiC.sub.1.0) can be produced at a deposition rate of 4 micrometers per minute at a source-to-substrate distance of 8 inches. Typical film thicknesses are in the range of 25 to 100 micrometers. Deposition rates of 1 to 12 micrometers per minute have been achieved. Higher and lower rates may be had by varying the parameters of the system. The metal evaporation rate may be controlled by varying the output of the electron gun 20 and the gas pressure may be controlled by adjusting a valve 41 in the gas line 40.

Claims

I claim:

1. In a process for deposition of a carbide film by reactive evaporation, the steps of:

supporting a substrate in a vacuum;

evaporating a metal by an electron beam directed to the metal, producing a metal vapor in a zone between the metal and the substrate;

introducing a hydrocarbon gas into said zone; and

generating a low voltage electric field in said zone and deflecting electrons to said zone ionizing the metal vapor and gas atoms in said zone,

so that the metal vapor atoms and gas atoms react in said zone to form a metal carbide which then deposits on said substrate.

2. The process as defined in claim 1 including generating the low voltage electric field with a potential of not more than about 200 volts.

3. The process as defined in claim 1 including generating the low voltage electric field with a D.C. potential.

4. The process as defined in claim 1 including generating the low voltage electric field with an A.C. potential.

5. The process as defined in claim 1 wherein said hydrocarbon gas is selected from the grop consisting of acetylene and ethylene.

6. The process as defined in claim 1 wherein said metal is selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium and tantalum and combinations thereof.

7. The process as defined in claim 1 wherein said hydrocarbon gas is acetylene and said metal is titanium.

8. The process as defined in claim 1 wherein the partial pressure of the gas atoms is at least about 10.sup.-.sup.4 torr.

9. The process as defined in claim 1 wherein the rate of deposition of the carbide film on the substrate is at least about 1 micrometer per minute.

10. The process as defined in claim 1 including the step of heating the substrate to a predetermined temperature to produce a predetermined density of the deposit on the substrate.

11. The process as defined in claim 1 including the step of controlling the relative amounts of metal vapor and gas in the zone to produce a metal-carbon compound of predetermined stoichiometry.