U.S. patent number 3,791,852 [Application Number 05/263,708] was granted by the patent office on 1974-02-12 for high rate deposition of carbides by activated reactive evaporation.
This patent grant is currently assigned to The Regents of the University of California. Invention is credited to Rointan F. Bunshah.
United States Patent |
3,791,852 |
Bunshah |
February 12, 1974 |
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.
Inventors: |
Bunshah; Rointan F. (Los
Angeles, CA) |
Assignee: |
The Regents of the University of
California (Berkeley, CA)
|
Family
ID: |
23002928 |
Appl.
No.: |
05/263,708 |
Filed: |
June 16, 1972 |
Current U.S.
Class: |
427/567; 204/164;
427/570; 427/530; 427/577; 427/249.17 |
Current CPC
Class: |
C23C
14/0635 (20130101) |
Current International
Class: |
C23C
14/06 (20060101); C23c 011/08 () |
Field of
Search: |
;117/16C,16R,16A,17.2R,93.1,93.2,93.3,93.31 ;118/49.1,49.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kendall; Ralph S.
Assistant Examiner: Massie; J.
Attorney, Agent or Firm: Harris, Kern, Wallen &
Tinsley
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.
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.
* * * * *