Controlled high rate deposition of metal oxide films

A process for depositing metal oxides by activated reactive evaporation (ARE) wherein deposition rate and film quality is controlled by reference to the relative amounts of metal and metal oxide present on the surface of the target material. The ratio of metal surface area to metal oxide surface area required to obtain high deposition rates is achieved by maintaining a relatively high concentration of oxygen in the reaction zone. This relative ratio of metal surface area to metal oxide surface area on target material provides a continuous indirect measure of film deposition rate and quality during the ARE process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to processes for depositing thin 
films of metal oxides onto a variety of substrates. More particularly, the 
present invention involves the fabrication of such metal oxide films with 
high deposition rates using the activated reactive evaporation (ARE) 
technique. 
2. Description of Related Art 
A wide variety of simple evaporation processes have been used to deposit 
thin films of materials on various substrates. In these processes, vapors 
are produces from a source material by the application of heat or other 
energy source. The vapors produced from the source are then condensed onto 
a substrate. One problem experienced with such simple evaporation involves 
partial dissociation of the compounds during evaporation which results in 
less than full stoichiometry. This causes the formation of films which are 
dificient in one or more elements and therefore non-stoichiometric. 
Another problem involves those compounds which have a high melting point. 
A very high power density is required in order to obtain appreciable 
evaporation rates of these compounds. The application of such high power 
densities to billets fabricated by powder metallurgy methods may result in 
disintegration of the billet. 
In response to the above problems, the process of reactive evaporation (RE) 
was developed. In reactive evaporation, metal atoms are evaporated from a 
thermally heated source in a vacuum chamber in the presence of a 
partial-pressure reactive gas, such as oxygen. The reactive gas and metal 
atoms react to form compounds which are deposited on the substrate. A 
modified form of reactive evaporation was developed in 1972 by R. F. 
Bunshah and A. C. Raghurham (J. Vac. Sci. Technol. 9, 1385 (1972)). The 
modified process involves introduction of a plasma between the evaporant 
source and the substrate. This modified evaporation process is commonly 
referred to as activated reactive evaporation (ARE). An advantage of ARE 
is that it enhances the probability of reactions taking place by 
activating or partially ionizing the evaporant atoms. In view of this 
advantage of the ARE technique, it would be desirable to extend the 
process to deposit high quality Al.sub.2 O.sub.3 films at very high 
deposition rates. 
In any process for forming films on a substrate, it is desirable to 
maximize deposition rates while not adversely affecting film quality or 
compound stoichiometry. This is particularly true for aluminum oxide films 
which are used in many high technology applications. For example, aluminum 
oxide films have played an important role in various fields as an 
insulating layer in metal-insulator-semiconductor field effect transistor 
(MISFET), gate insulators in solid state hydrogen sensors, X-ray and 
accelerator neutralizers in nuclear reactors, tunnel barriers in Josephson 
tunnel junctions, antireflection coatings in solar cells, optical wave 
guides and protective layers for metal reflectors. In view of these 
applications, a variety of techniques have been used to synthesize 
Al.sub.2 O.sub.3 films. For example, techniques such as chemical vapor 
deposition (CVD), metal organic chemical vapor deposition (MOCVD), glow 
discharge, electron beam evaporation of alumina, rf sputter deposition, 
reactive sputter deposition, ion beam sputter deposition and recently 
molecular layer epitaxy (MLE) have been used to synthesize aluminum oxide 
films. 
Aluminum oxide films have been prepared by chemical vapor deposition 
utilizing aluminum chloride as the major reactant (S. K. Tung and R. E. 
Caffrey, Tran. Metall. Soc. AIME, 233 (1965) 572; A. S. Wong, G. M. Michal 
and I. E. Locci, J. Mater. Res., 3(5) (1988) 572). In this technique, a 
uniform deposit of aluminum oxide was produced by pyrohydrolizing aluminum 
chloride with hydrogen and carbon dioxide gas at approximately 850.degree. 
C. Aluminum oxide films have also been fabricated at lower substrate 
temperatures utilizing metal organic chemical vapor deposition techniques 
(J. Fournier, W. DeSisto, R. Brusasco, M. Sosnowski, R. Kershaw, J. 
Baglio, K. Dwight and A. Wold, Mat. Res. Bull., 23 (1988) 31; C. 
Dhanavantri and R. N. Karekar, Thin Solid Films, 169 (1989) 271). In these 
MOCVD processes, volatilized aluminum isoperoxide was thermally decomposed 
at substrate temperatures of between 350.degree. C. and 500.degree. C. to 
deposit the aluminum oxide films. In another technique, a plasma was 
employed to enhance the CVD process (Y. Catherine and A. Talebian, J. 
Electrochem. Soc., 17(2) (1988) 127). The plasma was produced by means of 
either a 450 kHz or 13.56 MHz discharge. A mixture composed of 
Al(CH.sub.3).sub.3 (TMA) with helium or argon as the carrier gas and 
CO.sub.2 was used as the reactant. The aluminum oxide films were prepared 
at substrate temperatures between 25.degree. C. and 350.degree. C. 
High quality aluminum oxide films have also been prepared by evaporating 
aluminum oxide pellets in an environment of oxygen gas at substrate 
temperatures ranging from 25.degree. C. to 250.degree. C. (J. Saraie, S. 
Goto, Y. Kitao and Y. Yodoggawa, J. Electrochem. Soc. 134 (1987) 2805). 
Radio frequency sputter deposition and reactive sputter deposition have 
also been employed to produce aluminum films using aluminum oxide and 
aluminum metal targets, respectively. The ion beam sputter deposition 
technique utilized to deposit aluminum oxide films has used ionized argon 
gas which was accelerated and directed to the aluminum oxide target by an 
accelerator grid. The target material was sputtered off with oxygen gas 
being introduced to compensate for the partial dissociation of aluminum 
oxide during the sputtering process (C. Nishimura, K. Yanagisawa, A. Tago 
and T. Toshima, J. Vac. Sci. Technol., A5(3) (1987) 343; S. M. Arnold and 
B. E. Cole, Thin Solid Films, 165 (1988) 1). 
With respect to molecular layer epitaxy, single crystals of alpha-aluminum 
oxide films have been produced (G. Oya, M. Yoshida and Y. Sawada, Appl. 
Phys. Lett., 51(15) (1987) 1143). In the MLE technique, an anhydrous 
aluminum chloride vapor and a helium/oxygen gas mixture were alternatively 
supplied to the substrate through separate pipes by opening and closing 
valves attached to the pipes. Aluminum oxide has been deposited by 
activated reactive evaporation wherein a one inch diameter billet of 
aluminum metal was evaporated from a rod-fed electron beam source in the 
presence of partial pressures of oxygen varying from 2.times.10.sup.-5 
Torr to 2.times.10.sup.-4 Torr (R. F. Bunshah and R. J. Schramm, Thin 
Solids Film, 40 (1977) pp. 211-216). 
Although high quality aluminum oxide films have been synthesized utilizing 
the above described techniques, the deposition rates obtained have been 
relatively low. For example, deposition rates of 4.6 to 33.9 nm per minute 
for MOCVD and 18 nm per minute for electron beam evaporation of aluminum 
oxide have been reported. Deposition rates for techniques such as ion beam 
sputter deposition and glow discharge are typically below 15 nm per 
minute. Aluminum oxide deposition rates as high as 90 nm per minute have 
been obtained using modified ion source geometry in an ion beam sputter 
deposition technique. Deposition rates for ARE have typically been well 
below 200 nm per minute. 
An obstacle to increasing deposition rates of metal oxides, such as 
aluminum oxide, by the ARE technique is oxide poisoning of the metal 
billet or target material (i.e., formation of an oxide layer on top of the 
molten aluminum). Metal oxides, such as aluminum oxide, have a melting 
point which is much higher than the melting point of the metal. 
Accordingly, if a layer of aluminum oxide is allowed to form, the 
evaporation of metal from the source material is adversely affected. In 
the ARE technique, the oxygen which is introduced into the system to react 
with vaporized metal also tends to react with metal present on the billet 
surface. This leads to reduction of the deposition rate because of the 
lower evaporation rate of aluminum oxide as compared to pure aluminum. An 
increase in deposition rate can be achieved by increasing the electron 
beam current. However, this leads to deposition of substoichiometric metal 
rich films. 
There is a continuing need to develop and define process conditions for the 
deposition of metal oxides by ARE wherein the deposition rate is increased 
without sacrificing film quality. It would be desirable to provide a 
process for depositing metal oxide films by ARE wherein fully 
stoichiometric, transparent films of metal oxides are deposited at rates 
on the order of 10 to 20 .mu.m/hour or higher. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process is provided to deposit 
stoichiometric films of metal oxides at relatively high rates. The present 
invention is based on the discovery that deposition rates of aluminum 
oxides on the order of 10 to 20 .mu.m/hour and higher can be obtained 
utilizing activated reactive evaporation when the poisoning of the target 
material is controlled within certain parameters. 
The present invention involves an activated reactive evaporation process 
for depositing metal oxide films onto a wide variety of substrates. The 
process involves conventional ARE wherein a substrate is supported in a 
vacuum system and a source of metal having an exposed surface displaced 
away from the substrate is evaporated by heating. The evaporated metal 
forms a metal vapor in a zone located between the substrate and the metal 
target. An oxygen-containing gas is introduced into the zone where it 
mixes with the metal vapor to form a mixture of metal vapor and oxygen. 
The introduced oxygen also reacts with metal present in the target 
material to form one or more oxides of the metal on the target surface. 
The mixture of metal vapor and oxygen is energized with sufficient energy 
to form a plasma from which metal oxide is deposited onto the substrate. 
As a feature of the present invention, the amount of oxygen introduced into 
the zone is controlled to achieve a ratio of metal surface area on the 
target to metal oxide surface area which has been previously determined to 
provide rapid deposition of metal oxide on the substrate. It was 
discovered that the amount of oxygen required to achieve the desired ratio 
of metal surface area to metal oxide surface area on the target was 
surprisingly high in view of known problems with respect to oxide 
poisoning of targets. It was found that high deposition rates could be 
achieved, in spite of high oxygen levels, provided that the ratio of metal 
surface area to metal oxide surface area on the target was controlled to 
achieve the preselected ratio of metal surface area to metal oxide surface 
area. 
As another feature of the present invention, it was discovered that the 
ratio of metal surface area to metal oxide surface area on the metal 
target surface should correspond to the stoichiometric ratio of elements 
being deposited. For example, when depositing aluminum oxide films, the 
ratio of metal surface area to metal oxide surface area in accordance with 
the present invention should be about 2:3. In addition, the optimum 
relative ratio of metal surface area to metal oxide surface area can be 
determined experimentally by establishing the optimum deposition rate by 
varying the ratio of metal surface area to metal oxide surface area on the 
target and observing changes in deposition rate and film quality. 
The deposition process in accordance with the present invention allows one 
to obtain deposition rates on the order of 10 to 20 .mu.m/hour and higher 
for many different metal oxides utilizing the well known ARE technique. 
Further, these high deposition rates can be controlled and reproducibly 
obtained based upon reference to the ratio of metal surface area to metal 
oxide surface area as discussed in detail below.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention may be used in a variety of activated reactive 
evaporation processes (ARE) to deposit metal oxides on substrates. The ARE 
process is disclosed in U.S. Pat. No. 3,791,852, the contents of which are 
hereby incorporated by reference. The metal oxides which may be deposited 
in accordance with the present invention include aluminum oxide, titanium 
oxide, yttrium oxide, zirconium oxide, tantalum oxide, vanadium oxide, 
tungsten oxide, niobium oxide and ruthenium oxide. The process is 
especially well suited for depositing metal oxides wherein the metal has a 
high thermal conductivity. The following description of a preferred 
exemplary embodiment will be limited to the deposition of aluminum oxide 
films with it being understood that the present invention is not limited 
to deposition of this particular metal oxide. 
The apparatus which is used to carry out the process of the present 
invention is a conventional ARE apparatus as shown diagrammatically in 
FIG. 1. The ARE apparatus includes a vacuum chamber 10, an electron beam 
evaporating gun 12, a vacuum pump 14, a heating lamp or element 16, and a 
radio frequency antenna 18, which is powered by a radio frequency power 
unit 20 which is powered by a power supply 22. The vacuum chamber 10 is 
preferably a water-cooled stainless steel bell jar which is divided into 
two sections by a stainless steel pressure barrier or plate 24. The 
electron beam gun 12 is located in the lower section of the vacuum chamber 
below plate 24 where the pressure is maintained at less than 10.sup.-3 
Torr to avoid arcing. The electron beam gun 12 is preferably a 
self-accelerated 270.degree. deflection type, such as Airco Temescal model 
SFIH-200-2. The power supply for the electron gun 12 is preferably an 
Airco Temescal model CV14 unit which may be operated at a constant 
voltage, such as 10 kv, with a variable emission current. 
The aluminum metal which is to be evaporated is placed in a water-cooled 
copper hearth 26 which has a diameter of approximately two inches. A rod 
feed-type configuration can also be used for continuous operation if 
desired. However, use of the water-cooled copper hearth is preferred. The 
aluminum metal is placed in the copper hearth 26 as an ingot 28 which is 
preferably of high purity, i.e., 99.999% pure. The low pressures necessary 
to carry out ARE in the vacuum chamber 10 are maintained by the vacuum 
pump 14 which is preferably a fractionating diffusion pump with an 
anti-migration-type liquid nitrogen trap. 
In order to form the plasma required in ARE deposition processes, oxygen or 
an oxygen-containing gas is introduced through inlet 30 into the zone 32 
located between the aluminum target 28 and substrate 34. Introduction of 
oxygen into the chamber through inlet 30 is controlled by a valve 36. The 
radio frequency antenna 18 used to excite the plasma formed by the 
evaporated aluminum metal and oxygen is preferably an inductively-coupled 
stainless steel circular coil which is powered by a conventional 13.56 MHz 
radio frequency supply such as Bendix model AST350 via an L-type matching 
network or radio frequency unit 20. The antenna 18 may also be energized 
by direct current or electron-cyclotron-resonance (ECR). 
The aluminum oxide may be deposited on a wide variety of substrates 34 
including glass, quartz, molybdenum, stainless steel, silicon, sapphire 
and the like. A wide variety of substrate sizes and shapes may also be 
used, provided that they are amenable to conventional ARE deposition. The 
substrate temperature can be between room temperature and 1000.degree. C. 
A shutter 38 is provided between the aluminum target 28 and the substrate 
34. The shutter is operable between open and closed positions as is 
conventionally known to allow control of the migration of evaporated metal 
from the target 28 toward the substrate 34. 
The ARE process is carried out by evacuating the vacuum chamber 10 down to 
a base pressure of about 10.sup.-6 Torr utilizing vacuum pump 14. The 
electron beam gun 12 is then used to irradiate the aluminum ingot or 
target 28, as represented by arrow 40, to form a pool of molten aluminum 
metal on the target material surface from which aluminum metal is 
evaporated. Upon opening of shutter 38, the evaporated aluminum metal 
migrates into the zone 32 where it is mixed with oxygen which has been 
introduced through inlet 30. The resulting mixture of evaporated metal and 
oxygen is activated by the radio frequency antenna 18 to form a plasma 
from which aluminum oxide is deposited onto substrate 34. 
During the ARE process, oxygen comes in contact with the aluminum target 28 
and reacts with the aluminum metal to form aluminum oxide. Aluminum oxide 
has a melting point which is much higher than that of aluminum metal. 
Accordingly, the evaporation of aluminum metal from the target 28 in the 
area covered by aluminum oxide is reduced. A detailed view of the aluminum 
target material 28 during the ARE process is shown in FIGS. 2 and 3. 
During activated reactive evaporation, a pool of molten aluminum 42 is 
formed on the surface of the target. The remainder of the surface 44 is 
covered with a layer of aluminum oxide. 
In accordance with the present invention, it was discovered that the 
relative ratio of surface area covered by the molten metal 42 as opposed 
to the aluminum oxide layer 44 has a direct effect and relationship to the 
rate of deposition of metal oxide onto the substrate 34. It was further 
found that the size of the molten metal pool 42 could be controlled by 
varying the oxygen flow into the chamber 10. The amount of oxygen 
introduced into the chamber 10 is controlled to achieve a ratio of the 
molten metal surface area 42 to metal oxide surface area 44 which results 
in maximum deposition rates of acceptable quality aluminum oxide films. 
The preferred ratio of molten metal surface 42 to metal oxide surface area 
44 is approximately 2:3 for aluminum oxide. For other metals, the relative 
ratio of metal surface area to metal oxide surface area is close to the 
stoichiometric ratios. However, the actual ratio of molten metal surface 
area 42 to metal oxide surface area 44 which provides optimum deposition 
characteristics can be established easily by experimentation. 
Conventional vacuum chambers have windows through which the surface of the 
target material is visible. Accordingly, the relative surface areas 
covered by the molten metal and metal oxide can be easily measured, 
monitored and controlled. Once the electron beam current is above a 
threshold value, for example 350 mA, the ratio of pure metal to metal 
oxide on the target surface is conveniently controlled by varying the 
amount of oxygen introduced into the vacuum chamber 10. The oxygen rate 
can then be varied to change the ratio of metal surface area to metal 
oxide surface area. In this way, the optimum deposition rate and film 
quality can be correlated to the ratio of metal to metal oxide surface 
area. 
It is preferred that the oxygen be initially controlled to achieve a ratio 
of metal surface area to metal oxide surface area which approximately 
corresponds to the stoichiometric ratio of metal to oxide in the deposited 
metal oxide. The oxygen flow into the chamber can then be increased or 
decreased slightly with changes in rate of deposition and deposition film 
characteristics being compared to changes in the surface area of the 
target covered by metal and metal oxide. Once the desired deposition rate 
and deposition film characteristics are achieved, the ratio of metal 
surface area to metal oxide surface area on the target remains constant. 
Once this ratio has been determined, it may be used in subsequent ARE 
processes as a preselected or predetermined ratio which is achieved in 
order to reproducibly obtain high deposition rates for aluminum oxide 
films having acceptable characteristics. 
Once the system is in operation, the ratio of metal surface area to metal 
oxide surface area is monitored during the process via partial pressure of 
oxygen gas to ensure that the deposition rate does not change or that the 
character of the depositing species also does not change. In effect, this 
provides an indirect means for monitoring deposition rate and film quality 
on the substrate. 
In accordance with the present invention, it was discovered that in order 
to obtain ratios of metal surface area to metal oxide surface area on the 
target which provided high deposition rates, the oxygen level in the 
vacuum chamber must be raised to levels which are relatively high. Oxygen 
levels on the order of 1 mTorr to about 5 mTorr are typically used to 
achieve the desired ratio of metal surface area to metal oxide surface 
area on the target material. Further, the overall chamber pressure was 
found to determine stoichiometry of the aluminum oxide film. When the 
chamber pressure is kept at over 5 mTorr, near stoichiometric aluminum 
oxide films are obtained which are not dependent on the electron beam 
current. It is believed that a semi-equilibrium condition is established 
on the billet under these conditions which results in the increased 
deposition rates which were observed. 
Examples of practice are as follows: 
EXAMPLE 1 
The chamber 10 was pumped down to a base pressure of approximately 
10.sup.-6 Torr before turning on the substrate heater 16. The substrate 
heater was then turned on. The substrate was a glass plate having the 
dimensions 25 mm.times.1 mm.times.75 mm. A high purity (99.999% pure) 
aluminum ingot was placed in the water-cooled copper hearth whose diameter 
was two inches. After the substrate reached a temperature of 300.degree. 
C., the electron beam gun was turned on and the electron beam current was 
increased very slowly to clean the surface of the aluminum and degas the 
charge. The electron beam gun was the Airco Temescal model SFIH-200-2. The 
power supply for the electron beam gun was the Airco Temescal model CV14 
unit which was operated at a constant voltage of 10 kv. A very thin layer 
of aluminum oxide was present on the aluminum ingot. This layer of oxide 
was ruptured and an elliptical molten pool was formed as the power 
increased. The metal pool surface area continuously increased until the 
electron beam current reached approximately 350 mA. Beyond this point, the 
pool area did not increase even though the power was increased up to 600 
mA. 
Oxygen gas was introduced into the chamber after the metal pool size 
reached a maximum surface area. A plasma was formed near the billet at 
this time. The inductively coupled radio frequency generator was operated 
at 13.56 MHz with radio frequency power gradually being increased to about 
100 watts. The shutter was opened after a stable operating condition was 
obtained. Deposition of aluminum oxide films were performed for two hours 
at various oxygen pressures while the electron beam current was varied 
from 520 to 580 mA. Changes in deposition rates of the films ranged from 
22 to 5.8 .mu.m/hour as the oxygen pressure was changed from 1.5 mTorr to 
5 mTorr (see FIG. 4). 
As the oxygen pressure increased, the surface area of the molten pool of 
aluminum decreased. A relatively high deposition rate of 8 .mu.m/hour was 
obtained with good film quality when the ratio of metal surface area to 
metal oxide surface area was approximately 1:2. Even higher deposition 
rates were obtained when the ratio of metal surface area to metal oxide 
surface area was 1:1. At a ratio of 2:3, the deposition rate was 12 
.mu.m/hour. It was found that the ratio of metal surface area to metal 
oxide surface area is determined by electron beam current, plasma 
intensity and partial pressure of the oxygen gas. When chamber pressure 
was kept at over 5 mTorr, near stoichiometric films were obtained that 
were not dependent on electron beam current. 
Dark brown colored films which indicate excess aluminum in the films were 
obtained at 1 mTorr whereas clear and featureless films were obtained at 5 
mTorr. The color of the film prepared at 2 mTorr was light brown. The 
ratio of oxygen to aluminum is also shown in FIG. 4 for the various films 
which were deposited. The oxygen to aluminum ratio increased as the oxygen 
pressure increased. Aluminum rich films were formed at lower oxygen 
pressure due to an insufficiency of oxygen species at the substrate. The 
number of oxygen species increased with oxygen pressure, thereby producing 
near stoichiometric films at higher oxygen pressure. The composition of 
the films obtained at 5 mTorr was 42 atomic % aluminum and 58 atomic % 
oxygen. 
X-ray diffraction patterns of all the films showed a broad halo at near 
22.degree. indicating an amorphous structure. This broad halo shifted to a 
higher scattering angle as the aluminum content increased. This is 
believed to be due to the change in the chemical short range order of the 
films. The nearest neighbor distances may change resulting in change in 
the microscopic density of the film as the composition of the film varies. 
A sample which initially exhibited the amorphous phase was heat-treated at 
1200.degree. C. in an atmosphere of argon gas for two hours. Several X-ray 
diffraction peaks appeared after annealing, which were indexed as 
.alpha.-Al.sub.2 O.sub.3 which has a corundum structure, except for one 
peak at near 65.degree.. The peak at near 65.degree. was indexed as 
.theta.-Al.sub.2 O.sub.3. According to McArdle, et al. (J.L. McArdle and 
G.L. Messing, Advanced Ceramics Materials, 3(4) (1988) 387), 
.theta.-Al.sub.2 O.sub.3 transforms to .alpha.-Al.sub.2 O.sub.3 at about 
1200.degree. C. X-ray diffraction showed that the transformation from 
.theta. phase to .alpha. phase had not been completed at 1200.degree. C. 
for two hours. 
The refractive index was measured for the various films. The refractive 
index was found to decrease with increases in oxygen partial pressure. The 
observed variation in refractive index with oxygen pressure is believed to 
be due to variations in the stoichiometry of the film. For a given 
electron beam current and radio frequency excitation power, the oxygen to 
aluminum ratio decreased as the oxygen partial pressure decreased, thereby 
resulting in aluminum rich films. As the oxygen pressure was decreased, 
values for refractive index approached a constant value of about 1.5 which 
is close to the refractive index of the stoichiometric Al.sub.2 O.sub.3 
film. The lowest value observed for the deposited films was 1.47. This low 
refractive index is believed to be due to the lower density of the films 
resulting from the very high deposition rate. The index of refraction of 
the film could be increased by biasing and/or heating the substrate during 
deposition. 
In conducting subsequent ARE deposition, the above system can be set up and 
various parameters, such as chamber pressure, oxygen pressure and electron 
beam current, established to achieve a ratio of metal surface area to 
metal oxide surface area in order to provide a desired preselected 
deposition rate and film quality. Further, these same parameters can be 
monitored and varied to continually maintain the predetermined ratio of 
relative surface areas at a given value to provide deposition of the 
desired film at the desired rate. 
EXAMPLE 2 
This example is carried out in the same manner as Example 1 except that the 
substrate is heated to a temperature above 300.degree. C. The heating of 
the substrate to temperatures above 300.degree. C. allows the deposition 
rate to be increased up to about 12 .mu.m/hour without adversely affecting 
the deposited film quality. The ratio of metal surface area to metal oxide 
surface area on the aluminum ingot is maintained at approximately the same 
levels as in Example 1 to achieve desired film quality and 
characteristics. 
EXAMPLE 3 
Several aluminum oxide films were deposited at 5 mTorr oxygen pressure in 
accordance with Example 1. The electron beam current was varied between 
300 mA and 700 mA over a two-hour period. The deposition rates increased 
from 2 to 12 .mu.m/hour as the current was increased. All of the films 
were clear and colorless regardless of the electron beam current. 
EXAMPLE 4 
This example is carried out in the same manner as Example 1 except that the 
substrate is kept a room temperature. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations and 
modifications may be made within the scope of the present invention. 
Accordingly, the present invention is not limited to the specific 
embodiments as illustrated herein, but is only limited by the following 
claims.