Process for the chemical vapor deposition of aluminum

A method for forming aluminum films is provided comprising employing the techniques of chemical vapor deposition to thermally decompose a vapor comprising a aluminum hydride subsequent to the treatment of the substrate with a Group IVB or VB metal complex, so as to deposit a mirror-like coating of aluminum on the surface of a substrate.

FIELD OF THE INVENTION 
This invention relates to the chemical vapor deposition of mirror-like thin 
films of aluminum. 
BACKGROUND OF THE INVENTION 
A variety of physical and chemical deposition procedures have been used to 
prepare aluminum (Al) films. These methods are of interest, in part, 
because thin films of aluminum have many uses due to their high electrical 
conductivity, high reflectivity, mechanical strength, and their resistance 
to chemical attack. There is much current interest in generating thin 
films of aluminum using chemical vapor deposition (CVD), particularly 
resulting from applications in the microelectronics industry. In a typical 
CVD process, organoaluminum precursors are volatilized and then decomposed 
to yield aluminum, which is deposited as a film on the target substrate. 
The generation of aluminum films using triisobutylaluminum (TIBA) has been 
reported. Typical deposition conditions generally require relatively high 
substrate temperatures, however. For example, the low pressure CVD of 
aluminum on silicon or other substrates using TIBA requires a substrate 
temperature of 260.degree. C., TIBA temperature of 45.degree. C., with 
argon as the carrier gas at pressures up to 1 torr. These conditions 
resulted in Al deposition rates of up to 0.15 .mu.m/min. See M. J. Cooke 
et al., Solid State Technol., 25, 62 (1982). The films produced by such 
methods have low resistivities (2.8 to 3.5 .mu..OMEGA.-cm) and other 
properties that are comparable to the properties of aluminum films 
prepared by evaporative techniques. See R. A. Levy et al., J. Electrochem. 
Soc., 131, 2175 (1984). However, a major disadvantage observed using TIBA 
is the rough morphology of the Al film, which leads to poor reflective 
properties. 
Improved aluminum film uniformity, particularly on nonmetallic surfaces 
such as silicon, has been obtained by pretreating the substrate surface 
with TiCl.sub.4 prior to deposition of aluminum using TIBA as a precursor 
by chemical vapor deposition. See R. A. Levy et al., J. Electrochem. Soc., 
134, 37C (1987); M. J. Cooke et al., vide supra; M. L. Green et al., Thin 
Solid Films, 114, 367 (1984); and R. A. Levy et al., vide supra. Although 
the aluminum films are more uniform, they may not be highly reflective. 
Recently, a method of preheating TIBA to a temperature of 230.degree. C. 
with the substrates at 400.degree. C. has produced aluminum films with 
reflectivities of 90% and epitaxial Al&lt;111&gt; growth on Si&lt;111&gt;. See T. 
Kobayashi et al., Jap. J. Appl. Phys., 27, L1775 (1988) and T. Kobayashi 
et al., Abstracts of Papers, Fall Meeting, Boston, MA; Materials Research 
Society: Pittsburgh, Pa., E9.47 (1988). 
Other sources of aluminum have been used as precursors in aluminum plating 
processes. For example, D. L. Schmidt et al., U.S. Pat. No. 3,462,288 
(1969), describe the use of aluminum hydrides such as AlH.sub.2 Cl, 
LiAlH.sub.4, and AlH.sub.2 (i-C.sub.4 H.sub.9) with a decomposition 
catalyst in an electroless solution plating process. In C. B. Roberts et 
al., U.S. Pat. No. 3,787,225 (1974), aluminum hydride-ether complexes were 
contacted with trimethylamine vapor in the presence of a decomposition 
catalyst. 
A series of stable, volatile donor-acceptor complexes of alane (AlH.sub.3) 
have been known for many years. They can be generally represented by 
D.AlH.sub.3, and can be readily synthesized in one step from LiAlH.sub.4. 
These donor-acceptor complexes of alane are air sensitive, but they are 
not pyrophoric, as are the trialkylaluminums. Among the known donors (D) 
are Me.sub.3 N, Et.sub.3 N, Me.sub.3 P, Me.sub.2 S, and tetrahydrofuran 
(THF). See, for example, E. Wiberg et al., Hydrides of the Elements of 
Main Groups I-IV, Elsevier: Amsterdam, Ch. 5 (1971); and R. A. Kovar et 
al., Inorg. Synth., 17, 36 (1977). Trimethylamine is unique among these 
donors in its ability to form a bis complex with alane, i.e., (Me.sub.3 
N).sub.2 AlH.sub.3. 
The use of amine-alane complexes for the vapor phase deposition of aluminum 
have been described in T. P. Whaley et al., U.S. Pat. No. 3,206,326 
(1965), and in D. R. Carley et al., U.S. Pat. No. 3,375,129 (1968). These 
methods, however, do not produce mirror-like coatings. Rather, less 
reflective "shiny" and "metallic" surfaces result. Laser-induced 
deposition of aluminum using amine-alane complexes has been disclosed in 
T. H. Baum et al., Abstracts of Papers, Fall Meeting, Boston, Mass.; 
Materials Research Society: Pittsburgh, Pa., B4.12 (1988). 
Therefore, a need exists for a process to deposit a mirror-like coating of 
aluminum having low resistivity using precursors that will permit the use 
of substantially lower deposition temperatures in the CVD process.

BRIEF DESCRIPTION OF THE INVENTION 
The present invention provides a method for applying a mirror-like aluminum 
film to the surface of a substrate by employing the techniques of chemical 
vapor deposition in a flow-through system. Prior to deposition of the 
aluminum, the substrate surface is exposed to a first vapor comprising a 
Group IVB or VB metal complex. The source of aluminum is a second vapor 
comprising an aluminum precursor, which is thermally decomposed so as to 
deposit a mirror-like coating of aluminum on the substrate surface. The 
aluminum precursor is of the formula [(R.sup.1 R.sup.2 R.sup.3)Y].sub.x 
AlH.sub.3 wherein: Y is a Group VA element; R.sup.1, R.sup.2 and R.sup.3 
are independently an alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, 
alkaryl, aryl, or aralkyl group; and x is an integer having a value of 1 
or 2. 
Herein, a "mirror-like" finish refers to a highly reflective surface. 
Generally, this type of finish is associated with a specular reflectivity 
of greater than about 80%. That is, the intensity of the radiation 
reflected at the specular angle, i.e., the radiation that is reflected at 
an angle with the surface equal to the angle between the incident 
radiation and the surface, is much greater than the diffuse reflectivity. 
Mirror-like finishes of aluminum as described herein typically result from 
a generally uniform distribution of very small particles, i.e., with a 
grain size of less than about 0.2 .mu.m. 
The substrate upon which the aluminum is deposited according to the method 
of this invention is a material of use in the electronics, optics, or 
metals-on-polymers industries. In certain applications, the aluminum film 
will function as an electrically conductive interconnect in 
microelectronic circuits. For example, see K. L. Chopra et al., Thin Film 
Device Applications; Plenum, N.Y. (1983); M. L. Green and R. A. Levy, J. 
of Metals (June 1985) at page 63. Therefore, useful substrates for 
aluminum film coating include silicon, tin oxide (SnO.sub.2), gallium 
arsenide (GaAs), silica, glass, polyimide, polymethyl-methacrylate, and 
other polymers. Additional substrates that can be coated with aluminum 
using this method include cloth, paper, graphite, ceramic materials, and 
other metals. It is to be understood, however, that this method is not 
limited to the above-listed materials, since it is not substrate-specific. 
The CVD process is carried out in a flow-through, horizontal, low pressure 
chemical vapor deposition reactor equipped with a first vessel containing 
the pretreating agent and a second vessel containing the aluminum 
precursor. A substrate placed in the reactor is contacted with a first 
vapor of the pretreating agent, and then contacted with a second vapor of 
the aluminum precursor. Prior to exposing the substrate to the aluminum 
precursor, and subsequent to exposing it to the pretreatment agent, the 
reactor may be evacuated for a short period of time. The precursor and 
pretreating, or pretreatment, agent are maintained at a constant 
temperature, usually below their respective decomposition temperatures and 
generally at or below room temperature, and the substrate is maintained at 
about room temperature or above, preferably at about 25.degree. C. to 
200.degree. C., most preferably at about 50.degree. C. to 150.degree. C. 
Using the method of the invention, thin aluminum films of varying 
thicknesses can be prepared. For example, with the pretreatment agent 
maintained at 10.degree. C., the aluminum precursor at 25.degree. C., and 
using a substrate temperature of 180.degree. C., for a 4 minute deposition 
time, a 4 .mu.m film of Al can be deposited on Si&lt;100&gt;. Films of about 
0.05 .mu.m (500 A) to 5 .mu.m can be readily formed under these reaction 
conditions, but the thickness can be varied considerably, and is dependant 
upon the deposition time and rate. The rate of deposition can be varied in 
a number of ways. For example, the precursor temperature can be lowered to 
decrease the mass flow of the precursor upon vaporization, or an inert 
carrier gas can be used to dilute the precursor vapor. 
DETAILED DESCRIPTION OF THE INVENTION 
Reactor and Reaction Conditions 
As shown in FIG. 1, the CVD process is generally carried out in a 
flow-through, horizontal, low pressure chemical vapor deposition reactor 
(LPCVD) (1). A pretreatment agent comprised of a Group IVB or VB metal 
complex, e.g., TiCl.sub.4, contained in a first reservoir (2) at one end 
of the reactor is exposed to a vacuum by opening valves (3) and (11) and 
vaporized under an initial vacuum of no more than about 10.sup.-2 torr, 
e.g., at 10.sup.-7 to 10.sup.-3 torr for a sufficient time to produce a 
surface coating of the agent. The vacuum is provided by a suitable vacuum 
pump positioned at the opposite end of the reactor (not shown). Following 
treatment with TiCl.sub.4, valves (3) and (11) are closed. Optionally, the 
reactor (1) is evacuated for a short period of time to ensure removal of 
any vaporized pretreatment agent. The aluminum hydride precursor, e.g., 
(Me.sub.3 N).sub.2 AlH.sub.3, contained in a second reservoir (4) at one 
end of the reactor is then exposed to an initial vacuum by opening valves 
(3) and (12) and vaporized under a vacuum of no more than about 10.sup.-2 
torr, e.g., at 10.sup.-7 to 10.sup.-3 torr. A carrier stream of an inert 
gas (e.g., He and/or Ar) can optionally be employed by passing it through 
a liquid or solid precursor and into the reaction chamber (5). The 
aluminum precursor vapor (8) then passes into a reaction chamber (5) that 
contains one or more units of the substrate (6). The substrate, e.g., 
wafers of Si&lt;100&gt;, are preferably held in a vertical position by a 
suitable holder (7). The reaction chamber is maintained at a specified 
temperature, by means of an external furnace (10), which is effective to 
decompose the aluminum precursor vapor (8) so as to deposit a film of 
aluminum (9) on the exposed surfaces of the substrate units. Preferably, 
the reaction chamber is maintained at about 25.degree. C. to 200.degree. 
C. during the deposition process, most preferably at about 50.degree. C. 
to 150.degree. C. The total pressure within the reaction chamber is 
generally within the range of about 0.01 torr to 2 torr. However, this 
pressure does not appear to be highly critical to the deposition of the 
mirror-like films of aluminum. 
The pretreatment agent and precursor are generally maintained at a constant 
temperature during the vaporization process for ease of handling; however, 
this is not critical. The temperature of each is generally below their 
respective decomposition temperatures, but at a temperature such that they 
are sufficiently capable of being volatilized in the process of chemical 
vapor deposition. 
The substrate is exposed to the pretreatment agent for a sufficient period 
of time such that an effective amount of the pretreatment agent is coated 
onto the surface of the substrate. Herein, an "effective amount" refers to 
an amount that is effective to produce a mirror-like film upon exposure of 
the substrate to the aluminum precursor. Generally, this is dependent upon 
the temperatures at which the substrate and pretreatment agent are 
maintained. 
The substrate is exposed to the aluminum precursor for a sufficient period 
of time such that a mirror-like film of aluminum is deposited on the 
surface of the substrate. Generally, short deposition times, i.e., less 
than about 6 minutes, produce mirror-like finishes when the aluminum 
precursor is maintained at a temperature of about 25.degree. C. and the 
substrate is maintained at a temperature of about 100.degree. C. As with 
the pretreatment agent, the temperatures at which the substrate and 
aluminum precursor are maintained have an effect upon the exposure time of 
the substrate to the precursor vapor. 
It is believed that the deposition, or growth, rates can effect the quality 
of the aluminum coating, i.e., the grain size and reflectivity. With slow 
depositions the quality of the coating is enhanced. That is, the coating 
is made up of smaller grain sized particles, and hence exhibits better 
reflectivity. Aluminum films with high specular reflectivities, i.e., 
greater than about 80%, are generally obtained with the precursors of this 
invention if the deposition rate is maintained at less than about 0.5 
.mu.m/minute. 
Aluminum Precursors 
Precursors for use in the method of this invention have the general formula 
[(R.sup.1 R.sup.2 R.sup.3)Y].sub.x AlH.sub.3 wherein: Y is a Group VA 
element; R.sup.1, R.sup.2, and R.sup.3 are independently an alkyl, 
alkenyl, alkynyl, cycloalkyl, cycloalkenyl, alkaryl, aryl, or aralkyl 
group; and x is an integer having a value of 1 or 2. Preferably R.sup.1, 
R.sup.2, and R.sup.3 are independently lower molecular weight groups: 
(C.sub.1 -C.sub.6)alkyl, such as methyl, ethyl, propyl, butyl, pentyl, and 
hexyl; (C.sub.2 -C.sub.6)alkenyl, such as propenyl or butenyl; (C.sub.2 
-C.sub.6)alkynyl, such as propynyl; (C.sub.3 -C.sub.8)cycloalkyl, such as 
cyclopentanyl; (C.sub.4 -C.sub.8)cycloalkenyl, such as cyclopentenyl; 
(C.sub.1 -C.sub.6)alkaryl, such as styryl or tolyl; (C.sub.6 
-C.sub.10)aryl, such as phenyl; and aryl(C.sub.1 -C.sub.6)alkyl, such as 
benzyl. Included within this group of substituents are branched or 
straight chain hydrocarbon groups. 
The most preferred precursors of the aluminum films prepared in accord with 
the present method are trialkyl (Group VA) aluminum hydrides wherein the 
preferred Group VA elements are nitrogen (N) and phosphorus (P). Most 
preferably, a trialkyl amine aluminum hydride of the general formula 
[(R.sup.1 R.sup.2 R.sup.3)N].sub.x AlH.sub.3, wherein R.sup.1, R.sup.2, 
and R.sup.3 are independently a (C.sub.1 -C.sub.3)alkyl group and x is 1 
or 2, is used. That is, the alkyl group is preferably methyl, ethyl, 
propyl, or mixtures thereof, and most preferably, methyl or ethyl. 
Examples of the amine complexes of aluminum hydride are trimethylamine, 
triethylamine, tri-n-propylamine, triisopropylamine, methyl diethylamine, 
n-propyldimethylamine, and isopropyl diethylamine. 
These compounds can be prepared from readily available starting materials 
by known synthetic methods. Bis(trimethylamine)aluminum hydride has been 
prepared by the reaction of Me.sub.3 N.HCl with LiAlH.sub.4 by E. Wiberg 
et al., vide supra and R. A. Kovar et al., vide supra, the disclosures of 
which are incorporated herein by reference. Triethylamine aluminum hydride 
has been prepared by the reaction of triethylamine HCl with lithium 
aluminum hydride by the procedure of E. Wiberg et al., vide supra, the 
disclosure of which is incorporated herein by reference. 
Trimethylphosphine aluminum hydride has been prepared by the reaction of 
trimethylphosphine with (Me.sub.3 N).sub.2 AlH.sub.3 by the procedure of 
E. Wiberg et al., vide supra, the disclosure of which is incorporated 
herein by reference. Certain of the amine-alanes and phosphine-alanes are 
preferable precursors because they are generally easier to prepare and 
handle since they are solids that can be sublimed. 
(Me.sub.3 N).sub.2 AlH.sub.3 is an effective precursor for the formation of 
thin films of high reflectivity and purity, low resistivity aluminum in a 
low pressure chemical vapor deposition reactor. Gas phase studies by C. W. 
Heitsch, Nature, 195, 995 (1962) of (Me.sub.3 N).sub.2 AlH.sub.3 suggest 
that at 80.degree. C. the predominant species in the gas phase is 
(Me.sub.3 N)AlH.sub.3. Although less well-documented, higher temperatures 
are reported to cause dissociation of the second Me.sub.3 N to release 
AlH.sub.3. Studies of the thermal decomposition of liquid (Me.sub.3).sub.2 
AlH.sub.3 and (Me.sub.3 N)AlH.sub.3 have also suggested that dissociation 
of the Me.sub.3 N precedes hydrogen loss. See G. N. Nechiporenko et al., 
Izv. Akad. Nauk SSSR, Ser. Khimicheskaya, 1697 (1975). 
The reactor temperature is generally maintained within the range of 
25.degree. C. to 200.degree. C., preferably within the range of 50.degree. 
C. to 150.degree. C. However, the choice of precursor can determine the 
choice of reactor temperature. For example, when (Me.sub.3 N).sub.2 
AlH.sub.3 is the precursor, the most preferred range is 85.degree. C. to 
150.degree. C., and when (Et.sub.3 N)AlH.sub.3 is the precursor, the most 
preferred range is 25.degree. C. to 120.degree. C. The low temperatures of 
deposition make (Me.sub.3 N).sub.2 AlH.sub.3 and (Et.sub.3 N)AlH.sub.3 
especially attractive for forming aluminum coatings on temperature 
sensitive substrates. 
While the total pressure in the reactor of the invention is not directly 
measured, and this pressure does not appear to have a significant impact 
on the formation of mirror-like films of aluminum, the pressure is 
generally estimated to be within the range of 0.01 torr to 2 torr in the 
reactor. The direct pressure measurement is taken after the liquid 
nitrogen trap and the hydrogen pressure is monitored. For a reactor 
temperature of 180.degree. C., the hydrogen pressure is generally about 
0.2 torr. It is expected that the pressure in the reactor would be 
somewhat higher, but generally less than the equilibrium vapor pressure of 
the precursor, which for (Me.sub.3 N).sub.2 AlH.sub.3 is 1.8 torr at 
25.degree. C. 
Pretreatment Agent 
Transition metal complexes suitable for use as pretreatment agents in the 
method of preparing mirror-like aluminum films on various substrates are 
compounds of the Group IVB and VB transition metals. This includes 
compounds of titanium, hafnium, zirconium, vanadium, niobium, and 
tantalum. Included within this group are, for example, ZrCl.sub.4, 
NbCl.sub.5, VOCl.sub.3, VOCl.sub.2, TiCl.sub.3, TiCl.sub.4, 
TiCl.sub.4.20Et.sub.2, TiBr.sub.4, TiI.sub.4, VCl.sub.4, TiCl.sub.2 
(OEt.sub.2).sub.2, TiCl.sub.2 (i-OC.sub.3 H.sub.7).sub.2, 
Ti(BH.sub.4).sub.2.20Et.sub.2, and mixtures thereof. The chlorides, 
bromides, and oxychlorides of titanium, niobium, vanadium, and zirconium 
are generally thought to be more effective, with TiCl.sub.4 being one of 
the most effective in achieving a more even distribution of aluminum, 
smaller crystallites, and hence a mirror-like coating of aluminum. 
The interaction of the pretreatment agent with the substrate surface is not 
fully understood. It may be that the material is adsorbed onto the 
surface, and that less than a monolayer of TiCl.sub.4 is actually 
adsorbed. It should be kept in mind, however, that when the substrate is 
Si&lt;100&gt;, silica, or glass, there are available hydroxyl groups that offer 
a route, via a possible intermediate such as Si-O-TiCl.sub.3, for binding 
the titanium to the surface. 
While the role of the pretreatment agent, e.g., TiCl.sub.4, is not 
presently known, and the inventors do not wish to be held to any 
particular theory, it can be speculated that the advantages may result, at 
least in part, from the interaction of TiCl.sub.4 with alane, a powerful 
reducing agent. It is thought that a factor that may contribute to the 
surface roughness of films in the absence of a pretreating agent is the 
gas phase nucleation of particles of Al or (AlH.sub.3).sub.n, which would 
then coat the surface. Evidence of an interaction between TiCl.sub.4 and 
alane can be observed in the trap placed at the exit of the CVD reactor. 
Even at very low temperatures (-100.degree. C.), dark green to blue 
coloration is observed, and as the trap is further warmed towards room 
temperature, a very exothermic reaction takes place. While no spectral 
data regarding the products is yet available, it is anticipated that Ti-H 
[or perhaps aluminohydride, Ti(AlH.sub.4)] complexes form initially. 
Reductive elimination of H.sub.2 from the titanium would be expected to be 
facile, thus providing a route to metallic aluminum. 
Substrates 
The present method is described primarily by reference to examples 
involving the deposition of aluminum films on Si&lt;100&gt; surface, polyimide 
films, and glass. However, it is expected that the surfaces of other 
crystal plane orientations of silicon and/or other substrates can be 
effectively coated with a film of aluminum by the present method, for a 
variety of end-uses, as discussed hereinabove. Such substrates include, 
but are not limited to, Si&lt;311&gt;, Si&lt;111&gt;, Si&lt;110&gt;, GaAs&lt;110&gt;, GaAs&lt;111&gt;, 
GaAs&lt;311&gt;, SnO.sub.2, and various SiO.sub.2 glasses, paper, wood, 
synthetic fibers and fabrics, cotton, ceramic materials, cermets, 
nonmetallic refractories and polymers. Any substrate that is generally 
stable, i.e., does not substantially decompose, under the conditions 
employed in the method of the present invention, are suitable for use. 
That is, the aluminum films are generally adherent to a wide variety of 
materials and highly adherent to polyimide films. Therefore, it is to be 
understood that this method of aluminum deposition is not 
substrate-specific. 
The invention will be further described by reference to the following 
detailed examples, wherein X-ray diffraction data were obtained on a 
Siemens D500 diffractometer using graphite monochromatized Cu K.alpha. 
radiation and scintillation detection. Alignment was determined using the 
Si&lt;400&gt; reflection. All film thickness measurements were performed on a 
Tencor Alphastep stylus profilometer. Resistivities were measured using a 
Veeco FPP-5000 four-point probe. The microscopic surface structure of the 
films was examined by electron microscopy using a JEOL 840 II Scanning 
Electron Microscope. Auger electron spectra were measured on a Perkin 
Elmer Corporation/Physical Electronics Division model 555 electron 
spectrometer. Reflectivity measurements were made on a Perkin Elmer Lambda 
9 UV-Vis-NIR spectrophotometer. 
EXAMPLE 1 
Chemical Vapor Deposition of Thin Films of Aluminum 
The reactor employed in the low pressure chemical vapor deposition of 
aluminum was an all-glass, horizontal tube, low pressure CVD reactor. FIG. 
1 shows a schematic of the system. The pressure of the system (1) was 
maintained with an oil diffusion pump capable of base pressures of 
3.times.10.sup.-5 torr The reaction tube(s) itself was made of quartz and 
had an inside diameter of 2.6 cm. 
The temperature of the tube furnace was monitored by thermocouples placed 
above and below the quartz tube at 4.5 cm (in the middle of the region 
where the substrates were placed) from the edge of the tube furnace 
heating coils (on the precursor entry side of the tube). The substrate 
temperatures in the tube were not monitored during a deposition. The 
numbers quoted as the deposition temperatures were obtained by a separate 
calibration of the internal temperature against the external thermocouple 
readings. 
Si&lt;100&gt; wafers were degreased/etched by immersion in the following baths 
according to the order listed for 10 minutes each: tetrachloroethylene, 
ethanol, deionized water, dilute HF, and deionized water. Glass microscope 
slides and strips of polyimide film (Du Pont KAPTON Type H) were cut to 
fit in the tube and were treated in the same fashion as the silicon wafers 
with the exclusion of the HF etching treatment. After air drying, the 
substrates were placed in the tube in a Macor ceramic holder (7) to hold 
them in a vertical position during the deposition. The distance between 
the adjacent wafers was 6 mm. Alternatively, the substrate wafers can be 
laid flat on the bottom of the reactor tube. Masking of the wafers was 
achieved by placing two wafers in direct contact with one another. After 
placing the substrate into the reactor, the reactor was evacuated and 
heated to the desired reaction temperature. The reactor was then 
maintained at the desired temperature for a minimum of one hour, prior to 
initiating a run. 
The entrance to the quartz tube was fitted with a valve (3) leading to a 
vessel (2) containing the TiCl.sub.4, which was fitted with a valve (11), 
and a second vessel (4) containing (Me.sub.3 N).sub.2 AlH.sub.3, which was 
fitted with a valve (12). The vessel containing TiCl.sub.4 was cooled to 
10.0.degree. C. with a HAAKE A81 circulating bath (not shown) and opened 
to the reactor, by opening valves (3) and (11), for one minute while 
pumping was maintained. The tube was then evacuated for about 20 to 30 
minutes prior to opening the vessel containing (Me.sub.3 N).sub.2 
AlH.sub.3. 
In a typical deposition the vessel containing the (Me.sub.3 N).sub.2 
AlH.sub.3 precursor (4) was opened to the reaction chamber by opening the 
valves (3) and (12). The valves were allowed to remain open for four 
minutes during which time the inside of the quartz tube and the substrates 
were coated with an aluminum film. The vessel containing the (Me.sub.3 
N).sub.2 AlH.sub.3 was also maintained at a constant temperature, with a 
similar bath as above, usually at about 25.degree. C. Upon completion of 
the deposition the system was allowed to return to its original pressure 
before opening. The pressure was monitored with an Inficon capacitance 
manometer placed between the pump and the liquid nitrogen cooled trap 
located at the exit of the furnace. 
At the exit of the furnace a small amount of a crystalline deposit of 
(Me.sub.3 N).sub.2 AlH.sub.3 precursor was observed. Unreacted precursor 
was also found in the liquid nitrogen cooled trap placed between the 
reactor and the diffusion pump. Since the trap was located between the 
capacitance manometer and the reactor, the pressures measured resulted 
from the hydrogen expelled during the deposition. This appeared to be a 
sensitive measure of the reproducibility of a given set of reaction 
conditions. The behavior usually observed is best described with a 
specific example. With the precursor vessel at 25.degree. C. and the 
furnace at 180.degree. C., the pressure would stabilize at a constant 
value of approximately 0.2 torr. The observed pressures were decreased by 
both a lower furnace temperature and a lower precursor vessel temperature. 
The rate of aluminum deposition was determined by masking part of the 
wafers prior to the deposition and measuring the step height created for a 
given deposition time. With a precursor temperature of 25.degree. C. and 
the reaction tube at 100.degree. C., the deposition rate was about 0.06 
.mu.m/min, which produced a mirror-like film. The results of single 
depositions under different conditions are given in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Typical Deposition Conditions and Results of Deposition of Si&lt;100&gt; 
T (precursor) 
T (reactor) 
TiCl.sub.4 
Time (min) 
Growth rate 
Resistivity 
Surface 
(.degree.C.) 
(.degree.C.) 
Pretreatment 
of Al deposition 
(.mu.m/min) 
(.mu..OMEGA.-cm) 
Finish 
__________________________________________________________________________ 
25 280 No 4.0 2.9 3.9 Chalky 
25 180 No 4.0 0.8 3.3 Milky 
25 180 Yes 4.0 1.0 4.5 Milky 
25 100 Yes 2.0 0.06 3.5 Mirror 
25 180 Yes 6.0 1.0 3.2 Milky 
25 180 Yes 2.0 0.6 3.8 Milky 
__________________________________________________________________________ 
In depositions where the reactor and its contents were treated with 
TiCl.sub.4 prior to the alane precursor, a more even distribution of the 
aluminum layer was observed. On the edges of these films on the quartz 
reactor tube, a gray, chalky, nonreflective portion was observed. It is 
believed that this may be due to small particles of aluminum that have not 
yet formed a continuous film. 
EXAMPLE II 
Characterization of the Aluminum Films 
Auger Spectroscopy 
FIG. 2a shows the results of the Auger electron spectral profile, i.e., 
electron intensity, as a function of sputtering time. The top layers of 
the film exhibit the usual oxide coating along with carbon and nitrogen, 
all of which are absorbed from the atmosphere. All of these elements 
decreased rapidly to within the detection limits of the method as the 
sputtering proceeded. In the films where TiCl.sub.4 was used to pretreat 
the surface, no titanium or chlorine was detected in the aluminum films or 
at the interface with the silicon wafer. This is consistent with the 
absorption of less than a monolayer of TiCl.sub.4 during the pretreatment. 
FIG. 2b is a graph of intensity versus binding energy. The peak is a 
derivative of the number of electrons counted per unit time. After 15 
minutes of sputtering of the coated surface, the Auger spectrum of the 
surface displayed only Al (68 keV). 
X-Ray Diffraction 
The X-ray diffraction spectrum of polycrystalline aluminum films grown on 
the surface of Si&lt;100&gt; or glass slides at 280.degree. C. without 
TiCl.sub.4. pretreatment is shown in FIG. 3a. Thick films (approximately 
&gt;1 .mu.m) deposited without any TiCl.sub.4 pretreatment gave nearly the 
expected intensity distributions; see Powder Diffraction File, 
International Center for Diffraction Data, Swarthmore, PA, Card #4-0787. 
An increasing deviation from the polycrystalline aluminum pattern was 
observed for films as they became thinner, however. The 2.theta. values 
(deg), with their relative intensities and assignments for polycrystalline 
aluminum, are: 38.47 (1.00) &lt;111&gt;, 44.74 (0.47) &lt;200&gt;, 65.13 (0.22) &lt;220&gt;, 
78.22 (0.24) &lt;311&gt;, 82.432 (0.07) &lt;222&gt;. The reflection at 69.17.degree. 
is due to the substrate. FIG. 3b shows the striking effect caused by 
pretreating the surface with TiCl.sub.4 and growing the films at 
100.degree. C. The films show nearly complete preference for growing with 
the Al&lt;111&gt; face parallel to the surface. While FIG. 3b was taken from a 
film grown on Si&lt;100&gt;, similar patterns were obtained on simple glass 
slides. FIG. 4 clearly illustrates this same orientation preference, i.e., 
Al&lt;111&gt; parallel to the surface, on TiCl.sub.4 pretreated polyimide films 
with aluminum deposited at 100.degree. C. 
Electron Microscopy 
The microstructure of the films was examined using scanning electron 
microscopy (SEM), and, once again, a dramatic effect of pretreating the 
film with TiCl.sub.4 was observed. FIG. 5 shows the scanning electron 
micrograph of the surface of a rough film typical of those deposited 
without TiCl.sub.4 pretreatment. The grain size observed averages 2-3 
.mu.m. The photograph shows the large gaps between grains that are 
responsible for the roughness of the surface. For masked wafers, in the 
regions near the edge of the aluminum created by the mask, small 
crystallites of aluminum were visible on the silicon. Scanning towards the 
aluminum film revealed an increase in crystallite size, but not a 
corresponding increase in the number of small crystallites. This suggests 
that the rate of crystallite growth is greater than the rate of 
nucleation, which undoubtedly contributes to the surface roughness of the 
final film. Those films grown on Si&lt;100&gt; wafers or glass slides that were 
pretreated with TiCl.sub.4 exhibited a much higher number of small 
crystallites in this same near-edge region. On those films grown on 
pretreated surfaces at 180.degree. C. the grains averaged 1 .mu.m, whereas 
the films grown on pretreated surfaces at 100.degree. C. exhibited an 
average grain size of 0.15 .mu.m. FIG. 4 shows the scanning electron 
micrograph of this latter surface. 
Resistivity 
The resistivities of the films were evaluated using a four-point probe. The 
results are summarized in Table 1. The low resistivities observed are near 
that of bulk aluminum The sensitivity of the resistivity measurement to 
small concentrations of impurities verified the high purity of these Al 
films. 
Reflectivity 
The surface roughness of many of the films not subjected to TiCl.sub.4 
pretreatment caused their specular reflectivity to be very low. Specular 
reflectivity relates to the intensity of the radiation reflected at the 
specular angle, i.e., the angle between surface and the reflected beam 
that is equal to the angle between the surface and the incident radiation. 
Typically, the total reflectivity, i.e., specular plus diffuse, was 
greater than 90% at a wavelength of 550 nm, but the specular contribution 
to this value was less than 5%. These films wherein the substrate had not 
been treated with TiCl.sub.4 prior to aluminum deposition could be 
described as generally milky, cloudy, chalky, and in some situations shiny 
or metallic. For the films grown at 100.degree. C. on TiCl.sub.4 treated 
surfaces, the relative contributions of the diffuse and specular 
components to the total reflectivity were reversed, i.e., diffuse 
generally less than 5%. FIG. 7 illustrates the high specular reflectivity 
of these films (the actual value of the specular reflectivity of these 
mirror-like films at 550 nm was about 85%). Similar reflectivities were 
also obtained from aluminum deposited on treated polyimide films. 
The invention has been described with reference to various specific and 
preferred embodiments and techniques. However, it should be understood 
that many variations and modifications may be made while remaining within 
the spirit and scope of the invention.