Process for metal nitride deposition

A method for forming metal nitride films is provided comprising employing the techniques of chemical vapor deposition to thermally decompose a vapor comprising a dialkyl(Group III metal) azide, so as to deposit a film of the corresponding metal nitride on the surface of a substrate.

BACKGROUND OF THE INVENTION 
Methods for the deposition of thin films of solid state nitrides have been 
available for many years and are of interest for a variety of reasons. 
Among the nitrides of the Group III metals, aluminum nitride (AlN) has 
several potential applications. Its high melting point (2300.degree. C.) 
and hardness (between Al.sub.2 O.sub.3 and diamond), coupled with its 
large band gap (6.2 eV) make it a useful ceramic coating particularly for 
optical or optoelectronic devices. Its piezoelectric nature also renders 
it valuable for certain other applications. While not as robust as AlN, 
gallium nitride (GaN) and indium nitride (InN), both of which have the 
wurtzite structure analogous to AlN, have band gaps of 3.4 and 1.9 eV, 
respectively, which makes them of interest as semiconductors. There have 
been several reports describing the interesting properties of mixed 
compounds such as Al.sub.x Ga.sub.1-x N. 
By far the most common source of nitrogen for the preparation of nitrides 
is N.sub.2 and NH.sub.3. See L. E. Toth, in Transition Method Carbides and 
Nitrides, Academic Press, NY (1971). Because of the chemical inertness 
and/or the stability of these two species, high temperatures (1000.degree. 
C. or greater) are usually required to form the nitride using chemical 
vapor deposition techniques (CVD). In a CVD process, organometallic 
precursors are volatilized and then decomposed to yield the metal nitride 
which is deposited as a film on the target substrate. For example, 
trimethylaluminum can be decomposed in the presence of ammonia to yield 
methane and aluminum nitride. 
For AlN, lower deposition temperatures have recently been reported using 
the cyclic trimer, [R.sub.2 AlNH.sub.2 ].sub.3 which is formed by the 
pyrolysis of the donor-acceptor complex R.sub.3 Al--NH.sub.3. See L. V. 
Interrante et al., Mats. Res. Soc. Symp. Proc., 73, 359 (1986). The 
chemical vapor deposition of both AlN and GaN has also been reported using 
hydrazine as the N-source. See D. K. Gaskill et al., J. Crystal Growth, 
77, 418 (1986). Although these approaches have met with some success, a 
need exists for alternate nitrogen-containing precursors that will permit 
the use of substantially lower deposition temperatures in the CVD of 
nitride films.

BRIEF DESCRIPTION OF THE INVENTION 
The present invention provides a method for applying a metal nitride film 
to the surface of a substrate comprising employing the techniques of 
chemical vapor deposition in conjunction with organometallic compounds of 
Group III elements which comprise the azide (--N.sub.3) group as the 
nitrogen source. A vapor comprising an azide of the formula [(C.sub.1 
-C.sub.5)alkyl].sub.2 MN.sub.3, wherein M is a Group III metal, is 
thermally decomposed, so as to deposit a film of a metal nitride, MN, on 
said surface. 
Preferably, the substrate is a material of use in the electronic industry, 
e.g., the MN film will function as a dielectric layer or as a diffusion 
barrier in a variety of microelectronic devices. For example, see K. L. 
Chopra et al., Thin Film Device Applications; Plenum, NY (1983) and E. 
Doering, in Insulating films in Semiconductors; J. F. Verweij et al., 
eds., Elsevier, Amsterdam (1985) at page 208. Therefore, useful substrates 
for MN film coating include silicon, platinum, tin oxide (SnO.sub.2), 
aluminum oxide (Al.sub.2 O.sub.3), gallium arsenide (GaAs) or glass. 
As shown in FIG. 1, the CVD process will generally be carried out in a 
horizontal, low pressure CVD reactor (2). The axide precursor, e.g., 
Et.sub.2 AlN.sub.3, contained in reservoir (1) at one end of the reactor 
is vaporized at about 25.degree.-85.degree. C. under a vacuum of no more 
than about 10.sup.-3 torr, e.g., at 10.sup.-7 -10.sup.-4 torr. The vaccum 
is provided by a suitable vacuum pump positioned at the opposite end of 
the reactor (not shown). A stream of an inert gas (He and/or Ar) can 
optionally be employed to vaporize the precursor, e.g., by passing it 
through a liquid precursor and into the reaction chamber. The azide vapor 
(8) then passes into a reaction chamber (5) which contains one or more 
units of the substrate (6). The substrate, e.g., wafers of silicon &lt;100&gt;, 
are preferably held in a vertical position by a suitable holder (7). The 
reaction chamber is maintained at a temperature, by means of an external 
furnace (10), which is effective to decompose the azide vapor (8) so as to 
deposit a film of a metal nitride, MN, (9) on the exposed surfaces of the 
substrate units. Preferably, the reaction chamber is maintained at about 
400.degree.-800.degree. C. during the deposition process, most preferably 
at about 450.degree.-700.degree. C. Unreacted azide or volatile 
by-products then exit the chamber at exit port (10), and can be condensed 
in a liquid nitrogen trap (not shown). 
Using the conditions described above, thin MN films of varying thicknesses 
can be prepared. For example, using Et.sub.2 AlN.sub.3, reaction 
temperature of 480.degree. C. and a system pressure of 
.ltoreq.1.times.10.sup.-4 torr, a four-hour deposition time generated a 
0.4 .mu.m film on Si(100). The rate of deposition was about 1000 .ANG./hr. 
Films of about 0.1-1.5 .mu.m can readily be formed under these reaction 
conditions. 
DETAILED DESCRIPTION OF THE INVENTION 
Dialkylmetal Azides 
The preferred precursors of the metal nitride films prepared in accord with 
the present method are di(C.sub.1 -C.sub.5)alkyl metal nitrides. Most 
preferably, a dialkyl nitride of a Group III metal will be used, e.g., of 
the general formula [(C.sub.1 -C.sub.5)alkyl].sub.2 MN.sub.3 wherein M is 
aluminum, gallium or indium. Preferably the alkyl group will be methyl or 
ethyl, most preferably, ethyl. 
These compounds can be prepared from readily available starting materials 
by known synthetic methods. Diethylaluminum azide has been prepared by the 
reaction of sodium azide with Et.sub.2 AlCl by M. I. Prince et al., J. 
Organomet. Chem., 5, 584 (1966), the disclosure of which is incorporated 
by reference herein. 
However, even relatively weak acids are capable of cleaving the M-C bond in 
trialkyl metal complexes. [See I. Haiduc et al., Basic Organometallic 
Chemistry, de Gruyter, Berlin (1985)]. By carefully controlling the 
stoichiometry, and the rate of addition, hydrazoic acid could be used to 
form the azides. This reaction (eq. 1) would have the advantage of forming 
completely halide-free products. 
EQU MR.sub.3 +HN.sub.3 .fwdarw.R.sub.2 MN.sub.3 +H--R (1) 
EQU R.sub.2 MH+HN.sub.3 .fwdarw.R.sub.2 MN.sub.3 +H.sub.2 (2) 
Reaction 2 illustrates another mild route that makes use of hydrazoic acid. 
Other routes involving more exotic reagents such as ClN.sub.3 are known 
and can be used if necessary. (See J. Muller et al., J. Organomet. Chem., 
12, 37 (1968)). Trialkyl(Group III metal) compounds (MR.sub.3) are 
commercially available and include trimethylaluminum, trimethylgallium, 
trimethylindium, triethylaluminum, triethylgallium and triethylindium 
(Alfa Products, Danvers, MA). Dialkyl metal azides, such as Et.sub.2 
AlN.sub.3, are known to exist as cyclic trimers in the liquid state, but 
will be represented as monomers (R.sub.2 MN.sub.3) herein for ease of 
depiction. 
It is believed that a (C.sub.1 -C.sub.5) (Group III metal) azide thermally 
decomposes to yield a (Group III metal) nitride film as shown in the 
reaction (eq. 3), below, for Et.sub.2 AlN.sub.3. 
EQU Et.sub.2 AlN.sub.3 .fwdarw.AlN+N.sub.2 +(2-X)C.sub.2 H.sub.4 +XC.sub.2 
H.sub.6 +(1-X)H.sub.2 (3) 
Based on the relative amounts of C.sub.2 H.sub.4 and C.sub.2 H.sub.6 
detected by 1R and mass spectrometry, the value of X is about 0.3 
Substrates 
The present method is described primarily by reference to examples 
involving the deposition of metal nitride films on Si(100) surfaces. 
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 a Group (III) metal nitride 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;III&gt;, Si&lt;110&gt;, GaAs&lt;110&gt;, GaAs&lt;III&gt;, 
GaAs&lt;311&gt;, SnO.sub.2, Al.sub.2 O.sub.3 and various SiO.sub.2 glasses. 
The invention will be further described by reference to the following 
detailed examples, wherein diethylaluminum chloride and sodium azide were 
purchased from Aldrich Chemical Company, Milwaukee, WI. The sodium azide 
was carefully ground and placed in a vacuum to dry for two days. The 
diethylaluminum azide was prepared by the procedure of M. I. Prince et 
al., J. Organomet. Chem., 5, 584 (1966), the disclosure of which is 
incorporated by reference herein. 
All reactions were performed under an atmosphere of purified nitrogen. 
Benzene was purified by shaking with concentrated sulfuric acid until the 
washings were colorless, washed with water, saturated aqueous sodium 
bicarbonate and saturated aqueous sodium chrloride. It was then dried with 
sodium sulfate, and distilled under nitrogen from calcium hydride. 
Infrared (IR) spectra were obtained either on a Mattson Cygnus 25 or Sirius 
100 spectrophotometer equipped with HgCdTe detectors. NMR spectra were 
obtained on an IBM NR-200 AF instrument using dried benzene-d.sub.6 as the 
solvent. Mass spectra were obtained on a VG 7070E-HF spectrometer. X-ray 
diffraction data were obtained on a Siemens D500 diffractometer using 
graphite monochromatized Cu K.sub..alpha. radiation and scintillation 
detection. Alignment was determined using the Si&lt;400&gt; reflection. All film 
thickness measurements were performed on a Tencor alpha step stylus 
profilometer. 
The structure of the surface of the CVD AlN films were examined by optical 
microscopy using an Olympus BH microscope utilizing Nomarski interference 
contrast optics. Magnification was possible from 100.times. to 500.times.. 
The microscopic surface structure of the films was examined by electron 
microscopy using a JEOL 840II Scanning Electron Microscope with a Tracor 
Northern TN-5500 EDS system for doing the energy dispersive X-ray 
analysis. 
Example I. Chemical Vapor Deposition 
(CVD) of Aluminum Nitride (AlN) 
Except as noted below in the example describing X-ray photoelectron 
spectroscpy (XPS) studies, all depositions were conducted in an all-glass, 
horizontal tube, low pressure CVD reactor. FIG. 1 shows a schematic of the 
system. The reactor tube (5) can be isolated and removed from the system 
(2) and placed directly into a Vacuum Atmosperes glove box. This allows 
coated and uncoated substrate samples to be handled and stored under an 
inert atmosphere. During a deposition, the temperature of the precursor 
was maintained (typically at 40.degree. C.) using a constant temperature 
water bath (not shown). The pressure of the system was maintained with a 
triple-stage oil diffusion pump and was estimated to be 
.ltoreq.1.times.10.sup.-4 torr. A single-stage furnace (10) was used to 
heat the 26 mm o.d. pyrex tube (5) which contained the substrate (6). A 
typical deposition was conducted at 480.degree.-550.degree. C. 
The substrate consisted of Si&lt;100&gt; cut into approximately 1 cm squares. 
These were cleaned by sequentially rinsing them in tetrachloroethylene, 
ethanol, deionized water, dilute HF, and deionized water. After air-drying 
the substrates, they were placed in a Macor ceramic holder (7) to hold 
them in a vertical position during the deposition. The distance between 
adjacent wafers was 6 mm. Masking of the wafers was achieved by placing 
two wafers in direct contact with one another. 
After placing the substrate into the reactor, it was heated from 
200.degree.-480.degree. C. under vacuum to remove adsorbed water. When the 
temperature was stabilized at 480.degree. C., the valve (3) was opened to 
connect the reaction chamber to the flask (1) containing the precursor. As 
the reaction proceeded, the material was deposited on the tube wall and 
ceramic holder as well as the Si wafers. The deposition zone, which was 
approximately 20 cm in length, began about 5 cm into the heated region. 
all the diethylaluminum azide was decomposed in the hot zone of the 
reactor. No alkyl aluminum compounds were observed in the liquid nitrogen 
cooled trap located after the reactor. At the end of the deposition, 
byproduct gases were analyzed by high resolution mass spectrometry and 
infrared spectroscopy. Ethene was the major organic product observed along 
with minor amounts of ethane. When the deposition was conducted in the 
antechamber of the XP system described below, hydrogen was observed as a 
reaction byproduct using the system's residual gas analyzer. Under the 
standard deposition conditions described, AlN films were prepared ranging 
in thickness from 0.1 to 1.3 .mu.m. As an example, a deposition time of 4 
hr generated a 0.4 .mu.m film. The rate of deposition was about 1000 
.ANG./hr. 
Example II. X-Ray Photoelectron Spectroscopy (XPS) 
All XPS surrace analysis was performed on a Physical Electronics division 
of Perkin Elmer model 555 surface analysis system. The double pass 
cylindrical mirror electron energy analyzer was operated in the retarding 
mode for constant resolution. The excitation source was Mg K.sub..alpha. 
or Al K.sub..alpha. radiation. Any sputter etching of the sample done 
during the surface analysis was done with 5 KV aragon (Ar) ions. 
In the direct transfer experiments, the AlN deposition was done in a 
reactor appended directly to the ultra-high vacuum (UHV) surface analysis 
chamber so that sample transfers and film surface analysis could be 
performed without exposing the sample to the atmosphere. The base pressure 
of the reactor was 2.times.10.sup.-5 torr with the major contaminant being 
water. Prior to performing the CVD, at least three freeze-pump-thaw cycles 
were carried out on the diethylaluminum azide to a pressure of 
4.times.10.sup.-5 torr to remove dissolved gases. 
During the AlN deposition, the azide precursor dosing vessel was maintained 
at 80.degree. C. to provide a sufficient vapor pressure of the 
diethylaluminum azide. The dosing lines leading to the reactor were also 
heated to minimize condensation within those lines. The deposition was 
done in a flow-through mode where the precursor vapor was pumped through 
the reactor vessel. The azide pressure in the reactor during the 
deposition ranged from sub-millitorr levels to 10 millitorr for different 
experiments. AlN was deposited onto tin oxide, platinum, or single crystal 
silicon by resistively heating these substrates to 550.degree. C. The 
reaction times were greater than 45 min to insure growth of a film thick 
enough to exhibit bulk properties. Annealing of the AlN subsequent to the 
deposition took place in UHV at this same temperature. 
In the axide dosing experiments, the diethylaluminum azide was condensed 
onto a clean, gold substrate held at 77.degree. K. in the reaction 
chamber, and then transferred to the UHV chamber for XPS analysis. 
XPS Analysis of the Films 
We studied the elemental composition of the thin AlN films using X-ray 
photoelectron spectroscopy (XPS). This technique also gave us some 
important information regarding the chemical state of the elements. FIG. 2 
shows the surface XPS of a powdered sample of commercially available AlN. 
The most prominent feature is the O(1s) peak located at 531.5 eV. The size 
of the peak relative to the other elements, especially Al, suggests that 
there is oxygen or water adsorbed on the surface. The C(1s) peak at 284.6 
eV is as intense as the N(1s) peak at 396.7 eV. An X-ray powder 
diffraction scan of the AlN powder was in good agreement with the ASTM 
tables. See Powder Diffraction File, International Center for Diffraction 
Data, Swarthmore, PA, at Card #25-1133. The only detectable impurity was a 
small amount of Al metal itself. The spectrum is typical of samples of AlN 
that are exposed to the atmosphere for several days. In all of the samples 
prepared in the low pressure CVD reactor, this surface coverage could be 
removed by argon sputtering to a point where Al and N were the predominant 
atoms observed. The oxygen impurity, however, was always present. 
In order to minimize the problem of oxygen contamination, we conducted 
several depositions of AlN (using Et.sub.2 AlN.sub.3) directly in the 
antechamber of the XPS spectrometer. The system offered two important 
advantages, the base pressure in the antechamber was about 10.sup.-5 torr 
(lower than the pressure in the low pressure CVD reactor), and the sample 
could be moved directly into the UHV chamber for analysis without exposure 
to the atmosphere. The films obtained were superior to films prepared in 
the CVD reactor as demonstrated by the data summarized in FIG. 3. This 
particular deposition was conducted on tin oxide at 
500.degree.-550.degree. C. for about one hour. No peaks due to the 
substrate can be seen, and the prominent peaks are due to Al and N. The 
two impurities on the surface prior to sputtering are oxygen (about 2 
atomic percent) and carbon (23%). This scan also indicates tht the N/Al 
ratio is 1.3. When Ar ion sputtering is used to remove the topmost .ANG. 
of the surface, the oxygen level drops to about 1% whereas most of the 
carton is removed. Further sputtering decreases the carbon to a very small 
but observable peak, and does little to change the oxygen content. At this 
stage the N/Al ratio is 1.0. 
High resolution XPS scans made prior to sputtering revealed relatively 
symmetric, although slightly broad, peaks for O, Al, and C. Nitrogen, 
however, gave a much more complex pattern as shown in FIG. 4, along with 
the result of the curve-fitting procedure. The large peak at low energy 
(397.1 eV) is characteristic of AlN. The three high energy peaks located 
at 398.5, 399.9, and 404.1 eV are proposed to arise from residual, 
unreacted azide bound to the film. 
In a separate experiment, we deposited the Et.sub.2 AlN.sub.3 on an Au 
surface cooled to 77.degree. K. The C and Al peaks were symmetric with 
full-width-half-maximum (fwhm) values of 3.0 eV. FIG. 5 shows the two 
distinct bands observed for the nitrogen. The larger band at lower energy 
was curve fit to two peaks and these are assigned to the two terminal 
nitrogens of the azide. (Curve fitting the data with a constant fwhm value 
among all peaks suggeted that two distinct chemical states of nitrogen 
give peaks at 399.4 and 400.9 eV). This would be expected from the 
proposed structure of Et.sub.2 AlN.sub.3 which has only one of the 
terminal nitrogens bound to the Al. The high energy peak (404.2 eV) is 
assigned to the most electron difficient nitrogen at the center of the 
azide group. The similarity in chemical shifts of the three high energy 
peaks found on the AlN deposited using Et.sub.2 AlN.sub.3 with those 
observed in the precursor itself strongly suggest some azide is still 
present in the film. In subsequent experiments with films deposted in the 
antechamber of the spectrometer, the residual azide were always observed. 
Annealing these films at 550.degree. C. under UHV conditions did remove 
all detectable traces of the azide, giving one symmetric nitrogen peak at 
397.1 eV as expected for AlN. 
Infrared Spectroscopy of the Films 
For AlN films deposited on silicon wafers, transmission infrared spectral 
measurements were possible. All of the samples of AlN gave a broad 
absorption having a maximum at 700 cm.sup.-1. This compares favorably with 
other reports of AlN films prepared by alternative methods. [L. Xinjiao et 
al., Thin Solid Films, 139, 261 (1986)]. 
X-Ray Diffraction by the Films 
Many of the films were examined by X-ray powder diffraction. The only peak 
observed was a very broad feature centered at 34.degree. C. in two-theta. 
Annealing the film at 1000.degree. C. for several hours did not 
significantly change the peak. The three most intense reflections from 
polycrystalline AlN appear betwween 33.degree. C. and 36.degree. C. The 
lack of any identifiable peaks due to AlN suggests that the sample is 
basically amorphous. The broad peak may be due to the presence of very 
small crystallites within an amorphous matrix. 
Optical and Electron Microscopy 
The thin films were examined using both optical and electron microscopy. 
The optical microscope was fitted with Nomarski optics which are useful 
for examining small changes in relief of the sample by setting up 
interference patterns between two polarized rays reflected from the 
surface. Most of the AlN films were remarkably flat. Some depositions gave 
flaky-looking films that adhered poortly to the substrate. A correlation 
was observed between the length of time the substrate spent in the HF wash 
and the appearance of flakes. In many cases, the edge of the flakes would 
be parallel to one another and intersect others at right angles. The 
electron microscopy also gave pictures showing a flat surface. Most of the 
area was observed to be featureless. 
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.