Low-pressure chemical vapor deposition process for depositing thin titanium nitride films having low and stable resistivity

An improved process for creating thin titanium nitride films via chemical vapor deposition. The films deposited using the improved process are characterized by low and stable bulk resistivity. The deposition process is performed in a low-pressure chamber (i.e., a chamber in which pressure has been reduced to between 0.1 and 2 Torr), and utilizes ammonia and the metal-organic compound tetrakis(dimethylamido)titanium, Ti(NMe.sub.2).sub.4, as precursors. Ammonia flow rate in the deposition chamber is maintained at more than approximately thirty times the metal-organic precursor flow rate. Such flow rates result in deposited TiN films having low and relatively constant bulk resistivity over time when exposed to an aerobic environment. Oxygen content of films produced using the improved process is less than 5 percent. Additionally, the deposition process is performed at a substrate temperature of at least 200.degree. C., and ideally as high at 450.degree. C. in order to minimize bulk resistivity of the deposited TiN films.

FIELD OF THE INVENTION 
This invention relates to integrated circuit manufacturing technology and, 
more specifically, to processes for depositing titanium nitride through 
chemical vapor deposition. 
BACKGROUND OF THE INVENTION 
The compound titanium nitride (TiN) has numerous potential applications 
because it is extremely hard, chemically inert (although it readily 
dissolves in hydrofluoric acid), an excellent conductor, possesses optical 
characteristics similar to those of gold, and has a melting point around 
3000.degree. C. This durable material has long been used to gild 
inexpensive jewelry and other art objects. However, during the last ten to 
twelve years, important uses have been found for TiN in the field of 
integrated circuit manufacturing. Not only is TiN unaffected by integrated 
circuit processing temperatures and most reagents, it also functions as an 
excellent barrier against diffusion of dopants between semiconductor 
layers. In addition, TiN also makes excellent ohmic contact with other 
conductive layers. 
Until little more than a year ago, reactive sputtering, the nitrogen anneal 
of an already deposited titanium layer, and high-temperature atmospheric 
pressure chemical vapor deposition (APCVD), were the three principal 
techniques available for creating thin titanium nitride films. Reactive 
sputtering and nitrogen anneal of deposited titanium result in films 
having poor step coverage, which are not useable in submicron processes. 
Chemical vapor deposition process have an important advantage in that a 
conformal layers of any thickness may 
This is especially advantageous in ultra-large-scale-integration circuits, 
where minimum feature widths may be smaller than 0.5.mu.. Layers as thin 
as 10.ANG. may be readily produced using CVD. However, TiN coatings 
prepared used the high-temperature APCVD process must be prepared at 
temperatures between 900.degree.-1000.degree. C. using titanium 
tetrachloride, nitrogen and hydrogen as reactants. The high temperatures 
involved in this process are incompatible with conventional integrated 
circuit manufacturing processes. Hence, depositions using the APCVD 
process are restricted to refractory substrates such as tungsten carbide. 
The prospects for the use of TiN films in integrated circuits improved in 
1986, when Roy G. Gordon and Steven R. Kurtz, colleagues in the Department 
of Chemistry at Harvard University, announced at the Material Research 
Society Symposium that TiN could be deposited in a new APCVD process at 
lower temperatures (1986 Mat. Res. Soc. Symp. Proc. Vol. 140, p. 277). 
Using this process, titanium chloride is reacted with ammonia within a 
temperature range of 500.degree.-700.degree. C. However, even this 
temperature is incompatible with silicon chips metallized with aluminum, 
amorphous silicon solar cells and plastics. In addition, the presence of 
hydrogen chloride gas, a corrosive by-product of the reaction, is 
undesirable. 
Some thirty years ago, D. C. Bradley and I. M. Thomas showed that, in 
solution, Ti(NR.sub.2).sub.4 complexes undergo transamination reactions 
under very mild conditions (J. Chem. Soc. 1960, 3857). Then in 1988, D. 
Seyferth, and G. Mignani reported that polymeric titanium imides could be 
transaminated with NH.sub.3 and pyrolized to form titanium nitride in the 
form of a porous solid ceramic (J. Mater. Sci. Lett. 7, p. 487, 1988). 
Based on these earlier studies, Renaud M. Fix, Roy G. Gordon, and David M. 
Hoffman, colleagues at the Department of Chemistry of Harvard University, 
hypothesized that TiN might be synthesizeable with an APCVD process 
similar to that devised by Gordon and Kurtz, but using ammonia and 
Ti(NR.sub.2).sub.4 compounds as precursors. The results of their 
confirming experiments were presented in a paper delivered at the Material 
Research Society Symposium (Mat. Res. Soc. Symp. Proc. Vol. 168). Smooth 
(i.e., mirror-like), nonporous, gold-colored TiN films were produced at 
temperatures between 100.degree. and 400.degree. C., using ammonia and the 
metal-organic compound tetrakis(dimethylamido)titanium, 
Ti(NMe.sub.2).sub.4, as precursors. Flow rate ratios of less than 10:1 of 
ammonia to Ti(NMe.sub.2).sub.4 were utilized. 
A problem with TiN films produced with chemical vapor deposition using 
Ti(NMe.sub.2).sub.4 and NH.sub.3 as precursors at the aforementioned flow 
rates is that, in spite of the apparent high quality and golden hue of the 
deposited TiN films, the resistivity of the films is highly unstable, 
increasing rather dramatically as a function of the time the film is 
exposed to the atmosphere. Since TiN films are often used for conductive 
barrier layers in integrated circuit structures, a TiN film having high 
resistivity is unsuitable for such uses. Resistive instability of TiN 
films is demonstrated by depositing a TiN film on a wafer heated to 
300.degree. C. in a low-pressure chemical vapor deposition (LPCVD) 
chamber. Approximately 5 sccm of Ti(NMe.sub.2).sub.4 was introduced into 
the chamber from a bubbler heated to 50.degree. C. into which a helium 
carrier gas was introduced at 30 sccm. When ammonia flow was 30 sccm, 
sheet resistivity of the deposited TiN film was in excess of 1 
megohms/square. When ammonia flow was increased to 50 sccm, sheet 
resistivity of the deposited TiN film was initially approximately 300 
kilo-ohms/square. Within two hours, this value had increased by 30 
percent, and after 48 hours, the sheet resistivity was in excess of 1 
mega-ohms/square. It is postulated that the cause of this instability is 
the existence of unsaturated titanium bonds in the deposited TiN films. 
When the newly-created TiN films are exposed to the atmosphere, oxygen 
most likely diffuses into the films and forms titanium dioxide. Since 
titanium dioxide (TiO.sub.2) is an exceptionally good dielectric, even a 
small amount of it will dramatically increase the resistance of a TiN 
film. SIMS analysis of the deposited TiN films indicates that oxygen 
content increases from less than 5 percent upon removal from the 
deposition chamber to as much as 20 percent 48 hours later. 
What is needed is a chemical vapor deposition process for TiN which will 
result in highly conformal films of stable, low resistivity. 
SUMMARY OF THE INVENTION 
This invention constitutes an improved process for creating thin titanium 
nitride films via chemical vapor deposition. The deposited films are 
highly stable with regard to resistivity. The deposition process takes 
place in a low-pressure chamber (i.e, a chamber in which pressure has been 
reduced to between 0.1 and 2 Torr), and utilizes ammonia and the 
metal-organic compound tetrakis(dimethylamido)titanium, 
Ti(NMe.sub.2).sub.4, as precursors. It has been determined that initial 
bulk resistivity in TiN deposited using CVD is affected by both the ratio 
of precursors (i.e., ammonia and Ti(NMe.sub.2).sub.4, in the deposition 
chamber, and the temperature at which deposition occurs. Resistive 
stability of such TiN films is affected primarily by the ratio of 
precursors. 
It has been determined that when ammonia flow rate in the deposition 
chamber is maintained at more than approximately 30 times the 
metal-organic precursor flow rate, the oxygen content of deposited TiN 
films drops to below 5 percent and remains relatively constant over time 
in an aerobic environment. It has also been determined that resistivity of 
deposited TiN films decreases with deposition temperature, with the 
minimal acceptable temperature for depositions compatible with typical 
semiconductor manufacturing processes being at least 200.degree. C, and 
with the optimum temperature for least resistivity being approximately 
450.degree. C. Reactions occurring at the higher temperature are generally 
still compatible with most integrated circuit substrates.

PREFERRED EMBODIMENT OF THE INVENTION 
The improved process for creating thin titanium nitride films via chemical 
vapor deposition will now be described in detail. The improved process 
results in films having substantially constant low resistivity. Like the 
prior art process developed by Harvard University by Renaud M. Fix, et al, 
the improved process utilizes the metal-organic compound 
tetrakis(dimethylamido)titanium, Ti(NMe.sub.2).sub.4, and ammonia as 
precursors 
Low-temperature chemical vapor deposition of TiN has heretofore been 
performed in atmospheric pressure chambers. However, such chambers must be 
flushed with an inert gas such as helium for extended periods (usually 
several hours) in order to purge the system of oxygen. The presence of 
oxygen in the deposition chamber would result in the formation of titanium 
dioxide, as well as titanium nitride. Such extended purge periods are 
incompatible with production processes, since throughput is greatly 
hampered. In order to eliminate the requirement for extended purge 
periods, the improved deposition process is performed in a low-pressure 
chamber. 
Following introduction of the substrate on which the deposition is to be 
made into the deposition chamber, a vacuum is applied to the chamber in 
order to reduce internal pressure to between 0.1 and 2 Torr. 
Ti(NMe.sub.2).sub.4 is converted to a gaseous state in a bubbler heated to 
approximately 50.degree. C. Helium carrier gas, at a flow rate of 
approximately 50 sccm, is introduced into the bubbler, after which it is 
piped in to the deposition chamber, carrying the gaseous 
Ti(NMe.sub.2).sub.4, which is equivalent to approximately 5 sccm. Ammonia 
gas is also introduced into the low pressure deposition chamber. 
Initial resistivity (not to be confused with resistive stability, which 
will be discussed later) in TiN films has been shown to be significantly 
affected by two variables, one of which is the ratio of ammonia to 
Ti(NMe.sub.2).sub.4 in the deposition chamber. FIG. 1 depicts the initial 
bulk resistivity of deposited TiN films (i.e., immediately following 
deposition) as a function of the ammonia flow rate. With the flow of 
Ti(NMe.sub.2).sub.4 being maintained at a constant value of approximately 
5 sccm, the ammonia flow rate was varied between 50 sccm and 400 sccm. It 
will be noted that at an ammonia flow rate of 50 sccm, initial bulk 
resistivity was nearly 200,000 .mu.ohm-cm. An ammonia flow rate of 150 
sccm, the resistivity value had dropped to approximately 7000 .mu.ohm-cm. 
At a flow rate of 250 sccm, the resistivity had dropped to approximately 
1700 .mu.ohm-cm. Very little drop in resistivity was noted as the ammonia 
flow was increased beyond 250 sccm. It is hypothesized that at an ammonia 
flow rate of 250 sccm, TiN having a stoichiometry of close to 1:1 is 
deposited. Increasing the ammonia flow beyond the 250 sccm rate therefore 
will not result in a significant change from this stoichiometric 
relationship. 
Initial resistivity is also a function of deposition temperature. FIG. 2 
depicts initial bulk resistivity as a function of deposition temperature. 
It will be noted that resistivity of deposited TiN films drops 
dramatically between 150.degree. and 200.degree. C. Resistivity continues 
to drop at a fairly constant rate until 350.degree. C., at which 
temperature, the drop in resistivity accelerates up to 400.degree. C. The 
rate of drop then slows between 400.degree. and 450.degree. C. 
Temperatures around 450.degree. C. are considered optimum for conventional 
integrated circuit manufacturing processes, as aluminum melts at 
650.degree. C. 
FIG. 3 shows the effect of varying rates of ammonia flow on the resistive 
stability deposited TiN films. When ammonia flow was 50 sccm, bulk 
resistivity is highly unstable over time, increasing from a low of 
approximately 18,000 .mu.ohms-cm immediately following deposition to 
approximately 27,000 .mu.ohms-cm 43 hours later. On the other hand, when 
ammonia flow was 200 sccm, initial bulk resistivity (i.e., immediately 
following deposition) was 2,600 .mu.ohms-cm and increased by only about 10 
percent over the same 43 hour period. It was hypothesized that the cause 
of this instability was the existence of unsaturated titanium bonds in the 
deposited TiN films. Thus, when the newly-created TiN films are exposed to 
the atmosphere, oxygen most likely diffuses into the films and forms 
titanium dioxide. Since titanium dioxide (TiO.sub.2) is an exceptionally 
good dielectric, even a small amount of it will dramatically increase the 
resistance of a TiN film. This hypothesis was substantiated by SIMS 
analysis of the deposited TiN films, which indicated that oxygen content 
increases from less than 5 percent upon removal from the deposition 
chamber to as much as 20 percent 48 hours later. SIMS analysis also 
indicated that when ammonia flow is maintained at more than approximately 
thirty times the metal-organic precursor flow rate, the oxygen content of 
deposited TiN films drops to below 5 percent and remains relatively 
constant over time in an aerobic environment. 
Deposition temperature apparently does not markedly affect resistive 
stability of deposited TiN films, at least between 150.degree. and 
450.degree.. Most likely, the films deposited at the lower temperatures 
are either porous or have stable, but resistive impurities such as carbon 
imbedded in them. 
Although only a single embodiment of the inventive process has been 
disclosed herein, it will be obvious to those having ordinary skill in the 
art that modifications and changes may be made thereto without affecting 
the scope and spirit of the invention as claimed.