Method of low temperature plasma enhanced chemical vapor deposition of tin film over titanium for use in via level applications

A titanium/titanium nitride film stack can be formed with reduced amounts of impurity by depositing onto a substrate a film of titanium using plasma-enhanced chemical vapor deposition of titanium tetrachloride and hydrogen. This film is then subjected to a hydrogen/argon plasma which significantly reduces the chlorine content of the titanium film. The titanium film can then be subjected to an ammonia plasma which will form a thin layer of titanium nitride which is then coated with a thick layer of titanium nitride using plasma-enhanced chemical vapor deposition of titanium tetrachloride and ammonia. The hydrogen/argon anneal significantly reduces the chlorine content of the titanium film and thus the chlorine content at the titanium substrate interface, particularly when the substrate contains aluminum. This enhances the overall reliability of the formed product.

BACKGROUND OF THE INVENTION 
Chemical vapor deposition is currently used to form titanium/titanium 
nitride film stacks that can be used at the via level. There are several 
ways to initially deposit the titanium film, including sputtering and 
chemical vapor deposition. In chemical vapor deposition, the titanium 
precursor, typically titanium tetrachloride or other titanium tetrahalide, 
is energized to form elemental titanium which is then deposited on a 
substrate. A plasma can also be used to excite the titanium. In this 
method, the plasma, along With for example titanium tetrachioride and 
hydrogen, are formed into a plasma using RF energy. The plasma is then 
directed at a substrate and the titanium forms on the substrate. 
One problem associated with the deposition of titanium using chemical vapor 
deposition or plasma enhanced chemical vapor deposition of a titanium 
halide is the residual halide atoms on the surface of the titanium. 
Particularly when the titanium is deposited over aluminum, this halide can 
react with the aluminum forming aluminum halide which has a high 
resistance. This is a significant problem in via-level applications. This 
affects the operability, reliability and durability of the formed product. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of forming titanium/titanium nitride stacks in via-level applications 
wherein the halide impurity is minimized. 
The present invention, in turn, is premised on the realization that the 
halide impurities can be minimized in the formation of titanium/titanium 
nitride stacks wherein the titanium film is subjected to a hydrogen/argon 
plasma after the deposition of titanium by plasma-enhanced chemical vapor 
deposition of titanium tetrahalide. The hydrogen/argon plasma reacts with 
and removes residual chlorine species from the aluminum surface. 
Subsequent to the hydrogen plasma, a TiN film is deposited by 
plasma-enhanced chemical vapor deposition. Further, an ammonia-based 
plasma can be used to further remove chlorine and form a passivating 
nitride film that reduces the probability of reaction of chlorine with 
aluminum during the TiN deposition. 
The objects and advantages of the present invention will be further 
appreciated in light of the following detailed description and drawings in 
which:

DETAILED DESCRIPTION 
According to the present invention, titanium nitride film is deposited on a 
titanium film to form a titanium/titanium nitride stack. For use in the 
present invention, the titanium film is deposited using plasma-enhanced 
chemical vapor deposition of titanium tetrahalide, preferably titanium 
tetrachloride. Subsequent to the deposition of the titanium film, the film 
is subjected to an argon hydrogen plasma, and then the titanium nitride is 
deposited. This can be conducted in a single reaction chamber. 
Although not limited to any particular apparatus, one preferred apparatus 
for use in the present invention is a chemical vapor deposition reactor 20 
shown in FIG. 1. 
Reactor 20, and specifically reaction space 24 within housing 22, may be 
selectively evacuated to various different internal pressures--for 
example, from 0.5 to 100 Torr. The susceptor 26 is coupled to a variable 
speed motor (not shown) by shaft 30 such that the susceptor 26 and 
substrate 28 may be rotated at various speeds such as between 0 and 2,000 
rpm. Susceptor 26 includes a resistance heating element (not shown) 
coupled to the susceptor 26 to heat substrate 28. 
Extending downwardly from the top wall 32 of housing 22 is a cylinder 
assembly 34 which is attached to a gas-dispersing showerhead 36. 
Showerhead 36 is coupled to an RF energy source 38 by an appropriate RF 
feed line assembly 40 which extends through cover 46 which may, if 
necessary, include a heat pipe to dissipate unwanted heat. A sealing 
structure 49 seals the opening around feed line assembly 40. Plasma and 
reactant gases are introduced into flow passage 44 by concentric rings or 
halos 50, 52. The concentric rings 50, 52 include a number of holes which 
evenly dispense the gases around the flow passage 44. Ring 50 is connected 
to a gas supply through line 56, while ring 52 is connected to a supply by 
line 58. 
An insulator ring 62 separates cylinder 34 and showerhead 36. Cylinder 34 
is electrically grounded by ground line 61. 
The insulator ring 62 preferably has an outer diameter approximately the 
same as the outer diameter of showerhead 36 and a width dimension which 
ensures complete separation of cylinder 34 and showerhead 36 along the 
entire attachment interface between the cylinder and showerhead. The 
insulator ring is preferably made of quartz material approximately 0.75 
inches thick. 
Showerhead/electrode 36 contains a plurality of dispersion holes 64 which 
disperse the flow of gas over substrate 28. The showerhead 36 includes a 
stem 68. Stem 68 is formed integrally with the showerhead 36 and forms 
part of the RF line assembly 40 which connects to showerhead 36. The 
showerhead 36, including stem 68, is formed of an electrically conductive 
material, preferably Nickel-200. 
The RF power source, through RF feed line assembly 40, biases the 
showerhead 36 so that the showerhead functions as an RF electrode. The 
grounded susceptor 26 forms another parallel electrode. An RF field is 
created, preferably between showerhead 36 and susceptor 26. The RF field 
created by the biased showerhead/electrode 36 excites the plasma gases 
which are dispensed through holes 64 so that a plasma is created below 
showerheadlelectrode 36. 
The showerhead employed is about 0.25 inches thick, having a diameter of 
about 17.3 cm and 600 holes. The number of holes is not critical and could 
easily be varied from 100 holes to 1,000 or more holes. The holes are 
preferably less than 1.5 mm in diameter and are more preferably about 0.75 
mm. This prevents the plasma from being generated in the hole, thereby 
reducing efficiency. 
The gas flow from injector rings 50 and 52 is allowed to develop within the 
length of the cylinder 34 as it travels to the showerhead 36. It is 
desirable for the velocity profile of the incoming plasma gases passing 
through showerhead 36 to be fully developed before they reach the rotating 
surface of the substrate 28. Due to the proximity of the showerhead to the 
surface, that profile must develop in the cylinder 34. 
Utilizing cylinder 34 shown in FIG. 1, the showerhead-to-susceptor spacing 
may be reduced to approximately 30 to 20 mm or less because the velocity 
profile develops in cylinder 34. Therefore, the length of cylinder 34 from 
the injector rings 50 and 52 to showerhead 36 should be 40 to 100 mm. As 
the gases pass through the showerhead 36, the pressure drop across the 
showerhead 36 flattens out the velocity profile of the gases. As the gases 
approach showerhead/electrode 36 and pass therethrough, they are excited 
into a plasma which contacts surface 29. 
Preferably, the showerhead 36 can be from about 10 cm to about 10 
millimeters from the susceptor, with 20 mm preferred. It is preferred to 
have the showerhead as close as possible to the substrate while still 
permitting the substrate or wafer to be removed, although this is not 
critical for practicing the present invention. 
A pumping effect is created by the rotating susceptor 26. The plasma 
radicals and ions are drawn to the upper surface 29 of substrate 28. 
Generally, the rotation rate can vary from 0 rpm to 1500 rpm. About 100 
rpm is preferred. Further, matched flow does not appear to be critical but 
can be employed. 
With a spacing of about 25 mm between the showerhead and the substrate 28, 
the created plasma is much closer to the substrate surface 29. With the 
showerhead 36 acting as an RF electrode, a more uniform plasma is 
generated, therefore enhancing the uniformity of radical and ion density 
at the substrate 28 and thereby improving reaction rate. 
When employing this apparatus, the electrode is biased--generally at a 
frequency from about 13.56 MHz (a frequency which is authorized by the 
Federal Communication Commission)--down to about 55 KHz. The power of the 
electrode is generally set at about 250 watts. 
Using reactor 20, the titanium film is deposited by plasma-enhanced 
chemical vapor deposition, as disclosed in U.S. Pat. No. 5,567,243. 
According to this method, titanium tetrahalide is combined with a diluent 
gas and formed into a plasma using RF energy. This is then deposited upon 
a substrate. 
The substrate can typically be any semiconductor substrate such as silicon, 
thermal oxides, patterned wafers including metal layers and in particular 
aluminum layers. 
The titanium tetrahalide can be titanium tetrabromide, titanium tetraiodide 
or titanium tetrachloride. Titanium tetrachloride is preferred due to 
cost. This will be combined with an inert diluent gas, preferably 
hydrogen. Other inert diluent gases include helium, argon, neon and xenon. 
Generally, the molecular ratio of diluent to titanium tetrachloride is 
from about 1:1500 to about 5:1500. 
Adhesion between aluminum and titanium is promoted by minimizing corrosion 
of the aluminum layer. Corrosion is largely the result of exposure of the 
aluminum layer to halide ions released from the titanium tetrahalide 
during deposition. By reducing the flow rate of titanium tetrahalide, the 
corrosion of the aluminum layer is reduced and adhesion is promoted. 
Reduction of the titanium tetrahalide flow rate also reduces deposition 
rate, allowing dissociated titanium atoms additional time to locate stable 
sites in the underlying aluminum layer. This additional time is 
particularly beneficial due to the low thermal energy and reduced thermal 
motion of the titanium atoms at reduced process temperatures. 
The flow rate will vary, depending upon the particular reactor. With the 
present reactor, a flow rate of TiCl.sub.4 of 3 to 7 sccm is preferred; 
and a flow rate of hydrogen of 1000 to 5000 sccm is preferred. 
The RF energy can also be varied, depending upon the particular 
application. The power of the RF energy can be from about 200 watts to 
about 1 kilowatts at about 450 KHz to 1 MHz. 
The reaction chamber also provides for control of the pressure. Generally, 
the pressure will be from 500 millitorr up to about 10 torr. Under these 
conditions, the deposition rate should be about 50 .ANG./minute and 
therefore the deposition time can vary from about 30 seconds to about 90 
seconds, depending upon the desired application. 
As shown in the apparatus, the substrate is held on a susceptor 26 which 
can be rotated. The rotation rate can be from about 0 rpm up to about 1500 
rpm. This facilitates a pumping action which draws the plasma to the 
surface of the substrate. Also, using the present invention the substrate 
temperature can be adjusted by adjusting the temperature of the susceptor. 
Generally, to avoid damaging an underlying aluminum layer, the substrate 
temperature should be about 400 to about 450.degree. C. It is desirable to 
minimize the temperature in each separate step in order to avoid 
deformation of the aluminum layer. However, with lower temperatures 
increased halide formation occurs. 
The titanium film is subjected to a plasma immediately after deposition. 
Preferably, the plasma is formed from a gas selected from hydrogen, argon, 
mixtures thereof, as well as helium. It is desirable to have at least 1 to 
5% hydrogen to react with the halide to form the hydrogen halide or 
hydrogen chloride compound which is then vented from the reaction chamber. 
During the plasma treatment, the RF electrode will operate at about 200 to 
about 700 watts, with the frequency being from about 450 KHz to 1 MHz. In 
order to preserve the underlying titanium film and substrate, the 
temperature should be kept at from about 400 to about 450.degree. C. 
Generally, the flow rate should be about 1000 SCCM with the reaction 
pressure varying from about 500 millitorr to about 10 torr. This plasma 
treatment is continued for a period of 30 to 90 seconds, with about 60 
seconds being preferred. Subsequent to the hydrogen/argon plasma 
treatment, the titanium film can be nitrided using a nitrogen-containing 
plasma. 
Two nitriding gases can be used in the present invention. These are ammonia 
and nitrogen. Ammonia is preferred because of its better reactivity. The 
plasma Is created by simply subjecting the nitriding gas to an RF 
electrode at elevated temperature and reduced pressure. The titanium film 
is then contacted with this plasma, thereby forming titanium nitride. 
Preferably for use in this nitridization step, the RF electrode will be 
from 100 watts up to the power at which devices are damaged, i.e., about 5 
Kilowatts. Approximately 250 watts is adequate. The frequency of the RF 
electrode should be from about 55 MHz to about 33 KHz. As the frequency is 
lowered, the temperature of the treatment can also be reduced. The upper 
frequency is a function of Federal Communication Commission regulation and 
equipment availability. However, as described below, lower frequencies are 
generally preferred. 
In order to preserve the underlying titanium film and substrate, the 
temperature should be kept at 400 to 450.degree. C. As the frequency of 
the electrode is reduced, the temperature can also be reduced. These 
temperatures provide for excellent nitridization and reduce thermal 
degradation of the underlying substrate and titanium film. 
The time, pressure and flow rates, as well as temperature, can all be 
varied to increase or decrease the reaction rate of the nitridization. 
Generally, the minimum flow rate of the nitridization gas should not be 
less than about 10 sccm. At flow rates above 5,000 sccm there is increased 
unvented gas without any benefit although flow rates above 10,000 sccm 
will function. But precise flow rate is not critical for practicing the 
present invention. Therefore, about 1,000 sccm is preferred. The time can 
range from 20 seconds up to ten minutes, however 5 minutes is generally 
acceptable. 
The reaction pressure must be subatmospheric and generally will vary from 
about 500 millitorr to about 3 torr. If one desired to decrease the time, 
the flow rate and temperature could be increased. Likewise, with reduced 
temperature increased time is preferred. Likewise, when reducing the 
temperature, the RF frequency can also be reduced. Plasma power can be 
increased or decreased, likewise, to alter the time or reaction rate. 
The nitridization gas, preferably ammonia or a combination of nitrogen and 
hydrogen, is introduced through injectors 50 and 52 and flows through the 
cylinder 34 and through showerhead 36, which creates the plasma from the 
gas. The flow rate of the gas into cylinder 34 is generally about 1,000 
sccm and the pressure within the reaction chamber itself is maintained at 
about 1 to 3 torr (3 is preferred). The heated susceptor 26 is rotated at 
a rotation rate of about 100 rpm which, in effect, pumps gas laterally 
away from the titanium surface 29 as the plasma is forced downwardly 
toward the titanium surface. This reaction continues for about five 
minutes. Unreacted ammonia, along with hydrogen, will (as shown by arrows 
65) be pulled around baffles 27 and from the reaction chamber 24 through 
vent 53. 
The titanium film 29 will take on a gold luster, indicating the formation 
of titanium nitride. This nitridization step is optional and may be 
omitted. However, it does decrease further the halide content of the 
titanium film, further reducing aluminum corrosion and TiN film adhesion. 
Next, the titanium film is subjected to a plasma-enhanced chemical vapor 
deposition of titanium nitride. The film thickness of the titanium nitride 
should be from about 200 to about 500 .ANG.. 
In depositing the titanium nitride film, a plasma of reactant gases is 
created using apparatus 20 at showerhead 36. The reactant gases include 
titanium tetrachloride, ammonia and a diluent. Although diluents such as 
hydrogen, helium and argon can be employed, nitrogen is preferred. These 
are combined together and introduced into cylinder 34. 
Cylinder 34 is maintained at a pressure from about 0.5 to about 20 torr 
with about 5 torr being preferred. The substrate is maintained at a 
temperature of about 400 to about 500.degree. C. with about 450.degree. C. 
being preferred. The substrate is generally heated by providing heat from 
the support 30. The support itself is preferably rotated at about 100 rpm 
or more simply to provide for more even distribution. However, the 
substrate need not be rotated at all. 
The concentration of the gases is controlled by flow rate. Generally, the 
titanium tetrachloride will be introduced at a flow rate of about 1 to 
about 40 sccm, with about 10 sccm being preferred. The partial pressure of 
the TiCl.sub.4 must be sufficiently low to form TiN. If the TiCl.sub.4 
partial pressure becomes too high, a black powder is formed which is not 
TiN. When the total pressure is 5 torr, the partial pressure of TiCl.sub.4 
should be less than 0.02 torr, preferably 0.01 torr to 0.001 torr. At the 
lower pressures (i.e., 0.0001 torr), the reaction rate is significantly 
reduced and the step coverage can be unacceptable. As the total pressure 
increases from 5 torr, the partial pressure of TiCl.sub.4 can be increased 
accordingly. For TiN to be useful, the film on the substrate should be 
adherent and continuous. Films of this nature are gold in color. The black 
powder that forms is non-adherent (it can be wiped off readily). 
Therefore, the upper limits of the partial pressure of TiCl.sub.4 is that 
partial pressure at which a black powder begins to form on the substrate. 
This, of course, can vary depending on the total pressure. Generally, the 
molar ratio of ammonia to TiCl.sub.4 will be from 2:1 (ammonia to 
TiCl.sub.4) up to 100:1. At this higher rate, the reaction rate will be 
very low. Preferably, the ratio will be about 10:1. 
Generally the ratio of diluent to ammonia will range from about 10:1 up to 
about 10,000:1. 
EXAMPLE 1 
In order to demonstrate the present invention, a titanium film was 
deposited and covered with a titanium nitride film. The titanium was 
deposited under the following conditions: 
TiCl 3.5 SCCM 
H.sub.2 1500 SCCM 
Reaction Pressure 5 torr 
Substrate 
Rotation Rate 100 rpm 
Substrate Temp. 400.degree. C. 
Susceptor Temp. 420.degree. C. 
Titanium Thickness 150 .ANG. 
Reaction Rate 2.5 .ANG./second 
Pressure 5 torr 
RF Power 250 watts 
Frequency 450 KHz 
This was then subjected to an ammonia/plasma anneal under the following 
conditions: 
Ammonia flow rate 450 SCCM 
Susceptor Temperature 420.degree. C. 
Substrate Temperature 400.degree. C. 
Pressure 5 torr 
Reaction time 60 seconds 
RF Power 500 watts 
Frequency 450 KHz 
Then a TiN film was deposited under the following reaction conditions: 
TiCl Flow Rate 10 SCCM 
NH.sub.3 Flow Rate 100 SCCM 
H.sub.2 Flow Rate .0. 
Reaction Pressure 5 torr 
Rotation Rate 100 rpm 
Substrate Temp. 400.degree. C. 
Susceptor Temp. 420.degree. C. 
Reaction Time 180 seconds 
Reaction Rate 35 .ANG./second 
RF Power 300 watts 
Frequency 450 KHz 
FIG. 2 shows an AES spectra of the Ti/TiN film stack that was deposited 
without an intermediate argon/hydrogen plasma between the titanium and TiN 
deposition. The chlorine is accumulated at the interface of the titanium 
and aluminum films, bound as either TiCl or the thermodynamically favored 
Al--Cl species. This film stack was deposited without using the 
hydrogenlargon plasma treatment between the titanium and titanium nitride 
films. With this type of process, the chlorine concentration at the 
interface is measured to be 5-6 atomic percent by Auger. In this case, the 
film stack did receive a passivating ammonia plasma, but did not receive 
the additional argon/hydrogen plasma to remove additional chlorine 
species. 
EXAMPLE 2 
A titanium film was deposited under the conditions set forth in Example 1. 
This film was subsequently subjected to an argon/hydrogen plasma under the 
following conditions: 
Argon 300 SCCM 
H.sub.2 1500 SCCM 
Reaction Pressure 5 torr 
Substrate 
Rotation Rate 100 rpm 
Substrate Temp. 400.degree. C. 
Susceptor Temp. 420.degree. C. 
Reaction Time 60 seconds 
RF Power 250 watts 
Frequency 450 KHz 
FIG. 3 shows a similar AES depth profile (similar to FIG. 2) of chlorine 
deposited onto aluminum that has been treated with a post-deposition 
argon/hydrogen plasma. No TiN was deposited after the titanium, so no 
ammonia plasma nitridization was used forthis film stack. In this case, 
the chlorine measured at the interface has been reduced to less than 3 
atomic percent--one-half the value of Example 1, in spite of the fact that 
there was no ammonia plasma which should also remove chlorine. 
Accordingly, this demonstrates that subjecting the titanium film to a 
hydrogen/argon plasma significantly reduces chlorine content at the 
titanium/aluminum interface. This is very beneficial, particularly given 
the fact that it operates at approximately the same pressures and 
temperatures as the plasma-enhanced chemical vapor deposition of titanium. 
Thus, it can be conducted in the same module with no transfer steps. The 
time required is only approximately 30 seconds in duration, and it permits 
the immediate deposition by plasma-enhanced chemical vapor deposition of 
TiN. 
This has been a description of the present invention, along with the 
preferred method of practicing the present invention currently known. 
However, the invention Itself should only be defined by the appended 
claims, wherein we claim: