Method of forming a metal layer on a substrate, including formation of wetting layer at a high temperature

A wetting layer is formed on a substrate at a relatively high process temperature (e.g., the temperature of the substrate and/or the temperature within a process chamber in which the wetting layer is formed). A metallization layer that is subsequently formed on the wetting layer adheres to the wetting layer better than the metallization layer would adhere to the wetting layer if the wetting layer was formed at a lower process temperature. The high process temperature causes the density of the wetting layer to be increased, so that, consequently, the wetting layer has a smoother surface to which the metallization layer can adhere.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the formation of a layer of metal on a substrate 
and, in particular, to the formation of a wetting layer at high 
temperature that results in improved adhesion of a subsequently formed 
metallization layer to the wetting layer. Most particularly, the invention 
relates to the formation on a semiconductor substrate of a titanium (or 
titanium alloy) wetting layer at high temperature, and to the subsequent 
formation of an aluminum (or aluminun alloy) metallization layer on the 
titanium wetting layer. 
2. Related Art 
Formation of a layer of metal on a substrate (or on other material 
previously formed on the substrate) is a common step in the production of 
some devices, such as, for example, semiconductor devices. A metal layer 
can be formed as part of such a device to provide an electrically 
conductive trace, for example. Or, metal can be formed in a via or hole 
previously formed in the device to provide electrical contact between 
electrically conductive material (e.g., electrically conductive traces or 
other electrically conductive regions) formed adjacent to opposite ends of 
the via or hole. The formation of a metal layer can be subject to several 
problems, such as cusping and voiding, that may degrade the electrical 
performance of the metal layer. In particular, metal formed in vias having 
a high aspect ratio (i.e., ratio of the depth of the via to the width or 
diameter of the via) or over steps having a relatively large height has 
been subject to such problems. 
In some methods of forming a layer of metal on a substrate, a wetting layer 
is first formed on the surface on which the metal layer is to be formed. A 
metal material (i.e., the "primary" metal with which it is desired to form 
the metal layer) is then formed on the wetting layer. The material with 
which the wetting layer is formed (which is, itself, often a metal 
material) attracts the primary metal material better than would the 
material of the surface on which the wetting layer is formed, so that the 
primary metal material more completely covers the surface on which it is 
desired to form the metal layer. 
For example, in the production of semiconductor devices, aluminum is often 
formed (by, for example, deposition) on a semiconductor wafer. Before 
formation of the aluminum on a substrate surface, a wetting layer of a 
suitable material (such as titanium, titanium nitride, or a composition of 
titanium and tungsten) can be formed on that surface so that when the 
aluminum metallization layer is formed, the aluminum adheres more 
completely and uniformly to the substrate surface than would otherwise be 
the case. Previously, the wetting layer has been formed at relatively low 
process temperatures (e.g., while the process chamber and wafer are at a 
temperature less than about 200.degree. C. and, frequently, about 
40.degree. C.) using a standard deposition process. The low temperature 
has been used for several reasons. First, standard heaters (such as the 
heater that is part of the widely used Endura sputtering system made by 
Applied Materials of Santa Clara, Calif.) that are used to heat the 
semiconductor wafer during formation of the wetting layer are not designed 
for extended operation at high temperatures (e.g., the recommended maximum 
operating temperature for the Endura heater is 450.degree. C.). Operation 
of a standard heater at elevated temperatures undesirably shortens the 
life of the heater. Second, the wetting layer is typically formed 
immediately after and immediately prior to other process steps that occur 
at a relatively low temperature (e.g., at or below about 200.degree. C.). 
For example, typically, the wetting layer is formed right after a preclean 
step or a sputter etch step, each of which are performed at a low 
temperature as described above. Further, the aluminum metallization layer 
(or at least an initial portion of the aluminum metallization layer) 
formed on the wetting layer is often formed at a low temperature. (This 
may be done, for example, because aluminum attaches to a wetting layer of 
titanium better when the aluminum is deposited at a relatively low 
temperature.) Thus, absent a reason to do otherwise, it is desirable to 
form the wetting layer at a temperature that is approximately the same as 
the temperature at which the prior and subsequent process steps will be 
performed (e.g., a temperature below about 200.degree. C.), so that time 
is not unnecessarily spent heating and cooling the wafer between process 
steps. 
Standard deposition processes have been modified to produce other processes 
for depositing a wetting layer on a semiconductor wafer. Collimated 
deposition and ionized metal plasma (IMP) deposition are two such 
processes. 
In collimated (or "coherent") deposition, a collimator (honeycomb) is 
positioned in a sputtering chamber between the sputtering target and the 
wafer. The collimator directs the sputtered atoms of the wetting material 
in a direction perpendicular to the wafer, so that the likelihood that the 
atoms will fall to the bottom of a deep via, for example, is increased. 
Generally, collimated deposition produces a higher quality wetting 
layer--denser and smoother--than does the standard deposition process. 
However, because a collimator must be positioned in the sputtering 
chamber, the collimated deposition system is more complex and requires 
more frequent preventative maintenance than a standard deposition system. 
Additionally, the deposition rate of a collimated deposition process is 
slower than that for a standard deposition process (decreasing wafer 
throughput). These differences make collimated deposition a more expensive 
process than the standard deposition described above. 
In IMP deposition, radiofrequency (RF) power is used to control the 
directionality of the sputtered atoms of the wetting material. IMP 
deposition can produce a wetting layer that is even more dense and smooth 
than that produced by collimated deposition. However, like collimated 
deposition, IMP deposition is somewhat more expensive than a standard 
deposition process. Further, IMP deposition is still being developed; 
commercial IMP deposition systems are not yet available. Even when such 
systems are available, the high cost of replacing existing standard 
deposition systems with IMP deposition systems will represent a 
significant deterrent to the use of IMP deposition systems. 
In another previous process for forming a metal layer on a semiconductor 
wafer, tungsten (actually, a combination of tungsten and fluorine, 
WF.sub.6) is formed (e.g., deposited) as the primary metal material (i.e., 
as the metallization layer). Tungsten is sometimes used instead of 
aluminum as a metallization layer on a semiconductor wafer because, for 
example, tungsten can more easily fill in high aspect ratio vias. In a 
tungsten metallization process, titanium is first formed on the surface 
(which is typically silicon) on which the metal layer is to be formed. The 
titanium does not function as a wetting layer, but is formed, instead, 
because it provides better contact resistance with silicon than does 
tungsten. However, since titanium reacts with the fluorine that is 
combined with the tungsten, a barrier layer (typically made of titanium 
nitride, though other materials could be used) must be formed over the 
titanium. The tungsten (WF.sub.6) is then formed over the barrier layer. 
In a previous tungsten metallization process, titanium nitride has been 
deposited as a barrier layer while the process chamber and wafer are held 
at a relatively elevated temperature (e.g., the wafer can be at a 
temperature of about 375.degree. C.). The elevated temperature has been 
used to reduce film stress during deposition of the titanium nitride. In 
such a tungsten metallization process, the titanium (which has typically 
been deposited using a collimated deposition process) has been deposited 
at approximately the same temperature as that used for deposition of the 
titanium nitride. This is done not for any reason associated with the 
titanium deposition, but, rather, occurs incidentally so that there is no 
need to heat the wafer between the titanium deposition and the subsequent 
titanium nitride deposition. 
It is desirable to provide a method of forming a metal layer on a substrate 
that improves upon the capability of the above-described methods to 
produce a high quality metal layer, e.g., a metal layer having few or no 
voids. In particular, it is desirable to provide a method of forming a 
wetting layer for use in forming a metal layer that enhances the 
capability to produce such a high quality metal layer and that facilitates 
the implementation of the process steps necessary to produce a metal layer 
(e.g., enlarges the process window for those steps). 
SUMMARY OF THE INVENTION 
According to the invention, a wetting layer is formed on a substrate at a 
relatively high process temperature (e.g., the temperature of the 
substrate and/or the average temperature within a process chamber in which 
the wetting layer is formed). A metallization layer that is subsequently 
formed on the wetting layer adheres to the wetting layer better than the 
metallization layer would adhere if the wetting layers had been formed at 
a lower process temperature. The high process temperature causes the 
density of the wetting layer to be increased, so that, (consequently, the 
wetting layer has a smoother surface to which the metallization layer can 
adhere. 
In one embodiment of the invention, a method of forming a layer of metal on 
a substrate while the substrate is positioned in a process chamber 
comprises the steps of: i) forming a wetting layer on a first substrate 
surface while the process chamber temperature is greater than or equal to 
about 250.degree. C.; and ii) forming a metallization layer on the wetting 
layer. The method is more preferably performed by heating the process 
chamber to a temperature greater than or equal to about 275.degree. C., 
even more preferably to a temperature greater than or equal to about 
400.degree. C., and most preferably to a temperature greater than or equal 
to about 450.degree. C. The method can advantageously be used in forming a 
wetting layer of titanium. In particular, the method can advantageously be 
used in forming a wetting layer of titanium on which a metallization layer 
of aluminum is then formed. The method can be used with any method for 
forming a metallization layer, even a method in which, immediately 
subsequent to the formation of the wetting layer, some or all of the 
metallization layer is formed with the process chamber at a temperature 
that is lower than the process chamber temperature during the formation of 
the wetting layer, in contrast to the above-discussed method for 
depositing a titanium wetting layer followed by an aluminum metallization 
layer. 
In another embodiment of the invention, a method of forming a layer of 
metal on a substrate while the substrate is positioned in a process 
chamber comprises the steps of: i) forming a wetting layer on a first 
substrate surface while the process chamber temperature is greater than or 
equal to a predetermined temperature; and ii) forming at least a portion 
of a metallization layer on the wetting layer while the process chamber 
temperature is less than or equal to the predetermined temperature. In a 
particular embodiment, the predetermined temperature is about 250.degree. 
C. In another particular embodiment, the predetermined temperature is 
about 400.degree. C. In a still further embodiment based on this latter 
embodiment, at least a portion of the metallization layer is formed while 
the process chamber temperature is less than or equal to about 250.degree. 
C. 
In yet another embodiment of the invention, a method for use in forming a 
layer of metal on a substrate, the steps of the method being performed in 
a process chamber, comprises the steps of: i) heating the process chamber; 
and ii) forming a wetting layer on a substrate surface while the process 
chamber temperature is greater than or equal to about 250.degree. C. The 
method is more preferably performed by heating the process chamber to a 
temperature greater than or equal to about 275.degree. C., even more 
preferably to a temperature greater than or equal to about 400.degree. C., 
and most preferably to a temperature greater than or equal to about 
450.degree. C. The method can advantageously be used in forming a wetting 
layer of titanium. 
As discussed above, forming a wetting layer in accordance with the 
invention enables a metallization layer to be formed more smoothly on the 
wetting layer, improving the quality of the overall metal layer so formed. 
The improved wetting layer according to the invention can thus enlarge the 
process window for the formation of the metallization layer. For example, 
the process temperature (e.g., the substrate temperature) during a hot 
deposition step of a two-step aluminum deposition process can be reduced 
while still producing a metal layer of the same quality. This is 
particularly advantageous, since reduction in the substrate temperature 
reduces the likelihood that the substrate temperature will become high 
enough to cause damage to the substrate (e.g., previously deposited metal 
layers on the substrate). Alternatively, if the other aspects of the 
process of forming a metal layer are left unchanged, a wetting layer 
formed in accordance with the invention improves the quality of the 
overall metal layers formed using the wetting layer, thus enabling 
formation of more difficult metal layers (e.g., high aspect ratio vias). 
Consequently, it is possible to use aluminum deposition for situations in 
which it previously would have been necessary to use tungsten deposition, 
thereby enabling the elimination of process steps and, consequently, 
producing significant savings in cost and time in producing a metal layer 
in these situations. 
Further, using a relatively high process temperature in accordance with the 
invention enables formation of a wetting layer using a standard deposition 
process that equals or approaches the quality of a wetting layer produced 
using collimated or IMP deposition. Thus, a high quality wetting layer can 
be produced without need to modify or replace existing deposition 
processes and equipment. Moreover, even collimated and IMP deposition 
processes can be improved by increasing the process temperature in 
accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a simplified cross-sectional view of an apparatus with which the 
invention can be implemented. As shown in FIG. 1, the apparatus is a 
conventional sputtering chamber. However, generally, the apparatus can be 
any apparatus that is configured to enable a metal to be formed (e.g., 
deposited) on a surface of a substrate. For example, the invention could 
also be implemented using an apparatus for forming a metal layer by 
chemical vapor deposition. 
In FIG. 1, a substrate 101 (e.g., a semiconductor wafer) is positioned 
within a process chamber 102. As is well known to those skilled in the art 
of sputtering, the process chamber 102 is held at a vacuum pressure and a 
sputtering gas is injected into the process chamber 102 through one or 
more gas inlets (two gas inlets 103a and 103b are shown in FIG. 1), the 
gas is ionized, and the ions are accelerated toward a sputtering target 
104. A metal to be deposited on a surface of the substrate 101 (the upper 
surface, as shown in FIG. 1) is formed on the sputtering target 104 so 
that when the sputtering gas ions strike the sputtering target 104, atoms 
of the metal (shown generally by the arrows designated by the numeral 105) 
are dislodged from the sputtering target 104. Some of the dislodged metal 
atoms are deposited on the upper surface of the substrate 101, thus 
forming a metal layer on the substrate 101. 
Initially, the substrate 101 may be supported on a substrate support 
surface 106a of a substrate support 106. A heated gas (shown generally by 
the arrow designated by the numeral 107) can be flowed through a channel 
106b formed in the substrate support 106 to impact a surface (the bottom 
surface, as shown in FIG. 1) of the substrate 101, forcing the substrate 
101 away from the substrate support surface 106a and against substrate 
retention arms 106c and 106d, where the substrate 101 is held in place 
while metal is deposited on the upper substrate surface (this position is 
shown in FIG. 1). The temperature of the heated gas 107 is controlled so 
that the gas 107 heats the substrate 101 to a desired temperature during 
deposition of the metal. Though FIG. 1 shows a single heating gas inlet 
(channel 106b), it is to be understood that the heating gas can be 
supplied through any number of heating gas inlets, that the heating gas 
inlets can be positioned at any location proximate to the substrate bottom 
surface, and that the heating gas inlets can be configured to impinge the 
heating gas against the substrate 101 at any angle. 
In another embodiment of the invention, a sputtering chamber such as that 
shown in FIG. 1 can be used without heating the substrate with a heated 
gas as described immediately above. In such an embodiment, the Substrate 
101 remains on the substrate support surface 106a of the substrate support 
106 while the metal is deposited on the upper substrate surface. 
Typically, the process chamber 102 (and, in many cases, the substrate 101) 
is heated up as a consequence of injection of the sputtering gas--which is 
typically heated--into the process chamber 102. 
The invention can be implemented, for example, using an Endura sputtering 
system made by Applied Materials of Santa Clara, Calif. The Endura 
sputtering system can include one to four sputtering chambers as described 
generally above with respect to FIG. 1. Below, exemplary magnitudes are 
given for certain aspects of the invention: in particular, process chamber 
temperatures, heating gas temperatures and substrate temperatures. These 
magnitudes relate particularly to implementation of the invention using an 
Endura sputtering system. However, the magnitudes (both absolute and 
relative) are expected to be similar for other sputtering systems. 
FIG. 2 is a flow chart of a method 200, according to an embodiment of the 
invention, for forming a metal layer on a substrate. As used herein, 
"metal layer" refers generally to any formation of a metal on a substrate, 
without regard to the relative magnitudes of the dimensions of the metal 
formation (i.e., the term "metal layer" is not intended to be restricted 
by the connotation associated with the word "layer"). For example, in a 
semiconductor device, a metal layer can include, for example, an 
electrically conductive trace formed of metal, an electrically conductive 
ground or power plane formed of metal, or an electrically conductive 
contact (i.e., a filled via) formed of metal. 
In step 201 of the method 200, a wetting layer is formed on the substrate 
at a hot process temperature. Herein, a "wetting layer" is any layer of 
material which causes a subsequently formed metallization layer to adhere 
to the substrate better than would be the case if the metallization layer 
were formed directly on the substrate. Typically, the wetting layer is 
relatively thin, e.g., 100 to 300 angstroms. Thus, the wetting material is 
typically formed (e.g., deposited) for a relatively short period of time, 
only long enough to ensure that the wetting material covers all surfaces 
that are desired to be covered. "Process temperature" refers generally to 
the temperature at which a process occurs and can refer to, for example, 
the temperature of the substrate ("substrate temperature") and/or the 
average temperature within a process chamber ("process chamber 
temperature") in which the process takes place. Generally, a "hot process 
temperature" at which a wetting layer is formed is any process temperature 
higher than the process temperature at which such a wetting layer has 
previously been formed. What can constitute a "hot process temperature" is 
discussed in more detail below. To obtain an improved wetting layer in 
accordance with the invention, the wetting material can be deposited at 
any time after the process temperature has increased above the process 
temperature used in previous such processes for forming a wetting layer. 
In particular, the process temperature need not have risen to a steady 
state temperature; the wetting layer can be formed while the process 
temperature is increasing, through generally the highest quality wetting 
layer is achieved by forming the entire wetting layer after the process 
temperature has reached a predetermined hot process temperature. 
In step 202, a metallization layer is formed on the wetting layer. Herein, 
a "metallization layer" is a layer of metal which is the primary layer 
that it is desired to form on the substrate. Generally, the metallization 
layer can be formed using any appropriate process (some examples are 
discussed below). Both the wetting layer and the metallization layer can 
be formed in a sputtering chamber as described above with respect to FIG. 
1. Typically, the wetting layer and metallization layer are formed in 
different sputtering chambers, though this need not necessarily be the 
case. 
Forming a wetting layer at a hot process temperature in accordance with the 
invention produces a wetting layer that is denser than would be the case 
at a process temperature below the hot process temperature. It is believed 
that the elevated process temperature increases the mobility of the atoms 
of the material with which the wetting layer is formed; consequently, the 
atoms disperse more completely over the surface of the substrate, filling 
in any available spaces more completely to form a more compact wetting 
layer than is formed at lower process temperatures. Since the wetting 
layer is denser, the wetting layer has a smoother surface to which the 
subsequently formed metallization layer can adhere. Thus, the 
metallization layer is more likely to be formed smoothly and continuously, 
without voids, at the junction with the wetting layer. Such smooth and 
continuous formation of the deposited metal increases the likelihood that 
subsequently deposited metal will be formed smoothly and continously. 
Thus, the probability of forming a void-free metal layer is increased. 
Additionally, a wetting layer formed in accordance with the invention has 
improved barrier properties as compared to previous wetting layers formed 
at lower process temperatures. Generally, the principles of the invention 
are applicable to the formation of a wetting layer and metallization layer 
of any appropriate material or materials, i.e., increasing the process 
temperature before and/or during formation of a wetting layer to a higher 
level than previously used when forming such a wetting layer produces a 
wetting layer to which the subsequently formed metallization layer adheres 
better than has previously been the case when such a wetting layer has 
been used. 
Varying other process parameters may also affect the quality of the wetting 
layer produced. For example, decreasing the chamber pressure may produce a 
denser wetting layer. As the chamber pressure is decreased, then, this 
effect tends to decrease the process temperature necessary to produce a 
wetting layer of a given density. On the other hand, decreasing the 
chamber pressure causes the heating of the substrate to be less efficient. 
As the chamber pressure is decreased, this effect tends to increase the 
process temperature necessary to produce a wetting layer of a given 
density. Generally, the effect of variation in other process parameters 
within the range of acceptable values of the process parameters is not 
believed to significantly affect the magnitude of the process temperature 
necessary to produce the benefits of the invention. 
FIG. 3A is a cross-sectional view of a structure on which a metal layer can 
be formed according to a method of the invention. FIG. 3B is a 
cross-sectional view of the structure of FIG. 3A after a metal layer has 
been formed on the structure according to a method of the invention. The 
structure shown in FIG. 3B can be, for example, a portion of a 
semiconductor substrate (e.g., a semiconductor wafer) in which a 
conductive via makes electrical connection between two electrically 
conductive regions (e.g., electrically conductive traces). 
In FIG. 3A, an electrically conductive layer 302 is formed on a first 
dielectric layer 301. A second dielectric layer 303 is formed on the 
electrically conductive layer 302. A via is formed in the second 
dielectric layer 303 to expose a portion of the electrically conductive 
layer 302. 
As shown in FIG. 3B, a wetting layer 304 has been formed on the structure 
of FIG. 3A. The wetting layer 304 is formed in accordance with a method of 
the invention (e.g., by the step 201 of the method 200). The wetting layer 
304 covers the exposed portion of the electrically conductive layer 302, 
the surface or surfaces of the second dielectric layer 303 that form the 
walls of the via, and the surface of the second dielectric layer 303 
adjacent to the via. A metallization layer 305 has been formed on the 
wetting layer 304. The metallization layer 305 can be formed by any 
appropriate method. The metallization layer 305 fills in the via and 
covers the wetting layer 304 adjacent to the via. Together, the wetting 
layer 304 and metallization layer 305 are a metal layer formed in 
accordance with the invention. 
The invention can be used to form a metal layer on a semiconductor 
substrate (e.g., a semiconductor wafer). Electrical connection between 
electrically conductive regions of material on a semiconductor substrate 
is often made by forming a layer of metal (e.g., by forming an 
electrically conductive trace or via) on the substrate to connect the 
electrically conductive regions. Such metal layers are commonly formed by 
depositing aluminum (or an aluminum alloy) on the substrate. In some 
situations, the aluminum may not adhere as well as desired to the surface 
(e.g., silicon) on which it is desired to deposit the aluminum. As 
discussed above, a wetting layer can first be formed on the surface so 
that when the aluminum is formed, the aluminum adheres more completely and 
uniformly to the wetting layer than the aluminum would adhere to the 
substrate surface. Titanium can be used, for example, to form the wetting 
layer. Titanium nitride or a composition of titanium and tungsten could 
also be used to form the wetting layer. Even aluminum might be used to 
form the wetting layer. Previously, for the reasons discussed above, a 
relatively low process temperature (e.g., less than about 200.degree. C. 
and, frequently, about 40.degree. C.) has been used when a wetting layer 
of titanium is deposited prior to deposition of a metallization layer of 
aluminum. 
According to the invention, a higher process temperature is used when 
forming a wetting layer than has previously been used. In one embodiment 
of the invention, the substrate is heated to a hotter temperature during 
the formation of the wetting layer than has previously been used in the 
formation of such a wetting layer. In a particular embodiment of the 
invention, the substrate is heated so that the substrate is at a 
temperature of greater than or equal to about 225.degree. C. during 
formation of the wetting layer. In a further particular embodiment, the 
substrate is heated so that the substrate is at a temperature of greater 
than or equal to about 250.degree. C. during formation of the wetting 
layer. In yet a further particular embodiment, the substrate is heated so 
that the substrate is at a temperature of greater than or equal to about 
350.degree. C. during formation of the wetting layer. In a still further 
particular embodiment, the substrate is heated so that the substrate is at 
a temperature of greater than or equal to about 400.degree. C. during 
formation of the wetting layer. The substrate can be heated to these 
temperatures in any appropriate manner. For example, the substrate can be 
heated directly by, for example, impinging a heated gas on the substrate 
(such as described above with respect to FIG. 1). Or, the substrate can be 
heated indirectly as a consequence, for example, of the heating of the 
process chamber as a result of the introduction of a sputtering gas into 
the process chamber. Or, the substrate can be heated by a combination of 
these mechanisms. 
In another embodiment of the invention, the process chamber is heated to a 
hotter temperature during the formation of the wetting layer than has 
previously been used in the formation of such a wetting layer. In a 
particular embodiment of the invention, the process chamber is heated so 
that the process chamber is at a temperature of greater than or equal to 
about 250.degree. C. during formation of the wetting layer. In a further 
particular embodiment, the process chamber is heated so that the process 
chamber is at a temperature of greater than or equal to about 275.degree. 
C. during formation of the wetting layer. In yet a further particular 
embodiment, the process chamber is heated so that the process chamber is 
at a temperature of greater than or equal to about 400.degree. C. during 
formation of the wetting layer. In a still further particular embodiment, 
the process chamber is heated so that the process chamber is at a 
temperature of greater than or equal to about 450.degree. C. during 
formation of the wetting layer. As with the heating of the substrate, the 
process chamber can be heated to these temperatures in any appropriate 
manner. For example, the process chamber may be heated as a result of the 
introduction of a heated sputtering gas into the process chamber, as a 
result of the heating of the substrate (e.g., by impinging a heating gas 
against the substrate), or as a result of the combination of these two. 
In still another embodiment of the invention, the temperature of a heating 
gas used to heat a substrate (e.g., as described above with respect to 
FIG. 1) during formation of a wetting layer is greater than or equal to 
about 250.degree. C., more preferably greater than or equal to about 
275.degree. C., even more preferably greater than or equal to about 
400.degree. C. and most preferably greater than or equal to about 
450.degree. C. 
The substrate, process chamber and heating gas temperatures given 
immediately above for particular embodiments of the invention are 
appropriate, in particular, when the wetting layer is formed of titanium 
(i.e., titanium that is essentially pure except for the presence of 
incidental contaminants). However, these temperatures are also 
appropriate, with little or no change, for use with wetting materials that 
are a composition of titanium and some other material, such as titanium 
nitride or a composition of titanium and tungsten. 
The density of the wetting layer increases as the magnitude of the process 
temperature during formation of the wetting layer increases; therefore, 
generally, the process temperature is made as high as possible. However, 
the magnitude of the substrate temperature is limited by the fact that 
high substrate temperatures increase the likelihood of damaging the 
substrate (e.g., damaging previously deposited metal layers on the 
substrate by, for example, forming voids as a result of thermal stress). 
In view of this consideration, the substrate temperature is preferably 
maintained less than or equal to about 500.degree. C. when the material 
being used for the metallization layer is aluminum. When previous layers 
of aluminum have already been formed on the substrate, the maximum 
substrate temperature is preferably kept even lower, e.g., less than or 
equal to about 430.degree. C. If the metallization layer is formed of 
tungsten, then these maximum temperatures can be somewhat higher. 
In a system such as shown in FIG. 1 in which the substrate 101 is heated by 
flowing the heated gas 107 against the bottom surface of the substrate 
101, the gas 107 (and, therefore, the process chamber 102) is heated to a 
temperature that is somewhat greater than the temperature of the substrate 
101. The particular temperature to which the gas 107 must be heated to 
produce a particular substrate temperature can depend upon the sputtering 
system used (in particular, the heating apparatus of the sputtering 
system), the length of time that the substrate 101 is heated, and other 
process parameters (e.g., chamber pressure). Additionally, the temperature 
difference typically becomes greater as the substrate temperature 
increases. For example, the temperature of the substrate 101 is generally 
about 50.degree. C. to 75.degree. C. less than the temperature of the 
heated gas 107 (at substrate temperatures above 250.degree. C.) in an 
embodiment of the invention in which the Endura sputtering system is used 
to form the wetting layer, the chamber pressure is about 2 mTorr, the 
sputtering gas 105 is argon that is flowed into the process chamber 102 at 
a flow rate of about 25 sccm, and the heated gas 107 is argon that is 
flowed against the bottom surface of the substrate 101 at a flow rate of 
about 15 sccm. In particular, for a heating gas temperature of about 
40.degree. C., the wafer temperature is about 30.degree. C.; for a heating 
gas temperature of about 150.degree. C., the wafer temperature is about 
130.degree. C.; and for a heating gas temperature of about 450.degree. C., 
the wafer temperature is about 390.degree. C. 
The invention has particular utility when used in a metallization process 
in which a titanium wetting layer is formed prior to forming an aluminum 
metallization layer on the titanium wetting layer. As discussed above, in 
such metallization processes, aluminum that is formed immediately after 
formation of the wetting layer is often formed at a relatively cold 
process temperature (e.g., when the substrate temperature is below about 
200.degree. C.). For example, in some previous methods of forming an 
aluminum metallization layer on a semiconductor substrate, the aluminum 
layer is formed using a two step process. Several such methods are 
described in the commonly owned, copending U.S. patent application Ser. 
No. 08/693,978, entitled "Improved Hot Metallization Process," by Sam G. 
Geha, filed Aug. 1, 1996, the disclosure of which is incorporated by 
reference herein. In a first step (the "cold" deposition step), a first 
portion of aluminum is deposited at a relatively cold process temperature 
as described above. In the second step (the "hot" deposition step), a 
second portion of aluminum is deposited on the first portion of aluminum 
at a relatively hot process temperature (e.g., while the substrate is at a 
temperature of about 450.degree. C. to about 500.degree. C.). In previous 
such metallization processes, if a titanium wetting layer is to be formed, 
the titanium wetting layer is formed at a relatively low process 
temperature that approximates the process temperature to be used during 
the cold deposition step, so that time need not be spent heating up or 
cooling down the substrate between the titanium deposition and the cold 
aluminum deposition. In contrast, according to the invention, during 
formation of a titanium wetting layer, a process temperature is used which 
is significantly above that at which a cold aluminum deposition is 
typically performed. The improvement in the quality of the wetting layer 
so produced outweighs any detriment resulting from the need to cool the 
substrate after forming the wetting layer. 
While the invention is known to be particularly useful, as described above, 
when used to form a wetting layer prior to forming aluminum at a lower 
process temperature than that at which the wetting layer was formed, the 
invention can also be used when aluminum is subsequently formed at a 
higher process temperature than that at which the wetting layer was 
formed. 
FIGS. 4A and 4B are bar charts illustrating the improved quality of 
aluminum contacts formed on a titanium wetting layer as the process 
temperature at which the titanium wetting layer is formed is increased. 
The results shown in FIGS. 4A and 4B were obtained using an Endura 
sputtering system to deposit both the titanium wetting layer and the 
aluminum metallization layer. During deposition of the titanium wetting 
layer, the chamber pressure was about 2 mTorr, an argon sputtering gas was 
flowed into the process chamber at a flow rate of about 25 sccm, and an 
argon heating gas was flowed against the bottom surface of the wafer at a 
flow rate of about 15 sccm. The power applied to the magnetron of the 
Endura sputtering system was about 750 watts DC. During deposition of the 
aluminum layer, the chamber pressure was about 6 mTorr, an argon 
sputtering gas was flowed into the process chamber at a flow rate of about 
80 sccm, and an argon heating gas was flowed against the bottom surface of 
the wafer at a flow rate of about 30 sccm. The power applied to the 
magnetron of the Endura sputtering system was about 750 watts DC. The 
aluminum was deposited in two steps. In the first step, aluminum was 
deposited for about 10 seconds while the substrate was at a temperature of 
about 200.degree. C. In the second step, aluminum was deposited for about 
3 minutes while the substrate was at a temperature of about 450.degree. C. 
FIG. 4A illustrates the formation of aluminum contacts near the center of a 
semiconductor wafer, while FIG. 4B illustrates the formation of aluminum 
contacts near the edge of a semiconductor wafer. Each of FIGS. 4A and 4B 
show the contact filling probability (i.e., the probability that a contact 
is filled with no voids) for each of five contact geometries at each of 
three process chamber temperatures during deposition of a titanium wetting 
layer. The leftmost group of bars in each of FIGS. 4A and 4B show the 
contact filling probability when the process chamber temperature during 
deposition of the titanium wetting layer was about 40.degree. C., the 
center group of bars show the contact filling probability when the process 
chamber temperature during deposition of the titanium wetting layer was 
about 150.degree. C., and the rightmost group of bars show the contact 
filling probability when the the process chamber temperature during 
deposition of the titanium wetting layer was about 450.degree. C. 
Within each group of bars, individual bars represent the contact filling 
probability for contacts formed in vias having various aspect ratios. 
Moving from left to right in each group, the aspect ratios are 1.60 (via 
diameter of 0.5, via depth of 0.8 micrometers), 1.33 (via diameter of 0.6, 
via depth of 0.8 micrometers), 1.14 (via diameter of 0.7, via depth of 0.8 
micrometers), 1.00 (via diameter of 0.8, via depth of 0.8 micrometers) and 
0.89 (via diameter of 0.9, via depth of 0.8 micrometers). 
As can be seen, in accordance with the invention, the contact filling 
probability increased as the temperature of the process chamber (and thus 
the wafer) was increased during deposition of the titanium wetting layer. 
For all of the vias located near the edge of the wafer, and all but the 
highest aspect ratio vias located near the center of the wafer, depositing 
a wetting layer at a process chamber temperature of about 450.degree. C. 
produced a contact filling probability of 100%. 
As discussed above, forming a wetting layer at a relatively high process 
temperature produces a denser wetting layer having a smoother and more 
continuous surface on which to form the subsequent metallization layer. 
Thus, the metallization layer can be formed more smoothly on the wetting 
layer, improving the quality of the overall metal layer so formed. The 
improved wetting layer according to the invention thus enlarges the 
process window for the formation of the metallization layer. For example, 
when a two-step process described above for depositing aluminum is used, 
formation of a wetting layer according to the invention enables the 
process temperature during the second, hot deposition step to be reduced 
while still producing a metal layer of the same quality. Illustratively, 
when a process as taught by Geha in the above-referenced U.S. patent 
application Ser. No. 08/693,978 is used to deposit a metallization layer 
of aluminum, the substrate temperature during the second, hot deposition 
step can be reduced by about 50.degree. C. This is particularly 
advantageous, since such reduction in substrate temperature reduces the 
likelihood that the substrate temperature will become high enough to cause 
damage to the substrate (e.g., previously deposited metal layers on the 
substrate). Alternatively, if the other aspects of the process of forming 
a metal layer are left unchanged, a wetting layer formed in accordance 
with the invention improves the quality of the overall metal layers formed 
using the wetting layer, as illustrated in FIGS. 4A and 4B, thus enabling 
more formation of more difficult metal layers (e.g., high aspect ratio 
vias). Consequently, it is possible to use aluminum deposition for 
situations in which it previously would have been necessary to use 
tungsten deposition. Since tungsten deposition, as described above, 
requires more process steps than aluminum deposition, the invention 
enables the elimination of process steps and, consequently, significant 
savings in cost and time in producing a metal layer in these situations. 
As discussed above, collimated deposition and IMP deposition are two other 
processes for depositing a metal. Using a relatively high process 
temperature in accordance with the invention enables formation of a 
wetting layer using a standard deposition process that equals or 
approaches the quality of a wetting layer produced using collimated or IMP 
deposition. Thus, a high quality wetting layer can be produced without 
need to modify or replace existing deposition processes and equipment. 
The invention can improve any process for forming a wetting layer. For 
example, increasing the process temperature in accordance with the 
invention during either collimated or IMP deposition improves the quality 
of the deposited wetting layer, thereby enabling formation of more 
problematic metal layers (e.g., filling of high aspect ratio vias) and/or 
enlargement of the process window for the formation of the wetting layer 
or subsequent formation of the metallization layer. 
The steps of the method 200 described above are typically preceded and 
followed by a number of other steps. These other steps can be performed in 
processing chambers other than the processing chamber or chambers used to 
implement a method of the invention. For example, the following describes 
a process sequence that can encompass the steps of a method according to 
the invention. First, a conventional degassing procedure can be performed 
to remove moisture from a dielectric layer or layers on which a metal 
layer or layers are to be formed. Next, a conventional etching procedure 
(e.g., a conventional sputter etch procedure) can be performed to remove 
portions of the dielectric layer(s) to create vias or steps. Next, a 
method according to the invention is used to deposit a wetting layer and 
metallization layer in a desired location or locations. The metal 
deposition can be followed by further processing steps, such as the 
formation of a conventional antireflective coating (ARC) using 
conventional techniques. Finally, the substrate can be cooled according to 
a standard cooling procedure for a specified time (e.g., 30 seconds). It 
is to be understood that a method according to the invention is not 
confined to use with the process sequence described immediately preceding, 
and that a method according to the invention can be part of other process 
sequences that include some or all of the above-described steps, none of 
the above-described steps, and/or other steps not described above. 
The invention as described above can be used, for example, to form various 
types of metallization on a semiconductor substrate (e.g., a semiconductor 
wafer). FIG. 5 is a cross-sectional view of a semiconductor substrate on 
which various layers of metal have been formed, illustrating several 
applications of a method according to the invention. Each of the metal 
layers or contacts shown in FIG. 5 includes a wetting layer and 
metallization layer, as described above. For example, a metal layer 504 
(including a wetting layer 504a and a metallization layer 504b) formed on 
the dielectric layer 502 may be electrically connected to a polysilicon 
gate 509 formed on oxide 510 by a metal contact 506 (including a wetting 
layer 506a and a metallization layer 506b) that extends through a 
dielectric layer 502. Similarly, metal layer 504 may be electrically 
connected to an electrically doped region 511 of the silicon substrate 501 
by a metal contact 507 (including a wetting layer 507a and a metallization 
layer 507b) that extends through a dielectric layer 502. A second metal 
layer 505 (including a wetting layer 505a and a metallization layer 505b) 
formed on the dielectric layer 503 that overlies the first metal layer 504 
may be electrically connected to the first metal layer 504 by a metal 
contact 508 (including a wetting layer 508a and a metallization layer 
508b) that extends through the dielectric layer 503. When aluminum is used 
for the metallization layer 504b of the layer 504, then it may be 
necessary to form a barrier layer (e.g., a titanium-tungsten alloy or 
titanium nitride) on top of the metallization layer 504b to prevent or 
inhibit migration of silicon atoms into the aluminum. 
The invention is broadly applicable to the formation of a metal layer on 
any type of substrate or device. For example, formation of a metal layer 
according to the invention can be accomplished on any type of 
semiconductor substrate, such as a semiconductor wafer. Illustratively, 
the invention can be used to form metal layers in active electronic 
components (e.g., integrated circuit chips, transistors and diodes) and 
passive electronic components (e.g., resistors, capacitors and inductors). 
The invention can also be used to form metal layers in other types of 
devices, such as lead frames, medical devices, disks and heads and flat 
panel displays. 
Various embodiments of the invention have been described. The descriptions 
are intended to be illustrative, not limitative. Thus, it will be apparent 
to one skilled in the art that certain modifications may be made to the 
invention as described above without departing from the scope of the 
claims set out below.