Processing for polishing dissimilar conductive layers in a semiconductor device

A process of polishing two dissimilar conductive materials deposited on semiconductor device substrate optimizes the polishing of each of the conductive material independently, while utilizing the same polishing equipment for manufacturing efficiency. A tungsten layer (258) and a titanium layer (256) of a semiconductor device substrate (250) are polished using one polisher (10) but two different slurry formulations. The two slurries can be dispensed sequentially onto the same polishing platen (132) from two different source containers (111 and 112), wherein the first slurry is dispensed until the tungsten is removed and then the slurry dispense is switched to the second slurry for removal of the titanium. In a preferred embodiment, the first slurry composition is a ferric nitrate slurry while the second slurry composition is an oxalic acid slurry.

FIELD OF THE INVENTION 
The present invention relates generally to processes for polishing, and 
more particularly, to processes for polishing semiconductor device 
substrates. 
BACKGROUND OF THE INVENTION 
Chemical mechanical polishing (CMP) is presently used to polish a variety 
of materials found in semiconductor devices. Those materials include 
metals, such as tungsten, aluminum, and copper. Regardless of the type of 
material being polished, similar techniques are used. For example, a 
polishing system typically includes a polishing platen, on which is 
attached a polishing pad. While the platen is being rotated, a slurry is 
dispensed while a semiconductor wafer is pressed against the pad. A 
combination of the chemical reaction between the slurry and the layer 
being polished and the mechanical interaction between abrasives within the 
slurry and the layer being polished cause the planarization of the layer. 
In some instances, two layers of different materials are deposited on each 
other in a semiconductor substrate, and both materials need to be 
polished, preferably in a continuous polishing operation to minimize cycle 
time. Commercially available polishing slurries do not provide ideal 
properties for polishing two dissimilar materials during the same 
polishing operation. For example, when polishing tungsten that is 
deposited on a titanium/titanium nitride layer, the polishing properties 
of tungsten and the titanium layer differ greatly. Titanium is a 
relatively difficult material to polish using a slurry composition 
optimized for tungsten polishing. Slurry formulations that successfully 
polish titanium typically do not polish tungsten as fast as other 
slurries. In most cases, optimizing the polishing conditions for one 
material, for example tungsten, leads to a degradation of the polishing 
characteristics of the other materials, such as titanium. 
One known method for polishing a combination of tungsten and titanium is to 
use a relatively hard or abrasive polishing pad, such as a Suba 500 made 
by Rodel, Inc. of Delaware, with a slurry formulated for tungsten 
polishing (e.g. a ferric nitrate slurry). The polishing slurry does not 
significantly chemically react with the titanium, therefore use of a 
harder polishing pad is effective in mechanically removing titanium. 
However, problems with this method include 1) a lower tungsten polishing 
rate than if a softer pad is used; and 2) high oxide removal or erosion 
during polishing. Oxide removal or erosion is undesirable because it is 
generally non-uniform across the wafer, being faster in dense feature 
arrays and slower in peripheral areas. Use of a softer pad, such as a 
Politex pad, also by Rodel, Inc. of Wilmington Delaware, results in less 
removal of oxide, but inadequately removes titanium. 
Another method for overcoming the problem of polishing tungsten and 
titanium is to polish the tungsten away, but leave the titanium layer in 
place. An interconnect metal layer(s), such as aluminum, is then deposited 
on the remaining titanium layer, and the aluminum and titanium layers are 
simultaneously patterned and etched. By etching the titanium layer with 
the aluminum, the need to polish away the titanium is eliminated. However, 
the titanium layer is nonetheless exposed to the polishing process during 
the polishing of tungsten. Consequently, the quality of titanium under the 
aluminum is poor and reliability of the resulting aluminum interconnects 
is degraded. 
Accordingly, there is a need in the industry to establish a polishing 
process that can effectively polish two dissimilar conductive materials in 
a cost effective manner that is conducive to a manufacturing environment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Generally, the present invention provides a method of polishing two 
dissimilar conductive materials deposited on semiconductor device 
substrate. While prior art techniques for polishing sequentially deposited 
dissimilar materials use a common slurry and set of polishing parameters, 
the present invention optimizes the polishing of each of the conductive 
material independently, while utilizing the same polishing equipment for 
manufacturing efficiency. In one embodiment, a tungsten layer is deposited 
on a titanium layer, and the two layers are polished using one polisher 
but two different slurry formulations. A first slurry composition is 
optimized for polishing tungsten, and a second slurry composition is 
optimized for polishing titanium. The two slurries can be dispensed 
sequentially onto the same polishing platen and pad from two different 
sources, wherein the first slurry is dispensed until the tungsten is 
removed and then the slurry dispense is switched to the second slurry for 
removal of the titanium. In a preferred embodiment, the first slurry 
composition is a ferric nitrate based slurry while the second slurry 
composition is an oxalic acid based slurry. In this embodiment, both 
slurries have a similar pH (both ferric nitrate and oxalic acid being 
acidic). Use of an oxalic acid slurry as the second slurry composition not 
only polishes titanium more effectively than traditional tungsten slurry 
formulations, but also ties up iron atoms to prevent unwanted 
incorporation of iron into dielectric layers of the semiconductor device 
substrate. Preferably, polishing of the two conductive layers is 
accomplished using the same polishing pad, and while the semi-conductor 
substrate is continuously in contact with the rotating pad, but wherein 
the two different polishing slurries are sequentially deposited onto the 
same pad to polish the two different conductive materials. By merely 
dispensing the two different slurries onto the same polishing platen and 
pad, manufacturing time is not adversely affected since transporting of 
the wafers between polishings is unnecessary. 
These and other features, and advantages, will be more clearly understood 
from the following detailed description taken in conjunction with FIGS. 
1-5. It is important to point out that the illustrations are not 
necessarily be drawn to scale, and that there are likely to be other 
embodiments of the present invention which are not specifically 
illustrated. 
FIG. 1 includes a schematic view of a portion of a chemical-mechanical 
polisher 10 for use in practicing the present invention. The 
chemical-mechanical polisher 10 has two sections that include a feed 
section 11 and a polishing section 13. Within the feed section are two 
containers 111 and 112. The two containers 111 and 112 include two 
different polishing fluids for polishing two different conductive layers 
of a semiconductor device substrate in accordance with the present 
invention. For example, container 111 includes a tungsten polishing 
slurry, such as ferric nitrate, and container 112 includes a polishing 
slurry chosen to polish titanium, such as an oxalic acid slurry. While in 
the embodiment illustrated in FIG. 1, the two different polishing slurries 
are shown as being premixed (i.e. the three primary components of the 
slurry--oxidizing agent, abrasive particles, and water--are mixed together 
within the container), it is important to understand that the oxidizing 
components and the diluted abrasive components of the polishing slurries 
can be stored and delivered separately and then mixed near the point of 
dispensing onto the polishing pad. 
The slurries within the containers 111 and 112 flow through the feed lines 
113 and 114, respectively, to a manifold 121. Pumps 115 and 116 regulate 
the flow through the feed lines 113 and 114, respectively. Manifold 121 
regulates flow of the slurries to the polishing pad. Valves (not shown) 
are opened and closed by the polisher, depending upon which of the two 
slurries is to be delivered to the polishing pad. 
After passing through the manifold, the desired polishing slurry flows 
through a piece of dispense tubing 122 and is delivered to polishing 
section 13. The polishing section 13 includes a tub 131, a platen 132 and 
a polishing pad. For simplicity, the combination of the platen and 
polishing pad is just denoted as the platen 132. Above the platen 132 is a 
substrate holder 133 and a semiconductor device substrate 134. During 
polishing, the polishing slurry is dispensed from tubing 122 onto the 
platen 132, and excess slurry is eventually received by the tub 131, at 
which time the polishing slurry may be recycled or discarded. Substrate 
134 is held against the pad while the slurry is dispensed and while the 
platen is rotating to polish exposed layers of the substrate. 
FIGS. 2-5 illustrate, in cross-sectional views, an example of how a 
semiconductor device substrate 250 is processed using, for example, the 
polisher of FIG. 1 in practicing the present invention. Semiconductor 
device substrate 250 of FIG. 2 includes a metal interconnect 252 having an 
overlying anti-reflective coating (ARC) 254. Metal interconnect 252 can be 
formed of aluminum, an aluminum alloyed with copper or silicon, copper, a 
copper alloy, or the like. ARC 254 is typically a metal nitride such as 
titanium nitride, tantalum nitride, aluminum nitride, or the like. An 
interlevel dielectric (ILD) layer 255 is deposited over metal interconnect 
252 and ARC 254 and is etched to form a via opening (also known as a 
contact opening) which exposes a top portion of the metal interconnect 
252. ILD layer 255 is usually an oxide material that is chemically vapor 
deposited and can be doped or undoped. The via opening is etched using 
conventional anisotropic dry oxide etching techniques. As thus far 
described, semiconductor device substrate 250 can be formed using 
conventional semiconductor fabrication techniques. 
After forming a via opening, a plug layer is formed by sequentially 
depositing a titanium layer 256 over the upper surface of the ILD layer 
and within the via opening as shown in FIG. 2. After depositing titanium 
layer 256, the plug filling material is deposited. In one embodiment, this 
material is a tungsten layer 258 as shown in FIG. 2. In addition to 
titanium layer 256, a titanium nitride layer (not shown) can be used 
between the titanium layer 256 and tungsten layer 258. Titanium and/or 
titanium nitride layers function as adhesion and barrier layers in the 
interconnect metallization. The titanium layer is typically much thinner 
than the plug fill material. For example, titanium layer 256 is less than 
1,000 .ANG. thick while the tungsten layer is at least half the via 
opening width, and is usually between 2,000-10,000 .ANG.. 
Both the tungsten layer 258 and the titanium layer 256 outside the opening 
need to be removed to produce a plug within the via opening. This is 
achieved by polishing in accordance with the present invention. The 
tungsten layer is first removed using the polisher 10 previously 
described. Preferably, the polishing pad used is a relative soft one 
(wherein its Shore D hardness is less than 45, such as with the Politex 
polishing pad mentioned previously), but harder pad materials (having 
harnesses in excess of Shore D 45, such as Suba 500 pads) are also 
suitable. A first polishing slurry is pumped from container 111, through 
feed line 113, manifold 121, and is dispensed onto platen 132 through 
dispense tubing 122. In the case of polishing a tungsten layer, a 
preferred slurry is an acidic ferric nitrate (Fe(NO.sub.3).sub.3 chemistry 
in combination with water and alumina abrasive particles. Polishing is 
performed until tungsten layer 258 is removed from everywhere beyond via 
opening, as shown in FIG. 3, to produce a tungsten plug 260 within the via 
opening. Timed polishing or any type of end-point detection for polishing 
can be used to determine when the tungsten is adequately removed. Dispense 
rates, rotational speeds and polishing times and pressures to remove the 
tungsten are optimized within conventional ranges. As one example, a 6,000 
.ANG. tungsten layer is polished for about two minutes using a pressure of 
5 pounds per square inch (PSI) and a rotational speed of 30 revolutions 
per minute (RPM), and a slurry of 10% by weight ferric nitrate solution in 
a 1:1 ratio with 6% by weight aluminum abrasive in deionized water. Such a 
slurry will have a pH less than 2, and generally within the range of 
1.2-1.5. Under such conditions, tungsten is polished at a rate of about 
6,000 .ANG. per minute, while titanium is polished at a rate of only 
300-400 .ANG. per minute. 
After removing tungsten layer 258 from beyond the via opening as shown in 
FIG. 3, the slurry chemistry is switched to deliver the polishing slurry 
of container 112 to the polishing platen for removing titanium layer 256. 
Switching slurries is achieved by closing the valve of the manifold 
associated with the first slurry and opening the valve associated with the 
second slurry. The second slurry is then delivered from container 112, 
through feed line 114, manifold 121, and is dispensed onto platen 132 
through dispense tubing 122. During the switch from the first slurry to 
the second slurry, the platen continues to be rotated, and the substrate 
continues to be held against the polishing pad with some pressure. As the 
slurry chemistry change is made, the titanium layer is polished at a much 
faster rate than any titanium exposed during polishing of the substrate 
with the first polishing slurry. 
In a preferred embodiment where titanium is being polished, the second 
polishing slurry is an oxalic acid slurry having oxalic acid [(COOH).sub.2 
or HO.sub.2 CCO.sub.2 H] in combination with water and alumina abrasive 
particles. Polishing is performed until titanium layer 256 is removed from 
everywhere beyond via opening, as shown in FIG. 4. Again, timed polishing 
or any type of end-point detection for polishing can be used to determine 
when the titanium is adequately removed. Dispense rates, rotational speeds 
and polishing times and pressures to remove the tungsten are optimized 
within conventional ranges. As one example, a 400 .ANG. titanium layer is 
polished for about 30 seconds using a pressure of 5 PSI and a rotational 
speed of 30 RPM, and a slurry of 0.5% by weight oxalic acid solution in a 
1:1 ratio with 6% by weight aluminum abrasive in deionized water. Such a 
slurry will have a pH less than 2, and generally within the range of 
1.2-1.5. Since the oxalic acid slurry is not highly selective to ILD layer 
255, it is preferred that a soft pad (as explained above) be used for 
polishing titanium layer 256 to avoid excess, and possibly non-uniform, 
oxide removal. Under such conditions, tungsten is polished at a rate of 
less than 200 .ANG. per minute, while titanium is polished at a rate of 
about 1,500 .ANG. per minute. 
After removing the dissimilar conductive layers, substrate 250 is 
preferably moved to a finishing platen of the polisher (not shown) to 
remove residual particles from the surface of the substrate 250. In one 
embodiment, a short dielectric polish using a basic slurry may be 
performed on the finishing platen to provide a smooth surface to the ILD 
layer 255. A water rinse follows to remove any remaining basic slurry. In 
another embodiment, only water (without the basic slurry) is introduced 
over the finishing platen. The finishing platen typically has a soft pad, 
which may be identical to the polishing pad (see, e.g. the commonly 
assigned co-pending application by Kim et al. entitled "Process for 
Polishing a Semiconductor Device Substrate, Ser. No. 08/780,113, filed 
Dec. 26, 1996). Since most commercially available polishers have just two 
platens (typically one for polishing and one for buffing or finishing), 
and since a finishing or buffing polish of the ILD layer 255 is preferred 
after removing a metal layer, it is preferred that the polishing of the 
dissimilar conductive materials in accordance with the invention occur on 
the same platen to best utilize floor space and capital expenditures. As 
such, the second platen of the polisher can be used for finishing. 
Alternatively, a finishing step could be performed on a separate machine, 
and each of the two platens of polisher 10 could be used to remove a 
different one of the conductive layers, or the finishing step could be 
eliminated altogether. 
After plug formation is completed by removing tungsten layer 258 and 
titanium layer 256, the substantially completed semiconductor device 
substrate 250 is formed as shown in FIG. 5. Another titanium layer 262, or 
combination of titanium and titanium nitride is deposited, followed by a 
second level of metallization 264. Metallization 264 is similar to metal 
interconnect 252, for instance made of aluminum, an aluminum alloy, 
copper, a copper alloy, or the like. If the second level of metallization 
is the uppermost level of metallization for interconnects within a 
semiconductor device, a passivation layer 266 is deposited to complete the 
device. The passivation layer 266 is likely to be a doped oxide, nitride, 
silicon oxy-nitride, polyimide, or similar known passivation material. 
While the present invention has been described in reference to a specific 
embodiment wherein tungsten and titanium are polished, it is important to 
note the present invention is useful for polishing any two dissimilar 
overlying conductive materials. For example, the invention can be used in 
conjunction with polishing a metallization layer 264 of aluminum and an 
underlying titanium layer 256 in an inlaid interconnect application, as 
shown in a semiconductor device substrate 350 in FIG. 6. In inlaid 
applications, a plug to connect to the underlying metal interconnect 252 
is not formed of a material different than the interconnect metallization. 
Instead, the next interconnect metallization, e.g. aluminum or copper, 
fills both a via opening 266 and an interconnect trench or channel 268 
formed within the ILD 255. A single polishing operation is used to remove 
the aluminum and titanium from above the ILD, leaving the same metal in 
the via openings and the interconnect trenches as shown. Furthermore, the 
underlying metal layer need not be titanium to benefit from using an 
oxalic acid slurry composition. Other refractory-metal containing 
materials can be used as one of the dissimilar materials. 
The foregoing description and illustrations contained herein demonstrate 
many of the advantages associated with the present invention. In 
particular, it has been revealed that two dissimilar conductive materials 
can be polished, each using stable slurry compositions designed to 
optimize the polishing rates and conditions for each material. Moreover, 
the polishing process is easily integrated into existing polishing 
processes. Polishers today are already equipped for receiving more than 
one component through its distribution systems, and most have more than 
one polishing platen (although such is not a requirement for practicing 
the invention). Yet another advantage of the present invention is that the 
use of oxalic acid for polishing offsets some of the adverse affects of 
using slurries containing heavy metals (which include alkali metals, 
alkaline earth metals and transition elements). For instance, use of 
ferric nitrate to polish tungsten results in iron contamination of the ILD 
oxide layers. Using an oxalic acid following the use of a ferric nitrate 
slurry is advantageous in that the oxalic acid ties up the iron which may 
be left within the polishing pad or on the substrate, thereby preventing 
the iron from contaminating an oxide ILD. In practicing the invention, it 
was found that the iron concentration within the ILD was less than 1E12 
atoms/cm.sup.2 within the top 200 .ANG. of the exposed ILD surface, as 
compared to 1E14 atoms/cm.sup.2 if using ferric nitrate slurry alone. 
Thus it is apparent that there has been provided, in accordance with the 
invention, a process for polishing dissimilar conductive materials in a 
semiconductor device substrate that fully meets the need and advantages 
set forth previously. Although the invention has been described and 
illustrated with reference to specific embodiments thereof, it is not 
intended that the invention be limited to these illustrative embodiments. 
Those skilled in the art will recognize that modifications and variations 
can be made without departing from the spirit of the invention. Therefore, 
it is intended that this invention encompass all such variations and 
modifications as fall within the scope of the appended claims.