Chemical mechanical polishing slurry for metal layers and films

A polishing slurry for chemically mechanically polishing metal layers and films during the various stages of multilevel interconnect fabrication associated with integrated circuit manufacturing. The slurry includes an aqueous medium, an abrasive, an oxidizing agent, and an organic acid. The polishing slurry has been found to significantly lower or inhibit the silicon dioxide polishing rate, thus yielding enhanced selectivity. In addition, the polishing slurry is useful in providing effective polishing to metal layers at desired polishing rates while minimizing surface imperfections and defects. Also disclosed is a method for producing coplanar metal/insulator films on a substrate utilizing the slurry of the present invention and chemical mechanical polishing technique relating thereto.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a chemical mechanical polishing slurry for 
semiconductor integrated circuit manufacturing and, more particularly, to 
improved chemical mechanical polishing slurries for polishing metal layers 
and thin-films used in semiconductor integrated circuit manufacturing. 
2. Background of the Related Art 
A semiconductor wafer typically includes a substrate, such as a silicon or 
gallium arsenide wafer, on which a plurality of transistors have been 
formed. Transistors are chemically and physically connected into a 
substrate by patterning regions in the substrate and layers on the 
substrate. The transistors are interconnected through the use of well 
known multilevel interconnects to form functional circuits. Typical 
multilevel interconnects are comprised of stacked thin-films consisting of 
one or more of the following: titanium (Ti), titanium nitrite (TiN), 
tantalum (Ta), aluminum-copper (Al--Cu), aluminum silicon (Al--Si), copper 
(Cu), tungsten (W), and various combinations thereof. 
The traditional technique for forming interconnects has been improved by 
the disclosure of U. S. Pat. No. 4,789,648 to Chow et al. relating to a 
method for producing coplanar multilevel metal/insulator films on a 
substrate. This technique, which has gained wide interest and produces 
multilevel interconnects, utilizes chemical mechanical polishing (CMP) to 
planarize the surface of the metal layers or thin-films during the various 
stages of device fabrication. In general, CMP involves the concurrent 
chemical and mechanical polishing of an overlying first layer to expose 
the surface of a non-planar second layer on which the first layer is 
formed. One such process is described in U. S. Pat. No. 4,789,648 to Beyer 
et al., the specification of which are incorporated herein by reference. 
Briefly, Beyer et al. discloses a CMP process using a polishing pad and a 
slurry to remove a first layer at a faster rate than a second layer until 
the surface of the overlying material becomes coplanar with the upper 
surface of the initially covered second layer. For a more detailed 
explanation of chemical mechanical polishing, please see U. S. Pat. Nos. 
4,671,851, 4,910,155 and 4,944,836, the specifications of which are 
incorporated herein by reference. 
Polishing slurry composition is an important factor in providing a 
manufacturable chemical mechanical polishing process. Typical polishing 
slurries available for CMP processes contain an abrasive such as silica or 
alumina in an acidic or basic solution. For example, U. S. Pat. No. 
4,789,648 to Beyer et al. discloses a slurry formulation using alumina 
abrasives in conjunction with sulfuric, nitric, acetic acids and deionized 
water. Similarly, U. S. Pat. No. 5,209,816 to Yu et al. discloses a slurry 
for polishing aluminum using alumina abrasives in conjunction with 
phosphoric acid, hydrogen peroxide, and deionized water. U. S. Pat. Nos. 
5,391,258 and 5,476,606 to Brancaleoni et al. discloses a slurry for 
polishing a composite of metal and silica which includes an aqueous 
medium, abrasive particles and an anion which controls the rate of removal 
of silica. The anion contains at least two acid groups and the pKa of the 
first dissociable acid is not substantially larger than the pH of the 
polishing slurry, wherein the term substantially is defined as 0.5 units. 
Other polishing slurries for use in CMP processes are described in U. S. 
Pat. No. 5,354,490 to Yu et al., U. S. Pat. No. 5,340,370 to Cadien et 
al., U. S. Pat. No. 5,209,816 to Yu et al., U. S. Pat. No. 5,157,876 to 
Medellin, U. S. Pat. No. 5,137,544 to Medellin, and U. S. Pat. No. 
4,956,313 to Cote et al., the specifications of which are incorporated 
herein by reference. 
Although many of the slurry compositions are suitable for limited purposes, 
the slurries described above tend to produce poor film removal traits for 
the underlying films or produce deleterious film-corrosion which leads to 
poor manufacturing yield of typical multilevel metallization structures. 
In addition, the polishing slurries tend to exhibit unacceptable polishing 
rates and corresponding selectivity levels to the insulator media. 
Accordingly, a need remains for improved polishing slurries and processes 
related thereto which provide uniform metal layers and thin-films, free 
from undesirable contaminants and surface imperfections. In particular, it 
is highly desirous to produce a polishing slurry for multilevel 
interconnects having low selectivity to the barrier films, e.g., Ti, TiN, 
Ta, and high selectivity to the insulator media surrounding the multilevel 
interconnects, e.g., silica, spin on glass, and low-k dielectric 
materials, which are not hazardous or corrosive. A further need remains 
for a single slurry which is capable of providing both the low 
selectivities and high selectivities to the barrier and insulator films, 
respectively. 
SUMMARY OF THE INVENTION 
The present invention is directed to a chemical mechanical polishing slurry 
for polishing metal layers and thin-films. The polishing slurry includes 
an aqueous medium, an abrasive, an oxidizing agent, and organic acid. In 
one preferred embodiment, the abrasive is a metal oxide abrasive 
consisting of metal oxides aggregates having a size distribution less than 
about 1.0 micron, a mean aggregate diameter less than about 0.4 micron and 
a force sufficient to repel and overcome the van der Waals forces between 
abrasive aggregates themselves. In another preferred embodiment, the 
abrasive is a metal oxide abrasive consisting of discrete, individual 
metal oxide spheres having a primary particle diameter less than 0.4 
micron (400 nm) and a surface area ranging from about 10 m.sup.2 /g to 
about 250 m.sup.2 /g. Also disclosed is a method of polishing metal layers 
with the polishing slurry of the present invention. 
Depending on the choice of the oxidizing agent, the organic acid, and other 
desirable additives, the polishing slurry can be tailored to provide 
effective polishing to metal layers at desired polishing rates while 
minimizing surface imperfections, defects and uncontrollable corrosion. In 
addition, the polishing slurry of the present invention has been found to 
significantly lower or inhibit the silicon dioxide polishing rate, thus 
yielding enhanced selectivity with respect to the insulator layer. 
Furthermore, the polishing slurry may be effectively used to provide 
controlled polishing selectivities to other thin-film materials used in 
current integrated circuit technology, such as titanium, titanium nitride 
and the like.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to a chemical mechanical polishing slurry 
for polishing metal layers and thin-films which includes an aqueous 
medium, an abrasive, an oxidizing agent, and an organic acid. The 
polishing slurry has been found to yield high selectivity to the insulator 
layer. Preferably, the polishing slurry further provides low selectivity 
to the barrier metal layer or thin-film. 
The abrasive is typically a metal oxide abrasive characterized as having a 
surface area, as calculated from the method of S. Brunauer, P. H. Emmet, 
and I. Teller, J. Am. Chemical Society, Volume 60, Page 309 (1938) and 
commonly referred to as BET, ranging from about 5 m.sup.2 /g to about 430 
m.sup.2 /g and should be of a high purity. High purity means that the 
total impurity content, from sources such as raw material impurities and 
trace processing contaminants, is typically less than 1% and preferably 
less than 0.01% (i. e. 100 ppm). 
The metal oxide abrasive of the present invention is selected from the 
group of alumina, titania, zirconia, germania, silica, ceria and mixtures 
thereof. Preferably, the metal oxide is a fumed or precipitated abrasive 
and, more preferably is a fumed abrasive. The metal oxide abrasive may be 
produced utilizing techniques known to those skilled in the art. For 
example, the production of fumed metal oxides is a well-documented process 
which involves the hydrolysis of suitable feedstock vapor (such as 
aluminum chloride for an alumina abrasive) in a flame of hydrogen and 
oxygen. Molten particles of roughly spherical shapes are formed in the 
combustion process, the diameters of which are varied through process 
parameters. These molten spheres of alumina or similar oxide, typically 
referred to as primary particles, fuse with one another by undergoing 
collisions at their contact points to form branched, three dimensional 
chain-like aggregates. The force necessary to break aggregates is 
considerable and often considered irreversible. During cooling and 
collecting, the aggregates undergo further collision that may result in 
some mechanical entanglement to form agglomerates. Agglomerates are 
thought to be loosely held together by van der Waals forces and can be 
reversed, i.e. de-agglomerated, by proper dispersion in a suitable media. 
Precipitated abrasives may be manufactured utilizing conventional 
techniques and are typically formed by the coagulation of the desired 
particles from an aqueous medium under the influence of high salt 
concentrations, acids or other coagulants. The particles are filtered, 
washed, dried and separated from residues of other reaction products by 
conventional techniques known to those skilled in the art. In addition, 
the abrasive may be produced from other suitable technologies, such as 
sol-gel and plasma processing. 
In one preferred embodiment, the metal oxide abrasive consists of metal 
oxides aggregates having a size distribution less than about 1.0 micron, a 
mean aggregate diameter less than about 0.4 micron and a force sufficient 
to repel and overcome the van der Waals forces between abrasive aggregates 
themselves. Such metal oxide abrasive has been found to be effective in 
minimizing or avoiding scratching, pit marks, divots and other surface 
imperfections during polishing. The aggregate size distribution in the 
present invention may be determined utilizing known techniques such as 
transmission electron microscopy (TEM). The mean aggregate diameter refers 
to the average equivalent spherical diameter when using TEM image 
analysis, i. e. based on the cross-sectional area of the aggregate. By 
force is meant that either the surface potential or the hydration force of 
the metal oxide particles must be sufficient to repel and overcome the van 
der Waals attractive forces between the particles. 
In another preferred embodiment, the metal oxide abrasive consists of 
discrete, individual metal oxide spheres having a primary particle 
diameter less than 0.4 micron (400 nm) and a surface area ranging from 
about 10 m.sup.2 /g to about 250 m.sup.2 /g. 
Preferably, the metal oxide abrasive is incorporated into the aqueous 
medium of the polishing slurry as a concentrated aqueous dispersion of 
metal oxides, which aqueous dispersion of metal oxide abrasives typically 
ranges from about 3% to about 45% solids and, preferably, between 10% and 
20% solids. The aqueous dispersion of metal oxides may be produced 
utilizing conventional techniques, such as slowing adding the metal oxide 
abrasive to an appropriate media, for example, deionized water, to form a 
colloidal dispersion. The dispersion is typically completed by subjecting 
it to high shear mixing conditions known to those skilled in the art. The 
pH of the slurry may be adjusted away from the isoelectric point, as 
described below, to maximize colloidal stability. 
In a more preferred embodiment, the concentrated aqueous dispersion of 
metal oxides has a mean aggregate size distribution less than 0.3 micron 
and also have a maximum zeta potential greater than .+-.10 millivolts. 
Zeta potential (.zeta.) is the potential difference, measured in a liquid, 
between the shear plane and the bulk of the liquid beyond the limits of 
the electrical double layer. The zeta potential is dependent on the pH of 
the aqueous medium. For a given metal oxide abrasive composition, the 
isoelectric point is defined as the pH at which zeta potential is zero. As 
the pH is increased or decreased away from the isoelectric point, the 
surface charge is increased negatively or positively, respectively. As the 
pH continues to increase or decrease, the surface charge will reach an 
asymptote, the asymptote being referred to as the maximum zeta potential. 
It should be noted that the maximum zeta potential and isoelectric point 
are functions of the metal oxide composition and that the maximum zeta 
potential can be effected by the addition of salts to the aqueous medium. 
For a more complete discussion of zeta potentials, please see R. J. 
Hunter, Zeta Potential in Colloid Science (Academic Press 1981). 
The loading level of the abrasive in the polishing slurry may comprise 
between 0.5% and 55% of the slurry depending on the degree of abrasion 
required. The abrasion capability of the metal oxide, in turn, is a 
function of the particle composition, the degree of crystallinity and the 
crystalline phase, e. g. gamma or alpha. In order to achieve the desired 
selectivity and polishing rate, it has been found that the optimum surface 
area and loading level of the metal oxide abrasive may vary. For example, 
an alumina abrasive typically has a solids loading level in the final 
polishing slurry ranging between about 1% and about 12%, preferably 
between 2% and 8%, more preferably between 3% and 6%. 
The oxidizing agent of the present invention is added to the polishing 
slurry to oxidize the metal layer to its corresponding oxide or ions. For 
example, in the present invention, an oxidizing agent may be used to 
oxidize a metal layer to its corresponding oxide, such as aluminum to 
aluminum oxide or copper to copper oxide. The layer is mechanically 
polished to remove the respective oxide from the layer. Although a wide 
range of oxidizing agents may be used, suitable agents include oxidizing 
metal salts, oxidizing metal complexes, nonmetallic oxidizing acids such 
as peracetic and periodic acids, iron salts such as nitrates, sulfates, 
EDTA, citrates, potassium ferricyanide and the like, aluminum salts, 
sodium salts, potassium salts, ammonium salts, quaternary ammonium salts, 
phosphonium salts, or other cationic salts of peroxides, chlorates, 
perchlorates, nitrates, permanganates, persulfates and mixtures thereof. 
Furthermore, it is expected that water may also be used as an effective 
oxidizing agent in slurries when electronegative metals are used, such as 
aluminum. The standard electrochemical potential for the oxidation of, for 
example, aluminum to Al.sup.+3 -ions is: E.sub.0 =-1.663+0.0197 
log(Al.sup.+3) and for the oxidation to Al.sub.2 O.sub.3, E.sub.0 
=-1.550-0.0591 pH, expressed in Volts, V, against normal hydrogen 
electrode, NHE (as defined in "Atlas of Elechtrochemical Equilibria in 
Aqueous Solutions" by M. Pourbaix, Pergamon Press, New York, 1966). The 
standard potential for H.sub.3 O.sup.+ and H.sub.2 O reduction is 0 V on 
the same scale. The actual potential which can be measured during the 
abrasion of aluminum in some of the electrolytes, without the addition of 
oxidizers, is -1.4 V vs. NHE. This potential is low enough for vigorous 
reduction of both H.sub.3 O.sup.+ and H.sub.2 O. Electrochemical 
measurements indicate that the prevailing reduction reaction is that of 
water. 
Typically, the oxidizing agent is present in the slurry in an amount 
sufficient to ensure rapid oxidation of the metal layer while balancing 
the mechanical and chemical polishing components of the slurry. As such, 
the oxidizing agent is typically present in the slurry from about 0.5% to 
15% by weight, and preferably in a range between 1% and 7% by weight. 
In addition, it has further been found that inorganic acids and salts 
thereof may be added to the polishing slurry to improve or enhance the 
polishing rate of the barrier layers in the wafer, such as titanium and 
tantalum. Preferred inorganic additives include sulfuric acid, phosphoric 
acid, nitric acid, ammonium salts, potassium salts, sodium salts or other 
cationic salts of sulfates and phosphates. 
A wide range of conventional organic acids may be used in the present 
invention to enhance the selectivity to oxide polishing rate, such as 
monofunctional acids, difunctional acids, hydroxyl/carboxylate acids, 
chelating and non-chelating acids. Preferably, the organic acid is 
selected from the group of acetic acid, adipic acid, butyric acid, capric 
acid, caproic acid, caprylic acid, citric acid, glutaric acid, glycolic 
acid, formic acid, fumaric acid, lactic acid, lauric acid, malic acid, 
maleic acid, malonic acid, myristic acid, oxalic acid, palmitic acid, 
phthalic acid, propionic acid, pyruvic acid, stearic acid, succinic acid, 
tartaric acid, valeric acid and derivatives thereof. It is also believed 
that the organic acids of the present invention possess the ability to 
complex or associate with dissolving metals and improve the removal rate 
of metal thin-films such as aluminum, titanium and the like, during the 
CMP process. 
Typically, the organic acid is present in the slurry, individually or in 
combination with other organic acids, in an amount sufficient to enhance 
the oxide selectivity without detrimentally effecting the stability of the 
slurry. As such, the organic acid is typically present in the slurry from 
about 0.05 % to 15 % by weight, and preferably in a range between 0.5% and 
5.0% by weight. 
It has been found that an interrelationship exists between the metal oxide 
abrasive, the oxidizing agent and the organic acid of the present 
invention to improve or enhance the selectivity to oxide polishing rate of 
the polishing slurry. 
In order to further stabilize a polishing slurry containing an oxidizing 
agent against settling, flocculation and decomposition of the oxidizing 
agent, a variety of additives, such as surfactants, polymeric stabilizers 
or other surface active dispersing agents, can be used. The surfactant can 
be anionic, cationic, nonionic, amphoteric and combinations of two or more 
surfactants can be employed. Furthermore, it has been found that the 
addition of a surfactant may be useful to improve the 
within-wafer-non-uniformity (WIWNU) of the wafers, thereby improving the 
planarity of the surface of the wafer and improving yield. 
In general, the amount of an additive used, such as a surfactant, in the 
present invention should be sufficient to achieve effective steric 
stabilization of the slurry and will typically vary depending on the 
particular surfactant selected and the nature of the surface of the metal 
oxide abrasive. For example, if not enough of a selected surfactant is 
used, it will have little or no effect on stabilization. On the other 
hand, too much of the surfactant may result in undesirable foaming and/or 
flocculation in the slurry. As a result, additives like surfactants should 
generally be present in a range between about 0.001% and 10% by weight. 
Furthermore, the additive may be added directly to the slurry or treated 
onto the surface of the metal oxide abrasive utilizing known techniques. 
In either case, the amount of additive is adjusted to achieve the desired 
concentration in the polishing slurry. 
The polishing slurry may be produced using conventional techniques known to 
those skilled in the art. Typically, the oxidizing agent, organic acid and 
other desired additives, such as surfactants, are mixed into the aqueous 
medium, such as deionized or distilled water, at pre-determined 
concentrations under low shear conditions until such components are 
completely dissolved in the medium. A concentrated dispersion of the metal 
oxide abrasive, such as fumed alumina, is added to the medium and diluted 
to the desired loading level of abrasive in the final polishing slurry. 
The polishing slurry of the present invention may be used as one package 
system (metal oxide abrasive and oxidizing agent, if desired, in a stable 
aqueous medium), a two package system (the first package consists of the 
metal oxide abrasive in a stable aqueous medium and the second package 
consists of the oxidizing agent) or a multi-package system with any 
standard polishing equipment appropriate for use on the desired metal 
layer of the wafer. The two or multi package system is used when an 
oxidizing agent decomposes or hydrolyzes over time. In the two or multi 
package system, the oxidizing agent and other desirable additives may be 
added to the slurry just prior to polishing. 
The polishing slurry of the present invention has been found to 
significantly lower or inhibit the silicon dioxide polishing rate, thus 
yielding enhanced selectivity. In addition, the polishing slurry may be 
effectively used to provide controlled polishing selectivities to other 
thin-film materials used as underlayers or barrier films in current 
integrated circuit technology, such as titanium, titanium nitride and the 
like. The polishing slurry of the present invention may be used during the 
various stages of semiconductor integrated circuit manufacture to provide 
effective polishing at desired polishing rates while minimizing surface 
imperfections and defects. 
Non-limiting illustrations of the polishing slurry of the present invention 
follow. 
EXAMPLE 1 
Eight polishing slurries were prepared to investigate the interrelationship 
between the abrasive, the oxidizing agent and the organic acid on 
polishing and selectivity in accordance with the present invention. The 
slurries consisted of fumed alumina, an oxidizing agent, an organic acid, 
and the remainder deionized water. The properties of the slurries are 
described in Table I. The slurry was utilized to chemically-mechanically 
polish an aluminum layer having a thickness of approximately 12,000 .ANG. 
using a composite pad available from Rodel, Inc., Newark, Del. The 
polishing conditions and performance results are illustrated in Table II. 
TABLE I 
______________________________________ 
Abrasive Oxidizing Agent 
Organic Acid 
Fumed Alumina 
Ammonium Persulfate 
Succinic Acid 
Concentra- Concentra- Concentra- 
Sample tion (wt %) tion (wt %) tion (wt %) 
______________________________________ 
1 6 4 5 
2 6 4 0.05 
3 6 8 5 
4 6 8 0.05 
5 3 4 5 
6 3 4 0.05 
7 3 8 5 
8 3 8 0.05 
______________________________________ 
TABLE II 
______________________________________ 
Flow Al Pol- 
Pres- Rate Table Spindle 
ishing 
Oxide 
Sam- sure (ml/ Speed Speed Rate Rate 
ple (psi) min) (rpm) (rpm) (A/min) 
(A/min) 
Selectivity* 
______________________________________ 
1 5 200 100 125 4993 13 384:1 
2 5 200 100 125 4662 80 58:1 
3 5 200 100 125 4782 21 228:1 
4 5 200 100 125 4738 54 88:1 
5 5 200 100 125 4196 13 323:1 
6 5 200 100 125 4177 46 91:1 
7 5 200 100 125 4133 15 276:1 
8 5 200 100 125 4445 32 139:1 
______________________________________ 
(*Aluminum:Thermal Oxide Selectivity, i.e. the polishing rate ratio 
between the aluminum layer and the thermal oxide). 
As shown in Table II, increasing the alumina abrasive content from 3% by 
weight to 6% by weight in the polishing slurry enhanced the aluminum 
removal rate by approximately 500 A/minute. Increasing the oxidizing 
agent, ammonium persulfate, from 4% to 8% did not affect any of the 
response variables significantly. Increasing the organic acid, succinic 
acid, from 0.05% (samples 2, 4, 6 and 8) to 5% (samples 1, 3, 5 and 7) 
significantly lowered the oxide removal rate, thereby enhanced the 
selectivity to thermal oxide by approximately 200:1. This example 
demonstrates the interrelationship between the metal oxide abrasive, the 
oxidizing agent and the organic acid of the present invention to improve 
or enhance the selectivity to oxide polishing rate of the polishing 
slurry. 
EXAMPLE 2 
Seven polishing slurries were prepared to investigate the use of various 
organic acids at various concentrations and their effect on polishing and 
selectivity in accordance with the present invention. The slurries 
consisted of fumed alumina, an oxidizing agent, an organic acid, and the 
remainder deionized water. The properties of the slurries are described in 
Table III. The slurry was utilized to chemically-mechanically polish an 
aluminum layer having a thickness of approximately 12,000 .ANG. with a 
blown polyurethane felt pad (available from Rippey Corporation, El Dorado 
Hills, Calif.). The polishing conditions and performance results are 
illustrated in Table IV. 
TABLE III 
______________________________________ 
Slurry Surface Oxidizer Organic 
Sample 
Abrasive(wt %) 
Area (m.sup.2 /g) 
(wt %) Acid (wt %) 
______________________________________ 
1 Fumed 55 Ammonium acetic(0.5%) 
Alumina(5%) Persulfate(4%) 
2 Fumed 55 Ammonium acetic(3.0%) 
Alumina(5%) Persulfate(4%) 
3 Fumed 55 Ammonium tartaric(0.5%) 
Alumina(5%) Persulfate(4%) 
4 Fumed 55 Ammonium tartaric(3.0%) 
Alumina(5%) Persulfate(4%) 
5 Fumed 55 Ammonium phthalic(0.2%) 
Alumina(5%) Persulfate(4%) 
6 Fumed 55 Ammonium gluconic(0.5%) 
Alumina(5%) Persulfate(4%) 
7 Fumed 55 Ammonium gluconic(3.0%) 
Alumina(5%) Persulfate(4%) 
______________________________________ 
TABLE IV 
______________________________________ 
Flow Al Pol- 
Pres- Rate Table Spindle 
ishing 
Oxide 
Sam- sure (ml/ Speed Speed Rate Rate 
ple (psi) min) (rpm) (rpm) (A/min) 
(A/min) 
Selectivity* 
______________________________________ 
1 5 200 50 50 1963 4.3 457:1 
2 5 200 50 50 1451 0.2 7255 
3 5 200 50 50 1090 1.75 623 
4 5 200 50 50 1128 10.35 109 
5 5 200 50 50 1768 12.3 144 
6 5 200 50 50 613 17.7 34.6 
7 5 200 50 50 341 4.75 71.8 
______________________________________ 
(*Aluminum:Thermal Oxide Selectivity, i.e. the polishing rate ratio 
between the aluminum layer and the thermal oxide). 
Table IV illustrates the a number of different organic acids may be used in 
the polishing slurry of the present invention to suppress the oxide 
polishing rate, thereby significantly improving the selectivity. A 
desirable high selectivity is typically defined as having a removal rate 
equal or greater than 50:1 between the first layer (aluminum) and the 
second layer (thermal oxide). In contrast to the previously disclosed 
slurries of the prior art, the polishing slurry of the present invention 
exhibits a high degree of selectivity to the insulator layer, SiO.sub.2. 
It should further be noted that the Al rate may need to be improved 
depending on the desired polishing rate and in order to achieve acceptable 
wafer throughput. 
EXAMPLE 3 
The polishing slurries of samples 3 and 4 in Example 2 were further 
investigated to demonstrate the effect of the present invention on 
selectivity to other metal layers. The properties of samples 3 and 4 are 
reproduced in Table V. The slurry was utilized to chemically-mechanically 
polish a titanium layer having a thickness of approximately 12,000 .ANG. 
with a Rodel 28" pad (available from Rodel, Inc., Newark, Del.). The 
polishing conditions and performance results are illustrated in Table VI. 
TABLE V 
______________________________________ 
Slurry Surface Oxidizer Organic 
Sample 
Abrasive(wt %) 
Area (m.sup.2 /g) 
(wt %) Acid (wt %) 
______________________________________ 
1 Fumed 55 Ammonium tartaric(0.5%) 
Alumina(5%) Persulfate(4%) 
2 Fumed 55 Ammonium tartaric(3.0%) 
Alumina(5%) Persulfate(4%) 
______________________________________ 
TABLE VI 
______________________________________ 
Flow Al Pol- 
Pres- Rate Table Spindle 
ishing 
Ti 
Sam- sure (ml/ Speed Speed Rate Rate 
ple (psi) min) (rpm) (rpm) (A/min) 
(A/min) 
Selectivity* 
______________________________________ 
1 5 200 50 50 1090 259 4:1 
2 5 200 50 50 1128 598 2:1 
______________________________________ 
(*Aluminum:Titanium Selectivity, i.e. the polishing rate ratio between th 
aluminum layer and the titanium). 
Table VI demonstrates that, in addition to improving the aluminum to oxide 
selectivity, the polishing slurry of the present invention may further be 
used to increase the Ti removal rate, thereby lowering the selectivity to 
Ti. A desirable low selectivity is typically defined a having a removal 
rate equal or less than 10:1 between the first and second layers. More 
preferred is a removal rate equal or less than 5:1 between the first and 
second layers. This process can be effectively replicated while 
fabricating semiconductor circuits to provide the desired selectivity to 
the Ti cladding film accompanying the Al intra-chip wiring. 
It is believed that the buffering capacity or concentration of the organic 
acid in the polishing slurry has been found to play an important role in 
promoting low Ti selectivity. The buffering capacity, which may be 
represented by the free acid component of the slurry, may be determined by 
utilizing known techniques to calculate the free acid to total acid 
points. 
EXAMPLE 4 
A polishing slurry was prepared to evaluate the dissolution and 
self-passivation of aluminum. The slurry consisted of 3% by weight fumed 
alumina as the abrasive, 3% succinic acid as the organic acid, 4% ammonium 
persulfate as the oxidizing agent and the remainder deionized water. A 
rotating disk electrode (RDE) system setup, as illustrated in FIG. 1, was 
adopted to evaluate the dissolution rate of aluminum with a continuous 
renewal of the aluminum surface by abrasion, as well as the corrosion and 
passivation of the metal in the slurry immediately after the abrasion. A 
metallic sample consisting of aluminum and copper in the form of a slug 
was embedded in a stick-resistant sleeve and attached to a rotating motor. 
The electrode was placed into an electrochemical cell with a scouring pad 
at the bottom. With a controlled rotation of 500 rpm and downward pressure 
of 1200 grams, the metal surface of the slug was abraded by the slurry and 
its dissolution was determined by simultaneously calculating 
electrochemical data provided by the electrode. After the measurement with 
abrasion was complete (taking approximately 100 seconds), the electrode 
was raised away from the pad while continuing the rotation of the 
electrode and simultaneously recording the electrode potential. Once the 
potential was stabilized, after approximately 5 minutes, the 
potentiodynamic polarization was re-applied to determine the rate of 
dissolution in the absence of the abrasion. A sweep rate of 10 mV/sec and 
a sufficiently large potential range was allowed to provide an estimate of 
the rates and of the rate determining steps. 
The results obtained from the slurry are demonstrated in FIGS. 2 and 3. 
During the abrasion step, aluminum dissolves at a rate of 7.2.times.10-3 
.ANG./cm.sup.2 (1,728 .ANG./minute). However, as noted in FIG. 3, as soon 
as the abrasion is stopped, the aluminum potential readily increases and 
the repassivation of the surface sets in. The dissolution of aluminum 
after the abrasion (which is equivalent to wet etching) is low, that is at 
about 1.times.10.sup.-5 .ANG./cm.sup.2 or 2.4 .ANG./min. The shape of the 
potential time curve indicates that normally corrosion-sensitive aluminum 
readily repassivates. As a result, the organic acid was found to act as a 
corrosion inhibitor, thereby minimizing concerns of any uncontrollable 
corrosion loss during CMP processing. 
As described herein, the oxidizing agent, the organic acid, and other 
additives, of the polishing slurry can be tailored to provide effective 
polishing to metal layers at desired polishing rates while minimizing 
surface imperfections, defects and uncontrollable corrosion lost. In 
addition, the polishing slurry of the present invention has been found to 
significantly lower or inhibit the silicon dioxide polishing rate, thus 
yielding enhanced selectivity with respect to the dielectric layer. 
Furthermore, the polishing slurry may be effectively used to provide 
controlled polishing selectivities to other thin-film materials used in 
current integrated circuit technology, such as copper and titanium, as 
well as underlayers such as titanium, titanium nitride, titanium tungsten 
and similar alloys. 
It is further understood that the present invention is not limited to the 
particular embodiments shown and described herein, but that various 
changes and modifications may be made without departing from the scope and 
spirit of the invention.