Activated polishing compositions

Disclosed is a process for preparing activated compositions and the compositions derived therefrom which are suitable for polishing surfaces, particularly integrated circuits, wherein a base abrasive is activated by addition of a second cation whose oxide exhibits a higher polishing rate than the base abrasive alone. The activation is effected by chemical adsorption of the activating cation onto the base abrasive during cyclic impact in an aqueous medium whose pH is at a level which is favorable for adsorption of the activating cation onto the base abrasive surface.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates generally to the polishing of glasses, 
semiconductors, and integrated circuits. More particularly, this invention 
relates to the surface preparation of articles wherein a more rapid rate 
of polishing is desired. 
Description of the Prior Art 
Polishing solutions, or slurries generally consist of a solution which 
contains a concentration of abrasive particles. The part, or substrate, is 
bathed or rinsed in the slurry in conjunction with an elastomeric pad 
which is pressed against the substrate and rotated such that the slurry 
particles are pressed against the substrate under load. The lateral motion 
of the pad causes the slurry particles to move across the substrate 
surface, resulting in wear, volumetric removal of the substrate surface. 
The rate of surface removal (polishing rate) is largely determined by the 
magnitude of the applied pressure, the velocity of pad rotation and the 
chemical activity of the abrasive particle. While virtually any particle 
of sufficiently small size may be used for polishing, economically useful 
high polishing rates are exhibited by a relatively small number of 
compounds. For most substrates (e.g., SiO.sub.2 or silicates) the highest 
rates are found for formulations composed primarily of CeO.sub.2 or 
ZrO.sub.2. In consequence, there is a large body of prior art describing 
the composition and preparation of polishing slurries based on these two 
oxides. 
Extensive efforts have been made to develop additives which accelerate the 
rate of polishing in order to make the polishing process more economical. 
Such accelerants may be generally classified as etchants, which would by 
themselves dissolve the substrate, or polishing compound accelerants, 
which increase rates when added to the abrasive itself. Etchant 
accelerants, such as described in U.S. Pat. No. 4,169,337 are commonly 
employed in conjunction with SiO.sub.2 abrasives to polish silicon wafers. 
These additives can be classified into two categories; (1) additives that 
increase or buffer the solution pH (e.g. organic amines), or (2) organic 
compounds, generally amines, that may additionally increase the Si 
corrosion rate by complexing or sequestering Si (e.g., ethylene diamine or 
piperazine). These classes of etchant accelerants are distinctly different 
than the accelerants employed in the present invention. 
A variety of polishing compound accelerants have been described. They can 
be classified into two main categories; (1) Additives which are 
coprecipitated with the base abrasive prior to calcination, and (2) water 
soluble additives to the final polishing slurry. Examples of prior art 
belonging to the first category are found in U.S. Pat. No. 3,262,766 
(Nonamaker), U.S. Pat. No. 3,768,989 (Goetzinger and Silvernail) and U.S. 
Pat. No. 3,301,646 (Smoot). These examples am important, as they 
illustrate the primary prior art pathways for coprecipitating activating 
substances. 
Nonamaker teaches the incorporation of small amounts of SiO.sub.2 (&lt;5 % ) 
to a mixture consisting primarily of rare earth oxides (including 
CeO.sub.2) prior to calcination in order to accelerate polishing rates in 
the final calcined product. The precise mechanism of this effect is not 
understood. 
In a similar fashion, Goetzinger and Silvernail taught coprecipitation of 
rare earth carbonates, primarily cerium carbonate, together with 
Wollastonite (calcium metasilicate). The co-precipitate was subsequently 
calcined to yield an activated final product. Once again, the precise 
mechanism of the activation was not disclosed. 
Smoot taught the deliberate incorporation of Calcium or other divalent ions 
(e.g., Mg.sup.2 +) into zirconium oxide to produce calcium-stabilized 
cubic zirconium oxide, a material which is widely used as a structural 
ceramic. The process consisted of dry batch mixing of ZrO.sub.2 and the 
stabilizing compound, typically CaCO.sub.3, followed by calcination of the 
mixture at elevated temperature (.sup..about. 2100.degree. F.) to form a 
cubic ZrO.sub.2 product. The stabilized cubic zirconia was found to have 
an accelerated polishing rate relative to the normal monoclinic phase of 
zirconia obtained without addition of the calcium accelerant. 
The second pathway for activation is the activation of slurries by addition 
of water soluble additives to the final solution. As reviewed by 
Silvernail ("Mechanism of Glass Polishing", Glass Industry, vol. 52, pp. 
172-5, 1971), addition of Ce(OH).sub.4 to polishing slurries can produce 
significantly increased polishing rates. In particular, some previously 
inactive oxides, such as Tb.sub.4 O.sub.7 showed high polishing rates 
after Ce(OH).sub.4 addition. Other compounds have also been used as 
accelerants. Shlishevskii and Migus'kina (Sov. J. Opt. Technol., vol. 44, 
pp. 680-1, 1977) demonstrated as much as 2X inprovement in polishing rate 
when 2% ammonium molybdate, 1% Mohr's salt (NH.sub.4)SO.sub.4 FeSO.sub.4, 
or 1% zinc sulphate was added to a CeO.sub.2 -based polishing slurry. The 
basis of the effect was ascribed to complexation of silicate reaction 
products with the additive compounds so as to prevent their redeposition 
back onto the substrate surface. 
The prior art methods for enhancing polishing activity suffer from a number 
of deficiencies. First, while etchant additives may increase the overall 
rate of surface removal of the substrate, their action is isotropic, i.e., 
they attack all portions of the exposed substrate surface regardless of 
position. This leads to significantly degraded surface roughness and 
texture in the polished substrate. Their incorporation is therefore 
generally considered to be undesirable for slurries used to prepare high 
quality surfaces (i.e. final polishing). This is particularly true for the 
polishing of Si wafers. 
As regards polishing compound additives, the principal deficiency of adding 
additives prior to calcination is that rates cannot be adjusted subsequent 
to formation of the final polishing compound. An additional deficiency is 
that the technique cannot readily be applied to some polishing abrasives 
of technical importance, particularly SiO.sub.2, which is commonly used 
for Si wafer and integrated circuit polishing. Solution additives, such as 
Ce(OH).sub.4, have not given consistent activation and cannot be used with 
SiO.sub.2 -based polishing slurries due to gelation. 
Yet another obvious way of increasing the polishing rate of a slurry with 
low rate (e.g. SiO.sub.2) would be to simply add to it a portion of 
another slurry (e.g., CeO.sub.2). While this has not been the subject of 
prior art disclosures, it is a common practice in the polishing art. This 
technique suffers from two deficiencies. First, the rate of increase is 
linearly proportional to the amount of the second slurry added. Thus, to 
achieve a substantial amount of acceleration, a significant fraction of 
the second material must be added. Second and more critical, addition of 
the second slurry changes the particle size distribution of the original 
slurry unless the two particle size distributions are precisely matched. 
While this may be possible, it is generally not economically feasible. 
This is particularly true in the case of colloidal silica slurries such as 
are used in Si wafer polishing. These slurries have extremely small 
particle sizes, typically 50-100 nm. In contrast, all known commercial 
CeO.sub.2 -based slurries have mean particle diameters in excess of 1000 
nm. Incorporation of such larger particles would have a catastrophic 
effect on the quality of the Si wafer surfaces produced after polishing. 
From the above, it is clear that an additive which could increase 
polishing rams without increasing the static corrosion of the substrate, 
which could be applied to a variety of abrasive types, particularly 
SiO.sub.2, subsequent to particle formation, and which could be applied 
without alteration of the original slurry particle size would be a 
significant improvement over prior art. 
Accordingly, it is the object of this invention to provide an improved 
means of increasing the polishing rate of slurries without increasing the 
overall corrosiveness of the polishing solution, which can be easily 
applied to a variety of abrasive particles, particularly SiO.sub.2, and 
which can be employed in a manner which does not alter the original 
particle size. 
It is also an object of this invention to provide polishing slurries with 
significantly improved performance which are prepared by said means. 
These and other objects of the invention will become apparent to those 
skilled in the art alter referring to the following description and 
examples. 
SUMMARY OF THE INVENTION 
The object of this invention has been achieved by providing a process for 
preparing compositions suitable for polishing surfaces, particularly 
integrated circuits, wherein the base abrasive, e.g., SiO.sub.2, is 
activated by addition of a second cation, whose oxide exhibits a higher 
polishing rate than the base abrasive alone. The activation is effected by 
chemical adsorption of the activating cation onto the base abrasive. This 
adsorption is accomplished by co-milling the base abrasive and small 
amounts of an activating oxide in an aqueous medium whose pH is at a level 
which is favorable for adsorption of the activating cation onto the base 
abrasive surface. Alternatively, one may employ milling abrasives which 
themselves are made from or contain said activating cation under the same 
solution conditions to obtain the same result.

DESCRIPTION OF THE INVENTION 
This method of preparation differs substantially from prior art activation 
processes in that it is applicable to a variety of base abrasive 
particles, providing that certain minimum conditions for adsorption are 
met, no subsequent thermal processing of the abrasive is required, and, 
when the milling abrasives are used as the source of the activating 
cation, no foreign particles are added to the slurry which might 
negatively change the particle size distribution. Additionally, as will be 
shown in subsequent examples, only small quantities of activating cations 
are required for a substantial acceleration of polishing rate. This makes 
the technique of the present invention clearly different from the case of 
a simple addition of a second more active abrasive component. 
The basis for the effect is transfer of active cations to the surface of 
the base abrasive by cyclic impact with a solid source of said active 
cation. As a consequence of this adsorption, the abrasive particle takes 
on the surface characteristics of the activating ion itself. Thus a 
SiO.sub.2 particle treated with a ZrO.sub.2 source will exhibit surface 
charge characteristics similar to that of a ZrO.sub.2 particle. During 
polishing, the particle will, therefore, exhibit rates characteristic of a 
ZrO.sub.2 abrasive rather than a SiO.sub.2 abrasive. 
While there is extensive prior art technology existing for changing surface 
charge of solid or particulate surfaces by adsorption of a second cation 
(e.g. Al.sup.3+ on SiO.sub.2, see R. Iler, The Chemistry of Silica, 
Wiley-Interscience, NYC, 1979, pp. 667-76) it has been exclusively 
effected by adsorption from solution. In sharp contrast, the proposed 
mechanism for the present invention is that interparticle bonds are 
momentarily formed and broken during the impact process, resulting in 
retention of a surface concentration of activating cations on the base 
particle surface. 
When said cyclic impacts are performed under solution conditions favorable 
to adsorption of the activating cation of interest onto the base abrasive 
surface, retention of the activating ion may be enhanced. The recognition 
of the importance of controlling the solution pH to stabilize retention of 
activating cations onto the base abrasive is a key factor in the success 
of the present invention. For each combination of activating cation and 
base abrasive there will be a specific pH range which is optimal. This may 
be relatively narrow or quite broad. For cations of practical interest, 
particularly Zr.sup.4+, strong adsorption onto silica surfaces occurs over 
virtually the entire pH range, and precise control of pH during the 
milling process is of secondary importance. These cations are preferred, 
as it allows broader latitude in the manufacturing of activated silica 
polishing slurries. 
Examples of polishing slurries prepared by the present invention are set 
forth below to illustrate the essential features and results. They are not 
meant to be restrictive in any way. 
EXAMPLES 
Example 1 
A polishing slurry (composition 1.a) was prepared as follows. 30 kg of 
fumed silica was added to 70 kg. deionized water and blended using a high 
speed mixer until throughly dispersed. The mixture was then fed through an 
agitator mill which contained a zirconium silicate mill medium. The silica 
mixture was milled at a flow rate of 1.5 liter/min and passed to a second 
tank. After milling, sufficient water was added to dilute the milled 
product to 13% solids concentration, and ammonium hydroxide was added to 
adjust the final pH to 10.5. Chemical analysis of composition 1.a showed a 
ZrO.sub.2 content of 1.4 ppm in the final composition. Measurements of the 
surface potential (zeta potential) were made using acoustophoresis to 
assess the surface charge of the milled particles relative to a high 
purity silica sol (2355) and a high purity ZrO.sub.2 sol. As shown in FIG. 
1; the zeta potential for composition 1.a was markedly different from that 
of a high purity silica sol (2355). The isoelectric pH, or pH at which the 
zeta potential was zero, was shifted markedly to higher pH (from 2.2 to 
4). This value is intermediate between the silica and zirconia reference 
samples. 
An equavalent slurry (composition 1.b) was prepared in the same manner but 
without milling. Chemical analysis of composition 1.b indicated no 
ZrO.sub.2 present, confirming that the ZrO.sub.2 observed in composition 
1.a had originated from the zirconium silicate mill media. 
Both compositions were then used to polish samples of thermally grown 
SiO.sub.2 on Si substrates using a Strasbaugh model 6DS planarizer to 
assess polishing activity. Polishing conditions were 7 psi downforce, 20 
rpm table speed, and 150 ml/min. slurry flow, and an IC1000 polishing pad, 
with dressing between sample runs. Composition 1.a gave a polishing ram of 
1200 angstroms/min. In contrast, composition 1.b polished at only 600 
angstroms/min, a two-fold difference. 
Example 2 
Four lots of slurry (hereinafter designated as compositions 2.a-2.d) were 
prepared in the same manner as composition 1.a of the previous example. 
Composition 2.a was identical to composition 1.a in every respect. 
Compositions 2.b, 2.c, and 2.d were made with 1%. 2 %, and 4 % CeO.sub.2 
substitutions for SiO.sub.2 respectively. The CeO.sub.2 was added to the 
initial dispersion prior to milling. 
Following slurry preparation, all compositions, as well as a portion of a 
commercially available silica based polishing compound (SC-112, 
manufactured by Cabot Corp.), were used to polish samples of thermally 
grown SiO.sub.2 on Si substrates using a Strasbaugh model 6CA polishing 
machine for assessment of polishing activity. Polishing conditions were 7 
psi downforce, 20 rpm table speed, and 150 ml/min. slurry flow, and an 
IC1000 polishing pad. No pad conditioning was employed. Average polishing 
rates are summarized below. 
TABLE 1 
______________________________________ 
Sample Polishing rate (angstroms/min) 
______________________________________ 
SC-112 901 
Composition 2.a 
1184 
Composition 2.b 
1106 
Composition 2.c 
1658 
Composition 2.d 
1665 
______________________________________ 
Composition 2.a gave a polishing rate equivalent to 1.a, as expected. A 
significant amount of additional activation was observed with CeO.sub.2 
additions. However, the activation was clearly non-linear; a threshold 
concentration of .sup..about. 2 % CeO.sub.2 gave the most pronounced 
effect. Additional CeO.sub.2 addition did not give further increase in 
rate (2.c vs. 2.d). This threshold activation effect is quite different 
from the linear effect expected from simple addition of CeO.sub.2 to the 
slurry. Also, as was the case example 1, the quantity of CeO.sub.2 
required for activation is substantially below levels normally used to 
obtain rate enhancement in simply blending two slurries.