US20060240748A1

US20060240748A1 - Method of manufacturing abrasive composition

Info

Publication number: US20060240748A1
Application number: US10/545,370
Authority: US
Inventors: Yoshiharu Ohta; Yasuyuki Itai; Keiji Fukuda
Original assignee: Nitta Haas Inc
Current assignee: Nitta DuPont Inc
Priority date: 2003-02-14
Filing date: 2004-02-10
Publication date: 2006-10-26
Also published as: TW200426205A; WO2004072203A1; TWI306115B; JP4426192B2; JP2004262975A

Abstract

A process for producing a polishing composition excelling in dispersion stability wherein the amount of agglomerated particles is reduced. In step 1-1 thereof, ultrapure water is adjusted so as to have a pH value of 1.0 to 2.7. Under shearing force given by a high shear disperser, fumed silica powder of 50 to 200 m²/g specific surface area is charged therein until an initial silica concentration of 46 to 54 wt %, and the high shear disperser is operated so as to apply shearing force for 1 to 5 hours. In step 1-2, a small amount of ultrapure water is added to the silica dispersion so as to realize a silica concentration of 45 to 53 wt % and shearing force is applied for 10 to 40 minutes. In step 1-3, ultrapure water is added to the silica dispersion so as to realize a silica concentration of 33 to 44 wt % and shearing force is applied for 0.5 to 4 hours. In step 2-1, the silica dispersion is added to an aqueous basic substance solution prepared so that a pH value after mixing is in a range of 8 to 12 and so that silica concentration is in a range of 10 to 30 wt %.

Description

TECHNICAL FIELD

The present invention relates to a method of manufacturing an abrasive composition for use in a polishing step of semiconductor manufacturing processes.

BACKGROUND ART

In a field of manufacturing semiconductors, the planarization technique for semiconductor and metal layers is becoming an important constituent technology as semiconductor devices scale down and become more and more multilayered. When an integrated circuit is formed in a wafer, stacking layers without planarizing irregularities owing to electrode wiring, etc. will enlarge elevation changes and therefore worsen the planarity extremely. Also, enlarged elevation changes make it difficult to focus on both the reentrants and protrusions in lithography, thus making it impossible to realize scaling down. Therefore, there is a need to perform a planarization process for removing irregularities on a wafer surface at an appropriate stage of sacking. Such planarization process may be an etch-back method by which portions of irregularities are removed by etching, a film-growing method by which a planar film is formed by plasma CVD (Chemical Vapor Deposition), etc., a fluidizing method by which planarization is practiced by a thermal treatment, and a selective growing method by which reentrants are embedded by selective CVD etc.
The above methods have problems in that: whether or not they can be adopted depends on the kind of films, such as an insulating film or a metal film; and the area which can be planarized by them is remarkably small. As a technique for planarization process that enables these problems to be overcome, there is planarization by CMP.
In planarization process by CMP, a polish pad pressed against a silicon wafer is relatively moved to polish a surface thereof while a slurry with fine particles (whet grains) suspended therein is supplied onto a surface of the polish pad, whereby a widespread range of the wafer surface can be planarized with high precision.
A CMP apparatus for planarization by CMP is mainly constructed of: a rotary platen section; a carrier section; a slurry-supply section; and a dressing section. The rotary platen section has a top face on which a polish pad is stuck by an adhesive tape or the like, and a bottom face connected with a rotation-drive mechanism through a rotating shaft. The carrier section holds a silicon wafer on its bottom face, an object to be polished, by a backing material and a retainer ring, and brings a processed face of the silicon wafer into press-contact with the polish pad. the carrier section is connected with the rotation-drive mechanism through the rotating shaft on the side of the top face thereof.
The slurry-supply section supplies the surface of the polish pad with a slurry with particles of silica, ceria, alumina, etc. suspended in a medium. The dressing section includes a plate with industrial diamond particles electrodeposited thereon, and razes a portion on which polishing waste, etc. are deposited thereby to restore the surface of the polish pad degraded in polish property.
The CMP apparatus forces the rotation-drive mechanism to rotate the rotary platen section and carrier section in parallel with supplying a slurry to a substantially central portion of the polish pad and causing relative movement of the silicon wafer and polish pad, and thereby polishes the processed face of a silicon wafer.
In recent years, with scaling down of the design rule for IC (Integrated Circuit) chips, a microscratch ascribable to a slurry that appears on a polished surface of a silicon wafer has presented a problem. What can be considered to be a factor of such microscratch is an agglomerate of whet grains suspended in a medium or a bulky particle existing as a dispersion-failed material.
As a raw material of silica slurry, fumed silica or colloidal silica is used. Fumed silica allows a silica slurry with less impurities to be produced because of its higher purity in comparison to colloidal silica. However, it is hard to highly disperse it in a medium because of its high agglomerate property.
As a conventional method of manufacturing a silica slurry that aims at enhancing the dispersion stability of fumed silica, there are methods described in Japanese Examined Patent Publications JP-B2 2,935,125 and 2,949,633, and Japanese Unexamined Patent Publication JP-A 2001-26771. With any of the methods, the realization of stable dispersibility is attempted by specifying a shearing condition, the concentration of silica, etc.
While a silica slurry was made using fumed silica as a raw material by the manufacturing method described in the above-describe patent documents in fact, the ability of silica to disperse was insufficient and lots of agglomerates were present in the slurry.

DISCLOSURE OF INVENTION

It is an object of the invention to provide a method of manufacturing an abrasive composition that is superior in dispersion stability in which fewer agglomerated particles are produced.
The invention is a method of manufacturing an abrasive composition comprising:
a first step of preparing an acid fumed silica dispersion; and
a second step of adding the fumed silica dispersion to an aqueous basic substance solution which is prepared so that the abrasive composition to be obtained after the end of mixing with the fumed silica dispersion has predetermined pH and silica concentration, and mixing the fumed silica dispersion and the aqueous solution.
In addition, the invention is characterized in that the aqueous basic substance solution is prepared so that the abrasive composition has a pH of 8 to 12 and a silica concentration of 10 to 30 wt %.
Also, the invention is characterized in that the fumed silica has a relative surface area of 50 to 200 m²/g.
Further, the invention is characterized in that the aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.
According to the invention, first of all, an acid fumed silica dispersion is prepared in the first step. Incidentally, it is desirable that the relative surface area of the fumed silica to be used is 50 to 200 m²/g.
Then, in the second step, an aqueous basic substance solution is prepared. The aqueous basic substance solution is controlled in concentration and volume by mixing with the fumed silica dispersion prepared in the first step so that the intended abrasive composition has a pH of 8 to 12 and a silica concentration of 10 to 30 wt %. The aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.
While an aqueous basic substance solution is added to a fumed silica dispersion according to the conventional manufacturing method, a fumed silica dispersion is added to an aqueous basic substance solution that has been prepared according to the invention.
There is an excess of aqueous basic substance solution in an early stage of feeding fumed silica dispersion and as such, the mixture is strong alkaline and therefore the pH-shock is caused. However, the occurrence of agglomerate is suppressed because the silica concentration is extremely low. While the silica concentration of the mixture increases as the feeding is continued, feeding fumed silica dispersion makes the mixture weaker alkaline, which weakens the pH-shock and thus suppresses the occurrence of agglomerate.
Thus, an abrasive composition that is superior in dispersion stability and has fewer agglomerated particles can be achieved.
Also, the invention is characterized in that the first step includes the steps of:
feeding fumed silica into water which has been prepared to have a pH of 1.0 to 2.7 so that an initial silica concentration reaches 46 to 54 wt % and applying a high shearing force, thereby preparing a fumed silica dispersion;
adding water to the fumed silica dispersion so that its silica concentration reaches 45 to 53 wt %; and
further adding water to the fumed silica dispersion so that its silica concentration reaches 33 to 44 wt %.
According to the invention, first the fumed silica is fed into the water which has been prepared to be pH 1.0 to 2.7 so that its initial silica concentration reaches 46 to 54 wt %, and a high shearing force is applied, whereby a fumed silica dispersion is prepared. By controlling pH to 1.0 to 2.7, it is made possible to applying a high shearing force efficiently thereby to enhance the dispersibility.
Then, water is added to the fumed silica dispersion so that its silica concentration reaches 45 to 53 wt %. The addition of a small amount of water can lower the viscosity of the abrasive composition.
Finally, water is further added so that the silica concentration reaches 33 to 44 wt %. The generation of agglomerate can be suppressed by making the silica concentration 33 to 44 wt %.
Also, the invention is characterized in that the mixing of the fumed silica dispersion and the aqueous basic substance solution is finished in less than 5 hours in the second step.
According to the invention, the mixing of the fumed silica dispersion and aqueous basic substance solution is finished in less than 5 hours. By finishing the mixing in 5 hours, pH of the mixture can be lowered quickly to shorten the time during which the mixture is placed in a pH condition of fumed silica prone to agglomerate and therefore to suppress the occurrence of agglomerate.
Further, the invention is characterized by further comprising a third step of filtering the abrasive composition obtained in the second step with a filter having a filtering accuracy of 1 to 4 μm.
According to the invention, in the third step, the abrasive composition obtained in the second step is filtered with a filter having a filtering accuracy of 1 to 4 μm.
As described above, agglomerate can be removed efficiently by use of a filter having a filtering accuracy of 1 to 4 μm because there are few agglomerates in the abrasive composition obtained in the second step.

BRIEF DESCRIPTION OF DRAWINGS

Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:
FIG. 1 is a flow chart, as a mode embodying the invention;
FIG. 2 is a graph showing the influence of pH on the growth rate of agglomerated particles;
FIG. 3 is a graph showing the influence of initial silica densities of silica dispersion on the particle size distribution of the silica slurry;
FIG. 4 is a graph showing the influence of silica densities on the number of bulky particles in the silica slurry;
FIG. 5 is a graph showing the influence of mixing conditions on the particle size distribution of the silica slurry;
FIG. 6 is a graph showing the influence of the silica dispersion-feeding time on the particle size distribution of silica slurry;
FIG. 7 is a graph showing the influence of feeding rate of silica dispersion on pH of the mixture
FIG. 8 is a view showing the influence of the filtering accuracy of the filter on its performance to remove bulky particles;
FIG. 9 is a view showing the influence of the filtering accuracy of the filter on its processing-flow rate
FIG. 10 is a graph showing the numbers of bulky particles contained in Comparative examples 1 and 2 and Example 1;
FIG. 11 is an outline view showing the outline of a CMP apparatus 100;
FIG. 12 is a sectional view of the carrier section 122; and
FIG. 13 is a view showing the result of the polish process, for which Example 1 and Comparative examples 1, 2 were used.

BEST MODE FOR CARRYING OUT THE INVENTION

Now referring to the drawings, preferred embodiments of the invention are described below.
The method of manufacturing silica slurry has two processes, when roughly divided. The first step is making an acid silica dispersion; and the second step is mixing the silica dispersion with an aqueous basic substance solution.
In the first step, an acid, such as hydrochloric acid, is added to ultrapure water to make the water acid, e.g. pH 2; and fumed silica is fed into the resultant water while applying a shearing force, thereby to make a dispersion.
In the second step, an alkaline aqueous solution of potassium hydroxide or the like is dropped and mixed into the dispersion while stirring the silica dispersion.
In the second step, the pH-shock involved in the change of pH of the silica dispersion from acid to alkaline causes agglomerate of silica. Especially, in the silica dispersion, there is a high concentration of silica and as such, the agglomerate is more likely to occur.
In the invention, the making condition of silica dispersion and the mixing condition of the silica dispersion and alkaline aqueous solution are improved, thereby enabling the manufacture of silica slurry superior in dispersion stability.
FIG. 1 is a flowchart, as a mode embodying the invention.
First, the first step will be described in detail. The first step further includes small steps.
In Step 1-1, ultrapure water is prepared to be pH 1.0 to 2.7; and fumed silica powder having a relative surface area of 50 to 200 m²/g is fed into the resultant water until the water reaches an initial silica concentration of 46 to 54 wt % while applying a shearing force with a high-shear-dispersing apparatus for 1 to 5 hours.
In Step 1-2, a small amount of ultrapure water is added to the silica dispersion so that the water has a silica concentration of 45 to 53 wt %; and the shearing force is applied for 10 to 40 minutes.
In Step 1-3, ultrapure water is added to the silica dispersion so that the water has a silica concentration of 33 to 44 wt %; and applying a shearing force for 0.5 to 4 hours.
As the above, in the first step, the application of a high shearing force and the addition of ultrapure water in Step 1-2 can lower the viscosity of the silica dispersion sufficiently.
Now, the second step will be described.
In Step 2-1, silica dispersion is fed into aqueous basic substance solution that has been prepared so as to have pH 8 to 12 and a silica concentration of 10 to 30 wt % after the mixing. Unlike the conventional mixing, feeding silica dispersion into aqueous basic substance solution can suppress the generation of agglomerate in mixing. The reason is as follows.
There is an excess of aqueous basic substance solution in an early stage of feeding silica dispersion and as such, the mixture is strong alkaline of pH 12 to 14 and therefore the pH-shock is caused. However, the occurrence of agglomerate is suppressed because the silica concentration is extremely low. While the silica concentration of the mixture increases as the feeding is continued, feeding silica dispersion makes the mixture weaker alkaline at pH 8 to 12, which weakens the pH-shock and thus suppresses the occurrence of agglomerate.
In addition, it is preferable to feed all the silica dispersion into the aqueous solution within 5 hours. The aqueous basic substance solution is of pH 12 to 14, which represents a pH region such that the surface of fumed silica is eluted. Therefore, quickly feeding the silica dispersion into the solution can quickly shift the solution to PH 8 to 12, a dispersion-stable region of silica particles.
As the above, in the second step, the generation of agglomerate in mixing can be suppressed by feeding silica dispersion into the aqueous basic substance solution.
The silica slurry obtained through the first and second steps has a small amount of agglomerate and a low viscosity and as such, the agglomerate can be removed by a filter efficiently.
In Step 3-1, the resultant solution is filtered using a filter with an accuracy of 1 to 4 μm. This enables treatment at a flow rate of 2 to 10 l/min, and therefore bulky particles can be removed while keeping a sufficient processing flow.
The result of examination on conditions for the steps will be described below.
(1) About pH of Silica Dispersion
As for pH in Step 1-1, silica slurry was made in the conditions of pH 2, 3 and 7, provided that all the other conditions except for pH were identical.
FIG. 2 is a graph showing the influence of pH on the growth rate of agglomerated particles. The vertical axis shows the growth rate of agglomerated particles, and the horizontal axis shows the shaking time.
For the purpose of investigation of dispersion stability of the silica slurry, a shaking experiment was performed. In the shaking experiment, the following were performed: putting a 20 ml of the resultant silica slurry in a centrifugation tube with a capacity of 50 ml; setting the tube in a vertical shaking machine; shaking the tube at a shaking speed of 310 spm (stroke per minute), with a shaking stroke of 40 mm; unloading the centrifugation tube after a predetermined time has elapsed; and measuring a median particle diameter of the silica slurry with an apparatus for measurement of particle size distribution (Model LA-910 fabricated by HORIBA). The growth rate of agglomerated particles was calculated by: (post-shaking median particle diameter minuses pre-shaking median particle diameter) divided by pre-shaking median particle diameter and multiplied by 100(%).
The broken line 11 shows the case of pH 2, the broken line 12 shows the case of pH 3, and the broken line 13 shows the case of pH 7. In the case of pH 2, it was found that the silica slurry had high dispersion stability because the particle diameter did not change even after ten days shaking. The case of pH 3 showed a growth rate of about 18% after ten days, and the case of pH 7 showed a growth rate of about 88% after ten days. In both the cases, it was found that the agglomerate took place. The reason for this can be considered as follows. That is, because the isoelectric point of fumed silica lies in the vicinity of pH 2, the surface of particles is electrically neutral at pH 2 and therefore a high shearing force is prone to be applied to the particles.
It has been found from the foregoing that it is desirable that the silica dispersion is at pH 1 to 2.7.
(2) About Initial Silica Concentration of Silica Dispersion
With initial silica densities in Step 1-1, under the conditions of initial silica densities of 45, 50, 55 and 60 wt %, silica slurry was made, provided that all the other conditions except for initial silica densities were identical.
FIG. 3 is a graph showing the influence of initial silica densities of silica dispersion on the particle size distribution of the silica slurry. The vertical axis shows frequencies, and the horizontal axis shows particle diameters. The curve 14 shows the case of an initial silica concentration of 45 wt %, the curve 15 shows the case of an initial silica concentration of 50 wt %, the curve 16 shows the case of an initial silica concentration of 55 wt %, and the curve 17 shows the case of a silica concentration of 60 wt %.
It was found that the higher the dispersibility was, the higher the initial silica concentration was, since the particle size distribution of the silica slurry shifted leftward with an increase in initial silica concentration as seen from the graph. In the case of an initial silica concentration as low as 45 wt %, it can be considered that the shearing force of the high-shear-dispersing apparatus was not sufficiently transmitted and as such, the dispersibility was low. In the cases of 55 and 60 wt %, the dispersibility was high because of the shearing force transmitted sufficiently while the viscosities of the silica dispersion rose to make the load to the dispersing machine larger. Therefore, the cases are not adequate. In the case of an initial silica concentration of 50 wt %, it was found that the load to the dispersing machine was small and the dispersibility was high.
It has been found from the foregoing that it is desirable to make the initial silica concentration of the silica dispersion 46 to 54 wt %.
(3) About Addition of Small Amount of Ultrapure Water
As for the addition of ultrapure water in Step 1-2, silica slurry was made in the conditions when ultrapure water was added and when ultrapure water was not added, provided that all the other conditions except for the conditions concerning the addition of ultrapure water were identical.
In the case where a small amount of ultrapure water was not added, the median particle diameter of the silica slurry became larger than that in the case where the ultrapure water was added. In addition, the higher the silica concentration is, the more easily the shearing force is transmitted. On this account, the dispersibility in the case of no addition of ultrapure water is deteriorated and the viscosity of silica slurry rose by about 4%.
It has been found from the foregoing that it is desirable to add a small amount of ultrapure water to the silica dispersion to make the silica concentration 45 to 53 wt %.
(4) About Silica Concentration of Silica Dispersion
As for silica densities in Step 1-3, silica slurry was made under the conditions of silica densities of 32, 40, 45 and 49 wt % (with no addition of ultrapure water), provided that all the other conditions except for the silica densities were identical.
FIG. 4 is a graph showing the influence of silica densities on the number of bulky particles in the silica slurry. The vertical axis shows the number of bulky particles, and the horizontal axis shows a particle diameter.
The curve 18 shows the case of a silica concentration of 32 wt %; the curve 19 shows the case of a silica concentration of 40 wt %; the curve 20 shows the case of a silica concentration of 45 wt %; and the curve 21 shows the case of a silica concentration of 49 wt %. A Particle having a particle diameter larger than 0.5 μm was referred to as a bulky particle, and the number of particles were counted in particles having a different particle diameter.
In comparison to the cases of silica densities of 32, 45 and 49 wt %, the number of bulky particles in the case of a silica concentration of 40 wt % was the least. The reason for this can be considered as follows. That is, in the viscosity at the time of silica concentration of 40 wt %, the shearing force of the dispersing machine was applied most efficiently. Therefore, it can be considered that the viscosity was low in the case of a silica concentration of 32 wt %, and the viscosities in the cases of silica densities of 45 and 49 wt % were too high.
It has been found from the foregoing that it is desirable to make the silica concentration of the silica dispersion 33 to 44 wt %.
(5) About Mixing Condition of Silica Dispersion and Aqueous Basic Substance Solution
As for mixing conditions of silica dispersion and aqueous basic substance solution in Step 2-1, silica slurry was made under the conditions: where aqueous basic substance solution was fed into silica dispersion (first mixing condition); and where silica dispersion was fed into aqueous basic substance solution (second mixing condition). Potassium hydroxide was used as the basic substance. Moreover, all the other conditions except for the mixing conditions were identical.
FIG. 5 is a graph showing the influence of mixing conditions on the particle size distribution of the silica slurry, in which the vertical axis shows the frequency; and the horizontal axis shows the particle diameter. The curves 22 a, 22 b show the case of the first mixing condition; and the curve 23 shows the case of the second mixing condition.
As described above, in the first mixing condition, an agglomerate is prone to occur because of large pH-shock. Actually, with the particle size distribution, an agglomerate peak was observed in the vicinity of 10 μm. In contrast, it was found that a sharp peak was observed in the vicinity of a particle diameter of 0.1 μm in the second mixing condition which indicated an enhanced dispersibility. Also, the median particle diameter of silica slurry was measured. The median particle diameter of silica dispersion was 110 nm prior to the mixing. The median particle diameter of silica slurry reached 8082 nm when the silica dispersion and aqueous potassium hydroxide solution were mixed under the first mixing condition, and an extremely large agglomerate was present. In contrast, the median particle diameter of silica slurry was unchanged, 110 nm, when the mixing was performed under the second mixing condition, which showed that particles hardly agglomerate.
Therefore, it was found that it is proper to feed silica dispersion into aqueous basic substance solution in the case of mixing silica dispersion and aqueous basic substance solution.
(6) About Silica Dispersion-Feeding Time
As for the silica dispersion-feeding time into aqueous basic substance solution in Step 2-1, silica slurry was made in the conditions where all the silica dispersion was fed in 5 hours and in 20 minutes. Potassium hydroxide was used as the basic substance. Moreover, all the other conditions except for the feeding time were identical.
FIG. 6 is a graph showing the influence of the silica dispersion-feeding time on the particle size distribution of silica slurry, in which the vertical axis shows the frequency; and the horizontal axis shows the particle diameter. The curve 24 shows the case where the feeding time was 5 hours. While the curve 25 shows the case where the feeding time was 20 mins.
Taking a long time to feed silica dispersion, the pH-shock will generate agglomerates because the aqueous potassium hydroxide solution is strong alkaline. Shortening the feeding time can cause pH of the mixture to be quickly lowered to or below pH 12, a silica-stabilized region, and accordingly it becomes possible to suppress the generation of agglomerate.
It has been found from the foregoing that it is desirable to finish the feeding of silica dispersion into aqueous basic substance solution in less than 5 hours.
FIG. 7 is a graph showing the influence of feeding rate of silica dispersion on pH of the mixture, in which the vertical axis shows pH of the mixture and the horizontal axis shows the silica dispersion-feeding time. The curve 26 shows the case of a feeding rate of 25 l/min; the curve 27 shows the case of a feeding rate of 12.5 l/min; and the curve 28 shows the case of a feeding rate of 5 l/min.
Thus increasing the feeding rate can quickly lower pH of the mixture to or below pH 12, a silica-stabilized region.
(7) About Filtering Accuracy and Processing-Flow Rate of Filter
First, as for the filtering accuracy of the filter in Step 3-1, the filtering was performed under the conditions of filtering accuracies of 1, 3, 5, 7 and 10 μm, respectively. As the filter was used a depth-type filter that is small in pressure loss and allows a large flow to be obtained.
FIG. 8 is a view showing the influence of the filtering accuracy of the filter on its performance to remove bulky particles, in which the vertical axis shows the number of bulky particles in the silica slurry. In the drawing are shown the number of bulky particles before the filtering process and the number of bulky particles after the filtering process. The straight line 29 shows the change in the number of particles in the case of a filtering accuracy of 1 μm; the straight line 30 shows the change in the number of particles in the case of a filtering accuracy of 3 μm; the straight line 31 shows the change in the number of particles in the case of a filtering accuracy of 5 μm; the straight line 32 shows the change in the number of particles in the case of a filtering accuracy of 7 μm; and the straight line 33 shows the change in the number of particles in the case of a filtering accuracy of 10 μm.
In the cases of filtering accuracies of 5, 7 and 10 μm, the filtering accuracies were larger than the particle diameter of bulky particles and as such, the number of bulky particles after the filtering process hardly changed, and therefore adequate filtering performance could not be achieved. In contrast, in the cases of filtering accuracies of 1 and 3 μm, the number of bulky particles after the filtering process was reduced significantly.
Next, as for the processing flow of the filter in Step 3-1, under the same conditions as those described above, the filtering was performed.
FIG. 9 is a view showing the influence of the filtering accuracy of the filter on its processing-flow rate, in which the vertical axis shows the processing-flow rate in the filtering process. In the drawing is shown a processing-flow rate for each filtering accuracy. The symbol 74 shows a flow rate in the case of a filtering accuracy of 1 μm; the symbol 75 shows a flow rate in the case of a filtering accuracy of 3 μm; the symbol 76 shows a flow rate in the case of a filtering accuracy of 5 μm; the symbol 77 shows a flow rate in the case of a filtering accuracy of 7 μm; and the symbol 78 shows a flow rate in the case of a filtering accuracy of 10 μm.
As is clear from the drawing, the flow rate is reduced as the filtering accuracy is made smaller. In consideration of its actual manufacturing process, the flow rate of 2 l/min or faster is good for practical use and as such, even the filter with a filtering accuracy of 1 μm can be practically used.
Considering the ability to remove bulky particles and the processing-flow rate, it has been found that a filter with a filtering accuracy of 1-4 μm may be used. In such cases, the processing-flow rate is 2 to 10 l/min.
Next, described is the result of the comparison between conventional silica slurry made according to the prior art and the silica slurry (hereinafter referred to as “Example 1”) made according to the invention. The first conventional silica slurry (hereinafter referred to as “Comparative example 1”) was made according to the manufacturing method described in Japanese Examined Patent Publication 2,935,125. The second conventional silica slurry (hereinafter referred to as “Comparative example 2”) was made so as to have a viscosity comparable to that of the silica slurry made according to the invention, with its silica concentration lower than that of the first conventional silica slurry.
Example 1 was made according to the following procedures.
(a) Ultrapure water was poured in a high-shear-dispersing apparatus, followed by adding hydrochloric acid to control the water to pH 2.
(b) Fumed silica was fed therein while applying a high shearing force until the initial silica concentration reached 50 wt %.
(c) After the feeding of fumed silica, the high shearing force was applied to the silica dispersion for 2.5 hours.
(d) A small amount of ultrapure water was added so that the concentration of the silica dispersion was 49 wt %, and subsequently the high shearing force was applied for 30 minutes.
(e) Ultrapure water was added so that the silica concentration of the silica dispersion was 40 wt %, followed by applying the high shearing force for 1 hr.
(f) The silica dispersion was fed into the aqueous potassium hydroxide solution that had been controlled in potassium hydroxide concentration so that the silica slurry, as a final product, was going to have pH 11 and a silica concentration of 25 wt %.
(g) Further, bulky particles were removed using a depth-type filter with a filtering accuracy of 3 μm.
FIG. 10 is a graph showing the numbers of bulky particles contained in Comparative examples 1 and 2 and Example 1, in which the vertical axis shows the number of bulky particles; and the horizontal axis shows the particle diameter. The curve 39 shows Examle 1; the curve 40 shows Comparative example 1; and the curve 41 shows Comparative example 2.

It was found that the number of bulky particles of Example 1 was reduced significantly in comparison to Comparative examples 1 and 2. The results of comparisons among the three kinds of silica slurries in the number of bulky particles and values concerning other physical properties are shown in Table 1.

TABLE 1


				NUMBER	NUMBER
				OF	OF
				BULKY	BULKY
			MEDIAN	PARTICLES	PARTICLES
SILICA			PARTICLE	(>0.5 μm)	(>1 μm)
CONCENTRATION	pH	VISCOSITY	DIAMETER	[Particles/	[Particles/
[wt %]	[—]	[cP]	[nm]	0.5 ml]	0.5 ml]

EXAMPLE 1	25.7	11.0	3.7	112	100,000	2,000
COMPARATIVE	25.7	10.9	6.6	125	9,000,000	160,000
EXAMPLE 1
COMPARATIVE	20.7	11.0	3.7	125	500,000	25,000
EXAMPLE 2

AS shown in Table 1, Example 1 has a low viscosity in spite of its small median particle diameter and high silica concentration.
Further, these kinds of silica slurries were used to polish silicon wafers actually.
FIG. 11 is an outline view showing the outline of a CMP apparatus 100. The CMP apparatus 100 is constructed of: a polish pad 101; a rotary platen section 121; a carrier section 122; a slurry-supply section 123; and a dressing section 124. The polish pad 1 is brought in press-contact with a silicon wafer held by the carrier section 122 of the CMP apparatus 100, and is moved relatively to the silicon wafer hereby to polish the surface of the silicon wafer.
The rotary platen section 121 is supporting means that includes a platen 102 for sticking the polish pad 101 onto the substantially front surface of its top face with an adhesive tape or the like thereby to support the pad, and a rotation-drive mechanism 103 connected through a rotating shaft provided on the bottom face side of the platen 102. The rotational driving force of the rotation-drive mechanism 103 is transmitted to the platen 102 through the rotating shaft; the platen 102 rotates together with the polish pad 101 about a vertical axis line at a predetermined rotation number. The rotation number can be set freely, and a proper rotation number can be selected depending on the kind of a wafer or a film to be polished, the type of the polish pad 1, etc.
As shown in the sectional view of FIG. 12, the carrier section 122 includes a carrier body 104, a backing material 105, a retainer ring 106, and a rotation-drive mechanism 107, and thus is holding means for holding a silicon wafer 108, i.e. an object to be polished, and rotating the polish pad 101 and the silicon wafer 108 in a press-contact condition. The fixing of the silicon wafer 108 on the carrier body 104 is performed by moistening the backing material 105 and attracting the wafer with the surface tension of water. Further, in order to prevent the silicon wafer 108 from being disengaged during the polishing step, the retainer ring 106 holds an outer peripheral portion of the silicon wafer 108. The rotation-drive mechanism 107 is connected through a rotating shaft with the top face of the carrier body 104. The rotational driving force of the rotation-drive mechanism 107 is transmitted through the rotating shaft to the carrier body 104 and then the carrier body 104 rotates together with the silicon wafer 108 about a vertical axis line at a predetermined rotation number. The rotation number can be set freely, and a proper rotation number can be selected depending on the kind of a wafer or a film to be polished, the type of the polish pad 1, etc. like in the case of the rotary platen section 121. In addition, the carrier section 122 is urged in a direction to approach the rotary platen section 121, i.e. vertically downward, whereby the polish pad 101 and the silicon wafer 108 are brought in press-contact. The urge of the carrier section 122 may be performed by the rotation-drive mechanism 107, or an additional urging mechanism.
The slurry-supply section 123 is supply means that includes a nozzle 109, a slurry-supply pipe 110 and a slurry tank 111.
The silica slurry stored in the slurry tank 111 is forced to flow into the slurry-supply pipe 110 by a pump or the like and is supplied to the surface of the polish pad 101 through the nozzle 109 mounted above a substantially central portion of the rotary platen section 121 at a predetermined flow. As the silica slurry to be supplied, Example 1 and Comparative examples 1 and 2 were used.
As the polishing proceeds, fine holes in the vicinity of a polishing face of the polish pad 101 are filled with polishing waste, whet grains, etc., thereby polish properties such as a polishing rate being deteriorated. The dressing section 124 is restoring means constructed of: a plate 112 electrodeposited with industrial diamond particles, which are conditioners; and a rotation-drive mechanism 113 connected with the plate 112 through a rotating shaft. In dressing, the rotation-drive mechanism 113 rotates the plate 112, and brings the diamond particles into contact with the polishing face of the polish pad 101 to raze clogged portions, thereby restoring a polish property of the polish pad 101.
In regard to the operations of the respective sections during the polish process, the carrier section 122 is urged vertically downward, and the slurry-supply section 123 supplies silica slurry in the situation where the polish pad 101 and the silicon wafer 108 are in press-contact. The supplied silica slurry penetrates between the polish pad 101 and the silicon wafer 108. By rotating and relatively moving the rotary platen section 121 and the carrier section 122, the surface of the silicon wafer 108 is polished chemically by a medium and mechanically by whet grains with high accuracy.
As for the relative movements of the rotary platen section 121 and the carrier section 122, there are two or more patterns as follows.
(1) As shown in the drawing, the carrier section 122 is disposed so that the center of the carrier section 122 is located at a place that is spaced away from the rotational center of the rotary platen section 121 by substantially a half of the diameter in a direction of radius. The polish process is performed only by the rotations of the rotary platen section 121 and carrier section 122.
(2) In the case where the difference between the radius of the polish pad 101 and the radius of the silicon wafer 108 is not so large, (1) is sufficient. However, in the case where the radius of the polish pad 1 is larger than the grain size of the silicon wafer 9, a portion which is out of contact with the silicon wafer 108 exists in the surface of the polish pad 101 and as such, the carrier section 122 is forced to reciprocate in a direction of radius of the rotary platen section 121 in addition to the rotations of the rotary platen section 121 and the carrier section 122 of (1) so as to use the entire surface of the polish pad 1101.
(3) In addition to the rotations of the rotary platen section 121 and the carrier section 122 of (1), the carrier section 122 is forced to revolve around the center of the rotary platen section 121.
(4) In the case where the radius of the polish pad 101 is larger than the radius of the silicon wafer 108 like (2), the reciprocation in the radical direction and the revolution around the center of the rotary platen section 121 are combined. For example, the carrier section 122 may be forced to move so as to trace a helical orbit around the center of the rotary platen section 121.
Incidentally, the rotational directions of the rotary platen section 121 and the carrier section 122 may be identical or different. Also, the rotation speeds of the rotary platen section 121 and the carrier section 122 may be identical or different.
In regard to the dressing by the dressing section 124, there are the cases of performing the dressing after having polished one or more silicon wafers and of performing the dressing during the polish process. The radius of the diamond plate 112 of the dressing section 124 is often smaller than the radius of the polish pad 101 and as such, when the dressing is performed after the polish process, the dressing may be carried out substantially in the same way as in the above relative movement patterns (2) and (4) of the rotary platen section 121 and the carrier section 122. In the case where the dressing is performed during the polish process, the dressing section is disposed opposite to the carrier section 122 with the center of the rotary platen section 121 located therebetween, as shown in the drawing, and then the dressing may be carried out substantially in the same way as in the relative movement pattern (2).
The CMP apparatus 100 as describe above was used to perform the polish process.
As the object to be polished was used a TEOS wafer; and as the polish pad 101 was used an IC1400 K-Groove (manufactured by Rodel Nitta Co.). The rotational speed of the rotary platen section 121 was set at 60 rpm, and silica slurry was supplied at a speed of 100 ml/min. After the polish process for 1 minute, the number of scratches (having a size of 0.2 μm or larger) in the surface of the wafer was counted using a wafer surface-inspecting apparatus (LS6600) manufactured by Hitachi Electronics Engineering Co. Ltd.
FIG. 13 is a view showing the result of the polish process, for which Example 1 and Comparative examples 1, 2 were used. The vertical axis shows the number of scratches per wafer. The polish process was performed three times for each of Example 1 and Comparative examples 1, 2.
The number of scratches for Comparative example 1 was 261 to 399 (322 on average), and the number of scratches for the Comparative example 2 was 103 to 154 (123 on average), whereas the number of scratches for the Example 1 was significantly reduced to 28 to 63 (40 on average).
Thus, the silica slurry made according to the invention can reduce the number of scratches in a wafer surface in the polish process because of its high dispersibility and a small number of bulky agglomerated particles.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

INDUSTRIAL APPLICABILITY

As described above, according to the invention, an abrasive composition that is superior in dispersion stability and has fewer agglomerated particles can be obtained by adding a fumed silica dispersion to a prepared aqueous basic substance solution.
Also, according to the invention, a high shearing force can be applied efficiently, thereby to enhance the dispersibility.
Further, according to the invention, the viscosity of the abrasive composition can be reduced by adding a small amount of water.
Still further, according to the invention, the mixing is finished in less than 5 hours, whereby pH of the mixture can be lowered quickly to shorten the time during which the mixture is placed in a pH condition in which the fumed silica is prone to agglomerate and therefore to suppress the occurrence of agglomerate.
In addition, according to the invention, the abrasive composition obtained in the second step has fewer agglomerates and as such, the agglomerates can be efficiently removed by a filtering process.

Claims

1. A method of manufacturing an abrasive composition comprising:

a first step of preparing an acid fumed silica dispersion; and

a second step of adding the fumed silica dispersion to an aqueous basic substance solution which is prepared so that the abrasive composition to be obtained after the end of mixing with the fumed silica dispersion has predetermined pH and silica concentration, and mixing the fumed silica dispersion and the aqueous solution.

2. The method of claim 1, wherein the first step includes the step of:

feeding fumed silica into water which has been prepared to have a pH of 1.0 to 2.7 so that an initial silica concentration reaches 46 to 54 wt % and applying a high shearing force, thereby preparing a fumed silica dispersion;

adding water to the fumed silica dispersion so that its silica concentration reaches 45 to 53 wt %; and

further adding water to the fumed silica dispersion so that its silica concentration reaches 33 to 44 wt %.

3. The method of claim 1, wherein the aqueous basic substance solution is prepared so that the abrasive composition has a pH of 8 to 12 and a silica concentration of 10 to 30 wt %.

4. The method of claim 1, wherein the mixing of the fumed silica dispersion and the aqueous basic substance solution is finished in less than 5 hours in the second step.

5. The method of claim 1, wherein the fumed silica has a relative surface area of 50 to 200 m²/g.

6. The method of claim 1, wherein the aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.

7. The method of claim 1, further comprising a third step of filtering the abrasive composition obtained in the second step with a filter having a filtering accuracy of 1 to 4 μm.

8. The method of claim 2, wherein the aqueous basic substance solution is prepared so that the abrasive composition has a pH of 8 to 12 and a silica concentration of 10 to 30 wt %.

9. The method of claim 2, wherein the mixing of the fumed silica dispersion and the aqueous basic substance solution is finished in less than 5 hours in the second step.

10. The method of claim 3, wherein the mixing of the fumed silica dispersion and the aqueous basic substance solution is finished in less than 5 hours in the second step.

11. The method of claim 8, wherein the mixing of the fumed silica dispersion and the aqueous basic substance solution is finished in less than 5 hours in the second step.

12. The method of claim 2, wherein the fumed silica has a relative surface area of 50 to 200 m²/g.

13. The method of claim 3, wherein the fumed silica has a relative surface area of 50 to 200 m²/g.

14. The method of claim 4, wherein the fumed silica has a relative surface area of 50 to 200 m²/g.

15. The method of claim 2, wherein the aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.

16. The method of claim 3, wherein the aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.

17. The method of claim 4, wherein the aqueous basic substance solution contains at least any of ammonium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide or magnesium hydroxide.

18. The method of claim 2, further comprising a third step of filtering the abrasive composition obtained in the second step with a filter having a filtering accuracy of 1 to 4 μm.

19. The method of claim 3, further comprising a third step of filtering the abrasive composition obtained in the second step with a filter having a filtering accuracy of 1 to 4 μm.

20. The method of claim 4, further comprising a third step of filtering the abrasive composition obtained in the second step with a filter having a filtering accuracy of 1 to 4 μm.