US20050020188A1

US20050020188A1 - Polishing pad, method of producing same, and polishing method

Info

Publication number: US20050020188A1
Application number: US10/868,660
Authority: US
Inventors: Mitsuru Saito; Jun Tamura; Toshihiro Izumi; Toshihiro Kobayashi; Takuya Nagamine; Claughton Miller
Original assignee: Nihon Micro Coating Co Ltd
Current assignee: Nihon Micro Coating Co Ltd
Priority date: 2003-06-19
Filing date: 2004-06-14
Publication date: 2005-01-27
Also published as: JP2005007520A

Abstract

A polishing pad is formed with a non-foamed member shaped as a plate having a flat surface and abrading particles affixed inside and on the surfaces of this non-foamed member. Its average surface roughness is in the range of 0.5 μm-10 μm, and its Shore D hardness is within the range of 50-85. Grooves are formed on the surface of the polishing pad for collecting debris generated during the polishing operation and serving also as flow routes of a polishing liquid for distributing it uniformly over the surface of the polishing pad.

Description

Priority is claimed on Japanese Patent Application 2003-174258 filed Jun. 19, 2003.

BACKGROUND OF THE INVENTION

This invention relates to a polishing pad to be used for polishing the surface of a workpiece made of a metallic material, ceramics or glass, as well as a method of producing such a polishing pad. In particular, the invention relates to such a polishing pad used for polishing the surface of a workpiece such as a semiconductor wafer, a semiconductor device wafer, a liquid crystal display element, a thin-film imaging device, a magnetic disk substrate and an optical disk substrate, a method of producing such a polishing pad and a method of polishing.
Semiconductor devices and magnetic disks are used as principal electronic components of electronic apparatus such as telephones, cameras and computers in order to control their functions and to store or display data. A workpiece such as a semiconductor wafer used in such an electronic component is made into a product after having its surface subjected to a polishing process and going through required production processes such as a multi-layer wiring process and a thin film process and also an inspection process.
This series of processes required in the production stage of a workpiece is required to be carried out with an accuracy on the order to nanometers in order to provide the component characteristics and functions required on the design stage. For this reason, a very high level of accuracy is required of each process and the polishing process, too, is required to be carried out such that the surface of a workpiece can be made flat to a very high degree.
In such a polishing process required to be carried out on a workpiece at a very high level of precision, a lapping plate having a polishing pad attached to its surface is rotated, a polishing slurry having abrading particles dispersed therein is supplied to the surface of the polishing pad and the workpiece is pressed against it while the lapping plate is kept rotating.
In such a polishing process on a workpiece, a cloth pad comprising a woven or non-woven cloth and a foamed pad comprising a foamed material have been in use as a polishing pad, as disclosed, for example, in Japanese Patent Publication Tokkai 2000-239651. It was because such polishing pads are flexible and elastic and have gaps and voids due to air bubbles capable of taking debris thereinto and hence are capable of making the surface of a workpiece extremely flat.
When a new layer is formed on top of a surface of a lower layer with unevenness in a processes such as the aforementioned multi-layer wiring and thin film processes, there results a nearly identically uneven surface on the top layer. (This happens, for example, on the surface of a multi-layer wired structure of a semiconductor device wafer.) If a conventional flexible and elastic polishing pad is used to polish the surface of such a top layer, a gentle unevenness based on the unevenness of the layer below is formed and a workpiece with a completely flat surface cannot be obtained.
It is believed that this problem is caused as follows. As shown in FIG. 3, if a conventional flexible and elastic polishing pad is pushed against the uneven surface of an upper layer 32, the pad comes not only to contact the protrusions 33 but also to enter into the indentations 34. Thus, the abrading particles contained in the polishing slurry operate on the bottom surfaces of the indentations 34 as well and both the protrusions 33 and the bottom surfaces of the indentations 34 come to be abraded. Since the pressure of the polishing pad operating on the bottom surfaces of the indentations 34 is smaller that that on the protrusions 33, the bottom surfaces of the indentations 34 are abraded at a slower rate than the protrusions 33. In this case, the bottom surfaces of the indentations 34 may be polished beyond the target surface (shown by the straight broken line at 36) before the protrusions 33 of the upper layer 32. This may be considered to be the cause of having a gentle unevenness 35 on the upper layer 32 based on the unevenness of the lower layer 31. As will be demonstrated below by way of a comparison example, if a conventional flexible and elastic polishing pad is used, indentations are formed on a workpiece at positions corresponding to the indentations in the lower layer.

SUMMARY OF THE INVENTION

It is therefore an object of this invention in view of this problem to provide a polishing pad capable of polishing the surface of a workpiece uniformly to a high level of accuracy, a method of producing such a polishing pad and a method of polishing a workpiece.
It is another object of this invention to provide such a polishing pad capable of polishing the surface of a workpiece quickly and with a high degree of accuracy, a method of producing such a polishing pad and a corresponding method of polishing.
A polishing pad embodying this invention may be characterized as comprising a non-foamed member in a shape of a plate, having a flat surface, and abrading particles affixed inside and on the surfaces of this non-foamed member. Its average surface roughness is in the range of 0.5 μm-10 μm, and the abrading particles protruding from the flat surface of the non-foamed member act on and thereby polish the surface of the workpiece.
The Shore D hardness of the polishing pad of this invention is within the range of 50-85, and preferably within the range of 70-85. Since the workpiece is pressed against a polishing pad with such hardness during a polishing operation, the workpiece does not sink into the interior of the polishing pad and hence the polishing pad of this invention, unlike conventional polishing pads which are flexible and elastic, is not pulled by the workpiece to undergo large elastic deformations.
Grooves are formed on the surface of the polishing pad of this invention for collecting polluting or contaminating objects such as debris generated during the polishing operation such that such objects can be discharged outward through these grooves. This prevents the polishing pad from becoming easily clogged up. Such grooves also serve as flow routes of a polishing liquid for distributing it uniformly over the surface of the polishing pad.
The non-foamed member is a plate comprising a resin material such as polyurethane resins, polyethylene resins, polystyrene resins, vinyl polychloride resins and acryl resins, and the thickness of this plate is preferably in the range of 0.5 mm-3 mm.
The abrading particles may comprise primary particles, secondary particles or mixed particles of primary and secondary particles. The average diameter of the primary particles is 0.005 μm-0.5 μm and that of the secondary particles is 0.01 μm-20 μm.
The abrading particles may comprise particles of one or more materials such as cerium oxide, silicon oxide, alumina, silicon carbide, zirconia, iron oxide, manganese dioxide, titanium oxide and diamond. The volume percentage of the abrading particles in the non-foamed member is 10 volume %-60 volume %, and preferably 20 volume %-50 volume %.
The polishing pad of this invention may further comprise an adhesive sheet including a base sheet having a front surface and a back surface both coated with an adhesive material, the adhesive sheet being attached to a back surface of the non-foamed member through the adhesive material on the front surface of the adhesive sheet. This base sheet may comprise an elastic sheet.
Such a polishing pad of this invention may be produced by the steps of preparing a dispersed liquid by mixing a resin solution, abrading particles and a hardener, preparing a non-foamed dispersed liquid by reducing pressure and thereby defoaming the dispersed liquid, forming a block of plate-shaped non-foamed member (with thickness in the range of 1 mm-6 mm) having the abrading particles affixed inside and on surfaces by hardening the non-foamed dispersed liquid, and forming the block into a desired thickness. Grooves may further be formed on a surface of the block formed into the desired thickness.
A method according to this invention of polishing a surface of a workpiece is characterized as comprising the steps of providing a polishing machine having a lapping plate with the polishing pad of this invention attached to a surface of the lapping plate, means for rotating the lapping plate, a holder for holding the workpiece and means for supplying a polishing liquid or polishing slurry, causing the lapping plate to rotate, supplying the polishing liquid or polishing slurry on a surface of the polishing pad attached to the lapping plate, and pressing the surface of the workpiece held by the holder onto the surface of the polishing pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are sectional views of polishing pads embodying this invention.
FIG. 2 is a schematic diagonal view of a polishing machine according to this invention.
FIG. 3 is a schematic sectional view of a multi-layer wired structure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a polishing pad 10 embodying this invention, characterized as comprising a plate-shaped non-foamed member 11 having a flat surface and abrading particles 12 that are dispersed and affixed inside and on the surfaces of this non-foamed member 11. The average surface roughness of the surface of this polishing pad 10 is 0.5 μm-10 μm. During a polishing operation, those of the abrading particles 12 protruding out of the flat surface of the non-foamed member 11 serve to operate on and to thereby polish the surface of a workpiece W.
The Shore D hardness of the polishing pad 10 is within the range of 50-85, and preferably within the range of 70-85. According to this invention, since the workpiece W is pressed against the surface of a polishing pad with such hardness, the workpiece W does not sink into the interior of the polishing pad and the polishing pad does not undergo large elastic deformations by being pulled by the workpiece W.
The non-foamed member 11 is a plate comprising a resin material of the polyurethane, polyethylene, polystyrene, vinyl polychloride or acryl type and is preferably of thickness in the range of 0.5 mm-3 mm.
As shown in FIG. 1B, grooves 13 may be formed on the surface of the polishing pad of this invention such that contaminants such as debris generated during a polishing operation can be taken into them and caused to flow away to the exterior. This prevents the polishing pad 10 from easily becoming clogged within a relatively short time of use. These grooves 13 also serve as a flow route for a polishing liquid or polishing slurry such that the latter can be uniformly supplied over the entire surface of the polishing pad 10.
The planar design of the grooves 13 may be selected from straight lines, curves lines and geometrical patterns combining these such as radial, latticed, spiral and concentric patterns but spiral and concentric circular patterns are preferred. The cross-sectional shape of the grooves 13 is preferably rectangular because the planar design of the grooves 13 remains the same as the polishing pad 10 becomes worn. The pitch of the grooves 13 is within the range of 0.5 mm-10 mm, and the depth of the grooves 13 is within the range of 0.2 mm-1 mm which is less than ½ of the thickness of the plate-shaped non-foamed member 11. The ratio between the depth and the width of the grooves 13 is within the range of 3:5-4:1.
Primary particles, secondary (coagulated) particles and their mixtures may be used as the abrading particles. The average diameter of these primary particles is 0.005 μm-0.5 μm and that of the secondary particles is 0.01 μm-20 μm. If secondary particles come off the polishing pad 10 or become broken up, they become tiny free primary abrading particles to work on the surface of the workpiece W and hence do not damage the surface of the workpiece W.
The abrading particles 12 comprise particles of materials such as cerium oxide, silicon oxide, alumina, silicon carbide, zirconia, iron oxide, manganese dioxide, titanium oxide and diamond or particles of two or more of these materials. The content of the abrading particles attached to the non-foamed member 11 is within the range of 10 volume %-60 volume %, and preferably 20 volume %-50 volume %. If this content is too low, it is not possible to obtain a sufficient polishing efficiency. If it is too high, on the other hand, the abrading particles 12 tend to more easily become detached and drop off and may damage the surface of the workpiece W.
As shown in FIG. 1B, the polishing pad 10 may be further provided with an adhesive sheet 14 having both the front and back surfaces of its base sheet 15 coated with an adhesive material 16 and affixed to the back surface of the non-foamed member 11 through the adhesive material 16 on the front surface. An elastic sheet such as comprising a woven sheet, a non-woven sheet or a foamed member may be used as the base sheet 15. As a practical matter, a peelable paper sheet (not shown) is attached to the adhesive material 16 on the back surface of the adhesive sheet 14 in order to prevent it from drying so as to be peeled off immediately before the polishing pad 10 is attached to the surface of a lapping plate.
The polishing pad 10 as described above may be produced firstly by mixing a resin solution and the abrading particles 12 together to prepare a dispersion liquid. Secondly, this liquid having the abrading particles 12 dispersed is defoamed by reducing pressure to obtain a foamless dispersion liquid. In this production process, it is preferable to use secondary particles as the abrading particles because secondary particles are more easily dispersed uniformly inside the solution and break up as explained above during a polishing operation.
Next, this foamless dispersion liquid is hardened to form a block of a non-foamed structure in the shape of a plate, having the abrading particles 12 affixed both inside and on the surface. The hardening process is carried out by using a molding apparatus. The thickness of this block is preferably in the range of 1 mm-6 mm such that the abrading particles are dispersed uniformly in the direction of the thickness after the molding.
After the block is taken out of the molding machine, a polishing tool is used to polish both surfaces of the block until a desired thickness is obtained and the plate-shaped non-foamed member 11 having the abrading particles affixed both inside and on the surface is produced. Known technologies for using a lathe may further be employed to form the aforementioned grooves 13 on the surface of the non-foamed member 11. The aforementioned adhesive sheet 14 may also be attached to the bottom surface of the non-foamed member 11.
The polishing of a workpiece W is carried out by using a polishing machine 20 shown in FIG. 2, comprising a lapping plate 21, a means (not shown) for rotating the lapping plate 21, a holder (such as a chuck 22) for holding the workpiece W, means (such as a nozzle 23) for supplying a polishing liquid or slurry and means (not shown) for rotating the holder. When the workpiece W is polished, the polishing pad 10 according to this invention is pasted on the surface of the lapping plate 21 as shown in FIG. 2 and the lapping plate 21 is then rotated in the direction of the arrow R. As a polishing liquid is supplied onto the surface of this polishing pad 10 through the nozzle 23, the surface of the workpiece W held by the chuck 22 is pressed against the surface of the polishing pad 10 and this is rotated in the direction of the arrow r. When the surface of the polishing pad 10 has become worn, its surface condition can be corrected by carrying out a conditioning operation of a conventionally known kind.
Water may be used as the polishing liquid. In order to polish the surface of the workpiece W in a chemical-mechanical manner, a suitable chemical adapted to chemically react with the surface of the workpiece W may be added to this polishing liquid. Such a chemical may be selected suitably according to the material which comprises the surface (“target surface”) of the workpiece W to be polished. If the material that comprises the target surface of the workpiece W is silicon dioxide, for example, potassium hydroxide, tetramethyl ammonium hydroxide, fluoric acid and fluorides may be used. If the target surface is tungsten, iron nitride and potassium iodide may be used. If the target surface is copper, glycine, quinaldic acid, hydrogen peroxide and benzotriazol may be used.
According to this invention, a polishing slurry obtained by dispersing abrading particles as described above in water or a water-based solution may be used instead of a polishing liquid. In order to improve dispersion of the abrading particles, alcohols or glycols may be added to the polishing slurry. For carrying out a chemical-mechanical polishing of the surface of the workpiece W, a chemical that reacts with the surface of the workpiece W may be further added to the polishing slurry.
The invention is described next by way of test and comparison examples.

TEST EXAMPLE

A polishing pad according to this invention was produced as follows.
Sufficiently dried cerium oxide (50 parts) with average particle diameter 0.2 μm was added to HDI prepolymer (100 parts) heated to 60° C. After the mixture was sufficiently stirred, it was defoamed by reducing the pressure to obtain Liquid Mixture A. Separately, sufficiently dried cerium oxide (40 parts) with average particle diameter 0.2 μm was added to polyether polyol (80 parts) heated to 50° C., and after the mixture was sufficiently stirred, it was defoamed by reducing the pressure to obtain Liquid Mixture B. Next, Liquid Mixture B was added to Liquid Mixture A and the mixture was stirred for a short time by using a stirrer of the type undergoing a planetary motion in order not to introduce foams. Liquid Mixture C was obtained by reducing pressure to sufficiently defoam it. Next, Liquid Mixture C was filled into a mold and held therein for 10 minutes at 120° C. to obtain a plate with thickness about 3 mm. This plate was taken out of the mold and cooled naturally after being held inside a thermostatic tank at 100° C. for about 12 hours. Next, this plate was cut into a specified (circular) shape and polished to a thickness of 2 mm to obtain a disk with Shore D hardness 72. Next, a lathe was used to form a spiral groove (with pitch 2 mm, depth 0.5 mm and width 1 mm) on the surface of this polishing pad to obtain the polishing pad (Test Example) having abrading particles dispersed inside and on the surfaces of a plate-like non-foamed member comprising polyurethane and having a flat surface and grooves.

COMPARISON EXAMPLE

A commercially purchased foamed pad (product name IC1000, produced by Rodel, Inc.) comprising foamed polyurethane having independent air bubbles with thickness 1.27 mm was used as a polishing pad of Comparison Example. Abrading particles are not affixed to this pad of a foamed material.

Comparison Test 1

Both polishing pads of Test Example and Comparison Example were used to polish the surface of a workpiece. The workpiece was an 8-inch silicon wafer with a multi-layer wiring structure (having a SiO₂membrane of thickness 2 μm formed by CVD on a step pattern with width 50 μm, pitch 100 μm and depth 0.8 μm) formed on its surface and cut into the size of 20 mm×20 mm. The polishing was carried out by using a commercially available single-surface polishing machine as shown in FIG. 2 (product name SPL-15 produced by Okamoto Kosaku Kikai Seisakusho) with a lapping plate of size 380 mm. Comparison was made in terms of the remaining step difference (the depth of the step remaining on the surface of the SiO₂membrane) with respect to the stock removal (which is 100% when the step has been removed).

In the test by using a polishing pad of Test Example, a polishing liquid (with no abrading particles) with its pH value adjusted to 11.3 by adding potassium hydroxide to pure water was used and the polishing was carried out under the following conditions given in Table 1.

TABLE 1


Pressure of compression	350	gf/cm²
Rotational speed of lapping plate	60	rpm
Rotational speed of chuck	60	rpm
Supply rate of polishing liquid	35	ml/minute

In the test by using a polishing pad of Comparison Example, a polishing slurry obtained by diluting a commercially available slurry (product name SS-25 produced by Cabot, Inc.) commonly used for the polishing of a SiO₂membrane was used and the polishing was carried out under the following conditions given in Table 2.

TABLE 2


Pressure of compression	350	gf/cm²
Rotational speed of lapping plate	60	rpm
Rotational speed of chuck	60	rpm
Supply rate of polishing liquid	35	ml/minute

Results of Comparison Test 1

The results of Comparison Test 1 is shown in Table 3. When the step difference on the SiO₂membrane is removed (when the stock removal is 100%), the remaining step difference was 0.027 μm by Test Example (about one third of the value by Comparison Example) and this means that the target surface was made significantly flatter than by the prior art technology.

TABLE 3

Test Example Comparison Example

Stock Removal (Remaining step (Remaining step

(%) difference)(μm) difference)(μm)

0 0.600 0.800

30 0.503 0.600

50 0.254 0.355

100 0.023 0.073

150 0.013 0.028

Comparison Test 2

Both polishing pads of Test Example and Comparison Example were used to polish the surface of a workpiece. The workpiece was a 4-inch silicon wafer with an oxide membrane with thickness 1 μm formed on its surface. The polishing was carried out by using the same commercially available single-surface polishing machine as used in Comparison Example 1 (product name SPL-15 produced by Okamoto Kosaku Kikai Seisakusho) with a lapping plate of size 380 mm. Comparison was made in terms of the polishing rate (stock removal per unit time) and the average surface roughness.
In the test by using a polishing pad of Test Example, a polishing liquid (with no abrading particles) with its pH value adjusted to 11.3 by adding potassium hydroxide to pure water was used and the polishing was carried out under the conditions shown in Table 1.
In the test by using a polishing pad of Comparison Example, a polishing slurry obtained by diluting a commercially available slurry (product name SS-25 produced by Cabot, Inc.) commonly used for the polishing of a SiO₂membrane was used and the polishing was carried out under the conditions shown in Table 2.

Results of Comparison Example 2

The results of Comparison Example 2 are shown in Table 4.

	TABLE 4


	Polishing	Average Surface
	Rate	Roughness
	(nm/minute)	Ra (Å)

Polishing pad of Test Example	80	1.8
(with polishing liquid)
Polishing pad of Test Example	230	2.0
(with polishing slurry)
Polishing pad of Comparison Example	120	2.0
(with polishing slurry)

Table 4 shows that the surface of a workpiece can be polished by using a polishing pad of Test Example with a polishing slurry containing abrading particles at twice the polishing rate with a polishing pad of Comparison Example.

With a polishing pad according to this invention, the workpiece does not sink into the interior of the polishing pad. Since the polishing pad does not undergo large elastic deformations by being pulled by the workpiece, the surface of the workpiece can be made even at a high degree of accuracy by using a polishing liquid free of abrading particles. If a polishing slurry containing abrading particles is used, the polishing of the surface of a workpiece can be accomplished quickly and at a high level of accuracy.

Claims

1. A polishing pad comprising:

a non-foamed member in a shape of a plate, having a flat surface; and

abrading particles affixed inside and on surfaces of said non-foamed member.

2. The polishing pad of claim 1 having Shore D hardness within the range of 50-85.

3. The polishing pad of claim 1 having Shore D hardness within the range of 70-85.

4. The polishing pad of claim 1 wherein said non-foamed member comprises a resin material selected from the group consisting of polyurethane resins, polyethylene resins, polystyrene resins, vinyl polychloride resins and acryl resins.

5. The polishing pad of claim 1 wherein said abrading particles comprise particles selected from the group consisting of primary particles, secondary particles and mixed particles of primary and secondary particles.

6. The polishing pad of claim 5 wherein said primary particles have an average diameter of 0.005 μm-0.5 μm.

7. The polishing pad of claim 5 wherein said secondary particles have an average diameter of 0.01 μm-20 μm.

8. The polishing pad of claim 1 wherein said abrading particles comprise particles of one or more materials selected from the group consisting of cerium oxide, silicon oxide, alumina, silicon carbide, zirconia, iron oxide, manganese dioxide, titanium oxide and diamond.

9. The polishing pad of claim 1 wherein said non-foamed member contains said abrading particles at 10 volume %-60 volume %.

10. The polishing pad of claim 1 wherein said non-foamed member has thickness in the range of 0.5 mm-3 mm.

11. The polishing pad of claim 1 with average surface roughness in the range of 0.5 μm-10 μm.

12. The polishing pad of claim 1 having grooves formed on the surface thereof.

13. The polishing pad of claim 1 further comprising an adhesive sheet including a base sheet having a front surface and a back surface both coated with an adhesive material, said adhesive sheet being attached to a back surface of said non-foamed member through the adhesive material on the front surface of said adhesive sheet.

14. The polishing pad of claim 13 wherein said base sheet comprises an elastic sheet.

15. A method of producing a polishing pad, said method comprising the steps of:

preparing a dispersed liquid by mixing a resin solution, abrading particles and a hardener;

preparing a non-foamed dispersed liquid by reducing pressure and thereby defoaming said dispersed liquid;

forming a block of plate-shaped non-foamed member having said abrading particles affixed inside and on surfaces by hardening said non-foamed dispersed liquid; and

forming said block into a desired thickness.

16. The method of claim 15 wherein said desired thickness is in the range of 1 mm-6 mm.

17. The method of claim 15 further comprising the step of forming grooves on a surface of said block formed into said desired thickness.

18. A method of polishing a surface of a workpiece, said method comprising the steps of:

providing a polishing machine having a lapping plate with the polishing pad of claim 1 attached to a surface of said lapping plate, means for rotating said lapping plate, a holder for holding said workpiece and means for supplying a polishing liquid or polishing slurry;

causing said lapping plate to rotate;

supplying said polishing liquid or polishing slurry on a surface of said polishing pad attached to said lapping plate; and

pressing said surface of said workpiece held by said holder onto said surface of said polishing pad.