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, 2004, 46, . 4 Nano-polishing of silicon wafers using ultra-dispersed diamonds T. Kurobe, T. Fujimura, H. Ikeda Vision Development Company Limited, Tokyo 104-0031, Japan Institute of Development of Raw Materials and Industrial Technologies, Tokyo 104-0031, Japan E-mail: k-vision@rapid.ocn.ne.jp, knakanishi@proof.ocn.ne.jp In the present study, two new methods are proposed for polishing of silicon wafers using the ultra-dispersed diamonds (UDD). The first proposed polishing method uses a polishing tool with an ultra-fine abrasive material made by electrophoretic deposition of UDD onto a brass rod. Dry polishing tests showed that the surface roughness of the silicon wafer has been reduced from Ra = 107 to 4 nm after polishing for 30 min. The second method uses a new polishing pad with self-generating porosity. By polishing using the new pad in combination with the polycrystalline UDD in water suspension, it is possible to achieve the specified surface roughness of the silicon wafer rather faster than using a conventional pad made of foamed polyurethane. The tests showed that the surface roughness of the silicon wafer has been reduced from Ra = 107 to 2 nm after polishing for 90 min.

1. Introduction 2. Experimental equipment To improve the surface roughness and remove residual The polishing experiments conducted using laboratorydamages of subsurface layers of silicon wafers after cutscale equipment. The polishing machine consisted of two ting, conventional polishing technologies use the chemicalrotary shafts independently driven by electric motors and a mechanical polishing (CMP) methods. The CMP has container filled with abrasive materials. The silicon wafer comparatively low productivity because it uses abrasive was fixed on the bottom of the container associated with materials with soft grains, such as colloidal silica or oxidized Spindle B. The abrasive grinding wheels and the polishing cerium.

pads were fixed on Spindle A, where the polishing load To increase the efficiency of polishing of hard materials, of 2 N was applied. The profiles of finished surfaces several alternative approaches propose to use the ultra diswere measured using a diamond stylus (measuring machine:

persed diamonds (UDD) as abrasives. The UDD produced Taylor Hobson Talysurf 4).

by detonation of explosives with negative oxygen balance [1] can be obtained after chemical treatment in forms of pastes, powders, and water suspensions with various types and 3. Precise polishing using ultra-fine sizes of grains. For example, the typical sizes of the abrasive grinding wheels monocrystalline UDD grains in water suspensions lie in range of 4-50 nm, and the polycrystalline UDD in water 3.1. E x p e r i m e n t a l c o n d i t i o n s. In this series of suspensions have the grain sizes of several micrometers.

tests, we evaluated the characteristics of the ultra-fine The UDD-based slurries showed superior chemical stability abrasive grinding wheels for dry polishing and wet polishing and lubricity, however these slurries have not been applied of the silicon wafers.

widely for precise polishing on the mass-production basis.

The grinding wheels were made using the electrophoresis In the current study, we proposed two new methods phenomenon. The monocrystalline UDD grains, put into for precise polishing using the UDD in water suspensions sodium alginate or into a mixture of sodium alginate with and investigated its polishing characteristics by a number of carboxymethylcellulose (CMC), deposited during 60 min on experiments.

a brass electrode rotating with a speed of 50 rpm. An The first proposed method uses a new grinding wheel applied electric potential was varied within the range of with an ultra-fine abrasive material made by electrophoretic 10-100 V. The grinding wheel was dried and machined.

deposition of the monocrystalline UDD onto an elecUpon the type of solutions and the concentration of UDD trode [2]. In this series of experiments, we tested various in the solutions, the maximum thickness of abrasive UDD types of grinding wheels and evaluated its characteristics for material reached 1.5 mm. The manufacturing conditions are precise polishing of silicon wafers.

summarized in Table 1.

The second proposed method uses a new polishing pad, 3.2. Experi ment al resul t s and di scussi ons.

in which the porosity can be generated naturally during Polishing tests with manufactured ultra-fine grinding wheels polishing. In this series of experiments, we compared the polishing performances using the new polishing pad and were conducted in two different environments: wet and dry.

a conventional foamed polyurethane pad in combinations It was found that the surface roughness decreased gradually with the monocrystalline UDD in water suspension and with polishing time for both cases, however the absolute the polycrystalline UDD in water syspension for precise values of surface roughness were larger in the case of wet polishing of the silicon wafers. polishing than in the case of dry polishing.

Nano-polishing of silicon wafers using ultra-dispersed diamonds Table 1. Experimental conditions (ultra-fine abrasive grinding performance of this method was unsatisfactory due many wheel) uncertain factors, which influenced of the manufacturing process of the ultra-fine abrasive grinding wheel. It was Abrasive grain UDD (0.5 wt.%) found that the quality of the grinding wheels strongly affected by such parameters as the adhesiveness of UDD Binder a) Sodium alginate (0.25 wt.%) to the brass electrodes. The wet polishing was not found b) Sodium alginate (0.2 wt.%)+CMC (0.5 wt.%) effective in comparison with the dry polishing. Certainly, Work piece Silicon wafer this method requires many improvements.

Polishing load 2N Spindle A 450 rpm 4. Precise polishing using pads with Spindle B 165 rpm self-generating porosity Polishing a) Wet polishing conditions b) Dry polishing 4.1. Experi ment al condi t i ons. The polishing tests were conducted using the new-type polishing pad on the Polishing time 90 min (max) equipment described in Section 2 of the current paper. The experimental conditions are listed in Table 2.

Table 2. Experimental conditions (new polishing pad) In the first series of tests, we used the grinding wheels made with a mixture of the monocrystalline UDD and Pad a) Foamed polyurethane pad sodium alginate without addition of CMC. The test results b) New pad with self-generating porosity are shown in Fig. 1. The minimum surface roughness Work piece Silicon wafer achieved after 80 min of dry polishing was Ra = 6nm. In Slurry a) Monocrystalline UDD in water suspension the case of wet polishing, the grinding wheel broke suddenly b) Polycrystalline UDD in water suspension after 60 min.

UDD in slurry 5wt.% Polishing load 2N Spindle A 250 rpm Spindle B 50 rpm Polishing time 180 min (max) Made from petroleum pitch mixed with halite, the new pad with self-generating porosity has high manufacturability.

The molten petroleum pitch was mixed with the pulverized halite grains of the uniform size and later was solidified and flattened by turning operations in order to create a flat polishing pad. The optical micrographs of the pad are shown in Fig. 2. Self-generation of porosity was achieved in the boundary layer of the pad when the halite grains protruded of the surface of the pitch were dissolved by the UDD slurry during polishing. It was found that the pad with self-generating porosity and the foamed polyurethane Figure 1. Results of polishing tests using ultra-fine grinding pad had similar visco-elastic and mechanical characteristics.

wheels.

Another series of tests was performed with the grinding wheel made with the monocrystalline UDD grains in the mixture of sodium alginate and CMC. As it is shown in Fig. 1, the surface roughness of the silicon wafer in this case reduced from Ra = 107 to 4 nm after dry polishing for 30 min.

For small-size silicon wafers, the method showed rela- Figure 2. Optical micrographs of new pad with self-generating tively good polishing performance, however the overall porosity. a surface, b cross-section.

, 2004, 46, . 732 T. Kurobe, T. Fujimura, H. Ikeda 4.2. Experi ment al resul t s and di scussi ons.

The polishing tests showed that in the case of using the UDD in water suspensions with the new self-generating porosity pad, the surface roughness of the silicon wafer was rapidly decreased at the beginning and slowly decreased in the rest of the polishing process.

The test results for the pad with self-generated porosity are plotted in Fig.3 and the results for the foamed polyurethane pad are shown in Fig.4. After polishing for 90 min using the new polishing pad with the polycrystalline UDD in water suspension, the surface roughness of the silicon wafer reduced from Ra = 107 to 2 nm (Fig.3). Twice longer polishing time, about 180 min has been required to achieve the same surface roughness using the foamed polyurethane pad with the polycrystalline UDD in water suspension. Moreover, the surface was more scratched in this case than it was obtained in the case of the polycrystalline UDD in water suspension and the pad Figure 3. Surface roughness obtained using new polishing pad with self-generating porosity. The optical micrographs of with self-generating porosity.

the finally polished surfaces and the surface profiles are presented in Fig. 5 for the case of the pad with selfgenerated porosity.

Fast decrease of the surface roughness by polishing using the polycrystalline UDD in water suspension may be explained by a peculiar polishing mechanism when many new cutting edges can be created after cleaving of the polycrystalline UDD grains during polishing process.

Thus, using the polycrystalline UDD suspensions in conbination with the new polishing pad it is possible to achieve the specified surface roughness of the silicon wafer rather faster than using the foamed polyurethane pad.

Figure 4. Surface roughness obtained using foamed polyurethane pad.

In this series of experiments, we used the silicon wafer samples with initial surface roughness of Ra = 107 nm. The following combinations of polishing materials were tested:

1) the new pad with self-generating porosity and the slurry of the monocrystalline UDD in water suspension;

2) the new pad with self-generating porosity and the slurry of the polycrystalline IDD in water suspension;

3) the foamed polyurethane pad and the slurry of the Figure 5. Optical micrographs of surfaces polished by new monocrystalline UDD in water suspension;

polishing pad with self-generating porosity. a prior to polishing, 4) the foamed polyurethane pad and the slurry of the Ra = 107 nm; b polycrystalline UDD, t = 90 min, Ra = 2nm;

c monocrystalline UDD, t = 90 min, Ra = 12 nm.

polycrystalline UDD in water suspension.

, 2004, 46, . Nano-polishing of silicon wafers using ultra-dispersed diamonds 5. Conclusions In this study, we proposed two methods for precise polishing of silicon wafers and investigated the efficiencies of the methods using the UDD in water suspensions for the abrasives.

The fist polishing method uses ultra-fine abrasive grinding wheels manufactured with the monocrystalline UDD. The surface roughness of the silicon wafer has been reduced from Ra = 107 to 4 nm after 30 min of dry polishing by the ultra-fine abrasive grinding wheels. The method showed satisfactory results for dry polishing of silicon wafers, but the overall performance of the method was poor. We concluded that this method still needs improvements and it cannot be recommended at the current stage of the research.

The second method uses a new-type polishing pad with self-generating porosity. Polishing was done with monocrystalline and polycrystalline UDD in water suspensions. In addition, we used a conventional foamed polyurethane pad for comparison of the polishing performances. The tests showed that the surface roughness of the silicon wafer has been reduced from Ra = 107 to 2 nm after polishing by the new-type polishing pad and the polycrystalline UDD for 90 min. Comparing with polishing by the conventional foamed polyurethane pad, we showed that the same surface roughness of the silicon wafer could be achieved after twice shorter polishing time using the pad with self-generating porosity. Although cutting ability per one UDD grain is very small, we considered that many UDD grains per unit of polishing area are responsible for high polishing performance.

The results of silicon wafer polishing showed that the pad with self-generating porosity could be effectively used in combination with the polycrystalline UDD in water suspensions.

References [1] V.Yu. Dolmatov. Russian Chem. Rev. 70, 7, 607 (2001).

[2] J. Ikeno, Y. Tani, H. Sato. Ann. CIRP 39, 1, 341 (1990).




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