Grinding Behaviour of Silicon Wafer and Sintered Al2O3 By Constant Force Feeding Grinding System

Hyunjin Kim, Koji Matsumaru, Atsushi Takata and Kozo Ishizaki

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AZojomo (ISSN 1833-122X) Volume 2 August 2006

Topics Covered

Abstract

Keywords

Introduction

Experimental Procedure

Results and Discussion

Conclusions

Acknowledgement

References

Contact Details

Abstract

Excessive forces acting on grinding surfaces cause defects on specimen during grinding.  Removal depth and table feeding rate are the only controlling factors for a conventional grinding system.  A new grinding system is proposed modifying the feeding process to maintain feeding force constant instead of feeding rate.  Forces of a grinding wheel on a specimen are easily controlled by the present method.  We used 4 inch Si wafer and sintered Al₂O₃ (25 mm X 50 mm) to evaluate grinding forces, table-feeding rate for a given feeding force and ground defects.  We found that the forces to grind sintered Al₂O₃ and Si wafers behave in different way.  The table-feeding rate decreases as feeding depth increases.  Estimating the energy required to remove a given volume of specimen, i.e., specific grinding energy, shows a constant curve for any given feeding depth.  Surface roughness was evaluated by confocal laser microscopy, and did not change for different amounts of feeding depth from 1 μm to 50 μm.  Finally, we could grind the silicon wafer from 0.5 mm thickness to 40 μm and the sintered-Al₂O₃ from 1 mm thickness to 40 μm by only one-step of grinding in less than 50 min.  We found that the constant grinding force system allow to minimize defects.

Keywords

Constant-force-feeding, Grinding System, Reduced Damage, Specific Grinding Energy (SGE), Ceramics

Introduction

Most of the advanced materials, e. g., functional ceramics and high temperature materials are hard to machine.  Grinding is widely used, and an important machining operation for producing modern equipment using advanced materials.  At present, the percentage of machining cost would be about 80% or more of the total component cost in the industrialized world [1].  Silicon wafers are used for the production of most microchips.  Various processes are required to make a silicon wafer from a single crystal ingot.  Typical processes, lapping/ surface grinding, and polishing of silicon wafers consume a long time and a high cost [2].  In grinding process, damages on ground surfaces are not avoidable.  For removing the damaged layer, heat treatments and etching processes are required [3].  Excessive forces during grinding process generate defects such as chipping or cracking.  Grinding materials inevitably generates both brittle fractures and ductile flaws as the diamond abrasive grains cut into the specimen [4,5].  For conventional grinders, grinding depth and feeding rates are controlled.  Forces on the grinding plane increase to generate an excess stress on sample surface, and defects on ground surfaces.  Astute operators monitor vibration [6,7], sound [8], and acoustic emission [9-11] to estimate the grinding wheel conditions in conventional grinding systems.  In this study, we propose a new grinding equipment to achieve defect-reduced machining. This advanced machining has constant-force-feeding system. Hence, table-　feeding rate is altered depending on grinding conditions. The effects of grinding depth with grinding forces, table-feeding rate and specific grinding energy (SGE) on constant-force-feeding grinding system using Si wafer and sintered Al₂O₃ are evaluated.

Experimental Procedure

A newly developed constant-force-feeding grinding system is used in this study.  Figure 1 shows the schematic diagram of the setup for newly developed grinding system.

Figure 1. Schematic diagram of the constant-force-feeding grinding machine.

This system consists of an air sliding grinding-table, a vacuum vise and a cup-shaped diamond-grinding wheel.  The vacuum vise is made by porous ceramic material, and machined to guarantee parallel surface between the vacuum vise and the grinding wheel surface for high flatness ground specimen.  The specimen is placed on the porous vacuum vise without glue and air is evacuated to hold the specimen.  This attachment will not cause any damage to the specimen.  The grinding table is controlled by air cylinder and air slider for the constant-force-feeding system.  In this study, 4 inch Silicon wafer and sintered Al₂O₃ (98.1-98.5% relative density and 25 mm X 50 mm) are used for evaluation of grinding abilities.  Specimens are ground by a cup-shaped 600-grit grinding wheel (vitrified bond, 35-37% porosity with 30-40 μm diamond particles as abrasive grains, Nano TEM Co. Ltd., Nagaoka, Japan).  Normal and tangential forces to the sample surface are measured by 3-component piezoelectric dynamometer (9257B. Kistler Corp.) during grinding.  Effects of feeding depth (1 μm to 50 μm) per pass on grinding force and table feeding rate were evaluated.  Table feeding rates from the period while grinding forces were active.  The change of grinding wheel thickness between before and after grinding test is evaluated.  Finally, the ground surface roughness and morphology are evaluated by confocal laser microscopy (LSM VK 8500. Keyence Corp.).

Results and Discussion

Figures 2 (a) and (b) show the relationship between grinding forces (normal force; F_n, tangential force; F_t) and table feeding rate (v_f) versus feeding depth for sintered Al₂O₃ and Si wafer, respectively.

Figure 2. The influence of the various feeding depths on the grinding force (Tangential force; F_t, normal force; F_n) and table-feeding rate (v_f).  (a) Sintered Al₂O₃, (b) Si wafer.

Grinding forces of sintered Al₂O₃ increased continuously as the feeding depth increased.  The grinding forces for Si wafer increased as Al₂O₃ for feeding depth to 30 μm/pass, and decreased as the feeding depth increased over 30 μm/pass.  The cause of the decrease in forces is related to bonding strength between abrasive-grins and bonding matrix in the grinding wheels.  Figure 3 shows the relationship between wearing ratio of grinding wheels versus feeding depth. 

Figure 3. Wear ratio of grinding wheel increase as the feeding depth increase for sintered Al₂O₃. However, which is not the case of Si wafer.

The grinding wheel wear ratio was calculated by the following equation (1).

     (1)

Where Δt_w is the change of grinding wheel thickness between before and after all the grinding, Δt_m is effectively ground thickness, i. e., totally feeded depth (d_f), subtracted by grinding wheel change (Δt_w).  The grinding wheel wear for sintered Al₂O₃ increased rapidly as the feeding depth increased, but that for Si wafer did not change much. These results show the bonding strength of grinding wheels, e. g., bonding strength between abrasive-grains and bonding materials, or between bonding materials themselves is stronger than grinding Si wafer and weaker than sintered Al₂O₃.  Therefore, grinding wheel surface was removed at the same time as ground sintered Al₂O₃ surface.  For that reason grinding forces increased continuously; it is the effect of self-dressing to the grinding wheels.  On the other hand, the used grinding wheels have enough bonding strength to grind Si wafer even at a feeding depth of 50 μm/pass; this feeding depth is larger than the abrasive-grain size of the grinding wheels.  Therefore, Si wafer surface was removed and table-feeding speed decreased and grinding force decreased radically.  The table-feeding rate decreased as feeding depth increased the grinding wheels did not under go the self-dressing effects, the edge of the abrasive-grains become dull, and contact area between wheel and specimen increased. It means that the feeding rate was altered depending on grinding conditions.  These results demonstrate the assumption that constant-force-feeding grinding system controls feeding speed as grinding conditions varies.

Grinding forces between the grinding wheels and specimen are easily controlled.  Therefore, excessive forces on ground surface were eliminated.  Measured parameters allow to calculate the energy required to remove a given volume of specimen, i. e., specific grinding energy (SGE).  The SGE was calculated by the ratio of the product of measured grinding force and wheel speed to the volumetric removal rate as shown in equation (2) [12].

    (2)

Where F_t is tangential force, V_s is wheel velocity, and V_f is removal rate.  Figure 4 show the relationship between SGE versus feeding depth.  The SGE becomes constant at a feeding depth over 10 μm/pass Figure 5 shows the relationship between maximum roughness (R_y) versus feeding depth.  The surface roughness does not change at various feeding depths.  Moreover, this maximum surface roughness is the same value of pore size (Al₂O₃) and less than 4 μm (Si wafer) for any given feeding depth.  These results also demonstrate that the constant-force-feeding grinding system does not generate excessive forces of ground surface.

Figure 4: The specific grinding energy (SGE) becomes constant at a feeding depth over 10 µm/pass.

Figure 5: Similar maximum surface roughness (R_y) at different feeding depths (d_f).

Figure 6 shows SEM photograph of the side view of the silicon wafer (left) and sinterd-Al₂O₃ (right) before and after ground.  Finally, we could grind the silicon wafer from 0.5 mm thickness to 40 μm and the sintered-Al₂O₃ from 1 mm thickness to 40 μm by only one-step of grinding in less than 50 min.

Figure 6. SEM photographs of the side view of the silicon wafer and sintered Al₂O₃ before (left) and after one-pass of ground (right).  (a) Si wafer: One-pass grinding thinned to 40 µm from 520 µm thickness for only 50 min, (b) sintered Al2O3: 40 µm from 1mm thickness for 90min.

Conclusions

In this study, the constant-force-feeding grinding system was evaluated for reducing damages with high grinding rate for silicon wafer and sintered Al₂O₃.  Grinding forces of sintered Al₂O₃ increased continuously as the feeding depth increased.  The grinding force of Si wafer, however, decreased as the feeding depth increase over 30 μm/pass.  The different in forces is caused by the relative differenced of bonding strength of grinding wheels with respect to the force required to remove Al₂O₃ or Si wafer.  The grinding wheel wear for sintered Al₂O₃ increased rapidly as the feeding depth increased, but that for Si wafer did not change much. These results show the bonding strength of grinding wheels, e,. g., bonding strength between abrasive- grains and bonding materials, or between bonding materials themselves is stronger than grinding Si wafer and weaker than sintered Al₂O₃.  Therefore, grinding wheel surface was removed at the same time as ground sintered Al₂O₃ surface.  For that reason grinding forces increased continuously; it is the effect of self-dressing to the grinding wheels.  On the other hand, the used grinding wheels have enough bonding strength to grind Si wafer even at a feeding depth of 50 μm/pass; this feeding depth is larger than the abrasive-grain size of the grinding wheels.  The table-feeding rate decreased as feeding depth increased the grinding wheels did not under go the self-dressing effects, the edge of the abrasive-grains become dull, and contact area between wheel and specimen increased.  It means that the feeding rate altered depending on grinding conditions and excessive force on ground surface was not created.  The specific grinding energy becomes constant at a feeding depth over 10 μm/pass (Al₂O₃: about 17 GJ/m³, and Si wafer: about 7 GJ/m³).  Maximum surface roughness (R_y) was evaluated by confocal laser microscopy.  The maximum surface roughness does not change any feeding depths.  Finally, we could grind the silicon wafer from 0.5 mm thickness to 40 μm and the sintered-Al₂O₃ from 1mm thickness to 40 μm by only one-step of grinding in less than 50 min.

Acknowledgement

The authors wish to express their gratitude to the Japanese government for partially supporting this work through the 21st Century Center of Excellency (COE) Program of the Ministry of Education, Culture, Sports, Science and Technology

References

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