| IC chip is the core component of microprocessors used for intelligent terminal equipment such as computers, smartphones, and tablets. Currently, silicon wafer 90% of IC chips all over the world take as substrate material. It can be said that back-thinning grinding of IC chip silicon substrate material is directly related to development of microprocessors in the future. Rotating grinding of silicon wafer with high precision diamond grinding wheel is now the mainstream technology to manufacture IC chips, which requires high self-sharpness of the wheel. Ground surface of silicon wafer should meet standards of nanoscale surface roughness(Ra) and micron scale subsurface damage(SSD). However, as the self-sharpness of Chinese grinding wheel is quite low, and Ra and SSD of silicon wafer are so big that the wafers break easily, causing great economic loss. Especially for advanced IC chip manufacture, diameter of the wafer is over 200 mm. High precision diamond grinding wheels are listed as a strategic resource by developed countries, which rendered the export to China as impossible. At the same time, components, formula and process of high precision grinding wheels are strictly confidential. Thus, systematic studies on material and preparation of such grinding wheel are urgently needed.This thesis is focused on self-sharpness of the grinding wheel, as well as Ra and SSD of wafer surface. Pore-forming mechanisms of resin-bond and vitrified-bond grinding wheels are studied. A large number of pores should be made in the grinding wheel to weaken the holding force between adhesive and abrasive and finally improve the grinding wheel self-sharpness. New process of large porosity grinding wheel is developed on sample mixing, molding, sintering and dressing, as well as new types of vitrified bond and resin bond. Sample morphology is observed and properties such as mechanical strength are analyzed using SEM, TG-DSC, FTIR, and EDS. Material performances of grinding wheel are optimized, and the influence of material content on both mechanical strength and grinding performance of the wheels is explored. Silicon wafer grinding mechanism is researched and vitrified through industrial applications.Back grinding process of silicon wafer includes coarse grinding and fine grinding. Vitrified bond grinding wheel with good sharpness is used in coarse grinding and resin bond grinding wheel with good polishing performance in fine grinding. For vitrified bond grinding wheel, the bond is Na2O-Al2O3-SiO2-B2O3 multivariate borosilicate glass, whose sintering temperature is no more than 685 ℃ and liquidity 140%- 150%. It expansion coefficient is 4.46×10-6 m/K, close to that of diamond, namely 4.40×10-6 m/K. Resin powder BMI is prepared, particle size 10~40 μm, density 1.33 g/cm3, and water absorbency near 0.2%~0.3%. It is shown in TG analysis that the 5% themo-gravimetric point of the resin is 348.9 ℃.New processes are developed, including polymer powder to make pores in vitrified bond bridge and wet milling vitrified bond. Polymer powder is added into vitrified bond at volume fraction of 6%. There appear many 5- 8 μm pores on the bond bridge when the bond is sintered at temperature of 685℃. Such addition weakens the bonding force between vitrified bond and abrasives, so dull diamond abrasive particles falling off by themselves, thus improving the self-sharpness of the grinding wheel. Wheel pellets that contain polymer powder are manufactured through hot pressing at temperature of 215 ℃ and pressure of 10 MPa. Ceramic bond is designed according to a new formulation and repeatedly vitrified and broken. Self-made planetary ball mill machine is used and a wet milling process is developed to make the grain size of the bond powder less than 2 μm. Eight kinds of diamonds, particle sizes ranging from 90~2 μm, are used to prepare vitrified and resin bond grinding wheels, with which silicon wafer grinding tests are implemented. Results show that Ra of ground surface varies from 378 to 1.91 μm, SSD and MMR varying from 16.5 to 0.92 μm and from 35 to 0.3 μm. It’s also shown that grinding effect of polycrystalline is better than that of single crystalline, such as Ra, MMR, SSD and grinding electric current. Compared with resin bond diamond grinding wheel of the same particle size, MMR of vitrified diamond grinding wheel is 20% higher, SSD 20% higher, current 5% lower and Ra 100% higher.There are several pore forming mechanisms, such as removing placeholder after dissolving and gas foaming agents. Results show that grinding wheel made with gas foaming agents possesses better grinding ability. After the pore-forming agents are added, the sharpness of grinding wheel is obviously enhanced so there is no need for coercive dressing. SSD of silicon wafer decreases from 3.5 μm to 0.92 μm. Ra of the surface decreases from 6.72 nm to 1.91 nm. When grinding Ф200 mm silicon wafer at an allowance of 20 μm, the wheel could grind 25000 pieces.Coarse surface of polycrystalline diamond abrasive could enhance bending force between abrasive and bond, thus decreasing falling abrasive and reducing scratches caused by three body friction. The pores in grinding wheel could decrease contact area thus reducing friction force. Solid lubricant could decrease the friction coefficient between resin bond and silicon wafer as well as grinding power. After comparing chip size, width of ground scars, and particle size which is about to fall through EDS and SEM, 2 μm particles are conformed to be silicon chips. It is concluded that there are two removal mechanisms namely brittle crack and plastic deformation due to the existence of granular and filiform chips.Industrial application verifies that grinding wheel with continuous sharpness could be prepared using the method of adding a lot of pores thus weakening the combination between bond and abrasives. The pressure of grinding wheel on silicon could be controlled during grinding. With the increase of self-sharpness of porous grinding wheel, the surface of silicon wafer could meet the requirement of nanoscale roughness and micron grade subsurface damage. In addition, there is no fragment when grinding silicon wafer, suitable for IC chip substrate. |
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