Design And Development Of Ultra-precision Grinder For 300mm Wafer

During the process to turn silicon ingot into the integrated circuits (IC) chips, ultra-precision grinding is mainly used in planarization for planarization in silicon wafer preparation and back thinning in the backend of semiconductor manufacturing process. The ultra-precision grinding technology of the large size silicon wifer with ultra-smooth damage-free surface is facing many new challenges:first, the large size ground wafer is easy to warp so that it is difficult to achieve the high accuracy grinding surface shape. Second, the high machining efficiency is required because the material removal amount of the wafer with increasing of wafer diameter and decreasing of IC chip thicknes is increasing. Finally, as the wafer surface quality and processing efficiency is more sensitive to the grinding force, it is essential to monitor grinding force during grinding thinning wafer. Demand on ultra-precision grinding technology and equipment, aiming at improving wafer surface quality, the processing efficiency and precision, the key technologies of precision feeding, surface shape control and grinding force monitoring have been studied. Then the ultra-precision grinder for 300mm wafer is designed and developed.The main research contents and conclusions are as follows:(1)Based on the principle of the infeed grinding and control forces, the ultra-precision grinding technology for large-sized wafer was studied, and the ultra-precision automatic wafer grinder for silicon wafer with maximum diameter of 0300mm was developed. The layout structure of the wafer grinder, which has the characteristics of compact structure and high automation, etc., is constructed into one with double-spindle and triple-workstation. The wafer grinder has many functions including on-line measurement of the thickness and grinding force. All the process are performed automatically, such as wafer transmission, centering, soakage, rough grinding, fine grinding, cleaning and drying, etc.(2)The three-dimensional virtual prototyping of the ultra-precision wafer grinder is established, and then the static and dynamic analysis of the major components and grinding unit are carried out in order to perfect the design. The rigid-flexible coupling model of the infeed system is established, and kinematic characteristic of the infeed system is analyzed. The co-simulation with mechanical and electric model of the infeed system is established and the transfer function of the infeed system is derived. At last, the displacements, static and dynamic characteristics of the infeed system are verified by experiments.(3)According to the structural features of the ultra-precision grinder with double-spindle and triple-workstation, the influence law of the obliquity between wheel spindle and workpiece spindle on the grinding surface shape is studied, and then the mathematical model of the grinding surface shape for rough grinding unit and fine grinding unit are established. The adjustment method of the spindle obliquity for wafer grinder with double-spindle and triple-workstation is proposed, and the obliquity adjustment devices of the rough grinding spindle, fine grinding spindle are developed. At last, the model of the surface shape control is verified by self-grinding chuck and wafer grinding experiments.(4)The main grinding force Fc, Cut-in force Fp, Feeding force Ff in the process of the wafer grinding are analyzed. The piezoelectric quartz crystal-based dynamometer, integrated in 0300mm silicon ultra-precision grinder, is developed for measuring grinding force in three directions. The sensitivity, linearity, repeatability and other static and dynamic characteristics of the dynamometer are obtained by performing static calibration, dynamic performance measurement and on-line calibration. Aiming at processing efficiency and surface quality, an innovative process, which combines wheel feeds step by step and grinding based on force control, is put forward. At last, a series of the grinding experiments, which the grinding wheel feed rate is controlled by grinding force feedback, are conducted.(5)The ultra-precision automatic wafer grinder for silicon wafer with maximum diameter of 0300mm was developed, which is first in domestic. The results show that the total thickness variation of the ground wafer, the surface roughness of the wafer ground by #4800 grinding wheel and the thickness of the sub-surface damage layer are less than 4μm,1.4nm, 0.25μm, respectively. The ultra-precision grinding with high efficiency and low damage can be realized.