Measurement Methods And Technologies For Deformation Of Thinned Silicon Wafers

Large silicon wafers are widely used to increase productivity and reduce cost in the manufacturing of integrated circuits. Besides, three dimensional packages are adopted to increase packaging density and enhance the chip performance, which require thinned silicon wafers. Residual stress is induced on the wafer surface during thinning processes and would lead to wafer deformation. Wafers with large deformation tend to break easily during machining and transmission processes, leading to yield loss. The wafer deformation measurement plays an important role to assess the machining quality of wafers. Also, the wafer deformation could be used to analyze the residual stress and improve the processing technique. However, the precise measurement of the deformation of thinned silicon wafers is significantly affected by external forces (especially gravity) due to their low rigidity. There exist limits for current measurement methods which are used to cancel gravitational effect. Besides, the stress-deformation relations of the ground wafers are in the nonlinear range and there exist large errors when the existing equations are used to calculate the residual stress by the wafer deformation. In this dissertation, measurement methods which are used to cancel gravitational effect were proposed and a measurement apparatus was developed. Also the stress-deformation relationship was analyzed for ground silicon wafers. The main research works and conclusions are as follows:(1) Deformation measurement methods were proposed aiming for thinned silicon wafers. The silicon wafer is static and supported by three points during the measuring process. A non-contact displacement sensor mounted on a two dimensional motion stage was selected to scan the surface profile of the wafer. To cancel gravitational effect during the measurement process, the method to compute and separate gravity-induced deflection (GID) according to the supporting positions was proposed. Also the method in wich the wafer is immersed in a liquid to cancel gravitational effect was proposed.(2) The computing method to separate the GID was studied. The effect of support location on the GID of the wafer supported by three points was analyzed. The methods to acquire the positions of the supports and the wafer were proposed. The FEM model of the wafer was created and verified using the inverting method by measuring a double-sided polished wafer. The GID could be computed using the FEM model according to the actual locations of the supports. Then the true wafer shape could be obtained by subtracting the GID from the measured wafer profile.(3) The liquid immersion method to cancel the gravitational effect was studied. The wafer is immersed in the liquid of which the density is slightly smaller than that of silicon. The buoyancy force on the wafer cancels out most of the effect of gravity. The residual effect of gravity is further removed by subtracting the residual deflection calculated using the computing method from the measured result. The suitable liquids were selected through the comparison of density and safety of different liquids. The correction method for the measurement data was studied through analyzing the transismission of the laser light in different media. The measure to eliminate the effect of the fluctuation of liquid level was proposed. The correction factor was determined by comparing the measurement results of standard specimen in air and in the liquid.(4) A measurement apparatus and its control system were developed. Laser triangulation sensor and two dimensional motion stage based on air-bearing granitic guides were adopted to build the apparatus. The open control system was developed based on PC and motion control card. The motion control, data processing and image display software was developed using the Matlab and VC++ softwares. The combined uncertainty for the wafer deformation measurement is ±1.7 ?m through the analysis of the geometric accuracy of the developed apparatus and the linear accuracy of the laser sensor. It was verified by the surface profile meausurement result of a standard specimen by a commercial interferometer.(5) The relationship between the residual stress and the deformation of the ground silicon wafers was analyzed. The mathematical model between the wafer deformation and the residual stress was established using the FEM model which includes the effect of the anisotropy of monocrystalline silicon. The residual stress could be calculated by the wafer deformation. Besides, the effects of the residual stress on GID were researched. When the residual stress is smaller than the critical value, the wafer deformation and the GID could be superposed on each other. However, the effect of residual stress would lead to errors when the inverting method is used. The computing method could be adopted to correct the result. When the residual stress is larger than the critical value, the wafer bifurcates. The wafer deformation and the GID could not be superposed on each other. The liquid immersion method could be used to measure the wafer deformation.