| Monocrystalline silicon is the most commonly used substrate in integrated circuit manufacturing.In order to meet the packaging requirements of semiconductor chips,it needs to be thinned.Because of its high precision and high efficiency,ultra-precision grinding based on the principle of wafer rotation grinding is widely used in back-thinning of silicon wafers.However,in the uppermost layer of the ground wafer,a thin sub-surface damage layer is formed,and the residual stress in the damage layer causes the wafer to deform.The deformation of the wafer will increase the rate of debris during subsequent processing and transportation.Therefore,it is necessary to study the method for wafer deformation measurement and residual stress calculation to evaluate the wafer processing quality and optimize the processing technology.Due to the serious deformation of silicon and the significant deformation induced by gravity,the existing methods and equipment to measure the deformation of silicon wafer is difficult.There is no high-precision measurement methods,so the analysis for residual stress is also difficult.In this paper,a processing deformation detection method for the ground thinned silicon wafer is presented.The relationship between the deformation and residual stress of the silicon wafer is studied and based on the theory of thin plate bending and the finite element method,two methods of calculating residual stress based on wafer deformation are proposed.Finally,the distribution of residual stress on the surface of the thinned silicon wafer was analyzed by using the method of wafer deformation detection and the residual stress calculation method.The main contents and conclusions are as follows:A special three-point support platform was designed for the silicon wafer,and the detection method for the deformation of ground wafers is put forward.The wafer is supported by a three-point support platform in the method.The surface profile of the wafer was measured with a laser displacement sensor.Determined the relationship between the center of the wafer and the position of the three support spheres and established the finite element model to calculate the gravity additional deformation of the three-point supporting wafer.By eliminating the additional deformation induced by the gravity from the measured surface profile,the machining deformation of the ground thinned wafer was obtained.The theoretical relationship between the deformation of silicon wafer and the residual stress is studied.Based on the theory of thin plate bending and the finite element method,two methods of calculating residual stress based on wafer deformation are proposed.The influence of anisotropy on the deformation of silicon wafer,the Stoney formula and the theoretical formula suitable for the silicon wafer with large deformation are studied by finite element software.The experimental result show that the calculation accuracy of the finite element empirical equation was all better than that of theoretical equation of large deformation of thin plate.The machining stress of resin-bond diamond wheels of 600#,2000# and 3000# mesh size under certain parameters is determined and they are respectively 127.8 MPa,170.9 MPa and 999.7 MPa(The thickness of the damage layer is 1 ?m).The deformation of the ground wafer was measured by the wafer deformation detection method.Calculated the deformation curvature by radial subregion and based on the established residual stress calculation method,the distribution of residual stress on the surface of the thinned silicon wafer was analyzed.The results show that the deformation of the wafer without spark-out process increases from the central area to the edge,which indicates that the deformation is more and more serious,and the corresponding surface residual stress is getting larger and larger.At the same time,it is found that the crystal orientation of silicon wafer has a significant effect on the deformation of silicon wafer,which is due to the difference of sub-surface crack propagation. |
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