Fundamental Research on Silicon Wafer Thinning by Ultra-precision Grinding
|School||Dalian University of Technology|
|Course||Mechanical Manufacturing and Automation|
|Keywords||Silicon Wafer Thinning Diamond Grinding Subsurface Damage Mechanical-chemical Grinding|
The demands of high-performance, multi-function, miniaturization and low-cost for electronic products promote the rapid development of integrated circuit(IC) manufacturing technology. Especially the high-speed evolution of portable electronic products puts forward higher and higher requirements for IC packaging technology.3D stacked package technology becomes the main development direction of IC packaging technology for its advantages in lowering space occupation, elevating electrical stability, reducing costs, etc. To raise layer number of3D stacked package without enlarging the overall package thickness, the silicon wafer in each layer needs back thinning. Furthermore, the back thinned wafers are required to have high surface accuracy and free surface/subsurface damage. Although being widely used in back thinning of silicon wafers, ultra-precision grinding technology with diamond grinding wheel is facing the problems of subsurface damage, wafer warpage/bow, machining efficiency, etc with an increase in diameter and thickness of prime wafers,but a decrease in finical thinning thickness. To meet IC packaging demand for ultra-thin wafers and improve surface layer quality, surface accuracy and machining efficiency, this dissertation aims to investigate the subsurface damage characteristics, warping mechanism and edge chipping of silicon wafers in diamond grinding and the mechanical-chemical grinding(MCG) technology with soft abrasive grinding wheel. The research results provide practical guidance for realizing high-efficiency and low-damage silicon wafer thinning by ultra-precision grinding. The main research works and conclusions are as follows:(1) A mathematical model of the grain depth-of-cut in wafer rotation grinding was established and the formula of the grain depth-of-cut was derived, which correlated grain depth-of-cut with grinding parameters, dimensions of the diamond cup wheel and distance from wafer center to the sample location. As the increase in grain size, wheel feedrate, wafer speed and distance from wafer center to the sample location, the grain depth-of-cut increased, while the increase in circumference and teeth width of the diamond cup wheel, wheel speed would decrease the grain depth-of-cut. Using the angle cross-section microscopy, the subsurface damage distributions along radial and circumferential direction of silicon wafers ground by diamond grinding wheels were investigated, and the effect of spark-out process on the subsurface damage distribution was analyzed. On that basis, the effects of grinding parameters on subsurface damage depth were also studied and the reasons were analyzed based on the established mathematical model of the grain depth-of-cut. The experiment results showed that in the ground wafer without spark-out process, the subsurface damage depth in<110> crystal orientation was larger than that in<100> crystal orientation and the subsurface damage depth increased along the radical direction from the centre to the edge; but in the ground wafer with spark-out process, the subsurface damage depths in different circumferential and radial locations were almost the same. And the subsurface damage depth in ground silicon wafers with spark-out process was significantly smaller than that without spark-out process. The subsurface damage depth decreased with a decrease in grain size and wheel feedrate, and an increase in the wheel speed, but the effect of wafer speed on subsurface damage depth was less.(2) Wafer holding mechanics on a vacuum chuck and grinding-induced stress distribution during a wafer thinning process were investigated using Ansys simulation software, and the warping mechanism of silicon wafers after grinding thinning was revealed. Based on the deflection theory of the circular thin plate, a mathematical model was established for predicting the wafer warp in the wafer grinding thinning when wafer deflection was small in the elastic range. The model correlated wafer warping with subsurface damage depth, machining stresses, wafer thinning thickness and the mechanical properties of the monocrystalline silicon, and the model was verified through thinning experiments of silicon wafers.The results indicated that as the increase in subsurface damage depth and machining stresses, the wafer warping increased; while the increase in wafer thinning thickness would decrease the wafer warping. And the model also showed a good agreement with the experimental results.(3) Edge chipping profile and size distributions along circumferential direction of thinned silicon wafers in diamond grinding thinning were experimentally investigated, on that basis, the effects of grinding parameters such as grain size, wafer thinning thickness, grinding mode and wheel feedrate on edge chipping size were also studied. The study also discussed the edge chipping mechanisms based on crystal orientation and machining mechanics of a wafer in thinning. The experiment results indicated that the edge chipping profile in the <100> crystal orientation of a ground silicon wafer was almost a isosceles right triangle and that in the<110> crystal orientation similar to a rectangle, but the edge chipping size was independent on the crystal orientation and thus wafer edge location. The edge chipping size decreased with a decrease in grain size and down-feedrate, but an increase in the wafer thickness. And the edge chipping size was larger in the down-grinding mode than in the up-grinding mode. (4) Aiming at the surface layer damage of silicon wafers in diamond grinding, a mechanical-chemical grinding(MCG) technology with soft abrasive grinding wheel was proposed. Based on the material properties of single crystal silicon, the soft abrasive grinding wheels using CeO2, Fe2O23and MgO as abrasives for grinding silicon wafer were developed. The dressing method of soft abrasive grinding wheel was studied. Through measuring wafer surface roughness, surface/subsurface damage, dressing interval, spindle motor current, grinding ratio and material removal rate, and comparing with the machining effects of diamond wheel grinding and chemical mechanical polishing(CMP), the grinding performances of the soft abrasive grinding wheels were investigated. The surface component of silicon wafer ground with soft abrasive grinding wheel was inspected and the chemical reaction between abrasives, additives and single crystal silicon was analyzed. The material removal model in silicon wafer grinding with soft abrasive grinding wheel was established. The results demonstrated that the surface layer quality of silicon wafers ground by the soft abrasive grinding wheels were all much better than the diamond grinding wheel and comparable to CMP, but the material removal rate in wafer grinding with soft abrasive grinding wheels was higher than about CMP.(5) Building upon the above research on material removal rate, subsurface damage, edge chipping and wafer warping of diamond grinding and characteristics of MCG with soft abrasive grinding wheel, a novel high-efficiency and low-damage ultra precision grinding thinning process of silicon wafers by stages using#600diamond grinding wheel,#3000diamond grinding wheel and#3000MgO soft abrasive grinding wheel in one single clamping step was proposed. Using the proposed thinning process, the minimum thinning thickness of silicon wafer reached40μm.