| For quality control and reliability analysis in semiconductor manufacturing, it is crucial to access the localized stress in devices due to process integration in thin film deposition, etching, passivation and thermal treatment. Presented in this dissertation is the exploration of a new methodology to access localized stress in patterned microstructures. It is called the “micro-bending-beam method”.; In order to evaluate the residual stress distribution in a thin film pattern residing on a silicon wafer, the Si underlying the pattern was thinned down uniformly so that its deflection, caused by the residual stress, could be measured. If the etched-back surface remains optically flat and reflective, then the bending of the diaphragm would be equivalent to its surface profile, which could be readily measured by a Twyman-Green laser interferometer. A procedure called “numerical etching” was implemented to simulate the Si etching process, which linked the stress state of the microstructure on a bulk wafer to that on a Si diaphragm. An initial stress field in the pattern was assumed, its effect on the bending of the Si diaphragm beneath was calculated and compared to the measured value. The discrepancy between them was used to modify the initially assumed stress field and repeated until satisfactory matches were achieved at each diaphragm thickness. The applicability of the micro-bending-beam method was demonstrated by resolving the residual stress in an electroless Ni bump.; It was found that for a relatively thick diaphragm, the “plate” effect dominated; for a relatively thin diaphragm, the “membrane” effect dominated; at intermediate thickness, both effects existed. A general algorithm to solve non-linear equations where both bending stiffness and residual stress in a diaphragm must be considered was invented, and named “non-linear sequential analysis”. It was found that for a pre-stressed pattern sitting on a stress-free Si diaphragm starting at to and thinned down to tn, there existed an “optimum” thickness of the diaphragm where the deformation was maximum. For the same structure, if a residual tensile stress existed in the Si diaphragm, then this optimum thickness would shift to a smaller value and the magnitude of this deformation would decrease as well. Furthermore, the shape of the diaphragm deformation was also a strong function of the diaphragm thickness. |
