Effects of Thickness and Crystallographic Orientation on Tensile Properties of Thinned Silicon Wafers

Thinning silicon wafers for stacking in limited space is essential for the 3-D integration (3D!) technology of semiconductors. Due to the lack of research on the mechanical properties of thinned silicon wafers, it is difficult to assess and improve the mechanical reliability of 3D! semiconductor devices. This article reports the effects of thickness and crystallographic orientation on the tensile properties, such as Young's modulus, elongation, and strength, of the thinned silicon wafer. Tensile properties of a {100} silicon wafer are measured using a direct tensile testing system, where a digital image correlation method is adopted for accurate strain measurement. Femtosecond laser patterning for accurate shape control is used to fabricate dog-bone-shaped specimens with various thicknesses and crystallographic orientations. The effect of crystallographic orientation is investigated for (110), (320), (210), and (100) orientations. The Young's modulus of each orientation closely matches the theory of anisotropic elasticity. The surface energy ratios between crystallographic planes are calculated using fracture mechanics analysis. As the thickness decreases from 100 to 10 μm, the elongation and strength increase threefold, while Young's modulus is constant along the (110) direction. The strength results are analyzed with a Weibull statistical size effect model, where the Weibull modulus is calculated to be 2.35, which correlates strength only with thickness variation. Using this value and the Weibull size effect model, the expected strength of specific thickness can be calculated easily without additional experiments.