単結晶薄膜化シリコンウェーハの機械的性質

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In recent years, with the continuous development of semiconductor devices towards miniaturization and high integration, the three-dimensional stacking technology of silicon wafers has received widespread attention. To achieve the integration of more silicon wafers in a limited space, the thinning process of single crystal silicon wafers is particularly crucial. As the thickness of silicon wafers continues to decrease, their mechanical reliability becomes an important factor affecting device performance. Therefore, the systematic study of the mechanical properties of thinned single crystal silicon wafers is of great significance for improving the overall reliability of devices.

1. Sample Preparation of Thinned Silicon Wafers

The researchers conducted experiments using 300mm, {001} crystal orientation single crystal silicon wafers. Through a series of advanced thinning processes such as rough grinding, fine grinding, and chemical mechanical polishing (for stress relief), the silicon wafers were thinned from an initial thickness of 755um to 10um. Femtosecond laser processing was used to prepare dog bone shaped tensile specimens to ensure the uniformity of stress distribution during the experiment. Subsequently, through direct tensile testing, the mechanical behavior of thinned single crystal silicon wafers was systematically evaluated under different thicknesses and tensile directions, aiming to clarify the effects of size effect (thickness) and crystal orientation on key mechanical parameters such as Young’s modulus, strength, and elongation, and provide a basis for the design of high reliability semiconductor devices.

2. Influence of Wafer Thickness on Mechanical Properties

Perform tensile tests on silicon wafers with thicknesses of 10, 20, 50, and 100um along the <110> direction. The results showed that the Young’s modulus remained stable at around 169 GPa at different thicknesses, consistent with the theoretical value, indicating that there was no significant size effect on the Young’s modulus within this thickness range.

However, thickness has a significant impact on the strength of the material: when the thickness of the silicon wafer is reduced from 100um to 10um, its strength increases by about three times (Fig. 1). Due to the brittle nature of the thinned silicon material, its fracture behavior is mainly controlled by defects, and large-sized silicon wafers are likely to have more major defects. Therefore, after thinning, the silicon wafer exhibits higher strength.

Fig. 1 Silicon wafer strength variation with the thickness

Effectsof Crystal Orientation on Young’s Modulus

Tensile tests were conducted on {001} silicon wafers with a thickness of 100um along the <110>, <320>, <210>, and <100> directions. The measured Young’s modulus values were 171.9±5.8GPa, 160.7±4.5GPa, 154.7±7.6GPa, and 131.3±5.1GPa, which were consistent with the theoretical values calculated based on anisotropic elastic theory (Fig. 2).

Fig. 2 The Young’s modulus of silicon wafers varies with crystal orientation

The results indicate that the Young’s modulus remains stable at different thicknesses and does not exhibit size effects; The strength significantly increases with decreasing thickness, and the strength of a silicon wafer with a thickness of 10um is about 3 times higher than that with a thickness of 100um. The experimental values of Young’s modulus under different stretching directions are highly consistent with the theoretical values, proving that the influence of crystallographic orientation on Young’s modulus can be accurately described through theoretical models.

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参照：

Lee, S., Kim, J. H., Ohba, T., Kim, Y. S., & Kim, T. S. (2018, April). A study on mechanical properties of thinned single crystal silicon wafer: Effect of size and direction. In 2018 International Conference on Electronics Packaging and iMAPS All Asia Conference (ICEP-IAAC) (pp. 339-340). IEEE.