Thin monocrystalline silicon wafers are employed in the photovoltaic industry for the manufacture of solar panels with high conversion efficiency. Increased conversion efficiency and lower manufacturing costs require thin wafers. Micro-cracks on the surface of the thin and fragile wafers can be induced by the wafer cutting process during the panel manufacture. However, the minimum wafer thickness is in practice limited by the wafer breakage rates. High frequency guided waves allow for the monitoring of the wafers and detection and characterization of surface defects. The material anisotropy of the monocrystalline silicon leads to variations of the guided wave characteristics, depending on the guided wave mode and propagation direction relative to the crystal orientation.

 

Selective excitation of the first anti-symmetric A~0~ and S~0~ guided wave modes at 5 MHz center frequency was achieved experimentally using custom-made wedge transducers. Noncontact laser interferometer measurements allowed the accurate characterization of the Lamb wave propagation and scattering at artificial surface defects. The guided wave propagation and scattering at shallow surface defects were investigated using time explicit Finite Element modelling in ABAQUS. The dependence of the wave propagation characteristics on the material anisotropy is not well understood for guided waves. The phase slowness and beam skewing of the two fundamental Lamb wave modes A~0~ and S~0~ were investigated. Good agreement was found between the theoretically calculated angular dependency of the phase slowness and measurements for different propagation directions relative to the crystal orientation. Significant wave skew and beam widening was observed experimentally due to the anisotropy, especially for the S~0~ mode. Good agreement was found for the A~0~ mode, but a systematic discrepancy was observed for the S~0~ mode. These effects need to be considered for the non-destructive testing of silicon wafers using guided waves. The surface extent of the artificial defects was characterized using an optical microscope. The influence of defect size on the magnitude and directionality of the guided wave scattering was quantified. The characteristics of the scattered wave field were correlated to the defect size and the detection sensitivity discussed.


High Frequency Guided Wave Propagation and Scattering in Silicon Wafers

No proceedings papers in 2020. Video only.

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