Defect detection in monocrystalline silicon wafers using high frequency guided waves

Abstract

Monocrystalline silicon wafers are employed in the photovoltaic industry for the manufacture of solar panels with high conversion efficiency. Micro-cracks can be induced in the thin wafer surfaces during the cutting process. High frequency guided waves are considered for the testing of the wafers and the nondestructive characterization of the micro-cracks. Experimentally selective excitation of the fundamental Lamb wave modes was achieved using a custom-made angle beam transducer and holder to achieve a controlled contact pressure. The out-of-plane component of the guided wave propagation was measured using a noncontact laser interferometer, scanned parallel to the specimen surface using a positioning system. The material anisotropy of the monocrystalline silicon leads to variations of the guided ultrasonic wave characteristics depending on the propagation direction relative to the monocrystalline silicon orientation. In non-principal directions of the crystal, wave beam skewing occurs due to material anisotropy. Artificial defects were introduced in the wafers using a micro indenter with varying force. The defects were characterized from microscopy images to measure the indent depth and combined crack lengths. The scattering of the A0 Lamb wave mode was measured experimentally. The scattered wave field showed high amplitude peaks close to the defect location and an interference pattern indicative of a scattered wave, but was found to be not symmetric to the defect and crystallographic orientations. Characteristics of the scattered wave amplitudes were correlated to the defect size and the detection sensitivity discussed.