Micro-cracks can be induced in thin monocrystalline silicon wafers during the manufacture of solar panels. High frequency guided waves allow for the monitoring of wafers and characterization of defects. Selective excitation of the first anti-symmetric A0 guided wave mode was achieved experimentally using a custom-made wedge transducer. The Lamb wave scattered field in the vicinity of artificial defects was measured using a noncontact laser interferometer. The surface extent of the shallow defects varying in size from 30 μm to 100 μm was characterized using an optical microscope. The characteristics of the scattered wave field were correlated to the defect size and the detection sensitivity was discussed.
In the photovoltaic industry monocrystalline silicon wafers are employed for the manufacture of solar panels with high conversion efficiency. The cutting process induces micro-cracks on the thin wafer surface. High frequency guided ultrasonic waves are considered for the structural monitoring of the wafers and the nondestructive characterization of the micro-cracks. The material anisotropy of the monocrystalline silicon leads to variations of the wave characteristics depending on the propagation direction relative to the crystal orientation. In non-principal directions of the crystal, wave beam skewing occurs. 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. Artificial defects were introduced in the wafers using a micro indenter with varying loads. The defects were characterized from microscopy images to measure the indent size and combined crack length. The scattering of the A0 Lamb wave mode was measured experimentally and the characteristics of the scattered wave field were correlated to the defect size. The detection sensitivity is discussed.
Monocrystalline silicon wafers are widely used in the photovoltaic industry for solar panels with high conversion efficiency. The cutting process can introduce micro-cracks in the thin wafers and lead to varying thickness. High frequency guided ultrasonic waves are considered for the structural monitoring of the wafers. The anisotropy of the monocrystalline silicon leads to variations of the wave characteristics, depending on the propagation direction relative to the crystal orientation. Full three-dimensional Finite Element simulations of the guided wave propagation were conducted to visualize and quantify these effects for a line source. The phase velocity (slowness) and skew angle of the two fundamental Lamb wave modes (first anti-symmetric mode A0 and first symmetric mode S0) for varying propagation directions relative to the crystal orientation were measured experimentally. Selective mode excitation was achieved using a contact piezoelectric transducer with a custom-made wedge 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. Good agreement was found with the simulation results and theoretical predictions based on nominal material properties of the silicon wafer.
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