Micro defect detection on silicon carbide mirror with high frequency ultrasound array scanning

While silicon carbide (SiC) has been found to be a desirable mirror material due to its excellent thermal and mechanical properties, the optical performance of SiC mirrors might be limited by the internal defects introduced in fabrication. Therefore, it is necessary to evaluate the internal conditions of SiC mirror parts before integrating the mirror. This paper presents using a 64-element 30 MHz linear ultrasonic array to detect micro defects in SiC mirrors. To avoid scratching mirrors, instead of placing the array directly against the SiC mirror samples, water coupling was used in this ultrasonic nondestructive testing (NDT). A modified refraction beamforming scheme for the delay error correction was introduced to address the refraction and the delay errors in beamforming caused by the huge difference in sound speed between water and SiC. B-mode images of a pre-diced SiC sample were acquired and the defect detection ability of the system was evaluated. The imaging system paired with the high frequency ultrasonic array was shown to have the potential in evaluating the quality of SiC mirrors by utilizing the refraction beamforming methodology.