Thickness evaluation of mesoporous silicon layer using ultrasonic method
Résumé
The manufacturing processes of porous silicon (PoSi) now allow samples with variable depths and porosities to be obtained. Thickness can be critical to ensure reliability as for microelectronic applications. However, thickness measurement methods are generally destructive or strongly limited by thickness or pore. Therefore in this study a non-destructive ultrasonic method based on an immersion insertion-substitution technique is investigated. Yet wavelength in ultrasonic method is much larger than optical ones, allowing to observe porous silicon layers having larger pore diameter or higher thickness. Total thickness of the wafer (550 microns) and high sound speed in pure silicon (8450 m.s-1) require transducers with high centre frequencies. The acoustic parameters of these samples, such as velocity and attenuation, are measured using time domain analysis. These measurements are compared with those obtained through a one-dimensional multilayer model of the wafer, using a homogenization approach for the porous layer based on Biot’s Theory.