Ultrasonic Test Structures for Non-Destructive Measurement of Trapezoidal Angle in Bosch Processes

Abstract

This paper reports a new, simple and non-destructive approach based on ultrasonic interrogation to measure the trapezoidal angle of anisotropically etched silicon micropillars (e.g. by the Bosch Process). The approach is based on a test structure—a lattice of vertical micropillars—designed to resonate close to 5 MHz. Experiments and finite element simulations are utilized to explain the sensitivity of the resonance frequency of our test structure. With a design radius of 30 μm (45 μm), a max. (min.) sensitivity of 0.73 MHz/deg (0.47 MHz/deg) is demonstrated. We propose an analytical formula to relate the resonance frequency to the trapezoidal angle and radius of the micropillars.