Optical vibration analysis of MEMS devices with pm-resolution in x, y, and z directions

Abstract

Laser-Doppler vibrometry has become the state-of-the-art technique for broadband vibration analysis with picometer resolution in microelectromechanical systems (MEMS). Displacement or velocity is detected only in direction of the measurement beam and, thus, three impinging laser beams are necessary to investigate all components of a threedimensional (3D) motion. This requirement is not problematic for 3D-vibration measurements on macroscopic objects with scattering surfaces but for reflective microstructures. A general problem of measuring 3D vibrations with three laser beams is optical crosstalk. This problem is especially critical for MEMS applications because the three beams have to be positioned closely to achieve high lateral resolution. In this paper, we prove that it is possible to impinge the small laser focus of a single laser beam with 3.3 μm diameter on a proper edge, corner or etch hole of a MEMS device to obtain real-time, 3D-vibration measurements with picometer amplitude resolution without optical crosstalk. We present the first measurements of the 3D-vibrations in MEMS devices. We prove that our method can meet the requirement of the MEMS community for fast, full-3D, broad-bandwidth, vibration measurements with picometer amplitude resolution and micrometer spatial resolution.