Two-axis-scanning laser Doppler vibrometer for microstructure

Abstract

The study and characterization of the dynamic behavior of micro-scale objects like e.g. AFM cantilever is essential to assess their performance. Laser Doppler vibrometers are suitable to achieve the measurement of dynamic parameters like displacement and vibration. Most of the time, the measurement can be done at one point of the object. As a point measurement is often not sufficient to fully understand the behavior of the microstructures, parameters must be determined at several points along the surface, and with more measurements, a more comprehensive study can be deduced from the analysis of these informations. Consequently, to evaluate the dynamic behavior of the entire microstructure, a highly accurate measurement system coupled to an automatic and fast scanning system is necessary. Presently, some systems do provide scanning and the scanning measurement is achieved either by using deflecting/rotating mirror or by using a two-axis stage that drives the tested sample under a stationary laser beam emitted out from a vibrometer. The positioning resolution and accuracy of mechanically driven scanning mirrors and moving stages limit the current techniques to the measurement of features with sizes in the millimeter range. In this paper, a two-axis-scanning Laser Doppler Vibrometer for micro-scale object dynamic behavior characterization is described. The system employs a two-axis acousto-optic deflectors and a telecentric lens to achieve high-precision and high-speed scanning. The newly developed vibrometer was used to measure the dynamic behavior of an entire AFM cantilever operated in free air. The 120 μm long, 25 μm wide micro-cantilever dynamic response was measured at different positions with 2 μm spatial resolution.