Silicon Micromachined Device Testing by Infrared Low-Coherence Reflectometry

With the development of silicon micromachining technologies, non-contact measurement techniques for in-depth non-destructive inspection of layered and microstructured samples are becoming increasingly relevant. In this work, we apply optical low-coherence reflectometry (OLCR) to detect the optical path between the interfaces of several silicon devices with characteristic distance in the range 3 - 17 μm. The implemented configuration is based on a fiberoptic Michelson interferometer that exploits infrared broadband radiation in the wavelength range of 1.2 - 1.7 μm, with coherence length shorter than 2 μm, for performing spot tomographic measurements. OLCR enabled out-of-plane measurements on a MEMS linear accelerometer and in-plane measurements on vertical periodic silicon/air microstructures: the optical distance between hidden interfaces was found well in agreement with the design parameters.