Optical low-coherence reflectometry for non-destructive testing of silicon micromachined devices

In recent years the development of silicon micromachining technologies has required more efforts in research to find non-contact measurement techniques for in-depth, non-destructive inspection of layered and microstructured samples. In this work, we apply a optical low-coherence reflectometry for in-plane and out-of-plane measurements aimed at detecting the optical path between hidden interfaces of several silicon devices with characteristic distance in the range 3 - 17 μm. The implemented configuration is based on a fiberoptic Michelson interferometer and it used infrared broadband radiation in the wavelength range of 1.2 - 1.7 μm, exhibiting a coherence length shorter than 2 μm. Out-of-plane measurements were performed to detect the optical pathlength of the main structural layers of a MEMS gyroscope. Moreover, in-plane measurements on vertical periodic silicon/air microstructures allowed us to detect the optical path among several silicon/air interfaces. Arrays with different spatial period were tested and the optical distance between hidden interfaces was obtained with high in-depth resolution. The results were in good agreement with the design parameters of the specific device. The proposed spot optical technique is a powerful and highly versatile diagnostic tool for non-destructive testing of silicon devices.