Silicon-based surface micromachined interferometers for infrared wavelengths

The Fabry-Perot interferometer (FPI) is a tunable optical band-pass filter. The realization with micro-electro mechanics (MEMS) technology enables high-volume production of miniature-size FPI at a low unit cost. Some ten-euro unit cost, few-millimeter square size, and millions of units fabrication volumes could characterize the industrial fabrication. The MEMS FPI is a key component of a miniature spectrometer. The gas-phase transmission infrared (IR) spectroscopy is applied in the detection and analysis of hydrocarbons, carbon dioxide and more complicated organic compounds. Application areas include medical, agriculture, automotive, gas security, and interior air quality, to name but a few. This work introduces a capacitively tunable surface-MEMS FPI for mid- and thermal IR. The unique feature is the Si/air/Si Bragg reflector (DBR) structure enabled by point-wise anchoring of tensile-stressed poly-Si films. The Si-air DBR enables narrower pass-band (better resolution) and more uniform tuning behavior over a wider tuning range. The founding technology of the MEMS FPI at VTT additionally provides a wide optical area and generally provides the uniform tuning performance. The improved performance is based on the work hypothesis of the Si-air DBR optical properties. Along with the proof of concept in this work, further benefits in the mechanical behavior of our structure were found. The work of the thesis covered the design of the concept, structure, and fabrication process for the novel MEMS FPI. Several wafers of multiple device designs were fabricated. Dozens of devices were characterized for the optical and electrical performance. In the early phase of the work, process-test wafer lots were fabricated for testing the sacrificial-layer composition, poly-Si implanting and the DBR structure, fabrication and performance. The publications, included in the thesis, concentrate on the dissemination of the designs of the best performance and of their structure and fabrication. The earlier test runs are not emphasized in the publications but the final conclusions are merely rephrased. A paper on the DBR demonstration, and another on the implanting optimization, report process-test results that supported the development of the fully functional device. The scope of the work covers the development from the idea until the proof of the concept. The scope is limited such that the devices reliability under environmental loads is discussed but not experimentally studied.