NEW CO2 FILTERS FABRICATED BY ANODIC BONDING AT OVERPRESSURE IN CO2 ATMOSPHERE

New bulk micromachined CO2 infrared filters are presented. The filters consist of a chamber in which CO2 is encapsulated during an anodic bo nding procedure. The chamber is formed by three fusion bonded silicon wafers joined to a glass wafer by anodic bonding at overpressure in a CO2 atmosphere. The two central silicon wafers are etched in KOH to fo rm a cavity. The high temperature anodic bonding process (430 degrees C) is performed at overpressures up to 2 bar, enabling an optimized ga s absorption at a final CO2 chamber pressure up to 1 bar at room tempe rature. CO2 filters of different bonding pressures were fabricated and evaluated. An experimental investigation was conducted to improve the optical performances of the device at the wavelength of interest, i.e ., 4.23 mu m, where the absorption peak of CO2 is located. We studied both the influence of the glass thickness and antireflective coatings on optical losses. Theoretical calculations for transmission with anti reflective coating materials are presented. From these calculations, t he thickness of two antireflective coating layers (silicon nitride and silicon dioxide) was optimized to obtain a maximum transmission at 4. 23 mu m. Finally, an optimized filter with thinner glass and silicon d ioxide as antireflective coating is presented. (C) 1998 Elsevier Scien ce S.A. All rights reserved.