Infrared micro-spectrometers for low-cost, portable applications

Abstract

Infrared (IR) spectroscopy offers solutions to major challenges faced by a wide range of strategic applications including target identification in defence and security, biomedical instrumentation, and spectroscopic sensing for agriculture and processcontrol. By combining broadband IR detectors with the enhanced capabilities afforded by microelectromechanical systems (MEMS) technologies, new systems for the detection of IR radiation, spectral data collection, and spectral imaging will become available. While IR spectrometers exist for laboratory applications, the integration of wavelength tuning elements onto the IR detector chip will create new systems that are lower in cost, smaller in size, extremely robust, and ideally-suited to numerous portable applications. Micromachined tuning elements integrated with detectors and operating in the visibleto near-IR range have been demonstrated. 2 However, it is important to note that such technologies are either incompatible with compound semiconductor technology due to high-temperature processing, are unable to tune over the wavelength ranges required, and/or do not operate at the IR wavelengths required for spectroscopy. Here we describe the realisation of a Fabry-Perot device designed to select a narrow wavelength band in the range from 1.6 to 2.5μm within the short-wavelength infrared (SWIR) spectrum, with the ability to electrically-tune the selected wavelength using voltage levels that are low enough to be compatible with standard microelectronic devices. The fabrication process is compatible for monolithic integration of the MEMS tunable filter on any semiconductor-based photon detector (see Figure 1). The Fabry-Perot filter consists of a pair of verticallydistributed Bragg mirrors (which also act as the electrodes for filter tuning), a silicon nitride membrane, and an air-gap, which determines the narrow band of wavelengths of the infrared radiation that will pass to the detector. The filter tuning is achieved Figure 1. General concept of a Fabry-Perot filter formed on a detector.