Infrared micro-spectrometers for low-cost, portable applications
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 process-control. 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 visible- to near-IR range have been demonstrated.1,2 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 2.5μ m 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 (Figure 1).
The Fabry-Perot filter consists of a pair of vertically-distributed 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 by applying a bias voltage across the two distributed Bragg mirrors, which decreases the air-gap spacing and changes the wavelengths that are sensed by the detector. It is important to note that all process steps developed for the tunable MEMS filter need to be compatible with device processing involving temperature-sensitive compound semiconductors.
Following fabrication of HgCdTe photoconductor test structures and their associated optical characterisation, the lower Bragg mirror (Figure 1), consisting of a three-layer Ge-SiO-Ge stack (with layer thicknesses of 120nm-280nm-120nm) was deposited using a thermal evaporation process, with the first-deposited Ge layer being the doped conducting layer. Following lower mirror patterning, a polyimide spacer layer was deposited on the sample, which was then patterned to define the structural supports of the eventual MEMS filter structure. The samples were then coated with low-stress (< 20MPa intrinsic stress) silicon nitride3 deposited using a plasma-enhanced chemical-vapor-deposition (PECVD) system with the processing temperature limited to HgCdTe compatible levels (< 125°C). Stress control is critical, since too great a stress in the silicon-nitride membrane will result in structure distortion and possible collapse upon release. It is the ability to control the silicon-nitride stress while maintaining a low process temperature that has enabled the successful monolithic integration of the detectors/MEMS structure.
The upper mirror of the Fabry-Perot cavity was deposited in a similar manner to the lower mirror, and released along with the silicon-nitride membrane in order to form a free-standing flexible MEMS structure. Thus, the resultant stress of the combined membrane and upper mirror layers had to be controlled in order to prevent the collapse or rupture of the structure. The underlying silicon nitride membrane was then defined using a dry-etching process, and metal for the top electrode patterned onto the top mirror. The top membrane/mirror structure was then released by removing the spacer layer.
The room-temperature spectral-response data of the monolithic filter on a HgCdTe photoconductive detector, with released spacer layer to form an air cavity, is shown in Figure 2. The corresponding mirror displacement, at bias voltages in the range from 0–7.5V, is 1.2–0.7μ m and the peak spectral response shifts from 2.2–1.85μ m. The peak transmission of the filter is found to be > 60% over the entire range of mirror displacement. At the center wavelength of ∼ 1950nm, the full width at half maximum (FWHM) is ∼ 100nm, although much narrower values of approximately 35nm were achieved prior to release. Beyond 7.5V, a non-destructive filter snapdown was found to occur.
Here we presented preliminary results of a technology for the monolithic integration of SWIR detectors with a tunable MEMS optical filter. This is used to select a narrow band of wavelengths in the 1.6–2.5μ m range of the SWIR region of the IR spectrum using very-low applied voltages. The MEMS fabrication technology is a low-temperature (< 125°C, low-stress process that is fully compatible with any semiconductor technology and with the mechanical requirements of suspended MEMS devices. This technology has also been demonstrated to be applicable in the mid-wave infrared region from 3 to 5μ m. Further work is required in decreasing the FWHM in order to improve the spectral selectivity of the filter.
