John Bumgarner

MEMS-Based Tunable Fabry–Perot Filters for Adaptive Multispectral Thermal Imaging

This paper reports on a proof-of-concept microelectromechanical system-based Fabry-Perot filter that is capable of electrically tuning within the long-wave infrared thermal imaging band of 8-12 μm. The device employs a single-layer quarter-wavelength thick tensile germanium membrane for the suspended top mirror in order to achieve nanometer-scale as-released mirror flatness across an area of several hundred square micrometers without any extraneous stress management techniques. Mechanical and optical characterization of the tunable filters of various sizes are presented and compared. A 200-μm dimension square filter is demonstrated with <;100-nm top mirror bowing and near-theoretical spectral characteristics across the entire tuning range of 8.5-11.5 μm, namely, peak transmission above 80%, full-width at half-maximum of spectral passband of approximately 500 nm, and out-of-band rejection greater than 40:1. Optical modeling shows that this filter can achieve a pixel-to-pixel transmission peak wavelength variation of less than 1.2% across the entire 200 μm × 200-μm optical imaging area. These results exceed the optical performance requirements for passive multispectral thermal imaging applications based on large-area focal plane arrays. In comparison, the 500 and 1000-μm dimension filters are shown to exhibit significant mirror bowing with actuation and, thus, for a pixel-to-pixel transmission peak wavelength non-uniformity of <; 4%, demonstrate narrower usable spectral tuning ranges of 9.3-11.4 and 10.3-11.3 μm, respectively.

Large-Area MEMS-Based Distributed Bragg Reflectors for Short-Wave and Mid-Wave Infrared Hyperspectral Imaging Applications

We present the design, fabrication, and optical characterization of silicon-air-silicon-based distributed Bragg reflectors, or quarter wavelength mirrors, in sizes ranging from 200 μm × 200 μm to 5 mm × 5 mm. Such mirrors can be used in conjunction with either single-element photodetectors or large-area focal plane arrays to realize tunable multispectral sensors or adaptive focal plane arrays from the short-wave infrared wavelength ranges (1500-3000 nm) to mid-wave infrared wavelength (3000-6000 nm) ranges. Surface optical profile measurements indicate a flatness of the order of 20-30 nm in the fabricated structures across several millimetres. Single point spectral measurements on devices show excellent agreement with simulated optical models. The fabricated distributed Bragg reflectors show ~94% reflectivity, which is in close agreement with theoretical reflectivity. The demonstrated high reflectivity across a wide wavelength range renders them suitable as broadband reflectors. Finally, we present optical transmittance modeling results for Fabry-Pérot filters based on these distributed Bragg reflectors.

Ge/ZnS-Based Micromachined Fabry–Perot Filters for Optical MEMS in the Longwave Infrared

This paper reports on the successful demonstration of Ge/ZnS-based Fabry-Perot filters operating in the longwave infrared (LWIR). The suitability of thermally deposited Ge and ZnS as thin-film mirror materials for micromachined LWIR Fabry-Perot filters has been fully investigated, and it is shown that a film growth temperature higher than 150 °C is key to depositing durable ZnS films. The optical constants of Ge and ZnS films in the LWIR band reveal that the material pair possesses high refractive index contrast and excellent LWIR transparency. Fixed-cavity LWIR Fabry-Perot filters with a 150-μm circular single-layer Ge top mirror and a four-layer Ge/ZnS/Ge/ZnS bottom mirror were fabricated. Curvature in the suspended top mirror was corrected using a thin SiNx stress-compensation layer. After curvature correction, a mirror flatness of 550 nm was achieved, and the filter demonstrated a 60% peak transmission with a full-width at half-maximum of 700 nm as well as a out-of-band rejection of 24:1.

Silicon-Air-Silicon Distributed Bragg Reflectors for Visible and Near Infrared Optical MEMS

This letter presents the design, fabrication, and optical characterization of silicon-air-silicon-based surface micro-machined distributed Bragg reflectors (DBRs) for the visible to near infrared wavelength range (540-960 nm). The DBR (mirror) consisted of two quarter wave thick silicon films separated by a quarter-wave air gap. A mirror array was successfully fabricated, consisting of mirrors ranging in diameter between 270 and 420 μm. Calibrated optical measurements indicate that a peak reflectivity close to 92% has been achieved for visible wavelengths, despite the fact that silicon has strong absorbtion in the visible wavelength range. The mirrors are shown to be broadband reflectors, having 85% or more reflectivity over a 160-nm wavelength range. A spatially resolved optical transmission mapping and optical transmission profile of the mirrors demonstrates high uniformity across the fabricated array of DBRs.

Nanoindentation of Si 1−x Ge x thin films prepared by biased target ion beam deposition

The mechanical properties of Si_1-xGe_x thin films are studied via nanoindentation. The Si_1-xGe_x thin films are prepared with a biased target ion beam deposition (BTIBD) method. We investigate the effect of varying the Si to Ge composition ratio on the elastic modulus and hardness of the resulting alloyed films. In comparison to pure BTIBD Si (E_si = 154GPa, H_si = 9.9GPa), for Si_1-xGe_x a decreasing trend in Youngs modulus and hardness is observed to be associated with the increase in Ge content, and for Si_0.5Ge_0.5 the values of 139 GPa and 8.4GPa are found to represent the Youngs modulus and hardness, respectively.

Cryogenic optical profilometry for the calculation of coefficient of thermal expansion in thin films

This work focuses on the development of a cryogenic optical profilometry system for the measurement of material properties of thin films across a wide temperature range. A cryostat was machined and integrated with a Zygo NewView 600K optical profilometer and vacuum system. Curvature data were taken for a SiNx thin film on a GaAs substrate from 300 K down to 80 K. From the curvature data, the coefficient of thermal expansion was calculated. The cryogenic optical profilometry system was benchmarked with a three beam curvature technique, and demonstrated excellent agreement across the full temperature range from 300 K to 80 K

Towards MEMS-based long-wavelength infrared tuneable Fabry-Perot filters

This paper describes the realization of LWIR (8-12 μm) Fabry-Perot filters on basis of our previously demonstrated SWIR (1.6-2.5 μm) or MWIR (3-5 μm) filter technique. An innovative filter design employing a single layer of Ge as the top mirror is proposed and optical modelling shows that the filter can potentially achieve spectral characteristics required by the LWIR spectral imaging applications. As a proof of concept, filters with fixed air cavity were fabricated and bowing in the suspended mirror was corrected by depositing a thin SiNx stress compensation layer underneath the mirror. Transmission measurement was carried out on the filter exhibiting the least bowing, despite some spectral degradation compared to the theoretical model of the filer, showing not optimal but adequate transmission and bandwidth for multispectral imaging applications.