Dhirendra Kumar Tripathi

A versatile instrumentation system for MEMS-based device optical characterization

Future improvements in spectral imaging systems can be attained through the integration of MEMS-based optical transmission devices matched with pixelated arrays. Such integrated module designs will require a detailed knowledge of the MEMS device optical properties at high spatial resolution and over a wide range of operating conditions. A substantially automated low-cost optical characterization system has been developed, which enables the optical transmission of the MEMS device be measured with high spatial and spectral precision. This Optical Metrology System (OMS) can focus light on the device under test (DUT) to a spot diameter of less than 30 μm, and characterize devices at near infrared for wavelengths within the spectral band from 1.4 μm to 2.6 μm. A future upgrade to the OMS will enable measurements to be carried out across a wide range of DUT temperatures and with a spectral range from visible to long wave infrared wavelengths.

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.

Suspended Large-Area MEMS-Based Optical Filters for Multispectral Shortwave Infrared Imaging Applications

We present the design, fabrication, and optical and mechanical characterization of silicon-/silicon-oxide-based optical filters and distributed Bragg reflectors in sizes ranging from 500 μm × 500 μm to 5 mm × 5 mm. They are designed to 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 in the shortwave infrared wavelength range. Surface optical profile measurements indicate a flatness of the order of 30 nm in the fabricated structures across several millimeters. Single-point spectral measurements on devices show an excellent agreement with simulated optical models, and demonstrate Si-SiOx-Si fixed optical filters with a 94% transmission at 1940 nm with a full-width at half-maximum of 250 nm. Distributed Bragg reflectors demonstrate 90% reflectance across the 1560-2050-nm wavelength range, making them suitable as broadband reflectors. The optical spatial uniformity across a 3-mm × 3-mm device shows only a 3% variation across the entire optically active area. Finally, the mechanical resonance characteristic of a 1-mm x 1-mm fabricated device shows the lowest resonant frequency of the suspended structure to be 39 kHz, indicating excellent immunity to extraneous low-frequency vibrations.

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.

Optimization of ICPCVD Amorphous Silicon for Optical MEMS Applications

In this paper, we present the optimization of optical and mechanical properties of inductively coupled plasma chemical vapor deposited (ICPCVD) amorphous silicon thin films for fabrication of high-quality optical microelectromechanical systems-based devices operating from visible to short-wave infrared wavelengths (450-3000 nm). Our results indicate that, at relatively high deposition temperatures for plasma CVD, a decrease in the ICP power results in films with lower tensile stress, higher refractive index, and lower extinction coefficient. We show that hydrogen concentration alone is not a sufficient parameter for controlling optical and mechanical quality of the films. In particular, both the hydrogen concentration and the hydrogen-silicon bonding nature together play a vital role in determining the optical and the mechanical quality of the silicon thin films. As a demonstration vehicle, three layer silicon-silicon oxide-silicon-based distributed Bragg reflectors were fabricated for the visible (500-700 nm), near infrared (700-1000 nm), and short-wave infrared (2000-3000 nm) wavelength ranges using an optimized silicon fabrication recipe. The measured optical transmission spectra show close to 90% peak reflectivity. Finally, stress optimization was evaluated by fabricating 270-μm diameter circular suspended silicon membranes, which demonstrate a flatness variation on the order of <;6 nm across the entire lateral dimension.

Recent developments towards low-cost MEMS spectrometers

While optical spectroscopy has shown great promise in a multitude of applications, the cost, size, and fragility of spectrometer instruments have hindered widespread application of the technology. :tvfEMS microspectrometers offer great hope for low-cost, lightweight, and robust spectrometers, paving the way for pervasive use in many fields. In this invited paper, we report on nearly 15 years of development on MEMS spectrometers in our research group, beginning with devices designed for the shortwave infrared (SWIR) and midwave infrared (MWIR), and moving on to our most recent work towards MEMS spectrometers in the visible and near infrared (NIR) as well as the thermal long-wave infrared (LWIR) bands.

Compositional and mechanical properties of PECVD silicon for thin-film optical applications

In this paper we present Inductively coupled plasma chemical vapour deposition(ICPECVD) of amorphous Silicon (a-Si:H) thin films. By tuning the ICP power, RF power and pressure tensile and conformal a-Si:H films can be obtained. Such films are of great importance for the MEMS application. We also show that the optical absorption due to Si:H and Si-H2 bond in the a-Si:H can be reduced by annealing the film at low temperature (500°C)by removing the hydrogen. The rapid and low temperature annealing ensures smooth a-Si:H films for the NIR applications in optical MEMS.