K.K.M.B.D. Silva

Optical characterization of Fabry-Pe/spl acute/rot MEMS filters integrated on tunable short-wave IR detectors

Optical characterization of a microspectrometer based on a voltage tunable Fabry-Pe/spl acute/rot microelectromechanical optical filter monolithically integrated on a Hg/sub x/Cd/sub 1-x/Te infrared (IR) photoconductor is presented. Wavelength tuning from 1.7 to 2.35 /spl mu/m (650 nm) with less than 9 V is demonstrated. Bandwidths of less than 55/spl plusmn/5 nm and switching times of 60/spl plusmn/10 /spl mu/s have been achieved. Requirements to reduce the optical bandwidth and extend the tunable range are discussed. This technology has potential applications in hyperspectral imaging and spectroscopy across the entire IR band from 1 to 12 /spl mu/m.

A monolithically integrated HgCdTe SWIR photodetector and tunable MEMS-based optical filter

A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.

SWIR hyperspectral detection with integrated HgCdTe detector and tunable MEMS filter

Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array, integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the detector signal as well as to electrically tune the filter across the wavelength band. We report the first such demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode (EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200 K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP detectors exhibit FWHM of 140 nm.

Adaptive focal plane array (AFPA) technologies for integrated infrared microsystems

Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array, integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the detector signal as well as to electrically tune the filter across the wavelength band. We report the first such demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode (EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200 K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8×104 V/W. Initial results for the filters on HDVIP detectors exhibit FWHM of 140 nm.

MEMS-based microspectrometers for infrared sensing

Micro-electro-mechanical systems (MEMS)-based tunable optical filters, integrated with an infrared detector, select narrow wavelength bands in either the short-wavelength infrared (SWIR), or the mid-wavelength infrared (MWIR) region of the electromagnetic spectrum. The SWIR microspectrometer is based on monolithic integration of a parallel plate MEMS optical filter with a HgCdTe-based infrared detector. The fabrication process for the MEMS Fabry-Perot filter and the integral HgCdTe detector is completed while maintaining the processing temperature less than 125degC, as the performance of HgCdTe based detectors degrades at higher temperatrues. The preliminary MWIR microspectrometer result was based on a hybrid approach, fabricating the filter separately from the HgCdTe detector, however the process temperature control were maintained during fabrication of the MWIR filter, ensuring migration of this technology into an integrated solution. A tuning range of 900 nm with linewidths of 210 nm have been achieved for the MWIR, while maintaining a relatively low tuning voltage of 17 V.

Investigation of Thermal Expansion Effects on Si-Based MEMS Structures

This paper presents a study of the effects of stress and thermal expansion of inductively coupled plasma enhanced chemical vapor deposited (ICPCVD) amorphous Si thin films on low-temperature microelectromechanical systems test structures. Experimental data were used in conjunction with finite-element modeling (FEM) to predict deformation in simple microstructures across a wide temperature range from 85 to 300 K. Temperature dependence of residual stress and the coefficient of thermal expansion (CTE) of ICPCVD Si thin films was investigated by characterizing the curvature of bilayer thin-film samples through the use of optical profilometry at low temperature. Extracted parameters were used in an FEM package to confirm the experimental results by correlating with observed deformation of fabricated test structures. It is demonstrated that the experimentally determined CTE enables accurate modeling of the mechanical behavior of thin-film microstructures across a wide range of temperatures.

Optical read-out scheme based on grated waveguide cantilever cavity resonance for interrogation of cantilever sensor arrays

The design and analysis of an optical read-out scheme based on a grated waveguide (GWG) resonator for interrogating microcantilever sensor arrays is presented. The optical system consisting of a micro cantilever monolithically integrated in proximity to a grated waveguide (GWG), is realized in silicon optical bench platform. The mathematical analysis of the optical system is performed using a Fabry-Perot interferometer model with a lossy cavity formed between the cantilever and the GWG and an analytical expression is derived for the optical power transmission as a function of the cantilever deflection which corresponds to cavity width variation. The intensity transmission of the optical system for different cantilever deflections estimated using the analytical expression captures the essential features exhibited by a FDTD numerical model.

Effect of FIB milling on MEMS SOI cantilevers

Stress induced by Focused Ion Beam (FIB) milling of cantilevers fabricated on silicon-on-insulator (SOI) wafer has been studied. Milling induces stress gradients ranging from -10MPa/μm to -120MPa/μm, depending on the location of cantilevers from the point of milling. Simulations were done to estimate the stress in the milled cantilevers.

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.

Optical Performance of a MEMS Tunable IR Microspectrometer

The fabrication issues and optical performance of monolithically integrated microspectrometers are presented. The primary technical challenge in achieving the integration of a MEMS Fabry-Perot filter with the HgxCd1-xTe detector is to keep the processing temperature less than 125 degC. The results show successful device operation with tuning over 440 nm across the SWIR (1.6-2.5 mum) band, lineswidths of 55 nm, and switching times of 60 mus.