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High Density Vertically Integrated Photodiodes (HDVIP) MWIR detectors were fabricated in LPE-grown Mercury Cadmium Telluride material. Devices were fabricated with two different acceptor level concentrations. The low doped n-region was held at a single concentration but the dimensions are tailored to simultaneously maintain high quantum efficiency while minimizing dark current and 1/f noise. Since this study target was for operating at high temperatures, detector I-V data was collected between 120 K and 280 K for I-Vs and 180 to 280 K for noise to understand current mechanisms that limit device performance at these elevated temperatures. Noise as a function of frequency has also been collected over the same temperature range. 1/f noise has also been modeled for MWIR detectors as a function of temperature and will be covered.

An overview of recent improvements in our understanding of, and the maturity of, linear-mode photon counting with the HgCdTe electron-initiated avalanche photodiode is presented. In 2010 DRS fabricated an experimental 2 × 8 array with (64 μm)2 pixels which enabled, for the first time, linear-mode photon counting by use of the MWIR cutoff HgCdTe electron-initiated avalanche photodiode. The device had a high single-photon signal-to-noise ratio of 13.7, an excess noise factor of 1.3–1.4, a 7 ns minimum time between events, and a broad spectral response extending from 0.4 μm to 4.2 μm. DRS recently fabricated a new set of devices with improved yield and performance compared with the first device: the false event rate was reduced by a factor of almost 10 to 150 kHz, the photon detection efficiency was increased from 50% to >60%, and the APD gain was increased by a factor of 4 to over 1900.

Spectral response of detectors can be modeled using a stack matrix approach. The different material layers that make up the detector form the optical stack. Light intensity is proportional to the square of the amplitude of the electromagnetic field’s electric field component. Using the electric component of light, the model takes into account the reflective and transmission coefficients at the interface between two adjacent layers and the phase difference during propagation within each layer. The transition from one optical layer to another and the propagation within an optical layer can be formulated as 2 × 2 matrices; their multiplication through every optical layer makes up the stack matrix that defines the optical properties of the detector. Knowing the complex index of refraction for each layer in the detector, the intensity of light can be calculated at any point in the detector structure, which can then be used to determine the response of the detector as a function of wavelength. This model shows fairly good agreement with the experimental data, even revealing fine structure and etalon effects. Using this model, the detector can be designed to meet specific spectral characteristics. The spectral response model was used on high-density vertically interconnected photodiode front-side-illuminated photodiodes.

The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are related to the to the collection region fill factor. A 2D diffusion model calculates the time dependent response and steady state pixel point spread function for cylindrical diodes, and predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system was developed for point spread function and crosstalk measurements at 80 K. An electron diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current data, at high gain levels, indicate an effective gain normalized dark density count as low as 1000 counts per μs per cm2 at an APD gain of 444. A junction doping profile was determined from capacitance-voltage data. Spectral response data shows a gain independent characteristic.

Modern electronic applications demand more and more complex, multifunctional microsystems with performance characteristics which can only be achieved by using best-of-breed materials and device technologies. Three-dimensional (3-D) integration of separate, individually complete device layers provides a way to build complex heterogeneous microsystems without compromising the system performance and fabrication yield. In the 3-D integration approach, each device layer is fabricated separately using optimized materials and processes. The layers are bonded and interconnected through area array vertical interconnects with lengths on the order of microns. This paper will review bonding techniques for high density area array 3-D integration of integrated circuits, focusing on techniques suitable for die-to-die and die-to-wafer bonding configurations.

Microspectrometers based on the monolithic integration of a microelectromechanical system (MEMS) Fabry–Perot filter and a HgxCd1–xTe-based infrared detector are discussed and measured results presented. The microspectrometers are designed to operate in the 1.5 μm to 2.6 μm wavelength range. Design equations are presented which account for the mechanical and optical characteristics of the device. Measurements indicate linewidths as narrow as 55 nm, switching times of 40 μs, and a tuning range of 380 nm, which is limited by snap-down. Optical characterization of the distributed Bragg mirrors and the Fabry–Perot filter are presented, and these are shown to be in good agreement with simple first-order analytical models. Bowing of the movable Fabry–Perot mirror due to stress gradients is identified as the dominant source of linewidth broadening.

Time series and Fourier-transform data on high-density vertically integrated photodiodes (HDVIP) at 0 and 50 mV reverse bias in the dark have been studied. The detectors have a cutoff wavelength λc (60 K) of 10.5 μm. Examination of the detector current time series and Fourier-transform curves of these devices reveals a variety of interesting characteristics: (i) time series displaying switching between four states characteristic of random telegraph signal (RTS) noise, the noise current power spectrum having Lorentzian or double Lorentzian type characteristics, (ii) time series data exhibiting wave-like characteristics with the noise current power spectrum being 1/f2-like at low frequencies, (iii) time series having a mean value independent of time with the noise current power spectrum being white, and (iv) time series nearly independent of time with the noise current power spectrum having 1/f characteristics. Although from a single array, the excess noise characteristics at low (mHz) frequencies were varied, most of the detectors measured fell into one of these four categories. The predominance of detectors examined had minimal excess low-frequency noise down to ~ 10 mHz. Detectors that displayed RTS noise in reverse bias were repeatable under subsequent measurement. However, when measured at zero bias, the same detectors exhibited no RTS noise, the noise current power spectrum being white in nature.

Results of characterization data on linear mode photon counting (LMPC) HgCdTe electron-initiated avalanche photodiode (e-APD)focal plane arrays (FPA) are presented that reveal an improved understanding and the growing maturity of the technology. The first successful 2x8 LMPC FPA was fabricated in 2010 [1]. Since then a process validation lot of 2x8 arrays was fabricated. Five arrays from this lot were characterized that replicated the previous 2x8 LMPC array performance. In addition, it was unambiguously verified that readout integrated circuit (ROIC) glow was responsible for most of the false event rate (FER) of the 2010 array. The application of a single layer metal blocking layer between the ROIC and the detector array and optimization of the ROIC biases reduced the FER by an order of magnitude. Photon detection efficiencies (PDEs) of greater than 50% were routinely demonstrated across 5 arrays, with one array reaching a PDE of 70%. High resolution pixel-surface spot scans were performed and the junction diameters of the diodes were measured. The junction diameter was decreased from 31 μm to 25 μm resulting in a 2x increase in E-APD gain from 470 on the 2010 array to 1100 on one of the 2013 FPAs. Mean single photon signal to noise ratios of >12 were demonstrated at excess noise factors of 1.2-1.3. NASA Goddard Space Flight Center (GSFC) performed measurements on the delivered FPA that verified the PDE and FER data.

Staring infrared focal plane arrays (FPAs) require pixel-level, three-dimensional (3D) integration with silicon readout integrated circuits (ROICs) that provide detector bias, integrate detector current, and may further process the signals. There is an increased interest in ROIC technology as a result of two trends in the evolution of infrared FPAs. The first trend involves decreasing the FPA pixel size, which leads to the increased information content within the same FPA die size. The second trend involves the desire to enhance signal processing capability at the FPA level, which opens the door to the detector behaving like a smart peripheral rather than a passive component—with complex signal processing functions being executed on, rather than off, the FPA chip. In this paper, we review recent advances in 3D integration process technologies that support these key trends in the development of infrared FPAs. Specifically, we discuss approaches in which the infrared sensor is integrated with 3D ROIC stacks composed of multiple layers of silicon circuitry interconnected using metal-filled through-silicon vias. We describe the continued development of the 3D integration technology and summarize key demonstrations that show its viability for pixels as small as 5 microns.

A monolithically integrated low-temperature micro-electro-mechanical systems (MEMS) and HgCdTe infrared (IR) detector technology is introduced, implemented, and characterized. The ultimate aim of this project is to develop a MEMS-based optical filter, integrated with an IR detector, that selects narrow wavelength bands within the short-wavelength IR (SWIR) region of the spectrum. The entire fabrication process is compatible with two-dimensional IR focal plane array technology, and needs to be compatible with a proposed electrically tunable MEMS filter based on a Fabry-Perot optical cavity. The fabricated device consists of an HgCdTe SWIR photoconductor, distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, and a silicon nitride membrane for structural 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 1,780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1,950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8×104 V/W. The experimental results are in good agreement with the optical model, which takes into account mirror reflectivity, absorption within the cavity by the spacer material, and absorption by the silicon nitride support structure.

We report successful implementation of high-density 3D integration technology in the fabrication of advanced mixed-signal processors for infrared focal plane arrays. Separate analog and digital integrated circuits (ICs) were custom designed and fabricated in standard bulk CMOS technology in two different commercial foundries. The 3D interconnects were arrayed in a 256×256 format with a 30 micron pitch and had the form of through-silicon vias 4 micron in diameter and 30 micron deep. Completed 3D readout IC stacks were tested pixel by pixel, demonstrating operability of 99.9%. The 3D IC stacks were hybridized with infrared photodiode arrays and produced functional FPA imagers with 99.9% array operability and unprecedented performance. Such imaging arrays and other smart sensors enabled by 3D integration of sensing elements with sophisticated signal processing are of increasing interest for Internet-of-Things applications, from net-enabled surveillance to mobile health.

Infrared detector pixel pitch has been decreasing, driven by interest in higher resolution, larger displays, and decreased cost. Previous generations of focal plane arrays (FPAs) were on 50, 40, 30, and 20μm pitch. 12μm pitch FPAs are now available. DRS Network and Imaging Systems has developed ultra-small 5μm pitch infrared detectors for the long-wave infrared (LWIR) and medium-wave infrared (MWIR) bands as part of the DARPA AWARE Lambda Scale effort. The smaller pitch was achieved using DRS’ high-density vertically integrated photodiode (HDVIP®) architecture. This technology is a major advance in the state of the art for infrared imaging sensors. The pixel density of 4 million pixels/cm2 enables the production of lower cost FPAs from HDTV resolution up to many millions of pixels. Dark current, collection efficiency, cross-talk, and operability are similar to larger pitch HDVIP FPAs.

Over the past decade, the development of infrared focal plane arrays (FPAs) has seen two trends: decreasing of the pixel size and increasing of signal-processing capability at the device level. Enabling more capability within smaller pixels can be achieved through the use of advanced wafer-level processes for the integration of FPAs with silicon (Si) readout integrated circuits (ROICs). In this paper, we review the development of these wafer-level integration technologies, highlighting approaches in which the infrared sensor is integrated with three-dimensional ROIC stacks composed of multiple layers of Si circuitry interconnected using metal-filled through-silicon vias.

A 16 element HgCdTe e-APD detector has been developed for lidar receivers that has significant improvements in sensitivity in the spectral range from < 1μm to 4 μm. A demonstration detector consisting of a 4x4 APD detector array, with 80 μm square elements, a custom CMOS readout integrated circuit (ROIC), a closed cycle cooler-Dewar, and support electronics has been designed, fabricated, and tested. The custom ROIC design provides > 6 MHz bandwidth with low noise and 21 selectable gains. Ninety-six arrays were fabricated with 69% of the arrays meeting the dark current spec in the center 4 pixels at 10 V bias where the APD gain was expected to be around 150. Measurements to 12 V on one array showed APD gains of 654 with a gain normalized dark currents of 1.2 fA to 3.2 fA. The lowest dark current array showed a maximum dark current of 6.2 pA at 10 V and 77 K. The 4.4 μm cutoff detector was characterized at an operating temperature of 77K with a 1.55 μm, 1μs wide, laser pulse. The photon conversion efficiency at unity gain was 91%. The mean measured APD gain at 77 K was 308 at 11V, the responsivity was 782 μV/pW, the average NEP was 1.04 fW/Hz1/2. The bandwidth was 6.8 MHz, and the broadband NEP was 2.97 pW. This detector offers a wide spectral response, dynamic range, and substantially improved sensitivity and lifetime for integrated path differential absorption (IPDA) lidar measurements of atmospheric trace gases such as CO2 and CH4.

The photocurrent of High Density Vertically Integrated Photodiodes (HDVIP) manufactured in LPE grown SWIR (λc ~ 2.5 μm) HgCdTe material is modeled as a function of incident spot location using a Monte Carlo diffusion calculation in the p-type bulk. The Monte Carlo calculation assumes a 3 x 3 mini-array of detectors surrounded by guard detectors. Carriers generated in the n-regions are always collected. The result is a responsivity map that yields the individual detector "spot scan" profile that is then used to calculate the detector modulation transfer function (MTF). Fourier transforms of detector "spot scan" response profile provided experimental confirmation of MTF that corresponded to the Monte Carlo modeled MTF.

Detectors that have broadband response from the visible (∼ 400 nm) to near infrared (∼ 2.5 μm) have remote sensing hyper spectral applications on a single chip. Cut-off wavelengths in the 2.2 and 2.5 μm range permit detector operation in the 200 K regime. The DRS HDVIP detector technology is a front side illuminated detector technology. Consequently, there is no substrate to absorb the visible photons as in backside-illuminated detectors and these 2.2 and 2.5-μm-cutoff detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated using CdTe that absorbs the visible light photons. CdTe with a direct bandgap ∼ 1.6 eV strongly absorbs photons of wavelength shorter than about 800 nm. Detectors in 320 x 6 arrays with varying thickness of CdTe passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. The SWIR HDVIP detectors have well known high quantum efficiency (QE) in the near infrared region. The focus here was in acquiring array level data in the visible region of the spectrum. The 320 x 6 FPA QE and NEI data were acquired using a 642 nm narrow band filter with 50 % points at 612 nm and 698 nm. The array QE average is ∼ 70 % for the array with CdTe passivation thickness = 44.5 nm. The NEI is ∼ 5 x 10 10 ph/cm 2 /s at a flux Φ = 5.36 x 10 13 ph/cm 2 /s. QE for an array with CdTe passivation thickness = 44.5 nm is ∼ 10 % higher than an array with CdTe passivation thickness = 79.3 nm. In addition, a model that takes into account the complex optical properties of every layer in the HDVIP photodiode architecture was developed to predict the QE of the detectors in the near infrared and visible wavelength regions as a function of CdTe thickness. Measured QE as a function of wavelength is not a good match to the model QE probably due to limitations in the measured QE and knowledge of optical constants that are input into the model.

DRS LPE-grown SWIR, MWIR and LWIR HgCdTe material are fabricated in the High-Density Vertically Integrated Photodiode (HDVIP) architecture. Instruments manufactured for certain strategic applications have severe constraints on excess low frequency noise due to the effect the noise has on the image quality with subsequent consequences on the period of calibration. This paper will present data and analysis of excess low frequency noise in LWIR (lc ~ 10.5 mm @ 60 K) HDVIP HgCdTe detectors. The vehicle for noise measurements is a multiplexed 320 x 6 array of 40 mm x 50 mm, 10.5 mm cutoff, HgCdTe detectors. Noise has been measured on a column of 320 detectors, at 60 K, as a function of frequency at zero and 50 mV reverse bias. Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled every 10th or every 100th frame. 32,768 frames of time series data were collected for a total record length of 98 minutes. Since the total time for collecting the 32,768 time data series points is 98 minutes, the minimum frequency is 170 mHz. Time series and Fourier transform data on individual detectors at 0 mV and 50 mV reverse bias in the dark have been studied. Examination of the detector current time series and Fourier transform curves thereof, reveal a variety of interesting characteristics: (i) time series displaying switching between four states characteristic of random telegraph signal (RTS) noise, the noise current power spectrum having Lorentzian type characteristics; (ii) time series data exhibiting slight wave-like characteristics with the noise current power spectrum being 1/f-like at low frequencies; (iii) pronounced wave-like characteristics in the time series with the noise current power spectrum being 1/f2-like at low frequencies; and (iv) time series having a mean value independent of time with the noise current power spectrum being white. The predominance of detectors examined had minimal excess low frequency noise down to ~ 10 mHz. In addition some isolated diodes had characteristics that lay between the four main types outlined above.

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.

DRS uses LPE-grown SWIR, MWIR and LWIR HgCdTe material to fabricate High-Density Vertically Integrated Photodiode (HDVIP) architecture detectors. 2.5 μm, 5.3 μm and 10.5 μm cutoff detectors have been fabricated into linear arrays as technology demonstrations targeting remote sensing programs. This paper presents 320 x 6 array configuration technology demonstrations' performance of HDVIP HgCdTe detectors and single detector noise data. The single detector data are acquired from within the 320 x 6 array. Within the arrays, the detector size is 40 μm x 50 μm. The MWIR detector array has a mean quantum efficiency of 89.2% with a standard deviation to mean ratio, σ/μ = 1.51%. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz. Operability values exceeding 99.5% have been obtained. The LWIR arrays measured at 60 K had high operability with only ~ 3% of the detectors having out of family response. Using the best detector select (BDS) feature in the read out integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with 100% operability is obtained. For the 320 x 1 array constituted using the BDS feature, a 100% operable LWIR array with average NEI value of 1.94 x 1011 ph/cm 2/s at a flux of 7.0 x 1014 ph/cm2/s has been demonstrated. Noise was measured at 60 K and 50 mV reverse bias on a column of 320 diodes from a 320 x 6 LWIR array. Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled every 100th frame. 32,768 frames of time series data were collected for a total record length of 98 minutes. The frame average for a number of detectors was subtracted from each detector to correct for temperature drift and any common-mode noise. The corrected time series data was Fourier transformed to obtain the noise spectral density as a function of frequency. Since the total time for collecting the 32,768 time data series points is 98.0 minutes, the minimum frequency is 170 μHz. A least squares fit of the form (A/f + B) is made to the noise spectral density data to extract coefficients A and B that relate to the 1/f and white noise of the detector respectively. In addition noise measurements were also acquired on columns of SWIR detectors. Measurements were made under illuminated conditions at 4 mV and 50 mV reverse bias and under dark conditions at 50 mV reverse bias. The total collection time for the SWIR detectors was 47.7 minutes. The detectors are white noise limited down to ~10 mHz under dark conditions and down to ~ 100 mHz under illuminated conditions.

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.

This paper describes a technology for three-dimensional (3-D) integration of multiple layers of silicon integrated circuits. The technology promises to dramatically enhance on-chip signal processing capabilities of a variety of detector devices hybridized with Si electronics. The focus of the paper is on high performance infrared focal plane arrays based on HgCdTe, which offer the ultimate in infrared sensitivity and find application in high performance military systems. Performance data from test FPA devices with integrated multilayer Si stacks are discussed in this paper.

Remote sensing programs require detectors with a variety of wavelengths. One example of remote sensing applications is the GOES-ABI program that requires linear arrays of detectors with cutoff wavelengths ranging from the visible to the VLWIR (λc ~ 15 μm). In order to target the variety of remote sensing applications, an internal task was conducted to develop detectors and linear arrays operating under nominal remote sensing applications. SWIR [λc(295 K) ~ 2.5 μm] test detectors have been measured as a function of temperature between 170 K and 295 K. At 200 K the RoA values are in the 106 ohm-cm2 range. MWIR [λc(60 K) = 5.3 μm] and LWIR [λc(60 K) = 10.5 μm] HgCdTe detectors in a 320 x 6 array format have also been measured at 60 K. Within the arrays, the detector size is 40 μm x 50 μm. The MWIR detector array has a mean quantum efficiency of 89.2 % with a standard deviation to mean ratio, σ/μ = 1.51 %. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz. Operability values exceeding 99.5 % have been obtained. In addition, test diodes at the edge of the array that did not go through a read out integrated circuit (ROIC) were also measured and had quantum efficiency ~ 86 % that agreed well with the ~ 87 % quantum efficiency measured for detectors in the array that were located near the test detectors. The LWIR arrays, measured at 60K also had high operability with only ~ 3 % of the detectors having out of family response. Using best detector select (BDS) feature in the read out integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with 100 % operability is obtained. For the 320 x1 array constituted using the BDS feature, a 100 % operable LWIR array with average NEI value of 1.94x1011 ph/cm2/s at a flux of 7.0x1014 ph/cm2/s has been demonstrated.

A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting 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 integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.

The resistivity, Seebeck coefficient, superconducting transition temperature, and upper critical field have been measured in Cs/sub x/WO/sub 3/ for 0.19< or =x< or =0.30. In contrast to Rb/sub x/WO/sub 3/, no temperature-dependent anomalies were observed in either the normal resistivity or the Seebeck coefficient. The superconducting transition temperature, T/sub c/, increases monotonically with decreasing x from 2 K at x = 0.30 up to 6.7 K at x=0.20, with an inflection near 0.24. The resistivity as a function of x increases monotonically with decreasing x and also has an inflection near x=0.24.

The Tc and Hc2 of hexagonal single crystals Rb0.2WO3 were measured. Upper critical field rotation data in the plane perpendicular to the c-axis showed a large anisotropy with a 60° periodicity. Rotation in a plane containing the c-axis showed an even larger anisotropy having 180° symmetry.

Results of measurements of T/sub c/ and H/sub c2/ on single crystals of Cs/sub 0/./sub 1/WO/sub 2/./sub 9/F/sub 0/./sub 1/ are reported. The critical temperature varied from 4.87 to 4.94 K for different specimens. The upper critical field showed an anisotropy reflecting the symmetry; its temperature dependence was also measured. 11 refs.

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.