Timothy D. Pope

Micromachined, uncooled, VO2-based, IR bolometer arrays

Bulk silicon micromachined IR bolometer detectors operating at room temperature are presented. These devices are based on VO2 films typically exhibiting a thermal coefficient of resistance of the order of -3%/ degree(s)C. Detector sizes are 50 micrometers X 50 micrometers and 100 micrometers X 100 micrometers , and they are arranged in 1 X 64, 1 X 128 and 1 X 256 pixel linear arrays. A test bench for detector performance evaluation is described. The fabricated detectors exhibit responsivities of up to approximately 20,000 V/W, normalized detectivities typically exceeding 108 cmHz1/2 W-1, and response times typically below 20 ms, At 300 K and a frequency of 30 Hz, the noise equivalent temperature difference for these detectors is of the order of 3 X 10-2 degree(s)C. A bolometer simulation tool is also briefly described.

128 x 128 pixel uncooled bolometric FPA for IR detection and imaging

An uncooled IR camera making use of a 128 X 128 pixel bolometric FPA is presented. The reconfigurable bolometric focal plane array consist of 50 micrometer X 50 micrometer pixels and simple on-chip CMOS readout electronics which can be operated in random access, independent row and column clocking, and self-scanning modes. Depending on the selected pixel format and frame rate, the FPAs NETD varies from 0.52 degrees Celsius down to 0.10 degrees Celsius. The modular IR camera is software configured and provides RS170A analog video and 12-bit TTL format digital outputs.

Micromachined VO2-based uncooled IR bolometric detector arrays with integrated CMOS readout electronics

Uncooled IR bolometric detectors fabricated using surface silicon micromachining are presented. The detector fabrication process employs a polyamide sacrificial layer, and a VO2 thermistor layer exhibiting a thermal coefficient of resistance on the order of -3 percent/degrees C. Detector sizes are 100 micrometers X 100 micrometers and 50 micrometers X 50 micrometers , and 64 X 64 and 128 X 128 pixel arrays are fabricate. The detectors exhibit responsivities of up to 15 000 VW-1, normalized detectivities typically exceeding 108 cm Hz1/2W-1, and response times below 20 ms. Three integrated readout circuit designs for 64 X 64 and 128 X 128 pixel detector arrays, fabricated using a standard 1.5 micrometers CMOS process,a re described. These circuits include several test and detector nonuniformity correction features and can operate in either self scanning mode at a rate of 30 frames per second, or in the random access mode in which column and row addresses are input directly.

Commercial and custom 160x120, 256x1, and 512x3 pixel bolometric FPAs

INO has been active in microbolometer and FPA technology development since the early 1990s. Microbolometer detectors based on VO2 films with TCR above 3% are typically fabricated. VOx films with TCR above 2% have been developed for applications where FPA temperature is not stabilized. INO is continuing its development of high fill factor pixels with sizes down to 25 micrometers and new macro- and micro-packaging technology. All fabrication is done on six inch wafers in INOs newly expanded clean room facility. INO currently offers as standard products 256x1 and 160x120 pixel FPAs with 52 micrometers pixel pitch. Both arrays have simple, robust, and versatile CMOS readout integrated circuits (ROICs) that may be accessed in self-scanning or random access mode, and reference detectors for on-chip coarse offset and temperature drift compensation. Single frame NETDs (f/1, 300 K, 8-12 micrometers ) are on the order of 150 - 250 mK and may be reduced by frame averaging. Prototyping boards have been developed for both arrays, and the 160x120 FPA has been integrated in a number of thermal cameras and instruments. In collaboration with its clients, INO has developed several FPAs for specific space and terrestrial applications. Custom ROICs fabricated in several different CMOS processes from multiple foundries have been used. A 512x3 pixel microbolometer FPA with 39 micrometers pitch is being developed for the European Space Agency. The array is designed for multi-spectral pushbroom imaging applications and features a novel ROIC with very low 1/f noise, pixel by pixel offset and drift compensation, variable integration time, and digital output. Its single frame NETD (f/1, 300 K, 8-12 micrometers ) is nominally 80 mK.

Uncooled detector, optics, and camera development for THz imaging

A prototype THz imaging system based on modified uncooled microbolometer detector arrays, INO MIMICII camera electronics, and a custom f/1 THz optics has been assembled. A variety of new detector layouts and architectures have been designed; the detector THz absorption was optimized via several methods including integration of thin film metallic absorbers, thick film gold black absorbers, and antenna structures. The custom f/1 THz optics is based on high resistivity floatzone silicon with parylene anti-reflection coating matched to the wavelength region of interest. The integrated detector, camera electronics, and optics are combined with a 3 THz quantum cascade laser for initial testing and evaluation. Future work will include the integration of fully optimized detectors and packaging and the evaluation of the achievable NEP with an eye to future applications such as industrial inspection and stand-off detection.

Nonoptical characterization techniques for uncooled microbolometer infrared sensors

Infrared image sensors based on micromachined microbolometers can be integrated with CMOS addressing, readout, and signal processing circuitry. Testing of such sensors in the commercial environment must be rapid and cost effective, hence, other diagnostic techniques are preferred to optical testing wherever possible. Accordingly, this paper presents nonoptical techniques for characterising microbolometer-based IR sensors. From measurements of microbolometer resistance, thermal conductivity, and temperature coefficient of resistance, sensor fixed pattern noise and temperature response nonuniformity are determined. These measurements also provide feedback on the fabrication process parameters and defects.

Flexible micromirror linear array for high-resolution projection display

The visual displays of contemporary military flight simulators lack adequate definition to represent scenes in basic fast-jet fighter tasks. For example, air-to-air and air-to-ground targets are not projected with sufficient contrast and resolution for a pilot to perceive aspect, aspect rate and object detail at real world slant ranges. Simulator display geometries require the development of ultra-high resolution projectors with greater than 20 megapixel resolution at 60 Hz frame rate. A new micromirror device has been developed to address this requirement; it is able to modulate light intensity in an analog fashion with switching times shorter than 5 μs. When combined with a scanner, a microlaser and Schlieren optics, a linear array of these flexible micromirrors can display images composed of thousands of lines at a frame rate of 60 Hz. The approach selected for light modulation and the micromirror fabrication process flow are reviewed. Static and dynamic performances of these electrostatic MOEMS are described. Preliminary results following the integration of the described modulator into a projector prototype are reported. Developments toward a fully addressable 2000 × 1 flexible micromirror array are presented. The specifications and design of the CMOS circuit required to control this micromirror array are described. Packaging issues related to these large arrays are discussed.

Design, manufacturing, and qualification of an uncooled microbolometer focal plane array–based radiometric package for space applications

Uncooled microbolometer detectors are well suited for space applications due to their low power consumption while still exhibiting adequate performance. Furthermore, the spectral range of their response could be tuned from the mid- to the far-infrared to meet different mission requirements. If radiometric measurements are required, the radiometric error induced by variation of the temperature of the detector environment must be minimized. In a radiometric package, the detector environment is thermally stabilized by means of a temperature-controlled radiation shield. The radiation shield must be designed to prevent stray radiation from reaching the detector. A radiometric packaging technology for uncooled microbolometer FPAs is presented. The selection of materials is discussed and the final choices presented based on thermal simulations and experimental data. The radiometric stability with respect to stray light and variation of the temperature of the environment as well as the different noise components studied by means of the Allan variance are presented. It is also shown that the device successfully passed the prescribed environmental tests without degradation of performance.

Microbolometer Detector Array for Satellite-Based Thermal Infrared Imaging

An infrared imaging array based on micromachined bolometric detectors is under development. The array is designed to meet the requirements of satellite-based infrared imaging and detection applications, including cloud and earth surface temperature monitoring and forest fire monitoring. Its design makes it also suitable for terrestrial spectrometric and moderate speed imaging applications. The array consists of three parallel lines of 512 pixels on a 39 µm pitch. Each pixel includes both active and reference detectors to reduce pixel to pixel offset variation, eliminate common mode bias noise, and provide increased immunity to die temperature drift. The thin films making up the active detector structure are designed to provide high and uniform infrared absorption between 8.3 and 13 µm. The pixels are fabricated monolithically over a custom CMOS readout integrated circuit (ROIC) using a surface micromachined post-processing approach. The advanced ROIC integrates the signals of all pixels in parallel using switched capacitor correlated double sampling integrators and performs on-chip analog to digital conversion. Despite the small pixel size the projected thermal resolution over the 8-12 µm spectral band for f/1 optics is below 50 mK. In this paper we will describe the detector array design and present preliminary test results.

Miniature VO2-based bolometric detectors for high-resolution uncooled FPAs

Prototypes of VO2-based bolometric detectors with lateral dimensions of 25 X 25, 30 X 30, 35 X 35, 40 X 40 and finally 50 X 50 micrometers2 and fill factors approaching 90% are presented. These detectors are grouped in hardwired linear arrays as large as 512 X 1 pixels. Under DC biasing, the fabricated detectors, even the smallest ones, exhibit responsivities from 48,000 to 120,000 VW-1, detectivities in the range of 1.5 X 108 cm Hz1/2W-1 and response times in the range of 5 ms. These new bolometric detector structures contain hidden-legs placed completely underneath the bolometer platform. Results of simulations of the mechanical, optical and electrical properties of these new detector structures are presented. A complete detector fabrication process flow is described.