Christine Alain

Noise-equivalent power characterization of an uncooled microbolometer-based THz imaging camera

A THz camera based on an uncooled microbolometer 160X120 pixel array with nominal pitch of 52 μm has been developed at INO and initial transmission and reflection images showed promise. In the present paper, the characterization of both standard infrared and THz-optimized uncooled microbolometer pixel arrays are presented at both infrared and THz wavelengths. Measurements in the THz region has been performed using non-uniform low-power quantum-cascade laser (QCL) and uniform high-power far-infrared laser (FIR laser) beams at 3 THz and 4.25 and 2.54 THz, respectively. A measurement comparison has been achieved in the infrared using a blackbody radiation. Different methods for noise-equivalent power (NEP) measurements have been investigated. These characterization methods are promising especially for non-uniform laser beams irradiated on pixel arrays. The NEP results obtained from the different methods are in good agreement independent of the method used in the experiments. The results show a high sensitivity of the THz-optimized pixel array in the THz region. Large beam area reflection imaging of obscured materials at 2.54 THz have been performed at video rates of 30 frames per second using the THz-optimized pixel array equipped with a semi-custom fast THz objective, proving that the INO THz camera provides a promising solution for stand-alone imaging systems.

A microbolometer-based THz imager

THz imaging is a very promising field rapidly growing in importance. This expanding field is at its early stage of development but already a large number of applications are foreseen. THz imaging promises to be a key technology in various fields, such as defense & security where it can be used to defeat camouflage. Based on its many years of experience in uncooled bolometers technology, INO has developed, assembled and characterized a prototype THz imager. The cameras 160 × 120 pixel array consists of pixels with a 52 μm pitch that have been optimized for the THz region. Custom camera electronics and an F/1 THz lens barrel complete the imager design. Real-time imaging at video rate of 30 frame/sec has been performed with a 3 THz quantum cascade laser set-up. THz images of numerous object-obscurant combinations are presented, proving the feasibility of video imaging in security screening applications.

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.

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.

Video-rate THz imaging using a microbolometer-based camera

A THz 160×120 pixel array camera has been developed at INO. Real-time transmission and reflectance imaging at video rates of 30 frames/s were performed with a low-power 3 THz quantum cascade laser. Various hidden objects were imaged, proving feasibility of real-time THz imaging for security screening applications.

Hybrid wafer-level vacuum hermetic micropackaging technology for MOEMS-MEMS

Packaging constitutes one of the most costly steps of MEMS/MOEMS manufacturing. The package protects the MEMS devices and, in the case of MOEMS, it also provides light access to the device. In many cases, MEMS require a specific atmosphere for their proper functioning. The atmosphere should be kept invariable during the lifetime of the package in order to not degrade the performance of the device. Maintaining a constant atmosphere inside the package becomes more challenging as the cavity volume is decreased to the microliter and nanoliter range. Other packaging requirements are compatibility with wafer-level microfabrication techniques (cost reduction) and low temperature assembly in cases where temperature sensitive devices are to be packaged. In recent years, INO has performed a great amount of work towards the development of uncooled IR microbolometer detectors using VOx technology. Different pixel designs have been optimized for different applications. The bolometer pixels require a vacuum atmosphere below 10 mTorr to be maintained during the lifetime of the device in order to operate at their highest sensitivity. INOs micropackaging technology has been demonstrated to provide base pressures below 5 mTorr. An equivalent flow rate of 2.5×10-14 Torr.l/sec has been obtained for a device packaged without any getter. The advantages of INOs micropackaging technology are the possibility of achieving very low base pressures, the low temperatures required for the assembly (the package device is never exposed to a temperature above 150 °C) and its compatibility with hybrid wafer-level packaging. The technology has been developed for the micropackaging of INOs 160×120 pixel uncooled microbolometer FPA, but it is compatible with any other kinds of MOEMS-MEMS devices requiring vacuum hermetic packaging. In order to increase the lifetime of the package, knowledge of the gases outgassing inside the package is crucial. A hybrid approach has been chosen as it permits packaging only known-good dies and saving considerable quantities of IR window material. In INOs hybrid wafer-level packaging, dicing is performed only through one of the wafers, therefore reducing the risk of perturbing the vacuum during the separation of the different dies.

Influence of pixel geometry on the 1/f noise coefficient

This paper presents a systematic study of the 1/f noise coefficient as a function of pixel geometry for microbolometer structures. Structures with various VOx widths, electrode gaps, electrode widths and via hole sizes were fabricated and characterized. The experimental results show that the 1/f noise coefficient is adversely affected by current non uniformity, in agreement with model predictions. Design parameters that significantly impact current non uniformity are identified and approaches to minimize their importance are proposed.

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.

256 x 1 and 256 x 40 pixel bolometer arrays for space and industrial applications

Three types of uncooled IR bolometric detector arrays equipped with 256 X 1 and 256 X 40 VO2 thermistor pixels and on-chip readout electronics are presented. These reconfigurable arrays consist of 50 micrometer X 50 micrometer pixels and CMOS readout electronics that can be operated either in random access mode or in self-scanning mode. Depending on the operational conditions, the NETD of the arrays can be as low as 20 mK.