Stuart Horn

Vertically integrated sensor arrays: VISA

The VISA program has been sponsored by DARPA to enable a significant enhancement in signal conditioning, processing, and digitalization on the focal plane of visible and infrared sensors. The approach being developed builds on the traditional “hybrid” structure of a detector with a 2D array of indium-bump interconnects to a silicon readout. VISA will allow additional layers of silicon processing chips to be connected below the readout to provide more complex functionality. Connections will be fully arrayed two-dimensionally with one or more vias per pixel possible. The structural overview will be presented along with several application candidates that appear to be most promising to exploit this technology. These include active/passive sensors, expanded charge storage capacity for full flux utilization in the LWIR, cameras on a chip, high speed sub-frame collection to defeat pulsed laser interference, together with digital output with greater bit depth than currently possible from analog outputs. An A/D candidate circuit to achieve this performance within each pixel will be described.

Third generation imaging sensor system concepts

Second generation forward looking infrared sensors, based on either parallel scanning, long wave (8 - 12 um) time delay and integration HgCdTe detectors or mid wave (3 - 5 um), medium format staring (640 X 480 pixels) InSb detectors, are being fielded. The science and technology community is now turning its attention toward the definition of a future third generation of FLIR sensors, based on emerging research and development efforts. Modeled third generation sensor performance demonstrates a significant improvement in performance over second generation, resulting in enhanced lethality and survivability on the future battlefield. In this paper we present the current thinking on what third generation sensors systems will be and the resulting requirements for third generation focal plane array detectors. Three classes of sensors have been identified. The high performance sensor will contain a megapixel or larger array with at least two colors. Higher operating temperatures will also be the goal here so that power and weight can be reduced. A high performance uncooled sensor is also envisioned that will perform somewhere between first and second generation cooled detectors, but at significantly lower cost, weight, and power. The final third generation sensor is a very low cost micro sensor. This sensor can open up a whole new IR market because of its small size, weight, and cost. Future unattended throwaway sensors, micro UAVs, and helmet mounted IR cameras will be the result of this new class.

Progress in the development of vertically integrated sensor arrays

The demand continues to grow for small, compact imaging sensors, which include new capabilities, such as response in multiple spectral bands, increased sensitivity, wide high dynamic range, and operating at room temperature. These goals are dependant upon novel concepts in sensor technology, especially advanced electronic processing integrated with the sensor. On-focal plane processing is especially important to realize the full potential of the sensor. Since the area available for focal plane processing is extremely limited, a new paradigm in sensor electronic read-out technology is necessary to bridge the gap between multi-functional, high performance detector arrays and the off-focal plane processing. The Vertically Integrated Sensor Array (VISA) Program addresses this need through development of pixel-to-pixel interconnected silicon processors at the detector, thus expanding the area available for signal and image processing. The VISA Program addresses not only the array interconnection technology, but also investigates circuit development adapted to this new three-dimensional focal plane architecture. This paper reviews progress in the first phase of the program and outlines direction for demonstrations of vertically integrated sensor arrays.

Development of low dark current SiGe-detector arrays for visible-NIR imaging sensor

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPAs. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

Fused reflected/emitted light sensors

Fusion of reflected/emitted radiation light sensors can provide significant advantages for target identification and detection. The two bands -- 0.6 - 0.9 or 1 - 2 micrometer reflected light and 8 - 12 micrometer emitted radiation -- offer the greatest contrast since those bands have the lowest correlation, hence the greatest amount of combined information for infrared imaging. Data from fused imaging systems is presented for optical overlay as well as digital pixel fusion. Advantages of the digital fusion process are discussed as well as the advantages of having both bands present for military operations. Finally perception tests results are presented that show how color can significantly enhance target detection. A factor of two reduction in minimum resolvable temperature difference is postulated from perception tests in the chromaticity plane. Although initial results do not yet validate this finding, it is expected with the right fusion algorithms and displays that this important result will be proven shortly.

Design Considerations using APD Detectors for High-Resolution UV Imaging Applications

High resolution imaging in UV band has a lot of applications in Defense and Commercial systems. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPDs have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We will discuss an analytical model that can predict sensor performance in the UV band using p-i-n or APD detectors with and without gain and other detector and sensor parameters for a desired UV band of interest. SNRs can be modeled from illuminated targets at various distances with high resolution under standard MODTRAN atmospheres in the UV band and the solar blind region using detector arrays with unity gain and with high gain APD along with continuous or pulsed UV lasers. The performance can be determined by the signal level which results from the UV laser return energy (laser power, beam divergence, target reflectance and atmospheric transmittance), the optics f/number, the response of the detector (collection area, quantum efficiency, fill factor and gain), and the total noise which will be the sum of the dark current noise, the scene noise, and the amplifier noise. We also discuss trades as a function of detector response, dark current noise and the 1/f noise. We also present various approaches and device designs that are being evaluated for developing APDs in wide band gap semiconductors. The paper also discusses current state of the art in UV APD and the future directions for small unit cell size and gain in the APDs.

Uncooled IR technology and applications

Uncooled infrared cameras have made dramatic strides recently. Very low cost, lightweight, low power cameras have been built. Also low cost high performance uncooled cameras have been built. A discussion of this technology to make this happen and the resulting new applications will follow.

Microbolometer sensor model for performance predictions and real-time image generation of infrared scenes and targets

Presented is a comprehensive, physics-based model for microbolometer detector and sensor performance prediction. The model combines equations found in the literature and various standard models that generate NETD, MRTD, 3-D noise statistics and atmosphere characteristics (MODTRAN-based), with a comprehensive microbolometer model and HgCdTe model developed by the author to provide an end-to-end detector/FPA/sensor analysis and design tool, as well as a realistic image sequence generation tool. The model characterizes the individual pixel element based on the structure used, the various layer thicknesses, the electrical and thermal characteristics of the bolometer material and the biasing and readout circuit, and uses these results to calculate response and noise, NEP and NETD. The NETD, MTF and MRTD are predicted from the optics, detector and readout. Predicted NETD has been compared and verified with values found in literature, results from other models, and to uncooled camera measurements. The MRTD prediction has been verified with camera measurements and with standard industry MRTD model outputs. The model also calculates atmospheric path radiance and transmittance for horizontal paths based on MODTRAN outputs for the LWIR band at altitudes from 0 to 10km and ranges from 1 to 50km for assessments of air-to-air engagement SNRs. The model in matlab utililizes a 3-D noise model to provide accurate realistic imagery used to present realistic sensor images and to further validate the NETD and MRTD routines.(1) Images at 30Hz and 60Hz have been generated for visual assessment by the user and have mirrored industry model results and real-time camera images for MRTDs for the temporal noise case. The models 3-D noise generation feature allows the prediction of MRTD vs. frequency under any 3-D noise combination. This model provides an end-to-end performance prediction tool useful in bolometer element design, readout design and for system level trade studies.

CCD/CMOS hybrid FPA for low light level imaging

We present a CCD / CMOS hybrid focal plane array (FPA) for low light level imaging applications. The hybrid approach combines the best of CCD imaging characteristics (e.g. high quantum efficiency, low dark current, excellent uniformity, and low pixel cross talk) with the high speed, low power and ultra-low read noise of CMOS readout technology. The FPA is comprised of two CMOS readout integrated circuits (ROIC) that are bump bonded to a CCD imaging substrate. Each ROIC is an array of Capacitive Transimpedence Amplifiers (CTIA) that connect to the CCD columns via indium bumps. The proposed column parallel readout architecture eliminates the slow speed, high noise, and high power limitations of a conventional CCD. This results in a compact, low power, ultra-sensitive solid-state FPA that can be used in low light level applications such as live-cell microscopy and security cameras at room temperature operation. The prototype FPA has a 1280×1024 format with 12-um square pixels. Measured dark current is less than 5.8 pA/cm2 at room temperature and the overall read noise is as low as 2.9e at 30 frames/sec.

Design Considerations for SiGe-based Near-Infrared Imaging Sensor

Low cost IR Sensors are needed for a variety of Military and Commercial Applications. SiGe based IR Focal Plane Arrays offer a low cost alternative for developing near IR sensors that will not require cooling and can operate in the visible and NIR bands. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size and compatibility with the low power silicon CMOS circuits for signal processing. A feasibility study of an infrared sensor based on SiGe material system and its performance characteristics are presented. Simulations comparing the sensitivity of the SiGe detector with spectral cutoff wavelength of 1.6 micron to other IR Focal Plane arrays are discussed. Measured electrical and optical characteristics of Ge-on-Si photodetectors are also presented.