Raymond S. Balcerak

Uncooled IR imaging: technology for the next generation

Uncooled IR sensor technology has accelerated rapidly in the past few years. Higher performance sensor, electronics integration, and enhanced signal processing are generating new applications and increasing production volume. Uncooled sensor are being considered to replace cooled sensor in some applications, but most importantly, the unique characteristics of the uncooled sensor spawn novel uses of the technology. Very small, lightweight and lower power sensor are possible with the uncooled IR. However, moderate levels of performance are expected even from the smallest sensors. This demand for performance stimulates new ideas for thermal detector structures operating at or near the theoretical limit. This paper reviews the exciting new applications of the uncooled technology and investigates the novel technical approaches necessary to bring about a new generation of uncooled IR sensor technology.

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.

Advances in linear and area HgCdTe APD arrays for eyesafe LADAR sensors

HgCdTe APDs and APD arrays offer unique advantages for high-performance eyesafe LADAR sensors. These include: operation at room temperature, low-excess noise, high gain, high-quantum efficiency at eyesafe wavelengths, GHz bandwidth, and high-packing density. The utility of these benefits for systems are being demonstrated for both linear and area array sensors. Raytheon has fabricated 32 element linear APD arrays utilizing liquid phase epitaxy (LPE), and packaged and integrating these arrays with low-noise amplifiers. Typical better APDs configured as 50-micron square pixels and fabricated utilizing RIE, have demonstrated high fill factors, low crosstalk, excellent uniformity, low dark currents, and noise equivalent power (NEP) from 1-2 nW. Two units have been delivered to NVESD, assembled with range extraction electronics, and integrated into the CELRAP laser radar system. Tests on these sensors in July and October 2000 have demonstrated excellent functionality, detection of 1-cm wires, and range imaging. Work is presently underway under DARPAs 3-D imaging Sensor Program to extend this excellent performance to area arrays. High-density arrays have been fabricated using LPE and molecular beam epitaxy (MBE). HgCdTe APD arrays have been made in 5 X 5, 10 X 10 and larger formats. Initial data shows excellent typical better APD performance with unmultiplied dark current < 10 nA; and NEP < 2.0 nW at a gain of 10.

Development of high performance radiation hardened antireflection coatings for LWIR and multicolor IR focal plane arrays

High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation. In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures. We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.

A comprehensive model for bolometer element and uncooled array design and imaging sensor performance prediction

This paper reports on a model developed to predict bolometer performance in its environment, where the environment consists of thermal, optical and electrical components. Two complementary methods were employed to predict performance. The first solves the heat balance equation for the bolometer in its circuit and its thermal environment with known values of heat capacitance, thermal conductance, absorptance, temperature coefficient of resistance and the userdefined bias current (either constant or pulsed). This iteration yields a bolometer temperature rise, and a corresponding change in resistance and voltage. This is the signal part of the equation. The second method is required to calculate the total bolometer noise. It uses equations derived from the literature to predict bolometer noise, response, NEP and NETD from first principles for the four types of noise generated in thermal detectors (thermal fluctuation noise, background fluctuation noise, johnson noise and 1/f noise). Thermal conductance and heat capacities are calculated using all the elements of the bolometer structure such as the silicon nitride structure, the VOx coating, and the nichrome electrical leads. Using a calculation of the full spectral irradiance on the bolometer from the scene and the dewar and a userdefined bolometer element spectral absorption, the model will accurately assess performance in any environment. The model also employs a 3-D noise model and provides synthetic images of PSF-blurred bar targets for NETD and MRTD predictions.

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.

EO/IR sensor model for evaluating multispectral imaging system performance

This paper discusses the capabilities of a EO/IR sensor model developed to provide a robust means for comparative assessments of infrared FPAs and sensors operating in the infrared spectral bands that coincide with the atmospheric windows - SW1 (1.0-1.8&mgr;m), sMW (2-2.5&mgr;m), MW (3-5&mgr;m), and LW (8-12&mgr;m). The applications of interest include thermal imaging, threat warning, missile interception, UAV surveillance, forest fire and agricultural crop health assessments, and mine detection. As a true imaging model it also functions as an assessment tool for single-band and multi-color imagery. The detector model characterizes InGaAs, InSb, HgCdTe, QWIP and microbolometer sensors for spectral response, dark currents and noise. The model places the specified FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR) and creates two-point corrected imagery complete with 3-D noise image effects. Analyses are possible for both passive and active laser illuminated scenes for simulated state-of-the-art IR FPAs and Avalanche Photodiode Detector (APD) arrays. Simulated multispectral image comparisons expose various scene components of interest which are illustrated using the imaging model. This model has been exercised here as a predictive tool for the performance of state-of-the-art detector arrays in optical systems in the five spectral bands (atmospheric windows) from the SW to the LW and as a potential testbed for prototype sensors. Results of the analysis will be presented for various targets for each of the focal plane technologies for a variety of missions.

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.

Characterization of SiGe-detector arrays for visible-NIR imaging sensor applications

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays 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 characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPAs. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS Bolometers.