Robert A. Richwine

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.

Multispectral EO/IR Sensor Model for Evaluating UV, Visible, SWIR, MWIR and LWIR System Performance

Next Generation EO/IR Sensors using Nanostructures are being developed for a variety of Defense Applications. In addition, large area IRFPAs are being developed on low cost substrates. In this paper, we will discuss the capabilities of a EO/IR Sensor Model to provide a robust means for comparing performance of infrared FPAs and Sensors that can operate in the visible and infrared spectral bands that coincide with the atmospheric windows - UV, Visible-NIR (0.4-1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR (8-14μ). The model will be able to predict sensor performance and also functions as an assessment tool for single-color and for multi-color imaging. The detector model can also characterize ZnO, Si, SiGe, InGaAs, InSb, HgCdTe and Nanostructure based Sensors. The model can predict performance by also placing the specific FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR). This model has been used as a tool for predicting performance of state-of-the-art detector arrays and nanostructure arrays under development. Results of the analysis can be presented for various targets for each of the focal plane technologies for a variety of missions.

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.

EO/IR sensors development using zinc oxide and carbon nanostructures

EO/IR Sensors have been developed for a variety of Military Systems Applications. These include UV, Visible, SWIR, MWIR and LWIR Sensors. The conventional SWIR Sensors using InGaAs Focal Plane Array (FPA) can operate in 0.4 - 1.8 micron region. Similarly, MWIR Sensors use InSb and HgCdTe based FPAs that are sensitive in 3-5 and 8-14 micron region. DOD investments in the last 10 years have provided the necessary building blocks for the IR Sensors that are being deployed in the field. In this paper, we discuss recent developments and work under way to develop Next Generation nanostructure based EO/IR detectors that can potentially cover UV, Visible and IR regions of interest. The critical technologies being developed include ZnO nanostructures with wide band gap for UV detection and Carbon Nanostructures that have shown the feasibility for IR detection. Experimental results on ZnO based nanostructures demonstrate enhanced UV sensitivity and path forward for larger arrays. Similarly, recent works on carbon nanostructures have shown the feasibility of IR detection. Combining the two technologies in a sensor can provide multispectral capability.

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.

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.

Design and development of wafer-level short wave infrared micro-camera

Low cost IR Sensors are needed for a variety of Defense and Commercial Applications as low cost imagers for various Army and Marine missions. SiGe based IR Focal Planes offers a low cost alternative for developing wafer-level shortwave infrared micro-camera that will not require any cooling and can operate in the Visible-NIR band. The attractive features of SiGe based IRFPA’s will take advantage of Silicon based technology, that promises small feature size and compatibility with the low power silicon CMOS circuits for signal processing. SiGe technology offers a low cost alternative for developing Visible-NIR sensors that will not require any cooling and can operate from 0.4- 1.7 microns. The attractive features of SiGe based IRFPA’s will take advantage of Silicon based technology that can be processed on 12-inch silicon substrates, that can promise small feature size and compatibility with the Silicon CMOS circuit for signal processing. In this paper, we will discuss the design and development of Wafer-Level Short Wave Infrared (SWIR) Micro-Camera. We will discuss manufacturing approaches and sensor configurations for short wave infrared (SWIR) focal plane arrays (FPAs) that significantly reduce the cost of SWIR FPA packaging, optics and integration into micro-systems.

Development of III-N UVAPDs for ultraviolet sensor applications

High-resolution imaging in ultraviolet (UV) bands has many applications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. In past work, UV avalanche photodiodes (APDs) have been reported as discrete devices demonstrating gain. The next frontier is to develop UVAPD arrays with high gain to demonstrate highresolution imaging. We will discuss a model that can predict sensor performance in the UV band using APDs with various gain and other parameters for a desired UV band of interest. Signal-to-noise ratios (SNRs) can be modeled from illuminated targets at various distances with high resolution under standard atmospheric conditions in the UV band and the solar-blind region using detector arrays with unity gain and with high-gain APDs. We will present recent data on the GaN-based APDs for their gain, detector response, dark current noise, and 1/f noise. We will discuss various approaches and device designs that are being evaluated for developing APDs in wide-bandgap semiconductors. The paper will also discuss the state of the art in UVAPDs and the future directions for small unit cell size and gain in the APDs.

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.