Advances in the development of infrared image sensors with Schottky-barrier detectors (SBDs) are reviewed. SBD operation and design structure are described, including the responsivity and dark-current characteristics and SBD fabrication. The reported results for a variety of focal plane arrays (FPAs) are compared in terms of parameters such as pixel size, fill factor, and SBD type. The development of scanning PtSi FPAs with 4096 by 4 elements and 2048 by 16 TDI elements is reported for spaceborne remote sensing applications. Up to 512 by 512 elements and 640 by 486 elements have reportedly been developed for high-resolution staring PtSi FPAs. Useful thermal imaging is shown for some staring FPAs, and IrSi SBDs with cut-off wavelengths up to 10 microns is shown for some FPAs in the LWIR band. Because the silicon VLSI process is exclusively employed to produce Schottky-barrier FPAs, these FPAs are found to be useful for uniform, low-cost SWIR and MWIR applications.

The utilization of both cobalt and nickel silicides as Schottky detectors is presented with both theoretical and experimental supporting data. The uses considered are limited to those with a typical cut-off frequency of more than 2.2 microns, such as earth observation and satellite imaging spectroscopy. The theoretical calculations of key parameters of Schottky-barrier detectors are discussed, including quantum efficiency, the dark current, and the noise. Experimentally, Co and Ni layers were tested on silicon wafers at a variety of temperatures and layer thicknesses. Average values for the barrier height, activation analysis, and quantum efficiency are given and compared. The dark current for both Ni and Co is shown to be negligible below 140-160 K, and Ni quantum efficiencies are higher than those for Co, as is the measured barrier height. The theoretical and technological requirements are met by both cobalt and nickel silicides. The characteristics tested show that Ni and Co silicides are appropriate for realizing large focal planes with high dark current and responsivity homogeneity.

A 640 x 486 pixel monolithic focal plane array detector using PtSi Schottky barrier photodiodes was developed. This detector uses 1.2-micron design rules to achieve a 54-percent fill factor with 25-micron square pixels. The detector array used an interline CCD configuration with a progressive scan (noninterlaced) readout of the field, and two-phase clocking of both the vertical and horizontal registers.

A mid-infrared imaging camera head built around a 512 x 512 monolithic PtSi focal plane array is discussed. The pixel size of the focal plane array is 30 by 30 microns. The array uses a novel line-addressed charge-accumulation structure that allows a 54-percent fill factor using 2.5-micron minimum design rules. Effective frame rates exceeding 75 Hz are achieved using multiple readout registers (i.e., four each) when operated at 5 Mpixels/s. The array is cooled in a side-looking, liquid nitrogen dewar with a hold time greater than 8 hours. All the drive and signal processing circuitry necessary to operate the device and perform correlated-double sampling on the video signals are mounted in boxes attached to the dewar.

A new concept, the Direct Schottky Injection (DSI), is described for a three-dimensional construction of infrared imagers with a continuous Schottky-barrier-detector surface on one side of a thinned (10 to 25 microns) silicon substrate and p-type buried-channel CCD readout structure on the other side. The DSI structure provides a 100-percent fill factor, a large charge-handling capacity, and a high-density pixel design. The construction and operation are described for DSI imagers with frame-transfer CCD (FT-CCD) and interline-transfer CCD(IT-CCD) readout. The operation of the IT-CCD DSI imager was demonstrated with a 128 x 128 focal plane array (FPA) with 50 x 50-micron pixels.

The design of a 640 by 480 element PtSi IR sensor is presented which includes a low-noise MOS X-Y addressable readout multiplexer and an on-chip correlated double-sampling amplifier. The sensor is designed to load scan data into CMOS horizontal and vertical scanning registers by means of a multiplexed horizontal/vertical input address port and onchip decoding, allowing any element in the focal plane array to be randomly accessed. The FPA is shown to be operable in both the interlaced and noninterlaced formats, with variable exposure control. Enhanced noise performance is shown due to the use of buried channel source follower buffers in the horizontal signal lines. It was shown that 24 micron square pixels with a 1.5 micron double level metal CMOS process provide a fill factor of 38 percent. TTL compatibility and ESD protection diodes are key features of the digital inputs to the sensor's chip.

A 2048 x 16 pixel time delay and integrate (TDI) monolithic focal plane array detector using PtSi Schottky barrier photodiodes and an interline CCD architecture was developed. A high resolution was achieved by using 30-micron pixels, with a high sensitivity through the TDI mode of operation. A ripple clocked vertical shift register was used for higher charge capacity, allowing a 43-percent fill factor with a saturated charge capacity of 530,000 electrons.

The requirements, design, fabrication, and test results are presented for a high-density hybrid FPA based on platinum silicide IR detector technology. The hybrid Schottky FPA is intended to optimize detector and readout design and processing, achieve optimal fill factors, and reduce cell size. Schottky barrier detectors are employed in arrays of 24 micron pixel spacings, in a 244 by 400 array format. The readout and detector structure are detailed, as well as the fast-settling circuitry, and a fill factor of 84 percent is shown. The modeling used to predict optical performance is set forth, indicating detector response and noise level for specified conditions. The preamplified output of each detector was sampled in a performance test consisting of irradiation by an extended blackbody IR source. Results are presented for responsivity, RMS noise, dc uniformity and noise equivalent temperature distance. Sensitivity levels in low background conditions are shown to be suitable. The design permits low-cost readout fabrication and extension to larger arrays.

The platinum silicide power spectrum found on p-type silicon Schottky diodes was measured for the diodes available on an IR FPA. The noise from the diodes is shown to have a white power spectrum even at frequencies below 3.0 x 10 to the -5th Hz. The data generated from each pixel were digitized into twelve bits, and tranferred by a GPIO bus to a computer. The unit cell, camera system response, low frequency drift and mutual drift compensation techniques, and optimization of the charge transfer efficiency are explained. The modeled response, and the sample of the observed power spectrum for three diodes, are presented. 1/f noise is characterized as ubiquitous in nature and nonuniformity correction techniques are effective, but the inconsistency with current 1/f models elicits a discussion of potential flaws in the experiment. Sensitivities to two terms are found in the measurement technique, and if the product of the terms is more than the diode power spectrum the estimates of the power spectrum of an individual diode cannot be accurate. It is concluded that 1/f noise may be completely absent from PtSi Schottky diodes.

Monolithic silicon Schottky barrier IR FPAs are described which include new design features. The features are intended to optimize the sensitive diode area in each pixel. The nonimaging area of the pixels is reduced by a field plate approach to edge leakage suppression and by column multiplexing and automatic demultiplexing in the focal plane array. A four-phase meander channel CCD structure further minimizes readout area. A design figure of merit is defined and introduced to compare the fill factors of two arrays. The FPA fabrication and operation are discussed. With an NMOS compatible wafer process, fill factors of approximately 50 percent were obtained for small pixel dimensions. The new design features are shown to significantly increase the fill factor. The performance of the FPAs is shown to be adequate for both medium-wavelength PtSi (at 83 K) and long-wavelength IrSi (at 50 K) diodes.

This paper describes a staring Platinum Silicide (PtSi) medium wave infrared (MWIR) camera. The sensor is configured with a 244 x 400 hybrid focal plane array (HFPA), which has a 24 x 24 sq micron unit cell and an 84-percent fill factor. The PtSi HFPA exhibits high sensitivity with a single pixel NE-Delta-T less than 0.09 C at F/2.0 at 60 Hz. The HFPA performance characteristics, such as dc uniformity, signal responsivity, noise, and NE-Delta-T, are reviewed. The camera containing the HFPA is divided into two units: a camera head and camera processor. The camera head operates at a 60 Hz frame rate generating two 3.6 MHz video outputs. The camera processor converts these two outputs into RS-170 video. Nonuniformity correction and video reformatting are performed in the processor. The system architecture, operation, and system performance are described.

The specifications and performance are set forth for a multispectral linear array developed for SWIR remote sensing applications. The device comprises a monolithic quadruple-band image sensor with 4096 detectors in each band. The silicon Schottky barrier IR CCD has been utilized for the SWIR sensor. Operating at temperatures from 77 k to 82 k, the device also employs a SiC-Al2O3-SiC-based package constructed with epoxy adhesives, and four narrow bandpass filters attached to the device. A performance study shows that the dynamic range is 2500 with a noise floor of 74 electrons rms, and the signal to noise ratio is 41.6 dB for an irradiance of 9.4 microwatts per square centimeter, at a wavelength of 2.06 microns and an integration time of 3.46 ms. The response of the device demonstrates a nonuniformity of 6.4 percent peak-to-peak. The sensor is intended for the Japanese Earth Resources Satellite-1.

MWIR HgCdTe arrays have been prepared on Si substrates for hybrid FPAs. HgCdTe has also been grown directly on the Si readout for developing monolithic HgCdTe/Si FPAs. CdTe, Al2O3 and Si substrates were included in a 35 parameter model for estimating the cost of both monolithic and hybrid HgCdTe FPAs. The model shows the monolithic approach on large Si substrates to be the preferred approach for reducing HgCdTe FPAs cost.

The performance of a recently developed indium antimonide (InSb) two-dimensional multiplexed medium wavelength infrared (MWIR) hybrid focal plane array (FPA) is presented. The CMOS FET switch array multiplexer individually buffers each detector in a 64 x 64 element array through a source follower amplifier. This multiplexer was designed for demanding low-background, high sensitivity requirements. The detector array consists of InSb photodiodes spaced on 100 micron centers, bump bonded through indium columns to the silicon multiplexer, and is thinned for backside illumination. The array is responsive to radiation in the 1 to 5.5 micron region. The FPA has been demonstrated to have a near theoretical read-out noise performance of less than 500 electrons. The, charge storage capacity is approximately 4 million electrons giving a dynamic range of 78 dB. The device is linear to better than 99.95%over the lower 30% of its dynamic range, and greater than 90% over its total range, reflecting the capacitive discharge nature of the charge integration. Dark currents of less than 10 pA are obtained at 77K with reverse biases as great as 0.5V, and less than 0.2 fA at 25K. Quantum efficiency greater than seventy-five percent has been achieved at the peak wavelength. Functional element yields of 99% have been obtained.

The development of two 256 by 256 hybrid HgCdTe focal plane array (FPA) families is described, and their performance is discussed. The hybrid FPAs employ a PV HgCdTe detector array and custom Si CMOS readouts. The PACE-1 process was used to fabricate the detectors, whereby the liquid phase epitaxial growth of HgCdTe occurs on sapphire substrates buffered by a layer of CdTe. The performance characteristics of the detector arrays are given. A tactical 256 by 256 CMOS readout is tested, in which a high functional yield was achieved. Updated test results are given for a 256 by 256 readout circuit developed for use in an orbital replacement instrument for the Hubble Space Telescope. The characterizations of several MWIR and SWIR FPAs were thorough and shown to be reliable. The pixel yield, maximum FPA responsivity nonuniformity, and SWIR FPA read noise for the tests are given. The high contrast and insignificant fixed pattern noise of the imagery from the MWIR 256 by 256 FPA are emphasized. These qualities were obtained when the device was operating at 80 k and utilizing f/2 optics with an 8-in. focal length and a 4.4 micron high pass filter.

The effects of detector series resistance on the electrooptical performance of infrared detectors are calculated by means of a simple diode model with a parallel photocurrent source. Calculations are performed for a number of variables, and series resistance is shown to be an important parameter in the design of photovoltaic detectors because it can limit operation in high performance design applications.

Scene-based nonuniformity correction (NUC) was performed via two techniques, and the results are presented in terms of ability to allow increased sensitivity and to minimize scene degradation. The two techniques perform recalibration in real-time based on the radiance levels of the scene being viewed. The scene-based NUC was effected with an algorithm based on a temporal high-pass filter, and with one based on an artificial neural network. Recorded data from an MWIR staring array collecting different images were used for the experiments. The advantages of the two scene-based NUC techniques are summarized and compared to those of the traditional calibration technique. The potential for the elimination of spatial noise and the achievement of BLIP performance levels in high-quantum-efficiency FPAs are emphasized. Spatial noise can be eliminated by employing real-time signal processors.

The design and performance of an IR FPA test system are described. The parameters, techniques, and equipment required for an IR test system are discussed. An analysis of focal planes is presented to extract the last dB of performance, emphasizing the minimization of required device testing time. The CECOM test system is presented, including the instrument controller, the five source types for characterizing IR FPAs, the electronics, and the protective plexiglass enclosure. Critical test parameters and bias and clocking supply specifications are set forth. Printers which display the results are described, and a buffer/spooler is suggested to increase output rate. Data presentation is discussed. Tests on FPAs and readouts, such as dc uniformity, rms noise, 1/f noise, detectivity, linearity, and responsivity, can be conducted in an automated mode. The possibility of automated testing of optical crosstalk, spectral response, and optical area is mentioned. Future improvements to the facility are listed. Improved yields and increased FPA testing throughput in IRFPA production programs can result from the IRFPA test system facilities.

The results of tests for leakage performed on ten IR sensor dewars are presented, and the design principles of the new testing devices are discussed. The ultrasensitive leak detector used for testing is compared to conventional detectors. The superfine leak calibrator consisting of a tracer gas supply, an aliquot volume, a pressure transducer, temperature gage, and valves was used to measure leak rates in the E-4 to E-12 std cc He/s range. The testing method is explained, including the gases used, the quadrupole mass analyzer, the reference leak calibration, and the temperature coefficient of the reference leak. The test results of the IR sensor dewars are shown: seven showed leak rates in the E-15 std cc He/s range, two had no detectable leaks, and one had a mid-range E-14 leak. The shelf lives of the dewars are calculated based on the results. The vacuum integrity of small IR sensor dewars can be reliably tested to the range of 1E-15 std cc He/s using the ultrasensitive leak detector and the superfine leak calibrator.

The heteroepitaxy of narrow gap semiconductor (NGS) layers on Si substrates is described, and the fabrication of photovoltaic IR-sensor arrays in the NGS layers is explained. Lead chalcogenides are grown and employed to facilitate the fabrication of NGS material. Stacked intermediate CaF2-BaF2 bilayers are employed to overcome the lattice and thermal expansion mismatches, thereby attaining epitaxy in the NGS layers. Linear sensor arrays were built on Si substrates with cutoff wavelengths ranging from 3 to more than 12 microns. Performance testing shows that the sensitivity of the most effective PbTe on Si sensors equals that of Hg(1-x)Cd(x)Te, with possible improvement. The compositional homogeneity of NGS material is shown to be less critical with lead salts. The efficacious shielding of charges resulting from defects, a consequence of the high permittivity of lead salts, is shown to allow the development of more fault-tolerant IR sensors. The slower response time indicated in photovoltaic IR focal plane arrays for thermal imaging applications is shown to be insignificant.

Separate absorption and multiplication InGaAs/InP avalanche photodiodes (SAM-APDs) with a floating guard ring structure that is well-suited to array applications have been successfully demonstrated. Individual APDs have breakdown voltages greater than 80 V, multiplications over 40 at 100 nA dark current, and uniform spatial gain profiles. Uniform I-V characteristics and gains have been measured over linear dimensions as large as 1.2 cm. Gains over 10 at low multiplied dark currents were measured on 21 consecutive devices at the wafer level.

The design, fabrication, and operation of a two-phase meander-channel CCD imager on GaAs are described. The fabrication process is based on the use of anodic oxidation for producing thin dielectric isolations between close-packed Schottky-barrier metal electrodes and the employment of recessed gates for producing self-aligned potential barriers between the adjacent charge wells of the meander channel. Additionally, the semiinsulating property of GaAs is utilized in order to produce the high-speed photoconductive sensors. It is shown that such imagers can have unit cell sizes and pixel densities comparable to their silicon counterparts but offer the speed of performance higher by about one order of magnitude.

The field emission from III-V, II-VI, PbTe/PbSnTe, strained layer, and HgTe/CdTe superlattices with graded structures under magnetic quantization is investigated and compared with that of the bulk specimens of the constituent materials. The doping and magnetic-field dependences of the magneto-field emission of these superlattices are analyzed, and it is found that the field emission exhibits oscillatory dependences on the inverse magnetic field and electron concentration with more significant oscillations appearing in the HgTe/CdTe superlattices. Relations for the bulk specimens are obtained from the special cases of generalized formulations.

An attempt is made to determine an absolute lower limit on the noise of a quantum detector. A comparison is made between several types of detectors and the absolute limit curve. The equations of Kinch (1989) for XSi are used as well as those of Forrseter et al. (1989) for a superconductor.

The Improved Stratospheric and Mesospheric Sounder (ISAMS) employs two Oxford Stirling cycle refrigerators to cool eight infrared (6 MCT, 2 InSb) detectors to 80 — 100 Kelvin. Techniques to minimize heat loads on the displacer cold tip and attached cold stage conductor include unique radiation and conduction isolation designs. The designs of the cold stage conductor and detector mounts are coaxial so that all cold parts are loaded both radiatively and conductively only to an intermediate temperature. The outer stage of the detector cooling system is radiatively cooled to approximately 200 Kelvin by a recessed cold patch. This arrangement reduces total heat loads on each displacer cold tip to less than 700 milliwatts. Heaters are provided to de-gas the low emittance gold coatings if they become contaminated. A detailed nodal computer model was developed to thermally represent the hardware and sensitivity studies were performed to evaluate design parameters and orbital environmental effects. Thermal-vacuum testing at the Oxford Clarendon Laboratory showed excellent performance of the system and a correspondence with math model predictions to within 3 Kelvin. The ISAMS will be launched aboard the Upper Atmosphere Research Satellite (UARS) in 1991 .Its objectives include: measurement of the thermal structure of the atmosphere and its fluctuations in space and with season; investigation of the photochemistry of nitrogen-containing species; determination of the water vapor budget of the upper atmosphere. These objectives will be addressed by measurements of carbon dioxide, nitrous oxide, nitrogen dioxide, di-nitrogen pentoxide, nitric acid, ozone, water vapor, methane, and carbon monoxide.

Closed cycle Stirling coolers used in conjunction with infrared detectors operating at or near 80 Kelvin have undergone many design changes over the past several years. The need for increased lifetime and reliability as well as miniaturization for compact imagers has led to the Integrated Dewar Cooler Assembly (IDCA). In the IDCA, the detector is directly mounted to the cooler expander (cold finger) thus eliminating thermal losses from the dewar inner bore and interface losses associated with the use of fuzz buttons and bellows. The result is a significant reduction in thermal inertia (less mass to cool) and heat load (amount of heat to be removed from the system). These reductions have a direct impact on input power demands which indirectly influence the coolers size and weight. Design features such as all metal, ceramic coated clearance seals, isolation of the motor and electionics from the working gas volume and specially designed, long life bearings contribute to increased lifetime. Reliability and lifetime testing on three IDCA units is in progress and to date each unit has demonstxated greater than 5000 hours of operation without failure. IDCA integration and performance testing with discrete detectors and small HgCdTe arrays has been highly successful. The Kollmorgen MICRO-FUR® thermal imager incorporates the IDCA in systems requiring closed cycle cooling. Future plans call for IDCAIFPA integration (HgCdTe, InSb, PtSi) and subsequent performance testing at the system level.

Smoke detection systems as they are now configured in aircraft cargo bays do not meet the current FAA requirements. The Federal Aviation Regulations, Part 25.5851 includes a specification that a fire be detected within one minute after initiation. Data from full scale testing programs have provided conclusive evidence that smoke detection systems can take up to several minutes to provide a fire warning signal. This has been emphasized in a new FAA Airworthiness Directive requiring new fire detection systems on all combination passenger/cargo (combi) aircraft built by Boeing and McDonnell Douglas. In response to this situation Pacific Scientific has developed a fire detection system which meets the new FAA regulations. It is based on infrared detection technology coupled with a fresnel scanning optical system. The system uses a dual computer controller unit. The sensor is responsive to either over heat or fire conditions, has three levels of fire alarm signal, incorporate complete system redundancy, and has fire location capability. It is highly reliable and immune to false alarm stimuli. A description of the system will be presented. Performance characteristics, false alarm immunity, and compliance with the FAA regulations will be discussed.

Two cameras used as output transducers in a time-sequenced templet-matching optical correction system are compared. One camera, a charge-injection device (CID) camera, is equipped with an image intensifier, automatic gain control (AGC), and a standard video output, while the second camera is a charge-coupled device (CCD) camera without an intensifier or AGC but with a computer interface custom-designed to use the camera's digital output. The cameras are used at the output of the optical correlator in order to integrate high frame-rate time-sequenced output correlation responses derived from binary phase-only filters. It is concluded that an image intensifier is the most important feature to have in a CCD or CID camera because the use of it results in a shorter integration time.

A comparative evaluation of several advanced CCD imagers developed for use in acoustooptical systems is presented. A distinguishing feature of these detectors is the use of optical gain compression in the photo-site to increase the optical dynamic range well above that of conventional CCDs. Measurements are presented in the context of the AO systems application.

A proposal for a metal-silicide full Schottky IR detector array is presented. IR detection and charge transfer are conducted with metal silicide gates which lie directly on silicon, and a detector array designed as a frame transfer device. A half-Schottky CCD was developed to probe the concept, with one phase gate built as a Schottky silicide gate. A more advanced version with higher fill factors was also tested. IR sensitive Schottky CCD gates are connected to nonsensitive Schottky CCD gates, allowing frame storage; line by line readouts are made through a Schottky gate readout CCD. A new method for submicron patterning between the metal Schottky gates is presented to develop the fully integrated version. IR detection is facilitated by the use of low Schottky barrier height silicides, while non-IR-sensitive storage CCDs and readout CCDs use metal silicides with high Schottky barrier heights. The device is demonstrated to be highly radiation-resistant, with built-in antiblooming capacity, and the charge handling capacity is higher than that of conventional devices because no gate oxides are utilized.

The operation, performance, and applications are set forth for linear and area focal plane arrays which use PtSi Schottky barrier diodes to image in the 1 to 5.5 micron spectral range. Two linear FPAs, with pixels spaced on 25 micron centers, are considered. A digital scan readout is used in these arrays. A discussion of a 128 by 128 element PtSi FPA follows, where a MOS/CTD readout structure is employed. The digital shift register addresses diode rows one by one due to the parallel connection of the MOS multiplexer switches in this device. Two bucket brigade charge transfer registers are utilized for signal readout. Lastly, a high fill factor, low noise 512 by 512 PtSi FPA is discussed, which uses a new line-addressed charge-accumulation (LACA) CCD in an interline transfer configuration and a multiple-readout horizontal register. The digital scan readout allows high sensitivity and signal levels. The second device and the LACA structure permit a high fill factor and charge handling capacity. The lower cost and higher resolution of the latter devices are underscored, and applications for all the FPAs are listed.

Mercury cadmium telluride (MCT) photoconductive detectors which detect radiation in the 10.6 micron spectral region are considered with respect to specific applications at varying temperatures. Requirements such as sensitivity, time constant, and cooling constraints are discussed. MCT detectors employed in the 10 to 11 micron spectral region successfully detect fast laser pulses.

The effect of photochemical deposition of Si02 on the current•voltage characteristics of the InSb pn diodes was studied. By applying different voltages on the gate electrode over the p+n junction periphery, various kinds of current-voltage characteristics can be induced, including multiple negative differential resistance in forward bias, This strongly indicates that the major part of the current, especially the reverse leakage current, flows through the surface of the p+n junction. Reverse leakage current as low as 20 zA/cm2 at -1.1 V for a diode with n-type doping concentration of 2 x 1015 cm3 could be easily achieved by applying a gate voltage of -9 V. It is also found that diodes with similar performance can be fabricated by properly adjusting the photochemical vapor deposition passivation process.

This paper explores the noise characteristics of a 20 x 64 element cryo-SiCMOS multiplexed FPA readout of the switched MOSFET type over the temperature range 8-30 K. Both subtractive double sampling and correlated double sampling techniques were used to probe the multiplexer's output waveform during static and dynamic operation. The rms noise was observed to be a weak, decreasing function of temperature over the ranges 8-19 K and 24-30 K, while the apparent noise was excessively high in the region 19-24 K, owing to a device instability that is attributed to a fundamental property of silicon-based cryo-MOSFETs.

GaAs and related heterostructure charge-coupled devices (CCDs) for detector array readout multiplexer applications are described. Features of resistive-gate CCDs are compared with capacitive-gate CCDs for this application. Design examples of GaAs CCD readouts for linear and two-dimensional detector arrays with different detection methods and signal coupling schemes are described. Recent progress in two-dimensional electron gas (2DEG) Al(0.3)Ga(0.7)As/GaAs resistive-gate CCDs is also presented. The 2DEG resistive-gate CCD conventional buried-channel GaAs CCD.