Hybrid Focal Plane Arrays Research Articles

Analysis of design principles of silicon multiplexer circuits for multielement infrared focal plane arrays

Design principles of silicon multiplexers for linear and matrix infrared (IR) focal plane arrays (FPAs) are considered. Silicon multiplexers intended for operation with multielement mercury-cadmium-tellurium (MCT) photodiode detectors, with multielement photoresistor detectors based on multilayer structures with quantum wells, and with other types of photodetectors sensitive to radiation in the IR ranges from 3 to 5 and from 8 to 16 µm, are discussed. The type and size range of the multiplexers includes 19 models that differ in frame sizes, input circuits, charge capacity, and cell pitch. Around the designed multiplexers, hybrid and monolithic FPAs of various formats for medium and far IR ranges with a rather high temperature resolution (< 0.02 K) have been developed.

Development of solar-blind AlGaN 128×128 Ultraviolet Focal Plane Arrays

This paper reports the development of solar-blind aluminum gallium nitride (AlGaN) 128×128 UV Focal Plane Arrays (FPAs). The back-illuminated hybrid FPA architecture consists of an 128×128 back-illuminated AlGaN PIN detector array that is bump-mounted to a matching 128×128 silicon CMOS readout integrated circuit (ROIC) chip. The 128×128 p-i-n photodiode arrays with cuton and cutoff wavelengths of 233 and 258 nm, with a sharp reduction in response to UVB (280–320 nm) light. Several examples of solar-blind images are provided. This solar-blind band FPA has much better application prospect.

Infrared focal plane array based on GaAs/AlGaAs quantum-well multilayer structures

An infrared 320 × 256 focal plane array (FPA) for 7.5–10 μm was designed, fabricated and investigated. The photodetector matrix based on GaAs/AlGaAs quantum-well multilayer structures was grown by hydride epitaxy from organometallic compounds. The sensitivity peak was achieved at a wavelength of 9.3 μm and equaled 340 mA/W. The hybrid FPA was fabricated by cold welding indium bumps of the photodetector and silicon multiplexer. The obtained NETD was 29.4 mK at 68 K.

XRD measurement of thermal strain in InSb‐based devices

AbstractThe low temperature‐induced strain in the thin brittle InSb upper part of ready‐to‐work InSb‐based infrar‐ed photo‐detectors (Hybrid Focal Plane Arrays (HFPA) technology) has been measured using X‐ray diffraction (XRD). Assuming no shear component and no surface normal stress in the InSb upper part of the device and using linear elasticity laws lead to simple strain‐stress relations. It was shown that the maximum value of the thermal stress in the InSb part of the studied samples is well under the value of the yield stress of InSb at the working temperature of the devices. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High differential resistance type-II InAs∕GaSb superlattice photodiodes for the long-wavelength infrared

Type-II InAs∕GaSb superlattice photodiodes with a 50% cutoff wavelength ranging from 11to13μm are presented. Optimization of diffusion limited photodiodes provided superlattice structures for improved injection efficiency in direct injection hybrid focal plane array applications. Photodiodes with a cutoff wavelength of 12.9μm exhibit an R0A of ∼7Ωcm2 and a Johnson-limited detectivity of 4.03×1010cmHz1∕2W−1 operating at 77K. Quantum efficiency measurements indicate minority carrier diffusion lengths exceeding 3μm.

High-quality large-area MBE HgCdTe/Si

HgCdTe offers significant advantages over other similar semiconductors, which has made it the most widely utilized variable-gap material in infrared (IR) focal plane array (FPA) technology. HgCdTe hybrid FPAs consisting of two-dimensional detector arrays that are hybridized to Si readout circuits (ROIC) are the dominant technology for second-generation infrared systems. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially grown HgCdTe, which have been limited to 30 cm2 in production. This size limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to 2048×2048, and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be scaled to large wafer sizes and further offer a thermal-expansion-matched hybrid structure that is suitable for large format FPAs. This paper presents data on molecular-beam epitaxy (MBE)-grown HgCdTe/Si wafers with much improved materials characteristics than previously reported. We will present data on 4- and 6-in diameter HgCdTe both with extremely uniform composition and extremely low defects. Large-diameter HgCdTe/Si with nearly perfect compositional uniformity and ultra low defect density is essential for meeting the demanding specifications of large format FPAs.

Monolithically integrated HgCdTe focal plane arrays

The cost and performance of hybrid HgCdTe infrared (IR) focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an IR sensor technology based on monolithically integrated IR focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts; (2) the low-temperature cleaning of Si(001) wafers; (3) the growth of CdTe and HgCdTe layers on read-out integrated circuits; (4) diode creation, delineation, electrical, and interconnection; and (4) demonstration of high yield photovoltaic operation without limitation from earlier preprocessing such as substrate cleaning, molecular beam epitaxy (MBE) growth, and device fabrication. Crystallographic, optical, and electrical properties of the grown layers will be presented. Electrical properties for diodes fabricated on misoriented Si and readout integrated circuit (ROIC) substrates will be discussed. The fabrication of arrays with demonstrated I-V properties show that monolithic integration of HgCdTe-based IR focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the third generation of IR systems.

LPE HgCdTe on sapphire status and advancements

With the evolution of infrared arrays to over four million pixels, larger formats have demanded higher quality mercury cadmium telluride (MCT) wafers. Since single defects can easily degrade multiple diodes, high operability requires very homogeneous and nearly flawless epitaxial surfaces. Subsequent photolithography and hybridization also demand unprecedented levels of substrate flatness and low imperfections. To consistently and reliably produce large area arrays, Insaco Inc., The Boeing Company, and Rockwell International Corporation have developed major quality improvement procedures which address all three components of the infrared material wafer architecture. Centered on the producible alternative to cadmium telluride for epitaxy (PACE) process, technological advancements encompassed sapphire substrates, organometallic vapor phase epitaxy (OMVPE), cadmium telluride (CdTe) buffer layer growth, and liquid phase epitaxial (LPE) mercury cadmium telluride growth. Processed material from these runs mated to Conexant™ fabricated multiplexers have successfully produced 1024 1024 and the first 2048 2048 IR short-wave (2.5 m at 80 K) hybrid focal plane arrays. Operabilities in these implanted n-on-p junction devices reach 99.98% with near 70% quantum efficiency in the astronomy ‘K’ band (2.2–2.4 microns).

Spatial Noise Reduction by Optical Filters in Mercury Cadmium Telluride Hybrid Focal Plane Arrays

This paper discusses the role of optical filters in the reduction of spatial noise in Hgl-xCdxTe hybrid focal plane arrays (FPAs). It is shown that optical filters of appropriate cut-off wavelength can cause considerable reduction in the spatial noise of both mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) arrays. These filters can result in considerable improvement in the NETD performance of a MWIR HgCdTe FPA, whereas in LWJR FPAs, these can also be used to trade-off the requirement of composition uniformity of the HgCdTe substrate, depending upon the relative magnitude of temporal and spatial noise components

A new method for measurement of threshold voltage non-uniformity in CCD multiplexers for hybrid focal plane arrays

A new, simple and rapid method for determining threshold voltage ( V T) non-uniformity in two-dimensional CCD multiplexers (MUXs) for hybrid focal plane arrays is presented. The method is based on simple oscilloscopic measurement of non-uniformity in the output signal of CCD MUX. The non-uniformity in V T, measured by this method, is compared with conventional current forcing method. The results of the proposed new method agree within 7.8% with the conventional method. Additionally, intrinsic non-uniformity due to processing and material variations is also measured by this method.

Anomalous effect in readout of large size charge coupled device multiplexers for hybrid focal plane arrays

The multiplexer functionality of two sets of two-dimensional Charge Coupled Device (CCD) multiplexers (MUXs) viz. 16×16 and 100×100, using identical design tools, on-chip amplifiers, technology and foundry for fabrication, was measured under identical clocking conditions. It was found that the frame readout of the 100×100 CCD MUX shows an anomalous effect, in the sense that video output signal value monotonically decreases as the pixel number increases (from first to last pixel). This was contrary to the expectation of constant output from all the pixels—as was observed for 16×16 CCD MUXs. The problem was analysed by performing a number of experiments (with horizontal and vertical shift registers, input structure, etc.) to isolate the number of reasons/blocks that are possibly responsible for the cause of this effect. The origin of the problem was traced to a polysilicon bus used for interconnecting Store Gates (SGs) of the input structure. The total resistance, and consequently voltage drop, across this bus was found to be negligible for smaller arrays (16×16), but not for larger ones (100×100). By proper tuning of spill time of input injecting pulse, this anomalous effect could be eliminated completely. A simple model for the anomalous effect was developed and the predicted trends found to agree with various experimental observations.

InSb infrared photodetectors on Si substrates grown by molecular beam epitaxy

The InSb infrared photodetectors grown heteroepitaxially on Si substrates by molecular beam epitaxy (MBE) are reported. Excellent InSb material quality is obtained on 3-in Si substrates (with a GaAs predeposition) as confirmed by structural, optical, and electrical analysis. InSb infrared photodetectors on Si substrates that can operate from 77 K to room temperature have been demonstrated. The peak voltage-responsitivity at 4 μm is about 1.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> V/W and the corresponding Johnson-noise-limited detectivity is calculated to be 2.8×10/sup 10/ cm/spl middot/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> 2//W. This is the first important stage in developing InSb detector arrays or monolithic focal plane arrays (FPAs) on silicon. The development of this technology could provide a challenge to traditional hybrid FPA's in the future.

Spatial noise limited NETD performance of a HgCdTe hybrid focal plane array

This paper presents a model for theoretically estimating the residual spatial noise in a direct injection readout hybrid focal plane array (FPA) consisting of photovoltaic detectors. The procedure consists of computing the response of the pixels after taking into account the nonlinearity induced by the transfer function in the hybrid configuration and the estimated r.m.s. response nonuniformity from the known input parameters of the detector and readout arrays. A linear two point nonuniformity compensation algorithm is applied to the computed pixel responses to calculate the residual spatial noise. Signal-to-spatial noise ratio is then used to estimate the spatial noise limited NETD performance of MWIR and LWIR Hg 1− x Cd x Te hybrid FPAs.

Noise equivalent temperature difference performance of an IR detector in a hybrid focal plane array

Theoretical expressions to calculate the signal-to-noise ratio of an IR detector in a direct injection readout hybrid focal plane array (FPA) are developed in this paper. The theory takes into account the effect of shift of operating point of the pixel with small variations in scene temperature and contribution of shunt resistance due to surface leakage currents in the photodiode. These expressions are then used to calculate the noise equivalent temperature difference (NETD) performance of the IR detector in the FPA. The role of buffered direct injection (BDI) circuit in improving the NETD performance of hybrid pixels in a LWIR mercury cadmium telluride FPA array is analyzed. The results of our calculations show that the individual pixels in a LWIR HgCdTe FPA would not function to their ultimate performance and in certain conditions may even exhibit erratic behaviour in the absence of BDI interface.

Monolithic CCD imagers in HgCdTe

Charge-coupled device (CCD) infrared detector arrays in 5 /spl mu/m cutoff HgCdTe have been demonstrated for low background applications. These fully monolithic 128 by 28 element CCD arrays incorporate time-delay-and-integrate (TDI) detection, serial readout multiplexing, charge-to-voltage conversion and buffer amplification in the HgCdTe detector chip. Operation of these devices at 77 K have produced average detectivity values exceeding 3/spl times/10/sup 13/ cm-Hz/sup 1/2//W for a background flux level of 6/spl times/10/sup 12/ photon/cm/sup 2/-sec in the 3.0 /spl mu/m to 5.5 /spl mu/m spectral band. Overall performance data indicates the monolithic HgCdTe CCD to be a promising alternative to present midwave infrared hybrid focal plane array technology. >

Rapid measurement of threshold non-uniformity in ccd multiplexers for hybrid focal plane arrays

A new, simple and rapid method of measurement for threshold non-uniformity in CCD multiplexers for hybrid Focal Plane Arrays (FPAs) is presented. The method is based on simple oscilloscope measurement of non-uniformity in the readout of CCD multiplexers wherein identical charge packets are loaded on the whole array through background suppression drain.

Model for response nonuniformity calculations of a direct-injection readout hybrid focal plane array

A model for the calculation of rms response nonuniformity in a direct-injection readout mercury cadmium telluride hybrid focal plane array is presented. The model includes contributions from detector material composition variations, processing-induced variations, threshold voltage variations in the input structure of the readout multiplexer, and transfer efficiency variations in the case of CCD-based readouts. The model is then used to analyze the role of the buffered direct-injection (BDI) circuit in controlling the nonuniformity. It is predicted that a minimum BDI gain of 10 is required to reduce the nonuniformity of long-wavelength infrared focal plane arrays (LWIR FPAs) to its minimum value limited by the compositional variations in detector material. This gain requirement is expected to go up with a reduction in the resistance-area product of photodiodes resulting from either imperfect processing or increased cutoff wavelength (λ_<i>co</i> > 9.43 μm).

Long-wavelength infrared technology for astronomy at the Hughes Technology Center

The Hughes Technology Center (HTC) has developed a family of high-performance Si:As impurity-band conduction (IBC) hybrid focal plane arrays (FPAs) optimized for low background applications: 58×62 pixels (76-μm pitch), 128×128 pixels (75- and 120-μm pitch), and 256×256 pixels (30-μm pitch). These FPAs exhibit state-of-the-art low noise (<100e -) achieved by using readout arrays fabricated on HTC's CryoCMOS process line. The IBC detector arrays, also fabricated at HTC, exhibit high quantum efficiency over a wide waveband with operating temperature of 4–12 K. In addition, Hughes is developing a 256×256 Si:As IBC FPA for high background applications as well as a 512×512 FPA. Readout development includes design and fabrication of 256×256 readouts with large well size of 1×107 e - for the high-background FPA and 512×512 readouts with moderate well size (1×106 e -).

MOCVD grown CdZn Te/GaAs/Si substrates for large-area HgCdTe IRFPAs

Large-area HgCdTe 480×640 thermal-expansion-matched hybrid focal plane arrays were achieved by substituting metalorganic chemical vapor deposition (MOCVD)-grown CdZnTe/GaAs/Si alternative substrate in place of bulk CdZnTe substrates for the growth of HgCdTe p-on-n double-layer heterojunctions by controllably-doped mercury-melt liquid phase epitaxy (LPE). (100) CdZnTe was grown by MOCVD on GaAs/Si using a vertical-flow high-speed rotating disk reactor which incorporates up to three two-inch diameter substrates. Layers having specular surface morphology, good crystalline structure, and surface macro defect densities <50 cm−2 are routinely achieved and both the composition uniformity and run-to-run reproducibility were very good. As the composition of the CdZnTe layers increases, the x-ray full width at half maximum (FWHM) increases; this is a characteristic of CdZnTe grown by VPE techniques and is apparently associated with phase separation. Despite a broader x-ray FWHM for the fernary CdZnTe, the FWHM of HgCdTe grown by LPE on these substrates decreases, particularly for [ZnTe] compositions near the lattice matching condition to HgCdTe. An additional benefit of the ternary CdZnTe is an improved surface morphology of the HgCdTe layers. Using these silicon-based substrates, we have demonstrated 78K high-performance LWIR HgCdTe 480×640 arrays and find that their performance is comparable to similar arrays fabricated on bulk CdZnTe substrates for temperatures exceeding approximately 78K. The performance at lower temperatures is apparently limited by the dislocation density which is typically in the low-mid 106 cm−2 range for these heteroepitaxial materials.

A method for determining the yield of indium bump contacts for integration in hybrid focal plane arrays at room temperature

A simple electrical method for determining the yield of indium bump integration between the infrared photosensing chip and the silicon readout chip in hybrid focal plane arrays (FPAs) is proposed. In our case, the IR photosensing chip consists of a 16 × 16 HgCdTe (MCT) photovoltaic (PV) array and the readout chip is a 16 × 16 silicon-CCD multiplexer (MUX). However, the method can be used for even larger array sizes. The method allows one to determine the yield of integration at room temperature. In addition, it does not necessitate the vacuum sealing of hybrid FPA in the dewar and subsequent testing at 77 K for determining this yield. The proposed method essentially verifies the electrical connectivity between the MCT PV diodes and their corresponding input diffusions in the CCD MUX on pixel-to-pixel basis. A simple examination of the readout of the whole array on the oscilloscope at room temperature, initiated at the detector and through CCD MUX is used for determining exactly how many MCT PV diodes have been joined successfully to their corresponding CCD MUX pixels after indium bump integration.