Hybrid Focal Plane Arrays Research Articles

Future WSI technology: stacked monolithic WSI

Describes a methodology for stacking chips vertically and interconnecting them through the chips to achieve a three-dimensional (3D) circuit. This methodology involves the integration of two technologies: indium bump interconnect technology, historically used to fabricate hybrid focal plane arrays, and the precision thinning of bonded silicon wafers by a process called Acuthin. Substantial improvements in computing density, power dissipation, and signal propagation time can be realized. This paper describes some of the techniques and benefits of this 3D interconnect methodology.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Control and design of nonuniformities in a hybrid focal plane array

A model is presented to calculate response nonuniformity in a hybrid mercury cadmium telluride focal plane array in terms of material/ device parameters. The model predicts a minimum 10% intrinsic nonuniformity contribution from the IR detector array caused by compositional variations in detector material with the state-of-the-art HgCdTe material. In hybrid FPAs an additional contribution to the nonuniformity is caused by variations in the charge injection efficiency across the area of the array. The injection nonuniformity contribution can, however, be controlled to negligibly small values by working at very high injection efficiencies, on the order of 99%.

Injection efficiency of 8=to 14=um hybrid focal plane arrays

The injection efficiency of an 8- to 14-μm hybrid IR/CCD FPA is calculated from a transcendental equation involving the detector, CCD input currents, and the detector impedance Rd at the operating point. The latter has been varied either by changing the composition of x in the Hg1-x Cdx Te detector or by changing the tunneling current while x is kept constant. Calculations were carried out for both weak and strong inversion cases of the input MOSFET of the CCD. It is argued that for the 8- to 14-μm region, since the detector currents are high, operation of the input of the CCD in the strong inversion regime seems more reasonable.

LWIR 128*128 GaAs/AlGaAs multiple quantum well hybrid focal plane array

The authors describe the fabrication and performance of a new type of hybrid focal plane array (FPA). The hybrid consists of a 128*128 GaAs/AlGaAs superlattice multiple-quantum-well detector array with peak response at 7.7 mu m mated to a high-performance CMOS readout with direct injection input. The quantum-well infrared photodetector (QWIP) array was fabricated by molecular-beam epitaxy (MBE). Optical gratings were excluded to facilitate evaluation of the basic detector-technology. The mean D* at 78 K was 5.76*10/sup 9/ cm- square root Hz/W. Total FPA 1/f noise was negligible, as corroborated by imagery having minimum resolvable temperature (MRT) of 30 mK at 0.07 cycles/mrad. No gain nonuniformity correction was used in the imaging demonstration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum efficiency and crosstalk of an improved backside-illuminated indium antimonide focal-plane array

The quantum efficiency and crosstalk of backside-illuminated indium antimonide photodiodes in hybrid focal plane arrays are calculated. An improved structure with crosswise ohmic contacts at the backside of the thinned InSb substrate is described. The simulations predict a significant reduction in the crosstalk while retaining high quantum efficiency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

General noise processes in hybrid infrared focal plane arrays

The noise limitations of IR imaging systems are often described as arising from predominantly temporal sources, such as background, thermal, and readout noise. Actual systems can also be limited by structured pattern) noises and drift. This is especially true with the advent of highly integrated hybrid focal plane arrays, which consist of an infrared detector array, usually photovoltaic, mated to a silicon CMOS VLSI readout device. In these arrays the large detector count, high density, and complexity create new susceptibilities for image noise. Low natural backgrounds in some systems, and the need to save aperture area in others, often force the designer to work at low background levels and high quanturn efficiencies, eliminating options for simpler FPA architectures and making control of focal plane elements more challenging. The desire to dc couple the array output and use an infrequent detector equalization update further complicates the issue. We discuss several noise processes in low-background hybrid arrays, both observed and anticipated. These processes are described through example circuits, typical of proposed and assembled lR focal plane arrays.

Heteroepitaxial HgCdTe/CdZnTe/GaAs/Si Materials for Infrared Focal Plane Arrays

ABSTRACTThe structural properties of LPE-grown HgCdTe on heteroepitaxial MOCVD-grown CdZnTe/GaAs/Si substrates were evaluated using high-resolution x-ray diffraction techniques and TEM. Large tilts {up to 4°} between CdZnTe layers and GaAs/Si substrates are a general characteristic of this heteroepitaxial system and are are attributed to the interaction of closely spaced misfit dislocations that arrange to form a tilt boundary. Either {112}CdTe or {552}CdTe can be grown on {112}GaAs/Si; the {552} was shown to result from a first-order twinning operation of {112}. Lamnella {111} microtwins in {111}CdZnTe/{100}GaAs/Si substrates, measured by x-ray techniques, are not readily propagated into the LPE-grown HgCdTe layer. The x-ray FWHM of the LPE HgCdTe is typically at least a factor of two lower than that of the Si-based substrate from annealing and due to the increased thickness of the layer; both mechanisms promote dislocation interaction and annihilation. High performance MWIR and LWIR HgCdTe 128×128 hybrid focal plane arrays were fabricated on these Si-based substrates. An array average of ROAj = 17.8 ohmcm2 for a cutoff wavelength of 10.8 μm at 78K was demonstrated.

Blocked impurity band hybrid infrared focal plane arrays for astronomy

High-performance infrared hybrid focal plane arrays using 10*50-element Si:As blocked-impurity-band (BIB) detectors (cutoff wavelength=28 mu m) and matching switched MOSFET multiplexers have been developed and characterized for space astronomy. Use of impurity-band-conduction technology provides detectors which are nuclear-radiation-hard and free of the many anomalies associated with conventional silicon photoconductive detectors. Emphasis in the present work is on advances in detector material quality which have led to significantly improved detector and hybrid characteristics. Results demonstrating increased quantum efficiency (particularly at short-wavelength infrared), obtained by varying the BIB detector properties (infrared active layer thickness and arsenic doping profile), are summarized. Measured read noise and dark current for different temperatures are reported. The hybrid array performance achieved demonstrates that BIB detectors are well suited for use in astronomical instrumentation. >

Charge Coupled Devices for Multiplexing Applications in Hybrid Focal Plane Arrays

The review begins with a brief description of charge coupled device (CCD) as a signal processor. This is followed by description of various types of CCD's used in signal processing applications. We...

Infinite-melt vertical liquid-phase epitaxy of HgCdTe from Hg solution: Status and prospects

Liquid-phase epitaxy (LPE) has emerged as the predominant materials growth technology for the fabrication of HgCdTe infrared (IR) detectors in the IR community over the past decade. This paper reviews the current status as well as the evolution of one modification of LPE technology, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions. The backside-illuminated hybrid focal plane array (HFPA) approach for IR detection has been established in the past few years as the most attractive one for both scanning and staring modes in either tactical or strategic applications. Photodiode structures used in HFPA's with state-of-the-art performance in the entire 2–12 μm spectral region, including p-on-n as well as n-on-p double-layer heterojunctions (DLHJs), have been prepared by VLPE. This is mainly due to two unique characteristics inherent in the Hg-rich solution: (1) Low liquidus temperature ( <450 °C), which makes feasible the cap-layer growth step required for the DLHJ structure (2) Ease of intentionally incorporating temperature-stable impurity dopants, such as As, Sb, and In, in the HgCdTe layers during growth. It has been generally recognized that the DLHJ is the key element to further improvement in photovoltaic (PV) HgCdTe detector performance for LWIR applications. Knowledge of fundamental material properties for the Hg-Cd-Te system is reviewed in three main areas: phase diagram, defect chemistry, and impurity doping. The review concludes with a discussion of the prospects for use of the VLPE technology for investigating fundamental material properties of HgCdTe as well as fabricating advanced device structures of high performance.

Mercury Cadmium Telluride Short- And Medium-Wavelength Infrared Staring Focal Plane Arrays

Short-wavelength (1 to 2.5 µm) and medium-wavelength (1 to 4.0 µm and 1 to 4.8 µm) 64 x 64 hybrid focal plane arrays (FPAs) have been developed. The short-wavelength (SWIR) arrays were developed for use in a prototype Airborne Imaging Spectrometer system. The medium-wavelength (MWIR) arrays are suitable for tactical missile seekers and strategic surveillance systems. The detector material is HgCdTe LPE grown on a sapphire substrate. The unit cell size is 52 µm x 52 µm, and the detectors are ion-implanted planar structures. The multiplexer is a surface- or buried-channel, four-phase charge-coupled device. The unit cell has a direct-injection FET and a storage capacitor. The current-voltage characteristics of the 64 x 64 detector arrays were measured at near-zero background using Si fan-out chips. For the detector arrays with a 2.5 µm cutoff, the mean RoA for 29 random elements was 2.7 x 106 cm2 at T = 150 K. The mean RoA was 3 x 104 cm2 at 120 K for detector arrays with a 4.6 µm cutoff. Both results were adequate for specific system applications. The arrays were also characterized at different temperatures, and the results are presented herein. The 64 x 64 SWIR and MWIR detector arrays were mated to a multiplexer, and the resulting FPAs were characterized under different operating conditions. Detailed characterization results are presented.

HgCdTe on sapphire — A new approach to infrared detector arrays

Some of the limitations imposed by bulk CdTe substrates on epitaxial HgCdTe, such as wafer size, fragility, and uniformity, have led to the development of an alternate substrate to CdTe for epitaxial HgCdTe growths. Described here are the synthesis and some of the properties of an alternate hybrid CdTe/sapphire substrate, and the material and device properties of liquid phase epitaxial (LPE) grown HgCdTe on CdTe/sapphire substrates. Devices made in LPE grown HgCdTe layers on CdTe/sapphire have shown excellent electrical and optical properties and superior uniformity in diode-to-diode D * in midwave infrared (MWIR) focal planes at low temperature when compared to devices fabricated in HgCdTe grown on CdTe substrates. Diodes have typical resistance area product values of ⩾ 10 Ω cm 2 at 195 K (cutoff wavelength λ c = 4.2 μm), ⩾ 3 x 10 4 Ω cm 2 at 120 K (λ c = 4.45 μm) and ⩾ 1 x 10 6 Ω cm 2 at 77 K (λ c = 4.6 μm). Typical quantum efficiencies are 60–80% without anti-reflection coating. Analysis of the detectivity of a 1024 element MWIR hybrid focal plane array shows that the number of defective elements, even under low-to-moderate photon backgrounds (high 10 12 photons cm -2 s -1), is less than 5%.

HgCdTe hybrid focal plane

Second-generation IR systems, consisting of 2-D mosaics of IR detectors, have been under intense development for the last few years. One of the most successful architectures has been a HgCdTe hybrid focal plane array (FPA), using a Si charge-coupled device (CCD) readout chip interfaced to epitaxial HgCdTe. Detection is made by backside-illuminated photovoltaic detectors with high fill factors and quantum efficiency. The detectors are coupled into the CCD by In bumps which mass bond each detector in the mosaic to a CCD input. Advances have been made in uniform, large area HgCdTe detector material that can be grown with a bandgap from less than 0.1 eV to greater than 1 eV. CCD architectures have been developed with simple, linear inputs and dynamic ranges up to 80 dB. Hybrid FPAs are currently being tested in prototype imaging systems, for detecting thermal differences as well as reflected sunlight in the IR. In the 3–5μm region, these arrays have proven capable of noise-equivalent temperature differences as low as 0.01 K, acquired at a 400 Hz frame rate. In addition to improving current imaging systems, these area arrays allow new system concepts to be brought to fruition.

A method for routine characterisation of the hole concentration in p-type cadmium mercury telluride

Recently there has been growth of interest in the fabrication of photodiodes in CMT, for use in electronically scanned hybrid focal plane arrays. Most devices are currently fabricated using a p-type wafer with an n-type surface layer introduced either by ion implantation or diffusion. In both types of junction the &,A product is dependent on the net doping concentration of the p-type substrate. In ion-implanted junctions for example the performance can be limited even at zero bias by the junction tunnelling current if the substrate doping is too high. It has been determined that a carrier concentration of 1 x 1Ol6 cme3 or less is required to produce implanted detectors with high zero bias resistance-junction area, R,,A, products. (l) Consequently an accurate, and rapid, method of determining the carrier concentration of p-type CMT is required. A common method of determining the net carrier concentrations in routine characterisation of semiconductors is to measure the Hall coefficient, RH, at a single temperature in the exhaustion range of the extrinsic conduction region. This method, using a spot measurement at 77K, has been found to be satisfactory for n-type CMT in which carrier freeze-out is not observed. However, p-type CMT is much more difficult to characterise by Hall effect measurements because of the high electron-to-hole mobility ratio, b, (500 at 80K) and a relatively high intrinsic carrier density in this narrow gap semiconductor. As a result the mixed conduction region in lightly doped p-type samples extends below 80K and may overlap the freeze-out region so that no exhaustion region is observed. Determination of the net doping concentrations in p-type material has therefore required the interpretation of Hall data obtained over a range of temperatures extending down to about 20K. The interpretation may be further complicated since oxidation of the surface of the samples can lead to an n-type skin which shunts the sample at low temperatures and in extreme cases can cause a p-type sample to appear in n-type. To overcome these difficulties the possibility of using a measurement of bulk conductivity to characterise the hole concentration has been investigated. The two band expressions for conductivity, 0, and low field Hall coefficient, RH are:

MTF modelling of backside-illuminated PV detector arrays

Backside-illuminated detector arrays are used extensively in hybrid focal plane arrays. The MTF of the detector array is affected by the cross-diffusion of photo-generated carriers, especially for materials with long diffusion lengths such as HgCdTe. In this paper, the MTF of the backside-illuminated detector arrays, including the effect of carrier diffusion, are modelled. The MTFs are calculated as a function of detector thickness, absorption coefficient, pixel size and diffusion length. The trade-off of detector MTF and quantum efficiency is also analyzed.