The infrared spectral region has intrigued scientists for over a hundred years, but until recently it has been impossible to create infrared sensors analogous to the human eye. The recent breakthroughs have come about through the creation of large, two-dimensional solid-state arrays that have thousands of detectors distributed across the focal plane of an optical system. The days of scanning-type optical systems are numbered, as such systems give way to staring-type sensors.
This paper discusses the Navy program for infrared focal plane array (IR FPA) development as well as several ultimate IR FPA system applications. Of particular interest are applications involving missile seekers, threat warning, and search and track. Advantages of IR systems as Navy sensors are discussed, as are problems involved with developing high-performance IR FPAs. Emphasis is placed on specific IR FPA technologies currently being developed as part of the Navy program. These technologies include both hybrid and monolithic arrays (charge-coupled devices, charge-injection devices, and charge-imaging matrix devices). Other technologies are also discussed.
This paper discusses the evolution of military requirements, from a relatively simple scanned linear array of detectors, to a scanned two-dimensional array of detectors with time delay and integration, to two-dimensional staring arrays. We review the process that led to the concentration on one detector material, namely, mercury cadmium telluride, to satisfy most military requirements. The various material growth methods are described. These growth techniques are closely coupled to material characterization techniques, both as an analytical tool and for in-process control. Finally, we review focal plane array fabrication technology and the relationship between detector and on-focal-plane processing parameters.
Abstract. 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.
A 256 x 256 element PtSi Schottky-barrier 1R-CCD image sensor has been developed using a minimum design rule of 2 µm and a three-level polysilicon structure. The pixel size and chip size are 37 x 31 µm2 and 10 x 10 mm2, respectively. In spite of the small pixel size, a large fill factor of 25% has been obtained. The responsivity has been improved by use of a thin metal film and an optical cavity structure. The barrier height and quantum efficiency coefficient obtained from the array performance measurement are 0.23 eV and 0.15 eV -1, respectively. The noise equivalent temperature difference of about 0.1 K is obtained with f/1.4 optics and a 16.7 ms stare time. The noise in this case is limited by the shot noise of the detector. An infrared camera was also developed using the 256 x 256 element IR-CCD image sensor.
The construction, operation, and performance of a linear in-frared focal plane array employing platinum silicide Schottky-barrier detectors is described. The 1 x256 element array is fabricated with standard integrated-circuit-grade silicon and NMOS processing. This device uses a digital scan readout scheme consisting of a digital shift register and associated MOSFET switches. The inherent advantage of the digital scan read-out is that large signal levels can be handled without saturation. As a result, the pixel size can be made quite large (i.e., high aspect ratios are possible), thus maximizing sensitivity. This is particularly important in spectroscopy applications. In addition, the digital scan readout structure is less susceptible to freeze-out at temperatures below 40 K when compared with other types of readout structures. The array is operated with two-phase clocking, which is relatively simple and not critical to the performance of the device. The device exhibits a very high reverse breakdown voltage Vrb of 30 V, a quantum efficiency coefficient C1 of 42%/eV, and a long-wavelength cutoff Xc of 5.4µm.
In this paper D* is analyzed as a figure of merit for focal plane arrays. Spatial noise (nonuniformity) is incorporated into the signal-to-noise ratio, and its effects are examined for a Schottky barrier and for a standard photon detector (constant quantum efficiency) over the 3 to 5 µm spectral band. The requirements for radiometric source uniformities are presented to show their effect in reduction of fixed-pattern noise. Because of the D* inadequacies presented in this paper, the preferred figure of merit is the signal-to-noise ratio.
Maturity, proven producibility, and superior performance make indium antimonide the preferred choice for mid-wavelength (2 to 5.4 µm) infrared systems. Only at temperatures in excess of 130 K do other detector materials afford advantages over InSb. To realize the high performance inherent to the InSb detectors in large array formats, readouts need to be designed that provide low noise, buffer the InSb to off-focal-plane electronics, and provide the speed dictated by the specific application. For the highest performance the readout needs to be more than a simple matrix of switches. Reset-integrator and buffered-common-gate inputs are discussed and compared to an innovative chopper-stabilization design that uses a carrier modulation technique to upconvert detector signals to higher frequencies where MOSFET 1/f noise is low. Projections of focal plane performance are made for various qualities of InSb detectors interfaced to chopper-stabilized readouts specifically designed for the detector impedances achievable at elevated temperatures (> 100 K).
A new hybrid infrared array sensitive in the 1 to 5µm spectral region has recently become generally available to the astronomy community. It is a composite (hybrid) array using photovoltaic indium antimonide detector material bump-bonded to a silicon substrate that contains the MOS direct readout multiplexer. This paper describes the array, its operation, and preliminary results on quantum efficiency, dark cur-rent, noise, linearity, cosmic ray response, and imaging characteristics as they apply to ground-based astronomy. The tests were performed on an engineering device since scientific devices are not yet available. Based on our results, we believe that this array and others like it will affect infrared astronomy to an even greater extent than CCDs affected optical astronomy in the mid-1970s.
A buffered direct-injection (BDI) current readout for infrared detectors is described and analyzed. It is compared with the common direct-injection (DI) circuit with respect to injection efficiency, noise, and tolerance of low RoA product photovoltaic detectors. Power requirements and threshold control are also discussed. Throughout the analysis it is clear that much advantage is gained at relatively little cost by the use of a BDI structure for an integrated circuit focal plane.
This paper reviews recent developments in photodetector technology based on III-V compound superlattices. Of particular interest is the relatively unexplored InAsSb system, in which strained-layer superlattices are required to obtain very small bandgaps (predicted to be as low as 0.1 eV). Progress in the growth of these materials by both molecular beam epitaxy and metal-organic chemical vapor deposition is summarized. Experimental results for a variety of superlattice photodetector structures are presented, along with a discussion of processing technologies that are important for fabricating photodetector arrays.
We propose to implement systolic sampled-data lattice processors optically by means of linear integrated optics arrays of coupled-wave devices such as switched directional couplers and TE-TM mode converters. Potential signal processing applications include discrete time and analog frequency filtering and optical pulse compression.
The Diffuse Infrared Background Experiment (DIRBE) optics module will be assembled at ambient temperature but will operate at liquid helium temperature. The challenging task of ensuring that is possible to maintain alignment through cryogenic cooling and cryogenic vibration has been demonstrated in a DIRBE optics module breadboard program. This paper discusses the configuration of the DIRBE optics module breadboard, ambient temperature optical alignment methods used to assemble the breadboard, and cryogenic testing of the breadboard. The alignment of the optical assembly at ambient temperature, at 77 K, and after a 77 K vibration was within specification.
This paper develops a method for calculating the intensity distribution of a Gaussian laser beam that has been truncated by a centered circular aperture at some distance prior to the plane of interest.
Expectation values in the frequency domain are used to calculate the correlation peak of an optical matched filter. These calculations show that the phase-only and classical matched filters are equally sensitive to scale changes when modest high-pass filtering is used. Computer simulations were done for verification.
Characteristics of pulse-width modulation (PWM) were studied to provide an analog communication alternative to the predominantly digital techniques presently used in fiber optic links. Large analog signal dynamic range and high SNR, coupled with freedom from intermodulation, make PWM attractive for broadband local area network (LAN) applications, including future information-oriented offices and hospitals and automated factory floors. A simple video link was constructed using common laboratory equipment. The performance of the video link supports the PWM theory developed here and elsewhere. Analog transmission capability was experimentally demonstrated. SNR was limited only by the carrier noise, and no evidence of the intermodulation problem was observed.
A very important part of Optical Engineering is the Book Reviews section, in which reviews of usually two or three current books dealing with the general subject of optical engineering are published in each issue. From July 1981 until January 1986, Dr. Joseph L. Horner of Rome Air Development Center, Hanscom Air Force Base, Massachusetts, served as the Editor lof the SPIE Reports portion of Optical Engineering, which contains the Book Reviews section. Since January 1986, when the edito rial organization of the journal was changed, he has continued to serve as Book Reviews Editor. Joe has done a superb job in this volunteer position but has now decided to step down so that he can pursue other activities in SPIE; however, he has agreed to continue his duties as Book Reviews Editor until the reviews he has in progress have been published. On behalf of the officers, governors, and members of the Society, I would like to take this opportunity to express our appreciation to him for a job well done. Thanks, Joe, and best wishes.
This book is Vol I of a series titled Advances in Computer Vision and Image Processing, edited by Prof. Thomas S. Huang of the University of Illinois. This inaugural volume, devoted to problems in signal and image reconstruction in which the data are incomplete, includes contribution from a number of leaders in the field of signal and image processing.