This paper is a status report on recent scientific CMOS imager developments since when previous publications were
written. Focus today is being given on CMOS design and process optimization because fundamental problems affecting
performance are now reasonably well understood. Topics found in this paper include discussions on a low cost custom
scientific CMOS fabrication approach, substrate bias for deep depletion imagers, near IR and x-ray point-spread
performance, custom fabricated high resisitivity epitaxial and SOI silicon wafers for backside illuminated imagers,
buried channel MOSFETs for ultra low noise performance, 1 e- charge transfer imagers, high speed transfer pixels, RTS/
flicker noise versus MOSFET geometry, pixel offset and gain non uniformity measurements, high S/N dCDS/aCDS
signal processors, pixel thermal dark current sources, radiation damage topics, CCDs fabricated in CMOS and future
large CMOS imagers planned at Sarnoff.
MCT was discovered in the UK in 1958. This paper reviews key developments in the research and development of the
material and devices from the early days to the present. The growth of the material by Bridgman, LPE and MOVPE
methods is described. Fabrication techniques are described for SPRITES, two dimensional and long linear "loophole"
diode arrays and the more recent wafer scale technologies for very large arrays. The use of multilayer heterostructures in
Auger-suppressed diodes, two-colour detectors and negative luminescence devices is outlined. A brief glimpse of the
future potential for the growth of MCT directly onto silicon circuits is given.
A new class of CMOS imagers that compete with scientific CCDs is presented. The sensors are based on deep depletion
backside illuminated technology to achieve high near infrared quantum efficiency and low pixel cross-talk. The imagers
deliver very low read noise suitable for single photon counting - Fano-noise limited soft x-ray applications. Digital
correlated double sampling signal processing necessary to achieve low read noise performance is analyzed and
demonstrated for CMOS use. Detailed experimental data products generated by different pixel architectures (notably
3TPPD, 5TPPD and 6TPG designs) are presented including read noise, charge capacity, dynamic range, quantum
efficiency, charge collection and transfer efficiency and dark current generation. Radiation damage data taken for the
imagers is also reported.
There are five bolometric detector arrays for the SPIRE instrument on board of the Herschel Space Observatory. Our first report (Nguyen et al., 2004) presented the measurement of the two spectroscopic detector arrays. In this paper, we report the performance of the remaining three units for the Photometer, including the photometric long, medium and short wavelength (PLW, PMW and PSW). We note that all five SPIRE detector arrays meet the requirement.
In depth characterization of CMOS arrays is unveiling many characteristics not observed in CCD imagers. This paper is
the first of a series of papers that will discuss unique CMOS characteristics related to fundamental performance
differences between CMOS and CCD imagers with emphasis on scientific arrays. The first topic will show that CMOS
read noise is ultimately limited by a phenomenon referred to as random telegraph signal (RTS) noise. RTS theory and
experimental data discuss its creation, time and frequency domain characteristics, wide variance from pixel to pixel and
magnitude on the sensor's overall read noise floor. Specific operating parameters that control and lower RTS noise are
identified. It is shown how correlated double sampling (CDS) signal processing responds to RTS noise and demonstrate
that sub electron CMOS read noise performance is possible. The paper also discusses CMOS sensitivity (V/e-)
nonlinearity, an effect not familiar to CCD users. The problem plays havoc on conventional photon transfer analysis that
leads to serious measurement errors. New photon transfer relations are developed to deal with the problem. Nonlinearity
for custom CMOS pixels is shown to be beneficial for lowering read noise and extending dynamic range. The paper
closes with a section on the high performance CMOS array used to generated data products presented.
We present results of CCD radiation testing for a proposed Jovian mission. Samples of two candidate star tracker CCDs were irradiated with 10-MeV and 50-MeV electrons at Rensselaer Polytechnic Institute's Gaerttner LINAC. Differences in displacement damage effects on CCD parameters and star tracker performance are discussed for these two energies. Dark current, charge transfer efficiency (CTE), hot pixels, and flat-band voltage shifts are examined. Our electron data is compared to proton irradiation data taken by other experimenters. 10-MeV electron-induced transient data are also discussed.
The large imaging format, high sensitivity, compact size, and ease of operation of silicon-based sensors have led instrument designers to choose them for most visible-light imagers and spectrometers for space-based applications. This will probably remain the case in the near future. In fact, technologies presently under development will tend to strengthen the position of the silicon-based sensors. CCD-CMOS hybrids currently being developed may combine the advantages of both imagers and new high-gain amplifiers and could permit photon- counting sensitivity even in large-format imagers. Back- illumination potentially enables silicon detectors to be used for photometry and imaging applications for which front- illuminated devices are poorly suited. Successful detection by back illumination requires treatment of the back surface using techniques such as delta doping. Delta-doped CCDs were developed at the Microdevices Laboratory at the Jet Propulsion Laboratory in 1992. Using molecular beam epitaxy, fully- processed thinned CCDs are modified for UV enhancement by growing 2.5 nm of boron-doped silicon on the back surface. Named delta-doped CCDs because of the sharply-spiked dopant profile in the thin epitaxial layer, these devices exhibit stable and uniform 100% internal quantum efficiency without hysteresis in the visible and ultraviolet regions of the spectrum. In this paper we will discuss the performance of delta-doped CCDs in UV and EUV, applicability to electron- bombarded CCD (EBCCD), our in-house thinning capability, and bonding approaches for producing flat focal plane arrays. Recent activities on the extension of delta-doping to other imaging technologies will also be presented.
A versatile post-fabrication process to produce thinned, flat, back-illuminated charge-coupled devices (CCDs) has been developed at Jet Propulsion Laboratory's Microdevices Laboratory. This technique is compatible with many ultraviolet enhancement treatments and has been demonstrated with the delta doping process. The significance of this demonstration is that thinned, robust, and flat CCDs are produced without the use of epoxies or waxes using temperatures and materials that are compatible with standard CCD fabrication and delta doping processes. In our approach, the CCD is attached by thermocompression bonding to a specially-designed silicon substrate using gold-gold diffusion bonding prior to thinning. CCDs with optically flat membranes (10 - 20 micrometers ) were produced with excellent yield. These flat CCDs have been successfully delta doped. We will discuss the process of producing thinned flat CCDs, their delta doping, and our results to date.
There is a large variety of mining and manufacturing operations where process monitoring and control can benefit from on-site analysis of both chemical and mineralogic constituents. CHEMIN is a CCD-based instrument capable of both X-ray fluorescence (XRF; chemical) and X-ray diffraction (XRD; mineralogic) analysis. Monitoring and control with an instrument like CHEMIN can be applied to feedstocks, intermediate materials, and final products to optimize production. Examples include control of cement feedstock, of ore for smelting, and of minerals that pose inhalation hazards in the workplace. The combined XRD/XRF capability of CHEMIN can be used wherever a desired commodity is associated with unwanted constituents that may be similar in chemistry or structure but not both (e.g., Ca in both gypsum and feldspar, where only the gypsum is desired to make wallboard). In the mining industry, CHEMIN can determine mineral abundances on the spot and enable more economical mining by providing the means to assay when is being mined, quickly and frequently, at minimal cost. In manufacturing, CHEMIN could be used to spot-check the chemical composition and crystalline makeup of a product at any stage of production. Analysis by CHEMIN can be used as feedback in manufacturing processes where rates of heating, process temperature, mixture of feedstocks, and other variables must be adjusted in real time to correct structure and/or chemistry of the product (e.g., prevention of periclase and alkali sulfate coproduction in cement manufacture).
The use of multilayer heterostructures based on the narrow-gap semiconductor materials InSb/InAlSb and HgCdTe is leading to a range of IR and other devices which can operate without cooling. Work in DERA will be reviewed which has demonstrated uncooled detectors out to 12 micrometer; uncooled infrared LEDs for the 3 - 12 micrometer region, employing either positive or negative luminescence; diode injection lasers with output between 3.9 micrometer and 5.1 micrometer (operating up to 150 k); and uncooled very high speed, very low voltage transistors.
The incorporation of non-imaging optical concentrations in uncooled mid-IR LEDs is described. Novel micromachining methods are used to produce optical concentrators in the growth substrate of epitaxial InSb/InAlSb heterostructures. Resultant large area LED arrays, displaying both positive and negative luminescence, are shown to have optical gains of 3.5 over conventional mesas made form the same material. The LED technology shown also relies on the micromachined substrate being transparent to 3-5 micrometers radiation and to act as a low resistance common contact. The use of degenerate doping in InSb is described, resulting in a shift in the room-temperature transmission to the 3-5micrometers atmospheric window and providing high electrical conductivities.
We describe uncooled mid-IR light emitting and negative luminescent diodes made form indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The application of these devices to gas sensing, improved thermal imagers and imager testing is discussed.
Delta-doped CCDs have achieved stable quantum efficiency, at the theoretical limit imposed by reflection from the Si surface in the near UV and visible. In this approach, an epitaxial silicon layer is grown on a fully-processed CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30 percent of a monolayer of boron atoms are deposited nominally within a single atomic layer, resulting in the effective elimination of the backside potential well. In this paper, we will briefly discuss delta-doped CCDs and their application of low-energy electron detection. We show that modification of the surface this way can greatly improve sensitivity to low-energy detection. We show that modification of the surface this way can greatly improve sensitivity to low-energy electrons. Measurements comparing the response of delta-doped CCDs with untreated CCDs were made in the 50 eV-1.5 keV energy range.For electrons with energies below 300 eV, the signal from untreated CCDs was below the detection limit for our apparatus, and data are presented only for the response of delta-doped CCDs at these energies. The effects of multiple electron hole pair production and backscattering on the observed signals are discussed.
Multilayer, epitaxial, heterostructure devices have been fabricated in In1-xAlxSb by molecular beam epitaxy and in Hg1-xCdxTe by metallo-organic vapor phase epitaxy. The principal motivation was to produce devices which will operate with little or no cooling. Results are presented for InSb and MCT diode detectors operating in both equilibrium and non-equilibrium modes at ambient and near ambient temperatures. An uncooled MCT detector has demonstrated near shot-noise limited detection of carbon- dioxide laser radiation in a heterodyne receiver. Uncooled, light-emitting diodes have demonstrated useful power outputs in both positive and negative luminescence at wavelengths out to 11 micrometers. A diode injection laser has been demonstrated in InSb giving an output at 5.1 micrometer and 90 K.
This paper will describe in some detail a new large area CCD image sensor designed specifically to be used either as a single imager or assembled in large, tightly configured mosaics of CCDs. The device has 2048 X 4096, 15 micrometers pixels. Performance data are presented on both front- and back-illuminated parts.
Seven new CCDs are presented. The devices will be used in a variety of applications ranging from generating color cinema movies to adaptive optics camera systems to compensate for atmospheric turbulence at major astronomical observatories. This paper highlights areas of design, fabrication, and operation techniques to achieve state-of-the-art performance. We discuss current limitations of CCD technology for several key parameters.
Recent advances in the growth of cadmium mercury telluride (Hg1-xCdxTe or MCT) by metal organic vapor phase epitaxy (MOVPE) allow the fabrication of advanced device structures where both the alloy composition x and the doping concentration can be accurately controlled throughout the epitaxial layer. For p-type doping using arsenic, the acceptor concentration can be varied from 5 X 1015 cm-3 to 4 X 1017 cm-3 and for n-type doping using iodine, the donor concentration can be varied from 1 X 1015 cm-3 to 2 X 1017 cm-3. A number of diode arrays have been fabricated in this material and their properties assessed at 77 K, 195 K and 295 K. It has been found that the diffusion currents are at least ten times lower than in homojunctions. In addition, the devices exhibit negative resistance at temperatures above 190 K due to auger suppression. The successful demonstration of auger suppression in these structures has greatly improved the diode leakage currents at room temperature and will enable the development of new devices such as a room temperature laser detector.
We present data from a charge-coupled device (CCD), collaboratively designed by PSU/JPL/Loral, which incorporates several novel features that make it well suited for soft X-ray spectroscopy. It is a three-phase, front-side illuminated device with 1024x1024 pixels. Each pixel is 18 microns by 18 microns.The device has four output amplifiers: two conventional floating diffusion amplifiers (FDAs) and two floating gate amplifiers (FGAs). The FGA non-destructively samples the output charge, allowing the charge in each pixel to be measured multiple times. The readnoise of a given pixel is reduced as the square root of the number of readouts, allowing one to reduce the amplifier noise of these devices to well below the 1/f knee. We have been able to achieve sub-electron readnoise performance with the floating gate amplifier (0.9 e+-) rms with 16 reads per pixel). Using the FGA, the measured energy resolution at 5.9 keV is 120 eV (FWHM). The CCD also has a thin poly gate structure to maximize soft X-ray quantum efficiency. Two-thirds of the active area of the chip is covered only by an insulating layer (1000 angstrom) and a thin poly silicon electrode (400 angstrom). This design enhances the soft X-ray quantum efficiency, but retains the excellent charge transfer efficiency and soft X-ray charge collection efficiency of front-side illuminated devices. The measured energy resolution at 277 eV is 38 eV (FWHM) with a measured quantum efficiency of 15%. We also show that this device performs well below 100 eV, as demonstrated by the detection of Al L fluorescence at 72 eV with a measured FWHM of 16 eV.
The charge-coupled device (CCD) camera of the Soft X-ray Telescope (SXT) for the Japanese Solar-A Mission utilizes a 1024 X 1024 virtual phase CCD manufactured by Texas Instruments in Japan. This sensor will be subject to radiation in the form of trapped protons from the earth''s radiation belts and soft x-rays (0.2-4 keV) in the solar image. Proton damage produces ''dark spikes'' or pixels of enhanced dark current. This can be characterized in terms of the average increase in dark current as a function of proton fluence and predicted through proton transfer calculations. During the preparation of this camera it has been discovered that exposure to soft x-rays creates ''permanent'' ionization damage in the gate insulator, resulting in flat-band shift, dark current increase, loss of charge transfer efficiency, and, ultimately, total unpinning of the sensor. It has been found that ultra-violet, and to a lesser degree, visible-light flooding photo-emits free electrons into the gate oxide which ''anneals'' the damage, restoring proper operation of the CCD.
Recent analytical and experimental work has provided new insights into the production of damage sites in silicon Charge-Coupled Devices (CCDs) by energetic particles and into the effects of these sites on CCD performance. An approximate correlation is presented between experimental results and a prediction of proton-induced displacement damage, and possible explanations for remaining inconsistencies are discussed. As a consequence of this agreement, it is now possible to predict the effect of complicated space proton environments upon CCD charge transfer efficiency and other CCD performance parameters. This prediction requires evaluation of the damage resulting from only a small number (
InSb and related ternary alloys have many potential applications in addition to the conventional one of infrared detection provided that near ambient temperature operation can be achieved. The growth by MBE of n-type and p-type InSb has been established using silicon and beryllium dopants respectively. Multilayer diode structures have been studied up to 300K in order to determine carrier generation mechanisms and examine concepts for ambient temperature operation.
This paper reports on two new advancements in CCD technology. The first area of development has produced a special purpose CCD designed for ultra low-signal level imaging and spectroscopy applications that require sub-electron read noise floors. A nondestructive output circuit operating near its 1/f noise regime is clocked in a special manner to read a single pixel multiple times. Off-chip electronics average the multiple values, reducing the random noise by the square-root of the number of samples taken. Noise floors below 0.5 electrons rms are reported. The second development involves the design and performance of a high resolution imager of 4096 x 4096 pixels, the largest CCD manufactured in terms of pixel count. The device utilizes a 7.5-micron pixel fabricated with three-level poly-silicon to achieve high yield.