W. V. McLevige

A novel simultaneous unipolar multispectral integrated technology approach for HgCdTe ir detectors and focal plane arrays

In the last few years Rockwell has developed a novel simultaneous unipolar multispectral integrated HgCdTe detector and focal plane array technology that is a natural and relatively straightforward derivative of our baseline double layer planar heterostructure (DLPH) molecular beam epitaxial (MBE) technology. Recently this technology was awarded a U.S. patent. This simultaneous unipolar multispectral integrated technology (SUMIT) shares the high performance characteristics of its DLPH antecedent. Two color focal plane arrays with low-1013 cm−2s−1 background limited detectivity performance (BLIP D*) have been obtained for mid-wave infrared (MWIR, 3–5 m) devices at T>130 K and for long-wave infrared (LWIR, 8–10 m) devices at T∼80 K.

Micro-optic integration with focal plane arrays

The large detector size of conventional focal plane arrays (FPAs) often acts as a limiting source of noise currents and requires these devices to run at undesirably low temperatures. To reduce the detector size without reducing the detector’s quantum efficiency (QE), we have developed efficient on-focal-plane collection optics consisting of arrays of thin-film binary-optics microlenses and photoresist-based refractive microlenses on the back surface of hybrid detector array structures. Photodiodes of p/n polarity, of an unusual planar-mesa geometry, were fabricated in epitaxial HgCdTe deposited by molecular beam epitaxy (MBE) on the front side of a CdZnTe substrate. Diffractive (8- to 16-phase-level) Ge microlenses were deposited on 48-µm centers in a registered fashion (using an IR mask aligner and appropriate marks on the front surface of the CdZnTe) on the back side of the substrate using a lifting process. The lifting circumvents some of the process limitations of the more conventional chemical etching methods on diffractivemicrolens processing, allowing the microlenses to approach more closely their theoretical efficiency limit of .95%. Photoresist microlenses were fabricated by reflow of photolithographically defined photoresist islands. Prior to microlens deposition, but after diode fabrication, the test structures were flip-chip bonded or ‘‘hybridized’’ using indium interconnections to metallic striplines that had been photolithographically deposited on sapphire dice (a process equally compatible with a siliconintegrated- circuit readout). After hybridization, the CdZnTe was thinned to equal the focal length of the lenses in the CdZnTe material. Optical characterization has demonstrated that the microlenses combined with the detector mesas concentrate light sufficiently to increase the effective collection area. The optical size of the mesa detectors being larger than the theoretical diffraction limit of the microlenses precludes determining whether the lenses themselves produce the theoretical diffraction-limited gain, but they clearly decrease the required detector area by at least 3 to 6 times. To our knowledge, this is the first successful demonstration of IR detectors and binary optics and of photoresist refractive-microlens integration.

Prospects for large-format IR astronomy FPAs using MBE-grown HgCdTe detectors with cutoff wavelength > 4 μm

Rockwell Science Center has developed a double layer planar heterostructure (DLPH) detector array fabrication process with significant advantages over the PACE-1 process now being used to produce 256 X 256 and 1024 X 1024 FPAs for low background IR astronomy. The DLPH detectors are p- on-n photodiodes fabricated in a double layer of wide and narrow bandgap HgCdTe grown by molecular beam epitaxy on CdZnTe substrates. The double layer structure provides superior surface passivation while the lattice matched CdZnTe substrate reduces the defect density. DLPH FPAs have been fabricated in array sizes up to 640 X 480 and with cutoff wavelengths as long as 15 micrometers . Quantum efficiencies are typically in the 0.5 to 0.8 range. For a 256 X 256 array DLPH detectors with 5.3 micrometers cutoff wavelength at 50K, the median dark current was 0.39 e-/sec at 0.5V reverse bias. For 7 of 17 individual DLPH detector with 10.6 micrometers cutoff at 30K, the dark current was less than 104 e-/sec at 20 mV bias. For long cutoff wavelengths, the detector breakdown voltage is too low to permit signal integration directly on the reverse biased detector capacitance. Such detectors require a readout circuit that maintains the detector near zero bias and provides a separate capacitor to store the integrated signal.

Progress in development of H4RG-10 infrared focal plane arrays for WFIRST-AFTA

We describe progress in the development and demonstration of Teledyne’s new high resolution large format FPA for astronomy, the H4RG-10 IR. The H4RG-10 is the latest in Teledyne’s H×RG line of sensors, in a 4096×4096 format using 10 micron pixels. It is offered as a hybrid sensor using either a silicon p-i-n detector array (HyViSI) or a HgCdTe photodiode array with standard infrared cutoff wavelength of 1.75μm, 2.5μm, or 5.3μm (with custom cutoff wavelengths also available). The HgCdTe sensor arrays are fully substrate removed to provide high quantum efficiency, response to visible wavelengths, and minimize cosmic ray and fringing mitigation. Packaging using either CE6 or SiC bases is available. Teledyne is currently fabricating H4RG-10 SWIR FPAs for NASA’s WFIRST space telescope instrument. Initial array performance has been tested and will be presented. Key results include the demonstration of low dark current (array mean dark current of <0.01e-/s/pixel at 100K), low noise (<10 e-/CDS read noise), and high array operability (>99% pixels). The paper discusses the sensor configuration and features, the performance achieved to date including QE, dark current, noise maps and histograms, and the remaining challenges.

Performance of science grade HgCdTe H4RG-15 image sensors

We present the test results of science grade substrate-removed 4K×4K HgCdTe H4RG-15 NIR 1.7 μm and SWIR 2.5 μm sensor chip assemblies (SCAs). Teledyne’s 4K×4K, 15 μm pixel pitch infrared array, which was developed for the era of Extremely Large Telescopes, is first being used in new instrumentation on existing telescopes. We report the data on H4RG-15 arrays that have achieved science grade performance: very low dark current (<0.01 e-/pixel/sec), high quantum efficiency (70-90%), single CDS readout noise of 18 e-, operability >97%, total crosstalk <1.5%, well capacity >70 ke-, and power dissipation less than 4 mW. These SCAs are substrate-removed HgCdTe which simultaneously detect visible and infrared light, enabling spectrographs to use a single SCA for Visible-IR sensitivity. Larger focal plane arrays can be constructed by assembling mosaics of individual arrays.

LWIR, photovoltaic, Hg1-xCdxTe, and FPA performance for remote sensing applications

Focal plane arrays (FPAs), used for remote sensing applications, are required to operate at high temperatures and are subject to high terrestrial background fluxes. Typical remote sensing applications like cloud/weather imagery, sea- surface temperature measurements, ocean color characterization, and land-surface vegetation indices also require FPAs that operate from the visible through the LWIR portion of the spectrum. This combination of harsh requirements have driven the design of a unique LWIR FPA, that operates at 80 K under 300 K background conditions, with an operating spectral range from 11.5 micrometers to 12.5 micrometers , and a spectral cutoff of 13.5 micrometers . The FPA consists of 2 side by side arrays of 1 X 60 HgCdTe, (grown by molecular beam epitaxy) photovoltaic, detector arrays bump bonded to a custom CMOS Si readout. The 2 arrays are completely independent, and can be operated as such. The readout unit cell uses two, current-mode, analog building blocks; a Current Conveyor (CC1) and a dynamic current mirror. The CC1 has input impedance below 300 Ohms and an injection efficiency that is independent of the detector characteristics. This combination extracts high performance and excellent sensitivity from detectors whose average RoA values are approximately 1.7 Ohm-cm2 at T equals 80 K. The dynamic current mirror is used to subtract high background photocurrent while preserving excellent dynamic range. In addition to the performance enhancing readout, the detectors are manufactured with integral microlenses and operated in reverse bias to take advantage of their increased dynamic impedance. The dark currents associated with reverse bias operation are subtracted along with the background photocurrents by the dynamic current mirror. The expected and measured LWIR FPA performance will be presented. Measurements were performed on an LWIR FPA. Expected and measured FPA results are shown in the table below. The expected data are calculated from FPA models and compared to the measured values.

Characteristics and uniformity of group V implanted and annealed HgCdTe heterostructure

This work presents characterization of implanted and annealed double layer planar heterostructure HgCdTe for p-on-n photovoltaic devices. Our observation is that compositional redistribution in the structure during implantation/ annealing process differs from that expected from classical composition gradient driven interdiffusion and impacts the placement of the electrical junction with respect to the metallurgical heterointerface, which in turn affects quantum efficiency and RoA. The observed anomalous interdiffusion results in much wider cap layers with reduced composition difference between base and cap layer composition. The compositional redistribution can, however, be controlled by varying the material structure parameters and the implant/anneal conditions. Examples are presented for dose and implanted species variation. A model is proposed based on the fast diffusion in the irradiation induced damage region of the ion implantation. In addition, we demonstrate spatial uniformity obtained on molecular beam epitaxy (MBE) material of the compositional and implanted species profile. This reflects spatial uniformity of the ion implantation/annealing Processes and of the MBE material characteristics.

VLWIR HgCdTe photovoltaic detectors performance

Very Long Wavelength InfraRed (VLWIR; (lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detector operating in the 78 K range are needed for remote sensing applications. This temperature range permits the use of passive radiators in spacecraft to cool the detectors. VLWIR ((lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detectors in a range of sizes (8 micrometer diameter to 1000 micrometer diameter) have been fabricated and their performance measured as a function of temperature. Molecular Beam Epitaxy (MBE) was used to grow n-type VLWIR Hg1-xCdxTe on lattice matched CdZnTe. Arsenic was implanted and the wafer was annealed to provide the p-type regions. All the material was grown with wider bandgap cap layers and consequently the detector architecture is the Double Layer Planar Heterostructure (DLPH) architecture. Id - Vd versus temperature curves for 8 and 1000 micrometer diameter, (lambda) c equals 17 micrometer at 78 K detectors indicate that the 8 micrometer diameter detector is diffusion limited for temperatures greater than 63 K even at a -200 mV bias. There is no appreciable tunneling at T equals 50 K and at -200 mV applied bias. At T equals 40 K tunneling commences at a bias approximately equals -80 mV. Below T equals 30 K, the diode is tunneling limited. The 1000 micrometer diameter detector is diffusion limited at bias values less than -50 mV at 78 K. At zero bias, the detector impedance is comparable to the series/contact resistance. Interfacing with the low (comparable to the contact and series resistance) junction impedance detector is not feasible. Therefore a custom pre- amplifier was designed to interface with the large VLWIR detectors in reverse bias. The detector is dominated by tunneling currents at temperatures less than 78 K. The 1000 micrometer diameter, (lambda) c approximately equals 17 micrometer at 78 K detectors have dark currents approximately equals 160 (mu) A at a -100 mV bias and at 78 K. Detector non-AR coated quantum efficiency > 60% was measured at -100 mV bias in these large detectors and the response was constant across the (lambda) equals 7 micrometer to 15 micrometer spectral band. With AR- coating the quantum efficiency will be > 70%. Response was measured and non-linearity < 0.15% was calculated for the 1000 micrometer detectors. The flux values were in the 1017 ph/cm2/sec range and were changed by varying the blackbody temperature. In addition, a linear response was measured while varying the spot size incident on the 1000 micrometer detectors. This excellent response uniformity measured as a function of spot size implies that, low frequency spatial response variations are absent, for the 1000 micrometer detectors.

Growth of HgCdTe films on 7x7.5 cm2 CdZnTe substrates for science grade H4RG-15 image sensor applications (Conference Presentation)

HgCdTe films are grown by molecular beam epitaxy (MBE) on large area CdZnTe substrates to achieve low dark current, high quantum efficiency infrared image sensors with 1.7um and 2.5um cut-off respectively. We present the structural and optical characterization of our HgCdTe films with emphasis on spatial uniformity across 7x7.5cm2 wafer size. Science grade detectors are fabricated on these films and subsequently hybridized to our H4RG-15 4K x 4K readout integrated circuit (ROIC). Test results from these image sensors show low dark current ( 99%), less than 1.0% cross talk and a well capacity larger than 70,000e-. the operation temperature is between 80-110K. These image sensors are also responsive in the visible-IR region due removal of the CdZnTe substrate after hybridization. This feature enables spectrographs to use a single image sensor for both visible and IR regions. These image sensors are developed for extremely large telescopes and used in various telescopes around the world.

HgCdTe detectors for infrared remote sensing applications

Remote sensing applications including the National Polar Orbiting Environmental Satellite System (NPOESS) require imaging in a multitude of infrared spectral bands, ranging from the 1.58 micrometer to 1.64 micrometer VSWIR band to the 11.5 micrometer to 12.5 micrometer LWIR band and beyond. These diverse spectral bands require high performance detectors, operating over a range of temperatures; room temperature for the VSWIR band 100 K for MWIR, LWIR and VLWIR, these needs can all be met using molecular beam epitaxy (MBE) to grow HgCdTe. The flexibility inherent in the MBE growth technology is its ability to vary the HgCdTe materials bandgap within a growth run and from growth run to growth run, a capability necessary for remote sensing applications that require imaging in a wide variety of spectral bands. This bandgap engineering flexibility also permits tailoring the device architecture to the various specific system requirements. This paper combines measured detector optical and electrical data, with noise model estimates of ROIC performance to calculate signal to noise ratio (SNR), D* or noise equivalent temperature difference (NE(Delta) T), for each spectral band. The SNR, D* and/or NE(Delta) T are calculated with respect to system focal plane specifications, as required for the meteorological NPOESS.