KEYWORDS: Cameras, Modulation transfer functions, Mid-IR, Sensors, Reliability, Staring arrays, Accelerated life testing, Simulation of CCA and DLA aggregates, Failure analysis, Temperature metrology
Leonardo DRS (DRS) has developed High Operating Temperature (HOT) HgCdTe detector material, small-pitch focal plane arrays and ultra-compact Dewar/cooler assemblies. These breakthrough technologies are integrated into a high-definition version of our micro camera core. This report provides an update on the performance, reliability and maturity of the HexaBlu 1280 × 960, 6 μm pitch midwave camera core. The HexaBlu weighs <0.65 lbs., displaces <80 cm3, and achieves a Noise Equivalent Temperature Difference (NETD) of 27 mK at f/2.62 with full well at a 335 K scene. This sensitivity is complemented by a Modulation Transfer Function (MTF) within 2% of theoretical for DRS’s detector architecture. Together, low NETD and high MTF enable sharp longrange imagery. The Camera Core dissipates 4.5 W and achieves cooldown in ~2.5 minutes at 25°C, while providing onboard non-uniformity correction and bad pixel replacement. DRS predicts a mean time between failure of 23,900 hours in an environment cycling between 15°C and 64°C. In ongoing accelerated life testing, the cooler has averaged over 27,000 failure free hours in a 72 hour thermal cycle, and the Dewar has averaged 100,000 failure free hours at ≥70°C ambient. Now in limited rate production, the HexaBlu® leverages 95% design commonality with the Dewar/cooler assembly in fielded weapon sights.
High Density Vertically Integrated Photodiodes (HDVIP) MWIR detectors were fabricated in LPE-grown Mercury Cadmium Telluride material. Devices were fabricated with two different acceptor level concentrations. The low doped n-region was held at a single concentration but the dimensions are tailored to simultaneously maintain high quantum efficiency while minimizing dark current and 1/f noise. Since this study target was for operating at high temperatures, detector I-V data was collected between 120 K and 280 K for I-Vs and 180 to 280 K for noise to understand current mechanisms that limit device performance at these elevated temperatures. Noise as a function of frequency has also been collected over the same temperature range. 1/f noise has also been modeled for MWIR detectors as a function of temperature and will be covered.
Results of characterization data on linear mode photon counting (LMPC) HgCdTe electron-initiated avalanche photodiode (e-APD)focal plane arrays (FPA) are presented that reveal an improved understanding and the growing maturity of the technology. The first successful 2x8 LMPC FPA was fabricated in 2010 [1]. Since then a process validation lot of 2x8 arrays was fabricated. Five arrays from this lot were characterized that replicated the previous 2x8 LMPC array performance. In addition, it was unambiguously verified that readout integrated circuit (ROIC) glow was responsible for most of the false event rate (FER) of the 2010 array. The application of a single layer metal blocking layer between the ROIC and the detector array and optimization of the ROIC biases reduced the FER by an order of magnitude. Photon detection efficiencies (PDEs) of greater than 50% were routinely demonstrated across 5 arrays, with one array reaching a PDE of 70%. High resolution pixel-surface spot scans were performed and the junction diameters of the diodes were measured. The junction diameter was decreased from 31 μm to 25 μm resulting in a 2x increase in E-APD gain from 470 on the 2010 array to 1100 on one of the 2013 FPAs. Mean single photon signal to noise ratios of >12 were demonstrated at excess noise factors of 1.2-1.3. NASA Goddard Space Flight Center (GSFC) performed measurements on the delivered FPA that verified the PDE and FER data.
Staring infrared focal plane arrays (FPAs) require pixel-level, three-dimensional (3D) integration with silicon readout
integrated circuits (ROICs) that provide detector bias, integrate detector current, and may further process the signals.
There is an increased interest in ROIC technology as a result of two trends in the evolution of infrared FPAs. The first
trend involves decreasing the FPA pixel size, which leads to the increased information content within the same FPA die
size. The second trend involves the desire to enhance signal processing capability at the FPA level, which opens the door
to the detector behaving like a smart peripheral rather than a passive component—with complex signal processing
functions being executed on, rather than off, the FPA chip. In this paper, we review recent advances in 3D integration
process technologies that support these key trends in the development of infrared FPAs. Specifically, we discuss
approaches in which the infrared sensor is integrated with 3D ROIC stacks composed of multiple layers of silicon
circuitry interconnected using metal-filled through-silicon vias. We describe the continued development of the 3D
integration technology and summarize key demonstrations that show its viability for pixels as small as 5 microns.
Infrared detector pixel pitch has been decreasing, driven by interest in higher resolution, larger displays, and decreased cost. Previous generations of focal plane arrays (FPAs) were on 50, 40, 30, and 20μm pitch. 12μm pitch FPAs are now available. DRS Network and Imaging Systems has developed ultra-small 5μm pitch infrared detectors for the long-wave infrared (LWIR) and medium-wave infrared (MWIR) bands as part of the DARPA AWARE Lambda Scale effort. The smaller pitch was achieved using DRS’ high-density vertically integrated photodiode (HDVIP®) architecture. This technology is a major advance in the state of the art for infrared imaging sensors. The pixel density of 4 million pixels/cm2 enables the production of lower cost FPAs from HDTV resolution up to many millions of pixels. Dark current, collection efficiency, cross-talk, and operability are similar to larger pitch HDVIP FPAs.
Over the past decade, the development of infrared focal plane arrays (FPAs) has seen two trends: decreasing of the pixel
size and increasing of signal-processing capability at the device level. Enabling more capability within smaller pixels
can be achieved through the use of advanced wafer-level processes for the integration of FPAs with silicon (Si) readout
integrated circuits (ROICs). In this paper, we review the development of these wafer-level integration technologies,
highlighting approaches in which the infrared sensor is integrated with three-dimensional ROIC stacks composed of
multiple layers of Si circuitry interconnected using metal-filled through-silicon vias.
A 16 element HgCdTe e-APD detector has been developed for lidar receivers that has significant improvements in
sensitivity in the spectral range from < 1μm to 4 μm. A demonstration detector consisting of a 4x4 APD detector array, with 80 μm square elements, a custom CMOS readout integrated circuit (ROIC), a closed cycle cooler-Dewar, and
support electronics has been designed, fabricated, and tested. The custom ROIC design provides > 6 MHz bandwidth
with low noise and 21 selectable gains. Ninety-six arrays were fabricated with 69% of the arrays meeting the dark
current spec in the center 4 pixels at 10 V bias where the APD gain was expected to be around 150. Measurements to 12
V on one array showed APD gains of 654 with a gain normalized dark currents of 1.2 fA to 3.2 fA. The lowest dark
current array showed a maximum dark current of 6.2 pA at 10 V and 77 K. The 4.4 μm cutoff detector was characterized
at an operating temperature of 77K with a 1.55 μm, 1μs wide, laser pulse. The photon conversion efficiency at unity
gain was 91%. The mean measured APD gain at 77 K was 308 at 11V, the responsivity was 782 μV/pW, the average
NEP was 1.04 fW/Hz1/2. The bandwidth was 6.8 MHz, and the broadband NEP was 2.97 pW. This detector offers a
wide spectral response, dynamic range, and substantially improved sensitivity and lifetime for integrated path
differential absorption (IPDA) lidar measurements of atmospheric trace gases such as CO2 and CH4.
KEYWORDS: Sensors, Modulation transfer functions, Monte Carlo methods, Data modeling, Mercury cadmium telluride, Diffusion, Fourier transforms, Short wave infrared radiation, Photons, Metals
The photocurrent of High Density Vertically Integrated Photodiodes (HDVIP) manufactured in LPE grown SWIR
(λc ~ 2.5 μm) HgCdTe material is modeled as a function of incident spot location using a Monte Carlo diffusion
calculation in the p-type bulk. The Monte Carlo calculation assumes a 3 x 3 mini-array of detectors surrounded by
guard detectors. Carriers generated in the n-regions are always collected. The result is a responsivity map that yields
the individual detector "spot scan" profile that is then used to calculate the detector modulation transfer function
(MTF). Fourier transforms of detector "spot scan" response profile provided experimental confirmation of MTF that
corresponded to the Monte Carlo modeled MTF.
Detectors that have broadband response from the visible (~ 400 nm) to near infrared (~ 2.5 μm) have remote
sensing hyperspectral applications on a single chip. 2.2 and 2.5 μm cutoff detectors permit operation in the 200
K range. The DRS HDVIP detector technology is a front side illuminated detector technology. Consequently,
there is no substrate to absorb the visible photons as in backside-illuminated detectors and these 2.2 and 2.5-μm-cutoff
detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated
using CdTe that absorbs the visible light photons. CdTe with a direct bandgap ~ 1.6 eV strongly absorbs photons
of wavelength shorter than about 800 nm. Detectors in 320 x 6 arrays with varying thickness of CdTe
passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. The SWIR
HDVIP detectors have well known high quantum efficiency (QE) in the near infrared region. Focus here was in
acquiring array level data in the visible region of the spectrum. 320 x 6 FPA QE and NEI data was acquired
using a 642 nm narrow band filter with 50 % points at 612 nm and 698 nm. The array QE average is ~ 70 % for
the array with CdTe passivation thickness = 44.5 nm. The NEI is ~ 5 x 1010 ph/cm2/s at a flux Φ = 5.36 x 1013
ph/cm2/s. QE for an array with CdTe passivation thickness = 44.5 nm is ~ 10 % higher than an array with CdTe
passivation thickness = 79.3 nm. In addition, a model that takes into account the complex optical properties of
every layer in the HDVIP photodiode architecture was developed to predict the QE of the detectors in the near
infrared and visible wavelength regions as a function of CdTe thickness. Measured QE as a function of
wavelength is not a good match to the model QE probably due to limitations in the measured QE and knowledge
of optical constants that are input into the model.
The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection
avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are
related to the to the collection region fill factor. A 2D diffusion model calculates the time
dependent response and steady state pixel point spread function for cylindrical diodes, and
predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system
was developed for point spread function and crosstalk measurements at 80 K. An electron
diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown
that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current
data, at high gain levels, indicate an effective gain normalized dark density count as low as
1000 counts per μs per cm2 at an APD gain of 444. A junction doping profile was
determined from capacitance-voltage data. Spectral response data shows a gain independent
characteristic.
DRS LPE-grown SWIR, MWIR and LWIR HgCdTe material are fabricated in the High-Density Vertically
Integrated Photodiode (HDVIP) architecture. Instruments manufactured for certain strategic applications have
severe constraints on excess low frequency noise due to the effect the noise has on the image quality with
subsequent consequences on the period of calibration. This paper will present data and analysis of excess low
frequency noise in LWIR (&lgr;c ~ 10.5 &mgr;m @ 60 K) HDVIP HgCdTe detectors.
The vehicle for noise measurements is a multiplexed 320 x 6 array of 40 &mgr;m x 50 &mgr;m, 10.5 &mgr;m cutoff, HgCdTe
detectors. Noise has been measured on a column of 320 detectors, at 60 K, as a function of frequency at zero and 50
mV reverse bias. Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled
every 10th or every 100th frame. 32,768 frames of time series data were collected for a total record length of 98
minutes. Since the total time for collecting the 32,768 time data series points is 98 minutes, the minimum frequency
is 170 &mgr;Hz. Time series and Fourier transform data on individual detectors at 0 mV and 50 mV reverse bias in the
dark have been studied. Examination of the detector current time series and Fourier transform curves thereof, reveal
a variety of interesting characteristics: (i) time series displaying switching between four states characteristic of
random telegraph signal (RTS) noise, the noise current power spectrum having Lorentzian type characteristics; (ii)
time series data exhibiting slight wave-like characteristics with the noise current power spectrum being 1/f-like at
low frequencies; (iii) pronounced wave-like characteristics in the time series with the noise current power spectrum
being 1/f2-like at low frequencies; and (iv) time series having a mean value independent of time with the noise
current power spectrum being white. The predominance of detectors examined had minimal excess low frequency
noise down to ~ 10 mHz. In addition some isolated diodes had characteristics that lay between the four main types
outlined above.
Military applications demand more and more complex, multifunctional microsystems with performance characteristics which can only be achieved by using best-of-breed materials and device technologies for the microsystem components. Three-dimensional (3-D) integration of separate, individually complete device layers provides a way to build complex microsystems without compromising the system performance and fabrication yield. In the 3-D integration approach, each device layer is fabricated separately using optimized materials and processes. The layers are stacked and interconnected through area array vertical interconnects with lengths on the order of just tens of microns. This paper will review recent advances in development of 3-D integration technologies with focus on those which enable integration of heterogeneous materials (e.g. HgCdTe FPAs with silicon ROICs) or heterogeneous fabrication processes (e.g. resistive IR emitters with RIICs).
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array,
integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a
hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable
Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the
detector signal as well as to electrically tune the filter across the wavelength band. We report the first such
demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed
for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting
and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that
permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode
(EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200
K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity
and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will
form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width
at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully
integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of
approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP
detectors exhibit FWHM of 140 nm.
DRS uses LPE-grown SWIR, MWIR and LWIR HgCdTe material to fabricate High-Density Vertically Integrated
Photodiode (HDVIP) architecture detectors. 2.5 μm, 5.3 μm and 10.5 μm cutoff detectors have been fabricated into
linear arrays as technology demonstrations targeting remote sensing programs. This paper presents 320 x 6 array
configuration technology demonstrations' performance of HDVIP HgCdTe detectors and single detector noise data. The
single detector data are acquired from within the 320 x 6 array. Within the arrays, the detector size is 40 μm x 50 μm.
The MWIR detector array has a mean quantum efficiency of 89.2% with a standard deviation to mean ratio, σ/μ = 1.51%. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz.
Operability values exceeding 99.5% have been obtained. The LWIR arrays measured at 60 K had high operability with
only ~ 3% of the detectors having out of family response. Using the best detector select (BDS) feature in the read out
integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with
100% operability is obtained. For the 320 x 1 array constituted using the BDS feature, a 100% operable LWIR array
with average NEI value of 1.94 x 1011 ph/cm 2/s at a flux of 7.0 x 1014 ph/cm2/s has been demonstrated.
Noise was measured at 60 K and 50 mV reverse bias on a column of 320 diodes from a 320 x 6 LWIR array.
Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled every 100th frame.
32,768 frames of time series data were collected for a total record length of 98 minutes. The frame average for a
number of detectors was subtracted from each detector to correct for temperature drift and any common-mode noise.
The corrected time series data was Fourier transformed to obtain the noise spectral density as a function of frequency.
Since the total time for collecting the 32,768 time data series points is 98.0 minutes, the minimum frequency is 170 μHz.
A least squares fit of the form (A/f + B) is made to the noise spectral density data to extract coefficients A and B that
relate to the 1/f and white noise of the detector respectively. In addition noise measurements were also acquired on
columns of SWIR detectors. Measurements were made under illuminated conditions at 4 mV and 50 mV reverse bias
and under dark conditions at 50 mV reverse bias. The total collection time for the SWIR detectors was 47.7 minutes.
The detectors are white noise limited down to ~10 mHz under dark conditions and down to ~ 100 mHz under
illuminated conditions.
Remote sensing programs require detectors with a variety of wavelengths. One example of remote sensing applications is the GOES-ABI program that requires linear arrays of detectors with cutoff wavelengths ranging from the visible to the VLWIR (λc ~ 15 μm). In order to target the variety of remote sensing applications, an internal task was conducted to develop detectors and linear arrays operating under nominal remote sensing applications. SWIR [λc(295 K) ~ 2.5 μm] test detectors have been measured as a function of temperature between 170 K and 295 K. At 200 K the RoA values are in the 106 ohm-cm2 range. MWIR [λc(60 K) = 5.3 μm] and LWIR [λc(60 K) = 10.5 μm] HgCdTe detectors in a 320 x 6 array format have also been measured at 60 K. Within the arrays, the detector size is 40 μm x 50 μm. The MWIR detector array has a mean quantum efficiency of 89.2 % with a standard deviation to mean ratio, σ/μ = 1.51 %. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz. Operability values exceeding 99.5 % have been obtained. In addition, test diodes at the edge of the array that did not go through a read out integrated circuit (ROIC) were also measured and had quantum efficiency ~ 86 % that agreed well with the ~ 87 % quantum efficiency measured for detectors in the array that were located near the test detectors. The LWIR arrays, measured at 60K also had high operability with only ~ 3 % of the detectors having out of family response. Using best detector select (BDS) feature in the read out integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with 100 % operability is obtained. For the 320 x1 array constituted using the BDS feature, a 100 % operable LWIR array with average NEI value of 1.94x1011 ph/cm2/s at a flux of 7.0x1014 ph/cm2/s has been demonstrated.
A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.
A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.
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