The short wave infrared (SWIR) spectral band is an emerging domain thanks to its large potential. Close to VISible/Near Infrared wavelengths, SWIR images interpretation is made easier for the users. In this spectral region, new opportunities can be found in several fields of applications such as defense and security (night vision, active imaging), space (earth observation), transport (automotive safety), or industry (nondestructive process control, food and plastic sorting). In the frame of this paper, the development of a small pitch InGaAs technology for the SWIR band is described. The objective is to fuel an increase in focal plane arrays (FPA) resolution with the development of a10μm pitch process. A specific Read Out Integrated Circuit (ROIC) has been designed to address a high frame rate and various functions like the selection of lines or specific windowing. The ROIC exhibits 3 different gains. A full set of electro-optical characterizations for a VGA 10μm pitch focal plane array is presented.
The short wave infrared (SWIR) spectral band is an emerging domain thanks to its large potential. Close to VISible/Near Infrared wavelengths, SWIR images interpretation is made easier for the users. In this spectral region, new opportunities can be found in several fields of applications such as defense and security (night vision, active imaging), space (earth observation), transport (automotive safety), or industry (nondestructive process control, food and plastic sorting). In the frame of this paper, two different developments of the InGaAs technology addressing emerging fields of SWIR imaging are described: pixel pitch reduction and multi-spectral imaging. In pixel pitch reduction the obvious objective is to increase the imaging resolution without jeopardizing system cost. Multi-spectral resolution deals, on the other hand, with interposition of pixelated filters in the optical path right onto SWIR focal plane arrays (FPA) to enable a real time spectral analysis of recorded SWIR images.
SOFRADIR is the worldwide leader on the cooled IR detector market for high-performance space, military and security applications thanks to a well mastered Mercury Cadmium Telluride (MCT) technology, and recently thanks to the acquisition of III-V technologies including Indium Gallium Arsenide (InGaAs). As a result, strong and continuous development efforts are deployed to deliver cutting edge products with improved performances in terms of spatial and thermal resolution, dark current, quantum efficiency, low excess noise and operability. Recent years have evidenced significant progress with InGaAs process maturity enabling low noise detectors operable at room temperature, with no cooling requirement. This new generation of so called Tecless devices meets a surging interest for affordable Short-Wave Infrared (SWIR) detectors for use in volume applications such as industrial machine vision, science imaging or surveillance applications. For these systems, the latest uncooled InGaAs sensors allow major cost savings without sacrificing performance. In this talk we will review and analyze the impact of temperature on InGaAs detectors. We will also discuss implementation and optimization tradeoffs which should be considered when using uncooled InGaAs SWIR detectors.
Xenics has designed and manufactured a 1280*1024 pixel, 17 µm pitch InGaAs array for SWIR imaging in the [0.9 - 1.7 µm] range. It will report on the first characterization results of the device. As usual for this type of room temperature operated SWIR image sensors, the detector interface is based on a CTIA stage, yielding excellent linearity, a low detector bias and hence a low and stable dark current combined with low image lag. The charge to voltage conversion factor is 40 µV/e-. The pixel interface scheme contains a CDS circuit in order to reduce the kTC noise and common mode effects. The noise is expected to be below 30 e-rms in linear mode, resulting in a dynamic range < 60 dB. Additionally the linear dynamic range is complemented with a high dynamic range logarithmic response with a saturation level < 5 nA/pixel. The information in the pixel matrix can be read via 2, 4 or 8 outputs, yielding a maximum full frame rate between 50 and 200 Hz. Each output is operating at 40 MHz pixel rate. The outputs are differential with a common mode voltage of 0.9 V and an adjustable output swing of 2 Vptp. Nevertheless the power dissipation shall be below 330 mW.
We report on the design and development of the large linear SWIR focal plane arrays to be deployed in the multispectral
instrument of the Proba-V satellite. These sensors are based on mechanical butting of three InGaAs photodiode arrays
with 1024 pixels on 25 μm pitch, forming a nearly continuous line of 3072 pixels. A new read-out integrated circuit
(ROIC) for photocurrent integration and signal multiplexing with 1024 inputs was designed and manufactured by
stitching due to the length of the chip. The ROIC (XRO3508) includes both correlated double sampling (CDS) and autozero
features, enabling a very low Dark Signal Non Uniformity (DSNU) and Photoresponse Non-Uniformity (PRNU)
less than 0.5% of the available signal range.
InGaAs is the material of preference for uncooled imaging in the [0.9-1.7 μm] SWIR range, as it can be manufactured on
low cost InP substrates in a mainstream technology for optical telecommunications. By removing the substrate the
spectral range can be extended to the [0.6 - 1.7 μm] range. In this way low cost, room temperature operated FPAs
cameras for imaging and hyperspectral applications can be developed. The FPA is built around a low power CTIA stage
with 3 S&H capacitors in the 20*20 um2 unit cell. This approach results in a synchronous shutter operation, which will
support both ITR and IWR operation.
In IWR mode the integration dead time is limited to max. 10 μsec. The CDS operation yields in a high sensitivity
combined with a low noise: This presentation will focus on the development of a 20 μm pitch 320*256 device, with the
following main characteristics: 20 μV/e-sensitivity and < 60 e-noise. The 4 low-power, differential outputs are enabling
to drive an output load of > 30 pF at 40 Msamples/sec each, resulting in a > 1700 Hz frame rate, while at the same time
the overall nominal power dissipation is < 200 mW. The ROIC is realized in a 0.35 um technology and the outputs are
designed to drive directly a 3.3 V, 1.5 V VCM differential AD convertor. The circuit also supports a NDR operating
mode to further reduce the noise of the FPA. A small from factor camera with Cameralink output is built around this
FPA.
A high resolution, high frame rate InGaAs based image sensor and associated camera has been developed. The sensor
and the camera are capable of recording and delivering more than 1700 full 640x512pixel frames per second.
The FPA utilizes a low lag CTIA current integrator in each pixel, enabling integration times shorter than one
microsecond. On-chip logics allows for four different sub windows to be read out simultaneously at even higher rates.
The spectral sensitivity of the FPA is situated in the SWIR range [0.9-1.7 µm] and can be further extended into the
Visible and NIR range.
The Cheetah camera has max 16 GB of on-board memory to store the acquired images and transfer the data over a
Gigabit Ethernet connection to the PC. The camera is also equipped with a full CameralinkTM interface to directly
stream the data to a frame grabber or dedicated image processing unit. The Cheetah camera is completely under
software control.
Short wavelength infrared (SWIR) photovoltaic diode structures made of InGaAs material were grown on GaAs by means of molecular beam epitaxy. Growth quality and composition of the layers are determined by HRXRD. The electrical characterization is performed by Current-Bias characterization (proposal) and spectral resolved measurements to determine the resistance area product (R0A) and the spectral responsivity (R) of diodes. The processing is performed with standard photolithography and micro-structuring techniques aiming at the production of 1D and 2D infrared camera arrays. The diced IR sensor is flip chip assembled on a Silicon read out integrated circuit (ROIC). Linear arrays of 256 pixels with 25 μm pitch were fabricated as well as focal plane arrays (FPA) of 256 × 320 pixel with 30 μm pitch. Measures of electrical interconnection yield will be shown. Functionality is proven for different applications up to 2.5 μm wavelength.
High quality extended-wavelength InGaAs (80% In) has been grown heteroepitaxially on 3'-GaAs substrates by means of Molecular Beam Epitaxy (MBE). In contrast growth on InP, sensor manufacturing on GaAs substrates can be readily scaled to 6'. An additional benefit is the larger cut-off of the 80%- In material (2.5μm vs. 1.7μm). The photodiode layer design was optimized for maximum improvement of dark current with temperature. A high-uniformity and high- yield process was developed to manufacture bondable 128 pixel 50 μm pitch linear array sensors. To this end, a dedicated mesa-type process was developed assuring high uniformity in dark current and photoresponse. The sensors were designed for top-illumination. Sensors have been characterized electrically and optically at the intended thermo-electrical operation temperature (250K). The homogeneity of dark current and photoresponse was also evaluated at 250 K. The improvement in dark current as a function of temperature has been studied in more detail, yielding a zero bias resistance of approximately 2.5 MΩcm2 at 77K. A linear array has been bonded to a commercial CMOS read-out circuit and the functionality is shown. The sensor is suitable for spectroscopic and imaging applications in the short wavelength infrared (SWIR) region. Work is underway to develop sensors with smaller pitch and up to 512 pixels.
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