Sensors Unlimited Inc. (SUI), a Collins Aerospace Company, has developed a large-area, high-speed, short-wave infrared (SWIR) focal plane array (FPA) to meet the field-of-view (FOV) and bandwidth requirements of LiDAR applications. Modifications to SUI’s standard InGaAs photodiode array (PDA), include junction shape, dielectric thickness, and contact metallization. These changes allow for a reduction in the effective capacitance seen by the hybridized FPA’s readout integrated circuit (ROIC) while preserving the epitaxial structure that ensures the company’s industry-leading dark current. Compared to SUI’s standard device, significant capacitance reductions have been demonstrated. Enhancements of laser pulse detection performance arising from the capacitance improvement, and suitability of the resulting device for implementation in LiDAR systems, will be discussed.
Sensors Unlimited Inc. (SUI), a Collins Aerospace company, has developed a short wave infrared (SWIR) photodetector device structure using isolated mesa pixels to improve the detector modulation transfer function (MTF), an important parameter in determining the overall image quality of a camera system. A combination of device fabrication and simulation has been used to evaluate the design and manufacturability of various mesa morphologies. Because mesa formation entails both the removal of some portion of the active region of the photodetector and the introduction of non- planar surfaces, any MTF improvement must be balanced against a loss of quantum efficiency (QE) and potentially higher dark current. Focal plane arrays (FPAs) based on the optimal mesa morphology have been fabricated and compared for MTF and QE performance at the camera level to FPAs built using SUI’s standard pixel structure. The mesa structure described herein is implemented on the front side of the photodetector and could also be implemented across all of SUI’s backside-illuminated (i.e., VIS/SWIR, NIR/SWIR, SWIR) structures for applications where a premium is placed on MTF performance.
For ultra-fine pixel pitch focal plane array (FPA) applications, flip-chip hybridization has advantages including high I/O density and short distance between the photodiode array (PDA) and the readout integrated circuit (ROIC). Indium has become the primary interconnect material because of its high ductility at low temperature. Successful mating of large format die becomes increasingly difficult, however, for finer pitch applications where bumps are shorter, as tolerance for bowing is low. Simultaneously, the epoxy filling process for large image format, hybridized focal planes becomes more challenging. These constraints call for tall indium bumps with high aspect ratio to accommodate die bowing and provide larger openings for the flow of fill epoxy. A process for the fabrication of highly uniform, high aspect ratio (height:diameter) indium bumps has been developed by Sensors Unlimited Inc. (SUI), a Collins Aerospace Company. The grain size of the deposited indium metal is minimized by optimizing process parameters as well as introducing intermediate metal layers underneath the indium bumps. Anisotropic deposition has been achieved by optimizing deposition rate and controlling substrate parameters. Indium bumps with aspect ratios over 2:1 and flat bump heads have been achieved. The developed bump process has been successfully applied to the fabrication of high resolution indium gallium arsenide (InGaAs) FPAs. Key control parameters for bump formation will be discussed in this paper.
The increasing demand for short wave infrared (SWIR) imaging technology for soldier-based and unmanned
platforms requires camera systems where size, weight and power consumption are minimized without loss of
performance. Goodrich, Sensors Unlimited Inc. reports on the development of a novel focal plane (FPA) array for
DARPA's MISI (Micro-Sensors for Imaging) Program. This large format (1280 x 1024) array is optimized for
day/night imaging in the wavelength region from 0.4 μm to 1.7 μm and consists of an InGaAs detector bump bonded to a
capacitance transimpedance amplifier (CTIA)-based readout integrated circuit (ROIC) on a compact 15 μm pixel pitch.
Two selectable integration capacitors provide for high dynamic range with low (< 50 electrons) noise, and expanded onchip
ROIC functionality includes analog-to-digital conversion and temperature sensing. The combination of high
quality, low dark current InGaAs with temperature-parameterized non-uniformity correction allows operation at ambient
temperatures while eliminating the need for thermoelectric cooling. The resulting lightweight, low power
implementation is suitable for man-portable and UAV-mounted applications.
There are few choices when identifying detector materials for use in the SWIR wavelength band. We have exploited the
direct-bandgap InGaAs material system to achieve superior room temperature (293°K) dark current. We have
demonstrated sensitivity from 400nm through 2.6um with this material system and thus provide the opportunity to sense
not only the visible, but also the J-band (1.25um), H-band (1.65um) and K-band (2.2um) windows. This paper discusses
the advantages of our hybridized CMOS-InGaAs material system versus other potential SWIR material systems.
The monolithic planar InGaAs detector array enables 100% fill factor and thus, high external quantum efficiency. We
have achieved room-temperature pixel dark current of 2.8fA and shot noise of 110 electrons per pixel per second. Low
dark current at +300K allows uncooled packaging options, affording the system designer dramatic reductions in size,
weight (cameras <28grams), and power (<2.5W). Commercially available InGaAs pin arrays have shown diode lifetime
mean time between failures (MTBF) of 1011hours for planar InGaAs detectors1, far exceeding telecom-grade reliability
requirements. The use of a hybrid CMOS-InGaAs system allows best of breed materials to be used and permits efficient, cost-effective,
volume integration. Moreover, we will discuss how the InGaAsP material system is compatible with CMOS monolithic
integration. Taken together, these advantages, we believe, make InGaAs the obvious choice for all future SWIR