KEYWORDS: Aerospace engineering, Photodetectors, Diffusion, Modulation transfer functions, Avalanche photodetectors, Readout integrated circuits, Personal digital assistants, Metals, Short wave infrared radiation, Scanning electron microscopy
Increasing shortwave infrared (SWIR) sensor performance requirements have pushed traditional HOT detector technologies to their limits. Collins Aerospace Princeton, a Raytheon Technologies (RTX) Company, has answered this call by looking beyond dark current reduction, and leveraging its onshore foundry capabilities to develop unprecedented, high performance photodetector array (PDA) technologies to better serve both passive and active imaging applications. In this work, Collins Aerospace Princeton offers an update on several previously presented, PDA-specific development fronts as well as offering introductions into other novel efforts. In addition to presenting current state-ofthe-art (SOA) InGaAs dark current performance, results related to mesa-structure PDAs for modulation transfer function (MTF) improvement and hybridization capacitance reduction for active imaging noise equivalent irradiance (NEI) improvement are offered. Additionally, focal plane array (FPA) interconnection improvement techniques and results for yield improvement and cost reduction are presented. Finally, results related to the three most advanced and nascent development tracks, avalanche photodiode (APD), PDA metallization and single side bumped FPA, are introduced. Collins Aerospace Princeton’s APD technology is in direct response to the challenging size, weight, and power (SWaP) and NEI performance requirements of active imaging applications. Similarly, Collins Aerospace Princeton’s PDA metallization technology, inspired by silicon-based brethren, facilitates greater integration capability on the PDA itself, which subsequently allows for greater functionality and performance at every pixel location. The most recently developed single side bumped FPA will dramatically improve operability with reduced cost. Overall, these PDA-specific developments represent the most innovative SWIR technology portfolio known to date.
KEYWORDS: Sensors, Short wave infrared radiation, Personal digital assistants, Photodiodes, Modulation transfer functions, Sensor technology, Detector arrays, Capacitance
Sensors Unlimited Inc. (SUI), a Raytheon Technologies Company, has long been the vanguard of low-noise InGaAs/InP PiN back-side illuminated (BSI) planar-type photodiode technology. In addition to focusing on dark current reduction efforts, SUI has also initiated other photodiode detector array (PDA) improvement efforts to better serve its broad portfolio of sensor technology. In previous years, SUI has presented results related to mesa-structure PDAs for modulation transfer function (MTF) improvement and hybridization capacitance reduction for NEI improvement. An update to these technologies is offered. Additionally, SUI has more recently engaged in more advanced PDA development to better satisfy active imaging applications. Results of these efforts are also presented.
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.
Two-dimensional photo detector arrays with a cutoff wavelength of 2.5 μm were fabricated on InP/InGaAs
epitaxial wafers with graded buffer layers in a 320x256 geometry on a 12.5μm pitch. Novel growth and fabrication
techniques were employed to fabricate these arrays and optimize the performance. The dark current of the detector was
investigated for a wide range of temperatures. The fabricated detector array was mated with a ROIC and packaged with a
multi-stage TEC and investigated further at the FPA level. The effect of the graded buffer layers on the sensor
performance was investigated and the results were compared to other methods used to develop and fabricate 2D image
sensors on extended wavelength materials.
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