KEYWORDS: Charge-coupled devices, Semiconducting wafers, Germanium, Back end of line, Silicon, Atomic layer deposition, Sensors, Reticles, Hard x-rays, Back illuminated sensors
A germanium charge-coupled device (CCD) offers all of the advantages of silicon CCDs – notably excellent uniformity, high energy resolution, and noiseless on-chip charge summation – but covers an even broader spectral range, extending into the hard X-ray band. MIT Lincoln Laboratory has been developing germanium CCDs for applications in astrophysics, with the goal of realizing megapixel-class arrays with read noise less than a few electrons, sensitivity to both soft and hard X-rays, and high energy resolution. We recently realized our first small pixel arrays and have since been working to increase both the format and performance of these devices. In this article, we discuss performance improvements, identification of yield-limiting process steps in fabrication of frontside-illuminated devices, fabrication and analysis of our first backside-illuminated detectors, and design of prototypes which includes a 512 × 512 pixel frame-transfer CCD with 24 µm pixel pitch.
A germanium charge-coupled device (CCD) offers the advantages of a silicon CCD for X-ray detection – excellent uniformity, low read noise, high energy resolution, and noiseless on-chip charge summation – while covering an even broader spectral range. Notably, a germanium CCD offers the potential for broadband X-ray sensitivity with similar or even superior energy resolution than silicon, albeit requiring lower operating temperatures (≤ 150K) to achieve sufficiently low dark noise due to the lower band gap of this material. The recent demonstration of high-quality gate dielectrics on germanium with low surface-state density and low gate leakage is foundational for realization of high-quality imaging devices on this material. Building on this advancement, MIT Lincoln Laboratory has been developing germanium CCDs for several years, with design, fabrication, and characterization of kpixel-class front-illuminated devices discussed recently. In this article, we describe plans to scale these small arrays to megapixel-class imaging devices with performance suitable for scientific applications. Specifically, we discuss our efforts to increase charge-transfer efficiency, reduce dark current, improve fabrication yield, and fabricate backside-illuminated devices with excellent sensitivity.
We have developed a new approach for rapid die-level hybridization of backside-illuminated silicon avalanche photodiode (APD) arrays to CMOS readout integrated circuits (ROICs). APD arrays are fabricated on a custom silicon-on-insulator (SOI) wafer engineered with a built-in backside contact and passivation layer. The engineered APD substrate structure facilitates uniform APD substrate removal by selective etching at the die level after bump bonding. The new integration process has the following advantages over wafer-level 3D integration: 1) reduced cost per development cycle since a dedicated full-wafer ROIC fabrication is not needed, 2) compatibility with existing ROICs that are in chip-format from previous fabrication runs, and 3) accelerated schedule. The new approach is applied to produce 32×32 100-μm-pitch silicon GmAPD arrays. Electrical performance of the APD arrays show 100% pixel connectivity and excellent yield before and after substrate removal.
Silicon charge-coupled devices (CCDs) are commonly utilized for scientific imaging in wavebands spanning the near infrared to soft X-ray. These devices offer numerous advantages including large format, excellent uniformity, low read noise, noiseless on-chip charge summation, and high energy resolution in the soft X-ray band. By building CCDs on bulk germanium, we can realize all of these advantages while covering an even broader spectral range, notably including the short-wave infrared (SWIR) and hard X-ray bands. Since germanium is available in wafer diameters up to 200 mm and can be processed in the same tools used to build silicon CCDs, large-format (>10 MPixel, >10 cm2 ) germanium imaging devices with narrow pixel pitch can be fabricated. Furthermore, devices fabricated on germanium have recently demonstrated the combination of low surface state density and high carrier lifetime required to achieve low dark current in a CCD. At MIT Lincoln Laboratory, we have been developing germanium imaging devices with the goal of fabricating large-format CCDs with SWIR or broadband X-ray sensitivity, and we recently realized our first front-illuminated CCDs built on bulk germanium. In this article, we describe design and fabrication of these arrays, analysis of read noise and dark current on these devices, and efforts to scale to larger device formats.
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