The soft x-ray band covers the characteristic lines of the highly ionized low-atomic-number elements, providing diagnostics of the warm and hot plasmas in star atmospheres, interstellar dust, galaxy halos and clusters, and the cosmic web. High-resolution spectroscopy in this band is best performed with grating spectrometers. Soft x-ray grating spectroscopy with R = λ / Δ λ = > 104 has been demonstrated with critical-angle transmission (CAT) gratings. CAT gratings combine the relaxed alignment and temperature tolerances and the low mass of transmission gratings with high diffraction efficiency blazed in high orders. They are an enabling technology for the proposed Arcus grating explorer and were selected for the Lynx Design Reference Mission grating spectrometer instrument. Both Arcus and Lynx require the manufacture of hundreds to perhaps ~2000 large-area CAT gratings. We are moving toward CAT grating volume manufacturing using 200 mm silicon-on-insulator wafers, 4X optical projection lithography tools, deep reactive-ion etching, and KOH polishing. We have, for the first time, produced high-throughput 200 nm-period CAT gratings ~50% deeper than previously fabricated. X-ray diffraction efficiency is significantly improved in the ~1:25 - 1.75 nm wavelength range, peaking above 40% (sum of blazed orders). A new grating-to-grating alignment technique utilizing cross-support diffraction of visible light is presented, as well as the results of CAT grating emissivity measurements.λ
High-resolution (R = λ/Δλ >2000) x-ray absorption and emission line spectroscopy in the soft x-ray band is a crucial diagnostic for the exploration of the properties of ubiquitous warm and hot plasmas and their dynamics in the cosmic web, galaxy clusters, galaxy halos, intragalactic space, and star atmospheres. Soft x-ray grating spectroscopy with R > 10,000 has been demonstrated with critical-angle transmission (CAT) gratings. CAT gratings combine the relaxed alignment and temperature tolerances and low mass of transmission gratings with high diffraction efficiency blazed in high orders. They are an enabling technology for the proposed Arcus grating explorer and were selected for the Lynx design reference mission grating spectrometer instrument. Both Arcus and Lynx require the manufacture of hundreds to perhaps ≈ 2000 large-area CAT gratings. We are developing new patterning and fabrication process sequences that are conducive to large-format volume processing on stateof-the-art 200 mm wafer tools. Recent x-ray tests on 200 nm-period gratings patterned using e-beam-written masks and 4x projection lithography in conjunction with silicon pore focusing optics demonstrated R ≈ 104 at 1.49 keV. Extending the grating depth from 4 μm to 6 μm is predicted to lead to significant improvements in diffraction efficiency and is part of our current efforts using a combination of deep reactive-ion etching and wet etching in KOH solution. We describe our recent progress in grating fabrication and report our latest diffraction efficiency and modeling results.
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.
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.
Two scanning electron beam lithography (SEBL) patterning processes have been developed, one positive and one negative tone. The processes feature nanometer-scale resolution, chemical amplification for faster throughput, long film life under vacuum, and sufficient etch resistance to enable patterning of a variety of materials with a metal-free (CMOS/MEMS compatible) tool set. These resist processes were developed to address two limitations of conventional SEBL resist processes: (1) low areal throughput and (2) limited compatibility with the traditional microfabrication infrastructure.
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