Kevin K. Ryu

Mixed-Signal Organic Integrated Circuits in a Fully Photolithographic Dual Threshold Voltage Technology

Analog & digital circuits implemented in a dual threshold voltage (VT) p-channel organic technology are presented. The dual VT organic technology is compatible with large-area and mechanically flexible substrates due to its low processing temperature (≤ 95°C) and scalable patterning techniques. We demonstrate the first analog & digital organic integrated circuits produced by a dual-gate metal process. The analog circuits are powered by a 5-V supply and include a differential amplifier and a two-stage uncompensated operational amplifier (op-amp). A dynamic comparator is measured to have an input offset voltage of 200 mV and latching time of 119 ms. Both the comparator and the op-amp dissipate 5 nW or less. Area-minimized digital logic is presented. Inverters powered by a 3-V supply were measured to have positive noise margins and consumed picowatts of power. An 11-stage ring oscillator, also powered by a 3-V supply, swings near rail to rail at 1.7 Hz. These results demonstrate dual threshold voltage process feasibility for large-area flexible mixed-signal organic integrated circuits.

Wafer-Level Heterogeneous Integration of GaN HEMTs and Si (100) MOSFETs

This letter demonstrates a new technology for the heterogeneous integration of GaN and Si devices, which is scalable at least up to 4-in wafers and compatible with conventional Si fabrication. The key step in the proposed technology is the fabrication of a Si (100)-GaN-Si hybrid wafer by bonding a silicon (100) on insulator (SOI) wafer to the nitride surface of an AlGaN/GaN on Si (111) wafer. A thin layer of silicon oxide is used to enhance the bonding between the SOI and the AlGaN/GaN wafers. Using this technology, Si pMOSFETs and GaN high-electron-mobility transistors have been fabricated on a 4-in hybrid wafer. Due to the high-temperature stability of GaN as well as the high-quality semiconductor material resulting from the transfer method, these devices exhibit excellent performance. A hybrid power amplifier has been fabricated as a circuit demonstrator, which shows the potential to integrate GaN and Si devices on the same chip to enable new performance in high-efficiency power amplifiers, mixed signal circuits, and digital electronics.

Dual Threshold Voltage Organic Thin-Film Transistor Technology

A fully photolithographic dual threshold voltage (VT) organic thin-film transistor (OTFT) process suitable for flexible large-area integrated circuits is presented. The nearroom-temperature (<; 95 °C) process produces integrated dual VT pentacene-based p-channel transistors. The two VT s are enabled by using two gate metals of low (aluminum) and high (platinum) work function. The Al and Pt gate OTFTs exhibit nominally identical current-voltage transfer curves shifted by an amount ΔVT. The availability of a high-VT device enables area-efficient zero-Vos high-output-resistance current sources, enabling high-gain inverters. We present positive noise margin inverters and rail-to-rail ring oscillators powered by a 3-V supply-one of the lowest supply voltages reported for OTFT circuits. These results show that integrating nand p-channel organic devices is not mandatory to achieve functional area-efficient low-power organic integrated circuits.

Thin-Body N-Face GaN Transistor Fabricated by Direct Wafer Bonding

This letter presents a method to fabricate thin-body N-face GaN-on-insulator-on-Si (100) wafers. These new wafers are promising to increase the carrier confinement and reduce the contact resistance in AlGaN/GaN high electron mobility transistors (HEMTs). In the reported technology, a Ga-face AlGaN/GaN epilayer grown on Si (111) is transferred to a Si (100) wafer by direct wafer bonding and thinned down by selective dry etch to the device active layers. A GaN channel thickness as thin as 20 nm is obtained with the use of AlGaN etch-stop layers. Excellent transport characteristics are obtained in the fabricated thin-body N-face AlGaN/GaN structures, with a sheet resistance of 430 Ω/sq, an electron mobility of 1700 cm2/V · s, and a 2-D electron gas concentration of 9 × 1012 cm-2. HEMTs fabricated on these N-face thin-body epilayers shows excellent current-voltage characteristics and great potential for high-frequency applications.

Development of CCDs for REXIS on OSIRIS-REx

The Regolith x-ray Imaging Spectrometer (REXIS) is a coded-aperture soft x-ray imaging instrument on the OSIRIS-REx spacecraft to be launched in 2016. The spacecraft will fly to and orbit the near-Earth asteroid Bennu, while REXIS maps the elemental distribution on the asteroid using x-ray fluorescence. The detector consists of a 2×2 array of backilluminated 1k×1k frame transfer CCDs with a flight heritage to Suzaku and Chandra. The back surface has a thin p+-doped layer deposited by molecular-beam epitaxy (MBE) for maximum quantum efficiency and energy resolution at low x-ray energies. The CCDs also feature an integrated optical-blocking filter (OBF) to suppress visible and near-infrared light. The OBF is an aluminum film deposited directly on the CCD back surface and is mechanically more robust and less absorptive of x-rays than the conventional free-standing aluminum-coated polymer films. The CCDs have charge transfer inefficiencies of less than 10-6, and dark current of 1e-/pixel/second at the REXIS operating temperature of –60 °C. The resulting spectral resolution is 115 eV at 2 KeV. The extinction ratio of the filter is ~1012 at 625 nm.

Invited) Wafer Bonding Technology in Nitride Semiconductors for Applications in Energy and Communications

GaN-based semiconductors are excellent candidates for power devices at high frequencies due to their combination of large band gap (3.4 eV), and high electron mobility (1500 ~ 2000 cm/Vs) of the two-dimensional electron gas (2DEG). Current gain cut-off frequencies (fT) in excess of 160 GHz and power gain cut-off frequencies (fmax) of 300 GHz have been demonstrated [1], and RF transistors capable of more than 10 W/mm of output power at 40 GHz has been reported [2]. This power density is almost one order of magnitude higher than in any other semiconductor technology at these frequencies. In this paper we aim to demonstrate that wafer bonding can further expand GaN capabilities. It offers unprecedented freedom in the heterogeneous integration of GaN and other semiconductors, and allows novel device structures.

Oxide-bonded molecular-beam epitaxial backside passivation process for large-format CCDs

We describe recent advances in backside passivation of large-format charge-coupled devices (CCDs) fabricated on 200- mm diameter wafers. These CCDs utilize direct oxide bonding and molecular-beam epitaxial (MBE) growth to enable high quantum efficiency in the ultraviolet (UV) and soft X-ray bands. In particular, the development of low-temperature MBE growth techniques and oxide bonding processes, which can withstand MBE processing, are described. Several highperformance large-format CCD designs were successfully back-illuminated using the presented process and excellent quantum efficiency (QE) and dark current are measured on these devices. Reflection-limited QE is measured from 200 nm to 800 nm, and dark current of less than 1e- /pixel/sec is measured at 40°C for a 9.5 μm pixel.

Development of germanium charge-coupled devices

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.

Directly deposited optical-blocking filters for single-photon x-ray imaging spectroscopy

Abstract. Directly deposited optical-blocking filters (DD OBFs) have the potential to improve filter performance and lower risk and cost for future x-ray imaging spectroscopy missions. However, they have not been fully characterized on high-performance charge coupled devices (CCDs). This paper reports the results of DD OBFs processed on high-performance photon-counting CCDs. It is found that CCD performance is not degraded by deposition of such filters. X-ray and optical transmission through the OBF is characterized and found to match theoretical expectation. Light-leaks through pinholes and the side and back surfaces are found to lower the optical extinction ratio; various coating processes are developed to resolve these issues.

Directly-deposited blocking filters for high-performance silicon x-ray detectors

Silicon X-ray detectors often require blocking filters to mitigate noise and out-of-band signal from UV and visible backgrounds. Such filters must be thin to minimize X-ray absorption, so direct deposition of filter material on the detector entrance surface is an attractive approach to fabrication of robust filters. On the other hand, the soft (E < 1 keV) X-ray spectral resolution of the detector is sensitive to the charge collection efficiency in the immediate vicinity of its entrance surface, so it is important that any filter layer is deposited without disturbing the electric field distribution there. We have successfully deposited aluminum blocking filters, ranging in thickness from 70 to 220nm, on back-illuminated CCD X-ray detectors passivated by means of molecular beam epitaxy. Here we report measurements showing that directly deposited filters have little or no effect on soft X-ray spectral resolution. We also find that in applications requiring very large optical density (> OD 6) care must be taken to prevent light from entering the sides and mounting surfaces of the detector. Our methods have been used to deposit filters on the detectors of the REXIS instrument scheduled to fly on OSIRIS-ReX later this year.