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Technical Insight

NASA takes the helm of US SiC research (Wide Bandgap Devices)

Well known for its SiC research and development activities, NASA also fulfills a crucial role in transferring SiC technology to the commercial world and helping to co-ordinate the US SiC program. Martin Grado-Caffaro reports.
Military and aerospace environments are frequently hostile, subjecting equipment to large temperature variations, high mechanical stresses and high EM radiation exposures. Electronic components capable of operating effectively in these conditions are crucial to the military and aerospace industries. Applications requiring high temperature, high power and radiation-resistant operation include microwave communications, high-power radar, engine ignition and sensing systems, and space-based electronics. Silicon and GaAs devices do not work effectively at high temperatures. Above 350C, silicon requires cooling equipment to maintain its electronic properties, adding size and cost to circuitry. GaAs-based devices found in solid state microwave communications and radar electronics also suffer at high power densities and high temperatures. Silicon carbide Silicon carbide has a large bandgap and high breakdown voltage, properties that make it a good material for devices that may need to operate in the military and aerospace environments. The ability of devices made from SiC to operate at high temperatures and powers (see ), while at the same time being radiation hard, have made them the subject of intense research by military and aerospace agencies throughout the world. NASA Glenn Research Center (GRC) in Cleveland, Ohio is one such agency. SiC at NASA GRC SiC research forms part of the High Temperature Integrated Electronics and Sensors (HTIES) program within the Instrumentation and Control Division (ICD) at NASA GRC. The purpose of ICD is the development of electronics, photonics and controls technology for aerospace applications. Facilities at the disposal of the HTIES team include a SiC MOCVD epitaxial growth laboratory equipped with an Emcore Discovery 75 vertical flow reactor and an Aixtron 200/4HT horizontal flow reactor, both specifically designed for SiC growth. Also available are comprehensive materials and electrical characterization laboratories and a 1500 sq. ft clean room for fabricating prototype devices. NASA GRC has an in-house SiC research program going back over 25 years, and they can claim a number of firsts, including:
  • The first large-area epitaxial growth of SiC on silicon wafers in 1982;
  • The first kilovolt and multi-kilovolt SiC rectifier devices in 1991 and 1993, respectively;
  • The invention of dopant control by "site competition epitaxy" in 1993;
  • The identification of micropipes as crystal defects that provoke premature electrical breakdown in SiC power devices, also in 1993.
  • Developments in epitaxy Site-competition epitaxy is a growth process where Si and C compete with the dopant atoms for available substitutional lattice sites on the growing SiC crystal surface. Nitrogen acting as a donor and aluminum acting as an acceptor compete with carbon and silicon, respectively. Nitrogen concentration in the grown epilayer is proportional to the Si/C ratio during growth. Aluminum acceptor concentration is inversely proportional to this ratio. This technique has been applied to create very low-doped epilayers for the first 6H-SiC 2000 V and 3C-SiC 300 V diodes, and to fabricate JFETs able to operate at 600C in air for 30 hours. Another growth-related breakthrough has come with the granting of a patent in April 2001 for a method of growing atomically flat semiconductor surfaces. The method involves etching device-sized arrays of mesas into the wafers. By controlling growth conditions, crystal growth is limited to the side of each atomic step. The crystal at each step grows sideways until the step reaches the edge of the mesa, leaving behind an atomically flat surface. Mesas up to 0.4 mm2 have been flattened out by this sideways growth. One benefit of this method is that mesas containing screw dislocations are left unflattened and thus can be isolated as unsuitable areas for further processing. The ability to process devices on flat, dislocation-free material will improve device reliability and performance. NASA and commercialization One of NASA GRC s missions is to facilitate the uptake of SiC technology outside of the military and aerospace arenas. Currently it is the military/aerospace demand that is pushing SiC and other wide bandgap technologies. Getting the technology into the much larger and potentially very profitable commercial marketplace is proving difficult. Device performance mainly hampered by poor defect control is as yet not sufficient to make SiC an obvious choice for many applications. Presently there exists no obvious large volume commercial application to drive the development of SiC technology. One notable success in this role as facilitator has been the acceptance by industry of NASA s base technology for epitaxial growth of SiC as a de facto standard (see ). As Jih-Fen Lei, chief of ICD explains, "the site competition process enables better control of the doping level (from 1014 to 1020 cm3) and enhances the durability of the fabricated device". The adoption of this and other processes by a number of companies working alongside NASA GRC has brought some success stories. Lei continues, "GE has demonstrated the world s first 500C amplifier, and Kulite Semiconductor took advantage of the deep etching technique developed by GRC. This lead to the joint development and demonstration of the world s first SiC-based pressure sensor in an engine compressor under a NASA-funded project". Other industrial partners to whom technology has been transferred are Westinghouse, Motorola, Cree Research, Boston Microsystems, Northrop Grumman, ATMI (for crystal growth) and Sienna Technologies (for sensor and packaging applications). MEMS: a promising niche for SiC Micro-electro-mechanical systems (or MEMS) have carved themselves an important niche in the aerospace industry and are becoming prolific across the whole electronics industry. Traditionally based on silicon, MEMS have become a strategic area at NASA GRC, driven by the need to perform sensing functions in hostile environments where the properties of silicon make it unsuitable. SiC has become the material of choice because of its mechanical and electrical stability in hostile environments. Much SiC research to date has focussed on the 6H-SiC polytype as a material for high-power, high-temperature electronics. The difficulty of depositing the 6H polytype onto any other material has limited its development as a material for MEMS. Attention has turned to the 3C polytype because of the ability to epitaxially grow this material on single crystal silicon substrates. Because SiC is resistant to the common Si etches, such films are suitable for bulk micromachining. Polycrystalline SiC can be deposited onto polysilicon or silicon dioxide sacrificial layers to enable surface micromachining of SiC MEMS. Glennan Microsystems Initiative Work on SiC MEMS at NASA is facilitated by the Glennan Microsystems Initiative (GMI), a public/private partnership between NASA GRC, Case Western Research University, and other government agencies and private sector companies. These include Air Force Research Laboratories, Battelle, Steris Corp, General Electric and the Cleveland Clinic Foundation. Companies involved in GMI gain access to funding, expertise, research results, prototyping facilities and help with early commercialization. To date the initiative has supported $4 million in research projects and will support a further $17 million worth of projects through to 2003. The main goal of GMI is the development and commercialization of innovative microsystems for harsh environments. In this instance, harsh is defined as high temperature, corrosive and high mechanical loading through shock and vibration. As a result, most projects feature SiC as the base structural material. "One important component in the commercialization of SiC MEMS will be the development of a multi-user SiC surface micromachining process called MUSiC, which will provide access to SiC devices through a process quite similar to the multi-user MEMS processes (MUMPS) based on Si," says Walter Merrill, executive director of GMI. MUSiC promotes the general philosophy of GMI by making prototyping and small-volume manufacturing more economical by providing access to a multi-user process. Multiple users with similar goals share overhead costs for small volume production. In a single process run, designs from different users are fabricated on the same multi-project wafer. The foundry then dices the wafer and sends each user the relevant chips. This approach to small volume prototyping is already well established in silicon and SiGe microelectronics through services such as Europractice and even large companies, including IBM, offering multi-project wafer runs. Co-ordinating SiC projects NASA s relationship with the outside world where SiC is concerned can be viewed as one of technology transfer and co-operative research with universities, government agencies and industry. NASA GRC works with DARPA, US Naval, US Air Force and US Army Research Laboratories, all of them with interests in SiC. NASA acts very much as a co-ordinator for all these activities. As Lei points out, "NASA GRC has been solicited by the Air Force, Navy and Army to help them to set the direction for their related programs and provide evaluations for the industry proposals."
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