SiC TO SiC WAFER BONDING
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SiC TO SiC WAFER BONDING G. N. Yushin, A. V. Kvit, R. Collazo, Z. Sitar, North Carolina State University, Department of Materials Science and Engineering, Raleigh, NC 27695, U.S.A.
ABSTRACT SiC wafers with an RMS roughness of 1.5 nm were bonded in a dedicated ultrahigh vacuum bonding chamber. Successful fusion of wafers was observed at temperatures as low as 800ºC under a uniaxial mechanical stress of 20 MPa. Cross-section transmission electron microscopy (XTEM) of a specimen bonded at 1100ºC revealed parts of the interface where wafers were in intimate contact, while other parts contained an up to 3 nm thick amorphous carbon interlayer. The bonded SiC retained its high crystalline quality; no extended defects emanating from the interface were observed within the sampling region. Electrical measurements showed that the azimuthal orientation of the bonded couple significantly influences the electrical character of the junction.
INTRODUCTION SiC is a promising material for high-power and high-temperature electronics. Its chemical inertness, high thermal conductivity ( ~ 5 Wcm-1K-1), and wide band gap (2.4 eV to 3.3 eV, depending on the polytype) may lead to significant improvements in applications like high voltage switching in power distribution lines, powerful microwave devices in radars, high temperature sensors and control in aircraft engines [1]. Fabricated SiC metal oxide semiconductor and metal semiconductor field effect transistors as well as bipolar homojunction transistors showed that their performance exceeds even the theoretical limits of Si and GaAs counterparts [1]. Until recently, the idea of using different SiC polytypes in the fabrication of more efficient heterojunction devices was thought to be impractical due to difficulties in controlling the polytype change during growth. Koitzsch et al. [2] suggested a wafer-bonding technique [3] to fabricate an electronically useful interface between the polytypes. They performed empirical molecular dynamic studies on SiC (0001) surfaces and bonded interfaces and predicted the possibility of formation of contamination free SiC wafer-bonded interfaces in ultra high vacuum (UHV). Torvik et al. [4] experimentally demonstrated wafer bonding of SiC at temperatures above 1000ºC. In the present study we investigated the microstructure of the bonded interface and the influence of temperature and wafer orientation on the bonding process.
EXPERIMENTAL DETAILS In all bonding experiments, we used Si-face, on-axis (0001) 6H-SiC wafers with n-type doping level of 3·1018 cm-3 from Cree, Inc. The RMS surface roughness of as received wafers was measured by atomic force microscopy (AFM) using a Digital Instruments D3000 microscope. Following the roughness measurements, the wafers were cut into 2.5 X 1.2 cm2 rectangular pieces, ultrasonically cleaned in JTB-111 solution, 10% hydrofluoric acid and de-
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ionized water, placed in clean containers and sealed. This procedure was performed in a Class 100 particulate hood located in a Class 1000 clean-room. In order t
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