Specific contact resistance of ohmic contacts to n-type SiC membranes
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Specific contact resistance of ohmic contacts to n-type SiC membranes N.F. Mohd Nasir1, A.S. Holland1, G.K. Reeves1, P.W. Leech1, A. Collins1 and P. Tanner2 1 RMIT University, School of Electrical and Computer Engineering, Melbourne, Australia 2 Griffith University, Queensland Microtechnology Facility, Brisbane, Australia ABSTRACT Membranes of epitaxial SiC have been used as a means of eliminating the leakage current into the Si substrate during circular transmission line model (CTLM) measurements. In the n+3C-SiC/ Si wafers, the Si substrate was etched in a patterned window with dimensions up to 10 mm x 15 mm2. An array of CTLM metal contacts was then deposited onto the upper surface of the n+-SiC membrane. The CTLM contacts on the membrane have shown an ohmic current/ voltage response while electrodes located on the adjacent substrate were non-ohmic. Values of c were measured directly on the membranes. These results have shown a significant increase in the current flow below the metal contacts due to the presence of the Si substrate. INTRODUCTION Silicon carbide (SiC) has become an important structural material in micro-electromechanical systems (MEMs). The mechanical strength and thermal stability of SiC have enabled the fabrication of MEMs devices with significantly greater resistance to harsh environments than for silicon. SiC-based devices have shown a distinct advantage over silicon at elevated temperatures above 300 °C, in corrosive atmospheres and in components subject to mechanical sliding contact and wear. A further advantage of SiC in the fabrication of MEMs devices has been the compatibility of the micromachining processes with equivalent methods used for Si. The wide range of MEMs devices based on SiC has been reviewed by Camilla et al. [1] and Zappe [2]. Examples of SiC devices have included automotive pressure sensors used in harsh environments, electronics for high radiation applications and microswitches with improved power capability [1,2]. Other applications of SiC in biological and biomedical MEMs have been based on the chemical inertness and biocompatibility of the surfaces [1,3]. An essential requirement in the fabrication of many of these devices in SiC is the formation of ohmic contacts with low specific contact resistance ( c). For n-type SiC, the metal Ni has been reported to form low resistivity ohmic contacts after annealing [4,9-10]. Other metal systems including Pd [5,11] and Ti/Ni/Ti/Au [6] have resulted in ohmic contacts at significantly lower annealing temperatures than for Ni. However, in previously reported measurements of c in nSiC using the transmission line model (TLM), the magnitude of leakage current within the substrate has remained an uncontrolled variable. In this paper, we report for the first time on the use of a membrane of epitaxial n+-SiC as a means of eliminating leakage current in the substrate during TLM measurements. In these membrane structures, the Si substrate was removed from a window region of the n+-SiC/ Si wafer by backside etching. An array of circular TLM
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