Two-contact Circular Test Structure for Determining Specific Contact Resistivity
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Two-contact Circular Test Structure for Determining Specific Contact Resistivity Y. Pan1, G. K. Reeves1, P. W. Leech1, P. Tanner2, A. S. Holland1 1 RMIT University, School of Electrical and Computer Engineering, Melbourne, Australia 2 Griffith University, Queensland Microtechnology Facility, Brisbane, Australia ABSTRACT As ohmic contacts decrease in size and approach nanoscale dimensions, accurate electrical characterization is essential, requiring the development of suitable test structures for this task. We present here a new test structure derived from the standard three-contact circular transmission line model (CTLM) [1], for determining the specific contact resistivity of ohmic contacts. This test structure minimizes sources of error which arise from the CTLM by – (i) reducing the number of contacts within one test pattern from three to two, (ii) ensuring the assumption of equipotential metal contacts used in modelling is more easily attained experimentally, and (iii) allowing the fabrication of reduced geometrical dimensions essential for determining low specific contact resistivity values. The analytical expressions are presented and experiment results are undertaken to demonstrate the accuracy of the technique. There are no error corrections required for determining contact parameters using the presented test structure. INTRODUCTION Specific contact resistivity (ρc) is an important parameter, which quantifies a metal– semiconductor ohmic contact. Many methods were reported to determine ρc [2-4], and one commonly used method is the circular transmission line model (CTLM) [1]. The standard CTLM consists of a central dot contact with two circular concentric contact rings. The advantage of the CTLM is to simplify the sample preparation process (no mesa etch required or active area formation). However, the middle ring of the standard CTLM is difficult to probe and keep as an equipotential if it is too narrow. In this paper, a new method, using two-contact test structures for determining ρc is presented with experimental results for Ti-to-SiC contacts. The structure uses three pairs of circular metal contacts, each pair being concentric, thus removing the annular contact between the centre and outermost contact of the standard CTLM. This is an important modification as it removes the contact that is least likely to be an equipotential, and its removal significantly simplifies the electrical probing of the test structure. We used Finite Element modelling to show the extent to which each contact differs from being an equipotential for a variety of typical contact parameters. The modelling demonstrated that the annular contact is the contact least likely to satisfy the equipotential assumption. Its removal thus improves the accuracy and simplifies the analysis of the contact test structure. In comparison to the standard CTLM, the new two contact test structure does not use modification of the rather complex expressions used to describe current-voltage behaviour in circular electrodes. Rather, we show in this paper th
