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0989-A08-05

Reliability of Silicon Nitride Gate Dielectric in Vertical Thin-Film Transistors M. Moradi1, D. Striakhilev1, I. Chan1, A. Nathan2, N. I. Cho3, and H. G. Nam3 1 Electrical and Computer Eng., University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Canada 2 London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, United Kingdom 3 Electronic Eng., Sun Moon university, Asan-Si, Chungnam, 336-708, Korea, Republic of ABSTRACT This paper presents results of a systematic investigation of the impact of film thickness on leakage current and electrical breakdown of plasma enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx). We consider SiNx films of various thicknesses, in the range 50 to 300 nm, deposited on both planar and vertical sidewalls in resemblance to the structural topology of the vertical thin film transistor (VTFT). The electrical breakdown strength for 150300 nm thick films was approximately 7 MV/cm, while the value dropped to ~3 MV/cm for 50 nm thick films deposited under the same process conditions. In all cases, failure is inevitably accompanied by an increase in pinhole density. The results show that the reliability and leakage current of the gate dielectric in vertical thin film transistors depends on the step coverage of the SiNx on the vertical sidewall. INTRODUCTION Thanks to its inherent structural attributes of short channel length and small device area, the VTFT offers an excellent platform for a new generation of low-cost, high-speed, and highresolution flat-panel electronics [1]. While the device holds great promise, there are issues related to its electrical performance and reliability when scaled to smaller geometries. In particular, the thickness of the gate dielectric has a significant impact on the short channel effects in the device as channel lengths approach the nano-scale regime. An aggressive scaling of the gate dielectric is necessary to improve the short channel performance and sustain high ON/OFF current ratio [2, 3]. VTFTs with channel lengths of 1µm and below require gate dielectrics as thin as 50nm to comply with scaling rules. However, and as is well known, scaling down the gate dielectric thickness adversely impacts the electrical characteristics of the device. High gate leakage and, in particular, early breakdown are the major issues (see Fig. 1). EXPERIMENT To examine the impact of thickness on the physical properties and electrical characteristics of SiNx, we considered films deposited on both planar and vertical surfaces. The latter resembles the topology of the vertical thin film transistor. Fig. 2 shows a schematic diagram of the planar and vertical structures. For the electrical properties, current-voltage characteristics of the nitride with different thicknesses were measured to study the leakage current and the breakdown voltage. In both structures the silicon nitride film was sandwiched

Gate Leakage Current (A)

between two metal electrodes with different areas. The dependence of the leakage