Current Transport Modeling in an Amorphous Silicon Antifuse Structure
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CURRENT TRANSPORT MODELING IN AN AMORPHOUS SILICON ANTIFUSE STRUCTURE AJITH AMERASEKERA, S.PING KWOK, JEROLD SEITCHIK Semiconductor Process and Design Center, Texas Instruments Inc. P.O. Box 655012, MS 461, Dallas TX 75265, Tel: (214)-995-7985
ABSTRACT We have studied the transport mechanisms in thin film (nz2000 A ) hydrogenated a-Si structures used as programmable antifuses in Field-Programmable-Gate-Arrays (FPGA) The antifuse was simulated using a back-to-back Schottky model for a metal/Si/metal thin film incorporating the thermionic-emission diffusion model for the metal-semiconductor contacts. The model predicts the current transport in the low voltage and high voltage regions. In the intermediate voltage range a linear field dependence of the barrier lowering is observed in the experimental results which leads to higher currents than predicted with the image force barrier lowering in the simulations. An important observation is that carrier multiplication is essential for breakdown which is evidence of impact ionization in a-Si. The permanent breakdown condition is modelled thermally, based on the steadystate solution to the heat equation.
INTRODUCTION The objective of this work is to determine the current transport mechanism in an a-Si antifuse used in FPGAs [1], and the mechanisms governing the programming of these structures. There have been many studies of the switching behavior in amorphous and non-crystalline films [2] [3]. Mott [2], in a theoretical review of the mechanisms, concluded that the conduction was predominantly due to the Schottky effect at the contacts, and that breakdown was due to thermal instabilities resulting from carrier multiplication in the film. Recent studies [4] have shown that the thermionic emission-diffusion model can be used for current conduction in a-Si films up to 3 pm thick. We have used the thermionic-emission diffusion model implemented in TMA PISCES, in our simulations and examined its validity through comparison with experimental data. The test structures were via TiW/PECVD a-Si/TiW capacitors. Areas ranged from 0.8 x 0.8 /m 2 to 200 x 200 pm 2. The TiW was 3000 A thick and the a-Si film thickness was 2000 A . The as deposited hydrogen content of the a-Si was 14.8 atomic percent, and the devices were sintered at a temperature between 300 - 500'C.
EXPERIMENTAL RESULTS DC J - V curves, Figure 1, were measured on an HP 4145 using a current sweep. The current shows an exponential dependence for an applied voltage V.,,, > 1 V, and breakdown occurs for Vn,,,l > 13.5 V. The J - V curve on a log-log scale, Figure 2, has a slope of 1 in the region 10 mV < Vap,, < 1 V. For V,,,, > 13.5 V the current rises rapidly with voltage, and eventually the device breaks down, creating a low resistance path between the electrodes. In Figure 3, the current density, J, is shown as a function of the ambient temperature, T. The ln(J) vs. 1/T plot is given for V.W of 0.5 V, 2 V, 4 V, and 5 V. The slopes are linear and J is observed to increase with V,,,, for a given T. Since the dependence o
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