Hints, Tips and Solutions - August 2006


Q. How to obtain a stable grid and smooth doping profile in non-planar a-Si TFT using ATHENA/Elite?

A. Silvaco process and device simulation tools have been successfully used for many TFT applications. It has been shown that if accurate density of state distributions are specified the transfer characteristics of TFT devices show good agreement with experiments. The typical configuration of TFT includes a non-planar n+ doping source/drain region around the metal gate. Such a non-planar structure could be simulated only by the process simulator ATHENA because ATLAS cannot generate the regions with rounded shapes.

The accuracy of ATLAS device simulation strongly depends on grid quality and uniformity of doping along the surface of a-Si region. The internal SSuprem4 deposition algorithm does not produce a smooth grid and consequently uniform doping. To overcome this difficulty the advanced topography simulation module of ATHENA must be used for a-Si deposition step. Elite produces much better grid and avoids non-uniform doping as seen in the left plot of Figure 1.

Figure 1. Ssuprem4 n+ doping v.s Elite doping profile


Figure 2 shows that experimentally proved IV-curves can only be obtained when Elite module is used for simulation of the critical a-Si deposition step. If Elite is not used the reverse biased trend of the IV-curve disappears which means that these characteristics are very sensitive to the grid and doping quality. Therefore, we strongly recommend to use Elite deposition for simulation of non-planar TFT devices.

Figure 2. (experimental data is omitted) The top plot shows leakage current increase with a -Si thickness when simulating with ELITE. The bottom plot does not show a behavior if ELITE is not used.


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