High Precision Physical Model for Nickel MILC
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High Precision Physical Model for Nickel MILC C. F. Cheng, W. M. Cheung, K. L. Ng, P. J. Chan, M. C. Poon, Mansun Chan and C. W. Kok Department of Electrical and Electronic Engineering, The Hong Kong University of Science & Technology, Sai Kung, Hong Kong ABSTRACT Mechanism and growth rate of Metal-Induced-Lateral-Crystallization (MILC) with annealing temperature range from 550 oC to 625 oC were studied. Base on the MILC growth mechanism and effect of metal diffusion, a modeling on metal impurity distribution was developed. The modeling can be used to predict the distribution of metal impurity formed in the polysilicon layer after MILC annealing process. By applying the modeling, effects of annealing on the metal impurity distribution can be analyzed.
INTRODUCTION Recently, Metal-Induced-Lateral-Crystallization using nickel has been recognized as a high potential method for making large polysilicon grain size and high quality TFT [1] [2]. However, people doubt that existence of metal impurity in the polysilicon layer may degrade device’s lifetime and performances, such as having a larger leakage current and a gentler subthreshold slope. So, the nickel concentration in MILC region has attracted many attentions. In traditional case, mass spectrum analysis, such as secondary ion mass spectrometry (SIMS), is normally employed to obtain the details of metal impurity distribution [3]. However, the information can only be extracted after MILC formation. In this paper, a modeling on metal impurity distribution is proposed. Mechanism of MILC formation and effect of metal diffusion are considered in the modeling. It is believed that the modeling can be used to predict the impurity profile efficiently. Even though without fabricating a sample, this method still helps to provide reliable information on the metal impurity distribution.
PROCESS OF METAL-INDUCED-LATERAL-CRYSTALLIZATION As nickel is commonly used in MILC process, it was chosen as a metal impurity in this paper. In our MILC formation process, 1000 Å amorphous silicon (a-Si) was deposited on an buried oxide substrate layer first. Next, 4000 Å low-temperature oxide (LTO) was deposited on the top. Seed windows were then patterned on the LTO layer. After that, 50 Å nickel thin film was deposited. During MILC annealing with temperature range from 550 oC to 625 oC for 2 hours, nickel silicide and metal-induced-crystallization (MIC) region were formed under the seed window area instantly. Further annealing led nickel atoms diffuse laterally into the silicon layer. Thus, lateral-crystallization of amorphous silicon was resulted and MILC polysilicon was formed. Figure 1 illustrates the MILC process, and a photo of the silicon layer after MILC annealing is shown in Figure 2.
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Nickel silicide reactive interface Seed windows
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