Stress Effects on Nanocrystal Formation by Ni-Induced Crystallization of Amorphous Si
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A16.2.1
Stress Effects on Nanocrystal Formation by Ni-Induced Crystallization of Amorphous Si Yaocheng Liu, Michael D. Deal, Mahmooda Sultana*, and James D. Plummer Center for Integrated Systems, Stanford University, CA * Department of Chemical Engineering, University of Southern California, CA ABSTRACT Metal-induced crystallization (MIC) of amorphous Si is gaining increased interest because of its potential use for low-temperature fabrication of integrated circuits. In this work, the MIC technique was used to make Si nanocrystals and the effects of stress on the crystallization were studied. Amorphous Si films were deposited onto the Si substrate with thermal oxides on top by low-pressure chemical vapor deposition (LPCVD) and then patterned into nanoscale pillars by electron beam lithography and reactive ion etching. A conformal low-temperature oxide (LTO) layer was deposited to cover the pillars, followed by an anisotropic etch back to form a spacer, leaving only the top surface of the pillars exposed to the 5 nm Ni sputtering deposition afterwards. An HF dip was used to partially remove the LTO spacers on the pillars, leading to different LTO thicknesses on different samples. These samples were then annealed to crystallize the amorphous Si pillars, forming Si nanocrystals. Transmission electron microscope (TEM) observations after anneal found a clear dependence of the crystallization rate on the pillar size as well as the LTO thickness. The crystallization rate was lower for pillars with thicker LTO spacers, while for the same LTO thickness the crystallization rate was lower for pillars with narrower width. A model based on the stress in the pillars is proposed to explain this dependence. This model suggests some methods to control the nickel-induced crystallization process and achieve higher quality Si nanocrystals.
INTRODUCTION Metal-induced crystallization (MIC) has become an attractive technology due to its applications in three-dimensional integrated circuits and heterogeneous integration [1-4]. It has been reported to be possible to make high quality polycrystalline Si with low-temperature process by metal-induced crystallization [5, 6]. Among different metals, Ni is of particular interest because NiSi2 has a fluorite structure with a very small lattice mismatch with Si. Large Si grains with low defect density can be achieved through NiSi2-mediated solid-phase epitaxy [7, 8]. The main focus of this work is to obtain single-crystalline Si structures of sub-µm to nm scales and study the related mechanism. It was found in our experiments that stress plays an important role in metal-induced crystallization and can be used as a tool for process control to improve crystal quality.
EXPERIMENTAL DETAILS A thin oxide layer was first grown on (100) Si wafers by thermal oxidation. Then amorphous Si (a-Si) was deposited onto the wafers by low-pressure chemical vapor deposition
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