Etching Process of Al Oxide on Si Surface with HF Treatment
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easily get deposited in alkali solution with the formation of metal oxides [2]. Those metallic impurities must be removed later by acid cleaning solutions such as HCl/H 20 2 /H2 0 (called HPM or SC-2) and/or dilute HF (DHF). Therefore, clarifying the removal process of metal oxides is important for the development of future cleaning technology. In this work, we have performed a first-principles calculation to clarify the etching mechanism of metal oxide deposited on Si substrate. In particular, investigation is focused on the removal of A1O because Al is usually detected in a large volume on wafer surface after APM cleaning [3]. The present theoretical approach will provide the reaction path for removal of Al metal oxide by HF treatment and, also, offer the estimation of activation energy barriers that is helpful to evaluate reaction rates. METHOD OF CALCULATION A schematic illustration of Fig.1(a) shows the unreconstructed Si(111) surface. Since the atomic geometry is unknown for Al oxides adhering to a Si substrate, 201
Mat. Res. Soc. Symp. Proc. Vol. 492 ©1998 Materials Research Society
(b) Figure 1 : (a) Schematic representation of Si(11) surface. (b) Computational cluster model representing atomic structure around an Si rest-atom shown with hatched spheres in (a). Small solid spheres denote the embedded H atoms. an initial structure is postulated to be a simple configuration where an Al atom resides on a surface dangling bond with an incorporating 0 atom between them. In order to construct the cluster model to represent a surface dangling bond, we focused on a part consisting of an Si rest-atom and three Si atoms in the second layer as denoted by hatched spheres. Those four Si atoms were included in the computational cluster shown in Fig.l(b), and the outer and lower chemical bonds were terminated by H atoms to make it possible to perform the quantum chemical calculations. The Si 4H 9+O+Al system was used for the model structure as an initial target to be etched by HF. First principles calculations were performed with the density functional theory, using the split valence-type basis set :6-3 1G**. The exchange energy was computed by the gradient-corrected formula developed by Becke [4] and the correlation energy calculated by the formula derived by Lee, Yang, and Parr [5]. The spin electronic
state was singlet. The computational programs used were TurboMol and Gaussian94. In the initial geometry, Si-Si and Si-H bond lengths were set to 2.35 and 1.48A, respectively, and all bond angles around Si atoms are set to 109.47*. During the geometry optimization, all the Si, 0, and Al surface atoms were allowed to move freely, and only H atoms were fixed to keep the crystalline structure.
RESULTS Fig.2 shows the atomic configurations and the potential energy change during the etching reaction of HF with AlO-Si structure on Si(111) surfaces. Initially, there exists no interaction between the HF molecule and the AlO-Si surface, because the HF molecule is far apart from the surface. It is recognized that Al metal make
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