Mechanism for Breakage of Si-O Networks of SiO 2 Films in HF Solutions
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Tomoki Oku, Ryo Hattori and Kazuhiko Sato Mitsubishi Electric Corporation, Optoelectronic and Microwave Devices Laboratory, 4-1 Mizuhara, Itami City, Hyogo 664, Japan, [email protected] ABSTRACT An extended Htickel calculation was employed to calculate the total electronic energy and the electron density during the breakage of the Si-O networks of a (F3SiO) 2FSiOSiF(OSiF 3)2 cluster modelled on the ox-cristobalite structure. The Si-O networks are opened by the attack of F- ions on the silicon atoms, and the reaction is exthothermic by 2.7 eV through the attack of H+ ions on the oxygen atoms. Although the hydrogen termination of the oxygen atoms is an early reaction, the fluorine termination of the silicon atoms is a late reaction. The atomic bond population on the Si-O bonds decreases to zero by opening the Si-O networks. We conclude that the o•-cristobalite and ctquartz SiO 2 are dissolved in HF solutions since the Si-O networks are easily opened by the attack of F ions. Our conclusion indicates that both cosite and stishovite Si0 2 , which are not dissolved in the HF solutions, are composed of Si-O networks that can be hardly opened by the attack of Fions. Moreover, we propose the continual breakage of Si-O networks without the desorption of H 20 molecules as an etching mechanism of Si0 2 films. INTRODUCTION The wet chemical etching technique of silicon dioxide (Si02) films by hydrogen fluoride (HF) solutions has been used as a clean and selective etching process with respect to the substrate in the fabrication of the silicon ultralarge scale integrated circuits [1], micromachines [2] and GaAs integrated circuits [3]. The progress of microfabrication requires the clarification of the molecularlevel mechanism to ensure controllable etching on an atomic scale. However, the molecular-level mechanism is not so simple that the etching reaction can be fully described by SiO 2 + 2HF 2 - + 2H÷ -- SiF4 + 2H 20. In particular, the intermediate steps of the reaction have recently started to be clarified by some researchers [1, 2, 4, 5]. Trucks et.al applied an ab-initio molecular orbital calculation to the etching reaction on silicon surfaces [1]: H3 Si-SiF 3 + HF -- transition state SiH 4 + SiF 4 . They explained the removal of SiF 4 from silicon surface by the insertion of HF into the Si-Si back bond. However, most experimental research showed that SiO 2 films are etched through the nucleophilic HF 2 - attack on Si 6 + and the electrophilic H+ ion attack on 0 6- [6]. We proposed in a previous paper that the desorption of the SiF 4 molecules is a kind of SN 2 process (nucleophilic substitution) involving the dissociation of the HF 2 ion, the attack of the H÷ ion and hydrogen passivation [5]: (HO) 3Si-O-SiF 3 + HF 2- + H÷ -- (HO) 3Si-O-SiF 3 + F +H÷ + HF (HO) 3 SiOH + SiF 4 + HF. After the desorption of the SiF 4 molecules, the breakage of the Si-O networks proceeds to the etching of Si0 2 films in HF solutions. Monk et al. proposed that the etching of SiO 2 films is a two-step phenomenon [2]. In the first step,
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