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Electrochemical Investigation into the Dissolution Mechanism of Anodic Oxide Films on Silicon Dongqing Liu and Daniel J. Blackwood Department of Materials Science and Engineering, National University of Singapore, Block E3A, 7 Engineering Drive 1, Singapore 117574 ABSTRACT Electropolishing of p-type silicon has been investigated over a wide range of HF concentrations (0.01-11 wt. %) by potentiodynamic polarization. Oxide dissolution rates were determined from the plateau current densities observed in the electropolishing region during the reverse sweeps; i.e. where the growth and dissolution rates of the anodic oxide film are believed to be equal. Based on the shape of the CV curves the oxide dissolution process was treated as a corrosion process controlled by the dissolution of a salt film, that is its rate controlled by removal of dissolved products away from the surface rather than reactants to the surface as previously proposed. Although a contribution from HF from bulk to surface cannot be completely ruled out, because the removal of the initial dissolution product can be by either mass transport or further chemical reaction with HF species in solution this mechanism is capable of explaining the dependence of the dissolution rate on HF concentration for the whole range investigated.

INTRODUCTION Because of its excellent dielectric properties silicon dioxide films are widely used in the manufacturing of microelectronics. There are many methods for oxide formation, but electrochemical anodization in wet etchants based on HF offers the advantage of a lower deposition temperature compare with thermal oxides [1,2]. However, research on the anodic oxide dissolution rates has been limited compared with that for the dissolution of thermal and CVD formed oxides. Electrochemical potentiodynamic polarization experiments are an attractive way to study both the growth and dissolution mechanisms of anodic oxide films on silicon. HF does not dissolve bulk silicon at any appreciable rate, therefore within the electrochemical polishing region the anodic oxide dissolution rate should be related to the anodic oxide formation rate, which in turn is given by the anodic current density [3]. Here, cyclic voltammetry (CV) curves for p-type silicon are recorded in order to investigate the relationship between anodic oxide dissolution rate and HF electrolyte composition. A possible dissolution mechanism is proposed based on the generated data. EXPERIMENT A boron doped p-type silicon wafers of (100) orientation and 0.2 Ω cm resistivity was cleaved into 1 cm x 1 cm squares and the native oxide removed in 10 vol% HF, after which GaIn eutectic was applied to the back surface to ensure an Ohmic contact. The specimens were then mounted vertically as the working electrode, with an exposed surface area of 0.017 cm2, in a standard three electrode polytetrafluoroethylene electrochemical cell. A platinum mesh counter electrode and a saturated calomel reference electrode (SCE) completed the cell. The SCE

reference electrode was furnished with a