CHEMICAL AND ELECTROCHEMICAL ANISOTROPIC DISSOLUTION OF SILICON IN ETHYLENEDIAMINE PLUS PYROCATECHOL PLUS WATER MEDIA

Etching in ethylenediamine+pyrocatechol+water (EPW)baths occurs via a parallel chemical-electrochemical dissolution mechanism. From interfacial electrical capacitance measurements it is concluded that over the whole etching potential region of interest, i.e. up to - 0.75 V vs. SCE, the surface is depleted of majority carriers. A flat-band potential of approx. - 0.6 V has been estimated. The role of pyrocatechol is to act as a ligand to dissolve Si(IV). The ionization state of the ligand has been studied conductimetrically and the solution probably contains the monoanion, which further dissociates on complexation with Si(IV). The presence of water is essential for etching to occur either by a chemical or an electrochemical mechanism. The reason for this is the necessary involvement of H2O, as well as OH- (or F-), in the intermediate chemical steps following either chemical attack of a Si-Si bond or hole capture. The (100) and [111] surfaces have very similar values of the passivation potential. It is concluded that the origin of anisotropy is the difference in both the rates of the chemical reaction resulting from inductive effects on the Si-OH bond and the surface potentials of the different Si faces.