Electrochemical effects during anisotropic bulk-silicon etching with doped and undoped TMAHW solutions

Tetra-methyl ammonium hydroxide/water (TMAHW) solutions are gaining considerable interest as alternative anisotropic silicon etchant to the more usual KOH and EDP etchants because of their compatibility with CMOS processing and relatively low toxicity. TMAH exhibits a high etch rate for lower concentrations of solution, however this rate falls dramatically due to the inevitable formation of hillocks on the <100> surface. For TMAH concentrations above approximately 20 wt.%, hillock formation is markedly reduced, however the intrinsic etch rate at these concentrations is low. Using TMAH at lower concentrations has the advantage of being more economical, both in terms of cost and time. Furthermore, it has been shown that oxidizing agents can be added to TMAH, such as ammonium persulfate (NH4)2S2O8 to eliminate hillock formation at these reduced concentrations. Our investigations have the aim to determine the effects of this additives on the <100> silicon etch rate and anisotropy under different conditions, i.e. TMAH concentration, temperature, oxidizer concentration and frequency of oxidizer additions. In addition the role of the redox potential of the etchant solution is investigated and anomalous high <100> silicon etch rates under anodic polarisation with respect to OCP, are reported and discussed.