Experimental electrochemical investigation of silicon/aluminum in tetra-methyl ammonium hydroxide (TMAH

This thesis considers the micromachining of silicon for the fabrication of micro-sensors, microactuators, and microsystems. At present it is problematic to anisotropically etch silicon, in wet etchants such as TMAK while protecting deposited aluminum layers. This is especially a problem in CMOS-compatible micromachining, where the etch occurs as a post-process step after the creation of the microelectronic circuitry. It is suspected that this difficulty is related to electrochemical phenomena, and this work explores the electrochemistry of Si/TMAH and Al/Si/TMAH by measurements of open-circuit potentials (OCP) of a galvanic cell. It is found that the presence of Al in the etchant, electrically connected to the silicon, dominates the electrochemical reactions, and shifts the overall cell potential to a potential substantially more negative than the OCP of Si only. This change in potential reduces the Si etch reaction, as manifested in the relative rate of bubbling at the Si electrode, especially for n-type silicon, consistent with literature results from linear-sweep voltammetry. These results are compared with the result of Ashruf et at, and found to form a consistent electrochemical understanding. The variation of the cell behaviour with solution temperature and pH are analyzed, and found to be inconsistent with the Nernst equation, confirming the non-equilibrium nature (irreversibility) of the silicon anisotropic etch reaction.