(232t) Deep Electrochemical Etching of Silicon

Deep structures in silicon substrates are typically created by anisotropic etching in aqueous KOH solutions or in plasma processes such as Reactive Ion Etching (RIE). Electrochemical etching and photoelectrochemical etching are well known techniques and are commonly used to form porous silicon but have not been used for deep anisotropic etching. In this work we evaluate the potential for deep electrochemical etching of silicon without formation of porous silicon. In this process, silicon wafers are patterned using a mask. The wafer may be chemically etched using KOH to form an initiator pit, or inverted pyramid, revealing (111) surfaces. This initiator pit helps provide anisotropy in the subsequent electrochemical etch. Silicon is electrochemically etched in the presence of hydrofluoric acid (HF) to form a soluble silicon hexafluoride species. The etch mechanism is believed to include electrochemical and chemical components. In the electrochemical step, holes (or electrons missing from bonding orbital) at the surface initiate the attack of fluorine or oxygen species in the solution. In the chemical step, the breaking of Si backbonds may be favored in areas with weakly bonded sites such as ?hanging bonds? at the interface of (111) planes. In patterns with wide pitched structures, macropores with porous sidewall branching is typically observed. In patterns with tight pitches, there is no sidewall etching or pore branching. Sidewall etching may be prevented by tailoring the chemical reaction or interface such that there is a strong etch rate dependence on crystal plane.