Chemical Etching in Alkaline Solutions

Alkaline etchants are anisotropic. The etch rate for the (111) crystal planes of the Si crystal is smaller by about two orders of magnitude than the etch rate of any other crystal plane. The etch rate ratio between other crystal planes like (100) and (110) depends on etchant concentration and temperature, but doesn t usually exceed a factor of two [Sa6]. Addition of oxidizing agents reduces the anisotropy. The etch rate of (100) Si and Si02 in KOH at different temperatures is shown in Fig. 2.2. 

The etch rate of highly p-doped silicon ( 1019 cm4) is found to be reduced by orders of magnitude compared to the etch rate of low doped or highly doped n-type silicon of (100) orientation. This effect is shown in Fig. 2.3 for KOH note that the etch rate is significantly affected by the amount of dissolved oxygen 

In the temperature regime shown the etch rate of (110) silicon is roughly 1.5 times that 

The (100) etch rates show an Arrhenius dependency on temperature with activation energies of about 0.3-0.6 eV, depending on the alkaline solution used [Krl, Se3]. 

Etching of silicon in alkaline solutions occurs under evolution of hydrogen with a ratio of two molecules H2 per dissolved Si atom. This ratio is found to be reduced under positive bias [Pa6] or by addition of oxidizing agents like H202 [Sc6], If the anodic bias is increased beyond the passivation potential (PP), the dissolution rate is reduced by orders of magnitude. 

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Anisotropic treatment Chemical etching in alkaline solutions Lehmann (2007), Matthias et al. (2005b), Chemienko et al. (2013), Zharova etal. (2011)... 

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