Specific Process Knowledge/Etch/KOH Etch: Difference between revisions
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KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster. | KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster. | ||
KOH-etching is a highly versatile and cheap way to realize micromechanical structures if you can live with the necessary Si<math>_3</math>N<math>_4</math>- or SiO<math>_2</math> masking materials and the | KOH-etching is a highly versatile and cheap way to realize micromechanical structures if you can live with the necessary Si<math>_3</math>N<math>_4</math>- or SiO<math>_2</math>-masking materials and the potassium contamination of the surface. The latter necessitates in most cases a wet post-clean ('7-up' or RCA-clean) if the wafer is to be processed further. | ||
At Danchip we use as a standard a 28 wt% KOH. The etch rate - and the selectivity towards a SiO<math>_2</math>-mask - is depending on the temperature. We normally use T=80 <sup>o</sup>C but may choose to reduce this to e.g. 60 <sup>o</sup>C or 70 <sup>o</sup>C in case of a high-precission timed etch (e.g. defining a thin membrane). In some cases we recommend to saturate the standard 28 wt% KOH with IPA with an etch temperature at T=70 <sup>o</sup>C (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved compared to the standard etch. Key facts for the two solutions are resumed in the table: | At Danchip we use as a standard a 28 wt% KOH. The etch rate - and the selectivity towards a SiO<math>_2</math>-mask - is depending on the temperature. We normally use T=80 <sup>o</sup>C but may choose to reduce this to e.g. 60 <sup>o</sup>C or 70 <sup>o</sup>C in case of a high-precission timed etch (e.g. defining a thin membrane). In some cases we recommend to saturate the standard 28 wt% KOH with IPA with an etch temperature at T=70 <sup>o</sup>C (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved compared to the standard etch. Key facts for the two solutions are resumed in the table: | ||
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===Etch rates: Empirical formula (Seidl et al)=== | ===Etch rates: Empirical formula (Seidl et al)=== | ||
The | The following empirical formula can be used for concentrations in the range of 10-60 wt%: | ||
R = k<sub>0</sub> [H<sub>2</sub>O]<sup>4</sup> [KOH]<sup>0.25</sup> e<sup>-E<sub>a</sub>/kT</sup>, | R = k<sub>0</sub> [H<sub>2</sub>O]<sup>4</sup> [KOH]<sup>0.25</sup> e<sup>-E<sub>a</sub>/kT</sup>, | ||