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-etching is a highly versatile and cheap way to realize micro mechanical 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 ([[Specific Process Knowledge/Wafer cleaning/7-up & Piranha|'7-up']] or [[Specific Process Knowledge/Wafer cleaning/RCA|RCA-clean]]) if the wafer is to be processed further.
+KOH-etching is a highly versatile and cheap way to realize micro mechanical structures if you can live with the necessary Si<sub>3</sub>N<sub>4</sub>- or SiO<sub>2</sub>-masking materials and the potassium contamination of the surface. The latter necessitates in most cases a wet post-clean ([[Specific Process Knowledge/Wafer cleaning/7-up & Piranha|'7-up']] or [[Specific Process Knowledge/Wafer cleaning/RCA|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-precision 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<sub>2</sub>-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-precision 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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 |'''Chemical solution'''
 |
-:KOH:H<math>_2</math>O  - 500 g : 1000 ml
+:KOH:H<sub>2</sub>O  - 500 g : 1000 ml
 |
-:KOH:H<math>_2</math>O:IPA -  500 g : 1000 ml : ?? ml
+:KOH:H<sub>2</sub>O:IPA -  500 g : 1000 ml : ?? ml
 |-
 |'''Process temperature'''
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 |'''Possible masking materials'''
 |
-*Stoichiometric Si<math>_3</math>N<math>_4</math>
+*Stoichiometric Si<sub>3</sub>N<sub>4</sub>
-*Si-rich Si<math>_3</math>N<math>_4</math>
+*Si-rich Si<sub>3</sub>N<sub>4</sub>
-*PECVD Si<math>_3</math>N<math>_4</math>
+*PECVD Si<sub>3</sub>N<sub>4</sub>
-*Thermal SiO<math>_2</math>
+*Thermal SiO<sub>2</sub>
 |
-*Stoichiometric Si<math>_3</math>N<math>_4</math>
+*Stoichiometric Si<sub>3</sub>N<sub>4</sub>
-*Si-rich Si<math>_3</math>N<math>_4</math>
+*Si-rich Si<sub>3</sub>N<sub>4</sub>
-*PECVD Si<math>_3</math>N<math>_4</math>
+*PECVD Si<sub>3</sub>N<sub>4</sub>
-*Thermal SiO<math>_2</math>
+*Thermal SiO<sub>2</sub>
 |-
 |'''Etch rate in Si'''
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 |-
-|'''Etch rate in SiO<math>_2</math>'''
+|'''Etch rate in SiO<sub>2</sub>'''
 |
 *~6nm/min (80 <sup>o</sup>C) ''theoretical value''