Specific Process Knowledge/Etch/KOH Etch
KOH etch - Anisotropic silicon etch
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 SiFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _3} NFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _4} - or SiOFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} masking materials and the K 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 SiOFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} -mask - is depending on the temperature. We normally use T=80 oC but may choose to reduce this to e.g. 60 oC or 70 oC 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 oC (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved roughly a factor of 10 compared to the standard etch. Key facts for the two solutions are resumed in the table:
Comparing the two solutions
| 28 wt% KOH | 28 wt% KOH sat. with IPA | |
|---|---|---|
| General description |
Etch of Si(100) |
Etch of Si(100) with boron etch-stop |
| Chemical solution | KOH:HFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} O 500 g : 1000 ml | KOH:HFailed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle _2} O:IPA 500 g : 1000 ml : ?? ml |
| Process temperature | 60 oC
70 oC 80 oC |
70 oC
|
| Possible masking materials: |
Photoresist (1.5 µm AZ5214E) |
Photoresist (1.5 µm AZ5214E) |
| Etch rate |
~100 nm/min (Pure Al) |
~60(??) nm/min |
| Batch size |
1-25 wafers at a time |
1-25 wafer at a time |
| Size of substrate |
4" wafers |
4" wafers |
| Allowed materials |
|
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