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 Si $_{3}$ N $_{4}$ - or SiO $_{2}$ 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 SiO $_{2}$ -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 compared to the standard etch. Key facts for the two solutions are resumed in the table:

KOH 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:H $_{2}$ O 500 g : 1000 ml	KOH:H $_{2}$ O:IPA 500 g : 1000 ml : ?? ml
Process temperature	60 ^oC 70 ^oC 80 ^oC (standard - fast etch)	70 ^oC
Possible masking materials:	Stoichiometric Si $_{3}$ N $_{4}$ Si-rich Si $_{3}$ N $_{4}$ PECVD Si $_{3}$ N $_{4}$ Thermal SiO $_{2}$	Stoichiometric Si $_{3}$ N $_{4}$ Si-rich Si $_{3}$ N $_{4}$ PECVD Si $_{3}$ N $_{4}$ Thermal 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}
Etch rate	~0.4 µm/min (60 ^oC) ~0.7 µm/min (70 ^oC) ~1.3 µm/min (80 ^oC)	~0.2 µm/min (70 ^oC) in p⁺⁺ (doping level > 5x10¹⁹ cm^-3)
Roughness	Typical: 100-600 Å	May form hillocks (pyramidal)
Batch size	1-25 wafers at a time	1-25 wafer at a time
Size of substrate	4" wafers	4" wafers
Allowed materials	Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride	Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride