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.

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 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	Aluminium Etch 2
General description	Etch of pure aluminium	Etch of aluminium + 1.5% Si
Chemical solution	H $_{2}$ O:H $_{3}$ PO $_{4}$ 1:2	PES 77-19-04
Process temperature	50 ^oC	20 ^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	Aluminium Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist E-beam resist	Aluminium Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist E-beam resist