Specific Process Knowledge/Etch/Wet Polysilicon Etch

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Wet PolySi Etch

File:BHF-PolySi-Al-etch.jpg

Wet PolySilicon Etch (in the middle): positioned in cleanroom 4

The wet PolySi Etch is an isotropic silicon etch. This holds for both a poly-silicon thin-film as well as single-crystalline material such as a Si(100) surface. The PolySi Etch process is placed in a dedicated PP-tank in a laminar-flow bench in cleanroom 4.

The PolySi Etch is typically used for opening holes in poly-silicon thin-films, using photoresist as an etch mask. Due to its isotropic nature the under-etching (etch-bias) at least amounts to the thickness of the poly-silicon layer. Another example of usage is etching of "circular-shaped" holes in silicon substrates.

The PolySi Etch is based on the combined oxidation of silicon followed by dissolution of the silicon oxide. The etch solution consists of:

HNO₃ : BHF : H₂O - (20 : 1 : 20)

NB: The life time of the solution is only a few days.

PolySi Etch data

	PolySi Etch @ room temperature
General description	Etch of poly-si/Si(100)
Chemical solution	HNO3 : BHF : H2O (20 : 1 : 20)
Process temperature	Room temperature
Possible masking materials	Photoresist(min. 2.2 µm is recommended) LPCVD-oxide (TEOS)
Etch rate	RSi ~1000-2000 Å/min (depending on doping level) RSiO2 ~60 Å/min Photoresist (2.2 µm) withstand ~20-30 min
Batch size	1-25 wafers at a time
Size of substrate	2-4" wafers