Specific Process Knowledge/Etch/Wet Silicon Nitride Etch

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Wet Silicon Nitride Etch

Wet nitride etch: positioned in cleanroom 4

Wet Etching of silicon nitride - stoichiometric and Si-rich - is done in a dedicated laminar flow bench with an integrated quartz tank (Tiger Tank - TT-4). The quartz tank can take up to one 6" wafer carrier. The flow bench is placed in cleanroom 4. The process is mainly used to strip silicon nitride (maskless), but can also be used for masked etching of silicon nitride using some kind of silicon oxide as etch mask. However, the wet silicon nitride etch is isotropic - meaning that the under-etching (etch-bias) at least amounts to the thickness of the silicon nitride layer.

The etch solution is initially 85 wt% H₃PO₄ which is heated up to the boiling temperature - ca. 157 ^oC. Water is allowed to boil off thus raising the concentration and the boiling temperature of the solution until a boiling temperature of 180 ^oC is reached. Thereafter, the wafers are submerged into the bath and the water-cooled lid is closed to maintain the concentration and the boiling temperature. In some cases a lower boiling temperature is chosen - typically 160 ^oC - which lowers the etch rate and improves the selectivity R_Si3N4 / R_SiO2.

NB: Great care has to be taken in this process due to risk of "shock-boiling"

Comparison of Wet Silicon Nitride Etch

	Nitride Etch @ 180 oC	Nitride Etch @ 160 oC
Generel description	Etch/strip of silicon nitride	Etch/strip of silicon nitride
Link to safety APV and KBA	see APV here see KBA here	see APV here see KBA here
Chemical solution	H3PO4 (85 wt%)	H3PO4 (85 wt%)
Process temperature	180 oC	160 oC
Possible masking materials	Thermal oxide (converted si-rich surface) LPCVD-oxide (TEOS) PECVD-oxide	Thermal oxide (converted si-rich surface) LPCVD-oxide (TEOS) PECVD-oxide
Etch rate	~85 Å/min (stoichiometric Si3N4) ~60 Å/min (Si-rich Si3N4) ~30 Å/min (annealed Si-rich Si3N4) ~4 Å/min (Thermal oxide)	~26 Å/min (Si-rich Si3N4)
Selectivity RSi3N4 / RSiO2	~20	The selectivity (sirich nitride:oxide) is higher at 160 oC than at 180 oC
Batch size	1-25 wafers at a time	1-25 wafer at a time
Size of substrate	2-6" wafers	2-6" wafers
Allowed materials	Silicon Silicon nitrides Silicon oxides	Silicon Silicon nitrides Silicon oxides