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Wet Silicon Nitride Etch

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"

Nitride Etch data

	Nitride Etch @ 180 ^oC	Nitride Etch @ 160 ^oC
General 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 R_Si₃N₄ / R_SiO₂	~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

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Comparison of Platinum Etch Methods

	Pt wet etch	IBE (Ionfab300+)
Generel description	Wet etch of Pt	Sputtering of Pt
Etch rate range	~?nm/min	~30nm/min (not tested yet)
Etch profile	Isotropic	Anisotropic (angles sidewalls, typical around 70 dg)
Masking material	None (only used for stripping Pt)	Any material that is allowed in the chamber, photoresists included
Substrate size	Any size and number that can go inside the beaker in use	Smaller pieces glued to carrier wafer #1 50mm wafer #1 100mm wafer #1 150mm wafer #1 200mm wafer
Allowed materials	No restrictions cross contamination wise as long as you use the right beaker and make sure that they are safe to enter in the chemicals	Silicon Silicon oxides Silicon nitrides Metals from the +list Metals from the -list Alloys from the above list Stainless steel Glass III-V materials Resists Polymers Capton tape