Etching of very thin layer of SiO2

Pegasus 3 is not meant to be used as an SiO2 etcher. Please do not develop SiO2 recipes on this machine. Often SiO2 etch recipes are very polymerizing and that can disturb the chamber conditions for the silicon etching recipes.

The recipe developed here is only for etching through a very thin SiO2 layer and does not have a high selectivity to silicon to avoid the recipe to become too polymerizing. It is also not very uniform over the wafer, so when it is used for etching down to silicon it will also etch a lot in the silicon and in areas more than other.

Test work done

Unless otherwise stated, all content in this section was done by Berit Herstrøm, DTU Nanolab
The tests were done on 6" wafers. Silicon wafers with 1µm thermal SiO2 and mask less. Measurements were done on the ellipsometer before at after the etch to map the etch rate.

Tests with SiO2 etch
Wafer number	C4F8	SF6 flow	Ar	Platen power	Pressure	SiO2 Uniformity results	SiO2 Center Etch rate	Si Center Etch rate	Si Uniformity
Y34	50 sccm	50 sccm	0 sccm	100 W	80 mTorr	±23.3%	36.0 nm/min (highest)
Y35	25 sccm	75 sccm	0 sccm	100 W	80 mTorr	±20.8%	41.3 nm/min (highest)
Y36	25 sccm	75 sccm	0 sccm	100 W	10 mTorr	±22.1%	36.2 nm/min (lowest)
Y37	25 sccm	75 sccm	0 sccm	100 W	40 mTorr	±24.6%	24.4 nm/min (lowest)
Y38	25 sccm	75 sccm	0 sccm	100 W	60 mTorr	± 8.1%	34.0 nm/min (highest in center and at edge)	141.4@15s, 137.5@2min	± 13.9%@15s, ± 16.5%@2min
Y39	50 sccm	150 sccm	0 sccm	100 W	10 mTorr	±25.0%	34.8 nm/min (lowest)
Y40	8.33 sccm	50 sccm	100 sccm	100 W	10 mTorr	±15.1%	35.8 n/min (lowest)	70.8@15s, 107.5@5min	± 17.6%@15s, ± 11.1%@4min

Etch rate uniformity