Specific Process Knowledge/Etch/Etching of Silicon Oxide/SiO2 etch using ASE/More test with CF4/H2, CHF3 and C4F8/H2 - SiO2 etch

More test with CF4/H2 - SiO2 etch

Tests performed by Maria Farinha @DTU Nanolab

The gas flows were combined and the following results were achieved. It´s visible that with higher CF₄ the selectivity is not good, so by adding H2, until 1:1, the selectivity improves. Additionally, the uniformity gets slightly worse while going for 1:1 of gas chemistry.
The recipe CF4lowCP 22.5/22.5 seems to be the only one good enough to use. It is a slow etch recipe, with good selectivity with AZ5214E resist.

CF4ICP	SiO2 ER (nm/min)	Uniformity (SiO2 etch)	Selectivity (SiO2:resist)	Si3N4 ER (nm/min)	SixN ER (nm/min)
45 CF4 + 0 H2	71.4	11.7%	0.74	-	-
35 CF4 + 10 H2	69	12.6%	0.89	-	-
22.5 CF4 + 22.5 H2	58.6	13.3%	1.43	84.4	42.9

• The uniformity considers a 100mm wafer, calculated with 5 points.

CF4lowCP	SiO2 ER (nm/min)	Uniformity (SiO2 etch)	Selectivity (SiO2:resist)	Si3N4 ER (nm/min)	SixN ER (nm/min)
45 CF4 + 0 H2	24.9	11.5%	0.47	-	-
35 CF4 + 10 H2	22.6	11.6%	0.64	-	-
22.5 CF4 + 22.5 H2	17.8	13.7%	1.85	28	22.5

Profile pictures of some tests - April 2023

Even in the cases with the resist strip (plasma asher 2 processing for 10/15min), there are some traces of resist left on the edges of the features.

Also, it´s noticeable that the feature profile is trenching, being more evident with faster etch rates.

CF4ICP 45 CF4 + resist strip	CF4ICP 45 CF4 / 10 H2 + resist strip	CF4ICP 35 CF4/ 10 H2	CF4lowCP 45 CF4/ 10 H2	CF4lowCP 45 CF4/ 0 H2 + resist strip	CF4lowCP 35 CF4/ 10 H2 + resist strip	CF4lowCP 22.5 CF4/ 22.5 H2 + resist strip

Recipes and results - CF₄ / H₂ tests

Recipe	Recipe parameters	Process time	Date	SEM picture	Etch rate in SiO2	Etch rate in resist (AZ5214E inverse)	Selectivity (SiO2:resist)
CF4ICP 22.5/22.5	CF4= 22.5 sccm H2= 22.5 sccm Coil= 800W Platen= 15W Press= 2.5mTorr Temp= 20°C	11:00 min	11/09/2023		68 nm/min +/- 8.5%	60,2 nm/min +/- 9%	1.13
CF4ICP 35/10	CF4= 35 sccm H2= 10 sccm Coil= 800W Platen= 15W Press= 2.5mTorr Temp= 20°C	08:00 min	March 2023		69 nm/min +/- 11.1%	77,1 nm/min +/- 9.4%	0.9
CF4ICP 45/0	CF4= 45 sccm H2= 0 sccm Coil= 800W Platen= 15W Press= 2.5mTorr Temp= 20°C	08:00 min	March 2023		71.4 nm/min +/- 10.3%	96,5 nm/min +/- 6.0%	0.74
CF4ICP 45/10	CF4= 45 sccm H2= 10 sccm Coil= 800W Platen= 15W Press= 2.5mTorr Temp= 20°C	08:00 min	March 2023		67.8 nm/min +/- 14.9%	88,86 nm/min +/- 4.8%	0.76
CF4lowCP 22.5/22.5	CF4= 22.5 sccm H2= 22.5 sccm Coil= 150W Platen= 25W Press= 2.5mTorr Temp= 20°C	20:00 min	Sept 2023		23.8 nm/min +/- 11%	20,6 nm/min +/- 19.4%	1.16
CF4lowCP 35/10	CF4= 35 sccm H2= 10 sccm Coil= 150W Platen= 25W Press= 2.5mTorr Temp= 20°C	10:00 min	Feb 2023		22.65 nm/min +/- 10.2%	35.9 nm/min +/- 10.1%	0.63
CF4lowCP 45/0	CF4= 45 sccm H2= 0 sccm Coil= 150W Platen= 25W Press= 2.5mTorr Temp= 20°C	10:00 min	Feb 2023		24.9 nm/min +/- 10.1%	52,3 nm/min +/- 7.1%	0.47
CF4lowCP 45/10	CF4= 45 sccm H2= 10 sccm Coil= 150W Platen= 25W Press= 2.5mTorr Temp= 20°C	10:00 min	Feb 2023		29,4 nm/min +/- 13.7%	100,6 nm/min +/- 16.5%	0.29

Recent results - CF₄ / H₂ tests

The following results were processed on chips bonded to a 100mm wafer. They were patterned with 915 UVN resist (DUV negative) and 65nm of BARC. The SiO2 layer was 2um (deposited on the C1 furnace).
(25 jan 2024)

CF4ICP @ 30W platen, 8min
Values					SEM pictures	Comments
Structure size (nm)	structure pitch (nm)	SiO2 ER (nm/min)	Resist ER (nm/min)	Selectivity SiO2/resist
250	500	69.36	114.38	0.61		Across the sample, the resist was fully etched away
500	1000	87.04	114.38	0.76
1000	2000	98.83	114.38	0.86
2000	4000	100.03	114.38	0.87

CF4lowCP @ 45W platen, 15min
Values					SEM pictures	Comments
Structure size (nm)	structure pitch (nm)	SiO2 ER (nm/min)	Resist ER (nm/min)	Selectivity SiO2/resist
250	500	33.54	33.45	1.0		there was visible redeposition across smaller structures
500	1000	35.94	30.45	1.18
1000	2000	36.54	29.85	1.22
2000	4000	38.09	28.65	1.33