Specific Process Knowledge/Etch/Etching of Silicon Oxide/SiO2 etch using ASE/CF4 recipes for SiO2

*Unless otherwise stated, all content in this section was done by Maria Farinha@DTU Nanolab, April 2023

Due to the instability of older SiO₂ etch recipes, 1SIOICP1 and TSIO2_02, new recipes are being developed, with a CF₄/H₂ chemistry. Two new recipes were created, to replace the older ones, CF4ICP and CF4lowCP. The first one aims for thicker etches, with higher ER, while the other is a slower etch, dedicated to thinner layers.

Parameter	Recipe name: CF4ICP	Recipe name: CF4lowCP
Coil Power [W]	800	150
Platen Power [W]	15	25
Platen temperature [oC]	20	20
CF4 flow [sccm]	*	*
H2 flow [sccm]	*	*
Pressure [mTorr]	2.5	2.5

• CF₄ and H₂ flow values were changed in order to find the best recipe regarding selectivity between SiO₂ and the resist and also the uniformity.

Results with pat Si wafer, 1um of SiO2 and AZ5214E resist - April 2023

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

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

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

Profile pictures of some tests - April 2023 @mfarin

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

Further tests will be done, testing these recipes' etch rates regarding silicon and silicon nitride, since most of the time these are the adjacent layers.