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By Maria Farinha @nanolab, Jan 2023

Important! The pressure settings used below may no longer be permitted, always check with the Dry etch group.

As stated by Nguyen et al., the SF₆ and O₂ fluxes are only separated after 4 s during the C-step. Shorter time steps than that do not execute the function of the clearing. After testing recipes with only 2 s of clearing and with no clear step, it was understood it could be eliminated, showing more depth as well as less undercut when going for shorter cycles. From then on, not CORE but ORE recipes were applied D/E = O/RE.
Moreover, during the E-step, the MFC (mass flow controller) presented a delay to read the pressure when compared with the power, creating unwanted bias. To fight it, the E-step was divided into E₁ and E₂. The E₁ of only 2 s is enough to stabilize the pressure, and E₂ the profile is etch correctly, without the unwanted bias.

Going further, the process is set with 4 steps, Oxidize, Removal, E₁ and E₂ steps. In the E₁, the pressure will rise, without power applied, so that for the E₂ the pressure is already rising and the power starts to be applied.

Pillars

Using patterned samples of 1 μm pillars with 50 nm of Al₂O₃. After the tests, approximately 32nm of Al₂O₃ were still intact. During the recipe, Pegasus 2 conditions were: Outer EM=10A, T=20°C, no clamping, no He BGC, no coil power and all heaters OFF.

1μm Pillar recipe from March 2022 - #244 = 122 min
	Time (s)	Pressure (valve control)	O₂ flow (SCCM)	SF₆ flow (SCCM)	Platen power (W)
O-step	10	3%	200	0	40
R-step	10	100%	0	5	40
E₁-step	2	4%	0	350	40
E₂-step	7	4%	0	350	300

* Pillars profile effect when varying the etch time.

Holes

Using patterned samples of 1 μm holes with 50 nm of Al₂O₃. After the tests, approximately 28nm of Al₂O₃ were still intact. When using this recipe, by adjusting the number of cycles, approximately 10 μm were achieved maintaining a straight profile. When going for deeper profiles, 17 μm were also achieved, but the profile starts to get positive. Further work may solve the issue. During the recipe, Pegasus 2 conditions were: Outer EM=10A, T=20°C, no clamping, no He BGC and all heaters OFF.

1μm holes recipe from March 2022
	Time (s)	Pressure	O₂ flow (SCCM)	SF₆ flow (SCCM)	Platen power (W)	Coil power (W)
O-step	10	220 mTorr	200	0	40	0
R-step	10	100%	0	40	40	0
E₁-step	2	220 mTorr	0	1200	0	0
E₂-step	1-5	220 mTorr	0	1200	1	2000

* For #288 cycles, 120 min, the profile depth reaches 17.6 μm. For #144 cycles, 60 min, the profile depth reaches 10.1 μm. Width variation presented in the pictures.

Nanoholes

The nanoholes are 200nm wide, with 400nm pitch with 100 nm Al₂O₃ mask. During the recipe, Pegasus 2 conditions were: Outer EM=10A, T=20°C, no clamping, no He BGC and all heaters OFF.

200nm nanoholes recipe from March 2022
	Time (s)	Pressure (valve control)	O₂ flow (SCCM)	SF₆ flow (SCCM)	Platen power (W)
O-step	10	220 mTorr	200	0	40
R-step	10	100%	0	40	*
E₁-step	2	220 mTorr	0	350	0
E₂-step	0-15	220 mTorr	0	350	100-300

*The R-power value was changed and observed.

The most suitable removal power was 40 W, removing the bottom of the profile correctly without damaging the top part of the profile.

Isotropic etch

Some isotropic etches were performed, intercalated with anisotropic etches. The nanoholes are 200nm wide, with 400nm pitch with 100 nm Al₂O₃ mask. The recipe presented in the table was repeated either 2 or 3 times in order to achieve the picture results. During the recipe, Pegasus 2 conditions were: Outer EM=10A, T=20°C, no clamping, no He BGC and all heaters OFF.

**Table: nanoholes + isotropic etch from May 2022**
		Time (s)	Pressure	O₂ flow (SCCM)	SF₆ flow (SCCM)	Platen power (W)	Coil power (W)
Nanoholes etch	O-step	10	220 mTorr	200	0	40	0
	R-step	10	100%	0	0	40	0
	E₁-step	2	220 mTorr	0	350	0	0
	E₂-step	0-15	220 mTorr	0	350	100-300	0
Isotropic etch	O-step	30	200 mTorr	200	0	0	2000
	R-step	30	100%	0	40	40	0
	E-step	20	200 mTorr	0	1200	0	2000