Descumming Etch Residue

When using CHF3 and/or CF4 to etch GCA keys into a wafer, after removal of photoresist, often there is still a bumpy layer of crap on the surface (10~20 nm).

The bumps are from the plasma interacting with the photoresist to make a different polymer that’s a pain to remove.

I found oxygen plasma (the PT-72, Oxford, Branson) to be ineffective at removing the crap. However, a MOS level clean does successfully remove the crap.

I’m not sure if it’s the acid step or the base step, but the MOS clean goes as follows:

10 minute soak in (6L DI H2O, 1L H2O2, 1 L NH3OH), rinse till R > 16 MOhm/cm 10 minute soak in (6L DI H2O, 1L H2O2, 1 L HCl), rinse till R > 16 MOhm/cm

The optional step of 10 s etch in 20:1 HF, followed by a rinse, doesn’t seem to be required, but doesn’t hurt.

Follow it all with a spin dry, and have happy clean wafers that are good to go.