The Lam 4420 is a plasma etcher for the patterning of polysilicon layers. It may also be used for silicon nitride. The powered upper electrode generates a medium to low density plasma, and the silicon is etched by fluorine ions from SF6. Cl- and HBr based chemistries may also be used to obtain more vertical sidewalls. In the chamber, the wafers are mechanically clamped to a water cooled platen and helium backside gas provides coupling.
Allowed materials: silicon nitride, poly-silicon, TiW (and Mo)
This tool has a mechanical clamp so wafers must have a standard 56 mm primary flat. Secondary flats will be covered by an oversized ring. As a result of this reing, the edge of the wafer will not be etched. The size of the edge exclusion is yet to be determined for the oversized clamp rings. For patterned wafers, EBR must be sufficient to clear the wafer beneath the ring.