The Christmas tree🎄appearance is actually produced by standing waves of intensity variation caused by the interference of light reflected from the substrate surface and recorded in the photoresist on the wafer.

Having strong standing-wave oscillations in a photoresist profile is normally not a desirable outcome but their presence – especially as sharp as they are on this sample produced by EULITHA AG‘s PhableX Lithography system – points to the capability of the photoresist process that can resolve very high-resolution features!

The “Christmas trees” like patterns seen in this cross-section SEM image are about 400nm wide and the standing wave features are about 50nm wide.