Semi Additive Plating is a standard process name in pcb or substrate manufacturing whereby dry film is attached on barely 1-2um electroless Cu plating on dielectric layer. Then use Cu plating to build up the trace thickness. Subsequently the dry film is stripping, then Cu etching only for 1-2 um thickness. The process help to achieve fine line capability down to less than 20um.

Conventional pcb substraction method is etching on copper clad laminate (CCL) , with 0.5oz or 0.33oz Cu; this processs is limited to 30-50um line width only.

semi-additive process with photolithography is basically coating adhesion/seed layers on working substrate then having the sample go through photolithography process to pattern fine features for following electroplating of metals (usually Cu).

Typical adhesion layer can be done with ebeam evaporation or sputtering. Usually Cr and Ti are used.

Seed layer can be Cu, Au, Ni, etc.

Photoresist thickness for photolithography process can different from brand to brand, type to type. Let's say Shipley 1827 can be from 2um to 4 or 5um with single spin which defines the feature plated metal thickness.

Thank you all