A process for filling one or more etched holes defined in a frontside surface of a wafer substrate. The process includes the steps of: (i) depositing a layer of a thermoplastic first polymer onto the frontside surface and into each hole; (ii) reflowing the first polymer; (iii) exposing the wafer substrate to a controlled oxidative plasma; (iv) optionally repeating steps (i) to (iii); (v) depositing a layer of a photoimageable second polymer; (vi) selectively removing the second polymer from regions outside a periphery of the holes using exposure and development; and (vii) planarizing the frontside surface to provide holes filled with a plug comprising the first and second polymers, which are different than each other. Each plug has a respective upper surface coplanar with the frontside surface.