Implementations of the present disclosure generally relate to apparatus and methods for uniform deposition of thin films on substrates. In one implementation, a plasma-processing chamber comprises a chamber body including chamber walls, a chamber floor, and a lid support. The plasma-processing chamber further comprises a substrate support assembly at least partially disposed within the chamber body and configured to support a substrate. The plasma-processing chamber further comprises a lid assembly disposed over the support assembly and positioned on the lid support wherein the lid assembly and the chamber body define a first processing volume. The plasma-processing chamber further comprises a bottom isolation assembly that circumscribes at least a portion of the substrate support assembly and is vertically movable from a loading position to a processing position. A seal is formed between the bottom isolation assembly and the lid assembly when the bottom isolation assembly is in the processing position.