A gas distribution structure for supplying reactant gases and purge gases to independent process cells to deposit thin films on separate regions of a substrate is described. Each process cell has an associated ring purge and exhaust manifold to prevent reactive gases from forming deposits on the surface of the wafer between the isolated regions. Each process cell has an associated showerhead for conveying the reactive gases to the substrate. The showerheads can be independently rotated to simulate the rotation parameter for the deposition process.