1. A method of depositing a material onto a peripheral region of a substrate by capacitively coupled Plasma Enhanced Chemical Vapour Deposition (PE-CVD) comprising the steps of: providing an apparatus comprising: a chamber; a first electrode comprising a substrate support positioned in the chamber; a second electrode comprising a gas inlet structure positioned in the chamber, the gas inlet structure comprising: an edge region, a central region which protrudes farther into the chamber than the edge region by a depth from 5 mm to 45 mm, and one or more precursor gas inlets for introducing a PE-CVD precursor gas mixture to the chamber, the edge region and the central region both constituting part of the second electrode, wherein the precursor gas inlets are disposed in the edge region, and the central region is spaced apart from the substrate support to define a plasma dark space channel; and an RF power source connected to the gas inlet structure for supplying RF power thereto; positioning the substrate on the substrate support; and performing a capacitively coupled PE-CVD process in which RF power is supplied to the gas inlet structure to generate a plasma which has a dark space in the plasma dark space channel, thereby causing the material to be deposited only onto a peripheral region of the substrate.