Grooved backing plate for standing wave compensation

A plasma processing chamber, comprising:

• a chamber body defining a process volume;
• a substrate support disposed in the process volume;
• a radio frequency (RF) power source operable to provide power to generate a plasma within the process volume;
• and
• a backing plate positioned over the processing volume and comprising:
• a rectangular body having:
• a front surface;
• a back surface opposing the front surface;
• a first axis perpendicular to a center of the rectangular body;
• and
• a plurality of circular grooves formed in the front surface, concentric to the first axis and spaced apart from each other;
• a first RF return path orthogonal to the first axis and extending across the front surface of the rectangular body from the first axis across the plurality of circular grooves to a corner of the rectangular body;
• and
• a second RF return path orthogonal to the first axis and extending across the front surface of the rectangular body from the first axis across the plurality of circular grooves to a midpoint of an edge of the rectangular body;
• and
• wherein each groove of the plurality of circular grooves has a first groove profile defined along the first RF return path by a first depth and a first width and a second groove profile defined along the second RF return path by a second depth and the first width, wherein the second depth is greater than the first depth, and
• wherein the second groove profile is larger than the first groove profile such that a first length of the front surface defined along the first RF return path is equal to or substantially equal to a second length defined along the second RF return path.