Method and apparatus for measuring process kit centering

1. A sensor wafer, comprising:

a substrate, wherein the substrate comprises a first surface, a second surface opposite the first surface, and an edge surface between the first surface and the second surface; and

a plurality of sensors positioned along the edge surface, wherein each sensor is a capacitive sensor, and wherein each sensor comprises a first probe and a second probe, the first probe laterally adjacent to the second probe, wherein an output phase of the first probe is 180 degrees offset from an output phase of the second probe.

2. The sensor wafer of claim 1 , wherein the plurality of sensors comprises at least three sensors.

3. The sensor wafer of claim 1 , wherein each sensor is a self-referencing capacitive sensor.

4. The sensor wafer of claim 1 , further comprising:

a recess into the first surface of the substrate proximate to each sensor.

5. The sensor wafer of claim 4 , wherein the recess extends back from the edge surface at least 1.0 mm.

6. The sensor wafer of claim 1 , further comprising:

a plurality of second sensors on the second surface of the substrate.

7. The sensor wafer of claim 6 , wherein the second sensors are recessed from the second surface.

8. The sensor wafer of claim 1 , further comprising an electric field guard below each of the plurality of sensors.

9. The sensor wafer of claim 1 , further comprising:

a computing module, wherein the computing module is communicatively coupled to the plurality of sensors.

10. The sensor wafer of claim 9 , wherein the computing module comprises one or more of a power source, a processor, a memory, and a wireless communications module.

11. The sensor wafer of claim 1 , wherein the substrate has a diameter that is 300 mm.