Method and apparatus for measuring wafer bias potential

1. A device for use in a wafer processing chamber having a plasma forming volume, a hot edge ring, the hot edge ring having a first surface and a second surface, the first surface being in contact with the plasma forming volume, the second surface not being in contact with the plasma forming volume, said device comprising:

a detector separated from the plasma forming volume by the hot edge ring and being operable to contact the second surface of the hot edge ring, wherein said detector comprises indium,

wherein said detector is operable to detect a voltage of the hot edge ring and provide a detected signal based on the detected voltage.

2. The device of claim 1 , further comprising:

a coupling ring operable to contact the second surface of the hot edge ring, said coupling ring having cavity therein, the cavity having an opening facing the second surface of said hot edge ring,

wherein said detector is disposed within the cavity and is arranged to contact the second surface of the hot edge ring.

3. The device of claim 1 , further comprising:

an output portion operable to provide an output signal based on the detected signal; and a resistor disposed between and in series with said detector and said output portion.

4. The device of claim 3 , further comprising a heat sink disposed in contact with said resistor.

5. The device of claim 4 , wherein said heat sink comprises quartz.

6. The device of claim 1 ,

wherein said detector comprises a biasing portion and a contact portion, and

wherein said biasing portion is operable to provide a biasing force against said contact portion to keep said contact portion in contact with the second surface of the hot edge ring.

7. The device of claim 6 , wherein said biasing portion comprises a coil spring.