Eddy current gain compensation

1. A method of in-situ monitoring during processing of a substrate, comprising:

processing a conductive film on a substrate in a substrate processing apparatus;

generating a signal from an eddy current sensor during processing, the signal including a first portion generated when the eddy current sensor is adjacent the substrate, a second portion generated when the eddy current sensor is adjacent a metal body and not adjacent the substrate, and a third portion generated when the eddy current sensor is adjacent neither the metal body nor the substrate;

comparing the second portion of the signal to the third portion of the signal and determining a gain based at least on a result of the comparing; and

multiplying the first portion of the signal by the gain to generate an adjusted signal.

2. The method of claim 1 , further comprising making a plurality of sweeps with the eddy current sensor across the substrate and the metal body to generate a plurality of signals, each of the plurality of signals including the first portion, second portion and third portion.

3. The method of claim 2 , further comprising performing the comparing and multiplying at each sweep of the plurality of sweeps to generate a plurality of adjusted signals.

4. The method of claim 1 , wherein the substrate processing apparatus includes a chemical mechanical polisher that includes a polishing pad situated on a rotatable platen, the eddy current sensor situated in the platen and sweeping the substrate once per rotation of the platen.

5. The method of claim 4 , wherein the metal body is on the polishing pad.

6. The method of claim 4 , further comprising holding the substrate on the polishing pad with a carrier head having a retaining ring, and wherein the metal body comprises a portion of the retaining ring.

7. The method of claim 4 , wherein the metal body comprises a portion of a conditioning disk for conditioning the polishing pad.

8. The method of claim 1 , further comprising generating a first reference value from the second portion of the signal and generating a second reference value from the third portion of the signal.

9. The method of claim 8 , further comprising storing a first constant that represents a target value for the signal from the eddy current sensor when the eddy current sensor is adjacent the metal body, and a second constant that represents a target value for the signal from the eddy current sensor when the eddy current sensor is off-wafer or when the substrate is a bare substrate.

10. The method of claim 9 , wherein the gain is

(

K

1

-

K

2

)

(

S

1

-

S

2

)

where K 1 is the first constant, K 2 is the second constant, S 1 is the first reference value, and S 2 is the second reference value.

11. The method of claim 1 , wherein said signal is a phase difference signal.

12. A computer program product, tangibly encoded on a computer readable media, operable to cause a data processing apparatus to perform operations comprising:

receiving a signal from an eddy current sensor during processing of a conductive film on a substrate in a substrate processing apparatus, the signal including a first portion generated when the eddy current sensor is adjacent the substrate, a second portion generated when the eddy current sensor is adjacent a metal body and not adjacent the substrate, and a third portion generated when the eddy current sensor is adjacent neither the metal body nor the substrate;

comparing the second portion of the signal to the third portion of the signal and determine a gain based at least on a result of the comparing; and

multiplying the first portion of the signal by the gain to generate an adjusted signal.

13. The computer program product of claim 12 , wherein receiving a signal includes receiving a plurality of signals from a plurality of sweeps with the eddy current sensor across the substrate and the metal body, each of the plurality of signals including the first portion, second portion and third portion.

14. The computer program product of claim 13 , operable to cause the data processing apparatus to perform operations further comprising performing the comparing and multiplying at each sweep of the plurality of sweeps to generate a plurality of adjusted signals.

15. The computer program product of claim 12 , operable to cause the data processing apparatus to perform operations further comprising generating a first reference value from the second portion of the signal and generating a second reference value from the third portion of the signal.

16. The computer program product of claim 15 , operable to cause the data processing apparatus to perform operations further comprising storing a first constant that represents a target value for the signal from the eddy current sensor when the eddy current sensor is adjacent the metal body and a second constant that represents a target value for the signal from the eddy current sensor when the eddy current is off-wafer or when the substrate is a bare substrate.

17. The computer program product of claim 16 , wherein the gain is

(

K

1

-

K

2

)

(

S

1

-

S

2

)

where K 1 is the first constant, K 2 is the second constant, S 1 is the first reference value, and S 2 is the second reference value.