Methods and apparatus for measuring substrate edge thickness during polishing

1. A system for determining a substrate polishing endpoint, the system comprising:

a light source adapted to transmit light to an edge of a substrate;

one or more detectors adapted to detect an arrangement of light reflected from the substrate edge; and

a controller adapted to determine a polishing endpoint for the substrate edge based on the arrangement of reflected light, which indicates a thickness of a film on the substrate edge.

2. The system of claim 1 wherein the controller further comprises a spectrograph component adapted to measure wavelengths forming the arrangement of reflected light.

3. The system of claim 2 wherein the controller is adapted to determine a thickness of a film on the substrate edge based on the amount of the wavelengths present in the arrangement of reflected light.

4. The system of claim 3 wherein the controller is adapted to compare the wavelengths to a reference library to determine a polishing endpoint.

5. The system of claim 1 wherein the arrangement of light spans from the substrate edge to a center of the substrate.

6. The system of claim 1 further comprising:

a focusing lens adapted to focus the transmitted light to a point on the edge of the substrate.

7. The system of claim 1 comprising at least two detectors.

8. The system of claim 7 wherein each detector is positioned at a point along the arrangement.

9. The system of claim 8 wherein each point along the arrangement corresponds to a point on the substrate edge.

10. A method comprising:

obtaining a reflected light measurement, wherein the reflected light indicates a thickness of a film on a substrate edge;

comparing the reflected light measurement to a reference measurement; and

determining whether a desired polishing endpoint is reached based on the comparison.

11. The method of claim 10 further comprising:

transmitting light to an edge of the substrate prior to obtaining a reflected light measurement, wherein the transmitted light reflects off of the substrate edge.

12. The method of claim 11 further comprising:

scanning the light from the substrate edge towards a center of the substrate.

13. The method of claim 10 wherein obtaining a reflected light measurement further comprises:

detecting the reflected light with one or more detectors.

14. The method of claim 11 further comprising:

providing a uniform fluid medium between the substrate edge and the light source transmitting the light and the one or more detectors.

15. The method of claim 13 wherein providing a uniform fluid medium further comprises:

flowing a fluid through a jacket of a jacket assembly.

16. The method of claim 15 further comprising:

directing the fluid flow with a base of the jacket assembly.