Gathering spectra from multiple optical heads

1. A polishing apparatus, comprising:

a platen to hold a polishing pad having a plurality of optical apertures;

a carrier head to hold a substrate against the polishing pad;

a motor to generate relative motion between the carrier head and the platen; and

an optical monitoring system, the optical monitoring system including

a plurality of light sources, each light source of the plurality of light sources being a white-light source configured to output substantially the same spectrum,

a common detector, wherein the detector comprises a spectrometer, and

an optical assembly configured to direct light from each light source of the plurality of light sources to a respective different position of a plurality of separated positions in the platen, there being an equal number of separated positions as light sources, to direct light from each position of the plurality of separated positions to the substrate as the substrate passes over said each position, to receive reflected light from the substrate as the substrate passes over said each position, and to direct the reflected light from each of the plurality of separated positions to the common detector.

2. The polishing apparatus of claim 1 , wherein the platen is rotatable about an axis of rotation.

3. The polishing apparatus of claim 2 , wherein the plurality of separated positions are spaced equidistant from the axis of rotation.

4. The polishing apparatus of claim 3 , wherein the plurality of separated positions are spaced at equal angular intervals around the axis of rotation.

5. The polishing apparatus of claim 1 , wherein the optical assembly is configured such that an angle of incidence of the light on the substate from said each position is identical.

6. The polishing apparatus of claim 1 , wherein the plurality of separated positions consists of exactly three positions.

7. The polishing apparatus of claim 1 , wherein the optical assembly includes a plurality of optical fibers, each optical fiber of the plurality of optical fibers having a first end connected to an associated light source of the plurality of light sources and a second end configured to direct light to an associated position of the plurality of separated positions.

8. The polishing apparatus of claim 7 , wherein the optical assembly includes a bifurcated optical fiber having a trunk connected to the common detector and a plurality of branches, each branch of the plurality of branches configured to receive light from the associated position of the plurality of separated positions.

9. The polishing apparatus of claim 1 , further comprising an optical switch configured to pass light from a selected one of the plurality of separated positions to the detector.

10. The polishing apparatus of claim 1 , wherein the platen is configured such that relative motion between the carrier head and the platen causes each position of the plurality of separated positions to repeatedly sweep across the substrate.

11. The polishing apparatus of claim 10 , further comprising a controller configured to receive a group of spectrum measurements from the detector for each sweep of each position across the substrate and generate a sequence of spectra.

12. The polishing apparatus of claim 11 , wherein the controller is configured to determine at least one of a polishing endpoint or an adjustment to a polishing parameter based on the sequence of spectra.

13. The polishing apparatus of claim 1 , wherein the optical assembly is configured such that light reaches only one of the plurality of separated positions at a time.