Construction of reference spectra with variations in environmental effects
A method of generating a library of reference spectra includes storing at least one reference spectrum, storing a plurality of different transmission curves, and for at least two transmission curves from the plurality of different transmission curves, calculating a modified reference spectrum from the reference spectrum and the transmission curve to generate a plurality of modified reference spectra. The transmission curves represent distortion to a spectrum introduced by variations in components in an optical path before a substrate surface.
1. A method of controlling polishing, comprising:
generating a library of reference spectra, comprising:
storing at least one reference spectrum;
storing a plurality of different transmission curves, the transmission curves representing distortion to a spectrum introduced by variations in components in an optical path before a substrate surface;
for at least two transmission curves from the plurality of different transmission curves, calculating a modified reference spectrum from the reference spectrum and the transmission curve to generate a plurality of modified reference spectra;
polishing a substrate;
measuring a sequence of spectra of light from the substrate during polishing;
for each measured spectrum of the sequence of spectra, finding a best matching reference spectrum from the plurality of modified reference spectra to generate a sequence of best matching reference spectra; and
determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra.
2. The method of claim 1 , wherein the different transmission curves represent variations in transmission of one or more windows.
3. The method of claim 2 , wherein the different transmission curves represent distortion to a spectrum at different ages of a window of a polishing pad.
4. The method of claim 2 , wherein different transmission curves represent different windows.
5. The method of claim 1 , wherein the different transmission curves represent distortion to a spectrum at different ages of a bulb from a light source.
6. The method of claim 1 , further comprising receiving user input identifying the at least two transmission curves from the plurality of different transmission curves.
7. The method of claim 1 , further comprising generating the plurality of different transmission curves.
8. The method of claim 7 , wherein generating the plurality of different transmission curves comprises calculating the transmission curves from an optical model.
9. The method of claim 1 , wherein calculating the modified reference spectrum comprises multiplying the reference spectrum by the transmission curve.
10. The method of claim 9 , wherein the transmission curve is stored as a ratio between 0 and 1 as a function of wavelength.
11. The method of claim 1 , further comprising generating the at least one reference spectrum.
12. The method of claim 11 , wherein generating the at least one reference spectrum comprises measuring a test substrate during a polishing operation using the an optical monitoring system that includes the optical path.
13. The method of claim 11 , wherein generating the at least one reference spectrum comprises calculating the reference spectrum from an optical model.
14. A method of generating a library of reference spectra, comprising:
storing at least one reference spectrum;
storing a plurality of different transmission curves, the transmission curves representing distortion to a spectrum introduced by variations in components in an optical path before a substrate surface; and
for at least two transmission curves from the plurality of different transmission curves, calculating a modified reference spectrum from the reference spectrum and the transmission curve to generate a plurality of modified reference spectra;
wherein generating the plurality of different transmission curves comprises measuring a base spectrum from a test substrate using an in-situ optical monitoring system and measuring one or more additional spectra from the same test substrate or another test substrate of the same material using the in-situ optical monitoring system at different times, and calculating the transmission curves from the base spectrum and the one or more additional spectra.
15. The method of claim 14 , wherein the different times are at different ag of a window of polishing pad in an optical path between the test substrate and a light source or detector of the in-situ optical monitoring system.
16. The method of claim 14 , wherein the different times are at different ag of a bulb of a light source of the in-situ optical monitoring system.
17. The method of claim 14 , wherein calculating the transmission curves comprises a division operation in which the additional spectrum is in the numerator and the bas spectrum is in the denominator.
18. The method of claim 14 , further comprising measuring a first dark spectrum at the same age of the window as the base spectrum and measuring a second dark spectrum at the same age of the window as the additional spectrum.
19. The method of claim 18 , wherein calculating the transmission curves comprises calculating
T
=
A
-
D
A
B
-
D
B
where A is the additional spectrum, D A is the second dark spectrum, B is the base spectrum, and D B is the first dark spectrum.
20. The method of claim 14 , wherein the test substrate comprises a bare silicon wafer.