Method and system for measuring patterned substrates

1. A common path auto-correlation low coherence interferometer system comprising:

a light source having an output directed toward a first beam splitter;

the first beam splitter configured to direct at least a portion of a light beam output by the light source toward a sample having two reflective interfaces including a top surface reflective interface and a feature bottom reflective interface;

the first beam splitter further configured to pass toward a second beam splitter each of a reference light beam reflected from the top surface interface and a measurement light beam reflected from the feature bottom reflective interface;

the second beam splitter configured to direct the reference light beam to a first mirror and the measurement light beam to a second mirror, wherein the first mirror is a first distance from the second beam splitter and the second mirror is a second distance from the second beam splitter;

the second beam splitter further configured to combine a reflected measurement light beam reflected from the second mirror and a reflected reference light beam reflected from the first mirror and direct a resulting interference pattern toward a detector, wherein the first mirror is a scanning mirror such that a difference distance between the first distance and the second distance is equal to an optical separation between the feature bottom reflective interface and the top surface reflective interface; and

a filtering element disposed in a beam path between the second beam splitter and the first mirror.

2. The system of claim 1 , wherein the second distance is offset from the first distance a known distance.

3. The system of claim 1 , wherein the detector is a spectrograph.

4. The system of claim 1 , wherein the light source is an ultraviolet light source.

5. The system of claim 1 , further comprising a plate disposed in a beam path between the second beam splitter and the second mirror.

6. The system of claim 1 , wherein the light source is a polarizing light source and further comprising a selected polarized light beam reflective and transmissive element is disposed in a beam path between the first beam splitter and the sample and the second beam splitter is a polarizing beam splitter.

7. The system of claim 6 , further comprising a 45 degree polarizer element disposed in a beam path between the second beam splitter and the detector.

8. The system of claim 1 , wherein the detector has a per pixel bandwidth that can accommodate a coherence length that is about twice a feature depth.

9. A common path auto-correlation low coherence interferometer method comprising:

directing a light beam emitted from a light source output toward a first beam splitter;

directing at least a portion of the light beam output by the light source through the first beam splitter toward a sample having two reflective interfaces including a top surface reflective interface and a feature bottom reflective interface;

passing each of a reference light beam reflected from the top surface reflective interface and a measurement light beam reflected from the feature bottom reflective interface through the first beam splitter and toward a second beam splitter;

reflecting the reference light beam through the second beam splitter to a first mirror;

passing the measurement light beam through the second beam splitter to a second mirror, wherein the first mirror is a first distance from the second beam splitter and the second mirror is a second distance from the second beam splitter, wherein the first mirror is a scanning mirror;

combining a reflected measurement light beam from the second mirror and a reflected reference light beam from the first mirror in the second beam splitter;

directing a resulting interference pattern toward a detector; and

scanning the first mirror such that a difference distance between the first distance and the second distance is equal to an optical separation between the feature bottom reflective interface and the top surface reflective interface,

wherein the light source is a polarizing light source and further comprising a selected polarized light beam reflective and transmissive element is disposed in a beam path between the first beam splitter and the sample and the second beam splitter is a polarizing beam splitter.

10. The method of claim 9 , wherein the second distance is offset from the first distance a known distance.

11. The method of claim 9 , wherein the detector is a spectrograph.

12. The method of claim 9 , wherein the light source is an ultraviolet light source.

13. The method of claim 9 , wherein the light source is a visible light source.

14. The method of claim 9 , wherein the light source is an infrared light source.

15. The method of claim 9 , further comprising a filtering element disposed in a beam path between the second beam splitter and the first mirror.

16. The method of claim 15 , further comprising a plate disposed in a beam path between the second beam splitter and the second mirror.

17. The method of claim 15 , further comprising a 45 degree polarizer element disposed in a beam path between the second beam splitter and the detector.

18. The method of claim 9 , wherein the detector has a per pixel bandwidth that can accommodate a coherence length that is about twice a feature depth.