Layer detection for high aspect ratio etch control

1. A method for controlling an etch process applied to a multi-layered structure, the method comprising:

calculating a spectral derivative of reflectance of an illuminated region of interest of a multi-layered structure during an etch process applied to the multi-layered structure;

identifying in the spectral derivative a discontinuity that indicates that an edge of a void formed by the etch process at the region of interest has crossed a layer boundary of the multi-layered structure;

determining that the crossed layer boundary corresponds to a preselected layer boundary of the multi-layered structure; and

applying a predefined control action to the etch process responsive to determining that the crossed layer boundary corresponds to the preselected layer boundary of the multi-layered structure.

2. The method according to claim 1 wherein the calculating of the spectral derivative comprises calculating the spectral derivatives at each of multiple different wavelengths of the reflectance.

3. The method according to claim 2 wherein the calculating of the spectral derivatives further comprises calculating an average of the spectral derivatives at each of multiple different wavelengths of the reflectance.

4. The method according to claim 1 and further comprising:

illuminating the region of interest of the multi-layered structure during the etch process applied to the multi-layered structure; and

measuring the reflectance of the illuminated region of interest.

5. The method according to claim 1 wherein the identifying comprises identifying in the spectral derivative a plurality of discontinuities, wherein each of the discontinuities corresponds to a different layer boundary of the multi-layered structure.

6. The method according to claim 5 and further comprising determining, as any of the discontinuities in the spectral derivative are identified, a currently etched layer of the multi-layered structure based on a count of the discontinuities in the spectral derivative.

7. The method according to claim 5 and further comprising determining, as any of the discontinuities in the spectral derivative are identified, an etch rate based on an elapsed time between identifying any of the discontinuities in the spectral derivative.

8. The method according to claim 1 wherein the applying comprises effecting termination of the etch process.

9. The method according to claim 8 wherein the effecting comprises effecting termination of the etch process after a predefined delay.

10. The method according to claim 9 wherein the predefined delay is based on an etch rate of a currently etched layer of the multi-layered structure.

11. A system for controlling an etch process applied to a multi-layered structure, the system comprising:

an etch layer detector configured to

calculate a spectral derivative of reflectance of an illuminated region of interest of a multi-layered structure during an etch process applied to the multi-layered structure, and

identify in the spectral derivative a discontinuity that indicates that an edge of a void formed by the etch process at the region of interest has crossed a layer boundary of the multi-layered structure; and

an etch process controller configured to

determine that the crossed layer boundary corresponds to a preselected layer boundary of the multi-layered structure, and

apply a predefined control action to the etch process responsive to determining that the crossed layer boundary corresponds to the preselected layer boundary of the multi-layered structure.

12. The system according to claim 11 wherein the edge layer detector is configured to calculate the spectral derivatives at each of multiple different wavelengths of the reflectance.

13. The system according to claim 12 wherein the edge layer detector is configured to calculate an average of the spectral derivatives at each of multiple different wavelengths of the reflectance.

14. The system according to claim 11 and further comprising an optical profile monitor configured to illuminate the region of interest of the multi-layered structure during the etch process applied to the multi-layered structure, and measure the reflectance of the illuminated region of interest.

15. The system according to claim 11 wherein the etch layer detector is configured to identify in the spectral derivative a plurality of discontinuities, wherein each of the discontinuities corresponds to a different layer boundary of the multi-layered structure.

16. The system according to claim 15 wherein the etch layer detector is configured to determine, as any of the discontinuities in the spectral derivative are identified, a currently etched layer of the multi-layered structure based on a count of the discontinuities in the spectral derivative.

17. The system according to claim 15 wherein the etch layer detector is configured to determine, as any of the discontinuities in the spectral derivative are identified, an etch rate based on an elapsed time between identifying any of the discontinuities in the spectral derivative.

18. The system according to claim 11 wherein the etch process controller is configured to apply the predefined control action by effecting termination of the etch process.

19. The system according to claim 18 wherein the etch process controller is configured to effect termination of the etch process after a predefined delay.

20. The system according to claim 19 wherein the predefined delay is based on an etch rate of a currently etched layer of the multi-layered structure.