Phase Calibration for Attenuating Phase-Shift Masks

Abstract

A phase metrology pattern for attenuating phase masks. The phase error of this pattern can be determined to high accuracy by aerial image measurements. This pattern can be used to create an optical phase standard for calibrating phase metrology equipment for attenuated phase masks, or as a witness pattern on a product mask to verify the phase accuracy of that mask. The pattern includes an effective line to space ratio and can be tested using a microscope or stepper system or can be measured directly using a detector for the 0 order diffraction measurement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an alternating phase shift mask.

FIG. 2 a shows an alternating phase shift mask.

FIG. 2 b shows an aerial image formed by the ±1^st diffraction orders.

FIG. 3 is an example of an attenuated phase-shift test structure.

FIG. 4 a shows an attenuated phase shift test structure and the zero and ±1^st diffractive orders.

FIG. 4 b shows an aerial image of an attenuated phase grating.

FIG. 5 a illustrates an example of attenuating phase shift structure and the corresponding aerial image is shown in FIG. 5b.

FIG. 6 illustrates an example of an asymmetry parameter for measuring phase differential.

FIG. 7 is a plot of an asymmetry parameter vs. focus.

FIG. 8 a is an aerial image at negative defocus.

FIG. 8 b shows the aerial image at best focus.

FIG. 8 c shows an aerial image at positive defocus.

FIG. 9 is a method of measuring asymmetry using measured space widths in photoresist.

FIG. 10 a is a method of measuring asymmetry using amplitude differentials in an aerial image.

FIG. 10 b is a method of measuring asymmetry using width differentials in an aerial image.

FIG. 11 is an apparatus for performing attenuating phase shift calibration using a microscope.

FIG. 12 is an apparatus for performing attenuating phase shift calibration using a lithographic stepper.

FIG. 13 shows an apparatus for taking a direct measurement for calibrating an attenuating phase shift mask.

FIG. 14 is an exemplary graph showing asymmetry parameter vs. defocus for 6 space:line ratios at 350 nm pitch.

FIG. 15 is an exemplary graph showing several asymmetry/defocus slopes for gratings with different values of phase as measured on an MPM193 phase measurement system.

Claims

1. A method for measuring the phase error of an attenuated phase-shift mask, comprising the steps of: providing a test structure; andmeasuring the amplitude of the zero order diffraction of the structure.

2. The method of claim 1 wherein the intensity of the zero order diffraction is directly measured with an optical intensity meter

3. The method of claim 1 wherein the zero-order diffraction amplitude is measured by calculating the asymmetry of the aerial image through focus.

4. The method of claim 3 wherein the asymmetry is measured as the difference between at least one real and at least one ghost image peak divided by the average amplitude.

5. The method of claim 3 wherein the asymmetry is measured as the difference between the widths of the real and ghost images.

6. The method of claim 3 wherein the asymmetry is measured as the ratio of the width of the real image to the width of the ghost image.

7. The method of claim 3 wherein the asymmetry is measured as the difference between the widths of the real and ghost images printed in photoresist.

8. The method of claim 3 wherein the asymmetry is measured as the ratio between the width of the real image and the width of the ghost image printed in photoresist.

9. A test structure comprising: At least 2 regions having equal transmission amplitude; anda phase difference of about 180 degrees.

10. The structure of claim 9, wherein the light intensity transmitted through the first region differs from the light intensity transmitted through the second region and the transmission amplitude of the first region is equal to the transmission amplitude of the second region.

11. The structure of claim 9 wherein the test structure is patterned.

12. The structure of claim 9 wherein the test structure is linear.

13. The structure of claim 12 wherein the linear test structure is spatially periodic.

14. The structure of claim 9, wherein the structure comprises spaces and optically attenuating lines.

15. The structure of claim 14, wherein the ratio between the spaces and line widths is approximately equal to the optical transmission amplitude of the lines.

16. The test structure of claim 14, wherein the spaces are transparent.

17. A system for calibrating an attenuated phase-shift structure comprising: an illuminator; anda test structure;wherein the illuminator illuminates the test structure and the test structure produces one or more diffracted orders.

18. The system of claim 17, further comprising a detector which measures a zero order diffraction.

19. The system of claim 17, further comprising a lens which projects one or more of the diffracted orders to create an image.

20. The system of claim 19, wherein the image is an aerial image.

21. The system of claim 19, further comprising a computer and a monitor for analyzing and displaying the image.

22. The system of claim 19, wherein the lens is a stepper lens.

23. The system of claim 19, further comprising a detector array that captures the image.

24. The system of claim 19, further comprising a substrate that captures the image to produce a pattern.

25. The system of claim 24, wherein the substrate is a wafer comprising a layer of photoresist which records at least one real image and at least one ghost image.