Empirical detection of lens aberration for diffraction-limited optical system
A method for qualitatively detecting aberration and determine aberration types in a photolithography system is disclosed. The method includes using a digital micromirror device (DMD) pattern to project an optical signal on a reflective substrate, acquiring a return optical signal reflected from the substrate at different focus heights (ranging from above to below best focus), forming a through focus curve based off of the return optical signal at various focus heights, comparing the through focus curve to a predetermined curve—the predetermined curve being a function of focus, and determining if a lens aberration is present. By using the existing hardware of the photolithography system to determine if a lens aberration exists, costs are maintained at a minimum and the DMD pattern creates a through focus curve (TFC) image in less than five minutes allowing for quick correction.
1. A method of determining focus in a photolithography system, comprising:
using a digital micromirror device (DMD) pattern to project an optical signal on a substrate;
acquiring a return optical signal from the substrate;
measuring one or more focus heights of the return optical signal;
forming a through focus curve based off of the one or more focus heights of the return optical signal;
comparing the through focus curve to a predetermined curve, wherein the predetermined curve is a function of focus; and
determining if a lens aberration is present.
2. The method of claim 1 , wherein the DMD pattern is a Dots5×5 pattern.
3. The method of claim 1 , wherein the optical signal is a light beam.
4. The method of claim 3 , wherein the light beam has a wavelength in a range from about 360 nm to about 410 nm.
5. The method of claim 3 , wherein a combination of mirrors in on positions and off positions forms the DMD pattern.
6. The method of claim 1 , wherein the return optical signal is a light beam created from the reflection and transformation of the first optical signal.
7. The method of claim 1 , wherein the return optical signal is acquired by a camera.
8. A method of determining an aberration, comprising:
using a first digital micromirror device (DMD) pattern to project a first optical signal on a substrate;
acquiring a first return optical signal from the substrate;
forming a first through focus curve based off of the first return optical signal;
comparing the first through focus curve to a predetermined curve, wherein the predetermined curve is a function of focus;
determining if a lens aberration is present;
using a second DMD pattern to project a second optical signal on the substrate;
acquiring a second return optical signal from the substrate;
forming a second through focus curve based off of the second return optical signal;
and
comparing the first through focus curve to the second through focus curve to determine a lens aberration type.
9. The method of claim 8 , wherein the first DMD pattern is a horizontal stripes.
10. The method of claim 8 , wherein the optical signal is a light beam.
11. The method of claim 10 , wherein the light beam has a wavelength in a range from about 360 nm to about 410 nm.
12. The method of claim 9 , wherein the second DMD pattern is a vertical stripe pattern.
13. The method of claim 8 , wherein the first DMD pattern is a Dots5×5 pattern.
14. The method of claim 13 , wherein the second DMD pattern is a vertical stripe pattern.
15. A method of determining focus, the method comprising:
using a first digital micromirror device (DMD) pattern to project a first optical signal on a substrate;
acquiring a first return optical signal from the substrate;
forming a first through focus curve based off of the first return optical signal;
comparing the first through focus curve to a predetermined curve, wherein the predetermined curve is a function of focus;
using a plurality of DMD patterns to project a second optical signal on the substrate;
acquiring a second return optical signal from the substrate; and
determining a lens aberration type.
16. The method of claim 15 , further comprising:
forming a second through focus curve based off of the second return optical signal; and
comparing the second through focus curve to a predetermined curve to determine the lens aberration type.
17. The method of claim 15 , wherein the plurality of DMD patterns comprises a horizontal stripe pattern and a vertical stripe pattern.
18. The method of claim 15 , wherein the optical signal is a light beam.
19. The method of claim 15 , wherein the light beam has a wavelength in a range from about 360 nm to about 410 nm.
20. The method of claim 15 , wherein the light beam has with a wavelength in a range from about 400 nm to about 750 nm.