Lithographic systems and methods with extended depth of focus
View Patent ↗An optical lithography system that has extended depth of focus exposes a photoresist coating on a wafer, and includes an illumination sub-system, a reticle, and an imaging lens that has a pupil plane function to form an aerial image of the reticle proximate to the photoresist. The pupil plane function provides the extended depth of focus such that the system may be manufactured or used with relaxed tolerance, reduced cost and/or increased throughput. The system may be used to form precise vias within integrated circuits even in the presence of misfocus or misalignment.
1. An optical lithography system for exposing a photoresist coating on a wafer, the system comprising:
an illumination sub-system;
a reticle; and
an imaging lens having a pupil plane function and forming an aerial image of the reticle that is proximate to the photoresist and that has extended depth of focus.
2. The optical lithography system of claim 1 , the reticle being constructed such that a desired pattern is recorded by the photoresist from the aerial image.
3. The optical lithography system of claim 1 , wherein the pupil plane function codes a wavefront from the reticle to increase the depth of focus.
4. The optical lithography system of claim 3 , wherein the depth of focus is at least K*λ*f#/2 where K is greater than or equal to 1.5, f# is an f-number of the imaging lens and λ is a wavelength of optical radiation generated by the illumination sub-system.
5. The optical lithography system of claim 4 , wherein K is greater than 2.
6. The optical lithography system of claim 5 , wherein K is greater than 8.
7. The optical lithography system of claim 1 , the pupil plane function comprising a phase function from a cosine form family or a constant profile path family.
8. The optical lithography system of claim 1 , the aerial image being substantially unchanged over the depth of focus.
9. The optical lithography system of claim 1 , the illumination subsystem comprising an illumination source selected from the group of incoherent, annular, fourfold, separated, quadrupole, CQUAD, QUASAR and dipole.
10. The optical lithography system of claim 9 , wherein the reticle is dependent upon the type of illumination.
11. The optical lithography system of claim 1 , the photoresist coating having an exposure threshold, the pupil plane function altering the aerial image such that part of the aerial image is above the exposure threshold over the extended depth of focus.
12. The optical lithography system of claim 1 , the illumination source generating partially coherent illumination.
13. The optical lithography system of claim 1 , the pupil plane function comprising a phase function with a form of P(r)=Σa i r i , wherein r is a radius of the imaging lens and i is a number greater than 2.
14. The optical lithography system of claim 1 , wherein the reticle and the pupil plane function are jointly optimized to form the aerial image according to an image goal.
15. The optical lithography system of claim 1 , the extended depth of focus being sufficient to compensate for tolerances associated with one or more of assembly of the system, warpage of the reticle and nonuniformity of the wafer.