Fast Freeform Source and Mask Co-Optimization Method

• US 20110230999A1
• Filed: 11/20/2009
• Published: 09/22/2011
• Est. Priority Date: 11/21/2008
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: ASML Netherlands BV (ASML Holding NV)
• Inventors: Cao, Yu, Ye, Jun, Chen, Luoqi
• Granted Patent: US 8,584,056 B2
• Time in Patent Office: Days
• US Class Current: 700/105
• CPC Class Codes: G03F 1/36 Masks having proximity correction features; Preparation thereof, e.g. optical proximity correction [OPC] design processes
• CPC Class Codes: G03F 7/70083 Non-homogeneous intensity distribution in the mask plane
• CPC Class Codes: G03F 7/70125 Use of illumination settings tailored to particular mask patterns details of setting means G03F7/70091
• CPC Class Codes: G03F 7/70441 Optical proximity correction [OPC]
• CPC Class Codes: G03F 7/705 Modelling or simulating from physical phenomena up to complete wafer processes or whole workflow in wafer productions
• CPC Class Codes: G06F 30/39 Circuit design at the physical level physical level design for reconfigurable circuits G06F30/347
• CPC Class Codes: G06F 30/398 Design verification or optimisation, e.g. using design rule check [DRC], layout versus schematics [LVS] or finite element methods [FEM] optical proximity correction [OPC] design processes G03F1/36

Abstract

The present invention relates to lithographic apparatuses and processes, and more particularly to tools for optimizing illumination sources and masks for use in lithographic apparatuses and processes. According to certain aspects, the present invention significantly speeds up the convergence of the optimization by allowing direct computation of gradient of the cost function. According to other aspects, the present invention allows for simultaneous optimization of both source and mask, thereby significantly speeding the overall convergence. According to still further aspects, the present invention allows for free-form optimization, without the constraints required by conventional optimization techniques.

Claims

1. A method for optimizing a lithographic process, comprising:

receiving descriptions of an illumination source and a mask, the mask comprising a lithography pattern; and

until the source and mask are simultaneously optimized for a process window of the lithographic process, selectively repeating the steps of;

forming a cost function as a function of both the illumination source and mask;

calculating a gradient of the cost function, and

reconfiguring the source and mask descriptions depending on the calculated gradient.

2. The method of claim 1, wherein the source and mask are optimized when the calculated gradient has a value of zero.

3. The method of claim 2, further comprising characterizing the source as independent source points.

4. The method of claim 2, further comprising characterizing the mask as diffraction elements in a spatial frequency domain.

5. The method of claim 4, wherein reconfiguring the mask description includes:

using optical proximity correction;

placing sub-resolution assist features; and

recharacterizing the reconfigured mask description.

6. The method of claim 1, wherein the cost function is formulated in terms of worst case edge placement error over a given process window.

7. The method of claim 6, wherein the cost function F is expressed as:

14. A computer program product comprising a computer readable medium having instructions recorded thereon, the instructions. when executed by a computer, implementing the method of claim 1.

15. A computing platform for running the computer program product of claim 14, wherein the computing platform comprises specialized digital signal processors for processing cost functions and for calculating gradients.

8. A method for optimizing a lithographic process having an illumination source and a mask, comprising:

forming a cost function as a function of descriptions of both the illumination source and mask, wherein the cost function is formulated in terms of worst case edge placement error over a given process window; and

calculating a gradient of the cost function.

9. A method according to claim 8, further comprising, before the calculating step, forming a computationally efficient approximation of the cost function.

10. A method for optimizing a lithographic process having an illumination source and a mask, comprising:

a free-form optimization process;

placing SRAF seeds in a description of the mask based on a result of the free-form optimization process; and

a constrained optimization process, including growing the SRAF seeds while taking into account manufacturability constraints for both the illumination source and the mask.

11. A method according to claim 10, wherein the free-form optimization process includes designing an optimal illumination source that comprises a fully flexible set of illumination source points.

12. A method according to claim 11, wherein taking into account the manufacturability constraints for the illumination source includes matching the optimal illumination source to a diffractive optical element.

13. A method according to claim 10, wherein taking into account the manufacturability constraints for the mask includes constraining a mask transmission to a predetermined value.