Fast freeform source and mask co-optimization method

• US 8,584,056 B2
• Filed: 11/20/2009
• Issued: 11/12/2013
• 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: Chen, Luoqi, Ye, Jun, Cao, Yu
• Primary Examiner(s): Whitmore, Stacy
• Application Number:
  US13/130,548
• Publication Number:
  US 20110230999A1
• Time in Patent Office: 1,453 Days
• Field of Search: 716 50- 56
• US Class Current: 716/53
• 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 computer-implemented method for achieving a lithographic process with a desired process window, the method comprising:

receiving descriptions of an illumination source and a mask, the mask comprising a target pattern to be imaged by the lithographic process; and

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

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

calculating a gradient of the cost function, and reconfiguring the source and mask descriptions depending on the calculated gradient,

wherein one or more of the above steps are performed by the computer.

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:

8. A computer-implemented method for achieving a lithographic process with a desired process window, the lithographic process using an illumination source and a mask, the method comprising:

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

calculating a gradient of the cost function to simultaneously optimize both the illumination source and the mask for the desired process window,

wherein one or more of the above steps are performed by the computer.

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

10. The method of claim 9, wherein the computationally efficient approximation of the cost function comprises a linearized approximation based on aerial image intensity at a given process window condition.

11. The method of claim 8, wherein the illumination source arid the mask are optimized when the calculated gradient of the cost function has a value of zero.

12. The method of claim 11, further comprising characterizing the illumination source as independent source points.

13. The method of claim 11, further comprising characterizing the mask as diffraction elements in a spatial frequency domain.

14. A computer program product comprising a non-transitory computer readable medium having instructions recorded thereon, the instructions, when executed by a computer, implements a method for achieving a lithographic process with a desired process window by performing the steps of:

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 the desired process window of the lithographic process, selectively repeating the steps of;

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

calculating a gradient of the cost function, and

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

15. The computer program product of claim 14, wherein the computer program product runs on a computing platform, the computing platform comprising specialized digital signal processors for processing cost functions and for calculating gradients.