Soft defect printability simulation and analysis for masks

• US 20040052411A1
• Filed: 09/13/2002
• Published: 03/18/2004
• Est. Priority Date: 09/13/2002
• Status: Active Grant

• Current Assignee: Synopsys Incorporated
• Original Assignee: Numerical Technologies Inc. (Synopsys Incorporated)
• Inventors: Pang, Linyong, Qian, Qi-De
• Granted Patent: US 7,043,071 B2
• Time in Patent Office: Days
• US Class Current: 382/144
• CPC Class Codes: G06T 2207/30148 Semiconductor; IC; Wafer
• CPC Class Codes: G06T 7/0002 Inspection of images, e.g. flaw detection
• CPC Class Codes: G06T 7/001 using an image reference approach
• CPC Class Codes: G06T 7/571 from focus

Abstract

Masks that include optical proximity correction or phase shifting regions are increasingly being used in the manufacturing process. These masks, either initially or after repair, can have “soft” defects, e.g. phase and/or transmission defects. In accordance with one feature of the invention, soft defect information can be computed from standard test images of a mask. This soft defect information can be used to generate an accurate simulated wafer image, thereby providing valuable defect impact information to a user. Knowing the impact of the soft defect can enable a user to make better decisions regarding the mask. Specifically, a user can now with confidence accept the mask for the desired lithographic process, repair the mask at certain critical locations, or reject the mask, all without exposing a wafer.

Claims

1. A method of inspecting a mask, the method including:

obtaining a test image from the mask, wherein the test image includes a defect;

computing a transmission, a phase, and a shape of the defect; and

generating a simulated test wafer image based on the computing associated with the defect.

2. The method of claim 1, further including:

obtaining a reference image, wherein the reference image provides a defect-free image of the test image;

generating a defect image by subtracting the reference image from the test image, wherein computing the transmission, the phase, and the shape of the defect uses the defect image;

generating a host image by subtracting the defect image from the test image;

computing a transmission, a phase, and a shape of the host image; and

generating a corrected test image based on the computing associated with the host image and the defect image, wherein generating the simulated test wafer image uses the corrected test image.

3. The method of claim 2, wherein obtaining the reference image includes at least one of rendering the reference image from a database, inspecting a die image, and reconstructing the reference image.

4. The method of claim 2, further including:

computing a transmission, a phase, and a shape of the reference image;

generating a corrected reference image based on the computing associated with the reference image; and

generating a simulated reference wafer image based on the computing associated with the reference image.

5. The method of claim 4, further including:

processing the simulated test wafer image and the simulated reference wafer image to determine an impact of the defect.

6. The method of claim 2, further including:

calibrating the transmission of at least one of the defect, the host, and the reference images.

7. The method of claim 2, wherein computing the shape of the defect includes:

using diffraction compensation to correct for optical distortions in the defect image.

8. The method of claim 2, wherein computing the phase of the host image includes:

analyzing gray scale intensities along a cut line on the host image; and

identifying locations of high intensities, wherein bumps at the locations indicate approximately a 180 degree phase and no bumps at the locations indicate approximately a 0 degree phase.

9. The method of claim 2, wherein computing the phase of the defect includes:

generating multiple defect images, each defect image taken at a different defocus.

10. The method of claim 2, wherein computing the transmission of the defect includes:

determining a first transmission of an area immediately surrounding the defect using the test image; and

assigning the defect the first transmission.

11. The method of claim 2, wherein computing the transmission of the host image includes:

using a lookup table including gray scale intensities.

12. The method of claim 2, wherein if the mask is a phase shifting mask, then further including:

using a supplemental mask image in generating the simulated test wafer image.

13. A system for generating an accurate simulation image for a phase shifting mask, the system comprising:

a computer; and

a tool running on the computer, the tool including;

means for obtaining a test image from the phase shifting mask, wherein the test image includes a defect;

means for obtaining a reference image, wherein the reference image provides a defect-free image of the test image;

means for generating a defect image from the test image and the reference image;

means for generating a host image from the test image and the defect image;

means for computing a transmission, a phase, and a shape of the defect image and the host image;

means for providing a corrected test image based on the transmission, the phase, and the shape of the defect image and the host image; and

means for simulating a test wafer image based on the corrected test image.

14. A phase shifting mask used for forming an integrated circuit, the phase shifting mask comprising:

a plurality of pairs of phase shifters, each pair configured such that light transmitted by a first phase shifter is 180 degrees out of phase with light transmitted by a second phase shifter,

wherein at least one phase shifter includes a phase defect identified, but determined to be acceptable based on a simulated wafer image of the phase defect.

15. The phase shifting mask of claim 14, being one of an alternating phase shifting mask, an attenuated phase shifting mask, and a tri-tone attenuated phase shifting mask.

16. A method for determining an impact of a soft defect on wafer printability, the method comprising:

obtaining a test image from a mask, wherein the test image includes the soft defect;

obtaining a reference image, wherein the reference image provides a defect-free image of the test image;

generating a defect image from the test image and the reference image;

generating a host image from the test image and the defect image;

computing a transmission, a phase, and a shape of the defect image, the host image, and the reference image;

generating a corrected test image and a corrected reference image based on the computing;

simulating a test wafer image and a reference wafer based on the corrected test image and the corrected reference image, respectively; and

assigning a defect severity score based on the simulated wafer test wafer image and the simulated reference wafer image.

17. The method of claim 16, further including providing an indicator based on the defect severity score, the indicator representing one of accepting the mask, rejecting the mask, and repairing the mask.

18. The method of claim 16, wherein the mask is one of a binary mask, an alternating phase shifting mask, an attenuated phase shifting mask, and a tri-tone attenuated phase shifting mask.

19. The method of claim 16, wherein the mask is a repaired binary mask.

20. The method of claim 16, wherein the soft defect includes one of a phase defect and a transmission defect.

21. The method of claim 16, wherein computing the phase includes referring to a gray scale associated with at least one of the defect image, the host image, and the reference image.