Patent Number: 
Section: claims

1. A method comprising:providing a target mask for use in integrated circuit photolithography, wherein the target mask is nonpixelated, quantized tone, and in a spatial domain;performing a frequency domain transformation on the target mask to obtain a first mask, wherein the first mask is nonpixelated, continuous tone, and in a frequency domain;computing a first cost function for a first mask to obtain a first value;altering the first mask in a frequency domain to obtain a second mask, wherein the second mask is nonpixelated, continuous tone, and in the frequency domain;computing the first cost function for the second mask to obtain a second value; andrepeating the altering the first mask and the computing the first cost function for the second mask until the second value is less than the first value,wherein the performing a frequency domain transformation, computing a first cost function, and altering the first mask are performed using at least one electronic processor. 2. The method of claim 1 wherein the frequency domain transformation uses a Fourier basis. 3. The method of claim 1 wherein the frequency domain transformation uses a wavelet basis. 4. The method of claim 1 wherein the first mask comprises frequencies up to a multiple of a cutoff frequency. 5. The method of claim 1 comprising:performing an inverse frequency domain transformation on the second mask to transform the second mask to a first finalized mask, wherein the finalized mask is nonpixelated, continuous tone, and in the spatial domain; andquantizing the first finalized mask to obtain a second finalized mask, wherein the second finalized mask is quantized tone and in the spatial domain. 6. The method of claim 5 wherein the inverse frequency domain transformation uses a Fourier basis. 7. The method of claim 5 wherein the inverse frequency domain transformation uses a wavelet basis. 8. The method of claim 5 comprising:using the second finalized mask, generating subresolution assist features to obtain a third finalized mask. 9. The method of claim 8 wherein a simplified resist model is used for the first cost function. 10. The method of claim 8 wherein the subresolution assist features are generated according to preset rules. 11. The method of claim 1 wherein the first cost function comprises a term representative of a measure of variation with exposure. 12. The method of claim 1 wherein the first cost function comprises a term representative of a measure of variation with depth of focus. 13. The method of claim 1 wherein the first cost function comprises a term for weighting different portions of a printed image. 14. The method of claim 1 wherein the first cost function comprises terms representative of a measure of variation with exposure, representative of a measure of variation with depth of focus, and for weighting different portions of a printed image. 15. The method of claim 1 wherein the computing the first cost function for the second mask to obtain a second value comprises:clamping mask tone to minimum and maximum allowed mask values. 16. The method of claim 1 comprising:partitioning the target mask into a plurality of regions, each region comprising geometric shapes. 17. The method of claim 1 wherein the computing the first cost function for the second mask to obtain a second value comprises:adjusting high frequency coefficients in the first cost function while holding low frequency coefficients constant. 18. The method of claim 1 comprising:generating a second cost function by applying a quantization function to the second mask, wherein the quantization function regularizes a mask value of the second mask;computing the second cost function for the second mask to obtain a third value;altering the second mask in the frequency domain to obtain a third mask, wherein the third mask is nonpixelated, continuous tone, and in the frequency domain;computing the second cost function for the second and third masks to obtain a fourth value; andrepeating the altering the second mask and the computing the second cost function for the second and third masks until the fourth value is less than the third value. 19. The method of claim 18 comprising:performing an inverse frequency domain transformation on the third mask to transform the second mask to a first finalized mask, wherein the finalized mask is nonpixelated, continuous tone, and in the spatial domain; andquantizing the first finalized mask to obtain a second finalized mask, wherein the second finalized mask is quantized tone and in the spatial domain. 20. The method of claim 18 wherein the second cost function is different from the first cost function.