Simulation method using a simulation system that provides information on a transfer pattern of a predetermined mask pattern transferred to a wafer by optical photolithography and method of modifying mask pattern

1. A simulation method using a computer that provides, through simulation, information on a transfer pattern to be produced on a wafer upon transferring a predetermined mask pattern thereto by optical photolithography, comprising the steps of the computer:

accepting an input of a measured dimension of said transfer pattern;

calculating a light intensity at each of a plurality of positions on said transfer pattern;

calculating a light intensity gradient at each of said positions based on said light intensity;

multiplying said light intensity, a tentative optical reaction coefficient and said light intensity gradient to thus obtain a modified value, and adding said modified value to said light intensity thereby giving a modified light intensity; and

deciding a threshold value and an optical reaction coefficient by regression calculation such that a difference between said measured dimension and a calculated dimension becomes minimal under said modified light intensity, employing as a constant said threshold value of said light intensity at a pair of edges that define said calculated dimension of said transfer pattern in said simulation.

2. The simulation method according to claim 1 ,

wherein said calculating a modified light intensity includes calculating a modified light intensity through convolution integration of the sum of said light intensity and said modified value, and a Gaussian function having a tentative diffusion length; and

said deciding a threshold value and said optical reaction coefficient further includes deciding a diffusion length, in addition to said threshold value and said optical reaction coefficient.

3. The simulation method according to claim 1 , further comprising:

calculating a post-diffusion light intensity through convolution integration of said light intensity and a Gaussian function having a diffusion length; and

calculating a post-diffusion light intensity gradient at each of said positions based on said post-diffusion light intensity;

wherein said calculating a modified light intensity includes multiplying said post-diffusion light intensity, said optical reaction coefficient, and said post-diffusion light intensity gradient to thus obtain said modified value, and adding said modified value to said post-diffusion light intensity, thereby giving said modified light intensity; and

said deciding a threshold value and said optical reaction coefficient further includes deciding said diffusion length, in addition to said threshold value and said optical reaction coefficient.

4. A method of modifying a mask pattern, comprising utilizing a lithography model obtained through said simulation method according to claim 1 , to thereby modify said mask pattern.

5. A simulation system that provides, through simulation, information on a transfer pattern to be produced on a wafer upon transferring a predetermined mask pattern thereto by optical photolithography, comprising:

a unit accepting an input of a measured dimension of said transfer pattern;

a unit calculating a light intensity at each of a plurality of positions;

a unit calculating a light intensity gradient at each of said positions based on said light intensity;

a unit multiplying said light intensity, a tentative optical reaction coefficient and said light intensity gradient to thus obtain a modified value, and adds said modified value to said light intensity thereby giving a modified light intensity; and

a unit deciding a threshold value and an optical reaction coefficient by regression calculation such that a difference between a calculated dimension and said measured dimension becomes minimal under said modified light intensity, employing as a constant said threshold value of said light intensity at a pair of edges that define said calculated dimension of said transfer pattern in said simulation.