Proximity correction for electron beam lithography

Abstract

As the critical dimensions required in mask making and direct write by electron beam lithography become ever smaller, correction for proximity effects becomes increasingly important. Furthermore, the prob- lem is beset by the fact that only a positive energy dose can be applied with an electron beam. We discuss techniques such as chopping and dose shifting, which have been proposed to meet the positivity require- ment. An alternative approach is to treat proximity correction as an opti- mization problem. Two such methods, local area dose correction and optimization using a regularizer proportional to the informational entropy of the solution, are compared. A notable feature of the regularized prox- imity correction is the ability to correct for forward scattering by the gen- eration of a ''firewall'' set back from the edge of a feature. As the forward scattering width increases, the firewall is set back farther from the fea- ture edge. The regularized optimization algorithm is computationally time consuming using conventional techniques. However, the algorithm lends itself to a microelectronics integrated circuit coprocessor implementation, which could perform the optimization faster than even the fastest work stations. Scaling the circuit to larger number of pixels is best approached with a hybrid serial/parallel digital architecture that would correct for proximity effects over 10 8 pixels in about 1 h. This time can be reduced by simply adding additional coprocessors. © 1996 Society of Photo-Optical