Design of two-dimensional optical alignment signals robust to diffractive effects

Mask alignment is one of the most critical processes in photolithography. Prior to the shadow projection, the alignment between the lithographic mask and the silicon wafer is needed. In contact and proximity photolithography, a method to achieve the alignment with submicron or even nanometer resolution consists of superimposing two identical 2-D zero reference codes and registering the optical output signal. In order to increase the resolution of the system, the size of the code must be reduced and the diffractive effects become strong. The signal is then degraded and the precision of the alignment is reduced. In this paper, the effect of the diffraction in 2-D codes is analyzed, the degradation of the signal is characterized and its effect is modelled by means of a simple and fast computing parameter. Finally, we propose a genetic algorithm to optimize this parameter and design 2-D codes robust to diffractive effects. We propose the use of these codes to increase the resolution of alignment systems.