AN EXCIMER-LASER STEPPER WITH THROUGH-THE-LENS ALIGNMENT

A new excimer laser stepper at 248 nm wavelength has been developed. T he numerical aperture and field size of the all quartz 5x reduction le ns are 0.4 and 15mm square, respectively. A key feature of the system is a through-the-lens alignment system using an optical heterodyne met hod operating with a 633 nm He-Ne laser. The problem of the large foca l plane difference between the exposure and alignment wavelength has b een solved by the new alignment system using two separated grating mar ks on the reticle and a checkerboard grating on the wafer. Moreover, a wafer leveling mechanism and a yaw compensation mechanism have been d eveloped in order to realize precise positioning of the wafer stage. E xposure experiments have proved that the resolution is 0.35 mum and th e overlay accuracy is better than 0.1 mum.