Imaging control functions of optical scanners

For future printing based on multiple patterning and directed self-assembly, critical dimension and overlay requirements become tighter for immersion lithography. Thermal impact of exposure to both the projection lens and reticle expansion becomes the dominant factor for high volume production. A new procedure to tune the thermal control function is needed to maintain the tool conditions to obtain high productivity and accuracy. Additionally, new functions of both hardware and software are used to improve the imaging performance even during exposure with high-dose conditions. In this paper, we describe the procedure to tune the thermal control parameters which indicate the response of projection lens aberration and reticle expansion separately. As new functionalities to control the thermal lens aberration, wavefront-based lens control software and reticle bending hardware are introduced. By applying these functions, thermal focus control can be improved drastically. Further, the capability of prediction of reticle expansion is discussed, including experimental data from overlay exposure and aerial image sensor results.