Experimental study of effect of pellicle on optical proximity fingerprint for 1.35 NA immersion ArF lithography

Abstract

Pellicles are mounted on the masks used in ArF lithography for IC manufacturing to ensure defect-free printing. The pellicle, a thin transparent polymer film, protects the reticle from dust. But, as the light transmittance through the pellicle has an angular dependency, the pellicle also acts as an apodization filter. In the current work, we present both experimental and simulation results at 1.35 NA immersion ArF lithography showing the influence of two types of pellicles on proximity and intra-die Critical Dimension Uniformity (CDU). To do so, we mounted and dismounted the different pellicle types on one and the same mask. The considered structures on wafer are compatible with the 32 nm logic node for poly and metal. For the standard ArF pellicle (thickness 830 nm), we experimentally observe a distinct effect of several nm's due to the pellicle presence on both the proximity and the intradie CDU. For the more advanced pellicle (thickness 280 nm) no signature of the pellicle on proximity or CDU could be found. By modeling the pellicle's optical properties as a Jones Pupil, we are able to simulate the pellicle effects with good accuracy. These results indicate that for the 32 nm node, it is recommended to take the pellicle properties into account in the OPC calculation when using a standard pellicle. In addition, simulations also indicate that a local dose correction can compensate to a large extent for the intra-die pellicle effect. When using the more advanced thin pellicle (280 nm), no such corrections are needed.