The onset of lithographic technology involving extreme numerical aperture (NA) values introduces critical technical issues that are now receiving particular attention. Projection lithography with NA values above 0.90 is necessary for future generation devices. The introduction of immersion lithography enables even larger angles, resulting in NA values of 1.2 and above. The imaging effects from oblique angles, electric field polarization, optical interference, optical reflection, and aberration can be significant. This paper addresses polarization considerations at critical locations in the optical path of a projection system, namely in the illuminator, at the mask, and in the photoresist. Several issues are addressed including TE and azimuthal polarized illumination, wire grid polarization effects for real thin film mask materials, and multilayer resist AR coatings for high NA and polarization.
The objective of this paper is to study the polarization induced by mask structures. Rigorous coupled-wave analysis (RCWA) was used to study the interaction of electromagnetic waves with mask features. RCWA allows the dependence of polarization effects of various wavelengths of radiation on grating pitch, profile, material, and thickness to be studied. The results show that for the five different mask materials examined, the material properties, mask pitch, and illumination all have a large influence on how the photomask polarizes radiation.
Extreme ultraviolet (EUV) lithography is one of the leading technologies for 16nm and smaller node device patterning. One patterning issue intrinsic to EUV lithography is the shadowing effect due to oblique illumination at the mask and mask absorber thickness. This effect can cause CD errors up to a few nanometers, consequently needs to be accounted for in OPC modeling and compensated accordingly in mask synthesis. Because of the dependence on the reticle field coordinates, shadowing effect is very different from the traditional optical and resist effects. It poses challenges to modeling, compensation, and verification that were not encountered in tradition optical lithography mask synthesis.In this paper, we present a systematic approach for shadowing effect modeling and model-based shadowing compensation. Edge based shadowing effect calculation with reticle and scan information is presented. Model calibration and mask synthesis flows are described. Numerical experiments are performed to demonstrate the effectiveness of the approach.
A Talbot interference immersion lithography system that uses a compact prism is presented. The use of a compact prism allows the formation of a fluid layer between the optics and the image plane, enhancing the resolution. The reduced dimensions of the system alleviate coherence requirements placed on the source, allowing the use of a compact ArF excimer laser. Photoresist patterns with a half-pitch of 45 nm were formed at an effective NA of 1.05. In addition, a variable-NA immersion interference system was used to achieve an effective NA of 1.25. The smallest half-pitch of the photoresist pattern produced with this system was 38 nm.
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