The roughness present on the sidewalls of lithographically defined patterns imposes a very important challenge for advanced technology nodes. It can originate from the aerial image or the photoresist chemistry/processing . The latter remains to be the dominant group in ArF and KrF lithography; however, the roughness originating from the mask transferred to the aerial image is gaining more attention [2-9], especially for the imaging conditions with large mask error enhancement factor (MEEF) values. The mask roughness contribution is usually in the low frequency range, which is particularly detrimental to the device performance by causing variations in electrical device parameters on the same chip [10-12]. This paper explains characteristic differences between pupil plane filtering in amplitude and in phase for the purpose of mitigating mask roughness transfer under interference-like lithography imaging conditions, where onedirectional periodic features are to be printed by partially coherent sources. A white noise edge roughness was used to perturbate the mask features for validating the mitigation.
Line edge roughness (LER) is a common problem to most lithography approaches and is seen as the main resolution limiter for advanced technology nodes<sup>1</sup>. There are several contributors to LER such as chemical/optical shot noise, random nature of acid diffusion, development process, and concentration of acid generator/base quencher. Since interference-like lithography (IL) is used to define one directional gridded patterns, some LER mitigation approaches specific to IL-like imaging can be explored. Two methods investigated in this work for this goal are (i) translational image averaging along the line direction and (ii) pupil plane filtering. Experiments regarding the former were performed on both interferometric and projection lithography systems. Projection lithography experiments showed a small amount of reduction in low/mid frequency LER value for image averaged cases at pitch of 150 nm (193 nm illumination, 0.93 NA) with less change for smaller pitches. Aerial image smearing did not significantly increase LER since it was directional. Simulation showed less than 1% reduction in NILS (compared to a static, smooth mask equivalent) with ideal alignment. In addition, description of pupil plane filtering on the transfer of mask roughness is given. When astigmatism-like aberrations were introduced in the pupil, transfer of mask roughness is decreased at best focus. It is important to exclude main diffraction orders from the filtering to prevent contrast and NILS loss. These ideas can be valuable as projection lithography approaches to conditions similar to IL (e.g. strong RET methods).
The introduction of EUV lithography into the semiconductor fabrication process will enable a continuation
of Moore's law below the 22 nm technology node. EUV lithography will, however, introduce new and
unwanted sources of patterning distortions which must be accurately modeled and corrected on the
reticle. Flare caused by scattered light in the projection optics is expected to result in several nanometers of
on-wafer dimensional variation, if left uncorrected. Previous work by the authors has focused on
combinations of model-based and rules-based approaches to modeling and correction of flare in EUV
lithography. Current work to be presented here focuses on the development of an all model-based approach
to compensation of both flare and proximity effects in EUV lithography. The advantages of such an
approach in terms of both model and OPC accuracy will be discussed. In addition, the authors will discuss
the benefits and tradeoffs associated with hybrid OPC approaches which mix both rules-based and modelbased
OPC. The tradeoffs to be explored include correction time, accuracy, and data volume.
Research has been conducted to develop alternatives to chemically amplified 193 nm photoresist materials that will be
able to achieve the requirements associated with sub-32 nm device technology. New as well as older photoresist design
concepts for non-chemically amplified 193 nm photoresists that have the potential to enable improvements in line edge
roughness while maintaining adequate sensitivity, base solubility, and dry etch resistance for high volume manufacturing
are being explored. The particular platforms that have been explored in this work include dissolution inhibitor
photoresist systems, chain scissioning polymers, and photoresist systems based on polymers incorporating
formyloxyphenyl functional groups. In studies of two-component acidic polymer/dissolution inhibitor systems, it was
found that compositions using ortho-nitrobenzyl cholate (NBC) as the dissolution inhibitor and poly norbornene
hexafluoro alcohol (PNBHFA) as the base resin are capable of printing 90 nm dense line/space patterns upon exposure to
a 193 nm laser. Studies of chain scission enhancement in methylmethacrylate copolymers showed that incorporating
small amounts of absorptive a-cleavage monomers significantly enhanced sensitivity with an acceptable increase in
absorbance at 193 nm. Specifically, it was found that adding 3 mol% of α-methyl styrene (α-MS) reduced the dose to
clear of PMMA-based resist from 1400 mJ/cm<sup>2</sup> to 420 mJ/cm<sup>2</sup>. Preliminary data are also presented on a direct
photoreactive design concept based on the photo-Fries reaction of formyloxyphenyl functional groups in acrylic copolymers.