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Plasmonics based photolithography has been able to achieve pattern size beyond the typical diffraction limit by exploiting the surface plasmon (SP) [1]. However, film roughness is inevitable in practical fabrication, and can strongly impact the performance of the lithography. In our work, exemplary lithography systems including superlens and hyperbolic metamaterial (HMM) based approaches are considered, where the effects of films roughness and the defects on the mask are analyzed systematically.
For the superlens system, a chromium (Cr) mask with thickness of 50 nm and period of 90 nm on glass substrate, is placed above the superlens with a poly-methyl-methacrylate (PMMA) spacer layer. A silver (Ag) film with thickness of 20 nm transmits the transverse magnetic (TM) polarized light with wavelength 365 nm the photoresist (PR) film with thickness of 40 nm. A reflector composed of an Ag film with thickness of 50 nm is added at the bottom of the PR. For the HMM system, Cr mask is used with period of 360 nm and the multi-layer structure is composed with 9 layers of 15 nm thick aluminum (Al) and 30 nm thick silicon dioxide (SiO2) films. The PR is also 40 nm and reflector film is an Al film with thickness of 50 nm.
The broad optical transfer function (OTF) of the smooth Ag enables the evanescent waves of wide wave vector range to pass through [2]. While the OTF of HMM shows that only ±2nd order diffraction waves with the wave vector about 2k_0 can pass through [3]. Different degrees of roughness are introduced on the films and the photomask, and the performance of the two systems in terms of the intensity, pattern uniformity and line edge roughness are compared. As the roughness of the films grows, the OTF of the superlens degrades dramatically, while the OTF of the HMM maintains, as shown Figure 1 and 2. Accordingly, the field distribution of the HMM system is less affected by the film roughness, thereby can still produce high quality periodic patterns. By properly choosing the period of the mask, the desired transmission order coincides with the peak of the OTF function. Therefore, the light of desired spatial frequency transmits maximally, while the other diffraction orders induced by the rough surfaces is relatively suppressed, leading to a pattern with better uniformity. On the other hand, the superlens case does not offer such a utility because of the broad transmission function in OTF. But superlens can image arbitrary patterns; while the drawback is that line-edge roughness of the mask will also be imaged onto the photoresist. Therefore, the impact of a single defect on the mask pattern is much greater than that of the HMM approach.
In addition, the waveguide lithography system consists of aluminum (Al) layers acting as a filter shows similar behavior compared with that of the HMM system. Though the structure is almost identical to the superlens structure, but the thin metal film serves a very different function in this case, which confirms the distinctive advantage of the frequency selective scheme. Meanwhile, our simulation shows Al can be a good superlens at 193 nm, which indicates that the same metal can function differently in different schemes or wavelengths.
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