PMMA-Based Microsphere Mask for Sub-wavelength Photolithography
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PMMA‑Based Microsphere Mask for Sub‑wavelength Photolithography Wenhe Feng1 · Yin Chi Wan1 · Xincai Wang1 Received: 28 April 2020 / Revised: 17 June 2020 / Accepted: 30 June 2020 © International Society for Nanomanufacturing and Tianjin University and Springer Nature Singapore Pte Ltd. 2020
Abstract The authors present a polymethyl methacrylate (PMMA)-based, reusable microsphere mask used in the laser sub-wavelength photolithography. In order to overcome the diffraction limit to achieve nano-structuring using 1-μm laser wavelength, the photolithography technique was conventionally characterized by applying a one-off monolayer of silica microspheres serving as Mie scatterers. Addressing the major limitation of this technique, which was that the monolayer of microspheres must be prepared on the sample surface prior to fabrication, the proposed hot press approach could firmly fuse the 1.57-µm silica microspheres to the PMMA base without the use of adhesives. The PMMA-based microsphere mask could hence reduce the amount of work for the monolayer preparation and was proven reusable for at least 20 times without damage to top or bottom surfaces. Using the mask, dimples that were 0.7 µm in diameter and 40 nm in depth were produced on tool steel by a single pulse of picosecond laser irradiation. Keywords Ultrafast laser · Microspheres · Mie-scattering · Photolithography · Mask · Micro/nano-fabrication process
1 Introduction Photolithography is the leading technique for the semiconductor industry that enables compact and powerful electronic devices. The minimum feature size it can produce is only defined by the diffraction limit that is set to the half wavelength of laser source. To overcome this physical limitation and achieve nano-structuring, near-field optical enhancement based on Mie theory can be employed by applying a monolayer of silica microspheres with a size of about 1–10 times the laser wavelength on the substrate surface. When the laser beam irradiates on the monolayer, the laser intensity underneath the microsphere is high enough to melt or even ablate the substrate underneath, and a uniform texture of an array of sub-µm pits or bumps can be produced. The early discovery of the microsphere-based sub-wavelength photolithography phenomenon and theoretical modeling was demonstrated in [1]. Recent publications reported its application in OLED fabrication [2], different substrates’ response [3], two-color laser irradiation [4], nanofabrication * Wenhe Feng [email protected]‑star.edu.sg 1
Singapore Institute of Manufacturing Technology (SIMTech), 2 Fusionopolis Way, Innovis #08‑04, Singapore 138634, Singapore
on bearing steel [5], and working distance control of a single microsphere held on a pipette [6]. Of all the reported work on this topic, a self-assembled microsphere monolayer must be formed in the first place on the surface of the target substrate. The method is one-off as the monolayer is easily damaged by ablation ejecta, i.e., the attachment of spheres to the sample can only withsta
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