Patterning on reflective (Al) surface using auto alignment in immersion lithography to achieve tight overlay for reconfigurable metasurface devices

Metasurfaces have been used for efficient manipulation of high-frequency terahertz waves [1]. Nanogaps in terahertz metasurface play a critical role in the device performance in terms of higher Q, lower switching power, larger tuning range and/or higher sensitivity. Due to the need for nanogaps in micron-scale unit cell forming a centimeter-scale metasurface requires advance lithography tools that can provide both fine resolution and good alignment to make the process more suitable for manufacturing. The process involves patterning on highly reflective metals such as Aluminium, which can be a challenge in photolithography processing with good alignment. On a reflective surface, the minor difference on topography of the features can cause interference that leads to alignment failure, even with BARC (Back Anti-Reflection Coating) in advanced DUV (Deep Ultra-Violet) photolithography tool. The main objective of this study is to find the optimum condition that will make the tool recognize the alignment mark without any error or manual intervention. Subsequently, ensuring good overlay alignment in few nanometers.