Grafted Polymer Monolayer Brush as Electron Beam Resist
Electron beam lithography (EBL) is a versatile tool for nanofabrication widely used for research purpose and industry because of its high resolution and the capability of writing patterns without any mask. One of the important challenges in EBL is nanofabrication on non-flat or irregular surfaces. Although spin coating is the most popular technique for resist coating, it is not suitable for non-flat, irregular substrates because uniform film cannot be obtained on those surfaces. In this work, it was demonstrated that monolayer PMMA brush can be used as negative tone e-beam resist that can be applied for non-flat, irregular surfaces as well as flat, conventional surfaces.
Although it is well known that heavily exposed PMMA undergoes cross-linking and works as negative tone e-beam resist when it is developed by solvent, such as acetone, solvent does not work as a developer for negative tone monolayer PMMA brush because of the strong bonding between PMMA brush and substrate. Instead, thermal treatment was used to develop monolayer PMMA brush to achieve negative tone behavior. Since cross-linked PMMA has higher thermal resistance than uncross-linked one, only unexposed PMMA was vaporized at proper temperature. The line array patterns as thin as 14 nm width and 100 nm period were fabricated on a flat Si substrate by using PMMA brush. Also, the process was applied to a non-flat surface of an AFM cantilever to show that PMMA brush can be used on non-flat surfaces.