To address this issue, researchers at Princeton University, US, have developed a novel technique to speed up and simplify the patterning of metals. The method involves pressing a mould against a thin metal film deposited on a wafer, and applying a laser pulse for just 20 billionths of a second. The surface of the metal briefly melts for hundreds of nanoseconds and resolidifies around the mould. This ultrafast one-step process replaces the typical sequence of resist patterning, pattern transfer by etching and resist removal. Fabrication of metal (nickel, copper, gold and aluminium) gratings with a period of 200 nm and line widths in the region of 100 nm has been demonstrated by the team.
Targeted approach
The key turned out to be a tool called an excimer laser, which is commonly used in laser surgeries because it can heat just the thinnest surface layer of a material without causing damage underneath. For the same reason, there is minimal heating to the substrate, thus the mould and substrate may have different thermal expansion coefficients, such as a transparent quartz mould combined with a silicon substrate.
Numerous applications for this new fabrication technique are envisaged. For example, the current approach makes it straightforward to create a hole array in a noble metal film, which can be then be used as a biosensor based on so-called extraordinary optical transmission phenomena. Besides nano-structuring the metal, the metal surface is expected to be smoother when using a smooth quartz mould. This is an advantage for devices based on surface plasmon resonance, as a smooth surface is crucial for decreasing loss and improving sensitivity.
More details are available in the journal Nanotechnology.