(54) Nanoscale Patterning Laser Interference Lithography

Nanoscale patterning by means of laser interference lithography (LIL) is an easy, inexpensive process for creating periodic nanostructures over a large area on a substrate (typically glass or silicon). This process first... [ view full abstract ]

Nanoscale patterning by means of laser interference lithography (LIL) is an easy, inexpensive process for creating periodic nanostructures over a large area on a substrate (typically glass or silicon). This process first creates the inverse of the desired nanoscale patterns in a photosensitive polymer layer (photoresist) that is sensitive to 365-436 nm light, allowing for maskless patterning via exposure to two or more coherent laser beams in this wavelength range. The process I used to expose the photoresist to dark and bright interference fringes requires only one laser at 406 nm and a Lloyd’s mirror, with reflection off the latter serving as a virtual source that creates a periodic spacing, where the separation between adjacent features is determined by the laser wavelength and the incident angle on the Lloyd’s mirror. I chemically developed my samples to leave a diffraction grating with the desired periodicity, and then deposited on top a metallic thin film of silver by electron-beam evaporation. I tested the quality of my nanopatterns with a green laser (532 nm). The quality of the diffraction grating can be visually estimated by how many orders of diffraction are generated by the green laser beam. The final nanopatterns of silver, in combination with yet another laser source and a target substance to be detected, identified and/or analyzed, can enable a powerful "molecular fingerprinting" technique called surface-enhanced Raman spectroscopy (SERS), which relies on the strong surface plasmon resonances (SPR) of noble metals. The SPR effect is caused by the collective oscillations of the free electrons in metallic nanostructures. This effect allows for an enhancement of the Raman signal by factors as large as 10^10, which means SERS could even detect a single molecule in a sample. Thus far I have created nanopatterns with second-order diffraction and I am currently working toward finding optimal exposure and development times to make samples useful for SERS measurements. In this presentation I will discuss how the exposure time, development time, and incident angle determine the periodicity and fidelity of the nanoscale patterns.