Eliminate the vibration defect for laser interference lithography using an optical chopper system

Laser interference lithography (LIL) is a maskless lithography technique with many advantages such as simple optical design, low cost, maskless, infinite depth of focus, and large area patterning with single exposure. Compare to the tradition optical lithography, LIL is very suitable for applications which need periodic nanostructure, such as grating, light-emitting diode (LED), photonic crystals, etc. However, due to the principle of LIL, the exposure result is very sensitive to the light source and the environment vibration. Defects which perpendicular or parallel to the grating occurs when the LIL system is effect by the environment vibration. The reason that cause this defect is Moiré fringe. When the periodic structure is fabricated in an environment with vibration source, the grating structure will have a small angle rotational vibration and the Moiré fringe defect is formed. In order to eliminate the Moiré fringe defect, this paper developed a new LIL system with chopper and accelerometer. The accelerometer can measure the vibration frequency. And by setting the chopper frequency equal to the vibration frequency, the Moiré fringe defects can be eliminate. In this paper, we use a piezo stage to generate a stable vibration source with tunable frequency. In this way, we can produce a repeatable Moiré fringe defect. By setting the chopper frequency equal to the stage vibration frequency, the Moiré fringe defect can be eliminate. And we successfully fabricated large area periodic structure without any vibration defects. The periodic structure is 360nm pitch and the area is 2x3 cm² .