Interference lithography

Interference lithography (or holographic lithography) is a technique for patterning regular arrays of fine features, without the use of complex optical systems or photomasks. The basic principle is the same as in interferometry or holography. An interference pattern between two or more coherent light waves is set up and recorded in a recording layer (photoresist). This interference pattern consists of a periodic series of fringes representing intensity minima and maxima. Upon post-exposure photolithographic processing, a photoresist pattern corresponding to the periodic intensity pattern emerges. For 2-beam interference, the fringe-to-fringe spacing or period is given by , where λ is the wavelength and θ is the angle between the two interfering waves. The minimum period achievable is then half the wavelength. By using 3-beam interference, arrays with hexagonal symmetry can be generated, while with 4 beams, arrays with rectangular symmetry or 3D photonic crystals are generated. With multi wave interference (by inserting a diffuser into the optical path) aperiodic patterns with defined spatial frequency spectrum can be originated. Hence, by superimposing different beam combinations, different patterns are made possible. For interference lithography to be successful, coherence requirements must be met. First, a spatially coherent light source must be used. This is effectively a point light source in combination with a collimating lens. A laser or synchrotron beam are also often used directly without additional collimation. The spatial coherence guarantees a uniform wavefront prior to beam splitting. Second, it is preferred to use a monochromatic or temporally coherent light source. This is readily achieved with a laser but broadband sources would require a filter. The monochromatic requirement can be lifted if a diffraction grating is used as a beam splitter, since different wavelengths would diffract into different angles but eventually recombine anyway. Even in this case, spatial coherence and normal incidence would still be required.