Laser Interference Lithography

Yu F, Li P, Shen H, Mathur S, Lehr C, Bakowsky U, Miicklich F. (2005) Laser interference lithography as a new and efficient technique for micropatterning of biopolymer surface. 

Recent developments in the area of microengineered structures for chemical processing [24] made possible to manufacture meshes of various materials (i.e., steel, silicon nitride), by techniques such as standard mask lithography, or laser interference lithography [56,57]. 

Vlcek M., Schroeter S., Brueckner S., Fehling S., and Fiserova A., Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography, /. Mater. Set Mater. Electron., 20,290-293 (2009). 

Kuip)er, K., van Rijn, C. J. M., Nijdam, W., and Elwenspoek, M. C. (1998). Microsieves made with laser interference lithography for micro-filtration applications. J. Membr. Sci. 150(1), 1. 

MF membranes, in silicon nitride, showing high porosity and narrow pore size distribntion, conpled with very low flow resistance and minimal fouling tendency, were prodnced by laser interference lithography and silicon micromachining technology (Fignre 1.2) [8]. Snch membranes are often referred to as... 

FIGURE 1.2 Field emission scanning electron microscopy image of a membrane produced by laser interference lithography. (Data from S. Kuiper et al., Journal of Membrane Science, 150, 1-8, 1998.)... 

With interference lithography (IL) a resist layer is exposed by an interference pattern generated by two obliquely incident laser beams, which is used to expose a photoresist layer without the use of a mask (see section... 

Interference lithography makes use of the interference pattern which is formed when two or more coherent light waves are superposed. In a t3q>ical optical set-up, a laser is used as a source for UV radiation. The laser beam is spht into two beams. Each of the beams is directed by mirrors towards a substrate coated with photoresist where the beams are superposed after being expanded. Two interfering beams produce a ID grating with a sinusoidal intensity distribution. To this intensity pattern the UV-sensitive photoresist is exposed. After exposure the photoresist plate is developed where exposed or unexposed. Photoresist is removed depending on the type of photoresist. As the intensity profile is sinusoidal continuous microstructure profiles will result in general. 

In Fig. 11, a sketch of the optical set-up for interference lithography and in Fig. 12, a picture of the largest interference lithography laboratory at Fraunhofer ISE are shown. A laser beam is split, directed with mirrors and then spatially filtered and expanded. A sample holder with the photoresist plate is positioned where the expanded beams are superimposed. A shutter defines the exposure time. Behind the spatial filters, no optical components are in the optical path in order to avoid parasitic interference effects such as Airy patterns from dust particles. The nonplanarity of the interfering beams results in a small variation of the grating periods which is tolerated for the above mentioned appfications. If one assumes symmetrical angles of incidence a, then the... 

At the sample stage, a flat mirror is mounted perpendicular to the sample on a rotation stage. As part of the laser beam is reflected by the mirror, it interferes with the nonreflected part to form an interference pattern on the photoresist. The period P is given as P = where 6 is the incident angle [44]. After lithography and baking, the templates are then transferred into an electron-beam or a thermal evaporation thin film deposition system for metallization. As the thickness of the metal film is larger than the skin depth of metal, the metallic arrays are considered as semi-infinitely thick. 

An important characteristic of excimer lasers that sets them apart from traditional UV lasers is their lack of spatial coherence. The interference phenomena that result from the high spatial coherence of traditional singlemode continuous wave lasers produces a random intensity variation in projected patterns called speckle. This speckle phenomenon has historically made use of lasers in high-resolution lithography very difficult. The beam of excimer lasers is so highly multimode that speckles are, for all practical purposes, nonexistent in projected patterns. The application of excimer laser... 

Exciplex and excimer lasers have very poor spatial coherence and therefore do not produce speckles in lithography. Speckles are the random interference patterns that result when, on illumination of an object with a spatially coherent wavefront, any... 

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