Laser interference lithography is a pattern definition technique capable of defining micrometer and sub-micrometer large area periodic patterns. Interference lithography has two advantages compared with other fabrication techniques like electron beam lithography and laser writing:
1. Interference lithography is not a step-and-repeat process but allows for processing of a complete substrate with one single exposure (several full area exposures for generation of more complex patterns),
2. Interference lithography offers the possibility to realize homogeneous micro and sub-micrometer structured surfaces on areas more than one square meter in size either impossible or extremely time and cost consuming with other patterning technologies.
Laser interference lithography is made with a large scale interferometer setup. First step is to split the laser beam into two equal laser beams that are directed by mirrors and expanded by lenses. Behind the lenses spatial filters are mounted to avoid parasitic interference effects such as airy patterns which have their origin in dust particles or defects on the optical components. A photoresist coated substrate, size up to 1 m^2, is mounted in a holder and positioned in the area where the two single laser beams superimpose. Thus, the photoresist is exposed via the interference pattern and the interference pattern determines the surface relief profile of the resist after development.
There are many parameters in laser interference lithography to optimize and tweak and with perfect control of these it is possible to make advanced 3D shaped structures like micro lens arrays and anti-reflective structures.