Lloyd's mirror interferometer using a single-mode fiber spatial filter

An ultraviolet single-mode fiber is used for beam transport, spatial filtering, and beam expansion for a Lloyd's mirror interferometer for laser interference lithography. Polarized laser light at 325 nm from a HeCd laser was coupled to a nonpolarization-maintaining step-index fiber, which preserved the linear polarization with an extinction ratio exceeding 100:1. The linear polarization direction of the output beam was remotely adjusted by a half-wave plate in front of the laser. The output beam profile matched the predicted far-field distribution of the single LP01 mode step-index fiber, with a numerical aperture of 0.09 at 325 nm. By illuminating a Lloyd's mirror interferometer with the beam produced by a single fiber, line/space photoresist patterns with a pitch of 220 nm were demonstrated. Various mechanical and optical aspects that may be helpful to other research groups which are building a simple but stable interference lithography system of this technique are discussed.