Immersion lithography at ultraviolet (UV) wavelengths has been used extensively to create wire-grid polarizers consisting of parallel lines of aluminum or some other metal on an optically transparent substrate. To date, the period or "pitch" of these lines is limited to around 140 nm for visible-light applications using a 365 nm source. But a new immersion-lithography technique developed by Glenn Hutchinson, senior R&D officer at FFCP Consultants (Highland, UT), cuts the period in half to 70 nm, resulting in improved contrast ratio and operation at shorter wavelengths (into the UV) for these wire-grid polarizer devices.
In the frequency-doubling technique, a polarized 365 nm source is passed through a spatial filter and then divided (using a beamsplitter) just prior to the target substrate. The two beams are then overlapped and intersected to produce a ribbed pattern in the photoresist. Careful etching prevents the ribs from collapsing on each other and anisotropically transfers the mask rib pattern down to the underlying metal. Utmost attention must be paid to the gas composition, bias powers, and etch times to reduce the etch rate. Optical properties are almost exclusively determined by this etch step. Passivation of the metal wires is then needed to maintain integrity of the polarizer structure. Contact Glenn Hutchinson at [email protected].