Evanescent wave imaging in optical lithography

Abstract

New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference.

Mask enhancement using an evanescent wave effect

“…Prior work with EWAFs has focused on near-field enhancement of intensity at an aperture 1 . Expanding on these ideas, the enhancement effect is investigated more thoroughly and propagated to the far field.…”

“…The sub-wavelength grooves have been referred to as Evanescent Wave Assist Features, or EWAFs, since their primary purpose in the lithographic scope is to "assist" the aperture which they surround 1 . Previous work has demonstrated that these gratings, buried under the mask absorber, have the ability to enhance or inhibit the intensity at the center of a slit or contact.…”

“…The mathematical form of the wave is described in equation (1). The transmitted electric field, E 0t is modulated by an exponential.…”

“…Evanescent waves can be generated in a variety of ways, including Total Internal Reflection (TIR) 1,3 . As a beam at TIR approaches an interface to a lower index medium, all of its energy must be reflected back.…”

“…In this scenario, a 45nm (1X) contact was flanked in X and Y dimensions with EWAFs as in Figure 2. Assist geometries were restricted to Manhattan style layouts in order to simplify simulation, however several alternative EWAF layouts and CDEW generating structures have been proposed in literature 1,6 . Duty ratios investigated varied from 1:1.5 through 1:5.7 l/s.…”