A study of interferometric lithography: Approaching the linear system limits of optics

Conventional optical lithography is limited by the spatial frequency coverage ({dollar}{lcub}sim{rcub}NA/lambda){dollar} of an optical system. Interferometric lithography, the use of the aerial image formed by two coherent laser beams incident on a wafer at different angles to print extreme sub-{dollar}mu{dollar}m structures, provides a simple approach to the ultimate linear system spatial frequency limit of optics (2/{dollar}lambda).{dollar} Interferometric lithography process development efforts have demonstrated high aspect ratio resist patterns with near vertical sidewalls at the requisite scales for the next several ULSI generations. A sophisticated interferometric lithography process model has been developed and verified experimentally to provide a robust process development capability. The periodic nature of an interferometric pattern also lends itself directly to a convenient process control by allowing real-time exposure monitoring.; Several techniques are presented to address the pattern flexibility problem associated with interferometric lithography. Multiple-beam and multiple-exposure interferometric techniques have been demonstrated for the extension of interferometric lithography to more complex, but still repetitive patterns. Moire alignment techniques enable registration between these multiple exposures. Additional flexibility is attained by coupling of interferometric lithography, for very high spatial resolution, with optical lithography, for introducing the information-containing aperiodic features. This mix-and-match approach offers access to a wide array of complex nanoscale structures.; Imaging interferometric lithography, a true integration of the best features of optical and interferometric lithography, provides a novel route to arbitrary pattern fabrication with a resolution approaching the fundamental limits of optics. Off-axis illumination is used to shift the high frequency components of the mask pattern so that they are captured by a lens. After passing through the optical system, an interferometric beam is introduced to shift the high frequency components back to their original spatial frequencies. Modeling and simulation results show that arbitrary patterns with dense CDs extending to 120-nm at I-line and to 65-nm at a 193-nm exposure wavelength are possible. Initial experiments demonstrate that the coverage in frequency space is increased up to {dollar}({lcub}sim{rcub}3NA/lambda){dollar} for a 3-exposure imaging interferometric lithography configuration and the resolution is concomitantly increased by a factor of 3. Development of the optics-based imaging interferometric lithography technique may extend the life of familiar optical lithography well into sub-100-nm CD generations.