Double Exposure Method Allows Lithographic Steppers to Produce Half-Wavelength Features
Innovative double exposure by advanced lithography (IDEAL), a technique touted by Canon Inc. (Tokyo, Japan) at last week's SPIE Microlithography '99 conference (Santa Clara, CA), can reportedly allow lithographic steppers to image circuit patterns less than one-half the wavelength of light. According to Canon researchers, IDEAL achieves a process k1 factor of 0.3, in contrast to current lithographic processes, which typically achieve k1 factors of only 0.5 in production environments. To produce the 100-nm features that will be needed in future chips, argon fluoride (ArF; 193 nm) excimer-laser-based lithography will require k1 factors below 0.4. By reducing the k1 factor to 0.3, the IDEAL method will allow high-numerical-aperture ArF tools to print circuit features as fine as 80 nm, and extend 157-nm optical lithography to features as small as 60 nm or less.
Double exposure
The technique works by dividing critical-layer fine and coarse circuit pattern components between two reticles. By first using a simple alternating phase-shift mask with fine line and space patterns (k1=0.3), then a simple binary mask for rough outline patterning, IDEAL achieves a multi-level exposure dose at the wafer plane. Highly detailed resist patterns are exposed where the accumulated partial dose from the overlapping aerial images of the phase shift mask and the coarse reticles meets or exceeds the resist threshold level.
Canon researchers showed the results of using the new method on conventional krypton fluoride (KrF; 248 nm) excimer-laser-based steppers with 0.60 and 0.63 NA to achieve difficult 120 nm circuit features, equivalent to a k1 of 0.3. Data and SEM pictures showed 120 nm dense and isolated lines, 130-nm logic gate pattern array, and 130-nm contact holes with excellent profile.
The technology not only relaxes reticle design, but also allows significant alignment latitude between the two exposures. Data and SEM pictures demonstrate that misaligning the reticle by as much as ±20 nm produced no significant change in critical dimensions. The effect of misalignment between the two exposures on overlay is not a major concern either, since image placement on the wafer is mainly a function of the fine pattern reticle.