Based on Rayleigh's equation, smaller resolution of state-of-the-art high-numerical-aperture ArF lithography can only be achieved with resolution-enhancement techniques.
RETs are a cost-effective way to maintain the aggressive evolution to smaller dimensions in IC manufacturing and are becoming integral to manufacturing lithography solutions. Apart from the traditional binary masks, attenuated phase-shift masks and alternating phase-shift masks are widely available.
The combination of these reticles with off-axis illumination techniques, and the variation of NA/sigma (sigma being the coherence or fill factor) settings that the scanners permit, offer the optical scientist a wide range of possibilities when optimizing the printing conditions of a given design. Still, for each critical layer, such as poly or contact, the most preferred enhancement technique needs to be selected and implemented.
To explore the capability of optical extension techniques toward the 45-nanometer technology node, it is important to investigate the pitch range they can cover. To this end, IMEC has made a one-on-one comparison of the various optical enhancement techniques for gates.
Rayleigh's equation defines half-pitch resolution as R = k1 (lambda/NA), with lambda denoting the wavelength. Needed are RETs that reduce k1 toward its theoretical limit of 0.25.
Using an ASML PAS 5500/1100 ArF scanner with an NA of 0.75, IMEC researchers printed lines at a range of pitches starting at 160 nm (or a k1 factor of 0.31). We used several mask types, including traditional binary masks and attenuated phase-shift masks for single-exposure patterning. For double-exposure patterning, alternating phase-shift masks and double-dipole binary masks were selected. Different types of off-axis illumination, including annular, quadrupole and dipole, were used.
Subresolution assist features were applied to boost process capability of semidense through isolated lines, and sigma settings were selected to enhance the printability of the densest pitch or for good through-pitch pattern printability.
Same resist process
At each illumination condition and mask type, IMEC has identified the resolution limit, process capability and forbidden pitches based on process windows and mask error factors at different pitches.
All experiments have been done with the same ultrathin resist process, exposure tool and metrology. The outcome shows which enhancement technique is optimal for printing a given pitch range.
Through-pitch solutions are explored for three different resolution limits that have been defined based on the capabilities of the various enhancement techniques. The data shows that up to a 200-nm pitch (k1 = 0.39), Quasar quadrupole illumination in combination with a binary reticle represents the best through-pitch solution. Attenuated phase-shift masks and annular illumination provide a through-pitch, single-exposure solution if the densest pitch on the design is 180 nm (k1 = 0.35).
Finally, to improve the depth of focus, the 0.75-NA ArF imaging data was compared to experimental data of a 0.75-NA ArF full-field immersion scanner, which is among the most advanced tools available for the next technology node. With the immersion system, a gain in depth of focus of 40 percent has been achieved compared with the conventional ArF scanner.
We extrapolated the data to a 0.85-NA ArF immersion system to establish the possible pitch range for the 45-nm node. For the three through-pitch imaging solutions proposed for "dry" 0.75-NA ArF lithography, pitch resolution limits that can be achieved using 0.85-NA ArF immersion lithography are expected to be about 30 nm lower than the corresponding "dry" resolution limits.
Eric Hendrickx is senior researcher and Geert Vandenberghe is manager of the Optical Extensions and Imaging Group at IMEC (Leuven, Belgium).