|Xiong et. al. report a feasible design to achieve subdiffraction-limited patterns generated from a diffraction-limited mask using a hyperlens with flat input and output surfaces. This approach has demonstrated utility for generating arbitrary subdiffraction-limited pattern features.|
Reviewed by Jeff Morse, Ph.D, National Nanomanufacturing Network
- Xiong Y, Liu Z, Zhang X. 2009. A simple design of flat hyperlens for lithography and imaging with half-pitch resolution down to 20nm. Applied Physics Letters 94 203108. DOI: 10.1063/1.3141457.
A key driver in the progress of nanotechnology has been the evolution of photolithographic capabilities that persistently reduce feature size and pitch resolution to achieve increased density of electronic devices and circuits. Though conventional photolithography has kept pace with Moore’s Law by reducing the illumination wavelength, system complexity and associated costs increase significantly as the wavelength approaches the extreme UV range. As an alternative approach, plasmonic lithography allows the use of conventional UV wavelength sources to achieve higher resolution by breaking the diffraction limit of light.
Plasmonic lithography exploits the excitation of surface plasmon polaritons that have wavelengths much smaller than the excitation wavelength, thereby enabling high-resolution lithography. An imaging device known as a hyperlens enables subdiffraction-limited photolithography by magnifying the subdiffraction-limited objects, projecting the patterns to the far field. Therefore, the hyperlens, which consists of a curved stack of periodic metal-dielectric multilayers, could be used to generate arbitrary subdiffraction-limited patterns from diffraction-limited masks or via interference patterns of the illumination light, representing a versatile approach for masked or maskless lithography scenarios.
The original hyperlens concept had a cylindrical-shaped surface; a more desirable approach for photolithographic systems must include the versatility to generate patterns on arbitrarily shaped or nominally flat surfaces. Recently, Xiong et. al. reported a feasible design to achieve subdiffraction-limited patterns generated from a diffraction-limited mask using a hyperlens with flat input and output surfaces. Assuming a hyperlens multilayer material consisting of 10 nm Ag and 10 nm Al2O3 having input radius of 790 nm and output radius of 120 nm in their analysis, the authors demonstrated that at a working wavelength of 375 nm the dispersion relation of the electromagnetic waves has smaller hyperbolic curvature than for shorter working wavelengths—therefore enabling a flatter wavefront, which is more ideal for achieving high resolution optical imaging or lithography. As a result, the subdiffraction-limited features can be magnified by the hyperlens and then captured by conventional optics to form the image or pattern.
Numerical simulations by the authors showed that the reduction factor of the mask pattern was determined by the radius ratio of the input and output surfaces of the hyperlens. Thus, for a mask pattern having 280 nm period on the input surface, a resulting pattern having 40 nm period was generated at the output surface. Furthermore, the intensity contrast of the resulting image pattern was approximately 0.33 greater than that required for common photoresist patterning.
The authors point out the practical limitations of feature resolution will be dependent on geometrical parameters of the hyperlens design, which in practice must account for materials losses and imperfections due to fabrication processes. As such, phase mismatch and other artifacts will limit the half-pitch resolution for critical designs and are presently estimated to be on the order of 20 nm. Additional study is required as other parameters can be varied in order to reduce the resolvable pattern size, including working wavelength, multilayer material thickness and period. Ultimately this approach has demonstrated utility for generating arbitrary subdiffraction-limited pattern features using diffraction-limited masking.