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Posted: Jul 08, 2010
When top-down meets bottom-up: EUV and X-ray interference lithography for sub-20-nm features
(Nanowerk Spotlight) After achieving the 45-nm process, today's semiconductor industry is nearing the 20-nm process and looking for techniques that would enable sub-22-nm-half-pitch line patterns . Following the continuous increase in exposure tool numerical aperture (NA), researchers are pursuing reductions in exposure wavelengths. This effort had them look at extreme ultraviolet (EUV: 13.4 nm in wavelength) as an exposure light source. Unlike the numerical aperture engineering, change of a light source to EUV demands development of its related components, such as photoresist and optics. Until a reliable solution for EUV lithography is developed, EUV interference lithography (EUVIL) would not solely advance the lithographic technology but would also help to optimize photoresist materials for EUV.
EUVIL typically comprises 4 components: light source; diffraction; interference; and photoresist exposure. First, a coherent beam is generated in the EUV or soft X-ray range. Diffracted by a grating, this beam creates an interference pattern. Information in the form of the interference pattern is recorded on a photoresist-coated substrate. The physics of interference enables one to record a pattern that is as wide as a 20-cm wafer. This technique is, thus, widely used for testing photoresist materials.
EUVIL uses 13.4 nm as a canonical wavelength due to engineering considerations ("Extreme ultraviolet interference lithography at the Paul Scherrer Institut"). EUV can be generated by laser, plasma and synchrotron. It is possible to further decrease the wavelength to between 10 nm (120 eV) and 0.1 nm (12 keV), called soft X ray. Optical requirement of the light source includes spatial coherence and temporal coherence. Spatial coherence, or uniform marching of wavefronts, is deemed more important than temporal (monochromatic) coherence in the interference lithography. A diffraction mask is made of metal (Cr) lines on a transparent thin film (SiNx) by ebeam lithography or EUVIL itself. Photoresist acts as a crucial EUVIL component due to its conflicting requirements of high sensitivity, high resolution and low line-edge roughness. As an EUVIL resist, high-resolution resists used for electron beam lithography, such as PMMA, calixerene and HSQ, are often employed.
Fig. 1 Grating design determines the exposed pattern on the substrate. (a) Two gratings can produce line patterns, and (b) four gratings can pattern a rectangular array of holes and dots. (Image reproduced from ; permission pending)
Recently, researchers of Paul Scherrer Institut (PSI), Switzerland, reported a record-high resolution result of 11-nm half-pitch line/space patterns . PSI is using a beam line from their synchrotron facility (Swiss Light Source, SLS) and a diffraction grating for EUV and X-ray interference lithography. The third generation synchrotron facility of SLS can provide spatially coherent beams of up to 100 eV. In order to gain full spatial coherence, they filter the EUV/X-ray from the synchrotron through a pinhole. The coherent light is diffracted at a grating mask and creates interference patterns that are recorded on a substrate coated with photoresist . It is the grating design that can determine the final pattern of photoresist on the substrate: Fig. 1. With the typical flux of 20-50 mW/cm2, the PSI EUVIL setup can expose at once substrates from a few centimeters to 20 cm with line and dot patterns within 10 seconds.
The clearest advantage of EUVIL lies in the capability of patterning sub-20-nm features at large scales. A maskless, bottom-up patterning method, such as block copolymer lithography, can create similar dimensions as well .
However, 20-cm wafer scale fabrication has not been reported yet. Self-assembly-based lithography should achieve a wafer-level control of surface chemical properties. Besides, unlike hexagonal sub-20-nm features demonstrated by block copolymer lithography, EUVIL can create diverse geometries from line/space to hexagonal to rectangular array of dots. EUVIL is also advantageous in the sense of aspect ratio or feature depth. Solak et al.'s 11-nm line pattern (Fig. 2) is obtained on a 20-nm-thick HSQ photoresist , while state-of-the-art results of the block copolymer lithography can attain 3- to 20-nm features on ca. 5-nm-thick layers. A thicker layer of photoresist polymer may provide more leeway to the etching or deposition process following the lithographic patterning.
Fig. 2 EUVIL exposure in HSQ demonstrates the capability of making 11-nm half-pitch line/space. (Image reproduced from ; permission pending)
There are hurdles that EUVIL, and future XIL, should overcome in order to be a viable option for the next generation lithography. First, all the necessary compartments of EUVIL/XIL have to be developed together. Development of the light source, diffraction scheme and the photoresist materials, in particular, must be coming along at the same time. Second, a lack of interference lithographic capability to expose either localized or complex features such as alignment marks demands the development of a novel pattern-to-pattern alignment scheme. Third, in order to gain industrial acceptability, EUV and soft X-ray light sources need to relieve the coherence requirement. The coherence requirement limits the EUVIL/XIL to mostly synchrotron facilities. To resolve this issue, efforts are currently being made by use of incoherent and partially coherent EUV lithography techniques . Two of the most promising schemes under theoretical consideration include multi-grating interference and re-imaging interference.
With the benefit of large scale lithography of sub-20-nm patterns, EUVIL and XIL are noted as some of the next generation lithography techniques that are under active development. Demonstrated applications of these techniques encompass production of catalyst arrays, nanophotonic devices, nanoimprint stamps, holographic fabrication of Fresnel zone plates, guided self-assembly of block copolymers and colloidal particles, nanoparticle arrays, chemical patterning of self-assembled monolayers and radiation grafting of polymer nanostructures [3, 7]. Once appropriate grating schemes and photoresists are elucidated, X ray interference lithography may further pull down the top-down lithographic technique to the realm of its bottom-up counterpart.
 ITRS - International Technology Roadmap for Semiconductors