Using more than one light source has allowed three groups to overcome the diffraction limit in photolithography
It is widely assumed that diffraction places a limit on the smallest features that can be seen by an optical instrument, such as a microscope or a telescope, and on the smallest structures that can be produced by light-based fabrication techniques, such as photolithography. This diffraction limit is typically about one-half to one-quarter of the wavelength of light involved. Now three independent groups of researchers have shown that it is possible to shatter this diffraction limit by using two light sources for photolithography.
One group at MIT used two different wavelengths — 325 nm and 633 nm — to produce structures with linewidths as narrow as 36 nm (ref. 1). The team covered the surface they wanted to pattern with a film of photochromic molecules that becomes transparent when exposed to the shorter wavelength and transparent when exposed to the longer wavelength. When exposed to both wavelengths the film is opaque, apart from small regions where the 325-nm light can pass through to produce a pattern on the surface below (see figure).
In the other experiments, a group at the University of Colorado in Boulder also used a two-wavelength technique to perform subdiffraction photolithography in an approach based on photo-polymerization2, while researchers at the University of Maryland used two lasers — one continuous and one pulsed — operating at a wavelength of 800 nm to produce feature sizes as small as 40 nm, which is a factor of twenty less than the wavelength3.
References
Andrew, T. L., Tsai, H.-Y. & Menon, R. Confining light to deep subwavelength dimensions to enable optical nanopatterning. Science 10.1126/science.1167704 (2009).
Scott, T. F., Kowalski, B. A., Sullivan, A. C., Bowman, C. N. & McLeod, R. R. Two-color single-photon photoinitiation and photoinhibition for subdiffraction photolithography. Science 10.1126/science.1167610 (2009).
Li, L., Gattass, R. R., Gershgoren, E., Hwang, H. & Fourkas, J. T. Achieving γ/20 resolution by one-color initiation and deactivation of polymerization. Science 10.1126/science.1168996 (2009).
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Rodgers, P. What diffraction limit?. Nature Nanotech (2009). https://doi.org/10.1038/nnano.2009.115
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DOI: https://doi.org/10.1038/nnano.2009.115