Credit: © 2009 AAAS

It is widely assumed that diffraction places a limit on the smallest features that can be produced by light-based fabrication techniques, such as photolithography. This 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.

A group at MIT (Science doi: 10.1126/science.1167704; 2009) covered the surface they wanted to pattern with a film of photochromic molecules that becomes transparent when exposed to the light source with the shorter wavelength (325 nm) and transparent when exposed to the longer wavelength (633 nm). When exposed to both wavelengths the film is opaque, apart from small regions where the 325-nm light can pass through to produce structures with features as small as 36 nm. This scanning electron micrograph shows trenches with a width of 94 nm.

In the other experiments, a group at the University of Colorado in Boulder (Science doi: 10.1126/science.1167610; 2009) also used a two-wavelength technique to perform subdiffraction photolithography in an approach based on photo-polymerization, while researchers at the University of Maryland (Science doi: 10.1126/science.1168996; 2009) 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 20 less than the wavelength.