Optical lithography is an essential tool for fabricating high-density circuits and micron-scale devices. By exposing an organic material, called a photoresist, to intense light and etching away the exposed organic in a solvent, it is possible to print complex patterns that serve as masks or channels for later processing.
When it comes to patterning truly nanoscale devices, however, optical lithography hits the diffraction barrier, which limits the smallest feature light can write to about half the wavelength of the light source—or about –. In a paper appearing in Physical Review Letters, Zeyang Liao and M. Suhail Zubairy at Texas A&M University and Mohammad Al-Amri at King Abdulaziz City for Science and Technology (KACST) in Saudia Arabia propose a new way to pattern a photoresist that skirts this diffraction limit.
Unlike most optical lithography techniques, where one flash of light alters the photoresist directly, Liao et al. propose a two-step process that uses two light sources with different frequencies. The first source is a pair of focused laser pulses, tuned to a resonant frequency of the molecules in the photoresist, that combine to create a standing wave pattern that excites a fraction of the molecules. The second light source does the actual “patterning” by pushing the already excited molecules into a different state that is vulnerable to solvent. The key point is that the first light source controls the excitation of the molecules on a length scale that is less than half the wavelength of the source and it is this excitation that, with careful timing, can be imprinted by the second light source. Assuming a suitable resist can be found, this two-step process results in a much finer pattern than a single source could write. – Jessica Thomas