Illustration by Laura Figlewski
Artist’s interpretation of the
RAPID lithography process.
Over the past decade, researchers have shattered
the traditional view of the diffraction limit. Using
new techniques, they have obtained resolution far smaller
than the wavelength of light excitation or emission. Similar
concepts are now being applied to photolithography, making
it possible to create nanoscale features in a photoresist using
visible or near-infrared light.
t is well known that diffraction limits the resolution of far-field optical systems: In text-
books, this limitation is often described in terms of the Rayleigh criterion, which states that
the resolution of a far-field optical system is given by 0.61l/NA, where l is the wavelength
of light and NA is the numerical aperture of the system. Within this type of description, the
practical limit to the resolution of a far-field system is on the order of one-third of the wavelength
of the light used.
In photolithography, this criterion implies that, the smaller the features that one wishes to
create, the shorter the wavelength of radiation that must be used. Thus, in order to manufacture
state-of-the-art integrated circuits, it is necessary to perform photolithography with short-wavelength radiation—i.e., vacuum ultraviolet or soft X-rays. Radiation in this region of the spectrum
can be difficult to produce and manipulate, and thus it must be propagated in a vacuum. These
factors contribute significantly to the high cost of fabrication facilities for integrated circuits.
The Rayleigh criterion applies to far-field imaging or lithography performed with a single beam
of light. However, scientists have also explored schemes that rely on the photo-induced deactivation of photophysical or photochemical processes as a means of circumventing the Rayleigh limit.
For instance, researchers have demonstrated the photo-deactivation of atoms in metastable excited
states as a route to lithographic patterning.
The development of stimulated emission depletion (STED) fluorescence microscopy has led
the scientific community to recognize that far-field imaging need not be limited to resolution on
the order of the Rayleigh limit. In the most common implementation of STED, researchers use
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