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Posted: Apr 26, 2006

Thermolithography: Nanomanufacturing under heat

(Nanowerk News) Lithography is a critical enabling technology for manufacturing nanoscale devices and structures. Suppose nanolithography tools cost just a few thousand dollars a piece instead of a few million dollars. These cheap tools would wide open the fields of nanotechnology to practically every university and industry researcher interested in the field. Just like the availability of inexpensive computers and hard disk drives has been the very key to the success of the Internet, the availability of affordable nanomanufacturing capability will be crucial to really tap into innovative minds of scientists and engineers and realize huge potentials offered by various branches of nanotechnology.
Professor Yongho "Sungtaek" Ju's group at the Multiscale Thermosciences Laboratory at UCLA Henry Samueli School of Engineering and Applied Science has been exploring novel alternative micro- and nanomanufacturing techniques that utilize controlled and localized heating. Their ultimate goal is to develop highly economic three-dimensional nanolithography techniques and tools to facilitate scientific investigation and engineering development/optimization of a wide variety of nanoscale devices and structures.
"Thermolithography is very intriguing because we can base it on technologies similar to those found in hard disk drives and DVD drives – well-established consumer products" Ju explains to Nanowerk. "This makes is possible to exploit highly sophisticated and yet economic control and mechanics technologies."
Cross-sectional SEM images of the photoresist patterns obtained using thermolithography. A high dose of UV exposure 1000 mJ/cm2 is used for a c and a lower dose for d . The average heater temperatures are a 100, b 120, and c , d 140°C, respectively. (Reprinted with permission from AIP)
"Another critical aspect is that transport of heat or, to be more precise, thermal energy can be very distinct from the propagation of electromagnetic waves or energetic particles at nanoscales" says Ju. "In highly disordered materials, such as polymers used as resist layers in lithography processes, thermal energy transport does not exhibit interference, scattering, or other factors complicating existing photo or e-beam lithography techniques."
Ju points out that heat diffusion in polymeric materials is a very slow process compared with the propagation of electromagnetic waves. "In one nanosecond, for example, heat diffusion length in typical photoresist layers is of the order of 10 nm. During the same period of time, light travels approximately 300 million times the distance. By exploiting this attribute, we can envision creating three-dimensional features by simply changing the durations of heating pulses."
Ju's group published a recent article, titled "Exploration of thermolithography for micro- and nanomanufacturing", that appeared online on March 20, 2006 in Applied Physics Letters. This and further articles to be published from Ju's group report fundamental studies of heat transport and cross-linking kinetics of polymers, which are essential in developing and optimizing various thermolithography schemes.
"In one particular scheme called image reversal" explains Ju, "we pattern a photoresist layer by locally inducing thermo-photo-chemical crosslinking reactions. We have shown that we can use heating temperature, overall heating duration, and UV exposure dose as three independent process control variables."
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