Nanoimprint lithography could be simplified by making the circuitry-pattern molds using a new fracture-induced structuring process invented at Princeton University by electrical engineer Stephen Chou. As founder of Nanonex Corp. (Monmouth Junction, N.J.)--a nanoimprint-lithography tool maker--Chou expects his latest discovery to eventually lower the cost of making nanoimprint molds.
"Nanoimprint lithography is superb chip-duplication method, but making the first pattern in a mold is very expensive," said Chou. "Now we have found a cheap way to make large-area, fine-pitch grating molds."
Today, the molds for nanoimprint lithography are fabricated using state-of-the-art techniques, such as electron beams and ion beams, or by using dip-pens to "draw" lines with photoresist. These slow methods are limited to patterning small areas at a time, but are deemed worth the trouble, because once a mold is made, it can be used over and over to form fine patterns in many locations.
Chou's fracture-induced patterning method, however, is reportedly not only simpler and faster, but it enables patterning over much larger areas. "We put molten polymer between two plates, and when we pry them apart it fractures into a regular pattern of lines, whose pitch corresponds to the thickness of the polymer," said Chou.
Chou's group has succeeded in creating gratings using the technique that are measured in square centimeters, rather than the square millimeter or less achieved by older methods. Plus, the researchers predict that even larger areas eventually will be possible to pattern with the technique.
"You end up with two complementary sets of grating--one on each plate," said Chou. "So far we have varied the thickness from 13 nanometers to 40 microns thick, resulting in line half-pitches of from 60 nanometers to several hundred microns."
After testing several different polymers with the fracture-induced structuring method, the researchers discovered that the thickness of the polymer film relates to the subsequent line half-pitches, which are approximately four times the thickness of the polymer films sandwiched between the plates.
"The strict relationship between polymer film thickness and grating line pitch is very fortunate, because it is easy to control the thickness of films with very high precision," said Chou.
Next, the researchers plan to create even finer pitch gratings to test the limit of how fine the fracture-induced structuring method can go. Also working on the project is Professor William Russel, dean of the Graduate School for Chemical Engineering at Princeton, and his graduate student Leonard Pease. Chou's graduate students, Paru Deshpande and Ying Wang, also contributed to the work.
Princeton has filed for a patent on the process, which will be licensed to Nanonex for commercial use.