PORTLAND, Ore. Imaging the nanoscale interface between exposed and unexposed photoresist for advanced photolithography nodes has been nearly impossible due to their nearly infinitesimal size. Now the National Institute of Standards and Technology (NIST) has invented a technique using neutron beams that enables semiconductor makers to clearly depict these edges.
Photolithography involves coating a wafer with a polymer-based photoresist, then exposing it to 193-nanometer wavelength light through a mask, changing the solubility of the resist, which is then washed away with developer fluid leaving the pattern behind. However, the edges of this pattern are not perfect. At the 65-nm node, just how ragged the edges became was not so important. But as the semiconductor industry advances to 45 nm and beyond, this edge must be made sharper and sharpera variation of no more than 3 nm can be tolerated at the 32-nm node.
Unfortunately, until now it was nearly impossible to image this edge. Now NIST has invented a method using neutron beams to help semiconductor makers see what they are doing. The key to the process was substituting deuterium-based heavy water, enabling nanoscale resolution to be obtained when imaging the interface between the exposed and unexposed photoresist edge.
What they found is that this edge is much more ragged than current semiconductor models predict. During washing with developer, the fluid seeps under the edge for several nanometers, causing it to collapse when dried because of swelling that is much larger than the molecules in the resist. As a result, NIST predicts that current photoresist formulations will not be able to achieve the necessary sharpness for chips at the 32-nm node.
Armed with its new imaging method, NIST is investigating how to modify the resist chemistry to control this swelling so that by the time semiconductor makers reach the 32-nm node, they will have a photoresist formulation that produces sharper edges.
NIST's research was funded by SEMATECH (SEmiconductor MAnufacturing TECHnology), the U.S. nonprofit consortium of semiconductor makers that performs basic research into manufacturing methodologies.