Improvements in photoresist could help extend Moore's Law

July 16, 2014
In the past few years, the familiar Moore's law has been said to be coming to an end due to limits on optical lithography -- the standard method for patterning silicon chips.  However, improvements to lithographic photoresist may be able to at least postpone the demise of Moore's Law.

In the past few years, the familiar Moore's law has been said to be coming to an end due to limits on optical lithography -- the standard method for patterning silicon chips. However, improvements to lithographic photoresist may be able to at least postpone the demise of Moore's Law.

Manufacturing improvements possible by 2017
Chipmaker Intel (Santa CLara, CA) has partnered with researchers from the U.S. Department of Energy's Lawrence Berkeley National Lab (Berkeley, CA) to design an entirely new kind of photoresist. They have characterized the chemistry of photoresist, crucial to further improving performance in a systematic way. The researchers believe their results could be easily incorporated by companies that make photoresist, and could find their way into manufacturing lines as early as 2017.

The new resist effectively combines the material properties of two pre-existing kinds of photoresist, achieving the characteristics needed to make smaller features for microprocessors, which include better light sensitivity and mechanical stability, says Paul Ashby, staff scientist at Berkeley Lab's Molecular Foundry. "We discovered that mixing chemical groups, including cross linkers and a particular type of ester, could improve the resist's performance." The work is published this week in the journal Nanotechnology.

Finding a new kind of photoresist is "one of the largest challenges facing the semiconductor industry in the materials space," says Patrick Naulleau, director of the Center for X-ray Optics (CXRO) at Berkeley Lab. Moreover, there's been very little understanding of the fundamental science of how photoresist actually works at the chemical level, says Deirdre Olynick, staff scientist at the Molecular Foundry. The lack of fundamental understanding could potentially put Moore's Law in jeopardy, she adds.

The problem with today's photoresists is that they were originally developed for light sources that emit deep-ultraviolet (DUV) light at a wavelength of 248 or 193 nm. To create finer features on chips, the industry intends to switch to extreme-ultraviolet (EUV) light; EUV sources have already found their way into manufacturing pilot lines. Unfortunately, today's photoresist isn't yet ready for high-volume manufacturing.

Combining photoresists
Two types of photoresist were investigated. One is called crosslinking, composed of molecules that form bonds when exposed to UV. This type of photoresist has good mechanical stability and doesn't distort when it is developed. But achieving stability with excessive crosslinking results in long, expensive exposures. The second type of photoresist is highly sensitive, yet doesn't have the same mechanical stability.

When the researchers combined these two types of photoresist in various concentrations, they found they were able to retain the best properties of both; the researchers saw improvements in the smoothness of lines created by the photoresist even as they shrank the width. Through chemical analysis, they were also able to see how various concentrations of additives affected the crosslinking mechanism and resulting stability and sensitivity.

Future work includes further optimizing the photoresist's chemical formula for the extremely small components required for future microprocessors; yhe semiconductor industry is currently locking down its manufacturing processes for chips at the 10 nm node.

The research was funded by the Intel Corporation, JSR Micro, and the DOE Office of Science (Basic Energy Sciences).

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