New chip-chemistry process could help extend Moore’s Law

July 17, 2014

When a low concentration of crosslinker is added to resist (left), it is able to pattern smaller features and doesn’t require the longer, expensive exposures needed with a high concentration of crosslinker (right) (credit: Prashant Kulshreshtha, Berkeley Lab)

An Intel-Lawrence Berkeley National Lab (Berkeley Lab) collaboration has found a new way to create smaller features for future generations of microprocessors by modifying the chemistry of photoresists, which are used to generate the patterns on a chip.

The researchers believe their results could be easily incorporated by companies that make resist, and could be incorporated into manufacturing lines as early as 2017.

The new resist mixes chemical groups, including cross linkers and a particular type of ester. In addition to smaller features, it achieves better light sensitivity and better mechanical stability, says Paul Ashby, staff scientist at Berkeley Lab’s Molecular Foundry.

The work is published in the journal Nanotechnology.

There’s been very little understanding of the fundamental science of how resist actually works at the chemical level, says Deirdre Olynick, staff scientist at the Molecular Foundry.

“Resist is a very complex mixture of materials and it took so long to develop the technology that making huge leaps away from what’s already known has been seen as too risky,” she says. But now the lack of fundamental understanding could potentially put Moore’s Law in jeopardy, she adds.

Photoresist (Credit: Intel)

How a resist is used to make a chip

1. A silicon wafer is cleaned and coated with a layer of photoresist.

2. Ultraviolet light is used to project an image of the desired circuit pattern, including components such as wires and transistors, on the wafer, chemically altering the resist.

3. Depending on the type of resist, light either makes it more or less soluble, so when the wafer is immersed in a solvent, the exposed or unexposed areas wash away. The resist protects the material that makes up transistors and wires from being etched away and can allow the material to be selectively deposited.

4. This process of exposure, rinse, and etch or deposit is repeated many times until all the components of a chip have been created.

Extreme ultraviolet

The researchers explain that the problem with today’s resist is that it was originally developed for light sources that emit deep ultraviolet light (with wavelengths of 248 and 193 nanometers). But to gain finer features on chips, the industry intends to switch to a new ultraviolet light source with a shorter wavelength of just 13.5 nanometers.

Called extreme ultraviolet (EUV), this light source has already found its way into manufacturing pilot lines. Unfortunately, today’s photoresist isn’t yet ready for high-volume manufacturing.

To deal with that, the researchers investigated two types of resist processes. One is called crosslinking, composed of molecules that form bonds when exposed to ultraviolet light. This has good mechanical stability and doesn’t distort during development — that is, tall, thin lines made with it don’t collapse.

But if this is achieved with excessive crosslinking, it requires long, expensive exposures. The second kind of resist process is highly sensitive, yet doesn’t have the mechanical stability.

Best of both worlds

When the researchers combined these two types of resist processes in various concentrations, they found they were able to retain the best properties of both. These new materials were tested using the unique EUV patterning capabilities at the Center for X-ray Optics (CXRO) at Berkeley Lab.

The researchers saw improvements in the smoothness of lines created by the photoresist, even as they shrunk the width. Through chemical analysis, they were also able to see how various concentrations of additives affected the cross-linking mechanism and resulting stability and sensitivity.

The researchers say future work includes further optimizing the resist’s chemical formula for the extremely small components required for tomorrow’s microprocessors.

The semiconductor industry is currently locking down its manufacturing processes for chips at the 10-nanometer node. If all goes well, these resist materials could play an important role in the process and help Moore’s Law persist.

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

Abstract of Nanotechnology paper

Here we present a new resist design concept. By adding dilute cross-linkers to a chemically amplified molecular resist, we synergize entropic and enthalpic contributions to dissolution by harnessing both changes to molecular weight and changes in intermolecular bonding to create a system that outperforms resists that emphasize one contribution over the other. We study patterning performance, resist modulus, solubility kinetics and material redistribution as a function of cross-linker concentration. Cross-linking varies from dilute oligomerization to creating a highly networked system. The addition of small amounts of cross-linker improves resist performance by reducing material diffusion and redistribution during development and stiffening the features to avoid pattern collapse. The new dilute cross-linking system achieves the highest resolution of a sensitive molecular glass resist at 20 nm half-pitch and line-edge roughness (LER) of 4.3 nm and can inform new resist design towards patterned feature control at the molecular level.