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July 17, 2014

New Photoresist Could Add Years to Moore’s Law

Tiffany Trader

Over the last five decades, microprocessors have gotten cheaper and more powerful as predicted by Gordon Moore’s famous observation, which states that the number of transistors on an integrated circuit doubles every two years. However, the limits of miniaturization can only go so far before crossing the quantum threshold.

Currently there is some progress left to exploit as the industry heads beyond 22-nanometer process technology, but doing so requires new tools. One of these indispensable tools is a chemical called a photoresist, sometimes shortened to “resist,” a light-sensitive liquid plastic used to etch the lines and features on a chip.

Chip designers recognized that in order support ever-shrinking process geometries, a new kind of resist was needed. A collaboration formed between Intel and the US Department of Energy’s Lawrence Berkeley National Lab (Berkeley Lab) to address this challenge.

The research resulted a much-improved resist that combines the properties of two already-existing kinds of resist and retains the best properties of both, i.e., better light sensitivity and mechanical stability.

“We discovered that mixing chemical groups, including cross linkers and a particular type of ester, could improve the resist’s performance,” says Paul Ashby, staff scientist at Berkeley Lab’s Molecular Foundry, a DOE Office of Science user facility. The research is written up in the journal Nanotechnology.

The process of transferring images onto a substrate is known as lithography. In chipmaking, the wafer is first cleaned and then coated with a layer of photoresist. Then ultraviolet light is used to project an image of the desired circuit pattern including components such as wires and transistors. The resist exposed to the light hardens and the non-exposed part is chemically washed away.

The issue with today’s resist is that it isn’t compatible with the new light source, called extreme ultraviolet (EUVL), necessary for smaller process nodes. EUVL has a much shorter wavelength – just 13.5 nanometers – than the current standard, called deep ultraviolet light, which has wavelengths of 248 and 193 nanometers.

“The semiconductor industry wants to go to smaller and smaller features,” explains Ashby, adding that “you also need the resist materials that can pattern to the resolution that extreme ultraviolet can promise.”

The Intel and Berkeley Lab researchers combined two types of resists in various concentrations to create the new material, which is suitable for patterning smaller feature sizes in tandem with EUVL. Next, the researchers plan to further optimize the new resist for even smaller componentry, down to the 10-nanometer node.

The research project was funded by Intel, JSR Micro and the DOE Office of Science, all of whom have a vested interest in keeping Moore’s law alive as long as possible.

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