Manufacturers are always on the hunt for ways to make microchips, which underpin almost all electronic devices, cheaper to produce and more powerful. Now, researchers have discovered new methods for making even smaller microchips.
"Companies have their roadmaps of where they want to be in 10 to 20 years and beyond," says Michael Tsapatsis, professor of chemical and biomolecular engineering at Johns Hopkins University in the US.
"One hurdle has been finding a process for making smaller features, in a production line where you irradiate materials quickly and with absolute precision to make the process economical."
Microchips can be made by layering semiconducting materials like silicon and etching them to create microscopic circuits.
The process, called 'photolithography', involves printing a thin photosensitive layer, or 'resist', on top of a silicon waver and irradiating it with a high-powered laser.
This triggers chemical and physical changes in the resist which burns the patterns and circuity into the wafer. The shorter the wavelength of light used, the smaller the pattern that can be etched.
Currently, cutting-edge microchips are produced using 'extreme ultraviolet lithography' (EUVL) which can produce patterns smaller than 10nm. This mind-boggling nanometre scale is in the same realm as single antibodies (12nm) or transfer RNA molecules (7nm) within cells.
"While EUVL (13.5 nm) has become a leading candidate for sub-10-nm patterning in semiconductor fabrication, next-generation lithography tools are exploring even shorter wavelengths in the [beyond extreme ultraviolet lithography] range of 6.x nm (6.5-6.7 nm) to push resolution limits further," writes Tsapatsis and collaborators in a paper published this week in Nature Chemical Engineering.
There's just one problem. B-EUV doesn't interact strongly enough with traditional resists.
Previously, Tsapatsis' laboratory has shown that metals like zinc can absorb the B-EUV light, producing electrons which trigger chemical reactions that imprint patterns on an organic material called imidazole.
Researchers have created a new method called 'chemical liquid deposition' (CLD) for depositing the 'amorphous zeolitic imidazolate frameworks' (aZIFs) from solution onto silicon wafers.
"This study shows that precise control over the film thickness of high quality, mirror-like aZIF films can be achieved by using continuous CLD spin coating, with growth rates of 1 nm [per second]," the authors write.
The CLD method also allowed the researchers to explore different combinations of metals and imidazoles very quickly to create pairings specifically for B-EUV radiation, which they say will likely be used in manufacturing in the next 10 years.
"Because different wavelengths have different interactions with different elements, a metal that is a loser in one wavelength can be a winner with the other," Tsapatsis explains.
"Zinc is not very good for extreme ultraviolet radiation, but it's one of the best for the B-EUV."
"By playing with the 2 components (metal and imidazole), you can change the efficiency of absorbing the light and the chemistry of the following reactions.
"That opens us up to creating new metal-organic pairings. The exciting thing is there are at least 10 different metals that can be used for this chemistry, and hundreds of organics."
While the research mainly focused on resists for microchip fabrication, the researchers expect their findings will also benefit other patterning and thin film applications, such as sensors and separation membranes.