Paul van Gerven
14 November

Research at the University of Twente into manufacturing and characterizing free-standing thin films has been awarded a cum laude degree.

Research at the University of Twente (UT) may lead to better EUV pellicles by developing new synthetic techniques and methodologies to characterize the fracture properties of free-standing thin films. Focusing on metal silicides, PhD student Airat Shafikov combined modeling and real-life experiments to unravel synthetic procedures for manufacturing pellicles that are less likely to break. For his work, he’s been awarded a cum laude degree.

Pellicles are membranes that protect lithography masks from contamination. It’s particularly challenging to make them for EUV masks since extreme ultraviolet light is absorbed by just about anything. Less light means wafers need to be exposed longer to blast the same number of photons into the resist, which means lower throughput. That’s the last thing semiconductor manufacturers want, but the thought of a stray speck of dusk ruining their precious EUV mask isn’t exactly appealing either.

EUV pellicle
An EUV pellicle. Credit: ASML

And so, although it initially wasn’t even considered, chipmakers insisted on having at least the option of having an EUV pellicle available. At a time when successful industrialization of EUV technology still hung in the balance, no company stuck out its neck, prompting ASML to develop the mask-protecting technology internally about a decade ago.

By now, EUV pellicles are routinely used in manufacturing. There’s always room to improve their performance, however. One particularly challenging aspect is their fragility. To limit the absorption of light, EUV pellicles need to be extremely thin, typically around 20 nanometers. Such high size-to-thickness ratios make EUV pellicles extremely delicate.

Patent application

This is where Shafikov’s research comes in. He discovered that by controlling the metal-to-silicon ratio and average grain size, the fracture strength can be improved while reducing internal stress. This enables the fabrication of thin films that are less likely to break.

Additionally, the UT researcher developed a technique that makes it possible to directly observe and study the process of crack propagation in the delicate membranes. Thin film test structures were prepared on a silicon chip and mechanically loaded by bending the chip. Performing the whole test on-chip removes the need to handle the microscopic test structures and opens up the possibility to make many structures on one single test wafer. This improves data collection and also offers the possibility to test many variants in one go.

Pellicle fracture
High-speed recording of an on-chip metal silicide pellicle fracturing. Credit: University of Twente

“To illustrate how impactful the work of Airat has been, we note that next to four peer-reviewed papers, his work on pellicles has also led to one patent application in collaboration with ASML,” says Jos Benschop, Senior Vice President Technology at ASML and (by now former) professor of industrial physics at the UT, who supervised the research.

Shafikov’s on-chip test structures have sparked a collaboration with groups in Leuven and Antwerpen, which will use them for understanding the mechanical properties of a much broader family of materials. And his ultra-thin membranes are now also being used in other projects within XUV optics groups, for example in XFEL (Hamburg) and TEM (Saclay) experiments, where they serve as support windows.