One of the problems faced by developers of next-generation lithography tools is the debris that accumulates inside the delicate tools and interferes with wafer throughput. But now a team of researchers at the University of Illinois at Urbana-Champaign has come up with a way of gobbling up these vexing contaminants.
Extreme ultraviolet lithography (EUVL), which bounces 13.5-nanometer light off a mask containing the chip pattern and onto the silicon wafer, is the method of choice to make chips due out between 2015 and 2022. EUVL tools were supposed to be ready a few years ago, but technical problems, including debris, prevented commercialization. So far, toolmakers have focused on shielding the machinery from the debris generated during the process of creating EUV photons.
But light-source debris isn't the only kind of contamination. A lesser-known but equally vexing problem is the much smaller kind of gunk that collects inside the vacuum-sealed machines as a side effect of the lithography process itself. Carbon particles and nanoparticles are always present on masks, but the particles' size, tolerable on today's masks, becomes a problem when the features shrink below 16 nm. Martin J. Neumann, acting director at Urbana-Champaign's Center for Plasma-Material Interactions, graduate student Wayne Lytle, and plasma physicist David Ruzic developed a system to clean things up.
Roughly 30 to 200 nm in size, nanoparticles cling to the surfaces of masks like lint, causing defects, distortions, and other problems in the projected pattern. Lithography tool-maker ASML has invested "years and substantive R&D budget" to prevent particle buildup, using a combination of nested mask pods, automation, and other tricks.
But completely preventing their accumulation is impossible, short of redesigning the entire fabrication process, says Neumann. "Even the cleanest room will have nanoparticles." EUVL tools are vacuum-sealed, but they must still collect masks from external compartments exposed to the clean-room air. Multiple masks are used to expose the same wafer and are changed out using a jukeboxlike robot. Every time the robot handles the masks, taking them in and out of an external chamber, says Neumann, they are "just absolutely covered in debris."
The cleaning system developed by the Illinois researchers is called PACMAN (Plasma Assisted Cleaning by Helium Metastable Atom Neutralization). Like the cheery yellow 1980s game character, PACMAN literally eats the nanopellets.
The process has two steps. For particles on the order of 20 nm, a pulsed electric field in conjunction with a helium plasma confers a static charge on the nanoparticles, which are then repelled from the surface of silicon. But the remaining nanoparticles are too big for such treatment. These are vanquished by the plasma's cargo of metastable helium atoms (which are roughly halfway between ground state and excited ionized state). Because of its peculiar energy state, the helium can break up the particles and free them from the surface.