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Posted: Oct 11, 2011

Lithography using neon ions achieves efficient exposure and high resolution

(Nanowerk Spotlight) Cost of ownership has become a critical challenge facing future research in nanofabrication. As potential applications have broadened beyond the high-volume manufacture of integrated circuits, demand has increased for a robust tool capable of lithography at high pattern density and fidelity but also at low cost and thus suitable for scientific research, rapid prototyping, and low-volume manufacturing. Unfortunately, current manufacturing technologies employed in the chip industry are anything but 'low cost'.
A persistent goal in the information-technology hardware industry has been to increase the patterning density achievable by resist-based lithography while maintaining acceptable fidelity and throughput. To build microprocessors with more than one billion transistors, manufacturers still use the same technique – photolithography, the high-tech, nanoscale version of printing technology – that they have been using for the past 50 years. Since the wavelength of visible light limits the size of the transistors that photolithography can produce, it appears that this technology has reached its limit at 32 nm resolution. In order to keep shrinking chip features further, chip manufacturers are looking into other methods; one of them is electron-beam lithography.
Unfortunately, particle-beam-based lithography exhibits a trade-off between fidelity and throughput when scaling resolution to the 10 nm scale. Specifically, a particle source of given brightness can only focus a beam of so much current to a given spot size, thus limiting throughput.
Furthermore, statistical fluctuations of the particle current and placement of particles – shot noise – can result in unacceptable variation of written feature dimensions. The consequence of shot noise is that many more particles than might be theoretically necessary are nevertheless practically necessary to achieve acceptable patterning fidelity at high resolution.
Researchers at MIT, in collaboration with Carl Zeiss NTS, have now found that it is possible to write structures at a resolution and fidelity that normally requires a large number of incident particles per pixel – that is, high fluence – but that in this case was done with extraordinary efficiency of fluence.
"Our work advances the field from a scientific perspective because it calls into question common intuition about shot noise and how it affects achievable resolution and fidelity in lithography," Donald Winston, a PhD student in Karl Berggren's Quantum Nanostructures and Nanofabrication Group at MIT, tells Nanowerk. "From a technological perspective, our work introduces a lithographic technique that, with significant engineering effort, could be a future player in low-volume manufacturing."
Reporting their findings in a recent edition of Nano Letters ("Neon Ion Beam Lithography (NIBL)") this work, first-authored by Winston, demonstrates a new source for lithography that has both higher per-particle exposure efficiency and a higher brightness than the sources conventionally used for lithography at the 10 nm scale.
Scanning electron micrographs of developed gratings written into 16 nm thick hydrogen silsesquioxane on silicon
Scanning electron micrographs of developed gratings written into 16 nm thick hydrogen silsesquioxane on silicon using 20 keV Ne+. Left: The 19 nm pitch gratings written using a linear dose density of 7 ions/nm. Right: The 14 nm pitch gratings written using a linear dose density of 5 ions/nm. These images exhibit resolution insufficient to show residue between developed features, but adequate to emphasize the dose contrast achieved. (Reprinted with permission from American Chemical Society)
"Thus, although we have not 'solved' the fundamental trade-off between fidelity and throughput for arbitrary pattern generation at the 10 nm scale, we have demonstrated a system that, with significant engineering effort, could surpass conventional systems for this task," says Winston.
The high efficiency of ion beam writing has been known for decades. The team's contribution was to demonstrate such efficiency at a resolution near the state of the art for lithography.
The researchers note that they were motivated to conduct this work by the recent development of a bright source of neon ions. Commercially available gas field ionization source (GFIS) of helium ions for lithography has previously been investigated. However, this work did not demonstrate the resolution of helium-ion-beam lithography to be superior to electron-beam lithography (EBL). More recently though, an experimental GFIS system has been modified for operation with neon ("Gas field ion source and liquid metal ion source charged particle material interaction study for semiconductor nanomachining applications").
"Brightness is a fundamental property of a particle-beam source that constrains resolution given a requirement on fidelity, so a lithographer's ears perk up when hearing about a new source that offers higher brightness than conventional sources," says Winston. "Neon is of higher mass than helium and thus should lead to a smaller resist interaction volume for lithography. In addition, the higher-mass neon ion has a larger stopping power for a given landing energy, which should lead to higher efficiency in resist exposure. This system has been evaluated for nanomachining, but not for resist-based lithography."
In their experiments, the team demonstrated neon ion beam lithography with resolution comparable to state-of-the-art electron-beam lithography (7 nm half-pitch and line width) and exposure efficiency ∼1000 greater than EBL at comparable landing energies.
With further development and refinement, this work may be broadly applicable to the prototyping of nanoscale devices and the manufacture of masks and templates for higher-volume production.
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