Jul 11, 2008
Fullerene resist appeals to green scientists
The rising cost of waste disposal is putting pressure on chip makers to find processing materials that are easy to recycle or friendlier to the environment. Help could be at hand in the form of a water-soluble fullerene resist being developed by scientists at the University of Birmingham, UK.
"Typically, lithography resists are cast from an organic solvent and developed in either an aqueous base or an organic solvent," Alex Robinson of the University of Birmingham's Nanoscale Physics Research Laboratory told nanotechweb.org. "Eliminating these solvents in favour of water reduces the environmental impact of the resist."
The researchers claim that the use of a fullerene-based resist gives a significant improvement in resolution compared with other water soluble materials. To prove the point, the team has used its formulation to transfer feature sizes as small as 30 nm on to silicon wafers.
It turns out that fullerene derivatives are highly resistant to plasma etching – the technique used to transfer the pattern from the resist on to the semiconductor. "This means that it is possible to use thinner resist films for a given etch depth," explained Robinson. "As patterns shrink below 100 nanometre feature sizes, surface tension can cause the patterns to collapse as the developer dries, which limits the achievable aspect ratio of the resist. Thinner films therefore allow better resolution, as long as the etch durability is high enough to transfer the pattern successfully."
So far so good, but if the fullerene formulation is going to win over the chip makers then it needs to match the performance of today's commercial resists. And that means more work for the group.
"Our aqueous resist showed high resolution, but the sensitivity needs to be improved," said Robinson. "To do this we are currently synthesizing materials that can take advantage of a technique known as chemical amplification."
To enhance the affects of the exposing radiation, the team incorporates a photoacid generator into the patterning film. On exposure the photoacid generator releases an acid that reacts catalytically with the resist. As a result, the effect of each incident electron or photon is multiplied and thus a much lower dose of of electrons or light is required.
"The challenge is to introduce this mechanism without losing the resolution of the resist," he added. "Currently, we have demonstrated high-resolution, high-sensitivity chemically amplified fullerene resists that require organic developers. The next step is to modify the resist to allow the use of aqueous developers."
The researchers presented their work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.