Taking Nanolithography Beyond Semiconductors

A chemical patterning process combines molecular self-assembly with traditional lithography to create multifunctional surfaces in precise patterns at the molecular level. It allows scientists to create surfaces with varied chemical functionalities and promises to extend lithography to applications beyond traditional semiconductors.

The technique, which could have a number of practical chemical and biochemical applications, will be described in the Dec. 22 issue of the journal Advanced Materials by a team at Penn State led by Paul S. Weiss, distinguished professor of chemistry and physics, and Mark Horn, associate professor of engineering science and mechanics.

A schematic of the photolithography-assisted chemical patterning technique, using organic-acid molecules (COOH, red) as the first component of the self-assembled monolayer (SAM) and methyl-group-terminated molecules (CH₃, blue) as the second component. After the first SAM is placed, a robust lithographic resist is patterned on top of it. A section of the first component of the SAM is then removed only in the unprotected regions, and the second component of the SAM is deposited in the resulting open areas of the surface. The lithographic resist prevents movement of molecules between the SAM components.

They have used self-assembled monolayers (SAM) -- chemical films that are one molecule thick -- to build a layer on a surface, followed by the addition of a photolithographic resist that protects the covered parts of the film during subsequent processing. The resist acts as a shield during processing, allowing the cleaning and then self-assembly of different chemical functions on the unprotected parts of the surface.

"Other chemical patterning processes on surfaces suffer from cross-reactions and dissolution at their boundaries," said Weiss. "In our process, the resist provides a barrier and prevents interactions between the molecules already on the surface and the chemistry being done elsewhere. The resist is placed on top of the pattern by standard photolithographic techniques. After the resist is placed, molecules are removed from the exposed areas of the surface. Subsequent placement of a different SAM on the exposed surface creates a pattern of different films, with different functionalities."

Because the resist protects everything it covers, the layer under it does not have to be a single functionality. As a result, a series of pattern/protect/remove/repattern cycles can be applied, allowing complex patterns of functional monolayers on the surface of the substrate.

At right (top left), lateral-force microscopy image contrasting COOH-terminated regions of high friction (light) with CH₃-terminated regions (dark)

Top right: Field emission scanning electron microscope image contrasting the COOH-terminated regions (dark) and CH₃-terminated regions (light)

Bottom: 3-D--rendered FESEM image of a surface patterned with two chemical functionalities

"It allows us to work stepwise across a surface, building complex patterns," said Weiss. "We have demonstrated patterns at the micrometer scale and have the potential to go down to nanometer-scale patterns."

While the two processes used by the team -- molecular self-assembly and photolithography -- are individually well-developed, the team's innovation is the successful combination of the techniques to build well-defined surfaces.

Chemical functionalities are distributed across the surface in high-quality layers as a result of the self-assembly process and in high-resolution patterns due to the use of the specialized resists. Different chemical functionalities can be used to detect or to separate a variety of species from a mixture.

"The product of the process can be used to create a multiplexed, patterned, capture surface," said Weiss. "We could expose the entire surface to one mixture and capture different parts of the mixture in each region."

The work was a collaborative effort between the Weiss group, specializing in surface chemistry, and the Horn group, specializing in nanolithography. In addition to Weiss and Horn, the Penn State research team included graduate students Mary E. Anderson (now graduated), Charan Srinivasan and J. Nathan Hohman, and undergraduate researcher Erin Carter.

The work was performed as a part of both the National Science Foundation-supported Center for Nanoscale Science and Penn State's node of the National Nanofabrication Infrastructure Network.

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