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May 13, 2002
Volume 80, Number 19
CENEAR 80 19 p. 9
ISSN 0009-2347


MATERIALS SCIENCE

DOING CHEMISTRY IN THREE DIMENSIONS
Sensitive two-photon dye promises to advance 3-D device fabrication

RON DAGANI

A new organic dye that is much more efficient than earlier compounds in absorbing two photons and generating acid promises to bring three-dimensional microfabrication closer to commercial viability.

The creation of 3-D microstructures typically involves a series of lithographic steps in which the device is built up layer by layer. This approach works well for some device architectures but not for others. For example, microscale cantilevers, shaped like tiny diving boards, are challenging to fabricate using conventional litho- graphic methods, says Seth R. Marder, a professor of chemistry and optical sciences at the University of Arizona. It would be complicated to create a region without material underneath the platform of the cantilever, he tells C&EN.

But with 3-D microfabrication, the task is considerably simplified, he says. One can, for instance, use the focal point of a tightly focused laser beam to "write" or sculpt microscopic 3-D structures inside a polymer slab with submicrometer resolution. The laser beam actually performs chemistry at the focal point--for instance, making the illuminated material more soluble so that it can be rinsed away in a subsequent step.

DOUBLE WHAMMY Dye's conjugated core (black) absorbs two photons and transfers an electron to a sulfonium group (red), which ultimately leads to the formation of acid (H+).

One way to achieve high 3-D spatial resolution is to exploit molecules that efficiently absorb two photons simultaneously--an ability that falls off as the fourth power of the distance from the laser focal point. So only molecules located right at the laser beam's focal point absorb enough light to initiate the desired chemical reactions, Marder explains. This fact makes it possible to perform chemistry in three dimensions with submicrometer resolution.

In recent years, scientists have created materials that can more readily absorb two photons at lower laser intensities, thus avoiding damage to the material. These materials have opened up the possibility of reliable two-photon microfabrication of microfluidic and microoptical devices, including labs-on-a-chip and waveguides, according to Joseph W. Perry, an associate professor of chemistry and optical sciences, also at Arizona.

Now, Marder, Perry, and their colleagues have designed and synthesized a dye that allows two-photon microfabrication to be carried out with perhaps the greatest efficiency, reliability, and speed thus far [Science, 296, 1106 (2002)].

The new dye functions as an efficient "photoacid generator." When the molecule absorbs two photons, it jumps up to the second excited state. The excitation leads to cleavage of an S–CH3 bond, triggering a series of reactions that results in the expulsion of a proton--that is, formation of acid--in high quantum yield (0.5). The acid can then activate a wide variety of reactions that are useful in microfabrication.

The Arizona team, in collaboration with materials scientist Christopher K. Ober's group at Cornell University, has used one such reaction to demonstrate the dye's applicability to 3-D microfabrication. The researchers dispersed the dye in a 50-mm-thick film of a specially designed transparent copolymer. Using a computer-controlled near-infrared laser scanning instrument, they rastered the beam's focal point so as to define a fairly complex microchannel structure within the film. The dye molecules within this structure absorbed two photons and released acid, which cleaved certain nearby bonds in the copolymer, rendering the resin's light-exposed regions soluble in aqueous base. The exposed regions were washed away, leaving behind a complementary microstructure containing buried channels.

This process, in which the exposed resin is solubilized and then removed, is known as "positive-tone" microfabrication. "To our knowledge," Marder says, "no one previously has demonstrated two-photon 3-D microfabrication using positive-tone resists."

Marder and Perry expect to prepare dyes that will perform with even greater sensitivity, allowing microfabrication at higher spatial resolution. They have formed a company--Focal Point Microsystems--to move this technology to market.

HOLLOWED OUT In positive-tone 3-D microfabrication, achieved using a two-photon-absorbing dye, a laser beam is used to "write" the blue-and-red microstructure within a polymer film, and then the entire microstructure is dissolved away. The blue volumes become open, rectangular cavities with a sloped sidewall that are connected by 12 buried channels (here shown as red rods) lying 10 mm below the surface.
ADAPTED FROM SCIENCE



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