Using a femtosecond laser for exposure and hydrofluoric acid (HF) as an etching agent, a team of researchers at the Institute of Physical and Chemical Research (Riken; Wako, Saitama, Japan) have constructed 3-D microreactors known as micro-total-analysis systems (µ-TAS) within photostructurable glass. "µ-TAS enable high-efficiency, high-accuracy, high-performance chemical analysis in the field," says Koji Sugioka, team spokesman. The devices include microfluidic components such as valves, mixers, and pumps to control reagent flow and reaction, and micro-optical components like mirrors, lenses, gratings, waveguides, and micro-optical sensors for in-situ analysis of reactants. µ-TAS devices have reduced reaction and analysis time in human gene and protein analysis, medical inspection, and drug discovery, among others.
Current fabrication of µ-TAS relies on photolithography and semiconductor processing techniques, which are well adapted to surface microfabrication, multilayer, and multistep processes, but forming true 3-D structures requires stacking and bonding. Sugioka's team set out to create 3-D µ-TAS without these extra processes. "We know that ultrashort-pulse laser beams can modify transparent materials, which makes lasers a promising and simple way to embed 3-D microstructures in glass, for example. Also, direct laser writing is very flexible, as it is resistless and maskless, which makes it suitable for rapid prototyping," says Sugioka.
For its transparent material, Sugioka's team chose lithium aluminosilicate glass doped with trace amounts of silver and cerium (Foturan; Schott Glass; Duryea, PA). The process was to first write the structure with the ultrafast laser, then anneal the glass, and then etch it with HF aqueous solution to create the microreactor.
The first step was to ascertain the critical dosethat is, the lowest dose of laser light necessary to modify the photostructure of the glass at the point of exposure. If the photoreaction is induced by the m-photon process, the formula is Dc=F mc N, where Fc is the critical fluence and N is the number of pulses. The group determined the critical dose by using the femtosecond laser to irradiate several wafers of photostructurable glass, then annealing and etching them. The team then inspected the etching contrast ratio between exposed and non-exposed glass, confirming that the contrast ratio with 10% HF solution was 45 times higher for the exposed regions.
The first experiment aimed at fabricating a microreactor consisting of three open microcells measuring 300 µm square, joined by a Y-branched microchannel situated 200 µm below the surface. "Our data showed that the critical dose was 78 mJ/cm2, assuming a Gaussian beam profile of 26 µm diameter," says Sugioka. The group set the scanning velocity at 510 µm/s, which overlapped 50 pulses per position to reach the critical dose.
The resultant channel was 17-µm wide and 71-µm high, with an elliptical cross section produced by the longitudinal distribution of the spatial intensity of the focused beam produced by an objective lens with a 0.46 numerical aperture. "The high aspect ratio of the microchannel is not a disadvantage for µ-TAS," says Sugioka, "but we felt it was important to be able to control the cross-sectional shape for greater flexibility in µ-TAS fabrication. We developed a simple method of controlling the aspect ratio with a narrow slit."
Sugioka's team also fabricated a free-moving microplate inside a glass coupon, which demonstrated how a microvalve can switch fluid-flow direction in a microreactor. The team used the same process to embed a micromirror and a microbeamsplitter for optical analysis of reactants in a microreactor, and used refractive-index modification to embed a waveguide. Sugioka says the technique can construct microturbines and microgears, and he says that selective metallization of the glass with femtosecond lasers and Cu electroplating solution allows electric control of micromechanical components.