Sandia’s canals could expand uses for tomorrow’s ‘microfluidic’ devices

Better detectors for air or water pollutants or for chem-bio warfare agents may be possible as a result of recent research at Sandia, research that could lead to an elaborate microfluidic network on a chip that sorts, detects, and identifies individual chemicals or proteins.

The innovations at the Department of Energy’s Sandia National Laboratories involve a new microchip processing technique that creates raised, microscopic canals on chips, through which liquids or gases can flow from one chip feature to another. Such canals are useful for emerging families of minuscule gadgets called ‘microfluidic’ devices that make use of the chemical properties of liquids or gases and the electrical properties of semiconductors on a single chip or among nearby chips. Better detectors for airborne toxins, rapid DNA analyzers for crime-scene investigators, and new pharmaceutical testers for drug development are among the possible future uses for inexpensive microfluidic devices.

The technique’s compatibility with standard semiconductor batch-processing tools should allow future microfluidic devices to be made quickly and cheaply says co-developer Carolyn Matzke of Sandia’s Compound Semiconductor Research Laboratory. Using the patented new technique, the researchers have created raised, hemispherical canals on silicon, glass, and quartz surfaces that are 8 to 100 microns in diameter. They’ve also made canals with tight turns, creating hairpin curvatures with radii as small as 8 microns.

The smallest of the canals are one-tenth of the size of a human hair sliced lengthwise. The canals can be small enough and curvy enough that some liquids or gases pass easily through them and others pass more slowly. This ability to distinguish among fluidic materials is useful for chemical-separation applications, the most common use of microfluidic devices.

To make the Sandia canals, the researchers have moved away from the traditional ‘trench and seal’ method (where intense heat often damaged chip features); instead they pattern a thin layer of photoresist on the wafer’s surface using a conventional photo mask and light, then develop away areas of the photoresist exposed to the light, leaving a network of photoresist ridges on the wafer’s surface that eventually becomes the canals’ interiors. Next they heat the wafer to a relatively low 100°C for about 20 seconds, which causes the square-edged ridges to slump into a hemispherical shape.

A 2-micron-thick film of silicon oxynitride is deposited over the rounded photoresist, and the entire wafer is soaked in an acetone bath until the remaining photoresist is dissolved, leaving hollow tunnels on the wafer’s surface. The technique is 10 to 100 times faster than trench-and-seal techniques and the resulting tunnels are virtually indestructible.

Canal networks also could be used to separate and analyze DNA for crime-scene forensics, or proteins for biomedical research that seeks to understand human responses to toxins or diseases. In the pharmaceutical industry, such devices could help develop better drugs by analyzing responses of hundreds of proteins simultaneously.