FAYETTEVILLE, Ark. University of Arkansas researchers have demonstrated a sensor chip that harnesses semiconductor photolithography equipment to pattern 2-micron-deep wells for identifying chemicals in quantities as small as 1.2 picoliters. That represents sample sizes 10 times smaller than competing chemical sensor chips, according to professor Ingrid Fritsch and her research associate, Walter Vandeveer, who developed the system.
A recent report on the experimental chip describes sandwiching vapor deposition of metal electrode layers with low-temperature polymer insulating layers on a silicon substrate. The five-layer chip includes three application-specific electrodes that are pre-coated to test for specific chemicals in each test well. Two electrodes supply a known voltage, as measured at the third "reference" electrode, and the current flowing corresponds to the presence of the test chemical.
"Electrochemistry uses oxidation reduction to draw electrons off the molecules that can get through the coating on the electrodes. The amount of current tells us how many molecules have oxidized to a reduced form, so if you have one electron per molecule, then you can calculate how many molecules of the substance are in the sample," said Fritsch. The current experimental chip has four test wells, measuring 2, 5, 10 and 50 microns and can test substances in quantities as small as 1.2, 3, 6 and 30 picoliters, respectively. In most tests, however, the researchers submerge the chip into the test sample rather than place individual drops on the chip.
"We need to develop the technology to prevent the rapid evaporation of such small test samples; we can put drops on each well, but we need some sort of lid. So for now we are testing the chips by submerging the end with the wells," said Vandeveer. The chip measures 1.25 by 2.5 cm, yielding 27 chips on every 5-inch wafer. The wafers were fabricated in a semiconductor facility at the University of Arkansas.
Reactive ion etching was used to open up the test wells. The design locates a disk-shaped electrode at the bottom of each etched well along with two tubular-band shaped electrodes ringing each well. All three electrodes are then brought to the edge connector of the chip protruding from the test sample.
"Our design is so much smaller than previous designs, the smallest of which uses 200-micron features, because we used a different approach: We stack the electrodes vertically," said Fritsch. "Microfabrication patterns features about 1 micron in resolution, so we are interspersing layers of gold with layers of polymer insulators that are only angstroms thick.
"By stacking our electrodes vertically, their density can increase further as you decrease the feature size of the chip."
The team's next step, already under way, will characterize the specific coatings and electrode spacing requirements needed to build application-specific chips. The biggest obstacle to the specific coatings required comes from the poorly understood reactions of chemicals in such small volumes. The biggest obstacle to densely packing test wells is crosstalk between adjacent electrodes. "There is an interaction between the electrodes to take account of, especially when one electrode adversely affects other nearby electrodes. They need to be isolated," said Vandeveer. "We also need to characterize the electrochemistry of smaller volumes; the characterization is well-known for large volumes, but we had to build a simulation to predict what the response of small volumes should be."
The National Science Foundation recently started a sensor center at the University of Arkansas where a group of electrical engineers will work with chemical, mechanical and biochemical engineers to build sensor chips. The university's High Density Electronics Center and several corporations have also jumped on the bandwagon to support the sensor development work.
"We were a natural choice for the sensor center," said Fritsch. "We have two fabrication facilities and a very cooperative group of interdisciplinary researchers, including biochemistry experts as well as engineering talent for the electronic and micromechanical fabrication."