February 19, 2004 – Researchers at UC-Berkeley have created a miniature detector for testing everything from agricultural toxins to DNA, built with familiar IC manufacturing techniques.
The process begins with a laser-processing technique called “laser liftoff,” whereby thin-film components are separated and transferred from one substrate to another to build up the system-on-chip layers. An excimer laser shines pulses of UV light onto a GaN thin-film LED sitting atop a sapphire crystal; the UV light removes the sapphire substrate, leaving behind only the thin-film layer, which is transferred onto a filtering layer of cadmium sulfide to screen out the blue light. The material is then added to a photodetector on a single chip.
“It’s much the same in concept as going from separate transistors to an integrated circuit that includes many transistors on a single chip,” said Timothy Sands, team leader and now professor of engineering at Purdue U.
To create the miniature detector, a fluorescent dye is attached to specific molecules in a substance, and then illuminated by a laser; the dyed molecules absorb the light and emit a color (determined by the laser used — a blue laser results in green light emissions, green lasers generate red emissions) that is picked up by a sensor and analyzed.
The 5-micron-wide device fits on a centimeter-wide chip, and can be produced for less than $1. Scientists also have added a green LED to the device to analyze biological and chemical materials (one LED serves as a control to calibrate measurements, the other for detecting target molecules). Ultimately the researchers want to combine several colors and arrays of LED-filter pairs, creating a “spectrometer-on-a-chip.”
Tests are now underway to gauge the chip’s accuracy against traditional laser-based benchtop instruments in laboratories. Even if the chip’s performance falls short, its size “suggests a future as a portable, handheld device for chemical detection and bioassay applications in remote locations,” according to Sands.