Nanowire Arrays Help Make Less Expensive Image Sensors

Hank Hogan, hank@hankhogan.com

Berkeley, Calif. – Cheap and disposable sensors for use almost anywhere could be within reach, thanks to researchers from the University of California, Berkeley, and from Lawrence Berkeley National Laboratory, who combined nanowire photosensors and electronics on a single chip to build an image sensing array.

The large surface area and the exacting composition of nanowires make them many times more sensitive to their environment than bulk material counterparts. However, the small signals generated by nanowire sensors must be amplified with electronics. The best results would come from doing so on-chip, but achieving that intimate combination has proved challenging. The two components may not be made of the same material, making integration difficult. What’s more, successful fabrication requires that the placement and type of nanowire be carefully controlled.

An integrated nanowire sensor could result in cheaper sensing for a variety of applications. This image shows the sensor’s photosensor (NS) and amplifying transistors (T1, T2). Reprinted with permission from PNAS.

The researchers, led by Ali Javey, an assistant professor of electrical engineering and computer sciences at the university and a staff scientist at Lawrence Berkeley, achieved successful integration using a contact printing process. They grew a dense layer of cadmium-selenide nanowires, which are photosensitive, ensuring that they were uniform in composition. They slid the layer across a receiver substrate composed of silicon and 50-nm-thick silicon dioxide. They previously had coated the receiver with photoresist and patterned it. The sliding aligned the nanowires, a constancy of orientation that was vital, Javey noted.

An all-nanowire image sensor array is shown (top), with an artist’s impression of the light sensor circuitry below. Courtesy of Ali Javey, University of California, Berkeley.

Acetone removed the nanowires sitting atop patterned resists leaving the nanowires only in specific areas, where they served as photosensors.

The investigators repeated the process using germanium-silicon nanowires, thereby fabricating the needed electronics. They arranged the circuitry so that it amplified the weak signal from the photodetectors by five orders of magnitude.

With this method, they built a 13 × 20 image-sensing array consisting of independently addressable pixels. Testing showed that about 80 percent of the pixels worked, with only small variations in output among them. Although fairly high for a laboratory procedure, the yield must be improved for manufacturing. “We hope to enhance this yield in the future through process optimization,” Javey said.

The researchers’ results, reported in the Aug. 1 issue of PNAS, are only the beginning. “This work presents a baby step toward development of integrated sensors on virtually any imaginable substrate using a printing technology,” Javey added.