For a Cheaper Camera, Use Curved Silicon

Hank Hogan

For all the benefits silicon brings to imaging, it does have a drawback: Silicon sensors are flat. Now a group from Stanford University in California has demonstrated a way to curve monolithic silicon structures that are fabricated in a standard semiconductor process. The technique makes it possible to build curved focal plane arrays.

The result could be a boon for cameras, according to Peter Peumans, assistant professor of electrical engineering. “Since the optics can be a lot simpler for a curved focal plane array, you can save quite a bit of money there. In addition, it allows you to build a much more compact camera.”

Stretchable silicon could lead to curved focal plane arrays as well as to better performing and less expensive small cameras. Researchers fabricated membranes composed of small blocks of silicon connected by silicon ribbons in a process compatible with CMOS manufacturing. The membrane seen here has a layout tailored to increase the fill factor. Courtesy of Peter Peumans, Stanford University.

Earlier work by the group had shown that a suitably curved focal plane array results in better imaging. A case study showed that, for comparably sized flat and curved sensors, the field of view is larger and the image brighter for the curved sensor than for the flat one. Moreover, the curved sensor achieves these results with a single ball lens, whereas the flat sensor requires a larger, more complex and more expensive optimized triplet lens system.

The challenge lay in building the curved array. Silicon bends slightly under pressure but breaks long before reaching the 0.5- to 2.0-cm radius of curvature that is required. On the other hand, sensors made of monolithic silicon perform better than alternatives, and silicon transistors are cheaper than ones made from other materials.

The group overcame silicon’s reluctance to bend by using a divide-and-conquer approach. The researchers used a silicon-on-insulator substrate with a 30-μm-thick layer of silicon on a 5-μm-thick silicon dioxide layer sitting atop a silicon wafer. They used a reactive ion-etching process to form a pattern of silicon islands that were connected to one another by thin ribbons. Finally, they etched away the silicon dioxide layer.

They ended up with squares of silicon connected by springy silicon strands. The investigators transferred this silicon membrane onto a latex one and stretched it with a pushrod. The result was a membrane with a radius of curvature of up to 2 cm.

Although the group did not build functioning devices, Peumans explained why that step should not present too much of a problem. “Since the curving process works on CMOS-processed silicon, we don’t expect any fundamental problems.”

In a curved focal plane array, the individual pixels would be built on the silicon islands, with the interconnecting conductors running on top of the springs. These would be processed with the rest of the circuitry before stretching so that they would be integral to the structure.

>One problem that must be corrected is the fill factor, which ran from 30 percent in the center to 75 percent on the edges in the researchers’ model. That can be fixed with changes in the layout to account for the distortions induced by curving a flat surface.

The scientists are designing imagers with row decoders and pixel readout circuitry. They hope to find an industrial partner to help with sensor fabrication, and they are talking to camera companies. In addition to cameras, the technique could find use in retinal implants that would replace the photosensitive layer of the eye. More broadly, such approaches could be used to produce expandable silicon, with layouts that can be stretched in area by a factor of a thousand.

Applied Physics Letters, March 3, 2008, Vol. 92, 091114.