Scientists tout flexible silicon-based “eye”

August 7, 2008 – Researchers at the U. of Illinois/Urbana-Champaign and Northwestern U. say they have created a hemispherical “eye” camera using an array of single-crystalline silicon detectors and electronics in a stretchable, interconnected mesh, an achievement they say points the way toward advanced camera designs — and even the bionic eyes sported by sci-fi robots in Hollywood movies like Terminator.

“Conformally wrapping surfaces with stretchable sheets of optoelectronics provides a practical route for integrating well-developed planar device technologies onto complex curvilinear objects,” said John Rogers, prof. of materials science and engineering at Illinois, a statement. The work was published in the Aug. 7 issue of the journal Nature.

The human eye’s hemispherical detector geometry enables a wide field of view and low aberrations with simple, few-component imaging optics, but this configuration is difficult to replicate in optoelectronics due to the planar nature of fabrication steps (patterning, deposition, etch, materials growth, doping).

Using single-crystal silicon, the scientists’ approach forms optoelectronics in 2D compressible configurations and elastomeric transfer elements that transform the planar fabrication layouts into hemispherical geometries — i.e. they’re flexible. “This approach allows us to put electronics in places where we couldn’t before,” Rogers said. “We can now, for the first time, move device design beyond the flatland constraints of conventional wafer-based systems.”

Schematic illustration of steps for using compressible silicon focal plane arrays and hemispherical, elastomeric transfer elements to fabricate electronic eye cameras. (Source: U. of Illinois/Urbana-Champaign)

To make the camera (an integrated system like the human eye, with a hemispherical detector and cap and imaging lens), a thin rubber membrane is molded into a hemisphere shape and stretched to forma flat drumhead. A prefabricated focal plane array and other electronics (created by conventional planar fabrication methods) are transferred from a silicon wafer to the tensioned drumhead membrane (the silicon itself is not strained by the deformations, the scientists point out). Once the tension is released the membrane retracts to its original shape, compressing the focal plane array and causing specially designed interconnects to delaminate from the rubber, forming arcs pinned on the ends by detector pixels. The array package is then transfer-printed to a matching hemispherical glass substrate; assembled with a lens and connected to external electronics. The final system has the size and shape of a human eye.

Simulations and imaging studies suggests the hemispherical detector arrays offer a broader field of view, more uniform illumination, and fewer aberrations than flat cameras with similar lenses, Rogers said. Target application is for retinal implants as a better alternative than flat detectors. “The ability to wrap high-quality silicon devices onto complex surfaces and biological tissues adds very interesting and powerful capabilities to electronic and optoelectronic device design, with many new application possibilities,” he said.

Photograph of the electronic eye camera after integration with a transparent hemispherical cap and a simple, single component imaging lens. (Source: U. of Illinois/Urbana-Champaign)


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