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Electrode Improves CCD Sensitivity

Photonics Spectra
Jan 1999
Aaron J. Hand

Eastman Kodak Co. has developed a technology that increases the sensitivity of its charge-coupled device (CCD) image sensors in the blue and green regions of the spectrum. By incorporating a transparent electrode into its existing technology, the company has been able to improve efficiency significantly, reducing exposure times in a range of applications including microscopy, stellar imaging and x-ray diagnostics.

The KAF 1401E is the first of Kodak's Blue Plus CCD image sensors, with increased sensitivity in the blue and green regions of the spectrum.

"The drawback has always been the lack of sensitivity in blue and going into the green," said Bill Des Jardin, product engineer for Kodak. "This really solves that problem."

At 400 nm, the transparent electrode material improves the 2 percent quantum efficiency of Kodak's standard CCD image sensors to 30 percent. At 550 to 700 nm, efficiency improves from 40 to 65 percent. The range of the CCD is also extended from the 400- to 1000-nm range to 300 to 1000 nm.

Exposure times for stellar imaging, for example, could be considerably reduced depending on the part of the spectrum being imaged. In the blue (400 nm), that time could be cut by a factor of 10, Des Jardin said. X-ray systems also could benefit because the CCD images a screen that fluoresces in the green (550 nm) when exposed to x-rays. He estimated that a patient could be exposed to half of the usual x-ray radiation because of the reduced time needed.

The technique uses a standard double polysilicon gate structure, but simply replaces one of the electrodes with a transparent version. "A true two-phase process is very manufacturable, very reliable and allows us to make large arrays," Des Jardin said. "This technique maintains that." This is in contrast to previous attempts at increasing the sensitivity in this portion of the spectrum, such as virtual-phase and open-gate CCDs, which have been more complicated and not as easy to manufacture, he said.

The new technology presents no technical drawbacks, according to Des Jardin, causing no degradation in performance. For the time being, though, it comes at a price.

Single-unit orders will cost about 50 percent more than the less sensitive versions, said Keith Wetzel, sales and marketing manager for Kodak, although larger orders will be more on par with current prices. The company also expects the price to come down as the products gain more acceptance and yield improves, he said.

All of the manufacturer's full-frame sensors will be made available with the transparent electrode. The KAF 1401E is the first image sensor available in the Blue Plus CCD family. All of the products will be pin-for-pin compatible with previous CCD versions, Wetzel noted. The company will wait for customer feedback to decide if it will continue to produce both versions of its CCD image sensors, or move toward just the more sensitive version, he said.

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