Fuji alters pixel structure for CCD breakthrough
By Anthony Cataldo
EE Times
(10/20/99, 5:04 p.m. EDT)
TOKYO — Fuji Photo Film Co. Ltd. has developed a prototype
charged-coupled device that sports an array of honeycomb-shaped
photodiodes, and which the company said is better than conventional digital
camera CCDs by just about any performance measure. The device has the
potential to singlehandedly expand the range of digital imaging products, the
company said.
The new CCD structure, which was developed in collaboration with Fujifilm
Microdevices Co. Ltd., a Fuji subsidiary, is expected to be employed in a new
line of sub-$1,000 consumer digital cameras that the company will bring to
market sometime next year. Fuji said it plans to eventually sell the so-called
Super CCD devices on the merchant market — a first for Fujifilm, said
Yasuo Nagashima, assistant manager of technical service for the electronic
imaging products division at Fuji Photo Film (Tokyo).
The Super CCD chip, with its larger, more densely-packed photodiode
structure, is an improvement over today's CCDs in areas such as resolution,
color reproduction, signal-to-noise, dynamic range, sensitivity and power
consumption, according to the company.
The device used the human eye as a model for how to receive optical
information, company officials said. To mimic the eye, Fuji tilted to a 45°
angle the CCD's tiled pixel structures containing the light-sensitive photodiode
and its attached electrodes. In this way, the vertical and horizontal spacing
between the photodiodes becomes smaller than diagonal intervals. In
conventional CCDs, the opposite is true, the company said.
In addition, Fujifilm created a larger octoganal-shaped photodiode so that
more of the CCD area is used for soaking up incoming photons, further
enhancing horizontal/vertical resolution. A larger photodiode also helps offset
loss of sensitivity for each photodiode when the device undergoes a process
technology shrink, which is done to cram more pixels on a device. For
Fujifilm's 2-million-pixel, 0.5-inch CCD prototype, the pixel pitch is 4.5
microns, Nagashima said. He declined to describe the process technology
used to make the part.
Because the octagonal shape is closer to the circular microlens that sits on
top of the pixel, light can be collected more efficiently, the company said.
And with the new photodiode structure, Fujifilm was able to further maximize
the light-sensitive area of the CCD by eliminating the control signal path, so
that the pixel now only contains one large photodiode and a charge
transmission path around its periphery.
For comparison, Fuji Photo said a 1.9-million pixel CCD based on this
structure will have an equivalent resolution of a 3-million-pixel CCD using
standard rectangular pixels. And Fujifilm claims that the aggregate
photodiode charge in a 0.5-inch Super CCD will continue to increase as the
pixel count goes up, rather than staying constant, as with conventional CCDs.
The company also boasts greater than twofold improvements in
signal-to-noise, dynamic range and sensitivity, though the company did not
provide actual specifications. And because a Super CCD doesn't need as
many pixels as a regular CCD to provide the same resolution, digital camera
OEMs have an opportunity to reduce power consumption.
The company expects Super CCD to expand the range of functions and
products using CCDs. Better sensitivity will enable faster shutter speeds and
longer flash range. The simpler two-poly structure, four-packet photodiode
and wide charge transmission path allows the pixels to be read in one step
rather than the normal two, so no mechanical shutter is needed.
The part's smaller size in comparison to other CCDs with similar pixel counts
opens the door for the design of ultra-compact digital cameras. The CCD
can also be used for full-motion video. "This technology may be applied to a
movie camera, and any conventional CCD might be replaced using this
CCD," Nagashima said.
One challenge Fuji faces is making the device production-worthy, Nagashima
said. "Conventional CCDs use a vertical and horizontal layout, but to make
this work we need to have diagonal lines, so in that sense it might be a bit
more difficult to manufacture. But it has only two poly levels so in this sense
it is simpler," he said.
By next year, the company expects to introduce a digital camera using a
Super CCD with more than 3 million pixels, Nagashima said. And that's just
the start. "Theoretically, we can make CCDs with more than 10 million pixels
based on this technology," he said.
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