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Quantum dot-based image sensors: A picture of the future?

Photonics Spectra
Jun 2010
Marie Freebody, marie.freebody@photonics.com

MENLO PARK, Calif. – After operating in complete secrecy for the past three years, InVisage Technologies Inc. has revealed the reason behind its clandestine behavior: the development of a new type of image sensor material that promises to deliver four times better image quality than traditional silicon-based sensors.

Edward “Ted” Sargent, chief technology officer and lead researcher behind the project, said that the company’s QuantumFilm technology will be the first to combine “great imaging” and portability in one system. The Silicon Valley startup plans to provide it to select customers toward the end of the year and expects to see the material adopted into commercial devices by the end of 2011 or early 2012.

“Our aim was to create image sensors that go beyond the inherent limitations of today’s silicon-based sensors and to ultimately deliver cameras that provide dramatically enhanced sensitivity and resolution combined,” Sargent said. “For the last three years, we have kept quiet – not even had a Web site – keeping our heads down to focus on engineering great products. Now that we have succeeded, we are telling the world.”

The new sensor material uses quantum dot technology and a simple method of integration that will enable about 95 percent of an image to be captured. Applications include digital photography and video, with the first products expected to be incorporated into camera phones.


InVisage Technologies’ QuantumFilm image sensor, the first using quantum dots, is expected to bring high-quality images to camera phones.


Today’s digital cameras use CMOS image sensors, with silicon as the light sensor. The problem is that silicon image sensors exhibit light-sensing efficiencies of only about 25 percent; i.e., for every four photons incident on the sensor, only about one is converted into a photoelectron.

The losses result from the fact that silicon is not only a weak absorber of light, but it is also buried beneath layers of metal interconnect wires that serve to obscure half of the pixel area. This is known as the fill factor problem and accounts for an optical loss of ~50 percent.

Such losses mean that it is difficult to take a high-quality image using small devices such as a cell phone camera. To compensate for the lost data, you would need a large piece of silicon.

QuantumFilm, however, is spin-coated on top of the silicon wafer so that no lines obscure the pixel area. It also is engineered from the bottom up to be highly light-absorbing (black). Sargent said that all of these properties combined lead to complete conversion of photons to electrons, 100 percent fill factor and an increase in sensitivity by a factor of four.

“Today there are two classes of cameras, those that take great pictures, like digital SLRs, and those that are portable and that we therefore always have with us, like camera phones,” he said. “Great photography and superb portability urgently need to be merged into a single solution, and this is what InVisage has enabled.”

Because all of InVisage’s light conversion is performed using QuantumFilm and not silicon, Sargent said that image quality is only enhanced as pixels shrink in size to accommodate larger megapixel counts.


On the left: Today’s pixels capture only about 25 percent of light. InVisage Technologies’ QuantumFilm image sensor captures nearly all of the light.


“The design also means that InVisage can continue to take advantage of silicon’s superb electronic properties for all circuit engineering – the crucial analog, analog-to-digital and digital functions our customers expect,” he said.

With total worldwide image sensor sales reaching nearly $6 billion in 2009, some believe that QuantumFilm has the potential to capture a significant share of the market as well as to aid market growth through new applications.

“This is an extremely innovative technology. It overcomes a limit imposed by physics that was thought to be impossible to overcome,” said Morry Marshall, vice president of strategic technologies at semiconductor marketing and research firm Semico Research Corp. of Phoenix.

“QuantumFilm technology allows a smaller image sensor for a given number of pixels, or more resolution for the same image sensor size. This is particularly valuable for small, handheld devices such as smart phones. It could also increase resolution in digital still cameras.”


GLOSSARY
digital photography
A form of photography in which an electronic camera converts an image to an electronic signal that is stored in digital format on magnetic media or film.
fill factor
In solar energy technology, the percent of usable land covered by collectors. The horizontal collector is the only design that exhibits 100 percent fill factor.
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