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Quantum Dot Camera Phones

A layer of light-absorbing particles boosts digital image quality.
March 22, 2010

Cell phone cameras are famous for taking grainy, low-resolution pictures. Part of the problem is the lens, which is usually cheaply made and has limited resolution and ability to collect light. But another problem is the light sensor: a silicon chip containing photodetectors. When shrunk to fit in a phone, these chips are limited in the amount of light they can capture.

Quantum bits: This camera sensor chip contains a layer of quantum dots that absorbs light before it reaches the silicon.

Now, InVisage, a Menlo Park, CA-based startup, is demonstrating a way to improve the quality of pictures, at least on the sensor side, without adding size, significant complexity, or cost. At the DEMO conference in Palm Springs, CA, today, the company’s executives announced a new technology called QuantumFilm that lets small camera sensors, like those in cell phones, capture more light than ever before. QuantumFilm is simply a layer of quantum dots–tiny crystals that efficiently absorb light and emit either photons or electrons–in a top layer of the sensor. The electrons emitted by QuantumFilm are collected and sorted the chip’s circuitry.

The result is a sensor that collects twice the light of the standard chip, converts it to electricity twice as efficiently, and is just as cheap to make, says Ted Sargent, chief technology officer of InVisage and professor of electrical and computer engineering and the University of Toronto, where the early research for QuantumFilm began. “Silicon image sensors have a really severe problem in that they just throw away photons left right and center,” says Sargent. Quantum dots, he says, provide a “fundamental solution to the problem.”

In today’s digital cameras, a silicon sensor does double duty. It serves as a photodetector that absorbs incoming light and converts it to an electrical signal. But it also acts as the foundation for the electronics that store the signal from the photodetector and route it away from the chip, where it is processed by separate electronics. The problem is that the photodetector part of the silicon often sits below layers of transistors, metal wires, and a color filter. Because of these obstructions, only about half the original light reaches the photodetector.

There are some commercial technologies that try to tackle the problem of the obstructed photodetector. For instance, manufacturers have added microlenses that focus the light into a tiny space. But even with a bigger photodetector area, silicon still isn’t the best light collector: it registers less than half of the photons that hit it.

Instead of silicon, InVisage uses a layer of quantum dots as a light collector. Quantum dots are a relatively new technology that have only recently found their way into products. QD Vision, a startup that spun out of an MIT lab, is currently using quantum dots to improve the color of LED lighting. Quantum dots can also be used to improve the efficiency of liquid crystal displays.

In the case of QuantumFilm, a liquid layer of quantum dots consisting of lead and sulfide is added to the top of an image sensor, above the electronics and silicon, but below a color filter. Light passes through the color filter and is absorbed by the quantum dots, creating a negatively charged electron, and the other a positively charged “hole.” According to Sargent, the quantum dot layer is about twice as efficient as silicon, registering almost 100 percent of the photons. An electric field below the quantum dot layer separates the electrons from the holes, sweeping the electrons away to the circuitry below, where they are measured as an electrical signal.

The major consideration for adding a new layer or device into a product is to make sure it can be manufactured cheaply, says Seth Coe-Sullivan, chief technology officer at QD Vision. Integrating quantum dots into silicon manufacturing processes is where InVisage “is in uncharted territory,” Coe-Sullivan says.

InVisage is modifying a process already used to make the chips by applying a photoresist to a silicon wafer, to etch features into the silicon. After all the transistors have been made and metal interconnects have been laid down, a small amount of liquid containing quantum dots is spun onto the wafer in the same way as the photoresist. The solution dries and leaves behind a layer of quantum dots about a micron thick.

It’s a difficult task to completely overhaul the silicon sensor used in cell phone cameras because it’s a cost-sensitive device. While quantum dots, when prepared appropriately, have the ability to filter light, manufacturers are likely to keep the original color filters on the sensors, at least in the beginning. However, if QuantumFilm takes off, manufacturers could get rid of the filters completely, says Sargent.

Peter Catrysse, a researcher working on nanophotonic materials at Stanford University agrees that it’s good to start with small changes to the sensor design and not try to change everything at once. “While that might not deliver the greatest promise that quantum dot-based photodetectors have to offer, it may get their technology in the door,” he says.

InVisage, which has attracted more than $30 million in funding since it was founded in 2006, has partnered with Taiwan Semiconductor Manufacturing Company to integrate the quantum dots into the silicon chip-making process. The company expects to sample their camera sensors in 10 months, and the first quantum dot cameras could be on the market by the end of 2011.

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Illustration by Rose Wong

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