InVisage Technologies Inc. (Menlo Park, Calif.) is an image sensor startup that is nearly ten years old and that has taken in more than $100 million in venture capital. The company has been developing image sensors based on a quantum-dot material called QuantumFilm that replaces conventional silicon photodiodes but that is integrated on supporting CMOS technology.
EETE Q1: What is QuantumFilm made of and how does it differ from other quantum dot materials?
Jess Lee: "We have not said too much about the material structure of QuantumFilm. There are concerns over cadmium in quantum dots so the first thing to say is there is no cadmium. What we have said is that it is a metal-chalcogenide material, similar to a II-VI material surrounded by ligands in a matrix."
Lee added that it is the combination of the quantum dots and the carrier material that allows charge transfer. "It's because of the packing regime we are in." The dots are of a diameter of between about 3 and 5 nanometers and it is these dimensions that affect the electron band structure and govern the sensitivity to light.
In addition, InVisage has developed an automated process to control the diameter of quantum dots produced as part of film production, Lee said.
Q2: Quantum dots are known for having multiple applications besides sensing, such as displays and energy storage. Is InVisage only interested in image sensor applications?
JL: "Image sensors are our focus for the short- and medium-term future. It is where our expertise is. But semiconductor development is getting harder and More-than-Moore opportunities are increasing, so there is the possibility it [QuantumFilm] can be applied elsewhere in the future.
Q3: InVisage has shown that QuantumFilm has certain advantages over silicon photodiodes in a number of parameters, such as sensistivity, viewing angle, but how does it compare in terms of speed?
JL: Lee answered by saying that for machine vision applications it may well be speed of response, rather than resolution, and other features such as global shutter that determine design wins. Lee pointed out that QuantumFilm is not limited in terms of its speed of response.
"We are limited by the state of silicon I/O and analog-to-digital converters. As that rises we can go at hundreds, even thousands of frames per second. There is no difference between us and comparable CMOS image sensors.
However, Lee said that QuantumFilm has an additional advantage: that it is able to capture scenes very quickly, in periods of the order of a millisecond, and apply a "global shutter" electronically. This is not done easily in CMOS image sensors, which conventionally use a scheme of continuously rolling exposure and line-by-line read out. Although this is adequate for still images it can produce visual artifacts in images of fast moving objects.
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