“If cameras had a dark limit that was comparable to our own vision,” explains Dr. Dileepan Joseph, “we could shoot video in lower light without the video being grainy, or we could shoot still images without using a flash or without requiring really long exposures that result in blurred motion.” Anyone who has tried to take pictures in a low-light setting has experienced the difficulty of capturing the images that our eyes can see clearly.
Joseph, an Associate Professor in Electrical and Computer Engineering at the University of Alberta, has been tackling such difficulties since he arrived on campus in 2004 from the University of Oxford, where his work on electronic imaging began. He has been investigating new types of hardware and software that would enable image sensors to match the specifications of the human retina. The goal is to improve camera performance, especially in low light environments, and overcome current limitations.
“If the eye can do it, why can’t we do it with imaging systems?” he asks.
Joseph’s work has adopted a strategy of constructing different layers to improve imaging performance; additional space results in better circuits. “CMC was an early adopter of vertical integration,” he says. “If you put photodetectors above circuitry, it means you can design the circuitry for a finer process. We want to put more transistors in the pixel, but we don’t want to sacrifice the overall size of the pixels. You can achieve that with different processes using 3D integration.”
CMC is also playing a part in a related aspect of Joseph’s research, which investigates flip-chip assembly to improve sensors that respond to light outside the visible spectrum. This fabrication and packaging method results in a CMOS image sensor made up of two dies — one, made of silicon, accommodates circuitry, while the other is transparent and consists of photodetectors. The two structures are ultimately attached with metallic linkages, which are tightly aligned.
“The vertical integration allows one chip to be a photo detector for X-rays, and another chip to be just like what we’re already building,” says Joseph, noting that a lower dark limit will help reduce the amount of light necessary to yield a suitable image for medical purposes. “That could translate into lowering the dose for an X-ray image sensor, where you don’t need as much radiation to get the same image quality.”
Experimental results achieved using 3D assembly techniques have helped the group establish a case for their technology and have been instrumental in helping the lab forge their successes in a field dominated by industry.
Joseph’s research can be applied to a number of practical applications, although medical imaging is the focus. In collaboration with TEC Edmonton, he and his team are designing sensors that could make procedures safer for doctors and patients. The work is supported by an Idea-to-Innovation grant from the Natural Sciences and Engineering Research Council, allowing Joseph to further develop his prototypes and pursue commercialization.