Researchers at Dartmouth’s Thayer School of Engineering have unveiled a new Quanta Image Sensor (QIS) designed to significantly enhance low-light sensitivity.
Thayer professor Eric Fossum, the engineer and physicist who invented the CMOS image sensor used in a plethora of mobile devices, says QIS is targeted at a number of applications and platforms, including security cameras, astronomy and life science imaging.
Image Credit: Robert Gill (Dartmouth Now)
“Light consists of photons, little bullets of light that activate our neurons and make us see light,” Fossum explained. “The photons go into the semiconductor [the sensor chip] and break the chemical bonds between silicon atoms and, when they break the bond, an electron is released. Almost every photon that comes in makes one electron free inside the silicon crystal. The brighter the light, the more electrons are released.”
As Fossum notes, one of the challenges QIS addresses is counting how many electrons are set free by photons. This is particularly important for very low light applications, such as life science microscopy, photography, quantum cryptography and the burgeoning Internet of Things (IoT).
“When we build an image sensor, we build a chip that is also sensitive to these photons. We were able to build a new kind of pixel with a sensitivity so high we could see one electron above all the background noise.”
Indeed, the new pixels are considerably smaller than regular pixels, as they are designed to sense only one photon, although many more are placed on the sensor to capture the same number of total photons from the image.
“We’d like to have 1 billion pixels on the sensor and we’ll still keep the sensor the same size,” said Thayer PhD candidate Jiaju Ma.
According to Fossum, the new pixels are capable of sensing and counting a single electron for the first time – without resorting to extreme measures, such as cooling the sensor to minus 60 C and/or avalanche multiplication.
“Avalanche multiplication may be thought of as an electrically induced chain reaction, but the strong electric fields necessary lead to reliability issues and it is difficult to make small pixels,” he explained. “We deliberately wanted to invent it in way that is almost completely compatible with today’s CMOS image sensor technology so it’s easy for the industry to adopt.”
The question, says Fossum, was how to design QIS in a current, commercially accessible, not-too-expensive CMOS process.
“You use all the tricks you can think of. Being able to measure one electron is fundamental from a scientific point of view and we were able to do it without a ‘Manhattan Project.’ ”
Fossum also confirmed that the image sensor community seemed receptive to the new technology.
“Engineers in industry are continuously improving the state of the art. They have to worry about the next product or the product after the next product,” he continued. “They don’t have the luxury of thinking like, ‘What are we going to do 10 years from now?’ That’s where I’m happier thinking—in that timescale.”
It should be noted that the QIS project is funded by Rambus, where Jiaju Ma has served as an intern over the past two years.
“A company representative offered some extraordinarily high praise for [Ma], calling him a ‘superstar intern,’” Fossum added. “We hope to continue our collaborations with Rambus in the future.”