The inventor of the CMOS image sensor is developing an ultra-sensitive sensor capable of detecting single photons.
Eric Fossum is the engineer and physicist who invented the CMOS image sensor used in phones and digital cameras today. He worked with Jiaju Ma, a PhD candidate from Darmouth College's Thayer School of Engineering, to develop pixels for the new Quanta Image Sensor (QIS).
The co-inventors have worked together on the project for more than three years. The QIS could significantly enhance the low-light sensitivity of CMOS chips, allowing imaging capabilities in applications like security, astronomy or life science imaging, where there may only be a few photons.
Light consists of photons. When the photons go into the semiconductor of the sensor chip, they break the chemical bonds between silicon atoms cause the release of electrons. QIS works on the basis that almost every incoming photon frees one electron. The brighter the light, the more photons bombard the sensor, and thus the more electrons are released.
Fossum says one of the challenges in the QIS is to count how many electrons are set free by photons and thus effectively count photons. This is particularly important in very low light applications.
"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," he said.
The new pixels are considerably smaller than regular pixels since they are designed to sense only one photon, but many more are placed on the sensor to capture the same number of total photons from the image. Importantly, these pixels can sense and count a single electron, without having to be cooled to extreme temperatures, or using avalanche multiplication, which introduces strong electric fields and thus causes reliability issues.
The researchers engineered the solution so it is almost completely compatible with today's CMOS image sensor technology, allowing the industry to easily adopt the technology.
Besides developing a cost-effective, commercially accessible CMOS chip with pixels capable of measuring single electrons, the researchers also are working on technology to read out a billion pixels hundreds or thousands of times each second without dissipating too much heat, and also creating images from all the data that is collected.
The QIS project has been funded by the Silicon Valley company Rambus. The researchers hope to continue collaborating with Rambus in the future.