High-Speed CMOS Sensors Provide Better Images

A patented lateral drift field photodetector (LDPD) that boosts the speed of traditional CMOS sensors now can produce better-quality images for low-light applications such as spectroscopy, astronomy and x-ray photography.

Certain CMOS applications require pixels in excess of 10 µm, which compensate for low light in x-ray or astronomy image capture. The high-speed sensors could be used as 3-D sensors based on the time-of-flight process, whereby light sources emit short pulses that are reflected by objects. The time of flight of the reflected light is recorded by a sensor and used to create a 3-D image.

Pinned photodiodes (PPD) are used to convert light signals into electrical pulses, but they encounter speed problems when the pixels exceed 10 µm, said Werner Brockherde, department head at Fraunhofer Institute for Microelectronic Circuits and Systems IMS. When pixels exceed a certain size, the readout speed using PPD is too low for low-light applications.

To combat this problem, the Fraunhofer scientists developed an optoelectronic LDPD that integrates an internal electric field into the photoactive region of the component, Brockherde said.

“In this component, the charge carriers generated by the incident light move at high speed to the readout node,” Brockherde explained. “But by integrating an internal electric field into the photoactive region of the component, we have managed to accelerate this process by a factor of up to a hundred.”

The researchers improved upon the currently available CMOS chip manufacturing process based on the 0.35-µm standard, making sure not to impair the properties of the other components by adding the LDPD. Simulation calculations ensured that they met those requirements, and a prototype of the new high-speed CMOS sensor was developed. It is now available, and series production is expected to begin in about a year.

The team has already developed time-of-flight 3-D area sensors based on the unique pixel configuration for TriDiCam GmbH.

For more information, visit: www.fraunhofer.de/en