Fraunhofer fabs optoelectronic LDPD on CMOS image sensors for low light

January 4, 2012 — Fraunhofer Institute for Microelectronic Circuits and Systems IMS researchers developed a new optoelectronic component for low-light CMOS image sensor applications.

Certain CMOS applications require pixels in excess of 10µm, compensating for low light in X-ray or astronomy image capture, among other environments. The sensors could also be used as 3D 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 then recorded by a sensor and used to create a 3D image. Also read: CMOS image sensors see growth beyond cellphones

Pinned photodiodes (PPD) convert the light signals into electrical pulses. PPDs encounter a speed problem when the pixels exceed a certain size, said Werner Brockherde, head of department at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS. The read-out speed of PPD cannot keep up with the image-rate demands of these low-light applications, Brockherde explained.

The Fraunhofer optoelectronic component, a lateral drift field photodetector (LDPD) integrates an internal electric field into the photoactive region of the component, Brockherde said. "The charge carriers generated by the incident light move at high speed to the readout node;" with PPD, electrons simply diffuse to the exit comparatively slowly. The Fraunhofer LDPD accelerates this process "by a factor of up to a hundred."

The Fraunhofer researchers used an improved 0.35µm CMOS chip manufacturing process, making sure the additional LDPD component did not impair the properties of the other components. Simulation calculations ensured proper operation. A prototype of the new high-speed CMOS image sensor is available, and series production should begin in about a year.

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

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