High-Speed CMOS Sensors Provide Better Images

Photo: High-speed CMOS sensors

Yet these optical semiconductor chips are now reaching their limits: while miniaturisation in consumer electronics is leading to increasingly smaller pixels around 1 micrometer across, certain applications require larger pixels in excess of 10 micrometers. Particularly in areas where only minimal light is available, such as in X-ray photography or in astronomy, having a larger pixel area compensates for the lack of light. Pinned photodiodes (PPD) are used to convert the light signals into electrical pulses. These optoelectric components are crucial for image processing and are built into the CMOS chips. “Yet when the pixels exceed a certain size, the PPDs have a speed problem”, explains Werner Brockherde. Low-light applications tend to call for high image rates. “But the readout speed using PPD is too low”, says Brockherde.

The scientists have developed a new optoelectronic component, the lateral drift field photo detector (LDPD). “In this component, the charge carriers generated by the incident light move at high speed to the readout node,” explains the researcher. With the PPD the electrons simply diffuse to the exit; a comparatively slow process but which is sufficient for many applications. “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.”

To produce the new component, the Fraunhofer researchers improved upon the currently available CMOS chip manufacturing process based on the 0.35 µm standard: “The additional LDPD component must not be allowed to impair the properties of the other components,” says Brockherde. Using simulation calculations the experts managed to meet these requirements – and a prototype of the new high-speed CMOS image sensors is already available. “We expect to get approval for series production next year,” says Brockherde.

The high-speed CMOS sensors are ideal candidates for applications that require large pixels and a high readout speed: astronomy, spectroscopy or state-of-the-art X-ray photography are among the potential applications.

COMPAMED.de; Source: Fraunhofer Gesellschaft