High-speed CMOS sensors are used here to control production machinery. |
CMOS
image sensors have long since been the solution of choice for digital
photography. They are much cheaper to produce than existing sensors, and
they are also superior in terms of power consumption and handling.
Consequently, leading manufacturers of cell-phone and digital cameras
fit CMOS chips in their products almost without exception. This not only
reduces the demands made of the battery, it also makes increasingly
smaller cameras possible.
Yet
these optical semiconductor chips are now reaching their limits: while
miniaturization in consumer electronics is leading to increasingly
smaller pixels around 1 µm across, certain applications require larger
pixels in excess of 10 µm. 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, head
of department at the Fraunhofer Institute for Microelectronic Circuits
and Systems IMS. Low-light applications tend to call for high image
rates. “But the readout speed using PPD is too low,” says Brockherde.
The
Fraunhofer researchers have now come up with a solution to this
problem—it is unique and has already been patented. The scientists have
developed a new optoelectronic component, the lateral drift field
photodetector (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. But the sensors are also ideally suited for use as 3-D
sensors based on the time-of-flight process, whereby light sources emit
short pulses that are reflected by the objects. The time-of-flight of
the reflected light is then recorded by a sensor and used to create a
fully-fledged 3-D image. This technology is a compelling proposition for
applications such as crash protection, as the sensors can precisely
record their environment in three dimensions. The Fraunhofer researchers
have already developed this kind of area sensor based on the unique
pixel configuration for TriDiCam GmbH.
