3D Time-of-flight distance measurement with custom - Universität ...

SOLID-STATE IMAGE SENSING 51
out by sequentially connecting the low impedance pixel-amplifier to the output,
rather than the high-impedance nodes of the photodiodes. The APS principle,
sketched in Figure 3.1 d, offers low conversion capacitance and hence good
sensitivity and low noise performance, gradually approaching CCD performance.
Another advantage is the possibility to read out and reset single pixels by selecting
them with the proper address at the row- and column- address decoder. This
makes it possible to define so-called regions of interest (ROI) and to increase the
image frame rate locally. Also, in general each pixel can be operated with an
individual integration time. However, compared to CCDs, CMOS APS imagers often
have a poorer image quality. This is because they suffer from an effect known as
fixed pattern noise (FPN): due to variations in oxide thickness, size of gate area
and doping concentrations over the sensor, each pixel has a different offset (up to
some 10mV differences) and a slightly different gain, (which can usually be
neglected). Therefore, external FPN correction is required; an effort, which is not
necessary if using CCDs. FPN correction subtracts a locally stored dark image from
the actually acquired image, thus eliminating the fixed pattern noise. Recently,
approaches such as sensor-integrated correlated double sampling (CDS) or the
active column principle have been demonstrated, offering ways to overcome the
FPN problem [HOP, DEC, VOG].
The basic difference between CCD (charge domain signal processing) and CMOS
APS (in general voltage domain signal processing) is illustrated by the hydrostatic
equivalents in Figure 3.1 a+b. CCD imagers locally collect charge carriers to
transport them physically to the output stage, where they are converted into a
voltage. In contrast, CMOS APS sensors generate the signal voltage already in the
pixel. The information can then be directly connected to the output.
Both CCD and CMOS APS are realized in MOS-technologies (Metal Oxide
Semiconductor), mostly in silicon. The special processes, however, are optimized
towards different goals. So it is usually not possible to realize good CCDs with a
CMOS process or to realize efficient CMOS circuitry in a CCD technology. An
overview of the most important and relevant electrical and optical properties of
silicon for image sensing is given in Section 3.1. The following sections then focus
on specific principles and properties of CCD (Section 3.2) and CMOS APS sensors
(Section 3.3). More detailed information can be found in the references [TEU, BEY,
SEQ, SZ1, SZ2, PS1, PS2, FOS].