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

DEMODULATION PIXELS IN CMOS/CCD 109
and the necessary correction of the measured data appear to be impractical.
Summarizing, the 4-tap pixel approach is only partly suited to range measurements,
since, for modulation frequencies higher than 1 MHz, it suffers from transfer
inhomogeneities between the single transfer gates.
5.1.3 1-tap lock-in pixel
Both pixel concepts introduced so far suffer from a low optical fill factor. In-pixel
storage always occupies space, in proportion to the number of sampling points to
be stored. The multitap approach only works up to 1MHz modulation frequency
(realized with 4 storage sites) because the demodulation process happens in the
extended area of a CCD line with design-rule-limited long CCD gates. The four-tap
lock-in pixel accesses the photogate with four different transfer gates, resulting in
an inhomogeneous response of the four sampling points. All of these limitations can
be traced back to the requirement of realizing four independent in-pixel storage
sites. The one-tap lock-in pixel ignores this requirement and offers only one storage
site but attempts to maximize the fill factor. Therefore, the sampling points have to
be stored externally.
Figure 5.6 shows the layout and a cross-sectional view (including the potential
distribution in the semiconductor) of the 1-tap pixel. The left and the right light
sensitive photogates (PGL and PGR) have a length of 6 µm, the middle photogate
a length of 2.5 µm; the pixel width is 21 µm. Each pixel also includes a reset
transistor, a source follower transistor (buffer amplifier) and the select transistor
(cascode with source follower). This leads to a pixel size of 65 µm by 21 µm and an
optical fill factor of more than 20%. The pixel’s working principle is illustrated in
Figure 5.7: By applying proper gate voltages to the photogates, the potential
gradient in the semiconductor can be influenced (c.f. Chapter 3). If the control
voltages of the photogates are changed synchronously with the modulated
incoming light, optically generated charge carriers can either be integrated (if they
belong to the sampling interval) or they are dumped to the dump diffusion (if they
do not belong to the sampling interval). This process can be repeated until the
integration gate (IG) has accumulated a sufficiently large signal. The addition or
accumulation of single electrons under the integration gate is essentially noise free.
This sampling mechanism is shown in Figure 5.7 (a), whereas (b) describes the