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

80 CHAPTER 3
without causing short circuits. Actually, the first CCDs [BOY, AME] were produced
as one poly CCDs. Other successful implementations of CCDs with inter-poly gaps
are reported in [HOP], [FUR] and [KRA]. In most CMOS processes, however, it is
not possible to realize two neighboring gates without highly doped implant between
the gates (see Figure 3.22 a and b). Only a few processes offer special masks
(“non active implant” masks) to prohibit such a diffusion between the MOS gates
(Figure 3.22 d and e). Only if, additionally, the gap between the gates is small
enough (only a hundred to a few hundred nanometers) will the potential barrier
(Figure 3.22 c) disappear and a charge transfer become possible (Figure 3.22 f).
Today such CCDs with inter-poly gaps still require special processes but, with the
predictable future decrease in minimum feature size, sufficiently small inter-poly
gaps will become available and allows the implementation of CCDs with pure
CMOS processes. However, most of today’s CMOS processes only offer silicide
poly-layers for improved conductivity. Silicide gates are opaque and therefore not
suited for the realization of photogates.
The Orbit 2.0µm CMOS/CCD process additionally allows the realization of
overlapping polys over thin oxide, and hence makes possible the realization of
SCCDs. Also, as an option, a buried channel implant is available, which allows
BCCD realization. However, according to our measurements, the Orbit BCCDs are
only slightly better than the SCCDs. The Orbit 2.0µm CMOS/CCD process is
primarily a CMOS process, i.e. optimized for transistor structures rather than for
CCDs.
CCDs require relatively high control voltages to completely deplete the
semiconductor (in the case of SCCDs). Also the voltage amplitude defines the
charge-handling capability and hence the maximum signal amplitude (dynamic) that
the device can handle. Since many CCD gates are connected, the total gate
capacitance is often in the range of some nanofarads. Therefore, not only high
voltages but also high power is necessary to control CCD imagers, a serious
drawback for battery-powered systems. Charging and discharging the CCD gates
can require up to several amperes for some 10 MHz clocking speed.