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

SOLID-STATE IMAGE SENSING 83
3.4 Discussion
Both CCD and CMOS technologies are special MOS technologies. They differ in
optimizations of process parameters with different emphases. In a CCD line the
defect of a single CCD gate (oxide breakthrough) may make a complete column
unusable, whereas for CMOS APS only a single pixel would be unusable for a
comparable defect. CCDs therefore have thicker gate oxides and are characterized
by a certain process softness. In addition to the possibility of realizing overlapping
CCD gates for a barrier-free potential coupling in the semiconductor, today’s CCD
processes offer buried channel implants for enhanced CTE performance. Also the
substrate doping concentrations are usually lower than for CMOS processes,
leading to deeper depletion regions for the same gate voltages. Usually with CCD
processes only one type of transistor (p- or n-type) can be realized. Therefore only
limited electronics can be realized on-chip.
CMOS processes are optimized for fast reliable transistors with both n and p
channel. They are available in prototyping services - so called MPWs (Multi project
wafer). This allows the relatively cheap and uncomplicated realization of test
structures. Unfortunately such MPW services are not available for CCD processes.
Concerning this work, we would like to benefit from both the functionality of the
CCD principle (note: not necessarily CCD process) and the flexibility of CMOS
processes. As we will see in Chapter 5, we do not need CCD structures with a very
good CTE performance. This is because the demodulation principle only requires a
few charge transfers. The advantage of the CCD principle (also if realized in
CMOS) over CMOS circuitry is that it offers noise-free signal addition and defined
local and temporal charge separation, assuming proper CCD structures and timing.
In this way first signal processing steps (temporal sampling or demodulation in our
application) can already be realized in the pixel.