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

INTRODUCTION 7
Chapter 3 gives a short overview and comparison of CCD and CMOS image
sensors. It is pointed out that we have to distinguish between CCD principle and
CCD technology, since the latter is only a dedicated, optimized process for large
area CCD imagers. The CCD principle can also be realized in (slightly modified)
CMOS processes. We have used a 2.0µm CMOS/CCD process for all monolithical
implementations presented in this work. This process makes it possible to realize
CCDs with a charge transfer efficiency (CTE) between 96 % and 99.6 % at 20 MHz
transport frequency, depending on the number of electrons to be transported (96 %:
10,000 e - , 99.6 %: 1’000’000 e - ). Dedicated CCD processes reach typical CTE
values of 99.9998 % and better. In spite of the inferior CTE performance compared
with a CCD process, the availability of the CCD principle offers enormous
capabilities, such as virtually noise-free single electron level signal addition or fast
signal sampling. For these useful properties, which have not been demonstrated
with any transistor-based CMOS circuitry so far, a fair CCD performance is
sufficient. This is of essential importance for an economic product development,
since (1) CMOS processes are available as Multi-Project-Wafers (MPWs) for
affordable prototyping and (2) additional functionality such as A/D conversion or
any kind of signal processing can easily be implemented on chip. The advantages
of both CCD and CMOS will be pointed out. The chapter is completed by an
overview of those characteristics of silicon images sensors in general, that are of
special interest for the underlying TOF-application: spectral and temporal response,
optical fill factor and noise sources.
Since optical TOF ranging uses active illumination, an optical power budget is very
important. In Chapter 4 we show the relations between the optical power of the
light source, the number of electrons generated in a pixel of the imager and the
resulting output voltage swing. This budget is influenced by the power and
wavelength of the light source, the color, reflectivity and distance of the illuminated
and imaged surface, the choice of optics of the camera as well as the quantum
efficiency, integration time and internal electrical amplification of the image sensor.
Also an estimation of the resolution limit is carried out. For this calculation, the
quantum noise is mainly considered as a final theoretical limitation. Together with
the power budget, the range accuracy can then be calculated depending on the
nature of the target and the technical properties of the TOF-camera.