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

184 CHAPTER 7
suggested, by using two optically identical photosites per pixel, both accessed from
two highly symmetrical sides. The “optical identity” can be achieved by a microlens
array that equally distributes and focuses the incoming optical power per pixel to
both photosites. With such a structure it will be possible to integrate all four
sampling points in parallel.
In the near future it will be possible to fabricate CCD structures in sub-micron
standard CMOS processes without any special CCD process option. This is
because, for smaller technologies, the minimum gap between two transistor gates
becomes so small that the potential barrier between these gates can be removed
with relatively low gate voltages of a few volts. The possibility to change from a
modified (non-standard) 2.0µm CMOS/CCD process, as we used for the pixels
presented here, to a standard CMOS process brings several advantages: (1) The
devices will become cheaper, (2) it will be possible for the first time to realize the
complete bandwidth of CCD signal processing combined with additional on-chip
functionality in a single process, and (3), it will be possible to realize smaller
structures. This is an enormous advantage, because the diffusion-limited transfer
time of charge carriers from one CCD gate to another increases with the square of
the CCD gate’s length. With smaller technologies, shorter gates become possible,
enabling a faster CCD transport. In particular, all of the pixels presented here will
be realizable even with improved performance in such a technology. First results of
such inter-poly-gap CCDs are presented in [CAR, FUR, HOP, KRA]. An interesting
but more demanding alternative to the 2D-demodulation device introduced here
would be the use of a sensitivity-modulated array of avalanche photodiodes. Such a
device is introduced in [BIB]; however, as long as the observed problems with too
high dark current and much too large excess noise factor are not solved, this
approach remains impractical.
To summarize, we have shown that TOF is the range imaging technique of the
future. We were able to demonstrate robust, compact range cameras (2D and 3D)
with excellent performance that is only limited in a very predictable manner by
ubiquitous quantum noise of the employed light. The ongoing miniaturization in
microelectronics will lead to faster demodulation devices so that we can expect
devices for demodulation frequencies beyond 100 MHz in the future. Also, it will be
possible to realize additional functionality such as on-chip A/D conversion, distance