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

128 CHAPTER 5
right photogate, where a fraction of charge will always travel directly into the
dump diffusion and will never be detected.
(2) Charge carriers are not only collected under the transparent photogates but
also under the adjacent opaque photogates, because (a) the geometry and
position of the optical shield is never 100% precise. And (b), even more
important, depending on its wavelength the light penetrates deep into the
semiconductor before it generates electron hole pairs (c.f. Figure 3.4). Then it
diffuses in all directions, not only perpendicularly to the semiconductor surface.
This leads to the blurring effect, which we have already mentioned above.
Here ideal perpendicular diffusion has been assumed. Nevertheless 50% of the
charge generated under the left photo gate cannot be separated and will be
collected under the integration gate. Such a behavior would correspond to a
demodulation contrast of (5/6-1/6) / (5/6+1/6) = 4/6 = 67%.
This means that, for this simple model, we cannot expect the demodulation contrast
to be better than 67%, even for these idealized DC-conditions. So far, however, we
have neglected two important parameters: (1) the demodulation frequency (the
demodulation contrast will decrease with increasing frequencies) and (2) the
influence of the light’s wavelength.
Influence of the wavelength
As already mentioned, we expect the wavelength of the light source used to have
an influence on the achievable demodulation contrast. This is due to the
wavelength-dependent penetration depth of incoming light, introduced in
Section 3.1.1. Since, generally, electrical fields within a CMOS-processed
semiconductor exist only near the semiconductor surface, substrate-regions more
than about 2 µm - 5 µm away from the surface are nearly field-free. Charge carriers
that are optically generated in these regions move by thermal diffusion in any
random direction. Either they recombine within the substrate or they are collected
by an electrical field near the surface. However, they can travel up to 100 µm
before being captured by an electrical field (Section 3.1.3). Since red-light
photoelectrons are generated deeper in the semiconductor than blue-light
photoelectrons, their tendency to travel before being detected (crosstalk) is much
higher.