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

SOLID-STATE IMAGE SENSING 55
For light of long wavelength a large number of electron-hole pairs is generated far
from the depletion region in the semiconductor bulk. After their generation, these
charge carriers move by diffusion in random directions. The further away from the
surface they have been generated, the higher is the probability that they either (1)
are not detected at all, an effect lowering the spectral responsivity towards long
wavelengths, or (2) they are detected in neighboring pixels. This effect leads to an
increased crosstalk for long wavelengths.
On the other hand, for very short wavelengths, photons are already absorbed in the
covering thin-film layers (passivation layers for scratch protection and inter-layer
dielectrics, mostly of SiO2 or Si3N4 type) leading to a responsivity decrease towards
blue and UV.
Quantum efficiency
In the framework of this dissertation we have also realized some test diodes in a
0.5µm CMOS technology. The measured quantum efficiency curves for the
classical n+ and n-well diodes (sketched in Figure 3.2) are summarized in
Figure 3.5. Quantum efficiency is the ratio of the number of collected electron-hole
pairs to the number of incoming photons, hence the efficiency with which optical
radiation of a certain wavelength is converted into an electrical signal. In addition to
the decrease in efficiency towards short wavelengths (UV) and long wavelengths
(IR), one can observe interference peaks in the curves, which are caused by
reflections in the thin-film structure on top of the semiconductor [PS2, SP4].