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

DEMODULATION PIXELS IN CMOS/CCD 115
Thermal diffusion drift E-field drift (1 V)
Transport distance Transport time Transport distance Transport time
30 µm 270 ns 30 µm (1 V) 7 ns
15 µµµµm 68 ns 15 µm (1 V) 1.8 ns
10 µm 30 ns 10 µm (1 V) 0.8 ns
5 µm 7.5 ns 5 µm (1 V) 0.2 ns
3⋅10 µm 90 ns 3⋅10 µm (1 V=3·0.33 V) 7 ns
3⋅⋅⋅⋅5 µµµµm 23 ns 3⋅⋅⋅⋅5 µµµµm (1 V=3·0.33 V) 1.8 ns
Table 5.2 Transfer time estimation with pure diffusion and pure E-field model.
(p-substrate, NA=4⋅10 14 cm -3 , µn=1300 cm 2 /Vs, Dn=33.6 cm 2 /s).
The 1-tap structure benefits from stronger fringing fields, since the pixel architecture
allows the realization of shorter CCD gates. Also the transfer does not necessarily
require high charge transfer efficiency, as in the multitap approach. According to
Table 5.2 we can expect transfer times between 2 and 10 ns (depending on the
actual fringing field influence) corresponding to demodulation frequencies up to
100 MHz or even 500 MHz. The subdivision of the photogate into three separate
gates results in a speed improvement of at least a factor of three (considering only
diffusion), more probably a factor of 10 to 20, since with (1) smaller gate lengths
and (2) more gate junctions (three instead of one) the fringing field influence
increases. We demonstrate the 1-tap pixel with demodulation frequencies up to
20 MHz (actual operation frequency of the 3D cameras introduced in Chapter 6).
For higher frequencies it would be even more difficult to realize the highly accurate
control and driving electronics. But another fact also reduces the demodulation
performance at high modulation frequencies, the penetration depth of the
modulated light. So far we have only considered the transfer times of electrons that
are already captured by the electric field of the CCD gates. Generally, however,
depending on the wavelength (c.f. Figure 3.4), the electron hole pairs are
generated (far) away from the semiconductor surface. They will first have to diffuse
to the top, before being captured by the CCD-field. This random diffusion process
takes time and additionally limits the pixel-bandwidth, especially for modulation light
of long wavelength.