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

IMAGING TOF RANGE CAMERAS 161
n+ diffusion
poly 1
poly 2
cross sectional view of PGR modulation photogates
Pixel #1 #2 #3 #4 #5 #6
modulation input signal
from gate driver
oxide
cross section line for PGR gates
poly1 CCD
gates
substrate
Figure 6.6 Cross sectional view of the right modulation CCD gate PGR for a line
arrangement of 1-tap demodulation pixels. The sheet resistance of
the poly layer, together with the capacitance of each CCD gate,
results in an RC string that reduces the electrical bandwidth of the
pixel. For ease of understanding, the light-shield mask, the 16-stage
CCD line between demodulation region and sense circuitry and the
address decoder are not shown.
With a size of 16 µm by 6 µm, a sheet resistance of 21 Ω/square and a gate
capacitance of 85⋅10 -5 pF/µm 2 , each of the modulation gates has a typical ohmic
resistance of 56 Ω and a capacitance of 82 fF (82⋅10 -15 F). With these parameters,
we can simulate the time response and the amplitude spectrum of the RC string
using SPICE. The simulation results are in Figure 6.7. For this simulation, we
assume the polysilicon strip forming the modulated CCD gates to be connected
only from one side. One can see from the simulation results that, for the one-side
connected modulation gate, the low pass filter effect of the RC string prevents the
operation of the CCD gates beyond 2-5 MHz. The 3 dB cut-off frequency is only
5 MHz for the 108 th CCD gate, the last gate in the RC string.