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

106 CHAPTER 5
a lot of space and diminishes the pixel’s fill factor to only 2.2%. In spite of the lower
conversion gain the 2-side readout version is the preferable realization of both 4-tap
pixels, because it offers a three times higher fill factor (6.6%) and has a reduced
demand on a highly efficient CCD charge transport. This is because the
photoelectrons only have to pass three CCD gates from their generation site, the
photogate, to the detection diffusion. The influences of the unconventionally shaped
CCD gates of both realization versions have to be investigated thoroughly.
The array-friendlier aspect ratio of 1:2 (1:1 respectively for the wheel-shaped
version) and the short transport paths of the photoelectrons in the demodulation
process are the major advantages of this 4-tap device compared to the multitap-
CCD. However, it is not possible to realize more than four storage sites with this
pixel architecture without drastically enlarging the photogate. The photogate, on the
other hand, should be as small as possible for a fast and efficient charge
separation. This appeared to be a problem in the first implementation of this
technique [SP1], where, due to an over-dimensioned size of the photogate
compared to the relatively narrow transfer gates, an inhomogeneous sensitivity of
the single sampling points could be measured. By the modified, highly symmetrical
pixel layouts shown in Figure 5.3 this problem of inhomogeneities between the
sampling points was partly overcome, at least for low sampling frequencies.