Types of Image Sensors - Essential Principles of Image Sensors - page 66

barrier is adjusted by the voltage of V-sub, blooming is not suppressed in the case of a
V-sub voltage that is too low, that is, a barrier that is too high, while saturation level is very
low in the case of a V-sub voltage that is too high, that is, low dynamic range performance
is caused. Therefore, there is an optimal V-sub voltage. But the conditions depend on the
impurity concentration of the n -substrate, and processing conditions and their variations
to form the p -well and PD. Therefore, the best voltage is different at each image sensor
chip, so adjusted voltage is applied at each chip.
Looking at Figure 5.15b, although the blooming phenomenon is well suppressed, light
white vertical bars are seen above and below the lamp. This is a phenomenon named smear ,
caused by incident light or generated charges mixing directly into the VCCD channel, as
shown in Figure 5.19.
When image sensors receive high-intensity incident light, many charges are generated
around the PDs and smear becomes conspicuous. Since charges generated in the p -type sub-
strate diffuse isotropically because there is no electric field, there is a certain probability of
them flowing into the VCCD channel. Signal charges in the VCCD channel are transferred to
the perpendicular direction of a plane of paper. The quantity of smear charges flowing into
VCCD is determined by the product of [structure factor] and [time factor]. 12 Structure factor
means the ratio of charges flowing into the VCCD channel to total charges generated over a
certain length of time and is determined by the pixel structure and electric field distribution
in the pixel. On the other hand, time factor means the length of time it takes for the VCCDs
to accept the smearing charges determined by the structure factor. Therefore, the smearing
charge quantity is inverse proportion to the VCCD transfer frequency.
Generated charges in deep areas of the p -type substrate can contribute as signal charges
by entering the PD as well as smear charges by flowing into the VCCD channel. As men-
tioned in Figure 2.21, in the case of p -well structure image sensors, charges generated in
deeper regions under the dividing ridge of potential are discharged out of the chip through
the n -type substrate. Since they do not contribute to smearing charges, smear is drastically
suppressed. But smear charges entering the VCCD channel through other paths, such as
the gap between the shielding film and silicon surface, should be suppressed by different
methods, as shown in Figure 5.20.

5.1.2.2 Depleted Photodiode and Transfer Mechanism

In Figure 5.21, the same portion as Figure 5.18b is shown. The situations of signal charges
integrated in the PD just before readout operation, early state just after readout operation

Light

PD

n -

VCCD

n

p

Smear charge

p -sub

FIGURE 5.19
Mechanism of smear phenomenon at cross-sectional view of pixel. Generated charges are flowing in. Light
enters the VCCD channel directly.