Image Processing Reference
In-Depth Information
On-chip micro lens
[Sensitivity]
Color filter
[Colorization]
[Light shield film]
Inner lens
[Sensitivity, Smear]
Aperture
[VCCD transfer electrode]
Antireflection film
[Sensitivity]
Pinned photodiode
[Dark current]
n
-
[Buried-channel VCCD]
p
+
p
p
p
p
n
-
Depleted photodiode
[Lag]
[Electronic shutter]
p
-
Vertical overflow drain
[Blooming]
[Electronic shutter]
n
-substrate
p
-well
[Smear]
[Cross talk]
FIGURE 5.14
Cross-sectional view of IT-CCD representative pixel technology.
of the silicon substrate, VCCD transfer electrodes are formed on contact with the gate-
insulating material above the VCCD channel and covered with metal light-shielding mate-
rial. Looking from above the PD, an on-chip micro lens (OCL),
8
which effectively focuses
incident light toward the aperture area, is formed. If there was no OCL, only incident light
coming directly to the aperture area could generate signal charge, although no other light
could contribute to sensitivity. A color filter that passes only light belonging to a specific
wavelength range to the PD is formed under the OCL. Moreover, an inner lens is arranged
to direct incident light in a perpendicular direction to the PD,
9
and an antireflection (AR)
film is formed on the PD. A mirror-polished silicon wafer shows a silver-like color and
reflects about 30%-40% of visible light. The AR film reduces the reflection at the silicon
surface to increase the intensity of the light arriving at the PD. The thickness of the AR
film is set so that the phases of two kinds of light, reflecting at the AR surface and silicon
surface, have an antiphase relationship to cancel each other's amplitude by the interference
effect, using material such as SiO
2
and Si
3
N
4
.
As mentioned above, an OCL, inner lens, and AR film have been introduced to increase
the signal. It is clear that efforts to increase signal
S
by making much of light showering
onto the image sensor lead to PDs with lower loss. The techniques reducing noise
N
will
be shown next.
5.1.2.1 Vertical Overflow Drain Structure
The capacity of each PD is finite, although signal charges are kept generating by con-
tinuous incident light and integrated in each PD. If a pixel that is irradiated with high-
intensity light generates too much signal charge to be stored in the PD, it overflows to
VCCD channels and neighboring PDs. This phenomenon is called
blooming
and is shown
in Figure 5.15a. It can be seen that VCCD channels are flooded by excess charges and there
is no image information in the area.
