Image Processing Reference
In-Depth Information
Incident light
1. Reflection at both faces of sealing glass
Loss as light
1
Sealing glass
Package
2. Absorption through sealing glass (ultraviolet)
(a)
2
Sensor
3. Reflection at on-chip lens surface
4. Absorption in filter and related layers
5. Reflection and absorption by passivation film and
interlayer isolation film
6. Reflection at silicon surface
5
6
3
7
Color filter
Light shielding
4
(b)
7. Light collection loss of OCL
PD
8. Recombination in high-density surface hole layer
Si
9. Discharge to
n
-type substrate
Incident light
Potential
Generatedelectron density
Loss as charge
p
+
8
Shorter wavelength
PD
L
n
(c)
(c)
p
-well
Longer wavelength
9
n
-sub
Larger L
(d)
Wavelength
FIGURE 7.1
Loss factor of sensitivity: (a) packaged sensor; (b) sensor chip; (c) around PD; (d) diagram of
p
-well depth depen-
dency of spectral response.
(8) On accessing the silicon, while light absorption starts photoelectric conversion,
generated electrons are prone to annihilation by the recombination with high-density
holes in the
p
layer near the surface, as shown in Figure 7.1c. From the viewpoint of dark
current suppression, a high impurity concentration of the surface p
+
layer is desirable.
Conversely, from the viewpoint of repression of the signal charge extinction by recombi-
nation, it is preferable that the
p
+
layer has low density and is thin. Therefore, a balanced
design is necessary. (9) In the case of a sensor formed in a
p
-well on an
n
-type substrate as
shown in Figure 7.1c, the signal charges generated in the
n
-type substrate are discharged.
As the longer depth L, which is the distance from the surface to the electronic dividing
ridge, can collect more signal charges generated in deeper areas, the sensitivity of longer
wavelengths increases, as mentioned in Section 2.2.2. In the case of sensors formed on a
p
-type substrate, all the generated signal charges are capable of contributing, as shown
in Section 2.2.1.
The ratio of the number of signal charges, which contribute to sensitivity, to the number
of incident photons is called quantum efficiency (QE). Specifically, the ratio of the number
of incident photons to the image area or a pixel is called external QE, while the ratio of the
number of penetrating photons to silicon is called internal QE.
As shown in Figure 7.1, while there are many factors that impact on sensitivity, the
more cost-effective technologies have been adopted. The development of technologies
that make effective use of photons and signal charges, such as OCL proposed in the
early 1980s, AR film, backside illumination (BSI), and advanced front-side illumination
