Image Processing Reference
In-Depth Information
OCL
Color
filter
M
4
M
3
M
2
M
1
Photodiode
Photodiode
Si
Si
Lightguide
(filled with high refractive index material)
(a)
(b)
FIGURE 5.66
Schematic diagram of a CMOS sensor with a four-metal-layer wiring pixel: (a) without a lightguide; (b) with a
lightguide. (Reprinted with permission from Gambino, J., Leidy, B., Adkisson, J., Jaffe, M., Rassel, R.J., Wynne, J.,
Ellis-Monaghan, J. et al.,
Proceedings of the International Electron Devices Meeting
,
Technical Digest
, pp. 5.5.1-5.5.4,
San Francisco, CA, 2006.)
material,
52
as shown in Figure 5.66b. While OCLs correspond to sharpshooting, as it
were, in lightpipe sensors incident light is confined in high refractive index material and
guided to the PD, similar to a funnel. Using this method, loss of light and cross talk can
be suppressed.
5.3.3.3.2 Backside Illuminated Sensors
Backside illuminated sensors (BSIs) have a structure in which incident light can directly
reach the PD without passing through metal layers by receiving light from the backside
or the substrate side of sensors. Although BSIs were proposed with the first FT-CCD
4
in
1972, processing to fabricate thin silicon wafers is complicated and expensive. This was
adopted only for special applications such as scientific measurements
53
and was used only
for monochrome image sensors. When a BSI-type CMOS sensor
54
was proposed for a con-
sumer SCCC, a big trend
55
was started. Schematic diagrams of cross-sectional views of a
front-side illuminated sensor (FSI) and a BSI are compared in Figure 5.67.
While incident light has to pass through metal wiring layers in the FSI, light can arrive
at the PD directly in the BSI. Therefore, the BSI has some advantage in sensitivity over the
FSI. This is very easy to understand through instinct, that is, advantageous technology
for monochrome sensors. For this reason, not only people who are unfamiliar with image
sensor technology but also those who should be experts on it often misunderstand that the
BSI has an absolute advantage in sensitivity over the FSI without any conditions. This will
be described in detail in this section and Section 5.3.3.3.3.
From the viewpoint of pixel design, flexibility of metal layout is of great benefit in BSIs.
The process flow shown in Figure 5.68 is necessary to implement a BSI structure. (a) At
the first PD, a circuit and wiring are formed, as well as an FSI. (b) Then, a supporting sub-
strate is attached to the surface side. (c) Wafer thinning is followed by grinding and etch-
ing. (d) New backside surface is processed to suppress dark current and a light shield film
is formed. (e) Formation of color filters and OCLs completes the process flow. The steps
