Image Processing Reference
In-Depth Information
OCL
Separation
wall
Color filter
Light pipe
Copper wiring
layers
Photodiode
Si substrate
FIGURE 5.71
Cross-sectional view of advanced FSI. (Reprinted with permission from Watanabe, H., Hirai, J., Katsuno, M.,
Tachikawa, K., Tsuji, S., Kataoka, M., Kawagishi, S. et al.,
Proceedings of the IEEE International Electron Devices
Meeting, Technical Digest
, 8.3, pp. 179-182, Washington, DC, 2011.)
Stack technology might compensate for the disadvantageous cost-effectiveness of BSI and
may achieve high-speed processing.
5.3.3.3.3 Front Side Illuminated Sensor
In a recent advanced front side illuminated sensor (SmartFSI
®
*), it was shown
59
that FSIs
achieve performance comparable to that of BSIs. A cross-sectional view with an actual
aspect is shown in Figure 5.71.
The pixel pitch is 1.4 μm. The essential point is how effectively incident rays pass through
wiring layers and propagate toward the PD and are captured. To this end, the following
techniques are adopted: (1) Lightpipe capability is added to the color filter by forming a
low refractive index separation wall between each pixel color filter. Therefore, incident
light propagates downward. (2) The aperture area is enlarged and the optical stack height
is lowered. This is achieved by shrinking the thickness and width of low-resistance copper
layers. (3) The original lightpipe is made shallow and powered up due to the high refrac-
tive index SiN used on the lightpipe core. (4) A deep PD is formed to effectively capture
longer-wavelength light guided to the PD. By using these techniques, quantum efficiency
(QE) by simulation of green light (wavelength 520 nm) is 73.9%, while those of conven-
tional FSIs with a deep lightpipe and BSIs with a light shield film are 42.9% and 69.3%,
respectively.
Figure 5.72 shows the light energy attenuation along the light propagation direction from
the top of the OCL on the extreme right to the silicon surface on the far left. The coordinate
value of the abscissa axis shows the height from the silicon surface. The arrows indicate
the energy loss through a green filter. The reason they do not start from 100% is that light
reflected at the silicon surface and returned to the OCL is subtracted just above the OCL in
the simulation. The energy loss through the color filter for the SmartFSI is 3%, while that
for the BSI is 14%. The light confinement effect of the separation walls to the color filter
prevents energy attenuation at the color filter boundary. The light waves are effectively
guided and confined through the color filter between the separation walls and through the
lightpipe between the metal apertures and focused on the PD. Peak quantum efficiencies
*
SmartFSI is a trademark of Panasonic Corporation.
