Classic photo gate APS uses a MOS capacitor that can capture incident illumination with a potential well created under
the photogate. The major drawback of such a technology is the absorption of shorter wavelength by the polysilicon gate
resulting in a higher sensitivity in the red visible spectrum than in the blue range. To reduce this we previously had
experimentally shown that a multifinger photo gate APS designs with 0.72Νm fingers implemented in the 0.18 μm
CMOS technology have a significant increase in sensitivity of 1.7 times the standard photo gate APS. Using advanced
2-dimensional device simulations had shown that the fringing fields form the these fingers would create a potential well
shape that approached that of the standard fully covered photo gate, but with large open areas which would have less
optical absorption. Reducing the gate widths resulted in higher efficiency of photo carriers generated in the larger open
areas while keeping the potential well shape desired. In this work, we use optical simulation package on the 2D device
simulation tools to simulate the multi finger photo gate designs with white light illumination. Sensitivity of the pixel is
calculated as the count of total number of photocarriers that are collected by the potential well for a given exposure
cycle. All the multifinger designs achieved a significant increase in efficiency with respect to the standard photogate
APS design, with the peak sensitivity of 550% by the 7finger design with a gate width of 0.25μm.
A 2-dimensional device simulation of Multi finger active pixel sensors is investigated for obtaining enhanced pixel
sensitivity. Photo gate APS use a MOS capacitor that can capture incident illumination with a potential well created
under the photo gate. The major drawback of such a technology is the absorption of shorter wavelength by the
polysilicon gate resulting in a higher sensitivity in the red visible spectrum than in the blue range. In our previous work
we implemented 0.18μm CMOS standard and multi fingered photo gate design where the enclosed detection area is
divided by 3, 5 and 7 fingers. The experimental results showed that fringing field created potential wells for the 3 and 5
finger photo gate designs have 1.7 times higher collection of photo carriers over the standard photo gate. The device
simulation showed that fringing fields from the edges of the poly gates created potential wells that fully covered the open
silicon areas allowing light conversion without the optical absorption in the poly silicon gates. Extending simulations to
0.5 μm, 0.25 μm and 0.18 μm multifinger poly gates showed that the fringing fields stayed the same width as the gates
shrunk, so that as the number of fingers increased the potential well in the open areas became more uniform. The device
sensitivity based on the potential well locations, and previous experimental results, suggested peak efficiencies for the
0.5 μm design as 7 fingers, 0.25 μm at 9 fingers and 0.18 μm at 11 fingers. Peak efficiency was projected to be 2.2 times
that of a standard photogate.
Photogate APS pixels use a MOS capacitor created potential well to capture photocarriers. However, optical
absorption of the poly-silicon gate reduces photon transmission. We investigate multi-fingered photogates with
openings in the gate to increase photon collection. 0.18 μm CMOS standard and multi-fingered photogates were
implemented where the enclosed detection area is divided by 1, 3 and 5 poly fingers. Preliminary response comparison
with standard photogates suggested the sensitivity of 1-finger pixels dropped ~22% implying open areas collected 62%
of the photocarriers. The sensitivity of 3 and 5 finger pixels increased ~33 - 49% over standard, with open area
collection ~170 - 290% more photocarriers due to fringing field created potential wells. These results indicated at least 66% of the incident light is absorbed by the poly-silicon gate. In spectral response multi-fingered pixels showed an increase in sensitivity in the red (631 nm) - yellow (587 nm) - green (571 nm) wavelengths but a relative decline in the blue (470 nm) possibly due to more absorption in the Silicon Nitride insulator layers. Some Silicon Nitride (SixNy) compositions have higher absorption coefficients in the Blue than poly-silicon and thus may dominate the absorption in these photogates structures. Extended analysis on the potential well formation in the multi-fingered photogates was perform using 2-D device simulation. Simulated multi-fingered photogates showed the strength of the fringing field increased as the open area spacing between poly-fingered decreases; with the 5-finger having a nearly uniform depleted region over the entire photogate area.
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