Design of a CMOS image sensor pixel with embedded polysilicon nano-grating for near-infrared imaging enhancement

Elie Cobo; Sébastien Massenot; Alexandre Le Roch; Franck Corbière; Vincent Goiffon; Pierre Magnan; Jean-Luc Pelouard

doi:10.1364/AO.444673

Applied Optics
Vol. 61,
Issue 4,
pp. 960-968
(2022)
•https://doi.org/10.1364/AO.444673

Design of a CMOS image sensor pixel with embedded polysilicon nano-grating for near-infrared imaging enhancement

Elie Cobo, Sébastien Massenot, Alexandre Le Roch, Franck Corbière, Vincent Goiffon, Pierre Magnan, and Jean-Luc Pelouard

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Elie Cobo, Sébastien Massenot, Alexandre Le Roch, Franck Corbière, Vincent Goiffon, Pierre Magnan, and Jean-Luc Pelouard, "Design of a CMOS image sensor pixel with embedded polysilicon nano-grating for near-infrared imaging enhancement," Appl. Opt. 61, 960-968 (2022)

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Abstract

Complementary metal–oxide semiconductor (CMOS) image sensor sensitivity in the near-infrared spectrum is limited by the absorption length in silicon. To deal with that limitation, we evaluate the implementation of a polysilicon nano-grating inside a pixel, at the transistor gate level of a 90 nm standard CMOS process, through opto-electrical simulations. The studied pixel structure involves a polysilicon nano-grating, designed with the fabrication layer of the transistor gate, which does not require any modifications in the process flow. The diffraction effect of the nano-grating increases the length of the light path in the photosensitive area and thus increases the photoelectric conversion efficiency. The nano-grating is integrated in combination with deep trench isolations to reduce cross talk between pixels. Coupled optical and electrical simulations report 33% external quantum efficiency improvement and 7% cross talk reduction at 850 nm.

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The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Layer	Material	Thickness	Other
$P^{+ +} s u b$	$S i$	$27 µ m$	Doping: $10^{18} c m^{- 3}$
$P^{-} e p i$	$S i$	$3 µ m$	Doping: $10^{15} c m^{- 3}$
$N - w e l l$	$S i$	800 nm	Doping: $10^{18} c m^{- 3}$
Oxide stack	$S i O_{2}$	$3 µ m$
STI/DTI	$S i O_{2}$	$0.4 / 4 µ m$	Width: $1 / 0.15 µ m$
Passivation	$S i_{3} N_{4}$	200 nm
Filter	Custom	$1.05 µ m$
Micro-lens	$S i O_{2}$	$1.5 µ m$	Radius: $3 µ m$

		Epitaxial Thickness ( $µ m$ )
DTI	FOM	3	4	5	6	7	8	9	10
Reference	EQE CTK	26.6% 20.4%	29% 21%	31% 23.8%	32.7% 26.7%	34.1% 29.2%	35.1% 31%	35.9% 32.4%	36.6% 33.6%
$0 µ m$	EQE CTK	35.5% 26.6%	38.6% 28%	41.3% 30%	43.4% 31.7%	45% 32.9%	46.5% 33.9%	47.5% 34.6%	48.4% 35.3%
$2 µ m$	EQE CTK	35.5% 25.6%	38.6% 27%	41.3% 29.1%	43.5% 30.9%	45.1% 32.2%	46.4% 33.2%	47.5% 34%	48.4% 34.7%
$4 µ m$	EQE CTK	35.5% 19%	38.7% 16.7%	41.4% 21.3%	43.6% 24.5%	45.3% 27%	46.6% 28.9%	47.7% 30.2%	48.5% 31.2%
$6 µ m$	EQE CTK	35.7% 15.8%	38.7% 15%	41.7% 14.1%	44.5% 13.1%	46.2% 21.7%	47.5% 24.7%	48.5% 26.7%	49.3% 27.9%
$8 µ m$	EQE CTK	36% 12.7%	39% 12.5%	41.8% 12.3%	44.5% 12.1%	47.1% 11.8%	49.6% 11.4%	50.1% 22%	50.7% 24.4%

Layer	Material	Thickness	Other
$P^{+ +} s u b$	$S i$	$27 µ m$	Doping: $10^{18} c m^{- 3}$
$P^{-} e p i$	$S i$	$3 µ m$	Doping: $10^{15} c m^{- 3}$
$N - w e l l$	$S i$	800 nm	Doping: $10^{18} c m^{- 3}$
Oxide stack	$S i O_{2}$	$3 µ m$
STI/DTI	$S i O_{2}$	$0.4 / 4 µ m$	Width: $1 / 0.15 µ m$
Passivation	$S i_{3} N_{4}$	200 nm
Filter	Custom	$1.05 µ m$
Micro-lens	$S i O_{2}$	$1.5 µ m$	Radius: $3 µ m$

		Epitaxial Thickness ( $µ m$ )
DTI	FOM	3	4	5	6	7	8	9	10
Reference	EQE CTK	26.6% 20.4%	29% 21%	31% 23.8%	32.7% 26.7%	34.1% 29.2%	35.1% 31%	35.9% 32.4%	36.6% 33.6%
$0 µ m$	EQE CTK	35.5% 26.6%	38.6% 28%	41.3% 30%	43.4% 31.7%	45% 32.9%	46.5% 33.9%	47.5% 34.6%	48.4% 35.3%
$2 µ m$	EQE CTK	35.5% 25.6%	38.6% 27%	41.3% 29.1%	43.5% 30.9%	45.1% 32.2%	46.4% 33.2%	47.5% 34%	48.4% 34.7%
$4 µ m$	EQE CTK	35.5% 19%	38.7% 16.7%	41.4% 21.3%	43.6% 24.5%	45.3% 27%	46.6% 28.9%	47.7% 30.2%	48.5% 31.2%
$6 µ m$	EQE CTK	35.7% 15.8%	38.7% 15%	41.7% 14.1%	44.5% 13.1%	46.2% 21.7%	47.5% 24.7%	48.5% 26.7%	49.3% 27.9%
$8 µ m$	EQE CTK	36% 12.7%	39% 12.5%	41.8% 12.3%	44.5% 12.1%	47.1% 11.8%	49.6% 11.4%	50.1% 22%	50.7% 24.4%

Abstract

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Applied Optics