Browse Prior Art Database

CMOS imager with final passivation spacers

IP.com Disclosure Number: IPCOM000177914D
Original Publication Date: 2009-Jan-09
Included in the Prior Art Database: 2009-Jan-09
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Abstract

An important parameter for CMOS image sensor performance is quantum efficiency (efficiency of light conversion to an electrical signal), especially at low light levels. The quantum efficiency is degraded by reflections in the dielectric stack above the silicon, for any layers which introduce a discontinuity in the refractive index. In the prior art, the SiN cap layer that is deposited after each Cu layer is removed in the optical path in the array. In the prior art, SiN passivation is used on the top surface of the wafer to protect the devices and wires from moisture and contamination. However, this SiN passivation can degrade the optical performance of CMOS image sensors, because of the high index of refraction (2.1)compared to the lens (1.6) and the SiO2 (1.46). Hence, a method is needed to remove the SiN passivation from the optical path without degrading reliability of the chip. This invention uses an SiN spacer, either before or after the SiO2 passivation, to protect the final Al wiring layer from moisture and contamination. The lower wiring layers are protected by the SiN capping layer and TaN/Ta barrier surrounding the Cu wires. The dielectric coverage over the bottom corner of the wires is especially susceptible to cracking. By forming an SiN spacer, this weak region in the dielectric is sealed off from contaminants, while still allowing maximum light transmission in the optical path. The formation of the spacers requires one additional etch compared to the conventional process, but no extra lithograpy steps.

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CMOS imager with final passivation spacers

CMOS imager with final passivation spacersCMOS imager with final passivation spacers

The conventional process has SiN final passivation. The SiN protects that Al wiring from corrosion, but can cause reflections.

Problem: reflections from SiN

lens

Color filter

Al wire

Al wires

SiN

TV

Al Light shield MZ

Al Bond pad

SiO2

VW

M2

M2

V1

M1

M1

Cu

PC PC

CA

RX

STIN-WellRX

active pixel

Dark pixels

To improve the optical properties of the image sensor, the SiN passivation can be removed with a blanket directional etch, such as RIE. SiN spacers from along the sidewalls of the wiring, protecting the Al from corrosion.

The first option is to form the SiN spacers after the SiO2 passivation.

lens

Color filter

Al wire

Al wires

SiN

TV

Al Light shield

Al Bond pad

SiO2

VW

M2

M2

V1

M1

M1

Cu

PC PC

CA

MZ

RX

STI

N-Well RX

active pixel

Dark pixels

The second option is to form the SiN spacers before the SiO2 passivation.

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Page 2 of 2

lens

Al wire

Color filter

Al wires

SiN

TV

Al Light shield

Al Bond pad

SiO2

VW

M2

M2

V1

M1

M1

Cu

PC PC

CA

MZ

RX

STI

N-Well RX

active pixel

Dark pixels

Both approaches result in improved imager sensitivity,

while protecting the Al wiring.

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