Optical Sensor Wafer Bonding With MEMS and IR Cut-Off Filtering

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Solution Overview

Problem

Conventional methods for integrating biometric sensors in electronic devices face challenges due to high stress and thermal effects, leading to performance degradation in optical sensors, particularly in fingerprint recognition systems, where infrared cut-off filters and silicon microelectromechanical systems (MEMS) structures cause wafer bonding issues and affect the optical properties.

Innovation Solution

A novel sensing apparatus is developed with a silicon MEMS structure bonded onto a complementary metal oxide semiconductor (CMOS) image sensor (CIS) wafer, featuring a passive oxide layer, a buried oxide layer, and an infrared cut-off filter (IRCF) with a pattern, along with metal pads outside the IRCF pattern, to mitigate stress and thermal effects, and improve light sensitivity by filtering out infrared light.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If an infrared cut-off filter (IRCF) is deposited directly on a CMOS image sensor (CIS) wafer before wafer bonding, then the optical filtering function is achieved, but the high stress in the IRCF induces a high bow surface that makes smooth wafer bonding more difficult

Engineering Contradiction:

Improveinfrared light interferenceVSAvoidwafer bonding smoothness

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

The patent divides the wafer structure into multiple separate layers: the IRCF is deposited on a separate wafer rather than directly on the CIS wafer. This segmentation allows the IRCF to be formed with proper stress management, and the separate wafer can then be bonded to the CIS wafer through a controlled fusion bonding process, eliminating the bowing issue while maintaining the optical filtering function.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces an intermediate wafer structure that carries the IRCF. This intermediate wafer acts as a mediator between the IRCF deposition process and the final CIS assembly, allowing the filter to be formed with appropriate stress characteristics and then bonded to the CIS wafer through a controlled process that ensures smooth bonding surfaces.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of manufacture

If deep silicon etching is performed to form silicon MEMS structure after IRCF deposition, then the MEMS structure is created, but the processes after IRCF deposition impact optical property function by extra thermal and stress effect, inducing charging and stress degradation in CIS performance

Engineering Contradiction:

ImproveMEMS structure formationVSAvoidCIS performance stability

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The patent performs the deep silicon etching to form the MEMS structure on a separate wafer before the fusion bonding step. This preliminary action allows the MEMS structure to be created without exposing the CIS wafer to the thermal and stress effects of the etching process, thereby preserving the optical properties and performance stability of the CIS while still enabling MEMS functionality in the final assembled device.

Inventive Principle:

Principle #10Preliminary action

3Object-affected harmful factors

If the IRCF is deposited with high stress to achieve optical filtering, then the infrared cut-off function is improved, but the high stress induces high bow surface and makes subsequent processing more difficult

Engineering Contradiction:

Improveinfrared light transmissionVSAvoidwafer surface flatness

Core Design Contradiction:

Object-affected harmful factorsVSShape

Solution Approach 1:

The patent segments the device into separate functional layers, with the IRCF deposited on a separate wafer from the CIS. This allows the IRCF to be formed with the necessary stress characteristics for effective infrared filtering, while the separate wafer structure prevents this stress from causing bowing that would affect the overall device assembly and bonding processes.

Inventive Principle:

Principle #1Segmentation

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This solution enhances the signal-to-noise ratio (SNR) of incident light and reduces performance degradation, resulting in improved accuracy and reliability of biometric data detection, even in ambient light conditions, while maintaining smooth wafer bonding and minimizing stress-induced degradation.

Implementation Method 1

an infrared cut-off filter (IRCF) with a pattern... to mitigate stress and thermal effects, and improve light sensitivity by filtering out infrared light

Methodology Applied

Scientific EffectInfrared light filtering: Absorption (EM radiation)

Implementation Method 2

Methodology Applied

Scientific EffectOxide layer bonding: Adhesive

Implementation Method 3

an alignment fusion bonding is performed to bond the first wafer onto the second wafer

Methodology Applied

Scientific EffectFusion bonding: Welding

Data Source

PatentUS11861928B2Optical sensor and methods of making the same

Publication Date: 2024.01.02 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

Optical sensors and their making methods are described herein. In some embodiments, a described sensing apparatus includes: an image sensor; a collimator above the image sensor, wherein the collimator includes an array of apertures; and an optical filtering layer above the collimator, wherein the optical filtering layer is configured to filter a portion of light to be transmitted into the array of apertures.