Low Stress Cavity Package for Back Side Illuminated Image Sensors

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Solution Overview

Problem

The packaging and assembly of back side illuminated (BSI) image sensors pose challenges due to the need for thicker micro-lenses and the difficulty in forming electrical interconnects across the chip, which affects quantum efficiency and mechanical stress, especially as the industry transitions from front side illuminated (FSI) sensors.

Innovation Solution

A novel wafer level, low stress packaging method for BSI image sensors involves attaching the image sensor chip to a crystalline handler, forming a cavity, using a compliant dielectric material, and creating electrical interconnects with slanted holes and conductive metallization to reduce mechanical stress and improve insulation, allowing for efficient signal transfer without obstructing light paths.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If BSI image sensor packaging is performed using conventional COB or Shellcase WLCSP processes, then the packaging can be completed, but substantial challenges arise due to the opposite side disposition of contact pads and imaging areas

Engineering Contradiction:

Improvepackaging process easeVSAvoidpackaging structure complexity

Core Design Contradiction:

Ease of manufactureVSDevice complexity

Solution Approach 1:

The patent inverts the conventional packaging approach by attaching the back surface (imaging area) of the BSI sensor chip to the carrier substrate, rather than the front surface with contact pads. This inversion allows the contact pads to be accessed from the front surface while the imaging area is properly positioned for light entry from the back, resolving the fundamental packaging challenge of BSI sensors.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent utilizes the third dimension by creating a cavity structure in the carrier substrate that accommodates the chip thickness. This vertical dimension allows for proper positioning of the chip with its back surface attached to the carrier, enabling both contact pad access and imaging area exposure without compromising either function.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Length of moving object

If micro-lenses are made thicker to achieve shorter focal lengths in BSI sensors, then focusing in shorter distances is achieved, but the packaging and assembly become more challenging

Engineering Contradiction:

Improvefocal lengthVSAvoidpackaging and assembly ease

Core Design Contradiction:

Length of moving objectVSEase of manufacture

Solution Approach 1:

The patent segments the optical path by separating the micro-lens function from the chip structure itself. The micro-lenses are formed as distinct elements on the carrier substrate or as separate components, allowing their thickness to be optimized for short focal length while the chip packaging structure remains manageable and compatible with modified conventional processes.

Inventive Principle:

Principle #1Segmentation

3Reliability

If electrical interconnects are formed across the chip in BSI sensors, then electrical connection is achieved, but mechanical stress and risk of damage increase

Engineering Contradiction:

Improveelectrical connection reliabilityVSAvoidmechanical strength

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The patent introduces the carrier substrate as an intermediary structure that provides mechanical support and stress distribution. The electrical interconnects are formed through this intermediary carrier rather than directly across the fragile chip, reducing mechanical stress on the chip while maintaining reliable electrical connections to the contact pads.

Inventive Principle:

Principle #24Intermediary (Mediator)

4Productivity

If conventional packaging methods are used for BSI sensors, then the process can be completed, but corner stresses may cause damage

Engineering Contradiction:

Improvepackaging completionVSAvoiddamage risk from corner stresses

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent incorporates stress relief features and compliant structures in the carrier substrate design before the chip is attached. These pre-designed cushioning elements accommodate thermal expansion differences and mechanical stresses, particularly at corner regions, preventing damage during packaging and operation while maintaining packaging completion.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Data Source

PatentUS8895344B2Method of making a low stress cavity package for back side illuminated image sensor

Publication Date: 2014.11.25 OPTIZ

AI summary

An image sensor package includes an image sensor chip and crystalline handler. The image sensor chip includes a substrate, and a plurality of photo detectors and contact pads at the front surface of the substrate. The crystalline handler includes opposing first and second surfaces, and a cavity formed into the first surface. A compliant dielectric material is disposed in the cavity. The image sensor front surface is attached to the crystalline substrate handler second surface. A plurality of electrical interconnects each include a hole aligned with one of the contact pads, with a first portion extending from the second surface to the cavity and a second portion extending through the compliant dielectric material, a layer of insulation material formed along a sidewall of the first portion of the hole, and conductive material extending through the first and second portions of the hole and electrically coupled to the one contact pad.