Stud Bump Tail Region for CIS Flip-Chip Bonding
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Solution Overview
Problem
The existing interconnect structures for Backside Illumination (BSI) image sensor chips face challenges in reliable bonding to ceramic substrates, particularly due to the thinness of the image sensor chips and the risk of damage during the formation of stacked stud bumps, which can lead to reduced reliability and increased risk of chip breakage.
Innovation Solution
The implementation of a single stud bump with a tail region per metal pad, where the tail region is short enough to support itself against gravity and is electrically connected through a solder layer, reduces the risk of chip damage and enhances bonding reliability by using a soft solder layer to absorb bonding forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If stacked stud bumps are formed on metal pads, then electrical connection reliability is improved, but the risk of chip breakage increases due to bonding forces during formation
Solution Approach 1:
The stud bump structure is segmented into two distinct parts: a stud portion that provides electrical connection and a tail portion that acts as a mechanical stop. This segmentation allows the tail portion to prevent further bonding tool pressure from being transmitted to the thin chip, thereby reducing the risk of chip breakage while maintaining reliable electrical connection through the stud portion.
Solution Approach 2:
The tail portion of the stud bump is formed to extend beyond the metal pad surface before bonding occurs. This preliminary configuration of the tail portion serves as a pre-established mechanical barrier that stops the bonding tool's pressure before it can damage the chip, preventing chip breakage before it happens during the bonding process.
2Volume of moving object
If the image sensor chip is made thin to reduce size, then device compactness is improved, but structural strength during bonding deteriorates
Solution Approach 1:
The tail portion of the stud bump acts as an intermediary mechanical stop between the bonding tool and the thin chip. It absorbs and blocks the bonding pressure, preventing direct transmission of force to the fragile thin chip, thereby enabling thin chip design without compromising structural strength during bonding.
3Reliability
If multiple stud bumps are stacked to ensure electrical connection, then electrical conductivity is improved, but manufacturing complexity increases
Solution Approach 1:
The stud bump is segmented into a stud portion for electrical connection and a tail portion for mechanical stopping. This single-segment design with dual functionality replaces the need for multiple stacked stud bumps, simplifying the bonding process while ensuring reliable electrical connection through the stud portion.
Solution Approach 2:
The tail portion serves multiple functions: it acts as a mechanical stop to prevent chip breakage, provides alignment reference during bonding, and eliminates the need for multiple stacked bumps. This multi-functionality reduces manufacturing complexity while maintaining electrical connection reliability through the stud portion.
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 approach ensures reliable flip-chip bonding of image sensor chips to ceramic substrates with reduced risk of chip breakage and improved bonding reliability, while maintaining the efficiency of photon capture in BSI image sensor applications.
Implementation Method 1
using a soft solder layer to absorb bonding forces
Data Source
AI summary
A device includes a metal pad at a surface of an image sensor chip, wherein the image sensor chip includes an image sensor. A stud bump is disposed over, and electrically connected to, the metal pad. The stud bump includes a bump region, and a tail region connected to the bump region. The tail region includes a metal wire portion substantially perpendicular to a top surface of the metal pad. The tail region is short enough to support itself against gravity.