Gold-Bump Copper Wire Bonding to Protect Die Bond Pads
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Solution Overview
Problem
Copper wire bonding techniques can damage bond pads and underlying circuity of semiconductor dies due to copper's higher hardness and brittleness compared to gold, making gold wire bonds preferable in certain applications.
Innovation Solution
A semiconductor package design that includes a semiconductor die with a gold bump on the bond pad and a copper wire bond extending from the gold bump, where the gold bump is applied with a lower bond energy than the copper wire bond, protecting the bond pad during the higher-energy copper wire bond application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If copper wire bonds are used instead of gold wire bonds, then cost is reduced and electrical/thermal conductivity is improved, but damage to bond pads and underlying circuitry occurs due to copper's higher hardness and brittleness
Solution Approach 1:
A nickel under-bump barrier layer is introduced between the copper wire bond and the aluminum bond pad to prevent direct contact and damage. The nickel layer acts as a protective intermediary that absorbs the mechanical stress and prevents copper-induced damage to the aluminum bond pad and underlying circuitry, while still allowing electrical conduction.
Solution Approach 2:
The bonding structure uses a composite material system consisting of copper wire bond, nickel barrier layer, and aluminum bond pad. This composite structure combines the advantages of copper (high conductivity, low cost) while mitigating its disadvantages (hardness, brittleness) through the nickel interlayer that provides mechanical protection and compatibility.
2Ease of manufacture
If copper wire bonds are used instead of gold wire bonds, then manufacturing cost is reduced, but manufacturing precision is compromised due to copper's brittleness causing bond pad damage
Solution Approach 1:
The nickel barrier layer serves as a protective intermediary during the wire bonding process, absorbing mechanical stresses and preventing copper-induced damage to the aluminum bond pad. This allows copper wire bonds to be used without compromising bond pad integrity, enabling cost-effective manufacturing with maintained precision.
3Object-affected harmful factors
If gold wire bonds are used, then damage to bond pad and underlying circuitry is prevented, but cost increases and electrical/thermal conductivity decreases
Solution Approach 1:
The nickel barrier layer provides the protective function previously offered by gold wire bonds, preventing damage to the aluminum bond pad. Meanwhile, copper wire bonds replace gold to achieve superior electrical and thermal conductivity at lower cost, with the nickel layer ensuring compatibility between the dissimilar metals.
Solution Approach 2:
The multi-material structure of copper wire bond with nickel barrier on aluminum bond pad combines the protective properties needed for bond pad safety with the high conductivity properties of copper, eliminating the need for expensive gold while maintaining or improving performance.
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 design mitigates damage to the semiconductor die bond pad and underlying circuity while offering the advantages of copper wire bonds, such as reduced cost and higher electrical and thermal conductivity.
Implementation Method 1
The gold bump may be applied with a lower bond energy than the copper wire bond. During the higher-energy application of the copper wire bond, the gold bump protects the bond pad, mitigating damage to the semiconductor die bond pad and underlying circuity.
Implementation Method 2
Methods for forming the copper wire bond may include limited or no ultrasound current, which may limit growth of intermetallics between the bond pad and gold bump while forming the copper wire bond compared to alternative techniques utilizing higher levels of ultrasound current for forming the copper wire bond.
Data Source
AI summary
A semiconductor package includes a conductive pad, a semiconductor die with an aluminum bond pad over a dielectric layer of the semiconductor die, a gold bump on the aluminum bond pad, a first intermetallic layer of gold and aluminum between the aluminum bond pad and the gold bump, a copper ball bond on the gold bump, a second intermetallic layer of copper and gold between the copper ball bond and the gold bump, a copper wire extending from the copper ball bond to the conductive pad, a stitch bond between the copper wire and the conductive pad.