Semiconductor Metal Bump With Hardened Outer Shell

Solution Overview

Problem

Conventional semiconductor devices for high-frequency applications face challenges in achieving reliable and strong joining of Au stud bumps to Au electrode pads using ultrasonic joining at low temperatures, which results in increased resistance and deformation, compromising high-frequency characteristics.

Innovation Solution

The solution involves forming a metal bump with a soft inner part and a harder outer part, where the outer part is reinforced with a plating film of larger hardness, allowing for effective ultrasonic joining without excessive deformation, using a flip-chip mounting process and non-electrolytic plating to enhance junction strength and maintain high-frequency performance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Temperature

If ultrasonic joining is used to join Au stud bumps to Au electrode pads at low temperatures, then thermal damage to the mounting substrate is avoided, but the joining strength is insufficient and resistance increases

Engineering Contradiction:

Improvejoining temperatureVSAvoidjoining strength

Core Design Contradiction:

TemperatureVSStrength

Solution Approach 1:

The patent applies local quality by creating a gradient in metal hardness within the bump structure. The inner core remains soft (original Au stud bump material) to enable ultrasonic joining, while the outer shell is made harder through plating (e.g., Pd, Pt, or Rh) to prevent deformation and maintain structural integrity. This local differentiation of material properties resolves the contradiction between low-temperature joining and sufficient joining strength.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent employs composite materials by combining two different metal materials with distinct properties: a soft Au stud bump core and a harder plated outer layer. This composite structure allows the soft core to facilitate ultrasonic bonding at low temperatures while the hard outer layer provides mechanical strength and deformation resistance, thereby achieving both low-temperature processing and high joining strength simultaneously.

Inventive Principle:

Principle #40Composite materials

2Reliability

If the metal bump is made softer to facilitate ultrasonic joining, then joining reliability improves, but deformation increases compromising high-frequency characteristics

Engineering Contradiction:

Improvejoining reliabilityVSAvoidbump deformation

Core Design Contradiction:

ReliabilityVSShape

Solution Approach 1:

The patent applies local quality by creating a gradient in metal hardness within the bump structure. The inner core remains soft (original Au stud bump material) to enable ultrasonic joining, while the outer shell is made harder through plating (e.g., Pd, Pt, or Rh) to prevent deformation and maintain structural integrity. This local differentiation of material properties resolves the contradiction between low-temperature joining and sufficient joining strength.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent employs composite materials by combining two different metal materials with distinct properties: a soft Au stud bump core and a harder plated outer layer. This composite structure allows the soft core to facilitate ultrasonic bonding at low temperatures while the hard outer layer provides mechanical strength and deformation resistance, thereby achieving both low-temperature processing and high joining strength simultaneously.

Inventive Principle:

Principle #40Composite materials

3Temperature

If adhesive joining is used instead of thermal welding, then heat resistance of the mounting substrate is maintained, but resistance increases and wire snapping occurs under shock or thermal cycling

Engineering Contradiction:

Improveheat resistanceVSAvoidconnection reliability

Core Design Contradiction:

TemperatureVSReliability

Solution Approach 1:

The patent replaces the chemical bonding mechanism of adhesive joining with a mechanical bonding mechanism through ultrasonic welding. The ultrasonic vibration creates direct metal-to-metal bonding between the Au stud bump and Au electrode pad, eliminating the need for adhesive materials. This substitution provides superior mechanical strength and reliability under shock and thermal cycling while maintaining compatibility with low-temperature processing through the composite bump structure.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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 achieves a junction strength of 80 MPa, suppresses deformation, and maintains high-frequency characteristics within predetermined standards, ensuring reliable connection and performance under thermal cycling.

Implementation Method 1

ultrasonic joining technology that can achieve fusion between the Au stud bump and the Au electrode pad at low temperatures

Methodology Applied

Scientific EffectUltrasonic joining: Ultrasonic Vibration

Implementation Method 2

Au stud bumps 13A and 13B are formed respectively on the bump electrodes 12A and 12B by an electrolytic plating process

Methodology Applied

Scientific EffectElectrolytic plating: Electrodeposition

Data Source

PatentUS7429796B2Semiconductor device and fabrication process thereof

Publication Date: 2008.09.30 FUJITSU LTD

AI summary

A semiconductor device includes a mount substrate and a semiconductor chip mounted upon the mount substrate via a metal bump, wherein metal bump includes an inner part joined to the semiconductor chip and an outer part covering the inner part, the outer part having an increased hardness as compared with the inner part.