Passivated Copper Thermocompression Bonding for 3D Chip Stacking
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Solution Overview
Problem
Conventional 3D interconnect bonding techniques using solder reflow face challenges when adding subsequent chips, leading to alignment issues, thermal stress, and degradation of heat-sensitive components.
Innovation Solution
The method involves cold temperature bonding below the melting point of the conductive material, using oxide reduction and passivation steps to improve adhesion and conductance, and controlling the bond height through applied force, eliminating the need for solder reflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder reflow is used to bond chips, then initial bonding is achieved, but subsequent chip additions cause alignment loss and bonding instability
Solution Approach 1:
The patent changes the bonding temperature parameter from conventional high-temperature solder reflow (above melting point) to low-temperature bonding (below melting point, specifically 150-250°C). This parameter change allows multiple chips to be bonded sequentially without remelting previous joints, thereby maintaining alignment accuracy and bonding stability throughout the stacking process.
2Adaptability or versatility
If repeated solder reflow cycles are performed for multiple chip additions, then subsequent bonding is enabled, but thermal stress and component degradation increase
Solution Approach 1:
The patent implements a parameter change in bonding temperature (from above to below solder melting point) that enables multi-chip stacking capability while avoiding the harmful thermal stress associated with repeated high-temperature reflow cycles. The lower bonding temperature prevents degradation of heat-sensitive components.
3Strength
If high bonding temperature is used to ensure strong bonding, then bond strength is improved, but built-in stress and fatigue failure risk increase
Solution Approach 1:
The patent changes the bonding temperature parameter to a lower range (150-250°C below melting point) that maintains adequate bond strength through controlled plastic deformation of solder bumps, while significantly reducing the built-in thermal stress that causes fatigue failure. This optimized temperature parameter achieves both strong bonding and high reliability.
4Reliability
If solder reflow is used for bonding, then electrical connection is established, but process time and energy consumption increase
Solution Approach 1:
The patent changes the bonding temperature parameter to lower values (150-250°C), which reduces the time required for heating and bonding operations. The lower temperature enables faster thermal response and shorter process cycles while maintaining reliable electrical connections through effective solder bump deformation and contact.
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 enables reliable, high-density 3D chip stacking with improved alignment accuracy, reduced thermal stress, and increased process throughput, while maintaining equivalent bonding stability to conventional methods.
Implementation Method 1
oxide reduction and/or passivation steps are used to avoid native oxide and improve the adhesion and specific conductance
Implementation Method 2
oxide reduction and/or passivation steps are used to avoid native oxide and improve the adhesion and specific conductance
Implementation Method 3
The contacting metals are heated to a temperature sufficient to promote diffusion bonding
Data Source
AI summary
Methods and systems for low-force, low-temperature thermocompression bonding. The present application teaches new methods and structures for three-dimensional integrated circuits, in which cold thermocompression bonding is used to provide reliable bonding. To achieve this, reduction and passivation steps are preferably both used to reduce native oxide on the contact metals and to prevent reformation of native oxide, preferably using atmospheric plasma treatments. Preferably the physical compression height of the elements is set to be only enough to reliably achieve at least some compression of each bonding element pair, compensating for any lack of flatness. Preferably the thermocompression bonding is performed well below the melting point. This not only avoids the deformation of lower levels which is induced by reflow techniques, but also provides a steep relation of force versus z-axis travel, so that a drastically-increasing resistance to compression helps to regulate the degree of thermocompression.