Cold Thermocompression Bonding for Oxide-Free 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
A cold temperature bonding process is used to add subsequent chips without disrupting the bonding of previous chips, employing oxide reduction and passivation steps to improve adhesion and conductance, and controlling the bond height through applied force.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder reflow is used to bond chips, then electrical and mechanical connection is achieved, but alignment accuracy deteriorates when adding subsequent chips
Solution Approach 1:
The patent changes the bonding temperature parameter from high temperature (solder reflow) to low temperature (cryogenic or room temperature), enabling subsequent chip additions without remelting previous bonds. This parameter change resolves the contradiction by maintaining alignment accuracy while achieving reliable bonding through alternative mechanisms.
Solution Approach 2:
The patent replaces the thermal-melting mechanism with a mechanical compression mechanism. By applying controlled compressive force at low temperatures, the bonding interface achieves reliable contact without phase change, thereby preserving alignment accuracy across multiple bonding steps.
2Reliability
If solder reflow is used to add subsequent chips, then bonding is achieved, but thermal stress increases
Solution Approach 1:
The patent changes the bonding temperature from above solder melting point to below melting point (cryogenic or room temperature), eliminating repeated thermal cycling. This reduces differential thermal expansion and contraction between chips with different CTE, thereby minimizing thermal stress while maintaining bonding reliability through low-temperature compression.
3Reliability
If solder reflow is used for 3D stacking, then interconnect bonding is achieved, but heat-sensitive components degrade
Solution Approach 1:
The patent changes the bonding temperature parameter to low temperature (cryogenic or room temperature), avoiding exposure of heat-sensitive components to high temperatures. This enables reliable interconnect bonding through cold compression while preventing thermal degradation of sensitive devices such as MEMS, photodetectors, and other temperature-sensitive components.
4Reliability
If oxide layers are present on bonding surfaces, then native protection is maintained, but adhesion and conductance deteriorate
Solution Approach 1:
The patent applies preliminary oxide removal treatment (such as plasma cleaning, chemical etching, or in-situ reduction) to bonding surfaces before compression bonding. This preliminary action removes native oxide layers that would impede adhesion and electrical conductance, while the low-temperature process prevents rapid oxide regrowth, thereby achieving both surface protection and high adhesion quality.
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 method enables reliable, high-density 3D chip stacking with improved alignment accuracy, reduced thermal stress, and increased process throughput, while avoiding the need for solder reflow.
Implementation Method 1
directing plasma-activated radical-enriched gas flow at substantially atmospheric pressure both to first contacting metallizations on a first element and also to second contacting metallizations on a second element, both to reduce native oxides from said contacting metallizations
Implementation Method 2
passivate said contacting metallizations against re-oxidation
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.