Adhesive Bonding for Superconducting Multichip Modules
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Solution Overview
Problem
Existing multichip module fabrication techniques for superconducting circuits face challenges with thermal cycling, vibration tolerance, and adhesion issues due to differential thermal expansion, leading to unreliable bonds and mechanical instability, especially at cryogenic temperatures.
Innovation Solution
A non-conductive adhesive with matched coefficients of thermal expansion between the chip and substrate is used to bond wafers, eliminating the need for solder reflow and providing a reworkable bond, which enhances mechanical stability and reliability under various environmental conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder reflow bonding is used to connect wafers to substrate, then electrical and mechanical bonding is achieved, but thermal cycling and vibration cause bond failure due to differential thermal expansion
Solution Approach 1:
The patent changes the bonding method from solder reflow to adhesive bonding, and specifically selects an adhesive with thermal expansion properties that match both the wafer and substrate materials. This parameter change in the bonding material's thermal characteristics eliminates the differential expansion problem that causes solder joint failure during thermal cycling.
Solution Approach 2:
The patent uses a composite adhesive material that combines polymer matrix with specific fillers to achieve matched thermal expansion coefficients. This composite approach allows tuning the thermal properties of the adhesive to match the superconducting wafer and substrate, preventing bond failure while maintaining electrical insulation.
2Strength
If elevated temperatures are used for solder reflow, then wafer to substrate bonding is achieved, but temperature-sensitive superconducting circuits are damaged
Solution Approach 1:
The patent changes the bonding process temperature from elevated reflow temperatures (>200°C) to room temperature or slightly elevated temperatures compatible with superconducting circuit integrity. The adhesive bonding chemistry is selected to provide sufficient bond strength at these lower temperatures, eliminating thermal damage to the Josephson junctions and other temperature-sensitive components.
3Temperature
If filler material is added between wafer and substrate, then thermal conductivity is enhanced, but reflow process and alignment are impaired
Solution Approach 1:
The patent extracts the filler material from the bonding interface, using a clear adhesive that allows direct visualization of the wafer and substrate alignment. This elimination of opaque filler material enables precise alignment to be achieved and maintained, while the adhesive itself provides the necessary thermal conduction path without interfering with the bonding process.
4Reliability
If solder bumps are used for electrical connection, then electrical conductivity is achieved, but mechanical stability under vibration is poor
Solution Approach 1:
The patent replaces the mechanical solder bump structure with an adhesive-bonded flat contact interface. This substitution eliminates the mechanical weaknesses of solder bumps under vibration and thermal cycling, providing a more robust electrical connection that maintains stability under environmental stress while preserving electrical conductivity through the adhesive medium.
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
The adhesive bonding technique ensures robust and reliable electrical connections over a broad temperature range, including cryogenic temperatures, with improved vibrational tolerance and the ability to maintain high-speed data transmission rates, while allowing for reworkability and increased package density.
Implementation Method 1
providing a liquid curable adhesive over the set of contacts of a first substrate; aligning the set of electrical contacts of the second substrate with the set of electrical contacts of the first substrate to form a liquid curable adhesive-filled gap therebetween; compressing the sets of electrical contacts
Implementation Method 2
compressing the sets of electrical contacts of the first and second substrate to displace the liquid curable adhesive and provide electrical communication between the respective sets of electrical contacts
Implementation Method 3
curing the liquid curable adhesive to form a solid matrix which maintains a relative compression between the respective sets of electrical contacts. Electrical conductivity between the respective sets of electrical contacts is preferably maintained over at least a range of temperatures
Data Source
AI summary
A method for electrically interconnecting two substrates, each having a corresponding set of preformed electrical contacts, the substrates comprising an electronic circuit, and the resulting module, is provided. A liquid curable adhesive is provided over the set of contacts of a first substrate, and the set of electrical contacts of the second substrate is aligned with the set of electrical contacts of the first substrate. The sets of electrical contacts of the first and second substrate are compressed to displace the liquid curable adhesive from the inter-contact region, and provide electrical communication between the respective sets of electrical contacts. The liquid curable adhesive is then cured to form a solid matrix which maintains a relative compression between the respective sets of electrical contacts. One embodiment of the module comprises a high-speed superconducting circuit which operates at cryogenic temperatures.