DC SQUID Test Structure for Superconducting Bump Bond Verification
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Solution Overview
Problem
Existing methods fail to unambiguously verify the superconductivity of bump bonds in flip-chip superconducting integrated circuit fabrication, particularly for those with low resistance in the microohm to milliohm range, and cannot experimentally determine the inductance of individual bump bonds, which is crucial for ensuring manufacturing criteria are met and accounting for parasitic inductance in larger systems.
Innovation Solution
Incorporating a DC SQUID into the test structure that embeds superconducting bump bonds, allowing for the verification of superconductivity by observing periodic modulation of the critical current under flux bias current and extracting inductance through voltage measurements across the SQUID, thereby distinguishing true superconductivity from low resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional resistance measurement methods are used to characterize bump bonds, then measurement simplicity is maintained, but measurement precision is insufficient for low resistance bump bonds in the microohm to milliohm range
Solution Approach 1:
The patent introduces a DC SQUID as an intermediary device to indirectly measure the electrical properties of bump bonds. Instead of directly measuring resistance with conventional instruments, the SQUID acts as a sensitive mediator that converts the electrical state of the bump bond into a measurable magnetic flux signal, enabling precise characterization of low-resistance connections that are otherwise difficult to measure.
Solution Approach 2:
The patent replaces conventional electrical resistance measurement systems with a magnetically-based SQUID measurement system. By substituting the direct electrical measurement approach with a magnetic field detection approach, the system achieves significantly higher sensitivity for measuring low-resistance bump bonds, overcoming the limitations of traditional ohmmeters and multimeters.
2Reliability
If DC SQUID is incorporated into the test structure to verify superconductivity, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent segments the test structure into distinct functional components: the DC SQUID device under test, the bump bond interconnects, and the measurement system. This segmentation allows the SQUID to be tested in isolation for superconductivity verification while maintaining the ability to separately characterize the bump bond connections, thereby improving reliability without overwhelming complexity.
Solution Approach 2:
The DC SQUID test structure is designed to serve multiple functions: verifying superconductivity of the SQUID device itself, characterizing the electrical properties of bump bonds, and determining inductance values. This multi-functionality justifies the increased device complexity by providing comprehensive electrical characterization capabilities in a single integrated test structure.
3Productivity
If bump bonds are used to connect superconducting chips, then manufacturing efficiency is improved, but inductance characterization capability deteriorates due to inability to measure individual bump bond inductance
Solution Approach 1:
The patent incorporates test structures and measurement capabilities into the bump bond fabrication process itself, performing inductance characterization during or immediately after the bump bond formation. This preliminary action allows inductance values to be determined early in the manufacturing process, enabling design adjustments to be made before final assembly, thereby maintaining manufacturing efficiency while achieving precise inductance measurement.
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 unambiguously verifies superconductivity and determines the inductance of bump bonds, even for those with resistance below conventional measurement thresholds, ensuring accurate characterization and reducing uncertainty in superconducting circuit design.
Implementation Method 1
a DC SQUID that has a loop that electrically includes at least two of the bump bonds
Implementation Method 2
A modulated critical current of the DC SQUID can then be observed by measuring voltage across the DC SQUID
Data Source
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Figure 6
AI summary
Test structures and methods for superconducting bump bond electrical characterization are used to verify the superconductivity of bump bonds that electrically connect two superconducting integrated circuit chips fabricated using a flip-chip process, and can also ascertain the self-inductance of bump bond(s) between chips. The structures and methods leverage a behavioral property of superconducting DC SQUIDs to modulate a critical current upon injection of magnetic flux in the SQUID loop, which behavior is not present when the SQUID is not superconducting, by including bump bond(s) within the loop, which loop is split among chips. The sensitivity of the bump bond superconductivity verification is therefore effectively perfect, independent of any multi-milliohm noise floor that may exist in measurement equipment.