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Indium bumps are one of the key enabling technologies in superconducting quantum computing, providing superconducting electrical interconnects, low-loss microwave connections, mechanical bonding, and dense 3D integration. In flip-chip bonding of qubit chips, an array of indium bumps bonds a chip containing superconducting qubits face-to-face, with a second chip containing some combination of control wiring, resonators, shielding structures, and interposers.
In our video, Denton Vacuum Vice President of Marketing and Technology Dr. David Douglass explains why indium has become such an important material to researchers and manufacturers and discusses several ways smaller bumps can facilitate quantum’s ongoing advancement.
What Makes Indium the Right Metal for Superconducting Bumps?
Indium becomes superconducting below 3.4 K, and its extreme softness and low bonding temperature make it particularly well suited to forming the microscopic connections used in quantum devices. The material offers good thermal conductivity and vacuum compatibility with an oxide layer that breaks easily.
Properly deposited indium maintains its elasticity at superconducting temperature, and the metal’s ductility gives each joint very high shear strength. Silver and copper run into problems at temperatures an indium bump can handle without issue.
How Are Indium Bumps Below 10 Microns Possible at Production Scale?
At 25 microns and above, electroplating is the standard for creating indium bumps. Increasing wiring density in quantum packaging, however, means going below that line. At the sub-10-micron scale, with 2:1 height-to-width aspect ratios, thin film vacuum deposition picks up where electroplating falls short, delivering better lift-off yield and shear strength. Our Integrity evaporation system brings throughput as well, depositing bumps with excellent film crystallography at high-volume production rates.
Small bumps open new possibilities to manufacturers and their chip designers. With more connections between the chips themselves, lossy routing layers no longer occupy space that could be used for qubits. Quantum processors can be assembled from modular pieces instead of one large die, with multiple superconducting layers integrated into the design.
From Infrared Detectors to Qubit Chips
Our industry-leading indium deposition systems have been used extensively for indium flip-chip bonding of cooled infrared detectors, a thin film application similar to what quantum computing demands. Quantum packaging also requires superconducting temperatures, another area where we have experience.
Quantum hardware teams don’t have to start from zero on sub-10-micron indium bumps, because the deposition process is already established in cooled infrared detector manufacturing. In the video, Dr. Douglass explains indium’s growing role in quantum computing, and why chiplet-based designs built on indium bump bonding are considered one of the most likely paths toward million-qubit systems. Watch the full video, then reach out to talk about your application.
Watch Dr. Douglass’s talk and get a fuller picture of indium bumps.