Josephson Junction QC

Silicon Nitride Stencils Open New Superconducting Qubit Manufacturing Era

Karlsruhe Institute of Technology and Forschungszentrum Jülich researchers developed an on-chip stencil lithography technique using durable inorganic materials to overcome long-standing constraints in creating Josephson junctions, the building blocks of superconducting qubits. By making quantum devices more dependable, effective, and reproducible, this unique approach could accelerate quantum computing.

Quantum computing requires nanofabrication developments to create increasingly complex quantum circuits. Creating superconducting qubits, especially transmon qubits, requires sophisticated methods and materials, sometimes using organic resists.

Contamination and heat instability are drawbacks of conventional polymer-based lithography. These resists' fragility limits pre-growth cleaning and high-temperature processing, restricting substrate preparation. Oxidation, residual resist, and amorphous layers can compromise a qubit's coherence, which determines its quantum state.

Overcoming Inorganic Stencil Fabrication Issues

Roudy Hanna from Forschungszentrum Jülich and JARA, Sören Ihssen and Simon Geisert from Karlsruhe Institute of Technology, and his colleagues devised an on-chip stencil printing method to solve these challenges. Silicon nitride and silicon dioxide form a durable stencil mask for this method. These inorganic materials can withstand rigorous cleaning and 1200°C processing, unlike organic materials. Resilience is necessary for superconducting qubits to have few defects and greater coherence. Traditional organic resists' stability and contamination issues are solved by the revolutionary technique.

Removing polymer resists from critical production processes makes the new method cleaner and more reliable. Inorganic stencils reduce oxidation, residual resist, and amorphous layers, which weaken qubit coherence in conventional methods, allowing for more thorough substrate preparation. Unlike off-chip stencil approaches, the on-chip design ensures exact alignment and eliminates misalignment issues, improving scalability and reproducibility for future quantum device production. High temperature tolerance allows surface treatments before material deposition, which improves material discovery and interface optimisation during qubit manufacture.

Validating Transmon Qubit Performance

The study team validated their strategy with aluminium transmon qubits. The complex process of producing superconducting qubits begins with sapphire substrates, which have outstanding mechanical properties at cryogenic temperatures and low dielectric loss. Next are superconducting ground plane deposition and patterning, resonator structures, and most crucially, the Josephson connection.

The new Josephson junction approach uses sputtering and electron beam evaporation to create thin films, which requires atomic layer deposition to carefully regulate the insulating layer thickness. Aluminium deposition, controlled oxidation to construct the aluminium oxide barrier, and further aluminium deposition create the Josephson junction.

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The inorganic mask deposits metal in modern stencil lithography. The silicon dioxide and silicon nitride mask is carefully removed after metal deposition using vapour hydrofluoric acid, which selectively etches silicon dioxide without damaging the newly created qubit. Final qubit geometry is determined by junction and resonator geometry before packaging and testing at cryogenic temperatures. Electrical wiring, control and readout interconnects, and insulation follow.

This innovative method produced gadgets with amazing coherence. One device had a T1 relaxation period of 75 microseconds at 200 MHz, whereas another had 44. The study also showed millisecond coherence in varied cool-downs. This proves the strategy works with cutting-edge quantum devices.

An Exciting Quantum Hardware Future

This crucial finding allows more material study and optimisation in the quest for more powerful and reliable quantum computing. Inorganic stencils that can endure harsh processing conditions improve qubit coherence and reduce defects, which are essential for robust quantum processes.

Even though the study focused on aluminum-based qubits, the researchers recognise the need for more research to apply the technique to other superconducting materials and junction designs. Surface cleaning and film deposition must be optimised to improve qubit performance and coherence.