Paper TF-MoA3
Atomic Layer Deposition of Al₂O₃ for Quantum Computing

Monday, October 31, 2011, 2:40 pm, Room 107

Josephson junctions (JJ) are a primary building block of superconducting quantum computers. The JJ structure is a superconductor-dielectric-superconductor stack, with the conventional dielectric layer fabricated by thermal oxidation of an aluminum metal electrode. The coherence time of these qubits is limited by high loss tangents in the dielectric layer thought to be due to defect-related quantum two level systems (TLS) inherent in the Al₂O₃. We have identified the –OH rotor TLS associated with hydroxyl species in the Al₂O₃ as a prime defect candidate based on TLS energetics. Simulations indicate that replacement of the –OH defect with the –OD defect will reduce the dielectric loss in these systems. We demonstrate the fabrication of Al₂O₃ dielectric thin films using atomic layer deposition (ALD) from trimethylaluminum and both H₂O and D₂O precursors as a function of substrate temperature and precursor dose. ALD enables precise control of film growth at the atomic scale, while comparison of H₂O vs. D₂O as the oxidation precursor enables isotopic defect loading in the Al₂O₃ and replacement of the –OH defect with the –OD defect. We have developed the D₂O based ALD process, and characterized both D₂O and H₂O based ALD Al₂O₃ thin films by SIMS, XPS and spectroscopic ellipsometry. Our resulting Al₂O₃ films are physically identical with the exception of –OH and –OD defects, making this system an ideal platform for the study of TLS defect related dielectric loss in JJ qubits. To this end we have fabricated MIM capacitor test structures and superconducting resonators using Re and Al electrodes and performed electrical measurements. Correlation between these data and low temperature superconducting microwave resonator performance will be discussed.