Increasing the Fidelity of Qubit Operations

Summary

FabricationThis section provides a detailed description of our sample fabrication process, which is an adaptation of the recipe described in ref. 16 to the chemicals and equipment available to us. Notable differences include the choice of CF4 as the processing gas for Nb etching, which improves the reproducibility of the etch, and the pre-dicing of the sample between the Al evaporation and lift-off, which minimizes the exposure of the sample to the ambient atmosphere. The pieces of equipment used in our process are listed in Supplementary Table 2.Substrate and niobium patterningThe sample is fabricated using a 675-μs-m-thick 6-inch (100)-oriented high-resistivity ( > 10 kΩcm) intrinsic-silicon wafer sourced from Siegert Wafer. The pre-cleaning of the wafer follows ref. 31 and begins with an RCA solvent clean. The wafer then undergoes a dip in dilute hydrofluoric (HF) acid (1:100) for 1 min. After being rapidly transferred to the sputtering tool to minimize exposure to the ambient atmosphere, the wafer is baked at 300 °C under vacuum. The sputtering of a 200-nm Nb film is carried out near room temperature and at 2600-W power. The sputter target has a purity of 99.998%. After the sputtering, the wafer is coated with a protective layer of AZ 5214E photoresist and diced into 25 mm × 30 mm rectangular coupons with a dicing saw.The resonators, coplanar waveguides, ground plane, and transmon capacitors are patterned as described below. The selected coupon is sonicated in acetone and isopropyl alcohol (IPA) for 3 min each to remove the protective resist layer and dried with a nitrogen gun. The sample is then dehydrated on a hotplate at 110 °C for 1 min, spin-coated with AZ 5214E photoresist at 4000 rpm, and baked at 110 °C for 1 min to achieve a coating thickness of 1.4 μm. The photoresist is exposed using a maskless aligner with a laser wavelength of 405 nm and a dose of 130 mJ/cm2. Subsequently, the photoresist is developed in AZ 726MIF for 1.5 min and rinsed in deionized...