Recent result of ~650-GHz SIS-device fabrication at NRO K. Kikuchi 1, W. Shan 2, Y. Fujii 3, and J. Inatani 1 (1NASDA, 2 CRL, 3 Nittuki )

Introduction Purpose: Development of ~650-GHz SIS mixer devices for JEM/SMILES, B-SMILES, and SMA. Difficulties: - Just below the gap frequency of Nb. - Junction size should be as small as ~1mm2. Mixer performance is highly sensitive to fabrication parameters. This work is … - Improvement and optimization of the fabrication processes. - Understanding of controllability of the SIS device parameters.

SIS-device and Fabrication Process IF port 500 mm GND - SIS Junction: Nb/AlOx/Nb - Junction Size: ~1 x 1 mm2 (for JEM/SMILES) - Current Density: ~7 kA/cm2 (for JEM/SMILES) - RF Matching: PCTJ with Integrated Circuit

Cross-section Images by TEM TEM image of junction portion 500 nm Wire-Nb Al2O3 Upper-Nb SiO2 Nb2O5 Lower-Nb Quartz substrate Al-AlOX 20 nm “T96” device: - fabricated in 2001 - moderate RF performance (TRX,DSB ~ 300 K)

Nb-film: Superconductivity - Nb-film samples were fabricated with the parameter changing. - Film having a good superconductivity seems to be obtained by using LN2 shroud or gettering effect of sputtered Nb and Al. TC ~ 9.0 ± 0.1 K r10K ~ 5.0 ± 0.5 mW cm

Nb-film: Surface Roughness Measured by AFM +0.22 GPa, 184 nm Stress -0.49 GPa, 173 nm -0.52 GPa, 359 nm Thickness 1mm

Optimization of Etching Processes - Etching rates and stabilities are investigated in various conditions. Example: introduction of Ar-cleaning process: 50mm Un-developed photo-resist makes rough surface in some conditions. Ar-cleaning is effective to make flat. (SiO2 : Nb ~ 5:1 for 100 W, 50 mTorr.) Surface Profile (nm) Relative Distance (mm)

Improvement of Junction Lift-off Process Introduced paint brush, instead of cotton swab

Controllability: Film Thickness Surface profile is measured at each process to estimate the thickness. JEM/SMILES SIS device 640-GHz PCTJs f35 Test junctions Surface profile measurement (500-GHz PCTJs)

Controllability: Junction Size Rn distribution of KS17 devices ID = 1 76 Mean junction size with 1-s deviation in a wafer L ∝ (Jc・Rn) -0.5 fc ∝ L-1

Peak Structure in PIF-V Curve SIS having peak structure shows poor RF performance. Model Amplitude is affected by magnet. - Position varies with temperature.

Mixer Performance Mixer block for JEM/SMILES TRF~60K, TIF~15K

Summary Improvement of processes (Ar-cleaning, lift-off, …) Understanding of the film properties (TEM, AFM, superconductivity, …) Improvement of processes (Ar-cleaning, lift-off, …) Variation of device parameters: - Superconductivity of Nb film: DTC ~ 0.1 K, Dr10K ~ 0.5 mW cm - Film thickness: DT ~ 2-5 % - Junction size: DL ~ 5-10 % - Repeatability of JC is also poor (~ 10 %) Rough estimation of yield: - Survival rare in DC characteristics: pDC ~ 90 % - No peak structure in PIF-V curve: pno-peak ~ 70 % (?) If the band-width of SIS mixer DB/B = Db ~ 5 %, Db/DL ~ 1 s. Then the maximum “yield” from the view point of RF performance is… pDC x pno-peak x pGaussian(Db/DL) = 0.9 x 0.7 x 0.68 = 0.43