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Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Evaluation of SRF thin film properties using a microstrip disk resonator

Daniel Bowring

Lawrence Berkeley National Laboratory

July 19, 2012

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Overview

small samples SRF cavities

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Overview

small samples SRF cavities present work

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Overview

1 Motivation: Evaluating multilayer films 2 Disk resonator: operating principle, experimental design 3 Preliminary results 4 Future aspirations 5 Postscript: A note on NbTiN

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

We want a method to evaluate multilayer thin films.

100 200 300 400 500 600 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Film depth (nm) Normalized field strength H/H0

(Nb,Ti)N alumina Nb

Figure: Thin films screen the magnetic field from the bulk layer.

−60 −40 −20 20 40 60 −35 −30 −25 −20 −15 −10 −5 5 Depth of vortex in thin film (nm) Normalized free energy G/G0 symmetric currents no current asymmetric currents

Figure: This is why it works: asymmetric currents tilt the free energy profile of a flux vortex.

G/L = φ2 4πµ0λ2 ln

• d

1.07ξ cos πu d

• − φ0

d/2

u

J(z)dz

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Test this using NbTiN films.

50 100 150 200 250 300 350 1 2 3 4 5 6

H (mT) Log10Q0

Control resonator Thick film resonator Multilayer system

NbTiN has low Hc1. This simplifies testing, leaves basic physics unchanged. We didn’t try to exceed 180 mT right

• away. Walk before you run.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

What has been done?

Theory Modeling Design and fabrication Process development: reactive magnetron sputtering in JLab’s UHV system. Preliminary cryo/RF testing Material analysis Inconveniently-timed capital development at JLab → No clean multilayer films for testing. Yet.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Our experimental facility can evaluate multilayer thin films under cryogenic, RF conditions.

Experimental design goals: Reduce parameter space: evaluate multilayer physics using small, flat samples. Highest magnetic field must be on sample → sample is performance-limiting feature. Suppress multipacting, field emission as much as possible. Modular, easily-demountable samples → quick experimental turnaround. In the VTA, can get 2 measurements per day if you push. Small flat samples → easier & quicker post-facto surface analysis.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

A microstrip disk resonator satisfies the above requirements.

Pillbox-like TM01 fields supported between a circular disk (which sets frequency) and a superconducting ground plane.

4 cm

f = 2.8 GHz, Q0 ≈ 105 (very low U). Capacitive coupling. Simulations via CST Microwave Studio.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Other configurations are possible.

“Active” sample size set by frequency requirements. (1 cm radius → 6 GHz.) Rectangular ground plane not required. Can operate with circular samples (and minimal hardware changes).

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Resonator assembly

Currently, minimum sample size is 4 cm diameter.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Dewar insert for operation in JLab’s VTA

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Preliminary results

2.77 2.772 2.774 2.776 2.778 2.78 2.782 2.784 −60 −55 −50 −45 −40 −35 −30 −25 −20 −15

Frequency (GHz) S11

Figure: f = 2.78 GHz, Q0 ≈ 103.

• 75
• 50
• 25

25 50 75 0.25π 0.5π 0.75π π 1.25π 1.5π 1.75π

|S11| Φ

Figure: Tuners permit critical coupling.

Coupler upgrade, full system characterization must wait until TEDF is online.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Future aspirations

Several people at this workshop have indicated a desire to evaluate SRF properties of small samples. Upgrade couplers, amplifier. Add thermometry capabilities. This system is not only useful for multilayer films. Can also explore SRF films on various substrates; bulk properties from grain size, surface treatments; build large statistical database. Rs = Aω2

T e−∆/kT + R0 → at low temperatures, measure

R0 directly.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

In the future, use this system in Texas A&M’s cluster tool.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

A brief epilogue re: NbTiN

5 10 15 20 25 30 50 100 150 200 250 300

Temperature (K) Resistivity (µΩ−cm)

DB−04 DB−05 DB−06 DB−07 DB−08 DB−09

Tc = 13.2 ± 0.4 K RRR= 1.46 ± 0.58 (but this was during commissioning)

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

What is NbTiN?

Typically, literature mentions Tc with no further analysis. Everybody writes it in a different way: NbTiN (Nb,Ti)N δ-NbTiN Nb1−xTixN Nb-Ti-N

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

XRD comparisons to PDF

XRD measurements of JLab-made films with various P(N2), compared with PDF.

Powder Diffraction File, edited by S. Kabekkodu (International Centre for Diffraction Data, Newton Square, PA, USA, 2011).

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

XRD comparisons to PDF

Comparison of JLab-made film with other possible phases of the Nb-Ti-N system.

Powder Diffraction File, edited by S. Kabekkodu (International Centre for Diffraction Data, Newton Square, PA, USA, 2011).

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

δ−(Nb,Ti)N

Substitutional solid solution. B1 structure similar to NbN. Ti stabilizes the δ-phase.

Evaluation of SRF thin film properties using a microstrip disk resonator Daniel Bowring Motivation Disk resonator Preliminary results Future aspirations NbTiN postscript

Acknowledgements

• L. Phillips, J. Spradlin, A.-M. Valente-Feliciano, X. Zhao