Using HiPIMS to Deposit V3Si Superconducting Thin

FilmsF. Lockwood Estrin, J. W. Bradley, E. Seiler, G. Stenning and R. Valizadeh

28 June 2021 – 02 July 2021

Abstract Nearly all superconducting RF (SRF) cavities today use superconducting

niobium: whether it be bulk niobium or thin niobium films on copper.

However, alternatives are worthy of exploration; A-15 superconductors

are a promising avenue of investigation. V3Si is an A-15

superconductor, shown to have high critical temperature (17.1 K) and

RRR value up to 80, making it a promising candidate for SRF-cavities*.

The RRR and critical temperature of V3Si is closely linked to the

Stoichiometry, which in turns depends on substrate and deposition

temperature**. Previous experiments have relied on dual magnetron

sputtering or reactive sputtering, which have had problems achieving

the correct stoichiometry.

The magnetron sputtering technique HiPIMS has been shown toperform ion bombardment during deposition. This ion bombardmenthas been shown to have a similar effect to sample heating, allowinggreater control of stoichiometry***. This is combined with new heatingtechniques to improve the existing benchmarks of V3Si films.

DC Sputtered Samples

HiPIMS Samples

HiPIMS Samples Increase Temp.

• Single V3Si Target • Heater can reach up to 650 °C• Sapphire and copper substrates• Background pressure <1x10-8 mBar• Operating pressure 1.5 mTorr

* S. M. Deambrosis, G. Keppel, V. Ramazzo, C. Roncolato, R. G. Sharma, and V. Palmieri, “A15 superconductors: An alternative to niobium for RF cavities,” Phys. C Supercond. its Appl., vol. 441, no. 1–2, pp. 108–113, 2006.** V. Palmieri, “New Materials for Superconducting Cavities,” Proc. SRF 2001, no. 6, pp. 162–169, 2001.*** A. Anders, “A structure zone diagram including plasma-based deposition and ion etching,” Thin Solid Films, vol. 518, no. 15, pp. 4087–4090, 2010.

Setup

First a set of samples are sputtered in pulsed DC at roomtemperature (20 °C), 600 °C and 650 °C. Cross-sectional SEM of thedeposited films confirmed that increasing deposition temperatureincreases film density. Films were then analysed with VSM: resultsshown in Figure 2 showed that these denser films have a higher Tc.

The DC power supplied were then switched for HiPIMS. Three supplies wereused for this: a Starfire HiPIMS supply, an Ionautics HiPIMS supply and a‘Home-made’ supply from the University of Liverpool.Copper and sapphire substates are deposited at 600 °C and 650 °C. DC samplebias is also used to increase the energy that could be deposited into the films.

Figure5. Showing Sem images of the samples deposited on Cu, the EDX of the Starfire HiPIMS 650 C with 150 V bias Cu substrate is also shown.

Figure 6. Showing SEM of Starfire HiPIMS 650 °C with 150 bias sapphire substrate

Starfire HiPIMS 650 C with Cu substrate

Ionautics HiPIMS 650 C with Cu substrate

Home-made HiPIMS 650 C with Cu substrate

Cu Si V

Starfire HiPIMS 650 C 150 V bias with Cu substrate

EDX

Ionautics HiPIMS 650 °C with sapphire substrate

Home-made HiPIMS 650 °C with sapphire substrate

The VSM measurements showed that the films have a higher Tc when deposited by HiPIMS at the same temperature, with Tc between 10-14 °K. However, the Squid measurements indicate there are a significant number of defects in the film leading to flux pinning.

The SEM images of the Cu substrate shows the formation of non-mixed droplets around 1 µm in diameter. EDX in Figure 5 confirmed these droplets contain both Si and V, however the stoichiometry is not measurable. It is possible these are target droplets, caused by arcing. SEM images of the Sapphire deposited V3Si, shown in Figure 6, also show a rough surface

A new setup capable of depositing at temperature up to 700 °C was thendeveloped. SEM images of sapphire samples deposited at these highertemperature show a smoother surface, with smaller grain and have a TC upto 15.2 °K, measured by four point probe.

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Cu substrate 600 C

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Sapphire substrate RoomTemp.

Figure 1. Showing the deposition setup

Figure 2. Showing the Tc of the samples deposited using DC-MS as measured by VSM.

The TC of all films is significantlybelow that of the pure V3Si. XRD of600 °C sapphire substate sample isshown in Figure 3. The XRDconfirms that samples haveirregular crystal structure and non-ideal Stoichiometry.

Figure 3. XRD of the V3Si film sputtered on Sapphire at 600 °C compared to a reference V3Si sample (top). Lines indicating the presence of V3Si5 included.

To address this, we switch to HiPIMSsputtering .

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Figure 4. Showing the Tc of the samples deposited Via HiPIMS as measured by VSM with Squid Measurements taken at 4.2 K.

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Figure 7. Showing SEM images of V3Si on sapphire deposited around 700 °C using HiPIMS

Figure 8. Showing the TC of V3Si on sapphire deposited around 700 °C using HiPIMS

Figure 7 and 8 show that increasing the deposition temperature has led to smoother films with a higher TC.

AcknowledgementsThe authors gratefully acknowledge the Albert Crewe Centre for Electron Microscopy SEM SRF for their support & assistance in this research in addition we acknowledge contribution to this work by Dr. T. Sian of the Cockcroft Institute.

Future Plans are to further increase this temperature with the aim to increase film quality and Tc

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Starfire 650 C

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