A brief discussion of Conventional Sputtering and Energetic

Condensation for Superconducting Cavity Applications

L. Phillips, D. Bowring, C. Reece, J. Spradlin, Anne Marie Valente-Feliciano, and Xin Zhao

Motivation

Why do we need to develop thin niobium films with bulk-like performance?

• Total Linac Cost Reduction over linacs with bulk Nb cavities

• High Hc₁- best choice for the first SC Film in a multi-layer structure

Current Status of Sputtered Nb/ Cu Cavities

• Thin film niobium on copper cavity substrates have been successfully developed but have not been used in β=1 applications since LEP.

• This is only due to their well known increase in surface impedance with increasing surface field.

Figure 1

• Since LEP, this behavior has been studied extensively both at CERN and INFN with single cell cavities at 1.5 Ghz, and at other laboratories using strip-line resonator techniques

• All causes not known with certainty but nonlinear behavior has been correlated with increased total defect density in terms of electron scattering (RRR) leading to depressed Hc₁ and vortex entry.

Microstructure and Defect Density in Sputtered Films

Defect Sources:•Impurity scattering from background impurities and sputtering gas

-In CERN studies, somewhat better high field performance was obtained with Krypton than Argon raising the film RRR from 18 to 27

Figure 2

Grain boundary scattering

Two film structures explored:

1. fiber structure-

grain size ~ 0.1µ

2. heteroepitaxy-

grain size ~ 1µ

Figure 3: Cross sectional images of niobium films on oxidised (left) and oxide-free (right) copper substrates

• both are columnar with RRR= 11.5 and 29– not due to grain boundary scattering alone

• Intra- grain defect density– dislocations, impurities, voids, etc. due to shadowing

• Surface Roughness

- field enhancement

• Geometrical Effects

- angle of incidence

What We Do Not Know About Sputtered Niobium Cavity Films

• How does the RRR vary through the thickness of the film?

• What is the RRR within the penetration depth?

• What is the RRR variation through the grain for various copper niobium lattice mismatches with heteroepitaxial films?

• Global surface roughness on a prepared copper cavity surface prior to sputtering

Energetic Condensation

• Niobium-Films are deposited in vacuum from niobium ions at controlled energies.

Advantages

- control of films energy gives better control of film microstructure

- near normal angle of incidence

- impurity control in vacuum is easier to control than in a gaseous process

- simultaneous disposition at multiple energies

• Adding Energy to the condensing niobium atoms produces a denser, more ordered film structure with higher RRR.

• Things to consider with energetic condensation of niobium films.

- two common structures: 1. Heteroepitaxy 2. fiber structure

• In each of the CERN examples the defect density is substantially greater than that from grain boundary scattering alone.

What we might expect?

Figure 4

• low RRR near substrate

1. nucleation layer

2.fiber films small grains at substrate and growth through grain

3.heteroepitaxal films- misfit

dislocations

• for heteroepitaxy we need to know

the surface RRR for various

Nb-Cu grain combinations

What do we need to know?• maximum RRR in the film as a function of ion

energy for both film structures

• curve may differ for differing degrees of lattice mismatch in the case of heteroepitaxy

• How does a large degree of lattice mismatch affect RRR at the RF surface?

• How does the local RRR vary from substrate to RF surface for energies of interest?

  Figure 5

Discussion

The primary focus at JLAB is to understand the relationship between deposition process, film microstructure and RF performance. Activities are largely organized initially toward reliably producing clean films (RRR and small sample RF measurements as a metric) and carried out with a group of collaborating universities and laboratories which is well underway and shedding much light on the issues raised above.

In Particular:

• a large array of samples has been produced with niobium ion deposition in vacuum with a variety of ion energies and substrate materials representing different lattice mismatch conditions.

• defect densities have been measured as global RRR values for the whole film as high as 213

• film texture and micro structural properties have been measured using EBSD and XRD

• RRR values as high as 114 have been achieved on lattice mismatched surfaces.

 

• films have been deposited on substrates of single crystal copper with selected orientations

in order to understand the properties of a polycrystalline niobium film on a practical cavity substrate.

These topics are from the talks by Anne Marie Valente, Joshua Spradlin, and Krishnan Mahadevan in this workshop.

• The initial emphasis has been on heteroepitaxial deposition on clean crystalline substrates but the results obtained on ultimate defect density within the grain apply to the fiber growth mode as well. This mode is of interest and was used by CERN on oxidized copper substrates without the benefit of lower levels of intra grain defects using currently available techniques. The advantages of each mode as to desirability for cavity fabrication will be known when the small sample studies are near completion.

• We have now clearly demonstrated that the reduction in defect density by energetic condensation over conventional sputtering is substantial. This is a basis for confidence that niobium film cavities without Q slope can be produced.