This document discusses thin film techniques that could be applicable for superconducting radio frequency (SRF) cavities. It reviews various thin film deposition methods like sputtering, evaporation, and ion beam assisted deposition. Challenges in achieving high quality niobium films for SRF cavities are discussed, including issues like adhesion, purity, defects, grain size, stress. The document provides background on thin film nucleation and growth processes. It also summarizes some previous work done on niobium thin films at the College of William and Mary using DC magnetron sputtering and reactive sputtering.

1. A review of the thin film techniques
potentially applicable to cavities
Rosa A. Lukaszew, B. Burton, M. Beebe, William M. Roach,
College of William and Mary, Williamsburg, Virginia, USA
C. Clavero, LBNL
Grigory V. Eremeev, Charles E. Reece, Anne-Marie Valente-Feliciano, L. Phillips
Thomas Jefferson National Accelerator Facility [TJNAF], Newport News, Virginia,
USA

2. Why thin films?
In the 2012 workshop at Jlab L. Phillips pointed out:
• Cost is the main driver
– Thin film niobium cavity substrates are of castable metals, such as
copper or aluminum.
– Enables integration of many system functions in a single low-cost
structure, i.e. an aluminum casting.
– Integrated functions
– Cavity RF surface definition
– High thermal conductivity substrate
– Helium vessel
– Cryogenic manifold with heat exchanger
– Cavity stiffening, and many more……….
• Future applications of SRF linacs for which cost is a driver: ILC, or
LEP3, ADS, medical, light sources, ….

3. The Challenge of Nb-coated SRF Cavities:
Coatings have the promise of very substantial savings
• bulk Nb cavities are very expensive
• the gradient challenge: higher gradient
implies less cavities, significant savings
• potentially even greater savings with
cavities with Nb films
• so far mixed results for sputtered films
– adhesion issues
– low RRR
– Q-slope at high field
– breakdown even at relatively low field
typical bulk Nb cavities
best sputtered Nb films
on electropolished Cu
cavities (Benvenuti)
(Padamsee)
Fig. 4 of C. Benvenuti et al., Physica C:
Superconductivity 351, 421 (2001).
 Nb films on copper are considered a proven
technology for up to about 10 MV/m
 LEP (1996): 216 cavities with sputtered Nb,
6 MV/m with Q0 = 3.4 x 109 at 4.5 K
 the quality factor Q0 of magnetron-sputtered
cavities slopes down with
increasing RF electric field

4. Other advantages
• The micro-structure and contaminant levels of the RF
surface can –in principle- be controlled.
• Stable diffusion barriers can be added in situ impeding the
development of oxides or other forms of atmospheric
degradation after exposure to air.
• Other materials can be considered.
But:
• Practical issues that need attention:
– Control of film thickness over complex shapes
– Materials of relevance are compounds and techniques for
coating the interior of cavities while maintaining stoichiometry
and SRF proper ties within restricted limits of film thickness also
requires control.

5. Some History of thin films for SRF
cavities
• Since SRF is a surface phenomenon where only ~10
penetration depths are needed (=40 nm for niobium),
it was recognized for some time now that it would be
economically convenient to use thin film coated
cavities.
• Earlier attempts at CERN applied standard sputter PVD
methods, but the grain size for the CERN Nb/Cu films
was 100 nm, which is 10,000 times smaller than for
conventional SRF cavities with the ensuing problems
that appear at grain boundaries.
• Thus, these prior attempts showed higher surface
resistance and worst Q-slope than bulk.

6. Previous reviews
• A 2006* review by Sergio Calatroni of CERN discusses some
of the problems:
– Defects within 1 or 2  of the surface or on the surface.
Insufficient attention has been paid to this topic, including
trapping of impurities like oxygen in defects.
– The grain size for the CERN Nb/Cu films is 100 nm. This is 10,000
times smaller than for conventional SRF cavities, (for which grain
sizes are > 1 mm and are not important).Grain boundaries are
themselves one-dimensional defects. Grain boundary diffusion
is much faster than diffusion in the bulk Nb.
– Local thermal conductivity of the film itself may be poor
compared to bulk Nb.
– Interfacial thermal resistance, also known as thermal boundary
resistance, or Kapitza resistance at two interfaces: Nb/Cu and
Cu/LHe(II)
* Physica C: Superconductivity, Volume 441, Issues 1–2, 15 July 2006, 95–101

7. Coating Nb on SRF Cavities is promising but challenging
• Electroplating: not clean enough
• atomic layer deposition: promising but slow
• sputtering in UHV: low RRR, low Q
• filtered cathodic arc in UHV: tricky particle and geometry issues
• emerging: sputtering technology with ionization
 Film issues include
 adhesion
 purity
 defects (like substrate defects and
particulates)
 grain size and texture
 stress (intrinsic and thermal)
 thermal conductivity of base material
at cryogenic temperature can be better
than bulk niobium (copper!)
affecting SRF performance

8. Classify thin films
• Crystalline
– atoms show short and
long range order
• Polycrystalline
• Amorphous
– atoms show short range
order only
– Glasses; not stable state
for most pure metals;
generally less dense than
crystalline materials.
• Typical defects:
– grain boundaries
– Dislocations
– Point defects
– Surface roughness

9. Can we understand TF nucleation?
• Nucleation from a liquid phase to a solid depends on:
– Liquid phase instability (going through a phase change
from higher to lower T)
– Diffusion of atoms into clusters (increases with T)

10. Film formation
• Competing Processes
• adding to film:
– impingement
(deposition) on surface
• removing from film:
– reflection of impinging
atoms
– desorption (evaporation)
from surface
• Steps in film formation:
1. thermal
accommodation
2. binding
3. surface diffusion
4. nucleation
5. island growth
6. coalescence
7. continued growth

11. How do nuclei grow initially?

12. From kinetic theory of gases
• How many gas
molecules collide with a
surface each second ?
• How long does it take to
form a complete layer
of gas on a surface?
pressure tm
1 atm 2 x 10-9 sec
10-6 torr 2 seconds
10-9 torr 31 minutes

13. Contamination
• PROBLEM: residual gas
in chamber gives two
"sources" impinging
• evaporant:
• residual gas:
Impurity concentration
SOLUTION:
• better vacuum
• higher deposition rate

14. Sputter deposition
• target atoms and ions impinge
• electrons impinge
• Ar atoms impinge
– Ar pressure about 0.1 torr
• Ar may be incorporated into film
• energetic particles may modify
growth
• substrates heat up

15. Variations
• Ion assisted deposition (IBAD)
– with evaporation or sputtering (or
chemical vapor deposition)
• bombard surface with ions
– not necessarily same type as in film
• ions typically NOT incorporated in
film
• relatively low voltages (50 - 300 eV)
• leads to
– physical rearrangement
– local heating
• can change film properties
– for better or worse
• disruption of columnar (fiber) growth
requires about 20 eV of added
energy per depositing atom
• Reactive Sputter deposition
• add reactive gas to chamber during
deposition (evaporation or
sputtering)
– oxygen, nitrogen
• chemical reaction takes place on
substrate and target
• can poison target if chemical
reactions are faster than sputter rate
• adjust reactive gas flow to get good
stoichiometry without incorporating
excess gas into film.

16. Arc
• high current, low voltage
discharge initiate by
touching electrode surfaces
and then separating trigger
arc by high voltage
breakdown
• produces large numbers of
electrons
• very efficient ionization of
film atoms (almost 100 %)
• impinging ions may be high
energy
– enhanced chemical reactions
– film densification

17. Plasma sources
• plate electrodes
– low plasma densities (109 - 1010
charged particles per cm3)
– common in sputter deposition
• Inductively coupled plasma
(ICP)
– high plasma densities (1011 -
1012 charged particles per cm3)
– operates well at lower gas
densities (< 50 mTorr)
– can be used up to atmospheric
pressures (and beyond)
– couple RF energy inductively
into plasma (lossy electrical
conductor)
– produces more efficient
ionization
• Electron cyclotron resonance
(ECR)
– high plasma densities (1012 -
1013 charged particles per cm3)
– operates well at lower gas
densities (down to 0.1 mTorr)
– couples microwave energy to
electrons by matching
frequency to electron gyration
frequency
– produces more efficient
ionization
– control the plasma density with
microwave power and gas
pressure
– can also control ion species
created.

18. A note on metallic thin films
• The properties of thin films
depend on their
microstructure.
• The stability of thin metal films
depends on being deposited
on appropriate substrates.
• Important characteristics are
residual stress and strain,
which often develop in film-substrate
combinations An
unfavorable consequence of
high stress is crack formation,
local plastic deformation, and
layer delamination.
• Residual stresses, which are
commonly assumed to be
biaxial in thin films, result from
different thermal expansion
coefficients of substrate and
film (thermal stress) and/or
from stress formation during
film deposition (grown-in
stress)
• In polycrystalline films a
central mechanism that
governs stress relaxation by
inelastic deformation is
thought to be atomic
diffusion, predominantly along
grain boundaries.

19. Examples
• Quantitative, quasisimultaneous in situ
characterizations of the modification of
vacancy concentration and of residual
strain in metallic films have been carried
out for particular cases (e.g Pt thin films,
PRL 107, 265501, 2011).
• This work was based on based on x-ray
scattering techniques. This has the
advantage that the use of synchrotron
radiation becomes possible, which
allowed to carry out time-resolved
studies to measure fast relaxation
processes taking place on a time scale of
minutes.
• In order to detect directly the
modification of the vacancy
concentration, x-ray diffractometry (XRD)
was used to determine the of the out-of-plane
lattice parameter a and x-ray
reflectivity (XRR) was used to detect the
film thickness.

20. SRF Thin film coating approaches
• CVD (L. N. Hand, Cornell, USA) and ALD (T.
Proslier, ANL) have been explored.
• A hybrid physical-chemical vapor deposition
(HPCVD) technique at Temple University has
resulted in optimal MgB2 films.
• Energetic PVD processes such as ECR (A.-M.
Valente-Feliciano, Jlab; private companies),
HiPIMS (A. Anders, LBNL) vacuum-arc (R. Russo)
have also been reported as suitable techniques
for this application.

21. Opportunities for energetic PVD Niobium Films
• Two major limitations of conventional magnetron
sputtering are:
– Low deposition energy of arriving atoms limiting control of film
structure
– Presence of argon gas being incorporated into the growing film
in addition to spatial limitations of the plasma
• Both issues are eliminated by using niobium ions in vacuum
to deposit films.
– Control of film microstructure through energetic condensation
– High adatom surface mobility
– Sub-plantation
– Grain competition driven by incident ion energy selective
sputtering, channeling, surface energy of crystal face

22. Energetic Condensation
• Energetic condensation is a deposition process where a significant fraction of the condensate
has hyper‐thermal energies (energies 10 eV ). A number of surface and subsurface
processes are activated or enabled by the energy of the particles arriving at the surface (e.g.
desorption of adsorbed molecules, enhanced mobility of surface atoms, and the stopping of
arriving ions under the surface). The purpose of using energetic condensation deposition
methods is to improve film structure while keeping the substrates at lower temperature by
adding energy to the film during condensation to compensate for lack of thermally induced
growth processes.
• For example crystalline defects, grains connectivity and grain size may be improved with a
higher substrate temperature that provides higher surface mobility. However the substrate
used may not allow substantial heating and in such case the missing energy may be supplied
by ion bombardment. In bias sputter deposition a third electron accelerates the sputtering
gas ions, removing the most loosely bound atoms from the coating, while providing
additional energy for higher surface mobility
• One possible process, ion beam assisted deposition (IBAD), uses a secondary source of ions
to co‐bombard the film from conventional sources during growth.
• A second process, direct ion deposition, uses vacuum plasmas formed from the material
being deposited to produce a film grown from metal ions.

23. Approach:
Use a plasma-based technology for “Energetic Condensation”
A. Anders, Thin Solid Films 518 (2010) 4087
Generalized Structure zone Diagram (2010),
derived from Thornton’s diagram for
sputtering (1974)

24. High Power Impulse Magnetron Sputtering
( a form of IBAD)
Copper target
2” magnetron
A. Anders(LBNL)

25. Illustration of Self-sputtering
target

Self-sputtering runaway
    1


substrate
ions to substrate atoms to substrate
Sustained self-sputtering
    1
Probability
for ions
to return to
the target
Ionization
probability
yield
adapted from: A. Anders, J. Vac. Sci. Technol. A 28 (2010) 783

26. Our work using DC magnetron sputtering
and reactive sputtering at W&M
• We have investigated the
effect of microstructure and
morphology on the
superconducting properties
of Nb thin films deposited
onto different ceramic
surfaces and metallic
surfaces.
• In particular we studied a-plane
sapphire and (001)
MgO and Cu (001).
• We monitored the
microstructure of the films,
the morphology of the
surface and the
superconducting properties
as well as the DC properties.
• We explored several aspects
in the thin film deposition
parameters-space, such as
growth rate, substrate
temperature during growth,
annealing treatments, etc.

28. Nb growth on a-plane sapphire
• Nb can grow epitaxially on a-plane sapphire, with Nb(110)//Al2O3(11-20)
Comparison of RRR values obtained by different groups:
Group Nb film thickness
(nm)
RRR
Lukaszew 600 97
S. A. Wolf [1] 600 82
G. Wu [2] 235 50.2*
* RRR values for niobium thin films is highly dependent on thickness
[1]. S. A. Wolf et al., J. Vac. Sci. Tecnol. A 4 (3), May/June 1986
[2] G. Wu et al., Thin Solid Films, 489 (2005) 56-62

29. Early stages of growth
Nb thickness (nm)
[111]Nb ll[0001]Al O 2 3
bulk Nb bcc
hcp Nb
[1120]Nb ll
[0001]Al O 2 3
1 10 100
0.36
0.35
0.34
0.33
0.32
0.31
0.30
0.29
0.23 2.3 23
Lattice parameter (nm)
Nb atomic layers
a
bcc Nb
hcp+bcc Nb
a
• Using Reflection high energy
electron diffraction (RHEED), we
observed a hexagonal Nb
surface structure for the first 3
atomic layers followed by a
strained bcc Nb(110) structure and
the lattice parameter relaxes after
3 nm.
• RHEED images for the hexagonal
phase at the third atomic layer.
Patterns repeat every 60 deg.
0 deg 30 deg 60 deg

30. Susceptibility AC measurements
• The thinner Nb film exhibits two
steps in the χ’ susceptibility
transition accompanied by two
peaks in the χ’’ susceptibility due
to strained Nb layers at the
interface.
• Growth on a-plane sapphire
initially follows a hexagonal
surface structure to relax the
strain and to stabilize the
subsequent growth of bcc
Nb(110) phase.
• Such initial layers affect the
superconducting properties of
the films and these effects must
be taken into account in the
design of multilayers.
0.1
0.1
0.0
0
0
-1
0.0
-1
7 8 9 10
0.2
0.0
30 nm
100 nm
7 8 9 10
0
-1
''
'
600 nm
Temperature (K)
Temperature (K)
χ(ω)= χ’(ω)+i χ’’(ω)
Strain Effects on the Crystal Growth and Superconducting Properties of Epitaxial Niobium Ultrathin Films, C. Clavero, D. B. Beringer, W.
M. Roach, J. R. Skuza, K. C. Wong, A. D. Batchelor, C. E. Reece, and R. A. Lukaszew, Cryst. Growth Des., 12 (5), pp 2588–2593 (2012)

31. ( a ) 30 nm Nb
200 nm
( b ) 100 nm Nb
200 nm
( c ) 600 nm Nb
400 nm
30
20
10
0
Al 2O3[1100]
600 nm
100 nm
30 nm
Al2O3[0001]
0 500 1000
heigth (nm)
distance (nm)
( d )
Nb [110]
N b [ 001]
Biaxial anisotropy is observed for thicknesses up
to 100 nm while uniaxial anisotropy is observed.
For thicker films

32. Nb growth on (001) MgO
• Nb can also be
epitaxially grown on
(001) MgO surfaces.
• Unexpected findings:
We have found that
depending on the
deposition conditions
it is possible to tailor
different epitaxial
possibilities.

33. RHEED images for Nb(110) on MgO

34. Scaling of surface features
50 nm 600 nm
RRR = 46.5
RMS = 6.51 nm

35. Nb (001) on MgO
14.29 nm
0.00 nm
400nm
RRR = 165 RMS = 4.06 nm
>200 RRR values!

36. 30.00 nm
0.00 nm
1.0μm
RMS = 2.90 nm
10.00 nm
0.00 nm
400nm
10.00 nm
0.00 nm
200nm
RMS = 1.21 nm
RMS = 1.08 nm
D. B. Beringer, W. M. Roach, C. Clavero, C. E. Reece, and R. A. Lukaszew, "Roughness analysis applied to niobium thin films
grown on MgO(001) surfaces for superconducting radio frequency cavity applications," Phys. Rev. ST Accel. Beams 16,
022001 (2013).

37. 4 5 6 7 8 9 10
0.0
-0.2
-0.4
-0.6
-0.8
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
"
Temperature (K)
-1.0
4 5 6 7 8 9 10
'
SQUID characterization
Tc = 9.2 K!
Possible loss
due to interfacial
strain

38. Nb on Cu (111)
• Growth at room temperature and annealing at
350 ºC leads to the crystallization of Nb islands
in a hexagonal surface structure, even though
Nb is expected to growth tetragonal (110).
3.3 Å
3.30 Å
0.00 Å
0 Å
Cesar Clavero, Nathan P. Guisinger, Srivilliputhur G. Srinivasan, and R. A. Lukaszew, “Study of Nb epitaxial growth
on Cu(111) at sub-monolayer level”, J. Appl. Phys. 112, 074328 (2012).

39. Nb films on Cu (001) surfaces
(a) RHEED pattern for Nb(110)/Cu(100)/Si(100)
along the Si[100] and Si[110] azimuths. (b) A
representative 2 μm x 2 μm AFM scan for Nb
films on the Cu template.
Possible Nb/Cu(100) epitaxy:

41. SC properties for different growth T
• The films grown at 150 °C have
a very sharp transition from
the superconducting state to
the normal state that begins at
~9 K while films grown at RT
have a much more gradual
transition.
• Our results suggest that an
increased deposition
temperature of Nb onto Cu
leads to films with higher
crystalline quality (grain size)
and thus improved
superconducting properties
(HC1).
Niobium thin film deposition studies on copper surfaces for superconducting radio frequency cavity applications, W. M.
Roach, D. B. Beringer, J. R. Skuza, W. A. Oliver, C. Clavero, C. E. Reece, and R. A. Lukaszew, Phys. Rev. ST Accel. Beams 15,
062002 (2012).

42. Characterization
1. Property that matters (e.g. SRF impedance,
Q, etc)
2. Correlation with microstructure, surface
morphology, DC transport (RRR) and DC
magnetic properties (Hc1)

43. What do we want to know ? How do we find this out ?
What does the sample look like ?
•on a macroscopic scale
•on a microscopic scale
•on an atomic scale
•optical microscopy
•scanning electron microscopy (SEM)
•transmission electron microscopy (TEM)
•scanning probe microscopies (STM, AFM ...)
What is the structure of the sample ?
•internal structure
•density
•microscopic and atomic scales
•X-ray diffraction (XRD)
•low energy electron diffraction (LEED)
•reflection high energy electron diffraction (RHEED)
What is the sample made of ?
•elemental composition
•impurities
•chemical states
•Auger Electron Spectroscopy (AES)
•Energy Dispersive Analysis of X-rays (EDAX)
•X-ray Photoelectron Spectroscopy (XPS)
•Secondary Ion Mass Spectrometry (SIMS)
•Rutherford Backscattering (RBS)
What are the optical properties of the sample ?
•refractive index, absorption
•as a function of wavelength
•ellipsometry
What are the transport properties of the sample?
• resistance
• Surface impedance
•resistance - four point probe
•SIC
What are the mechanical properties of the sample ?
•internal stress in films / substrates
•adhesion
•stress curvature measurements
•adhesion tests

44. Important
• What exactly are we
probing?
– E.g. XRD typically probes
films in the growth
direction. It provides
average microstructure
information.
– E.g. the grain size extracted
from the width of peaks is
along the z-direction.
– RHEED, LEED, TEM provide
local microstructure
information.
– E. g. SEM provides coarser
information regarding
surface morphology than
AFM/STM.
– Optical techniques
(ellipsometry) can provide
information regarding
density of the films.
– It is important to correlate
more than one technique
for complementary
characterization and
acquire a more complete
description of the sample.

45. Good prospects for next SRF films:
Energetic condensation (ECR, HiPIMS)
• As a result of these fundamental
changes, energetic condensation
allows the possibility of
• controlling the following film
properties:
• the density of the film may be
modified to produce improved
optical and corrosion-resistant
coatings
• the film composition can be
changed to produce a range of
hard coatings and low friction
surfaces
• crystal orientation may be
controlled to give the possibility
of low‐temperature epitaxy.
• The additional energy provided by
fast particles arriving at a surface can
induce the following changes to the
film growth process:
– residual gases are desorbed from
the substrate surface
– chemical bonds may be broken
and defects created thus
affecting nucleation processes
and film adhesion
– film morphology changes
– microstructure is altered
– stress in the film is altered

47. Summary
• Higher energetic
condensation offers the
most promise for better
performing SRF films.
• There is sufficient
evidence of better
addition as well as
conformal growth.
• Still needs more R&D to
achieve real “bulk-like”
films!