Anne marie valente feliciano - nucleation of nb films on cu substrates

A-M Valente-Feliciano
A. Lukaszew (College William & Mary)
L. Phillips, C. Reece, J. Spradlin (JLab)
Nucleation of Nb on Cu

 Thin Film Nucleation overview
 The Nb – Cu system
 Hetero-epitaxy
 Fiber growth
 Conclusions
Outline
A-M Valente-Feliciano - TFSRF 2014 - 07/10/2014

Nucleation: Why do we care?
The thickness of interest for SRF applications corresponds to the RF
penetration depth, i.e. the very top 40 nm of the Nb film.
However the final surface is dictated from its origin, i.e. the
substrate, the interface, and deposition technique (ion energy,
substrate temperature…)
Heterogeneous nucleation.
Nucleation driven by nucleation
centers such as defect, impurities on
the substrate surface or the
orientation of the underlying
substrate in the case of hetero-epitaxy.

Crystalline Cu vs. Amorphous CuO Substrate
CERN magnetron sputtered 1.5GHZ Nb/Cu films (coated with Ar)
Standard films Oxide-free films
RRR 11.5 ± 0.1 28.9 ± 0.9
TC 9.51 ± 0.01 K 9.36 ± 0.04 K
Ar cont. 435 ± 70 ppm 286 ± 43 ppm
Texture -110 (110), (211), (200)
Hc1 85 ± 3 mT 31 ± 5 mT
Hc2 1.150 ± 0.1 T 0.73 ± 0.05 T
a0 3.3240(10)Å 3.3184(6) Å
a/a 0.636 ± 0.096 % 0.466 ± 0.093 %
Stress -706 ± 56 MPa -565 ± 78 MPa
Grain size 110 ± 20 nm > 1 μm
Columnar grains, size ~ 100 nm
In plan diffraction pattern: powder
Standard Oxide-free
CI=
CI
=
0.5 μm 0.5 μm
diagram
(110) fiber texture  substrate plane
Equi-axed grains, size ~ 1-5mm
In plan diffraction pattern: zone axis [110]
Heteroepitaxy
Nb (110) //Cu(010) , Nb (110) //Cu(111),Nb
(100) //Cu(110)
Courtesy of CERN, P. Jacob,FEI
Oxide –free films closer to bulk but Rres, standard < Rres, oxide-free

Film Nucleation & Growth
Thin film growth from the gas phase=non-equilibrium process phenomenon governed by a competition
between kinetics & thermodynamics. 3 stages can be distinguished in the nucleation & growth of films:
• Production of ionic, molecular or atomic species in the gas phase.
• Transport of these species to the substrate
• Condensation of the species onto the substrate directly or either by chemical or
Condensation from the vapor involves incident atoms becoming bonded adatoms which
diffuse over the film surface until they are trapped at low energy lattice sites. This atomic
odyssey involves 4 basic processes:
Shadowing
Surface diffusion
Bulk diffusion
Desorption
The last 3 are quantified by the characteristic diffusion and sublimation activation energies
(scaling with the melting point). Shadowing arises from the line of sight impingement of
arriving toms. The dominance of one or more of these processes as function of substrate
temperature is manifested by different structural morphologies.
electrochemical reaction.

Hetero-epitaxy of Nb on Cu
DC magnetron sputtered @ 150 °C
Lukaszew A. et al. , W&M
bcc Nb on fcc Cu system
hetero-epitaxial relationships:
[110]Nb || [100]Cu 4 domains
[100]Nb || [110]Cu
[110]Nb || [111]Cu 6 domains
[W. M. Roach et al. , PRSTAB 15, 062002 (2012)]
[Masek &Matolin, Vacuum 61(2001) 217-221]
Growth of Nb on CuO
The Cu oxide (CuO) is amorphous, the Nb growth is then not driven by the substrate orientation

Nb Film Nucleation at RT
Nb/Cu hetero-epitaxy by MBE at RT
A. Lukaszew et al. , W&M
Vacuum ~10-11 Torr
Substrate cleaned by in-situ Ar + etching and
annealing @600 °C
Topography STM maps of Nb islands deposited on
Cu(111) substrates at 300 K (RT) with coverages from
0.1 to 0.4 AL. Randomly distributed 2 AL high islands
primarily observed on substrate and close to the
terrace edges at very low coverage..
Irregular shaped islands → amorphous
microstructure at low temp. (RT)
[Study of Nb epitaxial growth on Cu (111) at sub-monolayer
level) C. Clavero et al., JAP 112, 074328
(2012)]

Annealing at 350 °C
STM/STS studies-Proximity effects
Annealing @ 350
°C leads to
rearrangement of
Nb atoms into
crystalline islands.
Atomic resolution
topography STM
images of islands
with hexagonal and
rhomboidal shape

Annealing at 600 °C
Further annealed Nb islands, which exhibit hexagonal and triangular shapes, with two
distinctive heights, namely 1 and 2 AL

MBE Growth @ 350 °C
Topography STM map for a nominally 1 AL
thick Nb film / Cu (111): 3D Volmer-Weber
growth mode with islands up to 3 AL height
훾푓,푁푏 2.983 퐽. 푚−2 > 훾푠,퐶푢 1.934 퐽. 푚−2
Lattice mismatch 9%
Further annealing @ 600 °C leads to
coalescence to larger islands with
reconstruction on their surface

Film Nucleation
Molecular Dynamics Simulation
large-scale atomic/molecular massively parallel
simulator (LAMMPS) code Visualization with
OVITO.
Initially circular Nb nano-islands
→Hexagonal shape as observed in
experiment
With 350 °C annealing
Higher temperatures → intermixing
[C. Clavero, M. Bode, G. Bihlmayer, S.
Bl€ugel, and R. A. Lukaszew, Phys.
Rev. B 82(8), 085445 (2010)]

Energetic Condensation
Condensing (film-forming) species : hyper-thermal & low energies (>10 eV).
Additional energy provided by
fast particles arriving at a surface
⇒number of surface & sub-surface
processes ⇒changes in
the film growth process:
Generalized Structure Zone Diagram
 residual gases desorbed from the
derived from Thornton’s diagram for sputtering
(1974)
substrate surface
 chemical bonds may be broken and
defects created thus affecting nucleation
processes & film adhesion
 enhanced mobility of surface atoms
 stopping of arriving ions under the
surface
 morphology
 microstructure
 stress
⇒ Changes in
A. Anders, Thin Solid Films 518 (2010) 4087
As a result of these fundamental changes, energetic condensation allows the possibility of
controlling the following film properties:
 Density of the film
 Film composition
 Crystal orientation may be controlled to give the possibility of low-temperature epitaxy

Nb on Cu single crystals
RRR=88 RRR=76 RRR=242
In the same run, Nb/fine grain Cu RRR=82
Nb/large grain Cu RRR=169
Structural Properties of Niobium Thin Films Deposited on Metallic Substrates by ECR Plasma Energetic Condensation
Joshua K. Spradlin, Anne-Marie Valente-Feliciano, Larry Phillips, Charles E. Reece, Xin Zhao (JLAB, Newport News, Virginia), Kang Seo (NSU, Newport News), Diefeng Gu (ODU, Norfolk,
Virginia) - to be submitted PRST-AB
A-M Valente-Feliciano - AVS Conference 2011– Nashville TN,11/01/2011

Nb films on polycrystalline Cu substrates
RRR=169 RRR=82
A-M Valente-Feliciano - TFSRF 2012, 07/18/2012
Nb/large grain Cu
(substrate heat treated ex-situ @ 1000°C, 12h)
Nb/fine grain Cu

150 x 150 μm, 1 μm
resolution, CI Avg. 0.71
Effect of Bias Voltage for Nb/Cu
120 x 150 μm, 1 μm resolution,
CI Avg. 0.23
50 x 75 μm, 1 μm resolution,
CI Avg. 0.16
Bias -120V, 2mm
Bias 0V , 4mm
Typical Cu substrate
Ab-normal growth
vith bias
vs.
columnar growth
without bias

Fiber Growth
1 0 0 n m
LAADF
Nb Cu
CL=12CM
ABF
Nb Cu
5 n m
interface
T bake = 200 °C
T coating = 200 °C
ENb ions = 64 eV
Thickness = 1.4 μm
RRR = 21
Tc= 9.41 ± 0.16 K

Fiber Growth
Nb
1 0 0 n m
Cu
LAADF
2 0 n m
Nb Cu
CL=12CM
ABF
2 nm
Nb
Amorphous
interface
Cu
T bake = 200 °C
T coating = 200 °C
ENb ions = 184 eV then 64 eV
Thickness = 1.7 μm
RRR = 15
Tc= 9.46 ± 0.19 K
Δ=1.71 meV

Spatial Drift
Spectrum Image
1 0 n m
EELS plot for Cu/Nb signal across interface
11.5 nm
Interface thickness
(e-1 of highest density)
Incident ion energy: 64 eV
Nb: 7.6 nm
Cu: 8.9 nm
Fiber Growth

Hetero-epitaxy, low Tcoating
ABF
2 n m
interface
CL=12CM
Continuous crystalline
Nb Cu
1 0 0 n m
Nb Cu
CL=12CM
T bake = 200 °C (long bake)
T coating = 200 °C
ENb ions = 184 eV
Thickness = 1.8 μm
RRR = 58
Tc= 9.43 ± 0.13 K
Δ=1.56 meV

Low versus high energy
Nb Cu
0V (64 eV)
120 V (184 eV)
Interface thickness
Ion energy 64 eV:
Nb: 3.89 nm
Cu: 1.89 nm
Ion energy 184 eV:
Nb: 3.67 nm
Cu: 7.33 nm
86: 6 nm
87: 11.4 nm

Hetero-epitaxy, High Tcoating
Nb Cu
2 0 n m
interface
CL=12CM
T bake = 500 °C
T coating = 360 °C
ENb ions = 184 eV
then 64 eV
Very thick film
Thickness = 4.5 μm
RRR = 305
Tc= 9.37 ± 0.12 K
Δ=1.53 meV (1.38
meV?)

Spectrum Image
5 0 n m
interface
Interface thickness
Nb: 12.5 nm
Cu: 20.1 nm
31.8nm
1 0 n m
Continuous crystal
interface

Substrate roughness and defects
Whatever the inherent nature of the film, the roughness of the substrate will dictate the
minimum roughness of the film (the final roughness depends as well on the coating
technique and other refinements).
Any defect (scratch, pin-hole) is duplicated and enhanced in the film as it grows.
E-beam evaporated Pt
Nb
Cu
Ion beam coated Pt
Nb
Cu
Section cut by FIB

CONCLUSIONS
 First atomic layers of Nb film constitute an adaptation layer to
the substrate, (differs from relaxed cubic Nb structure) , a
template for subsequent growth of more or less “relaxed” Nb
film.
Deposition at high energy : sub-implantation , enhanced with
higher temperatures (on-going studies)
Interface engineering : introduction of interlayers, buffer layer
to “erase the influence “ of the substrate (nature, grain
boundaries…)

Nb on Cu substrates
Influence of energy as function of substrate nature,
coating temperature
Nb/ single crystals Cu Nb/ large grain Cu Nb/ large grain Cu
Nb(100) always higher RRR
Nb/large grain Cu can achieve higher RRR due to underlying relaxed (heat treated) substrate
For Nb/fin grain Cu, less variation in RRR as preferred orientation (110)

Nb/a-Al2O3 - Early stages of growth
• 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.
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
0 deg 30 deg 60 deg
A-M Valente-Feliciano - SRF Conference 2011– Chicago, 07/26/2011

Hetero-epitaxy
184 eV continuous
500 C
360 C
RRR= 182
Tc = 9.39 ± 0.08 K
Δ = 1.62 meV

Zone Structure Model
• Zone 1
-lack of surface mobility
-random direction of incoming
vapor atoms
-shadowing
 loose fibrous structure, voids, porosity
• Zone T - transition between Zones 1
and 2 (Thornton)
- more tightly packed fibrous
grain structure but not fully dense
• Zone 2 - Ts ~ < 0.3 Tm - fully dense
columnar grain structure with long
columns extending from substrate to film
surface
• Zone 3 - Ts ~ > 0.45 Tm – no longer
columnar –recrystallized with random
orientation
J. A. Thornton, J. Vac. Sci.
Technol.11, 666, 1974

Thin Film Growth Modes
(i) 3-D or island growth mode, also known as Volmer–Weber (VW) mode
The adatoms have a strong affinity with each other and build 3-D islands that grow in all directions,
including the direction normal to the surface. The growing islands eventually coalesce and form a
contiguous and later continuous film.
(ii) 2-D or layer-by-layer growth, also known as Frank–van derMerwe (FVDM) mode
The condensing particles have a strong affinity for the substrate atoms: they bond to the
The film nucleation depends first and foremost on the nature of the material
deposited (metal... )
Niobium as most metals usually grows in the island mode.
substrate rather than to each other.
(i) a mixed mode that starts with 2-D growth that switches into island mode after one or more monolayers;
this mode is also known as the Stranski–Krastanov (SK) mode.

Effect of ion energy on film growth
A. Anders / Thin Solid Films 518 (2010) 4087–4090
Kinetic energy of arriving positive ions - an initial component from the plasma, E0
- a change due to acceleration in the sheath,
Ekin=E0+QeVsheath where Q =ion charge state number, e =elementary charge, and Vsheath is the voltage
drop between plasma and substrate surface.
Non-penetrating ions (or atoms) in the film bulk
 Promotion of surface diffusion of atoms.
 Between the surface displacement energy and bulk displacement energy: epitaxial
growth is promoted because no defects are created in the film bulk .
 Atomic displacement cascades if Ekin > Ebulk displacement, (12–40 eV)
Penetrating particles :
very short (∼100 fs) ballistic phase with displacement cascades followed by a thermal spike
phase (∼1 ps) (mobility of atoms in the spike volume very high) ~ transient liquid
large amplitude thermal vibrations still facilitate diffusion (migration of interstitials inside
grains & adatoms on the surface).
The driving force is the gradient of the chemical potential, leading to minimization of volume
free energy and surface free energy density with contributions of interface and elastic strain
energies and often resulting in a film where grains have a preferred orientation.
As Ekin increase, e.g. by biasing, the sputtering yield is increased and the net deposition rate is
reduced. Film growth ceases as the average yield approaches unity, for most elements
between 400 eV and 1400 eV
 Surface etching as Ekin is further increased.
Incident ion energy
L. Hultman, J.E. Sundgren, in: R.F. Bunshah (Ed.), Handbook of Hard Coatings, Noyes, Park Ridge, NY, 2001, p. 108.
D.K. Brice, J.Y. Tsao, S.T. Picraux, Nucl. Instrum. Methods Phys. Res. B 44 (1989) 68.
G. Carter, Phys. Rev. B 62 (2000) 8376.
M.M.M. Bilek, D.R. McKenzie, Surf. Coat. Technol. 200 (2006) 4345.
D.R. McKenzie, M.M.M. Bilek, Thin Solid Films 382 (2001) 280
W. Eckstein, Computer Simulation of Ion-Solid Interactions, Springer-Verlag, Berlin,1991.
D.K. Brice, J.Y. Tsao, S.T. Picraux, Nucl. Instrum. Methods Phys. Res. B 44 (1989) 68.

Effect of ion energy and substrate temperature
Energetic particle bombardment promotes competing processes of defect
generation and annihilation.
Kinetic energy → displacement and defects followed by re-nucleation
Release of potential energy & post-ballistic thermal spike → atomic
scale heating, annihilation of defects.
- Epot/Ekin per incident particle as well as the absolute
value of the kinetic energy will shift the balance and
affect the formation of preferred orientation and
intrinsic stress .
Maximum of intrinsic stress for Ekin ~100 eV; the actual
value depends on the material and other factors.
insertion of atoms under the surface yet still very little
annealing .
At higher temperature (higher homologous temperature or temperature
increase due to the process itself) the grains are enlarged because the increase
of adatom mobility dominates over the increased ion-bombardment-induced
defects and re-nucleation rates.
All energy forms brought by particles to the surface will ultimately contribute
to broad, non-local heating of the film and shift the working point of process
conditions to higher homologous temperature.
[M. M. Bilek, D. R. McKenzie “A
comprehensive model of stress generation
and relief processes in thin films deposited
with energetic ions” Surf. And Coat.
Techno. 200 (2006) 4345-4354]

Energetic Condensation Processes
Atomic Scale Heating
Each ion delivers significant kinetic and potential energy, and both
contribute to what may be called atomic scale heating (ASH).
Sub-implantation
- Energy transfer to knock-on atoms ~10E-13 sec.
- Collisional cascade, thermalization ~10E-11 sec.
- Fills sub-surface voids
At high energies the rate of defect creation can exceed defect
annealing
Secondary Electron Emission
Emitted secondary electrons are in the same electric field of the
sheath that accelerated the (positive) ions, but now it accelerates
the (negative) electrons in the opposite direction. The electrons
may interact with the arc plasma, especially with the colder
plasma electrons, as well as with the background.
self-sputtering & sticking coefficient
When an arriving atom becomes incorporated into the substrate, the collision
cascade under the surface can lead to the expulsion of one (or more) surface
atoms(sputtering). If the arriving ion and the sputtered atom are of the same
material, one speaks of self-sputtering. This reduces the effective film
deposition rate, and in case the yield exceeds unity, no film is grown.
Not all ions are incorporated on/into the substrate, rather, depending on the energy and incident angle of the arriving ion and the kind of
substrate material, some ions may ‘‘bounce’’ back as neutralized atoms and therefore contribute to the density of neutral atoms rather than to
film growth. Sticking probability for incoming energetic ions.

Nb/CuO – High Resolution TEM

Nb(110)//Cu(100) Nb(100)//Cu(110) (Nb(110)//Cu(111)
150mm x 150mm, 1mm
CI =0.49
500x
150mm x 150mm, 1mm
CI =0.24
500x
150mm x 150mm, 1mm
CI =0.21
500x
Nb hetero-epitaxy on Cu
0.733° 0.357 ° 0.432°
CI =0.58
CI =0.51 CI =0.59
Lattice mismatch with (100) 8.5% 50x
1500 mm x 1500mm, 25 mm step

Thin film growth from the gas phase=non-equilibrium process phenomenon governed by a competition
between kinetics & thermodynamics. 3 stages can be distinguished in the nucleation & growth of films:
• Production of ionic, molecular or atomic species in the gas phase.
• Transport of these species to the substrate
• Condensation of the species onto the substrate directly or either by chemical or electrochemical
reaction.
Competing Processes
• Addition to film – impingement (deposition) on surface
• Removal from film: – reflection of impinging atoms
– desorption (evaporation) from surface
sticking coefficient = mass deposited / mass impinging
The vapor atoms are continuously depositing on the surface. Depending on the atom’s energy and the
position at which it its the surface, the impinging atom could re-evaporate from the surface or adsorb to it,
becoming an adatom. Adsorption occurs either by forming a van der Waal’s bond with a surface atom -
physisorption- or by forming a covalent/ionic bond with a surface atom-chemisorption. When the atoms are
physisorbed they can migrate on the surface and interact with each other as well as with the substrate
atoms. These interactions determine the morphology of the growing film.

Anne marie valente feliciano - nucleation of nb films on cu substrates

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