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

Nucleation of Nb on Cu

A-M Valente-Feliciano

A. Lukaszew (College William & Mary)

L. Phillips, C. Reece, J. Spradlin (JLab)

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


CI=

CI=

Standard films Oxide-free films

0.5 µm 0.5 µm

Columnar grains, size ~ 100 nm

In plan diffraction pattern: powder 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

CERN magnetron sputtered 1.5GHZ Nb/Cu films (coated with Ar)

Standard Oxide-free

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

Oxide –free films closer to bulk but Rres, standard < Rres, oxide-free

Condensation from the vapor involves incident atoms becoming bonded adatoms whichdiffuse over the film surface until they are trapped at low energy lattice sites. This atomicodyssey involves 4 basic processes:

ShadowingSurface diffusionBulk diffusionDesorption

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 ofarriving toms. The dominance of one or more of these processes as function of substratetemperature is manifested by different structural morphologies.


Film Nucleation & GrowthThin 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.

Hetero-epitaxy of Nb on Cu


DC magnetron sputtered @ 150 °CLukaszew A. et al. , W&M

[W. M. Roach et al. , PRSTAB 15, 062002 (2012)][Masek &Matolin, Vacuum 61(2001) 217-221]

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

Growth of Nb on CuOThe Cu oxide (CuO) is amorphous, the Nb growth is then not driven by the substrate orientation

Vacuum ~10-11 TorrSubstrate cleaned by in-situ Ar + etching andannealing @ 600 °C

Topography STM maps of Nb islands deposited onCu(111) substrates at 300 K (RT) with coverages from0.1 to 0.4 AL. Randomly distributed 2 AL high islandsprimarily observed on substrate and close to theterrace edges at very low coverage..Irregular shaped islands → amorphousmicrostructure at low temp. (RT)[Study of Nb epitaxial growth on Cu (111) at sub-monolayer level) C. Clavero et al., JAP 112, 074328(2012)]

Nb Film Nucleation at RT


Nb/Cu hetero-epitaxy by MBE at RTA. Lukaszew et al. , W&M

STM/STS studies-Proximity effects


Annealing at 350 °C

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

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

𝛾𝑓,𝑁𝑏 2.983 𝐽.𝑚−2 > 𝛾𝑠,𝐶𝑢 1.934 𝐽.𝑚−2

Lattice mismatch 9%

Further annealing @ 600 °C leads to coalescence to larger islands with reconstruction on their surface

MBE Growth @ 350 °C


Film NucleationMolecular Dynamics Simulation


Initially circular Nb nano-islands→Hexagonal shape as observed in experimentWith 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)]

large-scale atomic/molecular massively parallel simulator (LAMMPS) code Visualization with OVITO.

Energetic Condensation


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 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

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

Condensing (film-forming) species : hyper-thermal & low energies (>10 eV).

⇒ Changes in

A. Anders, Thin Solid Films 518 (2010) 4087

derived from Thornton’s diagram for sputtering (1974)

Nb on Cu single crystals

A-M Valente-Feliciano - AVS Conference 2011– Nashville TN,11/01/2011

RRR=88 RRR=242RRR=76

In the same run, Nb/fine grain Cu RRR=82Nb/large grain Cu RRR=169

Structural Properties of Niobium Thin Films Deposited on Metallic Substrates by ECR Plasma Energetic CondensationJoshua 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 - TFSRF 2012, 07/18/2012

Nb films on polycrystalline Cu substrates

RRR=169 RRR=82

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

120 x 150 μm, 1 μm resolution,CI Avg. 0.23

50 x 75 μm, 1 μm resolution,CI Avg. 0.16

Bias -120V, 2mmBias 0V , 4mm

Typical Cu substrate

Effect of Bias Voltage for Nb/Cu


Ab-normal growthvith bias

vs.columnar growth

without bias

Fiber Growth


1 0 0 n m

LAADF

Nb Cu

CL=12CM

5 n m

ABF

Nb Cu

interface

T bake = 200 °CT coating = 200 °CENb ions = 64 eVThickness = 1.4 μm RRR = 21Tc= 9.41 ± 0.16 K

Fiber Growth


1 0 0 n m

Nb

Cu

2 0 n m

LAADF

Nb Cu

CL=12CM

CL=12CM

2 nm

ABF

Nb

Cu

Amorphous interface

T bake = 200 °CT coating = 200 °CENb ions = 184 eV then 64 eV

Thickness = 1.7 μm RRR = 15Tc= 9.46 ± 0.19 KΔ=1.71 meV

1 0 n m

Spectrum Image

Spatial Drift

EELS plot for Cu/Nb signal across interface

11.5 nm

Interface thickness(e-1 of highest density)

Incident ion energy: 64 eVNb: 7.6 nmCu: 8.9 nm


Fiber Growth

Hetero-epitaxy, low Tcoating

A-M Valente-Feliciano - TFSRF 2014 - 07/10/20142 n m

Nb Cu

Continuous crystalline

1 0 0 n m

ABF

Nb Cu

interface

CL=12CM

CL=12CM

T bake = 200 °C (long bake)T coating = 200 °CENb ions = 184 eVThickness = 1.8 μm RRR = 58Tc= 9.43 ± 0.13 KΔ=1.56 meV

Nb Cu

0V (64 eV)120 V (184 eV)



Ion energy 64 eV: Nb: 3.89 nmCu: 1.89 nm

Ion energy 184 eV:Nb: 3.67 nmCu: 7.33 nm

86: 6 nm

87: 11.4 nm


Low versus high energy

Hetero-epitaxy, High Tcoating

A-M Valente-Feliciano - TFSRF 2014 - 07/10/20142 0 n m

interface

CL=12CM

Nb Cu

T bake = 500 °CT coating = 360 °CENb ions = 184 eV then 64 eVVery thick filmThickness = 4.5 μm RRR = 305Tc= 9.37 ± 0.12 KΔ=1.53 meV (1.38 meV?)

interface

5 0 n m

Spectrum Image



Nb: 12.5 nmCu: 20.1 nm

31.8nm

1 0 n m

Continuous crystalinterface

Substrate roughness and defects

Cu

Nb

E-beam evaporated Pt

Ion beam coated Pt

Nb

Cu

Section cut by FIB

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.

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 substratesInfluence 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 RRRNb/large grain Cu can achieve higher RRR due to underlying relaxed (heat treated) substrateFor 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.

1 10 100

0.29

0.30

0.31

0.32

0.33

0.34

0.35

0.36

0.23 2.3 23

Nb thickness (nm)

Latt

ice

para

me

ter

(nm

)

Nb atomic layers

bulk Nb bcc

[111]Nb ll[0001]Al O2 3

[1120]Nb ll [0001]Al O2 3

a

hcp Nb

bcc Nbhcp

+b

cc N

b

a

0 deg 30 deg 60 deg

A-M Valente-Feliciano - SRF Conference 2011– Chicago, 07/26/2011

Hetero-epitaxy


184 eV continuous500 C360 CRRR= 182Tc = 9.39 ± 0.08 KΔ = 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

Film Nucleation & Growth


The film nucleation depends first and foremost on the nature of the materialdeposited (metal... )

Niobium as most metals usually grows in the island mode.

(i) 3-D or island growth mode, also known as Volmer–Weber (VW) modeThe 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) modeThe condensing particles have a strong affinity for the substrate atoms: they bond to the

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.

Thin Film Growth Modes

Effect of ion energy on film growth


A. Anders / Thin Solid Films 518 (2010) 4087–4090

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) 280W. 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.

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 liquidlarge 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.

Inci

den

t io

n e

ne

rgy

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.

Energetic particle bombardment promotes competing processes of defect generation and annihilation.

Kinetic energy → displacement and defects followed by re-nucleationRelease 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.

Effect of ion energy and substrate 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

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

self-sputtering & sticking coefficient

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.

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.

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.

Each ion delivers significant kinetic and potential energy, and both contribute to what may be called atomic scale heating (ASH).

Nb/CuO – High Resolution TEM


150mm x 150mm, 1mmCI =0.49500x

150mm x 150mm, 1mmCI =0.24500x

150mm x 150mm, 1mmCI =0.21500x

Nb(110)//Cu(100) Nb(100)//Cu(110) (Nb(110)//Cu(111)

Nb hetero-epitaxy on Cu

0.733° 0.357 ° 0.432°

CI =0.58CI =0.59CI =0.51

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.

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.

Film Nucleation & Growth


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

Science

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