sebastiani - nano-mechanical and microstructural characterization of ms-pvd nb thin films
DESCRIPTION
http://www.surfacetreatments.it/thinfilms Nano-mechanical and microstructural characterization of MS-PVD Nb thin films (Marco Sebastiani - 30') Speaker: Marco Sebastiani - Roma 3 University | Duration: 30 min. Abstract E. Bemporad1, M. Sebastiani1, F. Carassiti1 1Mechanical and Industrial Engineering Department, University of Rome ‘Roma Tre’, Via della Vasca Navale 79-00146 Rome, Italy The main objective of the present study was to identify the influence of the applied bias voltage on the microstructural and mechanical properties of magnetron sputtering physical vapour deposition (MS-PVD) niobium thin films for use in superconducting resonant cavities for particle accelerators. The microstructure and mechanical properties evolution as a function of the applied bias voltage and nature of the substrate (copper or quartz) were investigated by means of micro-hardness and nanoindentation testing, FIB/SEM, AFM and TEM techniques. The superconducting properties (critical temperature Tc and residual resistivity) were determined by a calibrated four-contact probe and a cryogenic apparatus and then correlated to the mechanical properties. Significant difference in terms of microstructure, surface roughness and mechanical properties were observed for biased coatings grown on different substrates. The observed differences are likely connected to the low conductivity of quartz that induces a re-sputtering effect and a consequent modification of the superconducting performances.TRANSCRIPT
Material Science and Technology Research Group (STM)Dipartimento di Ingegneria Meccanica e Industriale
Università Roma TRE
Nano-mechanical and microstructural characterization of MS-PVD Nb thin films
Edoardo Bemporad, Marco Sebastiani, Fabio CarassitiGiulia Lanza and Enzo Palmieri
The fourth international Workshop on:THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF
SUPERCONDUCTIVITYOctober 4-6, 2010
Legnaro National Laboratories (Padua) ITALY
www.stm.uniroma3.it
STM Roma TRE channel on YouTube Image Gallery Agilent Web seminar Interviews and videos on daily life at STM group
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www.lime.uniroma3.itN
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RESONANT CAVITIES FOR PARTICLE ACCELERATORS
Very low Surface electrical
resistance (n a 1,8 K)
Bulk Niobium
Niobium Coated (PVD)Copper cavity
Lower costsHigher thermal stabilityBut…Significantly lower superconducting performances– WHY??
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INTRODUCTION
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Many microstructural and morphological factors can affect the in-service performance of Nb coated OFHC Cu cavities (apart from impurities):
Roughness of the substrate; Morphology of coating/substrate interface; Coating thickness and thickness homogeneity; Coating density Grain size and crystals growth angle; Thickness and composition of the surface oxide layer Residual stress
All these aspects are related to the deposition parameters;
1999, Physica C 316 153–88
1997 , J. Appl. Phys. 81 (10) 6754
2005, J. Appl. Phys. 97 083904
2005, Thin Solid Films 489 56–62
1998, Appl. Surf. Sci. 126 219–30
2006, Physica C 441 79–82
INTRODUCTION
Quality control measurement (RRR and Tc) are usually performed on Nb film deposited on Quartz substrate RRR and Tc measured on Quartz are very useful
as threshold parameters;
Nevertheless, characterization methods are absolutely necessary to verify what are the micro-structural differences between coatings which are grown either on quartz or on copper
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Supercond. Sci. Technol. (21 (2008) 125026)
GAP OF KNOWLEDGE
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A comprehensive chemical, morphological, and mechanical surface characterization can be useful, in order to find out the appropriate correlation function among process parameters/microstructure/surface properties and superconducting properties
In particular, the influence of the applied bias voltage on the functional and nano-mechanical properties of sputtered Nb thin films still needs to be deeply investigated.
Supercond. Sci. Technol. (21 (2008) 125026)
AIM OF THE WORK
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To verify the influence of the applied bias voltage and nature of the substrate on the microstructural and nano-mechanical properties of MS-PVD Niobium thin films for use in superconducting resonant cavities for particle accelerators;
To establish a correlation between the functional performance (RRR and Tc) and the observed microstructural/mechanical properties;
To verify how nano-mechanical characterization can help in the characterization and quality control of such coatings
Supercond. Sci. Technol. (21 (2008) 125026)
Introduction and motivation
Brief Description of the adopted
characterization methodology
Results and discussion Conclusions
Evolution of characterization techniques and new potential of their synergic
uses
Nano-Indentatio
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HARDNESS – SCALE OF INDENTATION
Macro-hardness
Micro-hardness
Nano-hardness
10 N 10 mN Load
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ROMA TRE NANOHARDNESS TESTER
Berkovich tip
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early 2008- CSM option- Lateral force measurement- Nano- positioninng stage
INSTRUMENTED INDENTATION TESTING
Load (P)
ht hcψ
Ac = f(hc)
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DEPTH SENSING INDENTATION DESCRIBE MATERIALS BEHAVIOUR: ELASTIC TO PLASTIC DEFORMATION
Stress-Straincurves
Indentationcurves
Indentationprints
Plastic Elastic - PlasticElastic
F
h
F
h
F
h
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QUASI-STATIC INSTRUMENTED INDENTATION TESTING
cA
PH
)( cc hfA
S
Phhc
thhdh
dPS
W.C. Oliver and G.M. PharrJ . Mater. Res., Vol. 19, No. 1, J an 2004
INPUT
)(3
metalsH
y i
i
s
s
EEE
111*
OUTPUT
Fitting of the unloading curve and first derivative at h=ht
(+ Correction for frame stiffness)
ε = 0.75 for a Berkovich indenter
Area function is obtained by independent measurements on fused silica reference sample
cA
SE
2*
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NANO HARDNESS MAPSN
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Stiffness is determined NOT from the unloading curve, but continuously from the ratio of the amplitude of the imposed force oscillation (F0) to the
amplitude of the resulting displacement oscillation (z0)
CONTINUOUS STIFFNESS MEASUREMENT (CSM): IN DEPTH H AND E VARIATION, VISCOELASTICITY
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NANOINDENTATION WITH CSM
W. C. Oliver, G. M. Pharr, J. Mater. Res., Vol. 19, No. 1, Jan 2004
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80 nm thin ALD coating TiO2-Al2O3 coating on silver substrate (Roma Tre).
CSM: ANALYSIS OF THIN COATINGS
Intrinsic hardness of the top layer
TEM, 120kV, 380.000x dual-layer coating
Pt
TiO2
Al2O3
Silver
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Tip rounding Surface roughness and
chemistry Thermal Drift Initial penetration depth Instrument compliance Indenter geometry Specimen preparation and
mounting Piling-up / sinking-in Substrate effect Indentation size effect Residual stress
FACTORS AFFECTING RESULTSN
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NANO INDENTATION: WHAT’S REALLY HAPPENED?
The correct understanding of microstructural effects during nanoindentation is not straightforward; How to separate real microstructural effects from
artifacts? How to identify the actual deformation/failure
mechanisms? Must validate nanoindentation measurements materials behavior during indentation would not
be considered with just continuum theory The solution relies in the coupled and
synergic use of high resolution microscopy techniques and indentation testing
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Evolution of characterization techniques and new potential of their synergic
uses
Dual-beam microscopy
LIME LAB: DUALBEAM (FEI) HELIOS 600 NANOLAB
early 2008
- Omnprobe- GIS (4 gases)- STEM- EDS (Oxford)
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DUALBEAM (FEI)SEM Column: spatial resolution
0,69 nm @ 15 kVFIB Column: spatial resolution
5 nm @ 30 kV
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HIGH RESOLUTION ELECTRON COLUMN 0,69 nm resolution, up to more that 1 million X
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ION-SAMPLE INTERACTIONN
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PRINCIPAL FIB MODES:
Milling
Imaging
Deposition
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FIB MODES: ION IMAGING
• Voltage contrast: insulators appear dark while grounded conductors are bright.
• Materials contrast: differences in yield of secondary particles due to the way energy is lost in the material.
• Crystallographic orientation contrast (channeling contrast): incident ions are channeled down between lattice planes of the specimen.
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PRINCIPAL FIB MODES:
Milling
Imaging
Deposition
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FIB MODES: ION MILLINGN
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PRINCIPAL FIB MODES:
Milling
Imaging
Deposition
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FIB MODES: DEPOSITION (PT)N
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Possibility to reveal microstructure beneath the surface with very little or practically no artifacts
Accurately localize the region of interest(within few nanometers)
Excellent technique even on soft materials
Time to produce a TEM lamella (couple of hours)
EDS and Scanning Transmission Imaging within the DualBeam also available
USING A DUALBEAM TO CHARACTERIZE SAMPLES; WHAT IS NEW WITH RESPECT TO CONVENTIONAL CHARACTERIZATION ROUTE?
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CROSS SECTIONSN
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TEM LAMELLA PREPARATIONN
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Results of experimental activities and discussion
Introduction and motivation
Brief Description of the adopted
characterization methodology
Results and discussion Conclusions
DEPOSITION OF COATINGS
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Four different coatings are produced, by varying the applied bias voltage and the nature of the substrate.
R20 797 CERN type sputtering on QUARTZ
R20 796 CERN type sputtering on COPPER
R30 804 BIAS type sputtering (100V) on QUARTZ
R30 803 BIAS type sputtering (100V) on COPPER
NANOINDENTATION: HOW TO GET THE RIGHT NUMBERS?
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1000 nm 500 nm
200 nm 100 nm
NANOINDENTATION: HOW TO GET THE RIGHT NUMBERS?
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1000 nm 500 nm
200 nm 100 nm
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NANOINDENTATION (CSM) ON NB THIN FILMS
500 nmSubstrate Influence on measured Hardness
Substrate effects!!
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Hardness data are not affected by the substrate only for a penetration depth < t/10
NANOINDENTATION: SURFACE EFFECTS
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Inaccuracies on hardness and modulus at indentation depths lower than 30 nm, due to roughness and tip
rounding effects
Elastic modulus at 25 nm:93,2 ± 35,82 GPa
Elastic modulus at 100 nm:106,21 ± 13,35 GPa
NANOINDENTATION: DEFORMATION MECHANISMS
Pt deposition and FIB sectioning
500 nm
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Partial re-crystallization during plastic deformation;
Relative sliding of columnar grain
Direct measurement of piling-up
Analysis of deformation mechanisms
NANOINDENTATION: DEFORMATION MECHANISMS
200 nm
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Piling-up is still visible, but it is less pronounced
No more evidence of columns sliding and bending
Platic deformation is only given by dislocation nucleation and multiplication inside the grains.
NANOINDENTATION RESULTS SUMMARY
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Sample code Description NanoindentationStandard micro-
indentation
H (GPa) E (GPa)H (GPa)
Korsunsky model
796Nb on Cu BIAS
type3,10 ± 0,58 101,5 ± 23,61 2,51 ± 0,15
797Nb on Quartz
BIAS type1,63 ± 0,30 76,22 ± 48,99 1,75 ± 0,12
803Nb on CuCERN type
2,59 ± 0,35 108,68 ± 11,65 2,38 ± 0,15
804Nb on Quartz
CERN type2,19 ± 0,31 95,95 ± 26,31 2,01 ± 0.10
DEFORMATION MECHANISMS AT HIGH LOADS
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Strong pile-up at high indentation loads (> 1N)
Vickers 1 N
DEFORMATION MECHANISMS AT HIGH LOADS
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Very low plastic deformation of the substrate
Lateral plastic flow, with strong column bending
Complete re-crystallization due to plastic deformation at the top of piled-up material
MICRO-HARDNESS ON SOFT-ON-HARD SYSTEMS
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Fitting of models for extracting properties from MICRO-hardness testing is not straightforward in this case due to the observed deformation mechanisms;
COATINGS ON COPPER SUBSTRATE
BIASED (a-b)and UNBIASED
(CERN, c-d)
type coatings on COPPER substrate
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COATINGS ON COPPER SUBSTRATEN
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BIASED (a-b)and UNBIASED
(CERN, c-d)
type coatings on QUARTZ substrate
NANOINDENTATION: OXIDE LAYER
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Niobium film load-depth curves
Load On Sample vs Displacement Into Surface
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 50 100
Displacement Into Surface (nm)
Load
On
Sam
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mN)
0
0,005
0,01
0,015
0,02
0,025
0,03
0 5 10 15 20 25
Displacement Into Surface (nm)
Load
On
Sam
ple
(mN)
Niobium film load-depth curve
Niobium film: Detail of a load-depth curve
Indenter pop-in systematically observed at depth ~ 10 nm
TEM ANALYSIS OF THE OXIDE LAYER
768F BF 25000X film Nb
768F BF 380000X grain
boundary
768F BF 140000X grain
boundaries
768F BF 660000X oxide
layer
768F BF 140000X grain
boundaries
768F BF 660000X oxide
layer
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The control of surface oxide layer can be very important in determining the functional performances of Nb coatings for superconducting applications
52INFN Workshop, 10-2006
Bemporad et al.: High resolution morphological and mechanical characterization of niobium films obtained by MS and Bias-MS PVD
ResultsHalbritter, Journal Of Applied Physics 97, 083904 (April, 2005)
FIB-SEM MICROSTRUCTURAL ANALYSIS
An improper preparation of the copper substrate (usually electro-polishing + chemical polishing) dramatically decrease the interface and film quality, and the surface smoothness
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CORRELATION TO SUPERCONDUCTING PERFORMANCES
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A difference in terms of Tc is observed between biased and unbiased films on quartz;
This suggest a microstructural and morphological difference (density, grain size, lattice distortion, residual stress)
Should we expect the same differences in terms of Tc also for coatings on Copper?
DISCUSSION: WHAT ABOUT RESIDUAL STRESS?
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A difference in residual stress is expected both in case of different substrate and in case of applied bias voltage: Residual stress can reduce the elastic strain to
failure, thus enhancing the possibility of defect formation (dislocations, delamination, micro-cracking and debonding of grains)
Residual stress arises from a lattice distortion, thus influencing the electron-phonon coupling constant and the superconducting performance.
CONCLUSIONS
Nano-mechanical testing can be a valuable procedure for the assessment of basic quality indicators for Nb films on either Copper or Quartz;
Careful analysis of the raw data coming from nanoindentation allows to find out several important information: Actual elastic modulus and hardness of the coatings, leading to more reliable
comparisons between samples; Presence of a thin oxide layer Surface and substrate effects Indirect evaluation of coating density
A correlation is found between surface micro-roughness, coating density, mechanical properties and superconducting performance;
The coupled use of HR microscopy techniques and hardness testing has been the key point of all research activities
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FUTURE ACTIVITIES: FIB-DIC METHOD FOR MEASURING STRESS GRADIENTS IN COATINGS
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Lateral resolution: <1um; Depth resolution: <100 nm Semi-automated procedure
DIC
FEMDual Beam FIB/SEM
This is an example of a PVD Gold film on Silicon…
Stress gradient analysis in Niobium films is still an unexplored field…PLEASE SEND US SAMPLES!!!
THANK YOU FOR YOURTIME AND ATTENTION !
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Introduction and motivation
Brief Description of the adopted
characterization methodology
Results and discussion Conclusions
The starting point of the work here presented are the results published in a recent paper on Supercond. Sci. Technol. (21 (2008) 125026):
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0.2
0.3
0.4
0.5
0.60.70.80.9
1
1 10
DMA
Nanoindentation
Lo
ss F
act
or
(-)
Frequency (Hz)
3
4
5
6
789
10
1 10
DMA
Nanoindentation
Sto
rag
e M
od
ulu
s (M
Pa)
Frequency (Hz)
Dynamic characterization of polymers in the time and frequency domains using a flat punch
Highly plasticized polyvinylchloride,the complex modulus at 22 oC
Supeconducting properties of MS-PVD Nb films are significantly affected by Grain size surface roughness coating density interfaces integrity
A completely different behaviour is expected for the BIASED Nb coating deposited on Quartz substrate
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M. S
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pp
licatio
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f nanoin
denta
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te
chniq
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non-h
om
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ate
rials a
nd
structu
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MTS N
ano E
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Rom
a T
RE, A
pril 3
/04
/20
08
Surface Layers - Thin Film Effects
Film
Substrate
Hardness
Hfilm
Hsubstrate
Indentation Depth/Film Thickness
1 2
Soft film on hard substrate
Substrate
Film Hardness
Hfilm
Hsubstrate
Indentation Depth/Film Thickness
1
Hard film on soft substrate
Perfect pyramid or cone:
(C0 constant, 24.5 for Vickers)
Perfect tip shape do not exist in the reality (tip blunting)
Perfect tip
Real tip
Tip Calibration
AP AND TIP CALIBRATION
20p hCA
h
Ap
...hChChChCA 4
1
32
1
212
0p
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HARD INCLUSION IN A SOFT METALN
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