Tecnologie e strumentazioni per la SurfaceEngineering

Alberto Rota

Tecnologie avanzate per la surface engineering di materiali di interesse industriale – 19, 26 maggio Modena

Who

How many

Energy

Angle

Spin

Charging state

........

Composition

structure

morphology

chemical bond

hardness

adhesion

........

“excitation” of a solid

Collecting the “response”

spatially integrated

spatially resolved

e-

+e-

+

h

h

Modification and analysis of surfaces

Chemical analyses

sputtering

SIMSISS

XRD

XAS

AES

XPS

fluorescence

auger

sem

e-

hh

e-

+

h

+

++++

+

e-

h

e-

e-

++++

e-

a) Structural modifications: crystal amorphous;

b) Compositional modification: ion implantation; in multilayers, interface mixing;

c) Morphological modifications: surface roughness; generation of a crater.

Sputtering yield S = (ejected atoms)/(incident ion)

A50 + B50

A

B

ASA = 2SB

+

[C’A /C’B ] = [CA /CB ] . [SB/SA]

A33 + B67

A

B+

Polyatomic solids

ion energy (keV)0 5 10 15 20

0.1

1.0

Ion-surface interactionIon erosion (sputtering)

1 Primary incident ions2 Backscattered primary ions3 Implanted primary ions4 Sputtered ions in fast processes5 Sputtered ions in slow processes

Ion Scattering Spectroscopy (ISS)

Knowledge: in-depth atomic distribution, thickness, intermixing.

Properties: non-destructive; insensitive to the chemical state.

Ion energy in the range of MeV: Rutherford scattering

C A,B2

= K kinematic factor

E

N

KE0

DE

Primary ion: He+, Ne+, Ar+

Primary energy: 0-1keV

Elastic collision

Channeling

SEM image

FIB image

Sensitive to reconstruction, but not to relaxation.

Imaging of polycrystalline material

Depth profile - During SIMS analysis, the sample surface is slowly sputtered away. Continuous analysis obtains composition information as a function of depth. Depth resolution of a few Å is possible.

Mass spectrum - Identifies the elemental and ion composition of the uppermost 10 to 20 Å of analyzed surface from positive and negative mass spectra. The high resolution of the ToFanalyzer can distinguish species whose masses differ by only a few millimass units.

r = c{[(M/n)U]1/2}/B

Knowledge: 3D compositional map, thickness, intermixing.

Properties: destructive; insensitive to the chemical state

Secondary Ion Mass Spectroscopy (SIMS)

Primary ion: O+, Cs+, Ga+

Primary energy: 1-100keV

H

SiH K Ti

Si

W

N

WO

C4H7

Ar

Ionic microscope – Ions hit an extended area, the secondary ions reach the detector keeping their spatial distribution.

Secondary ion mapping - Measures the lateral distribution of elements and molecules on the surface. To obtain a SIMS map, a highly focused primary ion beam is rastered across the sample surface, and the secondary ions are collected at specific points. Image brightness at each point is a function of the relative concentration of the mapped element or molecule. Lateral resolution is less than 100 nm.

SIMS - mapping

Ionic microscope – Ions are collimated in a very collimated beam. The analyzer collects on a wide area on the surface.

Co deposition

normal incidence

Rdep ~ 3·10-3 MLE/s

Tdep = 300 K

Surface structuring: ripples on single crystals and films

Cu crystal preparation

1 keV Ar+ sputtering

annealing at 800 K

ion sputtering

2 μA of Ar+ at 1 keV

70o incidence angle to [1-10]

Tsputter= 180 K

Cu(001)Co film

Cu(001)

Cu(001)

d

h

Magnetic anisotropy vs. ion dose

Surface morphology vs. ion dose

Cu(001)

ion beam

Cu(001)

[110][1-10]

[001]20o

Co Ar+

1-2 nm

primary electron beam

Auger Electron Spectroscopy (AES) - fundamental

The backscattered and secondary electrons yield are almost independent from the composition and strongly depend on the morphology : imaging!

h

Ep

Ek(A) = [Eb(1) - Eb(2)] - Eb(3)

Ionization of a core level,relaxation (emission from the atom), emission from the solid (competionwith X-ray emission)

Ep-Eb(1)-Ek(S)

Ek(S)

Knowledge: composition, quantitative information.

Properties: non-destructive; conductive materials; very surface sensitive.

Disadvantage: UHV conditions

e-surface interaction

q

f

Angle integrating electron analyzer

(CMA)

electron beam

sample

The outgoing electrons are collected by a Cylindrical Mirror Analyzer (CMA), whose plates are biased. The entering electron has a trajectory which depends on its velocity, i.e. its energy. Tuning the bias it is possible to drive it to the collector. From the bias value it is possible to calculate its energy.

AES - elemental analysis

We can make a rough estimate of the KE of the Auger electron from the binding energies of the various levels involved. In this particular example,

KEKLL= (EK - EL1) - EL23 and KELMM= (EL – EM23) – EM45

There will be many possible Auger transitions for a given element - some weak, some strong in intensity. AUGER SPECTROSCOPY is based upon the measurement of the kinetic energies of the emitted electrons. Each element in a sample being studied will give rise to a characteristic spectrum of peaks at various kinetic energies.

The peaks are located on a high background which arises from the vast number of so-called secondary electrons generated by a multitude of inelastic scattering processes.

Auger spectra are also often shown in a differentiated form to get a better sensitivity for detection.

C

265eV

O

507 eV

Co

774-776eV

AES - quantitative analysis

C%

kinetic energy

inte

nsi

tyA

B

IA

accuracy 515%; es: (35±10)%

Cx = (Ix/Sx) / i(Ii/Si)

CA = (IA/SA) / [(IA/SA) + (IB/SB)]

in generale:

The intensity of each peak must be weighted by its “sensitivity”, a parameter which takes into account different processes related to Auger electron emission

AES – depth profile & maps

kinetic energy

inte

nsi

tyA

B

e- ions

providing qualitative/quantitative compositional information as a function of depth below the surface

e-

Scanning Auger Microscopy (SAM): providing spatially-resolved compositional information on heterogeneous samples

molibdenum in steel

SEM

Auger map

Auger Images: Fe Sb CrEmbrittlement of an aged

steel rotor.

Steel Fracture SurfaceSEM image, 10,000X

Ni InPT

Fe

Zn

Fe

Photoelectron spectroscopy (XPS-UPS)

When a photon X is adsorbed by an atom, its energy Ex can be spent to extract an electron from the atom. Only electrons with binding energy Eb lower than that or the photon can be extracted. The emitted photoelectron will have a kinetic energy Ec = Ex - Eb. If the kinetic energy of the photoelectron is analysed, maxima will be found for discrete value, corresponding to the discrete values of the binding energies. This technique enables to study the energetic level of the solid at the surface.

Due to the of the electrons, only those photoelectrons emitted close to the surface will be collected. Therefore this technique is very surface sensitive .h

e-

nm(s)

Knowledge: composition, quantitative information, chemical bonding.

Properties: non-destructive; very surface sensitive.

Disadvantage: UHV conditions

photon-surface interaction

Ec = Ex - Eb

The outgoing electrons are collected by a hemispherical analyzer (HA), whose plate are biased. The entering electron has a trajectory which depends on its velocity, i.e. its energy. Tuning the bias it is possible to drive it to the collector. From the bias value it is possible to calculate its energy.

F=qE=m(v2/R)

The HA works as an energy filter. Varying the bias continuously it is possible to get the energy spectrum of the material.

Survey spectrum qualitative and quantitative information

Narrow spectrum information on the bonds among the elements

Application of XPS: piston-pin bushing surface analysisIn

ten

sit

y [

a.u

]

120010008006004002000

Kinetic Energy [eV]

Piston-pin bushingAES spectra vs depthEp = 5 keV

Pb SCl

C

Ca

SnO

Cu

Na

0 nm

1.8 nm

36 nm

Cu

Cu

100

80

60

40

20

0

Co

nc

en

tra

tio

n [

%]

20151050

Thickness [nm]

25

20

15

10

5

0

Co

nc

en

tratio

n [%

]

Piston-pin bushingAES depth profileSputtering 0.5x0.5 mm

Ar+ 3 keV, 19A/cm

2

Cu

Sn

OIn

ten

sit

y [

a.u

.]

800 600 400 200 0

Binding Energy [eV]

Cu 2p

Cu A

Sn3dSn A

Piston-pin bushingXPS Spectra, Mg anode

SurfaceAfter sputtering (1.8 nm depth)

Pb4f

Cu3p

O1s

C1s

Pb4d

O A

C A

Application: oxidation in laser interference surfacetexturing

Survey spectrum

Narrow spectrum

Inte

nsity

[]a.u

.

Auger depth-profiling

XPS and Auger depth-profiling reveal that the laser interference processing induces the formation of a WO3 about 70nm thick.

Gachot et al., Tribol Lett (2013) 49:193–202

Bragg’s law

The Bragg’s law consists in the simple equation:

n = 2d senq [1]

In order to have the two reflected rays in phase, the difference of their optical distance must be an integer of the wavelength :

n = AB + BC [2]

From the trigonometry:

AB = d sen q [3]

As AB = BC, the equation [2] becomes:

n = 2 d sen q [4]

which is the Bragg’s law

The X-ray are diffracted by the reticular planes of the crystal. This process depend on the frequency of the X and on the interplanar distance. This technique is used for the qualitative and quantitative analysis of the crystalline phases of a solid.

X-ray diffraction (XRD) Knowledge: crystallographic structure, presence of phases, grain size, (chemical bonding).

Properties: non-destructive; possibility to analyse all kind of materials.

Cubic-spinelstructure

a≈8.15 Å

XRD: applications to materials science

macrostress (the deformation does not exceed the elastic limit)

Elongation or compression along the unit cell axis

Shift of the diffraction peaks

Generation of crystalline phases

Strengthening or disappearing of diffraction

peaks

TiO2

Hanaor, Mater Sci (2011) 46:855–874

Kr-implanted SiC

microstress (the deformation exceeds the

elastic limit)

Nucleation of an ensambleof local defects (vacancies,

dislocations, grain boundaries, ...)

Enlargement of the diffraction peaks

Xu, CHIN. PHYS. LETT. Vol. 28, No. 10 (2011) 106103

Contact angle test

Knowledge: hydrophobic/hydrophilic properties

Properties: non-destructive; possibility to analyse all kind of materials, fast

qc < 90° - hydrophilicqc > 90° - hydrophobic

Application: micro-texturing

Golden rule

Surface micro-texturing amplifies the natural tendency of the surface:

- it increases the hydrophilicity in already hydrophilic materials

- it increases the hydrophobicity in already hydrophobic materials

Wenzel model Cassie-Baxter model Hemi-Wicking model

Rose petal-inspired hierarchical PDMS+silane

Park et al., Thin Solid Films 520 (2011) 362–367

Hierarchical surface: superposition of 2 (or more) periodic structures of different scales

Applications

Superhydrophobic surfaces/coatings:

- self-cleaning windows, in order to reduce their cleaning maintenance

- limitation of corrosion

- fluidic applications

Superhydrophilic surfaces/coatings:

- anti-fog windows

- support for cell proliferation

Applications

Superhydrophobic surfaces/coatings:

- self-cleaning windows, in order to reduce their cleaning maintenance

- limitation of corrosion

- fluidic applications

Superhydrophilic surfaces/coatings:

- anti-fog windows

- support for cell proliferation

http://jncc.defra.gov.uk/page-5592

(a) water spray test

(b) the condensation test by an incident temperature increase from −17 to 25°C