Hamamatsu, November 2007

Methods and Tehniques in Surface Science

Prof. Dumitru LUCA

“Alexandru Ion Cuza” University, Iasi, Romania

Hamamatsu, November 2007

Ioni Spectroscopies

SIMS/SNMSSIMS/SNMS (Secondary Ion/Neutral Mass Spectroscopy)

– the most sensitive elemental.

Difficulties in interpretation of spectra…

Sputtering: incident ion ≠ emergent ion.

Requires HV/UHV conditions.

Detectioin: mass spectrometry

RBS RBS (Rutherford Backscattering Spectroscopy) – scattering of high-energy (MeV) incident ions on sample NUCLEI.

Probing depth – a copuple of m!

Scattering: incident ion = emergent ion

Sputtering effect is minor (sputtering cross-section sectiunea is almost nil at that energy values).

Detection: solid-state scintillators.

Equipment: particle accelerator. LEISS (ISS)LEISS (ISS) (Low-Energy Ion scattering Spectroscopy) – scattering of incident ions

imprastierea on the atoms in the topmost layer of the surface.

Requires UHV.

Equipment: Dedicated LEIS spectrometer

Detection – electrostatic analizer.

Hamamatsu, November 2007

Low Energy Ion Scattering Spectroscopy (LEIS)Low Energy Ion Scattering Spectroscopy (LEIS)

E1 – kinetic energy of scattered ions;

E0 – kinetic energy of the incident ion (100 – 10 000 eV);

M1 – mass of the incident iont;

M2 – mass of the scattering atom;

L – scattering angle.AAllows for qualitative and semi-quantitative llows for qualitative and semi-quantitative

analysis of surface composition.analysis of surface composition.

<< dij → Classical Mechanics

Interaction potential ??

2/3

0.850( )

2 ( )ion

ion

aZ e ZeV r

r r Z Z

Coulmb Potential Screening function

a0 – Bohr radius of the scattering atom.

Incident ions experience to a lesser extent the presence of nucleus due to electrostatic screening.

(in RBS the screening function = 1)

Hamamatsu, November 2007

The optimum performance in terms of mass discrimination involves:

Due to very high neutralization probability of the incident ions by impact with the atoms in the surface, the LEIS technique provides information on the nature of the ions in the topmost in the topmost

layer of the sample surface, exclusively. layer of the sample surface, exclusively.

Low Energy Ion Scattering Spectroscopy (LEIS) Low Energy Ion Scattering Spectroscopy (LEIS)

2

2 1 2 1

0 2 1 0 2 1

,f fE EM M M M

E M M E M M

For a scattering angle values of L = 900 (forward scattering) and L = 1800 (backward

scattering), the previous equation becomes even simpler:

150 180L

Hamamatsu, November 2007

The intensity of the detected current, I, is a function of the number of the atoms of a species k, Nk, via the equuation:

I = K Ip Nk Pi W

where:

- scattering cross-section (= probability that an incident ion be scattered towards the detector, after a collision with an atom of species k),

Ip – incident beam current,

Pi – the probability that an ion remains un-neutralized after a collision,

W – entrance solid angle of the detector. The above equation is seldom used for quantitative analysis, since the Pi parameter is hardly

known.

• Data processing involves to know the scattering cross-section and the probability for Data processing involves to know the scattering cross-section and the probability for impact neutralization/re-ionizationimpact neutralization/re-ionization. Usually we rather want to calibrate the LEISS machine by using standard samples.

Most frequently, LEISS is associated with complementary techniques.

Low Energy Ion Scattering SpectroscopyLow Energy Ion Scattering Spectroscopy

Hamamatsu, November 2007

LEIS LEIS - I- Instrumentnstrumentatationion

Schematics of the LEIS setup, using the TOF spectroscopy to detect forward- and backward scattered particles. Nuclear Instruments and Methods, Vol. 162, 1979, p

587.

Hamamatsu, November 2007

TOFTOF basics basics

In reality, correction factors should be taking into accountm ostly in the case of reflectron configuration :

Hamamatsu, November 2007

A LEIS spectrum showing the evolution of a topmost layer of the Ti

during titanium nitridation

The evolution of the LEIS Ti peak area with pressure. A Ti surface is exposed to a nitrogen atmosphere in UHV.

LEISS applications LEISS applications

Incident beam: 3 keV 3He

Detection angle = 1350

Hamamatsu, November 2007

N/O substitution at Ti surface

0 500 1000 1500 2000

0

500

1000

1500

2000

2500

3000

OXIDATION OF TiNLEIS study of outermost surface

LEIS

sig

nal (

cps/

nC*k

eV)

Time (s)

Nitrogen Oxygen

0 500 1000 1500 2000

0,0

0,2

0,4

0,6

0,8

1,0p

o2 = 5x10-9 mbar

frac

tion

Time (s)Time (s)

1000 1100 1200 1300 1400 1500 1600

100

120

140

160

180

200

220

240

260

280

300

N peakO peak

pO2 = 5 x 10-9 mbar

2015186017051550139512401085930775620

465310

155

0

Inte

ns

ity

(c

ps

/nC

)

Final energy (eV)