INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Secondary Ion Mass Spectrometry Spettrometria di massa a ioni secondari

Dr Cinzia Sada

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Outline

1. Principles of the technique SIMS (Secondary Ion Mass Spectrometry)Surface phenomena under ionic bombardmentSecondary ion emissionSputteringSputtering Yield

2. Experimental set-upThe spectrometer Analytical condition for the measurement

3. ApplicationsMass spectraDepth profilesImage acquisition

4. Ionic microscope: Cameca ims 4fExamples

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Study of the material properties

1. Problems in metallurgy connected to chemical and physical phenomena (segregation and oxidation);

2. Local variation in the elemental concentration;3. Detection of elements in trace.

Techniques with concentration resolution close to ppma and depth resolution close to nm

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Compositional analysis

Analysis of the surface In-depth profile

BasicsTo analyze the mass of particles emitted from the material under ion

bombardment

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The incident ions loose energy through binary collisions with the target atoms that therefore becomes “sources ” of collisions cascades

Sputtering process

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Collisional cascades Energy distribution

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Principle of the SIMS technique

Detection of the charged particles emitted from the surface after the ion bombardment

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Sputtering erosion

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

When the collisions sequences cross the solid surface one or more atoms can gain enough momentum to escape from the surface

The particle emission due to direct impact with the primary incident ion can be ignored

The ion bombardment in vacuum of the surface with a focused primary beam of energy E in the range of keV (0.1-20 keV)

Energy and momentum transfer in a very limited region of the material

Emission of secondary ions

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Collision cascade: simulation

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Angular distribution of the sputtered material

Emission of particles after ion bombardment

Electrons, photons, atoms and molecules (~1%), ions and charged molecules exit from the surface with a given angular

distribution

Result of the ion-solid interaction:

(i) Modification in the lattice structure

(ii) Erosion (sputtering) + ionization

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

When the incident ions loose all the energy, they implant into the substrate

The depth reached by the primary ions (typically 10-15 nm from the surface) depends on:

• Incident particle energy• Incident angle with respect to the surface normal

cos E 838.1 R:Cs

cos E 622.1 R:Ar

cos E 2.15 R:O

0.68

0.84

2

Experimental data:

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Stopping time:

• mean stopping time of the primary ions: ts~ 10–13 s• Mean time of the collision cascades: tc~ 10–12 s

ts, tc << arrival time of the next primary ion

Sequence of single events without overlapping of cascade

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Mean escape depth of atoms (~6Å), depends on:

• Atomic mass and number of the primary ions;• Primary ion energy;• Atomic mass and number of the material components;• Surface bond energy.

NB: emission from depth > 20Å has a low probability

Particles emission after ion bombardment

Erosion of the material (sputtering)

Atoms distribution sputtered by primary ions with energies E2>E1

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Sputtering

The generation of secondary ions is a very complex phenomenon. Up to now a complete theory does not exist.

The number of secondary ions emitted depends on:

• Incident beam: energy, type (inert or reactive);incident angle of the beam;

• Electronic and chemical properties of the surface;

• Crystallographic orientation of the bombarded material.

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

In general:

Sputtering Yield S:

p

emitted

n

n

particlesincident n

particles emitted n

S

Partial sputtering yield Si

pn

n

particlesincident n

emitted particles-i n iiS

Relative sputtering Yield of secondary ions gi+/- (ionization efficiency)

emitted particles-i n emitted ions)-(isecondary n

γ /i

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Relative sputtering Yield of secondary ions i+/- (ionization efficiency)

emitted particles-i n emitted ions)-(isecondary n

γ /i

Absolute Yield of secondary ions Si+/-

//iii SS

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

• The sputtering yield depends on the energy of the primary ions beam

• The sputtering yield depends on the incident angle of the primary beam with respect to the normal to the surface: S cos-1

• The sputtering yield depends on the incident chemical specie:

Inert (Ar+): does not modify the surface from a chemical point of view

Reactive (O+, Cs+): interact with the surface.

O+ enhances the positive secondary ions emission Cs+ enhances the negative secondary ions emission

Important:

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Sputtering yield increases with increasing atomic number of the incident beam

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Sputtering yield S in function of the incident angle of the primary beam with respect to the surface normal

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The secondary ions yield depends on the energy and incident angle of the primary beam

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The secondary ion yield depends on the energy of the primary beam

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The sputtering yield is function of the incident angle with respect to the normal to

the surface

Dependence on the sample potential

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The sputtering yield depends on the electronic and chemical properties of the surface

The binding energy at the surface determines the efficiency in the particle emission

The surface “reactivity” to the incident specie and to the molecule adsorbance influences the sputtering yield

The exploitation of reactive elements (O+, Cs+) enhances the emission of given secondary ion (+/- respectively)

Atomic number of the material

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Primary beam: inert

The processes occurring into the material have mainly a kinetic nature.

The emission of secondary ions occurs in three steps:

• Primary ions go into the samples and produce collision cascades;

• The excitation of electrons from the inner shells induced by the collisions;

• The target specie exits from the surface (de-excitation can occurs via Auger

processes);

• The secondary ion emission yield is low and depends on the emitted specie.

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Primary beam: reactive specie

The emission of secondary ions depends on the presence of reactive species (O, Cs) initially at the surface or introduced during the ion bombardment and depends on the efficiency of electron exchange between the emitted particles and the surface.

The probability of emission of a positive ion is related to:

Attitude of the emitted particles to give an electron (ionization potential); Attitude of the surface to accept the electron (presence of Oxygen).

Ai

+= adimensional constant dependent on the i-specie

Bi+= constant dependent on the material

PIi= ionization potential of the specie i

B

PI

ii

i

eA

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The probability of emission of a negative ion is related to

1. Attitude of the emitted particles to accept an electron (electron affinity);2. Attitude of the surface to give the electron (presence Cs).

Ai-= adimensional constant dependent on the i-specie

Bi-= constant dependent on the material

EAi=electron affinity of the specie i

B

EA

ii

i

eA

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

O+ and Ar+ beams impinging on a Fe substrate (normal

incidence primary energy 16.5 KeV)

Sputtering yield of 58Fe+ in function of the sputtering time.

Secondary ions yield depends strongly on the primary beam specie

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Secondary ion sputtering yield in function of the atomic number of the analyzed element

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Theory of Sigmund (1969)

The sputtering yield for a primary beam of energy E and incident angle

N = atom density within the material;

U0 = binding energy at the surface;

C0 = constant related to the scattering cross section

FD = mean energy density deposited on the surface of the material by the primary particle, dependent on the frction on the atom nuclei of the lattice: FD = N Sn(E)

Sn(E) = stopping nuclear cross section

= adimensional factor thant includes: electronic shileding, E and , ration between the the masses of the primary particle and the lattice atom;

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

For incident primary beam normal to the surface

• These relations work when linear collisional cascades occur (i.e. a small fraction of atom in the lattice is moved);

• When the incident beam is made of heavier elememts the fraction of atom moved from their position is higher: a loca amorphization can occur ("thermal spike“ regime);

• The typical experimental conditions are in between these two conditions it is not possible to quantify the emitted particles from theory only simulations are needed.

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Application of the Sigmund theory

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

In conclusion:

The sputtering yield varies with the material:element with low Ip = positive secondary ion spectrometry, element with high Ea = negative secondary ion spectrometry;

The yield of a given element depends on the matrix in which it is dispersed on:

1. Efficiency of keeping an electron attached to the matrix (to produce positive ions "+") or capability to be a donor of electrons (to produce negative ions "–")

2. The quantity of oxygen or Cs at the surface

“matrix effect"

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

Matrix effect

The yield of secondary ions emitted by a primary reactive beam depends on the material :

The matrix effect is related to the concentration of the reactive specie implanted into the material and oin lower extent on the chemical and electronic properties of the surface.

It is necessary to correct the yield of secondary ions for the possible variation in the sputtering yield and rate.

INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy

SIMS

The history

• The observation of particles emission after ion bombardment with primary ions: 1910 thanks to J.J. Thomson.

• The first experimental set-up for the ion bombardment of metals and oxides where the incident particles can be distinguished from the emitted ones: 1949 (Herzog e Viehbock).

• In the first ‘70s the need of detecting trace elements in materials coming from space induced researchers of Bedford labs (USA) to build the first commercial version of SIMS (Herzog).

SIMS is now commercially available (>1-2 millions Euro)