1. Secondary Ion Mass Spectrometry
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Secondary Ion Mass Spectrometry
Spettrometria di massa a ioni secondari
Dr Cinzia Sada

2. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Outline
1. Principles of the technique SIMS (Secondary Ion Mass Spectrometry)
Surface phenomena under ionic bombardment
Secondary ion emission
Sputtering
Sputtering Yield
2. Experimental set-up
The spectrometer
Analytical condition for the measurement
3. Applications
Mass spectra
Depth profiles
Image acquisition
4. Ionic microscope: Cameca ims 4f
Examples

3. Study of the material properties
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Study of the material properties
Problems in metallurgy connected to chemical and physical phenomena (segregation and oxidation);
Local variation in the elemental concentration;
Detection of elements in trace.
Techniques with concentration resolution close to ppma and depth resolution close to nm

4. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy

5. Analysis of the surface
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Basics
To analyze the mass of particles emitted from the material under ion bombardment
Compositional analysis
Analysis of the surface
In-depth profile

6. SIMS Sputtering process
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Sputtering process
The incident ions loose energy through binary collisions with the target atoms that therefore becomes “sources ” of collisions cascades

7. SIMS Collisional cascades Energy distribution
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Collisional cascades
Energy distribution

8. SIMS Principle of the SIMS technique
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Principle of the SIMS technique
Detection of the charged particles emitted from the surface after the ion bombardment

9. SIMS Sputtering erosion
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Sputtering erosion

10. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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 ( keV)
Energy and momentum transfer in a very limited region of the material
Emission of secondary ions

11. Collision cascade: simulation
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Collision cascade: simulation

12. Emission of particles after ion bombardment
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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
Angular distribution of the sputtered material

13. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
When the incident ions loose all the energy, they implant into the substrate
The depth reached by the primary ions (typically nm from the surface) depends on:
Incident particle energy
Incident angle with respect to the surface normal
Experimental data:

14. Sequence of single events without overlapping of cascade
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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

15. Atoms distribution sputtered by primary ions with energies E2>E1
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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

16. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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.

17. SIMS In general: Sputtering Yield S: Partial sputtering yield Si
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
In general:
Sputtering Yield S:
Partial sputtering yield Si
Relative sputtering Yield of secondary ions gi+/- (ionization efficiency)

18. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Relative sputtering Yield of secondary ions gi+/- (ionization efficiency)
Absolute Yield of secondary ions Si+/-

19. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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  cosq-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:

20. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Sputtering yield increases with increasing atomic number of the incident beam

21. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Sputtering yield S in function of the incident angle of the primary beam with respect to the surface normal

22. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
The secondary ions yield depends on the energy and incident angle of the primary beam

23. SIMS The secondary ion yield depends on the energy of the primary beam
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
The secondary ion yield depends on the energy of the primary beam

24. Dependence on the sample potential
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
The sputtering yield is function of the incident angle with respect to the normal to the surface
Dependence on the sample potential

25. The sputtering yield depends on the electronic
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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

26. The emission of secondary ions occurs in three steps:
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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.

27. Primary beam: reactive specie
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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

28. SIMS The probability of emission of a negative ion is related to
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
The probability of emission of a negative ion is related to
Attitude of the emitted particles to accept an electron (electron affinity);
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

29. Sputtering yield of 58Fe+ in function of the sputtering time.
SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Secondary ions yield depends strongly on the primary beam specie
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.

30. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Secondary ion sputtering yield in function of the atomic number of the analyzed element

31. SIMS Theory of Sigmund (1969)
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Theory of Sigmund (1969)
The sputtering yield for a primary beam of energy E and incident angle Q
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 = a N Sn(E)
Sn(E) = stopping nuclear cross section
a = adimensional factor thant includes: electronic shileding, E and Q, ration between the the masses of the primary particle and the lattice atom;

32. SIMS For incident primary beam normal to the surface
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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.

33. SIMS Application of the Sigmund theory
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
Application of the Sigmund theory

34. SIMS In conclusion: “matrix effect"
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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:
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 "–")
The quantity of oxygen or Cs at the surface
“matrix effect"

35. SIMS
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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.

36. SIMS is now commercially available (>1-2 millions Euro)
INFM e Physics Department, University of di Padova, Via Marzolo 8, Padova, Italy
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)