Secondary Ion Mass Spectrometry SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Secondary Ion Mass Spectrometry Spettrometria di massa a ioni secondari Dr Cinzia Sada

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

Study of the material properties SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

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

Analysis of the surface SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

SIMS Sputtering process INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Sputtering process The incident ions loose energy through binary collisions with the target atoms that therefore becomes “sources ” of collisions cascades

SIMS Collisional cascades Energy distribution INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Collisional cascades Energy distribution

SIMS Principle of the SIMS technique INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Principle of the SIMS technique Detection of the charged particles emitted from the surface after the ion bombardment

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

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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 (0.1-20 keV) Energy and momentum transfer in a very limited region of the material Emission of secondary ions

Collision cascade: simulation SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Collision cascade: simulation

Emission of particles after ion bombardment SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy 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 Experimental data:

Sequence of single events without overlapping of cascade SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

Atoms distribution sputtered by primary ions with energies E2>E1 SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

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

SIMS In general: Sputtering Yield S: Partial sputtering yield Si INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy In general: Sputtering Yield S: Partial sputtering yield Si Relative sputtering Yield of secondary ions gi+/- (ionization efficiency)

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Relative sputtering Yield of secondary ions gi+/- (ionization efficiency) Absolute Yield of secondary ions Si+/-

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

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Sputtering yield increases with increasing atomic number of the incident beam

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Sputtering yield S in function of the incident angle of the primary beam with respect to the surface normal

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy The secondary ions yield depends on the energy and incident angle of the primary beam

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

Dependence on the sample potential SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy The sputtering yield is function of the incident angle with respect to the normal to the surface Dependence on the sample potential

The sputtering yield depends on the electronic SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

The emission of secondary ions occurs in three steps: SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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.

Primary beam: reactive specie SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

SIMS The probability of emission of a negative ion is related to INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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

Sputtering yield of 58Fe+ in function of the sputtering time. SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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.

SIMS INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Secondary ion sputtering yield in function of the atomic number of the analyzed element

SIMS Theory of Sigmund (1969) INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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;

SIMS For incident primary beam normal to the surface INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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.

SIMS Application of the Sigmund theory INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 Padova, Italy Application of the Sigmund theory

SIMS In conclusion: “matrix effect" INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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"

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

SIMS is now commercially available (>1-2 millions Euro) INFM e Physics Department, University of di Padova, Via Marzolo 8, 35100 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)