Methods of surface analysis Nanomaterials characterisation I RNDr. Věra Vodičková, PhD.
Surface of solid matter: last monoatomic layer + absorbed monolayer physical properties are effected (crystal lattice ending defects, more complicated structure, environment interaction Surface analysis identification of particles in the surface layer crystal structure of layer chemical bonds General principle of methods: suitable selected primary probe sample surface interaction: material x probe analysis of signal emitted from the sample condition: signal has to originate from small depth 2
The most frequent methods of surface analysis AES (Auger Electron Spectroscopy) chemical composition of surface layer, segregation study, oxidation SIMS (Secondary Ion Mass Spectroscopy) chemical composition of surface layer, difussion processes study, corrossion PES (Photoelectron Spectroscopy) chemical composition of surface layer, chemical state of atoms APS (Appearance Potential Spectroscopy) spectroscopy of threshold potentials LEED (Low Energy Electron Diffraction) slow electron diffraction monocrystallic surfaces study And others. 3
AES Auger electron spectroscopy 1925: discovery of slow electron emission (Auger electrons, AE) at irradiation of solid matter by electron beam (energy of AE is characteristic for atoms of different elements) Using in1953 AE are slow (energy is affected by mass scatter) short mean free path information (chemical composition) from very thin layer (cca 1 nm) 4
5 Auger electrons
AES Auger electron spectroscopy 6 Principle: an electron from internal shell is ejected by primary electron (ionisation) transition of excited atom to ground state - two mechanisms: Vacancy is filled by electron transition from higher shell radiative transition emission of irradiation quantum (its energy by virtue of shell difference) Energy is handed over to other electron that is emitted radiationless transition
7 AES Auger electron spectroscopy
AES Auger electron spectroscopy Energy of AE does not depend on primary beam energy! Transition marking (shells between thats the transition has already), for example: KL 3 L 3 electron from L 3 fills in vacancy in K shell, electron is emitted from L 3 The most significant transitions shell of filling in electron = shell of emitted AE (KLL, LMM, apod.) Condition: 2 energy shells, 3 electrons 8
AES Auger electron spectroscopy Analysis of emitted electrons: Detection signal processing energy spectrum of electrons Accentuation of Auger peaks electronic derivation of signal from peak positions in spectrum qualitative and quantitative surface composition 9
10 AES Auger electron spectroscopy
AES Auger electron spectroscopy Auger microprobe = REM + Auger spectrometer - vacuum chamber - energy analyzer 11
AES Auger electron spectroscopy - using quantitative surface composition thin layers analysis (micro, nano) checking of coating quality corrosion study in-depth concentration profiles in multilayer systems 12
13 PES photoelectron spectroscopy (XPS, ESCA, UPS) one of the most used methods of surfaces study and thin layers study provides information about quantitative composition, atom bonds characters, layers thickness basic process photoeffect = emission of photoelectrons in consequence of the irradiation of matter by photons energy spectrum of these electrons is measured so called photoelectron spectrum
PES photoelectron spectroscopy Three different methods of PES are used according to the way of photoelectron spectra excitation XPS = X-Ray Photoelectron Spectroscopy using of soft X-ray (200-2000 ev) to examination of core-levels UPS = ultraviolet Photoelectron Spectroscopy using of vacuum UV (10-45 ev) to examination of valence-levels ESCA = Electron Spectroscopy for Chemical Analysis
X-ray photoelectron spectroscopy (XPS) Basic process: photoeffect emission of electrons as a result of photon irradiation of material Ionization of atom on the E 1 energy level by the photon with energy hν the emission of an electron with energy hν * = E 1 Energy of photoelectrons E F = hν E 1 atom transition to ground state by X-ray or Auger electron emission 15
X-ray photoelectron spectroscopy (XPS) Photoelectron spectrum kinetic energy of photoelectrons distribution Spectrum 16
X-ray photoelectron spectroscopy (XPS) Photoelectron spectra evaluation : Positions of photoemission lines are characteristic for certain elements element identification Integral intensities of lines correspond to concentration information about concentration of element on the surface Remark: if photoionized atom is a part of a molecule or solid matter then bond energy of electrons are affected by formation of chemical bonds with other atoms observation of chemical shifts in bond energy values it is possible to use XPS for chemical state of atoms study
type resolution of elements bonding in molecule fitting of bands by model profile functions X-ray photoelectron spectroscopy (XPS) Binding states resolution 18
X-ray photoelectron spectroscopy (XPS) Oxidation state resolution 19
PES (XPS) equipment 20 spectrometer = source + electron analyzer conditions of very low vacuum it is necessary for sufficiently long mean free path of electrons during their moving in system sample detector
PES (XPS) equipment Elektrostatic hemispherical analyzer 21
Photoelectron Spectroscopy Application Quantitative composition of surface Polymers and surface modification Oxides and dielectrics Surface cleaning and residuum evaluation Catalyzers Identification of defects and corrosion Information about chemical state of surface Oxide states of metalls Valence bonds structure Thin films analysis (depth profile) Thin films of semiconductors and binders Thin films of magnetic discs
Secondary ions mass spectroscopy (SIMS) physico - chemical method of atoms and molecule identification (ionisation and separation according to mass) principle: erosion of studied material surface by energy particle beam (0,2 kev 20 kev) information about studied surface of material is contained in emitted particles specific fraction of these particles is ionized during dust-laying process= secondary ions analysis via mass filter
SIMS source of ions - transmission of analysed matter to ionized state mass analyser (f.e. electromagnet) - dispersive element - separation of different mass ions mixture in space or in time detector analog signal is proportional to incident ions number signal digitalisation, mass spectrum evaluation Diagram of SIMS measuring equipment 1, 2 ion gun, 3 focusation, 4 sample surface, 5 ion lenses, 6 mass filter 7 multiplier (top), Farraday cup (bottom), 8 CCD screen
Magnetic mass analyzer SIMS Dispersive element construction: electromagnet, ions pass through its magnet pole pieces oldest, but the most perfect dispersive element according to resolution and mass range ions with different rate m/z describe trajectories with different radii space disperse of ions according to their mass
SIMS equipment
SIMS SSIMS static SIMS small quantity (< 10 12 primary ions per cm 2) analysis of surface molecules DSIMS dynamic SIMS large quantity measurement of concentration in-depth profiles Comment.: imaging of area distribution of elements on the surface in both modes
SIMS Mass spectra Mass spectrum of oxides on the tungsten powder surface
SIMS Analysis of multilayers In-depth profil of a multilayer 4 (Mo/Si)/Mo on Si(100) substrate. The thiskness of layers is 3 nm (1,5 kev, 60, Ar+, left saturation by oxide at pressure 5.10 5 Pa, right without present of oxide 9.10 6 Pa).
SIMS Advantages: low detection limit possibility to detect only one particle from million possibility of all elements determination including its isotopes, 3D analysis surface sensitivity at low primary ions beam possibility of light elements analysis Disadvantages: destructive method measurement can not be repeated on the same part of the sample sometimes it is not possible to resolve elements with similar mass measuring sensitivity decreases with increased depth resolution
31 Methods usability