AP 5301/8301 Instrumental Methods of Analysis and Laboratory

Transcription

1 1 AP 5301/8301 Instrumental Methods of Analysis and Laboratory Final Review (except XPS and AES) Prof YU Kin Man Tel: Office: P6422

2 2 Teaching and Learning Questionnaire (TLQ) - Course-end Evaluation, Semester A, 2016/17 Course-end teaching and learning evaluation for Semester A, 2016/17 will be carried out from 31 October to 4 December 2016 Prof K. M. Yu Prof Paul K. Chu

3 Introduction: Materials Characterization The broad and general process by which a material's structure and properties are probed and measured. It is a fundamental process in the field of materials science, without which no scientific understanding of engineering materials could be ascertained. The structure of a material is determined by its chemical composition and how it was synthesized (processed) A material s properties will determine what it can be used for (applications) and the performance of the final device. At the CORE of this tetrahedron is material characterization 3 Performance is the ultimate end use function of the material and is resulted from properly tuning properties of materials by optimizing the structure down to the atomic level through material processing (synthesis).

4 Materials characterization techniques There are many different ways to classify material characterization techniques: What information they produce: crystal structure, electronic structure, electrical conductivity, optical constants, chemical composition, nature of defects, etc. What probe they use: photons, electrons, ions, neutrons, x-ray, etc. What they measure: ions, electrons, photons, etc. 4 Electron probe X-ray probe Ion beam probe Optical probe

5 AP5301/AP Introduction 2. Optical microscopy 3. Electron microscopy: Scanning electron microscopy (SEM), scanning probe microscopy (SPM) 4. Electron microscopy: Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), electron probe microanalysis (EPM) 5. X-ray diffraction 6. Electrical measurements: Four point probe, Hall effect, Capacitance-voltage profiling, thermoelectric effect, minority carrier lifetime 7. Optical spectroscopy: Spectrophotometry, Spectroscopic ellipsometry (SE), Photoluminescence (PL), Modulation spectroscopy 8. Secondary ion mass spectrometry (SIMS) 9. Auger electron spectroscopy (AES) 10. X-ray photoelectron spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) 11. Ion beam analysis: Rutherford backscattering spectrometry (RBS), hydrogen forward scattering (HFS), particle induced x-ray emission (PIXE)

6 AP5301/AP8301 Chemical analysis diffraction 6 XPS RBS OM SPM TEM SEM Microstructure Crystalline defects Mapping Surface morphology STEM AES SIMS EDS,WDS EELS XRD SAD Spectrophotometry CBED Crystal structure Elemental composition Chemical Depth profiles composition Elemental mapping Chemical bonding PL SE PR Optical properties Band gap Crystalline defects 4-pt probe Hall CV/ECV lifetime Resistivity Carrier conc. Mobility Minority carrier lifetime Dopant profiles

7 Materials characterization: acronyms Microscopy OM optical microscopy NA numerical aperture BF bright field DF dark field SCOM scanning confocal optical microscopy SEM scanning electron microscopy SE secondary electron BES backscattered electron SPM scanning probe microscopy STM scanning tunneling microscopy AFM atomic force microscopy SNOM scanning near field optical microscopy TEM transmission electron microscopy SAD/ selected area electron SAED diffraction CBED convergent beam electron diffraction STEM scanning transmission electron microscopy EDS/ energy dispersive x-ray EDX spectroscopy X-ray techniques WDS wavelength dispersive x- ray spectroscopy ADF annular dark field imaging HAADF high angle annular dark field EPMA electron probe microanalysis EELS electron energy loss spectroscopy XRF x-ray fluorescence XRD x-ray diffraction HRXRD high resolution x ray diffraction XAS x-ray absorption XANES x-ray absorption near edge structure EXAFS extended x-ray absorption fine structure Electrical measurements SRP CV ECV spreading resistance profiling capacitance voltage electrochemical CV Optical spectroscopy SE PL PR ER TR spectroscopic ellipsometry photoluminescence photoreflectance electroreflectance thermoreflectance Chemical analysis SIMS secondary ion mass spectrometry ToF-SIMS time of flight SIMS RBS Rutherford backscattering spectrometry HFS hydrogen forward scattering PIXE particle induced x-ray emission AES Auger electron spectroscopy SAM scanning Auger microscopy XPS x-ray photoelectron spectroscopy ARPES angle resolved photoemission spectroscopy 7

8 Optical vs. electron microscopy 8 Glass ceramic transmission microscope image made with polarized light and full wave plate Optical Microscope Microstructure of steel D2 (Metal Ravne Steel Selector) Exfoliated molybdenum disulfide on a perforated grid SEM micrographs of SMNb0.05% Mat. Res. vol.6 no.2 São Carlos Apr./June Scanning electron microscopy (SEM) image of as-grown p-type gallium nitride (p-gan) nanowire arrays on a silicon (111) substrate Cross-section TEM image of MOCVD grown InGaAs/GaAs quantum dot superlattice solar cell (NREL) Electron Microscope Atomic resolution TEM image of nanocrystalline palladium. H. Rösner and C. Kübel et al., Acta Mat., 2011, 59,

9 Optical microscope 9 Uses visible light as the illumination source, Has lateral resolution down to 0.1 m Used for almost all solids and liquid crystals Typically nondestructive Mainly used for preliminary direct visual observation Microstructural features observed: grains, precipitates, inclusions, pores, whiskers, defects, twin boundaries, etc.

10 Bright field (BF) and dark field (DF) imaging Bright field (BF): using the full illumination of the light source Dark field (DF): illuminating the sample with peripheral light by blocking the axial rays, producing a dark, almost black, background with bright objects on it DF images are visually impressive but may be very sensitive to dirt and dust located in the light path. The intensity of the illumination system must be high 10

11 Advanced optical microscopy Polarized light microscopy involves illumination of the sample with polarized light for specimens that are visible primarily due to their optically anisotropic Phase contrast microscopy uses a special condenser and objective lenses to convert phase differences (not visible) into amplitude differences (visible) Differential interference contrast (DIC) microscopy enhances contrast by creating artificial shadows (pseudo three-dimensional) using polarized light as if the object is illuminated from the side A fluorescence microscope uses fluorescence to generate an image Only allows observation of the specific structures which have been labeled for fluorescence Confocal microscopy adds a spatial pinhole at the confocal plane to increase optical resolution and contrast Scanning confocal optical microscopy (SCOM) is a technique for obtaining high-resolution optical images with depth selectivity. (a laser beam is used) Blue-green Algae Phase contrast DIC 11

12 Optical and electron microscopies: a comparison 12 Probe Light source Condenser Specimen Objective Projector Eyepiece Source of electrons Magnetic lenses Specimen detector Optical microscope Transmission electron microscope (TEM) scanning electron microscope (TEM)

13 Electron beam microscopy/spectroscopy The types of signals produced by a SEM include secondary electrons (SEs), back-scattered electrons (BSEs), characteristic X-rays and photons (cathodoluminescence) (CL), absorbed current (specimen current) and transmitted electrons. Both SEs and BSEs are used for imaging 13 (1-50keV)

14 Scanning electron microscopy 14 Magnification Depth of Field Resolution OM x m ~ 0.2 m SEM x 4mm 0.4 m 1-10nm SEM has a large depth of field, producing an image that is a good representation of the three-dimensional sample SEM produces images of high resolution at a high magnification. SEM usually also equipped with analytical capability: electron probe microanalysis (energy dispersive x-ray analysis). The higher magnification, larger depth of field, greater resolution and compositional and crystallographic information makes the SEM one of the most useful instruments in various fields of research.

15 Secondary (SE) & backscattered (BSE) electrons 15 SEs are low energy electrons (<50eV) produced by inelastic interactions of high energy electrons with core electrons SE yield: δ = n SE /n B >1 independent of Z but depends on the angle of incidence. This gives rise to topographic contrast of the specimen Due to their low energy, only SE that are very near the surface (<10nm) can exit the sample and be examined (small escape depth). BSE are produced by elastic interactions (scatterings) of electrons with nuclei of atoms and they have high energy and large escape depth BSE yield: η = n BS /n B ~ increases with atomic number, Z and thus can be used to obtain images with atomic number contrast Dark Bright

16 SEM magnification & resolution 16 Magnification in an SEM can be controlled over a range of about 6 orders of magnitude from about 10 to 500,000x. Magnification results from the ratio of the dimensions of the raster on the specimen and the raster on the display device, Magnification=area scanned on the monitor/area scanned on the specimen The spatial resolution of the SEM depends on the size of the electron spot, which in turn depends on both the wavelength of the electrons and the electron-optical system that produces the scanning beam. the size of the interaction volume. The resolution can fall somewhere between less than 1 nm and 20 nm. Low M; Large x 40 m 2500X High M; small x 7 m 15000x

17 Scanning probe microscopy (SPM) Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. Scanning tunneling microscopy (STM): uses an atomically sharp metallic tip and records the minute tunneling current (I) between the tip and the surface. It maps out the sample topography and electrical properties. Atomic force microscopy (AFM): a cantilever with a sharp tip is scanned over the surface. Using the van der Waals forces or contact forces between a tip and the sample, the sample topography or mechanical properties can be measured. constant current mode constant height mode 17

18 STM: modes of operation 18 constant current mode STMs use feedback to keep the tunneling current constant by adjusting the height of the scanner at each measurement point the voltage applied to the piezoelectric scanner is adjusted to increase/decrease the distance between the tip and the sample The image is then formed by plotting the tip height vs. the lateral tip position. constant height mode Tunneling current is monitored as the tip is scanned parallel to the surface. There is a periodic variation in the separation distance between the tip and surface atoms. A plot of the tunneling current vs. tip position shows a periodic variation which matches that of the surface structure-a direct "image" of the surface.

19 STM vs. AFM 19 STM Measures local electron density of states, not nuclear positions not true topographic imaging High lateral and vertical resolution because of the exponential dependence of the tunneling current on distance Exponential dependence between tunneling current and distance Probe electronic properties (LDOS including spin states) Generally applicable only to conducting (and semiconducting) samples Writing voltage and tip-to-substrate spacing are integrally linked AFM Real topographic imaging Lower lateral resolution The force-distance dependence in AFM is much more complex Probe various physical properties: magnetic, electrostatic, hydrophobicity, friction, elastic modulus, etc Applied to both conductors and insulators Writing voltage and tip-to-substrate spacing can be controlled independently

20 Transmission electron microscopy 20 A disc of metal Control brightness, convergence Control contrast

21 TEM operation TEM offers two methods of specimen observation, diffraction mode and image mode. The objective lens forms a diffraction pattern in the back focal plane with electrons scattered by the sample and combines them to generate an image in the image plane. Whether the diffraction pattern or the image appears on the viewing screen depends on the strength of the intermediate lens. The image mode produces an image of the illuminated sample area In image mode, the post-specimen lenses are set to examine the information in the transmitted signal at the image plane of the objective lens. There are three primary image modes that are used in conventional TEM work, bright-field microscopy, dark-field microscopy, and high-resolution electron microscopy. 21

22 TEM: diffraction pattern 22 Polycrystalline materials The electron diffraction pattern is a set of rings, with some spots depending on the crystallite sizes. Al single crystal Polycrystalline Pt silicide (PtSi) Silicon with epitaxial nickel silicides ( Si - NiSi - NiSi 2 ) Polycrystalline nickel mono silicide (NiSi) on top of single crystalline silicon (Si). Nano to Amorphous materials As the crystal size get smaller (nm) the rings get more diffuse and eventually become halo-like when the material becomes amorphous nanocrystalline GaNAs Amorphous GaNAs

23 TEM: advantages and disadvantages 23 Advantages TEMs offer very powerful magnification and resolution. TEMs have a wide-range of applications and can be utilized in a variety of different scientific, educational and industrial fields TEMs provide information on element and compound structure. Images are high-quality and detailed. Chemical information with analytical attachments Disadvantages TEMs are large and very expensive (USD 300K to >1M) Laborious sample preparation. Operation and analysis requires special training. Samples are limited to small size (mm) and must be electron transparent. TEMs require special housing and maintenance. Images are black and white.

24 Comparison: SEM and TEM 24 TEM SEM Electron beam Broad, static beam Beam focused to fine point and scan over specimen Electron path passes through thin specimen. scans over surface of specimen Specimens Specially prepared thin specimens supported on TEM grids. Sample can be any thickness and is mounted on an aluminum stub. Specimen stage Located halfway down column. At the bottom of the column. Image formation Transmitted electrons collectively focused by the objective lens and magnified to create a real image Image display On fluorescent screen. On TV monitor. Image nature Image is a two dimensional projection of the sample. Magnification Up to 5,000,000x ~250,000x Resolution ~0.2 nm ~2-5 nm Beam is scanned along the surface of the specimen to build up the image Image is of the surface of the sample

25 Scanning Transmission Electron Microscopy (STEM) In a STEM the electron beam is focused into a narrow spot which is scanned over the sample in a rastering mode. The rastering of the beam across the EDX detector sample makes these microscopes suitable for analysis techniques such as mapping by I Z 2 X-rays luminescence SAED =0.26 o or ~6.4 mrads energy dispersive X-ray (EDX) spectroscopy electron energy loss spectroscopy (EELS) annular dark-field imaging (ADF). By using a high-angle detector (high angle annular dark-field HAADF), atomic resolution images where the contrast is directly related to the atomic number (z-contrast image) can be formed. 25

26 Electron probe microanalysis (EPMA) Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS) is an analytical technique used for the elemental analysis. In a SEM or STEM, the incident electrons are used as the excitation source creating characteristic x-rays from different elements in the target. Electron energy loss spectroscopy (EELS): Electrons lose energy through inner-shell ionizations are useful for detecting the elemental components of a material the detailed shape of the spectral profiles gives information on the electronic structure, chemical bonding, and nearest neighbor distances for each atomic species. Quantitative elemental concentration for the element 3 Z 35 26

27 EPMA: WDS vs EDS 27 WDS EDS Spectra acceptance One element/run Entire spectrum in one shot Collection time > 10 mins Mins Sensitive elements Better for lighter elements (Be, B, C, N, O) Resolution ~few ev ~130 ev Probe size ~200 nm ~5 nm Max count rate ~50000 cps <2000 cps Detection limits 100 ppm ppm Spectral artifacts rare Peak overlap

28 EPMA: EELS vs EDS 28 EELS Energy resolution ~0.1 ev ~130 ev Energy range ev 1-50 kev EDS Element range Better for light elements Better for heavy elements Ease of use Medium high Spatial resolution Good beam broadening Information Elemental, coordination, bonding Quantification Easy Easy Only elemental Peak overlap No Can be severe

29 X-ray powder diffraction 29 X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The sample holder and the x-ray detector are mechanically linked: the detector turns 2 when the sample holder turns so that the detector is always ready to detect the Bragg diffracted beam 2d sin θ = nλ Bragg-Brentano geometry Applications: Phase Composition of a Sample Unit cell lattice parameters and Bravais lattice symmetry Residual Strain (macrostrain) Epitaxy/Texture/Orientation Crystallite Size and Microstrain

30 XRD: alloy composition analysis 30 (0002) Diffraction peaks of ZnO 1-x S x alloy Increasing x ZnO is alloyed with ZnS to form ZnO 1-x S x alloy Wurtzite ZnO (c=0.52 nm) and ZnS (c=0.626 nm) As x increases (more S substituting in O sublattice), the lattice parameter increases Bragg law: λ = 2d sin θ increasing d means decreasing θ. Vegard's law: lattice parameter of a solid solution of two constituents is approximately equal to a rule of mixtures of the two constituents' lattice parameters c ZnO1 x S x = xc ZnS + (1 x)c ZnO Composition x can be derived from the measured lattice parameter c.

31 XRD: crystallite size 31 Crystallites smaller than ~120nm create broadening of diffraction peaks this peak broadening can be used to quantify the average crystallite size of nanoparticles using the Scherrer equation contributions due to instrument broadening should be known by using a standard sample (e.g. a single crystal) Scherrer equation: B 2θ = Kλ L cos θ where B is the 2θ FWHM peak broadening in radian, λ is the wavelength of the x- ray used, L is the grain size and K~0.9

32 XRD: lattice strain 32 d o No Strain 2 d 1 Uniform Strain: (d 1 -d o )/d o Peak moves, no shape changes d strain 2 Non-uniform Strain d 1 constant Peak broadens d Broadeing b 2 2 tan d 2

33 XRD: preferred orientation 33 Preferred orientation of crystallites can create a variation in diffraction peak intensities that can be qualitatively analyzed using a 1D diffraction pattern (powder pattern) quantitatively analyzed by a pole figure which maps the intensity of a single peak as a function of tilt and rotation of the sample

34 Resistivity: the four point probe 34 The four point probe is commonly used to determine the resistivity of semiconductor samples (wafers) The outer 2 probes are connected to a current source The two inner probes are high impedance voltage sensors The sample thickness δ is assumed to be constant

35 Hall Effect 35 The Hall effect describes the behavior of free carriers in a semiconductor when an electric and a magnetic field are applied. The Hall coefficient for electrons is R H = E H,e J x B = 1 ne (negative) For hole carriers: R H = E H,h J x B = 1 pe (positive) The van der Pauw method is a technique commonly used to measure the resistivity and the Hall coefficient of a sample of any arbitrary shape The free carrier concentration n = 1 is given by: n = 1 B er H δ e R 24,13 With known resistivity and carrier concentration, the mobility is given by: μ = 1 neρ = 2ln(2) R 24,13 1 πb R 12,34 + R 23,41 f

36 Variable Temperature Hall Effect For a semiconductor sample with both donors and acceptors N d and N a. A variable temperature Hall effect measurement plotted as ln n vs. 1 T Arrhenius plot can tell us a lot of information Arrhenius Plot T At high temperature: Intrinsic n i e E g/2k B T slope= E g 2k B T Medium Temperature: k B T E d Extrinsic or saturation regime 36 Half slope Full slope n = N d N a Low temperature: freeze out region n e E d/2k B T slope= E d (half slope) 2k B T At even lower temperature n e E d/k B T (full slope) The concentration at which the half slope turns into full slop corresponds to N a Variable temperature Hall effect measurements provide a convenient way to obtain donor or acceptor binding energies.

37 Electrical measurements: a comparison 37 4 point probe Hall effect CV ECV thermopower Information obtained Conduction type (n or p) Resistivity Free carrier concentration, mobility Net ionized dopant concentration Net ionized dopant concentration No Yes Yes Yes Yes Sample size From mm to wafer 0.5 to 2 cm square 0.5 to 2 cm square >0.5 cm >0.5 cm Depth profiling No No Yes Yes Mo Destructive Somewhat No Need to form a Schottky contact Equipment cost Low Low to high (from <20kUSD to >200k USD) Low Yes High (>150k USD) Seebeck coefficient No Low to medium

38 Spectrophotometry 38 Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength in the spectral range of visible light (Vis), near-ultraviolet (UV), and near-infrared (NIR). It is more commonly called UV-Vis-NIR spectroscopy. Transmission and reflection data are combined to find the absorption coefficient α λ I T (λ) = (I o λ I R (λ))e α(λ)x α λ = 1 x ln I o λ I R (λ) I T (λ) From the absorption coefficient of a solid (thin film), the electronic properties can be derived. I o I R x I T

39 Optical Absorption Measurement 39 x I o I T I R ZnO thin film α λ = 1 x ln I o λ I R (λ) I T (λ) αhν 1/n = A hν E g To determine if the material has a direct or indirect gap plot αhν 1/n vs. hν Notice that we can linearly extrapolate αhν 2 vs. hν (n = 1/2) to obtain a band gap E g = 3.3 ev αhν 1/2 vs. hν (n = 2) does not result in a straight line ZnO is a direct gap material.

40 Spectroscopic ellipsometry (SE) 40 The sample has a layered structure and each layer i has optical constants (n i, k i ) and a thickness t i. The measured signal is the change in polarization as the incident radiation (in a known state) interacts with the material structure of interest (reflected, absorbed, scattered, or transmitted). The polarization change is quantified by the amplitude ratio, Ψ, and the phase difference, Δ. ρ = R p R s = tanψe iδ = f(n i, k i, t i ) where R p R s is the Fresnel reflection coefficient R p = E p(reflected) E p (incident) ; R s = E s(reflected) E s (incident)

41 SE: advantages and limitations 41 Non-destructive technique Film thickness measurement, can measure down to <1 nm Can measure optical constants n and k for unknown materials Absorption coefficient, band gap, carrier concentration, mobility, effective mass, etc. Can also measure film composition, porosity and roughness Absolute measurement: do not need any reference. Rapid measurement: get the full spectrum (190nm up1700nm) in few seconds Can be used for in-situ analysis Small equipment footprint: do not require a lot of lab space Can only measure flat, parallel and reflecting surfaces Some knowledge of the sample is required: number of layers, type of layers, etc. SE is an indirect measurement: does not give directly the physical parameters A realistic physical model of the sample is usually required to obtain useful information

42 Photoluminescence 42 Spectral feature Peak energy Peak width Peak intensity Material parameter Compound identification Band gap/electronic levels Impurity or exciton binding energy Quantum well width Impurity species Alloy composition Internal strain Fermi energy Structural and chemical quality Quantum well interface roughness Carrier or doping density Relative quantity Polymer conformation Relative efficiency Surface damage Excited state lifetime Impurity or defect concentration

43 Photoreflectance: principles 43 Changes in reflectivity R can be related to the perturbation of the dielectric R function of the material, ε = ε 1 + iε 2 : R R = α ε 1, ε 2 ε 1 + β ε 1, ε 2 ε 2 where α and β are the Seraphin coefficients, ε 1 and ε 2 represent photo-induced changes of the real and imaginary parts of the dielectric function, respectively. The imaginary part ε 2 changes slightly in electric field, resulting in a sharp resonance ε 2 exactly at the energy of the optical transition. It can be shown that in a case of the bulk crystal, the shape of dielectric function changes is of the third derivative of the unperturbed dielectric function.

44 Secondary ion mass spectrometry (SIMS) 44 SIMS is generally used for surface, bulk, microanalysis, depth profiling, and impurity analysis. The technique involves bombarding the surface of a sample with a beam of ions, thus emitting secondary ions. These ions are later measured with a mass spectrometer to determine either the elemental or isotopic composition of the surface of the sample. Cs +, O 2+, Ar + and Ga + at energies ~ 1-30 kev negative, positive, and neutral charges with kinetic energies ranging from zero to a few hundred ev. Sputtered species: Monatomic and polyatomic particles of sample material (+ve, -ve or neutral) Re-sputtered primary species (+ve, -ve or neutral) Electrons photons

45 Secondary ion yields: primary ion beams Secondary ion yield depends critically on the primary ion beam species. Typically Cs + and O 2 + ion beams are used in SIMS measurements. 45 O 2 + ions beam: During secondary emission the Me-O bonds break thus generating Me n+ Cs + ions beam: Increased availability of electrons leads to increased negative ion formation especially for elements with high electron affinity. Selection of primary ions: Inert gas (Ar, Xe, etc.) Minimize chemical modification Oxygen Enhance positive ions Cesium Enhance negative ions Liquid metal (Ga) Small spot for enhanced lateral resolution Oxygen works as a medium which strips off electrons from the speeding sputtered atoms when they leave surface, while Cesium prefers to load an electron on the sputtered atoms.

46 SIMS: secondary ion yield The number of secondary particles (atoms/ions) emitted by the surface for each impinging primary ion is defined as sputtering yield and can range between 5 and 15. The fraction of ionized emitted particles is called secondary ion yield and ranges typically between 10-4 to In SIMS, it is the secondary ions that are eventually detected Secondary ion current of species m I s m = I p y m α + θ m η I p = primary particle flux y m = sputter yield α + = ionization probability to positive ions θ m = fractional concentration of m in the layer η = transmission of the analysis system Ion yield is influenced by Matrix effects Surface coverage of reactive elements Background pressure Orientation of crystallographic axes with respect to the sample surface Angle of emission of detected secondary ions 46

47 SIMS: modes of operation 47 According to the primary ion energy and current, the SIMS technique can be divided into two variants: Static SIMS: kev ions are employed, with current surface densities in the na/cm 2 range, Under these conditions the total erosion of the sample first monolayer (1 nm) may take even an hour. Dynamic-SIMS: kev ions, with current surface densities in the A-mA/cm 2 range, are used. Under these conditions the sample is eroded continuously and the acquired mass spectra enable the monitoring of constituting elements along the sample depth (depth profiling). Ultra surface analysis Elemental or molecular analysis Analysis completed before significant fraction of molecules destroyed Profiling Material removal Elemental analysis

48 Static SIMS Range of elements H to U: all isotopes Destructive Yes, if sputtered long enough Chemical bonding Yes Depth probed Outer 1 to 2 monolayers Lateral resolution Down to below 100 nm Imaging/mapping Yes Quantification Possible with suitable standard Mass range Typically up to 1000 amu, amu (ToF) Main application Surface chemical analysis, organics, polymers 48 Positive ion TOF mass spectrum of polydimethylsiloxane contaminated polyethylene terephthalate Silicon wafer contaminated with copper, iron and chromium

49 Dynamic SIMS Range of elements H to U: all isotopes Destructive Yes, material removed during sputtering Chemical bonding In rare cases only Depth probed Depth resolution 2-30 nm, probe into m below surface Quantification Standard needed Accuracy 2% Detection limits atoms/cm 3 (ppb-ppm) Imaging/mapping Yes Sample requirements Solid; vacuum compatible 49 Near surface B depth profiles from a 2.2 kev BF implant in Si using different energies O 2 + primary beam

50 Comparison: static and dynamic SIMS 50

51 SIMS: summary 51 SIMS can be used to determine the composition of organic and inorganic solids at the outer 5 nm of a sample. Can generate spatial or depth profiles of elemental or molecular concentrations. These profiles can be used for elemental mapping. To detect impurities or trace elements, especially in semiconductors. Secondary ion images have resolution on the order of 0.5 to 5 μm. Detection limits for trace elements range between to atoms/cc. Spatial resolution is determined by primary ion beam widths, which can be as small as 100 nm. SIMS is the most sensitive elemental and isotopic surface microanalysis technique (bulk concentrations of impurities of around 1 part-per-billion). However, very expensive.

52 Advantages and weaknesses of SIMS 52 Advantages Excellent sensitivity, especially for light elements High surface sensitivity Depth profiling with excellent depth resolution (nm) (dynamic) Good spatial resolution (<1-25 m) Small analyzed volume (down to 0.3 m 3 ) so little sample is needed Information about the chemical surface composition due to ion molecules (static) Elements from H to U can be detected with excellent mass resolution Weaknesses Destructive method Element specific selectivity Standards needed for quantification Sample must be vacuum compatible Sample mist have a flat surface High equipment cost (>1M-3M USD)

53 SIMS and other techniques Characteristic AES XPS SEM/EDS SIMS Elemental range Li and higher Z Li and higher Z Na and higher Z All Z Specificity Good Good Good Good Quantification With calibration With calibration With calibration Correction necessary Detection limits (atomic fraction) 10-2 to to to to 10-8 Mass resolution element element element <isotope Lateral resolution ( m) 0.05 ~ Depth resolution (nm) Organic samples No Yes Yes * Yes Insulator samples Yes * Yes Yes * Yes * Structural information Elemental Elemental and Chemical Destructiveness Low High (profiling) Very Low High (profiling) Elemental Medium * = yes, with compensation for the effects of sample charging Elemental and Chemical High(dynamic) Medium(static)

54 Ion Beam Analysis: an overview 54 Inelastic Nuclear Reaction Analysis (NRA) p,, n, g Incident Ions Elastic Rutherford backscattering (RBS), resonant scattering, channeling Particle induced x-ray emission (PIXE) Defects generation Elastic recoil detection Ion Beam Modification Absorber foil

55 RBS: basic concepts 55 Kinematic factor: elastic energy transfer from a projectile to a target atom can be calculated from collision kinematics mass determination Scattering cross-section: the probability of the elastic collision between the projectile and target atoms can be calculated quantitative analysis of atomic composition Energy Loss: inelastic energy loss of the projectile ions through the target perception of depth These allow RBS analysis to give quantitative depth distribution of targets with different masses

56 Strengths & weaknesses of RBS 56 Simple in principle Fast and direct Quantitative without standard Depth profiling without chemical or physical sectioning Non-destructive Wide range of elemental coverage No special specimen preparation required Can be applied to crystalline or amorphous materials Simultaneous analysis with several ion beam techniques Poor lateral resolution (~0.5-1mm) Moderate depth resolution (>50Å) No microstructural information No phase identification Poor mass resolution for target mass heavier than 70amu (PIXE) Detection of light impurities more difficult Data may not be obvious: require knowledge of the technique

57 Hydrogen Forward Scattering (HFS) 57 Generally known as Elastic Recoil Detection (ERD) Charles Evans and Assoc., RBS APPLICATION SERIES NO. 3 Quantitative hydrogen and deuterium profiling Good sensitivity (~0.01at% of H) Can be perform simultaneously with RBS and PIXE Profiling with any light element in solid (using heavy ion beam, ERD)

58 Particle Induced X-ray Emission (PIXE)

59 Ion Channeling 59 Kobelco Steel Group

60 Flux Channeling: Impurity Lattice Location 3 r r o Atomic row y/y 1 = Channel center Distance from atom row, r/r o

61 Strengths of Ion Beam Analysis Techniques 61 Simple in principle Fast and direct Quantitative (without standard for RBS) Depth profiling without chemical or physical sectioning Non-destructive Wide range of elemental coverage No special specimen preparation required Can be applied to crystalline or amorphous materials Simultaneous analysis with various ion beam techniques (RBS, PIXE, NRA, channeling, etc.)

62 Elemental detection techniques 62 Factors EDS SIMS RBS AES XPS Probe Electrons Ions (O, Cs) Ions (He) Electrons X-ray Measured species X-rays Sputtered ions Scattered ions Electrons Electrons Vacuum (Torr) < < Acquisition time minutes Seconds Minutes Seconds Minutes Depth profiling No Yes (dynamic SIMS) Yes Yes, with sectioning Yes with sectioning Detection limits (atomic fraction) ~ to to to to 10-3 Lateral resolution (mm) ~1000 Elemental range Z>6 All Z Z>6 Z>3 Z>3