Chemical and Physical Characterization S Postgraduate Course in Electron Physics I P

Page  2 Introduction  Probing with –Electron beams –Ion beams –X-rays  Measurands – Imaging –Composition –Impurities –Crystal structure –Thickness

Page  3 Outline  Introduction  Electron beam techniques  Ion beam techniques  X-ray techniques  Conclusions

Page  4  Loosely bound electrons kicked out from the sample (E < 50eV)  High-E beam -> several SE:s for each incident e - Secondary electrons

Page  5  Electrons from electron gun  The beam is focused on the sample  Secondary electrons ejected from the sample Scanning electron microscope (SEM)

Page  6  Raster scan over the sample  Secondary electrons from each spot -> intensity for each spot -> image Scanning electron microscope (SEM)

Page  7  Electron wavelength  10 kV acceleration voltage -> (compare to optical λ ≈ 400 nm) (Rayleigh: )  However:  Practical resolution ~1 nm (SEM in Micronova) 1 1: Scanning electron microscope (SEM)

Page  8  Low-E electrons escape only from surface  X-rays from larger area  Z+ -> depth –  E+ -> depth + SEM signals and scattering

Page  9 Auger electron spectroscopy (AES)  Auger electrons:  Characteristic energies -> element identification  Low energy ( eV)-> surface probing

Page  10 Auger electron spectroscopy (AES)  Scanning -> resolution ~10nm  Chemical analysis: –Detectable elements Z = 3 and up –Detection limit 0.1 – 1% –Chemical information (Si vs SiO2)

Page  11 SEM, Electron microprobe  X-ray generation:  Similar to Auger

Page  12 SEM, Electron microprobe  X-ray energies characteristic for elements  Detection limit 10 2 – 10 4 ppm  Detection: –X-rays create electron-hole pairs in a detector crystal –Which are detected and counted –N ehp ~ X-ray energy

Page  13 SEM, Electron microprobe  Detector types: –Fast: Energy-dispersive spectrometer (EDS) –Accurate: Wavelength-dispersive spectrometer (WDS) Energy – element identification Intensity – density

Page  14 Transmission electron microscopy (TEM)  Electron gun  Focused on a thin sample  Electrons pass through -> scattering in the sample -> image or diffraction pattern

Page  15  Atomic scale resolution  Diffraction pattern –> crystal structure and direction  Also electron microprobe available  EELS for accurate analysis TEM Images: Nature nanotechnology [ ] Caroff, P v:2008 vol:4 iss:1 s:50 Transmission electron microscopy (TEM)

Page  16 Electron microscopy  Advantages –High resolution –High depth of field in SEM –Analysis tools integrable –Crystalline structure in TEM  Drawbacks –Sample charging –> insulating samples difficult to probe –TEM sample preparation (sample thickness ~≤200 nm) –Beam damage especially in TEM –Vacuum required

Page  17 Secondary Ion Mass Spectrometry (SIMS)  Sample is bombarded with an ion beam  Sputtering  Fraction of sputtered material ionized  Measured by mass spectrometer

Page  18  Counts for mass/charge ratios  Possible overlap (e.g. N, O, H, C + molecules often present)  Sputtering -> depth profiling –Initially distorted by sputtering yield Secondary Ion Mass Spectrometry (SIMS)

Page  19  Two common modes: –Static: surface probed for complete mass spectrum –Dynamic: one mass/charge ratio is probed in a depth scan (sputtering ~10µm/h) Static scan: Surface and interface analysis [ ] Ogaki, R v:2008 vol:40 iss:8 s:1202 Depth profile: Applied physics letters [ ] Zolper, J C v:1996 vol:68 iss:14 s:1945 Secondary Ion Mass Spectrometry (SIMS)

Page  20  All elements detectable  Detection limit – cm -3 (~0.1 – 100 ppm) -> the most sensitive beam technique  Depth profiling  Lateral resolution 0.5 – 100 µm, depth nm  Cons: destructive, high vacuum needed, crater wall effects, preferential sputtering, knock-on effects... Secondary Ion Mass Spectrometry (SIMS)

Page  21  High-E ions incident on the sample  Ions collide with sample atoms losing energy  Energy of backscattered ions measured  Energy loss depends on the material  Additional energy loss due to interactions with electrons Rutherford Backscattering (RBS)

Page  22 Rutherford Backscattering (RBS)

Page  23  Non-destructive  Determination of –Masses -> elements –depth distribution (res. ~10nm) –crystalline structure (ions penetrate deeper between crystal planes) Rutherford Backscattering (RBS)

Page  24 X-ray fluorescence (XRF)  Same as electron microprobe with e - -> X-ray  Comparison to electrons: + no charging + no vacuum +- larger area +- deeper penetration - no imaging

Page  25 X-ray fluorescence (XRF)  Surface analysis with total reflection XRF (TXRF)  Small incident angle assures surface probing  XRF sensitivity: 100 ppm or 5x10 18 cm -3

Page  26 X-ray photoelectron spectroscopy (XPS)  High-energy version of photoelectric effect  Like XRF, but ejected electron is measured

Page  27 X-ray photoelectron spectroscopy (XPS)  Commonly used to inspect alloys

Page  28 X-ray photoelectron spectroscopy (XPS)  Surface technique –e - escape depth shallow  Elemental + chemical analysis –Measured energy depends on chemical surroundings  Sensitivity ~0.1% or cm -3

Page  29 X-ray topography (XRT)  Defect detection: –Take monochromatic X-rays –Diffract the X-rays from a crystal plane (Bragg) –Take an image of the diffracted intensity –See defects and strain

Page  30 X-ray topography (XRT)  Surface scan:  Through-sample scan:

Page  31 X-ray diffraction (XRD)  Sample tilted over θ-angle  Intensity peaks at diffraction  Structure and composition information 31

Page  32 Imaging techniques TechniqueResolutionDepth-of-fieldDamageCostComments Optical microscope 0,25 µmModerateNoLow SEM~1 nmGoodOrganicsMediumCharging TEM< 50 pm [2] PoorYesHighCharging XRT1 µmGoodNoMediumDefect imaging (AFM)Atomic bonds [3] PoorNoMedium 2: "Lithium Atom Microscopy at Sub-50pm Resolution By R005". JEOL News 45 (1): 2–7. 3: Science 337, 1326 (2012);

Page  33 Surface elemental / chemical characterization TechniqueSmallest element Lateral resolution Depth resolution Detection limit cm -3 Information type Scan time AES (scan)Z=310 nm2 nm10 19 elem+chem30min EMP-EDSZ≈111 µm Elemental30min EMP-WDSZ≈41 µm Elemental2h SIMSZ=11 µm1 nm10 9 cm -2 Elemental1h RBSZ=30.1 cm20 nm10 19 elemental30min TXRFZ≈60.5cm5 nm10 10 cm -2 Elemental30min XPSZ=3100 µm2 nm10 19 Elem+chem30min More complete table on book page 677

Page  34 Depth profile elemental / chemical characterization TechniqueSmallest element Lateral resolution Depth resolution Detection limit cm -3 Information type Scan time Depth by... AES (scan)Z=310 nm2 nm10 19 elem+chem30minSputtering SIMSZ=11 µm1-30 nm Elemental1hSputtering RBSZ=30.1 cm20 nm10 19 elemental30minEnergy scale XRF-EDSZ≈ cm1-10µm10 19 Elemental30minPenetration XRF-WDSZ≈40.1-1cm1-10µm10 18 Elemental30minPenetration XRTNone1-10µm µm -Cryst. struct. + strain + defect 45minPenetration More complete table on book page 677

Page  35 Tool availability in Otaniemi  Eds in nanotalo sem, tem, µnova low-res sem ToolAvailability in Aalto CommentsCost per tool Optical microscope All overLow SEMMicronova, Nanotalo In Nanotalo a ”proper” SEM with EDS Tens-hundreds € TEMNanotaloDifferent tools available (res. <1Å) ~1 M€ XRDMicronova~100 k€

Page  36 Conclusions  Electrons, ions and X-rays give extensive chemical and physical information  Suitable technique depends on application –Needed sensitivity, destructive/non destructive, contact/noncontanct, conductivity...

Page  37 Elemental / chemical characterization 37