Chemical and Physical Characterization

Chemical and Physical Characterization S-69.4123 Postgraduate Course in Electron Physics I P 16.11.2011

Introduction • Probing with • Electron beams • Ion beams • X-rays • Measurands • Imaging • Composition • Impurities • Crystal structure • Thickness

Outline Introduction Electron beam techniques Ion beam techniques X-ray techniques Conclusions

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

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

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

Scanning electron microscope (SEM) Electron wavelength 10 kV acceleration voltage ->(compare to optical λ≈ 400 nm) (Rayleigh: ) However: Practical resolution ~1 nm (SEM in Micronova)1 1: http://www.speciation.net/Database/Instruments/Carl-Zeiss-AG/SUPRA-40-;i666

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

Auger electron spectroscopy (AES) Auger electrons: Characteristic energies -> element identification Low energy (30-3000 eV)-> surface probing

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)

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

SEM, Electron microprobe • X-ray energies characteristic for elements • Detection limit 102 – 104 ppm • Detection: • X-rays create electron-hole pairs in a detector crystal • Which are detected and counted • Nehp ~ X-ray energy

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

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

Transmission electron microscopy (TEM) Atomic scale resolution Diffraction pattern –> crystal structure and direction Also electron microprobe available EELS for accurate analysis TEM Images: Nature nanotechnology [1748-3387] Caroff, P v:2008 vol:4 iss:1 s:50

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

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

Secondary Ion Mass Spectrometry (SIMS) • 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) Static scan: Surface and interface analysis [0142-2421] Ogaki, R v:2008 vol:40 iss:8 s:1202 Depth profile: Applied physics letters [0003-6951] Zolper, J C v:1996 vol:68 iss:14 s:1945 • Two common modes: • Static: surface probed for complete mass spectrum • Dynamic: one mass/charge ratio is probed in a depth scan (sputtering ~10µm/h)

Secondary Ion Mass Spectrometry (SIMS) All elements detectable Detection limit 1014 – 1018 cm-3 (~0.1 – 100 ppm)-> the most sensitive beam technique Depth profiling Lateral resolution 0.5 – 100 µm, depth 5 - 10 nm Cons: destructive, high vacuum needed, crater wall effects, preferential sputtering, knock-on effects...

Rutherford Backscattering (RBS) 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)

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

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

X-ray fluorescence (XRF) Surface analysis with total reflection XRF (TXRF) Small incident angle assures surface probing XRF sensitivity: 100 ppm or 5x1018 cm-3

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

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

X-ray photoelectron spectroscopy (XPS) • Surface technique • e- escape depth shallow • Elemental + chemical analysis • Measured energy depends on chemical surroundings • Sensitivity ~0.1% or 1019 cm-3

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

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

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

Imaging techniques 2: "Lithium Atom Microscopy at Sub-50pm Resolution By R005". JEOL News45 (1): 2–7. 3: Science 337, 1326 (2012);

Surface elemental / chemical characterization More complete table on book page 677

Depth profile elemental / chemical characterization More complete table on book page 677

Tool availability in Otaniemi Eds in nanotalo sem, tem, µnova low-res sem

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...

Elemental / chemical characterization

Chemical and Physical Characterization