E-beam Characterization: a primer Part 1. Matthew J Kramer 225 Wilhelm October 30,2009

Transcription

1 E-beam Characterization: a primer Part 1 Matthew J Kramer 225 Wilhelm mjkramer@ameslab.gov October 30,2009

2 Why and Which Method E-beam characterization IT IS the only reliable means for microstructural and microchemical analysis! E-beam Instruments Surface SEI From mm to nm, surface information only BSE Surface Z distribution Topography Transmission Diffraction contrast Microns to atoms Crystallography Both types can utilize the various spectroscopes

3 Forest or the Leaves

4 What do you need to know? Surface scanning Loose powders to polished surfaces Large depth of field Morphology will effect spectroscopy

5 Transmission Microscopy Region of interest needs to be electron transparent ~ nm, depending on Z How you thin can matter Significantly higher resolution Very different imaging contrast used

6 The SEM E beam is accelerated by a large voltage Lens forms a fine probe Rastered across the sample SEI BSE

7 Imaging Secondary Electrons Very near surface Backscattered Electrons Strongly Z dependent e(bs)/e(incident) Interactions at a depth Z Co-Sm-Fe alloy

8 Crystallography BSE can be induced to channel along crystallographic planes Electron Backscattered Diffraction (EBSD)

9 Si dendrites in Al matrix 10 mm (a) 1 mm

10 e interactions Auger electrons Two views of the Auger process. (a) illustrates sequentially the steps involved in Auger deexcitation. An incident electron creates a core hole in the 1s level. An electron from the 2s level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted. The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b) illustrates the same process using spectroscopic notation, K L1 L 2,3.

11 e interactions C and higher Z E-beam energy must exceed the binding energy

12 Auger e s vs X-rays Counter yield 200 x 200 μ XPS Auger e s low energy Depth: μ (2-10 nm) 5-20 atom layers Require a high vacuum Very near surface 1 μ 3 EDS 0.01 x 0.01 x μ (10 x 10 x 2 nm ) AES NOTE: Scaling is only approximate Sample being analyzed

13 Detecting and Quantifying X-rays Solid State detectors Energy Dispersive Spectroscopy (EDS)

14 Detecting and Quantifying X-rays Wavelength Dispersive Spectroscopy Much higher energy resolution S (WDS) Mo (WDS) S (EDS) counts (arbitrary units) Separation of Mo Lα from S Kα 14.3 ev EDS energy resolution 120eV FWHM WDS energy resolution 3.45eV FWHM E [kev]

15 Quantification and Spatial Resolution Average Z and E dependent Requires stable e-beam source Requires standardization Requires known geometry i.e., flat, parallel surface at % Al Ge distance (µm) 1 µm

16 Auger Ge Al KV 20 EDS Al-K Ge-L

17 JEOL 5910lv Resolution (SEI) mmwd Magnification ,000 x Probe Current 1pa 1 microamp Image modes: SEI 3 Backscatter Topo Compo Shadow EDS Line scans mapping

18 JEOL 5910lv Poor Vacuum mode: Resolution mmwd Backscattered mode only in the poor vac mode Adjustable chamber pressure Pa Tungsten filament with automatic adjustments Beam Blanked in Freeze mode Alignments and conditions automatically and individually saved for each user Specimen Chamber and Stage: 5 axes (X 125mm range, Y 100mm range, Z 43mm range, T 10 to 90 range, R 360 endless) Maximum specimen size 7 with full coverage Specimen position graphical indicator as well as chamber camera Absorbed current ( Specimen current ) measured Image memory selectable to 1,280 x 960 x 8 bits Can frame average up to 255 frames Image storage formats BMP, TIFF or JPEG. We recommend BMP with merged text.

19 JEOL JXA-8200 Superprobe combined WDS/EDS 5 wavelength-dispersive spectrometers, 10 crystals B through U four 140 mm Rowland circle one 100mm Rowland circle EDS, 10 mm 2 Si(Li) crystal, Be window Na through U Software quantitative analysis compositional mapping phase analysis integrated WDS/EDS operation. Crystal K lines L lines LIF (140mm) Ca Rb Sn U LIF (100mm) Ca Ge Sb Hg PET (140mm) Si Fe Rb Tb PET (100mm) Si Ti Rb Ba TAP O P Cr Nb LDE1 (2d=60Å) LDE2 (2d=90Å) C, N, O, F B, C, N, O

20 Overall Capabilities Electron Optical and Vacuum Systems W or LaB6 electron source Acceleration voltage kV Useable beam current to 10-5A Pneumatically driven Faraday cup for beam current measurement (serves as beam blanking) Turbomolecular vacuum pump All functions controlled through central computer system Imaging Capabilities Secondary electron detector Backscattered electron detector with composition/topography mode 10 user selectable scan speeds Magnification 40x to 300,000x Optimal imaging resolution 6nm Dedicated 18 inch flat panel display for electronic images Optical microscope for reflected light observation of sample Optical system coaxial with electron beam High resolution color video mini camera Automatic optical focus device

21 Sample Preparation Maximum sample size 150 x 150 x 50mm 4 x 1in. diameter samples Stage travel x = 100mm, y = 90mm, z = 3mm Stage tracking < ±1μm Requires flat, well polished surface Standardization Elemental or line compounds

22 Element Mapping

23 JAMP7830F Auger Microprobe Schottky field emission gun 0.5 to 25kV beam voltage to 1x10-7 A current Minimum probe size: 4 nm in SEM mode, 10 nm in Auger mode Chamber pressure 5 x 10-8 Pa (3 x torr) Stage movement: X,Y mm, Z + - 6mm Normal sample size: 12mm dia. 5mm thick

24 JEOL 7830F Schematic View Key Components that produce High Energy Resolution and Reliable KE Values JEOL 7830F AES Advantages & Capabilities ( HSA vs CMA, and FE vs LaB 6 )

25 Elemental Mapping Nb-Cu metalmetal composit e

26 High Energy Resolution AES Chemical State Map: Cu o vs Cu 2 O (Δ E = 0.9 ev) Cu o Cu 2 O Red = Cu o Green = Cu 2 O SEM

27 Chemical Shifts This sample was ion etched to remove all contamination and left in UHV at 3 x torr 14 hr. This reveals the reactive nature of clean surfaces. 14 hr - end O Gas Capture Study CrO x O hr - start Cr o CrO x Cr o Reactive Nature of the Clean Surface of a Co-Ni-Cr Alloy

28 Limitations of Auger Electron Spectroscopy Cannot detect hydrogen or helium Destructive depth profiles. Samples must be small and compatible with high vacuum. Elemental quantization depends on instrumental, chemical, and sample related factors. Chemical information is depended on quantity and element Most sample surfaces are contaminated must be cleaned by ion etching

29 Sample Preparation Best if sample is parallel polished Best sample size: < 10 mm dia. < 3 mm thick Please no potting of sample in plastic matrix For very small samples consult with us first Samples that are used in some type of process, need to be in proper form before processing Remember ---- do not touch the samples with your hands!

30 Conclusions 1 st determine what you need to know i.e., grain size or just average chemistry How precise do your measurements need to be? Will dictate sample preparation, obtaining standards etc. WDS vs EDS or is BSE good enough What elements are possibly present, including impurities? Are there overlaps? WDS vs EDS Chemistry of the surface or of the bulk? XPS and SAM vs SEM Decide which instrument(s) will be needed Discus techniques and sample preparation first with staff Not with your classmates (unless they are an expert)! Keep an open mind It is not unusual that your preconceived notions are wrong But artifacts are possible How does grinding and polishing affect the composition and possibly microsctucture Is your sample sensitive to air, moisture etc. How do these results mesh with other bulk measurements? Microscopy is a powerful tool, but one of many tools, it should be complimented with other techniques when appropriate. XRD, SQUID etc.