Transmissions Electron Microscopy (TEM)

Transcription

1 Transmissions Electron Microscopy (TEM) Basic principles Diffraction Imaging Specimen preparation A.E. Gunnæs MENA3100 V18

2 Electron interaction with the (thin) specimen e - Typical specimen thickness Absorbed electrons EBIC Auger electrons Cathodoluminescence Backscattered electrons Secondary electrons X-rays Specimen Gas Heating Cooling ~ 100 nm or less Elastically scattered electrons Inelastically scattered electrons Transmitted electrons Electrons interacts times stronger with matter than X-rays -need thin samples

3 perating modes Convergent beam Parallel beam Can be scanned (STEM mode) Spectroscopy and mapping (EDS and EELS) Specimen Example of EDS mapping in STEM mode. Cu 2 (sputtering, 600nm) Ti 2 (ALD, 10 nm) AZ (sputtering, ~200 nm) Quartz (1mm) S. Gorantla STEM: Scanning transmission electron microscopy HAADF: High angular annular dark field EDS: Energy dispersive spectroscopy EELS: Electron energy loss spectroscopy

4 TEM is based on three possible set of techniqes Diffraction From regions down to a few nm (CBED). Imaging With spatial resolution down to the atomic level (HREM and STEM) Spectroscopy Chemistry and elecronic states (EDS and EELS). Spatial and energy resolution down to the atomic level and ~0.1 ev. 200 nm SAD pattern CBED: Convergent beam electron diffraction SAD: Selected area diffraction BF TEM image HREM: High resolution electron microscopy BF: Bright field

5 Imaging and resolution Resolution of the eyes:~ mm Resolution in a visible light microscope: ~200 nm Modern TEMs with Cs correctors have sub Å resolution! A.E. Gunnæs

6 The interesting objects for TEM is local structure and inhomogeneities in specimens Important for material properties Defects Interfaces Cu Precipitates Local atomic structure and composition, HAADF image Zn Electronic structure and chemical bonding S. Gorantla Strain analysis around a dislocation core at the Cu-Zn interface

7 An example of a TEM study: Identification of an unknown phase in a thin film A.E. Gunnæs

8 PowderCell 2.0 Specimen: thin film of BiFe 3 + unknown phase Metal organic compound on Pt Ti 2 Pt BiFe 3 Lim Glue Heat treatment at 350 o C (10 min) to remove organic parts. Process repeated three times before final heat treatment at o C (20 min). (intermetallic phase grown) Si 2 Bi Bi Si Bi Fe Bi Bi Fe Fe 200 nm A.E. Gunnæs BF TEM image of the cross section of the specimen Fe Bi Bi Bi Fe Fe Fe Bi Bi Bi Fe Fe Goal to produce single phase: BiFe 3 with space grupe: R3C and celle dimentions: a= Å c= Å Fe Bi Bi Fe Fe Bi b c a Bi Bi

9 Determination of the Bravais-lattice of an unknown crystalline phase Tilting series around a dens row of reflections in the reciprocal space 0 o 50 nm 19 o A. E. Gunnæs Positions of the reflections in the reciprocal space 25 o 40 o 52 o Courtesy: Dr. Jürgen Thomas, IFW-Dresden, Germany

10 Electron Diffraction in TEM Elastic scattered electrons nly the direction of v is changing. (Bragg scattering) Elastic scattering is due to Coulomb interaction between the incident electrons and the electric charge of the electron clouds and the nucleus. (Rutherford scattering). The elastic scattering is due to the average position of the atoms in the lattice. Reflections satisfying Braggs law: 2dsinθ=nλ Electrons interacts times stronger with matter than X-rays -can detect weak reflections not observed with XRD technique Courtesy: Dr. Jürgen Thomas, IFW-Dresden, Germany

11 6.04 Å Bravais-lattice and cell parameters c b [011] [100] [101] a 100 d = L λ / R From the tilt series we find that the unknown phase has a primitive orthorhombic Bravias-lattice with cell parameters: a= 6,04 Å, b= 7.94 Å og c=8.66 Å 7.94 Å α= β= γ= 90 o

12 Chemical analysis by use of EDS and EELS Ukjent fase BiFe 3 BiFe 2 5 CCD CCD counts counts x 1000 x _1evprc.PICT Nr_2_1evprc.PICT - K Fe - L2,3 BiFe 3 Ukjent fase ev forskyvning, ev pr. 600 kanal Energy 400 Loss 600 (ev) Energy Loss (ev)

13 PowderCell 2.0 Published structure A.G. Tutov og V.N. Markin The x-ray structural analysis of the antiferromagnetic Bi 2 Fe 4 9 and the isotypical combinations Bi 2 Ga 4 9 and Bi 2 Al 4 9 Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy (1970), 6, Romgruppe: Pbam nr. 55, celleparametre: 7,94 Å, 8,44 Å, 6.01Å Bi x y z Bi 4g 0,176 0,175 0 Fe 4h 0,349 0,333 0,5 Fe 4f 0 0,5 0,244 4g 0,14 0, i 0,385 0,207 0,242 4h 0,133 0,427 0,5 2b 0 0 0,5 Fe Fe Fe Bi Fe Fe Bi Fe Bi Fe Bi Bi Fe Fe Bi Bi Fe Fe Fe b c a Celle parameters found with electron diffraction (a= 6,04 Å, b= 7.94 Å and c=8.66 Å) fits reasonably well with the previously published data for the Bi 2 Fe 4 9 phase. The disagreement in the c-axis may be due to the fact that we have been studying a thin film grown on a crystalline substrate and is not a bulk sample. The conditions for reflections from the space group Pbam is in agreement with observations done with electron diffraction. Conclusion: The unknown phase has been identified as Bi 2 Fe 4 9 with space group Pbam with cell parameters a= 6,04 Å, b= 7.94 Å and c=8.66 Å.

14 A.E. Gunnæs The construction of a TEM

15 A.E. Gunnæs

16 Basic TEM Electron gun Apertures 1. and 2. condenser lenses Sample Vacuum in the column better than 10-6 Pa Sample holder bjective lens Intermediate lenses Projector lens Recording media (Cameras, detectors) Fluorescence screen Similar components as a transmission light microscope

17 The electron source Two types of emission guns: Thermionic emission W or LaB 6 Field emission Cold FEG W Schottky FEG Zr/W Thermionic emission

18 Thermionic guns Filament heated to give thermionic emission -Directly (W) or indirectly (LaB 6 ) Filament negative potential to ground Wehnelt produces a small negative bias -Brings electrons to cross over

19 Field emission gun The principle: The strength of an electric field E is considerably increased at sharp points. E=V/r r W < 0.1 µm, V=1 kv E = V/m Lowers the work-function barrier so that electrons can tunnel out of the tungsten. Surface has to be pristine (no contamination or oxide) Ultra high vacuum condition (Cold FEG) or poorer vacuum if tip is heated ( thermal FE; Zr surface tratments Schottky emitters).

20 Resolution (JEL2100F: 0.19 nm) The point resolution in a TEM is limited by the aberrations of the lenses. -Spherical - Chromatic -Astigmatism

21 Electromagnetic lenses A charged particle such as an electron, is deflected by a magnetic field. The direction and magnitude of the force F, on the electron is given by the vector equation: F= -e(v x B)

22 Basic TEM Electron gun Apertures 1. and 2. condenser lenses Sample Vacuum in the column better than 10-6 Pa Sample holder bjective lens Intermediate lenses Projector lens Recording media (Cameras, detectors) Fluorescence screen Similar components as a transmission light microscope

23 c b a 3,8 Å Simplified ray diagram Si Parallel incoming electron beam Sample 1,1 nm PowderCell 2.0 bjective lense Diffraction plane (back focal plane) bjective aperture Image plane Selected area aperture

24 Selected area diffraction Parallel incoming electron beam Specimen with two crystals (red and blue) Diffraction from a single crystal in a polycrystalline sample if the aperture is small enough/crystal large enough. rientation relationships can be determined. ~2% accuracy of lattice parameters bjective lense XRD is much more accurate Diffraction pattern Pattern on the screen Selected area aperture Image plane

25 Electron Diffraction in TEM Amorphous phase Poly crystalline sample Single Crystals Interface between two different phases epitaxially grown The orientation relationship between the phases can be determined with ED. 25

26 Diffraction with large SAD aperture, ring and spot patterns SAD Similarities to XRD

27 Why do we observe many reflections in one diffraction pattern? 2dsinθ B =λ Cu K alpha X-ray: = 150 pm Electrons at 200 kv: = 2.5 pm Courtesy: Dr. Jürgen Thomas, IFW- Dresden, Germany

28 Illustration with the Ewald Sphere The radius of the Ewald sphere is 1/ (=k) Cu K alpha X-ray: = 150 pm => small k Electrons at 200 kv: = 2.5 pm => large k ko k Resiprocal lattice of a crystal

29 ED and form effects The dimensions of the specimen affects the shape of the resiprocal lattice poins Real space Resiprocal space

30 The intensity distribution around each reciprocal lattice point is spread out in the form of spikes directed normal to the specimen. 2d sinθ = nλ λ 200kV = nm Θ~1 o I(k -k)i=(2/λ)sinθ B =g

31 Zone axis and Laue zones Zone axis [uvw] (hkl) uh+vk+wl= 0

32 Indexing diffraction patterns The g vector to a reflection is normal to the corresponding (h k l) plane and IgI=1/d nh nk nl - Measure R i and the angles between the reflections - Calculate d i, i=1,2,3 (=K/R i ) - Compare with tabulated/theoretical calculated d-values of possible phases (h 2 k 2 l 2 ) - Compare R i /R j with tabulated values for cubic structure. - g 1,hkl + g 2,hkl =g 3,hkl (vector sum must be ok) - Perpendicular vectors: g i g j = 0 rientations of corresponding planes in the real space - Zone axis: g i x g j =[HKL] z - All indexed g must satisfy: g [HKL] z =0

33 A.E. Gunnæs TEM imaging with parallell incomming beam

34 Imaging / microscopy Amplitude contrast Phase contrast BiFe 3 Pt Si 2 Ti 2 Glue Si 200 nm The elctron wave can change both its amplitude and phase as it traverses the specimen Give rise to contrast We select imaging conditions so that one of them dominates.

35 Contrast Difference in intensity of to adjacent areas: C ( I 2 I1) I 1 I I 1 The eyes can not see intensity chanes that is less then 5-10%, however, contrast in images can be enhanced digitally. NB! It is correct to talk about strong and week contrast but not bright and dark contrast

36 Use of apertures Condenser aperture: Limits the number of electrons reaching the specimen (reducing the intensity), Affecting the convergent of the electron beam. Selected area aperture: Allows only electrons going through an area on the sample that is limited by the SAD aperture to contribute to the diffraction pattern (SAD pattern). bjective aperture: Allows certain reflections to contribute to the image. Increases the contrast in the image. Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution Images (several reflections from a zone axis).

37 c b a 3,8 Å Simplified ray diagram Si Parallel incoming electron beam Sample 1,1 nm PowderCell 2.0 bjective lense Diffraction plane (back focal plane) bjective aperture Image plane Selected area aperture

38 bjective aperture: Contrast enhancement Si Ag and Pb hole glue (light elements) No aperture used Amplitude contrast: Central beam selected Mass-Density contrast and Diffraction contrast A.E. Gunnæs

39 Mass-Density contrast in TEM Incoherent elastic scattering (Rutherford scattering): peaked in the forward direction, t and Z-dependent Areas of greater Z and/or t scatter electrons more strongly (in total). TEM variables that affect the contrast: -The objective aperture size. -The high tension of the TEM. Williams and Carter, TEM, Part 3 Springer 2009

40 Mass-density contrast A.E. Gunnæs

41 bjective aperture: Contrast enhancement Intensity: Dependent on grain orientation Diffraction contrast 50 nm Try to make an illustration to explain why we get this enhanced contrast when only the central beam is selected by the optical aperture.

42 Diffraction contrast Bright field image A.E. Gunnæs

43 Size of objective aperture Bright field (BF), dark field (DF) and High resolution EM (HREM) bjective aperture BF image DF image HREM image Amplitude/Diffraction contrast Phase contrast

44 Phase contrast: HREM and Moire fringes Long-Wei Yin et al., Materials Letters, 52, p HREM image Interference pattern A Moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes (rotational and parallel Moire patterns).

45 PART II Transmissions Electron Microscopy (TEM) Basic principles Diffraction Imaging Specimen preparation A.E. Gunnæs MENA3100 V18

46 A.E. Gunnæs Some repetiton.

47 TEM mode with parallel incoming electron beam Specimen Change the strength of the intermediate lens: The diffraction pattern or image of the specimen is magnified

48 TEM mode with parallel incoming electron beam Apertures (No: Blendere) Condenser bject SAD Specimen SAD aperture: used to select an area on the specimen one want diffraction date from (on the second intermediat image and projected).

49 TEM mode with parallel incoming electron beam Apertures (No: Blendere) Condenser bject SAD Specimen bjective aperture: used to select which electron beams will contribute to the image (on the second intermediat image and projected).

50 The objective aperture is used to controls the contrast in the image (enhances contrast). TEM mode with parallel incoming electron beam ne beam: Amplitude contrast (central (BF) or a scattered beam (DF)) Two or more beams: Phase contrast (+ amplitude) (HREM images (zone axis) or Moire) bjective aperture: used to select which electron beams will contribute to the image (on the second intermediat image and projected).

51 Size of objective aperture Bright field (BF), dark field (DF) and High resolution EM (HREM) bjective aperture BF image DF image HREM image Amplitude/Diffraction contrast Phase contrast

52 Phase contrast

53 Amplitude contrast Diffraction contrast and Mass-density contrast A TEM image will in most cases show both contrast types

54 Mass-Density contrast in TEM Incoherent elastic scattering (Rutherford scattering): peaked in the forward direction, t and Z-dependent Areas of greater Z and/or t scatter electrons more strongly (in total). Williams and Carter, TEM, Part 3 Springer 2009 TEM variables that affect the contrast: -The objective aperture size. -The high tension of the TEM.

55 Diffraction contrast in the TEM 50 nm The contrast is very sencitive to the specimen orientation. (In contrast to mass-density contrast) Bright field image A.E. Gunnæs

56 Effect of specimen tilt on diffraction contrast

57 Diffraction contrast

58 Where do you see a)mass-density contrast and b) Diffraction contrast? BiFe 3 Pt Ti 2 Glue Si 2 Si 200 nm

59 Crystal defects - Effect of bending - Dislocations - Wedges

60 Bending contours sample bj. lens bj. aperture BF image DF image DF image A.E. Gunnæs Solberg, Jan Ketil & Hansen, Vidar (2001). Innføring i transmisjon elektronmikroskopi

61 Bending contours

62 Dislocations

63 Double diffraction, extinction thickness Double electron diffraction leads to oscillations in the diffracted intensity with increasing thickness of the sample Incident beam No double diffraction with XRD, kinematical intensities Forbidden reflection may be observed t 0 : Extinction thickness Periodicity of the oscillations t 0 =πv c /λif(hkl)i Wedge shaped TEM sample Diffracted beam Doubly Transmitted diffracted beam beam t 0

64 Simplified kinematical theory for perfect crystals Basis of kinematical theory of electron diffraction for imperfect crystals: t Ψ g (t)= (πi/ξ g ) exp(-2πis g z)dz, 0 Ψ o =1, t: crystal thickess Ψ g (t)= (i/ξ g s g ) exp(-πits g ) sinπs g t Intensity of the scattered beam g (dark field): I g = l Ψ g (t) l 2 = sin 2 πs g t/(ξ g s g ) 2 Intensity of the unscattered beam 0 (bright field): I 0 = 1-I g = 1- l Ψ g (t) l 2 = 1 - sin 2 πs g t/(ξ g s g ) 2

65 Thickness fringes (s g konstant) In the two-beam situation the intensity of the diffracted and direct beam is periodic with thickness (I g =1- I o ) e 000 g I g =1- I o Sample (side view) t Hole Sample (top view) A.E. Gunnæs Intensity of the scattered beam g: I g = l Ψ g (t) l 2 = sin 2 πs g t/(ξ g s g ) 2 MENA3100 V10 Positions with max Intensity in I g

66 Thickness fringes, bright and dark field images Sample Sample BF image DF image A.E. Gunnæs MENA3100 V10

67 Kikuchi lines rigin and use

68 Line pairs in the diffraction plane Need to have a thick specimen region Close to a zone axis Zone axis pattern

69 Need two scattering events 1. Inelastic 2. Elastic 1. -Angular distribution of inelastic scattered electrons falls of rapidly with angle. I=I o cos 2 α

70 Kikuchi pattern. Incoherently and inelastically (ΔE~15-25 ev) scattered electrons give rise to diffuse background in the ED pattern 1.Ineleastic scattering + θb Deficient Excess 2. Bragg scattering event θb 2θB bjective lens Diffraction plane Deficient line 1/d Excess line What will happen if you tilt the specimen?

71 Kikuchi maps Kossel cones -g 000 g I g =I -g Sg<0 Sg=0 g and g Kikuchi lines Parabolas Effect of tilting the specimen

72 TEM specimen preparation

73 What to considder before preparing a TEM specimen Ductile/fragile Bulk/surface/powder Insulating/conducting Heat resistant Single phase/multi phase Etc, etc. What is the objectiv of the TEM work?

74 Specimen preparation for TEM Crushing Cutting saw, diamond pen, ultrasonic drill, ultramicrotomy Mechanical thinning Grinding, dimpling, Tripod polishing Electrochemical thinning Ion milling Coating Replica methods FIB (Focused ion beam) Etc.

75 Grids Several types Different materials (Mo, Cu, Ni ) Support brittle materials Support small particles 3 mm The grid may contribute to the EDS signal.

76 Preparation of self-supporting discs Top view specimens Cutting Ductile material or not? Grinding μm thick polish Cut the 3mm disc Dimple? Final thinning Ion beam milling Electropolishing

77 Cross section TEM sample preparation: Thin films Top view Cut out cylinder Grind down/ dimple Ione beam thinning Cut out slices Cut out a cylinder and glue it in a Cu-tube Grind down and glue on support rings Cross section Glue the interface of interest face to face together with support material or Focused Ion Beam (FIB) Cut a slice of the cylinder and grind it down / dimple Cut off excess material Ione beam thinning A.E. Gunnæs

78 Focused ion beam TEM specimen preparation