Advanced Materials Technology (FKMN20) Light Microscopy PDF

Advanced Materials Technology (FKMN20) Prof. Dmytro ORLOV Division of Materials Engineering, LTH [email protected] Microstructure Characterization and Analysi...

Advanced Materials Technology (FKMN20) Prof. Dmytro ORLOV Division of Materials Engineering, LTH [email protected] Microstructure Characterization and Analysis (Sections 5.1-5.8) Advanced Materials Technology (FKMN20) Prof. Dmytro ORLOV, Division of Materials Engineering, LTH, [email protected] GOALS 1. Understanding the properties of light and the basic goals and principles in the characterisation and analysis of material surface and structure; 2. Understanding principles and applications of optical microscopy for the characterisation and analysis of material surface and structure. 1  Characterisation of Material Structure Light Microscopy Electron Microscopy - Transmission Electron Microscopy - Scanning Electron Microscopy (SEM, ESEM, EDS) X-ray diffraction Neutron Scattering Thermal Analysis Elastic Scattering Electron Microscopy (EM): – Scanning Electron Microscopy (SEM): SEM-based Electron Channeling Patterns (ECP); – Transmission Electron Microscopy (TEM): Transmission Electron Diffraction (TED), Convergent Beam Electron Diffraction (CBED), Selected Area Electron Diffraction (SAED); – Scanning Transmission Electron Microscopy (STEM): Reflection High Energy Electron Diffraction (RHEED); Low Energy Electron Diffraction (LEED) X-ray Diffraction (XRD), Scanning Transmission X-ray Microscopy (STXM) Neutron Diffraction (ND). Inelastic Scattering Secondary Electron Imaging (SEI) Backscattered Electron Imaging (BEI/BSI) Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS), – EXtended Energy Loss Fine Structure (EXELFS), – Energy Loss Near Edge Fine Structure (ELNES), X-ray Emission Spectroscopy (XES), – X-ray Energy Dispersive Spectroscopy (XEDS), – Wavelength Dispersive Spectroscopy (WDS), Cathodoluminescence (CL) X-ray Photoelectron Spectroscopy (XPS) – X-ray Photoelectron Microscopy (XPM), Ultraviolet Photoelectron Spectroscopy (UPS), X-ray Absorption Spectroscopy (XAS), – EXtended X-ray Absorption Fine Structure (EXAFS), – X-ray Absorption Near Edge Fine Structure (XANES) X-Ray Fluorescence (XRF). 2  Scale vs Sensitivity The Electromagnetic Spectrum and Probe size / Resolution Limits Summary Energy vs Resolution in microscopic techniques 3  Characteristics of ‘light’ sources Elastic Mean Absorption Attainable Brightness Free Path Pathlength Probe Size Source (particles/sR/eV) (nm) (nm) (nm) Neutrons 1014 107 108 106 X-rays / photons 1026 103 105 ~ 3x101 Electrons 1029 101 102 < 10-1 Neutrons vs Photons (X-rays) Interaction of X-rays just depends on number of electrons; Interaction of neutrons with nuclei depends on isotope; Neutrons are more penetrating than X-rays (interact less with matter); Scattered intensity measured depends on which isotopes are in sample for neutrons, only on elements for X-rays; [Kardjilov et al Materials Today V.14 (6), 2011, pp.248-256] Optical vs Electron Microscopy Light Microscope Scanning Electron Transmission Microscope (SEM) Electron Microscope (TEM) Imaging Source Photons Electrons Electrons Wavelength, 300-700 nm ≤ 1 nm ≤ 1 nm Resolution 200 nm 1-10 nm ≤ 1 Angstrom Magnification ~1,000x 1,000,000x < 50,000,000x Sample Types Living/Dead Solid Solid Conductive Conductive Vacuum Compatible Vacuum Compatible < 25mm diameter 3mm diameter (typically) < 100nm thick Information Surface Topography, Surface Topography, Internal Structure, Provided Structure, Morphology Morphology, Composition, Composition, Orientation Orientation 4  Depth of Focus/Field Comparison of electron and X-ray properties (for SEM) Property Electron X-ray (photon) Mass 9.11x10-31kg (matter) None (electromagnetic radiation, l 10 to 0.01nm) Charge -1.60x10-19C None Speed Determined by energy Speed of light in medium Absorption Many mechanisms, frequent Approximation: X-rays travel in scattering in matter results a straight line until they are in loss of energy and/or destroyed in a single absorption change in direction event (the photoelectric effect) Diffraction Yes Yes and refraction Magnetic field Changes direction No effect Electric field Changes energy and/or No effect direction Electrons: In the SEM, electrons X-rays: described either by their energy E (eV) are usually described in terms of or by their wavelength l (nm): their energy (E) given in electron- l(nm) = 1239.6/E (eV), i.e. an X-ray with volts (eV) where: 1 eV = 1.6x10-19 J 0.1nm wavelength has an energy of 12,396 eV Property of Light Wave Vector Phase Beam 5  Resolution = f (Wavelength, NA) Resolution Limit θR is the angular position of the first order diffraction minimum (the first dark ring) λ is the wavelength of the incident light d is the diameter of the aperture 0.6 λ ρ= η sin(α ) λ is wavelength; η is the index of lens refraction; 2α is the angle through which the first- order beam is diffracted Resolution Limit Resolved features 0.6 λ ρ= η sin(α ) λ = wavelength of the imaging radiation η = index of refraction of the lens α= illumination semi- angle NA = numerical aperture = η sin (α) NON-resolved features 6  Chromatic Aberration Spherical Aberration Diffraction: 0D (single slit) Intensity I(x)=Iosinc2(πxw/(λL)) Diffraction patterns can be calculated mathematically. The operation that directly predicts the amplitude of the diffraction pattern from the mask is known as a Fourier Transform. 7  Diffraction: 1D series S=λL/X Diffraction: 2D series Imaging Bright Field 1/U+1/V=1/f Dark Field 8  5.3 X-ray diffraction analysis 171 30 Diffraction Atomic scattering factor Bragg demonstrated that diffraction is equivalent to Diffraction from crystal planes symmetrical reflection from the Incident Reflected 20 various crystal planes, provided Zinc rays rays certain conditions are fulfilled. If a beam of wavelength λ, impinging at an angle θ on a set N of crystal planes of spacing d. u 𝛳 u 𝛳 10 The beam reflected at the angle Aluminium θ can be real only if the rays from each successive plane P Q reinforce each other. For this to O Lattice d be the case, the extra distance planes a ray, scattered from each successive plane, has to travel, 0 0.2 0.4 0.6 0.8 i.e. the path difference, must be Sin u/ ! equal to an integral number of wavelengths, nλ. 𝑛𝜆 = 𝑃𝑂 + 𝑂𝑄 =(a)2𝑂𝑁 𝑠𝑖𝑛𝜃 = 2𝑑 𝑠𝑖𝑛𝜃 (b) 𝑎 FIGURE 5.8 2𝑎 𝑠𝑖𝑛𝜃 2𝑎 𝑠𝑖𝑛𝜃 𝑑 (" # $ ) = => 𝜆 = = ℎ & + 𝑘 & (a) 𝑛"ℎplanes. + 𝑙 &Diffraction from crystal " + 𝑛"𝑘 " + 𝑛"𝑙 " 𝑁 scattering curves for aluminium and zinc. (b) Form of the atomic N is a reflection number The lattice planes which give rise to a reflection at the smallest Bragg angle are those which are most widely spaced, i.e. those with a spacing equal to the cell edge, d100. The next planes in order of decreased spacing will be {1 1 0} planes for whichthan thethefirst d110=1/√2; rayplanes octahedral by the d111=1/√3.PO 1 OQ. The condition distance {111} have for reflection and reinforcement is then given by nλ 5 PO 1 OQ 5 2ON sin θ 5 2d sin θ (5.7) This is the well-known Bragg’s law, and the critical angular values of θ for which the law is sat- isfied are known as Bragg angles. The directions of the reflected beams are determined entirely by the geometry of the lattice, which in turn is governed by the orientation and spacing of the crystal planes. If for a crystal of cubic symmetry we are given the size of the structure cell, a, the angles at which the beam is dif- fracted from the crystal planes (hkl) can easily be calculated from the interplanar spacing The Reciprocal Space relationship pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dðhklÞ 5 a= ðh2 1 k2 1 l2 Þ (5.8) It is conventional to incorporate the order of reflection, n, with the Miller index, and when this is done the Bragg’s law becomes pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi λ 5 2a sin θ= ðn2 h2 1 n2 k2 1 n2 l2 Þ pffiffiffiffi (5.9) 5 2a sin θ= N where N is known as the reflection or line number. To illustrate this let us take as an example the second-order reflection from (1 0 0) planes. Then, since n 5 2, h 5 1, k 5 0 and l 5 0, this reflection is referred to either as the 2 0 0 reflection or as line 4. The lattice planes which give rise to a reflection at the smallest Bragg angle are those which are most widely spaced, i.e. those with a spacing equal to the Through theedge, cell Looking-Glass, and What next Alice Found There of decreased spacing will be {1 1 0} planes for which pdffiffiLooking-Glass (also known as Alice Through the ffi 0 0. The 1 planes in order or simply Through the Looking-Glass) pffiffiffi an 1871 novel by Lewis 5 a= and2,thewhile d110 Carroll sequel the octahedral to Alice's Adventures{1 1 1} planes in Wonderland will have a spacing equal to a= 3. The angle at (1865) The Reciprocal Lattice, fcc case Reason to use is constructed from the real lattice by the sets of lattice planes are replaced by a set drawing a line from the origin normal to of points, this being geometrically simpler. the lattice plane hkl under consideration of length, d*, equal to the reciprocal of the interplanar spacing: d* = 1/dhkl. üThe reciprocal lattice of a simple cubic lattice with primitive cell side a is again a simple cubic lattice, but with cell side 2π/a. üThe reciprocal lattice of an fcc Bravais lattice with conventional cubic cell side a is a bcc lattice with conventional cubic cell side 4π/a. üThe reciprocal lattice of a bcc lattice with conventional cell side a is similarly an fcc lattice with conventional cell side 4π/a. üThe reciprocal lattice on a simple hexagonal Bravais lattice with lattice constants a and c is also a simple hexagonal lattice but with lattice constants 4π/√3a and 2π/c, and rotated 30° around the c-axis. 9  First Optical Microscope 1665: “Micrographia” by Robert Hook Optical Microscope Objective lines Ocular lines Useful Magnification Range ‘rational’ approach Based on the selection of a set of lenses that is comprehensive and ‘useful’ (exempt from ‘empty’ magnification). Typically, the Michel series of 6.3x, 8x, 10x, 12.5x, 16x, 20x, 25x, etc. – a geometrical progression with a common ratio of ~1.25. Alternatively, a geometrical progression devised by a French military engineer, Colonel Charles Renard (1847-1905) in 1879 (originally for classifying cable diameters during the development of steerable balloons). A typical Renard series is 1.25, 1.6, 2.0, 2.5, 3.2, 4.0, 5.0, 6.4, 8.0, etc. 10 The Metallurgical Microscope Ocular Objective Köhler Illumination Reflected Light Microscopy refractive index refractive index air: n = 1 cedar wood oil: n = 1.5; monobromonaphthalene: n = 1.66 Use of oil to improve numerical aperture of objective Reflection of light from etched specimen 11  Light Microscopy Numerical Aperture (light-gathering power) NA = n · sina – typical NA range: 0.08-1.25; Resolution (Abbe formula) d = l /2 (NA) – Resolution limit d ≈ 200 nm; ‘useful range of magnifications (500-1000)´(NA) = 40x to 1250x Depth of field: 250µm (12x), 0.08µm (1200x) Objective Semi-apex angle a Refractive index = n Sample surface Optical Microscopy Methods Special illumination methods used in reflected light microscopy Used to reveal microstructural details of metallic samples even in the as polished condition Dark-field illumination, Polarized light, Phase contrast, Differential interference contrast Köhler illumination is used in all these methods Dark-field Illumination Regularly reflected light does not pass through the objective Light rays deflected by diffuse scattering are used for image formation Completely reversed contrast relative to bright-field illumination Surface regions normal to the optical axis appear dark, inclined surfaces appear light Reveals cracks, pores, voids, inclusions 12 Dark-field / Bright-field illumination Polarized Light Microscopy Incident light on the specimen is plane polarized and the reflected light is analysed by a polarizing unit Optically anisotropic metals / phases reflect plane polarized light as elliptically polarized light - rotation of the plane of polarization depends on crystal structure, orientation Grain contrast is seen as a variation in brightness and colour For revealing grain structure, identify phases Polarized Light Micrographs 13  Phase-Contrast Microscopy Small differences in height on the specimen surface are used to generate contrast A filter (phase-plate) generates the phase contrast Positive contrast: higher areas are brighter Min. Difference in height: 1 to 5 nm, optimal difference: 20 to 50 nm Phase identification: carbide, 𝜎-phase in chromium steel Differential Interference Contrast Plane polarized ray of light - through a bi-prism, split into 2 linearly polarized rays (perpendicular planes of polarization) Reflected rays are recombined and interference is produced in the analyser Height differences, optical properties of the specimen – light, dark...interference contrast Images have a 3-D appearance Differential Interference Contrast Image (U-33at%Al-25%Co alloy at 250X) 14  Examples of optical images - I Images of an fcc lattice of a copper alloy brightfield darkfield DIC in polarized light and from different angles [http://www.leica-microsystems.com/science-lab/metallography-with-color-and-contrast/ (last accessed on 2017.03.23)] Examples of optical images - II Polarization and interference contrast with colour etching Grain area etching and subgrain formation in tin Zinc with twinning Cross-section view of an electronic component, ceramics, metal and glass-fiber reinforced plastic [http://www.leica-microsystems.com/science-lab/metallography-with-color-and-contrast/ (last accessed on 2017.03.23)] Depth of field as a function of wavelength and Numerical Aperture 15  Confocal Microscopy Examples of optical images - III ‘scanning’ techniques in ‘digital microscopy’ Correction of surface height difference Enhanced depth of focus 3D surface reconstruction [Brochure of Digital Microscope VHX-5000 by Keyence (December 2016)] Summary The ‘light’ from any source has common basic properties allowing the characterization of surfaces and structures in various materials; Optical microscopy is a simple but powerful technique, which has been re- discovered recently with the introduction of digital ‘Scanning’ of samples; 16