More review problems for OM 13 What are

More review problems for OM 13. What are differences between the common transmitted (or reflected) OM and polarized OM? 14. Can a polarized OM be used to determine the orientation (or optical axis) of an anisotropic single crystal? State your reasoning. 15. Can you see something from eyepiece lens when an isotropic crystal is on the sample stage of a polarized OM? Why or why not?

Lecture-4 Scanning Electron Microscopy (SEM) • • • What is SEM? Working principles of SEM Major components and their functions Electron beam - specimen interactions Interaction volume and escape volume Magnification, resolution, depth of field and image contrast Energy Dispersive X-ray Spectroscopy (EDS) Wavelength Dispersive X-ray Spectroscopy (WDS) Orientation Imaging Microscopy (OIM) X-ray Fluorescence (XRF) http: //www. mse. iastate. edu/microscopy http: //www. youtube. com/watch? v=KYNkn. R-e 5 IU to 2: 40 Introduction http: //virtual. itg. uiuc. edu/training/EM_tutorial http: //science. howstuffworks. com/scanning-electron-microscope. htm/printable

Comparison of OM, TEM and SEM Probe Source of electrons Light source Condenser Magnetic lenses Specimen Objective Projector Eyepiece Specimen CRT Cathode Ray Tube detector OM TEM SEM Principal features of an optical microscope, a transmission electron microscope and a scanning electron microscope, drawn to emphasize the similarities of overall design. https: //www. youtube. com/watch? v=b 4 WOs. Yktdn 4 comparing microscopes to 0: 12

Optical Microscopy (OM ) vs Scanning Electron Microscopy (SEM) radiolarian 25 m OM Small depth of field Low resolution SEM Large depth of field High resolution

What is SEM http: //virtual. itg. uiuc. edu/training/EM_tutorial Column SEM is designed for direct studying of the surfaces of solid objects Sample Chamber Scanning electron microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a OM. https: //www. youtube. com/watch? v=s. FSFp. Xd. Ai. AM http: //www. youtube. com/watch? v=lr. XMIgh. ANbg How a SEM works ~1: 20 -2: 10 http: //www. youtube. com/watch? v=Saa. Va. ILUObg Operation of SEM

Advantages of Using SEM over OM Magnification Depth of Field Resolution OM 4 x – 1000 x SEM 10 x – 3000000 x 15. 5 m – 0. 19 m ~ 0. 2 m 4 mm – 0. 4 m 1 -10 nm The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time and produces an image that is a good representation of the three-dimensional sample. The SEM also produces images of high resolution, which means that closely features can be examined at a high magnification. The combination of higher magnification, larger depth of field, greater resolution and compositional and crystallographic information makes the SEM one of the most heavily used instruments in research areas and industries, especially in semiconductor industry.

Scanning Electron Microscope – a Totally Different Imaging Concept • Instead of using the full-field image, a point-topoint measurement strategy is used. • High energy electron beam is used to excite the specimen and the signals are collected analyzed so that an image can be constructed. • The signals carry topological, chemical and crystallographic information, respectively, of the samples surface. https: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I http: //www. youtube. com/watch? v=lr. XMIgh. ANbg https: //www. youtube. com/watch? v=n. Pskv. GJKt. DI at~4: 18 -4: 38 at~4: 16 -4: 42

Main Applications • Topography The surface features of an object and its texture (hardness, reflectivity… etc. ) • Morphology The shape and size of the particles making up the object (strength, defects in IC and chips. . . etc. ) • Composition The elements and compounds that the object is composed of and the relative amounts of them (melting point, reactivity, hardness. . . etc. ) • Crystallographic Information How the grains are arranged in the object (conductivity, electrical properties, strength. . . etc. )

http: //virtual. itg. uiuc. edu/training/EM_tutorial A Look Inside the Column http: //www. youtube. com/watch? v=c 7 EVTn. VHN-s at~1: 10 -2: 10 inside the column

A more detailed look inside http: //www. youtube. com/watch? v=s. FSFp. Xd. Ai. AM <72 o e- beam Electron Gun Source: L. Reimer, “Scanning Electron Microscope”, 2 nd Ed. , Springer-Verlag, 1998, p. 2 https: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 at~1: 06 -2: 40 https: //www. youtube. com/watch? v=GY 9 lf. O-t. Vf. E at~2: 38 -4: 45 - beam convergence

• What is SEM? • Working principles of SEM • Major components and their functions

How an Electron Beam is Produced? • Electron guns are used to produce a • fine, controlled beam of electrons which are then focused at the specimen surface. The electron guns may either be thermionic gun or field-emission gun

Electron beam Source La. B 6 w W or La. B 6 Filament Thermionic or Field Emission Gun http: //www. youtube. com/watch? v=fx. EVsn. ZT 8 L 8 http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at ~2: 10 -2: 20 at ~1: 05 -1: 40 thermionic gun

http: //www. matter. org. uk/tem/electron_gun/electron_sources. htm http: //www. matter. org. uk/tem/electron_gun_simulation. htm Thermionic Emission Gun • A tungsten filament heated by DC to approximately 2700 K or La. B 6 rod heated to around 2000 K • A vacuum of 10 -3 Pa (10 -4 Pa for La. B 6) is needed to prevent oxidation of the filament • Electrons “boil off” from the tip of the filament • Electrons are accelerated by an acceleration voltage of 1 -50 k. V http: //www. youtube. com/watch? v=ZIJ 1 j. I 1 x. Dh. Y + Electron gun detail

Field Emission Gun • The tip of a tungsten needle is made very sharp (radius < 0. 1 m) • The electric field at the tip is very strong (> 107 V/cm) due to the sharp point effect • Electrons are pulled out from the tip by the strong electric field • Ultra-high vacuum (better than 10 -6 Pa) is needed to avoid ion bombardment to the tip from the residual gas. • Electron probe diameter < 1 nm is possible http: //www. matter. org. uk/tem/electron_gun/electron_sources. htm

Source of Electrons E: >10 MV/cm Thermionic Gun T: ~1500 o. C Filament W (5 -50 m) (5 nm) W and La. B 6 Cold- and thermal FEG Electron Gun Properties Source Brightness Stability(%) Size Energy spread W 3 X 105 ~1 50 m 3. 0(e. V) La. B 6 3 x 106 ~2 5 m 1. 5 C-FEG 109 ~5 5 nm 0. 3 T-FEG 109 <1 20 nm 0. 7 Brightness – beam current density per unit solid angle Vacuum 10 -5 (t ) 10 -6 10 -10 10 -9

Why Need a Vacuum? When a SEM is used, the electron-optical column and sample chamber must always be at a vacuum. 1. If the column is in a gas filled environment, electrons will be scattered by gas molecules which would lead to reduction of the beam intensity and stability. 2. Other gas molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image. http: //virtual. itg. uiuc. edu/training/EM_tutorial external http: //www. youtube. com/watch? v=c 7 EVTn. VHN-s at ~4: 50 -5: 25 http: //www. mse. iastate. edu/research/laboratories/sem/microscopy/how-does-thesem-work/high-school/the-sem-vacuum/ SEM Vacuum

Major components and their functions Magnetic Lenses • Condenser lens – focusing controls the spot size and convergence ( ) of the electron beam which impinges on the sample. • Objective lens – final probe forming determines the final spot size of the electron beam, i. e. , the resolution of a SEM. http: //www. youtube. com/watch? v=lr. XMIgh. ANbg http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at~1: 30 -1: 53 at~0: 32 -1: 02 http: //www. matter. org. uk/tem/lenses/simulation_of_condenser_system. htm

http: //www. youtube. com/watch? v=G 9 Glw 3 BUTAQ Magnetic field in a solenoid http: //www. youtube. com/watch? v=a 2_w. UDBl-g 8 e- in a magnetic field https: //www. youtube. com/watch? v=fwi. KRis 145 E to~1: 15 and at~3: 08 -4: 15 F=-e(vx. B) How Is Electron Beam Focused? http: //www. matter. org. uk/tem/lenses/electromagnetic_lenses. htm A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution. (Beam diameter) Magnetic lens (solenoid) p F = -e(v x B) q Lens formula: 1/f = 1/p + 1/q Demagnification: https: //www. youtube. com/watch? v=s. CY X_XQgn. SA&feature=related 6: 13 -6: 23 Magnetic lens M = q/p f B o 2 f can be adjusted by changing Bo, i. e. , changing the current through coil. Bo - magnetic field -

Lens formula and magnification Objective lens ho f f i O Lens Formula hi 1 _ f = 1 _ O + 1_ i I 1 -Inverted image f-focal length (distance) O-distance of object from lens i hi Magnification m = = i-distance of image from o ho O by objective lens http: //www. youtube. com/watch? v=-k 1 NNIOzj. Fo&feature=related at~3: 00 -3: 40

The Condenser Lens • For a thermionic gun, the diameter of the first cross-over point ~20 -50µm • If we want to focus the beam to a size < 10 nm on the specimen surface, the magnification should be ~1/5000, which is not easily attained with one lens (say, the objective lens) only. • Therefore, condenser lenses are added to demagnify the cross-over points.

http: //www. matter. org. uk/tem/lenses/simulation_of_condenser_system. htm The Condenser Lens Demagnification: M = f/L change of f

The Objective Lens • The objective lens controls the final focus of the electron beam by changing the magnetic field strength • The cross-over image is finally demagnified to an ~10 nm beam spot which carries a beam current of approximately 10 -9 - 10 -12 A.

The Objective Lens – Aperture • Since the electrons Electron beam coming from the electron gun have Objective spread in kinetic lens energies and directions Narrow Wide of movement, they may aperture not be focused to the same plane to form a Narrow disc sharp spot. Wide disc of of least confusion • By inserting an aperture, the stray electrons are Large beam diameter Small beam diameter blocked and the striking specimen remaining narrow beam Better resolution will come to a narrow “Disc of Least Confusion” https: //www. youtube. com/watch? v=E 85 FZ 7 WLvao http: //www. matter. org. uk/tem/lenses/simulation_of_condenser_system. htm aperture

A Look Inside the Column Objective aperture A disc of metal

http: //www. matter. org. uk/tem/lenses/second_condenser_lens. htm The Objective Lens - Focusing • By changing the current in the objective lens, the magnetic field strength changes and therefore the focal length of the objective lens is changed. Objective lens Out of focus in focus out of focus lens current too strong optimized too weak Over-focused Focused Under-focused

The Scan Coil and Raster Pattern • Two sets of coils are used for scanning the electron beam across the specimen surface in a raster pattern similar to that on a TV screen. y-direction scanning • This effectively coil samples the specimen surface point by point over the scanned area. X-direction scanning coil Holizontal line scan Blanking Objective lens specimen http: //www. youtube. com/watch? v=lr. XMIgh. ANbg at ~4: 12

http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I http: //virtual. itg. uiuc. edu/training/EM_tutorial at~4: 45 internal Electron Detectors and Sample Stage Objective lens Sample stage https: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 at~2: 20 -2: 30

Scanning Electron Microscopy (SEM) • What is SEM? • Working principles of SEM • Major components and their functions • Electron beam - specimen interactions • Interaction volume and escape volume • Magnification, resolution, depth of field and image contrast • Energy Dispersive X-ray Spectroscopy (EDS) • Wavelength Dispersive X-ray Spectroscopy (WDS) • Orientation Imaging Microscopy (OIM) • X-ray Fluorescence (XRF)

Electron Beam and Specimen Interactions Sources of Image Information Electron/Specimen Interactions (1 -50 Ke. V) Electron Beam Induced Current (EBIC) https: //www. youtube. com/watch? v=F 9 qwf. Yww. CRM at~0. 58 -1: 38 http: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 at~2: 30 -2: 42

http: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 at~2: 52 -3: 52 https: //www. youtube. com/watch? v=F 9 qwf. Yww. CRM at~0: 58 -1: 14 Secondary Electrons (SE) Produced by inelastic interactions of high energy electrons with Primary valence (or conduction) electrons of atoms in the specimen, causing the ejection of the electrons from the atoms. These ejected electrons with energy less than 50 e. V are termed "secondary electrons". Each incident electron can produce several secondary electrons. SE yield: d=n. SE/n. B independent of Z Ba. Ti. O 3 d decreases with increasing beam energy and increases with decreasing glancing angle of incident beam Production of SE is very topography related. Due to their low energy, only SE that are very near the surface (<10 nm) can exit the sample and be examined 5 m (small escape depth). Z – atomic number Growthstep SE image

Topographical Contrast Everhart-Thornley SE Detector Bright lens polepiece e. SE Dark Scintillator light pipe PMT sample Quartz window Faraday cage +200 V +10 k. V Photomultiplier tube Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample. Thus local variations in the angle of the surface to the beam (roughness) affects the numbers of electrons leaving from point to point. The resulting “topographic contrast” is a function of the physical shape of the specimen. http: //www. youtube. com/watch? v=lr. XMIgh. ANbg https: //www. youtube. com/watch? v=GY 9 lf. O-t. Vf. E http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at ~2: 10 -3: 30 at~4: 35 -6: 00 at~3: 00 -3: 20 (3: 09~3: 18)

Everhart-Thornley SE Detector System Solid angle of collection Both SE and B electrons can be detected, but the geometric collection efficiency for B electrons is low, about 1 -10%, while for SE electrons it is high, often 50% or more.

http: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 at~3: 52 -4: 26 Backscattered Electrons (BSE) Primary BSE image from flat surface of an Al (Z=13) and Cu (Z=29) alloy BSE are produced by elastic interactions of beam electrons with nuclei of atoms in the specimen and they have high energy and large escape depth. BSE yield: h=n. BS/n. B ~ function of atomic number, Z BSE images show characteristics of atomic number contrast, i. e. , high average Z appear brighter than those of low average Z. h increases with tilt. http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at ~3: 20 -3: 35 https: //www. youtube. com/watch? v=F 9 qwf. Yww. CRM at~1: 14 -1: 34

Semiconductor Detector for Backscattered Electrons High energy electrons produce electronhole pairs (charge carriers) in the semiconductor, and generate a current pulse under an applied potential. A and B are a paired semiconductor detectors https: //www. youtube. com/watch? v=F 9 qwf. Yww. CRM at~4: 25 -4: 50

Semiconductor Detector for Backscattered Electrons

Effect of Atomic Number, Z, on BSE and SE Yield

Interaction Volume: I e- Monte Carlo simulations of 100 electron trajectories The incident electrons do not go along a straight line in the specimen, but a zig-zag path instead.

Interaction Volume: II The penetration or, more precisely, the interaction volume depends on the acceleration voltage (energy of electron) and the atomic number of the specimen and e- beam size

Escape Volume of Various Signals • The incident electrons interact with specimen atoms along their path in the specimen and generate various signals. • Owing to the difference in energy of these signals, their ‘penetration depths’ are different • Therefore different signal observable on the specimen surface comes from different parts of the interaction volume • The volume responsible for the respective signal is called the escape volume of that signal.

Escape Volumes of Various Signals If the diameter of primary electron beam is ~5 nm - Dimensions of escape zone of • Secondary electron: diameter~10 nm; depth~10 nm • Backscattered electron: diameter~1 m; depth~1 m • X-ray: from the whole interaction volume, i. e. , ~5 m in diameter and depth http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at ~3: 38 -4: 10

Electron Interaction Volume Pear shape 5 m a b a. Schematic illustration of electron beam interaction in Ni b. Electron interaction volume in polymethylmethacrylate (plastic-a low Z matrix) is indirectly revealed by etching

Escape Volumes of Various Signals Primary SE Lost SE BE Lost BE X-ray

Lecture-3 SEM • What is SEM? • Working principles of SEM • Major components and their functions • Electron beam - specimen interactions • Interaction volume and escape volume • Magnification, resolution, depth of field and image contrast • Energy Dispersive X-ray Spectroscopy (EDS) • Wavelength Dispersive X-ray Spectroscopy (WDS) • Orientation Imaging Microscopy (OIM) • X-ray Fluorescence (XRF) http: //www. youtube. com/watch? v=s. FSFp. Xd. Ai. AM

http: //www. youtube. com/watch? v=VWx. Ys. ZPt. Ts. I at ~4: 18 -4: 45 Image Formation & Magnification in SEM M= C/x A e- beam 10 cm Detector 10 cm Amplifier A Beam is scanned over specimen in a raster pattern in synchronization with beam in CRT. Intensity at A on CRT is proportional to signal detected from A on specimen and signal is modulated by amplifier. http: //www. youtube. com/watch? v=lr. XMIgh. ANbg at ~4: 10 -4: 40

https: //www. youtube. com/watch? v=Mr 9 -1 Sz_CK 0 ex at~11: 45 -11: 55 Magnification Low M Large x 40 m High M small x 7 m 1. 2 m 15000 x 2500 x The magnification is simply the ratio of the length of the scan C on the Cathode Ray Tube (CRT) to the length of the scan x on the specimen. For a CRT screen that is 10 cm square: M= C/x = 10 cm/x Increasing M is achieved by decreasing x. M 1000 x 1 mm 100 m M 100000 x 10 m 1 m

Image Magnification Example of a series of increasing magnification (spherical lead particles imaged in SE mode)

Resolution Limitations Ultimate resolution obtainable in an SEM image can be limited by: ~ 500 nm NA=nsin 1. Electron Optical limitations dmin = 0. 61 /NA for OM Diffraction: dd=1. 22 / for a 20 -ke. V beam, =0. 0087 nm and =5 x 10 -3 dd=2. 1 nm Chromatic and spherical aberrations: dmin=1. 29 3/4 Cs 1/4 A SEM fitted with an FEG has an achievable resolution of ~1. 0 nm at 30 k. V due to smaller Cs (~20 mm) and . 2. Specimen Contrast Limitations Contrast dmin 1. 0 0. 5 0. 1 0. 01 2. 3 nm 4. 6 nm 230 nm 3. Sampling Volume Limitations (Escape volume) http: //www. youtube. com/watch? v=SVK 4 Ok. UK 0 Yw at~1: 47 -3: 07 Cs – coefficient of spherical aberration of lens (~mm)

How Fine Can We See with SEM? • If we can scan an area with width 10 nm (10, 000×) we may actually see atoms!! But, can we? • Image on the CRT consists of spots called pixels (e. g. your PC screen displays 1024× 768 pixels of ~0. 25 mm pitch) which are the basic units in the image. • You cannot have details finer than one pixel! Pixel - In digital imaging, a pixel (picture element) is a physical point in a raster image, or the smallest addressable element in a display device; so it is the smallest controllable element of a picture represented on the screen. http: //en. wikipedia. org/wiki/Pixel

Resolution of Images: I • Assume that there the screen can display 1000 pixels/(raster line), then you can imagine that there are 1000 pixels on each raster line on the specimen. • The resolution is the pixel diameter on specimen surface. P=D/Mag = 100 um/Mag P( m) Mag P(nm) P-pixel diameter on specimen surface D-pixel diameter on CRT, Mag-magnification 10 x 1 kx 10 0. 1 10 kx 10 100 kx 1

Resolution of Images: II • The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size.

Resolution of Images: III • Signal will be weak if escape volume, which depends on beam size, is smaller than pixel size, but the resolution is still achieved. (Image is ‘noisy’)

Resolution of Images: IV • Signal from different pixel will overlap if escape volume is larger than the pixel size. The image will appeared out of focus (Resolution decreased)

Resolution of Images: V In extremely good SEM, resolution can be a few nm. The limit is set by the electron probe size, which in turn depends on the quality of the objective lens and electron gun. Magnification 10 100000 Pixel diameter on Specimen µm nm 10 10000 1 0. 01 0. 001 1000 10 1

Resolution of Images The resolution is the pixel diameter on specimen surface. The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size. Effect of probe size on escape volume of SE e 10 nm BSE X-ray P=D/Mag = 100 um/Mag 1 m 5 m

Depth of Field D= 4 x 105 W AM ( m) A/2 = To increase D Decrease aperture size, A Decrease magnification, M Increase working distance, W (mm) =5 x 10 -3=0. 005 (1 o=0. 0175) OM 72 o=1. 26

SE Images Image Contrast Image contrast, C is defined by C= SA-SB ____ SA S = ____ SA SA, SB Represent signals generated from two points, e. g. , A and B, in the scanned area. In order to detect objects of small size and low contrast in an SEM it is necessary to use a high beam current and a slow scan speed (i. e. , improve signal to noise ratio). SE-topographic and BSE-atomic number contrast

SE Images - Topographic Contrast 1 m Defect in a semiconductor device The debris shown here is an oxide fiber got stuck at a semiconductor device detected by SEM Molybdenum trioxide crystals

BSE Image – Atomic Number Contrast 2 m BSE atomic number contrast image showing a niobium-rich intermetallic phase (bright contrast) dispersed in an alumina matrix (dark contrast). Z (Nb) = 41, Z (Al) = 13 and Z(O) = 8 Alumina-Al 2 O 3

Field Contrast Electron trajectories are affected by both electric and magnetic fields • Electric field – the local electric potential at the surface of a ferroelectric material or a semiconductor p-n junction produce a special form of contrast (Voltage contrast) • Magnetic field – imaging magnetic domains

Voltage contrast +U -U 500 m Voltage contrast from integrated circuit recorded at 5 k. V. The technique gives a qualitative view of static (DC) potential distributions but, by improvements in instrumentation, it is possible to study potentials which may be varying at frequencies up to 100 MHz or more, and to measure the potentials with a voltage resolution of 10 m. V and a spatial resolution of 0. 1 m.

Magnetic Field Contrast (monolayer) + - t tc SE electrons emitted from a clean surface ferromagnet are spin-polarized, the sign of the polarization being opposite to the magnetization vector in the surface of the material. High resolution SEM image of a magnetic microstructure in an untrathin ‘wedge-shaped’ cobalt film.

Other Imaging Modes Cathodoluminescence (CL) Nondestructive analysis of impurities and defects, and their distributions in semiconductors and luminescence materials Lateral resolution (~0. 5 m) Phase identification and rough assessment of defect concentration Electron Beam Induced Current (EBIC) Only applicable to semiconductors Electron-hole pairs generated in the sample External voltage applied, the pairs are then a current – amplified to give a signal Image defects and dislocations

CL micrographs of Te-doped Ga. As a. b. a. Te=1017 cm-3, dark-dot dislocation contrast b. Te=1018 cm-3, dot-and-halo dislocation contrast which shows variations in the doping concentration around dislocations

EBIC Image of Doping Variations in Ga. As Wafer The variations in brightness across the material are due to impurities in the wafer. The extreme sensitivity (1016 cm-3, i. e. , 1 part in 107) and speed of this technique makes it ideal fro the characterization of as-grown semiconductor crystals.

Do review problems on SEM Study http: //science. howstuffworks. com/scanning-electron-microscope. htm/printable Next Lecture • • Energy Dispersive X-ray Spectroscopy (EDS) Wavelength Dispersive X-ray Spectroscopy (WDS) Orientation Imaging Microscopy (OIM) X-ray Fluorescence (XRF) • Scanning Probe Microscopy (STM and AFM)