Introduction of Microscopy and Microanalysis (1)

Dr. Chunlei Wang Oct 14, 2009

SEM Pictures Copyright@peggy wang

Instructor: Dr. Chunlei Wang Office hours: Tue and Thu 11:00 -12:00 Office location: EC 3463

Reference books: (1) Electron Microscopy and Analysis (3 rd edition) by Peter J. Goodhew , John Humphreys, and Richard Beanland , Taylor&Francis , 2001 (2) Transmission Electron Microscopy (I) Basics, by David B.Williams and C. Barry Carter, Springer, 1996

SEM Lab: Oct 16 and Oct 25, 1:00 -3:00pm TEM Lab: end of Nov Grading: Two homeworks (40%) + One final written exam (60%)

1 Why microscopy and microanaylsis?

Typical cleanroom equipments:

Photolithography tools Thin film deposition and material growth Dry etching Wet etching Furnaces and ovens Packaging Sample preparation Analytical equipment Critical dimension measurements of small features Topography and 3D microstructure Qualitative identification and quantitative elemental information Chemical composition and compositional profile Defect and impurity …

Typical Analytical Techniques

Optical Microscopy Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) Scanning Probe Microscopy (AFM, STM) Energy Dispersion X-ray Spectroscopy (EDS) X-ray Diffraction Spectroscopy (XRD) Photoelectron Spectroscopy (XPS, UPS) Auger Electron Spectroscopy (AES) Electron Energy Loss Spectroscopy (EELS) Rutherford Backscattering Spectroscopy (RBS) Focused Ion Beam (FIB) Secondary Ion Mass Spectrometry (SIMS) Mass Spectrometry (MS) Nuclear Magnetic Resonance spectroscopy (NMR) Copyright © 2003, Charles Evans & Associates. Photoluminescence Spectroscopy (PL) Cathodoluminescence Spectroscopy (CL) Raman Spectroscopy Infrared spectroscopy (IR, FTIR)

2 Microscope

There are many types of microscopes:

Bright-field microscope •≥2 optical lenses Dark-field microscope Phase-contrast microscope •Resolution: wavelength of light Fluorescence microscope UV, violet, or blue light

Confocal microscope laser

Scanning Electron Microscope (SEM) Electron beam Transmission Electron Microscope (TEM)

Scanning Probe Microscope (SPM): Atomic force microscope (AFM) Constant distance Scanning tunneling microscope (STM) Constant current

Ocular (eyepiece)Optical vs. Electron Microscopy

Objective

First Place, Nikon's Small World 1995 Competition, Christian Gautier, Larva of Pleuronectidae (20x), Rheinberg Illumination and Polarized Light

• Easy to use • Samples in air or water • Total magnification: ×100-1000 product of the magnifications of the First Place Winner, Nikon's Small ocular lens and the objective lens World 2005 Competition, Charles B. • Image processing by CCD Krebs, Muscoid fly (house fly) (6.25x) Reflected light http://www.microscopyu.com/articles/optics/components.html

3 Brief History • In 1801, Thomas Young passed a beam of light through two parallel slits in an opaque screen, forming a pattern of alternating light and dark bands on a white surface beyond. This led Young to reason that light was composed of waves. wave theory of light

Thomas Young (1773-1829)

Brief History

• In 1897, J.J.Thomson discovered “corpuscles”, small particles with a charge/mass ratio more than 1000 times greater than that of protons, swarming in a sea of positive charge (“plum pudding model”).

Discovery of the ELECTRON

Sir Joseph John Thomson Thomson’s 2 nd Cathode ray experiment (1856-1940) Nobel prize 1906

4 Brief History

In 1924, Louis de Broglie first theorized that the electron had wave -like characteristics. Application of the idea of particle – wave dualism (only known for photons up to then) for any kind of matter. (first person to receive a Nobel Prize on a PhD thesis )

Electron=Particle & Wave

h h λ = = p mv

Louis Victor de Broglie (1892-1987) Nobel prize 1929

Brief History

In 1926, Hans Busch discovered that magnetic fields could act as lenses by causing electron beams to converge to a focus (electro n lens).

5 Brief History

In 1927, Davisson and Germer , Thomson and Reid, independently carried out their classic electron diffraction experiments (demonstration of wave nature of electrons) Electron=Wave Electron gun detector

Ni Crystal

Interference peak ) Sir George Paget Thomson θ (1892 – 1975) GP Thomson Experimental Apparatus and Results I( Nobel Prize: 1937 o 0θ 60 (shared with C.J. Davison) Davisson-Germer experiment

Brief History

In 1931, Knoll (inventor of SEM, 1935) and Ruska co -invent electron microscope and demonstrated electron images.

Max Knoll Ernst Ruska (1897-1969) (1906-1988) Knoll and Ruska co-invent electron microscope Nobel Prize 1986

6 Brief History

1938: M. von Ardenne: 1st STEM 1936: the Metropolitan Vickers EM1, first commerical TEM, UK 1939z: regular production, Siemens and Halske , Germany After World War II: Hitachi, JEOL, Philips, RCA, etc 1945: 1nm resolution 1949: Heidenreich first thinned metal foils to electron transparency Cambridge group developed the theory of electron diffraction contrast Thomas pioneered the practical applications of the TEM for the solution of materials problems (1962) ……

Optical vs. Electron Microscopy

7 SEM

SEM permits the observation and characterization of heterogeneous organic and inorganic materials on a nm to µm scale. Imaging capabilities elemental analysis

In the SEM, the area to be examined or the microvolume to be analyzed is irradiated with a fine focused electron beam, which may be swept in a raster across the surface of the specimen to form images or maybe static to obtain an analysis at one position.

The types of signals produced from the interaction of electron beam with the sample include secondary electrons, backscattered electrons, characteristic x-rays, and other photons of various energies .

Optical vs. Electron Microscopy

(a) (b)

(a) Optical micrograph of the radiolarian Trochodiscus longispinus . (b) SEM micrograph of same radiolarian. (Taken from J.I. Goldstein et al., eds., Scanning Electron Microscopy and X-Ray Microanalysis, (Plenum Press,NY,1980).) Depth of Focus Resolution

8 Why electrons?

Optical microscope: 400-700nm, resolution? Electron microscope: ? Light e- • p = h/ λ (matter also) • p = E/c Wave Behaviors • E = hf = hc/ λ – images and diffraction patterns Matter – wavelength can be tuned by energies • p = h/ λ (light also) • e- p = 2mE Charged Particle Behaviors • E = h 2/2m λ2 – strong electron-specimen interactions h: planck’s constant – chemical analysis is possible

Matter Waves

DeBroglie (1924) proposed that, like photons, particles have a wavelength: electron gun detector λ = h/p Inversely proportional to momentum. θ

In 1927-8, it was shown (Davisson-Germer) that, like x-rays, ELECTRONS can also diffract off crystals ! Ni Crystal Interference peak !

Electrons can act like waves!! ) θ I( Particles as waves Electron Diffraction o 0θ 60

9 Matter wavelengths

What size wavelengths are we talking about? Consider a photon with energy 3 eV, and therefore momentum p = 3 eV/c. Its wavelength is: h 4.14 ×10 −15 eV ⋅ s λ = = ×c = (1.4×10 −15 s )× (3×10 8 m s/ )= 414 nm p 3 eV What is the wavelength of an electron with the same momentum?

λ = h/p Same relation for e = h/p e particles and photons.

Note that the kinetic energy of the electron is different from t he energy of the photon with the same momentum (and wavelength): 2 2 −34 2 p h (6.625 ×10 J ⋅ s) − KE = = = = 1.41 ×10 24 J 2m 2mλ2 2( 9.11 ×10 −31 kg )( 414 ×10 −9 m )2 ÷1.602 ×10 −19 J eV/ = 8.8×10 −6 eV Compared to the energy of the photon (given above): E = pc = 3eV

Wavelength of an Electron

The DeBroglie wavelength of an electron is inversely related to the electron momentum: λ = h/p

Frequently we need to know the relation between the electron’s wavelength and its kinetic energy E. p and E are related through the classical formula: p 2 E= m = 9.11 × 10-31 kg 2m e 2 h always -15 p = h/ λ E=2 h =×⋅ 4.14 10 eV s 2m λ true!

1.505 eV ⋅ nm 2 For m = m e: E = E in electron volts (electrons) λ2 λ in nanometers 1240 eV ⋅ nm Don ’t confuse with Ephoton = for a photon ! λ

10 Application of Matter Waves: Electron Microscopy

The ability to “resolve ” tiny objects improves as the wavelength decreases. Consider the microscope objective: Objects to be d D resolved α α diffraction f disks = focal length of lens if image (not interference plane is at a large distance. maxima) Critical angle for λ The minimum d for which we α = 22.1 can still resolve two objects f resolution: c D dmin ≈ fαc = 1.22 λ is αααc times the focal length: D

A good microscope objective has f/D ≅ 2, so with λ ~ 500 nm

the optical microscope has a resolution of dmin ≅ 1 µm. We can do much better with matter waves because, as we shall see , electrons with energies of a few keV have wavelengths less than 1 nm. The instrument is known as an “electron microscope ”.

Imaging a Virus*

Electron Microscopy of a Virus: electron gun

Electron You wish to observe a virus with a diameter of 20 nm, which is optics much too small to observe with an optical microscope. Calculate D the voltage required to produce an electron DeBroglie wavelength suitable for studying this virus with a resolution of f

dmin = 2 nm . The “f-number ” for an electron microscope is quite large: f/D ≈ 100 . object

11 Solution

Electron Microscopy of a Virus: electron gun

dmin = 2 nm . The “f-number ” for an electron microscope is quite large: f/D ≈ 100 . object

f d ≈ 1.22 λ min D  D   D  λ ≈ dmin   = 2nm   = 0.0164 nm 1.22 f  1.22 f 

h2 1.505 eV ⋅ nm 2 E = = = 5.6 eVk 2mλ2 ()0.0164 nm 2

To accelerate an electron to an energy of 5.6 keV requires 5.6 kilovolts

Resolution

For 100keV electron, we can find that λ~4pm (0.004 nm)

We are nowhere near building TEMs that approach this wavelength limit of resolution, because we can ’t make perfect electron lenses.

12 HVEM

HVEMs : High Voltage Electron Microscopes 1980s: only one HVEM (1MV) 1990s: three 1.25 MV machines

Intermediate voltage electron microscopes ( IVEMs ) were introduced in the 1980s. (300 -400 kV)

Ultra -High Voltage Electron Microscopy

The Research Center for Ultra-High Voltage Electron Microscopy was established in 1974 in Osaka University, Japan. The main purpose of this center is to utilize for scientific research a 3-MV class ultrahigh voltage electron microscope, which was originally installed in 1972 and renewed in 1995 through financial aid from the Ministry of Education

13 14 Electron Beam ---Specimen Interactions

What happens when the beam reaches the specimen?

How the signals produced by the EB -specimen interactions are converted into images and/or spectra that convey useful information? Size, shape, composition, certain properties, etc.

It is critical to understand the physics of electron beam- specimen interactions by David Joy and are based on the algorithms described in the book "Monte Carlo Modeling for Electron Microscopy and Microanalysis" published by Oxford University Press (1995). Monte Carlo Simulation of Electron Beam-Specimen Interactions

Interaction of high energy (~kV) electrons with (solid) materials

15 Secondary Electrons

Secondary electrons are specimen electrons that obtain energy by inelastic collisions with beam electrons. They are defined as electrons emitted from the specimen with energy less than 50ev.

Secondary electrons are predominantly produced by the interactions between energetic beam electrons and weakly bonded conduction-band electrons in metals or the valence electrons of insulators and semiconductors. There is a great difference between the amount of energy contained by beam electrons compared to the specimen electrons and because of this, only a small amount of kinetic energy can be transferred to the secondary electrons.

Electron Beam ---Specimen Interactions

There are two basic type of scattering:

– Elastic scattering which causes backscattering of electrons without loss of kinetic energy. This is particularly important in SEM.

– Inelastic scattering which results in transfer of energy from the beam electrons to the sample’s. This leads to generation of secondary electrons: Auger, X-rays, electron- hole pairs in conductors and insulators, long wavelength electromagnetic radiation or cathodoluminecence (vis, UV, IR), lattice vibrations (phonons), electron oscillations in metals (plasmons).

16 Electron Beam ---Specimen Interactions

Electron Beam ---Specimen Interactions ------Visualizing the interaction volume

•The interaction volume can be observed in certain plastic materials such as PMMA

•Undergo Molecular bonding damage during electron bombardment that renders the material sensitive to etching in a suitable solvent

•This phenomenon is the basis for • Polymethylmethacrylate (PMMA) EB lithography • e-beam: 20 keV, ~ 0.5µm

(Everhart et al., Proc. 6th Intl. Conf. on X-ray Optics and Microanalysis)

17 Electron Beam -Specimen Interactions • EB lithography

Kartikeya Malladi, Chunlei Wang, and Marc Madou, “Microfabrication of Suspended C-MEMS structures by EB Writer and Pyrolysis”, Carbon, 44(13), (2006) 2602-2607

SEM

High voltage electrons hit the sample and reflect off Elastic collisions, determine angles of reflection Interference from electrons at different angles create contrast Focus beam reflection onto a larger image Retains orientation and phase information Visualization of sample, with theoretical 1 Å resolution Surface topography if low energy secondary electrons are collected Atomic number or orientation information if higher energy backscattered electrons are used for imaging Differentiation between surface roughness, porosity, granular deposits, stress -related gross microcracks (often used in conjunction with microsectioning ) Observation of grain boundaries in unetched samples Critical dimension measurements

18 Homework

What is the basic principle of a Confocal Microscope?

non-confocal

confocal 3D projection

Drosophila melanogaster, FITC

Nikon ’s Small World Gallery

First Place, 2007 Competition Gloria Kwon Memorial Sloan-Kettering Insititute New York City, New York, USA Double transgenic mouse embryo, 18.5 days (17x) Brightfield, Darkfield, Fluorescence

19 Nikon ’s Small World Gallery

Second Place, 2007 Competition Michael Hendricks Temasek Life Sciences Laboratory National University of Singapore Kent Ridge, Singapore Zebrafish embryo midbrain and diencephalon (20x) Confocal

Third Place, 2007 Competition Fourth Place, 2007 Competition Wim van Egmond Charles Krebs Micropolitan Museum Charles Krebs Photography Rotterdam, Netherlands Issaquah, Washington, USA Testudinella sp. (400x) Marine diatoms attached to Polysiphonia alga (100x) Differential interference contrast Differential interference contrast