Scanning Electron Microscopy Scanning Electron Microscopy (SEM)(SEM)
Wacek SwiechJim Mabon
Vania PetrovaMike Marshall
Ivan Petrov
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Basic Comparison to Optical Microscopy
1
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de Broglie h peU m v p m v h m eU
hence h m eU
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OpticalOptical SEM (secondary electron)SEM (secondary electron)The higher resolution and depth of focus available with the SEM are clearly observed,electrons have small mass (m0).
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
Animation from A Guide to X-Ray Microanalysis, Oxford Microanalytical Instruments
Monte Carlo Calculations, CASINO
Simulations are very useful for testing if a measurement is possible or interpreting results
• Determine effective lateral or depth resolution for a particular signal in a defined sample
• Simulate X-ray generation / X-ray spectra in a defined sample
• Simulate image contrast
1 μm
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Sequential Image Acquisition in SEM
• The scan of the electron beam and the screen raster are synchronized with intensity proportional to the collected signal
• Magnification is given by the ratio of the length of the line on display device to length scanned on the real sample
Figure from Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
M = Ldisplay/Lspecimen
Animations from, The Oxford Guide to X-Ray Microanalysis, Oxford Instruments Microanalysis Group
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Typical SEM Column / Vacuum Conditions
JEOL 6060LV
Courtesy JEOL USA
• An SEM specimen chamber typically operates at high vacuum conditions:Vacuum <10-4 Pa (NOT UHV)
• Usually operate with beam voltages of a few hundred volts up to 30 kV typically
• Vacuum as low as <10-8 Pa in the gun area via differential pumping depending on type of electron source
• Special differentially pumped environmental or variable pressure systems for operation up to 20 Torr in specimen chamber are available for samples not compatible with high vacuum (wet, for example) and imaging uncoated non-conductive samples
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Thermionic Sources- Tungsten Filament / LaB6
Schematic of a generalized W-Hairpin electron source
LaB6W
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
http://www.feibeamtech.com/pages/schottky.html
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Cold Field Emission Electron SourceSharp Single Crystal (310) Tungsten Tip
(310)
Courtesy Hitachi Instruments From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
Courtesy Hitachi Instruments
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Thermal-Field (Schottky) Emission Source
http://www.feibeamtech.com/pages/schottky.html
ZrO2
(100)
~1800 K
From Rooks and McCord, SPIE Handbook of Microlithography
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
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Electron Lenses
Magnetic LensElectromagnet coilPrecision machined soft iron “pole piece”
Graphic from A Guide to X-Ray Microanalysis, Oxford Microanalytical Instruments
Electron optical brightness, β, is a constant throughout the column, thus is a very important electron source parameter
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Origin of Depth of Focus
Adapted from Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
Quantifying Depth of Focus
For an observer it is taken that image defocus becomes detectable when two image elements fully overlap, where an image element is given by the resolving power of the human eye (~0.1mm).
The depth of focus can then be described geometrically by:
D ~ 0.2 mm / αM
Distance above and below plane of focus that beam becomes broadened to a noticeable size “blurring” in the image
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Objective Aperture
Function of Condenser Lens
• Ray diagram for lens weakly excited
• Longer focal length, Small α1, Larger d1
• More beam accepted into objective aperture
• Higher probe current at specimen
• Larger focal spot at specimen
• Lower resolution
• Higher Signal Levels
• Ray diagram for lens strongly excited
• Short focal length, Small α1, Smaller d1
• Less beam accepted into objective aperture
• Lower probe current at specimen
• Smaller focal spot at specimen
• Higher Resolution• Lower Signal Levels
Adapted from Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
De-magnify the beam obtained from the source to enable a small spot to be obtained on the sample. Multiple lenses are more typically used in the condenser lens system. 1 1 1
u v f+ =
Object Image
u v/M v u=
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Objective Lens / Working Distance
• Focus the electron beam on the specimen with minimal lens aberrations
Adapted from Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
Astigmatism is caused by imperfections in the lens or other interference.It can be corrected using additional elements called stigmators contained inside the objective lens
Octupole lens stigmator
Magnetostaticquadrupole lens is basis of a stigmator
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Secondary Electron Detector / Imaging
• Faraday Cage (collector) is usually biased a few hundred volts positive (for collection efficiency)
• Scintillator is biased +10kV to accelerate electrons to sufficient energy to efficiently excite scintillating material
• Amplified output level is directly used to set brightness (offset) and contrast (gain) in corresponding pixel in image
Contrast from predominately angular dependence of secondary electron yield and edge effects.
Everhart-Thornley SE detector
Reactive ion etching of Al/Si(001)
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Secondary Electron Yield Dependence of SE yield with angle (local) of incidence with surface
Escape probability for SE’s as a function of depth of generation image resolution for SE imaging will approach the probe size
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
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Edge Effect on SE Yield
Adapted From L. Reimer, Scanning Electron Microscopy, 2nd edition, Springer Verlag
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Analogy to Oblique and Diffuse Optical Illumination
• Secondary electron yield is strongly dependent on local angle of incidence with beam
• Backscattered electrons are also directly and indirectly detected (image is not pure SE)
• Together this, along with the high depth of focus of the SEM, gives the familiar SEM images with a good perceptive sense of surface topography
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
100,000X original magnification10,000X original magnification
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Extremely Wide Range of Magnifications
Sputtered Au-Pd on Magnetic TapeMiniature Sensor Device -Calorimeter
12X original magnification 500,000X original magnification
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Backscattered Electron Detectors and Yield
• Solid State 4 quadrant Backscattered Electron detector placed annular to bottom of objective lens
• Composition image –electronically sum signal from all 4 quadrants
• BSE topographic images –differencing various detector quadrants
• Backscattered electron yield is a strongly dependent on sample mean atomic number
Graphic from, The Oxford Guide to X-Ray Microanalysis, Oxford Instruments Microanalysis Group
Typical 4 quadrant solid state BSE detector
Objective lens pole piece
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Backscattered Compositional Contrast
Secondary Electron Image Backscattered Electron Image
SnBi alloy• most useful on multi-phase samples• can be sensitive to < 0.01 average Z differences• flat-polished specimens essential
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Non-Conductive Specimens - Charging
=0Isc = Ib− ηIb − δIb = Ib (1−(η+δ))
Upper cross-over energy, E2,for several materials
Total emitted electron coefficientη+δ as a function of beam energyWhen η+δ=1 charge balance
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
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Variable Pressure (VP / LV / Environmental) SEM• A different and simpler solution to specimen charging of un-coated non-
conductive samples is to introduce a gas (air, etc.) into the specimen chamber.
• The high energy electrons ionize the gas, thus positive ions are available to dynamically neutralize any charge on the sample.
• Available in both Schottky and Thermionic (Tungsten) instruments
Uncoated Dysprosium Niobium Oxide Ceramic @ high vacuum
@ 20 Paair
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• Energy-Dispersive Spectroscopy (EDS) – solid state detector simultaneously measures all energies of X-ray photons
• Wavelength Dispersive Spectroscopy (WDS) – sequentially measures intensity vs X-ray wavelength (energy). Superior energy resolution and detection limits (P/B ratio).
• Electron Backscattered Diffraction (EBSD) – acquires electron diffraction information from surface of highly tilted bulk sample with lateral resolution of 10’s of nm
• Cathodoluminesence (CL) – optical emission spectrometer and imaging system for 300-1,700nm. Liquid He cooled stage module.
EDSEDS
WDSWDSCLCL
JEOL JSM-7000F Analytical Scanning Electron Microscope
EBSDEBSD
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Characteristic X-ray Generation
• A scattering event kicks out an electron from K,L,M, or N shell of atom in specimen
• An electron from an outer shell falls to fill in the vacancy
• Energy difference results in release of an X-ray of characteristic energy/wavelength or an Auger electron
Atomic Number
X-ray vs. Auger Generation
Graphic from, The Oxford Guide to X-Ray Microanalysis, Oxford Instruments Microanalysis Group
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Energies of Characteristic X-rays to 20 keV
From Scanning Electron Microscopy and X-RayMicroanalysis, Joseph I. Goldstein et al. Plenum Press
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Energy Dispersive X-ray Detector: Si(Li)
Collimator
Magnetic Electron Trap
X-ray Window
Si(Li) Detector
FET charge sensitive amplifier
Copper Rod (at Liq. N2 Temperature)
Graphics from A Guide to X-Ray Microanalysis, Oxford Microanalytical Instruments
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Mechanism of X-ray Energy Determination
• X-ray loses energy through inelastic scattering events creating electron / hole pairs
• High voltage bias keeps generated pairs from re-combining
• Charge sensitive amplifier “counts”pairs generated by X-ray
• Spectrometer calibration effectively multiplies by energy/pair (3.8 eV) to determine X-ray energy
Volta
ge
Time
Charge restore
Voltage stepX-ray event
Animations from, The Oxford Guide to X-Ray Microanalysis, Oxford Instruments Microanalysis Group
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EDS Spectral Resolution and Count Rates
Pulse processing time constants are used to adjust available count rate versus spectral peak resolution
• Long time constants are best for single acquisition analysis for best energy resolution.
• Short time constants are best for fast acquisition of X-ray elemental maps (elemental distribution images) or line-scans (intensity or concentration profiles).
• Compromise is often needed.
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X-ray EDS Microanalysis in the SEM
• Fast Parallel Detection• Qualitative elemental
analysis– From beryllium up on
periodic table– Sensitivities to <0.1 wt.%
depending on matrix and composition
• Quantitative analysis– Possible, has many
requirements / limitations• Digital elemental
distribution imaging and line-scans, full spectrum imaging
• Analysis of small volumes, from order of μm3 to << 1 μm3 depending on accelerating voltage, element analyzed, and matrix
Oxford Instr.: Link-ISIS software
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EDS Full spectrum imagingA full X-ray spectrum collected for each pixel X-ray elemental maps, phase maps, spectra, and quantitative analysis extracted from full spectrum image
+/- 0.0933.23La L
+/- 0.0730.57Mn K
+/- 0.0410.26Ca K
+/- 0.023.81Al K
---22.13 StoichiometryO K
Weight % ErrorWeight %Element Line
Cumulative Spectra and Quant Analysis for each extracted phase (ex. Phase 1)
Microtexture in Al-Li Alloys for Future Aerospace Applications
Investigation of crystallographic aspects grain morphology and delaminations
Complimentary to XRD texture determination
- gives local texture& misorientationsTrue Grain ID, Size and Shape Determination
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Channeling / Diffraction Contrast with a Forward Scatter detector
316L Stainless Steel
Before deformation: equiaxed grains with no deformation evident and lots of annealing twins
Deformed to uniform elongation @ 200 C, considerable strain contrast, individual grains no longer discernable, highly deformed structure
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Limit of Lateral Resolution for EBSD
5 μm step size = 0.01 μm Deformed Copper
Literature generally reports a lateral resolution limit (x-direction) of 5 – 50nm. Resolution in the y-direction is somewhat less due to high tilt of sample.
Actual resolution obtained depends on beam voltage, probe size, sample material (Z), specimen preparation, etc.
Defects (dislocations) are observed as points or lines of reduced emission
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Monochromatic CL imaging of GaN PyramidsSEM composite
550 nm
The strongest yellow emission comes from the apex of the elongated hexagonal structure.
CL Image
550 nmCL imaging of cross-sectional view
SEM
Results courtesy of Xiuling Li , Paul W. Bohn, and J. J. Coleman, UIUC
Typical CL spectra
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The Dual-Beam Focused Ion Beam (DB-FIB)Electron column
Ion column
Pt doser
The FEI Dual-Beam DB-235 Focused Ion Beam and FEG-SEM has a high resolution imaging (7nm) Ga+ ion column for site-specific cross-sectioning, TEM sample preparation, and nano-fabrication. The Scanning Electron Microscope (SEM) column provides high resolution (2.5 nm) imaging prior to, during, and after milling with the ion beam. The instrument is equipped with beam activated Pt deposition and 2 in-situ nanomanipulators: Omniprobe for TEM sample preparation and Zyvex for multiprobe experiments.
• site –specific cross-sectioning and imaging * Serial sections and 3-D reconstruction are an extension of this method
• site –specific preparation of specimens for Transmission Electron Microscopy (TEM) • site –specific preparation of specimens for EBSD• nano-fabrication (micro-machining and beam-induced deposition)• modification of electrical routing on semiconductor devices • failure analysis • mask repair
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Ion Microscopy: Ions and Electrons
The gallium ion beam hits the specimen thereby releasing secondary electrons, secondary ions and neutral particles (e.g. milling).
The detector collects secondary electrons or ions to form an image.
For deposition and enhanced etching: gases can be injected to the system.
Layout of the Focused Ion Beam column
Liquid Metal Ion Source
The Focused Ion Beam (FIB) Column
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Ion/Electron Beam Induced Deposition
Precursor molecules adsorb on surfacePrecursor is decomposed by ion or electron beam impinging on surfaceDeposited film is left on surfaceVolatile reaction products are released
IBID and EBID
Similarly, reactive gases, can be injected for enhanced etching in milling and improving aspect ratio for milled features
Pt, W, and Au are common metalsSiOx can be deposited as an insulator
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The DB-FIB Specimen Chamber
Pt Injection Needle
FIB Column
CDEM detector
Electron Column
ET & TLD electron detectors
Specimen Stage(chamber door open)
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Polished Cross–Section of Semiconductor1. Locate area of interest
2. Deposit Pt ProtectionLayer
3. Mill “stair step” trench (20nA), face (1nA) cross-section, and polish (0.1nA) - all with ion beam
4. Image features with SEM or Ion beam
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TEM Sample Preparation w/ Omniprobe
• Step 1 - Locate the area of interest (site – specific)
• Step 6 – Mill to release from substrate and transfer to grid
• Step 7 – "Weld" sample to a Cu TEM half-grid and FIB cut manipulator needle free
• Step 8 - FIB ion polish to electron transparency
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“Pre-Thinned” TEM Sample prepared by FIB
Diced Wafer with Thin Film Prethinned Section
Prethinned TEM Sample on Half Grid After Thinning
TEM Direction
TEM Direction
Ion Beam DirectionPt ProtectionLayer
Half Grid
Grind to 30 MicronsDice to 2.5 mm
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“Pre-Thinned” TEM Sample prepared by FIB
Drawing of typical “pre- thinned”specimen for FIBTEM sample preparation
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Software for Controlled Patterning in FEI–DB235
Developed by Jim Mabon
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Precisely controlled etching and deposition
500 nm
Pt Flower
30 nm Pt dot array
Etched or deposited structures using grey-scale bitmaps (more complex, parallel process) or scripting language (sequential, unlimited # of points).
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Photonic Array: A seed layer for photonic cell crystal growth nucleation fabricated (etching) with the FIB.J. Lewis, P. Braun
Pt Dot: This Platinum dot is used as an etch mask in porous silicon experiments.P. Bohn
FIB patterning by etching and/or depositionBitmap Script
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5μm
ϕθ [110]
ϕ[110]
θ
Step 1: Micro-structures by FIB fabrication
θ : 5.5º θ : 13.7º
1μm
: 75ϕ °
[110]ϕ
Step 2: Ge growth on extra large-miscut Si
Step 3: Isolate specified orientation
Exploration of novel orientations in Si crystalline structure:Ge nanostructure growth on Si
θ : 28.2º
FIB & AFM
K. Ohmori, Y.-L. Foo, S. Hong, J.-G. Wen, J.E. Greene, and I. Petrov, Nanoletters, 5 369 2005
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Ge nanowires on Si(173 100 373)
Long straight Ge nanowires on Si(173 100 373) period: 60 nmwidth: ~40 nm
azimuthal direction ϕd: 114º
500 nm
[110]dϕ
Z-contrast STEM image20 nm
[121]Si substrate
(517)(113) (111)
Hill-valley structure composed of (113)&(517) facets
Plane index: (k 100 200+k)
4 μm: 75pϕ °
θ : 28.2º [110]
K. Ohmori, Y.-L. Foo, S. Hong, J.-G. Wen, J.E. Greene, and I. Petrov, Nanoletters, 5 369 2005
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Summary: Scanning Electron Microscopy
• Remarkable depth of focus• Imaging from millimeters to a nanometer• Chemical composition with 0.1-1 μm resolution• Crystallography using electron EBSD• Optical properties on a micrometer scale (via CL)
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• Site –specific cross-sectioning imaging and EBSD– Serial sections and 3-D reconstruction are an
extension of this method• Site –specific preparation of specimens for
transmission electron microscopy (TEM) • Nano-fabrication (micro-machining and beam-induced
deposition)• Modification of the electrical routing on semiconductor
devices
Summary: Focus Ion Beam Microscopy
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References
• Scanning Electron Microscopy and X-ray Microanalysis by Joseph Goldstein, Dale E. Newbury, David C. Joy, and Charles E. Lyman (Hardcover - Feb 2003)
• Scanning Electron Microscopy: Physics of Image Formation and Microanalysis (Springer Series in Optical Sciences) by Ludwig Reimer and P.W. Hawkes (Hardcover - Oct 16, 1998)
• Energy Dispersive X-ray Analysis in the Electron Microscope (Microscopy Handbooks) by DC Bell (Paperback - Jan 1, 2003)
• Physical Principles of Electron Microscopy: An Introduction to TEM, SEM, and AEM by Ray F. Egerton (Hardcover - April 25, 2008)
• Advanced Scanning Electron Microscopy and X-Ray Microanalysis by Patrick Echlin, C.E. Fiori, Joseph Goldstein, and David C. Joy (Hardcover - Mar 31, 1986)
• Monte Carlo Modeling for Electron Microscopy and Microanalysis (Oxford Series in Optical and Imaging Sciences) by David C. Joy (Hardcover - April 13, 1995)
• Electron Backscatter Diffraction in Materials Science by Adam J. Schwartz, Mukul Kumar, David P. Field, and Brent L. Adams (Hardcover - Sep 30, 2000)
• Introduction to Texture Analysis: Macrotexture, Microtexture and Orientation Mapping by Valerie Randle and Olaf Engler (Hardcover - Aug 7, 2000)
• Electron backscattered diffraction: an EBSD system added to an SEM is a valuable new tool in the materials characterization arsenal: An article from: Advanced Materials & Processes by Tim Maitland (Jul 31, 2005)
• Cathodoluminescence Microscopy of Inorganic Solids by B.G. Yacobi and D.B. Holt (Hardcover - Feb 28, 1990)
• Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques and Practice by Lucille A. Giannuzzi and Fred A. Stevie (Hardcover - Nov 19, 2004)
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Acknowledgements
Frederick Seitz Materials Research Laboratory is supported by the U.S. Department of Energy
