Lecture-4 I Scanning Electron Microscopy • 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) II Scanning Probe Microscopy (SPM) http://www.youtube.com/watch?v=sFSFpXdAiAM http://www.youtube.com/watch?v=tDzInhT5zzQ at~0.55-1:35
Transcript
Lecture-4 I Scanning Electron Microscopy
• 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
II Scanning Probe Microscopy (SPM)http://www.youtube.com/watch?v=sFSFpXdAiAMhttp://www.youtube.com/watch?v=tDzInhT5zzQ at~0.55-1:35
Resolution of Images
The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size.
e-
Effect of probe size on escape volume of SE
10nm
1m
5m
BSE X-ray
The resolution is the pixel diameter on specimen surface.
P=D/Mag = 100um/Mag
X-ray Spectroscopy
• X-rays are produced when energetic electrons strike a solid sample
• By measuring and analyzing the energy and intensity distribution of the X-ray photons E (eV) = hc/(nm) elemental concentration or composition of the sample can be determined
There are two ways to measure the energy distribution of X-rays emitted from sample:
• Energy dispersive spectroscopy (EDS)• Wavelength dispersive spectroscopy (WDS)
-Chemical Analysis in SEM
How are X-rays produced?
h=4.136x10-15 eV sec
http://www.youtube.com/watch?v=IRBKN4h7u80 ~0:30
http://www.youtube.com/watch?v=K7G3U7rn5uU ~0:30 What is X-ray spectroscopy
If an incoming electron has sufficient kinetic energy for knocking out an electron of the K shell (the inner-most shell), it may excite the atom to an high-energy state (K state).
One of the outer electron falls into the K-shell vacancy, emitting the excess energy as a x-ray photon.
Characteristic x-ray energy:
Ex-ray=Efinal-Einitial
Excitation of K, L, M and N shells and Formation of K to M Characteristic X-rays
K L MN
K
KL
Energy K state(shell)
L state
M state
N state
ground state
K
K
L
L
K1
K2
I II III
M
subshells
EK>EL>EM
EK>EK
K excitation
L excitation M
K emission spectrum of copper
K
K
K
K
Energy (eV)
I
EDS - Basics E (eV) = hc/(nm) • In most of the modern EDS
system, a semiconductor detector is used for measuring the energy of the x-ray photon emitted from the specimen. The X-ray energy is displayed as a histogram of number of photons versus energy.
• Liquid nitrogen is for cooling the detector so as to reduce noise caused by thermal excitation.
• A window is therefore required for protecting the detector against condensation when the sample chamber is opened.
LN2
Si (Li) detector
Be windowspecimen
Owing to the use of Be window, only elements higher than Be can be detected.
I
Kevhttp://www.youtube.com/watch?v=gd-mzdIHnOc
Characteristic X-ray Spectrum of YBaCuO6.9 Superconductor
CuK
YK
• As Z increases the Kth shell line energy increases (Y vs Cu).• If K-shell is excited,then all shells are excited (Y, Cu, Ba) but they may not be detected.• Severe spectral overlap may occur for low energy lines.
BaL1
BaL2
BaL1
Applications of EDS: I. Microchemical Analysis
Quantitative Analysis – Thin Samples
Cliff-Lorimer Technique
Ca IA = Kab
Cb IBCa and Cb are weight fraction of element a and bIA and IB are the peak intensitiesKab is a constant depending on the two elements and the operating conditions, and can be obtained by using a standard sample.
NiSn alloy
Applications of EDS: II. X-ray line scan
This powerful 3D EDS solution combines EDS analysis software syncronised to the milling capabilities of a FIB (Focused Ion Beam)-SEM. FIB milling is followed by EDS X-ray map acquisition for each slice
showing the distribution of carbon, silicon and calcium
Applications of EDS: III. X-ray Mapping
EDS map of a sandstone
• A crystal of fixed d is moved along a circle to vary so that x-ray of different is recorded.
• No window needed, therefore light elements can be detected
• Peak very sharp• Very large S/N
ratio c.f. EDS.
WDS - Basics E (eV) = hc/(nm)
I
Bragg’s Law: 2d sin =
1 ~ 1
2 ~ 2
1
2
X-ray is diffracted by LiF crystal and detected by a proportional counter, and then amplified, processed. X-ray intensity is displayed as a function of .
Spectra one element entire spectrumAcceptance per run in one shot
Collection timetens of min mins
Resolution ~a few eV ~130eV
Probe size ~200nm ~5nm
Max. Count rate ~50000 cps <2000 cps
Detection limits 100ppm 1000ppm
Accuracy ~4-5wt% ~4-5wt%
Spectral artifacts rare peak overlap
absorption
etc.
cps - count/second on an X-ray line
WDS EDS
EDS vs WDS
EDS
WDS
Superposed EDS and WDS spectra from BaTiO3. The EDS spectrum shows the strongly overlapped Ba L-Ti K and Ba L1-Ti K peaks. The WDS peaks are clearly resolved.
FWHM=135-eV
FWHM=a few-eV
Wavelength (nm)
Orientation Imaging Microscopy (OIM)
e-
specimen
Phosphor screen
EBSP
Electron backscatter diffraction patterns (EBSP) are obtained in SEM by focusing e- beam on a crystalline sample.
Diffraction pattern is imaged on a phosphor screen and captured using a CCD camera.
OIM is based on an automatic indexing of EBSP and provides a complete description of the crystallographic orientations in polycrystalline materials.
Effects of the crystal orienta-tions on materials properties.
The bands in the pattern are referred to as Kikuchi bands and are directly related to the crystal lattice structure in the sampled region. As such, analyzing the pattern and bands can provide key information about the crystal structure for the measured phase:
•The symmetry of the crystal lattice is reflected in the pattern.
•The width and intensity of the bands are directly related to the spacing of the atoms in the crystal planes.
•The angles between the bands are directly related to the angles between planes in the crystal lattice.
Fast and Accurate Indexing of Any Crystal System
Cubic Hexagonal Monoclinic Orthorhombic
Tetragonal Triclinic Trigonal
Applications of OIM
•Orientation/misorientationPhysical properties are often orientation dependentYoung’s modulus, permeability, hardness, plasticity, etc.Fatigue mechanism, creep in superalloys, integrity of single crystals, in-service reliability of microelectronic, corrosion, cracking, fracture, segregation and precipitation, twinning and recrystallization, etc.
•Phase identification-coupled with chemical analysisDistinguish between phases having similar chemistriesDistinguish between body-centered cubic and face centered cubic form, etc.
•StrainStrain in superalloys and aluminum alloysAssessment of implantation damage in Si from Ge ions
A Grain boundary Map can be generated by comparing the orientation between each pair of neighboring points in an OIM scan. A line is drawn separating a pair of points if the difference in orientation between the points exceeds a given tolerance angle. An Orientation Map is generated by shading each point in the OIM scan according to some parameter reflecting the orientation at each point. Both of these maps are shown overlaid on the digital micrograph from the SEM.
OIM-Semiconductors
Effect of micro-texture on Mean
Time to Failure (MTF) of inter-
connect lines and thin films for
semiconductor applications.
Good film Bad film
SEM Specimen Preparation• Remove all water, solvents, or other materials
that could vaporize while in the vacuum.• Flat surface is required for BSE and OIM• Firmly mount all the samples.• Non-metallic samples, such as building
materials, insulating ceramics, should be coated with a thin conductive layer to eliminate image artifacts which arise from excess surface charge. Metallic and conducting samples can be placed directly into the SEM.
Sputter coater is used to coat insulating samplesAu and Al – good for SE yieldAuPd alloy – good for high resolutionC – used if X-ray microanalysis is required
Coating should have low granularity in order not to mask the underlying structure (<20nm thick).
http://www.youtube.com/watch?v=NlVCJG3bEwo
X-ray Fluorescence (XRF)
Main use Identification of elements; determi-nation of composition and thickness(for thin film samples)
Samples Solids, powders and liquids; 5.0cmin diameter
Range of elements All but low-Z elements: H, He, and LiAccuracy 1% composition, 3% thicknessDetection limits 0.1% in concentrationDepth sampled Normally in the 10nm range, but can
be a few nm in the Total-reflection XRFInstrument cost $400K-$2.4M
XRF is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, ceramics and building materials, and for research in geochemistry, forensic science and archaeology.
http://en.wikipedia.org/wiki/X-ray_fluorescence
http://www.amptek.com/pdf/xrf.pdf
X-ray Microfluorescence (XRMF) - Basics
1. X-ray irradiates specimen2. Specimen emits characteristic X-rays or XRF3. Detector measures position and intensity of XRF peaks
A fast, convenientway to examine sample chemistry and heterogeneity along a line of analysis points.
Ideal for studying diffusion profiles or layered materials.
Elemental Mapping
QualitativeQuantitativeSpatial distribution
Cumap
Pbmap
Scanning Probe Microscopy (SPM)
• What is SPM?• Working principles of SPM• Basic components and their functions• Scanning Tunneling Microscopy (STM)• Atomic force microscopy (AFM)• Advanced SPM techniques• Examples of SPM images
http://www.youtube.com/watch?v=WiFgwB_BADE A journey to the nanoworldhttp://www.youtube.com/watch?v=oOmxg4u9efs Advanced SPM Lecture
FESEM: “Seeing” materials at the nanoscale SPM: “feeling” materials at the nanoscale
Invented at IBM byGerd Binning andHeinrich RohrerNobel Prize in 1986
The world's first truly flexible SPM, providing a single system that could meet all of the the needs of most scientists and engineers at an affordable price.
Basic Principles of SPM (STM & AFM) SPM relies on a very sharp probe positioned within a few nanometers above the surface of interest. When the probe translates laterally (scanning) relative to the sample, any change in the height of the surface causes the detected probe signal to change. A 3-D map of surface height is carried out with a probe scanning over the surface while monitoring some interaction between the probe and the surface. Probe signals that have been used to sense surfaces include electron tunneling current (STM), interatomic forces (van der Waals force, AFM), magnetic force (MFM), capacitive coupling (SCM), electrostatic force (EFM), and thermal coupling (SThM), etc. The probe signals depend so strongly on the probe-sample interaction that changes in substrate height of ~0.1Å can be detected with a sub-nm lateral resolution.
sense of touch instead of light or electrons. Imagine drawing a picture of a computer keyboard without using your eyes. You could drag your fingertip over the surface to "feel" what the keyboard looks like. Instead of a fingertip, the SPM has a very tiny
sensor called a probe. The SPM can magnify an object up to
10,000,000 times. In the laboratory under ideal conditions,
the SPM can be used to look at individual atoms.
http://www.doitpoms.ac.uk/tlplib/afm/intro.php
Piezoelectric Scanner
PiezoelectricScanner
http://www.youtube.com/watch?v=MZb8C0f7Kdg
Basic SPM Components•Scanning System: Scanner - the heart of the microscope. It may scan the sample or the probe. A piezoelectric tube scanner can provide sub-Å motion increments.•Probe (tip): Very sharp tips are secured on the end of cantilevers which have a widerange of properties designed for a varietyof scanning probe technologies. There arealso many types of tips with varying shapesfor probing different morphologies andscales of surface features and materials(conducting, magnetized, very hard, etc.).
•Probe Motion Sensor: Senses thespacing between the probe and thesample and provides a correctionsignal to the piezoelectric scannerto keep the spacing constant. Thecommon design for this function iscalled beam deflection system asshown at right figure.
Actuators - Piezoelectric Scanner in Scanning Probe Microscopy (SPM)
STM uses a piezoelectric scanner to scan an electrical probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface.
A piezoelectric tube scanner can provide sub-Å motion increments
to ~0.:55
Piezoelectric materials convert mechanical to electrical energy (and vice versa).
Direct Converse D = d S = dE
D-Charge density or dielectric displacement (C/m2), -stress (N/m2), S-strain, E-electric field and d-piezoelectric constant (C/N or m/V) D3(P3)=d333 S3=d33E3
A piezoelectric tube scanner can provide sub-Å motion increments
Scanning Tunneling Microscope(STM)
STM uses a sharpened, conducting tip with a bias voltage applied between the tip and the sample. When the tip is brought within about 10Å of the sample, electrons from the sample begin to "tunnel" through the 10Å gap into the tip or vice versa, depending upon the sign of the bias voltage.
It varies with tip-to-sample spacing and is also a function of local electronic structure or surface state. It is the signal used to create a STM image. For tunneling to take place, both the sample and the tip must be conductors or semiconductors.
Two Scanning Modes in STMImaging of surface topology can be done in one of two ways:
constant height mode
Tunneling current is monitored as the tip is scanned parallel to the surface. There is a periodic variation in the separation distance between the tip and surface atoms. A plot of the tunneling current v's tip position shows a periodic variation which matches that of the surface structure-a direct "image" of the surface.
It It
constant current mode
Tunneling current is maintained constant as the tip is scanned across the surface. This is achieved by adjusting the tip's height above the surface so that the tunneling current does not vary with the lateral tip position. The image is then formed by plotting the tip height v's the lateral tip position.
Atomic Force Microscope (AFM)AFM senses interatomic forces that occur between a probetip and a sample.
Probe tip
Optical lever detection of cantilever deflection
a laser beam bounces off the back of the cantilever onto a photodiode. As the cantilever scans over a sample it bends and the position of the laser beam on the detector shifts. The cantilever deflection is regarded as the vertical force signal between the tip and the sample surface. The local height of the sample is measured by recording the vertical motion of the tip while keeping the cantilever deflection at constant.
3-D topographical maps of the surface are thenconstructed by plotting the local sample height versus horizontal probe tip.
The properties and dimensions of the cantilever play an important role in determining the sensitivity and resolution of the AFM.
cantilever
Contact, Non-Contact and TappingMode AFM Contact Non-contact Tapping
Contact mode imaging is heavily influenced by frictional and adhesive forces which can damage samples and distort image data. Non-contact imaging generally provides low resolution and can also be hampered by the contaminant layer which can interfere with oscillation. TappingMode imaging eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant layer. The graphs under the images represent likely image data resulting from the three techniques.
a b c
Measure topography by
a.Sliding the probe tip across surfaceb.Sensing Van der Waals attractive forces between surface and probe tip held above surface c.Tapping the surface with an oscillating probe tip
LiftMode is a two-pass technique for measurement of magnetic and electric forces above sample surfaces. On the first pass over each scan, the sample's surface topography is measured and recorded. On the second pass, the tip is lifted a user-selected distance above the recorded surface topography and the force measurement is made.
Topographic structure (a) and Magnetic force image (b) of a compact disk
a bTopographic structure results from surface preparation and exhibits striations from a polishing process (imaged using Tapping Mode) and magnetic force image (LiftMode and lift height 35nm) shows small magnetic domains that are unrelated to surface topography.
Image Insulating Surfaces at High Resolution in Fluid - AFM
Fluid cell for an AFM which allowsimaging in an enclosed, liquidenvironment.
Image of two GroES (protein) molecules positioned side-by-side in fluid, demonstrating 1nm lateral resolution and 0.1nm vertical resolution. Entire molecule measures 84Å across and a distinct 45Å “crown” structure protrudes 8Å above remaining GroES surface.
12
18nm
Advanced SPM Techniques• Lateral Force Microscopy (LFM)
measures frictional forces between the probe tip and the sample surface
• Magnetic Force Microscopy (MFM)measures magnetic gradient and distribution above the sample surface; best performed using LiftMode to track topography
• Electric Force Microscopy (EFM)measures electric field gradient and distribution above the sample surface; best performed using LiftMode to track topography
• Scanning Thermal Microscopy (SThM)measures temperature distribution on the sample surface http://www.youtube.com/watch?v=zSvm1KaEd6k&feature=related
measures carrier (dopant) concentration profiles on semiconductor surfaces
• Nanoindenting/Scratching measures mechanical properties of thin films and uses indentation to investigate hardness, and scratch or wear testing to investigate film adhesion and durability
• Phase Imagingmeasures variations in surface properties (stiffness, adhesion, etc.) as the phase lag of the cantilever oscillation relative to the piezo drive and provides nanometer-scale information about surface structure often not revealed by other SPM techniques
• LithographyUse of probe tip to write patterns
Applications of SPMTo resolve a wide range of surface properties on the nanometer scale including:Topography, mechanical, magnetic, electric, thermal, and etc.e.g., Nano lithography, inspecting defects of semiconductors, measuring physical and chemical properties of surface, DNA imaging, etc.
Advantages: Superior resolution and versatility of scanning probes
Limitations:Long imaging times due to slow scanning speedThe maximum imaging area is limited (<mm2)
Examples of AFM Images
Lateral force map of a patterned, monolayer, organic film deposited on a gold substrate. The strong contrast comes from the different frictional characteristics of the two materials. 30 µm scan.
80nm tall elevated features in a Si/Si3N4 substrate
Al2O3
4m
10m
3-D
Grain growth studies
Electric Force Microscopy (EFM) Mode :Ferroelectric Domains on BaTiO3 Surface
Piezoresponse force microscopy of ferroelectric domains on BaTiO3
surface. 5µm Scan courtesy of S. Kalinin, T. Alvarez, D. Bonnell, Department of Material Science Engineering, University of Pennsylvania.
1 m
- Ferroelectric domain
-
+
NanoindentationUsing a diamond tip to indent a surface and immediately image the indentation. Using indentation cantilevers, it is possible to indent various samples with the same force in order to compare hardness properties.
diamond tip
metal-foil
200nm
Indentations on two different diamond-like carbon films using three different forces (23, 34, and 45N) with four incidents made at each force to compare difference in hardness.Indentation depths are deeper for the
softer thin film (right).
IC Failure Analysis and Defect Inspection with Scanning Thermal Microscopy
a. Top-view image of surface topography of a failing IC where emission microscope detected two current leakage points but did not give the exact location of failure. The image shows no topographical features which suggest a problem. b. Scanning thermal microscopy (SThM) temperature distribution map of the same area showing a hot area on the surface above what was found to be a gate oxide short.
a b
Nanolithography
STM - Seeing Atoms
STM image showing single-atom defect in iodine adsorbate lattice on platinum. 2.5nm scan
Contact AFM images of the same area of a (001) oriented TiO2
thin film, acquired with a pyramidal Si3N4 tip (a) and a conical, etched Si tip (b). Both images are on the same scale and in the same surface orientation. Clearly, the surface morphology appears different in each image and is convoluted with the tip shape.
