1. 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)
at~0.55-1:35

2. Effect of probe size on escape volume of SE
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-
10nm
BSE
X-ray
P=D/Mag = 100um/Mag
1m
5m

3. How are X-rays produced?
~0:30 What is X-ray spectroscopy
X-ray Spectroscopy
-Chemical Analysis in SEM
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/l (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)
h=4.136x10-15 eV sec
How are X-rays produced?
~0:30

4. Principles of X-ray Production
(20keV)
X-ray are produced
by transitions
of electrons between shells of atoms
Shells correspond to particular energy level
for an atom
Transition between shells and energy levels are characteristic of element
1. Ionization-excitation L
Create a vacancy
in K shell
excited state K
E-E
2. Relaxation-deexicitation E K L
e- in L shell jumps
in to fill vacancy
The interaction of a beam electron with an inner-shell electron can result in the ejection of the bound electron which leaves the atom in an excited state with a vacancy in the electron shell. Deexcitation of an atom following ionization, which occurs when an electron is removed from a shell and ejected from the atom, takes place by a mechanism involving transitions of electrons from one shell or subshell to another. The transition may be radiative, that is, accompanied by the emission of a photon of electromagnetic radiation, or nonradiative, e.g., the Auger electron emission process. Since the energy of the emitted x-ray photon is related to the difference in energy between the sharply defined levels of the atom, it is referred as a characteristic x-ray. K
Auger Electron
Emission
nonradiative
radiative L
Process of inner-shell ionization and subsequent deexcitation

5. Excitation of K, L, M and N shells and Formation of K to M Characteristic X-rays
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 L K
K2
K1 K I II
III
K L M N L
subshells
K state
(shell)
Energy K K
Since the energy of the emitted x-ray photon is related to the difference in energy between the sharply defined levels of the atom, it is referred as a characteristic x-ray.
L state
K excitation L
M state
EK>EL>EM
EK>EK M
L excitation
N state
4/19/2017
ground state

6. K emission spectrum of copper
Energy (eV)

7. EDS - Basics E (eV) = hc/l (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.
Si (Li) detector
LN2
Be window
specimen
Owing to the use of Be window, only elements higher than Be can be detected.
e- interacts with the specimen with the emission of X-rays, X-rays pass through the window and absorbed by detector crystal Si(Li) (lithium-drifted silicon detector). Absorption of each individual x-ray photon leads to the ejection of a photoelectron which gives up most of its energy to the formation of electron-hole pairs. They in turn are swept away by the applied bias to form a charge pulse which is then converted to a voltage pulse by a charge-sensitive preamplifier. The number of charge carriers and hence the current pulse height, are proportional to the energy of the incoming photon. The signal is further amplified and shaped by a main amplifier and finally passed to a multichannel analyzer (MCA), where the pulses are sorted by voltage. Assigning the voltage to a channel in a multi-channel analyzer. The X-ray energy is processed into a digital signal that is displayed as a histogram of number of photons versus energy.
To obtain sufficiently low conductivity, the detector must be maintained at low temperature, and liquid-nitrogen must be used for the best resolution. I
Kev

8. Characteristic X-ray Spectrum of YBaCuO6.9 Superconductor
Applications of EDS: I. Microchemical Analysis   
BaL1 
BaL2 
CuK
BaL1
YK
Z(Y)=39 and Z(Cu)=29
Characteristic X-ray Spectrum of YBaCuO6.9 Superconductor
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.

9. Quantitative Analysis – Thin Samples
Cliff-Lorimer Technique
Ca IA
= Kab
Cb IB
NiSn alloy
Ca and Cb are weight fraction of element a and b
IA and IB are the peak intensities
Kab is a constant depending on the two elements
and the operating conditions, and can be obtained by using a standard sample.
A sample is considered a “thin” if its thickness dimension is less than the depth dimension of the electron interaction volume in a bulk solid of the same composition.

10. 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

11. showing the distribution of carbon, silicon and calcium
Applications of EDS: III. X-ray Mapping
Carbon
Calcium
Silicon
Overlaying
EDS map of a sandstone
showing the distribution of carbon, silicon and calcium

12. WDS - Basics E (eV) = hc/l (nm)
Wavelength Dispersive Spectroscopy ~3:35
Bragg’s Law: 2d sin = 
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. 1
1 ~ 1
2 ~ 2 I 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 l. 

13. EDS vs WDS WDS EDS Spectra one element entire spectrum
Acceptance per run in one shot
Collection time tens of min mins
Resolution ~a few eV ~130eV
Probe size ~200nm ~5nm
Max. Count rate ~50000 cps <2000 cps
Detection limits 100ppm ppm
Accuracy ~4-5wt% ~4-5wt%
Spectral artifacts rare peak overlap absorption
etc.
cps - count/second on an X-ray line

14. EDS vs WDS EDS WDS FWHM=135-eV FWHM=a few-eV
Measure X-ray peak intensity and get weight or atomic concentration of element
Wavelength (nm)
Superposed EDS and WDS spectra from BaTiO3. The EDS spectrum shows the strongly overlapped Ba La-Ti Ka and Ba Lb1-Ti Kb peaks. The WDS peaks are clearly resolved.

15. Orientation Imaging Microscopy (OIM)
OIM is based on electron backscatter diffraction
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.
Phosphor
screen e-
70o
specimen
When the electron beam strikes a crystalline material mounted at an incline around 70º, the electrons disperse beneath the surface, subsequently diffracting among the crystallographic planes. The diffracted beam produces a pattern composed of intersecting bands, termed electron backscatter patterns, or EBSPs. The patterns can be imaged by placing a suitable film or phosphor screen in close proximity to the sample in the SEM sample chamber.
EBSP
at~1:40-5:08

16. Information provided by EBSP
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.

17. Fast and Accurate Indexing of Any Crystal System
Hexagonal
Monoclinic
Orthorhombic
Cubic
Tetragonal
Triclinic
Trigonal

18. Applications of OIM Orientation/misorientation
Physical properties are often orientation dependent
Young’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 analysis
Distinguish between phases having similar chemistries
Distinguish between body-centered cubic and face centered cubic form, etc.
Strain
Strain in superalloys and aluminum alloys
Assessment of implantation damage in Si from Ge ions
Follow the evolution of orientation, and related property changes, as function of thermomechanical treatments.
at~6:58-9:18

19. OIM-Grain Boundary Maps
Orientation Map
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.

20. OIM-Semiconductors
Effect of micro-texture on Mean Time to Failure (MTF) of inter-connect lines and thin films for semiconductor applications.
Orientation effect
Good film
Bad film

21. 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.
Line by line charging
at~2:33-3:00
at~2:00-2:10

22. Coating Techniques Sputter coater is used to coat insulating samples
Au and Al – good for SE yield
AuPd alloy – good for high resolution
C – used if X-ray microanalysis is required
Coating should have low granularity in order not to mask the underlying structure (<20nm thick).

23. X-ray Fluorescence (XRF)
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.
Main use Identification of elements; determi-
nation of composition and thickness
(for thin film samples)
Samples Solids, powders and liquids; 5.0cm in diameter
Range of elements All but low-Z elements: H, He, and Li
Accuracy 1% composition, 3% thickness
Detection limits 0.1% in concentration
Depth sampled Normally in the 10nm range, but can be a few nm in the Total-reflection XRF
Instrument cost $400K-$2.4M
experimenter’s XRF Kit
When a primary x-ray excitation source from an x-ray tube or a radioactive source strikes a sample, the x-ray can either be absorbed by the atom or scattered through the material. The process of emissions of characteristic x-rays is called "X-ray Fluorescence," or XRF. Analysis using x-ray fluorescence is called "X-ray Fluorescence Spectroscopy

24. X-ray Microfluorescence (XRMF) - Basics
at~2:20-3:06
XRMF Spectrometer-Eagle II
The Eagle uses special capillary optics to focus x-rays onto a sample and can achieve spatial resolution for elemental composition down to tens of microns. As depicted in Figure 1, x-rays are collected at the top end of the capillary and directed by reflection or direct transmission to a focal point several millimeters away from the tip of the capillary. The x-ray fluoresence emitted from the sample as a result of the x-ray excitation is captured by the detector, processed and translated into elemental information which is displayed for the user. Compared to older aperture-based systems, a capillary delivers more of the tube's x-rays to the analysis area, while providing a greater analytical working distance. This means the user can take advantage of higher x-ray flux to achieve faster analysis times and have the ability to analyze a greater variety of sample shapes and contours.
Low and high magnification CCD cameras for easy sample positioning and rapid set-up of automated experiments
X-ray irradiates specimen
Specimen emits characteristic X-rays or XRF
Detector measures position and intensity of XRF peaks

25. XRMF Analysis - line scan
A fast, convenient
way to examine sample chemistry and heterogeneity along a line of analysis points.
Ideal for studying diffusion profiles or layered materials.
Line scan done on a printed circuit board

26. Elemental Mapping Qualitative Quantitative Spatial distribution Cu mapPb
map

27. 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
FESEM: “Seeing” materials at the nanoscale
SPM: “feeling” materials at the nanoscale
A journey to the nanoworld
Advanced SPM Lecture

28. 2nm Invented at IBM by Gerd Binning and Heinrich Rohrer
History of STM
Rohrer
Binning
Invented at IBM by
Gerd Binning and
Heinrich Rohrer
Nobel Prize in 1986
STM
A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986.[They shared the prize with Professor Ernst Ruska for the design of the first electron microscope in 1931.
STM is limited to the imaging of conductive materials. The AFM was invented to image both insulating and conducting materials.
2nm
STM of silicon-Si(111)
7x7reconstruction

29. Typical STM and AFM Dimension 3100 AFM Low temperature STM
Modules
Dimension 3100 AFM
The world's best selling SPM
T: 4K – 300K
UHV <5x10-11mbar
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.
Low temperature STM
atomic resolution at 5K
Cost: $500K(ambient) to $1.6M (ultrahigh vacuum)

30. 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.
In general, if the probe signal decreases, this means that the point on the surface directly beneath the probe is farther from the probe than the previous point was. Conversely, if the probe signal increases, then the point on the surface is closer to the probe than the previous point. It was the ability of the STM to image individual atoms on surfaces that won the inventors of the STM the Nobel Prize for Physics in 1986.
at~0:30

31. How Does SPM Work?
The SPM creates images with the 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.
Piezoelectric
Scanner
Piezoelectric
Scanner
Due to the extreme sensitivity of tunnel current to height, proper vibration isolation or an extremely rigid STM body is imperative for obtaining usable results.

32. 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 wide
range of properties designed for a variety
of scanning probe technologies. There are
also many types of tips with varying shapes
for probing different morphologies and
scales of surface features and materials
(conducting, magnetized, very hard, etc.).
Probe Motion Sensor: Senses the
spacing between the probe and the
sample and provides a correction
signal to the piezoelectric scanner
to keep the spacing constant. The
common design for this function is
called beam deflection system as
shown at right figure.
With today's sophisticated semiconductor technology, tips and cantilevers are produced in large quantity with consistently shaped, very sharp tips. These tips are secured on the end of cantilevers with a wide range of properties designed for a variety of scanning probe technologies. Cantilevers are available with spring constants less than interatomic bond strengths (about 1 Newton/m) and will therefore allow topographic imaging of surface atomic structure by sliding the tip/cantilever assembly across the surface and monitoring cantilever deflection (contact AFM). These cantilevers can be made with resonant frequencies >10khz to allow rapid scanning over surfaces with high spatial frequency roughness. At the other extreme, the cantilever oscillation techniques (e.g., non-contact AFM, MFM, TappingMode AFM, etc.) require very stiff cantilevers with high resonant frequencies. There are also many types of tips available with varying shapes (for probing different morphologies and scales of surface features) and materials (conducting, magnetized, very hard, etc.).
Beam deflection system-use a laser shining onto and reflecting off the back of the cantilever and onto a segmented photodiode to measure the probe motion.
Electronics: provides interfacing between the computer and the scanning system. It supplies the voltages that control the piezoelectric scanner, accepts the signal from the position sensing unit and contains the feedback control system for keeping the spacing between sample and tip constant.
Vibration Isolation: The microscope must be isolated from its surroundings vibration.
Computer: drive the system and process, display and analyze the image data.

33. 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.
Scanning Tunneling Microscopy (STM)
A piezoelectric tube scanner can provide sub-Å motion increments
Piezoelectric scanner
(PZT)
what is reconstruction?
to ~0.:55

34. Piezoelectric materials convert mechanical to electrical energy (and vice versa).
PZT - Pb(ZrTi)O3 3
Direct Converse 2
D = d S = dE 1
- Piezoelectricity in BaTiO3
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)=d33 S3=d33E3

35. How does a piezoelectric scanner work?
Electrode
Piezoelectric Material, such as Pb(ZrTi)O3 or PZT
A piezoelectric tube scanner can provide sub-Å motion increments
How does a piezoelectric scanner work?

36. Scanning Tunneling Microscope(STM)
It=Ve-Cd
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.
tip V
vacuum
90%It
d~10Å
-quantum tunnel effect
It-the tunnelling current, V-the bias voltage, C-a constant of the material
And d-the nearest spacing between the tip and the sample. It is also a function of local electronic structure so that atomic-scale spectroscopy is possible.
At 1V and d=1nm, E=10MV/cm.
A small bias voltage (mV to 3 V) is applied between a atomically sharp tip and the sample. If the distance between the tip and the sample is large no current flow. However, when the tip is brought very close (~10 Å) without physical contact, a current (pA to nA) flows across the gap between the tip and the sample. Such current is called tunneling current which is the result of the overlapping wavefunctions between the tip atom and surface atom, electrons can tunnel across the vacuum barrier separating the tip and sample in the presence of small bias voltage. The magnitude of tunneling current is extremely sensitivity to the gap distance between the tip and sample, the local density of electronic states of the sample and the local barrier height. The density of electronic states is the amount of electrons exit at specific energy. As we measure the current with the tip moving across the surface, atomic information of the surface can be mapped out.
STM does NOT probe the nuclear position directly, but rather it is a probe of the electron density, so STM images do not always show the position of the atoms, and it depends on the nature of the surface and the magnitude and sign of the tunneling current.
– sample problem
sample
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.
at~0:30

37. Two Scanning Modes in STM
Imaging of surface topology can be done in one of two ways:
Scan direction
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 height mode
Scan direction
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.
Constant height mode – constant height and constant applied bias are simultaneously maintained. A variation in current results as the tip scans the surface because topographic structure varies the sample-tip separation. In this case, the current is the image and can be related to charge density. Constant current mode – a feedback mechanism is enabled that maintains a constant current while a constant bias is applied between the sample and tip. These conditions require a constant sample-tip separation. As the tip is scanned over the sample, the vertical position of the tip is altered to maintain that constant separation. The motion in all three directions is controlled by piezoelectric elements. Simple voltage ramps applied to a piezoelectric tube cause the tip to scan the surface, and the voltage signal from a comparison circuit is directed to the z piezoelectric element. The signal required to alter the vertical tip position is the image, which represents a constant charge density contour of the surface. The image is recorded as a 2-D array of integers representing heights at a specific x, y positions. Altering the level of the current set point or the applied bias produces contours of different charge densities. Constant current mode produces contrast directly related to electron charge density profiles, whereas the constant height mode provides for faster scan rates not being limited by the response time of the vertical driver. Atomic resolution images are possible only under optimized sample and tip conditions. Larger sample-tip separations and blunt tips have the effect of smearing the localized structure and produce topographic images with somewhat lower resolution. To image a relatively rough surface, constant current mode is best suitable. It It
constant current mode

38. Atomic Force Microscope (AFM)
AFM senses interatomic forces that occur between a probe
tip and a sample.
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.
Photo diode
detector
Probe tip
Modules
Optical lever detection of cantilever deflection-The diagram illustrates how this works; as the cantilever flexes, the light from the laser is reflected onto the split photo-diode. By measuring the difference signal (A-B), changes in the bending of the cantilever can be measured.
Since the Cantilever obeys Hooke's Law for small displacements, the interaction force between the tip and the sample can be found. The movement of the tip or sample is performed by an extremely precise positioning device made from piezo-electric ceramics, most often in the form of a tube scanner. The scanner is capable of sub-angstrom resolution in x-, y- and z-directions. The z-axis is conventionally perpendicular to the sample.
Piezoelectric sample
scanner
Optical lever detection of cantilever deflection
3-D topographical maps of the surface are then
constructed by plotting the local sample height versus horizontal probe tip.

39. Silicon Nitride-Contact Mode AFM Probe
Substrate
Tip
Cantilever
Probes
Spring constants
(N/m)
The properties and dimensions of the cantilever play an important role in determining the sensitivity and resolution of the AFM.
One of the most important factors influencing the resolution which may be achieved with an AFM is the sharpness of the scanning tip. The best tips may have a radius of curvature of only around 5nm. The need for sharp tips is normally explained in terms of tip convolution. This term is often used (slightly incorrectly) to group together any influence which the tip has on the image. The main influences are: broadening, compression, interaction forces, aspect ratio. Tip broadening arises when the radius of curvature of the tip is comparable with, or greater than, the size of the feature trying to be imaged. As the tip scans over the specimen, the sides of the tip make contact before the apex, and the microscope begins to respond to the feature. This is what we may call tip convolution. Compression occurs when the tip is over the feature trying to be imaged. It should be born in mind that although the force between the tip and sample may only be nN, the pressure may be MPa. Interaction forces between the tip and sample are the reason for image contrast with the AFM. However, some changes which may be perceived as being topographical, may be due to a change in force interaction. Forces due to the chemical nature of the tip are probably most important here, and selection of a particular tip for its material can be important. Chemical mapping using specially treated or modified tips is another important aspect of current research in SPM. The aspect ratio (or cone angle) of a particular tip is crucial when imaging steep sloped features.
Contact Mode probes consist of cantilevers with low spring constants (<1N/m) to minimize the force between the tip and the sample during imaging. For greater reduction of tip-sample forces, contact mode imaging may also be performed in a fluid environment with the same probes. For TappingMode™ AFM in fluid, any of the contact mode probes can be used.
There are different types of scanning modes.
cantilever

40. Contact, Non-Contact and TappingMode AFM
Contact
Non-contact
Contact, Non-Contact and TappingMode AFM
Contact Non-contact Tapping
Measure topography by c
a.Sliding the probe tip across surface
b.Sensing Van der Waals attractive forces between surface and probe tip held above surface
c.Tapping the surface with an oscillating probe tip a b
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.
Detection of deflections of <0.1nm is readily achieved.

41. LiftMode AFM 2nd pass 1st pass Lift height Magnetic or electric
Field source
Lift height
Topographic image
Non-contact
Force image
The strength of various force interactions depend on the tip-sample separation and are related to surface topography. To distinguish the topography from electrostatic or magnetic forces, a combination contact and noncontact measurement is made. Fig. Illustrates the difference in topography based on short-range interactions and a force image based on long-range interactions. The example describes a magnetically or electrostatically inhomogeneous material with a rough surface. Typically, the topography is first obtained using dc or ac AFM, after which, the tip is separated a preset distance from the surface (chosen so that short-range forces contribute negligibly compared to the electrostatic or magnetic forces). At this preset sample-tip separation, long-range interactions are measured. This procedure results in the tip retracing the topography, but with a large sample-tip separation, effectively eliminating the topographic contribution from the force image.
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.

42. Topographic structure (a) and Magnetic force image (b) of a compact disk
a b
Topographic 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.

43. Image Insulating Surfaces at High Resolution in Fluid - AFM
18nm 1 2
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.
SEM is conducted in a vacuum environment, and AFM is conducted in an ambient or fluid environment to study hydrated samples in the fields such as biology and biomaterials. For SEM, hydrated samples are addressed by placing a specimen in an environmental chamber with either an electron transparent window or a small aperture for the beam to enter the chamber. Imaging samples in a hydrated state with an AFM is performed by enclosing the sample and probe in a fluid environment, as shown. The image and scanning mechanism are not disturbed by the presence of the fluid. The resolution of the image will be determined by the radius of the tip, the applied force, and the noise floor of the instrument. Because of these factors, this configuration allows the study of hydrated specimens at a lateral resolution of 1 to 5 nm and a vertical resolution down to 0.5 Å without sample damage.
Fluid cell for an AFM which allows
imaging in an enclosed, liquid
environment.

44. 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
Atomic Force Microscopy (AFM) Probes • Contact Mode Probes • TappingMode™ Probes • High Aspect Ratio Tapping Probes • Force Modulation Probes • Magnetic Force Microscopy (MFM) Probes • Laser Diode Interferometer-Based AFM Probes • LZT Probes • Scanning Thermal Microscopy Probes • Electrical Probes (EFM, SCM, TUNA, SSRM) • Nanoindenting/Scratching/Wear Testing Probes
Scanning Tunneling Microscopy (STM) Probes • NanoTips™ Probes

45. Advanced SPM Techniques
Scanning Capacitance Microscopy (SCM)
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 Imaging
measures 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
Lithography
Use of probe tip to write patterns

46. Applications of SPM
To 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 speed
The maximum imaging area is limited (<mm2)

47. Examples of AFM Images 3-D Al2O3 10m
80nm tall elevated features in a Si/Si3N4 substrate
4m
Grain growth studies
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.

48. Electric Force Microscopy (EFM) Mode : Ferroelectric Domains on BaTiO3 Surface- +
- Ferroelectric domain
1 m
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.

49. Nanoindentation metal-foil diamond tip 200nm
Using 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.
metal-foil
diamond tip
Indentations on two different diamond-like carbon films using three different forces (23, 34, and 45N) with four incidents made at each force to compare difference in hardness.
200nm
Indentation depths are deeper for the
softer thin film (right).

50. IC Failure Analysis and Defect Inspection with Scanning Thermal Microscopyb
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.
Example: page 6 SCM dC/dV image-bipolar transistors.
SPM application modules for electrical characterization of semiconductors: SCM, SSRM and TUNA, SCM-scanning capacitance microscopy for capacitance (dC/dV) imaging and 2-D dopant profiling inside semiconductor devises; SSRM-scanning spreading resistance microscopy for resistivity/conductivity imaging and 2-D dopant profiling inside semiconductor devises, and TUNA-Tunneling AFM for untral-low current imaging (60 fA – 120 pA) and study of thin films, oxide thickness and defects…

51. Nanolithography

52. STM - Seeing Atoms
STM image showing single-atom defect in iodine adsorbate lattice on platinum. 2.5nm scan
then click Movies see 3-D atomic lattice of graphite.
- Manipulation of Atoms: One innovative applications of STM recently found is manipulation of atoms. Here is an example, Iron atoms are placed on Cu(111) surface at very low temperature (4K), Iron atoms are first physisorbed on the Cu surface, then the tip is placed directly over a physisorbed atom and lowered to increase the attractive force by increasing the tunneling current, the atom was dragged by the tip and moves across the surface to a desired position. Then, the tip was withdrawn by lowering the tunneling current.
Iron on copper (111)

53. Factors Influencing the Resolution of SPM
Broadening
Aspect ratio
Compression
Interaction forces
Tip broadening arises when the radius of curvature of the tip is comparable with, or greater than, the size of the feature trying to be imaged.
Tip
Image
Surface
As the tip scans over the specimen, the sides of the tip make contact before the apex, and the microscope begins to respond to the feature. This is what we may call tip convolution.
Compression occurs when the tip is over the feature trying to be imaged. It should be born in mind that although the force between the tip and sample may only be nN, the pressure may be MPa. Interaction forces between the tip and sample are the reason for image contrast with the AFM. However, some changes which may be perceived as being topographical, may be due to a change in force interaction. Forces due to the chemical nature of the tip are probably most important here, and selection of a particular tip for its material can be important. Chemical mapping using specially treated or modified tips is another important aspect of current research in SPM. Contact Mode probes consist of cantilevers with low spring constants (<1N/m) to minimize the force between the tip and the sample during imaging. For greater reduction of tip-sample forces, contact mode imaging may also be performed in a fluid environment with the same probes. For TappingMode™ AFM in fluid, any of the contact mode probes can be used.
The aspect ratio (or cone angle) of a particular tip is crucial when imaging steep sloped features.

54. AFM Images Acquired with Two Different Tipsb
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.
Although AFM may be easier to characterize rougher surface than STM do, image interpretation can often be a challenge as shown in above example.

55. Do review problems for SEM and SPM
Next Lecture
X-ray Diffraction a b