1. Scanning Electron Microscopy (SEM)
SEM: A focused electron beam (2-10 keV) scans on the surface, several types of signals are produced and detected as a function of position on the surface. The space resolution can be as high as 1 nm. Different type signal gives different information: a. Secondary electrons: surface structure. b. Backscattered electrons: surface structure and average elemental information. b. X-rays and Auger electrons: elemental composition with different thickness-sensitivity.

2. The image formation
TV screen
e-beam
The selection of signal

3. Advantages of Using SEM over OM
Magnification Depth of Field Resolution
OM 4x – 1000x mm – 0.19mm ~ 0.2mm
SEM 10x – x 4mm – 0.4mm nm
The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time and produces an image that is a good representation of the three-dimensional sample. The SEM also produces images of high resolution, which means that closely features can be examined at a high magnification.
The combination of higher magnification, larger depth of field, greater resolution and compositional and crystallographic information makes the SEM one of the most heavily used instruments in research areas and industries, especially in semiconductor industry. OM
SEM

4. Schematic set-up of SEM
Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample.

5. Backscattered Electrons (BSE)
BSE yield: h=nBS/nB ~ function of atomic number Z.
BSE images show characteristics of atomic number contrast, i.e., high average Z appear brighter than those of low average Z.
BSE image from flat surface of an Al (Z=13) and Cu (Z=29) alloy

6. Different surface sensitivity
Auger electron and X-ray can be the signal source for SEM and obtain the elemental composition microscopy but with totally different surface sensitvity.

7. magnetic lens f  Bo2 Why suitable for high energy e M = q/p
(solenoid)
(Beam diameter)
F = -e(v x B) p q
Lens formula: 1/f = 1/p + 1/q
M = q/p
Demagnification:
f  Bo2
f can be adjusted by changing Bo, i.e., changing the current through coil.

8. TEM
binocular
screen
Control
brightness,
convergence
Control contrast

9. Interpretation of Images
Bent
foil
image
A buckled region of a thin foil and bend contour occurs where the Bragg condition is locally satisfied.
bend
contour
Dislocations
loop
Dislocations are readily analyzed and characterized by means of diffraction contrast.

10. Why TEM
The uniqueness of TEM is the ability to obtain full morphological (grain size, grain boundary and interface, secondary phase and distribution, defects and their nature, etc.), crystallographic, atomic structural and micro-analytical such as chemical composition (at nm scale), bonding (distance and angle), electronic structure, coordination number data from the sample.
TEM is the most efficient and versatile technique for the characterization of materials. It has many mechanism for contrast in TEM for different application purpose.
limitation
More or less bulk-like information, the sample cannot be too thick, Sample preparation can be difficult.

11. Comparison of OM,TEM and SEM
Source of
electrons
Light source
Condenser
Magnetic
lenses
Specimen
Objective
Specimen
Projector
CRT
Eyepiece
Cathode Ray Tube
detector OM
TEM
SEM
Principal features of an optical microscope, a transmission electron microscope and a scanning electron microscope, drawn to emphasize the similarities of overall design.

12. PEEM(Photoemission Electron Microscope)
Resolution reached < 20 nm
PEEM with integral sample stage for precise sample positioning via remote controlled piezo-drives. Secondary electrons generated by the incident X-rays are focused by the lenses to form a magnified image of the electron yield distribution emerging from the sample surface.
Topography contrast
Work funktion contrast
Chemical contrast (absorption edges)
Magnetic contrast (magnetic dicchroism)
Secondary have relatively long free path length (10nm), very suitable study multilayer

13. Low Energy Electron Microscopy (LEEM)a b
                                                                                                                                                     
                                                                                                                                                     
a) Si(001) atomic arrangement b) the corresponding diffraction pattern
                                                                                                         
                                                         
Green spots
Red spots
Basically same as PEEM but with e source
Localized structural information

14. Scanning Probe Microscopy (SPM) 1. Scanning tunneling Microscopy (STM)
I = U/d exp(-Kd f1/2)
STM is based on the tunneling current between a metallic tip and the sample surface, which depends on voltage (U), distance d, and work f average of the two work functions of both sides , K is a constant.
STM works only with conducting sample !
Inverse decay length K (reciprocal of distance for density to fall to
1/e): K = 2p/h (2me f)1/2

15. Tunneling between two metals
Without bias tunneling
Without bias tunneling

16. Spatial Resolution of STM
For a typical metal (K = 1 Å), current falls about an order of magnitude for an increase of 1.0 Å in d. very sensitive dependence of tunnel current on d - good "vertical“ resolution.
I = U/d exp(-Kdf1/2)
If one metal is sharp tip, most of Itunnel will travel through pinnacle atom - good "lateral" resolution

17. How STM works More compact
material changes length in applied electric field
to 10-7 % length change per V allows < 0.5 Å positioning for in-plane and vertical resolution < 0.05 Å.

18. The image formation height change I change How the scan works
Itunnel constant by moving tip up and down (feedback circuitry) - z
movement becomes image
Slower but works for rough surfaces
Most common
Constant Height Mode
Tip-sample distance fixed - variation in Itunnel forms image
Fast but only works for flat samples
How the scan works

19. Sharpness is essential !!!
The tip sharpness
Sharpness is essential !!!
bad
good
Etching: a common way +
Tip radius < 20 nm !!! -
NaOH
Etched not etched

20. STM
Vibration isolation of the STM (SPM) device is very important (spring, eddy current damping magnet assemblies, etc). The vertical resolution is 0.05 Å!! The electric isolation is also important, the tunneling current normally nA – pA.
tip
spring

21. STM on clean surface and with absorbent
Atom-resolved !!!
Ag(110) clean surface with step edge
O2 covered Ag(110)

22. Image of work function
Normal STM mode can not distinguish the change of morphology (height) from work function (see the formula before), however, from the formula , which able to tell the difference.
Au on Si(111) line scan of both z and f, the change of z is due to change of work function (where there is Au).

23. Tunneling from tip to substrate
Tunneling is current from occupied states of one side to unoccupied states of another side of the STM. The direction of current depends on bias. The current depends the filled or unfilled electron states near the Fermi surface.
Between metal tip to semiconductor

24. IV curve: Scanning Tunneling Spectroscopy
Unique peak at certain bias
When tip is biased negative, one gets information about empty electronic states on surface. When tip is biased positive, one gets information about filled electronic states on surface. With the same height, one record the current as function of bias voltage, in reality this measurement is often plotted as I/V, dI/dV, or d2I/dV2 curves. This kind of plot provides atom-resolved information about local electronic (and even vibrational) structure. Simply one can can do STM at changing bias to resolve "location" of individual electronic states.

25. Manipulation Several methods:
- field effect - under influence of high electric field, polarizable molecule or atom can be made to jump from surface to tip or vice versa.
- dragging - vdW forces between close tip and adsorbate can be used to
drag species.

26. Atomic Force Microscope
Tip
Atomic Force Microscope
Cantilever
Can work with insulator
Soft, flexible
(low spring constant)
The Force that is experience by the tip is mainly due to Van der Waals force, typically to 10-6 N at separation of ~ 1 Å.

27. Working modes
Contact Mode (Repulsive Mode): Tip makes a “soft physical” contact with the surface and scan. Repulsive force on tip about 10-6 – 10-8 N. operates in ambient or in liquid. frictional force that will distort the image. Works in two modes: constant force or constant height.
non-contact mode (attractive Mode): Tip-surface distance Å. Attractive force of N. uses AC driven oscillating cantilever ( Hz frequency, Å amplitude) to sense the changes in the resonant frequency of a cantilever which reflect changes in the tip-to-sample spacing. Fluid contamination may result in a quite different picture. best for soft or elastic surfaces. least contamination. least destructive. long tip life
intermittent contact (tapping) mode: Similar to non-contact using vibrating cantilever except at one extent tip "taps“(barely contact) into contact. The amplitude > 20 Å, the change of the amplitude due to tap will be sensed. Useful for soft surfaces - less prone to external vibration/noise than noncontact, but will not image water layers (pierce). Less destructive than contact AFM and can image rougher samples.

28. Working modes
a b c
Contact(repulsive) Non-contact (attractive) Tapping
a.Sliding the probe tip across surface, heavily influenced by frictional and adhesive forces, which can damage samples and distort image data.
b.Sensing Van der Waals attractive forces between surface and probe tip held above surface ( Å), low resolution and can also be hampered by the contaminant layer which can interfere with oscillation.
c.Tapping the surface with an oscillating probe tip, 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.

29. Fluid contamination
Non-contact AFM contact AFM

30. Protein Atom-resolved Creature with mouth AFM photos 50 x 50 mm
NaCl(100) surface using a variant of contact mode UHV AFM – arrow shows defects
Creature with mouth

31. More Scanning Probe techniques
AFM
MFM
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;
Electric Force Microscopy (EFM): measures electric field gradient and distribution above the sample surface;
Scanning Thermal Microscopy (SThM): measures temperature distribution on the sample surface.
Scanning Capacitance Microscopy (SCM): measures carrier (dopant) concentration profiles on semiconductor surfaces.
Spin-resolved STM: atom-resolved magnetic microscopy.