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Page 1Quang hoc ng Electron Microsco

Knh hien vi ien t

Phan loai: Electron in, Electron out:

Transmission/Reflection Electron Microscopy (TEM/REM)

Scanning Electron Microscopy (SEM) Scanning Probe Microscopy:

Scanning Tunneling Microscopy (STM)

Atomic Force Microscopy (AFM)


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Cac loai tng tac Electron

Cathod phat quang

e Th cap

e tan xa ngce

ti

eAuger

X-rays

e tan xa an hoie tan xakhong an hoi

e khong tan xa


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Electron Microscopy: TEM, REM

REM

Reflection EM

Mau quan sat trc thau knh t, toan bo anh c thu

cung luc

Electron Gun

Electron GunTEM

TransmissionElectron

Microscopy


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Electron Microscopy: Scanning SEM, STEM

Mau vat at sau thau knh t , lan lt quet chum tia

ien t e thu anh

Electron Gun Electron Gun

SEM

Scanning Electron Microscopy

STEM

Scanning Transmission EM


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Scanning Probe Microscopy

Family ofScanning Probe Microscopy Techniques Scanning Tunneling Microscopy (STM)

Atomic Force Microscopy (AFM); also called Scanning ForceMicroscopy (SFM)

Specialized techniques: Electric Force Microscopy (Magnetic ForceMicroscopy, Conductive AFM, Phase Imaging, Spectroscopy)

STM developed in 1982 by Binnig, Gerber, & Rohrer at IBM Zurich.

AFM developed in 1986 by Binnig, Quate (Stanford), and Gerber.


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KNH HIEN VI AU DO QUET

(Scanning Probe Microscope

SPM)

SPM

STM Scanning Tunneling Microscope

G.Binnig, H.Rohrer, 1981

AFM Atomic Force Microscope

G.Binner, 1986

MFM Magnetic Force Microscope

J.A.Sidles, 1992

SCM Scanning CapacitanceMicroscope

J.R.Matey, 1985


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Atomic Force Microscopy

Developed in 1986 by Binnig & Gerber (IBM), and Quate(Stanford).

Measures atomic forcesbetween probe (cantilever tip) andsample by using a laser to detect lever motion.

Image formed by scanning and using feedback loop to maintainconstant tip-sample force (contact mode) or constant tiposcillation (tapping mode) during scanning.

Images both insulators and conductors with nanometer

resolution.

Robust technique with simple sample preparation.


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Imaging Methods: Scanning Force Microscopy (SFM / AFM)

The atomic force microscope (AFM) probes the surface of asample with a sharp tip, a couple of microns long and often lessthan 100 in diameter. The tip is located at the free end of acantilever that is 100 to 200 m long. Forces (108 106 N)

between the tip and the sample surface cause the cantilever tobend, or deflect. A detector measures the cantilever deflection asthe tip is scanned over the sample, or the sample is scannedunder the tip. The measured cantilever deflections allow a

computer to generate a map of surface topography. AFMs can beused to study insulators and semiconductors as well as electricalconductors.


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au do

SEM images of Ultrasharp silicon cantilever tip


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SEM images of Ultrasharp silicon cantilever tip


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Atomic Force Microscopy

Measures forcesbetween a cantilever and sample. Forces detected by measuring small motions of a soft micromachined

cantileverusing a laser beam.

Spring constant kfor Si cantilevers = 20-100 N/m and Si3N4= 1 N/m.

Resonant frequencies of 50-500 kHz.

Cantilever

sample

Laser Beam

Tip

Cantilever

Substrate


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In this mode cantileverstouches the surface while

scanning in repulsive mode


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Allows to distinguish areas with different friction and also to obtain edge-enhancedimages of any surface. This capability may be used in conjunction withtopographical images during one scan to characterise your samples morecompletely. LFM has important uses for semiconductors, polymers, deposited films,data storage devices, investigative studies of surface contamination, chemicalspeciation and frictional characteristics, and a growing list of new applications.


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Relation between the types of the cantilever bending deformations (bottom)and the change of the spot position on the split photodiode (top)


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AFM: Schematic

Cantilever scanned across surface using x-y piezoelectric tube.

Bending motion of lever(or changing force) detected by laser beam reflectedfrom backside of lever into photodetector.

Feedback loop maintains constant force by moving lever up and down (z-piezo).

Topography image formed by displaying z-piezo voltage vs. position.

Cantilever & Tip

Sample

PhotodiodeDetector

FeedbackElectronics

PiezoelectricScanner

Laser

X, Y

Z


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Assembly of three tubes into one unit (Fig. 4) allows to organizeprecise movements of a tip of a microscope in three mutuallyperpendicular directions. Such scanning element is referred to astripod.


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The drawbacks of such scanner are the complexity of manufacturing and strongasymmetry of its construction. For today the scanners made on the basis of onetubular element are most widely used in the scanning probe microscopy. The generalview of a tubular scanner and arrangement of electrodes are presented on above Fig.The material of a tube has a radial direction of a polarization vector.


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AFM

LaserReflectionWindow

1. Mount Cantilever

Piezo Tube Scanner

OpticalMicroscope

Laser

Sample

2. Adjust Laserbeam on cantilever

3. Adjust Laserbeam on detector

4. Approach cantilever

to Sample


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AFM: Standard Contact Mode

Cantilever is in direct contact with sample at all times.

Feedback loop maintains constant cantilever deflection (or force) duringscanning by adjusting cantilever-sample distance.

Vertical motion of cantilever in Dimension and of sample in Multimode.

Tip-sample forces of 10-7 to 10-11N.

Lateral (shear) forces can damage soft or fragile samples.

Feedback increases cantilever-sampledistance over bump.


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AFM: TappingMode

Cantileveroscillates at resonant frequency and taps sample surface, wherefeedback loop maintains constant oscillation amplitude.

Reduces normal (vertical) forces and shear (lateral) forces, thereby reducingdamage of softer samples.

Water layer

Reduced Amplitude

Free Amplitude

Tapping

10-100 nm


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Contact vs. Tapping Mode AFM

CONTACT MODE

Scans cantilever (k = 0.01 - 1.0 N/m) across surface. Feedback loop maintains constant cantilever deflection during scanning.

Tip-sample force = 10-7 to 10-11N.

Advantage: Cost-effective (~$4/tip).

Disadvantage: Lateral (shear) forces can damage soft or fragile samples.

TAPPING MODE

Scans oscillating cantilever (~20 nm) across surface.

Feedback loop maintains constant oscillation amplitude. Advantages: Minimizes shear (lateral) forces and thereby minimizes sample

damage.

Disadvantages: More $$ for tips (~$20/tip).


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Simplified

schematic

of the AFM,

EFM, MFMcontrol

system


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AFM Data: Ga-Polar GaN

Ga-polarity indicates (0001) growth direction, with Ga termination on thetop surface layer.

Smooth surface with surface height rms ~3 nm.

Hexagonal islands with diameters ~300 nm and heights ~80 nm.

Gallium

Nitrogen10 Qm(z = 30 nm 2 Qm(z = 20 nm


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AFM Data: N-Polar GaN

N-polarity indicates (0001) growth direction, with N termination on the topsurface layer.

Rough, island surface with rms ~20 nm and island heights of 50-90 nm.

Gallium

Nitrogen5 Qm(z = 80 nm 2 Qm(z = 80 nm


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0.5 QmSTM15 QmAFM

AFM & STM Data: Si(111) with Normal Flash

High density of SiC pinning sites (~4 per 5 Qm2) on normal flashedSi(111) [5 min @ 1150rC].


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15 QmAFM 0.5 QmSTM

AFM & STM: Si(111) with High Temp. Flash

Lower density of SiC pinning sites (~1 per 5 Qm2) on highertemperature flashed Si(111) [10 min @ 1250rC].

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