1-1电子显微镜与电子光学(1)

• electron guns -provides electron source • condenser lens -control the electron beam
• The brightness is determined by the strength of electron emission • The size of the spot is determined by the properties of conderser lens system
Working modes
Incident Beam Sample
α
α
C2 Aperture
Objective Lens Back Focal Plane
Sample
ADF Detector Image Plane
Conventional TEM
Scanning TEM（STEM）
Imaging system
Comparison of electron sources
• FEG
– Highest brightness, low energy spread, good coherency – For all applications that require a bright, coherent source the FEG is the best. – need UHV, very expensive (>US$10,000)
Elements of a TEM
Electron optics system (1) illumination system (2) imaging system (3) image viewing and recording • electron guns • condenser lens
Electron Source －Electron Gun
φ Cold FEG
Cross-over size (nm) Vacuum (Pa)
thermal field emission
Elements of a TEM
Electron optics system (1) illumination system (2) imaging system (3) image viewing and recording • electron guns • condenser lens
Electron optics system (1) illumination system (2) imaging system (3) image viewing and recording • electron guns • condenser lens
Comprises:
• Objective lens • Intermediate lens • Projector lens
Structure of TEM
Electron optics system (1) illumination system (2) imaging system (3) image viewing and recording
illumination system
TEM
Vacuum system Power Supply and control system
TEM
Vacuum system
•
Power Supply and control system
illumination system
Two different ways to use the illumination system
– form a parallel beam (used for TEM imaging and diffraction) – form a convergent beam (used for STEM imaging, microanalysis, and microdiffraction)
LaB6 crystal
亮度高、光源尺寸 和能量发散小
•
• •
W filament
fine W needle
电子束亮度高，相干性好
Ideal Electron Source
• high brightness (high current density) • better coherency (small energy spread) – small chromatic aberration – good for modern TEM work • good stability • long lifetime
Comparison of electron sources
• Tungsten source
– low emission efficiency, large size of emission source in diameter – 5~7nm Spot size, poor resolution – cheap and easily replaceable.
• •
Condenser lens
The illumination system consists of two (three) condenser lenses The first condenser lens (C1,often controlled via a knob which is labeled spot size), is responsible for controlling the spot size and hence the current (sets the demagnification of the gun crossover). The second condenser lens (C2, often controlled via a knob which is labeled intensity) provides direct control of the illuminated areas at the specimen, and direct control of the convergence angle.
–
Power Supply and control system
Magnetic Lens
1. Coil of several thousand turns of wire through which a current of less than or equal to one amp is passed ‐‐‐ creates a magnetic field. 2. Electrons are deflected by magnetic field. 3. To avoid need for a large current passing through the coil the magnetic field is concentrated by encasing the coil in a soft iron cover with only a small ring‐ shaped gap in centre. The entire field is therefore concentrated in this gap. 4. To concentrate field further a soft iron pole piece is inserted into the bore of the objective lens – reducing the bore of the lens and width of the ring‐shaped gap. 5. To focus an electron beam onto a given plane the current through the coils must be set to a precise value. ↑ current – beam focus closer to lens ↓ current – beam focus further from lens
TEM
Vacuum system
• • Provides source of electrons to illuminate the specimen. There are two types of electron sources: – thermionic source
• tungsten filaments • lanthanum hexaboride (LaB6) crystals
Brightness 1010 (A/m2sr) at 100 kV
5x101 1700 1.5 <1 500 10k 10-4
φ Schottky FEG
Temperature (K)
2700
Energy spread ΔE 3.0 (eV) at 100 kV Emission Stability (%/hr) Lifetime (hr) <1 100 500k 10-2
电子显微镜与电子光学
Transmission Electron Microscope (TEM)
• 电子显微镜的结构 • 电子光学系统的像差 • 电子显微镜的工作模式 • 电子枪 • 磁透镜 • 像差
Hale Waihona Puke Baidu
Illumination system
Imaging system
Image viewing and recording
TEM
Vacuum system
Functions
– Uses objective lens to form the image or the electron diffraction pattern Uses intermediate lens and projector lens to magnify the image or the diffraction pattern produced by the objective lens and to focus them on the viewing screen.
Properties of Electron Guns
Work function, φ W，LaB6 Thermionic
Electron Source Φ (eV) W 4.5 LaB6 2.4
1
Schottky FEG 3.0 5x1012 1500 0.7 <1 2000 15 10-8
Cold FEG 4.5 1013 300 0.3 5 >1000 3 10-9
• LaB6 – high brightness, improves coherency and the energy spread, increases operating life – is a recommended thermionic source, – expensive (several hundred dollars each) – Resolution ~2Å
•
Electron Beam Sources: Field‐Emitter
The principle behind field emission is that the strength of an electric field E is considerable increased at sharp points, because if we have a voltage V applied to a (spherical) point of radius r then E=V/r. If a strong electric field is applied to the surface of metals, the barrier at the surface of metals will decrease a lot. Electrons inside metals overcome the barrier due to the tunneling effect and are emitted from the surface -> FEG One of the easiest materials to produce with a fine tip is tungsten wire. To allow field emission, the surface should be free of contaminants and oxide. This can be achieved by operating in UHV (better than 10-11 Torr)
Power Supply and control system
– field‐emission source (FEG): fine tungsten needles
• Schottky FEG • Cold FEG
Thermionic Emission
• If any material to be heated to a high enough temperature , i.e. 2000oC, the electrons gains sufficient energy to overcome the natural barrier (work function) that prevents them from leaking out to escape from the source. • Two thermionic sources used in practice are tungsten and LaB6.