Bài giảng Vật lý điện tử__em-lens | Môn Vật lý điện tử | Trường Đại học Bách Khoa Hà Nội
Bài giảng Vật lý điện tử__em-lens | Môn Vật lý điện tử | Trường Đại học Bách Khoa Hà Nội. Tài liệu gồm 45 trang giúp bạn tham khảo ôn tập đạt kết quả cao trong kỳ thi sắp tới. Mời bạn đọc đón xem.
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The Transmission Electron Microscope
Analogous to transmission optical microscopy. apertures Also z, tilt, rotation cuu duong than cong . com
1 in 1020 electrons collected
<1 in 106 photons collected CuuDuongThanCong.com
https://fb.com/tailieudientucntt Components of TEMs
• Source - filament plus anode plates with applied accelerating voltage.
• Condensor Lenses - electromagnetic lenses
adjusted by lens currents not position.
• Specimen Stage - allows translations and tilts.
• Objective Lens - usually < 50x.
• Imaging System – multiple electromagnetic
lenses below the objective: set magnification,
focal plane (image vs. diffraction pattern).
• Observation - fluorescent screen, plate film or cuu duong than cong . com CCD camera. CuuDuongThanCong.com
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The Scanning Electron Microscope
Uses secondary signals to obtain serial images: TEM SEM cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Components of SEMs
Source and condensor same as for TEM (lower V), but:
• Scanning Coils - raster the probe. • Probe Lens - lens forms a spot at the specimen surface. • Detector/Processing System
-Collects signals (X-rays, electrons etc.) as a
function of time (and thus position since probe scans with time).
-Gives digital “images” convenient for real-time cuu duong than cong . com
viewing, processing, and storage. CuuDuongThanCong.com
https://fb.com/tailieudientucntt Source: Electron Gun Electrons emitted from filament by thermionic emission (W or LaB6) or from tip by field emission Heater (W single crystal). Filament Wehnelt Accelerated through Cap potential difference V: Bias nearly monochromatic beam of fast electrons. Focused by grounded do Wehnelt Cap. cuu duong than cong . com Anode αo CuuDuongThanCong.com
https://fb.com/tailieudientucntt Thermionic Emission source • Heat a metal substantially (2700K). • Electrons are excited and drawn toward the Anode. • Many electrons transfer through cuu duong than cong . com a hole in the anode and down the beam path. CuuDuongThanCong.com
https://fb.com/tailieudientucntt (Cold) Field Emission source • Similar basic design, but now electrons tunnel out of a sharp tip due to strong electric fields. • Smaller emitting area so higher resolution is possible. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Condensor Lenses EM lenses focus light to as small of a spot as possible, equivalent to convex lenses in optics. cuu duong than cong . com CuuDuongThanCong.com
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• Use electrostatic or electromagnetic fields to focus beams of charged electrons.
• Electromagnetic - Most lenses of this type. Cu wire
coils around soft Fe cores. Sometimes use Fe pole-
pieces to “shape” the field.
– Diagram in book is not too descriptive
• Electrostatic - Unusual, only common example is
the Wehnelt aperture in the electron gun. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electrons in magnetic fields
• An electron moving through a perpendicular magnetic field
experiences a [different] perpendicular force – “right hand rule”
• An electron moving parallel to a magnetic field is unaffected.
• An electron moving nearly parallel is slightly deflected out of the
plane, then to the right, then into the plane, then to the left, etc.
– Takes helical path around the magnetic field direction (microscope axis). unaffected electron nearly aligned Force on Magnetic electron electron is field out of plane cuu duong than cong . com No Force Electron follows helical path CuuDuongThanCong.com
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Trajectories in Electromagnetic Lenses
• When adjusting magnification (and focal length), the lens
strength is modified by adjusting the current in the
electromagnetic lens windings.
• Since the magnetic field therefore changes, so do the helical trajectories.
• Ultimately, this leads to image rotation, which needs to be
corrected for, or at least calibrated. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Beam steering
• The electron beam can be positioned for fine
measurements (spot modes) or scanning (SEM, STEM) cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Sample stage • Lateral and normal control • Rotation • Tilt in two directions • Also, possibly integrated: – Heaters – Mechanical testing – Electrical testing cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Objective lens • Electromagnetic. • Usually only ~50x. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Objective Aperture
• Depending on the aperture position, the bright
field image or the dark field (scattered) image can be collected. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt STEM (scanning TEM)
• By selecting bright or dark field electrons, contrast
varies and provides information about crystallography. cuu duong than cong . com CuuDuongThanCong.com
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Imaging System (magnification lenses) • Electromagnetic.
• Various combinations employed for increasing magnification. cuu duong than cong . com CuuDuongThanCong.com
https://fb.com/tailieudientucntt Electron Wavelengths h λ = . 1 5 or λ = nm 2m eV 1 + eV m c V 0 ( 2 0 ) Where: • h - Planck’s constant • m0 - electron rest mass • e - charge on the electron
• V - accelerating voltage (in Volts) • c - speed of light SEM TEM V (kV) 5 20 100 200 300 1000 cuu duong than cong . com λ (nm) .017 .008
0.00370 0.00251 0.00197 0.00087 CuuDuongThanCong.com
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• As features on the surface separate, they may be resolved. cuu duong than cong . com CuuDuongThanCong.com
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Diffraction Limited - Defined by Raleigh criterion:
δ = 0.61λ /µ sin α ≈ 0.61λ /α d
Since λ varies with V, higher voltages give smaller δ (good) d
but at increased cost and beam damage (bummer).
Also, δ increases as α gets smaller. d
Aberration Limited – The wavelength is so small that
aberrations become extremely important, particularly spherical aberration. δ α3 ≈ s Cs cuu duong than cong . com
Where: Cs - Spherical aberration coefficient
Smaller α gives smaller δ (good), but also lower s intensity (bummer). CuuDuongThanCong.com
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