Electron Sources, Optics and Detectors

Transcription

1 Thomas LaGrange, Ph.D. Faculty Lecturer and Senior Staff Scientist Electron Sources, Optics and Detectors TEM Doctoral Course MS-637 April 16 th -18 th, 2018 Summary Electron propagation is only possible through vacuum. The vacuum level varies between regions of an electron microscope. The highest vacuum level (<10-7 Pa or 10-9 mbar) is the gun where electrons are emitted and accelerated and is required to keep source and accelerator clean. The specimen area also requires a high vacuum level to reduce contamination, which is especially important for chemical analysis that requires the electron beam to rest in the same area for a long time. Hydrocarbon build up (contamination) on the observed area often results from a low system vacuum level. Turbo-molecular and oil-diffusion pumps for high vacuum cannot work against atmospheric pressure and need a mechanical pre-vacuum pump to back them for proper and efficient operation. Electron beams are generated either by thermal emission or field emission. Field emission sources provide lower energy spread and high coherence required high resolution spatial resolution imaging and electron spectroscopy. Electrons are focused by simple round magnetic lenses which properties resemble the optical properties of a wine glass. Unlike in light optics the wavelength (2pm for 300kV) is not thelimiting factorin the spatial resolution. Lens aberrations and instabilities of the electronics (lens currents etc.) limit the resolution of even the best and most expensive transmission electron microscopes to about 50pm. Recording an image means detecting electrons. Depending on their energy electrons can be detected by different detectors. A high detector efficiency and a high signal to noise ratio allows faster recording and reduces the exposure (beam damage) of the sample to the electron beam. A high linearity and high dynamic range permits to quantify images and to record high and low intensities in one image (important for diffraction experiments)

2 Outline: This section of the course describes main components of the scanning electron microscope: pumping systems, electron sources, electron optics and detectors. 1) Pumping Systems 2) Electron sources 3) Electron optics A. Basics B. Magnetic Lens C. Aberrations and Spatial Resolution Limits in a TEM 4) Detectors :CCD, CMOS and Direct Detection cameras 3 Components of a scanning electron microscope High Vacuum Gun>Column>Detector 1. Electron propagation is only possible through vacuum! 2. Need a good vacuum system to reduce contamination! 4 4 2

3 1) Pumping system: block diagram Gun Ion Pump Electron Gun Primary vacuum (>0.1 Pa) Mechanical pump Column Ion Pump SEM Column Gun Valve Detector Valve Detect Chamber Sample Holder Column Valves Secondary vacuum (detector or viewing chamber) (<10-4 Pa) Oil diffusion pump Turbomolecular pump Diffusion or Turbomolecular Pump Specimen exchange pump valve High and ultra-high vacuum Gun & specimen area (<10-6 Pa) Buffer/ Reservoir Tank Primary pump Valve Air Ion getter pump Cold trap Mechanical Rough Pump Vent Valve 5 1) Pumping system: Primary vacuum Rotary vane pump Uses oil (wet) which can be a source of contamination Noisy but not active all of the time (buffer tank backing) Cheap and low maintenance Scroll pumps No oils or sources of contamination, Dry Expensive Lower ultimate vacuum pressure (10-2 torr) High maintenance, scrolls need to be yearly 6 3

4 1) Pumping system: Secondary vacuum Oil diffusion pump Vibration free cheap, reliable and low maintenance Possible Contamination sources: oil vapor High pumping capacity (>500 l/s) Best with cold trap (limits oil vapor migration) 7 1 ) Pumping system: Secondary vacuum Turbomolecular pumps Rotation speed 20-50k rpm Magnetic bearings Pumping volumes l/s No oils or sources of contamination 8 4

5 1 ) Pumping system: High / Ultra-high vacuum Ion getter pump (IGP) no vibrations No exit ; no pumping improves vacuum by trapping gas molecules in sputtered Ti layers Requires High Vacuum via secondary pumping before using and for proper operation 9 2) Electron Sources LaB 6 Cathode Tungsten Field Emission Tip

6 Increasing Temperature 300 K 1700 K 4/11/2018 2) Electron Sources: Comparison of Sources 11 2) Electron Sources: Emission of electrons metal vacuum (with electrical field) Increasing electric field Thermionic emission Schottky emission field-enhanced thermionic emission (10 8 V/m) Extended Schottky emission thermally assisted field emission Cold field emission tunnel effect (quantum tunnelling) Hot Schottky Cold Schottky Hot FE Cold FE 12 6

7 2) Electron Sources: thermionic gun Electron boil from surface Tungsten wire heated to ~2800K LaB 6 crystal heated to 1900K Main advantages: simple, cheap, no high vacuum required and maintenance friendly Disadvantages: low brightness, high energy spread and large source size ( m) 13 2) Electron Sources: thermionic gun Filament Image 14 7

8 2) Electron Sources: Field emission guns (emitter) Cathodes (tips) Cold field emission (E 10 9 V/m) W single crystal with a sharp tip (radius ~25nm) Advantages Small energy dispersion (<0.1eV) high coherence, high brightness -> higher resolution at lower energies Disadvantages: expensive, high vacuum necessary cold emission needs flushing (cleaning) after 8 hrs Thermally assisted emission: Schottky effect W/Zr tip at K ZrO complexes lower work function Continuous operation self-cleaning ZrO 2 reservoir ) Electron Sources: Field emission guns (extraction) First anode (extractor) Some kv V/m Second anode Final acceleration Grounded Characteristics Tip and anodes form an electrostatic condensor Cross-over (source) is virtual Ø~5nm

9 3) Electron Optics: Basics Object plane Focal plane Image plane Taken from Carter/Williams ) Electron Optics: basics 1. Over-focus 2. Focus 3. Under-focus Image plane crossover point used for the alignment of the TEM column and apertures Taken from Carter/Williams

10 3) Electron Optics: Basics Parallel beams are focused to one point in the focal plane Object plane Parallel beams are focused to one point in the focal plane Focal plane Image plane Taken from Carter/Williams ) Electron Optics: Basics Object plane Focal plane Image plane Taken from Carter/Williams TEM: transmitted beam, diffracted beams 20 10

11 3) Electron Optics: Basics Apertures Angle limiting apertures Used to change current, convergence angle, probe coherence and size in combination with condenser lens system Taken from Carter/Williams ) Condenser lens system First Condenser Lens Defines minimum probe size De-magnifies gun crossover Large spot <-> high current Used to change Probe Current (comprise with probe coherence) Second Condenser Lens Change illumination area sample by converging beam on sample C2 aperture used to change spot size, current convergence angle and beam coherence in combination with C1/C2 lenses Third or Mini Condenser Lens Change convergence angle independent of C1/C2 and illumination area You need at least 2 condenser lenses to get a parallel beam! 22 11

12 3) Electron Optics: Basics Apertures Taken from Carter/Williams Other Apertures Objective Located in back focal plane of the objective lens Used to isolate and limit scattered beams and form TEM images with enhance contrast Bright-field centered on unscattered beam Dark-field centered on diffracted beams Mass-thickness contrast centered on optic axis of objective lens and aperture diameter controls Z-contrast Selected Area Located in the forward image plane of the objective (object plane of the intermediate lens) Limit diffraction area on sample ) Electron Optics: Basics Apertures Object plane Back focal plane Image plane Intermediate lens Intermediate lenses are used to switch between imaging to diffraction mode Intermediate lenses are used to change magnification and camera length Three lenses are required to compensate for image rotation mode DIFFRACTION mode IMAGE Projector lens used to greatly magnify the last intermediate lens image plane onto the detector 24 12

13 3) Electron Optics: Lenses Two types of Lenses Electrostatic lenses Applied potenial and electric field lines define deflection force and focusing Magnetic lenses: Lorentz Force! Variable focus (no moving parts) Tunable correctors (astigmatism) Pole piece metal cone that confines the magnetic field No (simple) divergent lens! multi-poles lenses Image rotation! ) Magnetic Lens: Lorenz force F = Force on electron V = electron velocity B = Magnetic field strength (rotationally symmetric) e = charge of electron = angle to optic axis Interaction of an electromagnetic field on a moving electron: Lorenz force F = -e(vxb) Component of v B: v r << v Magnitude of Force F=-eVBsin Focal length depends on B increasing B lowers f To obtain the same focal at higher electron voltages, B must increase Spiral with radius r = mv r /eb All electrons crossing the axis in one point are focused into the same point,, at frequency (cyclotron frequency) =eb/m Image rotation!

14 3) Electron optics: Magnetic lens focal length Focal Length (f):point on Z axis where initially parallel rays cross the axis after passing through the lens HIGHER LENS STRENGTH = SHORTER FOCAL LENGTH Upper polepiece Lower polepiece High lens strength =>short focal length B = vector parallel to field B r = radial component of field (vector perpendicular to axis) B z = axial component of field (vector parallel to axis) B varies depending on position in lens Force strongest at center of the gap Low lens strength = long focal length 27 3) Magnetic lens: aberrations Field with rotational symmetry Lorenz Force : F=-eVBsin electrons on optical axis: F = 0 electrons not on optical axis : deviated **optical axis is the symmetry axis Scherzer 1936: Magnetic lens with rotational symmetry: Aberration coefficients: C s : spherical C c : chromatic Always positive!! iron Pole piece e-beam coil Resolution limit: D res 3/ 4 1/ Cs Example: = nm, C s = 1 mm D res = = 1.8Å

15 over focus astigmatic under focus astigmatic corrected! 4/11/2018 3) Electron Optics: Resolution Limits for a modern TEM 29 d ech d d 2 g 2 sph d 2 ch d Thermionic limited by source brightness FEG limited by lens aberrations 2 d Using an optimal convergence angle, resolution is, 2.5x kv =(4 /C s ) 1/4 <10 mrad d is nm 3) Electron Optics: Aberrations Lens aberrations Focus Astigmatism spherical and chromatic aberrations Condenser astigmatism affecting beam shape Can be corrected or minimized Physical limits Diffraction limited resolution

16 3) Electron Optics: Aberrations-astigmatism Astigmatism: focal length varies in different planes. This causes the image to be blurred along different directions depending on the focus ) Electron Optics: Astigmatism Correctors Astigmatism: Light optics: Correction with cylindrical lenses Electron optics: Correction with quadrupole lenses, 2 quadrupole lenses under 45 degree allow to control strength and direction of correction Electromagnet Beam corrected for Astigmatism Astigmatic beam Direction of Magnetic Force

17 2) Electron Optics: Chromatic Aberration Focal length varies with energy critical for nonmonochromatic beams (advantage for FE guns) ) Electron Optics: Spherical Aberration Focal length varies with distance from optical axis, i.e., rays from the center to edge of the lens have different focal points Image of the object is dispersed (or blurred) along the optical axis Circle of least confusion d s = ½ C s

18 3) Electron Optics: CS-aberration correction - HR-TEM Combination of standard radiallysymmetric convergent lenses with multipole divergent lenses (e.g. tetrapoles, hextapoles) to tune CS => Resolution jumps to sub-å! Example: Σ3 grain boundaries in Al Uncorrected CS-corrected CEOS corrector Oikawa, JEOL 36 4) Detectors 1. Phosphor screen 2. Film 3. Image plates 4. CCD or CMOS cameras 5. YAG scintillator multichannel plates 6. Phosphor photomultipler tube 7. X-ray detectors 8. BSE, SE, STEM detectors I will discuss more in the next lecture about electron guns, optics and detectors 18

19 4) Detectors Dynamic Range The full well capacity is the largest charge a pixel can hold before saturation. When the charge in a pixel exceeds the saturation level, the charge starts to fill adjacent pixels, known as Blooming. Larger pixels have lower spatial resolution but their greater well capacity offers higher dynamic range which can be important for some applications, e.g., electron diffraction Signal to noise The signal-to-noise ratio (SNR) determines the ultimate performance of the camera/tem. The SNR value is the ratio of the measured signal to the combined noise, which consists of undesirable signal components arising in the electronic system, and inherent natural variation of the incident photon flux, i.e, photon noise, dark noise, and read noise. 38 DQE Detection quantum efficiency is defined, DQE=(SNR) 2 out/(snr) 2 in and provides a measure of the quality with which incident electrons are recorded. A perfect detector has a DQE of unity and to achieve this all incident electrons must be detected with equal weight. The DQE of real detectors is always smaller than unity reflecting the fact that in practice incident electrons are recorded with different weights. MTF The resolution of a camera within an optical system can be characterized by a quantity known as the modulation transfer function (MTF), which is a measurement of the camera and optical system s ability to transfer contrast from the specimen to the intermediate image plane at a specific resolution. The important parameter for HRTEM simulations 4) Detectors: CCD cameras From CCD Camera (charge coupled device) 1kx1k, 2kx2k, 4kx4k Pixel + high dynamic range, linear, sensitive - Slow (because of readout procedure) - expensive Readout of pixels through charge transfer and 1 pixel readout 41 19

20 Electrons Phosphor Electrons Phosphor 4/11/2018 4) Detectors: CMOS Cameras Pixel Sensor photons to electrons conversion Electron charge to voltage conversion Voltage Signal Output Advantages of CMOS over CCD Larger pixel arrays Faster readout cheaper Direct Detection Disadvantages More thermal noise Lower image quality for same pixel resolution New cameras currently sold by vendors are based on CMOS technology which has almost completely replaced CCD cameras in the market 40 4) Detectors: Pixelated detectors are the future! CHEAP! Direct Detection of single electron events Dynamic Range Up to 24-bit (1:16.7 million), ideal for recording diffraction patterns. Effectively Noise Free Charge Summing Mode mitigate charge sharing effects for maximising both DQE and MTF. Rapid Readout Kilohertz frame rates, minimising effects such as sample drift, and enabling single shot and pump and probe dynamic experiments. Wide energy range and radiation tolerance Minimum 20 kev to 300 kev. No requirement for a beamstop radiation tolerant Merlin from Quantum Detectors (256X256 pixels-55µm ) 41 20

21 Summary 1. TEM require a vacuum for electron propagations High vacuum and clean vacuum system reduce sample contamination 2. Two types of Electron Sources: FEG and Thermionic FEG guns have higher brightness 3. Electrons are focused and manipulated using electromagnetic lens via Lorentz forces Electrons spiral in lens and thus images rotate with changes in lens strength need 3 lens to compensate image rotation with changes in magnification Condenser lens define the probe size and illumination area on the specimen Objective lens is the imaging lens diffraction pattern is formed in the back focal plane Intermediated lens change mode from imaging to diffraction and are used to change magnification and camera length Chromatic and Spherical lens aberrations limit resolution condenser and objective lens astigmatism can be corrected 4. Detectors TEM imaging Film, CCD and CMOS cameras 42 QUESTIONS? 43 21