1、Chapter 2 Transmission Electron Microscope (TEM),TEM,Electron optics system,Vacuum system,Power Supply and control system,(1) illumination system (2) imaging system (3) image viewing andrecording,Structure of TEM,Illumination system,Imaging system,Image viewing and recording,2.1 The illumination sys
2、tem,Comprises electron gun (provides electron source) condenser lenses (control the electron beam),2.1.1 Electron gun,provides source of electrons to illuminate the specimen. There are two types of electron sources: thermionic source tungsten filaments lanthanum hexaboride (LaB6) crystals field-emis
3、sion source (FEG) fine tungsten needles,Thermionic Emission,If any material to be heated to a high enough temperature, 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 p
4、ractice are tungsten and LaB6.,W filament,LaB6 crystal,Thermionic gun,A high voltage is placed between the filament (acting as a cathode) and the anode, modified by a potential on the Wehnelt which acts to focus the electrons into a crossover, with diameter d0 and convergence/divergence angle a0.,Fi
5、eld Emission,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. One of the easiest materials to produce with a fine tip is tungsten wire. To a
6、llow field emission, the surface should be free of contaminants and oxide. This can be achieved by operating in UHV (better than 10-11 Torr),An FEG tip (fine W needle),Field Emission Gun (FEG),In order to get an FEG to work, we make it the cathode with respect to two anodes. A fine crossover is form
7、ed by two anodes acting as an electrostatic lens,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,Characteristics of the three sources operating at 100kV,Comparison of
8、electron sources,Tungsten source the worst in most respects, but for routine TEM applications they are excellent, reliable sources and are cheap and easily replaceable. LaB6 high brightness, improved coherency and the energy spread, increased operating life is a recommended thermionic source, for al
9、l aspects of TEM, but particularly AEM expensive (several hundred dollars each),Comparison of electron sources,FEG For all applications that require a bright, coherent source the FEG is the best. (This is the case for AEM, HRTEM) For routine TEM, an FEG is far from ideal because the source size is s
10、o small. It is not possible to illuminate large areas of the specimen without losing current density, and therefore intensity on the screen. need UHV, very expensive (US$10,000),2.1.2 Condense lens,The illumination system consists of two (three) condense lenses The first condenser lens (C1,often con
11、trolled via a knob which is labeled spot size), 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 spot size at the specimen, and direct control of the convergence angle.,The illuminatio
12、n 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),Parallel Beam mode,Parallel illumination is essential to get the sharpest diffraction patterns as
13、 well as the best image contrast. In the traditional TEM mode the first two condenser lenses (C1,C2) are adjusted to illuminate the specimen with a parallel beam of electrons typically several micrometers across.,In the parallel-beam mode usually no need to change C1. adjust the C2 lens to produce a
14、n underfocused image of the C1 crossover.,Convergent-beam mode (FEG-TEM),For FEG-TEM, a focused C2 lens illuminates a small area of the specimen with a nonparallel beam. (Crossover of FEG is 10nm),Unless you have an FEG, it isnt possible to use just the C1 and C2 lenses as in the Figure shown here t
15、o converge the beam to as small a probe as you would like (10nm). This is because the C1 and C2 lenses cant demagnify the crossover of thermionic gun (W: 50mm, LaB6: 10mm) sufficiently.,Convergent-beam mode (non-FEG),The usual solution is to convert the upper polepeice of the objective lens into a t
16、hird condenser lens, which is called a condenser-objective lens C3. We make the C3 much stronger than usual and weaken C2 or turn it off, as shown in the Figure, to get the convergent-beam.,2.2 The Imaging system,Comprises mainly objective lens intermediate lens projector lens Functions Uses objecti
17、ve 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.,The Imaging system,The Imaging system is composed of Objective lens Intermed
18、iate lens Projector lens Objective aperture SAD aperture(The two apertures are never used in the same time),The Imaging system,the most important lens in TEM forms the first intermediate image and diffraction pattern (diffraction pattern is inevitably formed in the back focal plane of the lens) magn
19、ification of the objective lens Mo: 100-200,Objective lens,Intermediate lens,magnify the first image and diffraction pattern formed by the objective lens and project it to the objective plane of the projector lens control to get image or diffraction pattern control the total magnification of TEM a l
20、ens with a variable magnification (MI=0-20 ),Projector lens,magnify the second image and diffraction pattern formed by the intermediate lens and project it to the fluorescent screen magnification of the projector lens MP 100 ,The total magnification of the TEM,Mo : magnification of the objective len
21、s (fixed) MI : magnification of the intermediate lens (variable) Mp : magnification of the projector lens. (fixed),Total magnification is controlled by the magnification of intermediate lens,Aperture/diaphragm,The aperture is a circular hole in metal disk and the disk is made of either Pt or Mo. The
22、 metal surrounding the aperture is called the diaphragm. Aperture/diaphragm is normally simply called aperture. The diameter of the aperture is in the range of 10-50m.,Aperture/diaphragm,We use the aperture to allow certain electrons to pass through the lens and exclude others by causing them to hit
23、 the surrounding diaphragm, i.e., limits the collection angle of the lens.,Objective aperture,to control the image contrast small aperture, high contrast large aperture, low contrast the resolution of the image formed by the lens the collection angle of the EELS the angular resolution of the diffrac
24、tion pattern,Image mode/diffraction mode,We need to view image or diffraction pattern using TEM. This is achieved by adjusting the intermediate lens. The intermediate lens can be switched between two settings: the image mode the diffraction mode,Image mode,In the image mode, you adjust the intermedi
25、ate lens so that its object plane is the image plane of the objective lens. Then an image is projected onto the viewing screen.How to see diffraction patter?,Diffraction mode,In the diffraction mode, you have to adjust the intermediate lens so that its object plane is the back focal plane of the obj
26、ective lens. Then the diffraction pattern is projected onto the viewing screen.,Selected Area Diffraction (SAD),As you see from the figure in the above slide, the diffraction pattern contains electrons from the whole area of the specimen that we illuminate with the beam. Such a pattern is not very u
27、seful because(1) the specimen will often be buckled.(2) The direct beam is often too intense to damage the viewing screen. So we perform a basic TEM operation both to select a specific area of the specimen to contribute to the diffraction pattern and to reduce the intensity of the pattern falling on
28、 the screen.,Selected Area Diffraction (SAD),There are two ways we could reduce the illuminated area of the specimen contributing to the diffraction pattern,We could make the beam smaller by converge the beam at the specimen to form CBED (Convergent Beam Electron Diffraction ) pattern. Converging th
29、e beam destroys any coherence, and spots in the pattern are not sharply defined but spread into disks.,Method 1,ED,CBED,Selected Area Diffraction (SAD),If we wish to obtain a diffraction pattern with a parallel beam of electrons, the standard way is to insert an aperture above the specimen which wou
30、ld only permit electrons that pass through it to hit the specimen. This operation is called selected-area diffraction (SAD).,Method 2,If we insert an aperture in a plane conjugate with the specimen, i.e., in one of the image planes, then it creates a virtual aperture at the plane of the specimen. Th
31、is is exactly selected area diffraction does.,But we cant insert an aperture at the specimen plane, because the specimen is already there!,Selected Area Diffraction (SAD),The conjugate plane we choose is the image plane of the objective lens, as shown in the figure.,We insert the SAD aperture into t
32、he image plane of the objective lens and center the aperture on the optic axis in the middle of the viewing screen. It must be focused by adjusting the intermediate lens so it is conjugate with (i.e., exactly in the plane of ) the image of the specimen. Then any electron that hits the specimen outsi
33、de the area defined by the virtual aperture will hit the real diaphragm when it travels onto the image plane. It will thus be excluded from contributing to the diffraction pattern.,Selected area aperture,Another advantage of putting selected area aperture in the image plane of the objective lens is
34、that a much large aperture can be used instead to very small aperture (we cant make apertures smaller than 10 m). For example, if used in the image plane of the objective lens with magnification 100x, a 100m aperture will select a region optically equivalent to 1 m in specimen.,2.3 Image viewing and
35、 recording,Image viewing viewing screen TV image recording film CCD camera (show image on computer screen and the image can be processed digitally),viewing screen and camera chamber,Viewing screens,The viewing screen in a TEM is coated with a material such as ZnS, which emits light with a wavelength
36、 of 450nm. The ZnS is usually modified with impurities to give off green light at closer to 550nm. The resolution of the screen is dependent on the grain size of the screen coating. Typical screen coatings are made with a ZnS grain size of about 50m (10 m for the high resolution screen).,Viewing scr
37、een,For high magnification observation, TEM is also equipped with a small screen and an auxiliary focusing binoculars with magnification of 5-10. As some signals are also given off by the viewing screen, such as X-rays, and whenever you look at the screen you are protected from this lethal radiation
38、 flux by lead glass.,Image recording methods,Film TV cameras Charge-Coupled Devices (CCD) Imaging plate (IP),Film,Film has a resolution 4-5m, much higher than the viewing screen, so it is most commonly used image recording method (will be continued to be used in TEM). High Information density 107 pi
39、xels in a 10cm 10cm image,TV,A standard TV camera has a resolution of 500 lines/frame (high resolution TV camera has resolution of 1000 lines/frame). Advantage to use TV is that you can record dynamic in situ events. Low information density 2 106 pixels in a 10cm 10cm image,Charge-Coupled Devices (C
40、CD),CCDs are MOS devices that store charge generated by light or electron beams. CCD arrays consist of thousands or millions of pixels which are electrically isolated from each other by creating potential wells under each CCD cell so they can accumulate charge in proportion to the incident beam inte
41、nsity.,Charge-coupled Devices (CCD),The image recorded by CCDs can be processed digitally. Because this, CCDs get more and more popular. Information density 1k 1k pixels = 106 pixels (most common) 2k 2k pixels = 4 106 pixels 4k 4k pixels =1.6 107 pixels (latest CCDs) low Speed frame time: 0.01s qual
42、ity of image recorded using CCD (1k 1k pixels) is not as good as the film,Image Plate (IP),TEM image can be recorded on image plate (IP). IP could be processed using special machine (expensive) instantly. The quality of image stored on IP is better than the film expensive,2.4 Vacuum and control part
43、s,TEM can only work in vacuum because high speed electrons will interact with gas molecule resulting in scattering of random electrons which reduce the image contrast electron will ionizing and charging causing electron beam unstable residue gas can corrode filament of electron gun, shortening the l
44、ife of the filament, and contaminate the specimen seriously.The TEM is kept permanently under vacuum,Category of vacuum,Rough vacuum 1 _ 10-3 torr low vacuum 10-3 _ 10-6 torr high vacuum (HV) 10-6 _ 10-9 torr ultrahigh vacuum (UHV) 10-9 _ 10-11 torrSI unit: pascal (Pa) non-SI unit: torr, bar 1 torr
45、is 130 Pa 1 Pa is 7.5 10-3 Torr,Vacuum required for TEM,10-7 torr for TEMambient pressure: 103 torr. It is quite remarkable that we can transfer a specimen into the TEM, reducing the ambient pressure at its surface by 10 orders of magnitude in a matter of a few seconds. 10-9 torr for UHV TEM 10-11 t
46、orr for gun region of FEG TEM,Use pump to achieve required vacuum,Vacuum system of TEM,Other parts,High tension supply provide 100-200kV voltage computer controlled electronic system,Other requirements,high tension: the resolution reduction caused by the fluctuation of maximum lens current and chang
47、e of high tension should smaller than 10-6 mechanics: vibration free (make a good base) electromagnetism: TEM room should be electromagnetism interference free,2.5 Specimen holder,Specimen holder has two different designs top-entry holder mainly used for high resolution TEM in the past (is rarely us
48、ed now) side-entry holder is now the standard. Almost all the TEM nowadays use it,Side-entry holder,Principal parts of a side-entry holder is held in the stage. The specimen is clamped into the cup at the end of the rod. A small jewel at the end of the rod fits into another jewel bearing in the stag
49、e to provide a stable base for manipulating the specimen. The O-ring seals the end of the holder inside the vacuum. Manipulating the specimen is accomplished from outside the column via controls within the rod.,Side-entry holder,Specimen support grid,The actual cup that holds your specimen is either 2.3mm or 3.05 mm (most common) in diameter, so the specimen disk or support grid has to be the same dimension as shown in the figure. The grid is usually Cu but could be Ni, Au, etc.,A variety of specimen support grids of different mesh size and shape,
