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Transmission Electron Microscopy Demonstration TEM experiment – October 2008 Page 1 1 Objective In this demonstration you will learn how to form and interpret images with the transmission electron microscope (TEM). After becoming familiar with the image formation mechanism and the features of the microscope, a semiconductor specimen will be investigated. Information on the specimen will be found, using the diffraction pattern and high resolution images. Introduction Electron microscopes are used for studying materials at very high spatial resolution. In many ways the principles behind how they work are identical those in an optical microscope (figure 1). The smallest distance that can be seen as two distinct points of an image is a definition of the resolution of the microscope (figure 2) and is known as the point resolution. The reason for using electrons, instead of light photons, is that much higher resolutions can be obtained. In some cases it is even possible to view the individual columns of atoms in a material by using an electron microscope. Figure 1: Comparison of image formation in different microscopes
Transcript

Transmission Electron Microscopy Demonstration

TEM experiment – October 2008 Page 1 1

Objective In this demonstration you will learn how to form and interpret images with the transmission electron microscope (TEM). After becoming familiar with the image formation mechanism and the features of the microscope, a semiconductor specimen will be investigated. Information on the specimen will be found, using the diffraction pattern and high resolution images. Introduction Electron microscopes are used for studying materials at very high spatial resolution. In many ways the principles behind how they work are identical those in an optical microscope (figure 1). The smallest distance that can be seen as two distinct points of an image is a definition of the resolution of the microscope (figure 2) and is known as the point resolution. The reason for using electrons, instead of light photons, is that much higher resolutions can be obtained. In some cases it is even possible to view the individual columns of atoms in a material by using an electron microscope.

Figure 1: Comparison of image formation in different microscopes

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Aperture angular range (α α α α - rad)

Poin

t R

esolu

tion (nm

) .

aberration limited

diffraction limited

resolution

dmin

αopt

Figure 2: Point Resolution limit for JEM-3010.

Note aberration limit, ds=Csα3; diffraction limit, dd=0.6λ/α; resolution, d=(ds2+dd

2)1/2

; where Cs is the spherical aberration coefficient, λ the electron wavelength and α the aperture angle

In this demonstration you will investigate and use a transmission electron microscope (TEM). This is an instrument that accelerates an electron beam through a very thin material specimen (<100nm thick) and then uses electromagnetic lenses to form an image on a phosphor viewing screen, TV or CCD camera. How does such a microscope work? Because electrons don’t travel far in air, the first thing to note is that the microscope is enclosed in a vacuum chamber with the air pumped out. Typically, the pressure inside is of the order of 10-4 Pa or less (atmospheric pressure is around 105 Pa). In most TEMs the electrons are generated and accelerated to high speed at the top, and travel down the electron-optical column, encountering the specimen near the middle and forming an image at the bottom – figure 1. The electron gun produces the electrons by giving them enough energy to escape from a metal or crystal attached to a filament. This is most often, and most easily, done by heating the filament (similar to what happens at the back of a TV set). The filament is held at a very high negative voltage (electric potential) with respect to the rest of the microscope which results in the electrons being accelerated away from the filament, because of the negative electron charge. In

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practice, a series of electrodes with small holes at the center accelerate the beam in stages. Eventually, a small fraction of the emitted electrons reaches the column travelling downwards with a high kinetic energy, typically of a few hundred thousand electron volts (one electron volt (eV) is the energy an electron acquires when accelerated through a potential of 1 volt: 1 eV = 1.6×10-19 J).

Figure 3: Generation of electron beam in a single stage 100kV TEM with a thermal source.

In order to form an image in the microscope we use electromagnetic lenses to focus the electrons. It is possible to build an electron lens out of a coil of wire which behaves in a similar fashion to light lenses made of carefully shaped glass (figure 4). When wrapped around a carefully machined magnetic metal alloy (called a pole-piece), the coil can be used to bring electrons passing through the center to a focal point. Passing a current through a wire coil induces a magnetic field in the pole piece gap. Because of the electron charge, it is deflected, as it passes through the lens, by the Lorentz force:

( )BvF ×= e ,

e is the electron charge, v the velocity of the electron, and B the magnetic field vector. The vector cross product means that the direction of the force vector is at right angles to the velocity and magnetic field vectors. If the pole-piece is correctly designed, the beam is forced towards the optic axis as it traverses the lens; just as a light beam is by refraction at the edges of a glass lens. The electron lens has the advantage that its strength (called its excitation), and hence its focal length, can be controlled by varying the current in the coil.

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Figure 4: Types of magnetic lenses. a) Simple solenoid; b) A coil enclosed by a soft iron case to reduce flux leakage, containing a gap to concentrate the field; c) As b but with soft iron pole piece to concentrate magnetic field further

The TEM is now fairly easy to understand. A gun and lenses give us control over a high energy beam of electrons, within a vacuum chamber. After passing through the specimen, the lenses are used to focus images at the bottom of the microscope column. Generally, the specimen is mounted within a quite complex stage that allows tilting and shifting of the specimen to very high accuracy. To get specimens into the microscope there also has to be an airlock and specimen transfer system. Other major microscope components are the various apertures that can be inserted into the beam path to cut out sections of the beam. Usually the apertures are small holes a few tens of micro-meters in diameter. Only the central portion of the beam is allowed to pass. Finally, there are various alignment and stigmation coils throughout the column that are used to deflect and manipulate the beam slightly. The instrument The TEM you will be using in this demonstration is a JEOL JEM-3010. The electron acceleration energy will be 300 keV (notice the thick, high voltage cable attached to the gun at the top). The JEM-3010 has more lenses than shown in the schematic of figure 1 (fig 5), and a variety of apertures; however, forming images and diffraction patterns is relatively easy, following a set procedure. The TEM specimen is clamped at the end of a rod which is inserted into the microscope from the side. When inserted, the specimen sits right in the middle of the objective lens (OL). This is the most important lens because it is generally the most strongly excited and provides the greatest magnification. The optics are designed so that the performance of the OL is the main limit of resolution, and the OL is used to focus the image. Lenses above the OL are called condensers, they control how the beam illuminates the specimen. Below the OL are the projector lenses. Changing their excitation (usually with a selector switch) adjusts the final magnification, since

lensesprojectorOLtotal MMM ×=

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Having more than one projector lens increases the flexibility of the microscope. Both images and diffraction patterns of the specimen can be projected onto the screen, at a range of magnifications and, in newer microscopes, without any rotation.

Figure 5: Cross section of JEOL JEM-3010 Transmission Electron Microscope

There are two main types of specimen supported and self-supported. The first type (fig 5) consists of particles mounted on a thin carbon film which is supported on a copper grid. This is one of the easiest ways of preparing specimens for the

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TEM and can be used when the specimen consists of particles thin enough to be electron transparent (t < 100nm). The particles are dispersed on the film, for example, in a solvent such as Isopropyl Alcohol (IPA).

Figure 5: Typical supported specimen. Left) low magnification optical image of 3mm copper grid with carbon film. Right) higher magnification image showing spore located on the film for TEM imaging.

The second type (fig 6) is produced from bulk material. It consists of a 3mm disc

of material which has been ground down to be a few µm (10-6 m) thick. The final stage of thinning was then carried out in an ion-beam thinner. Here a beam of argon ions bombarded the specimen until a small hole began to appear. At the

edges of this hole, the specimen is thin enough for high-resolution TEM (0.1 µm and under).

Figure 6: Typical self supporting specimen. Left) low magnification image of 3mm specimen ion

beam thinned to perforation. Right) higher magnification image showing 20µm hole in the specimen

We will use both single tilt and double tilt holders in the JEM-3010. Before the specimens are inserted, take a look at it under an optical microscope. The specimen we will look at in the single tilt holder is a self supporting aluminum specimen. The specimen we will be using in the double tilt holder is a cross sectional specimen (fig 7) prepared so that we can look at SiGe interfaces grown in by Molecular Beam Epitaxy. This specimen has features of both supported and self supported specimens.

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Figure 7: Cross sectional TEM specimen of a ceramic after ion beam thinning so that the surface layer can be investigated. The specimen is sliced perpendicular to the surface, thinned mechanically then attached to a copper washer for support before ion beam thinning

The specimen rod can be inserted into the airlock as long as the ready light is on the left hand panel. After pumping in the air lock (which takes about 2 minutes) the red light on the airlock goes out and the specimen rod is rotated, and inserted into the main chamber. Wait a few minutes for the chamber pressure to recover. We are going to use the microscope at its maximum acceleration voltage of 300 kV. If the kinetic energy gained by an electron, accelerated through a potential difference, V is eV, work out, using classical mechanics, its wavelength. Use the DeBroglie equation of wave/particle duality, linking momentum, p, to

wavelength, λ. In fact, such an electron travels down the microscope column at

relativistic speeds: the calculation has to be modified and the value of λ is different to the classically derived one. Typical relativistic electron wavelengths and velocities are as follows:

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acceleration voltage, Eo

(kV) Wavelength, λ (pm) electron velocity (v/c)

100 3.7 0.55 200 2.5 0.70 300 2.0 0.78 400 1.6 0.83

(1 pm = 10-12 m) The limit to resolution in an optical instrument is given approximately by the wavelength of radiation used to form an image. Visible light wavelengths are around 500 nm (1 nm = 10-9 m). Therefore, an electron microscope of some kind can feasibly have a resolution about 500,000 times that of a light microscope. The spacing between atoms in many useful materials is between 0.1 nm and 0.3 nm (1 nm = 10-9 m), so atomic resolution ought to be possible. Unfortunately, as you will see, other factors mean that the limiting resolution is about 100 times worse than the wavelength limit. Atomic resolution imaging is possible, but difficult. When you are satisfied that the column pressure has recovered in the microscope, it is time to turn on the filament. On modern instruments the accelerating voltage is normally left on to improve stability. To get a beam the LaB6 filament (fig 8) must be heated. Check that the filament ready light is on. Slowly increase the filament control to 3. Wait 2 minutes and then increase the filament in ½ divisions every 2 minutes until the mechanical stop is reached. This stop is preset to give 5µA of beam current on top of the 114µA of HT current. DO NOT MOVE THE STOP OR CHANGE THE GUN BIAS COARSE OR FINE SETTINGS.

Figure 8 LaB6 filament

Familiarize yourself with the aperture mechanisms (each blade holds a number of apertures). There are three main aperture mechanisms in the column: the condenser aperture (top) controls the illuminating beam angles; the objective aperture (middle) limits the angle of scattering from the specimen that can contribute to the image; the selected-area aperture (bottom) controls which area of an image is projected onto the phosphor screen.

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Microscope Alignment Now that the beam is on, the microscope has to be aligned, making sure the beam passes through the optic axis of each lens. 1. Find the beam.

If the specimen does not have a large hole near its center or it is on a grid, it is still unlikely that you will see anything on the screen: if so, try moving the specimen around using the large, white stage position controls on either side of the viewing chamber. After a short search, it is possible you will not find the illumination. In this case, go to LOW MAG mode, using the relevant FUNCTION button. Also try turning the BRIGHTNESS control part of the way back in the clockwise direction. Find and center a transmission thin area in the specimen using the stage shifts. If you used LOW MAG for this, go back to normal magnification mode (M1). The procedure to find the specimen is below:

• The microscope should have been left by the last user with the beam just filling the screen at a magnification of around 20Kx. If you cannot see any beam, the most likely reason is that your specimen is in the way – try moving it around. DO NOT move apertures or gun tilt/shift or beam shift to find the beam!

• If you cannot see the beam try the following :

• Check that the emission current meter is reading around 118µA.

• Check that the spot size is set to 1 and the alpha selector 3 (1-3).

• Decrease the magnification and spread the beam (clockwise on BRIGHTNESS control).

• Make sure that the objective and selected area apertures are out. DO NOT MOVE the X and Y knobs!

• If you know you have a hole near the center of the specimen or you have a grid, move the specimen a bit until you see the hole. Watch the specimen position on the microscope status screen.

• Go to low mag. Mode and spread the beam.

• Move your sample to find the hole.

Figure 9 Left Hand Control Panel

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2. Center the Condenser Aperture

Now we will experiment with the BRIGHTNESS control and show how it controls the beam convergence by changing C3 lens current. Use the SHIFT X, Y knobs to center, on the screen, the position of the convergence center. As you adjust the BRIGHTNESS control, the illumination should converge and diverge concentrically about the screen center. Physically, the SHIFT X, Y knob energizes a coil, near the condenser lenses, that shifts the beam around slightly. What you have just done is to shift the beam onto the optic axis of the microscope. As you change MAGNIFICATION and SPOT SIZE, you will need to keep the beam centered with SHIFT X, Y, and control the illumination condition with BRIGHTNESS. Try changing the MAGNIFICATION control and keeping the phosphor screen evenly illuminated with the BRIGHTNESS control. If the aperture is not centered properly the illumination will swing across the screen and we have to mechanically align the aperture. Astigmatism in an electron lens causes the beam profile to depart from a round shape and, leads to blurring of the image in a directionally dependant way. Luckily we can easily correct for this using stigmator coils. Although fine adjustments are made with an objective lens stigmator coil, it is sometimes necessary to first stigmate the illumination system using the condenser stigmator. The procedure for centering and stigmating is:

• Insert the appropriate Condenser aperture and center it by obtaining the smallest electron beam using the BRIGHTNESS control. Move it to the center with the DEFLECTOR - X and Y SHIFT controls (X Fig 5, Y Fig 6). Expand the spot and correct for swing of the image with the aperture mechanical X and Y controls.

• Apertures fitted are 120µm, 70µm, 50µm and 10µm.

• Correct for any Condenser lens astigmatism with the condenser stigmator (Cond Stig button on left panel; DEF X and Y). Oscillate around the smallest beam position at a magnification of at least 50Kx. De-select the condenser stigmator button once you have corrected any astigmatism.

3. Optimum Objective Focus (Eucentric Height)

The specimen should be placed in the pole piece at a height such that as the specimen is tilted on the primary X axis the specimen does not significantly move.

• Set Objective lens current at 300kV to give DV=+/-0 on the status screen.

• Locate the specimen and center a feature at a magnification of 50Kx.

• Turn on WOBBLER IMAGE X or Y.

• Stop the image moving by changing the Z height controls (left hand side of column). This will also be focusing the specimen.

• After this alignment is complete, all coarse focusing should be done with the Z height adjustment – especially if you move away from the area where the eucentric height adjustment was done. The objective lens

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current should be kept as close to DV=0 as possible, if it is a long way off both the image magnification and the resolution will be affected.

Figure 10 Right Hand Control Panel

4. Condenser and Gun Alignment

Using shift coils at the bottom of the gun and in the condenser lens region, we ensure that the illumination stays on the optic axis of the microscope over a wide range of condenser lens settings.

Figure 11 Right hand drop down panel

• Change the SPOT SIZE to 4-3. This significantly increases the current through the C1 lens. Explain how and why the illumination on the screen has changed.

• Bring BRIGHTNESS to crossover and center the beam with the beam X and Y SHIFTS.

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• Change SPOT SIZE back to 1-3.

• Select DEFLECTOR-GUN on the right hand drawer (Fig 7.)

• Bring BRIGHTNESS to crossover and center the beam with the DEFLECTOR - GUN X and Y SHIFTS.

• Repeat until the beam remains centered for both spot sizes.

• Center the beam from now on using the Beam shifts not the Gun shifts. De-select DEFLECTOR – GUN.

5. Voltage Center Adjustment

It is now necessary to complete the alignment of the illuminating beam. This will prevent translation of the beam as the accelerating voltage (HT) varies. When performing very high resolution microscopy, any misalignment with respect to this voltage center will blur out features such as crystal lattice fringes. This is because the microscope high voltage is unstable by a few parts per million – enough to have an effect on what you see when alignment is poor.

• Increase the magnification to 100kX and adjust the BRIGHTNESS control to fill the entire screen.

• Turn on the WOBBLER-HT control (right panel).

• With the BRIT TILT (Bright Tilt) selected minimize the image movement with the X and Y DEF.

• Turn off the WOBBLER-HT and the BRIGHT TILT. 6. Correcting for Objective lens Astigmatism

To obtain the best image resolution it is necessary to compensate for astigmatism in the image that arises from asymmetry in the objective pole piece and contamination of the objective aperture. As in light optics, astigmatism concerns the departure of the beam profile from a round shape. Here this is caused by the objective lens field not being completely rotationally symmetric. The effect is easily seen because image features in perpendicular directions come into focus at different OL settings. An astigmatic image has a streaky appearance. The aluminum specimen will have very little amorphous material and we may leave the fine tuning of the objective stigmator until the SiGe specimen is loaded.

• Insert and center the objective aperture you intend to use. (For atomic resolution imaging, use the largest or no objective aperture; For phase contrast or mass thickness contrast, use a smaller objective aperture to select just the direct beam). This will select the part of the diffraction pattern from which the image is formed. Put in a large Selected Area Diffraction aperture (SAD). Select DIFF. You should now see a diffraction pattern on the screen. The brightest spot will be the directly transmitted spot, put in the second largest objective aperture and center it around that

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spot. Go back to MAG mode and remove the SAD. The image of the specimen should now show much more contrast (why?).

• Find an amorphous film on your specimen. A carbon support film is ideal, or look at the very edge of your specimen.

• Magnification at 200Kx or greater. Raise the screen to display the image on the TV monitor.

• Press the Objective Stigmator (Obj Stig) button (left panel). Adjust the astigmatism using DEF X & Y. (This can also be done using the Objective Stigmator button and deflector controls, in the right hand drawer.)

• Adjust the stigmators until the image quality improves and then vary the focus slightly either side of focus. The amorphous material should appear uniformly grainy either side of gaussian focus with no visible streaking. When the image is at focus it should have minimum contrast (fig 12).

a)

b)

c)

Figure 12 Correctly stigmated image of amorphous carbon at a) underfocus, b) Gaussian focus and c) overfocus showing minimum contrast at focus

When there are areas of the specimen that are amorphous (i.e. the atomic arrangement is not ordered, as in a crystal) a well stigmated bright-field image, at a magnification of a few 100k, will have a blotchy, speckled appearance. If astigmatism exists the speckle will become streaky at two focus points; the two sets of streaks having perpendicular directions. To stigmate, iterate between the two focal positions, and values in between, adjusting the OBJECTIVE STIGMATOR X and Y controls as you do so. As the astigmatism diminishes, the image will become less streaky and the number of focal steps between the focus conditions will decrease. Eventually, there will be just one focus point with amorphous speckle contrast and no streakiness. Most specimens have a thin amorphous surface layer of oxide and possibly of amorphous carbon. This is inevitable unless the specimen surface was exposed in ultra-high vacuum and never subsequently exposed to the atmosphere. The asbestos specimen is supported on an amorphous carbon film so finding an area should not be difficult! Try to stigmate the objective lens.

• If you do not have any amorphous areas a small hole can be used. Adjust the stigmators so that the Fresnell fringe is of even thickness around the

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hole. With a well aligned microscope, you should observe a fringe (bright or dark line) around the edge of the hole when near the minimum contrast focus condition. Try finely adjusting the focus and see how the fringe changes. Make sure you have a well spread beam: use of the small phosphor screen and binoculars is recommended. If you can’t see a fringe, try increasing the SPOT SIZE and going to a thinner specimen region. This Fresnel fringe is caused by interference between electrons scattered at the specimen edge and those passing through the hole. Whether it is bright or dark depends on the OL excitation relative to the in-focus value. For a well stigmated system, the fringe will be uniform all way around the hole. If you can find a small hole and observe the Fresnel fringe, experiment with the OBJECTIVE STIGMATOR controls and see what difference they make. Try to stigmate the microscope by making the fringe uniform.

The first image With the second largest condenser aperture inserted, spread the beam using the BRIGHTNESS control and move the stage until the edge of the specimen hole is visible. Approximately FOCUS the image and move around the specimen edge until a thin region comes into view. Very thin regions will be almost transparent to the electron beam. When you have found a thin area, zoom in using the MAGNIFICATION control to about 100k. FOCUS changes slightly the objective lens excitation. At the correct setting the specimen edge will almost disappear; it will be quite well defined at either side of this point. With the image focused, move the specimen so that a thin region, near the edge, is at the center of the viewing screen and insert the largest selected area diffraction aperture (SAD). Ensure the illumination is well spread. The disc of illumination should now be limited by the SAA. Adjust the aperture position so as to center it. All electrons reaching the phosphor screen have now gone straight through the thin region of the specimen and closely surrounding areas. Depress the FUNCTION button marked SA DIFF. This changes the projector lens excitation so that the back focal plane of the objective lens, not its image plane is focused at the viewing screen. You are therefore now looking at a diffraction pattern. Adjust the CAMERA LENGTH to 40 cm (as shown on the display that previously indicated magnification). Try to explain the form of the diffraction pattern. It will probably be quite complex, why is this? If the selected area of the specimen is thin enough, there should be a well defined central spot in the diffraction pattern, indicating that most of the electrons were unscattered, passing straight through the material. Try inserting different

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sizes of objective aperture, centering them with respect to the central spot and observing the limitation they impose on the diffraction pattern. Insert the second largest objective aperture and center it with respect to the diffraction pattern. Then go back to imaging mode by pressing the M1 button from the FUNCTION. Remove the SAD. The specimen should now be imaged with enhanced contrast. By adjusting the MAGNIFICATION, take a tour around the sample. You should see areas giving different levels of contrast and sharp boundaries between each area. These are the separate grains in the polycrystalline aluminum. Why do you think grains in different crystallographic orientations have different contrast? Bear in mind that you have just enhanced the contrast by putting in an objective aperture. Electrons scattered through large angles no longer contribute to the image. Select a magnification of 20k and spread and center the beam (ALIGNMENT TRANS). Also, find a recognizable feature at the edge of the specimen. You should still be in focus (the focus condition when contrast at the specimen edge is least. Grain boundaries and tilting the specimen Choose a SAD aperture that is small enough to fit within one grain. Go to diffraction. You will now have a diffraction pattern from a single crystal. You should see individual spots and diffuse lines formed from cones of inelastically scattered electrons that have been Bragg diffracted. These are known as Kikuchi lines and can be used as a road map around diffraction space. We only have a single tilt on this holder so aligning the crystal accurately on a zone axis will dificult. However try tilting the specimen until a narrow Kikuchi band (large atomic spacing) is selected, tilt until the Kikuchi band runs through the direct and diffracted beams (two beam condition). If the grain is small this may prove difficult and it may be necessary to frequently go back to MAG mode to recenter the grain (The stage is eucentric, this means we can adjust the Z height to minimise the movement as we tilt the specimen).

Figure 13 Low magnification TEM image of grains in Strontium Titanate

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Insert an objective aperture that only samples the direct spot. Go back to M1 mode and remove the SAD aperture. The grain that we have tilted up on should now appear darker than the surrounding grains as less intensity is in the direct beam and more in the diffracted beam which does not contribute to the image. Look carefully at the grain boundaries, you may see fringes in the boundary if it is tilted with respect to the beam. Also look in the grains themselves. You may be able to see defects such as dislocations (an additional incomplete plane of atoms) or stacking faults. Try tilting a little bit from the two beam condition – What happens to the image and why? High resolution TEM For the second part of this experiment we are going to look at a silicon specimen with germanium layers grown by MBE. This specimen consists of a cross sectional slice of the multilayer structure glued to a 3mm diameter copper ring for support (fig 7). The specimen has been ion-beam thinned again until a hole is formed and we will look around this hole in the thin area. Load the specimen into a double tilt holder and insert in the microscope. Obtain a diffraction pattern. On this instrument there are two ways to do this – first using an SAD aperture as we did before, and secondly by converging the beam down into a spot and selecting Diffraction (Convergent beam diffraction (CBD)). In the second case instead of spots we get discs and the Kikuchi lines show up more clearly. Within the discs is structure which is a result of the beam having a range of angles onto the specimen. The size of these discs depends on the size of the condenser aperture. The symmetry of these patterns can tell us a lot about the crystal structure of the specimen and can be used to determine crystallographic space and point groups. Record both SAD and CBD patterns on the CCD at 40cm. Using X and Y mechanical tilts tilt the specimen until you are exactly on a low index zone axis (eg 110). A low index axis has the diffraction spots closely spaced. The three lowest in a cubic structure are 100 – square, 111 – regular hexagon and 110 elongated hexagon. There are many other zone axes visible in the diffraction pattern but the lattice spacings contained in those patterns will be below the resolution limit of the microscope (0.14nm lattice resolution). In Appendix 1 there is a calibration diffraction pattern from a NiOX specimen and a pattern from Si <110> collected at the same camera length. Using the NiOX pattern to calculate the camera length, use this camera length to calculate the spot spacings in the Si pattern to determine which lattice planes the spots correspond to. Choose an objective aperture that contains the direct spot and the first ring of diffracted spots. It is the interference between these spots that will give rise to the

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atomic resolution image. Go back to image mode and note the location of the area where the tilt is correct. For atomic resolution images the specimen must be less than 10nm thick. As the specimen gets thinner it also frequently bends so setting the orientation up in a thick area is easier (more Kikuchi information) but when you go to the thinner areas the orientation is likely to have changed slightly.

Figure 14 Atomic resolution TEM image of Si <110> orientation and SAD pattern showing position of the objective aperture.

Check the objective stigmator on the amorphous carbon film around the hole. This can be done on the TV monitor or using the CCD camera an on line live FFT of the image can be taken which makes stigmation easier. At around 100,000 times indicated magnification focus the image on the TV monitor. The image should appear mottled due to the amorphous layer and local thickness variations. Turn the magnification up and carefully focus. If the orientation is correct atomic resolution images should appear from 500,000 times upwards. Record the image on the CCD camera. Slowly change the focus and watch the image. When going from overfocus to underfocus the image will change with bright areas going dark and vice versa. Although we have information at the atomic level we do not know wether the atoms are bright or dark without modelling the image which needs careful measurement of the microscope operating parameters (kV, focus etc). This specimen also contains Ge layers. Find one of these and collect an atomic resolution image. There is little contrast difference despite the change in atomic number. Compare this to the High Angle Dark Field Scanning Transmission Electron Microscope image at the end of this handout. Finally confirm the presence of Ge by inserting the X-ray detector and going to EDS mode, 25nm probe with a 70µm Condenser Aperture. Collect spectra from the quantum well area and compare it with that from the adjacent Si.

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Summary In this experiment we have looked at the principles of imaging and operation of a transmission electron microscope. We have primarily been interested with the interaction of the beam with the specimen and the resulting diffraction. From figure 15 we can see that this is only one possible interaction with the specimen. For instance, surface imaging of bulk specimens can be done using secondary electron and backscattered electrons in a Scanning Electron Microscope (SEM) and chemical analysis of the specimen is possible by measuring the characteristic X-ray signal from the specimen.

Figure 15. Electron beam / Specimen interaction

Other imaging techniques are also possible which can give unambiguous atom positions with some indication of the average atomic number of the column (Z contrast imaging in Scanning Transmission (STEM mode)). An example image is given below (fig 16).

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Figure 16. An example Z contrast STEM image of a Si/Ge quantum well. The Ge atomic columns are the heaviest and appear brightest in the image. For this technique the scanned probe must be less than the atomic spacing. In this case a 0.2nm probe was used (JEOL JEM-2010F)

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APPENDIX 1

Diffraction Pattern Calibration

The 40cm pattern from a NiO film comes from a fine grained polycrystal giving sharp rings. The first six rings are from:-

hkl dhkl spacing (nm) intensity 111 0.2408 strong 200 0.2085 strong 220 0.1474 strong 311 0.1257 medium 222 0.1204 medium 400 0.1043 weak

Bragg’s law and the definition of camera length L gives:

�λ/dhkl=Θ=Dhkl/2L where Θ is the scattering angle in radians, λ the electron wavelength

(0.00197nm @ 300keV) and Dhkl the ring diameter corresponding to the hkl ring. Camera length is given by

L=dhklDhkl/ 2 λ � Measure the ring diameters and calculate L for each ring. Take the average of the readings – remember you are in diffraction space so innermost ring is from largest spacing.

Take the Si_110_SAD_40cm pattern and calculate the d spacing for each of the six closest spots around the direct beam. Measure the distance from the direct spot to the diffracted spot ( R) and use the following equation:

dhkl= λ L/Rhkl

What spacing in Si do these spots correspond to?

Si hkl d spacing (nm) 111 0.3134 200* 0.2714 220 0.1919 311 0.1636 400 0.1357 331 0.1245 422 0.1108

* NB 200 reflection is forbidden in Diamond cubic structure but can occur by double diffraction.

Transmission Electron Microscopy Demonstration

TEM experiment – October 2008 Page 21 21

Figure A1.1 NiOX calibration pattern

Figure A1.2 Si 110 SAD pattern


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