扫描透射电子显微镜模式分析

A general introduction to STEM detector
1. BF detector
It is placed at the same site as the aperture in BF-TEM and detects the intensity in the direct beam from a point on the specimen.
2. ADF detector
The annular dark field (ADF) detector is a disk with a hole in its center where the BF detector is installed. The ADF detector uses scattered electrons for image formation, similar to the DF mode in TEM.The measured contrast mainly results from electrons diffracted in crystalline areas but is superimposed by incoherent Rutherford scattering.
3. HAADF detector
The high-angle annular dark field detector is also a disk with a hole, but the disk diameter and the hole are much larger than in the ADF detector. Thus, it detects electrons that are scattered to higher angles and almost only incoherent Rutherford scattering contributes to the image. Thereby, Z contrast is achieved.
In addition, there is the option to install a Secondary Electron Detector above the sample like in a SEM and thereby to obtain additional morphological information.
4. The central DF and BF mode in normal TEM
In normal TEM, we also mentioned the DF and BR mode. Actually, they are different with ADF and BF mentioned here completely. In normal TEM, the direct transmitted beam is blocked by the aperture and the diffracted beam is allowed passing. When the diffracted beam hits the
screen and forms a image which is called as DF. Otherwise, diffracted beam is blocked with aperture and transmitted beam forms BR image.
However, in STEM mode, they are totally different because the beam on sample is cone-shaped beam rather than parallel light.
STEM模式和TEM模式的对比图
Z-contrast actually uses an annular detector which can only collect the scattered electrons outside the beam cone. That is to say, the transmitted electrons inside beam cone are involved. This is good thing for other imaging mode because diffraction contrast was avoided and the imaging only has dependence on atomic structure. This imaging mode is called as STEM-HAADF. Here, two issues are not addressed yet. One is whether STEM-BF and STEM-ADF imaging involves diffraction contrast. Personally, I think STEM-ADF should involve phase contrast and Z contrast. For STEM-BR, phase contrast is greatly involved. However, in case of HAADF, only Z-contrast works.
Camera Length affects the Inner Collection angel. In our TEM, when camera length is 11 cm, the inner collection angel is 60 mrad. Once the camera length reducing to 5 cm, the inner collection angel is 110 mrad. Therefore, the collection angle is controlled by the camera length. Who
affects the camera length? According to equation (L=f*Mp*M l) where f is the focal distance of objective lens (f is related with the excited current, acceleration voltage, the number of coil), Mp and M l are the magnification of projective lens and intermediate lens respectively. Experimentally, the f can be changed significantly through adjusting the excited current. Besides, in order to change the camera length, you also can change the excited current of projective lens or intermediate lens. When inner collection angle is bigger than 100 mrad, the Rutherford scattering dominates the imaging.
The inner angles for HAADF detectors are at least three times the angle of electron-probe-forming aperture. In many cases of HAADF imaging, the annular detection angle was practically set to be 60 - 170 mrad due to the limitation of combinations between aperture sizes and camera lengths. Above image shows the positions of the detectors which can be installed in a STEM system. Depending on the scattering angle of the transmitted electrons, various signals can be detected as a function of the position of the
scanning probe: BF (bright-field)-STEM, DF (dark-field)-STEM or HAADF (high angle annular dark field)-STEM.The DF detectors are annularly shaped to maximize the collection efficiency and the range of the collected scattering angles can be adjusted through the magnification of the intermediate lenses.
FEI HAADF detector
Chapter 1. Introduction to STEM
Some TEMs have scanning coils which allow them to be used as a scanning transmission electron microscope (STEM). In STEM, a conical electron beam is focused through the specimen by a lens in front of the specimen (C2 and the objective lens pre-field which is also called mini lens), as shown below.
The optical configuration is roughly the same as simply forming an image of the filament by focussing C2 the very first experiment we did in the electron microscope. In fact, if your microscope has a twin objective lens including pre-field and backfield, the optics are a little bit more complicated, but lets start thinking about things as simply as possible.
The small beam cross-over at the specimen plane is usually called an Electron Probe. Images can be formed by moving the probe across the specimen (by the double- deflection shift coils) and detecting the transmitted electrons, which are either on the optic axis (to form a bright-field (BF)
image) or have been scattered to high angles, to form a dark-field image (DF).
In order to make an image, we have to display the signal coming out of one or more detectors in some way. This is usually done on a slow-scan television screen (although it is nowadays also done by computer). The signal detected is used to modulate the intensity of the image on the display screen, while the scan of the display is synchronised accurately with the position of the probe on the specimen. Usually, the same scan generator is used to control both the x-y position of the small beam (or probe) shift deflection coils and the coils that control the display screen. The principle is the same as a conventional scanning electron microscope.
A STEM doesn’t actually need any lenses below the specimen at all, because everything important happens before the electrons reach the sample. In practice, a TEM/STEM has all the usual objective lens and projector lenses below the sample (shown as a dotted box above), but in STEM mode these are just used to change the effective camera length (that is, the distance between the specimen and the detector plane). They can also be used to form an image of the electron probe.
Although there are certain benefits of STEM imaging (which have not been generally been recognised until quite recently), the main advantage of this geometry is the fact that the probe can be used to irradiate a very small volume of specimen in order to obtain all sorts of other signals such as characteristic X-rays, Auger or secondary electrons, or electron energy-loss spectra. All these signals can be spatially resolved at a resolution corresponding to the width of the probe, thus allowing for high resolution micro- analysis. For the material scientist, analytical signals like these can be much more useful than simply images, say to identify the elements present in an inhomogeneous sample.
Chapter 2.The Ronchigram（水纹相）
Experiment: Load a test specimen of polystyrene spheres, shadow coated with gold particles on a carbon film. Line up the microscope as usual in normal TEM mode. Select the largest condenser aperture and centre it.
Ask the demonstrator: Show me how to put the microscope into STEM mode, with the scan switched off. Please align the condenser lens, aperture and stigmators (消散器), and objective rotation centre (current and HT centre), so that I can see a reasonably well-aligned Ronchigram.
Let’s start by not actually scanning the probe at all. Examine what we see on the phosphor screen at the bottom end of the electron microscope. The rays coming out of the sample have conical shape, formed by the condenser aperture, and this cone eventually hits the phosphor screen, where it forms a bright circle. The circle is called the 'Gabor hologram', the 'Ronchigram' or the 'central zero-order disc of the convergent beam electron diffraction pattern' depending on the context and who is talking about it. I'm going to call it the Ronchigram. It provides the best way of lining up a STEM, and it will also teach us a lot about electron lens aberrations.
Experiment: Look at the Ronchigram. Start by going through focus on both C2 and the objective lens. Move the specimen.
You will find that the Ronchigram looks pretty strange, a sort of fish-eye view of the specimen. Go through focus with either the objective or C2 (Note: the objective lens is the pre-field objective lens). If you want to increase the contrast, go up in spot size. Weird shapes move in and out with over focusing and under focusing. If the microscope is well aligned, the Ronchigram from an amorphous specimen should look something the next figure, which has been calculated in a computer:
Well, why does the Ronchigram look like this? As a first approximation, what we should see is a shadow image cast by the specimen which reverses as we change the cross-over of the beam near the specimen, as shown below:
When the beam is crossing over exactly at the specimen, there will be a burred mess over the whole disc (middle picture above). If the specimen has some feature, like the curly P above, then above and below focus we see a shadow of that feature cast onto the central disc, and magnified according to how far away the beam cross-over is above or below the specimen.
Experiment: Turn C2 from a zero setting, slowly increasing its strength. You should see the image reversal. All this occurs at a rather low setting of C2. At higher settings, the Ronchigram may get focussed into a bright spot and undergo a second reversal. However, this reversal is to do with the way we are using the lower part of the microscope (below the specimen plane) to image the cone of illumination coming out of the specimen. Concentrate on the low settings of C2.
In fact, the behaviour of image is much more complicated than the figure above suggests because of the effects of aberrations in the lens. All
sorts of aberrations may be present in C2, but these tend to over-focussing
high angle beams (ones that pass well off-centre), bringing them to a premature focus, i.e. a focus that is nearer the lens than it should be.
To understand this, first consider a perfect lens. A perfect lens, by definition, focuses all beams from the source to a single point, as shown below.
If we have aberration presents, high-angle beams tend to be over-focussed, so that they focus at a plane above the plane of perfect focus, like this:
How is this extra complication going to affect what we see in the Ronchigram? Start by thinking about just two sets of beams: two which are virtually 'paraxial' (which means that they are traveling very close the centre of the lens) and which therefore come to the correct focal point of the lens (a point which is called the Gaussian focus); and two which are at very high angle, or close to the very edge of the condenser aperture, as shown below.
Now, when the probe-forming lens is highly over-focussed (which in this experiment means that C2 is moderately excited), all the beams,
including the paraxial and high- angle beams, cross-over before they reach the specimen. What we see is just a shadow-image Ronchigram as we would expect, although there will be a slight change in magnification between areas near the centre of the Ronchigram and its edge.
Similarly, when the lens is highly under-excited, we see a reversed shadow image, again slightly distorted in magnification. However, near focus there is a peculiar region which I have called the 'region of radial inversion' in the figure above. When the specimen is lying somewhere in this region (or the lens has been adjusted accordingly), paraxial beams are crossing below it, but high-angle beams are crossing above it. In the Ronchigram, the centre of the pattern has undergone a reversal; the edge of the pattern is still in the over-focused orientation.
Experiment: On a well-aligned Ronchigram, focus C2 so that the magnification of the image is at maximum. Under-focus slightly and move the specimen. You should be able to find a condition where the centre of image moves in one direction, while the outer area moves in the opposite direction. If you do, then you are within the region of radial inversion. If all you can see are streaking effects and asymmetric stretching of areas of the Ronchigram, then the lenses have not been lined up properly, or the astigmatism in the condenser lens has not been corrected.
Look again carefully at the artificial Ronchigram:
There are two characteristic rings, which you should be able to see experimentally as well. When the focus is set within the region of radial inversion, there is a central circular area where we see just a normal shadow image at rather high magnification. There is then a ring where everything is streaked out in the radial direction: this is called the ring of infinite radial magnification. At a higher radius, there is a ring where everything is streaked out into a circular pattern: this is called the ring of infinite azimuthal magnification. We don't have to understand why this happens like it does - it is all to do with the mathematics of how much the rays miss Gaussian focus as a function of their angle through the lens.
What is important is that the Ronchigram must be circularly symmetric if you want to get a good STEM image.
Experiment: Put the C2 stigmators onto their coarsest setting, and change them by a large amount. Watch the Ronchigram as a function of C2 defocus, especially at or near Gaussian focus (i.e. when you can see a highly magnified blob at the centre of the Ronchigram). You should see streaking shapes, which are like ovals or figures of eight.
If you can't see any of the effects we've talked about, then the lenses are not properly aligned. Let’s first discuss in more detail how the lenses are arranged.
Chapter 3.Lens geometry in STEM mode on a
TEM
In normal TEM mode, we have two cross-overs below C2, the lower one occurring within the objective pre-field, as shown in the elementary guide. One way of forming a probe would be to de-excite C2, as we did in the very first experiment in first section of the TEM guide, but then we still have a second cross-over which is happening somewhere odd within the pre-field.
Greater flexibility and an optimized probe focus can be obtained by avoiding the cross-over within the objective pre-field. In practice, this is achieved by either lowering the overall objective excitation or (depending on the make of microscope) using another small lens within the upper part of the objective twin lens, which is called either a mini-condenser or mini-lens. When the machine is being run in normal TEM mode, this lens is run in the same sense as the objective, making the pre-field very strong. When STEM mode is selected, the mini-lens is WEAKENED (or switched off), and so it cancels out the pre-field to a certain extent.
The next Figure illustrates conventional TEM imaging:
Where the top lens is C2, and the two lower lenses represent the objective pre-field (and/or the objective lens pre-field boosted by a condenser mini-lens or objective mini-lens).
Now when we go into STEM mode, most microscopes reduce the
influence of the pre-field, so now the ray-diagram looks like this:
Note that both C2 and the objective lens affect the focus of the electron probe. Because C2 is so weakly excited, you can see from the figure above that it will have a huge effect on the convergence angle of the illumination at the specimen plane. For this reason, many manufacturers fix the value of C2 in STEM mode. To obtain control of it, the user has to override the computer.
Remember: In STEM, the only electron optics that matter all happen before the specimen. Alignments you made relating to TEM imaging, especially the objective focus (this is the backfield objective focus below the specimen) and stigmation, are useless in STEM mode. We must alter the objective lens (thus altering the pre-field above the specimen) and C2 to control the alignment. The relevant stigmators for STEM mode are the condenser stigmators, not the objective stigmators.
Now that we know that two lenses are involved in the forming probe, we can work how to line them up with one another. The most sensitive mis-alignment arises from the double- deflection coils being on the wrong tilt setting. If the beam tilt is wrong, the beam from C2 passes into the objective pre-field off-axis and at an angle. It will come out below the specimen also at the wrong angle: this is most common reason for losing the beam in STEM or nano-probe mode at high camera lengths.
The first thing to get right in STEM mode is therefore the objective rotation centre. The alignment is almost certainly not the same as for TEM, because the objective is in a completely different state, and we are aligning with respect to the pre-field, not the main body of the lens below the specimen. Similarly, the condenser aperture is almost certainly off-line, even if you lined it up in TEM mode.
All the corrections are best done in the electron Ronchigram.
Experiment:
（A）Wobble the objective lens, and try to get the image moving in and out symmetrically with the condenser (we assume here that this is on line). As you alter the beam tilt (this should be the correction you are adjusting on the multi-function knobs when aligning in STEM mode), the whole Ronchigram will at the same time bodily move across the phosphor screen. That's because the conical beam cast by the condenser lens is rocking like a lever through the specimen, and moves all over the far-field plane, like the sweep of a torch beam. Start with a large condenser aperture. The aim is to get a stationary shadow image at the very centre of the disc you can see on the screen.
Just get the rotation centre roughly right, stop the objective wobbling, and then change C2. (Remember that on most machines you may have over-ride the C2 setting). Is the Ronchigram going in and out symmetrically around a point at the centre of the condenser aperture? If not, shift the condenser aperture onto that axis. If in any doubt at all, leave the condenser aperture where it was correct for TEM imaging. You will not get a perfect STEM image, but neither will you lose the beam, which is very easy at this stage.
Now go back and wobble the objective lens again. When both these centres are roughly co-incident with the condenser aperture, set C2 at the value you are going to use it in STEM mode.
（B）Focus the objective to get the Ronchigram to 'blow up' into infinitive magnification at its centre. Adjust the condenser stigmators so that the Ronchigram changes symmetrically as a function of defocus. This takes some practice. (Remember that the objective stigmators, which are below the specimen, are useless in STEM mode, although you should try to set them roughly right in TEM mode before switching to STEM mode.)
If in doubt, wait until you are in scanning mode and can see an image on the scanning monitors before you attempt to correct astigmatism. However, note that if the stigmators are wrong and you change them later, all the above alignments should be re-iterated if you want the best probe possible. Misalignment, tilt, astigmatism, defocus and spherical aberration (which is the thing that makes the Ronchigram look the way it does) are all just lens aberrations that add up and affect the focus of the probe. Change any one of these variables, and another one will no longer be optimal.
（C）Whatever you do, don't try turn C2 so high that what you see on the phosphor screen reaches a focus. This is bound to look astigmatic - but don't correct the condenser stigmators in this condition. Set C2 at its STEM mode setting. Now focus the objective lens: in other words, change the focus until the Ronchigram magnification is a maximum (i.e. the intensity distribution 'blows up' at its centre). If there is astigmatism present, the Ronchigram will not 'blow up' into a uniform infinite magnification blob. Instead, it will look stretched in one direction at one setting of objective defocus, and stretched roughly at right-angles at another setting of defocus. Set the objective between these two extremes, and then correct the condenser stigmators, turning them so that whatever you see spreads more uniformly over the Ronchigram. If the stretching of the image gets more extreme, you're turning the stigmator the wrong way.
As a general rule, for the best scanning probe resolution, you should select a condenser aperture size that just selects the middle 'flat' area of intensity in the Ronchigram when it is just in focus. 'Just in focus' means that as we decrease focus from above the beam cross-over setting, the very centre of the Ronchigram has just exploded into a blob, but has not yet become a clear inverted shadow image. Because changing spot size changes the value of C2 as well as C1 (in STEM mode), and this affects the angle of
convergence at the specimen, the size of the condenser aperture needed to fulfill this condition may also change.
Chapter 4.Pivot points
Before we begin to scan the probe to make a STEM image, it is essential to check the pivot points. (Sadly, this is not possible on all makes of machine because it is regarded as 'too difficult' by the manufacturers. However, you can usually obtain access to the control indirectly via the computer.) Think of the following diagram, where the pivot points are incorrectly adjusted:
In an ideal world, the probe continues to point in exactly the same direction as it is scanned over the specimen by the beam shift coils. However, if the rocking point of the scan coils is wrong, the beam tilts as well as shifts (see introductory guide if you have forgotten the meaning of tilt and shift). This has two consequences: the Ronchigram moves relative to the detectors as the probe is scanned, and the magnification of the STEM image will not be calibrated. At worst, the beam may be only tilting, and not moving laterally at all, in which case the apparent magnification of the STEM image will be much higher than the nominal value. Note that this pivot point adjustment is quite different to the pivot point values in TEM
mode. In the latter, we adjust for a stationary probe as a function of tilt; here we adjust for stationary tilt as a function of shift. We are aiming for shift purity.
There are different ways of making this adjustment on different machines. It is essential to correct it for each spot size and/or objective/condenser combinations of setting. The way the beam shift interacts with the objective pre-field means that the degree of tilt can be strongly correlated to strength of the objective.
Ask the demonstrator: How do I adjust the pivot points in STEM mode?
If in doubt, one way of doing this is to assume the objective aperture is truly in the back focal plane of the objective lens (Usually, it is not quite in the back focal plane). If we pretend it is in the back focal plane, then a way of testing the pivot points is to select diffraction mode and observe the Ronchigram. Insert a small objective aperture (smaller than the Ronchigram), which should cast a shadow over the Ronchigram, and switch on the pivot point adjustment.
If two objective apertures become visible, the diffraction lens is not focusing on the back focal plane; focus the diffraction lens until only one objective aperture is visible. Now remove the objective aperture and adjust the STEM pivot points until the two Ronchigrams are superposed. Note that in STEM mode, focusing the diffraction lens in diffraction mode does not mean making the beam make a sharp point (as in TEM diffraction), because there a very large range of angles present in the conical beam coming out of the specimen.
Why two objective aperture images are visible when diffraction lens is not focusing on the back focal plane?
Chapter 5.Aligning the detectors
Once we have corrected the Ronchigram and the pivots, then all we have to do is get the detectors lined up with the beam coming out of the bottom of the specimen. In a TEM/STEM, the STEM detectors are usually mounted below the phosphor screen, and normally you can't actually see them. Quite often, because of space constraints, they are not mounted on the centre line of the microscope, but are off to one side.
Ask the demonstrator: Please tell me roughly where the detectors are relative to the centre of the phosphor screen.
There are normally two detectors. A solid-state circular disc is used to collect the bright-field signal. Around it is mounted another solid-state detector in the shape of a circular annulus, which is used to collect all the dark- field electrons: that is, all the electrons which have scattered to a large angle outside the cone of the Ronchigram. Looking down on the two detectors, they look like this:
The STEM imaging will only work properly if we get three variables correctly adjusted: the camera-length (i.e. the effective distance or magnification between the specimen and the detector plane) and the x- and y-shift of the detector plane. It sounds simple, but there are a lot of things that can go wrong. This brings two questions. One is how to adjust the camera length and another is how to shift the detector plane? Another
important issue is why the camera length is one critical factor which affects the STEM imaging.
Suppose for a moment that the detectors are on the optic axis. What effect does changing the camera length have? Look at the next diagram.
At very short camera-lengths (This means the image magnification is very low), the disc of the Ronchigram will be much smaller than the disc of the bright-field detector. This condition is good for dark-field imaging, because the annular detector is effectively lying at a high angle which is what we want for easy image interpretation. The image contrast is roughly proportional to the Rutherford scattering of electrons from the atomic nuclei. NOTE: ABOVE STATEMENT IS JUST SUITED TO THE ALLIGNMENT.
As the camera length increases, the Ronchigram gets bigger and bigger, until it reaches a point where it covers the whole bright-field detector. In this condition, the signals on both the dark-field and bright-field detectors are at a maximum, although the contrast on both images will be rather poor.
Why the disc of the Ronchigram much smaller than the disc of the BF detector is good for DF imaging?
At even longer camera lengths, both the bright-field detector and the annular dark field detector are covered by the Ronchigram. In this condition, the bright-field image is very noisy and has low intensity, but it will have much more contrast and will be more like a conventional bright- field image in TEM mode.
However, for any one camera length, think of all the things that can go wrong if the detector is not properly aligned, as shown in the next figure:
Clearly, if the Ronchigram misses both detectors (which, by Sod's Law, is the most likely occurrence) then we will see nothing at all on the STEM monitors, because very few electrons will be hitting either of the detectors.
If the detector is almost aligned correctly, the central disc of the Ronchigram will hit the dark-field detector, but miss the bright-field detector. Under these circumstances, the signal displayed on the so-called dark-field monitor will be bright, and will look like a bright-field image, because it is collecting all the electrons which have passed through the specimen. Even worse, the so-called bright-field detector will be detecting high-angle scattered dark-field electrons, and so it will look like a dark-field image. Because the annular dark-field detector is so much larger than the bright-field detector, there are many more ways to get this inversion of signals to occur than to get the signals the right way round. For this reason, people often spend many happy hours thinking the monitors on the STEM are the wrong way round.。