Scanning Electron Microscopy

Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

1. Introduction The Scanning Electron Microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a light microscope. The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Preparation of the samples is relatively easy since most SEMs ony require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makes the SEM one of the most popularly used instruments in research areas today. A SEM may be equipped with an EDX (Energy Dispersive X-ray analysis) system to enable it to perform compositional analysis on specimens. EDX analysis is useful in identifying materials and contaminants, as well as estimating their relative concentrations on the surface of the specimen. In this lab session, we should have a basic understanding how SEM works and it is used. And then we are required to practice image acquisition using two basic imaging modes of SEM, SEI and BEI modes, respectively. 2. Background (1) SEM Setup The basic SEM setup is shown in Fig. 1. A beam of electrons is generated in the electron gun, located at the top of the column, where a filament is located. The most common used filament is Tungsten hairpin gun. This filament is a loop of tungsten which functions as the cathode. A voltage is applied to the loop, causing it to heat up. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode. This beam is attracted through the anode, condensed by one or two condenser lens, and focused as a very fine point on the sample by the objective lens. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron optical column. The scan coils are energized (by varying the voltage produced by the scan generator) and create a magnetic field which deflect the beam horizontally and vertically so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface. The size of the interaction volume depends on the electrons' landing energy, the atomic number of the specimen and the specimen's density. The energy exchange between the electron beam and the sample results in the emission of electrons and electromagnetic radiation, which can be detected to produce an image, as described below.

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(a) (b) Fig. 1 (a) Basic SEM Setup and (b) SEM Optics (2) Electron/Specimen Interactions When the electron beam strikes the sample, a large number of signals (shown in Fig. 2(a)) are generated. We can divide the signals into two broad categories: a) electron signals, b) photon signals. While all these signals are present in the SEM, not all of them are detected and used for information. The signals most commonly used by SEM are the Secondary Electrons, the Backscattered Electrons and X-rays. The specimen interaction volume (shown in Fig. 2(b)) is the volume inside the specimen in which interactions occur while interacting with an electron beam. Therefore we can know SEM is a kind of surface characterization.

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(a)

(b) Fig.2 (a) The signals generated when electron beam strikes the sample, and (b) Specimen interaction volume.

Secondary electrons are low energy electrons and when produced deeper within the interaction volume, will be absorbed by the sample. Only secondary electrons close to the surface will be able to escape the specimen. The weakly negative secondary electrons will be deflected by a positive pull exerted by the Faraday cage surrounding the secondary electron detector and therefore will contribute to the image formation, secondary electrons image (SEI). Different numbers of secondary electrons produced at different areas of the sample will provide image contrast. If at a certain spot on the sample more secondary electrons are produced, a bright spot will appear on the image. Often many secondary electrons are produced along raised areas of the sample; therefore many electrons will be detected, producing a bright spot on the image. This effect is called the edge effect. Some secondary electrons are produced in a valley of the sample and are difficult to deflect by the Faraday Cage. In this case only few electrons will be detected 3

Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

producing a dark spot on the image. Therefore, Image formation of surface structures using the SEM mainly depends on the production of secondary electrons. Backscattered electrons are also produced deep within the sample but have a much higher energy and because of this, are able to escape from deeper within the interaction volume. Because of their high energy, backscattered electrons will not be deflected by the Faraday Cage and therefore not contribute to the image formation. Backscattered electrons may be used to detect contrast between areas with different chemical compositions, especially when the average atomic number of the various regions is different, since the brightness of the backscattered electrons image (BEI) tends to increase with the atomic number. Backscattered electrons can also be used to form an electron backscatter diffraction image. This image can be used to determine the crystallographic structure of the specimen. There are fewer backscattered electrons emitted from a sample than secondary electrons. The number of backscattered electrons leaving the sample surface upward might be significantly lower than those that follow trajectories toward the sides. Additionally, in contrast with the case with secondary electrons, the collection efficiency of backscattered electrons cannot be significantly improved by a positive bias common on Everhart-Thornley detectors. This detector positioned on one side of the sample has low collection efficiency for backscattered electrons due to small acceptance angles. The use of a dedicated backscattered electron detector above the sample in a "doughnut" type arrangement, with the electron beam passing through the hole of the doughnut, greatly increases the solid angle of collection and allows for the detection of more backscattered electrons. Therefore, the quality of SEM image is affected greatly by the size of probing electrons and method of detecting electron emission. In case of SEI, the detector has a fast response and can work under TV rate. On the other hand, BEI detector works slow and thus cannot be used under TV rate. Therefore, it is customary to adjust all imaging conditions under SEI mode (using TV monitor), and create BEI image using slow scan rate imaging method.

(3) Using a Vacuum When a SEM is used, the column must always be at a vacuum. There are many reasons for this. If the sample is in a gas filled environment, an electron beam cannot be generated or maintained because of a high instability in the beam. Gases could react with the electron source, causing it to burn out, or cause electrons in the beam to ionize, which produces random discharges and leads to instability in the beam. The transmission of the beam through the electron optic column would also be hindered by the presence of other molecules. Those other molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image. (4) Application of SEM SEM could be used to characterize the information of sample below: (a) Topography 4

Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

The surface features of an object or "how it looks", its texture; detectable features limited to a few manometers

(b) Morphology The shape, size and arrangement of the particles making up the object that are lying on the surface of the sample or have been exposed by grinding or chemical etching; detectable features limited to a few manometers

(c) Composition The elements and compounds the sample is composed of and their relative ratios, in areas ~ 1 micrometer in diameter.

(d) Crystallographic Information The arrangement of atoms in the specimen and their degree of order; only useful on single-crystal particles >20 micrometers

3. Experiment The samples inside SEM are penny and nickel. Find the joint area and magnification this part, and take pictures under SEI mode and BEI mode. Besides, BEI mode has two different modes: COMP and TOPO mode. Present all the images in the report and compare them. Record the SEM setting of every image. 4. Results and Discussion (1) Joint area between Penny and Nickel The SEM setting of images (shown in Fig. 3) taken in joint area is: Magnification: ×100 Working distance: 15mm HT: 19.0kV Probe current: 3×10-9A

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(a) SEI of joint area between Penny and Nickel

(b) COMP mode image of joint area between Penny and Nickel

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(c) TOPO mode image of joint area between Penny and Nickel Fig. 3 Different SEM mode images of joint area between Penny and Nickel. (2) Magnification area on Penny The SEM setting of images (shown in Fig. 4) taken on Penny is: Magnification: ×2000 Working distance: 15mm HT: 19.0kV Probe current: 1×10-9A

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(a) SEI of some area on Penny

(b) COMP mode image of some area on Penny

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

(c) TOPO mode image of some area on Penny Fig. 4 Different SEM mode images of some area on Penny. (3) Comparison between images taken at different modes From the SEI images, we can tell the morphology of the sample’s surface, the shape of the coins, the gap between them, the size of the gap and the coins, and the arrangement of the coins, we can know which side is Penny and which side is Nickel. And also we can see clearly the dust (bright spot in the SEI image) on the coin. The COMP images can provide elemental composition variation in the sample. Backscatter imaging can distinguish elements with atomic number differences of at least 3, i.e., materials with atomic number differences of at least 3 would appear with good contrast on the image. That is because the efficiency of production of backscattered electrons is proportional to the sample material's mean atomic number, which results in image contrast as a function of composition. The materials with higher atomic number would be brighter than low atomic number materials in COMP image. Therefore, we can tell that the bright area and dark area in our COMP images have different elemental composition. Due to Penny and Nickel are made of copper and nickel, and the atomic number of copper is bigger than that of nickel, so the areas which contain copper is a little brighter than the areas which contain nickel in the COMP image. Besides, there maybe some dust on the coin, and that will also cause the different contrasts. From the TOPO images, we can tell the surface topography of sample: the surface features and its texture. Therefore, we can see clearly the texture on the joint area and on the Penny.

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

5. Problems a. The current of electron beam is the probe current. The current in the condenser lens changes the spot size or diameter of the beam of electrons that scans the sample, and therefore will affect the probe current. More detailed information will be collected when the electron beam scans the same area with a smaller spot size. An increased current or a higher number for the condenser lens setting, will produce a smaller spot size and in general will result in a better resolution. The disadvantage of obtaining a better resolution using a higher probe current is the effect on the beam current or number of electrons available to interact with the specimen. An increase in condenser lens current results in a lower beam current. The effect of a lower beam current is that fewer electrons interact with the sample and therefore fewer secondary and back scattered electrons become available to form an image. Electronically this signal can be increased to compensate for the low beam current but this will also result in an increase in electronic noise. Because of the increased noise, it will be more difficult to focus the specimen and the final image will start to look grainy as well. Therefore, a correct balance between spot size and beam current needs to be found for getting a good quality and high resolution image. b. The working distance is the distance between the final condenser lens and the specimen. Changing the working distance will have an effect on the spherical aberration of the imaging system and therefore will effect the resolution of the final image (Fig.5). Spherical aberration is the failure of the lens system to image central and peripheral electrons at the same focal point. Electrons passing along the axis of the electron beam refract less than electrons passing through the periphery of the electron beam creating more than one focal point and therefore resulting in an enlarged, unsharp spot. When the working distance decreases, this effect of spherical aberration will become less, the spot striking the specimen will become smaller and therefore will improve resolution.

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

Fig. 5 The effect of working distance. (A) If spherical aberration is present, the beam will scan with an enlarged, unsharp spot. (B) If spherical aberration is absent, the beam will scan with the sharpest spot possible. The working distance also has an effect on the depth in the sample that appears to be in focus or in other words, has an effect on the depth of field. At a short working distance the sample will be scanned with a wide cone of electrons resulting in an image with little depth of field. At a longer working distance the sample will be scanned with a narrow cone of electrons resulting in an image with an increased depth of field (Fig. 6). A longer working distance will not provide the optimal resolution. If a sample with large topographical variation needs to be scanned it may be important to use a longer working distance to bring as much of the image into focus as possible; however, some of the resolution will be lost. If a relative flat sample is scanned it is possible to benefit from a better resolution using a shorter working distance since depth of field becomes less important. Depending on the sample and the contained features, a correct balance between working distance and depth of field needs to be found for a good quality and high resolution image.

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Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

Fig. 6 The shape of the cone of electrons depends partly on the working distance and is important in determining the depth of field of a sample. (A) At a short working distance little of the sample appears to be in focus. (B) At a long working distance, more of the sample appears to be in focus because of a greater depth of field. c. Specimens need to be conductive when examined using a SEM. When a specimen is nonconductive a negative charge from the electron beam tends to accumulate, thus effecting the final image. Some of the effects are abnormal contrast, appearing of abnormal lines or shifts/breaks within the image. Biological samples are mainly nonconductive and need to be coated with a thin layer of metal making the surface conductive. Specimens like metals and most semiconductors are conductive and can be examined without coating. d. SEM is often equipped with analytical tools such as EDX and WDX for elemental characterization. (1) EDX Analysis stands for Energy Dispersive X-ray analysis. It is sometimes referred to also as EDS or EDAX analysis. It is a technique used for identifying the elemental composition of the specimen. The EDX analysis system works as an integrated feature of a scanning electron microscope (SEM), and can not operate on its own. During EDX Analysis, the specimen is striked by an electron beam inside the scanning electron microscope. The electrons collide with the specimen atoms' own electrons, knocking some of them off in the process. A position vacated by an ejected inner shell electron is eventually occupied by a higher-energy electron from an outer shell. To be able to do so, however, the transferring outer electron must give up some of its energy by emitting an X-ray. The amount of energy released by the transferring electron depends on which shell it is transferring from, as well as which shell it is transferring to. Furthermore, the atom of every element releases X-rays with unique amounts of energy during the 12

Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

transferring process. Thus, by measuring the amounts of energy present in the X-rays being released by a specimen during electron beam bombardment, the identity of the atom from which the X-ray was emitted can be established (Fig. 7). The output of an EDX analysis is an EDX spectrum. The EDX spectrum is just a plot of how frequently an X-ray is received for each energy level. An EDX spectrum normally displays peaks corresponding to the energy levels for which the most X-rays had been received. Each of these peaks are unique to an atom, and therefore corresponds to a single element. The higher a peak in a spectrum, the more concentrated the element is in the specimen. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well (Fig. 7). The pros of EDX is simple to operate and maintain, and it is cheap comparing other elemental characterization equipment. The cons of EDX is it has bigger noise and lower resolution (about 0.5 at%) comparing WDX.

Fig. 7 Elements in an EDX spectrum are identified based on the energy content of the X-rays emitted by their electrons as these electrons transfer from a higher-energy shell to a lower-energy shell (2) WDX Analysis stands for Wavelength Dispersive X-ray analysis. It is sometimes referred to also as WDS analysis. WDX analysis works in pretty much the same way as EDX analysis, except that its detector classifies and counts the impinging X-rays in terms of its characteristic wavelengths. The detector system uses an X-ray analyzing crystal that only allows the diffraction of desired wavelengths into the X-ray detector for counting. The pros of WDX is: 1) a much better energy resolution, preventing many peak overlap errors frequently encountered in EDX analysis; and 2) lower background noise allowing a more accurate quantitative analysis. The cons of WDX is: 1) higher time consumption; 2) greater sample damage and chamber contamination because of the high beam currents required; 3) high cost; and 4) bulky.. e. When the electron beam strikes the sample, a large number of signals (shown in Fig. 2(a)) are generated. Except secondary electrons and backscattered electrons, there 13

Name: Jie He

UTA ID#: 1000568567

Experiment: SEM

are some other signals could be detected, such as Auger electrons. Auger Emission Spectroscopy (AES), or Auger Analysis, is a failure analysis technique used in the identification of elements present on the surface of the sample. Like EDX and WDX analysis, AES involves the bombardment of the sample with an energetic primary beam of electrons. This process generates, among other things, a certain class of electrons known as Auger electrons. The energy content of the emitted Auger electron is unique to the atom where it came from. Thus, AES works by quantifying the energy content of each of the Auger electrons collected and matching it with the right element. AES has the ability to provide excellent lateral resolution, allowing reliable analysis of very small areas (less than 1 micron). It also offers satisfactory sensitivity, detecting elements that are less than 1% of the atomic composition of the sample. Coupled with ion-sputter milling capability and raster scanning, AES can even be used to generate 3-dimensional maps of elemental distributions of a volume of the sample. This analysis technique is known as scanning auger microscopy (SAM). The output of AES is referred to as an Auger spectrum. This spectrum would show peaks at Auger electron energy levels corresponding to the atoms from which the auger electrons were released. However, AES has the following limitations: 1) charging up of insulative surfaces when struck by the primary electron beam; 2) damage to certain materials, especially organic ones, when struck by the electron beam; 3) occurrence of matrix effects, i.e., signal alterations when some elements are present in particular matrices.

Note: I should be in the Monday morning group, but this time I attended the Thursday morning group, that is because that Monday morning I was out of town for research with my instructor. So the experiment data is the same with the Thursday morning group. Thanks.

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