A Brief Introduction to Scanning Electron Microscopy

Daniel Korchinski, Johannes Duereth, Robyn McNeil, Mia Au, Valentin Schmid(Dated: November 19, 2019)

Scanning Electron Microscopy (SEM) is a technique for image generation that, owing to its op-erational simplicity, short imaging time, and nanoscale spatial resolution, is now prevalent in manyfields of industry and research. Here, we review the key theoretical and experimental underpinningsof SEM, including pertinent details regarding data processing, image acquisition, and other exper-imental considerations. We also highlight recent developments in thin film studies, fractography,and nanomaterials as example applications of SEM in modern research.

I. INTRODUCTION

When speaking at a nanotechnology conference in late’59, Richard Feynman is reported to have said, “it isvery easy to answer many of these fundamental biolog-ical questions; you just look at the thing!” [1]. Thefirst commercial scanning electron microscope, sold in’65, aimed to realize that statement. Traditional opti-cal microscopes, have a resolution d limited by Abbe’sequation,

d =0.612λ

n sin(α), (1)

where d is the minimum resolvable distance, the 0.612comes from the radius of the Airy disk (cf. Supple-mentary Figure 11), λ is the associated wavelength, nis the optical system’s index of refraction, and α is halfthe aperture angle. The best case is n sinα ≈ 1, whichfor green light (500 nm) yields a maximum resolution of∼ 300 nm. To go beyond this limit, shorter wavelengthsare required. By using accelerated electrons, wavelengthsdown to 25 pm can be probed, with de Broglie’s relation.This motivates the use of electrons as a probe for short-scale physics, but only if one has a way to “see” usingelectrons. The principle is relatively straightforward, andhas not changed much since M. von Ardenne’s first SEMwas constructed in 1937. One begins with a narrow, col-limated, beam of electrons, and directs it at the surfaceof an object (preferably in vacuum, so that the electronsdo not scatter in the air). By reading off the currentscattered from the subject, one can build up an image byscanning the beam across the object.

The technique gains significant flexibility from the factthat the scattered electrons contain large amounts of in-formation about the surface on which they impinge. Thescattering angle and energy for an electron will dependon the scattering mode, as well as the impact energy. Byvarying the angle and energies to which the detector issensitive, as well as the impact energy, a broad array ofmaterial properties can be probed.

We will begin our review of this imaging technique witha theoretical exposition of the scattering modes for elec-trons impinging on a surface, before turning to practicalconsiderations relating to experimental set-up, specimenpreparation, and data acquisition and processing. Wewill conclude with several research applications for SEM.

II. THEORY OF BEAM-SAMPLEINTERACTIONS

The accelerated electrons interact with the nuclei andelectrons of the sample material through elastic and in-elastic mechanisms, creating reaction products that canthen be used for imaging. These include backscatteredelectrons (BSE), secondary electrons, Auger electrons,and a variety of X-rays.

II.1. Elastic Interactions

Electrons can interact through Coulomb or Rutherfordscattering with positively charged target nuclei withoutloss of energy. The initial energy of the electron beam iselastically converted into Coulomb potential energy witha dependence on the charge of the nucleus. The scatter-ing is therefore cylindrically symmetric about the beamaxis and characterized by a dependence on sin−4(θ/2),with an interaction probability directly proportional tothe Rutherford scattering cross section given by [2]:

dσ

dΩ=

(zZe2

4πε0

)2(1

4Ta

)21

sin4(θ/2)(2)

Where dσdΩ is the differential angular cross-section, Z

is the nuclear proton number, Ta is the incident kineticenergy, and the target nucleus is assumed stationary.The distribution of each elastic scattering interaction isforward-directed, but the fraction of incident electronselastically scattered at large ( > 90) angles increaseswith the sample thickness. Some of these large-anglescattered electrons can escape the sample material in thebackward direction as BSE, particularly from scatteringoff sample atoms on the surface or near-surface layers.These BSE have high energy, from several keV up to elas-tically scattered BSE with beam energy. Electrons thatare backscattered at the surface without penetrating thesample material are removed from the incident beam andprevented from undergoing further interactions in thesample. Electrons that are backscattered at a nonzerodepth within the sample can have additional interactionswith sample atoms as they exit the surface. Compre-hensive analysis of scattering events along electron tra-jectories can be studied through Monte Carlo codes im-plemented in simulation software such as CASINO, de-

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signed to simulate enough electron trajectories to repre-sent SEM conditions [3]. Figure 1 shows BSE trajectoriescompared to trajectories of beam electrons fully stoppedin the sample by inelastic interactions.

Figure 1. Monte Carlo simulation of a flat, bulk target of cop-per with 0 tilt for visualization of electron trajectories. Redtrajectories lead to backscattering events. (CASINO MonteCarlo simulation) [4]

II.2. Inelastic Scattering

In inelastic interactions, the beam electron energy isnot conserved. Some of these inelastic interactions pro-duce detectable signals for use in SEM. Radiative inter-actions produce a photon, often X-rays. Non-radiativetransitions can result in emission of secondary electrons(SE) or Auger electrons [5]. The incident beam electronscan interact with the electrons in atomic electron shells ofatoms in the sample material, transferring energy to theatomic electrons. The atomic electrons gain energy andcan be excited or even ionized by the interaction, whilethe beam electrons lose the corresponding amount of en-ergy per interaction. The energy required to displacean atomic electron depends on its electronic shell, andthese electrons are labeled with letters according to prin-ciple quantum number: K(n=1), L(n=2), M(n=3) andso on [2]. Valence electrons in the atom’s outer shells areweakly bound and may become ionized by interactionwith the beam electrons. If the ejected electrons haveenergy greater than the sample material’s work function,they can escape the sample surface as SE, with energy of0− 50 eV. Due to inelastic interactions of the low energySE, the probability of SE escaping and being detected de-creases depending how far from the surface the SE wascreated [6]:

p(z) = 0.5 exp

(− zλ

)(3)

Where p is the probability, z is the depth from the surfaceat which the SE was generated, and λ is the mean freepath of the SE in the sample material.

Inner or core electrons in the atom’s inner shells aremore tightly bound. They may become displaced by in-teraction with beam electrons, leaving a hole in an innerelectronic orbital. An outer electron can then transitionto the lower energy orbital, emitting a characteristic X-ray corresponding to the electronic shell. If these X-raysexit the sample material, they can be detected and ana-lyzed to provide information on the corresponding atomicorbital transition in the material being studied. If insteadof emitting an X-ray, the transition of the outer shell elec-tron to the inner shell hole transfers the correspondingenergy to an additional outer shell electron, the outershell electron can be ionized as an Auger electron. Nearthe sample surface, these low energy electrons can over-come the work function of the sample and escape towardsthe detectors. Additional interaction can occur if the X-rays produced by interaction with the electron beam haveenergies above the sample material electron excitationenergies. In this regime, the X-rays produced with elec-tron interaction can fluoresce atoms of the sample mate-rial, producing additional, lower energy X-rays. X-rayscan be produced by another method: bremsstrahlungor “braking radiation” in which beam electrons are re-pulsed by the sample electrons and undergo decelerationby emitting a photon that can range in energy from afew eV up to the beam energy [4].

The beam of electrons loses energy at a rate dependenton the density ρ of the sample material:

S = −(

1

ρ

)dE0

ds, (4)

where E0 is incident beam energy, s is the path lengthof the beam electron, and S is the rate of energy loss perdensity [6]. Though Monte Carlo simulations like thoseshown in Figure 1 are available for detailed descriptions,more practical approximations can be used as an alter-native. The energy lost by the electron can be describedby a continuous energy loss approximation as proposedby Bethe (1932) [4]:

dE

ds= −7.85

(Zρ

AE0

)ln

1.166E0

JeV/nm , (5)

where J is the mean ionization potential in keV. This“Bethe range” gives a description of the penetration ofbeam electrons into the sample, corresponding to thesample volume that can be analyzed using SEM. MonteCarlo results such as those displayed in Figure 2 can beapproximated using the Bethe range if individual trajec-tories are not required.

The interaction volume directly corresponds to the vol-ume of material that can be studied by the production

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Figure 2. Isocontours of energy loss showing fraction of inci-dent energy remaining; Cu, 20 keV, 0 tilt; 50,000 trajectories(CASINO Monte Carlo simulation) [4]

of detectable signals. Individual beam electrons can un-dergo a variety of reactions, producing BSE, SE, andX-Rays. SEM equipment is thus designed to detect andprocess these reaction products in different ways.

III. EXPERIMENTAL SET-UP

An SEM microscope consists of several components di-vided in three main groups according to their functions:the electron column, responsible for creation and align-ment of the electron beam; the specimen chamber, wherethe electrons interact with the sample and are detectedafterwards; and the computer control system, in whichimage processing is performed and beam scanning is con-trolled.

III.1. Electron Column

In a vacuum chamber at the beginning of the electroncolumn an electron source generates free electrons, whichare subsequently accelerated by a high voltage (typicallyin the kV range) to produce a beam. The most commonlyemployed sources are cold field emission guns, tungstenfilaments, Schottky field emissions, and LaB6 emitters.Generally favourable features are high beam stability,high brightness, long lifetime and low beam noise. Whichparticular type of source will be applicable to a specificset-up depends on the pivotal requirements as these gunsshow considerable differences in the aforementioned cri-teria.The diameter of the electron beam right after emission isup to 50µm in size. As the achieved spatial resolution de-pends on the beam diameter (also called probe diameter),focusing to around 10 nm via electromagnetic lenses andapertures is necessary. The former consist of copper coilsembedded in a cylindrical iron hull having pole pieceson the inside. Direct current running through these coils

Figure 3. Simple schematic of a standard SEM configuration[7]

produces a magnetic field constricted inside the iron shellexcept the area of the pole pieces. The electrons of thebeam travel through this radially symmetrical field andare focused as depicted in Figure 4. The Lorentz force,

Figure 4. Focusing of the incident electron beam inside theelectromagnetic lens. Due to interaction with the magneticfield produced by the pole pieces the electrons are deflectedtowards the optical axis and the beam diameter decreases [6].

resulting from the radial part of the magnetic field BR,that is perpendicular to the optical axis of the lens, causesthe electrons to circle around the optical axis. Then, theelectrons undergo deflection towards the optical axis be-cause of the longitudinal part of the magnetic field BL.This focusing effect is made particularly effective by the

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inhomogeneity of the field, so that at a further distancefrom the optical axis, a greater force results. The finalbeam diameter is given by

d0 =1

2B

√2mV0

e(6)

where B is the magnetic field strength, m the electronmass, e the electron charge and V0 the the voltage ac-celerating the electrons in the electron gun. Dependingon the initial beam size, two or three of these condenserlenses are used consecutively before the electrons reachthe specimen.

III.2. Specimen Chamber

Inside the specimen chamber detectors are locatedaround the specimen to collect electrons originating fromthe specimen after interaction. One detector capable ofdetecting both secondary (SE) and backscattered elec-trons (BSE), depending on the mode of operation, is theEverhart-Thornley detector. This fist-sized detector isbased on a collector-scintillator-photomultiplier interplayand faces the specimen under an angle of around 30 inrelation to the horizontal (Figure 5).

Figure 5. Working principle of an Everhart-Thornley detec-tor. Secondary and backscattered electrons are sucked awayby a positive bias and absorbed by a scintillator. The pho-tons emitted by the scintillator in turn are transported out ofthe vacuum and absorbed by photocathode, where they aredetected as a current.[6].

SE emitted by the specimen possess energy of 0−50 eVand BSE of several keV. By applying a positive potentialof 200 V to the grid (Faraday cage) all SE but only line-of-sight and low energy BSE are collected. To convey theinformation in these electrons beyond the vacuum cham-ber, they are converted into photons at a scintillator (amaterial that re-emits the energy of absorbed ionizingradiation in the from of of light). The resulting photonspass out of the vacuum chamber via a quartz window,where they arrive at a photomultiplier tube. There, thephotons trigger the ejection of electrons, which are at-tracted to dynodes (electrode in vacuum) at successivelyhigher voltage. As the electrons hit the dynode, moresecondary electrons than incident are emitted, as the in-cident electrons’ energy increases on approach. Eventu-ally, the electron shower so generated is large enough to

be detected by the anode as a voltage drop. This voltagedrop is passed on to the pre-amplifier for further pro-cessing. It is this voltage that corresponds to the thebrightness of individual areas in the image. By increas-ing the potential difference between the dynodes one canincrease the contrast of the resulting image as all areasgain signal strength by a certain factor. Changes in theoverall brightness are taken via pre-amplification.

By applying a negative potential of about −50V tothe Faraday cage instead of a positive one, the SE arerejected, because of their low energy, and only high ener-getic line-of-sight BSE are detected. This allows the pri-marily SE detector to detect BSE but rather inefficientlyas the majority of BSE possess a high energy and areejected upward along the beam direction after specimeninteraction. Therefore another Everhart-Thornley detec-tor in a doughnut shape concentric around the incidentelectron beam above the specimen is used for increaseddetection rate of BSE [6].

IV. SAMPLE PREPARATION

One of the reasons why SEM allows convenient and fastimaging of the surface topography is that it requires littleor no sample preparation depending on the material in-vestigated. General prerequisites on the sample are e.g.sufficient electrical conductivity so that grounding canprevent electrostatic charging that would deflect the in-cident electron beam. Non-conductive materials can becoated with a conductive material. In most cases goldis chosen for this due to its high conductivity and smallgrain size compared to other possible materials. Further-more the specimen must be stable in vacuum and ableto withstand the bombardment by the electrons withoutundergoing substantial degradation [4].

V. SIGNAL PROCESSING

V.1. Image Formation

In SEM imaging, like many other imaging techniques,the specimen is rastered using a collimated beam of elec-trons (also called electron probe) emerging from the mi-croscope column. The information about every discretelocation on the specimen is encoded in the intensity ofthe signal of the secondary electrons (SE) and backscat-tered electrons (BSE). To each pixel on the finished imageexists a corresponding picture element on the specimen.The size of those picture elements depends on the magni-fication during the imaging. If the signal generating area,which is dependent on probe diameter, is smaller than therespective picture element (dependent on magnification)the picture will be sharp. However, if the signal generat-ing area is bigger than the picture element, the image willappear blurry and out of focus because information fromneighbouring picture elements will overlap. So, to gain

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maximum performance, the spot size should be adjusteddepending on the magnification.

V.2. Signal-to-Noise Ratio

When a specimen is scanned several times, the inten-sity at a discrete location will show a Gaussian distribu-tion around a mean value. The uncertainty of this meanvalue is called statistical noise and is evaluated to be

√n

of the intensity (electrons per second) n. It follows thatthe signal-to-noise ratio (SNR) is n/

√n =√n.

One can increase the intensity by increasing the probecurrent, or increasing how long the SEM lingers over asite (i.e. reducing the scan rate). However, for non-conductive specimens, decreasing the scan rate will re-sult in the specimen accruing a net negative charge, aprocess called “charge up” (cf. Figure 12), which con-strains the scan-rate. For conductive specimens, this isnot a problem and slow scan-rates are possible, albeit atthe expense of imaging time. So in practice the fastestscan rate that gives a smooth image is chosen.

Additionally, a large solid angle of collection for thesignal and frame averaging, as well as editing softwarecan improve the quality of the resulting picture.

There are several models available that can be used todetermine the noise in SEM pictures to filter it. On topof the statistical noise described above are several morecontributors: secondary emission noise, primary beamnoise, and noise from the detection systems also corruptthe signal. These noise sources are well treated in stan-dard texts (e.g. [6]), and are beyond the scope of thepresent review.

V.3. Contrast Formation

In order to distinguish different features in a SEM pic-ture, the difference in intensity of a picture element com-pared to the background signal, has to be high. In thatway, contrast C can be described as

C =SA − SB

SA, (7)

where SA is the intensity of a picture element A andSB the signal of the background.

V.4. Spatial Resolution

In a given optical system that is only limited by diffrac-tion Abbe’s equation (1) can be used to determine theupper limit for the resolution d of the SEM. Here, nis the refractive index of the medium between specimenand column and α the half angle of the cone of electronsconverging on to the specimen in radians. Using the deBroglie wavelength and the kinetic energy of electrons

and substituting it into (1), the resolution d becomes afunction of accelerating voltage V :

d =0.753

α√V

(8)

For a maximum accelerating voltage of 30 keV andan α of around 0.01, the theoretical limit of resolutionis 0.435 nm. However, due to lens errors and aberra-tions the precision decreases. Using more advanced tech-niques like beam monochromators (which select for a sin-gle beam e− wavelength) and immersion optics a maxi-

mum resolution of 0.5 nm at 30 keV and 0.9 nm at 1keVcan be achieved.

V.4.1. Imaging Parameters

Due to the relatively large interaction volume of thebeam (cf. Figure 2), the high resolution part of the sig-nal comes from the SE signal (black spot in Figure 2)which is dependent on the spot size. Because of that, theresolution of the SEM can never be better than the diam-eter of the probe. So, in order to increase the resolutionat a given accelerating voltage, the excitation volume canbe further decreased by decreasing the distance betweenthe end of the electron column and the focal point, calledWorking Distance (WD). When the working distance isdecreased, the angle of convergence increases which inturn decreases the spot size. A decrease in probe sizeis always accompanied by a decrease in the number ofelectrons in the beam. The decrease in beam current ICcan lead to low contrast C: To evaluate this, the Rosecriterion can be used:

IC >4 · 10−12

qFC2(9)

where F is the scan time and q is the detector efficiencyand electron yield. A current of several tens of pA is usedfor high resolution imaging.

V.4.2. Accelerating Voltage

Higher accelerating voltages produce brighter imagesand smaller probe sizes, which is suitable for high-Z ma-terials, as well as thin film specimens. For bulk sampleswith lower atomic numbers, a lower accelerating voltage(500 V - 5 kV) is used to reduce low-resolution signals(BSE and multiply-scattered secondary electrons (SE2))caused by a large excitation volume in order to maximizethe SNR.

During low voltage microscopy, the excitation volumeof the SE2 signal gets comparably small to the SE sig-nal. This localization increases the amount of SE2 signalthat is emitted from the particular picture element thatis being probed and thus enhances the spatial resolution.

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Consequences are better performance, contrast and bet-ter resolution for BSE and SE signal.

Because the SE signal gain is stronger than the loss inbrightness, the SNR remains good. However, feature vis-ibility can be affected if acceleration voltage is too low.Apart from that, the amount of chromatic aberration in-creases in accordance to (10) due to larger energy spread∆E at low acceleration voltages:

d0 = Ccα

(∆E

E0

)(10)

where Cc is the chromatic aberration coefficient and E0

the beam energy. Along with that goes a loss in contrastas the energy spread alters the beam shape and lowers theintensity in the focal point. An example for the impactof chromatic aberration on the image quality is shown inFigure 13 in the appendix.

Due to the low beam energy the electrons are also moreeasily affected by electromagnetic fields and thus requirea shorter working distance.

In general, low acceleration voltages are more suited forsoft or fragile specimen and surface imaging while highacceleration voltages produce better resolution, brighterimages and are less susceptible to chromatic aberration.Figure 14 in the appendix shows an image taken with ofacceleration voltage of 2 keV and 15 keV respectively.

V.5. Depth of Field

A big advantage of SEM compared to other imagingtechniques is the large depth of field. Depth of fieldmeans that not only the plane of the focal point is infocus but also other planes above and below the focalplane. A high depth of field Df can be achieved becausethe convergence angles and the probe diameter are verysmall. It can also be expressed in terms of the magnifi-cation M , the radius Rap of the final aperture and theworking distance WD:

Df ≈WD · 200µm

RapM(11)

The influence of the aperture on the depth of field isshown in Figure 15 in the appendix. A large depth offield is important when observing topography, morphol-ogy and surface details of a specimen (for examples seeFigure 16 in the appendix). [6]

VI. APPLICATIONS

VI.1. Practical Motivation for SEM

As supported by the theory outlined above, SEM pro-vides a relatively robust and versatile tool from which

very highly magnified images can be extracted and anal-ysed. The following will describe how the unique capa-bilities of SEM can be applied to current research, en-gineering and manufacturing. As there exists a plethoraof SEM applications, a selection will be reviewed herewhich spans a broad range of fields including materialsengineering, nanotechnology, and thin film device man-ufacturing. Generally, SEM offers an early design stageimaging tool which provides primarily qualitative infor-mation.

VI.1.1. Cross Sectional Analysis of Thin Film Devices

Over the past few decades, clean energy production hasbecome a ubiquitous and invaluable research objective.Traditional inorganic photovoltaics have been proposedand studied as a solution in the past, however the cost,manufacturability and versatility of such devices has ledto alternative branches of photovoltaic research. Broadly,one such branch of alternatives is organic solar cells. Thedevices presented in Figure 6 and Figure 7 [8], are suchan organic solar cell, fabricated with a colloidal quan-tum dot active photovoltaic layer. Quantum dots areunique in the area of photovoltaic devices as they pro-vide room temperature manufacturing, flexibility and atunable photonic band gap due to the quantum size ef-fect.

Figure 6. Cross sectional SEM image of a Schottky colloidalquantum dot photovoltaic device. Both electrode layers andthe active quantum dot layer are clearly distinguishable. In-dividual quantum dots are also resolved.

Regardless of the specific composition of a proposedorganic photovoltaic device (or any thin film device forthat matter), it is essential to be able to visualize andimage the cross section of these devices. Although theseimages would not provide direct quantitative informationabout the device (other than an approximate film thick-ness), they provide early design stage insight into thegeneral morphology of the device, and any issues thatmay be present. As the entire structure is less than amicron thick, and each dot in the CQD layer is on thescale of a nanometer, high magnification and resolution isessential to visualize the device morphology. As depicted

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Figure 7. Cross sectional SEM image of a heterojunctioncolloidal quantum dot photovoltaic device. The line betweenhole transport and active quantum dot layers can be clearlyobserved.

in Figure 6, SEM at a magnification of 90 k and 5 kV ofpower was successful in imaging the full architecture ofthe device; from electrode to active layer to electrode.Impressively, the SEM was also able to sufficiently re-solve each quantum dot within the active layer (whichcan be seen in the magnified panel in Figure 6). A dif-ferent device (heterojunction as opposed to Schottky) isdisplayed in an SEM image in Figure 7, where a distinctline can be observed between two differently sized andtreated quantum dot layers (the hole transport layer andthe active layer).

VI.1.2. Fractography

Fractography is a primary subset of materials scienceand engineering, and describes the study of fracture sur-faces in materials post mechanical failure. From dentalimplants, to structural failure, to forensics, fractographypersists as an expanding research and engineering field.In terms of SEM usage, especially in the context of recentadvances in complex nanomaterials and composite mate-rials, the fracture surface must be magnified and resolvedto the nanometer scale for effective diagnoses of a failuremechanism. For example, Figure 8 [9] depicts the failuresurface of an alloyed metal hip implant post failure. Thestriations highlighted in red (somewhere around 500 nm- 1µm thick) show the crack propagation in the implantmoments before fracture, and indicate a fatigue mode offailure. This failure information is paramount to the fu-ture design of implants, and it only took one image todetermine, demonstrating the value of SEM in a designprocess.

VI.1.3. Nanostructures

In recent years, nanotechnology and the design ofnanomaterials has become a primary research goal in avariety of fields such as materials engineering, medicine,

Figure 8. SEM image of CrNiMo-steel hip implant post fail-ure. Red lines indicate striations caused by mechanical fa-tigue, indicating fatigue failure modes.

and physics due to the novel physical behavior and inter-actions these materials exhibit. However, to ever prop-erly characterize, fabricate and design with these mate-rials, there must exist an imaging method which can suf-ficiently resolve their features. With recent advances inSEM performance, this method has become the primarymethod in imaging and characterizing nanostructures.For example, carbon nanotubes (CNT’s) possess uniquemechanical, optical/photonic and conductive propertieswhich are being exploited in a variety of current scien-tific fields. One way of fabricating these structures isto form them within a porous anodic aluminum oxide(AAO) template. Figure 9 [10] depicts an SEM image

Figure 9. SEM images of porous anodic aluminum oxide tem-plate used for growing CNT arrays at a) a high magnificationb) a low magnification

Figure 10. SEM images of the CNT array once the templatehas been removed, qualitatively confirming a highly ordered,uniform and dense array.

of such a template at different magnifications. This tem-plate was used to fabricate the carbon nanotube arrayshown in Figure 10 [10]. In the case of that study, the goalwas to improve the template/fabrication process to pro-duce higher ordered/uniform CNT arrays. After adopt-ing a novel 2 step anodization process, the SEM imagesof the final product in Figure 10 were used to confirm theproposed hypothesis of increased order, uniformity and

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density. This example demonstrates the utility of SEMwhen designing and characterizing novel nanostructures.

VII. CONCLUSION

SEM provides an invaluable imaging tool in a varietyof fields of research and engineering primarily due to thepossibility of high magnification and resolution in com-parison to other methods. This capability is due to the

small wavelength range of the probing electron beam asopposed to the range of an optical microscope. Thereare a variety of extracted signals including SE, BSE andAuger electrons, which can be used to characterize dif-ferent features. SEM is relatively simple to operate, canbe quickly applied to a wide range of samples, and inter-pretation of the output images is intuitive. With recentadvances in small-scale physics and engineering, SEM hasbecome a routine tool in research and design alike. AsSEM technology continuously improves, so does the ca-pability and potential application of the method.

[1] R. P. Feynman, California Institute of Technology, Engi-neering and Science magazine (1960).

[2] K. Krane, Introductory Nuclear Physics (Wiley, 1987).[3] R. Gauvin, P. Hovongton, and D. Drouin, “Casino:

monte carlo simulation of electron trajectory in solids,”(2016).

[4] J. I. Goldstein, Scanning Electron Microscopy and X-RayMicroanalysis (Springer Science+Business Media LLC,2018).

[5] R. Mehta, Interactions, Imaging and Spectra in SEM,Scanning Electron Microscopy, Dr. Viacheslav Kazmiruk

(Ed.) (InTech, 2012).[6] A. Ul-Hamid, A Beginners’ Guide to Scanning Electron

Microscopy (Springer, 2018).[7] “High-resolution scanning electron microscopy,”.[8] Unpublished (2018).[9] M. Moeser (2013).

[10] J. S. Suh and J. S. Lee, Applied Physics Letters 75, 2047(1999), https://doi.org/10.1063/1.124911.

Appendix A: Appendix

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Figure 11. The Airy disks produced from two point sources, as they are moved closer together (in arbitrary units of separation).The bottom row are in log-scale, to improve the visibility of the higher order fringes. [6]

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Figure 12. Series of images that show the effect of increasing probe current on a specimen. Probe current increases in eachimages from (a) to (e). Along with this goes an improvement of SNR and smoothness. (f) Shows the effect of ”charge up” inthe upper left corner – a saturation in intensity is the consequence. [6]

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Figure 13. SEM image (a) that shows signs of chromatic aberration at low acceleration voltage of 5 keV which is corrected in(b) with a higher acceleration voltage of 20 keV. In (a) the the contours appear blurred and are sharper in (b).

Figure 14. SEM images composed of SE-signals showing the fracture surface of an aluminium specimen. (a) Most structuraldetails are visible at 2 keV acceleration voltage. (b) Surface structure is blurred and features are not clearly discernable, edgesare much brighter at 15 keV acceleration voltage [6].

12

Figure 15. SEM images of a fracture surface at 20 keV acceleration voltage with (a) big, (b) medium and (c) small aperture.As the aperture decreases, more features are in focus [6].

13

Figure 16. SEM images composed of SE-signals that show the large depth of field. (a) A screw fully in focus from bottom totop. (b-d) Different biological samples [6].