Introduction to the Scanning Electron Microscope (SEM)

Summary

This video provides an overview of Scanning Electron Microscopy (SEM) and Environmental Scanning Electron Microscopy (ESEM). It explains how SEM uses electrons instead of visible light to generate highly detailed images of samples, focusing on secondary and backscattered electrons. The script compares coated and uncoated samples, discusses issues like charging, and demonstrates the advantages of ESEM for imaging wet samples. Practical steps for sample preparation, imaging in different modes, and the differences between secondary electron and backscatter imaging are illustrated through a salt crystal experiment.

Takeaways

• 🔬 SEM and ECM use electrons instead of visible light for imaging, providing higher resolution due to the shorter wavelength of electrons.
• 💡 In a light microscope, glass lenses focus and detect light, whereas SEM uses an electron gun and electromagnetic lenses.
• 📸 SEM images are usually formed by detecting secondary electrons, which show surface features and produce a 3D-like effect.
• ⚡ Charging occurs in samples when the number of electrons hitting and leaving the sample is unequal, which can be prevented by coating with a thin layer of metal.
• 📏 SEM can image features as small as 1-2 nanometers and as large as 1-2 millimeters, with scale bars indicating sizes in images.
• 🎯 Backscatter electron images highlight differences in atomic number, with regions of higher atomic number appearing brighter.
• 💧 Traditional SEM requires a vacuum and dry samples, while ECM allows for imaging of wet samples by introducing water vapor.
• 🌿 ECM is particularly useful for imaging biological samples like cells and bacteria without distorting them by drying.
• ⚙️ Conductive samples do not require coating in SEM, while non-conductive samples are often coated to prevent charging issues.
• 🔄 SEM can operate in various modes, including high vacuum, low vacuum, and environmental SEM, offering flexibility for different sample types.

Q & A

What is the main difference between SEM and ECM?

The main difference between SEM (Scanning Electron Microscopy) and ECM (Environmental Scanning Electron Microscopy) is that SEM operates in a high vacuum, while ECM allows the introduction of water vapor into the vacuum chamber, making it suitable for imaging wet samples without drying them.

Why do electron microscopes use electrons instead of visible light?

Electron microscopes use electrons instead of visible light because electrons have a much shorter wavelength, allowing for higher resolution imaging that can visualize smaller features compared to traditional optical microscopes.

What is charging in the context of SEM, and why is it a problem?

Charging occurs when the number of electrons that strike the sample is not equal to the number of electrons that leave the sample. This causes a buildup of electrical charge, which negatively affects the quality of the image, often making it blurry or distorted.

How is charging prevented in SEM?

Charging is often prevented by coating non-conductive samples with a thin layer of metal, such as gold-palladium, to allow electrons to flow away from the sample, preventing charge buildup.

What are the two main types of electrons detected in SEM, and what information do they provide?

The two main types of electrons detected in SEM are secondary electrons and backscattered electrons. Secondary electrons provide information about the surface topology, offering 3D-like images, while backscattered electrons provide information on atomic number contrasts, often appearing flatter and showing brighter areas for materials with higher atomic numbers.

What advantages does ECM have over traditional SEM?

ECM allows for imaging wet or biological samples without needing to dry them out, preserving their natural state. It also helps prevent charging on non-conductive samples by introducing water vapor into the chamber.

Why is a vacuum necessary in SEM imaging?

A vacuum is necessary in SEM imaging because electrons used for imaging would scatter off gas molecules in the air, preventing the electron beam from being focused on the sample and resulting in poor image quality.

What is the purpose of the sputter coater in SEM sample preparation?

The sputter coater deposits a thin layer of conductive material, such as gold-palladium, onto non-conductive samples to prevent charging during SEM imaging, ensuring high-quality images.

What kind of samples benefit from being imaged in ECM mode?

Biological and wet samples, such as cells, bacteria, and plant tissues, benefit from being imaged in ECM mode, as they can remain hydrated and be imaged in their natural state without drying or coating.

What difference is seen between secondary electron and backscattered electron images?

Secondary electron images highlight the surface features of a sample, creating a more 3D appearance. Backscattered electron images emphasize atomic number contrasts, with regions of higher atomic numbers appearing brighter, and the images generally looking flatter.

Outlines

00:00

🔬 Introduction to SEM and ECM

Carrie Dumbly, the director of Chapel Hill Analytical and Nanofabrication Lab, introduces scanning electron microscopy (SEM) and environmental scanning electron microscopy (ECM). Unlike optical microscopes, which use visible light, SEM and ECM use electrons to create images, offering higher resolution due to the shorter wavelength of electrons. She explains how SEM and ECM use electron beams to interact with samples, producing backscattered and secondary electrons, which are key in generating detailed images. She also mentions the issue of sample charging and how coating samples with a thin metal layer prevents image distortion.

05:01

🔍 Imaging Wet Samples with ECM

This section focuses on the challenges and advantages of imaging wet samples using ECM. Traditional SEM requires samples to be dry due to the need for a vacuum, which distorts wet samples. However, ECM allows for a controlled amount of water vapor in the chamber, enabling the imaging of hydrated biological samples like cells and bacteria. Though ECM resolution is lower than that of traditional SEM due to electron scattering by water molecules, it allows for imaging without drying out the samples or requiring conductive coatings.

10:04

🧪 Sample Preparation for SEM Imaging

Catherine McInnis, a graduate student, demonstrates how to prepare a sample for SEM imaging. The process involves mounting the sample on a stub with conductive tape, usually carbon or copper tape. Non-conductive samples require a conductive coating, typically applied using a sputter coater that deposits a thin metal layer (gold-palladium) on the sample. McInnis walks through the sputter coating process and explains how a quartz crystal microbalance helps measure the amount of deposited material, ensuring that a sufficient coating is applied to prevent charging during SEM imaging.

15:05

📸 Imaging Salt Crystals Using SEM and ECM

This section covers the imaging of salt crystals using SEM in high vacuum, low vacuum, and environmental modes. The high vacuum mode produces clear images of coated salt crystals, while uncoated salt shows signs of charging, making imaging difficult. In low vacuum mode, introducing water vapor prevents charging, enabling clear imaging of uncoated salt. In ECM mode, the humidity level is adjusted to form water droplets, demonstrating how salt dissolves and recrystallizes. The section also compares images captured with secondary and backscatter electron detectors, highlighting differences in surface topology and atomic number contrasts.

🌧️ SEM and ECM Imaging Modes

This final section wraps up the demonstration of SEM and ECM imaging modes. The uncoated salt sample showed charging issues in high vacuum mode, but switching to low vacuum mode eliminated this effect. In ECM mode, increasing humidity dissolved the salt crystals, and reducing it caused recrystallization. The section also compares secondary electron and backscatter electron images of the same sample, showing the difference in surface detail and atomic number contrasts, with secondary electrons emphasizing surface features and backscatter electrons highlighting atomic number differences.

Keywords

💡Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a type of microscope that uses a focused beam of electrons to produce high-resolution images of samples. Unlike optical microscopes that use light, SEM provides detailed images of surface features at a nanometer scale. In the video, SEM is demonstrated as a tool to image various samples, such as salt crystals, and is highlighted for its ability to show surface topology in 3D.

💡Environmental Scanning Electron Microscopy (ECM)

Environmental Scanning Electron Microscopy (ECM) is a variant of SEM that allows for imaging wet or non-conductive samples by introducing water vapor into the chamber. This prevents charging and allows samples to remain hydrated, such as biological materials. In the video, ECM is used to image bacterial biofilms, demonstrating its capability to work with wet samples while avoiding distortion.

💡Electrons

Electrons are subatomic particles used in electron microscopy to produce images. In SEM and ECM, electrons interact with the sample, either scattering back or ejecting secondary electrons, which are detected to create an image. The video explains how electrons, with shorter wavelengths than light, allow for much higher resolution than light microscopes.

💡Backscattered Electrons

Backscattered electrons are electrons that bounce back from the sample after interacting with it. These electrons provide information about the atomic number of materials, with higher atomic number regions appearing brighter. In the video, backscattered electron images show contrasts in materials like polymer samples with barium titanate particles, which appear bright due to their higher atomic number.

💡Secondary Electrons

Secondary electrons are ejected from the surface of a sample when it is struck by the primary electron beam. These electrons provide high-resolution images of surface features, making SEM images appear three-dimensional. In the video, secondary electron images are used to visualize zinc oxide nanowires and salt crystals, emphasizing their surface structure.

💡Charging

Charging occurs when a sample accumulates excess electrical charge because the number of electrons entering the sample is not equal to the number leaving it. This leads to distortions in SEM images, especially for non-conductive samples. In the video, uncoated salt crystals show charging effects, with streaks and bright spots appearing in the images.

💡Vacuum

A vacuum is essential in SEM to prevent electrons from scattering off gas molecules, ensuring a focused electron beam. Traditional SEM requires a high vacuum to image dry samples, while ECM allows a controlled amount of water vapor for imaging wet samples. The video explains the role of the vacuum in achieving high-quality imaging and preventing gas interference.

💡Conductive Coating

Conductive coating is applied to non-conductive samples to prevent charging during SEM imaging. The video describes how a thin layer of gold-palladium is sputtered onto the sample using a sputter coater. This coating ensures that the sample can conduct electricity, allowing for clearer imaging without charge buildup.

💡Sputter Coating

Sputter coating is the process of applying a thin metal layer (e.g., gold-palladium) onto a sample to make it conductive. In the video, a sputter coater deposits this layer on salt crystals to prepare them for SEM imaging, which prevents charging and allows for better image clarity in traditional SEM mode.

💡Atomic Number Contrast

Atomic number contrast refers to the difference in brightness between materials with different atomic numbers in backscattered electron images. Regions with higher atomic numbers appear brighter because they scatter more electrons. In the video, this concept is demonstrated with a polymer sample embedded with barium titanate particles, where the higher atomic number particles appear much brighter than the surrounding material.

Highlights

Introduction to Scanning Electron Microscopy (SEM) and Environmental Scanning Electron Microscopy (ECM), focusing on using electrons instead of visible light for imaging.

Electrons have a much shorter wavelength than visible light, allowing SEM and ECM to achieve higher resolutions than traditional optical microscopes.

SEM uses backscattered and secondary electrons to produce images, highlighting how different electron interactions provide various imaging results.

Charging occurs when the number of electrons striking a sample differs from the number leaving it, potentially distorting the image. Coating non-conductive samples with a thin layer of metal can prevent charging.

Secondary electron images reveal surface features in great detail, showing three-dimensional structures, as seen in the zinc oxide nanowire image.

Backscattered electron images focus more on differences in atomic number, making regions with higher atomic numbers appear brighter, such as barium titanate particles in a polymer sample.

SEM is capable of imaging features as small as 1-2 nanometers and as large as 1-2 millimeters, making it versatile for various applications.

A comparison between secondary and backscatter SEM images of a polymer resin circuit board with soldered connections shows how each technique highlights different aspects of the same sample.

Environmental SEM allows imaging of wet samples like biological cells and bacteria without needing to dry them out, preserving their natural state.

Using an ECM with controlled water vapor can prevent charging for non-conductive samples without needing a conductive coating, which is advantageous for imaging.

Demonstration of how environmental SEM can dissolve and recrystallize salt crystals by adjusting the humidity in the chamber, showing the system’s versatility.

Switching between secondary electron detectors and backscatter electron detectors provides contrasting views of the same sample, enhancing analysis.

Uncoated samples can show streaks and artifacts due to charging, demonstrating the importance of proper sample preparation for SEM imaging.

Sputter coating with gold palladium is used to make non-conductive samples conductive, preventing charging during SEM imaging.

A discussion on optimizing SEM imaging conditions, including focusing the electron beam and adjusting stigmation, ensures high-quality images.

Transcripts

[Music]

welcome my name is Carrie dumbly and I'm

the director of the Chapel Hill

analytical and nanofabrication lab or

channel at UNC today we will be talking

about scanning electron microscopy often

called SEM an environmental scanning

electron microscopy often called ECM the

SEM and ECM are microscopes that produce

images using electrons instead of

visible light

remember the wavelength of light limits

the resolution in an optical microscope

today we're going to learn about using

electrons which have a much shorter

wavelength than visible light for

imaging purposes this is a diagram of a

light microscope the basic components

include a light source a way to focus

that light onto the sample a way to

collect the light that travels through

the sample and a way to detect that

light with a light microscope glass

lenses similar to magnifying glasses are

used to focus the light and collect the

light an electron microscope has many of

the same components as a light

microscope instead of a light source the

electron microscope uses an electron gun

to produce electrons electromagnetic

lenses are used to focus the electrons

and the detector is sensitive to

electrons instead of visible light

electrons can interact with a sample in

a number of ways today we will focus on

the backscattered and secondary

electrons that we can detect in an

electron microscope when an electron

beam strikes a sample some of the

electrons are absorbed other electrons

are back scattered and some sample

electrons can be ejected as secondary

electrons if the number of electrons

that strike the sample is not equal to

the number of electrons that leave the

sample then the sample will build up a

charge this is called charging and it

negatively affects the quality of the

resulting image in order to prevent

charging many SEM samples are coated

with a thin layer of metal

most SEM images are produced by

collecting secondary electrons this

image is a secondary electron image of

zinc oxide nanowires and nano flowers

secondary electron images show the

surface features of a sample and

therefore look very three-dimensional

the scale bar at the bottom right of the

image indicates that most of these

nanowires are a few microns long and

only 50 to 100 nanometers wide this is

another secondary electron image showing

cells that were cultured on top of

manufactured pillars notice that the

scale bar for this image is much larger

than the previous image scanning

electron microscopes can typically image

features as small as one or two

nanometers and as large as one or two

millimeters backscatter SCM images show

fewer surface features than secondary

electron images often back scatter

images look very flat the contrast that

we do see in a back scattered image is

due to differences in average atomic

number regions of the sample with higher

atomic number will produce more

backscattered electrons and appear

bright this image is of a polymer sample

sample with barium titanate particles

embedded in it since the barium titanate

has a much higher average atomic number

these particles appear much brighter

than the polymer that they are embedded

in many electron microscopes have both

secondary and backscatter electron

detectors and acquiring both images on

the same sample can illustrate the

differences between them this sample is

a polymer resin circuit board with some

soldered connections the secondary

electron image on the Left shows the

surface topology while the backscattered

image on the right shows the atomic

number contrasts it is clear that the

bright regions are from the higher

atomic number solder which is composed

primarily of tin both types of images

provide useful information

in a typical SEM vacuum is required

because the electrons that we use for

imaging will scatter off gas molecules

and prevent us from focusing the

electron beam on the sample as a result

we can only image dry samples in a

typical SEM if we want to look at wet

samples we need to dry them out first

and this often distorts their shape this

image shows how there are very few

residual gas molecules in the SEM

chamber of a traditional SEM and the

beam spot on the samples is relatively

small an environmental SEM often called

an ECM allows the operator to introduce

a controllable amount of water vapor

into the SEM vacuum chamber there is one

disadvantage of using an environmental

SEM when you introduce water molecules

into the chamber the electrons that are

traveling towards the sample will hit

these water molecules and scatter the

result is that the electron beam is not

as tightly focused as in a traditional

low-pressure SEM thus the resolution of

the images is not as good the main

advantage of an ECM is that you can

image wet samples without having to dry

them out these include many types of

biological samples such as cells

bacteria and plants in the ECM we can

keep these samples hydrated and image

them in their natural state in addition

we can use a small amount of water vapor

to also prevent charging this allows us

to image non conductive samples without

the need for a conductive coating this

is an ECM image of a bacterial biofilm

the bacteria are the rod shaped

particles that are about one micron long

the ECM is great for imaging these types

of wet samples thank you for joining

this discussion of scanning electron

microscopy and environmental scanning

electron microscopy

you

[Music]

welcome I'm Carrie Donnelly the director

of the Chapel Hill analytical and nano

fabrication lab or channel at UNC with

me today is Catherine McInnis a graduate

student at UNC hello everyone thanks for

joining us today I'm going to show you

how to prepare samples for scanning

electron microscopy or SEM let's head to

the law welcome to the SEM lab today we

will be imaging some salt crystals the

tools that we will need to prepare our

sample for imaging today include the

sample tweezers sample stubs conductive

tape two common types are carbon tape

and copper tape and possibly a sputter

coder first I'll mount the sample onto a

sample stub with some conductive tape

the tape I'm using is double-sided

carbon tape and I'll use it to hold the

salt in place

by gently tapping the sample stub on the

table I can remove any excess salt that

is not well attached samples that are

not conductive need to be coated with a

conductive coating for traditional SEM

imaging in order to prevent charging

problems our lab uses a sputter coder to

deposit a thin layer of gold palladium

but some labs use a carbon coating

instead I'll place the sample in the

sputter coder the metal that we will

deposit is actually held in the lid in

this small circular disk just next to

the sample is called a quartz crystal

microbalance it can measure how much

material has been deposited after

loading the sample into the chamber and

closing the lid I can evacuate the

chamber using a vacuum pump once the

chamber has been evacuated I can start

the deposition a plasma is generated

that removes material from the gold

palladium target and deposits it

everywhere in the chamber including on

my sample it's easier to see this plasma

if we turn off the lights the readout

from the quartz crystal microbalance

lets me know how much material has been

deposited a 2 nanometer coating is

usually sufficient to prevent charging

if your sample is conductive or if you

will be imaging in an environmental SEM

you can skip the sample coating step our

sample is now ready for SEM imaging

let's head to the imaging lab

[Music]

hello I'm Kerry Donnelly the director of

the Chapel Hill analytical and nano

fabrication lab or channel at UNC with

me today is Catherine McInnis a graduate

student at UNC in this section we will

show you how to image a sample with

scanning electron microscopy or SEM and

environmental scanning electron

microscopy or ECM I prepared our salt

sample for imaging and now I'm ready to

look at it with the SEM

welcome to the SEM lab this is the SEM

that we will be using today it can

operate in three different modes high

vacuum low vacuum and environmental SEM

mode we will image in all three modes

today remember the SEM operates under

vacuum so first I will bent the chamber

to atmosphere and then load the sample

the sample stub has a small post that

fits into the sample stage a small set

screw holds the sample in place I'm also

going to load a similar salt sample that

hasn't been coated with any metal so we

can compare the effect of the metal

coating on imaging most systems take

just a minute or two before they are

pumped down to a low enough pressure to

start imaging

I'll start by imaging in high vacuum

mode this is the standard operating mode

for most SEMS the first thing to do when

starting to image is to focus the

electron beam on the sample

I will also adjust the stig nation to

make sure that I'm imaging with a round

beam of electrons and not an ellipse and

make sure my imaging conditions are

optimized once these things are done it

is pretty easy to move around on the

sample and zoom in or zoom out the scale

bar at the bottom right of the image

lets me know how big the features in my

sample are these particles are on the

order of 100

of microns this sample is traditional

table salt and you can see from this

image I collected that it crystallizes

into nice square crystals common table

salt is sodium chloride and its crystal

structure is face centered cubic as

shown here it's easy to see how the

arrangement of atoms and sodium chloride

results in the small cubes that we see

by SEM imaging now let's move to the

uncoated salt crystals this image of the

uncoated salt shows some streaks and

some bright and dark spots that don't

quite look right this is the charging

effect that we discussed earlier because

the sample is not conductive it is

building up a charge and this makes it

very difficult to image it properly with

a charged beam of electrons it would be

impossible to collect a good image under

these conditions

now we're going to switch to low vacuum

mode and continue to image the uncoated

salt sample remember that in low vacuum

mode we introduce some water vapor in

the chamber that will help prevent

charging this is the same area of the

sample we were just looking at and you

can see that now I can collect a much

nicer image than I could in high vacuum

mode let's now operate the system in

environmental SEM mode for this we'll

need to use a different sample stage

that is capable of cooling the sample

and a slightly different sample holder

in which I can put some uncoated salt

crystals once the system has pumped down

to the pressure that will image at we

can start to adjust the humidity level

in the chamber you can see that at some

point the chamber becomes humid enough

to start forming water droplets on the

salt crystals if I let the water

continue to form on the sample the salt

crystal will eventually dissolve into

the water I can force the salt to

recrystallize by reducing the humidity

in the chamber and allowing the water to

evaporate once it does the salt will

form crystals again all of the imaging

I've done so far has all been with a

secondary electron detector if I switch

to the backscatter detector it's clear

that the conch

mechanism is different these two images

are of the same salt crystals but with

two different detectors the one on the

left is a typical secondary electron

image showing the surface topology the

image on the right is a backscatter

image which shows more atomic number

contrasts the salt crystals become

relatively dark because sodium and

chlorine have lower atomic numbers than

the metal sample holder that they are

sitting in this image is also much

flatter looking than the secondary

electron image thank you for joining us

for this demonstration of sem and ECM

imaging today we image coated and

uncoated salt samples in high vacuum

mode and saw charging on the uncoated

sample we successfully used low vacuum

mode to eliminate charging in isa mode

we increase the humidity enough to form

water droplets that could dissolve the

salt and then decrease the humidity in

the chamber and saw the salt

recrystallize finally we saw the

difference between images obtained with

a secondary electron detector and a

backscatter electron detector