Figure 3. SEM image of an integrated single
crystal silicon cantilever and tip which has an
end radius of 2 to 10nm. Tips for AFM aretypically made of silicon or silicon nitride.
Bar = 100µm.

(STEM).1, 2 In 1942, Zworkin et. al.
developed the first SEM for bulk
samples. This configuration contains
many of the basic principles oftoday’s SEMs.2, 3 Cambridge Scientific
Instruments produced the firstcommercial instrument in 1965.
A number of improvements haveoccurred since this time, resulting in
an increase in resolution from 50nm
in 1942 to ~0.7nm today. Besides
the development of morphologicalimaging, the SEM has beendeveloped to detect signals which
are used to determine compositional
information, such as X-rays, backscattered electrons, cathodoluminesce,
Auger electrons, and specimen current.
The development of the AFM waspreceded by the development of the
Scanning Tunneling Microscope (STM)
in 1981 at IBM Zurich ResearchLaboratory by Binnig and Rohrer.4
Its ability to view the atomic latticeof a sample surface earned theinventors the Nobel Prize in Physics
in 1986. Although the STM provides
subangstrom resolution in all three
dimensions, it is limited to conductive
and semiconductive samples. To image
insulators as well as conductors, theAtomic Force Microscope (AFM) was
developed in 1986, 5 and the firstcommercial AFMs were produced in
1989 by Digital Instruments, now
Veeco Instruments.

Figure 4. TappingMode AFM image of 1.4Åmonoatomic steps on epitaxial silicon
deposited on (100) Si. 1µm scan.

AFM provides three-dimensionalsurface topography at nanometer
lateral and subangstrom verticalresolution on insulators and conductors.
From this beginning, the field of
Scanning Probe Microscopy (SPM)
was born which consists of a familyof techniques that involves scanning a
sharp tip across the sample surface
while monitoring the tip-sample
interaction to form a high resolution
image. Although the AFM has become
the most commonly used form of SPM,

many other SPM techniques have beendeveloped which provide information
on differences in friction, adhesion,
elasticity, hardness, electric fields,
magnetic fields, carrier concentration,
temperature distribution, spreading
resistance, and conductivity.

Imaging Mechanisms
Scanning Electron Microscopy
The operation of the SEM consistsof applying a voltage between aconductive sample and filament,
resulting in electron emission from thefilament to the sample. This occurs ina vacuum environment ranging from
10-4 to 10 -10 Torr. The electrons are
guided to the sample by a series ofelectromagnetic lenses in the electron
column. A schematic of a typical SEM
is shown in Figure 2. The resolution
and depth of field of the image aredetermined by the beam current andthe final spot size, which are adjustedwith one or more condenser lenses
and the final, probe-forming objective
lenses. The lenses are also used toshape the beam to minimize theeffects of spherical aberration,
chromatic aberration, diffraction,
and astigmatism.
The electrons interact with the sample
within a few nanometers to several
microns of the surface, depending on
beam parameters and sample type.
Electrons are emitted from the sample
primarily as either backscatteredelectrons or secondary electrons.
Secondary electrons are the mostcommon signal used for investigationsof surface morphology. They are
produced as a result of interactions
between the beam electrons andweakly bound electrons in the
conduction band of the sample. Someenergy from the beam electrons istransferred to the conduction band
electrons in the sample, providing

a.

b.

Figure 5. (a) SEM image of rugged polysilicon thin film. 100,000x, Bar = 0.1µm; (b)TappingMode AFM image of the same with roughness measurement. 1µm scan.

enough energy for their escape fromthe sample surface as secondary
electrons. Secondary electrons are
low energy electrons (<50eV), so
only those formed within the first few
nanometers of the sample surface haveenough energy to escape and be
detected. High energy beam electrons
which are scattered back out of thesample (backscattered electrons) can
also form secondary electrons when
they leave the surface. Since theseelectrons travel farther into the sample
than the secondary electrons, they canemerge from the sample at a muchlarger distance away from the impactof the incident beam which makes their
spatial distribution larger. Once theseelectrons escape from the sample
surface, they are typically detectedby an Everhart-Thornley scintillatorphotomultiplier detector. The SEM
image formed is the result of theintensity of the secondary electron
emission from the sample at each x,ydata point during the rastering of theelectron beam across the surface.

Atomic Force Microscopy
AFM consists of scanning a sharp tip
on the end of a flexible cantileveracross a sample surface while
maintaining a small, constant force.
An integrated silicon tip and cantilevercan be seen in Figure 3. The tips
typically have an end radius of 2nm
to 20nm, depending on tip type. The
scanning motion is conducted by apiezoelectric scanner which scans thetip in a raster pattern with respect tothe sample (or scans the sample withrespect to the tip). The tip-sample
interaction is monitored by reflecting
a laser beam off the back of thecantilever into a split photodiodedetector. By detecting the differencein the photodetector output voltages,changes in the cantilever deflection oroscillation amplitude are determined.
A schematic can be seen in Figure 1.
The two most commonly used modesof operation are contact mode AFM
and TappingMode AFM, which
are conducted in air or liquid
environments. Contact mode AFM
consists of raster-scanning the probe(or sample) while monitoring thechange in cantilever deflection with the
split photodiode detector. A feedback

loop maintains a constant cantileverdeflection by vertically moving thescanner to maintain a constantphotodetector difference signal. The
distance the scanner moves verticallyat each x,y data point is stored by
the computer to form the topographic
image of the sample surface. This
feedback loop maintains a constantforce during imaging.
TappingMode AFM consists ofoscillating the cantilever at its
resonance frequency (typically~300kHz) and lightly “tapping” thetip on the surface during scanning.
The laser deflection method is used
to detect the root-mean-square (RMS)
amplitude of cantilever oscillation. A
feedback loop maintains a constantoscillation amplitude by moving thescanner vertically at every x,y data
point. Recording this movement
forms the topographical image.
The advantage of TappingMode overcontact mode is that it eliminates thelateral, shear forces present in contact
mode. This enables TappingModeto image soft, fragile, and adhesive
surfaces without damaging them,which can be a drawback of contact
mode AFM.

Comparison of Techniques
There are a number of different ways
to compare and contrast these twotechniques. Although investigations
that use both SEM and AFM tocharacterize a material are common,
there are just a few studies that
directly discuss the complementar ynature of the techniques. 6-13 A
comparison of these techniques will
be conducted with respect to 3
factors: (1) Surface Structure, (2)Composition, and (3) Environment.
The comparisons are presented for
typical equipment configurations andoperating procedures.

a.

Surface Structure
Although both SEM and AFM are
similar in lateral resolution, there aresituations in which one technique
can provide a more complete
representation of the sample surface,depending on the information desired.
One principle difference is in how
the two techniques process verticalchanges in topography. Below wewill discuss measurements of different
vertical scales of topography,
beginning with very smooth surfaces
and working up to very roughsurfaces to determine how the surface
topography affects the ability of eachtechnique to perform the measurement.
Atomically Smooth Surfaces
Atomically smooth surfaces can occur
either naturally, such as on mineralsurfaces, or by processing, such aspolishing and epitaxial growth on
semiconductor, data storage, andoptical surfaces. A TappingMode AFM
image of an epitaxial silicon surface isshown in Figure 4. Note that, unlike
SEM, the AFM can measure in all
three dimensions (x, y, and z) with asingle scan. Since the AFM has avertical resolution of <0.5Å, it canresolve the 1.4Å monoatomic siliconb.

c.

Figure 6. (a) SEM image of partially GaP-covered Si after chemical beam epitaxy deposition for
10 minutes. 30,000x, Bar = 1µm; (b) AFM image of the same sample as in figure 6a showing thepresence of nodules during the growth of GaP by chemical beam epitaxy. 10µm scan; (c) Crosssectional measurement with AFM across the image in Figure 6b showing 3 nodules which have aheight of approximately 70nm. 16

steps on the surface as well as
calculate an RMS roughness of 0.7Å. 14
On a sample this smooth, the SEM has
difficulty resolving these features due tothe subtle variations in height..
Thin Films
On most thin films, the SEM and AFM
produce a similar representation of thesample surface. A common applicationof surface investigations of thin films
consists of determining changes inmorphology with variations ofdeposition parameters, such astemperature, pressure, time, etc. Figure

5 shows SEM and AFM images of apolysilicon thin film at approximately
the same lateral magnification. The
two images show similar surface
structure, however, they differ in theother types of information that can beacquired on this sample. The threedimensional nature of the AFM can beused to calculate changes in roughness
and surface area variations due todifferences in deposition parameters.
For the SEM, a large area view of thevariations in surface structure can beacquired all at once (such as severalmm’s), whereas a 100µm x 100µm

area is typically the largest area
viewed by an AFM. These images are
an example of “rugged” polysiliconfilms which are used as capacitors inmemory devices. By making these films
rough, the surface area is increased
which makes it possible to hold morecharge without increasing the lateraldimensions of the capacitors on thechips. By adjusting the deposition
parameters and using the AFM to
analyze the surface area of the films,
the deposition parameters needed to
produce a film with the maximum
surface area were determined. 15
Another example of the difference
between the two techniques is ininterpreting subtle differences in height.
In the SEM image, changes in slope
can result in an increase in electron
emission from the sample surface,
producing a higher intensity in theimage. However, it can sometimes
be difficult to determine whether thefeature is sloping up or down. For
instance, in the SEM image in Figure
6a it is very difficult to determine
whether the small round structures arebumps or pits, even when tilting thesample stage in the SEM. The only
other option would be to cleave thesample through one of these featuresand look at the sample in crosssection, which would be tediousand time consuming. Since the AFM
data contains the height information,
determining whether a feature is abump or pit is straightforward. As canbe seen in Figures 6b and 6c, thefeatures on this sample are bumps.
This information was used in the studyof the growth mechanisms of GaP
on Si during chemical beam epitaxy
deposition.16 Determination of whether
these features were small bumps or
depressions would have changed how
the deposition process was altered to
produce an epitaxial GaP film.

High Aspect Ratio Structures
Semiconductor processing commonly
requires measurements of high aspectratio structures such as trenches andvia holes. In a SEM, these structures
are typically measured in cross section
by cleaving the wafer and imaging
the sample on end to obtain thedimensions of the structure. A common
example of this is seen in Figure 7a.
In contrast, the AFM image of a trenchor via is made by scanning over the
sample surface. The ability of the
AFM to measure these structuresnondestructively makes it possible
for the wafer to be returned to the
production line after the measurement

is acquired. An AFM image of viasin photoresist is shown in Figure 7b.
To image some higher aspect ratio
structures, the proper tip shape isneeded for the AFM to scan narrow
openings and steep sidewalls.
Although the SEM measurement is
destructive to the sample, the ability
to image the undercuts of these lines
is a useful application that AFMs are
not typically designed to perform withthe exception of the Dimension X3D
Automated AFM (please see Veeco
Instruments Application Notes on AFM
Metrology of undercuts and nearvertical side walls with model
Dimension X3D).

a.

b.

Figure 7. (a) Cross-sectional SEM image of polysilicon lines which shows undercutting due toreactive ion etching. Scale bar = 100nm; (b) Cross-sectional measurement of developed andincompletely developed vias in photoresist acquired by TappingMode AFM. In order to image thehigh aspect ratio structures on the sample, a silicon tip machined with a focused ion beam wasused to scan the vias. 6.2µm scan.

b.

a.

Figure 8. (a) SEM image of a non-woven textile sample of polyethylene oxide fibers. The large depth of field of the SEM makes it possible
to image fibers which are 10’s of µm’s below the upper layer of fibers. Bar = 10µm; (b) SEM image of Y2O3 crystal. Bar = 1µm.

Rough Surfaces
One of the key advantages of the
SEM with respect to other types of
microscopy is its large depth of field.
This ability makes it possible to image
very rough surfaces with millimetersof vertical information within a single
image. A SEM image of non-woven
polyethylene oxide fibers can be seenin Figure 8a. The depth of field andsmall beam size makes it possible toimage the fibers far below the top
layer. This ability also makes it possible
to measure very rough surfaces overlarger lateral areas as well. Although
the AFM can measure vertical surface
variations below 0.5Å, its ability tomeasure a tall structure comes fromhow far the scanner can movevertically. Standard scanners typicallyhave 5 to 6µm of vertical range,
however, in some configurations thevertical range approaches 10µm orlarger. For scanning areas that haveheights of greater than 5 to 10µm’s ofvariation, the SEM would be better
suited for the analysis.
Another example of a complex threedimensional surface structure whichshows how the SEM and AFM cancomplement each other can be seenin Figure 8b. The convoluted threedimensional Y 2O3 oxide crystal shown
growing out of a relatively flat Y2O3
thin film on a Si substrate is easily
imaged in the SEM (Figure 8b).
Although the AFM would haveproblems imaging the obtuse angles
and enclosed areas of this surface, theroughness of the Y 2O3 film can bemeasured whereas in the SEM image
the surface roughness is not evident.
Therefore, the two techniques together
give a more complete “picture“ ofthe sample.

the SEM, a number of other signals
are emitted as a result of the electron
beam impinging on the surface, as
shown in Figure 9. Each of thesesignals carries information about
the sample which provide clues to
its composition.
Two of the most commonly usedsignals for investigating composition
are x-rays and backscattered electrons.
X-ray signals are commonly used toComposition
SEM is the only one of the twotechniques which provides elemental
analysis, however, both SEM and AFM
are associated with techniques which
can provide compositional information
through analyzing materials andphysical properties of the sample.
Some of the most common of thesemethods are described next.
SEM
Along with the secondary electron
emission which is used to form amorphological image of the surface inFigure 9. Signals emitted from a sample
surface after interaction with an electron beam.

Figure 10. EDS X-ray spectrum of an AlGaN thin film on SiC
substrate showing the presence of N, Ga, and Al.

provide elemental analysis by theattachment of an Energy-Dispersive
Spectrometer (EDS) or WavelengthDispersive Spectrometer (WDS) to theSEM system. X-ray emission results frominelastic scattering between the beamelectrons and the electrons of thesample atoms. This interaction results inthe ejection of an inner shell electronfrom the atom, creating a vacancy thatis filled by an outer shell electron. Thisjump from an outer to inner shell results
in a change in energy that produces
either a x-ray or Auger electron. The
emitted x-ray has energy equal to thischange. The x-rays are then detectedby either a lithium-drifted silicondetector for an EDS system, or a gas
proportional counter detector for a
WDS system. A typical x-ray spectrum
collected with an EDS system is shownin Figure 10.
Backscattered electrons are the resultof beam electrons being scattered
back out of the sample. In this case,the incident beam electrons undergo anumber of scattering events within thespecimen in which very little energy islost, allowing these electrons to gomuch deeper into the sample thansecondary electrons and still emerge

Figure 11. Backscattered SEM image of an PbSn alloy showing
contrast based on the atomic number of the two components.
The brighter areas are Pb-rich. 5,000x, Scale bar = 1µm.

from the sample surface to be
detected. The percentage of beamelectrons that become backscatteredelectrons has been found to be
dependent on the atomic number ofthe material, which makes it a usefulsignal for analyzing the material
composition. Once these electrons
escape from the surface they aredetected by either the Everhart-Thornleydetector or a solid state detector. An
example of a backscattered image ofa PbSn alloy is shown in Figure 11.
AFM
Although an AFM does not provide
elemental analysis, it can supply
compositional information bydifferentiating materials based on
physical properties, such as stiffness,elasticity, compliance, friction,adhesion, magnetic and electrostatic
fields, carrier concentration,
temperature distribution, spreading
resistance, and conductivity. Manyof these techniques consist of looking
simultaneously at another signal while
performing standard AFM imaging.
One of the most common techniques
for mapping differences in materials
properties is PhaseImaging.
PhaseImaging is conducted during

TappingMode AFM operation bymonitoring the phase lag between theoscillating drive signal used to drive
the cantilever and the oscillating
detection signal from the photodiodedetector. This signal will indicate
differences in viscoelasticity and/or
adhesion across the imaged area.
This technique is commonly applied to
mapping the distribution of polymers ina heterogeneous system, or mapping
the distribution of filler, such as silica
or carbon black, in a polymer matrix.
An example of PhaseImaging on apolyethylene film is shown in Figure
12. Other ways to get similar
information include Force Modulation
AFM, which maps differences inelasticity across the sample surface,and Lateral Force Microscopy (LFM),
which maps differences in friction
across the sample surface.
There are also techniques that can beused to investigate long range forces
across the imaged area. Magnetic
Force Microscopy (MFM) and ElectricForce Microscopy (EFM) map themagnetic and electrostatic field
gradients, respectively, which extend
from the sample surface. These
techniques are performed by using

either a magnetic or conductive probe
to map the attractive and repulsive
forces between the tip and the sample.
MFM is commonly used to detect thedomain structure of magnetic bits
written on magnetic media, to evaluate
the performance of magnetic heads,
and to investigate the magnetic
structure of experimental materials.
This is conducted by a routine called
LiftMode in which a TappingMode
topographic image and a magnetic
image are acquired over the same
area. LiftMode consists of first collecting
a line scan in TappingMode of thesurface topography. The tip is thenlifted above the surface and a second
scan is made over the same line using
the saved topographic scan to maintaina constant tip-sample separation. The
long-range magnetic forces shift theresonance frequency of the oscillating
cantilever, which is detected to produce
the magnetic image. An example ofbits written on a textured hard disk isshown in Figure 13.
Although the AFM is applied so that it
is nondestructive to the sample surface,it can be used to study differences inmechanical properties by performing
nanoindention to investigate hardness
differences between materials. This

technique uses a diamond tip mounted
on a stiff, stainless steel cantilever. A
TappingMode AFM image is collectedwith the probe to determine the area ofinterest for indentation, the nanoindention is then made at a specified
force, and an image is then collectedof the indented area. An example ofcomparing the difference between
diamond like carbon films on a harddisk is shown in Figure 14. In thisexample, the two films demonstrate
a difference in hardness fromindentations made at the same forces,
producing different sized indents.
Scratching and wear testing may also
be conducted with this configuration toinvestigate adhesion and delaminationof films under a small applied force.

Environment
One of the primary differences
between these two types of microscopy
is the environment in which they are
performed, i.e., SEM is only conductedin a vacuum environment. In addition
to vacuum, AFM is conducted in an
ambient, gas or liquid environment.
There are several issues which makeenvironment an important issue. First,
there is a frequent need in fields suchas biology and biomaterials to study

Figure 12. Phase image of two components which are used to form apolyethelene (PE) film. The phase image (right) clearly shows the distributionof the two polymers due to differences in stiffness which is not evident fromthe topographic image (left). 2µm scan.

hydrated samples. These twotechniques compensate for this needby different means: an environmental
chamber for a SEM, and a fluid cellfor the AFM. Second, the SEM isrequired to work in a vacuum
environment due to the nature of thetechnique which brings up the issuesof vacuum compatibility of the sample,
the conductivity of the surface. To
image poorly conductive surfaces
without sample charging may require
conductive coatings or staining,which may alter or obscure thefeatures of interest; or it may
require low-voltage operation or anenvironmental chamber, which may
sacrifice resolution.
For SEM, hydrated samples are
addressed by placing a specimen in
an environmental chamber with either
an electron transparent window or asmall aperture for the beam to enter
the chamber. The chamber is typicallyflushed with an inert gas saturated withwater vapor. Common applications areto either investigate hydrated surfaces
to preserve their surface structure when
hydrated, or to reduce charging on
insulating samples. An example ofimaging of a pesticide film on skin canbe seen in Figure 15. For the electron

Figure 13. Magnetic Force Microscopy (MFM) image of overwritten
tracks on a textured hard disk. The topography (left) was imaged using
TappingMode; the magnetic force image of the same area (right) was
captured with LiftMode (lift height 35 nm) by mapping shifts in cantileverresonant frequency. 25µm scan. (17)Figure 14. Indentations on two different diamond-like carbon thin films
using three different forces (23, 34, and 45µN) with four indents madeat each force to compare differences in hardness. 500nm scans.

beam to interact with the surface inthis configuration, it must go though anenvironment of gas and water vapor.
One drawback of this configurationis that it will result in an increase in
scattering of the electron beam on theway to and from the surface, which
may result in the sacrifice of image
quality and resolution.
One of the primary attractions to the
AFM is its ability to image insulating
surfaces at high resolution in liquid.
Imaging samples in a hydrated statewith an AFM is commonly performedby enclosing the sample and probe
in a liquid environment, as shown in
Figure 16. Since AFM does not rely
on conductivity, the image andscanning mechanism are not disturbed
by the presence of the liquid. Common
applications for AFM investigations
in liquid are in the biological sciences,
biomaterials, crystal growth, force
interaction studies, and forinvestigating processes in-situ (Figures
17, 18). The resolution of the image
will be determined by the radius of thetip, the applied force, and the noise
floor of the instrument. Because of

Figure 15. Environmental SEM image of a pesticide film onskin. A hydrated environment was needed in order tomaintain the integrity of the pesticide layer and to reduce
charging. Bar = 1mm.

these factors, this configuration allows
the study of hydrated specimens at alateral resolution of 1 to 5nm and a
vertical resolution down to 0.5Å
without sample damage, as seen in
the image of the GroES chaperon(Figure 17). With the appropriate
accessories, AFM can also be used invaried gaseous environments and atelevated temperatures. The latter isparticularly important for research anddevelopment of polymers.

Further Discussion
One thing to keep in mind whencomparing these two techniques is that
although SEM and AFM appear very
different, they actually share a numberof similarities. Both techniques rastera probe across the surface to detectsome interaction with the surface toform an image. Both have a lateral
resolution which is similar in scale(although under certain conditions
AFM is superior). And both techniques
have image artifacts that the operator
is trained to identify. The SEM has had
a much longer time to mature as atechnique and we have developed
good understanding of how to identify
and avoid artifacts, but the rapid
adoption and implementation of AFM
has resulted in a similar understandingof artifacts. This article has avoided
discussing such artifacts unless they
are relevant to the comparison.
Furthermore, by using two techniques
which are complementar y, one
technique will often compensate for theimaging artifact of the other technique.
However, one should be wary of
combined systems in which an AFM isplaced inside the SEM chamber. Oneof the true advantages of the AFM is
its ability to perform high resolution
measurements outside of a vacuum
environment. Placing it inside a vacuum
environment reduces its flexibility andincreases its operating time. Combined
AFM/SEM systems often have reduced
capabilities and typically compromise
the performance of both instruments.

Figure 16. Fluid cell for an AFM
which allows imaging in an
enclosed, liquid environment.

Summary
SEM and AFM are complementary
techniques that provide a more
complete representation of a surface
when used together than if each were
the only technique available. These
techniques overlap in their capabilities
to provide nanometer scale lateral
information. However, they deviate
in the fact that the AFM can provide
measurements in all three dimensions,
including height information with avertical resolution of <0.5Å, whereas
the SEM has the ability to image very
rough samples due to its large depthof field and large lateral field of view.

The SEM can provide elemental
analysis using X-ray detection, whereas
the AFM can provide compositional
information based on physicalproperties. The fact that the twotechniques operate in different
environments can be a strength
when used together since the AFM
does not encounter vacuum issues(difficult sample preparation, sample
modification, etc.) and may image
samples in an enclosed fluid or otherenvironment. The vacuum environmentof the SEM makes it possible toconduct a number of techniques that
require vacuum, such as X-ray analysis.

Figure 17. Image of two GroES molecules
positioned side-by-side in physiologic fluid,
demonstrating 10Å lateral resolution and1Å vertical resolution. The entire molecule
measures 84Å across, and a distinct 45Å
heptameric “crown” structure protrudes 8Å
above the remaining GroES surface andsurrounds a central depression.
18nm scan. Image courtesy of Z. Shao,
University of Virginia.18

By having both techniques side-by-side
in an analytical facility, the overall
scope of analytical capabilities isbroadened, adding to the flexibilityof the facility.

Figure 18. Living human vascular endothelial cells imaged in culture media by TappingMode AFM. These images, collected at 30 minuteintervals, reveal the movement of living cells which were incubated with 200ng/ml vascular endothelia growth factor (VEGF). With
TappingMode, the nucleus as well as other submembraneous structures are visible. These cells appear flatter and more elongated compared
to control (untreated) cells. 65µm scans. Sample courtesy of Georges Primbs, Miravant Inc.

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