existence for over two decades,
SPMs are the newest entry into thesurface metrology field. As opposedto optical microscopes and electron
microscopes (SEMs, TEMs), SPMs
measure surfaces in all three
dimensions: X, Y, and Z. Like SEMs,SPMs image and measure thesurface of the sample. X and Y
topographic resolution for mostSPMs, including AFMs, is typically
2 to 10 nanometers (STM resolution
can be as good as 0.1nm). Z
resolution is about 0.1nm for awell-designed AFM or STM.
Optical microscopes and SPMs
are the easiest to use, with littleor no sample preparation andno vacuum required. Optical
microscopes and SEMs can havelarger fields of view, but SPMs
provide the highest magnifications
and resolution in 3D. Furthermore
only SPMs work on most samples
with minimal sample preparation.

Brief History
The scanning tunneling microscope(STM) was the first SPM technology
and was recognized as having
atomic resolution capability in 1981.
STM, in fact, still provides the best
resolution available (Figure 1). The
STM uses the tunneling current
between tip and sample to image
the sample surface. Unfortunately,there are some limitations, the mostFigure 1. STM image showing single-atom
defect in iodine adsorbate lattice on platinum.
2.5nm scan. Courtesy Purdue University.
2.

a.

c.

b.

d.

Figure 2. (a) Simplified diagram of a generic AFM. Photos show examples of (b) MultiModeSPM, (c) Dimension 3100 SPM, and (d) fully automated robotic Dimension X3D system forsemiconductor applications.

significant of which is that the surfaceof the sample must be conductors or
semiconductors. This limits thematerials that can be studied.
This limitation led to the invention
in 1986 of the first atomic force
microscope. The first commerciallyavailable AFM, the Digital®
Instruments NanoScope wasintroduced in 1989. Like the STM,
the AFM also uses a very sharp tip
to probe and map the morphology ofa surface. However, in AFM there is
no requirement to measure a current
between tip and sample. In thiscase, the tip is at the end of a
micro-fabricated cantilever with alow spring constant. In contact mode
AFM, the first AFM technique, thetip-sample force is held fixed bymaintaining a constant and very lowdeflection of the cantilever, pushing
the tip against the sample. This force
can be in the range of interatomic
forces in solids.
Next, we describe the basics ofAFM, including how the verticalmotion of the tip is detected andtransformed into topographic data.

Basic AFM Components
The basic AFM is relatively simple
in concept (Figure 2a). Its closest
predecessor is the stylus profiler.
AFM technology uses sharper probes
and lower forces than stylus profilers
to provide higher resolution
information without sample damage.
A generic AFM comprises thefollowing components:

Scanning SystemThe most fundamental componentof the AFM and the heart of themicroscope is the scanner. Dependingon the individual design, the scanner
may scan (move) the sample(Figure 2b, MultiMode™ SPM) if thesample is small enough, or it may
scan the probe over a larger sample(Figure 2c, Dimension™ 3100 SPM).
To accomplish the precision required,a piezoelectric tube scanner istypically used in order to provide
sub-Ångstrom motion control.

Probe
Another key component in the systemis the probe. As mentioned above,
the probe can be stationary and thesample can be scanned under it or
the probe can be scanned over thesample. With today’s sophisticatedtechnology, tip/cantilever assemblies
that make up the probe (Figure 3) canbe mass-produced with consistently
shaped, very sharp tips. These tips
are integrated into the end ofcantilevers, which have a wide rangeof properties designed for a varietyof applications.

Probe Motion Sensor
This unit senses the force between
the probe and the sample andprovides a correction signal to theZ portion of the piezoelectric scanner(Figure 2a) to keep the force constant.
The most common design for thisfunction is called the optical beam

deflection system, which is the lowest
noise, most stable, and most versatilesystem available. This design uses alaser beam shining onto and reflecting
off the back of the cantilever and onto
a segmented photodiode to measure
the probe motion.

Controller Electronics
This unit provides interfacing between
the computer, the scanning system,and the probe motion sensor. It
supplies the voltages that control thepiezoelectric scanner, accepts thesignal from the probe motion sensor,
and contains the feedback control
electronics for keeping the forcebetween sample and tip constant.

Noise Isolation

Figure 3. SEM micrograph of an etchedsingle-crystal silicon AFM tip andtip/cantilever assembly

To achieve the highest resolution, themicroscope must be isolated fromnoise in its surroundings. There arevery effective, yet simple systems for
isolating AFMs from floor vibrations
and from acoustic, electrical, andoptical noise sources.

Computer
Finally, scanning probe microscopy
and AFM would not be feasible
without the availability of powerful,high-speed PCs to drive the systemand to process, display, and analyzethe wealth of data produced.
Figure 4. Detailed
topography of three defects –two protrusions and a
depression – in a phase-shift
photolithography mask. Across section measures thesmaller of the two protrusions(A) ~140nm across in theplane of the image. The
depression defect (B)measures less than 6nm
deep. 1.5μm scan.

3.

Applications/Scanning
Techniquesa.

b.
Figure 5. AFM (a) and LiftMode MFM (b)images of pole tip region on magnetoresistive (MR) read/write head used incomputer hard drives. MFM image shows
domain structure and MR sensor that cannot
be seen in the AFM topography. 12μm scan.

In its short lifetime, SPM has already
added many variations to thefundamental scanning tunnelingtheme. Once the AFM overcame thesevere application limit of STM (thesample conductivity requirement), thevariety of techniques and the rangeof applications began to mushroom.
Although topographic mapping is still
the dominant application for AFM(Figure 4), commercially available
SPMs now provide some or all of thefollowing techniques:• Scanning Tunneling Microscopy(STM): Measures topographyusing the tunneling current
between the probe tip and a
conductive sample surface.
• Contact Mode AFM: Measures
topography with the probe
perpetually in contact withthe sample.
• TappingMode AFM (patented):
Measures topography by lightly
tapping the surface with an
oscillating probe tip. Eliminates
shear forces (present in contactmode). TappingMode is now thescanning mode of choice for mostapplications, particularly for softer
surfaces like polymers.
TMFigure 6. Scanning electrochemical
potential microscope (SECPM).

• Non-contact Mode AFM:
Measures topography bysensing Van der Waals attraction
between surface and probe tip.
It is less stable than either contact
or TappingMode.
• LiftMode (patented): Two-pass
technique that separately measures
topography and another selected
property (magnetic force, electricforce, etc.) using topographic
information to track the probe
tip at a constant distance above
the surface.
TMAngstrom scale

Figure 7. TRmode is a technique that uses
torsional oscillations of an AFM probe.

4.

• PhaseImaging (patented):
Maps surface composition
based on differences in local
mechanical or adhesive propertiesof the sample.
TM• Lateral Force Microscopy (LFM):
Maps frictional forces between theprobe tip and the sample surface.
The tip can be functionalized withchemical species for chemical
force microscopy.
• Magnetic Force Microscopy(MFM): Maps magnetic force
gradient and distribution above
the sample surface using LiftMode(Figure 5).
• Force Modulation (patented):
Maps relative stiffness of
surface features.
• Electric Force Microscopy (EFM):
Maps electric field gradient anddistribution above the sample
surface using LiftMode.
• Surface Potential Imaging:This is one of the few AFM
techniques that makes quantifiablemaps of a quantity other thansurface topography. Using
LiftMode, it maps the distributionof surface electric potential of
the sample. Recent applications
include corrosion studies of alloys.
• Electrochemical SPM:
Maps topographic changes in-situ
as induced by electrochemical
reactions in electrolyte solutions
simultaneously with electrochemicalcell potential control (e.g.,voltammetry). Can be performed
with AFM or STM.
• Scanning Electrochemical
Potential Microscopy (SECPM)(patented): In-situ imaging or
potential mapping of the electrodesurface by measuring thepotential difference between thepotentiometric probe and theFigure 8. Condensed deoxyribonucleic acid
(DNA) has been proposed as a gene delivery
mechanism for biotechnology applications.
Here, unfixed molecules were imaged in saltsolution. 20μm scan.

sample immersed in an electrolyte
solution or a polar liquid(Figure 6).
• Scanning CapacitanceMicroscopy (SCM) and Scanning
Spreading Resistance Microscopy(SSRM): Maps 2D carrier (dopant)concentration profiles insemiconductor materials.
• Scanning Thermal Microscopy(SThM): Maps surface
temperature distribution.
• Tunneling AFM and ConductiveAFM: Measure tip-sample current
for characterization of electricalconductivity and evaluation of thinfilm integrity.
• TRmode: Maps lateral forces andforce gradients. Interleaves with
TappingMode for complementary
lateral and verticalcharacterization (Figure 7).
• Nanoindenting: Measuresmechanical properties andwear characteristics (hardness,adhesion, durability) of thinfilms, polymers, etc. (e.g.,dielectrics, DLC).

These techniques are being applied
to a wide array of application areas,from biology to semiconductors, fromdata storage devices to polymers,
and from integrated optics tomeasurement of forces between
particles and surfaces. Other
applications include MEMS
fabrication, paints and coatings,
metals/alloys/platings,
plastics/polymers, biomaterials,
biotechnology, food and foodpackaging, optics/optical films,
optical disks, ceramics, thin films,
liquid crystals, cosmetics, andgeological and environmental studies.
In addition, AFM systems havealready been developed for highly
specific applications, including
automated robotic systems forhandling semiconductor wafers(Figure 2d). They have also beendeveloped with analysis routinesdesigned for specific applications
such as CD and DVD bump/pit
measurements, as well as pole tip
recession measurements for data
storage read/write head
manufacturing. These applications
continue to expand.

a.

b.
Figure 9. Successive phase images ofpoly(hexacyclodimethyl)siloxane at (a) 85ºC
and (b) 90ºC. Heating induces formation ofliquid islands within the amorphous polymers(a), which convert into arrays of small dots
on additional heating (b). 10μm scans.

Environmental Controls
AFM applications are carried out in avariety of environments. AFMs can be
operated in ambient air, in vacuum,and in liquids (Figure 8). Biologicalmeasurements, in particular, are often
carried out in-vitro in liquids. Electrochemical experiments are performedin liquid cells, allowing atomic-scaleobservation of electrochemical
processes. In some cases, surface
cleaning studies are done at
atmospheric pressure in the controlledenvironment of a dry glove box.

5.

Recent new products include heating
systems for biological and polymer
applications up to 250°C (Figure 9),complete with sophisticated sample
and environmental sensing. Systems
are also now available for controllingthe gaseous environment of the sample
under study (Figures 10a and b).

Recent Technology Advances

Figure 10a. (a.) The
Atmospheric Hood for theMultiMode SPM allows control of
the gaseous imaging environment
to vary humidity or image underinert gases.

Figure 10b. The EnviroScope offers highvacuum, heating, electrochemical cell potential
control, and purged gas environment.

New hardware and software haveextended the utility of high-end SPM
systems beyond measurement andcharacterization to include nanomanipulation and nanolithography.
Examples of in-plane and out-of-planenanomanipulation are shown in
Figures 11a and 11b. An example
of point-and-click nanolithography isseen in Figure 11c.
New controllers and electronics(e.g., the NanoScope IV and IVa
SPM controllers) have been designed

Figure 11a. AFM in-planenanomanipulation uses the AFM probe
to image, manipulate nanometer-scale
objects (carbon nanotubes), and image
again to see the results.

6.

Figure 11b. AFM out-of-planenanomanipulation uses the AFM probe
to image, pull a single biomoleculeout of the plane of the sample whilemeasuring the unfolding of themolecule, and image again to see theresults (in this case, the removal of one
molecule from an array).

Figure 11c. AFM nanolithography.

to enhance performance relative
to traditional designs. Some of therecent developments in AFM
technology include:

Combined Environmental Controls
The latest generation of SPMs offer
combinations of environmentalcontrols, including vacuum and hightemperature (Figure 12).

Higher lateral resolution
AFM systems now provide higher
data density to allow zooming into
the finest details, even on large scans.
This provides the resolution required
to characterize sidewalls on such
samples as DVD bumps/pits andsemiconductors. It also allows
observation and measurement of
nanoscale details on large scans —without the need to spend additionaltime re-scanning the sample with asmaller scan area (Figure 13).

“Q“– control
Controlling the quality factor, or Q,of the oscillating AFM probe allows
better control of the forces between
tip and sample and improves thesensitivity of measurements suchas with PhaseImaging and MFM(Figure 14).

a.

b.
Figure 12. Poly-sbs at room temperature inair (a) and at 180°C in 10-5 Torr pressure(b). Images captured with the Enviroscope(Figure 10b).

Figure 13. TappingMode+ height image
and zoom of a copolymer. The square
image is a zoom into the boxed area in theoriginal rectangular image. This detail isrevealed by simply zooming in with software
and without the need for time-consuming,
repetitive smaller scans. Without this higher
resolution scanning, the zoomed image
would not have the pixel resolution required
to view nanoscale details. 10μm x 1.24μm
scan and 1μm x 1μm zoom.

Figure 14. Images of the same area on magnetic recording tapescanned with and without Q-control. Phase detection MFM images andaverage cross-section measurements of the probe phase shift illustratenearly 4x enhanced signal-to-noise ratio for the Q-controlled image.
15μm scans.
7.