Bhushan B. Nanotribology and Nanomechanics: An Introduction

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110 Bharat Bhushan and Othmar Marti

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Probes in Scanning Microscopies

Jason H. Hafner

Summary. Scanning probe microscopy (SPM) provides nanometer-scale mapping of nu-

merous sample properties in essentially any environment. This unique combination of high

resolution and broad applicability has lead to the application of SPM to many areas of sci-

ence and technology, especially those interested in the structure and properties of materials

at the nanometer scale. SPM images are generated through measurements of a tip-sample

interaction. A well-characterized tip is the key element to data interpretation and is typically

the limiting factor.

Commercially available atomic force microscopy (AFM) tips, integrated with force sens-

ing cantilevers, are microfabricated from silicon and silicon nitride by lithographic and

anisotropic etching techniques. The performance of these tips can be characterized by imag-

ing nanometer-scale standards of known dimension, and the resolution is found to roughly

correspond to the tip radius of curvature, the tip aspect ratio, and the sample height. Although

silicon and silicon nitride tips have a somewhat large radius of curvature, low aspect ratio,

and limited lifetime due to wear, the widespread use of AFM today is due in large part to the

broad availability of these tips. In some special cases, small asperities on the tip can provide

resolution much higher than the tip radius of curvature for low-Z samples such as crystal

surfaces and ordered protein arrays.

Several strategies have been developed to improve AFM tip performance. Oxide sharp-

ening improves tip sharpness and enhances tip asperities. For high-aspect-ratio samples such

as integrated circuits, silicon AFM tips can be modiﬁed by focused ion beam (FIB) milling.

FIB tips reach 3 degree cone angles over lengths of several microns and can be fabricated

at arbitrary angles. Other high resolution and high-aspect-ratio tips are produced by electron

beam deposition (EBD) in which a carbon spike is deposited onto the tip apex from the back-

ground gases in an electron microscope. Finally, carbon nanotubes have been employed as

AFM tips. Their nanometer-scale diameter, long length, high stiﬀness, and elastic buckling

properties make carbon nanotubes possibly the ultimate tipmaterial for AFM. Nanotubes can

be manually attached to silicon or silicon nitride AFM tips or “grown” onto tips by chemical

vapor deposition (CVD), which should soon make them widely available. In scanning tun-

neling microscopy (STM), the electron tunneling signal decays exponentially with tip-sample

separation, so that in principle only the last few atoms contribute to the signal. STM tips are,

therefore, not as sensitive to the nanoscale tip geometry and can be made by simple mechan-

ical cutting or electrochemical etching of metal wires. In choosing tip materials, one prefers

hard, stiﬀ metals that will not oxidize or corrode in the imaging environment.

112 Jason H. Hafner

3.1 Introduction

In scanningprobe microscopy(SPM), an image is created by raster scanning a sharp

probe tip over a sample and measuring some highly localized tip-sample interaction

as a function of position. SPMs are based on several interactions, the major types

includingscanning tunnelingmicroscopy(STM), whichmeasures an electronic tun-

neling current; atomic force microscopy (AFM), which measures force interactions;

and near-ﬁeld scanning optical microscopy (NSOM), which measures local opti-

cal properties by exploiting near-ﬁeld eﬀects (Fig. 3.1). These methods allow the

characterization of many properties (structural, mechanical, electronic, optical) on

essentially any material (metals, semiconductors, insulators, biomolecules) and in

essentially any environment (vacuum,liquid, or ambient air conditions).The unique

combinationof nanoscale resolution,previously the domain of electron microscopy,

and broad applicability has led to the proliferation of SPM into virtually all areas of

nanometer-scale science and technology.

Several enabling technologies have been developed for SPM, or borrowed from

other techniques. Piezoelectric tube scannersallow accurate, sub-angstromposition-

ing of the tip or sample in three dimensions. Optical deﬂection systems and micro-

fabricated cantilevers can detect forces in AFM down to the picoNewton range. Sen-

sitive electronics can measure STM currents less than 1 picoamp. High transmission

ﬁber opticsand sensitive photodetectorscan manipulateand detect small opticalsig-

nals of NSOM. Environmentalcontrol has been developedto allow SPM imaging in

UHV, cryogenic temperatures, at elevated temperatures, and in ﬂuids. Vibration and

drift have been controlled such that a probe tip can be held over a single molecule

for hours of observation. Microfabrication techniques have been developed for the

mass production of probe tips, making SPMs commercially available and allowing

the development of many new SPM modes and combinations with other characteri-

zation methods. However,of all this SPM development over the past 20 years, what

has received the least attention is perhaps the most important aspect: the probe tip.

Interactions measured in SPMs occur at the tip-sample interface, which can

range in size from a single atom to tens of nanometers. The size, shape, surface

chemistry, electronic and mechanical properties of the tip apex will directly inﬂu-

ence the data signal and the interpretation of the image. Clearly, the better charac-

e–

Computer

Feedback

electronics

STM

SFM

NSOM

Tip

head

Sample

Piezo tube

scanner

Fig. 3.1. A schematic of the

components of a scanning

probe microscope and the

three types of signals ob-

served: STM senses electron

tunneling currents, AFM

measures forces, and NSOM

measures near-ﬁeld opti-

cal properties via a sub-

wavelength aperture

3 Probes in Scanning Microscopies 113

terized the tip the more useful the image information. In this chapter, the fabrication

and performance of AFM and STM probes will be described.

3.2 Atomic Force Microscopy

AFM is the most widely used form of SPM, since it requires neither an electrically

conductivesample, as in STM, noran optically transparentsample or substrate, as in

most NSOMs. Basic AFM modes measure the topographyof a sample with the only

requirement being that the sample is deposited on a ﬂat surface and rigid enough

to withstand imaging. Since AFM can measure a variety of forces, including van

der Waals forces, electrostatic forces, magnetic forces, adhesion forces and friction

forces, specialized modes of AFM can characterize the electrical, mechanical, and

chemical properties of a sample in addition to its topography.

3.2.1 Principles of Operation

In AFM, a probe tip is integrated with a microfabricated force-sensing cantilever.

A variety of silicon and silicon nitride cantilevers are commercially available with

micron-scale dimensions, spring constants ranging from 0.01 to 100N/m, and res-

onant frequencies ranging from 5 kHz to over 300 kHz. The cantilever deﬂection is

detected by optical beam deﬂection, as illustrated in Fig. 3.2. A laser beam bounces

oﬀ the back of the cantilever and is centered on a split photodiode. Cantilever de-

ﬂections are proportionalto the diﬀerence signal V

−V

. Sub-angstromdeﬂections

can bedetected and, therefore,forcesdownto tens of picoNewtonscan bemeasured.

A more recently developed method of cantilever deﬂection measurement is through

a piezoelectric layer on the cantilever that registers a voltage upon deﬂection [1].

A piezoelectric scanner rasters the sample under the tip while the forces are

measured through deﬂections of the cantilever. To achieve more controlled imaging

conditions, a feedback loop monitors the tip-sample force and adjusts the sample

Z-position to hold the force constant. The topographic image of the sample is then

Fig. 3.2. An illustration of

the optical beam deﬂection

system that detects cantilever

motionintheAFM.The

voltage signal V

−V

proportional to the deﬂection

114 Jason H. Hafner

taken from the sample Z-position data. The mode described is called contact mode,

in which the tip is deﬂected by the sample due to repulsive forces, or “contact”.

It is generally only used for ﬂat samples that can withstand lateral forces during

scanning. To minimize lateral forces and sample damage, two AC modes have been

developed. In these, the cantilever is driven into AC oscillation near its resonant fre-

quency (tens to hundreds of kHz) with amplitudes of 5 to tens of s. When the tip

approaches the sample, the oscillation is damped, and the reduced amplitude is the

feedback signal, rather than the DC deﬂection. Again, topography is taken from the

varying Z-position of the sample required to keep the tip oscillation amplitude con-

stant. The two AC modes diﬀer only in the nature of the interaction. In intermittent

contact mode, also called tapping mode, the tip contacts the sample on each cycle,

so the amplitude is reduced by ionic repulsion as in contact mode. In noncontact

mode, long-range van der Waals forces reduce the amplitude by eﬀectively shifting

the spring constant experienced by the tip and changing its resonant frequency.

3.2.2 Standard Probe Tips

In early AFM work, cantilevers were made by hand from thin metal foils or small

metal wires. Tips were created by gluing diamond fragments to the foil cantilevers

or electrochemically etching the wires to a sharp point. Since these methods were

labor intensive and not highly reproducible, they were not amenable to large-scale

production.To addressthis problem,and the need forsmaller cantileverswith higher

resonant frequencies, batch fabrication techniques were developed (see Fig. 3.3).

Building on existing methods to batch fabricate Si

cantilevers,Albrecht et al. [2]

etched an array of small square openings in an SiO

mask layer over a (100) silicon

surface. The exposed square (100) regions were etched with KOH, an anisotropic

etchant that terminates at the (111) planes, thus creating pyramidal etch pits in the

silicon surface. The etch pit mask was then removed and another was applied to

deﬁne the cantilever shapes with the pyramidal etch pits at the end. The Si wafer

was then coated with a low stress Si

layer by LPCVD. The Si

ﬁlls the etch

pit, using it as a mold to create a pyramidal tip. The silicon was later removed by

etching to free the cantilevers and tips. Further steps resulting in the attachment of

Apply

mask

Anisotropic

etch

Remove

mask

Deposit

Remove Si

tip fabrication

cantilever

Apply

mask

Under

etch

Remove

mask

tip fabrication

Fig. 3.3. A schematic

overview of the fabrication of

Si and Si

tip fabrication

as described in the text

3 Probes in Scanning Microscopies 115

the cantilever to a macroscopic piece of glass are not described here. The resulting

pyramidal tips were highly symmetric and had a tip radius of less than 30 nm, as

determined by scanning electron microscopy (SEM). This procedure has likely not

changed signiﬁcantly, since commercially available Si

tips are still speciﬁed to

have a curvature radius of 30nm.

Wolter et al. [3] developed methods to batch fabricate single-crystal Si can-

tilevers with integrated tips. MicrofabricatedSi cantileverswere ﬁrst prepared using

previously described methods, and a small mask was formed at the end of the can-

tilever. The Si around the mask was etched by KOH, so that the mask was under cut.

This resulted in a pyramidal silicon tip under the mask, which was then removed.

Again, this partial description of the full procedure only describes tip fabrication.

With some reﬁnements the silicon tips were made in high yield with curvature radii

of less than 10 nm. Si tips are sharper than Si

tips, because they are directly

formed by the anisotropic etch in single-crystal Si, rather than using an etch pit as

a mask for deposited material. Commercially available silicon probes are made by

similar reﬁned techniques and provide a curvature typical radius of < 10 nm.

3.2.3 Probe Tip Performance

In atomic force microscopy the question of resolution can be a rather complicated

issue. As an initial approximation, resolution is often considered strictly in geo-

metrical terms that assume rigid tip-sample contact. The topographical image of

a feature is broadened or narrowed by the size of the probe tip, so the resolution is

approximately the width of the tip. Therefore, the resolution of AFM with standard

commercially available tips is on the order of 5 to 10nm. Bustamante and Keller [4]

carried the geometrical model further by drawing an analogy to resolution in optical

systems. Consider two sharp spikes separated by a distance d to be point objects

imaged by AFM (see Fig. 3.4). Assume the tip has a parabolic shape with an end

radius R. The tip-broadened image of these spikes will appear as invertedparabolas.

There will be a small depression between the images of depth Δz. The two spikes

are considered “resolved” if Δz is larger than the instrumental noise in the z direc-

tion. Deﬁned in this manner, the resolution d, the minimum separation at which the

spikes are resolved, is

d = 2



(

Δz

)

, (3.1)

where one must enter a minimal detectable depression for the instrument (Δz)to

determine the resolution. So for a silicon tip with radius 5 nm and a minimum de-

tectable Δz of 0.5nm, the resolution is about 4.5 nm. However, the above model

assumes the spikes are of equal height. Bustamante and Keller [4] went on to point

out that if the height of the spikes is not equal, the resolution will be aﬀected. As-

suming a height diﬀerence of Δh, the resolution becomes:

d =

√



√

Δz+

√

Δz+Δh



. (3.2)

116 Jason H. Hafner

–1.0 1.0 r/h

0.80 h

Δz

Δh

Scan direction Scan direction

Sample spikes

Inverted

tip surfaces

Tip Tip

Point

sample

Tip motion

Needle tip

Fig. 3.4. The factors that determine AFM imaging resolution in contact mode (top) and non-

contact mode (bottom), adapted from [4]

For a pair of spikes with a 2 nm height diﬀerence, the resolution drops to 7.2nmfor

a 5 nm tip and 0.5nm minimum detectableΔz. While geometricalconsiderationsare

a good starting point for deﬁning resolution, theyignore factors such as the possible

compression and deformation of the tip and sample. Ve senka et al. [5] conﬁrmed

a similar geometricalresolutionmodel by imaging monodispersegold nanoparticles

with tips characterized by transmission electron microscopy (TEM).

Noncontact AFM contrast is generated by long-range interactions such as van

der Waals forces, so resolution will not simply be determined by geometry because

3 Probes in Scanning Microscopies 117

the tip and sample are not in rigid contact. Bustamante and Keller [4] have derived

an expression for the resolution in noncontact AFM for an idealized, inﬁnitely thin

“line” tip and a point particle as the sample (Fig. 3.4). Noncontact AFM is sensi-

tive to the gradient of long-range forces, so the van der Waals force gradient was

calculated as a function of position for the tip at height h above the surface. If the

resolution d is deﬁned as the full width at half maximum of this curve, the resolution

is:

d = 0.8h . (3.3)

This shows that even for an ideal geometry, the resolution is fundamentally lim-

ited in noncontact mode by the tip-sample separation. Under UHV conditions, the

tip-sample separation can be made very small, so atomic resolution is possible on

ﬂat, crystalline surfaces. Under ambient conditions, however, the separation must

be larger to keep the tip from being trapped in the ambient water layer on the

surface. This larger separation can lead to a point where further improvements in

tip sharpness do not improve resolution. It has been found that imaging 5 nm gold

nanoparticles in noncontact mode with carbon nanotube tips of 2nm diameter leads

to particle widths of 12nm, larger than the 7nm width one would expect assuming

rigid contact [8]. However, in tapping mode operation, the geometrical deﬁnition

of resolution is relevant, since the tip and sample come into rigid contact. When

imaging 5nm gold particles with 2nm carbon nanotube tips in tapping mode, the

expected 7 nm particle width is obtained [9].

The above descriptions of AFM resolution cannot explain the sub-nanometer

resolution achieved on crystal surfaces [10] and ordered arrays of biomolecules [11]

in contact mode with commercially available probe tips. Such tips have nominal

radii of curvature ranging from 5 nm to 30 nm, an order of magnitude larger than

the resolution achieved. A detailed model to explain the high resolution on ordered

membraneproteins has been put forth by [6]. In this model, the larger part of the sili-

con nitride tip apex balances the tip-sample interaction through electrostatic forces,

while a very small tip asperity interacts with the sample to provide contrast (see

Fig. 3.5). This model is supported by measurements at varying salt concentrations

to vary the electrostatic interaction strength and the observation of defects in the or-

dered samples. However, the existence of such asperities has never been conﬁrmed

by independent electron microscopy images of the tip. Another model, considered

Support

Tip

Sample

Electrostatic

repulsion

Van der Waals

attraction

Fig. 3.5. A tip model to

explain the high resolution

obtained on ordered samples

in contact mode, from [6]

118 Jason H. Hafner

especially applicable to atomic resolution on crystal surfaces, assumes the tip is

in contact with a region of the sample much larger than the resolution observed,

and that force components matching the periodicity of the sample are transmitted

to the tip, resulting in an “averaged” image of the periodic lattice. Regardless of

the mechanism, the structures determined are accurate and make this a highly valu-

able method for membrane proteins. However, this level of resolution should not be

expected for most biological systems.

3.2.4 Oxide-Sharpened Tips

Both Si and Si

tips with increasedaspect ratio and reduced tip radius can be fab-

ricated through oxide sharpeningof the tip. If a pyramidalor cone-shaped silicon tip

is thermally oxidized to SiO

at low temperature (< 1050

◦

C), Si-SiO

stress forma-

tion reduces the oxidation rate at regions of high curvature. The result is a sharper,

higher-aspect-ratio cone of silicon at the high curvature tip apex inside the outer

pyramidal layer of SiO

(see Fig. 3.6). Etching the SiO

layer with HF then leaves

tips with aspect ratios up to 10:1 and radii down to 1nm [7], although 5–10nm is

the nominal speciﬁcation for most commercially available tips. This oxide sharpen-

ing technique can also be applied to Si

tips by oxidizing the silicon etch pits that

are used as molds. As with tip fabrication, oxide sharpening is not quite as eﬀec-

tive for Si

.Si

tips were reported to have an 11nm radius of curvature [12],

while commercially available oxide-sharpened Si

tips have a nominal radius of

< 20 nm.

3.2.5 FIB tips

A common AFM application in integrated circuit manufacture and MEMs is to im-

age structures with very steep sidewalls such as trenches. To accurately image these

features,one mustconsider themicron-scaletip structure,rather thanthe nanometer-

scale structure of the tip apex. Since tip fabrication processes rely on anisotropic

etchants, the cone half-angles of pyramidal tips are approximately 20 degrees. Im-

ages of deep trenches taken with such tips display slanted sidewalls and may not

reach the bottom of the trench due to the tip broadeningeﬀects. To image such sam-

ples more faithfully, high-aspect-ratio tips are fabricated by focused ion beam (FIB)

20.0 nm

Fig. 3.6. Oxide sharpening

of silicon tips. The left image

shows a sharpened core of

silicon in an outer layer of

SiO

.Theright image is

a higher magniﬁcation view

of such a tip after the SiO

removed. Adapted from [7]

3 Probes in Scanning Microscopies 119

machining a Si tip to produce a sharp spike at the tip apex. Commercially available

FIB tips have half cone angles of < 3 degrees over lengths of several microns, yield-

ing aspect ratios of approximately 10:1. The radius of curvature at the tip end is

similar to that of the tip before the FIB machining. Another consideration for high-

aspect-ratio tips is the tip tilt. To ensure that the pyramidal tip is the lowest part of

the tip-cantilever assembly, most AFM designs tilt the cantilever about 15 degrees

from parallel. Therefore, even an ideal “line tip” will not give an accurate image of

high steep sidewalls, but will producean image that depends on the scan angle. Due

to the versatility of the FIB machining, tips are available with the spikes at an angle

to compensate for this eﬀect.

3.2.6 EBD tips

Anothermethodof producinghigh-aspect-ratiotips for AFMis calledelectronbeam

deposition (EBD). First developed for STM tips [13, 14], EBD tips were intro-

duced for AFM by focusing an SEM onto the apex of a pyramidal tip arranged

so that it pointed along the electron beam axis (see Fig. 3.7). Carbon material was

deposited by the dissociation of background gases in the SEM vacuum chamber.

Schiﬀmann [15] systematically studied the following parameters and how they af-

fected EBD tip geometry:

Deposition time : 0.5to8min

Beam current: 3−300 pA

Beam energy : 1−30keV

Working distance : 8−48mm .

EBD tips were cylindrical with end radii of 20–40nm, lengths of 1 to 5µm, and

diametersof 100to 200nm.Like FIBtips,EBD tipswere foundto achieveimproved

imaging of steep features. By controlling the position of the focused beam, the tip

geometry can be further controlled. Tips were fabricated with lengths over 5µm

and aspect ratios greater than 100:1, yet these were too fragile to use as a tip in

AFM [13].

Fig. 3.7. A pyramidal tip

before (left,2-µm-scale bar)

and after (right,1-µm-scale

bar) electron beam deposition,

adapted from [13]