REVIEW PAPER

Atomic Force Microscopy as a Nanometrology Tool: Some Issuesand Future Targets

G. Raina*

Center for Nanotechnology Research, VIT University, Vellore 632014, Tamil Nadu, India

Received: 04 October 2013 / Accepted: 23 November 2013 / Published online: 1 December 2013

� Metrology Society of India 2013

Abstract: Diverse novel nanomaterials are being developed for a wide range of applications nowadays. Atomic force

microscopy (AFM) assumes specific importance for the measurement of size and other related properties for such

nanomaterials. The different aspects related to AFM modes of operation, nanopositioning, sensing systems as well as

calibration for reliable characterization in order to meet the nanometrology requirements are discussed. The future targets

in this context, set by nanometrology institutes, are also highlighted.

Keywords: Atomic force microscope; Nanometrology; Scanning probe microscopy; Cantilever; Tips

1. Introduction

With the rapid development of novel nanomaterials and the

critical dependence of their chemical, electrical, optical

and magnetic properties on the size, the role of atomic

force microscope (AFM) instrument as a nanometrology

tool has become very important and a necessity. In this

article, attention is focused to some grey areas associated

with the basic AFM modes of operation, nanopositioning

systems, sensing systems and calibration standards with

regard to its use as a nanometrology tool. Some future

targets to be achieved, for the realization of AFM as a

nanometrology tool, are also discussed.

Since the invention of scanning tunneling microscope

(STM), in 1982, to investigate crystalline conducting surfaces

with atomic resolution [1] and followed by the development

of AFM as a complimentary technique for non-conducting

surfaces [2], a plethora of techniques have emerged as an

extension to the basic AFM technique. At the heart of the

basic AFM technique lie two important concepts, namely, a

‘nanopositioning device’ and the ‘near-field interaction’

being probed when a microscopic tip and sample are brought

extremely close to each other. The nanopositioner is com-

monly referred to as the ‘scanner’ which holds either the

sample or the AFM probe and is responsible for the nanoscale

positioning in x–y plane and z-direction of either the sample or

the tip. With the slight modification in the property of the

material used for the AFM probe and by varying the distance

between the probe and sample from fraction of angstroms to

few hundreds of nanometers, a variety of short-range and

long-range near-field interactions can be probed (see Fig. 1).

This unique concept of probing a wide variety of ‘near-field

interactions’ have broadened the scope of the basic AFM

technique and is hence referred to as a family of scanning

probe microscopy (SPM) techniques, in which a sharp probe

is scanned over the surface and the corresponding probe-

sample interactions are monitored. Accordingly, a 3-dimen-

sional image contrast showing nanometer-scale variations in

surface coulombic forces, frictional, van der Waals, electro-

static and magnetic forces can be mapped. This opens the

possibility of measurements for a wide range of materials

including metals, semiconductors, insulators, polymers,

nanocomposites and biological samples.

Another unique aspect of AFM techniques is the fact that

they can be employed under different environmental con-

ditions such as ambient, under a liquid surrounding, in ultra

high vacuum [3, 4], under low and high temperatures [5],

magnetic fields [6] as well as under externally applied

electric field [7]. This is possible owing to the fact that the

near-field interactions such as tunneling current or inter-

atomic forces have a measurable magnitude under each of

the corresponding ambient, liquid surrounding and ultrahigh

vacuum conditions. Such functionality under diverse con-

ditions can be achieved with relatively simple modifications

to the basic AFM set-up, while maintaining the compact

design of the whole instrument.*Corresponding author, E-mail: gargiraina@vit.ac.in

M �APAN-Journal of Metrology Society of India (December 2013) 28(4):311–319

DOI 10.1007/s12647-013-0085-6

123

In addition to mapping nanometer scale 3-dimensional

variations of various near field interactions, local modifi-

cation of the surface can also be performed using various

SPM-based nanolithography techniques such as local

anodic oxidation [8], dip-pen nanolithography [9] and

mechanical ploughing at the nanoscale [10].

As the SPM techniques are rapidly advancing from

being just a high-resolution qualitative imaging tool to

providing quantitative data from spectroscopy measure-

ments for currents and a broad range of forces in the nano-

regime and pico-regime, it becomes imperative to achieve

nanoscale accuracies in the sensing mechanisms and the

positioning devices along with calibration using appropri-

ate reference standards. One of the main issues, currently,

is that though we are able to measure forces and currents in

nano- to pico-regimes, our uncertainties in measurement

also lie in the same range. Hence, there has been a con-

siderable impetus towards the standardization and trace-

ability for the SPMs over the last few years. Towards this

aspect, various National Metrology Institutes (NMIs)

throughout the world developed metrological AFMs

(MAFMs) designed to provide the highest accuracy for

dimensional measurements along with traceability to the SI

unit of length. The first effort towards the design of a

metrological STM was initiated as early as in 1994 at

Physikalisch-Technische Bundesanstalt (PTB), Germany

[11]. Metrological AFMs of wide x–y range and z-range

starting from 100 and 16 lm [12] to 10,000 and 20 lm

[13], respectively, have been developed over the past

couple of years.

2. AFM Modes of Operations

The basic operation of the AFM is implemented in two

different types of modes of operation viz. the ‘‘static or

contact’’ mode and the ‘‘dynamic’’ mode.

2.1. Contact or Static Mode

In contact-mode AFM operation, the vertical deflection of

the cantilever has to be determined precisely. The deflec-

tion of the cantilever Dz is given by

Dz ¼ Fint=k ð1Þ

Fig. 1 Schematic depicting the

type of forces detected when the

distance of the AFM probe is

varied from the sample surface

from fraction of A to few

hundreds of nm. Courtesy of

Veeco Instruments

312 G. Raina

123

where Fint is the interaction force between the tip and the

sample surface and k is the spring constant of the cantilever.

Flexible cantilevers with small spring constant, minimal

noise, along with an improved detection system are required.

2.2. Dynamic Mode

In the dynamic-mode AFM, the cantilever is made to

oscillate at its resonance frequency m0 with a free amplitude

of A0 away from the sample surface, in the vertical direc-

tion, using a bimorph attached to the cantilever base end.

The cantilever spring constant k and resonance eigen-fre-

quency x1 are determined using the below Eqs. (2) and (3),

respectively [14]:

k ¼ 1

4E

bcd3c

L3ð2Þ

x1 ¼ 1:0149d

L2

ffiffiffiffi

E

q

s

ð3Þ

where bc, L, dc, E and q are the breadth, length, thickness,

Young’s modulus and mass density of the cantilever,

respectively.

As the tip interacts with the sample, either the change in

the amplitude of oscillation or the change in the frequency

of oscillation of the cantilever is monitored. In the ampli-

tude modulation (AM) mode, the position of the cantilever

is varied to maintain constant amplitude while as in the

frequency modulation (FM) technique, the cantilever’s

position is varied to maintain the cantilever frequency

constant. AM technique is used more commonly under

ambient conditions and, the frequency modulation is pre-

ferred under vacuum conditions. The various parameters

which affect the dynamic mode AFM operation are the

methods used to excite the cantilever, dynamics of a

vibrating tip near the surface and the existence of a variety

of interaction forces such as short-range repulsive forces,

attractive long-range forces, capillary and electrostatic

forces. These have been reviewed in detail by Garcıa and

Perez [15].

The motivation behind the origin of these modes of

operation has been the type of samples being probed using

each technique. For ‘hard’ surfaces, contact-mode AFM

operation does not cause significant modification to the

surface, while as ‘soft’ surfaces require a non-contact mode

technique to avoid modification of the surface during

scanning. The nature of the near-field interaction forces for

the two modes of operation varies from Coulombic or Pauli

repulsive forces to van der Waals forces. For example, true

atomic resolution on the cleavage ð1014Þ plane of calcite,

in a liquid cell, was first demonstrated by Ohnesorge and

Binnig in 1993 [16] by imaging point-like defects with a

net force of less than 10-11 N, while as for larger imaging

forces, the monoatomic steps on calcite surface vanished

and the surface showed perfectly ordered lattice structure.

Also, it was demonstrated that when a Si tip is brought in

contact with a silicon surface inside a transmission electron

microscope (TEM) [17], the tips wear off during scanning.

Hence, there is a need to evolve some guidelines to

define what classifies to be ‘hard’ or ‘soft’ surface and the

nature and type of material used for the AFM tip as well as

the cantilever, suitable for imaging in ‘‘Contact-mode’’ and

‘‘Dynamic mode’’, respectively. The upper limits of

imaging forces for a particular sample surface with a given

tip and cantilever material to obtain accurate morphologi-

cal and topography information need to be established and,

in addition, to be verified by other complimentary surface

techniques. This includes defining the environment under

which the measurement procedure has to be carried out. It

is only then, can we retrieve reliable morphological

information for a non-standard sample using the appro-

priate ‘‘static’’ and ‘‘dynamic’’ modes of operation of the

AFM.

Use of AFM-based spectroscopy viz. ‘force versus dis-

tance’ curves in both static and dynamic modes provides

additional possibility of measuring forces in the nano-

Newton to pico-Newton regime besides obtaining topog-

raphy information about the surface. Recently, Kasas et al.

[18] have reviewed the determination of local nanome-

chanical properties of biological samples with high reso-

lution by exploiting AFM cantilevers as sensitive force-

sensors using force-distance curves. Force-volume maps

are generated by collecting several force-distance curves

all across a given scan area of the sample surface. How-

ever, the drawback of requiring long acquisition times with

limited temporal and spatial resolution has been overcome

recently in commercial AFMs by using ‘‘peak force tap-

ping’’ (Bruker) in which the tip is oscillated with a small

amplitude and high frequency using a sinusoidal waveform

so as to reduce acquisition time and give better resolution.

Another new method called the ‘rate-jump’ method has

been recently proposed, to determine stiffness information

with high precision for biological samples, by performing

an indentation measurement at a given speed up to a

defined depth. The tip is then retracted also at the defined

speed and the deformation of the cantilever in the retract

cycle is used to determine the sample’s stiffness [19].

However, there are several issues that are not yet fully

resolved such as the modeling of various types of inter-

action forces between a real tip and variety of sample

surfaces, which is quite a demanding exercise. Though

metrological AFMs provide working in both static and

dynamic modes, it is very difficult to make the measure-

ments traceable. In this regard, establishment of a data-base

containing spectroscopy data for different sample surfaces

along with all details about the cantilever and tip, clear

Atomic Force Microscopy as a Nanometrology Tool 313

123

specifications about the ambient conditions as well as all

other experimental parameters would help towards estab-

lishing some uniformity in the data interpretation obtained

from these high resolution AFM techniques.

3. AFM Nanopositioning Systems

The piezoelectric materials, employed in the construction

of scanners, are used to generate sub-nanometer scale

motion of either the sample or the tip in the X–Y and

Z directions based on the geometry of piezo scanner,

dimensions of the piezo-electrode elements and the type of

piezoelectric material used. Unfortunately, piezoelectrodes

inherently suffer from problems associated with creep,

hysteresis and non-linearity in their behaviour. These non-

linearity problems become predominant when large area

scan sizes are chosen. Commercial AFMs are produced

with either ‘‘open-loop’’ or ‘‘closed-loop’’ scanners.

‘‘Open-loop’’ operation of the scanner implies operation of

the scanner without positional feedback to compensate for

inaccuracies caused by the piezos during the scanning.

Generally, open-loop scanners provide low noise and very

high resolution. Nowadays, most commercial AFMs pro-

vide the option of ‘‘closed-loop’’ scanning, which implies

the piezo-scanners elements (viz. X-, Y- and Z-electrodes)

are constantly monitored by additional position sensors to

keep them at the desired position and thus the topography

images reflect true dimensions of the features as well as

enable to return the scanner at the exact position each time.

There are different strategies to measure accurately the

position of the piezo-elements namely, fiber–optic laser

interferometers, capacitive detection and linear variable

differential transformers and strain gauges. Capacitive

detection offers a simple and inexpensive solution to

monitor position if the scanning head motion is usually

\100 lm. A sensitivity of better than 10-3 nm was

reported by Griffith [20].

With the present stringent demand from industry to be

able to measure atomic scale dimensions over larger areas,

the work towards the development of nanopositioning and

nanomeasuring (NPM) instruments has become important.

In this regard, the first NPM machine with a measuring

range of 25 mm 9 25 mm 9 5 mm with nanometer reso-

lution has been in the process of development at Ilmenau

University of Technology (Germany) over the past few

years. They used optical interferometer sensors where three

orthogonally arranged plane mirror laser interferometers are

kept in a fixed position and the position of the probing tip is

fixed at the intersection point of the three perpendicular

measurement axes. He–Ne lasers with high frequency sta-

bility of ±1.5 9 10-10/24 h have been employed with

thermally controlled laser operation. Additionally, the

traceability to the frequency standard is also accomplished

by the use of these lasers. With such arrangements, they

have been able to achieve a standard deviation of the noise

in all three axes of the order of 0.3 nm [21]. However, there

is need for better optical interferometers in metrological

AFMs, where the non-linearities can be reduced below

10 pm level.

4. AFM Sensor Systems

4.1. Cantilevers and Tips

The choice of cantilevers depends on the AFM mode of

operation being employed. For contact-mode, the spring

constant of the cantilever should be low, comparable to

interatomic force constants, to be able to provide high

sensitivity to surface forces and thus, should be typically in

the range of 0.1–5 Nm-1. For dynamic mode, the reso-

nance frequency of the cantilevers should be very high for

low sensitivity to mechanical disturbances, accordingly

resonance frequency of 10–1,100 kHz with the typical

k values of 15–40 Nm-1 are desirable. High quality factor

‘Q’ cantilevers are preferred for dynamic mode of

operation.

The tip shape and size critically affect the AFM mea-

surement of the dimensions and geometry of nanostructures

while as the tip material governs the nature of interaction

between the tip and the sample as well as its stability and

wear. There has been a consistent effort to reduce the

nominal end tip radius for the pyramidal shaped micro-

fabricated integrated silicon nitride, silicon and silicon

oxide tips. Some of the methods employed to fabricate high

aspect ratio tips are focused ion beam and electron beam

deposition. Carbon nanotubes tips have been employed as

high aspect ratio tips for high resolution dimensional

measurements of surfaces with deep and narrow trenches

by either gluing them or growing them inside pores on

flattened Si by CVD technique. Additionally, the CNT tips

have the advantage of longer lifetime on account of higher

wear resistance. Si tips functionalized with variety of

chemical or biological molecules are used for determina-

tion of intermolecular forces and chemical sensing. Tip

deformation and wear during the scanning precludes any

possibility of knowing the exact shape of the tip during the

measurement. Recently, nanoscale wear of a silicon diox-

ide tip sliding on a copper substrate has been studied by

AFM and molecular dynamics simulation by Hu et al. [22].

They found that nanoscale wear can occur both in adhesion

and abrasion mode. The dominant mode is dependent on

the sliding distance and the load applied and the material

314 G. Raina

123

transfer from the substrate to the tip occurs in both modes,

with the transfer being more when the adhesion is the

dominant mode.

4.2. Cantilever Spring Constant Determination

Generally, the spring constant of the cantilevers is supplied

by the manufacturers as a range of values, which can vary

over orders of magnitude. Knowledge of the exact value of

spring constant is an essential requirement for force mea-

surements related to molecules and nanoindentation exper-

iments. There are different methods used for the calibration

of spring constant of cantilever material namely dimensional

method (valid for w � L, where w and L are cantilever

width and length, respectively), the cantilever-on-cantilever

method, the Sader method, and the thermal noise method

with varying degree of calibration uncertainty ranging from

5 to 30 %. Most commercial AFMs employ the ‘‘thermal

noise’’ method for calibration of spring constant. In this

method, the cantilever is treated as a simple harmonic

oscillator, where the cantilever spring constant kc is inver-

sely proportional to mean square deflection due to thermal

fluctuations and is given by Eq. (4) below,

kc ¼kbT

DZ2c

� � ð4Þ

where kb is the Boltzmann constant, T the temperature and

hDZ2c i is mean square deflection. This is obtained by per-

forming power spectral analysis of the cantilever oscilla-

tions and integrating the area under the first peak of the

oscillation.

Recently, a nanoforce calibrator method, was proposed

to traceably measure the spring constant with uncertainties

of better than 1 % by Kim et al. [23]. For this a primary

standard for forces at micro- or nano-Newton level, based

on the mass standard, developed at Korea Research Insti-

tute of Standards and Science (KRISS) was used and the

deflection of the cantilever was measured using capacitive

sensor calibrated by laser interferometer.

4.3. Tip Characterization

Some ex situ and in situ techniques have been developed,

to characterize and reconstruct the shape as well as size of

the tip. Scanning electron microscopy (SEM) and TEM

have been used to examine the boundary of the tip shape.

However, these techniques only provide the projection

shape of the tip and not its actual three-dimensional shape.

Moreover the high energy electrons used in these tech-

niques may further modify the morphology of the nano-

scale tip end. Hence, such techniques have limited

usefulness in the determination of the actual tip shape and

size during the scanning of the corrugated sample.

The in situ tip characterization techniques involve using

a sample with well-defined nanoscale features to begin

with, followed by reconstruction of the tip shape using

mathematical algorithms. Some of these image analysis

options are available with commercial Scanning Probe

Image Processing SPIP (Image Metrology Inc.) software

tools. Tip characterizers are samples developed with fea-

tures which are either smaller or comparable to the size of

the AFM tip. Such tip characterizers are either artificially

generated sample surfaces or samples on which molecules

such as DNA or carbon nanotubes of well-known dimen-

sions are imaged with the AFM tip The exact tip shape, is

determined, depending on whether the tip radius is greater

or smaller than the nanoparticle, by using different models

based on Zenhausern model [24] or Garcia model [25],

respectively. Single wall carbon nanotubes (SWCNT) fixed

to a substrate have been used to determine the radii of the

AFM tips [26]. Sharp structures on samples with curvature,

have been produced by electron beam lithography followed

by etching and are used to determine the cone angle of the

tip besides the radius of curvature of the AFM tip.

Blind tip estimation is another technique employed to

determine the tip shape when the sample structure and

shape are unknown. The inputs required for the technique

are image of tip characterizer, upper limit for the tip shape

and threshold value for noise. Though this technique was

demonstrated to be a reasonable alternative [27] to other

mathematical models, it is a very time consuming and also

suffers from the effect of noise on AFM signal. Over time,

some improvements and guidelines were established to

obtain more reliable data from blind tip estimation method

[28] and is now a commonly adopted technique to estimate

tip size and shape.

4.4. Cantilever Deflection Detection Systems

There are a wide variety of cantilever types which can be

broadly classified as non-self-sensing and self-sensing

cantilevers and their corresponding deflection detection

systems. There are several types of self-sensing cantilevers

viz. piezo-resistive cantilevers, piezo-electric cantilevers,

quartz tuning fork sensors and cantilever arrays. Here, only

a few of the different cantilever types that have specific

relevance to high sensitivity and high speed AFM mea-

surement requirements are discussed.

4.4.1. Optical Beam Deflection

The most commonly employed technique to detect the

cantilever deflection for non-self-sensing cantilevers is the

‘‘Optical beam deflection’’ technique because of ‘ease of

use’ and equivalent sensitivity as interferometry techniques

[29]. This method is based on detecting the reflection of a

Atomic Force Microscopy as a Nanometrology Tool 315

123

tiny laser spot off the back of a cantilever onto a quadrant

photodetector diode as shown in Fig. 2. The major factors

affecting the deflection sensitivity for the optical beam

deflection technique is noise from the photodiode, scattered

light incident on photodiode and light reflected back from

the photodiode to laser, focusing elements and the canti-

lever. Using highly reflecting, opaque and smooth back

surface of the cantilever in combination with very high

numerical aperture lenses helps in minimizing the effect of

scattered reflections to the photodiode. Fukuma et al. [30]

showed that they could achieve deflection noise densities

of 16.7 and 38.9 fm (Hz)-1/2 with laser power of 2 and

0.5 mW. However, the optical beam deflection method has

some limitations with regard to usage for ultra high vac-

uum and low temperature AFM systems due the laser

alignments becoming difficult. They also do not provide

the best performance for high speed scanning metrological

AFMs.

4.4.2. Piezo-resistive Cantilevers

Self-actuating cantilevers provide an alternative solution to

the above limitation of optical beam deflection methods.

Doped-silicon based piezo-resistive cantilevers have been

used to detect deflection of a cantilever by measuring the

changes in the resistance caused by the change of charge

carriers in the doped region on account of lattice strain

produced in the doped-Si. Cantilever detections based on

‘‘piezo-resistive’’ sensor were first demonstrated by Tor-

tonese et al. [31]. Ultrathin (\100 nm) and short piezo-

resisitve cantilevers were made for increased force

resolution and bandwidth [32]. Force resolution of single

digit femto-Newton-/HHz was achieved as against the

force resolution for commercial AFM cantilevers of 0.5 nN

in 10–1 kHz bandwidth. However, such thin piezoresistive

cantilevers are very sensitive to 1/f noise, which can be

minimized by careful optimization of size, thickness of the

cantilever and its doping density. Recently, the mechanical

and electrical characteristics of MEMS-based thermally-

actuated cantilevers with resistive sensing were investi-

gated and shown to be promising for higher resolution and

high speed AFM imaging [33].

4.4.3. Quartz Tuning Fork Sensors

A high stability of frequency of oscillation of the cantilever

is required for frequency modulation-dynamic mode AFM,

in general. This becomes quite critical when low temper-

ature measurements have to be performed. In this context,

quartz-based tuning fork AFM sensors are helpful, owing

to the inherent great stability of the quartz material for

temperature changes. Generally, the tuning fork-type of

sensor is constructed by attaching the AFM tip to one of the

prongs and the mass of the tip is balanced with a coun-

terweight on the other prong [34]. A typical configuration

[35] is shown in Fig. 3. Recently, ‘qPlus sensor’ were

implemented by Giessibl et al. [36], where one of the

prongs was kept immobilized by attaching it to a heavy

substrate while the other prong acts as the cantilever, to

which a heavy tip can be attached without reducing the

Q of the cantilever. They measured and reported four dif-

ferent noise sources, namely deflection detector noise,

thermal noise, oscillator noise, and thermal drift noise for

the qPlus sensors. They found that for self-sensing qPlus

sensors, the deflection detector noise is independent of

sensor stiffness, while the thermal noise, oscillator noise,

and thermal drift noise increase strongly with sensor

stiffness.

4.5. Calibration Standards

A list of wide variety of nano- and micro-measurement

standards commonly used for AFM, STM, optical and

stylus profilers and SEM instruments have been compiled

by PTB [37]. Two-dimensional gratings (with etch pits) are

routinely used for x–y and z-axis calibration of AFMs.

Standards for calibrating tip radius, angle and probing force

standards are also available. Depending upon the range of

Fig. 2 Schematic of components of an AFM with ‘Optical beam

deflection’ system. Figure taken from [14]

Fig. 3 Configuration of the piezoelectric quartz tuning-fork sensor

showing that the AFM tip is fixed to one prong while the AC voltage

applied between the prongs drives the tuning fork into oscillations

near its resonance frequency. Figure taken from [35]

316 G. Raina

123

x–y periodicities and z-height required for measurements,

different methods are used to fabricate such reference

standards with traceability. Large lateral standards with

periodicities ranging from 1 to 10 lm are generally pro-

duced by holographic methods while as for spacings below

300 nm, electron beam methods are employed. Standards

with spacings of the order of 30 nm are made using

structuring of compound semiconductor materials such as

Ga30Al70As/GaAs using molecular beam epitaxy or

MOCVD [38]. For gratings and step-height standards of

sub-nanometer dimensions, standards based on atomic

lattices are used such as highly oriented pyrolytic graphite

and silicon. Atoms of Si (220) can be used for high accu-

racy line width measurements and are also traceable [39].

Of late, fullerene-based nanomaterials such as SWCNT and

multiwall carbon nanotubes are also being used for SPM

standardization and tip characterization [40]. Transfer

standards calibrated in metrological AFMs can be used to

calibrate the commercial or non-metrological AFMs in

industry and academic institutions. However, there is also a

need for establishing calibration services and guidelines for

nanoscale measurement to be adopted by all to ensure

uniformity of results obtained.

4.6. Future Targets

With the advent of nanotechnology worldover, the devel-

opment and synthesis of various novel nanomaterials as

well as the growing need of the industry for reproducible

process control has made the development of the required

nanometrology tools imperative for their reliable charac-

terization. This has necessitated some concerted effort

towards the support and proper use of nanotechnology.

A Co-nanomet (coordination of nanometrology in Europe)

consortium has been established, where five nanometrolo-

gy actions groups have been identified and formed on the

key areas of nanotechnology viz. engineered nanoparticles;

nanobiotechnology; thin films and structured surfaces;

critical dimensions and scanning probe techniques and

modeling and simulation for the dissemination of metrol-

ogy techniques as well as traceability to national standards

and internationally harmonized standard methods [41].

In 2010, Co-nanomet brought out a report on critical

dimensions, SPM and thin films [42] and some aspects

given below were identified, that need to be addressed

currently:

• Systematic investigations of the influence of control

parameters settings in different SPM modes on differ-

ent materials

• Development of improved probes and scanning strat-

egies to perform 3D measurements viz. at side-walls for

MEMS/NEMS, micro-bores, sidewalls on photomasks

• Fight the incompatibility of data formats, increase the

awareness for openness of formats and generally agreed

procedures

• Increase awareness for collaboration between NMIs,

funding organizations, industry for future studies of

Fig. 4 iMERA Roadmap for Dimensional Micro- and Nano-technologies. Figure taken from [43]

Atomic Force Microscopy as a Nanometrology Tool 317

123

probe-sample interaction as the key to progress in this

field

The iMERA roadmap [43] was developed by represen-

tatives of a delegation of European Metrologists Institutes

(EUROMET) which projects the needs for Nanometrology

for the future (see Fig. 4). Accordingly, some targets have

been laid out for the coming 10–12 years (until 2025). The

target for the next 5 years (up to 2018), is development of

‘Traceable 2D (3D) metrology with sub-nm accuracy over

sample measurement ranges of sub-mm range’, which is to

be further extended to sample measurement ranges of about

100 mm range by 2025. In order to meet these challenging

targets, efforts are needed for evolving instrumentation for

2D (3D) positioning systems with nm-level accuracy over

sub-mm range which will require high precision displace-

ment interferometry. Refined optical interferometry and

X-ray interferometry have been suggested as possible routes

to achieve this. The design of new types of probes which

would be capable of fast sensing and resistant to wear and

tear will be required. The study of the interaction forces

between the probe and surface depending upon the material

and dimension, using modeling and simulation, has been

identified as one important research area. To calibrate the

newly developed instrumentation, various nanostandards

over sum-mm to 100 s of mm will be required. This will

need development of novel nanofabrication techniques.

From the point of view of metrology and required trace-

ability, various measurement parameters will need to be

correlated. Hence, traceable single-probe 2D (3D) instru-

mentation will need to be evolved to multi-probe 2D (3D)

instrumentation. In addition, calibration techniques, strate-

gies and guidelines for nanoscale measurements will need to

be provided to standardize the measurement results.

5. Conclusion

Certain general aspects of AFM design that require further

development would be to make AFM system less sensitive

to mechanical, thermal and environmental factors. To

achieve the targets set forth by the European Metrology

Institutes, AFM instruments with large measurement range

together with high-speed measurement capabilities will

need to be developed, which would be especially relevant

in certain fields like surface science and biological science.

A multi-probe instrumentation by combining SEM, TEM,

Optical profiler and confocal laser scanning microscopy

and others will help towards verification of measurement

results using different techniques. A set of measurement

strategies in the contact mode and non-contact AFM modes

for different samples need to be established and made

available in the form of reference data as well as some

commonly adopted guidelines for calibration of the

instrument need to be set up, in order to bring uniformity in

the interpretation of AFM measurement results all across.

Advancement in the theoretical and simulation studies of

probe-sample interaction forces will be important for the

progress in the field of accurate quantitative determination

of intermolecular forces using AFM.

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