AFM is based on what is known as Scanning Probe Microscope

(SPM) technology which uses piezo-electric materials for the scanning

of sample surfaces with atomic precision. The first SPM was the

Scanning Tunneling Microscope (STM, Figure 1) which uses an

atomically sharp metal tip to read the surface morphology through

tunneling current reading. Even though STM opens a new horizon of

direct measurement of sample surface at the atomic level, it can only

analyze either conductive or semi-conductive samples and is unable

to be used for insulating materials. Another limitation of STM is that the

analysis can only be made for samples under ultra-high vacuum (UHV)

since the surface needs to be clean of the ambient organic

contaminants. AFM was invented to extend the SPM technique to

non-conducting materials by measuring the interatomic forces

between the tip and the sample instead of the tunneling current.

Currently, there are more than 40 different variations of

SPM technologies.

The basic mechanism of AFM is shown in Figure 2. Light from a

laser is bounced off from the cantilever into a photodetector, which

reflects the bending of the cantilever caused by the interaction

between the tip end and the surface right below it. The tip can be

designed to monitor various types of interactions. This includes

not only van der Waals force (typical application of AFM) but

also electric, magnetic, electrochemical, fluidic forces, and so

on. These days, AFM is even applied to spectroscopic applications

such as FTIR (nano-FTIR) and Raman (nano-Raman), which provides

extremely surface sensitive (~ 3 nm) material information.

There are three operation modes–contact mode, tapping mode (TM), and non-contact (NC) mode.

There are three operation modes – Tapping mode, contact mode, and non-contact mode.

Operation mode can be selected based on the sample type and analysis goals. For example,

contact mode is used for quick imaging of rough samples, tapping mode for fluid layer analysis,

and non-contact mode under UHV condition. As there are many different combinations of tip and

sample materials, the analyst usually tries all three modes and selects one that generates best

quality image data.

AFM is one of the most effective imaging techniques being used at the nanoscale analysis of

surfaces. It has been applied to multiple problems across the field of natural science and

engineering, since AFM can record a range of material surface properties materials in ambient,

liquid, and UHV condition. Disciplines areas includes:

● Semiconductor science and technology

● Thin film and coatings

● Tribology (surface and friction interactions)

● Surface chemistry

● Polymer chemistry and physics

● Cell biology

● Molecular biology

● Energy storage (battery) and energy generation (photovoltaic) materials

● Piezoelectric and ferroelectric materials

Another emerging application of AFM is the study and characterization of graphene composite

materials.

Other Varieties of AFM

Among more than 40 variations of AFM based technologies, we will briefly cover three advanced

technologies – 1. Scanning Capacitance Microscopy (SCM), 2. Scanning Spread Resistance

Microscopy (SSRM) and 3. Nano-IR Photo-Induced Force Microscopy (PiFM).

1. SCM measures the electrostatic capacitance between the surface and the probe end

generating line scan or mapping data. The probe tip end is coated with highly conductive metal

material (such as Co/CR or Pt/Ir) so that it forms either metal-insulator-semiconductor (MIS)

capacitor or Schottky capacitor with the sample surface depending on the existence of surface

oxide layer. With the alternating bias to the probe, the varying capacitance property between

the probe end and the surface is monitored as the surface capacitance changes.

Traditionally, the analysis of dopant type and distribution in reverse engineering of devices was

done by wet staining, which began to show its limitation as interfaces tends to be destroyed

during the wet process with the shrinking dimension of devices. As SCM reads the capacitive

property, it is now a perfect replacement of the wet staining in dopant analysis for nano-scale

device dimensions. The other applications are P-N junction delineation and front-end

failure/leakage analysis. In the SCM operation, you can collect both topography and capacitive

mapping together which helps the understanding of the dopant distribution through the

topography.

2. SSRM is a SPM version of Spreading Resistance Probe (SRP) which measures the

cross-sections as opposed to the beveled surface by SRP. Like SCM, SSRM also uses

conductive probes but with higher loading force to make the spreading resistance the dominant

contribution to the resistance. SSRM is an efficient way to measure the resistance profiles

defined by the distribution of dopant atoms to surface normal direction. It started to be used

for reverse engineering of semiconductor devices quite recently.

3. PiFM is a newly emerging technology that extends the SPM technology to the nano-scale

spectroscopy regime. It consists of a conventional AFM equipment coupled with a laser light

source that excites the specific polarization of the material between the probe tip and surface

which affects the metalized AFM tip in generating the spectra corresponding to the various

nanostructures. Considering the sampling depth of traditional FTIR is a few um, PiFM generates

highly surface sensitive signal (typically less than 5nm) and provides chemical mapping with

high spatial resolution (< 5 nm). Therefore, PiFM serves well for the characterization of

nano-scale thin films for applications in semiconductor industry. IR PiFM spectra agree well

with bulk FTIR spectra when the sample is homogeneous in nanoscale so that the existing IR

spectral database can be used to identify unknown nano-sized defects. However, if the sample

is heterogenous in nanoscale, then the FTIR spectrum will be an ensemble of the nanoscale IR

PiFM spectra from different regions.

Invention of STM

First Commercial AFM

Gerd Binnig was a visiting scientist at

Calvin Quate (USA 1923)’s group in

Stanford where they were able to

demonstrate first AFM together. By 1987,

IBM scientists Yves martin and Kumar

Wickramasinghe came up with a

non-contact AFM.

Scanning Capacitance

Microscope

3 years later, Martin

combined the existing

scanning capacitance

microscopy (SCaM)

from RCA’s J.R. Matey

with AFM and introduced

the technique now

known as SCM.

First Atomic

Force Microscope

Following the collaboration with Calvin Quate

at Stanford, Sang-il Park and Sung Park

founded Park Scientific Instruments and

offered the first commercial AFM. As of

today, both founders are still working in the

frontier of AFM technology development,

Sang-Il Park with Park Systems and Sung

Park with Molecular Vista.

1985

1988

1989

1981

1995

HOW DID WE GET HERE?

During his PhD at the University

of Leuven, Peter de Wolf

(pictured) worked under

Wilfried Vandervorst from IMEC

and together they combined

Spreading Resistance Profiling

(SRP) and AFM to create nano

SRP now known as Scanning

Spreading Resistance

Microscopy or SSRM.

SSRM

Atomic Force Microscope

HOW DOES IT WORK?

Technical Background

Figure 1. Typical STM head

on UHV flange

Figure 4. AFM images from various applications.

Life science (a) – (e), and materials and surface science (f) – (j)

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o NX20 from Park System for AFM/C-AFM/SCM/SSRM

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Good turnaround time of 3 – 4 days (for AFM)

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Advanced AFM modes (C-AFM, SCM, SSRM)

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APPLICATIONS

From their research at IBM Zurich, Gerd Binnig

(right-Germany, 1947) and Heinrich Rohrer

(left-Switzerland, 1933) invented scanning

tunneling microscopy (STM). They received

the Nobel Prize in 1986 for their work.

Sung Park Sang-il Park

Figure 2. Diagram of the

laser beam reflecting off

the cantilever surface

(a) A new AFM tip and

(b) a used AFM tip

Figure 3. (a) A new AFM tip and (b) a used AFM tip

(http://www.nanoscience.gatech.edu/zlwang/research/afm.html)

Figure 3. (a) Lennard -Jones potential curve of tip-sample separation illustrating

the main interaction during AFM scanning. (b)-(d) The AFM imaging modes for

each regime are also shown. (J. Funct. Biomater. 2017, 8, 7)

Table 1. AFM Modes of Operation

Figure 5. (a) SCM setup and (b) SRAM twin-well structure imaged by SCM

(Images from (a) Park Systems and (b) MA-tek)

Figure 6. (a) SRP vs SSRM (b) Comparison of topography and SSRM images

(Images from (a) folk.uio.no and (b) hitachi.com)

Figure 7. (a) working principle of PiFM and (b) analysis for chemical mixing at nanoscale

(Images from molecularvista.com)

N20 from Park System

VistaScope from Molecular Vista

(a) (b)