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Atomic force microscopy (AFM)

In recent years we have witnessed a trend toward the nanoscale and even to the atomic level in many areas of science and technology, such as electronics. We can expect this trend to continue at a fast pace. However, this development is not just about making things smaller. As material properties are ultimately determined by their atomic structure, in order to understand or modify materials it is necessary to go down to the nanoscale or atomic scale. This requires an atomic force microscope.

How it all began: The history of AFM

In 1959 in his visionary talk entitled “There’s plenty of room at the bottom,” R.P. Feynman postulated the possibility of nanotechnology even before the word “nanotechnology” had been invented. Although this field did not yet exist at the time, Feynman was the first one to see and predict its potential. Today, just 60 years later, nanotechnology is all around us.

In the 1980s a new field of microscopy called scanning probe microscopy (SPM) was invented. In SPM, a sharp probe tip is scanned over a surface and surface properties are sensed at the nano- or atomic scale. One of the first milestones in this field was the invention of the first STM, which stands for scanning tunneling microscope, in 1981/1982 by Binnig, Rohrer, and Gerber. In 1986 Binning and Rohrer received the Nobel Prize in Physics for this invention.

AFM technology belongs to the field of scanning probe microscopy (SPM) techniques and was invented in 1985 by Binnig, Quate, and Gerber.

What is atomic force microscopy (AFM)?

AFM stands for Atomic Force Microscopy and gathers information by “touching” the sample surface with a mechanical probe. AFM utilizes a sharp probe with the tip radius down to the nanometer scale to scan along the sample surface to acquire images. This technique allows us to see and measure surface structure with unprecedented resolution and accuracy. An atomic force microscope enables us, for example, to obtain images showing the arrangement of individual atoms in a sample. Compared to other microscopes, such as optical or electron microscopes, AFM provides higher resolution not only in the lateral direction but also in the vertical direction, so that very precise information about the surface topography is obtained. Another advantage of AFM compared to STM is that the sample does not have to be conductive, so AFM can also be used for insulating samples. The name Atomic Force Microscopy comes from the fact that with an atomic force microscope the force between the tip and the sample is measured.

AFM provides real 3D information on the surface topography and works in diverse environments, such as:

open air
vacuum
overpressure
diverse atmospheres (e.g. inert gas surroundings)
at various temperatures
humidity
liquids

AFM measures not only the surface topography but also a large number of surface properties such as:

friction
phase
electrical conductivity/resistance
thermal conductivity/resistance
glass transition temperature
melting temperature
stiffness
modulus
adhesion
surface potential
capacitance
magnetism
electrochemistry

The setup of an AFM instrument

AFM uses a cantilever with a sharp tip to scan over a sample surface usually following the surface profile line by line to record an image of the topography. In order to make sure that the tip constantly follows the surface profile during the scan, a feedback mechanism is applied to monitor and control the tip and sample interaction. A laser beam generated from a light source shines on the backside of the cantilever and reflects back onto a four-quadrant photodiode. While the cantilever is moving up and down to follow the sample surface profile, the position of the laser spot on the photodiode changes accordingly. Using the feedback control, the Z scanner extends or retracts due to the surface profile to maintain the laser spot at the same position on the photodiode.

Why apply feedback control? On the one hand, this actively extends the measurement range in the Z direction because usually the Z scanner has a much larger range than a photodiode. This means a sample with a deeper hole or a higher protrusion can possibly be measured as well. On the other hand, with the feedback control, the interaction between the tip and the sample can be actively manipulated by the user. This helps avoid excessive force which may damage the tip, the sample, or both.

Two different types of AFM

Depending on where the scanner is located, AFM can be classified into different types. For tip scanning, the cantilever moves while the sample is fixed on the sample stage (see Figure 1.1). For sample scanning, the scanner is placed under the sample and the sample moves while the cantilever is held at a fixed position (see Figure 1.2). With the decoupled XY & Z scanner, the XY scanner is placed under the sample while the Z scanner is placed in the head in order to reduce the cross talk between the scanners (see Figure 1.3).

scanning head (sample fixed in position) — Figure 1.1: Scanning head (sample in fixed position)

Figure 1.2: Scanning sample (cantilever in fixed position)

AFM modes

As the AFM technology has evolved, different modes have been developed so that a wide array of data types can be collected in order to gain different information on the examined sample. Some of the most well-known AFM modes are explained here.

Contact mode

Contact mode operates by scanning a cantilever across the surface with the tip in constant contact with the sample surface. The feedback mechanism controls the movement of the cantilever in the Z direction with the help of the Z scanner, following the change of the surface profile to maintain the laser deflection in the photodiode in a set deflection. This is known as the deflection set point. By maintaining the laser deflection at a set value, the force between the tip and the sample remains constant.

Tapping mode

Tapping mode has an intermittent contact between the tip and the sample and is often referred to as “non-contact mode”. In tapping mode, the cantilever oscillates at or near its resonance frequency with an amplitude range typically between 20 nm and 100 nm. The resonance frequency of a cantilever is the property of the cantilever itself and can change if the cantilever is broken or contaminated. The tip taps the sample surface at each scan point (X, Y) one after another. During the movement of the XY scanner from one point to the next point, the tip has no contact with the sample surface. The feedback mechanism controls the movement of the cantilever in the Z direction by the Z scanner to maintain a constant cantilever oscillation in terms of set amplitude.

Lateral force mode (LFM)

LFM is derived from contact mode. The tip maintains constant contact with the surface while the cantilever scans across the sample. Besides measuring the vertical deflection of the cantilever as in contact mode, the lateral deflection is usually also measured to determine the surface property of the friction. For this reason, LFM is sometimes also called friction force microscopy and the lateral signal is sometimes referred to as the friction signal, although the lateral signal may be not only decided by the factor of friction but also topography.

Phase imaging

Phase imaging is a secondary imaging mode derived from tapping mode. Phase imaging maps the phase lag between the signal that drives the cantilever oscillation and the output signal of the cantilever oscillation. The phase shift indicates the variations of the surface properties such as adhesion, friction, elasticity, etc., which may occur due to the variations of sample surface composition. In this way, in addition to topography, phase imaging provides very valuable information with respect to the inhomogeneity of the surface.

Force distance curve

Besides imaging, the other key capability of AFM is to measure the surface mechanical properties such as stiffness and adhesion via force distance curves. A typical force distance curve has a full cycle containing an approach curve and retract curve. This mode measures the cantilever deflection versus the tip sample distance. The cantilever starts from a free level position in ‘A’ and moves towards the surface. At position ‘B’ the cantilever is attracted to the surface without a real load on it. This surface area is called the ‘snap in’ region. While the cantilever is moving further into the sample, it starts to bend with an increasing loading force. When the cantilever reaches the maximum predefined loading force, the approach curve ends and the retract curve starts. The cantilever moves in a reversed direction to move away from the sample. Due to the adhesion, the tip may not leave the surface as it is attracted to the surface at position ‘B’. The cantilever remains in contact with the surface until the maximum adhesion force is reached and the tip is detached from the sample. With a quantitative calibration of detector sensitivity (nm/V) and cantilever spring constant k (N/m), using Hooke’s Law F=-kx (F is the force, k is the spring constant, x is the distance) the measured cantilever deflection can be converted to force. Therefore, a quantitative output of the maximum loading force, adhesion force, snap-in force, and stiffness (with further calculation) can be achieved by applying the force distance curve mode.

Force volume mode

The force volume mode contains a set of force curves measured in a 2D array. The size of the array and the number of the force curves in the X and Y directions can be predefined. With proper calibration, the acquired quantitative mechanical properties such as stiffness and adhesion can then be mapped. This mode could be considered as a simplified version of the PFM mode (see below) in which a force curve is performed at every single pixel.

Pulsed force mode (PFM)

The PFM electronics introduces a sinusoidal modulation to the Z-piezo of the AFM with amplitudes of 10 nm to 500 nm at a user-selectable frequency between 100 Hz and 2 kHz, far below the resonance frequency of the cantilever. A complete force-distance cycle is carried out at this repetition rate at every pixel in the image. The PFM allows a quantitative mapping of surface mechanical properties for adhesion and modulus, simultaneous with the acquisition of the surface topography in tapping mode. Therefore, the surface mechanical properties can be immediately and easily correlated to the features on the surface. The PFM can be done at normal scan rates because the system can work at up to several thousand pixels per second.

Force modulation mode (FMM)

FMM is an extension of AFM imaging in contact mode. It is used to detect the sample’s surface mechanical properties such as adhesion or elasticity in a qualitative way while the tip is in constant contact with the surface. Meanwhile, a periodic high frequency signal is applied to drive the cantilever to oscillate on the surface. The phase shift between the detection signal and the driving signal indicates the change in mechanical properties.

Contact resonance (CR)

CR-AFM is derived from contact mode. The tip scans along the sample surface to acquire the surface topography image. While the tip scans the sample in contact mode, the contact resonance is continuously changing depending on the sample’s mechanical properties. In order to measure the contact resonance, a very low-amplitude vertical modulation is introduced by driving either the cantilever or the sample at a relatively high frequency to avoid the impact on the deflection signal used in the contact mode feedback loop. By measuring the changes in frequency and the quality factor (Q), the sample surface stiffness and viscoelasticity can be quantitatively calculated and mapped. CR-AFM is often used to characterize stiffer samples with a modulus range from ~1 GPa to over 100 GPa.

Magnetic Force Microscope (MFM)

MFM is a two-pass imaging technique which measures the magnetic force gradient above the sample surface. A topography image is acquired in either contact or tapping mode during the first pass and the magnetic image is recorded during the second pass by maintaining the tip at a predefined fixed distance above the sample surface. MFM can be used to investigate magnetic recording materials, superconductors, magnetic nanoparticles, etc.

Conductive Atomic Force Microscope (C-AFM)

C-AFM is a secondary imaging mode derived from contact mode to measure the surface electrical conductivity. C-AFM uses a conductive cantilever to scan across the sample surface in contact mode to acquire the surface topography image. Meanwhile, a bias voltage is applied between the tip and the sample and the electric current flowing between the tip and the sample is measured and recorded as a current map along with the topography image. The current measured in C-AFM can vary from several μA down to a few pA.

Electrostatic Force Microscope (EFM)

EFM uses a conductive cantilever to measure the electric field gradient distribution with a so-called two-pass technique. The first pass records the surface topography and is done in either contact or tapping mode. The second pass is done by withdrawing the cantilever to a predefined distance above the surface to measure the surface’s electric properties. It is critical to detect the surface profile first and then maintain the tip at a fixed distance to measure the electric properties. EFM is used for electrical failure analysis, detecting trapped charges, mapping electric polarization, performing electrical read/write, etc.

Fig. 1.15: Electrostatic force microscopy

Kelvin Probe Force Microscope (KPFM)

Fig. 1.16: Kelvin probe force microscopy

KPFM uses a conductive tip to scan across the surface to record the surface potential. Thus, sometimes it is also called a surface potential microscope. KPFM works in two different ways. The first traditional way works with the same principle as EFM with a two-pass technique. The first pass is used to measure the surface topography in tapping mode and the second pass to measure the surface potential by lifting the tip at a fixed distance, following the surface profile. The improved way works with a single-pass technique to acquire the topography and the surface potential simultaneously, and to increase the measurement efficiency and accuracy. While the topography is being mapped in tapping mode, an AC voltage is applied to the tip at a frequency slightly lower than the cantilever resonance frequency. KPFM is often used to study metallic and semiconducting nanostructures in a variety of devices from biosensors to solar cells.

Additional parameters measured by AFM

Originally the AFM was designed and used to image the topography of surfaces. However, thanks to the continuous development of technology, it is now possible to measure other quantities, for example electrical and magnetic properties, chemical potentials, friction, phase, electrical conductivity, magnetism, and many others (see different AFM modes above).

Conclusion

Atomic force microscopy is a relatively new discipline which gives insights into the atomic structure of materials. Although AFM is considered to be very complex, it is based on a very simple idea: using the blindman’s stick principle all the way down to the atomic scale! Today the atomic force microscope is a standard tool not only in material science, physics, chemistry, biology, and engineering, but also in many industries such as the semiconductor industry, for polymers and coatings, in the metal industries, and many others.

References

Bert Voigtländer, Scanning Probe Microscopy: Atomic Force Microscopy and Scanning Tunneling Microscopy; Springer; 2015
Peter Eaton, Paul West, Atomic Force Microscopy; Oxford University Press, 2010
Edt. Dalia G. Yablon, Scanning Porbe Microscopy in Industrial Applications: Nanomaterial Characterization; Wiley, 2014

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