15 Atomic Force Microscopy-II

Dr. Ajit K. Mahapatro

Learning Objectives

From this module students may get to know about the following

i. Review of Basics of AFM

ii. Operating Modes of AFM

iii. Various Kinds of Force Microscopes

1. Introduction

Atomic force microscopy (AFM) includes various methods in for the interaction of probe and the specimen which helps to characterize various material properties. In this context AFM can characterize of mechanical, electrical, magnetic, and optical spectroscopic properties. In this article we explain in detail the different modes of operation in AFM and kinds of AFM systems.

2. Modes of Operation in AFM

(1) Contact AFM: Works at a probe-surface separation of less than 0.5 nm.

(2) Intermittent contact or tapping mode: Works at a probe-surface separation of 0.5-2 nm.

(3) Non-contact AFM: Works at a probe-surface separation of 0.1-10 nm.

The three modes are sketched in Figure 1 and the variations are tabulated in Table-1.

Figure 1. Different modes of operation in AFM

Table 1. Various modes of AFM

2.1. Contact Mode AFM

In this mode, the AFM tip is gently in contact with the surface of sample, and records the small force between the probe and the surface. The cantilever is held at a distance less than few angstroms away from the surface of the sample. This mode operates in the repulsive regime of the Van-der Waals curve i.e. the force on the tip is repulsive. When the spring constant of cantilever is less compared to the surface, i.e., lower than effective spring constant that binds the atoms of the sample together, the cantilever bending occurs. With a constant cantilever deflection (using the feedback loops) the force between the probe and the sample is maintained constant and an image of the surface is obtained.

In the contact AFM mode, the tip makes soft physical contact with the surface of the sample. Here, the deflection of the cantilever, (dx) is proportional to the force acting on the tip, via Hook’s law, F = -k. x, where k is the spring constant of the cantilever. In contact-mode, the tip either scans at a constant small height above the surface or under the conditions of a constant force. In the constant height mode the height of the tip is fixed, whereas in the constant-force mode the deflection of the cantilever is fixed and the motion of the scanner in the direction of height (z) is recorded. By using contact-mode AFM, the atomic resolution images could be captured.

For contact mode AFM imaging, it is necessary to have a cantilever soft enough to be deflected by very small forces and has a high enough resonant frequency to not be susceptible to vibrational instabilities. Generally, silicon nitride tips are used for contact mode.

Advantages of Contact Mode AFM

a) High scanning speed

b) Easier scanning for rough samples with abrupt changes in vertical topography.

c) Used in friction analysis

Disadvantages of Contact Mode AFM

a) Lateral forces can distort the image.

b) Capillary forces from a fluid layer can cause large forces normal to the tip sample interaction.

c) Combination of these forces reduces spatial resolution and can cause damage to soft samples. There are two contact scanning modes, as described below:

2.1.1. Constant Height

• In constant-height mode, the spatial variation of the cantilever deflection is used directly to generate the topographic data set because the height of the scanner is fixed as it scans.
• This mode is often used for taking atomic-scale images of atomically flat surfaces, where the cantilever deflections and variations in applied force are small. This mode is essential for recording real-time images of changing surfaces with high scan speed.

2.1.2. Constant Force

In constant force mode, the deflection of the cantilever can be used as input to a feedback circuit that scans up and down in z-direction and responds to the topography by keeping the cantilever deflection constant. With the cantilever deflection held constant, the total force applied to the sample is constant. The motion of the scanner in the z-direction helps generating image of the sample surface. The scanning speed is thus limited by the response time of the feedback circuit. Constant-force mode is generally preferred for most applications.

2.2. Intermittent (Tapping) Mode

For imaging biological samples, the constant force that the cantilever exerts on the sample in contact mode is quiet large. In such cases, it is advantageous to work in dynamic mode, also called tapping mode or intermittent contact mode. In this mode, the cantilever oscillates with an external piezo close to its resonant frequency (Figure 2). The probe lightly taps on the sample surface during scanning and contacts the surface at the bottom of its swing. By maintaining constant oscillation amplitude of 20-100 nm during scanning through an electronic feedback loop, a constant tip-sample interaction is achieved and an image of the surface is captured. Forces that act between the sample and the tip will not only cause a change in the oscillation amplitude, but also change the resonant frequency and phase of the cantilever. The amplitude is used for the feedback and the vertical adjustments of the piezo-scanner are recorded as a height image. Simultaneously, the phase changes are presented in the phase image (topography). Different components of the sample exhibit different adhesive and mechanical properties, and reflect different phase contrasts that allow compositional analysis of the materials. For a good phase contrast, larger tip forces are in advantage, while minimization of this force reduces the contact area and facilitates high- resolution imaging. Silicon probes are primarily used for tapping mode applications.

Figure 2. Amplitude dependence in tapping mode

The amplitude of the oscillation of cantilever decreases as it approaches the surface of the specimen (Figure 2a). This can be explained by observing the resonance behavior of the cantilever far away from the surface (black curve in Figure 2b) and close to the surface (red curve in Figure 2b). When the cantilever moves close to the specimen’s surface, an additional restoring force acts on the cantilever and the tip sample interaction pushes the cantilever back). This can increase the spring constant, which manifests itself in a shift of the cantilever resonance frequency to higher values (red curve). In tapping mode, the cantilever is always excited at a fixed frequency that is chosen to be just below the free resonance frequency (f₀). Far away from the surface, the cantilever will oscillate with amplitude of A0. When the cantilever comes closer to the surface, the resonance curve shifts, but the cantilever is still excited with frequency f₀. The resulting amplitude near the surface, A1, is reduced from A0. It is important to note that the cantilever still does touch the surface, but only at the bottom swing of the cantilever vibration.

This drop in amplitude can be used as the feedback parameter for AFM imaging, just like the cantilever deflection in contact mode (only that in tapping mode, decreasing the setpoint value increases the force on the sample, while in contact mode, decreasing the setpoint decreases the force on the sample). Using the change in amplitude as a feedback parameter is called operating the AFM in amplitude modulation (AM) mode. One can also track the shift of the resonance frequency of the cantilever when it approaches the surface. That is called frequency modulated (FM) mode. AM mode (or tapping mode) is the most common way to operate the AFM. Cantilevers for tapping mode are stiffer than cantilever for contact mode to allow for a higher resonance frequency. Typical values for k is 40 N/m and f0 in a range of 300-400 kHz.

2.2.1 Advantages and Disadvantages Advantages

a) Tapping mode allows high resolution of samples that are easily damaged and/or loosely held to a surface.

b) Good for biological samples.

c) Provides higher lateral resolution (1 nm to 5 nm).

d) Cause less damage to samples even with stiffer probes.

e) Eliminates a large part of permanent shearing forces.

Disadvantages

a) Difficult to produce images in liquid medium.

b) Slower scan speed

2.3 Phase Imaging: A Secondary Imaging Technique for Tapping Mode

Phase imaging is a powerful technique sensitive to specimen’s surface stiffness/softness, and adhesion between the tip and specimen’s surface, allowing the chemical mapping of surfaces based on these material differences. It monitors the phase lag between the signal that drives the cantilever to oscillate and the cantilever oscillation output signal (Figure 3). In Tapping Mode AFM, the cantilever is excited into resonance oscillation with a piezoelectric driver.

Figure 3. Amplitude dependence in tapping mode

As the sharp probe is brought into proximity with the specimen surface, it oscillates vertically near its mechanical resonance frequency. The amplitude of oscillation is reduced as the probe taps the surface, and this change in amplitude helps tracking the surface topography. The probe motion can be characterized by its phase relative to a driving oscillator. As  the  cantilever  oscillates,  it  will exhibit a phase shift (φ) between the drive and the response, as denoted by the equation:

d = A sin (2πft + φ)

where, d = deflection; A = amplitude; f = frequency; t = time; and φ = phase shift.

With any change in interaction between an oscillating cantilever and a sample, the resonance frequency of the cantilever also shifts either towards lower frequencies for attractive forces, or to higher frequencies for repulsive forces, and consequently, the phase at a fixed frequency shifts. Phase signal changes as the probe of the cantilever encounters regions of varied composition. However, the challenge with phase is that it shifts due to a convolution of multiple material properties such as adhesion, stiffness (modulus), dissipation, and viscoelasticity. Phase shifts are registered as bright and dark regions in phase images, comparable to the way height changes are indicated in height images. Phase detection images can be produced in any of the operational cantilever mode, such as tapping mode AFM, magnetic force microscopy (MFM), or electrostatic force microscopy (EFM). The phase lag and change in amplitude is monitored simultaneously such that images of topography and material properties can be collected giving direct correlation between surface properties and topographies.

2.3.1 Applications of Phase Imaging

• It is useful in identifying of contaminants, deposits, discontinuous or defective thin films.
• To  analyze  different  components  in  composite  materials.  It  can  examine  both  organic  and inorganic materials.
• It can differentiate regions of high and low surface adhesion or hardness.
• Mapping of magnetic and electrical properties with wide-ranging implications in data storage and semiconductor industries.
• It is used to map variations in surface properties such as elasticity, adhesion and friction as it causes phase lag.
• Phase imaging usually complements lateral force microscopy (LFM) and force modulation microscopy (FMM), often providing additional information more rapidly, conveniently and with higher resolution. It can generally achieve lateral resolution of 10 nm.
• Phase imaging is as fast and easy to use as tapping mode AFM, with all its benefits for imaging soft, adhesive, easily damaged or loosely bound samples.

2.4. Non-contact Mode

In this mode, the cantilever is held at a distance in the order of few tens of angstrom away from the sample and the probe tip uses the attractive forces to interact with the sample. The tip-sample interaction is minimized. Working in this mode allows scanning the surface of the sample without influencing it’s shape by the tip-sample forces.  The  cantilever  for  this  mode  are  having  high  spring  constant  of 20- 100 N/m so that it does not stick to the sample surface at small amplitudes. Cantilever oscillates near its resonant frequency (~200 kHz) to improve sensitivity. The tips used for this mode are silicon probes.

Advantages

a) Low force of magnitude in10-12 N is exerted on the sample surface.

b) No damage caused to soft samples.

c) Could be scanned with extended lifetime.

Disadvantages

a) It has lower lateral resolution.

b) Limited by tip-sample separation.

c) Slower scan speed to avoid contact with fluid layer.

d) Contaminant layer on surface can interfere with oscillation.

e) Need ultra-high vacuum (UHV) to have best imaging.

Atomic interaction at different tip-sample distances

Repulsion causes at very small tip-sample distance at few angstroms. Very strong repulsive force appears between the tip and sample atoms. Its origin is the exchange interactions due to overlapping of the electronic orbitals at atomic distances. When this repulsive force is predominant, the tip and sample are considered to be in contact.

Attractive  Van  der  Waals  force  causes  polarization  interaction  between  atoms.  An  instantaneous polarization of an atom induces a polarization in nearby atoms and therefore an attractive interaction.

2.5. Advanced imaging modes

Additional measurement modes often in combination with special cantilevers enable the measurement of sample properties beyond the topography. Examples are MFM and several electrical modes. Some of the modes depend on the detection of magnetic or electrical fields. Key in such measurements is to separate the short-range van der Waals forces from the longer range electrical or magnetic forces. A lifting mechanism enables the probing of longer range electrical and magnetic forces, and deconvoluting them from the short-range van der Waals forces that are present during topographic imaging.

3. Kinds of AFM Systems

3.1 Lateral Force Microscopy (LFM)

The LFM signal, which is related to the change in the surface friction on a sample surface, measures the deflection of the cantilever in the horizontal direction and can be represented as the difference in the signals recorded in the right cells (A+B) and the left cells (C+D).

Frictional information = (A+C) – (B+D)

Figure 4. Lateral force microscopy (LFM)

LFM  measures  lateral  deflections   (twisting)   of   the   cantilever   that   arise   from   forces   on the cantilever parallel to the plane of the sample surface. It images variations in surface friction, arising from inhomogeneity in surface material. Its imaging is also enhanced by edge deflection (slope variations) of surface feature. This differentiates from the imaging of different materials by two sharp changes (up/down) at both sides.

3.2. Force Modulation Microscopy (FMM): A Secondary Imaging Technique

In  FMM  mode,  the  tip  is  scanned  in  contact  with  the  sample,  and  the  z   feedback   loop maintains a constant cantilever deflection (as for constant-force mode AFM).

• A  periodic  vertical  oscillation  signal  is  applied  to  either  the  tip  or  the  sample.  The amplitude  of  cantilever  modulation   that   results   from   this   applied   signal   varies according   to the elastic properties of the sample.
• From  the  changes  in  the  amplitude  of  cantilever   modulation,   the   system   generates   a force modulation image — a map of the sample’s elastic properties.
• The  frequency  of  the  applied  signal  is  on   the   order   of   hundreds   of   kHz,   which   is faster than the rater scan rate (i.e. the z-feedback loop set up to track the scanning).
• Under the same force, a stiff area on  the  sample  deforms  less  than  a  soft  area;  i.e.,  stiffer areas  put  up  greater  resistance  to  the  cantilever’s   vertical   oscillation,   and, consequently cause greater bending of the cantilever. The variation in cantilever deflection amplitude at the frequency of modulation is a measure of the relative stiffness of the surface.
• Topographic information can be separated  from  local  variations  in  the  sample’s  elastic properties, and the two types of images can be collected simultaneously – direct correlation between topographic structure and elastic properties.

Figure 4. Force Modulation Microscopy (FMM)

3.3 Magnetic force microscopy (MFM)

MFM is a phase imaging mode that uses AFM cantilevers with a thin magnetic coating in order to probe the magnetic field between a sample and a magnetized tip. This method is commonly used to image any materials  with  heterogeneous  magnetic  properties   such   as   magnetic-based   hard   drives.   It can be operated in single, interlaced and dual scan line modes. Any of these modes require optimization of the height above the sample at which the MFM image is collected.

3.4 Conductive AFM (C-AFM)

3.4.1 Electrical measurement modes

AFM can probe a wide variety of electrical properties of materials and surfaces. These methods operate either in static mode or dynamic mode, depending on the information being sought. Probing properties such as current, conductance, surface potential, and capacitance are increasingly important in a number of applications including research on semiconductors, solar and battery cells, conductive polymers, and nanoelectronics.

This is a static mode method where both the current distribution and topography of a surface are mapped simultaneously. It is similar to scanning tunneling microscopy as in both modes a bias voltage is applied between tip and sample, and the tunneling current is measured between the two. However, the advantage of C-AFM, which uses a conductive cantilever as opposed to a sharp metallic wire, is that it provides topography information and current information independently. Single point measurements that measure the current verses voltage curves (I-V curves) can also be collected in this mode to probe the detailed electrical properties at a position.

3.5 Piezoelectric force microscopy (PFM)

This static mode based method is geared towards the study of ferroelectric or piezoelectric materials, which are materials that respond mechanically to the application of an electric field. This mode measures topography simultaneously with mechanical response of the material when an electric voltage is applied with a conductive AFM tip. A sharp conductive AFM tip is brought into contact with the sample and an AC voltage is applied between the tip and sample. The sample will either expand or contract oscillatory due to this applied voltage. The sample motion is then tracked by the cantilever defection, which is detected with a lock-in amplifier. The amplitude gives information on the piezoelectric tensor of the material and the phase provides information on the polarization direction.

3.6 Electrostatic force microscopy (EFM)

This mode is an electrical equivalence to MFM and operates in phase imaging mode, but now used for imaging variations in the electric field of the substrate. When scanning the tip lifted above the surface (typically only a few tens of nanometers), a voltage is applied between tip and sample to create a long-range electrostatic force. EFM images reveal information about surface potential and charge distribution from the phase image: with increasing magnitude of the potential difference between tip and sample, the resonance frequency drops, causing reduction in phase. Thus, a lower phase indicates a larger (absolute) potential difference. This also means that the contrast can be varied with the applied voltage.

3.7 Kelvin probe force microscopy (KPFM)

This mode images the surface potential distribution of a sample without direct electrical contact between the tip and the sample. It operates in dynamic mode with either a single or dual-pass setup. In the single- pass setup the tip is closer to the sample so there is higher sensitivity and resolution in the Kelvin force measurement, but the topography resolution may suffer. In the dual pass setup, the tip is farther away from the sample,  resulting  in  lower  sensitivity  and  resolution,  but  the  topography  can  be  sharper. In KPFM a combination of AC and DC voltage is applied to the cantilever causing an oscillating electrostatic force between tip and sample. The resulting defection oscillation is detected with a lock-in amplifier and minimized by the DC voltage. The DC voltage used is the local contact potential difference (CPD) between tip and sample. Applications of KPFM include imaging the Kelvin potential or work function of a surface and measuring applied voltage differences between conductors.

3.8 Electrochemical AFM (EC-AFM)

This mode enables AFM measurements while electrochemical reactions are taking place in electrolyte solutions on an electrode surface. Electrochemical reactions are processes in which electrons flow between solid (electrodes) and liquid (electrolyte), accompanying a reduction reaction (at the cathode) and an oxidation reaction (at the anode). These reactions are widely studied in applications such as corrosion and photovoltaics. EC-AFM measurements enable monitoring of the electrode structure during such reactions and establishing the relationship between the electrode structure/morphology and its electrochemical activity. As this mode occurs in aggressive liquid environments, excellent environmental control and protection of AFM electronics is necessary for effective imaging.

Summary

• The AFM can generate an accurate topographic map of the surface features.

• There are 3 operational modes in AFM: Contact AFM, Intermittent contact (tapping mode AFM), Non-contact AFM.

• Using the basic principles and different modes of operations, various kinds of force microscopes are developed.

References

• G. Binnig,   C.   F.   Quate,   and   Ch.   Gerber,“Atomic  Force  Microscope”,PhysicalReview Letters 56, 9 (1986).

• Franz J. Giessibl,“AFM’s path to atomic resolution”, Materials Today 8 (2005).

• Peter Eaton and Paul West, Atomic Force Microscopy. OUP Oxford.

• Werner Frammelsberger, GuentherBenstettera,JaniceKiely, and Richard  Stamp, “C-AFM-based  thickness  determination  of  thin  and  ultra-thin  SiO2 films  by  use  of  different conductive-coated probetips”,Applied Surface Science253 (7): 3615–3626.

• J. P. Cleveland et al. “Energy dissipation in tapping-mode atomic force microscopy”,Applied Physics Letters 72, 20 (1998