The Principles of Operation of an Atomic Force Microscope (AFM)

Despite of the great success of the Scanning Tunneling Microscopy it was obvious that STM has fundamental disadvantage - with STM one can investigate only the conductive or conductive layers coated samples.

This disadvantage was overcomed due to the invention of atomic force microscope by Binnig. He was first who have guessed that under interaction with sample surface macroscopic cantilever provided with sharp tip can be bended by atomic forces to sufficiently large amount to be measured by the common facilities. In first embodiment to measure tip displacement was used STM (Fig. 1).

Experimental setup of first AFM from paper Binnig, Quate and Gerber

Figure 1. Experimental setup of first AFM from paper Binnig, Quate and Gerber

For registration of cantilever bending many methods was used, but currently mostly useful and widely used is method invented by Amer and Meyer (Fig. 2). According to them an atomic force microscope includes a tip mounted on a micromachined cantilever. As the tip scans a surface to be investigated, interatomic forces between the tip and the sample surface induce displacement of the tip and corresponding bending of the cantilever.

Schematic sketch of AFM from Patent "Atomic Force Microscope"

Figure 2. Schematic sketch of AFM from Patent "Atomic Force Microscope"

A laser beam is transmitted to and reflected from the cantilever for measuring the cantilever orientation. The reflected laser beam is detected with a position-sensitive detector, preferably a bicell. The output of the bicell is provided to a computer for processing of the data for providing a topographical image of the surface with atomic resolution.

Currently used position-sensitive detectors are four-sectional that allows measuring not only longitudinal but torsion bending too.

Cantilever can be bended not only by the direct contact forces under the tip-sample surface interaction, but also by far-ranging forces: van der Waals, magnetic, electric etc. Cantilever can vibrate under scanning as firstly was proposed by Binnig. Vibrating can proceed in direct contact of the tip with the sample surface, without touching the surface under vibration and with Intermittent-contact ( Semicontact) under vibration. Scanning can be many-passing, each next pass can give additional information concerning sample under investigation.

All this abilities generate many techniques and modes of SFM operation. Below we will consider various dc and ac Contact , Semicontact, Noncontact and Many-pass techniques and modes.

dc Contact Techniques

In idealized experimental conditions (e.g. in ultrahigh vacuum) when the cantilever tip approaches the sample surface Van der Waals forces start acting upon it. They are sufficiently far-ranging and are felt at the distance of a few tens of angstroms. Then at the distance of several angstroms repulsive force starts acting.

In real conditions (in ambient air) practically always some humidity is presented in air and a water layer is adsorbed on the sample and tip surfaces. When cantilever approaches sample surface the capillary force arises that holds the tip in contact with the surface and increases the minimum achievable interaction force.

Electrostatic interaction between the probe and the sample may appear rather often. This can be both attraction and repulsion. Van der Waals attraction forces, capillary, electrostatic and repulsion forces at the point where the tip touches the sample and forces acting upon the tip from the deformed cantilever compensate each other in equilibrium.

In Contact mode of operation the cantilever deflection under scanning reflects repulsive force and is used as such, in feedback circuitry or in their combination to imagine the sample surface profile.

Simultaneously with topography acquisition under scanning one can imagine some other characteristics of the investigated sample. If cantilever with tip are conductive one can imaging spreading resistance of the sample. If scanning is carried out in direction perpendicular to the longitudinal axis of cantilever (lateral direction) the friction force causes cantilever twisting. By measuring this twisting using position-sensitive four-sectional detector one can simultaneously with topography imagine the friction forces distribution throughout sample surface.

ac Contact Techniques

Usage of Scanning Force Microscopy with oscillating cantilever was firstly anticipated by Binnig. This oscillations can take place in Non-Contact, Intermittent-Contact (Semicontact) and Contact (ac Contact) modes.

Mechanical equivalent including lateral and vertical tip-sample interaction forces.

Figure 3. Mechanical equivalent including lateral and vertical tip-sample interaction forces.

The peculiarities of ac Contact modes consist in that simultaneously with cantilever nearest area of the sample surface participates in oscillation. At that sample surface oscillations are not only normal (out-of-plane) but lateral also and cantilever can oscillate in higher harmonics.

Forcing of the cantilever oscillations can be provided by the scanner, piezodriver, special transducer under sample holder. Cantilever oscillations can be excited also by the tip-sample electric forces. Accordingly frequencies of oscillations can vary between tens kHz to several MHz.

Measurements in ac Contact modes are carried out simultaneously with topography Contact mode measurements and allow to determine contact stiffness, Young's module and other sample parameters.

Semicontact Techniques

Usage of Scanning Force Microscopy with oscillating cantilever was firstly anticipated by Binnig. Earlier experimental realizations of scanning with oscillated cantilever was realized in works. It was demonstrated influence of the force gradients on the cantilever frequency shift and possibility of non-contact scanning sample surface. It must be noted also that Durig studied frequency shift of oscillating cantilever under influence of STM tip.

Relatively small shift of oscillating frequency with sensing repulsive forces means that contact of cantilever tip with sample surface under oscillation is not constant. Only during small part of oscillating period the tip "feels" contact repulsive force. Especially it concerns to oscillations with relatively high amplitudes. Scanning sample surface with cantilever oscillated in this manner is not non-contact, but intermittent contact. Corresponding mode of Scanning Force Microscope operation (Intermittent Contact mode or Semicontact mode) is in common practice.

'Feeling" the contact repulsive forces under the scanning leads to the additional phase shift of cantilever oscillations relatively piezodriver oscillations. This phase shift depends on the material characteristics. Registration the phase shift under scanning (Phase Contrast Imaging mode) is very useful for nanostructured and geterogeneous materials. Similarly to Contact Error mode Semicontact Error mode can be employed for finding minor irregularities on large areas.

The Semicontac mode can be characterized by some advantages in comparison with dc Contact mode. First of all, in this mode the force of pressure of the cantilever onto the surface is less, that allows to work with softer and easy to damage materials such as polymers and bioorganics. The semicontact mode is also more sensitive to the interaction with the surface that gives a possibility to investigate some characteristics of the surface - distribution of magnetic and electric domains, elasticity and viscosity of the surface.

Semicontact Techniques

Figure 4. Semicontact Techniques

Non-Contact Techniques

The Non-Contact AFM (NC AFM), invented in 1987, offers unique advantages over other contemporary scanning probe techniques such as contact AFM and STM. The absence of repulsive forces (presenting in Contact AFM) in NC AFM permits it use in the imaging “soft” samples and, unlike the STM, the NC AFM does not require conducting samples.

Non-Contact Techniques

Figure 5. Non-Contact Techniques

Many-Pass Techniques

In general many-pass techniques are used in tasks where other than topography data are to be obtained at that topography unwanted side effects must be eliminated. As example on left side picture line scans across a single magnetic domain for various initial tip-sample spacing are presented. Analogous measurements can be carried out for determining thickness of the liquid film on the solid substrate, nanomanipulations (e.g. positioning single atoms), nanolithographies.

First pass can be carried out in Contact or Fmplitude modulation (Intermittent-Contact, Semicontact) modes. On the second pass one can measure electric forces and potentials, magnetic fields, dissipations, surface capacitance distributions. In some cases third pass can be necessary particularly to eliminate not only topography but surface electric fields influence.

Line scans across a single magnetic domainfor various initial tip-sample spacings are presented

Figure 6. Line scans across a single magnetic domainfor various initial tip-sample spacings are presented

This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.

For more information on this source, please visit NT-MDT Spectrum Instruments.

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