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AFM - ATOMIC FORCE MICROSCOPE


The Czech Metrology Institute offers the services of one of the most advanced technologies of surface analysis - microscopy using the forces occurring between atoms, the "interatomic forces" - available in its Laboratory of Modern Metrology, Regional Branch Brno.


principle of the method

application

more about the principle

prices


contact: pklapetek@cmi.cz


Atomic Force Microscopy - AFM is a modern experimental method allowing an easy characterization of solid body surfaces. The method uses a small probe that monitors the surface topography in the immediate vicinity of the surface; by this, like tunnel microscopy, it ranks among the family of scanning probe microscopy (SPM). To detect the distance between the probe and the surface, atomic force microscopy makes use of the interatomic forces that cause slight deformations of the probe holder. By means of laser-beam-based optical detection the position of the probe is then evaluated and the surface topography is further processed by the software. In addition, it is possible to measure the friction between the probe and the surface, the surface response to the available force (known as point spectroscopy), while in other modes of microscopy it is also possible to detect the magnetic properties of the surface or its heat-carrying capacity, all with the level of resolution that is near atomic resolution.



This technology is represented in laboratories of the Czech Metrology Institute by a commercial microscope ACCUREX II.L from the company TopoMetrix. The microscope uses the TrueMetrixT technology that makes possible the quantitatively correct measurement of linear dimensions of objects on the surface of solid substances. Following quantities can be characterized by means of the microscope ACCUREX II.L:

Surface topography - topography is monitored within the linear ranges of 100x100 to 10x10 nm, on surfaces with height differences smaller than 10 um, with the subsequent measurement of object dimensions, determination of profiles and definition of statistical properties.

Point spectroscopy - the interdependence of force and point location is measured to compare hardness and adhesion at arbitrary, exactly defined points on the sample.

Measurement of magnetic properties - magnetic induction is measured on the sample surface with the simultaneous possibility of obtaining the sample topography (in the Czech Metrology Institute since early April 2000).

Measurement of heat-carrying capacity, temperature and thermal capacity (in the Czech Metrology Institute since early April 2000).


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Application:

Surface analysis, surveillance of data carriers, application in semiconductor technology, measurement of biological preparations, etc.


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Prices

The measurement prices are contract prices based on hourly rates of metrological services. For commercial applications, the first test measurement is free of charge.


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Principle of Atomic Force Microscopy

The main element of atomic force microscopy is a tip, several micrometers long, attached to the free end of an elastic holder. The tip point radius is 10-50 um; however, owing to the long-reach forces available between several nearest atoms in the point and the surface, it is theoretically possible to achieve the atomic level of resolution by means of atomic force microscopy.



The tip, which is in the immediate vicinity of the surface, is primarily influenced by the short-reach repulsive forces of electrostatic origin (according to Pauli´s principle) and by long-reach Waals attractive forces (i.e., forces of dipole-dipole interaction). An exact calculation of these forces for the system of tip and surface atoms is rather complicated

Lennard-Jones potential for silicium atoms.



Nevertheless, the influence of both forces can be simulated, for instance, by the empirical Lennard-Jones potential (see Figure). In addition, when establishing the surface topography, the microscope works in the constant force mode, and it is therefore not necessary to know the exact size of these forces.

As a result of these forces, the tip deflects from the equilibrium position in the vicinity of the surface and, consequently, a deformation of its holder occurs. By means of a laser beam reflected from the tip holder the deformation can be easily detected. Through feedback, the microscope can then respond to these impulses. All motions along the three axes are monitored by piezoceramic scanners during the surface scanning process.

The microscope can work in two basic modes: contact and contactless. In the contact mode the tip, attached to the free end of a rather little rigid holder, follows directly the surface topography by detecting the repulsive forces occurring while the tip is in immediate vicinity of the surface. In this mode it is possible to detect, together with the topography, also the lateral force produced by friction, difference of material, and by other factors. The other - contactless - mode is based on a different principle. The tip is fixed on a holder of greater rigidity, and by means of piezoceramic scanners it keeps oscillating at its own frequency. The forces (attractive and repulsive) acting on the tip cause a shift of this own frequency which is then detected and evaluated. The advantage of this mode consists in the fact that the tip does not come into direct contact with the surface. As a result, tips with a smaller apex angle can be used and thus greater resolution achieved.

The microscope ACCUREX II L makes use of the TrueMetrixT feedback control and linearization system that minimizes the hysteresis of piezoceramics and thus ensures a quantitatively correct measurement of data. The quantitative accuracy of measurement by means of this microscope can be checked, for instance, by measuring optical grids whose period can be measured by means of laser light diffraction. Such comparative measurements have already been made for the range of 100-20 um.


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last updated: 2005-12-07
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