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Introduction into the AFM measuring technique
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| Visualisation of Chemical Contrasts with the Multiscan AFM 3000 Atomic Force Microscope |
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Introduction into the AFM measuring technique
A microscope is used to image, show and output magnified surface features of objects which otherwise could not be viewed using the naked eye or a camera. For a long time only the classical microscope was known to be capable of viewing objects in a magnified way. The prerequisite for obtaining an acceptable image is the presence of enough surface contrast under the given illumination conditions. This is not always easily accomplished and techniques have been developed to get magnified images of object surfaces. These are the interference, confocal, electron and optical near field microscopes and others. They have specific advantages for individual applications.
Strictly speaking the classical surface profilometry utilizing a stylus also represents a kind of microscopy. This simple device has been in use since the 1930’s and is still widely used today in industry. The object is scanned point-wise by a small stylus tip and the profile, after suitable magnification, can be assessed by the eye.
The AFM (Atomic Force Microscope) is a further development of this classical surface scanning technique employing a stylus with a radius of a few nano-meters which is a factor of 1,000 below that of industrially used diamond styli of approximately 5 µm. The replacement of the stylus, as used in roughness metrology, by a much smaller one represents a considerable technical challenge. On one hand such a miniaturized stylus is quite fragile on the other hand totally different physical conditions in the stylus tip/surface area interface are encountered. Both the technical challenges and the possibility of entirely new application areas of the AFM have resulted in new research fields and chronologically new instrument types.
With resolutions down to the atomic scale in both the vertical and lateral directions this new technology enables direct investigation in nature and technological processes. Often quantitative results are possible yielding an understanding of the events and the possibility of controlling these processes. Indeed the operation of an AFM requires specific care, skill and experience but the instrument can be used in a variety of applications under different environmental conditions which makes it more powerful than the SEM (Scanning Electron Microscope). A classical profilometer can only acquire a surface profile of a work-piece (convoluted by its transfer function). The AFM also provides the profile but in addition gives a variety of surface characteristics (see below). This is of great benefit when interpreting the surface properties and it considerably helps in discoveries in the micro world. The AFM closes the metrological gap between the working field of the classical tactile profilometer and the optical microscope from the micrometer range to the nanometer scale.
The Working Principle of an AFM
In the ideal case the tip of the stylus would consist of a single atom but this state is not stable for very long. Commercially available styli feature tip radii of approx. 5-20 nm and the geometrical shape of the tip determines, to a large extent, the achievable resolution.
The stylus including the miniaturized leaf spring (cantilever) is typically made of silicon nitride or silicon. As the stylus tip approaches a macroscopical surface it experiences various force interactions which have been known for a long time. These are quantum mechanical, electrostatical, electrodynamical and magnetic forces acting between the atoms of the stylus tip and the sample surface. In detail the classification is according to the following force components: van-der-Waals force (attractive), electrostatic force (attractive or repulsive), capillary force (attractive) and a repulsive quantum mechanical component.
For the measurement the work-piece is translated using a piezo actuator in x,y,z directions. By this one can get to any location with nanometer precision. The deflection of the beam carrying the stylus is measured by the deflection of a laser beam. This impinges on a four quadrant photo diode. With respect to the figure above the output voltage of the diode segments (1+2)-(3+4) represents the deflection amplitude of the stylus whereas the voltage (1+3)-(2+4) is a measure of the torsion of the beam and thereby represents the local friction.
In order to measure surface profiles there are three main operational modes with subgroups.
a) Contact Mode
In this mode the stylus tip is in direct contact with the sample surface. The force acting on the surface ranges around a few nano-Newtons and Hook’s law on the spring constant of the beam and its deflection is used. A potential disadvantage of this mode is the modification of delicate sample surfaces when the stylus touches the surface resulting in shear forces.
There are three subgroups:
- Constant Force Mode
During the scan the stylus force is kept constant by continuously adjusting the sample height position. The control voltage is directly proportional to the profile amplitude. This is the most commonly used mode. Benefits are that the results are precise, the instrument is easy to work with and the interaction force between the tip and sample is constant.
- Constant Height Mode
The height adjustment control is switched off. The beam deflects due to interacting forces with the sample surface and this to a large extent according to the profile amplitude. A benefit: of this mode is since the time constant of the controller is absent this mode can be used for time critical applications. It is especially suited for smooth and atomically ordered samples and also yields atomic resolution.
- Friction Force Microscopy
A decisive advantage of an AFM instrument is the potential of acquiring, along with the profile, other surface features, e.g. the local friction force. This facilitates the interpretation of the overall results. In this mode the movement of the sample is done perpendicular to the cantilever axis (a disliked method in the classical industrial roughness metrology since the hinge of the stylus beam is very stressed by the sidewise acting forces), whereby the stylus deflects sidewise as a result of the local friction.
b) Non Contact Mode
The stylus tip doesn’t touch the surface but is guided across the surface at a distance of approximately 1-40 nm. In this mode only the long reaching attraction forces are acting. Using an additional piezo actuator at the cantilever fixing point, the stylus is made to vibrate in vertical resonance oscillations at an amplitude < 10 nm. The resulting resonance frequency and effective spring constant depends on the attractive forces and gradients are produced. The local sample profile amplitude can now measured in two ways. Firstly, by the change of amplitude, or by the frequency of the oscillation. Since the work-piece surface isn’t touched, this mode is most commonly used for very delicate surfaces. A disadvantage is the slow measurement rate of 1s/line which is considerably slower than the contact mode scan. Also the mode does not produce as good a vertical resolution. The lateral resolution is better than 1 nm.
c) Intermittent Contact Mode (often called “tapping” mode)
Similar to the above described mode the cantilever oscillates on its resonance frequency (typically. 100 – 400 kHz) with an amplitude of 10 - 20 nm. The stylus tip slightly touches the sample surface at the lower reversal point of the oscillation. In this mode the interaction intervals between the tip and surface are very short and, therefore, there is practically no lateral force acting on the sample. The contact force is smaller than in the contact mode and this is why delicate samples can be investigated using this mode. The lateral resolution is approximately 1 - 5 nm. The scan time is naturally higher, the Z resolution of 0.01 nm is comparable to that of the contact mode.
Chemical Contrast Imaging (CCI-AFM)
In order to get useful information regarding surfaces and surface processes more data than the surface topography and surface roughness is required to obtain a full picture of the sample surface. In particular on heterogeneous surfaces a contrast enhancement of the local chemical surface properties and material properties is indispensable.
The recently developed and patented method, called Chemical Contrast Imaging, is an essential functional extension giving interpretative support. Employing a fine high frequency tip-sample oscillation at an amplitude of a few atom diameters, the smallest variations of the tip-sample interaction can be detected. Images can be obtained of extremely small differences in the surface structure with a local resolution of a few nano-meters. Even if only a fraction of the uppermost atomic layer within a locally very limited region is chemically modified such locally dependent variations of the surface condition of the sample can usually be easily detected.
Employing the Chemical Contrast Imaging two kinds of images may be simultaneously acquired:
(i) the image of the area surface topography and
(ii) the image of the chemical contrast. Topographic structures can then directly be attributed to variations of the surface material composition and surface properties.
There is a long list of possible applications. A few are mentioned here:
In the area of tribology quantitatively wear processes and local in-homogeneities of the material properties can be investigated.
For composites and nano-composites a clear indication where on the surface the existing material components can be found which is independent of the surface profile. Chemical Contrast Imaging does not show any amplitude variation.
Weak points and surface imperfections down to a few nano-meters can be detected when investigating surface coatings and refinements.
Precipitation of foreign substances on surfaces, which can frequently be observed in production processes, change the surface properties and often stronlgy influence the subsequent process reaction steps. Such precipitation of foreign substances on surfaces can be detected and displayed even if they extend over only a few nano-meters.
Elasticity Microscopy, Force Modulation Microscopy and Chemical Contrast Imaging
Modulating, with the help of a piezo element, the vertical position of the sample with a small amplitude and a frequency of a few kHz around its resting position, information about the local stiffness of the sample surface can be provided. At locations having a high stiffness of the sample surface almost the full modulation amplitude is transferred from the sample surface onto the cantilever. If the sample surface area under investigation is less stiff not only the cantilever deforms but also the surface itself and only a fraction of the height modulation is transferred onto the cantilever. Conclusions regarding the local sample stiffness from the modulation amplitudes, which are superimposed onto the original AFM measurement signal, can be obtained. The technique is called Elasticity (PFM) or Force Modulation Microscopy (FMM) and depends on the amplitude and frequency.
In addition allowing the cantilever to vibrate horizontally with a small amplitude the adhesion effects can be switched on or off. If the vertical and horizontal vibrations have suitable amplitude ratios, friction forces in an absolute (not relative) value using the torsional movement of the cantilever can be detected. This mode is very sensitive which enables the instrument to investigate atomic and molecular properties of surfaces. Therefore a new friction correlated detection method for chemical alterations of surfaces, the so-called Chemical Contrast Imaging (CCI-AFM), has been discovered. Using this method absolute chemical information (e.g. the kind of atom on the surface) with a relative locality of different elements or molecules on a surface, without resorting to other techniques, can be found.
Resolution limit of an AFM and artifacts.
Theoretically, atomic resolution is possible, but this requires the elimination of all external interference sources and all the internal parameters need to be optimized. Some of these are:Vibration: this ranges in industrial environments between 4-400 Hz and suitable vibration damping is used to eliminate it.Scanning step size: depending on the task larger or smaller scanning lengths are required. Using a translation length of several micrometers an atomic resolution cannot be reached.The number of pixels of a camera: this needs to be adapted to the required measurement area and resolution, respectively.Stylus tip geometry: the same considerations apply as in the industrial roughness metrology. Basically the tip geometry acts as a low pass filter. The measured surface structure is a convolution of the tip geometry and the surface structure. In the contact mode there is some jeopardy of damaging the tip. In the non contact mode surface films and dirt can modify the true surface profile.Impairments due to the instrument: the design of the measurement head and the electronics considerably influence the fidelity of the results.
Extensions of the standard AFM mode
Electrostatic Force Microscopy (EFM)
In this mode local electronic properties of a sample can be investigated. The cantilever vibrates with its resonance frequency and a voltage is applied between the tip and sample. The electrostatic forces, which depend on the distance between the tip and sample, are measured by monitoring the phase changes.
Kelvin Sample Force Microscopy (KSFM)
Analogous to the EFM the potential difference between the tip and sample is measured as the work function. It supplies information about the electronic surface states at the location of the measurement.
Conductivity Scanning Force Microscopy (C-AFM)
A voltage is applied between the cantilever and the sample and the resulting current is measured in the contact mode. Thus spatially highly resolved conductivity measurements can be conducted
Capacity Force Microscopy (SCM)
Spatially resolved measurements are done by the capacity between the sample and the tip in the contact mode. This can be used, for example, for measuring the degree of doping of a semiconductor.
Resistance Microscopy (SSRM)
By applying a DC voltage between the tip and a conductive sample the resulting current is measured in the contact mode and thereby the local charge carrier concentration of semiconductor samples can, for instance, be determined.
Thermal Conductivity
The thermo voltage of a micro contact of two thermo wires, depending on the location, can be measured simultaneously to the profile and thereby a thermal conductivity map can be established.
Surface Structuring
Vibration: this ranges in industrial environments between 4-400 Hz and suitable vibration damping is used to eliminate it.Scanning step size: depending on the task larger or smaller scanning lengths are required. Using a translation length of several micrometers an atomic resolution cannot be reached.The number of pixels of a camera: this needs to be adapted to the required measurement area and resolution, respectively.Stylus tip geometry: the same considerations apply as in the industrial roughness metrology. Basically the tip geometry acts as a low pass filter. The measured surface structure is a convolution of the tip geometry and the surface structure. In the contact mode there is some jeopardy of damaging the tip. In the non contact mode surface films and dirt can modify the true surface profile.Impairments due to the instrument: the design of the measurement head and the electronics considerably influence the fidelity of the results.
Interpretation of the results
In the field of the classical profilometry this can be very difficult, especially when measuring near the performance limits of the instrument. The aesthetic aspect of a surface profile is of limited practical benefit. When trying to interpret the data of an AFM there is the additional difficulty that the macroscopic overview is mostly missing.
In many cases it isn’t easy to attribute phenomena visible in the nano-world to our experience gained in normal life. Sometimes new ways of thinking are used to interpret the data and some experience is necessary.
This predicament is familiar in medical science where the interpretation of X-Ray, ultrasonic or NMR images requires experience. In the industrial environment therefore, sometimes, more effort is required to properly interpret the data gained by an AFM than the operator is used to from current instrument being used. In nano-world the same visual impressions are very different in many cases from the macroscopic world. It is therefore of a specific advantage for data interpretation of AFM results to use several measument methods for scanning the surface area of interest. Each of these delivers its own information segment which can be put together to give an overall picture of the sample surface properties. The analysis of the so-called Chemical Contrast (CCI-AFM) results are known to be particularly helpful in obtaining satisfactory surface determinations. Also it often extremely helpful to know the history of the surface to be investigated. In addition the extensive methods of characterizing a surface by an AFM the instrument offers the possibility of gaining detailed information of unknown sample surfaces, analyzing production process steps and optimizing materials and surfaces regarding their properties. The properties which can be measured and visualized by an AFM are in particular:
- topography
- friction
- contact stiffness
- chemical contrast (CCI-AFM)
- material contrast
- local wear properties
- adhesion
- damping
Literature and references
[1] S. Magonov und M. Whangbo, Surface Analysis with STM and AFM, VCH, Weinheim, 1996;
[2] G. Binnig, C.F. Quate and C. Gerber, Phys. Rev. Lett. 56 (1986) 930;
[3] Th. Schimmel, et al., Surface and Interface Analysis 23 (1995) 399;
[4] H. Fuchs und Th. Schimmel, Adv. Materials 3 (1991) 112;
[5] M.F. Crommie, C.P. Lutz und D.M. Eigler, Science 262 (1993) 218;
[6] V. Popp, R. Kladny, Th. Schimmel und J. Küppers, Surf. Sci. 401 (1998) 105;
[7] Th. Schimmel, P. von Blanckenhagen und W. Schommers, Appl. Phys. A 68 (1999) 263;
[8] R Kemnitzer, Th. Koch, J. Küppers, M. Lux-Steiner und Th. Schimmel, in: B. Bushan (ed.) “Fundamentals of Tribology and Bridging the Gap between Macro- and Micro/Nanoscale Tribology”, NATO-ASI Series, Kluwer, Dordrecht, 2001, S. 495-502;
[9] Ch. Obermair, Ch. Klinke und Th. Schimmel, Acta Physica Sinica (Intl. Ed.) 10 (2001) S151ff;
[10] A.Pfrang, K.J. Hüttinger und Th. Schimmel, Surface and Interface Analysis 33 (2002), 96-99;
[11] Küppers, Schimmel, Koch, Lux-Steiner: True Atomic Resolution under Ambient Conditions Obtained by [12] Atomic Force Microscopy in the Contact Mode, Appl. Phys. A. 68, 399, 1999.
www.uni-karlsruhe.de/~agschimmel
www.nano-world.org
www.nanonetz-bw.de
Chemical Contrast Imaging:
Visualisation of Chemical Contrasts with the Multiscan AFM 3000 Atomic Force Microscope
In the past Atomic Force Microscopes (AFMs’) have been valuable tools for basic research but now they are an accepted surface analysing instrument for industrial investigation. For example, heterogeneous samples, more often than not, require mapping of local chemical surface characteristics in addition to the usual topographic data. This is now possible using a recently patented AFM method now commercially available¹ called Chemical Contrast Imaging. This technique offers solutions to numerous new industrial applications.
1. Surface metrology on the nanometer scale
For successful research and development in nanotechnology suitable measurement procedures are required. These methods must provide measurement results which are a comprehensive description of the properties of nanostructures which in turn leads to accurate interpretation of the results. This, after all, is the main goal of any measurement effort. The investigation of material surfaces and electronic components on the micrometer and (true) nanometer scale has increasing technological relevance. Industrial examples, for instance, are for measurements on heterogeneous materials, surface microstructures and nanostructured systems used in the semiconductor industry. Valuable data may be obtained of the three dimensional surface topography and also associated local material characteristics such as inhomogeneties and chemical surface differentiation with a high lateral resolution using this new technique.
AFM instruments having conventional and well known features can quickly reach their useful limits for complex industrial samples.
This advanced Multiscan AFM 3000 instrument features un sur passed mechanical stability and new surface imaging methods for investigating materials with a spatial resolution on the atomic scale (1-5). An Atomic Force Microscope scans the sample surface line by line, similar to the well known stylus roughness instruments, using a probing tip ideally consisting of only a few atoms. The surface roughness, even at the atomic scale, displaces a cantilever which carries the probing tip.
Fig.1 Working principle of an AFM
The displacement of the cantilever is measured using a high resolution optical technique. Not only can the surface topography be measured but utilising various supplementary techniques such as elasticity, adhesion and friction microscopy can provide simultaneous information about the sample surface with one measurement scan. These methods provide nanometer resolution on surfaces that help to understand ongoing processes and the underlying chemical and physical properties of surfaces. Coarser investigation methods are unable to ascertain such information.
Besides the topography, the structure and composition of heterogeneous materials, such as polymer mixtures and composite materials can be investigated. Below is an example of how in an situ chemical reaction, in real time, can be measured and interpreted and even the smallest chemical changes on sample surfaces can be detected with extremely high lateral resolution.
2. The patented method of Chemical Contrast Imaging
This innovative new method (patent owned by Karlsruhe University of Germany) utilises a high frequency oscillation between the probing tip and the sample, with an amplitude of only a few atomic diameters. This detects tiny variations in the tip-sample interaction which provides nanometer spatial resolution of the surface characteristics of the sample. It gives a “chemical look” at the surface, not an elemental or molecular identification but a chemical differentiation of atoms on the surface.
The result is termed a “Chemical Contrast Image” (CCI). In a chemical reaction on a surface, typically, only a fraction of the uppermost atomic layer is locally chemically modified and such changes in the surface characteristics can, in most cases, be detected with ease using CCI.
Using this CCI AFM, two images are obtained simultaneously;
- a three dimensional surface topography, and;
- a map of the chemical contrast
There is a long list of potential application areas for CCI and a few are listed below:
• Chemical surface processes can be observed directly with high spatial resolution and the quality and homogeneity or deficiencies and defects of layer deposition and surface refinement processes can be analysed. This also holds true for corrosion processes.
• In the area of tribology, wear processes can be investigated quantitatively and local inhomogeneties in the material properties can be detected.
• Investigation of composites and nano composites can show where exactly one or other of a material component is located on the surface.
• Deposition of foreign substances on surfaces can, in a production process, change the surface properties and very often have a bad influence on any subsequent reaction steps, even if they occur to an extremely small extent. Such depositions of foreign substances can be easily detected and imaged with CCI even if they cover an area of only a few nanometers.
3. Application example: carbon islands on silicon
Carbon layers are hugely important as they are chemically inert and mechanically stable providing protection films that can be sliding and lubricating layers. In the use of these ultra thin carbon films, the micro technological investigation of the growing processes, the homogeneity, quality and local properties are vital to their successful technical application.
Fig. 2 Growing carbon film (dark) on silicon substrate (bright), visualised by Chemical Contrast Imaging
Fig. 2 shows the deposition of ultra thin carbon layers having nanometer thickness on a silicon wafer. The deposition was accomplished using methane gas at approximately 1100°C. The image sequence, from left to right, depicts various phases during the deposition process and shows an increasing covering status by carbon. Islands are produced and these are clearly visible in the Chemical Contrast Imaging (CCI) method as dark islands on the bright silicon substrate. While the first image shows an almost bare silicon wafer surface with a few tiny carbon nuclei (diameter 10-30 nm) images b and c show an increasing number of islands having diameters between 30 and 80 nm. Further stages of the deposition process show that more carbon is agglomerated on the existing islands forming islands with diameters up to 180 nm and do not produce more islands. Differentiating between the silicon surface and the carbon layer is done simultaneously with the topography measurement. The latter does not give any useful information in this application and conventional AFMs do not provide these results. Further useful information can also be obtained by analysing the bright brown colouring around the carbon islands using CCI.
4. Summary The Chemical Contrast Imaging method represents a significant supplement to the conventional AFM measurement techniques. Different materials may be visualised and even the smallest chemical surface changes induced by chemical or physical surface processes can be seen. These changes are invisible to standard AFMs not having this Chemical Contrast Imaging technology.
Bibliography (1) www.breitmeier.com
(2) G.Binnig, C.F.Quate, C.Gerber, Atomic Force Microscope, Phys.Rev.Lett. 56 (1986) 930
(3) R.Kemnitzer, Th.Koch, J.Küppers, M.Lux-Steiner, Th.Schimmel, Atomic-Scale Processes of Tribomechanical Etching Studied by Atomic Force Microscopy, B.Bushan(ed.), Fundamentals of Tribology and Bridging the Gap between Macro- and Micro/Nanoscale Tribology, NATO-ASI Series, Kluwer, Dordrecht, 2001, 495-502
(4) H.Gliemann, Y.Mei, M.Ballauff, Th.Schimmel, Adhesion of Spherical Polyelectrolyte Brushes on Mica: An in Situ AFM Investigation, Langmuir 22, 7254-7259 (2006)
(5) H.Gliemann, A.T.Almeida, D.F.S.Petri, Th.Schimmel, Nanostructure formation in polymer thin films influenced by humidity, Surf.Interface Anal. 39, 1-8 (2007)
(6) M.Müller, Th.Schimmel, Method and device for determining material properties, patent pending
