Applications
Chemical contrast application examples:
Automotive applications
Chemical Contrast Imaging of pyrolytic deposition of a carbon film on a silicon wafer using the probing tip of a scanning
atomic force microscope
An ultra thin carbon protective coating is grown from methane gas on a hot silicon surface. The ultra thin carbon deposits (dark) are contrasted against the bare silicon surface (bright) by applying the innovative technique of Chemical Contrast Imaging CCI-AFM™™. This technique distinguishes between different materials on a surface even in the case of ultra thin films. Coatings down to the thickness of single molecular layers may be seen. The left image shows a bare silicon surface immediately after the beginning of the carbon deposition, showing the first tiny nuclei of carbon (tiny dark spots in the image). The background colour pattern is due to an in-homogeneity of the silicon oxide which covers the silicon wafer. The image in the middle shows further progress of carbon film growth, exhibiting larger aggregates of carbon islands (dark), already covering more than 50% of the sample surface. Around the dark islands there are brownish hews which indicate the further growth of the islands. The islands are being added to continuously. The image on the top right shows the final stage of carbon deposition, the entire sample surface being covered by a thin film of carbon.
Again, the colour variation indicates that the film is not entirely homogeneous. Please note, that the above images do not represent the surface profile but the chemical contrast which distinguishes between different materials. Normal surface profile images do not show any of the above details since the profile amplitudes will be too small to be clearly detected. Instead AFM instruments not having the CCI feature will produce a similar image to the one on the right and will not show the progress of the growth status of the layers.
CCI feature will produce a similar image to the one on the right and will not show the progress of the growth status of the layers.
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It is evident that the CCI feature provides a necessary tool to help interpret physical properties of surfaces and chemical reactions on surfaces.
Courtesy Th. Schimmel et al., Karlsruhe University, Germany, www.pyrocarbon.de, www.schimmel-group.de
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Chemical Contrast showing surface modifications of a silicon wafer by a solvent using the probing tip of a scanning atomic force microscope
Chemical Contrast Imaging AFM-CCI™
Solvent droplets were placed on to a silicon surface. The solvent evaporated and a surface profile was taken using a BMT AFM as shown in the left image. The smooth silicon surface is shown in brown and there are also several brighter brown rings where the edges of the solvent were before evaporation. These rings are the result of impurities in the solvent which have left concentrated at the edges of the droplets. The question arises as to whether the solvent had any chemical interaction with the silicon. Using normal AFM topography, as in the left hand image, will not answer this question.
BMT AFM as shown in the left image. The smooth silicon surface is shown in brown and there are also several brighter brown rings where the edges of the solvent were before evaporation. These rings are the result of impurities in the solvent which have left concentrated at the edges of the droplets. The question arises as to whether the solvent had any chemical interaction with the silicon. Using normal AFM topography, as in the left hand image, will not answer this question.
On the right hand side the same surface is shown using the new Chemical Contrast Imaging AFM-CCI™ evaluation method. This method can distinguish between different materials even if the layers are very thin or monomolecular. The orange coloured silicon surface is seen showing many brownish spots. These cannot be seen in the conventional topography image and represent chemical inhomogenities of the surface. The dark brown rings correspond to the precipitation of the impurities which is seen as raised profile sections in the left image.
Of decisive relevance for the interpretation of chemical reactions on the surface is the observation that the silicon areas inside the rings (bright yellow), which have previously been wetted by the solvent, clearly differ from the areas outside the rings (dark brown). This is conclusive proof that the solvent modified the chemical properties of the silicon surface. In addition the surface properties underneath the solvent have been chemically homogenized.
From this example the method of Chemical Contrast Imaging™, which makes physical and chemical surface modifications induced by external influences clearly visible, represents a very valuable supplement to the conventional AFM measurement procedures.
Measurement data courtesy of Thomas Schimmel et al., Karlsruhe University, www.schimmel-group.de
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Chemical contrast of etching reactions on a graphite surface using the tip of an atomic force microscope
Chemical Contrasting by AFM Imaging
The understanding of chemical processes on surfaces and coatings is of central importance for many new technologies. The spectrum of applications extends from the production of processors via biocompatible coatings of cardiac valves and stents to scratch and corrosion proof paints and coatings. Even the very smallest defects have often fatal consequences for the function and life time of high tech surfaces.
A purely topographic investigation of the surface structure doesn’t provide appropriate information in most cases since such defects are mostly characterized by a local change of the chemical functionality rather than the topographic structure.
The chemical contrasting mode of a scanning atomic force microscope provides information about inhomogenities in the local material properties with a lateral resolution down to a few nanometers. Measuring the local elastic and tribologic surface properties shows where chemical reactions have locally modified the sample surface.
The profile image on the left shows a graphite surface which has been oxidized in air at 700°C. The arrows indicate steps in the graphite layers and these steps can also cross over each other. It is well known such surface discontinuities represent preferable chemical reaction centres. It is also important to know whether the steps are located right on the top of surface or remote from chemical attacks in a protected layer underneath the surface. This question is answered by the chemically contrasted image on the right. This image, which has been taken simultaneously with the left hand image, shows exactly the same surface area. It can be easily seen from the chemical contrasting mode, which shows different hues for different chemical species, that only the step marked by the arrow is located on the surface. In the left hand image only one atomic layer deep etching holes are seen which were brought into existence by the chemical attack of oxygen and are depicted by the circular shapes. Such holes start to grow at defects on the scale of single atoms. The chemically contrasting mode shows that at the boundaries of the holes 50 nm wide, reaction zones are formed and also that the graphite step itself is a nucleation zone for new defects (see arrow). Certainly the chemical contrasting mode plays an important role in determining the processes of surface chemical reactions.
Measurement data courtesy of Thomas Schimmel et al., Karlsruhe University of Excellence, www.schimmel-group.de
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Ground section sample of a Silitec cylinder face area
The task was to measure and evaluate the distribution of Si crystals and their average height and in particular the surface characteristics in the sub-?m range which are not accessible using alternative methods.
Using a short coherent interferometer surfaces may be measured quite well. Due to the large magnification a measurement area of approx. 0x70 ?m² is observed. A size distribution of the crystals from 20 x 20 ?m² down to 2 x 2 ?m² is seen and at this point the resolution limit of optical imaging is reached. |
Probe und Messposition
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Sample and measurement location
| By selecting a wrong colour sequence an inversion of the image impression is obtained. |
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Optical measurements show (independent on the measurement method) artifacts at surface discontinuities which are due to phase jumps and diffraction effects. The more sensitive the method the greater these spikes become (when using triangulation sensors or similar methods the spikes vanish in the noise).
From the above line zoom an average Si particle height of approx. 40 nm is seen.
Since this is a ground material specimen these values may not be the same as those of normal cylinder surfaces. The lateral resolution that can be obtained can be seen from the zoomed-in section. The Linnik Interferometer is superior in this respect to others such as the Mirau or Michelson as details down to less than 1 ?m may be seen. It is typical for this kind of measurement that aestethically good looking images are only obtained in the false colour mode. 3D graphic displays look noisy as this is close to the borderline of optical imaging capability.
More reliable data can be expected of a measurement method such as the AFM which is capable of atomic resolution.
At a x100 magnification an interferometer is at its’ limit of optical imaging.
Operating principle of an AFM:
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Opt. microscope image, bright field, x25
Inside the circle the following AFM profiles were taken. |
From 20 height measurements at arbitrary locations on the sample an averaged value for the particle height with respect to the Al matrix of 11 ± 2 nm is obtained. The error of the height calibration is less than 5%.
Surface structures in the micrometer range (lateral and vertical) have been investigated during the past decades in detail. This work was supported by the available metrology methods, initially tactile then supplementary optical methods as well as by the ‘spirit of the times’. Today it becomes more and more obvious that function related surface characteristics are on a scale of nanometers. With the AFM 3000 a new and very capable measuring device is available which lends itself for surface investigations covering several orders of magnitude from the mm range down to atomic resolution and it can, dependent on the setup, not only investigate static conditions but also dynamic ones, e.g. monitor the growth of layers.
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Profile measurement and surface characterisation of AluSil cylinder faces using an AFM 3000
A WLICyl White Light Interferometer can non-destructively measure the surface structure of engine block cylinder faces such as honing structure, broken Si crystals, wear laser pockets, etc.. The AFM has a vertical and lateral resolution which is an order of magnitude better than an interferometer. The following images show area surface profiles of both measurement techniques whereby AFM measurements of AluSil cylinder faces are presented for the first time. Unfortunately the most revealing results cannot be presented due to a confidentiality agreement with the customer, but we can say that the obtained information has proven of invaluable benefit for further developments. Using an AFM profile features in the nm range can be evaluated and measured. Artifacts as seen in optical measurements aren’t present. It is becoming very evident that the functional behaviour of surfaces is determined to a large extent by nanostructures. These can be studied using the AFM 3000.
Microscope image 100x
of a surface area.
The areas inside the circles were measured.
Bottom left: Microscope image 500x
Bottom right:
AFM image, tapping mode,
Image size approx.
100 ?m x 100 ?m |
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| Tapping mode height |
Tapping mode amplitude |
Microscope image 500x
Nanometer sized particles are seen which are firmly attached to the surface and which cannot be removed by cleaning.
The honing structure is clearly visible.
