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Why to use a combined SEM - AFM? This combination offers several benefits, yielding to a better AFM as well as a better SEM, when comparing the combined system with two standalone systems. Actually, this combination is even somehow "natural".
The AFM is a high-resolution-only microscope. When scanning, the AFM tip has to follow the surface in constant distance with nano meter precision. If it is not, in the best case it is too far away and one simply doesn't get all information from the surface, but in the worst case it is too close and both tip and sample are damaged.
From this results a maximum scan speed, which is that speed where the AFM tip just follows the surface without damaging the tip and the surface. This speed can be regarded as the "lowest resolution", as one will still see larger details on the surface but not as much as scanning with slower speed. This maximum speed is independent from the scan area size, i.e. magnification. With normal cantilevers, this speed is in the order of magnitude of 100 µm/s (see What determines the maximum scan speed of an AFM?).
When working with a light microscope, one commonly starts with a low magnification objective for easier focusing and for having an overview over the sample. Then one is using a higher magnification at the place of interest. If one works like that with an AFM and for example scans an area of 1 mm² with 500 x 500 pixels, which corresponds to a normal low magnified light microscope image, this takes more than 80 minutes with the speed mentioned above. Even more, if the sample is not absolutely clean and flat, there is a high probability that one has contaminated the tip with that long scan. If one finds now an interesting area in the image, one would like to zoom in further. But this makes only little sense with a contaminated tip, so, one is changing the tip and then starting a larger overview scan again because by changing the tip one commonly moves at least some micrometers on the sample and looses the interesting place. For mechanical setups, micrometer precision is quite good but for the high magnification of the AFM some micrometers are really a lot.
A large scan with an AFM takes very long and is bad for the tip. For an unknown sample, one is more likely working the opposite way,
starting with a high magnification and lowering it after one has an idea about the sample roughness and contamination. Therefore,
the AFM nearly always needs to be combined with a low magnification technique, to get an overview over the sample and find the
places of interest.
This is the reason why nearly all relevant AFM manufacturers deliver their AFMs together with an optical microscope for rough positioning. Unfortunately, the resolution of optical microscopes is strongly limited and some features on the sample surface like small topography changes etc. can not be seen at all with an optical microscope. This kind of features one can simply not find with a conventional AFM as long as they don't occur with high density all over the surface. With an optimized optical microscope with high numerical aperture one even compromises the AFM performance by reducing the stability of the whole system, but one can never close the resolution gap between the AFM and the optical rough positioning setup.
The electron microscope as a natural improvement of the rough positioning system
When one replaces the optical microscope with a scanning electron microscope (SEM), one can totally close the resolution gap, at least on a lateral scale. The SEM has the dramatic advantage that it works with low as well as with high magnification. With such a setup, one can find features that are simply impossible to find with another method. One can observe the sample condition, e.g. contamination etc., before contaminating and wasting the tip. One can even see the tip status itself and the tip interaction with the surface and does not unnecessarily work with used up tips which cause artifacts in the AFM image. This save a lot of time and money.
The advantages for an AFM user can be summarized as follows:
- Exact positioning of the tip, location of features that can not be found with standalone AFMs
- Sample condition evaluation before the AFM scan and reduction of tip consumption
- Live tip evaluation and observation of tip contamination during scanning
- Much faster and more efficient AFM work
- Possibility of doing combination measurements (e.g. influence of electron beam on surface potential)
- In-situ sample and tip preparation using a FIB
Not only the AFM user has advantages of putting the AFM into an SEM, also the SEM user benefits from an integrated AFM.
What information does a secondary electron beam image contain? It is more or less a qualitative information of different
material and charging behaviour. In many cases this is enough, but one does not obtain 3D height information and not at
all with atomic resolution. To measure a surface profile, one needs to cut the sample or make a FIB hole.
But with an AFM one obtains on a button press a 3D image with atomically resolved height scale and calibrated X,Y,Z scales without disturbing the sample. This is the ideal add-on to every SEM image. One can immediately see whether the bright spot on the surface is something sitting on top or a defect in the material. One can not obtain this information with any other method.
Another advantage: The AFM is an ideal sample manipulator: The tip approaches the surface by itself and standard cantilevers are sharp and more or less mass products. With an AFM tip one can easily mechanically manipulate the sample surface, compare stiffness and hardness, make electrical contact, measure temperature, and much more. And what does actually happen when one scans with an electron beam and measures at the same time the current through the AFM tip or vice versa scans with the AFM tip and reads out the SE detector, maybe with applied voltage on the tip?
The advantages for a SEM user can be summarized as follows:
- Calibrated 3D height information on button press
- Atomically resolved height information
- Ideal sample manipulator with tips as mass product and automatic sample approach
- Information about hardness, stiffness, temperature and more
- Combination measurements deliver information that cannot be observed with any other method
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