the nominal value at real time. Unfortunately, the position measuring system itself has a finite In}
resolution and introduces additional noise into the system, so that in most cases it cannot be applied pra
at small scan-ranges below approximately 1pm”. Zo.
Finally, tip geometry leads to image artefacts: the finite tip radius of curvature limits the lateral mie
resolution to typically the same scale as the radius. The finite opening angle of the tip limits the im-
aging of steep steps and the measure of large tilt angles. Often the tip apex is not spherical but el- AX
liptical or exhibits double tips resulting in deforming or doubling each surface feature (see figure 5). 3
The tip shape may be not constant within time. This is often the case in atomically resolved images [or
due to a lack in stability of single atomic tips or in contact mode atomic force microscopy (AFM) hE
due to wear of the tip. To recognise tip artefacts experience in interpretation of images is indispen- at
sable. Additionally, more than one tip should be used to image the surface, and tip deconvolution all
procedures (2,3,4) may be useful. .
In the following some examples will be given for special methods of material analysis by SPM. 2
Determination of nanometer size particle radii and distances
Metal containing diamond like carbon coatings (Me-DLC) are very useful as tribological protection
of tools, bearings, machine parts etc. to increase lifetime. The coatings are prepared by reactive DC-
or HF-sputtering within a hydrocarbon atmosphere,
resulting in the formation of small metallic or carbidic
nanoparticles embedded into an amorphous DLC ma-
rn trix (5,6). The metal content of the films can be varied
= by adjustment of deposition parameters in a wide
oo range, which results in a significant change of particle
al sizes and distances of the film, which in turn strongly
0% influences the macroscopic electrical, mechanical and
N tribological properties of the material. Therefore, par-
; ticle sizes and distances are of great interest for the
understanding of film properties.
Scanning tunnelling microscopy (STM) in air using EC
mechanically cut Pt-Ir tips is able to image the metal- "
Figure 1: STM image of Au particles of a Au- lic nanoparticles (see fig. b, but due to tip convolution
DLC coating (24at% Au) on silicon a direct measuring of particle radii leads to increased
values I’ =r + R,, where 1’ and r are the apparent and
280. real particle radii respectively and R, is the radius of
oO rx curvature of the tip. Consequently, to determine indi-
; | Tone vidual particle radii it is necessary to know the tip ra-
Cao) r TEM dius. A direct ex-situ imaging of the tip apex by
transmission electron microscopy (TEM) or STM of a
450 known test structure includes the risk that tip shape
P changes between tip determination and imaging of the
0 +5 S030 46 50 unknown sample. A tip deconvolution procedure on
Au [at%] the other hand, which determines tip radii from the
images of the unknown samples themselves, always
Figure 2: particle radii of gold containing guarantees that the tip radius present during imaging
DLC films determined by STM and other will be found.
analytical methods (XRD, SAXS, TEM) as Several authors (2,3,4) have provided mathematical
deconvolution procedures to determine tip shapes. In
