The main goal of this chapter, i.e. the positioning of single magnetic markers on top of small TMR elements, is achieved, although the TMR elements don't survive the drop of water, yet. The fabrication of a sample involves many steps (confer figure 6.5) and is, therefore, easily vulnerable to failures. The fabrication process needs several improvements until the TMR elements are functioning. The alignment for both optical lithography steps had to be improved several times, and a bottom contact over the whole sample area always lead to short-circuited TMR elements, because the insulating layer was not sealed everywhere.
But finally, it was shown that the samples work in principle. The TMR elements are measured right before the positioning of the magnetic markers, and show the wanted results. The positioning also works well, as shown for several examples.
To prevent the destruction of the TMR elements and to improve the positioning system, two suggestions for future experiments are made. Using e-beam lithography for the positioning system would greatly improve the accuracy of the alignment. The exposure of such big structures takes much longer with e-beam lithography than with optical lithography, but the better accuracy will be worth it. Especially for smaller beads that will be used in future experiments, the e-beam lithography will be mandatory.
A different coating technique is proposed to prevent the destruction of the elements. Using a Chemical Vapour Deposition (CVD) coating technique instead of sputtering, the protection layer probably has less defects and is, therefore, less vulnerable to the water molecules. A CVD coating machine will be soon available in our group, and so the CVD technique will be tried in future experiments.