Results and discussion

This chapter presents the development of a lithographic design that allows the positioning of four single magnetic markers at four predefined places. Taking all requirements into account, the design evolved over several steps (confer figure 5.1) into the final design that accomplishes all demands of the following experiments. For future developments of new lithographic designs, a finite-element simulation software could accelerate the development. The generated magnetic field can easily be calculated, and taking the friction in a liquid into account (see page for more information on STOKES' LAW), the whole path of a single magnetic particle can be simulated. Such simulations would shorten the development time a lot and give more insights into the newly developed system.

Figure 5.2: SEM image of the final design.

Although the final design works well for the intended application, there is still room for improvements. Figure 5.1(c) shows the final design during the experiments. Two single beads are very well positioned in the top corners, but there are also beads at the right edge of the structure. Thus, there are still some unwanted local maxima, and such additional maxima at the edges were observed in nearly all experiments. Examining the sample after the experiments in the SEM (see figure 5.2) exposes the reasons for the local maxima. There are still a lot of nooks and wrinkles at the edges of the structure. This is due to impurities in the optical resist or an mechanical oscillation of the laser lithography system (caused by a too small sample size). These small corners are responsible for the additional local maxima, to which some of the particles are drawn. So for very small or very exact structures, e-beam lithography should be used to minimise this problem.

This is another indication for the fact that the magnetic particles can be positioned within the same accuracy as the the lithography method. For bigger structures, the optical lithography is fine, but for very exact structures, e-beam lithography (which takes more time) has to be used.

The developed structure is used in the following chapter to position single magnetic markers on top of small rectangular TMR elements.