Development of the lithographic design

The lithographic design of the conducting lines determines the generated magnetic field, and with it the behaviour of the magnetic particles on the sample surface (see section 3.3 for several examples). So, before a new design is created, the requirements for the magnetic field and the sample have to be clear.

The aim of the new design is the positioning of a single magnetic particle directly on top of a TMR sensor (confer chapter 6). To get reproducible results within one experiment it is desirable to have a sensor array with a single bead on every sensor. On the one hand, it would be preferable to occupy several positions with one conducting line, because the used IC-socket has only limited contacts. On the other hand, it would be beneficial if every position could be controlled individually. Balancing both requirements, a rectangular conducting line was designed that allows the positioning at four predefined places (every corner of the square).

Figure 5.1: Three different designs to position single magnetic particles. The design is evolving from (a) to (c). See the CD for complete videos of the positioning experiments.

[First design tested for the positioning of single particles. Actual current is $I=101$mA.]     [Second tested design. Actual current is $I=102$mA.]     [Final design that was used for the positioning in chapter 6. Actual current is $I=92$mA.]

Figure 5.1(a) presents the rectangular design during a first positioning process. The magnetic particles in both top corners prove that this design works in principle, but some unwanted effects can also be seen. At the bottom left of the structure, a local maximum is at the round curve of the supply line(see respective particle agglutination). However, at the desired position in the corner, there are no particles. There are also several beads at the inner and outer edges of the rectangular design and not mainly in the corners.

Therefore, the design is changed to get rid of these problems. Figure 5.1(b) shows the second tested design to position four single particles at four predefined places. This design gets rid of the local maximum at the supply line, but there are still several particles on the outside of the structure. Also, the positioning is not really exact in this design. Most particles are near the inner corners, but not directly in the corner (see for example the bottom left corner).

This leads to a third design, presented in figure 5.1(c). Again, the supply line has no unwanted local maxima near the wanted positions, and the arched conducting lines allow very precise positioning of particles in the corners. Both top corners in figure 5.1(c) contain one single bead. The beads have a diameter of 1.5$\mu$m and are positioned within an area of the same size. There are still some particles at the outer edge of the structure, but no one near the defined positions. After the development of this design, it was used for all positioning experiments in chapter 6. Still, some particular conditions have to be met to get good results in the positioning experiments.