Problems during the measurements

Even for the first simple measurements, there were several problems that occurred during the measurements. Some were expected, others not.

Figure 3.3: Examples for the electromigration of a conducting line

[Optical image]         [SEM image]

Figure 3.4: Water is boiling because of an overheating conducting line

One major problem is electromigration [25] that is generally the result of momentum transfer from electrons, which move in an applied electrical field to the lattice of the conducting material [4]. Thus, the electromigration occurs when many electrons massively scatter inside the conducting material. This massive scattering moves material and thereby deteriorates the conductivity of the material. The predominant failure mechanism for conducting lines is the growing of voids over the entire line. Very small imperfections already amplify the scattering, and are often nucleation centers for the growth of large voids.

Figure 3.3 shows examples for disconnected conducting lines due to electromigration. In the image of the optical microscope (a), you can see clearly the part where the gold of the conducting line turned black and the electromigration has destroyed the line. The change of the material properties can also be seen in the SEM image (b).

Figure 3.5: Splintered glass on top of the conducting lines

A related problem often occurs before the electromigration. The conducting line is overheating, and with it, local boiling inside the water drop occurs. Initiated again by imperfections inside the conducting line, some points become very hot, and the water above these points starts boiling. As shown in figure 3.4, these hot spots trigger bubbles inside the water drop. It is observed that the bubbles nearly always occurred at the edges of the conducting line. This is an indication for the fact that there are more imperfections at the edges of the conducting lines due to nooks and ridges from the lithography.

Besides the problem that all magnetic particles are strongly pushed away by the rising bubbles, the bubbles can also destroy the SiO$_2$ protection layer and, therefore, destroy the sample. Figure 3.5 shows an optical image of a sample surface after those bubbles rose from the conducting lines. The protection layer is clearly destroyed, although no electromigration occurred.

For measurements with two or more conducting lines, it is important that there is no current flow between the lines. Because, if the current flows through the protection layer and the water drop you have unwanted electrolysis on the sample, and such a current often initiates overheating and electromigration. The resistance over the water drop or the Si-wafer between the lines is not very high ($\approx$ 50kOhm), so that a current can easily flow when the potential is switched from one to another line. Therefore, the potential has to be zero, when the power source is switched on to another conducting line.