Contacted Zigzag Lines

For the measurement of velocities and critical current densities of domain walls, zigzag lines were needed to nucleate domain walls and measure the displacement of domain walls. What is more, the elements have to be contacted to be able to apply current pulses. Magnetic elements and contacts are fabricated by pattern transfer using lift-off, but additionally an alignment procedure is needed to apply the contacts on the magnetic elements using electron beam lithography.

For measurements, PEEM (section 3.1) and different transmission electron microscopy methods, i.e. electron holography (section 3.3) and the Fresnel mode of Lorentz microscopy (section 3.2) were employed. In

Figure 2.12: SEM images (a) showing an overview of the design of the contacts. In the inset the exposure pattern of the alignment marks is shown. The area surrounded by the white frame is magnified in (b) showing different sets of 28 nm thick permalloy zigzag lines which are contacted with gold pads and wires. The set of wires surrounded by the white frame can be seen magnified in (c) showing 1000 nm-wide zigzag lines with kinks consisting of quarter rings.

the following it is described how samples for PEEM measurement are fabricated on silicon substrates. Second, it is explained what one has to take care of if membranes are needed for transmission electron mi-croscopy techniques.

2.3.4.1 Fabrication on Silicon Substrates

At first, a 130 nm-thick PMMA layer is spin coated on a2×2cm² silicon substrate. The choice of the silicon substrate is crucial for this kind of experiment. To avoid leakage through the substrate a material with high resistivity is preferred as a substrate. The fabrication of the magnetic el-ements is carried out according to section 2.3.1. Several sets of zigzag lines are written using electron beam lithography. The kinks of the lines consist of quarter rings and allow the positioning of domain walls in a controlled manner on application of a magnetic field. One set of lines comprises 4 lines each; 2 with one kink and 2 with 3 kinks (Fig. 2.12(c)) to observe more than one domain wall in the field of view of the PEEM. The lines with a single kink allow the measurement of large displacements of domain walls without them getting pinned at the next kink. The sets of zigzag lines are 220µm apart with each having a different linewidth ranging from 100 nm to 1500 nm (Fig. 2.12(b)). In addition, two align-ment marks are written on the top and the bottom of the area where the

zigzag lines are written (see inset of Fig. 2.12(a)). Permalloy is deposited by MBE or by sputtering. If the zigzag lines are fabricated, the contacts are written by an overlay procedure using electron beam lithography in PMMA resist. To position the electron beam, the coordinates of the align-ment marks are determined. A resist layer with thickness < 130 nm allows the observation of the Permalloy alignment mark below the resist using a 2.5 kV electron beam. Since the accuracy of this overlay procedure is in the order of 1µm, the measurement can be done before alignment of the electron beam. If the accuracy needs to be better than that, the measure-ment should be done directly before starting the exposure. Otherwise the changing of the sample holder which has to be done for beam alignment, introduces an error of about 1µm. A highly precise overlay is described later (section 2.3.5).

Once the position of the alignment marks is known, the position of the sets of zigzag lines and the rotation of the sample (only if the ro-tation is critical to the overlay) with respect to the first exposure can be calculated. The contacts consist of 70µm-wide and 90µm-high pads which overlap with the zigzag lines. 400µm-wide and 400µm-high pads close to the border of the substrate are meant for bonding, and 20 µm-wide wires connect the bondpads with the pads close to the zigzag lines (Fig. 2.12(a)). After development, the samples are shortly etched in oxy-gen (about 3 seconds) using the BMP reactive ion etcher. This can help to remove organic residues, e.g. the resist or isopropyl alcohol. One should note that it also can cause oxidation of the sidewalls of the magnetic el-ements which are not protected by the capping layer. In any case the sample should not be rinsed in water at this point.

The contacts are deposited by thermal evaporation. An 8 nm chromi-um layer is evaporated as an adhesion layer for a 50 nm gold layer. The gold layer is evaporated in the second step without breaking the vacuum.

Pattern transfer is completed by lift-off using ultrasound.

Figure 2.13: (a) Optical image of a Si3N4 membrane showing the gold contacts. The frames indicate the regions of (b-d). (b) SEM image of a contacted zigzag line with 3 kinks (Py 8 nm thick, 250 nm wide). (c) Exposure pattern of the corner of a big pad.

The inner part is exposed using wide lines and the edges using many narrow lines to improve lift-off. (d) Exposure pattern of an alignment mark consisting of a grating with 40µm spacing between the lines and 1µm linewidth. The lines consist of many narrow lines to improve lift-off.

2.3.4.2 Fabrication on Si3N4 Membranes

Zigzag lines on membranes are needed for current-pulse experiments us-ing the Fresnel mode of Lorentz microscopy and electron holography.

The high current densities required lead to considerable heating. As Si3N4 is a bad heat conductor, the thermal effects and current-induced effects cannot easily be distinguished and above all, the structures can get damaged. To solve this problem, a 30 nm aluminum layer was ther-mally evaporated on a 5 nm chromium layer on the back of the membrane substrate before processing to increase the thermal heat conductance.

The Si3N4 membranes being used are from Silson Ltd., UK. The size of the membrane sample is2.65×2.65mm²and the thickness 200µm. The size of the membrane window is 500×500µm²and the thickness 50 nm.

Because of the limited space for the contacts only one set of 4 zigzag lines can placed on one membrane (Fig. 2.13(a)). As the for silicon samples, the set consists of lines with 1 and 3 kinks (a line with 3 kinks is shown in Fig. 2.13(b)). The pads close to the zigzag lines are rhomboids with the parallel sides of 150µm and 310µm length being 80µm apart. The bondpads are 500 × 500µm² in size and the wires are 50µm-wide. To make lift-off possible without ultrasound, it is important to write frames

Figure 2.14: Schematic representation of the evaporation of gold contacts on the edges of the membrane sample if the resist layer is too thick to be exposed using electron beam lithography. Wafer pieces fixed with clamps cover the inner part of the membrane sample. The resulting contact pad can be seen in Fig. 2.13(a).

consisting of lines with small linewidth around the contacts, which gives sharp resist edges (Fig. 2.13(c)). Alignment marks located on the mem-brane frame are used to position the contacts (Fig. 2.13(d)). The resist close to the border of the membrane substrate is too thick to be exposed with a low energy electron beam. In order to fabricate bond pads the inner parts of the design are covered with a wafer piece and gold is de-posited directly on the substrate (to the left in Fig. 2.13(a) and schematic representation of the evaporation procedure in Fig. 2.14).