Contacted Notched Rings with Antenna

Domain walls trapped in a potential can be excited by an oscillating mag-netic field. The notch can serve as a pinning potential and the oscillating magnetic field can be provided by an alternating current. The behaviour of the domain wall can be investigated by measuring the magnetoresis-tance. The sample which was constructed to meet this requirement

con-Figure 2.15: Scheme of the exposed structures during each of 3 steps; ring with notches (a), small contacts applied to the ring with notches (b), big contacts and antenna (c).

sists of a ring with several notches to pin the domain wall (Fig. 2.15 and 2.17). Contacts close to the notch allow the magnetoresistance to be mea-sured. A gold wire with a round shape inside of the ring serves as an antenna providing an oscillating magnetic field.

In the first step, a ring with notches similar to the wavy lines with notches (Fig. 2.15(a)) is fabricated by pattern transfer using lift-off (see section 2.3.2). 23×23mm² silicon wafer substrates are cut with a wafer saw providing straight, orthogonal edges which improve the positioning of the substrate in the sample holder, facilitating the precise overlay pro-cedure required to create this device. Since the MBE deposited permalloy film is only 12.5 nm thick, the resist can be thin (52 nm). The outer ring diameter is 10µm and the line width 500 nm. Each ring has 7 notches, at the bottom a ring segment is missing giving an opening. This open-ing is important for magnetoresistance measurement and application of currents to obtain a well-defined current flow direction. It also serves as opening for the antenna. Arrays consisting of15×15rings each are fab-ricated with a 80µm period in each direction. Besides the rings several alignment marks were exposed having 5 mm separation from each other and being 5 mm away from the first row of ring arrays.

After the fabrication of the rings with notches, the substrate is spun

Figure 2.16: Scheme showing the position of the alignment marks with respect to the devices. The origin (”0”) of the coordinate system is the starting point of the GLOKOS routine. The detected regions are indicated by lines and the order of detection is given by the numbers. The magnification shows the geometry of an alignment mark.

with a resist layer of 130 nm PMMA. Care has to be taken that the alig-ment marks can be detected with the highest precison. Therefore a piece of silicon is put on top of the alignment marks during spin coating to pre-vent the marks to be covered by the resist. Any remaining resist can be removed with acetone. The best method here is to take a cleanroom tissue soaked with acetone and press it with a tweezer on the sample. Contact of the acetone with the resist close to the ring arrays must be prevented.

During the second lithography step small line contacts with 150 nm linewidth are written (Fig. 2.15(b)). The two contacts for each notch should be as close and as symmetrical as possible to each notch. This requires an accuracy of tens of nanometers which demands that any ro-tation of the sample with respect to the first exposure has to be compen-sated.

In the following it is described how the coordinate system of the elec-tron beam writer can be transformed to match it again with a rotated sample. With the so-called GLOKOS (Globales Koordinaten System) pro-cedure alignment marks on the sample are detected using sweep routines

of the electron beam and from this the rotation and position is calculated.

As mentioned before the alignment marks have to be free from resist be-cause otherwise the sweep routines might work not precisely enough or might simply fail. Then, the coordinate system of the electron beam writer is transformed accordingly. Therefore the exposure data needs to be changed. During a trial run of the GLOKOS routine it should be checked if the alignment marks can be detected at all positions (indicated in Fig. 2.16 in the same order as they are detected). This is important because remaining resist or particles can still disturb the detection proce-dure. In this case the routine can be modified to search at an alternative position. The starting position of the routine has to be saved which repre-sents the zero point of the coordinate system (”Origin” in Fig. 2.16). This means that all writing positions for the exposure have to be relative to this (arrow in Fig. 2.16). After the fundamental alignment of the electron beam, the holder is exchanged and this introduces an error of the beam position which is in the order of 1µm. Therefore it is essential to measure the starting position coordinates after the holder exchange and immedi-ately before starting the electron beam exposure. Finally the exposure can be started which includes the GLOKOS routine and the writing of the pattern.

The GLOKOS routine can not compensate a z-tilt of the sample, i.e. if the sample substrate does not lie flat, and inaccuracy of the stage. Espe-cially the tilt of the sample can significantly reduce the precision of the overlay to several 100 nm. The required precision can be achieved as fol-lows: an array of rings is produced where the positions of the small con-tacts with respect to the rings are shifted from ring to ring by multiples of 80 nm, e.g. -7×80 nm, -6×80 nm,..., 0×80 nm,..., 6×80 nm, 7×80 nm both in x and y direction. After exposure of the fine contacts, the ring with the most accurately positioned contacts is found using an optical microscope and only this ring is processed further. During the deposition of the 8 nm Cr/50 nm Au layer for the small contacts the alignment marks have to be covered with wafer pieces as they will be used again for the final step.

In the third and final step, large contacts and the antenna are written

Figure 2.17: SEM image of a notched permalloy ring of 10µm outer diameter, 500 nm linewidth, and 12.5 nm thickness. The ring includes 7 triangular notches with an opening angle of 70^◦ each forming a constriction. Small gold pads contact the ring close to the constrictions leading to wires which end in bigger pads. A microwave antenna inside of the ring provides an oscillating magnetic field.

in a resist layer of 130 nm PMMA (Fig. 2.15(c)). The alignment procedure is carried out in the same way as described in step 2. This step requires a high precision, and only one attempt can be made to put the big contacts on the fine contacts. Therefore the overlap of the large contacts with the small contacts has to be big enough, e.g. 2µm. For the arrangement of the contacts and antenna, attention has to be paid that none of the rings with contacts in the surrounding area short circuits the device. An SEM image of a device is shown in Fig. 2.17.