Samples patterning

The samples have been patterned by using UV Lithography methods, since the details of the desired planar dimensions are of the order of some microns.

In general, the first step of a lithography technique is to uniformly cover (usually through spin coating) a sample with a polymeric substance, known as resist, which varies its properties if irradiated by a radiation source (e.g. UV light, laser, or electron beam). In case of positive lithography, the exposed regions will be removed after the development, while in negative lithography the irradiated zones will remain on the sample and the others detaches. An important parameter to be considered, which varies from resist to resist, is the dose, meaning the quantity of energy absorbed for mass unity by the substance, which determines the correct outcome of the process.

As already mentioned, during this work UV lithography technique has been employed, since it has some advantages with respect to EBL, especially simplicity and velocity of execution; in fact it is not necessary to put the samples in high vacuum and it can be implemented by using a mask which creates some shadow zones between the lamp and the sample, which protect below-positioned area from the radiation (so without designing the layout pixel by pixel by the radiation source). Furthermore, the time saving with respect to laser-based lithography and EBL increases rapidly with the area to be patterned, since its exposure time is almost always the same if the sample can be located in the lamp field. The limit of the resolution depends on the frequency of the light employed, since it is determined by diffraction, so generally this phenomenon becomes significant at sub-micron scale. On the other hand, this technique is less versatile than the other mentioned, since the mask should be fabricated a priori. For this reason, the first step of the procedure has been to realize a hard mask for UV Lithography on a laboratory glass by employing the DWL mask-less system owned by the hosting institution.

The layout was realized with the software Autocad 2018 and exported as .dxf, since this format is the only readable by the DWL. It consisted of two arrays made by 9 𝜇𝑚 sized squares, where there will be the MTJs pillars and some alignment crosses for fabricating the readout pads. All the lithography steps must take place in a clean room, where the atmosphere is controlled to

38 have the lowest possible concentration of dust, but also illuminated with only yellow light, since this radiation doesn’t interfere with resist properties. The glass has been covered with resist AR-5350 by a pipette and then it was fixed over a spinner which rotated at 200 rpm for 4 seconds and at 4000 rpm for 60 seconds to obtain a uniform deposition with few 𝜇𝑚 of thickness. Then, it has been baked for 240 𝑠 at 100 °𝐶 above a hot plate before the exposure, which was performed overnight with the DWL machine, since it needed several hours. The day after, the samples has been developed into the specific solution provided with the resist and then a film of Ta and Au has been deposited over the glass. Ultrasonic cleavage was performed to remove the excess of resist, leaving the layout imprinted on the transparent substrate, ready to be used as hard mask for the sensors.

A similar procedure until the exposure was performed on any substrate after the stack deposition. Then, any sample has been aligned with the realized mask in order to have the structure near the center by using a home-made holder driven by a micrometer stage for any direction plus a possible in-plane sample rotation. This “system” has been put into the field of a UV lamp Thermo Oriel (Figure 2.5) and exposed for few seconds, necessary to give to the resist the right dose before proceeding with the development.

Figure 2.5: ThermoOriel UV lamp employed for lithography.

To define the structures on the samples, a process of ion beam etching was then performed in order to remove the films in zones outside the MTJs by a physical etching based on the sputtering principle to obtain vertical edges. However, in the case of etching compounds, it is not enough to know the sputtering yield for any of the constituent to obtain the resulting one,

39 since it depends on the interaction between them. In this case it’s necessary to use some workaround to control the process in real time. Specifically, in this work, the etching processes have been realized by an Argon gun Ion Tech S-3000 PBN connected with a spherical chamber for accommodating the samples, while the process was controlled in real time by a quadrupole mass spectrometer Leybold-Heraeus QMG 511. The system, shown in Figure 2.6 (a), is under the ownership of the hosting institution of my secondment period, the Physics Department, Center for Spinelectronic Materials and Devices of Bielefeld University. On the datasheet of the Argon gun, the supplier suggests applying a potential of 600 V to feed its device, since the cross section for the Argon at 600 eV shows a maximum for most part of the chemical species.

In order to avoid the mixing of different layer but maintaining the effectiveness of the lithography already performed, during the process the sample has been tilted at 60° with respect to the horizontal direction and the holder has put in rotation at 30 𝑟𝑝𝑚, values already optimized for this kind of processes by the hosting group.

Figure 2.6: Experimental setup used for the etching (a) and mass spectrometer response employed for the real time monitoring of the process (b).

The role of the mass spectrometer is to have an effective real time control of the status of the process, by visualizing which material is under removal at a specific instant. With reference with Figure 2.6 (b), it can be noticed that, if a peak for an element which constitutes a layer starts from a high baseline with respect to the other materials signal, it means the removal of that particular layer, while if the excursion is small and starts from the lower part of the graph, it represents only a moment during the rotation while the tilt let the lateral part of the sample to be exposed to the Argon gun. It has been chosen to stop the process while reaching a plateau

(a) (b)

40 from the second (from bottom) Ruthenium layer, to be sure that the magnetoresistive part of any stack was fully separated.

After this procedure, with still resist on top of the structures, a passivation of the samples was performed through the deposition of about 150 𝑛𝑚 of tantalum oxide 𝑇𝑎 𝑂 , which can easily be obtained through RF sputtering by creating a plasma cloud with a ratio 1:1 between Argon and Oxygen. The following lift off in Acetone under sonication was employed to remove the residual resist, leaving the last capping layer of ruthenium uncovered, since it hardly oxidizes in air. A further positive lithography step followed by sputtering of Ta and Au was also performed, to obtain gold pads bigger than the planar dimensions of the stack, to simplify the future characterization.

Figure 2.7: Three series of sensors fabricated, GMRs, PHE and TMRs (from left to right).

Two more series of sensors based on GMR and PHE have been fabricated with a similar procedure but using Permalloy (𝑁𝑖 𝐹𝑒 ) for the magnetic layers, which are shown in Figure 2.7. Regarding the layout, for GMR there was chosen a zig-zag layout made by some 5 𝜇𝑚 sized meander, while PHE structures were 20 𝜇𝑚 sized squares equipped with conductive pads arranged in crosses to provide current in one direction and to get the signal in the perpendicular one.