Along this thesis, metallic electrodes were required for either 4-point configuration measurements (LSMO, YBCO), to be used as a guide reference within the sam- ple, or to study the resistive switching effect in encapsulated Ag/CeO2/LSMO
heterostructures, as it will be shown in chapters 3 and 4. In some cases, lithog- raphy processes will be required to defined confined bridges. In this section, we present the photolithography process, etching techniques and metal evaporation methodologies used during this thesis. The optimization of these processes have been performed in our group along the years. All of them were performed in the clean room facilities at the Institut de Ciència de Materials de Barcelona (ICMAB- CSIC).
Photolithography
Lithography is a technique used to transfer a pattern onto a substrate/film. This pattern is subsequently used to etch an underlying thin film. In particular, pho- tolithography refers to one kind of lithography that uses a light source (ultraviolet (UV)) to define the desired pattern that is transferred into a substrate/film. The procedure to perform a photolithography process in our perovskite films is as fol- lows:
Pattern design: With an specific software (CleWin 5), the layout that should be
transferred into the film is created at will. In our setup, this layout can be easily modified, thus adapting it to the specific sample and experiment re- quirements.
Photoresist deposition: The photoresist is a polymeric photosensitive material
spinning speed and photoresist viscosity will determine the final resist thick- ness. In here, we have used the so called positive photoresist, which makes the UV-exposed areas to be dissolved in the subsequent development stage. In particular, we have employed positive photoresist MOCROPOSIT S181342. It is deposited by spin-coating at 5000rpm for 20 seconds, obtaining ∼ 1µm. After the spinning of the resist, it is soft-baked at 90◦Cfor 1 min in a hot plate in order to remove the solvents from the resist and improve adhesion.
UV exposure: Then, the exposure of the sample is performed in a Micro Writer
ML by Durham Magneto Optics through a direct-write lithography process [191]. The sample is placed in a chamber where an autofocus correction and alignment is performed prior the exposure. Once these tasks have been successfully completed, the layout is loaded through software, specifying the dose. Typical dose values for our metallic perovskite films are between 150 − 250 mJ/cm2 using a UV-laser of 1 µm. The time required to carry out this step is around 30-60 min.
Photoresist development: After the exposure, the photoresist is developed in a so-
lution composed of water 98% and tetramethyl ammonium 2% MICROPOSIT MF-319 for 45seg. The exposed areas are dissolved, leaving trenches in the photoresist, where the sample surface is ready for either metal deposition or etching process. At the same time, the undeveloped areas act as a protection where metal deposition or etching is undesirable.
Metallization or etching step: Metal deposition through DC sputtering or film
etching by either physical or chemical methods in the developed areas is car- ried out at this stage. These techniques are explained in detail in following subsection. After these processes, The unexposed photoresist and uncovered metal areas (if any) are removed by rinsing the sample in acetone.
Figure2.23shows schematically all the steps involved in the described process.
FIGURE2.23: Photolithography process followed in this thesis.
Metal deposition and lift-off
In this thesis, we have used DC sputtering to deposit Ag and Au metal electrodes in our metallic perovskite films. In sputtering [192], a target made out of the metal
to be deposited is bombarded with high-energy inert Ar ions in plasma ambient. As a result, individual atoms or clusters are removed from the surface and ejected towards the film. The Ar ions bombarding the target are produced in direct current plasma.
A sputtering system from Twente Solid State Technology (TSST) is used to sputter Au or Ag metal electrodes. The patterned structures range from 10 to 500 µm in size, depending on each experiment, as it will be shown in the following chapters. Each metal has been deposited with the conditions defined in table2.2.
Metal Vacuum (mbar) Ar flux (sscm) Gas pres- sure (mbar) Voltage (V) Current (A) Time (min) Thick- ness (nm) Au 10−6 20 0.05 425 0.09 5 50 Ag 10−6 20 0.05 500 0.08 10 50
TABLE2.2: Conditions employed for Au and Ag sputtering deposi- tion.
The final structures are attained by a lift-off process in an acetone bath, where all unexposed photoresist including the overlying metal coating is removed. In all cases, the sputtered metals present a very good adhesion to our metallic perovskite oxides, without the need to use an adhesive layer. In addition, the contact resis- tance has been demonstrated to be low enough avoiding the need of post annealing process [156]. Typical values of the resistances measured in Au and Ag sputtered samples are 800 and 600 Ω, respectively, which leads to contact resistance values of ∼ 10−2Ω · cm2 . In the case of superconducting transport measurements, a lower contact resistance is required and then a post annealing process of the metal con- tacts was carried out at 450◦ for 1h in oxygen atmosphere to decrease the contact resistance to a 10−8Ω · cm2.
Oxide etching
Etching is used to remove material in a selective way in order to create patterns. In our case, the pattern is previously defined by the photolithography process. The developed resist leads to unmasked areas that can be removed either by wet (chem- ical) or dry (physical) etching.
Wet etching consists of a liquid chemical attack to remove the material. It is strongly isotropic which limits its application for high-resolution patterning. In dry etching, plasmas or etchant gasses produce the material removal. It is highly anisotropic and therefore it is more capable for transferring small structures. However, the remaining material might be damaged during the process due to deoxygenation of the film. In this work, since we do not need to transfer sub-micron features and good performance of the films should be preserved, we have employed wet etching to pattern both YBCO and LSMO thin films.
In the LSMO, the wet etching is performed using a 0.2% diluted piranha solution (H2SO4+ H2O2 + H2O). An etching rate of 10-20 nm/min is obtained.
In the YBCO case, the wet etching is carried out by immersing the film in an acid solution of 0.1% H3PO4. This solution leads to an etching rate of 100 nm/min
approximately.