We are all surrounded by oxides. Oxygen is the most abundant element in the Earth’s crust and is highly reactive. This also means that many minerals are oxide minerals; oxides are employed to extract, for example, metals like iron. Many oxides are also of particular interest for their physical properties. Magnetite (Fe3O4) is a well-known magnetic (precisely a ferrimagnet) compound that has been employed in the ancient history for the fabrication of the first compasses. Silicon oxide, in its amorphous phase is used as insulating (dielectric) material in current transistors technology. In one of its crystalline phase, known as Quartz, silicon oxide is employed as mechanical oscillator for clocks and timing components. Its status of crystalline material allows exploiting an important feature of Quartz: piezoelectricity, or the properties to generate a voltage when deformed by an external pressure.
The converse piezoelectric effect makes this material changing its shape when subject to an external voltage signal. Other oxide-based piezoelectric compounds, such as the well-known PZT, or Pb(Zr,Ti)O3, are used todays in electronic devices and micromechanical components. Citing other peculiar properties of oxides, in 1986 some copper oxides were observed to be superconducting at high temperature. The most known compound used today is YBa2Cu3O7 – or YBCO - a complex oxide that has been widely employed for the realization of SQUID magnetometers working above the temperature of 77 K, the boiling point of liquid nitrogen>
Oxides offer a broad panorama of useful properties. Some of them are already used in electronic devices – see for example indium tin oxide (ITO) in displays. Meanwhile, the more fundamental research community is active to finely control their physical properties in different forms, especially in micro and nanoscale artificial structures such as thin films and heterostructures (layers of different sandwiched materials), nanopatterned devices and nanoparticles. A roadmap on oxide electronics with different examples of oxide-based devices can be found on the review paper “Towards Oxide Electronics: a Roadmap", Applied Surface Science Volume 482, 15 July 2019, Pages 1-93.
In the OXiNEMS project, we are using oxides for the realization of small structures with designed roles as mechanical devices. Our idea is to enrich the field of Microelectromechanical Systems, also known as MEMS, with this class of materials and add more functionalities to the existing materials library. The current MEMS field is broadly based on silicon technology. A paramount example is that of MEMS accelerometers and gyroscopes integrated in our mobile phones and tablet, but MEMS technology is also employed for the realization of microfluidic devices, infrared cameras, RF switches, oscillators, optical devices, microspeakers and microphones, pressure sensors, optical microdevices. Most of the MEMS devices require mechanical structures such as membranes, cantilevers or complex moveable structures clamped to the substrate by small joints, a sort of real mechanical engineering at the microscale. Nanoelectromechanical System (NEMS) take advantage of their extremely small dimensions to increase the sensitivity of mechanical-based resonant sensors. Despite different examples of NEMS sensors exist in literature, NEMS are still subject of laboratory research.
Oxides may show changes of their properties with temperature. They can abruptly change their volume, their magnetic or electrical and optical properties. This aspect is of peculiar interest for developing sensors and mechanical actuators (devices that transform a source energy into a controlled force or mechanical motion) based on local changes of temperature. See, for example, the micromechanical actuators based on Vanadium Dioxide previously developed in collaboration with Osaka University (www.vo2actuators.spin.cnr.it).
What we develop in OXiNEMS are basic mechanical structures with crystalline oxide thin films. We start from thin (about 100 nm of thickness) crystalline films of oxides and then study how to fabricate moveable structures. To do so, we deposit the films with a technique called Pulsed Laser Deposition and make the microstructures using optical lithography and selected chemical baths. We have developed special recipes to achieve the fabrication of suspended microstructures. As an example, the picture below shows a suspended microbridge structure made from a 100 nm thick magnetic oxide, called manganite or (La,Sr)MnO3.
One of the challenges in developing oxide-based micro and nanomechanical devices is the precise control of the internal stresses of these materials when they are grown in thin film form. To obtain thin layers of crystals (a crystal is a periodic arrangement of atoms), the oxide compound is grown on a polished piece of oxide crystal that works as “substrate”. The lattice of the oxide film matches with the growing substrate through a process called “epitaxy”. This matching may stress or compress the film with respect to its own natural distances between the atoms (lattice parameters) with the result of having stretched or bent freestanding structures, as it happens when stretching or wrinkling an elastic membrane by hands. The lattice parameters also depend on the precise ratio of the atomic composition of the oxide under study. An example is that related to (La,Sr)MnO3. The La/Sr ratio in (La,Sr)MnO3 changes the lattice parameters of this oxide compound and affects the stress with the substrate. We may grow La0.7Sr0.3MnO3 or La0.6Sr0.4MnO3 thin films on SrTiO3, a famous substrate for oxide deposition, and detect different levels of stress on the produced structures.
A) Double-clamped suspended 250 µm long microbridge fabricated from a 100 nm thick (La,Sr)MnO3 film grown on SrTiO3(110) substrate. B) Mechanical resonances of the microbridge. C) Zoom of the mechanical resonance of a LSMO microbridge measured at room temperature and vacuum conditions. Adapted from N. Manca, F. Remaggi, A. E. Plaza, L. Varbaro, C. Bernini, L. Pellegrino, D. Marré. Stress Analysis and Q-Factor of Free-Standing (La,Sr)MnO3 Oxide Resonators Small 18 2202768 (2022)
We also study the mechanical properties of these objects by measuring their “sound”, meaning their preferential mechanical vibrations as those achieved by the strings of a guitar. Figure 1a shows a doubly-clamped LSMO microbridge, the suspended regions are of yellow color. Figure 1b is a typical mechanical resonance detected on a 250 micrometer long LSMO microbridge centered at about 380 kHz (1 kHz means 1000 oscillations per second).
This sound also depends on the level of stress that microbridges have, so the spectra reported in figure 1c, which measures three different modes of vibrations of the microbridge, changes with the chemical composition of the original manganite film. Different systems for measuring the mechanical properties of our microstructures by optical methods have been developed in this project; some of them are specially designed to probe the mechanical properties of micro and nanostructures at different temperatures, including the temperature of boiling liquid nitrogen at ambient pressure (about 77 K, -196 °C). One of the core mechanical devices of our project requires the integration of magnetic mechanical structures and superconducting nanodevices, being able to work as very sensitive magnetic field detector. We are developing processes to integrate materials with magnetic and superconducting properties and oxides are a possible efficient route to achieve this goal, as they are often structurally and chemically compatible and exhibit magnetic and superconducting properties.