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New sensor devices based on soft chemistry assisted nanostructured functional oxides on Si integrated systems

Periodic Reporting for period 3 - SENSiSOFT (New sensor devices based on soft chemistry assisted nanostructured functional oxides on Si integrated systems)

Reporting period: 2022-01-01 to 2023-06-30

Piezoelectrics are the active elements of many everyday applications, from ink-jet printers to ultrasound generators, representing a billion euro industry. They are the key elements of motion sensors and resonators present in any wireless network sensor (WNS) node. However, an increased production of piezoelectrics in a sustainable way is to-date a milestone. In addition, if efficient piezoelectrics could be engineered from widely available and non-toxic compounds, their use in bio-sensor applications would become widespread. As a result, one could predict that piezoelectrics might radically contribute to attain a fully sensorized world, once the current toxicity and sustainability issues are solved. SENSiSOFT project proposes to come up with materials that can provide a solution to this problem: piezoelectric materials that are abundant, cheap and harmless. The aim of this project is to produce new piezoelectric devices of nanometer size with an unusual limit for wireless mechanical sensors, using direct and combined chemical integration of quartz, perovskite and hollandites materials as nanostructured epitaxial thin films on silicon. This is a major challenge that demands bridging the gap between soft-chemistry and microfabrication techniques. For industrial processing using MEMs technology (with the aim to reach prices of 1€/device), thin film deposition on Si is a must. Thus, the ambition of SENSiSOFT is the integration of highly-oriented oxide-based nanostructured piezoelectrics sensors devices on Si platforms using industrially scalable methods.
The first objective of SENSiSOFT project was the chemical solution preparation of epitaxial piezoelectric oxide thin layers on silicon and silicon–insulator–silicon (SOI) substrates with high-performance piezoelectric response. In this light, the ERC team intensively worked during the first two years on three main materials: on the one hand, the monolithic integration of piezoelectric α-quartz thin films, and novel hollandite oxides, and on the other hand, the study and integration of perovskites oxides on silicon technology. Many goals and significant results have been achieved.

Regarding the growth of epitaxial quartz on silicon :

1.1- The basics of crystallization and epitaxial processes of quartz thin films on silicon is understood. The SENSiSOFT team have disentangled the catalytic role of strontium devitrifying agent during the crystallization and epitaxial processes of quartz thin films on silicon.
1.2- The fabrication of high-quality epitaxial quartz films on silicon–insulator–silicon (SOI) substrate (up to 6 inch) in place of conventional silicon substrates has been attained.

Likewise, for the study of hollandites oxides on silicon:

1.3- The SENSiSOFT team have developped a cost-effective and scalable chemical method to modify the chemical composition of hollandite nanowires directly grown on Si. As a result, we integrated and stabilize a new room-temperature ferroelectric Sr1+δMn8O16 hollandite-like oxide in Si technology.

And finally, regarding the theme around the integration of perovskites on silicon substrates:

1.4- The SENSiSOFT team successfully established a hybrid chemical solution route to prepare nanostructured and dense epitaxial lead-free ferroelectric oxide materials on STO/silicon wafers. A combination of Chemical Solution Deposition methodology (CSD) and Molecular Beam Epitaxy (MBE) was employed to grow heterostructures of epitaxial BiFeO3 /La0.7Sr0.3MnO »/SrTiO3 on Si(001) wafers as a model system. This growth strategy permitted the direct integration on silicon of nanostructured epitaxial perovskite functional oxides by combining chemical and physical methods.

The second objective of the SENSiSOFT project was the nanostructuration of epitaxial oxide piezoelectrics into 1D wires, or rods to enhance its performance. The main goals of this aim have been achieved (listed below) :

2.1- The SENSiSOFT team have stablished an unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i) laser transfer lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films and (iii) self-assembled SrCO3 nanoparticles reactive nanomasks. With these results, we have obtained epitaxial α-quartz nanopillars with different diameters (from 1 µm down to 50 nm) and heights (up to 2000 nm).

The third objective of the SENSiSOFT project is to develop new sensors devices based on the previously integrated nanostructured piezoelectric oxides on silicon for monitoring mechanical parameters (mass, forces, pressure, torque, etc.). Until now, a part of these objectives have been achieved :

3.1- For the first time, The SENSiSOFT team have produced piezoelectric nanostructured high-quality factor epitaxial quartz based micro- and nanoelectromechanical cantilevers. We experimentally tested the mass and force resolution of nanostructured quartz-based cantilevers by applying different forces in the µN range with the AFM tip and recording in-situ the resonance frequency evolution.

With these promising results, we expect to enlarge the design of quartz nanoresonators in order to increase the sensitivity of these devices until the end of SENSiSOFT project. For instance an acoustic wave sensor (SAW) is under developing at tha moment. Moreover, these devices are expected to work also at the intrinsic quartz material frequency, which depends on the thickness of the quartz, i.e. around 10 GHz for a 800 nm thick resonator.

All of these results are internationally recognized, and have given rise to 7 high impact journal articles and other 5 papers under consideration, 1 Pantent, 3 covers pictures, 2 PhD thesis already defended, 10 international conferences and 2 invited conferences.
Three main achievements represent a progress beyond the state of the art in their corresponding field i.e. (i) the combination of chemical solution deposition techniques with soft-nanoimprint lithography and top-down microfabrication processes to develop the first nanostructured epitaxial (100)α-quartz/(100)Si piezoelectric cantilevers represent a breakthrough on this field. Indeed, The coherent Si/quartz interface and film thinness combined with a controlled nanostructuration on silicon–insulator-silicon technology (SOI) substrates provides high force and mass sensitivity while preserving the mechanical quality factor of the microelectromechanical systems. As a result, this work proves that biocompatible nanostructured epitaxial piezoelectric α-quartz based MEMS on silicon can be engineered at low cost by combining soft-chemistry and top-down lithographic techniques, (ii) the first chemical solution integration of a functional oxide using our epitxial quartz thin films on silicon as a buffer layer also represents a main advancement in the field and, (iii) We have discovered a new hollandite structure with the composition Sr1+δMn8O16. We identified the piezoelectric and ferroelectric properties by using different techniques confirming the non-centrosymmetric nature of this novel material. To exploit their interesting physical properties, we developed a chemical synthesis method to perform flat 1D single crystalline epitaxial Sr1+δMn8O16 nanowires films on silicon and SOI substrates.
Chemical Integration of Piezoelectric Oxides Nanomaterials for Improved Sensors