Structuration of piezotronic junctions for ultrasensitive strain sensors

Doctoral thesis (2021)

The piezotronic effect relies on the creation of piezoelectric polarization charges mechanically induced within a piezoelectric semiconductor to modulate the carrier dynamics across electronic contact ... [more ▼]

The piezotronic effect relies on the creation of piezoelectric polarization charges mechanically induced within a piezoelectric semiconductor to modulate the carrier dynamics across electronic contact interfaces. The field of piezotronics is a relatively new area of study, based on a mechanical signal triggering, which is one of the most common kind of interactions between the environment and electronic systems. It started to draw a considerable attention in the early 2010’s, by reaching higher electromechanical sensitivities when compared to conventional methods of sensing. The rapidly spreading Internet-of-Things is accelerating Micro-ElectroMechanical Systems (MEMS) industry to deliver highly sensitive and miniaturized self-sensing sensors with low power consumption and cost-effective production process. Within this context, strain sensors based on the piezotronic effect appear as promising candidates to address these needs. However, several crucial questions remain unanswered or need to be refined, concerning the design and integration of piezotronic junctions with its fabrication process into microsystems or MEMS, the optimal configuration for strain sensing as well as noise studies for such systems. This PhD thesis proposes to rationalize the piezotronic effect for strain sensors and presents a novel microfabrication process integrating for the first time piezotronic strain sensors in millimetre-sized cantilevers on flexible polymeric substrates by means of maskless laser lithography. The atomic layer deposition (ALD) technique was used for the deposition of ZnO polycrystalline thin films on high work function metals to obtain Schottky junctions. However, such ZnO-based Schottky junctions by ALD have never been post-processed and integrated into a strain sensor. We propose to rationalize the ALD processing to obtain wurtzite polycrystalline zinc oxide thin films with a privileged (002) orientation and to make it compatible with microfabrication processing on polymer. The difficulties linked with the integration of inorganic thin films onto a polymeric substrate within the developed microfabrication process will be highlighted. We propose appropriate adjustments of the sensor’s design and the process flow. Pt/ZnO/Pt back-to-back Schottky diode junctions have been shaped in interdigitated microelectrodes to get piezotronic strain sensing on the clamp area of the cantilever structure. The conduction mechanisms occurring within the piezotronic strain microsensors have been thoroughly studied, based on the thermionic emission model. The developed electrical model will be detailed, emphasizing the presence of interface trap states and their prominent impact on the electrical characteristics. The piezotronic strain sensors’ transducing properties will be detailed as well by the mean of force spectroscopy, leading to the expected Schottky barrier height modulation by the piezotronic effect. Furthermore, we investigated for the first time the noise figure of within strain sensors based on the piezotronic effect. These new insights about noise amplitudes and origins are promising matter of optimization to improve the signal-to-noise ratio of the sensor. Within the last section of this work, we will detail the piezotronic strain sensors size miniaturization for integration in microcantilevers in a full-SU8 body. The miniaturization of our strain sensors makes them more prone for AFM (Atomic Force Microscopy) scanning probe operations on commercial machines, with the aim of greatly improving the sensitivity to small mechanical deformations. The approach taken for the microfabrication of these miniaturized sensors is based on a reversed processing by the mean of a sacrificial layer. This raised new difficulties in terms of metal adhesion and electrical contact continuity, which will be reported. The results obtained are highly promising and pave the way towards the processing of ultrasensitive strain microsensors on MEMS structures, as well as their great potential for AFM scanning probe operations. [less ▲]