Dielectric Actuation Techniques at the Nanoscale: Piezoelectricity and Flexoelectricity (2019)

Micro- and nanoelectromechanical systems (MEMS/NEMS) have long shown their potential to disrupt the established technologies. Over the past 15 years, MEMS have become fundamental components in filters, accelerometers, gyroscopes and gas sensors. MEMS are now an essential part of most of the electronic devices we own. However, their smaller counterpart, NEMS, are still far from industrialization due to two challenges: their fabrication and the transduction of their motion. One transduction technique to control the motion of NEMS is piezoelectricity, which can be directly integrated, has low power consumption and exhibits linear transduction. Piezoelectric aluminum nitride (AlN) deposited by reactive sputtering has been applied in several MEMS applications and recently in NEMS. There is a good understanding of the sputtering parameters and substrate conditions needed to fabricate high quality micrometer thick AlN films, but less is known for nanometer thick films. One part of the thesis was focused on the impact of reactive sputtering parameters and substrate conditions on the c-axis texture and piezoelectric coefficients of 50-100 nm thick AlN films. We found that substrate temperature, sputtering power and type as well as quality of substrate have a strong impact on AlN c-axis texture. By optimizing these conditions, we were able to fabricate 50 nm thick AlN films with crystalline and piezoelectric properties similar to micrometer thick films. Recently, AlN doped with scandium (AlScN) has been developed due to its larger piezoelectric coefficients compared to undoped AlN. We found a strong influence of two sputtering parameters, argon gas concentration and substrate bias, on the c-axis texture and density of abnormal grains in 1 micrometer thick Al0.6Sc0.4N films. A fundamental issue in piezoelectric NEMS is that an asymmetric thickness cross section is necessary to create a flexural curvature and the asymmetry will reduce with decreasing system thickness. To avoid this issue, an alternative transduction technique based on flexoelectricity can be implemented, which has three advantages over piezoelectricity: it is theorized to exist in all dielectrics, it is predicted that its coefficients do not decrease with decreasing film thickness and any thickness cross section can be used to fabricate flexural NEMS. Most experimental work in flexoelectricity has been on ferroelectric or paraelectric materials; little research has been conducted on materials like CMOS-compatible, high-k dielectrics which could replace piezoelectric thin films in NEMS. Therefore, another part of this thesis focused on the fabrication and characterization of flexoelectric actuators based on amorphous hafnium oxide (HfO2). The fabricated 40 nm thick HfO2 films were non-ferroelectric and an effective flexoelectric coefficient of 37 pC/m was measured. The final part of the thesis analyzed the potential curvature generation of piezoelectric and flexoelectric films with nanoscale thicknesses, as well as provided a better understanding of the relationship between experimental flexoelectric coefficients and relative permittivity. Based on these analyses, we envision that flexoelectric thin films based on high-k dielectric materials with relative permittivities between 50-100 are a promising alternative to piezoelectric thin films in nanoscale actuation.