Since the mid 70’s, microelectromechanical systems (MEMS), have had a major impact on our modern, high-technological society. TFPMEMS carries on this MEMS-legacy, currently finding its way into factual technological applications, such as 3D cameras, gas-detectors, smart-homes, virtual and augmented reality devices (VR/AR), and smart hearing (Figure 1a).
Miniaturization drives the need for higher electromechanical robustness
Such devices rely on efficient electromechanical transduction – conversion between electrical and mechanical energy – which enables computers to interact with the physical world through sensors and actuators. The continued miniaturization of MEMS and TFPMEMS (Figure 1b) has driven the need for devices capable of withstanding ever higher electromechanical energy densities while maintaining long term stability and performance in harsh operating conditions. One such example is TFPMEMS THz metasurface-antennas for 6G-applications requiring electromechanically active device structures in the sub-micron range. This is more than 20 times smaller than the width of a human hair! In this size range it is crucial for these structures to electromechanically deform significantly at very low voltages, consuming virtually no power (nW). Today, the energy density is too high for the materials to handle, causing them to irreversibly break down at the field strength needed for novel applications. An important factor remains unsettled: it is unknown if devices break down electrically or mechanically! A multilayer approach has sparked the interest of industry, as it may only lower the operating voltage, but also increase the device’s overall “effective” piezoresponse. This however increases the energy density upon usage even further, driving the need for more electromechanically strong TFPMEMS materials.
Humidity-induced degradation as a critical reliability challenge
On the other end, within the existing application-range, state-of-the-art PZT-based TFPMEMS is robust enough to withstand even the harsh operating conditions of space, projecting lifetimes of several hundred years in a lab environment. Yet, their vulnerability to humidity induces rapid degradation and critical failure within seconds to minutes, after activating one particularly critical mechanism: the splitting of surface water into oxygen and hydrogen gas. As gas-bubbles form inside the material, the pressure increases to several hundred atm., cracking the material open as it tries to escape. Encapsulation using atomic layer deposition (ALD), has been proven effective in protecting against such humidity-induced degradation. Despite their widespread use as moisture barrier in electronics and MEMS, materials such as Al₂O₃, or HfO2 remains susceptible to long-term degradation due to hydrolysis and the conduction of protons along surfaces, interfaces and through the material itself. The presence of only a few nm of water at the surface onsets proton conduction and the subsequent splitting of water.
Robust TFPMEMS for harsh operating conditions
The HERO-project seeks to address both challenges by
- pushing the electrical and mechanical boundaries through compositional and multilayer engineering, and
- elucidate the factors driving surface proton conduction and degradation of the humidity barriers.
Together with Norwegian piezoMEMS industry and world leading research groups, SINTEF seeks to develop the robust TFPMEMS devices required in the futures advanced micro- and nanotechnological devices, and implement mitigation-strategies, preventing critical degradation in realistic to harsh operating conditions.