High performance piezoelectric AlN MEMS resonators for precise sensing in liquids

Abstract

Piezoelectric silicon micro-machined resonators triggered in the two-dimensional roof-tile shaped mode exhibit exceptionally high quality factors even in highly viscous liquids. In combination with an all-electric readout, this enables precise measurements in liquid media with high output signals, if the electrode design and the anchor geometry are tailored to match the selected order of the rooftile shaped mode. This paper provides an overview of this promising class of resonator modes for micro-electromechanical systems (MEMS) and highlights three application scenarios. In the food industry, the MEMS sensor can be used to monitor the fermentation process in wine by tracking the density and hence, the concentration of ethanol. In technical lubricants, the concentration of ferrous particles can be monitored by integrating a magnetic actuator on the resonator. Finally, even in highly viscous bitumen, the MEMS sensor is successfully used to measure the viscosity of this material. Piezoelektrische AlN-basierte MEMS-Resonatoren für hochgenaue Messungen in Flüssigkeiten.Piezoelektrische, durch Mikrofabrikation hergestellte Siliziumresonatoren, die in 2D-"Wellblech"-Schwingungsmoden angeregt werden, weisen selbst in hochviskosen Flüssigkeiten außergewöhnlich hohe Qualitätsfaktoren auf. In Kombination mit einer vollständig elektrischen Anregung und Auslesung ermöglicht dies präzise Messungen in flüssigen Medien mit hohen Ausgangssignalen, wenn das Elektrodendesign und die Ankergeometrie auf die ausgewählte Ordnung der "Wellblech"-Mode abgestimmt sind. Dieser Artikel bietet einen Überblick über diese vielversprechende Klasse von Resonatormoden für mikroelektromechanische Systeme (MEMS) und zeigt drei Anwendungsszenarien auf. In der Lebensmittelindustrie kann der MEMS-Sensor verwendet werden, um den Fermentationsprozess in Wein durch Verfolgung der Dichte und damit der Ethanolkonzentration zu überwachen. In technischen Schmierstoffen kann die Konzentration von Eisenpartikeln durch Integration eines magnetischen Aktuators auf dem Resonator überwacht werden. Abschließend kann der MEMS-Sensor auch in hochviskosem Bitumen erfolgreich zur Messung der Viskosität dieses Materials eingesetzt werden.

MEMS sensor for detection and measurement of ultra-fine particles

“…MEMS sensors can provide a direct mass weighing, as opposed to reference measuring instruments, such as optical particle counters, which indirectly estimate mass [31,32,52]. Moreover, MEMS sensors provide direct detection of particles rather than monitoring changes of parameters that could lead to the formation of particles [53]. This allows for instance, correlating with even subtle change in the condition of an oil used in the industrial machine, thus promoting further measures.…”

“…As depicted in figure 1, thin-film piezoelectric micro-resonant transducers have been widely used for particle mass detection and concentration measurements [20,24,37,53,79,42,45,81,82]. Piezoelectric resonant structure offers the following advantages.…”

“…For the resonator operating in the fluidic environment, the following equation can be used to quantify the fluidic mass [53]:…”

“…This mode was used due to its low viscous damping. The ferrous particles are considered to be responsible for the wear in the mechanical components used in the industry [53]. With a transducer as shown in figure 8, the concentration of the debris (ferrous particles) in the lubricant oil was measured [82].…”

Aluminum Nitride Out-of-Plane Piezoelectric MEMS Actuators

“…Aluminum nitride (AlN) is one of the most studied non-ferroelectric piezoelectric thin films in the last decades, as its crystal is isotropic in the x-y plane but anisotropic along the z-axis [ 42 ]. AlN has been used as a piezoelectric thin film in many applications, including energy harvesters [ 43 ], microphones [ 44 ], inertial sensors [ 45 ], and bulk acoustic wave resonators [ 46 ]. Despite the relatively low piezoelectric coefficients of AlN, it remains well suited in applications such as chip-to-chip alignment, where small out-of-plane displacements are required, due to its unique characteristics, such as its high Young’s modulus, MEMS-CMOS compatibility [ 25 ], ease of deposition [ 47 ], and good optical properties [ 48 ], which make it a good candidate for integrated photonic devices.…”