RF MEMS Resonators for Mass Sensing Applications (2015)

Sensing devices developed upon resonant microelectromechanical and nanoelectromechanical (M/NEMS) system technology have become one of the most attractive areas of research over the past decade. These devices make exceptional sensing platforms because of their miniscule dimensions and resonant modes of operation, which are found to be extremely sensitive to added mass. Along their unique sensing attributes, they also offer foundry compatible microfabrication processes, low DC power consumption, and CMOS integration compatibility. In this work, electrostatically and piezoelectrically actuated RF MEMS bulk resonators have been investigated for mass sensing applications. The capacitively-transduced resonators employed electrostatic actuation to achieve desired resonance mode shapes. These devices were fabricated on silicon-on-insulator (SOI) substrates with a device layer resistivity ranging from 0.005 Ω cm to 0.020 Ω cm. The electrode-to-resonator capacitive gap was defined by two different techniques: oxidation enabled gap reduction and sacrificial atomic layer deposition (ALD). For oxidation enabled gap reduction, a hard mask composed of silicon nitride and polysilicon is deposited, patterned, and defined using standard MEMS thin-film layer deposition and fabrication techniques. The initial lithographically-defined capacitive gap of 1 μm is further reduced to ~300 nm by a wet furnace oxidation process. Subsequently, the reduced gap is transferred to the device layer using a customized dry high-aspect-ratio dry etching technique. For sacrificial approach, a ~100 nm-thin ALD aluminum oxide sidewall spacer is chemically etched away as the last microfabrication step to define the ~100 nm capacitive gap. Small capacitive gaps developed in this work results in small motional resistance (Rm) values, which relax the need of the read-out circuitry by enhancing the signal transduction. Piezoelectrically-actuated resonators were developed using thin-film bulk acoustic resonant (FBAR or TFBAR) and thin-film piezoelectric-on-substrate (TPoS) technologies with reported Q factors and resonant frequencies as high as 10,638 and 776.54 MHz, respectively, along with measured motional resistance values as low as 169Ω. To the best of our knowledge, this work is the first one that demonstrated TPoS resonators using LPCVD polysilicon as an alternative low loss structural layer to single-crystal silicon with Q factors as high as ~3,000 (in air) and measured motional resistance values as low as 6 kΩ with an equivalent acoustic velocity of 6,912 m s-1 for a 7 μm thick layer. Polysilicon based TPoS single devices were measured with the coefficient of resonant frequency of -3.77 ppm/°C, which was the lowest ever reported for this type of devices. Also a novel releasing process, thin-piezo on single crystal reactive etched (TPoSCRE), allows us to develop of TPoS resonators without the need to SOI wafers. The fabricated devices using this technique were reported with Q factor exceeding ~1,000 and measured motional resistance values as low as 9 kΩ.\nThe sensitivity of a fourth-order contour mode ZnO-on-SOI disk resonator based mass sensor was determined by performing multiple depositions of platinum micro-pallets using a focus ion beam (FIB) equipped with gas injection system on strategically-chosen locations. It was found out that the sensitivity of the resonator on its maximal and minimal displacement points was of 1.17 Hz fg-1 and 0.334 Hz fg-1, respectively. Also, the estimated limit of detection of the resonator was found to be a record breaking 367 ag (1 ag = 10-18g) compared to devices with similar modes of resonance. Lastly, a lateral-extensional resonator was used to measure the weight of HKUST-1 MOF crystal cluster. The weight of it was found to be 24.75 pg and 31.19 pg by operating two lateral resonant modes, respectively.