Fabrication and Characterization of Nanomechanical Resonators as Highly Sensitive Mass Sensors
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Nanoelectromechanical (NEM) resonators have been used to detect masses of organic or inorganic particles in nanoscale or even atomic level. A reduction in the resonator mass can increase its mass sensitivity (frequency shift per loaded mass). However, the operation of most small resonators is restricted to vacuum or air since operation in liquid sharply decreases their quality factor (Q) due to the excessive damping resulting from liquid viscosity. Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is, and higher Q indicates a lower rate of energy loss relative to the stored energy of the resonator. Typically, large size resonators such as Quartz Crystal Microbalance (QCM) are used for mass detection in liquid to preserve a high Q factor that determines the resolution of measurements. However, as it was mentioned earlier, such resonators cannot offer high sensitivity due to their relatively large size and mass. Therefore, highly-sensitive resonators capable of real-time mass measurement with high Q both in air and liquid currently do not exist. Thermal piezoresistive disk resonator surface merely slides alongside the solid-liquid interface in the rotational mode, as opposed to paddling against the surrounding liquid offering high Q. In this dissertation, thermal-piezoresistive disk resonators with much smaller dimensions in the deep submicron range have been fabricated using electron beam lithography (EBL), and the effect of scaling on mass sensitivity, power consumption and quality factor (Q) is investigated. Disk resonators with diameter ranging from 2µm to 20µm with thermal actuator beams as narrow as 35nm have been fabricated via electron beam lithography. Mass sensitivity of disk resonators was characterized in air by formation of a self-assembled monolayer of hexa-methyl-disilazane (HMDS) on the surfaces. Frequency shifts as high as 318 Hz were measured for a calculated deposited mass of one attogram using a 2µm diameter disk resonator resonating at 221MHz. Operation in liquid was characterized by exposing a 20µm disk resonator to a 10mM solution of mercaptohexanol (MCH) diluted in ethyl alcohol (ethanol). For this experiment, frequency shift of 20 kHz for 2.8 pg of added MCH mass was obtained. In conventional rotational mode disk resonators, as the dimensions scale down, the mechanical losses including anchor loss increase. This adversely affects the detection of the resonance mode at higher frequencies. To alleviate this issue, donut-shaped resonators have been proposed potentially offering higher Qs while resonating at higher frequencies. Mass sensitivity of donut resonators with different sizes has been investigated with deposited 10nm gold nanoparticles (AuNPs) as added mass showing mass sensitivity of 36 Hz/attogram (712 Hz/AuNp) in air characterization. Due to the reduction of the surface area, the probability of adsorption of molecules or particulates of interest onto the NEM resonator surfaces diminishes. To address this issue, forests of multiwall carbon nanotubes (MWCNTs) have been used to enhance the effective surface area, which allows detection of much lower concentrations of analytes. Using this approach, average effective surface area enhancement as high as 9 times for organic and inorganic nanoparticles was demonstrated.