The asymmetric resonance responses of a thermally actuated silicon microcantilever of a portable, cantilever-based nanoparticle detector (Cantor) is analysed. For airborne nanoparticle concentration measurements, the cantilever is excited in its first in-plane bending mode by an integrated p-type heating actuator. The mass-sensitive nanoparticle (NP) detection is based on the resonance frequency (<i>f</i><sub>0</sub>) shifting due to the deposition of NPs. A homemade phase-locked loop (PLL) circuit is developed for tracking of <i>f</i><sub>0</sub>. For deflection sensing the cantilever contains an integrated piezo-resistive Wheatstone bridge (WB). A new fitting function based on the Fano resonance is proposed for analysing the asymmetric resonance curves including a method for calculating the quality factor <i>Q</i> from the fitting parameters. To obtain a better understanding, we introduce an electrical equivalent circuit diagram (ECD) comprising a series resonant circuit (SRC) for the cantilever resonator and voltage sources for the parasitics, which enables us to simulate the asymmetric resonance response and discuss the possible causes. Furthermore, we compare the frequency response of the on-chip thermal excitation with an external excitation using an in-plane piezo actuator revealing parasitic heating of the WB as the origin of the asymmetry. Moreover, we are able to model the phase component of the sensor output using the ECD. Knowing and understanding the phase response is crucial to the design of the PLL and thus the next generation of Cantor.
An improved nanoscale processing technique by using polystyrene (PS) nanoparticles as a mask is successfully implemented to produce vertically aligned silicon nanowire (SiNW) arrays. Lithographic microstructures with different shapes and opening sizes were applied to determine the fabrication area followed by deposition of a PSS/PDDA/PSS layer. Therefore, most of the substrate areas were covered and a large-range order of PS nanoparticles can be acquired by detailed investigation of spin-coating parameters and surface properties. Afterwards, the particle size was modulated resulting in feature diameters ranging from 459 ± 9 nm down to 248 ± 11 nm. Using this as a mask for inductively coupled plasma (ICP) cryogenic dry etching, a feature-size variation of high-density SiNWs from 225 ± 18 nm to 146 ± 7 nm can be achieved. Finally, a method with simple patterning steps has been developed and tested on more than 100 samples emerging as an alternative method for reliable nanostructure realization.
A transferable force calibration standard based on a silicon microelectromechanical sensor has been designed, fabricated, and characterized for micrometrology applications. Two essential elements of double-meander springs and full piezoresistive etched p-silicon-on-insulator Wheatstone bridges (WBs) are integrated to the sensor for enhancing the device’s sensitivity and eliminating the current leakage during an active sensing operation, respectively. The design process is supported by three-dimensional finite element modeling to select the optimal proposed sensors as well as simulating their mechanical and electrical properties in the desired force range (≤1000 μN). To fabricate the microforce sensors, a bulk micromachining technology is used by frequently involving an inductively coupled plasma deep reactive ion etching at cryogenic temperature. Several optical and electrical characterization techniques have been utilized to ensure the quality of the fabricated WBs, where their measured offset voltage can be down to 0.03±0.071 mV/V. In terms of its linearity, the fabricated device exhibits a small nonlinearity of <3%, which leads this sensor to be appropriate for precise microforce standard.