Maik Bertke

Gravimetric humidity sensor based on ZnO nanorods covered piezoresistive Si microcantilever

A ZnO nanorods film covered silicon resonant cantilever sensor is developed for atmosphere humidity detection by monitoring the resonant frequency shifts induced by the additional weight of adsorbed water molecules. Two different crystalline seed-layer deposition methods were applied to grow different nanorods films. The morphology of the ZnO films were characterized and the sensor sensitivities were measured under different relative humidity (RH) levels. The experiments results showed that this novel humidity sensor with ZnO nanorods has a sensitivity of 101.5 ± 12.0 ppm/RH% (amount of adsorbed water of 36.9 ± 4.4 ng/RH%), indicating its potential for portable sensing applications.

Asymmetric resonance frequency analysis of in-plane electrothermal silicon cantilevers for nanoparticle sensors

The asymmetric resonance frequency analysis of silicon cantilevers for a low-cost wearable airborne nanoparticle detector (Cantor) is described in this paper. The cantilevers, which are operated in the fundamental in-plane resonance mode, are used as a mass-sensitive microbalance. They are manufactured out of bulk silicon, containing a full piezoresistive Wheatstone bridge and an integrated thermal heater for reading the measurement output signal and stimulating the in-plane excitation, respectively. To optimize the sensor performance, cantilevers with different cantilever geometries are designed, fabricated and characterized. Besides the resonance frequency, the quality factor (Q) of the resonance curve has a high influence concerning the sensor sensitivity. Because of an asymmetric resonance behaviour, a novel fitting function and method to extract the Q is created, different from that of the simple harmonic oscillator (SHO). For testing the sensor in a long-term frequency analysis, a phase- locked loop (PLL) circuit is employed, yielding a frequency stability of up to 0.753 Hz at an Allan variance of 3.77 × 10-6. This proposed asymmetric resonance frequency analysis method is expected to be further used in the process development of the next-generation Cantor.

Size-selective electrostatic sampling and removal of nanoparticles on silicon cantilever sensors for air-quality monitoring

This paper reports the size-selective sampling and removal of airborne nanoparticles (NP) using a silicon resonant micro cantilever mass balance combined with a miniaturized electrostatic precipitator (ESP). This allows to design an overnight-refreshable, pocket-sized, autarkic monitor for NP mass concentration and size in the range below 300 nm.

Asymmetric resonance response analysis of a thermally excited silicon microcantilever for mass-sensitive nanoparticle detection

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 (f0) shifting due to the deposition of NPs. A homemade phase-locked loop (PLL) circuit is developed for tracking of f0. 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 Q 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.

Large-area fabrication of silicon nanostructures by templated nanoparticle arrays

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.