Stephan Merzsch

A resonant cantilever sensor for monitoring airborne nanoparticles

In this paper, a silicon resonant cantilever sensor is used for monitoring airborne nanoparticles (NPs) by detecting the resonant frequency shift that is directly induced by an additional NPs mass deposited on it. A piezoelectric stack actuator and a self-sensing technique using a piezoresistive strain gauge are involved in the sensor system in order to actuate and detect the oscillation of cantilever sensor, respectively. The dielectrophoresis (DEP) method is employed for trapping the airborne NPs in a stable carbon aerosol assessment. A thermal-induced frequency shift is also investigated with the purpose of observing the limitation imposed by thermal effects on the minimum detectable NPs mass. The proposed sensor reveals a mass sensitivity of 8.33 Hz/ng, a fundamental resonant frequency of 43.92 kHz, a quality factor of 1230, and a temperature coefficient of the resonant frequency (TCf) of −28.6 ppm/°C. The results demonstrate a possibility of using this resonant cantilever in mobile airborne sensor applications.

Low-weight electrostatic sampler for airborne nanoparticles

This paper presents a novel sampler for airborne nanoparticles (NPs). A synthetic aerosol is forced through a tiny tube containing a self-sensing cantilever for attached NPs mass determination. The sampler is miniaturized (tube mass = 39 g, volume = 18.85 cm3) to assure its applicability for personal NPs exposure monitoring Using the first prototype of the sampler an added mass 2.69 ± 0.02 ng was measured under typical test conditions, i.e. airborne carbon NPs of a concentration of 5941 ± 145 NPs/cm3 during a period of 15 min.

MEMS-based silicon cantilevers with integrated electrothermal heaters for airborne ultrafine particle sensing

The development of low-cost and low-power MEMS-based cantilever sensors for possible application in hand-held airborne ultrafine particle monitors is described in this work. The proposed resonant sensors are realized by silicon bulk micromachining technology with electrothermal excitation, piezoresistive frequency readout, and electrostatic particle collection elements integrated and constructed in the same sensor fabrication process step of boron diffusion. Built-in heating resistor and full Wheatstone bridge are set close to the cantilever clamp end for effective excitation and sensing, respectively, of beam deflection. Meanwhile, the particle collection electrode is located at the cantilever free end. A 300 μm-thick, phosphorus-doped silicon bulk wafer is used instead of silicon-on-insulator (SOI) as the starting material for the sensors to reduce the fabrication costs. To etch and release the cantilevers from the substrate, inductively coupled plasma (ICP) cryogenic dry etching is utilized. By controlling the etching parameters (e.g., temperature, oxygen content, and duration), cantilever structures with thicknesses down to 10 - 20 μm are yielded. In the sensor characterization, the heating resistor is heated and generating thermal waves which induce thermal expansion and further cause mechanical bending strain in the out-of-plane direction. A resonant frequency of 114.08 ± 0.04 kHz and a quality factor of 1302 ± 267 are measured in air for a fabricated rectangular cantilever (500x100x13.5 μm3). Owing to its low power consumption of a few milliwatts, this electrothermal cantilever is suitable for replacing the current external piezoelectric stack actuator in the next generation of the miniaturized cantilever-based nanoparticle detector (CANTOR).

Use of self-sensing piezoresistive Si cantilever sensor for determining carbon nanoparticle mass

A silicon cantilever with slender geometry based Micro Electro Mechanical System (MEMS) for nanoparticles mass detection is presented in this work. The cantilever is actuated using a piezoactuator at the bottom end of the cantilever supporting frame. The oscillation of the microcantilever is detected by a self-sensing method utilizing an integrated full Wheatstone bridge as a piezoresistive strain gauge for signal read out. Fabricated piezoresistive cantilevers of 1.5 mm long, 30 μm wide and 25 μm thick have been employed. This self-sensing cantilever is used due to its simplicity, portability, high-sensitivity and low-cost batch microfabrication. In order to investigate air pollution sampling, a nanoparticles collection test of the piezoresistive cantilever sensor is performed in a sealed glass chamber with a stable carbon aerosol inside. The function principle of cantilever sensor is based on detecting the resonance frequency shift that is directly induced by an additional carbon nanoparticles mass deposited on it. The deposition of particles is enhanced by an electrostatic field. The frequency measurement is performed off-line under normal atmospheric conditions, before and after carbon nanoparticles sampling. The calculated equivalent mass-induced resonance frequency shift of the experiment is measured to be 11.78 ± 0.01 ng and a mass sensitivity of 8.33 Hz/ng is obtained. The proposed sensor exhibits an effective mass of 2.63 μg, a resonance frequency of 43.92 kHz, and a quality factor of 1230.68 ± 78.67. These results and analysis indicate that the proposed self-sensing piezoresistive silicon cantilever can offer the necessary potential for a mobile nanoparticles monitor.

Silicon Nanowire Resonators: Aerosol Nanoparticle Mass Sensing in the Workplace

In this article, We focus on silicon nanowire (SiNW)-based resonators that were fabricated and employed to sense aerosol nanoparticles (NPs) by measuring resonant frequency shifts induced by the mass of stuck NPs. The fabrication of SiNW arrays was performed using inductively coupled plasma (ICP) cryogenic dry etching and multiple thermal oxidations. The SiNWs were coated with gold (Au) for contacting to the homebuilt electrostatic NP sampler to collect the flowing NPs. A piezoelectric shear actuator mounted in the frequency measurement system was used to excite the SiNW sensors into resonance. Tested in a titanium dioxide (TiO2) aerosol sampling with a total concentration of ~8,500 NPs/cm3, the sensor displayed its feasibility as a nanobalance to detect aerosol NPs in the femtogram scale with a mass sensitivity of 7.1 Hz/fg and a mass resolution of 31.6 fg. To extend the operating life of the sensor, an ultrasonic removal method was used to detach the adhered NPs.

Enhanced airborne nanoparticles mass sensing using a high-mode resonant silicon cantilever sensor

A high-mode resonant silicon cantilever sensor is developed for detection of airborne nanoparticles (NPs) by monitoring the change in resonant frequency induced by an additional trapped NPs mass. A piezoresistive bridge is integrated in the cantilever for signal sensing. An electrostatic method is employed to trap the NPs on the cantilever surface. The experimental results indicate that the cantilever sensor operated in the second resonant mode exhibits higher quality factor than the fundamental mode, i.e. 2100, implying that a higher sensitivity, i.e. 32.75 Hz/ng, can be attained by operation at higher resonant mode. The influences of thermal, pressure and relative humidity, respectively, on the sensor have also been investigated with the purpose of observing the limitation of sensor sensitivity imposed by the environment.

Design and fabrication of piezoresistive p-SOI Wheatstone bridges for high-temperature applications

For future measurements while depth drilling, commercial sensors are required for a temperature range from -40 up to 300 °C. Conventional piezoresistive silicon sensors cannot be used at higher temperatures due to an exponential increase of leakage currents which results in a drop of the bridge voltage. A well-known procedure to expand the temperature range of silicon sensors and to reduce leakage currents is to employ Silicon-On-Insulator (SOI) instead of standard wafer material. Diffused resistors can be operated up to 200 °C, but show the same problems beyond due to leakage of the p-njunction. Our approach is to use p-SOI where resistors as well as interconnects are defined by etching down to the oxide layer. Leakage is suppressed and the temperature dependence of the bridges is very low (TCR = (2.6 ± 0.1) μV/K@1 mA up to 400 °C). The design and process flow will be presented in detail. The characteristics of Wheatstone bridges made of silicon, n- SOI, and p-SOI will be shown for temperatures up to 300 °C. Besides, thermal FEM-simulations will be described revealing the effect of stress between silicon and the silicon-oxide layer during temperature cycling.

Self-exciting and self-sensing resonant cantilever sensors for improved monitoring of airborne nanoparticles exposure

Self-exciting and self-sensing resonant cantilever sensors for airborne nanoparticles (NPs) monitoring are investigated. A fabricated self-sensing piezoresistive cantilever sensor is actuated using a piezo stack and operated in the second resonant mode. It is able to detect a resonant frequency shift of 33.58 Hz that corresponds to an airborne carbon NPs mass of 1.03 ng deposited during a 15-min sampling. The quality factor is 2100 resulting in a mass resolution of the sensor of 9.8 pg. In order to realize a portable airborne NPs monitoring device, thermally excited silicon cantilevers vibrating in different modes are proposed. The integration designs of heating resistors, sensing piezoresistors and electrostatic NPs sampling are further analyzed using finite-element modeling (FEM). An electrode for electrostatic NPs sampling is placed on the free-end of the cantilever. The results indicate that a self-sensing silicon electrothermal cantilever can fulfill the requirements of portable monitoring of airborne NPs exposure.

Low-cost wearable cantilever-based nanoparticle sensor microsystem for personal health and safety monitoring

A low-cost pocket-sized airborne nanoparticle (NP) detector based on a silicon microelectromechanical cantilever is described oscillating at its fundamental in-plane resonance frequency. For direct reading of NP concentrations, the monitoring system consists of signal-conditioning electronics for NP sampling as well as tracking of frequency shift by the attached NP mass. Weight and cost of this first fully integrated personal gravimetric airborne NP monitor (Cantor-2) are comparable to low-cost optical fine-particle sensors but beyond this it can detect ultra-fine particles (UFPs) of less than 100 nm in diameter. Real-time measurements with engineered carbon NPs as well as carbon black NPs exhibited a zero-offset linear correlation to reference measurements using a standard fast mobility particle sizer (FMPS). A limit of detection (LOD) of better than 10 μg/m3 was found within a response time of 6 min, i.e., a typical short-term NP exposure duration.

In-plane-excited silicon nanowire arrays-patterned cantilever sensors for enhanced airborne particulate matter exposure detection

This paper presents the design, fabrication, and use of silicon nanowire (SiNW) arrays-patterned microcantilever sensors excited in the in-plane resonance mode to enhance the detection of airborne particulate matter (PM). Electrothermal excitation elements of p-diffused heating resistors were introduced in the current sensor system to replace the formerly used external piezoceramic stack actuator. The sensors exhibited high measured quality factors (Q-factors) of 4702 ± 102 during their in-plane mode operations in air, which are four times larger than those of the fundamental out-of-plane mode. To selectively define arrays of vertical SiNWs on the surface of the micromechanical cantilever, nanoimprint lithography (NIL) is combined with conventional photolithography. The diameter and position of the SiNWs can be adjusted depending on the nanoimprint stamp with the smallest cylindrical pattern possible down to 50 nm in diameter. By modifying the resonator surface, the PM sampling efficiency can be improved by a factor of 1.5 greater than that of a corresponding plain cantilever in a cigarette smoke exposure experiment because of the rise in collection surface area of the sensor given by the SiNWs.