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.
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.
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.
The paper summarize the PTB activities in the field of silicon sensors for dimensional metrology especially roughness measurements and silicon calibration standards developed during the past ten years. A focus lies in the development of 2D silicon microprobes which enable roughness measurements in nozzles as small as 100 μm in diameter. Moreover these microprobes offer the potential for very fast tactile measurements up to 15 mm/s due to their tiny mass and therefore small dynamic forces. When developing high precision tactile sensors care has to be taken, not to scratch the often soft surfaces. Small probing forces and well defined tip radii have to be used to avoid surface destruction. Thus probing force metrology and methods to determine the radius and form of probing tips have been developed. Silicon is the preferred material for the calibration of topography measuring instruments due to its excellent mechanical and thermal stability and due to the fabrication and structuring possibilities of silicon microtechnology. Depth setting standards, probing force setting standards, tip radius and tip form standards, reference springs and soft material testing artefacts will be presented.
Low-cost and low-power piezoresistive cantilever resonators with integrated electrothermal heaters are developed to support the sensing module enhancement of the second generation of handheld cantilever-based airborne nanoparticle (NP) detector (CANTOR-2). These sensors are used for direct-reading of exposure to carbon engineered nanoparticles (ENPs) at indoor workplaces. The cantilever structures having various shapes of free ends are created using silicon bulk micromachining technologies (i.e, rectangular, hammer-head, triangular, and U-shaped cantilevers). For a complete wearable CANTOR-2, all components of the proposed detector can be grouped into two main units depending on their packaging placements (i.e., the NP sampler head and the electronics mounted in a handy-format housing). In the NP sampler head, a miniaturized electrophoretic aerosol sampler and a resonant silicon cantilever mass sensor are employed to collect the ENPs from the air stream to the cantilever surfaces and measuring their mass concentration, respectively. After calibration, the detected ENP mass concentrations of CANTOR-2 show a standard deviation from fast mobility particle sizer (FMPS, TSI 3091) of 8–14%.
Silicon microforce sensors, to be used as a transferable standard for micro force and depth scale calibrations of hardness testing instruments, are developed using silicon bulk micromachining technologies. Instead of wet chemical etching, inductively coupled plasma (ICP) cryogenic deep reactive ion etching (DRIE) is employed in the sensor fabrication process leading to more precise control of 300 μm deep structures with smooth sidewall profiles. Double meander springs are designed flanking to the boss replacing the conventional rectangular springs and thereby improving the system linearity. Two full p-SOI piezoresistive Wheatstone bridges are added on both clamped ends of the active sensors. To realize passive force sensors two spring-mass elements are stacked using glue and photoresist as joining materials. Correspondingly, although plastic deformation seems to occur when the second spring is contacted, the kink effect (i.e., abrupt increase of stiffness) is obviously observed from the first test of the passive stack sensor.
Silicon microprobe tips are fabricated and integrated with piezoresistive cantilever sensors for high-speed surface roughness scanning systems. The fabrication steps of the high-aspect-ratio silicon microprobe tips were started with photolithography and wet etching of potassium hydroxide (KOH) resulting in crystal-dependent micropyramids. Subsequently, thin conformal wear-resistant layer coating of aluminum oxide (Al2O3) was demonstrated on the backside of the piezoresistive cantilever free end using atomic layer deposition (ALD) method in a binary reaction sequence with a low thermal process and precursors of trimethyl aluminum and water. The deposited Al2O3 layer had a thickness of 14 nm. The captured atomic force microscopy (AFM) image exhibits a root mean square deviation of 0.65 nm confirming the deposited Al2O3 surface quality. Furthermore, vacuum-evaporated 30-nm/200-nm-thick Au/Cr layers were patterned by lift-off and served as an etch mask for Al2O3 wet etching and in ICP cryogenic dry etching. By using SF6/O2 plasma during inductively coupled plasma (ICP) cryogenic dry etching, micropillar tips were obtained. From the preliminary friction and wear data, the developed silicon cantilever sensor has been successfully used in 100 fast measurements of 5- mm-long standard artifact surface with a speed of 15 mm/s and forces of 60–100 μN. Moreover, the results yielded by the fabricated silicon cantilever sensor are in very good agreement with those of calibrated profilometer. These tactile sensors are targeted for use in high-aspect-ratio microform metrology.
Nanopillar-based structures hold promise as highly sensitive resonant mass sensors for a new generation of aerosol
nanoparticle (NP) detecting devices because of their very small masses. In this work, the possible use of a silicon
nanopillar (SiNPL) array as a nanoparticle sensor is investigated. The sensor structures are created and simulated using a
finite element modeling (FEM) tool of COMSOL Multiphysics 4.3 to study the resonant characteristics and the
sensitivity of the SiNPL for femtogram NP mass detection. Instead of using 2D plate models or simple single 3D
cylindrical pillar models, FEM is performed with SiNPLs in 3D structures based on the real geometry of experimental
SiNPL arrays employing a piezoelectric stack for resonant excitation. In order to achieve an optimal structure and
investigate the etching effect on the fabricated resonators, SiNPLs with different designs of meshes, sidewall profiles,
lengths, and diameters are simulated and analyzed. To validate the FEM results, fabricated SiNPLs with a high aspect
ratio of ~60 are employed and characterized in resonant frequency measurements. SiNPLs are mounted onto a
piezoactuator inside a scanning electron microscope (SEM) chamber which can excite SiNPLs into lateral vibration. The
measured resonant frequencies of the SiNPLs with diameters about 650 nm and heights about 40 μm range from 434.63
kHz to 458.21 kHz, which agree well with those simulated by FEM. Furthermore, the deflection of a SiNPL can be
enhanced by increasing the applied piezoactuator voltage. By depositing different NPs (i.e., carbon, TiO2, SiO2, Ag, and
Au NPs) on the SiNPLs, the decrease of the resonant frequency is clearly shown confirming their potential to be used as
airborne NP mass sensor with femtogram resolution level.
Vertical silicon nanowire (SiNW) resonators are designed and fabricated in order to assess exposure to aerosol
nanoparticles (NPs). To realize SiNW arrays, nanolithography and inductively coupled plasma (ICP) deep reactive ion
etching (DRIE) at cryogenic temperature are utilized in a top-down fabrication of SiNW arrays which have high aspect
ratios (i.e., up to 34). For nanolithography process, a resist film thickness of 350 nm is applied in a vacuum contact mode
to serve as a mask. A pattern including various diameters and distances for creating pillars is used (i.e., 400 nm up to
5 μm). In dry etching process, the etch rate is set high of 1.5 μm/min to avoid underetching. The etch profiles of Si wires
can be controlled aiming to have either perpendicularly, negatively or positively profiled sidewalls by adjusting the
etching parameters (e.g., temperature and oxygen content). Moreover, to further miniaturize the wire, multiple sacrificial
thermal oxidations and subsequent oxide stripping are used yielding SiNW arrays of 650 nm in diameter and 40 μm in
length. In the resonant frequency test, a piezoelectric shear actuator is integrated with the SiNWs inside a scanning
electron microscope (SEM) chamber. The observation of the SiNW deflections are performed and viewed from the
topside of the SiNWs to reduce the measurement redundancy. Having a high deflection of ~10 μm during its resonant
frequency of 452 kHz and a low mass of 31 pg, the proposed SiNW is potential for assisting the development of a
portable aerosol resonant sensor.
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).
A closed loop circuit capable of tracking resonant frequencies for MEMS-based piezoresistive cantilever resonators is
developed in this work. The proposed closed-loop system is mainly based on a phase locked loop (PLL) circuit. In order
to lock onto the resonant frequency of the resonator, an actuation signal generated from a voltage-controlled oscillator
(VCO) is locked to the phase of the input reference signal of the cantilever sensor. In addition to the PLL component, an
instrumentation amplifier and an active low pass filter (LPF) are connected to the system for gaining the amplitude and
reducing the noise of the cantilever output signals. The LPF can transform a rectangular signal into a sinusoidal signal
with voltage amplitudes ranging from 5 to 10 V which are sufficient for a piezoactuator input (i.e., maintaining a large
output signal of the cantilever sensor). To demonstrate the functionality of the system, a self-sensing silicon cantilever
resonator with a built-in piezoresistive Wheatstone bridge is fabricated and integrated with the circuit. A piezoactuator is
utilized for actuating the cantilever into resonance. Implementation of this closed loop system is used to track the
resonant frequency of a silicon cantilever-based sensor resonating at 9.4 kHz under a cross-sensitivity test of ambient
temperature. The changes of the resonant frequency are interpreted using a frequency counter connected to the system.
From the experimental results, the temperature sensitivity and coefficient of the employed sensor are 0.3 Hz/°C and 32.8
ppm/°C, respectively. The frequency stability of the system can reach up to 0.08 Hz. The development of this system
will enable real-time nanoparticle monitoring systems and provide a miniaturization of the instrumentation modules for
cantilever-based nanoparticle detectors.
A method for die-attach based on sintering of micro- and nano-silver-particles, which is stable in harsh environments, was described. A modified flip-chip bonder providing high placement accuracy was used for precise pick and place die-attach. Components of sensors designed for data logging during deep drilling, i.e., a MEMS vibration sensor and a MOEMS pressure sensor, were assembled and tested at temperatures up to 250°C. Shear tests of bonded devices were performed before and after temperature load. Bonded silicon-on-insulator Wheatstone bridges and GaP-PD were tested by temperature cycling (50 cycles from 100°C up to 250°C).
Thermoelectric modules convert thermal energy into electrical energy and vice versa. At present bismuth telluride is the
most widely commercial used material for thermoelectric energy conversion. There are many applications where bismuth
telluride modules are installed, mainly for refrigeration. However, bismuth telluride as material for energy generation in
large scale has some disadvantages. Its availability is limited, it is hot stable at higher temperatures (>250°C) and
manufacturing cost is relatively high. An alternative material for energy conversion in the future could be silicon. The
technological processing of silicon is well advanced due to the rapid development of microelectronics in recent years.
Silicon is largely available and environmentally friendly. The operating temperature of silicon thermoelectric generators
can be much higher than of bismuth telluride. Today silicon is rarely used as a thermoelectric material because of its high
thermal conductivity. In order to use silicon as an efficient thermoelectric material, it is necessary to reduce its thermal
conductivity, while maintaining high electrical conductivity and high Seebeck coefficient. This can be done by
nanostructuring into arrays of pillars. Fabrication of silicon pillars using ICP-cryogenic dry etching (Inductive Coupled
Plasma) will be described. Their uniform height of the pillars allows simultaneous connecting of all pillars of an array.
The pillars have diameters down to 180 nm and their height was selected between 1 micron and 10 microns.
Measurement of electrical resistance of single silicon pillars will be presented which is done in a scanning electron
microscope (SEM) equipped with nanomanipulators. Furthermore, measurement of thermal conductivity of single pillars
with different diameters using the 3ω method will be shown.
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.
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.
Nanoparticles are easily attracted by surfaces. This sticking behavior makes it difficult to clean contaminated samples.
Some complex approaches have already shown efficiencies in the range of 90%. However, a simple and cost efficient
method was still missing. A commonly used silicone for soft lithography, PDMS, is able to mold a given surface. This
property was used to cover surface-bonded particles from all other sides. After hardening the PDMS, particles are still
embedded. A separation of silicone and sample disjoins also the particles from the surface. After this procedure, samples
are clean again. This method was first tested with carbon particles on Si surfaces and Si pillar samples with aspect ratios
up to 10. Experiments were done using 2 inch wafers, which, however, is not a size limitation for this method.
Aluminium nitride (AlN) reactively sputter deposited from an aluminium target is an interesting compound material due
to its CMOS compatible fabrication process and its piezoelectric properties. The crystal structure obtained during
sputtering is a very importance criterion to obtain a good piezoelectric performance. To demonstrate this, we focused our
investigations on two types of films. The first type shows a good c- axis orientation with round grain geometry. The
second type is (101) oriented having a triangular grain shape. For measuring the out-of-plane displacements for dij
determination, a MSV 400 Polytec scanning laser Doppler vibrometer was used. To obtain the piezoelectric constants d33
and d31 a fitting procedure between experimental and theoretical predicted results is used. Effective values for d33 and d31
in c-axis oriented films are about 3.0 pm/V and -1.0 pm/V, respectively. By contrast, films with (101) orientation show a
lower effective longitudinal piezoelectric coefficients, consistent with this different orientation.
Finally, both types of AlN layers were deposited on 640 μm long micro-cantilevers. The average displacement of the
first mode on the vertical axis was about 12 nm for the film with good c -axis orientation and 0.3 nm for that with (101)-
orientation when applying the same excitation.
Micro-cantilevers and micro-bridges actuated by sputter-deposited aluminium nitride (AlN) thin films were measured
with a scanning laser Doppler vibrometer up to 6 MHz, covering more than 10 resonance modes of different nature. A
finite element model (FEM) was used to simulate the modal response of the micromachined structures. The comparison
between experiment and simulation, regarding modal shapes and frequencies, resulted in an excellent agreement, what
confirmed the quality of the structures. Finally, we point out, and illustrate with the help of micro-bridges, the
importance for a locally tailored distribution of electrical excitation on the top surface of the device, in order to either
optimize or cancel out the displacement of a given mode.
We achieved to etch nano- and deep structures in silicon using ICP-cryogenic dry etching process. We etched nanopores
and nanocantilevers with an etch rate of 13 nm/min, nanopillars with an etch rate of 2.8 μm/min - 4.0 μm/min, membrane
and cantilever structures with an etch rate of 4 μm/min and 3 μm/min, respectively. Nanopores and nanocantilevers
are interesting structures for Bionanoelectronics. Nanopillars can be used as substrates/templates for the MOCVD
growth of GaN nanoLEDs. They are the basic constituents of a nanoparticle balance and also of a thermoelectric generator.
For the joining of the silicon wafers of the thermoelectric generator the low temperature joining technique can be
used. Cantilevers can be used for sensing, e.g. as tactile cantilevers. They can be used also as resonator for mass sensing
even in the subnanogram region. The actuation of the resonator can be done by using piezoelectric thin films on the
cantilevers. The mass detection depends on the resonance frequency shift caused by loaded mass on the cantilevers. Such
cantilevers are robust and easy to produce. The deep etching in silicon was done by using a photoresist mask and
creating perpendicular and smooth sidewalls.
A new tactile sensor with piezoresistive read-out is presented. The sensor is designed for measurements of high aspect ratio structures with a resolution of some ten nanometer and a measuring range of hundreds of micrometer. Possible applications of the sensor are suggested. The silicon micromachining fabrication process is shown in detail next to the finite element simulations we performed. First measurements and a calibration process are described and the results are shown. The implementation into a measuring system is indicated.
A piezoresistive silicon cantilever-type tactile sensor was described as well as its application for dimensional metrology
with micro components and as a transferable force standard in the micro-to-nano Newton range. As an example for
tactile probing metrology the novel cantilever sensor was used for surface scanning with calibrated groove and
roughness artifacts. Force metrology was addressed based on calibration procedures which were developed for
commercial stylus instruments as well as for glass pipettes designed for the characterization of the vital forces of isolated
cells.
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