Piezoresistive Cantilever Sensor Research Articles

Replicating Spectral Baseline for Unambiguous Frequency Locking in Resonant Sensors.

Electrothermal piezoresistive resonant cantilever sensors have been fabricated with embedded actuating (heating resistor) and sensing (piezo resistors) parts, with the latter configured in a Wheatstone bridge circuit. Due to the close spacing between these two elements, a direct thermal parasitic effect on the resonant sensor during the actuating-sensing process leads to asymmetric amplitude and reversing phase spectral responses. Such a condition affects the precise determination of the cantilever's resonant frequency, f0. Moreover, in the context of phase-locked loop-based (PLL) resonance tracking, a reversing phase spectral response hinders the resonance locking due to its ambiguity. In this work, a replica of the baseline spectral was applied to remove the thermal parasitic effect on the resonance spectra of the cantilever sensor, and its capability was simulated through mathematical analysis. This replica spectral was subtracted from the parasitized spectral using a particular calculation, resulting in optimized spectral responses. An assessment using cigarette smoke particles performed a desired spectral shifting into symmetrical amplitude shapes and monotonic phase transitions, subsequently allowing for real-time PLL-based frequency tracking.

Performance of an Electrothermal MEMS Cantilever Resonator with Fano-Resonance Annoyance under Cigarette Smoke Exposure.

An electrothermal piezoresistive cantilever (EPC) sensor is a low-cost MEMS resonance sensor that provides self-actuating and self-sensing capabilities. In the platform, which is of MEMS-cantilever shape, the EPC sensor offers several advantages in terms of physical, chemical, and biological sensing, e.g., high sensitivity, low cost, simple procedure, and quick response. However, a crosstalk effect is generated by the coupling of parasitic elements from the actuation part to the sensing part. This study presents a parasitic feedthrough subtraction (PFS) method to mitigate a crosstalk effect in an electrothermal piezoresistive cantilever (EPC) resonance sensor. The PFS method is employed to identify a resonance phase that is, furthermore, deployed to a phase-locked loop (PLL)-based system to track and lock the resonance frequency of the EPC sensor under cigarette smoke exposure. The performance of the EPC sensor is further evaluated and compared to an AFM-microcantilever sensor and a commercial particle counter (DC1100-PRO). The particle mass–concentration measurement result generated from cigarette-smoke puffs shows a good agreement between these three detectors.

Temperature drift-aware material selection of composite piezoresistive micro-cantilevers using Ashby’s methodology

In this paper, we devise an approach for material selection of constituent layers of a composite piezoresistive cantilever sensor using Ashby’s methodology. A rational choice of the constituent materials of a piezoresistive cantilever sensor becomes critical not only due to its multi-layered hetero structure but also due to the interdependence of material properties and geometrical parameters in governing its performance and reliability. Material selection is performed by identifying the performance metrics that govern the sensor performance and depict the interplay between the material constants and geometrical parameters. Here, apart from considering the coupled effects of electro-mechanical parameters we have also incorporated the impact of joule heating induced self-heating effects in the material selection process. Further, the material selection process is validated by computing the performance metrics of different sensors with same resonant frequency using analytical models. Results show that a silicon cantilever with a doped p-type piezoresistor, a silicon dioxide isolation layer and a gold immobilization layer is relatively better considering performance characteristics and reliability as compared to an SU-8 polymeric and other solid-state semiconductor based piezoresistive cantilever surface stress sensors, especially when joule heating induced inaccuracy is taken into account.

Improvement of frequency responses of an in-plane electro-thermal cantilever sensor for real-time measurement

Resonance-based sensors ordinarily show a symmetrical amplitude shape, which is accompanied by a monotonic transition as its phase response. At its resonance state, there is a 90° phase difference between the excitation force and the system response. Nevertheless, due to some parasitic factors, resonance-based sensors might show non-ideal frequency response as seen in in-plane electro-thermal piezoresistive cantilever sensors (EtPCSs). Because of a direct thermal parasitic coupling from the excitation part to the sensing part, the sensor output shows an asymmetric amplitude shape and a reversing phase response. In some EtPCS that have relatively lower thermal exposure on their sensing part, a symmetrical amplitude shape could be demonstrated but was still accompanied with a low magnitude of monotonical phase response. In this work, a method of reference subtraction was introduced to enhance the frequency response of the EtPCS at changing ambient conditions (e.g. humidity, temperature, smoke exposure). As a result, we obtained optimized frequency responses suitable for a phase-locked loop-based system to carry out resonant-frequency tracking in real-time.

Enhancement of real-time resonance tracking in electrothermally actuated cantilever sensor with optimized phase characteristic

Non-ideal phase responses on electro-thermally actuated piezoresistive cantilever sensors have led the phase-locked loop (PLL) systems into difficulties for real-time sensing applications. These outcomes are caused by thermal-parasitic coupling from the actuating part to the sensing part. Minimizing or eliminating parasitic effects is necessary to obtain an optimized phase response. To realize this, we adjusted the voltage supply of the sensing part, which is in form of a full Wheatstone bridge (WB). By increasing the WB supply voltage (VWB), the phase response can be enhanced. Alternatively, a reference signal that differentially eliminates the parasitic parameter from the sensor output was employed. To investigate the resulting optimized phase response under real-time measurement conditions, two different microcantilevers were connected to an MFLI lock-in amplifier + PLL system (Zurich Instruments). Measurement results exhibited a good sensor performance under varying humidity and temperature conditions.

Phase optimization of thermally actuated piezoresistive resonant MEMS cantilever sensors

Abstract. The asymmetric resonance response in thermally actuated piezoresistive cantilever sensors causes a need for optimization, taking parasitic actuation–sensing effects into account. In this work, two compensation methods based on Wheatstone bridge (WB) input voltage (VWB_in) adjustment and reference circuit involvement were developed and investigated to diminish those unwanted coupling influences. In the first approach, VWB_in was increased, resulting in a higher current flowing through the WB piezoresistors as well as a temperature gradient reduction between the thermal actuator (heating resistor: HR) and the WB, which can consequently minimize the parasitic coupling. Nevertheless, increasing VWB_in (e.g., from 1 to 3.3 V) may also yield an unwanted increase in power consumption by more than 10 times. Therefore, a second compensation method was considered: i.e., a reference electronic circuit is integrated with the cantilever sensor. Here, an electronic reference circuit was developed, which mimics the frequency behavior of the parasitic coupling. By subtracting the output of this circuit from the output of the cantilever, the resonance response can thus be improved. Both simulated and measured data show optimized amplitude and phase characteristics around resonant frequencies of 190.17 and 202.32 kHz, respectively. With this phase optimization in place, a phase-locked-loop (PLL) based system can be used to track the resonant frequency in real time, even under changing conditions of temperature (T) and relative humidity (RH), respectively. Finally, it is expected to enhance the sensitivity of such piezoresistive electro-thermal cantilever sensors under loading with any target analytes (e.g., particulate matter, gas, and humidity).

Optimization of a nano-cantilever biosensor for reduced self-heating effects and improved performance metrics

Over the last decade, surface stress-based piezoresistive cantilever biosensors have been extensively explored as a potential alternative for conventional clinical diagnostic techniques. The design of piezoresistive cantilever sensors is a complex multi-variable problem which involves the interplay between the thermal, electrical and mechanical design aspects of their constituent materials and geometry. Even though the literature includes examples where researchers have devised designs of piezoresistive cantilever biosensors, a majority of them have focused primarily on improving the electrical sensitivity by either dimensional optimization or material changes of their constituent layers. However, there are two important aspects of the sensor design which have been seldom addressed: (i) the negative impact of the transverse section of the U-shaped piezoresistor on the electrical sensitivity, and (ii) thermal drift in the output characteristics of the sensor. Although a few researchers have focused on the aforementioned factors, they have analysed them independently without considering their interdependence and cumulative impact on the sensor performance. In this paper, we devise a dual material multi-part U-shaped piezoresistor comprised of two p-type single crystalline silicon longitudinal sections and a p-type polysilicon transverse section. Evaluation of the proposed piezoresistor is performed in two stages using a finite element method-based numerical tool. In the first stage, the performance of the sensor with the multi-part piezoresistor is compared with the reported piezoresistors available in the literature. In the second stage, a comprehensive investigation of the multi-part piezoresistor is carried out to maximize the sensor performance. Results show that, compared to a conventional U-shaped piezoresistor, the proposed piezoresistor achieves improvement in the sensitivity ratio by 8.12 times.

Impact of Isolation and Immobilization Layers on the Electro-Mechanical Response of Piezoresistive Nano Cantilever Sensors.

In the last decade, piezoresistive nano cantilever sensors have been extensively explored, especially for chemical and biological sensing applications. Piezoresistive cantilever sensors are multi-layer structures with different constituent materials. Performance of such sensors is a function of their geometry and constituent materials. For a fixed material set, the pre-requisite for optimizing the performance of a composite piezoresistive cantilever sensor is careful geometrical design of its constituent layers. Even though, treatise encompasses various designs of such sensors, typically for computational simplicity the functional layers i.e., the isolation and immobilization layers are neglected in the modeling stages. In this paper, we elucidate the impact of the functional layers on the electro-mechanical response of composite piezoresistive nano cantilever sensors. Systematic and detailed computations are performed using theoretical models and numerical simulations. Results show that both the isolation and immobilization layers play a critical role in governing the sensor performance. Simulation results depict that compared to a sensor with an isolation layer of thickness 100 nm, a sensor without isolation layer has 36.29% and 42.51% better deflection sensitivity and electrical sensitivity respectively. Furthermore, it is found that when an immobilization layer of thickness 40 nm is added atop the isolation layer, the deflection sensitivity and electrical sensitivity reduces by 12.98% and 15.83% respectively. Through our investigation it is shown that the isolation and immobilization layers not only play a vital role in determining the stability and electro-mechanical response of the sensor but their negligence in the design stages can be detrimental. Apart from investigating the impact of the immobilization layer thickness, to model the sensor closer to real time operational conditions, we have performed analysis to understand the impact of non-uniformity in the immobilization layer thickness and non-uniform surface stress loading on the electro-mechanical response of the sensor. Results and inferences obtained from this study will help NEMS engineers to optimize the performance of piezoresistive nano cantilever sensors and to design multi-layer cantilever platform structures for other transducers.

Design, Simulation and Analysis of MEMS based Piezoresistive Cantilever sensor

Due to the increased industrial activities like burning of fossil fuels, use of refrigerant gases in air conditioning systems etc., the amount of Greenhouse gases(GHGs) are increasing in the atmosphere. Higher concentration of these gases result in trapping of more heat thus increasing the earth’s temperature leading to greenhouse effect. Hence release of GHG even at very minute concentration levels must be monitored strictly. MEMS cantilever sensor respond to mass loading effect which is useful to detect inert gases at low parts per million concentration levels. COMSOL Multiphysics Software is used for designing and simulating MEMS based piezoresistive cantilever sensor to enhance the sensitivity in terms of output voltage. The gas concentration is converted into voltage by the piezo resistors which are connected in the form of Wheatstone bridge. In addition, we have compared the outputs of sensors when different piezoresistive materials are used.

In silico modeling and investigation of self-heating effects in composite nano cantilever biosensors with integrated piezoresistors

Over the years, piezoresistive nano cantilever sensors have been extensively investigated for various biological sensing applications. Piezoresistive cantilever sensor is a composite structure with different materials constituting its various layers. Design and modeling of such sensors become challenging since their response is governed by the interplay between their geometrical and constituent material parameters. Even though, piezoresistive nano cantilever biosensors have several advantages, they suffer from a limitation in the form of self-heating induced inaccuracy which is seldom considered in design stages. Although, a few simplified mathematical models have been reported which incorporate the self-heating effect, several assumptions made in the modeling stages result in inaccuracy in predicting sensor terminal response. In this paper, we model and investigate the effect of self-heating on the thermo-electro-mechanical response of piezoresistive cantilever sensors as a function of the relative geometries of the piezoresistor and the cantilever platform. Finite element method (FEM) based numerical computations are used to model the target-receptor interactions induced surface stress response in steady state and maximize the electrical sensitivity to thermal sensitivity ratio of the sensor. Simulation results show that the conduction mode of heat transfer is the dominant heat transfer mechanism. Furthermore, the isolation and immobilization layers play a critical role in determining the thermal sensitivity of the sensor. It is found that the shorter and wider cantilever platforms are more suitable to reduce self-heating induced inaccuracies. In addition, results depict that the piezoresistor width plays a more dominant role in determining the thermal drift induced inaccuracies compared to the piezoresistor length. It is found that for surface stress sensors at large piezoresistor width, the electrical sensitivity to thermal sensitivity ratio improves.

Numerical study on the influence of buried oxide layer of SOI wafers on the terminal characteristics of a micro/nano cantilever biosensor with an integrated piezoresistor

Piezoresistive micro/nano cantilever sensors have been extensively utilized in the detection and quantification of miniscule forces generated due to the physical interaction of biomolecules. Due to the ultra-sensitive nature of piezoresistive cantilever sensors, they have found numerous applications, especially in the development of point-of-care testing systems for healthcare applications. Over the years, silicon-on-insulator (SOI) wafers have been widely utilized to realize silicon based piezoresistive cantilever sensors due to their performance and fabrication related advantages. Treatise encompasses examples where researchers have retained the buried oxide (BOX) layer of SOI wafers to realize piezoresistive cantilever devices. In this paper, we investigate the impact of BOX layer of SOI wafers on the performance of composite piezoresistive cantilever biosensors. Typically, the response of piezoresistive cantilever sensors is evaluated by considering only the electro-mechanical response. However, the design of such sensors is a multi-variant problem due to (i) their composite structure and (ii) interdependent nature of the electrical, mechanical and thermal characteristics. Therefore, in the present study, we devise a new figure of merit (FoM) defined as the product of sensitivity ratio and the square of resonant frequency of the sensor. The sensitivity ratio represents the ratio of the electrical sensitivity due to surface stress and thermal sensitivity of the sensor. The proposed FoM takes into account the interplay between the electrical, mechanical and thermal response of the sensor. Finite element method based numerical simulations are performed to model and investigate the influence of BOX layer on the thermo-electro-mechanical response of a square shaped silicon piezoresistive cantilever sensor. Simulation results show that the presence of the supporting BOX layer enhances the mechanical stability i.e. spring constant and resonant frequency of the sensor. In addition, it is found that the BOX layer reduces the zero bias deflection of the cantilever due to difference in TCE of the constituent layer materials. Furthermore, it is shown that by dimensional optimization of the thicknesses of the constituent layers, sensitivity ratio of the sensor with BOX layer can be improved up to 2.96.

Fabrication of a cost-effective MEMS-based piezoresistive cantilever sensor for gait movement analysis

Gait analysis measurement is a method to access and identify gait events and the measurements of motion parameters involving the lower part of body. This significant method is widely used in rehabilitation, sports as well as health diagnostic towards improving the quality of life. However, it is not routine practice due to costs involved in creating and using gait labs. Alternatively, inertial sensors such as micro cantilever accelerometer can be used in the development of cheap and wearable gait analysis systems. Human stride segmentation measurement based on micro-accelerometer cantilever is used in the study of the lower limb movement patterns that include walk, jump and run; and the measurements of the motion parameters. A complete system consists of a fabricated sensor, a Wheatstone bridge circuit and a signal amplifier tailored for real-time stride analysis measurement is proposed. The novel fabrication method of an accelerometer sensor using laser micromachining is introduced in order to develop a simple way in realizing the sensor formation. This study allows us to optimize the requirements of hard-mask and fabrication process steps by reduction of 30% and 25% respectively. In the general framework, this research activity is focused towards development of piezoresistive cantilever formation using laser micromachining for fast fabrication development for real life gait and stride segmentation measurement application.

Trace level detections of abrin with high SNR piezoresistive cantilever biosensor

In this paper, sensitive detections on abrin are demonstrated in real-time by piezoresistive (PZR) cantilever sensors. With attempts to optimize the design and fabrication of the PZR cantilevers, the voltage fluctuation of the PZR sensors was decreased to 1μV and the signal to noise ratio (SNR) was increased significantly. By using the biotin–streptavidin functionalized cantilevers, sensitive detections on abrin were realized. A linear relation between the concentration of abrin and the output voltage of PZR sensors was established within the range of 0.08–40ng/mL (1.2–600pM) and a limit of detection (LOD) of 80pg/mL (1.2pM) was obtained in PBS buffer. Meanwhile, detections on orange juice and skim milk samples spiked with abrin were also performed, and the detection sensitivity decreased by 6–17% compared with the one in PBS buffer at the same concentrations. At last, a model of pseudo-first-order kinetics was used to analyze the interaction mechanism of abrin and biotinylated polyclone antibody (bio-PcAb) against abrin, which was consistent with the detection results. The proposed PZR cantilever sensors provide an economical platform for on-site detections of food contaminants with the advantages of high SNR, portability, and rapid response.

Molecules sensing layer design of piezoresistive cantilever sensor for higher surface stress sensitivity

This paper reports on molecular sensing layer design of a piezoresistive cantilever sensor for higher surface stress sensitivity. The proposed analyses show that the previous understanding of piezoresistive cantilevers for surface stress measurement requires reconsideration for a cantilever utilizing polycrystalline silicon as a piezoresistor. The integration of the molecular sensing layer stripe pattern design to the cantilever effectively improves the piezoresistive output and utilizes the full sensing area of the cantilever surface. The proposed sensing layer design can be effectively integrated to current piezoresistive cantilever sensors to improve sensor performance in biochemical assays.

Axial-Stressed Piezoresistive Nanobeam for Ultrahigh Chemomechanical Sensitivity to Molecular Adsorption

This study proposes nanothickness piezoresistive double-clamped beams that are used in a double-side adsorbing mode. The axially stressed clamped beam exhibits continually increasing sensitivity as it is thinned down to nanoscale, and the thinning is theoretically without limitation. Sensing experiments to part-per-million levels of trimethylamine vapor well verify the proposal. A 93 nm thick beam sensor exhibits higher than 1 order of magnitude sensitivity compared to typical piezoresistive cantilever sensors, and its chemomechanical sensing resolution is comparable with that obtained by the off-cantilever optical detection method. With the nanobeam, a surprisingly ultrahigh sensitivity to surface molecular self-assembly induced surface stress is also obtained that is about 150 times higher than that obtained from a conventional cantilever. With additional advantages of elimination of single-sided adsorption induced bimetallic effect noise, tinier size, and easier fabrication, the ultrasensitive nanothick beam sensors show promise to replace the state-of-the-art piezoresistive cantilevers for bio/chemical nanomechanical detection.

Peizoresistive MEMS Cantilever based CO2 Gas Sensor

A study about the piezoresistive Micro-Electro-Mechanical Systems (MEMS) cantilever for a chemical sensitive mass based sensor has been carried out to enhance sensor sensitivity. The sensitive region attracts the CO2 molecules there by introducing the stress concentration region (SCR). Three types of SCR geometry designs were first analysed using Intellisuite software to study the effect of stress and its distribution when varying mass is applied at the SCR. The results showed that the rectangular SCR design has the highest stress and showed a linear relation too when compared to the other types. Then, the length of rectangular SCR is varied to study the stress distribution along the cantilever. The piezoresistive element was then placed at various positions to obtain the highest stress resulting due to the deflection of the cantilever. The impact due the SCR was also analysed. The testing results of this piezoresistive MEMS cantilever with rectangular SCR had successfully enhanced sensitivity compared to the piezoresistive MEMS cantilever without SCR when varying mass is applied. Therefore this SCR approach appears to be suitable for enhancing the sensitivity of a massbased piezoresistive MEMS cantilever sensor. General Terms CO2 Gas Sensor using MEMS

Design and processing of a cost-effective piezoresistive MEMS cantilever sensor for medical and biomedical use

In this special section article, cost-effective methods for fabrication of a piezoresistive cantilever sensor for industrial use are focused on. The intended use of the presented cantilever is a medical application. A closer description of the cantilever design is given. The low-cost processing sequence is presented and each processing step is explained in detail. The processing sequence is also compared to other low-cost fabrication techniques. Results from the electrical probing and mechanical strength test are given. The results demonstrate that the chosen low-cost processing route results in high yield and a mechanical robust device.

Determination of exposure to engineered carbon nanoparticles using a self-sensing piezoresistive silicon cantilever sensor

A novel MEMS-based cantilever sensor with slender geometry is designed and fabricated to be implemented for determining personal exposure to carbon engineered nanoparticles (NPs). The function principle of the sensor is detecting the cumulative mass of NPs deposited on the cantilever surface as a shift in its resonant frequency. A self-sensing method with an integrated full Wheatstone bridge on the cantilever as a piezoresistive strain gauge is introduced for signal readout replacing optical sensing method. For trapping NPs to the cantilever surface, an electrostatic field is used. The calculated equivalent mass-induced resonant frequency shift due to NPs sampling is measured to be 11.78 ± 0.01 ng. The proposed sensor exhibits a mass sensitivity of 8.33 Hz/ng, a quality factor of 1,230.68 ± 78.67, and a temperature coefficient of the resonant frequency (TC f ) of −28.6 ppm/°C. These results and analysis indicate that miniaturized sensors based on self-sensing piezoresistive microcantilever can offer the performance to fulfill the requirements of real-time monitoring of NPs-exposed personnel.

In situ Observation of Surface Reconstruction of Si(001) with Stress/Strain Field Scanning Probe Microscopy

In this study, the surface reconstruction of Si(001) was observed in situ by stress/strain field scanning probe microscopy (SF-SPM). This SF-SPM system includes two independent probes that can be operated as scanning tunneling microscope (STM) or atomic force microscope (AFM) freely. A special structure of the AFM probe is designed with a piezo-resistive cantilever sensor to make the probe head compact to be set in a limited space of ultrahigh-vacuum (UHV) chambers. With a low-noise direct current power source, the atomic-resolution STM and AFM images of the Si(111) and Si(001) surfaces at room temperature and high temperature were obtained. Using this system, the relationship between the reconstruction of the Si(001) surface with stress and time was investigated.

Enhancing the sensitivity of a mass-based piezoresistive micro-electro-mechanical systems cantilever sensor

A study on a fabricated piezoresistive micro-electro-mechanical systems (MEMS) cantilever for a mass-based sensor has been carried out to enhance sensor sensitivity by introducing the stress concentration region (SCR). Three types of SCR geometry designs were first analysed using finite element analysis (FEA) software ANSYS® to study the effect of stress and its distribution when varying mass is applied at the free end. FEA results show that the rectangular SCR design has the highest stress. Then, the length of rectangular SCR is varied from 1000 µm (A1) to 3000 µm (A3) to study the stress distribution along the cantilever. The piezoresistive MEMS cantilever with rectangular SCR A3 produced the highest stress and was thus selected for fabrication along with the piezoresistive MEMS cantilever without SCR. From the testing results, the piezoresistive MEMS cantilever with rectangular SCR A3 successfully enhanced sensitivity by 1.97 times as compared to the piezoresistive MEMS cantilever without SCR when varying mass is applied. Therefore this SCR approach appears to be suitable for enhancing the sensitivity of a mass-based piezoresistive MEMS cantilever sensor.