Silicon-based Pressure Sensors Research Articles

Capacitive pressure sensors based on microstructured polymer-derived SiCN ceramics for high-temperature applications

Traditional silicon-based pressure sensors cannot meet demand of pressure information acquisition in high-temperature extreme environments due to their low sensitivity, limited detection temperature and complex processing. Herein, a capacitive pressure sensor is fabricated using polymer-derived SiCN ceramics with convex microstructures via a sample replication strategy. Its performance is measured at different pressures (0–800 kPa) from room temperature to 500 °C. The results show that the SiCN ceramic capacitive pressure sensor exhibits low hysteresis, good non-linearity of 0.26 %, outstanding repeatability and high sensitivity of 0.197 pF/MPa under room temperature. When the test temperature reaches 500 °C, the performance of the prepared capacitive pressure sensor has no degradation, keeping competent sensitivity of 0.214 pF/MPa and nonlinear error of 0.24 %. Therefore, benefitting from the preeminent high-temperature properties, e.g., excellent oxidation/corrosion resistance and thermal stability, SiCN ceramics capacitive pressure sensors have great potential in the application of high-temperature and harsh environments.

Research on temperature drift mechanism and compensation method of silicon piezoresistive pressure sensors

Silicon piezoresistive pressure sensors have played a key role in the detection field due to their small size, high sensitivity, and high integration. However, the factors such as its material particularity, the temperature dependence of the piezoresistive coefficient, the variation of carrier motion in the material caused by heat-induced stress, doping concentration, and resistance mismatch may cause the sensor to generate temperature drift. In this paper, the mechanism of temperature drift of silicon-based pressure sensor is analyzed initially, and then a comparison between BP neural network and wavelet neural network is done, which shows the root mean square error of the wavelet neural network is much smaller than that of the BP neural network and better linearity.

Development of a free-form piezo-resistive pressure sensor using advanced printing methods

The use of printed pressure sensors is becoming a popular alternative to conventionally made silicon-based pressure sensors in niche applications, ranging from automotive and aerospace to the biomedical industry. In this paper, a piezo-resistive pressure sensor is produced onto a free-form Polyamide (PA) substrate, by combining aerosol-based jet printing and screen-printing techniques. The result is a first study towards the implementation of printed piezo-resistive pressure sensors for biomedical applications.The piezo-resistive pressure sensor consists of silver interconnects which are printed onto the substrate and are connected through a layer of piezo-resistive ink. The sensor is printed onto a customized additive manufactured PA substrate, which is produced using Selective Laser Sintering (SLS). First, the interconnects are printed using silver ink, CUR AG-001, from Agfa N.V. Belgium, which is deposited using an aerosol-based jet printer, the Nebula 5X-100s, developed at the Advanced Manufacturing Laboratory (AML) of the KU Leuven. Next, the sensitive elements are printed using a piezo-resistive material, Carbon EMS CI 2050, which is applied across the sensor interconnects using an adapted screen-printing technique. An optimal ink composition for the two-component piezo-resistive ink was found at 40wt% CI-2050LR: 60wt% CI-2050HR, which are respectively low and high resistivity ink components. Both silver ink and printed piezo-resistive elements were sintered in an oven at 150°C for 240 and 45 min, respectively. Finally, a coupling for the sensor readout was provided using a silver conductive epoxy adhesive, which cures at room temperature, so that electrical connections could be attached to the sensor contact pads.Validation of the piezo-resistive pressure sensors was conducted using fingertip actuation, which showed a typical piezo-resistive behavior where the resistance of the sensor decreases in function of the applied pressure, and a maximal decrease in electrical resistance of 30.5Ω has been measured. These results show the feasibility of employing advanced printing techniques to produce piezo-resistive sensing devices onto free-form and additive manufactured substrates, for biomedical applications.

Long-term Stability Enabling Technology of Silicon-Based Piezoresistive MEMS Pressure Sensor

Silicon-based piezoresistive microelectromechanical systems (MEMS) pressure sensor is one of the best studied and commercialized devices among all MEMS devices due to simple structure, easy reading circuit, strong batch fabrication capability and low price. The biggest technical bottleneck of this sensor for high-end applications such as aerospace is long-term stability after packaging. Therefore, long-term stability improvement technologies, as the common key technologies, have been widely concerned by academia and industry. In this paper, three common enabling technologies including pulsating fatigue, thermal-cold cycling and vibration aging are adopted to evaluate the effect of long-term stability of silicon-based piezoresistive MEMS pressure sensors by using control variable method and orthogonal test (OT) method. The comparative test results indicate that all these three technologies can improve the long-term stability of the pressure sensor. In addition, an optimized combination of environmental stress parameters is found to improve the stability more than 90% higher than before the test. The long-term stability of the sensors through the optimal parameter combination aging method is better than the original factory method. The research results can be used for reference to improve the long-term stability of various silicon-based pressure sensors.

Quantitative Analysis of MEMS Piezoresistive Pressure Sensors Based on Wide Band Gap Materials

Wide band gap materials such as silicon carbide and diamond are considered more suitable compared to silicon for the fabrication of piezoresistive pressure sensors for operation at high temperatures. We have carried out extensive FEM simulation study for theoretically assessing the response of pressure sensors incorporating these wide band gap materials. Response of sensors incorporating silicon micromachined diaphragm with oxide isolated piezoresistors of diamond or silicon carbide was studied using FEM simulations. The study was further extended for sensors incorporating diaphragm as well as piezoresistors of diamond or silicon carbide. The results in both cases were compared with those obtained for a single crystal or polycrystalline piezoresistor-based silicon pressure sensor. The results presented in this paper demonstrate that among the materials studied in this work, diamond-based sensors will have the least pressure sensitivity compared to silicon carbide or silicon-based sensors. However, considering the nonlinearity of response, diamond-based sensors will have the minimum nonlinearity of pressure response compared to silicon-based pressure sensors. Hence for the same nonlinearity, it would be possible to operate the diamond-based sensors up to much higher pressures.

Analytical modeling and simulation of MEMS piezoresistive pressure sensors with a square silicon carbide diaphragm as the primary sensing element under different loading conditions

Although silicon is the preferred choice for microelectromechanical systems (MEMS) piezoresistive pressure sensors, such devices are not preferred for application in harsh environmental conditions due to the exponential increase in leakage current with temperature. To alleviate such shortcomings of silicon-based pressure sensors in extreme conditions including elevated temperature and intense vibration, this study strives to shift focus from core complementary metal–oxide–semiconductor (CMOS) materials to silicon carbide. In this work, we adopt an analytical and simulation approach to model and analyze various characteristics of such silicon carbide piezoresistive sensors and determine an optimal design. A square diaphragm is modeled using the analytical expressions for a thin plate in combination with small-deflection theory, providing quick insight for estimation of critical parameters and thus the behavior of the pressure sensor. Both clamped and freely supported edge conditions of the diaphragm are explored. Although many studies and discussions are available on the rigidly supported loading condition, the freely supported edge condition for a square diaphragm has received little attention. The deflection, stress, strain, and sensitivity of the square diaphragm under both loading conditions are reported herein then compared to understand which of the two loading conditions results in more significant outputs.

Investigations of the impact of initial stresses on fracture and delamination risks of an avionics MEMS pressure sensor

Silicon based pressure sensors often take advantage of piezo-resistive gages which are normally embedded by multiple silicon oxide and silicon nitride layers where gold lines form a Wheatstone bridge (Meti et al., 2016; Bae et al., 2004 [2]). Because of manufacturing – stepwise deposition of multiple layers – significant layer residual stresses occur in the GPa range in tension and compression (Zhou et al., 2017 [3]). But also anodic bonding of the silicon MEMS device on usually glassy substrates results in additional initial stresses (Chou et al., 2009 [4] and Sandvan et al. [5]). Especially in avionics MEMS applications such stresses by far exceed the stresses arising under sensor operation and determine the major risks for cracking and delamination. Furthermore, those stresses could lead to a signal drift of the overall sensor over a long period of time — another important trustworthiness risk (Espinosa and Prorok, 2003 [6]).

Effects of residual stresses on cracking and delamination risks of an avionics MEMS pressure sensor

Silicon based pressure sensors often take advantage of piezo-resistive gages which are normally embedded by multiple silicon oxide and silicon nitride layers where gold lines form a Wheatstone bridge. As a result of manufacturing – stepwise deposition of multiple layers – significant layer residual stresses in the GPa range occur in tension and compression. Especially in avionics MEMS applications such stresses determine the major risks for cracking and delamination.To overcome the related reliability issues the authors performed experiments and nonlinear FEM-simulations. Basic information about the residual stresses in the gage stack were captured by a Focused Ion Beam (FIB) trench technique combined with digital image correlation. These results enriched the data base for finite element studies with the ABAQUS™. Especially delamination risks were investigated by a surface based cohesive contact approach which simulates the initiation and propagation of damage and cracking within and underneath the gage layer. The cracking risk is investigated by means of an Extended Finite Element Method (XFEM). Both, crack initiation location as well as crack path are results of XFEM simulations. Several design variations have been investigated and compared to give insights to potential crack initiation sites and to evaluate the risk of fracture during processing.

Offset stable piezoresistive high-temperature pressure sensors based on silicon

Abstract. The exploitation of new application fields and the drive to size reduction even in highly stable pressure sensing systems makes the extension of the operating temperature range of the microelectromechanical sensors (MEMS) essential. For this reason a silicon-based pressure sensor with an application temperature ranging up to 300 °C and the associated manufacturing technology was developed. With special design and manufacturing approaches mounting stress-insensitive sensors with high linearity, excellent offset stability, low hysteresis and low sensitivity changes over the entire temperature range were developed. At the moment, the sensors are tested till 300 °C at wafer level and between 135 and 210 °C as a first-level package.

Performance Optimization of a Capacitive Pressure Sensor

In this paper, the thermo-mechanical modelling of an entire sensor structure, under hydrostatic pressure, is carried out using commercial FEM software COMSOL multiphysics. A comparison of the obtained results with those found in the literature, allows us to corroborate the well-known models. Furthermore, the design of high performance conventional silicon-based pressure sensors for low-pressure biomedical applications has been achieved by optimizing the influencing parameters on the two features mentioned above.For a specific biomedical application, a complete analysis, considering the impact of the device thickness, embedding type and diaphragm material, has shown that the highest performances are reached for a silicon diaphragm thickness of 65μm, a circular shape having a radius of 1750μm and a cavity thickness of 4.6μm. Simulation results show that the sensor under investigation yields a quasi linear response with pressure values lying in the range [0 ÷70kPa]. The pressure sensitivity is about 2 pF/bar, and the thermal drift is about 16 ppm/°C for a temperature range [-20°C÷ 150°C]. The obtained results may well contribute to the optimization of the device’s performances and can provide to the designer several chosen criteria for a given application.

Abstract No. 416 - An integrated stent-graft for noninvasive 4-dimensional aneurysm sac pressure monitoring after endovascular aortic aneurysm repair (EVAR): First in vivo results in a porcine model

Purpose In vivo testing of a prototypic stent-graft with an integrated network of pressure sensors covering the external surface of the implant and capable of wireless digital data transmission for non-invasive measurement of aneurysm sac pressure following endovascular aortic aneurysm repair (EVAR). Materials and Methods Based on a commercially available aortic stent-graft we constructed a prototype with silicon-based pressure sensors fitting into a standard 24 F delivery sheath. Digital data conversion is performed by an integrated microcontroller, energy and data transfer by inductive coupling. All microelectronic devices are integrated on the implant: No further procedural steps are necessary. The prototype was tested in 4 swine by placing the (mono-iliac) graft across the iliac trifurcation with at least one sensor at the level of the occluded branches and performing invasive reference pressure measurement in the occluded (contra-lateral) arterial lumen following graft placement. Results Graft placement was successful in all 4 animals, transcutaneous wireless energy and data transmission was successfully demonstrated. The experiments revealed good correlation of the pressure-data changes acquired by the sensor at the level of the iliac trifurcation to the reference measurement. Conclusion The non-invasive acquisition of a sac pressure profile after EVAR by integrating the detectors on the external surface of the stent-graft allows detecting a regional pressure elevation, as in early endoleakage, and is a practical and efficient option for continous and direct pressure monitoring. Further development and implementation into a product is aimed.

Investigation of a Pressure Sensor with Temperature Compensation Using Two Concentric Wheatstone-Bridge Circuits

This study presents a silicon-based pressure sensor with temperature compensation. The eight piezoresistors were designed on the polycrystalline silicon membrane and constructed by two concentric Wheatstone-bridge circuits to perform two sets of sensors. The sensor in the central circuit measures the membrane deflection caused by the combined effects of pressure and temperature, while the outer one measures only the deflection caused by the working temperature. From this arrangement, it is reliable and accurate to measure the pressure by comparing the output signals from the two concentric Wheatstone-bridge circuits. The optimal positions of the eight piezoresistors were simulated by simulation software ANSYS. The investigated pressure sensor was fabricated by the micro electro-mechanical systems (MEMS) techniques. The measuring performance and an indication of the conventional single Wheatstone-bridge pressure sensor is easily affected under variation of different working temperature and causes a maximum absolute error up to 45.5%, while the double Wheatstone-bridge pressure sensor is able to compensate the error, and reduces it down to 1.13%. The results in this paper demonstrate an effective temperature compensation performance, and have a great performance and stability in the pressure measuring system as well.

Polyimide/SU-8 catheter-tip MEMS gauge pressure sensor

This paper describes the development of a polyimide/SU-8 catheter-tip MEMS gauge pressure sensor. Finite element analysis was used to investigate critical parameters, impacting on the device design and sensing characteristics. The sensing element of the device was fabricated by polyimide-based micromachining on a flexible membrane, using embedded thin-film metallic wires as piezoresistive elements. A chamber containing this flexible membrane was sealed using an adapted SU-8 bonding technique. The device was evaluated experimentally and its overall performance compared with a commercial silicon-based pressure sensor. Furthermore, the device use was demonstrated by measuring blood pressure and heart rate in vivo.

Sensitivity analysis of packaging effect of silicon-based piezoresistive pressure sensor

The silicon-based pressure sensor is one of the major applications in the MEMS device. Nowadays, the silicon piezoresistive pressure sensor is a mature technology in the industry, but its requirement in terms of sensing accuracy and stability is more rigorous than that of many advanced applications. The major factor affecting the sensing stability of the piezoresistive pressure sensor is its thermal and packing effects. For a packaged pressure sensor, silicone gel is usually used to protect the die surface, so the thermal and packaging effects caused by the silicone gel should be taken into consideration to obtain better sensing sensitivity and stability. For fast design and optimization purpose, a finite element method (FEM) is adopted for sensor performance evaluation, packaging-induced signal variation, and thermal/packaging effects will be examined in this research. Several experiments are also performed to validate the finite element model. After the simulation is validated, an optimization analysis is carried out under different packaged pressure sensor design parameters. The simulation results show that the different geometry of the protection gel will influence pressure sensitivity significantly; base on analysis results, this research will conclude a design guideline for pressure sensor packages with concave and convex type of protection gel.

A novel pressure sensor with a PDMS diaphragm

This paper describes a novel pressure sensor with a polydimethylsiloxane (PDMS) diaphragm, which is used to achieve much larger deformation than conventional silicon diaphragms under low pressure. The larger deformation of the PDMS diaphragm provides higher sensitivity than silicon-based pressure sensors because of the lower Young’s modulus of the PDMS diaphragm. The great advantage of the PDMS diaphragm with respect to sensitivity was confirmed through the use of the numerical simulation method and through the conduct of basic experiments. A carbon fibre 10μm in diameter was used to sense the strain change that had been induced on the diaphragm during its operation. The optimal position of the carbon fibre in the PDMS diaphragm was determined using the numerical analysis method. The carbon fibre was exactly formed across the Al electrodes in the diaphragm through dielectrophoresis. The replacement of the piezoresistor material provided some advantages, such as easy fabrication and a high gauge factor. Several types of pressure sensors with a PDMS diaphragm were fabricated, and the feasibility of such pressure sensors was evaluated.

Design optimization of a high performance silicon MEMS piezoresistive pressure sensor for biomedical applications

In this paper the design optimization of high performance conventional silicon-based pressure sensors on flat diaphragms for low-pressure biomedical applications has been achieved by optimizing the doping concentration and the geometry of the piezoresistors. A new figure of merit called the performance factor (PF) is defined as the ratio of the product of sensor sensitivity (S) and sensor signal-to-noise ratio (SNR) to the temperature coefficient of piezoresistance (TCPR). PF has been introduced as a quantitative index of the overall performance of the pressure sensor for low-range biomedical applications. A comprehensive analysis considering the impact of doping concentration, length of the piezoresistor and thickness of the diaphragm indicates that for achieving the highest figure of merit an optimum concentration of 1018 atoms cc−1 and geometry of 100 µm long piezoresistor and 10 µm thick membrane for the specified biomedical application are required.

Microfluidic MEMS for semiconductor processing

The advent of MEMS (microelectromechanical systems) will enable dramatic changes in semiconductor processing. MEMS-based devices offer opportunities to achieve higher performance and functionality, at lower cost, with decreased size and increased reliability. In this work, we describe the achievement of several important devices for use in the semiconductor equipment industry. They include a low-flow mass flow controller, a high-precision pressure regulator, and an integrated gas panel. Compared to current technology, the devices are ultra-small in size, thus minimizing dead volumes and gas contact surface areas. With wettable surfaces comprised of ceramic and silicon (or, silicon coated with Si/sub 3/N/sub 4/ or SiC), they are resistant to corrosion, and generate virtually no particles. The devices are created from modular components. The science and technology of these components will be detailed. The modules examined are: normally-open proportional valves; normally-closed, low leak-rate shut-off valves; critical orifices (to extract information of flow rate); flow models (to extract flow rate from pressure and temperature information); silicon-based pressure sensors; and, the precision ceramic-based packages which integrate these modules into useful devices for semiconductor processing. The work finishes with a detailed description of the low-flow mass flow controller.

Micromachined pressure sensors: review and recent developments

Since the discovery of piezoresistivity in silicon in the mid 1950s, silicon-based pressure sensors have been widely produced. Micromachining technology has greatly benefited from the success of the integrated circuit industry, borrowing materials, processes, and toolsets. Because of this, microelectromechanical systems (MEMS) are now poised to capture large segments of existing sensor markets and to catalyse the development of new markets. Given the emerging importance of MEMS, it is instructive to review the history of micromachined pressure sensors, and to examine new developments in the field. Pressure sensors will be the focus of this paper, starting from metal diaphragm sensors with bonded silicon strain gauges, and moving to present developments of surface-micromachined, optical, resonant, and smart pressure sensors. Considerations for diaphragm design will be discussed in detail, as well as additional considerations for capacitive and piezoresistive devices. Results from surface-micromachined pressure sensors developed by the authors will be presented. Finally, advantages of micromachined sensors will be discussed.

Small Silicon Pressure Transducers for Space-Time Correlation Measurements in a Flat Plate Boundary Layer

Silicon based pressure sensors have been used to measure turbulent wall-pressure fluctuations in a two-dimensional flat plate boundary layer, Reθ = 5072. The side lengths of the diaphragms were 100 μm (d+ = 7.2) and 300 μm (d+ = 21.6), giving a ratio of the boundary layer thickness to the diaphragm side length of the order of 240 and a resolution of eddies with wave numbers less than ten viscous units. Power spectra were measured for the frequency range 13 Hz < f < 13 kHz. Scaled in outer and inner variables a clear overlap region between the mid and high frequency parts of the spectrum is shown. In this overlap region the slope was found to be ω−1, while in the high frequency part it was ω−5. Correlation measurements in both the longitudinal and transversal directions were performed and compared to other investigations. Longitudinal space time correlations, including the high frequency range, indicated an advection velocity of the order of half the freestream velocity. A broad band filtering of the longitudinal correlation showed that the high frequency part of the spectrum is associated with the smaller eddies from the inner part of the boundary layer, resulting in a reduction of the correlation.

Gas development at the interface of directly bonded silicon wafers: investigation on silicon-based pressure sensors

Hydrophobic and hydrophilic silicon direct bonding (SDB) under vacuum conditions are compared with respect to the capability to form hermetically sealed cavities with minimal enclosed residual gas pressure. The quality of the enclosed vacuum inside the cavities of pressure-sensor test structures is monitored for different cavity layouts and annealing processes. The observed pressure increase during annealing is found to be strongly dependent on annealing temperature, subsequent storage time and on the bonded area surrounding each cavity. Trapped gaseous reaction products originating at the bonding interface are responsible for this effect. There is a significant difference in annealing-related gas evolution for hydrophobic and hydrophilic bonding surfaces, which can be explained by the existing theories of the bonding mechanism. Hydrophobic bonding leads to about 50% lower residual gas pressures compared with hydrophilic bonding. For hydrophilically bonded wafers a slow saturation of the gas pressure after annealing is observed.