Resonant Pressure Microsensor Research Articles

Stress isolations for resonant pressure micro sensors

This paper presents stress isolations for resonant pressure micro sensors that are supported by fixed isolation layers during assembly. While the resonant frequencies of the two resonators experience shifts due to measured pressures, they are susceptible to environmental perturbations. The proposed stress-isolated assembly in this study effectively mitigates the adverse effects of environmental noise and stabilizes the intrinsic frequencies of the resonators. Specifically, when subjected to temperature variations within the range of -55∼85 °C, the accuracy of the stress-isolated resonant pressure microsensor was determined to be ±0.01% FS. Similarly, during 3-axis random vibration tests, the accuracy of these stress-isolated resonant pressure microsensors was quantified as ±0.01% FS in response to environmental vibrations.

A High-Sensitivity Resonant Differential Pressure Microsensor Based on Wafer-Level Eutectic Bonding

This article presented a high-sensitivity resonant differential pressure microsensor based on wafer-level eutectic bonding, where dual resonators were embedded in the pressure-sensitive diaphragm to sense differential pressures, and a silicon cap was used to form a vacuum cavity. Differential pressures applied to the developed microsensor caused deflections of the pressure-sensitive diaphragm, resulting in shifts in resonant frequencies. This microsensor was designed based on finite element simulations with optimized structural parameters, which indicated high differential pressure sensitivities. Micro fabrications with key processes of deep reactive-ion etching (DRIE), lithography, and wafer-level eutectic bonding were adopted for device fabrication. The following experimental characterizations were conducted, producing the differential-pressure sensitivity of 181.53 Hz/kPa, the maxing fitting error of less than 80 Pa, under the differential pressure range of 0–100 kPa, the static pressure range of 110–200 kPa, and the temperature range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C to 60 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C. These results show that developed resonant differential pressure microsensors have excellent performances for differential-pressure measurements.

An All-Silicon Resonant Pressure Microsensor Based on Eutectic Bonding

In this paper, an all-Si resonant pressure microsensor based on eutectic bonding was developed, which can eliminate thermal expansion coefficient mismatches and residual thermal stresses during the bonding process. More specifically, the resonant pressure microsensor included an SOI wafer with a pressure-sensitive film embedded with resonators, which was eutectically bonded with a silicon cap for vacuum encapsulation. The all-Si resonant pressure microsensor was carefully designed and simulated numerically, where the use of the silicon cap was shown to effectively address temperature disturbances of the microsensor. The microsensor was then fabricated based on MEMS processes where eutectic bonding was adopted to link the SOI wafer and the silicon cap. The characterization results showed that the temperature disturbances of the resonant pressure microsensor encapsulated with the silicon cap were quantified as -0.82 Hz/°C of the central resonator and -2.36 Hz/°C of the side resonator within a temperature range from -40 °C to 80 °C, which were at least eight times lower than that of the microsensor encapsulated with the glass cap. Compared with the microsensor using the glass cap, the all-silicon microsensor demonstrated an accuracy improvement from 0.03% FS to 0.01% FS and a reduction in short-term frequency fluctuations from 3.2 Hz to 1.5 Hz.

A Low-Temperature-Sensitivity Resonant Pressure Microsensor Based on Eutectic Bonding

This paper presented a temperature-insensitive resonant pressure microsensor where silicon based resonators anchored on a pressure-sensitive diaphragm were vacuum packaged by a silicon cap based on eutectic bonding. Incoming pressures deformed the pressure-sensitive diaphragm and built stresses around resonators for frequency modulation while under temperature challenges, deformations of silicon based resonators and the vacuum cap were consistent and thus no stresses were generated on resonators. The temperature-insensitive resonant pressure microsensor was analyzed in both theoretical analysis and numerical simulations with confirmed high pressure sensitivities and low temperature disturbances. The resonant pressure microsensor was then fabricated by key steps of photolithography, deep reactive ion etching, and eutectic bonding and characterized in both open-loop and close-loop testing systems. Characterization results showed that the quality factors of resonators were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10000$ </tex-math></inline-formula> with pressure sensitivity of 82.98 Hz/kPa and temperature disturbance of −0.63 Hz/° (the lowest result among previously reported resonant pressure microsensors). In summary, the temperature-insensitive resonant pressure microsensor developed in this study exhibited a fitting accuracy better than 0.02% FS within the pressure range of 10 to 120 kPa and the temperature range of −45 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$85^{\circ }\text{C}$ </tex-math></inline-formula> .

A Resonant Pressure Microsensor with a Wide Pressure Measurement Range

This paper presents a resonant pressure microsensor with a wide range of pressure measurements. The developed microsensor is mainly composed of a silicon-on-insulator (SOI) wafer to form pressure-sensing elements, and a silicon-on-glass (SOG) cap to form vacuum encapsulation. To realize a wide range of pressure measurements, silicon islands were deployed on the device layer of the SOI wafer to enhance equivalent stiffness and structural stability of the pressure-sensitive diaphragm. Moreover, a cylindrical vacuum cavity was deployed on the SOG cap with the purpose to decrease the stresses generated during the silicon-to-glass contact during pressure measurements. The fabrication processes mainly contained photolithography, deep reactive ion etching (DRIE), chemical mechanical planarization (CMP) and anodic bonding. According to the characterization experiments, the quality factors of the resonators were higher than 15,000 with pressure sensitivities of 0.51 Hz/kPa (resonator I), −1.75 Hz/kPa (resonator II) and temperature coefficients of frequency of 1.92 Hz/°C (resonator I), 1.98 Hz/°C (resonator II). Following temperature compensation, the fitting error of the microsensor was within the range of 0.006% FS and the measurement accuracy was as high as 0.017% FS in the pressure range of 200 ~ 7000 kPa and the temperature range of −40 °C to 80 °C.

A Resonant Pressure Microsensor with Temperature Compensation Method Based on Differential Outputs and a Temperature Sensor.

This paper presents the analysis and characterization of a resonant pressure microsensor, which employs a temperature compensation method based on differential outputs and a temperature sensor. Leveraging a silicon-on-insulator (SOI) wafer, this microsensor mainly consists of a pressure-sensitive diagram and two resonant beams (electromagnetic driving and electromagnetic induction) to produce a differential output. The resonators were vacuum packaged with a silicon-on-glass (SOG) cap using anodic bonding and the wire interconnection was realized by sputtering an Au film on highly topographic surfaces using a hard mask. After the fabrication of the resonant pressure microsensor, systematic experiments demonstrated that the pressure sensitivity of the presented microsensor was about 0.33 kPa/Hz. Utilizing the differential frequency of the two resonators and the signal from a temperature sensor to replace the two-frequency signals by polynomial fitting, the temperature compensation method based on differential outputs aims to increase the surface fitting accuracy of these microsensors which have turnover points. Employing the proposed compensation approach in this study, the errors were less than 0.02% FS of the full pressure scale (a temperature range of −40 to 85 °C and a pressure range of 200 kPa to 2000 kPa).

Highly sensitive and wide-range resonant pressure sensor based on the veering phenomenon

We report a highly sensitive wide-range resonant pressure sensor. The concept is based on tracking multiple modes of vibration of an electrothermally heated initially curved micro-beam experiencing the veering phenomenon between its first and third vibration modes. For low values of pressure, the third resonance frequency is very sensitive, and thus its variation with pressure is monitored and recorded. As increasing pressure, the resonance frequency of the third mode decreases until reaching the veering phenomenon. At that point, the first mode exchanges role with the third mode, becoming very sensitive, and hence its frequency is tracked afterward as varying pressure. We show that using this concept, the sensitivity of the resonant pressure micro-sensor is significantly enhanced. Finite element method (FEM) simulations and experimental data show that the proposed micro-sensor becomes highly sensitive for wide-range of pressure from 38 mTorr to 200 Torr. The effect of various parameters on the performance of the proposed pressure sensor is investigated including the thickness of the micro-beam, the vacuum chamber size, and the thermal actuation load.

A Resonant Pressure Microsensor With a Stress Isolation Layer

This paper presents a resonant pressure microsensor composed of a glass cap for hermetic package, an SOI layer with sensing elements and a glass base for stress isolation. Tested pressure under measurement bends the pressure sensitive diaphragm, producing frequency shifts of underlining resonators while the stress isolation layer can work as a cushion layer to minimize environmental temperature and mechanical disturbances. Numerical simulations were conducted, finding that the stress isolation layer can decrease the axial stresses of the resonators from +16 to +2 MPa (high-temperature challenging), from −12 to −5 MPa (low-temperature challenging), and from 2.5 to 0.5 MPa (mechanical challenging). The proposed microsensor was fabricated based on conventional microfabrication and characterized with comparison to the counterpart without the stress isolation layer, producing an accuracy of 0.01% FS vs. 1% FS, a temperature shift of 37 ppm versus 10088 ppm, a pressure shift of 45 ppm versus 777 ppm, a temperature hysteresis error of 21 ppm versus 187 ppm, a pressure hysteresis error of 22 ppm versus 265 ppm, a temperature repeatability error of 15 ppm versus 703 ppm, a pressure repeatability error of 21 ppm versus 36 ppm, and a long-term stability of >7 months versus 69 days. These results validated the functionality of the proposed resonant pressure microsensor with the stress isolation layer, which may be potentially used as an effective tool in the fields of aerospace aviation, industrial control, and meteorological monitoring.

A Resonant Pressure Microsensor with the Measurement Range of 1 MPa Based on Sensitivities Balanced Dual Resonators.

This paper presents a resonant pressure microsensor with the measurement range of 1 MPa suitable for the soaring demands of industrial gas pressure calibration equipment. The proposed microsensor consists of an SOI layer as a sensing element and a glass cap for vacuum packaging. The sensing elements include a pressure-sensitive diaphragm and two resonators embedded in the diaphragm by anchor structures. The resonators are excited by a convenient Lorentz force and detected by electromagnetic induction, which can maintain high signal outputs. In operation, the pressure under measurement bends the pressure-sensitive diaphragm of the microsensor, producing frequency shifts of the two underlining resonators. The microsensor structures were designed and optimized using finite element analyses and a 4” SOI wafer was employed in fabrications, which requires only one photolithographic step. Experimental results indicate that the Q-factors of the resonators are higher than 25,000 with a differential temperature sensitivity of 0.22 Hz/°C, pressure sensitivities of 6.6 Hz/kPa, and −6.5 Hz/kPa, which match the simulation results of differential temperature sensitivity of 0.2 Hz/°C and pressure sensitivities of ±6.5 Hz/kPa. In addition, characterizations based on a closed-loop manner indicate that the presented sensor demonstrates low fitting errors within 0.01% FS, high accuracy of 0.01% FS in the pressure range of 20 kPa to 1 MPa and temperature range of −55 to 85 °C, and the long-term stability within 0.01% FS in a 156-day period under the room temperature.

A Resonant Pressure Microsensor Based on Double-Ended Tuning Fork and Electrostatic Excitation/Piezoresistive Detection.

This paper presents a resonant pressure microsensor relying on electrostatic excitation and piezoresistive detection where two double-ended tuning forks were used as resonators, enabling differential outputs. Pressure under measurement caused the deformation of the pressure sensitive membrane, leading to stress buildup of the resonator under electrostatic excitation with a corresponding shift of the resonant frequency detected piezoresistively. The proposed microsensor was fabricated by simplified SOI-MEMS technologies and characterized by both open-loop and closed-loop circuits, producing a quality factor higher than 10,000, a sensitivity of 79.44 Hz/kPa and an accuracy rate of over 0.01% F.S. In comparison to the previously reported resonant piezoresistive sensors, the proposed device used single-crystal silicon as piezoresistors, which was featured with low DC biased voltages, simple sensing structures and fabrication steps. In addition, the two double-ended tuning forks were used as resonators, producing high quality factors and differential outputs, which further improved the sensor performances.

A Lateral Differential Resonant Pressure Microsensor Based on SOI-Glass Wafer-Level Vacuum Packaging.

This paper presents the fabrication and characterization of a resonant pressure microsensor based on SOI-glass wafer-level vacuum packaging. The SOI-based pressure microsensor consists of a pressure-sensitive diaphragm at the handle layer and two lateral resonators (electrostatic excitation and capacitive detection) on the device layer as a differential setup. The resonators were vacuum packaged with a glass cap using anodic bonding and the wire interconnection was realized using a mask-free electrochemical etching approach by selectively patterning an Au film on highly topographic surfaces. The fabricated resonant pressure microsensor with dual resonators was characterized in a systematic manner, producing a quality factor higher than 10,000 (~6 months), a sensitivity of about 166 Hz/kPa and a reduced nonlinear error of 0.033% F.S. Based on the differential output, the sensitivity was increased to two times and the temperature-caused frequency drift was decreased to 25%.

A resonant pressure microsensor capable of self-temperature compensation.

Resonant pressure microsensors are widely used in the fields of aerospace exploration and atmospheric pressure monitoring due to their advantages of quasi-digital output and long-term stability, which, however, requires the use of additional temperature sensors for temperature compensation. This paper presents a resonant pressure microsensor capable of self-temperature compensation without the need for additional temperature sensors. Two doubly-clamped “H” type resonant beams were arranged on the pressure diaphragm, which functions as a differential output in response to pressure changes. Based on calibration of a group of intrinsic resonant frequencies at different pressure and temperature values, the functions with inputs of two resonant frequencies and outputs of temperature and pressure under measurement were obtained and thus the disturbance of temperature variations on resonant frequency shifts was properly addressed. Before compensation, the maximal errors of the measured pressure values were over 1.5% while after compensation, the errors were less than 0.01% of the full pressure scale (temperature range of −40 °C to 70 °C and pressure range of 50 kPa to 110 kPa).

A high-Q resonant pressure microsensor with through-glass electrical interconnections based on wafer-level MEMS vacuum packaging.

This paper presents a high-Q resonant pressure microsensor with through-glass electrical interconnections based on wafer-level MEMS vacuum packaging. An approach to maintaining high-vacuum conditions by integrating the MEMS fabrication process with getter material preparation is presented in this paper. In this device, the pressure under measurement causes a deflection of a pressure-sensitive silicon square diaphragm, which is further translated to stress build up in “H” type doubly-clamped micro resonant beams, leading to a resonance frequency shift. The device geometries were optimized using FEM simulation and a 4-inch SOI wafer was used for device fabrication, which required only three photolithographic steps. In the device fabrication, a non-evaporable metal thin film as the getter material was sputtered on a Pyrex 7740 glass wafer, which was then anodically bonded to the patterned SOI wafer for vacuum packaging. Through-glass via holes predefined in the glass wafer functioned as the electrical interconnections between the patterned SOI wafer and the surrounding electrical components. Experimental results recorded that the Q-factor of the resonant beam was beyond 22,000, with a differential sensitivity of 89.86 Hz/kPa, a device resolution of 10 Pa and a nonlinearity of 0.02% F.S with the pressure varying from 50 kPa to 100 kPa. In addition, the temperature drift coefficient was less than −0.01% F.S/°C in the range of −40 °C to 70 °C, the long-term stability error was quantified as 0.01% F.S over a 5-month period and the accuracy of the microsensor was better than 0.01% F.S.

Electrostatically driven and capacitively detected differential lateral resonant pressure microsensor

Presented is the design, fabrication and characterisation of an electrostatic-driving and capacitive-detection differential resonant pressure microsensor. The differential structure consists of two resonators immobilised on the diaphragm. In response to the pressure under measurement, the diaphragm deflection increases the intrinsic resonant frequency of one resonant beam and decreases the resonant frequency of the other. A differential frequency output reduces the common frequency drift caused by stresses and interferences, and thus improves the sensor's stability. The device geometries were optimised using numerical simulations and the fabrication process was based on a silicon-on-insulator wafer requiring only two masks with simplified microfabrication steps (e.g. sputter, wet etching and deep reactive ion etching). The sensor was quantified in an open-loop characterisation platform, producing a quality factor higher than 10 430 in vacuum (< 0.5 Pa). Closed-loop test results recorded a linear resonant frequency shift (a linear correlation of 0.9999) in response to applied pressure, with a sensitivity of 227 Hz/kPa. This resonant pressure microsensor has a simple fabrication process and reliable performance, and can be used for pressure monitoring in various locations including weather stations and aerospace.