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.


Introduction
The resonant pressure micro sensors find extensive applications in aviation and aerospace vehicles, process industry, atmospheric monitoring fields due to their remarkable features including high accuracies, high resolution, long-term stability, and quasi-digital outputs [1,2].These sensors operate based on the intrinsic frequency shifts of micro resonators caused by changes in axial stresses [3,4].However, the performances of these sensors can be significantly affected by environmental perturbations, such as temperature variations and vibrations, due to thermal mismatch and structural instabilities caused by assembly materials and methods [5].Consequently, the distribution of packaging-induced stresses within the resonators can significantly compromise measurement accuracies during temperature variations and vibrations.
Stress isolation is a common technique used in the design of MEMS (Micro Electro Mechanical Systems) devices to mitigate the side effects of temperature and vibrations [6], such as those found in gyroscopes [7], accelerometers [8], pressure sensors [9], resonators [10], and so on [11,12].These applications shown that stress isolation had been a critical technique in the design and operation of MEMS devices, as it helps to ensure the accurate and reliable measurements necessary for their intended usages.
This work presents a stress isolation method that utilizes an isolation layer to achieve permanent immobilization of the sensor chip on the pedestal.The simulation and characterization results clearly indicate significant decreases in thermal stresses and notable improvements in stability under random vibrations, attributed to the contributions of the isolation layer.

Design and modelling
Figure 1(a) shows the proposed stress-isolated resonant pressure micro sensor, including a sensor chip for pressure sensing, a stress isolation layer for fixed assembly and a Kovar pedestal for providing mechanical supports.Figure 1(b) shows the details of the sensor chip which consists of an SOI wafer and a glass cap.The components for temperature and pressure sensing were formed from the SOI wafer.Specifically, two identical H-shaped resonators were defined in the device layer of the SOI wafer, one in the center ('Resonator I') and the other on the side ('Resonator II').They were double ended fixed to the pressure-sensitive diaphragm (handle layer of the SOI wafer) by anchor structures (oxide layer of the SOI wafer).The resonators were vacuum packaged using a glass wafer that had a cavity and getter inside of it.Further stresses were isolated using the silicon isolation layer.In operation, resonator I experiences tensile axial strains, whereas resonator II experiences compressive axial stresses when the pressure-sensitive diaphragm is bent by the surrounding pressure under measurement.As a result, the stress isolation layer can reduce environmental perturbations by acting as a cushion support in reaction to external disturbances such random vibrations and temperature changes.
Finite element analyses were used in the design and optimization of the stress isolation layer of the resonant pressure micro sensor.Figure 2(a) shows the axial stress distributions of resonator I, which caused by the temperature decreasing from 350℃ to 25℃, in response to the thickness of stress isolation layer.The axial stress of resonator I decreased with the increasing of the thickness of the stress isolation layer.whereas, the thickness shown little improvement on the thermal stress isolations when the thickness of the stress isolation layer greater than 2 mm. Figure 2(b) shows the thermal stress distributions of the sensor chip when the ambient temperature changes from 25℃ to -55℃.The thermal stress was attenuated by the stress isolation layer, and finally transmitted to the resonator is about 1/2500 of the original.
Figure 2(c~f) show the equivalent stress distributions of the proposed micro sensor during random vibrations which followed the Chinses environmental test standard of GJB150.16A-2009.When the sensor was subjected to random vibrations in three axes of X, Y and Z, the maximum equivalent stresses around the whole sensor structure were less than 0.2 MPa, which were far lower than the fracture strength and yield point of the fabricated sensor materials.

Fabrication
Figure 3(a) shows the fabrication process, which closely aligns with our previous works [13,14] but with a few innovative additions (Figure 3 (a v-vii)).The main difference was that a silicon wafer was utilized as an isolation layer.The backside of this thick silicon wafer was patterned using photoresist as a mask and etched a certain depth using deep reactive ion etching (DRIE) to create a contact region for subsequent assembly (Figure 3(a v)).Subsequently, the thick silicon wafer and the bonded SOI-glass wafer were combined using a second anodic bonding process (Figure 3(a vi)).To establish electrical connections, a metal film was sputtered over the electrode vias (Figure 3(a vii)).
The fabricated sensor chip was then assembled to a Kovar pedestal by coating the adhesive on the bottom and rest for 7 days under room temperature for aging.The fabricated wafer and packaged prototypes are shown in Figure 3

Characterization
Figure 4(a~c) show the measurement errors of the resonant pressure micro sensors within the full pressure (10 kPa~1 MPa) and temperatures (-55℃~85℃) ranges.The maximum measurement error observed was less than 70 Pa (±0.01%FS).These results validate that the micro sensor equipped with an isolation layer can effectively mitigate temperature disturbances, ensuring highly accurate pressure measurements.

Conclusion
The present study focuses on the sensor design, micro fabrication, and device characterization of a stress-isolated resonant pressure micro sensor.Experimental results validate that the proposed isolation method effectively mitigates temperature disturbances and enhances stability in random vibrations, thereby demonstrating its potential as an invaluable tool in high-performance sensor assembly applications.

Figure 1 .
Figure 1.Schematic of the stress-isolated resonant pressure micro sensor, including the sensor in (a) assembly and (b) explosion diagram of the chip.

Figure 2 .
Figure 2. Simulation results: (a) the axial stress distributions of resonator I and (b) the equivalent stress distribution around the assembled sensor structure under thermal condition.(c) The conditions of random vibrations.The equivalent stress distributions around the assembled sensor structure in (d) X vibrations, (e) Y vibrations and (f) Z vibrations. (b).