0 Introduction Resonant quartz crystal pressure sensors are widely used in air pressure and altitude measurement, automatic pressure calibration, and precision process control due to their high precision and good long-term stability. These high-precision quartz crystal pressure sensors mainly come in two structural forms. One is the thick-film shear mode, where the quartz crystal is processed into a lens, and electrodes are fabricated on the lens. The pressure is detected by utilizing the relationship between the vibration frequency and the applied pressure. This type of sensor has very high requirements for the processing technology of the lens and electrodes, making it difficult to manufacture, and it has gradually been replaced by other structures. The other type uses a quartz resonant beam as the force-sensitive element, employing a Bourdon tube or metal diaphragm to sense the pressure and convert it into a force that acts on the resonant beam. The frequency of the resonant beam changes with the applied pressure, and the magnitude of the measured pressure is detected by utilizing the frequency change of the resonant beam. This type of pressure sensor has a more complex structure and requires high-quality materials and manufacturing processes. Currently, only a few companies internationally have mastered its key technologies and are able to provide products in batches. To address the manufacturing challenges of this sensor, this paper proposes the development of a pressure sensor with an all-quartz pressure-sensitive element. A quartz elastic diaphragm replaces the complex metal elastic element, reducing manufacturing difficulty and enabling sensor miniaturization. 1. Sensor Structure and Working Principle The quartz resonant pressure sensor mainly consists of a pressure-sensitive element and an excitation circuit. The sensor structure, as shown in Figure 1, includes a pressure interface, a pressure-sensitive element, and a housing. The pressure-sensitive element is the core of the sensor, comprising an elastic diaphragm and a tuning fork-type force-sensitive resonator. When pressure is applied to the elastic diaphragm, it deforms, causing tension or compression along the diameter direction. This force is then applied to the resonant beam, and the frequency of the resonant beam changes with the applied force. The magnitude of the measured pressure is detected by utilizing the frequency change of the resonant beam. All parts of the pressure-sensitive element are made of quartz crystal material. The quartz force-sensitive resonator is fabricated using photolithography and chemical etching processes, and then sintered onto an elastic diaphragm. Under vacuum conditions, the elastic diaphragm is sintered onto a base with a pre-ground cavity. This sintering of the base and diaphragm simultaneously creates a vacuum reference force cavity, meeting both the design requirements for an absolute pressure sensor and the vacuum operating environment requirements of the resonator. In the formula, L, t, and b represent the length, width, and thickness of the resonant beam, respectively; E and p represent the elastic modulus and density, respectively; and f0 is the center resonant frequency. In the first-order vibration mode, with both ends fixed, the constants a0 and as are 1.026 and 0.294, respectively. 2. Fabrication of the Quartz Force-Sensitive Resonator The quartz force-sensitive resonator consists of two outer support beams and two inner resonant beams. Electrodes are distributed on the resonant beams, specifically on the upper, lower, and side surfaces. There are no electrodes on the support beams. A copper plating layer is used for masking, and anisotropic etching is employed to etch the quartz crystal. The electrode layer is evaporated using a rotary evaporation method, and the side electrodes are photolithographically formed using a specially designed side light source, thus solving the problem of side electrode fabrication. The resonator after photolithography requires temperature cycling to reduce residual stress. The process flow of the force-sensitive resonator is shown in Figure 4. Through the above process, a force-sensitive resonator with a beam width of 190 μm, a beam spacing of 160 μm, and a beam length of 4.96 mm was fabricated. Its range is 0–150 gf, center frequency is (40±4) kHz, accuracy is 0.05%, and full-scale output is (1.2±0.24) kHz. 3. Pressure Sensing Element Assembly The structure of the pressure-sensitive element is shown in Figure 5. A circular cavity for the pressure-sensitive element base is machined using a grinding method. Since the diameter of the circular cavity and the thickness of the elastic diaphragm determine the sensor's range, the cavity diameter must be strictly controlled during processing. If the cavity diameter is too large, the range will be reduced; conversely, a smaller diameter will increase the range. After grinding, an isotropic etching solution is used to remove the damaged layer caused by grinding, reducing residual mechanical stress. The force-sensitive resonator and the elastic diaphragm are sintered together with glass powder to form a rigid connection. Since the position of the force-sensitive resonator on the diaphragm directly affects the sensor's output and accuracy, a special fixture is used during the sintering process to prevent slippage of the resonator. This is one of the key processes in the fabrication of the pressure-sensitive element. The base and the elastic diaphragm are sintered together with glass powder under vacuum conditions. This achieves both a rigid connection between the elastic diaphragm and the base and a vacuum seal of the pressure cavity, thus completing the vacuum encapsulation of the sensor. To ensure the sensor's accuracy, the resonator must remain centered in the cavity during sintering; therefore, a special fixture is used. The pressure-sensitive element, sensor excitation circuit, and housing are then assembled, completing the sensor fabrication. 4. Calibration Test Under room temperature conditions, a 10 V DC voltage was applied to the sensor for static calibration. Table 1 shows typical calibration data for sensor #1, and Table 2 shows static calibration data for sensor #2. From Tables 1 and 2, it can be seen that the full-scale output of the sensor is 1.1461 kHz and 1.1599 kHz, respectively. Using the average forward and reverse stroke values ​​at each measurement point as the standard value, the sensor accuracy is better than 0.05%. 5. Conclusion In the development of the resonant quartz crystal absolute pressure sensor, the use of quartz crystal material to fabricate the elastic diaphragm solved the technical difficulties of complex sensor structure and complex manufacturing process. The developed sensor has a simple structure, high accuracy, and small size. Its measurement range is 0–120 kPa, the center frequency is (40+4) kHz, and the accuracy is 0.05%, making it suitable for high-precision absolute pressure measurement requirements.