Development Of MEMS Capacitance Vacuum Sensor Based On Non Evaporable Getter Films

The capacitance vacuum sensor based on MEMS technology is one of the most widely used pressure sensors because of its high sensitivity,low temperature drift,small volume,low energy consumption,high precision,and independent measurement value and gas type.Due to the residual gas in the sensor cavity,the sensitivity and nonlinear error cannot be taken into account,and the electrode wire extraction process is complex,the vacuum sensor with high sensitivity lower than 10 Pa is not mature in China,which has hindered the development of deep space exploration and industrial field in China.In order to solve the above problems,based on non-evapotranspiration getter films a high sensitivity absolute pressure MEMS capacitance vacuum sensor with lower measurement limit of 1 Pa is proposed for the first time in this paper.The structure adopts large width thickness ratio pressure sensing film and contact mode,which improves the sensitivity and linearity of the sensor.In the preparation process,the problems of electrode extraction and residual gas in the vacuum cavity are eliminated by bonding technology.The main contents of the study are as follows:1.In this paper,Zr-Co-RE non-evaporable getter films were deposited on the rectangular silicon-based buffer structure by magnetron sputtering to maintain the high vacuum in the vacuum cavity of the sensor.On the one hand,it is proposed for the first time to improve the adsorption properties of Zr-Co-RE getter films by adding Ti transition layer on SiO2/Si substrate.The results show that the initial hydrogen absorption rate and hydrogen absorption capacity of the films with Ti transition layer increase from 472.32 mL s-1 cm-2 and 0.089 Pa L-1 cm-2 to 701.9 mL s-1 cm-2 and 0.104 Pa L-1 cm-2,and the film stress decreases from 1165.98 MPa to-384.13 MPa.On the other hand,the cuboid silicon-based buffer structure increases the specific surface area of the films and further improves the adsorption performance of the films2.Based on the requirement that the lower limit of measurement can be extended to 1 Pa,a new type of MEMS capacitance vacuum sensor with glass-silicon-glass structure is proposed in this paper.A high sensitivity silicon diaphragm with large width to thickness ratio is used as the pressure-sensitive diaphragm.The pressure-sensitive diaphragm is far away from the deformation of the fixed electrode.The rectangular silicon-based buffer structure is used to protect the pressure sensing film against load and act as the carrier of Zr-Co-RE non evaporable getter films.Based on the deflection theory,the relationship model between the input pressure and the output capacitance of the sensor is established.The influence of the size,thickness and electrode spacing of the pressure-sensitive diaphragm on the performance of the sensor is analyzed by ANSYS finite element method.The optimal size parameters of the sensor are determined,and the sensor is fabricated by MEMS technology.The sensor has a maximum sensitivity of 33.03 fF/Pa and a resolution of 0.5 Pa in the pressure range of(1～1000)Pa.In addition,the addition of rectangular silicon-based buffer structure improves the linearity of the sensor and makes the capacitance pressure curve piecewise linear.3.Based on the requirements of high sensitivity and wide measurement range,a silicon-silicon structure MEMS capacitance vacuum sensor is proposed.The high sensitivity borosilicate diaphragm with large width thickness ratio is also used as the pressure sensing film,which is close to the fixed electrode and deforms.Based on the large deflection theory and ANSYS finite element method,the relationship between pressure and deformation,stress and output capacitance of different shape pressure sensing membrane is analyzed.It is found that the dynamic measurement range and overload resistance of circular pressure sensing membrane are 1.6 times of that of square pressure sensing membrane.Silicon silicon direct bonding and boron doped etching self stop process were used to prepare high vacuum chamber and circular pressure sensing film.The sensor has a maximum sensitivity of 69.49 fF/Pa and a resolution of 1 Pa in the pressure range of(1～100000)Pa.In addition,the contact structure formed by the deformation of the pressure sensing film close to the fixed electrode not only protects the pressure sensing film,but also improves the linearity of the sensor.In this paper,aiming at the problem of restricting the wide application of the absolute pressure MEMS capacitance vacuum sensor,starting from the optimization of the structure design and preparation process,the performance of the vacuum sensor is greatly improved to meet the application in deep space exploration and industry.