Abstract: The main purpose of high temperature pressure sensor is to solve the problem of pressure measurement in high temperature harsh environments. SiC is an ideal material for manufacturing high temperature pressure sensors, combined with film technology and ceramic thick film technology, proposing a new 4H-SiC wireless Source-based high temperature pressure sensor design. Application ANSYS finite element analysis software is simulated, 600 ° C is 2.65 MHz / BAR, indicating that the sensor has high sensitivity at high temperatures, verifying the key process in the preparation process - SiC deeply etched, etching depth 124 m, meet the sensor preparation requirements.
0 Preface
High temperature pressure sensors have a wide range of uses in civil and military, but the elastic structural instability in high temperature environments, and the degradation of electrical lead performance is a key reason that the traditional MEMS pressure sensor cannot work properly. SiC is a wideband, high thermal conductivity, high thermal conductivity, and high electron saturation speed and good mechanical properties. These characteristics such as chemical stability and radiationability make the SiC's pressure sensor in manufacturing high temperature harsh environments. There is a significant advantage [1]. This article selects a SiC material preparation sensitive structure, and uses wireless passive detection techniques [2] to achieve high temperature pressure measurement. SiIC is a compound semiconductor having many tanomeric types, and 4H-SiC is selected herein, and Table 1 is the main characteristic of 4H-SiC and Si. Comparison.
1 Working principle
The pressure sensor mainly has a pressure-resistant and capacitive two structures, and the capacitive pressure sensor has the advantages of high sensitivity, high frequency sound, low temperature drift, and is the more potential research direction of SiC pressure sensors [3]. The basic structure of the capacitive pressure sensor is shown in Fig. 1, and when the pressure acts, the film generates a variable, and the upper and lower polar pitch changes, thereby changing the capacity of the capacitor.
Change capacitor [4]:
in
, W brown is the maximum deflection, and the a is the side length, and H is the sensitive film thickness, and D is the cavity spacing, and R is the relative dielectric constant of the SiC, and 0 is a vacuum dielectric constant.
Under the common action of flat thermal elasticity, under the common action of external pressure and temperature load [5] is as follows:
Where: P is an external pressure load, D is a bending stiffness,
, E is the Young's modulus Poisson ratio, X, Y, Z is 3 coordinate axes, respectively.
It is the coefficient of thermal expansion, which is the internal temperature distribution of the sensor, during steady-state heat transfer, the heat transfer campaign [6] is as follows:
Where, the seed is density, the CP is a specific heat capacity, and k is the heat transfer coefficient.
Due to the degradation of lead performance under high temperature environments, non-contact passive technology is subsequently tested. The capacitance to the inductor coil is connected to the LC resonant circuit, and the frequency signal is detected by the interaction coupling principle. The schematic is shown in Figure 2.
The scanning measurement is performed near the sensor by a coupling coil readout circuit (antenna), and resonance occurs when the measurement signal frequency is coupled to the inherent frequency of the sensor, resulting in a significant change in the input impedance, thereby estimating the inherent frequency associated with its sensor pressure. According to the pressure-displacement-capacitance of the pressure sensitive structure, the size of the pressure can be calculated [7].
2 structure design
Ceramics have high temperatures, self-encapsulation, insulation, low cost, etc., low-temperature co-burning ceramics (LTCC) processes have special advantages in making stereoscopic structures, using LTCC materials and processes to make pressure sensors, can satisfy 400 ~ 600 ° C Application in high temperature environments [8]. The glass slurry bonding is to coat the glass slurry on the bonding surface, melt the produced structure and contact the second substrate, and then form a stable mechanical connection.
Its advantage is that it has a sealed effect, high bonding strength, high production efficiency, and no special requirements for the surface of the sealing substrate [9]. The sensitive film made of the SiC chip is made into a capacitor with the LTCC ceramic bonding with a glass slurry, and the exhaust pipe is designed. Finally, in a vacuum environment, a glass bead is melted to form a vacuum capacitor cavity, and the inductive coil is printed on the ceramic. Finally, the lead bonding is formed in series to form an LC resonant circuit. The structure is shown in Figure 3.
2.1 SIC chip part design
First, the SiC chip is thinned to a certain thickness, etching a certain depth on a thinned chip, constitutes a cavity, etching a certain depth forming a sensitive film at the other side corresponding to the other. Oxidation of a layer of silica insulating layer, sputtering a layer of Ti as an adsorbing layer on an insulating layer, and then sputtering a layer of PT as a lead interconnect layer (a layer of TiN diffusion barrier layer can be prepared in the middle, relieving the layer and the layer Dynamically reactive) [10], graphical formation forming an upper electrode, as shown in Figure 4.
2.2 Ceramic Partial Design
By LTCC laser punch, thick film printing techniques, and multilayer laminated techniques, a capacitance underlayer lower plate and a platinum-inductive coil that meets the design requirements are prepared by appropriate process steps. Each layer of raw porcelain punch, printing is shown in Figure 5.
3 Simulation results and analysis
Simulation of the SiC film, due to the elastic modulus, Poisson ratio, density of the SiC material change [11], and the influence of thermal conductivity, thermal expansion, etc. [12], the sensor changes during temperature change during temperature changes. Simulation Analysis of Sensor Thin Films for Sensor Films. Simulation Analysis of Sensor Films. The characteristic parameters of silicon carbide at different temperatures are shown in Table 2, and the simulation displacement cloud is shown in Figure 6.
The maximum deflection of 2 atmospheres in 2 atmospheres at a temperature of 20 ° C, 200 ° C, 400 ° C, and 600 ° C, as shown in Table 3.
The design inductor is 2 h, which is determined by the formula (1), and the formula (4) can be seen that the resonant frequency change is shown in FIG.
The calculation can be obtained from 20 ° C, 200 ° C, 400 ° C, 600 ° C, 2.35 MHz / BAR, 2.4 MHz / BAR, 2.55 MHz / BAR, 2.65 MHz / BAR, and the sensor is known to have a high sensitivity at high temperatures. .
4 key process verification
The most critical process in this scheme is deep etching of SiC, since SiC chemical properties are very stable, there is no foundation of which acid or alkali can cause corrosion in room temperature, so dry in the processing process of SiC matrix French etching [13]. Since the selection of the Ni mask is relatively large, the step is straight and the surface condition is good, and the metal Ni is selected as a mask [14].
The SIC is more special is a C element. When using SF6 etching, it is necessary to take into account C, and because C and O can react, the addition of O2 is a better strategy:
SiC + O * + f * → SIF4 ↑ + CO ↑ + CO2 ↑
Therefore, SIC etch is generally adopted by SF6 + O2, then adds AR assist, enhances physical properties, and has a large rate, and the etch scanning electron microscope map is shown in FIG.
The etching depth is 124 m, which satisfies the sensor preparation requirements, and the bottom morphology has a "sub-Trench" phenomenon, follow-up to process optimization.
5 Conclusion
According to the design and simulation analysis of the SiC capacitive wireless passive high temperature pressure sensor, this sensor still has high sensitivity at a high temperature of 600 ° C, and verifies the key processes in the sensor preparation process - SiC. Sensor preparation requirements. The follow-up will be optimized, the preparation and testing of the sensor.
references
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