High-temperature-resistant MEMS pressure sensor chip and packaging structure thereof

Abstract

The invention belongs to the field of MEMS sensors, and discloses a high-temperature-resistant MEMS pressure sensor chip and a packaging structure thereof. The chip is provided with an SOI substrate, an AlN layer and a SiO2 layer from bottom to top, a plurality of P-type piezoresistors are arranged on the uppermost layer of the SOI substrate and are symmetrically distributed at the stress concentration position of the edge of the elastic diaphragm; a channel which continuously penetrates through the AlN layer and the SiO2 layer is arranged above the P-type piezoresistor; a metal Pad is arranged in the channel; the upper end of the metal Pad is located on the uppermost layer of the high-temperature-resistant MEMS pressure sensor chip, and the lower end of the metal Pad is in contact with the P-type piezoresistor. According to the invention, the relatively thick buried oxide layer can resist a high-temperature severe environment, and an AlN heat dissipation material with a high heat conductivity coefficient is deposited, so that rapid heat dissipation at a high temperature is ensured; and meanwhile, the stability and the reliability of the sensor are improved by adopting leadless packaging, and the packaging density is further optimized.

Description

Technical Field

[0001] The present invention belongs to the technical field related to sensors, and more specifically, relates to a high-temperature-resistant MEMS pressure sensor chip and its packaging structure. Background Art

[0002] With the development of MEMS technology, the market of micro pressure sensors has expanded rapidly. However, a problem has also emerged - characteristics such as device stability, reliability, and accuracy in high-temperature environments. When traditional pressure sensors are used in an environment above 120 °C, the performance of the sensors will decline sharply due to leakage of the internal PN junction, and then lead to failure. Pressure sensors used in industries, aerospace, and other fields need to meet two basic requirements: high temperature and high reliability. The most basic requirement for MEMS high-temperature pressure sensors is to operate in an environment of at least 125 °C. As the operating temperature of the sensor increases, due to factors such as increased leakage current, material creep, and component aging, its characteristics such as accuracy, linearity, and sensitivity also decrease, resulting in a reduction in the performance of MEMS piezoresistive pressure sensors and even device failure.

[0003] High-temperature pressure sensors have broad application requirements in fields such as aerospace, aviation, national defense construction, and energy development. Normal-temperature MEMS pressure sensors are mainly silicon (Si)-based pressure sensors. Within the working temperature range of 100 °C, the commercial Si pressure sensor process is mature, with small size and good performance. However, when it is used in an environment above 120 °C, leakage will occur in the internal PN junction, and the sensor performance will decline or even fail. In addition, the Si material will also undergo plastic deformation when the temperature is greater than 500 °C, which cannot meet the requirements of pressure measurement in high-temperature environments.

[0004] The packaging of the chip plays an important role in the sensor manufacturing process. The traditional wire bonding connection method has long wires. In high-frequency applications, it will generate large inductance and resistance, resulting in an increase in signal transmission delay and a decrease in signal integrity. For example, in fields with high requirements for high-frequency performance such as 5G communication and high-speed computers, wire bonding is difficult to meet the requirements. At the same time, the long wires are prone to electromagnetic radiation, interfering with the surrounding circuits, and are also vulnerable to external electromagnetic interference, affecting the stability and reliability of the system. Since a certain space needs to be reserved around the chip for arranging the wires, this also limits the further improvement of the packaging density and is not suitable for products with extremely high requirements for miniaturization, such as smart phones and wearable devices. Summary of the Invention

[0005] In view of the above deficiencies or improvement requirements of the prior art, the present invention provides a high-temperature-resistant MEMS pressure sensor chip and its packaging structure, which aims to design the MEMS pressure sensor chip by introducing an AlN layer with high thermal conductivity and combining with the SiO2 layer on the AlN, and at the same time achieve packaging by connecting the metal solder layer with the metal Pad and connecting the eutectic layer with the SOI substrate silicon, thereby solving the technical problem of insufficient reliability of the existing MEMS pressure sensor under high-temperature operation.

[0006] To achieve the above object, according to the first aspect of the present invention, a high-temperature-resistant MEMS pressure sensor chip is provided, which is provided with an SOI substrate, an AlN layer and an SiO2 layer from bottom to top; a concave area is provided at the center position of the bottom layer of the SOI substrate, and the chip area corresponding to the concave area is an elastic diaphragm;

[0007] A plurality of P-type piezoresistors are provided on the uppermost layer of the SOI substrate, and the plurality of P-type piezoresistors (7) are evenly distributed at the stress concentration points on the edge of the elastic diaphragm; a channel continuously penetrating through the AlN layer and the SiO2 layer is provided above the P-type piezoresistor; a metal Pad is provided in the channel; the upper end of the metal Pad is located on the uppermost layer of the high-temperature-resistant MEMS pressure sensor chip, and the lower end is in contact with the P-type piezoresistor.

[0008] As a preference of the present invention, the SOI substrate includes an SOI top silicon, an SOI buried oxide layer and an SOI substrate silicon from top to bottom; the plurality of P-type piezoresistors are symmetrically distributed in the SOI top silicon and form a Wheatstone bridge through electrical connection.

[0009] As a preference of the present invention, the piezoresistor crystal phases of each of the P-type piezoresistors on the SOI top silicon are the same.

[0010] As a preference of the present invention, the P-type piezoresistor is a folded-line piezoresistor strip, which is obtained by performing B ion implantation on the SOI top silicon.

[0011] As a preference of the present invention, the concentration of the B ions is 1*10 19 ~1*10 20 cm -3 .

[0012] As a preference of the present invention, the thickness of the SOI buried oxide layer is 1-3 μm.

[0013] As a preference of the present invention, the SOI substrate silicon is a silicon cup structure.

[0014] As a preference of the present invention, the thickness of the AlN layer (2) is 50-200 nm.

[0015] Preferably, the thickness of the SiO2 layer is 50 to 100 nm.

[0016] According to another aspect of the present invention, there is provided a high-temperature-resistant MEMS pressure sensor packaging structure, including a circuit board, a high-temperature-resistant MEMS pressure sensor chip as described in any one of the first aspects of the present invention, and a stainless-steel base; the circuit board is connected to the metal Pad through a metal solder layer, and the SOI substrate silicon is connected to the stainless-steel base through a eutectic layer; a through ventilation hole is provided at the center position of the stainless-steel base, leading directly to the bottom surface of the SOI substrate in the high-temperature-resistant MEMS pressure sensor chip.

[0017] Generally speaking, compared with the prior art by the above technical solutions conceived by the present invention, the following technical advantages are mainly possessed:

[0018] 1. By designing the layer structure of the chip, the present invention introduces an AlN layer with high thermal conductivity. As a heat dissipation layer and a protective layer, it can well improve the temperature characteristics of the pressure sensor. Compared with the traditional SiO2 heat dissipation material, AlN has a higher thermal conductivity. Therefore, the heat transfer characteristics of AlN are better, and the temperature will be quickly conducted to the top silicon of the SOI. In addition, the thermal expansion coefficient of the SiO2 material is much smaller than that of silicon, avoiding the phenomenon that the zero-point temperature drift is aggravated by a similar SiO2 layer. At the same time, a SiO2 inorganic coating is deposited or sputtered on the surface of the AlN as a protective layer to prevent the hydrolysis reaction from occurring due to the easy absorption of water molecules in the air by the AlN layer, which affects its thermal conductivity and insulation performance. Thus, the sensor of the present invention has high reliability under high-temperature operation.

[0019] 2. Preferably, four P-type piezoresistors are connected to form a Wheatstone bridge to output a voltage signal. The Wheatstone bridge adopts a constant current source power supply method, which has a smaller sensitivity temperature drift compared with the constant voltage source power supply.

[0020] 3. Preferably, the high-temperature-resistant pressure sensor chip of the present invention includes P-type piezoresistors that are evenly and symmetrically distributed at the stress concentration points on the edge of the elastic diaphragm. The P-type piezoresistors have the same crystal phase and reach the maximum sensitivity in the crystal phase.

[0021] 4. Preferably, the P-type piezoresistor of the present invention adopts a broken-line structure, which can better sense the stress concentration area to transfer stress. 5. Preferably, the P-type piezoresistor strip of the present invention adopts a high-concentration B ion implantation of 1*10 19 ~1*10 20 cm -3 and has a smaller temperature drift and high reliability.

[0022] 6. Preferably, the thickness of the buried oxide layer of the SOI in the present invention is 1-3 μm. Compared with the buried oxide layer of SOI prepared by traditional processes, which has a thickness of less than 450 nm, the buried oxide layer of the high-temperature resistant MEMS pressure sensor chip in the present invention has a greater thickness, which can effectively avoid the influence of leakage current under high-temperature conditions and ensure the stability and reliability of the sensor in harsh high-temperature environments.

[0023] 7. Preferably, a SiO2 inorganic coating is deposited or sputtered on the surface of AlN as a protective layer. Since the AlN layer also has some limitations, such as being prone to absorbing water molecules in the air and undergoing hydrolysis reactions, which may affect its thermal conductivity and insulation properties, a SiO2 inorganic coating is deposited or sputtered on the surface of AlN. The SiO2 inorganic coating can prevent moisture from reaching the AlN surface through physical barrier means; at the same time, it has good chemical stability and is not prone to reacting with water. Meanwhile, SiO2 has good insulation characteristics and will not have a negative impact on the insulation of AlN. Instead, it may enhance the overall insulation effect and further can be used as a protective layer to prevent environmental pollution and corrosion, etc.

[0024] 8. The high-temperature resistant MEMS pressure sensor chip of the present invention is connected to a circuit board and a stainless steel base. Specifically, on the one hand, the chip and the stainless steel base form a reliable bond through the eutectic bonding process. Its low-temperature characteristics effectively reduce the thermal damage to temperature-sensitive devices and reduce the thermal stress at the heterogeneous material interface. Secondly, the eutectic bonding has high airtightness, and the molten metal can fill the surface microdefects to form a pore-free interface, which is suitable for MEMS vacuum or pressure packaging; on the other hand, the high-temperature resistant MEMS pressure sensor chip and the circuit board form a reliable electrical connection through the flip-chip bonding process. Compared with the traditional wire bonding method, the connection distance between the flip-chip and the substrate is small, which can greatly reduce the length of the signal transmission path, reduce signal transmission delay and signal attenuation; at the same time, the high-temperature resistant MEMS pressure sensor chip and the circuit board are connected through a metal solder. The metal solder has good electrical conductivity and thermal conductivity, which can ensure the stable electrical performance and heat dissipation efficiency between the circuit board and the high-temperature resistant MEMS pressure sensor chip; in a high-temperature working environment, the physical properties of the metal solder will not change significantly, ensuring the reliability of the entire sensor under extreme conditions; in addition, by precisely controlling its composition and process, the metal solder can optimize its mechanical strength and fatigue resistance, thereby extending the service life of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic cross-sectional structure diagram of the high-temperature resistant MEMS pressure sensor chip according to an example of the present invention.

[0026] Figure 2 is the resistance distribution diagram and stress distribution diagram of the high-temperature resistant MEMS pressure sensor chip according to an example of the present invention; whereinFigure 2 In which, a is the distribution diagram of chip resistors, and b is the stress distribution diagram.

[0027] Figure 3 This is a comparison diagram of the heat dissipation performance of different thermal conductive materials in the examples of the present invention under the same conditions; among them Figure 3 In a, it is a schematic diagram of temperature diffusion when a SiO2 thin film with a thickness of 50 nm is applied with a 100 °C heat source at the bottom, in b, it is a schematic diagram of temperature diffusion when an AlN thin film with a thickness of 50 nm is applied with a 100 °C heat source at the bottom, and in c, it is a result diagram of the temperature change line of the SiO2 and AlN thin films along the thickness direction.

[0028] Figure 4 This is a comparison diagram of the heat dissipation performance of different thermal conductive materials in the examples of the present invention under different thickness conditions; among them, a is the result diagram of the temperature change line of SiO2 and AlN thin films with a thickness of 50 nm along the thickness direction, b is the result diagram of the temperature change line of SiO2 and AlN thin films with a thickness of 150 nm along the thickness direction, and c is the result diagram of the temperature change line of SiO2 and AlN thin films with a thickness of 200 nm along the thickness direction.

[0029] Figure 5 This is a schematic diagram of the packaging structure of a high-temperature resistant MEMS pressure sensor in the examples of the present invention.

[0030] Figure 6 This is a diagram of the sensor output - pressure relationship under constant voltage and constant current power supplies in the examples of the present invention.

[0031] In all the drawings, the same reference numerals are used to represent the same structures, where:

[0032] 1 - SiO2 layer; 2 - AlN layer; 3 - SOI top silicon; 4 - SOI buried oxide layer; 5 - SOI substrate silicon; 6 - metal Pad; 7 - P-type piezoresistor; 8 - pin terminal; 9 - circuit board; 10 - high-temperature resistant MEMS pressure sensor chip; 11 - eutectic layer; 12 - stainless steel base; 13 - metal solder layer. Detailed implementation manners

[0033] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0034] To achieve the above objectives, in the first aspect, the present invention provides a high-temperature resistant MEMS pressure sensor chip. As Figure 1As shown, the high-temperature resistant MEMS pressure sensor chip includes, from bottom to top: an SOI substrate, an AlN layer 2, and an SiO2 layer 1. A concave area extending in the upper layer direction is provided in the center of the bottom layer of the SOI substrate, and the chip area corresponding to the concave area forms a diaphragm;

[0035] On the top layer of the SOI substrate, a plurality of P-type piezoresistors are provided at the stress concentration points evenly distributed at the edge of the diaphragm; above the P-type piezoresistors, there is a channel continuously penetrating through the AlN layer and the SiO2 layer; a metal Pad is provided in the channel; the upper end of the metal Pad is located on the top layer of the high-temperature resistant MEMS pressure sensor chip, and the lower end is in contact with the P-type piezoresistor.

[0036] Among them, the SOI substrate includes, from bottom to top, an SOI top silicon 3, an SOI buried oxide layer 4, and an SOI substrate silicon 5. The AlN layer 2 is deposited on the upper surface of the SOI top silicon 3, and the AlN layer 2 is preferably 50 - 200 nm. Compared with the traditional SiO2 heat dissipation material, the AlN layer 2 has a higher thermal conductivity, and its thermal conductivity is generally 170 - 230 W / (m·K), which is about 14 times that of SiO2 under the same conditions; in addition, the thermal expansion coefficient of the SiO2 material is much smaller than that of silicon, and the SiO2 layer will exacerbate the zero-point temperature drift phenomenon. Therefore, selecting aluminum nitride thin film can well improve the temperature characteristics of the pressure sensor.

[0037] In some embodiments, the distribution of the P-type piezoresistors 7 on the SOI top silicon 3 is set according to the stress distribution map of the square diaphragm obtained by finite element analysis, so that the P-type piezoresistors 7 are evenly distributed in the stress concentration areas at the center positions of the four edges of the diaphragm. Usually, the piezoresistors are arranged at a distance of 10 - 50 μm inward from the midpoints of the four sides of the diaphragm.

[0038] The P-type piezoresistors 7 are symmetrically distributed in the SOI top silicon 3. The four P-type piezoresistors are all located at the positions with the maximum stress in each symmetric distribution area and are electrically connected to form a Wheatstone bridge. The Wheatstone bridge adopts a constant current source power supply method and has a smaller sensitivity temperature drift.

[0039] In some embodiments, the P-type piezoresistors have the same crystal phase and reach the maximum sensitivity in terms of crystal phase.

[0040] In some embodiments, the P-type piezoresistor 7 is a zigzag resistor bar, which is obtained by B ion implantation in the SOI top silicon 3. The ion concentration affects the resistance value and the resistance temperature coefficient of the piezoresistor. Usually, it is in the range of 1*10 19 ~1*10 20 cm -3 , so that the obtained P-type piezoresistor has a smaller temperature drift and high reliability.

[0041] In some embodiments, the P-type varistor adopts a zigzag structure with a length of 1000 - 1500 μm and a width of 10 - 15 μm.

[0042] For example, the zigzag resistor bar has a length of 1020 μm and a width of 10 μm. The stress distribution diagram of the square elastic diaphragm is obtained through finite element analysis. The varistors are evenly distributed in the stress concentration areas at the center positions of the four edges of the elastic diaphragm. Considering the limitations of actual processes such as photolithography and deep silicon etching, the varistors are arranged 30 μm inward from the midpoints of the four sides of the diaphragm.

[0043] In some embodiments, the SOI substrate is composed of an SOI top silicon 3, an SOI buried oxide layer 4, and an SOI substrate silicon 5 from top to bottom. Among them, the SOI substrate is prepared by a smart-cut process. The specific process is as follows: High-energy hydrogen ions are implanted on the surface of the silicon wafer to form a "microcrack layer" with a controllable depth; a layer of silicon dioxide (SiO2) is grown on another supporting silicon wafer as the buried oxide layer, and the two silicon wafers are bonded through hydrophilic bonding technology; through heat treatment at 400°C - 600°C, the hydrogen ion layer expands to form bubbles, causing the silicon wafer to break at the microcracks, and the upper silicon thin film (with a controllable thickness down to the nanometer level) is transferred to the supporting silicon wafer to form an SOI structure.

[0044] In some embodiments, the thickness of the SOI buried oxide layer 4 is 1 - 3 μm, and the thickness of the SOI buried oxide layer 4 is adjusted by controlling parameters such as ion implantation energy, concentration, and implantation angle. For example, the donor wafer is thermally oxidized before bonding to directly control the thickness of the SiO2 layer. For example, a SiO2 layer of 0.1 - 2 μm can be generated with an oxidation time of 10 minutes to 2 hours. Compared with the SOI buried oxide layer prepared by traditional processes with a thickness of less than 450 nm, the SOI buried oxide layer of the pressure sensor chip proposed in the present invention has a greater thickness, which can effectively avoid the influence of leakage current under high-temperature conditions and ensure the stability and reliability of the sensor in harsh high-temperature environments.

[0045] In some embodiments, the SOI substrate silicon 5 is a silicon cup structure formed by KOH wet etching. The formed angle is determined by the crystal structure of silicon, which is the angle between the (100) crystal plane and the (111) crystal plane, that is, the slope angle of the back of the silicon cup is 54.74°.

[0046] In some embodiments, a SiO2 layer 1 is deposited or sputtered on the surface of the AlN layer 2. The SiO2 layer 1 is an inorganic coating that prevents moisture from reaching the surface of the AlN layer 2 through physical barrier, serves as a protective layer, and at the same time, SiO2 has good insulation properties and serves as an overall insulating layer to protect against environmental pollution and corrosion, etc.

[0047] In the present invention, a method for preparing a high-temperature resistant MEMS pressure sensor chip is exemplified as follows:

[0048] (1) Each layer is prepared by using the smart-cut process, specifically including: injecting high-energy hydrogen ions on the surface of a silicon wafer to form a "micro-crack layer" with a controllable depth. A layer of silicon dioxide (SiO2) is grown on another supporting silicon wafer as a buried oxide layer, and the two silicon wafers are bonded together through hydrophilic bonding technology. Through heat treatment at 400 - 600 °C, the hydrogen ion layer expands to form bubbles, causing the silicon wafer to break at the micro-cracks, and the upper silicon thin film (with a controllable thickness down to the nanometer level) is transferred to the supporting silicon wafer to form an SOI structure.

[0049] (2) First, use RCA standard cleaning (H2O2:H2SO4 = 1:4, 80 °C) to remove the organic matter on the surface of the SOI, and then use HF (1%) rinsing to remove the native oxide layer.

[0050] (3) Photolithographic patterning is carried out on the top silicon of the SOI, and a piezoresistor is formed by high-concentration B ion implantation. The implantation concentration is 1*10 19 ~1*10 20 cm -3 , and the ion implantation angle is 7° inclined implantation to reduce the channeling effect. Rapid thermal annealing (RTA) is carried out for 10 - 30 s at 900 - 1000 °C in an N2 atmosphere to activate the B ions.

[0051] (4) In an N2 / Ar mixed gas (ratio 1:3) atmosphere, a 50 - 200 nm thick AlN thin film is prepared by magnetron sputtering of a high-purity Al target (99.999%). The radio frequency sputtering power is 200 W, and the temperature is 150 °C;

[0052] (5) A 50 - 100 nm thick SiO2 thin film is prepared by plasma-enhanced chemical vapor deposition (PECVD). Under the conditions of 100 W and 200 °C, through the chemical reaction of silane (SiH4) and an oxidant (such as N2O or O2), a dense silicon dioxide thin film is formed under the action of plasma.

[0053] (6) RIE etching of the film through-holes forms channels: The gas is a mixture of CF4 and CHF3 in a ratio of 1:2, the power is 150 W. Finally, under a vacuum of less than 2×10 -6 Torr, electrode materials (such as Au, Al, etc.) are electron beam evaporated to form metal electrodes and wires, and the excess metal is removed through the lift-off process.

[0054] To achieve the above object, on the other hand, the present invention provides a MEMS pressure sensor packaging structure with the characteristics of high temperature resistance, high precision, and low temperature drift. The sensor packaging structure includes a circuit board 9, the high-temperature-resistant MEMS pressure sensor chip 10 described in the present invention, and a stainless-steel base 12; the circuit board 9 is connected to the metal Pad 6 on the uppermost layer of the high-temperature-resistant MEMS pressure sensor chip 10 through a metal solder layer 13; a eutectic layer 11 is provided above the stainless-steel base 12 and is connected to the SOI substrate silicon 5 on the lowermost layer of the high-temperature-resistant MEMS pressure sensor chip 10 through the eutectic layer 11; and, a through ventilation hole is provided at the center position of the stainless-steel base 12, and the ventilation hole communicates with the back cavity side of the high-temperature-resistant MEMS pressure sensor chip 10, that is, the back of the SOI elastic diaphragm, so that the gas or liquid to be measured acts on the back of the elastic diaphragm (facing the SOI substrate side) through the ventilation hole.

[0055] In one embodiment, the stainless-steel base 12 is made of 17-4PH alloy. The high-temperature-resistant MEMS pressure sensor chip 10 and the stainless-steel base 12 are reliably bonded through an eutectic bonding process, so that the gas or liquid to be measured acts on the high-temperature-resistant MEMS pressure sensor chip 10 through the ventilation hole to directly sense the pressure magnitude. Since the gas or liquid does not directly act on the electrical structure layer on the front surface of the high-temperature-resistant MEMS pressure sensor chip 10, but directly contacts the back cavity passive layer, it can effectively avoid the corrosion and damage of the circuit structure, thereby affecting the pressure measurement accuracy.

[0056] The specific eutectic bonding process is as follows: First, a multi-layer structure containing an adhesion layer, a silicon diffusion barrier layer, a gold diffusion barrier layer, a gold film layer, and an ultra-thin germanium layer is deposited on the back of the high-temperature-resistant MEMS pressure sensor chip by vacuum sputtering. Then, the pressure inlet of the high-temperature-resistant MEMS pressure sensor chip is aligned with the through hole of the high-temperature alloy steel used for bonding and pasted on, and a certain pressure is applied and placed in a high-temperature furnace and heated to 450 °C. The two will form a solid bond through a short liquid-phase transition eutectic process.

[0057] In one embodiment, the high-temperature-resistant MEMS pressure sensor chip 0 and the circuit board 9 form a reliable electrical connection through a flip-chip bonding process. Compared with the traditional wire bonding method, it has disadvantages such as high transmission delay, susceptibility to electromagnetic interference, and limitation of the improvement of packaging density. The connection distance between the flip-chip and the substrate is small, which can greatly reduce the length of the signal transmission path, reduce signal transmission delay and signal attenuation; at the same time, flip-chip bonding realizes a more compact packaging structure. For example, in the chip packaging of mobile devices, the flip-chip bonding technology can reduce the size of the packaged chip by 20% - 30%, which helps to realize the miniaturization and thinness of the device.

[0058] In one embodiment, in terms of the encapsulation structure, in addition to traditional components such as a circuit board, a solder layer, a high-temperature-resistant MEMS pressure sensor chip, a eutectic layer, and a stainless-steel base, a housing made of a special alloy material with high temperature resistance, corrosion resistance, and good mechanical strength is particularly added. The design of this housing has multiple advantages. The cylindrical outer shape structure is not only convenient for processing and manufacturing but also provides good protection performance. The design with one end open facilitates the installation and debugging of internal components. The other end is closed and provided with a connection interface, which not only facilitates the connection and fixation with external devices but also ensures the stability of the entire sensor during operation. At the same time, the high-temperature-resistant sealant used between the inner wall of the housing and the internal components can effectively prevent external substances such as gases and liquids from invading the internal components, ensuring the normal operation of the sensor in a harsh environment.

[0059] In summary, the sensing principle of the MEMS sensor encapsulation structure of the present invention for the oil pressure or hydraulic pressure to be measured is as follows: The present invention utilizes the through ventilation hole in the center of the stainless-steel base, the elastic diaphragm, and the characteristics of the piezoresistors in the pressure sensor. Specifically, the external oil pressure or hydraulic pressure is transmitted to the ventilation hole through the connection interface and then acts on the back surface of the elastic diaphragm, causing the elastic diaphragm to deform; this deformation causes the piezoresistors distributed in the stress concentration areas at the center positions of the four edges of the elastic diaphragm to be stressed, thereby changing their resistance values; since the four piezoresistors are connected into a Wheatstone bridge, the change in the resistance value of the piezoresistor will cause a change in the output voltage signal of the Wheatstone bridge; by collecting and analyzing this voltage signal, the magnitude and change of the oil pressure or hydraulic pressure to be measured can be accurately sensed. This sensing method has high sensitivity and reliability and can meet the pressure measurement requirements in different industrial scenarios.

[0060] To further illustrate the high-temperature-resistant MEMS pressure sensor and its encapsulation structure provided by the present invention, the following further details the specific embodiments of the present invention in conjunction with the accompanying drawings, so that the technical solutions of the present invention are easier to understand and master.

[0061] Example 1:

[0062] Prepare a MEMS pressure sensor according to the above example process, as Figure 1 shown in the structure of the high-temperature-resistant MEMS pressure sensor chip. The size of the high-temperature-resistant MEMS pressure sensor chip is 4000μm×4000μm, the thickness of the SOI elastic membrane is 60 - 65μm, and the size is 2000μm×2000μm. The AIN layer 2 serves as a heat dissipation layer and a protective layer, with a thickness of 50 - 200nm. The SiO2 layer 1 serves as a protective layer to prevent the AlN layer 2 from being corroded, with a thickness of 50 - 100nm. The metal Pad 6 is Au or Al, with a thickness of 1 - 2μm. The P-type piezoresistor 7 is symmetrically distributed in a zigzag structure, with a thickness of 10μm.

[0063] See Figure 2 , the P-type piezoresistors 7 are all zigzag-shaped and are located at the edge stress concentration of the elastic diaphragm. When an external pressure is applied to the elastic diaphragm, the stress at the edge changes from negative to positive. Considering the limitations of actual processes such as photolithography and deep silicon etching, the P-type piezoresistors 7 are arranged 30 μm inward from the midpoints of the four sides of the diaphragm, which can avoid the influence of these edge effects and ensure the performance and dimensional accuracy of the P-type piezoresistors 7. Utilize the longitudinal and transverse piezoresistive effects of silicon resistors. Place two pairs of resistors at the edges of the horizontal and vertical elastic diaphragms respectively, then the resistance values of the two groups of resistors will change in opposite directions. The initial resistance values of the four P-type piezoresistors 7 are equal, and the relationship between the voltage signal converted by the Wheatstone bridge and the resistance change of the piezoresistor 7 is expressed by the formula:

[0064]

[0065] where, U o is the output voltage, ΔR1 is the resistance change of the P-type piezoresistor R1 caused by the pressure in Figure 2 , ΔR2 is the resistance change of the P-type piezoresistor R2 caused by the pressure in Figure 2 , R is the initial resistance value, and U bias is the applied voltage. By optimizing the resistor layout and diaphragm design, the sensitivity and measurement range of the sensor are significantly improved.

[0066] To ensure that the high-temperature-resistant MEMS pressure sensor chip has a good linear output, the deformation of the elastic diaphragm should satisfy the small deflection theory, that is, the maximum deflection is less than 0.3 of the diaphragm thickness. When a square diaphragm with a thickness of about 60 μm is used, the simulation analysis shows that the maximum deflection is 1.52 μm, which is located at the center point of the elastic diaphragm and satisfies the small deflection theory. Since the factors affecting the maximum deflection at the center of the sensitive diaphragm are pressure, diaphragm side length, diaphragm thickness, Young's modulus and Poisson's ratio of the material, and the maximum deflection is proportional to the pressure P, therefore, the relationship between deflection and stress can be expressed as:

[0067]

[0068] where, P is the pressure applied to the high-temperature-resistant MEMS pressure sensor chip 10, in Psi; ω is the maximum deflection generated under the pressure P, in μm; E is the elastic modulus of the material generated under the pressure P, E = 163.27 GPa; μ is the Poisson's ratio generated under the pressure P, μ = 0.2223; a is the side length of the diaphragm generated under the pressure P, in μm; h is the thickness of the diaphragm generated under the pressure P, in μm.

[0069] See Figure 3 , Figure 3In a, it is a schematic diagram of temperature diffusion when a heat source of 100 °C is applied to the bottom of a SiO2 thin film with a thickness of 50 nm (corresponding to the traditional SiO2 thin film replaced by the present invention). In b, it is a schematic diagram of temperature diffusion when a heat source of 100 °C is applied to the bottom of an AlN thin film with a thickness of 50 nm (corresponding to the solution of Example 1 of the present invention, that is, depositing a layer of AlN thin film 2 on the upper surface of the SOI top silicon 3). In c, it is a result diagram of the temperature change line of the SiO2 thin film and the AlN thin film along the thickness direction.

[0070] As can be seen from Figure 3 a in, the AlN material area shows a gradually changing blue tone, with a uniform temperature distribution and a gentle gradient, indicating that heat can quickly spread from the heat source to the surroundings. Figure 3 b in shows that the SiO2 area shows high-temperature aggregation, with obvious local hot spots and a steep temperature gradient, indicating serious heat accumulation. Combining Figure 3 c in shows that the heat dissipation performance of AlN is better than that of SiO2 under the condition of the same thickness.

[0071] Therefore, through Figure 3 it can be known that the AlN thin film 2, as a heat dissipation layer and a protective layer, has a higher thermal conductivity compared with the traditional heat dissipation material AlN. Its thermal conductivity is generally 170 - 230 W / (m·K), which is about 14 times that of SiO2 under the same conditions. Then, based on the good heat transfer characteristics of AlN, the temperature will be quickly conducted to the silicon layer, while the thermal conductivity of SiO2 is very low, which is not conducive to the heat dissipation of the pressure sensor. Using the AlN layer 2 as the sensor heat dissipation layer, heat can be quickly conducted out from the heat source, thereby achieving efficient heat dissipation, avoiding excessive heat accumulation in the local area of the sensor resulting in too high temperature, and further affecting the performance and service life of the sensor, etc. In addition, the thermal expansion coefficient of the SiO2 material is much smaller than that of silicon, so the SiO2 layer will exacerbate the zero-point temperature drift phenomenon. Therefore, choosing the AlN layer 2 can well improve the temperature characteristics of the pressure sensor.

[0072] Figure 4 In a, it is a result diagram of the temperature change line of the SiO2 and AlN thin films with a thickness of 50 nm along the thickness direction, Figure 4 In b, it is a result diagram of the temperature change line of the SiO2 and AlN thin films with a thickness of 150 nm along the thickness direction, Figure 4 In c, it is a result diagram of the temperature change line of the SiO2 and AlN thin films with a thickness of 200 nm along the thickness direction. It can be seen from the three comparison diagrams that the temperature curve of SiO2 is steep, while the temperature curve of AlN is relatively gentle, indicating that the heat dissipation performance of AlN is better than that of SiO2 under different thickness conditions. At the same time, as the thickness of the AlN thin film increases, the gradient of its temperature curve gradually becomes larger, indicating the influence of thickness on the heat dissipation performance of the AlN material. The larger the thickness, the less conducive it is to its heat dissipation.

[0073] Example Two:

[0074] Refer to Figure 5 , the stainless-steel base 12 is made of 17-4PH alloy. There is a through ventilation hole at the center of the stainless-steel base 12, and the ventilation hole communicates with the back cavity side of the high-temperature resistant MEMS pressure sensor chip 10, that is, the back of the SOI elastic diaphragm. The high-temperature resistant MEMS pressure sensor chip 10 and the stainless-steel base 12 form a reliable bond through the eutectic layer 11; the high-temperature resistant MEMS pressure sensor chip 10 and the circuit board 9 form a reliable electrical connection through the metal solder 13 (flip-chip bonding process).

[0075] Compared with the traditional wire bonding method, it has disadvantages such as high transmission delay, susceptibility to electromagnetic interference, and limitation of the improvement of packaging density. The connection spacing between the flip-chip and the substrate is small, which can greatly reduce the length of the signal transmission path, reduce signal transmission delay and signal attenuation. Flip-chip bonding can achieve a more compact packaging structure. The high-temperature resistant MEMS pressure sensor chip 10 and the circuit board 9 are connected through the metal solder 13. Pin terminals 8 are respectively provided at both ends of the circuit board 9. The metal solder 13 has good electrical conductivity and thermal conductivity, which can ensure stable electrical performance and heat dissipation efficiency between the circuit board 9 and the high-temperature resistant MEMS pressure sensor chip 10. In a high-temperature working environment, the physical properties of the metal solder 13 will not change significantly, ensuring the reliability of the entire sensor under extreme conditions. In addition, by precisely controlling its composition and process, the metal solder 13 can optimize its mechanical strength and fatigue resistance, thereby extending the service life of the sensor.

[0076] Refer to Figure 6 , which gives the relationship curve between the differential output voltage signal of the sensor and the input pressure and the schematic diagram of temperature drift under different power supply methods. Further analysis Figure 6 of the curve can find that the differential output voltage of the sensor is proportional to the pressure. As the input pressure increases, the growth trend of the output voltage signal is linear and stable, indicating that the sensor has high measurement accuracy and repeatability. The data of temperature drift reveals that under the constant voltage source power supply, the output voltage signal of the sensor is more sensitive to temperature, and there is a large sensitivity temperature drift at 150 °C. The relationship expression between the output voltage and temperature is:

[0077]

[0078] where ΔR t is the resistance change of the piezoresistor 7 caused by temperature, Uo is the output voltage, Ubias is the applied voltage; under the constant current source power supply, the output fluctuation of the sensor under different temperature conditions is extremely small, and the zero-point temperature drift and sensitivity temperature drift are almost zero, having good temperature self-compensation function. The relationship expression between its output voltage and temperature is: U o= I in ΔR, where I in is the current of the constant current source power supply.

[0079] When the oil pressure or hydraulic system to be tested is working, the oil pressure or hydraulic pressure is transmitted to the through ventilation holes of the stainless steel base through the connection interface. The ventilation holes directly transmit the pressure to the back surface of the elastic diaphragm, causing the elastic diaphragm to deform. The deformation of the elastic diaphragm causes the piezoresistor to be stressed, thereby changing its resistance value. The Wheatstone bridge outputs a corresponding voltage signal according to the change in the resistance value of the piezoresistor. Through a dedicated signal acquisition device, the voltage signal output by the Wheatstone bridge is collected in real time. The collected voltage signal is processed such as amplified and filtered, and then input into the data analysis system. The data analysis system analyzes and calculates the collected voltage signal according to the preset algorithm and calibration parameters, and finally obtains the magnitude and change of the oil pressure or hydraulic pressure to be tested, and displays the result on the corresponding display device.

[0080] In summary, a high-temperature-resistant MEMS pressure sensor and its packaging structure proposed by the present invention have materials with high thermal conductivity and low thermal expansion coefficient, which can reduce the influence of temperature fluctuations on the performance of the sensor. In addition, a temperature compensation mechanism is considered in the design. By optimizing the design of the internal circuit of the pressure sensor chip and combining the constant current source power supply technology, the measurement error caused by temperature changes is significantly reduced. Through these technical means, high-precision and stable measurement of the sensor under different ambient temperatures is ensured, meeting the strict requirements in industrial applications.

[0081] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the present invention and its equivalent technologies, the present invention also intends to include these changes and modifications. The above-described embodiments are merely preferred embodiments given to fully illustrate the present invention, and the scope of protection is not limited thereto. Equivalent substitutions or transformations made by those skilled in the art on the basis of the present invention are all within the scope of protection of the present invention.

Claims

1. A high temperature resistant MEMS pressure sensor chip, characterized in that: An SOI substrate, an AlN layer (2) and a SiO2 layer (1) are provided from bottom to top; a recessed area is provided at the center of the bottom layer of the SOI substrate, and a chip area corresponding to the recessed area is an elastic membrane; A plurality of P-type varistors (7) are arranged on the uppermost layer of the SOI substrate, and the plurality of P-type varistors (7) are evenly distributed at the stress concentration point at the edge of the elastic diaphragm; a channel is arranged above the P-type varistors (7) and continuously penetrates the AlN layer (2) and the SiO2 layer (1); a metal pad (6) is arranged in the channel; the upper end of the metal pad (6) is located at the uppermost layer of the high-temperature resistant MEMS pressure sensor chip, and the lower end is in contact with the P-type varistors (7).

2. The high temperature resistant MEMS pressure sensor chip according to claim 1, characterized in that: The SOI substrate comprises, from top to bottom, an SOI top silicon layer (3), an SOI buried oxide layer (4) and an SOI substrate silicon (5); a plurality of P-type varistors (7) are symmetrically distributed in the SOI top silicon layer and are electrically connected to form a Wheatstone bridge.

3. The high temperature resistant MEMS pressure sensor chip according to claim 2, characterized in that: The varistor crystal phase of each of the P-type varistors (7) on the SOI top silicon layer (3) is the same.

4. The high temperature resistant MEMS pressure sensor chip according to claim 2, characterized in that: The P-type varistor (7) is a zigzag varistor strip, which is manufactured by implanting B ions on the top silicon layer (3) of the SOI.

5. The high temperature resistant MEMS pressure sensor chip according to claim 4, characterized in that: The concentration of B ions is 1*10 19 ~1*10 20 cm -3 .

6. The high temperature resistant MEMS pressure sensor chip according to claim 2, characterized in that: The thickness of the SOI buried oxide layer (4) is 1-3 μm.

7. The high temperature resistant MEMS pressure sensor chip according to claim 2, characterized in that: The SOI substrate silicon (5) is a silicon cup structure.

8. The high temperature resistant MEMS pressure sensor chip according to claim 1, characterized in that: The thickness of the AlN layer (2) is 50-200 nm.

9. The high temperature resistant MEMS pressure sensor chip according to claim 1, characterized in that: The thickness of the SiO2 layer (1) is 50 to 100 nm.

10. A high temperature resistant MEMS pressure sensor packaging structure, characterized in that: It comprises a circuit board (9), a high temperature resistant MEMS pressure sensor chip (10) as claimed in any one of claims 1 to 9, and a stainless steel base (12); the circuit board (9) is connected to the metal pad (6) via a metal solder layer (13), and the SOI substrate silicon (5) is connected to the stainless steel base (12) via a eutectic layer (11); a through vent hole is provided at the center of the stainless steel base (12), which directly leads to the bottom surface of the SOI substrate in the high temperature resistant MEMS pressure sensor chip (10).

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