Anti-electromagnetic interference MEMS piezoresistive differential pressure sensor and manufacturing method thereof

Abstract

The invention discloses an anti-electromagnetic interference MEMS piezoresistive differential pressure sensor and a manufacturing method thereof, the sensor is based on a piezoresistive effect, a piezoresistor is manufactured on a sensitive film to form a Wheatstone bridge, and two sides of the sensitive film are respectively connected with two pressure inlets to realize differential pressure measurement. According to the sensor, two SOI silicon wafers are adopted as a first substrate and a second substrate, and the differential pressure sensor is formed through a micromachining and bonding technology; the piezoresistor is electrically connected with the outside through a silicon through hole so as to reduce electromagnetic coupling in a signal transmission process; the surface of the sensor is covered with a passivation layer, electrical isolation inside a chip is achieved through a buried oxide layer in an SOI silicon wafer, and leakage current and medium interference are effectively reduced. Shielding layers cover the upper and lower surfaces of the sensor and are connected to the same potential to form a Faraday cage structure to realize electromagnetic shielding. The sensor has the characteristics of simple structure, low cost, quick response and the like, and the reliability of the sensor in a strong electromagnetic environment is remarkably improved.

Description

Technical Field

[0001] This invention relates to a pressure sensor, specifically a MEMS piezoresistive differential pressure sensor. Background Technology

[0002] MEMS sensors offer advantages such as small size, low power consumption, high reliability, suitability for mass production, and ease of integration and intelligent implementation. Pressure sensors, as one of the largest market segments within MEMS sensors, are widely used in automotive, consumer electronics, industrial, medical, and aerospace fields. Currently, MEMS pressure sensors mainly include piezoresistive, capacitive, resonant, and piezoelectric types. Capacitive pressure sensors offer advantages such as high sensitivity, low power consumption, and low temperature drift, but their signal conversion is complex, leading to higher design difficulty and cost. Resonant pressure sensors offer high accuracy and strong anti-interference capabilities, but their complex structure and manufacturing difficulty are significant. Piezoelectric pressure sensors have extremely high requirements for manufacturing processes and are limited by piezoelectric materials. In contrast, piezoresistive pressure sensors, with their simple core sensing structure (based on the piezoresistive effect of silicon), mature processing technology, strong compatibility with CMOS processes, low cost, and ease of signal processing circuit integration, have become the mainstream technology in the current MEMS pressure sensor market.

[0003] However, the core sensing element of traditional MEMS piezoresistive pressure sensors is a semiconductor piezoresistor (usually fabricated on a single-crystal silicon substrate using ion implantation), whose conductivity is susceptible to external electromagnetic interference. In strong electromagnetic environments, electromagnetic radiation can induce parasitic currents in the piezoresistor, altering carrier mobility and causing abnormal, non-pressure-related fluctuations in its resistance. This leads to sensor output signal distortion, decreased measurement accuracy, and in severe cases, even complete sensor failure. This technological bottleneck is particularly prominent in complex electromagnetic environments such as industrial automation (e.g., pressure detection near frequency converters and motors), automotive electronics (e.g., pressure monitoring near high-voltage wiring harnesses in engine compartments), and aerospace (e.g., pressure detection in densely populated areas of airborne electronic equipment), and has become a core technical challenge restricting the application of piezoresistive pressure sensors in scenarios with high electromagnetic compatibility requirements.

[0004] Existing technologies often employ external metal shielding or the addition of filtering circuits in the signal link to suppress electromagnetic interference. The former blocks electromagnetic radiation through physical isolation, but this increases the overall size of the sensor, the complexity of the assembly process, and also increases the weight and cost of the device. The latter suppresses the transmission of electromagnetic interference signals through filtering circuits, but introduces additional circuit losses, affecting the sensor's response speed. Furthermore, the design of the filtering circuit needs to be precisely matched with the sensor's signal characteristics, increasing the difficulty of system integration, and it cannot fundamentally solve the direct interference of electromagnetic radiation on the varistor. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the shortcomings of existing pressure sensors that cannot work properly in strong electromagnetic interference environments, a MEMS piezoresistive differential pressure sensor and its fabrication method that is resistant to electromagnetic interference are proposed. This sensor can not only effectively isolate electromagnetic interference in specific frequency bands, but also achieve accurate differential pressure measurement.

[0006] Technical solution: A MEMS piezoresistive differential pressure sensor resistant to electromagnetic interference, comprising: Both the first substrate and the second substrate are SOI silicon wafers; the front sides of the first substrate and the second substrate are arranged facing each other; A back cavity is disposed on the back side of the first substrate, which serves as a pressure inlet; the top silicon layer above the buried oxide layer of the first substrate serves as the sensitive thin film of the sensor; a piezoresistor is fabricated on the sensitive thin film and connected by metal leads to form a Wheatstone bridge. A cavity is provided on the front side of the second substrate, and a vent is connected to the cavity and extends to the back side of the substrate, the vent serving as another pressure inlet; A first through-silicon via penetrates the first substrate and is used to bring the electrical connection terminal of the varistor to the back side of the first substrate. An electromagnetic shielding layer is applied to the bottom surface of the first substrate and the second substrate, and both electromagnetic shielding layers are connected to the same potential to form a Faraday cage structure. The second through-silicon via penetrates the first and second substrates and is used to electrically connect the two electromagnetic shielding layers. Both the front sides of the first substrate and the second substrate are covered with a passivation protective layer. The front sides of the first and second substrates are connected by a metal bonding process to form the differential pressure sensor with an integrated structure.

[0007] Furthermore, varistors are formed by ion implantation in the stress concentration areas on the four sides of the square sensitive film.

[0008] Furthermore, the back cavity is etched using the buried oxide layer of the first substrate as a stop layer.

[0009] Furthermore, the first and second through-silicon vias are formed by depositing an insulating layer on the inner wall of the through-silicon via and then filling the interior with conductive metal.

[0010] Furthermore, the electromagnetic shielding layer is a copper layer.

[0011] Furthermore, the metal bonding process employs hot-press bonding or eutectic bonding.

[0012] Furthermore, in the metal bonding structure, the top surface of the second through-silicon via in the first substrate is connected to a metal bump, and the top surface of the second through-silicon via in the second substrate is provided with a second contact hole. During bonding, the bump and the opposite second contact hole are interconnected.

[0013] Furthermore, a heavily doped region is provided on the surface of the second substrate, and the heavily doped region is electrically connected to the second contact hole.

[0014] The method for fabricating the MEMS piezoresistive differential pressure sensor includes: Step 1: Microfabricate the second substrate, including thermal growth of SiO2, photolithography of heavily doped regions, ion implantation, etching of through-silicon vias, copper electroplating, deposition of passivation protective layer, photolithography of metal interconnects, back-side grinding and thinning, and etching of cavities; Step 2: Perform microfabrication on the first substrate, including thermal growth of SiO2, photolithography of heavily doped regions and piezoresistive regions, ion implantation, etching of through-silicon vias, copper electroplating, deposition of passivation protective layer, photolithography of metal interconnects, backside grinding and thinning, and formation of bumps; Step 3: Align the first substrate and the second substrate and perform metal bonding; Step 4: Deposit a copper layer on the outer surface of the bonded assembly as an electromagnetic shielding layer; Step 5: Photolithography and etching of the pores on the back side of the second substrate; Step 6: After electroplating copper in the through silicon via, a chemical mechanical polishing process is used to planarize the upper surface.

[0015] Beneficial effects: (1) This invention innovatively combines the classic piezoresistive sensing principle with an integrated electromagnetic shielding design. While retaining the traditional advantages of piezoresistive sensors such as simple structure, stable output signal, and low cost, it effectively solves the problems of traditional piezoresistive sensors being susceptible to signal interference and decreased measurement accuracy in complex electromagnetic environments by optimizing the transmission path of the electrical signal generated by the sensor's sensitive unit and the electromagnetic shielding structure of the sensor, thus significantly improving measurement reliability.

[0016] (2) The differential pressure detection mechanism based on the piezoresistive effect of this invention directly outputs an electrical signal without the need for complex signal conversion. It has a fast response speed and good linearity. This invention adopts an integrated design of differential pressure chip, electrical isolation structure and electromagnetic shielding structure, which can be directly and safely applied to differential pressure measurement scenarios of two different pressure sources, such as fluid control, wind speed and wind pressure detection, filter monitoring, etc.

[0017] (3) This invention enhances the electromagnetic interference resistance and integrates differential pressure measurement function, which can be accurately adapted to fields such as industrial automation, automotive electronic systems, and aerospace equipment electronic systems where there is strong electromagnetic interference and high-precision differential pressure measurement is required. It fills the application gap of existing piezoresistive sensors in the field of differential pressure measurement with high electromagnetic interference and has broad market prospects. Attached Figure Description

[0018] Figure 1 This is a top view (excluding the outer copper layer) of a MEMS piezoresistive differential pressure sensor that is resistant to electromagnetic interference according to the present invention. Figure 2 This is a cross-sectional view along line A-A' of a MEMS piezoresistive differential pressure sensor that is resistant to electromagnetic interference according to the present invention. Figure 3 This is a B-B' cross-sectional view of a MEMS piezoresistive differential pressure sensor that is resistant to electromagnetic interference according to the present invention. Detailed Implementation

[0019] The invention will now be further explained with reference to the accompanying drawings.

[0020] like Figures 1 to 3 As shown, a MEMS piezoresistive differential pressure sensor resistant to electromagnetic interference uses two SOI silicon wafers as a first substrate 1 and a second substrate 2, with the front sides of the first substrate 1 and the second substrate 2 facing each other. The back side of the first substrate 1 has a back cavity 11; the front side of the second substrate 2 has a cavity 25, and the second substrate 2 also has vents 21 that are connected to the cavity 25 and extend to the back side of the substrate.

[0021] In this design, the back cavity 11 is etched using the buried oxide layer 12 of the first substrate 1 as a stop layer. The top silicon layer above the buried oxide layer 12 of the first substrate is the sensitive thin film of the sensor. Varistors 14 are formed by ion implantation in the stress concentration areas on the four sides of the square sensitive thin film, and are connected by metal leads 16 to form a Wheatstone bridge 7. A first silicon via 6 penetrates the first substrate 1 and connects to the metal leads 16, used to lead the electrical connection terminals of the varistor 14 to the back surface of the first substrate 1, i.e., the top surface of the complete structure after bonding.

[0022] The first substrate 1 also has a SiO2 layer 15 and a passivation protection layer 18 sequentially disposed on the front side; the second substrate 2 also has a SiO2 layer 24 and a passivation protection layer 27 sequentially disposed on the front side. Combined with the electrical isolation effect of the buried oxide layer in the SOI silicon wafer, this design achieves physical isolation between high and low voltage cavities while ensuring electrical insulation, which can effectively reduce the leakage current inside the sensor and prevent the conductive medium from causing signal short circuits or interference.

[0023] Electromagnetic shielding layer 4 covers the bottom surface of the first substrate 1 and the second substrate 2, that is, it covers the outer surface of the complete structure after bonding. The electromagnetic shielding layer 4 is connected to the same potential, forming a Faraday cage structure. The second silicon via 5 penetrates the first substrate 1 and the second substrate 2, and the upper and lower electromagnetic shielding layers 4, the second silicon via 5, and the bonding point achieve electrical connection.

[0024] Specifically, the first substrate 1 and the second substrate 2 are bonded together at their mid-face to form an integrated structure using a metal bonding process, which may employ hot-press bonding or eutectic bonding. In the metal bonding structure, the top surface of the second through-silicon via 5 of the first substrate 1 is connected to a metal bump 19, and the top surface of the second through-silicon via 5 of the second substrate 2 is provided with a second contact hole 26. During substrate bonding, the bump 19 and the corresponding second contact hole 26 are interconnected.

[0025] In the above structure, the sensitive film is used to sense differential pressure. The sensor is designed with dual pressure inlets, each connected to one of the two pressure sources to be measured: the lower pressure is transmitted through the vent 21 to the cavity 25 on the front side of the second substrate, and the upper pressure acts on the back cavity 11 of the first substrate. The two pressures act on both sides of the sensitive film, and the degree of deformation of the sensitive film is determined by the pressure difference between the two sides. When there is a differential pressure on both sides of the sensitive film, the sensitive film deforms, causing a change in the resistance value of the piezoresistor 14. The Wheatstone bridge 7 becomes unbalanced, outputting a differential voltage signal proportional to the differential pressure. The signal is amplified, linearized, and temperature compensated by external circuitry, ultimately outputting a stable differential pressure signal.

[0026] The upper and lower electromagnetic shielding layers 4 covering the outer surface of the sensor are electrically connected to maintain the same potential, forming a closed conductive shielding layer that blocks the intrusion of external electromagnetic fields. The area covered by the electromagnetic shielding layer 4 does not include the bottom area of ​​the first through-silicon via 6, that is, it does not include the pad area at the sensor signal output terminal.

[0027] A heavily doped region 23 is also provided on the surface of the second substrate 2, which is electrically connected to the second contact hole 26. Under the operating voltage, the Wheatstone bridge 7 on the sensitive film in the first substrate 1 is connected to the external circuit through the through-silicon via 6. When the sensor is working normally, the upper and lower device layers inside the sensor chip are simultaneously conductive, which can further enhance the electromagnetic shielding effect.

[0028] like Figure 3 As shown, the varistor 14 is directly connected to the external circuit via the first through-silicon via 6 at a short distance to amplify and process the weak bridge output signal and reduce electromagnetic interference coupling on the transmission path.

[0029] A method for fabricating a MEMS piezoresistive pressure sensor resistant to electromagnetic interference, the specific steps of which are as follows: 1. The processing flow for the second substrate 2 is as follows: (1) A 4-inch or 6-inch N-type SOI silicon wafer is selected as the second substrate 2, which includes a device layer, a buried oxide layer 22, and a substrate layer.

[0030] (2) A SiO2 layer is thermally grown on the front side of the second substrate 2 at 1050°C.

[0031] (3) Coat the SiO2 layer with photoresist, use ultraviolet light to define the pattern of the heavily doped region 23 of the second substrate, and etch the SiO2 of the heavily doped region with BOE solution.

[0032] (4) Boron ions are used for ion implantation to form a second substrate heavily doped region 23.

[0033] (5) Remove the surface SiO2 layer and re-grow a layer of SiO2 using dry oxygen thermal growth as the SiO2 layer 24 on the surface of the second substrate.

[0034] (6) Photolithography defines the TSV pattern, and the second silicon via 5 is etched using deep reactive ion etching (DRIE) process. During the etching process, parylene is deposited simultaneously as an insulating layer 3 on the inner wall of the silicon via.

[0035] (7) Electroplated copper fills the second silicon through hole 5, and then chemical mechanical polishing (CMP) is performed on the upper surface of the second silicon through hole 5 to make the surface flat.

[0036] (8) On the front side of the second substrate 2, 200~300nm silicon nitride is deposited using plasma enhanced chemical vapor deposition (PECVD) as a passivation protective layer 27 on the surface of the second substrate.

[0037] (9) Photolithography and etching of the contact hole 26 of the second substrate on the top of the second through-silicon via 5, and the same process is used to make the contact hole at the position opposite the heavily doped region 23 of the second substrate. Then, a 1~1.5 μm thick aluminum layer is sputtered on the two contact holes and the passivation protection layer 27 between the two contact holes and patterned to form interconnect metal.

[0038] (10) The back side of the second substrate 2 is thinned by grinding until the copper at the bottom of the second through-silicon via 5 is exposed.

[0039] (11) Photolithography and DRIE etching of the front side of the second substrate 2 to form cavity 25.

[0040] 2. The processing flow for the first substrate 1 is as follows: (1) The SOI silicon wafer with the same specifications as the second substrate 2 is selected as the first substrate 1, which also includes a device layer, a buried oxide layer 12, and a substrate layer.

[0041] (2) A SiO2 layer is thermally grown on the front side of the first substrate 1, photoresist is coated on the SiO2 surface, and boron ions are photolithographically etched and implanted to form a heavily doped region 13 of the first substrate.

[0042] (3) Remove the photoresist, recoat the photoresist in the heavily doped region 13 of the first substrate and photolithographically define the varistor region, etch the SiO2 of the varistor region with BOE solution, implant boron ions to form a P-type varistor 14 with a sheet resistance of 300~500Ω / sq.

[0043] (4) Remove the original SiO2 on the surface and re-grow a layer of SiO2 using dry oxygen thermal growth to serve as the SiO2 layer 15 on the surface of the first substrate.

[0044] (5) The TSV pattern is defined by photolithography. The first silicon via 6 is etched by deep reactive ion etching (DRIE) process. During the etching process, parylene is deposited simultaneously as an insulating layer 3 on the inner wall of the silicon via.

[0045] (6) Electroplated copper fills the first silicon through-hole 6, and then chemical mechanical polishing (CMP) is performed on the upper surface of the first silicon through-hole 6 to make the surface flat.

[0046] (7) On the front side of the first substrate 1, 200~300 nm silicon nitride is deposited using PECVD process as a passivation protective layer 18 on the surface of the first substrate.

[0047] (8) Photolithography and etching of the first contact hole 17, the first contact hole 17 penetrates the passivation protection layer 18 and the SiO2 layer 15 on the surface of the first substrate, and connects to the heavily doped region 13 of the first substrate.

[0048] (9) Photolithography is used to etch the metal lead area, and then metal Al is sputtered and patterned on the surface to form metal lead 16. The metal lead 16 connects the varistor 14 to form a Wheatstone bridge 7 through the first contact hole 17.

[0049] (10) The back side of the first substrate 1 is thinned by grinding until the copper at the bottom of the first through-silicon via 5 is exposed.

[0050] (11) Prepare metal bumps 19 on the upper surface of the first silicon via 5 region.

[0051] (12) Flip the first substrate 1, align the bump 19 with the contact hole 26 of the second substrate on the front side of the second substrate 2, and hot press or eutectic bond to form a solid electrical and mechanical connection.

[0052] (13) Photolithography and DRIE etching of the back cavity 11 of the first substrate 1 are performed from the back side of the first substrate 1 to release the sensitive film.

[0053] 3. Post-processing and shielding layer formation: (1) A copper layer is deposited on the outer surface of the bonded chip using a sputtering process as an electromagnetic shielding layer 4. This ensures that the copper layers on the upper and lower surfaces are electrically connected through the bonding bumps 19 to form an equipotential Faraday cage.

[0054] (2) Photolithography and etching of the pores 21 on the back side of the second substrate 2 as the lower pressure inlet.

[0055] (3) Double-sided film protection, and dicing to obtain independent sensor chips.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A MEMS piezoresistive differential pressure sensor resistant to electromagnetic interference, characterized in that, include: The first substrate (1) and the second substrate (2) are both SOI silicon wafers; The first substrate (1) and the second substrate (2) are arranged with their front surfaces facing each other; A back cavity (11) is disposed on the back side of the first substrate (1), the back cavity serving as a pressure inlet; the top silicon layer above the buried oxide layer (12) of the first substrate (1) serves as the sensitive thin film of the sensor; a piezoresistive resistor (14) is fabricated on the sensitive thin film and connected by metal leads (16) to form a Wheatstone bridge (7). A cavity (25) is provided on the front side of the second substrate (2), and a vent (21) is connected to the cavity (25) and extends to the back side of the substrate, the vent (21) serving as another pressure inlet; A first through-silicon via (6) penetrates the first substrate (1) and is used to bring the electrical connection terminal of the varistor (14) to the back side of the first substrate (1); An electromagnetic shielding layer (4) is applied to the bottom surface of the first substrate (1) and the second substrate (2). The electromagnetic shielding layer (4) is connected to the same potential to form a Faraday cage structure. The second through-silicon via (5) penetrates the first substrate (1) and the second substrate (2) to electrically connect the two electromagnetic shielding layers (4); The front sides of the first substrate (1) and the second substrate (2) are also covered with a passivation protective layer; The front sides of the first substrate (1) and the second substrate (2) are connected by a metal bonding process to form the differential pressure sensor with an integrated structure.

2. The MEMS piezoresistive pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, The varistor (14) is formed by ion implantation in the stress concentration areas on the four sides of the square sensitive film.

3. The MEMS piezoresistive differential pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, The back cavity (11) is etched with the buried oxide layer (12) of the first substrate (1) as the stop layer.

4. The MEMS piezoresistive differential pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, The first through-silicon via (6) and the second through-silicon via (5) are formed by depositing an insulating layer (3) on the inner wall of the through-silicon via and then filling it with conductive metal.

5. A MEMS piezoresistive differential pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, The electromagnetic shielding layer (4) is a copper layer.

6. The MEMS piezoresistive differential pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, The metal bonding process employs either hot-press bonding or eutectic bonding.

7. The MEMS piezoresistive differential pressure sensor with electromagnetic interference resistance according to claim 1, characterized in that, In the metal bonding structure, the top surface of the second through-silicon via (5) in the first substrate (1) is connected to a metal bump (19), and the top surface of the second through-silicon via (5) in the second substrate (2) is provided with a second contact hole (26). During bonding, the bump (19) and the opposite second contact hole (26) are connected to each other.

8. A MEMS piezoresistive pressure sensor with electromagnetic interference resistance according to claim 7, characterized in that, A heavily doped region (23) is also provided on the surface of the second substrate (2), and the heavily doped region (23) is electrically connected to the second contact hole (26).

9. The method for fabricating a MEMS piezoresistive differential pressure sensor according to any one of claims 1-8, characterized in that, include: Step 1: Microfabricate the second substrate (2), including thermal growth of SiO2, photolithography of heavily doped regions, ion implantation, etching of silicon vias, electroplating of copper, deposition of passivation protective layer, photolithography of metal interconnects, back-side grinding and thinning, and etching of cavities; Step 2: Microfabricate the first substrate (1), including thermal growth of SiO2, photolithography of heavily doped regions and piezoresistive regions, ion implantation, etching of silicon vias, electroplating of copper, deposition of passivation protective layer, photolithography of metal interconnects, back-side grinding and thinning, and formation of bumps; Step 3: Align the first substrate (1) and the second substrate (2) and perform metal bonding; Step 4: Deposit a copper layer on the outer surface of the bonded whole as an electromagnetic shielding layer (4); Step 5: Photolithography and etching of the pores on the back side of the second substrate (21); Step 6: After electroplating copper in the through silicon via, a chemical mechanical polishing process is used to planarize the upper surface.

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