Membrane-based pressure sensor and method of manufacturing the same
By introducing a new operating mode and structural optimization of the inverse piezoelectric unit into the MEMS piezoresistive pressure sensor, the nonlinear error, linearity and range constraints, and temperature drift problems of traditional sensors are solved, achieving higher measurement accuracy and sensitivity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NANJING GAOHUA TECH
- Filing Date
- 2023-08-07
- Publication Date
- 2026-07-31
AI Technical Summary
Traditional MEMS piezoresistive pressure sensors suffer from nonlinear errors, linearity and range constraints, temperature drift, and the difference in thermal expansion coefficients between silicon and glass substrates, which leads to bonding thermal stress and reduces measurement accuracy.
By employing a novel operating mode that includes first and second inverse piezoelectric units, the pressure-sensitive film does not require deformation. By combining the inverse piezoelectric effect and the piezoelectric effect, the substrate structure is optimized to reduce thermal stress and improve linearity and overload resistance.
The linearity and range of the sensor were improved, the overload resistance was enhanced, the temperature drift was reduced, and the measurement accuracy and sensitivity were improved.
Smart Images
Figure CN117007220B_ABST
Abstract
Description
Technical Field
[0001] The embodiments disclosed herein belong to the field of pressure sensor technology, specifically relating to a MEMS piezoresistive pressure sensor and its fabrication method. Background Technology
[0002] MEMS piezoresistive pressure sensors are widely used in the field of pressure measurement due to their many advantages, such as high sensitivity, good frequency response, high accuracy, good stability, small size and easy integration.
[0003] The fabrication of traditional MEMS piezoresistive pressure sensors first involves doping a piezoresistive strip on the surface of a silicon substrate, then etching a cavity on the back side of the piezoresistive strip to form a pressure-sensitive thin film in the square area where the piezoresistive strip is located, and finally bonding the unetched area on the back side of the piezoresistive strip to a glass substrate.
[0004] For the traditional MEMS piezoresistive pressure sensor structure described above, the deflection effect of its pressure-sensitive film causes a non-linear relationship between the sensor's output voltage and input pressure. This results in a non-linear error that increases with increasing input pressure, leading to a trade-off between sensor linearity and measurement range, thus limiting the overall improvement of sensor performance. Furthermore, the difference in thermal expansion coefficients between the silicon and glass substrates in traditional MEMS piezoresistive pressure sensors causes bonding thermal stress at the interface connecting the two substrates during normal sensor operation. This results in temperature drift and reduced measurement accuracy. Summary of the Invention
[0005] The embodiments disclosed herein aim to at least solve one of the technical problems existing in the prior art, and disclose a MEMS piezoresistive pressure sensor and its fabrication method.
[0006] In a first aspect, embodiments of this disclosure provide a MEMS piezoresistive pressure sensor, the MEMS piezoresistive pressure sensor comprising a first substrate, a second substrate, a first passivation layer, a second passivation layer, a first inverse piezoelectric unit, a piezoresistive unit, and a first lead and pad;
[0007] The first substrate has a cavity extending through its thickness and is fixed to the second substrate;
[0008] The first passivation layer is disposed on the side of the first substrate away from the second substrate, and the first passivation layer contains the piezoresistive unit corresponding to the cavity;
[0009] The first inverse piezoelectric unit is disposed on the side of the first passivation layer facing away from the second substrate; wherein, the first inverse piezoelectric unit, the first passivation layer, and the piezoresistive unit form a pressure-sensitive film in the region corresponding to the cavity;
[0010] The second passivation layer is disposed on the side of the first inverse piezoelectric unit away from the first substrate. The first lead and the pad are disposed on the side of the second passivation layer away from the second substrate. The first lead and the pad are electrically connected to the piezoresistive unit and the first inverse piezoelectric unit, respectively.
[0011] In some embodiments, the first reverse piezoelectric unit includes a first lower electrode plate, a first reverse piezoelectric layer, and a first upper electrode plate that are sequentially stacked on the first passivation layer.
[0012] In some embodiments, the MEMS piezoresistive pressure sensor further includes a second inverse piezoelectric unit and a second lead and a pad;
[0013] The second inverse piezoelectric unit is disposed on the side of the second substrate facing the first substrate and corresponds to the cavity;
[0014] The second lead and pad are disposed on the side of the second substrate away from the first substrate and are electrically connected to the second inverse piezoelectric unit.
[0015] In some embodiments, the second inverse piezoelectric unit includes a second lower electrode plate, a second inverse piezoelectric layer, and a second upper electrode plate sequentially stacked on the second substrate.
[0016] In some embodiments, the second substrate has a groove on the side facing the first substrate, and the groove accommodates the second inverse piezoelectric unit.
[0017] In some embodiments, a gap exists between the second inverse piezoelectric unit and the groove, and the MEMS piezoresistive pressure sensor further includes a filling layer filling the gap.
[0018] In some embodiments, the piezoresistive unit includes an N-type piezoresistive unit and a P-type piezoresistive unit;
[0019] The N-type piezoresistive unit includes an N-type piezoresistive strip, N-type electrode lead-out areas located at both ends of the N-type piezoresistive strip, and a P-type area disposed on the side of the N-type piezoresistive strip facing the second substrate;
[0020] The P-type piezoresistive unit includes a P-type piezoresistive strip, P-type electrode lead-out areas located at both ends of the P-type piezoresistive strip, and an N-type area disposed on the side of the P-type piezoresistive strip facing the second substrate.
[0021] Secondly, embodiments of this disclosure provide a method for fabricating a MEMS piezoresistive pressure sensor as described above, the method comprising:
[0022] An SOI wafer is provided; wherein the SOI wafer comprises a first substrate, an oxide layer and a device layer stacked sequentially.
[0023] The device layer of the SOI wafer is doped to form piezoresistive cells;
[0024] The device layer region outside the piezoresistive unit is oxidized to obtain a first passivation layer;
[0025] Forming a first inverse piezoelectric unit on the first passivation layer specifically involves: forming a first lower electrode on the first passivation layer; forming a first inverse piezoelectric layer on the first lower electrode; and forming a first upper electrode on the first inverse piezoelectric layer.
[0026] A second passivation layer is formed on the first inverse piezoelectric unit;
[0027] The second passivation layer is patterned to form the first via;
[0028] A second through hole is formed at the position of the first through hole corresponding to the first inverse piezoelectric unit;
[0029] A first lead and a pad are formed on the second passivation layer, and a metal layer is filled in the first through hole and the second through hole, so that the first lead and the pad are electrically connected to the first inverse piezoelectric unit and the piezoresistive unit, respectively.
[0030] A cavity is formed on the back side of the first substrate to form a pressure-sensitive thin film;
[0031] Provide a second substrate;
[0032] The second substrate is bonded to the first substrate to prepare a MEMS piezoresistive pressure sensor.
[0033] In some embodiments, before bonding the second substrate to the first substrate, the method further includes: forming a second inverse piezoelectric unit on the second substrate, specifically:
[0034] A deep through-hole is formed on the first surface of the second substrate;
[0035] A groove is formed on the second surface of the second substrate;
[0036] A second lower electrode plate is formed within the groove;
[0037] A second reverse piezoelectric layer is formed on the second lower electrode plate;
[0038] A filling layer is formed in the groove, and the filling layer is patterned to form through holes;
[0039] A second upper electrode plate and a lead wire in the through hole of the filling layer are formed on the surface of the groove.
[0040] Etch the deep via to expose the lead wire in the via of the filling layer;
[0041] A second lead and a pad are formed on the first surface of the second substrate, such that the second lead and the pad are electrically connected to the second inverse piezoelectric layer.
[0042] In some embodiments, doping the device layer of the SOI wafer to form piezoresistive cells includes:
[0043] The device layer of the SOI wafer is subjected to N-type doping and P-type doping to form N-type and P-type regions, respectively;
[0044] The surface regions of the N-type region and the P-type region are doped with P-type and N-type doping respectively to form P-type varistor strips and N-type varistor strips;
[0045] The two ends of the P-type varistor strip and the N-type varistor strip are heavily doped with P-type and N-type respectively to form P-type electrode lead-out regions and N-type electrode lead-out regions.
[0046] Compared with conventional MEMS piezoresistive pressure sensors, the MEMS piezoresistive pressure sensor of this disclosure has the following advantages:
[0047] 1. The MEMS piezoresistive pressure sensor of this disclosure adopts a new operating mode based on the first inverse piezoelectric unit. In this mode, when the sensor measures external pressure, its pressure-sensitive film does not need to deform, thereby eliminating the deflection effect of the pressure-sensitive film. The linearity and measurement range of the sensor are greatly improved, and the two do not restrict each other.
[0048] 2. When the external pressure input to the MEMS piezoresistive pressure sensor of this embodiment exceeds the measurement range, the pressure-sensitive film of the sensor can use its entire deformation range to accommodate the overload pressure. This greatly improves the overload resistance of the sensor compared to traditional MEMS piezoresistive pressure sensors, which can only use a small portion of the deformation range of the pressure-sensitive film to accommodate the overload pressure.
[0049] 3. In the MEMS piezoresistive pressure sensor of this embodiment, the resistance change of the piezoresistive strip comes not only from the piezoresistive effect but also from the piezoelectric effect of the first inverse piezoelectric layer region without electrodes. This effectively improves the sensitivity of the piezoresistive strip, enhances the negative feedback effect in the aforementioned new operating mode, improves the sensor's sensitivity, and further increases the sensor's linearity and range.
[0050] 4. In the structure of the MEMS piezoresistive pressure sensor of this embodiment, a second inverse piezoelectric unit is provided in the central square groove of the second substrate. When a suitable temperature-controlled voltage is applied to the upper and lower plates of the second inverse piezoelectric unit, an inverse piezoelectric effect is generated, thereby causing the structure to undergo temperature-controlled horizontal contraction, and driving the second substrate to also undergo temperature-controlled horizontal contraction, so as to reduce the difference in the degree of contraction between the two substrate layers, thereby reducing the bonding thermal stress, effectively reducing the temperature drift of the MEMS piezoresistive pressure sensor of this embodiment, and improving the measurement accuracy of the sensor. Attached Figure Description
[0051] The above and other features, advantages, and aspects of the embodiments of this disclosure will become more apparent from the accompanying drawings and the following detailed description. Throughout the drawings, the same or similar reference numerals denote the same or similar elements. It should be understood that the drawings are schematic, and elements are not necessarily drawn to scale.
[0052] Figure 1 This is a cross-sectional view of a MEMS piezoresistive pressure sensor according to an embodiment of this disclosure;
[0053] Figure 2 for Figure 1 The top view of the MEMS piezoresistive pressure sensor shown;
[0054] Figure 3 for Figure 1 The image shows a bottom view of a MEMS piezoresistive pressure sensor.
[0055] Figures 4 to 37 This is a process flow diagram of the fabrication method of the MEMS piezoresistive pressure sensor according to an embodiment of this disclosure. Detailed Implementation
[0056] To enable those skilled in the art to better understand the technical solutions of this disclosure, the disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments.
[0057] Embodiments of this disclosure will now be described in more detail with reference to the accompanying drawings. While some embodiments of this disclosure are shown in the drawings, it should be understood that this disclosure can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this disclosure. It should be understood that the accompanying drawings and embodiments of this disclosure are for illustrative purposes only and are not intended to limit the scope of protection of this disclosure.
[0058] It should also be noted that, for ease of description, only the parts relevant to this disclosure are shown in the accompanying drawings. Unless otherwise specified, the embodiments and features described in this disclosure can be combined with each other.
[0059] This disclosure will now be described in detail with reference to the accompanying drawings and embodiments.
[0060] like Figures 1 to 3 As shown, the embodiments of this disclosure relate to a MEMS piezoresistive pressure sensor, which includes a first lead and pad 1, a second passivation layer 2, a first inverse piezoelectric unit, a piezoresistive unit, a first passivation layer 12, a first substrate 13, a pressure-sensitive thin film 14, and a second substrate 16.
[0061] For example, such as Figure 1 and Figure 2 As shown, the first lead and pad 1 are located on the top layer of the sensor. In some embodiments, the material of the first lead and pad 1 is at least one of aluminum, copper, platinum, titanium, or gold, and the thickness of the first lead and pad 1 ranges from 50 nm to 500 nm. The first lead and pad 1 are electrically connected to the first inverse piezoelectric unit and the piezoresistive unit through the second passivation layer and the through-hole of the first inverse piezoelectric unit, respectively.
[0062] Continue to refer to Figure 1 The second passivation layer 2 is located below the first lead and the pad 1. In some embodiments, the material of the second passivation layer 2 is at least one of silicon oxide or silicon nitride, and the thickness of the second passivation layer 2 ranges from 50 nm to 2000 nm.
[0063] Next, as Figure 1 As shown, the first reverse piezoelectric unit is located below the second passivation layer 2. In some embodiments, the first reverse piezoelectric unit consists of a first upper electrode 3, a first reverse piezoelectric layer 4, and a first lower electrode 5 stacked sequentially from top to bottom. The first upper electrode 3 is located below the second passivation layer 2, and its material is at least one of aluminum, copper, platinum, titanium, or gold. The thickness of the first upper electrode 3 ranges from 50 nm to 500 nm. The first reverse piezoelectric layer 4 is made of one of aluminum nitride, lead zirconate titanate, or zinc oxide, and its thickness ranges from 5 μm to 100 μm. The material and thickness of the first lower electrode 5 are the same as those of the first upper electrode 3.
[0064] Continue to refer to Figure 1 and Figure 2 The first passivation layer 12 is located below the first inverse piezoelectric unit. In some embodiments, the material of the first passivation layer 12 can be silicon oxide or silicon nitride, etc. The thickness of the first passivation layer 12 ranges from 2 μm to 50 μm, and a piezoresistive unit is embedded in the first passivation layer 12. In some embodiments, the piezoresistive unit includes both N-type and P-type piezoresistive units, which are combined together. Figure 13The N-type piezoresistive unit comprises an N-type piezoresistive strip 9, N-type electrode lead-out areas 11 at both ends, and a P-type area 10 below it; the P-type piezoresistive unit comprises a P-type piezoresistive strip 6, P-type electrode lead-out areas 8 at both ends, and an N-type area 7 below it. Both the N-type and P-type piezoresistive units are made of either monocrystalline silicon or polycrystalline silicon, and there are two of each type, all distributed at the midpoints of the four sides of the pressure-sensitive film 14. These four piezoresistive strips are connected by metal leads to form a Wheatstone bridge.
[0065] For example, such as Figure 1 As shown, the first substrate 13 is located below the first passivation layer 12. In some embodiments, the material of the first substrate 13 is either monocrystalline silicon or polycrystalline silicon, and the thickness of the first substrate 13 ranges from 400 μm to 1000 μm. The first substrate 13 is provided with a sealed cavity 15 extending through its thickness, and the cavity 15 may be in the shape of a trapezoidal truncated cone. The area of the square upper surface of the sealed cavity 15 projected upwards onto the first passivation layer 12, the first inverse piezoelectric unit, and the second passivation layer 2 together forms the pressure-sensitive thin film 14.
[0066] Continue to refer to Figure 1 The second substrate 16 is located below the first substrate 13. In some embodiments, the material of the second substrate 16 is borosilicate glass or the like, and the thickness of the second substrate 16 ranges from 400 μm to 1000 μm. In some embodiments, the MEMS piezoresistive pressure sensor further includes a second inverse piezoelectric unit and a second lead and pad 21. The second inverse piezoelectric unit is disposed on the upper surface of the second substrate 16, and the second lead and pad 21 is disposed on the lower surface of the second substrate 16. The second lead and pad 21 are electrically connected to the second inverse piezoelectric unit.
[0067] For example, such as Figure 1 As shown, the second inverse piezoelectric unit includes a second upper electrode 17, a second inverse piezoelectric layer 18, and a second lower electrode 19. A square groove is provided at the center of the second substrate 16, and the second upper electrode 17, the second inverse piezoelectric layer 18, and the second lower electrode 19 are arranged sequentially from top to bottom within the square groove to form the second inverse piezoelectric unit. The gap between the second inverse piezoelectric unit and the square groove is filled with the filling layer 20.
[0068] In some embodiments, the second upper electrode 17 and the second lower electrode 19 are made of at least one of aluminum, copper, platinum, titanium, or gold, and have a thickness ranging from 50 nm to 500 nm. The second reverse piezoelectric layer 18 is made of one of aluminum nitride, silicon oxide, lead zirconate titanate, or zinc oxide, and has a thickness ranging from 50 μm to 200 μm. The filling layer 20 is made of one of silicon oxide or silicon nitride.
[0069] Furthermore, the performance of the sensor can be further improved by increasing the number of layers in both the first and second inverse piezoelectric units mentioned above.
[0070] The working principle of the MEMS piezoresistive pressure sensor according to the embodiments of this disclosure will be explained below:
[0071] like Figure 1 As shown, in the MEMS piezoresistive pressure sensor of this embodiment, the first inverse piezoelectric unit exhibits an inverse piezoelectric effect, and its upper and lower plates expand horizontally when a suitable voltage is applied. The square region in the middle of the first inverse piezoelectric unit forms part of the pressure-sensitive film 14; therefore, the horizontal expansion of the first inverse piezoelectric unit causes the pressure-sensitive film 14 to expand horizontally as well. Since the edges of the pressure-sensitive film 14 are fixed to the first substrate 13 through the first passivation layer 12, while the square region in the middle of the pressure-sensitive film 14 is suspended, a vertically upward force is generated in the suspended square region when the pressure-sensitive film 14 expands horizontally.
[0072] Based on the above, the MEMS piezoresistive pressure sensor of this embodiment adopts a novel operating mode: when external pressure is applied vertically downwards to the pressure-sensitive film 14, stress is concentrated in the area where the piezoresistive unit is located. The piezoresistive strip constituting the piezoresistive unit senses this stress, causing a change in resistance, which is then output as a voltage via a Wheatstone bridge. The external circuit of the sensor amplifies the output voltage and applies it to the upper and lower plates of the first inverse piezoelectric unit, causing the pressure-sensitive film 14 to generate an upward force, thereby counteracting the downward external pressure. Finally, by detecting the voltage value of the upper and lower plates of the first inverse piezoelectric unit, the value of the external pressure can be calculated to complete the measurement. In this process, the voltage of the upper and lower plates of the first inverse piezoelectric unit and the upward force generated by this voltage on the pressure-sensitive film 14 always follow the change in the downward external pressure experienced by the pressure-sensitive film 14. Because the transmission of the electrical signal and the response of the inverse piezoelectric effect are instantaneous relative to changes in external pressure, the pressure-sensitive film 14 does not deform during the sensor's detection of external pressure. This eliminates the deflection effect of the pressure-sensitive film 14, significantly improving the sensor's linearity and measurement range without mutual constraint. Furthermore, when the external pressure input to the sensor exceeds its range, the pressure-sensitive film 14 can utilize its entire deformation range to accommodate the overload pressure. This greatly enhances the overload resistance of traditional MEMS piezoresistive pressure sensors, which can only utilize a small portion of the film's deformation range to accommodate overload pressure.
[0073] When external pressure is applied to the pressure-sensitive film 14 and stress is concentrated in the piezoresistive unit region, not only will the resistance of the P-type piezoresistive strip 6 (and the N-type piezoresistive strip 9) decrease (increase) due to the piezoresistive effect, but the region of the first inverse piezoresistive layer 4 without electrodes covering the piezoresistive unit will also generate a piezoelectric effect, thereby generating a negative charge and an upward electrostatic field above the piezoresistive unit. This electrostatic field causes the PN junction at the interface between the P-type piezoresistive strip 6 and the N-type region 7 in the P-type piezoresistive unit to move downward, resulting in an increase in the thickness of the P-type piezoresistive strip 6 and a further decrease in resistance following the piezoresistive effect; conversely, it causes the PN junction at the interface between the N-type piezoresistive strip 9 and the P-type region 10 in the N-type piezoresistive unit to move upward, resulting in a decrease in the thickness of the N-type piezoresistive strip 9 and a further increase in resistance following the piezoresistive effect. This effectively improves the sensitivity of the piezoresistive strip, enhances the negative feedback effect in the new working mode, improves the sensor's sensitivity, and further increases the sensor's linearity and range.
[0074] The first substrate 13 and the second substrate 16 are joined together by anodic bonding to form a bonding interface. This process is typically carried out in a process environment above 400°C. After bonding, when the sensor is moved from the high-temperature process environment to the low-temperature normal operating environment, the first substrate 13 will shrink more than the second substrate 16 due to the greater thermal expansion coefficient of the first substrate 13, resulting in temperature-dependent bonding thermal stress at the bonding interface. This thermal stress is transmitted to the piezoresistive strip, causing temperature drift in the sensor and reducing its measurement accuracy. To address this issue, in the MEMS piezoresistive pressure sensor structure of this embodiment, the aforementioned second inverse piezoelectric unit is disposed in the central square groove of the second substrate 16. When a temperature-controlled voltage is applied to the upper and lower plates of the second inverse piezoelectric unit, a temperature-controlled inverse piezoelectric effect is generated, causing the structure to undergo temperature-controlled horizontal contraction. Since the second inverse piezoelectric unit is tightly connected to the second substrate 16 through the filling layer 20, the temperature-controlled horizontal contraction of the second inverse piezoelectric unit will also cause the second substrate 16 to undergo temperature-controlled horizontal contraction. This reduces the difference in the degree of contraction between the two substrate layers, effectively reduces bonding thermal stress, and thus effectively reduces the temperature drift of the sensor and improves the measurement accuracy of the sensor.
[0075] In summary, the MEMS piezoresistive pressure sensor of the present disclosure has the following advantages compared with conventional MEMS piezoresistive pressure sensors:
[0076] 1. The MEMS piezoresistive pressure sensor of this disclosure adopts a new operating mode based on the first inverse piezoelectric unit. In this mode, when the sensor measures external pressure, its pressure-sensitive film does not need to deform, thereby eliminating the deflection effect of the pressure-sensitive film. The linearity and measurement range of the sensor are greatly improved, and the two do not restrict each other.
[0077] 2. When the external pressure input to the MEMS piezoresistive pressure sensor of this embodiment exceeds the measurement range, the pressure-sensitive film of the sensor can use its entire deformation range to accommodate the overload pressure. This greatly improves the overload resistance of the sensor compared to traditional MEMS piezoresistive pressure sensors, which can only use a small portion of the deformation range of the pressure-sensitive film to accommodate the overload pressure.
[0078] 3. In the MEMS piezoresistive pressure sensor of this embodiment, the resistance change of the piezoresistive strip comes not only from the piezoresistive effect but also from the piezoelectric effect of the first inverse piezoelectric layer region without electrodes. This effectively improves the sensitivity of the piezoresistive strip, enhances the negative feedback effect in the aforementioned new operating mode, improves the sensor's sensitivity, and further increases the sensor's linearity and range.
[0079] 4. In the structure of the MEMS piezoresistive pressure sensor of this embodiment, a second inverse piezoelectric unit is provided in the central square groove of the second substrate. When a suitable temperature-controlled voltage is applied to the upper and lower plates of the second inverse piezoelectric unit, an inverse piezoelectric effect is generated, thereby causing the structure to undergo temperature-controlled horizontal contraction, and driving the second substrate to also undergo temperature-controlled horizontal contraction, so as to reduce the difference in the degree of contraction between the two substrate layers, thereby reducing the bonding thermal stress, effectively reducing the temperature drift of the MEMS piezoresistive pressure sensor of this embodiment, and improving the measurement accuracy of the sensor.
[0080] Based on the same inventive concept, embodiments of this disclosure also provide a method for fabricating a MEMS piezoresistive pressure sensor as described above. The specific structure of this MEMS piezoresistive pressure sensor can be found in the preceding description and will not be repeated here. The method specifically includes the following steps:
[0081] Step 1: Provide an SOI wafer; wherein the SOI wafer comprises a first substrate, an oxide layer and a device layer stacked sequentially.
[0082] Specifically, in this step, such as Figure 4 As shown, prepare an SOI wafer, which consists of a device layer, an oxide layer, and a first substrate 13 from top to bottom.
[0083] Step 2: Perform N-type doping and P-type doping on the device layer of the SOI wafer to form N-type and P-type regions, respectively.
[0084] Specifically, in this step, such as Figure 5 and Figure 6 As shown, N-type region 7 and P-type region 10 are formed by N-type doping and P-type doping of the device layer of SOI wafer, respectively.
[0085] Step 3: Perform P-type doping and N-type doping on the surface regions of the N-type region and the P-type region to form P-type varistor strips and N-type varistor strips, respectively.
[0086] Specifically, in this step, such as Figure 7 and Figure 8 As shown, the surface regions of N-type region 7 and P-type region 10 are doped with P-type and N-type, respectively, to form P-type varistor strip 6 and N-type varistor strip 9 and the PN junction below them.
[0087] Step 4: P-type heavy doping and N-type heavy doping are performed on both ends of the P-type varistor strip and the N-type varistor strip to form P-type electrode lead-out regions and N-type electrode lead-out regions, respectively.
[0088] Specifically, in this step, such as Figure 9 and Figure 10 As shown, the two ends of the P-type varistor strip 6 and the N-type varistor strip 9 are heavily doped with P-type and N-type respectively to form the P-type electrode lead-out region 8 and the N-type electrode lead-out region 11, thereby completing the fabrication of four piezoresistive units.
[0089] Step 5: Oxidize the device layer region on the outside of the piezoresistive unit to obtain the first passivation layer.
[0090] Specifically, in this step, such as Figure 11 As shown, the device layer region outside the four piezoresistive units is oxidized, thereby extending the oxide layer in the SOI wafer to form the first passivation layer 12.
[0091] Step 6: Form the first lower electrode on the first passivation layer.
[0092] Specifically, in this step, such as Figure 12 and Figure 13 As shown, metal is sputtered onto the upper surface of the first passivation layer 12 and photolithography is performed to form the first lower electrode plate 5.
[0093] Step 7: Form a first reverse piezoelectric layer on the first lower electrode plate.
[0094] Specifically, in this step, such as Figure 14 As shown, aluminum nitride is deposited on the upper surface of the first lower electrode 5 to form the first reverse piezoelectric layer 4.
[0095] Step 8: Form the first upper electrode on the first reverse piezoelectric layer.
[0096] Specifically, in this step, such as Figure 15 and Figure 16 As shown, metal is sputtered onto the upper surface of the first inverse piezoelectric layer 4 and photolithography is performed to form the first upper electrode plate 3.
[0097] Step 9: Form a second passivation layer on the first inverse piezoelectric unit.
[0098] Specifically, in this step, such as Figure 17 As shown, silicon oxide is deposited on the upper surface of the first upper electrode 3 to form the second passivation layer 2.
[0099] Step 10: Pattern the second passivation layer to form the first via.
[0100] Specifically, in this step, such as Figure 18 and Figure 19 As shown, photolithography is performed on the second passivation layer 2 to form through holes, in preparation for the electrical connection of the first upper electrode plate 3 and the formation of through holes in the first reverse piezoelectric layer 4.
[0101] Step 11: Form a second through hole at the position of the first inverse piezoelectric unit corresponding to the first through hole.
[0102] Specifically, in this step, such as Figure 20 and Figure 21 As shown, photolithography is performed on the first inverse piezoelectric layer 4 to form through holes, in preparation for the electrical connection of the first lower electrode plate 5, the N-type electrode lead-out area 11 and the P-type electrode lead-out area 8.
[0103] Step 12: Form a first lead and a pad on the second passivation layer, and fill the first through hole and the second through hole with a metal layer, so that the first lead and the pad are electrically connected to the first inverse piezoelectric unit and the piezoresistive unit, respectively.
[0104] Specifically, in this step, such as Figure 22 As shown, metal is sputtered on the upper surface of the second passivation layer 2 and photolithography is performed to form the first lead and pad 1. Metal is filled in the through holes of the second passivation layer 2 and the first reverse piezoelectric layer 4 to complete the electrical connection between the pad and the first upper electrode plate 3, the first lower electrode plate 5, the N-type electrode lead-out area 11 and the P-type electrode lead-out area 8.
[0105] Step 13: Form a cavity on the back side of the first substrate to form a pressure-sensitive film.
[0106] Specifically, in this step, such as Figure 23 and Figure 24As shown, the back side of the first substrate 13 is wet-etched to prepare a downward-facing "trapezoidal" cavity in the first substrate 13 to form the pressure-sensitive film 14.
[0107] Step 14: Provide a second substrate.
[0108] Specifically, in this step, such as Figure 25 As shown, a 500μm thick borosilicate glass is prepared as the second substrate 16.
[0109] Step 15: Form a deep through-hole on the first surface of the second substrate.
[0110] Specifically, in this step, such as Figure 26 As shown, two deep vias are formed by etching below the second substrate 16.
[0111] Step 16: Form a groove on the second surface of the second substrate.
[0112] Specifically, in this step, such as Figure 27 As shown, a square groove is formed by etching on the upper surface of the second substrate 16.
[0113] Step 17: Form a second lower electrode plate within the groove;
[0114] Specifically, in this step, such as Figure 28 and Figure 29 As shown, metal is sputtered at the bottom of the square groove of the second substrate 16 and photolithography is performed to form the second lower electrode plate 19.
[0115] Step 18: Form a second inverse piezoelectric layer on the second lower electrode plate.
[0116] Specifically, in this step, such as Figure 30 As shown, aluminum nitride is deposited on the surface of the second lower electrode 19 and photolithography is performed to form the second inverse piezoelectric layer 18.
[0117] Step 19: Form a filling layer in the groove, and pattern the filling layer to form a through hole.
[0118] Specifically, in this step, such as Figure 31 As shown, silicon oxide is deposited in a square groove to form the filling layer 20, and then photolithography is performed on the filling layer 20 to form through holes.
[0119] Step 20: Form a second upper electrode plate and a lead wire in the through hole of the filling layer on the surface of the groove.
[0120] Specifically, in this step, such as Figure 32 and Figure 33As shown, metal is sputtered on the upper surface of the groove and photolithography is performed to form the second upper electrode plate 17 and the lead wire in the through hole of the filling layer.
[0121] Step 21: Etch the deep via until the lead wire in the via of the filling layer is exposed.
[0122] Specifically, in this step, such as Figure 34 As shown, a deep through-hole is further etched to prepare for the formation of an electrical connection between the second upper electrode plate 17 and the second lower electrode plate 19.
[0123] Step 22: Form a second lead and a pad on the first surface of the second substrate, so that the second lead and the pad are electrically connected to the second inverse piezoelectric layer.
[0124] Specifically, in this step, such as Figure 35 and Figure 36 As shown, metal is sputtered and photolithography is performed on the lower surface of the second substrate 16 to form the second lead and pad 21, thus completing the electrical connection between the bottom pad and the second upper electrode plate 17 and the second lower electrode plate 19.
[0125] Step 23: Bond the second substrate to the first substrate to prepare a MEMS piezoresistive pressure sensor.
[0126] Specifically, in this step, such as Figure 37 As shown, the lower surface of the structure corresponding to step thirteen is bonded to the upper surface of the structure corresponding to step twenty-two through an anodic bonding process to form the sealed cavity 15, thereby completing the fabrication of the MEMS piezoresistive pressure sensor of this embodiment.
[0127] Compared with traditional MEMS piezoresistive pressure sensors, the MEMS piezoresistive pressure sensor prepared according to the embodiments of this disclosure has the following advantages:
[0128] 1. The MEMS piezoresistive pressure sensor of this disclosure adopts a new operating mode based on the first inverse piezoelectric unit. In this mode, when the sensor measures external pressure, its pressure-sensitive film does not need to deform, thereby eliminating the deflection effect of the pressure-sensitive film. The linearity and measurement range of the sensor are greatly improved, and the two do not restrict each other.
[0129] 2. When the external pressure input to the MEMS piezoresistive pressure sensor of this embodiment exceeds the measurement range, the pressure-sensitive film of the sensor can use its entire deformation range to accommodate the overload pressure. This greatly improves the overload resistance of the sensor compared to traditional MEMS piezoresistive pressure sensors, which can only use a small portion of the deformation range of the pressure-sensitive film to accommodate the overload pressure.
[0130] 3. In the MEMS piezoresistive pressure sensor of this embodiment, the resistance change of the piezoresistive strip comes not only from the piezoresistive effect but also from the piezoelectric effect of the first inverse piezoelectric layer region without electrodes. This effectively improves the sensitivity of the piezoresistive strip, enhances the negative feedback effect in the aforementioned new operating mode, improves the sensor's sensitivity, and further increases the sensor's linearity and range.
[0131] 4. In the structure of the MEMS piezoresistive pressure sensor of this embodiment, a second inverse piezoelectric unit is provided in the central square groove of the second substrate. When a suitable temperature-controlled voltage is applied to the upper and lower plates of the second inverse piezoelectric unit, an inverse piezoelectric effect is generated, thereby causing the structure to undergo temperature-controlled horizontal contraction, and driving the second substrate to also undergo temperature-controlled horizontal contraction, so as to reduce the difference in the degree of contraction between the two substrate layers, thereby reducing the bonding thermal stress, effectively reducing the temperature drift of the MEMS piezoresistive pressure sensor of this embodiment, and improving the measurement accuracy of the sensor.
[0132] It is understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of this disclosure, and this disclosure is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this disclosure, and these modifications and improvements are also considered to be within the scope of protection of this disclosure.
Claims
1. A MEMS piezoresistive pressure sensor, characterized by, The MEMS piezoresistive pressure sensor includes a first substrate, a second substrate, a first passivation layer, a second passivation layer, a first inverse piezoelectric unit, a piezoresistive unit, and a first lead and pad. The first substrate has a cavity extending through its thickness and is fixed to the second substrate; The first passivation layer is disposed on the side of the first substrate away from the second substrate, and the first passivation layer contains the piezoresistive unit corresponding to the cavity; The first inverse piezoelectric unit is disposed on the side of the first passivation layer facing away from the second substrate; wherein, the first inverse piezoelectric unit, the first passivation layer, and the piezoresistive unit form a pressure-sensitive film in the region corresponding to the cavity; The second passivation layer is disposed on the side of the first inverse piezoelectric unit away from the first substrate. The first lead and the pad are disposed on the side of the second passivation layer away from the second substrate. The first lead and the pad are electrically connected to the piezoresistive unit and the first inverse piezoelectric unit, respectively. The first inverse piezoelectric unit includes a first lower electrode plate, a first inverse piezoelectric layer and a first upper electrode plate stacked sequentially on the first passivation layer. The area of the first inverse piezoelectric layer without electrode plates on the piezoresistive unit generates a piezoelectric effect, thereby generating a negative charge and a vertically upward electrostatic field above the piezoresistive unit.
2. The MEMS piezoresistive pressure sensor of claim 1, wherein, The MEMS piezoresistive pressure sensor also includes a second inverse piezoelectric unit and a second lead and a pad. The second inverse piezoelectric unit is disposed on the side of the second substrate facing the first substrate and corresponds to the cavity; The second lead and pad are disposed on the side of the second substrate away from the first substrate and are electrically connected to the second inverse piezoelectric unit.
3. The MEMS piezoresistive pressure sensor of claim 2, wherein, The second inverse piezoelectric unit includes a second lower electrode plate, a second inverse piezoelectric layer, and a second upper electrode plate, which are sequentially stacked on the second substrate.
4. The MEMS piezoresistive pressure sensor of claim 2, wherein, The second substrate has a groove on the side facing the first substrate, and the groove accommodates the second inverse piezoelectric unit.
5. The MEMS piezoresistive pressure sensor of claim 4, wherein, The second inverse piezoelectric unit has a gap between itself and the groove, and the MEMS piezoresistive pressure sensor also includes a filling layer that fills the gap.
6. The MEMS piezoresistive pressure sensor according to any one of claims 1 to 5, characterized in that, The piezoresistive unit includes an N-type piezoresistive unit and a P-type piezoresistive unit; The N-type piezoresistive unit includes an N-type piezoresistive strip, N-type electrode lead-out areas located at both ends of the N-type piezoresistive strip, and a P-type area disposed on the side of the N-type piezoresistive strip facing the second substrate; The P-type piezoresistive unit includes a P-type piezoresistive strip, P-type electrode lead-out areas located at both ends of the P-type piezoresistive strip, and an N-type area disposed on the side of the P-type piezoresistive strip facing the second substrate.
7. A method of manufacturing a MEMS piezoresistive pressure sensor as claimed in any one of claims 1 to 6, characterized in that, The method includes: An SOI wafer is provided; wherein the SOI wafer comprises a first substrate, an oxide layer and a device layer stacked sequentially. The device layer of the SOI wafer is doped to form piezoresistive cells; The device layer region outside the piezoresistive unit is oxidized to obtain a first passivation layer; Forming a first inverse piezoelectric unit on the first passivation layer specifically involves: forming a first lower electrode on the first passivation layer; forming a first inverse piezoelectric layer on the first lower electrode; and forming a first upper electrode on the first inverse piezoelectric layer. A second passivation layer is formed on the first inverse piezoelectric unit; The second passivation layer is patterned to form the first via; A second through hole is formed at the position of the first through hole corresponding to the first inverse piezoelectric unit; A first lead and a pad are formed on the second passivation layer, and a metal layer is filled in the first through hole and the second through hole, so that the first lead and the pad are electrically connected to the first inverse piezoelectric unit and the piezoresistive unit, respectively. A cavity is formed on the back side of the first substrate to form a pressure-sensitive thin film; Provide a second substrate; The second substrate is bonded to the first substrate to prepare a MEMS piezoresistive pressure sensor.
8. The method according to claim 7, characterized in that, Before bonding the second substrate to the first substrate, the method further includes: forming a second inverse piezoelectric unit on the second substrate, specifically: A deep through-hole is formed on the first surface of the second substrate; A groove is formed on the second surface of the second substrate; A second lower electrode plate is formed within the groove; A second reverse piezoelectric layer is formed on the second lower electrode plate; A filling layer is formed in the groove, and the filling layer is patterned to form through holes; A second upper electrode plate and a lead wire in the through hole of the filling layer are formed on the surface of the groove. Etch the deep via to expose the lead wire in the via of the filling layer; A second lead and a pad are formed on the first surface of the second substrate, such that the second lead and the pad are electrically connected to the second inverse piezoelectric layer.
9. The method according to claim 7 or 8, characterized in that, The process of doping the device layer of the SOI wafer to form piezoresistive cells includes: The device layer of the SOI wafer is subjected to N-type doping and P-type doping to form N-type and P-type regions, respectively; The surface regions of the N-type region and the P-type region are doped with P-type and N-type doping respectively to form P-type varistor strips and N-type varistor strips; The two ends of the P-type varistor strip and the N-type varistor strip are heavily doped with P-type and N-type respectively to form P-type electrode lead-out regions and N-type electrode lead-out regions.