C-SOI-based resonant pressure sensor and its fabrication method

Abstract

The application designs a resonant pressure sensor based on C-SOI and a manufacturing method. The C-SOI with high device layer thickness uniformity is manufactured by gold-silicon eutectic bonding and mechanical chemical polishing processes, and the pressure sensitive membrane and resonator aligned with the pre-embedded groove are manufactured by using the bonding wafer through photoetching and deep etching processes, and the silicon wafer and the C-SOI are bonded together by using the gold-silicon eutectic bonding process to realize the vacuum packaging of the resonator. The silicon resonant pressure sensor designed by the application can realize high-precision measurement of gas pressure, and the differential output of the double resonator can greatly improve the pressure sensitivity of the sensor, reduce the temperature drift of the sensor and reduce the static pressure error of the sensor in the measurement process. The cavity thickness of the C-SOI is accurately controlled, the influence of side drilling on the performance of the resonator is eliminated, and the resonator failure caused by the release of hydrogen fluoride is eliminated.

Description

Technical Field

[0001] This invention relates to the field of silicon resonant pressure sensors, and more particularly to a resonant pressure sensor based on C-SOI (Cavity-SOI) and its fabrication method. Background Technology

[0002] Resonant silicon micromechanical pressure sensors are among the most accurate and stable pressure sensors currently available, making them suitable for precision measurement applications in aerospace, industrial process control, and other fields where high accuracy and long-term stability are required.

[0003] Silicon micromechanical resonant pressure sensors perform frequency detection, are insensitive to noise, and possess strong long-distance transmission and anti-interference capabilities. Their semi-digital output allows for connection to digital circuits without A / D conversion. Their characteristics are determined by the resonator's structural dimensions and material properties; single-crystal silicon exhibits excellent mechanical properties, resulting in high accuracy and good long-term stability. Furthermore, resonant silicon micromechanical pressure sensors also offer advantages such as fast response, wide bandwidth, compact structure, low power consumption, small size, light weight, and mass production capability.

[0004] Resonant silicon micromechanical pressure sensors primarily employ an integrated beam-diaphragm design, with the resonator fixed to the surface of the pressure-sensitive diaphragm via anchor points. When subjected to external pressure, the pressure-sensitive diaphragm deforms, transmitting stress to the resonator. Under this stress, the resonator changes its natural frequency, and the pressure is indirectly measured by detecting this frequency change. This design avoids external factors affecting the resonator's frequency, improving measurement accuracy. Furthermore, it allows for vacuum encapsulation of the resonator, increasing its Q value and resulting in higher detection resolution and faster system response speed for the resonant sensor.

[0005] Because beam-membrane resonant pressure sensors inherently possess multi-layered structures, they are often fabricated using SOI (Silicon-on-Ion) technology, which also has a multi-layered structure. The SOI-based comb-shaped electrostatic excitation / capacitive sensing silicon resonant pressure sensor developed by Northwestern Polytechnical University uses an SOI substrate layer to fabricate the pressure-sensitive membrane and a device layer to fabricate the resonator. Similarly, an electrostatic excitation / piezoresistive sensing silicon resonant pressure sensor developed by Hefei University of Technology is also based on SOI technology.

[0006] The intermediate oxide layer of SOI can be etched by hydrofluoric acid, while the silicon layers above and below do not react with it. This principle can be used in the release process after etching the resonant beam, which is the main method currently used to fabricate resonant sensors using SOI. However, this method has certain drawbacks. On the one hand, the fabrication method of SOI limits the thickness of its oxide layer. Compared to the small size of the resonator, adhesion failure may occur during the release process. On the other hand, the corrosion of silicon oxide by hydrofluoric acid is isotropic, resulting in side-drilling during the release of the resonant beam, which affects the performance of the resonator. Controlling the consistency of side-drilling is also challenging. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a C-SOI-based resonant pressure sensor and its fabrication method. Building upon pressure measurement, the invention optimizes the release process by pre-fabricating a cavity, addressing adhesion and side-drilling issues, thereby increasing the sensor's yield and reducing the impact of side-drilling on resonator performance.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A C-SOI-based resonant differential pressure sensor includes a bottom substrate layer, a middle device layer, and an upper silicon cover plate. A pressure-sensitive film is fabricated on the substrate layer using silicon wafer etching. A resonator is fabricated on the surface of the pressure-sensitive film through bonding and etching. The silicon layer containing the resonator is called the device layer. The resonator includes a first resonator and a second resonator. The device layer also includes first to eighth electrodes. The resonator is electrically connected to the first to eighth electrodes via silicon wires. The first to eighth electrodes are distributed at the frame of the resonant differential pressure sensor. The substrate layer, device layer, and silicon cover plate are bonded together via gold-silicon eutectic bonding.

[0010] Furthermore, the resonator and pressure-sensitive membrane are integrated into a beam-membrane structure through eutectic bonding. The cavity for releasing the resonator is prefabricated at the location of the resonator, eliminating the need for the resonator's release process during fabrication.

[0011] This invention also provides a method for fabricating a C-SOI-based resonant differential pressure sensor, comprising the following steps:

[0012] Step S1: Construct a three-dimensional model of a resonant pressure sensor using a silicon wafer;

[0013] Step S2: Use finite element simulation to analyze the three-dimensional model of the resonant pressure sensor and solve the problems of modal interference and sensitivity matching of the first and second resonators;

[0014] Step S3: Draw the photolithographic mask for the resonant pressure sensor;

[0015] Step S4: Using photolithography, etching, and thermal oxidation processes, a pre-embedded cavity is etched on one side of the silicon wafer, and alignment marks are etched on the other side;

[0016] Step S5: Use eutectic bonding process to bond the silicon wafer with the pre-embedded cavity to another silicon wafer, and use mechanical and chemical polishing to thin and polish it to prepare the C-SOI;

[0017] Step S6: Using photolithography and deep etching processes, a pressure-sensitive film, lead holes, a first resonator, a second resonator, and first to eighth electrodes are constructed on the prepared C-SOI to complete the chip body of the resonant pressure sensor.

[0018] Step S7: Vacuum encapsulate the fabricated chip body with the silicon cover using eutectic bonding;

[0019] Step S8: Assemble the sensor chip.

[0020] Further, step S1 includes:

[0021] Step S1a: Perform finite element simulation on the sensitivity of the first resonator and the second resonator when they are located at different positions on the pressure-sensitive membrane, and adjust the positions of the first resonator and the second resonator to make the sensitivity match each other;

[0022] Step S1b: Simulate the change in sensitivity of the resonant pressure sensor under different pressure-sensitive membrane thicknesses to determine the thickness of the pressure-sensitive membrane.

[0023] Step S1c: Optimize the symmetrical design of the resonant differential pressure sensor structure by simulating the static pressure sensitivity of the resonant pressure sensor, with the two resonators located at the stress matching position of the pressure-sensitive membrane.

[0024] Furthermore, step S2 includes simulating the pressure characteristics of the resonant pressure sensor within its operating range using finite element simulation software, which facilitates mutual verification of subsequent test results.

[0025] Further, step S4 includes:

[0026] Step S4a: Use photoresist as a mask to etch the silicon wafer to form a pre-embedded cavity;

[0027] Step S4b: Using photoresist as a mask, perform alignment photolithography etching on the other side of the silicon wafer to form alignment marks.

[0028] Further, step S5 includes:

[0029] Step S5a: Sputter metals Cr and Au onto the surface of the silicon wafer with the pre-embedded cavity to serve as an intermediate layer for eutectic bonding;

[0030] Step S5b: Eutectic bonding is performed between a silicon wafer sputtered with metals Cr and Au and a smooth silicon wafer to form C-SOI;

[0031] Step S5c: Thin the C-SOI using mechanical chemical polishing to achieve the preset thickness of the bonded silicon wafer;

[0032] Step S5d: Polish the C-SOI using mechanical chemical polishing to achieve the required surface roughness for bonding.

[0033] Beneficial effects:

[0034] This invention achieves C-SOI fabrication through gold-silicon eutectic bonding, solving the adhesion failure and side-drilling phenomena that occur during the release of the resonant beam, improving the yield of sensor fabrication, and reducing the impact of side-drilling on resonator performance. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the front and back of a resonant differential pressure sensor prototype.

[0036] Figure 2 This is a schematic diagram of the dimensions of an H-shaped double-ended fixed beam.

[0037] Figure 3 This is a schematic diagram of the SOI device layer structure;

[0038] Figure 4 This is a three-dimensional structural schematic diagram of the resonant differential pressure sensor designed in this invention;

[0039] Figure 5 This is a schematic diagram illustrating the assembly method of the resonant differential pressure sensor designed in this invention;

[0040] Figure 6 This is a process flow diagram of the resonant differential pressure sensor designed in this invention;

[0041] Wherein, 100-substrate layer; 110-pressure sensitive film; 120-electrode lead hole; 200-device layer; 211-first resonator; 212-second resonator; 221-first electrode, 222-second electrode, 223-third electrode, 224-fourth electrode, 225-fifth electrode, 226-sixth electrode, 227-seventh electrode, 228-eighth electrode; 300-silicon cover plate. Detailed Implementation

[0042] 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 specific embodiments and accompanying drawings.

[0043] The resonator of the silicon resonant pressure sensor designed in this invention is an H-shaped double-ended fixed beam. The resonators are distributed in different regions of the pressure-sensitive membrane and are referred to as the first resonator and the second resonator. The one located at the edge of the pressure-sensitive membrane is called the first resonator, and the one in the center is called the second resonator.

[0044] When the pressure-sensitive membrane deforms under external pressure, the area where the second resonator in the center is located becomes a tensile stress zone. Due to the tensile stress, its resonant frequency increases. At this time, the first resonator located at the edge experiences compressive stress, and its resonant frequency decreases. By adjusting the positions of the two resonators, the tensile and compressive stresses on the two resonators can be matched in absolute value, thereby making the changes in resonant frequency equal in absolute value.

[0045] This dual-resonator matching design offers two main benefits. First, it increases the sensor's sensitivity using differential output. Because the two resonators are subjected to different types of stress, their frequencies increase and decrease respectively. The difference in their resonant frequencies allows us to determine the magnitude of the pressure applied to the sensitive membrane. Second, the differential design of the dual resonators reduces errors caused by non-pressure factors. Besides external pressure, the resonant frequency is affected by temperature, assembly stress, and other factors. These influences cause changes in the resonant frequency, but differential output can significantly reduce these errors.

[0046] The structure of the silicon resonant pressure sensor designed in this invention is as follows: Figure 1 As shown. A pressure-sensitive film 110 is fabricated on the substrate layer 100 using silicon wafer etching. A resonator is then fabricated on the surface of the pressure-sensitive film 110 through bonding and etching. The silicon layer containing the resonator is called the device layer 200. An enlarged view of the resonator is shown below. Figure 2 As shown. The resonator is 1320 micrometers long and 20 micrometers wide, with a 60-micrometer long intermediate connector. The structure of device layer 200 is as follows. Figure 3 As shown, it mainly includes a first resonator 211, a second resonator 212, and first to eighth electrodes 221, 222, 223, 224, 225, 226, 227, and 228. Each resonator is electrically connected to the electrodes via silicon wires, which are distributed along the frame of the silicon resonant pressure sensor. The three-dimensional structure of the resonant differential pressure sensor is shown below. Figure 4 As shown, the structure consists of an upper silicon cover plate 300, a middle device layer 200, and a bottom substrate layer 100. The silicon cover plate 300 enables vacuum encapsulation of the resonator, and the device layer 200 fabricates the various resonators and electrodes, such as... Figure 1 As shown, a pressure-sensitive membrane 110 and an electrode lead hole 120 are fabricated on the substrate layer 100. Figure 4The diagram shows the structure unfolded along a fixed edge, with each layer bonded together via gold-silicon eutectic bonding.

[0047] like Figure 6 As shown, the fabrication method of the C-SOI-based resonant pressure sensor of the present invention includes the following steps:

[0048] Step a: Prepare a double-sided polished silicon wafer with a thickness of 300um, clean it, and then sputter a layer of 50nm chromium and a layer of 500nm gold.

[0049] Step b: On the sputtered metal side, use photoresist as a mask to pattern gold and chromium, then etch a 15µm deep trench as a pre-embedded chamber, and then clean the photoresist.

[0050] Step c: Prepare a double-sided polished silicon wafer, clean it, and then perform gold-silicon eutectic bonding with the silicon wafer from step b. Both wafers need to be dried before bonding to ensure the bonding surfaces are dry and clean. After bonding, C-SOI is obtained.

[0051] Step d: The bonding sheet obtained in step c is thinned by 260um through thinning and polishing. After thinning, it is polished to reduce the surface roughness and facilitate subsequent processes.

[0052] Step e: Etch both sides of the thinned C-SOI. The pressure-sensitive film and electrode lead holes are etched on the back side. The back side etching uses a two-step process: first, a 3µm thick layer of silicon oxide is deposited on the surface. The pattern of the film and holes is transferred onto the silicon oxide using photolithography, followed by photoresist cleaning. A layer of photoresist is then applied to the surface, patterning only the holes. The holes are deeply etched to a depth of 120µm, followed by photoresist cleaning. Using the patterned silicon oxide as a mask, both the holes and the film are etched simultaneously to a depth of 180µm. The device layer is etched on the front side, using photoresist as a mask, to a depth of 40µm.

[0053] Step f: Use gold etchant and chromium etchant in sequence to remove the metal layer exposed after the device layer etching is completed;

[0054] Step g: Prepare a double-sided polished silicon wafer with a thickness of 300um. After cleaning, sputter a layer of 50nm chromium and a layer of 500nm gold. Use photoresist as a mask to pattern the gold and chromium, then etch a trench 40um deep. After cleaning the photoresist, use it as a cover plate.

[0055] Step h: Use gold-silicon eutectic bonding to bond the C-SOI obtained in steps f and g to the cover plate to obtain the final device.

[0056] After the sensor chip is fabricated, it needs to be assembled. Assembly requires isolating the chip from the external environment using packaging, while ensuring connectivity with the measured air pressure and electrical connections. Therefore, this invention provides... Figure 5 The assembly method shown involves mounting the sensor chip on a Kovar alloy base and sealing it with a cap. The cap is connected to the outside gas through a small tube, and the chip is electrically connected to the outside through pins on the Kovar alloy base.

[0057] The embodiments of the present invention have now been described in detail with reference to the accompanying drawings. Based on the above description, those skilled in the art should have a clear understanding of the design and fabrication of the resonant differential pressure sensor of the present invention.

[0058] It should be noted that implementations not illustrated or described in the accompanying drawings or the main text of the specification are all forms known to those skilled in the art and are not described in detail. Furthermore, the definitions of the components above are not limited to the various methods mentioned in the embodiments; those skilled in the art can easily modify or substitute them, for example:

[0059] (1) The directional terms mentioned in the embodiments, such as "up", "down", "front", "back", "left", "right", etc., are only for reference to the directions in the accompanying drawings and are not intended to limit the scope of protection of the present invention;

[0060] (2) The above embodiments can be used in combination with each other or with other embodiments based on design and reliability considerations. That is, the technical features of different embodiments can be freely combined to form more embodiments.

[0061] The specific embodiments described above provide a detailed explanation of the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A C-SOI based resonant differential pressure sensor characterized in that: The C-SOI is made by gold-silicon eutectic bonding, including a substrate layer, a device layer and a silicon cover plate; a pressure sensitive membrane is made on the substrate layer by silicon wafer etching; a resonator is made on the surface of the pressure sensitive membrane by bonding and etching; the silicon layer where the resonator is located is called the device layer; the resonator includes a first resonator and a second resonator; the device layer further includes first to eighth electrodes; the resonator is connected to the first to eighth electrodes by silicon wires to realize electrical connection; the first to eighth electrodes are distributed at the frame of the resonant differential pressure sensor; the substrate layer, the device layer and the silicon cover plate are bonded by gold-silicon eutectic bonding.

2. The C-SOI based resonant differential pressure sensor according to claim 1, wherein: The resonator and the pressure sensitive membrane realize the beam membrane integrated structure by eutectic bonding; a cavity for releasing the resonator is made in advance at the position of the resonator.

3. The method for manufacturing a resonant differential pressure sensor based on C-SOI according to claim 1 or 2, characterized in that, The method includes the following steps: Step S1: constructing a three-dimensional model of the resonant pressure sensor using a silicon wafer; Step S2: using finite element simulation to analyze the three-dimensional model of the resonant pressure sensor, solving the modal interference and the sensitivity matching problem of the first resonator and the second resonator; Step S3: drawing a photolithography mask of the resonant pressure sensor; Step S4: using photolithography, etching or thermal oxidation process to etch a pre-embedded cavity on one side of the silicon wafer and etch alignment marks on the other side; Step S5: using eutectic bonding process to bond the silicon wafer with a pre-embedded groove to another silicon wafer, and using mechanical and chemical polishing to thin and polish, to make a prepared C-SOI; Step S6: using photolithography and deep etching process to construct a pressure sensitive membrane, lead holes, a first resonator, a second resonator and first to eighth electrodes on the prepared C-SOI, to complete the chip body of the resonant pressure sensor; Step S7: using eutectic bonding to vacuum package the prepared chip body and a silicon cover plate to form a sensor chip; Step S8: assembling the sensor chip.

4. The method of manufacturing according to claim 3, wherein, The step S1 includes: Step S1a: performing finite element simulation on the sensitivity of the first resonator and the second resonator at different positions on the pressure sensitive membrane, and adjusting the positions of the first resonator and the second resonator to match the sensitivity; Step S1b: simulating the change of the sensitivity of the resonant pressure sensor under different thicknesses of the pressure sensitive membrane, to determine the thickness of the pressure sensitive membrane; Step S1c: optimizing the symmetric design of the resonant differential pressure sensor structure by simulating the static pressure sensitivity of the resonant pressure sensor, and the two resonators are located at the stress matching position of the pressure sensitive membrane.

5. The method of manufacturing according to claim 4, wherein, The step S2 includes using finite element simulation software to simulate the pressure characteristics of the resonant pressure sensor within its working range, to facilitate the mutual verification of subsequent test results.

6. The method of manufacturing according to claim 5, wherein, The step S4 includes: Step S4a: using photoresist as a mask to etch the silicon wafer to form a pre-embedded cavity; Step S4b: using photoresist as a mask to perform alignment photolithography etching on the other side of the silicon wafer to form alignment marks.

7. The method of manufacturing according to claim 6, wherein, The step S5 includes: Step S5a: sputtering metal Cr and Au on the surface of the silicon wafer with the pre-embedded cavity as an intermediate layer for eutectic bonding; Step S5b: eutectic bonding of a silicon wafer sputtered with metal Cr and Au and a piece of clean silicon wafer to form a C-SOI; Step S5c: thinning the C-SOI using mechanical chemical polishing to make the bonded silicon wafer reach a preset thickness; Step S5d: polishing the C-SOI using mechanical chemical polishing to make the surface roughness reach the bonding requirement.

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