A MEMS pressure sensor and its manufacturing method

Abstract

The present invention discloses a MEMS pressure sensor and a manufacturing method thereof. The MEMS pressure sensor includes a support layer, an insulating layer, an N-type device layer, and a P-type heavily doped layer stacked in sequence along a first direction, where the first direction is the direction in which the support layer vertically points to the P-type heavily doped layer; four strip-shaped protrusions arranged on a first plane, and along the first direction, each strip-shaped protrusion includes a P-type heavily doped layer, and the first plane is a plane perpendicular to the first direction; a plurality of electrodes, each electrode is used for each strip-shaped protrusion to access and output an electrical signal; each electrode and each strip-shaped protrusion form a Wheatstone bridge; the support layer is provided with a first cavity, and a strain film layer is provided between the first cavity and the Wheatstone bridge. This structure can achieve the improvement of the sensitivity and stability of the pressure sensor and reduce the size of the pressure sensor.

Description

Technical Field

[0001] Embodiments of the present invention relate to the technical field of sensors, and in particular, to a MEMS pressure sensor and a manufacturing method thereof. Background Art

[0002] Pressure sensors are widely used in many industries such as national defense and military, automotive electronics, petrochemical, aerospace, medical devices, and consumer electronics. They account for one-third of the entire sensor market. According to different working principles, pressure sensors can be divided into piezoresistive, capacitive, piezoelectric, surface acoustic wave, Hall effect, and so on. Among them, piezoresistive pressure sensors fabricated based on MEMS technology are widely used due to their high sensitivity and low cost.

[0003] In traditional MEMS piezoresistive pressure sensors, a piezoresistor is formed by creating a certain area of P-type lightly doped region as a piezoresistive strip in an N-type lightly doped device layer, and then creating a P-type heavily doped region at the ohmic contact position of the P-type lightly doped region. The ohmic contact hole is formed in the P-type heavily doped region to achieve ohmic contact between the metal interconnection layer and the Wheatstone bridge. The P-type lightly doped region and the P-type heavily doped region formed by this traditional method are embedded in the N-type lightly doped device layer. This PN junction structure has problems such as a low breakdown voltage and a large leakage current due to many parasitic parameters and surface defects, which will cause problems in the measurement stability of the pressure sensor during long-term use. In addition, it is difficult to make the width of the piezoresistive strip very small in this structure. To ensure a certain resistance value for the arms of the Wheatstone bridge, only P-type lightly doped regions can be used to form the arm resistors, which will lead to a large temperature drift of the sensor sensitivity. Moreover, for traditional embedded piezoresistors, when the strain film is deformed by force, the stress cannot be well concentrated on the piezoresistor itself, resulting in a low sensitivity of the sensor. Finally, the traditional embedded PN junction structure requires at least 3 photomasks, resulting in a high cost. Summary of the Invention

[0004] The present invention provides a MEMS pressure sensor and a manufacturing method thereof to improve the sensitivity and stability of the pressure sensor and reduce the size of the pressure sensor.

[0005] To achieve the above object, the first aspect embodiment of the present invention proposes a MEMS pressure sensor, including:

[0006] A support layer, an insulating layer, an N-type device layer, and a P-type heavily doped layer stacked in sequence along a first direction; wherein, the first direction is the direction in which the support layer vertically points to the P-type heavily doped layer;

[0007] Four strip-shaped protrusions arranged on a first plane, and along the first direction, each of the strip-shaped protrusions at least includes the P-type heavily doped layer, and the first plane is a plane perpendicular to the first direction;

[0008] Multiple electrodes, each of which is used for each strip protrusion to access an input electrical signal and output an electrical signal; each of the electrodes and each of the strip protrusions form a Wheatstone bridge;

[0009] The support layer is provided with a first cavity, and a strain thin film layer is provided between the first cavity and the Wheatstone bridge.

[0010] According to an embodiment of the present invention, the first cavity is located on the surface of the support layer facing away from the insulating layer, and the first cavity is an open cavity;

[0011] Alternatively, the first cavity is located on the surface of the support layer adjacent to the insulating layer, and the first cavity is a vacuum-sealed cavity.

[0012] According to an embodiment of the present invention, along the first direction, each of the strip protrusions further includes: a P-type lightly doped layer, and the P-type lightly doped layer is located between the N-type device layer and the P-type heavily doped layer.

[0013] According to an embodiment of the present invention, each of the strip protrusions includes a varistor portion and two wire portions; both ends of the varistor portion are respectively connected to one of the wire portions; wherein, along the opposite direction of the first direction, the line width of the varistor portion is smaller than the line width of the wire portion.

[0014] According to an embodiment of the present invention, the four strip protrusions are arranged in parallel, and successively include a first strip protrusion, a second strip protrusion, a third strip protrusion, and a fourth strip protrusion; each of the strip protrusions includes a first wire portion and a second wire portion, the first wire portion and the second wire portion are symmetrically arranged, and both the first wire portion and the second wire portion are offset from the symmetry axis by a preset distance for connecting the varistor portion.

[0015] According to an embodiment of the present invention, the varistor portion in the first strip protrusion protrudes from the side of the first strip protrusion away from the second strip protrusion; the varistor portion in the second strip protrusion protrudes from the side of the second strip protrusion away from the first strip protrusion; the varistor portion in the third strip protrusion protrudes from the side of the third strip protrusion away from the fourth strip protrusion; the varistor portion in the fourth strip protrusion protrudes from the side of the fourth strip protrusion away from the third strip protrusion.

[0016] According to an embodiment of the present invention, the varistor portion includes a varistor; the shape of the varistor in each of the strip protrusions is one of a U shape, a V shape, or a cascaded shape composed of multiple V shapes or U shapes, and the arrangement shape of the varistors is axisymmetrically distributed with the symmetry axis as the axis.

[0017] According to an embodiment of the present invention, the shapes of the varistors in each of the strip-shaped protrusions are the same.

[0018] According to an embodiment of the present invention, the varistor portion includes a plurality of strip-shaped varistors and a third wire portion for connecting the plurality of strip-shaped varistors in series; the line width of the strip-shaped varistor is smaller than the line width of the third wire portion; the plurality of strip-shaped varistors are all parallel to the axis of symmetry and are symmetrically distributed with the axis of symmetry as the center, and the plurality of third wire portions are all perpendicular to the axis of symmetry.

[0019] According to an embodiment of the present invention, the four strip-shaped protrusions are arranged in parallel, and successively include a first strip-shaped protrusion, a second strip-shaped protrusion, a third strip-shaped protrusion, and a fourth strip-shaped protrusion; the plurality of electrodes include: a first input electrode, a second input electrode, a first output electrode, a second output electrode, a first grounding electrode, and a second grounding electrode;

[0020] One end of the first strip-shaped protrusion is connected to the first input electrode, and the other end is connected to the first output electrode; one end of the second strip-shaped protrusion is connected to the first output electrode, and the other end is connected to the first grounding electrode; one end of the third strip-shaped protrusion is connected to the second input electrode, and the other end is connected to the second output electrode; one end of the fourth strip-shaped protrusion is connected to the second output electrode, and the other end is connected to the second grounding electrode.

[0021] According to an embodiment of the present invention, the first input electrode, the second input electrode, the first output electrode, the second output electrode, the first grounding electrode, and the second grounding electrode are all metal electrodes.

[0022] According to an embodiment of the present invention, the first input electrode, the second input electrode, the first output electrode, the second output electrode, the first grounding electrode, and the second grounding electrode are all arranged on the same layer as the first strip-shaped protrusion, the second strip-shaped protrusion, the third strip-shaped protrusion, and the fourth strip-shaped protrusion.

[0023] According to an embodiment of the present invention, for the four strip-shaped protrusion regions, the first input electrode region, the first output electrode region, the second output electrode region, the second input electrode region, the first grounding electrode region, and the second grounding electrode region arranged on the first plane, each of the electrode regions is isolated by a trench.

[0024] According to an embodiment of the present invention, along the opposite direction of the first direction, the bottom surface of the trench at least contacts the surface of the N-type device layer facing away from the insulating layer.

[0025] According to an embodiment of the present invention, the support layer is a silicon layer, the insulating layer is a silicon dioxide layer, the N-type device layer is a lightly doped N-type silicon layer, the P-type heavily doped layer is a P-type heavily doped silicon layer, and the P-type lightly doped layer is a P-type lightly doped silicon layer.

[0026] To achieve the above object, a second aspect embodiment of the present invention proposes a manufacturing method of a MEMS pressure sensor, which is applied to the MEMS pressure sensor as described above, and includes the following steps:

[0027] Provide a substrate, wherein the substrate includes a support layer, an insulating layer, and an N-type device layer;

[0028] Perform P-type heavy doping on the entire surface of the side of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer;

[0029] Use an etching process to form four strip-shaped protrusions, wherein the side walls of the four strip-shaped protrusions expose at least the P-type heavily doped layer;

[0030] Form a plurality of electrodes, each of the electrodes is used for each of the strip-shaped protrusions to access and output electrical signals; each of the electrodes and each of the strip-shaped protrusions form a Wheatstone bridge;

[0031] Etch and form a first cavity on the surface of the support layer facing away from the insulating layer, the first cavity is an open cavity, and a strain film layer is formed between the first cavity and the Wheatstone bridge.

[0032] To achieve the above object, a third aspect embodiment of the present invention proposes a manufacturing method of a MEMS pressure sensor, which is applied to the MEMS pressure sensor as described above, and includes the following steps:

[0033] Includes the following steps:

[0034] Provide a substrate as the support layer;

[0035] Etch and form a first cavity on one surface of the support layer;

[0036] Provide an N-type device layer, and form an insulating layer on one surface of the N-type device layer;

[0037] The surface of the support layer etched with the first cavity is bonded and connected to the surface of the insulating layer away from the N-type device layer in a vacuum environment, so that the first cavity is a vacuum-sealed cavity;

[0038] Thin the N-type device layer;

[0039] Perform P-type heavy doping on the entire surface of the side of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer;

[0040] Four strip-shaped protrusions are formed by an etching process, wherein the side walls of the four strip-shaped protrusions at least expose the P-type heavily doped layer;

[0041] A plurality of electrodes are formed, and each of the electrodes is used for each of the strip-shaped protrusions to access and output an electrical signal; each of the electrodes and each of the strip-shaped protrusions form a Wheatstone bridge;

[0042] A strain thin film layer is formed between the first cavity and the Wheatstone bridge.

[0043] According to an embodiment of the present invention, before the entire surface of the side of the N-type device layer facing away from the insulating layer is subjected to P-type heavy doping to form a P-type heavily doped layer, it further includes:

[0044] The entire surface of the side of the N-type device layer facing away from the insulating layer is subjected to P-type light doping to form a P-type lightly doped layer;

[0045] After the entire surface of the side of the N-type device layer facing away from the insulating layer is subjected to P-type heavy doping to form a P-type heavily doped layer, it further includes:

[0046] Four strip-shaped protrusions are formed by an etching process, wherein the side walls of the four strip-shaped protrusions at least expose the P-type lightly doped layer and the P-type heavily doped layer.

[0047] According to an embodiment of the present invention, forming a plurality of electrodes includes:

[0048] A plurality of electrodes are formed by electroplating or sputtering.

[0049] According to an embodiment of the present invention, forming a plurality of electrodes includes:

[0050] A plurality of electrodes are formed by etching, and the side walls of the trenches between the electrodes at least expose the surface of the undoped N-type device layer facing away from the insulating layer.

[0051] According to an embodiment of the present invention, forming a plurality of electrodes and forming four strip-shaped protrusions are completed in one step.

[0052] The MEMS pressure sensor and its manufacturing method according to an embodiment of the present invention. The MEMS pressure sensor includes a support layer, an insulating layer, an N-type device layer, and a P-type heavily doped layer stacked in sequence along a first direction, where the first direction is the direction in which the support layer vertically points to the P-type heavily doped layer; four strip-shaped protrusions arranged on a first plane, and along the first direction, each strip-shaped protrusion includes a P-type heavily doped layer, and the first plane is a plane perpendicular to the first direction; a plurality of electrodes, each electrode is used to access and output electrical signals for each strip-shaped protrusion; each electrode and each strip-shaped protrusion form a Wheatstone bridge; the support layer is provided with a first cavity, and a strain thin film layer is provided between the first cavity and the Wheatstone bridge. Thus, by providing four strip-shaped protrusions, when the strain thin film layer is subjected to pressure, for the edge and the center position of the strain thin film layer, the stress is the largest, and the stress is more concentrated on the surface of the strip-shaped piezoresistor, with high sensitivity and good linearity. In addition, the parasitic parameters between the heavily doped layer and the substrate are smaller, and it is closer to an ideal PN junction with the substrate. Therefore, it has a higher breakdown voltage and lower leakage current, and thus has higher reliability and long-term stability. Due to the relatively high doping concentration on the surface of the strip-shaped protrusion, the device can achieve a lower sensitivity temperature drift. The structure of this MEMS pressure sensor is simple. The preparation process of the piezoresistor strip only requires 1 photomask, with low process cost and can be mass-produced. In addition, in the two embodiments of the present invention, the embodiment in which the first cavity is an open cavity can be used to measure the relative differential pressure; the embodiment in which the first cavity is a vacuum-sealed cavity can be used to measure the absolute pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 is a top view of the MEMS pressure sensor according to an embodiment of the present invention;

[0054] Figure 2 is Figure 1 a cross-sectional view along the AA' direction in one of the embodiments;

[0055] Figure 3 is Figure 1 a cross-sectional view along the AA' direction in another of the embodiments;

[0056] Figure 4 is Figure 1 a cross-sectional view along the AA' direction in yet another of the embodiments;

[0057] Figure 5 is a top view of the MEMS pressure sensor according to an embodiment of the present invention;

[0058] Figure 6 is a top view of the MEMS pressure sensor according to another embodiment of the present invention;

[0059] Figure 7 is a top view of the MEMS pressure sensor according to yet another embodiment of the present invention;

[0060] Figure 8 is the top view of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0061] Figure 9 is the top view of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0062] Figure 10 is the circuit schematic diagram of the Wheatstone bridge in the MEMS pressure sensor proposed in an embodiment of the present invention;

[0063] Figure 11 is the structural schematic diagram of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0064] Figure 12 is the top view of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0065] Figure 13 is the flowchart of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0066] Figure 14 is the flowchart of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0067] Figure 15 is the flowchart of the manufacturing method of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0068] Figure 16 is the flowchart of the manufacturing method of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0069] Figure 17 is the step diagram of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0070] Figure 18 is the step diagram of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0071] Figure 19 is the step diagram of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0072] Figure 20 is the step diagram of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0073] Figure 21 is the step diagram of the manufacturing method of the MEMS pressure sensor proposed in an embodiment of the present invention;

[0074] Figure 22 It is a step diagram of the manufacturing method of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0075] Figure 23 It is a step diagram of the manufacturing method of the MEMS pressure sensor proposed in another embodiment of the present invention;

[0076] Figure 24 It is a step diagram of the manufacturing method of the MEMS pressure sensor proposed in yet another embodiment of the present invention;

[0077] Figure 25 It is a step diagram of the manufacturing method of the MEMS pressure sensor proposed in yet another embodiment of the present invention;

[0078] Figure 26 It is a step diagram of the manufacturing method of the MEMS pressure sensor proposed in yet another embodiment of the present invention. Specific Embodiments

[0079] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only for explaining the present invention, rather than limiting the present invention. Additionally, it should be noted that for the convenience of description, only the parts related to the present invention are shown in the drawings, rather than all the structures.

[0080] Figure 1 It is a top view of the MEMS pressure sensor proposed in an embodiment of the present invention. Combining Figure 1 and Figure 2 as shown, the MEMS pressure sensor 100 includes:

[0081] A support layer 101, an insulating layer 102, an N-type device layer 103, and a P-type heavily doped layer 104 stacked in sequence along a first direction; wherein, the first direction is the direction in which the support layer 101 vertically points to the P-type heavily doped layer 104;

[0082] Four strip-shaped protrusions arranged on a first plane, and along the first direction, each strip-shaped protrusion at least includes the P-type heavily doped layer 104, and the first plane is a plane perpendicular to the first direction;

[0083] A plurality of electrodes 112, each electrode 112 is used for each strip-shaped protrusion to access an input electrical signal and an output electrical signal; each electrode and each strip-shaped protrusion form a Wheatstone bridge 105;

[0084] The support layer 101 is provided with a first cavity 106, and a strain thin film layer 107 is provided between the first cavity 106 and the Wheatstone bridge 105.

[0085] It should be noted that the vertical projection of the bottom surface of the first cavity 106 on the insulating layer 102 covers the vertical projection of the Wheatstone bridge 105 on the insulating layer 102. Further, when the strain film layer 107 deforms through the first cavity 106, the four strip-shaped protrusions in the Wheatstone bridge 105 located above the strain film layer 107 can also deform. Due to the deformation of the four strip-shaped protrusions, the resistance value changes, and the strip-shaped protrusions at the edge of the strain film layer 107 and the strip-shaped protrusions at the center position of the strain film layer 107 have opposite stress directions (as Figure 1 shown in. If the stress direction of the strain film layer 107 is upward, then the strip-shaped protrusion at the center is under tensile stress, and the strip-shaped protrusions at the edge are under compressive stress; conversely, the strip-shaped protrusion at the center is under compressive stress, and the strip-shaped protrusions at the edge are under tensile stress. Figure 2 Only one example is shown. The circle with a dot is perpendicular to the paper surface and outward, and the circle with a cross is perpendicular to the paper surface and inward), and the polarity of the resistance change is opposite. According to the characteristics of the Wheatstone bridge, there is a potential difference output at the two output ports of the Wheatstone bridge composed of each strip-shaped protrusion, and the output potential difference is proportional to the pressure received by the strain film layer 107, so as to detect the pressure value received by the strain film layer 107.

[0086] Among them, the support layer 101 can be a silicon layer, the insulating layer 102 can be a silicon dioxide layer, and the N-type device layer 103 can be a lightly doped N-type silicon layer. The lightly doped element can be boron. The P-type heavily doped layer 104 can be a P-type heavily doped silicon layer, where the heavily doped element can be boron.

[0087] It can be understood that along the first direction (i.e., Figure 2 the y direction in), the support layer 101, the insulating layer 102, and the N-type device layer 103 together form a substrate. As Figure 1 shown, the substrate can be rectangular. The four strip-shaped protrusions (the first strip-shaped protrusion 108, the second strip-shaped protrusion 109, the third strip-shaped protrusion 110, and the fourth strip-shaped protrusion 111) are arranged on the first plane. Among them, the resistance values of the four strip-shaped protrusions are equal, and the first plane can be the upper surface of the substrate. Since the four strip-shaped protrusions protrude from the substrate surface, after the strain film layer 107 is stressed and deformed, the induced stress is more concentrated on the P-type heavily doped layer on the surface of the piezoresistive part of the four strip-shaped protrusions, so that the sensitivity of the pressure sensor is higher and the linearity is better.

[0088] The higher the doping concentration on the surface of each strip-shaped protrusion, the lower the temperature coefficient of sensitivity of the pressure sensor, the smaller the temperature drift, and the higher doping concentration can achieve higher doping uniformity, thereby achieving better device consistency. Therefore, all four strip-shaped protrusions at least include a P-type heavily doped layer 104, achieving a lower temperature coefficient of sensitivity of the sensor and higher linearity. Thus, the P-type heavily doped layer 104 serves as a resistor in the Wheatstone bridge 105, making the MEMS pressure sensor less sensitive to temperature changes and less affected by ambient temperature changes. The low temperature coefficient of sensitivity and high linearity make the detection results more accurate and broaden the usage environment of the pressure sensor.

[0089] The PN junction formed between the P-type heavily doped layer 104 and the N-type device layer 103 is a parallel-plane junction, and the parasitic parameters between the protruding P-type heavily doped layer 104 and the N-type device layer 103 are smaller, which can be regarded as an ideal PN junction. This type of PN junction has a lower leakage current and a higher breakdown voltage. In the pressure sensor, the pressure sensor has better long-term reliability and a higher tolerated operating temperature, thereby broadening the usage environment of the pressure sensor.

[0090] According to an embodiment of the present invention, as Figure 2 shown, the first cavity 106 is located on the surface of the support layer 101 facing away from the insulating layer 102, and the first cavity 106 is an open cavity; furthermore, the MEMS pressure sensor with this structure can measure the open differential pressure.

[0091] Or, as Figure 3 shown, the first cavity 106 is located on the surface of the support layer 101 adjacent to the insulating layer 102. In this structure, the first cavity 106 is sealed by the insulating layer 102 and the support layer 101 to form a vacuum-sealed cavity, and the absolute pressure can be measured.

[0092] According to an embodiment of the present invention, as Figure 4 shown, along the first direction, each strip-shaped protrusion further includes: a P-type lightly doped layer 113, and the P-type lightly doped layer 113 is located between the N-type device layer 103 and the P-type heavily doped layer 104.

[0093] It can be understood that the P-type lightly doped layer 113 can be a P-type lightly doped silicon layer, and among them, the lightly doped element can be boron. Among them, on the first plane, a PN junction is formed between the P-type lightly doped layer 113, the P-type heavily doped layer 104 and the N-type device layer 103, and the junction depth of the PN junction including the P-type lightly doped layer 113 is deeper, so the breakdown voltage is higher.

[0094] According to an embodiment of the present invention, as Figure 5As shown, each strip-shaped protrusion includes a varistor portion 114 and two wire portions 115; both ends of the varistor portion 114 are respectively connected to a wire portion; wherein, along the reverse direction of the first direction, the line width of the varistor portion 114 is smaller than the line width of the wire portion 115.

[0095] It can be understood that the line width of the varistor portion 114 is much smaller than the line width of the wire portion 115, so that the resistance of the varistor portion 114 is much greater than the resistance of the wire portion 115, realizing the same-layer setting of the wire portion and the varistor portion.

[0096] According to an embodiment of the present invention, as Figure 5 shown, four strip-shaped protrusions are arranged in parallel, successively including a first strip-shaped protrusion 108, a second strip-shaped protrusion 109, a third strip-shaped protrusion 110 and a fourth strip-shaped protrusion 111; each strip-shaped protrusion includes a first wire portion 1151 and a second wire portion 1152, the first wire portion 1151 and the second wire portion 1152 are symmetrically arranged, and both the first wire portion 1151 and the second wire portion 1152 are offset by a preset distance relative to the symmetry axis for connecting the varistor portion 114.

[0097] The four strip-shaped protrusions are all symmetric with respect to the transverse axis of the substrate, so that the resistances on both sides of the Wheatstone bridge 105 are symmetrically distributed to achieve accurate detection results.

[0098] According to an embodiment of the present invention, as Figure 5 shown, the varistor portion 114 in the first strip-shaped protrusion 108 protrudes from the side of the first strip-shaped protrusion 108 away from the second strip-shaped protrusion 109; the varistor portion 114 in the second strip-shaped protrusion 109 protrudes from the side of the second strip-shaped protrusion 109 away from the first strip-shaped protrusion 108; the varistor portion 114 in the third strip-shaped protrusion 110 protrudes from the side of the third strip-shaped protrusion 110 away from the fourth strip-shaped protrusion 111; the varistor portion 114 in the fourth strip-shaped protrusion 111 protrudes from the side of the fourth strip-shaped protrusion 111 away from the third strip-shaped protrusion 110.

[0099] It can be understood that when the strain film layer 107 is deformed under pressure, the force on the surface of the strain film layer is mainly concentrated in the center and the middle of the edge of the strain film layer. Furthermore, the varistor portions 114 in the second strip-shaped protrusion 109 and the third strip-shaped protrusion 110 are arranged at the center as shown in Figure 5 , and the varistor portions 114 of the first strip-shaped protrusion 108 and the fourth strip-shaped protrusion 111 are arranged at the edge, so that the stress distribution is more concentrated on the varistor portion 114, which can improve the sensitivity of the pressure sensor.

[0100] According to an embodiment of the present invention, as Figure 6As shown in the figure, the varistor section 114 includes a varistor 1141; the arrangement shape of the varistors 1141 in each strip-shaped protrusion is one of a U shape, a V shape, or a cascade shape composed of multiple V shapes or U shapes, and the arrangement shape of the varistors 1141 is axially symmetrically distributed with the axis of symmetry as the central axis. It should be noted that the arrangement shape of the varistors 1141 in each strip-shaped protrusion can also be a straight line shape. The varistors 1141 in the first strip-shaped protrusion 108 and the fourth strip-shaped protrusion 111 cannot be in a straight line shape. If they are in a straight line shape and directly connect two wires, when the strain film layer 107 deforms, the stress direction at the varistor 1141 is along the axis of symmetry. If the varistor is arranged perpendicular to the axis of symmetry, there is no stress component in the axis of symmetry direction, and the deformation signal cannot be detected. Therefore, the varistors 1141 in the first strip-shaped protrusion 108 and the fourth strip-shaped protrusion 111 cannot be in a straight line shape and need to have a component in the direction along the axis of symmetry. The stress distribution of the varistors in the second strip-shaped protrusion 109 and the third strip-shaped protrusion 110 is in all directions. Therefore, the shape of the varistors in the second strip-shaped protrusion 109 and the third strip-shaped protrusion 110 can be a straight line shape.

[0101] In addition, the shape of the varistors 1141 in the first strip-shaped protrusion 108 and the fourth strip-shaped protrusion 111 is one of a U shape, a V shape, or a cascade shape composed of multiple V shapes or U shapes, and the varistors 1141 in the second strip-shaped protrusion 109 and the third strip-shaped protrusion 110 are one of a straight line shape, a U shape, a V shape, or a cascade shape composed of multiple V shapes or U shapes. The shapes of the varistors in the four strip-shaped protrusions can be freely matched and combined arbitrarily, as long as the resistance values of the resistors on both sides of the Wheatstone bridge 105 along the axis of symmetry are the same. Figure 6 This is just an example. That is, the shape of the varistors 1141 in the first strip-shaped protrusion 108 and the fourth strip-shaped protrusion 111 is a U shape, and the shape of the varistors 1141 in the second strip-shaped protrusion 109 and the third strip-shaped protrusion 110 is a straight line shape.

[0102] According to an embodiment of the present invention, as Figure 7 shown, the shapes of the varistors in each strip-shaped protrusion are the same.

[0103] In order to improve the detection accuracy of the pressure sensor, the resistance values of the four strip-shaped protrusions can be kept the same, that is to say, the shapes of the varistor sections of each strip-shaped protrusion are set the same. Figure 7 This is just an example. That is, the varistors 1141 in each strip-shaped protrusion are both cascades of 2 V shapes.

[0104] In other embodiments, the shape of the varistor portion 114 can also be wavy or zigzag. In this way, the resistance of the varistor 1141 in the varistor portion 114 can be increased, and being set as wavy or zigzag can further reduce the size of the pressure sensor.

[0105] According to an embodiment of the present invention, as Figure 5 shown, the varistor portion 114 includes a plurality of strip-shaped varistors and a third wire portion for connecting the plurality of strip-shaped varistors in series; the line width of the strip-shaped varistors is much smaller than the line width of the third wire portion; the plurality of strip-shaped varistors are all parallel to the symmetry axis and are symmetrically distributed with the symmetry axis as the central axis, and the plurality of third wire portions are all perpendicular to the symmetry axis.

[0106] Taking two strip-shaped varistors and one third wire portion as an example, as Figure 8 shown, the varistor portion 114 includes a first strip-shaped varistor 1142, a second strip-shaped varistor 1143, and a third wire portion 1144. The resistances of the first strip-shaped varistor 1142 and the second strip-shaped varistor 1143 are much larger than the resistance of the third wire portion 1144. The third wire portion 1144 can be regarded as a wire. In addition, the lengths of the first strip-shaped varistor 1142 and the second strip-shaped varistor 1143 along the symmetry axis direction can be adjusted according to actual needs. The greater the required resistance value, the longer the set length.

[0107] In other embodiments, a plurality of strip-shaped varistors can be provided and connected in series through the third wire portion.

[0108] Each strip-shaped varistor is parallel to the symmetry axis, and the third wire portions are all perpendicular to the symmetry axis, which is beneficial to adjusting the total resistance value of the entire strip-shaped protrusion through the length according to actual needs.

[0109] Next, the electrodes in the Wheatstone bridge 105 will be introduced.

[0110] According to an embodiment of the present invention, as Figure 9 shown, four strip-shaped protrusions are arranged in parallel, successively including a first strip-shaped protrusion 108, a second strip-shaped protrusion 109, a third strip-shaped protrusion 110, and a fourth strip-shaped protrusion 111; the plurality of electrodes 112 include: a first input electrode 1121, a second input electrode 1122, a first output electrode 1123, a second output electrode 1124, a first ground electrode 1125, and a second ground electrode 1126;

[0111] One end of the first strip-shaped protrusion 108 is connected to the first input electrode 1121, and the other end is connected to the first output electrode 1123; one end of the second strip-shaped protrusion 109 is connected to the first output electrode 1123, and the other end is connected to the first ground electrode 1125; one end of the third strip-shaped protrusion 110 is connected to the second input electrode 1122, and the other end is connected to the second output electrode 1124; one end of the fourth strip-shaped protrusion 111 is connected to the second output electrode 1124, and the other end is connected to the second ground electrode 1126.

[0112] Among them, the input electrical signals of the first input electrode 1121 and the second input electrode 1122 are the same and can be the same electrode. The first ground electrode 1125 and the second ground electrode 1126 can be the same electrode. As Figure 10 and Figure 11 shown, the four strip-shaped protrusions and each electrode form a Wheatstone bridge. Among them, the first input electrode 1121 and the second input electrode 1122 are Vcc electrodes, the first output electrode 1123 is the Vout+ electrode, the second output electrode 1124 is the Vout- electrode, and the first ground electrode 1125 and the second ground electrode 1126 are GND electrodes.

[0113] According to an embodiment of the present invention, the first input electrode 1121, the second input electrode 1122, the first output electrode 1123, the second output electrode 1124, the first ground electrode 1125, and the second ground electrode 1126 are all metal electrodes.

[0114] Among them, the material of the metal electrode can be Cu or Pt or Au, etc.

[0115] According to an embodiment of the present invention, the first input electrode 1121, the second input electrode 1122, the first output electrode 1123, the second output electrode 1124, the first ground electrode 1125, and the second ground electrode 1126 are all arranged on the same layer as the first strip-shaped protrusion 108, the second strip-shaped protrusion 109, the third strip-shaped protrusion 110, and the fourth strip-shaped protrusion 111.

[0116] According to an embodiment of the present invention, as Figure 12 shown, the four strip-shaped protrusion regions, the first input electrode 1121 region, the first output electrode 1122 region, the second output electrode 1123 region, the second input electrode 1124 region, the first ground electrode 1125 region, and the second ground electrode 1126 region arranged on the first plane are separated from each other by trenches 116. It can be seen that Figure 10 the regions surrounded by the solid lines in are all trenches. Since the P-type heavily doped layer 104 has good conductivity, the P-type heavily doped layer 104 can be directly used as an electrode to increase the material utilization rate.

[0117] According to an embodiment of the present invention, along the opposite direction of the first direction, the bottom surface of the trench 116 at least contacts the surface of the N-type device layer 103 that is not P-type doped and faces away from the insulating layer 102.

[0118] That is to say, the depth of the trench 116 is at least the same as the height of each strip-shaped protrusion. It should be noted that the depth of the trench 116 needs to be greater than the overall thickness of the doping layer. Thus, each strip-shaped protrusion can be separated.

[0119] According to an embodiment of the present invention, the support layer 101 can be a silicon layer, the insulating layer 102 can be a silicon dioxide layer, the N-type device layer 103 can be a lightly doped N-type silicon layer, the P-type heavily doped layer 104 can be a P-type heavily doped silicon layer, and the P-type lightly doped layer 113 can be a P-type lightly doped silicon layer. Among them, the setting of the insulating layer 102 can provide electrical insulation for the support layer 101. The support layer 101, the insulating layer 102, and the N-type device layer 103 can form an SOI substrate.

[0120] Figure 13 is a flowchart of a manufacturing method of a MEMS pressure sensor proposed by an embodiment of the present invention. This method is applied to the MEMS pressure sensor as described above. As Figure 13 shown, this method includes the following steps:

[0121] S101, provide a substrate, where the substrate includes a support layer 101, an insulating layer 102, and an N-type device layer 103; the substrate can be an SOI substrate.

[0122] S102, perform P-type heavy doping on the entire surface of the side of the N-type device layer 103 facing away from the insulating layer 102 to form a P-type heavily doped layer 104 (without a photomask); it should be noted that the formation of the P-type heavily doped layer 104 can be achieved by diffusion doping, ion implantation, or other doping methods well-known to those skilled in the art, and the present invention does not make specific limitations in this regard.

[0123] Among them, the doping element of the P-type heavily doped layer 104 can be boron element. It is beneficial to reduce the sensitivity temperature coefficient on the surface of the pressure sensor, reduce the temperature drift, and improve the detection accuracy of the pressure sensor.

[0124] S103, use an etching process to form four strip-shaped protrusions, where the side walls of the four strip-shaped protrusions at least expose the P-type heavily doped layer 104; among them, the etching process can be dry etching or wet etching. The formed four strip-shaped protrusions protrude from the surface of the N-type device layer 103, which is beneficial to the stress concentration on the four strip-shaped protrusions and improves the sensitivity of the pressure sensor. By etching, the line width of the four strip-shaped protrusions can be controlled, so that the piezoresistors in the four strip-shaped protrusion parts can be made very thin, increasing the resistance value of each strip-shaped protrusion, reducing the size of the pressure sensor, and being beneficial to cost reduction.

[0125] S104, form multiple electrodes, each electrode is used for each strip protrusion to access and output electrical signals; each electrode and each strip protrusion form a Wheatstone bridge 105; the Wheatstone bridge 105 can realize the conversion between force and electrical signals, thereby realizing the function of the pressure sensor.

[0126] S105, etch on the surface of the support layer 101 away from the insulating layer 102 to form a first cavity 106, and a strain film layer 107 is formed between the first cavity 106 and the Wheatstone bridge 105. The thickness of the strain film layer 107 is related to the size of the pressure sensor and the magnitude of the measured pressure. For example, if the pressure to be detected is relatively large, then the strain film layer 107 can be thicker; conversely, if the pressure to be detected is relatively small, then the strain film layer 107 can be thinner. The depth of the first cavity 106 is etched through the support layer 101 at most, and the preferred depth range is 20 microns - 800 microns. The side walls of the four strip protrusions expose at least the P-type heavily doped layer 104 and can be etched through the N-type device layer 103 at most. The specific values are set according to actual requirements. Furthermore, under the condition of the same measurement range, the chip area can be further reduced and the cost can be lowered, or a thicker strain film layer can be used under the condition of the same chip area to improve the linearity of the pressure sensor.

[0127] According to another embodiment of the present invention, the method includes the following steps:

[0128] Provide a substrate as the support layer 101;

[0129] Etch on one surface of the support layer 101 to form a first cavity 106;

[0130] Provide an N-type single crystal silicon wafer as the N-type device layer 103, and form an insulating layer 102 on one surface of the device layer 103;

[0131] The outer surface of the support layer 101 with the first cavity 106 etched is bonded to the surface of the insulating layer 102 away from the device layer 103 in a vacuum environment, so that the first cavity 106 is a vacuum-sealed cavity;

[0132] Thin the device layer 103;

[0133] Perform P-type heavy doping on the entire surface of the side of the N-type device layer 103 away from the insulating layer 102 to form a P-type heavily doped layer 104;

[0134] Use an etching process to form four strip protrusions, wherein the side walls of the four strip protrusions expose at least the P-type heavily doped layer 104;

[0135] Form multiple electrodes, each electrode is used for each strip protrusion to access and output electrical signals; each electrode and each strip protrusion form a Wheatstone bridge 105;

[0136] A strain film layer 107 is formed between the first cavity 106 and the Wheatstone bridge 105.

[0137] The difference between this embodiment and the previous one is that before manufacturing the Wheatstone bridge 105, the first cavity 106 is manufactured on one surface of the support layer 101 adjacent to the insulating layer 102. After manufacturing the first cavity 106, the support layer 101 and the insulating layer 102 are bonded and connected, an N-type device layer 103 is formed on the insulating layer 102, and then the Wheatstone bridge 105 is formed on the N-type device layer 103. This structure can measure the absolute pressure.

[0138] According to an embodiment of the present invention, as Figure 14 shown, before performing P-type heavy doping on the entire surface of the side of the N-type device layer facing away from the insulating layer in S102 to form a P-type heavy doping layer, it further includes:

[0139] S1011, performing P-type light doping on the entire surface of the side of the N-type device layer 103 facing away from the insulating layer 102 to form a P-type light doping layer 113 (without a photomask);

[0140] After performing P-type heavy doping on the entire surface of the side of the N-type device layer 103 facing away from the insulating layer 102 in S102 to form a P-type heavy doping layer 104, it further includes:

[0141] S1031, using an etching process to form four strip-shaped protrusions, wherein the side walls of the four strip-shaped protrusions expose at least the P-type light doping layer 113 and the P-type heavy doping layer 104.

[0142] The P-type light doping layer 113 can be a P-type lightly doped silicon layer, and the light doping element can be boron. Among them, on the first plane, a PN junction is formed between the P-type light doping layer 113, the P-type heavy doping layer 104, and the N-type device layer 103. The junction depth of the PN junction including the P-type light doping layer 113 is deeper, which is a bulk breakdown, and the breakdown voltage is higher, so that the MEMS pressure sensor can work in a higher temperature (for example, an ambient temperature of 175 degrees). In addition, the setting of the P-type heavy doping layer 104 makes the temperature drift smaller, thereby making the sensor calibration accuracy higher and the test calibration cost lower.

[0143] According to an embodiment of the present invention, as Figure 15 shown, S104 forming multiple electrodes includes:

[0144] S1041, forming multiple electrodes by electroplating or sputtering.

[0145] According to an embodiment of the present invention, as Figure 16 shown, S104 forming multiple electrodes includes:

[0146] S1042, form multiple electrodes by etching, and the sidewalls of the trenches between the electrodes are exposed at least to the surface of the N-type device layer 103 that has not undergone P-type doping and faces away from the insulator 102.

[0147] Among them, in this embodiment, forming multiple electrodes and forming four strip-shaped protrusions are completed in the same step, thereby saving process steps and simplifying the process flow. Among them, the structure of the four strip-shaped protrusions is the same as that in the structural embodiment.

[0148] Specifically, Figures 17 to 21 is a flowchart of a method for manufacturing a MEMS pressure sensor according to an embodiment of the present invention. Figure 17 、 Figure 18 、 Figure 19 、 Figure 22 、 Figure 23 is a flowchart of a method for manufacturing a MEMS pressure sensor according to another embodiment of the present invention. Figures 24 to 26 is a flowchart of a method for manufacturing a MEMS pressure sensor according to still another embodiment of the present invention. In this embodiment, the steps that are the same as those in the previous two embodiments are not shown. Please refer to the previous two embodiments. In each of the foregoing examples, only one photomask is used, which is very easy to achieve a smaller chip area, has a low process cost, and can be mass-produced.

[0149] In summary, according to the MEMS pressure sensor and its manufacturing method proposed by the embodiments of the present invention, the MEMS pressure sensor includes a support layer, an insulating layer, an N-type device layer, and a P-type heavily doped layer that are sequentially stacked along a first direction; wherein, the first direction is the direction in which the support layer vertically points to the P-type heavily doped layer; four strip-shaped protrusions arranged on a first plane, along the first direction, each strip-shaped protrusion includes a P-type heavily doped layer, and the first plane is a plane perpendicular to the first direction; multiple electrodes, each electrode is used for each strip-shaped protrusion to access and output an electrical signal; each electrode and each strip-shaped protrusion form a Wheatstone bridge; the support layer is provided with a first cavity, and a strain thin film layer is provided between the first cavity and the Wheatstone bridge. Thus, by setting four strip-shaped protrusions, when the strain thin film layer is subjected to pressure, for the edge middle and center positions of the strain thin film layer, the surface stress is the largest, and the stress is more concentrated on the surface of the strip-shaped piezoresistor, with high sensitivity and good linearity. In addition, the parasitic parameters between the P-type heavily doped layer and the substrate are smaller, and it is closer to an ideal PN junction with the substrate. Therefore, it has a higher breakdown voltage and lower leakage current. In addition, a P-type lightly doped layer is introduced between the P-type heavily doped layer and the N-type device layer, deepening the junction depth of the PN junction, and further improving the breakdown voltage, and thus having higher reliability and long-term stability. Since the surface doping concentration of the strip-shaped protrusion is relatively high, the device can achieve a lower sensitivity temperature drift. The MEMS pressure sensor has a simple structure, and the preparation process of the piezoresistor strip only requires 1 photomask, with a low process cost and can be mass-produced.

[0150] Note that the above is only a preferred embodiment of the present invention and the technical principles applied. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein. Various obvious changes, re-adjustments, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the concept of the present invention, it can also include more other equivalent embodiments, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A MEMS pressure sensor, characterized in that, include: A support layer, an insulating layer, an N-type device layer, and a P-type densely doped layer are sequentially stacked along a first direction; wherein the first direction is a direction in which the support layer is perpendicular to the P-type densely doped layer; Four strip-shaped protrusions are arranged on a first plane, and along the first direction, each of the strip-shaped protrusions includes at least the P-type densely doped layer, and the first plane is a plane perpendicular to the first direction; each of the strip-shaped protrusions includes a varistor portion and two wire portions; the four strip-shaped protrusions are arranged in parallel, and sequentially include a first strip-shaped protrusion, a second strip-shaped protrusion, a third strip-shaped protrusion, and a fourth strip-shaped protrusion; each of the strip-shaped protrusions includes a first wire portion and a second wire portion, the first wire portion and the second wire portion are symmetrically arranged, and the first wire portion and the second wire portion are both offset by a preset distance relative to the symmetry axis, and are used to connect the varistor portion; A plurality of electrodes, each of the electrodes is used for each of the strip-shaped protrusions to receive input electrical signals and output electrical signals; each of the electrodes and each of the strip-shaped protrusions constitutes a Wheatstone bridge; The support layer is provided with a first cavity, and a strained film layer is provided between the first cavity and the Wheatstone bridge.

2. The MEMS pressure sensor according to claim 1, wherein The first cavity is located on a surface of the support layer that is away from the insulating layer, and the first cavity is an open cavity; Alternatively, the first cavity is located on a surface of the support layer adjacent to the insulating layer, and the first cavity is a vacuum sealed cavity.

3. The MEMS pressure sensor according to claim 1, wherein Along the first direction, each of the strip-shaped protrusions further includes: a P-type lightly doped layer, wherein the P-type lightly doped layer is located between the N-type device layer and the P-type heavily doped layer.

4. The MEMS pressure sensor according to claim 1 or 3, characterized in that, The four strip-shaped protrusions are arranged in parallel, and include a first strip-shaped protrusion, a second strip-shaped protrusion, a third strip-shaped protrusion and a fourth strip-shaped protrusion in sequence; The plurality of electrodes include: a first input electrode, a second input electrode, a first output electrode, a second output electrode, a first ground electrode, and a second ground electrode; One end of the first strip-shaped protrusion is connected to the first input electrode, and the other end is connected to the first output electrode; one end of the second strip-shaped protrusion is connected to the first output electrode, and the other end is connected to the first ground electrode; one end of the third strip-shaped protrusion is connected to the second input electrode, and the other end is connected to the second output electrode; one end of the fourth strip-shaped protrusion is connected to the second output electrode, and the other end is connected to the second ground electrode.

5. The MEMS pressure sensor according to claim 4, characterized in that, The first input electrode, the second input electrode, the first output electrode, the second output electrode, the first ground electrode and the second ground electrode are all metal electrodes.

6. The MEMS pressure sensor according to claim 4, wherein The first input electrode, the second input electrode, the first output electrode, the second output electrode, the first ground electrode and the second ground electrode are all arranged in the same layer as the first strip-shaped protrusions, the second strip-shaped protrusions, the third strip-shaped protrusions and the fourth strip-shaped protrusions.

7. The MEMS pressure sensor according to claim 6, characterized in that, Four strip-shaped convex regions, a first input electrode region, a first output electrode region, a second output electrode region, a second input electrode region, a first ground electrode region, and a second ground electrode region arranged on the first plane, and trenches are used to isolate between the electrode regions.

8. The MEMS pressure sensor according to claim 7, characterized in that, In the opposite direction of the first direction, the bottom surface of the trench at least contacts one side surface of the N-type device layer facing away from the insulating layer.

9. The MEMS pressure sensor according to claim 3, wherein, The support layer is a silicon layer, the insulating layer is a silicon dioxide layer, the N-type device layer is a lightly doped N-type silicon layer, the P-type heavily doped layer is a P-type heavily doped silicon layer, and the P-type lightly doped layer is a P-type lightly doped silicon layer.

10. A manufacturing method of a MEMS pressure sensor, characterized in that, Applied to the MEMS pressure sensor according to any one of claims 1-9, including the following steps: Provide a substrate, wherein the substrate includes a support layer, an insulating layer, and an N-type device layer; Perform P-type heavy doping on the entire surface of the side surface of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer; Use an etching process to form four strip-shaped protrusions, wherein the side walls of the four strip-shaped protrusions at least expose the P-type heavily doped layer; Form a plurality of electrodes, each of the electrodes is used for accessing and outputting electrical signals for each of the strip-shaped protrusions; each of the electrodes and each of the strip-shaped protrusions form a Wheatstone bridge; Etch and form a first cavity on the side surface of the support layer facing away from the insulating layer, so that the first cavity is an open cavity, and a strain thin film layer is formed between the first cavity and the Wheatstone bridge.

11. A manufacturing method of a MEMS pressure sensor, characterized in that, Applied to the MEMS pressure sensor according to any one of claims 1-9, including the following steps: Provide a substrate as a support layer; Etch and form a first cavity on one side surface of the support layer; Provide an N-type device layer, and form an insulating layer on one side surface of the N-type device layer; The side surface of the support layer etched with the first cavity is bonded and connected to the side surface of the insulating layer away from the N-type device layer in a vacuum environment, so that the first cavity is a vacuum-sealed cavity; Thin the N-type device layer; Perform P-type heavy doping on the entire surface of the side surface of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer; Use an etching process to form four strip-shaped protrusions, wherein the side walls of the four strip-shaped protrusions at least expose the P-type heavily doped layer; Form a plurality of electrodes, each of the electrodes is used for accessing and outputting electrical signals for each of the strip-shaped protrusions; each of the electrodes and each of the strip-shaped protrusions form a Wheatstone bridge; A strain thin film layer is formed between the first cavity and the Wheatstone bridge.

12. The manufacturing method of the MEMS pressure sensor according to claim 10 or 11, characterized in that, Before performing P-type heavy doping on the entire surface of the side surface of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer, it further includes: Perform P-type light doping on the entire surface of the side surface of the N-type device layer facing away from the insulating layer to form a P-type lightly doped layer; After performing P-type heavy doping on the entire surface of the side surface of the N-type device layer facing away from the insulating layer to form a P-type heavily doped layer, it further includes: Use an etching process to form four strip-shaped protrusions, wherein the side walls of the four strip-shaped protrusions at least expose the P-type lightly doped layer and the P-type heavily doped layer.

13. The manufacturing method of the MEMS pressure sensor according to claim 10 or 11, characterized in that, Forming a plurality of electrodes includes: Form a plurality of electrodes by electroplating or sputtering.

14. The manufacturing method of the MEMS pressure sensor according to claim 10 or 11, characterized in that, Forming a plurality of electrodes includes: A plurality of electrodes are formed by an etching method, and the side walls of the trenches between the electrodes are exposed at least to the surface of the undoped N-type device layer facing away from the insulating layer.

15. The manufacturing method of the MEMS pressure sensor according to claim 14, characterized in that, Forming a plurality of electrodes and forming four strip-shaped protrusions are completed in one step.

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