A three-axis accelerometer chip with differential mode cancellation of common mode superposition and a manufacturing method thereof

Abstract

A three-axis accelerometer chip for canceling differential mode and superimposing common mode and a manufacturing method thereof belong to the field of three-axis accelerometer decoupling test of micro electro mechanical system. The chip integrates four measuring units, i.e. a first z-axis sensitive unit, a second z-axis sensitive unit, an x-axis sensitive unit and a y-axis sensitive unit, to realize detection of impact acceleration vectors of three orthogonal axes, i.e. x-axis, y-axis and z-axis. The sensitive units are isolated from each other by an outer frame of the MEMS chip. The z-axis sensitive unit abandons the traditional full diaphragm structure, forms a variable cross-section beam by hollowing out a stress concentration area, and symmetrically arranges two z-axis sensitive units along the diagonal line of the square chip, so that when the two z-axis sensitive units are subjected to lateral inertial acceleration, they generate equal and opposite symmetric mechanical responses, the lateral impact acceleration signal is converted into a differential mode signal, and the z-axis impact acceleration signal remains as a common mode signal. By superimposing the outputs of the two z-axis sensitive units, the lateral sensitivity of the z-axis sensitive unit is canceled, and the z-axis impact sensitivity is doubled.

Description

Technical Field

[0001] This invention belongs to the field of microelectromechanical systems (MEMS) triaxial accelerometer decoupling testing technology, and relates to a low lateral coupling, high sensitivity, and strong impact triaxial MEMS accelerometer chip with differential mode cancellation and common mode superposition, and its fabrication method. Background Technology

[0002] As a core sensing element for capturing transient impact signals in extreme environments, the high-impact triaxial MEMS accelerometer is widely used in extreme engineering information acquisition scenarios such as precision strikes against hard targets, aerospace, and weapon engineering. It is a key sensing element for capturing transient high-impact signals during ultra-high-speed collisions, deep penetrations, and explosive impacts. Its measurement accuracy directly determines the accuracy of dynamic parameter monitoring in extreme environments. It is also the core foundation for precision-guided weapons and high-overload inertial navigation systems to achieve precise control and effectiveness. It plays an irreplaceable and important role in the fields of national defense, military industry, and high-end equipment manufacturing.

[0003] Among the many design challenges of high-impact triaxial MEMS accelerometers, controlling the sensor's lateral sensitivity is a crucial factor affecting its measurement accuracy and a vital prerequisite for accurately capturing triaxial vector signals. In practical operation, triaxial accelerometers inevitably experience lateral impact interference in the off-axis direction. If the lateral sensitivity is too high, the lateral impact will be converted into parasitic electrical signals superimposed on the axial measurement signals, causing crosstalk between the three axis measurement signals and significantly reducing the measurement accuracy of the vector signals. This fails to meet the high-precision detection requirements of acceleration signals in extreme scenarios. Especially in ultra-high impact environments of 100,000g, the parasitic stress generated by lateral impacts will be very significant. If the lateral effect cannot be effectively suppressed, the sensor will be unable to accurately capture the true impact signal. Therefore, achieving extremely low lateral sensitivity and effectively isolating signal coupling between the three axes is a key technical bottleneck that must be overcome in the design of high-g triaxial MEMS accelerometers. This is of paramount importance for improving the overall measurement performance of the sensor and adapting to the application requirements of extreme impact scenarios. Summary of the Invention

[0004] To address the common problems of large lateral coupling and low measurement accuracy in existing high-impact triaxial MEMS accelerometers, this invention provides a triaxial accelerometer chip with differential-mode cancellation and common-mode superposition, and its fabrication method. By utilizing the differential-mode response characteristics of two diagonally distributed z-axis sensitive units to lateral impact signals, the lateral sensitivity of the z-axis is approximately zero, while the z-axis impact sensitivity is multiplied. At the same time, the low lateral coupling characteristics of the x / y axes are maintained, and the chip also features high linearity, high natural frequency, and high fabrication feasibility, thus meeting the high-precision measurement requirements under extreme impact environments.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] This invention discloses a triaxial accelerometer chip with differential mode cancellation and common mode superposition. The MEMS chip is square in shape and is divided into four equal parts. Z-axis sensing unit 1 and Z-axis sensing unit 2 are distributed along one diagonal of the square chip, and the two Z-axis sensing units are centrally symmetrical about the chip. X-axis sensing unit and Y-axis sensing unit are integrated on the other diagonal of the chip. Each sensing unit is physically isolated by the MEMS chip frame.

[0007] The z-axis sensing unit includes a first z-axis sensing unit and a second z-axis sensing unit. Its measurement axis is orthogonal to the xoy plane containing the square surface of the chip, and the measurement axis originates from one side of the pad and points outwards from the chip. The mass block of the first z-axis sensing unit is connected to the chip frame via a variable cross-section beam. Four piezoresistive and ohmic contact areas are etched at the root of the connection between the variable cross-section beam and the chip frame. These piezoresistive and ohmic contact areas are electrically connected via metal interconnects to form a Wheatstone bridge, which, together with the pads, forms the detection circuit for the first z-axis sensing unit. The first and second z-axis sensing units have identical structures and are symmetrical about the chip center. The input voltage and ground terminal of the detection circuit for the second z-axis sensing unit are exactly opposite to the input voltage and ground terminal of the detection circuit for the first z-axis sensing unit, ensuring that the triaxial accelerometer chip satisfies the decoupling condition of differential-mode cancellation and common-mode superposition.

[0008] The differential-mode cancellation and common-mode superposition mechanism refers to the ability of z-axis sensing unit 1 and z-axis sensing unit 2 to convert lateral impact into differential-mode signals and retain axial impact as common-mode signals. Therefore, by superimposing the differential outputs of z-axis sensing unit 1 and z-axis sensing unit 2, differential-mode cancellation of lateral signals can be achieved, enabling the chip to obtain extremely low lateral sensitivity. At the same time, when the chip is subjected to axial force along the z-axis, the axial impact responses generated by the two z-axis sensing units are the same, enabling common-mode superposition of axial signals and improving the chip's z-axis detection sensitivity and measurement accuracy.

[0009] The x-axis sensing unit has the same structure as the y-axis sensing unit, and their measurement axes are orthogonal and perpendicular to each other, forming an orthogonal coordinate system together with the measurement axis of the z-axis sensing unit. The x-axis sensing unit includes a central support beam, with separate x-axis sensing unit mass blocks arranged on both sides of the central support beam. The x-axis sensing unit mass blocks are connected to the chip frame via a sensing microbeam. At the root of the connection between the sensing microbeam and the chip frame, a piezoresistive resistor and an ohmic contact area are etched. The piezoresistive resistor and the ohmic contact area are electrically connected via metal interconnects to form a Wheatstone bridge, and the Wheatstone bridge, together with the pads, forms the x-axis sensing unit detection circuit.

[0010] The x-axis and y-axis sensitive units employ a central support beam structure. This separates the four sensitive microbeams housing the piezoresistive resistors from the central support beam to suppress lateral coupling and reduce the risk of structural failure. The central support beam is a beam that runs through the sensitive unit along the y-axis, with a thickness in the z-direction greater than that in the x-axis sensitive direction, exhibiting an overall configuration that is larger in the z-direction and smaller in the x-axis sensitive direction. The central support beam has a larger moment of inertia in the z-axis and a smaller moment of inertia in the sensitive axis, resulting in a smaller response to impact signals in non-sensitive axes and a larger response to impact signals in the sensitive axis. This allows it to initially suppress lateral coupling and simultaneously control the displacement of the sensitive structure to remain within the chip plane, reducing the risk of structural damage after a large off-axis impact.

[0011] The above-mentioned method for manufacturing a triaxial accelerometer chip with differential mode cancellation and common mode superposition includes the following steps:

[0012] Step 1: Perform double-sided thermal oxidation on the silicon wafer to form a thermally oxidized silicon dioxide layer on both the upper and lower surfaces of the silicon wafer.

[0013] Step two involves using a piezoresistive photomask. After spin-coating photoresist, the photoresist is etched using standard photolithography to pattern the piezoresistive resistor. Then, reactive ion etching is used to remove the oxide layer on the silicon wafer surface. Boron ion implantation is then used to form the piezoresistive resistor. Finally, the photoresist and oxide layer are removed. Next, a layer of silicon dioxide is deposited. Using an ohmic contact photomask, after spin-coating photoresist, the photoresist is etched using standard photolithography to pattern the ohmic contact area. Reactive ion etching is then used to remove the oxide layer on the silicon wafer surface. Boron ion implantation is then used to form the ohmic contact. Finally, the photoresist and oxide layer are removed.

[0014] Step 3: Using a motion gap mask, photoresist is spin-coated onto the back of the silicon wafer, and then etched using standard photolithography to pattern the motion gap. The oxide layer on the silicon wafer surface is then removed using reactive ion etching (RIE), and deep reactive ion etching (DRIE) is used to etch downwards to form the motion gap. Finally, the photoresist and oxide layer are removed.

[0015] Step four: Deposit a layer of silicon dioxide on the back side of the silicon wafer and spin-coat a layer of photoresist. Using a mass block mask on the back side of the silicon wafer, etch the photoresist using standard photolithography to pattern the mass block. Then, remove the oxide layer on the surface of the silicon wafer using reactive ion etching (RIE). Deep reactive ion etching (DRIE) is then used to etch downwards to form the mass block structure. Finally, remove the photoresist and oxide layer.

[0016] Step 5: Deposit a layer of silicon dioxide on the back side of the silicon wafer and spin-coat a layer of photoresist. Using a back-through mask, etch the photoresist on the back side of the silicon wafer using standard photolithography to pattern it. Then, remove the oxide layer on the silicon wafer surface using reactive ion etching. Deep reactive ion etching is then used to etch downwards to release the sensitive microbeam structure and variable cross-section beam structure. Subsequently, remove the photoresist and oxide layer.

[0017] Step six: Deposit a layer of silicon nitride on the front side of the silicon wafer and spin-coat a layer of photoresist. Using a leadhole plate, etch the photoresist on the front side of the silicon wafer using standard photolithography to pattern the leadholes. Then, remove the silicon nitride at the leadhole locations on the silicon wafer surface using reactive ion etching to form the leadholes. Finally, remove the photoresist.

[0018] Step 7: Deposit a layer of photoresist on the front side of the silicon wafer. Using a metal lead mask and a pad mask, etch the photoresist on the front side of the silicon wafer using standard photolithography to pattern the leads and pads. Then, deposit a layer of aluminum using sputtering. Subsequently, remove the photoresist and excess aluminum using a stripping process to form the leads and pads. The fabrication of the differential-mode canceling and common-mode superposition triaxial accelerometer chip is now complete.

[0019] Beneficial effects:

[0020] 1. This invention discloses a triaxial accelerometer chip with differential-mode cancellation and common-mode superposition, and its fabrication method. The chip includes one x-axis sensitive unit, one y-axis sensitive unit, and two z-axis sensitive units, enabling the measurement of acceleration components in three orthogonal directions. The z-axis measurement unit includes a first z-axis sensitive unit and a second z-axis sensitive unit, distributed along the diagonal of a square silicon wafer. The symmetrically arranged first and second z-axis sensitive units, when facing a lateral impact, will produce a symmetrical mechanical response under the structural influence of their respective sides along the lateral impact direction. This allows them to convert the lateral impact signal into a differential-mode signal, while the z-axis impact signal remains a common-mode signal. By superimposing the outputs of the first and second z-axis sensitive units, the lateral sensitivity of the z-axis sensitive unit can be canceled, achieving decoupling, and the axial sensitivity of the z-axis sensitive unit can be multiplied. It can achieve triaxial acceleration measurement with a z-axis range of 100,000 g and x- and y-axis ranges of 50,000 g. The natural frequency of each sensitive unit is greater than 1 MHz, ensuring high operating bandwidth and high overload resistance. At an input voltage of 5V, the decoupled z-axis sensitivity of this sensor is 0.72μv / g, and the decoupled lateral sensitivity is close to 0. Under the same input conditions, its x-axis and y-axis sensitivities are 1.03μv / g for a 50,000g range. Adjusting the chip parameters can achieve larger range triaxial acceleration measurements. This invention optimizes the problem of excessively high lateral sensitivity in traditional high-impact acceleration measurements by configuring the mechanical response in the chip structure, significantly improving the accuracy of impact measurements.

[0021] 2. The present invention discloses a triaxial accelerometer chip with differential mode cancellation and common mode superposition and its manufacturing method. The z-axis sensitive unit adopts an improved variable cross-section beam form to remove the stress concentration area of ​​the traditional full diaphragm and avoid structural failure under ultra-high impact.

[0022] 3. The present invention discloses a triaxial accelerometer chip with differential mode cancellation and common mode superposition and its manufacturing method. The x-axis and y-axis sensitive units adopt a structure of central support beam combined with sensitive microbeam, which separates the support beam, mass block and sensitive microbeam, effectively suppresses the lateral coupling in the x-axis and y-axis, increases its stiffness, significantly increases its natural frequency and achieves higher measurement bandwidth.

[0023] 4. The present invention discloses a triaxial accelerometer chip with differential mode cancellation and common mode superposition and its manufacturing method. The MEMS chip of the present invention realizes the lateral coupling cancellation and suppression of the z-axis sensitive unit, the lateral sensitivity is close to zero, which is almost perfectly eliminated, while ensuring high principal axis sensitivity and high natural frequency, which meets the requirements of accurately acquiring transient impact acceleration under extreme impact environment, thus greatly improving the measurement accuracy.

[0024] 5. The present invention discloses a method for fabricating a triaxial accelerometer chip with differential mode cancellation and common mode superposition. Based on the structural characteristics of the sensor chip, methods such as reactive ion etching, plasma chemical vapor deposition, and deep reactive ion etching are introduced to simplify the process flow and reduce the processing cost while ensuring the shape accuracy of the sensor chip. Attached Figure Description

[0025] Figure 1 This is a front view of the overall structure of the triaxial accelerometer chip with differential mode cancellation and common mode superposition according to the present invention.

[0026] Figure 2 This is a rear view of the overall structure of the present invention.

[0027] Figure 3 This is a circuit diagram of the detection circuit of the present invention.

[0028] Figure 4 for Figure 1 A magnified view of a portion of the image.

[0029] Figure 5 This is a schematic diagram of the deformation of each sensitive unit under inertial acceleration according to the present invention.

[0030] Figure 6 This is a schematic diagram illustrating the mechanical modeling principle of the z-axis sensitive unit of the present invention when subjected to lateral impact.

[0031] Figure 7 This is a schematic diagram of the MEMS chip fabrication process of the present invention.

[0032] Wherein: 1-Chip border; 2-Sensitive unit; 3-Pad; 4-Interconnect; 5-Piercing resistor; 6-Ohmic contact area; 7-Silicon wafer; 8-Silicon dioxide; 9-Silicon nitride; 2-1-Z-axis sensitive unit 1; 2-2-Z-axis sensitive unit 2; 2-3-X-axis sensitive unit; 2-4-Y-axis sensitive unit; 2-1-1-Z-axis sensitive unit 1 variable cross-section beam; 2-1-2-Z-axis sensitive unit 1 mass block; 2-2-1-Z-axis sensitive unit 2... Sensitive element variable cross-section beam; 2-2-2-z axis second sensitive element mass block; 2-3-1-x axis sensitive element central support beam; 2-3-2-x axis sensitive element mass block; 2-3-3-x axis sensitive element sensitive microbeam; 2-3-4-x axis sensitive element hinged beam; 2-4-1-y axis sensitive element central support beam; 2-4-2-y axis sensitive element mass block; 2-4-3-y axis sensitive element sensitive microbeam; 2-4-4-y axis sensitive element hinged beam. Detailed Implementation

[0033] To illustrate the technical problems solved by the present invention and its beneficial effects, the invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0034] Obviously, the embodiments described in this invention are only a part of the embodiments of the invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of this invention.

[0035] like Figure 1 and Figure 2 As shown, this embodiment discloses a triaxial accelerometer chip with differential mode cancellation and common mode superposition. Two z-axis sensing units, namely z-axis sensing unit 2-1 and z-axis sensing unit 2-2, are fixedly arranged along the diagonal of a square chip, along with x-axis sensing units 2-3 and y-axis sensing units 2-4. The four sensing units are isolated by a chip frame 1. The measurement axes of z-axis sensing units 2-1 and 2-2, and the measurement axis of y-axis sensing unit 2-4, are orthogonal to the measurement axis of x-axis sensing unit 2-3. Preferably, the z-axis sensing units improve upon the common full-diaphragm structure by hollowing out the stress concentration area, forming variable cross-section beams 2-1-1 and 2-2-1 for z-axis sensing units, thus reducing the risk of MEMS chip structure failure under strong impact.

[0036] like Figure 1 and Figure 2As shown, the z-axis sensing unit 2-1 includes a z-axis sensing unit mass block 2-1-2, which is connected to the chip frame 1 via the z-axis sensing unit variable cross-section beam 2-1-1. Figure 3 As shown, on the front of the z-axis sensing unit 2-1, four piezoresistive resistors 5 and ohmic contact areas 6 are etched at the root of the connection between the variable cross-section beam 2-1-1 of the z-axis sensing unit and the chip frame 1. They are connected by interconnecting lines 4 to form a Wheatstone bridge and, together with the pads 3, form the detection circuit of the z-axis sensing unit.

[0037] The second z-axis sensing unit 2-2 includes a second z-axis sensing unit mass block 2-2-2, which is connected to the chip frame 1 via a variable cross-section beam 2-2-1. For example... Figure 3 As shown, on the front of the second sensitive unit 2-1 of the z-axis, four piezoresistive resistors 5 and ohmic contact areas 6 are etched at the root of the connection between the variable cross-section beam 2-2-1 of the second sensitive unit of the z-axis and the chip frame 1. They are connected by interconnect line 4 to form a Wheatstone bridge and, together with pad 3, form the detection circuit of the second sensitive unit of the z-axis. The input voltage of the detection circuit of the first sensitive unit of the z-axis is exactly opposite to that of the ground terminal.

[0038] The x-axis sensing unit 2-3 includes a central support beam 2-3-1. Four separate x-axis sensing unit mass blocks 2-3-2 are arranged on both sides of the central support beam 2-3-1, connected to it by a hinge beam 2-3-4, and connected to the chip frame 1 through four sensing microbeams 2-3-3. Figure 3 and Figure 4 As shown, on the front of the x-axis sensing unit 2-3, four piezoresistive resistors 5 and ohmic contact areas 6 are etched at the root of the connection between the x-axis sensing unit's sensing microbeam 2-3-3 and the chip frame 1. They are connected by interconnect lines 4 to form a Wheatstone bridge and, together with the pads 3, form the x-axis sensing unit detection circuit.

[0039] The y-axis sensing unit 2-4 includes a central support beam 2-4-1. Four separate y-axis sensing unit mass blocks 2-4-2 are arranged on both sides of the central support beam 2-4-1, connected to it by a hinged beam 2-4-4, and connected to the chip frame 1 by four sensitive microbeams 2-4-3. Figure 3 and Figure 4 As shown, on the front of the y-axis sensitive unit 2-4, four piezoresistive resistors 5 and ohmic contact areas 6 are etched at the root of the connection between the sensitive microbeam 2-4-3 of the y-axis sensitive unit and the chip frame 1. They are connected by interconnect line 4 to form a Wheatstone bridge and, together with the pad 3, form the y-axis sensitive unit detection circuit.

[0040] In this embodiment, the overall dimensions of the sensor are: length Width thick 2000μm 2000μm 300μm;

[0041] The mass block dimensions of z-axis sensing element 1 and z-axis sensing element 2 are: length Width thick 300μm 300μm 250μm;

[0042] The variable cross-section beam of sensitive element 1 and the variable cross-section beam of sensitive element 2 on the z-axis are quadrangular prisms with a trapezoidal horizontal cross-section. The dimensions of the trapezoid are: upper base... bottom high 320μm 640μm 200μm, forming a square prism with a thickness of 80μm;

[0043] The dimensions of the central support beam for the x-axis sensitive unit are: length Width thick μm 700μm 250μm;

[0044] The dimensions of the hinged beam of the x-axis sensitive unit are: length Width thick μm 65μm 250μm;

[0045] The size of the mass block of the x-axis sensitive unit is: length Width thick μm μm 250μm;

[0046] The dimensions of the x-axis sensitive element microbeam are: length Width thick μm μm μm;

[0047] The dimensions of the central support beam for the y-axis sensitive unit are: length Width thick μm 100μm 250μm;

[0048] The dimensions of the hinged beam of the y-axis sensitive element are: length Width thick μm 75μm 250μm;

[0049] The mass block size of the y-axis sensitive unit is: length Width thick μm μm 250μm;

[0050] The dimensions of the sensitive microbeam of the y-axis sensitive element are: length Width thick μm μm μm.

[0051] The technical specifications that the chip in this embodiment can achieve are as follows:

[0052] 1. z-axis vector path 1.0 × 10 5 g; x-axis and y-axis vector range 5.0 × 10 4 g is the unit of gravitational acceleration, 1g = 9.81 m / s². 2 ;

[0053] 2. Under 5V power supply input conditions, the z-axis sensitivity is 0.72μv / g, and the x-axis and y-axis sensitivity is 1.03μv / g;

[0054] 3. Under 5V power supply input conditions, the lateral sensitivity of the z-axis sensing element after decoupling μv / g can be approximated as zero;

[0055] 4. The inherent frequency is better than 1MHz;

[0056] As can be seen from the above indicators, the sensor has extremely low lateral sensitivity and strong anti-lateral interference capability, which makes its lateral performance excellent and its measurement accuracy higher than that of current domestic and foreign high-impact MEMS accelerometer shelf products.

[0057] As can be seen from the above indicators, the sensor has an extremely high natural frequency, which makes its measurement bandwidth higher than that of current domestic and foreign high-impact MEMS accelerometer products.

[0058] The working principle of this sensor MEMS chip is as follows:

[0059] like Figure 5As shown, when each sensing unit is subjected to inertial acceleration along its measurement axis, the inertial force on its mass causes deformation of the MEMS sensing beam, which in turn changes the stress on the piezoresistive resistor on the sensing beam. The relationship between its resistance change and the applied stress is as follows:

[0060] (1)

[0061] in, This is the initial value of the piezoresistive resistor. This represents the change in the piezoresistive resistance. The resistivity of the piezoresistive material is... This represents the change in resistivity of the piezoresistive material. The piezoresistive matrix of the piezoresistive material. This refers to the stress on the piezoresistive resistor.

[0062] In practical MEMS chip design, the shear component of stress is usually zero, and due to the piezoresistive matrix of p-type silicon... The value is much larger than the other two. The piezoresistive coefficient, therefore the above equation can be further simplified to:

[0063] (2)

[0064] in, These are longitudinal stress and transverse stress, respectively.

[0065] Therefore, when subjected to inertial acceleration, the Wheatstone bridge composed of four piezoresistive resistors in the same sensitive unit will no longer remain in balance and will output a voltage signal proportional to the inertial acceleration.

[0066] Preferably, during the design process, to maximize sensitivity, the piezoresistive resistor should be arranged in... Within the range where the sign remains unchanged, i.e. within the maximum stress zone of the sensitive beam, the resistance of each micro-element of the overall resistor exhibits the same trend of resistance change when subjected to force.

[0067] like Figure 6 As shown, when the z-axis sensing element is subjected to lateral inertial acceleration, the lateral displacement of the mass block of the z-axis sensing element will cause additional torque and stress at its piezoresistive resistor. Such parasitic signals will cause lateral coupling in a single z-axis sensing element, resulting in a large measurement error.

[0068] Unlike the common approach of reducing lateral coupling by optimizing the structure of sensitive cells in MEMS chips, this invention focuses on designing a symmetrical mechanical response structure to reduce or even eliminate lateral coupling at the source.

[0069] like Figure 6As shown, when the sensor is subjected to lateral inertial acceleration, the strain of its z-axis sensing element can be divided into the overall lateral offset and the deflection of the mass block caused by inertial force.

[0070] in, and This is equivalent to the inertial force experienced by the mass block 2-1-2 of the first sensitive element along the z-axis and the second sensitive element 2-2-2 along the z-axis. and The inertial force equivalent to that experienced by the x-axis sensing element 2-3 and the y-axis sensing element 2-4 is discarded due to the small mass of the sensing beam. The piezoresistive resistors in the design are distributed in... Location, corresponding Figure 3 Medium resistance Assume its initial resistance is all R. This MEMS chip is in a statically indeterminate state, and the constraint forces at its two fixed ends can be determined using the force method to solve the statically indeterminate structure. Let l represent the axial stress, shear stress, and moment at the right fixed support of the first sensitive element 2-1 on the z-axis, respectively; l represent the chip side length; E represent the material elastic modulus; I represent the moment of inertia; and A represent the cross-sectional area. The following formula can be obtained:

[0071] (3)

[0072] (4)

[0073] (5)

[0074] Similarly, the constraint force of the second sensitive element along the z-axis can be calculated. Then, the forces at each piezoresistive sensitive element can be calculated as shown in the following equation:

[0075] (6)

[0076] (7)

[0077] From the above formula, it can be seen that since the z-axis sensing unit 2-1 and z-axis sensing unit 2-2 are symmetrically distributed along the diagonal, and are therefore affected by the x-axis sensing unit 2-3 and y-axis sensing unit 2-4 on their respective sides, the corresponding resistors in these two z-axis sensing units experience equal and opposite forces under the action of lateral inertial acceleration, exhibiting a centrally symmetrical distribution. Thanks to the identical structure of these two sensing units, combined with the piezoresistive effect mentioned earlier, it can be concluded that the resistance values ​​of the corresponding resistors of the two z-axis sensing units change in equal and opposite directions. Therefore, their lateral inertial acceleration signal can be converted into a differential mode signal, which can cancel out their lateral sensitivity by superimposing the outputs of the two z-axis sensing units and multiply the z-axis sensitivity. and The change in resistance is denoted as , and The change in resistance is denoted as The output of the first sensitive unit on the z-axis is... The output of the second sensing unit on the z-axis is Its lateral coupling output can be calculated:

[0078] (8)

[0079] Since the resistance value is much greater than its resistance change and its numerator is a second-order small quantity, the above equation can be approximated as zero, proving that the "differential mode cancellation common mode superposition" structure designed in this invention can eliminate lateral coupling.

[0080] like Figure 7 As shown, the method for manufacturing this chip includes the following steps:

[0081] Step 1, as follows Figure 7 As shown in (a), a double-sided polished silicon wafer is used, the silicon wafer comprising a silicon wafer 7 and silicon dioxide 8 deposited on the upper and lower surfaces of the silicon wafer. The silicon wafer 7 has a thickness of 300 μm, and the silicon dioxide 8 has a thickness of 0.1 μm.

[0082] Step two, as Figure 7 As shown in (b), a piezoresistive photomask was used. After spin-coating PR-AZ5214 photoresist, the photoresist was etched using a standard photolithography process to pattern the piezoresistive resistor. Then, reactive ion etching (RIE) was used to remove the silicon dioxide 8 oxide layer on the surface of the silicon wafer 7 in CHF3 at 30 sccm (standard cubic centimeters per minute), CF4 at 90 sccm, and He gas at 120 sccm. Boron ion implantation was then used to form the piezoresistive resistor 5, with an ion implantation depth of 0.3 μm. Subsequently, the photoresist and oxide layer were removed. A silicon dioxide layer 8 with a thickness of 0.1 μm is deposited using plasma chemical vapor deposition (PCVD). An ohmic contact mask is used, and photoresist is spin-coated followed by etching the photoresist using standard photolithography. The ohmic contact region 6 is then patterned, and the silicon dioxide 8 on the silicon wafer surface is removed by reactive ion etching. The ohmic contact region 6 is then formed by boron ion implantation to a depth of 0.3 μm. Subsequently, the photoresist and oxide layer are removed.

[0083] Step 3, as Figure 7As shown in (c), a new layer of silicon dioxide 8 is deposited on the back side of the silicon wafer. Using a motion gap mask, photoresist is spin-coated onto the back side of the wafer, and then etched using standard photolithography to pattern the motion gap. The silicon dioxide 8 at the motion gap locations on the silicon wafer surface is then removed using reactive ion etching (RIE). The remaining silicon dioxide 8 is retained as a mask, and RIE is used to etch downwards 50 μm in a Cl2 atmosphere at 35 sccm and a CF4 atmosphere at 15 sccm to form the motion gap. Subsequently, the photoresist and oxide layer are removed.

[0084] Step four, as Figure 7 As shown in (d), a layer of silicon dioxide 8 is deposited on the back side of the silicon wafer and a layer of photoresist is spin-coated. Using a mass block mask on the back side of the silicon wafer, the photoresist is etched using a standard photolithography process to pattern the mass block. After that, the unmasked silicon dioxide 8 is removed by reactive ion etching. The mass block structure is then released by etching down 170 μm using reactive ion etching. Subsequently, the photoresist and silicon dioxide 8 are removed.

[0085] Step 5, as Figure 7 As shown in (e), a layer of silicon dioxide 8 is deposited on the back side of the silicon wafer, and a layer of photoresist is spin-coated. Using a back-through mask, the photoresist is etched on the back side of the silicon wafer using standard photolithography. After patterning, the silicon dioxide 8 in the back-through area of ​​the silicon wafer is removed by reactive ion etching, while the silicon dioxide 8 in the remaining areas is retained as a mask. Using deep reactive ion etching (DRIE), the sensitive microbeam structure and variable cross-section beam structure are etched down 80 μm in SF6 at 130 sccm and C4F8 gas at 100 sccm. Subsequently, the photoresist and oxide layer are removed.

[0086] Step six, as Figure 7 As shown in (f), on the front side of the silicon wafer, a layer of silicon nitride 9 is deposited using low-pressure chemical vapor deposition (LPCVD) under conditions where the molar ratio of SiH2Cl2 gas to NH3 gas is 0.12. A layer of photoresist is then spin-coated. Using a leadhole plate, the photoresist is etched on the front side of the silicon wafer using standard photolithography to pattern the leadholes. Afterward, reactive ion etching is used to remove the silicon nitride at the leadhole locations on the silicon wafer surface, forming the leadholes. The photoresist is then removed.

[0087] Step seven, as Figure 7 As shown in (g), a layer of photoresist is deposited on the front side of the silicon wafer. Using a metal lead mask and a pad mask, the photoresist is etched on the front side of the chip using standard photolithography to pattern the leads and pads. Then, a layer of aluminum is deposited using a sputtering process in an argon atmosphere. Subsequently, a stripping process is used to remove the photoresist and excess aluminum, forming pads 3 and interconnects 4. The fabrication of the differential-mode canceling, common-mode superposition, low lateral coupling, high-sensitivity, high-impact triaxial MEMS accelerometer chip is thus completed.

[0088] This invention addresses the extreme penetration impact signal measurement requirements of precision-strike weapons targeting hard targets. It aims to provide a low-lateral-coupling, high-sensitivity, high-impact triaxial MEMS accelerometer chip with differential-mode cancellation and common-mode superposition, suitable for ultra-high impact load environments, and its fabrication method. This improves the measurement accuracy of transient impact signals during weapon penetration of hard targets and enhances the control capability for precise fuse detonation. The key technology utilizes the characteristic of two z-axis sensitive units symmetrically arranged around the center of a square MEMS chip to produce a symmetrical mechanical response to lateral impact signals, converting the lateral impact signal into a differential-mode signal, while the axial impact signal remains a common-mode signal. Based on this, a decoupling design method of differential-mode cancellation and common-mode superposition is employed. This allows the z-axis sensitive units, designed with a symmetrical structural layout, to almost completely cancel out the lateral sensitivity while simultaneously doubling the z-axis sensitivity. Furthermore, theoretical analysis and optimization of the mechanical and piezoresistive parameters of each sensitive structure are used. The differential-mode response characteristics of this MEMS chip structure to lateral impacts and the high stiffness of the supporting structure give the sensor the advantages of low lateral sensitivity, high axial sensitivity, and high natural frequency.

[0089] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A triaxial accelerometer chip with differential mode cancellation and common mode superposition, characterized in that: The MEMS chip is square in shape and divided into four equal parts. Along one diagonal of the square chip, there are two z-axis sensing units, and the two z-axis sensing units are centrally symmetrical about the chip. On the other diagonal of the chip, there are two x-axis sensing units and y-axis sensing units. All sensing units are physically isolated by the MEMS chip frame.

2. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 1, characterized in that: The z-axis sensing unit includes z-axis sensing unit 1 and z-axis sensing unit 2. Its measurement axis is orthogonal to the xoy plane where the square surface of the chip is located, and the measurement axis starts from one side of the pad and points to the outside of the chip. The mass block of the z-axis sensing unit 1 is connected to the chip frame through the z-axis sensing unit 1 variable cross-section beam. At the root of the connection between the z-axis sensing unit 1 variable cross-section beam and the chip frame, four piezoresistive and ohmic contact areas are etched. The piezoresistive and ohmic contact areas are electrically connected by metal interconnects to form a Wheatstone bridge. The Wheatstone bridge and the pad together form the detection circuit of the z-axis sensing unit 1. The z-axis sensing unit 1 and z-axis sensing unit 2 have the same structure and are symmetrical about the chip center. The detection circuit of z-axis sensing unit 2 sets the input voltage and ground terminal to be exactly opposite to the input voltage and ground terminal of the detection circuit of z-axis sensing unit 1, thus satisfying the decoupling condition.

3. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 2, characterized in that: by and This is equivalent to the inertial force experienced by the mass block of the first sensitive element along the z-axis and the second sensitive element along the z-axis. and This is equivalent to the inertial force experienced by the x-axis sensing element and the y-axis sensing element; each sensing element is equipped with a piezoresistive resistor, which is distributed in... At the point where no force is applied, the initial resistance is R for all piezoresistors. The forces applied to each piezoresistor are as follows: This MEMS chip is in a statically indeterminate state. The force method is used to solve the statically indeterminate structure and determine the constraint forces at its two fixed ends. Let l represent the axial stress, shear stress, and moment at the fixed support on the right side of the first sensitive element along the z-axis, respectively; l represent the chip side length; E represent the material elastic modulus; I represent the moment of inertia; and A represent the cross-sectional area. The following equation is obtained: （1） （2） （3） Find the constraint force of the second sensitive element along the z-axis; then the forces at each piezoresistive sensitive element can be calculated as shown in the following formula: （4） （5） Because the first and second z-axis sensing units are symmetrically distributed along the diagonal, and are thus influenced by the x-axis and y-axis sensing units on their respective sides, the corresponding resistors in these two z-axis sensing units experience equal and opposite forces under lateral inertial acceleration, exhibiting a centrally symmetrical distribution. Benefiting from the identical structure of these two sensing units, combined with the piezoresistive effect, the resistance changes of the corresponding resistors in the two z-axis sensing units are equal and opposite. Therefore, their lateral inertial acceleration signals can be converted into differential-mode signals, allowing the triaxial accelerometer chip to cancel out its lateral sensitivity in the z-axis measurement by superimposing the outputs of the two z-axis sensing units, and multiplying the z-axis sensitivity. and The change in resistance is denoted as , and The change in resistance is denoted as The output of the first sensitive unit on the z-axis is... The output of the second sensing unit on the z-axis is Find its lateral coupling output: （6） Since the resistance value is much larger than its resistance change and its numerator is a second-order small quantity, the above equation is equivalent to zero. Therefore, the centrally symmetrically distributed z-axis sensing unit 1 and z-axis sensing unit 2 have the ability to convert lateral impacts into differential-mode signals and retain axial impacts as common-mode signals. Thus, by superimposing the differential outputs of z-axis sensing unit 1 and z-axis sensing unit 2, differential-mode cancellation of lateral signals can be achieved, enabling the chip to obtain extremely low lateral sensitivity. At the same time, when the chip is subjected to an axial force along the z-axis, the axial impact responses generated by the two z-axis sensing units are the same, achieving common-mode superposition of axial signals.

4. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 1, characterized in that: The x-axis sensing unit has the same structure as the y-axis sensing unit, and their measurement axes are orthogonal and perpendicular to each other, forming an orthogonal coordinate system together with the measurement axis of the z-axis sensing unit. The x-axis sensing unit includes a central support beam, and separate x-axis sensing unit mass blocks are arranged on both sides of the central support beam. The x-axis sensing unit mass blocks are connected to the chip frame through the x-axis sensing unit sensing microbeam.

5. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 4, characterized in that: At the root of the connection between the sensitive microbeam of the x-axis sensitive unit and the chip frame, a piezoresistive resistor and an ohmic contact area are etched. The piezoresistive resistor and the ohmic contact area are electrically connected by metal interconnects to form a Wheatstone bridge. The Wheatstone bridge, together with the pads, forms the x-axis sensitive unit detection circuit. The x-axis sensitive unit and the y-axis sensitive unit adopt a central support beam structure, and the four sensitive microbeams on which the piezoresistive resistors are arranged are separated from the central support beam to suppress lateral coupling.

6. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 5, characterized in that: The central support beam refers to a beam structure that runs through the sensitive unit along the y-axis. The thickness of the central support beam structure along the z-direction is greater than the thickness along the x-axis sensitive direction, and the overall structure presents a configuration with a large dimension along the z-direction and a small dimension along the x-sensitive axis.

7. The triaxial accelerometer chip with differential mode cancellation and common mode superposition according to claim 6, characterized in that: The central support beam has a large moment of inertia in the z-axis and a small moment of inertia in the sensitive axis. This makes it less responsive to impact signals in the non-sensitive axis and more responsive to impact signals in the sensitive axis. This allows it to initially suppress lateral coupling and control the displacement of the sensitive structure to remain within the chip plane.

8. A method for fabricating a triaxial accelerometer chip with differential mode cancellation and common mode superposition as described in claim 7, characterized in that: Includes the following steps: Step 1: Perform double-sided thermal oxidation on the silicon wafer to form a thermally oxidized silicon dioxide layer on the upper and lower surfaces of the silicon wafer respectively. Step 2: Using a piezoresistive photoresistor mask, spin-coating photoresist and then etching the photoresistor using standard photolithography to pattern the piezoresistive photoresistor. Then, reactive ion etching is used to remove the oxide layer on the silicon wafer surface, leaving the mask. Boron ion implantation is used to form the piezoresistive photoresistor, and then the photoresist and oxide layer are removed. Another layer of silicon dioxide is deposited. Using an ohmic contact mask, photoresist is spin-coated and then etched using a standard photolithography process to pattern the ohmic contact area. The oxide layer on the silicon wafer surface is then removed by reactive ion etching. The ohmic contact is formed by boron ion implantation, and then the photoresist and oxide layer are removed. Step 3: Using a motion gap mask, spin-coat photoresist on the back of the silicon wafer, then etch the photoresist using standard photolithography to pattern the motion gap, and then remove the oxide layer on the silicon wafer surface using reactive ion etching. The motion gap is formed by etching downwards using reactive ion etching. Then the photoresist and oxide layer are removed; Step 4: Deposit a layer of silicon dioxide on the back of the silicon wafer and spin-coat a layer of photoresist. Using a mass block mask, etch the photoresist on the back of the silicon wafer using standard photolithography to pattern the mass block. Then, remove the oxide layer on the surface of the silicon wafer using reactive ion etching. Use reactive ion etching to etch downwards to form the mass block structure. Then the photoresist and oxide layer are removed; Step 5: Deposit a layer of silicon dioxide on the back of the silicon wafer and spin-coat a layer of photoresist. Using a back-through mask, etch the photoresist on the back of the silicon wafer using standard photolithography to pattern it. Then, remove the oxide layer on the surface of the silicon wafer by reactive ion etching. Use deep reactive ion etching to etch downwards to release the sensitive beam structure. Then the photoresist and oxide layer are removed; Step 6: Deposit a layer of silicon nitride on the front side of the silicon wafer and spin-coat a layer of photoresist. Using a lead hole plate, etch the photoresist on the front side of the silicon wafer using a standard photolithography process to pattern the lead holes. Then, remove the silicon nitride at the lead holes on the silicon wafer surface using reactive ion etching to form the lead holes. Then the photoresist is removed; Step 7: Deposit a layer of photoresist on the front side of the silicon wafer. Using a metal lead mask and a pad mask, etch the photoresist on the front side of the silicon wafer using standard photolithography to pattern the leads and pads. Then, deposit a layer of aluminum using sputtering. Finally, remove the photoresist and excess aluminum using a stripping process to form the leads and pads. This completes the fabrication of the triaxial accelerometer chip with differential mode cancellation and common mode superposition.

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