Micromechanical detection structure with low noise and design method

Abstract

The present invention provides a method for designing a micromechanical detection structure with low noise, which comprises: step one: respectively determining the influencing parameters of the mechanical Brownian noise and sensitivity that affect the micromechanical detection structure; step two: based on the principle of reducing the mechanical Brownian noise and ensuring that the sensitivity remains unchanged or increased, selecting the change parameters that need to be changed from the influencing parameters and the change strategy for the change parameters; step three: considering the influence of different processing techniques, improving the processing technique according to the change strategy when processing the micromechanical detection structure. The present invention can perform wafer packaging under normal pressure, avoids the complexity and high cost of vacuum packaging, and improves the reliability of the device. In addition, the mechanical Brownian noise of the accelerometer can be greatly reduced, and the overall performance of the sensor can be improved, while ensuring that important indicators such as sensitivity remain unchanged.

Description

Technical Field

[0001] The present invention relates to the technical field of MEMS sensors, and in particular to a micromechanical detection structure with low noise and a design method thereof. Background Art

[0002] The flat capacitive micromachined accelerometer is the most widely used inertial measurement device. The detection device is mainly composed of a micromachined detection structure and a signal conditioning circuit. The micromachined detection structure realizes the conversion of the acceleration signal to the response displacement signal, and the signal conditioning circuit converts the displacement signal into an electrical signal, and processes and outputs the electrical signal.

[0003] Compared with traditional accelerometers, flat capacitive accelerometers have larger initial capacitance and can achieve higher detection sensitivity. And because they use MEMS micromachining technology, they have the advantages of small size, light weight, low cost, low power consumption, high reliability, and high integration. They are widely used in consumer electronics, automotive electronics, general aviation, vehicle control and other fields.

[0004] Signal noise is one of the key factors affecting the application of accelerometers. Excessive signal noise will reduce the detection accuracy and signal-to-noise ratio of the sensor, affecting its overall performance. With the increase in application requirements, accelerometers are developing in the direction of low noise and high precision. Therefore, effective methods must be used to reduce signal noise and improve detection accuracy.

[0005] Signal noise mainly comes from circuit noise and mechanical Brownian noise of sensitive structures. Circuit noise can be optimized by low-noise signal conditioning circuits, while mechanical Brownian noise is the background noise of the sensor, which comes from the damping effect of the Brownian motion of gas molecules on the accelerometer. It cannot be reduced by increasing the sensitivity of the sensor or circuit signal processing methods. Therefore, to reduce the signal noise of the accelerometer, the key is to reduce the mechanical Brownian noise of the sensitive structure.

[0006] In order to reduce mechanical Brownian noise, the existing technology usually uses vacuum packaging methods to reduce the damping effect of air on accelerometers. However, the vacuum packaging of micro devices is complex and costly. In addition, it is difficult to maintain a constant vacuum degree for a long time in vacuum packaging, which causes the sensor performance to drift as the use time increases, reducing the reliability of the sensor. It is of great significance to study and develop low-cost and high-reliability methods to reduce the mechanical Brownian noise of accelerometers.

[0007] Therefore, the present invention provides a micromechanical detection structure with low noise and a design method thereof. Summary of the invention

[0008] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a micromechanical detection structure and design method with low noise, so as to effectively reduce the signal noise of the accelerometer and improve the overall performance of the sensor through process technology.

[0009] In order to solve the above problems in the prior art, the present invention provides a method for designing a micromechanical detection structure with low noise, the method comprising the following steps:

[0010] Step 1: respectively determine the influencing parameters of the mechanical Brownian noise and sensitivity that affect the micromechanical detection structure;

[0011] Step 2: Based on the principle of reducing mechanical Brownian noise and ensuring that the sensitivity remains unchanged or increases, select the change parameter that needs to be changed from the influencing parameters and the change strategy for the change parameter;

[0012] Step 3: Consider the impact of different processing techniques and improve the processing technique according to the change strategy when processing the micromechanical detection structure.

[0013] According to one embodiment of the present invention, the mechanical Brownian noise of the micromechanical detection structure is determined by the following formula:

[0014]

[0015] Among them, a noise represents mechanical Brownian noise, K B represents the Boltzmann constant, T represents the absolute temperature, c represents the damping coefficient, m represents the mass of the sensitive structure, and r c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

[0016] According to one embodiment of the present invention, the sensitivity of the micromechanical detection structure is determined by the following formula:

[0017]

[0018] Among them, S represents sensitivity, C0 represents initial capacitance value, s1 represents distance between electrode and rotating axis, s2 represents electrode width, d represents gap of plate capacitor, m represents mass of sensitive structure, k represents structural stiffness, r represents c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

[0019] According to one embodiment of the present invention, the step 1 specifically includes the following steps:

[0020] According to the calculation expression of mechanical Brownian noise of micro-mechanical detection structure, the damping coefficient, the mass of sensitive structure and the position of the mass center of sensitive structure from the rotation axis are determined as the influencing parameters affecting mechanical Brownian noise;

[0021] According to the sensitivity calculation expression of the micromechanical detection structure, the distance between the mass center of the sensitive structure and the rotation axis is determined as an influencing parameter affecting the sensitivity.

[0022] According to an embodiment of the present invention, the step 2 specifically includes the following steps:

[0023] Selecting the distance between the mass center of the sensitive structure and the rotation axis as the change parameter;

[0024] Increasing the distance between the mass center of the sensitive structure and the rotation axis position is used as the change strategy.

[0025] According to an embodiment of the present invention, the step three specifically includes the following steps:

[0026] For bulk silicon processing technology, it is not necessary to process damping holes on the eccentric mass block when processing the micromechanical detection structure. It is only necessary to process damping holes of the first size level on the symmetrical mass block including the left and right electrodes.

[0027] According to an embodiment of the present invention, the step three specifically includes the following steps:

[0028] As for the surface processing technology, when processing the micromechanical detection structure, it is necessary to process the damping holes of the second size level on the eccentric mass block, and to process the damping holes of the third size level on the symmetrical mass block including the left and right electrodes.

[0029] According to one embodiment of the present invention, the damping holes of the second size level are smaller than the damping holes of the third size level.

[0030] According to an embodiment of the present invention, the step three specifically includes the following steps:

[0031] Under different processing conditions, for the damping hole on the eccentric mass, the minimum size design principle is to ensure that the oxide layer under the sensitive structure is completely released.

[0032] Under different processing conditions, for the damping holes on the symmetrical mass block, the maximum size design principle is not to affect the overall strength of the structure and not to affect the initial capacitance value.

[0033] According to another aspect of the present invention, there is provided a micromechanical detection structure with low noise, characterized in that it is designed by any of the methods described above.

[0034] The low-noise micromechanical detection structure and design method provided by the present invention can be packaged in wafers under normal pressure, avoiding the complexity and high cost of vacuum packaging and improving the reliability of the device. In addition, the mechanical Brownian noise of the accelerometer can be greatly reduced while ensuring that important indicators such as sensitivity remain unchanged, thereby improving the overall performance of the sensor.

[0035] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0037] Figure 1 A flow chart of a method for designing a micromechanical detection structure with low noise according to an embodiment of the present invention is shown;

[0038] Figure 2 A flow chart of a method for determining a change parameter and a change strategy according to an embodiment of the present invention is shown;

[0039] Figure 3 A schematic diagram of a Z-axis detection structure in the prior art is shown; and

[0040] Figure 4 A schematic diagram of a Z-axis detection structure according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0041] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0042] Figure 1 A flow chart of a method for designing a micromechanical detection structure with low noise according to an embodiment of the present invention is shown.

[0043] like Figure 1 In step S101, influencing parameters of mechanical Brownian noise and sensitivity affecting the micromechanical detection structure are determined respectively.

[0044] like Figure 1 In step S102, based on the principle of reducing mechanical Brownian noise and ensuring that the sensitivity remains unchanged or increased, the change parameters that need to be changed and the change strategies for the change parameters are selected from the influencing parameters.

[0045] like Figure 1 In step S103, the influence of different processing techniques is considered, and the processing technique is improved according to the change strategy when processing the micromechanical detection structure.

[0046] The purpose of this application is to propose a micromechanical detection structure with low noise, therefore, it is necessary to consider the factors affecting the mechanical Brownian noise.

[0047] In one embodiment, for the Z-axis flat capacitive accelerometer, a "seesaw" structure is generally used, which is simple and easy to process. For the "seesaw" accelerometer, the mechanical Brownian noise of the micromechanical detection structure is determined by the following formula:

[0048]

[0049] Among them, a noise represents mechanical Brownian noise, K B represents the Boltzmann constant, T represents the absolute temperature, c represents the damping coefficient, m represents the mass of the sensitive structure, and r c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

[0050] From formula (1), we can see that the factors that determine the size of mechanical Brownian noise include the Boltzmann constant K B , absolute temperature T, damping coefficient c, sensitive structure mass m, distance between the mass center of the sensitive structure and the rotation axis r c At normal temperature and pressure, the Boltzmann constant K B and absolute temperature T are constants.

[0051] Therefore, to reduce mechanical Brownian noise, we can reduce the structural damping c, increase the mass m of the sensitive structure, or increase the distance r between the center of mass of the sensitive structure and the rotation axis. c However, increasing the mass m of the sensitive structure will increase the structural area, resulting in an increase in the structural damping c. Therefore, the most effective way to reduce mechanical Brownian noise is to increase the distance r between the center of mass of the sensitive structure and the axis of rotation. c .

[0052] Sensitivity is one of the most important indicators for evaluating the performance of an accelerometer. In one embodiment, for a “seesaw” type Z-axis accelerometer, the sensitivity of the micromechanical detection structure is determined by the following formula:

[0053]

[0054] Among them, S represents sensitivity, C0 represents initial capacitance value, s1 represents distance between electrode and rotating axis, s2 represents electrode width, d represents gap of plate capacitor, m represents mass of sensitive structure, k represents structural stiffness, r represents c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

[0055] In formula (2), the initial capacitance value C0, the distance between the electrode and the rotating axis s1, the electrode width s2 and the structural stiffness k are determined by the design, and the plate capacitor gap d is determined by the processing technology. When keeping other parameters unchanged, the sensitive structure mass m and the distance between the center of mass of the sensitive structure and the rotating axis position r can be increased. c To increase sensitivity.

[0056] By comparing formulas (1) and (2), it can be seen that by increasing the distance between the center of mass of the sensitive structure and the rotation axis position r c , which can effectively reduce mechanical Brownian noise while ensuring the sensitivity remains unchanged.

[0057] In step S103, it also includes: for the bulk silicon processing technology, when processing the micromechanical detection structure, it is not necessary to process the damping holes on the eccentric mass block, and only the damping holes of the first size level need to be processed on the symmetrical mass block including the left and right electrodes.

[0058] In step S103, it also includes: for the surface processing technology, when processing the micromechanical detection structure, it is necessary to process the damping holes of the second size level on the eccentric mass block, and process the damping holes of the third size level on the symmetrical mass block including the left and right electrodes.

[0059] Specifically, the damping holes of the second size class are smaller than the damping holes of the third size class.

[0060] In step S103, it also includes: under different processing conditions, for the damping hole on the eccentric mass, the minimum size design principle is to ensure that the oxide layer under the sensitive structure is completely released.

[0061] In step S103, it also includes: under different processing conditions, for the damping holes on the symmetrical mass block, the maximum size design principle is not to affect the overall strength of the structure and not to affect the initial capacitance value. Although a larger damping hole will have a certain impact on the initial capacitance C0 value, the capacitance edge effect around the damping hole will compensate for the capacitance. In addition, the initial capacitance C0 value can also be compensated in advance during the design.

[0062] By machining damping holes of different sizes, the mass m of the sensitive structure will be slightly reduced, but the distance between the mass center of the sensitive structure and the rotation axis position r c It will increase significantly, and the product of the mass m of the sensitive structure and the distance rc from the center of mass of the sensitive structure to the rotation axis will also increase significantly.

[0063] In addition, due to the fabrication of the damping hole, the damping effect of the gas molecules on the accelerometer will be significantly reduced, and the damping coefficient c will be greatly reduced. From formulas (1) and (2), it can be seen that by adjusting the above parameters, the mechanical Brownian noise a of the accelerometer noiseIt will be significantly reduced, and the sensitivity S can be further improved and optimized.

[0064] In summary, the present invention adjusts the structural parameters by machining damping holes of different sizes on the sensitive structure of the accelerometer, which can significantly reduce the mechanical Brownian noise of the accelerometer and improve the overall performance of the sensor while ensuring that the main performance parameters such as sensitivity remain unchanged.

[0065] In addition, the micromechanical detection structure designed according to the design method can be wafer packaged under normal pressure, avoiding the complexity and high cost of vacuum packaging, improving the reliability of the device, and has the advantages of simple principle, easy processing, low cost, and high reliability.

[0066] Figure 2 A flow chart of a method for determining change parameters and change strategies according to an embodiment of the present invention is shown.

[0067] like Figure 2 As shown, in step S201, based on the mechanical Brownian noise calculation expression of the micromechanical detection structure, the damping coefficient, the mass of the sensitive structure and the position of the mass center of the sensitive structure from the rotation axis are determined as influencing parameters affecting the mechanical Brownian noise.

[0068] Specifically, from formula (1), we can know that the damping coefficient c, the mass of the sensitive structure m, and the distance r from the center of mass of the sensitive structure to the rotation axis are c It can affect the size of mechanical Brownian noise. Reduce the structural damping c, increase the mass m of the sensitive structure, or increase the distance r between the mass center of the sensitive structure and the rotation axis. c Can reduce mechanical Brownian noise.

[0069] like Figure 2 As shown, in step S202, based on the sensitivity calculation expression of the micromechanical detection structure, the position of the center of mass of the sensitive structure from the rotation axis is determined as an influencing parameter affecting the sensitivity.

[0070] Specifically, from formula (2), we can know that the mass m of the sensitive structure and the distance r between the mass center of the sensitive structure and the rotation axis are c It can affect the sensitivity. Increase the mass m of the sensitive structure and the distance r between the mass center of the sensitive structure and the rotation axis. c Can improve sensitivity.

[0071] like Figure 2 As shown, in step S203, the distance between the mass center of the sensitive structure and the rotation axis is selected as the change parameter.

[0072] like Figure 2 As shown, in step S204, increasing the distance between the mass center of the sensitive structure and the rotation axis position is used as a change strategy.

[0073] Specifically, combining formula (1) and formula (2), it can be seen that based on the principle of reducing mechanical Brownian noise and ensuring that the sensitivity remains unchanged or increased, the distance between the center of mass of the sensitive structure and the rotation axis is selected as the change parameter, and increasing the distance between the center of mass of the sensitive structure and the rotation axis is used as the change strategy.

[0074] Figure 3 A schematic diagram of a Z-axis detection structure in the prior art is shown.

[0075] like Figure 3 As shown, the Z-axis detection structure in the prior art includes an eccentric mass block part and a symmetrical mass block part. The center of mass of the entire Z-axis detection structure is located in the symmetrical mass block area. Electrodes 1 and electrodes 2 are arranged on the left and right of the symmetrical mass block respectively.

[0076] The Z-axis detection structure in the prior art has large mechanical Brownian noise, which will reduce the detection accuracy and signal-to-noise ratio of the sensor and affect its overall performance.

[0077] Figure 4 A schematic diagram of a Z-axis detection structure according to an embodiment of the present invention is shown.

[0078] like Figure 4 As shown, the Z-axis detection structure (micromechanical detection structure with low noise) proposed in the present application has an eccentric mass block part and a symmetrical mass block part, the center of mass of the entire Z-axis detection structure is located in the eccentric mass block area, and the symmetrical mass block is respectively arranged with electrodes 1 and 2 on the left and right. Damping holes are processed on both the eccentric mass block and the symmetrical mass block, and the damping hole on the eccentric mass block is smaller than the damping hole on the symmetrical mass block.

[0079] like Figure 4 The Z-axis detection structure proposed in the present invention processes damping holes of different sizes at different positions of the sensitive structure, so that the distance between the center of mass of the sensitive structure and the rotation axis increases. Under the comprehensive influence of the damping holes, the damping effect of gas molecules on the sensitive structure is significantly reduced. By adjusting the above parameters, the mechanical Brownian noise of the accelerometer can be significantly reduced while ensuring that the main performance parameters such as sensitivity remain unchanged.

[0080] The present invention also provides a micromechanical detection structure with low noise, which is designed by any of the methods described above (such as Figure 4 ).

[0081] In summary, the low-noise micromechanical detection structure and design method provided by the present invention can be packaged in wafers under normal pressure, avoiding the complexity and high cost of vacuum packaging and improving the reliability of the device. In addition, the mechanical Brownian noise of the accelerometer can be greatly reduced while ensuring that important indicators such as sensitivity remain unchanged, thereby improving the overall performance of the sensor.

[0082] It should be understood that the embodiments disclosed in the present invention are not limited to the specific structures, processing steps or materials disclosed herein, but should be extended to equivalent substitutions of these features understood by ordinary technicians in the relevant field. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not meant to be limiting.

[0083] The "one embodiment" or "embodiment" mentioned in the specification means that the specific features, structures or characteristics described in conjunction with the embodiment are included in at least one embodiment of the present invention. Therefore, the phrases "one embodiment" or "embodiment" appearing in various places throughout the specification do not necessarily refer to the same embodiment.

[0084] Although the embodiments disclosed in the present invention are as above, the contents described are only embodiments adopted for facilitating the understanding of the present invention and are not intended to limit the present invention. Any technician in the technical field to which the present invention belongs can make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in the present invention, but the patent protection scope of the present invention shall still be subject to the scope defined in the attached claims.

Claims

1. A method for designing a micromechanical detection structure with low noise, characterized in that: The method comprises the following steps: Step 1: respectively determine the influencing parameters of the mechanical Brownian noise and sensitivity that affect the micromechanical detection structure, wherein the micromechanical detection structure adopts a seesaw structure; Step 2: Based on the principle of reducing mechanical Brownian noise and ensuring that the sensitivity remains unchanged or increases, select the change parameter that needs to be changed from the influencing parameters and the change strategy for the change parameter; Step 3: Considering the influence of different processing techniques, the processing technique is improved according to the change strategy when processing the micromechanical detection structure; The step 1 specifically includes the following steps: According to the calculation expression of mechanical Brownian noise of micro-mechanical detection structure, the damping coefficient, the mass of sensitive structure and the position of the mass center of sensitive structure from the rotation axis are determined as the influencing parameters affecting mechanical Brownian noise; According to the sensitivity calculation expression of the micromechanical detection structure, the distance between the mass center of the sensitive structure and the rotation axis is determined as an influencing parameter affecting the sensitivity.

2. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The mechanical Brownian noise of the micromechanical detection structure is determined by the following formula: Among them, a noise represents mechanical Brownian noise, K B represents the Boltzmann constant, T represents the absolute temperature, c represents the damping coefficient, m represents the mass of the sensitive structure, and r c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

3. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The sensitivity of the micromechanical detection structure is determined by the following formula: Among them, S represents sensitivity, C0 represents initial capacitance value, s1 represents distance between electrode and rotating axis, s2 represents electrode width, d represents gap of plate capacitor, m represents mass of sensitive structure, k represents structural stiffness, r represents c Indicates the distance between the center of mass of the sensitive structure and the rotation axis.

4. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The step 2 specifically includes the following steps: Selecting the distance between the mass center of the sensitive structure and the rotation axis as the change parameter; Increasing the distance between the mass center of the sensitive structure and the rotation axis position is used as the change strategy.

5. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The step three specifically includes the following steps: For bulk silicon processing technology, it is not necessary to process damping holes on the eccentric mass block when processing the micromechanical detection structure. It is only necessary to process damping holes of the first size level on the symmetrical mass block including the left and right electrodes.

6. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The step three specifically includes the following steps: As for the surface processing technology, when processing the micromechanical detection structure, it is necessary to process the damping holes of the second size level on the eccentric mass block, and to process the damping holes of the third size level on the symmetrical mass block including the left and right electrodes.

7. The method for designing a micromechanical detection structure with low noise according to claim 6, characterized in that: The damping holes of the second size class are smaller than the damping holes of the third size class.

8. The method for designing a micromechanical detection structure with low noise according to claim 1, characterized in that: The step three specifically includes the following steps: Under different processing conditions, for the damping hole on the eccentric mass, the minimum size design principle is to ensure that the oxide layer under the sensitive structure is completely released; Under different processing conditions, for the damping holes on the symmetrical mass block, the maximum size design principle is not to affect the overall strength of the structure and not to affect the initial capacitance value.

9. A micromechanical detection structure with low noise, characterized in that: The method is obtained by the method according to any one of claims 1 to 8.

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