A MEMS current sensor based on modal localization effect

Abstract

The present invention discloses a MEMS current sensor based on the modal localization effect, which belongs to the field of sensors. The sensor is based on the modal localization effect of a two-degree-of-freedom weak-coupled resonant system and a sampling resistor method, and is sensitive to external current signals by detecting the amplitude ratio of the weak-coupled resonant system. Its main structure includes: a weak-coupled resonant system, a sampling resistor and an electrode part. After the current I to be measured flows into the sampling resistor, a corresponding voltage V is obtained, and the voltage V is electrically connected to obtain an equipotential voltage V on the corresponding capacitor plate of an equivalent resonator 101 of the weak-coupled resonant system, so that the equivalent resonator 101 obtains an input stiffness disturbance Δk. The present invention is sensitive to current through a sampling resistor, and the resistance value of the sampling resistor is controlled by deposition and ion implantation processes. The MEMS current sensor based on the modal localization effect not only has extremely high measurement accuracy, but also has a large dynamic range. Compared with other MEMS current sensors, the present invention can achieve high-precision measurement of weak currents under a large dynamic range.

Description

Technical Field

[0001] The invention relates to a MEMS current sensor based on modal localization effect, belonging to the field of sensors. Background Art

[0002] Electric current is one of the seven basic physical quantities in the International System of Units. The accurate measurement of electric current is of great significance in modern scientific research and industrial production. MEMS (Micro-Electro-Mechanical System) sensors generally refer to sensor systems with internal structures at the micron or even nanometer level, including sensing, control and actuator elements. Due to the advantages of miniaturization, excellent performance, low cost and mass production, MEMS sensors are increasingly widely used.

[0003] Modern scientific research and industrial applications urgently need miniaturized and high-precision current sensors. Existing MEMS current sensors mainly detect current through magnetic effects, such as magnetic metal film current sensors, magnetic torsion current sensors, and optical torsion current sensors. Magnetic metal film current sensors use magnetic fields to soften the stiffness of the cantilever beam, thereby changing the resonant frequency of the cantilever beam, and the sensitivity reaches 5√Hz / A; magnetic torsion current sensors use magnetic torsion micromirrors to detect current, and the sensitivity reaches 0.024 / A. These two types of MEMS current sensors can only detect ampere-level currents, which is difficult to meet the detection application requirements of microamperes or smaller weak currents. Optical torsion current sensors combine optical detection methods with torsion micromirrors to achieve the measurement of weak currents, and their minimum resolution reaches 6.5nA. However, this type of sensor requires a complex optical path to be used, and the range is only 50nA, which has a limited application range.

[0004] In summary, existing MEMS current sensors cannot achieve accurate measurement of weak currents over a large dynamic range. In recent years, the modal localization mechanism has been used to improve the sensitivity of MEMS sensors. The basic principle is that external signals interfere with the resonant system through the input electrode, causing the system energy to be redistributed between the resonators, which manifests as a change in the amplitude of the resonator. The modal localization sensor has extremely high measurement accuracy and a large dynamic range, and can effectively suppress the impact of environmental noise on the measurement. Inspired by this mechanism, the present invention proposes a MEMS current sensor based on the modal localization effect (hereinafter referred to as the modal localization current sensor), which senses the current to be measured through a sampling resistor, and can achieve high-precision measurement of weak currents over a large dynamic range. Summary of the invention

[0005] The purpose of the present invention is to provide a MEMS current sensor based on the modal localization effect, which is based on the modal localization effect of a two-degree-of-freedom weakly coupled resonant system and a sampling resistor method, and detects the amplitude ratio of the weakly coupled resonant system to sense external current signals. Its main structure includes: a weakly coupled resonant system, a sampling resistor and an electrode part.

[0006] The modal localization principle of the weakly coupled resonant system is as follows: Figure 1 As shown, the equivalent resonator 101 is composed of an equivalent stiffness 103 and an equivalent mass block 104, and the equivalent resonator 2 102 is composed of an equivalent stiffness 105 and an equivalent mass block 106. The two equivalent resonators are connected by an equivalent coupling beam 107. 1 and k 2 Respectively represent the equivalent stiffness of resonator 1 101 and resonator 2 102, m 1 and m 2 They represent the equivalent masses of resonator 1 101 and resonator 2 102, respectively, and k c Represents the coupling stiffness. The ratio of coupling stiffness to resonator stiffness does not exceed 1:100, thus achieving weak coupling of the resonator. Ignoring the influence of damping, according to the dynamic equation, we can obtain:

[0007]

[0008] Assume a special solution to be:

[0009]

[0010] Among them A 1 , A 2 is the amplitude of the equivalent resonator 1 101 and the equivalent resonator 2 102, w is the frequency of the simple harmonic vibration, is the phase of simple harmonic oscillation. Substituting the equation into the equation, we get a linear equation system about the amplitude:

[0011]

[0012] Assume k 1 =k 2 = k, m 1 =m 2 =m, input stiffness disturbance Δk to the equivalent resonator 101, and obtain the output amplitude ratio formula:

[0013]

[0014] After the current I to be measured flows into the sampling resistor, a corresponding voltage V is obtained. The voltage V is electrically connected to obtain an equipotential voltage V on the corresponding capacitor plate of the equivalent resonator 101, so that the equivalent resonator 101 obtains an input stiffness disturbance Δk; the sampling resistor uses an ion implantation process, and gold is deposited on the surface of the structure around the sampling resistor.

[0015] The electrode part cooperates with the weak coupling resonance system to drive the weak coupling resonance system and obtain the output amplitude ratio of the equivalent resonator 1 101 and the equivalent resonator 2 102 by differential detection, thereby obtaining the current I to be measured.

[0016] Taking the first mode as an example, the working principle of the modal localization current sensor is explained. A typical structure of a modal localization current sensor is given as follows: Figure 2 As shown, the sensor comprises two resonators with exactly the same stiffness, resonator 1 201 and resonator 2 202. The two resonators are horizontally distributed and connected by a coupling beam 212, and the stiffness ratio of the coupling beam to the resonator is not higher than 1:100.

[0017] The resonators are fixed by a central anchor point 213, and the two ends are fixed by a left anchor point 214 and a right anchor point 215. Anchor points 214 and 215 apply a DC bias voltage to the entire weakly coupled resonator. A sampling resistor 203 and an input electrode 204 are provided at a certain gap outside the resonator 1 201, and an adjustment electrode 1 210 adjusts the initial working point. A detection electrode 1 206 and a detection electrode 2 207 at a certain gap on the other side of the resonator 1 201 form a differential detection electrode of the resonator 1 201 to detect the amplitude of the resonator 1 201; an AC driving electrode 205 is provided at a certain gap outside the resonator 2 202, and an adjustment electrode 2 211 adjusts the initial working point. A detection electrode 3 208 and a detection electrode 4 209 at a certain gap on the other side form a differential detection electrode of the resonator 2 202 to detect the amplitude of the resonator 2 202. The AC driving electrode 205, the anchor point 214 and the anchor point 215 apply a driving force to the entire resonator, causing the resonator to perform simple harmonic vibration.

[0018] The present invention adopts the sampling resistor method to introduce current disturbance, converts the input current into voltage to form a potential difference, and thus realizes the stiffness disturbance of the resonator. The sampling resistor value determines the sensitivity of the current sensor, and then determines the detection limit of the current sensor. The limit of the modal localization sensor's sensitivity to stiffness disturbance is about 0.0001N / m. If the sampling resistor value is too small, it is impossible to detect microampere-level weak currents, so the sampling resistor value should not be lower than 10Ω; if the sampling resistor value is too large, the thermal noise of the resistor will be increased, and a large voltage will be generated on the current input circuit, reducing the accuracy of the detection, so the sampling resistor value should not be higher than 1000Ω.

[0019] From the above analysis, it can be concluded that the design range of the sampling resistor value should be 10-1000Ω. Different sampling resistor values ​​correspond to different detection limits of the modal localized current sensor. The conductivity of pure single crystal silicon is very sensitive to temperature. For every increase of one degree Celsius, the conductivity increases by 8%, and the resistance stability is poor. The resistivity of single crystal silicon is generally greater than 50Ω·cm, and the processing scale of MEMS devices is in the micron level, which makes it impossible to directly use single crystal silicon to manufacture sampling resistors. In order to achieve resistance control of the sampling resistor, an ion implantation process is used to reduce the resistivity of single crystal silicon, and gold is deposited on the surface of the structure around the sampling resistor to reduce the resistance. Figure 2 As shown in the figure, it is assumed that the sampling resistor 203 has a length of 1500 μm, a width of 30 μm, and a thickness of 50 μm. When the ion implantation P element concentration is 7×10 19 / cm 3 , the resistivity of doped silicon is 0.001Ω·cm, and the corresponding sampling resistor is 10Ω; when the ion implantation P element concentration is 7×10 16 / cm 3 , the resistivity of doped silicon is 0.1Ω·cm, and the corresponding sampling resistor is 1000Ω. Gold is deposited on the input electrode 204, and the resistivity of gold is about 2.4×10 -6 Ω·cm, making the input electrode resistance much smaller than the sampling resistance and negligible. Compared with pure single crystal silicon, the use of ion implantation process to form an impurity semiconductor greatly reduces the resistivity of the sampling resistor, reduces the sensitivity to temperature, and improves the stability of the resistor.

[0020] The input current to be measured flows through the sampling resistor 203, causing the potential on the input electrode 204 to change, thereby applying an electrostatic negative stiffness disturbance to the resonator 201. Assuming that the resistance of the sampling resistor is R, the stiffness disturbance of the input current I on the resonator can be expressed as:

[0021]

[0022] Where ε is the dielectric constant of vacuum, A is the area facing the input electrode plate, V dc is the DC bias voltage, d is the plate spacing, R is the sampling resistor value, and I is the detected current. It can be found from the formula that the negative stiffness disturbance caused by the current is a quadratic function of the current, so it is necessary to consider linearizing the quadratic function so that the amplitude ratio output and input current are approximately linear within a certain current input range. To ensure the weak coupling connection of the resonator, the coupling stiffness k c Much smaller than the negative stiffness disturbance Δk caused by the current, where u 1 It can be simplified to:

[0023]

[0024] Substituting the formula into the formula, we can get the theoretical model of the ratio of input current to output amplitude:

[0025]

[0026] The beneficial effect of the present invention is to provide a MEMS flow sensor based on the modal localization effect. The invention designs a sampling resistor to be sensitive to current, and controls the resistance of the sampling resistor through deposition and ion implantation processes. The MEMS current sensor based on the modal localization effect not only has extremely high measurement accuracy, but also has a large dynamic range. Compared with other MEMS current sensors, the present invention can achieve high-precision measurement of weak currents under a large dynamic range. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the spring-mass block model of a weakly coupled system.

[0028] In the figure, 101 is equivalent resonator 1, 102 is equivalent resonator 2, 103 is equivalent stiffness 1, 104 is equivalent mass block 1, 105 is equivalent stiffness 1, 106 is equivalent mass block 2, and 107 is equivalent coupling stiffness.

[0029] Figure 2 It is a typical structural schematic diagram of the modal localization current sensor designed by the present invention.

[0030] In the figure, 201 is resonator one, 202 is resonator two, 203 is sampling resistor, 204 is input electrode, 205 is AC driving electrode, 206 is detection electrode one, 207 is detection electrode two, 208 is detection electrode three, 209 is detection electrode four, 210 is adjustment electrode one, 211 is adjustment electrode two, 212 is coupling beam, 213 is central anchor point, 214 is left anchor point, and 215 is right anchor point.

[0031] Figure 3 This is the working mode simulation result of the modal localization current sensor designed by the present invention.

[0032] Figure 4 It is the response simulation result of the mode localization current sensor when the current signal is input from the input electrode 204 through the sampling resistor 203.

[0033] Figure 5 It is a simulation result that when the current signal is input from the input electrode 204 through the sampling resistor 203, the output amplitude ratio of the modal localized current sensor changes with the input signal. DETAILED DESCRIPTION

[0034] according to Figure 2The typical structural diagram of the modal localized current sensor gives a specific implementation example of the modal localized current sensor. The two resonators have the same size parameters, and the overall parameters of the sensor are shown in Table 1. Resonator 1 201 and resonator 2 202 are horizontally connected through a coupling beam 212, fixed in the middle by a central anchor point 213, and the two resonators are horizontally distributed. The two ends are fixed at the left anchor point 214 and the right anchor point 215, and a DC bias voltage is applied through the anchor points 214 and 215. The AC driving electrode 205 outside the resonator 202 applies an AC driving signal to the weakly coupled resonant system to make the resonator perform simple harmonic vibration. Differential detection is performed by detection electrodes 206, 207, 208 and 209. The differential detection method can eliminate the interference of the feedthrough capacitance signal and make the detection of amplitude and frequency more accurate. The adjustment electrodes 210 and 211 are used to adjust the initial operating point of the sensor to avoid it working in a working area with poor nonlinearity. The resistance of the sampling resistor is controlled by ion implanting P elements and gold plating the surrounding electrode structure, and the resistance is about 100Ω.

[0035] Table 1 Design parameters of the modal localization current sensor embodiment

[0036]

[0037]

[0038] The detection steps using the modal localization current sensor are as follows:

[0039] The current is converted into voltage through the sampling resistor 203, so that the input electrode 204 forms a potential difference with the resonator 1 201, thereby generating an electrostatic negative stiffness disturbance, which reduces the stiffness of the resonator 1 201 by Δk. The two resonant beams that originally had completely symmetrical stiffness have an equivalent stiffness mismatch, inducing a modal localization effect. The differential detection electrodes 206, 207, 208 and 209 perform differential detection on the resonator 1 201 and the resonator 2 202, and output the amplitude signal. In the initial state, the vibration amplitude of the resonator 1 201 is equal to the vibration amplitude of the resonator 2 202, and the amplitude ratio output is 1; after being disturbed by the current, due to the occurrence of the modal localization phenomenon, the amplitude ratio output of the resonator changes significantly, thereby realizing the detection of the current signal.

[0040] The working principle of the modal localization current sensor is explained by formulas (1)-(9). However, because the structure of the modal localization current sensor is very complex, the sensitivity of the sensor cannot be accurately obtained by formula calculation under ideal conditions. Therefore, it is necessary to use the finite element analysis software Comsol Multiphysics (hereinafter referred to as Comsol) to perform finite element simulation calculation on the sensor. The modal simulation results are shown in Figure 3As shown, the resonant frequency w = 16057Hz. Further input current disturbance, the sensor response result is as follows Figure 4 Comsol finite element simulation calculation can prove that the modal localized current sensor designed by the present invention can effectively measure weak current signals. The finite element calculation results are shown in Table 2:

[0041] Table 2 Finite element calculation results of the in-phase modal response of the modal localization current sensor

[0042]

[0043]

[0044] Finite element calculations show that the sensor sensitivity reaches 656 / A, which is much higher than the 0.024 / A of the magnetic knob current sensor. At the same time, the range of the modal localization current sensor can reach 40mA while ensuring a certain linearity. In summary, the MEMS current sensor based on the modal localization effect can achieve high-precision measurement of weak currents in a large dynamic range.

[0045] In the description of the present invention, it should be noted that the modal localization current sensor proposed in the present invention is not only applicable to two-degree-of-freedom modal localization sensors, but also modal localization current sensors with three degrees of freedom, four degrees of freedom or higher degrees of freedom designed on the basis of the present invention all fall within the protection scope of the claims of the present invention.

[0046] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A MEMS current sensor based on modal localization effect, characterized in that: The main structure includes: weak coupling resonance system, sampling resistor and electrode part; The equivalent resonator 1 (101) of the weak coupling resonance system is composed of an equivalent stiffness 1 (103) and an equivalent mass block 1 (104), and the equivalent resonator 2 (102) is composed of an equivalent stiffness 2 (105) and an equivalent mass block 2 (106), and the two equivalent resonators are connected via an equivalent coupling beam (107); the electrode part cooperates with the weak coupling resonance system to obtain an output amplitude ratio of the equivalent resonator 1 (101) and the equivalent resonator 2 (102), thereby obtaining a current to be measured I; k1 and k2 represent the equivalent stiffness of the equivalent resonator 1 (101) and the equivalent resonator 2 (102), respectively; m1 and m2 represent the equivalent mass of the equivalent resonator 1 (101) and the equivalent resonator 2 (102), respectively; k c represents the coupling stiffness; The ratio of coupling stiffness to resonator stiffness does not exceed 1:100, thereby achieving weak coupling of the resonator; The modal localization principle of the weakly coupled resonant system is: Ignoring the influence of damping, according to the dynamic equation: Suppose a particular solution of (1) is: Where A1 and A2 are the amplitudes of the equivalent resonator 1 (101) and the equivalent resonator 2 (102), w is the frequency of the simple harmonic oscillation, is the phase of simple harmonic oscillation. Substituting equation (2) into equation (1) yields the linear equation system about the amplitude: Assuming k1=k2=k, m1=m2=m, input stiffness disturbance Δk to the equivalent resonator (101), the output amplitude ratio formula is obtained: According to formula (4), k c is a constant, and the only factor causing the change of the amplitude ratio u1 and u2 is the stiffness disturbance Δk; assuming that the resistance of the sampling resistor is R, the stiffness disturbance Δk of the resonator caused by the input current I is expressed as: Where ε is the dielectric constant of vacuum, A is the area facing the input electrode plate, V dc is the DC bias voltage, d is the plate spacing, R is the sampling resistor value, and I is the detected current. From equation (5), we can see that the negative stiffness disturbance caused by the current is a quadratic function of the current. Therefore, it is necessary to consider linearizing the quadratic function so that the amplitude ratio output and input current are approximately linear within a certain current input range. To ensure the weak coupling connection of the resonator, the coupling stiffness k c It is much smaller than the negative stiffness disturbance Δk caused by the current. u1 in equation (4) is simplified to: Substituting equation (5) into equation (6) yields the theoretical model of the ratio of input current to output amplitude: A MEMS current sensor based on modal localization effect is designed according to formulas (1)-(6) and a theoretical model of the ratio of input current to output amplitude, comprising two resonators with exactly the same stiffness, resonator one (201) and resonator two (202); the two resonators are horizontally distributed and connected by a coupling beam (212), and the stiffness ratio of the coupling beam to the resonator is not higher than 1:100; The resonators are fixed by a central anchor point (213), and the two ends are fixed by a left anchor point (214) and a right anchor point (215); the left anchor point (214) and the right anchor point (215) apply a DC bias voltage to the entire weakly coupled resonator; a sampling resistor (203) and an input electrode (203) are provided at a certain gap outside the resonator one (201); an adjustment electrode one (210) adjusts the initial working point; a detection electrode one (206) and a detection electrode two (207) at a certain gap on the other side of the resonator one (201) form a differential detection of the resonator one (201). The measuring electrode detects the amplitude of the resonator 1 (201); an AC driving electrode (205) is provided at a certain gap outside the resonator 2 (202); the regulating electrode 2 (211) adjusts the initial working point; the detecting electrode 3 (208) and the detecting electrode 4 (209) at a certain gap on the other side form a differential detecting electrode of the resonator 2 (202) to detect the amplitude of the resonator 2 (202); the AC driving electrode (205), the left anchor point (214) and the right anchor point (215) apply a driving force to the entire resonator to make the resonator perform simple harmonic vibration; The design range of sampling resistor value should be 10-1000Ω; The method for preparing the sampling resistor (203) comprises: using an ion implantation process to reduce the resistivity of single crystal silicon, and depositing gold on the surface of the structure around the sampling resistor to reduce the resistance; After the current I to be measured flows into the sampling resistor, a corresponding voltage V is obtained. The voltage V is electrically connected to obtain an equipotential voltage V on the corresponding capacitor plate of the equivalent resonator (101), so that the equivalent resonator (101) obtains an input stiffness disturbance Δk. The sampling resistor uses an ion implantation process, and gold is deposited on the surface of the structure around the sampling resistor.

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