WO2006062275A1 - Variable inductor type mems pressure sensor using magnetostrictive effect - Google Patents

Variable inductor type mems pressure sensor using magnetostrictive effect Download PDF

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Publication number
WO2006062275A1
WO2006062275A1 PCT/KR2005/001281 KR2005001281W WO2006062275A1 WO 2006062275 A1 WO2006062275 A1 WO 2006062275A1 KR 2005001281 W KR2005001281 W KR 2005001281W WO 2006062275 A1 WO2006062275 A1 WO 2006062275A1
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Prior art keywords
inductor
substrate
pressure sensor
array unit
thin film
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PCT/KR2005/001281
Other languages
French (fr)
Inventor
Wang Sup Kim
Bum Kyoo Choi
Jae Geun Oh
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Mdt Co., Ltd.
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Publication date
Application filed by Mdt Co., Ltd. filed Critical Mdt Co., Ltd.
Priority to JP2007545355A priority Critical patent/JP4621257B2/en
Priority to EP05740754A priority patent/EP1825236A4/en
Publication of WO2006062275A1 publication Critical patent/WO2006062275A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/12—Measuring force or stress, in general by measuring variations in the magnetic properties of materials resulting from the application of stress
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/14—Measuring force or stress, in general by measuring variations in capacitance or inductance of electrical elements, e.g. by measuring variations of frequency of electrical oscillators

Definitions

  • the present invention generally relates to a pressure sensor, and more specifically, to a pressure sensor obtained by embodying a variable inductance-type pressure sensor which has a magnetostrictive material as a core using a magnetostrictive effect which means that a magnetic permeability is remarkably changed depending on externally applied pressure through a Micro Electro Mechanical System (hereinafter, referred to as "MEMS") technology.
  • MEMS Micro Electro Mechanical System
  • a semiconductor pressure sensor which is a recently commercialized pressure sensor, has no hysteresis phenomenon where a characteristic curve when a pressure is applied is different from that when the pressure is decreased, and has excellent linearity. Also, even a miniaturized and light- weighted semiconductor pressure sensor is remarkably strong to vibration. In addition, the semiconductor pressure sensor has high sensitivity and reliability and excellent mass productivity than a mechanical sensor.
  • a micro-pressure sensor using a MEMS process technology includes a piezoresistive sensor using a piezoelectric/ piezoresistive effect, a capacitance sensor to measure change of capacitance by movement of a thin film depending on pressure application, and a vibration sensor to measure change of a resonant frequency of beam.
  • the piezoresistive pressure sensor detects the change amount of a resistive component using diffusion resistance constituted with a bridge pattern on a diaphragm of a silicon wafer, and then senses a pressure by detecting the change amount of the resistive component through a signal detecting unit.
  • the piezoresistive pressure sensor has been generally used, the sensor requires an additional circuit while manufactured with a wheatstone bridge method due to sensitivity of piezo materials depending on temperature, and telemetry is not facilitated.
  • the capacitance pressure sensor In the capacitance pressure sensor, an interval of electrode plates facing each other is changed by an external stress, and capacitance between the electrode plates is changed.
  • the capacitance pressure sensor converts the change into an electric signal to measure the stress.
  • the capacitance pressure sensor has low sensitivity depending on temperature, and can be manufactured with a MEMS technology and operated even at a low pressure change, so that the capacitance pressure sensor has been widely used for precise measurement.
  • the capacitance pressure sensor has a low FOM ( Figure of Merit) which represents sensitivity of the sensor, a span region is narrow band, and its manufacturing process is more difficult than that of the piezoresistive sensor.
  • a variable inductor type MEMS pressure sensor using a magnetostrictive effect comprises an inductor array unit and a capacitor unit.
  • the inductor array unit includes a coil unit having a plurality of serially connected circular electrodes formed on a first substrate and magnetostrictive material thin films which correspond one by one to the circular electrode and are formed on a second substrate opposite to the first substrate at a predetermined distance in parallel to form an inductor which has the magnetostrictive material thin film as a core of the coil unit for inducing change of magnetic permeability of the magnetostrictive thin film depending on external pressure to vary inductance of the inductor.
  • the capacitor unit constitutes a LC resonant circuit with the inductor array unit to convert magnetic energy discharged in the inductor array unit into a voltage.
  • FIG. 1 is a plane diagram illustrating a variable inductor type MEMS pressure sensor using a magnetostrictive effect according to an embodiment of the present invention.
  • FIG. 2 is a diagram illustrating an equivalent circuit of Fig. 1.
  • Fig. 3 is a cross-sectional diagram illustrating the sensor of Fig. 1 cut in an A- A'direction.
  • FIGS. 4-11 are cross-sectional diagrams illustrating a manufacturing process of an inductor array unit 100 of Fig. 3.
  • Fig. 12 is a diagram illustrating a general solenoid model having a magnetostrictive material as a core.
  • Fig. 13 is a diagram illustrating when a pressure is applied to one unit cell of the inductor array unit 100 of Fig. 1.
  • FIG. 14 is a circuit diagram illustrating how wireless telemetry works using mutual inductance. Best Mode for Carrying Out the Invention
  • Fig. 1 is a plane diagram illustrating a variable inductor type MEMS pressure sensor using a magnetostrictive effect according to an embodiment of the present invention.
  • a variable inductor type MEMS pressure sensor using a magnetostrictive effect comprises an inductor array unit 100 and a capacitor unit 200 which constitute a LC resonant circuit.
  • the inductor array unit 100 induces transformation of a magnetostrictive material thin film depending on an externally applied pressure to vary permeability of the magnetostrictive material thin film, thereby changing inductance of the sensor.
  • the inductor array unit 100 comprises a plurality of unit cells 130.
  • Each of the unit cells 130 includes a coil circular electrode 110 and a magnetostrictive material thin film 120 which corresponds to the coil circular electrode and is formed apart at a predetermined distance from the center of the corresponding coil circular electrode 110.
  • the plurality of coil circular electrodes 110 are serially connected on a glass substrate, and the magnetostrictive thin films 120 are formed on a dielectric thin film opposed in parallel to the glass substrate at a predetermined interval.
  • the inductor array unit 100 is embodied by solenoid having a magnetostrictive material as a core through a MEMS process technology.
  • the plurality of coil circular electrodes 110 formed on the substrate are electrically connected in series to form a coil unit of an inductor.
  • the magnetostrictive material thin films 120 are formed corresponding to the center of each circular electrode 110, so that a plurality of solenoids each having a magnetostrictive material as a core constitute circuits equivalent to a serially connected inductor network as shown in Fig. 2.
  • amorphous and single crystal alloys are used for the magnetostrictive material.
  • the capacitor unit 200 converts magnetic energy discharged in the inductor array unit 100 into a voltage and stores the voltage.
  • the capacitor unit 200 is connected to the coil circular electrodes of both ends among the serially connected coil circular electrodes 110, thereby constituting a LC tank circuit (LC resonant circuit) with the inductor array unit 100.
  • the energy change generated from the coil unit (primary winding coil) of the inductor array unit 100 by the external pressure is transmitted into the inductor (secondary winding coil) of an external measuring device (not shown) by mutual inductance effect.
  • the external measuring device (not shown) calculates the change of inductance measured by the second coil, thereby enabling power-free/wireless remote measurement of the pressure applied to the inductor array unit 100.
  • Fig. 3 is a cross-sectional diagram illustrating the sensor of Fig. 1 cut in an A-A' direction to show the detailed structure of the inductor array unit 100.
  • the inductor array unit 100 comprises a lower substrate 140 and an upper substrate
  • the circular electrodes are formed on the upper surface of the lower substrate 140 facing the upper substrate 150.
  • the circular electrodes 110 are made of Au or Cu through a electroplating or other process for forming a thick film metal. Pyrex or quartz glass is used for the lower substrate 140.
  • a backing plate 170 for absorbing external vibration and adhering a sensor to the housing 400 is formed on a lower surface of the lower substrate 400.
  • the backing plate 170 consists of soft polymer.
  • the magnetostrictive material thin films 120 corresponding to each circular electrode 110 are formed on a lower surface of the upper substrate 150 facing an upper surface of the lower substrate 140.
  • a pair of the circular electrode 110 and the magnetostrictive material thin film 120 form one unit cell 130.
  • the magnetostrictive material thin film 120 serves as an inductor core on each unit cell 130 of the inductor array unit 100.
  • the magnetomaterial thin film 120 is a thin film formed on the lower surface of the upper substrate 150 obtained by performing a metal film deposition method using magnetostrictive materials formed of amorphous or single crystal alloys.
  • a pressure phase dips 180 corresponding one by one to the magnetostrictive material thin film 120 are formed on the upper surface of the upper substrate so that an externally applied pressure may be easily transmitted into the magnetostrictive material thin film 120.
  • a dielectric thin film is used for the upper substrate 150.
  • the lower substrate 140 is separated at a predetermined distance from the upper substrate 150 by a spacer 160 so that the circular electrode 110 of the lower substrate 140 may not contact with the magnetostrictive material thin film 120 of the upper substrate 150 when the dielectric thin film is transformed by the externally applied pressure.
  • a space formed by the spacer 160 serves as a reference pressure chamber.
  • the spacer 160 is formed of silicon or its similar material.
  • a diaphragm 300 consisting of silicon rubber to intercept direct contact with external materials is formed on the upper surface of the sensor for receiving the external pressure.
  • the housing 400 for protecting the other surfaces except the upper surface of the sensor and fixing the sensor and diaphragm 300 is formed to cover the side surface of the lower substrate 140 and the upper substrate 150.
  • FIGs. 4 to 11 are cross-sectional diagrams illustrating a manufacturing process of the inductor array unit 100 of Fig. 3. In the embodiment, a manufacturing process of only one unit cell is described.
  • a silicon etch mask such as SiO or Si N is grown or deposited on a silicon wafer
  • a wet etching process is performed on a portion of the upper substrate 150 where the pressure phase dip 180 is formed, so that the corresponding region is formed to have a thickness t as shown in Fig. 4.
  • a magnetostrictive material is vacuum-deposited on the lower surface (upper surface of Fig. 5) of the upper substrate 150 corresponding to the pressure phase dip 180, and the deposited magnetostrictive thin film is etched so that the magnetostrictive material thin film 120 is formed on the upper substrate 150 as shown in Fig. 5.
  • a metal seed layer 142 is deposited on the pyrex or quartz glass
  • a thick film photoresist PR 144 is patterned to have a coil shape on the metal seed layer 142. After Au or Cu is electroplated depending on the patterned shape to form the circular electrode 110, the thick film PR 144 is removed as sown in Fig. 7.
  • an etching process is performed on the rest portion except a portion between the glass substrate 140 and the circular electrode 110 of the metal seed layer 142.
  • ER represents the etching ratio.
  • the spacer 160 formed of silicon is formed on the glass substrate 140 by anodic bonding.
  • the upper substrate 150 of Fig. 5 is bonded to the other surface of the spacer 160 so that the magnetostrictive material thin film 120 may correspond to the center of the circular electrode 110.
  • the bonding method includes a fusion bonding, an eutectic bonding and an organic bonding.
  • the sensor of Fig. 10 is safely positioned in the housing 170.
  • Equation 1 A differential equation of the magnetic permeability when a stress is applied to the magnetostrictive material under an AC condition is represented by Equations 1 and 2.
  • Equations 1 and 2 ⁇ : magnetostrictive constant, K : anisotropy constant, M : magnetization, ⁇ : applied stress, ⁇ : magnetic permeability in AC condition)
  • Equation 3 shows the inductance of the solenoid having the magnetostrictive material as a core.
  • Equation 1 represents the relationship between the stress and the magnetic permeability to the magnetostrictive material in Equation 3.
  • Fig. 13 is a diagram illustrating when a pressure is applied to one unit cell of the inductor array unit 100 of Fig. 1.
  • the dielectric thin film 150 is transformed by the pressure, so that a stress is applied to the magnetostrictive material thin film 120 deposited on the dielectric thin film 150 depending on the stress generated from the transformation.
  • the magnetostrictive material thin film 120 is mechanically transformed, so that the relative permeability of the magnetostrictive material thin film 120 is changed.
  • the inductance of the inductor array unit 100 is changed as shown in Equation 4.
  • the inductor array unit 100 is a primary winding inductor and an inductor (not shown) of the external measuring device is a secondary winding inductor
  • the energy change of the primary winding inductor depending on the inductance change of the inductor array unit 100 is transmitted into the secondary winding inductor by mutual inductance effect.
  • Fig. 14 is a circuit diagram illustrating how wireless telemetry works using mutual inductance. Generally used equations can be employed in the unit cell of Fig. 13 even when the principle of circuit of Fig. 14 is applied.
  • Equation 5 shows input impedance from the primary windings in a resonant frequency ⁇
  • Equation 6 represents mutual inductance M between the primary winding inductor and the secondary winding inductor.
  • Equation 6 k is the coupling coefficient between the two inductors, and represented by Equation 7. [70] [Equation 7]
  • the pressure applied to the sensor can be measured by calculating the input impedance as shown in Equation 5 to measure the change amount of the energy.
  • Equation 5 a large change is shown in the secondary winding impedance of Equation 5 only when the maximum energy is transmitted from the primary winding inductor 100 to the secondary winding inductor.
  • the condition where the maximum energy can be transmitted is when the resonant frequency f is generated.
  • Equation 8 The pressure phase dip 180 to each unit cell is represented by Equation 8, and its quality factor Q is represented by Equation 9.
  • Table 1 shows the relationship of parameters on the size of the pressure phase dip. As shown in Table 1, if the inductor array unit 100 comprises a plurality of inductors Ns connected serially to increase the entire inductance Ls and has a large magnetic permeability ⁇ , the pressure sensor according to an embodiment of the present invention is expected to show more excellent performance than a conventional MEMS LC resonant type pressure sensor.
  • variable inductor type MEMS pressure sensor using a magne- tostrictive effect has an excellent resolution because it is more sensitive than a conventional piezoresistive or capacitance sensor.
  • the variable inductor type MEMS pressure sensor using a magnetostrictive effect is manufactured using a MEMS process technology exchangeable with a semiconductor process, thereby enabling miniaturization and a mass package process to reduce the cost of production.
  • the above-described pressure sensor can be used as an implantable or real-time diagnosis system because the pressure sensor may be a power-free/wireless sensor to measure pressure at a power- free state without a power source.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Fluid Pressure (AREA)
  • Force Measurement Appropriate To Specific Purposes (AREA)
  • Micromachines (AREA)

Abstract

A variable inductor type MEMS pressure sensor using a magnetostrictive effect comprises an inductor array unit and a capacitor unit. The inductor array unit includes a coil unit having a plurality of serially connected circular electrodes formed on a first substrate and a magnetostrictive material thin film corresponding one by one to the circular electrode formed on a second substrate opposite to the first substrate at a predetermined distance in parallel to form an inductor which has the magnetostrictive material thin film as a core of the coil unit for inducing change of magnetic permeability of the magnetostrictive thin film depending on external pressure to vary inductance of the inductor. The capacitor unit constitutes a LC resonant circuit with the inductor array unit to convert magnetic energy discharged in the inductor array unit into a voltage. The variable inductor type MEMS pressure sensor has an excellent resolution because it is more sensitive than a conventional piezoresistive or capacitance sensor, and is manufactured using a MEMS process technology exchangeable with a semiconductor process, thereby enabling miniaturization and a mass package process to reduce the cost of production.

Description

Description
VARIABLE INDUCTOR TYPE MEMS PRESSURE SENSOR USING MAGNETOSTRICTIVE EFFECT
Technical Field
[1] The present invention generally relates to a pressure sensor, and more specifically, to a pressure sensor obtained by embodying a variable inductance-type pressure sensor which has a magnetostrictive material as a core using a magnetostrictive effect which means that a magnetic permeability is remarkably changed depending on externally applied pressure through a Micro Electro Mechanical System (hereinafter, referred to as "MEMS") technology. Background Art
[2] A semiconductor pressure sensor, which is a recently commercialized pressure sensor, has no hysteresis phenomenon where a characteristic curve when a pressure is applied is different from that when the pressure is decreased, and has excellent linearity. Also, even a miniaturized and light- weighted semiconductor pressure sensor is remarkably strong to vibration. In addition, the semiconductor pressure sensor has high sensitivity and reliability and excellent mass productivity than a mechanical sensor.
[3] A micro-pressure sensor using a MEMS process technology includes a piezoresistive sensor using a piezoelectric/ piezoresistive effect, a capacitance sensor to measure change of capacitance by movement of a thin film depending on pressure application, and a vibration sensor to measure change of a resonant frequency of beam.
[4] The piezoresistive pressure sensor detects the change amount of a resistive component using diffusion resistance constituted with a bridge pattern on a diaphragm of a silicon wafer, and then senses a pressure by detecting the change amount of the resistive component through a signal detecting unit. Although the piezoresistive pressure sensor has been generally used, the sensor requires an additional circuit while manufactured with a wheatstone bridge method due to sensitivity of piezo materials depending on temperature, and telemetry is not facilitated.
[5] In the capacitance pressure sensor, an interval of electrode plates facing each other is changed by an external stress, and capacitance between the electrode plates is changed. The capacitance pressure sensor converts the change into an electric signal to measure the stress. The capacitance pressure sensor has low sensitivity depending on temperature, and can be manufactured with a MEMS technology and operated even at a low pressure change, so that the capacitance pressure sensor has been widely used for precise measurement. However, since the capacitance pressure sensor has a low FOM (Figure of Merit) which represents sensitivity of the sensor, a span region is narrow band, and its manufacturing process is more difficult than that of the piezoresistive sensor.
[6] When the piezoresistive or the capacitance pressure sensor is used at a high frequency band (over UHF band), influence of high frequency parasite components and of resistive components of a semiconductor (Si group) becomes larger as a frequency becomes higher, so that a material/structure which can be used at a specific structure or a specific high frequency band is required. As a result, there is no high frequency pressure sensor that has been currently released.
[7] Meanwhile, although a method of measuring a pressure using change of inductance has been under way for about ten years, the method has been developed to application related to a RF tag rather than a result on the sensor itself. A general shape of a coil is too large to be applied to a micro-sensor, and requires a difficult process although it can be fabricated to have a MEMS structure, so that the method is not practical. Disclosure of Invention Technical Problem
[8] It is an object of the present invention to provide an inductor- type pressure sensor which is suitable for high-frequency application, able to be produced through a MEMS process and has high FOM (sensitivity of sensor) for precise measurement. Technical Solution
[9] In order to achieve the above-described object, a variable inductor type MEMS pressure sensor using a magnetostrictive effect comprises an inductor array unit and a capacitor unit. The inductor array unit includes a coil unit having a plurality of serially connected circular electrodes formed on a first substrate and magnetostrictive material thin films which correspond one by one to the circular electrode and are formed on a second substrate opposite to the first substrate at a predetermined distance in parallel to form an inductor which has the magnetostrictive material thin film as a core of the coil unit for inducing change of magnetic permeability of the magnetostrictive thin film depending on external pressure to vary inductance of the inductor. The capacitor unit constitutes a LC resonant circuit with the inductor array unit to convert magnetic energy discharged in the inductor array unit into a voltage. Brief Description of the Drawings
[10] Fig. 1 is a plane diagram illustrating a variable inductor type MEMS pressure sensor using a magnetostrictive effect according to an embodiment of the present invention.
[11] Fig. 2 is a diagram illustrating an equivalent circuit of Fig. 1.
[12] Fig. 3 is a cross-sectional diagram illustrating the sensor of Fig. 1 cut in an A- A'direction.
[13] Figs. 4-11 are cross-sectional diagrams illustrating a manufacturing process of an inductor array unit 100 of Fig. 3.
[14] Fig. 12 is a diagram illustrating a general solenoid model having a magnetostrictive material as a core.
[15] Fig. 13 is a diagram illustrating when a pressure is applied to one unit cell of the inductor array unit 100 of Fig. 1.
[16] Fig. 14 is a circuit diagram illustrating how wireless telemetry works using mutual inductance. Best Mode for Carrying Out the Invention
[17] The present invention will be described in detail with reference to the accompanying drawings.
[18] Fig. 1 is a plane diagram illustrating a variable inductor type MEMS pressure sensor using a magnetostrictive effect according to an embodiment of the present invention.
[19] In an embodiment, a variable inductor type MEMS pressure sensor using a magnetostrictive effect comprises an inductor array unit 100 and a capacitor unit 200 which constitute a LC resonant circuit.
[20] The inductor array unit 100 induces transformation of a magnetostrictive material thin film depending on an externally applied pressure to vary permeability of the magnetostrictive material thin film, thereby changing inductance of the sensor. The inductor array unit 100 comprises a plurality of unit cells 130. Each of the unit cells 130 includes a coil circular electrode 110 and a magnetostrictive material thin film 120 which corresponds to the coil circular electrode and is formed apart at a predetermined distance from the center of the corresponding coil circular electrode 110. Here, the plurality of coil circular electrodes 110 are serially connected on a glass substrate, and the magnetostrictive thin films 120 are formed on a dielectric thin film opposed in parallel to the glass substrate at a predetermined interval. That is, the inductor array unit 100 is embodied by solenoid having a magnetostrictive material as a core through a MEMS process technology. The plurality of coil circular electrodes 110 formed on the substrate are electrically connected in series to form a coil unit of an inductor. The magnetostrictive material thin films 120 are formed corresponding to the center of each circular electrode 110, so that a plurality of solenoids each having a magnetostrictive material as a core constitute circuits equivalent to a serially connected inductor network as shown in Fig. 2. Here, amorphous and single crystal alloys are used for the magnetostrictive material.
[21] The capacitor unit 200 converts magnetic energy discharged in the inductor array unit 100 into a voltage and stores the voltage. The capacitor unit 200 is connected to the coil circular electrodes of both ends among the serially connected coil circular electrodes 110, thereby constituting a LC tank circuit (LC resonant circuit) with the inductor array unit 100.
[22] The energy change generated from the coil unit (primary winding coil) of the inductor array unit 100 by the external pressure is transmitted into the inductor (secondary winding coil) of an external measuring device (not shown) by mutual inductance effect. The external measuring device (not shown) calculates the change of inductance measured by the second coil, thereby enabling power-free/wireless remote measurement of the pressure applied to the inductor array unit 100.
[23] Fig. 3 is a cross-sectional diagram illustrating the sensor of Fig. 1 cut in an A-A' direction to show the detailed structure of the inductor array unit 100.
[24] The inductor array unit 100 comprises a lower substrate 140 and an upper substrate
150 arranged in parallel apart at a predetermined distance from each other in a housing 400. The circular electrodes are formed on the upper surface of the lower substrate 140 facing the upper substrate 150. Here, the circular electrodes 110 are made of Au or Cu through a electroplating or other process for forming a thick film metal. Pyrex or quartz glass is used for the lower substrate 140.
[25] A backing plate 170 for absorbing external vibration and adhering a sensor to the housing 400 is formed on a lower surface of the lower substrate 400. The backing plate 170 consists of soft polymer.
[26] The magnetostrictive material thin films 120 corresponding to each circular electrode 110 are formed on a lower surface of the upper substrate 150 facing an upper surface of the lower substrate 140. A pair of the circular electrode 110 and the magnetostrictive material thin film 120 form one unit cell 130. The magnetostrictive material thin film 120 serves as an inductor core on each unit cell 130 of the inductor array unit 100. The magnetomaterial thin film 120 is a thin film formed on the lower surface of the upper substrate 150 obtained by performing a metal film deposition method using magnetostrictive materials formed of amorphous or single crystal alloys.
[27] A pressure phase dips 180 corresponding one by one to the magnetostrictive material thin film 120 are formed on the upper surface of the upper substrate so that an externally applied pressure may be easily transmitted into the magnetostrictive material thin film 120. A dielectric thin film is used for the upper substrate 150.
[28] The lower substrate 140 is separated at a predetermined distance from the upper substrate 150 by a spacer 160 so that the circular electrode 110 of the lower substrate 140 may not contact with the magnetostrictive material thin film 120 of the upper substrate 150 when the dielectric thin film is transformed by the externally applied pressure. A space formed by the spacer 160 serves as a reference pressure chamber. The spacer 160 is formed of silicon or its similar material.
[29] A diaphragm 300 consisting of silicon rubber to intercept direct contact with external materials is formed on the upper surface of the sensor for receiving the external pressure. The housing 400 for protecting the other surfaces except the upper surface of the sensor and fixing the sensor and diaphragm 300 is formed to cover the side surface of the lower substrate 140 and the upper substrate 150.
[30] Figs. 4 to 11 are cross-sectional diagrams illustrating a manufacturing process of the inductor array unit 100 of Fig. 3. In the embodiment, a manufacturing process of only one unit cell is described.
[31] A silicon etch mask such as SiO or Si N is grown or deposited on a silicon wafer
2 3 x to have a sufficient thickness so that the silicon etch mask may serve as an etch mask. A wet etching process is performed on a portion of the upper substrate 150 where the pressure phase dip 180 is formed, so that the corresponding region is formed to have a thickness t as shown in Fig. 4.
[32] A magnetostrictive material is vacuum-deposited on the lower surface (upper surface of Fig. 5) of the upper substrate 150 corresponding to the pressure phase dip 180, and the deposited magnetostrictive thin film is etched so that the magnetostrictive material thin film 120 is formed on the upper substrate 150 as shown in Fig. 5.
[33] As shown in Fig. 6, a metal seed layer 142 is deposited on the pyrex or quartz glass
140 apart from the upper substrate 150 where the magnetostrictive material thin film 120 is formed. Next, a thick film photoresist PR 144 is patterned to have a coil shape on the metal seed layer 142. After Au or Cu is electroplated depending on the patterned shape to form the circular electrode 110, the thick film PR 144 is removed as sown in Fig. 7.
[34] Referring to Fig. 8, an etching process is performed on the rest portion except a portion between the glass substrate 140 and the circular electrode 110 of the metal seed layer 142. Here, the selectivity to metal seed etching solution between the circular electrode 110 and the metal seed layer 142 is ER : ER = 1 : 10 or more, wherein coil seed
ER represents the etching ratio. [35] Thereafter, as shown in Fig. 9, the spacer 160 formed of silicon is formed on the glass substrate 140 by anodic bonding. [36] As shown in Fig. 10, the upper substrate 150 of Fig. 5 is bonded to the other surface of the spacer 160 so that the magnetostrictive material thin film 120 may correspond to the center of the circular electrode 110. Here, the bonding method includes a fusion bonding, an eutectic bonding and an organic bonding. [37] Next, after the internal bottom surface of the housing 400 formed through injection molding is covered with epoxy 170, the sensor of Fig. 10 is safely positioned in the housing 170. Then, the supper surface of the device is covered with a passivation film such as silicon rubber, as shown in Fig. 11. [38] Hereinafter, the operation of the variable inductor type MEMS pressure sensor according to an embodiment of the preset invention will be described. [39] A differential equation of the magnetic permeability when a stress is applied to the magnetostrictive material under an AC condition is represented by Equations 1 and 2. [40] [Equation 1]
[41] μ Q MZ μAC~ 2K-3λo + 1 [42] [Equation 2]
[43]
Figure imgf000007_0001
[44] (in Equations 1 and 2, λ: magnetostrictive constant, K : anisotropy constant, M : magnetization, σ: applied stress, μ : magnetic permeability in AC condition)
[45] Through the above-described Equations 1 and 2, it is known that the magnetic permeability is changed when the stress is applied to the magnetostrictive material. The change of the permeability induces change of the inductance. As a result, the pressure applied to the magnetostrictive material is obtained by measuring the change of the inductance.
[46] As shown in Fig. 12, Equation 3 shows the inductance of the solenoid having the magnetostrictive material as a core. When a compression stress is applied towards the surface A of the solenoid core, the change of the inductance can be obtained by substituting Equation 1 which represents the relationship between the stress and the magnetic permeability to the magnetostrictive material in Equation 3.
[47] [Equation 3]
[48]
_ μ 0 μ r N A
I
[49] (here, L : Inductance in H, μ :magnetic permeability of free space, μ : relative magnetic permeability of solenoid core, N : number of turns of conductor in solenoid, A : cross-sectional area of solenoid, 1 : length of solenoid over which conductor turns are arranged)
[50] [Equation 4]
[51]
Figure imgf000008_0001
[52] According to the above-described principle, when a pressure is applied to the diaphragm 300, the diaphragm 300 gets bent, and the pressure is transmitted into the dielectric thin film 150 through the pressure phase dip 180, so that the dielectric thin film 150 is transformed as shown in Fig. 13.
[53] Fig. 13 is a diagram illustrating when a pressure is applied to one unit cell of the inductor array unit 100 of Fig. 1. The dielectric thin film 150 is transformed by the pressure, so that a stress is applied to the magnetostrictive material thin film 120 deposited on the dielectric thin film 150 depending on the stress generated from the transformation. As a result, the magnetostrictive material thin film 120 is mechanically transformed, so that the relative permeability of the magnetostrictive material thin film 120 is changed.
[54] If the relative permeability of the magnetostrictive thin film 120 which is the solenoid core is changed, the inductance of the inductor array unit 100 is changed as shown in Equation 4. When the inductor array unit 100 is a primary winding inductor and an inductor (not shown) of the external measuring device is a secondary winding inductor, the energy change of the primary winding inductor depending on the inductance change of the inductor array unit 100 is transmitted into the secondary winding inductor by mutual inductance effect.
[55] Fig. 14 is a circuit diagram illustrating how wireless telemetry works using mutual inductance. Generally used equations can be employed in the unit cell of Fig. 13 even when the principle of circuit of Fig. 14 is applied.
[56] That is, Equation 5 shows input impedance from the primary windings in a resonant frequency ω , and Equation 6 represents mutual inductance M between the primary winding inductor and the secondary winding inductor.
[57] [Equation 5]
[58]
Figure imgf000008_0002
[59] R : Resistance at the primary winding (Parasitic)
[60] L : Inductance at the primary winding
[61] R : Resistance at the secondary winding (Parasitic)
[62] L : Inductance at the secondary winding
[63] C : Capacitance at the secondary winding (sensor) [64] M : Mutual inductance
[65] K : Coupling coefficient
[66] ω : Resonant frequency
[67] [Equation 6]
[68]
Figure imgf000009_0001
[69] In Equation 6, k is the coupling coefficient between the two inductors, and represented by Equation 7. [70] [Equation 7]
[71]
Figure imgf000009_0002
[72] z : distance between the two inductors
[73] r : radius of the secondary winding inductor (coil)
P
[74] r : radius of the primary winding inductor (coil)
[75] In the external measuring device, the pressure applied to the sensor can be measured by calculating the input impedance as shown in Equation 5 to measure the change amount of the energy.
[76] In the above-described Equations 5-7, a large change is shown in the secondary winding impedance of Equation 5 only when the maximum energy is transmitted from the primary winding inductor 100 to the secondary winding inductor. The condition where the maximum energy can be transmitted is when the resonant frequency f is generated.
[77] The pressure phase dip 180 to each unit cell is represented by Equation 8, and its quality factor Q is represented by Equation 9. tank
[78] [Equation 8]
[79]
- 1 2
Δφ= tan ( k Q bank)
[80] [Equation 9]
[81]
Figure imgf000009_0003
[82] Therefore, if the pressure phase dip 180 becomes larger, the quality factor also increases to be advantageous for high frequency or wireless sensor application. [83] Table 1 shows the relationship of parameters on the size of the pressure phase dip. As shown in Table 1, if the inductor array unit 100 comprises a plurality of inductors Ns connected serially to increase the entire inductance Ls and has a large magnetic permeability μ , the pressure sensor according to an embodiment of the present invention is expected to show more excellent performance than a conventional MEMS LC resonant type pressure sensor.
[84] Table 1
Figure imgf000010_0001
Industrial Applicability
[85] As described above, a variable inductor type MEMS pressure sensor using a magne- tostrictive effect according to an embodiment of the present invention has an excellent resolution because it is more sensitive than a conventional piezoresistive or capacitance sensor. Additionally, the variable inductor type MEMS pressure sensor using a magnetostrictive effect is manufactured using a MEMS process technology exchangeable with a semiconductor process, thereby enabling miniaturization and a mass package process to reduce the cost of production. Also, the above-described pressure sensor can be used as an implantable or real-time diagnosis system because the pressure sensor may be a power-free/wireless sensor to measure pressure at a power- free state without a power source.

Claims

Claims
[1] A variable inductor type MEMS pressure sensor using a magnetostrictive effect, comprising: an inductor array unit including a coil unit having a plurality of circular electrodes connected serially and formed on a first substrate and magnetostrictive material thin films which correspond one by one to the circular electrodes and are formed on a second substrate facing the first substrate at a predetermined distance in parallel to form an inductor which has the magnetostrictive material thin film as a core of the coil unit for inducing change of magnetic permeability of the magnetostrictive thin film depending on external pressure to vary inductance of the inductor; and a capacitor unit for constituting a LC resonant circuit with the inductor array unit to convert magnetic energy discharged from the inductor array unit into a voltage.
[2] The MEMS pressure sensor according to claim 1, wherein the magnetostrictive material thin film is an amorphous material or a single crystal alloy.
[3] The MEMS pressure sensor according to claim 1 or 2, wherein the inductor array unit comprises: the first substrate having the serially connected circular electrodes formed on a first surface; the second substrate having the magnetostrctive thin films which correspond one by one to the circular electrodes and are deposited on a second surface facing the first surface and having pressure phase dips formed on a third surface which is a backside of the second surface so that external pressure is transmitted into the magnetostrictive material thin films; and a spacer for maintaining the first substrate apart at a predetermined distance from the second substrate not to directly contact with the circular electrodes and the magnetostrictive material thin films.
[4] The MEMS pressure sensor according to claim 3, wherein the first substrate is pyrex or quartz glass.
[5] The MEMS pressure sensor according to claim 3, wherein the second substrate is a dielectric thin film.
[6] The MEMS pressure sensor according to claim 1, further comprising: a diaphragm for covering the upper surface of the sensor where an external pressure is applied; and a housing for covering the outer surface of the sensor except the upper surface of the sensor and fixing the sensor and the diaphragm. [7] The MEMS pressure sensor according to claim 1, further comprising a measuring device, which comprises a secondary winding coil unit for wirelessly receiving inductance change of the inductor array unit, for calculating inductance change of the inductor array unit received through the secondary winding coil to measure pressure applied to the inductor array unit.
PCT/KR2005/001281 2004-12-08 2005-05-03 Variable inductor type mems pressure sensor using magnetostrictive effect WO2006062275A1 (en)

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