JP4689542B2 - Membrane stiffness measuring apparatus and measuring method - Google Patents

Description

  The present invention relates to a measurement method for measuring the stiffness (stiffness) of a diaphragm in an electrostatic electroacoustic transducer such as a condenser microphone, and a measurement apparatus therefor, and in particular, provides a highly reproducible film stiffness measurement method. To do.

  An electrostatic electroacoustic transducer is an electroacoustic transducer that converts sound into an electric signal by using electrostatic energy as an intermediary, and conversely converts an electric signal into sound. An electret condenser microphone (ECM) and an electret condenser speaker are classified into this electrostatic electroacoustic transducer.

  The ECM can be miniaturized and is widely used in mobile phones. As shown in FIG. 6B, the conventional ECM includes a vibration plate 11 such as a metal conductor, a fixed electrode 12 on which an electret film 13 is formed, and a circuit element in a case 17 having a sound hole 15. The printed circuit board 18 is disposed, the distance between the diaphragm 11 and the fixed electrode 12 is held by the spacer 14, and the back air chamber 16 is formed between the fixed electrode 12 and the printed circuit board 18. Yes.

  The electret film is usually formed using FEP (fluorinated ethylene propylene resin), and keeps a given charge. In this ECM, when the diaphragm 11 vibrates due to sound pressure, the capacitance of the plate capacitor formed by the diaphragm 11 and the fixed electrode 12 changes, and is converted into a voltage change and output from the ECM.

  FEP, which is a polymer film, has the property of retaining charge semipermanently, but loses its charge retention property when exposed to high temperatures. Therefore, it is difficult for the ECM to be mounted by solder reflow.

  In recent years, in order to realize a heat-resistant ECM that can be reflowed, a technology for manufacturing an ultra-small MEMS (Micro Electro Mechanical Systems) microphone by applying an ultra-precision processing technique used in a semiconductor process to a silicon substrate has been developed. Yes.

  FIG. 6A shows an example of the MEMS microphone 20. On the silicon substrate, a large number of microphone chips are simultaneously manufactured using a semiconductor manufacturing technique, and finally divided into individual pieces. FIG. 6A shows a side view of one divided microphone chip. This MEMS microphone 20 has a vibrating membrane electrode 23 and an electret film 24 on a silicon substrate 21 via a first insulating layer 22, and a second insulating layer 25 is formed thereon. A fixed electrode 26 in which a sound hole 27 is formed is provided. Further, a back air chamber 28 is formed on the back surface of the vibrating membrane electrode 23 by etching the silicon substrate 21.

  The vibrating membrane electrode 23 is made of conductive polysilicon, the electret film 24 is made of a silicon nitride film or a silicon oxide film, and the fixed electrode 26 is made of conductive polysilicon and a silicon oxide film or silicon nitride. It is formed by laminating a film.

  In the MEMS microphone 20, when the vibrating membrane electrode 23 vibrates due to sound pressure, the capacitance of the plate capacitor formed by the vibrating membrane electrode 23 and the fixed electrode 26 changes and is taken out as a voltage change.

ここで、
ｆ０：振動膜の共振周波数 [Ｈｚ]
ｓ０：膜強さ（スチフネス） [Ｎ/ｍ]
ｍ０：振動膜の質量 [ｋｇ]
である。 By the way, in manufacturing sites of microphones and speakers, measurement of film strength (stiffness) of a vibrating membrane is performed in order to identify good and defective products. Conventionally, the vibration membrane is vibrated, the resonance frequency of the displacement is measured with an optical interferometer, and the value is substituted into the following equation (Equation 1) to calculate the membrane stiffness.

here,
f0: Resonance frequency of vibrating membrane [Hz]
s0: Film strength (stiffness) [N / m]
m0: Mass of vibrating membrane [kg]
It is.

The mass m0 can be uniquely derived if the structure of the diaphragm is determined. Therefore, the film stiffness can be calculated from (Equation 1) by measuring the resonance frequency f0 with an optical interferometer (see Non-Patent Documents 1 and 2).
optonor catalog micromap5000 Vibration / displacement measurement and surface shape measurement device Hisao Hayasaka et al. Introduction to Acoustical Engineering Nikkan Kogyo Shimbun

However, the optical interferometer used in the conventional film stiffness measurement method is extremely expensive, and it takes a lot of cost to prepare a set of measurement devices of this method.
Further, in the MEMS microphone, the diaphragm is made of silicon, and the diaphragm has a higher film stiffness than the conventional microphone and is difficult to vibrate. Therefore, when measuring the membrane stiffness of the MEMS microphone by applying a conventional method (a method of vibrating the diaphragm and obtaining the resonance point of the diaphragm from its movement), the measurement device is adjusted to obtain the measurement result. It takes a lot of time and it is difficult to obtain reproducible measurement results.

  The present invention was devised in view of such circumstances, and provides a membrane stiffness measurement method capable of easily and inexpensively measuring the stiffness of a diaphragm of an electrostatic electroacoustic transducer, Moreover, it aims at providing the measuring apparatus which enforces the method.

  In the present invention, the membrane stiffness measuring device includes a vacuum container that holds the electrostatic electroacoustic transducer in vacuum, an impedance measuring unit that measures the impedance of the electrostatic electroacoustic transducer held in vacuum, There is provided a stiffness calculation unit that obtains a resonance frequency from the impedance measured by the impedance measurement unit and calculates the film stiffness of the diaphragm of the electrostatic electroacoustic transducer from the resonance frequency.

  In this film stiffness measuring apparatus, instead of detecting a direct vibration displacement, the film stiffness is obtained from the impedance. Moreover, since the impedance is measured in a vacuum, it is not affected by the impedance of the air around the diaphragm.

  In the film stiffness measuring apparatus according to the present invention, the impedance measuring unit measures the absolute value | Z | of the impedance and the phase angle ∠θ.

  Therefore, an accurate measurement result can be obtained.

  In the membrane stiffness measuring apparatus of the present invention, the impedance measuring unit applies a DC voltage together with an AC voltage to the electrostatic electroacoustic transducer.

  By applying a DC bias, the impedance value can be set to an appropriate magnitude.

  In the membrane stiffness measuring apparatus of the present invention, the impedance measuring unit measures the impedance at each frequency in a predetermined frequency band, and the stiffness calculating unit calculates the resonance frequency from the frequency-impedance curve based on the measurement result of the impedance measuring unit. I want to ask.

  The resonance frequency appears as an inflection point of the absolute value of the impedance and the phase angle on the frequency-impedance curve.

  Moreover, the membrane stiffness measuring apparatus of the present invention further includes a display unit, and a frequency-impedance curve is displayed on the display unit.

Moreover, in the film | membrane stiffness measuring apparatus of this invention, the inside of a vacuum vessel is hold | maintained at the vacuum degree of 10 ^{< -1} > -10 ^<-2 > Torr.

  Since the degree of vacuum is not so high, the target vacuum state can be set in a short time.

  Further, the film stiffness measurement method of the present invention includes a first step of holding the electrostatic electroacoustic transducer in a vacuum, and applying an AC voltage to the electrostatic electroacoustic transducer to obtain a predetermined frequency band. A second step of measuring impedance at each frequency, a third step of extracting a frequency at which the absolute value of the impedance and the phase angle are inflected from the measurement result of the second step, and extraction at the third step And a fourth step of calculating the membrane stiffness of the diaphragm of the electrostatic electroacoustic transducer from the measured frequency.

  In this measurement method, instead of detecting a direct vibration displacement, the membrane stiffness is obtained from the impedance. Moreover, since the impedance is measured in a vacuum, it is not affected by the impedance of the air around the diaphragm.

  The measuring method and measuring apparatus of the present invention can measure the stiffness of the diaphragm of the electrostatic electroacoustic transducer at a low cost without using an expensive measuring instrument. Further, since the film stiffness is measured without directly detecting vibration displacement, a reproducible measurement result can be obtained in a short time even if the diaphragm is hard, such as a MEMS microphone.

  FIG. 1 is a diagram schematically showing a configuration of a membrane stiffness measuring apparatus according to an embodiment of the present invention, and FIG. 2 is a diagram specifically showing an impedance measuring circuit of the membrane stiffness measuring apparatus.

  In the membrane stiffness measuring apparatus, when the measurement target is, for example, a diaphragm of a MEMS microphone, as shown in FIG. 1, a vacuum container 30 that houses the MEMS microphone 20 and a MEMS placed in a vacuum while changing the measurement frequency. An impedance measurement unit 40 that measures the impedance of the microphone 20, a stiffness calculation unit 51 that calculates the membrane stiffness of the MEMS microphone 20 by obtaining the resonance frequency of the diaphragm from the frequency-impedance curve, and a display that displays a frequency-impedance curve and the like Part 52.

  The vacuum container 30 includes a mounting table for setting the MEMS microphone 20 therein, and also includes a terminal mechanism that enables energization of the MEMS microphone 20 from the outside of the vacuum container. The MEMS microphone 20 is set on the mounting table in a state where the conductive portions led out from the vibrating membrane electrode and the fixed electrode are exposed, and the terminal mechanism is connected so that the conductive portions can be energized.

Further, the vacuum container 30 is maintained in a vacuum state of about 10 ⁻¹ Torr or less (about 10 ⁻² Torr, that is, a vacuum state where sound is not transmitted) by a vacuum pump during impedance measurement. Since the degree of vacuum is not so high, the state can be reached in about 30 seconds after starting the vacuum pump.

  The impedance measurement unit 40 measures the impedance of the MEMS microphone 20 in a vacuum while sweeping the measurement frequency.

  The stiffness calculation unit 51 calculates the resonance frequency of the diaphragm from the frequency-impedance curve measured by the impedance measurement unit 40, and calculates the stiffness of the diaphragm of the MEMS microphone 20 by substituting this resonance frequency into (Equation 1).

  As described above, in this measuring apparatus, the resonance frequency for substituting into (Expression 1) is obtained from the measurement result of the impedance of the MEMS microphone 20.

  As shown in FIG. 2, the impedance measurement unit 40 that performs impedance measurement includes an AC voltage source 61 and a DC voltage source 62 that are excitation sources of the MEMS microphone 20, an operational amplifier 63 that forms an inverting amplifier, and a feedback resistor (Rs). ) 64, a phase detector 65 that decomposes the excitation AC voltage into a 0 degree component and a 90 degree component, a complex Rayshow detector 66 that outputs a ratio of the output voltage of the operational amplifier 63 and the excitation AC voltage, and a complex Rayshow detector And a quantizer 67 that quantizes the output of 66 and outputs the result to the stiffness calculation unit 51. The DC voltage source 62 is installed as necessary.

  As shown in FIG. 6A, the MEMS microphone 20 disposed in the vacuum container 30 has a capacitor unit including a fixed electrode 26 and a diaphragm (vibrating film electrode 23 + electret film 24). . The MEMS microphone 20 is applied with the AC voltage V1 of the AC voltage source 61 and the DC voltage Eb of the DC voltage source 6, and a current Is based on the AC voltage V1 flows to the capacitor unit.

（但し、Ｚｍはコンデンサ部のインピーダンス）
（数２）から、

となり、Ｒｓは既知であるから、Ｖ０とＶ１との比を検出すれば、インピーダンスＺｍが求まることになる。 The inverting amplifier using the operational amplifier 63 operates so that the negative input voltage is always zero (virtual zero point) by the action of negative feedback. For this reason, the current Is flowing through the capacitor section flows into the virtual zero point of the inverting amplifier, and all of it flows through the feedback resistor Rs64. As a result, the voltage applied to the capacitor unit becomes the same as the voltage (V1 + Eb) of the excitation source, and the output voltage V0 of the inverting amplifier is the current Is flowing through the capacitor unit and the feedback resistance as shown in the following (Equation 2). This is the product of Rs.

(However, Zm is the impedance of the capacitor)
From (Equation 2)

Since Rs is already known, the impedance Zm can be obtained by detecting the ratio between V0 and V1.

The complex Rayshade detector 66 obtains V0 and V1 as complex numbers using the phase information of the excitation source output from the phase detector 65, and outputs V0 / V1 (= Vy).
This Vy is a complex number expressed as the sum of a 0 degree component (Vy∠0 degree) and a 90 degree component (Vy∠90 degree). This Vy is proportional to the reciprocal (1 / Zm) of the impedance of the MEMS microphone 20, that is, the admittance (Ym), as shown in (Expression 4).

Next, it will be described that the impedance and admittance are due to the impedance and admittance of the diaphragm of the MEMS microphone 20 itself.
The mechanical impedance of the acoustic system of the MEMS microphone 20 can be expressed by an electrical equivalent circuit shown in FIG.

  Here, A is the mechanical impedance of the air chamber between the fixed electrode 26 and the diaphragm electrode 23 of the MEMS microphone 20 shown in FIG. 6A, and r1 is the mechanical resistance of the air present in the air chamber, m1 is the mass of the air, and c1 (= 1 / s1) is the compliance of the air (that is, the reciprocal of stiffness). B is the mechanical impedance of the diaphragm (the diaphragm electrode 23 + the electret film 24), m0 is the mass of the diaphragm, c0 (= 1 / s0) is the compliance of the diaphragm, and r0 is the mechanical resistance of the diaphragm. is there. C is the mechanical impedance of the back air chamber 28 on the back surface of the diaphragm electrode 23, r2 is the mechanical resistance of air existing in the back air chamber 28, m2 is the mass of the air, and c2 (= 1 / s2). Is the compliance of the air.

  Thus, since the MEMS microphone 20 in the air has mechanical impedance due to air in addition to the mechanical impedance of the diaphragm, when the MEMS microphone 20 is measured in the air, the machine of the diaphragm itself is used. It is very difficult to measure impedance.

Ｃ：音響コンプライアンス［ｍ^５/Ｎ］
γ：空気の体積比熱 ［Ｊ/ｍ³Ｋ］
Ｖ：気室の容積 [ｍ³]
Ｐ０：気圧 [Ｎ/ｍ²]
である。 Compliances c1 and c2 caused by the air in the front air chamber and the back air chamber 28 of the diaphragm are expressed by the following (Equation 5).

C: Acoustic compliance [m ⁵ / N]
γ: Volume specific heat of air [J / m ³ K]
V: Volume of air chamber [m ³ ]
P0: Barometric pressure [N / m ² ]
It is.

Ｚｍ ： 被測定振動板の機械インピーダンス
Ｅ_Ｂ ： 励振源の直流電圧 [Ｖ]
Ａ ： 力係数
ε０ ： 真空の誘電率 8.85E-12[Ｆ/ｍ]
Ｓ ： コンデンサ部の面積 [ｍ²]
Ｃｍ ： コンデンサ部の電気容量 [Ｆ]
ｄ０ ： エアギャップ間隔 [ｍ]
Ｖ ： 振動板の速度 [ｍ/ｓｅｃ]
ｓｎ ： 負スチフネス [Ｎ/ｍ] In this measuring apparatus, since the MEMS microphone 20 is disposed in a vacuum, the value of C (= c1, c2) becomes a very large value and is in a short circuit state. Therefore, the equivalent circuit of the capacitor part in the vacuum vessel is represented only by the mechanical impedance of the diaphragm itself.
The reversible expression of this capacitor part is expressed by the following expressions (Expression 6) (Expression 7).

Zm: Mechanical impedance of diaphragm to be measured E _B : DC voltage of excitation source [V]
A: Force coefficient ε0: Dielectric constant of vacuum 8.85E-12 [F / m]
S: Capacitor area [m ² ]
Cm: Capacitor capacitance [F]
d0: air gap interval [m]
V: Speed of diaphragm [m / sec]
sn: Negative stiffness [N / m]

Here, the mechanical impedance of the diaphragm to be measured is

The force coefficient is

で表される。 The capacitance of the capacitor part is

It is represented by

で表される。 Negative stiffness is

It is represented by

This (Equation 6) is an equation that focuses on the relationship of mechanical force of the diaphragm, and (Equation 7) is an equation that focuses on the electrical relationship of the diaphragm.
In this measuring apparatus, since the diaphragm of the MEMS microphone 20 is driven by an excitation source, the external force F is 0, and (Equation 6) is expressed as (Equation 12).

Therefore, (Equation 7) can be transformed into the following equation (Equation 13) using (Equation 12).

Therefore, the impedance and admittance are expressed by the following equation (Formula 14).

  As shown in FIG. 4, the impedance and admittance correspond to the impedance and admittance of an equivalent circuit in which the electric capacitance Cm and the diaphragm mechanical impedance are arranged in parallel.

  In the measurement circuit of FIG. 2, the same impedance and admittance of the equivalent circuit of FIG. 4 can be obtained by scaling Vy detected by the complex Rayleigh detector 66 by the stiffness calculation unit 51.

  Further, in the measurement circuit of FIG. 2, the frequency-impedance curve shown in FIG. 5A and the frequency-phase curve shown in FIG. can get. This is the same as the frequency-impedance curve obtained when the frequency of the AC voltage V1 in the equivalent circuit of FIG. 4 is swept.

For example, the frequency-impedance curve shown in FIG. 5A is displayed on the display unit 52.
The stiffness calculation unit 51 searches the frequency of the antiresonance point and the frequency of the resonance point from this curve, obtains the resonance frequency f0 by taking the average of the two frequencies, and substitutes this value in (Equation 1). The stiffness of the diaphragm of the MEMS microphone 20 is calculated.

  Alternatively, the stiffness calculation unit 51 obtains an intersection between the resonance-antiresonance part of the frequency-impedance curve and the frequency-electrical impedance (1 / wCm) curve, and substitutes the value into (Equation 1). To calculate the stiffness of the diaphragm.

  For example, the frequency-phase curve shown in FIG. The stiffness calculation unit 51 calculates the peak of this curve as the resonance frequency f0, and substitutes this value into (Equation 1) to calculate the stiffness of the diaphragm of the MEMS microphone 20.

  As described above, in this film stiffness measurement method, the resonance frequency is obtained from the impedance of the diaphragm having the capacitor structure, so that it can be measured at low cost. Further, even when a diaphragm having high stiffness such as a MEMS microphone is a target, a reproducible measurement result can be obtained in a short time.

  In addition, since the impedance is measured by placing the measurement object in a vacuum, the influence of the air around the diaphragm can be eliminated, and the impedance of only the diaphragm can be measured accurately.

The vacuum level at this time may be about 10 ⁻¹ to 10 ⁻² Torr, and can be set in a short time by a vacuum pump.
Further, in this measuring apparatus, since the impedance measuring unit measures the absolute value | Z | of the impedance and the phase angle, an accurate measurement result equivalent to the measurement in the equivalent circuit of FIG. 4 can be obtained. it can.

  Further, in this measuring apparatus, by adjusting the DC voltage Eb supplied from the DC voltage source 62, the force coefficient A in (Expression 14) is changed and the values of impedance and admittance are set to appropriate magnitudes. Can do.

  The impedance measurement may be performed using a commercially available impedance analyzer.

  Moreover, the display part 52 and the stiffness calculation part 51 are realizable using the function of a personal computer (PC). In this case, when the measurement frequency band is designated from the PC to the impedance measurement unit 40, and the impedance measurement unit 40 scans the designated frequency band and outputs the impedance measurement result to the PC, the absolute value of impedance | Z | and the phase angle can be displayed.

  Here, the case where the stiffness of the diaphragm of the MEMS microphone is measured has been described. However, the present invention can also use other electrostatic electroacoustic transducers as measurement targets.

  The film stiffness measuring method and measuring apparatus of the present invention can be used to distinguish whether a manufactured electrostatic electroacoustic transducer is a good product or a defective product, or to investigate the characteristics of a newly developed product. It can be widely used for inspection of various electrostatic electroacoustic transducers such as a MEMS microphone, an electret condenser microphone, and an electret condenser speaker.

The figure which shows the outline | summary of the film | membrane stiffness measuring apparatus in embodiment of this invention. The figure which shows the structure of the impedance measurement part in embodiment of this invention. The figure which shows the equivalent circuit of the mechanical impedance of a MEMS microphone The figure which shows the equivalent circuit of the film | membrane stiffness measuring circuit in embodiment of this invention The figure which shows the frequency-impedance curve measured with the film | membrane stiffness measuring method in embodiment of this invention. The figure which shows the frequency-phase curve measured with the film | membrane stiffness measuring method in embodiment of this invention (A) The figure which shows the structure of a MEMS microphone (b) The figure which shows the structure of an electret condenser microphone

Explanation of symbols

11 Diaphragm 12 Fixed electrode 13 Electret film 14 Spacer 15 Sound hole 16 Back air chamber 17 Case 18 Printed circuit board 20 MEMS microphone 21 Silicon substrate 22 Insulating layer 23 Vibrating film electrode 24 Electret film 25 Second insulating layer 26 Fixed electrode 27 Sound Hole 28 Back air chamber 30 Vacuum vessel 40 Impedance measurement unit 51 Stiffness calculation unit 52 Display unit 61 AC voltage source 62 DC voltage source 63 Operational amplifier 64 Feedback resistor 65 Phase detector 66 Complex Rayshade detector 67 Quantizer

Claims (7)

A vacuum vessel holding the electrostatic electroacoustic transducer in a vacuum;
An impedance measuring unit for measuring the impedance of the electrostatic electroacoustic transducer held in a vacuum;
A membrane stiffness measuring device comprising: a stiffness calculating unit that obtains a resonance frequency from the impedance measured by the impedance measuring unit and calculates a membrane stiffness of a diaphragm of the electrostatic electroacoustic transducer from the resonance frequency. .   2. The membrane stiffness measuring apparatus according to claim 1, wherein the impedance measuring unit measures an absolute value and a phase angle of impedance.   2. The membrane stiffness measuring apparatus according to claim 1, wherein the impedance measuring unit applies a DC voltage together with an AC voltage to the electrostatic electroacoustic transducer.   4. The membrane stiffness measuring apparatus according to claim 1, wherein the impedance measuring unit measures impedance at each frequency in a predetermined frequency band, and the stiffness calculating unit measures the impedance measuring unit. 5. A membrane stiffness measuring apparatus, wherein the resonance frequency is obtained from a frequency-impedance curve based on a result.   5. The membrane stiffness measuring apparatus according to claim 4, further comprising a display unit, wherein the frequency-impedance curve is displayed on the display unit. 2. The film stiffness measuring apparatus according to claim 1, wherein the inside of the vacuum vessel is maintained at a vacuum degree of 10 ⁻¹ to 10 ⁻² Torr. A first step of holding the electrostatic electroacoustic transducer in a vacuum;
A second step of applying an alternating voltage to the electrostatic electroacoustic transducer to measure impedance at each frequency in a predetermined frequency band;
A third step of extracting a frequency at which the absolute value of the impedance and the phase angle change from the measurement result of the second step;
A film stiffness measurement method comprising: a fourth step of calculating a film stiffness of a diaphragm of the electrostatic electroacoustic transducer from the frequency extracted in a third step.

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