JPH05187942A - Vibration type transducer - Google Patents

Abstract

PURPOSE:To measure a strain with high accuracy with a simple constitution by selecting a drain resistance, capacitance between a drain and substrate, and oscillating angular velocity so that their product can become extremely larger than '1'. CONSTITUTION:Since a negative potential is applied across a mobile electrode 30 from a DC power source E2, electrons are forcibly pushed toward the inside of a silicon substrate 24 from the lower surface of the vibrator 30 and, on the contrary, holes (p-type) are attracted toward the surface of the substrate 24. Because of the holes, a channel CNN composed of a p-type conductive layer is formed on the surface of the substrate 24 and an electric current id flows between a source S and drain D. The voltage the drain D generated by the current id is shifted in phase by a drain resistance RD and the capacitance CD formed between the drain and substrate 24 and, due to the potential change, the electrostatic attracting force between the electrode 30 and drain D changes, resulting in a change in the interval X between the electrode 30 and drain D. The oscillation caused by the potential change continues when the resistance RD, capacitance CD, and oscillating angular velocity omega are selected so that their product can become extremely larger than '1'. When a pressure PM is applied to the substrate 24, a strain is applied to the electrode 30 and its characteristic frequency varies. The pressure PM is detected by fetching the variation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vibration type transducer which detects applied strain by detecting a frequency which changes corresponding to the strain, and particularly to measure the strain with high precision by a simple structure. The present invention relates to a vibration type transducer improved so that

[0002]

2. Description of the Related Art FIG. 8 is a perspective view of a configuration using a conventional vibration type transducer as a pressure sensor, FIG. 9 is an enlarged view of portion A in FIG. 8 and a vibration detection circuit is connected to it, and FIG. Sectional drawing which shows the AA 'cross section in FIG.
FIG. 11 is an explanatory diagram showing the configuration shown in FIG. 9 as an electrically equivalent circuit.

As shown in FIG. 8, 10 has a crystal plane (100) on its upper surface, for example, an impurity concentration of 10 ¹⁵ atoms /
It is a silicon single crystal substrate having a conductivity type of p-type at a cm ³ or less. The diaphragm 11 is thinly formed on one surface of the substrate 10 by etching from the back surface.

A thick portion 12 around the diaphragm 11
Is joined to a pedestal 14 having a pressure guiding hole 13 in the center, and the pressure P _M to be measured is introduced into this pressure guiding tube 15. An n ⁺ diffusion layer (not shown) having an impurity concentration of about 10 ¹⁷ is partially formed on the surface of the diaphragm 11 on the non-etching side indicated by the symbol A, and the vibrator 16 is formed on a part of the n ⁺ diffusion layer. Are formed in the crystal axis <001> direction (FIG. 9). In this vibrator 16, for example, the n ⁺ diffusion layer and the p layer formed on the diaphragm 11 are processed by using photolithography and under etching techniques.

Reference numeral 17 designates the vibrator 1 at the upper center of the vibrator 16.
A magnet provided in a non-contact state orthogonal to 6
8 is a S _i O ₂ film as an insulating film (see FIG. 10). 1
9a, 19b is a metal electrode such as aluminum, the contact holes 2 and one end of the metal electrode 19a is that through the S _i O ₂ layer provided on the n ⁺ layer that extends from the transducer 16
0a, and the other end is connected via a lead wire to one end of a comparison resistor R ₀ substantially equal to the resistance value of the vibrator 16 and the input end of the amplifier 21.

An output signal is taken out from the output end of the amplifier 21 and is connected to one end of the primary coil L ₁ of the transformer 22. The other end of the primary coil L ₁ is connected to the common line. The other end of the comparison resistor R ₀ is connected to the secondary end of the coil L ₂ of the transformer 22 the midpoint of which is connected to the common line, the other end of the secondary coil L ₂ to the other end of the vibrator 16 A similarly formed metal electrode 19b,
It is joined to the n ⁺ diffusion layer through the contact hole 20.

In the above structure, the p-type layer (substrate 10)
When a reverse bias voltage is applied between the n-type diffusion layer and the n ⁺ diffusion layer (vibrator 16) for insulation, and an alternating current i is passed through the vibrating element 16, the impedance of the vibrating element 16 is represented as R in the resonance state of the vibrating element An equivalent circuit as shown in 11 is obtained.

In this way, a bridge is formed by the secondary coil L ₂ having the midpoint C ₀ connected to the common line, the comparison resistor R ₀ , and the impedance R. Therefore, the unbalanced signal by the bridge is amplified by the amplifier 21. When it is detected and its output is positively fed back to the primary coil L ₁ via the feedback line 23, the system causes self-oscillation at the natural frequency of the oscillator 16.

Here, when the pressure P is applied to the diaphragm 11 via the pressure guiding hole 13, the tension acting on the vibrator 16 is changed by this, and the resonance frequency is changed. Therefore, the natural frequency of the vibrator 16 changes, and the magnitude of the pressure P can be known from the change in the natural frequency.

[0010]

However, the vibration type sensor as described above has a drawback that it requires the magnet 17 having a large size, which makes the fixing structure complicated.
Therefore, as a configuration that does not use this magnet, for example, an electrostatic force is used as the excitation means, a piezoresistor is used as the detection element, and these are incorporated into the oscillation loop to cause self-excited oscillation, and the frequency generated in this oscillation loop is used. Although it is conceivable to detect the strain by the change of the temperature, this configuration causes the current to flow through the oscillator, so the temperature rises due to heat generation, which creates an error factor, and it is necessary to build a piezo gauge on the oscillator. Therefore, it has a drawback that the manufacturing process is complicated. A configuration in which piezoelectric driving is performed and detection is performed by a pressure sensitive element is also conceivable. However, according to this configuration, the piezoelectric material is limited, and since silicon does not have piezoelectricity in particular, PZT or the like must be formed on silicon. However, there is a problem that the fabrication is difficult.

[0011]

The present invention has, as a constitution for solving the above problems, a semiconductor substrate having a first conduction type and a second conduction type which is formed on the surface of the substrate and is opposite to the conduction type. A channel sandwiched between a drain and a source having a two-conduction type, and is disposed by covering at least one of the above drain and source and the above channel while maintaining a gap from the surface of these drain and source, and is displaced by oscillation. ; and a movable electrode which, ahead of the drain of drain resistance R _D in the previous drain the previous product of the oscillation angular velocity ω of the electrostatic capacitance C _D and the previous oscillation between the substrate (.omega.R _D C _D) is These values are selected so as to be extremely larger than 1.

[0012]

[Operation] A drain and a source are formed on the surface of a semiconductor substrate at a predetermined interval, and a predetermined voltage is applied between them. On the other hand, a channel is formed in the semiconductor substrate between the drain and the source by applying a predetermined potential to the vibrator as needed.

The oscillating voltage generated in the drain causes an electrostatic force between the movable electrode and the drain to change the distance between the movable electrode and the channel. The change in the distance changes the thickness of the channel to control the current flowing through the drain.

In this case, the oscillation is continued by selecting the product of the drain resistance R _D , the electrostatic capacitance C _D and the oscillation angular velocity ω (ωR _D C _D ) to be much larger than 1, so that the oscillation is continued. The frequency that changes corresponding to the strain applied to the semiconductor substrate is detected.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing the configuration of one embodiment of the present invention.

The silicon substrate 24 has, for example, a conductivity type of n.
The electrode 25 is fixed to the electrode 25.
Are connected to a common potential point COM. On the upper surface of the silicon substrate 24, p-type impurities are diffused and the source S
Is formed, and an electrode 26 for extracting the potential of the source S is formed here. In addition, this silicon substrate 24
Although not shown, a pressure P _M or the like to be measured is applied to the lower surface of the.

A p-type impurity is also diffused on the upper surface of the silicon substrate 24 at a predetermined distance W from the source S to form a drain D, and an electrode for extracting the potential of the drain D is formed there. 27 is formed.

Convex portions 28 and 29 are formed above the portion of the silicon substrate 24 at a predetermined distance L by a distance x, and both ends of the conductive plate-like movable electrode 30 are formed on the convex portions 28 and 2.
It is fixed at 9. That is, the movable electrode 30 and the silicon substrate 24 are arranged apart from each other by x, and the channel CN is provided between the drain D and the source S although not shown in the silicon substrate 24 corresponding to the movable electrode 30.
N is formed.

Between the electrode 27 and the common potential point COM,
The resistor R1 and the DC power source E1 are connected in series, and the potential of the drain D is held at a negative potential with respect to the common potential point COM. A direct current power source E2 is connected to the movable electrode 30 so as to have a negative potential with respect to the common potential point COM.

FIG. 2 is an explanatory diagram for explaining the operation of the embodiment shown in FIG. It is configured to include a cross section of the silicon substrate 24 as viewed from the longitudinal direction of the movable electrode 30. Since a negative potential is applied from the DC power source E2 to the movable electrode 30 functioning as a gate, electrons are transferred from the lower surface of the vibrator 30 to the inside of the silicon substrate 24 (lower side in FIG. 2) as shown in FIG. , And holes are attracted to the surface.

A channel CNN, which is a thin P-type conductive layer, is formed on the surface by the attracted holes (P-type), and the source S (P-type) and the drain D (P-type) are connected by the P-type. Therefore, the current i _d flows between the source S and the drain D.

Drain D generated by this current i _d
Is subjected to a phase shift by the drain resistance R _D and an electrostatic capacitance C _D formed between the drain and the silicon substrate 24, and the potential change caused by this phase shift causes the movable electrode 30 and the drain D to move. The electrostatic attraction force between the two is changed to change the interval x.

Due to this change in the interval x, the channel CNN
The thickness is changed so that the current i _dChange this
Causes a change in the potential of the drain. Repeat this
It oscillates, but this oscillation is caused by drain resistance R_DAnd drain D
Capacitance C between the silicon and the silicon substrate 24_DAnd oscillation angle of oscillation
Product with velocity ω (ωR_DC_D) Is much larger than 1
It will be continued by selecting the

When the pressure P _M is applied to the silicon substrate 24 as shown in the figure in the state where the self-excited oscillation is maintained as described above, the pressure P _M is applied via the convex portions 28 and 29 for fixing the movable electrode 30. Strain due to the pressure P _M is applied to the movable electrode 30, and the natural frequency changes correspondingly. Therefore, the value of the pressure P _M can be detected by extracting the change in the natural frequency.

Next, the above points will be described in detail using mathematical expressions. FIG. 3 shows the device configuration shown in FIG. 1 as an equivalent circuit. FIG. 4 shows the arrangement of the signs of the parts of the device configuration shown in FIG. 1 necessary for the following calculations, and FIG. 5 shows the transfer function by disassembling the parts of the equivalent circuit shown in FIG. FIG.

In FIG. 3, R _D is the drain resistance of the drain D, and C _D is the electrostatic capacitance between the drain D and the silicon substrate 24, and the movable electrode 30 is arranged so as to face the substrate 24 in an insulated manner. The movable electrode 30 oscillates toward and away from the substrate 24 at the oscillation angular velocity ω of oscillation. A DC power supply E2 is applied to the movable electrode 30. For simplicity, R ₁ will be described as zero.

For such an arrangement, the movable electrode 30 has a width b and a thickness h as shown in FIG.
Are separated by x. The movable electrode 30 is arranged so as to cover the drain D formed on the substrate 24 and the width a. By covering in this way, the electrostatic attraction force between the drain D and the movable electrode 30 can be effectively used for oscillation.

The distance between the source S and the drain D on the substrate 24, that is, the cannel width is the length of w.
FIG. 5 shows the process of causing oscillation in the configuration shown in FIGS. This is used for sequential analysis.

Where K₁Is the drain generated on the drain D
Change in voltage Δe_d1Drain D and movable electrode 3
Transfer function, which is the ratio of the change ΔF in electrostatic attraction force between 0 and
To K ₂Is the displacement of the movable electrode 30 with respect to the electrostatic attraction force ΔF.
Let K be the transfer function that is the ratio of Δx₃Is the displacement of the movable electrode 30
Drain current Δi of drain D with respect to Δx_dIn the ratio of
The transfer function_FourIs the drain current Δi_dAgainst dray
Voltage change Δe_d1Shows the transfer function which is the ratio of
ing.

[0030] Here, the conditions which such systems can cause oscillations, and _{G L = K 1 K 2 K} 3 K 4 (1), if the ∠G _L between the phase angle of G _L, G _L> 1 (2) ∠G _L = 0 (3) is the oscillation condition.

First, the electrostatic attraction force F generated per unit length of the movable electrode 30 is F = ε ₀ a (V / x) ² (4). Here, V = E ₁ −E ₂ , and ε ₀ is the dielectric constant. Electrostatic attraction force F that changes when V changes by Δe _d1
When Δ is expressed as a partial differential symbol, ΔF = (δF / δV) Δe _d1 = 2ε ₀ aV · Δe _d1 / x ² (5) ∴ K ₁ = ΔF / Δe _d1 = 2ε ₀ aV / x ² (6)

Secondly, the displacement Δx due to the distributed load ΔF of the beam fixed at both ends is Δx = ΔF · L ⁴ / 384EI (1 + jγω-τ ₀ ² ω ² ) (7). Here, L is the length of the movable electrode 30, I is the moment of inertia of the movable electrode 30, γ is the damping rate, and τ ₀ is the natural period. Note that E is E = E ₀ (1 + Sη) S = 0.24 (L / h) ² , E ₀ is the Young's modulus of the movable electrode 30, and η is strain. Therefore, K ₂ is K ₂ = Δx / ΔF = L ⁴ / 384E ₀ I (1 + jγω−τ ₀ ² ω ² ) (8).

Thirdly, the drain current i _d becomes i _d = (1/2) μC ₀ L (E ₂ −V _T ) ² / w (9) in the pinch-off region. Here, μ is the mobility of electrons, C ₀ is the capacitance per unit area between channels, and V _T is the threshold voltage. C ₀ is C ₀ = ε ₀ / x (10).

The change .DELTA.i _d of the drain current i _d when the movable electrode 30 approaches the [Delta] x silicon substrate 24 _{side, Δi d = (δi d /} δx) (- Δx) = (- 1/2) με 0 L [(E ₂ −V _T ) / x] ² (−Δx) / w. Therefore, _{K 3 = Δi d / Δx =} (1/2) με 0 L [(E 2 -V T) / x] 2 / w (11).

Fourthly, the change Δe _d1 of the drain voltage e _d1 with respect to the change Δi _d of the drain current i _d is obtained. This case will be described with reference to FIG. Since it can be regarded as the constant current source CC in the pinch-off region, this is illustrated in FIG.
As shown in. When this is viewed from the AC side, the equivalent circuit shown in FIG. 6B is obtained. From the equivalent circuit of FIG. _{6 (b), K 4 =} Δe d2 / Δi d = -R / (1 + jωC D R D) (12)

Therefore, G _L = K ₁ K ₂ K ₃ K ₄ = 2ε ₀ a (E ₁ −E ₂ ) / x ² · L ⁴ / 384EI (1 + jγω−τ ₀ ² ω ² ) · (1/2) _{με 0 L [(E 2 -V} T) / x] obtain ^{2 / w · [-R / (} 1 + jωC D R D)].

Here, ω = ω ₀ = 1 / τ _0, and ω ₀ C _D R
When _D is extremely large compared to ^{_{1, G L = -με 0 2}} L 5 a (E 1 -E 2) (E 2 -V T) 2 · (1 / jγω 0) (1 / jω 0 C D ) ・ (1 / 384EIx ⁴ )

If the sharpness of resonance is Q, then Q = 1
As demonstrated by _{/ γω 0, G L = -με} 0 2 L 5 a (E 1 -E 2) (E 2 -V T) 2 · (Q / j) (1 / jω 0 C D) · (1 / 384EIx ⁴ ) (12) is obtained.

Further, substituting I = bh / 12 E = E ₀ [1 + 0.24 (L / h) ² η] into the equation (12), G _L = Qμε ₀ ² L ⁵ a (E ₁ −E ₂ ) and a ^{_{(E 2 -V T) 2 /}} 32E 0 [1 + 0.24 (L / h) 2 η] bh 3 x 4 wω 0 C D (13).

[0040] ^{_{Here, E 0 = 1.6X10 11 N /}} m 2 ε 0 = 8.85X10 ^{over 12 / m μ = 0.1m 2 /} Vs ( impurity concentration: 1X10 ¹⁶ / cm ³⁾ Using the, G _L = 2.43X10 ^{over _{37 QL 5 a (E 1 -E}} 2) (E 2 -V T) 2 / E 0 [1 + 0.24 (L / h) 2 η] bh 3 x 4 wω 0 C D

As an example, for example, L = 150 × 1
0 ^over 6, a = w = 5X10 ^over 6, h = 1X10 ^over 6, b = 10
X10 ^over 6, x = 0.5X10 ^{over _{6, η = 0, f 0}} = 500
X10 ^3, C _D = 10X10 ^{over _{12, E 1 -E 2 = 1V}} , E 2 -
If V _T = 1V, the G _L = 61X10 ^{over 4} Q. Therefore, if Q = 10 ⁴ , then G _L = 61.

That is, if a vibration type transducer is constructed with a length of 150 μm, a width of 10 μm and a gap of 0.5 μm, G _L = 61 and the phase of G _L is in phase, which satisfies the oscillation condition sufficiently. In Equation (13), G _L is (E ₂
Since it is proportional to −V _T ) ² , by adjusting E ₂ ,
The amplitude of G _L can be controlled.

A concrete circuit configuration for controlling the amplitude is shown in FIG. Reference numeral 31 denotes the oscillator disclosed in FIG. 1, and the oscillator 31 itself continues to oscillate. However, this oscillation signal has an amplifier 32 whose input end is connected to the resistor R ₂ via the capacitor C _2. The frequency f ₀ is output to the output terminal.

This frequency f ₀ is output to the automatic gain control circuit 33 so as to correspond to this value, where the reference voltage V 0
_As compared with _ref , a DC voltage is applied to the movable electrode 30 so as to correspond to this reference voltage V _ref . Therefore, the amplitude of oscillation can be controlled by adjusting the value of this reference voltage V _ref .

The automatic gain control circuit 33 includes a rectifier circuit 34.
To change the amplitude to the corresponding DC voltage,
This direct current voltage is applied to the other end of the input of the comparator 35 where _ref is applied to one end of the input end,
The magnitude of the DC voltage applied to 0 is automatically adjusted.

[0046]

As described above in detail with reference to the embodiments, according to the present invention, a drain and a source are separately formed at a predetermined interval on a semiconductor substrate, and a channel is formed between them to form a movable electrode. Are arranged with a gap facing this channel and at least one of the drain and source, and continue oscillation in cooperation with the electrostatic attraction force between the movable electrode and the drain, the drain resistance and the capacitor between the drain and the substrate. Therefore, it is possible to realize a vibration type transducer with a simple structure by applying strain to the semiconductor substrate, and to manufacture by a CMOS compatible process, which has an advantage of simplifying the process. .. Therefore, it is not necessary to use a magnet with a large size as in the past, and therefore it is not necessary to accurately arrange the distance between the vibrator and the magnet, and a piezoelectric element such as a piezo gauge or PZT is formed on the vibrator. You don't have to dent.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is an explanatory diagram explaining an operation of the embodiment shown in FIG.

FIG. 3 is an equivalent circuit expressing the device configuration shown in FIG. 1 as an equivalent circuit.

FIG. 4 is an explanatory diagram showing a code agreement of each part of the device configuration shown in FIG. 1 necessary for calculation.

5 is an explanatory diagram for disassembling each part of the equivalent circuit shown in FIG. 3 to obtain a transfer function.

FIG. 6 is an equivalent circuit diagram in which the equivalent circuit shown in FIG. 3 is further simplified.

FIG. 7 is a configuration diagram showing a configuration of another embodiment of the present invention.

FIG. 8 is a perspective view of a configuration using a conventional vibration sensor as a pressure sensor.

9 is a configuration diagram in which an A portion in FIG. 8 is enlarged and a vibration detection circuit is connected thereto.

10 is a cross-sectional view showing a cross section taken along the line AA ′ in FIG.

FIG. 11 is an explanatory diagram showing the configuration shown in FIG. 9 as an electrically equivalent circuit.

[Explanation of symbols]

 10, 24 Silicon substrate 11 Diaphragm 12 Thick portion 13 Pressure guiding hole 15 Pressure guiding tube 16, 31 Transducer 17 Magnet 21 Amplifier 22 Transformer 23 Feedback line 30 Moving electrode 33 Automatic gain control circuit S Source D drain E1, E2 DC power supply CNN channel

Claims (2)

[Claims] 1. A semiconductor substrate having a first conductivity type, a channel formed on the surface of the substrate between a drain and a source having a second conductivity type opposite to the conductivity type, and a channel formed between the drain and the source. A drain electrode R _D of the drain, the drain, and the substrate, the drain resistance R _{D of the} drain and the source being disposed to cover at least one of the drain and the source and the channel and being displaced by oscillation while maintaining a gap from a surface of the source. A vibrating transducer characterized in that these values are selected so that the product (ωR CDD) of the electrostatic capacitance C _D between them and the oscillation angular velocity ω (ωR _D C _D ) becomes extremely larger than 1. 2. The vibration transducer according to claim 1, wherein a direct current voltage is applied to the movable electrode to control the amplitude of oscillation.