JPH0468576B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a pressure sensor that uses a silicon single crystal to detect strain corresponding to a measured pressure as a frequency signal, and particularly relates to a pressure sensor that uses a silicon single crystal to detect strain corresponding to a measured pressure as a frequency signal. Regarding pressure sensors.

<Prior art> A vibrating beam formed on a pressure-receiving diaphragm made of an elastic semiconductor and fixed at both ends;
A pressure sensor composed of an excitation means for exciting this vibrating beam and a vibration detection means for detecting the vibration generated by the excitation by this excitation means is, for example,
42632 ``Pressure sensor''.

The detection section of this pressure sensor will be outlined with reference to FIGS. 3 to 5. FIG. 3 is a perspective view showing the configuration of the detection part of this conventional pressure sensor with the cover removed, and FIG. 4 is a -
FIG. 5 is a cross-sectional view with a cover attached, and a plan view with some parts omitted.

In these figures, 1 is a cylindrical silicon substrate made of an elastic semiconductor. Reference numeral 2 denotes a pressure receiving diaphragm which is formed by digging the center of the silicon substrate 1 to form a thin wall portion to serve as a pressure receiving portion for receiving the measurement pressure Pm, and is made by etching the silicon substrate 1, for example.

3 and 4 are formed on the pressure receiving diaphragm 2,
It is a vibrating beam whose both ends are fixed to a silicon substrate 1, and the vibrating beam 3 is located approximately at the center of the pressure receiving diaphragm 2, and the vibrating beam 4 is located at the periphery of the pressure receiving diaphragm 2, respectively.

Specifically, these vibrating beams 3 and 4 are made by forming a first P ⁺ type epitaxial layer on an n-type silicon substrate 1, cutting a cut in the center of the layer to electrically separate the left and right sides, and After forming an n-type epitaxial layer thereon, a P ⁺ type epitaxial layer is further formed, and the top thereof is protected with an oxide film SiO ₂ . Then, the hollow portion at the bottom of the vibrating beam 3 is formed by under-etching this n-type epitaxial layer.

For example, the vibrating beam 3 formed in this way has a length of l, a thickness of h, and a width of d, then l=
The sizes are approximately 100 μm, h = 1 μm, and d = 5 μm.

Vibration beam 3 formed on pressure receiving diaphragm 2
The periphery of the diaphragm 2 is covered with, for example, a silicon cover 5 bonded to the pressure receiving diaphragm 2 by anodic bonding or the like, and the inside of the diaphragm 2 is maintained in a vacuum state. Although not shown, the vibrating beam 4 side is also formed in the same way.

In the above configuration, the first left and right vibrating beams 3 and 4, which are the second P ⁺ type epitaxial layers,
When an oscillation circuit is connected between the P ⁺ type epitaxial layers to cause oscillation, the vibrating beams 3 and 4 cause self-oscillation at their natural frequencies. In this case, cover 5
Since the inside of the vibrating beam is kept in a vacuum state and the vibrating beam is kept in a vacuum, the Q value indicating the sharpness of resonance becomes large, making it easy to detect the resonance frequency.

The frequencies of this self-oscillation change in opposite directions in response to the tensile or compressive stress applied to the vibrating beam. As shown in FIG. 4, when measurement pressure Pm is applied to the pressure receiving diaphragm 2, the vibrating beam 3 receives tensile stress and the vibrating beam 4 receives compressive stress.

Therefore, the natural frequencies of the vibrating beams 3 and 4 change differentially with respect to the measured pressure, and by calculating the difference between these, it is possible to know the measured pressure Pm with twice the sensitivity. can.

<Problems to be solved by the invention> However, such conventional pressure sensors
Since the thin pressure-receiving diaphragm 2 must be formed on the silicon substrate 1, its structure becomes complicated, and since it is necessary to maintain a large Q value of the vibrating beams 3 and 4, the cover 5 is bonded to the pressure-receiving diaphragm 2 by, for example, anodic bonding. Since the inside must be evacuated, it becomes a complicated structure and has the disadvantage of reducing reliability and precision. Furthermore, the existence of a thin pressure receiving diaphragm requires protection against excessive pressure, which is a problem.

<Means for solving the problems> In order to solve the above problems, the present invention has the following features:
A capsule formed by a semiconductor process to which measurement pressure is uniformly applied around the capsule, a hollow chamber formed inside this capsule, a capsule with both ends tipped at a predetermined tension inside this hollow chamber, and a semiconductor process only. The device is equipped with a silicon single crystal vibrating beam integrally formed with a silicon single crystal vibrating beam, and the previously measured pressure is measured from the change in resonance frequency based on the change in the tension applied to the vibrating beam due to the previously measured pressure. be.

<Effect> Measurement pressure is applied around the silicon single crystal capsule, and the resulting compressive strain in the capsule itself changes the tension applied to the vibrating beam, which changes the resonant frequency; Detects the measured pressure from.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. FIG. 1A is a block diagram showing the overall structure of the pressure sensor, FIG. 1B is a sectional view of the detection section in the negative direction, and FIG.

6 is a detection section having a rectangular outer shape for detecting the measured pressure, and 7 is an excitation section that excites the detection section 6 and extracts a change in its resonance frequency.

The detection unit 6 includes a capsule 8 made of a silicon single crystal, a hollow chamber 9 in a vacuum state formed inside the capsule, and both ends of the capsule 8 are fixed integrally with the capsule 8 in the hollow chamber 9, and an initial tension is applied thereto. vibrating beam 10,
and a sheet-shaped permanent magnet 11 fixed to the bottom of the capsule 8. Both ends of the vibrating beam 10 are integrally formed with the capsule 8, and lead wires l ₁ and l ₂ are drawn out from these ends.

Further, the permanent magnet 11 is fixed to the bottom of the capsule 8, and its DC magnetic field φ interlinks with the vibrating beam 10, as shown in FIG.

The excitation unit 7 includes a transformer 12 with a center point tap,
It is composed of an amplifier 13, a comparison resistor 14, and the like. Both ends of the vibrating beam 10 are connected to terminals T ₁ and T ₂ via lead wires l ₁ and l ² , respectively, and the voltage of the secondary winding n ₂ of the transformer 12 is connected to the series circuit of the vibrating beam 10 and the comparison resistor 14. is applied.

Furthermore, an amplifier 1 is connected between the connection point of the vibrating beam 10 and the comparison resistor 14 and the primary winding n ₁ of the transformer 12.
3 is connected to apply positive feedback, and the oscillation frequency is detected from the output terminal via terminal _T3 .

Since a permanent magnet 11 or a direct current magnetic field φ is applied to the vibrating beam 10, when the alternating current i of the vibrating beam 10 flows, an electromagnetic force acts on the vibrating beam 10, causing it to vibrate. Since the hollow chamber 9 is maintained in a vacuum state, the vibration beam 10 has a high Q value of several hundreds to tens of thousands, and has a large amplitude when it resonates. Therefore, when positive feedback is applied to the transformer 12 via the amplifier 13, the system self-oscillates at the natural frequency of the vibrating beam 10.

Measurement pressure Pm applied from around capsule 8
Compressive strain (approximately 3×
10 ^-7 /Kg/cm ² ), the tension in the axial direction of the vibrating beam 10 changes and the resonance frequency changes, so the measured pressure Pm can be determined from this change in the oscillation frequency.

Since the thickness of the vibrating beam 10 is configured to be negligibly small compared to the length l, the vibration can be regarded as string vibration. The resonance frequency ₀ is given by ₀ ² =Eε/(4l ₂ ρ) (1) where E is Young's modulus, ρ is the density of the vibrating beam 10, and ε is the strain.

On the other hand, the strain εp due to the measured pressure Pm of the silicon single crystal is εp=Pm(1-2ν)/E (2), where ν is Poisson's ratio, so assuming the initial strain is ε ₀ , (1), From equation (2), f ₀ ² = E (ε ₀ + εp) / (4l ² ρ) = E{ε ₀ +Pm (1-2ν) / E} / (4l ² ρ) (3) ∴Pm=E{ 4ρl ² ₀ ² /E−ε ₀ } /(1−2ν) (4).

Therefore, by measuring ₀ from this equation (4),
Pm can be measured.

Incidentally, the detecting section 6 of such a pressure sensor can be manufactured by the following steps.

FIG. 2 is a process diagram showing the process of manufacturing the detection section, and is shown based on the Y-Y direction in FIG. 1.

FIG. 2A shows a rectangular n-type silicon substrate 15 that is a base for making a detection section.

Next, this silicon substrate 15 is thermally oxidized to form an oxide film (SiO ₂ ) 16 on its surface (FIG. 2B).

Furthermore, in the step shown in FIG.
A groove 17 is formed.

In the process shown in Figure 2 (d), hydrogen gas is heated at a temperature of 1050°C.
A groove 18 is formed in the silicon substrate 15 via the groove 17 by etching in an atmosphere containing H ₂ and hydrochloric acid HCl.

Next, as shown in FIG.
10 ¹⁸ cm ^-3 in an atmosphere of hydrogen gas H ₂ at a temperature of 1050 ° C
A first epitaxial layer 19 is formed by selectively epitaxially using boron (P type) having a concentration of .

After this, as shown in FIG.
A second epitaxial layer 20, which will become the vibrating beam 10, is formed on the vibrating beam 10 by selectively epitaxially depositing boron (P type) at a concentration of 10 ²⁰ cm ^-3 in an atmosphere of hydrogen gas H ₂ at a temperature of 1050° C.

If the vibrating beam 10 is not given initial tension even when the measurement pressure P _M is zero, it will buckle due to the application of the measurement pressure P _M and become unable to be measured. Also, the covalent radius of silicon is 1.17 Å and that of boron is 0.88 Å, so when boron is partially implanted into silicon, that part experiences tensile strain.

Therefore, by utilizing this phenomenon, for example, by adjusting the concentration of boron in the second epitaxial layer 20, an initial tension can be applied to the second epitaxial layer 20, or by applying phosphorus in the n-type silicon substrate 15 (covalent bond radius is 1.10 Å). The initial tension is applied by adjusting the concentration and taking into account the relative strain between the silicon substrate 15 and the second epitaxial layer 20.

Next, as shown in FIG.
In an atmosphere of hydrogen gas _H2 at a temperature of 1050 °C above
A third epitaxial layer 21 is formed by selectively epitaxially using boron (P type) with a concentration of 10 ¹⁸ cm ^-3 .

Furthermore, as shown in FIG.
In an atmosphere of hydrogen gas _H2 at a temperature of 1050 °C above
A fourth epitaxial layer 22 is formed by selectively epitaxially using phosphorus (n-type) with a concentration of 10 ¹⁷ cm ^-3 .

FIG _. 2L shows that after the selective epitaxial process shown in FIG.
The figure shows an etching process for removing the first epitaxial layer 19 and the third epitaxial layer 21 in a state where the first epitaxial layer 19 and the third epitaxial layer 21 are removed by etching (the process is not shown).

In this etching process, although not shown,
The entire structure is immersed in an alkaline solution, and the fourth epitaxial layer 22 covers the n-type silicon substrate 15 and the p-type second silicon substrate 15.
A positive pulse voltage with a peak value of 5 V and a repetition period of about 0.04 Hz is applied from a DC pulse power source 23 so as to have a positive potential with respect to the epitaxial layer 20. By applying this voltage, the n-type silicon substrate 15
An insoluble film is formed on the surface of the fourth epitaxial layer 22 to make it passivated, and as a result, its etching rate is significantly slower than that of the first epitaxial layer 19 and the third epitaxial layer 21. to remove the first epi layer 19 and the third epi layer 21, and remove the second epi layer 20.
Utilizing the phenomenon that when the concentration of doped boron is greater than 4×10 ¹⁹ , the etching rate is significantly slower than the normal rate for undoped silicon, the entire structure is formed as shown in FIG.

Figure 2 shows the thermal oxidation process. In this step, oxide films (SiO ₂ ) 15a, 20a, and 22a are formed on all inner and outer surfaces of the silicon substrate 15, the second epitaxial layer 20, and the fourth epitaxial layer 22, respectively.

FIG. 2E shows the plasma etching process.
In this process, the silicon substrate 1 is
5. Oxide films 15a formed on the entire inner and outer surfaces of the second epitaxial layer 20 and the fourth epitaxial layer 22, respectively;
Among 20a and 22a, silicon substrate 15
Then, the oxide film formed on the outer surface of the fourth epitaxial layer 22 is removed by plasma etching to prepare for selective epitaxial growth in the next step.

In the process shown in FIG. 2W, n-type selective epitaxial growth is performed on the outer surfaces of the silicon substrate 15 and the fourth epitaxial layer 22 in an atmosphere of hydrogen H ₂ at a temperature of 1050° C. as a whole. By this selective epitaxial growth, the space between the silicon substrate 15 and the fourth epitaxial layer 22 is filled, and a capsule 23 having the vibrating beam 10 formed of the rod-shaped fourth epitaxial layer inside is formed.

In the process shown in FIG. 2, selective epitaxial growth was performed in an atmosphere of hydrogen H ₂ , so hydrogen H ₂ was sealed in the hollow chamber 9 inside the capsule 23 made of a silicon single crystal. . Therefore, 900℃
This capsule 23 is placed in a vacuum atmosphere, and the hydrogen H ₂ is degassed through the crystal lattice to create a vacuum. The degree of vacuum thus obtained is 1×10 ^-3 Torr or less.

The capsule 8 shown in FIG. 1 is formed by etching the bottom of the capsule 23 and embedding the permanent magnet 11 in the form of a sheet.

In FIG. 1, the shape of the detection section 6 is rectangular, but the shape is not limited to this and can be made into any shape.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the present invention, there is no need to form a thin pressure-receiving diaphragm on a silicon substrate, and the cover is bonded to the pressure-receiving diaphragm by anodic bonding or the like. Since there is no need to evacuate the pressure sensor, a pressure sensor with a simple structure and high reliability can be realized. Furthermore, the initial strain ε ₀
Since all values other than ₀ and the resonance frequency are physical property values of silicon, and all other than the resonance frequency are extremely stable constants, the relationship in equation (4) is strictly established, and a highly accurate pressure sensor can be realized.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a process diagram showing the process of manufacturing the detecting part shown in Fig. 1, Fig. 3 is a perspective view showing the structure of the detecting part of a conventional pressure sensor with the cover removed, and Fig. 4 is a - cross section in Fig. 3. FIG. 5 is a sectional view with a cover attached, and FIG. 5 is a plan view with a part omitted from FIG. 3. 1...Silicon substrate, 2...Pressure receiving diaphragm,
3, 4... Vibration beam, 5... Cover, 6... Detection unit, 7...
Excitation section, 8... Capsule, 9... Hollow chamber, 10... Vibration beam, 11... Permanent magnet, 12... Transformer, 13... Amplifier, 14... Comparison resistor, 15... Silicon substrate, 1
6... Oxide film, 19... First epi layer, 20... Second epi layer, 21... Third epi layer, 22... Fourth epi layer, 23
…capsule.

Claims (1)

[Claims] 1 A capsule formed by a semiconductor process and to which measurement pressure is uniformly applied around the capsule, a hollow chamber formed inside this capsule, and a chamber formed in this hollow chamber with a predetermined tension at both ends, which are formed only by the capsule and the semiconductor process. and a silicon single crystal vibrating beam integrally formed with a silicon single crystal, the pressure sensor is characterized in that the measured pressure is measured from a change in resonance frequency based on a change in tension applied to the vibrating beam due to the measured pressure.