JPH0519090B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a vibrating differential pressure sensor that detects strain corresponding to the differential pressure of a semiconductor vibrating beam as a frequency signal, and in particular has improved static pressure characteristics. Regarding a vibrating differential pressure sensor.

<Prior art> The present invention is based on a method of detecting pressure or differential pressure from changes in the resonant frequency of a vibrating beam formed on a silicon pressure receiving diaphragm, fixed at both ends, and excited by an excitation means. A known vibrating differential pressure sensor is disclosed, for example, in Japanese Patent Application No. 59-42632 entitled "Pressure Sensor". In this proposal, it is explained as a pressure sensor, but it can also be used as a differential pressure sensor, so in the following explanation, it will be explained as a differential pressure sensor.

Regarding this vibrating differential pressure sensor, Figures 7 and 8
The outline will be explained using figures.

FIG. 7 is a perspective view showing the structure of this conventional vibrating differential pressure sensor with the cover removed, and FIG. 8 is a sectional view taken along the line X--X in FIG. 7 with the cover removed.

In these figures, 1 is a cylindrical silicon substrate, and 2 is a pressure-receiving part that is dug in the center of this silicon substrate 1 to form a thin wall part that receives pressure S _p from the top surface and pressure (S _p +ΔP) from the bottom surface. The pressure receiving diaphragm is made by etching the silicon substrate 1, for example. Here, ΔP is the differential pressure to be measured.

A vibration beam 3 is formed on the pressure receiving diaphragm 2 and has both ends fixed to the silicon substrate 1, and the vibration beam 3 is provided approximately at the center of the pressure receiving diaphragm 2.

Specifically, this vibrating beam 3 is 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 then forming a layer on top of the first P + type epitaxial layer. After forming the n-type epitaxial layer, a P ⁺ -type epitaxial layer is further formed, and the top thereof is protected with an oxide film SiO ₂ . And vibration beam 3
The lower cavity 5 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 pressure-receiving diaphragm 2 is covered with, for example, a silicon cover 6 bonded to the pressure-receiving diaphragm 2 by anodic bonding or the like, and the internal space 7 is maintained in a vacuum state.

The vibrating beam 3, cover 6, cavity 5, internal space 7, etc. form a detection section D of the vibrating differential pressure sensor.

In the above configuration, the first left ^and right P ⁺
When an oscillation circuit is connected between the shaped epitaxial layers to cause oscillation, the vibrating beam 3 causes self-oscillation at its natural frequency.

In this case, since the internal space 7 of the cover 6 is brought into a vacuum state and the vibrating beam 3 is held in vacuum, the Q value indicating the sharpness of resonance becomes large, making it easy to detect the resonance frequency.

As shown in Figure 8, pressure S _p and pressure (S _p + ΔP)
When the pressure difference ΔP is applied to the pressure receiving diaphragm 2, the vibrating beam 3 receives tensile stress.

This tensile stress changes the natural frequency of the vibrating beam 3, allowing the differential pressure ΔP to be determined.

However, in such a conventional vibrating differential pressure sensor, a separately made silicone cover 6 is bonded to the top of the vibrating beam 3 formed on the thin pressure-receiving diaphragm 2 by anodic bonding or the like. Since the internal space formed by the pressure-receiving diaphragm 2 must be evacuated, there is a problem in that the pressure characteristics or temperature characteristics of the vibrating differential pressure sensor deteriorate, causing a decrease in accuracy.

Therefore, in order to solve this problem, the present applicant filed Japanese Patent Application No. 62-166175 on July 2, 1988 entitled "Title of the Invention: Method for Manufacturing a Vibratory Transducer"
has been submitted. The outline of this proposal will be explained below.

FIG. 9 is a process diagram showing the manufacturing process of the detection section, which is the main part of the vibratory differential pressure sensor according to this proposal.

This shows the process of the detection part D' shown in the axial cross section of the vibrating beam corresponding to the detection part D in FIG.

FIG. 9A shows an n-type silicon substrate 8 which will be the base for making the detection section D'.

Next, this silicon substrate 8 is thermally oxidized to form an oxide film (SiO ₂ ) 9 on its surface (FIG. 9b).

Furthermore, in the step shown in FIG. 9C, the central portion of this oxide film 9 is photoetched in a direction perpendicular to the plane of the paper to form a groove 10.

In the process shown in Figure 9 (d), hydrogen gas is heated at a temperature of 1050°C.
A groove 11 is formed in the silicon substrate 8 through the groove 10 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
The first epitaxial layer 12 is formed by selectively epitaxially using boron (P type) having a concentration of .

After this, as shown in FIG.
A second epitaxial layer 14, which will become the vibrating beam 13, is formed on the vibrating beam 13 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.

Even if the differential pressure ΔP is zero, the vibrating beam 13 will buckle due to the application of the differential pressure ΔP and become unmeasurable unless an initial tension is applied to it. 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 14, an initial tension can be applied to the second epitaxial layer 14, or by applying phosphorus in the n-type silicon substrate 8 (covalent bond radius is 1.10 Å). The initial tension is given by adjusting the concentration and taking into account the relative strain between the silicon substrate 8 and the second epitaxial layer 14.

Next, as shown in FIG.
In an atmosphere of hydrogen gas _H2 at a temperature of 1050 °C above
A third epitaxial layer 15 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 16 is formed by selectively epitaxially using phosphorus (n-type) with a concentration of 10 ¹⁷ cm ^-3 .

FIG. 9 (ri) shows that the _first epitaxial layer is removed after the selective epitaxial process shown in FIG. 12 and the third epitaxial layer 15 are shown.

In this etching process, although not shown,
The entire structure is immersed in an alkaline solution, and a peak voltage is applied from a DC pulse power source 17 so that the n-type silicon substrate 8 and the fourth epitaxial layer 16 have a positive potential with respect to the p-type second epitaxial layer 14. A positive pulse voltage with a value of 5V and a repetition period of about 0.04Hz is applied. By applying this voltage, an insoluble film is formed on the surfaces of the n-type silicon substrate 8 and the fourth epitaxial layer 16 and the etching rate is increased to the first level.
Since it is much slower than the epitaxial layer 12 and the third epitaxial layer 15, the first epitaxial layer 12 and the third epitaxial layer are
Epi layer 15 is removed. Furthermore, the second epi layer 14
Taking advantage of 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 second epitaxial layer 14 is left in place and the entire etching process is performed as shown in FIG. As shown in FIG. 3, it is formed to have an opening 18 in a part and a gap between the silicon substrate 8 and the second epitaxial layer 14.

FIG. 9 shows the thermal oxidation process. In this step, oxide films (SiO ₂ ) 8a, 14a, and 16a are formed on all inner and outer surfaces of the silicon substrate 8, the second epitaxial layer 14, and the fourth epitaxial layer 16, respectively.

FIG. 9E shows the plasma etching process.
In this step, oxide films 8a and 1 are formed on the entire inner and outer surfaces of the silicon substrate 8, the second epitaxial layer 14, and the fourth epitaxial layer 16, respectively, in the step of FIG.
Of 4a and 16a, the oxide films formed on the outer surfaces of silicon substrate 8 and fourth epitaxial layer 16 are removed by plasma etching to prepare for selective epitaxial growth in the next step.

In the process shown in FIG. 9W, n-type selective epitaxial growth is performed on the outer surfaces of the silicon substrate 8 and the fourth epitaxial layer 16 in an atmosphere of hydrogen H ₂ at a temperature of 1050° C. as a whole. By this selective epitaxial growth, the opening 18 formed between the silicon substrate 8 and the fourth epitaxial layer 16 is filled, a shell 20 is formed, and the vibration beam 13 formed of the rod-shaped fourth epitaxial layer is formed inside. A detection section of a vibrating differential pressure sensor is formed.

In the process shown in FIG. 9W, selective epitaxial growth was carried out in an atmosphere of hydrogen H ₂ , so that the epitaxial growth was carried out in the hollow chamber 21 formed between the silicon substrate 8 made of single crystal silicon and the shell 20. is filled with hydrogen _H2 .

Therefore, as shown in Figure 9 F, a vibrating differential pressure sensor with this detection part is placed in a vacuum atmosphere at 900°C, and this hydrogen H ₂ is degassed through the silicon crystal lattice. Make a vacuum. The degree of vacuum thus obtained is 1×10 ^-3 Torr or less.

<Problems to be Solved by the Invention> As described above, in the vibrating differential pressure sensor according to the prior application, both the vibrating beam and the cover can be manufactured integrally with the silicon substrate. Although the drawbacks of the sensor can be eliminated, it still has the following drawbacks.

Using such a vibrating differential pressure sensor, for example,
When measuring a minute differential pressure ΔP such as (10 mmH ₂ O to 10 Kg/cm ² ) under a static pressure Sp as high as 500 Kg/cm ² , one side of the pressure receiving diaphragm of the vibrating differential pressure sensor Static pressure Sp is applied to one surface, and static pressure (S _p + ΔP) is applied to the other surface, so this static pressure Sp causes the substrate, pressure-receiving diaphragm, and vibration beam to contract by ε _s = S _p l/E _s do. However, E _s is the volumetric compressibility, and l is the length of the vibrating beam.

Therefore, a compressive strain of ε _s is applied to the vibrating beam,
This causes a problem in that the resonant frequency of the vibrating beam changes, resulting in a static pressure error.

<Means for solving the problems> In order to solve the above problems, the present invention has the following features:
A semiconductor substrate having a fixed part around its periphery and a pressure receiving diaphragm deformed by differential pressure inside the semiconductor substrate, and at least one substrate formed on the substrate for measuring the strain generated near the surface of the pressure receiving diaphragm due to the differential pressure.
a support beam having one end fixed to the substrate and the other end connected at a predetermined angle to at least one end of the vibration beam, and a shell formed integrally with the substrate and surrounding the vibration beam and the support beam. and a semiconductor base having a pressure guiding hole which is connected to the fixed part and introduces one pressure forming a differential pressure.

<Operation> By fixing one end of the support beam fixed to the substrate and the other end of the vibration beam to at least one end of the vibration beam, the strain applied to the vibration beam due to static pressure is reduced by the strain produced in the support beam in the opposite direction due to static pressure. Cancel with distortion.

Embodiments of the present invention will be described below based on the drawings. Fig. 1 is a perspective view showing one embodiment of the present invention excluding the shell, Fig. 2 is a sectional view showing the configuration of the X-X cross section with the shell attached, and Fig. 3 is the vibrating beam in Fig. 1. FIG. 2 is an enlarged view showing the vicinity of FIG.

In these figures, 22 is a cylindrical silicon substrate, and 23 is a pressure-receiving part that is formed by digging the center of this substrate 22 to form a thin part to receive static pressure S _p , (S _p +ΔP). The diaphragm is made by etching silicon, for example, and has a thick annular fixing part 24 around the diaphragm.

25 is a vibrating beam section that detects distortion due to differential pressure;
6 is a shell that maintains the inside in a vacuum, for example;
These are manufactured in the same manner as the manufacturing method explained in FIG.

Details of the vibrating beam section 25 are shown in FIG. In this figure, 27 is a bar-shaped vibration beam with a rectangular cross section, one end 27a of which is integrally fixed to the pressure receiving diaphragm 23 of the substrate 22, and the other end 27b is one end of each of rod-shaped support beams 28 and 29 with a rectangular cross section. 28a,
It is integrally fixed to 29a. Support beam 28,2
9 is inclined at an angle α with respect to the vibrating beam 27, and the other ends 28b and 29b are integrally fixed to the pressure receiving diaphragm 23 of the substrate 22. Furthermore, spaces 30 and 31 are provided on the side surfaces of the vibration beam 27, and support beams 28 and
A space 32 is also formed on the side surface of the vibration beam 29, and a space 33 (FIG. 2) is formed on the bottom surface of the vibration beam 27, respectively.
Therefore, the vibrating beam 27 and the supporting beams 28, 29 are floating in space except that one end of each of them is fixed to the pressure receiving diaphragm 23.

A disk-shaped silicon base 34 having a pressure guiding hole 33 for introducing pressure (S _p +ΔP) in the center is bonded to the bottom surface of the fixed part 24 by, for example, anodic bonding.

Next, the operation of the embodiment shown in FIGS. 1 and 2 constructed as above will be explained using the operation explanatory diagrams shown in FIGS. 4 and 5. Figure 4 shows static pressure
Figure 5 shows the deformation of the vibrating beam section 25 when S _p is applied, and Fig. 5 shows the deformation of the vibrating beam section 25 when differential pressure ΔP is applied.
The deformation of each is shown.

First, the operation when static pressure S _p is applied will be explained using FIG. 4.

When static pressure S _p is applied to the pressure receiving diaphragm 23, the vibrating beam 2, which initially has a shape like a solid line,
7. The support beams 28 and 29 have a compressive strain of ε _s = S _p l ₁ /E _s depending on the length of the vibration beam 27.
The support beams 28 and 29 are connected to the vibration beam 27 at an angle α due to the compressive strain ε _s =S _p l ₂ /E _s generated in the beams 8 and 29, which is indicated by a dotted line. The right end of the vibrating beam 27 is displaced to the right by Δx ₁ .

Therefore, if the relationship of Δx ₁ =ε _s is selected, even if the static pressure S _p is applied to the pressure receiving diaphragm 23, no strain will be applied to the vibrating beam 27 as a result.

Next, the operation when a differential pressure ΔP is applied as a compressive stress σ _d in the direction of the vibrating beam will be explained using FIG. 5.

When the differential pressure ΔP is not applied, the vibrating beam 27,
The support beams 28 and 29 are shaped as shown by solid lines.

Next, when compressive stress σ _d is applied in the longitudinal direction of the vibrating beam 27 due to the differential pressure ΔP, one end 27a of the vibrating beam 27
A compressive strain ε _d is applied to the supporting beams 28 and 29 depending on their length, while a tensile strain ε _d ′=−l ₂ νε _d /l ₁ ( ν: Poisson's ratio) is added.

Due to the distortions of ε _d and ε _d ', the ends 28a and 29a of the support beams 28 and 29 are displaced by Δx ₂ . If each beam is designed so that Δx ₂ becomes Δx ₂ ≒ 0, then
Compressive strain ε _d is applied to the vibrating beam 27, resulting in a change in the resonance frequency corresponding to the differential pressure ΔP.
can be detected.

FIG. 6 is a longitudinal cross-sectional view showing the structure of a second embodiment of the vibrating beam section according to the present invention.

In this embodiment, one end 33a of the support beam 33 is connected to the support beam 27 in order to prevent the other end 27b of the vibration beam 27 shown in FIG.
8 and 29, and the other end 33b is fixed to the pressure receiving diaphragm 23. Even with this configuration, it operates in the same way as the embodiment shown in FIG.

In each of the embodiments, one end of the vibrating beam 27 is connected to the substrate, and a support beam is provided only at the other end. Operate.

Note that due to variations, these support beams 28,
If the static pressure compensation by 29 and 33 is insufficient, a strain gauge may be formed on the top surface or inside the fixed part 24 in FIGS. 1 and 2 by diffusion, or the vibrating beam 27 may be If you separately install a vibrating beam similar to the above, use it as a static pressure sensor, and use its output to compensate for variations in static pressure compensation in the vibrating beam using an electric circuit, more complete static pressure compensation can be achieved. . Further, this static pressure sensor may be provided on the base 34 side.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the present invention, a support beam is provided at at least one end of the vibrating beam to generate tensile strain against static pressure. Since compressive strain caused by static pressure is compensated for, static pressure errors can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of one embodiment of the present invention excluding the shell, FIG. 2 is a sectional view taken along the line X-X in FIG. 1, and FIG. 3 is a vibrating beam section in FIG. 1. FIG. 4 is an explanatory view of the operation of the embodiment shown in FIGS. 1 and 2 when static pressure is applied, and FIG. 5 is an enlarged view of the embodiment shown in FIGS. FIG. 6 is a sectional view showing the configuration of the second embodiment of the vibrating beam section of the present invention, and FIG. FIG. 8 is a perspective view showing the configuration of a known conventional vibrating differential pressure sensor; FIG. 8 is a sectional view taken along the line X-X in FIG. 7;
FIG. 9 is a process diagram illustrating the process of manufacturing the conventional vibrating differential pressure sensor of the prior application. 1...Silicon substrate, 2...Pressure diaphragm, 3
... Vibration beam, 5... Cavity, 6... Cover, 7... Internal space, 8... Silicon substrate, 12... First epi layer, 13
... Vibration beam, 14... Second epi layer, 15... Third epi layer, 16... Fourth epi layer, 18... Opening, 20... Shell, 21... Hollow chamber, 22... Substrate, 23... Pressure receiving diaphragm, 24... Fixed part, 25... Vibration beam part, 2
6... Ciel, 27... Vibration beam, 28, 29, 33...
Support beam.

Claims (1)

[Claims] 1. A semiconductor substrate having a fixed part around it and a pressure receiving diaphragm inside which is deformed by differential pressure, and at least a semiconductor substrate formed on this substrate to measure the strain generated near the surface of the pressure receiving diaphragm due to the differential pressure. one or more vibrating beams; a support beam having one end fixed to the substrate and the other end coupled at a predetermined angle to at least one end of the vibrating beam; and a support beam integrally formed with the substrate and connecting the vibrating beam and the support beam A vibrating differential pressure sensor comprising: a shell that covers the surrounding area; and a semiconductor base that is joined to the fixing part and has a pressure guiding hole that introduces one pressure forming the differential pressure.