JPH01138432A - Vibration type differential pressure sensor - Google Patents

Abstract

PURPOSE:To reduce a static pressure error and to detect differential pressure with high accuracy by using a material which is larger in bulk compressibility to pressure than a substrate as a base to be joined with the substrate. CONSTITUTION:When static pressure Sp is applied, compressive strain epsilons1=Spl/ Es1 (where Es1 is bulk compressibility and l is the length of the vibration beam) is generated in the vibration beam 25. The base 28, on the other hand, is also compressed with the static pressure Sp, but its bulk compressibility is larger than that of the substrate 22, so the base 28 shrinks more to deform projecting to the side of the substrate 22, thereby generating tensile strain epsilons2 in the vibration beam 25 provided nearby the surface of a pressure receiving diaphragm 23. For the purpose, the bulk compressibility of the base 28 is so selected that epsilons1=epsilons2, and consequently the static error of the vibration beam 25 is suppressed and high-accuracy differential pressure detection is performed.

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 basis of the improvement of the present invention is a type in which pressure or differential pressure is detected from changes in the resonance 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 turgor 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.

An outline of this vibrating differential pressure sensor will be explained using FIGS. 4 and 5.

FIG. 4 is a perspective view of this conventional vibrating differential pressure sensor with the cover removed, and FIG. 5 is a sectional view taken along the line XX in FIG. 4 with the cover attached.

In these figures, 1 is a cylindrical silicon substrate, and 2 is a pressure-receiving part that receives the pressure SP from the top surface and the pressure (Sp+ΔP) from the bottom surface by digging the center of the silicon substrate 1 to form a thin part. This is a pressure receiving diaphragm, and 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 vibration beam 3 includes, for example, an n-type silicon substrate 1
A first P+ type epitaxial layer is formed on top of the first P+ type epitaxial layer, a cut is made in the center of the layer to electrically separate the left and right sides, an n-type epitaxial layer is formed on this layer, and then a P+ type epitaxial layer is formed. This top is protected with an oxide film 5jO2.

The cavity 5 at the bottom of the vibrating beam 3 is formed by under-etching this n-type epitaxial layer.

The vibrating beam 3 formed in this way has a certain length, for example!
, if the thickness is h and the width is d, then! =100pm, h=L
The size is on the order of czm, d=5 μm.

The periphery of the vibrating beam 3 formed on 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 this 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 of the vibrating differential pressure sensor.

In the above a formation, when an oscillation circuit is connected between the first left and right P+ type epitaxial layers for the vibrating beam 3, which is the first and second P-type epitaxial layers, to cause oscillation, the vibrating beam 3 becomes Automatic oscillation occurs at its natural frequency.

In this case, the internal space 7 of the cover 6 is made into a vacuum state and the vibrating beam 3 is held in vacuum, so Q
The value increases, making it easier to detect the resonance frequency.

As shown in FIG. 5, when a pressure difference ΔP between the pressure sp and the pressure (Sp+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, with such conventional vibrating differential pressure sensors,
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, and the internal space formed by the cover 6 and the pressure receiving diaphragm 2 is evacuated. Therefore, 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 applicant
On July 2, 1982, the company filed Japanese Patent Application No. 1983-466175 titled ``Title of the invention: Method for manufacturing a vibrating transducer.'' The outline of this proposal will be explained below.

FIG. 6 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.

Figure 6 (a) is the base for making the detection part D-.
A shaped silicon substrate 8 is shown.

Next, this silicon substrate 8 is thermally oxidized to oxidize fl! on its surface. (SLO□) 9 is formed (FIG. 6(b)).

Further, in the step shown in FIG. 6(c), the central portion of this oxide film 9 is photo-etched in a direction perpendicular to the plane of the paper to form a blank 10.

In the step shown in FIG. 6(d), a groove 11 is formed on the silicon substrate 8 through the eraser 10 by etching in an atmosphere containing hydrogen gas H2 and hydrochloric acid HC2 at a temperature of -1050''C.

Next, as shown in Figure 6 (e), 105
IQ"cm" in an atmosphere of hydrogen gas H2 at a temperature of 00C
The first shrimp layer 12 is formed by selectively epitaxially epitaxially using boron (P type) having a concentration of "-".

After this, as shown in FIG.
Boron (P type) having a concentration of 0''cm-' is selectively epitaxially applied to form a second shrimp layer 14 that will become the vibrating beam 13.

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. In addition, the covalent bond radius of silicon is 1.178, and that of boron is 0.88A, so when boron is partially implanted into silicon, that part undergoes tensile strain. For example, by adjusting the concentration of boron in the second shrimp layer 14, an initial tension can be given here, or by adjusting the concentration of phosphorus (covalent bond radius is 1.10 μm) in the n-type silicon substrate 8. Initial tension is applied in consideration of the relative strain between the silicon substrate 8 and the second shrimp layer 14.

Next, as shown in FIG.
A third shrimp layer 15 is formed by selectively epitaxializing boron (P type) with a concentration of 0"cm"-'.

Furthermore, as shown in FIG.
A fourth shrimp layer 16 is formed by selectively epitaxializing phosphorus (n-type) having a concentration of 0110l7'.

Figure 6 shows 1. After the selective epitaxial process shown in FIG.
This figure shows an etching process for removing the first shrimp layer 12 and the third shrimp layer 15 in a state where the first shrimp layer 12 and the third shrimp layer 15 are removed by etching (step not shown).

In this etching process, although not shown, the entire silicon substrate 8 is immersed in an alkaline solution.
A positive pulse voltage with a peak value of 5cm and a repetition period of about 0.04H2 is applied from the DC pulse power supply 17 so that the fourth shrimp layer 16 has a positive potential with respect to the n-type second shrimp layer 14. has been done. As a result of this voltage application, an insoluble film is formed on the surfaces of the n-type silicon substrate 8 and the fourth layer 16 and the etching rate is significantly higher than that of the first epitaxial layer 112 and the third layer 15. Therefore, the first shrimp layer 12 and the third shrimp layer 15 are removed using this delay. Further, the second shrimp layer 14 is formed by leaving the second shrimp layer 14 by taking advantage of the phenomenon that when the concentration of doped boron is greater than 4×101 g, the etching rate is significantly slower than the normal speed for undoped silicon. As a whole, it is formed to have an opening 18 in a part and a gap between the silicon substrate 8 and the second shrimp layer 14, as shown in FIG.

FIG. 6(R) shows the process of thermal oxidation. In this process,
Silicon substrate 8, second shrimp layer 14, and fourth shrimp layer 1
Oxidized WA (St 02
) Form 8a, 14a, and 16a.

FIG. 6(E) shows a plasma etching process. In this process, in the process of FIG. 6(R), the silicon substrate 8, the second
Of the oxide films 8a, 14a, and 16a formed on the entire inner and outer surfaces of the shrimp layer 14 and the fourth shrimp layer 16, the oxide film formed on the outer surface portions of the silicon substrate 8 and the fourth shrimp layer 16. removed by plasma etching,
Prepare for selective epitaxial growth in the next step.

In the process shown in FIG. 6(W), the entire silicon substrate 8 and the fourth shrimp layer 16 are heated in an atmosphere of hydrogen H2 at a temperature of 1050'C.
N-type selective epitaxial growth is performed on the outer surface of the substrate. By this selective epitaxial growth, the silicon substrate 8
and a fourth shrimp layer 16 is filled to form a shell 20, and a detection part of a vibrating differential pressure sensor has a vibrating beam 13 formed of a rod-shaped fourth shrimp layer inside. or formed.

In the process shown in FIG. 6(W), selective epitaxial growth was performed in an atmosphere of hydrogen H2, so that the hollow chamber 21 formed between the silicon substrate 8 made of single crystal silicon and the shell 20 Hydrogen 142 is sealed.

Therefore, as shown in Figure 6 (force), a vibrating differential pressure sensor with this detection part is placed in a vacuum atmosphere at 900'C, and the hydrogen H2 is passed through the silicon crystal lattice.
Degas to create a vacuum. The degree of vacuum thus obtained is lXl0-"l'.

rr 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. The drawbacks of the sensor can be eliminated, but still await the drawbacks described below.

Using such a vibrating differential pressure sensor, for example, 500
When measuring a minute differential pressure ΔP such as (10 mmH20 to 10 Kgf/cm2) under a high static pressure Sp reaching K g f / cm ', one side of the pressure receiving diaphragm of the vibrating differential pressure sensor Static pressure SP is applied to the surface of
It contracts by spf/Es. However, ES is the volumetric compressibility,
2 is the length of the vibrating beam.

Therefore, a compressive strain of εS is applied to the vibrating beam, which causes a problem in that the resonant frequency of the vibrating beam changes and a static pressure error occurs.

Means for Solving the Problems> In order to solve the above problems, the present invention provides a semiconductor substrate having a fixed portion around the periphery and a pressure-receiving diaphragm that is deformed by differential pressure inside the substrate; At least one or more vibrating beams that are formed and measure the strain generated near the surface of the pressure receiving diaphragm due to the differential pressure; a shell that is integrally formed with the substrate and covers the periphery of the vibrating beam; and a shell that is joined to the fixed part and forms a differential pressure. The substrate has a pressure guide hole for introducing the pressure, and a base made of a material whose volumetric compression ratio with respect to pressure is larger than that of the substrate.

By using a base made of a material that has a higher volumetric compression ratio against pressure than the substrate as the base to be bonded to the substrate, the static pressure applied from the surroundings bends the base and the substrate A tensile stress is generated in the vibrating beam formed in the structure, and the compressive strain on the vibrating beam due to static pressure is canceled out by this tensile stress.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a perspective view showing one embodiment of the present invention without a shell, and FIG. 2 is a sectional view showing the structure of the embodiment taken along line X--X with the shell attached.

In these figures, 22 is a cylindrical silicon substrate, and 23 is a pressure-receiving diaphragm that is formed by digging the center of this substrate 22 to form a thin part and serving as a pressure-receiving part that receives static pressure Sp, (Sp+ΔP). , for example, is made by etching silicon, and its periphery is an annular fixed portion 24 with a thick thickness.

Reference numeral 25 is a vibrating beam for detecting strain due to differential pressure, and reference numeral 26 is a shell for maintaining the inside in a vacuum, and these are manufactured by the manufacturing method explained in FIG. 6.

A disk-shaped silicon base 28 having a pressure guiding hole 27 in the center for introducing pressure (Sp + ΔP) is bonded to the bottom surface of the fixing part 24 by, for example, anodic bonding. For the base 28, a material whose volumetric compression rate is larger than that of the substrate 22 is selected.

Next, the operation of the embodiment shown in FIGS. 1 and 2 constructed as above will be explained using the operation explanatory diagram shown in FIG. 3.

The operation when the differential pressure ΔP is applied is exactly the same as the conventional one, so in the following explanation, the operation when the static pressure sp is applied will be explained.

When the static pressure SP is applied, a compressive strain of εs + =SP f/Es 1 is generated in the vibrating beam 25 as in the conventional case shown in FIG.

On the other hand, the base 28 is compressed by the static pressure Sp, but its volumetric compression ratio is larger than that of the substrate 22, so the third
As shown in the figure, the base 28 shrinks more than the base 28, and the substrate 22
A tensile strain εs2 is generated in the vibrating beam 25, which is deformed so as to be convex to the side and is provided near the surface of the pressure receiving diaphragm 23.

Therefore, the relationship between compressive strain εs1 and tensile strain εs2 is εs1
By selecting the volumetric compression ratio of the base 28 with respect to the substrate 22 so that -εs2, the static pressure error with respect to the vibrating beam 25 can be suppressed to a small value.

Furthermore, by appropriately selecting the fi expansion coefficient of the base 28 with respect to the substrate 22, the temperature characteristics of the vibrating differential pressure sensor can be compensated.

If the degree of static pressure compensation by the base 28 is insufficient due to variations, a strain gauge may be formed on the top surface or inside the fixed part 24 in FIGS. 1 and 2 by, for example, diffusion. Alternatively, a vibrating beam similar to the vibrating beam 25 is separately provided and used as a static pressure sensor,
If the output is used to compensate for variations in the static pressure compensation of the base 28 using an electric circuit, more complete static pressure compensation can be achieved. Further, this static pressure sensor may be provided on the base 28 side.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the present invention, a material having a higher volumetric compression ratio against pressure than the substrate is used as the base to be bonded to the substrate, so that Since compressive strain caused by static pressure is compensated for by generating tensile strain, static pressure errors can be reduced and a highly accurate differential pressure sensor can be realized.

[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 cross-sectional view taken along the line x-x in Fig. 1, and Fig. 3 is a diagram showing Figs. 1 and 2. FIG. 4 is a perspective view showing the configuration of a known conventional vibrating differential pressure sensor excluding the shell, and FIG.
FIG. 6 is a cross-sectional view showing the X cross section, and is a process diagram illustrating the process of manufacturing the conventional vibrating differential pressure sensor of the prior application. 1... Silicon substrate, 2... Pressure receiving diaphragm, 3
... Vibration beam-5... Cavity part, 6... Cover, 7.
...Internal space, 8...Silicon substrate, 12...First
Shrimp layer, 13... Vibration beam, 14... Second shrimp layer, 1
5...Third shrimp layer, 16...Fourth shrimp layer, 18...
- Opening, 20... Shell, 21... Hollow chamber, 22
...Substrate, 23...Pressure diaphragm, 24...
Fixed part, 25... Vibration beam, 26... Shell, 28.
...base.

Claims (1)

[Claims] 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 or more devices formed on this substrate to measure strain generated near the surface of the pressure receiving diaphragm due to the differential pressure. a vibrating beam, a shell that is integrally formed on the substrate and covers the periphery of the vibrating beam, and a pressure guiding hole that is joined to the fixed part and introduces one pressure forming the differential pressure, and has a volumetric compression ratio with respect to pressure. A vibrating differential pressure sensor comprising a base made of a material larger than the substrate.