JPH0519089B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a vibrating strain sensor that detects strain corresponding to the measurement pressure of a vibrating beam formed on a substrate as a frequency signal, and in particular, the present invention relates to a vibrating strain sensor that detects strain corresponding to measurement pressure of a vibrating beam formed on a substrate as a frequency signal. This invention relates to a vibrating strain sensor with an added vibrating beam.

<Prior Art> A vibrating strain sensor using string vibration is disclosed, for example, in Japanese Patent Application Laid-Open No. 54-56880 entitled "Vibration Line Device."

What is disclosed herein is that when the differential pressure is received by a diaphragm and the differential pressure is detected from the change in tension applied to the vibrating line, a pair of flat wires surrounding the vibrating line are used to give initial tension to the vibrating line. It uses interconnected washer-like zero springs to tension the vibrating wire, and uses differences in the coefficients of expansion of the materials to prevent tension changes due to temperature.

Further, a vibrating strain sensor using a semiconductor is disclosed, for example, in Japanese Patent Application No. 59-42632 entitled "Pressure Sensor".

Regarding this vibration type strain sensor, Figures 7 to 9
The outline will be explained using figures.

Fig. 7 is a perspective view showing the configuration of this conventional vibrating strain sensor with the cover removed, Fig. 8 is a sectional view taken along the line X-X in Fig. 9 with the cover attached, and Fig. 7 is partially omitted. FIG.

In these figures, 1 is a cylindrical silicon substrate, and 2 is a pressure-receiving diaphragm that is formed by digging the center of this silicon substrate 1 to form a thin part to serve as a pressure-receiving part that receives the measurement pressure Pm. It is made by etching 1.

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, for example, n
A first P ⁺ -type epitaxial layer is formed on a silicon substrate 1 , a cut is made in the center to electrically separate the left and right sides, an n-type epitaxial layer is formed on this, and then a P + -type epitaxial layer is formed on top of the first P + -type epitaxial layer. ^{A +} type epitaxial layer is formed and the top thereof is protected with an oxide film SiO ₂ . The cavity 5 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 10 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. Although not shown, the vibrating beam 4 side is also formed in the same way.

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

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, 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 FIG. 8, when the 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 vibrating strain sensors, the former mechanical vibrating strain sensors, have a complicated tension application structure and are difficult to maintain tension stability. The latter semiconductor-type vibrating strain sensor has a simpler configuration than the former mechanical vibrating strain sensor, but in order to maintain the initial tension, many elements must be controlled. Since no special measures have been taken, there is a drawback that the sensitivity to pressure varies. Furthermore, there is a drawback that buckling occurs when subjected to compressive strain.

<Means for Solving the Problems> In order to solve the above problems, the present invention measures the resonant frequency of a semiconductor vibrating beam whose both ends are fixed to a semiconductor substrate. In a vibrating strain sensor that measures applied strain, other atoms with a covalent bond radius different from the covalent bond radius of the main atoms constituting the substrate are mixed into the substrate to give initial tension to the vibrating beam. This is what I did.

<Operation> Other atoms with a covalent bond radius different from that of the main atoms constituting the substrate are mixed into this substrate, and an initial tension is applied to the vibrating beam whose both ends are bonded to the substrate, and a predetermined sensitivity is achieved. maintain.

<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, and FIG. 1A is a longitudinal cross-sectional view thereof, and FIG. 1B is a cross-sectional view showing a Y--Y cross section in A.

Reference numeral 8 denotes a silicon substrate, and a silicon vibration beam 9 having a rectangular cross section is fixed to both ends of this silicon substrate 8, and is fixed at a predetermined distance from the silicon substrate 8 except for these two ends. In this silicon substrate 8, an initial tension is applied to the vibrating beam 9 by mixing other atoms having a smaller covalent bond radius with respect to atoms having a predetermined covalent bond radius mixed in the vibrating beam 9.

The top of this vibrating beam 9 is covered with a silicon shell 10 to form a hollow chamber 11, the inside of which is maintained in a vacuum.

A permanent magnet 12 is fixed to the bottom of the silicon substrate to apply a DC magnetic flux φ to the vibrating beam 9, so that the vibration of the vibrating beam 9 can be electrically detected.

FIG. 2 is an explanatory view of the manufacturing process showing an enlarged view of the essential parts of the vibrating strain sensor in FIG. 1. FIG.

Reference numeral 13 denotes a P-type silicon holding substrate with a boron B concentration of about 10 ¹⁷ cm ^-3 .

First, on top of this, 10 ²⁰ cm ^-3 of boron and 5 × 10 ²⁰ cm
A P ⁺ type silicon epitaxial layer doped with ^-3 or more germanium (Ge) is formed as the first epitaxial layer 14 .

Next, an N-type silicon epitaxial layer doped with 10 ¹⁸ cm ^-3 of phosphorus (P) is formed on the first epitaxial layer 14 as a second epitaxial layer 15 .

Thirdly, on this second epitaxial layer 15, a rectangular beam is doped with 10 ²⁰ cm ^-3 of boron by impurity diffusion, ion implantation, selective epitaxial, etc. to become the P ⁺ type vibrating beam 9. A region 16 is formed.

Fourth, a thermosetting film (SiO ₂ ) 17 is formed as a mask when etching the peripheral portion of the beam region 16, and then a window 18 is opened by photolithography.

In this manner, the intermediate step before etching shown in FIG. 2A is completed.

Next, proceed to the etching step shown in FIG. 2B.
When the semi-finished product obtained in the intermediate step shown in FIG. 2A is immersed in an alkaline solution and etched, a beam region 16 with a boron concentration of 10 ²⁰ cm ^-3 and a first epitaxial layer 14 (bottom surface) of P ⁺ are formed. Since the vibration beam 9 is not etched with alkaline solution, the vibrating beam 9 is formed so as to be spaced apart from the surroundings except for both ends of the rectangular shape shown in the figure.

Further, a portion of the holding substrate 13 is removed by etching from the back side, and the first epitaxial layer 14 is removed.
It is possible to obtain a silicon substrate 8 having a thickness equal to the thickness of the second epitaxial layer 15 plus the thickness of the second epitaxial layer 15.

This silicon substrate 8 can also be used as a diaphragm for detecting differential pressure or the like by removing the permanent magnet 12 and placing it in another position.

The vibrating beam 9 has a vibrating strain sensor that measures pressure, for example.
It is necessary to apply an initial tension to the vibrating beam 9 so that it does not buckle when Pm is applied, and the following explains how the initial tension is applied to the vibrating beam 9 with the above configuration. .

FIG. 3 shows the relationship of the covalent bond radius Ri/Rsi of various impurities with respect to the covalent bond radius Ri of various impurities and the covalent bond radius Rsi of silicon. FIG. 4 shows the change in lattice constant with respect to impurity concentration. From these figures, the covalent bond radius of silicon (Si) is 1.17 Å, phosphorus (P) is 1.10 Å, boron (B) is 0.88 Å, which is smaller than silicon, and germanium (Ge) is 1.22 Å, which is smaller than silicon. It turns out it's big.

Therefore, by taking the weighted average of the concentrations of the covalent radius of these dopants and silicon, the covalent radius of that part is calculated, and two different values of this value are calculated.
Strain will occur due to contact between the two layers. For example, when boron or phosphorus is implanted into silicon, this part shrinks, and the degree of shrinkage is shown in Figure 4. For example, when boron has a concentration of 10 ²⁰ cm ^-3 , the change in lattice constant is 2× 10 ^-3 Å,
On the other hand, since the lattice constant of silicon is 5.431 Å, the shrinkage is approximately 4×10 ⁻⁴ (=2×10 ⁻³ /5.431). 4
To give a shrinkage of ×10 ^-4 or more, boron can be implanted in proportion to the amount of implantation.

Considering the above points, since the beam region 16 is doped with 10 ²⁰ cm ^-3 of boron, the lattice is extremely shrunken. On the other hand, the second epitaxial layer 1
5 is doped with phosphorus to about 10 ¹⁸ cm ^-3 , so it shrinks less than the beam region 16, which causes tensile strain in the beam region 16. However, the second epitaxial layer 15 is P ⁺
Since it is an epitaxial layer formed on the epitaxial layer 14, it is influenced by the first epitaxial layer 14. If the first epitaxial layer 14 were a layer doped only with boron, the lattice of this layer would be compressed in the same way as the beam regions 16, and the second epitaxial layer 15 would also be compressed and there would be no sufficient space for the beam regions 16. Tension no longer occurs. For this reason, the ^first
The epitaxial layer 14 is simultaneously doped with germanium, which has a larger covalent bond radius than silicon, to expand the lattice and prevent tension from relaxing.

Furthermore, increasing the concentration of germanium,
If germanium is also mixed into the epitaxial layer 15, the tension can be increased.

As described above, any initial tension can be applied to the vibrating beam 9.

FIG. 5 is an overall configuration diagram showing a configuration in which the vibrating strain sensor shown in FIG. 1 is combined with an excitation circuit for exciting it.

19 is a vibrating strain sensor shown in FIG. 1 that detects the measured pressure, and 20 is an excitation circuit that excites the vibrating strain sensor 19 and extracts changes in its resonance frequency.

Lead wires l ₁ and l ₂ are drawn out from both ends of the vibrating beam 9 of the vibrating strain sensor 19 to terminals T ₁ and T ₂ , respectively.

The excitation unit 20 is a transformer 2 with a center point tap.
1, an amplifier 22, a comparison resistor 23, and the like. The voltage of the secondary winding n ₂ of the transformer 21 is applied to the series circuit of the vibrating beam 9 and the comparison resistor 23 connected via terminals T ₁ and T ₂ .

Furthermore, an amplifier 22 is connected between the connection point of the vibrating beam 9 and the comparison resistor 23 and the primary winding n ₁ of the transformer 21.
is connected to apply positive feedback, and the oscillation frequency is detected from its output terminal via terminal _T3 .

Since the DC magnetic field φ is applied to the vibrating beam 9 from the permanent magnet 12, the vibrating beam 9
When an alternating current i flows through the vibrating beam 9, an electromagnetic force acts on the vibrating beam 9, causing it to vibrate. Since the hollow chamber 11 is maintained in a vacuum state, this vibration has a high Q of the vibrating beam 9, ranging from hundreds to tens of thousands, and has a large amplitude when it resonates. Therefore, when positive feedback is applied to the transformer 21 via the amplifier 22, the system self-oscillates at different natural frequencies due to the distortion of the vibrating beam 9.

The tension in the axial direction of the vibrating beam 9 changes due to the compressive strain determined by the volumetric compressibility specific to the silicon Si constituting the silicon substrate 8 due to the measurement pressure Pm applied from the surroundings of the silicon substrate 8, etc., and the resonance frequency changes. Since the oscillation frequency changes, the measured pressure Pm can be determined from the change in the oscillation frequency.

The above explanation is mainly based on a configuration in which strain is detected from the difference in oscillation frequency caused by applying measuring pressure from the outside of the vibrating strain sensor and the vibrating beam shrinks. However, the configuration is not limited to this, and for example, /Can also be used for pressure sensors, acceleration sensors, etc.

Furthermore, although boron, phosphorus, and germanium have been described as examples of atoms to be implanted, the present invention is not limited to these.

Furthermore, the material of the vibrating beam or the substrate that fixes it is not limited to silicon; for example,
Even when a vibrating beam of GaxIn( ₁ -x)GaAs is formed, tension is generated.

<Effects of the Invention> As specifically explained above with reference to the embodiments, according to the present invention, it is possible to stably apply an initial tension of any magnitude simply by controlling impurities.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of one embodiment of the present invention, Fig. 2 is an explanatory diagram explaining the process of manufacturing the vibrating strain sensor shown in Fig. 1, and Fig. 3 is a covalent bonding of various impurities. A characteristic diagram showing the radius and the ratio of the covalent bond radius of various impurities to the covalent radius of silicon, Figure 4 is a characteristic diagram showing the change in lattice constant with respect to impurity concentration, and Figure 5 is the vibration shape shown in Figure 1. A block diagram showing an excitation circuit that excites the strain sensor; FIG. 6 is an explanatory diagram explaining the excitation state shown in FIG. 5;
Fig. 7 is a perspective view showing the configuration of a conventional vibrating pressure sensor, Fig. 8 is a sectional view taken along line X-X in Fig. 7 with a cover attached, and Fig. 9 is a partially omitted part of Fig. 8. FIG. 1, 8... Silicon substrate, 2... Pressure receiving diaphragm, 3, 4, 9... Vibration beam, 11... Hollow chamber, 12...
Permanent magnet, 13... Holding substrate, 14... First epitaxial layer, 15... Second epitaxial layer, 16...
Beam region, 19... Vibration type strain sensor, 20... Excitation circuit.

Claims (1)

[Claims] 1. In a vibrating strain sensor that measures the strain applied to both ends of a semiconductor vibrating beam by measuring the resonance frequency of a semiconductor vibrating beam whose both ends are fixed to a semiconductor substrate, A vibrating strain sensor characterized in that other atoms having a covalent bond radius different from the covalent bond radius are mixed into the substrate to apply initial tension to the vibrating beam.