JPH06289049A - Acceleration sensor - Google Patents

Abstract

PURPOSE:To provide a low-cost sensor structure having excellent temperature characteristics, high reliability of electric connection and sealing of an interior in an capacitance type silicon acceleration sensor. CONSTITUTION:A movable electrode 11 and stationary electrodes 22, 22' are all formed of silicon single crystal substrates. An acceleration sensor comprises a structure in which a thick porous silicon layer formed on the surface of an electrode chip is interposed between thermally oxidized thick oxide film layers 201 and 201', and diffused to be connected. In order to prevent the electrode 11 and the electrodes 22, 22' from short-circuiting on an outer periphery of a sensor chip, a structure in which spaces 3, 3' are formed on the periphery to spatially isolate both the electrode surfaces is provided. Accordingly, temperature characteristics are improved by equivalent thermal expansion coefficients, a gap accuracy between the electrodes is improved by uniform junction thickness, a capacitance preferable to the sensor is obtained by the oxide film thickness, a residual stress in the film is small due to short-time film formation, and a deformation after the connection is small. The sensor chip can be vertically mounted with small thermal deformation, and a short-circuit between boards can be prevented due to the space at the periphery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor having a minute sensor structure made of a single crystal silicon wafer.

[0002]

2. Description of the Related Art There is an increasing demand for small and high-performance acceleration sensors for purposes such as vehicle body control and collision detection systems for automobiles. In this field, in particular, the above requirements are satisfied, and even in mass production, quality uniformity, low production cost, reliability, etc. are required. One of the most powerful sensors developed so far is a sensor using silicon single crystal microfabrication technology. This method is further classified into a piezoresistive type and a capacitance type, depending on the difference in detection principle. The piezoresistive method has a drawback that temperature compensation of the output is considerably difficult because the piezoresistive element that detects strain greatly depends on the temperature of the environment. On the other hand, the capacitance type is suitable for mounting on an automobile in a harsh environment because the output has relatively small temperature dependence and high sensitivity. A typical example of this method is described in Japanese Patent Application Laid-Open No. 1-152369. In this example, a silicon single crystal movable electrode chip is sandwiched between two fixed electrodes from above and below. Glass having a conductive thin film formed on its surface is used as the fixed electrode.
Electrostatic bonding is used to bond silicon and glass,
The gap between the movable and fixed electrodes is created exactly.

[0003]

However, in the structure described in JP-A-1-152369 mentioned above, in order to take out the wiring from the fixed electrode on the glass surface to the outside, a hole is made in the glass to make a connection, and then the connection is made. , It is necessary to seal the hole. Therefore, there are problems in manufacturing cost and reliability of connection and sealing. Moreover, the temperature characteristics of the output tended to deteriorate due to the difference in the coefficient of thermal expansion among silicon, glass, and the sealing material.

As a method of preventing the characteristic deterioration due to the difference in the coefficient of thermal expansion by the method of sandwiching the silicon substrate between the two glass substrates as described above, instead of making the upper and lower glass substrates sufficiently thin, A structure in which a thicker silicon substrate is attached to the back surface has been proposed (Technical digest o
f the Transducers'87, p.385-398). With this method,
Since a total of five layers of substrates are laminated, there is a problem that the processing process becomes complicated.

On the other hand, Japanese Patent Laid-Open No. 2-134570 proposes a structure using a silicon single crystal substrate not only for the movable electrode but also for the fixed electrode for the same purpose.
In this method, for the purpose of insulation between three silicon substrates,
It is disclosed that a thermal oxide film formed on the surface of a silicon substrate is used, but since the film thickness that can be formed is limited to about 1 μm, there is a drawback that the parasitic capacitance on the joint surface between the substrates is large. . In order to reduce this parasitic capacitance, in this example, it is also disclosed that a low melting point glass having an appropriate thickness is further applied to the bonding surface and the substrate is bonded, but in this case, the low melting point glass is Control of layer thickness, ie
There is a drawback that it is difficult to control the gap between the movable and fixed electrodes.

Therefore, the inventors of the present invention have invented a structure in which a silicon single crystal is used for both the movable electrode and the fixed electrode, and a thick porous silicon layer formed on the surface of the electrode chip is diffusion-bonded with a thick oxide film layer thermally oxidized therebetween. did. Further, in order to prevent the movable electrode and the fixed electrode from being short-circuited on the outer peripheral surface of the sensor chip, the invention has invented a structure in which both electrode surfaces are spatially separated on the outer peripheral surface.

An object of the present invention is to solve the above problems in a capacitance type silicon acceleration sensor and to provide an inexpensive acceleration sensor having a highly reliable sensor structure of good temperature characteristics, electrical connection and internal sealing. To provide.

[0008]

In order to achieve the above object, the present invention provides a mass portion (movable electrode) that is displaced by acceleration, and an electrode portion (fixed electrode) that forms a capacitance facing the mass portion. ), A mass part that is displaced by the acceleration, and an electrode part that forms a capacitance facing the mass part are both made of silicon single crystal. It is characterized in that the bonding interface of the crystals is composed of a layer obtained by oxidizing porous silicon. In addition, each silicon single crystal has a structure in which it is directly bonded via an oxide on the surface, and the bonding interface does not exist on the end face of the outer circumference of the sensor chip, and the outer circumference of the bonded two members. It is characterized in that voids are formed on the end faces.

[0009]

According to the above structure, since the fixed electrode substrate itself is a conductor, there is no need to penetrate the fixed electrode for electrical connection or seal the through hole with resin or the like. Since the movable electrode and the fixed electrode are both made of silicon single crystal and have the same coefficient of thermal expansion, the temperature characteristics of the sensor output are excellent. The thick oxide film of porous silicon sandwiched between the movable electrode and the fixed electrode reduces the electrostatic capacitance (stray capacitance) between the two electrodes on the joint surface,
The sensitivity of the sensor is improved. Further, due to the gap on the outer peripheral end surface of the sensor chip, a short circuit on the end surface between the substrates can be completely avoided.

[0010]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows an exploded view of a gage portion of an acceleration sensor according to an embodiment of the present invention. 2 and 3 are sectional views of the sensor gauge portion. A movable electrode 11 that is displaced by feeling acceleration, and a fixed electrode 2 that is above and below the movable electrode 11 to detect movement of the movable electrode as a change in capacitance.
2, 22 ′ are three silicon single crystal substrates 1, 2,
2'is formed separately. The junction portion of the three-layer silicon substrate is made of a thick oxide film having a thickness of 10 to 20 μm. The oxide film is a film 1 formed on the surface of the central substrate 1.
01 and the films 201, 2 formed on the upper and lower substrates 2, 2 '.
01 'and the total thickness thereof is the value (1
0 to 20 μm).

When a three-layer structure of silicon is adopted, the peripheral portion of the sensor chip is exposed as a conductive surface at the surface cut by dicing. Even though the thick insulating film having a thickness of several tens of μm is interposed between the substrates, dust may adhere to the end faces to cause a short circuit between the substrates. In this embodiment, as shown in FIG. 1, notch shapes 13, 23, and 23 'are processed in advance on the outer peripheral portion of the surface of each silicon substrate. If such substrates are joined together, as shown in FIG. 2, the conductive portions of the three-layered substrates have an interval of about 100 μm or more on the end surface of the sensor chip. As a result, 3
Short circuits at the end faces of the layers between the substrates are completely avoided. The spaces 3, 3'on the end faces can be used as they are, but in order to further eliminate the risk of short circuit, as shown in FIG. 3, an insulating material 31 such as resin is used.
The filling of this space with can be appropriately performed for the purpose of improving reliability.

In this embodiment, the intermediate substrate 1 and the upper and lower substrates 2 and 2'are electrically insulated by a thick oxide film having a film thickness of 10 μm or more formed on the surface thereof. Such a thick oxide film has not been possible by conventional thermal oxidation of single crystal silicon. The growth rate of an oxide film by thermal oxidation of single crystal silicon is proportional to the square root of time, and the thickness of an oxide film that is usually formed is about 5 μm. In a silicon three-layer structure having an oxide film of this thickness, the inter-layer capacitance is large, and therefore the stray capacitance is too large for the purpose of capturing the displacement of the movable electrode as a change in capacitance. There was a flaw. Further, if the thickness of the oxide film is increased to about 5 μm, the internal stress of the oxide film causes the silicon substrate to warp and deform, resulting in a decrease in the bonding yield between the three-layer substrates.

The first feature of this embodiment is that a thick oxide film obtained by oxidizing the porous silicon layer is arranged at the bonding interface of the silicon substrate. As a result, it becomes possible to dispose an insulating layer having a uniform film thickness and sufficiently thick at the bonding interface, and at the same time, it becomes possible to manufacture a capacitance type sensor by an inexpensive processing process. The second characteristic of the present embodiment is that the bonding interface of laminated bonding does not exist on the outer periphery of the silicon chip, and the substrates have voids (spaces 3 and 3 ') in the outer peripheral portion.
Is to have. As a result, there is no longer an electrical short circuit between the directly stacked silicon substrates.

4 and 5 are sectional views showing the machining process of the acceleration sensor of this embodiment. FIG. 4 shows a processing process of the intermediate substrate 1 having the movable electrode shown in FIG. Using the nitride film (Si ₃ N ₄ ) 110 formed on the surface of the silicon substrate as an etching mask, the movable electrode 11, the beam 12, and the notch 13 on the periphery of the chip are formed by anisotropic etching using a KOH aqueous solution ( Fig.4 (a)-(e)).
After that, the nitride film on the surface where the oxide film is to be formed is removed, and the portion where the oxide film is not formed is covered with the protective film 14 (see FIG. 4).
(F)). As a material of the protective film 14, a film formed by vapor-depositing a metal resistant to hydrofluoric acid, for example, platinum is suitable. Next, the exposed portion of the single crystal silicon is made porous by the treatment described later (FIG. 4 (g)). The porous silicon layer 102 has a thickness of 10 to 50 μm. When the substrate is heated and oxidized after removing the protective film 14, the porous silicon layer 102 is easily oxidized and the thick oxide layer 10 is formed.
1 is formed.

FIG. 5 shows a process of processing the lower substrate on which the fixed electrode of FIG. 1 is formed. Using the nitride film (Si ₃ N ₄ ) 210 formed on the surface of the silicon substrate as an etching mask, K
The peripheral portion of the fixed electrode is etched by anisotropic etching using an OH aqueous solution (FIGS. 5A and 5B).
This etching depth 209 determines the gap between the fixed electrode and the movable electrode when it is later bonded to the intermediate substrate. Therefore, etching may not be necessary depending on the value of the gap between the fixed and movable electrodes, and conversely, the fixed electrode portion may be etched while leaving the periphery of the fixed electrode portion. Then, the periphery of the fixed electrode portion is covered with a thermal oxide film 205 (FIG. 5C). Since the role of the oxide film in this case is only as an etching mask, it is sufficient that its thickness is about 1 μm. After forming the groove pattern 202 for forming the notch 23 around the sensor chip (see FIG. 5).
(D)), the notch 23 is formed by anisotropic etching (FIG. 5E). Next, the nitride film and the oxide film 205 on the back surface of the silicon substrate are removed (FIG. 5F). At this time, a film (protective film) 203 formed by vapor-depositing a metal having resistance to hydrofluoric acid, for example, platinum, is formed on the back surface of the silicon substrate prior to the porous treatment of silicon, which will be described later in detail. Next, the exposed portion of the single crystal silicon is made porous by the treatment described later (FIG. 5 (g)). The thickness of the porous silicon layer 204 is 10 to 50 μm. When the substrate is heated and oxidized after removing the protective film 203, the porous silicon layer 204 is easily oxidized and the thick oxide layer 20 is removed.
1 is formed, and the processing of the lower substrate having the fixed electrode is completed (FIG. 5 (h)). The upper substrate is processed in the same process as the lower substrate.

For the bonding surfaces of the three silicon substrates, a sufficiently thick oxide film layer obtained by oxidizing porous silicon is used. As shown in FIG. 6, when the intermediate substrate 1 and the upper and lower substrates 2 and 2'are accurately positioned and overlapped and held in an oxygen atmosphere at about 1000 ° C., the three substrates are integrated by diffusion bonding. The second feature of this embodiment, that is, the spatial separation between the substrates at the outer peripheral portion of the sensor chip is that a groove having a depth of about 100 μm is formed in the portion corresponding to the outer peripheral portion of the chip in the processing step of each silicon substrate. It is realized by forming. That is, after the three substrates processed with such grooves are bonded by diffusion bonding,
When cut with a dicing saw along the lines -A 'and BB', the end faces, which are cut faces, can be processed into the shapes shown in FIGS.

The formation of a thick oxide film, which is the first feature of this embodiment, will be described below. In ordinary thermal oxidation, the diffusion rate of oxygen atoms in the oxide film is limited, and a sufficiently thick film cannot be formed. Moreover, the volume of silicon expands by about 2.2 times due to oxidation, resulting in high residual stress.
On the other hand, in this embodiment, the surface layer of the silicon is made porous in advance, and this layer is thermally oxidized to form a thick oxide film at high speed. By adopting this method, the residual stress of the oxide film layer due to the volume expansion can be reduced at the same time.

The porous silicon layer is formed by anodizing treatment. FIG. 7 is a diagram showing a cross section of the anodizing apparatus. When etching is performed in a 50% HF aqueous solution 41 while applying an electric field, a porous silicon layer is formed on the silicon surface. As shown in the figure, the electric field is applied so that the silicon substrate 42 is positive and the platinum electrode 4 is placed on the negative electrode side.
Connect 3 A DC power supply is connected between both poles.
A conductive wire may be directly connected to a part of the silicon substrate on the positive electrode side. However, as shown in FIG. 5 (f), platinum is vapor-deposited on the back surface of the surface to be made porous, and it is inserted through this. When an electric field is applied by applying the electric field, there is an advantage that the in-plane distribution of the thickness of the porous layer becomes uniform.

The thickness of the porous silicon layer can be increased or decreased depending on the length of processing time. A layer of porous silicon having a thickness of several tens of μm can be obtained in a short time of about several minutes. FIG. 8 shows the relationship between the anodization treatment time and the thickness of the porous silicon layer obtained by the experiment. When this porous silicon layer is thermally oxidized, the diffusion rate of oxygen in the porous layer is remarkable, so that a thick oxide film can be formed in a short time. An oxide film having a thickness of 10 μm was obtained by oxidation for about 10 hours. Furthermore, since the film obtained by oxidizing the porous silicon has a smaller internal stress than that of the single crystal, the amount of warpage occurring in the silicon substrate can be reduced. As a result, it was possible to improve the bonding yield in the process of diffusion bonding the three silicon substrates.

In the embodiment described above, the bonding surface of the three-layer silicon substrate is entirely covered with a thick oxide film. However, for example, a sufficiently thick oxide film is formed on the bonding surfaces of the upper and lower fixed electrode substrates, It is obvious that the object of the present invention can be achieved even if the bonding surface of the intermediate substrate is a normal thin oxide film. Further, the spatial separation method between the substrates on the outer peripheral end face of the silicon chip of the present invention is not limited to the direct bonding structure via the oxide of porous silicon, but is also a direct bonding structure via a normal thermal oxide film. It is self-evident that it can be generally applied to acceleration sensors to which the direct bonding structure is applied. As described above, according to the embodiment of the present invention, an acceleration sensor gauge having a silicon three-layer structure can be obtained, and a structure in which the floating capacitance between the three electrically insulated layers is extremely small can be obtained.

By inventing such a sensor structure, a sensor chip mounting structure as described below becomes possible. That is, the conventional sensor chip is shown in FIG.
As shown in (a), the surface of the ceramic substrate and the three-layer structure of glass / silicon / glass were adhered in parallel with each other. The electrodes drawn from each layer are
It is connected to the bonding wire 310 via a pad formed on the upper glass surface. On the other hand, in the structure of the present invention, since all the surfaces of each of the three layers of the substrate have conductivity, the wiring can be directly connected, and the flexibility of the mounting method and the connecting method is high.

That is, as shown in FIGS. 9B and 9C, the three-layer structure can be fixed vertically to the ceramic substrate 300. To take out the wiring, a conventional wire bonding method is adopted as shown in FIG. 2B, or metal wiring such as gold or aluminum plated on the ceramics substrate surface 300 as shown in FIG. 3C. 311
It is also possible to connect directly to 312 and 313. In this case, what is effective is that the spaces 3 and 3'are formed between the three layers of substrates in the outer peripheral portion of the chip, as described above. This eliminated the risk of electrical shorting of the three layers.

As described above, another effect of vertically bonding the three-layer structure to the ceramic substrate is that the influence of thermal deformation due to temperature change on the capacitance interval is larger than that of the conventional mounting form. Is small. In the conventional mounting form, the difference in thermal expansion between the ceramic substrate and the sensor chip induced bending deformation in the sensor chip.
This is avoided in the forms of (b) and (c).

The effects of the embodiments described above will be listed below. Since the single crystal silicon substrate has a three-layer structure, the temperature characteristics of the sensor are not deteriorated due to the difference in the coefficient of thermal expansion of the materials. Since three layers of substrates are joined through an oxide film having a uniform film thickness formed on the surface of the silicon substrate, the thickness of the joining portion between the silicon substrates is greater than the method of joining by sandwiching the glass substrate between the silicon substrates. Can be accurately and uniformly managed. This brings about the effect that the gap between the movable and fixed electrodes can be processed accurately. In addition, they can be joined by an inexpensive processing process. Since the porous silicon layer is oxidized, it is possible to form a thick oxide film compared to the formation of a thermal oxide film on the surface of a normal silicon substrate. The capacity can be reduced. The porous silicon oxide film is formed in a shorter time than the normal thermal oxidation of the surface of the silicon substrate, and the residual stress in the film is small, so that the deformation after joining the three layers is small. Since the conductive three-layer silicon substrate is spatially separated at the outer peripheral portion of the sensor chip, there is no risk of short circuit between the substrates due to adhesion of dust or the like. Since the sensor chip can be fixed vertically to the surface of the ceramic substrate, wire bonding becomes unnecessary.
Also, it is less likely to be affected by thermal deformation.

[0025]

As described above, according to the present invention, in the capacitance type silicon acceleration sensor, an inexpensive acceleration sensor having good temperature characteristics and having a highly reliable sensor structure of electrical connection and internal sealing is obtained. be able to.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an acceleration sensor structure of the present invention.

FIG. 2 is a cross-sectional view showing an example of a cross section of the acceleration sensor structure of the present invention.

FIG. 3 is a cross-sectional view showing another example of the cross section of the acceleration sensor structure of the present invention.

FIG. 4 is a sectional view of a process of processing an intermediate substrate having an acceleration sensor structure.

FIG. 5 is a sectional view of a process of processing a lower fixed electrode substrate of the acceleration sensor structure.

FIG. 6 is a cross-sectional view showing a process of joining three layers of silicon substrates to form a chip.

FIG. 7 is a diagram showing a configuration of an anodization process for forming a porous silicon layer.

FIG. 8 is a diagram showing a time required for forming a porous silicon layer.

FIG. 9 is a side view showing mounting forms of the acceleration sensor of the present invention and a conventional sensor.

[Explanation of symbols]

 1 Movable Electrode Substrate (Silicon Single Crystal Substrate) 2, 2'Fixed Electrode Substrate (Silicon Single Crystal Substrate) 3, 3'Space Provided on Outer Edge of Chip 11 Movable Electrode 12 Beams 13, 23, 23 'Notch 14 Protective Film 22, 22 'Fixed electrode 31 Insulating material 41 Hydrofluoric acid aqueous solution 42 Silicon substrate 43 Platinum electrode 44 DC power source 101, 201, 201' Thick oxide film 102, 204 Porous silicon layer 110, 210 Silicon nitride film 202 Groove pattern 203 Metal deposition Film (protective film) 205 Thin oxide film 209 Etching depth 300 Ceramics substrate 311 to 313 Metal wiring 310 Bonding wire

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masayoshi Suzuki 2520 Takaba, Katsuta City, Ibaraki Prefecture, Hitachi Ltd. Automotive Equipment Division, Hitachi, Ltd. (72) Masahide Hayashi 2520, Takaba, Katsuta, Ibaraki Hitachi Ltd. Factory Automotive Equipment Division

Claims (5)

[Claims] 1. A capacitance type acceleration sensor having a mass part that is displaced by acceleration, and an electrode part that forms a capacitance facing the mass part, and the mass part that is displaced by the acceleration, and the mass. An acceleration sensor, characterized in that each of the electrode portions facing each other to form a capacitance is made of a silicon single crystal, and a bonding interface of each silicon single crystal is made of a layer obtained by oxidizing porous silicon. 2. A capacitance type acceleration sensor having a mass part that is displaced by acceleration, and an electrode part that forms a capacitance facing the mass part, and the mass part that is displaced by the acceleration, and the mass. The electrode portion facing the portion to form a capacitance, both are made of silicon single crystal, each silicon single crystal is a structure directly bonded through the oxide of the surface, and, An acceleration sensor characterized in that a bonding interface does not exist on an outer peripheral end surface of a sensor chip, and a void is formed on an outer peripheral end surface of two bonded members. 3. A capacitance type acceleration sensor having a mass part that is displaced by acceleration, and an electrode part that forms a capacitance facing the mass part, and the mass part that is displaced by the acceleration, and the mass. The electrode portions that face each other and that form a capacitance are each made of a silicon single crystal, and each silicon single crystal is directly bonded via an oxide on the surface thereof, and An acceleration sensor characterized in that an interface does not exist on the outer peripheral end surface of the sensor chip, a void is formed in the outer peripheral end surfaces of the joined two members, and the void is filled with a different material. 4. The acceleration sensor according to claim 1, 2 or 3, wherein the bonding surface of the silicon single crystal is fixed perpendicularly to the surface of a mating substrate. . 5. The acceleration sensor according to claim 4,
An outer peripheral end face of each layer of the silicon single crystal stacked,
An acceleration sensor characterized in that it is electrically connected to a metal wiring formed on the surface of a board on which it is mounted by soldering or directly joining between metals.