JP2002214058A - Pressure sensor and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a touch mode type capacitive pressure sensor in which a breakdown strength of a diaphragm is higher than a conventional one. SOLUTION: A pressure sensor is provided where a silicon structure body (5) in which a conductive diaphragm (9) is formed is jointed on a substrate (6) where an electrode (7) and a dielectric film (8) for covering it are formed so that the diaphragm faces the electrode while a gap (10) is provided before the dielectric film, to detect change of a contact area by which the diaphragm contacts the dielectric film under a pressure, for measurement of the pressure. Related to the pressure sensor, the impurity concentration on the surface of the diaphragm is from 1×1019 to less than 9×1019 cm-3. A method for manufacturing the pressure sensor comprises a process for manufacturing the silicon structure body where an impurity is doped on a silicon surface at high concentration, and then etching is performed so that the impurity concentration on the surface (etching surface) is from 1×1019 to less than 9×1019 cm-3, thus forming the diaphragm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a touch-mode capacitive pressure sensor, and more particularly to a structure of a pressure sensor having a high pressure resistance and a method of manufacturing the same.

[0002]

2. Description of the Related Art An electrostatic capacitance type pressure sensor is arranged such that a substrate having a diaphragm formed thereon and a substrate having an electrode formed thereon are deformed in response to pressure so that the diaphragm and the electrode are opposed to each other with a certain gap. The pressure is detected from a change in the capacitance between the diaphragm and the electrode. Since a silicon or glass wafer can be used as a substrate for forming a diaphragm or an electrode, a large number of sensors can be manufactured on the wafer at one time, which is suitable for mass production at low cost.

Among the capacitive pressure sensors, for example, a touch mode capacitive pressure sensor disclosed in US Pat. No. 5,528,452 discloses a metal thin film on a glass substrate as shown in FIG. An electrode 1 is formed, and a dielectric film 2 is formed thereon, and a diaphragm 3 having at least a surface having conductivity is arranged to face each other with a slight gap 4 therebetween. As shown in FIG. 3B, the diaphragm is bent and is in contact with the dielectric film 2 (referred to as a touch mode).

The diaphragm 3 is a P ⁺ layer in which n-type silicon is doped with boron at a high concentration. If the diaphragm 3 is regarded as one electrode, the electrode 1, the dielectric film 2 and the diaphragm 3 are used when pressure is detected. Is formed. By detecting a change in the contact area between the diaphragm 3 and the dielectric film 2 as a change in capacitance between both electrodes (between the diaphragm 3 and the electrode 1), it is possible to measure the pressure applied to the diaphragm 3. The touch mode capacitive pressure sensor has many excellent characteristics, such as high sensitivity and high pressure resistance, and a linear relationship between pressure and capacitance as compared with other capacitive pressure sensors.

FIG. 4 shows the relationship between the capacitance of the touch mode capacitive pressure sensor and the applied pressure. Due to the characteristics of the touch-mode capacitive pressure sensor, the sensitivity is almost zero in a low-pressure region (non-contact region) before the diaphragm contacts the dielectric film. When the diaphragm contacts the dielectric film,
The capacitance of the sensor increases almost linearly with pressure within a certain range (linear region), and when the pressure further increases, the sensitivity gradually decreases and the change in capacitance saturates (saturation region). .

For forming a diaphragm using a silicon single crystal, an inorganic solution such as KOH or NaOH or an organic solution such as ethylenediamine / pyrocatechol (EDP) or tetramethylammonium hydroxide (TMAH) is used. It is often performed by anisotropic etching utilizing a difference in etching rate depending on the crystal orientation of the crystal. The etch stop effect in the P ⁺ layer, that is, the effect that the etching rate in the region where the boron concentration exceeds 10 ¹⁹ cm ⁻³ becomes several tenths to several hundredths in comparison with the silicon layer is used. Thus, a diaphragm having a thickness of several μm is usually formed. By controlling the thickness of the diaphragm and the dimension of the electrode interval, the above-described linear region can be adapted to a desired operating range of the sensor. For example, a sensor for detecting tire pressure has a pressure range of about 10 kgf / cm ^2. What is necessary is just to obtain a stable operation within.

[0007]

As described above,
The conventional touch-mode capacitive pressure sensor has characteristics of high sensitivity and high withstand voltage, and a stable operation can be obtained within a desired pressure range by changing the diaphragm thickness and the electrode interval. However, depending on the use condition of the sensor, a pressure that greatly exceeds the actual operating pressure range, for example, a pressure resistance as high as 4 to 5 times the upper limit of the measurable pressure range may be required. When added, the sensor may be broken, such as a diaphragm crack. This cracking of the diaphragm mainly occurs on the long side or the short side of the diaphragm where the rectangular diaphragm bends and is most stressed when in contact with the dielectric film.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a touch-mode capacitive pressure sensor having a diaphragm with higher pressure resistance than conventional products. The present inventors have conducted intensive studies to achieve the above object, and as a result, by controlling the boron concentration on the diaphragm surface, the pressure resistance can be greatly increased, especially the pressure resistance of the diaphragm is significantly improved, It has been found that the diaphragm does not break even when a pressure that greatly exceeds the actual operating pressure range is applied, and that it can be suitably used as a pressure sensor for automobile tire pressure detection where safety is required,
The present invention has been completed.

The pressure sensor of the present invention has a conductive
The electrode and the silicon structure with the diaphragm
The diaphragm and the electrodes are placed on a substrate on which a covering dielectric film is formed.
Join with the poles facing each other and leaving a gap with the dielectric film
And the diaphragm comes into contact with the dielectric film under pressure.
Pressure sensor that detects changes in the contact area and measures the pressure.
In the sensor, impurities on the surface of the diaphragm
Concentration 1 × 10¹⁹cm ^-3More than 9 × 10¹⁹cm^-3Less than
It is characterized by that. Further, the pressure sensor of the present invention
The manufacturing method includes the step of forming a conductive diaphragm.
An electrode and a dielectric film covering it were formed on the recon structure
On the substrate, the diaphragm and the electrode face each other and a gap is formed.
When the diaphragm receives pressure,
To detect the change in the contact area that contacts the dielectric film
A method for manufacturing a pressure sensor for measuring force, comprising:
The surface is highly doped with impurities and then the surface (air
1 × 10¹⁹cm^-39x above
10 ¹⁹cm^-3Etching to be less than
Forming a diaphragm and manufacturing the silicon structure.
It is characterized by including. Manufacture of pressure sensor of the present invention
In the method, KOH, Na is used as an etching solution.
OH, ethylenediamine pyrocatechol (EDP),
From the group of tetramethylammonium hydroxide (TMAH)
It is preferable to use at least one solution selected.
New The present invention also provides a diaphragm having conductivity.
The formed silicon structure is composed of an electrode and a dielectric film covering it.
On the formed substrate, the diaphragm and the electrode face and
It is bonded to the dielectric film with a gap left, and the diamond
Changes in the contact area where the flam contacts the dielectric film under pressure
Pressure sensor to detect the pressure and measure the pressure,
When the surface impurity concentration of the diaphragm is 1 × 10¹⁹cm
^-3More than 9 × 10 ¹⁹cm^-3Auto tire pressure detection
Pressure sensor.

[0010]

FIG. 1 shows an embodiment of a pressure sensor according to the present invention. FIG. 1 (A) is a plan view of the pressure sensor.
(B) is a side sectional view. This pressure sensor is composed of a silicon substrate provided with a conductive diaphragm 9 which is deformable according to pressure on a substrate 6 provided with an electrode 7 made of a metal thin film and a dielectric film 8 formed so as to cover the electrode 7. Structure 5
Are joined in a state where the diaphragm 9 and the electrode 7 face each other and a gap 10 is provided between the diaphragm 9 and the dielectric film 8.

The diaphragm 9 of the silicon structure 5 is formed by etching a silicon wafer to make it concave. In order to form the diaphragm 3 on a silicon wafer, a layer to which a high concentration of impurities is added can be formed on the silicon surface, and an etch stop technique by high concentration doping of an impurity such as boron can be used. In the present invention, the impurity concentration on the surface of the diaphragm is at least 1 × 10 ¹⁹ cm ^{−3 and} less than 9 × 10 ¹⁹ cm ⁻³ . The thickness of the diaphragm 9 and the diaphragm 9
The depth of the depression on the lower side of the diaphragm (on the side of the joint surface with the base 6) that defines the height of the gap 10 between the sensor and the dielectric film 8 is appropriately determined so that the linear region of the sensor matches the load pressure range to be measured. Can be set.

The constituent material of the base 6 is not particularly limited as long as the base 6 can maintain an electrical insulation state with the electrode 7.
For example, a glass plate, a ceramic plate, a hard plastic plate, a silicon wafer having a surface oxide film formed thereon may be used, and a glass plate is particularly preferably used. The electrodes 7 formed on the base 6 are made of Al, Cr, Au, Ag,
Various metals generally used as electrode materials such as Cu and Ti are deposited on the surface of the substrate 6 by vapor deposition, sputtering, C
The film is formed using a thin film forming method such as a VD method or an electroless plating method. The shape of the electrode 7 may be a method of covering the non-electrode film-forming portion on the surface of the base 6 with a mask and forming a metal film only on the electrode forming portion, or a method of uniformly forming a metal film on one surface of the base 6 and then performing photolithography. A pattern can be formed by a method of etching into a desired shape using a technique, and is usually formed in the same shape as the diaphragm 9. The dielectric film 8 formed on the base 6 so as to cover the electrode 7 is made of a material known as an insulating material such as glass (quartz glass) or ceramic by using a thin film forming means such as a sputtering method or a CVD method. It is formed by forming a film. The thickness of the dielectric film 8 is appropriately set according to the required sensitivity of the pressure sensor, and is usually about 0.1 to several μm.

A terminal 11 connected to the electrode 7 and extending to the periphery of the base 2 is provided on the base 6, and a terminal 11 provided on the dielectric film 8 and electrically connected to the structure 5. Are provided. These terminal portions 11 and 11 can be formed using the same metal material as the electrode 7 and the same kind of thin film forming means.

The gap 10 formed between the dielectric film 8 of the base 6 and the diaphragm 9 is formed by recessing the lower surface of the silicon structure 5 into a rectangular shape slightly larger than the electrode 7 and the diaphragm 9. . The height of the gap 10, that is, the dimension between the dielectric film 8 and the diaphragm 9 is appropriately selected according to the dimensions (length, width, and thickness) of the diaphragm 9. In this example, the inside of the gap 10 is evacuated.

Next, as an example of a method of manufacturing a pressure sensor according to the present invention, a method of manufacturing a pressure sensor having the above-described structure will be described. The substrate 6 is formed by connecting the electrode 7 and the terminal portion 11 connected thereto to a metal thin film by sputtering or the like.
A dielectric film 8 made of glass is formed by a sputtering method or the like, and covers the electrode 7 except for the terminal portion 11.
Further, the terminal portion 11 on the silicon structure 5 side is formed on the dielectric film 8.
Is formed in the same manner as the electrode 7.

The silicon structure 5 is doped with an impurity such as boron at a high concentration from the bonding surface side of the silicon wafer with the substrate 6 by a thermal diffusion method, and then the surface (etched surface) has an impurity concentration of 1 ×. 10 ¹⁹ cm ^-3 or more 9 × 10 ¹⁹
The diaphragm 9 is formed by performing etching from the side opposite to the bonding surface so as to be less than cm ⁻³ . This etching uses KOH, NaO as an etching solution.
H, ethylenediamine pyrocatechol (EDP), using at least one solution selected from the group of tetramethylammonium hydroxide (TMAH), utilizing an etch stop effect that leaves a portion doped with a high concentration of impurities, The etching is performed by controlling the etching time so that the thickness of the diaphragm 9 becomes a preset thickness. On this occasion,
By controlling the distribution of impurities in the depth direction, the thickness of the diaphragm can be changed. Then, the pressure sensor is manufactured by joining the base body 6 and the silicon structure 5 thus manufactured to each other in a vacuum by an appropriate bonding method, for example, by heat fusion or an inorganic adhesive.

This pressure sensor is operated by connecting a terminal portion 11 to a measuring device for measuring capacitance, applying an AC voltage between the diaphragm 9 and the electrode 7 and detecting a resonance frequency or a change in impedance thereof. Will be As shown in FIG. 4, when pressure is applied to the diaphragm 9 and comes into contact with the dielectric film 8, the capacitance of the sensor increases almost linearly with pressure within a certain range.

This pressure sensor is a touch mode capacitive pressure sensor having a diaphragm formed of silicon doped with an impurity such as boron.
The boron concentration on the surface of the diaphragm is 1 × 10 ¹⁹ c
m ^{−3 to} 9 × 10 ¹⁹ cm ⁻³ , especially for high pressures (for example, for detecting automobile tire pressure), the pressure resistance is significantly improved by setting it to less than 9 × 10 ¹⁹ cm ^−3. Can be. In the present invention, the impurity concentration is set to “9”.
The meaning of “less than × 10 ¹⁹ cm ⁻³ ” means that, in a pressure sensor for a high pressure (for example, a high pressure for detecting a tire pressure of an automobile), the impurity concentration is set to 1 ×, as described in an embodiment described later. It is particularly excellent to be 10 ¹⁹ cm ⁻³ or more and less than 9 × 10 ¹⁹ cm ^−3.
10 ¹⁹ cm ^-3 "is not completely out of range. Hereinafter, the effects of the present invention will be embodied by examples.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIGS. 1A and 1B, a touch mode having a structure in which a silicon structure 5 provided with a diaphragm 9 is joined to a substrate 6 on which an electrode 7 and a dielectric film 8 are formed. A capacitive pressure sensor was manufactured. An electrode 7 made of a Cr thin film and a dielectric film 8 made of glass which cover the electrode 7 made of a Cr thin film on a substrate 6 made of a glass plate are each 0.1 μm thick and 0.1 μm thick.
It was formed with a thickness of 4 μm. The formation of the electrode 7 and the dielectric film 8 was performed by a series of steps of film deposition by sputtering and patterning by photolithography.

The diaphragm 9 facing the electrode 7 (lower electrode) is doped with boron at a high concentration so as to function as an upper electrode.
It was a 5 mm rectangle. As disclosed in U.S. Pat. No. 5,528,452, by making the shape of the diaphragm 9 a rectangle having a ratio of the long side to the short side of 3: 1 or more, the pressure and the capacitance change within a certain range can be reduced. It is possible to obtain linearity. The gap 10 between the dielectric film 8 and the diaphragm 9 was 3 μm. A terminal 11 made of Al for contact with the outside was formed on a part of the base 6 so that one was connected to the upper electrode and the other was connected to the lower electrode. A series of manufacturing processes are performed using silicon wafers and glass wafers,
Each sensor was obtained by cutting both wafers after bonding.

FIG. 2 shows the detailed structure of the diaphragm of the manufactured touch mode capacitive pressure sensor. When the diaphragm 12 is formed, the silicon substrate 13 is doped with boron by thermal diffusion from the bonding surface (the back surface of the diaphragm) with the glass substrate 14. In this embodiment, the temperature at the time of diffusion is 1125 ° C.
The diffusion time was set to 10 hours so that the boron concentration at a depth of μm was 1 × 10 ¹⁹ cm ⁻³ . The diffusion depth at this time was substantially uniform in the silicon wafer, and was 9.4 μm. The boron concentration shows a constant value of about 2 × 10 ²⁰ cm ⁻³ at a depth of 5.5 μm from the back surface of the diaphragm, and is 5.5.
It gradually started to decrease around about μm, and was 2 × 10 ¹⁴ cm ⁻³ at the diffusion depth of 9.4 μm. The formation of the diaphragm 12 was performed by anisotropic etching from the surface (diaphragm surface) opposite to the bonding surface. The etching solution is a 24 wt% KOH solution at 80 ° C., and the thickness of the diaphragm 12 is 6 μm using an etch stop effect.
The etching time was controlled so as to be about the same.

A pressure resistance test was conducted to investigate cracks in the diaphragm after applying a constant pressure to the touch mode capacitive pressure sensor having the structure described above for a long time. In this embodiment, the number of samples is 1000, the pressure application time is 1 hour, and the pressure is 20 kgf / cm ² to 50 kgf.
/ Cm ^{2, the} pressure was increased by 5 kgf / cm ^{2 at a} time, and the presence or absence of a diaphragm crack after application of the pressure was examined. as a result,
No cracking of the diaphragm occurred at all until the applied pressure was 35 kgf / cm ² , but the applied pressure was 40 kgf / cm 2.
When it was set to ² , the diaphragm cracked in about 56% of the sensors. Then, when the applied pressure was increased to 50 kgf / cm ² , similar cracks finally occurred in about 68% of the sensor diaphragms. Most of the diaphragm cracks occurred in the long and short sides of the diaphragm where the most stress was applied when the diaphragm was bent and was in contact with the dielectric film.

Among the sensors used in the above-mentioned pressure resistance test, one in which the diaphragm was cracked and one in which the diaphragm was not cracked were each one.
When 00 pieces were extracted and the boron concentration on the diaphragm surface was examined, a frequency distribution as shown in FIG. 5 was obtained.
That is, the boron concentration on the diaphragm surface is 9 × 10 ¹⁹
No cracking was observed for the sensor less than cm ^-3 ,
Conversely, all sensors with cracks have a boron concentration of 9 × 10
^{It was 19} cm ^-3 or more. It should be noted that there is no significant difference in the diaphragm thickness between any of the sensors, and it has been confirmed that this crack is not caused by the difference in the thickness of the diaphragm.

Based on the results shown in FIG. 5, the etching time was adjusted again so that the boron concentration on the diaphragm surface was less than 9 × 10 ¹⁹ cm ^-3 , thereby producing a touch-mode capacitive pressure sensor. Here, considering that the boron concentration on the diaphragm surface is less than 1 × 10 ¹⁹ cm ⁻³ , a sufficient etch stop effect is not recognized, and the boron concentration is 1 × 10 ¹⁹ cm ^{−3 to} 9 × 10 ¹⁹ cm ^{−3. It} was set to be less than ^-3 . With respect to the 500 sensors thus obtained, the pressure resistance was evaluated by the same method as the above-described pressure resistance test. As a result, in all the sensors after the application of 50 kgf / cm ² , no cracks in the diaphragm were observed.

[0025]

As described above, the pressure sensor according to the present invention is a touch-mode capacitive pressure sensor having a diaphragm formed of boron-doped silicon, in which the boron concentration on the surface of the diaphragm is 1 × 10 The pressure resistance of the diaphragm can be greatly improved by setting it to be ¹⁹ cm ⁻³ or more and less than 9 × 10 ¹⁹ cm ⁻³ . Therefore, according to the present invention, it is possible to provide a touch-mode capacitive pressure sensor that is more excellent in pressure resistance of the diaphragm than conventional products. Further, the pressure sensor according to the present invention has a boron concentration of 1 × 1 on the surface of the diaphragm.
By setting the pressure to 0 ¹⁹ cm ⁻³ or more and less than 9 × 10 ¹⁹ cm ⁻³ , the pressure resistance of the diaphragm is greatly improved, and even when a pressure that greatly exceeds the actual operating pressure range is applied, the diaphragm breaks. Since it does not occur, it can be particularly suitably used as a pressure sensor for detecting the tire pressure of an automobile requiring safety.

[Brief description of the drawings]

FIG. 1 shows an example of a pressure sensor of the present invention.
(A) is a top view of a pressure sensor, (B) is a side sectional view.

FIG. 2 is an enlarged sectional view of a main part of the pressure sensor of the present invention.

FIG. 3 is a side view showing a conventional pressure sensor;
(A) shows a state where no pressure is applied, and (B) shows a state where the diaphragm contacts the dielectric film side due to the pressure.

FIG. 4 is a graph showing a relationship between pressure and capacitance in a conventional pressure sensor.

FIG. 5 shows the result of the example according to the present invention, in which the results of measuring the boron concentration on the diaphragm surface of the pressure sensor sample that was cracked and did not crack after applying high pressure to the sample were summarized. It is a frequency distribution graph.

[Explanation of symbols]

5 ... structure, 6 ... base, 7 ... electrode, 8 ... dielectric, 9 ... diaphragm, 10 ... gap, 12 ... diaphragm, 13 ... silicon substrate (structure), 14 ...
Glass substrate (substrate).

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Jin Nishimura 1440 Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant (72) Inventor Masahiro Sato 1440 Mutsuzaki Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant F term (reference) 2F055 AA12 BB19 CC02 DD05 EE25 FF38 GG01 GG11 4M112 AA01 BA07 CA03 CA04 CA11 DA04 DA06 DA08 DA09 DA12 EA03 EA10 EA13 EA20 FA07

Claims (4)

[Claims] 1. A silicon structure (5) having a conductive diaphragm (9) formed on a substrate (6) on which an electrode (7) and a dielectric film (8) covering the electrode (7) are formed. And the electrode are opposed to each other and are joined to the dielectric film with a gap (10) therebetween, and the pressure is measured by detecting the change in the contact area where the diaphragm contacts the dielectric film under pressure. In the pressure sensor, the impurity concentration on the surface of the diaphragm is 1 × 10 ¹⁹ cm ⁻³ or more and 9 × 1
A pressure sensor having a pressure of less than 0 ¹⁹ cm ^-3 . 2. A silicon structure (5) having a conductive diaphragm (9) formed on a substrate (6) on which an electrode (7) and a dielectric film (8) covering the electrode (7) have been formed. And the electrode are opposed to each other and are joined to the dielectric film with a gap (10) therebetween, and the pressure is measured by detecting the change in the contact area where the diaphragm contacts the dielectric film under pressure. A method for manufacturing a pressure sensor, wherein a silicon surface is doped with impurities at a high concentration, and then the impurity concentration on the surface (etched surface) is 1 × 10 ¹⁹ cm ⁻³ or more and 9 ×.
A method for producing a pressure sensor, comprising a step of producing a silicon structure by forming a diaphragm by etching so as to be less than 10 ¹⁹ cm ^-3 . 3. An etching solution comprising KOH, NaO
3. The method according to claim 2, wherein at least one solution selected from the group consisting of H, ethylenediamine pyrocatechol (EDP), and tetramethylammonium hydroxide (TMAH) is used. 4. A silicon structure on which a diaphragm having conductivity is formed is placed on a substrate on which an electrode and a dielectric film covering the electrode are formed, with the diaphragm and the electrode facing each other and a gap between the dielectric film and the dielectric structure. In a pressure sensor for detecting a change in a contact area where the diaphragm comes into contact with a dielectric film by receiving pressure and measuring the pressure, a surface impurity concentration of the diaphragm is 1 × 10 ¹⁹ cm ⁻ A pressure sensor for detecting a tire pressure of an automobile having a value of ³ or more and less than 9 × 10 ¹⁹ cm ⁻³ .