Abstract:
A pressure sensor includes a capacitive pressure sensor chip mounted with the intervention of a sealing member on a base adaptor so as to be detachable. The sensor chip being constructed in a manner that a second substrate with a diaphragm electrode is placed between a first substrate with a fixed electrode and a third substrate with a pressure inlet and is bonded to. the first and the third substrate so as to overlap the fixed electrode, the diaphragm electrode and the pressure inlet with a prescribed gap between the fixed electrode and the diaphragm electrode. Moreover, the third substrate is larger than the second substrate to make place which extends outside the second substrate and plays a role as a sealing surface to be in contact with the sealing member, and the sealing surface is pressed to the base adaptor with the intervention of the sealing member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The present application claims the priority of JP 2002-368867 filed in Japan Dec. 19, 2002,the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a capacitive pressure sensor and, more particularly, to a high accuracy pressure sensor which enables exchange of a multi-structural sensor chip.  
         2. Description of Related Art  
         [0004]    In recent years, the manufacturing techniques of semiconductor devices have been applied to the production of structural components which are essentially sub-millimeter-sized. This technology is called microelectromechanical systems (MEMS) and is widely used to fabricate light and small actuators, sensors and the like in many technical fields. Capacitive pressure sensors can also be produced by MEMS technology, and one example of them is described in JP2001-201417A.  
           [0005]    A conventional capacitive pressure sensor of which sensor chip is produced by MEMS technology generally has a configuration that the sensor chip is mounted on a base adaptor with an adhesive, and the base adaptor has a pressure inlet to conduct a gas pressure to the sensor chip. Here, the pressure sensor chip is composed of a silicon substrate and a glass substrate, and a diaphragm electrode and a fixed electrode are formed on the silicon substrate and the glass substrate, respectively. These two electrodes face each other in a sealed chamber which is formed by bonding the silicon substrate and the glass substrate. The pressure sensor having such a configuration is however disadvantageous because the base adaptor must be also replaced when the sensor chip is needed to be exchanged, and results in a wasteful use. Then, the present inventor investigated pressure sensors in which a sensor chip is mounted on a base adaptor detachably by use of a sealing member such as an O-ring, because it is th ought that this method enables the replacement of the sensor chip independently. This sensor is explained in detail with reference to FIGS. 8 and 9. FIGS. 8 and 9 are a schematic sectional view and a perspective view of a conventional capacitive pressure sensor, respectively.  
           [0006]    Here, a pressure sensor chip  1  is composed of three substrates, that is, a glass substrate  10  with a fixed electrode  13 , a silicon substrate  11  with a diaphragm electrode  15  and a glass substrate  12  with a pressure inlet  18 . A multi-structure is formed by bonding those three substrates. The fixed electrode  13  and the diaphragm electrode  15  are connected to an electric circuit (not illustrated) through lead wires  14  and  14 ′, respectively.  
           [0007]    A sealed chamber  16  is formed between a glass substrate  10  and a silicon substrate  11 , and the inside of it is kept at low pressure with the aid of a non-evaporable getter  17  which absorbs residual gas inside the sealed chamber  16 . The thickness of the diaphragm  15  of the silicon substrate  11  is usually several microns to dozens of microns. The thickness and the size of this diaphragm are determined according to a pressure range to be measured. The diaphragm deflects in response to the pressure difference between both sides of the diaphragm, and the electrostatic capacitance between the fixed electrode  13  and the diaphragm electrode  15  varies as a function of the degree of the diaphragm deflection. Thus, the pressure of the space to be measured  4  can be obtained from the relation between the electrostatic capacitance and the pressure.  
           [0008]    As shown in the drawings, the sensor chip  1  is placed on the base adaptor  3  with the intervention of the O-ring  30 , and a press plate  2  fixes the sensor chip  1  with plural screws and the like (not illustrated). The pressure sensor in this configuration is installed to a vacuum chamber and the like, and then is used to measure the actual pressure inside the vacuum chamber after zero point adjustment is carried out by adjusting the trimmer potentiometer of a pressure display unit.  
           [0009]    Thus, when the sensor chip  1  is damaged or deteriorated, the sensor chip  1  can be exchanged independently, while the base adaptor  3  and other parts are allowed to be reusable. Consequently, it makes possible to reduce the maintenance cost of the pressure sensor.  
         SUMMARY  
         [0010]    However, it has been found that the pressure sensors shown in FIGS. 8 and 9 have a disadvantage that the measurement accuracy can&#39;t be maintained since the measured pressure values change with the condition on which the sensor chip  1  is mounted. That is, in order to secure the vacuum seal, the sensor chip  1  should be pressed to such a degree that the O-ring  30  is deformed by the press plate  2 . However, the measured pressure values also change with the uniformity and the magnitude of the force applied to the press plate  2 .  
           [0011]    As a result of the investigation of this phenomenon, it has been proved that the force applied to the press plate  2  deforms the diaphragm  15  under the influence of the nonuniformity and the magnitude of the force, too. This is caused by the fact that the diaphragm  15  is designed to be thin and easily deflected. The deformation of the diaphragm induces a significant change in the electrostatic capacitance, and hence the errors in the measured pressure are caused. Therefore, in order to produce sensors which can measure accurate pressure, the conditions under which the screws are tightened must be adjusted so that the deviation of capacitance from the designed value should settle within the compensation limits of the electric circuit. However, this operation is very complicated and requires special skills.  
           [0012]    Then, further examination based on the above knowledge was carried out on various sensor configurations and the method for mounting a sensor chip in order to avoid the diaphragm deformation. Consequently, pressure sensors which realize reliable measurements have been completed. That is, an object of this invention is to provide a capacitive pressure sensor with a detachable sensor chip, which reduces the variation and the scatter in electrostatic capacitance due to the manner of attachment of the sensor chip. Another object of this invention is to provide a capacitive pressure sensor which realizes reliable measurements even in the case where ambient temperature varies.  
           [0013]    In order to attain the above-mentioned objects, embodiments of the capacitive pressure sensors of this invention are constructed as follows. That is, a pressure sensor comprises a capacitive pressure sensor chip mounted on a base adaptor so as to be detachable with the intervention of a sealing member, said sensor chip being constructed in a manner that a second substrate with a diaphragm electrode is placed between a first substrate with a fixed electrode and a third substrate with a pressure inlet and is bonded to said first and said second substrate so as to overlap said fixed electrode, said diaphragm electrode and said pressure inlet with a certain gap, wherein said third substrate is larger than said second substrate to make place which extends outside said second substrate and plays a role as a sealing surface to be in contact with said sealing member, and said sealing surface is pressed to said base adaptor with the intervention of said sealing member. Here, it is preferable to press said place of the third substrate from the surface opposite to said sealing surface by a press plate.  
           [0014]    Thus, the sensor chip is fixed by the force which is exerted not onto the entire sensor chip but only onto the third substrate with a pressure inlet, and hence, the diaphragm can be prevented from deformation caused by the attachment of sensor chip. This makes it possible to obtain the capacitance as designed, and moreover stable and accurate measurements over a prescribed pressure range are realized.  
           [0015]    In this invention, as the anodic bonding method is preferably employed to bond the substrates, insulating substrates are used for the first substrate with a fixed electrode and the third substrate with a pressure inlet, and conductive substrate is used for the second substrate with a diaphragm electrode. As the materials of insulating substrate and conductive substrate, the combination of Pyrex 7740 (registered trademark of Corning Glass, Corning) glass and silicon, or that of Pyrex 7740 glass and Fe-Ni alloy is preferably used since these materials have almost the same thermal expansion coefficient. The thickness and the size of the diaphragm are determined by the desired pressure range for measurements, and then the size of the second substrate is determined, too. In addition, the third substrate with a pressure inlet is preferably designed to be larger than the second substrate by 3 mm or more outside to form a sealing region. The thickness of the third substrate is usually 1-5 mm.  
           [0016]    It is also preferable to employ a guide member to prevent the destruction of the third substrate due to excessive tightening by screws. That is, the gap between the base adaptor and the press plate by which the peripheral region of the third substrate is pressed onto the base adaptor is kept to be larger than a prescribed value. This makes it easy and safe to mount the sensor chip.  
           [0017]    In addition, it is preferable to place a buffer member between said press plate and said third substrate. In particular, another same O-ring as being used as the sealing member is also placed as the buffer member at the symmetrical position on the opposite surface of the third substrate. The buffer member reduces the substrate strain due to the difference of thermal expansions and improves the temperature characteristic of electrostatic capacitance to carry out accurate pressure measurements.  
           [0018]    The buffer member embodiments of this invention can reduce the strain of the third substrate which is caused by the difference of thermal expansions between the press plate and the third substrate. Therefore, an elastic material such as rubber is preferably employed and even a rigid material may be also employed so long as its friction coefficient is small. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a schematic sectional view showing a capacitive pressure sensor of  
         [0020]    Embodiment 1.  
         [0021]    [0021]FIG. 2 is an exploded view of the pressure sensor shown in FIG. 1.  
         [0022]    [0022]FIG. 3 is a schematic diagram showing the stress distribution exerted on the diaphragm and the like of the pressure sensor shown in FIG. 1.  
         [0023]    [0023]FIG. 4 is a schematic diagram showing the stress distribution exerted on the diaphragm and the like of the pressure sensor shown in FIG. 8.  
         [0024]    [0024]FIG. 5 is a schematic sectional view showing a capacitive pressure sensor of  
         [0025]    Embodiment 2.  
         [0026]    [0026]FIG. 6 is a schematic sectional view showing a capacitive pressure sensor of  
         [0027]    Embodiment 3.  
         [0028]    [0028]FIG. 7 is a graph showing the relationship between the variation of electrostatic capacitance and the ambient temperature.  
         [0029]    [0029]FIG. 8 is a schematic sectional view showing one example of conventional capacitive pressure sensors in which the sensor element can be detachable.  
         [0030]    [0030]FIG. 9 is a schematic perspective view showing one example of conventional pressure sensors in which the sensor chip is detachable. 
     
    
       [0031]    Here, numeral  1  denotes pressure sensor chip,  2 ; press plate,  3 ; base adaptor,  4 ; space to be measured,  10 ; glass substrate (first substrate),  13 ; fixed electrode,  14 ,  14 ′; lead wire,  15 ; diaphragm electrode,  16 ; sealed (vacuum) chamber,  17 ; non-evaporable getter,  18 ; pressure inlet,  21 ; aperture,  30 ; O-ring,  31 ; groove,  32 ; guide member, and  33 ; buffer member.  
       DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]    The embodiments of this invention will be explained below with reference to the drawings in which the construction and the arrangement of members are roughly shown just for understanding the present invention. Moreover, the construction (material), various numerical values and the like described in the explanation are only the examples preferably used for this invention. Therefore, this invention is not limited to the embodiments mentioned below, but can be applied to a variety of forms based on the claims.  
         [0033]    (Embodiment 1)  
         [0034]    [0034]FIG. 1 is a schematic sectional view explaining one example of the capacitive pressure sensor on this invention, and FIG. 2 is an exploded view of the sensor.  
         [0035]    As shown in the drawings, the pressure sensor chip of this embodiment is composed of three substrates, that is, a glass substrate  10  with a fixed electrode  13  (first substrate), a silicon substrate (for example, boron-doped silicon)  11  with a diaphragm (electrode)  15  of which thickness is several microns to dozens of microns (second substrate) and a glass substrate  12  with a pressure inlet  18  (third substrate). The multi- structure is formed by bonding these three substrates. The glass substrate  12  is larger than both the glass substrate  10  and the silicon substrate  11 , and the peripheral region of glass substrate  12  is protruded from the endfaces of other substrates  10 ,  11 .  
         [0036]    The electrical signal of the fixed electrode  13  is taken out through lead wire  14  which runs through glass substrate  10 , and the signal of diaphragm electrode  15  is taken out through lead wire  14 ′which is connected to the silicon substrate  11 .  
         [0037]    The sealed (vacuum) chamber  16  is formed between the glass substrate  10  and the silicon substrate  11 , and the inside of it is always maintained at low pressure by a non-evaporable getter  17  which absorbs residual gas. If there is pressure difference between space  4  to be measured and the sealed chamber  16 , the diaphragm  15  deflects depending on. the magnitude of pressure difference, and results in the change in the electrostatic capacitance between the fixed electrode  13  and the diaphragm electrode  15 . Therefore, the pressure of the space  4  can be obtained from the measured values of electrostatic capacitance and the relationship between pressure and electrostatic capacitance.  
         [0038]    Furthermore, the methods for forming diaphragm and bonding substrates are described in, for example, JP2002-43585A.  
         [0039]    Next, the method for mounting the sensor chip  1  onto the base adaptor  3  is explained below.  
         [0040]    A groove  31  is formed on the base adaptor  3  to insert O-ring therein. The depth of the groove is determined so that the  0 -ring  31  inserted therein will be in contact with the peripheral region of glass substrate  12 , i.e., the region which protrudes from the endfaces of the glass substrate  10  and the silicon substrate  11 . Moreover, an aperture  21  is formed in the central portion of the press plate  2  so that the glass substrate  10  and the silicon substrate  11  fit into the aperture  21 , when the sensor chip  1  is mounted onto the base adaptor  3  with the press plate  2 .  
         [0041]    With such a configuration, the force is not exerted onto the silicon substrate  11 , but only onto the glass substrate  12 , even when the sensor chip  1  is pressed with a strong force to secure the vacuum seal or when the screw was tightened nonuniformly. Consequently, the sensor chip  1  is securely mounted onto the base adaptor  3  without strain on the diaphragm  15 . As a result, the electrostatic capacitance is obtained as is designed, this makes it possible to carry out accurate measurements.  
         [0042]    Then, it has been analyzed how the sensor chips shown in FIGS. 1 and 8 are deformed by the stress which relates to O-ring. This analysis was carried out by calculating the degree of deformation when the force of 24N, at which the O-ring becomes compressed. 20%, is applied to the contact surface between the O-ring and the sensor chip.  
         [0043]    The substrates  10  and  11  are 11.6×11.6×0.4 mm and 11.6×11.6×0.8 mm, respectively. The diaphragm is 4.2×4.2 mm ×7 μm. The substrates  12  of the sensors shown in FIGS. 1 and 8 are 20×20×2.0 mm and 11.6×11.6×2.0 mm, respectively. As the materials for the sensor chip, Pyrex 7740 glass (Corning) was used for the substrates  10  and  12 , and p-type silicon substrate was used for the substrate  11  with a diaphragm electrode. The characteristics of the materials are shown in Table 1.  
                                                 TABLE 1                                   Material   Pyrex glass   p-type silicon                                        Elastic modulus (GPa)   64   162           Poisson ratio   0.2   0.22           Shear coefficient (GPa)   26.18   0.7                      
 
         [0044]    The analytical results on the sensors shown in FIG. 1 and  8  are shown in FIGS. 3 and 4,respectively. In the drawings, the black region (the darkest region) and the white region (the brightest region) show the regions where the degree of displacement are 7×10 −13 m or more and 3.5×10 −13 m or less, respectively. The gray parts show the region where the degree of displacement is intermediate between them. In other words, the degree of displacement increases with the darkness in FIGS. 3 and 4.  
         [0045]    As is apparent from these two figures, only a limited part of the glass substrate  12 , which is in contact with O-ring  30 , suffers from the stress, and the rest is free from any stresses in the sensor chip  1  of this embodiment, while the diaphragm of the conventional sensor configuration (FIG. 8) suffers from a large amount of stress which would result in diaphragm deformation. That is, even if the sensor chip  1  is pressed down by the press plate  2  with rather strong force in order to secure the vacuum seal, the diaphragm  15  of the silicon substrate  11  does not become deformed, and this makes it possible to measure pressure with accuracy and reproducibility.  
         [0046]    (Embodiment 2)  
         [0047]    Next, another example of the present invention is shown in FIG. 5.  
         [0048]    [0048]FIG. 5 is a sectional view of a pressure sensor where a guide member  32  is equipped around the sensor chip  1 . By the equipment of the guide member  32 , excessive force is not exerted onto the glass substrate  12 , and it helps to avoid the destruction of the glass substrate and makes the mounting operation easy and safe.  
         [0049]    (Embodiment 3)  
         [0050]    As mentioned above, the pressure sensors of Embodiments 1 and 2 reduce the strain of the sensor chip which is attributed to the mounting condition, and therefore can suppress the variation and the scatter in the electrostatic capacitance. In contrast, it was found that the output signal from these sensor chips changes with the ambient temperature although the degree of change itself is very small as compared with the conventional sensors. From the experimental investigation, it was also found that temperature dependence of electrostatic capacitance increases with the deviation from the designed capacitance when the sensor chip is mounted on the base adaptor.  
         [0051]    This may be explained as follows. Since the press plate and the base adaptor are usually made of stainless steel whose thermal expansion coefficient is larger than those of silicon and Pyrex glass, it is likely that when the sensor chip is directly pressed and fixed by the press plate, the difference in thermal expansion may cause the strain between them and then the change in electrostatic capacitance. Furthermore, if the strain is large by some reason when the sensor chip is mounted on the base adaptor, the temperature dependence of electrostatic capacitance increases.  
         [0052]    Then, a variety of sensor configurations were examined in order to decrease the strain induced by mounting the sensor chip. Thus, a pressure sensor with a small temperature dependence of output signal has been invented. An embodiment of it is shown in FIG. 6.  
         [0053]    The pressure sensor of this embodiment shown in FIG. 6 has nearly the same configuration as the pressure sensor of Embodiment 1, except that an O-ring (a buffer member) is placed between the glass substrate  12  and the press plate  2 .  
         [0054]    The pressure sensors of this embodiment and Embodiment 1 were installed onto a vacuum chamber to evaluate the temperature dependence of electrostatic capacitance. The typical characteristics of them are shown in FIG. 7. Here, the ordinate denotes the variation in the electrostatic capacitance against the ambient temperature under a constant pressure. As is apparent from FIG. 7, the variation in the electrostatic capacitance can be drastically reduced by applying the sensor configuration of this embodiment. Moreover, although not shown in FIG. 7, it was also found that the temperature dependence of electrostatic capacitance (i.e., the inclination of the graph) increases and scatters in the wide range of 10-30 fF/° C. depending on the condition of screw tightening in the case of Embodiment 1, while the temperature dependence of electrostatic capacitance decreases to 1-2 fF/° C. and its scatter is hardly observed in embodiment 3.  
         [0055]    The same O-rings are used for both the buffer member  33  and the sealing member  30  in this embodiment. However, any material or shape can be employed as the buffer member so long as it may reduce the strain of the third substrate caused by the difference in thermal expansion between the press plate and the third substrate. Therefore, in addition to elastic materials such as rubber, even rigid materials such as fluorine-contained resins may be employed so long as they have a small friction coefficient.  
         [0056]    Moreover, the buffer member may be placed in fragments between the glass substrate and the press plate. However, the buffer member which has the same shape and material as the sealing member is preferably placed at the symmetrical position on the opposite surface of the glass substrate to the sealing member. This configuration makes it possible to realize a pressure sensor which has less strain and less variation in output signal. In addition, the groove for O-ring may be formed on the press plate in the same shape as that on the base adaptor.  
         [0057]    As has been mentioned, the substrate with a pressure inlet which is larger than the substrate with a fixed electrode and the substrate with a diaphragm electrode is employed to protrude the peripheral region from the two substrates. Then, a sealing member such as O-ring is placed on the protruded peripheral region to secure vacuum seal, and only the region for sealing with O-ring is pressed by a press plate. Thus, unexpected stress can be prevented from exerting onto the silicon substrate which is a critical member for pressure measurement, and this makes it possible to measure pressure with accuracy and reproducibility.  
         [0058]    Furthermore, the substrate strain due to the difference in thermal expansions is reduced by placing the buffer member between the press plate and the glass substrate, which makes it possible to carry out reliable, accurate pressure measurements even in the case where the temperature around the sensor chip changes.  
         [0059]    Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.