Abstract:
A covered acceleration sensor element includes a weight portion, a support frame portion surrounding the weight portion, a plurality of flexible beam portions for connecting the weight portion to the support frame portion to support the weight portion, piezoresistance elements provided on the beam portions, and wirings for connecting them. An upper cover and a lower cover enclosing the periphery of the weight portion together with the support frame portion are joined to the face and back of the support frame portion. Acceleration in the directions of three axes, i.e., a first axis in the joining thickness direction, a second axis in a plane perpendicular to the first axis, and a third axis in the plane and perpendicular to the second axis, or acceleration in the direction of any of the axes, is detected from changes in the resistances of the piezoresistance elements. The support frame portion is separated by separation grooves into an inner frame and an outer frame. The upper cover and the lower cover are joined to the outer frame. The inner frame is connected to the outer frame by a plurality of inner frame support portions having flexibility. The beam portions are connected to both sides of the weight portion along the second axis and the third axis. The inner frame support portions are connected to both sides of the inner frame in a direction in which they are rotated nearly 45 degrees from the second axis and the third axis.

Description:
[0001]    The entire disclosure of Japanese Patent Application No. 2009-130264 filed May 29, 2009 is expressly incorporated by reference herein. 
       TECHNICAL FIELD 
       [0002]    This invention relates to a semiconductor acceleration sensor for detection of acceleration, which is used in automobiles, aircraft, portable terminal equipment, toys, etc. 
       BACKGROUND ART 
       [0003]    An acceleration sensor is often used as a sensor for actuating an air bag of an automobile, and detects an impact in a collision of the automobile as acceleration. For the automobile, a one-axis (uniaxial) or two-axis (biaxial) detection function has been enough to measure acceleration on the X-axis and/or the Y-axis. The acceleration to be measured has been very great. Recently, the acceleration sensor has found frequent use in portable terminal equipment and robots, and there has been demand for a three-axis (triaxial) acceleration sensor for measuring accelerations in the X-, Y- and Z-axis directions, in order to detect spatial movements. Also, there has been demand for a high-resolution downsized sensor for detection of micro-acceleration. 
         [0004]    Many acceleration sensors adopt a configuration in which the movement of a weight portion or a flexible portion is converted into an electrical signal. Among the acceleration sensors with this configuration are those of the piezoresistor or piezoresistance element type which detect the movement of the weight portion from a change in the resistance of the piezoresistance element provided in the flexible portion coupled to the weight portion, and those of the electrostatic capacity type which detect the movement of the weight portion from a change in electrostatic capacity between the weight portion and a fixed electrode. 
         [0005]    Conventional triaxial acceleration sensors shown in Patent Document 1 and Patent Document 2 will be described below. In a triaxial acceleration sensor  101 , as shown in  FIGS. 11 and 12 , a triaxial acceleration sensor element  103 , and an IC  104  for control, which performs the amplification, temperature compensation, etc. of a sensor element signal, are laminated and fixed within a ceramic case  102 . A cover  105  and the case  102  are joined together to seal up the triaxial acceleration sensor element  103  and the IC  104  within the case  102 . As shown in  FIG. 12 , the triaxial acceleration sensor element  103  is secured to the case  102  with the use of a resin adhesive material  106 , and the IC  104  is secured onto the triaxial acceleration sensor element  103  with the use of a resin adhesive material  107 . 
         [0006]    The triaxial acceleration sensor element  103  has sensor terminals  108 , the IC  104  has IC terminals  109 , and the case  102  has case terminals  110 . The sensor terminals  105  and the IC terminals  109  are interconnected by wires  111 , and the IC terminals  109  and the case terminals  110  are interconnected by the same wires  111 , so that signals from the sensor are taken outwardly from output terminals  112  interconnected to the case terminals  110  provided in the case  102 . The cover  105  is secured to the case  102  by an adhesive material  102   a  such as a AuSn solder. 
         [0007]    In a plan view shown in  FIG. 13 , the triaxial acceleration sensor element  103  includes a square support frame portion  113 , a weight portion  114 , and paired beam portions sandwiching the weight portion  114 , the weight portion  114  being held in the center of the support frame portion  113  by the two pairs of beam portions  30 . Piezoresistance elements are provided in the beam portions  115 . 
         [0008]    X-axis piezoresistance elements  116  and Z-axis piezoresistance elements  118  are provided in the pair of beam portions  115 , and Y-axis piezoresistance elements  117  are provided in the other pair of beam portions  115 . The piezoresistance elements are arranged at the four bases of the pair of beam portions  115 , and they are interconnected to constitute a bridge circuit. By so doing, uniform resistance changes in the piezoresistance elements are cancelled. By changing the manner of connection of the bridge circuit, moreover, accelerations on the X-axis, the Y-axis and the Z-axis are separated and detected. The sensor terminals  108  are arranged on the support frame portion  113 . 
         [0009]    The principle of acceleration detection by the bridge circuit will be described by reference to  FIGS. 14A to 14D .  FIGS. 14A and 14B  show the movements of the weight portion  114  when accelerations are applied in the X direction and the Z direction by X-Z planes. When acceleration is applied in the X direction as in  FIG. 14A , for example, the weight portion  114  rotates about its site in the vicinity of its upper end center, whereupon the beam portions  115  deform. In accordance with the deformation of the beam portions  115 , stress imposed on four X-axis piezoresistance elements X 1  to X 4  provided on the upper surface of the beam portions  115  changes, and resistance also changes. In this case, X 1  and X 3  are subjected to tensile stress, while X 2  and X 4  are placed under compressive stress. As a result, a difference appears in the midpoint potential of a bridge circuit for X-axis detection shown in  FIG. 14C , so that an output conformed to the magnitude of acceleration is obtained. When acceleration in the Z direction is applied as shown in  FIG. 14B , on the other hand, tensile stress acts on piezoresistance elements Z 2 , Z 3  and compressive stress acts on piezoresistance elements Z 1 , Z 4 , with the result that an output is obtained by a bridge circuit for Z-axis detection in  FIG. 14D . 
         [0010]    The X-axis piezoresistance elements X 1  to X 4  and the Z-axis piezoresistance elements Z 1  to Z 4  are formed on the same beam portions  115 , but they are different in the configuration of the bridge circuit. Thus, even if the beam portions  115  deform, as in  FIG. 14A , in response to the X-direction acceleration, for example, the change in resistance is cancelled in the bridge circuit for Z-axis detection in  FIG. 14D , and no change occurs in the output. In this manner, the X-axis acceleration and the Z-axis acceleration can be separated and detected. Detection of the Y-axis acceleration is carried out by the piezoresistance elements formed on the other pair of the beam portions  115  orthogonal to the X-axis, as is done for detection of the X-axis acceleration. 
         [0011]    On the other hand, a method for realizing a downsized and inexpensive acceleration sensor by use of a resin protected package technology widely used in a semiconductor mounting technology is known, as shown in Patent Document 3. With this method, a technology for joining covers to the top and bottom of a triaxial acceleration sensor element  103  having movable portions to encapsulate it is used to protect the triaxial acceleration sensor element from a molding resin. 
         [0012]      FIG. 15A  shows a sectional view of the assembly structure of a triaxial acceleration sensor element having covers joined to the top and bottom thereof by the above-mentioned method, and  FIG. 15B  shows a plan view of a triaxial acceleration sensor element  120 . An upper cover  121  and a lower cover  122  are joined to the top and bottom of the triaxial acceleration sensor element  120  to encapsulate movable portions of the triaxial acceleration sensor element  120  in a closed space. Joining of the triaxial acceleration sensor element  120 , the upper cover  121 , and the lower cover  122  is carried out by various methods, such as metal bonding or anodic bonding. Here, metal bonding will be shown as an example. 
         [0013]    A joining metal region  123  as shown in  FIG. 15B  is formed on the face and back of the triaxial acceleration sensor element  120 . Joining metal regions are also formed in the upper cover  121  and the lower cover  122 . They are superposed, pressurized and heated for joining. With this joining step, before the triaxial acceleration sensor elements  120  are taken as individual pieces from a silicon wafer, the silicon wafer having many of the triaxial acceleration sensor elements  120  formed therein, an upper cover silicon wafer having many of the upper covers  122  formed therein with the same pitch, and a lower cover silicon wafer having many of the lower covers  123  formed therein with the same pitch are joined together. This step is called wafer level packaging (hereinafter referred to as WLP). After the closed space is formed by the WLP, the resulting composite is divided into individual chips by dicing. Hereinafter, the individual chip after encapsulation by the WLP will be termed a covered acceleration sensor element  124 . 
         [0014]    Next, a triaxial acceleration sensor  125  assembled into a package using resin will be described by reference to a sectional view in  FIG. 16 . An IC  127  for control is fixed onto a lead frame  126  with an adhesive material  128 , and the covered acceleration sensor element  124  is fixed onto the IC  127  with an adhesive material  129 . Sensor terminals  130  of the covered acceleration sensor element  124  and IC terminals  131  of the IC  127  are connected using wires  132 , and the IC terminals  131  and terminals of the lead frame  126  are connected by wires. A structure assembled from the covered acceleration sensor element  124 , the IC  127 , and the lead frame  126  is molded with a molding resin  133  by use of the transfer mold method. After the resin is cured within, a die, the product is withdrawn from the die to obtain the triaxial acceleration sensor  125 . There may be adopted a method in which a plurality of the triaxial acceleration sensors are handled collectively up to the stage of resin molding, released from the die, and then diced to separate them into the individual triaxial acceleration sensors. 
         [0015]    With the above-described acceleration sensor obtained using the WLP and resin mold packaging, the movable portions of the triaxial acceleration sensor element  120  can be protected in the silicon wafer stage. Thus, handling in subsequent steps is easy, and does not require strict control over foreign matter. Since the movable portions of the triaxial acceleration sensor element  120  are protected, moreover, the surroundings can be encapsulated by the transfer mold method. In this manner, package assembly can be performed by the resin mold packaging technology, which is often used for conventional IC chips, without the need to use an expensive ceramic package, whereby a small and inexpensive triaxial acceleration sensor can be realized. 
         [0016]    The triaxial acceleration sensor  125  shown in  FIG. 16 , however, poses the following problems in comparison with the triaxial acceleration sensor  101  shown in  FIG. 12 . 
         [0017]    The molding resin and the lead frame used in the triaxial acceleration sensor  125  are different from silicon, which is the material for the covered acceleration sensor element, in the coefficient of thermal expansion. Thus, a temperature change causes thermal stress, exerting external force on the covered acceleration sensor element, thereby changing piezoresistance. Furthermore, when the triaxial acceleration sensor  125  is installed by soldering on a product substrate of a subject product to be mounted with a sensor, the influence of thermal expansion of the product substrate is transmitted to the triaxial acceleration sensor  125  and the covered acceleration sensor element via the soldered region. 
         [0018]    With the triaxial acceleration sensor  101  of the ceramic package shown in  FIG. 12 , the triaxial acceleration sensor element  103  is held in the space within the package. By using a flexible material as the resin  107 , therefore, force from the product substrate can be minimally transmitted to the triaxial acceleration sensor element  103 . 
         [0019]    With the resin-packaged triaxial acceleration sensor  125  shown in  FIG. 16 , on the other hand, the covered acceleration sensor element  124  has its surroundings covered with the molding resin  133 , so that force from the product substrate is apt to be transmitted to the triaxial acceleration sensor element  120 . If nonuniform stress changes are caused to the four piezoresistance elements on each axis upon application of external force to the triaxial acceleration sensor element  120 , the zero-level of output fluctuates to change the output of the sensor (hereinafter, this zero-level fluctuation will be termed an offset change). 
         [0020]    The offset change responsive to the temperature change of the acceleration sensor can be corrected with the IC for detection before the sensor is installed on the product substrate. If the influence of force from the product substrate is exerted during mounting of the product, however, the outcome is produced that the sensor, when installed on the product substrate of various subject products, differs in the characteristics of the change responsive to the temperature. 
         [0021]    When the external force from the wiring substrate or the protective package is applied to the covered acceleration sensor element  124 , the disposition of the covered acceleration sensor element  124  near the center of the package allows its deformation due to the external force to be nearly bilaterally symmetrical, with the result that outputs on the X-axis and the Y-axis remain unchanged. 
         [0022]    However, if a difference occurs between the piezoresistance element near the frame portion (will hereinafter be termed the frame-side piezoresistance element) and the piezoresistance element near the weight portion (will hereinafter be termed the weight-side piezoresistance element), output on the Z-axis changes. 
         [0023]    Patent Document 4 describes an acceleration sensor whose output minimally changes under the influence of external force. In this acceleration sensor, stress separation grooves are formed in a frame body to separate it into an outer frame and an inner frame, and both frames are connected by stress relaxation beams having flexibility. The outer frame is connected to a support substrate, and the inner frame is joined to the support substrate by a partial junction. A cover body enclosing the inner frame and a weight portion together with the support substrate and the outer frame is joined to the outer frame. The area of joining of the inner frame to the support substrate is rendered relatively small, and the inner frame is connected to the outer frame by the stress relaxation beams. Thus, even if thermal stress occurs in the outer frame or the support substrate, the inner frame is minimally deformed, so that variations in output can be minimally caused. 
         [0024]    [Citation List] 
         [0025]    [Patent Literature] 
         [0026]    [Patent Document 1] JP-A-2003-172745 
         [0027]    [Patent Document 2] JP-A-2006-098321 
         [0028]    [Patent Document 3] JP-A-10-170380 
         [0029]    [Patent Document 4] JP-A-2005-337874 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0030]    Generally, the rigidity of the beam portion is designed to be low relative to the weight of the weight portion in order to realize a highly sensitive acceleration sensor. Thus, the beam portion is easily destructible under an impact or the like. In the aforementioned covered acceleration sensor element encapsulated by the WLP, the upper cover and the lower cover play the role of stoppers for regulating the excessive deviation of the weight portion. To obtain high impact resistance, the gaps between the weight portion and the upper cover and the lower cover are rendered very small. By so doing, the weight portion collides with the cover before being accelerated, so that stress generated in the collision can be decreased. The smaller the gap, the higher an air damping action can be made. An increase in the air damping action has the effect of reducing noise due to resonance of the sensor. 
         [0031]    In the acceleration sensor of Patent Document 4, the inner frame is joined to the support substrate at the one point. Thus, this acceleration sensor poses the problem that if the support substrate warps, the inner frame is displaced about the junction and tends to contact the support substrate or the cover body. In recent years, customers have expressed a strong demand for the thinning of the entire acceleration sensor. However, the support substrate thinned to meet this demand is liable to warpage, thereby aggravating the above-mentioned problem. It is an object of the present invention to realize an acceleration sensor whose output minimally changes in response to external force and which can achieve both of high sensitivity and impact resistance. 
       Solution to Problem 
       [0032]    The present invention provides a covered acceleration sensor element including a weight portion, a support frame portion surrounding the weight portion, a plurality of flexible beam portions for connecting the weight portion to the support frame portion to support the weight portion, piezoresistance elements provided on the beam portions, and wirings for connecting them, 
         [0033]    the covered acceleration sensor element being configured such that an upper cover and a lower cover enclosing a periphery of the weight portion together with the support frame portion are joined to a face and a back of the support frame portion, and 
         [0034]    the covered acceleration sensor element being adapted to detect acceleration in a direction of a first axis as a thickness direction in which the upper cover, the support frame portion, and the lower cover are stacked, and acceleration in a direction of at least one of a second axis in a plane perpendicular to the first axis, and a third axis in the plane and perpendicular to the second axis, from changes in resistances of the piezoresistance elements, 
         [0035]    wherein the support frame portion is separated by separation grooves into an inner frame and an outer frame surrounding a periphery of the inner frame, the upper cover and the lower cover are joined to the outer frame, and the inner frame is connected to and held by the outer frame via a plurality of inner frame support portions having flexibility, and 
         [0036]    the beam portions are connected to both sides of the weight portion along at least one of the second axis and the third axis, and the inner frame support portions are connected to both sides of the inner frame after being rotated through a predetermined angle from the at least one of the second axis and the third axis in a direction in which an influence of deformation of the outer frame is minimally transmitted to the beam portions. 
         [0037]    According to the above-described features, the inner frame is separated from the outer frame, the upper cover and the lower cover, and is supported by the inner frame support portions having flexibility. Thus, even if external force acts on the outer frame, the upper cover and the lower cover under thermal stress during assembly into the resin package and during mounting on the product substrate to deform them, this deformation is minimally transmitted to the inner frame, thus causing little output change. Deformation of the outer frame is somewhat transmitted to the inner frame via the inner frame support portions. However, the inner frame support portions are arranged in directions in which influence is minimally transmitted to the beam portions. Thus, deformation of the inner frame in the vicinity of the inner frame support portion causes little change to the stress on the piezoresistance elements on the beam portions. 
         [0038]    If the inner frame is supported from all directions in the surroundings, good symmetry is ensured. Thus, when the outer frame is deformed, relative displacement of the inner frame relative to the upper cover and the lower cover can be kept small, and the gap between the weight portion and the upper cover/lower cover can be decreased. Thus, under impact on the acceleration sensor, stress caused to the beam portion can be rendered low, and impact resistance can be enhanced, because of the effect of minimizing acceleration owing to the short distance until the collision of the weight portion with the upper cover or the lower cover, and the effect of increasing air damping. Since air damping can be increased, moreover, the effects are obtained that high frequency vibrations can be suppressed, vibrations of resonance of the weight portion can be kept down, and noise can be reduced. 
         [0039]    The covered acceleration sensor element may be one in which the beam portions are connected to both sides of the weight portion along the second axis, whereby accelerations in the directions of two axes, i.e., the first axis and the second axis, are detected. In this case, the inner frame support portions may be connected to both sides of the inner frame in directions in which they are rotated nearly 45 degrees from the second axis. The same effects are obtained even in the acceleration sensor element for biaxial detection which has the beam portions only in the second-axis direction. 
         [0040]    The covered acceleration sensor element may be one in which the beam portions are connected to both sides of the weight portion along the second axis, and accelerations in the directions of two axes, i.e., the first axis and the second axis, are detected. In this case, the inner frame support portions may be connected to both sides of the inner frame along the third axis perpendicular to the second axis. In the acceleration sensor element for biaxial detection which has the beam portions only in the second-axis direction, the inner frame support portions are arranged along the third axis. By so doing, the inner frame support portions are in the remotest arrangement, so that the influence of the deformation of the outer frame can be transmitted to the beam portions more difficulty. 
         [0041]    Desirably, the beam portion and the inner frame support portion have the same thickness, and they are thinner than the weight portion and the support frame portion. In order to increase the sensitivity of the acceleration sensor, it is desirable that the weight portion be heavy, and the rigidity of the beam portion be low. Such a configuration is easy to realize by composing the components of a thin silicon layer and a thick silicon layer such that the beam portion is formed only in the thin silicon layer, and the weight portion is formed in the thin silicon layer through the thick silicon layer. It is recommendable that the support frame portion be configured in the same manner as for the weight portion because it requires sufficient rigidity, and that the inner frame support portion be configured in the same manner as for the beam portion because it requires flexibility. 
         [0042]    Desirably, the flexural rigidity of the inner frame support portion is higher than the flexural rigidity of the beam portion. When weight portion resonance frequency determined by the rigidity of the beam portion and the weight of the weight portion is compared with inner frame resonance frequency determined by the rigidity of the inner frame support portion and the total weight of the inner frame and the weight portion, it is desirable that the inner frame resonance frequency be sufficiently high compared with the weight portion resonance frequency. Otherwise, in response to a relatively quick change in acceleration, it is likely that the inner frame will be displaced together with the weight portion, with the result that deformation of the beam portion may be hindered, failing to obtain correct sensitivity. It is advisable, at least, that the shape of the inner frame support portion be determined such that the resonance frequency of the inner frame will be higher than the resonance frequency of the weight portion. 
         [0043]    An acceleration sensor is constructed by adhering the above-mentioned covered acceleration sensor element onto a lead frame together with an IC chip for control, connecting the lead frame, electrodes on the IC chip, and electrodes on the covered acceleration sensor element by metal wires, and encapsulating the resulting composite with the use of a molding resin. A solder is formed on a surface of the lead frame exposed at the lower surface of the acceleration sensor, which is then reflow-soldered to a product substrate. In this manner, the acceleration sensor can be easily mounted. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0044]    According to the acceleration sensor of the present invention, the junctions of the inner frame support portions to the inner frame are arranged at positions as remote as possible from the junctions of the beam portions to the inner frame. This makes it possible to suppress output changes due to the influence of external force, such as thermal stress during assembly of the acceleration sensor into the resin package, or thermal stress during mounting of the resin-molded acceleration sensor on the product substrate. Furthermore, narrowing of the gap between the weight portion and the cover by the above stress can be suppressed. Since the gap can thus be rendered small, impact strength can be enhanced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0045]    [ FIG. 1 ] is a plan view showing the structure of an acceleration sensor element in an embodiment of the present invention. 
           [0046]    [ FIG. 2 ] is a sectional view taken along line k-k in  FIG. 1 , showing the structure of a covered acceleration sensor element. 
           [0047]    [ FIG. 3 ] is a sectional view taken along line m-m in  FIG. 1 , showing the structure of the covered acceleration sensor element. 
           [0048]    [ FIG. 4 ] is a schematic view showing a state in which an acceleration sensor assembled into a resin package is mounted on a product substrate. 
           [0049]    [ FIG. 5 ] is a plan view showing the acceleration sensor element having ring-shaped beam portions. 
           [0050]    [ FIG. 6 ] is a plan view showing the acceleration sensor element having ring-shaped inner frame support portions. 
           [0051]    [ FIG. 7 ] is a plan view showing the acceleration sensor element having beam portions and inner frame support portions rotated nearly 45 degrees with respect to a support frame portion. 
           [0052]    [ FIG. 8 ] is a plan view showing the acceleration sensor element having the inner frame support portions arranged only in one direction. 
           [0053]    [ FIG. 9 ] is a plan view showing the acceleration sensor element having the beam portions arranged only in one direction. 
           [0054]    [ FIG. 10 ] is a plan view showing the acceleration sensor element having the beam portions and the inner frame support portions arranged in directions perpendicular to each other. 
           [0055]    [ FIG. 11 ] is an exploded perspective view illustrating a conventional triaxial acceleration sensor. 
           [0056]    [ FIG. 12 ] is a sectional view illustrating the conventional triaxial acceleration sensor. 
           [0057]    [ FIG. 13 ] is a plan view illustrating an example of the structure of a conventional triaxial acceleration sensor element. 
           [0058]    [ FIG. 14A ] is an explanation drawing of the principle of detection of the conventional triaxial acceleration sensor element. 
           [0059]    [ FIG. 14B ] is an explanation drawing of the principle of detection of the conventional triaxial acceleration sensor element. 
           [0060]    [ FIG. 14C ] is an explanation drawing of the principle of detection of the conventional triaxial acceleration sensor element. 
           [0061]    [ FIG. 14D ] is an explanation drawing of the principle of detection of the conventional triaxial acceleration sensor element. 
           [0062]    [ FIG. 15A ] is a sectional view showing the conventional triaxial acceleration sensor element encapsulated using covers. 
           [0063]    [ FIG. 15B ] is a plan view showing the conventional triaxial acceleration sensor element encapsulated using the covers. 
           [0064]    [ FIG. 16 ] is a sectional view showing a protected package including a conventional triaxial acceleration sensor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0065]    An acceleration sensor according to an embodiment of the present invention will be described by reference to the accompanying drawings. 
       Embodiment 1 
       [0066]      FIG. 1  is a plan view showing the structure of an acceleration sensor element  10  in a covered acceleration sensor element  30  of Embodiment 1.  FIGS. 2 and 3  are sectional views of the covered acceleration sensor element  30  of Embodiment 1, in which  FIG. 2  is a sectional view taken along line k-k in  FIG. 1 , and  FIG. 3  is a sectional view taken along line m-m in  FIG. 1 . 
         [0067]    The acceleration sensor element  10  of Embodiment 1 can be applied, for example, to an acceleration sensor assembled into a resin protected package as shown in  FIG. 16  as a conventional example. In Embodiment 1, therefore, the covered acceleration sensor element  30 , in particular, will be mainly described in detail. 
       &lt;Basic Structure&gt; 
       [0068]    The acceleration sensor element  10  of Embodiment 1 has a weight portion  12  supported within a support frame portion  11  from all directions by four beam portions  13  having flexibility. The support frame portion  11  is separated by a first separation groove  14  into an inner frame  15  and an outer frame  16  surrounding the inner frame  15 , and the beam portions  13  are connected to the inner frame  15 . The inner frame  15  is supported on the outer frame  16  from all directions by inner frame support portions  17 . The weight portion  12  is separated from the inner frame  15  by a second separation groove  29 , and is composed of four body portions and a middle portion connected to the body portions and the beam portions  13 . 
         [0069]    The four beam portions  13  are termed a first beam portion  13   a , a second beam portion  13   b , a third beam portion  13   c , and a fourth beam portion  13   d . In the acceleration sensor element  10  of Embodiment 1, piezoresistance elements P are formed near the bases of the beam portions  13 , as explained in  FIG. 13 . The piezoresistance elements P for detecting accelerations in the X-axis and Z-axis directions are arranged in the first beam portion  13   a  and the second beam portion  13   b  extending in the X-axis direction, while the piezoresistance elements P for detecting acceleration in the Y-axis direction are arranged in the third beam portion  13   c  and the fourth beam portion  13   d  extending in the Y-axis direction. The piezoresistance elements P for detecting acceleration in the Z-axis direction may be arranged in the third beam portion  13   c  and the fourth beam portion  13   d . The respective piezoresistance elements P are interconnected by wirings (not shown) so as to form bridge circuits as shown in  FIGS. 14A to 14D . The wirings are pulled out onto the outer frame  16  over the inner frame support portions  17 , and are connected to electrode pads  18  formed on the outer frame  16 . 
         [0070]    An upper cover  19  is joined to a surface of the acceleration sensor element  10  where the piezoresistance elements P are formed. This joining is performed using a joining member  21  in conformity with an upper cover joining region  20  on the outer frame  16 . Similarly, a lower cover  22  is joined to the opposite surface of the acceleration sensor element  10  with the use of a joining material  23 . The upper cover  19  and the lower cover  22  are joined only to the outer frame  16 , and the outer frame  16 , the upper cover  19 , and the lower cover  22  enclose the periphery of the inner frame  15 . 
       &lt;Manufacturing Method&gt; 
       [0071]    A method of producing the acceleration sensor element  10  will be described briefly by reference to  FIG. 2 . The acceleration sensor element  10  was processed using an SOT wafer having a silicon layer with a thickness of about 6 μm on a silicon layer with a thickness of about 400 μm, with a silicon oxide layer about 1 μm thick being sandwiched therebetween. The silicon oxide film layer was used as an etching stop layer for dry etching, and a structure was formed in the two silicon layers. The thin first silicon layer was designated as a first layer  24 , and the thick second silicon layer was designated as a second layer  25 . A surface of the first layer which was not joined to the silicon oxide film layer was designated as a first surface  26 . A surface of the second layer which was not joined to the silicon oxide film layer was designated as a second surface  27 . A connected surface for connection via the silicon oxide film layer was designated as a third surface  28 . 
         [0072]    The shapes of semiconductor piezoresistance elements were patterned using a photoresist, and the first surface  26  was implanted with boron in a concentration of 1 to 3×10 18  atoms/cm 3  to form semiconductor piezoresistance elements. Similarly, boron was implanted in a higher concentration than for the piezoresistance elements to form P type wirings so as to be connected to the piezoresistance elements. Further, a silicon oxide film was formed on the first surface  26  to protect the piezoresistance elements. Metal wirings were formed by sputtering an aluminum-based metal on the silicon oxide film, and they were connected to the P type wirings via through-holes formed in the silicon oxide film. The silicon oxide film formed on the piezoresistance elements also worked as an insulating film between the silicon of the first layer  24  and the metal wirings. Further, a silicon nitride film was formed thereon by chemical vapor deposition as a protective film on the metal wirings. The silicon oxide film, the metal wirings, and the silicon nitride film were processed into desired shapes by photolithography. 
         [0073]    Then, after the photoresist pattern was formed in the first surface  26 , the shapes shown in  FIG. 1 , namely, the first separation groove  14  for separating the inner frame  15  and the outer frame  16 , and the second separation groove  29  for separating the weight portion and the inner frame  15 , were processed by dry etching, with the beam portions  13  and the inner frame support portions  17  being left. Further, a photoresist pattern was formed in the second surface  27 , whereafter the first separation groove  14  and the second separation groove  29  were processed by dry etching. Exposed parts of the silicon oxide film layer remaining between the first layer  24  and the second layer  25  were removed by wet etching. As a result, the first separation groove  14  and the second separation groove  29  penetrated the SOT wafer. According to the above-described manufacturing process, the weight portion  12 , the inner frame  15 , and the outer frame  16  were formed in the first layer  24  through the second layer  25 . The beam portions  13  and the inner frame support portions  17  were formed in the first layer  24 . 
         [0074]    Then, according to the WLP technology, the upper cover  19  and the lower cover  22 , each comprising silicon, were joined to the face and back of the acceleration sensor element  10  by metal bonding for encapsulation. For this purpose, in the acceleration sensor element before the above dry etching step, thin metal films for use in metal bonding were formed on the first surface  26  and the second surface  27  of the wafer. Two wafers to serve as the covers were provided with the same metal thin films and metal solders. These three wafers were superpose, and pressurized and heated for bonding. A gold-tin alloy was used for the metal solders. 
         [0075]    Then, the upper cover  19  and the lower cover  22  were ground to thin the entire composite. On a side of the upper cover  19  to be joined to the acceleration sensor element  10 , a groove was formed to a depth greater than the thickness of the upper cover after grinding so that after grinding, the electrode pads  18  of the acceleration sensor element  10  would be exposed. A side of the lower cover  22  need not be provided with such a groove, but may be structured similarly to the upper cover  19 . Moreover, cavities were formed in parts of the upper cover  19  and the lower cover  22  which will face the weight portion  12  on their surfaces to be joined to the acceleration sensor element  10 . A gap  31  between the weight portion  12  and the upper cover  19 /lower cover  22  is the sum of the depth of the cavity (cavity depth  32 ) and the thickness of the joining material (joining material thickness  33 ). If the joining material thickness  33  may be taken, unchanged, as the gap  31 , there is no need to form the above cavity. 
         [0076]    The foregoing steps up to the grinding step were performed, with the state of the wafer being maintained. Finally, the composite was diced to separate it into the individual covered acceleration sensor elements  30 . By the above-described manufacturing process, there was obtained the covered acceleration sensor element  30  having the inner frame  15  and the weight portion  12  supported in an airtight container composed of the outer frame  16 , the upper cover  19 , and the lower cover  22 . 
       &lt;Structure of Resin Package&gt; 
       [0077]      FIG. 4  shows a sectional schematic view of an acceleration sensor mounted structure  41  having an acceleration sensor  40  mounted on a product substrate  49 , the acceleration sensor  40  being produced by assembling the covered acceleration sensor element  30  of Embodiment 1 into a resin package. An IC chip  42  for control was adhered onto a lead frame  43  with the use of an adhesive material  44 , and the covered acceleration sensor element  30  was adhered onto the IC chip  42  with the use of an adhesive material  45 . Connections between the electrode pads  18  of the covered acceleration sensor element  30  and electrode pads  46  of the IC chip  42 , and connections between the electrode pads  46  of the IC chip  42  and the lead frame  43  were provided by wire bonding using metal wires  47 . Then, all the components were encapsulated with a molding resin  48  to obtain the acceleration sensor  40 . A die attach film (DAF), which functions concurrently as a dicing tape and an adhesive material, can be used for the adhesive materials  44  and  45 . A surface of the lead frame exposed at the lower surface of the acceleration sensor  40  was solder-plated, and joined to the product substrate  49  with the use of a solder  50  to obtain the acceleration sensor mounted structure  41 . 
       &lt;Inner Frame Support Portions&gt; 
       [0078]    In the covered acceleration sensor element  30  of the present invention, the inner frame  15  is separated from the outer frame  16 , the upper cover  19  and the lower cover  22 , as shown in  FIG. 2 , and is merely supported on the outer frame  16  at four locations in diagonal directions by the inner frame support portions  17  having flexibility, as shown in  FIG. 1 . Thus, even if external force acts on the outer frame  16 , the upper cover  19  and the lower cover  22  under thermal stress during assembly into the resin package and during mounting on the product substrate to deform them, this deformation is minimally transmitted to the inner frame  15 , thus causing little output change. Deformation of the outer frame  16  is somewhat transmitted to the inner frame  15  via the inner frame support portions  17 . However, the inner frame support portions  17  are arranged in the diagonal directions relative to the beam portions  13 . Thus, deformation of the inner frame  15  in the vicinity of the inner frame support portion  17  causes little change to the stress on the piezoresistance elements on the beam portions  13 . 
         [0079]    In  FIG. 1 , a change in the stress on the piezoresistance element by external force is apt to occur when compression or tension acts on the beam portion  13  in the longitudinal direction, or the beam portion  13  is warped. If stress on the inner frame  15  changes in a part near the junction of the beam portion  13 , moreover, only the piezoresistance element close to the inner frame  15  changes, whereas the piezoresistance element close to the weight portion scarcely changes, so that an offset change in the Z-axis is liable to occur. In Embodiment 1, the junction of the inner frame support portion  17  is remote from the junction of the beam portion  13 , and the stress change caused to the inner frame  15  by external force does not directly affect the beam portion  13 . Thus, a change in output can be rendered very small. 
       Rigidity of Inner Frame Support Portion&gt; 
       [0080]    To ensure the responsiveness of the acceleration sensor, it is desirable to make the rigidity of the inner frame support portion  17  higher than that of the beam portion  13 . When weight portion resonance frequency determined by the rigidity of the beam portion  13  and the weight of the weight portion  12  is compared with inner frame resonance frequency determined by the rigidity of the inner frame support portion  17  and the total weight of the inner frame  15  and the weight portion  12 , it is desirable that the inner frame resonance frequency be sufficiently high compared with the weight portion resonance frequency. Otherwise, in response to a relatively quick change in acceleration, the inner frame  15  is displaced together with the weight portion, with the result that deformation of the beam portion  13  is hindered, failing to obtain correct sensitivity. It is desirable that the shape of the inner frame support portion  17  be determined such that the phase characteristics and gain characteristics of the frequency characteristics are apart from each other to a degree to which they will not be coupled. 
       &lt;Symmetrical Support for Inner Frame&gt; 
       [0081]    Embodiment 1 ensures good symmetry, because the inner frame  15  is supported from all directions in the surroundings. If the inner frame  15  is supported by one of the inner frame support portions  17 , or is connected to the lower cover  22  at one point of the inner frame  15 , for example, the inner frame  15  is displaced in a cantilevered manner in response to the deformation of the Outer frame  16  and the lower cover  22 , so that the relative displacement of the inner frame  15  relative to the upper cover  19  and the lower cover  22  is prone to increase. In this case, the gap needs to be rendered large so that the inner frame  15  and the weight portion  12  make no contact with the upper cover  19  and the lower cover  22 . In the present embodiment, the inner frame  15  is supported from all directions in the surroundings, so that relative displacement of the inner frame  15  relative to the upper cover  19  and the lower cover  22  can be decreased, and the gap can be made small. Thus, under impact on the acceleration sensor, stress caused to the beam portion  13  can be rendered low, and impact resistance can be enhanced, because of the effect of minimizing acceleration owing to a short distance until the collision of the weight portion.  12  with the upper cover  19  or the lower cover  22 , and the effect of increasing air damping. Since air damping can be increased, there arise the effects that high frequency vibrations can be suppressed, vibrations of resonance of the weight portion  12  can be kept down, and noise can be reduced. 
       Joining of Cover Body&gt; 
       [0082]    Furthermore, Embodiment 1 is easy in terms of the manufacturing process as compared with the joining of the inner frame  15  to the lower cover  22 . As described above, in joining each of the upper cover  19  and the lower cover  22  with the use of the metal solder, it is necessary to perform heating during pressurization. When it is attempted to join the inner frame  15  only to the lower cover  22 , a sufficient pressurizing force cannot be exerted on the junction of the inner frame  15 , because the inner frame  15  is flexibly connected to the outer frame  16 . Thus, there is no choice but to perform a two-step procedure, such as joining the acceleration sensor element  10  to the lower cover  22  first, and then joining the upper cover  19  to the acceleration sensor element  10 . In joining the acceleration sensor element  10  to the lower cover  22 , the surface of the acceleration sensor element  10 , which is easily breakable, has to be directly pressurized. If the junctions exist only in the outer frame  16 , and the junctions of the upper cover  19  and the junctions of the lower cover  22  are different, as in Embodiment 1, sufficient press ing force can be applied to the junctions. 
       Embodiment 2 
       [0083]      FIG. 5  is a schematic plan view showing the structure of an acceleration sensor element  10  of Embodiment 2. The acceleration sensor element  10  is of a shape in which ring portions  51  are provided as compressive stress absorbing portions in the center of the beam portions  13 . The silicon oxide film formed on the surface of the acceleration sensor element  10  is smaller in the coefficient of thermal expansion than silicon, and annealing is carried out at a high temperature of, say, 950° C. during film formation. Thus, thermal stress occurs during cooling to ordinary temperature. The weight portion  12  and the inner frame  15  are formed in the first layer  24  through the second layer  25 , and the second layer  25  is so thick that it shrinks with nearly the same coefficient of thermal expansion as that of silicon. The beam portion  13  consists of the first layer  24  alone, and thus, its proportion of the silicon oxide film is high, and its thermal shrinkage is low. Thus, the beam portion  13  undergoes compression between the inner frame  15  and the weight portion  12 . If the beam portion  13  is thinned in order to increase the sensitivity of the sensor, the beam portion  13  may buckle under the above compressive force, causing an increase in the instability of the sensitivity or a great offset change. 
         [0084]    By providing the ring portions  51  in the beam portions  13 , as in Embodiment 2, the above compressive force can be absorbed, and buckling can be prevented, so that a high sensitivity acceleration sensor element can be designed. Various shapes are conceivable for the ring portion  51 , for example, a shape of three rings connected together. The shape may be determined so that the compressive force can be absorbed by deformation, and such that stress does not concentrate, for example, in the R-section of the ring. 
       &lt;Results of Analysis of Design Example&gt; 
       [0085]    A design, example in Embodiment 2 of  FIG. 5  will be shown. The acceleration sensor element  10  measured 1.32 mm in the X direction and 1.18 mm in the Y direction. The X- and Y-dimension of the weight portion were each 560 μm. The beam portion  13  had a length of 240 μm. The width of the piezoresistance element formation portion was 28 μm. The inner frame support portion  17  had a length of 50 μm, a connection width of 160 μm on the side of the outer frame  16 , and a connection width of 150 μm on the side of the inner frame  15 . The thickness of the first layer was 4 μm, and the thickness of the second layer was 400 μm. The width of the inner frame  15  was 70 μm. 
         [0086]    The acceleration sensor element  10  was assembled into the resin package to obtain the acceleration sensor  40 , which was mounted on the product substrate  49  with a thickness of 0.6 mm. Changes in the characteristics of the acceleration sensor  40  before and after its mounting on the product substrate  49  were evaluated using FEM analysis. In the conventional structure example in which the acceleration sensor element was of the same size and the support frame portion was not separated into the outer frame and the inner frame, the change in the Z-axis output before and after mounting was about 23% expressed as a proportion to the Z-axis sensitivity, whereas this change could be kept down to about 4% with the above-mentioned design example of the acceleration sensor. The weight portion resonance frequencies of the present design example were 2.0 kHz in each of the X direction and the Y direction, and 3.2 kHz in the Z direction, while the inner frame resonance frequency was about 46 kHz. Since the inner frame resonance frequency is sufficiently high, it does not affect sensor sensitivity. 
       Embodiment 3 
       [0087]      FIG. 6  is a schematic plan view showing the structure of an acceleration sensor element  10  of Embodiment 3. The acceleration sensor element  10  is of a shape in which ring portions  52  are provided as compressive stress absorbing portions in the inner frame support portions  17 . This configuration has the effect of preventing the inner frame support portion  17  from buckling, as in Embodiment 2. If the inner frame support portion  17  buckles, the inner frame  15  is displaced to approach the upper cover  19  or the lower cover  22 . Thus, it is difficult to make the gap  31  small. By forming the ring portions  52  in the inner frame support portions  17 , buckling can be prevented. Also, the formation of the ring portions  52  in the inner frame support portions  17  has the effect of absorbing the influence of the deformation of the outer frame  16 , thus making the output less changeable. 
       Embodiment 4 
       [0088]      FIG. 7  is a schematic plan view showing the structure of an acceleration sensor element  10  of Embodiment 4. In this acceleration sensor element  10 , the arrangements of the beam portions  13  and the inner frame support portions  17  are rotated nearly 45 degrees. The inner frame support portions  17  are arranged in the directions of the X-axis and the Y-axis, while the beam portions  13  are arranged in a direction at nearly 45 degrees with respect to the inner frame support portions  17 . Thus, the relative relationship between the inner frame support portions  17  and the beam portions  13  is maintained. By arranging the beam portions  13  in the diagonal direction of the square acceleration sensor element  10 , the beam portions  13  can be lengthened, thereby making it easy to increase the sensitivity of the sensor. 
         [0089]    In the structure of Embodiment 4, the number of the inner frame support portions  17  may be two. An example of the inner frame support portions  17  formed only at two locations in the Y direction is shown in  FIG. 8 . If the electrode pads are arranged on one side along the Y-axis, as in the present example, a shape of only this side protruding appears, thus resulting in poor symmetry with respect to the Y-axis. In assembling this structure into the resin package of  FIG. 4 , too, only the above side is subjected to wire bonding, so that the electrode pads are arranged in a shifted manner so as to render this side broad. In the case of Embodiment 4, as described above, the structure is symmetrical with respect to the X-axis, but its symmetry with respect to the Y-axis worsens. Thus, the inner frame  15  is connected to the outer frame  16  only in the Y-axis direction, whereby it becomes possible to improve the symmetry of the influence of external force transmitted to the beam portions  13  while minimally transmitting to the inner frame  15  the influence of poorly symmetrical deformation in the X direction. Deformations symmetrical with respect to the X-axis and the Y-axis do not influence X-axis output and Y-axis output, and thus they are effective, particularly, in suppressing changes in the outputs on the X-axis and the Y-axis. 
       Embodiment 5 
       [0090]      FIG. 9  is a schematic plan view showing the structure of an acceleration sensor element  10  of Embodiment 5. Embodiments 1 to 4 show examples of having four of the beam portions  13 , but the present invention can also be applied to an acceleration sensor element for biaxial detection which has only two of the beam portions  13  in one direction. Embodiment 5 involves two of the beam portions  13  in the Y-axis direction, and is designed to be capable of detecting accelerations in the Y-axis direction and the Z-axis direction. Similarly, the acceleration sensor element may have only two of the beam portions  13  in the X direction, and may be designed to be capable of detecting accelerations in the X-axis direction and the Z-axis direction. 
         [0091]    With the acceleration sensor element  10  having two of the beam portions  13 , the inner frame support portions  17  may be arranged in a direction at nearly 90 degrees with the respect to the beam portions  13 , as shown in  FIG. 10 . The main effects of the present invention are obtained by arranging the junctions of the inner frame support portions  17  to the inner frame  15  at positions as remote as possible from the junctions of the beam portions  13  to the inner frame  15 . If there are two of the beam portions  13  in the Y direction, as in Embodiment 5, therefore, two of the inner frame support portions  17  are provided in the X direction. By so doing, the remotest arrangement is ensured, so that the influence of the deformation of the outer frame  16  can be transmitted to the beam portions  13  more difficulty. 
       Modified Embodiments 
       [0092]    In the present invention, the inner frame support portions  17  are arranged in a direction at nearly 45 degrees or nearly 90 degrees with respect to the beam portions  13 . However, even if the angle is not accurately 45 degrees or 90 degrees, the same effects are obtained by arranging the junctions of the inner frame support portions  17  to the inner frame  15  to be sufficiently remote from the junctions of the beam portions  13  to the inner frame  15 . For example, if the inner frame support portions  17  are arranged in a direction at 45 degrees with respect to the beam portions  13 , certain effects are existent even when they are arranged symmetrically in a range at an angle of 45 degrees ±15 degrees. If they are arranged symmetrically in the 45±5 degree range, they can be used equivalently to their arrangement at 45 degrees, depending on the specifications and characteristics demanded. 
         [0093]    Moreover, the characteristics of the arrangement of the inner frame support portions  17  and the addition of the ring portions shown in Embodiments 1 to 5 can be used in combinations. 
         [0094]    [Reference Signs List] 
         [0095]      10  Acceleration sensor element,  11  Support frame portion,  12  Weight portion,  13  Beam portion,  13   a  First beam portion,  13   b  Second beam portion,  13   c  Third beam portion,  13   d . Fourth beam portion,  14  First groove portion,  15  Inner frame,  16  Outer frame,  17  Inner frame support portion,  19  Upper cover,  22  Lower cover,  29  Second separation groove,  30  Covered acceleration sensor element,  31  Gap,  32  Cavity depth,  40  Acceleration sensor,  41  Acceleration sensor mounted structure,  42  IC chip,  43  Lead frame,  44  Adhesive material,  45  Adhesive material,  47  Metal wire,  48  Molding resin,  49  Product substrate,  51  Ring portion,  52  Ring portion,  101  Triaxial acceleration sensor,  102  Case,  103  Acceleration sensor element,  104  IC,  105  Cover,  106  Resin adhesive material,  107  Resin adhesive material,  111  Wire,  113  Support frame portion,  114  Weight portion,  115  Beam portion,  116  X-axis piezoresistance element,  117  Y-axis piezoresistance element,  118  Z-axis piezoresistance element,  120  Triaxial acceleration sensor element,  121  Upper cover,  122  Lower cover,  123  Joining metal region,  124  Covered acceleration sensor element,  125  Triaxial acceleration sensor,  126  Lead frame,  127  IC,  132  Wire,  133  Molding resin,  134  Product substrate, P Piezoresistance element