Patent Publication Number: US-2009223292-A1

Title: Acceleration sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a piezo-resistor type acceleration sensor for detecting acceleration for use in automobiles, aircraft, home electric appliances, game machines, robots, security systems and the like. 
     BACKGROUND ART 
     Acceleration sensors have been used for purposes of detecting a large impact force for deploying an automotive air-bag and detecting a small acceleration for a vehicle control application such as a brake control system. For these automotive applications, an acceleration sensor with a one-axis or two-axis function is enough because only accelerations in the directions of the X-axis and/or Y-axis are measured. Recently, acceleration sensors have been put into practical use for new applications such as mobile terminal units, robots or various controls utilizing detection of movements of a human body. For these new applications, a three-axis acceleration sensor that can measure acceleration in the X-axis, the Y-axis, and the Z-axis is required in order to detect three-dimensional movement. Moreover, the three-axis acceleration sensor is also required to have high resolution to detect slight acceleration and have a small-sized thin profile. 
     Acceleration sensors are roughly classified as piezo-resistor type, capacitance type or piezoelectric type according to a way of converting movements of a flexible portion therein into electrical signals. Which of these types is used depends on the application, and for the purpose of detecting static acceleration, piezo-resistor type and capacitance type are employed. In these two types, a number of small-sized high sensitivity acceleration sensors can be produced at once by forming a three-dimensional structure on a silicon substrate with semiconductor technologies and micromachine technologies. In particular, an acceleration sensor of the piezo-resistor type allows easy construction of the structure and the manufacturing process, has a small-sized thin profile, and is suitable for price-reduction. Moreover, acceleration sensors are roughly classified as diaphragm type or beam (flexible arm) type according to the structure of the flexible portion. The ways of detecting electrical signals, the structures of the flexible portion and the number of detection axes may be combined to obtain various acceleration sensors. 
     There are many patent applications in relation to a piezo-resistor type three-axis acceleration sensor having flexible arms. Patent Documents 1 to 6 disclose a shape of a weight and flexible arms, an arrangement of piezo-resistors, connections of the piezo-resistors, a shape of a connection between the flexible arm and a support frame, and the like. In the three-axis acceleration sensor, a sensor chip and an upper regulation plate are spaced apart by a predetermined distance and adhered in a case thereof with an adhesive such as resin. A case lid is hermetically adhered on the case, for example, with an adhesive such as gold tin solder. The three-axis acceleration sensor element having the flexible arms is formed in the sensor chip, and the three-axis acceleration sensor element is constituted of a rectangular support frame, a weight and paired flexible arms, in which the weight is held in the center of the support frame with two pairs of flexible arms. Piezo-resistors are formed on the flexible arms. One pair of flexible arms has X-axis piezo-resistors and Z-axis piezo-resistors formed thereon, and the other pair of flexible arms has Y-axis piezo-resistors formed thereon, the resistors being connected to metal wires. A distance between an undersurface of the weight and an inner bottom surface of the case as well as a distance between a top surface of the weight and the upper regulation plate restrict a movement of the weight to prevent the thin flexible arms from breaking down when an excessive acceleration such as an impact is applied to the acceleration sensor. 
     Patent Documents 7 to 9 disclose a diaphragm structure of a three-axis acceleration sensor with a diaphragm, and an arrangement of piezo-resistors. As a flexible portion, a circular or polygonal diaphragm is attached to a support frame at outer edge of the diaphragm, and a weight is arranged at an inner edge of the diaphragm. When the weight is displaced by an external force, piezo-resistors provided in the diaphragm are deformed, so that an electric signal is obtained. Compared with a beam-type three-axis acceleration sensor element, the three-axis acceleration sensor with the diaphragm has an advantage of having a high degree of freedom for the arrangement of the piezo-resistors. The three-axis acceleration sensor with the diaphragm is constituted of a square support frame, a weight and a diaphragm, and the weight is held in the center of the diaphragm. In the diaphragm, piezo-resistors (X-axis piezo-resistors, Y-axis piezo-resistors and Z-axis piezo-resistors) are formed and connected to metal wires. 
     When the weight is moved under an external force, the flexible portion is deformed. The deformation of the flexible portion can be measured as a change in resistance of the piezo-resistor to determine the direction and magnitude of the external force. However, since the change in resistance of the piezo-resistor is slight, a full bridge circuit with four piezo-resistors arranged for each axis on the flexible portion is formed to detect such a slight change in resistance as a change in voltage. If the four piezo-resistors constituting the full bridge have equal electric resistance, there is no output from the bridge. However, an output of the bridge practically appears even with no acceleration applied or with no deformation of the flexible portion since the four piezo-resistors have different resistance due to various factors such as variations of an impurity concentration of the piezo-resistor, variations in dimension of the element, a difference in stresses applied to the element between the four piezo-resistors. This output voltage is referred to as an offset voltage. By providing an adjustment circuit in the acceleration sensor, the offset voltage is canceled to reduce the offset voltage to substantially zero. 
     In an effect confirmation test of the regulation plate, when an excessive impact was applied to the acceleration sensor, the offset voltage in excess of an allowable range may occur in such an acceleration sensor whose offset voltage is adjusted. This is attributed to the fact that the excessive impact applied to the acceleration sensor causes the flexible portion to collide with the upper regulation plate, deforming a part of metal wires provided in the flexible portion. A change in the electric resistance resulting from deformation in the metal wires causes an offset voltage. 
     In Patent Document 10, it is proposed to divide a piezo resistor into a plural of piezo sub-resistors to increase detection sensitivity without affecting electrical power consumption and impact resistance. For example, by dividing one piezo-resistor into two piezo sub-resistors and connecting the two piezo sub-resistors having the same width in series, the same resistance as one piezo-resistor can be obtained. By arranging two half-length piezo sub-resistors next to each other in a stress concentration zone of the flexible portion, the detection sensitivity thereof can be increased even when the flexible portion is deformed likewise. Dividing a piezo-resistor into a plural of piezo sub-resistors does not change resistance thereof, and therefore, detection sensitivity thereof can be increased while electrical power consumption thereof is not changed. However, since its division increases the number of the metal wires connecting the piezo sub-resistors, the metal wires collide with the upper regulation plate to be deformed, so that occurrence of the offset voltage is also increased. 
     It is possible to coat the metal wires thickly with a rigid and electrically insulating film such as alumina and silicon oxide such that the metal wires are not deformed even when the flexible portion collides with the upper regulation plate. However, when such film is thickly formed, the degree of deformation of the flexible portion is changed. The flexible portion is constituted of silicon and formed of a material with a coefficient of thermal expansion different from that of the electrically insulating film and the metal wires. Stresses applied to the piezo-resistors vary due to a difference between coefficients of thermal expansion of constituent materials, which is one of the causes of occurrence of the offset voltage. Further, when the metal wire is thickly covered with the rigid and electrically insulating film, the offset voltage is further increased.
     Patent Document 1: JP 2003-172745 A   Patent Document 2: JP 2003-279592 A   Patent Document 3: JP 2004-184373 A   Patent Document 4: JP 2006-098323 A   Patent Document 5: JP 2006-098321 A   Patent Document 6: WO 2005/062060 A1   Patent Document 7: JP Hei 3-2535 A   Patent Document 8: JP Hei 6-174571 A   Patent Document 9: JP Hei 7-191053 A   Patent Document 10: JP 2006-098321 A   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been made to solve the above-mentioned problems, and has an object to provide a small-sized thin three-axis acceleration sensor in which an offset voltage does not newly occur even if an excessive impact is applied after an adjustment of offset voltage. 
     Means for Solving the Problems 
     An acceleration sensor according to the present invention comprises: a weight in a center of the acceleration sensor; a support frame surrounding the weight and being at a predetermined distance from the weight; a flexible portion bridging an upper portion of the weight and an upper portion of the support frame and hanging the weight; a plurality of piezo-resistors formed in the flexible portion and adjacent to a top surface of the flexible portion; sensor terminals provided on a top surface of the support frame; and metal wires connecting between the piezo-resistors or between the piezo-resistors and the sensor terminals. A part of the metal wires disposed on the flexible portion is put in a groove having a rectangular cross-section or an inverted trapezoidal cross-section formed on the top surface of the flexible portion, and top surfaces of the metal wires put in the groove formed on the top surface of the flexible portion are lower than the top surface of the flexible portion. 
     In the acceleration sensor of the present invention, it is desirable that the metal wires which a top surface of the weight has is put in a groove having a rectangular cross-section or an inverted trapezoidal cross-section formed on the top surface of the weight, and top surfaces of the metal wires put in the groove formed on the top surface of the weight are lower than the top surface of the weight. 
     In the acceleration sensor of the present invention, it is desirable that a part of the metal wires which is on a weight side from an end on a support frame side of a piezo-resistor disposed on the support frame side among the plurality of piezo-resistors has a top surface lower than the top surface of the flexible portion. 
     In the acceleration sensor of the present invention, top surfaces of the metal wires put in the groove formed on the top surface of the flexible portion are preferably 0.05 μm to 0.5 μm lower than the top surface of the flexible portion. A top surface of the metal wires disposed in the groove formed on the top surface of the weight is preferably at least 0.05 μm lower than the top surface of the weight. 
     In the acceleration sensor of the present invention, it is preferable that the flexible arms are composed of a silicon layer and an electrically insulating layer covering the silicon layer, and that the electrically insulating layer covers the top surface of the flexible portion and both inner side walls and a bottom surface of the groove. The groove may be formed on the silicon layer. Alternatively, the groove formed on the top surface of the flexible portion may be formed in the electrically insulating layer covering the silicon layer. 
     In the acceleration sensor of the present invention, the weight preferably has a silicon layer and an electrically insulating layer covering the silicon layer, and the electrically insulating layer covers both inner side walls and a bottom surface of a groove formed on the top surface of the weight. The groove of the top surface of the weight may be formed on the top surface of the silicon layer. Alternatively, the groove of the top surface of the weight may be formed on the electrically insulating layer covering the silicon layer. 
     In the acceleration sensor of the present invention, the groove on the top surface of the flexible portion preferably extends to the top surface of the weight and to the top surface of the support frame. A plurality of metal wires may be formed in a part of the groove on the top surface of the weight or the support frame. 
     In the acceleration sensor of the present invention, it is preferable that the flexible portion is composed of a plurality of flexible arms bridging the upper portion of the weight and the upper portion of the support frame;
     each of the plurality of flexible arms has at least one of the grooves formed on the flexible portion, is constituted of a silicon layer and an electrically insulating layer covering the silicon layer, and the electrically insulating layer covers both inner side walls and a bottom surface; and   each of the plurality of flexible arms is structurally symmetric with respect to a centerline extending in a longitudinal direction of the flexible arm. Each of the plurality of flexible arms may have at least two grooves, and the piezo-resistors may be located between the grooves on the silicon layer.   

     Advantages of the Invention 
     In an acceleration sensor according to the present invention, since a metal wire disposed on a top surface of a flexible portion bridging a weight and a support frame is placed in a groove formed on the top surface of the flexible portion as well as a top surface of the metal wire is lower than the top surface of the flexible portion, the metal wire does not collide with an upper regulation plate when an excessive acceleration or impact is applied to the acceleration sensor. Accordingly, the metal wire does not deform and an offset voltage does not newly occur in the acceleration sensor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of an acceleration sensor device comprising an acceleration sensor of EXAMPLE 1 according to the present invention; 
         FIG. 2  is a plan view of the acceleration sensor of EXAMPLE 1; 
         FIG. 3  is an enlarged plan view of one of flexible arms extending in the X-axis direction of the acceleration sensor of EXAMPLE 1; 
         FIG. 4  is an enlarged plan view of one of flexible arms extending in the Y-axis direction of the acceleration sensor of EXAMPLE 1; 
         FIG. 5  is an enlarged cross-sectional view taken along V-V line of  FIG. 2 ; 
         FIG. 6  is an enlarged cross-sectional view taken along VI-VI line of  FIG. 2 ; 
         FIG. 7  is an enlarged cross-sectional view taken along VII-VII line of  FIG. 2 ; 
         FIG. 8  is an enlarged cross-sectional view taken along IIX-IIX line of  FIG. 2 ; 
         FIG. 9  is an enlarged cross-sectional view taken along IX-IX line of  FIG. 2 ; 
         FIG. 10  is an explanatory view of wires of the acceleration sensor of EXAMPLE 1; 
         FIG. 11  is an explanatory view of a full bridge circuit of X-axis piezo-resistors (Y-axis piezo-resistors) of  FIG. 10 ; 
         FIG. 12  is an explanatory view of a full bridge circuit of Z-axis piezo-resistors of  FIG. 10 ; 
         FIG. 13  is an enlarged plan view of two flexible arms extending in the Y-axis direction of an acceleration sensor of EXAMPLE 2; 
         FIG. 14  is an enlarged cross-sectional view taken along XIV-XIV line of  FIG. 13 ; 
         FIG. 15  is an enlarged cross-sectional view of a flexible arm of an acceleration sensor of EXAMPLE 3 extending in the Y-axis direction; 
         FIG. 16  is a longitudinal sectional view taken along XVI-XVI line of  FIG. 15 ; 
         FIG. 17  is a longitudinal sectional view taken along XVII-XVII line of  FIG. 15 ; and 
         FIG. 18  is a plan view of an acceleration sensor of EXAMPLE 4. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
       10 : weight 
       11   t,    12   t,    13   t,    14   t,    21   t,    23   t,    31   t,    33   t:  sensor terminal 
       16 ,  26 ,  26 ′,  36 : groove 
       21 ,  21 ′,  21 ″,  22 ,  22 ′,  23 ,  23 ′: flexible arm 
       24 : silicon layer 
       25 ,  25   c,    25   c ′,  25   d:  metal wire 
       28 ,  28 ′: electrically insulating layer 
       29 : diaphragm 
       30 : support frame 
     X 1 , X 2 , X 3 , X 4 , Y 1 , Y 2 , Y 3 , Y 4 , Z 1 , Z 2 , Z 3 , Z 4 : piezo-resistor 
     BEST MODE FOR CARRYING OUT OF THE INVENTION 
     Hereinafter, an acceleration sensor according to the present invention will be described in detail based on EXAMPLES with reference to drawings. 
     Example 1  
     An acceleration sensor device having an acceleration sensor of EXAMPLE 1 according to the present invention will be described with reference to  FIG. 1 to 12 .  FIG. 1  is an exploded perspective view of the acceleration sensor device of EXAMPLE 1 according to the present invention,  FIG. 2  is a plan view of the acceleration sensor for the acceleration sensor device of EXAMPLE 1,  FIG. 3  is an enlarged plan view of one of flexible arms extending in the X-axis direction of the acceleration sensor,  FIG. 4  is an enlarged plan view of one of flexible arms extending in the Y-axis direction of the acceleration sensor,  FIG. 5  is an enlarged cross-sectional view taken along V-V line of  FIG. 2 ,  FIG. 6  is an enlarged cross-sectional view taken along VI-VI line of  FIG. 2 ,  FIG. 7  is an enlarged cross-sectional view taken along VII-VII line of  FIG. 2 ,  FIG. 8  is an enlarged cross-sectional view taken along IIX-IIX line of  FIG. 2 ,  FIG. 9  is an enlarged cross-sectional view taken along IX-IX line of  FIG. 2 ,  FIG. 10  is an explanatory plan view of wires of the acceleration sensor of  FIG. 2 ,  FIG. 11  is an explanatory view of a full bridge circuit of X-axis piezo-resistors (Y-axis piezo-resistors) of  FIG. 10 , and  FIG. 12  is an explanatory view of a full bridge circuit of Z-axis piezo-resistors of  FIG. 10 . 
     In an acceleration sensor device of  FIG. 1 , an acceleration sensor  100  is adhered in a case  80  over a case inner bottom surface  84  with a bottom of a support frame  30  of the acceleration sensor  100  separated from an inner bottom surface  84  by a small gap, in which a small gap between the case inner bottom surface  84  and a weight  10  of the acceleration sensor  100  is formed. Each of sensor terminals  12   t,    11   t,    13   t,    31   t,    33   t,    23   t,    21   t,    14   t  of the acceleration sensor  100  is connected to a terminal  86  of the case  80  through a conductor  70 , and the terminal  86  of the case is connected to an external terminal  88  of the case inside the case. A voltage for measurement is applied to piezo-resistors of the acceleration sensor  100  from the external terminal  88 , or an output of the acceleration sensor  100  is taken out from the external terminal  88 . An upper regulation plate  60  is attached over the acceleration sensor  100  so as to cover its whole surface, with the upper regulation plate  60  separated from the acceleration sensor  100  by a small gap, preventing an excessive vibration/movement of the weight  10 . When an acceleration is applied to the weight  10 , the weight  10  vibrates and moves if the acceleration is within a certain range, but when an excessive acceleration is applied, the weight dose not vibrate beyond the amount of the small gaps or more between it and the upper regulation plate  60  as well as between it and the case inner bottom surface  84 . A case lid  90  is attached on the case  80 . 
     The acceleration sensor  100  has the weight  10  in a center of the acceleration sensor  100 , the support frame  30  surrounding the weight  10  and being at a predetermined distance from the weight  10 , and a flexible portion bridging an upper portion of the weight and an upper portion of the support frame and hanging the weight  10 . In this EXAMPLE, the acceleration sensor  100  has four flexible arms  21 ,  21 ′,  22 ,  22 ′ as the flexible portion. The acceleration sensor  100  is made of a silicon single crystal substrate with a SOI layer formed, i.e. a SOI wafer. The SOI is an abbreviation for Silicon On Insulator. In this EXAMPLE, a wafer, with a thin (for example, approximately 1 μm) SiO2 insulating layer serving as an etching stopper formed on the Si wafer having a thickness of approximately 410 μm and with an N-type silicon single crystal layer having a thickness of approximately 6 μm formed thereon, is used as a substrate. Four L-shaped through openings  150  are formed in the silicon single crystal substrate having a square shape that is as large as the support frame  30 ; the weight  10  in the center of the acceleration sensor  100 , the support frame  30  surrounding the weight  10 , and the flexible arms  21 ,  21 ′,  22 ,  22 ′ bridging therebetween are formed; as well as the thickness of a flexible arm portion therein is reduced. 
     The acceleration sensor  100  has piezo-resistors on the flexible arms and for each axis corresponding to two detection axes orthogonal to each other (the X-axis and the Y-axis) and a detection axis perpendicular to a top surface of the acceleration sensor (the Z-axis). Thus, piezo-resistors X 1 , X 2 , X 3 , and X 4  are provided on the flexible arms  21 ,  21 ′ extending in the X-axis direction, which detect an X-component of acceleration. Piezo-resistors Y 1 , Y 2 , Y 3 , and Y 4  are provided on the flexible arms  22 ,  22 ′ extending in the Y-axis direction, which detect a Y-component of acceleration. Further, piezo-resistors Z 1 , Z 2 , Z 3 , and Z 4  are provided on the flexible arms  21 ,  21 ′ extending in the X-axis direction, which detect a Z-component of acceleration. In this EXAMPLE, though the Z-component of acceleration is detected by the piezo-resistors provided on the flexible arms  21 ,  21 ′, piezo-resistors for detecting the Z-component of acceleration may be provided on the flexible arms  22 ,  22 ′. The piezo-resistors for detecting each axis component of acceleration constitute full bridge detection circuits as shown in  FIG. 11  or  12 , respectively. 
     In the acceleration sensor  100  of this EXAMPLE, the piezo-resistors X 1 , . . . , X 4 , Y 1 , . . . , Y 4 , Z 1 , . . . , Z 4  are respectively divided, so that divided piezo-resistors are constituted of piezo sub-resistors X 1   a,  X 1   b , . . . , X 4   a,  X 4   b,  Y 1   a,  Y 1   b , . . . , Y 4   a,  Y 4   b,  Z 1   a,  Z 1   b , . . . , Z 4   a,  Z 4   b.  Since the flexible arms connecting the weight  10  and the support frame  30  highly deform in a region adjacent to the weight  10  or the support frame  30  when an acceleration is applied to the weight, in order to increase sensitivity to an acceleration, each piezo sub-resistor is provided in a region adjacent to a boundary between the flexible arm and the weight or adjacent to a boundary between the flexible arm and the support frame where the flexible arm highly deforms. Their arrangements are shown in  FIG. 2 ,  FIG. 3 ,  FIG. 4  and  FIG. 10 . Each piezo sub-resistor is formed by implanting boron into the silicon layer constituting the flexible arms with its concentration of 1 to 3×10 18  atoms/cm 3 . High concentration diffusion layers X 1   c,  X 2   c,  X 3   c,  X 4   c,  Y 1   c,  Y 2   c,  Y 3   c,  Y 4   c,  Z 1   c,  Z 2   c,  Z 3   c,  and Z 4   c  are formed so as to connect between terminals on the center side of the flexible arm of two piezo sub-resistors constituting each piezo-resistor. These high concentration diffusion layers are formed with a concentration higher than the piezo sub-resistor, e.g. of 1 to 3×10 21  atoms/cm 3  by implanting boron. Since the piezo sub-resistor and the high concentration diffusion layer are formed by diffusing boron into the silicon layer, their mechanical properties are exactly the same as that of other portion of the flexible arm. Two piezo sub-resistor X 1   a  and X 1   b , . . . , Z 4   a  and Z 4   b  connected by the high concentration diffusion layers X 1   c , . . . , Z 4   c,  respectively, constitute the piezo-resistors X 1 , . . . , Z 4 . The piezo-resistors X 1 , X 2 , X 3 , and X 4  of the X-axis constitute a full bridge detection circuit as shown in  FIG. 11 , and a bridge output Vout is taken out from between the sensor terminals  11   t  and  13   t  by applying a measuring direct-current voltage Vcc between its sensor terminals  12   t  and  14   t.  The piezo-resistors Y 1 , Y 2 , Y 3 , and Y 4  of the Y-axis constitute a full bridge detection circuit as shown in  FIG. 11 , and a bridge output Vout is taken out from between the sensor terminals  21   t  and  23   t  by applying the measuring direct-current voltage Vcc between its sensor terminals  12   t  and  14   t.  The piezo-resistors Z 1 , Z 2 , Z 3 , and Z 4  of the Z-axis constitute a full bridge detection circuit as shown in  FIG. 12 , and a bridge output Vout is taken out from between the sensor terminals  31   t  and  33   t  by applying the measuring direct-current voltage Vcc between its sensor terminals  12   t  and  14   t.    FIG. 10  shows a plan view in which the piezo sub-resistors of the X-axis, the piezo sub-resistors of the Y-axis and the piezo sub-resistors of the Z-axis as well as the sensor terminals  11   t,    12   t,    13   t,    14   t,    21   t,    23   t,    31   t,    33   t  are represented on the top surface of the acceleration sensor  100 . Sensor terminals in  FIGS. 11 and 12  correspond to sensor terminals as shown in  FIG. 10 , respectively. The metal wires  25  such as aluminum are connected between the terminals of these piezo-resistors or between the terminals of the piezo-resistors and the sensor terminals. 
       FIG. 2  shows a view similar to  FIG. 10 , but  FIG. 2  represents that the metal wire  25  is placed, on the flexible arms  21 ,  21 ′,  22 ,  22 ′, in the groove  26  formed on the flexible arms  21 ,  21 ′  22 ,  22 ′. Moreover, the metal wire  25  is placed, on the top surface of the support frame  30 , in a groove  36  formed on the top surface of the support frame  30 , as well as placed, on the weight  10 , in the groove  16  formed on the top surface of the weight  10 .  FIG. 10  shows the same structure as that of  FIG. 2 , but illustrations of the grooves  16 ,  26 ,  36  are omitted in  FIG. 10  to show reference characters of the piezo sub-resistors.  FIGS. 3 and 4  show an enlarged plan view of the flexible arm  21  and the flexible arm  22  of  FIG. 2 , respectively.  FIGS. 5 to 9  show an enlarged cross-sectional view in respective V-V line, VI-VI line, VII-VII line, IIX-IIX line, and IX-IX line of  FIG. 2 . As is obvious from their cross-sectional views, cross-section shapes of the grooves  26 ,  36  are rectangular, but they may be an inverted trapezoid with an upper portion of the groove open. A cross-sectional view of the weight  10  is not shown, but a cross-section shape of the groove  16  on the top surface of the weight  10  is a rectangle. The cross-section shape of the groove  16  thereon may be an inverted trapezoid shape with the top portion of the groove  16  open. 
     As shown in the cross-sectional views of  FIGS. 5 to 7 , formed is an electrically insulating layer  28  of silicon dioxide surrounding the silicon layer  24  (including the piezo sub-resistors Z 1   a,  X 1   a,  X 1   b,  Z 1   b  as shown in  FIG. 5  and the high concentration diffusion layer Z 1   c  as shown in  FIG. 6 ) constituting the flexible arm  21 . Since the single-crystal silicon constituting the flexible arm is usually N-type or P-type and has small electric resistivity of 1 to 100Ω·cm, it is necessary to form the electrically insulating layer  28  on a bottom and side wall of the groove  26  in which the metal wire  25  is disposed and to insulate the metal wire  25  from the silicon layer  24 . The electrically insulating layer  28  has a thickness of 0.1 μm, but may have a thickness of 0.02 to 0.8 μm. The metal wire  25  of aluminum formed by sputtering is put in the groove  26 . In a connection portion of the piezo sub-resistor with the metal wire  25 , e.g. in a left end of the piezo sub-resistor X 1   a  of  FIG. 3 , a through hole is formed in the electrically insulating layer  28  on an end of the piezo sub-resistor to connect, and then a metal wire  25  of aluminum is formed by sputtering, so that the connection of the piezo sub-resistor with the metal wire  25  can be insured. In this EXAMPLE, the groove  26  has a bottom width of 4 μm and a depth of 0.3 μm. Since the electrically insulating layer  28  is also formed on the silicon layer  24  of the flexible arm  21 , the groove  26  has a depth of 0.3 μm from a top surface of the electrically insulating layer  28 . Since the metal wire  25  having a width of 3 μm and a thickness of 0.2 μm is formed in the groove  26 , a top surface of the metal wire  25  is 0.1 μm lower than the top surface of the electrically insulating layer  28  that is on the top surface of the flexible arm  21 . In the present invention, the top surface of the metal wire  25  is preferably at least 0.05 μm lower than the top surface of the electrically insulating layer  28  of the flexible arm. If the top surface of the metal wire  25  is at least 0.05 μm lower than the top surface of the electrically insulating layer  28  of the flexible arm, the metal wire  25  does not make contact with the upper regulation plate  60  when the flexible arm is deformed. There in no problem associated with preventing contact of the metal wire  25  with the upper regulation plate  60  even if the top surface of the metal wire  25  is lowered at any depth from the top surface of the flexible arm. However, it is necessary to increase the depth of the groove  26  to lower it. Therefore, it is preferable that the depth from the top surface of the flexible arm to the top of the metal wire  25  is within 0.5 μm. 
     In this EXAMPLE, the groove  26  has a depth of 0.3 μm. The flexible arm is formed of an N-type silicon single crystal layer, i.e. silicon layer having a thickness of 6 μm covering the SiO2 layer. A ratio of the depth of the groove to the thickness of the flexible arm is approximately 5%. In the present invention, the ratio of the depth of the groove to the thickness of the flexible arm is preferably 15% or less. If this ratio is more than 15%, the strength of the flexible arm decreases. 
     As above described, the metal wire  25  having the width of 3 μm is formed in the groove  26  having the bottom width of 4 μm. When the metal wire has contact with the side wall of the groove, a stress occurs in the metal wire by temperature change and the metal wire is disposed in a thin region of the electrically insulating layer at a corner of the bottom of the groove, which should be preferably avoided. Accordingly, in the present invention, a ratio of the bottom width of the groove to the width of the metal wire is preferably 110% or more. 
     As shown in a cross-sectional view of  FIG. 6 , though the groove  26  is formed across the high concentration diffusion layer Z 1   c,  since a depth of the high concentration diffusion layer Z 1   c,  formed in the silicon layer  24 , from the top surface of the flexible arm is 1 to 1.5 μm, which is considerably larger than a depth of the groove  26 , e.g. 0.3 μm, the high concentration diffusion layer Z 1   c  dose not cut by the groove  26 . Moreover, since the electrically insulating layer  28  is provided between the high concentration diffusion layer Z 1   c  and the metal wire  25 , electrical insulation is insured therebetween. 
     As shown in  FIGS. 4 and 8 , the flexible arm  22  in the Y-axis direction is provided with two piezo-resistors Y 1 , Y 2  (piezo sub-resistors Y 1   a,  Y 1   b,  Y 2   a,  Y 2   b ) and two metal wires  25 . Since  FIGS. 4 and 8  illustrating the flexible arm  22  are the same as  FIGS. 3 and 5  illustrating the flexible arm  21  in the X-axis direction except the number of the piezo sub-resistors and the number of metal wires, its explanation will be omitted. 
     The weight  10  in the center of the acceleration sensor  100  is composed of the SOI wafer where the Si wafer is covered with the SiO2 insulating layer and the N-type silicon single crystal layer (silicon layer). An electrically insulating layer is formed on a top surface of the silicon layer. As shown in  FIG. 2 ,  FIG. 3 ,  FIG. 4  and  FIG. 10 , the metal wire on the flexible arm extends to the top surface of the weight  10 , and then is connected with the metal wire and/or the high concentration diffusion layer on the top surface of the weight  10 . The groove  16  is formed along a section of the metal wires on the weight  10 , and the metal wire  25  is put therein. In this EXAMPLE, the groove is formed in the silicon layer on the weight. An electrically insulating layer is provided on both inner side walls and a bottom surface of the groove  16 , so that the metal wire and the silicon layer are electrically insulated from each other by the electrically insulating layer. When an acceleration is applied to the acceleration sensor, the top surface of the weight  10  of the acceleration sensor  100  is most likely to collide with the upper regulation plate. Therefore, the top surface of the metal wire on the top surface of the weight is lower than the top surface of the weight. In the top surface of the weight as well as in the top surface of the flexible arm, it is preferable that the top surface of the metal wire is at least 0.05 μm lower than the top surface of the electrically insulating layer of the weight. 
     In the section in which the metal wire  25  is drawn out to the top surface of the support frame  30 , as a cross-section of the support frame shown in  FIG. 9 , a broad groove  36  receiving a plurality of metal wires  25  may be provided, if desired. As flexible arms  21 ,  22  shown in  FIGS. 3 to 8 , each flexible arm has a structure symmetric with respect to a centerline CL extending in the length direction thereof. As can be understood from an arrangement of the metal wires  25  of  FIG. 3 , either a top metal wire or a second metal wire from the top metal wire is sufficient for electrical wiring, but two metal wires are respectively arranged on both sides of the centerline CL so as to have a structure symmetric with respect to centerline CL. The same applies to the flexible arm  21 ′ arranged on a right side in  FIG. 2 . 
     In the acceleration sensor  100  of EXAMPLE 1 described herein, since the metal wire on the weight and the flexible arm is put in the grooves formed in the weight or the flexible arm as well as the top surface of the metal wire put in the groove is lower than the top surface of the weight and the top surface of the flexible arm, even when the weight and the flexible arm collide violently with the upper regulation plate by an excessive acceleration or impact applied to the acceleration sensor, deformation in the metal wire does not occur and thus an offset voltage does not occur. 
     Example 2 
     An acceleration sensor of EXAMPLE 2 according to the present invention will be described with reference to  FIGS. 13 and 14 . The acceleration sensor of EXAMPLE 2 has flexible arms  23 ,  23 ′ in the Y-axis direction as shown in  FIGS. 13 and 14 , instead of the flexible arms  22 ,  22 ′ in the Y-axis direction that is included in the acceleration sensor  100  of EXAMPLE 1. Two flexible arms  23 ,  23 ′ as shown in  FIG. 13  have metal wires  25   c,    25   c ′ arranged in grooves  26  along their centerline CL. A terminal on a support frame side of a piezo sub-resistor Y 1   a  is connected, through the metal wire  25   c  along the centerline CL of the flexible arm  23  and then through the metal wire  25   c ′ along the centerline of the flexible arm  23 ′, to a sensor terminal  21   t  formed on an opposite support frame  30 . A metal wire  25   d  on a right side of the flexible arm  23  is a dummy, and one end of the metal wire  25   d  is opened. The grooves  26  and metal wires  25 ,  25   c,    25   c ′,  25   d  formed on the two flexible arms  23 ,  23 ′ are symmetric with respect to the centerline CL of the flexible arms  23 ,  23 ′. In EXAMPLE 1, the metal wire drawn out from a terminal on a support frame side of the piezo sub-resistor Y 1   a  travels halfway around the acceleration sensor on the support frame  30  and then is connected to the sensor terminal  21   t,  on the other hand, in EXAMPLE 2, the metal wire drawn out from the terminal on the support frame side of the piezo sub-resistor Y 1   a  is connected, through and on the two flexible arms  23 ,  23 ′ extending in the Y-axis direction, to the sensor terminal  21   t.    
     Example 3 
     Since an appearance of an acceleration sensor of EXAMPLE 3 is the same as shown in  FIG. 1 , the acceleration sensor will be described with reference to  FIG. 1 . The acceleration sensor of EXAMPLE 3 has an electrically insulating layer  28 ′ of silicon dioxide having a thickness of 0.8 μm on a top surface of a silicon layer  24 , and a groove  26 ′ is formed in the electrically insulating layer  28 ′.  FIG. 15  shows a cross-sectional view of the flexible arm  21 ″ extending in the Y-axis direction of the acceleration sensor. The groove  26 ′ has a shape of an inverted trapezoid having a bottom width of 6 μm and a depth of 0.4 μm, in which a metal wire  25  having a width of 3 μm and a thickness of 0.15 μm is provided. A top surface of the metal wire  25  is 0.25 μm lower than a top surface of the electrically insulating layer  28 ′. Also at a bottom of the groove  26 ′, there is the electrically insulating layer  28 ′ having a thickness of 0.4 μm between the bottom of the groove  26 ′ and the silicon layer  24 , so that the metal wire  25  and the silicon layer  24  are electrically insulated from each other. Piezo sub-resistors Y 1   a,  Y 1   b  are formed adjacent to the top surface of the silicon layer  24 , and a top of the piezo sub-resistors Y 1   a,  Y 1   b  are also covered with the electrically insulating layer  28 ′. As is the case with the flexible arm  21 ″, the electrically insulating layer  28 ′ of silicon dioxide having a thickness of 0.8 μm is formed on a top surface of a weight thereof, a groove is formed in the electrically insulating layer  28 ′, and the metal wire is disposed in the groove. The top surface of the metal wire in a top surface of the weight is 0.25 μm lower than the top of the electrically insulating layer. 
       FIG. 16  shows a longitudinal sectional view taken along XVI-XVI line of  FIG. 15 , and  FIG. 17  shows a longitudinal sectional view taken along XVII-XVII line of  FIG. 15 . As shown in  FIG. 16 , at an end on a side of the support frame  30  of the piezo sub-resistor Y 1   a,  a through hole is formed in the electrically insulating layer  28 ′ located on the piezo sub-resistor Y 1   a  and a part of a bottom of the metal wire  25  formed in the groove  26 ′ in the electrically insulating layer  28 ′ is connected to the end of the piezo sub-resistor Y 1   a  via the through hole. As shown in this figure, the top surface of the metal wire  25  is lower than the top surface of the electrically insulating layer  28 ′ at the end on the side of the support frame  30  of the piezo sub-resistor Y 1   a,  on the other hand, the top surface of the metal wire  25  is at the same level as the top surface of the electrically insulating layer  28 ′ on a center side of the support frame  30 . The weight and the flexible arm are displaced by an externally-applied acceleration, but the support frame  30  is not displaced. Therefore, even when the top surface of the metal wire  25  is at the same level as the top surface of the electrically insulating layer  28 ′ on the support frame  30 , the metal wire  25  does not collide with the upper regulation plate in an area of the support frame  30 .  FIG. 17  is a longitudinal sectional view of the central metal wire  25   c,  and shows that the electrically insulating layer  28 ′ lies between the high concentration diffusion layer Y 1   c  connecting two piezo sub-resistors Y 1   a  and Y 1   b  and the metal wire  25   c.    
     Example 4 
       FIG. 18  shows a plan view of an acceleration sensor  400  of EXAMPLE 4. The acceleration sensor  400  has a diaphragm  29  as a flexible portion, and a weight  10  is held in a center of a support frame  30  by the diaphragm  29 . Since the acceleration sensor  400  having the diaphragm  29  as the flexible portion instead of the flexible arms also serves as the acceleration sensor  100  of EXAMPLE 1, its detailed explanation will be omitted. 
     Example 5 
     One hundred acceleration sensor devices, each of which comprising the acceleration sensor of EXAMPLE 1, as well as one hundred acceleration sensor devices, each of which comprising conventional acceleration sensor with no groove on a top surface of the acceleration sensor and with a metal wire disposed on a top surface of a weight and a top surface of flexible arm thereof were produced. For their samples, following steps were performed: (a) measurement of an offset voltage (measurement of an output voltage with no applied acceleration); (b) application of an impact; and then (c) measurement of an offset voltage. In the measurement of the offset voltage after the application of the impact, samples in which the offset voltage was changed by ±10% or more as compared with an initial value were disassembled and then a state of a metal wire thereof was inspected. For samples in which the offset voltage was changed by less than ±10%, application of an impact and measurement of offset voltage were repeated 50 times. In the impact test, the acceleration sensor device is fixed on an iron jig having a thickness of 2 mm, and was naturally dropped from a height of 1 m on a wooden board having a thickness of 100 mm, so that an impact of 1500 to 2000 G was applied thereto. A direction of impact was Z-axis of the acceleration sensor. 
     In the acceleration sensor device of EXAMPLE 1, even when the impact tests were repeated 50 times, there was no acceleration sensor device in which the offset voltage was changed by ±10% or more. However, in six of the conventional acceleration sensor devices, the offset voltage fluctuated by more than ±10%. When these six acceleration sensor devices ware disassembled and inspected, all the metal wires was partially deformed. In five of them, metal wires were deformed on the flexible arm in a region adjacent to the weight, and in one of them, the metal wire on the weight was deformed. From this result, it can be confirmed that in the acceleration sensor according to the present invention, even when an excessive impact was applied, deformation in the metal wire does not occur, and thus occurrence of an offset voltage can be prevented. 
     One hundred acceleration sensor devices with an IC chip as the upper regulation plate using the acceleration sensor of EXAMPLE 1 were produced, and when an excessive acceleration was applied, whether or not a latch-up phenomenon occurred was evaluated. The same impact test as above was performed, confirming either a presence or absence of a latch-up phenomenon by measuring the output thereof. Even when impact tests were repeated 10 times, the latch-up phenomenon did not occur. From this, in the acceleration sensor according to the present invention, since the acceleration sensor had the structure that the metal wire did not make contact with the upper regulation plate, not only occurrence of an offset voltage depending on deformation of the metal wire, but also occurrence of a latch-up phenomenon can be also prevented. 
     INDUSTRIAL APPLICABILITY 
     The acceleration sensor for detecting an acceleration with the piezo-resistors is broadly used for automobiles, aircraft, home electric appliances, industrial machinery and the like. Even when an acceleration is not applied to the acceleration sensor, an output occurs. If the offset voltage is constant, the output can be canceled using a compensation circuit. However, when an excessive impact is applied to the acceleration sensor, the offset voltage may fluctuate. Since the acceleration sensor according to the present invention has a structure in which the metal wires on the weight or the flexible portion are placed in the groove or the groove formed on the flexible portion so that the metal wire does not collide with the upper regulation plate even when the weight collides with the upper regulation plate, fluctuation in the offset voltage can be prevented. The acceleration sensor having such structure is needed in industry.