Abstract:
A microminiature and thin semiconductor acceleration sensor with high sensitivity is provided. The acceleration sensor has a mass portion formed in a center part of a silicon semiconductor substrate, a frame formed at a perimeter portion of the substrate, thin elastic support arms, which are provided at upper part of the mass portion and the frame and connect the mass portion and the frame, and a plurality of pairs of piezoresistors disposed on top surface sides of the elastic support arms. At least one of the mass portion and the thick frame has a cross section vertical to a respective top surface, spreading in width from the respective top surface toward a respective bottom surface. Since a side length of the mass portion and/or a width of the frame at a site, where the elastic support arms each is connected, are made short, the elastic support arm is made long, hereby the sensitivity of the sensor is enhanced.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.  
           [0003]    2. Description of the Related Art  
           [0004]    Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.  
           [0005]    Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.  
           [0006]    As a conventional triaxial acceleration sensor, there is the one disclosed in, for example, Japanese Laid-Open Patent No. 63-169078, and its plan view is shown in FIG. 11, and a sectional view taken along the XII-XII line in FIG. 11 is shown in FIG. 12, and a perspective view is shown in FIG. 13. The acceleration sensor  200  has elastic support arms  230  each of a beam structure, constituted by a thin portion of a silicon single crystal substrate. A mass portion  220  in a center, which is constituted by a thick portion of a silicon single crystal substrate, and a frame  210  in a periphery thereof are connected by the elastic support arms  230 . A plurality of strain gauges  240  are formed in each axial direction on the elastic support arms  230 .  
           [0007]    An entire structure will be explained, referring to FIG. 11, FIG. 12 and FIG. 13. The sensor  200  has the mass portion  220  constituted by the thick portion of the silicon single crystal substrate, a frame  210  placed to surround the mass portion  220 , and two pairs of elastic support arms  230  in a beam form, which are perpendicular to each other and each constituted by the thin portion of the silicon single crystal substrate to bridge the mass portion  220  and the frame  210 . When the acceleration works, the mass portion moves in the frame to deform the elastic support arms, and thus the deformation is detected by the strain gauges provided on the elastic support arms to obtain the acceleration that works. The acceleration in an X-axis direction in FIG. 11 is measured by the four strain gauges  240  provided on the elastic support arms extending in the X-axis direction, and the acceleration in a Y-axis direction is measured by the four strain gauges  240  provided on the elastic support arms extending in the Y-axis direction. The acceleration in a Z-axis direction is measured by means of all the strain gauges  240 . By making four L-shaped through-holes  250  in the silicon single crystal substrate having the size of the frame  210 , the mass portion  220  in the center, the frame  210  in the periphery and the support arms  230  bridging them are formed, and by making the support arm portions thin, the acceleration sensor is constructed to be deformable and highly sensitive.  
           [0008]    In the above-described acceleration sensor, to enhance sensitivity, it is effective to increase the volume of the mass portion  220  to increase the mass, or to increase the length of the elastic support arms  230 , and as is generally well-known, the sensitivity increases substantially in proportion to the mass of the mass portion and the length of the support arms. That is, the volume of the mass portion  220  is increased, or the length of the elastic support arms  230  are increased, whereby the elastic support arm  230  becomes more deformable and the stress can be effectively transmitted to the strain gauges, thus enhancing sensitivity. However, increasing the mass portion  220  and increasing the length of the elastic support arms  230  are mutually contradictory, and both of them are not mutually compatible especially under the condition that the chip size is kept constant, or when reduction in size is planned. That is, if the mass portion  220  is made larger, the length of the elastic support arms  230  becomes smaller, and a great deal of improvement in sensitivity cannot be improved. Thus, glass pieces and the like are bonded on the back surface of the mass portion  220  in assembly process to increase the volume (that is, weight) of the mass portion  220 , whereby the sensitivity is enhanced. The length of the elastic support arms  230  cannot be made large, while the chip is made larger in the thickness direction (the thickness direction of the silicon single crystal substrate), whereby the mass of the mass portion  220  is increased to enhance the sensitivity. Accordingly, it is conventionally impossible to realize a compact and thin acceleration sensor with high sensitivity.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is accomplished in view of the above-described circumstances, and its object is to solve the above-described problem and provide a compact and thin acceleration sensor capable of enhancing sensitivity.  
           [0010]    In order to solve the above-described problem, the present invention adopts an acceleration sensor as follows. That is, the acceleration sensor of the present invention comprises a mass portion provided in a center; a thick frame surrounding the mass portion with a predetermined distance from the mass portion; a plurality of elastic support arms extending from an edge of a top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of a top surface of the thick frame and hanging the mass portion inside of the thick frame; and a plurality of strain gauges disposed on the elastic support arms. The mass portion has the top surface, a bottom surface opposite to the top surface and a plurality of side walls surrounding the mass portion between the top surface and the bottom surface of the mass portion. The thick frame has the top surface, a bottom surface opposite to the top surface and inside walls on inside surfaces, facing the mass portion, of the thick frame between the top surface and the bottom surface of the thick frame.  
           [0011]    The acceleration sensor is made of a silicon single crystal wafer, preferably a SOI (Silicon-on-insulator) wafer, and can be constructed to have a thick-walled frame with its plane shape being substantially a square, a mass portion which is provided at a center of the thick-walled frame and is formed to be substantially a square, and four elastic support arms which connect centers of sides of the square on the top surface of the mass portion and centers of inner sides of the thick-walled frame in a square shape which is on the top surface of the thick frame. In the acceleration sensor made of a silicon single crystal wafer or a SOI wafer, the top surface of the thick-walled frame, the top surface of the mass portion and the top surfaces of the four elastic support arms are formed by using the surface of one side of the wafer, and therefore they are on substantially the same surface. A bottom surface of the thick-walled frame and a bottom surface of the mass portion are formed by using a surface of the other side of the wafer, and therefore they are on substantially the same surface. The elastic support arm is formed to be thin by being cut by etching or the like from the surface of the other side of the wafer, and therefore when it is made of the SOI wafer, it is formed of a remaining SiO 2  layer, or a laminated product of the SiO 2  layer and a silicon layer.  
           [0012]    In the acceleration sensor of the present invention, at least either of the mass portion or the thick-walled frame expands in its width from the top surface toward the bottom surface, in the cross section perpendicular to its top surface. When the cross section perpendicular to the top surface of the square mass portion expands in the width from the top surface to its bottom surface, one side of the square on the bottom surface is longer than one side of the square on the top surface. When the vertical cross section of the thick-walled frame expands in its width from its top surface to its bottom surface, the width of the bottom surface is longer than the width of the top surface of the thick-walled frame. It is preferable that in the acceleration sensor of the present invention, the mass portion extends in the width from the top surface toward the bottom surface in the cross section perpendicular to its top surface, and the thick-walled frame expands in the width from its top surface toward its bottom surface in the cross section perpendicular to its top surface.  
           [0013]    The dimension of the mass portion and/or the thick-walled frame on the top surfaces is made small and the dimension on the bottom surfaces is made large, whereby the length of the elastic support arm can be made larger without reducing the mass of the mass portion, therefore making it possible to increase sensitivity with which acceleration is detected. If the dimension of the mass portion and the thick-walled frame on their top surfaces is made small and the dimension on their bottom surfaces is made large, the elastic support arms can be extended on the side of the mass portion and on the side of the thick-walled frame. Alternatively, as for only one of the mass portion and the thick-walled frame, the dimension on the top surface can be made small and the dimension on the bottom surface can be made large. However, when the dimension of one of them is changed, it is preferable to apply the present invention to the mass portion side. If the dimension of the thick-walled frame on its top surface is made small and the dimension on its bottom surface is made large, a kind of notch is formed in a portion of the thick-walled frame at which the elastic support arm is attached. If the dimension on its top surface is made extremely smaller as compared with the dimension on its bottom surface, the notch formed at the portion of the thick-walled frame at which the elastic support arm is attached is deepened, which causes the fear of reducing mechanical strength of a base supporting the elastic support arm.  
           [0014]    Expressing the acceleration sensor of the present invention in other words, on at least one of each side wall of the mass portion and each inner wall of the thick-walled frame, the angle, which is formed by a plane passing through the place on the wall at which the elastic support arm is attached and the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees. Preferably, the angle, which is formed by the plane passing through the place on the wall, at which the elastic support arm is attached, on each side wall of the mass portion and passing the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is  80  degrees or larger and smaller than 90 degrees, and the angle, which is formed by a plane passing through a place on the wall at which the elastic support arm is attached on each of the inner wall of the thick-walled frame and passing the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees.  
           [0015]    Further expressing the acceleration sensor of the present invention in other words, the angle, which is formed by at least one of each side wall of the mass portion and each inner wall of the thick-walled frame, and the bottom surface corresponding to the wall, is 80 degrees or larger, and smaller than 90 degrees. Preferably, the angle, which is formed by each side wall of the mass portion, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees, and the angle, which is formed by each of the inner walls of the thick-walled frame and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a plan view showing an acceleration sensor of EXAMPLE of the present invention;  
         [0017]    [0017]FIG. 2 is a sectional view of the acceleration sensor in FIG. 1 taken along the II-II line in FIG. 1;  
         [0018]    [0018]FIG. 3 is a graph showing a relationship between length of elastic support arms and angles of side walls and inside walls with bottoms in the acceleration sensor of the present invention;  
         [0019]    [0019]FIG. 4 is a graph showing a relationship between sensitivity and length of elastic support arms of the acceleration sensor of the present invention;  
         [0020]    [0020]FIG. 5 is a schematic view explaining deformation of the acceleration sensor when acceleration works in an X-axis (Y-axis) direction;  
         [0021]    [0021]FIG. 6 shows an example of a bridge circuit for measuring acceleration in the X-axis (Y-axis) direction;  
         [0022]    [0022]FIG. 7 is a schematic view explaining deformation of the acceleration sensor when acceleration works in a Z-axis direction;  
         [0023]    [0023]FIG. 8 shows an example of a bridge circuit for measuring acceleration in the Z-axis direction;  
         [0024]    [0024]FIGS. 9A through 9E are explanatory views of a production process of the acceleration sensor of EXAMPLE of the present invention;  
         [0025]    [0025]FIGS. 10A through 10D are sectional views of acceleration sensors according to other EXAMPLES of the present invention;  
         [0026]    [0026]FIG. 11 is a plan view showing an acceleration sensor disclosed in a Japanese Laid-Open Patent;  
         [0027]    [0027]FIG. 12 is a sectional view of the acceleration sensor in FIG. 11 taken along the XII-XII line in FIG. 11; and  
         [0028]    [0028]FIG. 13 is a perspective view of the acceleration sensor in FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    An acceleration sensor of an EXAMPLE of the present invention will be explained with reference to FIGS. 1 and 2. FIG. 1 is a plan view of the acceleration sensor. FIG. 2 is a sectional view taken along the line II-II in FIG. 1.  
         [0030]    The acceleration sensor of the present invention uses a silicon single crystal substrate with a SOI layer being formed via a SiO 2  insulation layer, namely, a SOI wafer, in order to make it possible to control the thickness of elastic support arms with high precision. The SOI is an abbreviation of a Silicon On Insulator. In this example, a wafer formed by thinly forming the SiO 2  insulation layer being an etching stopper (about 1 μm) on an Si wafer with thickness of about 625 μm, on which an N-type silicon single crystal layer with thickness of about 10 μm is formed, is used as a substrate. In an accelerator sensor  100  of the EXAMPLE, four L-shaped through-holes  150  in a square-shaped silicon single crystal substrate of the same size as a frame  110  are made, whereby a mass portion  120  in a center, a thick-walled frame  110  around it, and support arms placed between them are formed, and by making the portions of the support arms thin to make elastic support arms  131 ,  132 ,  133  and  134 . The acceleration sensor  100  has distortion gauges (in the explanation below, a “piezoresistor” as an example of a distortion gauge is used, and therefore they are called “piezoresistors”)  11 ,  12 , . . . ,  33 ,  34 , which are placed on the elastic support arms to be corresponding to two perpendicular detection axes (axes X and Y) and a detection axis (axis Z) perpendicular to a top surface of the acceleration sensor, with four distortion gauges for each axis. Namely, on the elastic support arms  131  and  133  extending in the X-axis direction, pairs of piezoresistors  11  and  12 , and  13  and  14  are provided respectively to detect acceleration in the X-axis direction. Pairs of the piezoresistors  21  and  22 , and  23  and  24  are provided respectively on the elastic support arms  132  and  134  extending in the Y-axis direction to detect acceleration in the Y-axis direction. Further, pairs of piezoresistors  31  and  32 , and  33  and  34  are provided respectively on the elastic support arms  131  and  133  extending in the X-axis direction to detect acceleration in the Z-axis direction. In this example, the acceleration in the Z-axis direction is detected with the piezoresistors provided on the elastic support arms  131  and  133 , but the element detecting the acceleration in the Z-axis direction may be provided on the elastic support arms  132  and  134 . The four piezoresistors for detecting the acceleration in each axis direction construct separate full bridge detection circuits.  
         [0031]    Mainly referring to FIG. 2, a shape of a section perpendicular to a top surface of the acceleration sensor will be explained. In the acceleration sensor  100 , the sectional shape of the mass portion  120  and the sectional shape of the thick-walled frame  110  are gradually expanding from a top surface where the elastic support arms  131  and  133  are connected toward a bottom surface with an angle θ. Since a side wall  123  of the mass portion and an inner wall  113  of the thick-walled frame, which opposes to the mass portion, each have the angle θ with respect to the bottom surfaces  122  and  112 , the length of the elastic portions  131  and  133  can be made longer.  
         [0032]    For explanation, in FIG. 2, a length of one side of an outer frame  110  of the acceleration sensor is w, a length of a side of the mass portion  120  on its top surface is f, a length of the side on its bottom surface is f′, a length of the side at intermediate height is f″, a length of each of the elastic support arms  131  and  133  is k, a width of the thick-walled frame  110  on its top surface is j, and the width on its bottom surface is j′. The one side of the thick-walled frame w is the total of the side length f of the top surface of the mass portion  120 , the length twice as long as the elastic support arm length k, and the width twice as long as the width j on the top surface of the frame portion. Since the side wall  123  of the mass portion extends at a certain angle with respect to the bottom surface  122 , f&lt;f″&lt;f′ holds good, and since the inner wall  113  of the thick-walled frame  110  extends at a certain angle with respect to the bottom surface  112 , j&lt;j′ holds good.  
         [0033]    If the length f′ of the side at the bottom surface of the mass portion  120  is fixed, the side wall  123  of the mass portion has an angle θ with respect to the bottom surface  122 , and thereby the length f of the side on the top surface of the mass portion becomes short. If the lower surface width j′ of the thick-walled frame  110  is fixed, the inner side wall  113  of the thick-walled frame has the angle θ with respect to the bottom surface  112 , and thereby the width j on the bottom surface of the thick-walled frame becomes short. If the length w of the one side of the thick-walled frame of the acceleration sensor is fixed, the side wall of the mass portion and the inner wall of the frame have the angle θ with respect to the bottom surface, whereby the length f of the top surface side of the mass portion and the bottom surface width j of the frame become shorter, and therefore the length k of the elastic support arm becomes longer to that extent.  
         [0034]    When the side wall  123  of the mass portion and the inner wall  113  of the thick-walled frame are inclined to the bottom surfaces  122  and  112  at the angle θ, the length f″ of the side at the intermediate height of the mass portion is fixed so as to keep the mass of the mass portion  120  substantially the same as the mass in the case where the side wall  123  is not inclined. And the side wall  123  of the mass portion  120  is inclined with an end of the side at the intermediate height as the center to shorten the length f of the top surface side of the mass portion and increase the length f′ of the bottom surface side of the mass portion. Even if the inner wall  113  is inclined and the volume of the thick-walled frame  110  is changed, it does not influence the detection sensitivity of the acceleration sensor, and therefore the inner wall  113  is inclined while the bottom surface width j′ of the thick-walled frame is fixed to form the angle θ with respect to the bottom surface  112 . When the side wall  123  of the mass portion  120  and the inner wall  113  of the thick-walled frame  110  are inclined at the angle  0  with respect to the bottom surfaces  122  and  112 , the length f of the top surface side of the mass portion and the top surface width j of the thick-walled frame become smaller, and therefore even if the side length w of the frame of the acceleration sensor is fixed, the length k of the elastic support arms  131  and  133  becomes longer to that extent.  
         [0035]    [0035]FIG. 3 shows the relationship between the length of the elastic support arm (relative value) and the angle θ of inclination of the side wall and the inner wall with respect to their bottom surfaces when the angle θ at which the side wall of the mass portion and the inner wall of the thick-walled frame are each inclined with respect to their bottom surfaces is changed from 90 degrees to 81 degrees in the acceleration sensor that is made with use of a silicon wafer of the thickness of 625 μm. In the acceleration sensor made with use of the silicon wafer of the thickness of 625 μm, the thickness of the elastic support arm on its top surface is about 10 μm, and therefore the height of the mass portion and the thick-walled frame becomes substantially 625 μm. Thus, when the inclined angle θ with respect to the bottoms of the side wall and the inner wall is made smaller to 84 degrees, the length of the elastic support arm can be made longer by about 130 μm if the bottom surface side length of the mass portion is fixed. If the length k of the elastic support arm of the acceleration sensor, in which the side wall of the mass portion and the inner wall of the thick-walled frame are perpendicular to their bottom surfaces, is assumed to be 400 μm, the length of each elastic support arm becomes about 1.3 times. The influence of the length of the elastic support arm on the sensitivity of the acceleration sensor is proportional to the length of the elastic support arm as shown in FIG. 4. In this EXAMPLE, the length of the elastic support arm can be increased by 30%, and therefore the detection sensitivity can be improved by about 30%.  
         [0036]    When the side length is fixed at the intermediate height of the mass portion and the mass portion side wall is inclined with the side end as the center to make the angle θ with respect to its bottom surface small to 84 degrees to fix the mass of the mass portion, the length of the elastic support arm can be made longer by about 100 μm. If the length k of the elastic portion is 400 μm when the side wall is perpendicular, the length of each of the elastic support arm becomes about 1.25 times.  
         [0037]    In the above explanation, the side wall of the mass portion and the inner wall of the frame are inclined at the same angle, but it is natural that the angle of the side wall of the mass portion and the angle of the inner wall of the thick-walled frame can be made different, and if they are made different, the aforementioned effect of the present invention can be also obtained.  
         [0038]    It is preferable that the side wall of the mass portion and the inner wall of the frame are each inclined at the angle of 80 degrees or more and less than 90 degrees. It is difficult to make the angle of inclination smaller than 80 degrees in forming the side wall by dry etching. If the angle of inclination is made smaller than 80 degrees, a notch formed between the elastic support arm and the side wall at the portion of the side wall where the elastic support arm is attached is deepened, and the mechanical strength may reduce at that portion. It is obviously preferable to make the angle of inclination smaller than 90 degrees, and it is further preferable to make it smaller than 88 degrees. If it is made smaller than 88 degrees, the length of the elastic support arm is increased by 1.1 times.  
         [0039]    The function of the piezoresistors provided at the elastic support arms will be explained here. The elastic support arms  131  and  133  extending in the X-axis direction are provided with the piezoresistors  11  and  12 , and  13  and  14  for measuring the acceleration in the X-axis direction. The elastic support arms  132  and  134  extending in the Y-axis direction are provided with the piezoresistors  21  and  22 , and  23  and  24  for measuring the acceleration in the Y-axis direction. The elastic support arms  131  and  133  extending in the X-axis direction are also provided with the piezoresistors  31  and  32 , and  33  and  34  for measuring the acceleration in the Z-axis direction.  
         [0040]    When the acceleration in the X-axis direction is exerted on the mass portion  120 , a force Fx works on the center of gravity of the mass portion  120 , and therefore as shown in FIG. 5, the left side of the top surface of the mass portion  120  tilts downward and the right side of the top surface thereof tilts upward. As a result, the piezoresistor  11  on the elastic support arm  131  extends, and the piezoresistor  12  contracts. Since, in the circuit in FIG. 6, the resistance of the piezoresistor  11  is expressed as R 11  and that of the piezoresistor  12  as R 12 , the resistors  11  and  12  are expressed as R 11  and R 12 , respectively, in FIG. 5 for convenience of understanding and extension and contract of the resistors are shown by “+” and “−”, respectively. In FIGS. 5 and 7, similar expressions are used. The piezoresistor  13  on the elastic support arm  133  extends, and the piezoresistor  14  contracts. Since the piezoresistors extend and contract like this, the resistances R 11 , R 12 , R 13  and R 14  are changed. When the resistances of the piezoresistors are combined to be a full bridge circuit and a constant voltage Vin is applied thereto as shown in FIG. 6, the change in the resistances of the piezoresistors can be measured according to an output Vout. As for the acceleration in the Y-axis direction, the elastic support arm extending in the Y-axis direction may be considered in the same manner as in the X-axis direction.  
         [0041]    When the acceleration in the Z-direction is exerted on the mass portion  120 , a force Fz works, and the left and right sides of the top surface of the mass portion  120  move upward as shown in FIG. 7. As a result, the piezoresistor  31  on the elastic support arm  131  contracts, and the piezoresistor  32  extends. In the same manner, the piezoresistor  33  on the elastic support arm  133  extends and the piezoresistor  34  contracts. When the resistances R 11 , R 12 , R 13  and R 14  of the piezoresistors are combined to be a full bridge circuit and a constant voltage Vin is applied thereto as shown in FIG. 8, the change in the resistances of the piezoresistors can be measured according to the output Vout. It is suitable to change the wire connection of the resistances in the bridge circuit for detecting the acceleration in the Z-axis direction from that in the bridge circuit for detecting the acceleration in the X-axis direction, and therefore other piezoresistors are used.  
         [0042]    A production method of the acceleration sensor of EXAMPLE will be explained below. FIGS. 9A through 9E show part (left half) of a section in the X-axis direction (II-II section) in FIG. 1 to explain a main process. As described above, an SOI wafer is a Si single crystal substrate constructed with a Si base substrate  60 , an SOI layer  80  on the top surface, which is a Si active layer, and a SiO 2  layer  70 , which is between the Si base substrate  60  and the SOI layer  80 , and is used as an etching stopper, as denoted by reference numerals in FIG. 9A. As for the thickness thereof, the base substrate  60  has thickness of 625 μm, SiO 2  layer has thickness of 1 μm and the SOI layer has thickness of about 10 μm.  
         [0043]    The first step of the production process is to make a pattern of a predetermined form with a photoresist or thermally oxidized SiO 2  film or the like on the surface of the SOI layer  80  and to make piezoresistors  11  and  12  with boron being diffused by an impurity diffusion process such as ion implantation (FIG. 9A). As surface impurity density, about 2×10 18  atoms/cm 3  is adopted, from the viewpoint of both the temperature characteristics and sensitivity.  
         [0044]    Next, to protect the piezoresistors  11  and  12 , a protection film  41  is produced (FIG. 9B). As the protection film  41 , a multilayer film of SiO 2  and PSG (Phosphorous silicated glass) that are generally used in a semiconductor technology are used to have a gettering effect of movable ion. Instead of the two-layer film of SiO 2  and PSG, a two-layer film of SiO 2  and SiN may be used. It is preferable that the thickness of the protection film  41  is made as thin as possible to decrease stress in terms of high sensitivity, and therefore it is made 0.3 μm to 0.5 μm.  
         [0045]    Next, through-holes  40   a  for connecting electrodes are formed in the protection film  41  on both ends of the piezoresistors  11  and  12  by a wet etching with hydrofluoric acid as a predominant ingredient (FIG. 9C).  
         [0046]    Next, to make electrode wiring, a film of aluminum alloy (aluminum, copper, silicon and the like are main ingredients) is formed by spattering. The thickness is 0.3 μm to 0.5 μm. Lead electrodes  40  are formed by photo etching (FIG. 9D).  
         [0047]    Next, though not shown, the SOI layer  80  is etched by a dry etching method or the like to form a through-hole pattern  150  to the thin portion shown in FIG. 1.  
         [0048]    Next, on the base substrate  60  on the back surface a photoresist mask is formed in the shapes of the mass portion  120  and the frame  110  with the positions of the piezoresistors  11  and  12  on the surface, the through-hole pattern  150  to the SOI layer  80  and the like are aligned with use of a double-side aligner device, the base substrate  60  is etched by the dry etching method, and the SiO 2  layer  70  as the etching stopper is removed by wet etching (FIG. 9E). In the dry etching step, an etching process mainly using SF 6 -gas and a process adhering polymer mainly containing C 4 F 8 -gas on the side walls and the inside walls are repeated alternately. As an etching solution to etch the SiO 2  layer, buffered hydrofluoric acid is used. Although the elastic support arms  131 ,  132 ,  133  and  134  are formed in the dry etching process, in some cases it is preferable to leave the SiO 2  layer  70  as the etching stopper without removing it in order to keep balance of the entire stress, forming a lamination of the SiO 2  layer and the silicon layer as the elastic support arms, and part of the SiO 2  layer  70  may be left as the etching stopper on the back side of the elastic support arms.  
         [0049]    Thereafter, a number of acceleration sensor elements formed on the wafer are cut into sensor chips one by one with use of a dicer or the like, and via the assembly process such as packaging and the like, the semiconductor acceleration sensor is completed.  
         [0050]    According to the acceleration sensor of the present invention, it is possible to increase the length of the elastic support arm by about 1.3 times while keeping the volume of the mass portion  120  at the center without changing the chip size, whereby the sensitivity of the sensor can be improved by about 30%.  
         [0051]    The acceleration sensor according to another embodiment of the present invention is shown in the sectional view in FIGS. 10A through 10D. The same reference numerals and symbols are used to show the same parts as in the aforementioned embodiment. In the acceleration sensor  100  shown in FIG. 10A, the mass portion side wall  123  and the frame side wall  113  are formed to have a concave curved surface. In the acceleration sensor  100  shown in FIG. 10B, the mass portion side wall  123  and the frame inner wall  113  are formed to have a convex curved surface. In the acceleration sensor  100  shown in FIG. 10C, the mass portion side wall  123  and the frame inner wall  113  are formed to be in a stepped shape. In the acceleration sensor  100  shown in FIG. 10D, the mass portion side wall  123  and the frame inner wall  113  are formed to have a multi-curved surface.  
         [0052]    Even if the side walls  123  and  113  of the mass portion  120  and the thick frame  110  are in the shape with projections and depressions as shown in FIGS. 10A through 10D, it is suitable if only the angle θ, which is formed by a plane (for example,  128 ) passing a place on the side wall  123  or the inner wall  113  at which the elastic support arms  131  and  133  are attached and the intersection lines  125  or  115  of the side wall  123  or the inner wall  113  and the corresponding bottom surfaces  122  or  112 , and the bottom surface  122  or  112  is 80 degrees to 90 degrees.  
         [0053]    As explained thus far, as the result that the cross section of the mass portion or the thick-walled frame is made larger on the bottom surface side than the top surface side which is connected to the elastic support arm, the length of the elastic support arm can be made larger even if the frame dimension is the same, and the volume of the mass portion can be made larger, thus making it possible to provide the compact and thin acceleration sensor with high sensitivity.