Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an acceleration sensor. More particularly, the present invention relates to an acceleration sensor that is capable of detecting the respective acceleration on three axes and implements an improvement in the temperature characteristic.  
         [0003]     2. Description of the Related Art  
         [0004]     In order to implement various types of control in moving bodies such as automobiles and ships, robots, mechanical fabrication devices, or mobile devices that are grasped and used by a user, such as notebook PCs, portable telephones, PDA (Personal Digital Assistant), DVC (Digital Video Camera), DSC (Digital Still Camera), for example, it is essential to detect the stress acting on all or part of the target body in correspondence with the objective of the control. By sensing the stress on the basis of the acceleration acting on all or part of the target, it is possible to sense the acceleration.  
         [0005]     A variety of devices for detecting this stress or acceleration have been conventionally developed, of which one device is known in which a gauge resistor is formed on the semiconductor substrate and which converts the mechanical strain produced on the basis of a force that is supplied from outside and acts on an weight portion to sway, into an electrical signal as a result of the piezoresistive effect (“Development of three-spindle sensor” ‘Invention’ Journal pages 52 to 63, published by the Invention Society on September 2003).  
         [0006]     In addition, the fact that it is also possible to use a change in the electrostatic capacity corresponding with strain instead of using the piezoresistive effect of the gauge resistor is explained in “Development of three-spindle sensor” ‘Invention’ Journal pages 52 to 63, published by the Invention Society on September 2003.  
         [0007]     In particular, the feature of the invention that is described in “Development of three-spindle sensor” ‘Invention’ Journal pages 52 to 63, published by the Invention Society on September 2003 is the fact that it is possible, by means of force sensors for a plurality of axial directions formed on a semiconductor substrate, to constitute sensors of a plurality of types, that is, sensors of four types, namely, a three-axis force sensor, a three-axis acceleration sensor, a three-axis angular speed sensor, and a six-axis motion sensor by means of one device.  
         [0008]     Proposals have been made for a variety of problems relating to semiconductor acceleration sensors that use the principles that appear in “Development of three-spindle sensor” ‘Invention’ Journal pages 52 to 63, published by the Invention Society on September 2003. One of these various problems is that a large unforeseen shock can sometimes be applied as a result of the environment in which the acceleration sensor is employed (depending on what kind of device the acceleration sensor is used in). In such a case, there is a risk that the weight portion will be displaced greatly and the gauge resistor will be accordingly damaged.  
         [0009]     In order to eliminate this inconvenience, a constitution in which a stopper that limits the displacement of the weight portion is provided has been proposed (Japanese Patent Application Laid Open No. 2000-187041).  
         [0010]     Meanwhile, there is the problem of detection errors caused by stress that arises from the difference in the thermal expansion coefficients of the parts constituting the acceleration sensor, for example, the difference in the thermal expansion coefficients of the semiconductor substrate and the stopper, or to the difference in the thermal expansion coefficients of the semiconductor substrate and electrode pads that are provided in order to lead the output of the detection element to the outside.  
         [0011]     Furthermore, one constitution for resolving the problem arising from this difference in the thermal expansion coefficients has been proposed (Japanese Patent Application No. 2000-304762). The invention appearing in Japanese Patent Application No. 2000-304762 is a constitution that forms a stopper by means of the tips of support portions that extend from the four corners of a rectangular support frame. As a result, the problem arising from the difference in the thermal expansion coefficients of the semiconductor substrate and the stopper is eliminated by reducing the area of the joint between the frame and the stopper.  
         [0012]     In the case of the invention appearing in Japanese Patent Application No. 2000-304762, because the stopper extends from the four corners, the pads used in the electrical connection on the frame are concentrated in the vicinity of a flexible or beam portion. As a result, a constitution results in which thermal stress arising from the difference in the thermal expansion coefficients of the frame and the pads is readily produced in the beam portion.  
       SUMMARY OF THE INVENTION  
       [0013]     Therefore, an object of the present invention relates to the problem of stress that arises from the difference in the thermal expansion coefficients and is that of providing an acceleration sensor that eliminates the inconvenience of this conventional invention.  
         [0014]     A first aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is an acceleration sensor that detects acceleration in three-axis directions in a three-dimensional coordinate system, and includes a frame having four sides that is integrally formed by a semiconductor substrate; a weight portion that is positioned within the frame; and four beam portions corresponding with each of the four sides one end of which is connected to the weight portion to support the weight portion and the other end of which is connected to the center of the corresponding side of the four sides, wherein a detection element that detects flexure of the beam portions that corresponds to the acceleration acting on the weight portion is formed on each of the four beam portions; a pair of joint portions that are formed with a gap there between are formed on each of the four sides of the frame; and the gap between the pair of joint portions formed on one side has a predetermined length with the center of the one side as the point of symmetry.  
         [0015]     A second aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is an acceleration sensor according to the first aspect, wherein each of the pair of joint portions comprises a plurality of electrode terminals connected to a predetermined electrode lead that is linked to the detection element and the plurality of electrode elements are disposed at equal intervals along the corresponding side.  
         [0016]     In addition, a third aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the first or second aspect, wherein the offset voltage that is detected by the detection element is adjusted by means of the size of the gap between the pair of joint portions.  
         [0017]     A fourth aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the first aspect, wherein the frame has a predetermined thickness and comprises, on each of the four sides thereof, a recess, in which a region that is narrower than the predetermined thickness is formed uniformly to a predetermined length along the side of the frame on the opposite side from the face where the pair of joint portions is formed; and the recess, which is joined to a mount opposite the opposite side, forms a space groove that is between the substrate and the frame.  
         [0018]     A fifth aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the first aspect, wherein the frame has a predetermined thickness and is formed by means of a recess in which at least a region where the pair of joint portions is formed is formed uniformly to a predetermined length that is narrower than the predetermined thickness along a side of the frame; and the recess, which is joined to a stopper substrate opposite the pair of joint portions, forms a space groove that is between the stopper substrate and the frame.  
         [0019]     A sixth aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the fourth aspect, wherein the mount that is joined to the frame is a circuit substrate, a package bottom portion or a package cover.  
         [0020]     Furthermore, a seventh aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the fourth or fifth aspect, wherein the offset voltage that is detected by the detection element is adjusted by means of the predetermined length of the recess.  
         [0021]     Moreover, an eighth aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the fourth or fifth aspect, wherein a groove portion is also formed in the reverse side of the frame in a direction that matches the direction of the beam portion, in the center of the recess of the frame.  
         [0022]     Further, a ninth aspect of the acceleration sensor according to the present invention that achieves the above object of the invention is the acceleration sensor according to the fourth or fifth aspect, wherein groove portions are formed in positions corresponding with the four corners of the frame on the surface side of the frame.  
         [0023]     The features of the present invention will become more evident from the embodiments of the invention that are described in accordance with the drawings herein below.  
         [0024]     The present invention makes it possible to provide an acceleration sensor that resolves the problem of stress arising from the difference in the thermal expansion coefficients by means of a simple constitution and which improves the temperature characteristic. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1A  shows an upper view and a lateral cross-sectional view of a first embodiment of the acceleration sensor according to the present invention;  
         [0026]      FIG. 1B  illustrates the joint between a sensor base and a mount of the first embodiment;  
         [0027]      FIG. 1C  is a temperature characteristic diagram of the first embodiment;  
         [0028]      FIG. 2A  is a diagram showing an upper view and lateral cross-sectional view of a second embodiment of the acceleration sensor according to the present invention;  
         [0029]      FIG. 2B  illustrates the joint between the sensor base and mount of the second embodiment;  
         [0030]      FIG. 2C  is a temperature characteristic diagram of the second embodiment;  
         [0031]      FIG. 3  shows an enlargement of the planar view (I) of  FIG. 2A ;  
         [0032]      FIG. 4A  is an example in which the interval of the individual joint portions  7   a  is determined so that this interval has the same size as the inter-joint gap  9  of the first embodiment;  
         [0033]      FIG. 4B  is an example in which the interval of individual joint portions  7   a  is determined under the condition of making the temperature drift  0  in the second embodiment;  
         [0034]      FIG. 5A  shows the third embodiment in an oblique view;  
         [0035]      FIG. 5B  illustrates the length of the space groove of the third embodiment;  
         [0036]      FIG. 5C  is a temperature characteristic diagram of the third embodiment;  
         [0037]      FIG. 6A  shows the fourth embodiment in an oblique view;  
         [0038]      FIG. 6B  illustrates the height of the space groove of the fourth embodiment;  
         [0039]      FIG. 7A  is a modified example of the fourth embodiment that shows an example in which a space groove is located on the surface side of the sensor base as a fifth embodiment;  
         [0040]      FIG. 7B  is a modified example of the fourth embodiment that shows an example in which space grooves are located on the lower and surface sides of the sensor base as a fifth embodiment;  
         [0041]      FIG. 8  is a sixth embodiment, in which a second groove is further provided;  
         [0042]      FIG. 9  is a seventh embodiment according to the present invention;  
         [0043]      FIG. 10A  shows a first example in which the acceleration sensor according to the present invention is packaged;  
         [0044]      FIG. 10B  shows a second example in which the acceleration sensor according to the present invention is packaged;  
         [0045]      FIG. 10C  shows a third example in which the acceleration sensor according to the present invention is packaged; and  
         [0046]      FIG. 10D  shows a fourth example in which the acceleration sensor according to the present invention is packaged. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     Embodiments of the present invention will be described herein below with reference to the drawings. Further, the embodiments permit an understanding of the present invention and the technological scope of the present invention is not limited to these embodiments.  
       First Embodiment  
       [0048]     A first embodiment of the present invention is shown in  FIGS. 1A  to  1 C.  FIG. 1A  shows an upper view ( FIG. 1A , I) of the acceleration sensor according to the present invention and a lateral cross-sectional view ( FIG. 1A , II) thereof along the line a-b. The acceleration sensor is produced by integrally forming a frame  1  having four sides that enclose a circumference, a weight portion  2  that is positioned within the frame  1 , and beam portions  3  that are formed between the frame  1  and weight portion  2  by means of a semiconductor substrate as a sensor base  10 . Further, the reverse side of the sensor base  10  is fixed mounted on a mount  4  constituting package material such as glass or alumina ceramic.  
         [0049]     In this constitution, when stress is applied to the weight portion  2  vertically or in a lateral direction, the beam portions  3  produce flexure that corresponds with the stress. It is possible to detect this flexure as strain by means of detection elements  5  that are formed on the beam portions  3  and thereby determine the acceleration that produces the stress.  
         [0050]      FIG. 1B  illustrates the joint between the sensor base  10  and the mount  4  of the first embodiment. The sensor base  10  and mount  4  are joined through anode bonding or a resin or the like by means of a joint portion  6  that is formed on the periphery of the mount  4 .  
         [0051]     Meanwhile, the surface side of the sensor base  10  is covered by a stopper substrate  8  of glass or alumina ceramic similarly via pads (joint portions)  7  of an electrically conductive material such as Al in order to draw out electrodes that are connected to leads (not shown) that are derived from the detection element  5 . The stopper substrate  8  may be a flat plate of a size corresponding with the base  10  or may be formed with a beam shape as shown in  FIG. 1B , the two ends of which may be fixed on the diagonals of the base  10 .  
         [0052]     Here, in this constitution, the mount  4  on which the sensor base  10  is mounted and fixed also has the function of limiting, together with the stopper substrate  8 , the movement of the weight portion  2  in a vertical direction.  
         [0053]     However, the difference in the thermal expansion coefficients of the joint portions  6  and  7  and the sensor base  10  constituting the semiconductor substrate is a problem. In other words, thermal stress is produced as a result of this difference in the thermal expansion coefficients. Accordingly, strain acts on the beam portions  3  even in a 0G state where acceleration is not applied and a signal (known as an ‘offset drift’ herein below) is produced by the detection element  5 . Further, although one method for resolving this offset drift has been proposed in Japanese Patent Application No. 2000-304762, which was described earlier as a conventional example, this constitution is one in which thermal stress that arises from the difference in the thermal expansion coefficients of the frame  1  and joint portions  7  is readily produced in the beam portions  3 .  
         [0054]     Therefore, the present invention resolves this inconvenience.  
         [0055]     In the case of the first embodiment, a feature of the present invention is that of a constitution having a pair of joint portions  7   a  and  7   b  that are formed with a gap there between on each of the four sides of the frame  1 .  
         [0056]      FIG. 1C  is a temperature characteristic diagram of the first embodiment and is a graph with plotted values that are rendered by normalizing, by means of the offset voltage when the temperature is 25° C., the maximum offset voltage when the temperature is changed from −45° C. to +85° C. in each of the gaps when the inter-joint gap  9  is widened from a state where there is no gap, that is from a 0 gap.  
         [0057]     As shown in  FIG. 1C , it can be seen that, the larger the inter-joint gap  9 , the smaller the normalized offset voltage becomes.  
         [0058]     Thus, in the case of the first embodiment of the present invention, the joint portion  7  is formed in the plane (X-Y plane) of the frame  1  and the inter-joint gap  9  is formed in a direction that is orthogonal to the asymmetrical center in the longitudinal direction of the beam portions  3 . As a result, the thermal stress produced by the difference in the linear expansion coefficients of the joint portions  7  and the semiconductor substrate can be suppressed and the temperature offset drift can be suppressed.  
         [0059]     Further, as a supplementary effect, the whole surface of the frame  1  can be fixed to the mount  4  by means of the joint portion  6  between the mount  4  and the sensor base  10  shown in  FIG. 1B . As a result, the first embodiment also exhibits superior shock resistance.  
       Second Embodiment  
       [0060]      FIGS. 2A, 2B , and  2 C show the second embodiment of the present invention and correspond to  FIGS. 1A, 1B , and  1 C respectively.  
         [0061]     Therefore, in  FIGS. 2A and 2B , reference numerals that are the same as those in  FIGS. 1A and 1B  denote the same parts. However, unlike the joint portions  7  of the first embodiment, the second embodiment has a form with a plurality of individual joint portions  7   a.    
         [0062]      FIG. 3  shows an enlargement of the planar view (I) of  FIG. 2A . Further,  FIG. 3  shows an example in which, unlike  FIG. 2B , the stopper substrate  8  is afforded an X shape and four terminals are fixed in the four corners of the base  10 .  
         [0063]     In  FIG. 3 , piezoresistors Rx 1  to Rx 4  for detecting the acceleration in the direction of the X axis, piezoresistors Ry 1  to Ry 4  for detecting the acceleration in the direction of the Y axis and piezoresistors Rz 1  to Rz 4  for detecting the acceleration in the direction of the Z axis are formed on the beam portions  3 .  
         [0064]     Further, the two terminals of these piezoresistors are connected to individual joint portions  7   a  constituting corresponding electrode pads via leads. Further, although there are also individual joint portions  7   a  that are not connected to piezoresistors in  FIG. 3 , these are dummy pads for holding the stress balance on the respective sides.  
         [0065]     In an external circuit (not shown) that is derived from the individual joint portions  7   a , a Wheatstone bridge is put together and the amount of strain of the beam portions  3  is sensed as acceleration. The circuit constitution for this detection is not directly related to the present invention and therefore an additional description is omitted here.  
         [0066]     The description will now continue by returning to  FIG. 2C . Similarly to  FIG. 1C ,  FIG. 2C  is a graph with plotted values that are rendered by normalizing, by means of the offset voltage when the temperature is 25° C., the maximum offset voltage when the temperature is changed from −45° C. to +85° C. in the respective inter-joint gaps  9 .  
         [0067]     In  FIG. 2C , graph A is the characteristic of the first embodiment and is the same as  FIG. 1C . On the other hand, graph B shows the characteristic of the second embodiment. It can be seen from the graph shown in  FIG. 2C  that, in the case of the second embodiment, the variation in the normalized offset voltage when the inter-joint gap  9  is increased is large. That is, it can be seen that, in the case of the second embodiment, the temperature drift can be suppressed more than in the case of the first embodiment even when the same inter-joint gap  9  is provided.  
         [0068]     Therefore, under the condition that the temperature drift should be 0, the second embodiment makes it possible to reduce the inter-joint gap  9  further than the first embodiment does (See the double arrow section C in  FIG. 2C ).  
         [0069]      FIGS. 4A and 4B  further illustrate the grounds for being able to increase the joint length L by being able to reduce the inter-joint gap  9  during a comparison with this first embodiment.  
         [0070]      FIG. 4A  is an example in which the inter-joint gap  9  of the first embodiment is of the same size and therefore the gap between the individual joint portions  7   a  is determined so that this interval has the same joint length L as the first embodiment. In this case also, as shown in  FIG. 2C , the second embodiment exhibits a smaller normalized offset voltage than the case of the first embodiment.  
         [0071]      FIG. 4B  is an example in which, in the second embodiment, the interval of individual joint portions  7   a  is determined to afford a smaller gap than the inter-joint gap  9  of the first embodiment under the condition of making the temperature drift  0 . Because the inter-joint gap  9  is small in comparison with  FIG. 4A , the joint length L can be increased.  
       Third Embodiment  
       [0072]      FIGS. 5A  to  5 C illustrate the third embodiment according to the present invention. The same reference numerals as those of the earlier first and second embodiments indicate the same parts.  
         [0073]     The third embodiment comprises the features of the earlier first or second embodiment and comprises a space groove  11  between the mount  4  and sensor base  10  as an additional feature. Here, as per the earlier first and second embodiments, the mount  4  comprises a package and a substrate with electrically conductive joint portions such as a circuit substrate.  
         [0074]     The joining of the sensor base  10  to the mount  4  is executed by joining column portions  12  formed in the corners of the four sides by means of anode bonding or resin or the like. The width of the space groove  11  that exists between the mount  4  and sensor base  10  as a result of the columns  12  is the length between two columns  12  in two corners.  
         [0075]     Here, as shown in  FIG. 5B , the length between two columns  12 , that is, the width of the space groove  11 , is W 1 , and the length of one side of the sensor base  10  is W.  FIG. 5C  is a graph showing the variation in the offset voltage with respect to the ratio W 1 /W. This is a graph on which the maximum offset variation (voltage) when the temperature is changed from −45° C. to +85° C. in the ratio between the respective W 1 /W is plotted.  
         [0076]     As may be seen from  FIG. 5C , in the third embodiment, by adjusting the ratio W 1 /W, the thermal stress that is produced and which arises from the difference in the linear expansion coefficients can be adjusted and the offset drift can be suppressed, as mentioned in the earlier embodiments.  
       Fourth Embodiment  
       [0077]      FIG. 6A  is the fourth embodiment according to the present invention. This embodiment resolves the problem arising from the difference in the thermal expansion coefficients by making the offset variation  0  by changing the height t 1  of the space groove  11 , the height T of the sensor base  10  and the ratio t 1 /T in the third embodiment.  
         [0078]     That is,  FIG. 6B  is a graph on which the maximum offset variation (voltage) when the length W 1  of the space groove  11  is set substantially equal to the width of the beam portions  3 , the ratio t 1 /T is changed, and the temperature is varied from −45° C. to +85° C. It can be seen from  FIG. 6B  that there is no effect on the temperature variation as a result of establishing the height t 1  of the space groove  11  at or above a fixed height.  
       Fifth Embodiment  
       [0079]      FIGS. 7A and 7B  are examples of extensions of the third embodiment in  FIG. 5A . An example in which a space groove  11   a  corresponding with the space groove  11  that is formed in the embodiment of  FIG. 5A  is formed in the surface side of the sensor base  10  and a space is provided between the surface side and the stopper substrate  8  is shown in  FIG. 7A . Further, the example shown in  FIG. 7B  is an embodiment that comprises both the space groove  11  and the space groove  11   a.    
         [0080]     Thermal stress that is produced as a result of the difference in the thermal expansion coefficients when a cover or circuit substrate, a package, and a substrate that comprises electrically conductive joint portions are electrically or mechanically joined to the surface side and reverse side of the frame  1  of the sensor base  10  affects the detection element, and an offset drift is produced. However, by reducing the thermal stress of the joint portions  7  and  7   a  by also forming the space grooves  11  and  11   a  in the frame according to the constitution shown in the  FIGS. 7A and 7B , it is possible to suppress the effect of thermal stress on the beam portions  3  and, therefore, on the detection elements  5  and suppress the temperature offset drift.  
       Sixth Embodiment  
       [0081]      FIG. 8  is yet another sixth embodiment according to the present invention. In the embodiment shown in  FIG. 8 , second grooves  11   b  are formed in the reverse side of the frame  1  in directions that match the vertical directions of the space grooves  11 , in the center of the space grooves  11  that constitute recesses in the frame  1  provided between the sensor base  10  and mount  4  shown in  FIG. 5A  to  FIG. 7B .  
         [0082]     As a result of this constitution, the above-mentioned thermal stress can be suppressed by the first space groove  11  and the effect of thermal stress on the detection element  4  can be suppressed further while retaining the overall strength of the frame  1  by means of the second space groove  11   b  with a narrow groove width. A third groove is similarly formed in a vertical direction in the space groove  11   a  in the surface as well as in the reverse side of the frame  1 . Here, the width of the space groove  11   b  is substantially the same as the width of the beam portions  3 .  
       Seventh Embodiment  
       [0083]      FIG. 9  is a further seventh embodiment according to the present invention. In the embodiment shown in  FIG. 9 , groove portions  11   c  are provided in correspondence with the column portions  12  in the four corners of the frame  1  on the surface side of the sensor base  10  shown in  FIGS. 5A  to  7 B. As a result of this constitution, the thermal stress of the four angular parts of the frame  1  in which thermal stress is readily produced is reduced. Accordingly, it is possible to suppress the effect of thermal stress on the beam portions  3  and, therefore, on the detection element  5  and suppress the temperature offset drift.  
         [0084]      FIGS. 10A  to  10 D show an embodiment in which the acceleration sensor according to the present invention is packaged.  
         [0085]      FIG. 10A  is an example in which the sensor of the second embodiment shown in  FIGS. 2A and 2B  is housed in a package that is formed by a package main body  110  and a package lid portion  111  and provides a lateral cross-sectional view thereof.  
         [0086]     This is a constitution in which electrical power is supplied from the outside by means of electrically conductive wire  101  that is connected to the block of the package main body  110  or a detection signal is extracted to the outside. The mount  4  is a circuit substrate and a processor element for determining the acceleration by processing the detection signal of the detection element  5  can also be installed.  
         [0087]     In addition, the mount  4  and the bottom face of the package  110  are electrically connected by means of electrically conductive resin  102 , and the circuit operation can be stabilized and the effects of external noise can be avoided by fixing the mount  4  and the bottom face of the package  110  at ground potential.  
         [0088]      FIG. 10B  is an example in which the sensor of the embodiment of  FIG. 5A  is housed in the package  110  and comprises the space groove  11  between the sensor base  10  and mount  4 . The remaining constitution is as described in  FIG. 10A .  
         [0089]      FIGS. 10C and 10D  correspond to  FIGS. 10A and 10B  respectively but are examples in which the stopper substrate  8  of the sensor is omitted.  
         [0090]     Further,  FIGS. 10A and 10D  have the space groove  11  between the sensor base  10  and mount  4  but it is understood that the same may be true of the other embodiments of  FIGS. 7A, 7B , and  FIG. 8 .  
         [0091]     As per the description of the embodiments above, the present invention is able to provide an acceleration sensor with reduced noise which resolves the problem of stress that arises from the difference in the thermal expansion coefficients. Therefore, when the acceleration sensor according to the present invention is integrated into a device performing control that is related to acceleration, highly reliable acceleration control is rendered possible and a substantial contribution to industry is accordingly provided.

Technology Category: 3