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 . a first embodiment of the present invention is shown in fig1 a to 1 c . fig1 a shows an upper view ( fig1 a , i ) of the acceleration sensor according to the present invention and a lateral cross - sectional view ( fig1 a , 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 . 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 . fig1 b 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 . 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 fig1 b , the two ends of which may be fixed on the diagonals of the base 10 . 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 . 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 . 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 . fig1 c 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 . as shown in fig1 c , it can be seen that , the larger the inter - joint gap 9 , the smaller the normalized offset voltage becomes . 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 . 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 fig1 b . as a result , the first embodiment also exhibits superior shock resistance . fig2 a , 2b , and 2 c show the second embodiment of the present invention and correspond to fig1 a , 1b , and 1 c respectively . therefore , in fig2 a and 2b , reference numerals that are the same as those in fig1 a 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 . fig3 shows an enlargement of the planar view ( i ) of fig2 a . further , fig3 shows an example in which , unlike fig2 b , the stopper substrate 8 is afforded an x shape and four terminals are fixed in the four corners of the base 10 . in fig3 , 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 . 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 fig3 , these are dummy pads for holding the stress balance on the respective sides . 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 . the description will now continue by returning to fig2 c . similarly to fig1 c , fig2 c 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 . in fig2 c , graph a is the characteristic of the first embodiment and is the same as fig1 c . on the other hand , graph b shows the characteristic of the second embodiment . it can be seen from the graph shown in fig2 c 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 . 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 fig2 c ). fig4 a 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 . fig4 a 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 fig2 c , the second embodiment exhibits a smaller normalized offset voltage than the case of the first embodiment . fig4 b 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 fig4 a , the joint length l can be increased . fig5 a 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 . 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 . 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 . here , as shown in fig5 b , 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 . fig5 c 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 . as may be seen from fig5 c , 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 . fig6 a 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 . that is , fig6 b 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 fig6 b 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 . fig7 a and 7b are examples of extensions of the third embodiment in fig5 a . an example in which a space groove 11 a corresponding with the space groove 11 that is formed in the embodiment of fig5 a 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 fig7 a . further , the example shown in fig7 b is an embodiment that comprises both the space groove 11 and the space groove 11 a . 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 fig7 a 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 . fig8 is yet another sixth embodiment according to the present invention . in the embodiment shown in fig8 , 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 fig5 a to fig7 b . 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 . fig9 is a further seventh embodiment according to the present invention . in the embodiment shown in fig9 , 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 fig5 a 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 . fig1 a to 10 d show an embodiment in which the acceleration sensor according to the present invention is packaged . fig1 a is an example in which the sensor of the second embodiment shown in fig2 a 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 . 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 . 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 . fig1 b is an example in which the sensor of the embodiment of fig5 a 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 fig1 a . fig1 c and 10d correspond to fig1 a and 10b respectively but are examples in which the stopper substrate 8 of the sensor is omitted . further , fig1 a 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 fig7 a , 7b , and fig8 . 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 .