HK1147805B - Acceleration sensor element and acceleration sensor having same - Google Patents

Description

Acceleration sensor element and acceleration sensor having the same

Technical Field

The present invention relates to a semiconductor acceleration sensor for detecting acceleration used in automobiles, aircrafts, portable terminal devices, toys, and the like.

Background

An acceleration sensor is often used as an airbag operation sensor for an automobile, and detects an impact of an automobile collision as an acceleration. In an automobile, it is sufficient to have a single-axis or double-axis detection function for measuring acceleration in the X-axis and the Y-axis. In addition, the measured acceleration is very large. Recently, acceleration sensors are increasingly used in portable terminal devices, robots, and the like, and a three-axis acceleration sensor for measuring X, Y, Z-axis acceleration is required in order to detect spatial motion. In addition, in order to detect a minute acceleration, a high-resolution and small-sized acceleration sensor is required.

Most acceleration sensors have a structure in which the motion of the weight portion and the flexible portion is converted into an electric signal. Among them, a piezo-resistance element type acceleration sensor that detects the movement of the weight portion based on the change in resistance of a piezo-resistance element provided on a flexible portion connected to the weight portion, and a capacitance type acceleration sensor that detects the movement of the weight portion based on the change in capacitance with a fixed electrode, and the like.

Next, a conventional three-axis acceleration sensor described in patent document 1 and patent document 2 will be described. In fig. 11 and 12, a triaxial acceleration sensor 101 is constructed by laminating and fixing an acceleration sensor element 103 and an IC104 for performing control such as amplification of a sensor element signal and temperature compensation in a ceramic case 102, and bonding a cover 105 and the case 102 to each other and sealing them in the case 102. As shown in fig. 12, the triaxial acceleration sensor element 103 is fixed to the housing 102 with a resin adhesive 106, and the IC104 is fixed to the triaxial acceleration sensor element 103 with a resin adhesive 107.

The triaxial acceleration sensor element 103 has sensor terminals 108, the IC104 has IC terminals 109, and the housing 102 has housing terminals 110. The sensor terminal 108 and the IC terminal 109, and the IC terminal 109 and the case terminal 110 are connected by a wire 111, and a sensor signal is output to the outside from an output terminal 112 connected to the case terminal 110 provided on the case 102. The cover 105 is fixed to the case 102 with an adhesive material 102a such as AuSn solder.

In the plan view shown in fig. 13, the triaxial acceleration sensor element 103 includes a square support frame portion 113, a weight portion 114, and a pair of beam portions 15 sandwiching the weight portion 114, and the weight portion 114 is held at the center of the support frame portion 113 by 2 pairs of beam portions 115. The beam 115 is provided with a piezoresistance element.

An X-axis piezoresistance element 116 and a Z-axis piezoresistance element 118 are provided on one pair of beam portions 115, and a Y-axis piezoresistance element 117 is provided on the other pair of beam portions 115. Piezoelectric resistance elements are arranged at four positions of each root of the pair of beam portions 115, and these are connected by wiring to constitute a bridge circuit, so that uniform resistance variation of the piezoelectric resistance elements is eliminated, and the acceleration in the X-axis, Y-axis, and Z-axis is detected separately by changing the connection method of the bridge circuit. Further, the sensor terminal 108 is disposed on the support frame portion 113.

Next, the acceleration detection principle of the bridge circuit will be described with reference to fig. 14A to 14D. Fig. 14A and 14B show the movement of the weight portion 114 when subjected to acceleration in the X direction and the Z direction on the XZ cross section, respectively. For example, as shown in fig. 14A, when acceleration is applied in the X direction, the weight portion 114 rotates around the vicinity of the center of the upper end, and the beam portion 115 deforms. As the beam portion 115 deforms, the stress applied to the four X-axis piezoresistance elements X1 to X4 provided on the upper surface of the beam portion 115 changes, and the resistance also changes. In this case, tensile stress is applied to X1 and X3, and compressive stress is applied to X2 and X4, so that a difference occurs in the midpoint potential of the X-axis detection bridge circuit shown in fig. 14C, and an output corresponding to the magnitude of acceleration is obtained. On the other hand, when subjected to acceleration in the Z direction as shown in fig. 14B, tensile stress is applied to the piezo-resistive elements Z2 and Z3, compressive stress is applied to Z1 and Z4, and an output is obtained by a Z-axis detection bridge circuit shown in fig. 14D.

The X-axis piezoresistance elements X1 to X4 and the Z-axis piezoresistance elements Z1 to Z4 are formed on the same beam portion 115, but because of the different configuration of the bridge circuit, for example, even if the beam portion 115 is deformed as shown in fig. 14A with respect to the acceleration in the X direction, the change in resistance is canceled and the output does not change in the Z-axis detection bridge circuit of fig. 14D. Thus, the X-axis acceleration and the Z-axis acceleration can be separated and detected. The detection of the Y-axis acceleration is performed by a piezoresistance element formed on another pair of beam portions 115 orthogonal to the X-axis, as in the X-axis.

On the other hand, as described in patent document 3, a method of realizing a small and inexpensive acceleration sensor by using a resin protective package technique which is often used in a semiconductor mounting technique is known. In this method, in order to protect the triaxial acceleration sensor element 103 having a movable portion with a mold resin, a technique of bonding and sealing caps to the upper and lower sides of the triaxial acceleration sensor element is employed.

Fig. 15A is a sectional view showing an assembly structure of the triaxial acceleration sensor element in which the cover is joined to the upper and lower parts by this method, and fig. 15B is a plan view showing the triaxial acceleration sensor element 120. The upper cover 121 and the lower cover 122 are joined to the upper and lower sides of the triaxial acceleration sensor element 120, and the movable portion of the triaxial acceleration sensor element 120 is sealed in a sealed space. The triaxial acceleration sensor element 120 is bonded to the upper cover 121 and the lower cover 122 by various methods such as metal bonding and anodic bonding.

The bonding metal region 123 shown in fig. 15B is formed on both the front surface and the back surface of the triaxial acceleration sensor element 120. A bonding metal region is also formed in the upper lid 121 and the lower lid 122, and these are stacked, pressed, and heated to be bonded. In this bonding process, before the triaxial acceleration sensor element 120 is singulated from a silicon wafer, the silicon wafer on which the plurality of triaxial acceleration sensor elements 120 are formed is bonded to an upper cap silicon wafer on which the plurality of upper caps 122 are formed at the same pitch and a lower cap silicon wafer on which the plurality of lower caps 123 are formed. This process is called a Wafer Level Package (WLP). After the closed space is formed by WLP, individual chips are singulated by dicing. Hereinafter, the chip singulated after being sealed by WLP is referred to as a capped acceleration sensor element 124.

Next, a triaxial acceleration sensor 125 assembled into a package by resin will be described with reference to a sectional view of fig. 16. The control IC127 is fixed to the lead frame 126 by adhesive materials 128 and 129, and the covered acceleration sensor element 124 is fixed to the IC 127. The sensor terminal 130 of the covered acceleration sensor element 124 and the IC terminal 131 of the IC127 are connected by a wire 132, and similarly, the terminal 131 and the terminal of the lead frame 126 are connected by a wire. The structure assembled by the capped acceleration sensor element 124, the IC127, and the lead frame 126 is molded with a molding resin 133 by transfer molding. After the resin is cured in the metal mold, the resin is taken out of the metal mold, and the triaxial acceleration sensor 125 is obtained. A method may be employed in which a plurality of triaxial acceleration sensors are collectively processed up to before resin molding, and after being released from a metal mold, the triaxial acceleration sensors are cut and separated into individual triaxial acceleration sensors.

In the acceleration sensor using the WLP and the resin mold package, since the movable portion of the triaxial acceleration sensor element 120 can be protected at the stage of the silicon wafer, handling is easy in the subsequent process, and strict foreign matter management is not necessary. In addition, in order to protect the movable portion of the triaxial acceleration sensor element 120, the periphery may be sealed by transfer molding. Thus, the package can be assembled by using a resin mold packaging technique commonly used for conventional IC chips without using an expensive ceramic package, and a small and inexpensive triaxial acceleration sensor can be realized.

However, the triaxial acceleration sensor 125 shown in fig. 16 has the following problems as compared with the triaxial acceleration sensor 101 shown in fig. 12.

Since the mold resin and the lead frame used for the triaxial acceleration sensor 125 have different thermal expansion coefficients from silicon, which is a material of the acceleration sensor element with a cover, thermal stress is generated due to a change in temperature, and an external force is applied to the acceleration sensor element with a cover, thereby changing the piezoresistance. When the triaxial acceleration sensor 125 is mounted on the product substrate of the object product to which the sensor is mounted by soldering, the influence of the thermal expansion of the product substrate is transmitted to the triaxial acceleration sensor 125 and the covered acceleration sensor element via the soldered portion.

In the ceramic package triaxial acceleration sensor 101 shown in fig. 12, since the triaxial acceleration sensor element 103 is held in the space inside the package, the resin 107 is made of a soft material, and thus a force from the product substrate is hard to be transmitted to the triaxial acceleration sensor element 103.

On the other hand, in the resin-encapsulated triaxial acceleration sensor 125 shown in fig. 16, since the periphery of the covered acceleration sensor element 124 is covered with the mold resin 133, the force from the substrate is easily transmitted to the triaxial acceleration sensor element 120. When a force is applied to the triaxial acceleration sensor element 120 from the outside, if uneven stress variation is caused in the four piezoresistive elements of each axis, the output level of the output varies at zero, and the output of the sensor varies (hereinafter, such zero-level variation is referred to as offset).

The change in the offset with respect to the change in the temperature of the acceleration sensor can be corrected by the detection IC before mounting on the product substrate. However, when the product is mounted on a product substrate, the product substrate is affected by a force from the product substrate, and when the product substrate is mounted on a product substrate of various target products, the characteristics of the product substrate change with respect to a change in temperature.

When an external force from the wiring board and the protective package is applied to the capped acceleration sensor element 124, if the capped acceleration sensor element 124 is disposed near the center of the package, the deformation caused by the external force is substantially symmetrical, and the outputs in the X-axis and the Y-axis do not change.

However, when a difference occurs between the piezoresistance element near the frame portion (hereinafter referred to as frame-side piezoresistance element) and the piezoresistance element near the weight portion (hereinafter referred to as weight-side piezoresistance element), the output in the Z axis changes.

Patent document 4 describes an acceleration sensor in which an output is not easily changed due to an influence of an external force. A stress separation groove is formed in the frame body to separate the outer frame and the inner frame, and the outer frame and the inner frame are connected by a stress relaxation beam having flexibility. The outer frame is connected to the support substrate, and the inner frame is joined to the support substrate by a partial joint portion. A cover body surrounding the support substrate, the outer frame, the inner frame, and the weight portion is joined to the outer frame. Since the bonding surface area between the inner frame and the support substrate is kept small and the inner frame and the outer frame are connected by the stress relaxation beam, even if thermal stress occurs in the outer frame and the support plate, the inner frame is not easily deformed and fluctuation in output is not easily generated.

[ Prior art documents ]

[ patent document ]

[ patent document 1 ] Japanese patent laid-open No. 2003-172745

[ patent document 2 ] Japanese patent laid-open No. 2006 and 098321

[ patent document 3 ] Japanese patent application laid-open No. H10-170380

[ patent document 4 ] Japanese patent laid-open No. 2005-337874

In general, in order to realize an acceleration sensor having high sensitivity, the beam portion is designed to have low rigidity with respect to the weight of the weight portion, and therefore, the beam portion is easily broken by an impact or the like. In the covered acceleration sensor element sealed by the WLP, the upper cover and the lower cover function as stoppers for restricting excessive displacement of the weight portion. In order to obtain high impact resistance, when the gap between the weight portion and the upper and lower covers is made very small, the cover is collided before the weight portion is accelerated, so that the stress generated at the time of collision can be reduced. In addition, the smaller the gap, the more the air damping effect can be increased. The increase in air damping also has the effect of reducing noise due to resonance of the sensor.

In the acceleration sensor of patent document 4, since the inner frame is joined to the support substrate at one point, when the support substrate is bent and deformed, the inner frame is displaced with the joint portion as a base point, and there is a problem that the inner frame is easily brought into contact with the support substrate or the lid body. In recent years, since customers strongly demand reduction of the entire thickness of the acceleration sensor, the support substrate is easily bent and deformed when it is made thin, and therefore, the influence of the above problem becomes large. The purpose of the present invention is to provide an acceleration sensor that is less likely to change in external force output and has high sensitivity and impact resistance.

Disclosure of Invention

The present invention is an acceleration sensor element with a cover, including: a weight portion, a support frame portion surrounding the weight portion, a plurality of beam portions 13 having flexibility for connecting and holding the weight portion to the support frame portion, a piezoresistance element provided on the beam portions 13 and a wiring for connecting them, an upper cover and a lower cover which surround the weight portion together with the support frame portion are joined to the front and back surfaces of the support frame portion, and acceleration in a first axial direction of the thickness direction of the stack of the upper cover, the support frame, and the lower cover and acceleration in at least one axial direction of a second axis in a plane perpendicular thereto and a third axis in the plane perpendicular to the second axis are detected based on a change in resistance of the piezoresistance element,

the support frame portion is divided into an inner frame and an outer frame surrounding the inner frame by a dividing groove, the upper cover and the lower cover are joined to the outer frame, the inner frame is connected and held to the outer frame by a plurality of flexible inner frame support portions, the beam portion is connected to both sides of the weight portion along at least one of the second axis and the third axis, and the inner frame support portions are connected to both sides of the inner frame such that a connecting portion between the inner frame support portion and the inner frame is separated from a connecting portion between the beam portion and the inner frame.

With the above configuration, since the inner frame is supported by the flexible inner frame support portion separately from the outer frame and the upper and lower covers, even if external force is applied to the outer frame, the upper and lower covers and deformed due to thermal stress at the time of assembling the resin package, thermal stress at the time of mounting the resin package on the product substrate, or the like, the deformation is not easily transmitted to the inner frame, and output variation is not easily caused. However, since the inner frame support portions are arranged in a direction in which the influence is hardly transmitted to the beam portions, the deformation of the inner frame near the inner frame support portions is less likely to cause a change in stress of the piezoresistance elements on the beam portions.

Further, since the inner frame is supported from the periphery in four directions, the symmetry is good, and therefore, when the outer frame is deformed, the relative displacement of the upper cover and the lower cover of the inner frame is suppressed, and the gap between the weight portion and the upper cover and the lower cover can be reduced. Therefore, when an impact is applied to the acceleration sensor, the effect of shortening the distance until the weight portion hits the upper cover or the lower cover to make acceleration difficult and the effect of enabling air damping to be increased are obtained, whereby the stress generated in the beam can be reduced, and the impact resistance can be improved. In addition, since the air damping can be increased, the high-frequency vibration can be suppressed, the resonance vibration of the weight portion can be suppressed, and the noise can be reduced.

The beam portion, which is connected to both sides of the weight portion along the second shaft, may be configured such that inner frame support portions are connected to both sides of the inner frame 15 in a direction rotated by substantially 45 degrees from the second shaft as covered acceleration sensor elements that detect both axial directions of the first shaft and the second shaft. The same effect is obtained also in the acceleration sensor element for biaxial detection having the beam portion only in the second axis direction.

The beam portion may be connected to both sides of the weight portion along the second axis, and may have a structure in which the inner frame support portion is connected to both sides of the inner frame along a third axis perpendicular to the second axis as a covered acceleration sensor element for detecting both axial directions of the first axis and the second axis. In the acceleration sensor element for biaxial detection having the beam portion only in the second axis direction, the inner frame support portion is disposed along the third axis, and the farthest arrangement is achieved, so that the influence of the deformation of the outer frame is hardly transmitted by the beam portion.

Preferably, the beam portion and the inner frame support portion have the same thickness and are thinner than the weight portion and the support frame portion. In order to improve the sensitivity of the acceleration sensor, it is preferable to increase the weight of the weight portion and reduce the rigidity of the beam portion. This structure can be easily realized by forming the beam portion only on the thin silicon layer and forming the weight portion in the range of the thin silicon layer and the thick silicon layer. Since the support frame portion needs to have sufficient rigidity, the inner frame support portion needs to be flexible as in the weight portion, and therefore, may be formed in the same manner as the beam portion.

In addition, the inner frame support portion preferably has a higher bending rigidity than the beam portion. When comparing the resonance frequency of the weight portion determined by the rigidity of the beam portion and the weight of the weight portion with the inner frame resonance frequency determined by the rigidity of the inner frame support portion and the total weight of the inner frame and the weight portion, it is desirable that the inner frame resonance frequency be sufficiently higher than the resonance frequency of the weight portion. Otherwise, the inner frame and the weight portion are displaced together with the change in relatively fast acceleration, which may prevent the deformation of the beam portion and may not provide accurate sensitivity. At least, the shape of the inner frame support portion may be determined so that the resonance frequency of the inner frame becomes higher than the resonance frequency of the weight portion.

The acceleration sensor element with the cap and the control IC chip are bonded together to a lead frame, and the lead frame, an electrode of the IC chip, and an electrode of the acceleration sensor element with the cap are connected by a metal wire, thereby forming an acceleration sensor sealed with a mold resin. The acceleration sensor is configured such that solder is formed on the surface of the lead frame exposed from the lower surface of the acceleration sensor, and the acceleration sensor can be easily mounted on a product substrate by reflow of the solder.

[ Effect of the invention ]

According to the acceleration sensor of the present invention, by disposing the connecting portion between the inner frame support portion and the inner frame at a position as far as possible from the connecting portion between the beam portion and the inner frame, it is possible to suppress a change in output due to the influence of external forces such as thermal stress when the acceleration sensor is assembled into a resin package and thermal stress when the acceleration sensor is mounted on a product substrate. Further, since the gap between the weight portion and the cover can be suppressed from being narrowed by the stress and the gap can be narrowed, the impact resistance can be improved.

Drawings

Fig. 1 is a plan view showing the structure of an acceleration sensor element in one embodiment of the present invention.

Fig. 2 is a cross-sectional view taken along the k-k line in fig. 1 showing the structure of the covered acceleration sensor element.

Fig. 3 is a cross-sectional view taken along the line m-m in fig. 1 showing the structure of the covered acceleration sensor element.

Fig. 4 is a schematic diagram showing a state in which an acceleration sensor element assembled into a resin package is mounted on a product substrate.

Fig. 5 is a plan view showing an acceleration sensor element having an annular beam portion.

Fig. 6 is a plan view showing an acceleration sensor element having an annular inner frame support portion.

Fig. 7 is a plan view showing the acceleration sensor element in which the beam portion and the inner frame support portion are rotated by substantially 45 degrees with respect to the support frame portion.

Fig. 8 is a plan view showing an acceleration sensor element in which inner frame support portions are arranged only in one direction.

Fig. 9 is a plan view showing an acceleration sensor element in which a beam portion is arranged only in one direction.

Fig. 10 is a plan view showing an element of an acceleration sensor in which a beam portion and an inner frame support portion are arranged in mutually perpendicular directions.

Fig. 11 is an exploded perspective view illustrating a prior art triaxial acceleration sensor.

Fig. 12 is a sectional view illustrating a prior art triaxial acceleration sensor.

Fig. 13 is a plan view showing an example of a structure of a conventional triaxial acceleration sensor element.

Fig. 14A is an explanatory diagram of the detection principle of the prior art triaxial acceleration sensor element.

Fig. 14B is an explanatory diagram of the detection principle of the prior art triaxial acceleration sensor element.

Fig. 14C is an explanatory diagram of the detection principle of the prior art triaxial acceleration sensor element.

Fig. 14D is an explanatory diagram of the detection principle of the prior art triaxial acceleration sensor element.

Fig. 15A is a cross-sectional view showing a conventional triaxial acceleration sensor element sealed with a cap.

Fig. 15B is a plan view showing a conventional triaxial acceleration sensor element sealed with a cap.

Fig. 16 is a sectional view showing a protective package of a three-axis acceleration sensor according to the related art.

Detailed Description

An acceleration sensor according to an embodiment of the present invention will be described below with reference to the drawings.

[ example 1 ]

Fig. 1 is a plan view showing the structure of an acceleration sensor element 10 in a covered acceleration sensor element 30 according to example 1. Fig. 2 and 3 are sectional views of a covered acceleration sensor element 30 according to embodiment 1, fig. 2 is a k-k sectional view of fig. 1, and fig. 3 is an m-m sectional view of fig. 1.

The acceleration sensor element with a cover 10 of example 1 can be applied to, for example, an acceleration sensor assembled into a protective package made of resin as shown in fig. 16 of the conventional example. Therefore, in embodiment 1, the acceleration sensor element 30 with a lid will be mainly described in detail.

< basic Structure >

In the acceleration sensor element 10 according to embodiment 1, the weight portion 12 is supported by four flexible beam portions 13 from four sides in the support frame 11. The support frame portion 11 is divided into an inner frame 15 and an outer frame 16 surrounding the inner frame by a dividing groove 14, and the beam portion 13 is connected to the inner frame 15. The inner frame 15 is held from all directions by the inner frame support portions 17 on the outer frame 16. The weight portion 12 is separated from the inner frame 15 by the second separating groove 29, and includes four main portions and an intermediate portion connected to the main portion and the beam portion 13.

The four beam portions 13 are respectively shown as a first beam portion 13a, a second beam portion 13b, a third beam portion 13c, and a fourth beam portion 13 d. In the acceleration sensor element 10 shown in fig. 1, as described with reference to fig. 13, the piezoresistive element P is formed in the vicinity of the root of the beam portion 13. Piezoresistance elements P for detecting acceleration in the X-axis and Z-axis directions are arranged on the first beam portion 13a and the second beam portion 13b extending in the X-axis direction, and piezoresistance elements P for detecting acceleration in the Y-axis direction are arranged on the third beam portion 13c and the fourth beam portion 13d extending in the Y-axis direction. The piezoresistance elements for detecting Z-axis acceleration may be arranged on the third beam portion 13c and the fourth beam portion 13 d. The respective piezoelectric resistance elements P are connected by a wiring not shown in the figure to form a bridge circuit shown in fig. 14. The wiring is led out to the outer frame 16 through the upper part of the inner frame support portion 17, and is connected to an electrode stage 18 formed on the outer frame 16.

The upper cover 19 is joined to a face on the side where the piezoresistance element P of the acceleration sensor element 10 is formed. The bonding is performed by the bonding material 21 in conformity with the upper cover bonding area 20 of the outer frame 16. Similarly, the lower cover 22 is joined to the opposite surface with a joining material 23. The upper cover 19 and the lower cover 22 are joined only to the outer frame 16, and the outer frame 16, the upper cover 19, and the lower cover 22 surround the inner frame 15.

< production method >

The method for manufacturing the acceleration sensor element 10 will be briefly described below with reference to fig. 2. The acceleration sensor element 10 was processed using an SOI wafer having a silicon layer of about 6 μm thickness sandwiching a silicon oxide layer of about 1 μm on a silicon layer of about 400 μm thickness. The silicon oxide film layer serves as an etching stopper for dry etching, and the structure is formed on the two silicon layers. Hereinafter, the first thin silicon layer is referred to as a first layer 24, the second thick silicon layer is referred to as a second layer 25, the surface of the first layer not bonded to the silicon oxide film layer is referred to as a first surface 26, the surface of the second layer is referred to as a second surface 27, and the connection surface via the silicon oxide film layer is referred to as a third surface 28.

Patterning the shape of the semiconductor piezoresistance element with photoresist, and patterning the shape on the first surface 26 by 1-3 × 10¹⁸Atom/cm³Boron is added to the mixture to form a semiconductor piezoresistance element. Similarly, a P-type wiring into which boron having a higher concentration than that of the piezoelectric resistance element is implanted is formed so as to be connected to the piezoelectric resistance element. Further, a silicon oxide film is formed on the first surface 26 to protect the piezoelectric resistance element. An aluminum-based metal is sputtered on the silicon oxide film to form a metal wiring, and the metal wiring is connected to the P-type wiring through a through hole formed in the silicon oxide film. The silicon oxide film formed on the piezoresistance element also functions as an insulating film between the silicon of the first layer 24 and the metal wiring. Further, thereon, as goldA silicon nitride film is formed on the protective film on the wiring by chemical vapor deposition. The silicon oxide film, the metal wiring, and the silicon nitride film are processed into a desired shape by photolithography.

Next, after a photoresist pattern is formed on the first surface 26, the shapes shown in fig. 1, that is, the beam portions 13 and the inner frame support portions 17 are left by dry etching, and the first separation grooves 14 for separating the inner frame 15 from the outer frame 16 and the second separation grooves 29 for separating the weight portions from the inner frame 15 are processed. Further, after a photoresist pattern is formed on the second surface 27, the first isolation groove 14 and the second isolation groove 29 are processed by dry etching. The exposed portion of the silicon oxide film layer remaining between the first layer 24 and the second layer 25 is removed by wet etching, and the first isolation groove 14 and the second isolation groove 29 penetrate the SOI wafer. Through the above-described manufacturing process, the weight portion 12, the inner frame 15, and the outer frame 16 are formed from the first layer 24 to the second layer 25. In addition, the beam portion 13 and the inner frame support portion 17 are formed on the first layer 24.

Next, the WLP technique is used to bond and seal the upper cover 19 and the lower cover 22 made of silicon to the front surface and the back surface of the acceleration sensor element 10 by metal bonding. Therefore, in the acceleration sensor element, before the dry etching step, the metal thin films for metal bonding are formed on the first surface 26 and the second surface 27 of the wafer, the same metal thin film and metal solder are provided on the two wafers constituting the lid, and the three wafers are stacked and pressed and heated to be bonded. For metallic solder, gold-tin alloy is utilized.

Next, the upper cover 19 and the lower cover 22 are ground to be entirely thinned. On the upper cover 19, a groove deeper than the thickness of the upper cover after grinding is formed on the side of the surface to be bonded to the acceleration sensor element 10, and after grinding, the electrode pad 18 of the acceleration sensor element 10 is exposed. The groove is not required on the lower cover 22 side, but may be configured to match the upper cover 19. In addition, on the upper cover 19 and the lower cover 22, a cavity is formed on the portion of the weight portion 12 on the side facing the surface to be joined with the acceleration sensor element 10. The gap 31 between the weight portion 12 and the upper and lower covers 19 and 22 is the sum of the depth of the cavity (cavity depth 32) and the thickness of the bonding material (bonding material thickness 33). When the thickness 33 of the bonding material can be used as the gap 31 as it is, the cavity is not necessarily formed.

The wafer is finally diced and separated into the acceleration sensor elements 30 with covers until the above-described grinding step. Through the above manufacturing steps, the lidded acceleration sensor element 30 supported by the inner frame 15 and the weight portion 12 is obtained in the airtight container constituted by the outer frame 16, the upper lid 19, and the lower lid 22.

< Structure of resin Package >

Fig. 4 is a schematic sectional view of an acceleration sensor mounting structure 41 in which the covered acceleration sensor element 30 of example 1 is assembled into a resin-sealed acceleration sensor 40 mounted on a product substrate 49. The acceleration sensor 40 is obtained by bonding the control IC chip 42 to the lead frame 43 with the adhesive 44, bonding the covered acceleration sensor element 30 to the IC chip 42 with the adhesive 45, connecting the electrode pad 18 of the covered acceleration sensor element 30 and the electrode pad 46 of the IC chip 42, and the electrode pad 46 of the IC chip 42 and the lead frame 43 with the metal lead 47 by wire bonding, and then sealing the entire structure with the mold resin 48. For the adhesive materials 44 and 45, a Die Attach Film (DAF) which is used as both a dicing tape and an adhesive material can be used. The surface of the lead frame exposed from the lower surface of the acceleration sensor 40 is plated with solder, and is bonded to the product substrate 49 with solder 50, thereby obtaining the acceleration sensor mounting structure 41.

< inner frame support part >

In the lidded acceleration sensor element 30 of the present invention, as shown in fig. 2, the inner frame 15 is separated from the outer frame 16 and the upper and lower covers 19 and 22, and as shown in fig. 1, the inner frame support portions 17 having flexibility are supported only by the outer frame 16 at four diagonal positions. Therefore, even if the outer frame 16, the upper cover 19, and the lower cover 22 are deformed by an external force due to a thermal stress when the resin package is assembled, a thermal stress when the resin package is mounted on a product substrate, or the like, the deformation is not easily transmitted to the inner frame 15, and a change in output is not easily caused. The deformation of the outer frame 16 is slightly transmitted to the inner frame 15 by the inner frame support portions 17, but since the inner frame support portions 17 are arranged diagonally with respect to the beam portions 13, the deformation of the inner frame 15 near the inner frame support portions 17 is less likely to cause a change in stress of the piezoresistance elements on the beam portions 13.

In fig. 1, the beam portion 13 is compressed and extended in the longitudinal direction, or the beam portion 13 is bent, whereby a stress change of the piezoresistance element due to an external force is likely to occur. In the vicinity of the connection portion of the beam portion 13, when the stress of the inner frame 15 changes, only the piezoresistance element on the side close to the inner frame 15 changes, and the piezoresistance element on the side close to the weight does not change greatly, so that the Z-axis offset change is likely to occur. In embodiment 1, the connection portion of the inner frame support portion 17 is separated from the connection portion of the beam portion 13, and the stress change of the inner frame 15 due to the external force does not directly affect the beam portion 13, so that the output change can be made very small.

< rigidity of inner frame support part >

In order to ensure the responsiveness of the acceleration sensor, the rigidity of the inner frame support portion 17 is preferably higher than the rigidity of the beam portion 13. When the weight resonance frequency determined by the rigidity of the beam portion 13 and the weight of the weight portion 12 is compared with the inner frame resonance frequency determined by the rigidity of the inner frame support portion 17 and the total weight of the inner frame 15 and the weight portion 12, it is preferable that the inner frame resonance frequency is sufficiently higher than the weight resonance frequency. Otherwise, the inner frame 15 and the weight portion are displaced together with the acceleration change relatively rapidly, which hinders the deformation of the beam portion 13, and accurate sensitivity cannot be obtained. Preferably, the shape of the inner frame support portion 17 is determined so that the phase characteristic and the gain characteristic of the frequency characteristic are separated to such an extent that they are not coupled to each other.

< symmetrical support of inner frame >

In addition, in embodiment 1, the inner frame 15 is supported from four sides around the periphery, and therefore, the symmetry is good. For example, when the inner frame 15 is supported by one inner frame support portion 17 or is connected to the lower cover 22 at one point of the inner frame 15, the inner frame 15 is supported and displaced by one arm with respect to the deformation of the outer frame 16 and the lower cover 22, and therefore, the relative displacement between the upper cover 19 and the lower cover 22 is likely to increase. In this way, in order to prevent the inner frame 15 and the weight portion 12 from contacting the upper cover 19 and the lower cover 22, it is necessary to increase the gap. In the present embodiment, since the inner frame 15 is supported from all around, the relative displacement between the upper cover 19 and the lower cover 22 is suppressed to be small, and the above-described gap can be reduced. Therefore, when an impact is applied to the acceleration sensor, by the effect that the distance before the weight portion 12 collides against the upper cover 19 or the lower cover 22 is short and acceleration is not easy, and the effect of air damping can be increased, the stress generated on the beam portion 13 can be reduced, and impact resistance can be improved. Further, since the air damping can be increased, the high-frequency vibration can be suppressed, the resonance vibration of the weight portion 12 can be suppressed, and the noise can be reduced.

< joining of cover >

Further, embodiment 1 is easier in manufacturing process than the case of joining the inner frame 15 to the lower cover 22. As described above, when the upper cover 19 and the lower cover 22 are joined by the metal solder, it is necessary to heat while applying pressure, but when the inner frame 15 is intended to be joined only to the lower cover 22, the inner frame 15 is flexibly joined to the outer frame 16, and therefore, a sufficiently large pressure force cannot be applied to the joined portion of the inner frame 15. Therefore, it is necessary to divide into two stages, that is, first, to join the acceleration sensor element 10 and the lower cover 22, and then to join the upper cover 19, and it is necessary to directly press the surface of the acceleration sensor element 10, which is easily damaged, when the acceleration sensor element 10 is joined to the lower cover 22. As described in embodiment 1, the joint portion is only provided on the outer frame 16, and if the positions of the joint portion of the upper cover 19 and the joint portion of the lower cover 22 coincide, a sufficient pressing force can be given to the joint portions.

[ example 2 ]

Fig. 5 is a schematic plan view showing the structure of an acceleration sensor element 10 according to embodiment 2. In the center of the beam portion 13, a ring portion 51 is provided as a compressive stress absorbing portion. The silicon oxide film or the like formed on the surface of the acceleration sensor element 10 has a smaller thermal expansion coefficient than silicon, and when the film is formed, annealing is performed at a high temperature of, for example, about 950 ℃. The weight portion 12 and the inner frame 15 are formed from the first layer 24 to the second layer 25, and the second layer 25 is thick, and therefore, the weight portion contracts substantially with the thermal expansion coefficient of silicon, but the beam portion 13 is formed only from the first layer 24, so that the proportion of the silicon oxide film is high, and the thermal contraction is small. Therefore, the beam portion 13 is compressed between the inner frame 15 and the weight portion 12. In order to improve the sensitivity of the sensor, when the beam portion 13 is thinned, the beam portion 13 is buckled by the compressive force, and there is a risk that instability in sensitivity increases or a large offset change occurs.

As described in example 2, by providing the ring portion 51 in the beam portion 13, the above-described compressive force is absorbed, buckling can be prevented, and an acceleration sensor element with high sensitivity can be designed. The shape of the annular portion 51 can assume various shapes such as three rings connected to each other. The shape is determined so that the compressive force can be absorbed by deformation and stress is not concentrated on the annular R portion or the like.

< analysis results of design example >

Next, a design example in embodiment 2 of fig. 5 will be described. The acceleration sensor element 10 had dimensions of 1.32mm in the X direction, 1.18mm in the Y direction, a weight portion of 560 μm in XY dimension, a beam portion 13 of 240 μm in length, a piezoresistance-forming portion of 28 μm in width, an inner frame support portion 17 of 50 μm in length, a connection width on the outer frame 16 side of 160 μm, and a connection width on the inner frame 15 side of 150 μm. The thickness of the first layer was 4 μm, and the thickness of the second layer was 400 μm. The width of the inner frame 15 is 70 μm.

The acceleration sensor 40 assembled into a resin package by the acceleration sensor element 10 was evaluated for a characteristic change before and after mounting by FEM analysis when mounted on a product substrate 49 having a thickness of 0.6 mm. In the conventional example of the structure in which the acceleration sensor elements have the same size and the support frame portion is not separated into the outer frame and the inner frame, the ratio of the Z-axis output change before and after mounting to the Z-axis sensitivity is about 23%, whereas the above-described example of the design of the acceleration sensor can be suppressed to about 4%. The resonance frequency of the weight portion in the present design example was 2.0kHz in the X, Y direction and 3.2kHz in the Z direction, while the inner frame resonance frequency was about 46kHz, the inner frame resonance frequency was sufficiently high, and the sensor sensitivity was not affected.

[ example 3 ]

Fig. 6 is a schematic plan view showing the structure of an acceleration sensor element 10 according to embodiment 3. The inner frame support portion 17 is formed with a ring portion 52 as a compressive stress absorbing portion. As in embodiment 2, the inner frame support portion 17 is prevented from buckling. When the inner frame support portion 17 is buckled, the inner frame 15 is displaced and approaches the upper cover 19 or the lower cover 22, and therefore, it is difficult to reduce the gap 31. By forming the annular portion 52 on the inner frame support portion 17, buckling can be prevented. Further, the effect of absorbing the influence of the deformation of the outer frame 16 is obtained, and the change in output is less likely to occur.

[ example 4 ]

Fig. 7 is a schematic plan view showing the structure of an acceleration sensor element 10 according to embodiment 4. The arrangement of the beam section 13 and the inner frame support section 17 is rotated by substantially 45 degrees. The inner frame support portion 17 is disposed in the direction X, Y, and the beam portion 13 is disposed substantially at 45 degrees to the inner frame support portion 17, and maintains the relative relationship between the inner frame support portion 17 and the beam portion 13. By arranging the beam portion 13 in the diagonal direction of the square acceleration sensor element 10, the beam portion 13 can be made longer, and the sensor sensitivity can be easily improved.

In the configuration of embodiment 4, the inner frame support portions 17 may be formed in two. Fig. 8 shows an example in which the inner frame support portion 17 is formed only at two positions in the Y direction. As shown in this example, when the electrode stage is disposed along one side along the Y axis, only the side protrudes, and therefore, symmetry with respect to the Y axis is deteriorated. When the resin package of fig. 4 is assembled, since only the side is wire-bonded, the side is arranged to be shifted so as to be enlarged on one side. As described above, in the case of example 4, symmetry with respect to the X axis is deteriorated, but symmetry with respect to the Y axis is deteriorated. Therefore, by connecting inner frame 15 and outer frame 16 only in the Y-axis direction, the influence of deformation in the X-direction, which is poor in symmetry, is less likely to be transmitted to inner frame 15, and the symmetry of the influence of external force transmitted to beam portion 13 can be improved. Since the symmetric deformation in the X axis and the Y axis has no influence on the X axis output and the Y axis output, it is particularly effective in suppressing the output variation in the X axis and the Y axis.

[ example 5 ]

Fig. 9 is a schematic plan view showing the structure of an acceleration sensor element 10 according to example 5. Although examples 1 to 4 have shown examples having four beam portions 13, the present invention can be applied to a biaxial-detection acceleration sensor element having only two beam portions 13 in one direction. In example 5, two beam portions 13 are provided in the Y-axis direction, and the acceleration in the Y-axis direction and the Z-axis direction can be detected. Similarly, only two beam portions may be provided in the X direction to detect the X-axis direction and the Z-axis direction.

In the acceleration sensor element 10 having two beam portions 13, as shown in fig. 10, the inner frame support portion 17 may be arranged in a direction substantially 90 degrees with respect to the beam portion 13. The main effect of the present invention can be obtained by disposing the connecting portion between the inner frame support portion 17 and the inner frame 15 at a position as far as possible from the connecting portion between the beam portion 13 and the inner frame 15. Therefore, as described in example 5, when the number of beam portions 13 is two in the Y direction, the inner frame support portions 17 are two in the X direction and are disposed farthest from each other, and the influence of the deformation of the outer frame 16 is hardly transmitted through the beam portions 13.

< modification example >

In the present invention, the inner frame support portion 17 and the beam portion 13 are arranged in the direction of substantially 45 degrees or the direction of substantially 90 degrees, but even if not precisely 45 degrees or 90 degrees, the same effect can be obtained by arranging the connecting portion between the inner frame support portion 17 and the inner frame 15 sufficiently apart from the connecting portion between the beam portion 13 and the inner frame 15. For example, when both are arranged along a direction of 45 degrees, even if they are symmetrically arranged within an angle range of 45 degrees ± 15 degrees, there is a certain effect. When the optical elements are symmetrically arranged within a range of 45 degrees ± 5 degrees, the optical elements can be used as in the case of 45 degrees in accordance with the required specification characteristics.

The arrangement of the inner frame support portions 17 and the additional features of the annular portions described in the first to fifth embodiments may be used in combination.

[ notation ] to show

10: acceleration sensor element, 11: support frame, 12: weight portion, 13: beam section, 13 a: first beam section, 13B second beam section, 13 e: third beam portion, 13 d: fourth beam portion, 14: first groove portion, 15: inner frame, 16: outer frame, 17: inner frame support portion, 19: upper cover, 22: lower cover, 29: second separation tank, 30: capped acceleration sensor element, 31: gap, 32: cavity depth, 40: acceleration sensor, 41: acceleration sensor mounting structure, 42: IC chip, 43: lead frame, 44: bonding material, 45: bonding material, 47: metal wire, 48: molding resin, 49: article substrate, 51: annular portion, 52: annular portion, 101: three-axis acceleration sensor, 102 housing, 103: acceleration sensor element, 104: IC, 105: cover, 106: resin binder, 107: resin binder, 111: wire, 113: support frame, 114: weight portion, 115: beam portion 13, 116: x-axis piezoresistor, 117: y-axis piezoresistor, 118: z-axis piezoresistor, 120: three-axis acceleration sensor element, 121: upper cover, 122: lower cover, 123: bonding metal region, 124: capped acceleration sensor element, 125: three-axis acceleration sensor, 126: lead frame, 127: IC, 132: wire, 133: molding resin, 134: article substrate, P: piezoelectric resistance element

Claims (9)

1. A capped acceleration sensor element, comprising: a weight portion, a support frame portion surrounding the weight portion, a plurality of beam portions having flexibility and connecting and holding the weight portion to the support frame portion, a piezoresistance element provided on the beam portions and a wire connecting them, an upper cover and a lower cover surrounding the weight portion together with the support frame portion are joined to the front surface and the back surface of the support frame portion, and acceleration in a first axial direction of a thickness direction of the upper cover, the support frame, and the lower cover stack and acceleration in at least one axial direction of a second axis within a plane perpendicular thereto and a third axis perpendicular to the second axis within the plane are detected based on a change in resistance of the piezoresistance element,

the support frame portion is divided into an inner frame and an outer frame surrounding the inner frame by a separation groove, the upper cover and the lower cover are connected to the outer frame, the inner frame is connected and held to the outer frame by a plurality of flexible inner frame support portions,

the beam portion is connected to both sides of the weight portion along at least one of the second axis and the third axis, and the inner frame support portion is connected to both sides of the inner frame such that a connection portion between the inner frame support portion and the inner frame is spaced apart from a connection portion between the beam portion and the inner frame.

2. The covered acceleration sensor element according to claim 1, characterized in that the beam portion is connected to both sides of the weight portion along both the second axis and the third axis, respectively, and the inner frame support portion is connected to both sides of the inner frame such that a connecting portion between the inner frame support portion and the inner frame is separated from a connecting portion between the beam portion and the inner frame.

3. The covered acceleration sensor element according to claim 2, characterized in that the inner frame support part is connected to both sides of the inner frame rotated substantially 45 degrees from both of the aforementioned second shaft and third shaft.

4. The covered acceleration sensor element according to claim 1, characterized in that the beam portion is connected to both sides of the weight portion along the second axis while detecting the acceleration of the first axis and the second axis, and the inner frame support portion is connected to both sides of the inner frame in the direction rotated substantially 45 degrees from the second axis in the plane.

5. The covered acceleration sensor element according to claim 1, characterized in that the beam portion is connected to both sides of the weight portion along the second axis while detecting the acceleration of the first axis and the second axis, and the inner frame support portion is connected to both sides of the inner frame in the direction of a third axis perpendicular to the second axis in the plane.

6. The covered acceleration sensor element according to any one of claims 1 to 5, characterized in that a compressive stress absorbing portion is provided on at least one of the aforementioned inner frame support portion or the beam portion.

7. The covered acceleration sensor element according to any one of claims 1 to 5, characterized in that the beam portion and the inner frame support portion have the same thickness, and the thickness thereof is thinner than the weight portion and the support frame portion.

8. The covered acceleration sensor element according to any one of claims 1 to 5, characterized in that the bending stiffness of the inner frame support portion is higher than the beam portion.

9. An acceleration sensor characterized in that the covered acceleration sensor element according to any one of claims 1 to 5 and a control IC chip are bonded to a lead frame, the lead frame, electrodes of the IC chip and electrodes of the covered acceleration sensor element are connected by metal wires, and they are sealed by a mold resin.