JP4637074B2 - Piezoresistive acceleration sensor - Google Patents

Description

The present invention relates to a piezoresistive acceleration sensor for acceleration detection used in automobiles, aircraft, home appliances, game machines, robots, security systems, and the like.

Accelerometers have been used to detect small accelerations for applications such as detecting large impact forces for automotive airbag actuation and vehicle control applications such as brake control systems. In these automotive applications, a 1-axis or 2-axis function is sufficient to measure the X-axis and Y-axis accelerations. Recently, it has been put into practical use for new applications such as portable terminal devices, robots, and various controls based on human body motion detection. In such a new application, a three-axis acceleration sensor capable of measuring accelerations in the X, Y, and Z axes is required to detect a three-dimensional movement. In addition, in order to detect minute accelerations, high resolution, small size, and thinness are also required.

The acceleration sensor is a method of converting the movement of the flexible portion into an electric signal, and is roughly classified into a piezoresistive type, a capacitance type, and a piezoelectric type. Depending on the application, the static acceleration detection application is limited to the piezoresistive type and the capacitance type. These two types are small by forming a three-dimensional structure on the silicon substrate using semiconductor technology or micromachine technology. High-sensitivity acceleration sensors can be manufactured in large quantities at once. In particular, the piezoresistive type is an acceleration sensor that is easy to construct a structure and a manufacturing process, and is small, thin, and suitable for cost reduction. Further, the structure of the flexible portion is roughly divided into a diaphragm type and a beam type. Various acceleration sensors can be obtained by combining the detection method of the electric signal, the structure of the flexible portion, and the number of detection axes.

Many applications have been filed for beam-type piezoresistive element type three-axis acceleration sensors. Patent Documents 1 to 6 disclose the shape of the weight portion, the shape of the beam portion, the arrangement of the piezoresistive elements, the connection method of the piezoresistive elements, the shape of the joint between the beam portion and the frame portion, and the like. 6 is an exploded perspective view of the triaxial acceleration sensor, FIG. 7a) is a cross-sectional view in the hh ′ direction of FIG. 6, and FIG. 7b) is a plan view of the sensor chip. In the triaxial acceleration sensor 20, the sensor chip 2 and the regulating plate 3 are fixed to the case 1 with an adhesive 16 such as resin at a predetermined interval. The chip terminal 4 of the sensor element 2 is connected to the case terminal 6 by a wire 5, and a sensor signal is taken out from the external terminal 7. A case lid 8 is fixed to the case 1 with an adhesive 17 such as gold-tin solder and sealed. In the sensor chip 2, a beam-type triaxial acceleration sensor element 9 ′ is formed. The beam-type triaxial acceleration sensor element 9 ′ is composed of a beam portion 12 that forms a pair with a rectangular frame portion 10 and a weight portion 11.
Is held at the center of the frame portion 10 by two pairs of beam portions 12. A piezoresistive element is formed on the beam portion 12. An X-axis piezo 13 and a Z-axis piezo 15 are formed on a pair of beams, and a Y-axis piezo 14 is formed on the other pair of beams, and are connected by metal wiring. The distance g4 between the lower surface of the weight part 11 and the inner bottom surface of the case 1 and the distance g3 between the upper surface of the weight part 11 and the regulating plate 3 are shown in FIG. 7a) when an excessive acceleration such as impact is applied to the sensor. The amount of movement of the portion 11 is regulated to prevent the beam portion 12 from being damaged.

JP 2003-172745 A Japanese Patent Laid-Open No. 2003-279592 JP 2004-184373 A JP 2006-098323 A JP 2006-098321 A WO2005 / 062060 A1

Patent Documents 7 to 9 disclose a diaphragm structure of a diaphragm type three-axis acceleration sensor element, an arrangement method of a piezoresistive element, and the like. The flexible part supports a circular or polygonal diaphragm with a support frame at the outer edge, and a weight is arranged on the inner edge of the diaphragm. When the weight is displaced by an external force, the piezoresistive element provided on the diaphragm is deformed and an electric signal is obtained. There is an advantage that the degree of freedom of arrangement of the piezoresistive elements is higher than that of the beam type triaxial acceleration sensor element 9 ′. FIG. 7 c) shows a plan view of the diaphragm type triaxial acceleration sensor element 9 ″. It is composed of a rectangular frame portion 10, a weight portion 11, and a diaphragm 19 that becomes a flexible portion, and the weight portion 11 is the center of the diaphragm 19. A piezoresistive element is formed on the diaphragm 19 by an X-axis piezo 13, a Y-axis piezo 14, and a Z-axis piezo 15, which are connected by metal wiring. Since the basic structure of the diaphragm type three-axis acceleration sensor element is the same as those of these patent documents, detailed description will be omitted.
A beam type and piezoresistive type triaxial acceleration sensor is used for explanation. Therefore, although a flexible part may be called a beam part, a flexible part and a beam part are used synonymously.

Japanese Patent Laid-Open No. 3-2535 JP-A-6-174571 Japanese Patent Laid-Open No. 7-191053

When the weight part 11 receives an external force and moves, the beam part 12 is deformed. By obtaining the deformation degree of the beam portion as a change in the resistance value of the piezoresistive element, it is possible to know from which direction and how much the external force is applied to the acceleration sensor. However, since the resistance change of the piezoresistive element is minute, a full bridge circuit is configured by arranging four piezoresistive elements for each axis on the beam portion, and the minute resistance change is detected as a voltage change. . If the electric resistance values of the four piezoresistive elements constituting the full bridge are the same, there is no output from the bridge. However, the resistance values of the four piezoresistive elements are actually different among the four piezoresistive elements due to various factors such as variations in piezoresistive element characteristics, dimensional variations in the elements, and differences in stress applied to the elements. , The output of the bridge comes out in the state where the acceleration is not applied and the beam is not deformed. This output voltage is
This is called an offset voltage. In order to cancel the offset voltage, the microcomputer attached to the acceleration sensor is provided with a correction circuit to store the offset value, and the product is shipped with the offset voltage set to substantially zero.

In the effect confirmation test of the regulation plate that applies excessive shock to the offset voltage corrected acceleration sensor, the offset voltage exceeds the allowable range. When the acceleration sensor was examined in detail, the beam part collided with the regulation plate due to excessive impact applied to the acceleration sensor, and a part of the metal wiring provided on the beam part was crushed and deformed. It was. The metal wiring was deformed and the cross-sectional area was changed, thereby changing the electric resistance, and an offset voltage was generated. Excessive impact is not applied to all the acceleration sensors used in the market, and the magnitude of the applied impact varies. In the market, it is considered that the rate of occurrence of defects in which an offset voltage exceeding the allowable range is generated due to deformation of a metal wiring due to an impact is almost zero. But,
The occurrence of even one failure leads to a decrease in reliability, and it is necessary to eliminate it.

Dividing the piezoresistive element into a plurality of pieces as a measure for increasing detection sensitivity without changing the power consumption and impact resistance is disclosed in Patent Document 10 and has been increasingly adopted. FIG. 8a) shows a state where the piezoresistive element is not divided, and FIG. 8b) shows a state where it is divided into two. In the case of the piezoresistive elements having the same width, the same resistance value as that in the case of one can be obtained by connecting two piezoresistive elements having a half length in series. By arranging half-length piezoresistive elements 13 'and 13 ", 15' and 15" next to the stress concentration part on the two beam parts 12, detection sensitivity can be improved even if the maximum stress of the beam parts is the same. Can be increased. Even if the piezoresistive element is divided into a plurality of elements, the resistance value does not change, so the power consumption does not change and the detection sensitivity can be increased. However, since the number of the metal wirings 22 is increased by dividing, there is an increased risk that the metal wirings 22 collide with the regulation plate 3 and are deformed to increase the occurrence rate of the offset voltage. In FIG. 8 a), the X-axis piezo 13 and the Z-axis piezo 15 are each connected to the metal wiring 22 at the connection portion 29. Since the piezoresistive element is formed by implanting an element having a different valence into silicon, the piezoresistive element has substantially the same color on the same surface as silicon. Therefore, in the figure, the piezoresistive element is indicated by a broken line. The same applies to the high concentration diffusion layer. In FIG. 8 b), the X-axis piezo is divided into 13 ′ and 13 ″ and the Z-axis piezo is divided into 15 ′ and 15 ″. Z-axis piezo 15
'And 15 "are arranged so as to sandwich the X-axis piezos 13' and 13". FIG. 8c) is similar to FIG.
It is a mm 'cross section of b). An electrical insulating layer 21 is formed on the frame portion 10, the beam portion 12, and the weight portion 11, and a metal wiring 22 is formed on the electrical insulating layer 21. A through hole is provided in the electrical insulating layer 21, and the piezoresistive element and the metal wiring 22 are electrically connected at the connection portion 29.

JP 2006-098321 A

In order to prevent the metal wiring from being deformed even when the flexible portion collides with the regulating plate, it is conceivable to coat the metal wiring thickly with a hard and electrically insulating film such as alumina or silicon oxide. However, the degree of deformation of the flexible portion changes by thickening such a film. The flexible part is formed of materials having different thermal expansion coefficients such as silicon, an electrical insulating film, and metal wiring. The stress applied to the piezoresistive element changes due to the difference in the thermal expansion coefficient of the constituent material due to the temperature change of the acceleration sensor, which is one of the causes of the generation of the offset voltage. Further, the offset voltage can be further increased by covering the metal wiring with a hard and electrically insulating film. For this reason, it is preferable to limit the constituent materials as much as possible, and it is preferable to avoid covering the metal wiring with a hard and electrically thick film.

The present invention has been made to solve the above-described problems, and provides a small and thin three-axis acceleration sensor that does not generate a new offset voltage even if an excessive impact is applied after offset voltage correction. The purpose is to do.

The piezoresistive acceleration sensor according to the present invention includes a frame, a weight held by the frame via the flexible part, a piezoresistive element provided on the flexible part, and a wiring connecting them. A piezoresistive acceleration sensor having a piezoresistive acceleration sensor element, the piezoresistive acceleration sensor element having at least one frame portion and a weight portion on the piezoresistive element forming surface side of the flexible portion. An electrical insulating layer is formed on at least the piezoresistive element surface and the groove bottom surface of the flexible portion, and a metal wiring is formed on the electrical insulating layer on the groove bottom surface. It is preferable to be recessed from the upper surface of the electrical insulating layer on the element.

In the diaphragm type in which the beam type and the beam are connected by a flexible material, the frame part, the weight part, the arrangement of the piezoresistive elements, and the arrangement of the metal wiring are substantially the same, and can be handled as the same flexible part. The present invention can be applied to both beam type and diaphragm type acceleration sensors.

A groove is formed in the flexible portion, and metal wiring is formed on the bottom surface of the groove. Since the metal wiring is continuously formed not only on the flexible part but also on the weight part and the frame part, it is preferable that the metal wirings of the weight part and the frame part are also formed on the bottom surface of the groove. Is preferably connected to the grooves formed in the weight portion and the frame portion. Since stress is concentrated most at the boundary between the flexible portion and the weight portion and between the flexible portion and the frame portion, one end portion of the piezoresistive element is disposed in the vicinity of the boundary portion. If the terminal end (tip) of the groove portion is brought near the boundary portion, the end portion of the piezoresistive element is aligned, and the stress distribution applied to the piezoresistive element may change. For this reason, it is preferable that the end (tip) of the groove is located at least the width of the groove in the weight portion or the frame portion from the boundary portion. if you can,
It is good to connect with the groove | channel formed in the weight part or the frame part.

Since metal wiring is formed on the bottom surface of the groove, the cross-sectional shape of the groove is preferably square or inverted trapezoid. A dry etching method and a wet etching method can be used to form the groove. When single crystal silicon is used as the substrate material, the cross-sectional shape of the groove becomes an inverted triangle by the wet etching method, and it is difficult to form a square or an inverted trapezoid. It is preferable to use a dry etching method that allows easy control of the groove depth and a high degree of freedom in forming the cross-sectional shape.

Since the commercial material of single crystal silicon forming the flexible part is doped P-type or N-type, the electrical resistance is as small as 1 to 100 (Ω-cm), so the bottom of the groove and the metal wiring are electrically insulated. There is a need to. Further, in order to protect the piezoresistive element from external ions, it is necessary to cover the surface of the piezoresistive element with an electrical insulating material. This can be dealt with by simultaneously forming an electrically insulating material such as silicon oxide with a thickness of 0.02 to 0.8 μm on the groove bottom surface and the surface of the piezoresistive element. When the piezoresistive element is divided into a plurality of pieces, the divided portions need to be connected. Since this connection is in the width direction of the beam portion, it intersects the metal wiring for assembling the piezoresistive element bridge. For this reason, it is preferable to connect with a high-concentration diffusion layer doped with an element having a different valence at a concentration 100 to 1000 times from the piezoresistive element portion instead of the metal wiring. However, since the high-concentration diffusion layer is also formed on the side wall and bottom surface of the groove, electrical insulation of the groove is indispensable.

It is not preferable to form the metal wiring so as to fill the groove. When the trench side wall and the metal wiring are integrated, if the temperature of the flexible portion rises, unnecessary stress is applied to the piezoresistive element due to the difference in thermal expansion coefficient between silicon and the metal wiring, causing an offset voltage. In order to reduce the generation of stress, it is preferable that the metal wiring is not fixed on the three surfaces of the groove cross section but only the groove bottom surface. Further, since it is inevitable that the thickness of the electrical insulating layer is thinner on the side surface of the groove than on the bottom surface of the groove, it is preferable to separate the metal wiring from the side surface of the groove.

By denting the upper surface of the metal wiring formed in the groove portion from the upper surface of the electrical insulating layer on the piezoresistive element, the metal wiring surface does not come into contact with the regulating plate even if the flexible portion is bent. Although the weight portion does not bend, the metal wiring of the weight portion is preferably formed in the groove portion because it is the place where the weight plate is most easily contacted.

When the piezoresistive element is divided into a plurality of pieces in order to increase the detection sensitivity, it is preferable that the divided piezoresistive elements are connected in series with a high concentration diffusion layer. The high-concentration diffusion layer has only different concentrations of elements with different valences, and the physical constant does not change from that of silicon forming the flexible part, so there is no need to consider the stress change caused by the wiring. The design freedom is high. Since there is a degree of freedom in wiring design, the divided piezoresistive elements can be arranged symmetrically with respect to the longitudinal center line of the beam. For example, when placing X-axis and Z-axis piezoresistive elements on one beam, X
Axis piezoresistive element-center line-Z axis piezoresistive element, but when divided, X axis piezoresistive element-Z axis piezoresistive element-center line-Z axis piezoresistive element-X axis piezoresistive element, center A symmetrical arrangement can be made with respect to the line, and a stable arrangement can be obtained with respect to torsion in the width direction of the beam.

In the piezoresistive acceleration sensor of the present invention, the dent amount of the upper surface of the metal wiring from the upper surface of the electrically insulating layer on the piezoresistive element of the piezoresistive acceleration sensor element is 0.05 μm or more and 0.5.
It is preferable that it is below μm.

By setting the distance between the upper surface of the metal wiring and the upper surface of the metal insulating layer on the piezoresistive element to be 0.05 μm or more, the metal wiring does not come into contact with the regulating plate even if the flexible portion is bent and deformed somewhat. Increasing the distance also increases the safety factor, but it is necessary to increase the depth of the groove.
μm or less is good. When the groove is deepened, the dry etching time for forming the groove becomes longer and the manufacturing cost increases.

It is conceivable that the amount of deflection partially changes due to the difference in the distance from the upper surface of the metal wiring to the upper surface of the electrical insulating layer on the piezoresistive element on the flexible portion, causing a distorted distribution of stress. It is preferable that the distance is constant at least in a portion parallel to the piezoresistive element.

In the piezoresistive acceleration sensor of the present invention, the depth d of the groove formed in the flexible portion of the piezoresistive acceleration sensor element is preferably 15% or less of the flexible portion thickness t.

The metal wiring formed on the bottom of the groove is preferably thin. By thinning the metal wiring, it is possible to reduce the twist applied to the width direction of the beam without hindering deformation in the thickness direction of the beam. Therefore, the groove depth is preferably 15% or less of the thickness of the flexible portion. By setting it to 15% or less, it is possible to shorten the groove processing time and to prevent the mechanical strength of the flexible portion from being lowered.

It is conceivable that the amount of deflection partially changes due to the difference in groove depth on the flexible part, causing a distorted distribution of stress. It is preferable that the groove depth is constant at least in a portion parallel to the piezoresistive element.

The groove width on the flexible portion is also the same, and it is preferable that the groove width of at least the portion parallel to the piezoresistive element is constant.

In the piezoresistive acceleration sensor of the present invention, the groove formed in the flexible portion of the piezoresistive acceleration sensor element is formed by a straight line or a curve, or a combination of a straight line and a curve, and at least a portion parallel to the piezoresistive element The grooves are preferably straight.

The groove formed in the flexible part is preferably a straight line in order to avoid stress concentration, but it is difficult to make a single straight line from the position of the connection part between the piezoresistive element and the metal wiring. The wiring can be a straight line or a curve, or a combination of a straight line and a curve, but these wirings are preferably symmetric with respect to the longitudinal center line of the beam. For example, if one side of the center line is a straight groove and the other side is a curve, the deflection of the beam becomes distorted, leading to generation of an unnecessary offset voltage.

The piezoresistive element has a rectangular shape with long sides in the longitudinal direction of the beam. By forming at least a portion of the groove parallel to the piezoresistive element as a straight line, unnecessary stress is not applied to the piezoresistive element, and generation of an offset voltage can be suppressed.

In the piezoresistive acceleration sensor of the present invention, the cross-sectional shape of the groove formed in the flexible portion of the piezoresistive acceleration sensor element is preferably symmetrical with respect to the center line in the groove width direction.

The groove on the flexible part preferably has a center line in the width direction of the groove perpendicular to the flexible part, and the groove wall surface is symmetric with respect to the center line. If an oblique groove is formed in the flexible part, normal bending of the flexible part is hindered. Further, an asymmetrical shape in which the wall surface on one side is vertical and the wall surface on the other side is in an overhang shape is not preferable because it inhibits normal bending of the flexible portion.

In the overhang state in which the groove top width is narrower than the groove bottom width, when an electrical insulating material is added, portions where no insulating film is formed on the groove bottom surface and the side wall surface are generated. In a structure in which the piezoresistive elements are divided and connected by a high concentration diffusion layer, the high concentration diffusion layer is formed on the groove surface so as to cross the groove. In order to prevent insulation failure between the metal wiring and the side wall surface of the groove, the cross section of the groove is preferably square or inverted trapezoidal.

In the piezoresistive acceleration sensor of the present invention, the bottom width w of the groove formed in the flexible portion of the piezoresistive acceleration sensor element is preferably 110% or more of the metal wiring width a.

When metal wiring is formed so as to fill the groove and the groove sidewall and the metal wiring are integrated, unnecessary stress is applied to the piezoresistive element due to the difference in thermal expansion coefficient between silicon and metal wiring when the temperature of the flexible part rises. In addition, an offset voltage is generated. In order to reduce the generation of stress, the metal wiring is preferably fixed only to the bottom surface of the groove. Moreover, since it is inevitable that the thickness of the electrical insulating material is thinner on the side surface of the groove than on the bottom surface of the groove, it is preferable to separate the metal wiring from the side surface of the groove. Groove bottom width w
And the metal wiring width a may be 110% or more of the metal wiring width a in consideration of alignment errors during photolithography and variations in the manufacturing process such as wraparound of the film during sputtering. preferable. When a plurality of metal wirings are formed in one groove, the total of the plurality of metal wiring widths is calculated as the metal wiring width a.

In the piezoresistive acceleration sensor according to the present invention, the grooves formed in the flexible portion of the piezoresistive acceleration sensor element are alternately arranged with n piezoresistive elements, each having (n-1) grooves. One wiring is preferably formed in the groove.

It is preferable that grooves and piezoresistive elements formed in the flexible portion are alternately arranged in the beam width direction, and one wiring is formed in each groove. Any number of piezoresistive elements-grooves-piezoresistive elements may be formed. Since the number of piezoresistive elements is one more than the groove, piezoresistive elements are arranged at both ends. By alternately arranging, the twisting force in the width direction of the beam becomes uniform, and normal deflection of the flexible portion is not hindered. The piezoresistive element and the groove are preferably equidistant. If it is not possible to equidistantly, it is preferable that the center line of the central groove be symmetric.

By arranging the piezoresistive element, the groove and the metal wiring so as to be symmetric with respect to the center line of the central groove, the influence of the stress applied to the beam width direction by the groove and the metal wiring formed in the groove is minimized. Can do. When the number of grooves and piezoresistive elements do not match and the number of grooves is insufficient, it is preferable to form dummy grooves and metal wirings that are not wired to the piezoresistive elements. By forming not only the dummy groove but also the dummy metal wiring, the stress in the beam width direction can be made uniform.

Even if the piezoresistive element, the groove and the metal wiring are arranged so as to be symmetric with respect to the center line of the center groove, if the center line of the center groove is greatly deviated from the center line in the beam width direction, it is arranged symmetrically. has no effect. Therefore, it is important to match the center line of the central groove with the center line in the width direction of the beam.

In some cases, the number of grooves and piezoresistive elements do not match and the piezoresistive elements are insufficient. A piezoresistive element simply implants an element having a different valence into silicon, and the mechanical characteristics of the piezoresistive element are almost the same as that of the original silicon. Therefore, it is not necessary to form a dummy piezoresistive element, and it is sufficient to secure a corresponding area. Is. Needless to say, a piezoresistive element dummy may be formed.

In the piezoresistive acceleration sensor of the present invention, the grooves formed in the flexible portion of the piezoresistive acceleration sensor element are alternately arranged with n piezoresistive elements, and (n + 1) grooves are provided in each groove. It is preferable that one wiring is formed.

It is preferable that grooves and piezoresistive elements formed in the flexible portion are alternately arranged in the beam width direction, and one wiring is formed in each groove. Any number of groove-piezoresistive element-groove structures may be formed. Since the number of grooves is one more than that of the piezoresistive element, grooves are arranged at both ends. By alternately arranging, the twisting force in the width direction of the beam becomes uniform, and normal deflection of the flexible portion is not hindered. In addition, the distance between the piezoresistive element and the groove is preferably equal. If it is not possible to equidistantly, it is preferable to make it symmetrical with respect to the center line of the central piezoresistive element.

By arranging the piezoresistive element, groove and metal wiring so as to be symmetric with respect to the center line of the central piezoresistive element, the influence of stress on the beam width direction by the metal wiring formed in the groove and groove is minimized. can do. If the number of grooves and the number of piezoresistive elements do not match and the number of grooves is insufficient, it is preferable to form dummy grooves and metal wiring that are not wired. By forming not only the dummy groove but also the dummy metal wiring, the stress in the beam width direction can be made uniform.

Even if the piezoresistive element, the groove and the metal wiring are arranged so as to be symmetric with respect to the centerline of the central piezoresistive element, the centerline of the central piezoresistive element is not greatly deviated from the centerline in the beam width direction. There is no effect of symmetrical arrangement. Therefore, it is important to match the center line of the central piezoresistive element with the center line in the width direction of the beam.

In some cases, the number of grooves and piezoresistive elements do not match and the number of grooves is insufficient. When the grooves are insufficient, it is preferable to form dummy grooves and metal wirings that are not wired. By forming not only the dummy groove but also the dummy metal wiring, the stress in the beam width direction can be made uniform.

In the piezoresistive acceleration sensor of the present invention, the groove formed in the flexible portion of the piezoresistive acceleration sensor element is formed between the piezoresistive elements, and one or more metal wirings are formed in the groove. It is preferable.

Instead of alternately arranging grooves and piezoresistive elements, forming grooves between piezoresistive elements,
A plurality of metal wirings can also be formed in the groove. For example, a combination of a piezoresistive element-groove-piezoresistive element, a piezoresistive element-piezoresistive element-groove-piezoresistive element-piezoresistive element, and the like are conceivable. By forming a plurality of metal wirings in one groove, the entire groove width can be reduced as compared with the case where the same number of metal wirings are provided in the groove. By reducing the groove width, if the beam width is constant, the degree of freedom of the spacing between the piezoresistive element and the groove increases. Since the beam width can be narrowed with the same arrangement interval, the beam can be easily bent, and the acceleration detection sensitivity can be increased. Conversely, since the groove width can be increased, the electrical resistance of the metal wiring can be lowered by increasing the cross-sectional area without changing the thickness of the metal wiring. Further, even if the difference between the number of piezoresistive elements and the number of electrical wirings is two or more, the difference in the number can be eliminated by forming a plurality of metal wirings in the groove.

By forming the metal wiring in the groove symmetrically with respect to the center line of the groove, the stress in the beam width direction can be made uniform. It is important that the center line of the central groove coincides with the center line in the beam width direction.

In the piezoresistive acceleration sensor of the present invention, the groove formed in the flexible portion of the piezoresistive acceleration sensor element is formed on both sides of the piezoresistive element, and one or more metal wirings are formed in the groove. Preferably it is.

A plurality of piezoresistive elements can be formed so as to be symmetric with respect to the center line in the width direction of the beam, and grooves can be provided on both outer sides of the piezoresistive elements. A plurality of metal wirings can be formed in the groove.
The center line of the plurality of central piezoresistive elements is aligned with the center line in the beam width direction. The piezoresistive element and the groove are formed symmetrically with respect to the center line of the plurality of piezoresistive elements arranged in the center. The metal wiring is symmetrically arranged with respect to the center line in the width direction of the groove. When the number of metal wirings differs between the grooves, it is preferable to form a dummy wiring to ensure symmetry in the beam width direction.

In the piezoresistive acceleration sensor of the present invention, the upper surface of at least a part of the metal wiring formed on the weight portion and the frame portion of the piezoresistive acceleration sensor element is recessed from the upper surface of the electrical insulating layer on the piezoresistive element. Preferably it is.

Since the weight portion is located at the central portion of the flexible portion, and the amount of movement of the weight portion is large, it is considered that the metal wiring of the weight portion has more opportunities to contact the regulating plate than the metal wiring of the beam portion. Therefore, it is preferable that the metal wiring upper surface of the weight portion is recessed from the upper surface of the electrical insulating layer formed in the weight portion. The metal wiring formed on the frame portion does not come into contact with the regulating plate even when an excessive impact is applied to the acceleration sensor, but it is preferable to form a groove on the bottom surface of the groove. By denting the upper surface of the metal wiring from the upper surface of the electrical insulating layer, it is possible to prevent the metal wiring from being damaged when the acceleration sensor element is gripped by air tweezers or the like in the piezoresistive acceleration sensor manufacturing process. By aligning the metal wiring formed on the bottom surface of the groove of the flexible portion and the metal wiring of the frame portion in the height direction, the metal wiring can be continuously formed. If there is a position shift (step) in the height direction without providing a groove in the frame,
Since there is a high risk that the thickness of the metal wiring is reduced in the portion, it is preferable to form a step in the frame portion.

The depth of the groove formed in the frame part may be the flexible part thickness. It is also possible to use the oxide layer as an electrical insulating layer by etching the frame portion silicon so as to penetrate the oxide layer. The deep frame groove can be formed simultaneously with the formation of the flexible portion. However, since the depth of the groove formed in the beam portion is too different from that of the groove portion, a step such as an inclined surface machining is required to eliminate the step at the portion where the groove is connected. When the thickness of the flexible part is as thin as several μm or less, the inclined surface processing or the like is not necessary to eliminate the step, and a through groove can be used.

The piezoresistive acceleration sensor of the present invention provides a small and thin piezoresistive acceleration sensor that does not generate new offset voltage due to deformation of metal wiring even if an excessive impact is applied after offset voltage correction. I was able to.

Hereinafter, the present invention will be described in detail based on examples with reference to the drawings. In order to make the explanation easy to understand, the same reference numerals are used for the same parts and parts.

FIG. 1a) is a perspective view of a beam-type piezoresistive acceleration sensor element 9, and FIG.
The A section enlarged view of is shown. 2a) to 2e) are sectional views taken along the line aa 'to ee' of FIG. The configuration of the acceleration sensor 20 is the same as that in FIG. The external dimensions of the beam-type triaxial acceleration sensor element 9 were 1.5 mm in length x 1.5 mm in width and the thickness 11 having a thickness of about 0.4 mm was 420 μm on a side. The beam portion 12 had a length of 280 μm, a width of 90 μm, and a thickness of 6 μm. The thickness of the weight part 11 and the frame part 10 was 410 μm.

FIG. 1b) is an enlarged perspective view of a portion including the Y-axis piezo at the connection portion between the beam portion 12 and the frame portion 10 in FIG. 1a). Three grooves 23 are formed in the width direction of the beam portion 12, and Y-axis piezos 14 ′ and 14 ″ are disposed between the grooves 23. A piezoresistive element is disposed at a central portion between the grooves 23. Metal wiring 22 was formed on the bottom surface of the groove, and the side wall surface of the metal wiring 22 was not in contact with the side wall surface of the groove 23. The groove 23 was also formed in the frame portion 10 and formed in the beam portion 12. The Y-axis piezos 14 ′ and 14 ″ are connected in series with a high-concentration diffusion layer 24. Since the high concentration diffusion layer 24 is formed so as to cross the central groove 23, the high concentration diffusion layer exists on the side wall surface and the bottom surface of the groove 23. Since the Y-axis of this embodiment has two piezoresistive elements and three metal wirings, the metal wiring is arranged to sandwich the piezoresistive elements. The metal wiring 22 plays the role of wiring in two rows, but by making the structure symmetrical with respect to the width direction of the beam, the twist in the width direction of the beam (that is, the left and right twist) is made equal.

FIGS. 2a) to 2e) show cross sections aa ′ to ee ′ of FIG. 2a) is a cross-section of the beam portion 12 including the Y-axis piezos 14 ′ and 14 ″, FIG. 2b) is a cross-section of the beam portion 12 where the Y-axis piezo is not present, and FIG. 2c) is a cross-section of the frame portion 10. 2d) is a cross-section of the weight portion 11, and FIG. 2e) is a cross-section of the beam portion 12 including the X-axis piezos 13 ′ and 13 ″ and the Z-axis piezos 15 ′ and 15 ″. In FIG. A method of forming a groove, a metal wiring, and a dimensional relationship will be described, and a high-concentration diffusion layer 24 connecting the piezoresistive element and the piezoresistive element of each axis is formed at a predetermined position on the silicon substrate. and the length 30μm in 3μm to. Pi substrate Ezo resistance element is formed, in the groove 23 4 [mu] m pitch depth 0.3μm in the groove bottom width 4 [mu] m, was formed three by dry etching, the piezoresistive element surface An electric insulating layer 2 of silicon oxide is formed on the silicon surface including the top and the bottom of the groove. A silicon oxide electrical insulating layer 21 was formed by sputtering while the substrate was tilted and rotated about 15 degrees so that an insulating layer was also formed on the side wall surface of the groove 23. An aluminum-based metal wiring 22 having a width of 3 μm and a thickness of 0.2 μm was formed on the bottom surface by sputtering. The upper surface of the metal wiring 22 formed on the bottom surface of the groove 23 and the electric insulating layer 21 on the Y-axis piezos 14 ′ and 14 ″. The distance g5 between the upper surfaces was 0.1 μm. In this embodiment, Y-axis piezos 14 ′ and 14 ″ are formed between the grooves 23 formed at a pitch of 4 μm. In the present embodiment, the central groove 23 has the same twist in the width direction of the beam. A dummy groove is provided in the central portion so as to be symmetrical with respect to the center line of the beam portion 12. A dummy metal wiring 22 ′ having no end connected thereto is formed on the bottom surface of the central dummy groove. The center line of the central dummy groove and the center line in the width direction of the beam portion 12 coincide with each other, and the center line in the width direction of the beam portion 12 and the center line of the combination of the groove having the piezoelectric element and the metal wiring. The grooves having the piezo elements and the metal wiring are symmetrically arranged with respect to the beam center line, so that unnecessary twisting does not occur in the width direction of the beam portion. To open the beam, the acceleration sensor is subjected to excessive impact and the beam portion 12 is enlarged. Even if the metal wiring 22 is deformed, the upper surface of the metal wiring 22 does not collide with the regulation plate, and therefore, it is possible to prevent generation of a new offset voltage due to the deformation of the metal wiring 22.

2b) is near the central portion of the beam portion 12 in the longitudinal direction, and therefore the Y-axis piezo 1 from FIG. 2a).
4 'and 14 "are removed. FIG. 2c) is a frame portion, so that there is no need to form a groove, but on the same surface as the metal wiring 22 formed in the groove 23 of the beam portion 12. In order to connect to the metal wiring of the beam portion 12, a groove 23 was formed in the beam portion 12 at the same depth as the beam portion.Because the frame portion is not deformed, the groove 23 is one piece having a width of 150 μm and is formed on the bottom surface of the groove. The width and arrangement of the metal wiring to be formed were arbitrary.
Further, the number of the metal wirings 22 in the frame portion differs depending on the location of the cross section. Since the metal wiring of the frame part, the weight part, and the beam part is formed at the same time, the metal wiring thickness of the frame part was set to 0.2 μm, which is the same as that of the beam part.
In FIG. 2d), since there is a high risk of contact with the regulating plate at the weight portion, the groove portion 23 is formed and the metal wiring 22 is formed on the bottom surface of the groove. In order to avoid a change in the position of the center of gravity of the weight part 11, the grooves 23 are arranged so as to be symmetrical with respect to the center of the weight part. FIG. 2 e) is a portion where the X-axis piezo and the Z-axis piezo are formed on the beam portion 12 .

A method of manufacturing the acceleration sensor element 9 other than the groove and metal wiring described above will be briefly described.
An SOI (Silicon on Insulator) wafer having a laminated structure of a silicon oxide layer of about 1 μm and a silicon layer of 6 μm was used on a 410 μm thick silicon plate. Patterning is performed with a photoresist, and boron is added to the silicon layer 1 to 3 × 10 ¹⁸ atoms /
After producing piezoresistive elements by implanting cm ³ , a high concentration diffusion layer connecting the piezoresistive elements was formed. The high-concentration diffusion layer was implanted with 1 to 3 × 10 ²¹ atoms / cm ³ of boron. In order to protect the piezoresistive element from external ions and to ensure insulation between silicon, metal wiring, and electrodes, an electrically insulating layer of silicon oxide was formed to a thickness of 0.1 μm. Patterning was performed with a photoresist, and a groove 23 was formed in the flexible portion from the frame portion to the weight portion using an ion milling device. The groove width was 4 μm and the groove depth was 0.3 μm. After forming the groove, an insulating film of silicon oxide was formed to a thickness of 0.1 μm. Metal wiring and electrodes connected to the piezoresistive elements, flexible portions, and the like were formed using a photoresist patterning and sputtering film forming apparatus, a dry etching apparatus, and the like. The thickness of the metal wiring film formed on the bottom surface of the groove was 0.2 μm, and the upper surface of the metal wiring was recessed by 0.1 μm from the upper surface of the electrical insulating layer on the piezoresistive element. Similarly to the flexible part, the metal wiring formed on the weight part and the frame part was recessed by 0.1 μm from the upper surface of the electrical insulating layer on the piezoresistive element. Since the silicon oxide layer serves as an etching stopper, only the silicon layer is etched.

With the piezoresistive element side of the wafer on which many acceleration sensor elements 9 are formed facing down,
A metal plate having a high thermal conductivity kneaded with a resin was adhered to the dummy substrate. In order to dry-etch 400 μm of silicon wafer part of SOI wafer, it is important to cool the work piece because it takes a long time in plasma with SF ₆ and oxygen introduced. It adheres to a high dummy substrate. Since only silicon is dry etched, the silicon plate is etched but the silicon oxide layer remains.
The silicon oxide layer was removed by chemical etching while immersed in a hydrofluoric acid solution while attached to the dummy substrate.
With the SOI wafer on which the beam part, the weight part, and the support frame part are formed bonded to the dummy substrate,
After separating into chips of the acceleration sensor element 9 using a cutting grindstone, the adhesive was removed with a solvent to obtain an acceleration sensor element alone.

The second embodiment of the present invention has a structure in which a dummy groove and a dummy metal wiring are not formed, and the number of piezos and the number of metal wirings are the same in the piezo part. FIG. 3b) shows the beam 1 of FIG. 3a).
FIG. 3c) shows the beam part 1 in FIG. 3a).
2 is a gg ′ cross section at a position without Y-axis piezo. FIG. 3 shows a structure in which the dummy grooves and dummy wirings in FIG. 1 are not formed. In FIG. 3b), since no dummy groove is formed, the Y-axis piezo 14 '
And 14 ″ is 8 μm. The center line between the Y-axis piezos 14 ′ and 14 ″ and the center line of the beam width coincide with each other. FIG. 3c) is the same as FIG. B) except that there is no piezo. The dimensions of the grooves and metal wiring are the same as those in the first embodiment. Although not shown in FIG. 3, the X-axis piezo and Z-axis piezo beams similarly did not form a dummy groove and dummy wiring in the center.

In the third embodiment of the present invention, the number of piezos is greater than the number of grooves. Even on the beam, the number of metal wiring may differ depending on the piezo wiring method. FIG. 4 a) shows the Y-axis piezo 1 of the beam part 12.
4b) is a cross-sectional view of the beam portion 12 without the Y-axis piezo, and FIG. 4c) is a cross-sectional view of the position including the X-axis and Z-axis piezo. The arrangement is such that the groove in which the metal wiring is formed by the piezoresistive element is sandwiched, and the dimensions of the groove, the piezoresistive element, the metal wiring, etc. are the same as in Example 1. The width of the piezoresistive element is 3 μm and the pitch is The groove width is 4 μm, the groove depth is 0.3 μm, and the silicon oxide electrical insulating film 2 is formed to a thickness of 0.1 μm after the groove formation.
1 was formed. An aluminum-based metal wiring 22 having a width of 3 μm and a thickness of 0.2 μm was formed on the bottom surface of the groove 23. The center line of the groove 23 and the center line in the width direction of the beam portion 12 in FIGS. In FIG. 4c), Z-axis piezos 15 ′ and 15 ″ are arranged so as to sandwich the X-axis piezos 13 ′ and 13 ″, and a groove 23 having a metal wiring 22 is arranged between the piezoresistive elements. The center line of the central groove 23 and the center line in the width direction of the beam portion 12 were made to coincide. By aligning the center line in the width direction of the beam portion 12 with the center line of the groove having the metal wiring in the center portion and arranging the piezo element and the groove having the metal wiring symmetrically with respect to the beam center line, the width of the beam portion Unnecessary twist in the direction does not occur. Further, since the upper surface of the metal wiring is spaced by g5, the upper surface of the metal wiring 22 does not collide with the regulation plate even if the acceleration sensor is subjected to an excessive impact and the beam portion 12 is greatly deformed. Therefore, generation of a new offset voltage due to deformation of the metal wiring 22 can be prevented.

In the fourth embodiment of the present invention, a plurality of metal wirings are formed in the groove of the beam portion. A cross section of a position including an X-axis piezo and a Z-axis piezo is shown. The embodiment varies depending on how many metal wirings are formed in one groove. Typical cross-sectional structures of four piezoresistive elements and four metal wirings are shown in FIGS. 5a) to 5d). ing. In any of the embodiments, the center line of the beam portion 12 in the width direction is matched with the center line of the combination of the groove having the piezoelectric element and the metal wiring, and the groove having the piezoelectric element and the metal wiring is symmetrical with respect to the beam center line. It is arranged. Further, a gap of g5 is provided between the upper surface of the metal wiring and the upper surface of the electrical insulating layer on the piezoresistive element. In FIG. 5 a), two X-axis metal wires 22 are arranged in the central groove 23, X-axis piezo and Z-axis piezo are arranged on both sides from the metal wire side, and metal wires 22 are arranged outside the Z-axis piezo. The groove | channel 23 which has was formed. In FIG. 5 b), all four metal wirings are arranged in the central groove 23. In FIG. 5c), four piezoresistive elements are arranged in the center, grooves 23 are formed on both sides of the piezo, and two metal wirings 22 are provided on the groove bottom surface. FIG. 5d) shows FIG.
) Of the piezoresistive element and the groove are reversed. In any case, the metal wiring 22 is symmetrically arranged with respect to the center line in the width direction of the groove 23.

The acceleration sensor of Example 1 (Sample # 1), the acceleration sensor of Example 2 (Sample # 2), the acceleration sensor of Example 3 (Sample # 3), and the acceleration sensors of FIGS. 5a) to 5d) of Example 4 (Samples # 4 to # 7) were manufactured, and whether or not a new offset voltage was generated due to deformation of the metal wiring when excessive acceleration was applied was investigated. For comparison, a conventional acceleration sensor (Sample # 8
) Was also tested. In order to measure the change rate of the offset voltage, the measurement of the offset voltage is a value that does not pass through the offset correction circuit. In addition, when measuring the offset voltage before and after the impact application, the measurement temperature fluctuation is suppressed to within 1 ° C., so that no offset voltage is generated due to temperature change.
Because the measurement conditions such as the holding angle of the acceleration sensor at the time of offset voltage measurement are fixed,
The change in the offset voltage before and after the impact application is considered to be due to the deformation of the metal wiring. Each 1,000 acceleration sensors were manufactured and tested. The evaluation was performed according to the following procedure. I)
Offset voltage measurement, b) Impact application, c) Offset voltage measurement, d) Offset voltage is ±
Samples that fluctuate by more than 10% are considered to be defective products.
For the fluctuation sample of less than 0%, the impact application in b) and the offset voltage measurement in c) were repeated. (B) By repeating c) 50 times, a maximum of 51 impacts were applied. In other words, about 51,000 acceleration sensors of samples # 1 to # 8 were tested. The occurrence rate of the offset voltage in the conventional product exceeding the allowable range is as low as 0.02%, so it is necessary to test a number of acceleration sensors. The test method was adopted. An occurrence rate of 0.02% means that one offset voltage exceeds the allowable range with 5,000 acceleration sensors. 51,000 samples # 1 to 1
By using the # 8 acceleration sensor, it is possible to obtain a result capable of determining the effect.

After fixing 100 accelerometers to a 2 mm thick steel jig and measuring the offset voltage, the sensor was naturally dropped from a height of 1 m on a 100 mm thick wooden board attached to the steel jig, giving an impact. It was. When dropped from this height, an impact of about 1,500 to 2,000 G is applied to the acceleration sensor. The direction in which the impact is applied is the Z-axis direction, and the gap g3 between the regulating plate 3 and the beam portion 12 is applied.
This is the direction in which becomes smaller. After applying an impact, the offset voltage is measured and the offset voltage is ±
Samples that fluctuated by more than 10% were removed from the iron jig as a defective product and examined for decomposition. The acceleration sensor with an offset voltage variation of less than ± 10% was subjected to another impact and the test was continued.

In the acceleration sensor of the invention of the present invention of sample # 1 to sample # 7, the offset voltage is ± 10%
None of these changed. In sample # 8 of the conventional acceleration sensor tested for comparison, the six acceleration sensors exceeded the offset voltage fluctuation ± 10%. Occurrence frequency is about 0.0
It was 12%. When the acceleration sensor was disassembled and investigated, a part of the metal wiring 22 provided on the beam portion 12 of the acceleration sensor element was deformed so as to be crushed. Of the 6 defective products of the conventional product, the part that was deformed as if the metal wiring was crushed was 5 parts near the weight part of the beam part.
There were one piece and one weight part. Also from this result, the metal wiring upper surface of the beam portion and the weight portion is recessed from the upper surface of the electrical insulating layer on the piezo element, the deformation of the metal wiring does not occur even if excessive impact is applied,
It was confirmed that the effect of preventing the generation of the offset voltage due to the deformation of the metal wiring is great.

In addition, an acceleration sensor using an IC chip was manufactured for the regulating plate 3 and it was evaluated whether or not a latch-up phenomenon occurred when excessive acceleration was applied. Using the acceleration sensor element shown in Example 1, 100 acceleration sensors were made and tested. The back surface of the IC chip facing the metal wiring is not insulated from the silicon surface. With the acceleration sensor attached to an iron jig, the electrical output was measured on a wooden board with a thickness of 100 mm and dropped naturally from a height of 1 m to determine whether there was a latch-up phenomenon. 100 acceleration sensors were tested 10 times each (1000 times), but no latch-up phenomenon occurred. The acceleration sensor of the present invention has a structure in which the metal wiring of the acceleration sensor element does not come into contact with the regulation plate 3 even when excessive acceleration is applied. Therefore, not only the occurrence of offset voltage due to the deformation of the metal wiring but also the latch-up phenomenon is prevented. The effect of was also obtained.

It is a perspective view of the acceleration sensor element of Example 1 of this invention. It is a-a 'to e-e' sectional drawing of the acceleration sensor element of Example 1 of this invention. It is the perspective view and sectional drawing of the acceleration sensor element of Example 2 of this invention. It is sectional drawing of the acceleration sensor element of Example 3 of this invention. It is sectional drawing of the acceleration sensor element of Example 4 of this invention. It is a disassembled perspective view of the conventional acceleration sensor. It is sectional drawing of the conventional acceleration sensor, and the top view of an acceleration sensor element. FIG. 6 is a layout diagram of piezoelectric elements and metal wiring of a conventional acceleration sensor.

Explanation of symbols

1 case, 2 sensor chip,
3 Regulatory plate, 4 chip terminal,
5 wires, 6 case terminals,
7 External terminal, 8 Case lid,
9, 9 'beam type triaxial acceleration sensor element, 9 "diaphragm type triaxial acceleration sensor element,
10 frame, 11 weight,
12 beam, 13, 13 ', 13 "X-axis piezo,
14, 14 ', 14 "Y-axis piezo, 15, 15', 15" Z-axis piezo,
16, 17 Adhesive, 19 Diaphragm,
20 3-axis acceleration sensor, 21 insulating layer,
22 metal wiring, 22 'dummy metal wiring,
23 grooves, 24 high-concentration diffusion layers,
29 Connection part.

Claims (6)

Piezo resistor having a frame portion, a weight portion is held in the frame portion via a flexible portion, and the piezoresistive elements provided on the flexible portion, a piezoresistive acceleration sensor element having a wiring connecting them In the piezoresistive acceleration sensor element, the piezoresistive element forming surface side of the piezoresistive acceleration sensor element has at least one groove extending from the frame part to the weight part. An electrical insulation layer is formed on the piezoresistive element surface and the groove bottom surface, and metal wiring is formed on the electrical insulation layer on the groove bottom surface. The upper surface of the metal wiring is recessed from the upper surface of the electrical insulation layer on the piezoresistive element. And
Furthermore, the bottom width w of the groove formed in the flexible portion of the piezoresistive acceleration sensor element is 110% or more of the metal wiring width a, and the piezoresistive acceleration sensor. Grooves formed in one section of the flexible portion of the piezoresistive acceleration sensor element are alternately arranged with n piezoresistive elements, and (n−1) grooves are provided, and one wiring is provided in each groove. The piezoresistive acceleration sensor according to claim 1 , wherein the piezoresistive acceleration sensor is formed. Grooves formed in one section of the flexible portion of the piezoresistive acceleration sensor element are alternately arranged with n piezoresistive elements, and (n + 1) grooves are formed, and one wiring is formed in each groove. The piezoresistive acceleration sensor according to claim 1 . A groove formed flexible portion of the piezoresistive acceleration sensor element is formed between the piezoresistive element, according to claim 1, characterized in that one or more metal wiring is formed in the groove Piezoresistive acceleration sensor. A groove formed flexible portion of the piezoresistive acceleration sensor element is formed on both sides of the piezoresistive element, to claim 1, characterized in that one or more metal wiring is formed in the groove The described piezoresistive acceleration sensor. At least a portion of the metal interconnect upper surface of the piezoresistive acceleration sensor weight portion and the frame portion to form metal wiring of the device, to claim 1, characterized in that recessed from electrically insulating layer upper surface of the piezo-resistive element The described piezoresistive acceleration sensor.

Images