Accelerometer

This application claims priority under 35 USC 119 from Japanese Patent Application No. 2003-379403, the disclosure of which is incorporated by reference herein.

1. Field of the Invention

The present invention relates to an accelerometer, and particularly to an accelerometer manufactured by applying semiconductor micro-machining technology, and a package structure for the accelerometer.

2. Description of the Related Art

In recent years, a technique for manufacturing a microscopic structure having a size of several hundred micrometers or thereabouts by use of micro-machining that applies semiconductor micro-machining technology has drawn much attention. Applications of such microscopic structures to various types of sensors, optical switches used in the field of optical communication, high-frequency components and the like have been studied. Generally, such components that apply micro-machining are manufactured by using a silicon process, so that these components can be integrated into the same chips as those of an integrated circuit of a signal processing system. As a result, a system having one certain function can be formed with only a single chip. Thus, an element having the above function is referred to as Micro Electrical Mechanical System (MEMS) or Micro System Technology (MIST).

Components utilizing MEMS include an accelerometer that is widely applied to an air bag control apparatus for an automobile, an information measuring system of an underground environment such as seismic activity or the like, an earthquake-resistant system of information communication components, and the like. Such accelerometers have been disclosed in, for example, JP-A No. 7-225240 and JP-A No. 11-248737.

FIG. 2 is a cross-sectional view showing the structure of a conventional accelerometer disclosed in JP-A No. 7-225240. This accelerometer has a sensor chip 1 that detects an acceleration. The sensor chip 1 is formed by a peripheral frame 1 a , an internal mass portion 1 b , and two beams 1 c that elastically support the mass portion 1 b on the frame 1 a in a cantilevered manner. These components are integrally formed with a silicon wafer by use of semiconductor manufacturing technology. A piezoelectric resistive element (not shown) whose resistance value varies depending on an amount of deflection occurring in the beam is formed on the upper surface of the beam 1 c . The piezoelectric resistive element is connected to a connection pad on the upper surface of the frame 1 a . A blocking portion 2 having the shape of a rectangular frame is provided in an outer peripheral region of the frame 1 a of the sensor chip 1 . Further, a glass cover 3 is bonded to the lower surface of the frame 1 a.

A sensor main body comprised of the sensor chip 1 , the blocking portion 2 , and the cover 3 is placed on a mounting substrate 4 , and thereafter, a connecting terminal of a detection circuit or the like, that is provided on the mounting substrate 4 , and a connecting pad of the sensor chip 1 are connected by a bonding wire 5 . Further, a resin portion 6 is formed extending from the blocking portion 2 in a manner to encapsulate the sensor chip 1 and the bonding wire 5 . The blocking portion 2 is provided around the frame 1 a so as to have the shape of a rectangular frame. Therefore, even when the accelerometer is molded by molding resin on the mounting substrate 4 , the molding resin does not come into the frame 1 a , and elastic displacement of the beams 1 c or the mass portion 1 b is not impeded.

In the aforementioned accelerometer, the sensor main body is protected by the resin portion 6 . Therefore, an impact caused by dropping of the accelerometer is alleviated, and it is unlikely that the sensor may be broken. As a result, reliability can be raised. Further, heat distortion due to the environment, or the like is restrained by the resin portion 6 , thereby improving the temperature characteristics.

However, in the aforementioned accelerometer, the sensor chip 1 is bonded to the glass cover 3 , and the cover 3 is placed on the mounting substrate 4 , and thereafter, these components are entirely sealed with the molding resin 6 . For this reason, there exist problems that the thickness of the accelerometer becomes larger and a process for bonding the cover 3 is required, thereby increasing manufacturing costs.

On the other hand, if the sensor chip 1 is directly bonded onto the mounting substrate 4 without using the cover 3 for the purpose of cost reduction, a problem newly arises that an adhesive seeps out from the frame 1 a and reaches the bottom of the mass portion 1 b , thereby causing the mass portion 1 b and the mounting substrate 4 to adhere to each other. If, in order to solve the aforementioned problem, a gap between the mass portion 1 b and the mounting substrate 4 is made larger (for example, 50 μm), another problem arises that the amount of movement of the mass portion 1 b due to an acceleration or the like may increase and the beams 1 c that support the mass portion 1 b may be broken.

It is an object of the present invention to provide an accelerometer having a small thickness and a high resistance to breakage and manufactured at a lower cost.

In accordance with a first aspect of the present invention, there is provided an accelerometer that comprises: a sensor chip including a weight portion that detects force imparted from outside, a frame portion that surrounds the weight portion, a beam portion that flexibly supports the weight portion by connecting an upper portion of the weight portion to an upper portion of the frame portion so that a bottom surface of the weight portion is placed at a position higher than the bottom surface of the frame portion by a predetermined amount, and a sensor element of which electric resistance varies depending on an amount by which the beam portion deflects; a mounting substrate on which the sensor chip is mounted by fixing the bottom surface of the frame portion of the sensor chip at a predetermined position by an adhesive portion having a predetermined thickness with an adhesive applied thereto; and a spacer that restricts an amount of movement of the weight portion in a downward direction, the spacer being formed by applying an adhesive similar to the adhesive portion at a position on the surface of the mounting substrate surface, which position corresponds to the central position of the weight portion of the sensor chip, by a thickness that is larger than the aforementioned predetermined thickness, with a predetermined gap being formed between the bottom surface of the weight portion and the spacer.

In accordance with a second aspect of the present invention, there is provided an accelerometer that comprises: a sensor chip including a weight portion that detects force imparted from outside, a frame portion that surrounds the weight portion, a beam portion that flexibly supports the weight portion by connecting an upper portion of the weight portion to an upper portion of the frame portion so that a bottom surface of the weight portion is placed at a position higher than the bottom surface of the frame portion by a predetermined amount, and a sensor element of which electric resistance varies depending on an amount by which the beam portion deflects, a receptacle that is formed by an exterior frame portion and a bottom portion and has a space that is as large as that allows accommodation of the sensor chip, in which the sensor chip is mounted and disposed within the space in such a manner that the bottom surface of the frame portion of the sensor chip is fixed to the bottom portion by an adhesive portion having a predetermined thickness with an adhesive applied thereto, and a spacer formed by applying an adhesive similar to the adhesive portion at a position on the bottom surface of the receptacle, which position corresponds to the central position of the weight portion of the sensor chip, by an amount of a predetermined thickness that is larger than the aforementioned predetermined thickness, with a predetermined gap being formed between the bottom surface of the weight portion and the spacer, the spacer being provided so as to restrict an amount of movement of the weight portion in a downward direction.

In accordance with a third aspect of the present invention, there is provided an accelerometer that comprises: a sensor chip including a square column-shaped central weight portion and square column-shaped peripheral weight portions respectively connected to four corners of the central weight portion, which weight portions are provided for detecting force imparted from outside, a rectangular parallelepiped-shaped frame portion that surrounds the central weight portion and peripheral weight portions, four beam portions which flexibly support the central weight portion by connecting upper portions of four sides of the central weight portion respectively to upper portions of four sides of the frame portion so that a bottom surface of the central weight portion is placed at a position higher than the bottom surface of the frame portion by a predetermined amount, and a sensor element whose electric resistance value varies depending on an amount by which the beam portions deflect, all of which components are integrally formed on a semiconductor substrate, a mounting receptacle formed by an exterior frame portion and a bottom portion and having a space that is about as large as that containing the sensor chip therein, the receptacle being provided in such a manner that the sensor chip is disposed and mounted in the space by fixing the bottom surface of the frame portion of the sensor chip at a predetermined position on the bottom surface by an adhesive portion having a first thickness and an adhesive applied thereto; and a spacer formed, by applying an adhesive similar to the adhesive portion of the frame portion, as one pattern, by an amount of a second thickness larger than the first thickness at the same time as with the adhesive portion, at a position on the bottom surface of the receptacle, which position corresponds to the central position of the central weight portion of the sensor chip, with a predetermined gap being formed between the bottom surface of the central weight portion and the spacer, the spacer being provided so as to restrict an amount of movement of the central weight portion in a downward direction.

In the present invention, a spacer is formed, by applying the same material as the adhesive by a predetermined thickness, on a mounting substrate to which the frame portion of the sensor chip is fixed and at a position corresponding to the central position of the weight portion of the sensor chip. As a result, even if the distance between the weight portion of the sensor chip and the mounting substrate is set to be larger, an amount of movement of the weight portion in the downward direction is limited by the spacer. Therefore, there is no danger of the beam portion being damaged. Due to the distance between the weight portion and the mounting substrate being set to be larger, there is no possibility that, when the sensor chip is fixed to the mounting substrate, the adhesive seeps out and reaches the lower region of the weight portion, and the weight portion and the mounting substrate may be bonded to each other. Accordingly, no conventional glass cover is required, and an accelerometer having a small thickness and a high resistance to breakage and manufactured at a lower cost can be obtained.

The foregoing and other objects and novel features of the invention will become apparatus more completely from the following description of preferred embodiments taken in connection with the accompanying drawings. However, these drawings are intended to be only illustrative, but are not intended to be construed as narrowing the scope of the invention.

FIG. 1A is a structural diagram of an accelerometer showing an embodiment of the present invention.

FIG. 1B is a cross-sectional view taken along the line B—B in FIG. 1A .

FIG. 2 is a cross-sectional view showing the structure of a conventional accelerometer.

Referring now to FIGS. 1A and 1B , an accelerometer according to one embodiment of the present invention is shown. This accelerometer has a sensor chip 10 that detects an acceleration, and a receptacle 20 in which the sensor chip 10 is accommodated. The sensor chip 10 is constructed in such a manner that a peripheral frame portion 1 , a central weight portion 12 a , a peripheral weight portion 12 b , and four beam portions 13 which flexibly support the central weight portion 12 a on the frame portion 11 are integrally formed by processing a silicon substrate using semiconductor manufacturing technology. The frame portion 11 may be formed into a rectangular parallelepiped that is approximately 1.5 mm by 1.5 mm by 0.6 mm.

The central weight portion 12 a has the shape of a square column, and a similar square column-shaped peripheral weight portion 12 b is connected to each of four corners of the central weight portion 12 a . The four sides on the surface of the central weight portion 12 a are connected to the surface of the frame portion 11 by the four beam portions 13 having elasticity, respectively. The frame portion 11 is formed so that the thickness thereof is larger than those of the central weight portion 12 a and the peripheral weight portions 12 b by 50 μm or thereabouts. In other words, the frame portion 11 is formed such that, when it is placed on a flat plate with the surface of the frame portion 11 facing upward, a gap of approximately 50 μm is produced between the bottom of the central weight portion 12 a and the peripheral weight portions 12 b , and the flat plate.

Although not illustrated in the drawings, a piezoelectric resistive element, of which electric resistance value varies depending on the amount of deflection occurring in the beam portion 13 , is formed on the surface of the beam portion 13 . The piezoelectric resistive element is connected to each of a plurality of pads 14 formed on the surface of the frame 11 .

The receptacle 20 is formed by an exterior frame portion 21 and a bottom portion 22 and is formed into an open-topped box having a space that is sufficiently large to accommodate the sensor chip 10 . A plurality of pads 23 each corresponding to the pad 14 of the sensor chip 10 is formed on the upper surface of the exterior frame portion 21 , and these pads 14 , 23 are connected to each other by a wire 31 .

The bottom surface of the frame portion 11 of the sensor chip 10 is bonded to the bottom portion 22 within the receptacle 20 by an adhesive portion 32 having a thickness of about 25 μm. As the adhesive portion 32 , an adhesive similar to that used as a sealing resin such as epoxy or the like is used. Further, a spacer 33 having a thickness of about 50 μm is simultaneously formed, by applying the same adhesive as the adhesive portion 32 , at the central region on the internal bottom portion 22 of the receptacle 20 at a position that faces the lower side of the central weight portion 12 a of the sensor chip 10 . Accordingly, a gap G of approximately 25 μm is formed between the central weight portion 12 a and the spacer 33 .

In the aforementioned accelerometer, the sensor chip 10 and the receptacle 20 are produced respectively by separate processes. Subsequently, the adhesive portion 32 and an adhesive that forms the spacer 33 are applied, as one pattern, to the internal bottom portion 22 of the receptacle 20 , and the sensor chip 10 is mounted in such a manner that the bottom surface of the frame portion 11 of the sensor chip 10 is disposed in opposing relationship with the adhesive portion 32 . Then, when the adhesive is hardened, the sensor chip 10 is fixed to the interior of the receptacle 20 , and the spacer 33 is formed at the lower side of the central weight portion 12 a with the gap G formed therebetween. Thereafter, the pads 14 and 23 are connected to each other by bonding the wire 31 thereto, thereby completing the accelerometer shown in FIG. 1B .

As described above, the accelerometer of the present embodiment is formed in such a manner that the spacer 33 is disposed in the internal bottom portion 22 within the receptacle 20 at a position directly below the central weight portion 12 a . Accordingly, even if the distance between the central weight portion 12 a of the sensor chip 10 , and the bottom portion 22 of the receptacle 20 is set so as to become large, the spacer 33 prevents movement of the central weight portion 12 a . Therefore, there is no danger that the beam portions 13 may be damaged. By setting the distance between the central weight portion 12 a and the bottom portion 22 at a large value, there is no possibility that, when the sensor chip 10 is mounted on the receptacle 20 , the adhesive seeps out and reaches the lower side of the central weight portion 12 a and the peripheral weight portions 12 b , thereby causing the weight portions 12 a and 12 b , and the bottom portion 22 of the receptacle 20 to adhere to each other. As a result, there are offered advantages in that no conventional glass cover is required any more, and an accelerometer that has a small thickness and a high resistance to breakage and that is manufactured at a lower cost can be obtained.

The aforementioned embodiment is intended to demonstrate the technical contents of the invention. Note that the invention should not be narrowly interpreted by being limited only to the aforementioned embodiment, but it could be put into practice using various modifications thereof within the scope of the appended claims. Such modifications are shown below.

(a) The structure of the sensor chip 10 is not limited to that as exemplified above. For example, the present invention can be applied to the structure shown in FIG. 2 as well. (b) The receptacle 20 on which the sensor chip 10 is fixedly mounted is not limited to the box-shaped structure as exemplified above. For example, the present invention can be applied to a mounting substrate formed into a flat plate as shown in FIG. 2 as well. (c) The aforementioned dimensions and materials are not limited to those as exemplified above.

The present invention can be widely used in the industrial fields that apply microelectronics technologies including various types of automatic control systems, measuring systems, information communication systems and the like.