AN ACCELEROMETER WITH A SYMMETRICALLY BONDED PROOF-MASS AND METHOD OF ITS FABRICATION METHOD

Abstract: This invention relates to a cantilever beam microaccelerometer symmetrically bonded proof-mass comprised of a lower portion having a cantilever (2), a lower proof-mass (1b) and a support (9) which are integrally formed on one side of the unique and homogeneous silicon plate so as to easily install a piezoresistive material (3) for detecting acceleration, self-diagnostic resistors (4) for determining whether the cantilever (2) is damaged or not, and a lead wire (14) on the other side of the plate unfabricated; and an upper portion includes a upper proof-mass (1a) which is same material and size as those of the lower proof-mass (1b) and symmetrically bonded on the unfabricated plane of the plate with respect to the cantilever.

Description: AN ACCELEROMETER WITH A SYMMETRICALLY BONDED PROOF-MASS AND METHOD OF ITS FABRICATION METHOD BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a cantilever beam accelerometer and its fabrication, and particularly to a cantilever beam microaccelerometer symmetrically bonded proof-mass and its fabrication method to increase fabrication ability, the accuracy of the size and simplability of the design. DESCRIPTION OF THE RELATED ART Recently, there has been growing interest in micromachined silicon accelerometers for applications to automotive electronic systems, such as airbag, anti-lock braking system, active suspension, electronic steering and guidance systems. Among them, airbag accelerometers open the most immediate and the large market, whose demand for high sensitivity, high reliability, low cost and mass production cannot be met easily by conventional electromechanical sensor technology. For automotive airbag applications, piezoresistive silicon accelerometers shows a strong potential due to simple detection circuitry and low cost for moderate performance characteristics. Generally, conventional cantilever beam microaccelerometer fabricated of the unique and homogeneuous silicon plate and its fabrication method to have a beam and proof mass can be classified 3 types as follows. Fig la shows conventional unsymmetrical microaccelerometers which includes a cantilever 2, a proof mass 1, and a support 9, all of which are integrally formed on the one side of the plate. On the other side of the plate which is flat plane unfabricated, mounted are a piezoresistive material 3 for detecting acceleration 3, self-diagnostic resistors for determining whether the cantilever 2 is damaged or not, and a lead wire 14. Fig lb shows a conventional symmetrical accelerometer. The symmetrical accelerometer includes a cantilever 2, a support 9 which is symmetrically formed with respect to the cantilever 2 at one end of the cantilever 2, and a proof mass 1 which is also symmetrically formed with respect to the cantilever 2 at the other end of the cantilever. There is provided a piezoresistive material 3, self-diagnostic resistors, and a lead wire 14 on the plane of the cantilever. Fig. lc is still to show a conventional accelerometer, in which a proof- mass 1 is symmetrically assembled on a cantilever 2. A support 9 is formed on the same plane as that of the cantilever, and the piezoresistive material 3, the self-diagnostic resistors and the lead wire 14 are mounted on the plane.. Generally, compared to the fixed beam type, the conventional cantilever type accelerometers have an advantage of having more sensitive to acceleration as well as less sensitive to packaging induced and thermally induced stresses. The unsymmetrical cantilever type accelerator shown in Fig. la has a drawback in that relatively large transverse sensitivity caused by the offset between the weight-center of the proof mass and that of the beam. In addition, the symmetrical accelerometer illustrated in Fig. lb solves the transverse sensitivity problem, but makes it difficult to install piezoresistor 3, self-diagnostic resistors 4 and electric circuit 14 on the highly stressed area in the accelerometer. In the accelerometer depicted in Fig. lc, since the proof-mass 1 is separately manufactured and then symmetrically assembled on the cantilever 2 and the support 9 is designed to have the same plane as that of the cantilever 2, the problems occurring in the accelerometers shown in Figs, la and lb can be solved. However, additional process for assembling the proof-mass 1 on the cantilever 1 is required, increasing manufacturing costs and, during this process, there may be possibility of damaging the cantilever 2 and misalignment of the proof-mass 1. SUMMARY OF THE INVENTION Therefore, the present invention has been made in an effort to solve the above described problems of the conventional arts. The object of the present invention is to provides a method of fabricating a cantilever microaccelerometer with a symmerically bonded proof mass comprised of processes: integrally fabricating a cantilever, a lower proof mass and a support on the one side of the unique and homogeneuous silicon plate so as to easily install a piezoresistive material for detecting acceleration, self diagnostic resistors for determining whether the cantilever is damaged or not, and a lead wire on the other side of the plate unfabricated; bonding a upper proof mass which is same material and size as those of the lower proof mass on the unfabricated side of the plate for the total prooof mass being symmetrically arranged with the respect to the cantilever and mass offset being not occured; and cutting an unit cantilever microaccrometer from the series of the microaccrometer fabricated according to the process described above by etching method so that the accuracy of the size and simplicity of design can be increased. The other object of the present invention is to provides a cantilever microaccelerometer comprised of: a lower portion having a cantilever, a lower proof mass and a support which are integrally formed on one side of the unique and homogeneuous silicon plate so as to easily install a piezoresistive material for detecting acceleration, self-diagnostic resistors for determining whether the cantilever is damaged or not, and a lead wire on the other side of the plate unfabricated; and an upper portion includes a upper proof mass which is same material and size as those of the lower proof mass and symmetically bonded on the unfabricated plane of the plate with respect to the cantilever. The cantilever microaccelerometer according to the present invention can process yield, chip size and reproducibility of the accelerometer, while solving the transverse sensitivity problem of the cantilever accelerometers as well as accommodating a self-diagnostic resistor for detecting structural failure of microbeam. The cantilever microaccelerometer according to the present invention can be applied to automobile electronics systems, as well as to consumer electronics, and industrial eletrical measurement system etc.. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and together with the description, serve to explain the principles of the invention: Fig. la is a sectional view illustrating a conventional cantilever accelerometer with an unsymmetrical proof -mass; Fig. lb is a sectional view illustrating another conventional cantilever accelerometer with a symmetrical proof-mass; Fig. lc is a sectional view illustrating still another conventional cantilever accelerometer with a separate proof-mass; Fig Id is a sectional view illustrating a cantilever accelerometer according to a first embodiment of the present invention; Fig. 2 is a perspective view illustrating a cantilever accelerometer according to a preferred embodiment of the present invention in a piezoresistive measuring manner; Figs. 3a through 3e are sectional views for showing the steps for fabricating the cantilever accelerometer depicted in Fig. 2; Fig 4a is a top view illustrating a cantilever accelerometer according to a second embodiment of the present invention; Fig 4b is a sectional view illustrating a cantilever accelerometer according to a second embodiment of the present invention; and Fig. 5 is a sectional view illustrating a cantilever accelerometer according to a preferred embodiment of the present invention in a piezoresistive measuring manner. DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. The cantilever microaccelerometer according to the present invention is composed of a lower portion having a cantilever 2, a lower proof mass lb and a support 9 which are integrally formed on one side of the unique and homogeneuous silicon plate so as to easily install a piezoresistive material 3 for detecting acceleration, self-diagnostic resistors 4 for determining whether the cantilever is damaged or not, and a lead wire 14 on the other side of the plate unfabricated; and an upper portion includes a upper proof mass la which is same material and size as those of the lower proof mass and symmetically bonded on the unfabricated plane of the plate with respect to the cantilever 2. Fig 2 shows a preferred embodiment of the present invention in a piezoresistive measuring manner, which the cantilever 2, the upper and lower mass la. lb, self-diagnostic resistors 4 for determining whether the cantilever is damaged or not , and the support 9 are mounted. Detailed description of the operation principle of the present invention is as follows. As shown in Fig 2, for the absolute displacement, z a , of the support 9, and the absoluter displacement, z 0 , of the proof mass la, lb the relative displacement between mass la, lb and the support 9 can be represented as z = z o - z a The cantilever beam 2 in Fig 2 acts as spring where the mass la, lb and the drag force of the fluid around the mass la, lb acts as damper respectively. For the spring constant, K, of the beam 2, total mass, M, of the proof mass and damping constant of the damper, C, the equation of motion for the accelerometer can be written as follows. Mz + C(z o - z a ) + K(z o - z a ) = 0 (1) For a sinusoidal input, z 0 =z a e"" ', the relative displscement Z = z 0 - z a = z e ,(ω * ~φ } can be obtained as follows, in the case of ω /ω n ( 1, Z 7 = 2 L z Z a (2) ω n where the resonant frequency ω n = / K (3) J M and, damping ratio ζ - C (4) 2Mω n Thus, from the equation (2), it can be seen that the magnitude of the acceleration z a of the support 9 can be measured from the magnitude of the relative displacement Z, that is the deflection of the mass. Fig 2 and 3 show an example of the implementation of the accelerometer based on above principle with piezoresistive detection of the deflection from the stress at the end of the beam. That is, on the end of the cantilever 2 is mounted the piezoresistive material 3 represents the stress caused by the deflection of the beam as the change of the electro resistance. The desired resonant frequency ω n can be obtained by controlling the size of the proof mass and the beam, and the desired damping ratio ζ can be obtained by controlling the viscosity and the pressure of the fluid around the mass la, lb. As shown in Fig 2, the upper proof mass la is symmetrically bonded to the lower proof mass lb, thus the offset of the total mass is eliminated and the transverse sensitivity is improved. And as the beam 2 and the lower mass lb aer fabricated on the one side of the plate, the piezoresistive material 3, self diagnosis 4, electric circuit 14 and electrodes 5, 6 can be easily installed on the unfabricatd of the plate. A piezoeletric material can be mounted instead of the piezoresistive material depicted in Fig 2 and Fig 3. Referring to Fig 5, as flat plates 16a, 16b having flat electrodes 17a, 17b respectively are mounted on both sides of the microaccelerometer in accordance with the present invention through an upper insert plate 18a and a lower insert plate 18b, a capacity type accelerometer which measures the displacement of the proof mass 1 caused by acceleration as change of electrostatic capacity and a piezoelecric accelerometer which measures the displacement of the proof mass 1 as voltage generated piezoelectric material can be constituted. Fig 3 shows an example of fabrication steps for fabricating the cantilever accelerometer depicted in Fig. 2 by using silicon as a substrated material. Detailed description are as follows. (a) after passivation films 13 is formed the under surface of the silicon plate for etching the beam 2, the lower proof mass lb and cutting groove 15, the piezoresistive material 3 and the self diagnosis material 4 are installed on the upper surface of the silicon plate 12. (b) after an electrode seperator 11 for separating the both electrodes, the electric circuit 14 are formed on the upper side of the silicon plate 12, a sensing electrode 5 of the piezoresistive material 3 and a electrode 6 of self diagnosis material 4 is formed on the upper plane. (c) under the condition that the upper surface of the silicon plate 12 is protected, the beam 2, the lower proof mass lb and the cutting groove 15 are fabricated on the under side of the silicon plate by using the passivation film 13. At this time, the control for the thickness of the beam 2 is accomplished by using the silicon wafer which is already etched to the desired thickness. When the etched wafer is throughly pierced, the work for the etching silicon wafer is finished. (d) after the pasivation film in Fig 3c is removed, the upper proof mass la which is separately fabricated is symmetrically bonded on the oppsite position of the lower mass lb. (e) cutting the microaccelerometer along the cutting groove 15 and the unit microacclerometer can be attained. The above fabrication processes can be applied to the accelerometer having both supports or multi supports. Advantages of the present invention due to its structural characteristics and unique fabrication method are as follows. 1) As the beam 2 and the lower proof mass lb are fabricated on one 5 side of the plate material, the piezoresitive material 3, the self diagnosis 4, electric circuit 14 and electrodes 5, 6 are easily mounted on the flat plane unfabricated. 2) As the upper proof mass la is symmetrically bonded to the oppsite side of the lower prooof mass lb with respective to the beam 2, the o mass offset and the transverse sensibility can be reduced. 3) The efficiency of the fabrication process and productability can be increased by using the method in accordance with the present invention. 4) The accuracy and the simplicity of the fabrication can be increased by 5 using the pre etched wafer in accordance with the present invention. 0 5

Claims: WHAT IS CLAIMED IS : 1. A method of fabricating a cantilever microaccelerometer with a symmerically bonded proof mass comprised of following process: integrally fabricating a cantilever, a lower proof mass and a support on the one side of the unique and homogeneuous silicon plate so as to easily install a piezoresistive material for detecting acceleration, self diagnostic resistors for determining whether the cantilever is damaged or not, and a lead wire on the other side of the plate unfabricated; bonding a upper proof mass which is same material and size as thoes of the lower proof mass on the unfabricated side of the plate for the total prooof mass being symmetrically arranged with the respect to the cantilever and mass offset being not occured; and cutting an unit cantilever microaccrometer from the series of the microaccrometer fabricated according to the process described above by etching method so that the accuracy of the size and simplicity of design can be increased. 2. A method of fabricating a cantilever microaccelerometer with a symmerically bonded proof mass according to claim 1, wherein the piezoresistive and the self diagnosis material mounted on the beam are used to correct or testing self characteristics of the accelerometer, and the self-testing function 3. A method of fabricating a cantilever microaccelerometer with a symmerically bonded proof mass according to claim 1, the microaccelerometer is used as a capacity type accelerometer which measures the displacement of the proof mass caused by acceleration as change of electric power interruption capacity and a piezoelecric accelerometer which measures the displacement of the proof mass as voltage generated piezoelectric material. 4. A method of fabricating a cantilever microaccelerometer with a symmerically bonded proof mass according to claim 1, both supports or multi supports is mounted on the beam. 5. A cantilever microaccelerometer comprised of: a lower portion having a cantilever, a lower proof mass and a support which are integrally formed on one side of the unique and homogeneuous silicon plate so as to easily install a piezoresistive material for detecting acceleration, self-diagnostic resistors for determining whether the cantilever is damaged or not, and a lead wire on the other side of the plate unfabricated; and an upper portion includes a upper proof mass which is same material and size as those of the lower proof mass and symmetically bonded on the unfabricated plane of the plate with respect to the cantilever. 6. A cantilever microaccelerometer according to claim 1, a piezoelecric material is mounted instead of the piezoresitive for sensing the acceleration.

Inventors: CHO YOUNG-HO, KWAK BYUNG MAN, LEE KWY RO, PARK KWAN HUM, CHO, YOUNG-HO, KWAK, BYUNG, MAN, LEE, KWY, RO, PARK, KWAN, HUM
Applicants: CHO YOUNG HO, HYUNDAI MOTOR CO LTD, KOREA INST SCIENCE TECHNOLOGY, KWAK BYUNG MAN, LEE KWY RO, PARK KWAN HUM, CHO, YOUNG-HO, HYUNDAI MOTOR COMPANY, KOREA ADVANCED INSTITUTE OF SCIENCE AND TECHNOLOGY, KWAK, BYUNG, MAN, LEE, KWY, RO, PARK, KWAN, HUM
Application year: 1996
Main classification: G01P15
IPC codes: G01P15, G01P21, H01L41