Capacitance type accelerometer and method of manufacturing the same

Abstract: The invention relates to a capacitance type accelerometer and a method of manufacturing the same. The capacitance type accelerometer has a first silicon plate (2,7) formed as having a movable electrode part (7) which is moved according to acceleration, two second silicon plates (1, 3) which are disposed on both sides of the first silicon plate with a certain separation distance, and thermal oxide films (4, 5) which are respectively disposed between the first silicon plate except the movable electrode part (7) and the two second silicon plates (1, 3) and stick the first silicon plate (2) except the movable electrode and the two second silicon plates together. The process of manufacturing the capacitance type accelerometer is made by processing and cutting a wafer by dicing saw without coming into the accelerometer of cutting chips or cutting water.

Description: (Background of the invention) The present invention relates to a capacitance type accelerometer and a method of manufacturing the same using a semiconductor manufacturing process, this capacitance type accelerometer being suitable for body control or engine control of an automobile. It is required for an accelerometer for an automobile to detect acceleration of relative-low level (0 through ± 1.0 G) and the low frequency (0 to 10 Hz) at high accuracy. In addition, 1G = 9.8 m/S² herein. Such accelerometer, as well-known, includes a piezoelectric sensor conventionally utilizing a piezoelectric effect of a piezoelectric material, an electromagnetic servo system one having an electromagnetic feedback mechanism, a magnetic one utilizing a differential transformer, an optical one utilizing a photo-interrupter, and a strain gauge system or a capacitance system one utilizing a fine processing technique of silicon. Among them the capacitance type accelerometer utilizing the fine processing technique of silicon is considered the most favorable, which can detect acceleration of the low frequency at high accuracy and provide an inexpensive sensor. The largest problem for packaging the capacitance type accelerometer is the package of the lead for electrode. Therefore, several methods are known. A first example, as disclosed in U.S. Patent 4,483,194, is a method to fetch the lead for the movable electrode or the fixed electrode through a P⁺ diffusion region. The method has the problem in that etching is complicated and yield when bonded is poor, the P⁺ diffusion region becomes a large stray capacitance, and its voltage dependency is large, thereby deteriorating the dejection system. A second example, as shown in Pages 336 to 339, Transducers '87 (the 4th International Conference on Solid-State Sensors and Actuators), is a method to fetch the lead from a gap at the bonding portion of the silicon plate and a glass base. The method, when a detected chip is diced by a usual dicing saw after the silicon plate and glass base are bonded in the wafer state, water content or chips enters into a space around the electrode through the gap, thereby creating the problem in that the performance as the accelerometer is not obtainable. A third example, as disclosed "Miniature Silicon capacitance absolute pressure sensor", Page 255 to 260, I Mech E, 1981, is a method to fetch the lead through a bore perforating a glass base. This example is applied to the pressure sensor, in which the problem is created in the poor lead connection atone bore end caused by variation in metallizing condition and the temperature characteristic deterioration caused by a large difference in coefficient of thermal expansion between the solder charged in the other bore end and the glass base. A fourth example, as shown in U.S. Patent 4,609,968, is a method of using a silicon plate to the fixed electrode to package the silicon plate itself as part of the lead. In this example, a silicon plate to be the fixed electrode is applied at the periphery thereof with glass powder and baked and then polished at the surface to be like a mirror. A movable electrode and a silicon plate having a beam for supporting the electrode are laminated and bonded with anodic-bonding. The anodic bonding is to apply high voltage at a high temperature to glass and silicon to be bonded, in which the movable electrode is displaced by an electrostatic force and contacts with the silicon plate of fixed electrode so as to be discharged, whereby both the members cannot be bonded. In order to avoid this, a gap between the movable electrode and the fixed electrode must be previously designed to be larger, or spring constant of the beam must be designed to be larger (in brief, deformation of the movable electrode with respect to the unit acceleration). As the result, the problem is created in that high sensitivity essentially possessed by the capacitance type accelerometer must largely be sacrificed. The above-mentioned conventional technique is not considered as to the electrode lead package method, thereby creating the problem in the productivity, detection accuracy or sensitivity. GB-A-2 158 945 discloses a silicon capacitive accelerometer having the features of the precharacterizing part of daim 1, having alternatively a notch between the layers through which the fixed electrode is contacted or insulating material, only exemplified by Pyrex glass, between the laminated layers, and having several more obligatory features. Summary of the Invention An object of the present invention is to provide a capacitance type accelerometer and a method of manufacturing the same so that the accelerometer is superior in productivity, high in performance, and inexpensive to produce. This object is accomplished with the features of the independent claims. Dependent claims are directed on preferred embodiments of the invention. [Brief Description of the Drawings] Fig. 1 is a diagrammatic cross-sectional view showing an embodiment of an accelerometer in the present invention. Fig. 2 is a diagrammatic view showing an embodiment of a manufacturing process of the accelerometer shown in Fig. 1. Figs. 3 and 4 are diagrammatic cross-sectional views showing another embodiments of the accelerometers in the present invention. Figs. 5, 6 and 7 are developed cross-sectional views of the accelerometer shown in Fig. 1. Figs. 8(A), 8(B) and 8(C) are developed plane views of the embodiment of the accelerometer shown in Fig.7. Fig. 9 is an elevation of an embodiment of the accelerometer shown in Figs. 8(A), 8(B) and 8(C). Figs. 10(A) and 10(B) are equivalent circuits of the accelerometer. Figs. 11(A) and 11(B) are illustrations of a positioning method for a silicon plate in the wafer state at the embodiment of the present invention. Figs. 12, 13, 14 and 15 are developed plane views of the another embodiments of the accelerometer, Fig. 13 and 14 describing technological background. Fig 16 is a circuit diagram showing an embodiment of a signal processing circuit of an electro-static capacitance type accelerometer. Fig. 17 is a voltage wave form of a pulse width modulator. Fig. 18 is an output characteristic view of the accelerometer. [Detailed Description of the Preferred Embodiments] Next, the accelerometer of the present invention will be detailed in accordance with an embodiment shown in the drawing. Fig. 1 shows an embodiment of the present invention which concretes the manufacturing method. A detection unit of the embodiment of the capacitance type accelerometer comprises a silicon plate 1, a silicon plate 2 and a silicon plate 3, which are directly laminated and bonded through electrical insulating thermal oxidation films 4 and 5. At the silicon plate 2, a silicon beam 6 (a bridge-like portion) and a movable electrode 7 are preformed by etching prior to the bonding. The movable electrode 7 functioning as a weight is supported by the silicon beam 6, and gaps 8 between the movable electrode 7 and the silicon plates 1 and 3 change in dimension corresponding to the magnitude of acceleration vertical in the drawing and acting on the movable electrode 7. The silicon plates 1 and 3 opposite to the movable electrode 7 are the fixed electrodes. The capacity between the electrodes at the gap 8 varies corresponding to the acceleration acting on the detection unit, so that the acceleration is detected by use of the variation in the capacity. The movable electrode 7 is electrically connected with an external electronic circuit (not shown) through the silicon beam 6 and a bounding pad 9. As above-mentioned, since the silicon plates 1 and 3 serve also as the fixed electrodes, they are usable as the lead-out terminals of the fixed electrodes, thereby extermely simplifying the lead package method. Meanwhile, in a case of this embodiment, electric capacity at the bonding portions (parts 4 and 5 of thermal oxidation film in the drawing) between the silicon plate 2 and the silicon plates 1 and 3 is inserted in parallel to the electric capacity at the gaps 8. The capacity of the bonding portion becomes stray capacitance not at all affected by variation in acceleration. Accordingly, the electric capacity at this part is desired to be as smaller as possible. For this purpose, an area of bonding portion need only be as small as possible, and on the other hand, the thermal oxidation films 4 and 5 need only be as large as possible. A schematic manufacturing process of the detection unit shown in Fig. 1 is shown in Fig. 2, in which the silicon plates 1, 2 and 3 in the wafer state after finished of etching process and other pre-processings are fully cleaned to remove stain or dust from the bonding portion. Next, the silicon plates in the wafer state are aligned and thereafter laminated, and subjected to heat treatment for 2 hours, at a temperature of 800°C, and in the oxygen atmosphere. The heat treatment can completely directly bond the silicon plates with each other through the thermal oxidation films. Lastly, the three-layer bonded wafer of silicon is cut by the dicing saw, thereby enabling a large number of detection parts shown in Fig. 1 to be simultaneously manufactured. In this embodiment of the sensor which directly bonds the silicon plates only by the heat treatment, differently from the anodic-bonding method to apply high voltage, even when the size of gap 8 or the spring constant of silicon beam 6 is reduced, any trouble is not generated during the manufacturing, thereby obtaining an accelerometer of high sensitivity with ease. Other modified embodiments of the detection unit of the invention will be shown in Figs. 3 and 4. In addition, in the following drawings, the same reference numerals show the components of the same functions. When the silicon plates are directly bonded with each other by the method shown in Fig. 2, gas filled in the space around the movable electrode 7 and silicon beam 6 expands depending on a rate of heating up to the heat treatment temperature and the kind of atmospheric gas, whereby a gap is generated at part of laminated silicon plates 1, 2 and 3 to be bonded, whereby the bonding in the wafer state during the heat treatment may partially be improper. Therefore, as shown in the Fig. 3 embodiment, it is extremely effective to form at the silicon plate 1 a bore 10 serving to escape the expanded gas to the atmosphere. Next when the silicon capacitance type accelerometer is intended to extremely improve its sensitivity, the construction of detection unit shown in Fig. 4 is effective. The Fig. 4 embodiment forms polycrystal silicon layers 11 and 12 at parts of silicon plates 1 and 3 opposite to the movable electrode 7. Here, the polycrystal silicon layers 11 and 12 are of conductive material and substantially become fixed electrodes opposite to the movable electrode 7. Accordingly, in the Fig. 4 embodiment, the gaps between the movable electrode 7 and the polycrystal silicon layers 11 and 12 are reduced in width, thereby enabling an accelerometer of high sensitivity to be obtained. Also, since each gap can be of smaller width than thickness of each thermal oxidation film 4 or 5, the stray capacitance at parts of thermal oxidation films 4 and 5 is relative-reduced. In addition, the polycrystal silicon layers 11 and 12 fundamentally need only be conductor layers, which may be constructed of a metal layer, such as platinum. Figs. 5 through 7 are developed views of the construction of detection unit shown in the Fig. 1 embodiment before lamination. In Fig. 5, electrically insulating thermal oxidation films 4 and 5 are preformed only at the bonded portion between the silicon plates 1 and 3, and of at least 1 micron or more in thickness. Conversely, in the Fig. 6 embodiment, the thermal oxidation films 4 and 5 are formed only at the bonding of silicon plate 2. In the Fig. 7 embodiment, the thermal oxidation films 4 and 5 are preformed at all the silicon plates 1, 2 and 3, which is more favorable than the embodiments in Figs. 5 and 6 in order to reduce the stray capacity at the parts after bonded. Figs. 8(A), 8(B) and 8(C) are developed plan views of the detection unit of the silicon capacitance type accelerometer of the present invention shown in Fig. 7 in which hatched portions show parts of thermal oxidation film as the bonding portion of silicon plates with each other. Reference numerals 9a, 9 and 9c designate bonding pads of the silicon plate 1 (functioning as the fixed electrode), movable electrode 7, and silicon plate 3 (functioning as the fixed electrode), the electrodes being connected with an external signal processing unit. Fig. 9 is an elevation view of the detection unit of the accelerometer of the invention, the detection unit being electrically connected with the external signal processing circuit through lead wires 15, 16 and 17. Fig. 10 shows an equivalent circuit of the detection unit. In Fig. 10(A), capacities 18 and 19 are capacities at the gap 8 between the electrodes, which change corresponding to magnitude of acceleration. While, capacities 20 and 21 are those by the thermal oxidation film at the silicon plate bonding portion, which are not at all affected by the acceleration. Therefore, the capacities 20 and 21 are made sufficiently smaller in magnitude than those 18 and 19, so that it is desirable to indicate the equivalent circuit substantially in Fig. 10(B). Therefore, it is effective to reduce an area of bounding, to make the thermal oxidation film larger in thickness, to enlarge the area of the movable electrode, and to reduce dimension of the gap between the electrodes. Next, explanation will be given on the positioning method of the silicon wafer when the silicon plate is directly bonded by the heat treatment in the wafer state. Fig. 11 (A) is a schematic view of a silicon plate 25 in the wafer state, in which reference numeral 23 designates detection chips, so that one wafercan produce several hundreds accelerometer, and,a plurality of through bores 22 are formed at the silicon plate 25 by etching. Next, as shown in Fig. 11(B), guide pins 24 are inserted into the through bores 22 so that the silicon plates 1, 2 and 3 are laminated and simultaneously positioned to each other. Accordingly, in this embodiment, the positioning accuracy between the silicon plates depends on tolerance between each through bore 22 and the guide pin 24, thereby enabling positioning by accuracy of ± several 10 microns or less. Other modified embodiments of the detection unit of the silicon capacitance type accelerometer of the invention are shown in Figs. 12 and 13 (sectional views) and 14 and 15 (plan views). At first, Fig. 12 shows an example in which a movable electrode 7 is held by a silicon beam 6 in a cantilever fashion, which is different from the Fig. 1 embodiment in that the silicon beam 6 is not disposed on the axis of gravity of movable electrode 7. Next, in Fig. 13, the movable electrode 7 is supported by two silicon beams 6 in a both-ends support fashion. This embodiment, and also that of Fig. 14, is technological background. Figs. 14 and 15 show the detection units in which the movable electrode 7 is supported by four silicon beams 6 in the both-ends support fashion and by two silicon beams 6 in the cantilever fashion. An example of connecting the silicon capacitance type accelerometer with a signal processing circuit to evaluate the characteristic thereof. Fig. 16 shows construction of the signal processing circuit. A detection unit shown in the drawing is essentially the same as that described in Fig. 1, in which the acceleration to be detected is defined by G, that in the direction of the arrow is defined to be positive, and that in the direction reverse to the arrow negative. Silicon plates 1,2 and 3 are connected to a ΔC detector 64 through conductive wires 15, 16 and 17. The ΔC detector 64 comprises switch and capacitors, and an amplifier 65 so controls a pulse width modulator 66 that a capacitance difference ΔC of the gaps 8 between the movable electrode 7 and the silicon plate 1 and 3 (serving also as the fixed electrode) becomes zero. Pulse-like output voltage V E is applied to the silicon plate 1 and output voltage -V E inverted by an inverter 67 to the silicon plate 3 in a feedback control manner. In addition, the silicon plate 2 is applied with constant DC voltage (In the drawing, 5 volts). A movable electrode 7 supported by a silicon beam 6 and functioning as a weight is intended to shift corresponding to magnitude of acceleration G. However, the position of movable electrode is controlled in a feedback manner by an electrostatic force acting between the electrodes. Therefore, electrostatic energy supplied between the electrodes leads to represent the acceleration G itself to be detected. Hence, the pulse-like output voltage waveform V E of pulse width modulator 66 is processed by a low-pass filter 68 and output voltage V O is fetched as the accelerometer through a differential amplifier 69. Fig. 17 shows the wave form of output voltage V E of the pulse width modulator. As seen from Fig. 17, the output voltage V E is pulse-like voltage waveform of a cycle period of 50 µS, and its low level,is 0 volt, its high level 5 volts. The pulse width is controlled to linearly decrease with respect to the positive acceleration and increase to the negative acceleration. An example of the output characteristic of the silicon capacitance type accelerometer is shown in Fig. 18, which result of measurement by the method shown in Fig. 16. As apparent from the above, acceleration of 0 to ± 1G could be detected with good linearity and in high accuracy. The present invention can easily and reliably connect the detection unit and the exterior without creating any problem in package of the electrode lead, whereby the capacitance type accelerometer or the semiconductor pressure sensor superior in productivity, of high performance, and inexpensive to produce, can be provided with ease.

Claims: A capacitance type accelerometer, having a first silicon plate (2) being a conductive electrode and having a movable electrode part (7) which is moved according to acceleration; and two second silicon plates (1, 3) being conductive electrodes which are disposed on both sides of the first silicon plate with a certain separation distance; characterized in that said movable electrode part (7) is supported by at least one silicon beam (6) in cantilever fashion, thermal oxide films (4, 5) are respectively disposed between the first silicon plate (2) except the movable electrode part (7) and the two second silicon plates (1, 3) and stick the first silicon plate (2) except the movable electrode part (7) and the two second silicon plates (1, 3) together, and in that on the first silicon plate (2) and the second silicon plates (1, 3) there are respectively formed bonding pads (9, 9a, 9c), said bonding pads (9, 9a, 9c) being connected to an external electronic circuit for obtaining an acceleration G from a signal generated between the silicon plates. A capacitance type accelerometer as defined in claim 1, wherein the thermal oxide films (4, 5) are previously formed on the first silicon plate (2) except the movable electrode part (7) or on the two second silicon plates (1, 3). A capacitance type accelerometer as defined in claim 1, further having polycrystal silicon layers (11, 12) formed on the parts of the second silicon plates (1, 3) which are opposite to the movable electrode part (7). A method of manufacturing a capacitance type accelerometer according to claim 1, comprising the steps of forming a first conductive silicon plate (2) having a movable electrode part (7) which is moved according to acceleration; and forming two second conductive silicon plates (1, 3) which are disposed on both sides of the first silicon plate with a certain separation distance; characterized by forming at least one silicon beam (6) in cantilever fashion to support the movable electrode part, respectively disposing thermal oxide films (4, 5) between the first silicon plate (2) except the movable electrode part (7) and the two second silicon plates (1, 3), and sticking the first silicon plate (2) at a high temperature except the movable electrode part (7) and the two second silicon plates (1, 3) together by the interposed thermal oxide films (4, 5), and respectively forming bonding pads (9, 9a, 9c) on the first and second silicon plates (1, 2, 3). A method of manufacturing a capacitance type accelerometer as defined in claim 4, wherein the thermal oxide films (4, 5) are previously formed on the first silicon plate (2) except the movable electrode part (7) or on the two second silicon plates (1, 3). A method of manufacturing a capacitance type accelerometer as defined in claim 4, wherein polycrystal silicon layers (11, 12) are formed on the parts of the second silicon plates (1, 3) which are opposite to the movable electrode part (7).

Inventors: MATSUMOTO MASAHIRO, MIKI MASAYUKI, SUZUKI SEIKO, TSUCHITANI SHIGEKI, MATSUMOTO, MASAHIRO, MIKI, MASAYUKI, SUZUKI, SEIKO, TSUCHITANI, SHIGEKI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: G01P15
IPC codes: G01P15, G01L9, H01L29