GB2194341A

GB2194341A - Capacitive acceleration sensors

Info

Publication number: GB2194341A
Application number: GB08716443A
Authority: GB
Inventors: Helmut Seidel
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1986-07-26
Filing date: 1987-07-13
Publication date: 1988-03-02
Anticipated expiration: 2007-07-13
Also published as: DE3625411A1; GB8716443D0; FR2602055A1; FR2602055B1; DE3625411C2; CH673897A5; GB2194341B

Abstract

A capacitive acceleration sensor, which is produced with micromechanical production technology and etching technique, has a preferably rectangular acceleration plate (2) suspended centrally in the interior of the acceleration sensor (1) and having an even number of bending bands (3, 15) arranged symmetrically with respect to the centre plane between the upper and lower side of the acceleration plate. The acceleration plate (2) itself or conductors applied to its surfaces form one of the capacitor electrodes of two plate capacitors. The counter-electrodes (7) for this purpose are arranged on opposite surfaces of cover plates (5) separated from the acceleration plate (2) by respective gaps (6). Preferably the acceleration plate (2), the bending bands (3) and a frame (4) are worked-out monolithically from a monocrystalline substrate of silicon. <IMAGE>

Description

SPECIFICATION Capacitive acceleration sensors This invention relates to capacitive acceleration sensors.

Such an acceleration sensor is known as a result of DE-OS 32 23 987. This has a flap which is fastened to a carrier by way of two torsion holders which are arranged symmetrically and in extension of one edge of the flap.

An electrode which is applied to a plate present underneath the flap makes it possible to measure the acceleration by measuring the corresponding change in the capacitance between the flap and the electrode. Such a sensor produces a capacitance change which is a relatively complicated function of the acceleration, because the air gap between the flap and the electrode changes only in wedge-shaped manner. Furthermore, in the case of this torsion suspension, relatively great transverse sensitivities are to be expected. As a result of the arrangement of the flap being unsymmetrical in the vertical direction, also no adequate overload protection of the sensor exists.

The problem underlying the invention is, in the case of an acceleration sensor of the kind mentioned at the beginning hereof, to improve the transverse sensitivity and the overload protection.

This problem is solved, in accordance with the invention, by the provision of a capacitive acceleration sensor, which is produced with micromechanical production technology and etching technique, characterised in that an acceleration plate, having an even number of bending bands arranged symmetrically with respect to the central plane between the upper and lower side of the acceleration plate, is suspended centrally-in the interior of the acceleration sensor, in which respect either the acceleration plate itself, or conductors applied to its surfaces, form one of the capacitor electrodes of two plate capacitors, counter-electrodes of which are applied to opposite surfaces which are separated by gaps from the acceleration plate.

As a result of the arrangement of the acceleration plate suspended centrally from bending bands, the acceleration sensor in accordance with the invention has a high sensitivity which is not achieved by known sensors, even by piezoresistive sensors. There is achieved capacitance changes which rise approximately uniformly and which correspond to the applied accelerations, in which respect a high degree of overload protection is achievable and thus the possibility of a short-circuit-is reduced.

The fully symmetrical construction of the structure guarantees a very accurately defined zero-point position. Moreover, as a result of the two-sided suspension with four, preferably eight, bending bands, the transverse sensitivity is extremely low. The two-sided arrangement of the rigid cover plates with the counter-electrodes ensures a high degree of electrical safety.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-section through a first preferred embodiment of the acceleration sensor of the invention; Fig. 2 is a perspective plan view of the acceleration sensor shown in Fig. 1 with its upper cover plate omitted in accordance with the arrows Il-lI; Fig. 3 is a view comparable with that of Fig.

1 but of a second preferred embodiment of the acceleration sensor of the invention; and Fig. 4 is a view comparable with that of Figs. 1 and 3 but of a third preferred embodiment of the acceleration sensor of the invention.

Referring now in detail to the drawings, the first preferred embodiment of the acceleration sensor 1 shown in Figs. 1 and 2 comprises a central acceleration plate 2, which is suspended by means of an even number of bending bands 3, preferably eight bending bands, from a frame 4. The bending bands 3 extend from the corners of the upper and lower side of the acceleration plate 2 to the frame 4 which is arranged at a spacing around the acceleration plate 2. Present above and below the acceleration plate 2 and the frame 4, which are the same thickness, are cover plates 5, which in each case rest on the frame 4. In the cover plates 5 opposite the acceleration plate 2 and the bending bands 3 are respective depressions 6, into each of which depressions 6 is applied a metal film 7.

If the acceleration plate 2, the bending bands 3 and the frame 4 are worked-out monolithically from a monocrystalline silicon, the surfaces of the acceleration plate 2 can serve as one of the electrodes and the metallic films 7 in the cover plates 5 can serve as the other electrodes of two plate capacitors with the depressions 6 as gaps between the electrodes. Upon the effecting of accelerations, the electrodes form variable capacitances, which serve for the measurement of the acceleration. The decrease in the changing or alternating current changes by changes in the gaps 6 upon different accelerations is effected in known manner for example by a power or lead strip 8 connected to the acceleration plate 2 (see Fig. 2) and power or lead strips 9 conducted out from the metallic films 6 between the cover plates 5 and the frame (see Fig. 1).Two possibilities are available for damping the sensor 1. Firstly, the inner space formed by the gaps 6 and free spaces 11 around the acceleration plate 2 can be evacuated, save for a defined residual pressure, through a duct 12 worked into the upper cover plate 5. The second possibility consists in leaving the duct 12 open as an air duct.

To produce the first preferred embodiment, in which the acceleration plate 2, the bending beams 3 and the frame 4 are worked monolithically out of a monocrystalline substrate.

Silicon with low doping and a crystal orientation (100) is used for the substrate. The bending beams 3, which may not lie parallel to a (111) crystal plane, have to be provided with a high edge doping ( > 7 x 10'9 cm-3), so that they are made etching-resistant for the subsequent structuring process. This can be achieved by means of an appropriately doped epitaxy layer, or respectively with ion implantation or diffusion. After that, the contours of the etching pit are lithographically defined and gnisotropically etched. Into the cover plates 5, which preferably consist of glass, the depressions 6 are likewise etched out and subsequently thereto the metallic films 7 and lead strips 9 are deposited by evaporation.Finally, the cover plates 5 are connected to the frame 4 with the aid of an anodic connection technique by applying an electrical voltage at increased temperature.

In the second preferred embodiment, shown in Fig. 3 of an acceleration sensor la, instead of the bending bands 3 worked out of the uniform substrate, separate bending strips 15 are present, which are inserted between the cover plates 5 and the acceleration plate 2.

The bending strips ' 1 5 can, as shown here, consist of a dielectric coated with a metal film 16, for example of Si3N4or SiO2. Furthermore, the bending strips 15 can be directly deposited by evaporation or applied directly from a metal film or respectively a metal foil. This arrangement has the advantage that the bending strips 15 serving as central capacitor electrodes or respectively the metal films 16 can be isolated dielectrically vis-a-vis the frame 4.

In the third preferred embodiment of an acceleration sensor 1b shown in Fig. 4, the acceleration plate 2 and the bending bands 3 are thinner, by the width of the gaps 6, than the frame 4. This reduced thickness can be worked out without difficulty upon the production from a uniform substrate. This arrangement has the advantage that the cover plates 5 do not have to be separately worked.

All three embodiments of the acceleration sensor 1, 1 a and 1 b can be used directly as capacitive signal transmitters, in which respect upon the effecting of accelerations two oppo sitely changing capacitances are available, which can be wired in a suitable bridge ar rangement. Furthermore, the two measuring capacitances can be supplemented by two rigid capacitances, which make possible, between frame 4 and the cover plates 5, a temperature compensation of the measuring capacitances.

Another possibility of the signal evaluation consists in the electrical fettering or impe dance of the acceleration plate 2, in which respect this can be kept in its zero position by application of a counter-voltage. In this respect, the counter-voltage occurring upon an acceleration is measured. With this arrangement, resonance effects can be compensated out.

The embodiments shown in the figures represent the most customary kind of the acceleration sensor in accordance with the invention. It also lies within the scope of the invention to arrange at least two bending bands, which connect in the centre of a longitudinal side the upper and lower sides of the acceleration plate and of the frame. A further embodiment which is usable in practice comprises four bending bands arranged in mirrorinverted manner at only one longitudinal side of the acceleration plate, and connecting the longitudinal side to the upper and lower side of the frame. This embodiment is suitable for particularly sensitive measurements, for example for measuring the inclination of an object relative to the gravitational field of the earth.

Claims

1. A capacitive acceleration sensor, which is produced with micromechanical production technology and etching technique, characterized in that an acceleration plate, having an even number of bending bands arranged symmetrically with respect to the central plane between the upper and lower side of the acceleration plate, is suspended centrally in the interior of the acceleration sensor, in which respect either the acceleration plate itself, or conductors applied to its surfaces, form one of the capacitor electrodes of two plate capacitors, counter-electrodes of which are applied to opposite surfaces which are separated by gaps from the acceleration plate.

2. An acceleration sensor as claimed in claim 1, characterized in that the acceleration plate, a frame surrounding it and the bending bands are covered on both sides, whilst leaving the gaps free, by cover plates to whose surfaces facing the acceleration plate the counterelectrodes are applied.

3. An acceleration sensor as claimed in claims 1 and 2, characterized in that the acceleration plate, the bending bands and the frame are worked monolithically from a mono crystalline substrate.

4. An acceleration sensor as claimed in claims 1 and 2, characterized in that only the acceleration plate and the frame consist of a monocrystalline substrate, and in that separate bending strips are inserted between the acceleration plate and the cover plates.

5. An acceleration sensor as claimed in claim 4, characterized in that the bending strips consist of a dielectric which is provided with a metal layer.

6. An acceleration sensor as claimed in claim 4, characterized in that the bending strips consist of a metal foil.

7. An acceleration sensor as claimed in one or more of claims 1 to 6, characterized in that the gaps between the acceleration plate with the bending bands and the cover plates are produced by etching-in of the cover plates.

8. An acceleration sensor as claimed in any one of claims 1 to 6, characterized in that the gaps between the acceleration plate with the bending bands and the cover plates are formed by a reduced thickness, of the acceleration plate opposite the frame.

9. An acceleration sensor as claimed in one or more of claims 1 to 8, characterized in that the gaps and free spaces between the acceleration plate, the cover plates and the frame are evacuated save for a defined residual pressure.

10. An acceleration sensor as claimed in one or more of claims 1 to 8, characterized in that from the gaps and the free spaces between the acceleration plate, the cover plates and the frame an air duct leads through one of the cover plates.

11. An acceleration sensor as claimed in any one of claims 1 to 10, characterized by four bending bands arranged in mirror-inverted manner on only one longitudinal side of the acceleration plate and connecting the longitudinal side to the upper and lower side of the frame.

12. An acceleration sensor as claimed in any one of claims 1 to 10, characterized by eight bending bands which are arranged in mirror-symmetrical manner at the corners of the acceleration plate and connect its upper and lower side to the frame.

13. Arí acceleration sensor as claimed in any of claims 1 to 10 wherein the acceleration plate is rectangular or square.

14. A capacitive acceleration sensor substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2, or in Fig. 3, or in Fig. 4 of the accompanying drawings.