DE3837883A1 - CAPACITIVE ACCELEROMETER AND METHOD FOR THE PRODUCTION THEREOF - Google Patents

Abstract

The accelerometer comprises two side electrode structures (15) incorporating fixed side electrodes (4) and, arranged between the fixed side electrodes, a centre electrode structure (16) which incorporates at least one centre electrode (17). The centre electrode structure (16) comprises a planar body element (3) providing a wall which, with the side electrode structures, defines a cavity. In the cavity a movable electrode portion (1) of centre electrode (17) is supported in cantilever by a stem portion (2) projecting from the wall. The device is symmetrical about plane S. The centre electrode structure is processed by etching a groove penetrating the body element (3) in a U-shape and delineating the cantilever beam-like centre electrode (17). Portion (1) of the centre electrode (17) is formed thinner than the wall section of the body element (3) so that electrode gaps (7) are formed between the side electrodes (4) and the centre electrode (17). The stem section (2) of the centre electrode structure (16), integral with the body element (3), is essentially thinner than the portion (1) in order to obtain a flexible centre electrode (17). The transducer construction in accordance with the invention is easily manufactured in mass production. <IMAGE>

Description

The present invention relates to a capacitive Accelerometer according to the preamble of the An saying 1.

The invention also relates to a method for the manufacture position of an accelerometer.

The accelerometer is actually a force sensor in miniature format, but its main application is in the Determination of the acceleration lies. The device can also be used for other purposes, e.g. to Determination of angles of inclination.

Accelerometer in miniature format according to the state The technology mostly work according to the piezoelectric or piezoresistive principle.  

A piezoelectric crystal develops a surface load, the size of which depends on the force acting on it the surface works. This charge is then under Ver measured using a charge-sensitive amplifier. The high input impedance of the amplifier used for measuring solution used makes the amplifier opposite sensitive to electrostatic interference. On the upper Surface-shaped charges are created by surfaces Leakage currents discharge spontaneously, making it impossible will measure this type of accelerometer of static or low frequency accelerations use.

A piezoresistive accelerometer becomes typi usually made of a semiconducting material, e.g. Sili manufactured in which resistors in suitable Crystal directions are diffused. If the Kri If the stall is bent, the resulting chips changes in the resistance values that it allow to determine the amount of bending. The Verbie size is the acting force and thus the Acceleration proportional.

Piezo resistors in miniature format can be made using silicon, for example, using the manufacturing methods of microelectronics and miniature machine technology. In order to obtain the maximum sensitivity, the voltage maxima must be brought to the resistance ranges. It follows that the dislocation amplitude of the elastic member becomes excessively large when considering the ratio to the thickness of the structure. Since silicon itself has a relatively low density, sensitive accelerometers must also be equipped with auxiliary weights that act as seismic masses of the accelerometer. The preparation of the auxiliary masses makes the process difficult to carry out. Piezoresistive accelerometers are also far more sensitive to temperature fluctuations than capacitive accelerometers, for example. Furthermore, the so-called strain factor for the piezoresistive accelerometer is smaller than for the capacitive accelerometer.
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Monocrystalline silicon has been used since the late 1960s used to manufacture accelerometers. Some of these solutions have been developed in scientific ar articles have been published and others have also been patented. Piezoresistive accelerometers are used, for example described in the following publications:

• (1) L.M. Roylance and J.B. Angell, "A batch fabricated silicon accelerometer, "IEEE Trans. on Electron De vices, ED-26, pp. 1911-1920 (1979)
• (2) W. Benecke et al., "A frequency selective piezore sistive silicon vibration sensor, "Transducers ′87, Pp. 406-409 (1987)
• (3) M. Tsugai and M. Bessho, "Semiconductor accelerome for automotive controls, "Transducers ′87, p. 403-405 (1987)
• (4) E.J. Evans, U.S. Patent No. 34 78 604 (1968)
• (5) A.J. Yerman, U.S. Patent 35 72 109 (1971)

Constructions according to documents (4) and (5) concern an accelerometer with a bend seed inertial body.  

Corresponding capacitive accelerometers are in described in the following publications:

• (6) H.W. Fischer, US Pat. No. 3,911,738 (1975)
• (7) W.H. Ficken, US-PS 40 09'607 (1977)
• (8) F.V. Holdren et al., U.S. Patent 4,094,199 (1978)
• (9) H.E. Aine, U.S. Patent 41 44 516 (1979)
• (10) K.E. Petersen et al., U.S. Patent 4,342,227 (1982)
• (11) R.F. Colton, U.S. Patent 4,435,737 (1984)
• (12) F. Rudolf, U.S. Patent 44 Q83 194 (1984)
• (13) L.B. Wilner, US-PS 45 74 327 (1986)

Document (6) describes an accelerometer using two capacities without a physika construction.

Document (7) is different except for one electrical version with printed text (6) identical.

Document (8) also deals with an acceleration diffraction meter that works according to the two-capacity principle worked.

Document (9) describes a micromechanical Accelerometer where the seismic mass is hung on leaf springs.

Document (10) describes an accelerometer  with a flexible inertial body, in which the Inertial body moved in the horizontal plane and one has an electrically asymmetrical structure.

Document (11) deals with an annular design.

Document (12) describes a twisted hanging Plate in which the capacity is only on one plate is arranged side.

Document (13) describes a construction in which the seismic mass is suspended from a diaphragm spring is.

The object of the present invention is one or several disadvantages of the aforementioned prior art to avoid and a completely new type of ka capacitive accelerometer and a method to create it.

This problem is solved by the character nenden features of claims 1 and 5.

The invention is based on the construction of a capacitive Accelerometer made up of two capacitors with one common movable electrode, which at the same time as Seismic mass of the accelerometer is used. The Center electrode forms a monolithic construction with a carrier made of the same material unit body. It is possible to have one or more of these to use elastic components. The seismic mass is applied according to an applied acceleration deflected a few microns and this deflection is determined from the changes in the capacitance values. The accelerometer is bilateral and symmetrical built up.  

The design according to the invention offers the following parts:

• - The principle of capacitive operation ensures a high sensitivity Δ C / C with a small deflection of the seismic mass.
• - According to the symmetrical design, the access system has level meter an extremely low not compen sized temperature sensitivity.
• - Since the seismic mass is made of the same material stands out like the flexible component, for example Silicon, a seismic auxiliary mass is not necessary.
• - The accelerometer's damping factor can can be designed variably by creasing the Capacitor electrodes as well by insertion a suitable pressure in the accelerometer construction during the manufacturing process.
• - The capacity changes are symmetrical around the Zero acceleration.
• - The accelerometer can be used in large series getting produced.
• - The accelerometer has a remarkable one high overload tolerance because of the seismic Mass is deflected only a few micrometers, be in front of them through the side electrodes is reset.

Further advantages and features of the invention result from the following description of several execution examples of the invention with reference to the drawing.  

It shows:

Fig. 1 is a side view of a longitudinal section of an accelerometer assembly according to the invention.

Fig. 2 is a side view of a longitudinal section of another embodiment of an accelerometer assembly according to the invention.

FIGS. 3a-3c, the various parts of the ge in Fig. 1 showed accelerometer structure in per spectral representation TiVi shear.

Figs. 4a-4c illustrate the various parts of the ge in Fig. 2 showed accelerometer structure in pers pektivischer representation.

FIG. 5 is a perspective view of a partial longitudinal section of the accelerometer assembly shown in FIG. 2.

Fig. 6 is a side view of a longitudinal section of a third accelerometer structure according to the invention.

Fig. 7a, a first phase of a masking erfindungsge MAESSEN accelerometer structure in top plan view.

Fig. 7b by Figs. Masking phase obtained result in plan view from above shown 7a.

Fig. 8a is a second masking phase of a fiction, modern accelerometer structure in top plan view.

Fig. 8b the end result obtained by the masking phase shown in Fig. 8a in plan view from above.

Fig. 9a shows a third phase of the masking erfindungsge MAESSEN accelerometer structure in top plan view.

Fig. 9b the end result obtained by the masking phase shown in Fig. 9a in plan view from above.

The accelerometer according to the invention can be produced, for example, from monocrystalline silicon using conventional methods known from micromechanical production. Fig. 1 illustrates the fundamental structure of the accelerometer loading. The accelerometer can be hermetically sealed such that the accelerometer housing can be closed and filled with gas of a suitably low vacuum. The pressure of the fill gas can be changed to obtain the desired damping factor of the seismic mass of the accelerometer. The filling gas used can be dry air, for example. A damping factor selected in a suitable manner leads to a favorable frequency behavior of the accelerometer.

However, a simple structure of the accelerometer is obtained if the accelerometer is constructed open, as shown in Fig. 2, so that the gas pressure inside the accelerometer is equal to the ambient pressure. The connection of the inner gas space with the environment takes place via a channel 8 . However, the damping factor of the seismic mass is exceptionally high, so that this type of accelerometer can only be used at low frequencies (ie a few Hertz) and for static acceleration measurements.

Fig. 1 shows the structure of the hermetically sealed accelerometer. The accelerometer has a layered structure which has planar, electrically interconnected side electrode structures 15 and between which a parallel arranged central electrode structure 16 is arranged, the seis mixed mass 1 is formed by the head of a clapper-like inertial body 17 . The clapper-type inertial body 17 is formed to the center electrode pattern 16, a U-shaped into operates in demman in the electrode structure 16 Mitt recess which extends over the entire structure of the center electrode sixteenth In addition, material is removed from the inertial body 17 to form a gap 7 , which causes a capacitance. Furthermore, the width of a stem area 2 of the inertial body 17 is reduced so that this area becomes flexible. Therefore, the entire Mittelelektro denstruktur 16 includes the inertial body 17 , which in turn comprises the seismic mass 1 , the flexible style area 2 and a body element 3 of the accelerometer, which surrounds the clapper-like inertial body 17 . The seismic mass 1 is advantageously produced a few micrometers thinner than the body element 3 . The center electrode structure 16 can be produced, for example, from the same monocrystalline silicon. The capacitances of the accelerometer are formed between the common movable electrode 1 of the center electrode structure 16 and the fixed side electrodes 4 of the side electrode structures 15 . The side electrodes 4 are made of monocrystalline silicon, for example, by etching the surfaces so that they correspond to an insulating layer 5 and thus leave an increased area in the silicon both in the seismic mass 1 and in a binding area 6 . The insulating layer 5 can be made of glass, for example. The glass-coated insulating layer 5 and the body element 3 can for example be firmly connected to one another using the so-called anodic bonding (anodic bon ding). The structure of the center electrode structure 16 is mirror-symmetrical with respect to a plane S (xy plane) indicated in FIG. 1. The seismic mass 1 is movably arranged in the z-direction. The structure of the accelerometer according to FIG. 1 is shown in detail in FIGS . 3a-3c. From top 3a and 4a of FIGS . 3 and 4 are shown in the reverse position to make both the metallized surfaces 6 and 6 ' and the silicon surface 4 more visible.

Fig. 2 shows an open design of an accelerometer supply. This structure differs from closed designs in the following details:

• - The side electrode structure 15 is made entirely of glass, the surface of which is covered by metallized surfaces 6 and 6 ', which are necessary for the electrical functions of the accelerometer.
• - The metallization 6 is arranged so that it opens into a channel 8 , for the purpose of preventing the short circuit of the metallization 6 and the body element 3 .

The structure of the accelerometer according to FIG. 2 is shown in detail in FIGS. 4a-4c. In both ge shown designs, the electrical contact surfaces 6 are preferably produced in the same plane, which makes it necessary to use an electrical via 10 shown in FIGS. 3 and 4. This part consists of the same material as the Körperele element 3 , but is isolated from it. The shape of the element 10 is described in more detail in FIG. 5. The purpose of a connection 9 shown in FIG. 4b is to make electrical contact from the body element 3 to the central contact surface 6 . The layout of the accelerometer is shown in FIG. 5.

The vibration resistance of the acceleration sensor can be improved by etching the flexible element 2 in such a way that such projections 11 , as shown in FIG. 6, form there and an excessive bending of the element 2 in the event of a severe vibration prevent. The dimensioning of the accelerometer depends strongly on the desired sensitivity and the desired capacitances. Typical dimensions for seismic mass 1 are, for example, 2 × 0.5 × 4 mm³ and for stem 2 2 × 0.07 × 4 mm³. The outer dimensions of the accelerometer are approximately 4 × 3 × 12 mm³ (width × height × length).

The manufacturing phases described below can to process both sides of a monocrystalline Semiconductor base material, for example one on both Sides of polished silicon wafers.

• 1. Both sides of the silicon wafer are up to one Oxidized depth of about 250 nm. The wafer thickness  can be, for example, 500 microns.
• 2. The wafer is coated with a photoresist and, as shown in FIG. 7a, is exposed in order to obtain telelectrode surfaces 12 , from which the oxide layer is etched away. Fig. 7 shows a silicon chip.
• 3. The center electrode surface 12 shown in FIG. 7b is then etched, for example with an aqueous solution of KOH, to a depth of approximately 4 μm.
• 4. The oxide layer of the wafer is ge using buffered HF is etched away and the wafer is up to egg at a depth of approximately 0.8 µm (1-4: process processing phase I).
• 5. The wafer is coated with photoresist and exposed to light at the corners of the center electrode area 12 to form a corner surface 13 and a contact area shown in Fig. 8a 'to obtain 13 and from which the oxide is removed. Then the photoresist is removed.
• 6. The surfaces 13 and 13 'are etched to a depth of approximately 50 microns, whereby a center electrode pattern 17 ' shown in Fig. 8b is formed (5-6: processing phase II).
• 7. The wafer is in turn coated with a photoresist and exposed in the stem area of the center electrode pattern 17 'in order to obtain a surface 14 , as shown in FIG. 9a. The oxide is then removed from this surface 14 .
• 8. The etching of the wafer is continued, for example, in an aqueous solution of KOH until a desired depth of area 14 is obtained, typically down to 40-100 µm, with, as shown in Fig. 9b, area penetration 13 occurs (7-8: processing phase III).

This method leads to element 16 , as shown in FIG. 3b.

The side electrode structures 15, shown in FIGS. 3a and 3c are prepared using the above-be signed art by etching a surface on the silicon wafer to a depth of about 150 microns, so that only the small (hatched) area 4, shown in Figures . 3a and 3c is shown, together with the surface 6 (hatched) which is electrically connected to the surface 4, remains at the original level. The etched surface of the wafer is then coated by melting a suitable glass layer, for example Schott Tempax, Corning 7070 or Corning 7740 grade glass, which is ground and polished to the same height as the original surface of the wafer. This method is known in the prior art from US Pat. No. 4,597,027 (A. Lehto). The glass-coated wafer is then processed further in order to obtain the metallizations 6 , using a lift-off process or an etching process. These processes are conventional processes and are therefore not described in detail. Finally, all three wafers are connected to one another, using the so-called anodic connection under a suitable pressure. The pressure range used depends on the desired damping factor of the accelerometer and is typically on the order of a few hPa. A detailed description of the methods used (as well as other methods) can be found in the book by Ivor Brodie and Julius J. Muray "The Physics of Microfabrication ", Plenum Press, New York, 1983.

The center electrode structures 16, which correspond to FIGS . 4a-4c, are manufactured using methods that are almost identical to those described above. The difference lies in an area that was enlarged in the area of the channel 8 and the contact area 9 during the 4 μm etching phase.

The center electrode structures 15 , according to FIGS. 4a-4c, are produced from glass, for example from Schott Tempax, Corning 7070 or Corning 7740 grade glass. The surfaces 6 and 6 'are metallized using the method described above. The accelerometer is constructed using anodic bonding at normal ambient pressure.

With both accelerometer designs, the Pre-cut wafer by breaking grooves are separated.

Claims (5)

1. Capacitive accelerometer made of silicon material with:
two mutually spatially arranged, parallel, substantially plate-like flat side electrode structures ( 15 ) which are arranged opposite one another, which are fixed side electrodes ( 4 , 6 '); and
a uniform, chen substantially plattenähnli arranged between the side electrode structures (15) center electrode structure (16), wherein the center electrode structure (16) comprises the following parts:
a body element ( 3 ) which is adjacent to the side electrode structures ( 15 ); and
at least one center electrode ( 17 ) which is in the vicinity of the side electrodes ( 4 , 6 ') angeord and which has a stem area ( 2 ) and a head ( 1 ), so that the stem ( 2 ) of the center electrode ( 17 ) this connects to the body element ( 3 ) which surrounds the center electrode ( 17 ) on the sides and the stem area ( 2 ) is substantially thinner than the head ( 1 ), characterized in that
the center electrode structure ( 16 ) is symmetrical with respect to the center plane ( S ) of the side electrode structures ( 15 );
the center electrode ( 17 ) has a clapper-like shape and is surrounded by a recess which extends in a U-shape through the body element ( 3 );
the head ( 1 ) of the center electrode ( 17 ) has a thickness which corresponds approximately to that of the body element ( 3 ), so that small electrode gaps ( 7 ) with a gap, which is determined by the thickness of the head ( 1 ), between the side electrodes ( 4 , 6 ') and the center electrode ( 17 ) are formed; and
the side electrode structures ( 15 ) via an electrically insulating layer ( 5 ) in a firmly binding manner on the body element ( 3 ) of the central electrode structure ( 16 ) are attached, each central electrode ( 17 ) remaining in a hermetically sealed space and the electrodes structures ( 15 , 16 ) are usually galvanically isolated from each other if no external electrical connections exist. 2. Accelerometer according to claim 1, characterized in that the central electrode structure ( 16 ) has a rectangular shape. 3. Accelerometer according to claim 1 or 2, characterized in that the central electrode structure ( 16 ) is processed to form a columnar, electrically conductively connected structure ( 1 ) electrically isolated from the rest of the central electrode structure ( 16 ), wherein connecting surfaces ( 6 ) the side electrodes ( 15 ) are arranged in the same plane. 4. A method of manufacturing a capacitive accelerometer comprising the following steps:
two side electrode structures ( 15 ) comprising the fixed side electrodes ( 4 , 6 ') are formed; and
a center electrode structure (16), the electrode is a means (17) and disposed the electrode structures between the sides (15), characterized in that
the center electrode structure ( 16 ) is manufactured during a processing phase I from a homogeneous monocrystalline semiconductor chip by etching a center electrode surface ( 12 ) on both sides of the center region of the chip ( FIG. 7);
Corner surfaces ( 13 ) of the central electrode surface ( 12 ) on both sides of the semiconductor chip are further etched during a processing phase II, so that from the central electrode surface ( 12 ) a clapper-like central electrode pattern ( 17 ') remains, the thickness of which is defined during the processing phase I and that stays connected to the unetched semiconductor chip ( Fig. 8);
a stem area ( 14 ) of the center electrode pattern ( 17 ') during processing phase III ( Fig. 9) is etched on both sides until the corner surface ( 13 ) is etched through and the stem area ( 14 ) has reached a desired thickness; and
the side electrodes ( 15 ) are attached to the central electrode structure ( 16 ) in a firmly connected manner. 5. The method according to claim 4, characterized in that in connection with the attachment of the side electrodes ( 15 ) on the central electrode structure ( 16 ) a suitable gas pressure in a space ( 7 ) is set, this hermetically from the side electrodes ( 15 ) is enclosed in order to achieve an advantageous frequency response of the sensor.