SE468067B - CAPACITIVE ACCELEROMETER AND SET FOR MANUFACTURING THEM - Google Patents

Description

468 067 can be detected. The magnitude of the bend is proportional to the force applied and consequently to the acceleration.

Piezoresistive devices of miniature size can be manufactured, for example, from silicon using the manufacturing methods used in microelectronics and microprocessing technology. To achieve the maximum sensitivity, stress maxima must be placed on the surfaces of the resistors. It follows that the displacement of the elastic part becomes too large in its amplitude in relation to the thickness of the structure, to which sensitive accelerometers must be made with additional weights, which act as seismic masses of the accelerometer, due to the silicon in itself has a relatively low density.

The production of the additives makes it difficult to apply.

Furthermore, the piezoresistive accelerometer is significantly more temperature sensitive than, for example, a capacitive accelerometer.

Furthermore, the so-called the gauge factor is lower for the piezoelectric accelerometer than for the capacitive accelerometer.

Monocrystalline silicon has been used in the manufacture of accelerometers since the late 1960s. Some of these solutions are published in scientific articles and some are even patented. Piezoresistive accelerometers are described i.a. in the following publications: (1) L.M. Roylance and J.B. Angell, "A batch fabricated silicon accelerometer", IEEE Trans. On Electron Devices, ED-26, pp. 1911-1920 (1979) (2) W. Benecke et al., "A frequency selective, piezoresistive silicon vibration sensor ", Transducers '87, p. 406-409 (1987) 468 067 (3) M. Tsugai and M. Bessho, "Semiconductor accelerometer for automotive controls", Transducers '87, p. 403-405 (1987) (4) E.J. Evans, U.S. Patent 3,478,604 (1968) (5) A.J. Yerman, U.S. Patent 3,572,109 (1971) The constructions according to publications (4) and (5) relate to an accelerometer of the type referred to in English as "flexible cantilever".

Capacitive accelerometers are described in the following publications: (6) H.W. Fischer, U.S. Patent 3,911,738 (1975) (7) W.H. Ficken, U.S. Patent 4,009,607 (1977) (8) F.V. Holdren et al., U.S. Patent 4,094,199 (1978) (9) 11.2. Aina, U.S. Patent 4,144,516 (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 4,483,194 (1984) (13) L.B. Wilner, U.S. Patent 4,574,327 (1986) Publication (6) discloses an accelerometer which utilizes two capacitances without describing a physical structure.

Publication (7) is in principle identical to publication (6) except that it relates to another electrical application. 468 067 Publication (8) also deals with the double capacitance principle of an accelerometer.

Publication (9) describes a micromechanical accelerometer in which the seismic mass is suspended on leaf springs.

Publication (10) describes an accelerometer of the flexible bracket type, in which the cantilever beam moves in the horizontal plane and has an electrical, non-symmetrical construction.

Publication (11) deals with an annular construction.

Publication (12) deals with a torsionally suspended plate, in which the capacitance is only placed on one side of the plate.

Publication (13) describes a construction in which the seismic mass is suspended on a diaphragm spring. The object of the present invention is to overcome the disadvantages of the above-mentioned prior art and to provide a completely new type of capacitive accelerometer and a method for manufacturing the same.

The invention is based on the manufacture of the accelerometer of the capacitive type of two capacitors, which have a common moving electrode, which acts as the seismic mass of the accelerometer. The center electrode forms a monolithic structure with a beam made of the same material. It is possible to use one or more of these elastic elements. As a result of an applied acceleration, the seismic mass is displaced a few micrometers and the displacement is detected as a result of the changes in the capacitance values.

The accelerometer design is double-sided and symmetrical.

More specifically, the accelerometer according to the invention is characterized by what is stated in the characterizing part of claim 1. The manufacturing method according to the invention is further characterized by what is stated in the characterizing part of claim 4.

The construction according to the invention offers the following advantages: In the capacitive operating principle, a high sensitivity [SC / C gives a small displacement of the seismic mass, due to the symmetrical construction, the accelerometer has an extremely low uncompensated temperature sensitivity, as it the seismic mass is of the same material as the flexible element, for example silicon, an extra seismic mass is unnecessary, the accelerometer attenuation factor can be varied by providing grooves on the capacitor electrodes or by providing a residual suitable pressure in the accelerometer structure in connection with manufacturing PIOCGSSGD , the capacitance changes are symmetrical about the zero value of the acceleration, the accelerometer can be manufactured by mass production, the accelerometer has an exceptionally high overload tolerance due to the fact that the seismic mass is only displaced a few micrometers before it is supported by the side electrodes. In the following, the invention will be described in more detail by means of the exemplary embodiments in the accompanying drawings. Fig. 1 shows a longitudinal sectional side view of an accelerometer construction in accordance with the invention.

Fig. 2 shows a longitudinal sectional side view of another accelerometer construction in accordance with the invention.

Figs. 3a-3c show in perspective view the different parts of the accelerometer construction according to Fig. 1.

Figs. 4a-4c show in perspective view the different parts of the accelerometer construction according to Fig. 2.

Fig. 5 shows a perspective view partly cut longitudinally over the accelerometer construction according to Fig. 2.

Fig. 6 shows a longitudinal sectional side view of a third accelerometer construction in accordance with the invention.

Fig. 7a shows a top view of a first masking phase in the accelerometer construction in accordance with the invention.

Fig. 7b shows a top view of the final result obtained by the masking phase shown in Fig. 7a.

Fig. 8a shows a top view of a second masking phase in the accelerometer construction in accordance with the invention.

Fig. 8b shows a top view of the final result obtained by the masking phase shown in Fig. 8a.

Fig. 9a shows a top view of a third masking phase in the accelerometer construction in accordance with the invention.

Fig. 9b shows a top view of the final result obtained by the masking phase shown in Fig. 9a.

The accelerometer according to the invention can be manufactured, for example, from monocrystalline silicon using conventional methods known in the micromechanical field of manufacture. Fig. 1 illustrates the basic construction of the accelerometer. The accelerometer can be hermetically sealed, so that the accelerometer housing can be sealed and filled with gas with a suitably low negative pressure. The pressure of the filling gas can be varied to obtain the desired attenuation factor for the seismic mass of the accelerometer. The filling gas used can, for example, consist of dry air. An appropriately selected attenuation factor results in an advantageous frequency response for the accelerometer.

However, a simpler construction of the accelerometer is achieved by making the accelerometer open, as illustrated in Fig. 2, so that the internal gas pressure in the accelerometer is equal to the ambient pressure. The communication between the internal gas space and the surroundings takes place via a channel 8.

In this case, however, the attenuation factor for the seismic mass becomes extremely high, which means that this type of accelerometer can only be used for low frequencies (ie a few hertz) and static acceleration measurements.

Fig. 1 illustrates the hermetically sealed accelerometer construction. The accelerometer is layered and comprises flat, electrically connected side electrode devices 15 and between them a parallel oriented center electrode device 16, the seismic mass 1 of which is formed with an end portion of a cantilever beam 17. The cantilever beam 17 is formed in the center electrode device 16 by the beam occupies a U-shaped groove, which extends through the entire center electrode 16. In addition, material has been removed from the beam 17 to form a gap 7, giving rise to a capacitance, and moreover, the width of a shaft portion 2 of the beam 17 has been reduced to give this part flexibility. The integral center electrode device 16 thus comprises the beam 17, which further comprises the seismic mass 1 and the flexible shaft portion 2 as well as an accelerometer body element 3 surrounding the cantilevered beam 17. The seismic mass 1 is preferably formed a few micrometers thinner than the body element 3. The center electrode device 16 may, for example, be made of the same monocrystalline silicon. The accelerometer capacitances are formed between the common, moving electrode 1 of the center electrode device 16 and the fixed side electrodes 4 of the side electrode devices 15. The side electrodes 4 are made, for example, of monocrystalline silicon by etching the surfaces corresponding to an insulating layer 5 with a residual insulating layer 5. raised area of silicon at the seismic mass 1 as well as at a connecting area 6. The insulating layer may, for example, be of glass. The glass-covered area 5 and the frame element 3 can be hermetically bonded to each other using, for example, anode connection technology. The construction of the center electrode device 16 is mirror-symmetrical with respect to a plane s (xy-plane) shown in Fig. 1. The seismic mass 1 is movable in the Z-direction.

The accelerometer assembly of Fig. 1 is shown in detail in Figs. 3a-3c. The upper images 3a and 4a in Figs. 3 and 4 are shown in inverted position to more clearly show the metallized areas 6 and 6 'as well as the silicon area 4.

Fig. 2 illustrates an open accelerometer construction.

This construction differs from the closed construction in the following details: - the side electrode device 15 is made entirely of glass, its surface being covered with metallized areas 6 and 6 'for the electrical functions of the accelerometer, and - the metallization 6 is placed so that it runs in a channel 8 for the purpose of preventing short circuit of the metallization 6 to the body element 3.

The accelerometer construction of Fig. 2 is illustrated in detail in Figs. 4a-4c. In the two constructions shown, the electrical contact areas 6 are preferably made in the same plane, which thus necessitates the use of an electrical bushing 10, shown in Figs. 3 and 4. This detail is of the same material as the frame element 3 but is electrically insulated from the frame element. The embodiment of the element 10 is shown in more detail in Fig. 5. The purpose of a body 9 shown in Fig. 4b is to form an electrical contact between the body element 3 and the center contact area 6.

The accelerometer design is shown in Fig. 5.

The impact strength of the accelerometers can be improved by leaving protrusions 11, as shown in Fig. 6, when etching the flexible element 2, which prevent an excessive bending of the element 2 as a result of a strong impact. The dimensions of the accelerometer become strongly dependent on the desired sensitivity and capacitances. Typical dimensions are, for example, 2 x 0.5 x 4 mm for the seismic mass 1 and 2 x 0.07 x 4 m3 for the cantilevered slide 2.

The external dimensions of the accelerometer are approximately 4 x 3 x 12 m3 (width x height x length).

The manufacturing phases described below are applicable for machining both sides of a base material which constitutes a monocrystalline semiconductor, e.g. a silicon wafer polished on both sides. 1. Both sides of the silicon wafer are oxidized to a depth of approximately 250 nm. The washer thickness can be, for example, 500 μm. 2. The wafer is coated with a photoresist and exposed as illustrated in Fig. 7a over the center electrode regions 12 from which the oxide is to be etched away. Fig. 7 illustrates a silicon wafer. 468 067 10 3. Then, as shown in Fig. 7b, the center electrode region 12 is etched to a depth of approximately 4 μm in, for example, an aqueous solution of KOH. 4. The oxide layer on the wafer is etched away using buffered HF and the wafer is oxidized to a depth of approximately 0.8 μm. The wafer is coated with a photoresist and exposed at the edges of the center electrode region 12 over an edge region 13 and a contact region 13 ', shown in Fig. 8a, from which regions the oxide is removed. Then the photoresist is removed. The areas 13 and 13 'are etched to a depth of approximately 50 .mu.m, whereby a center electrode pattern 17' is obtained in accordance with Fig. Bb. 7. The wafer is recoated with a photoresist and exposed at the shaft portion of the center electrode pattern 17 'over an area 14, in accordance with Fig. 9a, from which area the oxide is removed. The etching of the tray continues in, for example, an aqueous solution of KOH until the desired depth is reached at the area 14, typically down to 40-100 μm, with a penetration of the area 13 taking place, as illustrated in Fig. 9b.

This process results in the element 16 shown in Fig. 3b.

The side electrode devices 15 of Figs. 3a and 3c are fabricated using the technique described above by etching a surface of the silicon wafer to a depth of approximately 150 .mu.m while leaving only the small (dashed) area 4, shown in Figs. 3a and 3c. 3c, together with the area 6 (dashed) which is in electrical connection I; C) 468 067 11 with area 4, with original height. The etched surface of the tray is then coated by melting a suitable layer of glass, for example Schott Tempax, Corning 7070 or Corning 7740 type glass, which is ground and polished to the same level as the original surface of the tray. This method is previously known from U.S. Patent 4,597,027 (A. Lehto). The glass-coated washer is then treated with respect to the metallizations 6 using a lifting method or an etching process. These methods are conventional and no detailed description of them is made. Finally, all three washers are connected together using so-called anode connection at a suitable pressure.

The pressure level used depends on the desired damping factor for the accelerometer and is typically of the order of a few hPa. A detailed description of applied methods (as well as other methods) can be found in the book The Physics of Microfabrication, Plenum Press, New York, 1983, Ivor Brodie and Julius J. Muray.

The center electrode devices 16 of Figures 4a-4c are fabricated using nearly identical methods to those described above. The difference lies in an area which is made further during the 4 μm etching phase at the sections of the channel 8 and the contact area 9.

The center electrode devices 15 according to Figs. 4a-4c are made of glass, for example glass of the type Schott Tempax, Corning 7070 or Corning 7740. The areas 6 and 6 'are metallized using the method described above. The accelerometer is assembled using an anode connection at a normal ambient pressure.

In both accelerometer constructions, the washers can be separated by breaking with the help of pre-cut grooves.

Claims (5)

12,468,067 Patent claims: Capacitive accelerometer with silicon body, comprising - two substantially disc-shaped, parallel side electrode structures (15) spaced apart and comprising fixed side electrodes (4, 6 '), - a substantially disc-shaped center electrode structure (16) arranged between the fixed side electrode structures (15), the center electrode structure comprising - a body element (3) connected to side electrode devices (15), and - at least one center electrode arranged in the vicinity of the side electrodes (4, 6 ') (17) with a shaft portion (2) and a tip portion (1), the center electrode (17) being connected to the body member (3) via its shaft portion (2) so as to be surrounded from the sides of the body members, and the shaft portion (2) is substantially thinner than the tip portion (1), characterized in that - the center electrode structure (16) is symmetrical with respect to the median plane (S) of the side electrodes (15), - the center electrode (17) is substantially beam-shaped and is surrounded by a U-shaped groove, which passes through the body element, - the tip portion (1) of the center electrode (17) is almost as thick as the body element (3), on such a 'ä å? B 468 067 that the narrow electrode gaps (7) defined by the thickness of the tip portion r (1) are formed between the side electrodes (4, 6 ') and the center electrode (17), and the side electrode structures (15) are joined by means of an electrically insulating layer (5). hermetically with the frame element (3) of the center electrode structure (16), each center electrode (17) being in a hermetically closed space and the electrode structures being normally galvanically separated from each other without external connections. Accelerometer according to Claim 1, characterized in that the center electrode device (16) is rectangular in shape. Accelerometer according to claim 1 or 2, characterized in that a column-like electrical connection means (10). with which the contact areas (6) of the side electrodes can be brought to the same level, are electrically separated from the center electrode construction. A method for manufacturing a capacitive accelerometer, according to which method - two disc-shaped side electrode structures (15) comprising fixed side electrodes (4, 6 ') are formed, and - a center electrode structure (16) comprising a center electrode (17) is arranged between the fixed the side electrode structures (15), characterized in that - the center electrode structure (16) is made of a homogeneous single crystal semiconductor piece by etching a center electrode region (12) Um¶H7e fl process phase I on both sides of its center section 14 the central electrode area (12) is further etched during a process phase II on both sides of the semiconductor piece to leave on the center electrode area (12) a beam-like, (17 '), non-etched semiconductor piece and having the thickness defined during phase I ( Fig. 8), center electrode pattern smn is joined to - a shaft portion (14) of the central electrode pattern (17 ') is further etched during a process phase III (fig. 9) on both sides together with the edge area (13) until the edge area (13) is etched through and the shaft portion (14) obtains the desired thickness, and - the side electrodes (1) are hermetically attached to the center electrode structure (16). Method according to claim 4, characterized in that when the side electrodes (15) are attached to the center electrode device (16), a suitable gas pressure is arranged in the space delimited by the side electrodes to guarantee good frequency properties of the accelerometer. n v,