FR2622975A1 - CAPACITIVE ACCELEROMETER AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

a) Capacitive accelerometer. b) The accelerometer comprises two side electrodes 15 and a central electrode 7. The latter comprises a cantilever beam made up of a tip 17 connected to the body 3 of the central electrode 16 by a rod 2. There is in this way obtains a flexible central electrode 17. c) The seismic mass of the accelerometer consists of the central electrode itself, the displacement of which is detected by changes in capacitance.

Description

The present invention relates to an accelerating

  capacitive capacitance meter made of silicon, comprising: two spaced-apart, parallel, essentially planar electrode-like structures, such as platelets, positioned facing each other and having the fixed side electrodes, and an electrode structure a uniform core, essentially like a wafer, disposed between the side electrode structures and having - a body member adjoining the side electrode structures, and

  at least one central electrode placed in the vicinity

  swimming side electrodes and comprising a section

  forming a rod and a tip such as the rod of the elec-

  central trode joins the central electrode to the body element, which itself encloses the central electrode by the sides, and it being specified that the section forming

  rod is substantially thinner than the tip.

  The invention also relates to a method of

manufacture of an accelerometer.

  The accelerometer is actually a detector of

  a miniature force, the main application of which

  cation, is the detection of an acceleration. The provision

  tif can also be used for other jobs, for example

  example for the detection of an angle-of inclination.

  The miniature dimension accelerometers known in the art are for the most part based on the

  principle of piezoelectric or piezoelectric operation

  resistant. A piezoelectric crystal creates a surface charge whose value depends on the source applied to the surface. This load is then measured using a load sensitive amplifier. The high impedance of the amplifier used in the measurement makes

  the amplifier sensitive to electrostatic disturbance

  tick. The charges created on the surface are discharged spontaneously via surface leakage currents, which makes it impossible to use this type of accelerometer for the measurement of static accelerations

or low frequency.

  A piezoresistive accelerometer is typical-

  made of a semiconductor material, for example

  in silicon, in which resistors are created

  by diffusion in appropriate directions of the crystal.

  When the crystal bends, the induced stresses

  cause changes in the values of resistance

  tance, which allows the value of flexion to be detected.

  The value of flexion is proportional to the force

  applied and therefore to acceleration.

  Piezor accelerometers can be manufactured

  miniature dimensions, for example, in sili-

  using the manufacturing processes of microelectronics and micromachining. To obtain maximum sensitivity, the maximum stress values must apply in the resistance zone. As a result, the displacement of the elastic portion becomes excessively large in amplitude with respect to the thickness of the structure; to which is added that the sensitive accelerometers must be equipped with auxiliary weights acting as seismic masses, or masses sensitive to the acceleration, of the accelerometer, the fact that

  that silicon itself is of a relative density-

  weak. The manufacture of the auxiliary masses makes

  difficult the method of realization. In addition,

  piezoresistive is far more sensitive to sudden changes in temperature than, for example, a capacitive accelerometer. In addition, the so-called calibration factor is less for the accelerometer

  piezoresistive only for the capacitive accelerometer.

  Since the end of the 1960s, we have used

  monocrystalline silicon in the manufacture of

  léromètres. Some of these solutions are published in scientific articles and some are also patented. Piezoresistive accelerometers are described, inter alia, in the following publications: [1] L. M. Roylance and J. B. Angell, "A batch fabricated

  silicon accelerometer, / a silicon accelerometer

  Batch Manufactured IEEE Trans .on Electron Devices, ED-26, pp 1911 ... 1920 (1979)

  [2] W. Benecke and collaborators, "A frequency selec-

  tive, piezoresistive -silicon vibration sensor /

  a piezoresistive silicon vibration detector

  tif, frequency selective, "Transducers '87, pages 406 ... 409 (1987) [3] M. Tsugai and M. Bessho," Semiconductor accelerometer

  for automotive controls / semi-accelerometer

  Driver for Automotive Controls "Transducers '87, pages 403 ... 405 (1987) [4] E.J. Evans, US Patent 3,478,604 (1968)

  [5] A. J. Yerman, US Patent 3,572,109 (1971).

  Achievements in accordance with the publications

  [4] and [5] relate to an accelerometer of the type "in

overhang "flexible.

  Capacitive accelerometers are respectively

  described in the following publications: [6] H.W. Fischer, US Patent 3,911,738 (1975) [7] W.H. Ficken, US Patent 4,009,607 (1977) [8] F.V. Holdren et al., US Patent

4,094,199 (1978)

  [9] H.E. Aine, US Patent 4,144,516 (1979) [10] K. E. Petersen et al., US Patent

3,342,227 (1982)

  [11] RF Colton, US Patent 4,435,737 (1984) [12] F. Rudolf, US Patent 4,483,194 (1984) [13] LB Wilner, US Patent 4,574,327 (1986) Publication [6] discloses a accelerometer using two capabilities, without describing the physical construction. The publication [7] is in principle identical to the publication [6] except for one

different electric.

  The publication [8] also concerns the

  principle of double capacity of an accelerometer.

  The publication [9] describes an accelerometer

  micromechanical system in which the seismic mass is suspended

due to leaf springs.

  The publication [10] describes a 1s accelerometer of the flexible cantilever type, in which the cantilevered powder moves in a horizontal plane and presents

  an electrically unsymmetrical construction.

  The publication [11] relates to a

ring.

  The publication [12] relates to a plate suspended in torsion, in which the capacity is not

  placed only on one side of the wafer.

  The publication [13] describes a construction in which the seismic mass is suspended on an elastic membrane. The object of the present invention is to overcome the disadvantages of

  the aforementioned prior art and to propose a complete type

  a new capacitive accelerometer and a method of

for its manufacture.

  The invention is based on the construction of the capacitive-type accelerometer from two capacitors which have a common mobile electrode acting as a mass sensitive to the acceleration or seismic mass of the accelerometer. The central electrode forms a monolithic construction with a beam made of the same material. It is possible to use one or

  several of these elastic elements. As a result of

  When applied to it, the seismic mass moves a few microns, and the displacement is detected from the modifications of the value of the capacitance. The construction of the accelerometer is double-sided and symmetrical. More specifically, the accelerometer according to the invention is characterized in that the central electrode structure is symmetrical with respect to the median plane of the lateral electrode structures, in that the central electrode is in the form of a beam and is surrounded by a U-shaped groove which extends through the body member; in that the tip of the central electrode has a thickness close to that of the body member; in that between the lateral electrodes and the central electrode, fine clearances are formed, with a spacing determined by the thickness of the tip, and - in that the side electrode structures are mounted, hermetically on the body member of the central electrode structure via an electrically insulating layer, it being specified that each central electrode is in a hermetically sealed space and that the electrode structures are, under normal conditions, galvanically isolated from each other if

  there is no external electrical connection.

  Furthermore, the manufacturing method according to the invention is characterized in that in connection with the mounting of the side electrodes on the central electrode structure, a correct gas pressure is created in a hermetically sealed space by the side electrodes, in order to obtain an answer in

  advantageous frequency of the detector.

  The construction in accordance with the invention offers the following advantages: the capacitive operating principle makes it possible to reach a high AC / C sensitivity for a small displacement of the seismic mass; due to the symmetrical construction, the accelerometer

  has an extremely low sensitivity to a change in

  uncompensated temperature, - because the seismic mass is of the same material as the flexible element, for example silicon, an auxiliary seismic mass is not necessary, - the damping factor of

  the accelerometer by making grooves on the elec-

  trodes capacitors or by establishing a correct pressure in the construction of the accelerometer during the manufacturing process, - the variations of the capacitance are symmetrical around the zero value of the acceleration, - the accelerator can be manufactured in production mass, - the accelerator has a remarkably high tolerance to overload, because the seismic mass moves only a few microns before leaning

on the side electrodes.

  The invention will be discussed below in more detail by way of example embodiments,

according to the accompanying drawings.

  - Figure 1 is a side view in longitudinal section of an accelerometer embodiment according to the invention; FIG. 2 is a longitudinal sectional side view of another embodiment of an accelerometer according to the invention; - Figures 3a ... 3c are perspective views

  different parts of the realization of the acceleration

  meter illustrated in Figure 1; FIGS. 4a ... 4c are perspective views

  different parts of the construction of the acceler-

  rometer illustrated in Figure 2;

  FIG. 5 is a perspective view, partially

  in longitudinal section, of the realization of the accelerometer illustrated in Figure 2; FIG. 6 is a sectional side view

  of a third embodiment of the accelerator

  meter according to the invention; FIG. 7a is a view from above of a

  first stage of masking the achievement of

  lerometer according to the invention;

  FIG. 7b is a view from above of the result

  final state obtained by the masking phase illustrated in FIG. 7; FIG. 8a is a view from above of a

  second phase of masking the realization of accelerator

meter according to the invention;

  FIG. 8b is a view from above of the result of

  final state obtained by the masking phase illustrated on

Figure 8a; -

  FIG. 9a is a view from above of a

  third phase of masking the realization of accelerator

meter according to the invention;

  FIG. 9b is a view from above of the result

  final state obtained by the masking phase illustrated on

Figure 9a.

  The accelerometer according to the invention, for example, made of monocrystalline silicon, can be manufactured in

  using the conventional methods known in

  micromechanical Figure 1 illustrates the construction

  base of the accelerometer. The accelerometer can be sealed so that the accelerometer housing can be filled with gas tightness under a suitable low vacuum. We can

  vary the pressure of the filling gas to obtain

  the desired damping factor of the sys-

  than the accelerometer. The filling gas used

  can be for example dry air. A depreciation factor

  correctly chosen results in an answer in

  advantageous frequency of the accelerometer.

  However, a simpler construction of the accelerometer is obtained by producing an open accelerometer as shown in FIG. 2, so that the internal gas pressure of the accelerometer is equal to the ambient pressure. The communication between the internal gas space and the atmosphere is done by means of a channel 8. Then, the damping factor of the seismic mass is, however, extremely high, which makes it impossible to use this type of channel. accelerometer only for low frequency acceleration measurements (i.e.

  a few Hz) and static acceleration only.

  Figure 1 shows the construction of the hermetically sealed accelerometer. The accelerometer is a multilayer structure with structures

  , forming side electrodes, electrically inter-

  connected, and, between them, a structure 16 which forms the central electrode, parallel and aligned, and whose seismic mass 1 is provided by the tip of a

  cantilever beam 17. The beam is

  cantilever 17 the structure 16 of the central electrode by machining in the beam a U-shaped groove which

  extends over the entire structure of the central electrode 16.

  In addition, beam 17 has been removed from the material to form a clearance 7 which achieves a capacity and in addition the width of the section 2 forming the rod has been reduced.

  the beam 17 to obtain flexibility in this section.

  Therefore, the structure 16, which forms the central one-piece electrode, comprises the beam 17, which further comprises the seismic mass 1 and the flexible rod section 2, as well as an element 3 which forms the body. the accelerometer and which surrounds the cantilever beam 17. It is advantageous that the seismic mass 1 be made a few microns finer than the element 3

  forming the body. The structure 16 can be made

  the central electrode, for example, in the same silicone

  monocrystalline cium. The capabilities of the accelerometer are formed between the moving common electrode 1 of the

  structure 16 forming the central electrode and the electrodes

  fixed lateral 4 structures 15 forming lateral electrodes. The side electrodes 4 are manufactured by

  monocrystalline silicon, for example, by attacking

  corresponding faces to give an insulating layer 5 so as to leave in the silicon a zone protruding in front of the seismic mass 1 and at the location of the electrical connection 6. The insulating layer 5 may for example be glass. The glass-coated zone 5 and the body-forming element 3 can be sealed to one another using, for example, an anode bond. The embodiment of the structure 16 forming the central electrode is symmetrical with respect to the plane s (xy plane) shown in Figure 1. The seismic mass 1 is movable in the z direction. The construction of the accelerometer according to Figure 1 is shown in detail in Figures 3a ... 3c. The upper diagrams 3a and 4a of FIGS. 3 and 4 are shown in the inverted position, to make the metallized zones 6 and 6 'and the

Silicon area 4.

  Figure 2 shows a construction of access

  open lerometer. This construction differs from the

  truction closed by the following details: the structure 15 forming side electrode is made entirely of glass whose surface is

  covered by metal zones 6 and 6 'to ensure

  In order to avoid a short circuit between the metallized zone 6 and the element 3 forming the body, the metallized zone 6 is placed continuously in a channel 8 in order to avoid a short circuit between the metallized zone 6 and the element 3 forming the body. The construction of the accelerometer according to the

  Figure 2 is shown in detail in Figures 4a ... 4c.

  In the two constructions shown, the zones 6 of electrical contact are preferably made in the same plane, which makes it necessary to use a hole

  electric metallized 10 shown in Figures 3 and 4.

  This detail is made of the same material as the element 3 forming the body, however it is electrically isolated from the element forming the body. The structure of the element is further detailed in Figure 5. The role of the buffer 9 shown in Figure 4b is to form an electrical contact between the element 3 forming the body and

  the median contact zone 6. The structure of the accelerator

  meter is shown in Figure 5.

  The impact resistance of the accelerometers can be improved by attacking the flexible element 2 so as to leave projections 11 as shown in FIG.

  Figure 6 to prevent excessive bending of the

  2 under a powerful shock. The sizing of

  the accelerometer is highly dependent on the desired sensitivity

  and capabilities. The typical dimensions are for example 2 x 0.5 x 4 mm for the seismic mass 1 and 2 x 0.07 x 4 mm for the cantilever beam 2. The external dimensions of the accelerometer are approximately

  4 x 3 x 12 mm (width x height x length).

  The manufacturing phases described below can be applied to treat both sides of this base material, which is a monocrystalline semiconductor, for example a polished silicon chip on

two faces.

  1. Both sides of the silicon wafer are oxidized to a depth of about 250 nm. The thickness of the wafer may for example be 500 Nm. 2. The wafer is coated with a photoresist and

  it is exposed as shown in Figure 7a to obtain

  the zones 12 forming the central electrode, from which the oxide must be removed by etching. Figure 7 represents

a silicon wafer.

  3. Then attack the zone 12 forming the electrode

  center, shown in FIG. 7b, to a

  melter of about 4 Nm in, for example a solution

aqueous KOH.

  4. The etching layer is removed by etching

  using buffered HF acid and

  the wafer to a depth of about 0.8 -m.

  5. The wafer is coated with a photoresist and the edges of the zone 12 forming the central electrode are exposed to obtain an edge zone 13 and a contact zone 13 'shown in FIG.

  the oxide goes away. Then remove the phosterone product.

  6. Areas 13 and 13 'are attacked to a depth of approximately 50 gm, giving the pattern 17' forming

  the central electrode according to Figure 8b.

  7. The wafer is coated again with a photoresist and the section forming the rod of the pattern 17 'forming the central electrode is exposed to obtain a zone 14, according to FIG. 9a, where the oxide

go away.

  8. The wafer attack is continued in, for example

  an aqueous solution of KOH until a desired depth of zone 14 is reached, typically up to

  a depth of 40 ... 100 Am, which gives a penetration

  zone 13 as shown in FIG. 9b.

  This process finally gives the element 16

according to Figure 3b.

  The side electrode structures according to FIGS. 3a and 3c are manufactured using the technique described above by etching a surface of

  the silicon wafer to a depth of about

  150 Dm, leaving only a small (hatched) area 4 shown in FIGS.

  and 3c, and zone 6 (hatched) communicating electrically

  Then, the etched surface of the wafer is coated by melting a suitable layer of glass, for example, Schott Tempax quality glass, Corning 7070 or Corning 7740, which is

  brings abrasion to the same level as the surface of origin

  the wafer and polish it. This process is known in the art from US Patent 4,597,027 (A. Lehto). The glass-coated tray is then treated to obtain the metallizations 6 by means of a

  removal or chemical etching process. These

  assigned are conventional processes and can be

  omit the detailed description. Finally, we gather

  together the three plates by the so-called anodic bond under appropriate pressure. The value of the pressure used depends on the desired damping factor of the accelerometer and is typically

  the order of a few hPa. A description

  of the processes used (as well as other

  ceded) in Ivor Brodie and Julius J. Muray, The Physics of Microfabrication, Plenum Press, New York,

1983.

  The structures 16, forming the central electrode, according to FIGS. 4a.

  4c are manufactured using .DTD: processes almost identical to those described above.

  The difference lies in an area that is wider during the attack phase at 4 gm,

  of the channel 8 and the contact zone 9.

  The structures 15, forming the central electrode, according to FIGS. 4a ... 4c are made of glass, for example Schott Tempax, Corning 7070 or Corning 7740 quality glass. The zones 6 and 6 'are metallized using

  the process described above. The meeting is

  the accelerometer using the anodic bond

under normal ambient pressure.

  In both embodiments of the accelerometer, one can cut the plates by breaking them using

pre-cut grooves.

Claims (5)

  1. Capacitive silicon accelerometer, comprising   both - two lateral electrode structures, spaced apart from one another, and parallel, essentially flat   platelets, positioned face to face and   both the fixed lateral electrodes (4, 6 ') and - a uniform central electrode structure (16), essentially like a wafer, arranged between the   side electrode structures (15) and comprising   both - a body member (3) adjoining the side electrode structures (15) and   at least one central electrode (17) placed in the vicinity   swimming lateral electrodes (4, 6 ') and comprising a rod section (2) and a tip (1) such that the rod (2) of the central electrode (17) joins the electrode   (17) to the body member (3), which itself   same, encloses the central electrode (17) by the sides, and   it being specified that the stem section (2) is sensitive   thinner than the tip (1), characterized in that the central electrode structure (16) is symmetrical with respect to the median plane (s) of the lateral electrode structures (15), in that the central electrode (15) is 17) is in the form of a beam and is surrounded by a U-shaped groove which extends through the body member (3), in that the tip (1) of the central electrode (17) ) has a thickness close to that of the body member (3), so that, between the side electrodes (4, 6 ') and the central electrode (17),   fine games (7), with a spacing determined by the thick   of the nozzle (1), and - in that the side electrode structures (15) are hermetically sealed to the body member (3) of the central electrode structure (16) by the intermediate of an electrically insulating layer (5), it being specified that each   central electrode (16) is in a hermetically sealed space   tically closed and that the structures forming electrodes   (15, 16) are, under normal conditions, galvanically   isolated from each other if there is no connection outdoor electric.   2. Accelerometer according to claim 1, characterized in that the electrode structure   central (16) has a rectangular shape.   3. Accelerometer according to claim 1   or 2, characterized in that the structure forming   central trunk (16) is treated to form a structure   electrical connection (1) of the column type, electrically   isolated from the rest of the central electrode structure (16), it being specified that the zones (6) of electrical connection of the lateral electrodes (15) are arranged in the same plane.   4. A method of manufacturing a capacitive accelerometer, in which - are formed two structures (15) forming side electrodes consisting of fixed side electrodes (4, 6 '), and   a central electrode structure (16) constituting   of a center electrode (17) is disposed between the side electrode structures (15), characterized in that the central electrode structure (16) is fabricated from a homogeneous single crystal semiconductor chip. during phase I of the method, an area (12) forming a central electrode on both sides of the median section of the chip,   - in the course of phase II of the process,   than the edge regions (13) of the central electrode region (12) on both sides of the semiconductor chip so as to leave a pattern (17 ') in the central electrode region (12), forming a central electrode in the form of a beam, of a thickness defined during phase I and remaining integral with the unattacked portion of the semiconductor chip, in that during phase III of the process, a stem section (14) of the central electrode pattern (17 ') is then etched on both sides until the edge region (16) is etched through and the section (16) is 14) to a desired thickness, and - in that the side electrodes (15) are hermetically sealed to the structure (16) forming central electrode.   5. Process according to claim 4, characterized   in that in connection with the mounting of the side electrodes (15) on the central electrode structure (16), a correct gas pressure is created in a space (7) hermetically sealed by the electrodes   lateral sections (15) so as to obtain a frequency response advantage of the detector.