JPH01259265A - Capacitive accelerometer and manufacture thereof - Google Patents

Abstract

PURPOSE: To ensure high sensitivity from a small variation in oscillatory mass by constituting a capacitive accelerometer of two capacitors having a common movable electrode acting as the oscillatory mass of accelerometer. CONSTITUTION: The accelerometer has a layer structure of side face electrode structure 15 and central electrode structure interconnected electrically wherein the oscillatory mass 1 of central electrode structure 16 is provided by a chip part of a cantilever beam 17. The integral central electrode structure 16 includes the cantilever beam 17 comprising an oscillation unit 1, a flexible stem part 2, and an accelerometer body element 3 surrounding the cantilever beam 17. Capacitance of the accelerometer is formed between the common movable electrode 1 in central electrode structure 16 and the side face electrode 4 secured to side face electrode structure 15. The stem part 2 is essentially made thinner than the chip part.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a capacitive accelerometer and a method for manufacturing the same.

BACKGROUND OF THE INVENTION Accelerometers are actually small force sensors or are primarily used to detect acceleration. This device also has other uses,
For example, it is also used to detect tilt angles.

Miniature accelerometers known to date are mostly based on piezoelectric or piezoresistive operating principles.

A piezoelectric crystal generates a surface charge whose magnitude depends on the force applied to its surface. This charge is measured using a charge sense amplifier. Due to the high input impedance of the amplifier used for measurements, the amplifier is susceptible to electrostatic interference. The charge generated on the surface naturally discharges through the surface, and because of leakage currents this type of accelerometer cannot be used for static or low frequency acceleration measurements.

Piezoresistive accelerometers are typically made of a semiconductor material, such as silicon, into which a resistor is formed by diffusion in the appropriate crystal orientation. The stress induced when the crystal is bent causes a change in resistance such that the magnitude of the bend can be detected. The amount of bending is proportional to the applied force,
Therefore, it is proportional to acceleration.

Small piezoresistive accelerometers can be manufactured, for example, from silicon using microelectronics and micromachining techniques.

For maximum sensitivity the maximum stress must be applied to the area of resistance. The displacement of the elastic part thereby has a very large amplitude with respect to the thickness of the structure, and in addition, a sensitive accelerometer must be provided with an auxiliary weight that acts as a vibrating mass of the accelerometer. This is because silicon itself has a relatively low density. The fabrication of auxiliary masses complicates the fabrication of the device. Additionally, piezoresistive accelerometers are more sensitive to temperature changes than capacitive accelerometers, for example. Furthermore, piezoresistive accelerometers have a low so-called gauge factor compared to capacitive accelerometers.

Single crystal silicon has been used in the manufacture of accelerometers since the late 1960s. Some of these methods are described in the literature.

For example, [1] I E E E Trans, on Ele
ctron DeviceE D 26. 1911-
1920 pages (1979) [2] W, B, enec
ke et al. Tra'n5ducers1987, 406-409
Page [3] Tsugai and Bescho Transducers 1987, '403-40
Page 5 [4] E, J, Evans U.S. Patent No. 3,478,604 [5] A, J, Yer
The structure according to US Pat. No. 3,572,109 [4] and [5] relates to a bent cantilever type accelerometer.

Capacitive accelerometers are described in the following documents:

[6] H.W. Pischer U.S. Pat. No. 39117
Specification No. 38 [7] W, H, F 1cken U.S. Patent No. 4009607 [8 CoF, V, l1
oldren US Pat. No. 4,094,199 [9]
H, E, Aine U.S. Pat. No. 4,144,516 [1
0koK, E, P etersen
Other US Pat. No. 4,342,227 [11] R. F. Colton US Pat. No. 4,435
No. 737 Specification [12] F, Rudolf U.S. Patent No. 4,483,194 [13 CoL, B, WNn
er US Pat. No. 4,574,327 [6] describes an accelerometer that uses two capacitances without indicating the physical structure.

Document [7] has a different electrical configuration or is basically the same as Document [6].

Document [8] describes an accelerometer according to the double capacitance principle.

Reference [9] describes a micromechanical accelerometer in which a vibrating mass is suspended from a leaf spring.

Literature [lO] moves in a cantilever beam or in a horizontal plane,
A bent cantilever accelerometer with an electrically asymmetric structure is described.

Reference [11] shows a cyclic structure.

Document [12] describes a type with torsionally suspended plates in which the capacitance is arranged on only one side of the plate.

Reference [13] describes a structure suspended on a vibrating mass or a thin film spring.

[Problem to be Solved by the Invention] The object of the invention is to overcome the drawbacks of the prior art and to provide a capacitive accelerometer with complete and excellent characteristics and a method for manufacturing the same.

SUMMARY OF THE INVENTION The present invention constructs a capacitive type accelerometer from two capacitors having a common movable electrode that acts as the vibrating mass of the accelerometer.

The central electrode forms an integral structure with the beam made of the same material. It is also possible to use more than one of these elastic members. The applied acceleration causes a displacement of several microns in the vibrating mass, which displacement is detected from a change in capacitance value. The structure of the accelerometer is double-sided and symmetrical.

More specifically, in the accelerometer according to the present invention, the central electrode structure is symmetrical with respect to the central plane of the side electrode structures;
The central electrode has a beam shape and is surrounded by a groove extending through the U-shaped body element, and the tip portion of the central electrode has an electrode gap with a spacing determined by the thickness of the tip portion with the side electrodes. The side electrode structure has a thickness close to that of the main body element so as to be formed between the center electrode structure and the side electrode structure is attached to the main body element of the center electrode structure via an electrically insulating layer with a hermetic seal. The electrodes are held in a hermetically sealed space, and the electrode structures are characterized by a structure in which they are electrically isolated from each other under normal conditions when there is no external electrical connection.

The invention also provides a method for manufacturing a capacitive accelerometer in which two side electrode structures are formed that constitute fixed side electrodes, and a center electrode structure with a center electrode is disposed between the side electrode structures. The electrode structure is formed from a uniform single crystal semiconductor chip by etching a central electrode area on both sides of the central part of the chip during processing phase I, and edge areas of the central electrode are further etched during processing phase I.
The stem portion of the center electrode is further etched during the processing phase on both sides of the semiconductor chip to leave a beam-shaped center electrode pattern of a defined thickness, and further etched during the processing phase until a predetermined thickness is obtained. The side electrodes are hermetically sealed to the central electrode structure.

[Effects of the Invention] The structure of the accelerometer according to the present invention produces the following effects.

* Capacitive operating principle obtains high sensitivity ΔC/C with small displacements of the vibrating mass.

* Due to the symmetrical construction, this accelerometer has very low temperature sensitivity with no need for compensation.

* Since the vibrating mass is made of the same material as the bending member, e.g. silicon, no auxiliary vibrating mass is required.

* The damping coefficient of the accelerometer can be varied by forming grooves in the capacitor electrodes or by leaving an appropriate pressure in the accelerometer structure during the manufacturing process.

* The capacitance change is symmetrical around the upper acceleration value.

*This accelerometer can be manufactured in mass production.

* The accelerometer has a significantly high overload tolerance, since the vibrating mass undergoes only a few microns of displacement before contacting the side electrodes.

[Example] Hereinafter, the present invention will be described in more detail by way of example with reference to the accompanying drawings.

The accelerometer of the invention is fabricated, for example, from single crystal silicon using conventional methods known in micromachining technology. FIG. 1 shows the basic structure of an accelerometer.

The accelerometer is hermetically sealed, so the accelerometer case may be gas filled at a suitably low pressure.

The pressure of the fill gas can be varied to obtain the desired damping coefficient of the vibrating mass of the accelerometer. The fill gas used can be, for example, dry air. A properly selected damping coefficient produces an effective frequency response of the accelerometer.

However, a simpler construction of the accelerometer is obtained with an open accelerometer as shown in FIG. 2, in which the internal gas pressure of the accelerometer is equal to the pressure of the surrounding atmosphere. Communication between the internal gas space and the surrounding atmosphere is provided by channels 8. However, the damping coefficient of the vibrating mass is then very high and cannot be used for measurements of low frequencies (of the order of a few cycles) and static accelerations.

FIG. 1 shows the structure of a hermetically sealed accelerometer. The accelerometer is a layered structure with flat, electrically interconnected side electrode structures 15 and a central electrode structure 16 arranged parallel therebetween;
The vibrating mass 1 of is given by the tip portion of the cantilever beam 17. Cantilever beam 17 is formed in central electrode structure 16 by forming a U-shaped groove that extends across central electrode structure 16 . A portion of the material of the beam 17 is removed to form the gap 7 to provide further capacitance, and the width of the stem portion 2 of the beam 17 is reduced to provide flexibility in this portion. The integral central electrode structure 16 comprises a cantilever beam 17 comprising a vibrating mass 1 and a flexible stem portion 2, as well as an accelerometer body element 3 surrounding the cantilever beam]7. The vibrating mass 1 is made several micrometers thinner than the accelerometer main body element 3. The central electrode structure 16 is made, for example, from the same single-crystal silicone. The capacitance of the accelerometer is formed between the common movable electrode 1 of the central electrode structure 16 and the fixed side electrodes 4 of the side electrode structures 15. The side electrode 4 is
It is manufactured, for example, from monocrystalline silicon by etching the surface corresponding to the insulating layer 5 so as to leave a raised area in the vibrating mass 1 and the coupling area 6. The insulating layer 5 may be made of glass, for example. The glass-covered area 5 and the body element 3 can be hermetically sealed to each other, for example using an anodic bond.

The structure of the central electrode structure 16 is the plane S~1 shown in FIG.
3- Mirror symmetry with respect to (xy plane). Vibration quality jul is 2
It is possible to move in the direction.

The structure of the accelerometer shown in FIG. 1 is shown in FIGS. 3a to 3C.
This is shown in detail in the figure. 3a and 4a, the metallized parts 6 and 6' and the silicon area 4 are shown in an inverted position for better visibility.

FIG. 2 shows an open structure accelerometer. This structure differs from the closed structure in the following points. That is, the side electrode structure 15 consists entirely of glass with a surface covered by metallized areas 6 and 6' for the electrical functions of the accelerometer.

The metallized area 6 is connected to the metallized area 6 and the body element 3
It is arranged to pass through channel 8 in order to prevent short circuits with.

The structure of the accelerometer according to FIG. 2 is shown in detail in FIGS. 4a to 4c. In both structures shown, the metallization areas 6 for the electrical connections are preferably made in the same plane, so it is possible to use electrical feedthroughs 10 as shown in FIGS. 3 and 4. is necessary.

This part is made of the same material as the main body element 3, but is electrically insulated therefrom. The structure of the penetrating body 1o is shown in more detail in FIG. The purpose of the ridge 9 shown in FIG. 4b is to form an electrical connection from the body cord 3 to the central contact area 6. The layout of the accelerometer is shown in FIG.

The shock resistance of the accelerometer is improved by etching the stem portion 2 to leave protrusions 11 as shown in FIG. 6 to prevent excessive flexing of the stem portion 2 under strong impacts. The dimensions of acceleration 31 strongly depend on the desired sensitivity and capacitance. Typical dimensions are, for example, a vibrating mass 1 of 2 x 05 x 4 mIn3 and a cantilever beam 17 of 2 Xo, 07 x 4 m+n3. The external dimensions of the accelerometer are approximately 4 x 3 x 12 mm (width x height x length).

The manufacturing face described below can be applied to process both sides of a base material that is a single crystal semiconductor, for example a silicon wafer polished on both sides.

(2) Both sides of the silicon wafer are oxidized to a depth of about 250 nm. The thickness of the wafer is, for example, 500 μm.

2. The wafer is coated with photoresist to expose the central electrode area to be etched away with oxide as shown in Figure 7a. Figures 7a and 7b show a single silicon chip.

3. Then the central electrode area 12 shown in FIG.
It is etched to a depth of about 4 μm in an OH aqueous solution.

4. The oxide layer of the wafer is removed by etching with buffered HF, and the wafer is oxidized to a depth of approximately 0.8 μm.

5. The wafer is coated with photoresist and the edges of the central electrode area 12 are exposed for the edge areas 13 and contact areas 13° to be removed with oxide as shown in FIG. 8a. The photoresist is removed.

6. The edge area 13 and the contact area 13° are etched to a depth of approximately 50 μm, thereby forming a central electrode pattern 17''
or as shown in FIG. 8b.

7. The wafer is again coated with photoresist and the stem portion of the central electrode pattern 17° is exposed for areas 14 where the oxide is to be removed as shown in Figure 9a.

8. The etching of the wafer is continued in an aqueous solution of KOH until the desired depth (typically 40-100 μm) of the area 14 is obtained, thereby infiltrating the area 13 as shown in FIG. 9b. arise.

This step results in the element or central electrode structure 16 of FIG. 3b.

The side electrode structure 15 shown in FIGS. 3a and 3c is electrically connected to the small (shaded) area 4 shown in FIGS. 3a and 3c using the technique described above. It is fabricated by etching one surface of a silicon wafer to a depth of about 150 μm, leaving only areas 6 (shaded) of the original height that are interconnected. The etched surface of the wafer is then coated with a suitable glass layer, such as Schott Te
mpax), Corning 707
0 or Corning 7740 grade fused glass. It is polished to the same level as the original surface of the wafer. This method is described in U.S. Patent No. 4,59
No. 7,027. The glass-coated wafer is formed with metallization 6 by a lift-off method or an etching process. Since these methods are conventional, their details will be omitted. Finally these 3
All the wafers are bonded to one another with appropriate pressure, for example using so-called anodic bonding. The level of pressure used depends on the desired damping coefficient of the accelerometer and is typically a few bPa. A detailed description of the applied method (as well as other methods) can be found in the literature (lvor
Brodie and Julius J, Muray,
Physics of Microfabrjcati
on.

Plenum Press, New York, 19
1983).

The central electrode structure 16 shown in Figures 4a-4c is manufactured using the same method as described above. The difference lies in the size of the areas, which are widened in the 4 μm etched face in the areas of channels 8 and contact areas 9.

The side electrode structure 15 shown in FIGS. 4a-4c is fabricated from Nyoto Tempax, Corning 7070 or Corning 7740 grade glass. Area 6
and 6° are metallized in the manner described above.

Accelerometer construction is performed using anodic coupling in normal ambient atmosphere.

In both accelerometer configurations, the wafer can be cut by folding with pre-cut grooves.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of an accelerometer structure according to an embodiment of the present invention. FIG. 2 is a longitudinal cross-sectional view of an accelerometer structure according to another embodiment of the invention. 3a-3c are perspective views of different parts of the accelerometer structure shown in FIG. 1; FIG. Figures 4a-4c are perspective views of different parts of the accelerometer structure shown in Figure 2; 5 is a perspective view, partially in section, of the accelerometer structure shown in FIG. 2; FIG. FIG. 6 is a cross-sectional view of a third embodiment of the accelerometer structure of the present invention. FIG. 7a is a top view of the accelerometer structure of the present invention during the first mask phase. Figure M7b is a top view of the final result obtained by the mask phase shown in Figure 7a. Figure 8a shows a top view at the second mask face of the accelerometer structure of the present invention. Figure 8b shows a top view of the final result obtained by the mask phase shown in Figure 8a. Figure 9a shows a top view of the accelerometer structure of the present invention during the third mask phase. Figure 9b shows a top view of the final result obtained by the mask phase shown in Figure 9a. DESCRIPTION OF SYMBOLS 1... Oscillating mass, 2... Stem part, 3... Main body element, 4... Fixed electrode, 5... Glass covered area, 6...
...Coupling area, 15. Side electrode structure, 16. Central electrode structure, 17. Cantilever beam. al'fl

Claims (5)

[Claims] (1) Two substantially plate-shaped flat side electrode structures arranged in parallel and facing each other with a distance from each other forming a fixed side electrode, and a side electrode structure arranged between these side electrode structures. a uniform, generally plate-shaped central electrode structure having a body element coupled to a side electrode structure and one or more central electrodes disposed proximate the side electrodes and having a stem portion and a tip portion; an electrode structure, a stem portion of the central electrode couples the central electrode to a body element, the body element surrounds sides of the central electrode, and the stem portion is substantially thinner than the tip portion. In a capacitive accelerometer constructed of silicon material, the central electrode structure is symmetrical with respect to the central plane of the side electrode structures, and the central electrode has a beam shape and a groove extending through the U-shaped body element. and the tip portion of the central electrode approaches the thickness of the body element such that an electrode gap is formed between the side electrodes and the center electrode with a spacing determined by the thickness of the tip portion. The side electrode structure is mounted on the main body element of the center electrode structure via an electrically insulating layer with a hermetic seal, holding the center electrode in a hermetically sealed space, and the electrode structure is connected to external electrical connections. Capacitive accelerometers characterized in that they are electrically isolated from each other in normal conditions when not in use. (2) The accelerometer according to claim 1, wherein the central electrode structure has a rectangular shape. (3) A patent in which the central electrode structure is treated to form a columnar conductive connection structure electrically insulated from the rest of the central electrode structure, whereby the connection areas of the side electrodes are arranged in the same plane. The accelerometer according to claim 1 or 2. (4) A method for manufacturing a capacitive accelerometer, in which two side electrode structures forming a fixed side electrode are formed, and a center electrode structure comprising a center electrode is disposed between these side electrode structures, the center electrode structure comprising: is formed from a uniform single crystal semiconductor chip by etching a central electrode area on both sides of the central part of the chip during processing phase I, and the edge areas of the central electrode are further etched with a defined thickness in processing phase I. It is further etched during Processing Phase II on both sides of the semiconductor chip to leave a beam-like center electrode pattern, the stem portion of the center electrode is further etched during Processing Phase III until the desired thickness is obtained, and the side electrode structure is A method for manufacturing a capacitive accelerometer, characterized in that it is attached to a central electrode structure in a hermetically sealed state. (5) At the same time as the side electrode structure is attached to the central electrode structure, an appropriate gas pressure is applied to the space hermetically sealed by the side electrode structure in order to obtain a predetermined frequency characteristic of the sensor. Claim No. 4
Manufacturing method described in section.