WO1994001782A1

WO1994001782A1 - Acceleration sensor

Info

Publication number: WO1994001782A1
Application number: PCT/DE1993/000570
Authority: WO
Inventors: Jiri Marek; Martin Willmann; Frank Bantien; Dietrich Schubert; Michael Offenberg; Félix Rudolf
Original assignee: Robert Bosch Gmbh
Priority date: 1992-07-09
Filing date: 1993-06-30
Publication date: 1994-01-20
Also published as: DE4222472A1; JPH07508835A; EP0649538A1; DE4222472C2

Abstract

The proposal is for an acceleration sensor consisting of an upper plate (1), a central plate (2) and a lower plate (3). The sensor has a movable electrode (4) built out from the central plate (2). The assembly of the plates is simplified by the use of a closed dielectric layer (10) on the upper plate (1) and the lower plate (3) since the plates do not have to be mutually adjusted. The use of stops (17) prevents the jamming of the seismic mass (33) on the upper (1) or lower plate (3). The use of local oxidation (18) reduces the stray capacitance.

Description

Acceleration sensor

State of the art

The invention is based on an acceleration sensor according to the type of the main claim. EP 0 369 352 already discloses acceleration sensors in which thin dielectric layers made of silicon oxide are used for the connection of silicon wafers Silicon and subsequent structuring manufactured.

Advantages of the invention

The acceleration sensor according to the invention has the advantage in comparison that the manufacture and evaluation of the sensors is simplified. It is particularly advantageous that the sensors can be closed without adjusting the lower or upper plate.

Advantageous further developments of this sensor are possible through the measures listed in the dependent claims. The damping of the movable electrode is influenced by the defined pressure in the cavity. By manufacturing the sensors by dividing three silicon wafers, mass production is particularly simple and therefore inexpensive. If one of these wafers has no structure, the adjustment of this wafer is not necessary. The sensor with the features of claim 5 has the advantage that the movement of the movable electrodes is limited by the stops. In particular, the stops prevent the movable electrode from sticking to the top or bottom plate. The measures listed in dependent claim 6 show an advantageous development of this sensor, in which the layers for connecting the plates are also used for the stops.

The sensor with the features of claim 7 has the advantage that the ratio of useful to stray capacitance is improved. The measures listed in the dependent claims allow advantageous developments of this acceleration sensor. The silicon oxide layer produced by local oxidation can be connected particularly easily to a silicon oxide layer produced over the entire surface. The connection with two silicon oxide layers produced by local oxidation further improves the ratio of useful capacity to stray capacity, in particular if the thickness of one of the two silicon oxide layers is reduced by processing after the oxidation. This processing is carried out particularly easily mechanically or by using appropriate etching techniques.

The acceleration sensor with the features of the subordinate claim 13 has the advantage that it has stops. A further advantage is that all processes take place on the middle plate, and thus the processes for producing the middle plate are also used for producing the stops. Advantageous further developments of this acceleration sensor are possible through the measures listed in the dependent claims. Through the The use of dielectric layers which are produced by local oxidation of silicon improves the ratio of useful capacitance to stray capacitance. The sensors can also be closed by an upper or lower plate which, owing to their closed dielectric film, does not have to be adjusted relative to the middle plate.

Advantageous further developments and improvements of the specified acceleration sensors are possible through the measures listed in subclaims 16 to 18. The possible range of materials for the connecting layers and the stops is expanded by the abovementioned separation techniques. It is particularly advantageous that these deposition techniques are carried out at significantly lower temperatures than in the oxidation of silicon. In addition to silicon oxide, dielectric layers made of silicon nitride can also be used.

drawings

Exemplary embodiments of the invention are shown in the drawings and explained in more detail in the description below. FIG. 1 shows the manufacture of an acceleration sensor with a closed dielectric layer on the top and bottom plate, FIG. 2 shows an acceleration sensor with stops, FIGS. 3 and 4 shows the local oxidation of silicon, FIG. 5 shows a reworked local oxidation point, FIG. 6a a connection point of two structured dielectric layers applied over the entire surface, FIG. 6b a connection point with a local oxidation and a surface dielectric film, FIG. 7 a connection point between two local oxidations, FIG. 8 a connection point between two local oxidations, one of which afterwards and FIG. 9 shows an acceleration sensor with dielectric layers of different thicknesses on the central plate. Description of the embodiments

In FIG. 1, 5 denotes three silicon wafers. These are shown in an exploded view in Figure 1 to illustrate the manufacturing process of an acceleration sensor. As indicated by the arrows, the wafers 5 are connected to one another in order to produce acceleration sensors. The sensors are separated by cutting along the lines 31 to form a single sensor with an upper plate 1, a middle plate 2 and a lower plate 3. A movable electrode 4 is structured out of the middle plate 2 and can consist of a spiral spring 32 and a seismic mass 33. The connecting layers 20, 19 ensure that the seismic mass 33 maintains a certain distance from the top plate 1 and bottom plate. Bending spring 32 and seismic mass 33 are designed such that the seismic mass 33 is deflected from its rest position by an acceleration. The capacitance between the central plate 2 and the upper plate 1 and the lower plate 3 is measured by an electrical connection of the upper plate 1, middle plate 2, lower plate 3 and corresponding electronics, not shown here. The change in these capacities is then a measure of the acceleration. However, stray capacitances are connected in parallel to these measuring capacitances and do not change their value depending on the acceleration. In order to ensure good measurability of the acceleration, the useful capacity, that is to say the capacity which changes with the acceleration, should be large compared to the stray capacity. An essential step in the manufacture of the acceleration sensors is the connection of the wafers and thus the closing of the cavity 6. A vacuum or vacuum is preferably enclosed in the cavity 6 so that the movement of the movable electrode 4 is not prevented by damping the movement in air hinder. Because the surfaces of the wafers 5 are very smooth and by a chemical Pretreatment are activated, as soon as the wafers 5 are placed one on top of the other, adhesion forms such a firm connection that a subsequent shifting of the wafers 5 placed on top of one another is not possible. The use of an upper plate 1 or a lower plate 3 with a closed dielectric layer 10 thus simplifies the manufacturing process since it is no longer necessary to adjust the upper plate 1 or lower plate 3 relative to the central plate 2. The layers of the middle plate, which are also provided for connection, can be designed over the whole area like the layer 20 or can only be present in a small area, like the layer 19. The actual connection of the wafers 5 or the top plate 1, middle plate 2, bottom plate 3 takes place by laying one on top of the other and subsequent heat treatment. Before the stacking, the surfaces of the wafers are activated chemically, for example by immersing the wafers in ammonia solutions or nitric acid. In addition to silicon oxide layers, silicon nitride or silicon oxynitride can also be used as materials for the dielectric layers.

FIG. 2 shows an acceleration sensor with an upper plate 1, a middle plate 2 and a lower plate 3 made of single-crystal silicon. A movable electrode 4 is structured out of the middle plate. The three plates 1, 2, 3 are again drawn in an exploded view. The connection between top plate 1, middle plate 2 and bottom plate 3 is established by dielectric layers 11. A vacuum, preferably a vacuum, is again enclosed in the cavity 6. The function of the sensor shown here corresponds to the sensor as described in FIG. 1. In contrast to the sensor according to FIG. 1, the sensor shown here still has stops 7. The movement of the movable electrode 4 or the seismic mass 33 is limited by the stops 7, in particular the stops 7 prevent that the seismic mass 33 can lie flat against the top plate 1 or bottom plate 3. The stops 7 are designed so that there can only be a small contact area between the seismic mass 33 and the top or bottom plate. It is irrelevant whether the stops 7 are arranged on the seismic mass 33 or on the top plate 1 or the bottom plate 3. Contact between the seismic mass 33 and the plates 1 and 2 is problematic, in particular during the connection of the plates, since in this process step the surfaces of the plates 1, 2, 3 and the seismic mass 33 are chemically activated, and therefore upon contact very strong adhesive forces occur.

The local oxidation of silicon is shown in FIGS. 3 and 4. For this purpose, a silicon substrate 41 is covered with an approximately 150 nanometer thick silicon nitride layer 43. To improve the adhesion of the silicon nitride, a thin, approximately 50 nanometer thick silicon oxide layer 42 can also be located between the silicon 41 and the silicon nitride 43. The silicon nitride layer 43 has an opening at the points at which silicon oxide is to be generated locally. The exposed surface of the silicon 41 is oxidized by heating the wafer to a temperature of over 800 degrees in an oxygen-containing atmosphere. The resulting local silicon oxide partially grows into the surface of the silicon 41 and partially protrudes from the surface of the silicon 41. This is because the silicon oxide has approximately twice the volume as the amount of silicon required for its production. The local silicon oxide 44 extends approximately 55% above the original surface of the silicon 41 and approximately 45% into the silicon 41. The thickness of the local silicon oxide is approximately 1 micron, for example.

FIG. 5 shows a piece of silicon 41 with a post-processed local oxidation 44. The part of the local silicon 44 which protrudes above the surface of the silicon 41 was removed by the postprocessing. This processing is done either by mechanical means, chemical etching or a combination of both methods. The corresponding processes are known from wafer production.

The use of local silicon oxide to improve the ratio of useful to stray capacities is illustrated in FIG. 6a and FIG. 6b. In FIGS. 6a and b, two silicon wafers 5 are shown as cutouts which are connected to one another. In FIG. 6a, the two wafers 5 are connected by dielectric layers 45, which were produced by applying the dielectric layers over the entire area and subsequent structuring. This corresponds, for example, to acceleration sensors such as were shown in FIG. 1 or FIG. 2. The stray capacitance and the useful capacitance are each formed by plate capacitors. The capacitance in a plate capacitor is inversely proportional to the distance between the capacitor plates, that is to say the further apart the capacitor plates are, the smaller the capacitance is. The aim is to increase the useful capacity and to reduce the stray capacity. The distance 47 between the capacitor plates and the stray capacitance is shown by arrow 47. The arrow 47 shows the distance between the capacitor plates for the useful capacity. As can be seen from FIG. 6a, the spacing of the stray capacitance is equal to the spacing of the useful capacitance when the wafers 5 are connected by the layers 45 shown here. FIG. 6b shows the connection of two wafers 5 via two dielectric layers 12 and 13. The dielectric layer 12 has been produced by local oxidation of silicon. As can be seen from FIG. 6b, the relative spacing of the stray capacitance 47 compared to the spacing of the useful capacitance 48 was improved by using the local oxide 12. The ratio is no longer 1 to 1 as in FIG. 6a, but approx. 1 to 1.5. With a constant connection area between the two wafers 5, the ratio of useful capacity to stray capacitance was thus improved. FIG. 7 shows the connection of two silicon wafers 5 with two local oxidations of silicon 14 and 15. In this case, the relative distance from useful capacity 48 to stray capacitance 47 has improved to approximately 1 to 2, that is to say the stray capacitance has been reduced again for the same connecting area.

FIG. 8 shows two silicon wafers 5 which are connected by a local oxidation 15 and a post-processed local oxidation 16. In this case, the ratio of the relative distance of the useful capacity 48 to the stray capacity 47 has again shifted to 1 to 3 in favor of the useful capacity.

FIG. 9 shows an acceleration sensor consisting of an upper plate 1, a middle plate 2 and a lower plate 3 made of monocrystalline silicon. A movable electrode 4 is structured out of the middle plate 2. The operation of the acceleration sensor shown in FIG. 9 corresponds to the acceleration sensors shown in FIG. 1 and FIG. 2. The top plate 1 and the bottom plate 3 each have a closed dielectric layer 10 on the side facing the middle plate 2, ie no adjustment is necessary for joining the plates 1, 2 and 3. Furthermore, the seismic mass has 33 local oxidations

17, which can be used as stops. Furthermore, the middle plate 2 has local oxidations 18, which are used for connecting the middle plate 2 to the top plate 1 and bottom plate 3. The local oxidations 17 are thinner than the local oxidations 18. This is achieved in the manufacturing process in that the openings in the silicon nitride are initially only available for the local oxidations 18. The local oxidation is then stopped after a remaining time and further openings are made in the silicon nitride layer in order to produce the local oxidations 17. The oxidation is then continued for a while. Due to the time advantage, the local oxidations

18 thicker than the local oxidations 17. The sensor according to FIG. 9 can thus be closed without adjustment, it has stops and the ratio of useful to stray capacitance is favorable.

Claims

Expectations

1. Acceleration sensor with an upper (1), middle (2) and lower plate (3) made of single-crystalline silicon, which are connected to one another by superficial dielectric layers (10-20), with an electrode (4 ), which is structured out of the central plate (2) and in which the respective capacitance between the upper (1) and lower plate (3) and the central plate (2) is measured, characterized in that the upper plate (1) and / or the lower plate (3) has a closed dielectric layer (10) on the side facing the central plate (2).

2. Acceleration sensor according to claim 1, characterized in that the plates (1,2,3) include a cavity (6), the movable electrode (4) is in this cavity (6) and in the cavity (6) a defined Pressure, preferably a negative pressure, is included.

3. Acceleration sensor according to one of the preceding claims, characterized in that the acceleration sensor is made by dividing three silicon wafers (5).

4. Acceleration sensor according to claim 3, characterized in that at least one of the wafers (5) has no structure.

5. Acceleration sensor with an upper (1), middle (2) and lower plate (3) made of single-crystalline silicon, which are connected to one another by superficial dielectric layers (10-20), with an electrode (4 ), which is structured out of the central plate (2) and in which the respective capacitance between the upper (1) and lower plate (3) and the central plate (2) is measured, characterized in that the lower plate (3) and / or the top plate (1) in the region of the movable electrode (4) and / or the movable electrode (4) of the middle plate (2) have stops (7) made of dielectric material.

6. Acceleration sensor according to claim 5, characterized in that all the plates (1, 2, 3) have dielectric layers (11) for connecting the plates (1, 2, 3), and that on the respective plates (1,2,3) arranged stops (7) have the same thickness as the layers (11) for connecting the plates (1,2,3).

7. Acceleration sensor with an upper (1), means (2) and lower plate (3) made of single-crystal silicon, which are connected to one another by superficial dielectric layers (10-20), with an electrode (4) movable by acceleration, which is structured out of the central plate (2) and in which the respective capacitance between the upper (1) and lower plate (3) and the central plate (2) is measured, characterized in that the dielectric layers (10-20) consist of silicon oxide and are partially structured, and that at least part of the structured silicon oxide is produced by local oxidation of silicon.

8. Acceleration sensor according to claim 7, characterized in that for the connection of the plates, a silicon oxide layer (12) generated by local oxidation is connected to a silicon oxide layer (13) produced over the entire surface.

9. Acceleration sensor according to claim 7, characterized in that for the connection of the plates two silicon oxide layers (14, 15, 16) generated by local oxidation are connected to one another.

10. Acceleration sensor according to claim 9, characterized in that the thickness of one of the two silicon oxide layers (15, 16) is reduced by processing after the oxidation.

11. Acceleration sensor according to claim 10, characterized in that the machining is carried out mechanically.

12. Acceleration sensor according to claim 10, characterized in that the processing is carried out by etching.

13. Acceleration sensor with an upper (1), middle (2) and lower plate (3) made of single-crystal silicon, which are connected to one another by superficial dielectric layers (10-20), with an electrode (4 ), which is structured out of the central plate (2) and in which the respective capacitance between the upper (l) and lower plate (3) and the central plate (2) is measured, characterized in that the central plate (2) has structured dielectric Layers (17, 18) has that part of these layers (17) can be used as stops and another part (18) for connection to the top (1) and bottom plate (3), and that the thickness of the layers (18) for the connection to the upper (1) and lower plate (3) is greater than the thickness of the layers (17) which can be used as stops.

14. Acceleration sensor according to claim 13, characterized in that the dielectric layers (17, 18) consist of silicon dioxide and are produced by local oxidation of silicon.

15. Acceleration sensor according to claim 13 or 14, characterized gekenn¬ characterized in that the upper (l) and lower plate (3) on the Mittel¬ plate (2) facing side have a closed silicon oxide film.

16. Acceleration sensor according to one of the preceding claims, characterized in that the dielectric layers (10-20), which are not produced by local oxidation of silicon, by application over the entire surface by sputtering, chemical deposition from the gas phase or plasma-assisted chemical deposition from the gas phase are manufactured.

17. Acceleration sensor according to claim 16, characterized in that the dielectric layers (10-20) consist of silicon oxide.

18. Acceleration sensor according to claim 16, characterized in that the dielectric layers (19-20) consist of silicon nitride.

19. Acceleration sensor according to claim 16, characterized in that the dielectric layers (19-20) consist of a borosilicate glass.