GB2246635A

GB2246635A - Acceleration sensor

Info

Publication number: GB2246635A
Application number: GB9113281A
Authority: GB
Inventors: Jiri Marek
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1990-07-14
Filing date: 1991-06-19
Publication date: 1992-02-05
Anticipated expiration: 2011-06-19
Also published as: CH684029A5; DE4022464A1; DE4022464C2; JPH04232875A; GB2246635B; JP3199775B2; GB9113281D0

Description

:2 1 1 is d- -1 CE, CE. t3 E; Acceleration sensor

Prior art

The invention proceeds from a sensor for measuring accelerations,, particularly angular accelerations, according to the preamble of the main claim.

An acceleration sensor which is produced from a monocrystal line, twolayer carrier is described in German Patent Specification 4,,000j,903. It exhibits a blade, capable of vibration parallel to the surface of the carrier, which blade is arranged opposite a fixed electrode in the direction of vibration. In the case of this sensor, the acceleration is detected via the capacitive change between the moving blade and the fixed electrode.

Seismic masses which are suspended on Archimedes I spirals and are provided with an electrostatic comb drive are already known from "Laterally Driven Polysilicon Resonant Microstructures"; W.C. Tang, T.H. Nguyen, R.T. Howe; Sensors and Actuators, 20 (1989) 25 - 30. This article describes the implementation of such structures in polysilicon technology.

Advantages of the invention The sensor according to the invention having the characterising features of the main claim has the advantage that rotational movements can be distinguished from linear accelerations by detecting the deflection of the seismic mass on a plurality of sides of the seismic mass.

1 R. 23633 Depending on the type of the acceleration, the measurement signals which are detected on different sides of the seismic mass vary either in the same sense or in the opposing sense. Thus, by comparing the signals, it is possible to distinguish in a particularly simple manner between a rotational movement and a linear acceleration. It is also advantageous that the seismic mass of the sensor can be deflected preferably in the plane of the carrier and does not thereby jut out of the surface of the carrier. The carrier itself is advan-tageously used thereby as protection against mechanical overload. A suspension of the seismic mass an thin webs on at least two sides increases the stability of the sensor against overload and, at the same time, ensures a very high measurement sensitivity. At the same time, the sensitivity to lateral accelerations is reduced. With regard to lateral sensitivity, a four-sided symmetrical suspension of the seismic mass is also particularly advantageous. The production of the sensor out of a silicon carrier is particularly advantageous, since particularly small types of construction can be achieved using standard methods. It is moreover advantageous, that sensor evaluation circuits can also be integrated on a silicon carrier.

By means of the measures cited in the subclaims, advantageous further developments of the sensor stated in the main claim are possible.

The detection of deflections of the seismic mass is produced particularly advantageously piezoresistively', with the aid of, in each case, two piezo electric resistors, which are applied to the right and left of the web axis. In the case of linear accelerations of the seismic mass, the at least two piezoresistors change in the same sense on each of the suspension webs. In the case of a rotational movement, one side of each suspension web is stretched, while the other side is compressed. That leads to an opposingly-directed change of the resistance values of each suspension web. Piezoresistive signal detection can also advantageously be used in the case of sensors 1; f is R. 23633 which are produced from a single-layer carrier. The formation of narrow suspension webs in the complete thickness of the carrier is advantageous since, in this way, deflections of the seismic mass within the plane of the carrier are given preference while deflections of the seismic mass perpendicular to the plane of the carrier are suppressed.

For the production and for insulation of partial structures of the sensor, it is particularly suitable to use two-layer silicon carriers, a doping junction existing between the upper layer and the lower layer, preferably a pn-junction. The carrier can be monocrystalline, it being possible to produce the upper layer by diffusion of impurity atoms or for it to be an epitaxial layer deposited on a carrier. Depending on the sensor structure, it is of advantage to use a silicon carrier having a polysilicon layer deposited thereon. In this case, the insulation is provided,, for example, , by a silicon oxide layer between the monocrystalline and polycrystalline silicon layers.

A further advantageous possibility is'capacitive signal detection. To this end, it is advantageous to structure out of the silicon carrier fixed electrodes originating f rom two opposite sides of the frame and arranged parallel to the suspension webs. Together with the suspension webs used as moving electrodes, these fixed electrodes in each case form a capacitor. Alternatively, or for the amplification of the signal, it is advantageous to structure further fixed electrodes, originating from the frame, and moving electrodes parallel thereto, which originate from the seismic mass and,, together with the fixed electrodes, form interdigital capacitors. A further advantageous refinement is also to use these capacitors for position regulation,, by the application of a variable voltage, with which the seismic mass is restored to its original position. This represents a particularly suitable possibility for protecting against overload. A combination of capacitive position regulation, capacitive signal detection and is Drawing R. 23633 piezoelectric resistive signal detection is also advantageous. The insulation of the moving electrodes with respect to the fixed electrodes can be implemented particularly suitably if the suspension webs of the seismic mass are formed only in the upper layer. The pnjunction between upper layer and lower layer then represents an insulation of the electrodes with respect to the lower layer; the insulation in the upper layer can be provided advantageously either by isolation diffusions or by etched trenches penetrating completely through the upper layer.

A further advantageous refinement of the sensor according to the invention consists of the seismic mass being suspended on two Archimedes, spirals, running within each other, which are provided with moving webs on the outermost turns. The moving webs exhibit comb-shaped finger structures which, together with finger structures originating from fixed webs, form electrostatic reluctance drives. These interlocking finger structures can advantageously be used either for picking off a signal or else for position regulation. An additional signal tapping can advantageously be performed piezoresistively using piezoresistors arranged on the spirals.

For increasing the sensitivity of the sensor, the seismic mass can either (sic) be configured in the complete thickness of the carrier. In the case of configuration of the seismic mass in the same thickness as that of the suspension, either the moment of inertia or the lateral sensitivity can be optimised.

Exemplary embodiments of the invention are represented in the drawing and are explained in more detail in the following description. Figure 1 shows the plan view of a sensor having a piezoresistive signal pick-off, Figure 2 shows the plan view of a sensor having a capacitive signal pick-off, Figure 3 shows the section through this sensor, Figure 4 shows the plan view of a

1 1 1 i i i i i n f k R. 23633 sensor having interdigital capacitors, Figure 5 shows the plan view of a sensor having a suspension by means of Archimedes, spirals and Figures 6a and b show sections through this sensor along the A and B axes.

Description of the invention is Figure 1 represents a sensor having a fixed frame 10 and a deflexible seismic mass 20 mounted therein. The seismic mass 20 is in this case suspended symmetrically via four thin webs 21 to 24. This structure can be structured from a single-layer or two-layer silicon carrier. The carrier can be monocrystalline or can be provided with a polysilicon layer. The webs 21 to 24 and the seismic mass 20 can not only be configured in the full thickness of the carrier but also can be reduced in their thickness. For increasing the sensitivity, it is useful to make the seismic mass 20 as large as possible, that is to say to configure it in the full thickness of the carrier. In the case of seismic masses 20, whose webs 21 to 24 are thickec than they are wide, that is to say, for example, having the complete thickness of the carrier, deflections within the plane of the carrier are given preference over deflections perpendicular to the plane of the carrier. In the case of the exemplary embodiment represented in Figure 1, two piezoresistors, 81, 82 are in each case applied to each web 21 to 24. They are arranged in each case right and left of the web axes. A linear acceleration of the sensor in the plane of the carrier, or perpendicular thereto, always leads to a change in length of the two halves of the suspension webs right and left of the web axis in the same sense and thus to a change in resistance of the piezoresistors on one web in the same sense. In contrast thereto, a rotational movement about an axis of rotation perpendicular to the surface of the carrier leads to a bending of the halves of the web in opposite senses and therewith to a change in the resistance of the piezoresistors on one web in opposite senses. Linear accelerations can thus easily be R. 23633 distinguished from rotational movements by comparison of the resistance values of the resistors on one web or by appropriate interconnection.

Figure 2 represents a sensor structure which is comparable to that represented in Figure 1. However, in this case, the signal pick-off is performed capacitively rather than piezoresistively. To this end, fixed electrodes 11 to 14 are structured out of the carrier,, originating from the fixed frame 10, parallel to the suspension webs 21 to 24. Together with the suspension webs 21 to 24, used as moving electrodes, these fixed electrodes 11 to 14 form capacitors. The fixed electrodes 11 to 14 are arranged with respect to the moving electrodes 21 to 24 such that a linear acceleration in the plane of the carrier leads to capacitance changes in opposing directions on the two opposite capacitors. Only a rotational movement about an axis of rotation perpendicular to the plane of the carrier leads to a capacitance change in the same sense on at least two opposite capacitors. This sensor structure is structured out of a two-layer silicon carrier 1, a doping junction existing between the upper layer 2 and the lower layer 3 of the silicon carrier 1. The webs 21 to 24 are configured only in the upper layer 2. Isolation diffusions 30 are introduced into the frame 10 about the mouth regions of the webs. However, the isolation diffusions 30 can also be introduced in a suitable manner at the points on the frame 10 from which the fixed electrodes 11 to 14 originate. In conjunction with the pn-junction between the upper layer 2 and the lower layer 3, these are used to insulate electrically the suspension webs 21 to 24, used as moving electrodes, from the fixed electrodes 11 to 14. Figure 3 represents a section through this sensor in the region of the webs 22 and 24. The fixed frame 10 is configured in the full thickness of the carrier, as is the seismic mass 20. However, it is also possible to reduce the seismic mass 20 completely or partially in its thickness or even to configure it only in the upper layer 2.

i i i 1 i 1.

1:

i 4r k is R. 23633 A further possibility for the insulation of structural parts and for signal pick-off is represented in Figure 4. Etched trenches, which completely penetrate the upper layer 2, are denoted by 45. In this way, in Figure 4, fixed electrodes 41 originating from the frame, are electrically isolated from the moving electrodes 42, originating from the seismic mass 20. With the fixed electrodes 41, the moving electrodes 42 form parallelconnected interdigital capacitors, which cause amplification of the signal. The method of operation of the sensor represented in Figure 4 corresponds to that of the sensor represented in Figures 2 and 3. All combinations of the signal detection methods represented are also possible, such as interdigital capacitors having the diffusion isolations represented in Figure 2 and/or in conjunction with piezoresistive signal detection as represented in Figure 1. In addition,, it is conceivable to use the capacitor structures represented in Figures 2 and 4 not only for signal detection but also for position regulation of the seismic mass 20, by application of a variable voltage. In this manner, overload situations can be prevented better, which increases the service life of the sensor. The linearity of the output signal is also improved by this means.

Figure 5 represents the plan view of a sensor which is structured out of a silicon carrier 1 consisting of a substrate 3, an insulating layer 5 applied thereto and a polysilicon layer 2 applied to the insulating layer S. Figures 6a and b show sections through the sensor on the axes denoted A and B in Figure 5. Out of the polysilicon layer 2 there is structured an anchor point 55 which is firmly connected via the insulating layer 5 to the substrate 3. From this anchor point 55 as the centre point, there originate two spirals 50, 60,, running within each other, which are configured only in the polysilicon layer 2 and are not connected to the substrate 3, other than via the anchor point 55, and are thus can move like spiral springs. In each case on the outermost turns of the spirals 50, 60, moving masses 51, 61 are configured, i - a - 0 R. 23633 also only in the polysilicon layer 2, which are arranged in a star shape with respect to the anchor point 55. They exhibit comb-shaped finger structures 511,, 611 on two sides. Also arranged in a star shape about the anchor point 55, between the moving masses 51, 61, there are fixed electrodes 71, which are connected to the substrate 3 and/or a frame not represented in Figures 5. 6a and b. The fixed electrodes 71 also exhibit comb-shaped finger structures 711. The finger structures 511, 611 of the moving masses 51, 61 and the finger structures 711 of the fixed electrodes 71 interlock with each other. These finger structures 511, 611, 711 together form interdigital capacitors or electrostatic reluctance drives, which can be used for position regulation as well as for signal detection. In the case of this structure, signal detection is possible, moreover, using piezoresistors arranged on the spirals 50 and 60.

Using this sensor, angular accelerations about an axis perpendicular to the surface of the carrier can be detected particularly well. For this purpose, the Archimedes, Lpirals 50, 60 act like spiral springs, which become stretched or compressed depending on the direction of rotation, the position of the moving masses 51, 61 being changed with respect to the fixed electrodes 71, which leads to changes of the electrical characteristics at the interdigital capacitors.

z - 1 j 1 j j 1 1. i i 1 1 1 1.

1 J 1 1 1 i 0 1 1

Claims

Claims is 1. Sensor for measuring accelerations, particularly angular

accelerationsi which is produced from a silicon carrier, at least one fixed frame and at least one deflexible seismic mass,, mounted in the frame, being structured out of the silicon carrier, and means of detecting deflections of the at least one seismic mass In the plane of the carrier being provided, characterised in that the seismic mass (20) is connected to the frame (10) via at least two symmetrically arranged webs (21 to 24)r bendable in the plane of the carrier, and in that the deflection of the seismic mass (20) in the plane of the carrier is detected on at least two opposite sides of the seismic mass (20).
2. Sensor according to Claim 1,, characterised in that the frame (10) is square, in that the seismic mass (20) has a square upper side, In that the seismic mass (20) Is connected on four sides or on two sides to the frame (10), such that the edges of the square upper side of the seismic mass (20) are oriented parallel to the inner surfaces of the frame (10) and such that the seismic mass (20) Is suspended in the centre of the frame (10),, and in that the suspension webs (21 to 24) originate perpendicularly from the centres of the edges of the upper side of the seismic mass (20).
3. Sensor according to Claim 1 or 2. characterised in that on the webs (21 to 24) in each case at least two piezoresistors (81. 82) are applied to the right and left of the web axis or are integrated into the webs (21 to 01,R- R. 23633 is 24).
4. Sensor according to one of the preceding claims, characterised in that the webs (21 to 24) and/or the seismic mass (20) are configured completely or partially in the total thickness of the silicon carrier (1).
5. Sensor according to one of the preceding claims, characterised in that the silicon carrier (1) exhibits an upper layer (2) and a lower layer (3).
6. Sensor according to Claim 5, characterised in that a doping junction, preferably a pn-junction, exists between the upper layer (2) and the lower layer.
7. Sensor according to Claim 5 or 6. characterised in that the silicon carrier (1) is monocrystalline.
8. Sensor according to Claim 5 or 6. characterised in that the upper layer (2) is a polysilicon layer.
9. Sensor according to Claim 8,, characterised in that the silicon carrier (1) exhibits an insulating layer (5), preferably a silicon oxide layer. between the upper layer (2) and the lower layer (3).
10. Sensor according to one of Claims 5 to 9, charac terised in that at least ty:o fixed electrodes (11 to 14) are structured out of the silicon carrier (1), originat ing from two opposite sides of the frame (10), which are arranged in each case parallel to a suspension web (21 to 24) and which. together with the suspension webs (21 to 24), configured as moving electrodes, in each case form a capacitor.
11. Sensor according to one of Claims 5 to 10,, characterised in that structured out of the silicon carrier (1) there are at least two fixed electrodes (41), originating from two opposite sides of the frame (10) and, parallel thereto,, there are at least two moving electrodes (42), originating from the seismic mass (20), which together in each case form a capacitor.
12. Sensor according to Claim 10 or 11. characterised in that the suspension webs (21 to 24) are configured only in the upper layer (2) and in that the fixed elec trodes (11 to 14. 41) are insulated from the moving electrodes (21 to 24r 42) by a pn-junction or an T i 1 i i 1 R. 23633 insulating layer (5) between the upper layer f2) and the lower layer (3) and by isolation diffusions (30) in the upper layer (2) and/or by etched trenches (45) completely penetrating the upper layer (2).
13. Sensor for measuring accelerations, particularly angular accelerations, which is produced from a silicon carrier (1). which exhibits a substrate (3), an insulating layer (5) applied thereto and a polysilicon layer (2) applied to the insulating layer (5), characterised in that out of the polysilicon layer (2) there is structured an anchor point (55) which is firmly connected via the insulating layer (5) to the substrate (3), in that, originating from the anchor point (55) as the centre point,, two spirals (50, 60) are structured in the polysilicon layer (2),, running within each other and not connected to the substrate (3). in that the spirals (50, 60) in each case exhibit at least one moving mass (51, 61) on the outermost turn, which masses are arranged in a star shape about the anchor point (55),, in that the moving masses (51, 61) exhibit finger structures (511, 611) on one side or two sides, in that arranged in a star shape about the anchor point (55),, between the moving masses (51, 61), there are fixed electrodes (71), which are connected to the substrate (3) via the insulating layer (5), which exhibit finger structures (711) on one side or two sides, and in that the finger structures (511t 611) of the moving masses (51, 61) and the finger structures (711) of the fixed electrodes (71) interlock with each other.
14 Sensor according to Claim 13, characterised in that piezoresistors are arranged on the spirals (50, 60).
15. Any of the acceleration sensors substantially as herein described with reference to the accompanying drawings.

Published 1992 at Ile Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmif;clinfach. Cross Keys. Newport, NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cra Kent.