DE10350536B3 - Method for reducing effect of substrate potential on output signal of micromechanical sensor e.g. capacitive acceleration sensor, using application of opposite voltages to capacitor outer electrodes during compensation clock - Google Patents

Abstract

The method has a positive voltage (U Q4)) and a negative voltage (-U Q4) applied to respective outer electrodes of a micromechanical sensor with 2 capacitors, during the measuring clock (Q4) of a clock sequence (Q1-Q4) having at least one compensation clock (Q2), during which the positive voltage and the negative voltage are applied to the opposite outer electrodes.

Description

The The invention relates to methods for reducing the influence of Substrate potential on the output signal of a micromechanical Sensor, in particular a capacitive acceleration sensor, according to the preambles of the independent claims.

One Micromechanical sensor element is based on a mechanical deformation or deflection of a part of its structure, which in an electric Signal is transferred. For example, in a capacitive acceleration sensor a movable structure as a capacitor electrode of a capacitance whose change a measure of the acceleration is.

From the DE 100 49 462 A1 there is known a method and a device for a micromechanical component for detecting dynamic variables such as acceleration, wherein a first and a second capacitor electrode fixedly mounted over a substrate and a third capacitor electrode arranged therebetween and provided in a spring-deflectable manner are provided. Due to this arrangement, the middle, deflectable electrode forms together with the two fixed outer electrodes each have a first and a second partial capacitance. The sensor signal is detected according to a differential capacitive measuring principle. In this case, an external force causes a relative change in position of the movable capacitor electrode, whereby one partial capacity increases while the other decreases. For the formation of the sensor output signal, the difference of the partial capacitances is determined.

typically, become the electrical potentials required to detect the differential capacitance clocked applied to the electrodes. Be in a first beat all electrodes are applied with the same potential, and in a second clock, the actual measuring cycle, the potential of an outer electrode enlarged and the potential of the other outer electrode reduced. The caused thereby at the movable center electrode charge change is used as a measure of the capacity difference evaluated.

The However, the output signal can be corrupted by a change of substrate potential: Spontaneous, uncontrollable charge changes lead on the substrate to a relative shift of the electric potential of Substrate surface with respect to Potentials of over electrodes attached to the substrate. This then works together with the different potentials on the electrodes during the Measuring clock on an average of an electrostatic force the movable electrode. As a result, the electrode is deflected and finally thereby changing the output signal.

Furthermore It is known from the prior art, by varying the the substrate applied potential an electrical zero balance perform.

Next will be in DE 197 50 350 C1 a micromechanical acceleration sensor is described in which also a movable capacitor electrode between a first and a second fixed electrode is arranged and, as already explained above, by a deflection of the movable electrode, a dynamic variable as the acceleration is determined by the differential capacitive measuring principle. The movable electrode is realized by a finger structure on a movable seismic test mass. From the test mass protrude further finger structures, which are respectively associated with opposite finger plates fixed electrodes. The movable finger structures together form a movable comb structure. Since the fixed finger plates respectively belonging to the first and second fixed electrodes are electrically connected to each other, they together form a first and a second fixed comb structure. The comb structures are arranged so that their electrodes at least partially engage and a deflection of the movable electrodes is perpendicular to the side surfaces of the finger plates of the fixed electrodes. In this document, suitable measuring methods that allow a trouble-free determination of the differential capacity, not treated.

Advantages of invention

The two methods of the invention for Reduction of the influence of the substrate potential on the output signal of a micromechanical sensor, in particular a capacitive acceleration sensor, with the characterizing features of the independent claims in a simple way, it largely depends on the substrate potential independent To achieve sensor output signal. Hard to control charge distributions in the substrate thus have no effect on the sensor output signal. In particular, drifting phenomena resulting from uncontrollable changes the amount of electrical charge time, significantly reduced. Advantageous developments the inventive method are in the subclaims specified.

drawing

embodiments The inventions will become apparent from the drawings and the description below explained in more detail. It demonstrate:

1 a cross section of an acceleration sensor in a schematic representation,

2 a timing diagram for detecting the position of the movable sensor structure with the compensation clock Q2 according to the invention,

3 a timing scheme in the test mode with the inventive compensation clock Q7 and

4 a plan view of an acceleration sensor with electrodes in comb-shape.

description the embodiments

In 1 is a cross-section of an acceleration sensor produced by the technology of surface micromechanics shown schematically.

As known in the art, are sub two over a substrate fixed external electrodes S1, S2 and a movable center electrode positioned between them B arranged side by side. Due to this arrangement forms the Center electrode B together with the outer electrodes S1, S2 respectively a first partial capacity C1 or a second partial capacity C2. As can be seen from the direction of the arrow, the deflection takes place and thus the detection direction D of the movable center electrode B perpendicular to the side surfaces the outer electrodes S1, S2. In a deflection of the center electrode B takes the one partial capacity C1 or C2 to, while the other C2 or C1 from. At a diffenzkapazitiven measuring principle is the difference of the two partial capacitances C1, C2 determined and used for signal formation. To form the differential capacitance the electrodes are electrically connected to a differential capacitance detection device (not shown) connected.

In order to be able to read out the partial capacitances C1, C2, clocked potentials U _S1 , U _S2 , U _{B are} applied to the corresponding electrodes S1, S2, B. At the substrate Sub, the potential U _{Sub is} present.

Problematic for a reliable Sensor signal is the occurrence of uncontrollable fluctuations electric charge amounts on the electrodes S1, S2, B, in particular on the substrate Sub, which is a change in the relative position the electrical potential of the substrate surface based on the position of caused electrical potential of the other electrodes. are in addition to the electrodes S1, S2, B applied different potentials, such as for example during a measuring cycle, then acts an electrostatic force in the detection direction D, whereby the movable center electrode B deflected and thus the output signal is corrupted becomes.

The inventive method is using a in the 2 illustrated clock schemes Q1, Q2, Q3, Q4 explained.

The time profile of the voltages U _S1 , U _S2 , U _B applied to the electrodes S1, S2, _B can be seen from the timing diagram Q1, Q2, Q3, Q4. The dashed lines represent a reference potential. In order to determine the position of the movable center electrode B, a voltage U _Q4 and -U _{Q4 is applied} to the outer electrode S1 or S2 abruptly in a measuring clock Q4, ie the potential of the first outer electrode S1 increases, that of the second outer electrode S2 decreases. According to the invention, the timing diagram Q1, Q2, Q3, Q4 has a compensation clock Q2, a negative voltage -U being applied to the first outer electrode S1, S2 and a positive voltage U to the second outer electrode S2, S1. As a result, a change in the substrate potential during the additional clock Q2 will cause a horizontal force change on the center electrode B, which is opposite to the change in force during the measuring clock Q4. On average, a reduction of the influence of the substrate potential on the horizontal electrostatic force is achieved.

The introduction A compensation clock can be used in an existing clock scheme without an increase the number of bars of the clock scheme done. If necessary, but can the introduction a compensation clock due to an additional clock extension the Increase number of cycles.

The method according to the invention is particularly advantageous when the potentials U _S1 , U _S2 on the two external electrodes S1, S2 are respectively inverted relative to the measuring clock Q4 during the compensation cycle Q2, ie are of opposite magnitude. The horizontal force change due to a change in the substrate potential during the additional clock Q2 is then approximately identical in magnitude to a change in force during the measuring cycle Q4.

Further advantages arise when the potential U _B on the movable center electrode B during the compensation cycle Q2 is the same opposite to the measuring cycle Q4. Thus, the amount of electrical voltage between the fixed outer electrodes S1, S2 and the be movable center electrode B in the measuring clock Q4 and the compensation clock Q2 equal.

Due to the introduction of the compensation clock Q2, the clocks Q1, Q3, in which the potentials U _S1 , U _S2 , U _B are the same on all electrodes S1, S2, B, are no longer absolutely necessary for detecting the position of the movable center electrode B. However, such clocks Q1, Q3 can still be introduced into a clocking scheme as needed and their number can be arbitrarily increased.

In a particular embodiment it is provided, the compensation clock Q2 as an additional Measuring rate as in Q4 or vice versa. For example allows redundant the position of the movable center electrode B via a across from to evaluate the first measuring clock Q4 inverted signal. During the Clock Q4 will be a certain voltage swing at the moving center electrode B determined as a measurement signal during the Clock Q2 is an amount approximately identical Voltage swing of opposite signs determined. By subtraction both voltage strokes, the amount approximately identical, however, are associated with opposite signs, in addition to the primary Effect of oppression Charge-related drifts also systematic errors or asymmetries the evaluation circuit is eliminated and the primary signal approximately doubled in terms of amount. For this embodiment it is only necessary, the voltage strokes during the clocks Q2 and Q4 respectively store separately, for example, in two separate and respectively Q2 and Q4 allocated capacities, and subsequently to supply a difference in a differential amplifier or instrumentation amplifier.

A second method according to the invention, whose timing scheme Q1 to Q7 exemplifies in 3 allows to reduce the influence of the substrate potential on the output signal of a micromechanical sensor during a test mode. In addition to the in 2 already described clocks Q1 to Q4, the clock scheme is extended by the clocks Q6 and Q7. In the test mode, the clock scheme is designed such that the movable center electrode B is deflected from the rest position due to a horizontal electrostatic force. If the deflection is then determined for this test measurement, the mobility of the deflectable structure can be assessed.

The test mode is realized in that during a test clock Q6, a test voltage U _{T is} applied to one of the two outer electrodes S1, S2, whereby the electrical potential of the respective electrode changes with respect to the electrical potential of the other two electrodes and thus an electrostatic force on the movable center electrode B is applied.

As while of the measuring clock Q4, the potentials are also at the test clock Q6 different from the electrodes S1, S2, i. also in the test mode becomes a uncontrolled change the substrate potential to a change lead the electrostatic force.

According to the invention, it is proposed that the timing diagram Q1 to Q7 have a corresponding compensation clock pulse Q7, wherein the test voltage UT applied to the electrode S1 during the compensation clock Q7 with the negative test voltage -U _{T is} applied. This causes a change in the substrate potential during the compensation clock Q7 a horizontal force change, which is opposite to the force change during the test clock Q6. Accordingly, the introduction of the compensation clock Q7 in the timing scheme Q1 to Q7 causes on average over time, a reduction in the horizontal force change with a changed substrate potential.

Also in the test mode, additional clocks Q5, in which the potentials U _S1 , U _S2 , U _B on all electrodes S1, S2, B are equal, can be introduced.

4 FIG. 12 shows a top view of an exemplary acceleration sensor in which the electrodes S1, S2, B as individual finger electrodes are part of a respective comb structure. The movable center electrode B is mounted together with other center electrodes on a seismic mass M and forms with them the movable comb structure BKS, which is connected via mechanical spring elements and an insulating layer to the substrate Sub. All center electrodes are electrically contacted with each other. The movable comb structure bKS can be deflected in the detection direction D.

The center electrodes B are each surrounded by two fixed outer electrodes S1, S2. All outer electrodes S1, which belong to the first capacitor C1, are electrically connected to one another and form a fixed comb structure KS1, the same applies to the outer electrodes S2 belonging to the second capacitance C2. From the 4 shows that the electrodes of the two fixed comb structures KS1, KS2 and the electrodes of the movable comb structure bKS at least partially mesh and are arranged such that a deflection of the electrodes of the movable comb structure bKS is perpendicular to the side surfaces of the electrodes of the solid comb structures KS1, KS2 ,

Claims (9)

Method for reducing the influence of the substrate potential on the output signal of a micromechanical sensor, in particular an acceleration sensor, with a first capacitor (C1) and a second capacitor (C2) arranged above a substrate (Sub), wherein the two capacitors (C1, C2) have a common, movably mounted center electrode (B), wherein for detecting a dynamic quantity such as an acceleration due to a force on the center electrode (B) clocked by a differential capacitive measuring principle clocked potentials on electrodes (S1, S2, B) of the capacitances (C1, C2 ) are applied in such a way that in a clock scheme (Q1 to Q4) having at least one measuring clock (Q4) during the measuring clock (Q4) a positive voltage (U) is applied to the first outer electrode (S1, S2) and a negative voltage (-U) to the second outer electrode (S2, S1) is acted upon, characterized in that the clock scheme (Q1 to Q4) at least one Compensation clock (Q2), wherein during the at least one compensation clock (Q2) a negative voltage (-U) to the first outer electrode (S1, S2) and a positive voltage (U) to the second outer electrode (S2, S1) is applied. Method according to claim 1, characterized in that that the potentials on the two outer electrodes (S1, S2) during the at least one compensation clock (Q2) with respect to the measuring clock (Q4) reversed are. Method according to claim 1 or 2, characterized that the potential on the movable center electrode (B) during the at least one compensation cycle (Q2) opposite to the measuring cycle (Q4) is the same size. Method according to one of claims 1 to 3, characterized that the potentials during the at least one measurement (Q4) and compensation clock (Q2) respectively stored separately and then a difference be supplied in a differential or instrumentation amplifier to the at least a compensation clock (Q2) as an additional measuring cycle or vice versa consulted. Method for reducing the influence of the substrate potential on the output signal of a micromechanical sensor, in particular an acceleration sensor, with a first capacitor (C1) and a second capacitor (C2) arranged above a substrate (Sub), wherein the two capacitors (C1, C2) have a common, movably mounted center electrode (B), and wherein in a at least one test clock (Q6) having clocking scheme (Q1 to Q7) during the test clock (Q6) for deflecting the movable center electrode (B) has an outer electrode (S1) with respect to the other electrodes (S2, B) is subjected to a test voltage (U _T ), characterized in that the clock scheme (Q1 to Q7) at least one compensation clock (Q7), wherein during the at least one compensation clock (Q7) with the test voltage (U _T ) applied to the electrode (S1) with the negative test voltage (-U _T ) is applied. Method according to one of claims 1 to 5, characterized that extra Clocks (Q1, Q3, Q5) in which the potentials on all electrodes (S1, S2, B) are the same size, in the respective clock scheme (Q1 to Q4, Q1 to Q7) are introduced. Method according to one of claims 1 to 6, characterized that the movable center electrode (B) together with others with the center electrode (B) electrically connected finger electrodes attached to a seismic mass (M) and a movable Comb structure (UCS) forms. Method according to claim 7, characterized in that in that the outer electrode belonging to the first or second capacitance (C1, C2) (S1, S2) electrically with further to another first and second, respectively capacity (C1, C2) external electrodes (S1, S2) and together each have a fixed comb structure (KS1, KS2) next to the movable comb structure (BKS) forms. Method according to claim 8, characterized in that the electrodes (S1, S2) of the two fixed comb structures (KS1, KS2) and the electrodes (B) of the movable comb structure (BKS) at least partially interlock and are arranged such that a deflection the electrodes (B) of the movable comb structure (BKS) perpendicular to the side surfaces the electrodes (S1, S2) of the solid comb structures (KS1, KS2) takes place.