DE102017205979A1 - Micromechanical capacitive sensor - Google Patents

Abstract

Micromechanical capacitive sensor (100) comprising:
- A MEMS device (10) having a movable ground electrode (CM) and a defined number of readout electrodes (C0N, C0P), which are cooperatively formed with the movable ground electrode (CM) to electrical charge shifts on the readout electrodes (C0N, C0P) due to movement of the ground electrode (CM); and
- A drive device (20) which is designed to control the ground electrode (CM) with at least two different frequencies (f1, f2) and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process.

Description

The invention relates to a micromechanical capacitive sensor. The invention further relates to a method for producing a micromechanical capacitive sensor.

State of the art

Micromechanical capacitive sensors (accelerometers, pressure sensors, etc.) are normally operated on a single frequency during normal operation.

A micromechanical pressure sensor, in which a pressure difference is measured as a function of a deformation of a sensor membrane is, for example DE 10 2004 006 197 A1 known.

Disclosure of the invention

It is an object of the present invention to provide an improved micromechanical capacitive sensor.

• - eine MEMS-Einrichtung mit einer beweglichen Masseelektrode und einer definierten Anzahl von Ausleseelektroden, die mit der beweglichen Masseelektrode zusammenwirkbar ausgebildet sind, um elektrische Ladungsverschiebungen auf den Ausleseelektroden aufgrund einer Bewegung der Masseelektrode zu erfassen; und
• - eine Ansteuereinrichtung, die ausgebildet ist, die Masseelektrode mit wenigstens zwei unterschiedlichen Frequenzen anzusteuern und elektrische Ladungen zu erfassen; wobei die erfassten elektrischen Ladungen mit erfassten elektrischen Ladungen eines Kalibrierprozesses vergleichbar sind.

The object is achieved according to a first aspect with a micromechanical capacitive sensor, comprising:

• a MEMS device having a movable ground electrode and a defined number of sense electrodes cooperatively formed with the movable ground electrode for detecting electrical charge shifts on the sense electrodes due to movement of the ground electrode; and
• a drive device which is designed to drive the ground electrode with at least two different frequencies and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process.

• - Bereitstellen einer MEMS-Einrichtung mit einer beweglichen Masseelektrode und einer definierten Anzahl von Ausleseelektroden, die mit der beweglichen Masseelektrode zusammenwirkbar ausgebildet werden, um elektrische Ladungsverschiebungen auf den Ausleseelektroden aufgrund einer Bewegung der Masseelektrode zu erfassen; und
• - Bereitstellen einer Ansteuereinrichtung, die ausgebildet ist, die Masseelektrode mit wenigstens zwei unterschiedlichen Frequenzen anzusteuern und elektrische Ladungen zu erfassen; wobei die erfassten elektrischen Ladungen mit erfassten elektrischen Ladungen eines Kalibrierprozesses vergleichbar sind.

The object is achieved according to a second aspect with a method for producing a micromechanical capacitive sensor, comprising the steps:

• - providing a MEMS device having a movable ground electrode and a defined number of sense electrodes, which are cooperatively formed with the movable ground electrode to detect electrical charge shifts on the sense electrodes due to movement of the ground electrode; and
• - Providing a drive device which is designed to drive the ground electrode with at least two different frequencies and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process.

In this way, by operating the capacitive sensor with two different frequencies, the fact is exploited that an electric current flow due to the different frequencies of the drive signal is permanent, whereas the current flow occurs due to capacitive interaction of the electrodes once. In this way, a delta between calibration data and useful data in the field can be determined and thereby an effect of a resistive shunt can be easily compensated.

Preferred embodiments of the micromechanical capacitive sensor are the subject of dependent claims.

A preferred development of the micromechanical capacitive sensor is characterized in that a drive signal with the at least two different frequencies can be supplied to a switch for switching an electrical read-out voltage by means of the drive device.

In this way, the supply of the drive signal with the different frequencies can be realized electronically in a simple manner. The supply of the different frequencies can be realized for example by means of a physical switch. A selection of the frequency being used may be e.g. be performed by means of an electronic control machine.

A preferred further development of the micromechanical capacitive sensor is characterized in that a conductance of a molding compound is such that between connecting lines of the readout electrodes and a connecting line of the ground electrode to the drive means a defined low conductivity and / or optimal stress decoupling is realized. In this way, an influence of the molding compound on a measurement behavior of the sensor is advantageously minimized.

Another preferred embodiment of the micromechanical capacitive sensor is characterized in that the at least two frequencies are distributed in a defined manner within a defined frequency band. In this way, the principle of so-called spread spectrum clocking is realized, which supports that error in the system is limited only locally to some frequencies, whereby the system can suppress noise at the exact operating frequency.

A further preferred embodiment of the micromechanical capacitive sensor provides that an offset measured value of a measurement with the two frequencies is stored in the sensor. In this way, a stroke of a measurement acquired at two different frequencies can be stored and used in the operative normal operation of the sensor.

The invention will be described below with further features and advantages with reference to several figures in detail. Identical or functionally identical elements have the same reference numerals therein. The figures are particularly intended to illustrate the principles essential to the invention and are not necessarily to scale. For better clarity, it can be provided that not all the figures in all figures are marked.

Disclosed device features result analogously from corresponding disclosed method features and vice versa. This means, in particular, that features, technical advantages and embodiments relating to the micromechanical capacitive sensor result analogously from corresponding embodiments, features and technical advantages relating to the method for producing a micromechanical capacitive sensor and vice versa.

• 1 eine prinzipielle Darstellung eines konventionellen mikromechanischen kapazitiven Sensors;
• 2 ein zeitliches Diagramm zum Erläutern eines Auslesens eines konventionellen mikromechanischen kapazitiven Sensors;
• 3 eine prinzipielle Darstellung eines resistiven Nebenschlusses eines konventionellen mikromechanischen kapazitiven Sensors;
• 4 ein zeitliches Diagramm zum Erläutern eines Auslesens eines vorgeschlagenen mikromechanischen kapazitiven Sensors;
• 5 ein Blockschaltbild einer Ausführungsform eines mikromechanischen kapazitiven Sensors; und
• 6 einen prinzipiellen Ablauf einer Ausführungsform eines Verfahrens zum Herstellen eines vorgeschlagenen mikromechanischen kapazitiven Sensors.

In the figures shows:

• 1 a schematic representation of a conventional micromechanical capacitive sensor;
• 2 a timing diagram for explaining a readout of a conventional micromechanical capacitive sensor;
• 3 a schematic representation of a resistive shunt of a conventional micromechanical capacitive sensor;
• 4 a timing diagram for explaining a readout of a proposed micromechanical capacitive sensor;
• 5 a block diagram of an embodiment of a micromechanical capacitive sensor; and
• 6 a basic sequence of an embodiment of a method for producing a proposed micromechanical capacitive sensor.

Description of embodiments

1 shows a schematic diagram of a conventional micromechanical capacitive sensor 100 in the form of a MEMS acceleration sensor. One recognizes a MEMS device 10 with a movable ground electrode CM which is movable along an x-axis. It further recognizes readout electrodes C0N, C0P, between which a comb of the ground electrode CM is movable. It goes without saying that this is only a schematic representation and that in practice substantially more readout electrodes C0N, C0P are provided.

As a result, by moving the ground electrode CM between the sense electrodes C0N, C0P, the capacitance between the sense electrodes C0N, C0P and the ground electrode CM is affected, and electric charges can be transmitted through connection lines 12 . 13 in the form of an electrical current to an evaluation circuit 21 a control or evaluation 20 be transmitted.

Ideally, the ground electrode CM and the sense electrodes C0N, C0P form an ideal plate capacitor which, in the case of an acceleration sensor, is detuned by external acceleration influences. For an ordinary acceleration measurement, the readout electrodes C0N, C0P are at a fixed electrical potential V _BIAS , while the electrical potential on the ground electrode CM is clocked with an alternating with a frequency f1 voltage VCM, the resulting electric charge flow measured and to determine a Acceleration value is evaluated. It can be seen for this purpose within the control device 20 a drive circuit 22 with a switch 23 which is switched at the frequency f1 between GND and the voltage VREAD.

mit:

Q_ACC ....

elektrische Ladung pro Messung

ΔVCM ...

Hub der elektrischen Spannung an der Masseelektrode CM

ΔCACC ...

Verstimmung des Sensor-Kondensators

VCM ...

elektrische Spannung an der Masseelektrode CM

ΔC_ACC ...

Beschleunigungssignal in Form eines Differenzkapazitätssignals

The acceleration-dependent charge per measurement is calculated in this way according to the following relationship: $$Q_{ACC}=\Delta VCM\cdot \Delta C_{ACC}$$

With:

Q _ACC ....

electrical charge per measurement

ΔVCM ...

Stroke of the electrical voltage at the earth electrode CM

ΔCACC ...

Detune the sensor capacitor

VCM ...

electrical voltage at ground electrode CM

ΔC _ACC ...

Acceleration signal in the form of a differential capacitance signal

2 shows a time course with N read cycles for reading the conventional micromechanical capacitive sensor 100 from 1 , Shown are time courses of electrical voltages at the readout electrodes C0N, C0P and at the ground electrode CM. As a result, this results in a time course Q _{ACC of} the electrical charge, which is proportional to the electrical voltage on the readout electrodes C0N, C0P. It is indicated that during a read cycle TREAD electric charges QACC1, QACC2, etc. are transported.

ΔC/g ....

Empfindlichkeit des Sensors auf externe Beschleunigung in g-normiert, wobei gilt: ΔC/g = Δx/g* ΔC/Δx mit:

Δx/g ...

Auslenkungsempfindlichkeit

ΔC/Δx ...

Kapazitätsempfindlichkeit

wobei diese Ladungsmenge ca. 30.000 Elektronen entspricht. Aus diesem geringen Signal resultiert eine nachteilig hohe Empfindlichkeit gegenüber resistiven Nebenschlüssen zwischen den Ausleseelektroden C0N, C0P und der Masseelektrode CM.Typical MEMS sensors have sensitivities in the range of less fF / g, for example 5 fF / g. At this sensitivity, with an acceleration of 1 g and an electrical voltage VREAD of 1 V, a charge Q _{ACC is transported} according to the following equation: $$Q_{ACC}=VReAD\cdot \frac{\Delta C}{G}\cdot 1G=5\text{fC}$$

ΔC / g ....

Sensitivity of the sensor to external acceleration in g-normalized, where: ΔC / g = Δx / g * ΔC / Δx with:

Δx / g ...

deflection sensitivity

ΔC / Δx ...

capacity sensitivity

this charge amount corresponds to about 30,000 electrons. From this small signal results in a disadvantageous high sensitivity to resistive shunts between the readout electrodes C0N, C0P and the ground electrode CM.

The following 3 shows an equivalent circuit diagram of the micromechanical capacitive sensor 100 from 1 , which is supplemented by a parasitic resistive shunt GL1L, GL1R. The variable capacities are intended to indicate that the capacitance values of the sensor 100 detune at an acting acceleration.

There is a defined electrical voltage difference across the resistance or conductance GL1L, GL1R of the shunt: $$\Delta V_{GL}=VReAD-VBIAS$$

As a result of this electrical voltage difference .DELTA.V _GL flows via the conductance GL1R a parasitic electric current I _GL according to the following relationship: $$I_{GL}=\Delta V_{GL}\cdot {\mathrm{<figure-callout\; id="1"\; label="G\; L"\; filenames="DE102017205979A1\_0013.png"\; state="\{\{state\}\}">G\; </figure-callout>}}_L$$

This electrical current I _GL causes a parasitic charge flow to the readout electrodes C0N, C0P, which in a conventional micromechanical capacitive sensor 100 in its effect can not be distinguished from a common measurement signal Q _ACC .

wobei gilt: $$\Delta {\text{I}}_{\text{GL}}=\Delta {\text{V}}_{\text{GL}}\times \left({\text{G}}_{\text{L1L}}-{\text{G}}_{\text{L1R}}\right)$$

The absolutely introduced differential charge due to the introduced shunt is calculated as: $$Q_{ACC.GL}=\Delta I_{GL}\cdot \frac{T_{ReAD}}{2}$$

where: $$\Delta {\text{I}}_{\text{GL}}=\Delta {\text{V}}_{\text{GL}}\times \left({\text{G}}_{\text{L1L}}-{\text{G}}_{\text{L1R}}\right)$$

This calculation is based on the assumption that the shunt is sufficiently high-impedance, so that the readout electrodes C0N, C0P are at a substantially constant electrical potential during the measurement time.

Offset_GL ...

Offset in g (z.B. 10mg)

ΔG_L1 ...

Differenz der beiden Nebenschlussleitwerte, wobei Gleichtaktsignale per se unterdrückt werden und nur Differenzsignale bewertet werden

ΔV_GL ...

über dem Sensor anliegende elektrische Spannung

A transmission ratio between resistance and a faulty, read-out acceleration signal can be estimated as follows: $$\mathrm{<figure-callout\; id="1"\; label="\Delta \; G\; L"\; filenames="DE102017205979A1\_0013.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{G}_L{1}_{}=\frac{Offse{t}_{GL}}{1G}\cdot \frac{1}{\raisebox{1ex}{$T_{ReAD}$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\cdot \frac{VReAD\cdot \frac{\Delta C}{G}}{\Delta V_{GL}}$$

Offset _GL ...

Offset in g (eg 10mg)

ΔG _L1 ...

Difference of the two shunt conductance, wherein common mode signals are suppressed per se and only difference signals are evaluated

ΔV _GL ...

electrical voltage applied across the sensor

Assuming an allowable resistance-related offset error of 10mg, it can be estimated that the electrical resistance must be> 50 GΩ, which is an extremely high value.

The following parameters are used: $$\text{TREAD = 10}\mathrm{\mu }\text{s}\text{, VREAD = 1V}\text{.}\Delta {\text{V}}_{\text{GL}}=0.5\text{V}$$

A core idea of the present invention is, in particular, a provision of an improved micromechanical capacitive sensor, in which an effect of a parasitic resistive shunt is substantially completely compensated or eliminated.

The resistive shunts described above can be suppressed by a specific micromechanical capacitive sensor. The following relationships are exploited:

The electric charge Q _ACC shown in formula (1), which serves as a useful signal, depends only on the electric read voltage V _READ . In formula (5), the parasitic signal Q _{ACC, GL shows} a direct dependence on the electrical voltages and the readout time or frequency. The signal charge is essentially invariant with the read-out frequency, whereas the parasitic signal QACC, GL is directly proportional to the read-out frequency f _READ = 1 / T _READ .

It is therefore proposed that the micromechanical capacitive sensor 100 to suppress the parasitic shunt resistance is no longer operated or read only with a single frequency f1, but alternately with at least two different frequencies f1 and f2. A change in the mean acceleration value between the two read-out frequencies f1, f2 allows a direct inference to the parasitic component of the acceleration, which is then eliminated from the measured value.

mit:

Offset_Jetzt ....

Aktueller Offset

Offset_Trimmed ...

Offset nach Abgleich

Offset Parasitär,Gemessen ...

Gemessener parasitärer Offset

The parasitic offset value can be in the electronic drive device 20 and then use it in an appropriate form according to the following relation to a correction of the offset: $$Offse{t}_{Jetzt}=Offse{t}_{Trimmed}+Offse{t}_{Parasitär.Gemessen}$$

With:

Offset _Now ....

Current offset

Offset _Trimmed ...

Offset after adjustment

Offset Parasitic, Measured ...

Measured parasitic offset

The proposed measuring method can be either continuously during normal operation, periodically at suitable time intervals, or only after a system start of the micromechanical capacitive sensor 100 be carried out once. In continuous operation of the sensor 100 it is necessary to implement a suitable algorithm to ensure that the useful signal (ie a change of a detected quantity, eg an acceleration) is not affected by the measurement and vice versa.

4 shows in a time sequence an operation of the proposed method with N read cycles.

It can be seen that during the entire N read cycles an electrical voltage VBIAS is applied to the readout electrodes C0N, C0P. It can also be seen that in measuring phases M, in which the electrical voltage VREAD is applied to the ground electrode CM, electrical charges are continuously transported in the form of an electric current. In reset phases R, in which the read electrical voltage VREAD the ground electrode CM is at GND, a reset is performed in each case.

Due to the at least two different frequencies f1, f2, the read-out cycles TR1, TR2 have different lengths, whereby different amounts of charges QACC1, QACC2 are transported in the read-out cycles. These different values can be compared with values of a calibration process of the sensor in a manufacturing process, which can be concluded that a resistive shunt. The stated values of the calibration process can be used, for example, in the micromechanical capacitive sensor 100 be deposited and used in operative normal operation to determine the resistive shunt and its elimination.

The proposed micromechanical capacitive sensor 100 offers several advantages:

Parts in the field that result in a resistive shunt, for example due to the penetration of moisture into the package, are no longer classified as faulty parts. Advantageously, a number of moisture losses can be reduced in this way.

A quality or type of dielectric, which is located between the MEMS device 10 and the drive device 20 can be replaced by an actually unsuitable, slightly resistive material, if this brings other benefits, such as in terms of cost or behavior under mechanical stress. For this purpose, a molding compound between the MEMS device 10 and the drive device 20 arranged the connecting lines 11 . 12 . 13 encloses and causes the lowest possible resistive influence and / or an optimized mechanical stress decoupling.

If the measurement is carried out continuously, then the suppression of supply voltage noise is improved as the Measurement is no longer performed on a single frequency, but on several. The underlying principle is known as so-called "spread spectrum clocking". It ensures a low emission of electromagnetic interference of the micromechanical capacitive sensor into the system, but also increases its own robustness against external interference.

The sensor is advantageously not limited to micromechanical capacitive acceleration sensors, but can also be used generally for any micromechanical capacitive sensors, e.g. for pressure sensors, humidity sensors, temperature sensors, etc.

5 shows a block diagram of a proposed micromechanical capacitive sensor 100 , It can be seen that in the control device 20 a drive device 22 is arranged, which is a switch 25 which is driven with at least two different frequencies f1, f2. To control the switch 25 is a digital control element 24 intended. The output signal of the switch 25 is used to control the switch 23 used for switching the read voltage VREAD. For example, a difference of an acceleration value at the first frequency f1 and the second from f1 different frequency f2, which is eg 5 mg, is recorded. As a result, the said difference can be compared with a calibration value and, if the measurement deviates in the field, it can be concluded that there is a resistive shunt, which is eliminated during normal operation and thus advantageously does not appear. Alternatively, to select the frequencies f1, f2 instead of the switch 25 also a suitable other selection agent can be used (not shown).

6 shows a basic sequence of a method for producing a micromechanical capacitive sensor 100 ,

In one step 200 becomes a provision of a MEMS device 10 performed with a movable ground electrode and a defined number of detection electrodes, which are formed cooperatively with the movable ground electrode to detect electrical charge shifts on the readout electrodes due to movement of the ground electrode.

In one step 210 a provision is made of a drive device which is designed to drive the ground electrode with at least two different frequencies and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process.

The order of steps 200 and 210 is advantageous interchangeable.

The person skilled in the art can also realize embodiments of the invention which are not disclosed or are only partially disclosed, without deviating from the gist of the invention.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102004006197 A1 [0003]

Claims (8)

Micromechanical capacitive sensor (100) comprising: - A MEMS device (10) having a movable ground electrode (CM) and a defined number of readout electrodes (C0N, C0P), which are cooperatively formed with the movable ground electrode (CM) to electrical charge shifts on the readout electrodes (C0N, C0P) due to movement of the ground electrode (CM); and - A drive device (20) which is designed to control the ground electrode (CM) with at least two different frequencies (f1, f2) and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process. Micromechanical capacitive sensor (100) after Claim 1 , characterized in that by means of the drive device (20) a drive signal with the at least two different frequencies (f1, f2) a switch (23) for switching an electrical read voltage (VREAD) can be fed. Micromechanical capacitive sensor (100) after Claim 1 or 2 , characterized in that a conductance of a molding compound is such that between connecting lines (12, 13) of the readout electrodes (C0N, C0P) and a connecting line (11) of the ground electrode (CM) to the drive means (20) has a defined low conductivity and / or the best possible stress decoupling is realized. Micromechanical capacitive sensor (100) according to one of Claims 1 to 3 , characterized in that the at least two frequencies (f1, f2) are distributed in a defined manner within a defined frequency band. Micromechanical capacitive sensor (100) according to one of the preceding claims, characterized in that an offset measured value of a measurement with the two frequencies (f1, f2) is deposited in the sensor (100). A method of making a capacitive micromechanical sensor (100), comprising the steps of: - Providing a MEMS device (10) with a movable ground electrode (CM) and a defined number of readout electrodes (C0N, C0P), which are cooperatively formed with the movable ground electrode (CM) to electrical charge shifts on the readout electrodes (C0N, C0P ) due to movement of the ground electrode (CM); and - Providing a drive device (20) which is designed to control the ground electrode (CM) with at least two different frequencies (f1, f2) and to detect electrical charges; wherein the detected electrical charges are comparable to detected electrical charges of a calibration process. Method according to Claim 6 , wherein an offset measured value of a measurement with the two frequencies (f1, f2) is stored in the sensor (100). Use of a micromechanical capacitive sensor (100) according to Claim 1 as an acceleration sensor or as a pressure sensor or as a temperature sensor.

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