WO2004107570A1

WO2004107570A1 - Method for polarising an electromechanical microcircuit at a constant electrical charge value

Info

Publication number: WO2004107570A1
Application number: PCT/FR2004/001158
Authority: WO
Inventors: Andreas Kaiser; Dominique Collard
Original assignee: Centre National De La Recherche Scientifique (C.N.R.S); Galayko, Dimitri
Priority date: 2003-05-26
Filing date: 2004-05-12
Publication date: 2004-12-09
Also published as: FR2855681A1

Abstract

The invention relates to a method and a circuit for polarising an electromechanical microcircuit, such as an electrostatic transducer, at a constant electrical charge. An essentially constant polarisation voltage is applied (A) to a non-operating electromechanical microcircuit for a certain duration (Ti), in order to charge the residual electrical capacity of the microcircuit at a constant electrical charge value (qi), i.e. the initialisation charge, and the microcircuit is disconnected (B) from the polarisation voltage in order to maintain the initialisation charge at the constant value (qi), independently of the variation of the value of the residual electrical capacity during the operation of the microcircuit. The invention can be applied to microcircuits such as electrostatic transducers and adjustable pass-band filters.

Description

Method of polarizing an electromechanical micro-circuit at a constant electric charge value. The invention relates to a method of biasing an electromechanical micro-circuit at a constant electrical charge value and to an electromechanical microcircuit comprising a bias circuit with corresponding constant electrical charge. At present, the problem of the polarization of electro-mechanical micro-circuits, such as electrostatic transducers, arises in the context of research on electromechanical filter architectures, using micromechanical resonators with electrostatic actuation.

As basic elements for the design of filters, such resonators can be assimilated to conventional quartz resonators.

Resonators of the two aforementioned types use a mechanical resonant element and electromechanical transducers with electrostatic or piezoelectric actuation to couple the mechanical resonant element with the electrical signals. Many filter architectures are known based on this type, as shown in Figure 1a) and 1b) according to two examples.

However, unlike quartz resonators, electrostatic actuated resonators require continuous polarization for linear operation. Thus, the bias voltage and the signal are superimposed on each of the electrostatic transducers.

To isolate the signals from the sources of polarization, one uses an inductance or a resistance of high value, presenting a significant impedance at the operating frequencies, as represented in figure 1c, the insulation of the continuous source of polarization being necessary on the nodes signal intermediates N.

However, neither high value resistances, greater than 10 MΩ, nor high value inductors can be integrated on a substrate. Therefore, it is necessary to use components external to the latter, which has two major drawbacks:

- need for a large number of external connections;

- Introduction to the signal node, by the latter, of a large parasitic capacity, limiting the degree of freedom for the optimization of the filtering circuits. Furthermore, the electrostatic actuation being based on an exchange of loads, high parasitic capacities then reduce the efficiency of the latter, the same charge variation, on a larger capacity, resulting in a lower voltage.

An example of an architecture which is particularly vulnerable with respect to this problem represented in FIG. 1d relates to a filter with coupled resonators. In the above architecture, three elementary resonators Res. 1, Res. 2 and Res. 3 are coupled by their electrostatic transducers. The common nodes, N and M, are polarized by means of high value inductances, so as to isolate the above-mentioned nodes from the reference voltage at the operating frequencies.

Consequently, the quantity of electric charge on nodes M and N is constant during the vibration of the resonators. The bias voltages E ₀₁ and E ₀₂ applied to high value inductors can be zero.

The coupling mechanism can be explained from the resonators Res. 1 and Res. 2.

If the resonator Res. 1 vibrates and moves, the capacity of its transducer varies. As the charge at node N is constant, this variation of the capacity modifies consequently the distribution of the electric charge between the capacities of the two transducers. This charge variation generates a voltage variation on the node N, translated by the transducers into a force variation generated on the resonators. Thus, a displacement of one of the resonators generates a force on the other and vice versa. The resonators are therefore coupled.

It is constant that the coupling voltage generated on the node N is proportional to the ratio between the variation of capacity and the total capacity connected to this last. If, in addition, a stray capacitance is present on the node, this voltage is then weakened, the efficiency of the coupling being reduced accordingly. To compensate for this attenuation, higher bias voltages are then necessary. Due to the fact that the capacities of the transducers are very low, a few tens of femtofarads at most, external connections can create parasitic capacities greater by one or two orders of magnitude than the latter, and therefore significantly disturb the operation of the filter. The object of the present invention is to remedy the drawbacks of the methods of biasing electromechanical micro-circuits, such as electrostatic transducers, of the prior art.

In particular, the present invention relates to a method of polarizing an electromechanical micro-circuit, of the electrostatic micro-actuator type, having a substantially linear operation.

Such an operating mode is achieved by charging the transducer of such an electromechanical micro-circuit, and / or the constituent nodes of the latter, at a predetermined electric charge value, then by maintaining this electric charge at the aforementioned value. , during operation, independently of variations in the capacities of the transducers connected to the latter.

The polarization process of an electromechanical micro-circuit at a constant charge, object of the present invention, is remarkable in that it consists in applying to the electromechanical micro-circuit at rest a substantially constant polarization voltage for a determined period, to charge the residual electric capacity of this electromechanical micro-circuit to the value of constant electric charge, constituting an initialization electric charge, and to disconnect the electromechanical micro-circuit from the substantially constant bias voltage, which makes it possible to keep the initialisation electric charge at the constant electric charge value, independently of the variation of this residual electric capacity during the operation of the electromechanical micro-circuit.

The electromechanical micro-circuit, object of the invention, comprises in a common node an electrical capacity, residual formed by an intrinsic electrical capacity with respect to the reference voltage and by at least one elementary capacity of polarization of this microphone -electromechanical circuit electrically connected to this node. It is remarkable in that it further comprises a bias circuit with constant electric charge comprising at least one controlled switch connecting to this node a connection terminal of this electromechanical microcircuit, which makes it possible to apply to the common node of this microphone -electromechanical circuit at rest a substantially constant bias voltage for a determined period, to charge the residual capacity of this electromechanical micro-circuit has an electric charge value found, constituting an initialization electric charge.

The method and the electromechanical micro-circuit which are the subject of the present invention make it possible to polarize the transducers constituting the latter èans to introduce an external electrical connection and therefore to minimize the residual capacitances on the signal nodes.

Such a procedure appears particularly important for systems incorporating such electromechanical micro-circuits and for which signal processing, based on an interaction of several actuators, is carried out in the mechanical and electrical fields.

They find application in the implementation of micro-mechanical T-filters and their use in industrial fields as varied as mobile telecommunication modules, remote measurement devices in confined spaces, biomedical instruments, in the form of integrated circuits. The process and the electromechanical micro-circuit which are the subject of the present invention will be better understood on reading the description and the observation of the drawings below in which, in addition to FIGS. 1a to 1d, relating to the prior art,

- Figure 2a shows, by way of illustration, a flow diagram of the steps for implementing the method which is the subject of the present invention;

- Figure 2b shows, by way of illustration, a flowchart of a specific mode of implementation of the method object of the present invention shown in Figure 2a, capable of being implemented when the residual electrical capacity of the electromechanical micro-circuit has an intrinsic electrical capacity connected between a node of the electromechanical circuit and the reference voltage;

- Figure 3 shows, by way of illustration, a block diagram of an electromechanical micro-circuit conforming to the object of the present invention;

- Figure 4a shows, by way of illustration, a drawing of an integrated circuit mask of the switch implemented in an electromechanical micro-circuit according to the object of the present invention, as shown in Figure 3; - Figure 4b shows, by way of illustration, a block diagram of the self-polarization mechanism of a common node of an electromechanical micro-circuit object of the invention as shown in Figure 3;

FIG. 5 represents a series of diagrams of the transmission characteristics as a function of the frequency, the bias voltage values being taken as a parameter for variation of the passband of an electromechanical micro-circuit conforming to the subject of the invention, as shown in Figure 3;

- Figure 6 shows by way of illustration a T-filter filter diagram with constant charge polarization, in accordance with the object of the present invention.

The method of biasing an electromechanical micro-circuit at a constant electric charge value in accordance with the object of the present invention will now be described in conjunction with FIGS. 2a and 2b.

With reference to FIG. 2a, the method which is the subject of the invention consists in applying, in a step A, to the electromechanical micro-circuit at rest a substantially constant bias voltage at a signal node Ni for a determined duration denoted Tj for charging the residual electrical capacity of the electromechanical micro-circuit at node H, aforementioned at the value of constant electrical charge qi. This electrical charge constitutes an electrical charge for initializing the electromechanical micro-circuit.

Step A is then followed by a step B consisting in disconnecting the electromechanical micro-circuit from the substantially constant bias voltage Vj. This keeps the initialization electrical charge qi at the constant electrical charge value, independently of the variation in the value of the residual electrical capacity during the operation of the electromechanical microcircuit.

In general, with reference to FIG. 2a, it is indicated that the method which is the subject of the invention also consists in controlling the duration of the charging of the residual electrical capacity at a specific value of initial charging voltage of this capacity. during the duration Tj. This makes it possible to adjust, as a function of the initial charge voltage Vj, the attenuation / frequency transfer function, central frequency, bandwidth of the circuit of the electromechanical micro-circuit considered. It is conceivable, in particular, that from the initial voltage Vi and the duration of charge Ti, the electrical charge of initialization q * verifies the relation: Application Vj, T

In addition, as shown in FIG. 2b, for a residual electrical capacity of the electromechanical micro-circuit comprising an intrinsic electrical capacity connected between a node Nj of the electromechanical micro-circuit and the reference voltage of this electromechanical micro-circuit, the object method of the invention advantageously consists of a step C, following the disconnection of this electromechanical micro-circuit of the substantially continuous bias voltage in step B, to bring the electrical potential of the node Nj to the reference voltage to discharge completely the aforementioned intrinsic capacity and bringing the initialization electrical charge qi to a reduced initialization electrical charge value qj _{r as a} function of the aforementioned residual capacity.

In general, it is indicated that by residual electrical capacity of the electro-mechanical circuit is meant any elementary polarization capacity of the electromechanical micro-circuit electrically connected to the node as well as any intrinsic electrical capacity formed for example by an external or internal parasitic capacity connected between a node of the electromechanical circuit and the reference voltage of the aforementioned electromechanical micro-circuit.

Consequently, step C of FIG. 2b consists in bringing the voltage Vj substantially to the value 0 in order to bring the electrical initialization charge qj to a residual value denoted qι _r .

An example of implementation of an electromechanical micro-circuit in accordance with the object of the present invention will now be given in connection with FIG. 3.

Referring to Figure 3 above, we consider the electromechanical micro-circuit object of the invention comprising in a common node Ni a residual electrical capacity formed for example by an intrinsic capacity with respect to the reference voltage and by the minus an elementary polarization capacity of this electromechanical micro-circuit electrically connected to the node As shown in FIG. 3 above, the electromechanical micro-circuit further comprises a bias circuit with constant electric charge comprising at least one controlled switch, denoted I, connecting the node Nj to a connection terminal denoted E01 of the micro-circuit electromechanical considered. The common node Nj in fact comprises two transducers denoted resonator Res. 1 and resonator Res. 2, to which the bias voltages Ep1 respectively Ep2 and an input signal Vj _n are applied via a resistor Rs while the resonator 1 is charged by a load resistor R. The switch I makes it possible to apply to the common node Nj of the electromechanical microcircuit at rest a bias voltage Vj which is substantially constant during the period T determined to charge the residual capacity of the electromechanical micro-circuit at a constant electrical charge value, constituting an initialization electric charge, as mentioned in connection with the process which is the subject of the invention described in FIGS. 2a and 2b.

As shown in Figure 3, the micro-circuit object of the invention further comprises a control module Ml of the charging time of the residual electrical capacity at an initial charge voltage value of this capacity. This makes it possible to adjust, as a function of the initial charge voltage, the attenuation / frequency transfer function, central frequency, bandwidth width of the electronic circuit.

In FIG. 3, the module for controlling the charging time of the residual electrical capacity at an initial charging voltage value is denoted M1 and shown in dotted lines so as not to overload the drawing. Indeed, the aforementioned control module M1 and the switch I can be constituted by a controlled switch timed on opening, this switch can for example be a programmable switch.

Finally, the electromechanical micro-circuit which is the subject of the invention, as shown in FIG. 3, may further comprise a controlled timer switch with opening noted I ', this switch being connected between the common node Nj, and the voltage of reference, in parallel on the intrinsic electrical capacity for example. This second optional switch is also shown in dotted lines in FIG. 3. This allows, following the charge of the residual capacity at the electrical initialization charge qj to reduce the electrical initialization charge to a reduced initialization charge qι _r , this reduced initialization charge being a function of the elementary capacity of polarization and the polarization voltage applied to this elementary polarization capacity. This operating mode makes it possible to adjust, as a function of the initial charge voltage and the reduced initialization charge, by the duration T'i of closing of the switch I ', the attenuation / frequency transfer function, central frequency width bandwidth of the electromechanical circuit object of the invention considered. The electromechanical micro-circuit, object of the invention as shown in FIG. 3 implements the process of reducing the "gaps" or air gaps of the post-manufacturing transducers described in the article entitled "High frequency high-Q micro-mechanical resonators in thick epipoly technology with post-process gap adjustment "MEMS 2002, Las Vegas, January 2002 published by D. Galayko, A. Kaiser, B. Legrand, C. Combi, D. Colas, L. Buchaillot.

The electrodes of the resonator transducers are connected to a spring and are attracted to the resonators by an electro-static actuation. Thus, the distance between the electrodes and the resonators is reduced. The motor of the common electrode reduces the "gap" or air gap separating the two transducers. A more detailed description of the electrostatic actuating switch constituting the switch I, if necessary the switch I ', shown in FIG. 3 is given in connection with FIG. 4a.

The aforementioned switch advantageously comprises a motor electrode denoted EM, a spring R and safety stops denoted BS. The contact electrodes Eci and Ec ₂ are of course separated and secured to a fixed contact respectively to a movable contact secured to the spring R.

The operation of the switch I or of the switch I ′ is similar to the operation of the gap reduction or air gap motor, except that the geometry of the latter is designed so that, following the deformation of the spring R, the contact electrodes Eci and Ec2 are brought into physical electrical contact so as to ensure the connection of a node Nj either to the bias voltage E ₀ ι, or to the reference voltage in the case of the switch I '.

The polarization mechanism of the electromechanical micro-circuit as represented in FIG. 3 will now be explained in connection with FIG. 4b. In the aforementioned figures, only the capacities of the transducers denoted C ₁₀ respectively C ₂ o are shown and correspond to the capacities of the resonators Res. 1 and Res. 2 previously described in connection with FIG. 3. The parasitic capacitance on the common node is represented and designated by C _p . Once the switch I is closed, the abovementioned capacities are electrically charged, so that the voltage of the common node Nj is substantially equal to that of the source Eoι. The load on the common node Nj distributed between the three capacities is therefore substantially zero.

When the source E ₀ ι is disconnected from the common node Nj by means of the switch I, the aforementioned charge remains constant whatever the displacements of the resonators and whatever the variation in capacity of the transducers C ₁₀ and C ₂₀ - The polarization thus obtained, for the electromechanical micro-circuit considered, is designated polarization at constant charge. In the electromechanical micro-circuit as shown in FIG. 3, the parasitic capacitance Cp has proved to be much greater than the capacitances of the transducers C-10 and C ₂₀ due to the presence of a large motor spring.

In such a case, and with reference to the diagrams of FIG. 4b, the voltage on the common node Ni is essentially determined by the initial charge of the capacitor Cp and depends little, ultimately, on the bias voltages of the resonators VBI and Vβ2-

By discharging the capacitance C _p substantially substantially by means of the second switch I ′ for example, it is thus possible to ensure a substantially zero polarization of the common node Nj. The electromechanical micro-circuit thus implemented has shown stable and robust operation.

FIG. 5 presents examples of transmission characteristics obtained for an electromechanical micro-circuit conforming to the object of the invention at points a, b and c. The graphs represented in the points mentioned above in FIG. 5 are represented in transmission level in dB for the ordinate axis respectively in frequency for the abscissa axis. Frequencies are measured in MHz. On each of the diagrams represented in points a, b and c of FIG. 5, the above parameters denote:

- BW the bandwidth expressed in Hz for the transfer function;

- Vbi and Vb2 designate the bias voltages applied to each of the resonators Res. 1, respectively Res. 2;

- P denotes the pressure in Torr of the environment of the electromechanical micro-circuit considered.

By observing the diagrams represented in points a, b, c of FIG. 5, it can be seen that the bandwidth of the corresponding circuit varies according to the bias voltages.

In accordance with the polarization method which is the subject of the present invention, it is thus possible to adjust the frequency characteristics of the processing circuit obtained, and, in particular, the value of bandwidth depending on the polarization mode at constant charge implemented in accordance with process which is the subject of the present invention.

A specific architecture of an electromechanical micro-circuit in accordance with the object of the present invention specially adapted to the implementation of an electromechanical filter, in particular a T-filter, will now be given in conjunction with FIG. 6 In general, to form an electromechanical filter, the electromechanical micro-circuit object of the invention advantageously comprises a plurality of electrostatic transducers denoted Res. 1, Res. 2, Res. 3 in FIG. 6, these electrostatic transducers being connected to a common node

It further comprises at least one controlled switch, the switch

I connecting an input terminal of the electromechanical filter and the common node Ni, the terminal of the electromechanical filter being denoted E ₀ ι.

The switch I makes it possible of course to apply at rest a substantially constant bias voltage for a determined period. In the embodiment shown in FIG. 6, it is indicated that the three electrostatic transducers Res. 1, Res. 2 and Res. 3 are mechanically connected to the common node Nj to form a micromechanical T filter.

The first Res. 1 receives, on an excitation electrode, an input signal denoted Vj _n by means of a signal resistance Rs, the second transducer Res. 2 comprises, on an output electrode, a charge impedance RL intended to deliver an output signal and the third transducer Res. 3 comprises an electrode connected to the reference voltage. The free electrode of each transducer is connected to the common node Ni as shown in Figure 6. Of course, the switch I can be implemented as shown in Figure 4a.

Finally, the common node Nj can be connected to a first input terminal connected to a DC voltage source, the voltage V respectively to the reference voltage by means of a timed controlled switch denoted C. This makes it possible to adjust the electric charge and the electric voltage applied to the common node Nj by charge control respectively for discharging the residual electric capacity of the electromechanical micro-circuit in accordance with the object of the present invention.

In FIG. 6, the switch I is thus represented connected to an external switch C making it possible to apply either the direct voltage, or on the contrary the reference voltage by means of the switch I previously described in the description. This embodiment is in no way limiting, the direct integration of a switch in place of the switch I and the external switch C can, if necessary, be envisaged.

We have thus described a method of polarizing an electromechanical micro-circuit at a constant electric charge value and a corresponding electromechanical micro-circuit which is particularly efficient, insofar as, applied to resonators with electrostatic actuation, these make it possible to check the operating parameters of these via bias voltage.

Many applications can be foreseen from such a remarkable property, in particular that of the application to filters with variable bandwidth, or at the very least adjustable, by simple adjustment of the bias voltage as shown in connection with Figure 5 at points a, b and c thereof.

Claims

1. A method of polarizing an electromechanical micro-circuit at a constant electric charge value, characterized in that it consists:

to apply to said electromechanical micro-circuit at rest a substantially constant bias voltage for a determined period, to charge the residual electric capacity of said electromechanical micro-circuit to said constant electric charge value, constituting an initialization electric charge and controlling the charging time of said residual electrical capacity at a specific value of initial charging voltage of said residual electrical capacity, to adjust, as a function of said initial charging voltage, the attenuation / frequency, central frequency, width transfer function bandwidth of said electromechanical micro-circuit;

- disconnecting said electromechanical micro-circuit from said substantially constant bias voltage, which allows said initialization electrical charge to be kept at said constant electrical charge value, independently of the variation of the value of said residual electrical capacity during the operation of said electromechanical micro-circuit.

2. Method according to claim 1, characterized in that for a residual electrical capacity of said electromechanical micro-circuit comprising an intrinsic electrical capacity connected between a node of said electromechanical circuit and the reference voltage of this electromechanical micro-circuit, said residual electrical capacity comprising at least one elementary polarization capacity of said electromechanical micro-circuit electrically connected to said node, said method further consists, following the disconnection of this electromechanical micro-circuit of said substantially continuous bias voltage, in bringing the electrical potential of said node to said reference voltage for fully discharging said intrinsic capacitance and bringing said initialization electrical charge to a reduced initialization electrical charge value, a function of said at least one elementary bias capacitance and of the tensi of polarization applied to it.

3. Electromechanical micro-circuit comprising in a common node a residual electrical capacity formed by an intrinsic capacity with respect to the reference voltage and by at least one elementary capacity of polarization of said electromechanical micro-circuit electrically connected to said node, characterized in that it further comprises:

- a bias circuit with constant electric charge comprising at least one controlled switch connecting said node to a connection terminal of said electromechanical micro-circuit, which makes it possible to apply to the common node of said electromechanical micro-circuit at rest a substantially bias voltage constant for a determined period, to charge the residual capacity of said electromechanical micro-circuit to a constant electric charge value, constituting an initialization electric charge, and - means for controlling the charging time of said residual electric capacity at an initial charge voltage value of said residual electrical capacity, which makes it possible to adjust, as a function of said initial charge voltage, the attenuation / frequency transfer function, center frequency, bandwidth of said electronic circuit .

4. Electromechanical micro-circuit according to claim 3, characterized in that said means for controlling the charging time of said residual electrical capacity comprise at least one controlled switch timed on opening.

5. Electromechanical micro-circuit according to claim 3 or 4, characterized in that said bias circuit further comprises a controlled switch with time delay on opening, connected between said node and the reference voltage in parallel on said intrinsic electrical capacity, which allows, following the charge of said residual capacity to said electrical initialization charge to reduce said electrical initialization charge to a reduced initialization charge, a function of said at least one elementary polarization capacity and of the voltage of polarization applied to this elementary polarization capacity, which makes it possible to adjust, as a function of said initial charge voltage and of said reduced initialization charge, the attenuation / frequency transfer function, center frequency, bandwidth of said circuit electromechanical.

6. Electromechanical micro-circuit according to one of claims 3 to 5, characterized in that to form an electromechanical filter, it comprises a plurality of electrostatic transducers connected to a common node, and in that it comprises at least a controlled switch connecting a terminal input of said micromechanical filter and said common node and making it possible to apply at rest a substantially constant bias voltage for a determined period.

7. Electromechanical micro-circuit according to claim 6, characterized in that it comprises:

- three electrostatic transducers connected in a common node, to form a micromechanical T-filter, a first transducer receiving on an excitation electrode an input signal, a second transducer comprising on a output electrode a charge impedance intended to deliver an output signal and a third transducer comprising an electrode connected to the reference voltage, the free electrode of each transducer being connected to said common node.

8. Electromechanical micro-circuit according to claim 6 or 7, characterized in that said common node is connected to a first input terminal connected to a source of direct voltage respectively at the reference voltage via a switch timed controlled, which makes it possible to adjust the electric charge and the electric voltage applied to said common node by charge control respectively discharge of the residual electric capacity of said electromechanical micro-circuit.