FR2855681A1 - POLARIZATION METHOD OF AN ELECTROMECHANICAL MICRO-CIRCUIT AT A CONSTANT ELECTRIC CHARGE VALUE - Google Patents

Abstract

The invention relates to a method and a biasing circuit of an electromechanical microcircuit, of the electrostatic transducer type, at a constant electric charge. The electromechanical microcircuit at rest is applied (A) to a substantially constant bias voltage for a period of time. (Ti) to charge the residual electric capacity of the microcircuit to a constant electric charge value (qi), initialization charge, and the microcircuit is disconnected (B) from the bias voltage, to keep the charge d 'initialization at the constant value (qi), regardless of the variation of the value of the residual electric capacitance during the operation of the microcircuit. Application to microcircuits such as electrostatic transducers and filters with adjustable passband.

Description

The invention relates to a method of polarizing a micro-circuit

  electromechanical to a constant electric charge value and to an electromechanical microcircuit comprising a bias circuit with corresponding constant electric 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 represented in FIG. 1 a) and 1 b) 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 polarization sources, a high value inductance or resistance is used, having a high impedance at the operating frequencies, as shown in FIG. 1c, the insulation of the DC polarization source being necessary on the signal intermediate nodes N. However, neither the high value resistances, greater than 10 MD, nor the 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. In addition, the electrostatic actuation being based on an exchange of charges, high stray capacitances 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 amount of electrical charge on the nodes M and N is constant during the vibration of the resonators. The bias voltages E0o1 and E02 applied to high value inductors can be zero.

  The coupling mechanism can be explained from the resonators 15 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 25 is proportional to the ratio between the variation in capacity and the total capacity connected to the latter. If, in addition, a stray capacitance is present on the node, this voltage is then weakened, the coupling efficiency 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 subject of the present invention is a method for biasing an electromechanical micro-circuit, of the electrostatic microactuator 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 nodes constituting the latter, at a predetermined electric charge value, then by maintaining this electric charge at the value above, during operation, independently of variations in the capacities of the transducers connected to the latter.

  The method of biasing 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 bias voltage for a determined duration. , to charge the residual electrical capacity of this electromechanical microcircuit to the value of constant electrical charge, constituting an initialization electrical charge, and to disconnect the electromechanical micro-circuit from the voltage 20 of substantially constant bias, 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 a residual electrical capacity formed by an intrinsic electrical capacity with respect to the reference voltage and by at least one elementary polarization capacity of this micro -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 electromechanical micro-circuit at rest a substantially constant bias voltage for a determined period, to charge the residual capacity of this electromechanical micro-circuit to a value of observed electrical charge, constituting an initialization electrical charge.

  The electromechanical method and micro-circuit which are the subject of the present invention make it possible to polarize the transducers constituting the latter without introducing 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 method 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, - the FIG. 2a represents, by way of illustration, a flow diagram of the steps 20 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 comprises an intrinsic electrical capacitance 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; FIG. 4a represents, by way of illustration, a mask drawing of an integrated circuit of the switch implemented in an electromechanical micro-circuit in accordance with the object of the present invention, as shown in FIG. 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; - Figure 5 shows a series of diagrams of the transmission characteristics 5 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 object of l invention, as shown in Figure 3; - Figure 6 shows by way of illustration a T-filter filter with 10 polarization by constant charge, 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 Ti for charge the residual electrical capacity of the electromechanical micro-circuit at the aforementioned node Ni to the value of constant electrical charge qi. This electrical charge constitutes an electrical charge 20 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 Vi. 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 for the duration Ti. This makes it possible to adjust, as a function of the initial charge voltage Vi, 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 initialization electrical charge qi verifies the relation Application Vi, Ti qi = f (Vi, Ti) Furthermore, as well as shown in FIG. 2b, for a residual electrical capacity of the electromechanical micro-circuit comprising an intrinsic electrical capacity connected between a node Ni of the electromechanical micro-circuit and the reference voltage of this electromechanical micro-circuit, the method which is the subject of the invention consists advantageously in a step C, following the disconnection of this electromechanical micro-circuit from the substantially continuous bias voltage in step B, to bring the electrical potential of the node Ni to the reference voltage to fully discharge the aforementioned intrinsic capacity and bring the initialization electrical charge qi to a reduced initialization electrical charge value qir as a function of the capacity 15 aforementioned residual.

  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 parasitic capacity or internal 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 Vi substantially to the value 0 in order to bring the electrical charge 25 for initialization qi to a residual value denoted qir.

  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.

  With reference to FIG. 3 above, we consider the electromechanical micro-circuit 30 which is the subject of the invention comprising in a common node Ni a residual electrical capacitance formed for example by an intrinsic capacitance with respect to the reference voltage and by at least one elementary polarization capacity of this electromechanical micro-circuit electrically connected to the node Ni.

  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 1, connecting the node Ni to a connection terminal denoted E01 of the micro-circuit electromechanical 5 considered. The common node Ni in fact comprises two transducers denoted resonator Res. let resonator Res. 2, to which the bias voltages Epl respectively Ep2 are applied as well as an input signal Vin via a resistor Rs while the resonator 1 is charged by a load resistor RL.

  O10 The switch I makes it possible to apply to the common node Ni of the electromechanical microcircuit at rest a bias voltage V1 substantially constant during the duration Ti determined to charge the residual capacity of the electromechanical micro-circuit at a constant electrical charge value, constituting d an electrical initialization charge, as mentioned in connection with the method 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 switch 30 with time delay on opening denoted 1 ′, this switch being connected between the common node Ni, 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 qi, to reduce the electrical initialization charge to a reduced initialization charge qir, this reduced initialization charge being a function of the elementary polarization capacity and of the bias voltage 5 applied to this elementary bias capacitance. This operating mode makes it possible to adjust, as a function of the initial load voltage and the reduced initialization load, by the duration T'i of closing of the switch l ', 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 15 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 electrostatic 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 1, if appropriate the switch 1 ', 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 Ec1 and Ec2 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 1 'is similar to the operation of the motor reduction of "gap" or air gap, except that the geometry of the latter is designed so that, following the deformation of the spring R, the contact electrodes Ec1 and Ec2 are brought into physical electrical contact so as to ensure the connection of a node Ni either at the bias voltage E01, or at the reference voltage in the case of the switch 1 '.

  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 Cl0 respectively C20 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 Cp. 5 Once the switch I is closed, the abovementioned capacities are electrically charged, so that the voltage of the common node Ni is substantially equal to that of the source Eo1. The load on the common node Ni distributed between the three capacities is therefore substantially zero.

  When the source E01 is disconnected from the common node Ni by means of the switch 1, the aforementioned charge remains constant whatever the displacements of the resonators and whatever the variation in capacity of the transducers C10 and C20. 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 stray capacitance Cp has proved to be much greater than the capacitances of the transducers Cl0 and C20 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 VBî and VB2 .

  By discharging the capacitance Cp substantially completely by means of the second switch 1 ′ for example, it is thus possible to ensure a substantially zero polarization of the common node Ni.

  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 aforementioned parameters designate: - BW the bandwidth expressed in Hz for the transfer function; - Vbl 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 10 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 bandwidth width value as a function of the polarization mode with constant charge implemented in accordance with to the 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 microcircuit 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 denoted Ni.

  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 E0o.

  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 Ni to form a micromechanical T-filter. The first transducer Res. 1 receives, on an excitation electrode, an input signal denoted Vin by means of a signal resistor 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 5 Ni as shown in FIG. 6. Of course, the switch I can be implemented as shown in FIG. 4a.

  Finally, the common node Ni can be connected to a first input terminal connected to a DC voltage source, the voltage Vi respectively to the reference voltage by means of a timed controlled switch 10 denoted C. This allows d 'Adjust the electric charge and the electric voltage applied to the common node Ni by charge control respectively discharge of the residual electric capacity of the electromechanical microcircuit according to 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 of the external switch C can, if necessary, be envisaged.

  We have thus described a method of biasing an electromechanical micro-circuit at a constant electric charge value and a corresponding electromechanical microcircuit which is particularly efficient, in the measure o, applied to resonators with electrostatic actuation, these allow to control the operating parameters of the latter via bias voltage.

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

Claims (10)

  1. A method of polarizing an electromechanical micro-circuit at a constant electric charge value, characterized in that it consists in: applying to said electromechanical micro-circuit at rest a substantially constant polarization voltage for a determined period, for charge the residual electrical capacity of said electromechanical micro-circuit to said constant electric charge value, constituting an initialization electric charge - to disconnect said electromechanical micro-circuit from said voltage 10 of substantially constant bias, which makes it possible to conserve said initialisation electric charge at said constant electric charge value, independently of the variation of the value of said residual electric capacity during the operation of said electromechanical micro-circuit.   2. Method according to claim 1, characterized in that it further consists in controlling the charging time of said residual electrical capacity at a specific value of initial charging voltage of said residual electrical capacity, which makes it possible to adjust as a function of said initial charge voltage, the attenuation / frequency transfer function, central frequency, bandwidth of said electromechanical micro-circuit.   3. Method according to claim 1 or 2, 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 microcircuit, 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 electric potential of said node at said reference voltage for fully discharging said intrinsic capacitance 30 and bringing said initialization electrical charge to a reduced initialization electrical charge value, a function of said at least one elementary bias and the bias voltage applied thereto.   4. 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 polarization capacity of said electromechanical micro-circuit electrically connected to said node, characterized in that that it further comprises a bias circuit with constant electric charge comprising at least one controlled switch 5 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 to rest a substantially constant bias voltage 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.   5. Micro-circuit according to claim 4, characterized in that it further comprises means for controlling the charging time of said residual electrical capacity at an initial charge voltage value of said residual electrical capacity, which allows adjusting, as a function of said initial charge voltage, the attenuation / frequency transfer function, center frequency, bandwidth width of said electronic circuit.   6. Electromechanical micro-circuit according to claim 5, characterized in that said means for controlling the charging time of said residual electrical capacity comprise at least one controlled switch timed on opening.   7. Electromechanical micro-circuit according to claim 4, 5 or 6, characterized in that said bias circuit further comprises a controlled switch timed on opening, connected between said node and the reference voltage in parallel on said electrical capacity intrinsic, which makes it possible, 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 and the bias voltage applied to this elementary bias capacitance, which makes it possible to adjust, as a function of said initial charge voltage and said reduced initialization charge, the attenuation / frequency transfer function, center frequency, width of bandwidth of said electromechanical circuit.   8. Electromechanical micro-circuit according to one of claims 4 to 7, 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 an input terminal of said micromechanical filter and said common node and making it possible to apply at rest a substantially constant bias voltage for a determined duration.   9. Electromechanical micro-circuit according to claim 8, characterized in that it comprises: - three electrostatic transducers connected in a common node, to form a micromechanical T-filter, a first transducer receiving on 10 an excitation electrode an input signal, a second transducer comprising on an output electrode a load 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 knot.   10. Electromechanical micro-circuit according to claim 8 or 9, 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.