Abstract:
An electroacoustic transducer including a first electrode formed on a substrate capable of transmitting ultrasounds, a membrane formed above the first electrode and separated therefrom by a cavity, a second electrode formed on the membrane, a first insulating layer on the second electrode, and a third electrode formed on the first insulating layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 10/58294, filed on Oct. 12, 2010, entitled “ACOUSTIC GALVANIC ISOLATION DEVICE,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to electroacoustic transducers and to acoustic galvanic isolation devices. 
         [0004]    2. Discussion of the Related Art 
         [0005]    An electroacoustic transducer converts an A.C. electric signal into acoustic waves (transmission) or conversely (reception). By associating electroacoustic transducers respectively operating in transmit and in receive mode on either side of a substrate, a galvanic isolation can be achieved. 
         [0006]      FIG. 1  illustrates a device of galvanic isolation by acoustic connection described in unpublished French patent application 09/58864 filed on Dec. 11, 2009. 
         [0007]    This device comprises a substrate  10  made of any material capable of transmitting acoustic waves, for example, a glass or silicon wafer. As an example, on a first surface of a silicon substrate  10 , a first layer  12 A of a heavily-insulating material, for example, thermal silicon oxide or another thermal oxide, extends. On the first layer of insulating material  12 A is formed of a first array  14 A of ultrasonic transducers connected in parallel. These transducers comprise a conductive layer  16 A formed on insulating layer  12 A and forming a first electrode common to all transducers. Conductive layer  16 A may be made of heavily-doped polysilicon or of a metal. 
         [0008]    Above conductive layer  16 A is formed a layer  17 A of a dielectric material, for example, silicon nitride. Membranes  18 A are defined in layer  17 A above cavities  20 A. 
         [0009]    On membranes  18 A and opposite to cavities  20 A are formed second electrodes  22 A. These electrodes may be made of aluminum. 
         [0010]    One or several contacts  24 A are formed on first electrode  16 A. Electrodes  22 A are connected to a node  26 A. 
         [0011]    Symmetrically, the second surface of substrate  10  comprises elements  12 B,  14 B,  16 B,  17 B,  18 B,  20 B,  22 B,  24 B and  26 B homologous to elements  12 A,  14 A,  16 A,  17 A,  18 A,  20 A,  22 A,  24 A and  26 A. 
         [0012]    The device of  FIG. 1  operates as follows. An A.C. input signal which is desired to be transmitted from the electroacoustic transducers of array  14 A to the electroacoustic transducers of array  14 B and a bias voltage are applied between contact  24 A and  26 A of first array  14 A of transducers. The A.C. input voltage causes the oscillation of the different membranes  18 A of the transducers of first array  14 A. 
         [0013]    Substrate  10  propagates the ultrasound acoustic waves created by the oscillation of membranes  18 A towards second array  14 B. Advantageously, in the case of a silicon substrate  10 , the path of the acoustic waves is highly directional, which enables the waves to be well received at the level of second array  14 B. 
         [0014]    The acoustic waves transmitted by substrate  10  reach the second array of ultrasonic transducers  14 B, which causes the vibration of membranes  18 B. A D.C. voltage generator is placed between contact terminals  24 B and  26 B of the second array. The vibration of membranes  18 B then causes a variation of the voltage across an electric output circuit connected to contact  26 B. 
         [0015]    Such an isolation may, for example, be used to isolate a control circuit referenced to ground from the control terminals of a power circuit referenced to a higher voltage, for example, the mains. 
       SUMMARY OF THE INVENTION 
       [0016]    An embodiment provides an electroacoustic transducer comprising a first electrode formed on a substrate capable of transmitting ultrasounds, a membrane formed above the first electrode and separated therefrom by a cavity, a second electrode formed on the membrane, a first insulating layer on the second electrode, and a third electrode formed on the first insulating layer. 
         [0017]    Another embodiment provides a galvanic isolation acoustic device. 
         [0018]    Thus, an embodiment provides an electroacoustic transducer comprising a first electrode formed on a substrate capable of transmitting ultrasounds, a membrane formed above the first electrode and separated therefrom by a cavity, a second electrode formed on the membrane, a first insulating layer on the second electrode, and a third electrode formed on the first insulating layer. 
         [0019]    According to an embodiment, the membrane and the insulating layer are made of silicon nitride. 
         [0020]    According to an embodiment, the transducer comprises contacts capable of connecting a D.C. voltage generator between the first and second electrodes and an A.C. source between the first and third electrodes. 
         [0021]    According to an embodiment, the substrate is a silicon wafer having a second insulating layer extending over at least one of its surfaces. 
         [0022]    According to an embodiment, the substrate is a glass plate. 
         [0023]    An embodiment provides an acoustic galvanic isolation device comprising transducers such as hereabove, respectively formed on the first and second surfaces of a substrate. 
         [0024]    According to an embodiment, the second and third electrodes are connected according to a matrix structure. 
         [0025]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1 , previously described, shown a device of galvanic isolation by acoustic connection; 
           [0027]      FIG. 2  shows a device of galvanic isolation by acoustic connection according to an embodiment connected to a power switch; and 
           [0028]      FIG. 3  is a top view showing a matrix implementation of a transducer array. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    As usual in the representation of integrated circuits, the various drawings are not to scale. 
         [0030]      FIG. 2  shows a device of galvanic isolation by acoustic connection using electroacoustic transducers integrating a capacitor. 
         [0031]    Like the device of  FIG. 1 , this device comprises, symmetrically, first and second arrays  14 A and  14 B of ultrasonic transducers, in parallel, formed on either side of a substrate  10 . 
         [0032]    Instead of comprising a single electrode  22  on each membrane  18 , each of the transducers of  FIG. 2  comprises a pair of electrodes  23  and  28  separated by an insulating layer  30 . Thus, a D.C. bias voltage can be applied between electrode  16  and one of electrodes  23  and  28 , the high-frequency signal being applied or used between electrode  16  and the other one of electrodes  23  and  28 . 
         [0033]    Electrodes  23  and  28  thus form the two electrodes of capacitors. As previously, all electrodes  23 A, as well as all electrodes  28 A,  23 B and  28 B, are interconnected. The capacitors enable to decouple the biasing portion from the high-frequency portion and their advantage will especially appear from the following description of an application of the galvanic isolation device described herein. 
         [0034]    In the example shown in  FIG. 2 , on the transmit side, a generator  34 A applies a D.C. bias voltage between electrodes  16 A and  23 A. A microcontroller  38  controls a high-frequency source  40 , which applies high-frequency voltage bursts between electrodes  16 A and  28 A of array  14 A. On the receive side, a generator  34 B biases electrodes  23 B with respect to electrode  16 B, and a high-frequency signal resulting from the vibration of membrane  18 B is received on output contact  26 B connected to electrodes  28 B. The high-frequency signal is applied to a gate terminal  48  of a triac or any other power switch  46 . The triac belongs to a power circuit comprising a load  44  connected to an A.C. power source  42 , for example, the mains. Thus, the conduction of the triac is determined by the high-frequency signal controlled by microcontroller  38  and this microcontroller is isolated from the triac by a galvanic isolation acoustic device. 
         [0035]    Gate  48  of the triac or other power switch  46  is triggered by the high-frequency signal and must not be connected to a D.C. voltage. The problem of the risk for the D.C. biasing of the transducer membranes to be found on gate  48  of the triac or other power switch however remains. The capacitors described herein have the function of avoiding for D.C. voltage  34 B to be present on gate  48  and to affect the operation of power switch  46 . 
         [0036]    As described previously, capacitor structures between electrodes  23 A and  28 A are also provided on the transmit side. Such capacitors have the additional advantage of decoupling D.C. voltage source  34 A from high-frequency generator  40 . They are essentially provided to ensure the symmetry of the device, which provides the advantage of simplifying its manufacturing process and its use. 
         [0037]    The total capacitance of the parallel capacitors depends on the insulator thickness, on the surface area of the capacitors, and also on the number of capacitors. As an example, for a capacitor having an insulator  30  made of silicon nitride with a dielectric constant equal to 7.5, a 100-nm thickness, and a 400-μm 2  surface area, the capacitance of the integrated capacitor is 0.26 pF. For an acoustic galvanic isolation device having a surface area of 7×7 mm 2  on which are formed 35,546 transducers, each being provided with a capacitor having a 0.26-pF capacitance, the total capacitance of the parallel capacitors is 10 nF. Such a value enables to smoothly transmit the high-frequency signal (greater than 1 MHz) generated by membranes  18 B of the transducers of array  14 B. 
         [0038]      FIG. 3  is a top view showing a matrix implementation of a transducer array such as described previously. For simplification, an array of 4×4 transducers only has been shown, but this array will in practice have a much larger dimension, as indicated hereabove. Above electrode  16  are arranged electrodes  23  connected to one another along columns by conductive tracks  43 , all the columns of conductive tracks  43  ending at a common contact  32 . Similarly, above electrodes  23 , all the electrodes  28  of a same line are interconnected by conductive tracks  48  to a common contact  26 . Although this is not visible in the drawing, it should be clear that first electrodes  23  are arranged above membranes  18  and that an insulating layer  30  is interposed between electrodes  23  and  28 . This matrix arrangement enables to decrease stray capacitances. 
         [0039]    Further, although  FIGS. 2 and 3  have shown each electrode  28  as having a surface area smaller than that of the corresponding electrode  23 , it should be noted that this has been essentially done to ease the representation and that the two electrodes may have same surface areas. 
         [0040]    The thicknesses of the metal electrodes will be taken into account in the general thickness of membranes  18  and of insulating layers  30  to optimize the introduction, respectively the reception, of acoustic waves at the high-frequency signal oscillation frequency of source  40  towards, respectively from, the substrate. As an example, the thickness of the silicon nitride membrane ranges between 250 and 400 nm, the thickness of insulating layer  30  being selected by those skilled in the art, for example, between 80 and 250 nm. 
         [0041]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the dimensions and shapes of the transducers, the dimensions of the galvanic isolation device, the thickness of the electrodes, the acoustic frequency, the frequency of the A.C. source and the D.C. bias voltage will be selected by those skilled in the art according to the desired performance. 
         [0042]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.