Abstract:
A structure for protecting an integrated circuit against electrostatic discharges, including a device for removing overvoltages between first and second power supply rails; and a protection cell connected to a pad of the circuit including a diode having an electrode, connected to a region of a first conductivity type, connected to the second power supply rail and having an electrode, connected to a region of a second conductivity type, connected to the pad and, in parallel with the diode, a thyristor having an electrode, connected to a region of the first conductivity type, connected to the pad and having a gate, connected to a region of the second conductivity type, connected to the first rail, the first and second conductivity types being such that, in normal operation, when the circuit is powered, the diode is non-conductive.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 09/55757, filed on Aug. 24, 2009, entitled “STRUCTURE FOR PROTECTING AN INTEGRATED CIRCUIT AGAINST ELECTROSTATIC DISCHARGES” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to structures for protecting integrated circuits against electrostatic discharges. 
     2. Discussion of the Related Art 
       FIG. 1  is a simplified top view of an integrated circuit chip. The integrated circuit comprises a central portion  1  connected to an assembly of metal pads  3  arranged at the chip periphery and intended to provide connections to the outside. Central portion  1  comprises all the components enabling the integrated circuit to perform desired functions. Some of pads  3  are intended to receive positive (V DD ) and negative (V SS ) supply voltages. Positive and negative supply rails  5  and  7  are generally provided all around the circuit. The other pads  3  are especially intended for receiving and/or for providing input-output signals. The entire circuit is covered with an insulating layer which only leaves access to terminals connected to pads  3 , and may be placed in a package comprising lugs connected to pads  3  or balls connected to these pads. 
     Such a circuit generally receives and/or provides signals of low voltage level (for example, from 0.6 to 3 V) and low current intensity (for example, from 1 μA to 10 mA), and is likely to be damaged in the occurrence of overvoltages or overcurrents between terminals of the package. Overvoltages may occur during the manufacturing or assembly phase, before the circuit is assembled in a device (for example, on a printed circuit board), in the occurrence of electrostatic discharges linked to the manipulation of the circuits by tools or by hand. Such overvoltages may reach several thousands of volts and destroy elements of the circuit. 
     It is thus provided to associate, with each pad  3 , a protection structure which generally takes up a ring  9  arranged between pads  3  and central portion  1  of the chip. The protection structure should be capable of rapidly draining off large currents, which may appear when an electrostatic discharge occurs between two pads or two terminals of the package, and in case of an overvoltage on a terminal of a device connected to a circuit. 
       FIG. 2  shows the electric diagram of an example of a protection structure  10 , associated with an input-output pad  3  of an integrated circuit. A block  11  connected to pad  3  and to positive and negative power supply rails  5  and  7  symbolizes circuit elements protected by structure  10  against possible electrostatic discharges. 
     A diode  12  is forward-connected between pad  3  and positive supply rail  5 . A diode  13  is reverse-connected between pad  3  and negative supply rail  7 . A MOS transistor  15 , used as a switch, is connected between rails  5  and  7 . An overvoltage detection circuit  17 , connected in parallel on MOS transistor  15 , provides this transistor with a trigger signal. Overvoltage detection circuit  17  may, for example, be an edge detector comprising a resistor in series with a capacitor, the connection node between the resistor and the capacitor switching state in the occurrence of an abrupt overvoltage. MOS transistor  15  especially comprises a parasitic diode  16  forward-connected between rail  7  and rail  5 . 
     The operation of protection structure  10  in case of an overvoltage occurring on an input-output pad (now simply called “pad”) or on a pad connected to a power supply rail (now simply called “rail”) will be disclosed hereafter. 
     In normal operation, when the chip is powered, that is, when rail  5  is positive with respect to rail  7  and the input-output pads are at an intermediary level, diodes  12  and  13  are both reverse-biased and conduct no current. Further, detection circuit  17  turns off MOS transistor  15 . 
     In case of a positive overvoltage between positive and negative supply rails  5  and  7 , circuit  17  turns on transistor  15 , which removes the overvoltage. 
     In case of a negative overvoltage between rails  5  and  7 , parasitic diode  16  of transistor  15  turns on and the overvoltage is removed. 
     In case of a positive overvoltage between a pad  3  and positive supply rail  5 , diode  12  turns on and the overvoltage is removed. 
     In case of a negative overvoltage between a pad  3  and rail  5 , circuit  17  turns on transistor  15 , and the overvoltage is removed through transistor  15  and diode  13 . 
     In case of a positive overvoltage between a pad  3  and negative supply rail  7 , diode  12  turns on and the positive overvoltage is transferred onto rail  5 , which corresponds to the above-discussed case of a positive overvoltage between rails  5  and  7 . 
     In case of a negative overvoltage between a pad  3  and negative supply rail  7 , diode  13  turns on and the overvoltage is removed. 
     In case of an overvoltage between two pads  3 , diode  12  associated with most positive pads turn on, and the overvoltage is transferred to positive supply rail  5 . This corresponds to the above-discussed case of a negative overvoltage between a pad  3  (the most negative pad) and rail  5 . 
     A disadvantage of such a protection structure lies in the fact that diodes  12  and  13  have significant stray capacitances. In normal operation, the characteristics of the input/output signals of the circuit are degraded by such stray capacitances. 
     Further, to be able to drain off the currents induced by electrostatic discharges, diodes  12  and  13  should have a significant surface area (typically, a junction perimeter of 200 μm per diode). As a result, ring  9  ( FIG. 1 ) takes up a significant silicon surface area, to the detriment of central portion  1  of the chip. 
     Further, diodes  12  and  13  are separate components, which makes the manufacturing of ring  9  more complex. The separate components should further be insulated from one another, which increases the total silicon surface area of an integrated circuit. 
     SUMMARY OF THE INVENTION 
     Thus, an object of an embodiment of the present invention is to provide a structure of protection against electrostatic discharges which overcomes all or part of the disadvantages of conventional protection structures. 
     An object of an embodiment of the present invention is to provide a structure of protection against electrostatic discharges of decreased stray capacitance. 
     An object of an embodiment of the present invention is to provide such a structure which takes up a small silicon surface area. 
     An object of an embodiment of the present invention is to provide such a structure which is easy to implement. 
     Thus, an embodiment of the present invention provides a structure for protecting an integrated circuit against electrostatic discharges, comprising a device for removing overvoltages between first and second power supply rails; and a protection cell connected to a pad of the circuit comprising a diode having an electrode, connected to a region of a first conductivity type, connected to the second power supply rail and having an electrode, connected to a region of a second conductivity type, connected to the pad and, in parallel with the diode, a thyristor having an electrode, connected to a region of the first conductivity type, connected to the pad and having a gate, connected to a region of the second conductivity type, connected to the first rail, the first and second conductivity types being such that, in normal operation, when the circuit is powered, the diode is non-conductive. 
     According to an embodiment of the present invention, the protection cell comprises first to fifth regions of alternated conductivity types having, in top view, the shape of concentric rings of increasing respective diameters, wherein the first to third regions are formed in a central well of the first conductivity type; the first region is of the second conductivity type; the fourth and fifth regions are formed in a peripheral well of the second conductivity type; and the first and fourth regions are connected to said pad, the second and third regions are connected to the second power supply rail, and the fifth region is connected to the first power supply rail. 
     According to an embodiment of the present invention, the first and second conductivity types respectively correspond to a P-type doping and to an N-type doping and, in normal operation, the first power supply rail is more positive than the second power supply rail. 
     According to an embodiment of the present invention, the first and second conductivity types respectively correspond to an N-type doping and to a P-type doping and, in normal operation, the second power supply rail is more positive than the first power supply rail. 
     According to an embodiment of the present invention, said regions are insulated from one another by silicon oxide regions. 
     According to an embodiment of the present invention, the device for removing overvoltages between first and second power supply rails comprises a MOS transistor having first and second conduction terminals respectively connected to the first and second power supply rails and, between the power supply rails, an overvoltage detector having its output controlling the MOS transistor. 
     According to an embodiment of the present invention, the overvoltage detector comprises a resistor in series with a capacitor, the connection node between the resistor and the capacitor being connected to the gate of the MOS transistor. 
     According to an embodiment of the present invention, said rings have square or rectangular contours. 
     According to an embodiment of the present invention, the first region has the shape of a filled ring. 
     According to an embodiment of the present invention, the first region has the shape of a ring with a hollowed central portion. 
     The foregoing 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 
         FIG. 1 , previously described, is a simplified top view of an integrated circuit chip; 
         FIG. 2 , previously described, shows the electric diagram of an example of a structure of protection against overvoltages associated with a pad of an integrated circuit; 
         FIG. 3  shows the electric diagram of an embodiment of a structure of protection against overvoltages associated with an integrated circuit pad; 
         FIG. 4A  is a cross-section view schematically showing an embodiment of a portion of the protection structure described in relation with  FIG. 3 ; 
         FIG. 4B  is a top view of  FIG. 4A ; 
         FIG. 5  is a cross-section view in the same plane as  FIG. 4A , showing an alternative embodiment of a portion of the structure described in relation with  FIG. 3 ; 
         FIG. 6  shows the electric diagram of an alternative embodiment of the structure of  FIG. 3 ; and 
         FIG. 7  is a cross-section view in the same plane as  FIG. 4A  showing an embodiment of a portion of the structure described in relation with  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       FIG. 3  shows the electric diagram of an example of a structure of protection against overvoltages  21 , associated with an input/output pad  3  of an integrated circuit. A block  11  connected to pad  3  and to positive and negative power supply rails  5  and  7 , symbolizes circuit elements protected by structure  21  against possible electrostatic discharges. 
     Like protection structure  10  described in relation with  FIG. 2 , structure  21  comprises a MOS transistor  15 , used as a switch, connected between rails  5  and  7 . An overvoltage detection circuit  17 , connected in parallel with MOS transistor  15 , provides a trigger signal to this transistor. MOS transistor  15  especially comprises a parasitic diode  16  forward-connected between rail  7  and rail  5 . 
     A diode  23  is reverse-connected between pad  3  and negative rail  7 . A thyristor  25  is forward-connected between pad  3  and rail  7 . The anode gate of thyristor  25  is connected to positive rail  5 . 
     In normal operation, that is, when the chip is powered, the signals on pad  3  and rails  5  and  7  are such that diode  23  conducts no current and detection circuit  17  turns off MOS transistor  15 . The anode gate of thyristor  25  is more positive than its anode and this thyristor thus remains off. 
     In case of a positive overvoltage between rails  5  and  7 , circuit  17  turns on transistor  15 , which removes the overvoltage. 
     In case of a negative overvoltage between rails  5  and  7 , parasitic diode  16  of transistor  15  turns on and the overvoltage is removed. 
     In case of a positive overvoltage between a pad  3  and rail  5 , a current flows between pad  3  and rail  5 , through the anode and the anode gate of thyristors  25 . This current turns on thyristor  25  and the overvoltage is removed through thyristor  25  and diode  16 . 
     In case of a negative overvoltage between a pad  3  and rail  5 , circuit  17  turns on transistor  15  and the overvoltage is removed through transistor  15  and diode  23 . 
     In case of a positive overvoltage between a pad  3  and rail  7 , the anode of thyristor  25  is positive with respect to its anode gate. Part of the overvoltage is thus transferred to rail  5  and circuit  17  turns on MOS transistor  15 . There thus is a conduction path between pad  3  and rail  7 , through the anode and the anode gate of thyristor  25  and through MOS transistor  15 . This current turns on thyristor  25 , which then removes the overvoltage. 
     In case of a negative overvoltage between a pad  3  and rail  7 , diode  23  becomes conductive and removes the overvoltage. 
     In order to explain the removal of an overvoltage between two input/output pads, the pad receiving the highest voltage is called the positive pad and the pad receiving the lowest voltage is called the negative pad. In case of an overvoltage between pads, a current flows between the anode and the anode gate of thyristor  25  associated with the positive pad. Part of the overvoltage is thus transferred onto rail  5 , and circuit  17  turns on MOS transistor  15 . There thus is a conduction path between the positive pad and the negative pad, through the anode and the anode gate of thyristor  25  associated with the positive pad, through MOS transistor  15 , and through diode  23  associated with the negative pad. This current turns on thyristor  25 . The overvoltage is then removed through thyristor  25  associated with the positive pad and through diode  23  associated with the negative pad. 
     Protection structure  21  thus enables to remove all types of overvoltages likely to occur between pads and/or rails of the circuit following an electrostatic discharge. 
     An advantage of structure  21  lies in the fact that one of the two diodes of conventional protection structures is replaced with a thyristor. Now, for an equal current drain-off capacity, a thyristor exhibits a stray capacitance at least twice smaller than that of a diode. Protection structure  21  thus has a decreased stray capacitance with respect to the structure with two diodes of  FIG. 2 . 
     Further, for an equal current drain-off capacity, a thyristor will have a smaller surface area than a diode due to its smaller on-state voltage drop. 
     To further decrease the surface area of the protection structure, a specific integration mode of thyristor  25  and of diode  23  of a protection structure is provided. 
       FIG. 4A  is a cross-section view schematically showing an embodiment of a protection cell  31  comprising thyristors  25  and diode  23  of protection structure  21  described in relation with  FIG. 3 . 
       FIG. 4B  is a top view of  FIG. 4A . 
     As an example, the context of CMOS technology is considered, which especially enables forming N-channel MOS transistors in P-type doped wells, and P-channel MOS transistors in N-type doped wells. The N and P wells are generally formed in an N-type layer  35 , resting on a P-type doped substrate  33 . As an example, the doping level of substrate  33  is on the order of from 10 14  to 10 15  atoms/cm 3 , the doping level of layer  35  is on the order of 10 18  atoms/cm 3 , and the doping level of the N and P wells is on the order of from 10 16  to 10 17  atoms/cm 3 . 
     A P-type doped central well  37  is formed in the upper portion of layer  35 . Well  37  is surrounded with a peripheral N-type doped well  39 , which extends from the periphery of the central well. In top view, well  39  has the shape of a ring with an inner contour in contact with the outer contour of central well  37 . 
     In the upper portion of wells  37  and  39 , five heavily-doped regions  41   a  to  41   e  of alternated conductivity types having the shape of concentric rings of increasing respective diameters in top view, that is, in a plane parallel to the main surfaces of the substrate. In the shown example, the rings have square contours, and the ring  41   a  of smaller diameter is a filled ring. 
     Rings  41   a  to  41   c  are formed in central well  37 , and the ring of smaller diameter  41   a  is of the conductivity type opposite to that of well  37 , that is, of type N in this example. Rings  41   d  and  41   e  are formed in peripheral well  39 . 
     As an example, the doping level of N-type regions  41   a ,  41   c , and  41   e  is on the order of from 10 19  to 10 21  atoms/cm 3 , and the doping level of P-type regions  41   b  and  41   d  is on the order of from 10 18  to 10 20  atoms/cm 3 , which corresponds, in CMOS technology, to the doping levels of the source and drain regions of MOS transistors. 
     In this example, regions  41   a  to  41   e  are insulated from one another by grooves  43  filled with silicon oxide (STI). 
     Region  41   e  is connected to positive power supply rail  5 . Regions  41   d  and  41   a  are connected to an input/output pad  3  of the circuit. Regions  41   c  and  41   b  are connected to negative power supply rail  7 . As an example, the above-mentioned connections comprise metallizations, shown in  FIG. 4A  by hatched areas, forming ohmic contacts with heavily-doped regions  41   a  to  41   e.    
     As schematically illustrated in  FIG. 4A , protection cell  31  forms, between pad  3  and power supply rails  5  and  7 , a diode  23  and a thyristor  25 , connected as described in relation with  FIG. 3 . 
     Thyristor  25 , corresponding to PNPN regions  41   d - 39 - 37 - 41   c , can be found between pad  3  and rail  7 . The anode gate of this thyristor, corresponding to region  41   e , is connected to rail  5 . Diode  23 , corresponding to N + PP +  regions  41   b - 37 - 41   a , can be found between pad  3  and rail  7 . 
     According to an advantage of the above-described embodiment, diode  23  and thyristor  25  are integrated in a single protection cell  31 . Thus, the silicon surface area useful to remove overvoltages is optimized with respect to structures comprising two separate diodes ( FIG. 2 ) to be insulated from each other. As an example, for an equivalent overvoltage removal ability, cell  31  takes up a surface area which is approximately six times smaller than that of the two diodes of a conventional structure. 
     According to another advantage of such an integration with concentric rings, the surface of the contact metallizations between diode  23  and thyristor  25  is small with respect to a protection structure comprising only discrete components. This limits the parasitic capacitance due to the protection. In particular, in the present cell, a single common contact metallization is provided for the anode of diode  23  and the cathode of thyristor  25 . For example, in the known implementations, due to the parasitic capacitance associated with the protection structure, the frequency of the useful signal emitted or received on the pads of the protected circuit cannot be over 10 GHz. The tests made by the inventor have shown that the disclosed protection structure can be associated with circuits having pads emitting and/or receiving signals at frequencies close to 20 GHz. 
       FIG. 5  is a cross-section view in the same plane as  FIG. 4A  showing an alternative embodiment of a protection cell  51  comprising thyristor  25  and diode  23  of structure  21  described in relation with  FIG. 3 . 
     Protection cell  51  is identical to protection cell  31  described in relation with  FIGS. 4A and 4B , but for the fact that heavily-doped region  41   a , of smaller diameter, has in top view the shape of a ring with a hollowed central portion, and not of a filled ring, which decreases the stray capacitance of diode  23 . 
     As an example, for an equivalent overvoltage removal ability, cell  51  has a stray capacitance which is approximately one half than that of the two diodes of  FIG. 2 . 
       FIG. 6  shows the electric diagram of an alternative embodiment  61  of protection structure  21  described in relation with  FIG. 3 . Structure  61  comprises the same components as structure  21  with an inverted biasing. In structure  61 , diode  23  is forward-connected between pad  3  and positive rail  5 . Further, thyristor  25  is forward-connected between positive rail  5  and pad  3 , and the cathode gate (and not the anode gate) of thyristor  25  is connected to negative rail  7 . 
     Like protections structure  21 , structure  61  enables removing all types of overvoltages likely to occur between pads and/or rails of the circuit after an electrostatic discharge. 
       FIG. 7  is a cross-section view in the same plane as  FIG. 4A , schematically showing an embodiment of a protection cell  71  comprising thyristor  25  and diode  23  of protection structure  61  described in relation with  FIG. 6 . 
     Protection cell  71  is identical to protection cell  31  described in relation with  FIGS. 4A and 4B , but for the fact that the conductivity types of regions  41   a  to  41   e  and of wells  37  and  39  have been inverted. Further, the connections to positive and negative rails  5  and  7  have been permuted. 
     According to an advantage of embodiments of the present invention, the protection cells provided hereabove may be formed according to conventional manufacturing methods, for example, in the context of a CMOS technology, and require no additional step with respect to these methods. 
     Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, positive and negative voltages have been mentioned in the above description. It should of course be understood that “positive” designates values greater than the values designated as “negative”, and conversely. Often, the negative potential will be the ground. 
     Further, embodiments of protection cells  31 ,  51 , and  71  comprising heavily-doped regions having, in top view, the shape of concentric rings with square contours, have been described in relation with  FIGS. 4A ,  4 B,  5 , and  7 . The present invention is not limited to this specific shape. Any other closed contour shape may be used. 
     Moreover, in the embodiments described hereabove, the concentric ring regions are insulated from one another by silicon oxide. Some of the shown insulating layers are optional. The insulation provided between rings  41   b  and  41   c  may for example be omitted ( FIGS. 4A ,  4 B,  5 , and  7 ). Other insulating modes may also be provided. 
     Further, values of the doping levels of the different semiconductor regions forming the protection cells have been provided. These values are provided as an example only and are not limiting. They have been given in the context of a specific CMOS technology and will be easily adapted by those skilled in the art to other technological processes. 
     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.