Abstract:
The forming of an integrated circuit including at least one element of electronic protection of the circuit formed of at least one switch for short-circuiting supply conductors arranged in a rail, the switch being integrated in the rail, under said conductors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the protection of an integrated circuit against electrostatic discharges and other overvoltages. The present invention more specifically relates to the integration of the circuits necessary to such protection. 
     2. Discussion of the Related Art 
     Circuits of protection against electrostatic discharges (ESD) aim at protecting an electronic circuit against electrostatic discharges that come from its terminals. 
     FIG. 1 schematically illustrates a conventional example of an electronic protection circuit  1 . The function of this circuit especially is to short-circuit supply lines  2 ,  3  of the integrated circuit when an overvoltage due to an electrostatic discharge applied on a pad  4  of the circuit occurs. This pad is intended to connect the core of the integrated circuit (not shown) to the outside. Protection circuit  1  includes two diodes D 1 , D 2  in series connecting supply lines  2  and  3 . Midpoint  5  of this series connection is electrically connected to pad  4 . The biasing of diodes D 1  and D 2  is such that, in normal operation, they are off. Accordingly, in the example of FIG. 1, line  2  forms the most positive supply line while line  3  forms the most negative supply line (generally, the ground). Circuit  1  also includes a MOS transistor MOSSWI interconnecting supply lines  2  and  3  and having its gate connected to at least one ESD protection control circuit  6 . 
     Disturbances of electrostatic type likely to be received by pads  4  may be formed of positive or negative charges. When such charges appear, one of diodes D 1  or D 2  becomes conductive. Transistor MOSSWI must then be turned on to short-circuit supply lines  2  and  3  and thus conduct away the excess charges. By passing the charges through transistor MOSSWI, it is avoided for them to damage the core. To ensure that the electric path formed by diode D 1  or D 2  is less resistive than the passing through the circuit core, a resistor  7  is provided between each pad  4  and the integrated circuit core. 
     Circuit  6  (CT) thus sets a time constant (τ) of triggering of transistor MOSSWI upon occurrence of an electrostatic discharge, that is, of a current flowing through one of diodes D 1  or D 2 . Circuit  6  is generally formed of a resistive and capacitive circuit (RC cell). 
     Transistors MOSSWI for short-circuiting the supply conductors may also be used as a protection against possible overvoltages with respect to the maximum value admissible by the integrated circuit technology. In this case, an additional control circuit  8  (OVT) has the function of detecting possible overvoltages (for example, with respect to a predetermined voltage threshold Vref) and of turning on the transistor MOSSWI to which the circuit is associated. The integrated circuit is thus not only protected against electrostatic discharges but also against any overvoltage, whatever its origin. 
     Reference will be made hereafter to electronic protection circuits to encompass the protection against electrostatic discharges (ESD) and, unless otherwise mentioned, the protection against other overvoltages. 
     Among the features to be respected by an electronic protection circuit  1 , one should note: 
     the fact that transistor MOSSWI must exhibit the smallest possible on-state resistance (RdsON) to properly fill its protection function and avoid for the overvoltage to propagate to the rest of the circuit. As a result, transistor MOSSWI (generally, an N-channel MOS transistor) has a significant size to be able to rapidly conduct the excess charges. 
     Control circuit  6  intended to detect the overvoltages must generally have a low time constant (most often smaller than 200 ns). 
     The foregoing results in that it is generally desired to place the transistor MOSSWI of a protection circuit as close as possible to the pad or to pads  4  that it is supposed to protect. This, to minimize the resistance brought by the actual supply lines and by the sections present between the pads and the transistor terminals. 
     Further, the control circuit(s) themselves must conventionally be placed as close as possible to transistor MOSSWI, here again, to minimize the resistance due to the conductive path between these circuits and the gate of transistor MOSSWI generally forming the capacitive element of control circuit  6 . 
     FIG. 2 illustrates, in a very simplified partial top view, a conventional example of implementation of an integrated circuit  10  having pads  4  associated with ESD protection circuits. In the example of FIG. 2, three pads  4  (PAD) and two ESD protection circuits  1  have been illustrated. For the proximity reasons discussed hereabove, protection circuits  1  are generally placed in what is called a crown of the integrated circuit. This crown surrounds the circuit core integrating the different functions linked to the specific application of the integrated circuit. 
     The crown of circuit  10  generally includes what is called a supply rail  11  (BUS) which includes at least two conductors  12 ,  13  conveying respectively more positive and more negative supply voltages VP and VN of the integrated circuit. If necessary, supply rail  11  may include other conductors, for example, if the integrated circuit has a positive supply voltage, a ground and a negative supply voltage. 
     The supply bus may be only partial at the periphery or be arranged differently in the integrated circuit (for example, at the center). The notion of core encompasses, whatever its position, the integrated elements performing the different functions linked to the application specific to the integrated circuit and supplied by a bus having any form. 
     Each transistor MOSSWI of a circuit  1  is connected, respectively by a conductor  14 ,  15 , to conductor  12 ,  13  of the supply bus. Similarly, each pad  4  is connected to conductors  12  and  13  via diodes (D 1 , D 2 , FIG.  1 ), not shown in FIG.  2 . 
     From the point of view of the connection pads, two large integrated circuit families can generally be distinguished. 
     A first family includes circuits generally designated as “pad limited” circuits. It gathers the integrated circuits in which the high number of connection pads to the outside from the integrated circuit core conditions the size of the actual integrated circuit due to the perimeter required to align all pads. 
     A second circuit family includes circuits generally designated as “core limited” circuits, the size of which is limited by the core surface area and not by the perimeter required to align the pads. 
     In the circuits of the first family, all the room necessary to form, in sufficient numbers, MOSSWI transistors and their control circuits to correctly protect the integrated circuit against electrostatic discharges is available in the integrated circuit core. 
     However, in circuits having their size already determined by the integrated circuit core, it is prejudicial to the general circuit bulk to have to increase this size of the core to form the MOSSWI transistors of the ESD protection circuits. Typically, the protection circuits may amount to up to 10% of the chip core size, which adversely affects the generally desired miniaturization. 
     SUMMARY OF THE INVENTION 
     The present invention aims at providing an integrated circuit with an electronic protection element of reduced bulk. 
     The present invention more specifically aims at reducing the general bulk of an integrated circuit of core limited type equipped with electronic protection elements. 
     The present invention also aims at providing a solution which does not adversely affect the quality of the ESD protection or which even improves it. 
     The present invention also aims at providing a solution which enables protecting the integrated circuit against any type of overvoltage. 
     The present invention further aims at providing a solution which is compatible with current integrated circuit manufacturing techniques. 
     To achieve these and other objects, the present invention provides an integrated circuit including at least one element of electronic protection of said circuit formed of at least one switch for short-circuiting supply conductors arranged in a rail, said switch being integrated in said rail, under said conductors. 
     According to an embodiment of the present invention, several switches are controlled by at least one first common circuit. 
     According to an embodiment of the present invention, said first circuit detects the occurrence of electrostatic charges. 
     According to an embodiment of the present invention, the circuit further includes at least one second control circuit detecting the occurrence of overvoltages between said supply conductors. 
     According to an embodiment of the present invention, the control circuit(s) of the short-circuit switches are located in a core of the integrated circuit. 
     According to an embodiment of the present invention, the switches are formed of MOS transistors having their respective drains and sources connected to one of two supply conductors of the integrated circuit. 
     According to an embodiment of the present invention, an additional conductor for controlling the gates of the MOS transistors is provided in the supply rail. 
     The present invention also provides a method for integrating switches for short-circuiting supply conductors arranged in a rail, comprising forming the switches in the form of MOS transistors under the rail supply conductors. 
     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, shows the electric diagram of a circuit of protection against electrostatic discharges and other overvoltages; 
     FIG. 2, previously described, illustrates an example of a conventional arrangement of circuits of protection against overvoltages in an integrated circuit; 
     FIG. 3 shows, in a very simplified top view, an arrangement mode of electronic protection circuits according to the present invention; 
     FIG. 4 is a partial perspective cross-section view illustrating a first embodiment of MOSSWI transistors associated with supply rails of an integrated circuit of the present invention; and 
     FIG. 5 illustrates in a very simplified to view a second embodiment of MOSSWI transistors associated with supply rails of an integrated circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Same elements have been designated with same references in the different drawings. For clarity, only those elements useful to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the application function of the integrated circuit has not been detailed. The invention applies whatever the application of the integrated circuit. 
     A feature of the present invention is to take advantage of the silicon surface present under the supply rail of an integrated circuit to integrate therein the MOSSWI transistors of its elements of electronic protection (against electrostatic discharges and/or overvoltages). 
     To save room, it could have been devised to integrate under the supply rail core elements specific to the integrated circuit application. However, such a solution would have appeared to be unsatisfactory since the connection of the core circuits of the application would have required metallization levels to form the interconnections. Now, in the supply rail, all metallizations levels are most often used to form the actual supply conductors ( 12 ,  13 , FIG. 2) to increase their thickness to reduce the width of the supply rail and thus the circuit bulk. 
     The present invention takes advantage of the fact that the forming of the switches (MOSSWI) of the electronic protection elements (especially ESD) and their connection only require the positive and negative supply signals and a control signal. Now, the supply conductors are already present in the rail, and thus above the silicon surface in which the present invention provides integrating the MOSSWI transistors. 
     FIG. 3 illustrates, in a very simplified top view to be compared to that of FIG. 2, an embodiment of an integrated circuit  20  according to the present invention. 
     It includes pads  4  of connection of the core of integrated circuit  20  to the outside, as well as a supply rail  11  (in this example, peripheral) including a conductor  12  conveying a more positive supply voltage VP and a conductor  13  conveying a relatively more negative supply voltage VN (generally, the ground). 
     According to the present invention, the MOSSWI transistors of the electronic protection circuits (ESD and/or overvoltage) are arranged in supply rail  11 , under conductors  12  and  13 . The connections of the MOSSWI transistors to conductors  12  and  13  have been symbolized in FIG. 3 by connections  21  and  22  which are to be compared to connections  14  and  15  of FIG.  2 . The control of the MOSSWI transistors of the circuit of the present invention is provided by a conductor  23  conveying control signal CTRL provided by a control circuit  6  (CT) formed, as previously, of a delay circuit of RC cell type. Circuit  6  is conventionally connected to the supply conductor, although this has not been shown either in FIG. 2 or in FIG.  3 . 
     A feature of the present invention is that a single ESD protection control circuit  6  is enough for the entire integrated circuit. Although these control circuits are not the elements taking up most space in conventional solutions, this further helps reducing the circuit bulk. The use of several control circuits could however be maintained, for example, if the control signal access resistance needs being reduced. All control circuits are then connected in parallel to control conductor  23 . 
     Similarly, a single control circuit  8  of protection against overvoltages is enough for the entire integrated circuit. 
     The price to pay for the implementation of the present invention is to add a conductor in the rail to convey control signal CTRL. However, this conductor does not need to have a large cross-section. Accordingly, the space that it takes up is small. Further, since all metallizations levels are available in the rail, it is not disturbing to assign a small section of one of the metallization levels to form this control signal. The two signals conventionally present in the rail are those which, for the implementation of the present invention, require a significant cross-section (the supply conductors). 
     FIG. 4 illustrates in a cross-section perspective view the forming of MOSSWI transistors according to a first embodiment of the present invention. As illustrated in this drawing, the transistors are formed perpendicularly to conductors  12  and  13  of the supply rail. This eases the connections of the drain and source (not detailed) of each transistor by contacts, and possibly vias, respectively  25  and  26  from conductors  12  and  13 . Thus, in the embodiment of FIG. 4, a great number of MOS transistors of small dimension are distributed under supply rail  11 . Their small size is compensated for by a considerable increase in their number and their connection in parallel, so that the desired transistor surface area to have a small series resistance is obtained. 
     An additional advantage of the present invention which here appears is that each pad  4  of the integrated circuit necessarily has a set of very close transistors. 
     In the example of FIG. 4, the connection of gate G of the transistors is performed by means of conductor  23  conveying signal CTRL laterally arranged with respect to conductors  12  and  13 , that is, at the internal or external periphery of the supply rail, conversely to what is illustrated in FIG.  3 . The connection of conductor  23  to gate G is performed by one or several contacts, and possibly vias  27 . In FIG. 4, only thick oxide regions  29  have been symbolized on either side of gate G. The transistors may be implemented in silicon in accordance with conventional techniques. 
     FIG. 5 shows in a very simplified partial top view a second embodiment of the present invention. This embodiment illustrates the case where control conductor  23  of the MOSSWI transistors is at the center of supply rail  11 . In the example of FIG. 5, only rail  11  has been shown and it has, for example, a structure such that positive supply conductor  12  is surrounded with two ground conductors  13 . 
     As in the embodiment of FIG. 4, gates G of the MOSSWI transistors are perpendicular to the conductors of supply rail  11 . In the example of FIG. 5, several vias  27  for connecting gates G to conductor  23  have been shown, while a single via  25 , respectively  26 , for connecting the drain or the source of each transistor to the supply conductors has been shown. It should however be noted that the number of vias is to be adapted case by case according to the currents and to the application. In the example of FIG. 5, another series of vias  28  which are intended to bias the integrated circuit substrate to ground have been illustrated. 
     An advantage of the present invention is that the surface area taken up by the electronic protection elements (especially ESD) with respect to the core surface area intended for the application is much smaller. 
     Another advantage of the present invention is that the distribution of the MOSSWI transistors of the protection elements is more homogeneous than in a conventional circuit where, to minimize the surface area loss, reducing the number of protection elements may be a temptation. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the practical implementation of the electronic protection elements (ESD or overvoltage) and more specifically the MOSSWI transistor is within the abilities of those skilled in the art according to the application and based on the functional indications given hereabove. Account will especially be taken, for the sizing of control circuit  6 , of the fact that the gate capacitance of the MOSSWI transistors of the present invention is, as compared to the transistor surface area, greater than for conventional MOSSWI transistors. Further, although the forming of diodes D 1 , D 2  of the protection circuits has not been illustrated in detail in the drawings, said diodes are, conventionally, placed outside of the rail, as close as possible to pads  4 . 
     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.