Abstract:
The present invention relates to a structure for ground connection on a component including a vertical MOS power transistor and logic components, the substrate of a first type of conductivity of the component corresponding to the drain of the MOS transistor and the logic components being formed in at least one well of the second type of conductivity and on the upper surface side of the substrate. In the logic well, a region of the first type of conductivity is formed, on which is formed a metallization, to implement, on the one hand, an ohmic contact, and on the other hand, a rectifying contact.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to components associating in the same substrate vertical diffused-Type MOS power transistors (VDMOS) and logic circuits, and more specifically relates to the use of such components in circuits supplied by a battery, such as automobile circuits. 
     2. Discussion of the Related Art 
     FIG. 1 very schematically shows a portion of such a component. This component includes an N-type substrate generally formed of an N-type epitaxial layer 1 formed on an N +  -type substrate 2. A power transistor is formed in the right-hand portion and a logic well is formed in the left-hand portion. 
     The power transistor includes a set of identical cells connected to one another, such as cell 3. Each cell includes a P-type well 4, the central portion 5 of which is more heavily doped. An N-type ring 6 is formed in the upper portion of the well. The portion separating the external periphery of ring 6 from the external periphery of well 4 is coated with an isolated gate 8. N-type ring 6 as well as central portion 5 of the well are coated with a metallization 9. All gates 8 are connected to a gate terminal G and all metallizations 9 are connected to a source terminal S. The rear surface of the structure is coated with a drain metallization D. Thus, when a gate signal is applied, a current is likely to flow from terminal D to terminal S from N regions 1 and 2 to N regions 6, via a channel formed under the insulated gates. This structure is generally used so that the drain is biased to a positive potential with respect to the source. 
     Elements of logic circuits are formed in one or several wells 10. An elementary MOS transistor 11 having drain, source, and gate terminals g, d, and s has been shown in a well 10. This is only an example of a component that could be formed in a logic well. 
     The voltages on the several components formed in the logic well must be applied with respect to a reference. The simplest way to provide this reference, that is, to implement a ground connection, is illustrated in FIG. 1 and corresponds to the use of a well contacting area 12 formed on a region 13 of same type (P) as the well and more heavily doped. Contact 12 may be connected directly to the ground in simple embodiments. Generally, it should be understood that, for example, high voltages VDD are applied to some drains of the MOS transistors of the logic circuit which have their sources connected to contact 12 and to the ground. 
     FIG. 2 shows an example of an assembly of a component of the type shown in FIG. 1. The component is generally designated by block 20 surrounded with a frame in dotted lines. A reverse diode D1 is illustrated in parallel between the drain and the source of MOS power transistor T and corresponds to the junction between N-type substrate 1 and P-type region 5. Well 10 is represented by a block and it is assumed that it is connected to drain D of the MOS transistor via a diode D2 corresponding to the junction between substrate 1 and well 10. 
     In a very simple example of assembly, contact 12 of the well is grounded by a connection 21; and the source of the power transistor is connected to the ground via a load L, the switched supply of which is desired to be performed by the power transistor. A supply source such as a battery 23 is connected between the ground and drain terminal D of power transistor T. Thus, in normal operation, diodes D1 and D2 are reverse biased. According to its control, transistor T will be turned on or not and no current flows from the ground (contact 12) to the rear surface of the component (terminal D) due to the existence of reverse-biased diode D2. 
     Two incidents likely to occur in battery-powered circuits, and more specifically in automobile circuits, should however be considered. 
     The first incident corresponds to a biasing inversion of the battery. Diodes D1 and D2 are then forward biased. The current in diode D1 is limited by the presence of load L. Thus, the current will essentially flow through diode D2, as indicated by arrow 24. This current is likely to be destructive. 
     A second incident corresponds to a supply interruption, or battery disconnection, likely to occur when, due to vibrations or for any other reason, a lead wire of the battery breaks or operates intermittently. Then, if load L is inductive, the current will continue to flow therethrough according to the path designated by arrow 26. It should be noted that this current will necessarily exist, load L having to be considered as a current source. In the case of the simple assembly of FIG. 1, this current flow raises no specific problem. The problems result from the use of known protections against battery voltage inversions, as will be seen hereafter. 
     A first conventional solution to solve the battery inversion problem consists of inserting a diode, biased in a direction opposite to that of diode D2, in series with the well. The inserting of a diode may for example be performed in the way illustrated in FIG. 1, by adding an N +  -type region 15, grounding this region 15 by a connection 17, and suppressing the connection to ground 21. Various solutions have been provided to optimize the operation of this diode, and to have a well referenced to the ground when the circuit is in a normal operation state. Reference will especially be made to U.S. Pat. No. 5,099,302 (Antoine Pavlin) relating to an active diode which is incorporated herein by reference. There still remains the problem that, in the case of a battery disconnection, current 26 will have to cross an avalanching diode and will dissipate a high power therein, which can cause a destruction of the component, unless a diode having a large surface is provided, which unduly increases the cost of the component. 
     A second conventional solution to the battery inversion problem consists of placing in series in connection 21 a resistor, a terminal of which will form the ground connection. But a new dilemma, difficult to solve, arises. Indeed, in normal operation, the resistance must be as low as possible to limit the voltage drop thereacross caused by the consumption of the elements of the logic circuit. Conversely, to solve the problems linked to the battery inversion case, this resistance must be as high as possible to limit the current flowing through the well. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to provide a structure for ground connection of the logic well of a component integrating a power transistor and logic elements which does not adversely affect the normal state operation, which prevents the current flow in the logic circuit in case of a battery inversion, and which lets through the current resulting from a battery disconnection. 
     To achieve these and other objects, the present invention provides a structure for ground connection on a component including a vertical MOS power transistor and logic components, the substrate of a first type of conductivity of the component corresponding to the drain of the MOS transistor and the logic components being formed in at least one well of the second type of conductivity and on the upper surface side of the substrate. This structure includes, in the logic well, a region of the first type of conductivity on which is formed a metallization, to implement, on the one hand, an ohmic contact, and on the other hand, a rectifying contact. 
     According to an embodiment of the present invention, the rectifying contact corresponds to a contact with regions of the second type of conductivity. 
     According to an embodiment of the present invention, the rectifying contact corresponds to a Schottky contact. 
     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 shows a conventional structure of a component associating a vertical MOS transistor and logic circuits; 
     FIG. 2 shows an example of assembly of the component of FIG. 1 associated with a battery; 
     FIG. 3 shows a logic well ground connection structure according to the present invention; 
     FIGS. 4A and 4B show equivalent diagrams associated with the ground connection structure according to the present invention; 
     FIG. 4C shows a current-voltage characteristic; 
     FIG. 5 shows a simplified top view of an example of a ground connection structure according to the present invention; and 
     FIG. 6 shows an alternative of a ground connection structure according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the various drawings, same elements are referred to with same references. Further, in the various cross-sectional views of semiconductor components, as is usual, the various dimensions are not drawn to scale but are arbitrarily expanded to facilitate the readability of the drawings. 
     The ground connection structure according to the present invention includes an N-type region 30 formed in logic well 10. Alternate N +   and P +  -type regions, respectively designated by references 31 and 32 are formed in this N-type region, for example, in the form of a grid. Regions 31 and 32 are altogether coated with a metallization 33, itself normally connected to the circuit ground by a connection corresponding to connection 21 of FIG. 2. 
     FIGS. 4A and 4B show two forms of an equivalent diagram of the ground connection structure, seen between connection 21 and terminal D (rear surface of the component). It can be considered that this structure corresponds to a thyristor Th, the anode of which corresponds to metallization 33 and the cathode of which corresponds to rear surface metallization D. The thyristor anode will correspond to P +  -type regions 32, and a resistive region will be located between the anode and the anode gate of this transistor, this resistor being referred to by reference r be . FIG. 4B shows the same diagram, but in which the thyristor has been shown conventionally in the form of its equivalent transistors. P +  -type region 32, N well 30, and P-type well region 10 form a PNP transistor T1, the emitter of which is connected to metallization 33 and the base of which is connected via resistor rbe to metallization 33. Region 30, well 10, and substrate 1 form an NPN transistor T2, the emitter of which is connected to drain metallization D, the collector of which is connected to the base of transistor T1, and the base of which is connected to the collector of transistor T1. It should be noted that the common base region of transistor T2 and collector region of transistor T1 corresponds to well 10, that is, to reference potential VSS of the elements of the logic circuit formed in well 10. 
     Thus, during normal circuit operation, thyristor Th is reverse biased and no current can flow through the junction between substrate 1 and well 10. 
     The current-voltage characteristic of the structure appears in FIG. 4C in which a voltage V on metallization 33 is assumed to be positive with respect to terminal D which is then grounded, which occurs in case of a battery inversion. The junction between N well 30 and P well 10 is reverse biased and no current can flow as long as the applied voltage is lower than breakdown voltage V BR  of this junction. The structure must be implemented so that voltage V BR  is higher than the reverse battery voltage, to protect the logic portion of the circuit formed in well 10. 
     In case of a battery disconnection, inductive load L causes a current to flow which forces the junction between N well 30 and P well 10 to start an avalanche. When the current flowing through resistor r be  reaches a threshold value, noted I B  R on FIG. 4C, such that the value of the voltage drop across the resistor is higher than 0.6 V, transistor T1 turns on. This supplies the base of transistor T2, which also turns on. In other words, this causes the break-over of thyristor Th and the voltage drop across this thyristor becomes very low, which allows dissipation of high currents without any risk of destruction in case of a battery disconnection under an inductive load. Break-over threshold I BR  must be optimized so that it is high enough not to cause any untimely break-over of the characteristic in case of battery inversion, and low enough for the structure to be able to dissipate current I BR  under breakdown voltage V BR . 
     It should be noted, according to a first advantage of the present invention, that the triggering threshold of the protection circuit according to the present invention is easily adjustable. Indeed, considering a top view of this protection structure, such as shown in FIG. 5, P-type regions 32 can be considered as a grid separated by N-type regions 31. The surface ratio between P and N regions 32 and 31 determines the value of resistance rbe. The surface ratio between the P and N regions will for example be on the order of 15 to 1, the triggering current being lower when this ratio is higher. In an example of embodiment, each of squares 32 can have a side on the order of 8 μm and the distance between two squares can be 2 μm. 
     According to another advantage of the present invention, metallization 33 does not have a larger surface than the surface normally provided to establish a contact pad on P well 10. Thus, there is no surface increase with respect to the normal implementation of the well. It will again be stressed that the representations of the various drawings are not to scale. 
     The operating modes in battery inversion and disconnection states have been described hereabove. The logic well also has to be at the ground potential in normal operation. For this purpose, a structure such as illustrated in FIG. 3 may for example be used, taking inspiration from the structure described in above-mentioned U.S. Pat. No. 5,099,302. For example, a heavily-doped P-type region 35 connected to the ground via a MOS transistor 37 is provided. The gate of transistor 37 is connected to terminal D, whereby MOS transistor 37 is on when the battery is properly connected (normal operation) and off in all other cases (battery inversion or disconnection state). 
     FIG. 6 shows an alternative of the circuit according to the present invention in which the same elements as in FIG. 3 appear, designated by the same references. The difference between the two drains is that P +  -type regions 32 have been suppressed and the doping level of N-type region 30 is chosen to form a Schottky diode with metallization 33, which will for example be made of aluminum. The Schottky diode causes a limited injection of minority carriers, which is less efficient than P +  -N junction 32-30, but sufficient to create a bipolar effect similar to that created by transistor T1 of FIG. 4B. This structure has the advantage of eliminating one P +   masking level. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. 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.