Abstract:
A semiconductor device includes a semiconductor substrate and an internal circuitry which is formed on the semiconductor substrate and which executes a predetermined operation. The device also includes a terminal which is connected to the internal circuitry and which receives an external signal and a protection circuitry which is formed on the semiconductor substrate. The protection circuitry includes a transistor having a first region of a first conductivity type, a second region of the first conductivity type and a third region of a second conductivity type. The first region is connected to the terminal. The second region is provided at a scribe line of the semiconductor substrate. The third region is defined by a region between the first region and the second region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a semiconductor device, and more particularly, to an electrostatic discharge protection circuit that protects an internal circuitry from an electrostatic discharge damage. The present invention also relates to a method of fabricating the semiconductor device. 
     This application relies for priority on Japanese patent application, Ser. No. 228642/1999, filed Aug. 12, 1999, which is incorporated herein by reference in its entirety. 
     2. Description of the Related Art 
     An electrostatic discharge occurs in various steps such as an assembly step, a wafer processing step or a carrying step of an IC (Integrated Circuit) chip. 
     As an integration of the IC has been enhanced, the size of elements formed on the IC have been small in recent years. As a result, an amount of a withstanding voltage of the IC against the electrostatic discharge decreased. 
     In conventional IC, ESD (Electro-Static Discharge) protection circuits, which prevent an internal circuitry from breaking due to a surge voltage input from connection terminals, are provided at respective connection terminals connected to the internal circuitry. 
     However, since the conventional IC has the ESD protection circuits each of which is connected to corresponding connection terminal, a circuit area per one connection terminal increases. That is, a relatively large circuit area for the ESD protection circuit is needed. Therefore, the conventional IC is not suitable for a higher integration. 
     Consequently, there has been a need for an improved semiconductor device and method of fabricating the same. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention is to provide a semiconductor device having a smaller size. 
     It is another object of the present invention to provide a semiconductor device which includes a protection circuit having a smaller size. 
     According to an aspect of the present invention, for achieving one or more of the above objects, there is provided a semiconductor device that includes a semiconductor substrate and an internal circuitry which is formed on the semiconductor substrate and which executes a predetermined operation. The device also includes a terminal which is connected to the internal circuitry and which receives an external signal and a protection circuitry which is formed on the semiconductor substrate. The protection circuitry includes a transistor having a first region of a first conductivity type, a second region of the first conductivity type and a third region of a second conductivity type. The first region is connected to the terminal. The second region is provided at a scribe line of the semiconductor substrate. The third region is defined by a region between the first region and the second region. 
    
    
     The above and further objects and novel features of the invention will more fully appear from the following detailed description, appended claims, and accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view showing a part of the semiconductor device  1  according to the preferred embodiment of the present invention. 
     FIG. 2 is a cross sectional view showing the protection circuit  5  and the periphery of the protection circuit  5  of the semiconductor device  1 . 
     FIG. 3 is a schematic view showing a variation of the semiconductor device according to the preferred embodiment of the present invention. 
     FIG. 4 is a schematic view showing a variation of the semiconductor device according to the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A semiconductor device according to the present invention will be explained hereinafter with reference to figures. In order to simplify explanations, like elements are given like or corresponding reference numerals through this specification and figures. 
     First, a semiconductor device  1  according to the preferred embodiment is explained with reference to FIG.  1  and FIG.  2 . 
     FIG. 1 is a top plan view showing a part of the semiconductor device  1  according to the preferred embodiment of the present invention. More particularly, FIG. 1 is a top plan view showing a protection circuit  5  and the periphery of the protection circuit  5  of the semiconductor device  1 . 
     FIG. 2 is a cross sectional view showing the protection circuit  5  and the periphery of the protection circuit  5  of the semiconductor device  1 . 
     As shown in FIG. 1, an internal circuitry  7  and an aluminum pad  9  are formed on a major surface of a semiconductor substrate  3 . The internal circuitry  7  is a circuit to be protected from the electrostatic discharge. The aluminum pad  9  is connected to the internal circuitry  7 . 
     A protection circuit  6 , which protects the internal circuitry  7  from a surge voltage applied to the aluminum pad  9 , is formed on and in the semiconductor substrate  3 . 
     As shown in FIG. 2, the protection circuit  5  includes a transistor  11  which is formed in a surface region  3   b  of the major surface  3   a  and a control terminal  13  which is provided on the major surface  3   a  over the transistor  11 . 
     The transistor  11  has a first region  11   a  of an n-type active region which serves as source/drain or collector/emitter, a second region  11   b  of the n-type active region which serves as drain/source or emitter/collector and a third region of a p-type active region  11   c  which separates the first region  11   a  and the second region  11   b  and which serves as a base. 
     The transistor  11  is an n-type MOS transistor having the control terminal as a gate and also is a parasitic NPN transistor having the first region  11   a  of relatively small length as the emitter. 
     An insulating film  15  is formed between the first region  11   a  and the control terminal  13  and is formed between the second region  11   b  and the control terminal  13 . Therefore, the first region  11   a  is protected from directly contacting the control terminal  13 . The second region  11   b  is protected from directly contacting the control terminal  13  as well. 
     The third region  11   c  is formed directly under the control terminal  13  so as to contact with the control terminal  13 . On/Off states of the transistor  11  are controlled by applying a control voltage to the control terminal  13 . 
     The second region  11   b  is formed in a surface region  3   b  of the periphery of a grid line  17 . The grid line  17  is used as a scribe line for the semiconductor substrate  3 . The first region  11   a  is formed in the surface region  3   b  of the periphery of the aluminum pad  9  so that a part of the first region  11   a  is contact with the aluminum pad  9 . 
     Furthermore, a well region  21  of the n-type is formed under the second region  11   b . The well region  21  prevents the semiconductor substrate  3  from bending. However, the well region  21  may not been provided if the bend of the semiconductor substrate  3  can be prevented by another way. 
     In a case where the well region  21  is not formed in the semiconductor substrate  3 , since spreading an depletion layer can be prevented compare to a case where the well region  21  is formed, a parasitic capacitance between the semiconductor substrate  3  and the second region  11   b  become large. Therefore, it is possible to drop a peak voltage of the surge voltage applied to the second region  11   b . As a result, such this structure can improve the effect of the ESD protection. 
     Furthermore, in the semiconductor device  1 , there is a possibility that the surge voltage occurs around the grid line  17  during the semiconductor substrate  3  is being scribed. Therefore, if an impedance element for buffering the surge voltage is included in a region between the grid line  17  and the aluminum pad  9 , the transistor  11  can be easily protected from damage during the scribing. 
     However, when a distance E between the aluminum pad  9  and the control terminal  13  increases, a relatively long time is needed to transfer an electrical charge occurred around the aluminum pad  9  to the second region  11   b . Thus, there is a possibility that maintaining the effect of the ESD protection to the internal circuitry  7  is difficult. In order to overcome the difficulty, it is preferable that giving am appropriate resistive element to the second region  11   b  by adjusting a distance D between the grid line  17  and the control terminal  13 . For example, the distance D is set longer than the distance E. A structure to connect a capacitor to the second region  11   b  may also be possible. A structure to utilize a parasitic capacitance existed around the second region  11   b  may also be possible. 
     In the preferred embodiment, as shown in FIG. 3, a structure that a plurality of protection circuits  5  share one grid line  17  may be used. In this structure, a surge voltage applied to one of the protection circuits  5  may be distributed to other protection circuits  5  through the grid line  17 . Therefore, the surge buffered by one protection circuit  5  may be reduced. 
     In FIG. 3, furthermore, when the number of protection circuits  5  which share one grid line  17  is set to a relatively large number, sufficient ESD protection can be obtained without connecting a voltage source to the grid line  17  through the electrode  19 . That is, in the preferred embodiment, a floating structure as illustrated in FIG. 4 may also be adopted. 
     Next, a method of fabricating the semiconductor device  1  according to the preferred embodiment of the present invention is explained with reference to the FIG.  2 . 
     The method of fabricating the semiconductor device  1  includes a step for forming the protection circuit  5  and a step for ESD protecting the internal circuitry  7 . The method also includes a step for forming the internal circuitry and a step for scribing the semiconductor substrate  3  to utilize the grid line  17  as the scribe line. 
     In the step for forming the protection circuit  5 , the n-type impurity is implanted into the p-type semiconductor substrate  3  from the major surface  3   a  by ion implantation technique and thus the well region  21  is formed below the grid line  17 . 
     Next, the n-type impurity is implanted into the surface region  3   b  of the semiconductor substrate  3  from the major surface  3   a  by the ion implantation technique. Thus, the first region  11   a  and the second region  11   b  are formed at a predetermined interval. The concentration of the first region  11   a  and the second region  11   b  is set to a predetermined value higher than that of the well region  21 . As a result, the third region  11   c  is formed between the first region  11   a  and the second region  11   b  and a junction structure of the transistor  11  made up of the first region  11   a , the second region  11   b  and the third region  11   c  is obtained. 
     In this preferred embodiment, the second region  11   b  is formed below the grid line  17  utilized as the scribe line of the semiconductor substrate  3 . Therefore, in this preferred embodiment, a part of the transistor  11  of the protection circuit  5  is formed at a location where the scribe line will be provided. 
     Next, the insulating film  15  is formed on the major surface  3   a .The insulating film  15  is removed so as to expose the third region  11   c  and a part of and the internal circuitry  7  side of the first region  11   a . Thereafter, the control terminal  13  is formed over the exposed major surface  3   a  of the third region  11   c . The aluminum pad  9  is formed on the exposed major surface  3   a  of the first region  11   a . As a result, the protection circuit  5  according to the preferred embodiment is formed on and in the semiconductor substrate  3  of the semiconductor device  1 . 
     In the method of fabricating the semiconductor device according to the preferred embodiment, the ESD protection to the internal circuitry is executed by the protection circuit  5  in a predetermined step. When the protection circuit  5  protects the internal circuitry  7  from the ESD, an H level is set to the grid line  17  through the electrode  19 . Thereby, a junction between the second region  11   b  and the third region  11   c  is fixed at a backward biased state, In this time, since a potential of the control terminal  13  is set at an L level, the transistor  11  enters into an off state. 
     In this state, even if a normal voltage having a positive polarity is applied to the aluminum pad  9 , current does not flow to the protection circuit  5  side and thus the normal voltage is transferred to the internal circuitry  7 . 
     On the other hand, when an abnormal voltage having the positive polarity (an excessive voltage), i.e., the surge voltage, is applied to the aluminum pad  9 , the charge occurred by the excessive voltage is passed through the third region  11   c , the second region  11   b  and grid line  17 . And finally, the excessive voltage is transferred to the electrode  19 . That is, the internal circuitry  7  is protected from the excessive voltage applied to the aluminum pad  9 . 
     The aluminum pad is explained as an example of the connection terminal in this preferred embodiment, however, the present invention is not limited to this feature. A variety of connection terminals, e.g., Copper, Gold or other metal pad or the like, may be used in the semiconductor device  1 . 
     In this preferred embodiment, since the semiconductor device and method of fabricating the same having the second region formed under the grid line are explained as an example, however, the present invention is not limited to this feature. That is, the semiconductor device and method of fabricating the same having the second region formed under a aluminum wiring applied to a ground potential or under a well region may be used as the present invention. 
     In the present invention, a part of the protection circuit is formed at a location where the scribe line is provided. The scribe line is a region which is cut away from the semiconductor substrate. That is, the scribe line is unnecessary region for a circuit element. Therefore, the present invention ensures enough regions to form internal circuitry and can protect the internal circuitry from the ESD. 
     Furthermore, since a plurality of protection circuits share the second region of the transistor, a circuit area per one connection terminal can decrease. 
     While the preferred form of the present invention has been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. 
     The scope of the invention, namely, is to be determined solely by the following claims.