Abstract:
Provided is a semiconductor device which uses a comb-like N-type MOS transistor as an ESD protection element and is capable of uniformly operating the entire comb-like N-type MOS transistor. By adjusting a length L of a gate electrode of the N-type MOS transistor used as the ESD protection element in accordance with the distance from a contact for fixing a substrate potential, which is provided on a guard ring around an outer periphery, respective portion of N-type MOS transistor represented as a comb teeth uniformly enter snap-back operation, permitting avoidance of local concentration of current and obtainment of a desired ESD tolerance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device, in particular, to a semiconductor device using an N-type MOS transistor as an ESD protection element. 
     2. Description of the Related Art 
     In a semiconductor device including a MOS transistor, in order to prevent electrostatic breakdown of an internal circuit by static electricity from a pad for external connection (VDD), an off transistor provided under a state in which a gate potential of an N-type MOS transistor is fixed to a ground potential (Vss) so as to be in an off state is often used as an ESD protection element. 
     The off transistor should have a channel width (W) which is larger than that of an ordinary internal MOS transistor structure since an off transistor must flow the entire current caused by the static electricity in a short time and in a large amount. As a result, it is often the case that the entire channel width of an off transistor is increased by adopting a structure of a multi-finger type in which a plurality of drain regions, source regions, and gate regions are combined in the shape of a comb. 
     However, due to the structure in which a plurality of transistors are combined, it is difficult for the entire MOS transistor for ESD protection to operate uniformly, and there is a problem in that only a comb tooth which first enters parasitic bipolar operation cannot receive the entire ESD surge, resulting in local breakdown. A substrate contact for fixing the gate potential of the off transistor to a ground potential is provided for a guard ring which is provided around the off transistor. Since a guard ring is generally provided to surround an ESD protection element, the distance from the substrate contact provided around the comb-like ESD protection element to the respective unit tooth ESD protection elements, that is, to the respective bases of the parasitic bipolar transistors varies. Specifically local voltage difference as a trigger at which the parasitic bipolar transistor formed between a source region and a channel region after avalanche breakdown is turned on varies because the base resistance varies among the comb teeth. The timing of operation of the parasitic bipolar transistor varies among the respective unit ESD protection elements. Accordingly current concentrates on the unit ESD protection element having the parasitic bipolar transistor which has been turned on the earliest, resulting in local breakdown. 
     As a measure for preventing this phenomenon, a layout method has been proposed in which, by decreasing the distance between a contact in a drain region and a gate electrode as an external connection element becomes farther so as to increase the speed of operation of the transistor, the entire comb teeth uniformly enter a snap-back state that is the beginning of bipolar operation (see, for example, Japanese Published Patent Application No. H07-45829). 
     However, the above-mentioned method has problems in that, as the width of the drain region is reduced, it becomes more difficult to secure a desired position of the contact, and, as the resistance of the interconnecting line becomes lower due to the interconnecting line containing a high-melting point metal in recent years, the propagation speed of the surge becomes further faster, and there are cases in which only the distance between the contact and the gate electrode cannot make a complete adjustment. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-mentioned problems, according to one embodiment of the present invention, a semiconductor device is structured as follows. 
     In an N-type MOS transistor for ESD protection having an integral structure of a plurality of transistors in which a plurality of drain regions and a plurality of source regions are alternately formed, a gate electrode is formed between each of the drain regions and each of the source regions, and a substrate contact for fixing a potential of the gate electrode to a ground potential is formed on a guard ring around an outer periphery, the drain regions are electrically connected to an external connection terminal and the source regions are electrically connected to a ground potential supply line, and a length L of a single comb tooth of the gate electrode is reduced as the distance from the substrate contact, whose potential is fixed at the ground potential and provided around the outer periphery, becomes smaller, and is increased as the distance becomes larger. 
     According to one embodiment of the present invention, by increasing the L length of the gate electrode as the distance from the substrate contact whose potential is fixed at the ground potential becomes larger, that is, as the comb tooth of the gate electrode is located more inward, the respective comb teeth of the gate electrode uniformly enter snap-back operation, which enables avoidance of local concentration of current and obtainment of a desired ESD tolerance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a first embodiment of the present invention. 
         FIG. 2  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a second embodiment of the present invention. 
         FIG. 3  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a third embodiment of the present invention. 
         FIG. 4  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, embodiments for carrying out the invention are described in detail with reference to the attached drawings. Further, in the following description, only a contact connected to a ground potential via metal interconnecting line is referred to as a substrate contact. 
     First Embodiment 
       FIG. 1  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a first embodiment of the present invention. 
     A first source region  101  and a first drain region  301 , which are N-type high concentration impurity regions, are formed. A gate insulating film, which is a silicon oxide film or the like, is provided between the first source region  101  and the first drain region  301 . A gate electrode  201  formed of polysilicon or the like is formed on an upper surface of the gate insulating film. A second source region  102  is formed next to the first drain region  301  with the gate electrode  201  interposed therebetween. A second drain region  302  is formed next to the second source region  102  with the gate electrode  201  interposed therebetween. In a pattern in which the above is repeated similarly, a third source region  103 , a third drain region  303 , and a fourth source region  104  are formed with the electrode  201  interposed thereamong. In the first embodiment, a case is described in which four source regions, three drain regions, and a gate electrode having six comb teeth are provided. In this case, the drain regions  301 ,  302 , and  303  are connected to an external connection terminal  701  via a first metal interconnecting line  601 . The six comb teeth of the gate electrode are in the shape of a comb as a whole. Source regions and drain regions are alternately provided among the comb teeth. The transistor is of a multi-finger type in which six MOS transistors are combined. 
     A ground potential is supplied to the first source region  101 , the second source region  102 , and the third source region  103  from metal interconnecting line in an upper layer formed of a material containing a high-melting point metal or the like, the metal interconnecting line being connected to a ground potential supply line  401  formed of a metal material containing a high-melting point metal or the like. Substrate contacts  501  arranged in a direction in parallel with a channel width and substrate contacts  502  arranged in a direction in parallel with a channel length are provided at regular intervals on a guard ring around an outer periphery of the N-type MOS transistor for ESD protection. 
     In this case, a length L of the comb tooth of the gate electrode  201  is reduced as the distance from the substrate contact  501  provided in the direction in parallel with the channel width (lateral direction in  FIG. 1 ) to the comb tooth becomes smaller, and the length L is the largest in the innermost comb tooth of the gate electrode which is located the farthest from the substrate contact  501 . Specifically, in  FIG. 1 , setting is made so that, among the six comb teeth of the gate electrode, the uppermost and the lowermost comb teeth have the smallest length L, and the two comb teeth in the middle have the largest L. This is because bipolar operation is less liable to occur as the potential of a substrate region in which a channel is formed is more firmly fixed by a nearby substrate contact to the ground potential. By setting the lengths L of the respective comb teeth of the gate electrode  201  as described above, difference in timing of the start of bipolar operation which occurs in the channels under the respective comb teeth of the gate electrode  201  due to difference in potential of the substrate caused by the difference in distance to the substrate contact can be eliminated to cause the timing to be uniform, thereby enabling uniform operation of the entire N-type MOS transistor for ESD protection. In the embodiment illustrated in  FIG. 1 , in order to eliminate the difference in timing of the start of bipolar operation caused by the difference in distance between the substrate contacts  501  arranged in the direction in parallel with the channel width of the N-type MOS transistor for ESD protection and the comb teeth of the gate electrode  201 , the lengths L of the comb teeth of the gate electrode  201  are adjusted. A unit gate electrode corresponding to one comb tooth has a fixed width, and is basically in the shape of a rectangle. 
     Second Embodiment 
       FIG. 2  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a second embodiment of the present invention. Like reference numerals are used to designate members corresponding to those illustrated in  FIG. 1 . The second embodiment is different from the first embodiment illustrated in  FIG. 1  in that the length L of the unit gate electrode is not fixed. In  FIG. 2 , not only the substrate contacts  501  arranged in the direction in parallel with the channel width which is the lateral direction in the figure but also the substrate contacts  502  arranged in the direction in parallel with the channel length which is a longitudinal direction in the figure are provided. In consideration of this, the lengths L around distal ends and proximal ends at which the distance to the substrate contact  502  is small of the comb teeth of the gate electrode  201  are set smaller than the lengths L around intermediate portions, respectively. 
     Such a structure enables elimination of difference in timing of the start of bipolar operation caused by the difference in distance between the substrate contact  501  arranged in the direction in parallel with the channel width and the comb tooth of the gate electrode  201  and by the difference in distance between the substrate contact  502  arranged in the direction in parallel with the channel length and the comb tooth of the gate electrode  201  of the N-type MOS transistor for ESD protection. 
     Third Embodiment 
       FIG. 3  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a third embodiment of the present invention. Like reference numerals are used to designate members corresponding to those illustrated in  FIG. 1 . The third embodiment is different from the first embodiment illustrated in  FIG. 1  in that, by omitting the substrate contacts arranged in the direction in parallel with the channel length of the N-type MOS transistor for ESD protection, the influence of difference in substrate potential in the direction in parallel with the channel length is eliminated, and only the substrate contacts  501  arranged in the direction in parallel with the channel width are provided on the guard ring around the outer periphery. The comb teeth of the gate electrode  201  in this case are provided so that, similarly to the case of the first embodiment, the length L of the comb tooth of the gate electrode is reduced as the distance from the substrate contact  501  arranged in the direction in parallel with the channel width becomes smaller, and the length L is the largest in the innermost comb tooth of the gate electrode which is located the farthest from the substrate contact  501 . The unit gate electrode is in the shape of a rectangle. 
     Fourth Embodiment 
       FIG. 4  is a schematic plan view illustrating an N-type MOS transistor for ESD protection of a semiconductor device according to a fourth embodiment of the present invention. Like reference numerals are used to designate members corresponding to those illustrated in  FIG. 1 . 
     In this embodiment, only the substrate contacts  502  arranged in the direction in parallel with the channel length are provided on the guard ring around the outer periphery, and the shape of the gate electrode is formed in consideration of this arrangement. Specifically, the lengths L around the distal ends and the proximal ends at which the distance to the substrate contact  502  is small of the comb teeth of the gate electrode  201  are set smaller than the lengths L around intermediate portions, respectively, and all the six comb teeth of the gate electrode  201  are in the same shape. 
     The fourth embodiment is different from the first embodiment illustrated in  FIG. 1  in that the L length of the unit gate electrode is not fixed. The fourth embodiment is different from the second embodiment illustrated in  FIG. 2  in that the lengths L of the respective unit gate electrodes are identical to each other. 
     Such a structure enables elimination of difference in timing of the start of bipolar operation caused by the difference in distance between the substrate contact  502  arranged in the direction in parallel with the channel length and the comb tooth of the gate electrode  201 .