Abstract:
An electrostatic discharge (ESD) protection circuit for protecting an internal device from an ESD is disclosed. The ESD protection circuit includes an NMOS transistor connected to a ground voltage terminal having silicide layers on a gate electrode and on source/drain regions thereof; and a PMOS transistor having a gate electrode connected to a ground voltage terminal and connecting the NMOS transistor to a pad.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a semiconductor integrated circuit; and, more particularly, to an electro static discharge (ESD) protection circuit for protecting an internal device from an ESD.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    In a semiconductor integrated circuit, as integration is increased, a source-to-drain channel length is decreased, thereby causing unintentional electric phenomena, such as an ESD characteristic deterioration, a hot carrier effect and a punch-through, etc. to occur.  
           [0003]    An electrostatic discharge caused by static electricity largely includes two kinds. First is a machine model in which static electricity is caused by unstable connection of a device when the device is equipped with a test device with a package form. h this case, the static electricity has a low voltage of about 250 V, but has small impedance so the amount of electric charge is relatively high. Second is a human body model in which static electricity induced within a human body is discharged when a user touches a device. In this case, the static electricity is discharged at a high voltage of about 2000 V, but has high impedance, so the amount of electric charge is low.  
           [0004]    When a MOS device is exposed to a static electricity discharge, a gate oxide rupture and a junction spiking, etc, can be generated so that the device can be entirely demolished or have minute damage with serious effects on its reliability.  
           [0005]    In view of the above, an ESD protection circuit is inserted in a coupling region between internal and external circuits of an integrated device to protect it from damage caused by an influx of static electricity. The static electricity is removed by the ESD protection circuit through a ground line and a power line, thereby keeping the internal circuit safe.  
           [0006]    The ESD protection circuit is provided with a MOS transistor between a ground and a pad to extract the static electricity toward the ground. A drain region (the pad connection part) of the MOS transistor should have high resistance to achieve enhancement of the ESD characteristic for the following reasons.  
           [0007]    First, when the MOS transistors are turned on in the ESD situation, all of the transistors can be equally turned on without turning on specific MOS transistors at both ends of the protection circuit if each drain has an equal resistance.  
           [0008]    Second, in the ESD situation, when the MOS transistor enters a snap-back mode, impedance of the drain must be increased in order to make the second breakdown voltage higher than the snap-back voltage, and, by doing so, various transistors can be turned on at the same time.  
           [0009]    On the other hand, for a ULSI/VLSI MOS device requiring a high speed operation, a salicide (self-aligned silicide) process is well known as an effective method to obtain a low contact resistance and then this process is practically used.  
           [0010]    However, the salicide process deteriorates the ESD characteristics by decreasing the drain resistance of an NMOS transistor in the ESD protection circuit. Also, the thicker the silicide layer is, the weaker the ESD protection is.  
           [0011]    In a conventional method, even if the salicide process is applied to the integrated circuit for high speed processing, this process is not applied to a MOS transistor in an ESD protection region. This conventional method is illustrated in U.S. Pat. No. 5,994,176, assigned to Texas Instruments and Acer Incorporated, and issued on Nov. 30, 1999.  
           [0012]    [0012]FIG. 1 is a cross-sectional view showing the aforementioned conventional method. As shown in FIG. 1, in a transistor  110  in a functional region, silicide layers  113  are formed on a gate electrode  111  and a source/drain region  112 . However, in a transistor  150  in an ESD protection region, a silicide layer is not formed on a gate electrode  151  and a source/drain region  152  which are covered with a barrier layer  170 .  
           [0013]    Likewise, in the conventional method, the functional region and the ESD protection region are divided. The functional region has the salicide layer to obtain characteristic enhancement, such as a high speed operation, etc., and the ESD protection region realizes a ballast resistance which does not apply salicide layers to the MOS transistor for making the ESD protection characteristics better.  
           [0014]    However, the conventional method requires an additional process, such as a barrier layer formation, etc., to realize the ESD protection circuit. In this case, a leakage current is generated by such an additional process, and the process is complicated and the unit cost is increased.  
           [0015]    A design of a transistor is important to show good characteristics in an ESD protection circuit. It is especially important to prevent the formation of the suicide in the region of the NMOS transistor, passing through a current toward a ground at an ESD situation, during the formation of a semiconductor substrate.  
           [0016]    In the fully silicide process that applies the suicide process to a source region, a drain region and a gate electrode of the ESD protection transistor, source/drain regions of the NMOS transistors in a functional region form a little resistance to thereby increase operation speed of a device. However, in such an ESD protection circuit, when an ESD situation is generated, a little resistance of drain regions brings out worse results. Therefore, for good ESD characteristics, a high resistance value of the drain region of the ESD protection transistor is required.  
           [0017]    Since the conventional method uses a mask to prevent formation of a silicide in a drain region, the processing steps are complicated and unit price is increased. Also, as a result of the additional process, a leakage current is generated.  
         SUMMARY OF THE INVENTION  
         [0018]    In accordance with an aspect of the present invention, an ESD protection circuit is provided comprising an NMOS transistor connected to a ground voltage terminal, the NMOS transistor having a gate electrode, a source region and a drain region, and further including silicide layers on the gate electrode, the source region and the drain region; and a PMOS transistor having a gate electrode connected to a ground voltage terminal, the PMOS transistor connecting the NMOS transistor to a pad.  
           [0019]    In accordance with another aspect of the present invention, there is provided an ESD protection circuit comprising a P-type semiconductor substrate; an N-well formed in the P-type semiconductor substrate; an NMOS transistor having an N+ source junction, an N+ drain junction and a gate polysilicon which is in contact with a ground voltage line, wherein the N+ source junction, the N+ drain junction and the gate polysilicon are formed on the P-type semiconductor substrate; a PMOS transistor having a P+ drain junction, a P+ source junction and a gate polysilicon, the PMOS transistor being connected to the N+ source junction of the NMOS transistor, wherein the P+ drain junction, the P+ source junction and the gate polysilicon of the PMOS transistor are formed in the N-well; an N+ junction connected to the P+ source junction of the PMOS transistor and to a pad; and silicide layers formed on each of the gate polysilicon layers, the source and drain junctions of the PMOS and the NMOS transistors, and the N+ junction.  
           [0020]    In accordance with yet another aspect of the present invention, there is provided a semiconductor integrated circuit, comprising a pad; an internal circuit for performing a function in response to a signal inputted to the pad and including a plurality of first MOS transistors having silicide layers on gate electrodes and on source and drain junctions; a peripheral circuit having a plurality of second MOS transistors having silicide layers on gate electrodes and on source and drain junctions thereof; and an ESD protection circuit forming a current path between the pad and the internal circuit for protecting the internal circuit from static electricity, wherein the ESD includes: an NMOS transistor connected to a ground voltage line; and a PMOS transistor having a gate electrode connected to the ground voltage line, and connecting the NMOS transistor to the pad. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a cross-sectional view illustrating a conventional method in which a suicide layer is applied to a functional region and not to an ESD protective region;  
         [0022]    [0022]FIG. 2 is a schematic block diagram showing an exemplary configuration of an integrated circuit constructed in accordance with the teachings of the invention;  
         [0023]    [0023]FIG. 3A is a plan view showing the ESD protection circuit of FIG. 2 in more detail;  
         [0024]    [0024]FIG. 3B is a cross-sectional view showing the ESD protection circuit of FIG. 3A;  
         [0025]    [0025]FIG. 4A is a circuit diagram showing an equivalent circuit and a current path in a normal operation of the ESD protection circuit; and  
         [0026]    [0026]FIG. 4B is a circuit diagram showing an equivalent circuit and a current path in an ESD situation of the ESD protection circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Hereinafter, ESD protection transistors will be described in detail referring to the accompanying drawings.  
         [0028]    [0028]FIG. 2 is a schematic block diagram showing a configuration of an exemplary integrated circuit constructed in accordance with the teachings of the invention.  
         [0029]    Referring to FIG. 2, an ESD protection circuit  230  is provided between a pad  210  and an internal circuit  220 . Circuit  220  is a normal operating circuit portion. Different from the conventional method, a PMOS transistor  232 , which plays a role of a resistor in an ESD situation, is added between a drain edge and the pad  210  of an NMOS transistor  231 . Also, a silicide process is applied to the ESD protection transistors  231  and  232  and to the transistors of the internal circuit  220 .  
         [0030]    If a resistance of a drain region of the NMOS transistor  231  is decreased by the salicide process, the PMOS transistor  232  compensates for the decreased resistance. In other words, as described in the conventional method, static electricity or an error signal input to a pad is bypassed through the NMOS transistor  231  to a ground voltage line. If a drain resistance of the NMOS transistor  231  is decreased by a suicide layer, an ESD characteristic is deteriorated. So, instead of preventing application of the salicide process to the drain of the NMOS transistor  231 , in the disclosed device, the PMOS transistor  232  is connected to the drain to carry out a high speed operation, reduce the complexity of the manufacturing processes, and prohibit the ESD characteristic deterioration.  
         [0031]    In the PMOS transistor  232 , a gate is connected to a ground voltage line, therefore the PMOS transistor is turned on in a normal operation, but in an ESD situation, the PMOS transistor  232  is turned off. Accordingly, a drain resistance of the NMOS transistor  231  is increased through a well of the PMOS transistor  232 .  
         [0032]    [0032]FIG. 3A is a plan view showing the ESD protection circuit of FIG. 2 in more detail, and FIG. 3B is a cross-sectional view taken along the line A-A′ of FIG. 3A. Referring to FIGS. 3A and 3B, there are shown two NMOS transistors ( 231  in FIG. 2), two PMOS transistors ( 232  in FIG. 2) and a contact region.  
         [0033]    Referring to FIGS. 3A and 3B, an N-well  302  is formed within a portion of a P-type substrate (or well)  301 , and a gate electrode  303  of an NMOS transistor and an N+ source junction  304 A and an N+ drain junction  304 B are formed within the P-type substrate  301 .  
         [0034]    A gate electrode  305  of a PMOS transistor, a P+ source/drain junction  306  and N+ junctions  307  for contact pad conduction lines are formed on the substrate of the N-well  302 . Ground conduction line contact nodes  309  are formed on the N+ source junction  304 A of the NMOS transistor.  
         [0035]    The N+ source junction  304 A and the N+ drain junction  304 B of the NMOS transistor are formed. The N+ drain junction  304 B is in contact with the P+ source (or drain) junction  306  of the PMOS transistor, and the P+ drain (or source) junction  306  of the PMOS transistor is formed, being in contact with the N+ junction  307 .  
         [0036]    A silicide layer  308  (in FIG. 3B) is formed on each of the gate electrodes and source/drain junctions of the PMOS and the NMOS transistors and the N+ junction  307 .  
         [0037]    The above-mentioned device manufacturing method may be introduced to normal semiconductor integrated circuit manufacturing technologies. A barrier layer is not used in the ESD protection circuit region, and a normal salicide process is applied to the source/drain regions.  
         [0038]    A silicide layer is a transition metal, such as Ti, Co, W, Ni, Pt, etc., which undergoes a rapid thermal processing (RTP) to react an active region (silicon substrate) and a silicon of a gate polysilicon with the transition metal, thereby forming metal silicide layers such as TiSi, CoSi, WSi, NiSi, PtSi, etc. The RTP is carried out at a temperature of 350° C. to 750° C. and, after removing an unreacted transition metal, only the silicide layer remains on the gate and the active region.  
         [0039]    [0039]FIG. 4A is a circuit diagram showing an equivalent circuit and a current path in an normal operation of an ESD protection circuit. FIG. 4B is a circuit diagram showing an equivalent circuit and a current path in an ESD situation of an ESD protection circuit. In FIG. 4A, the gate electrode of the PMOS transistor  232  is connected to the ground voltage line and turned on so that a low resistance path  410  is formed. In FIG. 4B, the PMOS transistor is in the turned off state and, thus, it forms a large resistance path  420  through the N-well ( 302  of FIG. 3).  
         [0040]    The disclosed devices make it possible to apply a salicide process to the whole region of an integrated circuit including an ESD protection circuit and have good ESD protection characteristics. The disclosed devices decrease processing time and cost by eliminating a silicide protection or a silicide blocking which is needed in the prior art ESD protection circuit manufactured via conventional salicide processing. Also, the disclosed devices can prevent characteristic deterioration and also can obtain good circuit and structure for an ESD protection circuit, by inserting a PMOS transistor into a drain region of an NMOS transistor for an ESD protection transistor with low resistance.  
         [0041]    The disclosed device applies a suicide process to a source region, a drain region and a gate electrode of a MOS transistor, and inserts a PMOS transistor into a drain region of an NMOS transistor for an ESD protection transistor. That is, in the disclosed device, the drain region of the NMOS transistor is connected to a pad through an N-well and the PMOS transistor is formed in the N-well.  
         [0042]    In the ESD protection circuit of the disclosed device, a ground voltage level is applied to the gate of the PMOS transistor, so the PMOS transistor is turned on in a normal operation, thereby forming a small resistance through the PMOS transistor. In an ESD situation, the PMOS transistor is turned off, so a path is formed through the N-well of the bottom portion of the PMOS transistor, which forms a large resistance. Therefore, in the ESD situation, current inputted from outside passes through the N-well, so an essentially required resistance is obtained by the N-well.  
         [0043]    An ESD protection circuit and an integrated circuit, which have good ESD characteristics, and which simplifies the manufacturing process are disclosed.