Abstract:
An ESD protection device. The ESD protection device is incorporated with a gap structure in a laterally diffused metal oxide semiconductor (LDMOS) field effect transistor, isolating a doped region and a field oxide region. When a parasitical semiconductor controlled rectifier (SCR) of LDMOS is turned off, ESD current is discharged distributively through several discharge paths, avoiding ESD current focus in a signal narrow discharge path and the danger therefrom.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a protection device, and more particularly, to a high voltage electrostatic discharge (ESD) protection device.  
         [0003]     2. Description of the Related Art  
         [0004]     As the semiconductor manufacturing process develops, ESD protection has become one of the most critical reliability issues for integrated circuits (IC). In particular, as semiconductor process advances into the deep sub-micron stage, scaled-down devices and thinner gate oxides are more vulnerable to ESD stress. Generally, the input/output pads on IC chips must at least sustain 2 kVolt ESD stress of high Human Body Mode (HBM) or 200 V of Machine Mode. Thus, the input/output pads on IC chips usually include ESD protect devices or circuits protecting the core circuit from ESD damage.  
         [0005]      FIG. 1  shows an ESD protection device as disclosed in U.S. Pat. No. 6,459,127. This Esb protection device is also a laterally diffused metal oxide semiconductor (LDMOS) field effect transistor. The LDMOS is N-type MOS with a gate  110  on a P substrate  100 . An N+ region  112  acts as a source of the NMOS and an N well  102  a drain of the NMOS. An N+ region  106  is an electrical contact point of the well  102 . The gate  110  controls the electrical connection of N+ region  112  and the N well  102  and is also coupled to a ground line VSS or a pre-driver according to circuit requirements.  
         [0006]     The P substrate  100  is coupled to the grounded line VSS through the P+ region  116 . The N+ region  112  is also coupled to the grounded line VSS. Through the N+ region  106 , the drain is coupled to a pad. One parasitical SCR is composed with a P+ region  104 , the N well  102 , P substrate  100 , and N+ region  112 .  
         [0007]     The parasitical SCR is turned on when positive ESD voltage is applied to the pad and the ground line VSS is grounded. Beginning at the pad, ESD current flows through the P+ region  104 , N well  102 , P substrate  100 , and N+ region  112  and finally to the grounded line VSS to release ESD stress.  
         [0008]     When ESD stress is not high enough to turn on the parasitical SCR, a secondary ESD current is discharged through the N+ region  106 , N well  102 , P substrate  100 , and P+ region  116  to the grounded line VSS.  
         [0009]     Since doped concentration of the N+ region  106  is higher, the impedance of the N+ region  106  is lower. On the contrary, the doped concentration of the N well  102  is lower such that the impedance of the N well  102  is higher. Most of the secondary ESD current discharges through a discharge path having minimum impedance. In  FIG. 1 , discharge path A has minimum impedance between the N+ region  106  and the N well  102 . Thus, the secondary ESD current is released along the discharge path A to the grounded line VSS when the parasitical SCR is still turned off.  
         [0010]     In the discharge path A, the secondary ESD current, reaching a field oxide region  108 , changes direction. Since the secondary ESD current stays large, the change in direction generates a higher temperature in the turning point, easily damaging the field oxide region  108  and the discharge path.  
       SUMMARY OF THE INVENTION  
       [0011]     It is therefore an object of the present invention to provide an electrostatic discharge (ESD) protection device to avoiding excess current focus along a signal discharge path.  
         [0012]     The ESD protection device according to the present invention comprises a first substrate of a first conductive type, a well of a second conductive type, a first doped region of the second conductive type, a gate, a second doped region of the third conductive type, a field oxide region, and a gap. The well and the first doped region are formed in the substrate. The gate controls the electrical connection of the first doped region and the well. A field effect transistor comprises the gate, the first doped region, and the well. The second doped region, field oxide region, and gap are formed in the well. The field oxide region is located between the gate and the second doped region. The gap is located between the field oxide region and the second doped region. The first and the third conductivity types can be either N or P type. The second conductivity type can be either P or N type.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention can be more fully understood by reading the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:  
         [0014]      FIG. 1  shows an ESD protection device as disclosed in U.S. Pat. No. 6,459,127;  
         [0015]      FIG. 2  is a cross-section of an ESD protection device according to a first embodiment of the present invention;  
         [0016]      FIG. 3  is a cross-section of another ESD protection device according to the present invention;  
         [0017]      FIG. 4  is a cross-section of another ESD protection device according to the present invention;  
         [0018]      FIG. 5  is a cross-section of another ESD protection device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 2  is a cross-section of an ESD protection device according to a first embodiment of the present invention. The ESD protection device is an N-type LDMOS field effect transistor. The NMOS comprises gate  210 , N+ region  212 , and N well  202 . N+ region  212  is a source of the NMOS and N well  202  a drain of the NMOS. An N+ region  206  formed in the N well  202  acts as an electrical contact for the N well  202 . The gate  210  controls the electrical connection of N+ region  212  and the N well  202 , and is also coupled to a grounded line VSS or pre-driver according to circuit requirements.  
         [0020]     The P substrate  200  is coupled to the grounded line VSS through a P+ region  216 . The drain is coupled to a pad through the N+ region  206 .  
         [0021]     A field oxide region  214  isolates the N+ region  212  from P+ region  216 . In order to protect a gate-oxide layer under the gate  210  from overstress, a field oxide region  208  is formed between an N+ region  206  and gate  210  isolating the gate  210  from N well  202 . The field oxide region  208  and  214  are formed by shallow trench isolation (STI) or local oxidation of silicon (LOCOS). A gap separates field oxide region  208  and the N+ region  206 .  
         [0022]     The P+ region  204  is formed in the N well  202  and coupled to pad. The P+ region  204  can be located between the gap and the N+ region  206 , or the N+ region  206  can be located between the gap and the P+ region  204 . Since the P+ region  204  is formed, a parasitical SCR is also formed. The P+ region  204 , N well  202 , P substrate  200 , and N+ region  212  all constitute the parasitical SCR.  
         [0023]     A pn junction is formed between the P substrate  200  and the N well  202 . The P substrate  200  is coupled to the grounded line VSS through the P+ region  216  and the N well  202  is coupled to the pad through the N+ region  206 . When negative ESD voltage is applied to the pad and the grounded line VSS is grounded, the PN junction between the P substrate  200  and the N well  202  is forward biased and the pad and the grounded line VSS act as equivalent shorts, allowing release of ESD stress.  
         [0024]     When positive ESD voltage is applied to the pad and the grounded line VSS is grounded, the parasitical SCR is turned on. ESD current flows through the pad, P+ region  204 , N well  202 , P substrate  200 , N+ region  212 , and finally to the grounded line VSS.  
         [0025]     When ESD occurs in the pad but is insufficient to turn on the parasitical SCR, a secondary ESD current is discharged through the N+ region  206 , the N well  202 , the P substrate  200 , and the P+ region  216  to the grounded line VSS as discharge paths B and C.  
         [0026]     Since the gap is located between the field oxide region  208  and the N+ region  206 , the secondary ESD current does not contact the field oxide region  208 . If all region sizes are the same in  FIGS. 1 and 2 , in  FIG. 1 , the secondary ESD current is focused at discharge path A such that the field oxide region  108  is easily damaged, and in  FIG. 2 , the ESD protection device of the present invention disperses the secondary ESD current to the grounded line VSS through multiple discharge paths B and C.  
         [0027]     The gap is defined by mask pattern. After the field oxide region  208  is formed, the N+ region  206  is formed by a mask pattern, defining the N+ region separated from the field oxide region  208 . If the gap is doped with positive P+, a high impedance region between the field oxide region  208  and the N+ region  206  further avoids secondary ESD current contact with the field oxide region  208 .  
         [0028]      FIG. 3  is a cross-section of another ESD protection device according to the present invention. The same elements utilize the same symbols as in  FIGS. 2 and 3 . A dummy gate  218  is formed by a mask pattern and located between the field oxide region  208  and the N+ region  206 . The dummy gate  218  may be a floating gate uncoupled to any direct current signal. The gate  220  is located between the field oxide region  208  and the N+ region  212 , and part of the gate  220  extends to cover the field oxide region  208 .  
         [0029]      FIG. 4  is a cross-section of another ESD protection device according to the present invention. A dummy gate  222  is formed between the gate  220  and the N+ region  206  and part of the dummy gate  222  covers the field oxide region  208 .  
         [0030]      FIG. 5  is a cross-section of another ESD protection device according to the present invention. The ESD protection device is a P-type LDMOS. An N-type buried layer  501  is formed in a P substrate  500 . The N-type buried layer  501  and an N well  503  are as an N substrate of the P-type LDMOS. The grounded line VSS in  FIG. 3  is a power line VDD in  FIG. 5  and N-type and P-type doping regions are reversed.  
         [0031]     Additionally, N-type and P-type elements are formed on P substrate shown in  FIGS. 3 and 5 . Nonetheless, the present invention can be also applied with N-type or P-type elements formed on P substrate. Conversion between P-type and N-type components is well known to those skilled in the field and therefore is not discussed.  
         [0032]     Since, according to the present invention, a gap exists between a field oxide region and an N+ region, secondary ESD current occurring at the outset of an ESD event before activation of a parasitic SCR, is not focused along a single discharge path, such that danger of burnout along the path is avoided.  
         [0033]     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.