Abstract:
A high voltage device. A high voltage MOS transistor is applied in the ESD protection device to the structure of which a doped region is added, generating a parasitic semiconductor controlled rectifier (SCR) having a shorter discharge path such that the SCR has faster response enhancing ESD protection.

Description:
This Non-provisional application claims priority under 35 U.S.C. § 119(a) on patent application Ser. No(s). 092131637 filed in Taiwan, Republic of China on Nov. 12, 2003, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high voltage device with electrostatic discharge (ESD) protection and more particularly to high voltage device with a parasitic silicon controlled rectifier (SCR) which has a shorter discharge path. 
     2. Description of the Related Art 
     As semiconductor manufacturing evolves, ESD protection has become one of the most critical reliability issues for integrated circuits (IC). Several ESD test modes, such as machine mode (MM) or human body mode (HBM), have been proposed to imitate the circumstances under which an ESD event occurs. The ability to withstand certain levels of ESD is successful commercialization essential for an IC. 
     ESD protection circuits are generally located at input/output ports or between power rails, to release electrostatic stress before the electrostatic stress damages interior or core electronic circuits in an IC. ESD protection circuits are typically designed to be switched off during common/normal signal operation and switched on during an ESD event to release accumulated electrostatic charge. 
     Among ESD protection circuits, silicon controlled rectifiers (SCR) have been recognized as an effective ESD protection device.  FIG. 1  shows characteristic curves of a silicon controlled rectifier (SCR). Due to the low holding voltage (V hold , about 1V in a CMOS process) of the SCR, power (Power=I ESD ×V hold ) generated by the SCR device during ESD stress is lower than other ESD protection devices (such as diode, MOS, BJT, or field-oxide device) in CMOS technologies. Therefore, the SCR device can sustain much higher ESD stress within a smaller layout area in the CMOS IC. 
       FIG. 2  shows a conventional high voltage device with a parasitic SCR as ESD protection device disclosed in U.S. Pat. No. 6,459,127. The high voltage device is also a N-type metal oxide semiconductor (MOS) transistor. A gate  110  of the NMOS is formed on a P substrate  100 . An N +  region  112  is a source of the NMOS and an N well  102  a drain of the NMOS. An N +  region  106  is a contact point for the drain. The gate  110  controls the electrical connection of N +  region  112  and the N well  102 . The gate  110  is coupled to a ground line or a pre-driver according to circuit requirements. 
     The P substrate  100  is coupled to the ground line through the P +  region  116 . The N +  region  112  is coupled to the ground line. The drain is coupled to a pad through the N +  region  106 . Due to low dosage concentration, the junction between the N well  102  and P substrate  100  has a very high breakdown voltage, such that high voltage signal can be input from the pad into the N well  102  and does not cause junction breakdown. 
     The parasitic SCR comprises a P +  region  104 , the N well  102 , P substrate  100 , and N +  region  112 . P +  region  104  is located beside N +  region  106  and beyond the gate  110 . The parasitic SCR is turned on when positive ESD voltage occurs in the pad and the P substrate  100  is grounded. In  FIG. 2 , a discharge path A while the SCR is turned on is shown as a dotted line. The majority of the ESD current flows from the P +  region  104 , the N well  102 , P substrate  100 , and N +  region  112  to the ground line releasing ESD stress. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an electrostatic discharge (ESD) protection device having quick turn-on and superior ESD tolerance. 
     The electrostatic discharge (ESD) protection device of the present invention comprises a 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 second conductive type, and a third doped region of the first conductive type. The well and the first doped region are located in the substrate. The gate controls the electrical connection of the first doped region and the well. The gate, the first doped region, and the well make up a field effect transistor. The second doped region is located in the well as a contact point thereof. The third doped region is located in the well between the second doped region and the gate. The third doped region, the well, the substrate, and the first doped region make up a parasitical semiconductor controlled rectifier (SCR), and the well and the third doped region are connected to a pad. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with reference made to the accompanying drawings, wherein: 
         FIG. 1  shows characteristic curves of a SCR; 
         FIG. 2  shows a conventional high voltage NMOS device; 
         FIG. 3  is a cross-section of a high voltage NMOS device according to a first embodiment of the present invention; 
         FIG. 4  is a cross-section of a high voltage NMOS device according to a second embodiment of the present invention. 
         FIG. 5  is a cross-section of a high voltage NMOS device according to a third embodiment of the present invention. 
         FIG. 6  is a cross-section of another high voltage NMOS device according to the present invention; 
         FIG. 7  is a cross-section of another high voltage NMOS device according to the present invention; and 
         FIGS. 8 to 10  show three cross-sections of high voltage P-type transistors of the present invention, respectively corresponding to  FIGS. 3 ,  6 , and  7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  is a cross-section of a high voltage NMOS device according to a first embodiment of the present invention. This high voltage NMOS device in  FIG. 3  acts as a switch during normal operation. The NMOS transistor comprises gate  210 , N +  region  212 , N well  202  and P substrate  200 . The N +  region  212  is a source of the NMOS transistor and the N well  202  a drain, while the gate  210  controls the connection between the drain and the source. An N +  region  204  formed in the N well  202  acts as an electrical contact for the N well  202 . The gate  210  may be coupled to a ground line or a pre-driver according to circuit requirements. 
     The P substrate  200  is coupled to the ground line through a P +  region  216 . The drain is coupled to a pad through the N +  region  204 . Due to low dosage concentrations in the N well  202  and the P substrate  200 , the junction between the N well  202  and the P substrate  200  has a very high breakdown voltage, making this NMOS transistor tolerant to a high voltage input signal. 
     A field oxide region  214  isolates the N +  region  212  from the P +  region  216 . In order to avoid overstress across the gate-oxide layer under the gate  210 , a field oxide region  208  is formed between a P +  region  206  and gate  210 , lessening the induced voltage on the gate-oxide layer. The field oxide regions  208  and  214  are formed by shallow trench isolation or local oxidation of silicon. 
     The P +  region  206  is located between the N +  region  204  and the gate  210  and coupled to the pad. As shown in  FIG. 3 , the P +  region  206 , N well  202 , P substrate  200 , and N +  region  212  make up a parasitic SCR. In  FIG. 3 , the P +  region  206  contacts the N +  region  204 . 
     The pad can accommodate or accept input of a high voltage signal during normal operation. Since both doping concentrations of the P substrate  200  and the N well  202  are low, the PN junction between the P substrate  200  and N well  202  has a higher breakdown voltage. The PN junction is, therefore, strong enough to isolate the P substrate  200  and the N well  202  when no ESD event occurs in the pad. Thus, the SCR is turned off during normal operation. 
     The N well  202  is coupled to the pad through the N +  region  204  and the P substrate  200  to the ground line through the P +  region  216 . When negative ESD voltage occurs in the pad, the PN junction is forward biased to turn on, and the pad and the ground line are short, allowing ESD stress to be released. 
     When positive ESD voltage from an ESD event occurs in the pad, leakage current from the N well  202  to the P substrate  200  may trigger on the SCR.  FIG. 3  shows a discharge path B as a dotted line. The majority of ESD current flows through the pad, P +  region  206 , N well  202 , P substrate  200 , and N +  region  212  to the ground line. 
     In comparison with the discharge path A in  FIG. 2 , the discharge path B in  FIG. 3  is shorter than the discharge path A. Discharge path A flows around the N +  region  206  but discharge path B does not. The length of the discharge path represents the triggering time required by the SCR to release ESD stress. The shorter discharge path has the quicker triggering time. Quicker triggering time means quicker response to an ESD event and better ESD protection. In comparison with the discharge path A, the shorter discharge path B provides quicker triggering time to rapidly release ESD stress, such that internal/core circuit has better ESD protection. 
       FIG. 4  is a cross-section of a high voltage NMOS device according to a second embodiment of the present invention. The high voltage NMOS device is similar to that shown in  FIG. 3  except that the high voltage NMOS device comprises a region  203 . The field oxide regions  208  and N well  202  are replacing for the region  203 . The region  203  is a NDD region. Known manufacture techniques are used to form NDD region. See U.S. Pat. No. 6,590,262. 
       FIG. 5  is a cross-section of a high voltage NMOS device according to a third embodiment of the present invention. The high voltage NMOS device is similar to that shown in  FIG. 3  except that the high voltage NMOS device comprises a region  203  formed after the N well  202 . The region  203  replaces for the field oxide region  208  to isolate the P +  region  206  from the gate  210  and is a NDD region. 
       FIG. 6  is a cross-section of another high voltage NMOS device according to the present invention, in which a field oxide region  222  is formed between the P +  region  206  and the N +  region  204 .  FIG. 7  is a cross-section of another high voltage NMOS device according to the present invention, in which a dummy gate  224  on the N well  220  separates the P +  region  206  from the N +  region  204 . The dummy gate  224  can be a floating gate and receives no signal. The dummy gate  224  can alternatively be connected to the pad. 
     Conversion between P-type and N-type components is well known to those skilled in the art. The present invention can also be applied in PMOS transistors.  FIGS. 8 to 10  show three cross-sections of high voltage P-type transistors of the present invention, respectively corresponding to  FIGS. 3 ,  6 , and  7 . Each ground line in  FIGS. 3 to 5  is a power line VDD in  FIGS. 8 to 10  and each NMOS transistor in  FIGS. 3 ,  6 , and  7  is a PMOS transistor in  FIGS. 8 to 10 . 
     The provided structure of the present invention not only generates a parasitic SCR but also supplies a shorter discharge path for faster release of ESD stress, providing increased ESD tolerance in IC products. 
     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.