Abstract:
A novel low-voltage-triggered semiconductor controlled rectified (LVTSCR) as an ESD protection device is provided in this invention. The ESD protection device of the present invention has a lateral SCR (LSCR) structure with two electrodes and a MOS for triggering the LSCR. A dummy gate and a doped region are used to isolate the MOS from one of these two electrodes. The dummy gate is designed to block the formation of field-oxide layer formed in the device structure of the lateral SCR. Therefore, the proposed SCR device has a shorter current path in CMOS process, especially in the CMOS process with shallow trench isolation (STI) field-oxide layer. During an ESD, the current path in the ESD protection device is much shorter, and the turn-on speed and the ESD tolerance level are thereby enhanced.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a low voltage triggered electrostatic discharge (ESD) protection device and its applications. In particular, the present invention relates to a modified lateral silicon controlled rectifier especially suited to the process of shallow trench isolation (STI).  
           [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 in terms of ESD stress. Thus, the input/output ports on IC chips are usually designed to include ESD protect devices or circuits for protecting the devices in IC chips from ESD damage.  
           [0005]    [0005]FIG. 1 shows a cross-section view of a conventional Low-Voltage-Triggered lateral Semiconductor Control Rectifier (LVTSCR), triggered by an NMOS. The LVTSCR in FIG. 1 is constructed by a lateral SCR (LSCR) composed of P+ region  14 , N well  10 , P substrate  12  and N+ region  16 , and an NMOS composed of a gate  20 , a drain of N+ region  18  and a source of N+ region  16 . In FIG. 1, P+ region  14  and N+ region  16  are respectively used the anode and the cathode of the LVTSCR. NMOS is used to lower the trigger voltage of the LSCR, such that the combined device is named LVTSCR. While an LVTSCR is implemented by a process flow with a conventional field oxide process, the doped regions in the surface of the substrate are isolated from each other by field oxide layers  26 . The arrow and the dash line in FIG. 1 illustrates the ESD current path while the LVTSCR is triggered to release ESD stress. When positive ESD stress pulses on the anode of the LVTSCR in FIG. 1 and the cathode is relatively coupled to ground, the ESD current conducts from P+ region  14  (anode), detours under the field oxide  26 , reaches N+ region  16 , and is released to the coupled ground.  
           [0006]    However, semiconductor process progress has begun to replace the field oxide layers in ICs with shallow trench isolation (STI) regions. FIG. 2 is a cross-section view of the LVTSCR in FIG. 1 wherein the field oxide layers  26  in FIG. 1 are replaced by STI regions  30 . One of the strongest advantages of employing the STI process in an IC is that the substrate surface of the IC will become more even, and the subsequent electric connections are more easily fabricated on the substrate surface. To perform electrical isolation between devices, however, STI regions require a certain depth, usually deeper than that of a diffusion region, as shown in FIG. 2. When a positive ESD stress pulses on the anode of the LVTSCR in FIG. 2 and the cathode is relatively coupled to ground, the ESD current conducts from P+ region  14  (anode), detours under the STI region  30 , reaches N+ region  16 , and is released to the coupled ground. The ESD current path in FIG. 2, in comparison with that in FIG. 1, is distinctly longer due to the increased depth of the STI region  30 . Therefore, it is more difficult for the ESD current path in FIG. 2 to release ESD stress than in FIG. 1, such that the LVTSCR in FIG. 2 has a longer turn-on time and a lower ESD tolerance. Thus, replacing field-oxide structure with STI structure may degrade the ESD tolerance of an ESD protection device.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide an ESD protection device having a quick turn-on speed and superior ESD tolerance even though the ESD protection device is fabricated with the STI process.  
           [0008]    Another object of the present invention is to provide ESD protection circuits employing the ESD protection device of the present invention.  
           [0009]    As mentioned above, the ESD protection device of the present invention comprises a first well of a first conductive type, a second well of a second conductive type, a MOS of the first conductive type, a first doped region of the second conductive type, a second doped region of the first conductive type and a dummy gate. The second conductive type is opposite to the first conductive type. The second well contacts the first well to form a junction. The MOS comprises a control gate, a first drain/source region of the first conductive type and a second drain/source region of the first conductive type. The control gate is positioned on the second well. The first drain/source region is formed on the junction. The second drain/source region is formed on the second well and coupled to a first pad. The first doped region is coupled to a second pad and formed on the first well. The first doped region associates with the first well, the second well and the second drain/source region to construct a lateral semiconductor controlled rectifier (LSCR). The second doped region is formed on the surface of the well and between the first doped region and the first drain/source region. The dummy gate is positioned between the first drain/source region and the second doped region and on the first well.  
           [0010]    The LSCR has an anode and a cathode respectively coupled to a first pad and a second pad.  
           [0011]    The ESD protection device of the present invention has the advantage of quicker turn-on speed and higher ESD tolerance, in comparison with the prior art, since no STI structure stands between the anode and the cathode to lengthen the ESD current path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 shows a cross-section view of a conventional Low Voltage Triggered lateral Semiconductor Control Rectifier (LVTSCR);  
         [0014]    [0014]FIG. 2 is a cross-section view of the LVTSCR in FIG. 1 wherein the field oxide layers in FIG. 1 are replaced by STI regions;  
         [0015]    [0015]FIG. 3 a  shows a cross-section view of an NMOS-triggered LVTSCR according to the present invention;  
         [0016]    [0016]FIG. 3 b  is the symbol denoting the NMOS-triggered LVTSCR in FIG. 3 a;    
         [0017]    [0017]FIG. 4 a  shows a cross-section view of a PMOS-triggered LVTSCR according to the present invention;  
         [0018]    [0018]FIG. 4 b  is the symbol denoting the PMOS-triggered LVTSCR in FIG. 4 a;    
         [0019]    [0019]FIG. 5 a  is an ESD protection circuit according to the invention;  
         [0020]    [0020]FIG. 5 b  is an embodiment of the ESD protection circuit in FIG. 5 a;    
         [0021]    [0021]FIG. 6 a  shows two ESD protection circuits according to the present invention, wherein one is applied between the I/O pad and VSS and the other is applied between VDD and the I/O pad;  
         [0022]    [0022]FIG. 6 b  is an embodiment of FIG. 6 a;    
         [0023]    [0023]FIG. 7 a  is a VDD-to-VSS ESD protection circuit employing the nSCR of the present invention;  
         [0024]    [0024]FIG. 7 b  is an embodiment of the VDD-to-VSS ESD protection circuit in FIG. 7 a;    
         [0025]    [0025]FIG. 7 a  is a VDD-to-VSS ESD protection circuit employing the pSCR of the present invention; and  
         [0026]    [0026]FIG. 8 b  is an embodiment of the VDD-to-VSS ESD protection circuit in FIG. 8 a.   
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    The essence of the present invention is to replace the STI region through which the ESD current must detour in the prior art with a dummy gate structure. The dummy gate structure provides sufficient isolation to separate doped (diffusion) regions. Additionally, the dummy gate does not lengthen the ESD current path. The turn-on speed and the ESD tolerance level of the present invention are thus enhanced.  
         [0028]    [0028]FIG. 3 a  shows a cross-section view of an NMOS-triggered LVTSCR according to the present invention. FIG. 3 b  is the symbol denoting the NMOS-triggered LVTSCR in FIG. 3 a . The NMOS-triggered LVTSCR, named nSCR in short, in FIG. 3 a  is formed on a P substrate  40  and has an N well  42  and a P well  44  adjacent to each other.  
         [0029]    A P+ region  58  formed in the P well  44  is used as an electric contact for the P well  44 . The NMOS for triggering the nSCR is also positioned on the P well  44 . The NMOS has a control gate  56  and two N+ region ( 60  and  54 ), where N+ regions  60  and  54  respectively are the source and the drain of the NMOS. N+ region  54  is located on the junction formed by the contact between P well  44  and the N well  42 . N+ region  60  is isolated from P+ region  58  by a STI region  62 .  
         [0030]    There is an N+ region  46  used as an electric contact for the N well  42 . P+ region  48  in the N well  42  is isolated from N+ region  46  by a STI region  62 . Dummy gate  52  and N+ region  46  are located in the N well  42  to separate the P+ region  48  from N+ region  54 . Both the STI structure and field oxide are isolated from the ESD current path (the arrow and the dash line in FIG. 3 a ) so the length of the ESD current path is shorter than that in the prior art, such that the nSCR according to the present invention has a quicker turn-on speed and a better ESD tolerance level.  
         [0031]    As shown in FIG. 3 a , P+ region  48 , N well  42 , P well  44  and N+ region  60  construct a lateral SCR. Where N well  42  and P+ region  48  are coupled together as an anode and P well  44  and N+ region  60  are coupled together as a cathode. FIG. 3 b  illustrates the symbol of the nSCR. Furthermore, in FIG. 3 b , the letter “C” beside a P layer indicates that the control gate  56  is on P well  44 . The letter “D” beside an N layer indicates that the dummy gate  52  is on N well  42 .  
         [0032]    The control gate  56  of the NMOS can be coupled to an ESD detection circuit. The ESD detection circuit is responsive to an ESD event. When the ESD detection circuit detects an ESD, it drives the control gate  56  to trigger on the nSCR to release ESD stress.  
         [0033]    The dummy gate  52  can function in an electrically floating state (coupled to nothing) or can be coupled to VDD or VSS. Neither condition will influence the ESD current path in the nSCR.  
         [0034]    By employing the same concept, a cross-section view of a PMOS-triggered LVTSCR, named pSCR in short, according to the present invention is shown in FIG. 4 a . A control gate  56 ′ is located on N well  42 ′ and a dummy gate  52 ′ is located on P well  44 ′. The corresponding symbol of the pSCR in FIG. 4 a  is shown in FIG. 4 b , where “C” denotes the control gate and “D” denotes the dummy gate. As shown by the arrow and the dash line in FIG. 4 a , ESD current flows from the anode to the cathode without approaching any STI structure or field oxide structure.  
         [0035]    P substrate  40  in FIG. 3 a  or FIG. 4 a  can be replaced by an N substrate without any functional impact on the nSCR or pSCR according to the present invention. Furthermore, the present invention is further suitable to SOI (silicon on insulator) structure, in which an insulator layer is put under N well ( 42  or  42 ′) and P well ( 44  or  44 ′) to prevent interference between wells.  
         [0036]    [0036]FIG. 5 a  is an ESD protection circuit according to the invention. The cathode of the nSCR of the present invention is coupled to VSS. The anode and the dummy gate of the nSCR are coupled to an I/O (input/output) pad  80 . ESD detection circuit  84  is responsible for detecting if an ESD event occurring across the I/O pad  80  and VSS. When an ESD event is occurring across the I/O pad  80  and VSS, the ESD detection circuit  84  temporally drives the control gate to a relative voltage level to trigger the turn-on of the nSCR and conduct ESD current, and the inner circuit  82  is protected. FIG. 5 b  is an embodiment of the ESD protection circuit in FIG. 5 a . A RC-coupled circuit, having a resistor and a capacitor connected in series, composes an ESD detection circuit  84 . The control gate is coupled to the connection node between the capacitor C and the resistor R.  
         [0037]    As well as protection against ESD stress across the I/O pad and VSS, the present invention also can be applied to protect the inner circuits against ESD stress across the VDD and I/O pad, as shown in FIG. 6 a . FIG. 6 a  shows two ESD protection circuits according to the present invention, wherein one is applied between the I/O pad and VSS and the other is applied between VDD and the I/O pad. The ESD protection circuit between the I/O pad  80  and VDD includes an ESD detection circuit  86  and a PSCR. The anode of the PSCR is coupled to VDD. The dummy gate and the cathode of the pSCR are coupled to the I/O pad  80 . The ESD detection circuit  86  is responsible for driving the control gate of the pSCR. While an ESD event is occurring across VDD and the I/O pad  80 , the ESD detection circuit  86  will drive the control gate to a relative-low voltage to trigger the PSCR and conduct ESD current, thereby ESD stress is released and the inner circuit  82  is protected. FIG. 6 b  is an embodiment of FIG. 6 a . The ESD detection circuit  86  is composed of a resistor and a capacitor connected in series. The control gate of the pSCR is coupled to the connection node between the resistor and the capacitor in the ESD detection circuit  86 .  
         [0038]    The present invention also provides an ESD protection circuit for protecting an IC from ESD damage due to ESD events across power rails. FIG. 7 a  is a VDD-to-VSS ESD protection circuit employing the nSCR of the present invention. FIG. 7 b  is an embodiment of the VDD-to-VSS ESD protection circuit in FIG. 7 a . The anode and the dummy gate of the nSCR in FIG. 7 a  are coupled to VDD. The cathode of the nSCR is coupled to VSS. ESD detection circuit  90  is composed of an RC-base circuit and an inverter. The RC-base circuit is composed of a resistor and a capacitor connected in series and normally carries a time constant of about 0.1-1 microsecond to distinguish an ESD event from normal operation. The input of the inverter INV is connected to the connection node in the RC-base circuit while the output of the inverter INV is connected to the control gate of the nSCR. When a positive ESD pulse crosses the VDD and VSS is grounded, due to the RC time delay effect, the input of the inverter is temporally kept at a relativly low voltage to cause the inverter INV driving the control gate to a relative high voltage, such that the nSCR is triggered on to release ESD stress.  
         [0039]    The pSCR of the present invention can also be applied to be a major ESD protection device in a VDD-to-VSS ESD protection circuit, as shown in FIG. 8 a . FIG. 8 b  is an embodiment of the VDD-to-VSS ESD protection circuit in FIG. 8 a . The anode of the pSCR of the present invention is coupled to VDD. The cathode and the dummy gate of the pSCR are coupled to VSS. The ESD detection circuit  94  is composed of an RC-base circuit and two cascade inverters (INV 1  and INV 2 ). The RC-base circuit has a resistor and a capacitor connected in series and normally carries a time constant of about 0.1 to 1 microsecond. In FIG. 8 b , two cascade inverters INV 1  and INV 2  are used as an amplifier to drive the control gate of the pSCR according to the voltage at the connection node between the resistor R and the capacitor C.  
         [0040]    Finally, while the invention has been described by way of examples and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On 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.