Abstract:
An electrostatic discharge (ESD) power clamp circuit including a RC-delay element coupled to a plurality of serialized inverter elements with a power clamp element and an ESD-triggered keeper device coupled to the plurality of inverters. During negative mode ESD events, the ESD-triggered keeper device is activated and assists the power clamp element to pull up and strongly conduct current to protect the circuit. Additionally, a method of ESD protection in a circuit is provided. The method includes coupling a RC-delay element to an input of a plurality of serialized inverter elements, coupling an output of the plurality of serialized inverters with an ESD-triggered keeper device and a power clamp element, triggering the ESD-triggered keeper device to turn on during negative ESD events and conducting current by the power clamp element with assistance of the ESD-triggered keeper device to protect the circuit as a result of negative ESD events.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to electrostatic discharge (ESD) protection circuits. In particular, the present invention is directed to a RC-triggered power clamp capable of suppressing negative mode ESD stress. 
   BACKGROUND OF THE INVENTION 
   Electrostatic discharge (ESD) is a momentary and sudden electric current that flows when an excess of electric charge, stored on an electrically insulated structure, finds a path to a structure at a different electrical potential, such as ground. ESD is particularly a serious concern with microelectronic devices. The integrated circuits (IC) in these devices are made from semiconductor materials, such as silicon, and insulating materials, such as silicon dioxide, which can break down when exposed to high voltages. 
   ESD stress occurs in two modes: positive mode ESD and negative mode ESD. Positive mode ESD occurs when there is positive ESD stress at voltage supply (VDD) and ground line is at GND (ground) or when ground line is at VDD and there is negative ESD stress at GND. Negative mode ESD occurs when there is negative ESD stress at VDD and ground line is at GND or when ground line is at VDD and there is positive ESD stress at ground line. 
   Conventional ESD protection may be integrated onto chips using two types of snapback MOSFET-based strategies: gate-grounded NMOSFET (GGNMOS) and gate-tied to VDD PMOSFET (GVPMOS). These snapback strategies trigger the snapback mechanism to conduct the large amount of ESD current for ESD protection. Both snapback strategies use a RC-triggered power clamp to protect the whole chip from ESD events. These snapback strategies are effective for positive mode ESD stress where the conventional RC-triggered power clamps are fully on during positive mode ESD stress. However, these RC-triggered power clamps typically turn on only weakly and perform poorly during negative mode ESD stress events. Designing to suppress negative mode ESD stress is a major design concern for silicon on insulator (SOI) technology, with floating-body devices. 
   Another strategy for negative mode ESD protection is to add a parasitic diode into the circuit. However, such parasitic diodes occupy valuable silicon area on a chip and are not optimized for ESD stress. Moreover, parasitic diodes are often not found in floating-body devices. 
   SUMMARY OF THE DISCLOSURE 
   In one aspect, an electrostatic discharge (ESD) power clamp circuit is disclosed. The circuit comprises a RC-delay element coupled to a plurality of serialized inverter elements, a power clamp element and an ESD-triggered keeper device coupled to the plurality of inverters. At least one of the power clamp element and the ESD-triggered keeper device is activated as a result of an ESD event. 
   The disclosure also provides an ESD power clamp circuit. The circuit comprises a RC-delay element coupled to a plurality of serialized inverter elements having an output, an ESD-triggered keeper device and a power clamp element coupled to the output of the plurality of inverters. The ESD-triggered keeper device is designed to turn on during a negative mode ESD event with the result that the ESD-triggered keeper device assists the power clamp element to pull up and strongly conduct current to protect the circuit. 
   The disclosure also covers a method of ESD protection in a circuit. The method comprises coupling a RC-delay element to an input of a plurality of serialized inverter elements, coupling an output of the plurality of serialized inverters with an ESD-triggered keeper device, triggering the ESD-triggered keeper device to turn on during a negative ESD event, and conducting current by the power clamp element with assistance of the ESD-triggered keeper device to protect the circuit as a result of the negative ESD event. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments. However, it should be understood that the present disclosure is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
       FIG. 1  illustrates a conventional power clamp circuit; 
       FIG. 2  illustrates a RC-triggered power clamp circuit in accordance with one embodiment of the present disclosure; 
       FIG. 3  illustrates a RC-triggered power clamp circuit in accordance with another embodiment of the present disclosure; 
       FIG. 4  illustrates a RC-triggered power clamp circuit in accordance with yet another embodiment of the present disclosure; and 
       FIG. 5  illustrates a RC-triggered power clamp circuit in accordance with still another embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION 
   Referring now to the figures,  FIG. 1  illustrates a conventional power clamp circuit  6 . Power clamp circuit  6  includes an RC-delay or triggering element  22  having a resistor  30  connected in series with a capacitor  32 . RC-delay element  22  is connected in series with a plurality of serialized inverters  24  connected in series with a power clamp element  28 . Power for circuit  6  is typically supplied by a voltage supply referred to as VDD with ground represented by GND. During all ESD events, inverters  24  trigger on power clamp element  28  to conduct current. Power clamp circuit  6  is generally designed for positive mode ESD events. However, power clamp circuit  6  conducts poorly during negative mode ESD events. 
     FIG. 2  illustrates one embodiment of the disclosure showing a floating-body NFET-based power clamp circuit  20  for detecting and controlling both negative and positive modes of electrostatic discharge (ESD) stress. Circuit  20  protects against all types of ESD stress events in accordance with a human-body model (HBM), machine model (MM), and charged-device model (CDM), as well known in the art. 
   Power clamp circuit  20  includes an RC-delay or triggering element  22  having a resistor  30  connected in series with a capacitor  32  at node  38 . RC-delay element  22  is connected in series with a plurality of serialized inverters  24 . It should be noted, that any commercial or conventional RC-delay circuit and inverter could be utilized without departing from the scope and spirit of the present disclosure. Inverters  24  are connected in series with an ESD-triggered keeper device  26  and a power clamp element  28 . Power for circuit  20  is typically supplied by a voltage supply referred to as VDD, which has a voltage level dependent on the process used, with ground represented by GND. 
   In the embodiment illustrated in  FIG. 2 , plurality of serialized inverters  24  includes a first inverter  24 ′ connected in series with a second inverter  24 ″ connected in series with a third inverter  24 ′″. The number of inverters utilized in plurality of inverters  24  is variable according to the application requirements. It should be noted that less or more inverters may be utilized without departing from the scope and spirit of the disclosure. Plurality of inverters  24  may be implemented using CMOS inverters, as illustrated in  FIG. 2 . CMOS inverters  24 ′- 24 ′″ each include a floating-body PFET element  34  chained to a floating-body NFET element  36 . The gate terminals of PFET  34  and NFET  36  of first inverter  24 ′ are connected to RC-delay element  22  at node  38 . The drain terminals of PFET  34  and NFET  36  of third inverter  24 ′″ are connected to a source terminal of ESD-triggered keeper device  26  and a gate terminal of power clamp element  28 . ESD-triggered keeper device  26 , also referred to as an ESD-triggered pull up device, may include a floating-body PFET element. Power clamp element  28 , also referred to as the main conducting device or big FET, may include a large floating-body NFET element. 
   During normal operation and positive mode ESD events, ESD-triggered pull up device  26  is turned off and has no effect on circuit  20 . Positive mode ESD events generally occur where a fast rise voltage/current pulse is applied onto VDD causing the voltage across capacitor  32  to remain at zero. The voltage at node  38  is at a relatively low voltage level, “LOW.” “HIGH” and “LOW” voltage levels are relative to the voltage level to switch inverter  24 . “HIGH” would be in the range of VDD to VDD minus VT, and LOW would be in the range of 0 to VT, where VT is the threshold voltage of the MOSFET. The gate terminal of power clamp element  28  is at voltage level “HIGH.” Inverters  24  trigger on power clamp element  28  to conduct current. After the delay provided by RC-delay element  22  ends, which generally lasts as long as the ESD event, capacitor  32  is charged to higher than a threshold voltage of inverters  24 , which are at voltage level “HIGH.” Inverters  24  then switch and trigger the gate terminal of power clamp element  28  to voltage level “LOW.” Power clamp element  28  then turns off. 
   When a fast rise voltage/current pulse is applied onto GND, a negative mode ESD event occurs in floating-body power clamp circuit  20 . The voltage across capacitor  32  remains at zero. The voltage at node  38  is at voltage level “HIGH.” The output of inverter  24 ′ is at voltage level “HIGH-VT,” where VT is a threshold voltage of the NFET of the inverter. The outputs of inverters  24 ″ and  24 ′″ are at voltage levels “HIGH-2VT” and “HIGH-3VT,” respectively. Because the output of inverter  24 ′″ is connected to the gate terminal of power clamp element  28 , as discussed above, power clamp element  28  turns on weakly and conducts current poorly during negative mode ESD events if a keep device  26  is absent. However, since the gate terminal of power clamp element  28  is also connected to the source terminal of keeper device  26 , and the keeper device (PMOSFET) is turned on given its gate at “LOW” and its drain at “HIGH”, it acts as a pull-up device to pull the gate terminal of  28  to “HIGH”, and strongly turns on power clamp element  28 . After the delay provided by RC-delay element  22  ends, capacitor  32  is charged so that the voltage at node  38  is at voltage level “LOW.” Inverters  24  then switch and trigger the gate terminal of power clamp element  28  to voltage level “LOW.” The negative mode ESD event triggers keeper device  26  to turn on. Keeper device  26  assists power clamp element  28  to pull up and strongly conduct current. 
   ESD-triggered keeper device  26  boosts the performance of circuit  20  by clamping the circuit to lowest voltage during negative mode ESD stress events. Conventional circuits generally require the addition of a parasitic diode for protection during negative mode ESD events. Keeper device  26  relaxes the requirement for adding a parasitic diode or additional ESD diode for negative mode ESD stress protection. Such diodes occupy valuable space on a chip. Moreover, a parasitic diode is often not optimized for negative mode ESD stress and floating-body devices generally do not utilize parasitic diodes. 
   In another embodiment, a floating-body PFET-based power clamp circuit  100  is illustrated in  FIG. 3 . Circuit  100  includes an RC-delay or triggering element  122  having a resistor  130  connected in series to a capacitor  132  at a node  138 . As previously stated, any conventional or commercial RC-delay circuit can be employed while keeping with the scope and spirit of the disclosure. In addition, circuit  100  includes a plurality of serialized inverters  124  connected in series with RC-delay element  122 , an ESD-triggered keeper device  126  and a power clamp element  128 . A power supply for circuit  100  is typically supplied by a voltage supply referred to as VDD. 
   In this embodiment, power clamp element  128  includes a floating-body PFET element for conducting current during normal operation and positive mode ESD events. ESD-triggered keeper device  126  includes a floating-body NFET element for helping power clamp element  128  strongly conduct current during negative mode ESD stress events. Each inverter  124 ′ and  124 ″ includes a floating-body PFET element  134  chained to a floating-body NFET element  136 . 
   During a negative mode ESD stress event, circuit  100  performs in a similar manner to the example described above for floating-body NFET-based circuit  20 , as well known to one of ordinary skill in the art. However, keeper device  126  is configured with a floating-body NFET element with the gate terminal at GND and the drain terminal at VDD to complement the floating-body PFET element of power clamp element  128 , such that keeper device  126  pulls up during negative mode ESD stress to assist power clamp element  128 . 
   In yet another embodiment, a tied-body NFET-based power clamp circuit  200  is illustrated in  FIG. 4 . Circuit  200  includes an RC-delay or triggering element  222  having a resistor  230  connected in series to a capacitor  232  at a node  238 . RC-delay element  222  is connected in series to a plurality of serialized inverters  224 , which is connected in series an ESD-triggered keeper device  226  and a power clamp element  228 . A power supply for circuit  200  is typically supplied by a voltage supply referred to as VDD. 
   In the embodiment illustrated in  FIG. 4 , power clamp element  228  includes a tied-body NFET-based element with the body tied to the source terminal for conducting current during normal operation and positive mode ESD events in a similar manner to the example described above for power clamp circuit  20  as well known in the art. Keeper device  226  assists power clamp element  228  to pull up and strongly conduct current during negative mode ESD stress events. Each inverter  224 ′,  224 ″ and  224 ′″ includes a tied-body PFET element  234  chained to a tied-body NFET element  236 . 
   When a fast rise voltage/current pulse is applied onto GND, a negative mode ESD event occurs in tied-body NFET-based power clamp circuit  200 . The voltage across capacitor  232  of RC-delay element  222  remains at zero. The voltage at node  238 , between resistor  230  and capacitor  232 , is at voltage level “HIGH.” The output of first inverter  224 ′ is at voltage level “HIGH-VT,” where VT is the threshold voltage of NFET of the inverter. The output of second inverter  224 ″ is at voltage level “HIGH-2VT.” The output of third inverter  224 ′″ is voltage level “HIGH-3VT.” The output of third inverter  224 ′″ is connected to the source terminal of keeper device  226  and the gate terminal of power clamp element  228 . Keeper device  226  turns on and assists power clamp element  228  to pull up and strongly conduct current during the negative mode ESD event. 
   In still another embodiment, a tied-body PFET-based power clamp circuit  300  is illustrated in  FIG. 5 . Circuit  300  includes an RC-delay or triggering element  322  having a resistor  330  connected in series to a capacitor  332  at a node  338 . RC-delay element is connected in series with a plurality of serialized inverters  324 , which are connected in series with an ESD-triggered keeper device  326  and a power clamp element  328 . A power supply for circuit  300  is typically supplied by a voltage supply referred to as VDD. 
   In this embodiment, power clamp element  328  includes a tied-body PFET-based element with the body tied to the source terminal for conducting current during normal operation and positive mode ESD events. ESD-triggered keeper device  326  includes a tied-body NFET-based element with the body tied to the drain terminal for conducting current during negative ESD stress events. Each inverter  324  includes a tied-body PFET element  334  chained to a tied-body NFET element  336 . 
   During positive and negative mode ESD stress events, circuit  300  performs in a similar manner to the example described above for tied-body NFET-based power clamp circuit  200 , as is well known to one of ordinary skill in the art. Keeper device  326  is configured with a tied-body NFET element with the gate terminal at GND to complement the tied-body PFET-based element of power clamp element  328  such that the keeper device pulls up for negative mode ESD stress events to assist the power clamp element. 
   Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.