Abstract:
In a ESD protection device, hot carrier degradation and soft leakage are reduced by introducing a dynamic driver that includes a RC circuit for keeping the triggering circuit of the ESD device in an on-state for a certain period of time. This allows the current through the ESD protection device to be reduced during the RC delay time.

Description:
FIELD OF THE INVENTION 
     The invention relates to ESD protection devices and their protection against hot carrier degradation. 
     BACKGROUND OF THE INVENTION 
     In order to avoid damage to electronic circuits during electrostatic discharge (ESD) protection circuits have been devised to shunt current to ground and limit exposure to excessive voltages. A variety of devices have been developed including grounded gate NMOS, LVTSCR, and BJT triggering clamps. BJTs typically rely on avalanche breakdown and may be enhanced through the use of zener diodes to trigger the base of the BJT. These triggering clamps typically have relatively small dimensions with an active device width of 100-500 mm. However, they typically have high-triggering voltage levels and display breakdown characteristics that are process sensitive and in most cases, result in degradation, especially after exposure to multiple ESD pulses. 
     Normal operation mode clamps such as Merrill clamps are also known. These differ from triggering clamps insofar as they do not rely on avalanche injection conductivity modulation, but instead make use of a driver to be switched on. These CMOS normal operating mode clamps display large dimensions of the order of 10-20 mm since they have to deal with normal operating conditions where the current density is much lower. They find use in power devices, involving a whole chip design approach with rail protection strategies and wide metal buses where the increase in size due to the space consuming devices is less significant. On the other hand, they have the benefit of displaying excellent ESD protection characteristics. 
     A particular problem experienced by triggering clamps is so-called hot carrier degradation (HCD) and soft leakage. HCD has been ascribed to post-ESD stress caused by residual high voltage levels after triggering off. These residual high voltage levels approximately equal the triggering or breakdown voltage of the triggering structure. In the case of low leakage circuits, these voltage levels may be stored for a long time thereby causing long term overload, causing HCD of the gate oxide. 
     Examples of the prior art triggering clamps are illustrated in  FIGS. 1-4  where  FIG. 1  shows a grounded gate NMOS (GGNMOS) device  10  in which the gate is connected to ground via a resistor  12 .  FIG. 2  makes use of a low voltage triggering silicon controlled rectifier (LVTSCR)  20 .  FIG. 3  shows the use of a NPN BJT  30  that makes use of avalanche breakdown to provide ESD protection.  FIG. 4  also makes use of an NPN transistor  40 , but is supplemented with a zener diode  42  which feeds base current into the NPN transistor  40 . All of these triggering circuits make use of snapback characteristics of the devices. 
     The present invention seeks to address the problem of HCD and soft leakage increase displayed by triggering clamps due to multiple ESD spikes. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a dynamic driver is introduced to hold the controlling electrode of the triggering device in its on-state for a defined period of time by virtue of an RC circuit. 
     Further, according to the invention, there is provided a method of reducing hot carrier degradation in an ESD protection circuit, comprising the use of a dynamic driver in conjunction with a triggering clamp. The triggering clamp may be a GGNMOS, LVTSCR, or BJT device. Typically the dynamic driver includes a RC circuit. Thus the reduction in HCD is achieved by decreasing the current during an RC delay time. 
     Further, according to the invention there is provided a method of achieving early triggering in a triggering ESD protection device, comprising providing a RC circuit connected to a controlling electrode of the triggering device. This reduces the triggering voltage, thus providing early triggering. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-4  are schematic circuit diagrams of prior art triggering clamps; 
         FIG. 5  is a schematic circuit diagram of one embodiment of the invention; 
         FIG. 6  is a schematic circuit diagram of another embodiment of the invention; 
         FIG. 7  is a schematic circuit diagram of yet another embodiment of the invention; 
         FIG. 8  is a schematic circuit diagram of yet another embodiment of the invention; 
         FIG. 9  is a schematic circuit diagram of yet another embodiment of the invention; 
         FIG. 10  is a schematic circuit diagram of yet another embodiment of the invention; 
         FIG. 11  shows voltage curves for a NMOS device used in accordance with the invention; 
         FIG. 12  shows voltage curves for a LVTSCR used in accordance with the invention, and 
         FIG. 13  shows voltage curves for a BJT used in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 5  shows one implementation of the invention, in which a NMOS device  50  is connected between V dd  and V SS  and has its gate connected to ground through a resistor  52 . The NMOS device  50  serves as the triggering circuit, the controlling electrode of which is held in an on-state during a RC time defined by a driver circuit that includes a resistor  54  and a capacitor  56 . The NMOS device  50  is connected to the RC circuit through a PMOS device  58  and the resistor  52 . Initially, the dynamic driver presents an essentially discharged capacitor  56 . The resistor  54  and capacitor  56  are chosen to provide a delay time that is greater than the duration of a ESD pulse. During the voltage increase of the ESD pulse, the triggering structure remains in an open channel state. This results in early triggering since the triggering voltage is decreased due to the increase of the gate bias. Notwithstanding this benefit of a decreased triggering voltage, the operating characteristics of the circuit remain substantially unaffected. The triggering structure of the invention provides similar operation to a conventional circuit during the major part of the ESD stress (approximately 150 ns). In addition, the present invention provides for a current decrease at the back end of the ESD pulse, as dictated by the RC delay time of the driver circuit of the invention. This causes the current gradually to decrease down to the triggering off condition. However, the triggering structure (NMOS device  50 ) initially remains in a conductive state, thus discharging the rest of the ESD pulse during the RC delay time. This is illustrated in  FIG. 11  by curves  110  and  112 . Curve  110  shows the gate voltage while curve  112  shows the voltage across the clamp (the voltage across the drain and source of the NMOS device  50 .) 
       FIG. 6  shows another implementation of the invention in which a LVTSCR  60  is connected to the driver circuit, which includes an RC circuit (comprising a resistor  64  and a capacitor  66 ) connected to a PMOS device  68 . The output of the PMOS device  68  is connected to the gate of the LVTSCR  60  and to ground through a resistor  62 . The gate voltage curve  120  and clamp voltage curve  122  are shown in  FIG. 12  for the embodiment of  FIG. 6 . 
     Yet another embodiment of the invention is illustrated in  FIG. 7  in which the triggering device is a BJT in the form of a NPN transistor  70 . The driver circuit is similar to those in the embodiments of  FIGS. 5 and 6 , and comprises a RC circuit consisting of a resistor  74  and the capacitor  76 . These are connected to the gate of a PMOS device  78 , the output of which is connected through a resistor  72  to ground and to the base of the NPN transistor  70 .  FIG. 13  shows the base voltage curve  130  and the clamp voltage  132  for the device illustrated in  FIG. 7 . 
       FIGS. 8 ,  9 , and  10  illustrate further embodiments of the invention in which clamp inverters are used for triggering the snapback circuits. While the  FIG. 8  embodiment may appear similar to a Merrill clamp when represented as a schematic, it is structurally and functionally quite different. The triggering device  80  is a snapback NMOS device which is 10-100 times smaller than a normally operating NMOS device used in a Merrill clamp. Due to the fact that bipolar avalanche-injection conductivity modulation is involved in the snapback device of the present invention, a device of 100-400 um size, instead of 10-20 mm, is achieved. In  FIG. 9  the snapback circuit is a LVTSCR  90 , and in  FIG. 10  the snapback circuit is a BJT in the form of a NPN transistor  100 . The RC circuits comprise resistors  82 ,  92 ,  102  and capacitors  84 ,  94 ,  104 . The inverter comprises a PMOS-NMOS pair comprising PMOS transistors  86 ,  96 ,  106  and NMOS transistors  88 ,  98 ,  108 . 
     The effect of the dynamic driver of the present invention, is to provide residual conductivity of the protection structure, whether that be a GGNMOS, a LVTSCR, or an enhanced zener bi-polar clamp, thereby discharging the circuit capacitance below the residual voltage after triggering off once the ESD pulse has passed. 
     The effect of this is shown in the curves of  FIGS. 11 ,  12  and  13 . The prior art curves  114 ,  124  for the voltage across the gate of a conventional clamp goes down to zero volt very quickly, thus preventing discharge of the rest of the ESD pulse during the total RC delay time. This is evident from the large voltages across the clamp as indicated by the curves  116 ,  126  after long delay times. In contrast, curves  110 ,  120 ,  130  remain above zero V for substantial periods of time, thereby keeping the triggering structure in conduction as indicated by the clamp voltage (curves  112 ,  122 ,  132 ) for the dynamic structure of  FIGS. 5 ,  6 , and  7 , respectively, and curves  118 ,  128  for the structures of  FIGS. 8 and 9 , respectively. 
     It will be appreciated that the embodiments illustrated in  FIGS. 5-10  are by way of example only. Different triggering clamps can be used with different dynamic drivers that have the effect of holding the triggering electrode of the triggering clamp in an on-state for an extended period of time.