Abstract:
In an SCR-based ESD protection clamp, the voltage overshoot during an ESD event is reduced by separately controlling the voltage pulse to the drain and emitter contacts of the SCR. The voltage pulse to the drain is preferably delayed using a delay circuit such as an RC circuit. This allows double conductivity modulation to be achieved with lower voltage overshoot.

Description:
FIELD OF THE INVENTION 
     The invention relates to a new semiconductor structure and sub-circuit design for an ESD protection clamp for CMOS, BiCMOS, BCD integrated circuits. In particular, it relates to a silicon controlled rectifier (SCR) such as a low voltage silicon controlled rectifier (LVTSCR) ESD protection clamp. 
     BACKGROUND OF THE INVENTION 
     Due to double injection conductivity modulation LVTSCRs handle approximately 10 times the pulse power after snapback compared to Grounded Gate NMOS (GGNMOS) or NPN BJT structures of similar size. One of their main benefits is the ability to support high current densities which allows them to be made smaller than GGNMOS or BJTs with similar current handling capabilities. This also has the effect of providing lower parasitic capacitance. This makes LVTSCRs promising devices for IC protection. 
     However, they also suffer from drawbacks such as low holding voltage which makes them susceptible to latch-up. They also display a high resistance between the floating drain and drain contact, resulting in low saturation current under normal operation. Thus they have limited application in self-protecting applications. Also, they require a high triggering current. Further, LVTSCRs provide high voltage overshoot. This is due to an internal NMOS and NPN BJT structure and high triggering current. Some of these characteristics are better understood when considering a typical LVTSCR structure.  FIG. 1  shows a dual gate LVTSCR, but the workings of the dual gate LVTSCR discussed below are applicable also to single gate structures. The following discussion of an LVTSCR also serves to define the terminology used for certain regions referred to in describing the invention in the detailed description of the invention section. The LVTSCR  100  includes a p-well  102 , a p+ region  104  acting as p-well contact region (also referred to as the bulk  104 ), source contact region  106 , a first polygate  108 , a second polygate  110 , a floating drain region  112 , a p+ emitter contact region  114 , a n+ drain contact region  116  and corresponding n-well isolation  118  of the emitter junction. The n+ floating drain  112  and p-well  102  define a blocking junction under the dual polygate  108 ,  110 .  FIG. 1  also shows a contact  130  to the bulk  104  and source contact region  106 . It also shows an emitter contact  132  to the emitter contact region  114 , and a drain contact  134  to the drain contact region  116 . 
     Triggering of the structure  100  is dictated by the breakdown voltage. The first stage involves avalanche breakdown of the blocking junction. When the voltage is sufficiently high for impact ionization to occur, the internal NPN BJT (defined by n+ source contact region  106 , p-well  102 , and n-well  118 ) triggers causing forward injection of carriers into the n-well  118 . This forward biases the junction between the p+ emitter contact region  114  and the n-well  118 , to switch on the PNP BJT defined by p+ emitter contact region  114 , n-well  118  and p-well  102 , which in turn, injects positive charge carriers into the p-well. These are largely swept across to the n+ source contact region  106 . The downside with this structure is that there is limited ability to control the triggering. While the gate voltage can be controlled to achieve some effect on triggering, this provides very limited control over the triggering of the structure. As a result Merrill clamps are often used. Merrill clamps, however, are highly space consuming. 
     SUMMARY OF THE INVENTION 
     The invention defines a SCR or LVTSCR with dynamically controlled blocking junction. The invention also includes a sub-circuit and method for dynamically controlling the blocking junction. For ease of use, the term SCR will be used for both SCRs generally and LVTSCRs, and although the detailed description specifically shows LVTSCRs, it is to be understood that SCRs could be used instead of the LVTSCRS. 
     According to the invention there is provided a SCR-based protection clamp for protecting a node of a circuit against ESD events, which includes a SCR with separately connected drain contact and emitter contact in which the drain and emitter contacts are connected to the node but the drain contact is connected to the node through a triggering control circuit. The triggering control circuit may include a delay circuit to delay a voltage pulse to the drain contact. The delay circuit may include an RC circuit, or an RC circuit and a dual inverter to present a lower load to the RC circuit. 
     Further, according to the invention, there is provided a method of controlling the triggering of a SCR that includes an anode, which includes a drain contact and an emitter contact, and a cathode, comprising controlling the respective times at which a voltage pulse applied across the anode and cathode is applied to the drain and emitter contacts. This may include delaying the voltage pulse to the drain contact using a delay circuit (e.g. by means of an RC circuit) relative to the emitter contact. The method may include providing a high input impedance to the SCR as seen by the RC circuit. This may include providing a double inverter between the RC circuit and the SCR. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a typical dual gate LVTSCR known in the art, 
         FIG. 2  is one embodiment of a LYTSCR-based circuit of the invention, 
         FIG. 3  is another embodiment of a LVTSCR-based circuit of the invention, 
         FIG. 4  shows a set of drain voltage versus time graphs for different capacitor values of the embodiment of  FIG. 2 , 
         FIG. 5  is a set of drain current versus time graphs for different capacitor values of the embodiment of  FIG. 2 , and 
         FIG. 6  is a sectional view of a typical SCR known in the art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows one embodiment of the invention in which the LVTSCR  200  includes an anode  202  and a cathode  204 . The anode  202  is connected to a pad  206  which is connected to VDD in this embodiment, and the cathode  204  is connected to ground. Both the drain contact  210  and emitter contact  212  of the LVTSCR  200  are connected to the pad  206  and thereby define the anode of the structure. The source contact  214  and bulk contact  216  are connected to ground. As can be seen in  FIG. 2 , the drain contact is not connected directly to the pad  206  but is connected to an RC circuit, comprising a resistor  220  and a capacitor  222 . This has the effect of delaying a voltage pulse applied to the pad. Therefore, a voltage pulse, such as an ESD pulse will first appear at the emitter through the emitter contact  212  before appearing at the drain through the drain contact  210 , since the RC circuit will delay the voltage pulse to the drain. Thus, initially, before the capacitor  222  charges up, the drain contact or drain electrode potential is held close to ground. As a result the LVTSCR emitter is opened and injection starts earlier. If the capacitor  222  is big enough, the injection of carriers from the emitter is large enough to bring the LVTSCR into double injection conductivity mode, thereby reducing voltage overshoot. 
     This can be seen in  FIGS. 4 and 5 . For a 100 pF capacitor, the voltage peak is only about 6 V (curve  400 ) compared to about 15 V without a capacitor (curve  408 ). As the capacitor is increased from 1 pF to 10 pF to 50 pF to 100 pF (curves  406 ,  404 ,  402 ,  400 , respectively), the voltage overshoot can clearly be seen to decrease. Corresponding current curves for capacitors of 10 pF, 50 pF, 100 pF can be seen in  FIG. 5  (curves  500 ,  502 ,  504 , respectively). 
     In order to reduce the size of the capacitor that is needed to achieve the desired drop in voltage overshoot, another embodiment includes a double inverter, thereby presenting the RC circuit with a high input impedance. This is shown in  FIG. 3  where a first inverter  300  and a second inverter  302  are provided between the RC circuit  304  and the LVTSCR  306 . The first inverter  300  includes a PMOS transistor  310  and a NMOS transistor  312 , and the second inverter  302  includes a PMOS transistor  320  and a NMOS transistor  322 . The RC circuit  304  includes a resistor  330  and a capacitor  332 . Typically the second inverter  302  is larger than the first inverter  300  and can have a contact width of 100 μm to 1000 μm compared to the first inverter which may have a contact width of 1 μm to 10 μm. While the embodiment of Figure three involved the use of two inverters, it will be appreciated that any even number of inverters could be used. In yet another embodiment, the positions of the resistor and capacitor were reversed to have the capacitor at the top. In such an embodiment the drain will be initially held high and then be pulled down as the capacitor charges up. Thus, to achieve the delay of the voltage pulse to the drain contact, any odd number, e.g., 1 inverter, was used with the RC circuit. 
     It was found that the use of a double inverter as in  FIG. 3  provided similar voltage overshoot with a small RC circuit of 1 pF capacitor and 100 k resistor as was achieved using a large 100 pF capacitor without a double inverter. 
     In both embodiments, however, a clear benefit is achieved by separately controlling the drain and emitter to delay the voltage pulse to the drain. Under normal operation the capacitor will be fully charged. Thus the potential on the drain electrode will be equal to the emitter potential. This ensures low leakage operation. 
     While the embodiments described above dealt specifically with LVTSCRs, the invention also applies to the use of SCRs in general. For ease of claiming, the term SCR will be used generally to refer to any kind of SCR, including LVTSCRs. For completeness,  FIG. 6  shows an SCR  600  with its drain  602  and emitter  604  formed in n-well  606 , source  608 , and bulk  610 . It will also be appreciated that the invention can be implemented using different delay circuits other than RC circuits. In fact the invention could make use of any timer circuit to control the voltage pulse to the drain contact. For example, the timer circuit could comprise a mixture of NMOS and BJT devices. It could also be a more complex circuit with amplification that delivers a dynamic low potential on the LVTSCR anode n-region (drain contact region) to create a temporary condition for biasing the junction in a forward direction during the ESD transient pulse (˜10–100 ns) and then bring the region potential to Vdd