Abstract:
The present invention provides an electrostatic discharge (ESD) protection circuit with a silicon controlled rectifier (SCR) having a plurality of SCR fingers (SCRs) with the advantages to couple the different fingers or SCRs to decrease the multi-triggering problem and to increase the ESD-performance of the circuit. Additionally, a boost circuit can be introduced or additionally multiple SCRs can be coupled inherent through a common base.

Description:
CROSS REFERENCES  
       [0001]     This patent application claims the benefit of U.S. Provisional Application Ser. No. 60/666,445 filed Mar. 30, 2005, the contents of which are incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention generally relates to the field of electrostatic discharge (ESD) protection circuitry, and more specifically, improvements for silicon controlled rectifier (SCR) structures in the protection circuitry of an integrated circuit (IC).  
       BACKGROUND OF THE INVENTION  
       [0003]     Integrated circuits (IC&#39;s) and other semiconductor devices are extremely sensitive to the high voltages that may be generated by contact with an ESD event. As such, electrostatic discharge (ESD) protection circuitry is essential for integrated circuits. An ESD event commonly results from the discharge of a high voltage potential (typically, several kilovolts) and leads to pulses of high current (several amperes) of a short duration (typically, 100 nanoseconds). An ESD event is generated within an IC, illustratively, by human contact with the leads of the IC or by electrically charged machinery being discharged in other leads of an IC. During installation of integrated circuits into products, these electrostatic discharges may destroy the IC&#39;s and thus require expensive repairs on the products, which could have been avoided by providing a mechanism for dissipation of the electrostatic discharge to which the IC may have been subjected.  
         [0004]     In order to protect against these over-voltage conditions, protection devices such as silicon controlled rectifiers (SCR) or MOS devices have been incorporated within the circuitry to provide a discharge path for the high current produced by the discharge of the high electrostatic potential. Prior to an ESD event, the protection device is in a non-conductive state. Once the ESD event is detected, the protection device then changes to a conductive state to shunt the current to ground. The protection device maintains this conductive state until the voltage is discharged to a safe level.  
         [0005]     When protecting an IC (Integrated Circuit) against ESD (Electro Static Discharge) stress, the classic approach is to use a number of independently triggered placed power clamps (PC 1  to PCn).  FIG. 1A  shows an illustration of the classical approach in which four independently triggered power clamps PC 1   120 , PC 2   122 , PC 3   124 , and PC 4   126  are used. PC 1   120  and PC 3   124  are placed in a Vdd  128  power pad cell, and PC 2   122  and PC 4   126  are placed in a Vss  130  ground pad cell. In between the power and ground pad cells the power and ground busses have a certain amount of bus resistance RVdd  132  and RVss  134 . When using this approach, the voltage over an I/O or core element is not only dependent on the characteristics of the clamps itself but also on the bus resistance between this element and the clamp. Usually the ESD designer assumes a worst case scenario in which only one clamp triggers, and in which this clamps will take all the current. Because one cannot be sure which and how many clamps will trigger, this is a necessary approach. However, this approach leads to less area efficient, and sometimes over dimensioned or unrealistically big ESD protection device sizes. Especially in technologies which have a decreased ESD design windows, increased dynamic on resistance of protection devices and decreased heat dissipation characteristics of the active silicon film, the need to ensure more clamps to trigger is high.  
         [0006]     When using the classical approach of uncoupled clamps, triggering of multiple clamps highly depends on the Vt2/Vt1 relationship, with Vt2 the failure voltage and Vt1 the trigger voltage of the clamp, and the resistance of the busses in between the individual clamps. Imagine e.g. in  FIG. 1A  that we stress the Vdd 1   128  pin positively to Vss 2   130 . In such a case all four clamps have theoretically the same chance of triggering.  
         [0007]     Let&#39;s assume that PC 1   120  triggers first. Then the voltages over the other clamps are given by: 
 
 V   PC 2= V   PC 1+ R   VSS*I  
 
 V   PC 3= V   PC 1+ V   SS*I  
 
 V   PC 4= V   PC 1+ 2*R   Vss*I  
 
 V   PC 4= V   PC 1+ 3*R   Vss*I  
 
         [0008]     Note that I is the current and V is the voltage with * being multiplication sign. Also,  V PC 1 ,  V PC 2 ,  V PC 3  AND  V PC 4  are voltages at clamps  1 ,  2 ,  3  and  4  respectively. Clamps PC 2   122  PC 3   124  and PC 4   126  will also trigger when the following voltage relationship becomes true: 
 
 V PC2&gt; V t1 
 
 V PC3&gt; V t1 
 
 V PC4&gt; V t1 
 
         [0009]     The clamp, PC 4   126  which is closest to the ground pad has the biggest chance to trigger next. However, whether PC 4   126  and other clamps will trigger depends greatly on two factors. First, Vt2 being greater than Vt1 or not, and the second on the amount of bus resistance between the different clamps.  
         [0010]     Now let&#39;s assume another case (again positive stress from  V dd1 to  V ss2) in which power clamp PC 2   122  first triggers. Then the voltages over the other claims are given by: 
 
 V   PC 1= V   PC 2+ V   dd*I  
 
 V   PC 3= V   PC 2+ R   Vss*I  
 
 V PC4= V   PC 2+ 2*R   Vss*I  
 
         [0011]     Clamps PC 1   120 , PC 3   124  and PC 4   126  will also trigger when the following voltage relationship becomes true: 
 
 V PC1&gt; V t1 
 
 V PC3&gt; V t1 
 
 V PC4&gt; V t1 
 
         [0012]     The clamp PC 4   128 , which is closest to the ground pad has the biggest chance to trigger next. However, whether PC 4   126  and other clamps will trigger depends greatly on two factors. First, Vt2 being great than Vt1 or not, and second on the size of the bus resistance between the different clamps.  
         [0013]     Especially when Vt2&gt;Vt1, we can more safely assume that more than one clamp will take the current and in such a case the individual clamps can be downsized. However, many technologies produce ESD protection devices which have a deep snapback, and which have Vt2&lt;Vt1. In such cases, we cannot assume that multiple clamps will take the ESD discharge current. Moreover, in technologies which are characterized by a low heat dissipating efficiency (low It2), high Ron, and decreased ESD design windows (decreased GOX breakdown voltages), the demand to couple the ESD is high. In such a case one needs to ensure or initiate the simultaneous triggering of multiple clamps to ensure multi-clamp triggering.  
         [0014]     This problem isn&#39;t limited to different clamps but it&#39;s also possible in one clamp that consists of many separate fingers.  FIG. 1B  depicts a schematic diagram of a prior art multi-fingered SCR ESD protection circuit  100 , which serves as protection circuitry for an integrated circuit (not shown). The circuit  100  having multiple SCR fingers, and is illustratively depicted in  FIG. 1B  having three SCR “fingers”  102 ,  104  and  106 . Each finger works as a separate clamp, but is layouted as one whole clamp. The SCR protection circuit  100  comprises a first trigger device  108 , a first SCR  102  (i.e. “first finger”), a second SCR  104  (i,e, “second finger”) and a third SCR  106  (i.e. “third finger”). The first SCR  102  further comprises PNP transistor and an NPN transistor. In particular, the first SCR  102  includes an anode  108 , which is connected to a pad (not shown) and to one side of a resistor  114 . The resistor  114  represents the resistance of the N-well (or an external resistor), which is seen at the base of the PNP transistor of the SCR  102 . Also, included is a cathode  112  which is connected to a ground (not shown) and to one side of a resistor  110 . The resistor  110  represents the resistance of the P-well (or an external resistor) which is seen at the base of NPN transistor. The second and third SCRs  104  and  106  are formed exactly in the same manner as described with regard to the first SCR  102 . When SCRs  102 ,  104 ,  106  are placed in parallel as shown in  FIG. 1B  multifinger triggering is a potential issue. The typical solution is to connect a first triggering device G 1   116  and/or a second triggering device G 2   118 , as shown in  FIG. 1B  such that the voltage drop seen by all anode/G 2  respectively G 1 /Cathode diodes is the same. However, when the SCR goes into high injection mode, the structure acts like a PIN diode, such that the G 1  and G 2  taps do not control the voltage at the Nwell/Pwell junction anymore. This renders the multifinger triggering solution of connection the gates of the different SCRs ineffective. Therefore, there is a need in the art for a multi-fingered SCR protection device having an enhanced and reliable triggering mechanism.  
         [0015]     A SCR in its basic form is depicted as a prior art in  FIG. 1C  with an anode  136  and cathode  138 . It is regarded as a PNPN structure, formed by P+, N− well, P-substrate and N+. When using SCR&#39;s to protect a chip against ESD, one SCR is needed for each possible current path. As seen in  FIG. 1C , each SCR takes some area to implement. The large number of clamps (each current path needs its own clamp) increases the needed area for the ESD protection. So, there is a need in the art to incorporate different clamps into one clamp and also to couple these clamps to overcome the disadvantages of the prior art.  
       SUMMARY OF THE INVENTION  
       [0016]     The disadvantages heretofore associated with the prior art are overcome by one embodiment of the present invention of an electrostatic discharge (ESD) protection circuit comprising at least a clamp having at least one first anode coupled to a first voltage potential and at least one first cathode coupled to a second voltage potential. Also included is at least a second clamp having at least one second anode coupled to a third voltage potential and at least one second cathode coupled to the fourth voltage potential. In the preferred embodiment the clamps are scr&#39;s. In this case the first and second cathodes have at least one first high-doped region and the first and second anodes have at least one second high-doped region. The circuit further includes at least one first trigger tap disposed proximate to the first high-doped region of the first cathode and at least one second trigger-tap disposed proximate to the first high-doped region of he second cathode. Additionally, at least one first low ohmic connection is coupled between the first and second trigger-tap to connect the first and second silicon controlled rectifiers.  
         [0017]     In another embodiment of the present invention, an electrostatic discharge (ESD) protection circuit is provided comprising a silicon controlled rectifier (SCR) having a plurality of SCR fingers. Each SCR finger includes an anode and cathode. A boost circuit is connected to the anode or cathode. The scr comprises at least one first trigger-tap. Additionally, at least one first low-ohmic connection is respectively coupled between the at least one trigger tap of each SCR finger.  
         [0018]     In even further embodiment of the present invention, an electrostatic discharge (ESD) protection circuit is provided in a semiconductor integrated circuit comprising at least a first silicon controlled rectifier including at least one first region having a first conductive type formed in a second region having a second conductive type opposite to the first conductive type and at least one third region having a second conductive type formed in a fourth region having a first conductive type, said first region type coupled to a first voltage potential and said third conductive element coupled to a second voltage potential The circuit further comprises at least a second silicon controlled rectifier including at least one fifth region having a first conductive element formed in a sixth region having a second conductive type and at least one seventh region having a second conductive element formed in a eighth region having a first conductive type, said fifth conductive element coupled to a third voltage potential and said seventh conductive element coupled to a fourth voltage potential; 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1A  depicts an illustration of a prior art classical approach of uncoupled ESD protection clamps.  
         [0020]      FIG. 1B  depicts a schematic diagram of a prior art multi-fingered SCR ESD protection circuit.  
         [0021]      FIG. 1C  depicts a layout of a cross-section diagram of a prior art SCR  
         [0022]      FIG. 2  depicts an illustrative schematic diagram of different possible schematics for interconnecting ESD clamps in another embodiment of the present invention.  
         [0023]      FIG. 3  depicts an illustrative schematic diagram of different possible schematics for connecting SCR clamps with enhanced coupling technique in another embodiment of the present invention.  
         [0024]      FIG. 4  depicts an illustrative schematic diagram of different possible schematics for connecting SCR clamps with an enhanced coupling technique with reference to  FIG. 3  in an alternate embodiment of the present invention.  
         [0025]      FIG. 5  depicts an illustrative schematic diagram of an alternate embodiment with reference to  FIG. 4  of the present invention.  
         [0026]      FIG. 6  depicts an illustrative schematic diagram of one embodiment of a multi-fingered SCR ESD protection circuit of the present invention.  
         [0027]      FIG. 7  depicts an illustrative schematic diagram of an alternate embodiment with reference to  FIG. 6  of the present invention.  
         [0028]      FIG. 8A  depicts an illustrative cross-section diagram of a structure for ESD protection according to one embodiment of the present invention.  
         [0029]      FIG. 8B  depicts an illustrative cross-section diagram of a structure for ESD protection according to an alternate embodiment of the present invention.  
         [0030]      FIG. 9A  and  FIG. 9B  depicts an illustrative schematic diagram of an alternate embodiment with reference to  FIG. 8A  of the present invention.  
         [0031]      FIG. 10  depicts an illustrative cross-section diagram of an SCR for ESD protection according to another embodiment of the present invention.  
         [0032]      FIG. 11A  and  FIG. 11B  depict an illustrative circuit diagram of the SCR for ESD protection according to an alternate embodiment of the present invention.  
         [0033]      FIG. 12A  and  FIG. 12B  depict an illustrative circuit diagram of an alternate embodiment with reference to  FIG. 11A  and  FIG. 11B  respectively.  
         [0034]      FIG. 13  depicts an illustrative cross-section diagram of an SCR for ESD protection according to an another embodiment of the present invention.  
         [0035]      FIG. 14  depicts an illustrative cross-section diagram of an SCR for ESD protection according to an another embodiment of the present invention.  
         [0036]      FIG. 15A  and  FIG. 15B  depicts an illustrative circuit diagram of an alternate embodiment with reference to  FIG. 14  of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     In one embodiment of the present invention, a novel coupled clamping technique is shown which ensures multi-clamp triggering. The novelty of this embodiment is the coupling of the trigger gates of the separate clamps through low-ohmic connections, such as metal lines, or preferably feeding the triggering signal simultaneously to the trigger gates of the different clamps, or preferably making the anode and/or cathode of the clamps in the same active well, so as to ease the triggering of a network of clamps. The invention relates to the principle in which, when one certain ESD clamp triggers, it enables or triggers another ESD clamp or a group of other ESD clamps. When applying this technique to a number of ESD clamps in a certain IC protection scheme, all those clamps will trigger nearly simultaneously, therefore limiting the potential differences on the whole IC. This is very advantageous for an ESD protection strategy, especially against CDM stress, where it is critical to limit voltage drops anywhere on the IC as fast and efficient as possible. This is critical because for instance the capacitance related to two different power domains can be quite different so that large voltage differences can be built up during the discharge of a CDM event. If the power clamps related to these different power domains trigger together, this problem is less severe.  
         [0038]     Referring to  FIG. 2 a  generic representation of the invention is shown illustrating different possible schematics  200  for interconnecting ESD clamps to provide simultaneous triggering. There are a number of clamps (n)  202  which are interconnected in order to trigger each other. The anodes and cathodes (not shown) are connected to nodes(n)  204  such as node 1 , node 2 , node 3  and node 4 . They are connected preferably to different protected nodes (n)  204 , as well as the same nodes (n)  204  as illustrated in  FIG. 4A  and  FIG. 4B  respectively. So, the clamps  202  can be preferably be connected between any possible nodes  204 . They can have separate nodes  204  or common nodes  204 , or a combination of both. Whenever one clamp  202  triggers, it provides a voltage or current to the other clamps  202  it is connected with to trigger those other clamps  202 . Principally when a first clamp triggers, a part of the current will be tapped and used as a (current) signal or converted to a (voltage) signal which enables a second clamp to trigger, as well as any number of other clamps. In one example, the clamps are the power clamps of different power domains. There are several implementations possible. In the case of GGNMOS based clamps, they can be connected in a fashion as used with enhanced multi-finger triggering techniques as like domino-triggering. In the case of SCR clamps, their triggering gates G 1  or G 2  can be connected together. This can by done by hard wiring them with metal liners, or by placing the anodes of the SCR clamps in the same well, or by placing the cathodes of the SCR clamps in the same well. This is shown in  FIG. 3  as described in detail below. This invention is not limited to these two cases, which are only meant to illustrate the concept.  
         [0039]      FIG. 3  depicts an illustrative schematic diagram of different possible schematics for connecting SCR clamps with enhanced coupling technique in another embodiment of the present invention. Referring to  FIG. 3  an ESD protection circuit  300  is shown, having a first SCR clamp  302  and a second SCR clamp  304 . SCR clamp  302  includes a first anode  306  coupled to a first voltage potential Vdd  308  connected to a pad of the circuitry (not shown) and a first cathode  310  coupled to a second voltage potential Vss  312  preferably connected to ground (not shown). Also, a first trigger tap G 1   a    314  connected to a triggering device/element (not shown) is disposed proximate to the first cathode  310  and a third trigger tap G 2   a    316  is disposed proximate to the first anode  306  as shown in  FIG. 3 . Additionally, a first resistor Rg 1   a    318  is connected parallel to the first cathode  310  and a second resistor Rg 2   a    320  is connected parallel to the first anode  306 . Similarly, SCR clamp  304  includes a second anode  322  coupled to the Vdd  308  and a second cathode  324  coupled to the Vss  312 . Also, a second trigger tap G 1   b    326  connected to a triggering device/element (not shown) is disposed proximate to the second cathode  324  and a fourth trigger tap G 2   b    328  is disposed proximate to the second anode  322  as shown in  FIG. 5B .  FIG. 5C  illustrates connecting the first trigger tap G 1   a    314  to third trigger tap G 2   a    316  and connecting second trigger tap G 1   b    326  to fourth trigger tap G 2   b    328 .  
         [0040]     Note that this circuit shown in  FIG. 3  is not limited to two clamps, but can be applied to any number of clamps. Although, the first trigger tap G 1   a    314  and second trigger tap G 1   b    326  are shown as two separate trigger taps, however, they are essentially one trigger tap G 1 . Similarly, the third trigger tap G 2   a    316  and fourth trigger tap G 2   b    328  are shown as two separate trigger taps, but the are considered essentially one trigger tap G 2 . Moreover, no trigger elements or devices are drawn in  FIG. 3 , however any of the SCR&#39;s can have a trigger element apart from the shared trigger line, i.e. an external on-chip triggering devices coupled to the trigger taps. An example of this could be a number of GGNMOS triggered SCR clamps (GGSCR&#39;s) where the GGNMOS is connected between G 2  tap and ground of any or some SCR&#39;s and where the G 1  tap of any SCR is connected to G 1  of any other SCR. Or, a number of GGSCR clamps where the GGNMOS is connected between G 2  and ground of any or some SCR&#39;s, and where the G 2  tap of any SCR is connected to G 2  of any other SCR.  
         [0041]     Referring to  FIG. 3  ( a ), for example, let&#39;s consider the two SCR devices  302  and  304 . The trigger taps/gates G 1   a    314  and G 1   b    326  of both SCR&#39;s  302  and  304  respectively, are connected together. When the first SCR  302  now triggers due to an ESD event, a certain voltage will occur at the G 1   a    314  node. Due to this voltage drop, some current will be able to flow to the G 1   b    326  node of the second SCR  304 . This current will forward bias the G 1   b    326 -cathode  324  diode of this SCR  304 , thus triggering it. If the G 1   a    314 -G 1   b    326  connection  330  is fabricated from low ohmic connection, preferably metal lines, the current will flow through the metal lines. If the SCR&#39;s have a shared Pwell, carriers generated from the first SCR  302  in the Pwell will also trigger the other SCR  304 . Note that all of this can also be done by alternatively by connecting the G 2  gates of SCR&#39;s together with a low ohmic connection  330  in the same fashion as shown in  FIG. 3   b . Furthermore, in another alternative embodiment, both G 1  nodes of SCR  302  and G 2  nodes of SCR  304  can be connected with low ohmic connection  330  as shown in  FIG. 3   c  in order to stimulate triggering of the clamps. This technique can be applied to any number of SCR&#39;s.  
         [0042]     A possible problem that could occur with this technique is that the voltage which is built up by the triggered element is not high enough to trigger a neighboring device. In this case, some boost circuitry can be inserted into the schematic. This boost circuit causes the trigger voltage to increase, thus triggering other SCR&#39;s more easily. Depicted in  FIG. 4  are some possible implementations of this boost circuit as described in detail below.  
         [0043]     Referring to  FIG. 4 , there is illustrated a schematic diagram of different possible schematics for connecting SCR clamps of  FIG. 3  with an enhanced multi-fingering technique in an alternate embodiment of the present invention. In  FIG. 4 ( a ), a first boost circuit  402   a  connected in series with the first cathode  310  of the first SCR  302  and a second boost circuit  402   b  connected in series to the second cathode  324  of the second SCR  304 . When the first SCR  302  triggers, the boost circuit  402   a  will have a certain voltage drop over it, thus effectively increasing the voltage on node G 1   a    314 . This increased voltage will ease triggering of the other SCR  304 . Alternatively, as shown in  FIG. 6 ( b ), only one boost circuit  402  is connected in series with the G 1   a    314 -G 1   b    326 . This circuit amplifies the signal coming from one SCR  302  or  304 , boosting the other SCR  304  or  302  respectively. The amplifier can be constructed to operate in a single direction as well as to operate in both directions. Note that although, not shown more implementations are possible, for example where a boost circuit  402  would be connected in series with the Vdd line and the anode of each SCR  302  and  304 .  
         [0044]     A possible practical implementation of  FIG. 4 ( a ) can be found as an alternative embodiment in  FIG. 5  of the present invention. The boost circuit  402  is here a diode  404  as shown in  FIG. 6A  or a string of diodes  404  as shown in  FIG. 6B . When an SCR  302  or  304  is inactive (high resistive state), no current will flow through its series diode(s)  404 , thus no voltage drop will exist over the diode(s)  404 . When an SCR  302  or  304  is active (low resistive state), high ESD current will flow through the SCR and it&#39;s series diodes  404 . In this case, every diode  404  will build up by approximately 1V. The voltage on the G1 connection line will be boosted by 1V*number of series diodes. (i.e. 1V multiplied by number of series diodes). This condition will facilitate the triggering of other SCR&#39;s. In order to tune the performance of the whole circuit, the number of diodes can be altered.  
         [0045]     One skilled in the art will also understand that this boost circuit  402  could also comprise one of the devices such as a MOS, resistor, capacitor, inductor or any other device that has a resistance Also, each of the boost circuit  402  may preferably be included in only one of the SCR fingers or in any possible combination of two or more SCR fingers.  
         [0046]     It is to be noted that coupling multiple clamps (as described above) can preferably be used, for example, in multiple SCR fingers to simulate synchronous triggering of the clamps. In another embodiment of the present invention, there is shown that the problem of triggering is not only with different clamps but also in one clamp, provided by a multi-fingered SCR ESD protection circuit as described herein below.  
         [0047]      FIG. 6  depicts an illustrative schematic diagram embodiment of a multi-fingered SCR ESD protection circuit  600  of the present invention which serves as protection circuitry for an integrated circuit (not shown). Similar to  FIG. 1B , the SCR circuit  600  comprises multiple SCR fingers, and is illustratively depicted in  FIG. 6  having three SCR “fingers”  102 ,  104  and  106 . The SCR protection circuit  600  comprises first SCR  102  (i.e. “first finger”), a second SCR  104  (i,e, “second finger”) and a third SCR  106  (i.e. “third finger”). The first SCR  102  further comprises PNP transistor and an NPN transistor. In particular, the first SCR  102  includes at least one anode  108 , as known in the art, is one interspersed high-doped first region formed within a first lightly doped region. The anode  108  is connected to a first voltage potential, preferably a pad (not shown) and to one side of a resistor R 1   114 . The resistor R 1   114  represents the resistance of the N-well (or an external resistor), which is seen at the base of the PNP transistor of the SCR  102 . Also, included is a at least one cathode  112 , as known in the art, is a interspersed high-doped second region formed within a second lightly doped region. The cathode  112  is connected to a second voltage potential, preferably ground (not shown) and to one side of a resistor R 2   110 . The resistor R 2   110  represents the resistance of the P-well (or an external resistor) which is seen at the base of NPN transistor  106 . Furthermore, the circuit  600  comprises a boost circuit  602  connected to the cathode  112  or alternatively to the anode  108  as shown in  FIG. 6 . The boost circuit provides an additional voltage drop at a trigger tap ( 116  or  118  in  FIG. 6 ) as the trigger current runs through the boost circuit. The boost circuit  602  of  FIG. 6  may preferably be one or more diodes  702  as shown in an alternate embodiment of a multi-fingered SCR ESD protection circuit  700  in  FIG. 7 . One skilled in the art will also understand that this boost circuit  602  could also comprise one of the devices such as a MOS, resistor, capacitor, inductor or any other device that has a resistance The second and third SCRs  104  and  106  are formed exactly in the same manner as described with regard to the first SCR  102 .  
         [0048]     As shown in  FIG. 6 , a first triggering device (not shown in the figure) represented by a node G 1   116  is connected to the cathode  112  for supplying current to each of the SCR fingers  102 ,  104 ,  106 . Alternatively, there may be included a second triggering device represented by a node G 2   118  connected to the anode  108  such that the voltage drop seen by all anode/G 2  respectively G 1 /Cathode diodes is the same. Referring to  FIG. 6  where the boost circuit  602  is connected to the cathode  112 , In this case scenario, the G 1  node  116  will be pushed higher with respect to ground. Therefore, fluctuations in G 1 -Cathode voltage will be relatively smaller. Since the G 1  node  116  will be pushed higher, the current will be uniformly distributed over all the cathodes. In other words, the current flowing through the boost circuit  602  at the cathode  112  of the SCR finger  102  will build up enough voltage to be more uniformly distributed over all the other SCR fingers  104  and  106  to trigger. Also, as G 1  node  116  has a higher potential, more current will flow through the R 1   110  resistor. Since more current will flow through the R 1   110  resistor, less current will initially flow through the cathode of the SCR. This gives the other fingers more time to trigger, relaxing the multifinger triggering issue. Note that although not shown here, G 2  connections  118  can be also made. Anyone skilled in the art will understand that pushing G 2   118  lower by adding a boost-like circuit  602  between the first voltage potential (not shown) and anode  108  will create a similar effect.  
         [0049]     It is important to note that each boost circuit  602  may preferably be included in only one of the SCR fingers or in any possible combination of two or more SCR finger.  
         [0050]     In a further embodiment of the present invention, there is proposed a structure  800  illustrated as a cross-section diagram in  FIG. 8A  for ESD protection, based on SCR operation. It can be placed at any pin of a chip (not shown). The structure  800  is basically an SCR with preferably at least two anodes  802  and  804  or at least two cathodes  806  and  808 . The purpose is if one of the inherent SCR&#39;s get triggered, the other SCR&#39;s in the structures will tend to trigger as well because all SCR parasitics share the same well (the base of all parasitic bipolars are connected by the well resistance). This behavior is especially wanted for CDM stress. Beside this advantage, the protection structure is also an element that can protect the chip against ESD stress along several current paths at the same time. For example, to protect an input pin (not shown), placing such a structure could not only protect the chip for stress from input to the first voltage potential Vdd  508 , but also for stress from input to the second voltage potential Vss  512 . With conventional solutions, two elements were needed to achieve this protection, one element for each current path.  
         [0051]     Alternatively, a generic cross section of the SCR structure  800  is shown in  FIG. 8B  with three anodes  802 ,  804  and  810  and three cathodes  806 ,  808  and  812 . An inherent or parasitic SCR is shown in dashed line as will be described in greater detail below with reference to  FIG. 8A . Note that the number of anodes and cathodes doesn&#39;t need to be three. Neither does the number of anodes and cathodes need to be equal. There can for example be two anodes and one cathode, or one anode and four cathodes or any number of combinations can be possible.  
         [0052]     In order to ensure the desired operation of the invention, additional elements may be added as well. This includes, but is not limited to trigger elements or structures that alter the holding voltage such as diodes in series with the invention.  
         [0053]     Referring back to  FIG. 8A , there is shown another embodiment of the present invention. The structure  800  consists of two P+ regions  801  in the same N− well  803 , located next to two N+ regions  805  in the P− substrate  807 . This structure comprises 4 parasitic bipolars, bipoloar 1   814 , bipolar 2   816 , bipolar 3   818  and bipolar 4   820 , creating  3  inherent SCR&#39;s. The first SCR is created by parasitic bipolar 2   816  &amp; parasitic bipolar 4   820  and exists between Vdd  508  and Vss  512 . The second one is an SCR created by parasitic bipolar 2   816  and parasitic bipolar 1   814  between Vdd  508  and a PAD  840 . PAD  840  represents the bonding pad of an IO pin (not shown). The third one is an SCR created by parasitic bipolar 3   818  &amp; parasitic bipolar 4   820  between the PAD  840  and Vss  512 . Both the second SCR and the third SCR act as a local clamp here.  
         [0054]     The advantage of this structure is that when one of the three SCRs gets triggered, the other ones can trigger as well, if current is supplied to the anodes. For the trigger speed of the different SCR&#39;s, the placement of the N+  805  and P+  801  regions is of big importance. One can place both N+  805  regions on one side of the N− well  803 , or one N+ region  805  on each side of the well as is shown in  FIG. 8A . Both solutions will differ on trigger speed, resistance during conducting state and other factors such as trigger voltage. Those who are skilled in the art will know how to design the structure in such a way to get optimal ESD performance from it.  
         [0055]     When one wants to add holding diodes to the structure in order to make the holding voltage higher, this is possible in many different ways.  FIG. 9  illustrates a schematic representation  900  of the structure from  FIG. 8A  with addition of holding diodes  902 . Depicted in  FIG. 9A  are the addition of holding diodes  902 . Depicted in  FIG. 9B  is a possible example where each of the three possible ESD paths  904  has two holding diodes  902  in series. The three paths  904  are shown in dashed lines in  FIG. 9B . The protection structure of  FIG. 9  represents the one from  FIG. 8A  and consists of an SCR with 2 anodes  802  and  804 , and two cathodes  806  and  808 .  FIG. 9  shows one configuration, but many different configurations are possible. For example the path from Vdd  508  to Vss  512  can have 2 holding diodes  902 , while the paths from Vdd  508  to IO  906  and from IO  906  to Vss  512  may preferably have no diodes. In this case, the anti-parallel diodes at the IO  906  line can be left out.  
         [0056]     In an even further embodiment of the invention, there is shown a cross-section diagram of the SCR structure  1000  in  FIG. 10 . It is made by having the invention inherent in an output driver due to parasitic elements. The SCR structure  1000  with two anodes and two cathodes is created in an output driver. The structure  1000  is totally inherent to the driver. The layout of this driver can be altered for an optimal working of the structure. Both the NMOS  1002  and PMOS  1004  from the driver stage create the SCR&#39;s. By removing the sides of each of P+ guardband  1003  and N+ guardband  1005  band in between the two MOS transistors  1002  and  1004 , a structure that can easily latch is created. The structure has two anodes, formed by the drain and source of the PMOS  1004 , as well as two cathodes formed by the drain and source of the NMOS  1002 . An SCR between Out PAD  1008  and Vss  512  is created and uses the Drain of the PMOS as anode. Another SCR between Vdd  508  and Out PAD  1008  uses the drain of the NMOS cathode. This embodiment thus shows the intended creation of an SCR with multiple anodes and cathodes in an output buffer to create an ESD protection structure, which simultaneously works as a power clamp between Vdd  508  and Vss  512  and as a local ESD protection for the output pad.  
         [0057]     In prior arts, all these parasitic SCR&#39;s were seen as a problem for LU (latchup) issues. In the present invention, it is a way to create ESD protection. To avoid any LU issues using this SCR during normal operation of the chip, two approaches can be used. First, the holding voltage of the parasitic SCR between Vdd and Vss can be above the normal Vdd voltage. For LV technologies, such as 1V 65 nm CMOS, this is easily achieved since second, the trigger current can be increased above the latch up current (I latch ). This can be done by making the G 2  (N+ in N− well) to Vdd connection and the G 1  (P+ in P− well) to VSS connection low ohmic. In other words, the bulk ties in N− well and/or P− well need to be well placed in order to lower the well resistances.  
         [0058]     In order to improve the ESD capabilities of the inherent SCR from Vdd to Vss, the drain/source regions can be swapped, both for the NMOS as for the PMOS driver. This would reduce the length Anode/Cathode (LAC) spacing of the inherent SCR and thus improve its speed LAC is the distance between the anode and the cathode. Note that this will also affect the performance of the SCR&#39;s between Vdd and PAD, and PAD and Vss.  
         [0059]     Triggering of the intrinsic SCR can be done by adding a trigger circuit  1102  to the bulk ties of MOS devices as illustrated in the circuit  1100  of  FIG. 11 .  FIG. 11A  illustrates adding a trigger circuit  1102  via gate G 2   1104  to the PMOS and  FIG. 11B  illustrates adding a trigger circuit  1102  via gate G 1   1106  to the NMOS. In  FIG. 12 , the trigger circuit  1102  preferably consists of four diodes  1202 . However, in general this trigger circuit  1102  can consist of any elements, both passive (diodes, resistors, inductances, capacitances, etc.) and/or active elements (MOS devices, SCRS, etc.).  FIG. 12A  shows a possible implementation including the trigger circuit  1102  of  FIG. 11A  with four diodes  1108 .  FIG. 12B  includes the trigger circuit  1102  of  FIG. 11B  with four diodes  1108 . The resistance R 2  of  FIG. 12A  and the resistance R 1  of  FIG. 12B  can preferably be both intrinsic or externally added. The value of these resistances will determine the trigger current of the SCR. Making these resistances small will increase the latch up immunity of the clamp.  
         [0060]     An even further embodiment of the present invention can be seen in  FIG. 13 .  FIG. 13  illustrates a cross-section diagram consisting of an SCR  1300  with two anodes  1302  &amp;  1304  and two cathodes  1306  &amp; 1308 . It is used in a chip (not shown) with two power domains. The first domain is connected at nodes of a first voltage potential Vdd 1   1310  and second voltage potential Vss 1   1312 . The second domain is connected to nodes of a third voltage potential Vdd 2   1314  and a fourth voltage potential Vss 2   1316 . The first and third voltage potentials Vdd  1310  and Vdd 2   1314  respectively, have equivalent values, preferably connected to a pad of the circuitry (not shown). The second and the fourth voltage potentials Vss 1   1312  and Vss 2   1316  respectively, have equivalent values, preferably connected to a ground (not shown). When the power clamp activates at one domain, the power clamp at the other domain tends to trigger as well when current is flowing there.  
         [0061]     For example, this is especially advantageous for a Charge Device Model (CDM) event. CDM is know in the art as a model used to simulate a kind of ESD-stress. The different power domains on a chip have mostly a different capacitance. This means that during CDM, one domain can discharge faster than another domain. Such situation can possibly cause too much voltage difference between power domains on a chip. Using the invention, the Vdd and Vss line of all domains can be clamped tightly together, preventing too much potential difference between them.  
         [0062]     Holding diodes can be added in series with the Vdd 1   1310  and/or Vdd 2   1314  terminal as desired to raise the holding voltage of the power clamp for a certain power domain. This can be done for each power domain independently. Even though, the present invention shows an embodiment with two power domains as shown in  FIG. 13 , it can also be applied for chips with more than two power domains.  
         [0063]     An even further embodiment of the present invention includes a triggering scheme for triggering of the SCR as shown in cross section diagram  1400  of  FIG. 14 . Triggering the structure can happen by sending current through the N-well. In order to be able to do this, an N+ region is added to the N− well. The triggering scheme includes a string of two diodes  1402  and  1404  connected in series from the newly created N− well connection to Vss  512 . This is similar to the diode triggering scheme of a conventional SCR. When the voltage Vdd  508 -Vss  512  reaches about 3 V, the diodes  1402  and  1404  will conduct and current will flow from Vdd  508  to Vss  512  through the P+/N− diode and the two external diodes. This is indicated by dashed line “ 1 ” in  FIG. 14 . The voltage at which current starts to flow is dependent on the number of trigger diodes. The trigger current will forward bias the base of the parasitic transistor in the N-well and thus turn on the SCR between Vdd  508  and Vss  512 . Triggering can also happen due to an excess voltage on the PAD  840  with respect to Vss  512  as shown by dashed line “ 2 ” in  FIG. 14 . The same trigger mechanism is applied here, only the other parasitic PNP in the N− well gets forward biased now. The P− substrate is connected to Vss  512  by a resistor  1406  in order to prevent unwanted triggering by substrate noise, etc. Note that this resistor will have influence on the trigger speed of the circuit. A low ohmic resistor will cause slow triggering.  
         [0064]     A schematic representation of these triggering paths of  FIG. 14  is illustrated in  FIG. 15A . The two possible trigger paths marked in dashed line, indicated by “1” and “2”. The numbers refer to the same trigger paths as indicated in  FIG. 14 .  FIG. 15B  a variation of the trigger scheme as an alternate embodiment of the present invention. Only trigger diode  1502  is added here between the N− well and the P− well of the invention. There are now three possible trigger paths. The first one is for excess voltage between Vdd  508  and Vss  512  and is indicated by “ 1 ” similar to  FIG. 15A . The second one is indicated by “ 2 ” and current will flow here for an over voltage between PAD  840  and Vss  512  similar to  FIG. 15A . An additional third trigger path marked with “ 3 ” will start to conduct current and trigger the structure for excess voltage between Vdd  508  and PAD  512 .  
         [0065]     Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit and the scope of the invention.