Abstract:
In an ESD protection device using a LVTSCR-like structure, the holding voltage is increased by placing the p+ emitter outside the drain of the device, thereby retarding the injection of holes from the p+ emitter. The p+ emitter may be implemented in one or more emitter regions formed outside the drain. The drain is split between a n+ drain and a floating n+ region near the gate to avoid excessive avalanche injection and resultant local overheating.

Description:
FIELD OF THE INVENTION 
     The invention relates to ESD protection devices. More particularly, it relates to LVTSCR-like devices for protecting CMOS and Bi-CMOS integrated circuits against electrostatic discharge and electrical overstress. 
     BACKGROUND OF THE INVENTION 
     Analog circuits typically display sensitivity to excessive voltage levels. Transients, such as electrostatic discharges (ESD) can cause the voltage handling capabilities of the analog circuit to be exceeded, resulting in damage to the analog circuit. Clamps have been devised to shunt current to ground during excessive voltage peaks. 
     One of the difficulties encountered in designing such protection circuitry is that the specifications for these clamps have to fit within a relatively small design window that, on the one hand, must take into account the breakdown voltage of the circuit being protected, and, on the other hand, avoid latch-up under normal operation. Thus, the clamp must be designed so as to be activated below the breakdown voltage of the circuit that is to be protected. At the same time, the latch-up or holding voltage must exceed the normal operating voltage of the protected circuit. 
     Some protection clamps employ avalanche diodes such as zener diodes to provide the bias voltage for the it base of a subsequent power bipolar junction transistor (BIT). 
     Grounded gate NMOS devices (GGNMOS) have also been used as ESD protection devices. However, GGNMOS devices are not only large, consuming a lot of space on a chip, they also suffer from the disadvantage that they support only limited current densities. The protection capability of an ESD protection device can be defined as the required contact width of the structure required to protect against an ESD pulse amplitude, or, stated another way, as the maximum protected ESD pulse amplitude for a given contact width. Thus, the smaller the contact width for a given ESD pulse amplitude protection, the better. One possible ESD protection solution is to use a silicon-controlled rectifier (SCR). 
     A silicon-controlled rectifier (SCR) is a device that provides an open circuit between a first node and a second node when the voltage across the first and second nodes is positive and less than a trigger voltage. When the voltage across the first and second nodes rises to be equal to or greater than the trigger voltage, the SCR provides a low-resistance current path between the first and second nodes. Further, once the low-resistance current path has been provided, the SCR maintains the current path as long as the voltage across the first and second nodes is equal to or greater than a holding voltage that is lower than the trigger voltage. As a result of these characteristics, SCRs have been used to provide ESD protection. When used for ESD protection, the first node becomes a to-be-protected node, and the second node is typically connected to ground. The SCR operates within an ESD protection window that has a maximum voltage defined by the destructive breakdown level of the to-be-protected node, and a maximum voltage (also known as a latch-up voltage) defined by any dc bias on the to-be-protected node. 
     Thus, when the voltage across the to-be-protected node and the second node is less than the trigger voltage, the SCR provides an open circuit between the to-be-protected node and the second node. However, when the to-be-protected node receives a voltage spike that equals or exceeds the trigger voltage, such as when an ungrounded human-body discharge occurs, the SCR provides a low-resistance current path from the to-be-protected node to the second node. In addition, once the ESD event has passed and the voltage on the to-be-protected node falls below the holding voltage, the SCR again provides an open circuit between the to-be-protected node and the second node. 
     FIG. 1 shows a cross-sectional view that illustrates a conventional SCR  100 . As shown in FIG. 1, SCR  100  has a n-well  112  which is formed in a p-type semiconductor material  110 , such as a substrate or a well, and a n+ region  114  and a p+ region  116  which are formed in n-well  112 . The n+ and p+ regions  114  and  116  are both connected to a to-be-protected node  120 . As further shown in FIG. 1, SCR  100  also has a n+ region  122  and a p+ region  124  formed in semiconductor material  110 . The n+ and p+ regions  122  and  124  are both connected to an output node  126 . 
     In operation, when the voltage across nodes  120  and  126  is positive and less than the trigger voltage, the voltage reverse biases the junction between n-well  112  and p-type material  110 . The reverse-biased junction, in turn, blocks charge carriers from flowing from node  120  to node  126 . However, when the voltage across nodes  120  and  126  is positive and equal to or greater than the trigger voltage, the reverse-biased junction breaks down due to avalanche multiplication. 
     The breakdown of the junction causes a large number of holes to be injected into material  110 , and a large number of electrons to be injected into n-well  112 . The increased number of holes increases the potential of material  110  in the region that lies adjacent to n+ region  122 , and eventually forward biases the junction between material  110  and n+ region  122 . 
     When the increased potential forward biases the junction, a npn transistor that utilizes n+ region  122  as the emitter, p-type material  110  as the base, and n-well  112  as the collector turns on. When turned on, n+ (emitter) region  122  injects electrons into (base) material  110 . Most of the injected electrons diffuse through (base) material  110  and are swept from (base) material  110  into (collector) n-well  112  by the electric field that extends across the reverse-biased junction. The electrons in (collector) n-well  112  are then collected by n+ region  114 . 
     A small number of the electrons injected into (base) material  110  recombine with holes in (base) material  110  and are lost. The holes lost to recombination with the injected electrons are replaced by holes injected into (base) material  110  by the broken-down reverse-biased junction and, as described below, by the collector current of a pnp transistor, thereby providing the base current. 
     The electrons that are injected and swept into n-well  112  also decrease the potential of n-well  112  in the region that lies adjacent to p+ region  116 , and eventually forward bias the junction between p+ region  116  and n-well  112 . When the decreased potential forward biases the junction between p+ region  116  and n-well  112 , a pnp transistor formed from p+ region  116 , n-well  112 , and material  110 , turns on. 
     When turned on, p+ emitter  116  injects holes into base  112 . Most of the injected holes diffuse through (base) n-well  112  and are swept from (base) n-well  112  into (collector) material  110  by the electric field that extends across the reverse-biased junction. The holes in (collector) material  110  are then collected by p+ region  124 . 
     A small number of the holes injected into (base) n-well  112  recombine with electrons in (base) n-well  112  and are lost. The electrons lost to recombination with the injected holes are replaced by electrons flowing into n-well  112  as a result of the broken-down reverse-biased junction, and n-well  112  being the collector of the npn transistor. Thus, a small part of the npn collector current forms the base current of the pnp transistor. 
     Similarly, as noted above, the holes swept into (collector) material  110  also provide the base current holes necessary to compensate for the holes lost to recombination with the diffusing electrons injected by n+ (emitter) region  122 . Thus, a small part of the pnp collector current forms the base current of the npn transistor. 
     Thus, n+ region  122  injects electrons that provide both the electrons for the collector current of the npn transistor as well as the electrons for the base current of the pnp transistor. At the same time, p+ region  116  injects holes that provide both the holes for the collector current of the pnp transistor as well as the holes for the base current of the npn transistor. 
     One of the advantages of SCR  100  over other ESD protection devices, such as a grounded-gate MOS transistor, is the double injection provided by n+ region  122  and p+ region  116  of SCR  100 . With double injection, SCR  100  provides current densities (after snapback) that are about ten times greater than the densities provided by a grounded-gate MOS device. 
     One of the disadvantages of SCR  100 , however, is that a very large positive voltage, e.g., 50 volts, must be dropped across nodes  120  and  126  before the junction between p-type material  110  and n-well  112  breaks down. As a result, SCR  100  can not be used to protect devices, such as MOS transistors, that can be permanently damaged by much lower voltages, e.g., 15 volts. 
     One solution proposed in the past, was to use low voltage silicon controlled rectifiers (LVTSCRs) which are not only smaller but allow the reaching of current densities, after snap back, that are some ten times higher than the current densities of traditionally used grounded gate NMOS devices (GGNMOS), thus increasing the ESD protection capability for CMOS circuits. 
     An LVTSCR incorporates a NMOS transistor into SCR  100 . FIG. 2 shows a cross-sectional diagram that illustrates a conventional LVTSCR  200 . LVTSCR  200  and SCR  100  are similar and, as a result, utilize the same reference numerals to designate the structures that are common to both devices. 
     As shown in FIG. 2, LVTSCR  200  differs from SCR  100  in that LVTSCR  200  has a n+ (drain) region  230  that is formed in both material  110  and n-well  112 , and a channel region  232  that is defined between n+ (source) region  122  and n+ (drain) region  230 . In addition, LVTSCR  200  includes a gate oxide layer  234  that is formed on material  110  over channel region  232 , and a gate  236  that is formed on gate oxide layer  234 . N+ (source and drain) regions  122  and  230 , gate oxide layer  234 , and gate  236  define a NMOS transistor  238  which is typically formed to be identical to the to-be-protected MOS transistors in the circuit. 
     In operation, when the voltage on the drain of a conventional NMOS transistor spikes up, the drain-to-substrate junction of the NMOS transistor breaks down, for example, at 7 volts, while the gate oxide layer that isolates the gate from the drain destructively breaks down at, for example, 10-15 volts. 
     Since NMOS transistor  238  is formed to be identical to the to-be-protected MOS transistors, the junction between n+ region  230  and material  110  breaks down at the same time that the to-be-protected MOS transistors experience junction break down as a result of an ESD pulse. Once the reverse-biased junction between region  230  and material  110  breaks down, the break down triggers LVTSCR  200  to operate the same as SCR  100 . 
     Since junction break down occurs before the MOS transistors experience destructive gate oxide break down, LVTSCR  200  turns on before destructive gate oxide breakdown occurs, thereby protecting the MOS transistors. Thus, the junction breakdown voltage, which is less than the voltage level that causes destructive gate oxide break down, functions as the trigger voltage. In addition, other techniques, such as reducing the width of channel region  232 , can be used to lower the trigger voltage so that the region  230  to material  110  junction breaks down before the to-be-protected MOS transistors experience junction breakdown. 
     Thus, LVTSCR  200  provides a SCR with a significantly lower turn-on voltage that allows MOS transistors to be protected from ESD events with an SCR. However, one disadvantage of LVTSCR  200 , and, for that matter, any SCR is that it suffers from a holding voltage that is often less than the minimum (or latch-up) voltage of the ESD protection window. The low holding voltage of the LVTSCR which lies in the range of less than two volts, is due to the double junction injection of its conductivity modulation mechanism. While the p+ emitter allows one to define how many holes are injected, the injection of the holes leads to greater space charge neutralization and thus a lower holding voltage. As a result, standard LVTSCRs are unattractive candidates for providing ESD protection to power supply pins. 
     As mentioned above, the major requirement when designing ESD protection circuits, is that the circuit operate within a so-called “ESD protection window” that is usually limited by both the maximum voltage in the protected line (which is related to the breakdown of the protected circuits) and the latch-up voltage when the DC bias is presented in the protected line. 
     In the LVTSCR, when the minimum (or latch-up) voltage of the ESD protection window is equal to a dc bias, such as the power supply voltage, LVTSCR  200  cannot turn off (thus latching up) after the ESD event has passed. Thus, power must be cycled after the ESD event, to switch off the LVTSCR. 
     For example, assume that node  120  is a power supply pin at 3.3 volts, node  126  is a ground pin, the junction breakdown voltages of the to-be-protected MOS transistors are 0.7 volts, and the holding voltage is 1.8 volts. In this example, LVTSCR  200  is turned off under normal operating conditions when the voltage on node  120  is 3.3 volts. When the voltage on node  120  spikes up to a value equal to or greater than the trigger voltage (7 volts in this example), LVTSCR  200  turns on, thereby protecting the MOS devices that receive power from node  120 . However, once the ESD event has passed, since the normal operating voltage on node  120  is 3.3 volts, and it takes only 1.8 volts on node  120  to keep LVTSCR  200  in this example turned on, LVTSCR  200  remains turned on (latched up) after the ESD event has passed. 
     Thus, in spite of higher current availability from an LVTSCR after snap back, conventional CMOS integrated circuits are usually protected by grounded gate NMOS snap back structures (GGNMOS) due to the latch-up limitations of LVTSCRs. 
     What is needed is a device that fills the void between low current GGNMOS devices and low holding voltage, high current SCR and LVTSCR devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides an LVTSCR-like structure having an increased holding voltage. 
     Further, according to the invention, there is provided a LVTSCR-like ESD protection structure with a two-stage snapback triggering characteristic. The present invention seeks to increase the holding voltage by reducing the carrier injection from the p+ emitter region. This is achieved by effectively reversing the locations of the p+ emitter and n+ drain regions. The emitter is located outside the drain region so that at least part of the drain contact region lies between the gate and emitter region. The n+ drain region is typically split to comprise a floating n+ region and a n+ contact region. The structure may include multiple emitters outside the drain region. 
     Further according to the invention, there is provided a method of reducing local heating in a LVTSCR-like structure, comprising splitting the n+ drain region between a floating n+ region near the gate and a n+ contact region, wherein at least part of the n+ drain region is formed between the p+ emitter and the gate of the structure. Preferably the p+ emitter comprises a plurality of emitter regions. 
     Still further, according to the invention, there is provided a high holding voltage LVTSCR-like structure, comprising an emitter located so that at least part of the drain region is located between the gate and emitter region. Preferably, the n+ drain region is split into at least one first drain region located near the gate, and at least one second drain region, wherein first drain region may comprise a floating n+ region, and the second drain region may comprise a n+ contact region. The emitter may comprise a plurality of emitter regions, and the first and second drain regions are preferably separated by a shallow trench isolation region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view illustrating a conventional SCR; 
     FIG. 2 is a cross-sectional view illustrating a conventional LVTSCR. 
     FIG. 3 is a cross-sectional view of one embodiment of a LVTSCR-like structure of the invention; 
     FIG. 4 is a cross-sectional view of another embodiment of the structure of the invention; 
     FIG. 5 is a plan view of yet another embodiment of the invention; 
     FIG. 6 is a plan view of yet another embodiment of the invention; 
     FIG. 7 is a plan view of yet another embodiment of the invention; 
     FIG. 8 shows drain current against drain voltage curves for a conventional LVTSCR and an embodiment of a LVTSCR-like structure of the invention; 
     FIG. 9 shows drain-source voltage as well as lattice temperature curves against log time for a conventional LVTSCR device and one embodiment of a LVTSCR-like structure of the invention; 
     FIG. 10 shows the temperature distribution across a LVTSCR without a STI region splitting the drain, and 
     FIG. 11 shows the temperature distribution across one embodiment of a LVTSCR-like structure of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 shows one embodiment of a LVTSCR-like structure  300  of the invention. As in the case of a conventional LVTSCR, the device  300  includes a p-substrate  302  with a p-epitaxial layer  304  formed on top of the p-substrate  302 . A p-well  306  and an n-well  308  are formed in the epitaxial layer  304 . Formed in the p-well  306  is a n+ source  310 . The p-well  306  further includes a p+ well  312  formed between shallow trench isolation regions  314 . The n-well  308  has a n+ drain region  316  formed in it and a floating drain  318  which bridges the junction between the p-well and the n-well  308 . In contrast to the prior art LVTSCR, the present structure includes a p+ emitter  320  formed outside the drain region  316 ,  318 . Thus, the drain regions  316 ,  318  block some of the space charge injected by the p+ emitter  320 . The low holding voltage of a conventional LVTSCR can be ascribed to the double injection conductivity modulation mechanism as described above, in which holes are injected from the p+ emitter of a PNP transistor defined by the p+ emitter, the n-well and the p epitaxial layer. The second injection comes from electrons injected from the emitter of an NPN transistor defined by the n+ source, the p-well, and the n-well. In the present invention, the injection of holes from the p+ emitter  320  is delayed due to the emitter  320  lying outside the drain region  316 ,  318 . The effect of this is that the emitter injection turns on only after a sufficiently high critical current density is reached. As a result, the holding voltage of the structure  300  corresponds substantially to that of a snapback NMOS structure. Nevertheless, it still retains characteristics of a LVTSCR insofar as the emitter injection finally turns on to provide a higher current in the saturation region. This also has the effect of dissipating heat more effectively. In contrast, a NMOS device which includes a narrow channel region, produces a high electric field with correspondingly high heat dissipation in a gap of approximately 0.5 μm. 
     It will be appreciated that the invention can be implemented in a variety of ways without departing from the scope of the invention. FIG. 4 illustrates another embodiment of the invention in which the p+ emitter  400  is located between two n+ drain portions  402 . The n+ drain regions may take the form of continuous drain regions such as those illustrated in the plan view of FIG.  5  and indicated by reference numeral  500 . It will be appreciated that the embodiment of FIG. 5 does not show the n+ drain portion outside the p emitter  502 . Thus it is a slightly different embodiment but also displays the concept of providing a drain region  500  between the p emitter  502  and the gate  504 . As shown in FIG. 4, the p+ emitter  400  is located between two shallow trench isolation regions  404 ,  406 . Similar trench isolation regions are indicated in FIG. 5 by reference numerals  504 ,  506 . Like the STI  408 , this embodiment includes a STI region  508  that splits the drain into the drain contact region  500  and the floating drain region  510 . As discussed below, this provides for better heat dissipation. A solid drain region would cause excessive avalanche injection and local overheating. 
     Yet another embodiment of the invention is illustrated in FIG. 6 in which the p+ emitters  600  are formed as isolated islands in a shallow trench isolation region  602 . Thus the p+ emitters  600  are effectively placed between shallow trench isolation portions much as the p+ emitter  400  is located between the shallow trench isolation regions  404 ,  406 . The drain  604  is located between the shallow trench isolation regions  602  and a shallow trench isolation region  606 , much as the drain  402  in FIG. 4 is located between the shallow trench isolation region  404  and a shallow trench isolation region  408 . 
     Yet another embodiment of the invention is illustrated in FIG.  7 . In this embodiment, the p+ emitters  700  are also formed as individual islands. However, in this embodiment, the p+ emitter islands  700  are formed in the n+ drain  702 , which is formed between shallow trench isolation regions  704 ,  706 . As can be seen in all of the embodiments, the drain region is split into a n+ drain and a floating drain. In FIG. 3, the floating drain is indicated by reference numeral  318 , while in FIGS. 4,  5 ,  6 ,  7 , the floating drain is indicated by reference numerals  410 ,  510 ,  610 ,  710 , respectively. 
     It will be appreciated that the p+ emitter  400  need not be a continuous emitter as in the embodiment of FIG. 5, but could equally well be formed as individual islands as indicated in the embodiments of FIGS. 6 and 7. Thus, the p+ emitter can take the form of one or more emitter regions which are located outside at least part of the drain region. 
     The marked effects on the holding voltage of the present invention structure are evident in FIG. 8 which shows the drain current against drain voltage curve  800  of one embodiment of the invention as compared to the drain current against drain voltage curve of a conventional LVTSCR as indicated by reference  802 . The curve  800  displays a latch-up voltage of about 4.5 V compared to a latch-up voltage of about 2.5 for a conventional device. 
     The effects on drain-source voltage and lattice temperature in response to a human body model (HBM) pulse of 2 kV for a 50 μm contact width is illustrated in FIG. 9 for a conventional device and a device of the invention. Curve  900  shows the voltage curve for a device of the invention which shows much higher voltage handling capabilities over time than the conventional device, as indicated by reference  902 . The temperature profile of the device of the invention, as illustrated by the curve  904 , shows higher temperatures as compared to the curve  906  of the conventional device. However, as discussed below, STI separation of the drain into a n+ contact region and a floating n+ region, causes the temperature to be distributed over a large area, thereby avoiding local overheating. 
     The embodiments of the present invention include a shallow trench isolation (STI) region between the n+ drain and the floating drain. The effect of this is to provide more efficient heat dissipation as illustrated in the temperature profile for a device without STI separation, as illustrated in FIG. 10, compared to the temperature profile of an embodiment of the invention having STI separation, as illustrated in FIG. 11. A 2 kV HBM pulse was applied and the temperature profile determined after 12 ns.