Abstract:
In an NLDMOS, DMOS and NMOS device, the ability is provided for withstanding snapback conditions by providing one or more p+ emitter regions interdigitated between drain regions having drain contacts and electrically connecting the drain contacts to contacts of the emitter regions.

Description:
FIELD OF THE INVENTION 
     The present invention deals with high voltage devices that can withstand ESD events. In particular, it deals with high voltage devices that can reversibly withstand snapback mode. 
     BACKGROUND OF THE INVENTION 
     Numerous devices have been developed for handling electrostatic discharge (ESD) events. These ESD protection devices may be categorized as falling into two groups: the active devices that work in normal operating mode, and the snapback devices which are designed to be triggered and operate in snapback mode during an ESD event and then turn off again as voltage drops below the holding voltage of the device. 
     NLDMOS and DMOS devices are intended to be used in normal mode and will be destroyed if they go into snapback. Even high voltage NLDMOS and DMOS devices will only survive if the voltage they are handling does not exceed the capabilities of the device. While these devices typically are meant not to go into snapback, local overstresses due to current crowding can cause these devices to go into snapback, thereby damaging the device. Thus, in the case of an ESD event, unless the device is made extremely large, the device is pushed past its capabilities and goes into snapback, causing irreversible breakdown. Typically the margin is rather small before the devices go into snapback. This problem is exacerbated by the fact that the snapback voltage is dependent on gate bias and in practice high-voltage devices used for voltage regulation to provide a low voltage to internal circuits are often not directly connected to the power pad and ground. Thus they fail to provide local clamping of the high voltage pad and ground. 
     A typical NLDMOS, more correctly referred to as a drain extended MOS (DeMOS) is shown in cross-section in  FIG. 1 , which includes an n-epitaxial layer  100  in which an n-well  102  is formed. In the case of a BiCMOS process an n-buried layer (NBL)  103  may also be formed in the n-epi  100 . An n+ drain  104  is formed in the n-well  102 , and an n+ source  106  is formed in a p-well  108  in the n-epi  100 . A polysilicon gate  110  is formed on top of the n− and p-wells  102 ,  108 , the extended portion of the gate  110  being isolated from the n-well  102  by an isolation oxide  112 . As shown in  FIG. 1 , the drain  104  includes a drain contact  114 , the source  106  includes a source contact  116 , and the gate  110  includes a gate contact  120 . 
       FIG. 2  shows another prior art device in cross-section, namely an NLDMOS-SCR, which is capable of operating in snapback mode but suffers from considerable on-state resistance losses during normal mode. This device includes an n-epitaxial layer  200  grown on a p-substrate  201 . An n-well  202  is formed in the n-epi  200 . In the case of a BiCMOS process an n-buried layer (NBL)  203  may also be formed in the n-epi  200 . In the n-epitaxial layer  200 , an n+ drain  204  is formed, and an n+ source  206  is formed in a p-well  208  in the n-epi  200 . A polysilicon gate  210  is formed on top of the n- and p-wells  202 ,  208 , the extended portion of the gate  210  being isolated from the n-well  202  by an isolation oxide  212 . As shown in  FIG. 2 , the drain  204  includes a drain contact  214 , the source includes a source contact  216 , and the gate  210  includes a gate contact  220 . Unlike the NLDMOS of  FIG. 1 , this NLDMOS-SCR further includes a p-emitter region  222  formed under the drain contact. This device functions well insofar as it moves the hot spot (shown by region  130 ) away from the drain contact  214 . However, the inclusion of the p-emitter region  222  introduces additional process steps that are typically not required for the devices it supports. Also, the inclusion of the p-emitter region  222  results in a significant saturation NWELL resistor. Thus, the device on-state current is rather low since only the bottom portion of the NWELL under the p-emitter  222  can conduct the current. 
     The present invention seeks to provide an alternative solution for devices that will not only operate well during normal mode but are also capable of surviving a snapback scenario. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an NLDMOS, DMOS or NMOS device (both extended and low voltage device) that provides good normal mode operation and is capable of performing a reversible snapback operation, comprising a drain with a plurality of n+ drain pickup contacts, and at least one p+ emitter region, wherein each p+ emitter region is formed between two drain contacts. Each p+ emitter region may include at least one emitter contact that is electrically connected to the n+drain pickup contacts. The device may include multiple p+ emitter regions each with at least one emitter contact, the p+ emitter regions being are formed between the n+ drain pickup contacts. Preferably, the emitter contacts and n+ drain pickup contacts are electrically interconnected by a common metal layer. 
     In one embodiment, the device comprises an array with multiple drain regions and multiple p+ emitter regions, at least one of the drain regions being provided with multiple drain contacts, and the p+ emitter regions being formed between the drain contacts. All of the drain regions of the array may include multiple drain pickup contacts, and the p+ emitter regions may be interdigitated between the drain pickup contacts of the drain regions. The p+ emitter regions may be interdigitated between each of the drain pickup contacts of each of the drain regions. The p+ emitter regions are typically each provided with at least one emitter contact, the emitter contacts and drain pickup contacts being electrically connected to each other. The emitter contacts and drain pickup contacts may for example be electrically connected to each other by a common metal layer. 
     Further, according to the invention, there is provided a method of making an NLDMOS, DMOS or NMOS device capable of withstanding snapback mode, comprising providing a first current path between source and drain for normal mode operation, and providing a discharge current path for handling dual injection current. The discharge current path may include a path through one or more p+ emitter regions formed between drain contacts. The p+ emitter regions preferably include emitter contacts electrically connected to the drain contacts. The discharge current path may include a path through multiple p+ emitter regions formed between the drain contacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-section through a prior art NLDMOS device; 
         FIG. 2  a cross-section through a prior art NLDMOS-SCR device; 
         FIG. 3  a top view of one embodiment of an NLDMOS device of the invention; 
         FIG. 4  a top view of another embodiment of an NLDMOS device of the invention, 
         FIG. 5  shows a top view of yet another embodiment of an NLDMOS device of the invention, and 
         FIG. 6  shows a top view of yet another embodiment of an NLDMOS device of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the invention is show in  FIG. 3 , which provides for the inclusion of a p+ poly-emitter  300  with contact  302  formed between the drain contacts  304  of n+ drain  306 . In this embodiment the other n+ drain  316  with its drain contacts  314 , formed on the other side of n+ source  320 , does not include any p+ emitter regions. The source  320  includes a bulk  322  and contacts  324 . The poly gate  330  with its gate contacts  332  is shown surrounding the source  320 . 
     During normal operation, the current will flow from the source to the drains  306 ,  316  to be collected by the drain contacts  304 ,  314 . However, under ESD conditions, the emitter region  300  provides the device with SCR characteristics and provides a second current path for dual injection current. 
     Another embodiment of the invention is shown in  FIG. 4 , which shows an array  400  with drain fingers  402  and source fingers  404 . The drain fingers include drain contacts  406  with interdigitated p+ emitters  408 . The p+ emitters  408  include emitter contacts  410  which are electrically connected to the drain contacts by a common metal layer  414 . The rest of the structure includes a bulk  420  with bulk contacts  422  and source contacts  424  for the source fingers  404 . The gates  430  with their gate contacts  432  are shown surrounding the source fingers  404 . During normal mode, current flows from the source fingers  404  to the drain fingers as shown by the arrows  450 . During an ESD event the p+ emitters  408  provide an SCR operation mode. Thus, when the voltage drop due to avalanche drain current opens the p-emitter junction, dual injection current will flow to the p+ emitters  408 , the emitters  408  thus providing another current path for the dual injection current and allowing the device to go into reversible snapback mode. 
     In the case of an array such as the one described above with respect to  FIG. 4 , the p+ emitters can be included in each of the drain fingers or only in one or some of the fingers. 
       FIG. 5  shows an embodiment in which the array  500  has five drain fingers  502  but only drain finger  504  is provided with p+ emitters  506 . The rest of the structure is substantially the same as that discussed with respect to  FIG. 4  and is therefore not discussed in detail again. For instance, the source fingers  508  are formed between the drain fingers  502  and are surrounded by polygates  510 . As shown in  FIG. 5 , the p+ emitters  506  are interdigitated between the drain contacts  512 , although at the top of the matrix two of the drain contacts are shown without an interdigitated p+ emitter. It will be appreciated that other embodiments could be manufactured in which only some or one pair of adjacent drain contacts includes an interdigitated p+ emitter. 
     Yet another embodiment of the invention is shown in  FIG. 6  in which all of the drain fingers  600  include multiple drain contacts  602  and interdigitated p+ emitters  604  with contacts  606 . The source fingers  610  are formed between the drain fingers  600  and are surrounded by polygates  612 . During normal mode, current flow between source and drain is as shown by arrows  620 , while during snapback mode, the dual injection current makes use of the second current path provided by the p+ emitters  604 , as shown by the arrows  630 . 
     In each of the embodiments, the contacts to the p+ emitters are electrically connected to the drain contacts, e.g., by connecting them using a common metal layer. 
     The present invention is applicable in very large high voltage devices e.g. 50 V devices where the device can be entirely self protecting, and is also applicable in smaller devices such as 24 V devices where it may function as a second stage together with a local ESD clamp. 
     While the invention has been described with respect to a few exemplary embodiments, it will be appreciated that these were included by way of illustration only and are not intended to limit the scope of the invention as defined by the claims.