Abstract:
A semiconductor dual guardring arrangement is provided which is useful during electrostatic discharge (ESD) events as well as during normal operating conditions. In particular, an inner guard that is located closer to an active area provides desirable performance during normal operating conditions, while an outer guardring located further from the active area provides desirable performance during an ESD event.

Description:
FIELD OF INVENTION 
   The present invention relates generally to the art of semiconductor devices, and more particularly to a dual guardring arrangement that is useful during both electrostatic discharge (ESD) events as well as during normal operating conditions. 
   BACKGROUND OF THE INVENTION 
   Electrostatic discharge (ESD) is a continuing problem in the design, manufacture and utilization of semiconductor devices. Integrated circuits (ICs) can be damaged by ESD events stemming from a variety of sources, in which large currents flow through the device in an uncontrolled fashion. In one such ESD event, a packaged IC acquires a charge when it is held by a human whose body is electrostatically charged. An ESD event occurs when the IC is inserted into a socket, and one or more of the pins of the IC package touch the grounded contacts of the socket. This type of event is known as a human body model (HBM) ESD stress. For example, a charge of about 0.6 μC can be induced on a body capacitance of 150 pF, leading to electrostatic potentials of 4 kV or greater. HBM ESD events can result in a discharge for about 100 nS with peak currents of several amperes to the IC. Another source of ESD is from metallic objects, known as the machine model (MM) ESD source, which is characterized by a greater capacitance and lower internal resistance than the HBM ESD source. The MM ESD model can result in ESD transients with significantly higher rise times than the HBM ESD source. A third ESD model is the charged device model (CDM), which involves situations where an IC becomes charged and discharges to ground. In this model, the ESD discharge current flows in the opposite direction in the IC than that of the HBM ESD source and the MM ESD source. CDM pulses also typically have very fast rise times compared to the HBM ESD source. 
   ESD events typically involve discharge of current between one or more pins or pads exposed to the outside of an integrated circuit chip. Such ESD current flows from the pad to ground through vulnerable circuitry in the IC, which may not be designed to carry such currents. Many ESD protection techniques have been thusfar employed to reduce or mitigate the adverse effects of ESD events in integrated circuit devices. Many conventional ESD protection schemes for ICs employ peripheral dedicated circuits to carry the ESD currents from the pin or pad of the device to ground by providing a low impedance path thereto. In this way, the ESD currents flow through the protection circuitry, rather than through the more susceptible circuits in the chip. 
   Such protection circuitry is typically connected to I/O and other pins or pads on the IC, wherein the pads further provide the normal circuit connections for which the IC was designed. Some ESD protection circuits carry ESD currents directly to ground, and others provide the ESD current to the supply rail of the IC for subsequent routing to ground. Rail-based clamping devices can be employed to provide a bypass path from the IC pad to the supply rail (e.g., VDD) of the device. Thereafter, circuitry associated with powering the chip is used to provide such ESD currents to the ground. Local clamps are more common, wherein the ESD currents are provided directly to ground from the pad or pin associated with the ESD event. Individual local clamps are typically provided at each pin on an IC, with the exception of the ground pin or pins. 
   SUMMARY OF THE INVENTION 
   The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
   A semiconductor dual guardring arrangement is provided which is useful during electrostatic discharge (ESD) events as well as during normal operating conditions. In particular, an inner guardring that is located closer to an active area provides desirable performance during normal operating conditions, while an outer guardring located further from the active area provides desirable performance during an ESD event. 
   To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which one or more aspects of the present invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the annexed drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating an exemplary dual guardring arrangement suitable for use with one or more NMOS transistors. 
       FIG. 2  is a schematic diagram illustrating an exemplary dual guardring arrangement suitable for use with one or more PMOS transistors. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One or more examples are described with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more examples. It may be evident, however, to one skilled in the art that one or more examples may be practiced with a lesser degree of these specific details. In other instances, known structures and devices are illustrated to facilitate describing one or more examples. 
   Turning to  FIG. 1 , an exemplary dual guardring arrangement  100  is illustrated that is suitable for use with one or more NMOS transistors. The illustration is a top view of the arrangement  100 , which is formed on a semiconductor substrate  102 , where the substrate  102  may comprise any type of semiconductor body (e.g., silicon, SiGe, SOI) such as a semiconductor wafer or one or more die on a wafer, as well as any other type of semiconductor and/or epitaxial layers grown thereon and/or otherwise associated therewith. Since the illustrated arrangement  100  has application to one or more NMOS transistors, the illustrated portion of the substrate  102  may be doped to have a p type electrical conductivity, such that the arrangement  100  can be said to be formed in a p type well in the substrate  102 . 
   An active area  104  is centrally located in the arrangement  100 . The active area  104  comprises a region of the substrate  102  wherein one or more semiconductor devices can be formed. As such, since this arrangement has application to NMOS devices, the active area  104  is doped to have an n type electrical conductivity, and can thus be said to comprise an n type well. Additionally, one or more regions of electrically conductive material  106 , such as patterned polysilicon, for example, are formed over the active area  104  to serve as, at least part of, one or more NMOS transistor gates, for example. 
   A first guardring  108  is formed in the substrate  102  around the active area  104  and the conductive regions  106 . The first guardring  108  is doped to have p type electrical conductivity. The guardring  108  generally extends down to a subsurface or underlying substrate layer, such as a backgate region of the one or more NMOS transistors, for example. The first guardring  108  is situated relatively close to the active area  104  to satisfy normal operating requirements. Although the distance  110  between the first guardring  108  and the active area  104  may be technology dependent, the first guardring  108  and the active area  104  are generally separated by a distance  110  of between about 0.25 and about 1.5 microns, for example. 
   A second guardring  112  is formed in the substrate  102  around the first guardring  108 . Like the first guardring  108 , the second guardring  112  comprises an area of the substrate  102  that is doped to have a p type electrical conductivity. The second guardring  112  also generally extends down to a subsurface or underlying substrate layer, such as a backgate region of the one or more NMOS transistors, for example. The second guardring  112  is distanced away from the active area  104  to satisfy requirements during an ESD event. The second guardring  112  and the active area  104  are generally separated by a distance  114  of between about 2.5 and about 25 microns, for example. 
   A schematically illustrated inverter  120  is operatively coupled to the first guardring  108 . The inverter  120  comprises first and second transistors  122 ,  124 , where the gates (G) of the transistors are coupled to a first voltage Vdd, which generally comprises a supply voltage. The source (S) of the first transistor  122  is also coupled to the supply voltage Vdd, while the source (S) of the second transistor  124  is coupled to second voltage Vss, which generally corresponds to ground. The respective drains (D) of the first  122  and second  124  transistors are operatively coupled to the first guardring  108 . The second guardring  112  is operatively coupled to the second voltage Vss. 
   As previously mentioned, the first or inner guardring  108  operates during normal operating conditions, while the second or outer guardring  112  becomes operational during an ESD event. The first guardring  108  serves as a tap or input/output buffer for the n well active area  104  by inhibiting hot carriers and/or other undesirable particles or contaminants from entering into and exiting out of the active area  104 . The distance  110  between the first guardring  108  and the active area  104  is accordingly kept small to minimize any such adverse effects. For example, keeping the first guardring  108  close to the one or more NMOS transistors that are touching pads mitigates well/ground bounce effects wherein well or ground voltages can be inadvertently changed. This orientation also mitigates noise injection where noise can be undesirably introduced into the circuitry. This orientation further facilitates appropriate latch up robustness whereby desired current flow is generated within and/or between devices. 
   The distance  114  between the first guardring  108  and the active area  104  is kept large, on the other hand, to facilitate desired operation during ESD events. For example, during ESD conditions it is desirable to have tap guardrings placed far from the one or more NMOS transistors touching the pads. During human body model (HBM) ESD events, for example, locating the guardring far from the devices facilitates increased resistance from the substrate  102  which is beneficial for the uniform conduction of the one or more NMOS transistors being protected. Similarly, during charged device model (CDM) ESD events the separation between the guardring  108  and the devices facilitates enhanced gate to bulk oxide breakdown. 
   By way of further example, during normal operation Vdd is applied to the respective gates of the first  122  and second  124  transistors of the inverter  120 . As such, the first guardring  108  is pulled down to Vss through the connection to the respective drains of the devices  122 , 124 . As a result, both guardrings  108  and  112  are at Vss which is desirable for normal operating conditions. During an ESD event, however, Vdd is floating such that the respective drains of the first  122  and second  124  transistors of the inverter  120  are floating as well. As such, the first guardring  108  floats accordingly due to the coupling to the respective drains of the devices  122  and  124 . Thus, merely the second or outer guardring  112  is connected during an ESD event, which is desirable for the reasons described above. 
   Turning to  FIG. 2 , an exemplary dual guardring arrangement  200  is illustrated that is suitable for use with one or more PMOS transistors. The illustration is similar to that depicted in  FIG. 1 , except that electrical conductivities are reversed/opposite. Accordingly, the substrate  202  may be doped to have an n type electrical conductivity, such that the arrangement  200  can be said to be formed in an n type well in the substrate  202 . 
   An active area  204  is centrally located in the arrangement  200 . The active area  204  comprises a region of the substrate  202  wherein one or more semiconductor devices can be formed. As such, since this arrangement has application to PMOS devices, the active area  204  is doped to have a p type electrical conductivity, and can thus be said to comprise a p type well. Additionally, one or more regions of electrically conductive material  206 , such as patterned polysilicon, for example, are formed over the active area  204  to serve as, at least part of, one or more PMOS transistor gates, for example. 
   A first guardring  208  is formed in the substrate  202  around the active area  204  and the conductive regions  206 . The first guardring  208  is doped to have and n type electrical conductivity. The guardring  208  generally extends down to a subsurface or underlying substrate layer, such as a backgate region of the one or more PMOS transistors, for example. As with the arrangement illustrated in  FIG. 1 , the first guardring  208  is situated relatively close to the active area  204  to satisfy normal operating requirements. In particular, the first guardring  208  and the active area  204  are generally separated by a distance  210  of between about 0.25 and about 2.5 microns, for example. 
   A second guardring  212  is formed in the substrate  202  around the first guardring  208 . Like the first guardring  208 , the second guardring  212  comprises an area of the substrate  202  that is doped to have an n type electrical conductivity. The second guardring  212  also generally extends down to a subsurface or underlying substrate layer, such as a backgate region of the one or more PMOS transistors, for example. The second guardring  212  and the active area  204  are generally separated by a distance  214  of between about 2.5 and about 25 microns, for example. 
   A schematically illustrated inverter  220  is operatively coupled to the first guardring  208 . The inverter  220  comprises first and second transistors  222 ,  224 , where the source (S) of the first transistor  222  is coupled to a first voltage Vdd, which generally comprises a supply voltage. The source (S) of the second transistor  224  is coupled to a second voltage Vss, which generally corresponds to ground. The respective gates (G) of the transistors  222 ,  224  are also coupled to Vss. The respective drains (D) of the first  222  and second  224  transistors are operatively coupled to the first guardring  208 . The second guardring  212  is operatively coupled to the source voltage Vdd. 
   As with the NMOS related arrangement  100  illustrated in  FIG. 1 , the first or inner guardring  208  operates during normal operating conditions, while the second or outer guardring  212  becomes operational during an ESD event. During normal operation, the application of Vss to the gates of the first  222  and second  224  transistors causes Vdd to be at the respective drains of the transistors  222 ,  224  such that the first guardring  208  is pulled up to the Vdd through the connection to the drains of the devices  222 ,  224 . As a result, both guardrings  208  and  212  are at Vdd which is desirable for normal operating conditions. During an ESD event, however, Vdd is floating such that the respective drains of the first  222  and second  224  transistors of the inverter  220  are floating as well. As such, the first guardring  208  floats accordingly due to the coupling to the respective drains of the devices  222  and  224 . Thus, merely the second or outer guardring  212  is connected during an ESD event, which is desirable for the reasons set forth above. 
   Although the invention has been shown and described with respect to one or more examples, equivalent alterations, modifications and/or implementations may occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The invention includes all such modifications and alterations and is limited only by the scope of the following claims. In addition, while a particular feature or aspect of the invention may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” Also, the term “exemplary” is merely meant to mean an example, rather than the best. Further, while the guardrings and other features have been illustrated as being substantially square or rectangular, they are not intended to be limited to these exact shapes.