Patent Publication Number: US-10332871-B2

Title: Area-efficient and robust electrostatic discharge circuit

Description:
BACKGROUND 
     Electrostatic discharge (ESD) is the sudden discharge of electric charge between two electrically charged nodes (e.g., Input-Output (IO) pins coupled to an Integrated Circuit (IC)). This sudden discharge typically produces a large current that passes through the IC in a short duration of time, which may result in damage or destruction of the IC, if not properly handled or protected. This large current is bypassed by circuits called ESD protection circuits. Typically, an ESD protection circuit provides a current path to ground and/or supply when an ESD event occurs so that the high current resulting from the ESD event bypasses the ESD sensitive circuitry in the IC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure, which, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only. 
         FIGS. 1A-B  illustrate part of an electrostatic discharge (ESD) input-output (IO) driver using discrete resistor ballasts. 
         FIG. 2  illustrates a part of an ESD resistant IO driver with local ballast gate devices, according to some embodiments of the disclosure. 
         FIG. 3  illustrates a cross-section of two transistors, where one of the transistors is weakly biased to provide local resistance ballast during an ESD event, in accordance with some embodiments of the disclosure. 
         FIG. 4A  illustrates a plot showing voltage robustness (Vt 2 ) as a function of gate bias for a conventional planar device. 
         FIG. 4B  illustrates a plot showing Vt 2  as a function of gate bias for a FinFET device configured to provide local resistance ballast during an ESD event, in accordance with some embodiments. 
         FIG. 5  illustrates a plot showing ESD design window and the benefit of smaller Vt 2  achieved from the devices of various embodiments using local ballast elements compared to Vt 2  for a traditional planar device. 
         FIG. 6  illustrates a high level schematic of local ballasting of Metal Oxide Semiconductor Field Effect Transistor (MOSFET), in accordance with some embodiments of the disclosure. 
         FIG. 7A  illustrates a top view of a layout showing a local ballast resistor formed using trench contact (TCN) layer and gate contact (GCN) layer, in accordance with some embodiments. 
         FIG. 7B  illustrates a portion of the side view of  FIG. 7A  showing the hot spot and region where a dummy via is added for cooling, in accordance with some embodiments. 
         FIG. 8  illustrates a layout and corresponding schematic for a compact ESD MOSFET with local ballast, according some embodiments of the disclosure. 
         FIG. 9  illustrates a plot showing effectiveness of the local ballast elements compared to conventional discrete resistors, in accordance with some embodiments of the disclosure. 
         FIG. 10  illustrates a part of an ESD resistant IO driver showing transistors using local ballast elements, in accordance with some embodiments of the disclosure. 
         FIG. 11  illustrates a plot showing improvement in Vt 2  using local ballast elements that result in smaller layout footprint, in accordance with some embodiments of the disclosure. 
         FIG. 12  illustrates a smart device or a computer system or a SoC (System-on-Chip) having an ESD circuit local ballast elements, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-B  illustrate part of electrostatic discharge (ESD) resistant IO drivers  100  and  120 , respectively, using discrete resistor ballasts. Drivers  100  and  120  illustrates diodes d 1  and d 2  such that the anode of diode d 2  is coupled to a pad (which may be coupled to a pin), the cathode of diode d 2  is coupled to a power supply node (e.g., 1.8 V), anode of diode d 1  is coupled to ground, and the anode of diode d 1  is coupled to the pad. Driver  100  includes two coupled switchable transistors—p-type transistor MP 1  and n-type transistor MN 1 . The drain terminals of transistors MP 1  and MN 1  are coupled together and to a discrete ballast resistor, which is also coupled to the pad. The p-type transistor MP 1  and the n-type transistor MN 1  are driven by pre-drivers that are controlled by logic not shown. 
     Driver  120  is similar to driver  100  except that an additional p-type transistor MP 2  is coupled in series with transistor MP 1 , and an additional n-type transistor MN 2  is coupled in series with transistor MN 1 . Here, transistors MN 2  and MP 2  are coupled in series such that their drain terminals are coupled together and to a discrete ballast resistor, which is also coupled to the pad. In this example, transistors MP 2  and MN 2  are biased by a fixed bias (e.g., 1.8 V when the power supply is 3.3 V) to be operational at 3.3 V input-output (IO) signal and supply voltages, while the transistors are natively for 1.8 V and cannot handle higher voltages due to reliability constraints. In other words, the transistors MP 2  and MN 2  form cascodes with MP 1  and MN 1 , respectively. 
     Discrete ballast resistors are normally used to limit current localization, thereby increasing the parasitic lateral NPN or PNP bipolar junction transistor (BJT) second breakdown failure current. These ballast resistors (also referred to as drain ballast resistors) are used to achieve tuning of Vt 2  for both ESD victim and protection element. Here, the term “Vt 2 ” refers to a voltage at which a device permanently fails due to thermal effects. For an ESD protection element, Vt 2  should be higher than the operating voltage (e.g., Vdd or power supply), and It 2  should be as high as possible to provide area-efficient ESD protection. Here, the term “It 2 ” refers to the current at which thermal destruction occurs. For an ESD victim element, Vt 2  should be higher than Vdd but also higher than the Vt 2  of the ESD protection element. In addition, Vt 2  for the ESD protection element and the ESD victim element should be lower than the destructive breakdown or gate dielectric reliability limit. 
     In mature planar technologies, ballast resistance is normally accomplished by silicide blocking on the drain (and source) side(s) of the transistor. Alternatively, conventional metal ballasting is employed using low levels of metals (e.g., Metal layer 1 (M1)) and/or vias as ballasting elements with silicide transistor diffusions. Discrete resistors (as shown in drivers  100  and  120 ) continue to be the typical mechanism of providing ballast resistance. 
     However, in advanced (FinFET) technologies, silicide blocking is not feasible due to patterning, process, and cost restrictions. As such, discrete resistors are typically added in series to the transistor drain terminals. These discrete resistors merely generate a voltage drop but provide no substantial improvement in It 2 . The ESD currents still spread non-uniformly, and may form hot spots, and the devices continue to face early failure due to local damage. Moreover, these discrete resistors are typically accomplished in a large gate pitch for better precision and lower variability. This means that problems are encountered with the implementation in layout in case of a core transistor at a different pitch due the need for a transition region between the different pitches. All in all, the discrete resistors add undesirable overhead in layout footprint area. 
     Some embodiments describe two mechanisms to tune Vt 2  and improve It 2 . These two mechanisms are referred to as local ballast gate (LBG) and local ballast resistor (LBR). Both these mechanisms result in reduced layout footprint and higher ESD performance thus provide better ESD protection. 
     In LBG, ballasting is achieved by an additional transistor gate coupled in series with the ESD victim element, in accordance with some embodiments. The basic principle is to skip connection of one or more gate fingers in a stacked MOSFET (Metal Oxide Semiconductor Field Effect Transistor) layout to decrease the current in the effective drain-source path (e.g., achieve a lateral de-bias of the active transistor finger used in the stack which results in a larger average ballast resistance for improved ESD robustness). In some embodiments, an apparatus is provided which comprises: a pad; a first transistor (e.g., LBG) coupled in series with a second transistor (e.g., ESD victim element) and coupled to the pad; and a self-biasing circuit to bias the first transistor such that the first transistor is to be weakly biased during an electrostatic discharge (ESD) event. 
     In LBR, a pro-elongated contact line is used that extends from each single transistor block to form a local ballast resistor, in accordance with some embodiments. In some embodiments, the contact line may be implemented with trench contact (TCN) and/or gate contact (GCN) materials and may extend in different shapes (e.g. semi-loop, loop, straight line, etc.) before being contacted through vias to metal layers above the TCN and/or GCN materials. As such, the LBR for ballasting exploits the materials for TCN/GCN both in their reacted form (i.e., silicide) and in their non-reacted form (i.e., metallic), in accordance with some embodiments. Silicide is formed in trenches. For instance, a metal (e.g., Co, Ti, Ni, etc.) is deposited which reacts with the silicon diffusions and forms a silicide. Metallic form (e.g., high Ohmic metal resistor region) of TCN/GCN is achieved when the metal (e.g., Co, Ti, Ni, etc.) is deposited by the same process step over an insulator (e.g., Shallow Trench Isolation (STI)). Being in immediate contact to the physical transistor (e.g., each local drain region), in some embodiments, this LBR is more effective than the discrete resistor template connected by a metal line to the drain regions of multiple transistor blocks. 
     In the following description, numerous details are discussed to provide a more thorough explanation of embodiments of the present disclosure. It will be apparent, however, to one skilled in the art, that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring embodiments of the present disclosure. 
     Note that in the corresponding drawings of the embodiments, signals are represented with lines. Some lines may be thicker, to indicate more constituent signal paths, and/or have arrows at one or more ends, to indicate primary information flow direction. Such indications are not intended to be limiting. Rather, the lines are used in connection with one or more exemplary embodiments to facilitate easier understanding of a circuit or a logical unit. Any represented signal, as dictated by design needs or preferences, may actually comprise one or more signals that may travel in either direction and may be implemented with any suitable type of signal scheme. 
     Throughout the specification, and in the claims, the term “connected” means a direct connection, such as electrical, mechanical, or magnetic connection between the things that are connected, without any intermediary devices. The term “coupled” means a direct or indirect connection, such as a direct electrical, mechanical, or magnetic connection between the things that are connected or an indirect connection, through one or more passive or active intermediary devices. The term “circuit” or “module” may refer to one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” may refer to at least one current signal, voltage signal, magnetic signal, or data/clock signal. The meaning of “a,” “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.” 
     The terms “substantially,” “close,” “approximately,” “near,” and “about,” generally refer to being within +/−10% of a target value. Unless otherwise specified the use of the ordinal adjectives “first,” “second,” and “third,” etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking or in any other manner. 
     For the purposes of the present disclosure, phrases “A and/or B” and “A or B” mean (A), (B), or (A and B). For the purposes of the present disclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C). The terms “left,” “right,” “front,” “back,” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. 
       FIG. 2  illustrates a part of an ESD resistant I/O driver  200  with LBG devices, according to some embodiments of the disclosure. It is pointed out that those elements of  FIG. 2  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     Driver  200  comprises diodes d 1  and d 2  and transistors MN 1  and MP 1  (as described with reference to  FIGS. 1A-B ), n-type LGB device MN 2 , p-type LGB device MP 2 , self-bias circuit  201 , and logic  202 . Here, the discrete resistor ballast of  FIGS. 1A-B  is removed. In some embodiments, the n-type LGB device MN 2  is coupled in series with an n-type transistor MN 1  and also coupled to the pad. In some embodiments, the source terminal (i.e., node n 1 ) of the LGB device MN 2  is coupled to the drain terminal of transistor MN 1  while the drain terminal of the LGB device MN 2  is coupled to the pad and the p-type LGB device MP 2 . In some embodiments, the p-type LGB device MP 2  is coupled in series with the p-type transistor MP 1  and also coupled to the pad. Here, the source terminal (i.e., node n 4 ) of the LGB device MP 2  is coupled to the drain terminal of transistor MP 1  while the drain terminal of the LGB device MP 2  is coupled to the pad and the n-type LGB device MN 2 . In one case, the source terminal of MP 1  is coupled to a first power supply node which provides a first power supply (e.g., 3.3 V) while the source terminal of transistor MN 1  is coupled to ground. 
     In some embodiments, self-bias circuit  201  comprises diode connected p-type transistor MP 3  and diode connected n-type transistor MN 3 . A diode connected MOSFET (metal oxide semiconductor field effect transistor) device is a two terminal device which is configured to be in saturation region when a conductive inversion channel is formed, and in blocking mode when an opposite polarity is formed. In some embodiments, the source/drain terminals of the transistors MP 3  and MN 3  are coupled to the second power supply node. The second power supply node may provide a lower power supply than a power supply provided by the first power supply node. For example, the second power supply is 1.8 V while the first power supply is 3.3 V. In some embodiments, one terminal (i.e., node n 3 ) of the diode connected p-type transistor MP 3  is used to bias p-type LGB device MP 2 . In some embodiments, one terminal (i.e., node n 2 ) of the diode connected n-type transistor MP 2  is used to bias n-type LGB device MN 2 . 
     In some embodiments, during an ESD event (e.g., when first and second power supplies are absent and a large current or voltage occurs on the pad), capacitive coupling effects (shown by parasitic capacitors Cgs (gate-to-source capacitor), Cgd (gate-to-drain capacitor), and Cbg (gate to bulk capacitor)) provide gate biasing to the quasi-floating gate terminals of LGB devices MP 2  and MN 2 . For example, the voltage on the gate terminal of LGB device MP 2  is weakly pulled down or the voltage on the gate terminal of LGB device MN 2  is weakly pulled up during an ESD event (depending on the different polarities at which the ESD stress event may occur at the IO pad). 
     As such, during an ESD event, the LGB devices MP 2  and MN 2  are weakly biased and provide the ballast resistance without the need for any discrete ballast resistor as shown in  FIGS. 1A-B . In some embodiments, the gate bias for transistors MN 2  and MP 2  may be derived with a capacitive voltage divider. The term “weakly bias” here generally refers to a transistor gate voltage Vgate which is at least a threshold voltage (Vt) which is less than approximately Vdrain/2 (for an n-type transistor), where “Vdrain” is the voltage on the drain terminal of the n-type transistor (e.g., Vt&lt;Vgate&lt;Vdrain/2). In contrast, strongly biased generally refers to a transistor gate voltage which is at least approximately Vdrain/2 less than Vdrain (e.g., Vdrain/2&lt;Vgate&lt;Vdrain). Strongly biased transistor is also a transistor having a gate voltage which is greater than the Vdrain (e.g., Vgate&gt;Vdrain). 
     Referring back to  FIG. 2 , during normal operation (e.g., when first and second power supplies are present and providing their expected power supplies), the gate terminals of LGB devices MP 2  and MN 2  are biased to normal levels (e.g., to 1.8V-Vt) via the diode connect devices (where Vt is a threshold voltage). In some embodiments, logic  202  is any suitable known timing logic which is used to turn on transistors MN 1  and MP 1  via pre-drivers for transmitting data to the pad. 
     While the embodiment of  FIG. 2  illustrates a full IO driver combination of n-type and p-type transistors (e.g., transistors MN 1  and MP 1 ), in some embodiments, one LGB device serves two transistors—n-type transistor coupled to ground and p-type transistor coupled to the power supply. For example, transistor MP 2  can be removed and transistor MN 2  can couple to transistors MN 1  and MP 1 . In another example, transistor MN 2  is removed and transistor MP 2  can couple to transistors MN 1  and MP 1 . 
       FIG. 3  illustrates a cross-section view  300  of two transistors, where one of the transistors is weakly biased to provide local gate ballasting during an ESD event, in accordance with some embodiments of the disclosure. It is pointed out that those elements of  FIG. 3  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     Cross-section view  300  illustrates two FinFETs (e.g., transistors MN 1  and MN 2 ) P-substrate  301 , P-well region  302 , shallow trench isolations (STIs)  303 , N+ diffusion regions  304 , fully depleted intrinsic fins  305 ; gate dielectric, TCN  306  ( 306   a  and  306   b ), Via contact (VCN)  307 , and (dummy) TCN  308  for floating N+ diffusion region  310 , and metal gate  309  (e.g., Metal Gate  1  for transistor MN 2  and Metal Gate  2  for transistor MN 1 ). In some embodiments, transistor MN 1  and transistor MN 2  have a shared drain/source region which represents node n 1  in  FIG. 2 . 
     Referring back to  FIG. 3 , in some embodiments, the shared N+ diffusion  310  region is floated and may be coupled to trench contact (TCN) layer  308 . Here, TCN layer is any suitable layer below a first metal layer or at the same level as the first metal layer that can be used for connecting various nodes. In some embodiments, the gate  309  of LGB device MN 2  is weakly turned on (or partially turned on) during an ESD event and fully turned on during normal operation. A device is weakly turned on when it is not fully on (on-state by applying supply level to gate of an n-type transistor) and not fully off (off-state by applying ground level to gate of an n-type transistor). 
     Compared to the regular stacked MOSFET layout of  FIG. 1B  where the two channels are either fully off (e.g., grounded-gate NMOS (GGNMOS)) or fully inverted (e.g., gate terminals tied high), the LGB device MN 2  can be modeled as if part of the double gate channel is not fully inverted but some current still flows by drift and spreads in a larger volume under the dummy gate rather than being confined in an thin inversion layer right under the gate. This current path is shown by the resistor under the dummy gate. In some aspects, the LGB device MN 2  is equivalent to a transistor with an extended drain that provides ballasting for current spreading that leads to higher voltage robustness Vt 2  of the device during ESD. 
       FIG. 4A  illustrates plot  400  showing voltage robustness Vt 2  as a function of gate bias for a conventional planar device such as the one used in  FIGS. 1A and 1   n    FIG. 1B . Here, x-axis is Vgs (gate-to-source voltage) across driver transistor MN 1  of  FIG. 1A  or  FIG. 1B  and y-axis is voltage Vt 2 .  FIG. 4B  illustrates plot  420  showing voltage robustness Vt 2  as a function of gate bias Vgs 1  of MN 1  for a FinFET device of  FIG. 2  configured to provide local resistance ballast during an ESD event, in accordance with some embodiments. Plot  420  shows an inverse behavior to plot  400 , and thus provides move ESD margin (e.g., as Vgs across LGB device MN 2  increases, Vt 2  decreases after reaching a maximum point). 
       FIG. 5  illustrates plot  500  showing an ESD design window and the benefit of smaller voltage robustness Vt 2  achieved from the devices of various embodiments using local ballast elements compared to voltage robustness Vt 2  for a traditional planar device. Here, x-axis is voltage and y-axis is ESD Current. Plot  500  shows the safe ESD protection area between IC operating area, IC reliability area, and thermal failure effects area. The margins for designing within the ESD design window is shown by the double arrow lines. Given these constraints, it is challenging to design an ESD protection circuit. The local ballast based design of various embodiments achieves the IV curve ‘A’ while the conventional discrete resistor ballast designs of  FIGS. 1A-B  achieve the IV curve ‘B’. The local ballast based design of the various embodiments provides passage for higher ESD current (thus better ESD performance) with reference to the voltage compared to the conventional discrete resistor ballast design of  FIGS. 1A-B . 
       FIG. 6  illustrates a high level schematic  600  of local ballasting of Metal Oxide Semiconductor Field Effect Transistor (MOSFET), in accordance with some embodiments of the disclosure. It is pointed out that those elements of  FIG. 6  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     In some embodiments, local ballast resistor  602  is intimately coupled to a drain terminal of a transistor block to form a local ballast element  601 . A number of local ballast elements can be coupled in parallel such that the source terminals of each transistor of the ballast elements form a common source terminal while one terminal of the local ballast resistors of each ballast element forms a common drain terminal. In some embodiments, local ballast resistor  602  is an LGB device. In some embodiments, local ballast resistor  602  is a TCN/GCN based resistor, where GCN is a gate contact layer. Here, GCN layer is any suitable layer below a first metal layer or at the same level as the first metal layer that can be used for connecting various nodes. In this case, local ballast resistor  602  is placed in series with each of the transistor blocks. In some embodiments, for local source ballasting, the resistor would connect to ground and not to the pad. 
       FIG. 7A  illustrates a top view  700  of a layout showing a local ballast resistor formed using TCN layer and GCN layer, in accordance with some embodiments. It is pointed out that those elements of  FIG. 7A  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     Top view  700  illustrates a multi-fin FinFET block with a local ballast resistor formed of TCN metallic strips and GCN layers. The ballast resistor may comprise TCN and/or GCN material and may extend in different shapes (e.g., semi-loop, loop, straight line, etc.) before being contacted through vias to the above metal layers. These various structural mechanisms for ballasting exploit TCN/GCN materials both, in reacted form (i.e., silicide) and in non-reacted form (i.e., metallic). Being in immediate contact to the physical transistor block (e.g., each local drain region), this LBR (local ballast resistor) is more effective than the discrete resistor template connected by a metal line to the drain regions of multiple transistor blocks as shown with reference to  FIGS. 1A-B . 
     Referring back to  FIG. 7A , here, the TCN and GCN layers are coupled in a loop to form a ballast resistor such that one end of the ballast resistor forms a drain contact (which is coupled to a higher metal layer (e.g., M0)). In some embodiments, the second terminal of the ballast resistor is additionally coupled to a dummy via contact (e.g., VCN contact). Here, VCN layer is any suitable layer that can be used for providing a via function. In some embodiments, the VCN contact is coupled to a segment of a higher metal layer (e.g., M0). The VCN contact coupling the ballast resistor loop is placed on top of or in direct vicinity to the hot spot of this layout. It is expected that the strongest heating may be at the interface of the dummy VCN contact and the TCN layer over STI (e.g., deposited metal composite) and TCN over diffusion (e.g., temperature-sensitive silicide).  FIG. 7B  illustrates a portion  720  of the top view  700  of  FIG. 7A  showing the hot spot and region where a dummy via will be (not shown in  FIG. 7A ) added for cooling, in accordance with some embodiments. 
     Referring back to  FIG. 7A , in some embodiments, the VCN contact and associate metal layer or metal stub (e.g., M0) provides cooling to the hot spot which would otherwise break down (e.g., by electro-migration or thermal breakdown of the transistor). In some embodiments, additional dummy metals can be stacked with corresponding vias on top of the dummy VCN contact for further cooling on top of the dummy VCN contact/via. The vertical arrows along the diffusion show the density of the current. For example, the larger arrow of the top implies large current and smaller arrows imply smaller current. The horizontal arrows along the active diffusion regions represent current in the individual fins. Other elements of layout  700  include poly strips, TCN strips (or layers) formed on silicon diffusion. By intimately integrating the ballast resistor as TCN and/or GCN layers, there may be no need for discrete ballast resistor as shown in  FIGS. 1A-B . 
       FIG. 8  illustrates layout  800  and corresponding schematic  820  for a compact ESD MOSFET with local ballast, according some embodiments of the disclosure. It is pointed out that those elements of  FIG. 8  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
     The various embodiments of using local ballast resistor avoids the metal wiring needed to connect the discrete transistor blocks to individual resistor elements (both often arranged in separate large arrays). Layout  800  is a MOS configuration layout of a local ballast resistor LBR (e.g.,  602 ) with contact lines extended uni-directionally from the contact orientation (e.g., y-direction) on the transistor, in accordance with some embodiments. The extended contact line can be drawn on any existing transistor layout gate pitch resulting in area savings and smallest parasitic capacitances are possible. In some cases, the TCN is extended over (usually dummy) diffusion regions, a desired increased resistance compared to TCN over STI gets created due to silicide regions formed by the TCN material being on contact with silicon diffusion. The corresponding schematic  820  shows the four MOS transistors of layout  800  and their associated local ballast resistor  602  (note that the transistor and the resistor are grouped as  601 ). 
       FIG. 9  illustrates plot  900  showing effectiveness of the local ballast elements compared to conventional discrete resistors, in accordance with some embodiments of the disclosure. Here, x-axis is Transmission-Line Pulse (TLP) voltage (in Volts (V)) and y-axis is TLP Current (in Amperes (A)). 
     Plot  900  shows three IV characteristics. The solid line is for conventional discrete ballast resistor as described with reference to  FIGS. 1A-B . The regular dashed line is the IV curve for the case using local ballast resistor of the various embodiments. The dashed-dot curve is an IV curve when no ballast resistor for an IO driver. Both the cases of conventional discrete ballast resistor and no ballast resistor show early failure. Plot  900  demonstrates the effectiveness of using local ballast elements (LBE) applied to GGNMOS transistors, here shown for local ballasting resistance (LBR). The IV-slope can be tuned like with conventional ballasting (e.g., up to 7V/0.05 mA) but then the additional advantage becomes clear—strong bipolar action is enabled by avoiding localized early failure and pushing It 2  up which is in contrast to conventional ballasting or in contrast to no ballasting. The embodiments allow usage of the LBR-hardened devices either as local ESD clamps and/or as ESD-robust drivers. The LBR of various embodiments saves layout area and also reduces parasitic capacitance. 
       FIG. 10  illustrates part of ESD resistance IO driver  1000  showing transistors using local ballast elements, in accordance with some embodiments of the disclosure. It is pointed out that those elements of  FIG. 10  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. The ESD robust driver  1000  is similar to  FIG. 2  except that the self-biasing circuit is removed, the LBG devices MP 2  and MN 2  are removed (and so the corresponding transistors MP 3  and MN 3  are removed too) and transistors MP 1  and MN 1  are replaced with LBR integrated MOS devices (e.g.,  601 ). Functionally, ESD robust driver circuit  1000  is similar to ESD robust driver circuit  200 . 
       FIG. 11  illustrates plot  1100  showing improvement in Vt 2  using local ballast elements that result in smaller layout footprint, in accordance with some embodiments of the disclosure. Here, x-axis is TLP voltage (V) and y-axis is TLP Current (A). The first IV curve (STK) from the left is of the stacked transistor configuration of  FIG. 1B  but without the discrete ballast resistor. The second IV curve (i.e., STK+ballast resistor) from the left is of the stacked transistor configuration of  FIG. 1B . The third IV curve (i.e., SKG) is of the LGB based circuit  200  of  FIG. 2 . 
     Vt 2  increases as expected on stacked transistors once discrete ballast resistance is added (e.g., +0.5 kOhm→+1V in Vt 2  for STK→STK+BallastRes). The same Vt 2  increase is achieved by Local Ballasting Gate (SKG device) without the use of ballast resistance and hence with significant layout footprint savings. An equivalent Vt 2  is achieved. Compared to LBR approach, one advantage of the LGB is the footprint savings of having no ballast resistor at all. 
       FIG. 12  illustrates a smart device or a computer system or a SoC (System-on-Chip) having an ESD circuit local ballast elements, according to some embodiments. It is pointed out that those elements of  FIG. 12  having the same reference numbers (or names) as the elements of any other figure can operate or function in any manner similar to that described, but are not limited to such. 
       FIG. 12  illustrates a block diagram of an embodiment of a mobile device in which flat surface interface connectors could be used. For purposes of the embodiments, the transistors in various circuits, modules, and logic blocks are metal oxide semiconductor (MOS) transistors, which include drain, source, gate, and bulk terminals. The transistors also include Tri-Gate and FinFET transistors, Gate All Around Cylindrical Transistors, Tunneling FET (TFET), Square Wire, or Rectangular Ribbon Transistors or other devices implementing transistor functionality like carbon nano tubes or spintronic devices. MOSFET symmetrical source and drain terminals i.e., are identical terminals and are interchangeably used here. A TFET device, on the other hand, has asymmetric Source and Drain terminals. Those skilled in the art will appreciate that other transistors, for example, Bi-polar junction transistors—BIT PNP/NPN, BiCMOS, CMOS, eFET, etc., may be used without departing from the scope of the disclosure. 
     In some embodiments, computing device  2100  represents a mobile computing device, such as a computing tablet, a mobile phone or smart-phone, a wireless-enabled e-reader, or other wireless mobile device. It will be understood that certain components are shown generally, and not all components of such a device are shown in computing device  2100 . 
     In some embodiments, where each block shown in computing device  2100  is an individual chip, ESD circuits with local ballast elements are provided at the pads (coupled to pins) of the individual chip. In some embodiments, where the SoC is a single large chip, ESD circuits with local ballast elements are provided at the pads (coupled to the pins) of the SoC. 
     In one embodiment, processor  2110  can include one or more physical devices, such as microprocessors, application processors, microcontrollers, programmable logic devices, or other processing means. The processing operations performed by processor  2110  include the execution of an operating platform or operating system on which applications and/or device functions are executed. The various embodiments of the present disclosure may also comprise a network interface within  2170  such as a wireless interface so that a system embodiment may be incorporated into a wireless device, for example, cell phone or personal digital assistant. 
     The processing operations include operations related to I/O (input/output) with a human user or with other devices, operations related to power management, and/or operations related to connecting the computing device  2100  to another device. The processing operations may also include operations related to audio I/O and/or display I/O. 
     In one embodiment, computing device  2100  includes audio subsystem  2120 , which represents hardware (e.g., audio hardware and audio circuits) and software (e.g., drivers, codecs) components associated with providing audio functions to the computing device. Audio functions can include speaker and/or headphone output, as well as microphone input. Devices for such functions can be integrated into computing device  2100 , or connected to the computing device  2100 . In one embodiment, a user interacts with the computing device  2100  by providing audio commands that are received and processed by processor  2110 . 
     Display subsystem  2130  represents hardware (e.g., display devices) and software (e.g., drivers) components that provide a visual and/or tactile display for a user to interact with the computing device  2100 . Display subsystem  2130  includes display interface  2132 , which includes the particular screen or hardware device used to provide a display to a user. In one embodiment, display interface  2132  includes logic separate from processor  2110  to perform at least some processing related to the display. In one embodiment, display subsystem  2130  includes a touch screen (or touch pad) device that provides both output and input to a user. 
     I/O controller  2140  represents hardware devices and software components related to interaction with a user. I/O controller  2140  is operable to manage hardware that is part of audio subsystem  2120  and/or display subsystem  2130 . Additionally, I/O controller  2140  illustrates a connection point for additional devices that connect to computing device  2100  through which a user might interact with the system. For example, devices that can be attached to the computing device  2100  might include microphone devices, speaker or stereo systems, video systems or other display devices, keyboard or keypad devices, or other I/O devices for use with specific applications such as card readers or other devices. 
     As mentioned above, I/O controller  2140  can interact with audio subsystem  2120  and/or display subsystem  2130 . For example, input through a microphone or other audio device can provide input or commands for one or more applications or functions of the computing device  2100 . Additionally, audio output can be provided instead of, or in addition to display output. In another example, if display subsystem  2130  includes a touch screen, the display device also acts as an input device, which can be at least partially managed by I/O controller  2140 . There can also be additional buttons or switches on the computing device  2100  to provide I/O functions managed by I/O controller  2140 . 
     In one embodiment, I/O controller  2140  manages devices such as accelerometers, cameras, light sensors or other environmental sensors, or other hardware that can be included in the computing device  2100 . The input can be part of direct user interaction, as well as providing environmental input to the system to influence its operations (such as filtering for noise, adjusting displays for brightness detection, applying a flash for a camera, or other features). 
     In one embodiment, computing device  2100  includes power management  2150  that manages battery power usage, charging of the battery, and features related to power saving operation. Memory subsystem  2160  includes memory devices for storing information in computing device  2100 . Memory can include nonvolatile (state does not change if power to the memory device is interrupted) and/or volatile (state is indeterminate if power to the memory device is interrupted) memory devices. Memory subsystem  2160  can store application data, user data, music, photos, documents, or other data, as well as system data (whether long-term or temporary) related to the execution of the applications and functions of the computing device  2100 . 
     Elements of embodiments are also provided as a machine-readable medium (e.g., memory  2160 ) for storing the computer-executable instructions (e.g., instructions to implement any other processes discussed herein). The machine-readable medium (e.g., memory  2160 ) may include, but is not limited to, flash memory, optical disks, CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards, phase change memory (PCM), or other types of machine-readable media suitable for storing electronic or computer-executable instructions. For example, embodiments of the disclosure may be downloaded as a computer program (e.g., BIOS) which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals via a communication link (e.g., a modem or network connection). 
     Connectivity  2170  includes hardware devices (e.g., wireless and/or wired connectors and communication hardware) and software components (e.g., drivers, protocol stacks) to enable the computing device  2100  to communicate with external devices. The computing device  2100  could be separate devices, such as other computing devices, wireless access points or base stations, as well as peripherals such as headsets, printers, or other devices. 
     Connectivity  2170  can include multiple different types of connectivity. To generalize, the computing device  2100  is illustrated with cellular connectivity  2172  and wireless connectivity  2174 . Cellular connectivity  2172  refers generally to cellular network connectivity provided by wireless carriers, such as provided via GSM (global system for mobile communications) or variations or derivatives, CDMA (code division multiple access) or variations or derivatives, TDM (time division multiplexing) or variations or derivatives, or other cellular service standards. Wireless connectivity (or wireless interface)  2174  refers to wireless connectivity that is not cellular, and can include personal area networks (such as Bluetooth, Near Field, etc.), local area networks (such as Wi-Fi), and/or wide area networks (such as WiMax), or other wireless communication. 
     Peripheral connections  2180  include hardware interfaces and connectors, as well as software components (e.g., drivers, protocol stacks) to make peripheral connections. It will be understood that the computing device  2100  could both be a peripheral device (“to”  2182 ) to other computing devices, as well as have peripheral devices (“from”  2184 ) connected to it. The computing device  2100  commonly has a “docking” connector to connect to other computing devices for purposes such as managing (e.g., downloading and/or uploading, changing, synchronizing) content on computing device  2100 . Additionally, a docking connector can allow computing device  2100  to connect to certain peripherals that allow the computing device  2100  to control content output, for example, to audiovisual or other systems. 
     In addition to a proprietary docking connector or other proprietary connection hardware, the computing device  2100  can make peripheral connections  1680  via common or standards-based connectors. Common types can include a Universal Serial Bus (USB) connector (which can include any of a number of different hardware interfaces), DisplayPort including MiniDisplayPort (MDP), High Definition Multimedia Interface (HDMI), Firewire, or other types. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments. The various appearances of “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. If the specification states a component, feature, structure, or characteristic “may,” “might,” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the elements. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive 
     While the disclosure has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations of such embodiments will be apparent to those of ordinary skill in the art in light of the foregoing description. The embodiments of the disclosure are intended to embrace all such alternatives, modifications, and variations as to fall within the broad scope of the appended claims. 
     In addition, well known power/ground connections to integrated circuit (IC) chips and other components may or may not be shown within the presented figures, for simplicity of illustration and discussion, and so as not to obscure the disclosure. Further, arrangements may be shown in block diagram form in order to avoid obscuring the disclosure, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present disclosure is to be implemented (i.e., such specifics should be well within purview of one skilled in the art). Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the disclosure, it should be apparent to one skilled in the art that the disclosure can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting. 
     The following examples pertain to further embodiments. Specifics in the examples may be used anywhere in one or more embodiments. All optional features of the apparatus described herein may also be implemented with respect to a method or process. 
     For example, an apparatus is provided which comprises: a pad; a first transistor coupled in series with a second transistor and coupled to the pad; and a self-biasing circuit to bias the first transistor such that the first transistor is to be weakly biased during an electrostatic discharge (ESD) event. In some embodiments, the first and second transistors are n-type FinFETs. In some embodiments, the apparatus comprises a first diode coupled to the pad and the second transistor via a ground node. 
     In some embodiments, the self-biasing circuit comprises a diode-connected transistor with a source terminal coupled to the first transistor and a drain terminal coupled to a first power supply node. In some embodiments, the apparatus comprises a third transistor coupled in series with a fourth transistor and coupled to the pad. In some embodiments, the third and fourth transistors are p-type FinFETs. In some embodiments, the self-biasing circuit is to bias the third transistor such that the third transistor is to be weakly biased during the ESD event. In some embodiments, the self-biasing circuit comprises a diode-connected transistor with a source terminal coupled to third transistor and a drain terminal coupled to the first power supply node. In some embodiments, the apparatus comprises a second diode coupled to the pad and the fourth transistor via a second power supply node. In some embodiments, the second power supply node is to supply a power higher than a power supplied by the first power supply node. In some embodiments, the third and fourth transistors are p-type FinFETs. 
     In another example, a system is provided which comprises: an integrated circuit including: an input-output (I/O) pad; and an electrostatic discharge (ESD) circuit coupled to the I/O pad, wherein the ESD circuit comprises: a first transistor coupled in series with a second transistor and coupled to the I/O pad; and a self-biasing circuit to bias the first transistor such that the first transistor is to be weakly biased during an ESD event; and an interface allowing the integrated circuit to communicate with another device. 
     In some embodiments, the self-biasing circuit comprises a diode-connected transistor with a source terminal coupled to the first transistor and a drain terminal coupled to a first power supply node. In some embodiments, the ESD circuit comprises an apparatus to the apparatus described above. 
     In another example, an apparatus is provided which comprises: a first transistor; and a first local ballast resistor formed of a trench contact (TCN) layer, the first local ballast resistor having a first terminal coupled to either the drain or source terminal of the first transistor. In some embodiments, the first transistor is an n-type FinFET, and wherein a source terminal of the transistor is coupled directly or indirectly to a ground node. In some embodiments, the first local ballast resistor has a second terminal coupled to a pad. 
     In some embodiments, the first local ballast resistor is coupled to a dummy via contact node (VCN) for cooling. In some embodiments, the dummy via is coupled to a metal stub. In some embodiments, the apparatus comprises: a second transistor, and a second local ballast resistor formed of a TCN layer, the second local ballast resistor having a first terminal coupled to either drain or source terminals of the second transistor. In some embodiments, the second transistor has a source terminal which is directly or indirectly coupled to a power supply node, and wherein the second local ballast resistor has a second terminal coupled to the pad, wherein the second local ballast resistor is coupled to a dummy via contact node (VCN) for cooling, and wherein the dummy via is coupled to a metal stub. In some embodiments, resistance of the first local ballast resistor increases when the TCN is extended over one or more diffusion regions. 
     In another example, a system is provided which comprises: a memory; a processor coupled to the memory, the processor having an apparatus according to the apparatus described above; and a wireless interface for allowing the processor to communicate with another device. 
     In another example, a method is provided which comprises: weakly biasing a first transistor, coupled in series with a second transistor and coupled to a pad, during an electrostatic discharge (ESD) event. In some embodiments, the first and second transistors are n-type FinFETs. In some embodiments, the method comprises coupling a first diode to the pad and the second transistor via a ground node. 
     In another example, an apparatus is provided which comprises: means for weakly biasing a first transistor, coupled in series with a second transistor and coupled to a pad, during an electrostatic discharge (ESD) event. In some embodiments, the first and second transistors are n-type FinFETs. In some embodiments, the apparatus means for coupling a first diode to the pad and the second transistor via a ground node. 
     An abstract is provided that will allow the reader to ascertain the nature and gist of the technical disclosure. The abstract is submitted with the understanding that it will not be used to limit the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.