Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/690,701, filed Jun. 15, 2005, the entirety of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to circuits, and in particular, to electrostatic discharge protection for circuits. Embodiments of the invention apply to all fields using CMOS processes (including BICMOS where CMOS ESD clamps are used). 
     2. Description of the Related Art 
     Electrostatic Discharge (ESD) is a major source of reliability failures in integrated circuits (ICs). For example, ESD arises when electrostatic charge accumulated on an object, for example a human body or a piece of equipment, is conducted onto another object, for example, a circuit board. This conduction of charge often results in damage to ICs, whether through electrical over-voltage stress or through thermal stress caused by large currents. 
     While it is possible to reduce the severity of an ESD event by reducing the build up of electrostatic charge potential, by, for example, controlling humidity in lab environments, the potential is difficult to completely mitigate. As a result, ICs incorporate ESD protection structures, allowing them to tolerate a certain level of ESD in order not to create reliability hazards. How an integrated circuit is assembled along with how the ESD protection structures are assembled is often referred to as an ESD protection strategy. 
     A representative ESD protection strategy, illustrating the protection of a device incorporating a single input/output (I/O) bond site and a single power and ground rail, is shown in  FIG. 1 . Three bond sites, one for the positive power supply VDD, one for the negative power supply VSS, and one for a signal labeled Signal are drawn at the left. Signal is protected to both VDD and VSS by primary clamp structures. A series resistor R ESD  and secondary positive and negative clamps further reduce the voltage applied as input to the I/O circuitry. The I/O circuitry is operatively coupled to the device core circuitry. A power clamp is placed between the VDD and VSS rails. Not all ESD protection strategies will include all of these components, and more complex strategies involving multiple positive and/or negative supply rails can include even more components. However the schematic of  FIG. 1  illustrates the major components of an ESD protection strategy. 
     The various clamps of the circuit in  FIG. 1  shunt the ESD current away from the I/O circuitry and the core circuitry, providing a low-impedance path through the device, thereby avoiding over-voltage stress on the I/O and core circuitry. In addition, the clamps themselves should be able to handle the ESD current without damage from thermal over-stress caused by large ESD currents. 
     However, including the ESD protection strategy into an IC comes at a significant cost, both in area (cost) and in performance (speed and signal integrity). Because the various clamps are often physically large, the ESD protection strategy can be a significant fraction of the total area for an integrated circuit. Therefore, the cost of the integrated circuit is directly impacted by the requirement for ESD tolerance. Additionally, because the clamps are physically large, they can exhibit significant parasitic capacitances, which act to reduce the speed at which a signal can be driven. This parasitic capacitance also can cause signal integrity issues on signal traces due to increased reflections. Accordingly, it is desirable to reduce the size of an ESD clamp. 
     In CMOS circuitry, four main structures are commonly used for constructing ESD clamps. These structures are (1) Diodes; (2) “Big FET” MOS devices; (3) “Snapback” MOS devices; and (4) Silicon Controlled Rectifiers (SCRs). 
     The simplest structure is the diode. A diode is commonly fabricated as a simple P-N junction (for example, a p-type diffusion region in an n-well or an n-type diffusion region in a p-well). As used herein, a well is a lightly doped region and a diffusion region is a heavily doped region. A diode structure is simple, has a very high current carrying capacity per unit area, and is easy to simulate. In many respects, the diode is close to an ideal clamp, but usually only in one direction (the other direction can also clamp depending on the type of diode—for example a Zener diode can clamp in both directions, but this is not commonly used in a CMOS process). As a result, most realizable ESD protection strategies that use diodes also use one or more of the other structures. 
     A structure using MOS devices for ESD clamps, known as the “Big FET” approach, uses a trigger circuit to turn on a relatively large MOS device (MOSFET) to conduct current during the ESD event. This approach is particularly attractive from a simulation perspective because no parasitic devices are involved, so that a Big FET ESD Clamp can be readily simulated in a standard SPICE-compatible simulator. However, a MOS device is a surface conduction device (unlike the bulk conduction of the snapback and SCR devices) and its current carrying capacity is relatively small per unit area. Because of this, the area used by a Big FET structure is often significantly larger than that used for either the snapback or SCR structures. 
     Another approach for ESD clamping with MOS devices uses what is called the “snapback” device. This approach makes use of the parasitic lateral NPN bipolar device that is inherent to an NMOS device. During an ESD event, the parasitic NPN bipolar transistor turns on, conducting the ESD current. This bulk conduction permits a snapback device to conduct more current per unit area than a surface-conduction device, such as used with the Big FET approach. The use of a snapback device is also attractive because the snapback device can be made self-triggering and can also be used as the output device for standard CMOS I/O structures, thereby making so-called “self-protecting” I/O&#39;s. However, the snapback device has a weakness: the parasitic bipolar transistor has significant variation from device to device within a multi-finger structure, and also across the width of a single finger. These variations mean that current is typically not equally distributed across the device. Furthermore, these types of devices have a tendency to become more conductive the hotter they get, which means that unless steps are taken to prevent it, the hotter spots in the clamp will conduct relatively more ESD current, which can cause a localized failure in the clamp. In order to prevent this, a current ballast structure can be inserted into the drain of the NMOS device to ensure even spreading of the ESD current. This ballast structure can increase the size and cost of the snapback device. 
     Another structure commonly used for ESD clamps is a Silicon-Controlled Rectifier (SCR). The SCR makes use of two parasitic bipolar transistors, an NPN and a PNP, and as a result it too has relatively large current conduction ability, potentially higher than snapback devices. However, unlike the snapback device, the self-triggering voltage for a typical SCR is typically in the 10-20V range, which is too high for the majority of applications in fine-geometry ICs. In order to overcome this limitation, SCRs use a trigger circuit to turn on during the ESD event, which complicates the design significantly. In addition, SCRs normally require more simulation and testing than snapback devices, which also complicates their use. 
     As a result of all this, the most common structure (after the diode) used in the industry is the snapback device. The remainder of this disclosure will focus on techniques to reduce the area used by the current spreading ballast associated with this device, which reduces the IC cost and potentially increases its performance. 
     The snapback device commonly used in ESD protection schemes is the parasitic NPN bipolar device that is inherent to all NMOS devices, as is shown in  FIG. 2 . The base of the NPN transistor is also the bulk of the NMOS device, while the emitter and collector of the NPN are the drain and source of the NMOS device, respectively. 
       FIGS. 3A and 3B  illustrates a layout and cross sectional view of a single-finger snapback device  309  in a P-type substrate  305  complementary MOS (CMOS) technology with Shallow Trench Isolation (STI). N-type diffusion regions form a drain  301  and a source  302  for the NMOS device, while poly-silicon (also called “poly”) forms a gate  303 . A P-type diffusion forms the bulk tap  304 , allowing connection to the P-type substrate  305 . Metallic contacts  306 ,  307 , and  308  connect to the Drain, Source, and Bulk diffusion regions, allowing connection to the rest of the circuit. The parasitic NPN transistor of the snapback device  309  is a bulk device formed from the drain  301 , the bulk and the source  302  of the NMOS device, while the parasitic resistance  310  connects the base of the NPN device to the bulk tap  304 . 
     An idealized I-V curve of a snapback device  309  of an NMOS transistor is shown in  FIG. 4 . During an ESD event, current into the snapback device  309  suddenly increases the voltage at the drain  301 . This causes avalanche multiplication of current across the Drain/Bulk junction, which causes current to be injected into the substrate  305 . This current in turn builds up a voltage across the parasitic resistance  310  until the Bulk/Source junction becomes forward-biased, turning on the parasitic NPN transistor of the snapback device  309 . This happens at a voltage and current given by V T1  and I T1 , known as the snapback trigger point. After the NPN transistor of the snapback device  309  turns on, the voltage across the device drops to V H , known as the “Hold Voltage” while current flows from the drain  301  to the source  302  via the NPN transistor of the snapback device  369 . The avalanche multiplication current continues to flow, maintaining the snapback. As the current continues to increase, the voltage across the NPN transistor of the snapback device  309  increases according to the device on resistance, R ON . Unless otherwise limited, the power dissipation within the device can reach a point (V T2 , I T2 ) where the heat generated by the ESD current causes thermal breakdown, which is also known as second breakdown. At this point, the device reaches its thermal limits, and undergoes destructive breakdown. 
     The various points in  FIG. 4  are highly dependent upon several factors, including edge rate of the ESD event, dopant densities, physical dimensions, the circuitry connected to the gate  303 , and the parasitic resistance  310 . 
     Parameters of interest in the snapback I-V curve are the snapback trigger voltage V T1  and the second breakdown current I T2 . V T1  is the maximum voltage that the circuit being protected should experience, while I T2  determines the ESD current protection limit. 
     ESD testing can be performed according to any of many discharge models, selected to model common ESD events. Models include the Human Body Model (HBM), the Charged Device Model (CDM), the Machine Model (MM), as well as some other application-specific models. Each model includes a high voltage source (typically measured in hundreds or thousands of volts) together with a discharge network that sets the shape and edge rate of the ESD current pulse. Different ESD models have different peak currents for a given ESD voltage. For example, a 2 kilovolts (kV) HBM discharge gives approximately 1.33 amps (A) of peak current, while 500 volts (V) CDM can give up to 2 amps (A) peak current (dependent on package size and type). 
     So long as the peak ESD current is less than the second breakdown current I T2 , the snapback device  309  should be robust and remain undamaged from an ESD event. In order to reduce the size of the snapback device  309 , the scaling of the second breakdown current I T2  with device width should be considered. Depending upon the specific characteristics of the snapback device and processing, the second breakdown current I T2  values are normally in the 3 mA/μm to 8 mA/μm range. For example, for a 2 A peak ESD current, the width of the ESD protection device should be in the 250 to 670 micron range. However, this width is applicable for ESD protection when the entire device is conducting simultaneously, i.e., evenly. 
     In practical snapback ESD protection devices, there are significant variations across the width of a single-finger NMOS device. This can be modeled by segmenting the snapback device  309  into a number of smaller devices in parallel, as shown in  FIG. 5 . In  FIG. 5 , the snapback device  309  has been modeled as a number of smaller devices  501 , together with drain parasitic resistances  502  and source parasitic resistances  503 . 
     Because of both random and systematic mismatches between the individual snapback devices  501  and the parasitic resistances  502  and  503 , one of the devices of the model will snap back before the others. In a single-finger snapback device, this corresponds to snapback happening on only a small portion of the device. Because of the positive feedback nature of the snapback operation, this first region to enter snapback will tend to conduct the majority of the ESD current at that point. As the current continues to grow, this first region will tend to continue to take the majority of the ESD current, holding the drain/source voltage low, which will tend to prevent the rest of the device from entering snapback. This contributes to localized current crowding and reduces the effective width of the snapback device, decreasing the overall second breakdown current I T2  of the structure and increasing the likelihood of an ESD failure. 
     The parasitic resistances  502  and  503  act in a negative feedback manner to limit the current that each snapback device (or region thereof) will conduct. Each device (or region)  501  that undergoes snapback increases the current flow through its parasitic resistances  502  and  503 . The drain parasitic resistances  502  increase the voltage on the common drain node above V H , thereby increasing the likelihood that other devices will undergo avalanche multiplication, triggering snapback, thereby providing additional low-impedance paths for the ESD current. This negative feedback effect only helps over a limited area, based on the values of resistances, the amount of diode leakage that gets multiplied (avalanche multiplication), and local die temperature. Typically, the more compact the ESD structure, the better the current distribution due to the ballasting resistances  502 ,  503 . There are many ways to implement these ballasting resistances  502 ,  503 . 
     For example, in  FIG. 6 , which illustrates a conventional two-finger snapback device, the parasitic ballasting resistances  502 ,  503  are implemented by a diffusion area between the contacts  601 ,  602  for the drain and/or source and the poly gate  603 . Approximate values for the two ballasting resistors are expressed in Equations 1 and 2. 
     
       
         
           
             
               
                 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ≈ 
                   
                     
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         con 
                         eff 
                       
                     
                     + 
                     
                       
                         D 
                         W 
                       
                       * 
                       
                         ρ 
                         diff 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     R 
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                   Equation 
                   ⁢ 
                   
                       
                   
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                   2 
                 
               
             
           
         
       
     
     In Equations 1 and 2, ρ diff  is the sheet resistance of the diffusion region  600  and Rcon eff  is the contact resistance divided by the number of contacts in the drain (for Equation 1) or the source (for Equation 2). Sheet resistance is often written Rs. Usually, resistance R 1  is much bigger than resistance R 2  and is typically connected to the pad for providing ESD protection. The values of A, B, D, L and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. For example: A is typically made to be ≧1.5 times the minimum contact to gate spacing design rule of the process; L is typically between 1 to 1.2 times the minimum poly gate length for the MOS device; W is typically between 20 μm and 200 μm; B typically is ≧2 times the minimum diffusion overlap of contact; and D is made such that the resistance value is large enough to spread the current relatively evenly through the device. 
     The style of ballasting described in connection with  FIG. 6  works on non-salisided diffusion regions, i.e., a diffusion that has values of sheet resistance ρ diff  typically over 100 Ω/square, whereas salisided diffusions typically have sheet resistance ρ diff  under 10 Ω/square), otherwise the size of D would become fairly large, and a corresponding transistor would be relatively large. 
     In  FIG. 7 , which illustrates a two-finger snapback device, the parasitic drain ballasting resistances (R 1 ) are implemented by the diffusion area  700  between the drain contacts  701  and the non-salisided diffusion region  704  in the area between the drain contacts  701  and the poly gate  703 . The parasitic source ballasting resistances (R 2 ) are similar to those described earlier in connection with  FIG. 6  (implemented by a diffusion area between the contacts  702  and the poly gate  703 ). Values for the two ballasting resistances are expressed in Equations 3, 4, and 5. 
     
       
         
           
             
               
                 
                   
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                   ⁢ 
                   
                       
                   
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                   3 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     C 
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                   4 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     R 
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                   Equation 
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                   5 
                 
               
             
           
         
       
     
     In Equations 3, 4, and 5, ρ diff  is the sheet resistance of the diffusion region  700 , ρ ex-diff  is the sheet resistance of the non-salisided diffusion region  704  and Rcon eff  is the contact resistance divided by the number of contacts in the drain (for Equation 3 or Equation 4) or the source (for Equation 5). Usually resistance R 1  is much bigger than resistance R 2  and is typically connected to the pad for which ESD protection is to be provided. The values of A, B, C, D, L and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. It should be noted that some of the variable letters used for referencing dimensions are reused in other figures and correspond to other dimensions. A is typically made to be greater than or equal to about 1.5 times the minimum contact to gate spacing design rule of the process; L is typically between 1 and 1.2 times the minimum poly gate length for the MOS device; C can be the minimum design rule for the distance between poly and the salisided exclusion layer (it should be noted that in some processes this number can be negative or zero, that is the salisided exclusion mask can overlap or touch the poly hence Equation 3 simplifies to Equation 4); W is typically between 20 μm and 200 μm; E is typically the minimum separation for a contact to a salisided exclusion; the sum of B and G is greater than or equal to the minimum width for a salisided exclusion; B is typically greater than or equal to 2 times the minimum diffusion overlap of a contact; and D is made such that the resistance value is large enough to spread the current relatively evenly through the device. 
     The style of ballasting described in connection with  FIG. 7  can be used with devices that have salisided diffusion regions to reduce the physical size of the ballasting resistor, which reduces the cost of the corresponding integrated circuit. 
       FIG. 8  illustrates the use of resistors for providing ballasting resistance for a conventional two-finger snapback device. As will be described later, the principles and advantages of these resistors can also be applied with embodiments of the invention. The ballasting resistances for the circuit illustrated in  FIG. 8  are more complicated than those described earlier in connection with  FIG. 7 . 
       FIG. 9  is a schematic of a more detailed model for an equivalent circuit for a snapback device that will be used to describe the conventional circuit of  FIG. 8 , as well as the embodiments of  FIGS. 10-13 . For the snapback device illustrated in  FIG. 8 , the ballasting resistances R 1 , R 12 , R 2 , and R 22  modeled in  FIG. 9  includes resistances such as the resistance of a resistor  805 , a resistance of the diffusion area between the drain contacts  801  and the poly gate  803 , and a resistance between the source contacts  802  and the poly gate  803 . As illustrated in  FIGS. 8 and 9 , the “drain” for the device is connected to the drain contacts  801  via the resistors  805 . The resistance values for the ballasting resistances and for parallel resistances are expressed in Equations 6-13.
 each  R 1 ≈R   R   +Rcon   Equation 6 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
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                     11 
                   
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                       F 
                       
                         G 
                         + 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
             
           
         
       
     
                     each   ⁢           ⁢   R   ⁢           ⁢   13     ≈       F     2   *   B       *     ρ   diff               Equation   ⁢           ⁢   9               each R2≈Rcon  Equation 10 
     
       
         
           
             
               
                 
                   
                     each 
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                   11 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
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                       A 
                       
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                   12 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
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                     23 
                   
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   13 
                 
               
             
           
         
       
     
     In Equations 6-13, ρ diff  is the sheet resistance of the diffusion region  800 , R R  is the resistance of a resistor  805 , which can be implemented by a poly resistor as indicated in  FIG. 8  or can another type of resistor such as a diffusion resistor, and Rcon is a single contact resistance. Usually R 1  is much larger than R 2 , and typically, R 1  is typically connected to the pad for providing ESD protection. The values of A, B, C, D, E, F, G, H, L and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. A and B are typically made to be greater than or equal to 1.5 times the minimum contact to gate spacing design rule of the process; L is typically between 1 and 1.2 times the minimum poly gate length for the MOS device; W typically is between 20 μm and 200 μm; E typically is greater than or equal to 2 times the minimum diffusion overlap for a contact; F is the contact to contact spacing, which is usually defined by the spacing of the real resistor  805 ; H is the contact width, which is usually a value defined by the process; G is typically equal to the minimum diffusion overlap for a contact; C and D set the R R  resistance, which is made such that the resistance value is large enough to spread the current relatively evenly through the device. 
     The style of ballasting described in connection with  FIG. 8  reduces the overall size of the snapback device as compared to the ballasting described earlier in connection with  FIG. 7 . 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is an apparatus including: a semiconductor substrate assembly; a gate disposed on the semiconductor substrate assembly; a first area on a first side of the gate, the first area having both salisided diffusion areas and at least one non-salisided area, wherein there is no polysilicon on the non-salisided areas; a second area on a second side of the gate; and contacts electrically coupled to the salisided diffusion areas, wherein the at least one non-salisided area is disposed between contacts that are spaced apart along a side of the gate. 
     One aspect of the invention is a method of forming a semiconductor device, where the method includes: providing a semiconductor substrate assembly; forming a patterned mask for excluding saliside diffusion from corresponding portions of the substrate assembly; performing a saliside diffusion process to form a salisided diffusion for a source and a salisided diffusion for a drain, and leaving non-salisided areas corresponding at least to the patterned masks; and forming contacts on the substrate assembly, the contacts including two or more contacts of the same drain or source, wherein the contacts are spaced apart along a side of the gate; wherein the mask is formed such that the non-salisided regions remain between the two or more contacts of the same drain or source. 
     One aspect of the invention is an apparatus including: a semiconductor substrate assembly; a gate disposed on the semiconductor substrate assembly; gate material extending from a first side of the gate, such that the gate and the gate material are at the same voltage potential; a first area on the first side of the gate, the first area having with diffusion areas and at least one area covered by the gate material extending from the first side of the gate; a second area on a second side of the gate; and contacts electrically coupled to the diffusion areas, wherein the gate material is disposed between contacts that are spaced apart along a side of the gate. 
     One aspect of the invention is a method of forming a semiconductor device, where the method includes: providing a semiconductor substrate assembly; patterning a polysilicon structure for the semiconductor device such that a first portion forms a gate for the semiconductor device, and such that one or more second portions form a mask for excluding diffusion from corresponding portions of the substrate assembly, wherein the first portion and the one or more second portions have electrical continuity; performing a diffusion process thereby forming at least a diffusion region for a drain and at least a diffusion region for a source, wherein the one or more second portions of the polysilicon structure extend outward from the first portion and over a portion of at least one of the drain or the source; and forming contacts on the substrate assembly, wherein the contacts and the second portions of the polysilicon structure are formed such that second portions are disposed between two or more contacts of the same drain or source region. 
     One aspect of the invention is an apparatus including: a semiconductor substrate; a gate disposed on the semiconductor substrate; a first area on a first side of the gate, the first area having diffusion areas and areas covered by gate material; a second area on a second side of the gate; contacts for the first area electrically coupled to the salisided diffusion areas, wherein gate material is disposed between contacts that are spaced apart along a side of the gate; one or more contacts electrically coupled to the gate material; and metallization electrically coupling the contacts for the salisided diffusion areas to the one or more contacts for the gate material. 
     One aspect of the invention is a method of forming a semiconductor device, where the method includes: providing a semiconductor substrate assembly; patterning polysilicon for the semiconductor device to form a gate structure for the semiconductor device, and to form one or more masks for excluding diffusion, wherein the gate structure and the one or more masks are electrically isolated; performing a diffusion process thereby forming at least a diffusion region for a drain and at least a diffusion region for a source, wherein the one or more masks are formed overlying a portion of at least one of a drain or source such that the masked portions are diffusion excluded; and forming contacts on the substrate assembly such that the one or more polysilicon masks are tied to the same potential as the underlying region for a mask. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings and the associated description herein are provided to illustrate specific embodiments of the invention and are not intended to be limiting. 
         FIG. 1  is a schematic diagram illustrating a representative electrostatic discharge (ESD) protection strategy. 
         FIG. 2  illustrates a parasitic NPN bipolar transistor for an NMOS device. 
         FIGS. 3A and 3B  illustrate a layout (top view) and cross sectional view of a single-finger snapback device in a P-type substrate CMOS technology with Shallow Trench Isolation (STI). 
         FIG. 4  illustrates an I-V curve for a snapback device. 
         FIG. 5  is a schematic of an equivalent model for variations across a snapback device. 
         FIG. 6  illustrates a conventional two-finger snapback device using a diffusion area between contacts to implement ballasting resistances. 
         FIG. 7  illustrates a conventional two-finger snapback device with a non-salisided diffusion region between the drain and the gate, and a salisided region between the source and the gate. 
         FIG. 8  illustrates a conventional two-finger snapback device with resistors for ballasting resistance for the drain. 
         FIG. 9  is a schematic of an equivalent model. 
         FIG. 10  illustrates an embodiment of a two-finger snapback device with a segmented saliside ballast. 
         FIG. 11  illustrates an embodiment of a two-finger snapback device with a first type of gate material-segmented ballast. 
         FIG. 12  illustrates an embodiment of a two-finger snapback device with the first type of gate material-segmented ballast, with a dimension for a gap D set to zero. 
         FIG. 13  illustrates an embodiment of a two-finger snapback device with a second type of gate material-segmented ballast. 
         FIG. 14A  illustrates an embodiment of a snapback device with the second type of gate material segmented ballast for a MOS transistor with multiple fingers arranged side by side. 
         FIG. 14B  illustrates an embodiment of a snapback device with the second type of gate material segmented ballast for a MOS transistor with a single finger. 
         FIG. 15  is a flowchart generally illustrating a process for fabricating a device with a segmented saliside ballast. 
         FIG. 16  is a flowchart generally illustrating a first process for fabricating a device with a poly-segmented ballast. 
         FIG. 17  is flowchart generally illustrating a second process for fabricating a device with a poly-segmented ballast. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Although particular embodiments are described herein, other embodiments of the invention, including embodiments that do not provide all of the benefits and features set forth herein, will be apparent to those of ordinary skill in the art. In addition, while illustrated in  FIGS. 10-13  in the context of NMOS, ballasting resistance structures in the proximity of the drain and the gate, and with two-finger implementations, those of ordinary skill in the art will appreciate that the principles and advantages described herein are applicable to both NMOS and PMOS, to ballasting resistance structures in the proximity of the source and the gate, and to single-finger snapback devices or even to snapback devices with more than 2 fingers. In addition, the structures disclosed herein can be used with or without the resistors  805  described earlier in connection with  FIG. 8 . 
     A desirable characteristic is to spread the current flow relatively evenly throughout a device. In the context of ballasting in the proximity of the drain and the gate, it is desirable for the values of the parasitic resistances modeled by R 11  and R 13  to be as high as practical to reduce the current from one contact passing into an area being served by another contact, which could otherwise lead to current crowding into a weak spot, which could then cause damage. It is desirable to use a relatively large number of contacts to allow relatively large overall currents to flow, such as, for example, 2 Amps for ESD and 20 to 100 mA for drivers, while still occupying a relatively small area for cost. 
     The conventional technique described earlier in connection with  FIG. 8  results in an increase in the spacing between contacts (“F” in  FIG. 8 ) to increase the resistances modeled in  FIG. 9  as R 11  and R 13 . This makes the device relatively large and expensive to be able to handle the relatively large currents or can limit the amount of current the device can handle in a non-ESD event since the contacts have long-term current limits. Also to be able to make very compact layouts, the type of resistor  805  used rather than optimizing R 11  and R 13  defines the distance F. 
     As described earlier in connection with  FIG. 4 , the trigger for snapback is avalanche multiplication of current in the reverse-biased diode (drain to bulk in the NMOS device) to raise the voltage drop across the resistance modeled as R 3   910  in  FIG. 9 . The overall area of the snapback device should be relatively small so that the IR drop across the resistance R 3   910  is high enough to turn on all the parasitic NPN transistors rather than just a few fingers (or parts of fingers) of the parasitic NPN transistors. 
     The embodiments of the invention that will be described in the following provide a high ESD tolerance in a relatively small overall device size. In addition, characteristics for current paths modeled by parasitic resistors R 11  and R 13  are improved, i.e., relatively higher resistance, while providing for a large number of contacts (for high current capabilities). 
     Segmented Saliside Ballast 
     The segmented saliside ballast arrangement illustrated in  FIG. 10  can be used with a salisided diffusion process, which is typical of modern CMOS processes. As used herein, saliside (or salisided) includes saliside, silicide, salicide, etc. The segmented saliside ballast layout style substantially increases the resistances modeled by resistors R 11  and R 13  in  FIG. 9  with respect to the arrangement described earlier in connection with  FIG. 8 . 
     A two-fingered configuration is shown in  FIG. 10 . The arrangement can be extended to single-fingered configurations and to configurations with more than two fingers.  FIG. 10  illustrates a diffusion region  1000  of a semiconductor substrate, a drain region generally in the center, two gates  1003  with a gate  1003  to the left and a gate  1003  to the right of the drain region, and source regions outside the gates  1003 . Drain contacts  1001  provide electrical connection for the drain. Source contacts  1002  provide electrical connection for the source. 
     A non-salisided diffusion region  1004  between contact rows  1001  provides additional resistance between contact rows  1001 . In one embodiment, the non-salisided diffusion regions  1004  are protected from a saliside diffusion by a salisided exclusion mask. In the illustrated embodiment, there is one contact row  1001  for each “segment” of the device. The illustrated configuration typically also permits the removal of the resistor R R    805  described earlier in connection with  FIG. 8 , which decreases the overall size and cost of the structure. However, it will be understood that the resistor R R    805  can still be used if desired. In addition, while a single contact can be used, multiple contacts are typically placed within each segmented region for current handling as illustrated by the multiple drain contacts  1001  in  FIG. 10 . The use of multiple contacts decreases the resistance R 1  as expressed in Equation 14. Too low of a resistance R 1  can be undesirable because the sum of resistances R 1  and R 12  should be high enough to spread the current out among multiple contacts relatively evenly. Rectangular boxes above the contacts indicate conductive metal for the contacts. 
     To handle current relatively evenly, the number of contacts for the source side should be about equal to or relatively close to the number of contacts for the drain side. However, the total number of drain contacts  1001  and the total number of source contacts  1002  can vary as long as each side has enough contacts for current handling. This can be done by either having multiple columns of contacts in the source side, by placing contacts on opposite sides of the non-salisided diffusion regions  1004 , by using both multiple columns and opposing contacts, and the like. Equations 14-22 express resistance values for the ballasting resistances and for the parallel resistances as modeled in  FIG. 9 . 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ≈ 
                   
                     Rcon 
                     
                       # 
                       ⁢ 
                       ofContacts 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ≈ 
                   
                     
                       
                         
                           2 
                           * 
                           E 
                         
                         J 
                       
                       * 
                       
                         ρ 
                         diff 
                       
                     
                     + 
                     
                       
                         F 
                         J 
                       
                       * 
                       
                         ρ 
                         
                           ex 
                           - 
                           diff 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   15 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ≈ 
                   
                     
                       D 
                       
                         
                           2 
                           * 
                           E 
                         
                         + 
                         H 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   16 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     C 
                     &gt; 
                     0 
                   
                   ; 
                   
                     
                       each 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       13 
                     
                     ≈ 
                     
                       
                         
                           
                             2 
                             * 
                             E 
                           
                           + 
                           F 
                         
                         C 
                       
                       * 
                       
                         ρ 
                         diff 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   17 
                 
               
             
           
         
       
     
                     C   ⁢     &lt;   _     ⁢   0     ;       each   ⁢           ⁢   R   ⁢           ⁢   13     ≈           2   *   E     D     *     ρ   diff       +       F   D     *     ρ     ex   -   diff                     Equation   ⁢           ⁢   18               each R2≈Rcon  Equation 19 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     21 
                   
                   ≈ 
                   
                     
                       K 
                       
                         M 
                         + 
                         H 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   20 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     22 
                   
                   ≈ 
                   
                     
                       A 
                       
                         2 
                         * 
                         K 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   21 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   ≈ 
                   
                     
                       K 
                       
                         2 
                         * 
                         A 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   22 
                 
               
             
           
         
       
     
     In Equations 14-22, ρ diff  is the sheet resistance of the diffusion region  1000 , ρ ex-diff  is the sheet resistance of the non-salisided diffusion region  1004 , #ofContacts is the number of drain contacts  1001  between non-salisided blocking regions  1004 , and Rcon is a single contact resistance. R 1 , i.e., all of the drain contacts  1001 , can be connected to the pad for providing ESD protection. The values of A, B, C, D, E, F, G, H, J, K, L, M and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. It should be noted that some of the variable letters used for referencing dimensions are reused in other figures and correspond to other dimensions. 
     Examples of ranges for these values are provided in the following. Other values will be readily determined by one of ordinary skill in the art. In one embodiment: A and D are typically made to be greater than or equal to 1.5 times the minimum contact to gate spacing design rule of the process; L is typically between 1 and 1.2 times the minimum poly gate length for the MOS device; W typically is between 5 μm and 60 μm; M typically is the minimum diffusion overlap for a contact; G+B and F are typically greater than or equal to the minimum width of the salisided diffusion exclusion region; H is the contact width, which is usually a single value defined by the process; C is typically the minimum design rule for the distance between poly and the salisided exclusion layer, and in some processes, C can be negative or zero, such that the salisided exclusion mask can overlap or touches the poly, and hence, Equation 17 would simplify to Equation 18; K is typically the minimum contact to contact spacing; G is typically the minimum overlap of salisided exclusion layer over diffusion; and J is the width of the contacts  1001  in the drain and made such that the resistance value of R 1 +R 11  is large enough to spread the current relatively evenly through the device. A relatively large value for R 11  helps to balance current across the width of a finger. A relatively large value for R 12  helps to balance current across the width of a finger and among multiple fingers. 
     It will be appreciated that many variations are possible. For example, the dimensions referenced with “E” do not have to be symmetric, e.g., the dimensions could be E 1  and E 2 . In another example, the width of and/or the number of contacts  1001  for the drain region can vary among fingers. 
     Gate Material Segmented Ballasting Type I 
       FIGS. 11 ,  12 , and  13  illustrate segmented ballasting techniques using gate material as a mask to prevent source or drain diffusion, which thereby generates the segmented ballasting. The gate material or materials selected should prevent diffusion during fabrication. In one embodiment, the gate material or materials also prevent saliside diffusion during fabrication. In the illustrated embodiments of  FIGS. 11 ,  12 , and  13 , the gate material is polysilicon or “poly,” and will be described in that context. In the embodiments illustrated in  FIGS. 11 and 12 , the gate material is extended from the gate to create masks that increase the resistances modeled as R 11  and R 13  in  FIG. 9 . In the “Type I” embodiments illustrated in  FIGS. 11 and 12 , the poly  1103 ,  1203  for the gate and for the mask are connected to the gate potential. In the “Type II” embodiment that will be described later in connection with  FIG. 13 , poly for masks  1304  is formed between contact rows to increase the resistances modeled as R 11  and R 13  in  FIG. 9 . The poly for the mask  1304  is not connected to the same potential as the poly for the gate  1303 . 
     Returning now to the “Type I” embodiments, the Gate Material Segmented Ballasting arrangement illustrated in  FIG. 11  uses a configuration of polysilicon or poly  1103  to increase the resistances modeled as R 11  and R 13  in  FIG. 9  relative to the conventional configuration described earlier in connection with  FIG. 8 .  FIG. 11  illustrates a two-finger configuration with a diffusion region  1100  of a semiconductor substrate, a drain region generally in the center, two gates formed from the poly  1103  to the left and to the right of the drain region, and source regions outside the gates. Drain contacts  1101  provide electrical connection for the drain. Source contacts  1102  provide electrical connection for the source. 
     In one embodiment, the poly  1103  is laid down, such as via a patterning process, then the source/drain diffusion is performed, then the saliside diffusion is performed, and the poly  1103  masks the underlying regions from becoming salisided, and then an oxide layer for the gate oxide is grown through the poly  1103 . It will be understood by the skilled practitioner that the order of steps in the fabrication of MOS transistor and the selection of materials can be varied in many ways. For example, a gate oxide layer can be formed first, and then the polysilicon gate can be patterned over the oxide later. In another example, a silicon oxynitride layer is used instead of silicon oxide for a gate oxide. It will also be understood that other materials that prevent saliside diffusion can be used instead of polysilicon. 
     The poly  1103  includes a first portion  1104  between source and drain regions functioning as a gate, and in the illustrated embodiment, a second portion  1105  extending from the first portion into the drain region, which functions as a mask for the saliside diffusion. The portions of the semiconductor substrate masked by the poly  1103  do not become salisided, and accordingly exhibit relatively higher resistivity than salisided regions. This increases the resistances of  FIG. 9  modeled as R 11  and R 13 . 
     As illustrated in  FIG. 11 , the poly  1103  is extended between rows of drain contacts  1101 . Accordingly, the process for forming the poly  1103  should be compatible with 90° bends in a diffusion region or active area. In the illustrated embodiment, the resistor R R    805  structures described earlier in connection with  FIG. 8  are not used to decrease the size and cost of the overall structure. However, the resistor R R    805  structures can also be used if desired. In general, while a single contact can be used, multiple contacts are typically placed within each segmented gate region to allow for higher current handling. The use of multiple contacts decreases the resistance R 1  as expressed in Equation 23. The sum of resistances R 1  and R 12  described earlier in connection with  FIG. 9  should remain high enough to spread the current out among the multiple contacts relatively evenly. 
     To handle current relatively evenly, the number of contacts for the source side should be about equal to or relatively close to the number of contacts for the drain side. However, the total number of drain contacts  1101  and the total number of source contacts  1102  can vary as long as each side has enough contacts for current handling. For example, multiple source contacts  1102  can be provided increased by either having multiple columns of contacts in the source side, by placing contacts opposite to the gate regions, by using both multiple contacts and contacts on opposite sides, and the like. Rectangular boxes above the contacts indicate conductive metal for the contacts. 
       FIG. 12  illustrates an embodiment that is a variation of the embodiment described in connection with  FIG. 11 . It is possible to “close” the poly, i.e., set D=0, as illustrated in  FIG. 12 , but this is often not done due to the size of minimum poly hole allowed by the process rules.  FIG. 12  illustrates a two-finger configuration with a diffusion region  1200  of a semiconductor substrate, a drain region generally in the center, a gate  1203  to the left and to the right of the drain region with left and right portions connected via the poly, and source regions outside the gates  1203 . Drain contacts  1201  provide electrical connection for the drain. Source contacts  1202  provide electrical connection for the source. 
     Equations 23-31 express resistance values for the ballasting resistances and for the parallel resistances as modeled in  FIG. 9  for both the embodiments of  FIGS. 11 and 12 . 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       1 
                     
                   
                   ≈ 
                   
                     Rcon 
                     
                       # 
                       ⁢ 
                       ofContacts 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   23 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ≈ 
                   
                     
                       
                         
                           2 
                           * 
                           K 
                         
                         G 
                       
                       * 
                       
                         ρ 
                         diff 
                       
                     
                     + 
                     
                       
                         F 
                         G 
                       
                       * 
                       
                         ρ 
                         
                           gate 
                           - 
                           diff 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   24 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       12 
                     
                   
                   ≈ 
                   
                     
                       C 
                       
                         
                           2 
                           * 
                           K 
                         
                         + 
                         H 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   25 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     D 
                     &gt; 
                     0 
                   
                   ; 
                   
                     
                       
                         each 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         13 
                       
                       ≈ 
                       
                         
                           
                             
                               2 
                               * 
                               K 
                             
                             + 
                             F 
                           
                           C 
                         
                         * 
                         
                           ρ 
                           diff 
                         
                       
                     
                     || 
                     
                       ( 
                       
                         
                           
                             
                               2 
                               * 
                               K 
                             
                             C 
                           
                           * 
                           
                             ρ 
                             diff 
                           
                         
                         + 
                         
                           
                             F 
                             C 
                           
                           * 
                           
                             ρ 
                             
                               gate 
                               - 
                               diff 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   26 
                 
               
             
           
         
       
     
                     D   =   0     ;       each   ⁢           ⁢   R   ⁢           ⁢   13     ≈     2   *     (         2   *   K     C     *     ρ   diff     ⁢     F   C     *     ρ     gate   -   diff         )                 Equation   ⁢           ⁢   27               each R2≈Rcon  Equation 28 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     21 
                   
                   ≈ 
                   
                     
                       E 
                       
                         J 
                         + 
                         H 
                       
                     
                     * 
                     
                       p 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   29 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R22 
                   
                   ≈ 
                   
                     
                       A 
                       
                         2 
                         * 
                         E 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   30 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       23 
                     
                   
                   ≈ 
                   
                     
                       E 
                       
                         2 
                         * 
                         A 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   31 
                 
               
             
           
         
       
     
     In Equations 23-31, ρ diff  is the sheet resistance of the diffusion region  1100 , ρ gate-diff  is the sheet resistance of the diffusion region under the gate, #ofContacts is the number of drain contacts between gate blocking regions, and Rcon is a single contact resistance. R 1  is typically connected to the pad for providing ESD protection. Typical ranges for the sheet resistance ρ gate-diff  are about 250 to about 1500 ohms/square. 
     The values of A, B, C, D, E, F, G, H, J, K, L and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. Example ranges for these values will be described in the following. Other values will be readily determined by one of ordinary skill in the art. In one embodiment: A and C are typically made to be about 1.5 times the minimum contact to gate spacing design rule of the process; B is typically greater than or equal to 2 times the minimum diffusion overlap of contact; E is typically the minimum contact to contact spacing allowed; L and F are typically between 1 and 1.2 times the minimum poly gate length for the MOS device; W typically is between 5 μm and 60 μm; K is typically the minimum contact to gate spacing; J is typically the minimum diffusion overlap of contact; H is the contact width, and is usually a single value defined by the process; D can be the minimum design rule for the distance between poly and can be zero, and if zero, then Equation 26 changes to Equation 27; and G is the width of the contacts in the drain and made such that the resistance value R 1 +R 11  is large enough to spread the current relatively evenly through the device. 
     Gate Material Segmented Ballasting Type II 
     The Gate Material Segmented Ballasting arrangement illustrated in  FIG. 13  uses gate material, such as poly, for masks  1304  between rows of drain contacts  1301  as a mask against source/drain diffusion, but does not electrically couple the poly for the masks  1304  to the voltage potential of the gates  1303 . In one embodiment, the gate material also masks saliside diffusion. 
       FIG. 13  illustrates a two-finger configuration with a diffusion region  1300  of a semiconductor substrate, a drain region generally in the center, two gates  1303  to the left and to the right of the drain region, and source regions outside the gates  1303 . Drain contacts  1301  provide electrical connection for the drain. Source contacts  1302  provide electrical connection for the source. 
     In the illustrated embodiment, the poly regions  1304  segment the drain regions of the underlying transistor. The poly regions  1304  mask the underlying regions to prevent source/drain diffusion and saliside diffusion, which keeps the resistivity of the underlying semiconductor material relatively high. The fabrication of such poly regions  1304  between contacts to separate the contact rows breaks an old process rule “can only contact gate poly on field oxide,” which is no longer necessary to follow given modern polyside or salisided processes. The illustrated configuration typically also permits the removal of the resistor R R    805  described earlier in connection with  FIG. 8 , which decreases the overall size and cost of the structure. However, it will be understood that the resistor R R    805  can still be used if desired. 
     In addition, while a single contact can be used, multiple contacts are typically placed within each gate separated region for greater current handling. The use of multiple contacts decreases the resistance R 1  as expressed in Equation 32. The sum of resistances R 1  and R 12  should be high enough to spread the current out among multiple contacts relatively evenly. 
     To handle current relatively evenly, the number of contacts for the source side should be about equal to or relatively close to the number of contacts in the drain side. However, the total number of drain contacts  1301  and the total number of source contacts  1302  do not have to be the same. Multiple contacts for the source contacts  1302  can be provided by, for example, having multiple columns of contacts in the source side, by placing contacts opposite to the gate regions, by both of the foregoing, and the like. 
     Equations 32-39 express resistance values for the ballasting resistances and for the parallel resistances as modeled in  FIG. 9  for the embodiment illustrated in  FIG. 13 . 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       1 
                     
                   
                   ≈ 
                   
                     Rcon 
                     
                       # 
                       ⁢ 
                       ofContacts 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   32 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       11 
                     
                   
                   ≈ 
                   
                     
                       
                         
                           2 
                           * 
                           K 
                         
                         G 
                       
                       * 
                       
                         ρ 
                         diff 
                       
                     
                     + 
                     
                       
                         F 
                         G 
                       
                       * 
                       
                         ρ 
                         
                           gate 
                           - 
                           diff 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   33 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       R 
                       ⁢ 
                       12 
                     
                   
                   ≈ 
                   
                     
                       C 
                       
                         
                           2 
                           * 
                           K 
                         
                         + 
                         H 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   34 
                 
               
             
           
         
       
     
                     each   ⁢           ⁢     R   ⁢   13       ≈         2   *     (       2   *   K     +   F     )       C     *     ρ   diff               Equation   ⁢           ⁢   35               each R2≈Rcon  Equation 36 
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     21 
                   
                   ≈ 
                   
                     
                       E 
                       
                         J 
                         + 
                         H 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   37 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     22 
                   
                   ≈ 
                   
                     
                       A 
                       
                         2 
                         * 
                         E 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   38 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     each 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     23 
                   
                   ≈ 
                   
                     
                       E 
                       
                         2 
                         * 
                         A 
                       
                     
                     * 
                     
                       ρ 
                       diff 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   39 
                 
               
             
           
         
       
     
     In Equations 32-39, ρ diff  is the sheet resistance of the diffusion region  1300 , ρgate-diff is the sheet resistance of the diffusion region under the gate, #ofContacts is the number of drain contacts between gate blocking regions, and Rcon is a single contact resistance. R 1  is typically connected to the pad for providing ESD protection. 
     The values of A, B, C, D, E, F, G, H, J, K, L, M and W can be varied to control the amount of current that the snapback device can safely handle while still taking manufacturing design rules into account. It should be noted that some of the variable letters used for referencing dimensions are reused in other figures and correspond to other dimensions. Example values are provided in the following. Other applicable values will be readily determined by those of ordinary skill in the art. In one embodiment: A and C are typically made to be greater than or equal to 1.5 times the minimum contact to gate spacing design rule of the process; B is typically greater than or equal to 2 times the minimum diffusion overlap for a contact; E is typically the minimum contact to contact spacing allowed; L is typically between 1 and 1.2 times the minimum poly gate length for the MOS device; W typically is between 5 μm and 60 μm; K is typically the minimum contact to gate spacing; J is typically the minimum diffusion overlap of contact; H is the contact width, which is usually a single value defined by the process; M is typically the minimum poly overlap of contact; F is typically twice the sum of M and H; G is the width of the contacts in the drain and made such that the resistance value R 1 +R 11  is large enough to spread the current relatively evenly through the device; and D is typically the minimum design rule for the distance between poly. 
     The poly  1304  should be connected, i.e., not floating, as otherwise the poly  1304  could potentially charge up and create a channel underneath the poly  1304  that would undesirably reduce R 11 . As illustrated in  FIG. 13 , in the illustrated embodiment, the poly  1304  is connected to the drain via a poly contact  1305  and metallization  1306 . Even if the contact  1305  is “punched through” the poly  1304 , or if the poly  1304  is shorted to the diffusion regions  1306  on either side, the poly  1306  is not the gate of a MOSFET device and the device should be operable. 
     Other Embodiments 
     Many variations exist. For example, the embodiments described in connection with  FIGS. 10 ,  11 ,  12 , and  13  illustrate examples of devices with a drain region in the middle, gate regions on opposing sides, and two source regions outside the gates, i.e., a finger to the left and a finger to the right. 
     For example,  FIG. 14A  illustrates an example of a snapback device with the second type of gate material segmented ballast for a MOS transistor described earlier in connection with  FIG. 13 . In the illustrated configuration of  FIG. 14A , the snapback device has four fingers arranged side by side. The techniques disclosed herein can be applied to a broad number of fingers. 
       FIG. 14B  also illustrates an example of a snapback device with the second type of gate material segmented ballast for a MOS transistor described earlier in connection with  FIG. 13 . In the illustrated configuration of  FIG. 14B , the snapback device has a single finger. 
     In another example, while the embodiments described earlier in connection with  FIGS. 10-13  were described in the context of ballasting on the drain side, the described techniques can also be used on the source side in addition to or instead of on the drain side. In addition, while illustrated between single rows of contacts, the ballasting techniques can be used for groups of more than one row or more than one column of contacts within a segmented area. The described techniques are applicable to both N type and to P type of MOS devices. 
     These techniques can be applied at any inputs, outputs, or power supplies terminals of a CMOS Integrated Circuit (IC). For example, these techniques can be used to connect to a CMOS die to a printed circuit board (PCB), such as via a package or direct bonding of the die to the PCB. These techniques can be use to connect a CMOS die to another integrated circuit (IC), such as within a package. Examples include system in package (SIP), die-on-die stacking in chip scale packaging (CSP), and the like, or directly bonding of the CMOS die to another die, and the like. In one example, the snapback devices are combined with diodes to provide ESD protection in a first direction with the diodes, and in a second direction with the snapback devices. For example, with reference to  FIG. 1  and the block to the right labeled “power clamp,” a diode can be used to protect the circuits from a negative voltage transient that generates current in the direction from VSS to VDD, and a snapback device can be used to protect the circuits from a positive voltage transient that generates current in the direction from VDD to VSS. These techniques can also be applied in any of the blocks labeled as “primary positive clamp,” “secondary positive clamp,” “primary negative clamp,” or “secondary negative clamp.” 
     Output Current Balancing 
     The described techniques are also useful for the balancing of current passing through a MOS transistor (NMOS or PMOS) as opposed to the parasitic bipolar transistor that is inherent to a MOS transistor. For example, the described techniques are also useful to balance current for output or driver stages, which can have relatively high output currents. The techniques can also be used where ballasting of output stages, which may or may not go off-chip, is useful. For example, it can be useful to balance the current in a relatively high-current output stage. These high current output stages can, but do not necessarily go off-chip, so that the output stages may or may not be subjected to ESD. However, the current balancing techniques are still applicable. 
     Fabrication Processes 
       FIG. 15  is a flowchart generally illustrating a process for fabricating a device with a segmented saliside ballast. For example, the process can be used to fabricate the snapback device described earlier in connection with  FIG. 10 . It will be appreciated by the skilled practitioner that the illustrated process can be modified in a variety of ways. 
     The description of the process begins with the gates already fabricated. The process forms  1502  a patterned mask for saliside exclusion. For example, in the embodiment described earlier in connection with  FIG. 10 , these masked regions correspond to the non-salisided diffusion region  1004 . For example, the patterned mask can be formed using photoresist techniques. For example, the mask can be made out of photoresist and can be used to mask the deposition of metal from which the saliside layer is formed, and the mask can be removed, which removes the metal via a lift off process. In another embodiment, a hard mask, such as a mask of silicon oxide, can also be used and can remain for further processing. 
     The process performs a saliside diffusion  1504 . For example, the saliside diffusion  1504  can follow a source/drain diffusion. In a typical saliside diffusion, a metal such as titanium, tantalum, tungsten, or the like is deposited in areas for which saliside is desirably formed, and then the substrate assembly is heated to diffuse the metal to generate the saliside. It should be noted that the term “saliside” is referred to as “self-aligned silicide” in the art, but is used herein to include both saliside and silicide, including where the saliside is patterned via a mask as shown in the illustrated process. If a photoresist mask is used, the photoresist mask should be removed prior to thermal annealing. If a hard mask is used, it can later be removed if desired. 
     The process then proceeds to form  1506  contacts to the underlying structures. For example, the contacts can be formed through a layer of silicon oxide. In one embodiment, the contacts are formed such that contacts  1001  straddle non-salisided or saliside-excluded regions  1004  of the same region. For example, in the embodiment described earlier in connection with  FIG. 10 , the contacts  1001  and the saliside-excluded regions  1004  are formed such that rows of contacts  1001  are approximately parallel to a desired direction of current flow for the drain region, and the saliside-excluded regions  1004  are formed in relatively narrow strips that are oriented such that the lengthwise axis of the narrow strips are also approximately parallel to the desired direction of current. In one embodiment, each row of contacts  1001  is separated by a saliside-excluded region  1004 . The contacts  1001  should be contact the salisided region. 
       FIG. 16  is a flowchart generally illustrating a first process for fabricating a device with a gate material-segmented ballast, such as a poly-segmented ballast. For example, the process can be used to fabricate the snapback devices described earlier in connection with  FIGS. 11 and 12 . It will be appreciated by the skilled practitioner that the illustrated process can be modified in a variety of ways. 
     The description of the process begins with the gates already fabricated. The process will be described in the context of using polysilicon for the gate material. The polysilicon and the underlying oxide prevent the saliside from forming in underlying silicon. The process proceeds to pattern the polysilicon structure for use as a gate and a mask. In one embodiment, the mask portion made from poly is a 90 degree extension away from the poly that forms the gate. Examples of these extensions or members are illustrated in  FIGS. 11 and 12 . The process then performs  1604  a diffusion for at least the source and drain. In one embodiment, the process further includes a saliside diffusion after the source/drain diffusion. The mask portion of the poly prevents the source/drain diffusion and the saliside diffusion from growing underneath the poly. 
     The process then proceeds to form  1606  contacts to the underlying structures of the substrate assembly. In the illustrated embodiment, specific contacts to the mask portions of the poly are not needed because the mask poly is at the same potential as the gate poly. 
     In one embodiment, the contacts are formed such that contacts  1101 / 1201  straddle poly-masked of the same region. For example, in the embodiments described earlier in connection with  FIGS. 11 and 12 , the contacts  1101 / 1201  and the mask portions of the poly are formed such that rows of contacts  1101 / 1201  are approximately parallel to a desired direction of current flow for the drain region, and the extended poly from the gates are formed in relatively narrow strips that are oriented such that the lengthwise axis of the extensions are also approximately parallel to the desired direction of current. For example, each row of contacts  1101 / 1201  can be separated by an extension of poly. 
       FIG. 17  is flowchart generally illustrating a second process for fabricating a device with a gate material-segmented ballast, such as a poly-segmented ballast. For example, the process can be used to fabricate the snapback device described earlier in connection with  FIG. 13 . It will be appreciated by the skilled practitioner that the illustrated process can be modified in a variety of ways. The description of the process begins with the gates already fabricated. The process will be described in the context of using polysilicon for the gate material. 
     The polysilicon and the underlying oxide prevent the saliside from forming in underlying silicon. The process proceeds to pattern  1702  polysilicon into gate structures  1303  and mask structures  1304  as illustrated in  FIG. 13 . 
     The process then performs  1704  at least a source/drain diffusion. The source/drain diffusion is prevented from forming in the silicon underlying the gate structures  1303  and the mask structures  1304 . In one embodiment, the process further includes a saliside diffusion, and the gate structures  1303  and the mask structures  1304  also prevent saliside diffusion from forming in the underlying silicon. 
     The process then proceeds to form  1706  contacts to the underlying structures of the substrate assembly, such as to the drains, sources, bulk, gates and the like. In addition, the contacts formed include contacts  1305  for the mask structure  1304 , which is maintained at a different potential from the gate structures  1303 . In one embodiment, the mask structure  1304  is tied to the voltage potential of the underlying region, such as the drain region as illustrated in  FIG. 13 . For example, the contact  1305  can be electrically coupled to the same potential as the drain by coupling to the same metallization  1306 . It will be understood that the segmentation techniques can be applied to drain and/or to source regions. 
     Various embodiments of the invention have been described in this document. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those familiar with the subject without departing from the true spirit and scope of the invention as defined in the appended claims.

Technology Category: 5