Abstract:
This invention discloses a ballasting resistor for an electrostatic discharge (ESD) device that comprises at least one first active region forming a source/drain of an ESD discharge transistor, at least one resistive element with a serpentine shape formed in a single layer of a semiconductor structure, wherein the resistive element has a first terminal coupled to the first active region and a second terminal coupled to a bonding pad including power supply (Vdd or Vss) pads.

Description:
BACKGROUND 
   The present invention relates generally to an electrostatic discharge (ESD) protection device, and more particularly to a multi-finger ESD protection device with ballasting resistance for reducing the voltage stress on input/output pads of integrated circuits (ICs) during an ESD event. 
   As the feature sizes of semiconductor devices are being reduced to the nanometer level, semiconductor devices are getting more susceptible to ESD events. ICs formed of MOS (metal-oxide semiconductor) transistors are especially prone to ESD damages. A common technique to prevent ICs from being damaged by ESD events is using a multi-finger ESD protection device on the input/output pads of ICs. 
   A multi-finger ESD protection device is a series of transistors placed in parallel like fingers across the input/output pads of an IC so that it can have relatively large device widths to discharge ESD currents to ground potential Vss. To function properly, the trigger voltage of the multi-finger ESD protection device should be smaller than the trigger voltage of the other devices not used for ESD protection. Moreover, the multi-finger ESD protection device should not turn on during normal operation of an input/output circuit. During the conducting state, the multi-finger ESD protection device should provide a low resistance and have a high current handling capability. 
   A well-known problem with the multi-finger ESD protection device is the possibility of non-uniform triggering of the fingers. To ensure uniform turn-on of the multi-finger ESD protection device, an approach is to add ballasting resistors to each finger to increase the trigger voltage of the subsequently triggered finger, or to increase the substrate resistance of the MOSFET (Metal-Oxide Semiconductor Field Effect Transistors). For instance, the substrate resistance can be increased by increasing the distance of the substrate contact from the source/drain region of the MOSFET, or by increasing the P-well or N-well sheet resistance. 
     FIG. 1  depicts a multi-finger ESD protection device according to a conventional art. The ESD protection device is formed by NMOS (N-channel Metal-Oxide Semiconductor) multi-finger transistors placed in parallel in a driver block  100 . Each finger transistor has a MOS structure with a source  120   a , a drain  130   a  and a gate electrode  110   a . Two adjacent fingers share the same source or drain regions. Triggering the first finger may propagate and trigger adjacent fingers in the driver block  100 . 
   To increase the sheet resistance or the trigger voltage of the subsequently triggered finger, resist protective oxide (RPO) film  140   a  and  140   b  are formed on the drain regions  130   a . Alternatively, the RPO films  140   a  can also be formed on the source region  120   a . The RPO film  140   a  or  140   b  is usually applied on the I/O portion of an IC as a protection layer while forming electrical contacts to the bonding pads. During a typical salicide category of fabrication technology, a layer of RPO film is first deposited over the active area (OD). Then, a resist mask is formed over the area covered by the RPO film to protect the field effect transistor area from subsequent process steps. The RPO film in the exposed areas of the IC is then etched. The remaining RPO films function as ballasting resistance for ESD protection. 
   Nevertheless, there are several disadvantages with this approach. First, forming the RPO film may have an adverse influence on the yield. When wet etching is applied, the process will create undercut profiles near the edge of the resist mask, resulting in poor dimensional control and resist mask peeling and even mask lift-off. Second, the RPO area may increase the size of the drain/source region and cause the mechanical stress effect, known as LOD (Length of Oxide) effect, to each finger of the ESD protection device. 
     FIG. 2  depicts another ballasting resistor structure as disclosed in U.S. Pat. No. 5,721,439 that uses polysilicon strips as ballasting resistors to impose a gate delay. The ballasting resistors  203  are formed by polysilicon blockage and evenly distributed throughout the drain region  220  to provide substantially uniform diffusion resistance between the drain contact  202  and the gate electrode  201  while increasing the diffusion resistance of the drain region  220 . However, the disadvantage of this structure is that the polysilicon  204  are floating gates that may create reliability issues, such as punch-through or short. Moreover, the drain region  220  with the ballasting resistors  203  is considered relatively large because they may suffer area efficiency on the input/output of an IC. 
     FIG. 3  shows another approach as described in U.S. Pat. No. 6,587,320, called “back-end-ballasting”. In this embodiment, the ESD ballasting is formed by a ballasting network consisting of “back-end” elements, such as contact-to-silicon, contact-to-poly and silicided polysilicon. As shown in  FIG. 3 , the approach uses a meandering strip  302  extending from the common terminal  301  to the drain region  303  of the ESD device  320 . The meandering strip  302  creates a resistance path that connects a plurality of metallization layers M 1 ˜M 3 , polysilicon layer P 1  and interconnecting vias V 1 ˜V 2  to form ballasting resistance. 
   It is known that any additional layer or via can add resistance to the ballasting resistance. By making vertical interconnections to form back-end ballasting resistors, this approach can solve the problems induced by the LOD effects. However, the tradeoff is the increased cost and complexity in the manufacturing process due to the vertically formed resistance path. 
   As such, what is needed is a new structure of the multi-finger ESD protection device with the ballasting resistance that can increase area efficiency of MOS transistors in fully silicided technologies, and uniformly turn on each finger of the multi-finger ESD protection device. 
   SUMMARY 
   This invention discloses a ballasting resistor for an electrostatic discharge (ESD) device that comprises at least one first active region forming a source/drain of an ESD discharge transistor, at least one resistive element with a serpentine shape formed in a single layer of a semiconductor structure, wherein the resistive element has a first terminal coupled to the first active region and a second terminal coupled to a bonding pad including power supply (Vdd or Vss) pads. 
   The structure design of the invention, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional ESD protection device using RPO as ballasting resistance; 
       FIG. 2  illustrates a conventional ESD protection device with island-shaped ballasting resistance; 
       FIG. 3  illustrates another conventional ESD protection device with back-end-ballasting resistors; 
       FIG. 4  illustrates a preferred embodiment of the present invention with spiral ballasting resistors; 
       FIG. 5  illustrates another preferred embodiment of the present invention with zigzag ballasting resistors; and 
       FIG. 6  illustrates yet another preferred embodiment of the present invention with meandering ballasting resistors and common contacts. 
   

   DESCRIPTION 
   The present invention is presented to ensure the uniform turn-on of the multi-finger ESD device by increasing ballasting resistance in the source/drain regions without causing LOD effects. The purpose of increasing the ballasting resistance is to ensure that the trigger voltage of the subsequently triggered finger can be increased and eventually each finger can be turned on in a uniform manner. Moreover, as memory and logic devices are tending to be formed on the same integrated circuit, the present invention is also presented to form the ballasting resistors using the salicide fabrication process same as forming the electrical contacts to the FET elements. 
     FIGS. 1 ,  2  and  3  have already been described and discussed as the relevant background to the present invention. They require no further discussion here. 
     FIG. 4  depicts the ESD protection device having resistant elements in a spiral shape according to a preferred embodiment of the present invention. The ESD protection device includes a driver block  410  with multi-finger transistors  430   a˜d . The multi-finger transistors  430   a˜d  are fully silicided NMOS transistors formed on silicided active area  420 , which is formed as N+OD inside either Psubstrate or P-well. For instance, the finger transistor  430   a  consists of a source region  402 , a drain region  403   a  and a gate electrode  401 . The gate electrode  401  is formed by a polysilicon line. The source region  402  and the drain region  403   a  are formed by a typical silicided process. The multi-finger transistors  430   a˜d  include multiple channels to discharge ESD currents. Each channel is defined by a contact  406  in the source region  402  and a corresponding contact  405  in one terminal of the serpentine ballasting resistor. The other terminal of the serpentine ballasting resistor is connected to the drain of the transistor  430   a  through a piece of active region (OD). 
   Referring to  FIG. 4 , the resistive element  404  surrounds the contact  405  in a spiral shape with one end coupled to the drain region  403   a  and the other end coupled to the contact  405 . Therefore, the elements  404  form resistors coupled between a bonding pad and the ESD transistors  430   a  with desired resistance yet occupy relative small areas. 
   Although the element  404  shown in  FIG. 4  winds clockwise, it is obvious to persons skilled in art that counter-clockwise winding can produce the same effect. 
   The resistive elements  404  are formed in a substrate material, such as silicided material, or nickel silicide and each on the same layer. The advantage of using silicided material is that the process for forming the ESD protection device can be easily integrated with the same manufacturing process for making integrated circuits. Moreover, it is known that silicided material has better electron migration performance than metallization material. Alternatively, the resistive elements  404  can also be formed from polysilicon, or metallization material as long as it can provide high resistance in a predetermined area. 
   As intended by such design, the ESD current will route around the spiral resistive elements, resulting in the increase of the trigger voltage of the subsequently triggered finger transistor. In this embodiment, the LOD effect is almost resolved because the distance from poly gate to shallow trench isolation (STI) of each finger is substantially the same. The serpentine shape of the resistive element helps to increase the resistance path. There are various modifications of the serpentine shape to extend the resistance path, including various zigzag shapes as shown in  FIGS. 5 and 6 . 
   In  FIG. 5 , the multi-finger transistor layout is substantially the same as that in  FIG. 4 . The major difference is in the layout of resistive elements  504   a˜b.  Refer to  FIG. 5 , in this embodiment, the resistive element  504   a  is in a zigzag shape with one end coupled to the drain region  503   a  and the other end coupled to the contact  505   a . The resistive element  504   a  functions as ballasting resistors. The contact  507   a  in the source region  502   a  and its corresponding contact  505   a  define a channel for discharging ESD current. Please note that the contact  505   a  is slightly offset to the drain region  503   b  of finger transistor  510   b  for the purpose of extending the resistance path. For the same reason, the contact  505   b  is also slightly offset to the drain region  503   a  of finger transistor  510   a.    
     FIG. 6  shows another layout of resistive elements with a meandering shape and a common contact according to another preferred embodiment of the present invention. The multi-finger transistor layout is substantially the same as those in  FIGS. 4 and 5 . In this embodiment, the resistive elements  604   a  and  604   b  are in a meandering shape and share the same contact  605  to save space. The contact  607   a  in the source region  602   a  and its corresponding contact  605  define a channel for discharging ESD current. The resistive elements  604   a  and  604   b  provide ballasting resistance. On the other hand, the contact  607   b  in the source region  602   b  and its corresponding contact  605  also define a channel for discharging ESD current. 
   As persons skilled in the art can appreciate that different values of resistance can be obtained by adjusting the length and width of the resistant element, the distance between the contact and the gate, as well as the number of resistant elements in a row. To determine the ballasting resistance for the ESD protection device, we can apply the following formula:
 
 Rb= ( L/W )× Rsh/N,  
 
where “Rb” stands for ballasting resistance for a resistive element, “Rsh” for sheet resistance, “L” for length, “W” for width, “N” for the number of resistive elements on a drain/source side.
 
   The following table shows the sheet resistance per-square (Rsq) in various manufacturing processes with respect to various line-width and space requirements for the resistive elements under the minimum design rules for core functional elements of the IC: 
                                                                     scale                    term   0.18 μm   0.13 μm   0.09 μm   0.065 μm                       Rsq    4.1 Ω   7.06 Ω   8.41 Ω   16.88 Ω           Width   0.22 μm   0.15 μm   0.11 μm    0.08 μm           Space   0.28 μm   0.21 μm   0.14 μm    0.11 μm                        
Please note that “Width” means line width of the serpentine resistive element; “Space” means the distance between two line segments of the serpentine resistive element.
 
   From this table, we can see that the value of the square resistance increases as the line width shrinks. In other words, resistance works more efficiently in a nanometer semiconductor device than in a sub-micron semiconductor device. 
   Following the rules stated above, we can easily get the resistance value for a given finger transistor with 20 contacts on its drain region and made by a 65 nanometer process. In this instance, the resistive element has a given finger width of 33 μm and length of 0.48 μm. By looking up the table, we can find that the square resistance for a 65 nanometer process is 16.88 Ω, and the minimum design rules for the length and space of the resistive element. Then, applying the formula, we can get the ballasting resistance with the given length of only 0.48 um:
 
 Rb= ( L/W )× Rsh/N= (0.48  μm/ 0.08  μm )×16.88 Ω/20=5.06 Ω.
 
Accordingly, the ballasting resistance for the given ESD protection device is 5.06 Ω per finger. If the ballasting resistance does not meet the requirement of a certain IC, the length and other variables can then be adjusted.
 
   The physical dimensions in the embodiment of  FIGS. 4˜6  are only exemplary and not intended to limit the scope of the invention. The total device width depends on the required ESD strength. The number of contacts in each row over each source and drain region depends on the size of the active area. The number of fingers of the MOS ESD devices also depends on the size of the bonding pads of each MOS ESD device. 
   Based on the above discussion, there are many possible embodiments for designing the layout of resistive elements with a serpentine shape. The serpentine shape includes any meandering shape that can extend the resistance path from the drain to the contacts. Please note that the present invention is discussed in terms of. NMOS ESD devices. However, the present invention is also applicable to PMOS ESD devices in a similar manner. Various modifications are known to those skilled in the art without extensive discussions. 
   The above illustration, provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.