Abstract:
An circuit device includes a stack of electrostatic discharge protection devices shared between a plurality of contact pads on the device. In addition, each contact pad is coupled to its own individual protection device. Together, the individual protection devices and the stack establish a trip point for shunting a charge from the contact pads in the event that any of the pads reach a voltage potential greater then the trip point. In doing so, the stack protects internal operations circuits from damage. At the same time, the shared stack conserves die space.

Description:
RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/209,210, filed on Dec. 10, 1998 now U.S. Pat. No. 6,181,540; which is a continuation of application Ser. No. 08/802,665, filed Feb. 19, 1997 and issued as U.S. Pat. No. 5,889,644. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electrostatic discharge protection arrangements for circuit devices and, more specifically, to a system and method for diverting the transmission of electrostatic discharges. 
     BACKGROUND OF THE INVENTION 
     It is well known to those skilled in the art that, during handling and testing of circuit devices, electrostatic charges can be applied to the contact pads of such a device. These charges may be subsequently discharged through the device, thereby causing damage. One solution known in the prior art for preventing damage from these electrostatic discharge (ESD) events is to connect protection devices, such as p-n junction diodes, between each contact pad and its corresponding operations circuit. Typically, one such diode leads to a voltage source V CC  and another leads to ground. In the event a sufficient charge builds up on a contact pad, one of the two diodes will activate and direct the charge away from the operations circuit. Further, the activation of the diodes will be based on their threshold voltage (V t ). For example, a positive charge will be diverted towards the voltage source V CC  when the contact pad&#39;s potential exceeds one V t  above V CC . Alternatively, a negative charge will be diverted toward ground when the contact pad&#39;s potential is one V t  below ground. 
     Occasionally, it is desirable to drive an operations circuit at voltages beyond the one V t  parameters. Those of ordinary ski in the art know that the trip point for shunting charges can be changed by adding diodes to the configuration described above. Nevertheless, based on prior art teachings, changing the trip point for several operations circuits would require adding diodes for each contact pad. Unfortunately, these diodes, as well as other ESD protection devices, require a significant amount of die space. Thus, it would be desirable to provide ESD protection, while at the same time allowing for the alteration of the trip point without the space requirements necessary in the prior art. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides for ESD protection and trip point alteration of a circuit device. Several embodiments of the present invention are designed to accommodate a circuit device having a plurality of terminals, each leading to a different operations circuit. In one preferred embodiment of the present invention, an ESD protection device is connected to each terminal. These ESD protection devices, in turn, are commonly coupled to a stack of additional ESD protection devices. The number of devices in this stack depends upon the particular trip point to be established. Thus, in addition to protecting the device from ESD, the shared stack in this preferred embodiment has the added advantage of reducing the number of ESD protection devices needed to safeguard the operations circuits. Further, this configuration allows for simultaneous trip point alteration for different operations circuits, once again reducing the number of ESD protection devices needed to do so. The present invention also encompasses various methods for achieving these advantages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an ESD protection circuit as found in the prior art. 
     FIG. 2 demonstrates a prior art example of an ESD protection circuit using multiple diodes to alter the trip point at which current is diverted. 
     FIG. 3 demonstrates one exemplary embodiment of an ESD protection device in accordance with the present invention. 
     FIG. 4 demonstrates a second exemplary embodiment in accordance with the present invention. 
     FIG. 5 depicts a contact pad configuration found on a particular device in the prior art. 
     FIG. 6 is a close-up, top-down view of the device in FIG.  5  and demonstrates a third exemplary embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A standard ESD protection circuit is illustrated in FIG. 1, wherein a first diode  10  is coupled to the supply voltage V CC  and is provided for a first contact pad  12 . The first contact pad  12 , in turn, leads to a first operations circuit of a semiconductor device. For purposes of this application, the term “contact pad” refers to any conductive surface configured to permit temporary or permanent communication with an eternal operations circuit or node. A second diode  14  is coupled between the first contact pad  12  and ground. Similarly configured diodes  11  and  15  can be coupled to a second contact pad  16 , which leads to a second operations circuit. In fact, generally every contact pad on the device can be configured in a similar manner. For purposes of clarity, however, only two such pads are illustrated in FIG.  1 . Should a sufficient charge build up on the first contact pad  12 , one of the two diodes  10  or  14  will turn on and shunt the charge away from the operations circuit. As discussed in the background section, this configuration allows an operations circuit to accommodate voltages ranging from one V t  greater than the supply voltage V CC  to one V t  less than ground before a charge is diverted. As an example of the operation of this prior art configuration, the first diode  10  will turn on should contact pad  12  have a charge with a voltage potential that is one V t  greater than V CC . At that point, the first diode  10  turns on, thereby creating a path of lesser electrical resistance than the path leading to the operations circuit. As a result, the first diode  10  shunts the charge through itself and to the voltage source. Similarly, the second diode  14  has a trip point of one V t  less than ground. When the voltage potential of a charge at the first contact pad  12  reaches that point, the second diode  14  turns on and draws the current through itself toward ground. Charges at the second contact pad  16  are handled in the same manner. 
     FIG. 2 highlights a prior art configuration in which the trip point of the ESD protection circuits is increased. Four diodes  42 ,  44 ,  46 , and  48  are coupled between an input contact point IN and the source voltage which, in this case, is labeled V DD . As a result, a charge at IN will not be diverted until the voltage potential reaches four V t &#39;s above V DD . It is noteworthy that this multiple diode protection circuit is repeated for a second operations circuit, wherein four diodes  50 ,  52 ,  54 , and  56  are electrically interposed between an input contact point IN′ and V DD . Further, the same configuration is used for every input terminal in this prior art example. Thus, this example teaches that changing the trip point for multiple operations circuits requires adding diodes for each contact point. 
     A preferred embodiment of the invention using fewer gates to achieve the same effect appears in FIG. 3. A first contact pad  12  and a second contact pad  16  are again shown, although it is understood that a pad plurality of any size will benefit from this invention. The first contact pad  12  is coupled to an exclusive diode  18  that is configured to protect only the first contact pad  12 . Similarly, the second contact pad  16  is also coupled to an exclusive diode  28 . Exclusive diodes  18  and  28  share a first common node  20 . A positive ESD diode stack  22  is coupled to the first common node  20  and is further configured to receive a voltage source, V CC , although the voltage source itself is not a part of the invention. In FIG. 3, the positive ESD diode stack  22  comprises three diodes. It is understood, however, that the positive ESD diode stack  22  could contain any number of diodes coupled in series, cathode-to-anode, so that current is conducted predominantly in only one direction through the stack. Moreover, the diode stack as generally used in this and other embodiments may comprise only one diode. Given the stack size in FIG. 3 combined with the exclusive diode  18 , a charge on the first contact pad  12  reaching a voltage potential of four V t &#39;s above V CC  will be drawn away from the first operations circuit towards the voltage source. Moreover, the orientation of the exclusive diode  28  isolates the second contact pad  16  from a positive ESD event transmitted through the first contact pad  12 . Conversely, exclusive diode  18  will isolate the first contact pad  12  from any ESD at the second contact pad  16 . As a result, this embodiment has established a trip point of four V t &#39;s above V CC  for protecting both contact pads  12  and  16  using only five diodes, whereas prior art systems require eight diodes—four for each contact pad. 
     FIG. 4 shows another exemplary embodiment in which three contact pads A, B, and C, each coupled to a separate operations circuit, are protected from a positive ESD event by three exclusive diodes  18 ,  28 , and  30 , as well as a positive ESD diode stack  22  comprising two diodes. This second embodiment also illustrates that a shared stack of diodes can be used to protect against negative ESD events as well. Three exclusive diodes  32 ,  34 , and  36  are provided, one at each contact pad, and share a second common node  24 . A negative ESD diode stack  26  is coupled to the second common node  24  and is configured to connect to ground, although ground is not claimed as part of the current invention. As with the positive ESD diode stack  22 , the negative ESD diode stack  26  may comprise one or more diodes. In FIG. 4 the negative ESD diode stack  26  comprises a series of three diodes, thereby demonstrating that the size of the negative ESD diode stack  26  may differ in size from the positive ESD diode stack  22 . The negative ESD diode stack  26  and the exclusive diodes  32 ,  34 , and  36  coupled to it are configured to draw an electric charge from any of the contact pads having a negative voltage potential exceeding four V t &#39;s below ground. In addition, the orientation of the three exclusive diodes  32 ,  34 , and  36  serve to isolate each contact pad from negative ESD events originating at the other pads. Thus, the operations circuits in FIG. 4 are protected from a negative ESD event that might be transmitted through the pads shown. For such an event, this exemplary embodiment also establishes a trip point of four V t &#39;s below ground using only  6  diodes, whereas the prior art would require  12  diodes. 
     From the exemplary embodiments in FIGS. 3 and 4, it follows that more space on the circuit device is saved as the number of pads sharing diode stacks increases. The invention would be particularly useful given the contact pad configurations of certain circuit devices in current use. FIG. 5 illustrates such a device, wherein the contact pads are located in the center of a die  60  and extend in two columns down its length. The close-up view in FIG. 6 demonstrates how four contact pads on this device may be wired to share a positive ESD diode stack  22 . Given the proximity of each contact pad to the others in this device, even more contact pads may share the positive ESD diode stack  22  for a greater saving of space. Further, the contact pads could also share a negative ESD diode stack. 
     One of ordinary skill in the art can appreciate that, although specific embodiments of this invention have been described above for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. For example, protection devices other than p-n junction diodes may be used to shunt current away from the operations circuits, such as metal-oxide semiconductor field effect transistor (MOSFEST) diodes and bipolar junction transistors (BJT&#39;s). In addition, it should be noted that the contact pads discussed above could serve as either input pads or output pads. Accordingly, the invention is not limited except as stated in the claims.