Abstract:
Methods and circuits are disclosed for providing distributed ESD protection switchable between a capacitive decoupling state and an ESD protection state. The invention provides electronic circuitry with a selectable capacitive decoupling path and an ESD shunting path responsive to the detection of the presence or absence of an electrostatic discharge event. Circuits of the invention include one or more control circuits, electrostatic discharge devices, and control nodes operably coupled to responsively switch the circuit from a decoupling state to an electrostatic discharge state.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to semiconductor devices and microelectronic circuitry. More particularly, it relates to electrostatic discharge (ESD) protection devices and methods.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electrostatic discharge (ESD) events can cause damage to elements of circuitry due to excessively high voltages or currents. For example, the propagation of an ESD event through a circuit may cause a transistor to greatly exceed its voltage or current capacity, suffer physical damage, and subsequently fail. The potential for failure increases as circuitry becomes smaller and as operating voltage levels are reduced. ESD events may occur due to a relatively short period of relatively high voltage or current imposed on a device. For example, ESD events are sometimes caused by contact with the human body, by machinery such as manufacturing or test equipment, or in electrically active environments, as may be incurred in many consumer applications. A variety of ESD events can occur in electronic devices, including discharge between the pads of an integrated circuit, discharge between voltage supply terminals, and discharge between pads and voltage supply terminals. Various kinds of ESD protection circuitry are used in the art to protect ICs from the damage due to the occurrence of ESD events during manufacture, testing, and operation. In general, ESD protection circuitry is designed to protect the input/output circuitry and internal circuitry of an integrated circuit from excessively large and sudden discharges of electrostatic energy.  
         [0003]     One known approach is to provide ESD protection external to the microelectronic circuit. Among other problems, this approach can adversely affect the load capacitance, resistance, speed, linearity, frequency response, stability, or slew rate of the circuitry. Another potential solution is to make the circuitry better able to withstand higher voltages Of ESD event. This solution can also adversely affect the performance of the circuit, and is unacceptable for many applications. External ESD protection circuitry can also be placed within the functional circuit path. In many applications, however, extra loading and capacitance is unacceptable. Additional challenges to providing ESD protection in IC devices lie in the ever-present desire to reduce die area, and to reduce current leakage.  
         [0004]     Due to these and other problems, a need exists for circuits and methods that provide microelectronic circuits with the ability to withstand ESD events without adversely impacting the performance of the functional circuit path during normal operation.  
       SUMMARY OF THE INVENTION  
       [0005]     In carrying out the principles of the present invention, in accordance with preferred embodiments thereof, methods and circuits of the invention are described with reference to exemplary embodiments providing distributed ESD protection switchable between a capacitive decoupling state and an ESD protection state.  
         [0006]     According to one aspect Of the invention, a method is provided by which a circuit may be endowed with a selectable capacitive decoupling path and an ESD shunting path responsive to the detection of the presence or absence of an electrostatic discharge event.  
         [0007]     According to another aspect of the invention, a circuit embodiment includes an control circuit, an electrostatic discharge device, and a control node operably coupled to switch the circuit from a decoupling mode to an electrostatic discharge mode responsive to an electrostatic discharge event.  
         [0008]     According to another aspect of the invention, a circuit embodiment includes a plurality of electrostatic discharge responsive subcircuits. The electrostatic discharge responsive subcircuits further include control circuits, electrostatic discharge devices, and a control node. The configuration is operative to switch the electrostatic discharge responsive subcircuits from a decoupling mode to an electrostatic discharge mode responsive to an electrostatic discharge event providing a shunting path for dissipation of the electrostatic discharge.  
         [0009]     The invention provides technical advantages including but not limited to providing selectable capacitive decoupling and ESD shunting responsive to the occurrence of ESD events. These and other features, advantages, and benefits of the present invention can be understood by one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will be more clearly understood from consideration of the following detailed description and drawings in which:  
         [0011]      FIG. 1  is a schematic diagram of an example of a preferred embodiment of an ESD protection cell according to the invention;  
         [0012]      FIG. 2A  is a schematic diagram of an example of a preferred embodiment of the invention illustrating operation in a normal mode;  
         [0013]      FIG. 2B  is a schematic diagram of an example of a preferred embodiment of the invention illustrating operation in a protection mode;  
         [0014]      FIG. 3  is a schematic diagram depicting an example of preferred embodiments of the invention deployed at the input/output nodes of a device;  
         [0015]      FIG. 4  is a graphical representation of an example of changes in voltages during the operation of the circuit of  FIG. 3 ;  
         [0016]      FIG. 5  is a graphical representation of an example of changes in currents during the operation of the circuit of  FIG. 3 ;  
         [0017]      FIG. 6  is a graphical representation of an example of power dissipation during the operation of the circuit of  FIG. 3 ;  
         [0018]      FIG. 7  is a graphical representation of an example of energy dissipation during the operation of the circuit of  FIG. 3 ;  
         [0019]      FIG. 8  is a graphical representation of a further example of changes in voltages during the operation of the circuit of  FIG. 3 ;  
         [0020]      FIG. 9  is a graphical representation of a further exampie of cnanges in current during the operation of the circuit of  FIG. 3 ;  
         [0021]      FIG. 10  is a graphical representation of a further example of power dissipation during the operation of the circuit of  FIG. 3 ;  
         [0022]      FIG. 11  is a graphical representation of a further example of energy dissipation during the operation of the circuit of  FIG. 3 ; and  
         [0023]      FIG. 12  is a graphical representation of an example of the range of capacitance over a selected frequency spectrum during the operation of the circuit of  FIG. 3 . 
     
    
       [0024]     References in the detailed description correspond to the references in the figures unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, top, bottom, side, etc., refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     In general, the methods and circuits of the invention provide improved ESD protection in microelectronic circuits. During normal operation, the invention functions as a decoupling capacitor between the power supply nodes of an associated electronic circuit. The invention responds to the occurrence of an ESD event by providing a local power clamp for dissipating the ESD current.  
         [0026]     Referring primarily to  FIG. 1 , a schematic circuit diagram illustrates an example of a preferred embodiment of an ESD protection cell  10  according to the invention. A control circuit  12  is provided between the power rails Vdd, Vss within the context of an associated circuit (not shown). Preferably, as shown in  FIG. 1 , the control circuit  12  uses a first PMOS transistor  14  and a second NMOS transistor  16  coupled to a control node  18  in a suitable configuration for activation by a control signal as further described herein. It should be apparent to those skilled in the arts that parasitic resistance will be inherent in the components; these resistances are denominated in the figures as R vddk , and R vssk . An ESD device  20 , preferably a PMOS transistor as shown, is coupled between the rails Vdd, Vss, and the transistors  14 ,  16 , of the control circuit  12  in the operable configuration shown. Although one preferred embodiment of an ESD rail clamp circuit  10  of the invention is shown by way of example, it will be recognized by those skilled in the arts that alternative circuit configurations may also be used without departure from the invention, provided the described functionality is achieved.  
         [0027]      FIG. 2A  is a schematic diagram further illustrating the operation of the circuit of  FIG. 1  in an example of operation in a “normal,” i.e. non-ESD, state. In this example, the control circuit  12  is configured to select the normal mode responsive to a “ 0 ” at the control node  18  provided by an ESD detection circuit (not part of the invention). The invention may be used with various ESD detection circuits available in the arts. The first transistor  14  of the control circuit  12  is allowed to conduct, as is the ESD device  20 . The second transistor  16  of the control circuit  12  is inactive. Examination of the current paths indicated by arrows  22 ,  24 , reveals that in this state the ESD cell  10  functions as a decoupling capacitor isolating the Vdd and Vss sides of the ESD cell  10 . It may be seen in  FIG. 2A  that it is contemplated to use multiple implementations of the ESD cell  10  of the invention in parallel to provide multiple isolation/rail clamp circuits  10  as further described.  
         [0028]      FIG. 2B  is a schematic diagram of the example of the preferred embodiment of the invention depicted in  FIGS. 1 and 2 A illustrating operation of the ESD protection clamp  10  in an ESD protection mode. In this example, a high control signal indicated by “ 1 ” is provided at the control node  18 . In this state, the first transistor  14  of the control circuit  12  is inactive. Current paths, indicated by arrows  26 , and  28 , are provided through the ESD device  20  and the second transistor  16  of the control circuit  12 . It may be appreciated that in this state, the ESD cell  10  provides a path for shunting current between the circuit rails Vdd, Vss. As in  FIG. 2A , multiple implementations of the ESD cell  10  of the invention are shown in parallel providing multiple rail clamps for distributing ESD current.  
         [0029]     A depiction representative of an implementation of a preferred embodiment of the invention is shown in  FIG. 3 . Multiple ESD cells  10 , are shown in the context of a larger circuit  30 . Each of the ESD cells  10  is configured as shown and described with respect to  FIG. 1 , and functions as either a decoupling circuit or rail clamp circuit in response to a control signal at the control node  18 , as shown and described with respect to  FIGS. 2A and 2B . In this case, a conventional rail clamp  32  is aiso coupied between Vdd and Vss. The conventional rail clamp  32  is preferably an “active rail clamp” known in the arts for shunting ESD currents in a circuit  30 . Other rail clamps, including conventional rail clamps known in the arts, may alternatively be used in combination with the invention.  
         [0030]     Understanding of the operation of the circuit  30  and of the individual rail clamps  10 ,  32 , therein may be further enhanced by reference to  FIGS. 4 through 12 . For the purposes of this example, a 2 kV human body model (HBM) ESD event is used.  
         [0031]     Assuming for the sake of example, a circuit  30  as diagrammed in  FIG. 3  is subjected to an ESD event consisting of the application of a HBM voltage on the left side of the figure, and a left side ground,  FIG. 4  is a graphical representation of an example of changes in voltages across each ESD discharge element location in the circuit  30  during the ESD event. The changes in voltages across each of the ten successive ESD cell rail clamps  10   a ,  10   b , . . . ,  10   h , and the active rail clamp  32  are shown. It may be seen that the ESD voltage is shunted across the various clamps  10 i a,  10   b , . . . ,  10   h ,  32 .  
         [0032]      FIG. 5  is a graphical representation of an example of the cumulative discharge of current among the clamps  10   a ,  10   b , . . . ,  10   h ,  32  during the operation of the circuit  30  of  FIG. 3 . It may be seen that in this example, approximately one-half of the current is discharged by the ESD cells  10   a ,  10   b , . . . ,  10   h , and about one-half in the active rail clamp  32 . Of course, the circuit  30  of this example is a representative example only, and the actual distribution of current within a circuit used to practice the invention may be adjusted by changing the components or configuration of the circuit.  FIG. 6  is a graphical representation of an example of the cumulative power dissipation in the circuit  30  of  FIG. 3  and its component clamps  10   a ,  10   b , . . . ,  10   h ,  32 . Similarly,  FIG. 7  is a graphical representation of an example of cumulative energy dissipation during the operation of the circuit  30  of  FIG. 3 .  
         [0033]     For the purposes of an additional example, again assuming the circuit  32  diagrammed in  FIG. 3  undergoing the occurrence of an ESD event with an application of a HBM voltage on the left side of the figure, and a right side ground,  FIG. 8  is a graphical representation of an example of changes in voltages during the operation of the circuit  30  of  FIG. 3 . The changes in voltages across each of the ten successive ESD cell rail clamps  10   a ,  10   b , . . . ,  10   h , and the Active rail clamp  32  are shown. It may be seen that the ESD voltage is uniformly distributed across the various clamps  10   a ,  10   b , . . .  10   h ,  32 .  FIG. 9  is a graphical representation of an example of the cumulative discharge of current among the clamps  10   a ,  10   b ,  10   h ,  32  during the operation of the circuit  30  of  FIG. 3 . It may be seen that in this example, approximately one-third of the current is discharged by the ESD cells  10   a ,  10   b , . . . ,  10   h , and about two-thirds in the active rail clamp  32 .  FIG. 10  is a graphical representation of an example of the cumulative power dissipation in the circuit  30  of  FIG. 3  and its component clamps  10   a ,  10   b , . . . ,  10   h ,  32 .  FIG. 11  is a graphical representation of an example of cumulative energy dissipation during the operation of the circuit  30  of  FIG. 3 .  
         [0034]     The invention, in addition to providing ESD discharge capabilities, operates to provide an effective decoupling capacitance during conditions when no ESD event is evident, i.e., the majority of the time.  FIG. 12  shows a graphical representation of an example of the range of capacitance over a selected frequency spectrum, from  10   7  Hz to  10   10  Hz, during the operation of the circuit  30  of  FIG. 3  for three ranges of transistor strengths R 1 , R 2 , R 3  implicit to the statistical variation in the manufacturing process.  
         [0035]     Thus, the invention provides methods and circuits that provide microelectronic circuits with the ability to withstand ESD events without adversely impacting the performance of the functional circuit path during normal operation. The methods and devices of the invention provide advantages including but not limited to savings in die area, selectable capacitive decoupling and ESD protection capabilities. While the invention has been described with reference to certain illustrative embodiments, the methods and apparatus described are not intended to be construed in a limited sense. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the art upon reference to the description and claims.