Abstract:
Methods and apparatus are provided an electrostatic discharge (ESD) protection device having a first terminal and a second terminal. The ESD protection device comprises a vertical transistor having a collector coupled to the first terminal, a base, and an emitter coupled to the second terminal. A zener diode has a first terminal coupled to the first terminal of the ESD protection device and a second terminal coupled to the base of the vertical transistor. Subsurface current paths are provided to redistribute current from a surface of the vertical transistor in an ESD event. The method comprises generating an ionization current when a zener diode breaks down during an ESD event. The ionization current density from a surface zener diode region is reduced. The ionization current enables a transistor to dissipate the ESD event.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to electrostatic discharge protection, and more particularly relates to an electrostatic discharge structure having a zener diode triggered bipolar transistor. 
     BACKGROUND OF THE INVENTION 
     An integrated circuit is prone to damage by electrostatic discharge (ESD) throughout its life cycle, from manufacture to use in a product. An electrostatic discharge event is a high energy impulse having a voltage amplitude typically measuring thousands of volts that can physically and permanently damage a semiconductor device. In general, an electrostatic discharge event is coupled to circuitry of an integrated circuit through the input/output (I/O) of the device. The I/O are the interface of the integrated circuit for receiving/sending signals from off chip. The circuitry of the integrated circuit that couples to an I/O can be damaged unless protected by an electrostatic discharge protection circuit or device. ESD protection of an integrated circuit has always been a critical issue as the semiconductor industry continues the trend of shrinking transistor geometries to maximize circuit density per square inch of silicon. Forming a smaller transistor requires wafer processing having reduced critical dimensions and thinner material layers making it more sensitive to damage from ESD. 
     ESD devices are disabled during normal operation of an integrated circuit. An ESD event at one of the I/O triggers a corresponding ESD circuit to dissipate the energy of the ESD event before damage occurs to the integrated circuit. Typically, this is achieved by enabling a device or devices having an extremely low impedance coupled to ground. In other words, the high voltage impulse is shunted to ground through the ESD circuit. The ESD circuit is disabled once the energy is dissipated or the voltage at the I/O falls to a safe level. Ideally, an ESD circuit does not load the I/O significantly when disabled, responds rapidly to an ESD event, takes up minimum area, and has a low impedance when enabled. 
       FIG. 1  is a schematic diagram of a prior art electrostatic discharge (ESD) protection circuit  10 . ESD protection circuit  10  is a model of an integrated device structure that is coupled to the I/O pads of an integrated circuit to protect against electrostatic discharge. ESD protection circuit  10  has a terminal  15  and a terminal  20 . Terminal  15  couples to circuitry of an integrated circuit and corresponds to an I/O of the device. The terminal  20  of ESD protection circuit  10  is coupled to ground. ESD protection circuit  10  comprises a transistor  25 , a zener diode  30 , and a resistor  35 . Transistor  25  includes a collector coupled to terminal  15 , a base, and an emitter coupled to terminal  20 . Zener diode  30  has a first terminal coupled to terminal  15  and a second terminal coupled to the base of transistor  25 . A resistor has a first terminal coupled to the base of transistor  25  and a second terminal coupled to terminal  20 . In an embodiment of ESD protection circuit  10 , zener diode  30  is integrated into the structure of transistor  25 . 
     An integrated circuit typically has a specified voltage range under which it can operate. A voltage applied to terminal  15  within the specified voltage range will not damage the internal circuitry of the integrated circuit coupled to the I/O. In general, the specified voltage range is conservative and a greater voltage (but less than BV DSS ) can be applied to terminal  15  without damage. Zener diode  30  is designed to have a breakdown voltage greater than the specified voltage range. Zener diode  30  conducts no current when a voltage applied to terminal  15  is within the specified voltage range. Resistor  35  couples the base of transistor  25  to ground under this condition. Transistor  25  is off because the base-emitter junction is not forward biased having both the base and the emitter coupled to the same voltage potential (ground). Thus, transistor  25  and zener diode  30  are disabled and conduct no current under normal operation of the integrated circuit. 
     An ESD event coupled to terminal  15  is a fast rise time, voltage impulse, having a magnitude that measures thousands of volts if the impulse is not attenuated. ESD protection circuit  10  is enabled by the voltage impulse and dissipates the energy corresponding to the electrostatic discharge before damage to the circuitry coupled to terminal  15  occurs. An impact ionization current is generated in zener diode  30  when the voltage impulse exceeds the breakdown of the device. 
     The impact ionization current increases as avalanche multiplication occurs in zener diode  30 . The impact ionization current from zener diode  30  couples through resistor  35  and generates a rising voltage at the base of transistor  25 . Transistor  25  is enabled when the base voltage rises to a voltage that forward biases the base-emitter junction of the device. Transistor  25  is a high current gain device. Once enabled, transistor  25  rapidly sinks current from the ESD event thereby dissipating the energy of the impulse which results in the voltage being clamped to a level that protects the circuitry coupled to terminal  15 . 
       FIG. 2  is a cross-sectional view of the prior art electrostatic discharge (ESD) protection circuit  10  of  FIG. 1 . In general, ESD protection circuit  10  couples to and is formed in proximity to an I/O pad of an integrated circuit. ESD protection circuit  10  has terminal  15  coupled to the I/O pad of the integrated circuit and terminal  20  coupled to ground. An n-type epitaxial layer  110  overlies a p-type substrate  105 . An isolation region  120  defines the active area in which ESD protection circuit  10  is formed. Isolation region  120  is a p-type region that is formed in a ring shape and is coupled to ground. The active area is interior to the ring shape. 
     Transistor  25  of  FIG. 1  comprises a base region  130 , an emitter region  145 , and a collector. The collector is epitaxial layer  110  in the active area. Transistor  25  is a high current gain vertical transistor. Base region  130  is p-type and is formed in epitaxial layer  110 . Emitter region  145  is n-type and is formed in base region  130 . A p-type region  140  is formed at the surface of base region  130  in a ring shape and surrounds emitter region  145 . P-type region  140  is a low resistance base contact to base region  130 . P-type region  140  and emitter region  145  are coupled to terminal  20 . As mentioned previously, terminal  20  is coupled to ground. 
     A buried layer  115  and n-type region  125  combine to form a low resistance path for collector current of transistor  25 . Buried layer  115  underlies base region  130  and is formed at the interface between substrate  105  and epitaxial layer  110 . N-type region  125  is a ring shaped region that surrounds base region  130 . N-type region  125  is a deep n+ region formed in epitaxial layer  110  in the active area that forms a low resistance path from a surface of epitaxial layer  110  to buried layer  115 . A heavily doped n-type region  135  is formed in region  125  that couples to terminal  15  of ESD protection circuit. 
     Zener diode  30  of  FIG. 1  comprises a p-type region  150 , epitaxial layer  110 , and n-type region  125 . P-type region  150  overlies a boundary of base region  130  and epitaxial layer  110 . P-type region  150  couples to base region  130  and is formed in a ring shape around the periphery of base region  130 . P-type region  150  has a higher doping concentration than base region  130 . As shown in  FIG. 1 , zener diode  30  is coupled in parallel with the collector-base of transistor  25 . The spacing between p-type region  150  and n-type region  125  and the doping concentrations of the device components of zener diode  30  play a role in determining what voltage zener diode  30  breaks down. 
     Resistor  35  of  FIG. 1  corresponds to the inherent resistance of base region  130 . In an ESD event, impact ionization current is generated by zener diode  30  when the breakdown voltage of zener diode  30  is exceeded. The impact ionization current is coupled to base region  130 . Note that both emitter region  145  and p-type region  140  are coupled to ground through terminal  20  of ESD protection circuit  10 . The inherent resistance of base region  130  produces a voltage drop as impact ionization current is conducted that forward biases the base-emitter junction of transistor  25 . Upon enabling transistor  25 , a portion of the impact ionization current is base current for the device. The base current is multiplied by the current gain of transistor  25  which rapidly dissipates the energy of the ESD event and clamps the voltage from exceeding a value that can damage circuitry coupled to terminal  15 . The voltage at terminal  15  falls as transistor  25  dissipates the energy of the ESD event. Zener diode  30  stops conducting current when the voltage at terminal  15  falls below the breakdown voltage of the device. Transistor  25  is disabled when deprived of the current from zener diode  30  thus returning to the state prior to the ESD event with no current being conducted by zener diode  30  and transistor  25 . 
       FIG. 3  is a graph of a transmission pulse line characteristic corresponding to ESD protection circuit  10  of  FIG. 2 . Transmission pulse line testing provides a pulse similar to an ESD event to an ESD protection circuit. The voltage and current coupled to the ESD protection circuit is monitored. A curve on the graph relates to measurements on an ESD protection circuit similar in structure to that shown in  FIG. 2  and measuring 52.5 microns on a side. Voltage is displayed on the x-axis and current on the y-axis. 
     The voltage impulse is clamped to a voltage magnitude less than 50 volts as the zener diode breaks down providing impact ionization current to enable the transistor. The voltage rapidly falls to approximately the breakdown voltage of the zener diode plus a base-emitter junction voltage. The test equipment measures the maximum current that can be handled by ESD protection circuit before failure. The point of failure is represented by dot  210  on the curve which corresponds to a current slightly less than 4000 milliamperes. 
     Although not indicated by the graph, the failure mechanism typically results in damage at the base terminal of the ESD protection circuit. As mentioned previously, the transistor tested corresponds to the device shown in  FIG. 2 . The transistor tested is a vertical device comprising emitter region  145 , base region  130 , and epitaxial layer  110  (collector). The device structure shown also has parasitic lateral transistor component that is inherent to the design. It is believed that the failure at the base terminal of the device occurs due to high currents flowing near the surface of the transistor due to currents from the zener diode and the lateral transistor that couple to the base terminal. An ESD event of substantial energy produces a current at the base terminal due to the circuit structure that causes a failure in the ESD protection device. 
     Accordingly, it is desirable to provide an electrostatic discharge protection circuit capable of suppressing higher energy electrostatic events. It would be beneficial if the electrostatic discharge protection circuit had a smaller footprint. It would be of further benefit if the electrostatic discharge protection circuit did not require any special manufacturing steps. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a schematic diagram of a prior art electrostatic discharge circuit; 
         FIG. 2  is a cross-sectional view of a prior art electrostatic discharge (ESD) protection circuit corresponding to the schematic diagram of  FIG. 1 ; 
         FIG. 3  is a graph of a transmission pulse line characteristic corresponding to the ESD protection circuit of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of an electrostatic discharge (ESD) protection circuit in accordance with the present invention; 
         FIG. 5  is a graph of a transmission pulse line characteristic corresponding to ESD protection circuit of  FIG. 4 ; and 
         FIG. 6  is a top view of an ESD protection circuit in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     A zener diode triggered bipolar transistor electrostatic discharge (ESD) protection circuit similar to that shown in  FIG. 1  is capable of being formed on both existing and future wafer process flows. In general, cost is a factor in the design of an ESD protection circuit. Typically, an electrostatic discharge protection circuit is made from process steps and masks that exist in the wafer flow. The use of extra mask or wafer process steps to optimize performance of the ESD protection circuit performance is rarely justified due to the increased cost of manufacture. Another variable affecting the cost of ESD protection circuit is the size of the structure for a given performance level. ESD protection circuits can take up a substantial amount of silicon area. A reduction in the area of an ESD protection circuit can decrease the cost to manufacture by minimizing the die area. 
       FIG. 4  is a cross-sectional view of an electrostatic discharge (ESD) protection circuit  300  in accordance with the present invention. ESD protection circuit  300  prevents an electrostatic discharge from damaging an integrated circuit. In general, an ESD protection circuit is placed at each input/output (I/O) of an integrated circuit. ESD protection circuit  300  is enabled by an ESD event to dissipate the energy of the discharge before circuitry of the integrated circuit is damaged. For example, complementary metallic oxide semiconductor (CMOS) transistors have a thin gate oxide that is easily damaged by a high voltage pulse. In an embodiment of the structure, an ESD event is shunted to ground through ESD protection circuit  300  such that the peak voltage and duration of the ESD event is reduced to a level that does not damage circuitry of the integrated circuit. 
     ESD protection circuit  300  includes a terminal  400  and a terminal  410 . In an embodiment of the structure, terminal  400  couples to an I/O of the integrated circuit and terminal  410  couples to ground. ESD protection circuit  300  comprises a zener diode and a bipolar transistor. The zener diode is coupled across the collector-base junction of the bipolar transistor similar to that shown in  FIG. 1 . In an embodiment of ESD protection circuit  300 , the bipolar transistor is a vertical npn transistor having a collector coupled to terminal  400 , a base, and an emitter coupled to terminal  410 . The zener diode has a cathode and anode respectively coupled to the collector and the base of the transistor. 
     In an embodiment of ESD protection circuit  300 , the integrated structure is formed in an n-type epitaxial layer  320  that overlies a p-type substrate  310 . An isolation region defines the active area for ESD protection circuit  300 . In an embodiment of ESD protection circuit  300 , the isolation region is a p-type region  340  formed in epitaxial layer  320 . P-type region  340  is a deep p-type region that extends from a surface of epitaxial layer  320  into substrate  310 . In an embodiment of the device, p-type region  340  is formed in a ring shape that isolates and defines the active area of ESD protection circuit  300  as epitaxial layer  320  interior to the ring shape. It should be noted that ESD protection circuit  300  is not limited to p-type region  340  but can be fabricated using other isolation strategies such as a deep trench filled with dielectric material or undoped polysilicon that are well known to one skilled in the art. Substrate  310  and p-type region  340  are coupled to ground. 
     An n-type buried layer  330  partially underlies epitaxial layer  320  in the active area. Buried layer  330  is formed at the approximate interface between substrate  310  and epitaxial layer  320 . An n-type region  350  is formed in epitaxial layer  320 . N-type region  350  extends from a surface of epitaxial layer  320  to n-type buried layer  330 . Epitaxial layer  320  within the boundary set up by n-type region  350  and n-type buried layer  330  is the collector of the transistor. N-type region  350  and n-type buried layer  330  are heavily doped to form a low resistance path for collector current of the vertical npn transistor. In an embodiment of ESD protection circuit  300 , n-type region  350  is formed in a ring shape. Buried layer  330  underlies epitaxial layer  320  interior to the ring shape of n-type region  350 . In an embodiment of the transistor, an n-type region  360  is formed in n-type region  350  for coupling to metal interconnect that couples terminal  400  of ESD protection circuit  300  to an I/O of the integrated circuit. 
     A p-type base region  430  is formed in the active area within the interior of the ring shape of n-type region  350 . Base region  430  is formed in epitaxial layer  320  and spaced a predetermined distance from n-type region  350 . A p-type region  390  is formed at a surface of epitaxial layer  320  overlying a boundary between base region  430  and epitaxial layer  320 . P-type region  390  couples to p-type base region  430  and has a higher doping concentration than p-type base region  430 . The zener diode of ESD protection circuit  300  comprises n-type region  350 , epitaxial layer  320 , and p-type region  390 . 
     An n-type emitter region  370  is formed in base region  430 . A p-type region  420  is formed into base region  430 . In an embodiment of the transistor, p-type region  420  is formed in a ring shape that surrounds emitter region  370 . P-type region  420  extends from the surface into base region  430 . The depth of p-type region  420  is selected to redirect current from a lateral surface flow to a more vertical current flow. In general, p-type region  420  redistributes the current from an ESD event much deeper and more uniformly through base region  430  thereby reducing failure due to non-uniform current flow and current crowding. The resistance in the current flow path is also reduced by p-type region  420 . In general, the depth of p-type region  420  is greater than 30% of the depth of base region to ensure that a substantial amount of the current flow is redistributed below the surface. Reducing the current density greatly increases the energy that ESD protection circuit  300  can dissipate before failure as will be shown hereinbelow. In an embodiment of the transistor, a p-type region  380  is formed in p-type region  420  for coupling to metal interconnect that couples to emitter region  370  and ground. A doping concentration of region  420  is higher than base region  430 . Similarly, a doping concentration of region  380  is higher than region  420 . 
     One embodiment of the transistor is described hereinbelow. Base region  430  is formed having a depth of approximately 2.8 microns and a doping concentration of approximately 2E16 atoms/cm 3 . P-type region  420  is formed to a depth of approximately 2 microns into base region  430 . This gives p-type region  420  a substantial subsurface area that more uniformly distributes current flow in base region  430  to prevent high current densities at the surface. P-type region  420  has a doping concentration intermediate to base region  430  and p-type region  380  of approximately 3E16 atoms/cm 3 . P-type region  380  is heavily doped having a doping concentration of approximately 1E20 atoms/cm 3  or higher. P-type region  380  is formed on the surface of p-type region  420 , typically having a depth of approximately 0.2 microns. 
     Under normal operating conditions (no ESD event), the transistor and zener diode are disabled. Normal operating voltages applied to terminal  400  are insufficient to break down the zener diode. The base-emitter junction of the transistor comprising base region  430  and emitter region  370  are both coupled to ground. The base-emitter junction is not forward biased in this state, thus the transistor is off. In general, ESD protection circuit  300  does not represent a significant load to signals applied to the I/O common to terminal  400 . 
     The zener diode sets a voltage at which ESD protection circuit  300  is enabled. As mentioned previously, the zener diode comprises n-type region  350 , epitaxial layer  320 , and p-type region  390 . The breakdown voltage of the zener diode is a function of doping concentration and the spacing between n-type region  350  and p-type region  390 . Epitaxial layer  320  is fully depleted prior to the zener diode voltage breakdown. The breakdown voltage of the zener diode is selected based on the type of transistors or devices being protected on the integrated circuit wafer process flow. Typically, the breakdown voltage is selected to be greater than the operating voltage of the integrated circuit to prevent false triggering under normal operation. 
     In general, ESD protection circuit  300  acts as a voltage clamp to an ESD event. An ESD event couples a voltage impulse that can measure thousands of volts to circuitry coupled to an I/O of an integrated circuit. ESD protection circuit  300  clamps the voltage to a value that does not damage the circuitry of the integrated circuit and dissipates the energy of the pulse in a short period of time. An ESD event coupled to terminal  400  couples the voltage impulse to n-type region  350 . P-type region  390  is initially coupled to ground through base region  430 . An impact ionization current is generated as the voltage across the zener diode approaches the zener breakdown voltage of the device. The impact ionization current causes avalanche breakdown to occur in the zener diode (at the breakdown voltage of the device). The impact ionization current is coupled from p-type region  390  into base region  430 . P-type region  420  provides subsurface current paths that uniformly redistributes the impact ionization current from the surface of base region  430 . Current crowding is greatly reduced. P-type region  420  creates a redistribution of bipolar currents from the surface to flow in a more vertical manner thereby reducing power dissipation in the surface region. 
     The impact ionization current from the zener diode increases corresponding to the rising voltage of the ESD event. The impact ionization current in base region  430  produces a voltage that forward biases the base-emitter junction of the transistor due to the inherent resistance of the region. The enabled transistor is a high current gain device. A portion of the impact ionization current is base current to the transistor. The transistor multiplies the base current by the current gain (β) of the vertical transistor and sinks current corresponding to the ESD event. The enabled transistor clamps the voltage of the ESD event from rising and dissipates the energy the impulse. 
       FIG. 5  is a graph of a transmission pulse line characteristics corresponding to ESD protection circuit  300  of  FIG. 4 . In general, transmission pulse line testing provides a pulse similar to an ESD event to the ESD protection circuit under test. The data shown is for an ESD protection circuit measuring 52.5 microns on a side. In particular, the ESD protection circuit has parameters similar to that described hereinabove. More specifically, base region  430  is approximately 2.8 microns deep. P-type region  430  is formed approximately 2 microns deep in base region  430 . The doping concentration of p-type region  430  is approximately an order of magnitude more than the doping concentration of base region  430 . The voltage and current coupled to the ESD protection circuit is monitored. Voltage is displayed on the x-axis of the graph and current on the y-axis of the graph. 
     An initial voltage impulse is clamped to a voltage magnitude less than 50 volts as the zener diode comprising n-type region  350 , n-type epitaxial layer  320 , and p-type region  390  breaks down providing impact ionization current to base region  430 . The impact ionization current enables the transistor by creating a voltage drop in base region  430  that forward biases the base-emitter junction. A portion of the impact ionization current is base current that is multiplied by the current gain of the transistor thereby rapidly shunting current of the ESD event through a low impedance path to ground. The voltage at terminal  400  continues to fall to approximately the breakdown voltage of the zener diode plus a base-emitter junction voltage. The test equipment measures the maximum current that can be handled by ESD protection circuit  300  before failure. The point of failure is represented by dot  510  on the curve which corresponds to a current slightly greater than 8000 milliamperes. 
     ESD protection circuit  300  as tested has the same area as the prior art ESD protection circuit tested in  FIG. 3 . Note that ESD protection circuit  300  has greater than twice the current handling capability of the prior art ESD protection circuit. Moreover, a difference in failure mechanism occurs that shifts from the base to the collector of the transistor (at 8000 milliamperes the failure occurs at the collector of the transistor). The increase in maximum current that can be handled by ESD protection circuit  300  directly translates to better protection against higher energy ESD events. A further benefit of ESD protection circuit  300  is that the cell size can be reduced while providing the same benefit of the prior art ESD protection circuit thereby reducing the die size of the integrated circuit. ESD protection circuit  300  is easily implemented in many common wafer process flows without the need of extra processing steps. Furthermore, the design is robust and scalable from a processing perspective thereby allowing it to be used in future generation process flows. 
       FIG. 6  is a top view of an ESD protection circuit  300  in accordance with the present invention. The top view is representative of the ring shapes described in  FIG. 4 . P-type region  340  is formed in a ring shape that isolates ESD protection circuit  300  from other devices (not shown) of the integrated circuit. The active area in which ESD protection circuit  300  is formed is interior to p-type region  340 . P-type region  340  is coupled to ground. 
     N-type region  350  is formed in a ring shape in the active area and contacts the buried layer (not shown) underlying the base region of the transistor. P-type region  390  is formed in a ring shape interior to the ring shape of n-type region  350 . The zener diode comprises p-type region  390 , the epitaxial layer (not shown), and n-type region  350 . P-type region  390  couples to the base region (not shown). P-type region  420  is interior to the ring shape of p-type region  390 . Finally, emitter region  370  is interior to the ring shape of p-type region  370 . In general, ESD protection circuit  300  is a symmetrical structure. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and ate not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.