Abstract:
An electrostatic discharge (ESD) protection circuit, suitable for an input stage circuit including a first N channel metal oxide semiconductor (NMOS) transistor, is provided. The ESD protection circuit includes an P channel metal oxide semiconductor (PMOS) transistor and an impedance device, in which the PMOS transistor has a source coupled to a gate of the first NMOS transistor, and a drain coupled to a source of the first NMOS transistor, and the impedance device is coupled between a gate of the PMOS transistor and a first power rail to perform a initial-on ESD protection circuit. The ESD protection circuit formed by the PMOS transistor and the resistor is capable of increasing the turn-on speed of the ESD protection circuit and preventing the input stage circuit from a CDM ESD event.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 12/705,339, filed on Feb. 12, 2010, now pending, the entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an electrostatic discharge (ESD) protection circuit, in particular, to a Charged-Device Model (CDM) ESD protection circuit. 
     2. Description of Related Art 
     An ESD event refers to a phenomenon of electrical discharge of a current for a short duration during which a large amount of current is provided to a semiconductor integrated circuit (IC). ICs are generally susceptible to ESD events, which may damage or destroy the integrated circuit. Thus, ESD protection of ICs is a critical factor in obtaining higher yield and stable IC characteristics. The susceptibility of a device to ESD can be determined by testing each one of three models which include Human Body Model (HBM), Machines Model (MM) and Charged-Device Model (CDM). 
     With regard to a CDM ESD event, electrostatic charge could be stored within the body of an IC product due to induction or tribocharging and most of the charge is accumulated in a substrate, including a base, a body or a well of the devices disposed on the IC, and is uniformly distributed in the substrate. Once a certain pin of the IC is suddenly grounded, the electrostatic charge originally stored within the IC will be discharged through the grounded pin. This is called the CDM ESD event. The CDM ESD event delivers a large amount of current in a very short period of time, and in general, the entire ESD event can take place in less than 2 nanoseconds (ns). Current levels can reach several tens of amperes during discharge of the electrostastic charge, which are remarkably greater than those of the HBM and MM models. 
     Additionally, there are many situations where the pins of an IC may become grounded; for example, the pin may touch grounded metallic surface or the pin may be touched by grounded metallic tools. Different ICs have different die sizes, so their equivalent parasitic capacitances are totally different from one another. Thus, different ICs have different peak current and different CDM ESD levels. When a device under test (DUT) with the equivalent capacitance of 4 pF is put under a 1-kV CDM ESD test, the CDM ESD current can rise to more than 15 A within several nanoseconds. Compared with HBM and MM ESD events, the discharging current in a CDM ESD event is not only larger, but faster. Since the duration of CDM ESD events is much shorter than HBM and MM ESD events, the internal circuit may be damaged during CDM ESD events before the ESD protection circuit is turned on. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to ESD protection circuits with an initial-on mechanism. The ESD protection circuits are implemented by an additional P channel metal oxide semiconductor (PMOS) transistor and a resistor coupled between a gate of the PMOS transistor and a power rail. The ESD protection circuits can provide efficient CDM ESD protection for input stages in general nanoscale CMOS process. 
     The present invention is directed to an electrostatic discharge (ESD) protection circuit, suitable for an input stage circuit including a first N channel metal oxide semiconductor (NMOS) transistor. The ESD protection circuit comprises a PMOS transistor and an impedance device (for example, a resistor), wherein the PMOS transistor has a source coupled to a gate of the first NMOS transistor, and a drain coupled to a source of the first NMOS transistor, and the impedance device is coupled between a gate of the PMOS transistor and a first power rail. 
     According to an embodiment of the present invention, the drain of the PMOS transistor is directly coupled to a heavily doped N-type (N+) diffusion region used to form the source of the first NMOS transistor, and coupled to a first ground rail through the N+ diffusion region. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises a capacitor coupled to the impedance device and a first ground rail, wherein the gate of the PMOS transistor is coupled to a common node of the impedance device and the capacitor. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises an inverter and a transistor, wherein the inverter is coupled to the first power rail and the first ground rail, and has an input terminal coupled to the common node of the capacitor and the impedance device, and an output terminal coupled to a gate of the transistor which has a source and a drain coupled to the first power rail and the first ground rail respectively. The capacitor, the impedance device, the inverter and the transistor form a power-rail ESD clamp circuit. 
     According to an embodiment of the present invention, the source of the first NMOS transistor is coupled to a first ground rail, and the gate of the first NMOS transistor is coupled to an input pad. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises a diode which has an anode coupled to the source of the first NMOS transistor, and a cathode coupled to a first ground rail. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises a second NMOS transistor which has a drain coupled to the source of the first NMOS transistor, and a source coupled to a first ground rail, and a gate coupled to the first power rail. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises a resistor coupled between the gate of the first NMOS transistor and an input pad. 
     According to an embodiment of the present invention, the ESD protection circuit further comprises a first ESD clamp circuit, a second ESD clamp circuit and a third ESD clamp circuit. The first ESD clamp circuit is coupled to a second power rail and the input pad. The second ESD clamp circuit is coupled to a second ground rail and the input pad. The third ESD clamp circuit is coupled to a first ground rail and the second ground rail. 
     According to an embodiment of the present invention, the input stage circuit further comprises a first PMOS transistor which has a source coupled to the first power rail, and a drain coupled to a drain of the first NMOS transistor, and a gate coupled to the gate of the first NMOS transistor. 
     According to an embodiment of the present invention, wherein the PMOS transistor has a body coupled to the source of the PMOS transistor. 
     As described above, in the present application, a circuit formed by a PMOS transistor and a resistor serves as a CDM ESD protection circuit. The ESD protection circuit has an initial-on mechanism by connecting a gate of the PMOS transistor to a power rail through the resistor so that the turn-on speed of the ESD protection circuit is enhanced and efficient CDM ESD protection performance is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  shows a circuit diagram of a circuit capable of ESD protection in accordance with a first embodiment of the present application. 
         FIG. 2A  shows a circuit diagram of a circuit capable of ESD protection in accordance with a second embodiment of the present application. 
         FIG. 2B  shows a schematic diagram of the power-rail clamp circuit  210  in accordance with the second embodiment of the present application. 
         FIG. 3  shows a circuit  300  capable of ESD protection in accordance with the third embodiment of the present application. 
         FIG. 4  shows another circuit  400  capable of ESD protection in accordance with the third embodiment of the present application. 
         FIG. 5  shows a circuit  500  capable of ESD protection in accordance with the third embodiment of the present application. 
         FIG. 6  shows a circuit  600  capable of ESD protection in accordance with the third embodiment of the application. 
         FIG. 7  shows a schematic diagram of ESD current paths of the circuit  200  under ESD stress in accordance with the third embodiment of the present application. 
         FIG. 8  and  FIG. 9  show the simulated wave forms of the aforementioned ESD protection circuit under CDM-like transitions. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     First Embodiment 
       FIG. 1  shows a circuit diagram of a circuit  100  capable of ESD protection in accordance with a first embodiment of the present application. Referring to  FIG. 1 , the circuit  100  includes an input stage circuit  110 , a PMOS transistor P 2 , two resistors R 1  and R 2 , and a plurality of ESD clamp circuits  120 ,  130  and  140 , wherein the input stage circuit  110  further includes a first PMOS transistor P 1  and a first NMOS transistor N 1 . The first PMOS transistor P 1  has a source and a body both coupled to a first power rail VDD 1 , and a drain coupled to a drain of the first NMOS transistor N 1 . The first NMOS transistor N 1  has a source and a body both coupled to a first ground rail VSS 1 , and a gate thereof coupled to a gate of the first PMOS transistor P 1 . The resistor R 2  is coupled between the gate of the first PMOS transistor P 1  and an input pad PD. It is noted that the resistor R 2  is an optional element in the circuit  100 , and the input stage circuit  110  can be directly connected to the input pad PD without the resistor R 2  present. 
     The ESD clamp circuit  120  is coupled between a second power rail VDD 2  and the input pad PD. The ESD clamp circuit  130  is coupled between the input pad PD and a second ground rail VSS 2 , and the ESD clamp circuit  140  is coupled between the first ground rail VSS 1  and the second ground rail VSS 2 . The ESD clamp circuits  120 ,  130 ,  140 , which are disposed near the input pad PD, are capable of providing HBM and MM ESD protection for the input stage circuit  110 . The PMOS transistor P 2  has a source and a body both coupled to the gates of the first NMOS transistor N 1  and the first PMOS transistor P 1 , and a drain thereof directly coupled to a heavily-doped N-type (N+) diffusion region which is used to form the source of the first NMOS transistor N 1 . Therefore, the drain of the PMOS transistor P 1  is coupled to the first ground rail VSS 1  through the N+ diffusion region which functions to serve as a resistor between the drain of the PMOS transistor P 1  and the first ground rail VSS 1 . The resistor R 1  is coupled between the gate of the PMOS transistor P 2  and the first power rail VDD 1  to perform a self-biased PMOS transistor P 2 . 
     When no power is supplied to the first power rail VDD 1 , the gate of the PMOS transistor P 2  has a low voltage level, and the PMOS transistor P 2  would be immediately turned on and conducts a portion of an ESD current to the first ground rail VSS 1  or the input pad PD to avoid the ESD current damaging an internal circuit, as the input pad PD is suddenly grounded and a CDM ESD event occurs. The PMOS transistor P 2  has an initial-on mechanism by connecting the gate of the PMOS transistor P 2  to the first power rail VDD 1  through the resistor R 1 . Therefore, the PMOS transistor P 2  has the effect of high turn-on speed and instantly conducting of the ESD current as the CDM ESD event occurs. When an operation voltage is supplied to the first power rail VDD 1 , the gate of the PMOS transistor P 2  has a high voltage level and then the PMOS transistor P 2  would be turned off automatically to avoid affecting the signal transmitted through the input pad PD. Accordingly, a self-biased circuit is formed by the PMOS transistor P 2  and the resistor RI and is capable of preventing the CDM ESD from damaging the internal circuit. Accordingly, by having the ESD protection circuit formed by the ESD clamp circuits  120 ,  130 ,  140 , the PMOS transistor P 2  and the resistor R 1  are capable of providing a HBM and MM and CDM ESD protection for the input stage circuit  110 . It is noted that the ESD protection circuit formed by the PMOS transistor P 2  and the resistor R 1  can be applied to the circuit  100  by itself to provide CDM ESD protection. 
     Second Embodiment 
     The resistor R 1  of  FIG. 1  can be replaced by a resistor of a power-rail ESD clamp circuit.  FIG. 2A  shows a circuit diagram of a circuit  200  capable of ESD protection in accordance with a second embodiment of the present application. Referring to  FIG. 2A  and  FIG. 1 , the difference between the circuit  200  and the circuit  100  is mainly having a power-rail ESD clamp circuit  210  in the circuit  200 , as shown in  FIG. 2A . The power-rail ESD clamp circuit  210  is coupled between the first power rail VDD 1  and the first ground rail VSS 1 , and capable of providing ESD protection for the first power rail VDD 1  and the first ground rail VSS 1 . The power-rail ESD clamp circuit  210  includes a resistor  212  and a capacitor  214 , in which the resistor  212  and the capacitor  214  are connected in series between the first power rail VDD 1  and the first ground rail VSS 1 . The gate of the PMOS transistor P 2  is coupled to a common node of the resistor  212  and the capacitor  214 , and therefore coupled to the first power rail VDD 1  through the resistor  212 . 
     The circuit structure of the power-rail ESD clamp circuit  210  can be implemented in a variety of ways. Please refer to the  FIG. 2B .  FIG. 2B  shows a schematic diagram of the power-rail clamp circuit  210  in accordance with the second embodiment of the present application. The power-rail clamp circuit  210  includes the resistor  212 , the capacitor  214 , an inverter  220  and an NMOS transistor  230 . The inverter  220  has an input terminal coupled to a common node of the resistor  212  and capacitor  214 , and an output terminal coupled to a gate of the NMOS transistor  230 . The NMOS transistor  230  has a drain coupled to the first power rail VDD 1  and a source coupled to the first ground rail VSS 1 . It is noted that  FIG. 2B  is merely provided as an example in the form of the second embodiment, and the present invention is not limited thereto. 
     Third Embodiment 
     According to a third embodiment of the present application, the ESD protection designs can be implemented by an additional PMOS circuit in conjunction with a source loading (for example, resistor, diode, or MOS), as shown in  FIGS. 3-6 . Referring to  FIG. 3 ,  FIG. 3  shows a circuit  300  capable of ESD protection in accordance with the third embodiment of the present application. The difference between the circuit  300  of  FIG. 3  and the circuit  100  as shown in  FIG. 1  is mainly having a diode  310  in the third embodiment, the diode  310  has a anode coupled to the source of the first NMOS transistor N 1 , and a cathode coupled to the first ground rail VSS 1 . Similarly, the diode  310  can also be added to the circuit  200  as shown in  FIG. 2A . Referring to  FIG. 4 ,  FIG. 4  shows another circuit  400  capable of ESD protection in accordance with the third embodiment of the present application. The circuit  400  includes the diode  310  and the power-rail clamp circuit  210 , wherein the diode  310  is coupled between the first NMOS transistor N 1  and the first ground rail VSS 1 . 
       FIG. 5  shows another circuit  500  capable of ESD protection in accordance with the third embodiment of the present application. The difference between the circuit  500  of  FIG. 5  and the circuit  100  as shown in  FIG. 1  is mainly having a second NMOS transistor N 2  which has a drain coupled to the source of the first NMOS transistor N 1 , and a source and a body both coupled to a first ground rail VSS 1 , and a gate coupled to the first power rail VDD 1  for the circuit  500 . Similarly, the second NMOS transistor N 2  can also be added to the circuit  200  as shown in  FIG. 2A . Referring to  FIG. 6 ,  FIG. 6  shows another circuit  600  capable of ESD protection in accordance with the third embodiment of the present application. The circuit  600  includes the second NMOS transistor N 2  and the power-rail ESD clamp circuit  210 , wherein the second NMOS transistor N 2  is coupled between the first NMOS transistor N 1  and the first ground rail VSS 1 , and has a gate coupled to the first power rail VDD 1 . 
     Next, please refer to  FIG. 7 .  FIG. 7  shows a schematic diagram of ESD current paths of the circuit  700  under ESD stress in accordance with the second embodiment of the present application. When the input pad PD is suddenly grounded (connected to ground GND), charges are discharged from the substrate to the grounded input pad PD through several paths (dotted lines), as shown in  FIG. 7 . The ESD current is discharged through the ESD clamp circuit  130  and the PMOS transistor P 2  when a CDM-like ESD stress occurs at the input pad PD. Since the gate voltage of the PMOS transistor P 2  is at a low voltage level, so the PMOS transistor P 2  can be turned-on immediately and instantly conducts the ESD current to the first ground rail VSS 1  to prevent the ESD current flowing through the first NMOS transistor N 1 . 
       FIG. 8  and  FIG. 9  show the simulated wave forms of the aforementioned ESD protection circuit under CDM-like transitions. In  FIG. 8  and  FIG. 9 , the ±5 V voltage pulses with rise time of 0.3 ns are provided between the input pad PD and the P-substrate to simulate the fast transient voltage of a CDM ESD event. With a limited voltage height of 5 V in the voltage pulse, the voltage drop across the gate of the NMOS transistor N 1  of all the aforementioned ESD protection circuits can be simulated to check the clamping performance. As shown in  FIG. 8 , when a negative CDM-like pulse is provided at the input pad PD, the voltage across the gate of the NMOS transistor Ni is not greater than 3 V during 0.2 ns. As shown in  FIG. 9 , when a positive CDM-like pulse is provided at the input pad PD, the voltage across the gate of the NMOS transistor N 1  is not greater than 2 V during 0.2 ns. Therefore, with the proposed CDM ESD protection circuit as recited in above embodiments, the core circuit can be clamped at a lower voltage level under CDM stresses. It is noted that in  FIGS. 8 and 9 , the proposed design A and the proposed design B represent the ESD protection circuits as shown, for example, in  FIGS. 1 and 2A , respectively. 
     As described above, the novel CDM ESD protection circuits with the initial-on mechanism and source pumping design have been proposed in the present application. Since only a transistor and a resistor are required to implement the CDM ESD protection circuit, the present application can effectively protect the core circuits from CDM ESD damages and further enhance the turn-on speed of the ESD protect circuit without increasing the layout region and circuit complexity. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.