# Patent

## Title
High ESD stress sustaining ESD protection circuit

## Abstract

An ESD protection circuit suitable for application to an IC. The ESD protection circuit includes a primary discharging component and an ESD detection circuit. The primary discharging component has a control end. The ESD detection circuit comprises a capacitor, a first resistor and a second resistor. The capacitor and the first resistor are formed in series and coupled between the control and a first pad of the IC. The second resistor is coupled between a second pad of the IC and the control end. The primary discharging component is closed during normal power operations, and triggered by the ESD detection circuit during an ESD event. During an ESD event, the capacitance becomes short-circuited, leaving voltages at the control end dominated by a resistive voltage divider formed by the first resistor and the second resistor. The ESD detection circuit provides a suitable voltage to the control end allowing the primary discharging component to release the optimum amount of ESD stress.

## Background
Pursuant to 35 U.S.C. § 119(a)–(d), this application claims priority from Taiwanese application no. 90125931, filed on Oct. 19, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an electrostatic discharge (ESD) protection circuit. In particular, the present invention relates to an ESD protection circuit suitable for application in an integrated circuit (IC).
2. Description of the Related Art
FIG. 1 shows a conventional ESD protection circuit of an IC. The ESD protection circuit 10 has an NMOS NESD coupled between a pad 12 and a power line VSS. A RC coupling circuit is formed by a capacitor C and a resistor R in the circuit. When an ESD voltage, positive with respect to the power line VSS occurs at the pad 12, a positive voltage is coupled to a gate of the NMOS NESD enhancing the triggering rate of the NMOS NESD. Generally, an IC chip with ESD protection circuits must sustains more than the minimum human-body-mode (HBM) ESD stress, 2 k volts. However, when the gate of the NMOS NESD is over- or under-stressed, ESD protection of ICs provided by the ESD protection circuit in FIG. 1 is compromised. FIG. 2 shows a fixed voltage applied to the gate of the NMOS NESD in FIG. 1. An experimental result of voltages at the gate of the NMOS NESD against the highest voltage NESD can sustain is shown in FIG. 3. Voltages applied to the gate of the NMOS NESD allow a triggering voltage of the NMOS NESD to decreaseer. As shown in FIG. 3, when bias voltages at the gate of the NOS NESD is small and is increased, ESD stress sustainable by the NMOS NESD increases. Nevertheless, when bias voltages at the gate of the NMOS NESD passes an optimized point and is overly increased, large ESD current crossing a thin channel under the NMOS NESD easily damages the NOS NESD and causes ESD level sustained by the NMOS NESD to drop. ESD protection provided by the circuit in FIG. 2 is optimized by bringing a bias voltage at the gate of the NMOS NESD to a specified amplitude, such as VGopt in FIG. 3. Therefore, it has become an object for circuit designers to bring bias voltages at the gate of the NMOS NESD in FIG. 1 to the specified amplitude VGopt during an ESD event.
The ESD protection circuit in FIG. 1 may be unreliable during an ESD event. Capacitances of parasitic capacitors, such as Cgd and Cgs, formed in the NMOS NESD sway during different manufacturing processes. Voltages coupled from ESD stress at the pad 12 to the gate of the NMOS NESD are undoubtedly affected by the changing capacitances. In addition, ESD stresses occur on the pad 12 at different rates. Therefore, voltages coupled to the gate of the NMOS NESD are unpredictable, resulting in unreliable performance. For instance, the circuit in FIG. 1 might have sustained a HBM ESD stress of 5 kv, but fails to sustain a HBM ESD stress of 2 kv. Voltages at the gate of the NMOS NESD in the circuit of FIG. 1 are thus hard to control and are unpredictable during a manufacturing process.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an ESD protection circuit having an ESD detection circuit. The ESD detection circuit provides an appropriate gate voltage to a MOS optimizing ESD protection provided by the ESD protection circuit.
In accordance with the object described, the present invention provides an ESD protection circuit, suitable for application in an IC. The ESD protection circuit comprises a primary discharging component and an ESD detection circuit. The primary discharging component has a control end. The ESD detection circuit comprises a capacitor, a first resistor, and a second resistor. The capacitor and the first resistor are formed in series and coupled between the control and a first pad of the IC. The second resistor is coupled between a second pad of the IC and the control end. The primary discharging component is closed during normal power operations, and triggered by the ESD detection circuit during an ESD event.
The combination of the first pad and the second pad may be a power line and a pad, or two power lines. The primary discharging component can be an NMOS or a PMOS.
The ESD protection circuit of the present invention is suitable for application to a primary or secondary ESD protection circuit of an I/O pad.
Capacitance of the capacitor must be properly selected. The capacitance, on one hand, should be low enough during normal power operations that voltages coupled to the control end are not too large to trigger the discharging component, and on the other hand should be high enough that the capacitance resembles a short circuit during an ESD event.
During an ESD event, the ends of the capacitance are short-circuited, leaving voltages at the control end dominated by a resistive voltage divider formed by the first resistor and the second resistor. Control of voltages at the control end allows ESD protection provided by the primary discharging component to be optimized.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:
FIG. 1 is a conventional ESD protection circuit in an IC;
FIG. 2 is a schematic diagram showing a fixed voltage applied on the gate of the NMOS NESD in FIG. 1;
FIG. 3 is a diagram showing gate voltages against ESD stress sustained;
FIG. 4 is an ESD protection circuit of the present invention having an NMOS;
FIG. 5 is an ESD protection circuit of the present invention having a PMOS;
FIG. 6 is a diagram of voltages against current when the bias voltage at the gate of the NMOS NESD is VGopt;
FIG. 7 is a two stage ESD protection circuit of the present invention implemented by ESD protection circuits;
FIG. 8 is a power-rail ESD clamping circuit implented by an NMOS in accordance with the present invention; and
FIG. 9 is a power-line ESD clamping circuit implemented by a PMOS in accordance with the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 4 is an ESD protection circuit of the present invention having an NMOS. The ESD protection circuit 20 in FIG. 4 is coupled between an anode and a cathode and comprises an NMOS NESD and an ESD detection circuit 22. The ESD detection circuit 22 distinguishes an ESD event from normal operations. The NMOS NESD is closed during normal power operations and turned on during an ESD event. The ESD detection circuit 22 has a capacitor Cn, a resistor Rn1, and a resistor Rn2. The capacitor Cn and the resistor Rn1 are coupled between the anode and a gate of the NMOS NESD with flexible sequence. The resistor Rn2 is coupled between the gate of the NMOS NESD and the cathode.
The anode and the cathode are respectively coupled to two pads, such as I/O pads or power lines. The anode is coupled to a high voltage with respect to the cathode during normal power operations.
The capacitance of the capacitor Cn, on one hand, has to be low enough so that voltages coupled to a gate of the NMOS NESD are not too high to trigger the NMOS NESD during normal power operations (having low operational frequency), and on the other hand, has large enough so that the capacitor Cn resembles a state of short circuit with respect to the resistors Rn1 and Rn2 during an ESD event (having high operational frequency). This is realized using the impedance (=1/(2*pi*Cn*f)) of the capacitor Cn fluctuating with operational frequencies f.
During normal power operations, the gate of the NMOS NESD is coupled to the cathode having a low voltage through the resistor Rn2 so that the NMOS NESD is turned off to avoid current leakage.
When a positive ESD stress with respect to the cathode occurs at the anode, voltages (VG) at the gate of the NMOS NESD can be represented with an equation (1):

VG=VA-C*Rn2/(Rn1+Rn2+1/(2*pi*Cn*f))  (1)

VA-C represents the voltage difference between the anode and the cathode. The capacitor Cn resembles a short circuit with respect to the resistors Rn1 and Rn2 (Rn1, Rn2>>1/(2*pi*Cn*f)). The equation (1) can thus be simplified to an equation (2) as follows:

VG=VA-C*Rn2/(Rn1+Rn2)  (2)

As shown in FIG. 3, the optimum amount of ESD stress sustainable by the ESD protection circuit 20 is recorded when the ESD protection circuit 20 is triggered at a bias voltage of VGopt at the gate of the NMOS NESD. FIG. 6 shows a diagram of voltages against current for the protection circuit 20 when a bias voltage at the gate of the NMOS NESD in FIG. 4 is VGopt. Once voltage at the anode reaches Vbreak—opt in a condition that the bias voltage at the gate of the NMOS NESD is VGopt, snapback occurs at the NMOS NESD to release the ESD current. Namely, when the bias voltage at the gate of the NMOS NESD is VGopt, the maximum voltage at the anode is Vbreak—opt. Such a character is applied into the equation (2) to optimize the ESD protection circuit 20 in FIG. 4 and result in the following equation:

VGopt=Vbreak—opt*Rn2/(Rn1+Rn2)  (3)

Once such a relationship between the resistors Rn1 and Rn2- is acquired, the NMOS NESD is triggered when the voltage across the anode and the cathode reaches Vbreak—opt and the voltage at the gate of the NMOS NESD is at VGopt. The ESD stress is released and the optimum ESD protection is performed.
Similarly, the ESD protection circuit in the present invention can be implemented by a PMOS in FIG. 5. As in FIG. 4, connections of components in FIG. 5 are displayed without further descriptions.
FIG. 7 is a schematic diagram of a two-stage ESD protection circuit implemented by ESD protection circuits of the present invention. The two-stage ESD protection circuit comprises a primary ESD protection circuit 24, a resistor Rbuff and a secondary ESD protection circuit 26. An ESD protection circuit of the present invention is formed in the primary ESD protection circuit 24 by an NMOS NESD1, a resistor Rn1, a resistor Rn2 and a capacitor Cn1 between a pad 30 and a power line VSS. Similar structures are formed between the pad 30 and a power line VDD, an internal circuit 28 and the power line VDD, and the internal circuit 28 and the power line VSS. The secondary ESD protection circuit 26 is used to clamp voltages received by the internal circuit 28. The primary ESD protection circuit 24 is used to release most of the ESD stress. Therefore, a PMMOS PESD2 and an NMOS NESD2 in the secondary ESD protection circuit 26 are relatively small in comparison with a PMOS PESD1 and the NMOS NESD1 in the primary ESD protection circuit 24.
The ESD protection circuit of the present invention is also suitable for application to ESD clamping circuits formed between power lines, as shown in FIGS. 8 and 9. An ESD clamping circuit between power lines implemented by an NMOS in accordance with the present invention is shown in FIG. 8. An ESD clamping circuit between power lines implemented by a PMOS in accordance with the present invention is shown in FIG. 9.
Voltages at gates in a conventional ESD protection circuit are difficult to maintain at an appropriate level. In comparison, the ESD protection circuit of the present invention formed by a short-circuited capacitor and a resistive voltage divider provides an appropriate voltage for NMOSs or PMOSs to release the optimum amount of ESD stress.
Finally, while the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. A two-stage electrostatic discharge (ESD) protection circuit, comprising:
a primary ESD protection circuit comprising:
a first primary discharging component, having a first control end; and
a first primary ESD detection circuit, comprising:
a first capacitor and a first resistor formed in series, and coupled between the first control end and a input/output pad of an IC; and
a second resistor, coupled between the first control end and a first pad of the IC;
a buffering resistor coupled between the input/output pad of the IC and an internal circuit of the IC; and
a secondary ESD protection circuit comprising:
a second primary discharging component, having a second control end; and
a second ESD detection circuit, comprising:
a second capacitor and a third resistor formed in series, and coupled between the second control end and the internal circuit of the IC; and
a fourth resistor, coupled between the secoud control end and the first pad;



wherein the first and the second primary discharging components are closed during normal power operations, and triggered by the ESD detection circuit during an ESD event.

2. The ESD protection circuit in claim 1, wherein the primary ESD protection circuit further comprises:
a third primary discharging component, having a third control end; and
a third primary ESD detection circuit, comprising:
a third capacitor and a fifty resistor formed in series, and coupled between the third control end and the input/output pad of the IC; and
a second resistor, coupled between the third control end and a second pad of the IC; and
the secondary ESD protection circuit further comprises:
a fourth primary discharging component, having a fourth control end; and
a fourth ESD detection circuit, comprising:
a fourth capacitor and a sixth resistor formed in series, and coupled between the fourth control end and the internal circuit of the IC; and
a seventh resistor, coupled between the fourth control end and the second pad of the IC;



wherein the third and the fourth primary discharging components are closed during normal power operations, and triggered by the ESD detection circuit during the ESD event.

3. The ESD protection circuit in claim 2, wherein the first and the second primary discharging components are NMOS and the third and the fourth primary discharging components are PMOS.