Abstract:
A semiconductor layout structure for an electrostatic discharge (ESD) protection circuit is disclosed. The semiconductor layout structure includes a first area, in which one or more devices are constructed for functioning as a silicon controlled rectifier, and a second area, in which at least one device is constructed for functioning as a trigger source that provides a triggering current to trigger the silicon controlled rectifier for dissipating ESD charges during an ESD event. The first area and the second area are placed adjacent to one another without having a resistance area physically interposed or electrically connected therebetween, such that the triggering current received by the silicon controlled rectifier is increased during the ESD event.

Description:
RELATED APPLICATION  
       [0001]     This application is related to and claims the priority benefit of U.S. patent application Ser. No. 11/091,131 entitled “ESD PROTECTION CIRCUIT WITH LOW PARASITIC CAPACITANCE” filed Mar. 28, 2005. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to an integrated circuit (IC) design, and more particularly to a semiconductor layout structure for electrostatic discharge (ESD) protection circuits.  
         [0003]     A gate oxide of a metal-oxide-semiconductor (MOS) transistor of an IC is most susceptible to damage. The gate oxide may be destroyed by being contacted with a voltage only a few volts higher than a supply voltage of the IC. It is understood that a regular supply voltage for an IC is about 5.0, 3.3 volts or even lower. Electrostatic voltages from common environmental sources can easily reach thousands, or even tens of thousands of volts. Such voltages can be destructive even though the charge and any resulting current are extremely small. For this reason, it is of critical importance to discharge any static electric charge before it damages the IC.  
         [0004]     An ESD protection circuit is typically added to an IC at its bond pads. Such protection circuit must allow normal operation of the IC. It means that the protection circuit is effectively isolated from the normally operating core circuit by blocking a current flow through itself to ground or other pads. In an operating IC, electric power is supplied to a VCC pad, electric ground is supplied to a VSS pad, electronic signals are supplied from outside to some pads, and electronic signals generated by the core circuit of the IC are supplied to other pads for delivery to external circuits or devices. In an isolated, unconnected IC, all pads are considered to be electrically floating, or of indeterminate voltages. In most cases, this means the pads are at ground, or zero voltage.  
         [0005]     The ESD can arrive at any pad. This can happen, for example, when a person touches some of the pads on the IC. This is the same static electricity that may be painfully experienced by a person who walks across a carpet on a dry day and then touches a grounded metal object. In an isolated IC, the ESD acts as a brief power supply for one or more pads, while the other pads remain floating, or grounded. Because the other pads are grounded, when ESD acts as a power supply at a randomly selected pad, the protection circuit acts differently than it does when the IC is operating normally. When an ESD event occurs, the protection circuit must quickly become conductive so that the electrostatic charge is dissipated to VSS or ground.  
         [0006]     The ESD protection circuit, therefore, has two modes: normal operation mode and ESD mode. When an IC is in the normal operation mode, the ESD protection circuit has no effect to the IC. During the ESD mode, the ESD protection circuit serves its purpose of protecting the IC by conducting an electrostatic charge quickly to VSS or ground before it damages the IC.  
         [0007]     As technology in circuit designs continue to grow and lower supply voltages are being used, circuits become more vulnerable to early stages of ESD events. Even voltage slightly higher than the supply voltage can damage the IC and the protection circuit itself. It has been found that parasitic four layer PNPN devices called parasitic silicon controlled rectifier (SCR) can be one of the most effective devices in preventing ESD damage due to its low turn-on impedance, low capacitance, low power dissipation, and high current sinking/sourcing capabilities. By using a NMOS transistor to trigger an SCR, a low triggering voltage ESD protection circuit can be achieved.  
         [0008]     The conventional SCR ESD protection circuit may have parasitic capacitance and resistance that may create undesired consequences when the SCR ESD protection circuit is used in a high frequency IC. The shortcomings of the parasitic capacitance and resistance include, for example, noise, signal reflection, and reduced power gain.  
         [0009]     It is therefore desired to provide a semiconductor layout structure for a SCR ESD protection circuit with a reduced parasitic capacitance and resistance.  
       SUMMARY  
       [0010]     This invention provides a semiconductor layout structure for an electrostatic discharge (ESD) protection circuit. In one embodiment of the present invention, the semiconductor layout structure includes a first area, in which one or more devices are constructed for functioning as a silicon controlled rectifier, and a second area, in which at least one device is constructed for functioning as a trigger source that provides a triggering current to trigger the silicon controlled rectifier for dissipating ESD charges during an ESD event. The first area and the second area are placed adjacent to one another without having a resistance area physically interposed or electrically connected therebetween, such that the triggering current received by the silicon controlled rectifier is increased during the ESD event.  
         [0011]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  illustrates a circuit diagram of an ESD protection circuit.  
         [0013]      FIG. 2A  illustrates a circuit diagram of an ESD protection circuit in accordance with one embodiment of the present invention.  
         [0014]      FIG. 2B  illustrates a semiconductor layout structure for the ESD protection circuit shown in  FIG. 2A  in accordance with one embodiment of the present invention.  
         [0015]      FIG. 2C  illustrates a detailed circuit diagram equivalent of the semiconductor layout structure shown in  FIG. 2B .  
         [0016]      FIG. 3  illustrates a graph representing test results of ESD protection circuits constructed by various layout structures. 
     
    
     DESCRIPTION  
       [0017]      FIG. 1  illustrates a circuit diagram  100  of a conventional ESD protection circuit system, which is used for providing ESD protection for a high frequency IC. The conventional ESD protection circuit system is implemented with an SCR circuit  105 , which typically includes a trigger source (not shown in this figure). During an ESD event, the trigger source produces a triggering current to turn on the SCR circuit  105  for dissipating an ESD current from the pad  102  to ground. The trigger source, which typically includes a set of transistors, may be easily damaged by a high voltage caused by the ESD event. In order to protect the trigger source from damage, one or more resistors  106  are provided between the pad  102 , from which the ESD current may enter in to the system  100 , and an I/O pad  104 , which is tied to a protected circuit or device. The resistors  106  may be provided in a parasitic or non-parasitic form. Due to the resistance provided by the resistors  106 , the ESD current from the pad  102  is limited and not able to flow through to damage internal transistors of the trigger source.  
         [0018]     One drawback of the ESD protection circuit system  100  occurs when it is applied in high frequency ICs. With the resistance provided by the resistors  106 , issues such as signal reflection, reduced power gain, and induced noise can be observed. Thus, it is desired to provide a semiconductor layout structure for the ESD protection circuit system  100  that eliminates the resistors  106  while it protects the trigger source within the SCR circuit  105  from damage.  
         [0019]      FIG. 2A  illustrates a circuit diagram  200  of an ESD protection circuit system in accordance with one embodiment of the present invention. This proposed system includes an SCR circuit  203  coupled between a pad  202 , from which the ESD current enters into the system  200 , and an I/O pad  204 , which is tied to a protected circuit or device. With a semiconductor layout structure that will be further explained by the following embodiments of the invention, no resistors need to be provided between the pad  202  and the I/O pad  204  in order to protect a trigger source (not shown in the figure) within the SCR circuit  203 . The proposed layout structure provides the SCR circuit  203  with a very low parasitic resistance and capacitance, so that the SCR circuit  203  can be turned on much sooner as opposed to that shown in  FIG. 1  during the ESD event.  
         [0020]      FIG. 2B  illustrates a semiconductor layout structure  206  for the ESD protection circuit system shown in  FIG. 2A  in accordance with one embodiment of the present invention. The layout structure  206  includes an SCR area  208  and a trigger source area  210  placed within a P+ guard ring  212  and an N+ guard ring  214 . Within the SCR area  208 , one or more devices are constructed by a combination of different material layers of various doping conditions for collectively functioning as an SCR circuit. Within the trigger source area  210 , a set of transistors are constructed for providing a triggering current for turning on the SCR circuit during the ESD event. In this embodiment, a set of fully silicided ground-gate NMOS transistors are constructed within the trigger source area  210  for providing such triggering current. The trigger source area  210  may include one or more poly-silicon gate regions  213  extending through one or more doped source/drain regions  215 . A spacing gap  216  is left opened between the SCR area  208  and the trigger source area  210  without any resistance area physically interposed or electrically connected therebetween. The distance of the spacing gap  216  is crucial since it affects the amount of the triggering current reaching the SCR area  208 . In one embodiment, the shortest distance between the SCR area  208  and the trigger source area  210  ranges from about 2 to 10 μms.  
         [0021]     The SCR area  208  and the trigger source area  210  are placed within the P+ guard ring area  212 , which is further placed within the N+ guard ring area  214 . No part or segment of the P+ guard ring area is present between the SCR area  208  and the trigger source area  210 . The P+ guard ring area  212  is constructed to collect the holes and should be connected to a substrate potential. The N+ guard ring area  214  is constructed to collect the electrons and is connected to a relatively high-voltage potential. A diode area  218  is placed above the SCR area  208  within the P+ guard ring area  212 . Within the diode area  218 , an N-well region  219  is placed between P-well regions  221  for constructing a diode to clamp the ESD voltage during the ESD events.  
         [0022]     During the ESD event, the grounded gate NMOS transistors, which are represented by the poly-silicon gate regions  213  and doped source/drain regions  215 , within the trigger source area  210  will experience junction breakdown and produce the triggering current for the SCR circuit (not shown) within the SCR area  208 . Due to the absence of any resistance area between the SCR area  208  and the trigger source area  210 , the triggering current reaching the SCR area  208  is increased for turning on the SCR circuit earlier during the ESD current. The distance of the spacing gap  216  affects the amount of triggering current that can reach the SCR area  208  from the trigger source area  210 . This allows the SCR circuit within the SCR area  208  to be triggered on earlier.  
         [0023]     By removing the resistors between the SCR area  208  and the trigger source area  210 , the issues of using the conventional ESD protection circuit system in a high frequency IC, such as signal reflection, reduced power gain, and induced noise, are improved. The layout structure improves the applicability of the SCR ESD protection circuit in high frequency ICs.  
         [0024]      FIG. 2C  illustrates an exemplary equivalent circuit diagram  220  of the layout structure  206  shown in  FIG. 2B . The circuit diagram  220  includes an SCR circuit  208 ′, a trigger source  210 ′, and a diode  218 ′. The SCR circuit  208 ′, the trigger source  210 ′, and the diode  218 ′ are all coupled between a pad  222  and an I/O pad  224 . The diode  218 ′ is designed to clamp the ESD voltage generated during an ESD event. The SCR circuit  208 ′ includes two parasitic bipolar transistors  226  and  228  and a parasitic resistance represented by resistors  230  and  232 . The trigger source  210 ′ typically includes a set of grounded-gate NMOS transistors (not shown in this figure).  
         [0025]      FIG. 3  illustrates a graph representing the test results of the ESD performance of various devices. A curve  302  represents the ESD performance of a protection circuit with both a SCR and a ground-gate NMOS transistor constructed according to the proposed layout structure, while a curve  304  represents the ESD performance of a protection circuit with only a ground-gate NMOS transistor. Both circuits have a total width of 80 μms and total a length of 0.25 μm. For the circuit with the SCR, the curve  302  snaps back at a voltage level of about 3.8 V, and the triggering current keeps increasing beyond about 300 mA, which is usually the benchmark for a sufficient triggering current. For the circuit without the SCR, the curve  304  never snaps back, and the voltage level keeps increasing after the triggering current reaches about 300 mA. This increasing voltage may cause damage to the grounded-gate NMOS transistor.  
         [0026]     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.  
         [0027]     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.