Abstract:
The invention provides systems and methods for ESD protection for an integrated circuit (IC) having multi-power domains. The IC comprises a first device in a first power domain having a first power line and a first ground line and a second device in a second power domain having a second power line and a second ground line. A clamp circuit having a first node and a second node is coupled to the first device and the second device to provide cross-domain protection. Alternatively, two clamp circuits are used to couple with the first device and the second device to provide cross-domain ESD protection.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to integrated circuit design, and more particularly to electrostatic discharge (ESD) protection systems and methods for an integrated circuit having multi-power domains. 
       BACKGROUND 
       [0002]    Electrostatic discharge (ESD) has been always an issue for integrated circuit (IC) reliability and the issue becomes more of concern with technology scaling and high system integration. The ESD phenomenon may induce transient voltage as high as a few thousand volts at IC pins and cause large transient current that may damage the internal circuit. In order to protect the device from ESD damage, dedicated external devices were used to provide the needed protection. However, in recent years, on-chip ESD protection becomes a common practice in IC design due to the cost and space saving and the convenience. Depending on the intended level of ESD protection, either partial pins or all pins of an IC chip are equipped with ESD protection circuits. Furthermore, there exist several ESD models being widely used in the industry for ESD protection assessment, including Machine Model (MM), Human Body Model (HBM) and Charged Device Model (CDM). There are also several voltage levels associated with each test model to designate the ESD protection capability of a device under test. 
         [0003]    In light of the advancement in IC technology, more circuits corresponding to various devices for a system are packed into a single integrated circuit. The IC may contain not only mixed types of circuits, such as analog circuit and digital circuits, but also circuits having multiple power supply voltages. In a conventional ESD protection approach, the design has been focused on the ESD protection within a single device. For example, the ESD protection may be only individually considered for an analog circuit having 1.1V supply, such as a variable gain amplifier circuit within an integrated television receiver chip without addressing the possible ESD damage across different devices within the IC. A typical ESD protection circuit used between the power and ground lines is a clamp circuit that can cause the large ESD current to flow through the clamp circuit and quickly reduce the induced high voltage on the power line. Consequently, the circuits connected between the power and ground lines are protected. In order to provide an ESD current path for the ground lines corresponding to different power domains, the ground lines usually are connected through a diode module wherein a pair of diodes is connected back to back. The diode module may also contain two strings of series-connected diodes and the two strings are connected back to back. The diode module allows the passage of large ESD current while avoid noise coupling between the two coupled devices. 
         [0004]    For an IC device having multi-power domains, any interface circuit coupled to circuits in the different power domains may be subject to cross-domain ESD damage. In a technical paper, entitled “ESD Protection Design to Overcome Internal Damage on Interface Circuits of a CMOS IC With Multiple Separated Power Pins,” by Ming-Dou Ker, Chyh-Yih Chang, and Yi-Shu Chang, published in  IEEE TRANS. on Components and Packaging Technologies , Vol. 27, No. 3, pp. 445-451, September 2004, a gate-grounded NMOS (or PMOS) is disclosed to improve cross-domain ESD protection. The method will require the gate-grounded NMOS (or PMOS) device added to all interfaces between the two devices having separate power domains. In the US Patent Application Publication, entitled “ESD Protection System for Multi-Power Domain Circuitry,” Publication No. US 2007/0091523 A1, dated Apr. 26, 2007, by Ker-Min Chen, an interface buffer module is disclosed to overcome the ESD problem associated with multi-power domains. The interface buffer module can increase the impedance in the ESD condition to prevent large current from flowing through the interface circuit. Again, the interface buffer module has to be used for all interfaces. Furthermore, the effectiveness of the interface buffer module regarding cross-domain ESD protection is unclear. Therefore, it is desirable to provide effective and reliable protection systems and methods to avoid cross-domain ESD damage. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    An integrated circuit (IC) having on-chip electrostatic discharge (ESD) protection is disclosed. According to one embodiment of the present invention, the IC comprises a first device, a second device, and a protection module. The first device is in a first power domain having a first high power line and a first low power line, and the second device is in a second power domain having a second high power line and a second low power line. The protection module has a first node and a second node, wherein the first node is coupled to the first high power line and the second node is coupled to the second low power line to provide ESD protection between the first device and the second device. In an IC according to one embodiment of the present invention, the first device is a digital device and the second device is an analog device. Yet in an IC according to another embodiment of the present invention, the first device is an analog device and the second device is a digital device. According to the present invention, the first node of the protection module can be alternatively connected to a pad or an internal bus of the first high power line. Similarly, the second node of the protection module can be alternatively connected to a pad or an internal bus of the second low power line. 
         [0006]    According to another embodiment of the present invention, the IC comprises a first device, a second device, a first protection module and a second protection module. The first device is in a first power domain having a first high power line and a first low power line, and the second device is in a second power domain having a second high power line and a second low power line. The first protection module has a first node and a second node, wherein the first node is coupled to the first high power line and the second node is coupled to the second low power line to provide ESD protection between the first device and the second device. The second protection module has a third node and a fourth node, wherein the third node is coupled to the second high power line and the fourth node is coupled to the first low power line to provide ESD protection between the first device and the second device. 
         [0007]    According to another embodiment of the present invention, a method is disclosed for providing electrostatic discharge (ESD) protection to a first device in a first power domain having a first high power line and a first low power line and a second device in a second power domain having a second high power line and a second low power line. The method comprises steps of (a) providing a first protection module having a first node and a second node, (b) coupling the first node of the first protection module to the first high power line of the first device, and (c) coupling the second node of the first protection module to the second low power line of the second device. In an alternative ESD protection method, the method includes further steps of (d) providing a second protection module having a third node and a fourth node, (e) coupling the third node of the second protection module to the second high power line of the second device, and (f) coupling the fourth node of the second protection module to the first low power line of the first device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an ESD protection arrangement where a clamp circuit is used and the VSS for the I/O circuit is used as a common local ESD ground bus. 
           [0009]      FIG. 2  illustrates an ESD protection arrangement where a clamp circuit is used and the VSS for the core circuit is used as a common local ESD ground bus. 
           [0010]      FIG. 3  illustrates an ESD protection arrangement where a clamp circuit is used and the joined VSS for the core circuit and the I/O circuit is used as a common local ESD ground bus. 
           [0011]      FIG. 4  illustrates a scenario of ESD failure where interface circuits are coupled to two devices without cross-domain ESD protection and the points of ESD failure are indicated. 
           [0012]      FIG. 5  illustrates noise coupling in a device having multiple power domains and conventional ESD protection without cross-domain ESD protection. 
           [0013]      FIG. 6  illustrates possible routes of ESD failure where an interface circuit is coupled between two devices without cross-domain ESD protection. 
           [0014]      FIG. 7  illustrates an embodiment of the present invention where a pair of protection circuits is coupled between two devices to provide cross-domain ESD protection. 
           [0015]      FIG. 8  illustrates an embodiment of the present invention where a protection circuit is coupled between two devices to provide cross-domain ESD protection. 
           [0016]      FIGS. 9A-F  illustrate various configurations of connecting a protection circuit to the high power line and the low power line. 
           [0017]      FIG. 10  illustrates an example of integrated circuit containing an embodiment of the present invention where a protection circuit is coupled between a digital device and an analog device to provide cross-domain ESD protection. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    In order to protect a device from possible ESD damage, ESD protection circuit is often used. For integrated circuit application, it has been a technology trend to incorporate on-chip ESD protection circuits to reduce system cost as well to system space. In a typical ESD protection design, a type of protection circuit is included between the high power supply line VDD and the low power supply line, often also called ground line VSS. The high power supply line may also be called high power line or simply VDD for short. The low power supply line may also be called low power line or ground line for short. To prevent the possible damage to the circuits attached between the VDD and VSS caused by the high transient voltage associated with the ESD, a fast-response protection circuit, typically a clamp circuit is often used as the protection element. The clamp circuit is capable of quickly reducing the high transient voltage, which could be as high as several hundred or several thousand volts, across the VDD and VSS to a much lower voltage. The implementation of an ESD protection element such as the clamp circuit is well known to those skilled in the field and the details of the protection element are not further described herein. In the IC industry, there exist some ESD models and standard test procedures to measure the ESD protection capabilities of a device under test. There are three ESD models often used to set up the test environment: Machine Model (MM), Human Body Model (HBM) and Charged Device Model (CDM). There are several test voltage levels used as designation of ESD protection capability. For example, a device is qualified for HBM 2 kV is referring to a device that can sustain the ESD test using the HBM up to 2,000 volts. It is very desirable to provide high ESD protection capability to a device so that the device can survive in various hostile environments during packaging, shipping, assembly and usage. 
         [0019]    In light of the advancement in semiconductor technology, more circuits are integrated onto the same substrate or same package. Furthermore, the complexity of integrated circuit has also grown rapidly so that an integrated circuit may contain many difference devices on the same die. For example, a highly integrated television receiver chip may contain analog front end (AFE) circuit which is essentially all high-frequency (RF and/or IF) analog circuits, mixer, analog-to-digital converter, digital signal processor, video DAC, audio DAC, various analog and digital peripheral circuits, clock generation circuit, and etc. These devices may have different power requirement such as 2.5V and 1.2 V. Furthermore, to reduce noise coupling, the digital power domain usually is isolated from the analog power domain. While ESD protection may be incorporated in individual devices, it does not necessarily provide adequate ESD protection across different devices in different power domains on the same integrated circuit. 
         [0020]      FIG. 1  illustrates an exemplary device  100  having a common local ESD ground bus VSS_IO 1   130  and ESD protection circuits,  110   a  and  120   a - c . The clamp circuit  110   a  is placed between the high core power supply line VDD_core 1  and the local ESD ground bus VSS_IO 1   130 . In a typical application, the low power supply VSS may be coupled to the substrate of the semiconductor device and the substrate is connected to the ground of a power supply. Therefore, sometime the low power supply line is also referred to as ground line for convenience. Also shown in  FIG. 1  are diode pairs,  120   a - c , connected back to back to provide conductive paths for large surge current while avoid noise coupling among ground lines of different power domains. For example, a diode pair  120   b  is placed between the low power supply line VSS_core 1  for the core circuit and the low power supply line VSS_IO 1   130  for the I/O circuit. Both diode pairs  120   a  and  120   c  are used to connect the ESD ground bus VSS_IO 1   130  to low power supply lines of other power domains not shown in  FIG. 1 . The example in  FIG. 1  illustrates the use of the VSS for the I/O circuit as the common ground bus. While a diode pair is shown in  FIG. 1  for connecting ground lines with reduced noise coupling, a diode module may be used, where one or more series-connected diodes on each side is used instead of a single diode. 
         [0021]      FIG. 2  illustrates another exemplary ESD protection arrangement  200  having a common local ESD ground bus VSS_core 2   230  and ESD protection circuits,  110   a  and  120   a - c . Similar to  FIG. 1 , the clamp circuit  110   a  is placed between the high core power supply line VDD_core 2  and the local ESD ground bus VSS_core 2   230 . Unlike the device in  FIG. 1  where the I/O ground line is used as the ground bus, the device in  FIG. 2  uses the core ground line VSS_core 2  as the ground bus.  FIG. 3  illustrates yet another exemplary ESD protection arrangement  300 , where the joined VSS for I/O the circuit and VSS for the core circuit, VSS_IO 3 &amp;core 3   330 , is used as a ground bus. A clamp circuit  110   a  is placed between the high core power supply line VDD_core 3  and the local ESD ground bus VSS_IO 3 &amp;core 3   330 . 
         [0022]      FIG. 4  illustrates a scenario of ESD failure where interface circuits are coupled with two devices without cross-domain ESD protection. The device  400  comprises a first device  401  in the upper part of  FIG. 4  and a second device  402  in the lower part of  FIG. 4 . The first device  401  has a ground bus  430 - 1  based on the ground line for the I/O circuit VSS_IO 1  while the second device  402  has a ground bus  430 - 2  based on the ground line for the I/O circuit VSS_IO 2 . As is practiced in a conventional approach, the ground buses for the two devices are connected through a diode pair  120   a  to provide ESD current path across devices while reducing noise coupling. The diode pairs  120   c  and  120   e  are used to further connect the ground buses to additional ground buses. However, this arrangement is not sufficient to provide the needed ESD protection across the two devices. Interface circuits  420   a - d , typically buffers consisting of inverters, are illustrated as an example to demonstrate possible ESD damage routes across the two devices  401  and  402 . Inverters  420   a  and  420   b  serve as buffers for signal flowing from device  402  to device  401 . Similarly, inverters  420   c  and  420   d  serve as buffers for signal flowing from device  401  to device  402 . There are several possible ESD damage paths between the two devices. For example, a high ESD voltage induced on the VDD_core 2  may flow through inverters  420   a  and  420   b  to the VSS_IO 1 , and cause damage to the gate-oxide of the input buffer  420   b  for the first device  401 . Similarly, a high ESD voltage induced on the VDD_core 1  may flow through inverters  420   c  and  420   d  to VSS_IO 2 , and cause damage to the gate-oxide of the input buffer  420   d  for the second device  402 . There is also a potential ESD damage route between the VDD_core 1  and VDD_core 2  which may cause damage to the gate-oxide of inverter  420   b  or  420   d . Therefore, a conventional ESD protection scheme for a multiple power-domain device does not provide sufficient protection for interface circuits coupled to different power domains. 
         [0023]      FIG. 5  illustrates noise coupling in a device having multiple power domains and conventional ESD protection without cross-domain ESD protection. In this example, the VSS_core 2  is used as the common ground bus  530  for the whole device  500 . The first device  501  in the upper part of  FIG. 5  represents the ESD protection scheme for an analog device while the second device  502  in the lower part of  FIG. 5  represents the ESD protection scheme for a digital device. As is well known in the field of electronic circuits, an analog circuit usually is more prone to noise than a digital circuit. When the digital VSS_core 2  is used as the common ESD ground bus, the digital noise in the digital supply line VSS_core 2  will be coupled to the analog supply lines VSS_core 1  and VSS_IO 1  through the paths  532  and  534 . 
         [0024]      FIG. 6  illustrates a scenario of integrated circuits having three power domains corresponding to devices  601 ,  602  and  603 . There exists an interface circuit  610  between devices  602  and  603  and there is no interface between devices  601  and  603 . As described in the discussion associated with  FIG. 5 , there are several possible ESD damage paths between devices  602  and  603  in a conventional ESD protection arrangement. The potential ESD damage paths  632   a - b  and  634   a - b  are shown in  FIG. 6 . The ground lines  630 - 1 ,  630 - 2  and  630 - 3  are connected together through diode pairs  120   a  and  120   b . In the case that no interface circuit exists between the device  601  and device  603 , there is no ESD damage path exists between the two devices. Therefore, there is no need to protect the cross-domain ESD between devices  601  and  603  if there is no interface circuit exists between the two associated power domains. 
         [0025]      FIG. 7  illustrates one embodiment of the present invention where a pair of clamp circuits,  110   b  and  110   c , is coupled with the two devices in separate power domains to provide cross-domain ESD protection. The integrated circuit  700  of  FIG. 7  comprises a device  701  having a first power domain and a second device  702  having a second power domain, where the two devices are coupled to an interface circuit  710 . The first device uses a clamp circuit  110   a  between the power supply lines  736 - 1  and  730 - 1  and the second device uses a clamp circuit  110   d  between the power supply lines  736 - 2  and  730 - 2  to provide ESD protection within the device. The possible ESD damage paths are indicated as  732 / 733  and  731 / 734 . To provide cross-domain ESD protection, a pair of clamp circuits  110   b  and  110   c  is coupled with the two devices. As shown in  FIG. 7 , the clamp circuit  110   b  is coupled to the high power line  736 - 2  of device  702  and to the low power line  730 - 1  of the device  701 . An ESD voltage induced on the high power line  736 - 2  of device  702  will be clamped by the clamp circuit  110   b  so that the current surge will flow through the clamp circuit  110   b  to the ground bus  730 - 1  instead of the interface circuit  710 . Similarly, an ESD voltage induced on the high power line  736 - 1  of device  701  will be clamped by the clamp circuit  110   c  so that the current surge will flow through the clamp circuit  110   c  to the ground bus  730 - 2  instead of the interface circuit  710 . Consequently, the cross power domain ESD protection is accomplished. 
         [0026]      FIG. 8  illustrates another embodiment of the present invention where a clamp circuit  110   b  is coupled between the two devices to provide cross-domain ESD protection. The integrated circuit  800  of  FIG. 8  comprises a device  801  having a first power domain and a second device  802  having a second power domain, where the two devices are coupled to an interface circuit  810 . The first device uses a clamp circuit  110   a  between the power supply lines  836 - 1  and  830 - 1  and the second device uses a clamp circuit  110   c  between the power supply lines  836 - 2  and  830 - 2  to provide ESD protection within the device. The possible ESD damage paths are indicated as  832 / 833  and  831 / 834 . To provide cross-domain ESD protection, the clamp circuit  110   b  is coupled with the two devices as shown in  FIG. 8 . An ESD voltage induced on the high power line  836 - 1  of device  801  will be clamped by the clamp circuit  110   b  so that the current surge will flow through the clamp circuit  110   b  to the ground bus  830 - 2  instead of the interface circuit  810 . Similarly, if an ESD voltage is induced on the high power line  836 - 2 , the current will go to the high power line  836 - 1  through clamp circuits  110   c  and  110   b  or go to the low power line  830 - 1  through clamp circuit  110   c  and ESD protection circuit  120   a . Consequently, the cross power domain ESD protection is accomplished. 
         [0027]    The clamp circuit can be coupled to the power lines of a device in several means as shown in  FIG. 9 . In typical IC layout, the clamp circuit for ESD protection can be connected to a pad of a high power line, i.e., the VDD PAD and to a pad of a low power line, i.e., the VSS PAD as shown in  FIG. 9A . Nevertheless, the clamp circuit may also be connected to internal power lines to achieve the purpose of cross power domain ESD protection. For example,  FIG. 9B  shows a clamp circuit connected between an internal VDD line and a VSS PAD,  FIG. 9C  shows a clamp circuit connected between an internal VDD line and an internal VSS line, and  FIG. 9D  shows a clamp circuit connected to a VDD PAD and an internal VSS line. In this case, part of core areas may be blocked by other circuits and there are no VDD or VSS pads nearby. For IC layout, a technique call “feed through” is often used as a means to allow traces for power/ground lines to go through a large circuit block. Otherwise, the traces would have to be routed around the large circuit block which will unnecessarily increase the routing length. This will degrade signal quality in the particular area. When a high power supply line is fed through a circuit block, such as the clamp circuit, the clamp circuit may be coupled to the feed through power line for ESD protection purpose.  FIGS. 9E and 9F  shows clamp circuits coupled to a feed through power/ground line. 
         [0028]      FIG. 10  illustrates an example of integrated circuit  1000  containing an embodiment of the present invention where a protection circuit  110   b  is coupled between a digital device  1001  and an analog device  1002  to provide cross-domain ESD protection. The digital device includes a clamp  110   a  coupled between the supply lines DVDD 12  and DVSS 12  to provide ESD protection for the digital circuit. The analog device includes a clamp circuit  110   c  coupled between power lines AVDD 12  and AVSS 12  to provide ESD protection for the analog circuit. The cross-domain ESD protection arrangement shown in  FIG. 10  has been implemented in a mixed signal television chip using 0.13 μm technology. The television chip has been tested for cross-domain ESD protection to sustain HBM at 2 kV, MM at 200V and CDM at 1 kV. 
         [0029]    The above drawings, examples, and illustrations provide many different embodiments or embodiments for implementing different features of the present invention. Specific embodiments of components and processes are described to help explain and clarify the invention. These are not intended to limit the invention from that described in the claims. Furthermore, while the invention is illustrated and described herein as embodied in one or more specific examples, it is not intended to be limited to the details shown, since various modifications and structural changes may be practiced therein by those skilled in the art without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Therefore, the appended claims should be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.