Abstract:
The present invention provides a charged-device model (CDM) electrostatic discharge (ESD) protection circuit for an integrated circuit (IC). The ESD protection circuit comprises a substrate of first conductivity type; a MOS component of second conductivity type formed on a first well on the substrate, and coupled to a pad; an isolating well/region having the second conductivity type being formed between the first well and the substrate to separate the first well and the substrate. Additionally, the circuit comprises an ESD clamp coupled to the isolated well/region. Under normal power operation, the ESD clamp is open. During a CDM ESD event, the CDM charges accumulated in the substrate and the MOS component are removed by the ESD clamp to prevent damage to the IC.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to the field of electrostatic discharge (ESD) protection circuitry and, more specifically, improvements against Charged Device Model (CDM) stress cases in the protection circuitry of the integrated circuit (IC). 
       BACKGROUND OF THE INVENTION 
       [0002]    Integrated circuits (ICs) and other semiconductor devices are extremely sensitive to the high voltages that may be generated by contact with an ESD event. As such, electrostatic discharge (ESD) protection circuitry is essential for integrated circuits. An ESD event commonly results from the discharge of a high voltage potential (typically, several kilovolts) and leads to pulses of high current (several amperes) of a short duration (typically, 100 nanoseconds). An ESD event can occur within an IC, illustratively, by human contact with the leads of the IC or by electrically charged machinery being discharged in other leads of an IC. During installation of integrated circuits into products, these electrostatic discharges may destroy or impair the function of the ICs and thus require expensive repairs on the products, which could have been avoided by providing a mechanism for dissipation of the electrostatic discharge to which the IC may have been subjected. When the IC itself is charged, discharge can happen even through a single pin of the IC substrate. This type of stress is modeled as the Charged Device Model (CDM). 
         [0003]    There are various types of physical and chemical process to manufacture an IC. Many different processes exist, having many different process options. In many cases, one or more of these process options allow the creation of an isolated well. A well is considered ‘Isolated’ when it is possible to create a voltage difference between the well and the substrate. 
         [0004]    To protect an IC against ESD, many different type of clamps exist. In general, these clamps exhibit low leakage (i.e. extremely high resistivity) during normal operation, and low resistivity during ESD. These clamps are connected to power pads and/or IO pads. Any pad which is connected to an outside pin should have some kind of ESD clamp attached to it. Also, even some pins inside the chip need some ESD protection. Some typical examples of pins are drivers and receivers connected between different power domains. 
         [0005]    U.S. Pat. No. 6,885,529 discloses a CDM protection design using deep N-Well structure solving a CDM threat. The CDM threat in this patent is introduced because the functional device is placed directly in the substrate (not in an isolated well). Under CDM conditions, the substrate is filled with many electrostatic charges. This issue is solved by isolating the functional device from the substrate by introducing an isolating well. The functional device is placed within said isolating well, such that the charges in the substrate do not damage the functional device. A clamp between substrate and pad is placed to discharge the substrate. The U.S. Pat. No. 6,885,529 states that the charges in the isolated well in which the functional device is placed are ‘too few to damage the gate oxide’. This is however not true. Although the number of charges is limited, they can damage the gate oxide. 
         [0006]      FIG. 1A  illustrates a prior art cross-section diagram of an Integrated Circuit  100  for CDM ESD protection. The circuit  100  comprises a lightly doped region, such as a P-substrate  102  having a first conductivity type and first lightly doped regions, such as deep N-well  108  and the N-well  110  of the second conductivity type. The circuit further comprises a second lightly doped isolated region  106 , preferably a P-well of the first conductivity type formed within the first lightly doped regions deep N-well  108  and N-well  110 . Thus, as shown in  FIG. 1A , the region  110  preferably forms a ring structure around the isolated region  106  and together with the N-well region  108  isolates the P-well region  106  from the substrate  102 . 
         [0007]    Referring back to  FIG. 1A , the circuit further comprises a semiconductor device  104  such as a transistor, an exemplary MOSFET as shown in  FIG. 1A . The transistor  104  is preferably formed in the second lightly doped isolated region  106 , i.e. the isolated Pwell of the first conductivity type. The transistor  104  comprises a first heavily doped region  104   a , a second heavily doped region  104   b  and a gate  104   c . The gate is connected to a sensitive node  118  such as an input/output (I/O) pad leading to a periphery external to the circuit  100 . The transistor  104  comprises a first heavily doped region of the second conductivity type in the case of the  FIG. 1A  N+  104   a  and a second heavily doped region N+  104   b , also of the second conductivity type formed in the isolated well  106  of a the first conductivity type. 
         [0008]    As shown as an example scenario in  FIG. 1A , the N-well  110  and the Deep N-well are coupled to a first power supply, i.e. first voltage potential,  122 , for example VDD. The P-substrate  102  is connected to a second power supply, i.e. second voltage potential  124 , for example ground through a heavily doped region, P+  120 . The isolated P-well region  106  is connected to the second potential,  124  through a core circuitry  114 . Thus, a heavily doped region P+  116  is added. The region  116  will make a low ohmic path between the isolated region  106  and the core circuitry  114 . The transistor  104  is preferably connected to the potentials  122  and  124  through the core circuitry  114 . The core circuitry  114  may preferably be transistors, resistors, inductors, capacitors, metals, etc. The core circuitry  114  is placed accordingly to fulfill requirement for the normal operation and its function depends on the application. 
         [0009]    Additionally as illustrated in  FIG. 1A , clamps represented as diodes  126  are placed between the sensitive node, I/O pad  118  and the power supply  122  or  124 . The diodes are added to protect the gate  104   c  for ESD stress. Although, not shown in this figure, but other ESD protection elements such as local clamps can preferably be placed between the node  118  and the power supply  122  or  124 . The failure under CDM stress conditions is possible for this diagram as described herein below. 
         [0010]    Referring to  FIGS. 1B ,  1 C and  1 D, there is shown a working example for the IC circuit  100  of  FIG. 1A . Specifically,  FIG. 1B  illustrates an explanation of CDM for the IC circuit  100  of  FIG. 1A  before CDM. Before the CDM event happens, the IC is charged up. This means that charges  132  (i.e. positive charges for positive CDM, negative charges for negative CDM) are stored all over the IC  100 , and thus also in the isolated p-well region  106 . During CDM, the charges inside the P-substrate  102  and deep Nwell  108  typically have a low resistive path to the supply lines  122  and  124 . So, during CDM, the charges  132  from the P-substrate  102  and deep N-well  108  can typically flow easily to supply lines  122  or  124  as illustrated in  FIG. 1C . However, this case scenario does not occur for the charges  132  inside the isolated P-well region  106  as shown in  FIG. 1D . These charges  132  will either flow through a core circuitry  114  or through the gate oxide  104   c , depending on the resistivity of the core circuitry  114 , thickness of the gate oxide and CDM stress level. If the charges  132  flow through the core circuitry  114 , damage of the IC  100  is possible due to inefficient ESD protection from the core circuitry  114 . If the charges  132  flow through the gate oxide, damage of the IC  100  is also almost certain. As illustrated in  FIG. 1D , the gate oxide of the gate  104   c  will be damaged. Therefore, these isolated wells, exemplary, P-well isolated region  106  can pose a threat to the IC  100  during CDM stress. 
         [0011]    Thus, there is a need in the art to provide an improved electrostatic discharge (ESD) protection circuitry, specifically, improvement against Charged Device Model (CDM) stress cases in the protection circuitry of the integrated circuit (IC). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1A  depicts an illustration of a prior art cross-section diagram of an Integrated Circuit for CDM ESD protection 
           [0013]      FIG. 1B  depicts an illustrative prior art cross-section diagram of  FIG. 1A  when the chip is charged 
           [0014]      FIG. 1C  depicts an illustrative prior art cross-section diagram of  FIG. 1A  during CDM. 
           [0015]      FIG. 1D  depicts an illustrative prior art cross-section diagram of  FIG. 1A  during CDM. 
           [0016]      FIG. 2A  depicts an illustrative cross-section diagram of an Integrated Circuit with CDM ESD protection in accordance with one embodiment of the present invention. 
           [0017]      FIG. 2B  depicts an illustrative cross-section diagram of  FIG. 2A  during CDM in accordance with the embodiment of the present invention. 
           [0018]      FIG. 2C  depicts an illustrative exemplary cross-section diagram of  FIG. 2A  in accordance with alternate embodiment of the present invention. 
           [0019]      FIG. 2D  depicts an illustrative cross-section diagram of  FIG. 2A  in accordance with another alternate embodiment of the present invention. 
           [0020]      FIG. 2E  depicts an illustrative cross-section diagram of a further alternate embodiment with reference to  FIG. 2A  of the present invention. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0021]    In one embodiment of the present invention, there is provided a circuit having charged-device model (CDM) electrostatic discharge (ESD) protection comprising a substrate, a semiconductor device isolated from the substrate and an ESD clamp device coupled to the device to discharge the charges located in the device. 
         [0022]    In a preferred embodiment of the present invention, there is provided a circuit having charged-device model (CDM) electrostatic discharge (ESD) protection comprising a substrate of first conductivity type, a first lightly doped region of second conductivity type formed within the substrate and a second lightly doped region formed within the first lightly doped region. The second lightly doped region of the first conductivity type. The circuit further comprises a semiconductor device formed in the second lightly doped region and an ESD clamp device coupled between the second lightly doped region and a reference node. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The invention relates to a technique to increase the CDM performance of an IC by connecting additional ESD clamps to isolated wells (or junctions).  FIG. 2A  illustrates a cross-section diagram of an Integrated Circuit IC  200  for CDM ESD protection in accordance with one embodiment of the present invention. The IC  200  illustrates a cross-section diagram of the transistor  104  formed in the isolated P-well region  106  with the deep N-well  108  and N-well  110  forming a ring structure around the isolated region to isolate/separate the P-well region  106  from the P-substrate  104 . Furthermore, an additional ESD clamp  202  is coupled to the isolated P-well,  106  as shown in  FIG. 2A . Specifically, the ESD clamp  202  is placed between the isolated P-well  106  and a reference node. The selection of the reference node depends on the normal operation requirements such as noise, cross-coupling, and other ESD elements. Preferably for ESD and in this example of  FIG. 2A , the terminal to the isolated well  106  is coupled to the second potential  124  (i.e. the reference node) with the ESD clamp  202 . Depending on the normal operation requirements the ESD clamp  202  may preferably comprise one of: SCR (with or without trigger device), MOS, diode, resistor, or other elements. As discussed above, one implementation is that the second potential  124  is one of the ground lines. However, there exist a lot of cases where the isolated well  106  is coupled to another ground besides the ground potential  124 . This is preferably due to normal operation requirements such as noise. Now the voltage of the isolated well  106  is nearly equal to the second potential  124  and so one or more diodes in series can be utilized as ESD clamp  202 . However there are also other possible cases where the voltage difference between the isolated well  106  and the second potential  124  is larger during normal operation or there are some other more severe requirements. In those cases, other elements such as SCR, transistor, resistor, capacitor or inductor are preferably utilized as the ESD clamp  202  to remove the charges of the isolated P-well  106 . 
         [0024]    Referring to  FIG. 2B , there is illustrated a cross-section diagram of IC  200  of  FIG. 2A  during CDM in accordance with the embodiment of the present invention. As shown in  FIG. 2B , the ESD clamp  202  is added to remove the charges from the isolated P-well  106 . Thus, during CDM, as shown in  FIG. 2B , the charges  132  in the isolated P-well  106  are allowed to flow through the dedicated ESD path i.e. via the ESD clamp  202  to prevent the damage to either the core circuitry  114  or the gate oxide thus, avoiding the damage to the IC  100 . As shown earlier in  FIG. 1  C, the charges in the substrate  102  and in the N-Well  110  (and Deep N-Well Well  108 ) will flow easily to the node potentials  124  and  122  respectively. In an initial stage of the ESD discharge, the charges will remain in the isolated Well  106 . Due to the difference in discharging between the substrate  102  and N-well  110  at one side and the isolated P-well  106  at the other side, a voltage difference will be created between the I/O pad  118  and the substrate  102 . In the prior art the voltage built up will be large enough to damage the gate, but in this invention the ESD clamp  202  will turn on at a voltage below the gate oxide breakdown or the failure of the core circuitry  114 . The triggering of the clamp  202  will further limit the voltage built-up over the gate oxide, thus protecting it, and will discharge the charges of the isolated well  106  to the reference node, (i.e. node potential  124  in  FIG. 2A  and  FIG. 2B ) and then ultimately to the I/O pad  118 . 
         [0025]    Note that the invention is not limited to the placement of the ESD clamp  202 .  FIG. 2C  shows an exemplary cross-section diagram of IC  200  of  FIG. 2A  where the ESD clamp  202  is placed between the isolated P-well  106  and the first potential  122  instead of the second potential  124 . Thus, in this example of  FIG. 2C , the terminal to the isolated well  106  is coupled to the first potential  122  (i.e. the reference node) with the ESD clamp  202 . For negative CDM this can be advantage such that if the ESD protection of the sensitive node comprises only the ESD diodes  126   a  and  126   b  and no local clamps, the charges in  FIG. 2B  will flow to the second potential  124 . A power clamp (not shown) is always located between the first potential  122  and the second potential  124 . Thus the charges in  FIG. 2B  will need to travel through the power clamp to the first potential  122 , then, they will go through the diode  126   a  to the I/O pad  118 . However, in this embodiment of the present invention, the charges will flow directly to the first potential  122 , without any need to go through the power clamp anymore. The voltage built over the gate  104   c  will be now lower, i.e. having a less resistive path. 
         [0026]    Referring to  FIG. 2D  there is shown an illustrative exemplary cross-section diagram of IC  200  of  FIG. 2A  utilizing the invention for the isolated well inside the core of the IC. In this example, the isolated well, i.e. P-well  106  is placed in the core of the IC  100 , instead of in the periphery as illustrated in  FIG. 2A . In the prior art, during CDM stress the internal node can discharge with a different speed than the isolated well  106 , which creates as in the I/O pad  118 , a voltage built-up over the gate  104   c . So, in order to prevent gate damage, in the present embodiment, the charges in the isolated well  106  are preferably discharged also with an ESD clamp  202  coupled to another internal node. One example in  FIG. 2D  shows that the another internal node is one of the potentials, i.e. second potential  124  as described in  FIG. 2A . Thus, in this application, the charges of the substrate  102  and the isolated well  106  will be discharged at the same rate. Although, as shown in  FIG. 2D  of the present embodiment, the gate  104   c  of the transistor  104  is connected to a core circuitry  114 , it can also preferably be connected to the internal node. 
         [0027]    Now referring to  FIG. 2E , there is shown an illustrative exemplary cross-section diagram of IC  200  of  FIG. 2A  utilizing protecting another device, for example, a capacitance used to show the advantage of the technique described in the present invention. Thus, the problem that the isolated well  106  can not be discharged and will damage a device is not limited to transistors only.  FIG. 2E  illustrates a scenario where the device within the isolated Well, i.e. device  106  is a capacitance  204 , instead of a transistor  104 . The ESD clamp  202  is shown to be coupled between the potential node  124  and the isolated P-Well  106 . In this case, the connection to the isolated well  106  (and  204   a ) is not a separate tap  116  but a part of the device. The charges will flow during the stress through the tap region  204   a  (or even through  204   b , in this case these two taps are coupled together) to the ESD clamp  202 . Further the charges will flow to the potential Vss  124  which in this figure is the output. When the charges has reached this potential, they can flow to the stressed pin (not shown) internal to the chip as described in the previous embodiments. It is important to note that those skilled in the art can utilize many other devices to utilize the above-described invention technique. 
         [0028]    Although the invention is illustrated for an NMOS component, those skilled in the art would appreciate that a PMOS structure device can preferably be utilized. Furthermore, the present invention is not restricted for the use for an Isolated Pwell. Any well which is isolated from the Vss or Vdd busses or only connected to those busses through some core circuitry, requires the protection as described in this invention. 
         [0029]    A typical case where this kind of protection might be appropriate beside technologies with deep n-well (or buried layer), is the case of silicon-on-insulator (SOI) integrated circuit, where the body region of the transistor is easily isolated from Vss and Vdd bus, since there is no substrate connection between the body region of the transistor (i.e. the well) and a ground connection. Other processes are for example bipolar technologies (BCD, HV technologies), where a lot of isolated wells are used. 
         [0030]    Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit and the scope of the invention.