Abstract:
A method and apparatus for improving the latchup tolerance of circuits embedded in an integrated circuit while avoiding the introduction of noise from such tolerance into the power rails.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to integrated circuits, and more particularly to providing latchup and noise suppression in such integrated circuits. 
   2. Description of the Related Art 
   As electronic components are getting smaller and smaller along with the internal structures in integrated circuits, it is getting easier to either completely destroy or otherwise impair electronic components from latchup. Latchup is when a pnpn structure transitions from a low current high voltage state to a high current low voltage state through a negative resistance region (i.e. forming an S-Type I-V (current/voltage) characteristic). 
   Latchup is typically understood as occurring within a pnpn structure, or silicon controlled rectifier (SCR) structure. Interestingly enough, these pnpn structures can be intentionally designed, or even unintentionally formed between structures. Hence, latchup conditions can occur within peripheral circuits or internal circuits, within one circuit (intra-circuit) or between multiple circuits (inter-circuit). 
   Latchup is typically initiated by an equivalent circuit of a cross-coupled pnp and npn transistor. With the base and collector regions being cross-coupled, current flows from one device leading to the initiation of the second (“regenerative feedback”). These pnp and npn elements can be any diffusions or implanted regions of other circuit elements (e.g. P-channel MOSFETs, N-Channel MOSFETs, resistors, etc) or actual pnp and npn bipolar transistors. In CMOS, the pnpn structure can be formed with a p-diffusion in a n-well, and a n-diffusion in a p-substrate (“parasitic pnpn”). In this case, the well and substrate regions are inherently involved in the latchup current exchange between regions. 
   The condition for triggering a latchup is a function of the current gain of the pnp and npn transistors, and the resistance between the emitter and the base regions. 
   This inherently involves the well and substrate regions. The likelihood or sensitivity of a particular pnpn structure to latchup is a function of spacings (e.g. Base width of the npn and base width of the pnp), current gain of the transistors, substrate resistance and spacings, the well resistance and spacings, and isolation regions. 
   System-on-a-chip (SOC) solutions have been used for solving the mixed signal (voltage) and radio frequency (RF) requirements of high-speed data rate transmission, optical interconnect, wireless and wired marketplaces. Each of the noted applications has a wide range of power supply conditions, number of independent power domains, and circuit performance objectives. Different power domains are established between digital, analog and radio frequency (RF) functional blocks on an integrated chip. Part of the SOC solution has resulted in different circuit and system functions being integrated into a common chip substrate. The integration of different circuits and system functions into a common chip has also resulted in solutions for ensuring that noise from one portion or circuit of the chip does not affect a different circuit within the chip. 
   In internal circuits and-peripheral circuitry, latchup and noise are both a concern. Latchup and noise are initiated in the substrate from overshoot and undershoot phenomenon. These can be generated by CMOS off-chip driver circuitry, receiver networks, and ESD devices. In CMOS I/O circuitry, undershoot and overshoot can lead to injection in the substrate. Hence, both a p-channel MOSFET and n-channel MOSFET can lead to substrate injection. Simultaneous switching of circuitry where overshoot or undershoot injection occurs, leads to injection into the substrate which leads to both noise injection and latchup conditions. Supporting elements in these circuits, such as pass transistors, resistor elements, test functions, over voltage dielectric limiting circuitry, bleed resistors, keeper networks and other elements can be present leading to injection into the substrate. ESD elements connected to the input pad can also lead to noise injection and latchup. ESD elements that can lead to noise injection, and latchup include MOSFETs, pnpn SCR ESD structures, p+/n-well diodes, n-well-to-substrate diodes, n+ diffusion diodes, and other ESD circuits. ESD circuits can contribute to noise injection into the substrate and latchup. 
   Unfortunately, the solutions for reducing noise often result in increasing the likelihood of latchup, and are not reviewed or their effects contemplated with the solutions used for latchup. Conversely, solutions for improving latchup tolerance fail to contemplate the effects on noise. 
   It would, therefore, be a distinct advantage to have a method and apparatus that improved both noise suppression and latchup tolerance in an integrated circuit. The present invention provides such an apparatus and method. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus for improving the latchup tolerance of circuits embedded in an integrated circuit while avoiding the introduction of noise from such tolerance into the power rails. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
       FIG. 1  is a cross section diagram illustrating a parasitic latchup structure environment in an integrated circuit; 
       FIG. 2  is a cross section diagram illustrating an example of a parasitic latchup structure in an integrated circuit; 
       FIG. 3  is a cross section diagram illustrating an example of the how the present invention can be implemented within an integrated circuit; 
       FIG. 4  is a cross section diagram illustrating an example of how the noise suppression circuit of  FIG. 3  can be implemented according to the teachings of the present invention; and 
       FIG. 5  is a cross section diagram illustrating an additional example of how the noise suppression of  FIG. 3  can be implemented according to the teachings of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Detailed Description of a Preferred Embodiment of the Invention 
   Reference now being made to  FIG. 1 , a cross section diagram is shown illustrating a parasitic latchup structure environment in an integrated circuit. The integrated circuit includes a substrate  18  having an n-well region  8  with a well contact  10 . 
   N-well region  8  can represent a diffused well, a retrograde well, a subcolector, or other vertical modulated wells. Substrate  18  can represent a p-well, a p− epi/p+ substrate, a p− wafer with a p+ buried layer, or other known substrate doping profiles. 
   Located within n-well region is p-doped shape  12  that can be used to form a p-channel MOSFETs, p-resistors, p/n diode, an ESD element or a base region of a npn bipolar. Also located within substrate  18  is n-doped region  14  that can be used to form an n-channel MOSFETs, a n-resistor, a diode, an ESD element or a collector of a npn transistor, or any other structure requiring a n-doped region. Substrate  18  also includes a substrate contact  16 . It should also be noted that power (VDD) and ground (VSS) are applied to well contact  10  and substrate contact  16 , respectively. The structure illustrated in  FIG. 1  demonstrates the ease with which a parasitic pnpn structure can be formed in either a CMOS or BiCMOS technology from the p-doped shape  12 , the n-well  8 , substrate  18  and n-region  14 . The parasitic pnpn structure can cause undesirable latchup and/or noise problems. 
   In the above configuration where there are connections ( 10  and  16 ) directly to the power grid, noise is able to leave the area of the circuit in which it is generated and is injected into the power domain (e.g. VSS or VDD) of the chip. Unfortunately, the noise level can become significant enough to impact the noise floor of adjacent circuits on the same power grid. Furthermore, allowing the injection into the substrate region or well region can initiate a latchup state. For example, if region  14  is grounded and a positive pulse is applied to p-region  12 , latchup can occur. In further example, negative pulses can initiate a latchup between region  12  and  14 . In addition, a negative undershoot on region  14  can lead to noise injection into the substrate that can be absorbed by n-well contact region  10  and by substrate contact region  16 . 
   Reference now being made to  FIG. 2 , a cross section diagram is shown illustrating an example of a parasitic latchup structure in an integrated circuit. This example is similar to that shown in  FIG. 1  with the addition of a new guard ring structure  34  to the N-well. Guard ring structure  34  is intended to improve latchup tolerance or sensitivity. Guard ring structure  34  will typically be connected to a power rail (e.g. VDD) with a salicided contact area and contacts to provide a low resistance path to VDD. In this example, the guard ring  34  improves the latchup tolerance, however, the overshoot noise which can initiate the latchup is injected into the ground rail. (VSS), and possible spread to other circuits. 
   As illustrated in the prior  FIGS. 1-2 , solutions for improving latchup tolerance have been used, however, these circuits introduce noise into the power rails (e.g. VDD or VSS) which is also undesirable. The present invention provides a solution for improving latchup tolerance and at the same time limiting the amount of noise introduced into the power rails. 
   Reference now being made to  FIG. 3 , a cross section diagram is shown illustrating an example of the how the present invention can be implemented within an integrated circuit. The integrated circuit includes a p+ region  54  contained with a well region  55 , a substrate  64 , an n-region  58  in the substrate, and a substrate contact  60  (noise suppression collecting structure). In this particular embodiment, a p-region  56  (noise suppression collecting structure) is placed within the parasitic pnpn structure ( 60 ,  58 ,  54 ). In addition, an active noise suppression circuit  62  is also added with an input connected to p-region  56 , and an output connected to substrate contact  60 . 
   P-region  56  can be a guard ring which is not connected to the ground potential, or any additional shape that improves latchup tolerance. A guard ring is a structure which collects minority carriers or obstructs the flow of minority carriers in the substrate. Any current or voltage signal intiated on p-region  56  is directed to the active noise suppression circuit  62 . Noise suppression circuit  62  inverts the directed signal and applies the inverted signal to the region of the substrate where voltage increase or decrease has occurred locally in the substrate  64 . By using the Noise suppression circuit  62 , the signal produced from electrical overshoot or noise injection does not directly feed directly into the ground or power rails. 
   Reference now being made to  FIG. 4 , a cross section diagram is shown illustrating an example of how the noise suppression circuit  62  of  FIG. 3  can be implemented according to the teachings of the present invention. In this example, the noise suppression circuit  62  is illustrated as an inverting amplifier  94  coupled to resistors  100 ,  101  and  102  as shown. 
   Reference now being made to  FIG. 5 , a cross section diagram is shown illustrating an additional example of how the noise suppression circuit  62  of  FIG. 3  can be implemented according to the teachings of the present invention. The noise suppression circuit  62  is identical to that shown in  FIG. 4  with the addition of capacitive elements  102  and  103 . 
   It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description. While the method and system shown and described has been characterized as being preferred, it will be readily apparent that various changes and/or modifications could be made wherein without departing from the spirit and scope of the present invention as defined in the following claims.