Abstract:
A method and apparatus for analyzing an integrated circuit design for pnpn structures which are likely to latchup or cause injection of noise into the substrate. Once qualifying pnpn structures are identified, the method and apparatus automatically inserts a noise and latchup suppression circuit of the designers&#39; choice into the pnpn structure to eliminate the latchup and/or noise concerns.

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 designers of the circuits often fail to anticipate or recognize the appearance of parasitic pnpn structures. Even when the circuit designer does recognize or anticipate parasitic pnpn structures, the solutions for reducing the latchup tolerance often result in unnecessarily increasing the introduction of noise into the power rails. 
   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. It would be further advantages if the method and apparatus would be integrated into a software tool such that the tool searched for these parastic pnpn structures and automatically inserted a solution for reducing latchup and noise suppression. The present invention provides such a method and apparatus. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus for analyzing an integrated circuit design and recognizing parasitic pnpn structures. Upon such recognition, the present invention would automatically insert a noise and latchup suppression circuit of the designers choice. In addition, further changes to the substrate can be specified to increase the effective resistance. 

   
     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; 
       FIG. 5  is a cross section diagram illustrating an additional example of how the noise suppression circuit of  FIG. 3  can be implemented according to the teachings of the present invention; 
       FIG. 6  is a diagram illustrating a data processing system in which the present invention can be practiced; and 
       FIG. 7  is a flow chart is shown illustrating the execution, on the data processing system of  FIG. 6 , of a preferred embodiment of software for automatically recognizing parasitic pnpn structures and inserting a latchup noise suppression circuit 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 subcollector, 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  300  contained with a well region  310  a substrate  312 , an n-region  304  in the substrate, and a substrate contact  306  (noise suppression collecting structure). In this particular embodiment, a p-region  302  (noise suppression collecting structure) is placed within the parasitic pnpn structure ( 306 , 304 , 300 ). In addition, an active noise suppression circuit  308  is also added with an input connected to p-region  302 , and an output connected to substrate contact  306 . 
   P-region  302  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  302  is directed to the active noise suppression circuit  308 . Noise suppression circuit  308  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  308 , 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  308  of  FIG. 3  can be implemented according to the teachings of the present invention. In this example, the noise suppression circuit  308  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  308  of  FIG. 3  can be implemented according to the teachings of the present invention. The noise suppression circuit  308  is identical to that shown in  FIG. 4  with the addition of capacitive elements  502  and  504 . 
   Reference now being made to  FIG. 6 , a diagram is shown illustrating a data processing system  20  in which the present invention can be practiced. The data processing system  20  includes processor  22 , keyboard  82 , and display  96 . Keyboard  82  is coupled to processor  22  by a cable  28 . Display  96  includes display screen  30 , which may be implemented using a cathode ray tube (CRT) a liquid crystal display (LCD) an electrode luminescent panel or the like. The data processing system  20  also includes pointing device  84 , which may be implemented using a track ball, a joy stick, touch sensitive tablet or screen, track path, or as illustrated a mouse. The pointing device  84  may be used to move a pointer or cursor on display screen  30 . Processor  22  may also be coupled to one or more peripheral devices such as modem  92 , CD-ROM  78 , network adapter  90 , and floppy disk drive  40 , each of which may be internal or external to the enclosure or processor  22 . An output device such as printer  100  may also be coupled with processor  22 . 
   The present invention can be embodied within various types of software including but not limited to Computer Aided Design (CAD) software executing on the processing system  20  of  FIG. 1 . In general, the software identifies parasitic pnpn structures and inserts a latchup noise suppression circuit that raises the resistance of the substrate. In the preferred embodiment of the present invention, the latchup suppression circuit (s) illustrated and described above in connection with  FIGS. 3–5  are used. The execution of the software is explained in connection with the flow chart of  FIG. 7 . 
   Reference now being made to  FIG. 7 , a flow chart is shown illustrating the execution, on the data processing system  20  of  FIG. 6 , of a preferred embodiment of software for automatically recognizing parasitic pnpn structures and inserting a latchup noise suppression circuit according to the teachings of the present invention. The execution of the software is illustrated in  FIG. 7  with an example illustrated therewith using  FIGS. 1 and 3 . 
   The execution of the software begins executing (step  700 ) by identifying the following (step  130 ): (1) any p shapes that are connected to a power supply (VDD or VSS); (2) whether there is an associated parasistic pnp structure for the identified p shape(s); (3) a ground substrate contact connected to VSS; (4) the parasitic npn structure; and (5) an identification of the parasitic pnpn structure. 
   The identification is accomplished by evaluating the localness of the emitter and collector regions for each parasitic structure, and then determining if any two of the parasitic structures are cross-coupled (e.g. sharing common regions) and spatially local to one another. The identification of shapes connected to the power supplies can use a logical-to-physical check, and spacings can be verified by spatial ground rule check systems. 
   For example, using the structure illustrated in  FIG. 1 , the following would be identified: (1) p+  16 ; (2) p+  16 , substrate  18 , and n+  14 ; (3) p+  16 ; (4) n+  10 , p+  12 , and n-well  8 ; and (5) pnpn structure (p doped substrate  18 , n+  14  and p doped substrate  18 ), pnpn structure (p doped substrate  18 , n-well  8 , p+  12 ). 
   Once a parasitic pnpn shape(s) has been identified, the software datermines whether a p+ ring has been inserted in the pnpn parasitic structure (step  704 ) (In the case of  FIG. 1 , no p+ ring exists). If the p+ ring exists, then it is converted to a noise reduction connection (step  706 ). It however, the p+ ring does not exists, ten a noise reduction connection is inserted in the pnpn parasitic structure (step  708 ). Referencing  FIG. 3 , the noise reduction connection for the example is p+  302 . 
   The software provides the designer with a plurality of noise suppression circuits to select depending upon the particular design and/or requirements. The selection can be made upon each discovery of the pnpn structure or prior to the identification and used throughout automatically. Alternatively, the designer could also select a default noise suppression circuit that can be used automatically, unless certain criteria exists. Obvious variations on the selections and criteria could also be provided but are not discussed in detail hereinafter. 
   In this particular case, it can be assumed that the designer has selected the default automatic option, and the input of the selected noise suppresion circuit is connected to the noise reduction connection (Step  710 ) (p+  302  of  FIG. 3 ). The output of the noise suppression circuit can be connected in various manners all of which reduce the resistance of the substrate (Step  712 ). For example, the existing structure coupled to VSS could be used or converted to only be coupled to the noise suppression circuit. In addition, a noise reduction connection can also be added before the existing structure coupled to VSS. Provided the noise suppression circuit is coupled to receive the latchup current and inject the inversion of the lathup current locally (within 100 microns of where the latchup current was received), any manner that meets this criteria can be used. In addition, it would be further advantageous if the use of the noise suppression circuit would increase the effective resistance of the substrate. This can be accomplished by replacing some of the substrate contacts when the noise suppresion circuit is inserted as previously described. 
   For the current example, the p+  16  VSS contact is converted to serve as an output to the active latchup noise suppression circuit. 
   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.