Abstract:
An electrostatic discharge (ESD) protective structure is configured to protect an integrated circuit, which is connected between a first voltage bus with a first supply voltage and a second voltage bus with a second supply voltage. The ESD protective structure includes a plurality of laterally designed bipolar transistors, whose load lines are arranged parallel to one another and between the voltage buses, and whose control connections are connected to one of the voltage buses. A single track resistor is co-integrated into the semiconductor body and precedes every control connection of the bipolar transistors.

Description:
CROSS REFERNCE TO RELATED APPLICATIONS  
       [0001]    This application contains subject matter related to co-pending application designated Ser. No. ______ , filed May 8, 2001 and entitled “Electrostatic Protective Structure”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to the field of semiconductor devices, and in particular to an electrostatic discharge (ESD) protective structure to protect an integrated semiconductor circuit against electric discharge.  
           [0003]    During the operation of integrated circuits undesirably high voltage peaks may occur, which are coupled into the integrated circuit through the supply lines. Without suitable protective measures, parasitic overvoltages of a few volts and the electrostatic discharges caused thereby can damage and even destroy the integrated circuit. This makes the failure rate of such an integrated circuit unacceptably high.  
           [0004]    To reduce their failure rate, circuits located on an integrated circuit often contain co-integrated ESD protective structures. These ESD protective structures can be inserted as clamping circuits between the supply lines of the integrated circuit and, in case of an electrostatic discharge, drain off the parasitic overvoltage to one of the supply lines. As a result, the parasitic overvoltage between the supply lines is reduced to a tolerable value (i.e., to a value that does not damage the integrated circuit).  
           [0005]    An important boundary condition in the production of ESD protective structures derives from the requirement that under operational conditions (as these are described for example in the product specification) the ESD protective structures must not impair the function of the integrated semiconductor circuit that is being protected (or impair it only insignificantly). Thus the breakdown or switch-through voltage of the ESD protective element must be beyond the nominal signal voltage range of the circuit being protected. As a result, the ESD protective element should break down before the critical circuit path, but beyond the signal voltage range. As a rule this requires an exact adjustment of the breakdown and switch-through voltage of the particular ESD protective element.  
           [0006]    ESD protective elements often include semiconductor components that are blocking at least part of the time, such as, for example, thyristors, bipolar transistors, field effect transistors, or diodes.  
           [0007]    However, ESD diodes have the disadvantage that at very high currents the voltage drop across the diode may become so large that sensitive components of the protected circuit may be irreversibly damaged (e.g., their gate oxides or diffusion regions). Furthermore, in reverse operation, diodes consume a great deal of power.  
           [0008]    A disadvantage of ESD thyristors is that once it fires it retains its firing state in an undesirable manner. This firing state may last arbitrarily long and cause a short circuit in the voltage supply and the destruction of the ESD protective structure, thus damaging the component.  
           [0009]    For the above reasons, ESD protective structures are often used that have a plurality of parallel, bipolar field oxide transistors as ESD protection. These ESD protective structures are especially well suited to limit parasitic overvoltages. With a reverse bias voltage in the characteristic, they first show a blocking behavior, until a first voltage breakthrough is reached, at which the current-voltage characteristic snaps back to a lower voltage than the breakthrough voltage. This process of snapping back to the lower, so-called holding voltage, is also called snapback behavior. After the holding voltage has been reached, the current-voltage characteristic has an essentially linear slope until it reaches a second voltage breakthrough, at which the ESD protective structure is destroyed, and obviously must be avoided. The plurality of ESD transistors arranged in parallel consequently should switch through as uniformly and as simultaneously as possible in response to a coupled-in overvoltage pulse.  
           [0010]    However, ESD protective structures are at risk that one or a few bipolar transistors will pull the entire ESD current to themselves, and thus the second voltage breakthrough will be reached prematurely resulting in a short circuit. This short circuit caused by an inhomogeneous current flow may in the same manner short circuit the supply voltage of the entire integrated semiconductor circuit. Without additional protective measures this can have catastrophic consequences (i.e., the destruction of the integrated circuit). When a plurality of transistors is used as ESD protection, there is a need to distribute an ESD current caused by electrostatic overvoltage uniformly in the semiconductor body of the ESD protective structure, so that the ESD protective structure can be loaded to the greatest extent and thus damage of the ESD protective structure is prevented.  
           [0011]    Therefore, there is a need for an ESD protective structure that assures that the transistors of the ESD protective structure switch on as uniformly as possible.  
         SUMMARY OF THE INVENTION  
         [0012]    Briefly, according to an aspect of the present invention, an electrostatic discharge (ESD) protective structure is configured to protect an integrated circuit, which is connected between a first voltage bus with a first supply voltage and a second voltage bus with a second supply voltage. The ESD protective structure includes a plurality of laterally designed bipolar transistors, whose load lines are arranged parallel to one another and between the voltage buses, and whose control connections are connected to one of the voltage buses. A single track resistor is co-integrated into the semiconductor body and precedes every control connection of the bipolar transistors.  
           [0013]    The ESD protective structure has a single common track resistor that actuates each bipolar transistor. The track resistor is created by a deep implantation region or diffusion region inserted in the semiconductor body. These deep doping regions act like an artificially expanded distance or a barrier between the base zone and the region in which the collector zones and emitter zones are situated. Charge carriers are thus artificially prevented from diffusing directly to the base zones and take a detour into the depth of the substrate. The charge carriers created at a first voltage breakthrough are predominantly held back in a region below the emitter zones and collector zones, which favors a subsequent firing process. Since the region below the collector and emitter contains a defined and essentially uniformly distributed quantity of charge carriers the bipolar transistors of the ESD protective structure fire quickly and uniformly.  
           [0014]    In a preferred embodiment, the well-like regions are connected to the outermost zones of the first conduction type (e.g., the outermost emitter zones), and they have the same conduction type as these (emitter) zones. The well-like regions then lie at the same potential as the (emitter) zones. This assures that the charge carriers traverse a much longer path through the substrate of the semiconductor body in order to reach the base zones. However, the well-shaped regions typically have a much lower doping concentration than the connected (emitter) zones. It is contemplated that the well-shaped regions may not be doped, or may include a dielectric.  
           [0015]    In a preferred embodiment the well-shaped regions extend much deeper into the semiconductor body than those (emitter) zones that they adjoin or to which they are connected. In this case, the well-shaped regions can be produced, for example, by multiple ion implantation or by diffusion with a subsequent deep drive-in step. It is contemplated that a conventional trench etching process and filling up with doped or undoped polysilicon or with a dielectric may be performed.  
           [0016]    The emitter zones and the collector zones typically are surrounded laterally by the base zones and the well-shaped regions. This assures better latch-up protection for the ESD protective structure. This latch-up protection may be improved further if the zones of the ESD structure (i.e., the emitter zones, the collector zones, the base zones, and the well-shaped regions) are embedded in an additional well and thus are isolated from the substrate potential.  
           [0017]    In one embodiment, the emitter zones and collector zones are designed in strips. The strip-shaped collector zones and emitter zones are arranged parallel to one another, such that the collector zones and emitter zones alternate. The ESD protective structure may be configured and arranged in an essentially square layout, which assures significant homogenization of the current of the ESD structure. Further homogenization of the current can be achieved in that the emitter zones and collector zones are through-contacted by the respective electrodes.  
           [0018]    In one embodiment each of the strip-shaped emitter electrodes and collector electrodes forms a finger-like structure with the conductor track to which they are connected. These finger-like structures are preferably staggered with one another. As a result, large current densities and large voltage drops occur at opposite sides of the respective conductor tracks, thus reducing scorching at the conductor tracks caused by voltage breakthroughs.  
           [0019]    The ESD protective structure is especially suited for bipolar transistors designed as field oxide transistors.  
           [0020]    These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0021]    [0021]FIG. 1 illustrates a circuit arrangement with an inventive ESD protective structure;  
         [0022]    [0022]FIG. 2 illustrates a circuit diagram of the inventive ESD protective structure illustrated in FIG. 1, including three bipolar transistors;  
         [0023]    [0023]FIG. 3 illustrates a partial section through an embodiment of the ESD protective structure illustrated in FIG. 2;  
         [0024]    [0024]FIG. 4 is a top view of an arrangement for contacting the emitter, base, and collector of the ESD protective structure illustrated in FIG. 3; and  
         [0025]    [0025]FIG. 5 is a top view of an arrangement of the conductor tracks for contacting the emitter electrodes and collector electrodes.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    [0026]FIG. 1 illustrates an integrated circuit  1 . The integrated circuit  1  is connected via supply voltage inputs  2 ,  3 , to a first voltage bus  4  with a first supply voltage VCC and to a second voltage bus  5  with a second supply voltage VSS, respectively. In the present example VSS is reference ground.  
         [0027]    The integrated circuit  1  includes a circuit  6  that is being protected (e.g., a logic circuit, a program-controlled unit, a semiconductor memory, a power circuit, etc.). The integrated circuit  1  also includes an ESD protective element  7  to protect the circuit  6 . The circuit  6  that is being protected and the ESD protective element  7  are connected between the voltage buses  4 ,  5 .  
         [0028]    [0028]FIG. 2 illustrates a circuit arrangement of the inventive ESD protective  7 . The ESD protective structure  7  includes a plurality of (e.g., three) bipolar field oxide transistors T 1 , T 2 , T 3 . The bipolar transistors T 1 -T 3  are arranged in parallel across their load lines and are connected between the voltage buses  4 ,  5 . In the present embodiment, the respective emitter connections E are connected to the voltage of the reference ground VSS, and the respective collector connections C are connected to the supply voltage VCC. According to an aspect of the invention, the base connections B of the bipolar transistors T 1 -T 3  are short-circuited with one another and are coupled, via a track resistor RB to the reference voltage VSS and thus the emitter connections E.  
         [0029]    [0029]FIG. 3 illustrates a partial section of the ESD protective structure of FIG. 2. In one embodiment a semiconductor body  10  includes a weakly p-doped silicon substrate. The semiconductor body  10  has a wafer front side  11  in which are embedded the first and second n-doped regions  12 ,  13 . The first n-doped regions  12  are the collector zones in the present embodiment; the second n-doped regions  13  are the emitter zones. The collector zones  12  and the emitter zones  13  are arranged alternately on the surface  11 , and they are spaced apart from one another by a substrate zone  10 ′ or field oxide  14 . P-doped regions  15  that form the base zones are embedded in the semiconductor body  10 . The base zones  15  enclose the collector zones  12  and emitter zones  13  and are spaced apart from these for example by a field oxide  16 .  
         [0030]    The collector zones  12 , the emitter zones  13 , and the base zones  14  are respectively contacted through contact electrodes  18 ,  19 ,  20  at the surface  11  of the semiconductor body  10 . The collector electrodes  18  are short-circuited with one another and are connected to the first supply voltage VCC. The emitter electrodes  19  and the base electrodes  20  are also short-circuited with one another and are connected to the voltage of the reference ground VSS.  
         [0031]    According to an aspect of the present invention, well-shaped regions  17  are introduced into the semiconductor body  10  between the base zones  15  and the outer n-doped regions  12 ,  13  (in the present embodiment these are the outer emitter zones  13 ). These well-shaped regions are connected to the outer emitter zones  13 . The well-shaped regions  17  have the same conduction type as the emitter zones  13 , but a lower dopant concentration. However, they may also be undoped or can contain a dielectric. Furthermore, the regions  17  do not necessarily have to be well-shaped, but, in the context of the technology used to produce the particular integrated semiconductor circuit, they can also be for example trench-shaped.  
         [0032]    The lateral and/or vertical dimensions of the well-shaped regions  17  are such that the effective drift path of charge carriers between the p-doped base zones  15  and the substrate zones  10 ′ in the semiconductor substrate  10  are markedly lengthened. The regions  17 , therefore, are advantageously introduced much deeper into the semiconductor body  10  than the corresponding emitter zones  13  and/or collector zones  12 .  
         [0033]    The function of the inventive ESD protective structure  7  will be explained in more detail below. According to an aspect of the invention, the weakly n-doped well-shaped regions  17  are melded together with the outer n-doped emitter zones  13  and consequently are at the reference voltage VSS. Depending on the technological process used these well-shaped regions  17  can be created by high-energy ion implantation or by deep diffusion. They hinder the charge carriers created at a first voltage breakthrough from diffusing away rapidly into the substrate and thus into the base zones, since the effective path to these base zones  15 , which act as substrate contact, has been markedly extended through the well-shaped region  17 . Thus, the region  10 ″ below the emitter zones  13  and collector zones  12  is filled up relatively quickly with charge carriers, which significantly favors a subsequent firing process. Because a defined and essentially uniformly distributed quantity of charge carriers is situated in the region  10 ″, it can be assured that the bipolar transistors T 1 -T 3  of the ESD protective structure  7  fire quickly and uniformly. In this way, a more uniform and much greater common base resistance RB is achieved for all the bipolar transistors T 1 -T 3  of the ESD protective structure  7 .  
         [0034]    [0034]FIG. 4 illustrates a top view of a layout of an ESD protective structure  7  corresponding to FIG. 2. The entire ESD protective structure  7  preferably has an essentially square layout, which favors uniform current distribution among the various bipolar transistors T 1 -T 3 . In this square layout the emitter zones  13  and the collector zones  12  are formed as strips that are arranged parallel to one another. This assures that none of the bipolar transistors T 1 -T 3  in principle “sees” different substrate effects than the respectively adjoining bipolar transistor T 1 -T 3 . Furthermore, a square layout prevents the voltage drop of the contact metallization from becoming too large and the creation of a non-uniform current distribution, as would be the case for example for a rectangular non-square layout.  
         [0035]    The invention is not limited to square layouts of the ESD protective structures  7 . It is contemplated that non-square layouts such as rectangular, round, oval, or hexagonal or similar layouts may also be used. Furthermore, the emitter and collector zones  12 ,  13  need not necessarily be arranged as strips next to one another, but rather may be arranged in circles, squares, serpentines, fanned-out, or the like. The emitter zones  13 , collector zones  12 , and base zones  15  are through-contacted as much as possible by the respective electrodes  18 ,  19 ,  20  to promote homogenization of the current flow.  
         [0036]    [0036]FIG. 5 illustrates a top view of an arrangement of the conductor tracks for contacting the emitter electrodes and the collector electrodes. This view illustrates two metallizations for the voltage busses  4 ,  5 . Furthermore, FIG. 5 shows the respective strip-shaped emitter electrodes  19  and collector electrodes  18 , arranged next to one another. These electrodes  18 ,  19  are connected, via finger-shaped metallizations  18 ′,  19 ′, to their respectively associated voltage busses  4 ,  5 . In this arrangement the metallization fingers  18 ′,  19 ′ are staggered and are connected to the mutually opposite voltage buses  4 ,  5 , respectively.  
         [0037]    During operation of the ESD protective structure  7  there are unavoidable voltage drops at its electrodes  18 ,  19 , such that usually the greatest voltage difference between emitter and collector appears at the front ends of the electrodes  18 ,  19 . Consequently, the current densities are highest there and the electrodes  18 ,  19  may break down due to scorching. The arrangement of the metallization fingers  18 ′,  19 ′ shown in FIG. 5 essentially precludes this problem, since the metallization fingers  18 ′,  19 ′ are connected only to one of the opposite voltage buses  4 ,  5 . Voltage drops still occur along the respective metallizations, but with an alternating collector electrode  18  and emitter electrode  19  they occur in opposite directions so that a voltage difference between the collector electrode  18  and the emitter electrode  19  remains the same. In addition, the layout measure of FIG. 5 favors a more homogeneous current distribution on the entire surface of the ESD protective structure  7 .  
         [0038]    The ESD protective structure may be arranged next to one another arbitrarily often. It should be noted here that the p-doped base zones  15  are preferably arranged outside, and the collector zones  12  and emitter zones  13  are surrounded by them. Because the base zones  15  surround the zones  12 ,  13 , the ESD protective structure  7  is held at a defined voltage thus assuring improved latch-up protection. However, it is contemplated that the base zones  15  do not completely enclose the zones  12 ,  13 , but such a structure offers lesser latch-up protection.  
         [0039]    The ESD protective structure described in the embodiments has only three bipolar transistors T 1 -T 3 ; in the layout, this ESD protective structure consequently has three emitter fingers and two collector fingers. Of course, the present invention can also be implemented with more or fewer bipolar transistors T 1 -T 3 . An important point here is symmetry, that is the emitter zones  13  and collector zones  12  are arranged as symmetrically to one another as possible. Furthermore, the emitters should lie outside as much as possible, so as to exclude parasitic edge effects.  
         [0040]    The base zones  15 , collector zones  12 , and emitter zones  13  illustrated in FIG. 3 may be introduced into the semiconductor body  10  by diffusion or by ion implantation. The selected doping method is essentially guided in accordance with the particular manufacturing processes for the integrated circuit.  
         [0041]    With respect to the embodiments illustrated in FIGS. 3 and 4, it is contemplated that conductively types n and p may be interchanged. Furthermore, for the sake of completeness, it should be pointed out that the lateral and vertical dimensions in FIG. 3 naturally have not been shown to scale. For example, for the sake of ease of illustration, the vertical dimensions are shown greatly magnified compared to the lateral dimensions.  
         [0042]    The ESD protective structure  7  described above avoids the problems of the inhomogeneous current flow with concomitant rapid destruction of the ESD protective structure. A solution of the problem is achieved almost exclusively by device layout control, and consequently requires no expensive special processes or special masks. Only a very slight extra expense for chip surface is needed here; however, the self-protective effect of the above-described ESD protective structure can be substantially increased thereby.  
         [0043]    The inventive ESD protective structure is especially suited for MOS-/CMOS-integrated semiconductor circuits, which require protection of the supply voltage lines and also of the inputs and outputs. This includes integrated circuits with analog and digital functions with their own analog supply voltage, especially such circuits as operate at a high operating voltage and thus do not have inherent protection.  
         [0044]    The inventive ESD protective structure constructed and operated as described assures optimal ESD protection by the insertion of merely one deep implantation or diffusion for the well-shaped region, without at the same time having to accept the disadvantages of the prior art ESD protective structure with a large number of bipolar transistors.  
         [0045]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.