Abstract:
An electrostatic discharge (ESD) protective structure is configured and arranged to protect an integrated semiconductor circuit that is located between a first potential bus with a first supply potential and a second potential bus with a second supply potential. The ESD protective structure includes a laterally shaped ESD diode having a first region with a first conduction type and a second region of a second conduction type spaced apart from the first region. The ESD protective structure is located between the potential busses and is provided with a gate electrode, such that the first region and the second region are adjusted with respect to the gate electrode, and the spacing between the first region and the second region corresponds to the length of the gate electrode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    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.  
           [0002]    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.  
           [0003]    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).  
           [0004]    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 nominal the 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.  
           [0005]    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.  
           [0006]    When thyristors or transistors are used, fast noise voltage pulses may cause these devices to switch on or fire inappropriately, even though their switch-on or firing voltage, as determined by characteristic measurements in the low-current region, lies outside the specified signal voltage range. This so-called transient latch-up, which may last an arbitrarily long time, causes a short circuit of the operating voltage and, as a rule, destroys the ESD protective element. As a result, the function of the integrated circuit is no longer assured. In the extreme case, the integrated circuit and even its voltage supply can be irreversibly damaged from strong heating (of the integrated circuit). Due to their transient latch-up, ESD protective elements that have a decided snap-back behavior, for example thyristors or bipolar transistors, are not well suited as ESD protective elements, despite their high ESD strength and good protective action.  
           [0007]    ESD diodes, which up to now have been used for integrated semiconductor circuits, do not share these problems. Such a generic ESD diode is described in the DE 41 35 522 A1.  
           [0008]    However, a problem with using pn diodes as ESD protection is that pn diodes cannot be integrated or can be integrated only with great expense in standard CMOS or MOS processes, since they are generally present as parasitic structures in CMOS or MOS processes. When such pn diodes are produced for ESD protection the process parameters must be tightly controlled, since in principle these are parasitic pn structures in a CMOS process. In addition, prior art pn diodes have an inadequate clamping action that can only be overcome by using special masks, epitaxial wafers, etc. Finally, parasitic surface charges result in leakage currents that cause a walk-out effect or diode leakage current, especially at high voltages. In addition, prior art pn diodes have a poor blocking characteristic (i.e., an undefined blocking characteristic) so that pn diodes as ESD protection suffer from considerable reliability problems.  
           [0009]    Therefore, there is a need for an improved and easily manufactured ESD protective structure.  
         SUMMARY OF THE INVENTION  
         [0010]    Briefly, according to an aspect of the present invention, an ESD protective structure includes a laterally shaped ESD diode having a first region doped with a first conduction type and a second region doped with a second conduction type spaced apart from the first region. The ESD protective structure is located between first and second voltage potentials and the structure includes a gate electrode, such that the first region and the second region are adjusted with respect to the gate electrode, and the spacing between the first region and the second region corresponds to the length of the gate electrode.  
           [0011]    The ESD diode preferably has a relatively simple structure that is easily manufactured, since the layout of a known MOS transistor is employed wherein the conductivity type is exchanged at the source region or the drain region.  
           [0012]    Since the inventive ESD protective structure is preferably configured as a pn diode, problems with permanent short circuits which, for example, damage the voltage supply or the integrated circuit, are eliminated. The ESD diode adequately limits the voltage such that the integrated circuit is not damaged. Therefore, the ESD diode is especially suitable for high voltage applications.  
           [0013]    Significantly, the ESD diode requires no special process steps or additional masks. In addition, since the ESD diode is derived from a conventional MOS transistor, which is a main element of an MOS or CMOS process, the diode “inherits” the properties of the MOS transistor, whose process engineering is well known. In particular, critical pn junctions to the substrate and the regions of the gate oxide near the surface fortunately do not differ from those of known MOS transistors.  
           [0014]    Diffusion or implantation regions over the gate electrode may be produced in a self-adjusting manner. In MOS and CMOS processes, production of the gate oxide and of the gate electrode is subject to stringent process control. In this way, ESD diodes with well defined lateral dimensions can be produced, since dimensional deviations would be detected within the framework of the continuous MOS parameter measurements. Other potential problems of the diode, such as for example leakage currents, surface charges, impurities, etc. would affect the MOS transistors of the integrated circuit in the same measure, and as a result would be detected with standard process control techniques. Consequently, the ESD diodes require no additional measurements on the wafer.  
           [0015]    The gate electrode is short-circuited with the electrode that contacts the deliberately wrongly doped region. Accordingly, it can be assured that undesirable surface effects and the resulting leakage currents are essentially avoided.  
           [0016]    In one embodiment the ESD structure is embedded in its own well, and the appropriate conductivity types are matched to it. Therefore, the embedded ESD structure is insulated from the substrate.  
           [0017]    One region of the ESD diode, typically the anode region, may be completely surrounded by the region that forms the cathode to provide increased latch-up protection.  
           [0018]    The integrated circuit and the ESD diode are preferably produced in MOS or CMOS process technology, and a gate dielectric that includes silicon dioxide is present and separates the semiconductor body from the gate electrode. The gate electrode is may be designed as a polysilicon gate. The invention may be applied to other process technologies (i.e., the gate electrode and the gate oxide may include other materials). It is contemplated that ESD diodes without gate oxide may also be used.  
           [0019]    Advantageously, the ESD diode is easily manufactured since additional process steps are not required (e.g., implantation to produce an ESD Zener diode or the production of a buried layer or an epitaxial layer is not required). As a result, the ESD diode may be economically produced.  
           [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 diode;  
         [0022]    [0022]FIG. 2 illustrates a partial section of a first embodiment of an inventively integrated ESD diode;  
         [0023]    [0023]FIG. 3 illustrates a partial section of a second embodiment of an inventive integrated ESD diode; and  
         [0024]    [0024]FIG. 4 illustrates a partial section of a third embodiment of an inventive integrated ESD diode. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    [0025]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.  
         [0026]    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 .  
         [0027]    [0027]FIG. 2 illustrates a partial section of a first embodiment of an inventive integrated ESD diode. A semiconductor body  10  includes a weakly-doped silicon substrate. The semiconductor body  10  has a wafer front side  11  within which are embedded a first, a  5  doped region  12  and a second, ,-doped region  13 . The regions  12 ,  13  are spaced apart from one another by a spacing distance W that defines a drift region  14 . The drift region  14  includes the background doping (i.e., the doping of the semiconductor body  10 ). The first region  12  forms a cathode and the second region  13  together with the drift region  14  form an anode of the ESD diode. The second region  13  has a higher doping concentration than the drift region  14 . Electrodes  17 ,  18  are attached to the surface  11  in the first region  12  and the second region  13 , respectively. The electrode  17  is connected to VCC, while the electrode  18  is connected to VSS.  
         [0028]    A gate oxide  15  is disposed over the drift region  14 , and a gate electrode  16  is disposed on the gate oxide  15 . The gate oxide  15  may include silicon dioxide, and the gate electrode  16  may include polysilicon. The gate electrode  16  is electrically connected to the electrode  18  of the second region  13 , and thus to the second supply potential VSS.  
         [0029]    The ESD diode  7  is established through the regions of the elements  12 - 16 . The regions outside the ESD diode  7  adjoining the surface  11  are covered by a field oxide  19  that laterally passivates the semiconductor body  10 . Furthermore, the entire ESD diode  7  may be covered by a passivation (not shown). The function of the inventive lateral ESD diode  7  shall now be explained in more detail below.  
         [0030]    A parasitic noise signal may arise on the semiconductor chip through a potential bus  2 ,  3 , for example during transport or during handling, or also through a fluctuation in the supply voltage. If a parasitic noise signal is coupled into the integrated circuit  1 , and the signal exceeds the breakdown voltage of the ESD diode  7 , then the space charge zone at pn junction  20  between the first zone  12  and the drift zone  14  breaks down. This results in a current path from the first potential bus  4 , via the first zone  12 , the drift zone  14 , and the second zone  13 , to the second potential bus  5 . The ESD diode  7  thus protects the integrated circuit  1  from a parasitic overvoltage by draining this voltage off to the potential busses  5 , so the overvoltage does not damage the integrated circuit.  
         [0031]    Significantly, undesirable surfaces are eliminated (e.g., parasitic surface charges and surface leakage currents resulting therefrom), since the gate electrode  16  is disposed above the drift zone  14  and is connected to the second supply potential VSS. This increases the reliability of the ESD diode.  
         [0032]    Under normal operating conditions the ESD protective element(s) must not impair the function of the integrated semiconductor circuit  1 . Consequently, the breakdown voltage of the ESD diode  7  must be located within a voltage range whose lower limit is the maximum signal voltage coupled into the integrated circuit  1 , and whose upper limit is characterized by the minimum breakdown voltage of the integrated circuit  1 . The breakdown voltage of the ESD diode  7  is adjusted/set, so that despite process variations in the production of the integrated circuit  1  and the ESD diode  7 , the system still functions properly.  
         [0033]    The breakdown voltage of the ESD diode  7  can be adjusted through the doping concentrations in the zones  12 ,  13 , and  14 . However, there is an important boundary condition in the production of ESD protective structures, in that the process management should not be changed through the insertion of the ESD protective structures, of course if at all possible. Thus, the doping concentrations and/or the thickness of the gate oxide  15  should not be changed, if possible.  
         [0034]    The layouts of customary lateral MOSFETs that are also used for the corresponding MOS or CMOS circuit are preferably used as the ESD elements. However, in the layout of these MOSFETs, the conductivity type of the source region or of the drain region is exchanged. The pn diode  7  as ESD protection is thus produced by a relatively simple technique, and it is produced in MOS or CMOS technology. Using the MOS or CMOS technology of the protected circuit  6  assures that the process parameters of the ESD diode  7  likewise are easily controlled and adjusted.  
         [0035]    A conventional field oxide, for example produced in LOCOS technology, or also any other masking (e.g., a photoresist) can be used as a doping mask. It is preferred that the gate oxide  15  is applied first, and then the gate electrode  16  is applied on the gate oxide  15 . Then, using the gate electrode  16  as a mask, the first zone  12  and the second zone  13  are attached to the semiconductor body  10 .  
         [0036]    Using the above doping mask, the doping atoms for the a-doping of the first zone  12  and the P-doping for the second zone  13  are introduced into the semiconductor  1  by a diffusion process, preferably in two separate process steps. When diffusing, the doping atoms distribute themselves laterally and vertically homogeneously over the corresponding regions  12 ,  13 . However, it is also conceivable to introduce the doping atoms into the semiconductor bodies  1  by ion implantation. Ion implantation introduces an accurately measurable dopant dose into the semiconductor body  1 , to provide the switching threshold of the ESD diode. However, ion implantation must be followed by a suitable temperature step so the dopant atoms are electrically activated and crystal damage created by the ion bombardment is healed. Whether ion implantation or diffusion is used to produce the regions  12 ,  13  typically follows from the particular process that is utilized to manufacture the integrated circuit  1 .  
         [0037]    [0037]FIG. 3 illustrates a partial section through a second embodiment of an ESD diode. A weakly α-doped well  21  is inserted into the semiconductor body  10 , for example by diffusion. The mutually spaced zones  12 ,  13  are embedded in this well  21 . In contrast to the embodiment illustrated in FIG. 2, in the embodiment of FIG. 3 the conductivity types of the first zone  12  and of the second zone  13  are exchanged (i.e., the first zone  12  is now P-doped and the second zone  13  is a-doped). This establishes the ESD diode  7  embedded in the well  21  and electrically insulated from the substrate of the semiconductor body  10 .  
         [0038]    [0038]FIG. 4 illustrates a third embodiment of the ESD diode  7 , corresponding to FIG. 2, in which the first zone  12  is laterally enclosed by the second zone  13 . In this way, the substrate all around the ESD diode  7  is maintained at a defined potential to provide improved latch-up protection.  
         [0039]    The ESD structure can have a circular, annular, square, serpentine, finger-shaped, fanned-out, or similar layout.  
         [0040]    In the present embodiment a laterally shaped ESD diode  7  has been described. However, it is contemplated that a vertically formed ESD diode  7  may also be used, although this would be more complicated. Vertically designed ESD structures, using an epitaxial layer and/or a buried layer may be used especially for integrated semiconductor circuits  1  that are designed in trench technology.  
         [0041]    Possible new embodiments naturally can be created from all the embodiments of FIGS.  2 - 4  by interchanging the a and P conductivity types.  
         [0042]    The ESD diodes are especially suitable for MOS/CMOS-integrated circuits, which require protection of the supply voltage networks against ESD overvoltages. This includes integrated circuits with analog functions and having their own analog supply voltage, especially analog circuits that operate with a high operating voltage and thus practically have no inherent protection. The invention is also suited for digital integrated semiconductor circuits with a high-voltage supply network.  
         [0043]    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.  
         [0044]    What is claimed is: