FIELD OF THE INVENTION 
     The invention relates to an ESD protection structure. In particular, it deals with a high voltage structure that has bi-directional characteristics. 
     BACKGROUND OF THE INVENTION 
     Recent trends in analog product design include the combination of all typical functional blocks of an analog circuit inside a single chip. Ideally, analog chips have to be compatible with both the input/output information signals as well as the power supply of the system. For instance, an analog chip may have to provide the power supply for an output device such as a USB device. When dealing with inductive loads, for example, it is not uncommon to require a 40–60V ESD protection structure to protect the input pads of the integrated circuit (IC) while the core of the circuit uses a 5V process. A typical inductive load situation is the pulse width modulator circuit shown in  FIG. 1 , which includes an inductive load  100  and a switching NMOS transistor  102 . The output signal is derived by means of an operational amplifier  104  connected across a resistor  106 . A free-wheeling diode  108  provides continuity for current flow when the transistor  102  switches off. 
     The need for a high voltage solution is becoming particularly acute in the motor vehicle industry where the number of electronic components is not only increasing but a new high voltage standard of 42V is being promoted. 
     Clearly high voltage technology exists to accommodate high voltage applications, however this high voltage technology is expensive and therefore not always viable, especially in low cost applications such as imaging and low cost sensors. Also, the option of using multiple chips to convert to different voltage levels is not an optimal solution. 
     Apart from the cost issue associated with high voltage technology, and the use of multiple chips to convert between voltage levels, an additional consideration is the ability to provide bi-directional ESD protection. One prior art bi-directional structure is the NPN triggered structure, which has a triggering and breakdown voltage of about 30V. 
     As pointed out above, the motor vehicle industry is moving toward a 42V standard, thus there is a need to provide a cost effective high voltage bi-directional ESD protection structure. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a bi-directional ESD protection device comprising at least two DIAC devices, each DIAC device comprising a first and a second p-well separated by an n-well, the p-wells and n-well being formed in a p-substrate and separated from the substrate by an n-isolation layer, wherein each p-well has a p-buried layer formed under it and each p-well includes a p+ region and an n+ region. The n+ regions in the two p-wells are preferably on the inside to define a p+, n+, n+, p+ configuration, however other configurations may be used to achieve different voltage distributions. 
     Preferably at least one additional p+ region in at least one p-well is formed between the two DIAC devices. Preferably said additional p+ region comprises a p+ ring formed in a p-well surrounding each of the DIAC devices. Typically said p+ region is connected to ground. 
     Typically all of the p+ and n+ regions in the first and second p-wells are connected together, e.g., by means of a first metal layer. The n+ and p+ regions in the first p-well of the one DIAC are typically connected to an input pad, e.g., by means of a second metal layer. The n+ and p+ regions in the second p-well of the other DIAC are typically connected to a ground pad, e.g., by means of the second metal layer. The n+ and p+ regions of the second p-well of the first DIAC are typically connected to the n+ and p+ regions in the first p-well of the other DIAC, e.g. by means of the second metal layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art schematic circuit diagram of an inductive load device; 
         FIG. 2  is a section through a symmetrical DIAC structure known in the art; 
         FIG. 3  is a section through one embodiment of a cascaded structure of the invention; 
         FIG. 4  is a simplified plan view of the structure of  FIG. 3 , and 
         FIG. 5  shows I–V curves for a device of the invention compared to a prior art device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows and symmetrical p-n-p structure  200 , which, for convenience, will be referred to here as a DIAC, comprising two p-wells  202 ,  204 , separated by an n-well  206 , and which are all formed in a p-substrate  208 . In order to achieve the bi-directional characteristics, a n+ and a p+ region are formed in each of the two p-wells,  202 ,  204 . Thus p-well  202  has n+ region  212  and p+ region  214 , while p-well  204  has n+ region  216  and p+ region  218 . 
     To avoid p-well punch through, a p-buried layer (PBL)  220  is formed under each p-well  202 ,  204 . Also, since the p-substrate  208  is grounded, an n-isolation layer (NISO)  222  is included under the p-wells  202 ,  204 , and n-well  206  to avoid n-well punch through. 
     In accordance with the present invention, a high voltage bi-directional structure is achieved without making use of expensive high voltage processes, by providing a cascaded structure, one embodiment of which is shown in  FIG. 3 . 
       FIG. 3  shows a simplified representation of a sectional view through a cascaded DIAC structure of the invention. The structure  300  comprises two DIAC structures  302 ,  304 , each having two p-wells, separated by an n-well. Thus DIAC  302  has p-wells  306 ,  308  separated by n-well  310 . DIAC  304  has p-wells  312 ,  314  separated by n-well  316 . As in the DIAC structure shown in  FIG. 2 , p-buried layers  320  are formed beneath each of the p-wells. Also, the p-wells and n-well of each DIAC are separated from the p-substrate  330  in which they are formed, by an n-isolation layer (NISO)  332 . It will be appreciated that even though structurally the device of  FIG. 3  is similar to two DIACs side by side, this does not suggest that the two DIACs are separately formed first and then connected. They form part of a single structure, the DIAC elements being formed simultaneously. Thus, for example, it will be apparent that the p-substrate simply continues across where the one DIAC extends into the other one. The NISO  332  also does not continue across both structures. 
     The bi-directional nature is retained by providing a symmetrical structure with n+ regions  334  and p+ regions  336  in each of the p-wells  306 ,  308 ,  312 ,  314 . 
     In order to ground the p-substrate  330 , a p+ring  340  is formed in an additional p-well  342 . Since the p+ring  340  is simply connected to ground, the additional p-well  342  does not have a p-buried layer (PBL) in this embodiment. However, in another embodiment a PBL was included. 
     The configuration of the p+ ring is more clearly shown in  FIG. 4  which shows the structure in plan view.  FIG. 4  also shows the p+ regions  336  and n+ regions  334  in the p-substrates  306 ,  308 ,  312 ,  314 , and the p-buried layers  220  under the p-substrates. The n-well  310  extends between and around the p-wells  312 ,  314 , and is isolated by the NISO  332 . Similarly the n-well  316  extends around the p-wells  306 ,  308 , and is isolated by the other NISO  332 . The p+ ring  340  is, in effect two rings around each of the DIACs, as shown in  FIG. 4 . Thus there are actually two p+ regions side by side between the two DIACs. As shown in  FIG. 4 , the p+ rings are formed in a p-wells  342  which also surrounds the DIACs. For ease of understanding, the two DIACs are labeled with reference numerals  350  and  352  in  FIGS. 3 and 4 . 
     Referring again to  FIG. 3 , the n+ and p+ regions  334 ,  336  formed in the p-well  306  are connected together by a first metal layer (metal  1 ) and are connected to the input pad by a second metal layer (metal  2 ) Metal  1  also connects the n+ and p+ regions in the other p-wells  308 ,  312 ,  314 . Metal  2  also connects the n+ and p+ regions in p-well  314  to the ground pad, and connects the n+ rings  340  in the p-wells  342  to ground, as shown in  FIG. 3 . 
     The high voltage, bi-directional characteristic of the cascaded device of  FIGS. 3 and 4  is shown by the Current-Voltage curve in  FIG. 5  (curve  500 ) compared to the non-cascoded structure of  FIG. 2  (curve  502 ). 
     While the invention was described with respect to a particular embodiment, it will be appreciated that the invention covers all configurations within the scope of the claims.
In a high voltage ESD protection solution, a plurality of DIACs are connected together to define a cascaded structure with isolation regions provided to prevent n-well and p-well punch through. An p-ring surrounds the DIACs and provides a ground for the substrate in which the DIACs are formed.