Abstract:
A high voltage ESD protection diode wherein the p-n junction is defined by a p-well and an n-well and includes a RESURF region, the diode including a field oxide layer formed on top of the p-well and n-well, wherein the parameters of the diode are adjustable by controlling one or more of the junction width, the length of the RESURF region, or the length of the field oxide layer.

Description:
FIELD OF THE INVENTION 
     The invention relates to an ESD protection device for high voltage applications. In particular, it relates to an ESD protection device for high voltage switching regulators that is implemented in a high voltage BiCMOS process. 
     BACKGROUND OF THE INVENTION 
     In the case of ESD protection devices having a control gate, operation mode typically involves either normal mode, wherein the control gate is controlled by a bias voltage to define an active clamp, or snapback mode based on punch-through effect, which occurs when the forward blocking voltage is reached. 
     Apart from these control gate devices, some ESD protection solutions make use of Zener diodes, which operate in avalanche breakdown mode. 
     However the use of Zener diodes becomes problematic when dealing with ESD protection of high voltage, fast switching devices such as fast switching voltage regulators, which during normal operation display switching times in the range of 10 ns-200 ps. Thus they can have switching times that are faster than the ESD rise time (10 ns for HBM pulse). 
     Not only is it difficult to provide a high voltage Zener diode that is suited for such high voltage node ESD protection, they rely on passive switching due to a triggering voltage that exceeds the forward blocking voltage. Thus it is difficult to provide a Zener diode with the desired breakdown characteristics and which avoids being triggered under normal high speed, high voltage operation. 
     The present invention seeks to provide a diode ESD protection solution for high voltage, high speed applications. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a diode defining a p-n junction, the diode comprising a p+ region formed in a p-well, an n+ region formed in an n-well, the p-well and n-well being formed in an n-material that has a lower dopant concentration than the n-well or in a p-material that has a lower dopant concentration than the p-well, wherein the p-n junction includes a low doped region. The diode may further comprise an oxide layer formed on top of the p-well and n-well. The n-material or p-material in which the n-well an p-well are formed may comprise an epitaxial layer. The n-well and p-well may be spaced apart (typically by 0.5 um or less) so that the low doped region of the p-n junction is defined by part of the epitaxial layer. The n-well and p-well may instead overlap so that the low doped region of the p-n junction is defined by virtue of counter-doping. 
     Further, according to the invention, there is provided a method of increasing the breakdown voltage of a diode, comprising forming a p-n junction by forming a p-well and an n-well in a low doped material, and forming a low doped region between the n-well and p-well. The low doped region may be formed by separating the n-well and p-well, or by overlapping the n-well and p-well. The method may include forming an oxide layer on top of the n-well and p-well. 
     Still further, according to the invention, there is provided a method of controlling the parameters of a diode, comprising forming a p-n junction by forming a p-well and an n-well in a low doped material, forming a low doped region between the n-well and p-well, and adjusting the length of the low doped region. The low doped region may be formed by separating the n-well and p-well, or by overlapping the n-well and p-well. The method may include forming an oxide layer on top of the n-well and p-well and adjusting the length of the oxide layer. The method may also include varying the p-n junction length and operating the diode either in avalanche breakdown mode or dual avalanche breakdown mode (conductivity remodulation regime) 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view through one embodiment of an ESD protection structure of the invention, 
         FIG. 2  shows I-V curves for different embodiments of the invention, 
         FIG. 3  shows I-V curves for two different embodiments of the invention operating in avalanche breakdown mode, 
         FIG. 4  shows I-V curves for two different embodiments of the invention operating in conductivity remodulation mode, 
         FIG. 5  is a sectional view through another embodiment of an ESD protection structure of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One type of diode, referred to as a PIN (P+ material-Insulator-N+ material) diode makes use of two avalanche multiplication regions. At a high enough current density one avalanche multiplication region forms near the n+/Insulator junction and one forms near the p+/Insulator junction. The two avalanche regions result in space charge neutralization of the injected carriers, which provides a positive feedback that produces an S-shaped I-V characteristic. This dual avalanche breakdown effect will also be referred to in this application as the conductivity remodulation regime. 
     The advantage of a PIN diode over a Zener is that it produces almost vertical I-V characteristics, which are useful when dealing with high voltage, high speed applications as is the case with ESD protection of switching regulators. Furthermore, since the double avalanche multiplication of the PIN diode is avalanche based it does not have dV/dt side effects such as those found in NPN BJT or SCR devices. 
     However the implementation of such PIN diodes presents a challenge in a non-self-aligned process. 
     The present invention provides for a new type of diode that can be implemented in a high voltage BiCMOS process. The diode of the present invention includes a p+ region and an n+ region separated by a low doped or diluted or RESURF (reduced surface electric field) region. 
     One embodiment of the invention is shown in  FIG. 1 , which shows a cross-section of an ESD structure in accordance with the invention. The structure  100  has a p+ region  102  formed in a p-well  104 , and an n+ region  106  formed in an n-well  108 . The p-well  104  and n-well  108  are formed in an epitaxial layer  110  which is formed according to one embodiment of the invention to extend between the p-well  104  and n-well  108 . As shown in  FIG. 1 , an n-buried layer (NBL)  130  is formed beneath the n-epi  110 . Since the epi region  110  has a lower dopant concentration than the n-well  108  it forms a low doped or diluted or RESURF region  112  between the p-well  104  and n-well  108 . This resultant diluted p-n junction provides for a high breakdown voltage. Also, as is discussed in greater detail below, the p-well/n-well separation can be adjusted to adjust this breakdown voltage, thereby allowing the desired breakdown voltage to be achieved. 
     As shown in  FIG. 1 , the device  100  further includes a field oxide  120 , which serves to avoid surface breakdown. Tests have shown that by adjusting the separation between the p-well  104  and n-well  108 , and by adjusting the length of the filed oxide (LOX) the holding voltage can be adjusted to be only a few volts higher than the breakdown voltage. This is best illustrated by the I-V curves of  FIGS. 2 to 4 . 
       FIG. 2  shows current density against voltage across the device for a p-well/n-well separation of 0.25 um. 
     As shown in  FIG. 2 , as the field oxide length (LOX) increases for a given p-well/n-well separation (in this example a p-well/n-well separation of 0.25 um), the breakdown voltage VBR increases from 47 V for LOX of 1.5 um to 56 V for a LOX of 4. The snapback voltage also increases from 52 V for LOX of 1.5 um to more than 100 V for a LOX of 4. Also, the holding voltage increases from 50 V for LOX of 1.5 um to 66 V for a LOX of 4. In fact, it was found that diodes with a gate oxide length (LOX) in the range of 2-3 um had a holding voltage that was only 5-9 V higher than the breakdown voltage. As shown in  FIG. 2 , curve  200  shows the curve for a LOX of 1.5 um, curve  202  shows the curve for a LOX of 2 um, curve  204  shows the curve for a LOX of 2.5 um, curve  206  shows the curve for a LOX of 3 um, curve  208  shows the curve for a LOX of 3.5 um, curve  210  shows the curve for a LOX of 4 um. 
     As mentioned above, the diode of the present invention can be operated in the conductivity remodulation regime. This is of particularly importance when the device has to be small. This allows a p-well to n-well junction width of 100 um to be achieved while providing for operating current levels of 2-3 A. In the conductivity remodulation regime, once maximum avalanche current is reached in the low doped region between the p-well and the n-well, the peak of the electric field shifts from the center of this low doped region to the junction between the p+ region  102  and the p-well  104 , and to the junction between the n+ region  106  and the n-well  108 . This increases the conductivity of the low doped region and of the diode overall, which accounts for the near vertical I-V characteristic shown in  FIG. 3 . 
       FIG. 3  shows the I-V characteristics for a diode of the invention with a p-n junction of 100 um, operating in the conductivity remodulation regime. It also shows the effect of changing the field oxide length from 2 um to 2.5 um. Curve  300 , which shows the I-V curve for a LOX of 2.5 um has a breakdown voltage of about 73 V compared to a breakdown voltage of only 63V at a LOX of 2 um (curve  302 ). 
     While the above discussion dealt with the conductivity remodulation regime in which there is a dual avalanche breakdown, the diode of the invention can instead be operated in normal avalanche breakdown regime, like a Zener diode. This has the advantage that the operating characteristics are very stable and the device is easy to implement since it is based on p-n junction breakdown. However, in avalanche mode the resistance is very large, therefore to accommodate the ESD current levels of 2 A the width of the device has to be significantly larger than in the case of the conductivity remodulation regime. In particular, in one embodiment a p-n junction width of 10 mm was used. The I-V characteristics for this embodiment are shown in  FIG. 4 , which shows the less steep I-V characteristics of the single avalanche breakdown. Again the breakdown voltage increases as the LOX increases from 2 um (curve  400 ) to 2.5 um (curve  402 ). However, the breakdown voltage is lower for both than for the conductivity remodulation regime shown in  FIG. 3 . Also the impact of increasing the LOX is less significant. As shown in  FIG. 4 , at a LOX of 2 um the avalanche breakdown occurs at about 50 V, while at a LOX of 2.5 um the breakdown occurs at about 54V. In the embodiment discussed with respect to  FIG. 1 , the low doped or RESURF region was formed by separating the p-well from the n-well using an n-epi region having a lower doping level than the n-well. 
     In the embodiments discussed above, the low doped regions are formed by spacing the p-well and n-well apart in a lower doped epi material. In another embodiment the low doped region is formed by overlapping the p-well and n-well, which has the effect of reducing the doping level in the overlap region due to counter doping. Such an embodiment is shown in  FIG. 5 . In this embodiment, the device  500  includes a p+ region  502  formed in a p-well  504 , and an n+ region  506  formed in an n-well  508 . The p-well  504  and n-well  508  are formed in an n-epitaxial layer  510  and overlap in the region indicated by reference numeral  512  to form a low doped region. While in this embodiment the p-well  504  and n-well  508  were formed in an n-epi layer, it will be appreciated that the wells could instead be formed in a p-epi layer or in an n-substrate or p-substrate. As in device  100 , a field oxide  520  is formed between polysilicon regions  522 ,  524 . The operation of the device  500  remains similar to that of device  100 . Thus the LOX and spacing between p-well  104  and n-well  108  can again be adjusted to control the breakdown voltage and, in the case of conductivity remodulation regime, also the snapback and holding voltage. 
     It will be appreciated that while specific embodiments were discussed for implementing the diode of the invention, these were by way of example only and other embodiments could be implemented without departing from the scope of the invention.