Abstract:
In an avalanche structure, different breakdown voltages are achieved by making use of a polygate and forming a highly doped p-n junction beneath the polygate, and adjusting the gate length and optionally the bias voltage of the gate.

Description:
BACKGROUND OF THE INVENTION 
   Avalanche diodes, which are sometimes loosely referred to as Zener diodes are commonly used for providing a reference voltages for analog circuits. One particular application is as part of an ESD protection clamp. 
   An important consideration in designing semiconductor circuits, however, is the need to avoid introducing special process steps that would increase the overall cost. Thus it is desirable to be able to include so-called free structures by making use of existing process steps. In a CMOS process, however, only limited variations can be made to available regions in order to form diodes. For instance, diodes can be created using n+/p-well, p+/n-well, p-well/n-well, and, to some extent, n+/p+ junctions by spacing the n+ and p+ regions far apart to avoid tunneling. In a 0.18 um process these combinations typically provide breakdown voltages of approximately 12V, 12V, 17V, and 4V, respectively. As a result avalanche diodes are available only with discrete breakdown voltage values. 
   In the case of power supply electrostatic discharge protection (ESD) clamps, breakdown voltages in the range of about 5V–10V are, however, required. Conventional diodes thus fail to provide the requisite breakdown voltages. 
   One proposed prior art solution is to make use of n+/p+ as the diode and make use of a blocked space such as a shallow trench isolation region (STI)  100  between the n+ region  102  and p+ region  104  as shown in  FIG. 1 . However, this is usually not possible, especially in the case of small dimension devices, due to inadequate tolerance in the mask alignment. 
   The present invention seeks to address the problem of providing suitable breakdown voltages for avalanche diodes without adding additional process steps to the CMOS process. 
   SUMMARY OF THE INVENTION 
   The present invention comprises an avalanche diode structure, wherein the structure is adjustable to provide for a wide range of breakdown voltages. In particular, by adjusting the blocking junction, different breakdown voltages can be realized. This is achieved by forming n+ and p+ regions and making use of a polygate in a CMOS process to form an abrupt junction. The gate can, further, be provided with a contact and its voltage adjusted. For instance, the gate can be connected to the cathode or anode or to an external bias circuit to adjust the breakdown voltage. 
   Thus, according to the invention, there is provided an avalanche diode structure comprising a p+ and a n+ region under a polysilicon region. For ease of description, the polysilicon region will be referred to as a polygate since it is formed in a CMOS process in the same way as any other polygate would be formed. However, the polygate of the present diode structure need not necessarily be provided with a contact. 
   The p+ and n+ regions are typically formed in lightly doped regions, referred to as PLDD (p-lightly doped region) and NLDD (n-lightly doped region), respectively. 
   Further, according to the invention, there is provided a method of forming an avalanche diode, comprising providing a polygate and using the polygate as a self aligned mask during doping of the p-n junction of the diode. The masks for the oppositely doped regions of the junction are preferably positioned so as to overlap with the polygate. Preferably the doping of the p-n junction comprises forming n+ and p+ regions in corresponding lightly doped regions. The lightly doped regions are preferably formed during a high voltage portion of the CMOS process. The method may include adjusting the gate length. 
   The invention, further, provides for adjustment of the breakdown voltage of an avalanche diode of the invention by suitably biasing the polygate. The gate may be connected to either the anode or the cathode of the diode structure, or may be connected to a driver circuit that biases the polygate to provide dynamic breakdown voltage control. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section through a prior art avalanche diode; 
       FIG. 2  shows a cross section through one embodiment of the invention; 
       FIG. 3  shows a cross section through another embodiment of the invention; 
       FIG. 4  shows graphs of drain current against source-drain voltage for different gate lengths, and 
       FIG. 5  shows graphs of drain current against source-drain voltage for different gate bias. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One embodiment of the invention is shown in  FIG. 2  which shows the p-n junction of the diode  200  formed between a p+ region  210  formed in a lightly doped region  212  referred to as a p-lightly doped drain (PLDD), and a n+ region  220  formed in a lightly doped region  222  referred to as a n-lightly doped drain (NLDD). 
   The need for the PLDD and NLDD regions  212 ,  222  can be ascribed to the CMOS process. In order to avoid contamination across the junction during the high doping process, the formation of the p+ and n+ regions  210 ,  220  is typically preceded by the formation of lightly doped regions, referred to as PLDD (p-lightly doped region) and NLDD (n-lightly doped region), respectively. 
   As is evident from  FIG. 2 , a polygate  230  is formed over the region where the p-n junction will be located. The polygate  230  has a self aligning mask function. Even though separate masks are used during the doping of the p+ and PLDD and n+ and NLDD regions, it becomes difficult to properly align them, especially at small dimensions. By making use of the polygate as a mask, the exact alignment of the masks is no longer critical. The masks for the oppositely doped regions (not shown) applied to the opposite sides of the polygate  230 , are preferably positioned so as to overlap with the polygate  230 , using the polygate  230  as a mask that provides a certain amount of self-alignment. 
   In one embodiment the lightly doped regions, PLDD  212  and NLDD  222  are formed during a high voltage portion of the CMOS process to provide for greater flexibility in achieving the desired breakdown voltage. A typical semiconductor circuit may include a core and an I/O structure. These two portions typically operate at different voltages. The core typically operates at a lower voltage dictated by the process, e.g. for a 0.18 μm process the voltage is 1.8V±10%, while the I/O structure may operate at a higher voltage of 3.3V or 5V. For a 0.25 μm process the core voltage is 2.5V±10%, while the I/O voltage will again be at a higher voltage of 3.3V or 5V. These different portions will be implemented by varying the process steps in order to accommodate the low and high voltage levels, respectively. For instance, in the case of a high voltage structure, the gate oxide has to be thicker and is typically implemented by making use of a dual or triple oxide. For example in the case of a 0.18 μm process, the gate oxide for the low voltage part has a length of 0.18 μm and a thickness of 30 Å, while the gate oxide for the high voltage part has a length of 0.35–0.4 μm and a thickness of 70 Å. Also, the doped regions will be adapted to the different operating voltage. During a high voltage process, more dopant extends under the gate from either side of the gate  230 . Thus, for example in a 0.18 μm process, a junction width between the p+ and n+ regions  210 ,  220  of approximately 0.15 μm is achieved even with a polygate length of 0.35 um. In contrast, for a low voltage implantation in a 0.18 μm process the junction width will remain rather large (approximately 0.1 μm) even with a polygate length of only 0.18 μm. 
   Thus by reducing the length of the polygate in a high voltage process, smaller distances and even overlaps between the PLDD and NLDD can be achieved. 
   This provides an abrupt junction with minimum breakdown voltage of approximately 5V in a 0.18 um process. On the other hand, the gate length can be increased to provide for a more gradual doping distribution near the p-n junction region. This allows the breakdown voltage to be increased up to the well-to-well breakdown voltage level. Thus, the invention provides an avalanche diode structure for which the breakdown voltage can be adjusted in relation to the polygate length. In other embodiments, the PLDD and NLDD regions were formed during a high voltage portion of the process, while the polygate was formed during a low voltage portion of the process. Other embodiments formed some of the doped regions (n+, p+, NLDD, PLDD, n-well, p-well) during a high voltage portion of the process and others during a low voltage portion. Thus, for example the PLDD may have been formed during a low voltage portion of the process, while the NLDD was formed during a high voltage portion of the process. 
   The effect of gate length changes is illustrated by the graphs of  FIG. 4 , which show TCAD analyses for different gate lengths for diodes of the invention. In particular,  FIG. 4  shows curves of drain current against source-drain voltage for different gate lengths, while biasing the gate to a voltage of 1V. Curve  400  shows the current voltage curves for a 0.1 μm gate length, curve  402  shows the curves at a polygate length of 0.5 μm, and curve  404  shows the curve for a polygate length of 0.8 μm. Thus, it is clear that the breakdown voltage can be reduced by reducing the polygate length of the diode. For instance, in the embodiment of  FIG. 2 , the polygate length is reduced or increased to provide for breakdown voltages as low as approximately 5V. 
   Another embodiment of the invention is shown in  FIG. 3  which shows the p+ region  310  and PLDD  312  formed in a p-well  340 , while the n+ region  320  and NLDD  322  are formed in a n-well  350 . In this embodiment the junction between the p-well  340  and n-well  350  is located under the polygate  330 . However the junction could be shifted to the left or the right. Thus the  FIG. 2  embodiment can be seen as the extreme case where the junction has been moved all the way to the left so that the n-well covers both the cathode and the anode regions, and the p-well is eliminated altogether. Simulation results show that increasing the gate length of the polygate  330  allows breakdown voltages of up to the well to well breakdown voltage to be achieved. It will be appreciated that where the p-well is eliminated altogether the upper limit to the breakdown voltage will be the breakdown voltage of the PLDD-n-well junction. Below some critical gate length the breakdown voltage will essentially be a function of the polygate length. As mentioned above, reducing the polygate length allows breakdown voltages to be reduced to approximately 5V. 
   The invention, further, provides for adjustment of the breakdown voltage by suitably biasing the polygate. In one embodiment the gate is connected to either the anode or the cathode of the diode structure to act as a field electrode. This allows the breakdown voltage to be further decreased or increased. 
   In another embodiment the polygate is connected to a driver circuit that biases the polygate to provide dynamic breakdown voltage control. This, in turn, allows the triggering of the diode to be controlled. 
   The effect of changing the gate bias on the breakdown voltage is shown in  FIG. 5 .  FIG. 5  shows TCAD analyses of different devices of the present invention, showing the effect of changing the polygate bias. Curves  500 – 506  show drain current against source-drain voltage curves for a gate length of 0.8 μm. In curve  500  the polygate is biased to a voltage of −5V; in curve  502  the gate bias is −1V; in curve  504  the gate bias is 0V, and in curve  506  the gate bias is 1V. Thus, it can be seen that the breakdown voltage can be increased by increasing the bias of the gate. 
   While the invention has been described with respect to a few specific embodiments, it will be appreciated that different configurations could be provided without departing from the scope of the invention.