Abstract:
In an integrated circuit, dopant concentration levels are adjusted by making use of a perforated mask. Doping levels for different regions across an integrated circuit can be differently defined by making use of varying size and spacings to the perforations in the mask. The diffusion of dopant is completed by making use of an annealing stage.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method of controlling doping levels in an integrated circuit. It also relates to a method of controlling the breakdown voltage between the active region and the substrate of a snapback device. 
   BACKGROUND OF THE INVENTION 
   Recent trends in integrated circuits is the inclusion of all analog functional blocks in a single chip. In doing so, however, consideration has to be given to the fact that there are different voltage requirements for different functional blocks on a chip. For instance, the power supply voltage will typically be quite different from the signal voltage of the input and output signals. The analog blocks may therefore be required to provide the necessary power supply for certain output devices such as USB devices. This is seen, for example, in the automotive industry where new standards require a supply voltage level of 42 V. Thus integrated circuits, which now abound in motor vehicles, have to be compatible with the particular power supply level. The need for a cost-effective solution becomes particularly significant in the case of low cost electronics such as imaging and low cost sensors. Using high voltage processes for such applications is therefore economically not feasible. An alternative approach is to use a multiple chip solution to perform the voltage conversion. However, this is not only cumbersome but also costly. 
   A cost effective solution to handling different voltage levels is therefore required. For example, National Semiconductor Corporation deals with bi-directional ESD protection devices implemented in a 5 V process that have to provide 60V ESD protection at the input pads while the core of the chip still uses 5V. 
   More generally, it is desirable to be able to provide a semiconductor chip with different functional blocks operating at different voltage levels or having different breakdown voltages. Furthermore it is desirable to achieve this without having to incur additional process steps, such as additional mask or doping steps. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of varying doping levels across an integrated circuit (IC) by using a perforated mask with varying ratios of masked portion to unmasked portion. Thus some regions of the mask will have more unmasked portions to allow more dopant to pass through the mask, and some regions will have more masked portions and block more of the dopant. 
   Further, according to the invention, there is provided a method of varying the breakdown voltages of snapback devices on an integrated circuit by using a mask of varying degrees of perforation during formation of isolation layers. This allows the dopant concentration to be varied for isolation layers between, for example, a well and a substrate. 
   Still further, according to the invention, there is provided a method of controlling the breakdown voltage of a snapback device, comprising controlling doping levels of an isolation region by using a perforated mask during doping of the isolation region. 
   Still further, according to the invention, there is provided a method of increasing the breakdown voltage of a snapback device (e.g., an ESD protection or high voltage device) comprising forming an isolation layer between active regions (e.g. n-wells) and substrate of the device, by means of spotted implants. The spotted implants are typically achieved by making use of a mask with intermittent openings. Preferably the spotted implants are provided before one or both of an epitaxial layer being grown and high diffusion drive taking place. 
   Still further, according to the invention, there is provided a method of controlling the doping level of a doped region in an integrated circuit, comprising using a perforated mask during doping of the doped region, with the mask having a predefined ratio of perforation to mask material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a sectional view of a typical DIAC ESD device without isolation layer, 
       FIG. 2  shows a sectional view of a typical DIAC ESD device with isolation layer, 
       FIG. 3  shows a sectional representation of a snapback device showing a process step of the invention involving a perforated mask, 
       FIG. 4  shows a sectional representation of a snapback device showing a process step of the invention illustrating the dopant implantation, 
       FIG. 5  shows the device of  FIG. 4  after high diffusion drive, 
       FIG. 6  shows the device of  FIG. 5  after the remaining process steps have been completed, and 
       FIG. 7  shows current against voltage curves for a device of the invention compared to prior art devices. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a way of controlling the doping level for a particular region of a device. For example, an isolation region with a particular doping density can be formed between an active region and a substrate of a snapback device such as a DIAC. This is achieved by making use of a perforated mask to act like a sieve or filter, thereby limiting the amount of dopant that the region is exposed to. By appropriately choosing the ratio of unmasked to masked portion over the device region (i.e. by controlling the size and density of the perforations in the mask), the present invention achieves a desired doping density. 
   In choosing the perforation size and spacing between perforations, the typical distribution profile of dopant in the semiconductor material is taken into consideration. As the dopant diffuses into the semiconductor material it assumes a Gaussian distribution profile in all three dimensions, as a function of time and temperature. Thus, annealing the doped region causes the dopant to spread out in Gaussian manner. In order to ultimately achieve a relatively uniform continuous doped region under the perforated mask of the invention, the perforations (unmasked portions) have to be closer than the ultimate doping diffusion. Thus process steps such as high diffusion drive and subsequent epitaxial growth, contribute to the distribution of the dopant. 
   As mentioned above, the Gaussian distribution profile extends not only laterally, but also vertically. Therefore shallower implants with less room for vertical diffusion cannot be exposed to as much annealing. Hence, to achieve a continuous doped layer, the perforations in the mask have to be closer together than is possible with deep implants. 
   The invention also provides a way of achieving different doping levels across different regions of a single integrated circuit. In particular, it achieves this by using a single mask and providing different size perforations in the mask. In other words the mask is etched differently in different regions to provide for larger or more numerous perforations (unmasked portions) in some regions than in others. By appropriately choosing the ratio of unmasked to masked portion over the device region, the present invention achieves a desired doping density. This will be described in more detail with reference to  FIGS. 3 and 4 . 
   As mentioned above, one application of the invention is in providing the appropriate doping level to a particular isolation region. Also it allows different regions for different devices on a single integrated circuit to be provided with different doping levels by making use of different masks or using a single mask with different numbers and sizes of perforations per unit area. In accordance with the invention, this allows the breakdown voltage of devices to be controlled. When compared to a prior art DIAC device (see  FIG. 2 ) which has a solid isolation region, the present invention allows the breakdown voltage to be increased by reducing the doping level of the isolation region. The effect is shown in the curves of  FIG. 7 , which will be discussed in greater detail below. It will be appreciated that the breakdown voltage can thus be adjusted by adjusting the size and density of the perforations in the mask during doping of the isolation region. This will become clearer in the discussions that follow below. 
     FIG. 1  shows a prior art DIAC  100  that has p-wells  102  formed in a substrate  104 . No isolation region is provided in this device, and the device therefore suffers from substantial leakage. In contrast, the prior art device shown in  FIG. 2  shows a DIAC  200  with an isolation region  202  between p-wells  204  and substrate  206 . This provides a snapback device with current density versus voltage characteristics as shown by curve  702  in  FIG. 7 . As can be seen by the curve  702 , the breakdown voltage for the device  200  is approximately 35 V. 
   According to the present invention, this breakdown voltage can be increased by making use of a periodically masked isolation layer. In one embodiment, which made use of ratio of 2000/500 for masked portion to unmasked portion, a breakdown voltage of about 50V was achieved, as shown by the curve  704  in  FIG. 7 . 
   One approach to achieving such a periodically masked isolation layer is shown in  FIGS. 3–6 .  FIG. 3  shows a p-substrate material  300  before n-epitaxial growth. In accordance with the invention, a periodic mask  302  (comprising photoresist with perforations or openings) is formed over the substrate  300 . In this embodiment the photoresist material  306  periodically alternates with perforations or openings  308 . By adjusting the width W of the photoresist regions  306  and the gap size G of the openings  308 , the density and number of gaps or perforations can be varied per unit area. This will effect the amount of dopant that passes into the substrate as will become clear from  FIG. 4 . In this case phosphorus was used as the dopant to form spotted implant regions  400  of n-material in the substrate  300  ( FIG. 4 ). 
   As shown in  FIGS. 5 and 6 , by making use of high temperature process steps, such as long-term high temperature diffusion during the drive of phosphorus impurities, or by subsequent annealing, the spotted implants form a more uniform region to define an isolation layer  600  between the p-substrate  300  and active regions in the form of p-wells  602 . As mentioned above, the Gaussian distribution of the diffusion is a function of time and temperature. Thus, all steps following the diffusion will change the profile. However, an important aspect in determining the final diffusion dimensions is determined by the high diffusion drive and any subsequent epitaxial growth. As also discussed above, in the case of shallow implants, the anneal time and/or the temperature will have to be reduced. It will be appreciated that there may be more than one annealing step, each at a different temperature and performed for a different time. The annealing steps may be additional steps performed or may form part of the regular process of forming the device, such as epitaxial growth of a region. 
   While the embodiment discussed above provided for forming of the isolation layer before epitaxial growth, the isolation layer could also be formed after epitaxial growth by making use of high diffusion drive. 
   In the embodiment of  FIGS. 3–6  a certain ratio of W to G was used. As part of the invention, the doping density of the isolation layer can be adjusted by adjusting this ratio. The table below shows the results for different ratios, showing how breakdown voltage changes. The table also shows a prior art fully blocked device (mask has no perforations and therefore no isolation layer is formed, which accounts for resistive leakage at 0V) and a prior art unblocked or fully opened device (mask is eliminated and a highly doped isolation region is formed) for comparison. 
   By providing an isolation layer, the present invention also provides for a device that is bi-directional. Some of the breakdown voltages for negative pad voltages, are also shown in the table below. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
             
             
                 
                 
               Blocked region W 
               Unblocked (open) 
             
             
               Number 
               Breakdown voltage 
               (μm) 
               region (μm) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               Leakage from 0 V 
               Fully blocked 
               0 
             
             
               2 
               44.2 
               1 
               1 
             
             
               3 
               45.7 
               0.5 
               0.5 
             
             
               4 
               48.9 
               0.6 
               0.4 
             
             
               5 
               43.3/−43.2 
               2 
               2 
             
             
               6 
               49.3 
               2 
               1 
             
             
               7 
               39.7/−39.6 
               1 
               2 
             
             
               8 
               51.4/−51   
               0.2 
               0.2 
             
             
               9 
               Leakage from 4 V 
               2 
               0.2 
             
             
               10 
               Soft pumch through 
               3 
               0.5 
             
             
                 
               from 40 V 
             
             
               11 
               64.4/−25   
               2 
               0.5 
             
             
               12 
               35/−35 
               0 
               Fully open 
             
             
                 
             
           
        
       
     
   
   The embodiment discussed above dealt with forming an isolation layer of controlled impurity density. It will be appreciated that the invention could also be used to control the density of impurity implantation into any other regions in one or more devices on an integrated circuit.