Patent Publication Number: US-5837571-A

Title: High voltage transistor for sub-micron CMOS processes

Description:
This is a division of application Ser. No. 08/359,509, filed Dec. 20, 1994 now U.S. Pat. No. 5,501,577. 
    
    
     This invention relates to methodology to resolve the problem of low drain/source breakdown voltage (BVdss) in small geometry devices with thin gate oxide. 
     BACKGROUND OF THE INVENTION 
     Conventional solutions to improve BVdss require either thicker gate oxide or employ overlapping NWELL diffusions which generally leads to higher threshold voltages (Vt) or higher series resistance, which in turn limits the drive capability of the transistor. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a semiconductor device comprising a control electrode capable of establishing a channel region in use; and two doped regions each electrically connected to said channel region such that a current can flow from one doped region to the other when the channel region is established and wherein at least one of said doped regions has a graded profile the thickness of which decreases towards the channel region; whereby in use the device has a relatively high breakdown voltage. 
     This has the advantages of providing improved drain diffusion profile implanting through disjoint NSD/NWELL windows in the extended drain region. This provides essentially an improved lightly diffused (LDD) structure. 
     Further this invention relates to alternative methods to resolve the problem of low drain/source breakdown voltage in other structures which can be achieved by, for example, building a number of side wall oxide layers, impurity compensation or oxygen implantation. 
     The improved LDD structure to which this invention relates has a number of advantages when compared with other solutions. It enables high voltage transistors to be fabricated with high drive capability, without additional process steps being required to implement the structure. 
     The inventions will find applications wherever a high voltage capability is required to interface with the outside world. Such designs would include automotive applications; programming transistors for Field Programmable Gate Arrays; Robust I/O&#39;s and ESD Protection Circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Reference will now be made, by way of example, to the accompanying drawings in which: 
     FIG. 1, is a diagram of the cross-section of the arrangement for achieving high voltage breakdown after implant diffusions. 
     FIG. 2, is a plan representation of FIG. 1 showing, in particular, the disjoint strips through which diffusions are implanted. 
     FIG. 3, is a diagram showing this graded profile achieved after diffusion and its relationship to the disjoint strips (21) shown in FIG. 2. 
     FIG. 4, is a graph of I-V characteristics of experimental structures at low currents with reference to particular PIN numbers. 
     FIG. 5, is a graph of I-V characteristics of experimental structures at high currents with reference to the same PIN numbers identified in FIG. 4. 
     FIG. 6, is a cross-section view of a second embodiment of the present invention; and 
     FIG. 7 and 8, are plan views of third and fourth embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     High voltage transistors find many useful applications in circuit design, which may range from a programming transistor for Field Programmable Arrays to an output transistor in an automotive application. To achieve high voltage withstand capability in a transistor the electric field near the gate/drain region, for an N-channel transistor, must be reduced. The high electric field region, which must be reduced, is caused by the curvature effects of shallow source/drain diffusions in the substrate. These effects are more pronounced in sub-micron geometry as the source/drain diffusions tend to be even more shallow than in other geometries. 
     The conventional methodology, of increasing gate oxide thickness, to achieve a reduced value in the high electric field region near to the gate/drain region requires additional process steps. With reference to FIGS. 1 and 2, the prime methodology of this invention increases the BVdss with a design which produces a graded diffusion profile in the gate/drain region without additional process steps being required. 
     FIG. 1 is a diagram, in section, of the arrangement for achieving high voltage breakdown, after implant diffusion, by the methodology of this invention. A P+ substrate (19) has a deposited, or grown, P epitaxial layer (18) covering it. Into which an NSD diffusion (15 and 17) creates the base for the source (11) and drain (14) contact points. The high concentration source/drain profile is gradually terminated with a profile from under the gate region (12) with the LDD (16) towards the drain diffusion (17) under the drain contact area (14). The metal overhang (13) over part of the gate is retained, as device simulation suggests its retention will lower the peak electric field. 
     FIG. 2 is a diagram of the plan of the arrangement shown in FIG. 1. The metallisation (23), (26) has been removed from over the gate region (24) and to create bonding windows to the drain region (28) and source regions (29) allowing direct contact to be made to the NSD diffusion (22), (27). Metal has been retained to form the metal overhang (20). The disjoint strips (21) are formed by NWELL/NSD diffusion through mask windows. The number of disjoint strips (or rings) could be changed. Increasing the number of strips would produce, what may be considered, a more perfect profile. The distance between these strips is arranged so that they join up after diffusion. Bonding windows have been shown in plan only. 
     FIG. 3 is a diagram showing a graded profile of a type which may be achieved using three disjoint strips. An increasing depth of diffusion resulting from a wider disjoint strip. 
     As illustrated in FIGS. 1 and 2, the high concentration source/drain profile is gradually terminated under the gate with the LDD, helped with the implant diffusions through the disjoint strips. The distance between the disjoint strips is arranged so that they join up after diffusion to produce a graded profile of a type shown in FIG. 3. 
     Since the lateral diffusion is mostly dependent on the volume of dopant dissolved prior to diffusion drive or thermal anneal. The widths of the diffusion strips are made progressively smaller as they move from the drain area towards the gate. The reduction in width is to achieve the required graded profile. The separation between the strips will normally be less than twice the desired vertical diffusion. Increasing the number of diffusion strips will produce a more regular profile. 
     Employing NSD diffusions in the extended drain region has the effect of reducing the drain resistance and consequently increasing transistor drive capability. In sub-micron geometries, with the possibility of smaller moat-to-moat spacing, NSD diffusions may be sufficient to form the required graded junction. If not sufficient then NWELL diffusions will most likely be required, carried out from mask windows common to NWELL and NSD. 
     Device simulations using a Semiconductor Device Simulation software package show that in metal overhang structures, such as discussed in this disclosure, the equipotential lines are more sparse than in the standard case where metal does not overhang the poly-silicon of the gate. The consequence of this is that devices with metal overhang will have a lower peak electric field. 
     To investigate and record the effects of the graded profile on BVdss, device structures of the type within this disclosure are implanted on a bar processed in DMOS-4 in Gizs flow. For convenience a 44 Pin JLCC package is selected for assembly. The following table details the relevant Pin-outs and describes these structures. Only structures which relate to this disclosure are listed. 
     
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                                 NSD/NWELL                                
                 W/L      Metal  In The Extended                          
Pin No.                                                                   
      Function   μm/μm                                              
                          Overhang                                        
                                 Drain Region                             
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 7    Ground                                                              
38    NCH Trans  40/0.8   Yes    Standard                                 
37    NCH Trans  40/1.4   Yes    Standard                                 
36    NCH Trans  40/0.8   No     Standard                                 
35    NCH Trans  40/1.4   No     Standard                                 
34    NCH Trans  40/1.4   Yes    Continuous NWELL                         
33    NCH Trans  40/1.4   Yes    1 NWELL Strip                            
32    NCH Trans  40/1.4   No     Continuous NWELL                         
31    NCH Trans  40/1.4   No     1 NWELL Strip                            
30    NCH Trans  40/1.4   Yes    1 NSD Strip                              
29    NCH Trans  40/1.4   No     1 NSD Strip                              
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     For these purposes only one strip of NWELL or NSD is used, although more strips could be employed--but at the expense of the drain area. As the objective is to investigate and record the effect of the graded profile on BVdss, the poly-gate on these structures is left floating. 
     The reverse breakdown characteristics (BVdss) of the structures listed in the proceeding table are shown in FIGS. 4 and 5. FIG. 4 shows the BVdss at low currents, identified are specific Pin Numbers from the proceeding table. Pin 35 being the typical breakdown voltage of a standard structure. FIG. 5 shows the BVdss at high currents and identifies Pin numbers of the structures previously listed. 
     The values shown in FIGS. 4 and 5 clearly show that additional diffusions in the drain region beneficially increase the BVdss, with sharper I-V characteristics when compared with standard transistors. All structures with NSD or NWELL diffusion strips have a positive temperature coefficient of breakdown voltage (negative temperature coefficients for standard structures) indicating that these structures exhibit true avalanche breakdown characteristics. 
     The impact of metal overhang on these structures may not be fully assessed from their I-V characteristics. However, since the devices with metal overhang have relaxed electric field distribution this is likely to improve hot electron reliability. 
     The use of NSD diffusions in the extended drain regions gives lower series drain resistance and consequently increases the drive capability of the transistor. This is not demonstrated when the poly-gate is left floating, however, the breakdown characteristics at higher currents (FIG. 5) shown the difference in device impedance at breakdown. 
     The conclusive beneficial effects of &#34;improved lightly diffused drains (ILDD) on BVdss characteristics are demonstrated within this disclosure. By devices achieving a 20 volt breakdown in a process with 180 Å oxide. Further this type of structure does not require additional process strips on the use of thick oxide to build the devices on structure. 
     In technologies where the required fine control of well-well and moat-to-moat dimensions cannot be maintained. The combination of NWELL and NSD diffusions could achieve the required results. 
     The most distinguishing feature of this process is the use of NSD diffusions in the extended drain region, offering the advantages of higher voltage capability without any sacrifice of drive capability. 
     The graded profile shown in FIG. 3 can also be produced by the other methods. 
     FIG. 6 shows a possible method of achieving the desired diffusion profile using successive side wall oxide deposition followed by N-implant diffusion. To produce one step in the diffusion profile will require a first side wall oxidation and one implant diffusion to produce side wall oxide 60 and implant 62. An etch step is then carried out to remove the oxide. Subsequent side wall oxidation and implant diffusions produce side wall oxide 64 and 68 and implants 66 and 70. The source 72 and drain 74 diffusions are achieved in a standard manner. 
     Alternatively, the desired diffusion profile can be produced by impurity compensation i.e.: counter doping. A p-type impurity is diffused through mask windows 100 as shown in FIG. 7. The width of the windows will depend n-type impurities. The net dopant will still be n-type but its volume density will decrease going from drain to gate regions giving a diffusion profile similar to that in FIG. 3. The p-type diffusion can be done either before or after the N-type diffusion. 
     The desired diffusion profile can also be produced by using oxygen implant as diffusion barrier. By implanting oxygen through disjoint strips 102 shown in FIG. 8 followed by blanket n-type implant diffusion, doping profile as shown in FIG. 3 can be produced. Here oxygen atoms act as barriers for the n-type impurities thus decreasing the n-type dopant density in the regions where oxygen is implanted. 
     
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Tables for FIG. 1                                                         
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       11         Source                                                  
       12         Gate                                                    
       13         Metal Overhang                                          
       14         Drain                                                   
       15         NSD                                                     
       16         LDD Diffusion                                           
       17         NSD                                                     
       18         P-EPI                                                   
       19         P + Substrate                                           
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Table for FIG. 2                                                          
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20            Metal Overhang                                              
21            NSD or NWELL disjoint strips                                
22            NSD                                                         
23            Metal                                                       
24            Poly Gate                                                   
25            N Moat                                                      
26            Metal                                                       
27            NSD                                                         
28            Bonding windows drain                                       
29            Bonding windows source                                      
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Table for FIG. 6                                                          
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       60          Sidewall oxide                                         
       62          Implant                                                
       64          Sidewall oxide                                         
       66          Implant                                                
       68          Sidewall oxide                                         
       70          Implant                                                
       72          Source                                                 
       74          Drain                                                  
       18          P-EPI                                                  
       19          P + Substrate                                          
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