Abstract:
CMOS I/O structures are described which are latchup-immune by inserting p+ and n+ diffusion guard-rings into the NMOS and PMOS source side of a semiconductor substrate, respectively. P+ diffusion guard-rings surround individual n-channel transistors and n+ diffusion guard-rings surround individual p-channel transistors. These guard-rings, connected to voltage supplies, reduce the shunt resistances of the parasitic SCRs, commonly associated with CMOS structures, from either the p-substrate to p+ guard-ring or the n-well to n+ guard-ring. In a second preferred embodiment a deep p+ implant is implanted into the p+ guard-ring or p-well pickup to decrease the shunt resistances of the parasitic SCRs. The n+ and p+ guard-rings, like the guard-rings of the first preferred embodiment, are connected to positive and negative voltage supplies, respectively. In either of the two preferred embodiments the reduced shunt resistances prevent the forward biasing of the parasitic bipolar transistors of the SCR, thus insuring that the holding voltage is larger than the supply voltage.

Description:
This is a division of patent application Ser. No. 09/507,646, filing date Feb. 22, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to structures and methods of fabricating integrated CMOS circuits, and more particularly to making those CMOS circuits highly immune to latchup. 
     2. Description of the Related Art 
     Latchup is a phenomenon of CMOS circuits well known to circuit designers and is described by S. Wolf in  Silicon Processing for the VLSI Era,  Volume 2, by Lattice Press, copyright 1990, 6.4 LATCHUP IN CMOS. The inherent and self-destructive latchup effect in CMOS circuits which has always been a major problem has become even more so a problem as device dimension continue to shrink. The latchup phenomenon creates a low resistance path between the positive and negative voltage supplies of a CMOS circuit and enables the flow of large currents through the affected circuit. When latchup occurs the circuit stops functioning and may even get destroyed because of the heat developed by the large currents. 
     Latchup is caused by the presence of parasitic bipolar pnp and npn transistors in the structure of PMOS and NMOS transistors. The closer those complementary MOS transistors are to each other the more there is a likelihood of those parasitic transistors to interact electrically to form a pnpn diode, equal to a silicon controlled rectifier (SCR). Internal voltages across the anode and cathode of that SCR which exceed a breakover voltage cause the SCR to reach a low impedance state with the possibility of a resultant high current. This state can be maintained indefinitely if an external circuit can supply a necessary holding current, i.e., the SCR stays latched up and the circuit cannot recover. 
     FIG. 1 shows a cross-sectional view of a prior art CMOS device layout  100 . Embedded in a p-substrate  102  is an n-well  104  containing a plurality of p+ regions  106  which are both source and drain. An n+ guard-ring  108  located at the perimeter of the n-well surrounds regions  106 . Similarly, a plurality of n+ regions  116 , of both source and drain, are formed in the p-substrate  102  and are surrounded by a p+ guard-ring  118 . Gates  109  and  119  are indicated straddling sources and drains of regions  106  and  116 , respectively. The parasitic SCR inherent in CMOS structures is comprised of transistor Q 1  and Q 2 . Q 1  is a vertical bipolar pnp parasitic transistor structure and Q 2  is a lateral bipolar npn parasitic transistor structure resulting from the arrangement of the PMOS transistors of regions  106  and of the NMOS transistor of regions  116 . The emitter of Q 1  comprises the sources of regions  106 , the base comprises n-well  104  and the collector comprises p-substrate  102 . Analogous the emitter of Q 2  comprises the sources of regions  116 , the base comprises p-substrate  102  and the collector comprises n-well  104 . Between guard-ring  108  and the base of Q 1  is the bulk n-well resistance  130 . Between guard-ring  118  and the base of Q 2  is the bulk p-substrate resistance  132 . Bulk resistances  130  and  132  each have a value of about 100 Ohms. 
     FIG. 2 is an equivalent circuit diagram of the parasitic transistors of FIG.  1  and represents the above mentioned pnpn diode or SCR. One terminal of resistor  130  (equal to guard-ring  108 ) and the emitter of Q 1  (equal to the source of region  108 ) is connected to a positive voltage V cc . One terminal of resistor  132  (equal to guard-ring  118 ) and the emitter of Q 2  (equal to the source of region  116 ) is connected to a negative voltage V ss , typically ground. 
     The above described arrangement for I/O devices with guard ring structures is latchup free as long as NMOS and PMOS transistors are 15 micron or more apart. At distances below 15 micron these structures start exhibiting latchup. Another way to avoid latchup is to use EPI wafers to reduce the resistivity of the substrate resistor with a resultant higher cost. 
     Attempts by device designers to overcome the latchup problem are legions and of a great variety, each providing solutions applicable to the then current technological restraints and requirements. As circuit dimensions continue to shrink new device structures become necessary. The inventions described subsequently address and solve the latchup problem. 
     The following three U.S. Patents may be considered relating to the present invention: 
     U.S. Pat. No. 5,023,689 (Sugawara) illustrates a complementary integrated circuit device having a guard ring region surrounding a region having transistors that are larger than those in a second region do. The guard ring region is supplied with a power voltage via a conductor line, which is formed separately from a conductor line supplying the power voltage to each of the larger transistors. 
     U.S. Pat. No. 5,406,513 (Canaris et al.) shows a CMOS circuit formed in a semiconductor substrate having improved immunity to radiation induced latch-up and improved immunity to a single event upset. A continuous P+guard ring is formed surrounding the n-channel transistors and between the n-channel transistors and an N-Well. Similarly, a continuous N+guard ring is formed surrounding the p-channel transistors and between a p-channel transistors and the p-type substrate. In the event of a radiation hit, the guard rings operate to reduce the parasitic impedance in the collector circuits of the parasitic bipolar transistors forming a parasitic SCR and also act as additional collectors of radiation induced current. 
     U.S. Pat. No. 5,895,940 (Kim) describes integrated circuits having built-in electrostatic discharge protection thyristors. Guard rings are formed in a first well region and a second well region to complete the structures of a pair of thyristors. The guard rings are preferably electrically connected to reference potentials so that damage caused by excessive voltage can be inhibited upon latch-up of the built-in thyristors. 
     It should be noted that none of the above-cited examples of the related art reduce sufficiently the n-well or p-substrate resistance at decreased circuit dimensions or reduced NMOS-to-PMOS spacings to avoid latchup. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide new CMOS I/O structures and methods which improve their latchup immunity. 
     Another object of the present invention is to decrease the spacing between the NMOS and PMOS devices to 5 micron while maintaining improved latchup immunity. 
     These objects have been achieved in a first preferred embodiment by inserting p+ and n+ diffusion guard-rings into the NMOS and PMOS source side of a semiconductor substrate, respectively. P+ diffusion guard-rings surround individual n-channel transistors and n+ diffusion guard-rings surround individual p-channel transistors. These guard-rings, connected to voltage supplies, reduce the shunt resistances of the parasitic SCRs, commonly associated with CMOS structures, from either the p-substrate to p+ guard-ring or the n-well to n+ guard-ring. 
     In a second preferred embodiment of the present invention a deep p+ implant is implanted into the p+ guard-ring or p-well pickup to decrease the shunt resistances of the parasitic SCRs. The n+ and p+ guard-rings, like the guard-rings of the first preferred embodiment, are connected to positive and negative voltage supplies, respectively. In either of the two preferred embodiments the shunt resistances are reduced to less than 3 Ohms, thereby preventing the forward biasing of the parasitic bipolar transistors of the SCR. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a CMOS structure of the prior art with parasitic transistors shown schematically. 
     FIG. 2 is an equivalent circuit diagram of the parasitic transistors and resistors of FIG.  1 . 
     FIG. 3 is a cross sectional view of a CMOS structure of a first preferred embodiment of the present invention with parasitic transistors shown schematically. 
     FIG. 4 is an equivalent circuit diagram of the parasitic transistors and of the reduced parasitic resistances of FIG.  3  and FIG.  5 . 
     FIG. 5 is a cross sectional view of a CMOS structure of a second preferred embodiment of the present invention with parasitic transistors shown schematically. 
     FIG. 6 is a graph of the deep p+ boron concentration of the guard-ring for the CMOS structure of FIG.  5 . 
     FIG. 7 is a top view layout of the CMOS structure of FIG.  5 . 
     FIG. 8 is a block diagram of a first preferred method of providing a latchup-immune CMOS structure. 
     FIG. 9 is a block diagram of a second preferred method of providing a latchup-immune CMOS structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     We now proceed with the description of a first preferred embodiment of the present invention by directing attention to FIG.  3 . FIG. 3 shows a cross-sectional view of CMOS device layout  300 . Embedded in a p-substrate  102  of a semiconductor wafer is an n-well  104  containing p+ source regions  106  and p+ drain regions  107 . N+ guard-rings  108  surround individual regions  106  and  107 , where regions  106  and  107  make up one or more p-channel transistors. N+ guard rings  108  and p+ source regions  106  are tied to a voltage supply V cc . Similarly, n+ source regions  116  and n+ drain regions  117  are formed in the p-substrate  102 . P+ guard rings  118  surround individual regions  116  and  117 , where regions  116  and  117  make up one or more n-channel transistors. P+ guard rings  118  and n+ source regions  116  are tied to voltage supply V ss  which is more negative than V cc . Gates  109  are indicated straddling regions  106  and  107 . Gates  119  similarly straddle regions  116  and  117 . Q 1  is a vertical bipolar pnp parasitic transistor structure and Q 2  is a lateral bipolar npn parasitic transistor structure resulting from the arrangement of the PMOS transistors of regions  106  and  107 , and of the NMOS transistor of regions  116  and  117 . The emitter of Q 1  comprises the sources of regions  106 , the base comprises n-well  104  and the collector comprises p-substrate  102 . Analogous the emitter of Q 2  comprises the sources of regions  116 , the base comprises p-substrate  102  and the collector comprises n-well  104 . Between guard-rings  108  and the base of Q 1  are the bulk n-well resistances  130  which are parallel and therefore reduce the total n-well resistance. Between guard-rings  118  and the base of Q 2  is the bulk p-substrate resistance  132 . 
     The pattern of guard-ring  108  enclosing regions  106 ,  107 ,  106 , and of guard-ring  118  enclosing regions  116 ,  117 ,  116  keeps repeating as indicated by dots in n-well  104  and in p-substrate  102  in FIG.  3 . 
     The deep p+ implant may also be implanted into the p-well pick-up and the p+ guard-rings and the n+ regions may be located in a p-well of the p-substrate. When bulk n-well resistances  130  are combined into resistance R nW1  and when bulk p-substrate resistances  132  are combined into resistance R pW1  they each typically measure from 1 to 2 Ohms but may range from 0.5 to 10 Ohms. Refer to FIG. 4 for resistances R nW1  and R pW1 . 
     FIG. 4 is an equivalent circuit diagram of the parasitic transistors of FIG.  3 . FIG. 4 is similar to that of FIG. 2, except that the FIG. 4 well or substrate resistors R nW1  and R pW1  represent a lower resistance and thus prevent latchup. It is understood that components shown in the figures which are similar are identified by the same reference numbers. 
     In a second preferred embodiment of the present invention, as illustrated in FIG. 5, the shunt resistance is reduced by adding a deep p+ implant to the p+ guard-ring. FIG. 5 shows a cross-sectional view of CMOS device layout  500 . Embedded in a p-substrate  102  of a semiconductor wafer is an n-well  104  containing p+ region  506  which is comprised of sources and drains (not shown) and which make up p-channel transistors. An n+ guard-ring  108  diffused at the inside perimeter of n-well  104  surrounds region  506 . N+ guard ring  108  is tied to a voltage supply V cc , as are the sources within region  506  (not shown). An n+ region  516  is formed in the p-substrate  102 . Region  516  also comprises sources and drains (not shown) which make up n-channel transistors. A p+ guard ring  118 , diffused into p-substrate  102 , surrounds region  516 . P+ guard ring  118  is tied to voltage supply V ss , which is more negative than voltage supply V cc . A deep p+ implant  518  is implanted into the p+ guard-ring  118  to decrease the shunt resistance of the parasitic SCR (comprising transistors Q 1  and Q 2 ). Q 1  is a vertical bipolar pnp parasitic transistor structure and Q 2  is a lateral bipolar npn parasitic transistor structure resulting from the arrangement of the PMOS transistors of region  506 , and of the NMOS transistor of region  516 . The emitter of Q 1  comprises the sources of region  506 , the base comprises n-well  104  and the collector comprises p-substrate  102 . In a similar manner, the emitter of Q 2  comprises the sources of region  516 , the base comprises p-substrate  102  and the collector comprises n-well  104 . Between guard-ring  108  and the base of Q 1  are bulk n-well resistors  130  which are paralleled and, therefore, reduce the total n-well resistance. Between guard-ring  118  and the base of Q 2  are the shunt resistances  532  of p+ implant  518 . Because of the deep ion implant  518 , these shunt resistances are of a much lower value than the shunt resistances of the prior art. The shunt resistance  532  is typically 1 to 2 Ohms but may range from 0.5 to 10 Ohms. The deep p+ implant may also be implanted into the p-well pick-up and the p+ guard-rings and the n+ regions may be located in a p-well of the p-substrate. 
     FIG. 4 also serves as an equivalent circuit diagram of the parasitic transistors of FIG. 5 since the value of the shunt resistances of both the first and second preferred embodiment of the present invention are the same. In FIG. 4, resistor R nW1  represents the sum of all paralleled resistors  130 , and resistor R pW1  represents shunt resistance  532 . 
     FIG. 6 is a graph of the boron implant concentration in atoms/cm 3  as a function of the depth in the silicon up to 0.6 microns (μm). Curve  1  shows the boron concentration for deep p+ implants with an energy of 6 keV and a boron dose of 3.5×10 15 . Curve  2  shows the boron concentration for deep p+ implants with an energy of 6 keV and a boron dose of 3.5×10 15  followed by a second implant at 20 keV and a boron dose of 2×10 15 . Curve  3  shows the boron concentration for deep p+ implants with an energy of 6 keV and a boron dose of 3.5×10 15  followed by a second implant at 25 keV and a boron dose of 2×10 15 . It can be seen that the boron concentration after the second implant is raised significantly ranging from 10 21  atoms/cm 3  to about 3×10 17  atoms/cm 3  (Curve  2 ) and to about 6×10 17  atoms/cm 3  (Curve  3 ) from the silicon surface down to 0.35 μm. 
     FIG. 7 is a top view of the cross-section of FIG. 5 showing n-well  104  and embedded in it a one micron wide n+ guard-ring  108 . The n+ guard-ring is separated from the p+ region  506  by 2 microns all around. The p+ region itself is about 100 by 90 micron. The same dimensions apply to the p+ guard-ring  118  and the n+ region  516 . W indicates the distance between p+ region  506  and n+ region  516 . Varying W also changes the holding voltage. This is demonstrated by the results of an I/O test pattern comparing the latchup susceptibility of the related art with that of both the first and second preferred embodiment of the present invention, where the distance W was varied and the holding voltage determined. These results are summarized in Table 1. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 W (spacing) 
                 5 micron 
                 10 micron 
                 15 micron 
               
               
                   
                   
               
             
             
               
                   
                 hold. volt. 
                 1.86 V 
                 2.1 V 
                 no latchup 
               
               
                   
                 related art 
               
               
                   
                 hold. volt. 
                 no latchup 
                 no latchup 
                 no latchup 
               
               
                   
                 invention 
               
               
                   
                   
               
             
          
         
       
     
     Table 1 shows that the condition of “no latchup” occurs in the related art only when the spacing W is 15 micron (or above), whereas in the present invention there in no latchup at all down to a spacing of 5 micron. The holding voltage depends on the design and on the power supply voltage. The condition for preventing latchup is that the holding voltage V H  needs to be larger than the power supply voltage V cc , i.e.: 
     no latchup when: V H &gt;V cc    
     We now illustrate in the block diagram of FIG. 8 a first preferred method of providing a latchup-immune CMOS I/O structure, comprising the steps of: 
     1) providing a semiconductor wafer having a p-substrate and forming an n-well in the p-substrate, see Block  81 . 
     2) forming p+ source and drain regions in the n-well, see Block  82 . 
     3) diffusing n+ guard-rings around each of the p+ source and drain regions to provide a parallel resistive path to the n-well ranging from 0.5 to 10 Ohms, see Block  83 . 
     4) forming n+ source and drain regions in the p-substrate, see Block  84 . 
     5) diffusing p+ guard-rings around each of the n+ source and drain regions to provide a parallel resistive path to the p-substrate ranging from 0.5 to 10 Ohms, see Block  85 . 
     The p+ source and drain regions constitute one or more p-channel transistors and the n+ source and drain regions constitute one or more n-channel transistors. N+ guard-rings are connected to a first voltage supply and p+ guard-rings are connected to a second voltage supply, where the first voltage supply is more positive than the second voltage supply. The p+ guard-rings and the n+ regions may also be formed in a p-well of the p-substrate. 
     We now illustrate in the block diagram of FIG. 9 a second preferred method of providing a latchup-immune CMOS I/O structure, comprising the steps of: 
     1) providing a semiconductor wafer having a p-substrate and forming an n-well in the p-substrate, see Block  91 . 
     2) forming a p+ region in the n-well and diffusing an n+ guard-ring around the p+ region, see Block  92 . 
     3) forming an n+ region in the p-substrate and diffusing a p+ guard-ring around the n+ region, see Block  93 . 
     4) implanting a low-resistance, deep p+ implant into the p+ guard-ring so that the resistance from the p+ guard-ring to the p-substrate ranges from 0.5 to 10 Ohm, see Block  94 . 
     The n+ guard-ring is connected to a first voltage supply and the p+ guard-ring is connected to a second voltage supply, where the first voltage supply is more positive than the second voltage supply. The deep p+ implant may also be implanted into the p-well pick-up and the p+ guard-rings and the n+ regions may be located in a p-well of the p-substrate. 
     It is understood by those knowledgeable in the related art that all − and p-type materials mentioned in the preceding description can be replaced by p- and n-type materials, respectively, without affecting the operation of the present invention. 
     Advantages of the described invention are not only the increased latchup immunity but also the decrease of required NMOS and PMOS space, which imparts this invention with great economical benefits. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.