Abstract:
A silicon-on-insulator (SOI) gated diode and non-gated junction diode are provided. The SOI gated diode has a PN junction at the middle region under the gate, thus providing more junction area than a normal diode. The SOI non-gated junction diode has a PN junction at the middle region thereof, and thus also has more junction area than a normal diode. The SOI diodes of the present invention improve the protection level offered for electrical overstress (EOS)/electrostatic discharge (ESD) due to the low power density and heating for providing more junction area than normal ones. The I/O ESD protection circuits, which comprise primary diodes, a first plurality of diodes, and a second plurality of diodes, all of which are formed of the present SOI diodes, could effectively discharge the current when there is an ESD event. And, the ESD protection circuits, which comprise more primary diodes, could effectively reduce the parasitic input capacitance, so that they can be used in the RF circuits or HF circuits. The proposed gated diode and non-gated diode can be fully process-compatible to general partially-depleted or fully-depleted silicon-on-insulator CMOS processes.

Description:
This is a division of U.S. patent application Ser. No. 09/783,870, filed Feb. 15, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to diodes in a silicon-on-insulator (SOI) CMOS process, and more particularly, to ESD protection circuits with the diodes in silicon-on-insulator CMOS process. 
     2. Description of the prior Art 
     Silicon-on-insulator technology is a prime contender for low voltage, high speed applications because of its advantages over bulk-Si technology in reduced process complexity, latch-up immunity and smaller junction capacitance. However, electrostatic discharge (ESD) is a major reliability concern for SOI technology. 
     The protection level provided by an ESD protection device is determined by the amount of current that it can sink. The device failure is initiated by thermal runaway and followed by catastrophic damage during an ESD pulse. In SOI devices, the presence of the buried oxide layer having a thermal conductivity {fraction (1/100)} th  of Si causes increased device heating, which in turn accelerates thermal runaway. 
     FIG. 1 depicts a cross-sectional view of a prior SOI diode, called a Lubistor diode, published in the article of the Proc. Of EOS/ESD Symp., 1996, pp. 291-301. If the silicon layer above the buried oxide layer  100  is doped N type dopant, the junction of the SOI diode is P+  102 /N well  101 . The two terminals of this junction diode are V 1  connected to P+  102  and V 2  connected to N well  101 . If V 1  is positive relative to V 2 , the SOI diode is under forward biased. However, if V 1  is negative relative to V 2 , the diode is under reverse biased. If the P+  102 /N well  101  (or N+/P well) junction area in which the power is generated during an ESD event is smaller, then it will increase power density and heat. The heat is generated in a localized region at the P-N junction and the dominant component of the heat at the junction is Joule heat. Second breakdown is assumed to occur when the maximum temperature in the SOI diode reaches the intrinsic temperature (T intrinsic ). In order to get better ESD protection level, one should reduce the power density and Joule heat. 
     Accordingly, it is a desirable to provide a diode with lower power density in a silicon-on-insulator CMOS process for ESD protection. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a silicon-on-insulator diode with more junction area than a normal one, thereby a lower power density and heating are obtained, and the protection level offered for electrical overstress (EOS)/electrostatic discharge (ESD) is improved. 
     It is another object of the present invention to provide a silicon-on-insulator diode with more junction area than a normal one, which could be used in the I/O ESD protection circuit and the Vdd-to-Vss ESD protection circuit under forward biased conditions. 
     It is a further object of the present invention to provide an I/O ESD protection circuit having SOI diodes with more junction area than normal ones, which can reduce the parasitic input capacitance, and could serve as the I/O ESD protection circuit in the RF circuits or HF circuits. 
     In order to achieve the above objects, the present invention provides a silicon-on-insulator diode and ESD protection circuit thereof. The silicon-on-insulator diode comprises a substrate, an insulating layer, two shallow trench isolations, and a PN junction diode formed of a first well with a first conductive type having either of N type and P type and a second well with a second conductive type opposite to the first conductive type. The insulating layer is formed on the substrate and then the two shallow trench isolations are formed thereon. The PN junction diode is formed between the two shallow trench isolations. The ESD protection circuit having the SOI diodes comprises an electrically conductive pad, a conductor segment, a first voltage supply rail, a second voltage supply rail, a first diode, a second diode, a first plurality of diodes and a second plurality of diodes, all of which are fabricated on the insulating layer. The conductor segment connects the pad directly to a first node. The first diode connects between the first node and the first voltage supply rail, and the second diode connects between the first diode and the second voltage supply rail. The first plurality of diodes connect between the first node and the first voltage supply rail, and which are opposite to the first diode&#39;s direction. The second plurality of diodes connect between the first node and the second voltage supply rail, and which are opposite to the second diode&#39;s direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     FIG. 1 is a cross-sectional view of the structure of a prior SOI polysilicon-bounded diode called a Lubistor diode; 
     FIG. 2 is a cross-sectional view of a diode with the junction at the middle region under the gate according to the present invention; 
     FIG. 3 is a cross-sectional view of another diode structure with the junction at the middle region under the gate according to the present invention; 
     FIG. 4 is a cross-sectional view of another diode structure on a SOI wafer with integrated source/drain implants and the junction is at the middle region under the gate; 
     FIG. 5 is a cross-sectional view of the structure of a gated diode in the fully-depleted SOI CMOS process; 
     FIG. 6 is a cross-sectional view of a gated diode with the junction at the middle region under the gate; 
     FIG. 7 is a cross-sectional view of a non-gated junction diode with the junction at the middle region; 
     FIG.  8  and FIG. 9 are schematic diagrams of SOI ESD protection circuits for I/O pins in accordance with alternative embodiments of FIG. 2 to FIG. 7 of the present invention; 
     FIG.  10  and FIG. 11 are schematic diagrams of SOI ESD protection circuits in accordance with alternative embodiments of FIG. 2 to FIG. 7 of the present invention; and 
     FIG.  12  and FIG. 13 respectively are variations of FIG.  10  and FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a cross-sectional view of a gated diode according to the present invention. The structure of FIG. 2 comprises a substrate  200 , for example, a P− substrate or P+ substrate, and an insulating layer  201 , such as, a buried silicon dioxide layer, formed thereon. Two shallow trench isolations  202  are formed on the insulating layer  201 , and a P well  203  based on a silicon layer and an N well  204  based on a silicon layer are formed on the insulating layer  201  between the two shallow trench isolations  202 . The P well  203  and the N well  204  constitute a PN junction. A first highly doped P+ diffusion region  205  is formed at the upper-corner of the P well  203  adjacent to the one shallow trench isolation  202 , and a second highly doped N+ diffusion region  206  is formed at the upper corner of N well  204  adjacent to the other shallow trench isolation  202 . A MOS-like gate  207  is formed on the P well  203  and the N well  204 , and the junction of the P well  203  and the N well  204  is at the middle region under the MOS-like gate  207 . The MOS-like gate  207  comprises a dielectric layer  208 , a polysilicon gate formed on the dielectric layer  208 , consisting of a third highly doped P+ diffusion gate region  209   a  and a fourth highly doped N+ diffusion gate region  209   b , and a dielectric spacer  210  formed on each side of the MOS-like gate  207 . The third highly doped P+ diffusion region  209   a  and the fourth highly doped N+ diffusion region  209   b  are connected together electrically by a conductor layer (not shown in the figure) formed on the polysilicon gate, preferably a silicide layer. Besides, the first highly doped P+ diffusion region  205  and the second highly doped N+ diffusion region  206  are respectively self-aligned with the third highly doped P+ diffusion region  209   a  and the fourth highly doped N+ diffusion region  209   b.    
     The SOI diode is formed by the P well  203  and the N well  204 , and the PN junction of the SOI diode is at the middle region under the MOS-like gate  207 . Since the present diode with the P well  203 /N well  204  junction has more junction area than the normal Lubistor diode with P+/N well or N+/P well in FIG. 1, the ESD protection level are raised by the present diode due to the low power density and heating. 
     FIG. 3 is a cross-sectional view of an alternate embodiment that is a variation of FIG. 2. A first lightly doped P− diffusion region  305  is formed at the upper corner of the P well  303  adjacent to one shallow trench isolation  302 , and a second lightly doped N− diffusion region  306  is formed at the upper corner of the N well  304  adjacent to the other shallow trench isolation  302 . The MOS-like polysilicon gate  307  comprises a third lightly doped P− diffusion gate region  309   a  and a fourth lightly doped N− diffusion gate region  309   b . The third lightly doped P− diffusion region  309   a  and the fourth lightly doped N− diffusion region  309   b  are connected together electrically with a conductor layer (not shown in the figure) formed on the MOS-like polysilicon gate  307 , preferably a silicide layer. 
     The SOI diode is also formed by the P well  303  and the N well  304 . The PN junction of the diode is at the middle region under the MOS-like polysilicon gate  307 . 
     FIG. 4 is a cross-sectional view of an alternate embodiment that is a variation of FIG.  3 . In this alternate embodiment, a fifth highly doped P+ diffusion region  410  is formed at the upper corner of the P well  403  between one shallow trench isolation  402  and the first lightly doped P− diffusion region  405 , and a sixth highly doped N+ diffusion region  411  is formed at the upper corner of the N well  404  between the other shallow trench isolation  402  and the second lightly doped N− diffusion region  406 . The MOS-like polysilicon gate  407  comprises a third lightly doped P− diffusion region  409   a  and a fourth lightly doped N− diffusion region  409   b . The third lightly doped P− diffusion gate region  409   a  and the fourth lightly doped N− diffusion gate region  409   b  are connected together electrically by a conductor layer (not shown in the figure) formed on the MOS-like polysilicon gate  407 , preferably a suicide layer. 
     The SOI diode is formed by the P well  403  and the N well  404 . The PN junction of the diode is at the middle region under the MOS-like polysilicon gate  407 . 
     FIG. 5 is a cross-sectional view of an alternate embodiment that is a variation of FIG.  2 . The silicon thickness in this silicon-on-insulator (SOI) structure is fully depleted by a first highly doped P+ diffusion region  505  and a second highly doped N+ diffusion region  506 . The SOI diode is also formed by the P well  503  and the N well  504 , and the PN junction of the diode is at the middle region under the MOS-like polysilicon gate  507 . 
     FIG. 6 is a cross-sectional view of an alternate embodiment that is a variation of FIG.  2 . In this alternate embodiment, there is no diode in the MOS-like polysilicon gate  607 . However, the MOS-like polysilicon gate  607  can be a highly doped or lightly doped P type diffusion region or N type diffusion region. The SOI diode is also formed by the P well  603  and the N well  604 , and the PN junction of the diode is at the middle region under the MOS-like polysilicon gate  607 . 
     FIG. 7 is a cross-sectional view of an alternate embodiment that is a variation of FIG.  2 . In this embodiment, there is no gate structure and named as non-gated junction diode. The SOI diode is also formed by the P well  703  and the N well  704 . 
     FIG. 8 is one embodiment of an SOI ESD protection circuit comprising SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The ESD protection circuit  800  comprises an electrically conductive input pad  801 , two primary diodes D 1   803  and D 2   804 , a Vdd voltage supply rail  805 , a Vss voltage supply rail  806 , an input resistor  807 , a first plurality of diodes (Du 1  to Dun)  808  connected in series and a second plurality of diodes (Dd 1  to Ddn)  809  connected in series. All of these diodes are formed by the SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The input pad  801 , the Vdd voltage supply rail  805 , the Vss voltage supply rail  806 , and the input resistor  807  are fabricated on the insulating layer the same with the SOI diodes. 
     The input pad  801  is directly connected to a first node  802  through a conductor segment. The primary diode D 1   803  is connected between the first node  802  and the Vdd voltage supply rail  805 , and the primary diode D 2   804  is connected between the first node  802  and the Vss voltage supply rail  806 . The first plurality of diodes (Du 1  to Dun)  808  are connected between the first node  802  and the Vdd voltage supply rail  805 , and these diodes&#39; direction is opposite to the primary diode D 1   803 . The second plurality of diodes (Dd 1  to Ddn) are connected between the first node  802  and the Vss voltage supply rail  806 , and these diodes&#39; direction is opposite to the primary diode D 2   804 . The input resistor  807  is connected between the first node  802  and a portion of the internal circuit  810  to be protected by the ESD protection circuit  800 . While, the input resistor  807  can also be coupled to an input buffer of the internal circuit  810 , and then a second node is located between the input resistor  807  and the input buffer. 
     When the ESD event involves the application of a positive voltage to the input pad  801  relative to the Vdd voltage supply rail  805 , the primary diode D 1   803  is forward biased and the primary diode D 2   804  is not active because the Vss voltage supply rail  806  is floating. As a result, the associated ESD current is discharged to the Vdd voltage supply rail  805  through the primary diode D 1   803 . 
     Similarly, when the ESD protection event involves the application of a negative voltage to the input pad  801  relative to the Vss voltage supply rail  806 , the primary diode D 2   804  is forward biased and the primary diode D 1   803  is not active because the Vdd voltage supply rail  805  is floating. The ESD event is discharged to the Vss voltage supply rail  806  through the primary diode D 2   804 . 
     When the ESD event involves the application of a voltage to the input pad  801 , which is negative with respect to the Vdd voltage supply rail  805 , the primary diode D 1   803  is reversed. At this condition,the Vss voltage supply rail  806  is floating. The first plurality of diodes (Du 1  to Dun)  808  is forward biased under this ESD zapping condition, therefore the ESD current is discharged through the first plurality of diodes (Du 1  to Dun). 
     When the ESD event involves the application of a voltage to the input pad  801  which is positive with respect to the Vss voltage supply rail  806 . The primary diode D 2   804  is reverse biased. At this condition, the Vdd voltage supply rail  805  is floating. The secondary plurality of diodes (Dd 1  to Ddn)  809  is forward biased under this ESD zapping condition, therefore, the ESD current is discharged through the secondary plurality of diodes (Dp 1  to Dpn). 
     FIG. 9 is another embodiment of an SOI ESD protection circuit comprising the SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The ESD protection circuit  900  comprises an electrically conductive output pad  901 , primary diodes D 1   903  and D 2   904 , a Vdd voltage supply rail  905 , a Vss voltage supply rail  906 , a first plurality of diodes (Du 1  to Dun)  908  connected in series, and a second plurality of diodes (Dd 1  to Ddn)  909  connected in series. All of these diodes are formed of the SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . And, the output pad  901 , the Vdd voltage supply rail  905 , and the Vss voltage supply rail  906  are fabricated on the insulating layer the same with the SOI diodes. 
     The output pad  901  is directly connected to a node  902  by a conductor segment. The primary diode D 1   903  is connected between the node  902  and the Vdd voltage supply rail  905 , and the primary diode D 2   904  is connected between the node  902  and the Vss voltage supply rail  906 . The first plurality of diodes (Du 1  to Dun)  908  are connected between the node  902  and the Vdd voltage supply rail  905 , and these diodes&#39; direction is opposite to the primary diode D 1   903 . The second plurality of diodes (Dd 1  to Ddn)  909  are connected between the node  902  and the Vss voltage supply rail  906 , and these diodes&#39; direction is opposite to the primary diode D 2   904 . The node  902  is connected to the output terminal of an output buffer formed of a P-channel transistor  910  and an N-channel transistor  911 . And, the input terminal of the output buffer is connected to a pre-driver  912 . 
     When the ESD event involves the application of a positive voltage to the output pad  901  relative to the Vdd voltage supply rail  905 , the primary diode D 1   903  is forward biased and the primary diode D 2   904  is not active because the Vss voltage supply rail  906  is floating. As a result, the associated ESD current is discharged to the Vdd voltage supply rail  905  through the primary diode D 1   903 . 
     Similarly, when the ESD event involves the application of a negative voltage to the output pad  901  relative to the Vss voltage supply rail  906 , the primary diode D 2   904  is forward biased and the primary diode D 1   903  is not active because the Vdd voltage supply rail  905  is floating. The ESD event is discharged to the Vss voltage supply rail  906  through the primary diode D 2   904 . 
     When the ESD event involves the application of a voltage to the output pad  901 , which is negative with respect to the Vdd voltage supply rail  905 , the primary diode D 1   903  is reverse biased. The Vss voltage supply rail  906  is floating under this condition. The first plurality of diodes (Du 1  to Dun)  908  is forward biased under this ESD-zapping condition, therefore the ESD current is discharged through the first plurality of diodes (Du 1  to Dun). When the ESD event involves the application of a voltage to the output pad  901  which is positive with respect to the Vss voltage supply rail  906 , the primary diode D 2   904  is reverse biased. The Vdd voltage supply rail  905  is floating during this ESD event. The secondary plurality of diodes (Dd 1  to Ddn)  909  is forward biased under this ESD-zaping condition, therefore, the ESD current is discharged through the secondary plurality of diodes (Dd 1  to Ddn)  909 . 
     FIG. 10 is further another embodiment of an SOI ESD protection circuit comprising the SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The ESD protection circuit comprises an electrically conductive input pad  1001 , primary diodes D 1   1003 , D 2   1004 , D 3   1005  and D 4   1006 , an input resistor  1010 , an n-channel transistor  1011 , a Vdd voltage supply rail  1007 , a Vss voltage supply rail  1008  and an ESD clamp circuit  1009 . The primary diodes D 1   1003  and D 2   1004  are connected in series, and the primary diodes D 3   1005  and D 4   1006  are connected in series. All of these diodes are formed by the SOI diodes in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The input pad  1001 , the input resistor  1010 , the Vdd voltage supply rail  1007 , and the Vss voltage supply rail  1008  are fabricated on the insulating layer the same with the SOI diodes. 
     The input pad  1001  is directly connected to a first node  1002  through a conductor segment. The primary diodes D 1   1003  and D 2   1004  are connected between the first node  1002  and the Vdd voltage supply rail  1007 . The primary diodes D 3   1005  and D 4   1006  are connected between the first node  1002  and the Vss voltage supply rail  1008 . The input resistor  1010  and the n channel transistor  1011  are coupled in series between the input pad  1001  and the Vss voltage supply rail  1008 . The input resistor  1010 , the n channel transistor  1011  and the internal circuit  1013  are coupled through a second node  1012 . The gate and source of the n channel transistor  1011  are coupled to the Vss voltage supply rail  1008 . The ESD clamp circuit  1009  is connected between the Vdd voltage supply rail  1007  and the Vss voltage supply rail  1008 . 
     Two primary diodes D 1   1003  and D 2   1004  are connected between the input pad  1001  and the Vdd voltage supply rail  1007  instead of one diode D 1  in FIG. 8, and other two diodes D 3   1005  and D 4   1006  are connected between the input pad  1001  and the Vss voltage supply rail  1008  instead of one diode D 2  in FIG.  8 . If diode D 1 &#39;s parasitic junction capacitance is C 1 , diode D 2 &#39;s parasitic junction capacitance is C 2 , diode D 3 &#39;s parasitic junction capacitance is C 3 , and diode D′ 4  parasitic junction capacitance is C 4 . The input capacitance is Cin=C 1 +C 2  in FIG. 8, but in this embodiment, the input capacitance becomes Cin′=[C 1 C 2 /(C 1 +C 2 )]+[C 3 C 4 /(C 3 +C 4 )]. If the diodes (D 1 , D 2 , D 3 , D 4 ) are identity, that means C 1 =C 2 =C 3 =C 4 =C, then Cin= 2 C in FIG.  8  and Cin′=C in FIG.  10 . Therefore, the parasitic input capacitance of this embodiment is reduced, and then the RC time constant is also reduced. By the lowering of the input delay, the ESD protection circuit of this embodiment could be applied in RF circuits or in HF circuits. 
     FIG. 11 is an alternative of FIG.  10 . The Vdd-to-Vss ESD clamping circuit comprises a plurality of first SOI diodes (Dp 1  to Dpn)  1109  and a second SOI diode  1110  connected in parallel between the Vdd voltage supply rail and the Vss voltage supply rail. All of the diodes used in this ESD protection circuit are in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . 
     FIG. 12 is a variation of FIG.  10 . In this ESD protection circuit, there are three diodes D 1   1203 , D 2   1204 , and D 3   1205  in series between the Vdd voltage supply rail  1209  and the input pad  1201 , and three diodes D 4   1206 , D 5   1207 , and D 6   1208  in series between the Vss voltage supply rail  1210  and the input pad  1201 . All of the diodes used in this ESD protection circuit are in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . The input capacitance becomes Cin′=[C 1 C 2 C 3 /(C 1 C 2 +C 2 C 3 +C 1 C 3 )]+[C 4 C 5 C 6 /(C 4 C 5 +C 5 C 6 +C 4 C 6 )]=⅔ C, which further to be reduced. 
     FIG. 13 is an alternative of FIG.  12 . The Vdd-to-Vss ESD clamping circuit comprises a plurality of first SOI diodes (Dp 1  to Dpn)  1311  and a second SOI diode  1312  connected in parallel between the Vdd voltage supply rail and the Vss voltage supply rail. All of the diodes used in this ESD protection circuit are in accordance with the alternative embodiments of FIG. 2 to FIG.  7 . 
     According to the foregoing, the present invention provides the following advantages: 
     1.The present invention provides a SOI diode with low power density due to increasing the PN junction area. 
     2. The present invention provides a SOI diode with improved ESD protection level. 
     3. The present invention provides a SOI diode could be used in mixed-voltage and analog/digital circuits. The present SOI diodes also could serve as the I/O ESD protection circuit, and the Vdd-to-Vss protection circuit under forward biased condition. 
     4. The present invention provides an ESD protection circuit with the reduced input capacitance, and could serve as the I/O ESD protection circuit in the RF circuits or HF circuits. 
     The preferred embodiments are only used to illustrate the present invention, not intended to limit the scope thereof. Many modifications of the preferred embodiments can be made without departing from the spirit of the present invention.