Abstract:
Circuits are described that provide electrostatic discharge protection for I/O circuits that support the low voltage differential signaling (LVDS) and on-chip termination (OCT) standards. At least one additional transistor is connected across an I/O transistor. In the case of LVDS, a pair of stacked transistors is used in which the distance between the source/drain region and a well tap is considerably greater for the transistor connected to the I/O pad. A PMOS transistor and an NMOS transistor may also be connected in series between a first node such as a power supply node and the I/O pad. An OCT circuit is also disclosed in which the spacing between the source/drain region and a well tap in the OCT transistor is smaller than that in the I/O transistor.

Description:
The present invention relates generally to electrostatic discharge (ESD) protection circuits, and more particularly to ESD protection circuits and structures that support input/output (I/O) standards such as the low voltage differential signaling (LVDS) standard and the on-chip termination (OCT) standard. 
   BACKGROUND 
   The LVDS and OCT standards are widely accepted among I/O standards that support high data rates in electronic and opto-electronic systems. LVDS has been used in applications that require low voltage, high speed, low noise, low power, and lower electromagnetic interference. In addition, LVDS supports the high data throughput necessary for high-speed interfaces such as those in backplane circuits. LVDS compliant I/O interfaces have several advantages compared to other known interface levels, including differential signals with good noise margin and compatibility over different supply voltage levels, etc. But LVDS interfaces need precise line termination resistors. 
   OCT compliant I/O interfaces include series, parallel, and/or differential terminations on chip, where OCT resisters are placed adjacent to I/O buffers to eliminate stub effect and to help prevent reflections. OCT provides the benefit of high signal integrity, simpler board design, lower cost systems and good system reliability. OCT also allows system designers to use fewer resistors, fewer board traces, smaller board space, and fewer excess components on printed circuit boards. 
   A common LVDS compliant I/O interface includes an I/O buffer and stacked transistors coupled in parallel with the I/O buffer. Since the same type of devices are typically used to form the stacked transistors and the I/O buffer, the LVDS stacked transistors are stressed at the same time as the I/O buffer during an ESD event. 
   OCT compliant I/O interfaces also have ESD issues because OCT transistors are often connected to the I/O pads. These OCT transistors are typically far narrower than the ones used in the I/O buffers. As such, the OCT transistors have even lower ESD threshold voltages than the transistors in the I/O buffers. 
   Therefore, there is a need for improved ESD protection for the LVDS and OCT compliant interface circuits. 
   SUMMARY 
   The present invention provides electrostatic discharge protection for I/O circuits that support the low voltage differential signaling (LVDS) and on-chip termination (OCT) standards. At least one additional transistor is connected across an I/O transistor. In the case of LVDS, a pair of stacked transistors is used in which the distance between the source/drain region and a well tap is substantially greater for the transistor connected to the I/O pad. A PMOS transistor and an NMOS transistor may also be connected in series between a first node such as a power supply node and the I/O pad. An OCT circuit is also disclosed in which the spacing between the source/drain region and a well tap in the OCT transistor is smaller than that in the I/O transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a circuit schematic of a LVDS I/O circuit according to one embodiment of the present invention. 
       FIG. 1B  is a circuit schematic of a LVDS I/O circuit according to an alternative embodiment of the present invention. 
       FIG. 2  is a layout drawing of two LVDS transistors in the LVDS I/O circuit. 
       FIG. 3  is a circuit schematic of an OCT I/O circuit according to one embodiment of the present invention. 
       FIG. 4  is a layout drawing of I/O pull-down and OCT transistors in the OCT I/O circuit. 
       FIG. 5  is a circuit schematic of a parallel OCT I/O circuit according to one embodiment of the present invention. 
       FIG. 6  is a layout drawing of parallel OCT transistors in the parallel OCT I/O circuit. 
       FIG. 7  is a circuit schematic of a differential OCT I/O circuit according to one embodiment of the present invention. 
       FIGS. 8A and 8B  are layout drawings of stacked PMOS and NMOS transistors, respectively, in the OCT I/O circuit. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an LVDS-compliant interface circuit  100 A, according to one embodiment of the present invention. The LVDS interface circuit  100  can be part of an I/O interface of an integrated circuit chip. As shown in  FIG. 1 , the LVDS interface circuit  100 A includes an I/O pad  110  and an I/O buffer having a pull-down transistor  120  and a pull-up transistor  130  serially connected with each other between an I/O power line VCCIO and an I/O ground line VSSIO. The LVDS interface circuit  100  further includes stacked transistors  140  and  150  connected in parallel with the pull-down transistor  120  of the I/O buffer. The gates of transistors  120 ,  130 ,  140 , and  150  are connected to other parts of the integrated circuit via inverters  121 ,  131 ,  141 , and  151 , respectively. The substrates of the stacked transistors  140  and  150  are tied to VSS, which is the ground line for a core circuit in the integrated circuit. A decoupling device  105  is used to separate the core ground VSS from the I/O ground VSSIO. 
   To allow the I/O buffer to function as an ESD protection device, a parasitic bipolar transistor associated with the stacked transistors should not turn on in the event of an ESD pulse on the I/O pad  110 . The turn on of the parasitic bipolar transistor can be prevented by placing the stacked transistors  140  and  150 , which are usually NMOS (N-type metal-oxide-semiconductor) transistors in different P-wells separated by a trench isolation. This way, a very high voltage (about 15V) is required between the I/O pad  110  and the I/O ground VSSIO to simultaneously turn on the parasitic bipolar transistors associated with the stacked transistors  140  and  150 . 
   Although the parasitic bipolar transistors are unlikely to turn on, the drain-substrate diode of  140  can breakdown when there is a positive ESD potential between the I/O pad  110  and the I/O ground VSS. The breakdown current associated with the drain-substrate diode should be limited to protect the drain-substrate diode from ESD damage. This can be achieved by using the layout of  140  and  150  shown in  FIG. 2 . 
     FIG. 2  illustrates how the stacked transistors are laid out on a semiconductor substrate according to one embodiment of the present invention. As shown in  FIG. 2 , transistor  140  and  150  are formed in different P-wells  210  and  230 , respectively. P-wells  210  and  230  are separated by an isolation region such as a trench isolation (not shown). Transistor  140  includes at least one gate  240  and at least one pair of N-type source/drain diffusion regions  242  on two opposite sides of gate  240 . Transistor  150  includes at least one gate  250  and at least one pair of N-type source/drain diffusion regions  252  on opposite sides of gate  250 . To prevent the parasitic bipolar transistors associated with the stacked transistors  140  and  150  from turning on in the event of an ESD pulse on the I/O pad  110 , the N-type source/drain diffusion regions  242  of transistor  140  are separated from the N-type source/drain diffusion regions  252  of transistor  150  by their location in two different P-wells separated by the trench isolation. 
   Transistor  140  further includes a P-well tap region  215 , and transistor  150  also includes a P-well tap region  235 . To prevent the drain-substrate junction(s) from being damaged by an ESD pulse on the I/O pad  110 , the P-well tap region  215  for transistor  140  is placed far from the source/drain diffusion region(s)  242 . In particular, this placement should be such that the minimum distance between tap region  215  and source/drain diffusion regions  242  is about twice the minimum separation required by the design rules associated with the technology used to fabricate the integrated circuit. This raises the substrate resistance between the N+ diffusion regions  242  and the P-well tap  215  and thus limits any breakdown current from the drain-substrate junction(s) in transistor  140 . To further increase the substrate resistance and reduce the breakdown current, transistor  140  may also include a P-well block region  220  between the N-type source/drain diffusion regions  242  and the P-well tap region  215 . The presence of the P-well block region makes it possible to reduce the spacing between the N-type source/drain diffusion regions  242  and the P-well tap region  215  and thus makes the layout of  140  more compact. 
   In one embodiment of the present invention, the N-type source drain diffusion regions  242  and  252  are doped with N+ or N++ dopant concentrations, the P-well tap regions  215  and  235  are doped with P+ or P++ dopant concentrations, and the P-well block region  220  is undoped silicon substrate that has high resistivity. 
   Transistor  150  may be laid out the same as transistor  140 , but such a layout for transistor  150  is usually not necessary because transistor  150  is not connected directly to the I/O pad  110  and because the decoupling device  105  provides a low-voltage clamp between VSS and VSSIO. In particular, a P-well block region is not necessary. In practice, transistor  150  can be made small by requiring the distance d 2  between the N+ diffusions  252  and the P-well tap  235  to be equal to or not much larger than the minimum separation required by the design rules associated with the technology used to fabricate the integrated circuits. Thus, the distance d 1  between the N+ diffusions  242  and the P-well tap  215  in transistor  140  will be significantly greater than d 2 . 
   To minimize any stress voltage at the drain-substrate junction(s) in transistor  140 , it is preferred that decoupling device  105  of  FIG. 1A  be eliminated and the substrate near transistor  140  be tied to the I/O ground line VSSIO, as in an I/O interface circuit  100 B shown in  FIG. 1B . In other respects, the components of I/O interface circuit  100 B are the same as those of I/O interface circuit  100 A and have been numbered the same. In the absence of decoupling device  105 , the parasitic bipolar transistor in the I/O pull-down transistor  120  can be turned on at a lower voltage when a positive ESD voltage is across the I/O pad and the core ground VSS because the additional voltage drop across the decoupling device  105  is not present. 
   Furthermore, transistor  140  should be placed as far away from the I/O pad  110  as other design considerations allow so that the interconnect resistance and inductance between the I/O pad  110  and transistor  140  can be used to help limit the ESD current. 
     FIG. 3  illustrates a series OCT interface circuit  300  according to another embodiment of the present invention. The series OCT interface circuit  300  can be part of an I/O interface of an integrated circuit chip. As shown in  FIG. 3 , the series OCT interface circuit  300  includes an I/O pad  310  and an I/O buffer having a pull-down transistor  320  and a pull-up transistor  330  serially connected with each other between an I/O power line VCCIO and an I/O ground line VSSIO. The series OCT interface circuit  300  further includes a narrow OCT transistor  340  connected in parallel with the pull-down transistor  320  of the I/O buffer. The gates of transistors  320 ,  330 , and  340  are connected to other parts of the integrated circuit via inverters  321 ,  331 , and  341 , respectively. To protect the series OCT transistor  340  during an ESD event, the parasitic bipolar transistors associated with the I/O buffer should have a lower triggering voltage than the series OCT transistor  340 . This can be achieved by laying out the I/O pull-down transistor  320  and the series OCT transistor  340  according to the layout drawing in  FIG. 4 . 
   As shown in  FIG. 4 , the series OCT transistor  340  includes at least one gate  440  and at least one pair of N-type source/drain diffusion regions  442  that are formed in a P-well or P-substrate  450 . The series OCT transistor  340  may further include a P-well tap region  460  surrounding the N-type source/drain diffusion region  442 . In one embodiment of the present invention, the N-type source drain diffusion regions  442  are doped with N+ or N++ dopant concentrations, while the P-well tap region  460  is doped with a P+ or P++ dopant concentration. The spacing between the P-well tap region  460  and the source/drain diffusion regions  442  for the series OCT transistor  340  is small, and in many cases should be as small as the minimum spacing between N+ (or N++) and P+ (or P++) regions allowable by design rules associated with the fabrication technology for making the integrated circuit chip. 
   The I/O pull-down transistor  320  includes at least one gate  420  and at least one pair of N-type source/drain diffusion regions  422  that are formed in an isolated P-well  425 , which is surrounded by a deep N-well  430 . The I/O pull-down transistor  320  further includes a P-well tap region  435  between the N-type source/drain diffusion regions  422  and the deep N-well  430 . In one embodiment of the present invention, the N-type source drain diffusion regions  422  in the I/O pull-down transistor  320  are doped with N+ or N++ dopant concentrations, the P-well tap region  435  is doped with a P+ or P++ dopant concentration, and the deep N-well region  430  is doped with a N-well dopant concentration, which is much lower than the dopant concentrations in the N-type source/drain regions  422 . The P-well tap region  435  is laid out such that it is spaced far from the N-type source/drain regions  422  and, in particular, is at least twice the minimum spacing required by the design rules associated with the technology used to fabricate the integrated circuit. In many cases, the spacing between the P-well tap region  435  and the N-type source/drain regions  422  should be as wide as space in the integrated circuit chip allows. Thus, the spacing d 4  between the P-well tap region  435  and the N-type source/drain regions  422  for the I/O pull-down transistor  320  should be significantly wider than the spacing d 3  between the P-well tap region  460  and the N-type source/drain regions  442  in the series OCT transistor  340 . 
   The wider spacing between the P-well tap region  435  and the N-type source/drain regions  422  enables the I/O pull-down transistor  320  to be triggered by a lower substrate current generated by the breakdown of the junction between the drain diffusion  422  and the isolated P-well  425 . By isolating the P-well  425  for the I/O pull-down transistor  320  using the deep N-well  430 , the P-well  425  can also charge up faster to forward-bias the source/P-well junction, which forward-biasing is required for triggering the parasitic bipolar transistor in the event of a ESD pulse on the I/O pad  310 . This, when combined with the lower triggering current, provides a lower trigger voltage for the I/O pull-down transistor  320 . If possible, the series OCT transistor  340  should be placed far away from the I/O pad so that the higher resistance and inductance associated with the interconnect between the series OCT transistor  340  and the I/O pad  310  can be used to limit the ESD current through the series OCT transistor  340 . 
     FIG. 5  illustrates a parallel OCT interface circuit  500  according to another embodiment of the present invention. The parallel OCT interface circuit  500  can be part of an I/O interface of an integrated circuit chip. As shown in  FIG. 5 , the parallel OCT interface circuit includes an I/O pad  510  and an I/O buffer having a pull-down transistor  520  and a pull-up transistor  530  serially connected with each other between an I/O power line VCCIO and an I/O ground line VSSIO. The parallel OCT interface circuit  500  further includes two cascaded NMOS transistors  540  and  550  connected between the I/O pad  510  and the I/O ground VSSIO, and one pair of PMOS and NMOS transistors  560  and  570 , respectively, that are connected serially with each other between the I/O power line VCCIO and the I/O pad  510 . The gates of transistors  520 ,  530 ,  540 ,  550 ,  560 , and  570  are connected to other parts of the integrated circuit via inverters  521 ,  531 ,  541 ,  551 ,  561  and  571 , respectively. 
   ESD protection for the two cascaded NMOS transistors  540  and  550  and the pair of PMOS and NMOS transistors  560  and  570  can be achieved by laying out transistors  540  and  550  similar to LVDS transistors  140  and  150 , respectively, as shown in  FIG. 2 , and by laying out transistors  560  and  570  according to the layout drawing shown in  FIG. 6 . 
   As shown in  FIG. 6 , the PMOS transistor  560  includes at least one gate  610  and at least one pair of P-type source/drain diffusion regions  612  that are formed in a N-well  620 . PMOS transistor  560  further includes a N-well tap region  630  that is spaced from the P-type source/drain diffusion regions  611  by a distance d 6 . On the other hand, the NMOS transistor  570  includes at least one gate  640  and at least one pair of N-type source/drain diffusion regions  642  that are formed in a P-well  650 . The NMOS transistor  570  may further include a P-well tap region  660  that is spaced from the N-type source/drain diffusion regions  642  by a distance d 7 . 
   In one embodiment of the present invention, the P-type source/drain diffusion regions  612  and the N-type source drain diffusion regions  642  are doped with P+ (or P++) and N+ (or N++) dopant concentrations, respectively; the N-well  620  and P-well  650  are doped with a N-well dopant concentration and a P-well dopant concentration lower than the dopant concentrations of the source/drain diffusion regions in these wells; and the N-well tap region  630  and the P-well tap region  660  are doped with a N+ (or N++) and P+ (or P++) dopant concentrations, respectively. 
   In one embodiment of the present invention, the distance d 7  between the P-well tap region  660  and the N-type source/drain diffusion regions  642  is made large to protect the drain-substrate diode associated with the transistor, which is connected directly to the I/O pad  510 . In particular, d 6  should be at least about twice the minimum spacing allowed by the design rules associated with the technology for fabricating the integrated circuit. On the other hand, since the substrate of transistor  560  is connected to VCCIO, the associated drain-substrate diode has no potential drop during an ESD event. Thus, the layout for transistor  560  can be made compact, requiring only that the distance d 6  between the P+ diffusion regions  612  and the N-well tap  630  to be equal to or not much larger than the minimum spacing between a P+ diffusion region and a N-well tap allowed by the design rules associated with the technology for fabricating the integrated circuit. Alternatively, if the substrate of transistor  560  is not tied to VCCIO, the spacing d 6  must be made larger to protect the drain-substrate diode in transistor  560 . 
     FIG. 7  illustrates a differential OCT interface circuit  700  according to one embodiment of the present invention. The differential OCT interface circuit  700  can be part of an I/O interface of an integrated circuit chip. As shown in  FIG. 7 , the differential OCT interface circuit  700  includes two I/O pads  711  and  712  and two I/O buffers coupled to the respective ones of the I/O pads. The I/O buffer coupled to the I/O pad  711  includes a pull-down transistor  721  and a pull-up transistor  731  that are serially connected with each other between an I/O power line VCCIO and an I/O ground line VSSIO. Likewise, the I/O buffer coupled to the I/O pad  712  includes a pull-down transistor  722  and a pull-up transistor  732  that are serially connected with each other between the I/O power line VCCIO and the I/O ground line VSSIO. The gates of transistors  721 ,  731 ,  722  and  732  are connected to other parts of the integrated circuit through inverters  725 ,  735 ,  726  and  736 , respectively. 
   The differential OCT interface circuit  700  further includes a pair of stacked PMOS transistors  740  and  750  connected between the I/O pads  711  and  712 , and a pair of stacked NMOS transistors  760  and  770  connected between the two I/O pads  711  and  712 . The gates of transistors  740 ,  750 ,  760  and  770  are connected to other parts of the integrated circuit through inverters  741 ,  751 ,  761  and  771 , respectively. When the I/O pads  711  and  712  are used as input pads, signals from the I/O pads  711  and  712  are fed to a differential amplifier  780 . In one embodiment of the present invention, the substrates of transistors  740  and  750  are connected to input pads  711  and  712 , respectively, or to VCCIO if VCCIO has a higher voltage than the input voltages on the input pads. The substrates of transistors  760  and  770  are tied to the core ground VSS. ESD protection for the differential OCT interface circuit  700  can be achieved with a layout similar to transistor  560  in  FIG. 6  for the PMOS transistors  740  and  750  but with a larger d 6  and a layout similar to transistor  570  in  FIG. 6  for the NMOS transistors  760  and  770 . 
   In particular, as shown in  FIG. 8A , PMOS transistor  740  or  750  includes at least one gate  810  and at least one pair of P-type source/drain diffusion regions  812  that are formed in N-well  820 . PMOS transistor  740  or  750  further includes an N-well tap region  830  that is spaced from the P-type source/drain diffusion regions by a distance d 8 , which is substantially larger than the minimum distance between a P-type source/drain diffusion region and a N-well tap region allowed by the design rules for the technology used to fabricate the integrated circuit. As shown in  FIG. 8B , the NMOS transistor  760  or  770  includes at least one gate  840  and at least one pair of N-type source/drain diffusion regions  842  that are formed in P-well  850 . NMOS transistor  760  or  770  further includes a P-well tap region  860  that is spaced from the N-type source/drain diffusion regions by a distance d 10 , which is substantially larger than the minimum distance between a N-type source/drain diffusion region and a P-well tap region allowed by in the design rules for the technology used to fabricate the integrated circuit. By substantially larger is meant at least twice the minimum spacing allowed between the diffusion regions and the tap region by the design rules associated with the technology for fabricating the integrated circuit. 
   As will be apparent to those skilled in the art, numerous embodiments of the invention may be devised within the spirit and scope of the claims.