PATENT ABSTRACT
A microchip includes at least one I/O area surrounding at least one core circuit area. The I/O area further includes a first I/O cell having at least one first post-driver device connected to a first I/O pad; a second I/O cell having at least one second post-driver device connected to a second I/O pad; and an electrostatic discharge (ESD) cluster shared by the first I/O cell and the second I/O cell for protecting the same against ESD current during an ESD event, thereby reducing a total width of the first I/O cell and the second I/O cell.

PATENT DESCRIPTION
BACKGROUND 
   The present invention relates generally to integrated circuits, and more particularly to an ultra fine pitch I/O designs for microchips. 
   Advancement of semiconductor processing technology has caused devices implemented in core circuit areas of a microchip to shrink in size. It is estimated that the core circuit area of a particular integrated circuit is reduced by one half as the technology evolves from one generation to the next. Referring to  FIG. 1 , a floor plan  100  of a microchip manufactured with 90 nm semiconductor processing technology is shown to have a core circuit area  102  where a large number of core devices are implemented and an I/O area  104  where a plurality of I/O devices are disposed. A floor plan  110  of a microchip manufactured with 65 nm technology has a core circuit area  112  and an I/O area  114 , and a floor plan  120  of a microchip manufactured with 45 nm technology has a core circuit area  122  and an I/O area  124 . These three floor plans  100 ,  110  and  120  are deigned for implementing the same circuit schematics on semiconductors with various generations of semiconductor processing technology. As shown in the drawing, the core circuit area  112  is about half the size of the core circuit area  102 , and the area  122  is about half the size of the area  112 . 
   Although the core circuit areas  102 ,  112 , and  122  continue to shrink in size as the technology evolves, the I/O areas  104 ,  114  and  124  remain in about the same size, and therefore become the bottleneck for further reducing the size of microchips. One of the reasons that the I/O areas  104 ,  114  and  124  cannot be further reduced in size is that the pin count of a particular microchip remains unchanged regardless generations of technology. Another reason is that narrowing the width of the I/O areas  104 ,  114  and  124  can cause the I/O devices to be ineffective in functioning as electrostatic discharge (ESD) protection mechanism. For example,  FIG. 2A  illustrates three I/O cells  202 ,  204  and  206  arranged adjacent to each other, forming part of an I/O ring surrounding a core circuit area. Each cell  202 ,  204  or  206  includes a post-driver NMOS transistor area  208 , a post-driver PMOS transistor area  210  and a pre-driver area  212 , wherein devices implemented in these areas  208 ,  210  can function as ESD protection device during an ESD event.  FIG. 3A  illustrates a cross-sectional view  214  of the devices in the post-driver NMOS transistor area  208  where they function as ESD protection devices. In order for these ESD protection devices to provide a threshold voltage that distinguishes a normal operation state from an ESD protection state, the width D 1  of the substrate underlying the polysilicon gates  216  and between ESD pick-up contacts  218  needs to be sufficient in order to provide enough substrate resistance.  FIG. 2B  shows a layout view of narrowed I/O cells  220 ,  224 ,  226 , and  FIG. 3B  shows a cross-sectional view  240  of devices in a post-driver NMOS transistor area  230  in the I/O cell  220 . The width D 2  of the substrate underlying polysilicon gates  232  and between ESD pick-up contacts  244  is much narrower than D 1  shown in  FIG. 2A . As a result, this causes insufficient substrate resistance, such that the devices in the post-driver NMOS transistor area  230  cannot function properly as ESD protection devices during an ESD event. 
     FIG. 4  illustrates another conventional deign that splits I/O cells into two rows  402  and  404  in order to reduce the overall size of the I/O area. However, such design may cause unexpected ESD issues between the two rows of I/O cells, require complex routing of conductive lines, and may not be suitable for ball grid array (BGA) packaging. 
   As such, what is needed is a layout deign for I/O areas with reduced size in order to allow microchips for further shrinkage as semiconductor processing technology advances. 
   SUMMARY 
   The present invention discloses a microchip having at least one I/O area surrounding at least one core circuit area. In one embodiment of the present invention, the I/O area includes a first I/O cell having at least one first post-driver device connected to a first I/O pad; a second I/O cell having at least one second post-driver device connected to a second I/O pad; and an electrostatic discharge (ESD) cluster shared by the first I/O cell and the second I/O cell for protecting the same against ESD current during an ESD event, thereby reducing a total width of the first I/O cell and the second I/O cell. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates several floor plans of a microchip manufactured based on various generations of technology. 
       FIGS. 2A and 2B  illustrate layout designs of conventional I/O cells in a microchip. 
       FIGS. 3A and 3B  illustrate cross-sectional views of conventional I/O cells in a microchip. 
       FIG. 4  illustrates another layout design of conventional I/O cells in a microchip. 
       FIG. 5  illustrates a layout design of I/O cells in accordance with one embodiment of the present invention. 
       FIG. 6  partially illustrates an enlarged layout design of I/O cells in accordance with one embodiment of the present invention. 
       FIG. 7  illustrates a cross-sectional view of I/O cells in accordance with one embodiment of the present invention. 
   

   DESCRIPTION 
   This invention describes an I/O cell design that makes possible an ultra fine cell pitch in accommodation to continuous shrinkage of core circuit areas of microchips as semiconductor processing technology advances. The following merely illustrates various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art of integrated circuit deign and semiconductor manufacturing will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention. 
     FIG. 5  partially illustrates a layout design  500  of an I/O ring comprised of a plurality of I/O cells in accordance with one embodiment of the present invention. The pitch of each I/O cell is virtually defined by an elongated rectangle, such as  504  shown in the drawing. The pitch, however, does not necessarily represent the exact physical location where the I/O cell is implemented. Two or more neighboring I/O cells can share at least one common post-driver device area. In this exemplary embodiment, two neighboring I/O cells  504  and  506  share a common post-driver NMOS transistor area  508  and a common post-driver PMOS transistor area  510 , and have separate pre-driver device areas  512  and  514 . A number of NMOS transistors and PMOS transistors are constructed in the post-driver NMOS transistor area  508  and the post-driver PMOS transistor area  510 , respectively. A first conductive line  516  overlying the I/O cell  504  selectively connects some of the NMOS transistors disposed in the post-driver NMOS transistor area  508  to a first I/O pad (not shown in this figure), and a second conductive line  518  overlying the I/O cell  506  connects the rest of the NMOS transistors in the area  508  to a second I/O pad (not shown in this figure) that functions separately from the first I/O pad. Similarly, the first conductive line  516  selectively connects some of the PMOS transistors disposed in the post-driver PMOS transistor area  510  to the first I/O pad, and the second conductive line  518  connects the rest of the PMOS transistors in the area  510  to the second I/O pad. 
     FIG. 6  partially illustrates an enlarged view  600  of the post-driver NMOS transistor area  508  shown in  FIG. 5  in accordance with one embodiment of the present invention. A number of gate conductive lines G are constructed on a P-type substrate (no shown in the figure). A plurality of source doped regions S 1 , S 2 , S 3 , S 4  and S 5  are disposed adjacent to the gate conductive lines G on the P-type substrate. Similarly, a plurality of drain doped regions D 1 , D 2 , D 3 , D 4  and D 5  are disposed between two neighboring gate conductive lines G on the P-type substrate. Each combination of consecutive source doped region, gate conductive line and drain doped region constitutes an NMOS transistor, and each drain doped region is shared by two neighboring NMOS transistors. The first conductive line  516  disposed above the gate conductive layers G connects the drain doped regions D 2  and D 4  via drain contacts to the first I/O pad. The second conductive line  518  disposed above the gate conductive layers G connects the drain doped regions D 1  and D 3  via drain contacts to the second I/O pad. The drain doped regions D 2  and D 4  connected to the first conductive line  516  are interwoven with the drain doped regions D 1  and D 3  connected to the second conductive line  518 , such that every two drain doped regions connected to the first conductive line  516  are separated by at least one drain doped region connected to the second conductive line  518 . 
   A network of ground buses  602  disposed vertically between the gate conductive layer G and the conductive lines  516  and  518  are arranged along with the source doped regions S 1 , S 2 , S 3 , S 4  and S 5 , for connecting the same to ground via source contacts. The post-driver NMOS transistor area is surrounded by electrostatic discharge (ESD) pick-up doped regions  604 , which are shown at the bottom, left and right sides in the drawing, with the top side truncated. The ESD pick-up doped regions  604  are also connected to the ground bus network  602  via ESD pick-up contacts for switching the NMOS transistors implemented in the post-driver NMOS transistor area from a normal operation mode to an ESD protection mode. Details of such mode switching will be explained in detail below. 
     FIG. 7  illustrates a cross-sectional view  700  of the NMOS transistors implemented in the post-driver NMOS transistor area shown in  FIG. 6 . Source doped regions S 1 , S 2 , S 3 , S 4  and S 5  are connected to ground via the ground bus network  602 . The drain doped regions D 2  and D 4  are connected to the first I/O pad PAD_A, and the drain doped regions D 1  and D 3  are connected to the second I/O pad PAD_B. The NMOS transistor  702  at the right to a truncation mark shows an end of this transistor chain. A first ESD pick-up doped region  604 ′ is implemented at the left end of the post-driver NMOS transistor area, and a second ESD pick-up doped region  604 ″ is implemented at the right end of the post-driver NMOS transistor area. The width W between the first and second ESD pick-up doped regions  604 ′ and  604 ″ is crucial for these NMOS transistors to function properly during an ESD event. 
   In normal operation, the NMOS transistors that share common drain doped regions D 1  and D 3  function in a way that allows signals from the second I/O pad PAD_B to be sent to core circuit devices (not shown in the figure). Similarly, the NMOS transistors that share common drain doped regions D 2  and D 4  function in a way that allows signals from the first I/O pad PAD_A to be sent to core circuit devices (not shown in the figure). 
   During an ESD event, ESD current are passed to the P-type substrate via the ESD pick-up doped regions  604 ′ and  604 ″ to the bases of parasitic bipolar transistors formed by the source doped regions, the drain doped regions and the P-type substrate. The ESD current triggers on the parasitic bipolar transistors, such that the ESD current can flow from the drain doped regions to the source doped regions and dissipate to ground. 
   In order for these NMOS transistors, collectively referred to as an ESD cluster, function properly (or effectively) during an ESD event, the width W between the first and second ESD pick-up doped regions  604 ′ and  604 ″ needs to be sufficiently long. If the width W is too short, there will not be sufficient substrate resistance, and therefore the ESD cluster will be easily triggered and interfere with proper functioning of core circuit devices. Generally, the width W cannot be shorter than 30 um in order for the ESD cluster to function properly. 
   In the embodiment of the invention, since one ESD cluster can be shared by more than one I/O cells, its width can be kept sufficiently long, while the I/O cells can be made narrower. For example, the width of the ESD cluster can be kept longer than 40 um, and the pitch of an I/O cell can be made as short as 20 nm, with two I/O cells sharing one common ESD clusters. As another example, the width of the ESD cluster can remain no less than 30 um, and the pitch of an I/O cell can be made as short as 15 nm, with two I/O cells sharing one common ESD cluster. As a result, the overall size of the I/O ring that is made up by I/O cells can be reduced as semiconductor processing technology advances. 
   It is understood by people skilled in the art that the principles of the proposed embodiment of the invention can also be applied to post-driver PMOS transistor areas. For example, the layout design  600  in  FIG. 6  can be seen as the design for PMOS transistors, with the modification that the bus  602  is connected to a power supply instead of ground. It is also noted that the number of I/O cells that can share one common ESD cluster can be two or more. It is further noted that the gate conductive lines, the source doped regions and the drain doped regions can be arranged in parallel in a horizontal or vertical direction. 
   The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.