Abstract:
An improved layout pattern for electrostatic discharge protection is disclosed. A first heavily doped region of a first type is formed in a well of said first type. A second heavily doped region of a second type is formed in a well of said second type. A battlement layout pattern of said first heavily doped region is formed along the boundary of said first heavily doped region and said second heavily doped region. A battlement layout pattern of said second heavily doped region is formed along the boundary of said first heavily doped region and said second heavily doped region. By adjusting a distance between the battlement layout pattern of a heavily doped region and a edge of well of said second type, i.e. n-well, a first distance will be shorter than what is typically required by the layout rules of internal circuit; and a second distance will be longer than the first distance to ensure that the I/O device have a better ESD protection capability. Accordingly, by properly adjusting the breakdown voltage of ESD device within I/O circuit, i.e. adjusting the distance between the edge of n-well and the battlement layout pattern of heavily doped regions, it will help to reduce the chip area and improve the ESD reliability.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to electrostatic discharge protection for semiconductor integrated circuitry and more particularly to an improved layout structure/pattern for electrostatic discharge protection 
     2. Background of the Invention 
     Static electricity has been an industrial problem for centuries. Since ancient time, people employed basic grounding and flame ionization techniques to dissipate static electricity and to refrain from ignition to combustible objects. The age of electronics brought with it new problems associated with static electricity and electrostatic discharge (“ESD”). And as electronic devices become faster and smaller, their sensitivity to ESD also increases. 
     Static electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. ESD is defined as the transfer of charge between bodies at different electrical potentials. ESD can change the electrical characteristics of a semiconductor device by either degrading or destroying it. The ESD damage may be either a catastrophic failure or a latent defect which may cause the semiconductor device no longer function or be partially degraded and experience premature failure. It will increase the associated costs for repair, replacement, and et al. 
     The protection of integrated circuits from ESD has received a lot of attention. 
     Many researchers in this field have proposed solutions to protect submicron devices without requiring any increase of silicon chip area. Because die size is the major cost factor for silicon fabricated products, layout rules followed by many modern time ICs need to be adjusted. According to conventional layout rules, the distance between two adjacent regions, such as between a well of a conductivity type and a heavily doped region, takes a lot of circuit area especially within high voltage area of, for example, a TFT Driver IC. The outermost ESD circuit device within an input/output circuit (“I/O”) will induce ESD disaster if the ESD circuit device follows the same conventional layout rules. It may not be able to safeguard the whole chip against ESD because the breakdown voltage of internal circuit will be lower than the breakdown voltage of I/O circuit. On the other hand, if the ESD circuit device follows that same layout rules as internal circuit, it will affect the durability of the ESD circuit when the IC is not connected to the circuit board and/or power-off and/or floating. 
     Referring now to  FIG. 1 , an example of a prior art layout pattern of a heavily doped region in a well is shown. A p type heavily doped region  30  and an n type heavily doped region  40  are formed in a p-well  10  and an n-well  20 , respectively. A distance between a p type heavily doped region edge  300  and an n type well edge  200  is S 1 . A distance between an n type heavily doped region edge  400  and the n type well edge  200  is S 2 . Generally, S 1  and S 2  will be maintained at the same distance. 
     However, in order to reduce silicon chip area, the layout rules of the internal circuit need to be adjusted. As a result, the ESD device within I/O circuit will not be able to function properly to protect the internal circuit from power noise damage. On the other hand, reducing the distance S 1  and S 2  to safeguard the internal circuit will affect the ESD current clamping ability when the IC is not connected to the circuit board and/or power-off and/or floating. 
     SUMMARY 
     Therefore, it is an object of this invention to solve the problem where, within an internal circuit, the distance between a well of a conductivity type and a heavily doped region is shorter than the distance between a well of a conductivity type and a heavily doped region of the I/O circuit. The problem causes the p-n well junction breakdown voltage of internal circuit to be lower than the p-n well junction breakdown voltage of I/O circuit. Hence, the internal circuit will suffer from the damage of abrupt voltage pulse. 
     The above problem can be resolved by applying two different distances between a well of a conductivity type and a heavily doped region within a layout structure. A first distance will be shorter than the layout rules of internal circuit, and a second distance will be longer than the first distance, in order to achieve a better ESD protection capability. 
     By properly adjusting the breakdown voltage of I/O circuit, i.e. adjusting the distance between a well of a conductivity type and a heavily doped region according to the proposed method, it will help to reduce the chip area as well as curtailing the ESD reliability issue. The present invention will improve a product&#39;s ESD durability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a prior art guard-ring layout pattern of ESD device based on the layout rules specified by a foundry. 
         FIG. 2(   a ) illustrates the measurement of breakdown I-V curve for a p-well and n-well junction of the present invention in a top layout view where a positive voltage is applied to an n type heavily doped region and a negative voltage is applied to an p type heavily doped region. 
         FIG. 2(   b ) illustrates the measurement of breakdown I-V curve for a p-well and n-well junction of the present invention in a cross-section view where a positive voltage is applied to an n type heavily doped region and a negative voltage is applied to a p type heavily doped region. 
         FIG. 2(   c ) illustrates the DC measurement of breakdown I-V curve with various distances between a well of a conductivity type and a heavily doped region. 
         FIG. 2(   d ) illustrates the ESD measurement of breakdown I-V curve with various distances between a well of a conductivity type and a heavily doped region. 
         FIG. 3  illustrates the first preferred embodiment of the present invention in a top layout view where the first heavily doped region and the second heavily doped region have battlement layout patterns. 
         FIG. 4  illustrates the second preferred embodiment of the present invention in a top layout view where the second heavily doped region has a battlement layout pattern. 
         FIG. 5  illustrates the third preferred embodiment of the present invention in a top layout view where the first heavily doped region has a battlement layout pattern. 
     
    
    
     DETAILED DESCRIPTION 
     The preferred embodiments of the present invention disclose a battlement layout pattern of a heavily doped region in a well having improved ESD performance. By applying a shorter distance between a well and a heavily doped region, the breakdown voltage of I/O circuit will be lower than the breakdown voltage of internal circuit during the DC operation. Also, a p-n well junction will turn on and a discharging ESD current will protect the internal circuit from power noise damage. By applying a longer distance between a well and a heavily doped region, the I/O circuit will be able to clamp the ESD current when the IC is not connected to a circuit board and/or power-off and/or floating. It should be clear to those experienced in the art that other variations can be applied without deviating from the scope of the present invention. 
     In  FIGS. 2(   a ) and  2 ( b ), a p type heavily doped region is formed in a p-well and an n type heavily doped region is formed in an n-well. The distance between an n-well edge and an n type heavily doped region is S; the distance between the same n-well edge and a p type heavily doped region is S. A positive voltage is applied to the n type heavily doped region and a negative voltage is applied to the p type heavily doped region in order to measure the I-V curve of a p-n well junction breakdown with various distances at DC operation and/or off power. 
     In  FIG. 2(   c ), during a DC operation a smaller distance S will have a relatively small breakdown voltage of ESD circuit device within I/O circuit compared to the breakdown voltage of internal circuit. Once an abrupt power noise pulse occurs, the p-n well junction breakdown of ESD circuit device will clamp the voltage at the corresponding value according to the I-V curve of  FIG. 2(   c ), and discharge the current. It will help to protect the internal circuit from power noise damage. 
     In  FIG. 2(   d ), when an IC is not connected to a circuit board and/or power-off and/or floating, the I-V curve of  FIG. 2(   d ) will describe the characteristics of ESD discharging current. In order to get the best ESD characteristics, the distance S will be increased and usually will at least have to meet the layout rules specified by the foundry. Once an abrupt voltage occurs, the p-n well junction, with a larger distance S e.g. S=2.0 μm, of ESD circuit device will turn on and discharge the current. In other words, when other p-n well junctions with a relatively smaller distance S have not reached their breakdown voltages, the p-n well junction with largest distance S already turns on and discharges the current to protect the IC. 
     First Embodiment 
     In  FIG. 3 , a first well  10  of a first type is formed on a semiconductor substrate. A second well  20  of a second type, alongside the first well  10 , is also formed. According to one embodiment, a first heavily doped region  31  and a second heavily doped region  41  are formed in the wells  10  and  20 . Particularly, the first heavily doped region  31  of the first type is formed in the first well  10 . The second heavily doped region  41  of the second type is formed in the second well  20 . In one embodiment, the first heavily doped region  31  comprises a p+ type region formed in the p-well  10  having a battlement layout pattern along the boundary between the first and the second wells. The second heavily doped region  41  comprises an n+ type region formed in the n-well  20  having a battlement layout pattern along the boundary between the first and the second wells. In one embodiment, those concave shape regions in the second heavily doped region  41  are opposite to those concave shape regions in the first heavily doped region  31  as shown in the figure. 
     Furthermore, a distance between a top of the first heavily doped region battlement layout  311  and the second well edge  200  is S 3 . A distance between a top of the second heavily doped region battlement layout  411  and the second well edge  200  is S 4 . A distance between a bottom of the first heavily doped region battlement layout  312  and the second well edge  200  is S 1 . A distance between a bottom of the second heavily doped region battlement layout  412  and the second well edge  200  is S 2 . The distances S 3  and S 4  are arranged to be less than what is typically required by the layout rule of internal circuit. If the DC power noise occurs, the abrupt power noise pulse will breakdown the p-n well junction of ESD circuit device and discharge ESD current to protect the internal circuit. However, S 3  and S 4  may not be curtailed unlimitedly. S 3  and S 4  have to maintain a certain length to ensure the breakdown voltage of the p-n well junction of ESD circuit device to be greater than 1.5 times of DC operation voltage of the IC. The distance S 1  and S 2  are arranged to have a better ESD durability. Generally, S 1  and S 2  will be greater than S 3  and S 4 , and more specifically S 1  is equal to S 2  and S 3  is equal to S 4 . 
     Second Embodiment 
     In  FIG. 4 , a first well  10  of a first type is formed on a semiconductor substrate. A second well  20  of a second type, alongside the first well  10 , is also formed. According to one embodiment, a first heavily doped regions  30  and a second heavily doped region  41  are formed in the wells  10  and  20 . Particularly, the first heavily doped region  30  of the first type is formed in the first well  10 . The second heavily doped region  41  of the second type is formed in the second well  20 . In one embodiment, the first heavily doped region  30  comprises a p+ type region formed in the p-well  10 . The second heavily doped region  41  comprises an n+ type region formed in the n-well  20  having a battlement layout pattern along the boundary between the first and the second wells. 
     In  FIG. 3 , a first well  10  of a first type is formed on a semiconductor substrate. A second well  20  of a second type, alongside the first well  10 , is also formed. According to one embodiment, a first heavily doped region  31  and a second heavily doped region  41  are formed in the wells  10  and  20 . Particularly, the first heavily doped region  31  of the first type is formed in the first well  10 . The second heavily doped region  41  of the second type is formed in the second well  20 . In one embodiment, the first heavily doped region  31  comprises a p+ type region formed in the p-well  10  having a battlement layout pattern including a plurality of recessed concave regions  313  and ridges  311  along a side facing the boundary between the first and the second wells  10  and  20 . As shown, the first well  10  of the first doping type occupies entirely each of the recessed concave regions  313 , and extends continuously from each of the recessed concave regions  313  and ridges  311  to the boundary between the first and second wells  10  and  20 . The second heavily doped region  41  comprises an n+ type region formed in the n-well  20  having a battlement layout pattern including a plurality of recessed concave regions  413  and ridges  411  along a side facing the boundary between the first and the second wells  10  and  20 . In the same manner, the second well  20  of the second doping type occupies entirely each of the recessed concave regions  413 , and extends continuously from each of the recessed concave regions  413  and ridges  411  to the boundary between the first and second wells  10  and  20 . In one embodiment, the recessed concave regions  413  in the second heavily doped region  41  are respectively opposite to the recessed concave regions  313  in the first heavily doped region  31  as shown in the figure. 
     Third Embodiment 
     Furthermore, a distance between each ridge  311  of the first heavily doped region  31  and the edge  200  of the second well  20  facing the ridges  311  is S 3 . A distance between each ridge  411  of the second heavily doped region  41  and the edge  200  of the second well  20  is S 4 . A distance between a bottom  312  of each recessed concave region  313  and the edge  200  of the second well  20  is S 1 . A distance between a bottom  412  of each recessed concave region  413  and the edge  200  of the second well  20  is S 2 . The distances S 3  and S 4  are arranged to be less than what is typically required by the layout rule of internal circuit. If the DC power noise occurs, the abrupt power noise pulse will breakdown the p-n well junction of the ESD circuit device and discharge ESD current to protect the internal circuit. However, S 3  and S 4  may not be curtailed unlimitedly. S 3  and S 4  have to maintain a certain length to ensure the breakdown voltage of the p-n well junction of the ESD circuit device to be greater than 1.5 times the DC operation voltage of the IC. The distances S 1  and S 2  are arranged to have a better ESD durability. Generally, S 1  and S 2  will be greater than S 3  and S 4 , and more specifically S 1  is equal to S 2  and S 3  is equal to S 4 . 
     In  FIG. 4 , a first well  10  of a first type is formed on a semiconductor substrate. A second well  20  of a second type, alongside the first well  10 , is also formed. According to one embodiment, a first heavily doped region  30  and a second heavily doped region  41  are formed in the wells  10  and  20 . Particularly, the first heavily doped region  30  of the first type is formed in the first well  10 . The second heavily doped region  41  of the second type is formed in the second well  20 . In one embodiment, the first heavily doped region  30  comprises a p+ type region formed in the p-well  10 . The second heavily doped region  41  comprises an n+ type region formed in the n-well  20  having a battlement layout pattern including the recessed concave regions  413  and ridges  411  along a side facing the boundary between the first and the second wells  10  and  20 . Like in the previous embodiment, the second well  20  of the second doping type occupies entirely each of the recessed concave regions  413 , and extends continuously from each of the recessed concave regions  413  and ridges  411  to the boundary between the first and second wells  10  and  20 . 
     Furthermore, a distance between the edge  300  of the first heavily doped region  30  and the edge  200  of the second well  20  facing each other is S 1 . A distance between each ridge  411  of the second heavily doped region  41  and the edge  200  of the second well  20  is S 4 . A distance between the bottom  412  of each recessed concave region  413  and the edge  200  of the second well  20  is S 2 . The distance S 4  is arranged to be less than what is typically required by the layout rule of internal circuit. If the DC power noise occurs, the abrupt power noise pulse will breakdown the p-n well junction of the ESD circuit device and discharge ESD current to protect the internal circuit. However, S 4  may not be curtailed unlimitedly. S 4  has to maintain a certain length to ensure the breakdown voltage of the p-n well junction of the ESD circuit device to be greater than 1.5 times the DC operation voltage of the IC. The distances S 1  and S 2  are arranged to have a better ESD durability. Generally, S 1  and S 2  will be greater than S 4 , and more specifically, S 1  is equal to S 2 . 
     In  FIG. 5 , a first well  10  of a first type is formed on a semiconductor substrate. A second well  20  of a second type, alongside the first well  10 , is also formed. According to one embodiment, a first heavily doped region  31  and a second heavily doped region  40  are formed in the wells  10  and  20 . Particularly, the first heavily doped region  31  of the first type is formed in the first well  10 . The second heavily doped region  40  of the second type is formed in the second well  20 . In one embodiment, the first heavily doped region  31  comprises a p+ type region formed in the p-well  10  having a battlement layout pattern including the recessed concave regions  313  and ridges  311  along a side facing the boundary between the first and the second wells  10  and  20 . Likewise, the first well  10  of the first doping type occupies entirely each of the recessed concave regions  313 , and extends continuously from each of the recessed concave regions  313  and ridges  311  to the boundary between the first and second wells  10  and  20 . The second heavily doped region  40  comprises an n+ type region formed in the n-well  20 . 
     Furthermore, a distance between each ridge  311  of the first heavily doped region  31  and the edge  200  of the second well  20  is S 3 . A distance between the region edge  400  of the second heavily doped region  40  and the edge  200  of the second well  20  is S 2 . A distance between the bottom  312  of each recessed concave region  313  and edge  200  of the second well  20  is Sl. The distance S 3  is arranged to be less than what is typically required by the layout rule of internal circuit. If the DC power noise occurs, the abrupt power noise pulse will breakdown the p-n well junction of the ESD circuit device and discharge ESD current to protect the internal circuit. However, S 3  may not be curtailed unlimitedly. S 3  has to maintain a certain length to ensure the breakdown voltage of the p-n well junction of the ESD circuit device to be greater than 1.5 times the DC operation voltage of the IC. The distances S 1  and S 2  are arranged to have a better ESD durability. Generally, S 1  and S 2  will be greater than S 3 , and specifically, S 1  is equal to S 2 .