Abstract:
A semiconductor device includes a first doped region disposed on a first well in a semiconductor substrate; a second doped region disposed on a second well adjacent to the first well in the semiconductor substrate, the second doped region having a dopant density higher than that of the second well; and a gate structure overlying parts of the first and second wells for controlling a current flowing between the first and second doped regions. A first spacing distance from an interface between the second doped region and the second well to its closest edge of the gate structure is greater than 200 percent of a second spacing distance from a center point of second doped region to the edge of the gate structure, thereby increasing impedance against an electrostatic discharge (ESD) current flowing between the first and second doped regions during an ESD event.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to an integrated circuit (IC) design, and more particularly to a laterally diffused metal-oxide-semiconductor (LDMOS) device with improved electrostatic discharge (ESD) performance.  
         [0002]     As semiconductor devices continue to shrink in size, their susceptibility to ESD damage becomes an increasingly important reliability concern. An ESD event occurs when electrostatic charge is transferred between one or more pins of an IC and another object in a short period of time. The rapid charge transfer often generates voltages large enough to break down insulating films of semiconductor devices, thereby causing permanent damages. In order to protect the semiconductor devices from ESD damages, various protection circuits can be implemented at the input and output pins of the IC to shunt ESD currents away from sensitive internal structures.  
         [0003]     The LDMOS transistor is one kind of the semiconductor devices that are particularly susceptible to damages caused by the ESD event. The LDMOS device, featured by its extended source or drain doped region, is often found in circuits operating in high voltages, such as 5, 12, 40, 100 and 1000 volts. Conventionally, the LDMOS transistor requires some additional devices or circuit modules to protect it from ESD damages. These ESD protection devices and circuit modules typically require a separate set of fabrication process steps different from those for the LDMOS transistors. Thus, the addition of these ESD protection devices and circuit modules increases the costs of manufacturing ICs that have LDMOS transistors implemented thereon.  
         [0004]     As such, it is desirable to have a LDMOS transistor with improved ESD protection performance, thereby eliminating the need for other additional ESD protection devices and circuit modules.  
       SUMMARY  
       [0005]     The present invention discloses a semiconductor device with improved ESD protection performance. In one embodiment of the invention, the semiconductor device includes a first doped region disposed on a first well in a semiconductor substrate; a second doped region disposed on a second well adjacent to the first well in the semiconductor substrate, the second doped region having a dopant density higher than that of the second well; and a gate structure overlying parts of the first and second wells for controlling a current flowing between the first and second doped regions. A first spacing distance from an interface between the second doped region and the second well to its closest edge of the gate structure is greater than 200 percent of a second spacing distance from a center point of second doped region to the edge of the gate structure, thereby increasing impedance against an electrostatic discharge (ESD) current flowing between the first and second doped regions during an ESD event.  
         [0006]     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  
       [0007]      FIG. 1A  illustrates a cross-sectional diagram of a conventional LDMOS device set.  
         [0008]      FIG. 1B  illustrates a layout diagram of the conventional LDMOS device set shown in  FIG. 1A .  
         [0009]      FIG. 2A  illustrates a cross-sectional diagram of a LDMOS device set in accordance with one embodiment of the present invention.  
         [0010]      FIG. 2B  illustrates a layout diagram of the LDMOS device set shown in  FIG. 2A  in accordance with one embodiment of the present invention. 
     
    
     DESCRIPTION  
       [0011]      FIG. 1A  illustrates a cross-sectional diagram  100  of a conventional LDMOS device set, which includes two transistors  102  and  104 . The transistors  102  and  104  can be used as ESD protection devices or normally functioned devices. Both the transistors  102  and  104  are implanted on an N-tub  106  above a P-type substrate  108 . Each of the transistors  102  and  104  includes a gate structure  110 , an N+ doped source region  112 , and a shared N+ doped drain region  114 .  
         [0012]     The N+ doped source regions  112  for both the transistors  102  and  104  are formed on P-type wells  119 . A P+ doped region  115  is also formed on one of the P-type wells  119  to provide a substrate contact. The N+ doped drain region  114  is formed on an N-type well  117  having a dopant density lower than that of the region  114 . The gate structures  110  for both transistors  102  and  104  are formed on the surface that overlies parts of the P-type wells  119 , the N-tub  106  and the N-type well  117 . A set of source contacts  116  and  118  are implemented respectively at the N+ doped source regions of the transistors  102  and  104 , while a drain contact  120  is implemented at the N+ doped drain region  114 .  
         [0013]      FIG. 1B  illustrates a layout diagram  122  of the conventional LDMOS device set shown in  FIG. 1A . The layout diagram  122  shows the two LDMOS transistors  102  and  104  constructed by the gate structures  110 , the N+ doped source regions  112 , and the shared N+ doped drain region  114 , within a P+ guard ring  124 . The layout diagram  122  further illustrates the small spacing distances  121  between the N+ doped drain region  114  and the gate structures  110 , which are critical for the transistors  102  and  104  to withstand ESD currents.  
         [0014]     Referring simultaneously to both  FIGS. 1A and 1B , the spacing distances  121  are not long enough to provide sufficient impedance against an electrical current flowing between the N+ doped drain region  114  and the N+ doped source regions  112  when the transistors  102  and  104  are at the off state. During an ESD event occurring at the contact  120 , there is a high possibility that the ESD current would break down the transistors  102  and  104  and flow from the N+ doped drain region  114  to the N+ doped source regions  112 . As a result, the LDMOS transistors  102  and  104  are very susceptible to ESD damages. Additional ESD protection devices or circuit modules may be needed to protect the transistors  102  and  104 , thereby increasing manufacturing costs.  
         [0015]      FIG. 2A  illustrates a cross-sectional diagram  200  of a LDMOS device set in accordance with one embodiment of the present invention. The LDMOS device includes two transistors  202  and  204 , which are implanted on an N-tub  206  above a P-type substrate  208 . Each of the transistors  202  and  204  includes a gate structure  210 , an N+ doped source region  212 , and a shared N+ doped drain region  214 . The gate structure  210  can be constructed by a dielectric layer such as silicon oxide or nitride, and a conductive layer such as polysilicon or other metal materials. The P-type substrate  208  can be made of, for example, silicon, germanium, silicon-germanium alloys, or silicon on insulation (SOI) structures.  
         [0016]     The N+ doped source regions  212  for both the transistors  202  and  204  are formed on P-type wells  230 . A P+ doped region  215  is also formed on one of the P-type wells  230  to provide a substrate contact. The N+ doped drain region  214  is formed on an N-type well  232  having a dopant density lower than that of the region  214 . In this embodiment, the dopant density of the N+ doped drain region  214  ranges approximately from 1×10 14  (1/cm 2) to  1×10 17 ( 1/cm 2 ), while the dopant density of the N-type well  232  ranges approximately from 1×10 11  (1/cm 2) to  1×10 14  (1/cm 2 ). The gate structures  210  for both transistors  202  and  204  are formed on the surface that overlies parts of the P-type wells  230 , the N-tub  206  and the N-type well  232 . A set of source contacts  216  and  218  are implemented respectively at the N+doped source regions  212  of the transistors  202  and  204 , while a drain contact  220  is implemented at the N+ doped drain region  214 .  
         [0017]      FIG. 2B  illustrates a layout diagram  222  of the LDMOS device set shown in  FIG. 2A  in accordance with the embodiment of the present invention. The layout diagram  222  shows the two LDMOS transistors  202  and  204  constructed by the gate structures  210 , the N+ doped source regions  212 , and the shared N+ doped drain region  214 , within a P+ guard ring  224 . The layout diagram  222  further illustrates the small spacing distances  221  between the N+ doped drain region  214  and the gate structures  210 , which are critical for the transistors  202  and  204  to withstand ESD currents.  
         [0018]     Referring to  FIGS. 2A and 2B  simultaneously, unlike the conventional LDMOS device shown in  FIG. 1A , the N+ doped drain region  214  is designed to be much smaller in physical size, thereby allowing the spacing distances  221  between the gate structures  210  and the N+ implanted drain  214  to be increased. This, in turn, increases the impedance between the N+ doped drain region  214  and the N+ doped source regions  212 . When an ESD event occurs at the contact  220 , it would be more difficult for the ESD current to pass from the N+ doped drain region  214  to the N+ doped source region  212 . Thus, the transistors  202  and  204  can withstand ESD better than their conventional counterparts. As such, the need for ESD protection devices or circuit modules that are particularly designed for the LDMOS device can be eliminated, thereby significantly reducing the manufacturing costs.  
         [0019]     The determination of the value of spacing distance  221  is a matter of optimization, depending on, for example, the device dimensions. As a general rule the spacing distance  221  measured from an interface between the N+ doped drain region  214  and the N-type well  232  to its closest edge of the gate structure  210  should be greater than 200 percent of a reference spacing distance  234  from a center point of N+ doped drain region  214  to the edge of the gate structure. In a specific embodiment, the spacing distance  221  should be greater than 1.5 μm. The LDMOS transistors  202  and  204  can be dedicated ESD protection devices as opposed to other normally functioned LDMOS transistors. The LDMOS transistors that function as ESD protection devices should have a longer spacing distance than that of the normally functioned LDMOS transistors.  
         [0020]     While the above-mentioned embodiment uses N-type LDMOS transistors for descriptive purposes, it is understood by those skilled in the art that they can also be P-type LDMOS transistors. Specifically, the doped regions  212  and  214 , and the well  232  should be doped with the same type of dopant, while the well  230  should be doped with a different type of dopant. It is noted that silicide layers interfacing the contacts  216  and  220  and the doped regions  212  and  214  can be alternatively formed in another embodiment of the present invention.  
         [0021]     Table I below provides a set of test data demonstrating how ESD performance of a LDMOS device can be improved by increasing the spacing between its doped drain region and gate structure.  
                                                                         TABLE I                       De-           Total       +HBM/           vice   PO—N+   N+—CO   Width   IT   Vss   +MM/Vss                                A   0.4 μm   0.25 μm   360 μm   0.2 A   0.5 kV   Below 50   V       B   1.0 μm   0.25 μm   360 μm   0.6 A   2.0 kV   100   V       C   1.5 μm   0.25 μm   360 μm   5.1 A   3.0 kV   350   V       D   2.0 μm   0.25 μm   360 μm   5.2 A   7.5 kV   500   V                  
 
         [0022]     Specifically, Table I shows specifications and test results of four different LDMOS devices: A, B, C, and D. All four LDMOS devices A, B, C, and D are designed to have a total width of 360 μm and a spacing of 0.25 μm between the edges of the drain region and the drain contact. The LDMOS device A is designed to the specifications of the conventional LDMOS device shown in  FIG. 1A . More specifically, the LDMOS device A has a small spacing of 0.4 μm between the drain region and the gate structure. The LDMOS devices B, C, and D are designed to the specifications of the LDMOS device shown in  FIG. 2A . More specifically, the LDMOS devices B, C, and D have corresponding spacing distances of 1.0, 1.5, and 2.0 μm between the drain region and the gate structures. From the results of the test, it can be shown that by setting the spacing between the gate structures and doped drain region higher, ESD performance of the LDMOS device can be improved.  
         [0023]     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.  
         [0024]     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.