Abstract:
A lateral double diffused metal-oxide-semiconductor device includes: a semiconductor substrate; an epitaxial semiconductor layer disposed over the semiconductor substrate; a gate structure disposed over the epitaxial semiconductor layer; a first doped region disposed in the epitaxial semiconductor layer at a first side of the gate structure; a second doped region disposed in the epitaxial semiconductor layer at a second side of the gate structure; a third doped region disposed in the first doped region; a fourth doped region disposed in the second doped region; a trench formed in the third doped region, the first doped region and the epitaxial semiconductor layer under the first doped region; a conductive contact formed in the trench; and a fifth doped region disposed in the epitaxial semiconductor layer under the trench.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to integrated circuit (IC) devices, and in particular to a lateral double diffused metal-oxide-semiconductor (LDMOS) device and a method for fabricating the same. 
     2. Description of the Related Art 
     Recently, due to the rapid development of communication devices such as mobile communication devices and personal communication devices, wireless communication products such as mobile phones and base stations have been greatly developed. In wireless communication products, high-voltage elements of lateral double diffused metal-oxide-semiconductor (LDMOS) devices are often used as radio frequency (900 MHz-2.4 GHz) related elements therein. 
     LDMOS devices not only have a higher operating frequency, but they are also capable of sustaining a higher breakdown voltage, thereby having a high output power so that they can be used as power amplifiers in wireless communication products. In addition, due to the fact that LDMOS devices can be formed by conventional CMOS fabrications, LDMOS devices can be fabricated from a silicon substrate which is relatively cost-effective and employs mature fabrication techniques. 
     In  FIG. 1 , a schematic cross section showing a conventional N-type lateral double diffused metal-oxide-semiconductor (LDMOS) device applicable in a radio frequency (RF) circuit element is illustrated. As shown in  FIG. 1 , the N-type LDMOS device mainly comprises a P+ type semiconductor substrate  100 , a P− type epitaxial semiconductor layer  102  formed over the P+ type semiconductor substrate  100 , and a gate structure G formed over a portion of the P− type epitaxial semiconductor layer  102 . A P− type doped region  104  is disposed in the P− type epitaxial semiconductor layer  102  under the gate structure G and a portion of the P− type epitaxial semiconductor layer  102  under the left side of the gate structure G, and an N− type drift region  106  is disposed in a portion of the P− type epitaxial semiconductor layer  102  under the right side of the gate structure G. A P+ type doped region  130  and an N+ type doped region  110  are disposed in a portion of the P type doped region  104 , and the P+ doped region  130  partially contacts a portion of the N+ type doped region  110 , thereby functioning as a contact region (e.g. P+ type doped region  130 ) and a source region (e.g. N+ type doped region  110 ) of the N type LDMOS device, respectively, and another N+ type doped region  108  is disposed in a portion of the P− type epitaxial semiconductor layer  102  at the right side of the N− type drift region  106  to function as a drain region of the N type LDMOS device. In addition, an insulating layer  112  is formed over the gate structure G, covering sidewalls and a top surface of the gate structure G and partially covering the N+ type doped regions  108  and  110  adjacent to the gate structure G. Moreover, the N type LDMOS further comprises a P+ type doped region substantially disposed in a portion of the P− type epitaxial semiconductor layer  102  under the N+ type doped region  110  and the P− type doped region  104  under the N+ type doped region  110 . The P+ type doped region  120  physically connects the P− type doped region  104  with the P+ type semiconductor substrate  100 . 
     During operation of the N type LDMOS device shown in  FIG. 1 , due to the formation of the P+ type doped region  120 , currents (not shown) from the drain side (e.g. N+ type doped region  108 ) laterally flow through a channel (not shown) underlying the gate structure G towards a source side (e.g. N+ type doped region  110 ), and are then guided by the P− type doped region  104  and the P+ type doped region  120 , thereby arriving at the P+ type semiconductor substrate  100 , such that problems such as inductor coupling and cross-talk between adjacent circuit elements can be avoided. However, the formation of the P+ type doped region  120  requires the performance of ion implantations of high doping concentrations and high doping energies and thermal diffusion processes with a relatively high temperature above about 900° C., and a predetermined distance D1 is kept between the gate structure G and the N+ type doped region  110  at the left side of the gate structure G to ensure good performance of the N type LDMOS device. Therefore, formation of the P+ type doped region  120  and the predetermined distance D1 kept between the gate structure G and the N+ type doped region  110  increase the on-state resistance (Ron) of the N type LDMOS device and a dimension of the N type LDMOS device, which is unfavorable for further reduction of both the fabrication cost and the dimensions of the N type LDMOS device. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, an improved lateral double diffused metal oxide semiconductor (LDMOS) device and method for fabricating the same are provided to reduce size and fabrication cost. 
     An exemplary lateral double diffused metal oxide semiconductor (LDMOS) device comprises a semiconductor substrate, having a first conductivity type. An epitaxial semiconductor layer is formed over the semiconductor substrate, having the first conductivity type. A gate structure is disposed over a portion of the epitaxial semiconductor layer. A first doped region is disposed in a portion of the epitaxial semiconductor layer adjacent to a first side of the gate structure, having the first conductivity type. A second doped region is disposed in a portion of the epitaxial semiconductor layer adjacent to a second side of the gate structure opposing to the first side, having a second conductivity type opposite to the first conductivity type. A third doped region is disposed in a portion of the first doped region, having the second conductivity type. A fourth doped region is disposed in a portion of the second doped region, having the second conductivity type. A trench is formed in a portion of the third doped region, the first doped region, and the epitaxial semiconductor layer under the first doped region. A conductive contact is formed in the trench. A fifth doped region is disposed in a portion of the epitaxial semiconductor layer under the first doped region, having the first conductivity type, wherein the fifth doped region physically contacts the semiconductor substrate and surrounds portions of sidewalls and a bottom surface of the conductive contact. 
     An exemplary method for fabricating a lateral double diffused metal oxide semiconductor (LDMOS) device comprises providing a semiconductor substrate, having a first conductivity type. An epitaxial semiconductor layer is formed over the semiconductor substrate, having the first conductivity type. A gate structure is formed over a portion of the epitaxial semiconductor layer. A first doped region is formed in a portion of the epitaxial semiconductor layer adjacent to a first side of the gate structure, having the first conductivity type. A second doped region is formed in a portion of the epitaxial semiconductor layer adjacent to a second side of the gate structure opposite to the first side, having a second conductivity type opposite to the first conductivity type. A third doped region is formed in a portion of the first doped region, having the second conductivity type. A fourth doped region is formed in a portion of the second doped region, having the second conductivity type opposite to the first conductivity type. An insulating layer is formed over the second doped region, the gate structure, and a portion of the third doped region. A trench is formed in a portion of the third doped region, and the epitaxial semiconductor layer under the first doped region adjacent to the insulating layer. An ion implantation process is performed, implanting dopants of the first conductive type in the epitaxial semiconductor layer exposed by the trench, thereby forming a fifth doped region, wherein the fifth doping region physically contacts the semiconductor substrate. A conductive contact is formed in the trench, wherein the conductive contact physically contacts the fifth doped region. 
     An exemplary method for fabricating a lateral double diffused metal oxide semiconductor (LDMOS) device comprises providing a semiconductor substrate, having a first conductivity type. A first epitaxial semiconductor layer is formed over the semiconductor substrate, having the first conductivity type. A first trench is formed in a portion of the first epitaxial semiconductor layer. An ion implantation process is performed, implanting dopants of the first conductivity type in the first epitaxial semiconductor layer exposed by the first trench, thereby forming a first doped region, wherein the first doped region physically contacts the semiconductor substrate. A second epitaxial semiconductor layer is formed in the first trench. A gate structure is formed over a portion of the epitaxial semiconductor layer, adjacent to the second epitaxial semiconductor layer. A second doped region is formed in a portion of the first epitaxial semiconductor layer adjacent to a first side of the gate structure, surrounding the second epitaxial semiconductor layer, having the first conductivity type. A third doped region is formed in a portion of the first epitaxial semiconductor layer adjacent to a second side of the gate structure opposing to the first side, having a second conductivity type opposite to the first conductivity type. A fourth doped region is formed in a portion of the second doped region, having the second conductivity type. A fifth doped region is formed in a portion of the third doped region, having the second conductivity type. An insulating layer is formed over the fourth doped region, the gate structure, and a portion of the fifth doped region. The second epitaxial semiconductor layer is partially removed to form a second trench, wherein the second trench partially exposes a portion of the second and fourth doped regions. A conductive contact is formed in the second trench, wherein the conductive contact physically contacts the second epitaxial semiconductor layer. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is schematic cross section of a conventional lateral double diffused metal-oxide-semiconductor (LDMOS) device; 
         FIGS. 2-6  are schematic cross sections showing a method for fabricating a lateral double diffused metal-oxide-semiconductor (LDMOS) device according to an embodiment of the invention; and 
         FIGS. 7-11  are schematic cross sections showing a method for fabricating a lateral double diffused metal-oxide-semiconductor (LDMOS) device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIGS. 2-6  are schematic cross sections showing an exemplary method for fabricating a lateral double diffused metal-oxide-semiconductor (LDMOS) device applicable for a radio frequency (RF) circuit element. 
     Referring to  FIG. 2 , a semiconductor substrate  200  such as a silicon substrate is first provided. In one embodiment, the semiconductor substrate  200  has a first conductivity type such as a P type, and a resistivity of about 0.001-0.005 ohms-cm (Ω-cm). Next, an epitaxial semiconductor layer  202 , for example an epitaxial silicon layer, is formed over the semiconductor substrate  200 . The epitaxial semiconductor layer  202  can be in-situ doped with dopants of the first conductivity type such as P type during the formation thereof, and has a resistivity of about 0.5-1 ohms-cm (Ω-cm). In one embodiment, the resistivity of the epitaxial semiconductor layer  202  is greater than that of the semiconductor substrate  200 . 
     Referring to  FIG. 3 , a patterned gate structure G is formed over a portion of the epitaxial semiconductor layer  202 , and the gate structure G mainly comprises a gate dielectric layer  204  and a gate electrode  206  sequentially formed over a portion of the epitaxial semiconductor layer  202 . The gate dielectric layer  204  and the gate electrode  206  of the gate structure G can be formed by conventional gate processes and related gate materials, and are not described here in detail for the purpose of simplicity. Next, a plurality of suitable masks (not shown) and a plurality of ion implant processes (not shown) are then performed to form a doped region  208  in a portion of the epitaxial semiconductor layer  202  at the left side of the gate structure G, and a doped region  210  in a portion of the epitaxial semiconductor layer  202  at the right side of the gate structure G. In one embodiment, the doped region  208  has the first conductivity type such as P type and a dopant concentration of about 1*10 13 -5*10 14  atom/cm 2 , and the doped region  210  has a second conductivity type such as N type opposite to the P type and a dopant concentration of about 5*10 11 -5*10 13  atom/cm 2 , and the ion implant processes (not shown) for forming the doped regions  208  and  210  can be ion implant processes with tilted implantation angles. 
     Next, another suitable implant mask (not shown) and an ion implantation process (not shown) are performed to form a doped region  212  and a doped region  214  in a portion of the doped regions  208  and  210  on opposite sides of the gate structure G, respectively, and the configuration shown in  FIG. 3  is formed after performing a thermal diffusion process (not shown). In one embodiment, the doped region  212  formed in a portion of the doped region  208  and the doped region  214  formed in a portion of the doped region  210  respectively have the second conductivity type such as N type and a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 , and the ion implantion process (not shown) for forming the doped regions  212  and  214  can be an ion implantation vertical to a surface of the epitaxial semiconductor layer  202 . In one embodiment, the doped region  210  may function as a drift region, and the doped regions  212  and  214  may function as source and drain regions. 
     Referring to  FIG. 4 , an insulating layer  216  is next formed over the epitaxial semiconductor layer  202 , and the insulating layer  216  conformably cover a plurality of sidewalls and a top surface of the gate structure G. Next, a patterning process (not shown) is performed to form an opening  218  in a portion of the insulating layer  216 . As shown in  FIG. 4 , the opening  218  exposes a portion of the doped region  212  such that other portions of the epitaxial semiconductor layer  202  and surfaces of the gate structure G are still covered by the insulating layer  216 . In one embodiment, the insulating layer  216  may comprise insulating materials such as silicon oxide and silicon nitride, and can be formed by methods such as chemical vapor deposition. 
     Referring to  FIG. 5 , an etching process (not shown) is performed next, using the insulating layer  216  as an etching mask, thereby forming a trench  220  in the epitaxial semiconductor layer  202  exposed by the opening  218 . As shown in  FIG. 5 , the trench  220  is formed with a depth H1 which mainly penetrates the doped region  212 , the doped region  208  and the epitaxial semiconductor layer  202 . Next, an ion implantation process  222  is performed, using the insulating layer  216  as an implantation mask, to implant dopants of the first conductivity type such as P-type to a portion of the epitaxial semiconductor layer  202  exposed by the trench  220 , and then a thermal diffusion process (not shown) is performed, thereby forming a doped region  224  in a portion of the epitaxial semiconductor layer  202  and the semiconductor substrate  200  as shown in  FIG. 5 . In one embodiment, the doped region  224  may have the first conductivity type such as P-type and has a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 . In one embodiment, the dopant concentration in the doped region  224  may be greater than that in the epitaxial semiconductor layer  202 . 
     Referring to  FIG. 6 , a conductive layer  226  and another conductive layer  228  are then sequentially deposited, wherein the conductive layer  226  is conformably formed over surfaces of the insulating layer  216  and the bottom surface and the sidewalls of the epitaxial semiconductor layer  202  exposed by the trench  220 , and the conductive layer  228  is formed over the surfaces of the conductive layer  228 , thereby filling the trench  220 . Next, a suitable implant mask (not shown) and a patterning process (not shown) are performed to pattern the conductive layers  226  and  228 . 
     As shown in  FIG. 6 , the patterned conductive layers  226  and  228  are formed over the insulating layer  214  adjacent to the trench  220 , extending over the bottom surface and the sidewalls of the trench  220 , thereby covering surfaces of the epitaxial semiconductor layer  202 , and the doped regions  208 ,  212  exposed by the trench  220 , and the conductive layers  226  and  228  also cover the gate structure G and a portion of the doped region  210  adjacent to the gate structure G. However, the conductive layers  226  and  228  do not cover the doped region  214 . The portion of the conductive layers  226  and  228  formed in the trench  220  may function as a conductive contact. At this time, the doped region  224  partially encircles the bottom surface and the sidewalls of the conductive layers  226  and  228  formed in the trench  220 . 
     In one embodiment, the conductive layer  226  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  228  may comprise conductive materials such as tungsten. Next, a dielectric material such as silicon oxide or spin-on-glass (SOG) is deposited over the conductive layer  228 , such that the dielectric material covers the conductive layer  228 , the insulating layer  216 , and the gate structure G, thereby forming an inter-layer dielectric (ILD) layer  230  with a substantially planar top surface. Next, a patterning process (not shown) comprising photolithography and etching steps is performed to form a trench  236  in a portion of the ILD layer  230  and the insulating layer  216  over a portion of the doped region  214 , and the trench  236  exposes a portion of the doped region  214 . Next, a conductive layer  238  and another conductive layer  240  are sequentially deposited, and the conductive layer  238  conformably forms over the surfaces of the ILD layer  234  and the sidewalls exposed by the trench  236 , and the conductive layer  240  is formed over the surface of the conductive layer  238 , thereby filling the trench  236 . The portion of the conductive layers  238  and  240  formed in the trench  236  may function as a conductive contact. In one embodiment, the conductive layer  238  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  240  may comprise conductive materials such as tungsten. Therefore, an exemplary LDMOS device is substantially fabricated, as shown in  FIG. 6 . 
     In one embodiment, the gate structure G and the doped regions  212  and  214  of the LDMOS device shown in  FIG. 6  may be properly electrically connected (not shown), and the regions with the first conductivity type can be P type regions, and the regions of the second conductivity type can be N type regions, such that the formed LDMOS device herein is an N type LDMOS device. At this time, the doped region  212  may function as a source region and the doped region  214  may function as a drain region. 
     In this embodiment, during operation of the LDMOS device shown in  FIG. 6 , currents from the drain side (e.g. the doped region  214 ) may laterally flow through a channel (not shown) under the gate stack G and toward the source side (e.g. doped region  212 ), and then arrive at the semiconductor substrate  200  by the guidance of the doped region  208 , the conductive layers  226  and  228 , and the doped region  224 , such that undesired problems such as inductor coupling and cross-talk between adjacent circuit elements can be prevented. 
     In this embodiment, due to the formation of the conductive layers  226  and  228  formed in the trench  220  and the doped region  224  embedded in the epitaxial semiconductor layer  202  and being in contact with the semiconductor substrate  200 , such that ion implantation with high dosages and high energies for forming the P+ type doped region  120  as shown in  FIG. 1  can be avoided, a predetermined distance D2 between the gate structure G and the doped region  212  at the right side of the trench  220  can be less than the predetermined distance D1 as shown in  FIG. 1 . 
     Therefore, when compared with the N type LDMOS device as shown in  FIG. 1 , the N type LDMOS device shown in  FIG. 6  may have the advantages of reduced size and fabrication cost, and formation of the doped region  224  and the conductive layers  226  and  228  also helps to reduce the on-state resistance (Ron) of the N type LDMOS device. 
     In addition, in another embodiment, the regions with the first conductivity type of the LDMOS device shown in  FIG. 6  can be N type regions, and the regions of the second conductivity type can be P type regions, such that the formed LDMOS device herein can be an P type LDMOS device. 
       FIGS. 7-11  are schematic cross sections showing another exemplary method for fabricating a lateral double diffused metal-oxide-semiconductor (LDMOS) device applicable for a radio frequency (RF) circuit element. 
     Referring to  FIG. 7 , a semiconductor substrate  300  such as a silicon substrate is first provided. In one embodiment, the semiconductor substrate  300  has a first conductivity type such as a P type, and a resistivity of about 0.001-0.005 ohms-cm (Ω-cm). Next, an epitaxial semiconductor layer  301 , for example an epitaxial silicon layer, is formed over the semiconductor substrate  300 . The epitaxial semiconductor layer  301  can be doped in-situ with dopants of the first conductivity type such as P type during the formation thereof, and has a resistivity of about 0.5-1 ohms-cm (Ω-cm). In one embodiment, the resistivity of the epitaxial semiconductor layer  301  is greater than that of the semiconductor substrate  300 . Next, a patterned mask layer  302  is formed over the semiconductor substrate  300 , and the patterned mask layer  302  comprises an opening  303  exposing a portion of the epitaxial semiconductor layer  301 . The patterned mask layer  302  may comprise materials such as photoresists and the opening  303  can thus be formed by conventional photolithography and etching processes. Next, an etching process (not shown) is performed, using the patterned mask layer  302  as an etching mask, thereby forming a trench  304  in the epitaxial semiconductor layer  301  exposed by the opening  303 . 
     As shown in  FIG. 7 , the trench  304  is formed with a depth  112 . Next, an ion implantation process  306  is performed, using the patterned mask layer  302  as an implantation mask, to implant dopants of the first conductivity type such as P-type to a portion of the epitaxial semiconductor layer  301  exposed by the trench  304 , and then a thermal diffusion process (not shown) is performed, thereby forming a doped region  308  in a portion of the epitaxial semiconductor layer  301  and the semiconductor substrate  300  as shown in  FIG. 7 . In one embodiment, the doped region  308  may have the first conductivity type such as P-type and a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 . In one embodiment, the dopant concentration in the doped region  308  may be greater than that in the epitaxial semiconductor layer  301 . 
     Referring to  FIG. 8 , after removal of the patterned mask layer  302 , an epitaxial growth process (not shown) is next performed to grow epitaxial semiconductor materials over surfaces of the portions of the epitaxial semiconductor layer  301  exposed by the trench  304  and the top surface of the epitaxial semiconductor layer  301 . The epitaxial semiconductor materials can be in-situ doped with dopants of the first conductivity type such as P type during the formation thereof. Next, a planarization process (not shown) is performed to remove the portion of the epitaxial materials over a top surface of the epitaxial semiconductor layer  301 , thereby forming an epitaxial semiconductor layer  310  of doped epitaxial semiconductor materials in the trench  304  as a conductive layer. In one embodiment, the epitaxial semiconductor layer  310  has a resistivity of about 0.001 ohms-0.05 cm (Ω-cm). 
     Referring to  FIG. 9 , a patterned gate structure G is formed over a portion of the epitaxial semiconductor layer  301 , and the gate structure G mainly comprises a gate dielectric layer  312  and a gate electrode  314  sequentially formed over a portion of the epitaxial semiconductor layer  301 . The gate dielectric layer  312  and the gate electrode  314  of the gate structure G can be formed by conventional gate processes and related gate materials, and are not described here in detail for the purpose of simplicity. Next, a plurality of suitable masks (not shown) and a plurality of ion implant processes (not shown) are then performed to form a doped region  316  in a portion of the epitaxial semiconductor layer  301  at the left side of the gate structure G, and a doped region  318  in a portion of the epitaxial semiconductor layer  301  at the right side of the gate structure G. In one embodiment, the doped region  316  has the first conductivity type such as P type and a dopant concentration of about 1*10 13 -5*10 14  atom/cm 2 , and the doped region  318  has a second conductivity type such as N type opposite to the P type and a dopant concentration of about 5*10 11 -5*10 13  atom/cm 2 , and the ion implant processes (not shown) for forming the doped regions  316  and  318  can be ion implant processes with tilted implantation angles. 
     Next, another suitable implant mask (not shown) and an ion implantation process (not shown) are performed to form a doped region  320  and a doped region  322  in a portion of the doped regions  316  and  318  on opposite sides of the gate structure G, respectively, and the configuration shown in  FIG. 9  is formed after performing a thermal diffusion process (not shown). In one embodiment, the doped region  320  formed in a portion of the doped region  316  and the doped region  322  formed in a portion of the doped region  318  respectively has the second conductivity type such as N type and a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 , and the ion implant process (not shown) for forming the doped regions  320  and  322  can be an ion implantation vertical to a surface of the epitaxial semiconductor layer  301 . In one embodiment, the doped region  318  may function as a drift region, and the doped regions  320  and  322  may function as source and drain regions. As shown in  FIG. 9 , the doped regions  316  and  320  formed in the epitaxial semiconductor layer  301  at the left side of the gate structure surround a portion of the epitaxial semiconductor layer  310 . 
     Referring to  FIG. 10 , an insulating layer  324  is next formed over the epitaxial semiconductor layer  301 , and the insulating layer  324  conformably cover a plurality of sidewalls and a top surface of the gate structure G. Next, a patterning process (not shown) is performed to form an opening  325  in a portion of the insulating layer  324 . As shown in  FIG. 10 , the opening  325  exposes a portion of the epitaxial semiconductor layer  310  surrounded by the doped region  320  such that other portions of the epitaxial semiconductor layer  301  and surfaces of the gate structure G are still covered by the insulating layer  324 . In one embodiment, the insulating layer  216  may comprise insulating materials such as silicon oxide and silicon nitride, and can be formed by methods such as chemical vapor deposition. Next, an etching process (not shown) is performed, using the insulating layer  324  as an etching mask, thereby forming a trench  326  in the epitaxial semiconductor layer  310  exposed by the opening  325 . As shown in  FIG. 10 , the trench  325  is formed with a depth H3, and the trench  326  mainly exposes portions of the doped region  320 , the doped region  316  and the epitaxial semiconductor layer  310 . 
     Referring to  FIG. 11 , a conductive layer  328  and another conductive layer  330  are then sequentially deposited, wherein the conductive layer  328  is conformably formed over surfaces of the insulating layer  324  and the bottom surface and the sidewalls of the doped regions  320 ,  316 , and the epitaxial semiconductor layer  310  exposed by the trench  326 , and the conductive layer  330  is formed over the surfaces of the conductive layer  328 , thereby filling the trench  326 . Next, a suitable implant mask (not shown) and a patterning process (not shown) are performed to pattern the conductive layers  328  and  330 . 
     As shown in  FIG. 11 , the patterned conductive layers  328  and  330  are formed over the insulating layer  324  adjacent to the trench  326 , extending over the bottom surface and the sidewalls of the trench  326 , thereby covering surfaces of the epitaxial semiconductor layer  310 , and the doped regions  316 ,  320  exposed by the trench  326 , and the conductive layers  328  and  330  also cover the gate structure G and a portion of the doped region  318  adjacent to the gate structure G. However, the conductive layers  328  and  330  do not cover the doped region  322 . The portion of the conductive layers  328  and  330  formed in the trench  326  and the epitaxial conductive layer  310  thereunder may function as a conductive contact. 
     In one embodiment, the conductive layer  328  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  330  may comprise conductive materials such as tungsten. Next, a dielectric material such as silicon oxide or spin-on-glass (SOG) is deposited over the conductive layer  330 , such that the dielectric material covers the conductive layer  330 , the insulating layer  324 , and the gate structure G, thereby forming an inter-layer dielectric (ILD) layer  332  with a substantially planar top surface. Next, a patterning process (not shown) comprising photolithography and etching steps is performed to form a trench  336  in a portion of the ILD layer  332  and the insulating layer  324  over a portion of the doped region  322 , and the trench  336  exposes a portion of the doped region  322 . Next, a conductive layer  338  and another conductive layer  340  are sequentially deposited, and the conductive layer  338  conformably forms over the surfaces of the ILD layer  332  and the sidewalls exposed by the trench  336 , and the conductive layer  340  is formed over the surface of the conductive layer  338 , thereby filling the trench  336 . The portion of the conductive layers  338  and  340  formed in the trench  336  may function as a conductive contact. In one embodiment, the conductive layer  338  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  340  may comprise conductive materials such as tungsten. Therefore, an exemplary LDMOS device is substantially fabricated, as shown in  FIG. 11   
     In one embodiment, the regions with the first conductivity type in the LDMOS device shown in  FIG. 11  can be P type regions, and the regions of the second conductivity type can be N type regions, such that the formed LDMOS device herein is an N type LDMOS device. At this time, the doped region  320  may function as a source region and the doped region  322  may function as a drain region. 
     In this embodiment, during operation of the LDMOS device shown in  FIG. 11 , currents from the drain side (e.g. the doped region  322 ) may laterally flow through a channel (not shown) under the gate stack G and toward the source side (e.g. doped region  320 ), and then arrive at the semiconductor substrate  300  by the guidance of the doped regions  320  and  316 , the epitaxial semiconductor layer  310 , the conductive layers  328  and  330 , and the doped region  308 , such that undesired problems such as inductor coupling and cross-talk between adjacent circuit elements can be prevented. 
     In this embodiment, due to the formation of the conductive layers  328  and  330 , the epitaxial layer  310  formed in the trench  326 , and the doped region  308  embedded in the epitaxial semiconductor layer  301  and being in contact with the semiconductor substrate  300 , such that ion implantation with high dosages and high energies for forming the P+ type doped region  120  as shown in  FIG. 1  can be avoided, a predetermined distance D2 between the gate structure G and the doped region  212  at the right side of the trench  220  can be less than the predetermined distance D1 as shown in  FIG. 1 . Therefore, when compared with the N type LDMOS device as shown in  FIG. 1 , the N type LDMOS device shown in  FIG. 11  may have the advantages of reduced size and fabrication cost, and formation of the doped region  308 , the epitaxial semiconductor layer  310 , and the conductive layers  328  and  330  also helps to reduce the on-state resistance (Ron) of the N type LDMOS device. 
     In addition, in another embodiment, the regions with the first conductivity type of the LDMOS device shown in  FIG. 11  can be N type regions, and the regions of the second conductivity type can be P type regions, such that the formed LDMOS device herein can be a P type LDMOS device. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.