Patent Abstract:
A LDMOS device includes a substrate having opposite first and second surfaces; a well region in a portion of the substrate; a gate structure over a portion of the substrate; a first doped region disposed in a portion of the well region from a first side; a second doped region disposed in the well region from a second side; a third doped region disposed in the first doped region; a fourth doped region disposed in the second doped region; a first trench in the third doped region, the first doped region, the well region, and the substrate adjacent to the first surface; a conductive contact in the first trench; a second trench in the substrate adjacent to the second surface; a first conductive layer in second trench; and a second conductive layer over the second surface of the substrate and the first conductive layer.

Full 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 developed greatly. 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 operation 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 a 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 a 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 the P+ type semiconductor substrate  100 , such that problems such as inductor coupling and cross-talk between adjacent circuit elements can be avoided. However, formation of the P+ type doped region  120  needs to perform 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 opposite first and second surfaces and a first conductivity type; a well region formed in a portion of the semiconductor substrate adjacent to the first surface thereof, having the first conductivity type; a gate structure disposed over a portion of the first surface of the semiconductor substrate; a first doped region disposed in a portion of the well region adjacent to a first side of the gate structure, having the first conductivity type; a second doped region disposed in a portion of the well region 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 disposed in a portion of the first doped region, having the second conductivity type; a fourth doped region disposed in a portion of the second doped region, having the second conductivity type; a first trench formed in a portion of the third doped region, the first doped region, the well region, and the semiconductor substrate; a conductive contact formed in the first trench; a second trench formed in a portion of the semiconductor substrate adjacent to the second surface thereof, wherein the second trench exposes a portion of the conductive contact; a first conductive layer formed in second trench, contacting the conductive contact; and a second conductive layer formed over the second surface of the semiconductor substrate and the first conductive layer. 
     An exemplary method for fabricating a lateral double diffused metal oxide semiconductor (LDMOS) device comprises: performing a semiconductor substrate, having opposite first and second surfaces and a first conductivity type; performing an ion implantation process, forming a well region in a portion of the semiconductor substrate adjacent to the first surface thereof, having the first conductivity type; forming a gate structure over a portion of the well region; forming a first doped region in a portion of the well region adjacent to a first side of the gate structure, having the first conductivity type; forming a second doped region in a portion of the well region at a second side of the gate structure opposite to the first side, having a second conductivity type opposite to the first conductivity type; forming a third doped region in a portion of the first doped region, having the second conductivity type; forming a fourth doped region in a portion of the second doped region, having the second conductivity type; forming a trench in a portion of the third doping region, the first doped region, the well region, and the semiconductor substrate; forming a conductive contact in the first trench; thinning the semiconductor substrate from the second surface thereof; after thinning the semiconductor substrate, forming a second trench in a portion of the semiconductor substrate adjacent to the second surface thereof, exposing a portion of the conductive contact; forming a first conductive layer in the second trench; and forming a second conductive layer over the second surface of the semiconductor substrate and the first conductive 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; and 
         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. 
     
    
    
     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 a method for fabricating a lateral double diffused metal-oxide-semiconductor (LDMOS) device applicable for a radio frequency (RF) circuit element according to an embodiment of the invention. 
     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 conductivity, and a resistivity of about 5 ohms-cm (Ω-cm)-15 ohms-cm (Ω-cm). The semiconductor substrate  200  has opposing surfaces A and B. Next, a sacrificial layer  202  is formed over the surface A of the semiconductor substrate  202 . In one embodiment, the sacrificial layer  202  may comprise materials such as silicon oxide and may be formed by a deposition process (not shown) such as thermal oxidation. Next, an ion implantation process  204  is performed to the semiconductor substrate  200  to implant dopants of the first conductivity type through the sacrificial layer  202  and into a portion of the semiconductor substrate  200 , thereby forming a doped region  206 . In one embodiment, the dopants of the first conductive type implanted by the ion implantation process  204  can be, for example, dopants of P-type conductivity. 
     In  FIG. 3 , a thermal process (not shown) is performed to diffuse the dopants in the doped region  206 , thereby forming a well region  208  in the semiconductor substrate  200 . Herein, the well region  208  comprises dopants of the first conductivity type, and has a resistivity of about 0.5 ohms-cm (Ω-cm)-1 ohms-cm (Ω-cm). In one embodiment, the resistivity of the well region  208  is lower than the resistivity of the semiconductor substrate  200 . Next, the sacrificial layer  202  over the surface A of the semiconductor substrate  200  is removed, and a patterned gate structure G is formed over a portion of the surface A of the semiconductor substrate  200 . The gate structure G mainly comprises a gate dielectric layer  210 , a gate electrode  212 , and a hard mask layer  214  sequentially formed over a portion of the semiconductor substrate  200 . The gate dielectric layer  210 , the gate electrode  212 , and the hard mask layer  214  of the gate structure G can be formed by conventional gate processes and related 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  216  in a portion of the semiconductor substrate  200  at the left side of the gate structure G, and a doped region  218  in a portion of the semiconductor substrate  200  at the right side of the gate structure G. In one embodiment, the doped region  216  has a first conductivity type such as P type, and the doped region  218  has a second conductivity type such as N type opposite to the P type, and the ion implant processes (not shown) for forming the doped regions  216  and  218  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  220  and a doped region  222  in a portion of the doped regions  216  and  218 , respectively, on opposite sides of the gate structure G, and the configuration shown in  FIG. 3  is formed after performing a thermal diffusion process (not shown). In one embodiment, the doped region  220  formed in a portion of the doped region  216  and the doped region  222  formed in a portion of the doped region  218  respectively has the second conductivity type, such as N type, and the ion implant process (not shown) for forming the doped regions  220  and  222  can be an ion implantation vertical to the surface A of the semiconductor substrate  200 . In one embodiment, the doped region  218  may function as a drift region, and the doped regions  220  and  222  may function as source and drain regions, respectively. 
     In  FIG. 4 , an insulating layer  224  is next formed over the semiconductor substrate  200 , and the insulating layer  224  conformably covers the surface A of the semiconductor substrate  200  and a plurality of sidewalls and a top surface of the gate structure G formed thereover. Next, a patterning process (not shown) is performed to form an opening  226  in a portion of the insulating layer  224 . As shown in  FIG. 4 , the opening  226  exposes a portion of the doped region  220  such that other portions of the semiconductor substrate  200  and surfaces of the gate structure G are still covered by the insulating layer  224 . In one embodiment, the insulating layer  224  may comprise insulating materials such as silicon oxide and silicon nitride, and can be formed by a method such as chemical vapor deposition (CVD). Next, an etching process (not shown) is performed, using the insulating layer  224  as an etching mask, thereby forming a trench  228  in the semiconductor substrate  200  exposed by the opening  226 . The trench  228  is formed with a depth H which mainly penetrates a portion of the doped region  220 , the doped region  216 , the well region  208 , and the semiconductor substrate  200 . A conductive layer  230  and another conductive layer  232  are then sequentially deposited, wherein the conductive layer  230  conformably forms over surfaces of the insulating layer  224  and the bottom surface and the sidewalls of the semiconductor substrate  200  exposed by the trench  228 , and the conductive layer  232  is formed over the surfaces of the conductive layer  230 , thereby filling the trench  228 . Next, the conductive layers  230  and  232  are patterned by using a suitable patterning mask and a patterning process (both not shown). As shown in  FIG. 4 , the patterned conductive layers  230  and  232  are formed over the insulating layer  224  adjacent to the trench  228 , extending over the bottom surface and the sidewalls of the trench  228 , thereby covering surfaces of the well region  208 , and the doped regions  216 ,  220  exposed by the trench  228 , and the conductive layers  230  and  232  also cover the gate structure G and a portion of the doped region  218  adjacent to the gate structure G. However, the conductive layers  230  and  232  do not cover the doped region  222 . The portion of the conductive layers  230  and  232  formed in the trench  228  may function as a conductive contact. In one embodiment, the conductive layer  230  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  232  may comprise conductive materials such as tungsten. 
     In  FIG. 5 , a dielectric material such as silicon oxide or spin-on-glass (SOG) is deposited over the conductive layers  230  and  232 , such that the dielectric material covers the conductive layer  232 , the insulating layer  224 , and the gate structure G, thereby forming an inter-layer dielectric (ILD) layer  234  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  234  and the insulating layer  224  over a portion of the doped region  222 , and the trench  236  exposes a portion of the doped region  222 . 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. 
     In  FIG. 6 , a handling substrate (not shown) is used to bond with a surface of the conductive layer  240  and then the structure shown in  FIG. 5  is reversed, and a thinning process (not shown) comprising steps such as etching, polishing or combinations thereof are then performed to reduce the thickness of the semiconductor substrate  200  from the surface B thereof. Herein, after the thinning process, the thinned semiconductor substrate  200  is assigned with a reference number  200 ′, and a thinned surface B′ of the thinned semiconductor substrate  200 ′ has a distance X to the bottom surface of the conductive layer  230  in the trench  228 . In one embodiment, the distance X is about 50-300 μm. 
     Next, a patterning process (not shown) is performed by using a suitable patterned mask layer (not shown), thereby forming a trench  242  in the surface B′ of the thinned semiconductor substrate  200 ′, and the trench  242  exposes the bottom surface and portions of the sidewalls of the conductive layer  230 . Next, a deposition process (not shown) is performed to form a conductive layer  244  in the trench  242 . In one embodiment, the conductive layer  244  may comprise conductive materials such as Ti—TiN alloy, tungsten, AlCu alloy, AlSiCu alloy, and may be formed by a deposition process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). The formed conductive layer  244  may be processed by a planarization process (not shown), such that a surface of the conductive layer  244  is coplanar with the surface B′ of the thinned semiconductor substrate  200 . Next, another deposition process (not shown) is performed to form a blanket conductive layer  246  over the surface of the conductive layer  244  and the surface of the thinned semiconductor substrate  200 ′. In one embodiment, the conductive layer  246  may comprise conductive materials such as Ti—Ni—Ag alloy, and may be formed by a method such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Therefore, after removal of the handling substrate (not shown), an exemplar LDMOS device is substantially fabricated, as shown in  FIG. 6 . 
     In one embodiment, the gate structure G and the doped regions  220  and  222  of the LDMOS device shown in  FIG. 6  may be properly electrically connected (e.g. through conductive layers  230 ,  232 ,  238 , and  240 ), 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  220  may function as a source region and the doped region  222  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  222 ) may laterally flow toward the source side (e.g. doped region  220 ), and then arrive at the surface B′ of the thinned semiconductor substrate  200 ′ by the guidance of the doped region  216 , the conductive layers  230  and  232 , and the conductive layer  244 , and are then dissipated by the conductive layer  246 , 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  230  and  232  formed in the trench  228  and the conductive layer  244  embedded in the thinned semiconductor layer  200 ′ contacting the conductive layer  246 , 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  234  at the right side of the trench  232  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 conductive layers  244  and  246  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. 
     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.

Technology Classification (CPC): 7