Abstract:
A lateral double diffused metal-oxide-semiconductor device includes: an epitaxial semiconductor layer disposed over a semiconductor substrate; a gate dielectric layer disposed over the epitaxial semiconductor layer; a gate stack disposed over the gate dielectric layer; a first doped region disposed in the epitaxial semiconductor layer from a first side of the gate stack; a second doped region disposed in the epitaxial semiconductor layer from a second side of the gate stack; a third doped region disposed in the first doping region; a fourth doped region disposed in the second doped region; an insulating layer covering the third doped region, the gate dielectric layer, and the gate stack; a conductive contact disposed in the insulating layer, the third doped region, the first doped region and the epitaxial semiconductor layer; and a fifth doped region disposed in the epitaxial semiconductor layer under the conductive contact.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     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 D 1  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 D 1  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 formed over the semiconductor substrate, having the first conductivity type; a gate dielectric layer formed over the epitaxial semiconductor layer, having a step-like cross-sectional structure; a gate stack conformably disposed over the gate dielectric layer; a first doped region disposed in a portion of the epitaxial semiconductor layer adjacent to a first side of the gate stack, having the first conductivity type; a second doped region disposed in a portion of the epitaxial semiconductor layer adjacent to a second side of the gate stack 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; an insulating layer covering the third doped region, the gate dielectric layer, and the gate stack; a conductive contact formed in a portion of the insulating layer, the third doped region, the first doped region, and the epitaxial semiconductor layer; and a fifth doped region disposed in a portion of the epitaxial semiconductor layer under the conductive contact, having the first conductivity type, wherein the fifth doped region physically contacts the semiconductor substrate and the conductive contact. 
     An exemplary method for fabricating a lateral double diffused metal oxide semiconductor (LDMOS) device comprises: performing a semiconductor substrate, having a first conductivity type; forming an epitaxial semiconductor layer over the semiconductor substrate, having the first conductivity type; forming a first doped region in a portion of the epitaxial semiconductor layer, having a second conductivity type opposite to the first conductivity type; forming a first dielectric layer over the first doped region in the epitaxial semiconductor layer; forming a second dielectric layer over a portion of the epitaxial semiconductor layer, being adjacent to the first dielectric layer and contacting thereof, wherein the first dielectric layer and the second dielectric layer have different thicknesses; forming a gate stack over a portion of the first dielectric layer and a portion of the second dielectric layer; forming a second doped region in a portion of the epitaxial semiconductor layer adjacent to a first side of the gate stack, having the first conductivity type; forming a third doped region in a portion of the second doped region at the first side of the gate stack, having the second conductivity type opposite to the first conductivity type; forming an insulating layer over the first dielectric layer, the gate stack, and the second dielectric layer; forming a first trench at the first side of the gate stack, wherein the first trench penetrates a portion of the insulating layer, the second dielectric layer, the third doped region, the first doped region, and the epitaxial semiconductor layer; performing a first ion implantation process, forming a fourth doped region in a portion of the epitaxial semiconductor layer exposed by the first trench, wherein the fourth doping region contacts the semiconductor substrate and has the first conductivity type; forming a first conductive contact in the first trench, contacting the fourth doped region; forming an interlayer dielectric layer over the insulating layer and the first conductive contact; forming a second trench at the second side of the gate stack opposite to the first side, wherein the second trench penetrates a portion of the interlayer dielectric layer, the insulating layer, and the second dielectric layer and exposes a portion of the first doped region; performing a second ion implantation process, forming a fifth doped region in a portion of the first doped region exposed by the second trench, wherein the fifth doped region has the second conductivity type; and forming a second conductive contact in the second trench, contacting the fifth doped region. 
     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. 2-8  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 
         FIG. 9  is schematic cross section of 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. 2-8  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, and a resistivity of about 0.001-0.005 ohms-cm (Ω-cm). Next, an epitaxial semiconductor layer  202  is formed over the semiconductor substrate  200 . The epitaxial semiconductor layer  202  may comprise epitaxial materials such as silicon, and 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 pad oxide layer  204  and a pad nitride layer  206  are sequentially formed over the epitaxial semiconductor layer  202 . The pad oxide layer  204  may comprise materials such as silicon dioxide, and the pad nitride layer  206  may comprise materials such as silicon nitride. Next, a patterning process (not shown) comprising photolithography and etching steps is performed, and an opening  208  is formed in a portion of the pad nitride layer  208 . The opening  208  exposes a portion of the underlying pad oxide layer  204 . Next, an ion implantation process  210  is performed on the region exposed by the opening  208 , using the pad nitride layer  206  as an ion implantation mask. The ion implantation process  210  implants dopants of a second conductivity type such as N-type through the portion of the pad oxide layer  206  exposed by the opening  208 , thereby entering into a portion of the epitaxial semiconductor layer  202 . 
     Referring to  FIG. 4 , after performing the ion implantation process  210  (see  FIG. 3 ), a doped region  212  is formed in a portion of the epitaxial semiconductor layer  202 , having the second conductivity type opposite to the first conductivity type of the epitaxial semiconductor layer  202  and a dopant concentration of about 5*10 11 -5*10 13  atom/cm 2 . Herein, the doped region  212  functions as a drift-region. Next, an etching process (not shown) such as dry etching is performed, using the pad nitride layer  206  as an etching mask, to remove the portion of the pad oxide layer  204  exposed by the opening  208 , thereby exposing a top surface of the doped region  212  in the epitaxial semiconductor layer  202 . Next, a deposition process (not shown) is performed, forming a dielectric layer  214  over the epitaxial semiconductor layer  202  exposed by the opening  208 . Herein, a top surface of the dielectric layer  214  is slightly above the top surface of the pad nitride layer  206 . However, the top surface of the dielectric layer  214  may be slightly below or planar with the top surface of the pad nitride layer  206  in other embodiments. In one embodiment, the dielectric layer  214  may comprise materials such as silicon oxide, and can be formed by, for example, thermal oxidation. 
     Referring to  FIG. 5 , an etching process (not shown) is performed, using the dielectric layer  214  as an etching mask, to sequentially remove the pad nitride layer  206  and the pad oxide layer  204  over the epitaxial semiconductor layer  202 , thereby exposing a surface of the other portion of the epitaxial semiconductor layer  202 . Herein, during removal of the pad oxide layer  204 , a portion of the dielectric layer  214  may be partially removed. Next, a deposition process (not shown) is performed to form another dielectric layer  216  over the top surface of the epitaxial semiconductor layer  202  not covered by the dielectric layer  214 . In the deposition process for forming the dielectric layer  216 , dielectric materials are also formed over the surface of the dielectric layer  214 , thereby increasing the thickness of the dielectric layer  214 . In one embodiment, the dielectric layer  216  may comprise the same materials as that of the dielectric layer  214 , such as silicon dioxide, and can be formed by a deposition process such as thermal oxidation. 
     Referring to  FIG. 6 , a conductive layer  218  and a mask layer  220  are sequentially and conformably formed over surfaces of the dielectric layer  214  and the dielectric layer  216 , and are then patterned by a patterning process (not shown) comprising photolithography and etching steps into a plurality of separated patterned conductive layers  218  and mask layers  220 . These separated patterned conductive layer  218  and mask layer  220  are respectively illustrated as a gate stack G′. In one embodiment, the conductive layer  218  may comprise conductive materials such as doped polysilicon, and the mask layer  220  may comprise masking materials such as silicon dioxide and silicon nitride. In addition, a plurality of separated openings  222  are formed between these gate stacks G′, respectively. As shown in  FIG. 6 , the openings  222  respectively expose a portion of the dielectric layer  216  and a portion of the dielectric layer  214 , and one of the gate stacks G′ partially extends over a portion of the adjacent dielectric layers  214  and  216 . The conductive layer  218  in the gate stack G′ that extends over a portion of the adjacent dielectric layers  214  and  216  may function as a gate electrode layer, and the portion of the dielectric layers  214  and  216  covered by this conductive layer  218  may function as a gate dielectric layer and may have a step-like cross-sectional structure. Next, an ion implantation process (not shown) is performed, using the patterned conductive layer  218 , the patterned mask layer  220 , and the dielectric layer  214  as an implantation mask, to implant dopants of the first conductivity type such as P type to penetrate the dielectric layer  216  exposed by one of the openings  222  into the epitaxial semiconductor layer  202 , thereby forming a doped region  224  in the epitaxial semiconductor layer  202 . Herein, the doped region  224  has the first conductivity type such as p-type and has a dopant concentration of about 1*10 13 -5*10 14  atom/cm 2 . Next, a layer of dielectric layer is conformably deposited and an etching-back process (both not shown) is performed, thereby forming a spacer  226  in each of the openings  222  and on a sidewall of the gate stacks G′. Formation of the spacers  226  reduce the openings  222  into other openings  228  of a smaller size. Next, an ion implantation process (not shown) is performed, using the spacers  226 , the gate stack G′ and the dielectric layer  214  as an ion implantation mask, to implant dopants of the second conductive type such as N-type to penetrate the dielectric layer  216  exposed by one of the openings  222 , thereby forming a doped region  230  in a portion of the doped region  224 . Herein, the doped region  230  may function as a source/drain region, and the bottom surface and sidewall surfaces of the doped region  230  are surrounded by the doped region  224 . The doped region  224  may have a second conductivity type such as N-type and has a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 . 
     Referring to  FIG. 7 , an insulating layer  232  is conformably formed over the dielectric layer  200  to cover the surfaces of the gate stacks G′, the spacers  226 , the dielectric layer  216  and the dielectric layer  214 . The insulating layer  232  may comprise insulating materials such as silicon dioxide, and can be formed by a process such as chemical vapor deposition (CVD). Next, a patterning process (not shown) comprising photolithography and etching steps is performed to form a trench  234 . As shown in  FIG. 7 , the trench  234  has a depth H (to a top surface of the epitaxial semiconductor layer  202 ) and penetrates a portion of the insulating layer  232 , the doped regions  224  and  230 , and the epitaxial semiconductor layer  202  thereunder. Next, an ion implantation process  236  is performed, using the insulating layer  232  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  234 , thereby forming a doped region  238  therein. After performing a thermal diffusion process (not shown), the doped region  238  physically contacts the semiconductor substrate  200  and covers the bottom surface and portions of sidewalls of the trench  234 . Herein, the doped region  238  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 . 
     Referring to  FIG. 8 , a conductive layer  240  and another conductive layer  242  are then sequentially deposited over the structure shown in  FIG. 7 , wherein the conductive layer  240  is conformably formed over surfaces of the insulating layer  232  and the bottom surface and the sidewalls of the epitaxial semiconductor layer  202  exposed by the trench  234 , and the conductive layer  242  is formed over the surfaces of the conductive layer  240 , thereby filling the trench  234 . In one embodiment, the conductive layer  240  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  242  may comprise conductive materials such as tungsten. Next, an etching process (not shown) is performed to remove the portion of the conductive layers  240  and  242  above the insulating layer  232 , thereby leaving the conductive layers  240  and  242  in the trench  234  as a conductive contact. Next, an inter-layer dielectric (ILD) layer  244  is blanketly deposited to cover top surfaces of the insulating layer  232  and the conductive layers  240  and  242 . The ILD layer  234  may comprise dielectric materials such as silicon oxide or spin-on-glass (SOG), and may be planarized to have a planar surface. Next, a patterning process (not shown) comprising photolithography and etching steps is performed to form a trench  246  in a portion of the dielectric layer  214 , the insulating layer  232  and the ILD layer  244  over a portion of the doped region  212 , and the trench  246  exposes a portion of the doped region  212 . Next, an ion implantation process (not shown) is performed, using a suitable implantation mask, to implant dopants of the second conductivity type such as N-type, thereby forming a doped region  248  in a portion of the doped region  212 . Herein, the doped region  248  may function as a source/drain region, and the bottom surface and sidewalls thereof are surrounded by the doped region  212 , and the doped region  248  may have the second conductivity type such as N-type and has a dopant concentration of about 1*10 15 -5*10 15  atom/cm 2 . Next, a conductive layer  250  and another conductive layer  252  are sequentially deposited, and the conductive layer  250  is conformably formed over the surfaces of the ILD layer  244  and the sidewalls exposed by the trench  246 , and the conductive layer  252  is formed over the surface of the conductive layer  250 , thereby filling the trench  246 . The portion of the conductive layers  250  and  252  formed in the trench  246  may function as a conductive contact. In one embodiment, the conductive layer  250  may comprise conductive materials such as Ti—TiN alloy, and the conductive layer  252  may comprise conductive materials such as tungsten. Therefore, an exemplar LDMOS device is substantially fabricated, as shown in  FIG. 8 . 
     In addition, as shown in  FIG. 9 , another exemplar LDMOS device is illustrated. The LDMOS device in  FIG. 9  is similar to the LDMOS device shown in  FIG. 8 , and can be formed by the processes shown in  FIGS. 1-8 . Herein, the etching process for removing the conductive layers  240  and  242  shown in  FIG. 8  is replaced by a patterning process comprising photolithgraphy and etching steps, such that the conductive layers  240  and  242  are patterned and portions thereof now remain over the insulating layer  232 . 
     In one embodiment, one of the gate stacks G′ and the doped regions  230  and  248  of the LDMOS device shown in  FIGS. 8-9  may be properly electrically connected, and the regions of 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  230  may function as a source region and the doped region  248  may function as a drain region. In this embodiment, during operation of the LDMOS device shown in  FIGS. 8-9 , currents from the drain side (e.g. the doped region  248 ) may flow laterally toward the source side (e.g. doped region  230 ) by the guidance of the doped region  224 , the conductive layers  240  and  242 , and the doped region  238 , and then arrive at the semiconductor substrate  200 , such that 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  240  and  242  formed in the trench  234  (see  FIG. 7 ) and the doped region  238  embedded in the epitaxial semiconductor layer  202  and contacting the semiconductor substrate  200 , such that an 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 D 2  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 D 1  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  FIGS. 8-9  may have the advantages of reduced size and fabrication cost, and formation of the doped region  238 , and the conductive layers  240  and  242  also helps to reduce the on-state resistance (Ron) of the N type LDMOS device. Moreover, since the portion of the dielectric layers  214  and  216  (i.e. the gate dielectric layer) covered by the gate stack G′ formed between the doped regions  230  and  248  of the LDMOS device shown in  FIGS. 8-9  has a step-like cross-sectional structure, such that reduction of the parasitic capacitance and increase of the breakdown voltage of the LDMOS device shown in  FIGS. 8-9  can be achieved. 
     In addition, in another embodiment, the regions of the first conductivity type of the LDMOS device shown in  FIGS. 8-9  can be N type regions, and the regions of the second conductivity type can be P type regions, such that the LDMOS device formed 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.