Patent Publication Number: US-11380779-B2

Title: Semiconductor device and manufacturing method thereof

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     The present application claims priority to China Application Serial Number 202010894515.6, filed Aug. 31, 2020, which is herein incorporated by reference. 
     BACKGROUND 
     The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are desired to maintain the electronic components&#39; performance from one generation to the next. For example, low on-resistance and high breakdown voltage of transistors are desirable for various high power applications. 
     As semiconductor technologies evolve, metal oxide semiconductor field effect transistors (MOSFET) have been widely used in today&#39;s integrated circuits. MOSFETs are voltage controlled devices. When a control voltage is applied to the gate of a MOSFET and the control voltage is greater than the threshold of the MOSFET, a conductive channel is established between the drain and the source of the MOSFET. As a result, a current flows between the drain and the source of the MOSFET. On the other hand, when the control voltage is less than the threshold of the MOSFET, the MOSFET is turned off accordingly. 
     According to the conductivity type difference, MOSFETs may include two major categories. One is n-channel MOSFETs; the other is p-channel MOSFETs. On the other hand, according to the structure difference, MOSFETs can be further divided into three sub-categories, planar MOSFETs, lateral diffused MOS (LDMOS) FETs and vertical diffused MOSFETs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1A and 1B  illustrate a block diagram of a method of forming a semiconductor device in accordance with some embodiments. 
         FIGS. 2 to 20  illustrate a method for manufacturing a semiconductor device in different stages in accordance with some embodiments. 
         FIGS. 21 and 22  illustrate a method for manufacturing a semiconductor device in different stages in accordance with some embodiments. 
         FIGS. 23A and 23B  illustrate a block diagram of a method of forming a semiconductor device in accordance with some embodiments. 
         FIG. 24  illustrates a method for manufacturing a semiconductor device in a stage in accordance with some embodiments. 
         FIGS. 25 to 28  illustrate a method for manufacturing a semiconductor device in different stages in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     As used herein, “around”, “about”, “approximately”, or “substantially” shall generally mean within 20 percent, or within 10 percent, or within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about”, “approximately”, or “substantially” can be inferred if not expressly stated. 
     The lateral diffused (LD) MOS transistor has advantages. For example, the LDMOS transistor is capable of delivering more current per unit area because its asymmetric structure provides a short channel between the drain and the source of the LDMOS transistor. However, it has been appreciated that the LDMOS transistor suffers some issues as described below. The LDMOS transistor formed with field oxide (FOX) would result in an excessively large device size and an excessive high specific on-resistance (R sp ) because a large FOX structure is present between the source region and the drain region of the LDMOS transistor. On the other hand, if the LDMOS transistor is formed without FOX, a non-self-aligned implant is employed to form source/drain regions of the LDMOS transistor, and a resist protect layer (RPO) is employed to define a desired silicide region within the drain region of the LDMOS transistor. However, the non-self-aligned implant may result in un-doped region in a poly gate of the LDMOS transistor because the photolithography technique used in the non-self-aligned implant may suffer from misalignment, and the RPO may result in un-silicide region in the poly gate because the RPO may be formed over a top surface of the poly gate. In addition, the drift region of the LDMOS transistor may not be scaled down due to the RPO minimal length constraint. 
     The present disclosure will be described with respect to embodiments in a specific context, a LDMOS transistor manufactured using an improved process flow to address the foregoing issues resulting from the FOX and RPO. The embodiments of the disclosure may also be applied, however, to a variety of metal oxide semiconductor transistors. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings. 
     Referring now to  FIGS. 1A and 1B , illustrated is an exemplary method M 1  for fabrication of a semiconductor device in accordance with some embodiments, in which the fabrication includes a self-aligned implantation process of fabricating a semiconductor device. The method M 1  includes a relevant part of the entire manufacturing process. It is understood that additional operations may be provided before, during, and after the operations shown by  FIGS. 1A and 1B , and some of the operations described below can be replaced or eliminated for additional embodiments of the method. The order of the operations/processes may be interchangeable. The method M 1  includes fabrication of a semiconductor device  100 . However, the fabrication of the semiconductor device  100  is merely example for describing the self-aligned implantation process used in fabricating the semiconductor device  100  according to some embodiments of the present disclosure. 
     It is noted that  FIGS. 1A and 1B  have been simplified for a better understanding of the disclosed embodiment. Moreover, the semiconductor device  100  may be configured as a system-on-chip (SoC) device having various PMOS and NMOS transistors that are fabricated to operate at different voltage levels. The PMOS and NMOS transistors may provide low voltage functionality including logic/memory devices and input/output devices, and high voltage functionality including power management devices. For example, transistors that provide low voltage functionality may have operating (or drain) voltages of about 1.1 V with standard CMOS technology, or voltages of about 1.8/2.5/3.3 V with special (input/output) transistors in standard CMOS technology. In addition, transistors that provide medium/high voltage functionality may have operating (or drain) voltages of about 5 V or greater (e.g., about 20-35 V). It is understood that the semiconductor device  100  in  FIGS. 2-21  may also include resistors, capacitors, inductors, diodes, and other suitable microelectronic devices that may be implemented in integrated circuits. 
       FIGS. 2 to 20  illustrate a method for manufacturing the semiconductor device  100  in different stages in accordance with some embodiments. The method M 1  begins at block S 10  where an isolation structure  142  is formed in a semiconductor substrate  110 , as illustrated in  FIG. 2 . The semiconductor substrate  110  may include a semiconductor wafer such as a silicon wafer. Alternatively, the semiconductor substrate  110  may include other elementary semiconductors such as germanium. The semiconductor substrate  110  may also include a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, indium phosphide, or other suitable materials. Moreover, the semiconductor substrate  110  may include an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, and gallium indium phosphide, or other suitable materials. In some embodiments, the semiconductor substrate  110  includes an epitaxial layer (epi layer) overlying a bulk semiconductor. Furthermore, the semiconductor substrate  110  may include a semiconductor-on-insulator (SOI) structure. For example, the semiconductor substrate  110  may include a buried oxide (BOX) layer formed by a process such as separation by implanted oxygen (SIMOX). In various embodiments, the semiconductor substrate  110  may include a buried layer such as an n-type buried layer (NBL), a p-type buried layer (PBL), and/or a buried dielectric layer including a buried oxide (BOX) layer. In the some embodiments, illustrated as an n-type MOS, the semiconductor substrate  110  includes a p-type silicon substrate (p-substrate). For example, p-type impurities (e.g., boron) are doped into the semiconductor substrate  110  to form the p-substrate. To form a complementary MOS, an n-type buried layer, i.e., deep n-well (DNW), may be implanted deeply under the active region of the p-substrate  110 . In some embodiments, arsenic or phosphorus ions are implanted to form the DNW. In some other embodiments, the DNW is formed by selective diffusion. The DNW functions to electrically isolate the p-substrate. 
     In  FIG. 2 , an isolation structure  142  such as shallow trench isolation (STI) or local oxidation of silicon (LOCOS) (or field oxide, FOX) including isolation regions may be formed in the semiconductor substrate  110  to define and electrically isolate various active regions so as to prevent leakage current from flowing between adjacent active regions. As one example, the formation of an STI feature may include dry etching a trench in a substrate and filling the trench with insulator materials such as silicon oxide, silicon nitride, silicon oxynitride, or other suitable materials. The filled trench may have a multi-layer structure such as a thermal oxide liner layer filled with silicon nitride or silicon oxide. In some other embodiments, the STI structure may be created using a processing sequence such as: growing a pad oxide, forming a low pressure chemical vapor deposition (LPCVD) nitride layer, patterning an STI opening using photoresist and masking, etching a trench in the substrate, optionally growing a thermal oxide trench liner to improve the trench interface, filling the trench with CVD oxide, using chemical mechanical polishing (CMP) processing to planarize the CVD oxide, and using a nitride stripping process to remove the silicon nitride. In some embodiments where the flowable CVD is used in forming the CVD oxide of the STI region  142 , an anneal process can be performed to cure the deposited oxide. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 11  where a gate dielectric layer is formed over the semiconductor substrate. With reference to  FIG. 3 , in some embodiments of block S 11 , a gate dielectric layer  162 ′ is formed over the semiconductor substrate  110 . The gate dielectric layer  162 ′ may include a silicon oxide layer. Alternatively, the gate dielectric layer  162 ′ may include a high-k dielectric material. The high-k material may be selected from metal oxides, metal nitrides, metal silicates, transition metal-oxides, transition metal-nitrides, transition metal-silicates, oxy-nitrides of metals, metal aluminates, zirconium silicate, zirconium aluminate, hafnium oxide, other suitable materials or combinations thereof. Alternatively, the gate dielectric layer  162 ′ may include oxide and/or nitride material. For example, the gate dielectric layer  162 ′ may include silicon oxide, silicon nitride, silicon oxynitride, SiCN, SiCxOyNz, other suitable materials, or combinations thereof. For example, the gate dielectric layer  162 ′ may include silicon oxide. In some embodiments, the gate dielectric layer  162 ′ may have a multilayer structure such as one layer of silicon oxide and another layer of high k material. The gate dielectric layer  162 ′ may be formed using chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), thermal oxidation, other suitable processes, or combinations thereof. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 12  where a conductive layer is formed over the gate dielectric layer. With reference to  FIG. 4 , in some embodiments of block S 12 , a conductive layer  164 ′ is formed over the gate dielectric layer  162 ′. The conductive layer  164 ′ may include polycrystalline silicon (interchangeably referred to as polysilicon). Alternatively, the conductive layer  164 ′ may include a metal such as Al, Cu, W, Ti, Ta, TiN, TaN, NiSi, CoSi, other suitable conductive materials, or combinations thereof. The conductive layer  164 ′ may be formed by CVD, PVD, plating, and other proper processes. The conductive layer  164 ′ may have a multilayer structure and may be formed in a multi-step process using a combination of different processes. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 13  where the conductive layer is patterned to form a gate electrode. With reference to  FIG. 5 , in some embodiments of block S 13 , the conductive layer  164 ′ in  FIG. 4  is patterned to form a gate electrode  164  on the gate dielectric layer  162 ′. In some embodiments, a patterned mask layer (not shown) is formed over the conductive layer  164 ′ in  FIG. 4 . The patterned mask layer may be formed by a series of operations including deposition, photolithography patterning, and etching processes. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. The etching processes may include dry etching, wet etching, and/or other etching methods (e.g., reactive ion etching). Then, one or more etching processes are performed to form a gate electrode  164  on the gate dielectric layer  162 ′ using the patterned mask as an etching mask, and the patterned mask layer is removed after the etching. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 14  where an n-type doped region and a p-type doped region are formed in the semiconductor substrate. With reference to  FIG. 6 , in some embodiments of block S 14 , an n-type double diffused (NDD) doped region  152  is formed in the semiconductor substrate  110  and in the vicinity of a top surface  112  of the semiconductor substrate  110 . In this context, the “double diffused” doped region is a doped region that experiences dual implantation processes with dopants of same conductivity type during fabrication of the LDMOS transistor. For example, the region  152  is implanted with n-type dopant at the step as illustrated in  FIG. 6 , and then a part of the region  152  will be implanted with n-type dopant again at the step as illustrated in  FIG. 17 , and thus this region is referred to as a double diffused doped region (e.g., double diffused drain region in this embodiment). It is noted the double diffused doping profile will be formed at the step as illustrated in  FIG. 17 , not formed at the step as illustrated  FIG. 6 , and the terminology “double diffused region” recited at this step is merely used to distinguish from body region of LDMOS transistor. 
     In some embodiments, the NDD region  152  is formed by ion-implantation, diffusion techniques, or other suitable techniques. For example, an ion implantation utilizing an n-type dopant, such as arsenic or phosphorus, may be performed to form the NDD region  152  in the semiconductor substrate  110  through the gate dielectric layer  162 ′ using a first patterned mask layer (e.g., first patterned photoresist mask) and a portion of the gate electrode  164  as an implant mask. In  FIG. 6 , the NDD region  152  has a portion under the gate electrode  164  because of the implantation tilt angle of the ion-implantation for forming the NDD region  152 . For example, a first mask layer (e.g., patterned photoresist mask) is formed to cover a left portion of the gate electrode  164  and a region of the semiconductor substrate  110  in the vicinity of the left portion the gate electrode  164 , while leaving a right portion of the gate electrode  164  and another region of the semiconductor substrate  110  in the vicinity of the right portion of the gate electrode  164  exposed. In some embodiments, the first mask layer may be formed by a photolithography process. The photolithography processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. Then, an implantation process is performed to implant an n-type dopant at a tilt angle (as indicated by the arrows A 1 ) using the first mask layer and the gate electrode  164  as an implant mask, thus forming the NDD region  152  in the semiconductor substrate  110  and extending to directly below the gate electrode  164  due to the tilt angle. The first mask layer is then removed after the forming of the NDD region  152 . In some embodiments, the dopant concentration of the NDD region  152  is in a range, by way of example and not limitation, from about 10 16  and about 10 18  per cubic centimeter, and other dopant concentration ranges are within the scope of the disclosure. In some embodiments, the isolation structure  142  has a depth D 1 . In some embodiments, the NDD region  152  has a depth D 4  less than the depth D 1  of the isolation structures  142 . By way of example and not limitation, a ratio of the depth D 4  of the NDD region  152  to the depth D 1  of the STI  142  is in a range from about 0.2 to about 1. In some other embodiments, the depth D 4  of the NDD region  152  may be greater than the depth D 1  of the isolation structure  142 . 
     Then, a p-type doped region (interchangeably referred to as a p-body region)  156  is formed in the semiconductor substrate  110  and in the vicinity of the top surface  112  of the semiconductor substrate  110 . Specifically, the p-body region  156  is formed between the NDD region  152  and the isolation structure  142 . In some embodiments, the p-body region  156  is formed by ion-implantation, diffusion techniques, or other suitable techniques. For example, an ion implantation utilizing a p-type dopant, such as boron, may be performed to form the p-body region  156  in the semiconductor substrate  110  through the gate dielectric layer  162 ′ using a second patterned mask layer (e.g., second patterned photoresist mask) and the left portion of the gate electrode  164  as an implant mask. In  FIG. 6 , the p-body region  156  has a portion under the gate electrode  164  because of the implantation tilt angle of the ion-implantation for forming the p-body region  156 . For example, a second mask layer is formed to cover a right portion of the gate electrode  164  and the NDD region  152 , while leaving a left portion of the gate electrode  164  and the region of the semiconductor substrate  110  in the vicinity of the left portion the gate electrode  164  exposed. In some embodiments, the second mask layer may be formed by a photolithography patterning process. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. Then, an implantation process is performed to implant a p-type dopant at a tilt angle (as indicated by the arrows A 2 ) using the second mask layer and the gate electrode  164  as an implant mask, thus forming the p-body region  156  in the semiconductor substrate  110  and extending to directly below the gate electrode  164  due to the tilt angle. The second mask layer is then removed after the forming of the p-body region  156 . An ion implantation utilizing a p-type dopant, such as boron and/or boron difloride (BF 2 ), may be performed to form the p-body region  156  in the semiconductor substrate  110 . In some embodiments, the dopant concentration of each of the p-body region  156  may be between about 10 17  and about 10 19  per cubic centimeter, and other dopant concentration ranges are within the scope of the disclosure. In some embodiments, the dopant concentration of the p-body region  156  may be greater than the dopant concentration of the NDD region  152 . Although the embodiments discussed above include forming the p-body region  156  after forming the NDD region  152 , the p-body region  156  can be formed before forming the NDD region  152  in some other embodiments. 
     In some embodiments, the p-body region  156  has a depth D 5  less than the depth D 1  of the isolation structure  142  and greater than the depth D 4  of the NDD region  152 . In some other embodiments, the depth D 5  of the p-body region  156  may be less than the depth D 4  of the NDD region  152 . In some other embodiments, the depth D 5  of the p-body region  156  may be greater than the depth D 1  of the isolation structure  142 . 
     It is understood that order of the operations/processes shown by  FIGS. 1A and 1B  may be interchangeable. In some embodiments, the NDD region  152  may be formed prior to forming the gate dielectric layer  162 ′ and after forming the isolation structure  142 . For example, the NDD region  152  is formed by ion-implantation, diffusion techniques, or other suitable techniques through a patterned photoresist layer. A photoresist layer is coated on the semiconductor substrate  110 , and a photomask is then used to pattern the coated photoresist layer in a photolithography process or other suitable process. An exemplary photolithography process may include processing steps of photoresist coating, soft baking, mask aligning, exposing, post-exposure baking, developing, and hard baking. The patterned photoresist layer exposes a region of the semiconductor substrate  110 . Thereafter, an ion implantation utilizing an n-type dopant, such as arsenic or phosphorus, may be performed to form the NDD region  152  in the semiconductor substrate  110  using the patterned photoresist layer as an implant mask. 
     In some embodiments, the p-body region  156  is formed prior to forming the gate dielectric layer  162 ′ and after forming the isolation structure  142 . For example, the p-body region  156  may be formed by ion-implantation, diffusion techniques, or other suitable techniques through a patterned photoresist layer. The photoresist layer used to define the NDD region  152  is stripped by ashing, and then another photoresist layer is coated on the semiconductor substrate  110 . Next, another photomask with the pattern of the p-body region  156  is used to pattern the photoresist layer in a photolithography process or other suitable process. An exemplary photolithography process may include processing steps of photoresist coating, soft baking, mask aligning, exposing, post-exposure baking, developing, and hard baking. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 15  where a first spacer layer is deposited over the gate electrode and the gate dielectric layer. With reference to  FIG. 7 , in some embodiments of block S 15 , a first spacer layer  170 ′ is blanket deposited over the structure in  FIG. 6  (i.e., over the NDD region  152 , the p-body region  156 , the gate dielectric layer  162 ′, the gate electrode  164 , and the isolation features  142 ). In some embodiments, the first spacer layer  170 ′ may include silicon oxide, silicon nitride, silicon oxynitride, SiCN, SiC x O y N z , other suitable materials, or combinations thereof. For example, the first spacer layer  170 ′ may be a dielectric material such as silicon nitride. In some embodiments, the first spacer layer  170 ′ includes a material different than the gate dielectric layer  162 ′. In some embodiments, the first spacer layer  170 ′ may have a multilayer structure. The first spacer layer  170 ′ can be formed using a deposition method, such as plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD), or the like. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 16  where the first spacer layer is etched to form a first gate spacer. With reference to  FIG. 8 , in some embodiments of block S 16 , first gate spacers  170  are formed on opposite sides of the gate electrode  164 . In greater detail, an anisotropic etching process P 1  is performed to remove the horizontal portions of the first spacer layer  170 ′. The remaining vertical portions of the first spacer layer  170 ′ form first gate spacers  170 . The first gate spacers  170  have a height H 2  measured from the top surface of the semiconductor substrate  110 , and the gate electrode  164  has a height H 1  measured from the top surface of the semiconductor substrate  110 . In some embodiments, the height H 2  of the first gate spacers  170  may be lower than the height H 1  of the gate electrode  164  due to the nature of the anisotropic etching process that selectively etches the material of first gate spacers  170  at a faster etch rate than it etches the polysilicon gate  164 . The height H 2  of the first gate spacers  170  depends on process conditions of the anisotropic etching process P 1  (e.g., etching time duration and/or the like). Moreover, the first gate spacers  170  each have a vertical portion  170   v  vertically extending along the vertical sidewall of the gate electrode  164  and a lateral portion  1701  laterally extending a small length L 1  from an outermost sidewall of the vertical portion  170   v . The length L 1  of the lateral portion  1701  also depends on the process conditions of the anisotropic etching process P 1  (e.g., etching time duration or the like). In some embodiments, the etching process is performed using an isotropic etching process. In some embodiments, the first spacer layer  170 ′ is etched using, by way of example and not limitation, phosphoric acid (H 3 PO 4 ). 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 17  where the blanket gate dielectric layer is etched to form a patterned gate dielectric layer. With reference to  FIG. 9 , in some embodiments of block S 17 , the blanket gate dielectric layer  162 ′ as shown in  FIG. 8  is patterned to form a gate dielectric layer  162  remaining below the gate electrode  164  and the first gate spacers  170 . In greater detail, another etching process P 2  is performed to pattern the gate dielectric layer  162  using the gate electrode  164  and the gate spacers  170  as an etching mask. By way of example and not limitation, the gate dielectric layer  162  can be patterned using a liquid hydrogen fluoride (HF) or vapor HF as an etchant, in some cases where the gate dielectric layer  162  is silicon oxide. The gate dielectric layer  162  and the gate electrode  164  in combination serve as a gate structure  160  with a vertical symmetrical axis A. As illustrated in  FIG. 9 , the gate structure  160  overlies portions of the NDD region  152  and the p-body region  156 . 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 18  where a second spacer layer is deposited over the semiconductor substrate. With reference to  FIG. 10 , in some embodiments of block S 18 , a second spacer layer  180 ′ is blanket deposited over the structure as shown in  FIG. 9  (i.e., over the NDD region  152 , the p-body region  156 , the gate dielectric layer  162 , the gate electrode  164 , first gate spacers  170 , and the STI region  142 ). In some embodiments, the second spacer layer  180 ′ may include silicon oxide, silicon nitride, silicon oxynitride, SiCN, SiC x O y N z , other suitable materials, or combinations thereof. For example, the second spacer layer  180 ′ may be a dielectric material such as silicon oxide. In some embodiments, the second spacer layer  180 ′ may include a material different than the first gate spacers  170 . In some embodiments, a material of the second spacer layer  180 ′ may be the same as a material of the gate dielectric layer  162  (e.g., silicon oxide). In some embodiments, the second spacer layer  180 ′ may have a multilayer structure. The second spacer layer  180 ′ can be formed using a deposition method, such as plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD), or the like. 
     Returning to  FIG. 1A , the method M 1  then proceeds to block S 19  where a third spacer layer is deposited over the second spacer layer. With reference to  FIG. 11 , in some embodiments of block S 19 , a third spacer layer  182 ′ is blanket deposited over the second spacer layer  180 ′. In some embodiments, the third spacer layer  182 ′ may include silicon oxide, silicon nitride, silicon oxynitride, SiCN, SiC x O y N z , other suitable materials, or combinations thereof. For example, the third spacer layer  182 ′ may be a dielectric material such as silicon nitride. In some embodiments, the third spacer layer  182 ′ may include a material different than the second spacer layer  180 ′. In some embodiments, a material of the third spacer layer  182 ′ may be the same as a material of the first gate spacers  170 . In some specific embodiments, the first gate spacers  170  and the third spacer layer  182 ′ are formed of silicon nitride, and the second spacer layer  180 ′ is formed of silicon oxide. 
     In some embodiments, the third spacer layer  182 ′ may have a multilayer structure. In some embodiments, a thickness T 1  of the second spacer layer  180 ′ may be less than a thickness T 2  of the third spacer layer  182 ′. For example, the thickness T 1  of the second spacer layer  180 ′ may be in a range from about 10 nm to about 50 nm, and the thickness T 2  of the third spacer layer  182 ′ may be in a range from about 50 nm to about 300 nm, and other thickness ranges are within the scope of the disclosure. In some embodiments, if the thickness T 1  of the second spacer layer  180 ′ is less than about 10 nm, the second spacer layer  180 ′, polysilicon gate  164 , first gate spacer  170 , and semiconductor substrate  110  may be damaged in the following etch process P 3  (as shown in  FIG. 12 ), and thus the yield may reduce. If the thickness T 1  is large than about 50 nm, the top surface of the second spacer  180 ′ may be unwantedly flat so that, when the third spacer etch process P 3  is complete, the laterally width of the third spacer  182  may be unable to act as an implantation mask for the following self-aligned implantation process P 7  (as shown in  FIG. 17 ). In some embodiments, if the thickness T 2  of the third spacer layer  182 ′ is less than about 50 nm, when the third spacer etch process P 3  is complete, the laterally width of the third spacer  182  may be unable to act as a mask for the self-aligned implantation process P 7 . If the thickness T 2  is large than about 300 nm, the process time of the etching process P 3  may increase and may further damage second spacer layer  180 ′, polysilicon  164 , first gate spacer  170 , and semiconductor substrate  110 , and thus the yield may reduce. Stated differently, the ratio of the thickness T 2  to the thickness T 1  is in a range from about 1 to about 30, and other thickness ranges are within the scope of the disclosure. In some embodiments, if the ratio of the thickness T 2  to the thickness T 1  is less than about 1, when the third spacer etch process P 3  is complete, a laterally width of the third spacer  182  may be unable to act as a mask for the self-aligned implantation process P 7 . If the ratio of the thickness T 2  to the thickness T 1  is greater than about 30, the process time of the etching process P 3  may increase and may further damage second spacer layer  180 ′, polysilicon  164 , first gate spacer  170 , and semiconductor substrate  110 , and thus the yield may reduce. The thickness T 1  of the second spacer layer  180 ′ and the thickness T 2  of the third spacer layer  182 ′ are selected depending on a desired location of subsequently formed drain region (e.g., the drain region  174  as illustrated in  FIG. 18 ) and the silicide region (e.g., silicide region  220  as illustrated in  FIG. 20 ) formed on the drain region. In other words, the thickness T 1  of the second spacer layer  180 ′ and the thickness T 2  of the third spacer layer  182 ′ are selected to achieve a desired drift region length (e.g., the drift region length S 1  as illustrated in  FIG. 18 ). Stated differently, if the thicknesses T 1  and T 2  are excessively smaller than the selected range, a drift region length may be unwantedly short and thus results in low device breakdown voltage; if the thicknesses T 1  and T 2  are excessively large than the selected range, a drift region length may be unwantedly long and thus results in poor resistance. In some embodiments, the third spacer layer  182 ′ can be formed using a deposition method, such as plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD), or the like. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 20  where the third spacer layer is etched to form third gate spacers over the second spacer layer. With reference to  FIG. 12 , in some embodiments of block S 20 , an anisotropic etching process P 3  is performed to remove the horizontal portions of the third spacer layer  182 ′. The etching operation P 3  etches the third spacer layer  182 ′ at a faster etch rate than it etches the second spacer layer  180 ′. By way of example and not limitation, a ratio of the etch rate of the third spacer layer  182 ′ to the etch rate of the second spacer layer  180 ′ may be greater than about 2. If the ratio of the etch rate of the third spacer layer  182 ′ to the etch rate of the second spacer layer  180 ′ is less than about 2, the etching operation P 3  would significantly consume the second spacer layer  180 ′, and thus the second spacer layer  180 ′, polysilicon  164 , first gate spacer  170 , and semiconductor substrate  110  may be damaged, and thus the yield may reduce. In some embodiments, a ratio of the etch rate of the third spacer layer  182 ′ to the etch rate of the second spacer layer  180 ′ may be greater than about 10. In some embodiments, the etching process is performed using an isotropic etching process. In some embodiments, the third spacer layer  182 ′ is etched using, for example, phosphoric acid (H 3 PO 4 ). 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 21  where a first mask layer is formed, in which the first mask layer covers a portion of the gate structure, the first gate spacer, the third gate spacer, and the second spacer layer on a side of the symmetrical axis of the gate structure, and exposes another portion of the gate structure, the first gate spacer, the third gate spacer, and the second spacer layer on another side of the symmetrical axis of the gate structure. With reference to  FIG. 13 , in some embodiments of block S 21 , a mask layer  190  is formed over the semiconductor substrate  110  and then patterned to form separated mask portions to cover a portion of the gate structure  160 , the first gate spacer  170 , the third gate spacer  182 , and the second spacer layer  180 ′ on a right side of the symmetrical axis A of the gate structure  160  shown in  FIG. 13 , and exposes another portion of the gate structure  160 , the first gate spacer  170 , and third gate spacer  182 , and the second spacer layer  180 ′ on a left side of the symmetrical axis A of the gate structure shown in  FIG. 13 . 
     In some embodiments, the mask layer  190  may be formed by a photolithography patterning process. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 22  where the third gate spacer exposed by the first mask layer is removed. With reference to  FIG. 14 , in some embodiments of block S 22 , one or more etching processes are performed to remove the third gate spacer  182  on the second spacer layer  180  using the mask layer  190  as an etching mask. For example, an etching process P 4  is performed to remove the third gate spacer  182  exposed by the mask layer  190 . The etching process P 4  is a selective etching process that uses an etchant etching the nitride spacer  182  at a faster etch rate than it etches the oxide spacer layer  180 ′. For example, the etch rate of the etching process P 4  to the nitride spacer  182  is greater than about twice the etch rate of the etching process P 4  to the oxide spacer layer  180 ′. If the etch rate of the etching process P 4  to the nitride spacer  182  is lower than about twice the etch rate of the etching process P 4  to the oxide spacer layer  180 ′, the etching process P 4  may excessively consume the second spacer layer  180 ′ and thus the second spacer layer  180 ′, polysilicon  164 , first gate spacer  170 , and semiconductor substrate  110  may be damaged, and thus the yield may reduce. In this way, the oxide spacer layer  180 ′ remains substantially intact after removing the nitride spacer  182  from the left side of the symmetrical axis A of the gate structure  160 . In some embodiments, the etching process is performed using an isotropic etching process. For example, the etchant used in the etching process P 4  includes phosphoric acid (H 3 PO 4 ). 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 23  where the first mask layer is removed. With reference to  FIG. 15 , in some embodiments of block S 23 , the mask layer  190  is removed after the etching of the third gate spacer  182  and the dielectric layer  180  exposed by the mask layer. For example, the mask layer  190  is stripped by ashing if it is photoresist. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 24  where portions of the second spacer layer not covered by the third gate spacer  182  are removed to form a second gate spacer sandwiched between the first gate spacer and the third gate spacer. With reference to  FIG. 16 , in some embodiments of block S 24 , an etching operation P 6  is performed to remove a portion of the second spacer layer  180 ′ on the left side of the symmetrical axis A of the gate structure  160  and to remove the horizontal portions of the second spacer layer  180 ′ on the right side of the symmetrical axis A of the gate structure  160  using the third gate spacer  182  as an etching mask, so as to form a second gate spacer  180  on only one side of the gate structure  160  (e.g., only on right side of the gate structure  160 ). The etching operation P 6  is a selective etching process that etches the oxide spacer layer  180 ′ at a faster rate than it etches the nitride spacer  170 , the polysilicon gate  164 , and the third gate spacer  182 . For example, the etch rate of the etching operation P 6  to the oxide spacer layer  180 ′ is greater than about ten times the etch rate of the etching operation P 6  to the nitride spacer  170 , the polysilicon gate  164 , and the nitride spacer  182 . If the etch rate of the etching operation P 6  to the oxide spacer layer  180 ′ is lower than about ten times the etch rate of the etching operation P 6  to the nitride spacer  170 , the polysilicon gate  164 , and the nitride spacer  182 , the etching operation P 6  may excessively consume the nitride spacer  182  on the right side of the polysilicon gate  164  and the nitride spacer  170  on the left side of the polysilicon gate  164 , and thus the excessively consumed nitride spacers  170  and  182  may be unable to act as an implantation mask for the following self-aligned implantation process P 7  (as illustrated in  FIG. 17 ), which in turn adversely affects the desired drift region length. In this way, the nitride spacers  170  and  182  and the polysilicon gate  164  remain substantially intact after removing the portions of the second spacer layer  180 ′. 
     In  FIG. 16 , the sidewall of the resulting second gate spacer  180  has a notched corner and the third gate spacer  182  is embedded in the notched corner of the second gate spacer  180 . In some embodiments, an outermost sidewall of the third gate spacer  182  is coterminous with an outermost end surface of the second gate spacer  180 . The third gate spacer  182  is vertically spaced apart from the substrate  110  by the second gate spacer  180 , and laterally spaced apart from the gate electrode  164  by the second gate spacer  180  and the first gate spacer  170 . The second and third gate spacers  180  and  182  may in combination function as a silicide blocking layer during a subsequent self-aligned silicidation (salicide) process, which will be discussed in greater detail below. The device area that is intentionally precluded from the silicide process is covered with the second and third gate spacers  180  and  182 . This protects the areas under the second and third gate spacers  180  and  182  from the subsequent silicide formation. The NDD region  152  provides a resistive path which acts as a voltage drop in the channel region, and thus the semiconductor device  100  has an improved blocking voltage ability. 
     The second gate spacer  180  can be defined by applying, for example, an anisotropic etch that partially removes the second spacer layer  180 ′ exposed by the third gate spacer  182 . The third gate spacer  182  can thus act as an etching mask during the etching operation P 6 . Hence, a width W 1  of a remainder of the second spacer layer  180 ′ (i.e., the second gate spacer  180 ) can be controlled by the thickness of the third gate spacer  182 , which in turn will control a drift region within the NDD region  152  (i.e., the region in the NDD region  152  except for the subsequently formed drain region), thus facilitating scaling down the drift region length. 
     The etching operation P 6  etches the second spacer layer  180 ′ at a faster etch rate than it etches the third gate spacer  182 . By way of example and not limitation, a ratio of the etch rate of the second spacer layer  180 ′ to the etch rate of the third gate spacer  182  may be greater than about 10. If the ratio of the etch rate of the second spacer layer  180 ′ to the etch rate of the third gate spacer layer  182 ′ is less than about 10, the etching operation P 6  would significantly consume the third gate spacer  182 , and thus the third gate spacer  182  may be unable to act as an etching mask during the etching operation P 6 , which in turn adversely affects the desired drift region length. In some embodiments, a ratio of the etch rate of the second spacer layer  180 ′ to the etch rate of the third gate spacer  182  may be greater than about 10. In some embodiments, the second spacer layer  180 ′ is etched using, for example, liquid hydrogen fluoride (HF) or vapor HF in case silicon oxide is used as the oxide spacer layer  180 ′. In some embodiments where the gate dielectric layer  162  is silicon oxide, the left end of the gate dielectric layer  162  may be recessed by the etchant used in the etching process P 6 , as indicated by the dash line DL. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 25  where N-type source and drain regions are formed in the NDD or the p-body region. With reference to  FIG. 17 , in some embodiments of block S 25 , a self-aligned implantation process P 7  is performed to dope an N-type dopant into the p-body region  156  and the NDD region  152 , thus forming an N-type source region  172  in the p-body region  156  and the N-type drain region  174  in the NDD region  152 . Moreover, the self-aligned implantation process P 7  also dopes the N-type dopant into the polysilicon gate  164 . Because the oxide spacer  180  has been removed from the top surface of the polysilicon gate  164  in the previous etching process P 6  as illustrated in  FIG. 16 , the polysilicon gate  164  can be implanted through the entire top surface of the polysilicon gate  164 , which in turn will reduce un-doped region in the polysilicon gate  164 . Before performing the self-aligned implantation process P 7 , a mask layer  194  is formed over the semiconductor substrate  110  and then patterned to form separated mask portions to cover a portion of the p-body region  156  adjacent to the isolation structure  142 , thus defining a target location of the N-type source region  172 , and the mask layer  194  is removed after the formation of the N-type source region  172  and the N-type drain region  174 . For example, an ion implantation may be performed to implant an n-type dopant, such as arsenic or phosphorus, at a vertical angle to form the N-type drain region  174  in the NDD region  152  using the spacers  170 ,  180  and  182  and the patterned mask layer  194  as an implant mask. Because ions of n-type dopant are directed at a vertical angle (i.e., perpendicular to the top surface of substrate  110 ), the resulting N-type source region  172  has a left boundary substantially aligned with the patterned mask layer  194  and a right boundary substantially aligned with the left-side nitride spacer  170 , and the N-type drain region  174  has a left boundary substantially aligned with the outermost end surface of the oxide spacer  180  and an outermost end of the right-side nitride spacer  182 . Because in the implantation process P 7  the left boundary of the N-type drain region  174  is self-aligned to the outermost end surface of the oxide spacer  180 , and the right boundary of the N-type source region  172  is self-aligned to the outermost end surface of the left-side nitride spacer  170 , the implantation process P 7  is referred to as a self-aligned implantation process in this context. 
     In greater detail, the second and third gate spacers  180  and  182  may function as an implanting blocking layer during the self-aligned implantation process P 7  with a vertical implantation angle, and thus the N-type drain region  174  is self-aligned with outermost sidewalls of the second and third spacers  180  and  182 . As such, an outermost end surface of the second gate spacer  180  is coterminous with a boundary between the N-type drain region  174  and the NDD region, and thus the second and third gate spacers  180  and  182  may not overlap with the N-type drain region  174 . However, in some embodiments, the N-type drain region  174  may laterally extends past the outermost end surface of the second gate spacer  180  due to unintentional thermal diffusion occurring in following steps of the front-end-of-line (FEOL) process and the back-end-of-line (BEOL) process. 
     In some embodiments, the N-type drain region  174  is separated from a channel region  110   c  in the substrate  110  by a distance S 1  (interchangeably referred to as a drift region length). The drift region length S 1  depends on the width W 1  of the second and third gate spacers  180  and  182 . As such, the drift region can be scaled down by controlling the width W 1  of the second and third gate spacers  180  and  182 , which depends on the thickness of the second spacer layer  180 ′ and the thickness of the third spacer layer  182 ′, as illustrated in  FIG. 11 . Therefore, the drift region length S 1  can be controlled by the thicknesses of the second and third spacer layers  180 ′ and  182 ′. For example, the drift region length S 1  can be in a range from about 0.05 um to about 0.5 um, and other drift region length ranges are within the scope of the disclosure. In some embodiments, if the drift region length S 1  is less than about 0.05 um, it may result in excessively low device breakdown voltage, and if the drift region length S 1  is large than about 0.5 um, it may result in excessively high resistance. 
     In some embodiments, the mask layer  194  may be formed by a photolithography patterning process. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. 
     The N-type source region  172  and the N-type drain region  174  are N+ regions (interchangeably referred to as heavily doped N-type regions) having n-type impurity concentration greater than that of the NDD region  152  and the p-body region  156 . In some embodiments, the N-type source region  172  and the N-type drain region  174  include n-type dopants such as P or As. A rapid thermal annealing (RTA) process may be performed after the self-aligned implantation process P 7  to activate the implanted dopant in the polysilicon gate  164  and the N-type source/drain regions  172  and  174 . 
     As illustrated in  FIG. 17 , a depth D 8  of the N-type drain region  174  may be less than the depth D 4  of the NDD region  152  and/or the depth D 1  of the isolation structure  142 . For example, the depth D 8  of the N-type drain region  174  may be in a range of about 0.1 um to about 0.5 um, and other depth ranges are within the scope of the disclosure. In some embodiments, the depth D 8  of the N-type drain region  174  may be greater than the depth D 4  of the NDD region  152  and/or the depth D 1  of the isolation structure  142 . In  FIG. 17 , a depth D 9  of the N-type source region  172  may be less than the depth D 5  of the p-body region  156  and/or the depth D 1  of the isolation structure  142 . For example, the depth D 9  of the N-type drain region  174  may be in a range of about 0.1 um to about 0.5 um, and other depth ranges are within the scope of the disclosure. In some embodiments, a depth D 9  of the N-type source region  172  may be greater than the depth D 5  of the p-body region  156  and/or the depth D 1  of the isolation structure  142 . In some embodiments, the depth D 9  of the N-type source region  172  is comparable to the depth D 8  of the N-type drain region  174 , because they are formed using a same implantation process P 7 . 
     In some embodiments, the dopant concentration of each of the N-type source region  172  and the N-type drain region  174  may be between about 1020 and about 1021 per cubic centimeter, and other dopant concentration ranges are within the scope of the disclosure. As illustrated in  FIG. 17 , a lateral distance between the N-type drain region  174  and the gate structure  160  is greater than a lateral distance between the N-type source region  172  and the gate structure  160 , and thus the LDMOS transistor has source/drain regions  172  and  174  asymmetric with respect to the gate structure  160 . Moreover, the drain region  174  has a width W 3  that is greater than the width W 4  of the source region  172 . By way of example and not limitation, a ratio of the width W 3  of the drain region  174  to the width W 4  of the source region  172  is greater than 2, and other ranges of ratio are within the scope of the disclosure. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 26  where a P-type body contact region is formed in the p-body region. With reference to  FIG. 18 , in some embodiments of block S 26 , for example, the mask layer  194  shown in  FIG. 17  is stripped by ashing if it is photoresist, and then a mask layer  192  is formed over the semiconductor substrate  110  and then patterned to cover the N-type source and drain regions  172  and  174  and the N-doped polysilicon gate  164 . Then, an implantation process P 8  is performed to implant a p-type dopant in the p-body region  156  using the mask layer  192  as an implant mask, thus forming a P-type body contact region  176  in the p-body region  156 . The mask layer  192  is removed after the formation of the P-type body contact region  176 . In some embodiments where the mask layer  192  is photoresist, the mask layer  192  is stripped by ashing after the formation of the P-type body contact region  176  is complete. 
     In some embodiments, the mask layer  192  may be formed by a photolithography patterning process. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. 
     The P-type body contact region  176  may be P+ or heavily doped regions having p-type impurity concentration greater than the P-body region  156 . In some embodiments, the P-type body contact region  176  includes p-type dopants such as boron or boron difluoride (BF 2 ). The P-type body contact region  176  may be formed by a method such as ion implantation or diffusion. A rapid thermal annealing (RTA) process may be performed after the implantation process P 8  to activate the implanted dopant. As illustrated in  FIG. 18 , the P-type body contact region  176  is formed in the p-body region  156  and between the N-type source region  172  and the isolation structure  142 . In the depicted embodiments, the P-type body contact region  176  is formed after forming the formation of the second and third gate spacers  180  and  182 . In some other embodiments, the P-type body contact region  176  may be formed before the formation of the N-type source region  172  and the N-type drain region  174 . 
     In some embodiments, a depth D 10  of the P-type body contact region  176  may be less than the depth D 5  of the P-body region  156  and the depth D 1  of the isolation structure  142  shown in  FIG. 3 . In some other embodiments, the depth D 10  of the P-type body contact region  176  may be greater or less than the depth D 9  of N-type source region  172 . In some embodiments, the depth D 10  of the P-type body contact region  176  may be comparable to the depth D 9  of N-type source region  172 . For example, the depth D 10  of the P-type body contact region  176  may be in a range from about 0.1 um to about 0.5 um, and other depth ranges are within the scope of the disclosure. In some embodiments, the dopant concentration of each of the P-type body contact region  176  may be between about 10 20  and about 10 21  per cubic centimeter, and other dopant concentration ranges are within the scope of the disclosure. 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 27  where metal alloy layers are respectively formed above the gate structure, the N-type source/drain region, and the P-type source/drain region. With reference to  FIG. 19 , in some embodiments of block S 27 , metal alloy layers  220  may be formed by self-aligned silicidation (salicide) process. In an exemplary salicide process, a metal material (e.g., cobalt, nickel or other suitable metal) is formed over the substrate, then the temperature is raised to anneal and cause a reaction between the metal material and the underlying silicon/polysilicon so as to form silicide layers  220 , and the un-reacted metal is etched away. The silicide material is self-aligned with the N-type source region  172  and the N-type drain region  174 , the P-type body contact region  176 , and/or the gate electrode  164  to reduce contact resistance. 
     In  FIG. 19 , one of the metal alloy layers  220  is in contact with an entirety of a top surface of the N-type drain region  174  within the NDD region  152  and the outermost end surface of the second gate spacer  180 . Other region of the NDD region  152  that is intentionally precluded from the silicide process is covered with the second and third gate spacers  180  and  182 . This protects the NDD region  152  below the second and third gate spacers  180  and  182  from the silicide formation. One of the metal alloy layers  220  is in contact with an entirety of a top surface of the gate electrode  164  to lower a resistance of the gate. One of the metal alloy layers  220  is in contact with an entirety of a top surface of N-type source region  172  and a top surface of the P-type body contact region  176  and thus extends across an interface between the N-type source region  172  and the P-type body contact region  176 . 
     Returning to  FIG. 1B , the method M 1  then proceeds to block S 28  where contacts are respectively formed above the metal alloy layers. With reference to  FIG. 20 , in some embodiments of block S 28 , an interlayer dielectric (ILD) layer  196  is formed above the structure in  FIG. 19 . In some embodiments, the ILD layer  196  includes a material having a low dielectric constant such as a dielectric constant less than about 3.9. For example, the ILD layer  196  may include silicon oxide. In some embodiments, the dielectric layer includes silicon dioxide, silicon nitride, silicon oxynitride, polyimide, spin-on glass (SOG), fluoride-doped silicate glass (FSG), carbon doped silicon oxide, Black Diamond® (Applied Materials of Santa Clara, Calif.), Xerogel, Aerogel, amorphous fluorinated carbon, Parylene, BCB (bis-benzocyclobutenes), SiLK (Dow Chemical, Midland, Mich.), polyimide, and/or other suitable materials. The ILD layer  196  may be formed by a technique including spin-on coating, CVD, or other suitable processes. 
     Then, a plurality of contacts  242 ,  244 , and  246  are formed in the ILD layer  196  to contact the respective metal alloy layers  220  (i.e., silicide layers  220 ). For example, a plurality of the openings are formed in the ILD layer  196 , and conductive materials are then deposited in the openings. The excess portions of the conductive materials outside the openings are removed by using a CMP process, while leaving portions in the openings to serve as the contacts  242 ,  244 , and  246 . The contacts  242 ,  244 , and  246  may be made of tungsten, aluminum, copper, or other suitable materials. In some embodiments, the contact  242  is electrically connected to the gate structure  160  via the metal alloy layer  220  atop the gate structure  160 , the contact  244  is connected to the P-type body contact region  176  and the N-type source region  172  by the metal alloy layer  220  spanning the to the P-type body contact region  176  and the N-type source region  172 , and the contact  246  is connected to the N-type drain region  174  by the metal alloy layer  220  atop the N-type drain region  174 . In the depicted embodiments, the P-type body contact region  176  and the N-type source region  172  share a same contact  244 . In some other embodiments, the P-type body contact region  176  and the N-type source region  172  may be separated from each other and electrically connected to separate contacts. 
     Reference is made to  FIGS. 21 and 22 .  FIGS. 21 and 22  illustrate a method for manufacturing a semiconductor device  200  in different stages in accordance with some embodiments. Operations for forming the semiconductor device  200  are substantially the same as the operations for forming the semiconductor device  100  described in foregoing descriptions and thus are not repeated herein for the sake of clarity.  FIGS. 21 and 22  illustrates a more practical profile of the LDMOS manufactured using the method M 1  of  FIGS. 19 and 20 . 
       FIG. 21  illustrates a semiconductor device  200  at a stage corresponding to  FIG. 19  according to some alternative embodiments of the present disclosure. As shown in  FIG. 21 , a top end of the third gate spacer  282  and a top end of the second gate spacer  280  may be lower than a top surface of the gate electrode  264 . Moreover, the top end of the third gate spacer  282  may be lower than the top end of the second gate spacer  280  due to the nature of the etching processes P 3  and P 6 . Moreover, the gate dielectric layer  262  has a left end set back from an outermost end of the left first spacer  270 , due to the nature of the etching process P 6  that etches oxide materials. However, the right end of the gate dielectric layer  262  may be still coterminous with an outermost end of the right first spacer  270 , because the right end of the gate dielectric layer  262  is covered and thus protected by the photoresist mask  190 . 
       FIG. 22  illustrates a semiconductor device  200  at a stage corresponding to  FIG. 20  according to some alternative embodiments of the present disclosure. As shown in  FIG. 22 , one of the metal alloy layers  220  is in contact with an entirety of a top surface of the N-type drain region  174  within the NDD  152  and an outermost end surface of the second gate spacer  280 . The area of the NDD region  152  that is intentionally precluded from the silicide process is covered with the second and third gate spacers  280  and  282 . This protects the NDD region  152  below the second and third gate spacers  280  and  282  from the silicide formation. Hence, the NDD region  152  provides a resistive path which acts as a voltage drop in the channel region, and thus the semiconductor device  200  has an improved blocking voltage ability. One of the metal alloy layers  220  is in contact with an entirety of a top surface of the gate electrode  264  to lower a resistance of the gate. One of the metal alloy layers  220  spans the N-type source region  172  and the P-type body contact region  176 . 
     Referring now to  FIGS. 23A and 23B , illustrated is an exemplary method M 2  for fabrication of a semiconductor device in accordance with some embodiments, in which the fabrication includes a self-aligned implantation and silicidation process of a semiconductor device.  FIG. 24  illustrates an LDMOS transistor fabricated using the method M 2 . The method M 2  includes a relevant part of the entire manufacturing process. It is understood that additional operations may be provided before, during, and after the operations shown by  FIGS. 23A and 23B , and some of the operations described below can be replaced or eliminated for additional embodiments of the method. The order of the operations/processes may be interchangeable. The method M 2  includes fabrication of a semiconductor device  400 . However, the fabrication of the semiconductor device  400  is merely example for describing the self-aligned process of the semiconductor device  400  according to some embodiments of the present disclosure. 
     With reference to  FIG. 24 , at block S 40 , an isolation structures  142  such as shallow trench isolations (STI) or local oxidation of silicon (LOCOS) (or field oxide, FOX) including isolation features may be formed in a semiconductor substrate  110  to define and electrically isolate various active regions so as to prevent leakage current from flowing between adjacent active regions. 
     At block S 41 , a gate dielectric layer is formed over the semiconductor substrate  110 . At block S 42 , a conductive layer is formed over the gate dielectric layer. In some embodiment, the conductive layer may include polycrystalline silicon (interchangeably referred to as polysilicon). At block S 43 , the conductive layer is patterned to form a gate electrode  464  on the gate dielectric layer. At block S 44 , a p-type double diffused doped (PDD) region  452  is formed in the semiconductor substrate  110  and in the vicinity of the top surface  112  of the semiconductor substrate  110 , and an n-type doped region  456  (interchangeably referred to as a n-body region) is formed in the semiconductor substrate  110  and in the vicinity of the top surface  112  of the semiconductor substrate  110 . At block S 45 , a first spacer layer is blanket deposited over the PDD region  452 , the n-body region  456 , the gate dielectric layer, the gate electrode  464 , and the isolation features  142 . At block S 46 , the first spacer layer is etched to form a first gate spacer  170 . At block S 47 , the gate dielectric layer is patterned to form a gate dielectric layer  462  below the gate electrode  464 , and the gate dielectric layer  462  and the gate electrode  464  are defined as a gate structure  460 . 
     At block S 48 , a second spacer layer is blanket deposited over the PDD region  452 , the n-body region  456 , the gate dielectric layer  462 , the gate electrode  464 , the first gate spacer  170 , and the isolation features  142 . At block S 49 , a third spacer layer is blanket deposited over the second spacer layer. At block S 50 , an etching operation is performed to remove the horizontal portions of the third spacer layer. 
     At block S 51 , a first mask layer is formed over the semiconductor substrate  110  and then patterned to form separated mask portions to cover a portion of the gate structure  460 , the first gate spacer  170 , the third gate spacer  482 , and the second spacer layer on a right side of the symmetrical axis A of the gate structure  160  shown in  FIG. 24 , and exposes another portion of the gate structure  460 , the first gate spacer  170 , and third gate spacer  482 , and the second spacer layer on a left side of the symmetrical axis A of the gate structure shown in  FIG. 24 . At block S 52 , one or more etching processes are performed to remove the third gate spacer  482  on the second spacer layer using the first mask layer as an etching mask. 
     At block S 53 , the first mask layer is removed after the etching of the third gate spacer  482 . At block S 54 , an etching operation is performed to remove a portion of the second spacer layer on the left side of the symmetrical axis A of the gate structure  460  and to remove the horizontal portions of the second spacer layer on the right side of the symmetrical axis A of the gate structure  460  using the third gate spacer  482  as an etching mask, so as to form a second gate spacer  480  on only one side of the gate structure  460  (e.g., only on right side of the gate structure  460 ). At block S 55 , P-type source and drain regions  474  and  472  are formed in the PDD region  452  and the n-body region  456  by a self-aligned process. At block S 56 , an N-type body contact region  476  is formed in the n-body region  456 . 
     At block S 57 , metal alloy layers  220  may be self-aligned to be formed on various features such as the P-type source and drain regions  472  and  474 , the N-type body contact region  476 , and/or the gate electrode  464  to reduce contact resistance. At block S 58 , an interlayer dielectric (ILD) layer  196  is formed above the semiconductor substrate  110 , and a plurality of contacts  242 ,  244 , and  246  are formed in the ILD layer  196  to contact the respective metal alloy layers  220  (i.e., silicide layers  220 ). For example, the contact  242  is connected to the gate structure  460 , the contact  244  is connected to the N-type body contact region  476  and the P-type source and drain regions  472 , the contact  246  is connected to the P-type source/drain regions  474  (i.e., the drain region of the semiconductor device  400 ). 
       FIGS. 25-28  illustrate exemplary cross sectional views of various stages for manufacturing a semiconductor device  600  according to some embodiments of the present disclosure. It is understood that additional operations can be provided before, during, and after processes shown by  FIGS. 25-28 , and some of the operations described below can be replaced or eliminated, for additional embodiments of the method. The order of the operations/processes may be interchangeable. The same or similar configurations, materials, processes and/or operation as described with  FIGS. 2-20  may be employed in the following embodiments, and the detailed explanation may be omitted. 
     After the structure as shown in  FIG. 11  is formed, an anisotropic etching process P 9  is performed to remove the horizontal portions of the third spacer layer  182 ′. The resulting structure is shown in  FIG. 25 . The etching operation P 9  etches the third spacer layer  182 ′ at a faster etch rate than it etches the second spacer layer  180 ′. For example, a ratio of the etch rate of the third spacer layer  182 ′ to the etch rate of the second spacer layer  180 ′ may be greater than about 2. If the ratio of the etch rate of the etching operation P 9  to the third spacer layer  182 ′ to the etch rate of the etching operation P 9  to the second spacer layer  180 ′ is less than about 2, the etching operation P 9  would significantly consume the second spacer layer  180 ′, and thus the second spacer layer  180 ′, polysilicon gate  164 , first gate spacer  170 , and semiconductor substrate  110  may be damaged, and thus the yield may reduce. In some embodiments, a ratio of the etch rate of the third spacer layer  182 ′ to the etch rate of the second spacer layer  180 ′ may be greater than about 10. In some embodiments, the etching process is performed using an isotropic etching process. In some embodiments, the third spacer layer  182 ′ is etched using, for example, phosphoric acid (H 3 PO 4 ). 
     As illustrated in  FIG. 25 , the remaining vertical portions of third spacer layer  182 ′ serve as third gate spacers  682 . The third gate spacers  682  have a height H 4  measured from the top surface of the second spacer layer  180 ′. In some embodiments, the height H 4  of the third gate spacers  682  may be substantially the same as or comparable to the height H 1  of the gate electrode  164 . The height H 4  of the third gate spacers  682  depends on process conditions of the anisotropic etching process P 9  (e.g., etching time duration and/or the like). For example, the etching time duration of the etching process P 9  can be controlled such that the resulting third gate spacers  682  has a topmost position substantially level with a topmost position of the second spacer layer  180 ′. 
     Afterwards, as illustrated in  FIG. 26 , a planarization process P 10  such as chemical mechanical polish (CMP) is performed to remove the excess third gate spacer  682  and the second spacer layer  180 ′ over the gate electrode  164  such that a top surface of the gate electrode  164  is exposed. In some embodiments, the planarization process stops when the gate electrode  164  is exposed, and the gate electrode  164  may act as the etch stop layer in the planarization. Thus, the second spacer layers  180 ′ may not overlap with a top surface of the gate electrode  164 . In some embodiments, after the planarization process, a top surface of the second spacer layers  180 ′ may be level with the top surface of the gate electrode  164 . 
     It is noted that the sequence of the etching process P 9  shown in  FIG. 26  and the planarization process P 10  shown in  FIG. 27  mentioned above is an example, and is not used to limit the present disclosure. In some other embodiments, the planarization process P 10  can be performed before the etching process P 9 . 
     Afterwards, as illustrated in  FIG. 27 , a mask layer  690  is formed over the semiconductor substrate  110  and then patterned to form separated mask portions to cover a portion of the gate structure  160 , the first gate spacer  170 , the third gate spacer  682 , and the second spacer layer  180 ′ on a right side of the symmetrical axis A of the gate structure  160 , and exposes another portion of the gate structure  160 , the first gate spacer  170 , and third gate spacer  682 , and the second spacer layer  180 ′ on a left side of the symmetrical axis A of the gate structure  160 . 
     As illustrated in  FIG. 27 , the mask layer  690  is in contact with a top surface of the gate structure  160 , because the top surface of the gate structure  160  is free from coverage by the second spacer layer  180 ′ due to the CMP process P 10  as illustrated in  FIG. 27 . In some embodiments, the mask layer  690  may be formed by a photolithography patterning processes. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), and/or other applicable processes. 
     Thereafter, as illustrated in  FIG. 28 , an etching process P 4  is performed to remove the third gate spacer  682  exposed by the mask layer  690 . The etching process P 4  is a selective etching process that uses an etchant etching the nitride spacer  682  at a faster etch rate than it etches the oxide spacer layer  180 ′. In this way, the oxide spacer layer  180 ′ remains substantially intact after removing the nitride spacer  682  from the left side of the symmetrical axis A of the gate structure  160 . For example, the etchant used in the etching process P 4  includes phosphoric acid (H 3 PO 4 ). 
     Afterwards, the process steps as illustrated in  FIGS. 15-20  continue to complete fabrication of the LDMOS transistor. 
     According to the aforementioned embodiments, it can be seen that the present disclosure offers advantages in fabricating semiconductor devices. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein. An advantage is that field oxide (FOX) can be omitted in the NDD/PDD region of the LDMOS transistor, which in turn will reduce the device size and the specific on-state resistance (Rsp). Another advantage is that the un-doped and/or un-silicide regions in the polysilicon gate can be reduced. Yet another advantage is that the drift region length can be precisely controlled by the oxide spacer thickness and the nitride spacer thickness, which in turn will be aid in scaling down the drift region length. Moreover, trade-off among drain-source breakdown voltage (BVDSS), specific on-state resistance (Rsp), and switch speed of the LDMOS transistor can be improved, and excellent figure of merit (FOM) can be achieved. 
     In some embodiments, a semiconductor device includes a gate structure, a double diffused region, a source region, a drain region, a first gate spacer, and a second gate spacer. The gate structure is over a semiconductor substrate. The double diffused region is in the semiconductor substrate and laterally extends past a first side of gate structure. The source region is in the semiconductor substrate and is adjacent a second side of the gate structure opposite the first side. The drain region is in the double diffused region in the semiconductor substrate and is of a same conductivity type as the double diffused region. The first gate spacer is on the first side of the gate structure. The second gate spacer extends upwardly from the double diffused region along an outermost sidewall of the first gate spacer and terminates prior to reaching a top surface of the gate structure. The second gate spacer has an outermost end surface substantially aligned with a boundary of the drain region. 
     In some embodiments, a semiconductor device includes a semiconductor substrate, a double diffused region, a gate structure, a drain region, a first gate spacer, a drain silicide layer, and a second gate spacer. The double diffused region is in the semiconductor substrate. The gate structure overlaps at least a portion of the double diffused region. The drain region is in the double diffused region and is of a same conductivity type as the double diffused region. The first gate spacer is alongside the gate structure and over the double diffused region. The drain silicide layer laterally extends from an outermost end surface of the first gate spacer along a top surface of the drain region. The second gate spacer is over the first gate spacer and has an outermost end substantially aligned with a boundary of the drain region. 
     In some embodiments, a method for manufacturing a semiconductor device includes forming a body region of a first conductivity type and a doped region of a second conductivity type in a semiconductor substrate; forming a gate structure over a portion of the body region and a portion of the doped region, and first gate spacers respectively on first and second sides of the gate structure; depositing, in sequence, a second spacer layer and a third spacer layer over the gate structure; patterning the third spacer layer into third gate spacers respectively on the first and second sides of the gate structure; removing a first one of the third gate spacers from the first side of the gate structure, while leaving a second one of the third gate spacers on the second side of the gate structure; patterning the second spacer layer into a second gate spacer by using the second one of the third gate spacers as an etching mask; and after patterning the second spacer layer, forming a source region of the second conductivity type in the body region and a drain region of the second conductivity type in the doped region. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.