Abstract:
A method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate; forming a gate structure on the substrate; forming a lightly doped drain in the substrate; and performing a first implantation process for implanting fluorine ions at a tiled angle into the substrate and part of the gate structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 14/275,858 filed May 12, 2014 U.S. Pat. No. 9,196,726, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of using ion implantations to form interface layer in a gate structure. 
     2. Description of the Prior Art 
     In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers. 
     Nevertheless, as semiconductor technology advances, gate structures employing work function materials soon reaches their physical and electrical limitation, causing side-effects including electrical instability and negative bias temperature instability (NBTI) effect. 
     NBTI effect is typically caused by accumulation of electrical potentials between silicon substrate and silicon oxide layers, which induces an effect when gate electrode is negatively biased. As PMOS transistors apply negative bias to generate electrons on metal gate adjacent to gate oxide, reject electrons on n-type substrate, and generate electron holes on n-type substrate and electron hole channel under gate structure thereby inducing electron holes of the source/drain region to be transmitted through this channel, NBTI effect is especially influential in CMOS devices containing PMOS structures. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a method of fabricating semiconductor device for improving issues caused by NBTI in current process. 
     According to a preferred embodiment of the present invention, a method for fabricating semiconductor device is disclosed. The method includes the steps of: providing a substrate; forming a gate structure on the substrate; forming a lightly doped drain in the substrate; and performing a first implantation process for implanting fluorine ions at a tiled angle into the substrate and part of the gate structure. 
     According to another aspect of the present invention, a semiconductor device is disclosed. The semiconductor device includes a substrate, a gate structure on the substrate, and a source/drain region in the substrate adjacent to the gate structure. The gate structure includes a first interface layer on the substrate, an interfacial layer on the first interface layer, and a conductive layer on the interfacial layer. Preferably, the first interface layer includes a first region and a second region surrounding the first region, and the concentration of the second region is higher than the concentration of the first region. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-4  illustrate a method for fabricating semiconductor device according to a first embodiment of the present invention. 
         FIGS. 5-8  illustrate a method for fabricating semiconductor device according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-4 ,  FIGS. 1-4  illustrate a method for fabricating semiconductor device according to a first embodiment of the present invention. As shown in  FIG. 1 , a substrate  12 , such as a wafer or silicon-on-insulator (SOI) substrate is provided. A stack structure  14  is then formed on the substrate  12 , in which the stack structure  14  may be fabricated by first forming an interfacial layer  16  on the substrate  12  and then forming a sacrificial layer  18  on the interfacial layer  16 . In this embodiment, the interfacial layer  16  is preferably composed of silicon material such as silicon dioxide (SiO 2 ), silicon nitride (SiN), silicon oxynitride (SiON), or other dielectric material with high permittivity or dielectric constant. The sacrificial layer  18  is preferably composed of single crystal silicon, doped polysilicon, or amorphous polysilicon, but could also be composed material selected from silicides or other metal material. 
     According to an embodiment of the present invention, a hard mask (not shown) could be selectively formed on the surface of the sacrificial layer  18  after the formation of the sacrificial layer  18 , in which the hard mask could be selected from the group consisting of SiC, SiON, SiN, SiCN and SiBN, but not limited thereto. Since the hard mask is a selectively formed element, it will be omitted in the following embodiment for the sake of brevity. 
     Next, an implantation process  20  is conducted to implant fluorine ions into an interface between the interfacial layer  16  and the substrate  12 . This forms an interface layer  22  between the substrate  12  and the interfacial layer  16 . 
     Next, as shown in  FIG. 2 , a patterned mask, such as a patterned resist (not shown) is formed on the sacrificial layer  18 , and a pattern transfer process is conducted by using the patterned resist as mask through single or multiple etching processes to remove part of the sacrificial layer, interfacial layer, and interface layer not covered by the patterned resist for forming a gate structure  24 . In other words, the gate structure  24  preferably includes a patterned interfacial layer  16 , a patterned interface layer  22 , and a patterned sacrificial layer  18 . 
     Next, a 1  liner (not shown) could be selectively formed on the sidewall of the gate structure  24 , and a lightly doped implantation process is conducted to forma lightly doped drain  26  in the substrate  12  adjacent to two sides of the liner or the gate structure  24 . Preferably, the dopants implanted during the lightly doped implantation process are adjusted according to the type of transistor being fabricated. For instance, if a NMOS transistor were to be fabricated, n-type dopants are implanted into the substrate  12  whereas if a PMOS transistor were to be fabricated, p-type dopants are implanted into the substrate  12 . 
     After forming the lightly doped drain  26 , another implantation process  28  is conducted to implant fluorine ions at a tilted angle into the substrate  12  and part of the gate structure  24 . In this embodiment, the concentration of the fluorine ions implanted during the implantation process  28  is substantially the same as the concentration of the fluorine ions implanted during the implantation process  20 , but not limited thereto. For instance, the concentration of the fluorine ions implanted during the implantation process  28  could also be higher or lower than the concentration of fluorine ions implanted during the implantation process  20  depending on the demand of the product, which are all within the scope of the present invention. 
     It should be noted that since the implantation process  28  is conducted at a tilted angle so that some of the fluorine ions implanted during implantation process  28  would overlap some of the fluorine ions implanted during the previous implantation process  20 , a first region  30  and a second region  32  surrounding the first region  30  are preferably formed in the interface layer  22  between the substrate  12  and the interfacial layer  16  after the implantation process  28 . Moreover, as the second region  32  includes fluorine ions implanted from both implantation processes  20  and  28  while the first region  30  only includes fluorine ions implanted from a single implantation process, the concentration of fluorine ions in the second region  32  is preferably higher than the concentration of fluorine ions in the first region  30 . It should also be noted that even though the first region  30  and the second region  32  appeared to have a rectangular shaped cross-section, the present invention could also adjust the boundary and position of ion implantation to form a substantially triangular cross-section for the second region  32  and a substantially trapezoid cross-section for the first region  30  in the interface layer  22  according to the demand of the product, and in such instance the concentration of fluorine ions in the second region  32  would also be higher than the concentration of fluorine ions in the first region  30 . 
     Next, as shown in  FIG. 3 , a spacer  34  is formed on the sidewall of the gate structure  24 , and a source/drain region  36  is formed in the substrate  12  adjacent to two sides of the spacer  34 . In this embodiment, the formation of the spacer  34  could include formation of an offset spacer and a main spacer, and despite the spacer  34  is formed after the implantation process  28 , an embodiment involving an order of first forming an offset spacer on the sidewall of the gate structure  24 , conducting the implantation process  28  to form the aforementioned first region  30  and second region  32 , and then forming a main spacer on the sidewall of the offset spacer could also be employed according to the demand of the product, which is also within the scope of the present invention. Next, a contact etch stop layer (CESL)  38  is formed on the gate structure  24 , and an interlayer dielectric (ILD) layer  40  is formed on the CESL  38 . It should be noted that elements including epitaxial layer and silicides could also be formed before the deposition of the CESL  38  according to the demand of the product, and as the fabrication of these elements are well known to those skilled in the art, the details of which are not explained herein for the sake of brevity. In addition, if the sacrificial layer  18  in the gate structure  24  were composed of metal material, a fabrication of MOS transistor could be completed at this stage. 
     Next, a replacement metal gate (RMG) process could be selectively conducted along with a high-k last process to transform the gate structure  24  into a metal gate. As shown in  FIG. 4 , the RMG process could be accomplished by first using a planarizing process to partially remove the ILD layer  40  and the CESL  38  to expose the surface of the sacrificial layer  18  of the gate structure  24 , and then performing a selective dry etching or wet etching process, such as using etchants including ammonium hydroxide (NH 4 OH) or tetramethylammonium hydroxide (TMAH) to remove the sacrificial layer  18  from the gate structure  24  for forming a recess (not shown). Next, a U-shaped high-k dielectric layer  50  and a conductive layer  46  including a work function metal layer  42  and low resistance metal layer  44  is deposited into the recess, and another planarizing process is conducted thereafter to form a metal gate  48 . 
     In this embodiment, the work function metal layer  42  is formed for tuning the work function of the metal gate  48  so that the device could be adapted in an NMOS or a PMOS transistor. For an NMOS transistor, the work function metal layer  42  having a work function ranging between 3.9 eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or titanium aluminum carbide (TiAlC), but is not limited thereto. For a PMOS transistor, the work function metal layer  42  having a work function ranging between 4.8 eV and 5.2 eV may include titanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC), but is not limited thereto. A barrier layer (not shown) could be formed between the work function metal layer  42  and the low resistance metal layer  44 , in which the material of the barrier layer may include titanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN). Furthermore, the material of the low resistance metal layer  44  may include copper (Cu), aluminum (Al), titanium aluminum (TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof. 
     According to an embodiment of the present invention, as shown in  FIG. 4 , another implantation could be selectively conducted after forming the high-k dielectric layer  50  to implant fluorine ions into an interface between the high-k dielectric layer  50  and interfacial layer  16  for forming another interface layer  52 . After forming the interface layer  52 , the conductive layer  46  containing both the work function metal layer  42  and low resistance metal layer  44  could be formed thereafter. 
     Referring to  FIGS. 5-8 ,  FIGS. 5-8  illustrate a method for fabricating semiconductor device according to a second embodiment of the present invention. As shown in  FIG. 5 , a substrate  62 , such as a wafer or silicon-on-insulator (SOI) substrate is provided. A stack structure  64  is then formed on the substrate  62 , in which the stack structure  64  may be fabricated by sequentially forming an interfacial layer  66 , a high-k dielectric layer  68 , a bottom barrier metal (BBM) layer  70 , and a sacrificial layer  72  on the substrate  62 . 
     In this embodiment, the interfacial layer  66  is preferably composed of silicon material such as silicon dioxide (SiO 2 ), silicon nitride (SiN), or silicon oxynitride (SiON), or other dielectric material with high permittivity or dielectric constant. The sacrificial layer  72  is preferably composed of single crystal silicon, doped polysilicon, or amorphous polysilicon, but could also be composed material selected from silicides or other metal material. 
     As the present embodiment is preferably accomplished by the employment of a high-k first process from gate last process, the high-k dielectric layer  68  preferably has a “I-shaped” cross section and preferably be selected from dielectric materials having dielectric constant (k value) larger than 4. For instance, the high-k dielectric layer  68  may be selected from hafnium oxide (HfO 2 ), hafnium silicon oxide (HfSiO 4 ), hafnium silicon oxynitride (HfSiON), aluminum oxide (Al 2 O 3 ), lanthanum oxide (La 2 O 3 ), tantalum oxide (Ta 2 O 5 ), yttrium oxide (Y 2 O 3 ), zirconium oxide (ZrO 2 ), strontium titanate oxide (SrTiO 3 ), zirconium silicon oxide (ZrSiO 4 ), hafnium zirconium oxide (HfZrO 4 ), strontium bismuth tantalate (SrBi 2 Ta 2 O 9 , SBT), lead zirconate titanate (PbZr x Ti 1-x O 3 , PZT), barium strontium titanate (Ba x Sr 1-x TiO 3 , BST) or a combination thereof. 
     In this embodiment, the high-k dielectric layer  68  may be formed by atomic layer deposition (ALD) process or metal-organic chemical vapor deposition (MOCVD) process, but not limited thereto. 
     Similar to the first embodiment, a hard mask (not shown) could be selectively formed on the surface of the sacrificial layer  72  after the formation of the sacrificial layer  72 , in which the hard mask could be selected from the group consisting of SiC, SiON, SiN, SiCN and SiBN, but not limited thereto. Since the hard mask is a selectively formed element, it will be omitted in the following embodiment for the sake of brevity. 
     Next, an implantation process  74  is conducted to implant fluorine ions entirely into the stack structure  64 . This forms a first interface layer  76  between the substrate  62  and the interfacial layer  66  and a second interface layer  78  between the interfacial layer  66  and the high-k dielectric layer  68 . 
     Next, as shown in  FIG. 6 , a patterned mask, such as a patterned resist (not shown) is formed on the sacrificial layer  72 , and a pattern transfer process is conducted by using the patterned resist as mask through single or multiple etching processes to remove part of the sacrificial layer  72 , BBM layer  70 , high-k dielectric layer  68 , second interface layer  78 , interfacial layer  66 , and first interface layer  76  not covered by the patterned resist for forming a gate structure  80 . In other words, the gate structure  80  preferably includes a patterned first interface layer  76 , patterned interfacial layer  66 , patterned second interface layer  78 , patterned high-k dielectric layer  68 , patterned BBM layer  70 , and patterned sacrificial layer  72 . 
     Next, a 1  liner (not shown) could be selectively formed on the sidewall of the gate structure  80 , and a lightly doped implantation process is conducted to form a lightly doped drain  82  in the substrate  62  adjacent to two sides of the liner or the gate structure  80 . Preferably, the dopants implanted during the lightly doped implantation process are adjusted according to the type of transistor being fabricated. For instance, if a NMOS transistor were to be fabricated, n-type dopants are implanted into the substrate  62  whereas if a PMOS transistor were to be fabricated, p-type dopants are implanted into the substrate  62 . 
     After forming the lightly doped drain  82 , another implantation process  84  is conducted to implant fluorine ions at a tilted angle into the substrate  62  and part of the gate structure  80 . Similar to the first embodiment, the concentration of the fluorine ions implanted during the implantation process  84  is substantially the same as the concentration of the fluorine ions implanted during the implantation process  74 , but not limited thereto. For instance, the concentration of the fluorine ions implanted during the implantation process  84  could also be higher or lower than the concentration of fluorine ions implanted during the implantation process  74  depending on the demand of the product, which are all within the scope of the present invention. 
     It should be noted that since the implantation process  84  is conducted at a tilted angle so that some of the fluorine ions implanted during implantation process  84  would overlap some of the fluorine ions implanted during the previous implantation process  74 , a first region  86  and a second region  88  surrounding the first region  86  are preferably formed in the first interface layer  76  between the substrate  62  and the interfacial layer  66  and the second interface layer  78  between the interfacial layer  66  and the high-k dielectric layer  68 . Moreover, as the second region  88  includes fluorine ions implanted from both implantation processes  74  and  84  while the first region  86  only includes fluorine ions implanted from the first implantation process  74 , the concentration of fluorine ions in the second region  88  is preferably higher than the concentration of fluorine ions in the first region  86 . It should also be noted that even though the first region  86  and the second region  88  appeared to have a rectangular shaped cross-section, the present invention could also adjust the boundary and position of ion implantation to form a substantially triangular cross-section for the second region  88  and a substantially trapezoid cross-section for the first region  86  in the first interface layer  76  and second interface layer  78  according to the demand of the product, and in such instance the concentration of fluorine ions in the second region  88  would also be higher than the concentration of fluorine ions in the first region  86 . 
     Next, as shown in  FIG. 7 , a spacer  90  is formed on the sidewall of the gate structure  80 , and a source/drain region  92  is formed in the substrate  62  adjacent to two sides of the spacer  90 . Similar to the first embodiment, the formation of the spacer  90  could include formation of an offset spacer and a main spacer, and despite the spacer is formed after the implantation process  84 , an embodiment involving an order of first forming an offset spacer on the sidewall of the gate structure  80 , conducting the implantation process  84  to form the aforementioned first region  86  and second region  88 , and then forming a main spacer on the sidewall of the offset spacer could also be employed according to the demand of the product, which is also within the scope of the present invention. Next, a contact etch stop layer (CESL)  94  is formed on the gate structure  80 , and an interlayer dielectric (ILD) layer  96  is formed on the CESL  94 . It should be noted that elements including epitaxial layer and silicides could also be formed before the deposition of the CESL  94  according to the demand of the product, and as the fabrication of these elements are well known to those skilled in the art, the details of which are not explained herein for the sake of brevity. 
     Next, a replacement metal gate (RMG) process could be conducted to transform the gate structure  80  into a metal gate. As shown in  FIG. 8 , the RMG process could be accomplished by performing a selective dry etching or wet etching process, such as using etchants including ammonium hydroxide (NH 4 OH) or tetramethylammonium hydroxide (TMAH) to remove the sacrificial layer  72  from the gate structure  80  for forming a recess (not shown). Next, a conductive layer  102  including a U-shaped work function metal layer  98  and low resistance metal layer  100  is deposited into the recess, and another planarizing process is conducted thereafter to form a metal gate  104 . 
     In this embodiment, the work function metal layer  98  is formed for tuning the work function of the metal gate  104  so that the device could be adapted in an NMOS or a PMOS transistor. For an NMOS transistor, the work function metal layer  98  having a work function ranging between 3.9 eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or titanium aluminum carbide (TiAlC), but is not limited thereto. For a PMOS transistor, the work function metal layer  98  having a work function ranging between 4.8 eV and 5.2 eV may include titanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC), but is not limited thereto. A barrier layer (not shown) could be formed between the work function metal layer  98  and the low resistance metal layer  100 , in which the material of the barrier layer may include titanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN). Furthermore, the material of the low resistance metal layer  100  may include copper (Cu), aluminum (Al), tungsten (W), titanium aluminum (TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof. 
     Overall, the present invention preferably conducts two ion implantation processes to inject fluorine ions into a gate structure before and after forming the gate structure on a substrate. The first implantation process is preferably conducted to implant fluorine ions into the stack structure entirely before forming the gate structure whereas the second implantation process is conducted to implant fluorine ions at a tilted angle into the gate structure after the gate structure is formed. Preferably, the two implantation processes are carried out to form a first region and a second region surrounding the first region in a first interface layer between substrate and interfacial layer and a second interface layer between interfacial layer and high-k dielectric layer, in which the concentration of the fluorine ions in the second region is substantially higher than the concentration of fluorine ions in the first region. 
     Typically, the bond strength of Si—O bond and Si—H bond between material layers such as substrate, interfacial layer, and high-k dielectric layer is around 3.18 eV (taking Si—H bond as an example) or 4.8 eV (taking Si—O bond as an example). A weak bond strength created under these circumstances induces NBTI effect easily, which further affects the performance of the device substantially. By injecting fluorine ions into the interface layer to form Si—F bonds through aforementioned implant processes, the present invention is able to utilize the Si—F bond created to boost up the bond strength between material layers to about 5.73 eV, thereby increasing stability between the material layers and ultimately improving side effects caused by NBTI. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.