Abstract:
A MOSFET formed using asymmetric silicidation between source and drain induces higher leakage between the body and the source than between the body and the drain. Implementation of such a MOSFET on an SOI substrate reduces or eliminates floating body effect for consistent on-current and turn-on time. The asymmetry between the source and the drain is introduced by forming different silicides between the source and the drains with a thicker silicide on the source, or by recessing the source material so that the source silicide is formed closer to the buried oxide layer than the drain silicide.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to semiconductor devices, and particularly, to metal oxide semiconductor filed effect transistors (MOSFETs) with an asymmetric silicide between source and drain, or “asymmetric silicide MOSFETs”. 
       BACKGROUND OF THE INVENTION  
       [0002]    On one hand, a metal oxide semiconductor field effect transistors (MOSFET) built on a silicon-on-insulator (SOI) substrate in general offer advantages over a MOSFET with comparable dimensions that is built on a bulk substrate by providing a higher on-current and lower parasitic capacitance between the body and other MOSFET components. On the other hand, a MOSFET built on an SOI substrate tends to have less consistency in the FET operation due to history effect, or floating body effect, in which the potential of the body, and subsequently, the timing of the turn-on and the on-current of the SOL MOSFET are dependent on the past history of the SOI MOSFET. Furthermore, the level of leakage current also depends on the voltage of the floating body, which poses a challenge in the design of a low power SOI MOSFET. 
         [0003]    Therefore, there exists a need for a structure that provides the advantages of SOI MOSFET devices while minimizing or eliminating the history effect of the SOI MOSFET devices. 
         [0004]    Furthermore, there exists a need for a structure that provides the advantages of SOI MOSFET devices while minimizing the variations in the leakage current to enable a low power SOI MOSFET design. 
       SUMMARY OF THE INVENTION  
       [0005]    The present invention addresses the needs described above by providing SOI MOSFET structures with an asymmetric silicide between the source and the drain and methods of fabricating the same. 
         [0006]    Specifically, the present invention provides an SOI MOSFET with a thicker silicide in the source than in the drain according to a first embodiment. 
         [0007]    The present invention provides an SOI MOSFET with a recessed silicide in the source and a non-recessed silicide in the drain according to a second embodiment. 
         [0008]    According to the first embodiment of the present invention, a metal-oxide-semiconductor field effect transistor (MOSFET) structure is disclosed, which comprises: 
         [0009]    a body located within a semiconductor substrate; 
         [0010]    a source metal silicide located in a source and in a portion of the body; and 
         [0011]    a drain metal silicide located in a drain and not contacting the body. 
         [0012]    Preferably, the MOSFET structure further contains a portion of the source that is not silicided and directly contacts a spacer. Also, preferably, the source metal silicide is thicker than the drain metal silicide. 
         [0013]    The MOSFET structure may comprise a bulk substrate, a silicon-on-insulator (SOI) substrate, or a hybrid substrate which comprise a bulk substrate portion and an SOI substrate portion. Preferably, the MOSFET structure comprises a silicon-on-insulator substrate. 
         [0014]    Optionally, the source metal silicide and the drain metal silicide may be two different materials. In one version of the present invention, the source metal silicide is a cobalt silicide and the drain silicide is a nickel metal alloy silicide, for example, a nickel platinum silicide (Ni 1-x Pt x Si) with an atomic ratio between nickel and platinum of about 19:1 (x˜0.05). 
         [0015]    According to the second embodiment of the present invention, a metal-oxide-semiconductor field effect transistor (MOSFET) structure is disclosed, which comprises a source metal silicide having a first portion located at a level lower than an extension implant region. 
         [0016]    Preferably, the source metal silicide has a second portion, wherein the second portion contacts a spacer and is contiguous with the first portion. 
         [0017]    More preferably, the source metal silicide has a third portion having a vertical sidewall, wherein the second portion contacts the first portion and the third portion. 
         [0018]    The MOSFET structure may comprise a bulk substrate, a silicon-on-insulator (SOI) substrate, or a hybrid substrate which comprise a bulk substrate portion and an SOI substrate portion. Preferably, the MOSFET structure comprises a silicon-on-insulator substrate. If an SOI substrate is utilized, the first portion may or may not contact a buried oxide layer. 
         [0019]    The MOSFET structure preferably comprises a source metal contact that directly contacts the first portion. The MOSFET structure preferably further comprises a drain metal contact that directly contacts a drain, wherein a bottom of the source metal contact is located at a lower level than a bottom of the drain metal contact. 
         [0020]    While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]      FIG. 1A  is a top-down view of an SOI MOSFET structure according to the first and the second embodiments of the present invention. 
           [0022]      FIG. 1B  is a cross-sectional view of an SOI MOSFET structure taken in the plane of A-A′ in  FIGS. 1A-8A  according to the first and the second embodiments of the present invention. 
           [0023]      FIGS. 2A-8A  are sequential top-down views of an SOI MOSFET structure according to the first embodiment of the present invention. 
           [0024]      FIGS. 2B-8  are sequential cross-sectional views of an SOI MOSFET structure taken in the plane of A-A′ in  FIGS. 2A-8A  according to the first embodiment of the present invention. 
           [0025]      FIGS. 9A-13A  are sequential top-down views of an SOI MOSFET structure according to the second embodiment of the present invention. 
           [0026]      FIGS. 9B-13B  are sequential cross-sectional views of an SOI MOSFET structure taken in the plane of A-A′ in  FIGS. 9A-13A  according to the second embodiment of the present invention. 
           [0027]      FIG. 14A  is a top-down view of an alternative SOI MOSFET structure in a case in which the thickness of the source metal silicide is increased compared to the structure in  FIGS. 13A-13B . 
           [0028]      FIG. 14B  is a cross-sectional view of the alternative SOI MOSFET structure taken in the plane of A-A′ in  FIG. 13A  in a case wherein the thickness of the source metal silicide is increased compared to the structure in  FIGS. 13A-13B . 
           [0029]      FIG. 15  is a magnified view of the structures around the source in  FIG. 13B . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Referring to  FIGS. 1A and 1B , a MOSFET structure at an initial stage of manufacturing according to the first and the second embodiments of the present invention is shown.  FIG. 1A  is a top-down view and  FIG. 1B  is a cross-sectional view along the plane A-A′ in  FIG. 1A . Throughout the accompanying figures, the relationship between each pair of figures with the same figure number is the same as the relationship between  FIG. 1A  and  FIG. 1B . While the present invention is described using an SOI substrate, implementation of the present invention on other substrates are straightforward. 
         [0031]    As shown in  FIGS. 1A and 1B , an SOI substrate with a handler wafer  10 , a buried oxide layer  20 , and a top semiconductor layer  33  contacting the buried oxide layer  20  is provided. Shallow trench isolation (STI)  40  is formed in the top semiconductor layer  33  to define an active area of an SOI MOSFET. A gate dielectric  50 , a gate conductor  52 , and an optional gate cap  54  are deposited and patterned to form a gate stack. Extension implant regions  60  are formed under the surface of the top semiconductor layer  33  to define the extensions for the source and for the drain. The body  30  at this point is the volume of the active area excluding the volume of the extension implant regions  60 . Spacers  56  are formed around the gate stack. The spacers  56  contact the extension implant regions  60 . The top semiconductor layer  33  at this point comprises the body  30 , the STI  40 , and the extension implant region  60 . 
         [0032]    Referring to  FIGS. 2A and 2B , a subsequent structure according to the first embodiment of the present invention is shown. A source and drain implantation is performed at this point preferably with amorphization implants. Halo implant or other extension implants may be used to tailor device performance. Optionally, embedded silicon alloy materials may be introduced to the source and drain regions at this point. In a PMOSFET, p-type dopants are introduced into the source  62  and into the drain  64 . In an NMOSFET, n-type dopants are introduced into the source  62  and into the drain  64 . Preferably, a single implant is used to deliver the dopants to both the source  62  and the drain  64 . The source and drain implants deliver a substantially higher doping, preferably by more than one order of magnitude, into the source  62  and into the drain  64 . After the source and drain implantation, the extension implant regions  60  are reduced in size to form actual source and drain extensions in an operational MOSFET as shown in  FIGS. 2A and 2B . 
         [0033]    Referring to  FIGS. 3A and 3B , a dielectric layer  70  is deposited over the entire top surface of the semiconductor structure above. The dielectric layer  70  is preferably conformal. The dielectric layer  70  may comprise a silicon nitride, a silicon oxide, a silicon oxynitride, or a stack thereof. The dielectric layer  70  is preferably a silicon nitride. The thickness of the dielectric layer  70  is in the range from about 10 nm to about 100 nm, and preferably in the range from about 20 nm to about 60 nm. Various methods of deposition including chemical vapor deposition (CVD) may be utilized to form the dielectric layer  70 . 
         [0034]    Subsequently, a photoresist  72  is applied over the top surface of the semiconductor structure and patterned as shown in  FIGS. 3A-3B  to expose a portion of the dielectric layer  70  on the side of the source  62 . If a gate cap  54  is employed and is present in the structure, the edge of the photoresist  72  may be placed between the first edge E 1 , which is defined by the outer edge of the bottom of the spacer  56  on the side of the source  62 , and the third edge E 3 , which is defined by the boundary of the gate conductor  52  with the adjoining spacer  56  on the side of the drain  64 . Subsequent etching of the dielectric layer  70  exposes semiconductor material only from the source  62 , but does not expose the gate conductor  52 , e.g., gate polysilicon from underneath the gate cap  54 . If a gate cap  54  is not employed and therefore not present in the structure at this point, the edge of the photoresist  72  may be placed between the first edge E 1  and the second edge E 2 , which is defined by the boundary of the gate conductor  52  with the adjoining spacer  56  on the side of the source  62 . 
         [0035]    Referring to  FIGS. 4A and 4B , the pattern formed on the photoresist  72  is transferred into the dielectric layer  70  by a reactive ion etching (RIE). The RIE forms a temporary spacer  70 ′ out of the dielectric layer  70  on the side of the source  62 . The RIE may expose a portion of the spacer  56  on the side of the source  62  depending on the overlay of the edge of the photoresist  72  over the gate structure. The RIB may also expose a portion of the gate cap  54  depending on the overlay of the edge of the photoresist  72  over the gate structure. The RIE is preferably selective to the underlying layers, that is, selective to the semiconductor material in the source  62 , to the dielectric in the shallow trench isolation  40 , to the spacer  56 , and to the optional gate cap  54 . 
         [0036]    Referring to  FIGS. 5A and 5B , the photoresist  72  is removed and a first metal  74  is deposited over the top surface of the semiconductor structure above. The first metal  74  may be nickel, nickel platinum alloy, cobalt, tantalum, tungsten, molybdenum, titanium, another refractory metal, or an alloy thereof. The thickness of the first metal  74  is selected to provide enough material to form a thick silicide in subsequent processing steps. The first metal  74  is reacted with the exposed semiconductor material in the source  62  in  FIG. 5B . 
         [0037]    Referring to  FIGS. 6A and 6B , the first metal  74  during a silicidation process consumes all of the doped semiconductor material directly underneath the exposed surface of the source  62  and to form a first portion  76 A of the source. The first portion  76 A of the source is silicided at this point. Furthermore, a portion  76 B of the body  30  in  FIG. 5B  is also silicided by the reaction of the first metal  74  with the semiconductor material in the body  30  to form a silicided portion  76 B of the body  30 . The unreacted first metal  74  is removed to form a structure shown in  FIGS. 6A and 6B . 
         [0038]    Preferably, a portion of the source  62  does not subsequently react with the first metal  74  to form a silicide. The unsilicided portion  63  of the source contacts a lower surface of spacer  56  as shown in  FIGS. 6A and 6B . At this point, the source ( 63 ,  76 A) comprises an unsilicided portion  63  and a silicided portion  76 A, which is “the first portion”  76 A of the source. 
         [0039]    According to the first embodiment of the present invention, the body  30  at this point comprises an unsilicided portion  32  of the body  30  and a silicided portion  76 B of the body  30 . The silicided portion  76 B of the body  30  may or may not touch the underlying buried oxide layer  20 . 
         [0040]    The two silicides ( 76 A,  76 B) are formed by the reaction of the first metal  74  with the semiconductor material only on the side of the source ( 63 ,  76 A) and are therefore, designated as “source metal silicide”  76 . In other words, the source metal silicide  76  comprises the first portion  76 A of the source ( 63 ,  76 A) and the silicided portion  76 B of the body ( 32 ,  76 B). 
         [0041]    Thereafter, the patterned insulator layer  70  and the temporary spacer  70 ′ are removed either by a RIE or by a wet etch, If an optional gate cap  54  is present in the structure, the gate cap  54  is also removed by a RIE or by a wet etch. After a suitable preclean of semiconductor surfaces, particularly, the surfaces of the gate conductor  52  and of the drain  64 , a second metal  84  is deposited as shown in  FIGS. 7A and 7B . The second metal  84  may be nickel, nickel platinum alloy, cobalt, tantalum, tungsten, molybdenum, titanium, other refractory metal or an alloy thereof. Preferably, the second metal  84  is a different material from the first metal  74 . The thickness of the first metal  84  is selected to form a thin silicide, that is, to form a silicide with less thickness (t 2  in  FIG. 5B ) in subsequent processing steps than the thickness t 1  of the source metal silicide  76  as shown in  FIG. 7B . 
         [0042]    The second metal  84  is reacted by a silicidation process with the underlying semiconductor material in the drain  64  and in the gate conductor  54  in  FIGS. 7A and 7B  to from a drain metal silicide  86  and a gate metal silicide  88  as shown in  FIGS. 8A and 8B . The reaction of the second metal on the source side is minimal due to the presence of the silicided portion  63 , i.e., due to a lack of unsilicided semiconductor material on the source side. The unreacted second metal  84  is thereafter removed The drain ( 86 , 65 ) at this point comprises a drain metal silicide  86  and an unsilicided drain  65 . Thereafter, a middle-of-the-line (MOL) dielectric (not shown) is deposited and source and drain metal contacts  90  are formed as shown in  FIGS. 8A and 8B . 
         [0043]    The structure according to the first embodiment of the present invention at this point comprises: 
         [0044]    the body ( 32 ,  76 B) located within the semiconductor substrate; 
         [0045]    the source metal silicide ( 76 A,  76 B) located in the first portion  76 A of the source ( 63 ,  76 A) and in the silicided portion  76 B of the body ( 32 ,  76 B); and 
         [0046]    the drain metal silicide  86  located in the drain ( 86 ,  65 ) and not contacting the body ( 32 ,  76 B). 
         [0047]    Furthermore, the structure according to the first embodiment of the present invention further comprises the second portion  63  of the source ( 63 ,  76 A), wherein the second portion  63  is not silicided and directly contacts a spacer  56 . 
         [0048]    According to the second embodiment of the present invention, a semiconductor structure as shown in  FIGS. 1A and 1B  are provided first. 
         [0049]    Referring to  FIGS. 9A and 9B , a dielectric layer  70  is deposited over the entire top surface of the semiconductor structure above. A photoresist  72  is applied over the top surface of the semiconductor structure and patterned as shown in  FIGS. 9A-9B  to expose a portion of the dielectric layer  70  on the side of the source to be formed. In  FIGS. 9A and 9B , the side of the source is to the left of the gate structure and does not have an overlying photoresist  72 . A gate cap  54  may optionally be present. The structural and methodical aspects of the dielectric layer  70 , of the photoresist  72 , of the optional gate cap  54 , and the three edges E 1 , E 2 , and E 3  according to the second embodiment of the present invention are identical to those according to the first embodiment as described in the paragraphs accompanying  FIGS. 3A and 3B . 
         [0050]    Referring to  FIGS. 10A and 10B , the pattern formed on the photoresist  72  is transferred into the dielectric layer  70  by a first reactive ion etching (RIE). The first RIE forms a temporary spacer  70 ′ out of the dielectric layer  70  on the side of the source  62 . The first RIE may expose a portion of the spacer  56  on the exposed side depending on the overlay of the edge of the photoresist  72  over the gate structure. The first RIE may also expose a portion of the gate cap  54  depending on the overlay of the edge of the photoresist  72  over the gate structure. The first RIE is preferably selective to the underlying dielectric layers, that is, to the dielectric in the shallow trench isolation  40 , to the spacer  56 , and to the optional gate cap  54 . Preferably, however, the first RIE is not selective to the semiconductor material in the extension implant regions  60  or to the semiconductor material in the body  30 . 
         [0051]    Referring to  FIGS. 11A and 11B , a second RIE is employed after the first RIE described above to etch a source recess region  160 . Preferably, the second RIB etches the semiconductor material in the extension implant regions  60  and some of the semiconductor material in the body  30  selective to the dielectric in the shallow trench isolation  40 , to the spacer  56 , and to the optional gate cap  54 . Preferably, the depth of the source recess region is deeper than the thickness of the extension implant region  60 . 
         [0052]    Referring to  FIGS. 12A and 12B , the dielectric layer  70  is thereafter removed either by a wet etch or a third RIE, preferably by a wet etch. A source and drain implantation is performed at this point preferably with amorphization implants. Halo implant or other extension implants may be used to tailor device performance at this point. Optionally, embedded silicon alloy materials may be introduced to the source and drain regions at this point. Preferably, a single implant is used to deliver the dopants to both the source  162  and the drain  64 . The source and drain implantation deliver a substantially higher doping than the doping in the extension implant region, preferably by more than one order of magnitude, into the source  162  and into the drain  64 . After the source and drain implantation, the extension implant regions  60  are reduced in size to define actual source and drain extensions in an operational MOSFET as shown in  FIGS. 12A  and  12 B. Due to the recess present in the source recess region  160 , the source  162  is formed deeper, that is, closer to the buried oxide layer  20  and vertically farther away from the gate dielectric  50 , than the drain  60 . The source  162  has at this point two levels of top surfaces, a first top surface that is recessed substantially below the bottom surface of the gate dielectric  52  and a second top surface that is substantially at the same level as the bottom surface of the gate dielectric  52 . 
         [0053]    Referring to  FIGS. 13A and 13B , the optional gate cap  54  is removed either by a wet etch or by a RIE. A metal is deposited on the top surface of the semiconductor structure and reacted with the exposed semiconductor surfaces, i.e., the semiconductor surfaces of the source  162 , of the gate conductor  52 , and of the drain  64  in  FIGS. 12A and 12B . The unreacted metal is removed from the semiconductor structure. Thereafter, a middle-of-the-line (MOL) dielectric (not shown) is deposited and a source metal contact  190  and a drain metal contact  90  are formed. 
         [0054]      FIGS. 13A and 13B  are resulting structures if the silicidation of the first portion  186 A of the source ( 186 ,  163 ) does not contact the buried oxide layer  20 .  FIGS. 14A and 14B  are resulting structures if the silicidation of the first portion  186 A of the source ( 186 ,  163 ) proceeds to and contacts the top of the buried oxide layer  20 .  FIG. 15  is a magnified view of the structure in  FIG. 13B  around the source showing the details of the structure according to the second embodiment of the present invention. 
         [0055]    The source ( 186 ,  163 ) at this point comprises: 
         [0056]    a first portion  186 A of source metal silicide  186  that is located at a level lower than an extension implant region  60 ; 
         [0057]    a second portion  186 B of the source metal silicide  186 , wherein the second portion contacts a spacer  56  and is contiguous with the first portion  186 A; 
         [0058]    a third portion  186 C having a vertical sidewall, wherein the third portion  186 C contacts the first portion  186 A and the second portion  186 B; and 
         [0059]    an unsilicided portion  163  of the source that contacts a spacer  56 . 
         [0060]    The drain ( 86 ,  65 ) at this point comprises a drain metal silicide  86  and an unsilicided portion  65  of the drain ( 86 ,  65 ). 
         [0061]    The structure according to the second embodiment of the present invention also comprises the source metal contact  190  that directly contacts the first portion  186 A of the source ( 186 A,  163 ) and the drain metal contact  90  that directly contacts the drain ( 86 ,  65 ), wherein the bottom of the source metal contact  190  is located at a lower level than the bottom of the drain metal contact  90 . 
         [0062]    The asymmetric silicide MOSFETs according to the first and the second embodiments of the present invention enable asymmetric leakage current flow between the body-source junction and the body-drain junction. Specifically, a higher leakage at the body-source junction compared to a leakage at the body-drain junction reduces or eliminates the floating body effect by making the potential of the body approach the potential at the source. 
         [0063]    The source according to the present invention is the terminal of a MOSFET from which the carriers are supplied. In an n-type MOSFET (NMOSFET), of the two terminals that are connected to the body of the NMOSFET, the source is the terminal that is connected to a lower voltage, and therefore, supplies electrons to the channel, i.e., electrons flow out of the source. In a p-type MOSFET (PMOSFET), of the two terminals that are connected to the body of the NMOSFET, the source is the terminal that is connected to a higher voltage, and therefore, supplies holes to the channel, i.e., holes flow out of the source. 
         [0064]    The asymmetric silicide MOSFETs may be employed in conjunction with regular SOI devices, i.e., SOI devices with substantially symmetric source and drain silicidation, in a circuit comprising SOI devices to provide MOSFETs with high immunity to floating body effects, or history effects. For example, both the source and the drain of the regular SOI devices may have the same silicidation as the drain of the MOSFETs with an asymmetric silicide. Alternatively, the MOSFETs with an asymmetric silicide maybe employed in conjunction with regular SOI devices in a circuit comprising SOI devices to provide MOSFETs with low power consumption by reducing leakage currents due to unstable body potential. In another application, the reduced floating body effect may be utilized to reduce an uncertainty window in a critical timing circuit.