Abstract:
The cross-sectional area of polysilicon lines is increased by selectively epitaxially growing an upper portion of the polysilicon line in the presence of a dielectric layer exposing the upper portion. Thus, a substantially T-shaped line is obtained, allowing a minimum bottom-CD while insuring a sufficient high conductivity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of fabrication of integrated circuits, and, more particularly, to a semiconductor device such as a field effect transistor having a gate electrode structure with a metal silicide layer formed thereon.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, a silicon-based field effect transistor comprises highly doped silicon regions, also referred to as drain and source regions, that are embedded in a lightly and inversely doped silicon region, a so-called N-well or P-well, depending on the type of transistor. The drain and the source regions are spaced apart by a channel region interposed therebetween, wherein a conductive channel forms between the drain and source regions in the channel region upon application of an appropriate voltage to a gate electrode that is usually formed over the channel region and is separated therefrom by a gate insulation layer, often provided as a gate oxide layer.  
           [0005]    Thus, in the most common field effect transistors, the gate electrode structure essentially comprises the gate electrode formed above the gate insulation layer, with polysilicon often being selected as the material for forming the gate electrode for several reasons.  
           [0006]    For instance, polysilicon exhibits high compatibility with the subsequent high temperature processes that will be performed in completing the manufacture of the integrated circuit device. Moreover, the polysilicon interface with thermal silicon dioxide is well understood and electrically stable. Furthermore, polysilicon is more reliable than, for example, aluminum gate materials, and can be deposited conformally over a step-like topography. However, problems arise when polysilicon is used as a gate electrode material due to its higher electric resistance compared to metals, such as aluminum. Even when doped at the highest practical concentration, a 0.5 μm thick polysilicon film has a sheet resistance of about 20 Ωsq. compared to 0.05 Ωsq. for a 0.5 μm thick aluminum film. The resulting high values of interconnect line resistance can therefore lead to relatively long RC time constants (i.e., long propagation delays) and DC voltage variations within VLSI (very large scale integration) circuits.  
           [0007]    To overcome this drawback, several solutions have been proposed and developed in the art. Among these solutions, the formation of metal silicides on the top of the polysilicon gate structure has proven to be the most reliable one for obtaining the lowest resistance values.  
           [0008]    A typical prior art method of forming metal silicides on silicon-containing regions, such as the gate electrode of a MOS transistor, will be described in the following with reference to FIGS. 1 a - 1   d.    
           [0009]    [0009]FIG. 1 a schematically shows a MOS transistor  100  to be formed on a substrate  1 , such as a silicon wafer. Isolation structures  2  define an active region of the transistor  100 . Moreover, reference  3  relates to a polysilicon gate electrode of the MOS transistor  100 . Reference  4  relates to oxide side spacers formed on the sidewalls of the polysilicon gate electrode  3 . Reference  6  denotes a gate insulation layer and reference  5  relates to the source and drain regions of the MOS transistor  100 . FIG. 1 b  shows the MOS transistor  100  with a refractory metal layer  7  deposited thereon. In FIG. 1 c , reference  8  relates to metal silicide layers formed on top of the polysilicon gate electrode  3  and on the source and drain regions  5 .  
           [0010]    Starting with the MOS transistor as depicted in FIG. 1 a , in a first step, the refractory metal layer  7  is deposited on the MOS transistor  100 , as depicted in FIG. 1 b . Usually, either titanium or cobalt is used as a metal for forming the metal layer  7 , and typically a physical vapor deposition (PVD) process, e.g., a sputtering process, is carried out for depositing the refractory metal layer  7 .  
           [0011]    Once the refractory metal layer  7  has been deposited, a low temperature thermal step (approximately 450° for copper or 650° for titanium, respectively) is carried out to react the metal in contact with silicon on the source/drain regions  5  and the polysilicon gate electrode  3 . During the thermal step, inter-diffusion of the polysilicon and metal occurs at the polysilicon/metal interface on top of the polysilicon gate electrode  3  as well as on the source/drain regions  5 . As a result, the metal silicide layers  8  are formed as depicted in FIG. 1 c , whereby the refractory metal layer  7  is at least partially consumed.  
           [0012]    In a subsequent step, as depicted in FIG. 1 d , the non-reacted metal is selectively removed with a selective wet-etching step, leaving behind the metal suicide layers  8  on top of the silicon gate electrode  3  and on the source/drain regions  5 .  
           [0013]    Commonly, a further heat treatment (not depicted in the figures) is carried out at a higher temperature than the previous heat treatment to transform the metal silicide layers  8  into a more stable phase that exhibits a lower resistance than the metal silicide layers formed during the previous lower temperature heat treatment. For example, if cobalt is used, a cobalt mono-silicide is formed during the first heat treatment, which is then converted into a cobalt disilicide.  
           [0014]    Since the finally-obtained metal silicide layers  8  exhibit a sheet resistance that is much lower compared to the sheet resistance of polysilicon, the total resistance of the gate electrode  3  including the metal silicide layers  8  is decreased.  
           [0015]    The prior art method described above has accomplished satisfactory results for devices having minimum feature sizes of 0.5 μm and more. The above method, however, is not completely adequate to compensate for the increase of the polysilicon sheet resistance which arises in the case of deep-sub-micron devices, i.e., with feature sizes equal to or smaller than 0.25 μm.  
           [0016]    The reasons for this can be explained as follows. As a general rule, decreasing the transistor size, i.e., the channel length, that is, in FIGS. 1 a - 1   d , the horizontal distance between the source/drain regions  5 , requires reducing the thickness of the gate insulation layer  6  and necessitates shallower source/drain regions which, in turn, restrict the achievable thickness of the metal silicides  8 . As the metal silicide layer  8  for the gate electrode  3  is simultaneously formed with the metal silicide layers  8  of the drain and source regions  5 , the thickness and, thus, the reduction in resistance, of the gate silicides is also restricted.  
           [0017]    As the cross-sectional dimensions of the polysilicon gate electrode  3  decrease as a result of the continuous miniaturization of the devices, the resistance of the polysilicon portions of the gate structures increases and represents the dominant contribution of the resistance of the polysilicon gate electrode  3 . The final, total resistance of the gate electrodes is, therefore, only scarcely influenced by the silicide layer but practically corresponds to the resistance of the polysilicon portion of the gate structure.  
           [0018]    Since the trend toward an increasing miniaturization of the devices manufacturable on a substrate will continue in years to come, it clearly results that the formation of metal silicide layers on the top of gate polysilicon lines according to the prior art methods will render it very difficult to realize gate structures featuring resistances in conformity with the electrical performance required.  
           [0019]    Accordingly, in view of the problems explained above, it would be desirable to provide a technique that may solve or reduce one or more of the problems identified above.  
         SUMMARY OF THE INVENTION  
         [0020]    In general, the present invention is based on the consideration that gate structures featuring a low sheet resistance can be realized by “decoupling” the effective gate length from the gate resistance in that the cross-sectional area of polysilicon lines is increased in an upper portion that is at least partially silicided, while the desired bottom-CD (bottom critical dimension) of the lines is maintained.  
           [0021]    According to one embodiment, the present invention relates to a method of forming at least one feature of a semiconductive material. The method comprises forming at least one initial feature of a semiconductive material having sidewalls and an upper surface. Moreover, the method comprises forming a dielectric layer adjacent to a portion of the sidewalls, wherein the dielectric layer covers less than all of the sidewalls and defines an exposed portion of the sidewalls of the initial feature. The method further comprises forming a layer of a semiconductive material on at least the upper surface and the exposed portion of the sidewalls of the initial feature so as to form at least one final feature of a semiconductive material having an upper portion extending beyond the sidewalls of the initial feature. Finally, the method comprises removing the dielectric layer so as to substantially expose all sidewalls of the initial feature.  
           [0022]    According to another embodiment, the present invention relates to a method of forming at least one conductive silicon-containing feature, the method comprising forming at least one initial silicon-containing feature having sidewalls and an upper surface. Moreover, the method comprises forming a dielectric layer adjacent to a portion of the sidewalls, wherein the dielectric layer covers less than all of the sidewalls and defines an exposed portion of the sidewalls of the initial feature. The method further comprises forming a silicon-containing layer on at least the upper surface and the exposed portion of the sidewalls of the initial feature so as to form at least one intermediate silicon-containing feature having an upper portion extending beyond the sidewalls of the initial feature. Additionally, the method comprises removing the dielectric layer so as to substantially expose all sidewalls of the initial feature and forming a metal silicide on the upper portion of the intermediate feature.  
           [0023]    According to a further embodiment of the present invention, there is provided a method of forming the gate electrode of at least one field effect transistor to be formed on a substrate, the method comprising forming at least one initial polysilicon line above an active region of the transistor. The method further comprises forming a dielectric layer on a lower portion of the sidewalls of the initial polysilicon line by depositing a layer of a dielectric material on the upper surface and the sidewalls of the initial polysilicon line and partially removing the dielectric material to expose the upper surface and the upper portion of the sidewalls of the initial polysilicon line. The method also comprises forming a polysilicon layer on at least the upper surface and the exposed portion of the sidewalls of the initial polysilicon line so as to define at least one intermediate polysilicon line having an upper portion extending laterally beyond the initial polysilicon line and removing the dielectric layer so as to substantially expose all sidewalls of the initial polysilicon line. Finally, the method comprises forming a metal silicide on at least the upper portion of the intermediate polysilicon line.  
           [0024]    In still another embodiment of the present invention, there is provided a field effect transistor comprising at least one silicon-containing electrode having an upper portion and a lower portion, wherein the upper portion has a width that is greater than a width of the lower portion. The transistor further comprises a metal silicide layer covering at least the upper surface of said upper portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:  
         [0026]    [0026]FIGS. 1 a - 1   d  represent a typical process sequence of a prior art method of forming metal silicides;  
         [0027]    [0027]FIGS. 2 a - 2   f  represent a process sequence for forming gate structures according to one illustrative embodiment of the present invention; and  
         [0028]    [0028]FIGS. 3 a - 3   d  represent an example for modifying the process sequence depicted in FIGS. 2 a - 2   f  according to another embodiment of the present invention. 
     
    
       [0029]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0031]    The present invention will now be described with reference to the attached figures. Although the various regions and structures of a semiconductor device are depicted in the drawings as having very precise, sharp configurations and profiles, those skilled in the art recognize that, in reality, these regions and structures are not as precise as indicated in the drawings. Additionally, the relative sizes of the various features and doped regions depicted in the drawings may be exaggerated or reduced as compared to the size of those features or regions on fabricated devices. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.  
         [0032]    The present invention is understood to be of particular advantage when used for forming the gate structures of MOS transistors. For this reason, examples will be given in the following in which corresponding embodiments of the present invention are related to the formation of the gate structures of a MOS transistor. However, it has to be noted that the present invention is not limited to the formation of gate structures of MOS transistors but can be used in any other situation in which the realization of semiconductive lines exhibiting low sheet resistance is required. The present invention is, therefore, also applicable to these semiconductive lines, and the gate structures described in the following illustrative embodiments are to represent any semiconductive line, irrespective of the crystalline structure and the degree and type of dotation.  
         [0033]    With reference to FIGS. 2 a - 2   f , illustrative embodiments of the present invention will now be described. In FIG. 2 a , reference  1  relates to a substrate of a MOS transistor  200  to be formed, for instance a silicon wafer. Reference  2  relates to isolation structures defining an active region. The isolation structures  2  are provided as shallow trench isolation (STI) structures. However, other isolation structures, for instance local oxidation of silicon (LOCOS) structures, could have been formed instead of STI structures. The isolation structures  2  essentially comprise an insulating material, such as silicon dioxide, silicon nitride or the like. References  3  relates to a polysilicon gate electrode (in the following also referred to as gate polysilicon lines) formed on a gate insulation layer  6  formed on the active region of the substrate  1 .  
         [0034]    In FIG. 2 b , reference  9  relates to a dielectric layer formed on the substrate  1 . In FIG. 2 c , the dielectric layer  9  is partially removed to form a layer  9 ′. In FIG. 2 d , reference  3 ′ denotes a polysilicon cap layer formed on the polysilicon line  3 . In FIG. 2 e , reference  3 ″ relates to a polysilicon line having an having an upper portion  3   u  extending beyond sidewalls  3   m . In FIG. 2 f , reference  4  relates to sidewall spacers formed at the sidewalls  3   m , reference  5  relates to the source and drain regions of the MOS transistor  200 , and reference  8  denotes metal silicide layers formed on the polysilicon line  3 ″ and drain and source regions  5 .  
         [0035]    A typical process flow for forming the MOS transistor  200  in accordance with illustrative embodiments of the present invention may include the following steps. Starting from FIG. 2 a , the layer  9  of dielectric material is formed on the substrate  1  (FIG. 2 b ). For example, a layer of silicon oxide can be deposited according to a low pressure chemical vapor deposition (LPCVD) technique.  
         [0036]    Alternatively, a layer of silicon nitride or any other dielectric material can be formed on the substrate  1  according to any known technique. The layer  9  is blanket-deposited to a thickness that exceeds the height of the polysilicon line  3  so that the polysilicon line  3  is completely covered by the dielectric layer  9 .  
         [0037]    Subsequently, the dielectric layer  9  is partially removed, as depicted in FIG. 2 c . This step aims to expose an upper surface  10  and, at least partially, upper portions  11  of the sidewalls of the polysilicon line  3 . To this end, a first polishing step, for instance a chemical mechanical polishing step, is carried out until the upper surface  10  of the polysilicon lines  3  is exposed. Subsequently, the dielectric layer  9  is etched back anisotropically in order to reliably expose at least the portion  11  of the sidewalls of the polysilicon lines  3 , thereby forming the dielectric layer  9 ′. Thus, the upper surface  10  of the polysilicon line  3  and at least the upper portions  11  of the sidewalls of the polysilicon lines  3  are substantially completely exposed.  
         [0038]    As shown in FIG. 2 d , the polysilicon cap layer  3 ′ is formed on the polysilicon line  3 , for instance, by selective epitaxial growth. Since the upper surface  10  of the polysilicon line  3  and at least the upper portion  11  of the sidewalls of the polysilicon line  3  are substantially completely exposed, the growth of polysilicon will occur in both vertical and lateral directions, i.e., on both the upper surface  10  and the exposed portion  11  of the sidewalls of the polysilicon line  3  to form the cap layer  3 ′ covering the upper surface  10  and the exposed portion  11  of the sidewalls of the polysilicon line  3 .  
         [0039]    After forming the polysilicon cap layer  3 ′ on the polysilicon line  3 , as described above, the dielectric layer  9 ′ is removed by isotropic etch techniques until the underlying substrate and the sidewalls  3   m  are exposed. For instance, a wet-etching step can be carried out to this end.  
         [0040]    As shown in FIG. 2 e , the polysilicon line  3 ″ formed on the gate insulation layer  6  comprises a central portion defined by the sidewalls  3   m  and an upper portion  3   u  extending laterally beyond the sidewalls  3   m . The polysilicon line  3 ″ represents a T-shaped line having an increased cross-sectional area compared to a conventional polysilicon line. Thus, the polysilicon line  3 ″ formed according to the present invention has a desired small bottom-CD (bottom critical dimension), while at the same time providing an increased cross-sectional area and thus a reduced resistance. Therefore, metal silicide layers of increased dimension can be formed on the polysilicon lines  3 ″. The enlarged upper portion  3   u  of the polysilicon line  3 ″ allows enlarged metal layers to be deposited on the upper portion  3   u . Accordingly, the final sheet resistance of the polysilicon line  3 ″ is strongly influenced by the metal silicide layer  8 , so that the resistance thereof is substantially determined by the metal silicide layer  8  substantially without being limited by the bottom-CD. Accordingly, polysilicon lines can be formed exhibiting, on the one hand, a reduced sheet resistance, but, on the other hand, still accomplishing the dimensional requirements arising from the miniaturization of the devices manufacturable on a substrate.  
         [0041]    Once the polysilicon line  3 ″ of FIG. 2 e  is formed, the MOS transistor  200  is completed on the substrate  1  according to well-known techniques, for example, by forming lightly doped regions on the exposed portions of the substrate  1 , followed by a step for forming the sidewall spacers  4  depicted in FIG. 2 f . During the implantation step for forming the lightly doped regions (not depicted in FIG. 2 f ), an angle of incidence may be varied to allow dopants to be implanted in those regions of the substrate shielded and/or shaded by the protruding upper portion of the polysilicon line  3 ″.  
         [0042]    The formation of the sidewall spacers  4  is followed by a further heavy ion implantation step allowing formation of the source/drain regions  5 , as depicted in FIG. 2 f . Finally, the metal silicide layers  8  are formed on the polysilicon line  3 ″ as well as on the source and drain regions  5 . As stated above, due to the T cross-sectional shape of the polysilicon line  3 ″, the MOS transistor  200  has a gate electrode exhibiting a final sheet resistance that is lower compared to the sheet resistance of gate structures formed according to prior art methods.  
         [0043]    According to a further embodiment, the sequence described above may be modified, as will be described with reference to FIGS. 3 a - 3   d . In FIG. 3 a , the light implantation step for forming a lightly doped region can be carried out prior to the formation of the polysilicon line  3 ″ having the enlarged upper portion  3   u . Once the gate dielectric layer  6  and the overlying initial polysilicon line  3  have been formed according to well-known methods, including deposition of a polysilicon layer, DUV-lithography, resist-trim, anisotropic etching, etc., a light ion implantation step can be performed for the purpose of forming lightly-doped regions  5 ′ on those regions of the substrate  1  where the source and drain regions  5  will be formed. Forming the lightly-doped regions  5 ′ at this moment during the manufacturing process, i.e., after the formation of the initial polysilicon line  3 , has the advantage that dopants can be easily implanted into the regions of the substrate  1  adjacent to the gate dielectric layer  6 . The sequence is then continued with forming the final polysilicon line  3 ″ according to the process flow described above with reference to FIGS. 2 a - 2   f.    
         [0044]    As shown in FIG. 3 c , after forming the final polysilicon line  3 ″, the sequence is resumed by forming the sidewall spacers  4 . Subsequently, as depicted in FIG. 3 d , a next heavy ion implantation step is carried out resulting in the formation of the source and drain regions  5  depicted in FIG. 3 d . The transistor is then completed by forming the metal silicide layers (not depicted in FIGS. 3 a - 3   d ).  
         [0045]    In conclusion, the present invention allows realizing polysilicon lines featuring a required small bottom-CD and an increased cross-sectional area, thereby allowing a significantly larger portion of the polysilicon to be converted in a highly conductive metal silicide layer. Thus, scalable gate electrodes may be provided with a process flow that is highly compatible with conventional CMOS processing.  
         [0046]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.