Abstract:
A technique for forming a sub-0.05 μm channel length double-gated/double channel MOSFET structure having excellent short-channel characteristics as well as the double-gated/double channel MOSFET structure itself is provided herein. The inventive technique utilizes a damascene process for the fabrication of a MOSFET device with double-gate/double channel structure. The gates are present on opposite sides of a silicon film having a vertical thickness of about 80 nm or less which is present in the gate region. The silicon film serves as the vertical channel regions of the structure and connects diffusion regions that are abutting the gate region to each other. In the inventive device, the current is double that of a conventional planar MOSFET with the same physical width due to its dual channel feature.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to semiconductor devices, and more particularly to a method of fabricating a double-gated metal oxide semiconductor field effect transistor (MOSFET) structure that has sub-0.05 μm channel lengths associated therewith. The present invention also relates to a double-gated MOSFET structure having vertical channel regions. The structure of the inventive MOSFET comprises a silicon film having a vertical thickness of about 80 nm or less which forms the vertical body, i.e., vertical channels, of the structure.  
         BACKGROUND OF THE INVENTION  
         [0002]    Over the past twenty-five years or so, the primary challenge of very large scale integration (VLSI) has been the integration of an ever-increasing number of MOSFET devices with high yield and reliability. This was achieved mainly in the prior art by scaling down the MOSFET channel length without excessive short-channel effects. As is known to those skilled in the art, short-channel effects are the decrease of threshold voltage V t  in short-channel devices due to two-dimensional electrostatic charge sharing between the gate and the source/drain regions.  
           [0003]    To scale down MOSFET channel lengths without excessive short-channel effects, gate oxide thickness has to be reduced while increasing channel-doping concentration. However, Yan, et al., “Scaling the Si MOSFET: From bulk to SOI to bulk”, IEEE Trans. Elect. Dev., Vol. 39, p. 1704, July 1992, have shown that to reduce short-channel effects for sub-0.05 μm MOSFETs, it is important to have a backside-conducting layer present in the structure that screens the drain field away from the channel. The Yan, et al. results show that double-gated MOSFETs and MOSFETs with a top gate and a backside ground plane are more immune to short-channel effects and hence can be scaled to shorter dimensions than conventional MOSFETs.  
           [0004]    The structure of the prior art MOSFETs consists of a very thin insulating layer for the channel, with two gates, one on each side of the channel. The two gates are electrically connected so that they serve to modulate the channel. Short-channel effects are greatly suppressed in such a structure because the two gates very effectively terminate the drain field line preventing the drain potential from being felt at the source end of the channel. Consequently, the variation of the threshold voltage with drain voltage and with gate length of a prior art double-gated MOSFET is much smaller than that of a conventional single-gated structure of the same channel length.  
           [0005]    To date, there are no adequate means for fabricating double-gated MOSFET structures, thus there is a continued need for developing a new and improved method of fabricating double-gated MOSFETs in which the variation of the threshold voltage with drain voltage and with gate length is substantially less than that of a single-gated structure of the same channel length. Moreover, there is a continued need for developing a method of fabricating a double-gated structure having an on-current that is double that of a single-gated structure with the same channel length.  
         SUMMARY OF THE INVENTION  
         [0006]    One object of the present invention is to provide a method of fabricating a double-gated/double channel MOSFET structure that has sub-0.05 μm channels length associated therewith.  
           [0007]    Another object of the present invention is to provide a method of fabricating a double-gated/double channel MOSFET structure that has excellent short-channel characteristics.  
           [0008]    A further object of the present invention is to provide a method of fabricating a double-gated/double channel MOSFET structure in which the variation of threshold voltage with drain voltage and with gate length is substantially less than that of a single-gated structure of the same channel length.  
           [0009]    A yet further object of the present invention is to provide a method of fabricating a double-gated/double channel MOSFET structure which has double the on-current as compared with conventional single-gated structures of the same channel length.  
           [0010]    These and other objects and advantages are achieved in the present invention by utilizing a damascene process for the fabrication of a MOSFET structure with a double-gated/double channel structure; the gate of the inventive device is located on each side of a silicon film having a vertical thickness, i.e., height, on the order of about 80 nm or less. The silicon film, which is formed on top of an insulating region, functions as the vertical channel regions of the MOSFET structure and it is surrounded with a gate forming a MOSFET structure with two parallel channels and a single-gate.  
           [0011]    Short-channel effects are greatly reduced with sub-0.05 μm channel lengths because the device body, i.e., channel regions, has a very thin horizontal thickness which allows the termination of the drain field lines on the surrounding gate. This, in turn, prevents the drain potential from being felt at the source end of the channel. The current of the inventive structure is double that of a conventional planar MOSFET with the same physical channel length due to its dual channel feature.  
           [0012]    One aspect of the present invention thus relates to a method of fabricating a double-gated/double channel MOSFET structure having sub-0.05 channel lengths, said method comprising the steps of:  
           [0013]    (a) forming a patterned hard mask on a surface of a substrate, said substrate comprising at least a silicon layer formed on top of an insulating region;  
           [0014]    (b) forming a patterned dummy gate stack on a portion of said silicon layer and a portion of said patterned hard mask;  
           [0015]    (c) forming source/drain extensions by removing said silicon layer not protected by said hard mask and said patterned dummy gate stopping on said insulating region and oxidizing exposed sidewalls of said silicon layer protected by said hard mask and said patterned dummy gate region;  
           [0016]    (d) forming an oxide layer on exposed surfaces of said insulating region and planarizing said oxide layer stopping on an uppermost polysilicon surface of said patterned dummy gate;  
           [0017]    (e) removing said patterned dummy gate stopping on said hard mask so as to provide an opening in said oxide layer;  
           [0018]    (f) forming a gate stack in said opening; and  
           [0019]    (g) removing said oxide layer and said hard mask abutting said gate stack so as to expose said insulating region and portions of said silicon layer abutting said gate stack.  
           [0020]    In the above-described method, the silicon layer that remains under the hard mask in the opening provided in step (e) represents the vertical channel regions of the inventive double-gated/double channel MOSFET structure.  
           [0021]    Following step (g) above, the present invention also contemplates one or more of the following steps:  
           [0022]    forming activated source/drain regions in portions of said exposed silicon layer abutting said gate stack.  
           [0023]    subjecting said gate stack to oxidation (required if the gate stack comprises polysilicon).  
           [0024]    forming spacers on exposed sidewalls of said gate stack (optional for polysilicon-containing gate stacks, but required for non-polysilicon gate stacks).  
           [0025]    saliciding said source/drain regions, or forming raised source/drain regions on portions of said exposed silicon layer abutting the gate stack and then saliciding the raised source/drain regions.  
           [0026]    further back-end-of the line (BEOL) processing.  
           [0027]    Another aspect of the present invention comprises a double-gated/double channel MOSFET structure which is formed from the above-mentioned processing steps. Specifically, the inventive double-gated/double channel MOSFET structure comprises:  
           [0028]    a bottom Si-containing layer;  
           [0029]    an insulating region present on said bottom Si-containing layer;  
           [0030]    a top silicon layer present on a portion of said insulating region, wherein a portion of said top silicon layer functions as vertical channel regions and other portions of said top silicon layer abutting said vertical channel regions contain diffusion regions therein;  
           [0031]    a hard mask formed on top of said vertical channel regions; and  
           [0032]    a gate region formed surrounding said vertical channel regions, wherein said gate region includes at least a gate oxide formed on exposed sidewalls of said vertical channel regions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    FIGS.  1 - 8  show the formation of the inventive double-gated/double channel MOSFET structure through various processing steps of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    The present invention, which relates to a method of fabricating a double-gated/double channel MOSFET structure as well as the MOSFET structure itself, will now be described in more detail by referring to the drawings that accompany the present application. It is noted that in the accompanying drawings, like and/or corresponding elements are referred to by like reference numerals.  
         [0035]    Reference is first made to FIG. 1 which illustrates an initial structure that is employed in the present invention in fabricating the inventive double-gated/double channel MOSFET structure. Specifically, the initial structure shown in FIG. 1 comprises a substrate  10  having hard mask  16  formed on an upper surface thereof. The substrate shown in FIG. 1 comprises a bottom Si-containing layer  11 , an insulating region  12  present on layer  11 , and silicon layer  14  present on insulating region  12 .  
         [0036]    The initial structure shown in FIG. 1 is composed of conventional materials well known in the art and conventional processing techniques also well known in the art are employed in forming the same. For example, the substrate may be a silicon-on-insulator (SOI) wafer in which insulating region  12  is a buried oxide layer which is sandwiched between a silicon layer (i.e., silicon layer  14  of FIG. 1) and a bottom Si-containing layer (i.e., layer  11  shown in FIG. 1), or alternatively, the substrate includes a Si-containing wafer in which a thin oxide layer, i.e., insulating region  12 , is formed between a lightly doped silicon top layer, i.e., silicon layer  14 , and a heavily doped bottom Si-containing layer (i.e., layer  11  shown in the FIG. 1); the heavily doped bottom Si-containing layer is the ground plane region of the substrate. Note that in the case of a ground plane substrate, the insulating region depicted in FIG. 1 would be much thinner than that shown. The term “Si-containing” as used herein denotes a material such as Si, SiGe, SiGeC, SiC, polysilicon (i.e., polySi), epitaxial silicon (i.e., epi-Si), amorphous silicon (a:Si), multilayers thereof or other like Si-containing materials.  
         [0037]    In accordance with the present invention, top silicon layer  14  is a thin silicon layer which has a vertical thickness, t v , i.e., height, of from about 20 to about 80 nm, with a vertical thickness of about 50 nm being more highly preferred. The thickness of insulating region  12  and the underlying Si-containing layer is not critical to the present invention; however, when a ground plane substrate is employed, the insulating region typically is thinner than the corresponding insulating layer of an SOI substrate.  
         [0038]    When an SOI substrate is employed in the present invention, the SOI substrate is fabricated using techniques that are well known in the art. For example, the SOI substrate may be fabricated by a thermal bonding process, or alternatively the SOI substrate may be formed by an implantation process which is referred to in the art as separation by ion implantation of oxygen (SIMOX).  
         [0039]    When the substrate includes a ground plane region, the substrate containing the ground plane region is formed by conventional bonding processes well known in the art and doping of the Si layers is carried out utilizing conventional ion implantation.  
         [0040]    Notwithstanding the type of substrate employed in the present invention, hard mask  16  is formed on a surface of silicon layer  14  of substrate  10  utilizing conventional processes well known in the art. For example, the hard mask may be formed by a conventional deposition process such as chemical vapor deposition (CVD), plasma-assisted CVD, evaporation, sputtering, atomic layer deposition, chemical solution deposition, or alternatively, the hard mask is formed by a conventional thermal growing process such as oxidization, nitridation or oxynitridation. Of these techniques mentioned hereinabove, it is highly preferred that bard mask  16  be formed by a thermal growing process.  
         [0041]    The hard mask employed in the present invention is composed of an insulating material such as an oxide, nitride, oxynitride, or combinations thereof. A highly preferred material for hard mask  16  is an oxide such as SiO 2 . In accordance with the present invention, a layer of hard mask material is formed on the uppermost surface of substrate  10 . The layer of hard mask material employed in the present invention has a physical thickness of from about 30 to about 70 nm, with a physical thickness of about 50 nm being more highly preferred.  
         [0042]    Following the formation of the layer of hard mask material on the uppermost surface of the substrate, the layer of hard mask  16  is then patterned so as to form patterned hard mask  18  on a portion of silicon layer  14 ; See FIG. 2. Although only one patterned hard mask region is depicted in drawings of the present application, the present invention works equally well when more than one patterned hard mask region is formed on the uppermost surface of substrate  10 . Note that in FIG  2 , layer  11  has been omitted for clarity.  
         [0043]    Patterned hard mask  18  is formed on portions of silicon layer  14  by utilizing lithography and etching. The lithography step employed in this patterning process includes applying a photoresist (not shown in the drawings) to a surface of hard mask  16 , exposing said photoresist to a pattern of radiation and developing the pattern in the exposed photoresist by employing a conventional developer solution. The pattern is then transferred to the hard mask by employing a conventional etching process such as reactive-ion etching (RIE), plasma etching, ion beam etching or other like etching processes and following the etching step, the patterned resist is stripped providing patterned hard mask  18  such as shown in FIG. 2.  
         [0044]    A dummy gate stack  20  is then formed on portions of patterned hard mask  18  and portions of silicon layer  14  so as to provide the structure shown in FIG. 3. Specifically the structure shown in FIG. 3 is formed by first depositing a layer of polysilicon on the surface of the structure shown in FIG. 2. The layer of polysilicon is formed by a conventional deposition process such as CVD, plasma-assisted CVD, evaporation, sputtering, atomic layer deposition, or chemical solution deposition.  
         [0045]    Since this polysilicon layer is employed as a dummy gate material, the thickness of the polysilicon formed at this step of the present invention should be at least as thick as that of the desired gate region to be subsequently formed. Specifically, the thickness of the polysilicon material employed at this point of the present invention is from about 100 to about 200 nm, with a thickness of from about 120 to about 180 nm being more highly preferred.  
         [0046]    An oxide layer having a thickness of about 50 nm is then formed on a portion of uppermost surface of the polysilicon layer which lies on top of portions of pattern hard mask  18 . The oxide layer may be formed utilizing a conventional deposition process such as CVD, or a conventional oxidation process may be employed in forming the same. After forming the oxide layer, the oxide layer is patterned utilizing conventional lithography and etching forming hard mask  21  on the surface of the polysilicon layer. Note that the hard mask is used in the formation of the patterned dummy gate region.  
         [0047]    Following the formation of hard mask  21  on the structure, the exposed portions of the polysilicon not containing hard mask  21  is patterned by etching through those regions of the polysilicon layer that do not include the protective oxide layer thereon so as to form patterned dummy gate  20  shown in FIG. 3. Note that the patterned dummy gate is formed on portions of the patterned hard mask and portions of the silicon layer.  
         [0048]    Following patterning of the dummy gate region, the structure shown in FIG. 3 is subjected to a removal step in which the exposed portions of silicon layer  14 , not covered by dummy gate  20  and patterned hard mask  18 , are removed utilizing a conventional dry etching process such as RIE which is capable of stopping on the underlying insulating region (i.e., insulating region  12  of substrate  10 ). Note that the removal process employed at this junction of the present invention maintains a portion of the silicon layer that is protected by the patterned hard mask and the patterned dummy gate. The remaining silicon has exposed sidewalls which are then subjected to a conventional oxidation process which results in the formation of source/drain extension regions  22  that abut the patterned dummy gate region, and pad oxide region  23  that underlies dummy gate  20 , See FIG. 4.  
         [0049]    The oxidation process employed in the present invention includes any conventional sidewall oxidation process that is performed at a temperature of about 700° C. or above in an oxygen-containing atmosphere. More preferably, the sidewall oxidation process is performed at a temperature of from about 800° to about 900° C. The duration of the sidewall oxidation step employed in the present invention is not critical and thus may vary within ranges that are well known to those skilled in the art.  
         [0050]    [0050]FIG. 5 shows the resultant structure that is obtained after planarized oxide layer  24  is formed in the structure. Specifically, planarized oxide layer  24  is formed by first depositing an oxide layer over all exposed surfaces of the structure shown in FIG. 4 and thereafter planarizing the deposited layer down to the uppermost polysilicon surface of dummy gate region  20 . Note that the top protective oxide layer is removed during the planarization step. The oxide layer formed in this step of the present invention may be comprised of TEOS (tetraethylorthosilicate) which is deposited by a conventional low-pressure CVD process or other like materials that are formed by conventional deposition processes.  
         [0051]    The planarization process employed in this step of the present invention includes any conventional planarization technique such as chemical-mechanical polishing (CMP) or grinding. It is noted that the oxide material employed as layer  24  has an etch rate that is less than the dummy gate polysilicon so that the next step of the present invention can be performed.  
         [0052]    The next step of the present invention, which is shown, for example, in FIG. 6, includes selectively removing dummy gate region  20  so as to provide opening  26  which extends down to the top surface of patterned hard mask  18 . Note that during this step of the present invention, silicon layer  14  beneath the dummy gate polysilicon that is not protected by the patterned hard mask is also removed. In accordance with the present invention, the dummy gate region and silicon layer  14  beneath the dummy gate polysilicon not protected by the hard mask are removed by a conventional dry etching process such as RIE, plasma-etching or ion beam etching. Note that this step of the present invention results in the formation of channel regions within the remaining silicon layer  14  that is present on top of insulating region  12 . In FIG. 6, reference numeral  25  denotes one of the vertical channel regions of the inventive structure. Note that the vertical channel regions are composed of thin silicon layer  14  which is present atop of insulating region  12 .  
         [0053]    Nitride or other insulating spacers (not shown) may be optional formed in the opening at this point of the inventive process utilizing conventional deposition processes well known to those skilled in the art. The spacers are preferably employed in cases wherein extremely small channel lengths are desirable.  
         [0054]    Next, a gate stack  28  is formed within the opening providing the structure shown in FIG. 7. The formation of gate stack  28  includes the steps of: forming gate oxide  30  on exposed sidewalls of the remaining silicon layer  14  that lies beneath the patterned hard mask within opening  26 , filling said opening with gate conductor  32  and planarizing to planarized oxide layer  24 . Specifically, the gate oxide is formed by utilizing a conventional sidewall oxidation process as mentioned above, or the gate oxide may be formed by a deposition process such as CVD.  
         [0055]    The gate conductor-filling step employed in the present invention includes the use of any conventional deposition process such as CVD, plasma-assisted CVD, sputtering, plating, evaporation, atomic layer deposition or chemical solution deposition. Gate conductor  32  employed in the present invention includes any conductive material, including, but not limited to: polysilicon, elemental metals such as Cu, W, Pt, Ag, Au, Ru, Re, and alloys, silicates or oxynitrides of these elemental metals. A highly preferred conductive material employed in the present invention is polysilicon.  
         [0056]    CMP or another like planarization process may be employed in forming the planarized structure shown in FIG. 7.  
         [0057]    When polysilicon is employed as the gate conductor, the planarized structure containing the polysilicon gate conductor region may be subjected to a conventional ion implantation step so as to implant dopant ions (n-type or p-type) in the polysilicon gate conductor and thereafter a conventional annealing process may be employed which serves to diffuse and activate the dopant ions within the polysilicon gate conductor. Note this step may be omitted, if the polysilicon layer is formed within the opening utilizing a conventional in-situ doping deposition process.  
         [0058]    After forming the gate region within the opening, planarized oxide layer  24  as well as patterned hard mask  18  not within the gate region are removed utilizing a conventional etching process which selective removes oxide as compared to gate conductor. Specifically, this etching process of the present invention includes a wet etch process wherein a chemical etchant such as HF is employed. The resultant structure after removing the oxide layers is shown, for example, in FIG. 8.  
         [0059]    After removing the oxide layers from the structure, the structure shown in FIG. 8 may be subjected to one or more of the following additional processing steps: (i) forming activated diffusion (source/drain) regions in portions of said exposed silicon layer abutting said gate stack; (ii) subjecting the gate stack to oxidation (required if the gate stack comprises polysilicon); (iii) forming spacers on exposed sidewalls of said gate stack (optional for polysilicon-containing gate stacks, but required for non-polysilicon gate stacks); (iv) saliciding said source/drain regions, or forming raised source/drain regions via selective Si deposition and then saliciding the raised source/drain regions; and (v) further back-end-of the line (BEOL) processing.  
         [0060]    Each of the above mentioned additional processing steps include techniques that are well known in the art. For example, activated source/drain regions are formed by ion implantation and annealing; poly gate oxidation is carried out utilizing a conventional sidewall oxidation process; insulating, e.g. nitride, spacers are formed by deposition and etching; source/drain salicidation is performed by depositing a refractory metal such as Ti, Co, or W on the source/drain regions utilizing a conventional deposition process well known in the art and thereafter the same is annealed under conditions whereby a refractory metal silicide such as TiSi 2  is formed over the diffusion regions.  
         [0061]    It is noted that when polysilicon gate conductors are employed, a problem of high “off” current may exist due to the workfunction of the polysilicon gate and the very thin body of the device. This problem can be alleviated in the present invention by switching to the other conductors mentioned herein, or by using a substrate that contains a ground plane that can adjust threshold voltage. In cases wherein a ground plane is employed it may be necessary to form a reach-through implant in the substrate which provides a connection from the outer surface of the structure to the underlying heavily doped Si-containing layer.  
         [0062]    While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the present invention.