Abstract:
An asymmetric field effect transistor (FET) that has a threshold voltage that is compatible with current CMOS circuit designs and a low resistive gate electrode is provided. Specifically, the asymmetric FET includes a p-type gate portion and an n-type gate portion on a vertical semiconductor body; an interconnect between the p-type gate portion and the n-type gate portion; and a planarizing structure above the interconnect.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to semiconductor devices, and more particularly to an implanted asymmetric doped polysilicon-containing gate FinFET structure that is integrated with a thick polysilicon-containing gate. The present invention is also directed to a method to integrate the implanted asymmetric polysilicon-containing gate FinFET with a thick polysilicon-containing gate for manufacturing integration.  
         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 metal oxide semiconductor field effect transistor (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 diffusion 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 a typical prior art double-gated MOSFET consists of a very thin vertical Si layer (Fin) for the channel, with two gates, one on each side of the channel. The term “Fin” is used herein to denote a semiconducting material which is employed as the body of the FET. 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]    One problem with prior art structures which comprise symmetric polysilicon gates on a FinFET is that the symmetric polysilicon gate FinFET structure will result in threshold voltages that are not compatible with existing CMOS circuit designs. For example, the NFET threshold voltage will be negative and the PFET will be quite positive. A potential solution to this problem is using a symmetric metal gate. Integration and processing difficulty has, however, made the development of a metal gate FinFET quite slow.  
           [0006]    Another possible solution is an asymmetric doped polysilicon gate where one side of the Fin (i.e., thin film semiconducting layer) includes an N+ doped polysilicon gate and the other side of the Fin includes a P+ doped polysilicon gate. This asymmetry has been shown to shift threshold voltages to CMOS compatible levels in planar double-gate devices as well as FinFETs. Most prior art methods to integrate this asymmetry require sidewall image transfer and other difficult processing schemes. Moreover, in the prior art asymmetric polysilicon gate structures, thin polysilicon gates are employed. A problem with such structures is that thin polysilicon gates result in highly resistive gate electrodes. Additionally, the aspect ratio of the structures having thin gate electrodes makes the gate etch extremely difficult.  
           [0007]    In view of the above-mentioned problems with prior art FinFET structures; there is a continued need for developing a new and improved asymmetric polysilicon (polySi)-containing gate. FinFET structure in which the threshold voltage is compatible with current CMOS circuit designs and where low resistive gate electrodes are realized.  
         SUMMARY OF THE INVENTION  
         [0008]    One object of the present invention is to provide a FinFET structure containing asymmetric polySi-containing gates which make the threshold voltage of the structure compatible with current CMOS circuit designs. The term “polysilicon-containing” is used herein to denote materials that are comprised of polySi or polySiGe.  
           [0009]    Another object of the present invention is to provide an asymmetric FinFET structure wherein low resistive gate electrodes are employed.  
           [0010]    A still further object of the present invention is to provide an asymmetric FinFET structure in which the asymmetric polySi-containing gates are interconnected by an interconnect layer.  
           [0011]    A yet further object of the present invention is to provide an asymmetric FinFET structure wherein a planarizing structure is present atop the asymmetric FinFET structure.  
           [0012]    These and other objects and advantages are achieved in the present invention by providing a structure where an implanted asymmetric polySi-containing gate FinFET is integrated with a thick polySi-containing outer gate electrode (i.e., the planarizaing structure). The integrated FinFET/thick polySi-containing gate structure allows for the production of a FinFET structure in which the threshold voltage is compatible with current CMOS circuit designs and the resistivities of the gate electrodes are lower than conventional asymmetric FinFETs.  
           [0013]    One aspect of the present invention relates to a method of forming a plurality of conductive structures on a substrate. Specifically, the inventive method includes the steps of:  
           [0014]    forming a first semiconductor structure of a first conductivity type, a second semiconductor structure of a second conductivity type, and a third semiconductor structure on a substrate, said third semiconductor structure is disposed between said first and second semiconductor structures and is separated therefrom by an insulator structure;  
           [0015]    depositing an interconnect layer over at least said first, second and third semiconductor structures;  
           [0016]    forming a planarizing conductor on said interconnect layer, said planarizing conductor having etch characteristics similar to those of said interconnect layer and said first and second semiconductor structures, but different from those of said insulator structure; and  
           [0017]    patterning and etching said planarizing conductor, said interconnect layer, and said first and second semiconductor structures so that each has at least one lateral dimension that is substantially the same.  
           [0018]    Another aspect of the present invention relates to an asymmetric field effect transistor (FET) which includes:  
           [0019]    a p-type gate portion and an n-type gate portion on a vertical semiconductor body;  
           [0020]    an interconnect between said p-type gate portion and said n-type gate portion; and  
           [0021]    a planarizing structure above said interconnect. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIGS. 1-9 are pictorial representations (including A: top view; and B: cross-sectional view) showing the various processing steps employed in fabricating the inventive asymmetric FET structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention, which provides an asymmetric FET structure and method of fabricating the same, will now be described in greater detail by referring to the drawings that accompany the present application.  
         [0024]    Reference is first made to the initial structure shown in FIGS. 1A (top view) and  1 B (cross-sectional view) which is employed in the present invention in fabricating the implanted asymmetric doped polySi-containing gate FinFET structure. Specifically, FIGS. 1A and 1B show a structure that includes substrate  10  having a patterned stack comprising a semiconductor body region, i.e., Fin,  12 , and hard mask  14  present atop substrate  10 .  
         [0025]    The substrate includes upper portion  10   u  which is comprised of an insulating material such as an oxide, nitride, oxynitride or multilayers thereof, and bottom portion  10   b  which is comprised of a semiconducting material such as Si. Note that regions  10  and  12  may be part of an SOI (silicon-on-insulator) material wherein region  10   u  is a buried oxide layer which is sandwiched between semiconducting materials  10   b  and  12 . Alternatively, layers  10   u  and  12  are layers which are separately deposited atop a semiconductor substrate.  
         [0026]    The semiconductor body region, i.e., Fin region  12 , is comprised of any semiconducting material such as single crystalline Si and the hard mask is comprised of an oxide, nitride, oxynitride or multilayers thereof. The vertical thicknesses of semiconducting material layer  12  (i.e., Fin  12 ) and the hard mask are not critical to the present invention. Typically, semiconducting material layer  12  has a vertical thickness of from about 300 to about 2000 Å, whereas the hard mask has a vertical thickness of from about 100 to about 1000 Å.  
         [0027]    The structure shown in FIGS  1 A- 1 B is fabricated utilizing conventional processes well known to those skilled in the art. For example, hard mask  14  is formed atop a layer of semiconducting material by utilizing a conventional deposition process such as chemical vapor deposition (CVD), plasma-assisted CVD, or chemical solution deposition. Alternatively, hard mask  14  may be grown on semiconductimg material layer  12  (i.e., Fin  12 ) utilizing a conventional thermal oxidation, nitridation or oxynitridation process well known to those skilled in the art.  
         [0028]    Following formation of the hard mask material on semiconducting material layer  12 , the structure is subjected to conventional lithography (including applying a photoresist to the hard mask, exposing the photoresist to a pattern of radiation, and developing the pattern into the photoresist using a conventional resist developer) and dry etching such as reactive-ion etching, ion beam etching, plasma-etching or laser ablation. The etching step may include a single etching process or multiple etching processes using one or more of the above mentioned dry etching processes may be employed to provide the structure illustrated in FIGS  1 A- 1 B. After etching, the photoresist is removed from the structure utilizing a conventional stripping process well known to those skilled in the art.  
         [0029]    Next, the structure shown in FIGS. 1A-1B is then subjected to a conventional oxidation, nitridation or oxynitridation process which is capable of forming gate dielectric  16  on exposed sidewalls of Fin layer  12 . Following formation of the gate dielectric on the exposed sidewalls of Fin  12 , thin polysilicon-containing layer  18  is formed over the entire surface of the structure utilizing a conventional deposition process such as CVD providing the structure shown in FIGS. 2A-2B. The term “polysilicon (i.e., polySi)-containing” is used herein to denotes a layer that is comprised of polySi or polySiGe. In-one highly preferred embodiment of the present invention, the polySi-containing layer  18  is comprised of polySi. Polysilicon-containing layer  18  is a thin layer whose thickness is significantly thinner than the height of the Fin to provide the appropriate shadow angles required in the subsequent angled implant step. In the present invention, Fin  12  typically has a height from about 300 to about 2000 Å and polysilicon-containing layer  18  has a thickness of from about 150 to about 1000 Å. More preferably, Fin  12  has a height from about 900 to about 1200 Å and polysilicon-containing layer  18  has a thickness of from about 500 to about 800 Å.  
         [0030]    It is noted that Fin region  12  represents a third semiconductor structure which is surrounded by insulating structure (layers  10   u ,  14 , and  16 ) of the inventive asymmetric FinFET structure.  
         [0031]    Next, and as shown in FIGS. 3A-3B, asymmetric implants are performed which are capable of forming first semiconductor structure  24  of a first conductivity type (e.g., N+ implant region or n-type gate portion) on one side of Fin  12  and second semiconductor structure  26  of a second conductivity type (e.g., P+ implant region or p-type gate portion) on the other side of Fin  12 . Specifically, an n-type dopant  20  and a p-type dopant  22  are implanted into polysilicon-containing layer  18  using a, double angled implant process which is capable of forming implant regions having a final dopant concentration on the order of from about 1E19 to about 1E21 atoms/cm 3 . Due to the shadowing of the structure, the gate sides (i.e., vertical portions of polySi-containing layer  18 ) remain doped (either N+ or P+) with the species implanted from that side, while the horizontal portions of polySi-containing layer  18  are counterdoped, i.e., double implanted, such that those areas are not doped N+ or P+. The double implant regions are labeled as  28  in FIGS. 3A-3B.  
         [0032]    Following the asymmetric implant process described above, metallic layer  30  is formed atop all exposed surfaces shown in FIGS. 3A-3B so as to provide the structure illustrated, for example, in FIGS. 4A-4B. The metallic layer, which is formed utilizing a conventional deposition process such as CVD, sputtering or plasma-assisted CVD, has a thickness,of from about 20 to about 1000 Å, with a thickness of from about 100 to about 500 Å being more highly preferred. The metallic layer formed at this point of the inventive method may comprise any conductive metal or metal alloy such as, but not limited to: W, TiN, and Ta. Note that metallic layer  30  contacts both gate regions, i.e., regions  24  and  26 , hence metallic layer  30  is referred herein as an interconnect layer. In some embodiments of the present invention, the metallic layer may be transferred into a metallic silicide at this point of the inventive process. The transformation is carried out using a conventional annealing process well known to those skilled in the art (including, for example, the annealing conditions mentioned hereinbelow). In yet another embodiment, which is described hereinbelow, the transformation annealing occurs later in the inventive process at the same time as activation of the source and drain regions. In still another embodiment of the present invention, the metallic layer is not converted into a metallic silicide layer.  
         [0033]    After forming the metallic layer on the structure so as to interconnect the first and second semiconductor structures to each other, a planarizing polySi-containing layer (or other conductive material such as a conductive metal or metal alloy)  32  is formed atop metallic layer  30  so as to provide the structure shown in FIGS. 5A-5B. Specifically, a thick polySi-containing layer composed of polySi or polySiGe is formed by a conventional deposition process and thereafter the thick polySi-containing layer is planarized by a conventional planarization process such as chemical-mechanical polishing (CMP). The term “thick polySi-containing layer” denotes a polySi-containing layer having a thickness of from about 500 to about 2000Å, with a thickness of from about 800 to about 1500 Å being more highly preferred. It is noted that the planar conductor formed at this point of the inventive process has etch characteristics similar to those of metallic interconnect layer  30  and the first and second semiconductor structures (regions  24  and  26 ), yet different from those of insulating structure.  
         [0034]    Conventional patterning (i.e., lithography) and etching are then performed so as to provide the structure shown in FIG. 6A-6B. Note that portions of planarizing conductor  32 , metallic interconnect  30 , first and second semiconductor structures  24  and  26 , respectively, are patterned and etched such that those regions have at least one lateral dimension that is substantially the same. That is, this etching step is capable of etching the above layers such that each of the etched layers has the same final shape.  
         [0035]    Next, a conventional implantation process is performed so as to form source/drain implant regions  33  in the structure in areas adjacent to the Fin region; See FIGS. 7A-7B. Note that during this implant process, polysilicon-containing layer  32  is converted into doped polySi-containing layer  34 , but metallic layer  30  serves as a diffusion barrier preventing dopant from being implanted into the oppositely doped polySi-containing gate regions. The doping at this point may be either n or p-type. In one embodiment of the present invention the exposed areas adjacent to Fin  12  are doped using different implant species such as As and B so as to form source/drain regions  33  which have donor or acceptor impurities, respectively.  
         [0036]    Following formation of the structure shown in FIGS. 7A-7B, that structure is subjected to an activation annealing process which is capable of activating the source/drain regions as well as converting the metallic interconnect layer into metallic silicide (or metallic nitride) layer  36  which bridges the two polySi-containing layers. This eliminates the pn-junction in the gate and provides a contact between the top doped polySi-containing layer, i.e., layer  34 , and the gate electrodes (i.e., double implanted polySi-containing regions  28 ). The resultant structure after performing the activation annealing step is shown, for example, in FIG. 8A-8B.  
         [0037]    Specifically, activation annealing is carried out at a temperature of about 700° C. or higher in the presence of an inert gas atmosphere such as He, N 2 , Ar, Xe and Kr, that may be optionally admixed with H 2 . More preferably, the activation annealing is performed at a temperature of from about 850° to about 1000° C. in the presence of He or Ar.  
         [0038]    Following the activation annealing step, standard FinFET finishing processing steps including formation of gate contact  42 , and source/drain contacts  44 , are performed providing the structure shown in FIGS. 9A-9B.  
         [0039]    In some embodiments of the present invention, double implanted gate regions  28  and doped region  34  are composed of a semiconducting material instead of a polySi-containing material.  
         [0040]    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 spirit and scope of the appended claims.