Abstract:
A field effect transistor is formed having wrap-around, vertically-aligned, dual gate electrodes. Starting with an silicon-on-insulator (SOI) structure having a buried silicon island, a vertical reference edge is defined, by creating a cavity within the SOI structure, and used during two etch-back steps that can be reliably performed. The first etch-back removes a portion of an oxide layer for a first distance over which a gate conductor material is then applied. The second etch-back removes a portion of the gate conductor material for a second distance. The difference between the first and second distances defines the gate length of the eventual device. After stripping away the oxide layers, a vertical gate electrode is revealed that surrounds the buried silicon island on all four side surfaces.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to field-effect transistors and, more particularly, to dual-gated field-effect transistors.  
       BACKGROUND OF THE INVENTION  
       [0002]     A field-effect transistor (FET)is a type of transistor commonly used in Ultra Large Scale Integration (ULSI). In the FET, current flows along a semiconductor path called the channel. At one end of the channel, there is an electrode called the source. At the other end of the channel, there is an electrode called the drain. The physical dimensions of the channel are fixed, but its number of electrical carriers can be varied by the application of a voltage to a control electrode called the gate. The conductivity of the FET depends, at any given instant in time, on the number of electrical carriers of the channel. A small change in gate voltage can cause a large variation in the current from the source to the drain. This is how the FET amplifies signals. In one popular type of FET, known as a MOSFET, the channel can be either N-type or P-type semiconductor. The gate electrode is a piece of metal whose surface is insulated from the channel by an oxide layer between the gate electrode and the channel. Because the oxide layer acts as a dielectric, there is little current between the gate and the channel during any part of the signal cycle. This gives the MOSFET an extremely large input impedance.  
         [0003]     As semiconductor devices, such as FETs, have become smaller, a number of techniques have been employed to ensure that performance, speed, and reliability of the devices are not adversely affected. One technique, useful for a number of different devices, includes Silicon-On-Insulator (SOI) structures in which a silicon layer has a buried oxide layer (BOX) between it and a handle wafer. The active elements (e.g., transistors) are fabricated in the silicon layer over the BOX. The BOX is present to provide thick, robust vertical isolation from the substrate thereby resulting in better turn-off characteristics and low capacitance. One method of forming an SOI substrate is to bond two oxidized wafers, then thin one of those wafers so as to form a silicon layer of a thickness appropriate for device fabrication. This structure leaves a thin silicon layer above a layer of oxide.  
         [0004]     Another technique, specifically for improving field-effect transistors, involves using dual-gates. In a dual-gated transistor, a top gate and a bottom gate are formed around an active region. Specifically, the advantages for dual gate devices over their single gate counterparts include: a higher transconductance and improved short-channel effects. The improved short-channel effects circumvent problems involving tunneling breakdown, dopant quantization, and dielectric breakdown associated with increasingly high channel doping of shrinking single gate devices. These benefits depend on the top and bottom gates being similar in construction and properly aligned in the vertical direction and aligned with the source/drain regions.  
         [0005]     SOI techniques have been used in previous attempts at forming dual-gated devices. In these attempts, the buried oxide layer under a portion of the SOI island is removed, usually by dipping in an etchant, to gain access to the bottom surface of the silicon. Once exposed, a dielectric can be grown on this bottom surface and a gate conductor material deposited. One significant shortcoming of this technique is that the top gate and the bottom gate are not precisely aligned. Accordingly, the advantages of dual-gating are diminished or lost.  
         [0006]     One recent attempt to form dual-gated devices that have self-aligned gates is the FinFET. Unlike traditional devices, FinFETs are constructed vertically rather than horizontally and, thus, requires a difficult-to-perform directional etch to determine the device gate length. As gate length is one of the most critical characteristics of a device and its behavior, the fabrication steps that define gate length should be easy to control, very reliable, and easy to duplicate.  
         [0007]     Accordingly, there remains a need for a dual-gated device formed horizontally that has self-aligned top and bottom gates. Additionally, there remains a need for a method of forming these gates that simply, accurately, and reliably controls the gate length during fabrication.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, embodiments of the present invention use an SOT structure to form a wrap-around gate electrode for a FET. By wrap-around gate, it is meant that the gate electrode material encircles the periphery, or a majority thereof, of the silicon channel used to form the source and drain regions. In particular, a vertical reference edge is defined, by creating a cavity within the SOI structure, and used during two etch-back steps that can be reliably performed. The first etch-back removes a portion of an oxide layer, for a first distance, over which a gate conductor material is applied. The second etch-back removes a portion of the gate conductor material for a second distance. The difference between the first and second distances defines the gate length of the eventual device. After stripping away the oxide layers, a vertical gate electrode is revealed that surrounds the buried silicon island on all four side surfaces.  
         [0009]     One aspect of the present invention relates to a method for forming a wrap-around-gate field-effect transistor, gated on all four active surfaces by a self-aligned electrode, on a handle wafer. In accordance with this aspect, an SOI structure is formed on the handle wafer and then a cavity is formed in this structure extending from its top surface to the handle wafer. Within the cavity, an oxide material is etched back so as to expose the sides of a buried SOI island. With the sides of the SOI island exposed, a gate conductor material can be deposited thereon. This gate conductor material can then, itself, be etched back thereby forming a self-aligned gate electrode that surrounds the SOI island on its four sides.  
         [0010]     Another aspect of the present invention relates to a portion of a wrap-around-gated field-effect transistor. This portion includes a handle wafer, an SOI island and a gate electrode. More particularly, the SOI island includes four side surfaces and extends, for its length, in the horizontal direction. The gate electrode surrounds and supports the SOI island. The gate electrode extends in a vertical direction from the handle wafer and has a thickness smaller than the SOI island&#39;s length. In other words, the gate electrode includes a first portion below the SOI island, a second portion on one side of the SOI island, a third portion on another side of the SOI island, and a fourth portion above the SOI island such that the gate  
         [0011]     Yet another aspect of the present invention relates to a field-effect-transistor that includes a silicon-on-insulator (SOI) island having a top surface, a bottom surface, a right-side surface, a left-side surface, and two edge faces, wherein the SOI island is oriented substantially in a horizontal direction. This transistor also includes a wrap-around gate electrode oriented in substantially a vertical direction intersecting with the SOI island in-between the two edge faces such that the SOI island surrounds the SOI island along a portion of the top surface, the bottom surface, the right-side surface and the left-side surface. Additionally, the transistor includes a source region formed on a first part of the SOI island on one side of the gate electrode; and a drain region formed on a second part of the SOI island on another side of the gate electrode.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  illustrates a perspective view of a SOI structure having a silicon island surrounded by an oxide.  
         [0013]      FIG. 2  illustrates a cross-sectional view of the structure of  FIG. 1 .  
         [0014]      FIG. 3  illustrates the structure of  FIG. 1  with an etching boundary defined on its top surface.  
         [0015]      FIG. 4  illustrates a cross section of an SOI structure after a cavity has been formed along its entire thickness.  
         [0016]      FIG. 5  illustrates the structure of  FIG. 4  after the oxide has been etched-back a predetermined distance.  
         [0017]      FIG. 6  illustrates a top view of the structure of  FIG. 5  showing the outline of the oxide.  
         [0018]      FIG. 7  illustrates a cross-sectional view of an intermediate stage of the structure of  FIG. 5  after a gate dielectric, a gate conductor material and fill material have been formed within the cavity.  
         [0019]      FIG. 8  illustrates the structure of  FIG. 7  after the gate conductor material has been etched-back a predetermined distance.  
         [0020]      FIGS. 9 and 10  illustrate different views of the structure of  FIG. 8  that has been cut into separate devices and had the fill material within the cavity removed.  
         [0021]      FIGS. 11 and 12  illustrate different views of the structure of  FIG. 10  after a hard mask layer has been removed from the top of that structure.  
         [0022]      FIGS. 13 and 14  illustrate different views of the structure of  FIG. 12  after the oxide layers outside of the gate conductor material has been removed.  
         [0023]      FIGS. 15A and 15B  illustrate different views of an alternative structure to that of  FIG. 13 .  
         [0024]      FIG. 16  illustrates the structure of  FIG. 13  along with electrodes connected to the gate, source and drain regions.  
     
    
     DETAILED DESCRIPTION  
       [0025]     The structure  100  shown in the perspective view of  FIG. 1  is a typical SOI structure formed using conventional patterning and etching techniques. A wide variety of methods of forming the SOI structure  100  can be employed. This SOI structure  100  includes a handle wafer  102  that in many applications can be a non-silicon material such as a nitride and will have a thickness of approximately 200 nm-1mm. Alternatively, if a silicon handle wafer is employed, it can be capped with a nitride layer (not shown) to prevent interaction of the silicon with other materials.  
         [0026]     The central region of the structure  100  is an oxide such as, for example silicon dioxide. This central region includes a buried oxide (BOX) layer  104  and a cap oxide layer  105 . Within the oxide layers  104 ,  105  is a silicon-on-insulator (SOI) island  108 . On top of the cap oxide layer  105 , a hard mask  106  is formed.  
         [0027]      FIG. 2  illustrates a cross-sectional view of the exemplary structure  100  taken along the plane A-A depicted in  FIG. 1 . From this view, it is apparent that in this embodiment the SOI island  108  extends the entire length of the structure  100  and that the oxide layers  104 ,  105  are approximately as thick above the island  108  as below the island  108 . However, these relative dimensions can vary without departing from the scope of the present invention. In practice, the BOX layer  104  is typically between 100 to 1000 nm thick as is the cap oxide layer  105 . the SOI island  108  generally ranges between 20 to 250 nm thick.  
         [0028]     It is from this structure  100  that the wrap-around-gate of the present invention is formed. A cavity  402  is formed in the structure  100  as shown in  FIGS. 3 and 4 . In particular, conventional photolithography techniques, such as a photo resist layer, are used to print a etching region  302  on the hard mask  106  to define the boundaries of an etching step. Once the boundaries are defined, the cavity  402  is etched through the hardmask layer  106 , the cap oxide layer  105 , the island  108  and the BOX layer  104  below the island  108 . After etching, the resist layer is stripped off the hard mask  106 . One of ordinary skill would recognize that a variety of etching compounds are available that can remove these layers in one step or in a plurality of steps. Furthermore, the etching can be performed in a timed-manner or simply by relying on selectivity between the various materials to ensure that only portions of the desired layers are removed.  
         [0029]     As shown in the cross-sectional view of  FIG. 4 , the etching step to form the cavity  402  is performed so as to form substantially vertical sidewalls  404 ,  406  As a result, an SOI island  108  is created on each side of the cavity  402 . The width  306  of the etching region  302 , and therefore of the cavity  402  as well, is approximately between 50 to 200 nm. The length  304 , however, depends on the application. For example, the structure of  FIG. 1  only has a single SOI island  108  and the length  304  would typically only need to be enough to overlap each edge of the island  108  by around 20 nm. If however, a plurality of side-by-side SOI islands were formed between oxide layers  104  and  105 , then the length  304  would typically need to be enough to overlap the outside islands by around 20 nm. Thus, as SOI islands can vary between 25-2000 nm, the length  304  can vary widely based on the size of the island and the number of buried SOI islands.  
         [0030]     The next step in the process is to use the cavity  402  to etch the cap oxide layer  105  and the BOX layer  104 . For example, buffered hydrofluoric acid (BHF) can be used to etch the oxide (layers  104 ,  105 ) but it will not remove any of the SOI island  108 , the hard mask  106 , or the handle wafer  102 . The etch of the BOX layer  104  is timed or controlled so as to create the cross section profile shown in  FIG. 5 . Because the etch of the oxide layers  104 ,  105  occurs in three dimensions, the sides, top and bottom of each SOI island  108  are exposed.  
         [0031]      FIG. 6  is a top view of the structure  500  of  FIG. 5  with some of the visible features omitted. In particular,  FIG. 6  highlights the shape  602  of the cap oxide layer  105  and the BOX layer  104  after the etching step with BHF is completed. While not shown in  FIG. 6  for clarity, the hardmask  106  and island  108  also would extend into the region  602  and be visible from a top view. Dotted lines  604  and  606  depict the outline of the buried island  108 .  
         [0032]      FIG. 7  illustrates a cross-sectional profile of the SOI structure after completion of a number of intermediate steps. The first step is to form gate dielectric material  703  on all the exposed surfaces of each SOI island  108 . Once this gate dielectric  703  is formed, a gate conductor material  702 , such as polysilicon, is conformally deposited over the hard mask  106  and within the cavity  402  at a thickness of about 50 nm. This material coats the exposed surfaces of all the layers within the cavity  402 . In particular, the conformal gate conductor material  702  coats the top, bottom, face, and sides of the SOI island  108 , which are coated with the gate dielectric  703 . In one embodiment of the present invention, the gate conductor material  702  substantially fills the cavity  402  and no other material-depositing steps are used.  
         [0033]     However, the cross-sectional view of  FIG. 7  illustrates an alternative embodiment, in which the gate conductor material  702  does not fill the cavity  402 . In this embodiment, a gap-fill material  704 , usually an organic material, is used to substantially fill the cavity  402  once the gate conductor material  702  is deposited. Using the hardmask  106  as the guide, directional etching, such as reactive ion etching (RIE), is used to remove some of the gap-fill material  704  within the cavity  402  to create substantially vertical sidewalls. The etching of the gap-fill material  704  is continued until a portion  706  of the gate conductor material  702  on the edge face of each SOI island  108  is exposed within the cavity  402 . At this point, the SOI structure  100  is as illustrated in  FIG. 7 .  
         [0034]     Next, referring to  FIG. 8 , the gate conductor material  702  is isotropically etched back as shown by region  802 . Throughout the cavity  402 , all exposed gate conductor material  702  is uniformly etched back. Referring back to  FIG. 5 , the oxide layers  104 ,  105  were isotropically etched-back a first distance, such as 100 to 500 nm. Now, the gate conductor material  702  is being etched back a second distance  802 , such as 90 to 400 nm. The difference between these two distances is what determines the channel length (i.e., the length of the region between the source and drain, of the resulting transistor) and will be approximately 10 to 120 nm.  
         [0035]     The structure of  FIG. 8  is then modified by stripping the organic gap-fill material  704  from within the cavity  402 . A perspective view of the resulting structure is depicted in  FIG. 9 . From  FIG. 6  and  FIG. 7 , it can be determined that the gate conductor material  702  follows the profile of the oxide layers  104 ,  105  and, therefore, is substantially annular in shape. Thus, the gate conductor material  702  contacts both buried islands  108 . To form discrete structures, the sides of the annular gate conductor material can be trimmed, as shown in  FIG. 9 , so as to create two separate gates  904  and  906 . Of particular interest, the gates  904 ,  905  have a conductor region, such as  902 , that wraps around the respective island  108 .  FIG. 10  shows a cross-sectional profile of the structure of  FIG. 9 . The C-shaped profile of the portions  904  and  906  is a result of using the gap-fill material  704  in previous fabrication steps. An alternative embodiment is illustrated later that does not use the gap-fill material and has solid portions in place of the C-shaped profiles of gates  904 ,  906 . The gate dielectric  703  can be trimmed back now, as shown in  FIG. 10 , or etched away at a later stage to expose the surfaces of the SOI island  108 .  
         [0036]     After the hardmask  106  has been stripped, the structure is nearing its final form as shown in  FIG. 11 .  FIG. 12  is a cross-sectional profile view of  FIG. 11  and shows that one side of each island  108  still has oxide layers  104  and  105  present. Accordingly, it would be difficult to connect a contact, or other material layer, to this section  1202  as depicted in  FIG. 12 . Accordingly, the oxide layers  104  and  105  can be stripped, as depicted in  FIG. 13 , to result in two wrap around gates  904  and  906  that each surround a respective portion of the SOI island  108 .  
         [0037]     As more clearly seen in the coss-sectional profile of  FIG. 14 , the top and bottom portions of each gate  904  and  906  are aligned with each other and with the source and drain regions. The source and drain regions  1402 ,  1404  are exposed, and contacts to all regions can be easily formed. As understood, by one of ordinary skill, the exposed source/drain regions  1042 ,  1404  are doped with group  3  or group  5  elements before the contacts are formed. Thus, an SOI device having self-aligned wrap-around gates is formed in such a manner that channel length can be easily controlled using two etch-back steps instead of a difficult long directional etch.  
         [0038]      FIGS. 15A and 15B  illustrate an alternative embodiment of the device of  FIG. 14 . In particular, the mechanical strength of the SOI island  108  can be enhanced by stripping away all the BOX material  104  except that under the SOI island  108 . A directional etching method, such as RIE, could be used to effect such a result. Even in this embodiment, the top of the SOI island  108  remains exposed to facilitate later processing steps such as passivation or salicidation. A second difference illustrated in  FIGS. 15A and 15B  involves the gate structures  1502  and  1506 .  
         [0039]     Referring back to  FIG. 7 , gate conductor material  702  and gap-fill material  704  were used to fill the cavity  402 . However, if only gate conductor material  702  had been used, then the subsequent etching steps would have resulted in the gate structures  1502  and  1506 . In particular, these structures  1502  and  1506  do not have the C-shaped profile that is exhibited by the gate structures  904  and  906  of  FIG. 10 .  
         [0040]      FIG. 16  illustrates the wrap-around gate structure of  FIG. 13  with contact formed on the source/drain regions as well as on the gates  904 ,  906 . For example, the contact  1606  provides connectivity with the gate  906 ; the contact  1602  provides connectivity with one of the source/drain regions of the island  108 ; and contact  1604  provides connectivity with the other source/drain region of the island  108 .  
         [0041]     One of ordinary skill would recognize that there are still further modifications and variations that can be made to the disclosed exemplary embodiments without deviating from the intended scope of the present invention. For example, the exemplary silicon island  108  herein described includes substantially a rectangular cross-sectional profile. In addition to this particular shape, other styles of islands, such as circular, trapezoidal, and polygonal, can be adapted to wrap-around gates as well. Additionally, the wrap-around gate does not have to completely encircle the silicon island as herein described. Performance improvements are still achieved if the wrap-around gate encircles more than a majority around the periphery of the silicon island. By encircling the silicon island by at least that much, the wrap-around gate is able to act as two gate electrodes on opposite sides of the silicon island. Also, the semiconductor island within the SOI structure can include other semiconductor materials in conjunction with, or in replacement of, the exemplary silicon island herein described.