Abstract:
A multiple-channel semiconductor device has fully or partially depleted quantum wells and is especially useful in ultra large scale integration devices, such as CMOSFETs. Multiple channel regions are provided on a substrate with a gate electrode formed on the uppermost channel region, separated by a gate oxide, for example. The vertical stacking of multiple channels and the gate electrode permit increased drive current in a semiconductor device without increasing the silicon area occupied by the device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor devices and manufacture, and more particularly, to multi-channel devices. 
     BACKGROUND OF THE INVENTION 
     A conventional MOSFET operates by driving current through the channel region between the source and drain of a device. The conductivity of the channel region is modulated by the application of a voltage on the conducting gate above the channel surface and insulated from it. Efforts are ongoing within many MOS integrated circuit manufacturing companies as well as at many universities and government laboratories to improve the speed and available drive currents with MOSFETs to reduce their power consumption, and to improve their reliability and radiation hardness for applications in harsher remote environments, including space. 
     One of the goals in semiconductor processing is to maximize the use of the available silicon area. This allows increased miniaturization of the electronic circuitry. In particular, it is desirable to maximize the drive current for a given silicon area. This has included devices in which multiple gates are provided. For example, dual gate devices in which the drive current is doubled for a given silicon area have been created. Further improvements in maximizing the drive current for a given silicon area are desirable. 
     SUMMARY OF THE INVENTION 
     There is a need for providing a MOSFET in which the transistor drive is increased, while reducing gate leakage current and gate capacitance. Due to the requirements for miniaturization, such as the push towards sub-45 nm ULSI (ultra large scale integration). This increase in current drive should be obtained without an increase in device size or change in layout design. 
     These and other needs are met by embodiments of the present invention that provide a multiple-channel semiconductor device comprising a first insulator layer on a substrate and a first channel region on the first insulator layer. A second insulator layer is provided on the first channel region. A second channel region is on the second insulator layer, and a third insulator layer is on the second channel region. A gate electrode is provided on the third insulator layer. 
     The present invention thus provides a device that has a gate electrode and multiple channel regions which allow for increased drive current without an increase in device size. 
     The earlier stated needs are also met by embodiments of the present invention which provide a method of forming a multiple-channel semiconductor device comprising the steps of forming a stack on a substrate, this stack including at least two lightly doped channel regions vertically separated from each other and from the substrate by insulator layers. A gate electrode of the stack is separated from the channel regions by a insulator layer. An oxide layer is formed on the sidewalls of the gate electrode and source and drain regions are formed that contact sidewalls of the channel regions. Gate electrode spacers are formed on the oxide liner on the gate electrode. 
     The methods of the invention allow for formation of a multiple-channel device that does not occupy a greater amount of real estate than previous devices, but yet provides more drive current than conventional devices. The method may find particular utility in sub-45 nm applications, for example. 
     The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a stack formed in accordance with embodiments of the present invention after an etching of layers has been performed to create the stack. 
         FIG. 2  depicts the structure of  FIG. 1  following a thermal oxidation process and a source and drain extension implant, in accordance with embodiments of the present invention. 
         FIG. 3  depicts the structure of  FIG. 2  following a lightly doped silicon deposition and a heavily doped silicon deposition, in accordance with embodiments of the present invention. 
         FIG. 4  shows the structure of  FIG. 3  after a silicon spacer etch has been performed, in accordance with embodiments of the present invention. 
         FIG. 5  depicts the structure of  FIG. 4  following the formation of an oxide or nitride spacer on the gate electrode and silicon spacers. 
         FIG. 6  shows the structure of  FIG. 5  after a source and drain implantation and the formation of a silicide region in the gate electrode, in accordance with embodiments of the present invention. 
         FIG. 7  shows an alternative embodiment of the present invention that follows  FIG. 2  in a process flow, and in which raised source and drain structures are formed in accordance with embodiments of the present invention. 
         FIG. 8  depicts the structure of  FIG. 7  after formation of an oxide or nitride spacer and silicide region in the gate electrode, in accordance with embodiments of the present invention. 
         FIG. 9  depicts layers in an alternate embodiment of the present invention prior to etching of the layers. 
         FIG. 10  shows the structure of  FIG. 9  after the gate electrode and the hard mask have been etched, in accordance with embodiments of the present invention. 
         FIG. 11  shows the structure of  FIG. 10  after the formation of a thermal oxide liner on the gate electrode and the deposition of gate electrode spacer material over the gate electrode and the hard mask. 
         FIG. 12  depicts the structure of  FIG. 11  following a spacer etching to form gate electrode spacers in accordance with embodiments of the present invention. 
         FIG. 13  shows the structure of  FIG. 12  after a dry etch has been performed to create the stack, in accordance with embodiments of the present invention. 
         FIG. 14  shows the structure of  FIG. 13  following an extension implantation process, with a wet etch that recesses the stack and a silicon deposition over the stack, in accordance with embodiments of the present invention. 
         FIG. 15  shows the structure of  FIG. 14  following matching of the silicon layers to form silicon spacers and the formation of a silicide region on the gate electrode, in accordance with embodiments of the present invention. 
         FIG. 16  depicts the structure of  FIG. 15  following the deposition of nitride or oxide, the etching of the deposited nitride or oxide to form spacers, and source and drain implantation, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses and solves problems related to maximizing the drive current for a given silicon area in the formation of MOSFETs and other semiconductor devices. This is achieved, in part, by the creation of a multiple-channel device having multiple channel regions formed on a substrate, with insulator layers separating the channel regions vertically and from the substrate. The gate electrode is provided on the uppermost channel region, with an insulator layer interposed between the gate electrode and the uppermost channel region. Source and drain regions are formed vertically to contact the multiple channel regions. Verticality of the semiconductor device thus formed provides increased drive current through the multiple channels without increasing the silicon area required for the semiconductor device. 
       FIGS. 1–6  describe a method of making a multiple-channel device in accordance with embodiments of the present invention. The description will discuss certain materials and process steps in an exemplary manner, but it should be recognized that these materials and process steps are exemplary only as other materials and process steps may be employed without departing from the scope of the present invention. 
       FIG. 1  depicts a stack  10  that has been created on a substrate  12  by a dry etching of layers that have been previously formed. The stack  10  of  FIG. 1  includes a first oxide layer  14   a , a lightly doped polysilicon (hereafter silicon) layer  15 , another oxide layer  14   b , a second lightly doped silicon layer  15 , and a third oxide layer  14   c . The third oxide layer  14   c  forms a gate oxide layer in  FIG. 1 . The stack  10  includes a heavily doped polysilicon channel  16  and a hard mask  18 , on the silicon nitride, for example, or other hard mask material. 
     In preferred embodiments of the invention, the silicon in each layer  15  is doped prior to the formation of the next layer. The silicon layers  15  are doped with a first conductivity type, such as p conductivity type, and gate electrode  16  is doped with a second conductivity type, such as n conductivity type. Alternatively, the gate electrode  16  is doped with p type dopants while the silicon in channel regions  15  are doped with n type dopants. Furthermore, the silicon regions  15  and  16  may comprise other semiconductor materials, such as silicon germanium (SiGe). 
     Following the formation of the stack  10 , a thermal oxidation process is performed, the results of which are depicted in  FIG. 2 . For example, the stack may be exposed to temperatures of between about 900° to 1000° for approximately less than 10 minutes in an environment suitable for oxidation, as is well known. Strict control of the thermal oxidation process is necessary, especially in applications such as sub-45 nm ULSI devices, to prevent the gate electrode  16  from becoming over oxidized. This may readily occur since heavily doped silicon, such as that in the gate electrode  16 , allows oxide to grow much faster than in lightly doped silicon, such as in the channel regions  15 . 
       FIG. 2  also shows the formation of source and drain extensions by source and drain extension implants  21 . This may be performed in a conventional manner. 
       FIG. 3  shows the structure of  FIG. 2  following the sequential deposition of a lightly doped silicon layer  22  and a heavily doped silicon layer  24 . The layers  22  and  24  are doped with the same conductivity type dopant as in the gate electrode  16 . Hence, in the exemplary embodiment being described, the dopant is an n-type dopant. Deposition of the silicon layers  22  and  24  may be performed by chemical vapor deposition (CVD) for example. It is preferred to deposit doped silicon layers rather than attempting to dope by ion implantation the silicon layers after they have been deposited, since control of the implantation process to form a lightly doped region and heavier doped regions is difficult to control in this arrangement. However, it is also possible to perform implanting to achieve the desired doping, and certain embodiments of the invention perform such implanting. 
     A silicon etch is then performed to form silicon spacers  26  that include the lightly doped regions  22  and the heavily doped regions  24 . The silicon spacers  26  contact the first and second channel regions  15  but are electrically isolated from the gate electrode  16  by the thermal oxide  20 . 
       FIG. 5  shows the structure of  FIG. 4  following the deposition of an insulating spacer material and a dry etch procedure that forms spacers  28  over the silicon spacers  26 . The insulating material may be an oxide or nitride or other suitable material, for example. 
       FIG. 6  depicts the structure of  FIG. 5  after the hard mask  18  has been removed by etching, for example. A portion of the gate electrode  16  is silicided to form a silicide region  30  by conventional silicide techniques, including the deposition of a refractory metal layer and annealing to silicidize a portion of the gate electrode  16 . 
     A source and drain implant process is performed in a conventional manner to create source and drain regions  32  in a substrate  12 . The spacers  26 ,  28  serve as a mask during the source and drain implant process. 
     The arrows in  FIG. 6  represent an exemplary electron flow through the multiple channels created in the device of the present invention. Hence, while not increasing the width of the semiconductor device, the multiple channels (three such channels in  FIG. 6  embodiment) provide increased electron flow and drive current. Further, gate leakage current and gate capacitance are reduced. 
     In certain embodiments of the invention, the gate electrode  16  may be formed of a metal or completely silicidized. The metal gate may be a replacement metal gate, or may be formed initially. Furthermore, one or more of the insulator layers  14   a – 14   c  may consist of a high k gate dielectric material, rather than a relatively lower k oxide or other material. 
       FIGS. 7 and 8  show structures constructed in accordance with another embodiment of the present invention. Following the thermal oxidation and source and drain extension implant steps of  FIG. 2 ,  FIG. 7  provides for a raised source and drain formation process. This is performed, for example, by growing silicon on the substrate  12  in a known manner, and then etching the silicon to form the raised source and drains  40 . 
     In  FIG. 8 , an insulating spacer  42  has been formed over the raised source and drains  40 . Spacers  42  are formed from an oxide or nitride, for example. A silicide region  48  is formed on the gate electrode  16 , in a manner as described earlier. 
       FIGS. 9–16  depict still another embodiment of the present invention that may be considered especially preferred since it avoids the risks of thermal oxidation that are especially present in very small devices. 
       FIG. 9  shows a number of layers on a substrate  50 . The layers include insulator layers  52 , made of an oxide, for example. Other materials may be used, such as high K gate dielectrics, for example. Lightly doped semiconductor channel regions  54  are provided between the insulator layers  52 . A gate electrode layer  56  is provided on the uppermost insulator layer  52 . A hard mask layer  58  is provided on the gate electrode layer  56 . As in earlier described embodiments, the gate electrode layer  56  may be formed from a heavily doped silicon or silicon germanium layer, and the channel layers  54  may be formed of a lightly doped silicon or silicon germanium material. The conductivity types of the dopants in the gate electrode layer  56  and the channel layers  54  are of opposite types. In the described exemplary embodiment, for purposes of explanation and illustration, the gate electrode layer  56  is considered to have been heavily doped with n type dopants, while the channel layers  54  will be considered to have been lightly doped with p-type dopants. 
     Following the formation of the layers in  FIG. 9 , a dry etching is performed to shape the gate, the results of which are depicted in  FIG. 10 . The gate electrode  56   a  is thus created, with the dry etching stopping on the gate insulator layer  52 , which forms the uppermost insulator layer in the stack. The hard mask  58   a  is also etched at this time. A conventional dry etching technique may be employed. 
       FIG. 11  shows the structure of  FIG. 10  after a thermal oxide liner  60  has been formed on the gate electrode  56   a  to serve as protection for the gate electrode  56   a . Subsequent to the formation of the thermal oxide liner  60 , a second insulator layer, such as nitride, is deposited by CVD, for example. 
     A spacer dry etch step is then performed, the results of which are depicted in  FIG. 12 . The spacer dry etch stops on the top insulator layer  52 . The dry etch creates gate electrode spacers  64  on the thermal oxide liner  60 . A conventional dry etch process may be employed in this step. 
     A second etching procedure is then performed, using a dry etch, to create a stack  66 . The etching employs the gate electrode spacers  64  as a mask, while insulator layers  52  and channel layers  54  are etched. The dry etch is stopped on the bottom insulator layer  52  for control purposes to prevent damage to the silicon substrate  50 . 
       FIG. 14  shows the structure of  FIG. 13  following an extension implant of the same type dopants as provided in the gate electrode  56   a . The implants create extensions  68  in a substrate  50 . Following the extension implant, a wet etch process is performed that recesses the stack underneath the gate electrode spacer  64 . It also serves to remove a portion of the bottom insulator layer  52 . However, a second wet etch may be performed to remove more of the bottom insulator layer  52  if necessary to cause it to form the structure depicted in  FIG. 14 . The recessing of the stack  66 , including channel regions  54   a  and insulator regions  52   a , allows the diffusion of the extension implants to reach the channel under the gate electrode  56   a.    
     Following the recessing of the stack  66 , a multi-step polysilicon deposition process is performed to subsequently deposit a lightly doped polysilicon layer  70  over the stack  66 , followed by a heavily doped polysilicon layer  72  over the lightly doped polysilicon layer. The dopant conductivity type is the same as in gate electrode  56   a  and in the extension implants  68 . In this example, the dopant conductivity type is n-type dopant. The deposition of the polysilicon layer  70 ,  72  may be by chemical vapor deposition (CVD) or other appropriate methodologies. 
       FIG. 15  shows the structure of  FIG. 14  following a dry etch of the silicon layer  70 ,  72  to form silicon spacers  76 , having a lightly doped region and a heavily doped region. At this time, silicide can then be formed on the silicon spacers  76  and in the gate electrode  56   a , once the hard mask  58   a  has been removed by appropriate etching techniques. 
       FIG. 16  depicts the structure of  FIG. 15  following the formation of an insulator spacer  80 , made of nitride or oxide or other suitable material, for example. A source and drain implant is then performed to create source and drain regions  82  in the substrate  50 . 
     Three separate channel regioins  54   b  are decipited in the structure of  FIG. 16 , providing for a total of four channels (including the channel formed in the substrate  50 ). Hence, it should be clear to those of ordinary skill in the art that the number of channel regions may be varied in different embodiments. 
     The present invention thus provides for a semiconductor device and method for making the same that has a greater drive current than previous devices, but without occupying a greater amount of silicon area than conventional devices. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claim.