Abstract:
A method of forming a double-gated transistor having a rounded active region to improve GOI and leakage current control comprises the following steps, inter alia. An SOI substrate is patterned and a rounded oxide layer is formed over the exposed side walls of a patterned upper SOI silicon layer. A dummy layer, having an opening defining a gate, is formed over the exposed patterned top oxide layer and the exposed portions of the upper SOI silicon layer. An undercut is formed into the undercut lower SOI oxide layer and the exposed gate area portion of the oxide layer is removed. The portion of the rounded oxide layer within the gate area is removed and a conformal oxide layer is formed over a part of the structure. A gate is formed within the second patterned dummy layer opening and the patterned dummy layer is removed to form the double gated transistor.

Description:
BACKGROUND OF THE INVENTION 
   Double-gated transistors offer greater performance compared to conventional planar transistors. However, a problem has been how to fabricate such double-gated transistors. Current techniques being examined today include epitaxial growth to form the channel after gate oxidation and fin field effect transistors (FET). (“Fin” usually means a vertical silicon piece that is the gate, but there are many variations of fin-FETs.) 
   U.S. Pat. No. 6,451,656 B1 to Yu et al. describes a double-gated transistor on semiconductor-on-insulator (SOI). 
   U.S. Pat. No. 6,413,802 B1 to Hu et al. describes a double-gated FinFFET on semiconductor-on-insulator (SOI). 
   U.S. Pat. No. 6,365,465 B1 to Chan et al. also describes a process for a double-gated MOSFET on semiconductor-on-insulator (SOI). 
   U.S. Pat. No. 6,396,108 B1 to Krivokapic et al. describes a process for a double-gated MOSFET on semiconductor-on-insulator (SOI). 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide methods of forming double-gated silicon-on-insulator (SOI) transistors having improved gate oxide integrity (GOI) and leakage current control. 
   Other objects will appear hereinafter. 
   It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a substrate having an SOI structure formed thereover is provided. The SOI structure including a lower SOI oxide layer and an upper SOI silicon layer. A top oxide layer is formed over the SOI structure. A first top dummy layer is formed over the top oxide layer. The first top dummy layer, top oxide layer, and upper SOI silicon layer are patterned to form a patterned first top dummy layer/top oxide layer/upper SOI silicon layer stack having exposed side walls. The patterned upper SOI silicon layer including a source region and a drain region connected by a channel portion. A rounded oxide layer is formed over the exposed side walls of the patterned upper SOI silicon layer which also rounds the patterned upper SOI silicon layer. The patterned first top dummy layer is removed, exposing the patterned top oxide layer. A second patterned dummy layer is formed over the exposed patterned top oxide layer and the exposed portions of the upper SOI silicon layer. The second patterned dummy layer having an opening that defines a gate area exposing: a portion of the oxide layer within the gate area; portions of the upper surface of the lower SOI oxide layer within the gate area; and a portion of the rounded oxide layer within the gate area. The exposed gate area portions of the upper surface of the lower SOI oxide layer are etched into the lower SOI oxide layer to: form an undercut into the undercut lower SOI oxide layer exposing a bottom portion of the patterned upper SOI silicon layer within the gate area; remove the exposed gate area portion of the oxide layer exposing a top portion of the patterned upper SOI silicon layer within the gate area; and remove the portion of the rounded oxide layer within the gate area exposing a portion of the side walls of the patterned upper SOI silicon layer within the gate area. A conformal oxide layer is formed over: the exposed bottom portion of the patterned upper SOI silicon layer within the gate area; the exposed top portion of the patterned upper SOI silicon layer within the gate area; and the exposed portion of the side walls of the patterned upper SOI silicon layer within the gate area. A gate is formed within the second patterned dummy layer opening and includes an upper gate above the patterned upper SOI silicon layer within the gate area and a lower gate under the upper SOI silicon layer within the gate area. The second patterned dummy layer is removed to form the double-gated transistor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions and in which: 
       FIGS. 1  to  10  schematically illustrate a preferred embodiment of the present invention. 
       FIG. 11  is a cross-sectional view taken along line  11 — 11  of FIG.  10 . 
       FIG. 12  is a cross-sectional view taken along line  12 — 12  of FIG.  10 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   This invention provides an alternative way to fabricate double-gated transistors  80  using a silicon-on-insulator (SOI) substrate as the starting material. Since the technique of the present invention uses the top and bottom sides of the SOI to form the gate, surface mobility is not degraded as in the case of fin FETs. The SOI transistors formed in accordance with the present invention have improved gate oxide integrity (GOI) and leakage current control due to the corner rounding effect on the edge of the active region, the ‘fin.’ 
   Initial Structure— FIG. 1   
     FIG. 1  schematically illustrates a structure  10  having a fully depleted silicon-on-insulator structure (SOI)  16  formed thereover. 
   Structure  10  is preferably a semiconductor substrate comprised of silicon or germanium and is more preferably a silicon semiconductor substrate. 
   SOI  16  includes: a lower SOI silicon oxide (SiO 2 ) layer  12  having a thickness of preferably from about 1000 to 5000 Å and more preferably from about 2000 to 4000 Å; and an overlying SOI silicon (Si) layer  14  having a thickness of preferably from about 300 to 2000 Å and more preferably from about 500 to 1500 Å. 
   Formation of Top Oxide Layer  18  and First Top Dummy Layer  20 — FIG. 2   
   As shown in  FIG. 2 , a top oxide (silicon oxide —SiO 2 ) layer  18  is formed over SOI  16  to a thickness of preferably from about 90 to 110 Å, more preferably from about 95 to 105 Å and most preferably about 100 Å. 
   Then, a first top dummy layer  20  is formed over top oxide layer  18  to a thickness of preferably from about 450 to 1050 Å and more preferably from about 500 to 1000 Å. The first top dummy layer  20  is preferably comprised of nitride, silicon nitride (Si 3 N 4 ) or silicon oxynitride (SiON) and is more preferably comprised of nitride as will be used hereafter for illustrative purposes. 
   The first top dummy layer has a thickness of preferably from about 450 to 1050 Å and more preferably from about 500 to 1000 Å. 
   Patterning of First Top Dummy Nitride Layer  20 /Top Oxide Layer  18 /Overlying SOI Si Layer  14 — FIG. 2   
   As shown in  FIG. 3 , the first top dummy nitride layer  20 , top oxide layer  18  and overlying SOI silicon layer  14  of the SOI  16  are patterned down to the lower SOI oxide layer  12  to define: a patterned first top dummy nitride layer  20 ′/top oxide layer  18 ′/overlying SOI silicon layer  14 ′ stack  30  with exposed side walls  31 ; the active region as shown and exposed portions  22  of lower SOI silicon oxide layer  12 . The top nitride layer  10 , top oxide layer  18  and overlying SOI silicon layer  14  may be patterned, for example, using an overlying patterned mask layer (not shown) preferably comprised of photoresist. 
   Patterned stack  30  may be roughly in the shape of a dumbbell with the opposing ends of the patterned overlying SOI silicon layer  14 ′ being a opposing source region  34  and drain region  36  connected by a channel region  38 . 
   Formation of Rounded Oxide Layer  28 — FIG. 4   
   As shown in  FIG. 4 , a rounded oxide layer  28  is formed over the exposed side walls  31  of the patterned overlying SOI silicon layer  14  to help protect the patterned overlying SOI silicon layer  14  from subsequent processing and completes formation of an oxide encased patterned overlying SOI silicon layer  32  comprising: the patterned overlying SOI silicon layer  14 ′, the rounded oxide layer  28  and the patterned top oxide layer  18 ′. As shown in  FIG. 4 , rounded oxide layer  28  has rounded corners  26 . Rounded oxide layer  28  has a thickness of preferably from about 80 to 550 Å and more preferably from about 100 to 500 Å. 
   Rounded oxide layer  28  is preferably formed by an oxidation growth process at a temperature of preferably from about 800 to 1200° C. and more preferably from about 900 to 1100° C. for from about 10 to 120 minutes and more preferably from about 20 to 100 minutes. 
   Removal of Patterned First Top Dummy Nitride Layer  20 ′— FIG. 5   
   As shown in  FIG. 5 , the patterned first top dummy nitride layer  20 ′ is removed from the patterned stack  30  to expose the oxide encased patterned overlying SOI silicon layer  32 . The patterned first top dummy nitride layer  20 ′ is preferably removed employing a hot phosphoric acid etch selective to oxide/silicon oxide. 
   Formation of Second Patterned Dummy Layer  40 — FIG. 6   
   As shown in  FIG. 6 , a second patterned dummy layer  40  is formed over the structure of FIG.  5 . Second patterned dummy layer  40  may be patterned, for example, by using an overlying patterned gate reverse mask (not shown) and employing an anisotropic etch to form opening  42  opening up the gate area  50 . Second patterned dummy layer  40  includes opening  42  that exposes: a gate portion  38 ′ of the channel region  38  of the oxide encased patterned overlying SOI silicon layer  32  within gate area  50 ; and gate portions  22 ′ of lower SOI silicon oxide layer  12  within gate area  50 . 
   It is noted that the gate portion  38 ′ of the oxide encased patterned overlying SOI silicon layer  32  has an overall rounded characteristic due to the formation of rounded oxide layer  28 . 
   Second patterned dummy layer  40  is preferably comprised of nitride, silicon nitride (Si 3 N 4 ) or silicon oxynitride (SiON) and is more preferably comprised of nitride as will be used hereafter for illustrative purposes. 
   Second patterned dummy layer  40  has a thickness of preferably from about 1000 to 3000 Å and more preferably from about 1500 to 2500 Å. 
   Etching of SOI Oxide Layer  12 — FIG. 7   
   As shown in  FIG. 7 , an etch, preferably a dilute HF etch, is used to: (1) remove the portion of the patterned top oxide layer  18 ′ overlying the patterned overlying SOI silicon layer  32 ′ within gate area  50 ; (2) remove the portion of the rounded oxide layer  28  overlying the side walls  31  of the patterned overlying SOI silicon layer  32 ′ within gate area  50 ; and (3) etch the SOI oxide layer  12  exposed within opening  42  to form an undercut  44  within etched SOI oxide layer  12 ′ extending below the now denuded patterned overlying SOI silicon layer  32 ′ within gate area  50 . 
   The SOI oxide layer  12  is preferably etched using a dilute HF etch or a buffered oxide etch (BOE) and more preferably using a dilute HF etch. 
   Undercut  44  of the etched SOI oxide layer  12 ′ is preferably from about 500 to 3000 Å and more preferably from about 1000 to 2000 Å deep and preferably protrudes from about 500 to 3000 Å and more preferably from about 500 to 1000 Å under the leading edges of opening  42  of second patterned dummy nitride layer  40 . 
   Formation of Conformal Gate Oxide Layer  32 — FIG. 7   
   As further shown in  FIG. 7 , a conformal oxide layer  46  is formed, preferably by growth, around the denuded patterned overlying SOI silicon layer  32 ′ within gate area  50  to form a conformal oxide rounded gate portion  48  of the patterned overlying SOI silicon layer  14 ′. Conformal oxide layer  46  is grown on the exposed top, bottom and sides of the gate portion of the patterned overlying SOI silicon layer  14 ′ to a thickness of preferably from about 5 to 200 Å and more preferably from about 10 to 50 Å. 
   Formation of Gate  52 — FIG. 8   
   As shown in  FIG. 8 , a gate layer is formed over the patterned dummy nitride layer  40 , filling opening  42  and is planarized to remove the excess of the gate layer from over the top of patterned dummy nitride layer  40  to form a planarized gate  52  within opening  42 . Gate  52  is preferably comprised of polysilicon (poly), tungsten (W), W—Si x , silicon germanium (SiGe) or aluminum (Al) and is more preferably polysilicon (poly) as will be used hereafter for purposes of illustration. 
   Poly gate  52  includes upper gate  56  and lower gate  54  separated by the conformal oxide rounded gate portion  48  of the patterned overlying SOI silicon layer  14 ′ as more clearly shown in  FIGS. 11 and 12 . 
   Since polysilicon, for example, has good gap filling properties and the poly growth is conformal, poly gate  52  wraps completely around the conformal oxide layer  46  previously grown around the channel region  38  of patterned SOI silicon layer  14 ′ within gate area  50 . 
   The gate layer is preferably planarized by a chemical mechanical polishing (CMP) process to form the poly gate  52 . 
   Removal of the Patterned Dummy Nitride Layer  40 — FIG. 9   
   As shown in  FIG. 9 , the patterned dummy nitride layer  40  is removed from the structure of  FIG. 8  to expose: the side walls  79  of upper poly gate  56 ; and the rounded oxide layer  28 ′ of oxide encased patterned overlying SOI silicon layer  32  outside gate area  50 . The patterned dummy nitride layer  40  is preferably removed using hot phosphoric acid. 
   LDD Implantation, Formation of Spacers  60  and Source-Drain Implants— FIG. 9   
   As shown in  FIG. 10 , conventional SDE or LDD implants  100  are performed and will exist under the spacers  60  (see below) and overlap the gate by a small portion. The silicon substrate is thin enough such that the SDE/LDD extend from top to bottom of the Si substrate. 
   Spacers  60  are then formed over the sidewalls  79  of upper poly gate  56 , and spacers  62  are formed over the exposed rounded oxide layer  28 ′ of oxide encased patterned overlying SOI silicon layer  32  outside gate area  50  as shown in FIG.  10 . The rounded oxide layer  28  and the exposed portions of the patterned top oxide layer  18 ′ overlying the patterned overlying patterned SOI silicon layer  14  are removed during the spacer etching in the formation of spacers  60 ,  62 . 
   Source-drain (S/D) implants are then respectively formed into source region  34  and drain region  36 , for example, to form source  34 ′ and drain  36 ′ to complete formation of double-gated transistor  80  with a conformal oxide rounded gate portion  48  of the patterned overlying SOI silicon layer  14 ′. 
     FIG. 11  is a cross-sectional representation of  FIG. 10  along line  11 — 11  and illustrates upper gate  56  and lower gate  54  of poly gate  52  separated by the conformal oxide rounded gate portion  48  of the patterned overlying SOI silicon layer  14 ′. Sidewall spacers  60  extend over the side walls  42  of upper gate  56  of poly gate  52 . Sidewall spacers  62  extend over the exposed rounded oxide layer  28 ′ of oxide encased patterned overlying SOI silicon layer  32  outside gate area  50 . 
     FIG. 12  is a cross-sectional representation of  FIG. 10  along line  12 — 12 , perpendicular to line  11 — 11 , and illustrates upper gate  56  and lower gate  54  of poly gate  52  separated by the conformal oxide rounded gate portion  48  of the patterned overlying SOI silicon layer  14 ′. Sidewall spacers  60  extend over the side walls  79  of upper gate  56  of poly gate  52 . As most clearly shown in  FIG. 12 , the formation of rounded oxide layer  28  (see  FIG. 4 ) causes additional portions of the patterned SOI silicon layer  14 ′ to be oxidized: (1) proximate top oxide layer  18 ; and (2) proximate SOI silicon oxide layer  12  to form a rounded patterned SOI silicon layer  14 ′. 
   Optionally, a standard salicidation process may then be used (TiSi, CoSi, etc., e.g.). 
   Advantages of the Invention 
   The advantages of one or more embodiments of the present invention include the rounded active region improves gate oxide integrity (GOI) and leakage current control. 
   Some notable qualities of this invention include: 
   1) making use of the top and bottom surfaces of the SOI for gate oxide channel formation—due to the fact that both surfaces have low surface roughness, the mobility is improved; 
   2) gate oxide is formed all around the channel which is more like a circular FET rather than a planar FET; and 
   3) W or WN may be used to form a metal gate instead of using polysilicon. 
   While particular embodiments of the present invention have been illustrated and described, it is not intended to limit the invention, except as defined by the following claims.