Abstract:
A method for forming transistors with mutually-aligned double gates. The method includes the steps of (a) providing a wrap-around-gate transistor structure, wherein the wrap-around-gate transistor structure includes (i) semiconductor region, and (ii) a gate electrode region wrapping around the semiconductor region, wherein the gate electrode region is electrically insulated from the semiconductor region by a gate dielectric film; and (b) removing first and second portions of the wrap-around-gate transistor structure so as to form top and bottom gate electrodes from the gate electrode region, wherein the top and bottom gate electrodes are electrically disconnected from each other.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to double-gate FETs (Field Effect Transistors), and more specifically, to double-gate FETs whose double gates are electrically disconnected from each other. 
   2. Related Art 
   Dopant fluctuations are becoming a serious problem in Vt (threshold voltage) control in advanced semiconductor devices. As semiconductor devices become smaller and smaller in size, Vt control becomes more difficult. A known solution is to use back gates (in addition to front gates) in the semiconductor devices to control Vt. Without precise alignment between the top and bottom gates, however, the performance advantage of dual gates is decreased or lost completely. 
   As a result, there is always a need for new methods for forming transistors with aligned double gates. The present invention provides such a new method. 
   SUMMARY OF THE INVENTION 
   The present invention provides a semiconductor structure fabrication method, comprising the steps of (a) providing a wrap-around-gate transistor structure, wherein the wrap-around-gate transistor structure includes (i) a semiconductor region, and (ii) a gate electrode region wrapping around the semiconductor region, wherein the gate electrode region is electrically insulated from the semiconductor region by a gate dielectric film; and (b) removing first and second portions of the wrap-around-gate transistor structure so as to form top and bottom gate electrodes from the gate electrode region, wherein the top and bottom gate electrodes are electrically disconnected from each other. 
   The present invention also provides a semiconductor structure fabrication method, comprising the steps of (a) providing a semiconductor block embedded in a dielectric block; (b) etching a first trench through the semiconductor block and the dielectric block so as to form first and second semiconductor regions from the semiconductor block; (c) removing portions of the dielectric block such that a larger surface of the first semiconductor region is exposed to the atmosphere than before the portions of the dielectric block are removed; (d) forming a gate dielectric film on exposed-to-atmosphere surfaces of the first semiconductor region; (e) forming a gate electrode layer on the gate dielectric film, wherein the gate electrode layer is electrically insulated from the first semiconductor region by the gate dielectric film; (f) forming a gate electrode region from the gate electrode layer, wherein the gate electrode region, the gate dielectric film, and the first semiconductor region form a wrap-around-gate transistor structure, wherein the gate electrode region wraps around the first semiconductor region; and (g) removing first and second portions of the wrap-around-gate transistor structure so as to form top and bottom gate electrodes from the gate electrode region, wherein the top and bottom gate electrodes are electrically disconnected from each other. 
   The present invention also provides a new method for forming transistors with double gates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1-8  illustrate a fabrication method for forming a semiconductor structure, in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-8  illustrate a fabrication method for forming a semiconductor structure  100 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1A , in one embodiment, the method can start out with a substrate  110 , 120 , 130 . Illustratively, the substrate  110 , 120 , 130  can comprise a silicon layer  110 , a nitride (e.g., silicon nitride) layer  120  on top of the silicon layer  110 , and an oxide (e.g., silicon dioxide) layer  130  on top of the nitride layer  120 . 
   Next, a silicon region  140  can be formed on top of the oxide layer  130 . In one embodiment, the silicon region  140  can be formed by first bonding the face of another semiconductor wafer (not shown) to the top surface  132  of structure  110 , 120 , 130 , and then thinning that top wafer to its desired thickness using techniques known to experts in the field. Then, the bonded thinned silicon layer can be masked and etched to form the silicon region  140 .  FIG. 1A  shows a perspective view of the resulting structure  100  after the silicon region  140  is formed.  FIG. 1B  shows a cross-section view of the structure  100  of  FIG. 1A  in the plane defined by the line  1 B. 
   Next, with continued reference to  FIG. 1A , silicon dioxide can be deposited on top of the entire structure  100  by, illustratively, CVD (chemical vapor deposition). The newly deposited oxide material merges with the oxide layer  130  to form a new oxide layer  210  ( FIG. 2A ). As a result, the silicon region  140  becomes buried (embedded) in the oxide layer  210  ( FIG. 2A ). This oxide is then planarized by, for example, chemical mechanical polishing (CMP) in order to provide a flat top surface. 
   Next, with reference to  FIG. 2A , a hard mask  220  can be formed on top of the planarized oxide layer  210 . In one embodiment, the hard mask  220  can comprise a nitride material (e.g., silicon nitride). The nitride hard mask  220  can be formed by, illustratively, CVD. A perspective view of the resulting structure  100  after the formation of the hard mask  220  is shown in  FIG. 2A .  FIG. 2B  shows a cross-section view of the structure  100  of  FIG. 2A  in the plane defined by the line  2 B. 
   Next, with reference to  FIG. 3A  (a top view), a patterned photoresist layer  310  can be formed on top of the structure  100  of  FIG. 2A .  FIG. 3A  shows a top-down view of the resulting structure  100  after the patterned photoresist layer  310  is formed. The patterned photoresist layer  310  can cover the entire top surface of the structure  100  except an opening  320 . As a result, the hard mask  220  can be seen exposed to the atmosphere through the opening  320 . In one embodiment, the patterned photoresist layer  310  can be formed by a conventional photolithography process. 
   Next, the patterned photoresist layer  310  can be used as a mask to etch (etch process # 1 ) vertically down through different layers and regions of the structure  100  of  FIG. 2A , stopping at the nitride layer  120 . More specifically, etch process # 1  etches through the hard mask layer  220 , the oxide layer  210 , and the embedded silicon region  140 . Next, the patterned photoresist layer  310  can be removed. 
   If the structure  100  of  FIG. 3A  (after etch process # 1  is performed and the patterned photoresist layer  310  is removed) were cut vertically along the line  3 B into left and right portions,  FIG. 3B  shows a perspective view of the left portion. With reference to  FIG. 3B , etch process # 1  cuts the silicon region  140  ( FIG. 2A ) into two physically separated silicon regions  140   a  and  140   b  of which only the silicon region  140   a  is shown in  FIG. 3B  (the silicon region  140   b  can be seen in  FIG. 3C ). 
     FIG. 3C  shows a cross-section view of the structure  100  of  FIG. 3A  (after etch process # 1  is performed and the patterned photoresist layer  310  is removed) along the line  3 C- 3 C. As seen in  FIG. 3C , a trench  330  is formed as a result of etch process # 1 . 
   Next, with reference to  FIG. 3C , the oxide layer  210 , which is exposed to the atmosphere on side walls of the trench  330 , can be isotropically etched (etch process # 2 ). In one embodiment, etch process # 2  can comprise a wet etch.  FIG. 4A  shows the structure  100  of  FIG. 3C  after etch process # 2  is performed. With reference to  FIG. 4A , as a result of etch process # 2 , the trench  330  is expanded laterally (horizontally) at the oxide layer  210 . More specifically, the side wall portion  410  of the trench  330  corresponding to the oxide layer  210  before etch process # 2  is performed becomes side wall portion  420  as a result of etch process # 2 . If the structure  100  of FIG.  4 A were cut along the line  4 B into left and right portions,  FIG. 4B  shows a perspective view of the left portion, with the hard mask  220  being omitted for simplicity. 
   Next, with continued reference to  FIG. 4A , the exposed-to-atmosphere surfaces of the silicon regions  140   a  and  140   b  can be thermally oxidized so as to form gate dielectric films  510   a  and  510   b , respectively ( FIG. 5 ). Alternatively, a thin layer of metal oxides or metal silicates can be deposited on the entire structure  100  of  FIG. 4A  to form the gate dielectric films  510   a  and  510   b  on the exposed-to-atmosphere surfaces of the silicon regions  140   a  and  140   b , respectively. 
   Next, with reference to  FIG. 5 , a gate electrode layer  520  can be formed on exposed-to-atmosphere surfaces of the structure  100  (including walls of the trench  330 ). In one embodiment, the gate electrode layer  520  can be formed by CVD of polysilicon. 
   Next, an organic material can be deposited to completely fill the trench  330 , including the spaces created by etch process # 2 . Then, an anisotropic etch process # 3  can be performed to etch vertically down the filled trench  330  to remove some of the deposited organic material, essentially without affecting the gate electrode layer  520 . The remaining portions of the deposited organic material form organic regions  530 .  FIG. 5  shows the structure  100  of  FIG. 4A  after etch process # 3  is performed. 
   Next, an isotropic etch process # 4  can be performed to remove portions of the gate electrode layer  520  essentially without affecting the organic material. As a result of etch process # 4 , the polysilicon gate electrode layer  520  is reduced to a gate electrode region  520 ′ ( FIG. 6A ). 
   With reference to  FIG. 6A , the gate electrode region  520 ′ wraps around the silicon regions  140   a  and  140   b . For that reason, the gate electrode region  520 ′ can also be referred to as the wrap-around gate electrode region  520 ′. 
   If the structure  100  of  FIG. 6A  (after etch process # 4  is performed) were cut along the line  6 B into left and right portions,  FIG. 6B  shows a perspective view of the left portion, (with the organic regions  530  and the hard mask  220  being omitted for simplicity). 
   Next, with reference to  FIG. 7A  (a top view), a patterned photoresist layer  710   a , 710   b  comprising two photoresist stripes  710   a  and  710   b  can be formed on top of the structure  100  of  FIG. 6A .  FIG. 7A  shows a top-down view of the resulting structure  100  after the patterned photoresist layer  710   a , 710   b  is formed. Then, an anisotropic etch process # 5  can be performed using the patterned photoresist layer  710   a , 710   b  as a mask to etch vertically down through different regions of the structure  100  of  FIG. 6A . Etch process # 5  will be described further below. 
   After etch process # 5 , photoresist  710   a ,  710   b , hard mask  220 , and organic material  530  are removed. If the structure  100  of  FIG. 7A  (after photoresist  710   a ,  710   b , hard mask  220 , and organic material  530  are removed) were cut along the line  7 B into left and right portions,  FIG. 7B  shows a perspective view of the left portion. 
   In one embodiment, the patterned photoresist layer  710   a , 710   b  (more specifically, the photoresist stripe  710   a ) has size and shape such that after etch process # 5  is performed the resulting gate electrode region  520 ′ no longer wraps around the silicon regions  140   a  and  140   b  ( FIG. 6A ). More specifically, for the silicon region  140   a , two polysilicon portions are removed from two opposing sides of the polysilicon ring of the gate electrode region  520 ′ ( FIG. 6A ) that wraps around the silicon region  140   a  as a result of etch process # 5 . As a result, the polysilicon ring of the gate electrode region  520 ′ ( FIG. 6A ) that wraps around the silicon region  140   a  is cut into two physically separated gate electrode regions  520   a ′ and  520   b ′ (which can be referred to as the top and bottom gate electrode regions  520   a ′ and  520   b ′, respectively). To achieve this result, the stripe  710   a  ( FIG. 7A ) can be positioned directly above the silicon regions  140   a  and  140   b  ( FIG. 6A ) and have a width  720  ( FIG. 7A ) less than the width  150  ( FIG. 1A ) of the silicon region  140 . Similar structures are formed around the silicon region  140   b  ( FIG. 6A ) as a result of etch process # 5 . 
   With reference to  FIG. 7B , to achieve the structure  100  of  FIG. 7B , the etch process # 5  can comprise different etching steps that etch through different materials of different regions of the structure  100  of  FIG. 6A . In one embodiment, etch process # 5  can comprise etching through nitride of the hard mask layer  220 , through oxide of the oxide layer  210 , through polysilicon of the gate electrode region  520 ′, through the organic material of the organic regions  530 , and stopping after the portion of the gate electrode region  520 ′ that wraps around the silicon region  140   a  is completely cut through but before the portion  520   b   1 ′ of the gate electrode region  520 ′ that rests on the nitride layer  120  is completely cut through. Also as a result, the bottom gate electrode region  520   b ′ is still electrically connected to the bottom gate electrode portion  520   b   2 ′ via the bottom gate electrode portion  520   b   1 ′. Also, the top and bottom gate electrode regions  520   a ′ and  520   b ′, respectively, are electrically disconnected from each other. 
   In one embodiment, etch process # 5  can comprise RIE (reactive ion etching) steps having ion bombardments in a vertical downward direction. In one embodiment, the photoresist stripe  710   a  ( FIG. 7A ) has two parallel sides  722  and  724  so that etch process # 5  cuts down on the structure  100  along two parallel cutting surfaces. 
   Next, exposed sidewalls of silicon regions  140   a  and  140   b  are passivated, preferably employing thermal oxidation to grow 2 nm to 8 nm of oxide, and source/drain regions  812  and  822  ( FIG. 8 ) formed via implantation and activation anneal. Then, the entire structure  100  is filled by depositing a thick dielectric layer (not shown), preferably silicon dioxide or doped silicon dioxide, and planarizing the thick dielectric layer. This thick dielectric layer is omitted from drawings for clarity. 
   Next, with reference to  FIG. 8 , contact regions  810 ,  820 ,  830 , and  840  can be formed in the planarized dielectric layer (using any conventional process) to electrically connect different regions of the structure  100  of  FIG. 7B  to an upper interconnect level (not shown). For simplicity, only the active silicon region and the gate electrode regions are shown. More specifically, the source/drain (S/D) regions  812  and  822  in the active silicon region  140   a  can be electrically connected to the upper interconnect level via the contact regions  810  and  820 , respectively. The top gate electrode region  520   a ′ can be electrically connected to the upper interconnect level via the contact region  830 , whereas the bottom gate electrode region  520   b ′ can be electrically connected to the upper interconnect level via the contact region  840 . The resulting structure  100  after the contact regions  810 ,  820 ,  830 , and  840  are formed is shown in  FIG. 8 . It should be also noted that similar contact regions (not shown) can be formed for the right half of the structure  100  (also not shown) so that the resulting structure  100  has two symmetric transistors only the left transistor of which is shown in  FIG. 8 . 
   While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.