Abstract:
A semiconductor structure and the associated method for fabricating the same. The semiconductor structure includes (a) a semiconductor substrate, (b) a back gate region on the semiconductor substrate, (c) a back gate dielectric region on the back gate region, (d) a semiconductor region on the back gate dielectric region comprising a channel region disposed between first and second source/drain (S/D) regions, (e) a main gate dielectric region on the semiconductor region, (f) a main gate region on the main gate dielectric region, (g) a first contact pad adjacent to the first S/D region and electrically insulated from the back gate region, and (h) a first buried dielectric region that physically and electrically isolates the first contact pad and the back gate region, and wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to field effect transistors (FETs), and more specifically, to planar dual-gate FETs (i.e., planar FETs that have main gates and back gates). 
   2. Related Art 
   The switching speed of a typical planar dual-gate FET depends on, among other things, the capacitances between the S/D regions and the back gate region of the typical planar dual-gate FET. The higher such capacitances, the longer it takes for switching (because it takes time for charging and discharging the capacitors), which is undesirable. 
   Therefore, there is a need for a planar dual-gate FET (and a method for fabricating the same) in which capacitances between the S/D regions and the back gate region are relatively lower than that of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides a semiconductor structure, comprising (a) a semiconductor substrate; (b) a back gate region on the semiconductor substrate; (c) a back gate dielectric region on the back gate region in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate region and the semiconductor substrate; (d) a semiconductor region on the back gate dielectric region in the first direction, wherein the semiconductor region comprises a channel region and first and second source/drain (S/D) regions, wherein the channel region is disposed between the first and second S/D regions, and wherein the semiconductor region is electrically insulated from the back gate region by the back gate dielectric region; (e) a main gate dielectric region on the semiconductor region in the first direction; (f) a main gate region on the main gate dielectric region in the first direction, wherein the semiconductor region is electrically insulated from the main gate region by the main gate dielectric region; and (g) a first contact pad adjacent to the first S/D region in a second direction and in direct physical contact with the first S/D region, wherein the second direction is perpendicular to the first direction and pointing from the second S/D region to the first S/D region; and (h) a first buried dielectric region directly beneath the first contact pad in the first direction and in direct physical contact with the first contact pad and the back gate region, wherein the first buried dielectric region physically and electrically isolates the first contact pad and the back gate region, and wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region. 
   The present invention also provides a semiconductor structure fabrication method, comprising providing a semiconductor substrate, a back gate layer on the semiconductor substrate, a back gate dielectric layer on the back gate layer in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate layer and the semiconductor substrate, a semiconductor layer on the back gate dielectric layer in the first direction, wherein the semiconductor layer is electrically insulated from the back gate layer by the back gate dielectric layer, and a main gate dielectric layer on the semiconductor layer in the first direction; removing portions of the main gate dielectric layer, the semiconductor layer, the back gate dielectric layer, and the back gate layer so as to form first and second trenches such that a back gate dielectric region disposed between the first and second trenches results from the back gate dielectric layer, a semiconductor region disposed between the first and second trenches results from the semiconductor layer, and a main gate dielectric region disposed between the first and second trenches results from the main gate dielectric layer; forming first and second buried dielectric regions in the first and second trenches, respectively, and in direct physical contact with the back gate dielectric layer, wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region; and forming first and second contact pads on the first and second buried dielectric regions and in the first and second trenches, respectively, wherein the first and second contact pads are in direct physical contact with the semiconductor region, wherein the first and second contact pads are electrically insulated from the back gate region 
   The present invention also provides a planar dual-gate FET (and a method for fabricating the same) in which capacitances between the S/D regions and the back gate region are relatively lower than that of the prior art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1-5  illustrate cross-section views of a semiconductor structure going through different fabrication steps, in accordance with embodiments of the present invention. 
       FIGS. 6A-6D  illustrate a process for forming the semiconductor structure of  FIG. 1 , in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-5  illustrate cross-section views of a semiconductor structure  100  going through different fabrication steps, in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1 , the fabrication steps can start out with a structure  100  comprising (a) a semiconductor (e.g., silicon, germanium, etc.) substrate  110 , (b) a buried oxide (BOX) layer  120  on top of the semiconductor substrate  110 , (c) a back gate layer  130  on top of the BOX layer  120 , (d) a back gate dielectric layer  140  on top of the back gate layer  130 , and (e) a semiconductor (i.e., silicon, germanium, etc.) layer  150  on top of the back gate dielectric layer  140 . In one embodiment, the BOX layer  120  can comprise silicon dioxide, the back gate layer  130  can comprise silicon, and the back gate dielectric layer  140  can comprise silicon dioxide. 
   In one embodiment, the structure  100  of  FIG. 1  can be formed using a double SOI (silicon on insulator) process.  FIGS. 6A-6D  illustrate the formation of the structure  100  of  FIG. 1  using the double SOI process. With reference to  FIG. 6A , the double SOI process can start with a silicon substrate  610 . 
   Next, the BOX layer  120  can be formed on top of the silicon substrate  610  by, illustratively, chemical vapor deposition (CVD). 
   Next, hydrogen II ions can be implanted in the silicon substrate  610  so as to form a hydrogen ion layer  612  embedded in the silicon substrate  610 , resulting in the structure  100  of  FIG. 6A . The portion of the silicon substrate  610  above the hydrogen II ion layer  612  will become the back gate layer  130 . 
   Next, with reference to  FIG. 6B , the semiconductor substrate  110  can be bonded to BOX layer  120  resulting in the structure  100  of  FIG. 6B . 
   Next, the structure  100  of  FIG. 6B  can be annealed so that the structure  100  of  FIG. 6B  splits along the hydrogen ion layer  612 . The upper portion of the structure  100  of  FIG. 6B  after the split is turned upside down resulting in the structure  100  of  FIG. 6C . 
   Next, a structure similar to the structure  100  of  FIG. 6A  can be turned upside down and bonded to the semiconductor layer  130  of  FIG. 6C  resulting in the structure  100  of  FIG. 6D . 
   Next, the structure  100  of  FIG. 6D  can be annealed so that the structure  100  of  FIG. 6D  splits along the hydrogen ion layer  622 . The lower portion of the structure  100  of  FIG. 6D  after the split can be used as the structure  100  of  FIG. 1 . 
   Next, with reference to  FIG. 2 , shallow trench isolation (STI) regions  160   a  and  160   b can be formed in the semiconductor layer  150  using any conventional method. 
   Next, an oxide layer  210  can be formed on top of the entire structure  100  of  FIG. 1  by, illustratively, thermal oxidation or chemical vapor deposition (CVD). Next, a nitride layer  220  can be formed on top of the oxide layer  210  by, illustratively, CVD. 
   Next, with reference to  FIG. 3 , two trenches  310   a  and  310   b  can be formed in the structure  100  of  FIG. 2 . More specifically, in one embodiment, the trenches  310   a  and  310   b  can be formed by directionally etching (e.g., using reactive ion etching or RIE etch) in turn the nitride layer  220 , the oxide layer  210 , the semiconductor layer  150 , the back gate dielectric layer  140 , the back gate layer  130 , and the buried oxide layer  120  in that order. In one embodiment, the directional etch that forms the trenches  310   a  and  310   b  etches completely through the back gate layer  130  but does not etch completely through the buried oxide layer  120 . It should be noted that portions of the nitride layer  220 , the oxide layer  210 , the semiconductor layer  150 , the back gate dielectric layer  140 , the back gate layer  130  that are disposed between the trenches  310   a  and  310   b  can be referred to as the nitride region  220 ′, the oxide region  210 ′ the semiconductor region  150 ′, the back gate dielectric region  140 ′, and the back gate region  130 ′, respectively. 
   Next, buried dielectric regions  320   a  and  320   b  can be formed in the trenches  310   a  and  310   b , respectively. In one embodiment, the buried dielectric regions  320   a  and  320   b  can comprise silicon dioxide (SiO 2 ), and can be formed by, illustratively, (a) depositing a SiO 2  layer (not shown) on top of the entire structure  100  of  FIG. 3  such that the trenches  310   a  and  310   b  are completely filled with SiO 2  material, then (b) etching back the deposited SiO 2  layer until top surfaces  322   a  and  322   b  of the buried dielectric regions  320   a  and  320   b , respectively, are at the level of the semiconductor region  150 ′. The process of forming the buried dielectric regions  320   a  and  320   b  by filling the trenches  310   a  and  310   b  and etching back can be referred to as a fill and recess process. 
   Next, with reference to  FIG. 4 , contact pads  410   a  and  410   b  can be formed on top of the buried dielectric regions  320   a  and  320   b  in the trenches  310   a  and  310   b  ( FIG. 3 ), respectively. In one embodiment, the contact pads  410   a  and  410   b  can comprise polysilicon. The polysilicon contact pads  410   a  and  410   b  can be formed by (a) depositing polysilicon on the entire structure  100  of  FIG. 3  and then (b) planarizing using a conventional chemical mechanical polishing (CMP) process until a top surface  212  of the oxide region  210 ′ is exposed to the atmosphere. The process of forming the contact pads  410   a  and  410   b  by depositing polysilicon and then planarizing can be referred to as a deposit and planarize process. Because the top surfaces  322   a  and  322   b  of the buried dielectric regions  320   a  and  320   b , respectively, are at the level of the semiconductor region  150 ′, the contact pads  410   a  and  410   b  are in direct physical contact with the semiconductor region  150 ′. Next, the oxide layer  210  (including the oxide region  210 ′) is removed. 
   Next, with reference to  FIG. 5 , a main gate stack  510 , 505  comprising a main gate dielectric region  505  and a main gate region  510  can be formed on top of the semiconductor region  150 ′. In one embodiment, the main gate region  510  can comprise polysilicon. The main gate stack  510 ,  505  can be formed by (a) depositing a main gate dielectric layer (not shown) on top of the entire structure  100  of  FIG. 4  (after the oxide layer  210  is removed), then (b) depositing a main gate layer (not shown) on top of the main gate dielectric layer, and then (c) etching back the deposited main gate layer and the main gate dielectric layer to form the main gate stack  510 ,  505  using any conventional photolithography process. 
   Next, main gate spacers  512   a  and  512   b  can be formed on side walls of the main gate stack  510 , 505 . The main gate spacers  512   a  and  512   b  can comprise silicon dioxide and can be formed using any conventional method. 
   Next, the main gate stack  510 ,  505  and the main gate spacers  512   a  and  512   b  can be used as a mask to dope the semiconductor region  150 ′ so as to form source/drain (S/D) regions  520   a  and  520   b  in the semiconductor region  150 ′. In one embodiment, ion implantation can be used for this doping process. The portion  530  of the semiconductor region  150 ′ directly beneath the gate stack  510 ,  505  and disposed between the S/D regions  520   a  and  520   b  can be referred to as the channel region  530 . 
   Next, a silicide region  540   a  can be formed on top and in direct physical contact with both the contact pad  410   a  and the S/D region  520   a , while a silicide region  540   b  can be formed on top and in direct physical contact with both the contact pad  410   b  and the S/D region  520   b . 
   The silicide regions  540   a  and  540   b  can be formed by (a) depositing a metal material (e.g., cobalt, titanium, etc.) on top of the entire structure  100  of  FIG. 5  (without the metal vias  550   a ,  550   b , and  550   c , and the silicide regions  540   a  and  540   b  at this time), then (b) heating up the metal material such that the metal material chemically reacts with silicon of the contact pads  410   a  and  410   b  and the S/D regions  520   a  and  520   b  to form the silicide regions  540   a  and  540   b , and then (c) removing the remaining metal material by a wet etch step. 
   Next, a dielectric layer  545  can be formed on top of the entire structure  100  by, illustratively, CVD. Next, metal vias  550   a  and  550   b  can be formed in the dielectric layer  545  and on top of the silicide regions  540   a  and  540   b , respectively, while a metal via  550   c  can be formed in the dielectric layer  545  and on top of the main gate region  510 . The vias  550   a ,  550   b ,  550   c  can be used to electrically couple the structure  100  to an upper interconnect layer (not shown). 
   It should be understood that, although not shown, a contact to the back gate region  130 ′ may be formed, using any conventional method, to provide an electric connection between an upper interconnect layer (not shown) and the back gate region  130 ′. 
   In summary, because of the buried dielectric region  320   a , the first S/D block (including the S/D region  520   a , the contact pad  410   a , and silicide region  540   a , and the metal via  550   a ) forms with the back gate region  130 ′ a first capacitance relatively lower than that of the prior art. Similarly, because of the buried dielectric region  320   b , the second S/D block (including the S/D region  520   b , the contact pad  410   b , the silicide region  540   b , and the metal via  550   b ) forms with the back gate region  130 ′ a second capacitance relatively lower than that of the prior art. As a result, the structure  100  can switch faster than devices of the prior art. 
   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.