Abstract:
A semiconductor transistor with an expanded top portion of a gate and a method for forming the same. The semiconductor transistor with an expanded top portion of a gate includes (a) a semiconductor region which includes a channel region and first and second source/drain regions; the channel region is disposed between the first and second source/drain regions, (b) a gate dielectric region in direct physical contact with the channel region, and (c) a gate electrode region which includes a top portion and a bottom portion. The bottom portion is in direct physical contact with the gate dielectric region. A first width of the top portion is greater than a second width of the bottom portion. The gate electrode region is electrically insulated from the channel region by the gate dielectric region.

Description:
This application is a divisional application claiming priority to Ser. No. 11/275,514, filed Jan. 11, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to semiconductor transistors, and more particularly, to semiconductor transistors with expanded top portions of gates. 
     2. Related Art 
     In the fabrication process of a typical semiconductor device, if a gate is small it is very difficult to form silicide in the top portion of the gate. Therefore, there is a need for a semiconductor transistor with an expanded top portion of a gate (and a method for forming the same). 
     SUMMARY OF THE INVENTION 
     The present invention provides a semiconductor structure, comprising (a) a semiconductor region including a channel region, a first source/drain region, and a second source/drain region, wherein the channel region is disposed between the first source/drain region and the second source/drain region; (b) a gate dielectric region in direct physical contact with the channel region; and (c) a gate electrode region including a top portion and a bottom portion, wherein the bottom portion is in direct physical contact with the gate dielectric region, wherein a first width of the top portion is greater than a second width of the bottom portion, wherein the gate electrode region is electrically insulated from the channel region by the gate dielectric region, and wherein a first upper portion and a second upper portion of the first and second source/drain regions, respectively, are compressively strained. 
     The present invention provides a semiconductor structure, comprising (a) a semiconductor region including a channel region, a first source/drain region, and a second source/drain region, wherein the channel region is disposed between the first source/drain region and the second source/drain region; (b) a gate dielectric region in direct physical contact with the channel region; (c) a gate electrode region including a top portion and a bottom portion, wherein the bottom portion is in direct physical contact with the gate dielectric region, wherein a first width of the top portion is greater than a second width of the bottom portion, and wherein the gate electrode region is electrically insulated from the channel region by the gate dielectric region; and (d) an ion beam incident on the gate electrode region, wherein the ion beam comprises ions of a material selected from the group consisting of germanium and arsenic. 
     The present invention provides a semiconductor transistor with an expanded top portion of a gate or an expanded top portion of a source or drain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-10  show a first fabrication process of a semiconductor transistor with an expanded top portion of a gate, in accordance with embodiments of the present invention. 
         FIGS. 11-20  show a second fabrication process of a vertical semiconductor transistor with an expanded top portion of a gate, in accordance with embodiments of the present invention. 
         FIGS. 21-30  show a third fabrication of another semiconductor transistor with an expanded top portion of a gate, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-10  show a first fabrication process for forming a transistor structure  100 , in accordance with embodiments of the present invention, wherein  FIGS. 1-10  show cross-section views of the transistor structure  100 . 
     More specifically, with reference to  FIG. 1 , in one embodiment, the first fabrication process starts out with a silicon substrate  110 . 
     Next, with reference to  FIG. 2 , in one embodiment, two trenches  210  and  220  are formed in the silicon substrate  110 . Illustratively, the trenches  210  and  220  are formed using a conventional lithographic and etching process. 
     Next, with reference to  FIG. 3 , in one embodiment, two STI (Shallow Trench Isolation) regions  310  and  320  are formed in the two trenches  210  and  220  ( FIG. 2 ), respectively, using a conventional method. Illustratively, the two STI regions  310  and  320  comprise silicon dioxide. 
     Next, with reference to  FIG. 4 , in one embodiment, a gate dielectric layer  410  is formed on a top surface  111  of the silicon substrate  110 . Illustratively, the gate dielectric layer  410  comprises silicon dioxide. In one embodiment, the gate dielectric layer  410  is formed by thermal oxidation. 
     Next, with reference to  FIG. 5 , in one embodiment, a gate electrode region  510  is formed on the top surface  111  of the silicon substrate  110 . In one embodiment, the gate electrode region  510  is formed by (i) CVD (Chemical Vapor Deposition) of polysilicon everywhere on a top surface  412  of the structure  100  ( FIG. 4 ) to form a polysilicon layer (not shown), and then (ii) a conventional lithographic and etching process to etch the deposited polysilicon layer, resulting in the gate electrode region  510 , as shown in  FIG. 5 . 
     Next, with reference to  FIG. 6 , in one embodiment, extension regions  610  and  620  are formed in the silicon substrate  110 . Illustratively, the extension regions  610  and  620  are formed by ion implantation using the gate electrode region  510  as a blocking mask. 
     Next, with reference to  FIG. 7 , in one embodiment, halo regions  710  and  720  are formed in the silicon substrate  110 . Illustratively, the halo regions  710  and  720  are formed by ion implantation using the gate electrode region  510  as a blocking mask. 
     Next, with reference to  FIG. 8 , in one embodiment, dielectric spacers  810  and  820  are formed on side walls of the gate electrode region  510 . Illustratively, the dielectric spacers  810  and  820  are formed by (i) CVD of an insulating material, such as silicon dioxide or silicon nitride, or a composite, everywhere on top of the structure  100  of  FIG. 7 , and then (ii) directional etching back until the top surface  111  of the silicon substrate  110  and a top surface  511  of the gate electrode region  510  are exposed to the surrounding ambient. 
     Next, in one embodiment, source/drain regions  840  and  850  are formed in the silicon substrate  110 . Illustratively, the source/drain regions  840  and  850  are formed by ion implantation using the gate electrode region  510  and the dielectric spacers  810  and  820  as a blocking mask. 
     Next, in one embodiment, germanium atoms are implanted in a top portion  512  of the gate electrode region  510  by ion implantation in a direction indicated by arrows  830 . Hereafter, the implantation of germanium atoms in the top portion  512  of the gate electrode region  510  of  FIG. 8  can be referred to as a germanium implantation step  830 . Illustratively, the germanium implantation step  830  uses germanium atoms at a high dose (10 16  Ge atoms/cm 2 ) and at a low energy. The directions  830  can be vertical or tilted less than 10 degrees from vertical. As a result of the germanium implantation step  830 , the top portion  512  expands laterally, as shown in  FIG. 9A . 
     With reference to  FIG. 9A , it can be seen that as a result of the lateral expansion of the top portion  512 , a width  517  of the top portion  512  is greater than a width  516  of the bottom portion  515 . In one embodiment, the top portion  512  of the gate electrode region  510  is expanded laterally at least 20%. In other words, the width  517  is at least 120% of the width  516 . 
     Next, with reference to  FIG. 9B , in one embodiment, a metal (e.g., nickel, etc.) layer  910  is formed on top of the structure  100  of  FIG. 9A . Illustratively, the nickel layer  910  is formed by sputtering of nickel everywhere on top of the structure  100  of  FIG. 9A . 
     Next, with reference to  FIG. 10 , in one embodiment, silicide regions  512 ,  1010 , and  1020  are formed on top of the gate electrode region  510 , the source/drain regions  840  and  850 , respectively. Illustratively, the silicide regions  512 ,  1010  and  1020  comprise nickel silicide. In one embodiment, the silicide regions  512 ,  1010  and  1020  are formed by first annealing the whole structure  100  of  FIG. 9B  so that nickel of the nickel layer  910  chemically reacts with silicon of the gate electrode region  510 , the source/drain regions  840  and  850 , resulting in the silicide regions  512 ,  1010  and  1020 . Then, in one embodiment, unreacted nickel is removed by a wet etching step, resulting in structure  100  of  FIG. 10 . In one embodiment, the entire top portion  512  ( FIG. 9B ) of the gate electrode region  510  chemically reacts with Ni of the Ni layer  910  resulting in the silicide region  512  as sown in  FIG. 10 . 
     As can be seen in  FIGS. 8 ,  9 B, and  10 , because of the germanium implantation step  830  ( FIG. 8 ), an interfacing surface  514  between the nickel layer  910  and the top portion  512  of the gate electrode region  510  ( FIG. 9B ) is larger than the case in which the implantation step  830  is not performed. Therefore, it is easier for nickel (of the nickel layer  910 ) to react with silicon of the top portion  512  ( FIG. 9B ) than in the case the top portion of the gate electrode region  510  is not expanded. Also as a result of the top portion  512  being expanded laterally, the silicide region  512  ( FIG. 10 ) is more conductive than the case in which the top portion  512  of the gate electrode  510  is not expanded. 
       FIGS. 11-20  show a second fabrication process for forming a transistor structure  200 , in accordance with embodiments of the present invention. 
     More specifically, with reference to  FIG. 11 , in one embodiment, the second fabrication process starts out with an SOI (Silicon on Insulator) substrate  1110 . Illustratively, the SOI substrate  1110  comprises a silicon layer  1120 , a buried oxide layer  1130  on the silicon layer  1120 , and a silicon layer  1140  on the buried oxide layer  1130 . Illustratively, the SOI substrate  1110  is formed by a conventional method. In one embodiment, the SOI substrate  1110  may comprise an Ultra-Thin SOI wherein the silicon layer  1140  is less than 15 nm in thickness. 
     Next, in one embodiment, a dielectric hard mask layer  1150  is formed on top of the silicon layer  1140 . Illustratively, the dielectric hard mask layer  1150  is formed by CVD of silicon nitride or silicon dioxide, or a composite of the two, everywhere on top of the silicon layer  1140 . 
     Next, in one embodiment, a lithographic and etching step is performed to etch the dielectric hard mask layer  1150  and then the silicon layer  1140  so as to form a dielectric cap region  1151  and a fin region  1141 , respectively, as shown in  FIG. 12 . 
     With reference to  FIG. 12  (a front view of the structure  200 ), it should be noted that the dielectric cap region  1151  and the fin region  1141  are farther away from the viewer than the silicon layer  1120  and the buried oxide layer  1130 . 
     Next, with reference to  FIG. 13A , in one embodiment, a silicon dioxide layer  1310  is formed on side walls of the fin region  1141  of  FIG. 12 . Illustratively, the silicon dioxide layer  1310  is formed by thermal oxidation.  FIG. 13A  shows a front view of the structure  200  after the silicon dioxide layer  1310  is formed. In alternative embodiments,  1310  may comprise a high-k gate dielectric, such as hafnium silicate, deposited, for example, by means of CVD, MOCVD, ALD. 
     Next, with reference to  FIG. 13B , in one embodiment, a gate electrode region  1320  is formed on top of the dielectric cap region  1151  and on side walls of the silicon dioxide layer  1310 . Illustratively, the gate electrode region  1320  comprises polysilicon. In one embodiment, the gate electrode region  1320  is formed by (i) CVD of polysilicon everywhere on top of the structure  200  of  FIG. 13A , and then (ii) a conventional lithographic and etching process.  FIG. 13B  shows a front view of the structure  200  after the gate electrode region  1320  is formed. So, it should be noted that the silicon dioxide layer  1310  and the dielectric cap region  1151  are farther away from the viewer than the gate electrode region  1320 . 
     Next, in one embodiment, extension regions  1410  and  1420  and halo regions  1430  and  1440  (not shown in  FIG. 13B  but can be seen in  FIG. 14 ) are formed in the fin region  1141  of  FIG. 12  by ion implantation using the gate electrode region  1320  as a blocking mask. 
       FIG. 14  shows a top down view of the structure  200  of  FIG. 13B  along a line  14 - 14  after the formation of the extension regions  1410  and  1420  and halo regions  1430  and  1440 . 
     Next, in one embodiment, germanium atoms are implanted on a top portion  1321  ( FIG. 13B ) of the gate electrode region  1320  by ion implantation. Illustratively, germanium atoms are implanted at a high dose (10 16  Ge atoms/cm 2 ) and at a low energy. As a result of the germanium implantation in the top portion  1321  ( FIG. 13B ) of the gate electrode  1320 , the top portion  1321  expands laterally as shown in  FIG. 15 . 
     With reference to  FIG. 15 , it can be seen that as a result of the lateral expansion of the top portion  1321 , a width  1326  of the top portion  1321  is greater than a width  1325  of a bottom portion  1322 . In one embodiment, the top portion  1321  of the gate electrode region  1320  is expanded laterally at least 20%. In other words, the width  1326  is at least 120% of the width  1325 . 
     Next, with reference to  FIG. 16 , in one embodiment, a silicon dioxide layer  1610  is formed on top and side walls of the gate electrode region  1320 . Illustratively, the silicon dioxide layer  1610  is formed by thermal oxidation. Hereafter, expanded top portions  1620  and  1630  of the gate electrode region  1320  are referred to as overhangs  1620  and  1630 .  FIG. 16  shows a front view of the structure  200  after the silicon dioxide layer  1610  is formed (except for the silicon dioxide layer  1610  and the gate electrode region  1320  whose cross section view is shown). It should be noted that, the silicon dioxide layer  1310  and the dielectric cap region  1151  are farther away from the viewer than the silicon dioxide layer  1610  and the gate electrode region  1320 . 
     Next, with reference to  FIG. 17 , in one embodiment, dielectric spacers  1710  and  1720  are formed on side walls of the gate electrode region  1320  and under the overhangs  1620  and  1630 . Illustratively, the dielectric spacers  1710  and  1720  are formed by (i) CVD of a dielectric material, such as silicon dioxide, silicon nitride, or a composite of the two, everywhere on top of the structure  200  of  FIG. 16  to form a dielectric layer (not shown), and then (ii) directionally etching back the deposited dielectric layer. More specifically, the deposited dielectric layer is over etched so that the dielectric spacers  1710  and  1720  remain on side walls of the gate electrode region  1320  but no dielectric material remains on side walls of the silicon dioxide layer  1310 .  FIG. 17  shows a front view of the structure  200  after the dielectric spacers  1710  and  1720  are formed (except for the silicon dioxide layer  1610 , the gate electrode region  1320  and the dielectric spacers  1710  and  1720  whose cross section view is shown). 
     Next, in one embodiment, source/drain regions  1810  and  1820  (not shown in  FIG. 17  but can be seen in  FIG. 18 ) are formed in the fin region  1141  of  FIG. 18  by ion implantation using the gate electrode region  1320  and the dielectric spacers  1710  and  1720  as a blocking mask. 
       FIG. 18  shows a top down view of the structure  200  of  FIG. 17  along a line  18 - 18  after the formation of the source/drain regions  1810  and  1820 . 
     Next, with reference to  FIG. 19 , in one embodiment, the dielectric cap region  1151  of  FIG. 17  is removed by a Reactive Ion Etch (RIE), or a wet etching step, resulting in the structure  200  of  FIG. 19 . 
     Next, with reference to  FIG. 20 , in one embodiment, silicide regions  2010 ,  2020 , and  2030  are formed on top of the gate electrode region  1320  and the source/drain regions  1810  and  1820  ( FIG. 18 ). Illustratively, the silicide regions  2010 ,  2020 , and  2030  comprise silicide nickel. In one embodiment, the silicide regions  2010 ,  2020  and  2030  are formed by (i) sputtering of nickel everywhere on top of the structure  200  ( FIG. 19 ) to form a nickel layer (not shown), then (ii) annealing so that nickel of the deposited nickel layer chemically reacts with silicon of the gate electrode region  1320  and the source/drain regions  1810  and  1820  ( FIG. 18 ) resulting in the silicide regions  2010 ,  2020 , and  2030 . Then, unreacted nickel is removed by a wet etching step, resulting in structure  200  of  FIG. 20 . 
     Similar to the structure  100  of  FIG. 10 , the structure  200  of  FIG. 20  has an advantage of the enlarged silicide region  2010  which is more conductive than in the case in which the top portion  1321  of the gate electrode  1320  is not expanded laterally by the germanium implantation. Moreover, because the top portion  1321  of the gate electrode  1320  ( FIG. 19 ) is enlarged, it is easier for nickel of the deposited nickel layer (not shown) to chemically react with silicon of the gate electrode region  1320  to form the silicide  2010 . 
       FIGS. 21-30  show a third fabrication process for forming a transistor structure  300 , in accordance with embodiments of the present invention, wherein  FIGS. 21-30  show cross-section views of the transistor structure  300 . 
     More specifically, with reference to  FIG. 21 , in one embodiment, the third fabrication process starts out with an SOI substrate  2110 . In one embodiment, the SOI substrate  2110  comprises a silicon layer  2120 , a buried oxide layer  2130  on the silicon layer  2120 , and a silicon layer  2140  on the buried oxide layer  2130 . Illustratively, the SOI substrate  2110  is formed by a conventional method. 
     Next, with reference to  FIG. 22 , in one embodiment, a trench  2210  is formed in the silicon layer  2140 . In one embodiment, the trench  2210  is formed by a conventional lithographic and etching process. 
     Next, with reference to  FIG. 23 , in one embodiment, an STI region  2310  is formed in the trench  2210  ( FIG. 22 ) using a conventional method. Illustratively, the STI region  2310  comprises silicon dioxide. 
     Next, with reference to  FIG. 24 , in one embodiment, a gate dielectric layer  2410  is formed on top of the structure  300  ( FIG. 23 ). The gate dielectric layer  2410  may be formed (a) by oxidation and nitridation of a top portion of the silicon layer  2140 , to form a silicon oxinitride dielectric, or (b) by deposition of a high-k material such as hafnium silicate by CVD, MOCVD, or ALD. 
     Next, with reference to  FIG. 25 , in one embodiment, a polysilicon layer  2510  is formed on top of the structure  300  ( FIG. 24 ) by CVD. 
     Next, in one embodiment, the polysilicon layer  2510  is selectively etched, resulting in a gate electrode region  2511  as shown in  FIG. 26 . 
     Next, with reference to  FIG. 26 , in one embodiment, extension regions  2610  and  2620  and halo regions  2630  and  2640  are formed in the silicon layer  2140 . Illustratively, the extension regions  2610  and  2620  and halo regions  2630  and  2640  are formed by ion implantation using the gate electrode region  2511  as a blocking mask. Hereafter, a silicon region of the silicon layer  2140  which is disposed between the extension regions  2610  and  2620  and the halo regions  2630  and  2640  is referred to as a channel region  2140 . 
     Next, with reference to  FIG. 27 , in one embodiment, dielectric spacers  2710  and  2720  are formed on side walls of the gate electrode region  2511 . Illustratively, the dielectric spacers  2710  and  2720  are formed by (i) CVD of a dielectric layer, such as silicon dioxide or silicon nitride, or a composite of both, everywhere on top of the structure  300  of  FIG. 26 , and then (ii) directional etching back. Any remaining gate dielectric layer  2410  in the etched-back regions is completely removed by either sufficient over etch, or by and additional etching process, resulting in a gate dielectric region  2411 . 
     Next, with reference to  FIG. 28A , in one embodiment, silicon regions  2810  and  2820  are epitaxially grown on the extension regions  2610  and  2620 , respectively. 
     It should be noted that the silicon is also epitaxially grown on top of the gate electrode region  2511 . But to make the description simple, this is not shown. Alternatively, in one embodiment, before the formation of the silicon regions  2810  and  2820  by epitaxial growth, a cap region (not shown) can be formed on top of the gate electrode region  2511 . In one embodiment, the cap region (not shown) comprises a silicon dioxide layer and a silicon nitride layer (not shown). More specifically, the silicon dioxide layer and the silicon nitride layer (not shown) can be formed in that order on top of the polysilicon layer  2510  of  FIG. 25 . After that, the silicon dioxide layer and the silicon nitride layer (not shown) can be patterned at the same time that the gate electrode region  2511  is formed. As a result, portions of the silicon dioxide layer and the silicon nitride layer (not shown) still remain on top of the gate electrode region  2511 . Therefore, the cap region (not shown) can prevent epitaxial growth of the silicon on top of the gate electrode region  2511 . 
     Next, in one embodiment, the gate electrode region  2511  and the dielectric spacers  2710  and  2720  are used as a blocking mask to ion implant the silicon regions  2810  and  2820 , the extension regions  2610  and  2620  and the halo regions  2630  and  2640  so as to form source/drain regions  2811  and  2821  (as shown in  FIG. 28B ). 
     Next, in one embodiment, with reference to  FIG. 28B , germanium atoms are implanted in a top portion  2512  of the gate electrode region  2511  by ion implantation in a direction indicated by arrows  2830 . Hereafter, the implantation of germanium atoms in the top portion  2512  of the gate electrode region  2511  can be referred to as a germanium implantation step  2830 . Illustratively, the germanium implantation step  2830  uses germanium atoms at a high dose (10 16  Ge atoms/cm 2 ) and at a low energy. As a result of the germanium implantation step  2830 , the top portion  2512  expands laterally, as shown in  FIG. 29 . 
     With reference to  FIG. 29 , it can be seen that as a result of the lateral expansion of the top portion  2512 , a width  2519  of the top portion  2512  is greater than a width  2518  of a bottom portion  2514 . In one embodiment, the top portion  2512  of the gate electrode region  2511  is expanded laterally at least 20%. In other words, the width  2519  is at least 120% of the width  2518 . In one embodiment, the germanium implantation step  2830  also implants Germanium atoms in upper portions  2811   a  and  2821   a  of the source/drain regions  2811  and  2821 , respectively. As a result, the upper portions  2811   a  and  2821   a  are expanded laterally and compressively strained. Therefore, the channel region  2140  is tensile strained. 
     Next, with reference to  FIG. 30 , in one embodiment, silicide regions  2513 ,  2812  and  2822  are formed on top of the gate electrode region  2511 , the source/drain regions  2811  and  2821 , respectively. Illustratively, the silicide regions  2513 ,  2812 , and  2822  comprise silicide nickel. In one embodiment, the silicide regions  2513 ,  2811  and  2821  are formed by (i) CVD of nickel everywhere on top of the structure  300  ( FIG. 29 ) to form a nickel layer (not shown), then (ii) annealing so that the deposited nickel layer chemically reacts with silicon on top portions of the gate electrode region  2511 , the source/drain regions  2811  and  2821  so as to form the silicide regions  2513 ,  2812  and  2822 . Then, unreacted nickel is removed by a wet etching step, resulting in structure  300  of  FIG. 30 . 
     In the embodiments described above, germanium ions/atoms are implanted in the gates so as to expand the top portions of the gates. Alternatively, arsenic can be used instead of germanium. Also, in one embodiment, the germanium and arsenic ion implantations can be carried out at room temperature with the ions being at an energy of 25 KeV such that the ions can reach as deep as 23 nm in the gates. 
     In one embodiment, as a result of the Ge implantation in the top portion  512  ( FIG. 9A ), the top portion  1321  ( FIG. 13B ), the top portion  2512  ( FIG. 29 ), and in the top portions  2811   a  and  2821   a  ( FIG. 29 ), each of these portions  512 ,  1321 ,  2512 ,  2811   a , and  2821   a  is at least 0.5% compressively strained, meaning the average atom spacing of the resulting Si—Ge lattice is 0.5% less than the average atom spacing of a Si—Ge mixture of the same composition ratio in a relaxed/unstrained condition. 
     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.