Abstract:
A semiconductor structure. The structure includes (a) a fin region having (i) a first source/drain portion having a first surface and a third surface, wherein the first and third surfaces are (A) parallel to each other and (B) not coplanar, (ii) a second source/drain portion having a second surface and a fourth surface, wherein the second and fourth surfaces are (A) parallel to each other and (B) not coplanar, and (iii) a channel region; (b) a gate dielectric layer; (c) a gate electrode region, wherein the gate dielectric layer (i) is sandwiched between, and (ii) electrically insulates the gate electrode region and the channel region; and (d) first second strain creating regions on the third and fourth surfaces, respectively, wherein the first and second strain creating regions comprise a strain creating material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to transistors, having asymmetrically strained source/drain portions. 
     BACKGROUND OF THE INVENTION 
     In a conventional transistor, source/drain regions are usually etched, and then SiGe (silicon-germanium) or SiC (silicon-carbon) is epitaxially grown on source/drain portions of the fin region to provide strain into a channel region of the FET. However, the resulting structure usually does not have the optimum strain in the channel region. Therefore, there is a need for a transistor structure that provides strain in the channel region higher than that of prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a semiconductor structure, comprising (a) a fin region, wherein the fin region includes (i) a first source/drain portion having a first surface and a third surface, wherein the first and third surfaces are (A) parallel to each other and (B) not coplanar, (ii) a second source/drain portion having a second surface and a fourth surface, wherein the second and fourth surfaces are (A) parallel to each other and (B) not coplanar, and (iii) a channel region disposed between the first and second source/drain portions; (b) a gate dielectric layer in direct physical contact with the channel region; (c) a gate electrode region in direct physical contact with the gate dielectric layer, wherein the gate dielectric layer (i) is sandwiched between, and (ii) electrically insulates the gate electrode region and the channel region; and (d) a first semiconductor strain creating region and a second semiconductor strain creating region on the third and fourth surfaces, respectively, wherein the first and second semiconductor strain creating regions comprise a semiconductor strain creating material, and wherein no portion of the first and second surfaces is in direct physical contact with the semiconductor strain creating material. 
     The present invention provides a transistor structure in which the channel region has higher strain than that of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS.  1 A- 1 Eb (perspective and top-down views) illustrate a fabrication process for forming a vertical transistor (FinFET) structure, in accordance with embodiments of the present invention. 
       FIGS.  2 A- 2 Db (cross-section and top-down views) illustrate another fabrication process for forming another vertical transistor (FinFET) structure, in accordance with embodiments of the present invention. 
         FIGS. 3A-3E  (cross-section views) illustrate a fabrication process for forming a planar transistor structure, in accordance with embodiments of the present invention. 
         FIGS. 4A-4D  (cross-section views) illustrate another fabrication process for forming another planar transistor structure, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS.  1 A- 1 Eb (perspective views) illustrate a fabrication process for forming a vertical transistor (FinFET) structure  100 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1A , the fabrication of the vertical transistor structure  100  starts out with a structure including (i) a silicon (Si) substrate  110 , (ii) a BOX (Buried Oxide) layer  120  on top of the Si substrate  110 , (iii) a fin region  130  (comprising silicon in one embodiment) on top of the BOX layer  120 , (iv) a hard mask  140  (comprising silicon nitride in one embodiment) on top of the fin region  130 , (v) a gate electrode region  150  (comprising polysilicon in one embodiment) on top of the hard mask  140  and the BOX layer  120 , (vi) a dielectric cap region  151  (comprising SiO2 in one embodiment) on top of the gate electrode region  150 , and (vii) nitride spacers  160   a  and  160   b  (comprising silicon nitride in one embodiment) on side walls of the gate electrode region  150  and the dielectric cap region  151 . The vertical transistor structure  100  of  FIG. 1A  is formed by using a conventional method. 
     Next, with reference to  FIG. 1B , in one embodiment, a patterned dielectric (e.g., silicon nitride, etc.) covering layer  170  is formed on top of the structure  100  of  FIG. 1A . More specifically, the patterned nitride covering layer  170  is formed by using conventional lithographic and etching processes. 
     Next, in one embodiment, the fin region  130  is etched with the patterned nitride covering layer  170 , the hard mask  140 , and the nitride spacers  160   a  and  160   b  serving as a blocking mask. The etching of the fin region  130  is performed essentially without affecting the BOX layer  120 . As a result of the etching, the fin region  130  of  FIG. 1B  is reduced to a fin region  132  of  FIG. 1C . In other words, exposed surfaces of the fin region  130  are moved in a direction  133 . The etching of the fin region  130  can be isotropic. 
     Next, in one embodiment, SiGe (silicon-germanium) material can be epitaxially grown on exposed silicon surfaces of the structure  100  of  FIG. 1C , resulting in SiGe regions  180   a  and  180   b  of  FIG. 1D . 
     FIG.  1 Ea shows a cross-section view of the vertical transistor structure  100  of  FIG. 1D  along a plane defined by a line  1 Ea, in accordance with embodiments of the present invention. With reference to FIG.  1 Ea, the vertical transistor structure  100  comprises gate dielectric layers  132   a  and  132   b  between the gate electrode region  150  and the fin region  132 , extension regions  192   a  and  192   b  implanted in the fin region  132 , halo regions  194   a  and  194   b  implanted in the fin region  132 , and a channel region  196  between the two extension regions  192   a  and  192   b . It should be noted that the gate dielectric layers  132   a  and  132   b , the extension regions  192   a  and  192   b , the halo regions  194   a  and  194   b , and the channel region  196  are already present in the structure  100  of  FIGS. 1A-1D  but these layer and regions were not shown or mentioned above with reference to  FIGS. 1A-1D  for simplicity. In FIG.  1 Ea, in one embodiment, the SiGe regions  180   a  and  180   b  are on the same side of the fin region  132 . In the embodiments described above, the extension regions  192   a  and  192   b  and the halo regions  194   a  and  194   b  are formed early and are present even in  FIG. 1A  (though not shown in  FIG. 1A  for simplicity). Alternatively, the extension regions  192   a  and  192   b  and the halo regions  194   a  and  194   b  can be formed after the formation of the SiGe regions  180   a  and  180   b  ( FIG. 1D ). 
     The SiGe regions  180   a  and  180   b  will be parts of source/drain regions, which comprise source/drain portions  134   a  and  134   b , respectively, of the fin region  132  of the vertical transistor structure  100 . The presence of the SiGe regions  180   a  and  180   b  (also called strain creating regions  180   a  and  180   b ) in the vertical transistor structure  100  creates strain in the channel region  196  of the vertical transistor structure  100 . As a result, the strain in the channel region of the vertical transistor structure  100  is improved. It should be noted that this strain in the channel region  196  is created because the crystal lattice of the material of the strain creating regions  180   a  and  180   b  (i.e., SiGe) does not match the crystal lattice of the material of the channel region  196  (i.e., Si). In an alternative embodiment, the strain creating regions  180   a  and  180   b  can comprise SiC (mixture of silicon and carbon) for an NFET transistor. 
     FIG.  1 Eb shows a cross-section view of a vertical transistor structure  100 ′, in accordance with embodiments of the present invention. The vertical transistor structure  100 ′ is similar to the vertical transistor structure  100  in FIG.  1 Ea, except that SiGe regions  180   a ′ and  180   b ′ are on opposite sides of the fin region  132 . In order to form the SiGe regions  180   a ′ and  180   b ′ on opposite sides of the fin region  132 , the patterned nitride covering layer  170  (similar to the patterned nitride covering layer  170  in  FIG. 1C ) is formed in two opposite sides, and the step of etching the fin region  130  (similar to the etching step to form the structure of  FIG. 1C ) is performed such that exposed surfaces of the fin region  130  are moved in two opposite directions. It should be noted that a gate dielectric layer, extension regions, halo regions, and a channel region of the vertical transistor structure  100 ′ are omitted in FIG.  1 Eb for simplicity. The presence of the SiGe regions  180   a ′ and  180   b ′ in the vertical transistor structure  100 ′ improves the strain in the channel region of the vertical transistor structure  100 ′ and thereby improves the operation of the vertical transistor structure  100 ′. 
     FIGS.  2 A- 2 Db (cross-section views) illustrate a fabrication process for forming a vertical transistor (FinFET) structure  200 , in accordance with embodiments of the present invention. More specifically, the fabrication process starts out with a structure which is similar to the structure  100  of  FIG. 1C .  FIG. 2A  shows a cross-section view of the vertical transistor structure  100  of  FIG. 1C  along a plane defined by a line  2 A. It should be noted that similar regions of the structure  200  of  FIG. 2A  and the structure  100  of  FIG. 1C  have the same reference numerals, except for the first digit, which is used to indicate the figure number. For instance, a patterned dielectric (e.g., silicon nitride) covering layer  270  ( FIG. 2A ) and the patterned nitride covering layer  170  ( FIG. 1C ) are similar. 
     Next, in one embodiment, the patterned nitride covering layer  270  is removed by an etching step which is essentially selective to a hard mask  240 , a fin region  232 , and a BOX layer  220 , resulting in the structure  200  of  FIG. 2B . In one embodiment, the removal of the patterned nitride covering layer  270  can be achieved by an isotropic etch such as a wet etch or a plasma etch. 
     Next, in one embodiment, SiGe material can be epitaxially grown on exposed silicon surfaces of the structure  200  of  FIG. 2B , resulting in SiGe regions  280   c  and  280   d  of  FIG. 2C . 
     FIG.  2 Da shows a top-down view of structure  200  of  FIG. 2C , in accordance with embodiments of the present invention. For illustration and simplicity, only the SiGe regions  280   c ,  280   d ,  280   c ′, and  280   d ′, the fin region  232 , the BOX layer  220  are shown in FIG.  2 Da. The formations of SiGe regions  280   c ′ and  280   d ′ are similar to the formations of the SiGe regions  280   c  and  280   d . As can be seen FIG.  2 Da, in one embodiment, the fin region  232  was recessed at two places on one side (left side) of the fin region  232 . 
     The SiGe regions  280   c ,  280   d ,  280   c ′, and  280   d ′ will be parts of source/drain regions of the vertical transistor structure  200 . The presence of the SiGe regions  280   c  and  280   c ′ in the vertical transistor structure  200  creates strain in the channel region of the vertical transistor structure  200 . As a result, the strain in the channel region of the vertical transistor structure  200  is improved thereby improving the operation of the vertical transistor structure  200 . 
     FIG.  2 Db shows a top-down view of a vertical transistor structure  200 ′, in accordance with alternative embodiments of the present invention. The vertical transistor structure  200 ′ is similar to the vertical transistor structure  200  of FIG.  2 Da, except that the fin region  232  was recessed at two places on opposite sides of the fin region  232 . 
     The SiGe regions  280   c ,  280   c ′,  280   d , and  280   d ′ (also called expansion regions  280   c ,  280   c ′,  280   d , and  280   d ′) will be parts of source/drain regions of the vertical transistor structure  200 ′. The source/drain regions of the vertical transistor structure  200 ′ comprise source/drain portions  234   a  and  234   b  of the fin region  232  of the vertical transistor structure  200 ′. The source/drain portion  234   a  has surfaces  281  and  282  on which the SiGe regions  280   c  and  280   d  reside, respectively. The source/drain portion  234   b  has surfaces  281 ′ and  282 ′ on which the SiGe regions  280   c ′ and  280   d ′ reside, respectively. The surface  281  is not coplanar with surface  282 ′. Similarly, the surface  282  is not coplanar with either the surface  281 ′ or the surface  282 ′. The presence of the SiGe regions  280   c  and  280   c ′ in the vertical transistor structure  200 ′ creates strain in the channel region of the vertical transistor structure  200 ′. 
       FIGS. 3A-3E  (cross-section views) illustrate a fabrication process for forming a planar transistor structure  300 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 3A , the fabrication of the planar transistor structure  300  starts out with a structure including (i) a silicon substrate  310 , (ii) a gate dielectric layer  320  (comprising silicon dioxide in one embodiment) on top of the Si substrate  310 , (iii) a gate electrode region  330  (comprising polysilicon in one embodiment) on top of the gate dielectric layer  320 , (iv) a dielectric cap region  331  (comprising SiO2 in one embodiment) on top of the gate electrode region  330 , (v) nitride spacers  340   a  and  340   b  (comprising silicon nitride in one embodiment) on side walls of the gate electrode region  330  and the dielectric cap region  331 , and (vi) a dielectric covering layer  332  (comprising SiO2 in one embodiment) on top of all. The planar transistor structure  300  of  FIG. 3A  is formed by using a conventional method. 
     Next, with reference to  FIG. 3B , in one embodiment, a patterned photo-resist layer  311  is formed on top of the Si substrate  310 . In one embodiment, the patterned photo-resist layer  311  is formed by using a conventional lithographic process. 
     Next, in one embodiment, portions of the dielectric covering layer  332  not covered by the patterned photo-resist layer  311  are removed by a wet etching process. Then, the Si substrate  310  is etched with the patterned photo-resist layer  311 , the dielectric cap region  331 , and the nitride spacers  340   a  and  340   b  serving as a blocking mask, resulting in a trench  312  in the Si substrate  310 . The etching of the Si substrate  310  can be dry etching. The trench  312  is formed aligned with the nitride spacer  340   a . It should be noted that during the etching of the Si substrate  310 , the gate electrode region  330  is not etched because the gate electrode region  330  is protected by the dielectric cap region  331 . After that, the patterned photo-resist layer  311  can be removed using a wet etching process. 
     Next, in one embodiment, SiGe material can be epitaxially grown on exposed silicon surface of the trench  312  of  FIG. 3B , resulting in a SiGe region  350   a  of  FIG. 3C . It should be noted that SiGe material does not grow on the right side of the gate electrode region  330  because the Si substrate  310  on this right side is still covered/protected by the dielectric covering layer  332 . 
     Next, in one embodiment where the depths and thicknesses of the grown SiGe on both sides are asymmetric,  FIG. 3C  is modified to form  FIG. 3D . More specifically, with reference to  FIG. 3D , a patterned photo-resist layer  313  is formed on top of the SiGe region  350   a . In one embodiment, the patterned photo-resist layer  313  is formed by using a conventional lithographic process. 
     Next, in one embodiment, the remaining portions of the dielectric covering layer  332  ( FIG. 3C ) are removed. Then, the Si substrate  310  is etched with the patterned photo-resist layer  313 , the dielectric cap region  331 , and the nitride spacers  340   a  and  340   b  serving as a blocking mask, resulting in a trench  314  in the Si substrate  310 . The etching of the Si substrate  310  can be dry etching. The trench  314  is formed aligned with the nitride spacer  340   b . It should be noted that during the etching of the Si substrate  310 , the gate electrode region  330  is not etched because the gate electrode region  330  is protected by the dielectric cap region  331 . After the trench  314  is formed, the patterned photo-resist layer  313  can be removed using a wet etching process. 
     Next, in one embodiment, SiGe material can be epitaxially grown on exposed silicon surface of the trench  314  of  FIG. 3D , resulting in SiGe regions  350   a ′ and  350   b  of  FIG. 3E . In one embodiment, a thickness  352  of the SiGe region  350   a + 350   a ′ is greater than a thickness  354  of the SiGe region  350   b . It should be noted that during the formation of SiGe region  350   b , SiGe material also grows on the SiGe region  350   a  (on the left) resulting in the SiGe region  350   a′.    
     With reference to  FIG. 3E , the planar transistor structure  300  comprises extension regions  312   a  and  312   b  implanted in the Si substrate  310 , halo regions  314   a  and  314   b  implanted in the Si substrate  310 , and a channel region  316  between the two extension regions  312   a  and  312   b . It should be noted that the extension regions  312   a  and  312   b , the halo regions  314   a  and  314   b , and the channel region  316  may already be present in the structure  300  of  FIGS. 3A-3D  (in one embodiment) but these layer and regions are not shown or mentioned above with reference to  FIGS. 3A-3D  for simplicity. The SiGe regions  350   a + 350   a ′ and  350   b  will be parts of source/drain regions of the planar transistor structure  300 . The presence of the SiGe source/drain regions  350   a + 350   a ′ and  350   b  in the planar transistor structure  300  creates strain in the channel region  316  of the planar transistor structure  300 . As a result, the operation of the planar transistor structure  300  is improved. 
     In the embodiment described in  FIGS. 3A-3E  above, the source/drain regions  350   a + 350   a ′ and  350   b  comprise SiGe. Alternatively, one of the source/drain regions  350   a + 350   a ′ and  350   b  comprises Si, whereas the other of the source/drain regions  350   a + 350   a ′ and  350   b  comprises SiGe. 
       FIGS. 4A-4D  (cross-section views) illustrate a fabrication process for forming a planar transistor structure  400 , in accordance with embodiments of the present invention. More specifically, the fabrication process starts out with the structure  400  of  FIG. 4A . In one embodiment, the structure  400  of  FIG. 4A  is similar to the structure  300  of  FIG. 3A  (without the dielectric covering layer  332 ). It should be noted that similar regions of the structure  400  of  FIG. 4A  and the structure  300  of  FIG. 3A  have the same reference numerals, except for the first digit, which is used to indicate the figure number. For instance, a gate dielectric layer  420  ( FIG. 4A ) and the gate dielectric layer  320  ( FIG. 3A ) are similar. 
     Next, with reference to  FIG. 4B , in one embodiment, an extra nitride spacer  450  is formed on side wall of a nitride spacer  440   b . Illustratively, the extra nitride spacer  450  is formed by using a conventional process. 
     Next, with reference to  FIG. 4C , in one embodiment, the Si substrate  410  is etched with the extra nitride spacer  450 , the nitride spacers  440   a  and  440   b , and the dielectric cap region  431  serving as a blocking mask resulting in trenches  412  and  414  in the Si substrate  410 . The etching of the Si substrate  410  can be dry etching. The trenches  412  and  414  are formed aligned with the nitride spacer  440   a  and the extra nitride spacer  450 , respectively. 
     Next, in one embodiment, SiGe material can be epitaxially grown on exposed silicon surface of the trenches  412  and  414  of  FIG. 4C , resulting in SiGe regions  460   a  and  460   b , respectively, of  FIG. 4D . It should be noted that thicknesses  462  and  464  of the SiGe regions  460   a  and  460   b , respectively, are the same. 
     The SiGe regions  460   a  and  460   b  will be parts of source/drain regions of the planar transistor structure  400 . The presence of the SiGe source/drain regions  460   a  and  460   b  in the planar transistor structure  400  creates strain in channel region  422  of the planar transistor structure  400 . As a result, the operation of the planar transistor structure  400  is improved. 
     In the embodiment described in  FIGS. 4A-4D  above, the source/drain regions  460   a  and  460   b  comprise SiGe. Alternatively, one of the source/drain regions  460   a  and  460   b  comprises Si, whereas the other of the source/drain regions  460   a  and  460   b  comprises SiGe. 
     In the embodiments described above, SiGe material is used. Alternatively, compound semiconductors including silicon, carbon, germanium, etc are used. 
     It should be noted that in the embodiments described above, the SiGe regions will later be doped with dopants so that they can serve as parts of the source/drain regions of the transistors. 
     In the embodiments described above, SiGe is epitaxially grown resulting the regions  180   a  and  180   b  ( FIG. 1D ), regions  280   c  and  280   d  ( FIG. 2C ), region  350   a  ( FIG. 3C ), regions  350   a ′ and  350   b  ( FIG. 3E ), and regions  460   a  and  460   b  ( FIG. 4D ). Alternatively, instead of SiGe, any other strain-creating material can be used provided that the resulting regions create strain in the corresponding channel regions. For example, SiC (a mixture of silicon and carbon) can be used instead of SiGe to create optimal strain if the structure is to be an NFET. 
     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.