Abstract:
A field effect transistor and method of fabricating the field effect transistor. The field effect transistor, including: a gate electrode formed on a top surface of a gate dielectric layer, the gate dielectric layer on a top surface of a single-crystal silicon channel region, the single-crystal silicon channel region on a top surface of a Ge including layer, the Ge including layer on a top surface of a single-crystal silicon substrate, the Ge including layer between a first dielectric layer and a second dielectric layer on the top surface of the single-crystal silicon substrate.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of field effect transistors (FETs); more specifically, it relates to high-mobility p-channel field effect transistors (PFETs) and methods of fabricating high-mobility PFETs. 
   BACKGROUND OF THE INVENTION 
   Complimentary metal-oxide-silicon (CMOS) technology is used in many integrated circuits. CMOS technology utilizes n-channel metal-oxide-silicon field effect transistors (n-MOSFETs) often shortened to NFETs and p-channel metal-oxide-silicon field effect transistors (p-MOSFETs) often shortened to PFETs. Conventional NFETs and PFETs are well known in the art and comprise a source region and a drain region on opposite sides of a channel region formed in single-crystal silicon with a gate electrode formed on top of a gate dielectric layer which is itself formed on top of the channel region. 
   When NFETs and PFETs are used in high performance circuits, the PFETs need to be larger than the NFETs to overcome the difference in carrier mobility between NFETs and PFETs so as not to let the PFETs limit overall circuit switching speed. The hole mobility in PFETs is about 25% that of the electron mobility of NFETs. Larger PFETs require more silicon area and more power in a time when modern integrated circuits need to be smaller and consume less power in very many applications. 
   Therefore there is a need for both an improved PFET with high switching speed at reduced silicon area and power consumption compared to conventional PFETs and an NFET that may be fabricated simultaneously with the improved PFET. 
   SUMMARY OF THE INVENTION 
   The present provides both an improved PFET with high switching speed at reduced silicon area and power consumption compared to conventional PFETs by inducing stress in the PFET channel as well as an NFET that may be fabricated simultaneously with the improved PFET. 
   A first aspect of the present invention is a field effect transistor, comprising: a gate electrode formed on a top surface of a gate dielectric layer, the gate dielectric layer on a top surface of a single-crystal silicon channel region, the single-crystal silicon channel region on a top surface of a Ge comprising layer, the Ge comprising layer on a top surface of a single-crystal silicon substrate, the Ge comprising layer between a first dielectric layer and a second dielectric layer on the top surface of the single-crystal silicon substrate. 
   A second aspect of the present invention is a method of fabricating a field effect transistor comprising: (a) providing a single-crystal silicon substrate having a single-crystal Ge comprising layer formed on a top surface of the single-crystal silicon substrate and a single-crystal silicon layer formed on a top surface of the single-crystal Ge comprising layer; (b) forming a gate dielectric layer on a top surface of the single-crystal silicon layer; (c) forming a gate electrode on a top surface of the dielectric layer; (d) removing the single-crystal silicon layer to form a single crystal-silicon island and removing a less than whole portion of the single-crystal Ge comprising layer to form an island of single-crystal silicon under the gate electrode where the single-crystal silicon layer and the single-crystal Ge comprising layer are not protected by the gate electrode; (e) oxidizing an entire remaining portion of the single-crystal Ge comprising layer not protected by the gate electrode, and a less than whole portion of the single-crystal Ge comprising layer under the gate electrode to form a single-crystal Ge comprising island under the single-crystal silicon island and having a first dielectric layer on a first side and a second dielectric layer on second and opposite side of the single-crystal Ge comprising island, the first dielectric layer and the second dielectric layer each extending under the gate electrode; and (f) forming a polysilicon source region over the first dielectric layer and forming a polysilicon drain region over the second dielectric layer, the polysilicon source region and the polysilicon drain region abutting opposite sides of the single-crystal silicon channel island. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a cross-sectional view of a PFET  100  according to the present invention; 
       FIGS. 2A through 2P  are cross-sectional views illustrating fabrication of PFET  100  of  FIG. 1 ; 
       FIGS. 3A through 3D  are cross-sectional views illustrating fabrication of an NFET  300  of  FIG. 4  that may be fabricated alone or simultaneously with PFET  100  of  FIG. 1 ; and 
       FIG. 4  is a cross-sectional view of NFET  300  that may be fabricated alone or simultaneously with PFET  100  of  FIG. 1  according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a cross-sectional view of a PFET  100  according to the present invention.  FIG. 1  is a cross-section along the channel length direction of PFET  100 . In  FIG. 1 , PFET  100  includes a gate  105 ; an N-doped single-crystal silicon region  110  under gate  105 , a P-doped polysilicon source region  120 A abutting shallow trench isolation (STI)  115  (that bounds PFET  100  but is technically not part of PFET  100 ); a single-crystal silicon source region  125 A extending into single-crystal silicon region  110  (bounded by the dashed lines) and extending under gate  105 ; a P-doped polysilicon drain region  120 B abutting STI  115 ; and a P-doped single-crystal silicon drain region  125 B abutting polysilicon drain region  120 B and extending into single-crystal silicon region  110  (bounded by the dashed lines) and extending under gate  105 . PFET  100  further includes a buried dielectric layer  130 A under polysilicon source region  120 A and single-crystal silicon source region  125 A and extending from STI  115  to under gate  105 ; and a buried dielectric layer  130 B under drain region  120 B and single-crystal silicon drain region  125 B and extending from STI  115  to under gate  105 . PFET  100  still further includes a single-crystal Ge comprising layer  135  under single-crystal silicon region  110  and extending between buried dielectric layers  130 A and  130 B; an N-well  145  under buried dielectric layers  130 A and  130 B and Ge comprising layer  135 ; and a retrograde N-type ion-implant peak  140  in a single-crystal silicon N-well  145  (in a silicon substrate  150 ) under buried dielectric layers  130 A and  130 B and Ge comprising layer  135  and bounded by STI  115 . 
   It should be understood that polysilicon source region  120 A and single-crystal silicon source region  125 A are physically and electrically in contact and structurally and electrically comprise the source of PFET  100 . Likewise, it should be understood that polysilicon drain region  120 B and single-crystal silicon drain region  125 B are physically and electrically in contact and structurally and electrically comprise the drain of PFET  100 . 
   Gate  105  includes a gate dielectric layer  155  on a top surface  160  of single-crystal silicon region  110  and a P-doped or undoped polysilicon gate electrode  165  on a top surface  170  of gate dielectric layer  155  and a capping layer  175  on a top surface  180  of gate electrode  165 . Optional sidewall insulation layers  185 A and  185 B are formed on opposing sidewalls  190 A and  190 B respectively of gate electrode  165  and dielectric spacers  195 A and  195 B are formed on outer surfaces  200 A and  200 B respectively of corresponding sidewall insulation layers  185 A and  185 B. Gate dielectric layer  155  is illustrated in  FIG. 1  extending under spacers  195 A and  195 B. Alternatively, gate dielectric layer may extend partially or not at all under spacers  195 A and  195 B. 
   A channel region  205  is defined in single-crystal silicon region  110 . Channel region  205  may include a portion of adjacent to top surface  160  of substrate  150  between single-crystal silicon source region  125 A and single-crystal silicon drain region  125 B or channel region  205  may include all of single-crystal silicon region  110  between single-crystal silicon source region  125 A and single-crystal silicon drain region  1250 B. Single-crystal silicon region  110  extends under spacers  195 A and  195 B as illustrated in  FIG. 1  or may extend under and past spacers  195 A and  195 B toward STI  115 . 
   Buried dielectric layer  130 A includes a first region  210 A and a second region  215 A. Second region  215 A is thicker than first region  210 A. First region  210 A extends under polysilicon source region  120 A from STI  115  to meet second region  215 A under spacer  195 A. Second region  215 A extends from first region  210 A from under spacer  195 A to Ge comprising layer  135  under gate  105 . 
   Buried dielectric layer  130 B includes a first region  210 BA and a second region  215 B. Second region  215 B is thicker than first region  210 B. First region  210 B extends under polysilicon drain region  120 B from STI  115  to meet second region  215 A under spacer  195 B. Second region  215 B extends from first region  210 B from under spacer  195 A to Ge comprising layer  135  under gate  105 . 
   A top surface  220 A of second region  215 A slopes upward (toward surface  160  of substrate  150 ) from Ge comprising layer  135  to meet polysilicon source region  120 A under spacer  195 A. A bottom surface  225 A of second region  215 A slopes downward (away from surface  160  of substrate  150 ) from Ge comprising layer  135  to meet first region  210 A under spacer  195 A. A top surface  220 B of second region  215 B slopes upward from Ge comprising layer  135  to meet polysilicon drain region  120 B under spacer  195 B. A bottom surface  225 B of second region  215 B slopes downward from Ge comprising layer  135  to meet first region  210 B under spacer  195 B. 
   The upward slope of top surface  220 A of second region  215 A of buried dielectric layer  130 A and of top surface  220 B of second region  215 B of buried dielectric layer  130 B which is in the order of 50% percent from flat (relative to top surface  160  of substrate  150 ) imparts a stress of about 50 mega-pascals to about 1000 mega-pascals to the crystal lattice of single-crystal silicon region  110  and channel region  205 . Stress on silicon the silicon lattice of PFETs has been shown to increase the hole mobility and thus the drain current of the PFET which can be advantageously used to reduce the silicon area of a PFET required for a given PFET drain current rating. 
     FIGS. 2A through 2P  are cross-sectional views illustrating fabrication of PFET  100  of  FIG. 1 . In  FIG. 2A , single-crystal silicon substrate  150  has a Ge comprising layer  135  formed on a top surface  230  of a single-crystal silicon substrate  150  and a single-crystal silicon layer  240  formed on a top surface  235  of Ge comprising layer  135 . Single-crystal silicon substrates are also called mono-crystalline silicon substrates or bulk silicon substrates. In a first example, Ge comprising layer  135  comprises Si (1-X) Ge x  where X equals about 0.15 to about 0.5. In a second example, Ge comprising layer  135  comprises Si (1-X-Y) Ge X C Y  where X equals about 0.15 to about 0.5 and Y equals about 0 to about 0.1. A single-crystal SiGe layer may be epitaxially formed by low pressure chemical vapor deposition (LPCVD) using SiH 4  and GeH 4 . A single-crystal SiGeC layer may be epitaxially formed by LPCVD using a combination of SiH 4 , GeH 4  and CH 3 SiH 3  or C 2 H 6 . In one example Ge comprising layer  135  is about 10 nm to about 100 nm thick. A single-crystal silicon layer may be epitaxially formed by LPCVD using SiH 4  and/or H 2 . In one example single-crystal silicon layer  240  is about 5 nm to about 50 nm thick. 
   In  FIG. 2B , STI  115  is formed. STI  115  extends from a top surface  245  of single-crystal silicon layer  240  through single-crystal silicon layer  240 , through single-crystal Ge comprising layer  135  into substrate  150 . STI  115  may be formed by reactive ion etching (RIE) trenches through single-crystal Ge comprising layer  135  into substrate  150 , depositing an insulator such as SiO 2  or tetraethoxysilane (TEOS) oxide to fill the resultant trench and chemical-mechanical polishing (CMP) down to top surface  245  of single-crystal silicon layer  240  to remove excess insulator. 
   In  FIG. 2C , N-well  145  is formed in substrate  150  by ion implantation of an N-dopant such as arsenic or phosphorus. While N-well  145  is illustrated as extending below STI  145 , N-well  145  may be about even with or shallower than the STI. 
   In  FIG. 2D , a retrograde ion implantation is performed using an N-dopant such as arsenic. A retrograde ion implant is defined as an ion implant having a peak concentration below a surface of the material into which the ion implantation is performed. Peak  140  of the retrograde ion implant is located a distance D below top surface  235  of Ge comprising layer  240 . 
   In  FIG. 2E , gate dielectric layer  155  is formed on top surface  245  of single-crystal silicon layer  240 . In one example, gate dielectric layer  155  comprises deposited or thermal SiO 2 , but may be any gate dielectric known in the art. An N-doped or undoped polysilicon layer  250  is formed on top surface  170  of gate dielectric layer  155 . Polysilicon may be formed by CVD using SiH 4  (and optionally AsH 4  or PH 4  if the gate is to be doped at this point in the fabrication). Capping layer  175  is formed on a top surface  255  of polysilicon layer  250 . In one example, capping layer  175  comprises a TEOS oxide layer over a thermal SiO 2  layer. 
   In  FIG. 2F , a photolithography process is performed and capping layer  175  is patterned and used as a hard mask to etch away undesired portions of polysilicon layer  250  (see  FIG. 2E ) to form gate electrode  165  under remaining capping layer  175 . 
   In  FIG. 2G  an optional sidewall isolation layer  185  is formed on sidewalls  190  of gate electrode  165 . Then an optional P-dopant extension ion implant using, for example, boron and/or an optional N-dopant halo ion implant using, for example, arsenic is performed to form extension/halo regions  260  in single-crystal silicon layer  240 . Extension and halo implants may be performed at an angle of other than 90° relative to top surface  245  of single-crystal silicon layer  240 . The halo and extension implants are performed such that, while they extend under gate electrode  165 , they will not extend as far as thick regions  215 A and  215 B of respective buried dielectric layers  130 A and  130 B extend under the gate electrode (see  FIG. 1 ). The halo and extension implants are shallow implants and do not extend below Ge comprising layer  135 . 
   Alternatively, the extension and/or halo ion implants may be performed after formation of gate electrode  165  but before formation of sidewall isolation layer  185 . 
   In  FIG. 2H , spacers  195  are formed on outer surfaces  200  of sidewall insulation layer  185 . Spacers  195  may comprise Si 3 N 4 , SiO 2 , or combinations thereof. For example, spacers  195  may comprise a multiple overlaid spacers, each spacer formed from either SiO 2  and Si 3 N 4 . Further, one or both of the halo and extension ion implants discussed supra, may be alternatively, performed after formation of spacers  195 . Spacers are formed by depositing a conformal layer of material and then performing an RIE process. Gate dielectric layer  155 , not protected by gate electrode  165  and spacers  195  may also be removed by the RIE process or another process. 
   In  FIG. 21 , portions of single-crystal silicon layer  240  not protected by gate electrode  165  and spacers  195  are removed. Also Ge comprising layer  135  is etched to recess the Ge comprising layer in regions where single-crystal silicon layer  240  were removed, so that Ge comprising layer  135  is thinner in these regions than under gate electrode  165  and spacers  195 . In one example, Ge comprising layer  135  is thinned to half its original thickness where not protected by gate electrode  165  and spacers  195 . In a second example, Ge comprising layer  135  is thinned to between about 5 nm to about 50 nm where not protected by gate electrode  165  and spacers  195 . The etching of single-crystal silicon layer  240  and Ge comprising layer  135  may be accomplished using an RIE process that selectively etches Si, SiGe and SiGeC relative to the material of capping layer  175 , spacers  195 , and STI  115 . In the example that capping layer  175  spacers  195  and STI  115  are forms of silicon oxide, a suitable RIE process would utilize a mixture of CF 4  and O 2 . 
   In  FIG. 2J , Ge comprising layer  135  is oxidized to form buried dielectric layer  130  which comprises oxides of Si and Ge. In one example, an oxidation at about 600° C. or less using a mixture of H 2 O vapor and O 2  is performed. Under these conditions, single-crystal SiGe and single-crystal SiGeC oxidize about 40 times faster than single-crystal silicon. During oxidation, the volume of the oxidized SiGe or SiGeC about doubles with about 40% of the volume being below the original surface and about 60% of the volume being above the original surface. Also, Ge comprising layer  135  oxidizes horizontally under spacers  195  and gate electrode  165  a distance equal to the thickness of oxidized SiGe or SiGeC formed where Ge comprising layer  135  was not protected by gate electrode  165  and spacers  195 . It should also be remembered that Ge comprising layer  135  was thicker under spacers  195  and gate electrode  165  than where the Ge comprising layer was exposed. Therefore, buried dielectric layer  130  includes a thick region  215  under spacers  195  and extending partially under gate electrode  165  and a thin region  210  where buried dielectric layer  130  is not under spacers  195  and gate electrode  165 . In one example thin region  210  of buried dielectric layer  130  is about 10 nm to about 100 nm thick, thick region  215  of buried dielectric layer  130  is about 10 nm to about 200 nm thick and extends under spacers  195  about 10 nm to about 200 nm. 
   After the oxidation, the only remaining Ge comprising layer  135  is an island under gate electrode  165 . Also a thin layer of SiO 2    265  is formed on exposed edges of single-crystal silicon layer  240 . A effect of the oxidation process is that regions of single-crystal silicon layer  240  between thick region  215  of buried dielectric layer  130  and gate dielectric layer  155  under spacers  195  are strained, that is, the crystal lattice is distorted from normal. 
   In  FIG. 2K , thin layer of SiO 2    265  (see  FIG. 2J ) is removed to expose edges  270  of single-crystal silicon region  240 . 
   In  FIG. 2L , epitaxial silicon regions  275  are grown on edges  270  (see  FIG. 2K ) of single-crystal silicon region  240 . As described supra, epitaxial Si may be grown by LPCVD using SiH 4 . 
   In  FIG. 2M , a polysilicon layer  280  is formed of sufficient thickness to cover capping layer  175  and spacers  195 . As described supra, polysilicon layer  280  may be doped P-type or undoped. Epitaxial regions  275  on single-crystal silicon layer  240  (see  FIG. 2L ) may increase in size slightly and single-crystal silicon region  110  results (see also  FIG. 1 ). 
   In  FIG. 2N , a CMP process is performed so that a top surface  285  of polysilicon layer  280  is coplanar with a top surface  290  of capping layer  175 . 
   In  FIG. 20 , a RIE etch back process is performed, so that polysilicon layer  280  (see  FIG. 2N ) is removed from spacers  195 , exposed ends of gate dielectric layer  155  and atop surface  295  of STI  115 . Polysilicon layer  280  remains in the space defined by single-crystal silicon region  110 , buried dielectric layer  130  and STI  115 . 
   In  FIG. 2P , an optional P-type (for example boron) ion implantation is performed to form P-doped polysilicon source/drains  120  in remaining polysilicon layer  280  (see  FIG. 280 ). The P-type ion implant may also be used to dope gate electrode  165 . If polysilicon layer  280  was P-doped as deposited, this P-type ion implantation may be eliminated or not depending upon whether it is desired to P-type ion implant gate electrode  165 . 
   Returning to  FIG. 1 , the structure of PFET  100  improves several operational parameters of the PFET. First, the relatively shallow single-crystal silicon region  110  under gate electrode  165 , particularly near sidewalls  190 A and  190 B of the gate electrode, result in improved short channel characteristics such as decreased sub-threshold voltage swing (S SWING ), decreased drain induced barrier loading, and more precise threshold voltage (V T )control. Second, the relatively deep polysilicon source and drain regions  120 A and  120 B result in lower source/drain resistance. Third, buried dielectric layers  130 A and  130 B lower source/drain capacitance (compared to a conventional bulk silicon PFET). Fourth, Ge comprising layer  135  between second region  215 A of buried dielectric layer  130 A and second region  215 B of buried dielectric layer  130 B (because of the high Ge doping levels) allows control of V T  by voltage biasing N-well  145 . These improved operating parameters have been experimentally shown to result in a significantly faster PFET (when compared to a conventional bulk silicon PFET of about the same channel width and channel length as a PFET of the present invention) and results in up to about a 42% increase in drain region current at saturation (I DSAT ) on short channel length devices. Fabrication of a PFET according to the present invention is essentially complete. 
     FIGS. 3A through 3D  are cross-sectional views illustrating fabrication of an NFET  300  (see  FIG. 4 ) that may be fabricated alone or simultaneously with PFET  100  (see  FIG. 1 ). by several changes to the PFET process described supra. Before describing these changes, it should be understood, that it is well known in the art, that when both PFETs and NFETs are being fabricated on the same substrate, that the PFETs are protected from ion implantation during ion implants required only for the NFETs and that NFETs are protected from ion implantation during ion implants required only for the PFETs. Often this protection is provided by a photo resist layer. Thus, it should be understood in the description that follows, that such steps have taken place relative to the PFET and that such steps would also have taken place relative to an NFET is the previous description of formation of a PFET if PFETs and NFETs are being simultaneously fabricated according to the present invention. 
   Fabrication of NFET  300  (see  FIG. 4 ) alone or simultaneously with PFET  100  (see  FIG. 1 ) is similar to the fabrication of PFET  100  (see  FIG. 1 ) illustrated in  FIGS. 2A through 2M  and described supra, with the differences described immediately infra 
   In  FIG. 2C , N-well  145  is replaced by a P-Well formed by ion implantation of a P-dopant such as boron. In  FIG. 2D , the N-doped retrograde ion implantation is replaced with a P-dopant retrograde ion implantation using a P-dopant species such as boron. In  FIG. 2G , the P-dopant extension ion implantation is replaced with an N-dopant extension ion implantation using an N-dopant species such as arsenic and the optional N-dopant halo ion implantation is replaced with a P-dopant ion extension ion implantation using a P-dopant species such as boron. 
   Between the processes illustrated in  FIGS. 21 and 2J , the processes illustrated in  FIGS. 3A and 3B  are performed. In  FIG. 3A , a directional RIE is performed to remove thin region  210  of buried dielectric layer  130  not protected by spacer  195 , capping layer  175  and gate electrode  165 . Also capping layer  175  may be alternatively formed from Si 3 N 4  or layers of Si 3 N 4  and SiO 2 . In  FIG. 3B , a isotropic silicon etch is performed to remove exposed portions of silicon substrate and undercut thick regions  215  of dielectric layer  130 . STI  115  is not undercut. Removing silicon from under undercut thick regions  215  of dielectric layer  130  removes most or all of the stress previously induced into single-crystal silicon region  110  and channel region  205  (see  FIG. 4 ). 
   For an NFET,  FIG. 2L  is replaced with  FIG. 3C  and  FIG. 20  is replaced with  FIG. 3D . In  FIG. 3C , epitaxial silicon regions  275  are grown on edges  270  (see  FIG. 2K ) of single-crystal silicon region  240  and an epitaxial layer  285  is grown on exposed surface of silicon substrate  215 . As described supra, epitaxial Si may be grown by LPCVD using SiH 4 . In  FIG. 3D , a RIE etch back process is performed, so that polysilicon layer  280  (see  FIG. 2N ) is removed from spacers  195 , exposed ends of gate dielectric layer  155  and a top surface  295  of STI  115 . A polysilicon layer  290  remains in the space defined by single-crystal silicon region  110 , thick region  215  of buried dielectric layer  130 , epitaxial layer  285  and STI  115 . 
   In  FIG. 2P , the optional P-type ion implantation is replaced with an optional N-type ion implantation (for example using arsenic) to form N-doped source/drains  120 . Fabrication of an NFET according to the present invention is essentially complete. 
     FIG. 4  is a cross-sectional view of NFET  300  that maybe fabricated alone or simultaneously with the PFET  100  of  FIG. 1 , according to the present invention.  FIG. 4  is similar to  FIG. 1 , except for several differences. First, single crystal region  110  is P-doped, instead of N-doped, source and drain regions  120 A and  120 B are N-doped instead of P-doped, single crystal regions  125 A and be are N-doped instead of P-doped, N-well  145  is replaced with a P-well  145 . Second, structurally, only thick regions  215 A and  215 B of respective dielectric layers  130 A and  130 B, epitaxial layers  285 A and  285 B intervene between respective polysilicon source/drain regions  120 A and  120 B and silicon substrate  150  rather than respective thin regions  210 A and  210 B (see  FIG. 1 ) of dielectric layers  130 A and  130 B, and epitaxial layers  285 A and  285 B extend under respective thick regions  215 A and  215 B of dielectric layers  130 A and  130 B. Source/drain dopant species from source  120 A and drain  120 B may or may not extend into respective epitaxial layers  285 A and  285 B. 
   Thus the present invention provides both an improved PFET with high switching speed at reduced silicon area and power consumption compared to conventional PFETs and an NFET that may be fabricated simultaneously with the improved PFET. 
   The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.