Abstract:
Field effect transistors and method for forming filed effect transistors. The field effect transistors including: a gate dielectric on a channel region in a semiconductor substrate; a gate electrode on the gate dielectric; respective source/drains in the substrate on opposite sides of the channel region; sidewall spacers on opposite sides of the gate electrode proximate to the source/drains; and wherein the sidewall spacers comprise a material having a dielectric constant lower than that of silicon dioxide and capable of absorbing laser radiation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor devices and methods of fabricating semiconductor devices; more specifically, it relates to field effect transistors with low-k sidewall spacers and the method of fabricating field effect transistors with low-k sidewall spacers. 
     BACKGROUND 
     A metal-oxide-semiconductor (MOS) field effect transistor (FET) comprises a gate electrode separated from a semiconductor substrate by a gate dielectric which is above a channel region in the substrate. A source region and a drain region are located at the ends of the channel region. The “ON” or “OFF” states of the transistor is controlled by the voltage applied at the gate electrode. Sidewall spacers are formed at the sidewalls of the gate electrodes to isolate the gate electrode from the source/drain (S/D) contacts. The parasitic capacitance between the gate and the S/D contacts adversely affects the transistor performance. As CMOS transistor feature sizes are shrunk, the sidewall spacer thickness is reduced as well, resulting in increased degradation of device performance due to the parasitic capacitance. Accordingly, there exists a need in the art to mitigate the deficiencies and limitations described hereinabove. 
     SUMMARY 
     A first aspect of the present invention is a device, comprising: a gate dielectric on a channel region in a semiconductor substrate; a gate electrode on the gate dielectric; respective source/drains in the substrate on opposite sides of the channel region; sidewall spacers on opposite sides of the gate electrode proximate to the source/drains; and wherein the sidewall spacers comprise a material having a dielectric constant lower than that of silicon dioxide and capable of absorbing laser radiation. 
     A second aspect of the present invention is a method, comprising: forming a gate dielectric on a channel region in a semiconductor substrate; forming a gate electrode on the gate dielectric; after forming the gate, forming respective source/drains in the substrate on opposite sides of the channel region; after forming the source/drains, depositing a conformal absorption layer having a dielectric constant lower than that of silicon dioxide and capable of absorbing laser radiation; irradiating the absorption layer with laser radiation; and after the irradiating, forming sidewall spacers on opposite sides of the gate electrode proximate to the source/drains from the conformal absorption layer. 
     These and other aspects of the invention are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE 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: 
         FIGS. 1 through 8  are cross-sectional views illustrating fabrication of a field effect transistor (FET) according to embodiments of the present invention; and 
         FIG. 9  is cross-sectional view illustrating the structure of  FIG. 8  when fabricated on a silicon-on-insulator (SOI) substrate according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Laser annealing is a process for dopant activation, while maintaining shallow source/drain junctions. An absorption layer covering the substrate is needed to facilitate a uniform heating of the substrate. The laser energy is absorbed by the absorption layer which heats up and then the heat is transferred into the substrate allowing dopant species to diffuse through the crystal lattice of the substrate and insert into the crystal lattice of the substrate. In embodiments of the present invention the absorption layer is a low dielectric constant (low-k) material that is used, after the laser annealing, to form sidewall spacers on the sidewalls of the gate electrodes, which not only reduces the gate to source/drain parasitic capacitance but also eliminates the need to completely remove the absorption layer which is extremely difficult, if not impossible to do without damaging other device structures. A low-k material is a material that has a relative permittivity of about 2.4 or less. 
       FIGS. 1 through 8  are cross-sectional views illustrating fabrication of a field effect transistor (FET) according to embodiments of the present invention. In  FIG. 1 , a semiconductor substrate  100  having a top surface  105  is provided. Top surface  105  defines a horizontal direction and a vertical direction is defined as a direction perpendicular to top surface  105 . Horizontal surfaces are surfaces in planes parallel to a plane defined by top surface  105 . Vertical surfaces are surfaces in planes perpendicular to the plane defined by top surface  105 . In one example, semiconductor substrate  100  is a bulk single-crystal silicon substrate. Formed in substrate  100  is trench isolation  110 . A top surface  112  of trench isolation  110  is coplanar with top surface  105  of substrate  100 . Trench isolation  110  may be formed, for example, by etching trenches into substrate  100 , blanket depositing a dielectric material (e.g., silicon dioxide (SiO 2 )) to fill the trenches and performing a chemical-mechanical polishing (CMP) to remove excess dielectric from top surface  105 . Formed in substrate  100  is an optional well  113 . Well  113  may be doped P type when p-channel FETs (PFETs) are to be fabricated or N-type when n-channel FETs (NFETs) are to be fabricated. When well  113  is not present, substrate  100  may be doped P type when PFETs are to be fabricated or N-type when NFETs are to be fabricated. An example of a P-type dopant is boron. Examples of N-type dopants are arsenic and phosphorous. 
     Formed on top surface  105  is a gate dielectric layer  115  and formed on gate dielectric layer  115  is a gate electrode  120 . In one example, gate dielectric layer  115  may comprise SiO 2 , silicon nitride (Si 3 N 4 ), silicon oxynitride (SiON), a high dielectric constant (high-k) material or combinations thereof. Examples of high k materials include but are not limited to metal oxides such as Ta 2 O 5 , BaTiO 3 , HfO 2 , ZrO 2 , Al 2 O 3 , or metal silicates such as HfSi x O y  or HfSi x O y N z . A high K dielectric material has a relative permittivity above about 10. In one example, gate electrode  120  may comprise amorphous silicon, polycrystalline silicon a metal (e.g., aluminum, tungsten and titanium), metallic compounds (e.g., tungsten nitride, titanium nitride, tungsten silicide, nickel silicide, tungsten silicide and cobalt silicide) or combinations thereof. Metal silicides may be a layer on a top region on the gate electrode or the entire gate conductor may be formed of a metal silicide. Metal silicides may be doped with N or P type or they may be undoped. In one embodiment, the gate electrode comprises polycrystalline silicon. 
     In  FIG. 2 , an optional conformal dielectric layer (e.g., SiO 2 )  125  is formed on top surface  105  (not protected by gate dielectric layer  115 ), top surface of trench isolation  110 , a top surface  126  and sidewalls  127  of gate electrode  120  and edges of gate dielectric layer  115 . Dielectric layer  125  may be formed by thermal oxidation or chemical-vapor-deposition (CVD). An angled (an acute angle relative to top surface  105 ) ion implantation of a dopant species X is performed to form pre-source/drain extension regions  130  in well  113  (or substrate  100 , if no well is present) while substrate  100  is rotated about an axis  135  that is perpendicular to top surface  105 . Dopant species X is an opposite dopant type than the dopant type of well  113  or substrate  100 , if no well is present). The angled implantation allows pre-source/drain extensions  130  to extend under gate electrode  120 . Pre-source/drain extensions  130  are separated by a channel region  140  under gate electrode  120  (which is doped an opposite type to the pre-source/drain extensions). Alternatively, pre-source/drain extensions  130  may be formed by plasma doping or plasma immersion ion implantation. 
     Ion implantation is a process wherein ions are extracted from a plasma source, accelerated by a high voltage, passed through a magnetic field to select specific dopant ions of a particular weight, discussed into a beam and then the bean scanned across a wafer. Plasma immersion ion implantation is a process wherein ions, including dopant ions are formed in a plasma positioned over the wafer, and accelerated to the wafer by a DC voltage. Plasma immersion ion implantation is not as selective as ion implantation as to species implanted and cannot implant to the depths capable by ion implantation. Therefore, an ion implantation process as used herein and in the claims does not include plasma immersion ion implantation processes as used herein and in the claims despite the similarity of names. 
     Preferably, dopants are confined in a region with a depth D 1  measured from top surface  105  in well  113  (or substrate  100 , if well  113  is not present). In one example D 1  is less than about 100 nm, more preferably, is less than about 50 nm, and most preferably is less than about 20 nm. Dopants may also be incorporated into gate electrode  120  during the doping process. It is well known that integrated circuit chips include many FETs and many integrated circuit chips are fabricated on a single integrated circuit. Across-chip and across-wafer uniformity for these values for D 1  is extremely difficult or nearly impossible to achieve by ordinary activation processes such as rapid thermal anneal (RTA), other rapid thermal processes (RTP) and direct laser anneal because of the non-uniform heating of the substrate do to varying topology across the integrated circuit chips. 
     In  FIG. 3 , sidewall spacers  145  (e.g., Si 3 N 4 ) are formed on the vertical surfaces of dielectric layer  125  (or are formed on vertical surfaces of gate electrode  120  if dielectric layer  125  is not present). Si 3 N 4  sidewall spacers  145  may be formed by a deposition (e.g., CVD) of a conformal Si 3 N 4  layer, followed by an anisotropic etch (e.g., a reactive ion etch (RIE)) to remove the Si 3 N 4  layer from horizontal surfaces (those parallel to top surface  105 ). A perpendicular (relative to top surface  105 ) ion implantation of a dopant species Y is performed to form pre-source/drains regions  150  in well  113  (or substrate  100 , if no well is present). Dopant species Y is a same dopant type as pre-source/drain extensions  130 ). Pre-source/drains  150  do not extend under gate electrode  120 . Alternatively, pre-source/drains  150  may be formed by plasma doping or plasma immersion ion implantation. Again dopants may also be incorporated into gate electrode  120  during the doping process. 
     In  FIG. 4A , sidewall spacers  145  (see  FIG. 3 ) are removed, for example, using hot phosphoric acid, and an absorption layer  155  is deposited. In one example, the material of absorption layer  155  has a dielectric constant lower than the dielectric constant of silicon dioxide. Absorption layer  155  will absorb electromagnetic radiation in the laser annealing step of  FIG. 5  to activate the dopants in pre-source/drain extensions  130  and pre-source/drains  150 . Preferably, absorption layer  155  has a dielectric constant of less than about 3.4, more preferably less than about 3.0, and most preferably less than about 2.6. In one example, absorption layer  155  comprises amorphous carbon deposited by plasma enhanced chemical vapor deposition (PECVD) as described in US patent applications 2005/0074956 and 2005/0074986 and hereby incorporated by reference. In one example, absorption layer  155  is a carbon film deposited by high density plasma chemical vapor deposition (HDPCVD) as described in U.S. Pat. No. 6,423,384 and hereby incorporated by reference. In one example, absorption layer  155  is between about 800 Å and about 1500 Å thick. In one example, absorption layer  155  is between about 800 Å and about 1200 Å thick. 
       FIG. 4B  illustrates an optional step in the event that a thicker absorption layer is required. In  FIG. 4B , a thin dielectric liner  160  (e.g., SiO 2 ) is deposited on absorption layer  155  and then a second absorption layer  165  is deposited on dielectric liner  160 . Preferably, absorption layer  165  has a dielectric constant of less than about 3.4, more preferably less than about 3.0, and most preferably less than about 2.6. In one example, absorption layer  165  comprises amorphous carbon deposited by PECVD as described in US patent applications 2005/0074956 and 2005/0074986. In one example, absorption layer  165  is a carbon film deposited by HDPCVD as described in U.S. Pat. No. 6,423,384. Absorption layer  165  may be the same material or a different material as absorption layer  155 . In one example, absorption layer  165  is between about 800 Å and about 1500 Å thick. In one example, absorption layer  165  is between about 800 Å and about 1200 Å thick. Absorption layer  165  may be formed by the same method or a different method as absorption layer  155 . 
       FIG. 5  continues from  FIG. 4A . In  FIG. 5 , a laser annealing process is performed to activate dopants in pre-source/drain extension  130  and pre-source/drains  150 . Absorption layer  155  absorbs laser radiation  157  and transfers the thermal energy to the underlying structures. The power, pulse, and dose of the laser irradiation may be configured to activate the dopants with minimal out-diffusion. In one example, the wavelength of the laser irradiation is between about 600 nm and about 1000 nm. In one example, the wavelength of the laser irradiation is between about 808 nm and about 810 nm. Dopants in gate electrode  120  may also be activated during the same laser annealing process. 
     It should be understood that the laser annealing described in reference to  FIG. 5  as applied to the structure of  FIG. 4A  may also be applied to the structure of  FIG. 4B . 
       FIG. 6  continues from  FIG. 5  and shows the effect of the laser annealing. After laser annealing source/drains  170  having source/drain extensions  175  have been formed from the pre-source/drain extensions  130  and pre-source/drains  150  of  FIG. 4A  or  4 B. Also, if dielectric layer  160  and absorption layer  165  have been formed (see  FIG. 4B ) they are removed in  FIG. 5 . Absorption layer  165  may be removed, for example, using an RIE process selective over oxide and dielectric layer  160  may be removed using an RIE process selective over carbon. 
     In  FIG. 7 , absorption layer  155  of  FIG. 4A  or  4 B has been etched (e.g., using an RIE) to form low-k final sidewall spacers  155 A on vertical surfaces of dielectric layer  125  (or gate electrode  120  if dielectric layer  125  is not present) and expose horizontal surfaces of dielectric layer  125  (or source/drains  170  and trench isolation  110  if dielectric layer  125  is not present). 
     In  FIG. 8 , if dielectric layer  125  is present, it is removed where not protected by sidewall spacers  155 A. Then a metal silicide layer  180  is formed on exposed surfaces of gate electrode  120  (in the event it is silicon) and source/drains  170 . Metal silicide is not formed ion sidewall spacers  155 A or trench isolation  110 . A metal silicide is formed by depositing a metal layer on silicon, heating the silicon to a temperature sufficient to cause a reaction between the metal and silicon, followed by an etch process to remove unreacted metal layer. It is advantageous that the deposition, heating and etching be performed in inert or reducing environments and not oxidizing environments to prevent damage to sidewall spacers  155 A. 
       FIG. 9  is cross-sectional view illustrating the structure of  FIG. 8  when fabricated on an SOI substrate according to embodiments of the present invention.  FIG. 9  is similar to  FIG. 8  except bulk substrate  100  of  FIG. 8  is replaced with an SOI substrate  185 , SOI substrate  185  includes an upper silicon layer  190  separated from a support substrate  195  by a buried oxide (BOX) layer  200 . Upper silicon layer  190  is single-crystal silicon and source/drains  170 , source/drain extensions  175  and channel region  140  are formed in upper silicon layer  190 . Upper silicon layer (and channel  140 ) are doped opposite type from source/drains  170  and source/drain extensions  175 . Trench isolation extends from a top surface  105 A of upper silicon layer  190  to abut a top surface  205  BOX layer  200 . Note source/drains  170  abut BOX layer  200 . Alternatively, a region of upper silicon layer  190  may intervene between source/drains  170  and BOX layer  200 . 
     Thus, the embodiments of the present invention provide field effect transistors with low-k sidewall spacers and the method of fabricating field effect transistors with low-k sidewall spacers that mitigate parasitic gate to source/drain capacitance (due to the low dielectric constant of the final sidewall spacers) and do not require complete removal of laser absorption layer(s) which facilitate downward scaling of MOSFETs. 
     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.