Abstract:
A thin-silicon-on-insulator transistor with borderless self-aligned contacts includes a buried oxide layer above a substrate. A silicon layer overlays the buried oxide layer. A gate stack is on the silicon layer. The gate stack includes a gate oxide layer on the silicon layer and a gate electrode on the gate oxide layer. An off-set spacer surrounds the gate stack. Raised source/drain regions each have a first part overlying a portion of the silicon layer, a second part adjacent to off-set spacer, and a third part extending about a top portion of the gate stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of and claims priority from U.S. patent application Ser. No. 12/193,392 filed on Aug. 18, 2008, now ______, the entire disclosure is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to the field of semiconductors, and more particularly relates to thin body field-effect transistors with electrical contacts on semiconductor substrates. 
       BACKGROUND OF THE INVENTION 
       [0003]    Complementary Metal Oxide Semiconductor (“CMOS”) Field Effect Transistors (“FETs”) are employed in almost every electronic circuit application, such as signal processing, computing, and wireless communications. One known type of FET is a Silicon-On-Insulator (“SOI”) FET. The formation of electrical contacts to electronic and memory devices is a considerable challenge as the integration density of these devices is increased as a consequence of technology scaling. 
         [0004]    For example, the projected contact pitch for 32, 22, and 15 nm nodes are 130, 100, and 80 nm, respectively. In order to fit the contact between adjacent gates, contacts must be made at dimensions approaching the gate length of the device unlike previous technologies where the contacts were many times larger than the gate. Definition of this contact is a lithographic challenge. Alignment of the contact to the source, drain, and gate of the device is critical. In particular, misalignment of the source and drain contacts with respect to the gate can cause electrical shorts, rendering the device inoperable. 
       SUMMARY OF THE INVENTION 
       [0005]    Briefly, in accordance with the present invention, disclosed in one embodiment is a thin-silicon-on-insulator transistor with borderless self-aligned contacts is disclosed. The thin-silicon-on-insulator transistor includes a buried oxide layer above a substrate. A silicon layer overlays the buried oxide layer. A gate stack is situated on the silicon layer. The gate stack includes a gate oxide layer on the silicon layer and a gate electrode on the gate oxide layer. An off-set spacer surrounds the gate stack. Raised source/drain regions each have a first part overlying a portion of the silicon layer, a second part adjacent to off-set spacer, and a third part extending about a top portion of the gate stack. 
         [0006]    In another embodiment a circuit supporting substrate is disclosed. The circuit supporting substrate includes a thin-silicon-on-insulator transistor with borderless self-aligned contacts. The thin-silicon-on-insulator transistor includes a buried oxide layer above a substrate. A silicon layer overlays the buried oxide layer. A gate stack is situated on the silicon layer. The gate stack includes a gate oxide layer on the silicon layer and a gate electrode on the gate oxide layer. An off-set spacer surrounds the gate stack. Raised source/drain regions each have a first part overlying a portion of the silicon layer, a second part adjacent to off-set spacer, and a third part extending about a top portion of the gate stack. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1 to 7  are cross-sectional views of a circuit supporting substrate illustrating a process for forming a thin silicon-on-insulator field-effect transistor with borderless self-aligned electrical contacts according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Embodiments of the present invention provide thin silicon-on-insulator field-effect transistors with borderless self-aligned electrical contacts. Contacts that are self aligned to the source and drain are highly desirable to overcome the misalignment problem discussed above. Generally, an epitaxial layer of Si is grown in the source and drain region of thin body devices, often referred to as a raised source drain (“RSD”). The RSD lowers the external resistance of the device by mitigating the so called “current crowding” effect. It simultaneously provides the requisite volume of Si to form a silicide without fully siliciding the source and drain. The RSD causes an increased capacitance from the source and drain to the gate. This capacitance is present regardless of the gate height. 
         [0009]    However, various embodiments of the present invention provide an advantageous method for forming self-aligned borderless contacts to thin body FET devices. These contacts are formed by the epitaxial Si RSD. The RSD process is selective and does not grow on silicon oxide or silicon nitride. By scaling the gate height to match the thickness of the RSD, the parasitic capacitance of the device structure and contacts can be optimized. The RSD-to-gate capacitance remains the same as thin body FET devices fabricated with taller gates. Unlike devices fabricated with taller gates, the capacitance from metal contacts landing on the source and drain is eliminated. This enables more freedom in the design of the contacts to the RSD. Bar contacts can be used without causing a significant increase in the gate to contact capacitance. The RSD can be grown slightly about the height of the gate. This enables the size of the contact made to the RSD to have a relaxed dimension without causing a short to the gate. This type of growth also allows for increased overlay budget in the definition of this contact with respect to the gate. 
         [0010]      FIGS. 1 to 7  illustrate a process for forming self-aligned borderless contacts to thin body FET devices according to one embodiment of the present invention. The process begins with an SOI wafer that is formed by a silicon substrate (not shown for simplicity), a buried oxide layer (“BOX”)  102  comprising a material such as SiO 2 , and a thin SOI layer  104  comprising a material such as Si. The thin SOI layer  104  can have any thickness that is suitable for building short gate length devices. 
         [0011]    The SOI layer  104  is patterned into discrete islands to form individual transistors. The discrete SOI islands are electrically isolated by the BOX layer  102 . In addition, the BOX layer  104  can be recessed and a dielectric material can be used to form a shallow trench isolation. For example,  FIG. 2  shows a process for defining an active area for a transistor. In this embodiment, the active area is defined through pad-film deposition, photolithography, and reactive-ion etching (“RIE”). However, the present invention is not limited to these techniques. In particular, a pad oxide  206  (e.g., having a thickness of 2-10 nm) is formed in a conventional oxidation furnace, and a pad nitride  208  (e.g., having a thickness of 30-150 nm) is deposited using low-pressure chemical vapor deposition (“LPCVD”) or rapid-thermal chemical vapor deposition (“RTCVD”). Photolithography and a nitride-oxide-silicon RIE are then performed to define the active area. 
         [0012]    Next, the active area is optionally isolated, such as through shallow trench isolation (“STI”). In this embodiment, STI is obtained through deposition of an STI oxide/dielectric, densification anneals, and chemical-mechanical polishing (“CMP”) that stops on the pad nitride  208 . This forms an STI region  210  over the BOX layer  102  that is continuous around the active area, as shown in  FIG. 1 . The pad nitride  208 , along with any STI oxide remaining on the pad nitride  208 , and the pad oxide  206  are then removed (e.g., through wet etching using hot phosphoric acid and HF), as shown in  FIG. 3 . 
         [0013]    A gate stack  312  is deposited onto the SOI layer  104 . This gate stack  312  is composed of a gate oxide  314 , a gate electrode  316 , and a SiN-based hardmask  318 , as shown in  FIG. 3 . The gate oxide  314  can be (but not limited to) SiO2, SiON, or a metal oxide such as (but not limited to) HfO 2 , HfSiO x , HfSiO x N y , Ta 2 O 5 , TiO 2 , Al 2 0 3 , Y 2 O 3  and La 2 O 5 . In some embodiments, the metal oxide creates a high-k layer. The material comprising the gate electrode  316  is determined by the choice of the gate oxide  314 . For example, in the case of Si-based oxides, polysilicon can be used. In the case of metal oxides, a metal can be used such as (but not limited to) TiN, Ta, TaN, TaCN, TaSiN, TaSi, AlN, W and Mo. The gate oxide  314  and gate electrode  316  can be deposited using any conventional deposition process such metal-organic chemical vapor deposition (“MOCVD”) or atomic-layer deposition (“ALD”) and physical vapor deposition (“PVD”), MOCVD, or ALD, respectively. 
         [0014]    The gate stack  312  can also include an optional deposition of an amorphous Si or a poly Si layer  320 , which is deposited using conventional processes such as LPCVD or silicon sputtering. The silicon nitride cap  318  is deposited to allow the later formation of a silicon (or SiGe) raised source/drain through epitaxy. In particular, the nitride cap  318  protects the polysilicon gate during epitaxy to avoid forming an unwanted polysilicon (or poly-SiGe, also referred to as a mushroom) on the gate electrode, which would adversely affect transistor performance as well as transistor yield. (In some embodiments, an oxide cap is deposited to act as a hardmask for gate etch.) 
         [0015]    The transistor gate is then defined using photolithography, RIE, and wet cleaning, as shown in  FIG. 4 . The wet cleaning removes any polymers formed during the RIE and an oxide cap if one was previously formed on top of the nitride cap  318 . As shown in  FIG. 4 , the resulting gate stack  412  is formed by the gate oxide layer  314 , the gate electrode  316 , the optional polysilicon cap layer  320 , and the silicon nitride cap layer  320 . 
         [0016]    An offset spacer  522  is formed using a combination of thin film deposition and etching, as shown in  FIG. 5 . The spacer  522  can include two or more layers  524 ,  526 . For example, the layers  524 ,  526  can be a thin silicon oxide layer  524  followed by a thin SiN layer  526 . The SiN layer  526 , in one embodiment, prevents unwanted epitaxial growth on the sidewall of the gate  412 . If silicon oxide is used alone it will be attacked by the epitaxial growth preclean, which etches oxide. It should be noted that the outer layer  526  can be replaced by any suitable dielectric material that can withstand the epitaxial preclean process. Also, the etching of the spacer  522 , in one embodiment, is performed so as to not expose the gate  316 . This can be done by minimizing the over etch, keeping the spacer  522  from pulling down below the thickness of the nitride hard mask  318   
         [0017]    Once the spacer  522  has been at least partially completed, ion implantation can be performed to provide extension doping. For example, halo and source/drain extensions are formed through implantation. Photolithography is used to selectively define the NFET and PFET areas for source/drain extension and halo implants, and then ions are implanted. For an NFET, the halo implants are performed with a p-type species such as B, BF 2 , or In and the extension implants are performed with an n-type species such as As, P, or Sb. For a PFET, the halo implants are performed using an n-type species As, P, or Sb, and the extension implants are performed using p-type species such as B, BF 2 . An anneal (e.g., millisecond laser anneal or flash anneal) is performed after implantation to heal the damage to the thin SOI layer due to ion implantation. This annealing process also activates the halo and extension implants without diffusing them into the buried oxide layer  104 . Diffusion of the halo or extension implants would degrade performance due to dose loss into the underlying buried oxide layer. 
         [0018]    Raised source/drain growth is then performed to create a raised source/drain  628 , as shown in  FIG. 6 . This process can include the incorporation of dopant gases and can be performed at sufficiently low temperatures, pressures and gas flows to prevent agglomeration of the patterned SOI  104 . The raised source/drain  628  acts as a borderless self-aligned contact to the source and drain. The raised source/drain  628 , in one embodiment, is formed using epitaxy. 
         [0019]    To form the raised source/drain  628 , an initial pre-cleaning removes any oxide and liners and exposes the silicon surface  104  in the source/drain areas. In this embodiment, the pre-cleaning is performed using an HF wet etch or HF-vapor based chemical oxide removal (“COR”). Next, epitaxy that is selective with respect to oxide nitride is used to form the raised source/drain  1446 , so there is no deposition on the nitride cap  318 , the oxide and SiN spacers  524 ,  526 , and the optional STI oxide  210 . In this embodiment, the raised source/drain  628  is formed of silicon (or SiGe or SiC or SiGeC). Dopants can be introduced into the epitaxial growth to produce in situ source/drain regions. For example, by mixing dilute phosphine into the growth gasses, N type source/drain regions will be produced. Likewise, the incorporation of diborane during growth creates P type source/drain regions. This type of processing eliminates the need for further ion implantation. 
         [0020]    If in situ doping is not used during the time of RSD formation, deep source/drain implantation is performed. In this embodiment, the deep implantation is done by using photolithography to selectively define NFET and PFET areas for deep source/drain implants, and then ions are implanted. N-type species are implanted for NFETs, while p-type species are implanted for PFETs. A thermal anneal is then performed to activate and diffuse the ions that have been implanted. 
         [0021]    Next, silicide areas  620  and  620  are formed for contacts. In this embodiment, this is achieved by removing the oxide (e.g., through a wet etch using HF), depositing a metal, performing an anneal to form silicide, and then selectively removing the metal but leave the silicide untouched (e.g., through an aqua regia wet etch). In this exemplary embodiment, the metal is NiPt, CoSi2, or the like. A dielectric layer  732  is deposited onto the substrate and then planarized as shown in  FIG. 7 . 
         [0022]    Contacts  734  to the RSD  628  are created using lithography and RIE followed by metallization. The metallization can involve CVD, PVD, ALD, or electroplating processes or some combination of these processes. The contacts  734  can be defined as to overlap the gate stack  412 , as shown in  FIG. 7 . The remaining nitride hard mask  318  on the gate prevents the RIE process from shorting the contact to the gate electrode  316 . 
         [0023]    As can be seen, self-aligned borderless contacts are created by the epitaxial Si RSD. The RSD process is selective and does not grow on silicon oxide or silicon nitride. By scaling the gate height to match the thickness of the RSD, the parasitic capacitance of the device structure and contacts can be optimized. The RSD-to-gate capacitance remains the same as thin body FET devices fabricated with taller gates. Unlike devices fabricated with taller gates, the capacitance from metal contacts landing on the source and drain is eliminated. This enables more freedom in the design of the contacts to the RSD. Bar contacts can be used without causing a significant increase in the gate to contact capacitance. The RSD can be grown slightly about the height of the gate. This enables the size of the contact made to the RSD to have a relaxed dimension without causing a short to the gate. This type of growth also allows for increased overlay budget in the definition of this contact with respect to the gate. 
         [0024]    It should be noted that some of the features of the examples of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles, teachings, examples, and exemplary embodiments of this invention, and not in limitation thereof. 
         [0025]    It should be understood that these embodiments are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality. 
         [0026]    The circuit as described above is part of the design for an integrated circuit chip. The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0027]    The method as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare chip, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard, or other input device, and a central processor. 
         [0028]    Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.