Abstract:
A thin BOX ETSOI device with robust isolation and method of manufacturing. The method includes providing a wafer with at least a pad layer overlying a first semiconductor layer overlying an oxide layer overlying a second semiconductor layer, wherein the first semiconductor layer has a thickness of 10 nm or less. The process continues with etching a shallow trench into the wafer, extending partially into the second semiconductor layer and forming first spacers on the sidewalls of said shallow trench. After spacer formation, the process continues by etching an area directly below and between the first spacers, exposing the underside of the first spacers, forming second spacers covering all exposed portions of the first spacers, wherein the pad oxide layer is removed, and forming a gate structure over the first semiconductor wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to ETSOI MOSFETS, and more specifically to robust isolation for ETSOI MOSFETS. 
     2. Description of Related Art 
     Metal Oxide Semiconductor Field Effect Transistors (MOSFETS) are commonly used today in almost all electronic circuit applications. An emerging technology in the field of semiconductor-on-insulator (SOI) FET devices is the extremely thin semiconductor-on-insulator (ETSOI) MOSFET. Such a device shows excellent short channel control, which is desirable. With the trend toward continued scaling of MOSFET devices, ETSOI is a promising candidate for next generation technology. 
     There are, however, several manufacturing issues that can limit yield. Specifically, as the STI divot erodes as a function of the HF based cleaning and other process steps during manufacturing, the back gate or substrate wafer is exposed, leaving it susceptible to shorting due to source/drain epitaxial growth and unwanted metal gate connections, among other processes. 
     Referring now to  FIG. 1A , a problem inherent in some current ETSOI devices is illustrated.  FIG. 1A  illustrates a wafer including a substrate  102 , a buried oxide layer  104  and an ETSOI layer  106 . It also includes raised source/drain regions overlying the ETSOI layer  106  and adjacent to the gate structures  108 . Also shown is a dielectric  110  within the illustrated shallow trench isolation region. During normal processing, epitaxially grown silicon can develop along the sidewall of the shallow trench isolation region. In  FIG. 1A , this is illustrated as epitaxially grown silicon  114 . As illustrated in  FIG. 1A , epitaxially grown silicon  114  forms an unwanted connection from the substrate  102  to the raised source/drain region  112 . 
     Referring now to  FIG. 1B , another problem inherent in some current ETSOI devices is illustrated.  FIG. 1B  illustrates the device after the deposition of a pre-metal dielectric  116  and the formation of contact holes. In the case of a mis-aligned contact hole  118 , the hole can be etched partially into the shallow trench. This presents a problem when metal is later added, as it can also create a short between the substrate  102  and a raised source/drain region 
     BRIEF SUMMARY OF THE INVENTION 
     To overcome these deficiencies, the present invention provides a method of manufacturing a semiconductor device, including: providing a wafer including at least a pad layer overlying a first semiconductor layer overlying an oxide layer overlying a second semiconductor layer, wherein the first semiconductor layer has a thickness of 10 nm or less; etching a shallow trench into the wafer, extending partially into the second semiconductor layer; forming first spacers on the sidewalls of the shallow trench; etching an area directly below and between the first spacers, exposing the underside of the first spacers; forming second spacers covering all exposed portions of the first spacers, wherein the pad oxide layer is removed; and forming a gate structure over the first semiconductor wafer. 
     According to another aspect, the present invention provides a semiconductor device, including: a first semiconductor layer overlying an oxide layer overlying a second semiconductor layer, wherein the first semiconductor layer has a thickness of 10 nm or less; at least one shallow trench isolation region extending partially into the second semiconductor layer, separating regions of the semiconductor device; a first sidewall spacer adjacent to the sidewall of the shallow trench isolation region, extending from the first semiconductor layer to the semiconductor layer, wherein a gap exists between the bottom of the first sidewall spacer and the bottom of the shallow trench isolation region; a second sidewall spacer covering the first sidewall spacer and completely filling the gap; and a gate structure overlying the first semiconductor layer. 
     According to yet another aspect, the present invention provides a method of manufacturing a semiconductor device, including: providing a wafer including at least a pad layer overlying a first semiconductor layer overlying an oxide layer overlying a second semiconductor layer, wherein the first semiconductor layer has a thickness of 10 nm or less; etching a shallow trench into the wafer, extending until the second semiconductor layer; forming first spacers on the sidewalls of the shallow trench; etching an area directly below and between the first spacers, exposing the underside of the first spacers; forming second spacers covering all exposed portions of the first spacers, wherein the pad oxide layer is removed; and forming a gate structure over the first semiconductor wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  is a prior art illustration of an ETSOI device with epitaxial growth shorting. 
         FIG. 1B  is a prior art illustration of an ETSOI device with a mis-aligned contact and shorting due to the metal filling. 
         FIG. 2A  is a starting wafer according to an embodiment of the invention. 
         FIG. 2B  illustrates the formation of shallow trenches according to an embodiment of the invention. 
         FIG. 3A  illustrates the formation of first spacers according to an embodiment of the invention. 
         FIG. 3B  illustrates an alternative embodiment with larger shallow trenches. 
         FIG. 4A  illustrates additional etching of the shallow trenches according to an embodiment of the invention. 
         FIG. 4B  illustrates an alternative embodiment with large shallow trenches and additional etching. 
         FIG. 5A  illustrates the addition of insulating materials to the device according to an embodiment of the invention. 
         FIG. 5B  illustrates an alternative embodiment with the addition of insulating materials. 
         FIG. 6A  illustrates the device after removing several layers. 
         FIG. 6B  illustrates an alternative embodiment after removing several layers. 
         FIG. 7A  illustrates an alternative embodiment with an extended spacer. 
         FIG. 7B  illustrates an alternative embodiment with additional substrate undercutting. 
         FIG. 7C  illustrates an alternative embodiment without a second spacer. 
         FIG. 8A  illustrates the device with the addition of gate structures. 
         FIG. 8B  illustrates an alternative embodiment with added raised source/drain regions. 
         FIG. 9  illustrates the device with contact holes added. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 2A , a starting wafer according to an embodiment of the invention is presented. This embodiment includes a substrate layer  200 , a thin buried oxide (thin BOX) layer  202 , an extremely thin silicon-on-insulator layer (ETSOI)  204  and a pad layer  206  as the starting wafer. The present invention is not limited to this setup, and other starting wafer setups containing alternative layers can be used. 
     In an embodiment, the substrate layer  200  is silicon. In other embodiments, different semiconductor materials can be used, including but not limited to strained silicon, silicon germanium, silicon alloys, germanium, germanium alloys. 
     In an embodiment, a thin buried oxide layer (thin BOX)  202 , i.e. an insulating layer, overlies the substrate layer  200 . In an embodiment, thin BOX layer  202  can be deposited or grown prior to the formation of ETSOI layer  204 . In other embodiments, wafer bonding techniques can be used, using glue, adhesive polymer, or direct bonding. In yet another embodiment, a high energy dopant can be implanted into the substrate  200  and annealed to form thin BOX layer  202 . 
     In an embodiment, ETSOI layer  204  includes silicon. In other embodiments, ETSOI layer  204  can include any known semiconductor material, including but not limited to strained silicon, silicon germanium, silicon alloys, germanium, germanium alloys, and the like. ETSOI layer  204  can be reduced to the desired thickness by any method as is known in the art, including planarization, grinding and etching. In an embodiment, ETSOI layer  204  has a thickness ranging from 1 to 10 nm. 
     In an embodiment, pad layer  206  ETSOI layer  204 . In an embodiment, pad layer  206  is pad oxide layer  206 . Pad oxide layer  206  includes, for example, silicon dioxide. In an embodiment, pad oxide layer  206  has an overall thickness of 2 to 10 nm. 
     Referring now to  FIG. 2B , a shallow trench isolation (STI) region  208  is defined. The dimensions of the trench formed are relevant to the size of the device and dependent on the technology for which the device is being used. In an embodiment, the width of the STI region  208  is in a range of 30-60 nm. As illustrated in  FIG. 2B , the STI can be formed by selectively removing portions of the pad oxide  206 , the ETSOI layer  204 , the thin BOX  202 , and the substrate  200 . In one embodiment, as shown in  FIG. 2B , the bottom  210  of the STI  208  extends partially into substrate  200 , e.g. by approximately 5 nm. In other embodiments, it extends until reaching the substrate  200  without etching into it. In an embodiment, the shallow trench isolation is formed by known techniques of lithography, masking and etching. 
     Referring now to  FIG. 3A , furnace silicon nitride  300  is deposited using low pressure chemical vapor deposition (LPCVD). In an alternative embodiment, it can be deposited using plasma enhanced chemical vapor deposition (PECVD). In other embodiments, alternative spacer materials can be used as are known in the art. 
     In a next processing step, the furnace silicon nitride is removed from all horizontal surfaces using an anisotropic etch process. In an embodiment this is done using plasma Reactive Ion Etching (RIE), a highly directional etching process where the ions are normal to the surface, a preferred direction, which facilitates the removal of the silicon nitride from the horizontal surfaces but leaves a layer on the vertical surfaces. The end result of this process is the formation of spacers  300  on the sidewalls of the STI  208 . In the embodiment illustrated in  FIG. 3A , the bottom  210  of the STI  208  extends about 5 nm into the substrate  300 . As a result of this, the spacer  300  covers the 5 nm on the sidewall of the substrate  200 . In an embodiment where the bottom  210  of the substrate does not extend into the substrate  200 , the spacers  300  would not cover any of the sidewall of the substrate  200 . 
     Referring now to  FIG. 3B , an alternative embodiment is shown. In this embodiment, large pads are used with the transistors built as isolated features as opposed to separated by shallow trenches. In this embodiment, the spacing between semiconductor layers can be between 40 and 100 μm, as is illustrated in  FIG. 3B . The processing steps taken to this point, however, remain the same. In this embodiment, feature  208  represents the large void in between the transistor locations. 
     Referring now to  FIG. 4A , an additional etching step is performed. In this step, the STI  208  is etched further into the substrate  200 , opening up a void below the spacers  300 . In a first etching step, an isotropic etch is performed, done primarily to etch the space between and below the recently formed spacers  300 . In an embodiment, a wet, isotropic etch is performed using potassium hydroxide (KOH) as the wet etchant. In other embodiments, different types of wet etchants can be used, as are known in the art. 
     Following the isotropic etch, a dry, anisotropic etch can be performed to clear out the space  402  underneath the spacers  300 . In an embodiment, plasma RIE can be used to etch the space  402  as illustrated in  FIG. 4A . This anisotropic etching process uses the hole created by the isotropic etch to carve out sidewalls below the spacers  300 , and will later be filled with another insulating material. In an embodiment, these two etching processes combine to extend bottom of the previously etched STI  208  an additional 10-100 nm into the substrate. In another embodiment, this same process can be carried out on a wafer with large pads/isolated features, as illustrated in  FIG. 4B . 
     Referring now to  FIG. 5A , this embodiment continues with the addition of two more materials. In one processing step, a layer of insulating material  502  is deposited over the device. In an example, a hydrogen rich nitride, such as hydrogen rich silicon nitride is deposited. Insulating material  502  can be deposited by any number of known techniques, including but not limited to PECVD and LPCVD. In the embodiment shown in  FIG. 5A , insulating material  502  completely fills the STI locations including the recently opened void  402 . 
     In a next processing step, an oxide material  500  is deposited over the device. In an embodiment, high density plasma oxide can be used. In other embodiments, spin-on oxide or spin-on glass can be used. 
     In a next processing step, a chemical mechanical polishing (CMP) is performed, stopping on the insulating material  502 . This removes most of the oxide material  500  except for on the outer edges, away from the transistor regions. This is illustrated in  FIG. 5A .  FIG. 5B  is an alternative embodiment using large pads/isolated features. The process to get to this point, however, is the same. Notably, in the embodiment of  FIG. 5B , there is more oxide material  500  remaining due to the larger void between features. 
     Referring now to  FIG. 6A , some of the insulating material  502  and oxide  500  are removed. In an embodiment using hydrogen rich silicon nitride as the insulating material  502 , a directional nitride etch can be performed to remove it, for example a directional RIE process. Due in part to the directional nature of the etching process, the insulating material  502  inside the STI  208  remains. Other processes to remove the excess hydrogen rich nitride as are known in the art can be used here as well. 
     In a next processing step, the pad oxide  206  is stripped from the device, exposing the ETSOI layer  204 . In an embodiment, a hydrofluoric acid (HF) etch can be performed to strip the pad oxide. The oxide material  500  can also be reduced to a level equal with the ETSOI layer  204  in this or a separate etching step, depending on the embodiment and the materials chosen. 
     Following the stripping of pad oxide  206 , a high temperature densification anneal can be performed on the insulating layer  502 . In an embodiment with hydrogen rich silicon nitride as the insulating layer  502 , the high temperature densification anneal will densify the nitride. 
     As a result of these etching steps, the STI  208  now contains a spacer  300  covering the sidewalls of the ETSOI layer  204  and the thin BOX layer  202 . In one embodiment, the sidewall spacer terminates at the substrate sidewall. This is dependent on, if during the initial STI process, the substrate was etched into or if the etching was terminated on contact with the substrate. In the embodiment shown, the substrate was etched into, e.g. by about 5 nm, and the spacer  300  will cover these additional 5 nm of the substrate sidewall. 
     In addition to the spacer  300 , insulating material  502  has been added into the STI  208  and underneath the spacers  300 . Insulating material  502  will later act as a second spacer when gate structures are added, adding an extra layer of protection. In this embodiment, the sidewall spacer  300  extends from ETSOI layer  204  into substrate layer  200 . There is a gap between the bottom of sidewall spacer  300  and the bottom of STI  208 . Insulating material  502  covers sidewall spacer  300 , including completely filling the gap between the bottom of sidewall spacer  300  and the bottom of STI  208 . 
     Referring now to  FIG. 6B , an alternative embodiment of the device is shown. In this embodiment with large pads/isolated features, the processing steps remain the same as in  FIG. 6A . 
     Referring now to  FIGS. 7A-7C , several different embodiments of the present invention are illustrated. All three of these illustrations represent further embodiments of the present invention when having large pads/isolated features.  FIG. 7A  illustrates an embodiment of the invention with a spacer  300  that extends to the bottom of the substrate  200 . In this embodiment, the initial etch, illustrated in  FIG. 2B , extends to the bottom of the substrate  200 . 
     In the embodiment illustrated in  FIG. 7B , there is an additional undercutting of the substrate. Prior to the deposition of the insulating material  502 , additional isotropic etching is performed which cuts into substrate  200  underlying the thin BOX layer  202 . From this, the process continues as the embodiment previously described. As a result, extra protection from shorting and epitaxial growth from the substrate to a later added raised source/drain region is provided by the extra insulation. 
     In the embodiment illustrated in  FIG. 7C , the additional etching as in  FIG. 7B  is performed; the additional nitride layer, however, is omitted. For some applications, this may be suitable as the additional etching of the substrate can provide enough protection from the potential shorting problems that can occur in ETSOI devices. 
     The remaining  FIGS. 8A ,  8 B, and  9  illustrate an embodiment of the invention with a gate structure added. The following example is for illustrative purposes only, and does not represent the only embodiment in which a gate structure can be added. Other gate structures that are known in the art and can be built over an ETSOI layer can serve the same purpose in the present invention. 
     Referring now to  FIG. 8A , one embodiment of a gate structure that can be fabricated over the ETSOI layer is illustrated. The gate structure can be formed using known techniques of deposition, photolithography and etching. A pattern is created over the deposited materials by first applying a photoresist to the surface to be etched. Next the photoresist is exposed to a pattern of radiation which is developed into the desired pattern using a resist developer. This allows removal of the photoresist in areas that overly the portions of the device that are to be etched. After the completion of the patterning, the portions covered by the photoresist are protected from etching while the uncovered regions are etched using a selective etching process. A hard mask can be deposited over the device, and it can include silicon nitride, silicon dioxide, and the like. 
     The gate structure can include at least a gate conductor  802  overlying a gate dielectric. The gate conductor  802  can include any metal known in the art to act as a conductor. The gate structure can additionally include a second conductive material (not shown) overlying the gate conductor  802 . This additional conducting material can include a doped semiconductor material, including a doped silicon material, such as doped polysilicon. The gate dielectric can be a dielectric material, such as silicon dioxide. Additionally, the gate dielectric can include a high-k dielectric material, such as hafnium oxide, hafnium silicate, hafnium silicon oxynitride, zirconium silicate, zirconium oxide, and the like. 
     A set of first spacers  804  can be formed adjacent to and in direct contact with the sidewalls of the gate structure. In this embodiment, this first set of spacers is typically narrow, with a thickness under 15 nm. First spacers  804  can be formed using known techniques of deposition and etching. First spacers  804  can include, for example, silicon nitride. 
     Raised source/drain regions  808  can be formed adjacent to the first spacers  804 . In one embodiment, the extension regions are formed using an epitaxial growth process over the ETSOI layer. In an embodiment, the raised source/drain regions are formed by epitaxial growth of silicon germanium over the ETSOI layer. In other embodiments, the raised source/drain regions are formed by epitaxially grown carbon doped silicon. 
     In an embodiment, second spacers  806  can be formed adjacent to and in direct contact with first spacers  804 , so as to prevent any contact from the raised source/drain regions and the gate structure. The second spacers  806  can be formed by depositing a conformal film and using a highly directional etch. Second spacers  806  can include a dielectric, such as silicon dioxide. In a next step, silicides can be formed over the raised source/drain regions. 
     Referring now to  FIG. 9 , high-k liner  902  can be deposited over the device. High-k liner  902  can act to protect the insulating material  502  that is acting as a protective spacer from the formation mis-aligned contacts. Following this, a pre-metal dielectric is additionally deposited over the device. In an embodiment, a high-density plasma oxide is used as the pre-metal dielectric, and it is deposited, for example, by high-density plasma chemical vapor deposition. In another embodiment, spin on glass is deposited over the device as the pre-metal dielectric. In another embodiment, silicon dioxide is deposited over the device. Other dielectric materials can also be used as the pre-metal dielectric. 
     In an embodiment, the deposited dielectric is patterned and etched to form the holes needed to contact the source/drain regions and gate conductor regions of the device. The contact holes are filled with a metal, for example tungsten, silver, copper, gold, and the like, and then a CMP is performed until flat. In the event of mis-aligned contacts  906 , the sidewall of the device covered by the spacers  300  is protected. Due to the presence of first spacers  300 , second spacers  502 , and in some embodiments the high-k liner  902 , the formation of mis-aligned contact holes does not cause the sidewalls to become exposed. Such exposure can lead to shorting between the substrate and the raised source/drain regions when the metal is added to the contact holes. Additionally, epitaxial growth of silicon on the sidewalls which can also lead to shorting between the substrate and the raised source/drain regions is prevented by the presence of the first spacers  300  and second spacers  502 . Aligned contacts  904  illustrate correctly aligned contacts with no etching into the STI  208 . 
     Referring now to  FIG. 8B , an embodiment with large pads and isolated features is illustrated. Raised source/drain region  810  can be formed over ETSOI layer  204  in the same manner as raised source/drain region  808 . 
     The method as described above can be used in the fabrication of integrated circuit chips. In an embodiment, many field effect transistors are fabricated by this method, separated by at least one shallow trench isolation region, and electrically connected to form an integrated circuit. 
     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 die, 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. 
     While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.