Abstract:
A semiconductor device includes a spacer adjacent a gate structure. A protection layer covers oxide portions of the spacer surface such that subsequent manufacturing operations such as wet oxide etches and strips, do not produce voids in the spacers. A method for forming the semiconductor device provides forming a gate structure with adjacent spacers including an oxide liner beneath a nitride section, then forming the protection layer over the structure, and removing portions of the protection layer but leaving other portions of the protection layer intact to cover and protect underlying oxide portions of the spacer during subsequent processing such as the formation and removal of a resist protect oxide (RPO) layer. The protection layer is advantageously formed of a nitride film and an oxide film and produces a double spacer effect when partially removed such that only vertical sections remain.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates, most generally, to semiconductor devices and methods for manufacturing such devices. More particularly, the present invention relates to a method and structure for preventing void formation in spacers commonly used along gate structures in semiconductor devices.  
       BACKGROUND  
       [0002]     In today&#39;s rapidly advancing semiconductor manufacturing industry, there is a push for higher and higher levels of integration and smaller and smaller device features. Various structures and techniques have been developed to enable the increased levels of integration. For example, spacers are typically used alongside (transistor) gate structures formed on semiconductor substrates, to isolate gate contacts from source and drain contacts. Spacers formed of both an oxide liner and a bulk nitride portion are commonly favored in today&#39;s semiconductor manufacturing industry. One particularly favored spacer includes an “L-shaped” oxide liner that typically extends along the side of the gate structure and on the semiconductor substrate beneath a nitride portion, with the nitride portion forming the greater portion of the spacer. Various techniques have been developed which enable the production of gate structures of smaller dimensions and associated spacers.  
         [0003]     When the other processes used in semiconductor manufacturing operations, however, attack the gate structure and/or the spacers, device integrity and device yield are compromised and device failure may be the result. For example, an RPO is commonly formed over gate structures including the spacers, to protect the underlying structure during subsequent processing operations performed on the other structures of the semiconductor substrate. The RPO may be patterned to expose portions of the underlying structures to be silicided, while protecting other portions from being silicided, for example. The etching processes used to pattern the RPO may attach underlying structures such as oxide portions of spacers. Additionally, the RPO must eventually be removed, typically using a combination of dry and wet processing operations that preferentially attack oxides. When the RPO is etched or removed from over a conventional gate structure using conventional processing operations, underlying oxides may be attached and voids or divots may be produced at the corners of spacer structures in which the oxide liner extends to the outer spacer surface. In particular, when voids occur along the oxide liner formed as the bottom portion of a spacer, and which lies along the semiconductor substrate surface, the voids can result in leakage when subsequent implanting operations, silicidation operations or other similar operations are carried out. Such leakage can cause device failures, or at the least, degrade yield and compromise device integrity.  
         [0004]     It would therefore be desirable to produce a gate structure including a spacer, that includes an oxide liner that is immune to damage when an RPO layer is formed over the structure, patterned, and subsequently removed.  
       SUMMARY OF THE INVENTION  
       [0005]     To achieve these and other objects and in view of its purposes, the present invention provides, in one aspect, a method for forming a semiconductor device comprising providing a semiconductor substrate and forming a gate structure on the surface of the semiconductor substrate. The method further includes forming at least one spacer along a corresponding side of the gate structure, the spacer including a spacer oxide portion and a spacer nitride portion. The method further includes forming a protection layer over the gate structure and the at least one spacer and etching to remove portions of the protection layer but leaving further portions of the protection layer in place such that no portions of the spacer oxide portion are exposed.  
         [0006]     In another aspect, the invention provides a semiconductor device comprising a gate structure disposed over a substrate surface of a semiconductor substrate. The device includes at least one inner spacer disposed over the substrate surface and along a corresponding side of the gate structure. The inner spacer includes at least one spacer oxide portion and a spacer nitride portion. The inner spacer has an outer surface with at least one spacer oxide surface portion, and the semiconductor device also includes an outer spacer covering each spacer oxide surface portion and formed of an oxide layer and a nitride layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0007]     The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing.  
         [0008]      FIGS. 1-7  are cross-sectional views illustrating a process sequence according to the method of the invention and further illustrating device embodiments of the invention.  
         [0009]     In particular,  FIG. 1  shows a gate structure with spacers covered by a protection layer;  
         [0010]      FIG. 2  shows the structure of  FIG. 1  after portion of the protection layer has been removed;  
         [0011]      FIG. 3  shows the structure of  FIG. 2  after outer spacers have been formed from the protection layer;  
         [0012]      FIG. 4  shows the structure of  FIG. 3  after an RPO layer has been formed over the structure;  
         [0013]      FIG. 5  shows the structure of  FIG. 4  with a photoresist pattern formed thereover;  
         [0014]      FIG. 6  shows the structure of  FIG. 5  after it has been etched to remove the RPO layer in exposed areas; and  
         [0015]      FIG. 7  shows the structure of  FIG. 6  after RPO removal and silicidation.  
         [0016]      FIG. 8  is a cross-sectional view with similarities to the structure shown in  FIG. 7 , but according to the PRIOR ART and including undesirable voids/divots therein.  
     
    
     DETAILED DESCRIPTION  
       [0017]     The present invention provides a method and structure that protects conventional spacers that include an oxide portion, during subsequent oxide removal operations which, using conventional technology, result in attack of the oxide portion and may create voids or divots at locations where the oxide portions intersect the outer surface of the spacer. The present invention prevents any attack of the oxide liner portion of the conventional spacer during such subsequent oxide removal operations, by providing outer spacers that cover the otherwise exposed portions of the oxide liner of the conventional spacer. It may be stated that the present invention provide an inner spacer adjacent gate structure and an outer spacer adjacent the inner spacer.  
         [0018]      FIG. 1  shows gate structure  7  formed over surface  5  of substrate  3 . Conventional semiconductor substrates such as silicon, gallium arsenide, or other suitable materials, may be used as substrate  3 . Gate structure  7  may be formed of polysilicon or other suitable materials and includes sides  9  and top  11 . Adjacent each of opposed sides  9  are spacers  13 . Spacers  13  include a nitride portion and an oxide portion and in the illustrated embodiment spacers  13  each include a bulk nitride portion  15  and underlying oxide portion  17 . Nitride portion  15  may be silicon nitride in stoichiometric or other ratios but other nitrides may be used in other embodiments. Oxide portion  17  may be an SiO 2  liner formed using a TEOS (tetraethyl orthosilicate) precursor in one exemplary embodiment, but other oxide liners may be used in other exemplary embodiments. The dimensions of gate structure  7  and the components of spacer  13  may vary according to application and levels of device integration, in various exemplary embodiments. Spacers  13  include curved outer surface  19  which extends from surface  5  to side  9  but outer surface  19  does not extend up to top  11 . Although oxide portion  17  is a generally “L-shaped” liner in the illustrated embodiment, it should be understood that such is exemplary only, and in other exemplary embodiments, various other structural examples of oxide portion  17  and nitride portion  15  may be used. For example, spacer  13  may include multiple oxide portions  17 .  
         [0019]     Covering gate structure  7  and spacers  13  is protection layer  21  which consists of nitride layer  25  formed over oxide layer  23  in the illustrated exemplary embodiment. Protection layer  21  may consist of various other arrangements of films in other exemplary embodiments. Protection layer  21  includes vertical portion  26 . Protection layer  21  may also be referred to as a barrier structure Nitride layer  25  may be formed of various suitable materials such as stoichiometric or other silicon nitrides and oxide layer  23  may be formed of various suitable materials such as silicon dioxide.  
         [0020]     A conventional spacer dry etching process may be used to remove comparatively thin portions of nitride layer  25 , leaving thicker vertical portions  26  of nitride layer  25 . An anisotropic etch process may be advantageously used. The selective etching process does not appreciably remove even the exposed portions of oxide layer  23 . The structure in  FIG. 2  then undergoes an oxide removal etching operation such as an HF (hydrofluoric acid) wet dip to remove exposed portions of oxide layer  23  and produce the structure shown in  FIG. 3 .  
         [0021]     In  FIG. 3 , top  11  of gate structure  7  is exposed and portions of outer surface  19  of spacer  13  are also exposed.  FIG. 3  also illustrates an aspect of the invention that no portions of oxide portion  17  are exposed. Rather, at the locations where oxide portion  17  forms part of outer surface  19 , spacer  13  is covered by remaining portions of protection layer  21 , i.e., nitride layer  25  and oxide layer  23 . In essence, vertical portions  26  serve as spacers and therefore  FIG. 3  shows outer spacers  29  that are disposed outside spacers  13  which therefore serve as inner spacers.  
         [0022]     Although not illustrated in the figures, it should be understood that conventional source/drain regions may be formed in surface  5  of substrate  3  beneath spacers  13  and outer spacers  29  and extending essentially inwardly to about the intersection of sides  9  of gate structure  7 , and substrate  5 . Conventional methods such as self-aligned techniques may be used to form the source/drain region after definition of gate structure  7  and prior to formation of the films that form spacer  13 .  
         [0023]      FIG. 4  shows RPO (resist protect oxide) layer  31  formed over the structure previously shown in  FIG. 3 . Various formation methods may be used and RPO layer  31  may be pure stoichiometric silicon dioxide or other suitable oxide films. RPO layer  31  may be formed to various thicknesses and is formed over gate structure  7 , spacers  13  and outer spacers  29 . In the illustrated embodiment, RPO layer  31  is formed on outer surface  19 , but additional intermediate films may be used in other exemplary embodiments  
         [0024]      FIG. 5  shows the structure of  FIG. 4  after a conventional photoresist film  33  has been formed over the structure in  FIG. 4  then developed, i.e. patterned, to form opening  35  in which RPO layer  31  is exposed. Conventional methods may be used.  
         [0025]      FIG. 6  shows the structure of  FIG. 5  after an etching procedure has been used to remove portions of RPO layer  31  exposed in opening  35  and not covered by photoresist layer  33 , and after the etching procedure has been followed by a subsequent photoresist removal process to remove photoresist layer  33 . Various etching procedures may be used. In one embodiment, a dry etching procedure may be followed by a wet etching procedure.  FIG. 6  shows uncovered portion  37  in which RPO layer  31  has been removed exposing portions of outer spacer  29  including oxide layer  21 , outer surface  19  and surface  41  formed at top  11  of gate structure  7 . Because no portions of oxide portion  17  are exposed to the oxide wet etching solution used to remove RPO layer  31 , structural integrity is maintained as protected oxide portions  17  are not attacked. The locations on spacer  13  where oxide portion  17  intersects outer surface  19 , are divot-free. A silicidation process may then be carried out on the structure shown in  FIG. 6 .  
         [0026]      FIG. 7  shows the structure of  FIG. 6  after silicidation has taken place on portions of exposed silicon not covered by RPO layer  31 , and after remaining portions of RPO layer  31  have been subsequently removed. For example, silicide  43  is formed in top surface  41  of gate structure  7  and silicide  45  is formed in surface  5  of substrate  3  in locations not covered by gate structure  7 , spacer  13  or outer spacer  29 . Silicide  45  may advantageously provide contact to a source/drain layer formed in that region of the substrate  3  and extending laterally to gate structure  7 . The silicidation process may be a conventional process in which a metal film such as cobalt or titanium is formed over the structure, and annealed to engender the silicidation of the metal film with exposed silicon portions. The unreacted metal portions are then removed.  
         [0027]     The remaining portions of RPO layer  31  are then removed using conventional oxide stripping operations, typical, wet isotropic processes. Again, since oxide portion  17  of spacers  13  are not exposed, oxide portions  17  are not attacked during the oxide stripping operation used to remove RPO layer  31 .  
         [0028]     In comparison,  FIG. 8  shows a comparable structure according to the prior art but a structure in which novel protection layer  21  (oxide layer  23  and nitride layer  25 ) was not used. Spacers  113  of gate structure  107  formed on surface  105  of substrate  103 , include large divots  110  (i.e., voids) at locations where oxide portions  117  are coextensive with outer surface  119  within region  137  from which an RPO layer (not shown) was initially removed and smaller divots  120  (i.e., voids) in region  138  in which the RPO layer was not etched away but only removed later by stripping. Larger voids  110  are greater than smaller divots  120  because they represent portions of oxide portion  117  that were exposed during two wet operations used to remove oxide materials.  
         [0029]     An advantage of the inventive structure such as shown in  FIG. 7  is that such divots are not present because oxide portions  17  are protected from the wet etching solutions used to initially etch and then remove the RPO layer  31 .  
         [0030]     The preceding merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.  
         [0031]     This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the device be formed or used in a particular orientation.  
         [0032]     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.