Abstract:
To reduce the width of a MOSFET gate, the gate is formed with a hardmask formed thereupon. An isotropic etch is then performed to trim the gate in order to reduce the width of the gate. The resulting gate may be formed with a width that is narrower than a minimum width achievable solely through conventional projection lithography techniques.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application Ser. No. 60/400,384, filed Jul. 31, 2002. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     Embodiments of the present invention relate to semiconductor fabrication, and in particular, to methods for reducing the width of MOSFET gate structures. 
     2. Related Art 
     The demand for increased performance and greater functionality in semiconductors has driven the semiconductor industry to develop processes that pack more and more devices onto a substrate to create higher device densities. One approach to packing more devices onto a substrate is to shrink the size of the individual devices and their components. For example, a MOSFET may be made to operate faster by reducing the width of the gate line that drives it&#39;s gate, and hence the channel width of the device. 
     Improving the lithographic techniques that are used to define device patterns is one focus of this approach. Conventional photolithography techniques such as projection lithography define devices by transferring their patterns to a photoresist material that is coated on a semiconductor substrate. Using a series of optics, an image of a pattern representing an integrated circuit device is projected onto the photoresist. Current projection lithography is able to reduce critical geometries of the projected images to approximately 100 nm. However, because the demand for higher packing densities and increased performance requires that MOSFET gate widths be reduced below 100 nm, to 50 nm or less, conventional projection lithography techniques for gate formation will soon be inadequate. 
     Accordingly, recent approaches to further reduction of device dimensions have focused on supplementing conventional projection lithography techniques through additional photoresist processing. One such technique involves trimming a photoresist mask by isotropic etching to reduce the size of the mask beyond the minimum size achievable through projection lithography. An example of this technique is illustrated in FIG.  1 . As shown in  FIG. 1 , a photoresist mask  2  form defining the shape of a MOSFET gate is formed by projection lithography over a multi-layered structure that includes an antireflective (ARC) layer  4 , a gate conductive layer  6  formed of polysilicon, and a gate insulating layer formed of SiO 2 . The multi-layered structure is formed over a semiconductor substrate  10  that includes field isolations  12  that define the boundaries of MOSFET source and drain regions. For purposes of this example, it is assumed that the photoresist mask  2  has the minimum width achievable through projection lithography alone. It can be seen in  FIG. 1  that through subsequent anisotropic etching using the photoresist mask  2  as an initial etch mask, the width of structures formed by etching the conductive gate material  6  and gate insulator  8  will be limited to the width of the photoresist mask  2 . However, in accordance with the gate trim technique, the photoresist mask  2  is subjected to an isotropic etch that removes photoresist material, leaving a trimmed photoresist mask  14 . The trimmed photoresist mask  14  is then used to pattern the underlying layers, resulting in a gate that has a width that is less than the minimum width achievable through projection lithography. 
     The resist trim technique is limited by the fact that a minimum thickness of photoresist is required in order to successfully transfer the shape of the photoresist mask to underlying layers. Accordingly, the improvements in gate size provided by the resist trim technique are also limited. 
     Consequently, there is a need for further techniques for providing further reductions in gate widths. 
     SUMMARY OF THE DISCLOSURE 
     It is an object of the present invention to provide a method for reducing MOSFET gate dimensions. 
     In accordance with a preferred embodiment of the invention, a gate having a self-aligned hardmask on its top surface is formed by a primary anisotropic etch that uses a photoresist mask as an initial etch mask. A secondary isotropic etch is then performed to trim the gate. The hardmask is then removed, leaving a gate that is narrower than the initial photoresist mask. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and in which: 
         FIG. 1  shows a semiconductor structure formed in accordance with a conventional resist trim technique; 
         FIGS. 2   a ,  2   b ,  2   c  and  2   d  show structures formed during processing in accordance with a preferred embodiment of the invention; and 
         FIG. 3  shows a process flow encompassing the preferred embodiment of the invention and alternatives thereto. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Structures formed during processing in accordance with a preferred embodiment of the invention are shown in  FIGS. 2   a - 2   d .  FIG. 2   a  shows a structure similar to that of  FIG. 1 , including a photoresist mask  2 , ARC layer  4 , gate conductive layer  6 , gate insulating layer  8 , semiconductor substrate  10 , and field isolations  12 . In the preferred embodiment the ARC is formed of SiON or silicon rich nitride, the gate conductive layer is doped polysilicon, and the gate insulating layer is SiO 2 . 
       FIG. 2   b  shows the structure of  FIG. 2   a  after a primary anisotropic etch comprising one or more anisotropic etches has been performed to etch the ARC layer and gate conductive layer to form a gate  16  having a self-aligned hardmask  14  thereupon. The ARC layer is typically etched using a combination of Ar and CF 4 , while the polysilicon is typically etched using a combination of HBr, CL, HE, oxygen and CF 4 . The photoresist mask is stripped during or after the primary etch. 
       FIG. 2   c  shows the structure of  FIG. 2   b  after a secondary etch of the gate has been performed to narrow the gate  16 . The secondary etch is isotropic and is selective with respect to the gate material so that the ARC  14  is relatively unaffected. This causes the trimming effect of the secondary etch to be approximately uniform along the height of the gate  16 . The resulting structure has a trimmed gate that is narrower than the overlying hardmask, such that the edges of the hardmask extend beyond the edges of the gate. 
       FIG. 2   d  shows the structure of  FIG. 2   c  after removal of the hardmask. 
     After the structure of  FIG. 2   d  is formed, additional conventional processing may be performed, such as formation of source and drain diffusions, deposition of an interlevel dielectric, formation of source and drain contacts, and replacement of the polysilicon gate with a metal gate. 
     While the process flow of  FIGS. 2   a - 2   d  is presently considered to be the preferred embodiment of the invention, a wide variety of alternative embodiments in accordance with the invention may be formulated. For example, the materials used for the ARC/hardmask, conductive gate layer, and gate insulating layer may be varied in accordance with the particular implementation. The ARC/hardmask may be formed of silicon oxide, silicon nitride or silicon oxynitride, while the gate insulating layer may be formed of alternative high k materials such as ZrO. 
     Further, additional processing may be performed to trim the photoresist mask using the conventional trim process described above prior to etching the ARC and gate conductive layer. 
     In addition, it will be apparent to those having ordinary skill in the art that the tasks described in the preferred embodiment are not necessarily exclusive of other tasks, but rather that further tasks may be incorporated in accordance with the particular structures to be formed. For example, processing tasks such as seed layer formation, seed layer enhancement, formation and removal of passivation layers or protective layers between processing tasks, formation and removal of photoresist masks and other masking layers such as antireflective layers, formation of isolation structures, as well as other tasks, may be performed along with the tasks specifically described above. Further, the process need not be performed on an entire substrate such as an entire wafer, but rather may be performed selectively on sections of the substrate. 
       FIG. 3  shows a process flow for forming a MOSFET that encompasses the preferred embodiment and its aforementioned alternatives, as well as other alternative embodiments that are not explicitly discussed here but will be apparent to those of ordinary skill in the art. As shown in  FIG. 3 , substrate is initially provided ( 30 ). The substrate has stacked thereupon a gate insulating layer, a gate conductive layer and an antireflective layer. A primary anisotropic etch is then performed on the antireflective coating and the gate conductive layer to form a gate having a self-aligned hardmask thereupon ( 32 ). A secondary isotropic is then performed on the gate to narrow the gate ( 34 ). 
     While the invention has been described with reference to its preferred embodiments, those skilled in the art will understand and appreciate from the foregoing that variations in equipment, operating conditions and configuration may be made and still fall within the spirit and scope of the present invention which is to be limited only by the claims appended hereto.