Patent Publication Number: US-2007096200-A1

Title: Self-aligned conductive spacer process for sidewall control gate of high-speed random access memory

Description:
This application is a Divisional of co-pending application Ser. No. 11/148,342, filed on Jun. 9, 2005, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120. 
    
    
     TECHNICAL FIELD  
      The present invention relates to high-speed random access memory (RAM) applications, and particularly to a self-aligned conductive spacer process for fabrication sidewall control gates.  
     BACKGROUND  
      The demands for highly efficient digital equipment continue to increase, and there is strong need to embed high-speed and low-power-consumption memory into SoC (system-on-chip). Since conventional memories such as SRAM, DRAM, and flash are unsuitable choices for these applications, one unique memory, Direct Tunneling Memory (DTM), with ultra-thin tunnel oxide and novel depleted floating gate is developed for cost-effective and scalable embedded RAM applications. The features of the DTM structure include sidewall control gates fabricated on both sides of a floating gate, and offset source/drain regions without overlapping the floating gate. Its simple fabrication process is fully compatible with the existing CMOS logic technology, and its simple structure can obtain sufficient tunnel current at higher speed operation and lower operating voltage than conventional flash memory.  
       FIG. 1  is a cross-sectional diagram illustrating a high-speed RAM with sidewall control gates fabricated by a conventional polysilicon spacer process. In general, a gate oxide layer  12 , a first polysilicon layer  14  and a hard mask layer  16  are successively deposited on a semiconductor substrate  10 , and then the first polysilicon layer  14  is patterned to form a floating gate  14  from the use of photolithography and etching process. Next, a thin silicon oxide layer  18  is thermally grown on the substrate  10  and sidewalls of the floating gate  14 . Then a second polysilicon layer  20  is deposited followed by an anisotropic etch-back process, which forms sidewall control gates  20  on both sides of the floating gate  14 . The thin oxide layer  18  is also etched through, leaving this thin oxide layer  18  only underlying the sidewall control gates  20 .  
      The conventional anisotropic polysilicon etch process, however, can not well control the dimensions and profiles of the sidewall control gates to facilitate proper device design. The width of polysilicon spacer, referred to the control gate width, may vary to a large extent in a wafer or from wafer to wafer. The variation in the spacer width is also unfavorable to subsequent contact process, and therefore an additional polysilicon line is needed for the contact formation, which leads to increase in memory cell size and difficulty in layout design. Moreover, the anisotropic etch back may round corners of the polysilicon spacers, thus a subsequent silicidation process, for reducing the RC time constant and improving operations of reading, programming, and erasing, can not be perfectly performed in the rounded-shaped control gates.  
     SUMMARY OF THE INVENTION  
      Embodiments of the present invention include a self-aligned conductive spacer process for fabricating sidewall control gates on both sides of a floating gate for high-speed RAM applications, which can well define dimensions and profiles of the sidewall control gates.  
      In one aspect, the present invention provides a conductive spacer process including the following steps. A floating gate is formed on a semiconductor substrate, and a dielectric layer is then formed on the semiconductor substrate to cover sidewalls of the floating gate. A conductive layer is formed on the dielectric layer, and then an oxide spacer is formed on the conductive layer and located adjacent to the sidewalls of the floating gate. Performing an anisotropic etch process on the conductive layer and using the oxide spacer as a hard mask, a conductive spacer is formed adjacent to the sidewalls of the floating gate, serving as a sidewall control gate.  
      In another aspect, the present invention provides a conductive spacer process including the following steps. A semiconductor substrate is provided with a stack structure of a first dielectric layer, a floating gate and a hard mask layer formed thereon. A second dielectric layer is formed on the semiconductor substrate to cover sidewalls of the floating gate. A conductive layer is formed on the second dielectric layer and the stack structure, and then an oxide layer is formed on the conductive layer. Performing a first anisotropic etch process on the oxide layer, an oxide spacer is formed adjacent to the sidewalls of the floating gate. Performing a second anisotropic etch process and using the oxide spacer as a sidewall hard mask, a conductive spacer is formed adjacent to the sidewalls of the floating gate to serves as a sidewall control gate.  
      In another aspect, the present invention provides an random access memory comprises a floating gate formed on a semiconductor substrate; a dielectric layer formed on the semiconductor substrate to cover sidewalls of the floating gate; two control gates formed on the dielectric layer and located at two sides of the floating gate respectively; and two oxide spacers formed on sidewalls of the control gates respectively, wherein the profile of the combination of the control gate and the oxide spacer at one side of the floating gate is substantially oblong. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The aforementioned objects, features and advantages of this invention will become apparent by referring to the following detailed description of the preferred embodiments with reference to the accompanying drawings, wherein:  
       FIG. 1  is a cross-sectional diagram illustrating a high-speed RAM with sidewall control gates fabricated by a conventional polysilicon spacer process; and  
       FIG. 2A  to  FIG. 2F  are cross-sectional diagrams illustrating an exemplary embodiment of a self-aligned conductive spacer process for fabricating sidewall control gates; 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
      Embodiments of the present invention provide a self-aligned conductive spacer process to fabricate sidewall control gates on both sides of a floating gate for high-speed RAM applications, which can well define dimensions and profiles of the conductive spacers to form the control gates of an uniform width in a wafer or from wafer to wafer. Thus in subsequent processes, borderless contact formation and silicide formation can be well performed on the self-aligned control gates. Particularly, embodiments of the present invention provide a self-aligned polysilicon spacer process using a polyoxide spacer as an etch back mask to well define the width, thickness and shape of the sidewall control gate. Embodiments of the self-aligned conductive spacer process are simple and fully compatible with the existing CMOS logic technology.  
      Reference will now be made in detail to the present embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. In the drawings, the shape and thickness of one embodiment may be exaggerated for clarity and convenience. This description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. Further, when a layer is referred to as being on another layer or “on” a substrate, it may be directly on the other layer or on the substrate, or intervening layers may also be present.  
      Herein, cross-sectional diagrams of  FIG. 2A  to  FIG. 2F  illustrate an exemplary embodiment of a self-aligned conductive spacer process for fabricating sidewall control gates. In  FIG. 2A , an example of a substrate  30  may comprise an elementary semiconductor such as silicon, germanium, or a compound semiconductor. The substrate  30  may include an epitaxial layer overlying a bulk semiconductor, a silicon germanium layer overlying a bulk silicon, a silicon layer overlying a bulk silicon germanium, or a semiconductor-on-insulator (SOI) structure. The substrate  30  may comprise an isolation feature to separate different devices formed thereon. The isolation feature may comprise different structure and can be formed using different processing technologies. For example, the isolation feature may comprise a dielectric isolation such as local oxidation of silicon (LOCOS), shallow trench isolation (STI), junction isolation, field isolation, and/or other suitable isolation structures. A first dielectric material, a first gate material and a hard mask material are successively deposited on an active area of the substrate  30 , and then those stacked layers are patterned to form a first dielectric layer  32 , a floating gate layer  34  and a hard mask layer  36 . The patterning step may be accomplished using photolithography and etching to transfer the pattern defined by a photomask to the deposited materials. The photolithography process may include photoresist coating, soft baking, mask aligning, exposing, post-exposure baking, developing photoresist, hard baking, and photoresist stripping. The etching process may include wet etch, dry etch, ion-reactive-etch (RIE), and other suitable processing.  
      The first dielectric layer  32  may comprises a silicon oxide layer, referred to a tunnel oxide, which may be formed through a thermal oxidation process or a chemical vapor deposition (CVD) process. The first dielectric layer  32  is also to be appreciated other well-known dielectric material such as nitrides, oxynitride, high-k materials, any other non-conductive materials, and combinations thereof The floating gate layer  34  may comprise a polysilicon layer formed through, for example Low Pressure CVD (LPCVD) methods, CVD methods and Physical Vapor Deposition (PVD) sputtering methods employing suitable silicon source materials. If desired the polysilicon layer may be ion implanted to the desired conductive type. It is to be appreciated other well-known gate electrode material such as metal, metal alloys, single crystalline silicon, or any combinations thereof The hard mask layer  36 , which acts as a bottom anti-reflective layer underneath the photoresist and protects the floating gate layer  34  from damages in subsequent etching and implanting, may comprise a silicon oxide layer, a silicon nitride layer, an oxynitride layer, or combinations thereof formed by a method such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD).  
      In  FIG. 2B , a second dielectric layer  38  and a second gate material layer  40  are successively formed on the patterned structure as depicted in  FIG. 2A . The second dielectric layer  38  is formed along sidewalls of the floating gate  34  in order to insulate the floating gate layer  34  from the second gate material layer  40 . In some embodiments, the second dielectric layer  38  may comprise a silicon oxide layer grown on the surfaces of the floating gate layer  34  and the substrate  30  through a thermal oxidation process. It is to be appreciated other non-conductive material to form the dielectric layer  38  through CVD methods. The second gate material layer  40  may comprise a polysilicon layer formed through, for example Low Pressure CVD (LPCVD) methods, CVD methods and Physical Vapor Deposition (PVD) sputtering methods employing suitable silicon source materials. If desired the polysilicon layer may be ion implanted to the desired conductive type. Other well-known gate electrode material such as metal, metal alloys, single crystalline silicon, or any combinations thereof, is also appreciated to form the second gate material layer  40 .  
      In  FIG. 2C , an oxide layer  42  is formed on the second gate material layer  40  through methods including, but not limited to, thermal, rapid thermal oxidation (RTO), chemical vapor deposition (CVD) or other advanced oxide-grown technology. In some embodiments, as the second gate material layer  40  is a polysilicon layer, the exposed polysilicon is oxidized using wet-oxidation at a temperature between about 800 to 100 degree C., resulting in a polyoxide layer. This polysilicon re-oxidation process can thin down the second gate material layer  40 , and provide the polyoxide layer as a sidewall hard mask for defining sidewall control gates in subsequent etch back process. The oxide layer  42  preferably has a thickness between about 1100 to 1300 Angstroms.  
      In  FIG. 2D , the oxide layer  42  is etched back to expose the second gate material layer  40 , leaving oxide spacers  42   a  only on sidewalls of the second gate material layer  40 . In detail, certain regions (horizontal portions) of the oxide layer  42  are etched and stopped on the second gate material layer  40  to form the oxide spacers  42   a  along the vertical portions of the second gate material layer  40 . The etch back process may use anisotropic etch process, such as dry etch, RIE (Reactive Ion Etching), or other plasma etching processes.  
      In  FIG. 2E , the second gate material layer  40  is etched back through an anisotropic etch process to expose the hard mask layer  36  and the substrate  30 , leaving the oxide spacers  42   a  and the conductive spacers  40   a  along sidewalls of the floating gate layer  34 . The second dielectric layer  38  is also etched through, leaving this second dielectric layer  38  only underlying the conductive spacers  40   a . The conductive spacers  40   a  therefore act sidewall control gates  40   a  isolated from the floating gate layer  34  by the second dielectric layer  38  for high-speed RAM applications. The profile of the conductive spacer  40   a  may be an L-like shape at one side of the floating gate layer  34 . The profile of the combination of the conductive spacer  40   a  and the oxide spacer  42   a  at one side of the floating gate  34  may be substantially rectangular. In detail, using the oxide spacers  42   a  as the sidewall hard mask, certain regions (horizontal portions) of the second gate material layer  40  are etched, and portions of the oxide spacers  42   a  and the second dielectric layer  38  are also removed till the hard mask layer  36  and the substrate  30  are exposed. The etch back process may use dry etch, RIE (Reactive Ion Etching), or other plasma etching processes. In the self-aligned conductive spacer process, the oxide spacers  42   a  used as the sidewall hard mask for the conductive spacer  40   a , can well define the width and profile of the conductive spacers  40   a  to form the control gates  40   a  of a uniform width in a wafer or from wafer to wafer. Also, the self-aligned conductive sparer process can form the sidewall control gate  40   a  with a flat top, without rounded corners, thus borderless contact formation and silicide formation can be well performed in subsequent processes.  
       FIG. 2F  illustrates the borderless contact formation on the sidewall control gates  40   a . A source/drain ion implantation process is performed to implant various dopant species into the semiconductor substrate  30 , resulting in source/drain regions  44  laterally adjacent to the sidewall control gates  40   a  respectively. An inter-layer-dielectric (ELD) layer  46  is deposited on the substrate  30 , and then a contact process is performed in the ILD layer  46  to form borderless contact  46   a  on the sidewall control gates  40   a  and source/drain contacts  46   b  on the source/drain regions  44 . Since the inventive self-aligned conductive spacer process forms the sidewall control gates  40   a  with flat tops, the contact formation can be borderless on the sidewall control gates  40   a , thus saving an extra polysilicon line employed in conventional method.  
      Although the present invention has been described in its preferred embodiments, it is not intended to limit the invention to the precise embodiments disclosed herein. Those skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.