Abstract:
An improved dual gate oxide process for dual-gated devices using oxygen ion implantation to vary the thickness of gate oxide layers. The desired layers are identified by photoresist layer patterning prior to an ion implantation. A subsequent heat treatment oxidizes the implanted region.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a method for fabricating a dual gate oxide for deep submicron integrated circuit systems (ICS) using non-critical photolithographic masking to select implantation areas followed by oxygen ion implantation into the selected areas.  
           [0003]    2. Description of Related Art  
           [0004]    The increasing diversity of design requirements for metal oxide semiconductor (MOS) devices, such as dual voltage requirements on chip input/output, the need to tolerate higher voltages, and the need to handle higher power supply inputs, has driven the semiconductor industry towards the increased use of gate oxide layers of different thicknesses. A number of methods have been proposed to meet this demand. For example, U.S. Pat. No. 5,273,921 (Neudeck, et al.) proposes several methods for fabricating a dual-gated semiconductor-on-insulator field effect transistor. By in large, however, present techniques for dual-gated oxide processes have several complex processing steps, some of which entail high processing temperatures. The heat treatment steps typically expose the device to temperatures in the range of 600 to 1000° C. for as long as 4 hours. The present methods for producing dual-gate oxides are, therefore, undesirable not only for the additional time and expense added to the processing due to the complexity of the procedures, but also because of the undesirability of the high processing temperatures, which are difficult to accommodate in device designs in light of the undesired diffusion of impurities caused by heat exposure. The criticality of what wet etch processing and other complexities in the various present art dual-gate oxide processes further makes the uniformity of the oxide layers difficult to precisely control, thus leading to quality control problems.  
           [0005]    Accordingly, a need exists for a relatively simple method of accomplishing a dual-gate oxide device architecture. Preferably, this method should reduce the criticality of wet clean/oxide etch back steps as well as reduce the need for high processing temperatures. A desirable dual-gate oxide process would reduce the complexity of creating MOS integrated circuits with two or more different gate oxide thicknesses to a few simple steps and allow for control of the final thickness and uniformity independent of ambient flow pattern characteristics and the variable delivery of oxidizing species to the surface of the wafer.  
         SUMMARY OF THE INVENTION  
         [0006]    A method for fabricating a dual-gate oxide for MOS devices is disclosed using a photoresist masking layer applied after the first layer of gate oxide is grown on the wafer. The photoresist layer is removed from areas where thicker gate oxide material is desired. A low energy implantation of oxygen ions is then performed. The remaining photoresist is removed and the surface is cleaned followed by a heat treatment, thus resulting in the oxidation of the silicon of the surface underneath the gate oxide material.  
           [0007]    This invention reduces the complexity of creating MOS integrated circuits with two different gate oxide thicknesses to a simple photolithography step combined with an ion implantation technique. The criticality of wet etch processing of present state of the art processes is eliminated or substantially reduced. The final thickness uniformity is independent of ambient flow pattern characteristics and variable delivery of oxidizing species to the surface of the wafer. Since the invention does not involve several high processing temperature steps, the undesired diffusion of impurity is avoided.  
           [0008]    The above as well as additional features and advantages of the present invention will become apparent in the following written detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 illustrates a cross-sectional side view of a selected section of an MOS device showing the active areas defined;  
         [0011]    [0011]FIG. 2 illustrates a cross-sectional side view of a selected section of an MOS device showing the growth of a thin gate oxide;  
         [0012]    [0012]FIG. 3 illustrates a cross-sectional side view of a selected section of an MOS device showing a thin gate oxide covered with a photo resist pattern;  
         [0013]    [0013]FIG. 4 illustrates a cross-sectional side view of a selected section of an MOS device showing the implantation of oxygen ions to the thick gate active areas;  
         [0014]    [0014]FIG. 5 illustrates a cross-sectional side view of a selected section of an MOS device showing the photoresist cleaned from the patterned areas; and,  
         [0015]    [0015]FIG. 6 illustrates a cross-sectional side view of a selected section of an MOS device showing the wafer heat treated to oxidize the implanted silicon.  
     
    
     DETAILED DESCRIPTION  
       [0016]    The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention.  
         [0017]    [0017]FIG. 1 shows a cross-sectional side view of a selected portion of a MOS device. The embodiment illustrated shows a portion of the device comprising a shallow trench  10  filled with cvd oxide  12  within two pedestals  20 ,  21  of the silicon substrate. In this illustration, a design determination has been made to deposit a thick gate oxide on a surface  30  above one pedestal  20 , while a thin gate oxide is to be deposited on the surface  31  of the other pedestal  21 .  
         [0018]    [0018]FIG. 2 shows the same section of the MOS device with a thin gate oxide or insulating layer  40  grown on the exposed surface of the device. Corresponding reference numerals are used to represent corresponding elements unless indicated otherwise. Following a normal cleaning process, this insulating layer  40  is grown on the wafer using one of the well known methods of the present art. For example, if a nitrided gate oxide is preferred as the insulating layer  40 , such a film can be deposited on the wafer by growing the film in an ambient of nitrous oxide, perhaps including chlorine from one of the usual precursors, including trichloroethylene (TCE). The typical thickness of such film would be between about 40 to about 50 Angstroms.  
         [0019]    Referring now to FIG. 3, next, a photoresist masking layer  50  is applied and patterned in the usual way. The area  30  where the thicker gate oxide material is desired is exposed by removing the photoresist from this location. Conversely, the area  31  where the thin gate oxide is desired remains masked by the photoresist masking layer  50 .  
         [0020]    The next step, that of a low energy implantation of oxygen ions, is illustrated in FIG. 4. The oxygen ion  60  implantation will occur on the area  30  where the thicker gate oxide material is desired, while the area  31  where the thinner gated oxide material is desired remains masked by the photo resist material  50 . The low energy implantation of oxygen ions  60  is preformed using an accelerating voltage typically in the range of 200 to 800 eV. A preferred source of these ions is an oxygen plasma. A magnetic field, typically in the range of about 1 to about 100 gauss is used to increase the density of oxygen ions in the plasma. A DC voltage is used to extract the ion from the plasma, imparting a single direction to the extracted ions towards the desired surface  30 . Alternatively, conventional ion implant technology can be employed. The peak range of the implanted ion  60  will be from about 20 to about 50 Angstroms, with a range straggle of about 10 to about 25 Angstroms.  
         [0021]    [0021]FIG. 5 illustrates the removal of the photoresist and cleaning of the surface. At this point, the area  30  designated for the thick gate oxide layer and the area  31  designated for the thin gate oxide are still covered by the thin gate oxide layer  40  grown earlier as illustrated in FIG. 2. However, below the thin gate oxide layer  40  in the area  20  where the thick gate oxide is desired resides a zone  70  where the implanted ions described above reside. The dose of the implanted ions should result in an ion concentration in this layer  70  in the range of about 2×10 16  to about 1×10 17  ions per square centimeter.  
         [0022]    Following the removal of the photoresist and cleaning of the surface, a subsequent heat treatment will result in the oxidation of the silicon at the surface underneath the nitrided gate oxide material  40 , as shown in FIG. 6. The heat treatment is conducted at a temperature range of about 400 to about 600° C. for a period of about 30 to about 120 minutes. This treatment will result in a thickened gate oxide region  41 , yet is performed at a lower temperature than prior art procedures. The thickness of oxide added in the area  30  where the thick gate oxide is desired will depend on the dose of oxygen ions. Doses of the suggested density levels disclosed above will result in about 10 to about 40 Angstroms of additional silicon dioxide underneath the silicon nitride, respectively.  
         [0023]    In another embodiment of the invention, an additional region of yet another thickness of gate oxide could be accomplished by repeating the above steps and varying the parameters as desired.  
         [0024]    While the invention has been particularly shown and described with reference to preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.