Abstract:
A metal carbide film may be etched in an etchant bath using sonication. The sonication may drive the reaction and, particularly, the gaseous byproducts in the form of carbon dioxide. Thus, the use of sonication invokes a favorable equilibrium to pattern metal carbide films, for example, for use as metal gate electrodes.

Description:
BACKGROUND 
   This invention relates generally to the fabrication of integrated circuits. 
   Metal carbide films may be utilized in various applications in connection with integrated circuit fabrication. One possible application is as part of the gate electrode of a metal gate field effect transistor. In order to utilize the films in many semiconductor applications, they need to be etched. 
   Thus, there is a need for ways to etch metal carbide films. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an enlarged, cross-sectional view at an early stage of manufacture in accordance with one embodiment of the present invention; 
       FIG. 2  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
       FIG. 3  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
       FIG. 4  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
       FIG. 5  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
       FIG. 6  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; and 
       FIG. 7  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a semiconductor substrate  10  may be covered by a high dielectric constant dielectric  12 . As an example, the dielectric  12  may be formed of hafnium dioxide. The dielectric  12  may be covered by a metal carbide layer  14 . The metal carbide layer  14  may be made of any metal carbide including carbides of titanium, zirconium, tantalum, hafnium, aluminum, ruthenium, tungsten, and nitrides of those metals. In one embodiment, the layer  14  will become the gate electrode of an n-type or p-type field effect transistor. 
   The metal carbide layer  14  may be covered by a patterned etch stop layer  15  as indicated in FIG.  2 . Etching may proceed down to, but not through, the dielectric  12  using the patterned etch stop layer as a mask. The structure shown in  FIG. 2  may then be covered with another metal carbide layer  17  as shown in FIG.  3 . The metal carbide layer  16  may be a p-type metal carbide layer, such as platinum, ruthenium, or lead carbide. However, in some embodiments, only one of the layers  14 ,  16  may be metal carbide. 
   Referring to  FIG. 4 , the layers  14  and  16 , where n-type transistors will be formed, may be covered by a patterned etch stop layer  18 . The layer  18  may also be patterned over the locations on the substrate  10  where the p-type transistors will be formed. Then, as shown in  FIG. 5 , the metal carbide layer  16  may be etched using the layer  18  as a mask. The layers  14  and  16  may be between 25 and 300 Angstroms in one embodiment. 
   The etching of the metal carbide layers  14  and  16  may be done in a bath of wet etchant under the application of sonic energy. The sonic energy may be ultrasonic energy in the range of 10 to 100 kilohertz or megasonic energy in the range of 0.7 to 1.3 megaHertz, in one embodiment of the present invention. The power may be from 0.5 to 5 Watts per square centimeter in one embodiment of the present invention. The wafers may be immersed in a bath of wet etching solution. In one embodiment, the etching solution may be aqua regia, which includes hydrochloric and nitric acid. 
   The application of sonic energy to an otherwise benign oxidative etchant/metal carbide film drives the formation and dissipation of carbon dioxide, which is the byproduct of the carbide portion of the metal carbide, invoking a favorable equilibrium to pattern the metal carbide layer for use as a metal gate electrode in one embodiment. The removal of gaseous byproducts is according to LeChatelier&#39;s Principle. When sonication is employed, gas bubbles indicating the formation of carbon dioxide in solution may be observed. The sonic energy may add the correct amount of energy to drive the following reaction:
 
TiC+4 HNO3→TiO2+CO2+4 HNO2 
 
under low pH conditions. In one embodiment, the metal carbide layers  14  and  16  may be on the order of 25 to 300 Angstroms in thickness.
 
   After etching the layers  14  and  16 , the structure may be covered by a polysilicon layer  20  as shown in FIG.  6 . The layer  20  may then be covered by patterned etch stop layer  22  to etch the n-type and p-type stacks  24   a  and  24   b , respectively, as shown in FIG.  7 . Thereafter, standard fabrication processes may be utilized, including the formation of epitaxial source/drains, silicidation, and doping of polysilicon to complete the gate electrode stack. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.