Abstract:
A technique in accordance with the invention includes obtaining a semiconductor structure that has a metal disposed thereon. At least a portion of the metal is etched using an etching fluid while sonic energy is applied to the etching fluid.

Description:
BACKGROUND  
         [0001]    The invention generally relates to etching metal using sonication.  
           [0002]    In a variety of different circumstances, it may be desirable to selectively etch metal in the formation of a semiconductor device. For example, the etching of metal may be related to the formation of a metal silicide layer (a nickel silicide layer, for example), a layer used to reduce metal-to-semiconductor contact resistances in a semiconductor device.  
           [0003]    To form a metal silicide layer, a metal layer (nickel, for example) typically is deposited on a semiconductor structure. In this manner, the deposited metal reacts with exposed silicon of the structure to form the metal silicide layer. Not all of the deposited metal layer typically reacts. In this manner, the regions in which the metal layer does not react form excess or un-reacted metal regions that typically are removed by wet etching. As a more specific example, FIG. 1 depicts a semiconductor structure  9  that represents a particular stage in a process to form a complimentary metal oxide semiconductor (CMOS) transistor. For this example it is assumed that the CMOS transistor is formed on a silicon substrate  12 . As shown in FIG. 1, the polysilicon layer  18  resides on top of a gate oxide layer  16 , and vertically extending nitride spacers  20  may be located on either side of the polysilicon layer  18 .  
           [0004]    For purposes of creating a nickel silicide layer, a nickel layer  22  may be blanket deposited over existing layers of the structure  9 . As depicted in FIG. 1, the deposited nickel layer  22  extends over portions of the silicon substrate  12  as well as extends over a polysilicon layer  18 . The regions in which the nickel layer  22  contacts the silicon substrate  12  form parts of the source and drain of the transistor, and the region in which the nickel layer  22  contacts the polysilicon layer forms part of the gate of the transistor in this example.  
           [0005]    Thus, the deposited nickel layer  22  contacts the polysilicon layer  18  and the silicon substrate  12 , and in these contacted regions, the nickel layer  22  reacts with the polysilicon layer  18  and the silicon substrate  12  to form the nickel silicide layer that extends into regions  26  of a resulting structure  10  that is depicted in FIG. 2. As a more specific example, a particular nickel silicide region  26   a  may be associated with a drain of the transistor, another nickel silicide region  26   b  may be associated with a source of the transistor, and another nickel silicide region  26   c  may be associated with a gate of the transistor.  
           [0006]    The deposited nickel does not react everywhere, leaving regions  24  of excess or unreacted nickel. To remove these regions  24 , selective wet etching is used to target the nickel but not other substances (such as nickel silicide, for example) to remove the nickel to form a structure  11  that is depicted in FIG. 3. Thus, after the selective wet etching, the unreacted nickel portions  24  (see FIG. 2) are removed, leaving only the regions  26  of nickel silicide film, as depicted in FIG. 3.  
           [0007]    The wet etching typically involves submersing a wafer that contains the structure  10  into a nickel selective etchant, or etching fluid, that typically includes both an acid, such as sulfuric acid, and an oxidant, such as hydrogen peroxide or nitric acid. At room temperature, the use of sulfuric acid by itself to etch the nickel is not sufficient due to the potential energy barrier that prevents the oxidation of the nickel in accordance with the Pourbaix chart for nickel. Therefore, an oxidant typically is introduced into the etching fluid to supply the needed energy to oxidize the nickel into an aqueous derivative and thus, dissolve the nickel.  
           [0008]    For certain semiconductor devices, an oxidant in the etching fluid may undesirably oxidize and thus, etch substances that are not meant to be etched. For example, elemental germanium substrates, germanium-doped silicon substrates and germanide films are examples of germanium-based substances that typically are highly susceptible to oxidants that are used in the etching of nickel. The etch rates for these germanium substances may be the same or even higher than the etch rate for nickel in the presence of such an oxidant. Therefore, when germanium-based substances are present, the use of conventional etching fluid to etch nickel may undesirably dissolve significant portions of these germanium-based substances.  
           [0009]    Thus, there is a continuing need for a better way to selectively etch metal that is disposed on a semiconductor structure that contains certain semiconductor substrates, films and/or layers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]    [0010]FIGS. 1, 2 and  3  are cross-sectional views of semiconductor structures depicting different stages in the formation of a semiconductor device according to the prior art.  
         [0011]    [0011]FIG. 4 is a flow diagram depicting a technique to form a semiconductor structure according to an embodiment of the invention.  
         [0012]    [0012]FIGS. 5, 6,  7  and  8  are cross-sectional views of semiconductor structures in accordance with embodiments of the invention depicting different stages in the formation of a semiconductor device. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Germanium-based substances (herein called “germanium substances”), such as germanide films, germanium-doped regions and elemental germanium substrates, may be highly susceptible to the etchant, or etching fluid, that is conventionally used to etch nickel. In this manner, a typical etching fluid for nickel contains an acid, such as sulfuric acid, and an oxidant, such as hydrogen peroxide or nitric acid, which are highly oxidizing in nature. Although this etching fluid may be used in a standard silicon-based process, the etching fluid undesirably etches germanium substances because germanium is highly soluble in a low pH, aqueous solution that contains an oxidant (hydrogen peroxide or nitric acid, as examples).  
         [0014]    Thus, if such an oxidant-containing etching fluid is used to etch nickel that is disposed on a semiconductor structure that includes germanium substances, the germanium substances may be undesirably dissolved. However, an etching fluid that lacks an oxidant is not by itself sufficient to etch nickel due to the potential energy barrier that exists for dissolving nickel (i.e., oxidizing nickel to some aqueous nickel derivative) in a low pH solution. To address this problem, an embodiment of a technique in accordance with the invention overcomes the potential energy barrier by applying sonic energy to an oxidant-free etching fluid. Thus, with the application of sonic energy to oxidant-free etching fluid during etching of nickel, the nickel may be selectively etched while germanium substance(s) of the semiconductor structure remain intact.  
         [0015]    Therefore, referring to FIG. 4, an embodiment  100  of a technique in accordance with the invention includes depositing (block  102 ) a metal layer on a semiconductor structure. This metal layer may be, for example, a nickel layer, that reacts with germanium regions of the structure to form a nickel germanide film, or layer. This nickel germanide layer, in turn, may be located between germanium substances of the structure and source and drain metal contacts for purposes of reducing contact resistances between the germanium substances and these contacts. The nickel layer may also be deposited for purposes of forming a nickel silicide layer between a polysilicon layer and a gate metal contact for purposes of reducing a contact resistance between the polysilicon layer and the gate metal contact.  
         [0016]    After the metal layer to form the germanide layer (and possibly a silicide layer) is deposited in accordance with the technique  100 , the resulting metal germanide and silicide regions are annealed, as depicted in block  104 . Subsequently, in accordance with the technique  100 , the structure is selectively wet etched with an oxidant-free etchant, or etching fluid, to remove the excess or unreacted metal regions (unreacted or excess nickel regions, for example) while sonic energy is applied to the etching fluid to supply sufficient energy to facilitate oxidation of the metal being etched, as depicted in block  106 . The etching fluid may include sulfuric acid, for example. Due to the lack of an oxidant in the etching fluid, undesirable etching of germanium substances of the structure does not occur.  
         [0017]    As a more specific example, FIGS. 5, 6,  7  and  8  depict semiconductor structures that represent different stages in the formation of a CMOS transistor, in accordance with some embodiments of the invention. More specifically, FIG. 5 depicts a semiconductor structure  118 , in accordance with an embodiment of the invention, that is formed on a germanium substrate  122 . The substrate  122  may be an elemental germanium substrate. Alternatively, the substrate  122  may be a silicon substrate that is doped with germanium in the source and drain regions of the transistor.  
         [0018]    Regardless of how the germanium is introduced, the germanium substrate  122  includes a first region  125  that may be associated with a source of the transistor and another region  127  that may be associated with a drain of the transistor. The germanium substrate  122  is isolated on either side by insulating oxide regions  124 .  
         [0019]    The germanium substrate  122  may also include a region  129  that is associated with a gate of the transistor. A gate oxide layer  134  is deposited directly on the germanium substrate  122  on the gate region  129 , and a polysilicon layer  128  is formed on top of the gate oxide layer  134 . Nitride spacers  126  may extend upwardly on either side of the polysilicon layer  128 . Alternatively, the polysilicon layer  128  may be replaced by a germanium-based, germanium-silicon-based or metal-based layer, as just a few examples.  
         [0020]    As depicted in FIG. 5, a layer  130  of nickel is blanket deposited on the structure  118  and covers the otherwise exposed germanium substrate  122  and the otherwise exposed polysilicon layer  128 . Reactions occur with the nickel to form a structure  119  that is depicted in FIG. 6.  
         [0021]    Referring to FIG. 6, in this manner, the nickel reacts with the exposed polysilicon  128  and the exposed germanium substrate  122  to form nickel germanide regions  142  over the exposed germanium substrate  122  and a nickel silicide region  140  over the exposed polysilicon layer  128 . Thus, the nickel silicide region  140  is formed from the reaction of silicon (in the polysilicon layer  128 ) with the nickel, and the nickel germanide regions  142  are formed by the reaction of germanium (in the germanium substrate  122 ) with the nickel. Therefore, the reactions with the deposited nickel layer  130  form one nickel germanide region  142   a  that is associated with the drain of the transistor, another nickel germanide region  142   b  that is associated with source of the transistor and the nickel silicide region  140  that is associated with the gate of the transistor.  
         [0022]    As illustrated in FIG. 6, not all of the nickel reacts, thereby leaving unreacted or excess nickel regions, such as the depicted regions  146 . A next step in the process to form the transistor may be the annealing of the nickel silicide region  140  and the nickel germanide regions  142   a  and  142   b . After the annealing, the structure  119  is selectively wet etched in an oxidant-free etchant, or etching fluid, such as sulfuric acid, for example. During this etching, sonic energy (in lieu of the inclusion of an oxidant in the etching fluid) is applied to the etching fluid for purposes of overcoming the high energy barrier that is associated with the dissolution of nickel in solutions of low pH.  
         [0023]    As a more specific example, in some embodiments of the invention, ultrasonic sonic energy in the frequency range between approximately 10 kilohertz (kHz) and 100 kHz may be applied to the etching fluid during the etching of the unreacted nickel. Alternatively, in some embodiments of the invention, megasonic energy in the range of approximately 500 to 1000 kHz may be applied to the etching fluid during the etching of the nickel. The sonic energy may be applied via transducers that are located in, on or near an immersion tank in which the structure  119  is immersed and the wet etching is performed.  
         [0024]    The result of the etching is a structure  120  that is depicted in FIG. 7. In this manner, the etching removes the unreacted nickel regions  146  (FIG. 6) to leave the nickel silicide region  140  located above the polisilicon layer  128  and the nickel germanide regions  142   a  and  142   b  of the drain and source regions, respectively.  
         [0025]    Many other steps may be performed in the process to form the transistor from the structure  120 . As an example of one out of possibly many more steps that may be performed, in some embodiments of the invention, an oxide layer  160  may be subsequently deposited on the structure  120  to form a structure  121  that is depicted in FIG. 8. The oxide layer  160  is polished back and then selective etching is performed to create contact holes so that a metal layer may be deposited to form corresponding transistor contacts  162  with the germanide and silicide films. For example, as depicted in FIG. 8, the structure  121  may include a source metal contact  162   a  that extends through a contact hole in the oxide layer  160  to the nickel germanide region  142   b , and the structure  121  may include a drain metal contact  162   b  that extends through another contact hole in the oxide layer  160  to contact the nickel germanide region  142   a . A gate metal contact may be also formed to the nickel silicide region  140 , although such a contact is not depicted in the cross-section illustrated in FIG. 8.  
         [0026]    Thus, due to the intervening nickel germanide and silicide layers, contact resistances are decreased between upper metal layers and the germanium substrate  122  and polysilicon layer  128 . As an example, tungsten may be used to form the metal contacts  162 . Other metals may be used.  
         [0027]    In the context of this application, although the preceding description may have used such terms as “over” and “on” to describe the relative positions or locations of certain substances, materials or layers these terms do not necessarily mean that the substances, materials or layers contact each other, unless otherwise stated.  
         [0028]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, 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.