Abstract:
In some embodiments, a damascene structure may be formed with metal lines separated by a dielectric layer. Portions of the dielectric layer may be ion implanted with carbon and/or inert species to lower selectively the dielectric constant, while leaving the bulk of the dielectric layer unaffected by the implant. As a result, suitably low dielectric constants can be achieved in damascene dielectric layers with sufficient mechanical strength.

Description:
BACKGROUND 
     This relates generally to the fabrication of integrated circuits. 
     In the fabrication of integrated circuits, metal lines may be formed to carry signals between integrated components. One factor that adversely affects the signals carried along such metal lines is capacitance between proximate metal lines. The capacitance is a function of the dielectric constant of the dielectric material between the metal lines. Thus, it is advantageous to form dielectrics between metal lines with relatively low dielectric constants. 
     For example, in the widely used damascene process, trenches are formed in a dielectric and these trenches may be filled with metal layers to form metal lines. Thus, the dielectric between the metal lines is actually formed before the lines themselves. 
     Existing techniques for forming dielectric layers of lower dielectric constant generally result in relatively low mechanical strength. Thus, the resulting integrated circuits may be more prone to failure because of the poor mechanical characteristics of the lower dielectric constant dielectric layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged, cross-sectional view at an early stage of manufacture in accordance with one embodiment; 
         FIG. 2  is an enlarged, cross-sectional view at a subsequent stage to that shown in  FIG. 1  in accordance with one embodiment; 
         FIG. 3  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 2  in accordance with one embodiment; 
         FIG. 4  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 3  in accordance with one embodiment; 
         FIG. 5  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 1  in another embodiment; 
         FIG. 6  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 5  in accordance with one embodiment; 
         FIG. 7  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 6  in accordance with one embodiment; and 
         FIG. 8  is an enlarged, cross-sectional view at a stage subsequent to that shown in  FIG. 7  in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with some embodiments, a dielectric may be formed which has both low dielectric constant and suitable mechanical strength. In some embodiments, implants may be done in a way that provide the desired low dielectric constant in certain critical areas, while leaving other areas of the dielectric unaffected by the implants, so that overall the dielectric layer exhibits suitable mechanical strength. 
     Referring to  FIG. 1 , an etch stop layer  12  may be formed over an integrated circuit substrate  10  in one embodiment. The etch stop layer  12 , in one embodiment, may be silicon nitride. A dielectric layer  14  may be formed over the etch stop layer  12 . The dielectric layer  14  may be silicon dioxide and, in one embodiment, may be high density plasma or HDP oxide. 
     The deposition of the dielectric layer  14  can be by HDP deposition or plasma enhanced chemical vapor deposition, to mention two examples. It may be silicon dioxide, in one embodiment, or fluorinated glass as another example. Furnace thermal growth may also be used. 
     In accordance with one embodiment, shown in  FIG. 2  a plurality of trenches  16  may be formed through the dielectric layer  14 , down to the etch stop layer  12 . The trenches  16 , in some embodiments, may be formed by conventional etching techniques using conventional damascene technology to form damascene structures. In the embodiment shown in  FIG. 2 , the trenches have a wider upper region and a narrower lower region. 
     Then, referring to  FIG. 3 , the trenches  16  may be subjected to an ion bombardment I to reduce the dielectric constant of the bombarded portion of the dielectric layer. As used herein, the term “bombardment” includes any process that propels ions into a surface including ion implanting, sputtering, bombarding, or energetic deposition, as in the case of the use of an HDP deposition tool. In some embodiments, an angled implant may be utilized. For example, an angle of approximately 5 to 10 degrees may be used in some embodiments. In one embodiment, the angled implant or bombardment I may be carbon dioxide, carbon monoxide, or carbon. Alternatively or in addition, fluorine may be used. As another example, the ions may actually be in the form of an ion deposition or bombardment by the tool used to form the dielectric layer  14 , such as an HDP tool. 
     As a result, all exposed dielectric surfaces are doped by carbon, followed by carbon diffusion into the dielectric. “Doping” is used to indicate that carbon enters the dielectric structure even though the doping species may not be substitutional in the molecular structure of the dielectric. Thus, as indicated at  18 , as a result of the angled implant or bombardment, the carbon enters the exposed upper planar surfaces of the dielectric  14  and the walls of the trenches  16  to a greater or lesser extent. Generally, the wider portion of the trench receives more doping than the narrower lower portion. 
     Then, referring to  FIG. 4 , the etch stop layer  12  aligned under the trench  16  may be removed and the trench  16  may be filled with a metal, such as copper, according to the conventional damascene procedure. 
     In some embodiments, dielectric regions of different quality are formed in the resulting structure. For example, the dielectric material along the periphery of the metal lines  20  may be of a desirably lower dielectric constant, caused by carbon ions. The remainder of the dielectric may maintain sufficient strength in the bulk of the material that was not damaged by ion bombardment. Moreover, the dielectric material proximate to the upper, wider portion of the metal line  20  may be higher in carbon concentration than the material along the thinner, lower portion on a line. 
     In some embodiments, in addition to carbon bombardment, the exposed dielectric surface may also be bombarded by inert species, such as argon or xenon, on an implanter or an HDP tool. Surface heating during the inert species bombardment may result in diffusion of inert species, such as argon or xenon impurities, as well as carbon impurities, into the dielectric layer  14 . Angled bombardment increases efficiency of sidewall bombardment while also reducing damage on the etch stop layer  12  at the bottom of the exposed trenches  16 . The inert species bombardment may occur before, during, or after the carbon bombardment. However, advantageously, it occurs before or during the carbon or fluorine bombardment. In some cases, an anneal may be used to drive the bombarded impurities into the dielectric layer. The inert species bombardment may cause damage to the dielectric layer, reducing its dielectric constant. 
     In some cases, a relatively low energy, low dose bombardment is possible due to heating of the bombarded substrate. In one embodiment, the heating is due to bombardment by inert species. The bombardment energies for both carbon or fluorine and inert species may be 5 KeV or less, readily available in HDP tools. The heating causes diffusion which compensates for use of lower dose (e.g. 10 15  to 10 17  atoms per square centimeter) and lower energy bombardment. In some cases, the bombarded carbon or fluorine reacts with silicon in the dielectric layer  14 , due to the effect of heating. In some cases, the inert species bombardment contributes to such a reaction due to surface amorphization caused by the inert species bombardment. Temperatures in the range of 500 to 650° C. may be advantageous in some cases. In some embodiments, heating is controlled or limited to avoid damaging underlying metallic layers. 
     In accordance with another embodiment, the structure shown in  FIG. 1  is formed. Thereafter, a mask  22  is patterned and defined on the upper surface of the dielectric  14 . For example, the mask  22  may be photoresist, designed to shield the region between two proximate trenches  16  in part, but not in whole, so that only the part of this region proximate to the trenches  16  is subjected to bombardment J, shown in  FIG. 5 . In one embodiment, the bombardment J may use carbon dioxide or carbon impurities. All exposed surfaces end up being doped by the carbon with ensuing carbon diffusion into the dielectric. 
     Then, as shown in  FIG. 6 , the mask  22  is removed and the entire exposed surface is subjected to a second bombardment. The second bombardment K may be of an inert species, such as argon or xenon, on an ion implanter or an HDP tool, as two examples. Surface heating during the inert species bombardment may provide diffusion of both the argon or xenon, as well as carbon impurities. Thus, as indicated at  26 , in some locations, both the implants J and K were received (circles with interior dots) in the dielectric surface, while only the second implant of the inert species is effective to dope the central area (open circles) masked by the mask  22 . 
     Then, as shown in  FIG. 7 , the trenches  16  may be formed with the same shape described previously in one embodiment. Between two proximate trenches  16 , is a first region that was originally masked by the mask  22  and which received only the implant K. Two regions on either side of the first region, proximate to the trenches  16 , received both the implant J and the implant K. 
     Next, as shown in  FIG. 8 , the trenches may be filled with metal to form metal lines  20 , after removing the etch stop layer at the bottom of the trenches  16 , and planarized. As a result, the bulk of the dielectric layer  14  has good mechanical strength, but the region proximate to the metal lines  20  exhibits a low dielectric constant. 
     References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
     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.