Abstract:
Methods for planarizing layers of a material, such as a dielectric, and interconnect structures formed by the planarization methods. The method includes depositing a first dielectric layer on a top surface of multiple conductive features and on a top surface of a substrate between the conductive features. A portion of the first dielectric layer is selectively removed from the top surface of at least one of the conductive features without removing a portion the first dielectric layer that is between the conductive features. A second dielectric layer is formed on the top surface of the at least one of the conductive features and on a top surface of the first dielectric layer, and a top surface of the second dielectric layer is planarized. A layer operating as an etch stop is located between the top surface of at least one of the conductive features and the second dielectric layer.

Description:
BACKGROUND 
     The invention relates generally to semiconductor device fabrication and, in particular, to methods for planarizing layers of dielectric material during semiconductor device fabrication and structures formed by the planarization methods. 
     The manufacture of integrated circuit chips generally involves the deposition and patterning of a series of layers composed of different types of materials. In the semiconductor industry, chemical mechanical polishing (CMP) is a commonly used technique for smoothing, planarizing, and/or removing layers from a wafer during manufacture. To execute a CMP process, a wet abrasive and corrosive slurry is dispersed between the surface to be polished and a flexible polishing pad. The polishing pad has a contacting relationship with the surface and the polishing pad and wafer are spun relative to the other to smooth and planarize (i.e., flatten) the contacted surface on the wafer by material removal. The CMP system typically includes a polishing head that presses the rotating wafer against the flexible polishing pad. The CMP process combines the chemical removal effect of an acidic or basic fluid solution operating as the carrier of the slurry with the mechanical removal effect provided by an abrasive material of the slurry that is suspended in the carrier. 
     Improved methods and structures are needed for CMP processes, especially CMP processes employed to planarize material layers used in semiconductor device fabrication and, in particular, CMP processes employed to planarize interlayer dielectric layers used in fabricating back-end-of-line (BEOL) interconnect structures. 
     BRIEF SUMMARY 
     In one embodiment of the invention, a planarization method is provided that includes forming a plurality of conductive features on a substrate, depositing a first dielectric layer on a top surface of the conductive features and on a top surface of the substrate between the conductive features, and selectively removing a first portion of the first dielectric layer from the top surface of at least one of the conductive features without removing a second portion of the first dielectric layer that is located between the conductive features. The planarization method further includes forming a second dielectric layer on the top surface of the at least one of the conductive features and on a top surface of the second portion of the dielectric layer. A top surface of the second dielectric layer is then planarized. 
     In another embodiment of the invention, an interconnect structure includes a first dielectric layer having a top surface, a second dielectric layer directly on the top surface of the first dielectric layer, a plurality of conductive features in the first dielectric layer, and an etch stop layer on the top surface of at least one of the conductive features. Each of the conductive features has a top surface substantially coplanar with the top surface of the first dielectric layer. The etch stop layer is disposed between the top surface of at least one of the conductive features and the second dielectric layer. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the embodiments of the invention. 
         FIGS. 1-5  are diagrammatic cross-sectional views of a portion of a substrate at successive fabrication stages of a processing method in accordance with an embodiment of the invention. 
         FIGS. 6-8  are diagrammatic cross-sectional views of a portion of a substrate at successive fabrication stages of a processing method in accordance with an embodiment of the invention. 
         FIGS. 9-13  are diagrammatic cross-sectional views of a portion of a substrate at successive fabrication stages of a processing method in accordance with an embodiment of the invention. 
         FIGS. 14-16  are diagrammatic cross-sectional views of a portion of a substrate at successive fabrication stages of a processing method in accordance with an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1  and in accordance with an embodiment of the invention, a plurality of relatively small width features  10 ,  12 ,  14 ,  16 , a medium width feature  18 , and a relatively large width feature  20  are directly supported on a top surface  22  of a substrate  24 . Substrate  24  may be any suitable substrate that a person having ordinary skill in the art would recognize as used in the fabrication of an integrated circuit. For example, the substrate  24  may be an interlayer dielectric (ILD) layer of an underlying metallization or via level in a back-end-of-line (BEOL) interconnect structure and may include wires and/or stud-filled vias (not shown) as understood by a person having ordinary skill in the art. 
     The features  10 ,  12 ,  14 ,  16 ,  18 ,  20  may be wires of the BEOL interconnect structure as these structures are understood by a person having ordinary skill in the art. The features  10 ,  12 ,  14 ,  16 ,  18 ,  20  may be composed of a conductive material, such as aluminum or aluminum alloyed with another material like silicon or copper. A top surface of each of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  is covered by a bottom antireflective coating (BARC) layer  26  and an etch stop layer  28  of a dielectric material separated in each instance from the top surface by the BARC layer  26 . The etch stop layer  28  may be composed of a dielectric material, such as silicon nitride (Si 3 N 4 ). The BARC layer  26 , which provides a reduced reflectivity for prospective lithography processes, is composed of an organic or inorganic material that is characterized by a lower reflectivity than the conductive material of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . Common inorganic materials for the BARC layer  26  include, but are not limited to, metal oxides like titanium dioxide (TiO 2 ) and chromium oxide, and metal nitrides like titanium nitride (TiN). The BARC layer  26  and the etch stop layer  28  may be applied with physical vapor deposition (PVD), chemical vapor deposition (CVD), or another conventional deposition technology. The etch stop layer  28  is deposited to a thickness sufficient that the resultant layer will act as an effective etch stop to a subsequent dielectric etch process, as described below. Generally, the BARC layer  26  and etch stop layer  28  are significantly thinner than the height of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  and are only applied to the top surfaces of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     In one embodiment, a layer of the conductive material contained in the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  and overlying layers of the dielectric materials in the BARC layer  26  and etch stop layer  28  are serially deposited in the proper sequence on the top surface  22  of the substrate  24 . A conventional lithography and subtractive etching process is then used to pattern these serially deposited conductive and dielectric layers. The lithography portion of the process applies a radiation-sensitive resist (not shown) on the deposited layers, exposes the resist to a pattern of radiation (e.g., light, x-rays, or an electron beam), and develops the latent transferred pattern to define openings in the resist that expose portions of the conductive material and dielectric material layers to be removed from the top surface  22 . The masked portions of the conductive material layer coincide with the prospective locations of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The openings in the resist have a line width selected to provide the different widths for the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  and a selected line spacing between adjacent features. The features  10 ,  12 ,  14 ,  16  define a low density pattern containing multiple features with a relatively small line spacing. 
     The subtractive etching portion of the process involves an anisotropic dry etch process, like reactive ion etching (RIE) or plasma etching, that transfers the pattern in the resist to the conductive material and dielectric material layers. The subtractive etching process, which relies on one or more etchant chemistries to remove the different conductive and dielectric materials, stops on the constituent material contained in the substrate  24 . The etchant chemistry used to remove the conductive material removes the conductive material selective to (i.e., at a higher etch rate than) the material constituting the substrate  24 . The features  10 ,  12 ,  14 ,  16 ,  18 ,  20  represent residual portions of the conductive material layer that are masked by the patterned resist layer during the subtractive etching process. Features  10 ,  12 ,  14 ,  16  have the smallest line width, such as the minimum line width, feature  20  has the largest line width, and feature  18  has an intermediate line width. After etching, the resist is removed by ashing or solvent stripping. 
     In an alternative embodiment, the etch stop layer  28  may be conformally deposited on the top surfaces of features  10 ,  12 ,  14 ,  16 ,  18 ,  20  following the conclusion of the subtractive etching process, instead of before the subtractive etching process is executed. 
     A dielectric layer  30  is formed on the exposed portions of the top surface  22  and across the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The dielectric material contained in the dielectric layer  30  may be composed of, for example, an oxide of silicon, such as silicon dioxide (SiO 2 ), or a fluorosilicate glass (FSG) deposited by a high density plasma chemical vapor deposition (HDPCVD) process. Silicon dioxide may be deposited by a HDPCVD process using a mixture of process gases including silane (SiH 4 ), oxygen (O 2 ), and argon (Ar). Because of the addition of a sputter component to a plasma deposition process, HDPCVD processes can be controlled to promote gap-filling during deposition in a manner superior to conventional CVD processes. Generally, a dielectric layer deposited by HDPCVD possesses superior density, moisture resistance and planarization properties in comparison with conventional CVD dielectric layers. HDPCVD processes are characterized by a reduced level of conformality (i.e., non-conformality) with the topography of the top surface  22  and features  10 ,  12 ,  14 ,  16 ,  18 ,  20  in comparison with conventional CVD processes. The dielectric layer  30  is highly non-conformal with significantly greater thicknesses of dielectric material residing on horizontal surfaces of the substrate  24  and features  10 ,  12 ,  14 ,  16 ,  18 ,  20  as opposed to vertical surfaces. 
     Portions of the dielectric layer  30  fill the gaps between features  10 ,  12 ,  14 ,  16 ,  18 ,  20  to define a metallization level of the BEOL interconnect structure. Other portions of the dielectric layer  30 , which deposit on the top surfaces of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 , are separated from these top surfaces by the BARC layer  26  and the etch stop layer  28 . Because of the nature of the surface topography, the dielectric material in the dielectric layer  30  includes irregularities in the form of shallow recesses  31  between the features  10 ,  12 ,  14 ,  16 . Portions  32 ,  34  of the dielectric material reside on the top surfaces of features  18 ,  20 . The thickness of the dielectric layer  30  is slightly larger than the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20  measured relative to the top surface  22  and, in one embodiment, the thickness of the dielectric layer  30  is approximately equal to about 110% of the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     The presence of etch stop layer  28  promotes the thickness reduction and eliminates RIE variability inherent in conventional reverse mask dielectric etch back processes that are applied during the chemical mechanical planarization of a dielectric layer of much greater thickness. The dielectric material contained in the etch stop layer  28  has good etch selectivity in comparison with the dielectric material contained in the dielectric layer  30 . 
     The lines and spaces (L/S) layout rules for the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  may be selected to aid in subsequent planarization of the dielectric layer  30 . Specifically, the spacing for the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  may be selected to be four (4) times the minimum spacing, or larger, which is believed to reduce the occurrence of voids in the dielectric layer  30  in the gaps between adjacent features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . In addition, the maximum width for the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  may be selected to be at least two (2) times the height of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 , which may aid in planarization during deposition of dielectric layer  30 . 
     In certain embodiments, the height of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  is 4 μm or taller. The selective reverse planarization process described herein is particularly applicable to such tall features in comparison with a conventional reverse planarization process that includes chemical mechanical planarization or with conventional chemical mechanical planarization alone. These conventional planarization approaches result in an acceptable via height variability for features with heights of 1 μm or less. However, the via height variability scales for these conventional planarization approaches upwardly with increasing feature height. These conventional planarization approaches fail to provide an acceptable via height variability for comparatively tall features with heights greater than 1 μm and, in particular, for features with heights greater than 4 μm. 
     A reverse mask layer  36  is formed from a radiation-sensitive resist that is applied on the dielectric layer  30  and patterned with a conventional photolithography process. The resist is applied by a spin coating process and then heated in a soft baking or pre-baking process to drive off excess solvent and to promote partial solidification. In the photolithography process, the resist is exposed to radiation imaged using a photomask, baked after exposure, and developed to define residual areas of resist that operate as the reverse mask layer  36 . In photolithography, a pattern of radiation is generated using a photomask and an optical stepper of a lithography tool and then imaged onto the resist. If the resist is, for example, a positive resist, regions exposed to the radiation become chemically less stable to promote removal when the resist is developed and regions that are not exposed to the radiation remain chemically stable so that they remain intact when the resist is developed. 
     After patterning, the reverse mask layer  36  includes a feature pattern with apertures or openings  38 ,  40  that extend to the depth of the dielectric layer  30 . The openings  38 ,  40  operate as windows that substantially coincide with the locations of the portions  32 ,  34  of dielectric layer  30  that covers the top surfaces of features  18 ,  20 . The widths of the openings  38 ,  40  are slightly narrower than the widths of the respective portions  32 ,  34  of the dielectric layer  30  to ensure that the subsequent etching process will stop on the etch stop layer  28 . The reverse mask layer  36  covers and protects the remainder of the dielectric layer  30 . The reverse mask layer  36  is a substantial reverse image of the mask used to pattern the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     With reference to  FIG. 2  in which like reference numerals refer to like features in  FIG. 1  and at a subsequent fabrication stage, an anisotropic dry etch process, such as a reactive ion etching (RIE) process, is used to partially remove the portions  32 ,  34  of dielectric material exposed by the openings  38 ,  40  in the reverse mask layer  36 . The dry etch process relies on an etchant chemistry that removes the dielectric material of the dielectric layer  30  selective to (i.e., at a higher etch rate than) the material constituting the etch stop layer  28  and stops on the etch stop layer  28 . After the resist is removed by ashing or solvent stripping, the top surface  22  is cleaned using a conventional cleaning process. The reverse mask process removes a thickness of dielectric material in the mounds  32 ,  34  exposed by the openings  38 ,  40  approximately equal to the deposited thickness of the dielectric layer  30  because of the presence of the etch stop layer  28 . 
     With reference to  FIG. 3  in which like reference numerals refer to like features in  FIG. 2  and at a subsequent fabrication stage, a dielectric layer  42  is conformally deposited on the dielectric layer  30  and across the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The dielectric material constituting the dielectric layer  42  may be, for example, composed of an oxide of silicon, such as SiO 2 , or FSG deposited conformally by a plasma enhanced chemical vapor deposition (PECVD) process. The dielectric layer  42  is highly conformal (i.e., of approximately the same thickness, for example, within ±2-10% of the average thickness) on vertical and horizontal surfaces. In one embodiment, the dielectric layers  30  and  42  are composed of the same dielectric material. The thickness of the dielectric layer  42  is less than the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20  measured relative to the top surface  22 . The top surface  44  of the dielectric layer  42  has an undulating or rippled surface profile with protrusions  46 ,  48 ,  50 ,  52  that coincide with the unetched remainder of the portions  32 ,  34  of the dielectric layer  30 . In one embodiment, the thickness of the dielectric layer  42  is approximately equal to about 50% of the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . Generally, the dielectric layer  42  is thinner than dielectric layer  30  and, in certain embodiments, the thickness of dielectric layer  42  is approximately one-half or less of the thickness of dielectric layer  30 . 
     With reference to  FIG. 4  in which like reference numerals refer to like features in  FIG. 3  and at a subsequent fabrication stage, a polishing operation, preferably a chemical mechanical polishing (CMP) process, is conducted to polish the dielectric layer  42  until the top surface  44  is substantially planar and flat. As the top surface  44  of the dielectric layer  42  is polished, the CMP process removes the protrusions  46 ,  48 ,  50 ,  52 . For example, the CMP process may remove a thickness of the dielectric layer  42  equal to 25% of the height of the conductive features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . A buffered etchant is used to clean residual slurry from the top surface  44  that remains following the CMP process. The use of the reverse mask layer  36  in a preceding fabrication stage to partially remove the portions  32 ,  34  of dielectric layer  42  improves the surface uniformity of the CMP process. After polishing, the dielectric layer  30  defines an intermetal dielectric (IMD) layer in which the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are embedded and the dielectric layer  42  defines at least a partial thickness, if not an entire thickness, of an ILD layer. 
     With reference to  FIG. 5  in which like reference numerals refer to like features in  FIG. 4  and at a subsequent fabrication stage, a dielectric layer  54  is deposited on the planarized top surface  44  of dielectric layer  42 . The dielectric material in dielectric layer  54  may be composed of, for example, SiO 2  or FSG deposited by a PECVD process. In one embodiment, dielectric layer  54  is composed of the same dielectric material as either or both of the dielectric layers  30 ,  42 . The dielectric layer  54  represents the remaining thickness for the interlayer dielectric, in combination with the remaining thicknesses of dielectric layers  30 ,  42 , to be used in subsequent fabrication stages to contact the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  and to potentially support an additional interlayer dielectric. Dielectric layer  54  and the residual thickness of dielectric layer  42  after planarization of top surface  44  define the ILD layer in which stud-filled vias (not shown) are formed to contact the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . Dielectric layer  54  is optional is dielectric layer  42  provides an adequate layer thickness for the ILD layer. 
     In conventional reverse mask planarization processes, the entire thickness of the interlayer dielectric is initially deposited across the conductive features. As an example of a conventional reverse mask planarization process for 4 μm high AlCu wires, a 7 μm thick layer of SiO 2  may be applied, 4 μm of the thickness of the SiO 2  layer may be removed with a reverse mask RIE, and then 4 μm of the thickness of the SiO 2  layer may be removed with a CMP process. Using nominal variabilities of 5% of the thickness for the dielectric deposition, 25% of the removed dielectric thickness for the reverse mask RIE, and 40% of the removed thickness for the CMP process, the root sum square (RSS) of the variations is equal to ±1.2 μm. Hence, for feature heights of 4 μm, the final thickness of the resulting interlayer dielectric layer may vary by ±1.2 μm, which leads to significant variations in via heights for contacting the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     In embodiments of the invention, the deposition of the dielectric material for the IMD and ILD is split into multiple distinct layers at different junctures in the fabrication process. The total sum of the individual thickness of the dielectric layers  30 ,  42  used in the inventive selective reverse mask planarization process is significantly less than the thickness of the interlayer dielectric applied and etched in a conventional reverse mask planarization process. As an example of a selective reverse mask planarization process for 4 μm high AlCu wires, a 4 μm thick layer of SiO 2  may be applied as dielectric layer  30 , 4 μm of the thickness of the SiO 2  layer reflected in the portions  32 ,  34  may be removed using the reverse mask layer  36  and stopping on the etch stop layer  28 , another 4 μm thick layer of SiO 2  may be applied as dielectric layer  42 , and then 4 μm of the thickness of the SiO 2  layer may be removed with a CMP process. Using the same nominal variabilities for dielectric deposition and the CMP process, the RSS of the variations using the selective reverse mask planarization is equal to ±0.5 μm. The presence of the etch stop layer  28  eliminates the variability in physical layer thickness arising from the reverse mask RIE. The ability to reduce the variation in the thickness of the interlayer dielectric reduces the via height variability for contacting the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  in comparison with the interlayer dielectric resulting from a conventional reverse mask planarization process. 
     With reference to  FIG. 6  in which like reference numerals refer to like features in  FIG. 1  and in accordance with an alternative embodiment, the etch stop layer  28  may be omitted from the initial structure if the material of the BARC layer  26  is capable of providing the etch stop functionality during the reverse mask etch process. Accordingly, the top surfaces of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are initially covered only by the BARC layer  26  before the reverse mask layer  36  is applied and the portions  32 ,  34  of the dielectric layer  30  are partially removed by etching. Alternatively, the etch stop layer  28  may provide the functionality of a bottom antireflection coating layer so that the BARC layer  26  may be omitted from the layer stack. 
     With reference to  FIG. 7  in which like reference numerals refer to like features in  FIG. 6  and at a subsequent fabrication stage, the anisotropic dry etch process is used to remove the portions  32 ,  34  of the dielectric layer  30  exposed through the openings  38 ,  40  in the reverse mask layer  36 . The dry etch process is similar to the process performed in conjunction with the fabrication stage of  FIG. 2 , other than the use of the BARC layer  26  as an etch stop layer. Processing continues at the fabrication stage of  FIG. 3 , as described above, and results in an interconnect structure as shown in  FIG. 8 . 
     With reference to  FIG. 9  in which like reference numerals refer to like features in  FIG. 1  and in accordance with an alternative embodiment, a dielectric layer  62  is formed on the top surface  22  of the substrate  24  and across the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The dielectric layer  62  may be, for example, composed of an oxide of silicon, such as SiO 2 , or FSG, conformally deposited by a PECVD process. Gaps between features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are filled by portions of the dielectric layer  62  and the top surfaces of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are covered by other portions of the dielectric layer  62 , which are separated from these top surfaces by the BARC layer  26  and the etch stop layer  28 . The dielectric layer  62  has an irregular topography with a surface profile that includes recesses  64 ,  65 ,  66 ,  67 ,  68 ,  69 ,  70  defined between the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  and portions  72 ,  73 ,  74 ,  75 ,  76 ,  77  of the dielectric material on the top surfaces of features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The thickness of the dielectric layer  62  is slightly larger than the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20  measured relative to the top surface  22  and, in one embodiment, the thickness of the dielectric layer  62  is approximately 125% of the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     A reverse mask layer  80 , which is similar to reverse mask layer  36 , is formed on the dielectric layer  62 . The reverse mask layer  80  includes a feature pattern with apertures or openings  82 ,  83 ,  84 ,  85 ,  86 ,  87  that are registered with the locations of the portions  72 - 77  of the dielectric layer  62 . The widths of the openings  82 - 87  are slightly narrower than the widths of the respective portions  72 - 77 . 
     With reference to  FIG. 10  in which like reference numerals refer to like features in  FIG. 9  and at a subsequent fabrication stage, an anisotropic dry etch process, such as a RIE process, is used to remove the portions  72 - 77  of dielectric layer  62  exposed by the openings  82 - 87  in the reverse mask layer  36 . The dry etch process relies on an etchant chemistry that removes the dielectric material of dielectric layer  62  selective to (i.e., at a higher etch rate than) the material constituting the etch stop layer  28  and stops on the constituent material of the etch stop layer  28 . Each of the portions  72 - 77  of dielectric layer  62  is partially removed by the dry etch process. The resist is subsequently removed by ashing or solvent stripping and the top surface  22  is cleaned with a conventional cleaning process. The reverse mask process removes a thickness of dielectric material in the portions  72 - 77  exposed by the openings  82 - 87  approximately equal to the height of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  because of the presence of the etch stop layer  28 . 
     With reference to  FIG. 11  in which like reference numerals refer to like features in  FIG. 10  and at a subsequent fabrication stage, a dielectric layer  88 , which is similar to dielectric layer  42  ( FIG. 3 ), is deposited on the dielectric layer  62  and across the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The dielectric material constituting the dielectric layer  88  may be composed of an oxide of silicon, such as SiO 2 , or FSG deposited by a PECVD process. In one embodiment, the dielectric layers  62  and  88  are composed of the same dielectric material. The thickness of the dielectric layer  88  is less than the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20  measured relative to the top surface  22 . The top surface  90  of the dielectric layer  88  has an undulating or rippled appearance with protrusions  91 ,  92 ,  93 ,  94 ,  95 ,  96 ,  97 ,  98 ,  99  that coincide with the remainder of the portions  72 - 77  of the dielectric layer  30 . In one embodiment, the thickness of the dielectric layer  42  is approximately equal to about 50% of the height of features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . 
     With reference to  FIG. 12  in which like reference numerals refer to like features in  FIG. 11  and at a subsequent fabrication stage, a polishing operation, preferably a CMP process, is conducted to polish the dielectric layer  88  until the top surface  90  is substantially planar and flat. As the top surface  90  is recessed, the CMP process removes the protruding structures  91 - 99 . For example, the CMP process may remove a thickness of the dielectric layer  88  equal to 25% of the height of the conductive features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . A buffered etchant is used to clean residual slurry from the top surface  90  that remains following the CMP process. After polishing, the dielectric layer  62  defines an IMD layer in which the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are embedded and the dielectric layer  88  defines at least a portion, if not all, of an ILD layer. Generally, the dielectric layer  88  is thinner than dielectric layer  62 . In certain specific embodiments, the thickness of dielectric layer  88  is approximately one-half or less of the thickness of dielectric layer  62 . 
     With reference to  FIG. 13  in which like reference numerals refer to like features in  FIG. 12  and at a subsequent fabrication stage, a dielectric layer  100 , which is similar to dielectric layer  54 , is deposited on the planarized top surface  90 . The dielectric material in the dielectric layer  100  may be, for example, composed of SiO 2  or FSG deposited by a PECVD process. Dielectric layer  100  may be composed of the same dielectric material as either or both of layers  62 ,  88 . The dielectric layer  100  represents the remaining thickness for the interlayer dielectric to be used in subsequent fabrication stages. Dielectric layer  100  and the residual thickness of dielectric layer  88  after planarization of the top surface  90  define an ILD layer in which stud-filled vias (not shown) are formed to contact the features  10 ,  12 ,  14 ,  16 ,  18 ,  20 . The ability to split the deposition of the IMD and ILD into multiple dielectric layers  62 ,  88 ,  100  and the use of the etch stop layer  28  has benefits, as explained above with regard to the embodiment of  FIGS. 1-5 . Dielectric layer  100  is optional is dielectric layer  88  provides an adequate layer thickness for the ILD layer. 
     With reference to  FIG. 14  in which like reference numerals refer to like features in  FIG. 13  and at a subsequent fabrication stage in accordance with an alternative embodiment, the etch stop layer  28  may be omitted if the material of the BARC layer  26  is capable of providing the etch stop functionality during the reverse mask etch process. Specifically, the top surfaces of the features  10 ,  12 ,  14 ,  16 ,  18 ,  20  are initially covered only by the BARC layer  26  before the reverse mask layer  80  is applied. Alternatively, the etch stop layer  28  may provide the functionality of a bottom antireflection coating layer so that the BARC layer  26  may be omitted from the layer stack. 
     With reference to  FIG. 15  in which like reference numerals refer to like features in  FIG. 14  and at a subsequent fabrication stage, the anisotropic dry etch process is used to remove the portions  72 - 77  of dielectric layer  62  exposed through the openings  82 - 87  in the reverse mask layer  80 . The dry etch process is similar to the process performed in conjunction with the fabrication stage of  FIG. 10  and the fabrication stage of  FIG. 2 , other than the use of the BARC layer  26  as an etch stop layer. Processing continues at the fabrication stage of  FIG. 11 , as described above, to produce the interconnect structure shown in  FIG. 16 . 
     The methods, as described above, are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multi-chip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term “horizontal” as used herein is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The term “vertical” refers to a direction perpendicular to the horizontal, as just defined. Terms, such as “on”, “above”, “below”, “side” (as in “sidewall”), “upper”, “lower”, “over”, “beneath”, and “under”, are defined with respect to the horizontal plane. It is understood that various other frames of reference may be employed for describing the invention without departing from the spirit and scope of the invention. It is also understood that features of the invention are not necessarily shown to scale in the drawings. Furthermore, to the extent that the terms “composed of”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
     It will be understood that when an element as a layer, region or substrate is described as being “on” or “over” another element, it can be directly on or over the other element or intervening elements may also be present. In contrast, when an element is described as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is described as being “attached”, “connected”, or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is described as being “directly attached”, “directly connected”, or “directly coupled” to another element, there are no intervening elements present. 
     The fabrication of the structures herein has been described by a specific order of fabrication stages and steps. However, it is understood that the order may differ from that described. For example, the order of two or more fabrication steps may be swapped relative to the order shown. Moreover, two or more fabrication steps may be conducted either concurrently or with partial concurrence. In addition, various fabrication steps may be omitted and other fabrication steps may be added. It is understood that all such variations are within the scope of the present invention. It is also understood that features of the present invention are not necessarily shown to scale in the drawings. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.