Abstract:
A method for forming Dual Damascene structures wherein a via is etched to an element to be contacted, a non-photoreactive protective layer is deposited in the via, and an intersecting trench is formed. The protective layer is then removed, together with any residual debris resulting from the trench formation. The protective layer enhances reliability of the electrical contact at the bottom of the via.

Description:
FIELD OF THE INVENTION 
     This invention relates to semiconductor processing methods for imparting electrical contacts and multi-level electrical interconnection to integrated circuits. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of ultra large scale integrated circuits (ULSI), such as 4 megabit and up dynamic random access memories (DRAMs), one approach is to use an inlaid wiring technology known in the art as “Dual Damascene” technology, as described in Kaanta, C. W., et al., “Dual Damascene: A ULSI Wiring Technology,” IBM General Technology Division, Essex Junction, Vermont,  VMIC Conference,  Jun. 11-12, 1991, pp. 144-152. 
     One Dual Damascene process utilizes first and second successive etching steps in order to arrive at a trough and via geometry within a surrounding insulating layer formed on the surface of a silicon wafer. The first etch step forms a trough which extends down to a controlled depth within the insulating layer. The second etch step extends the depth of the trough down to the active devices within the silicon substrate to form the via. 
     Another Dual Damascene process utilizes a first etch step to form a via through the insulating layer to the active devices within the substrate. To form the trench, a second layer of resist is then patterned over the insulating layer leaving the via exposed. The insulating layer is again etched, although not completely, thereby creating a trench in the insulating layer but no additional contacts to the substrate. 
     In each of the above Dual Damascene processes, after formation of the via and trench geometry a layer of conductive material is then blanket deposited over the surface of the insulating layer, and the wafer is planarized to leave conductive material within the via and trench. 
     Various problems are associated with the processes described above. One problem arises because the insulating layer is first etched to completely, or partially, form the via and then a second patterned resist layer is formed and the insulating layer is again etched. The subsequent etch results in the formation of non-volatile carbon-based debris in the bottom of the via. Due to the small size of the via, it is very difficult to completely remove the debris, and thus the conductive material which contacts the active device within the substrate may not make adequate electrical contact. In addition, two-step via fabrication processes, wherein the via is partially completed with the first etch, and then fully etched to expose the substrate during a subsequent trough etch, are inherently prone to producing non-uniform vias. 
     An approach to avoiding the above problems is to first etch a via to expose the substrate below a first insulating layer, then deposit and planarize a first metal layer to form a metal plug to the substrate. A second insulating layer having a trench is then patterned over the first metal layer and the first insulating layer. Next, a second metal layer is formed over the second insulating layer and then planarized. This approach, however, requires the formation and planarization of two insulating layers and two metal layers, thus adding multiple additional steps and an additional metal-to-metal interface, which also can be difficult to form reliably. 
     What is needed is a reliable and efficient Dual Damascene process, which provides uniform vias and avoids via debris and other problems which can result in inadequate electrical contact. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for forming vias and trenches for metalization and multi-level electrical interconnection in ULSI using a single metal deposition and a minimum of process steps for each interconnection. 
     According to the method of the invention, an insulator layer is deposited over a conductive substrate or device to be contacted. A via is then etched in the insulator layer to outwardly expose a surface of the conductive substrate. A non-photoreactive protective layer, preferably an organic anti-reflective coating, is then deposited in the via, followed by a photoreactive layer to pattern the line. A trench is then patterned and etched in the insulator layer and in communication with the via. The protective layer is then removed from the via, together with any residual debris resulting from the trench etch. A metal or other conductive material is then deposited in the via and trench, and then planarized. 
     The above process steps can be repeated to form multiple levels of via contacts and trough interconnects using a non-photoreactive layer to protect the via during the trench etch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at an early processing step according to one embodiment of the present invention. 
     FIG. 2 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  1 . 
     FIG. 3 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  2 . 
     FIG. 4 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  3 . 
     FIG. 5 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  4 . 
     FIG. 6 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  5 . 
     FIG. 7 is a diagrammatic cross-sectional view of a portion of a semiconductor wafer at a processing step subsequent to that shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary construction of a fabrication process for forming via contacts and trough interconnects according to one embodiment of the present invention is described below. It is to be understood, however, that this process is only one example of many possible processes. For example, while in the description below, the via contact forms an electrical communication to a first metal layer atop a substrate, the via may also make electrically contact directly with active devices or other operable regions of the substrate. In addition, the protective layer may be any non-photoreactive material, although an anti-reflective coating is preferred. Negative photoresists and any other materials that do not develop out during the trench fabrication steps may be used. The invention is not intended to be limited by the particular process described below. 
     FIGS. 1-7 present a sequence of steps for constructing a wafer as shown in fragmentary view in FIG.  7 . The wafer comprises an electrically conductive path  10  and an electrically conductive layer  11 , the path  10  includes a horizontal interconnect  12  and a vertical contact  13 , the contact  13  providing electrical connection between the interconnect  12  and the electrically conductive layer  11 . The wafer also includes a substrate (not shown) which supports the foregoing components of the wafer, and an insulator layer or dielectric  14  which rests upon the electrically conductive layer  11 . The term “substrate” herein shall be understood to mean one or more insulative, conductive or semiconductive layers or structures which may include active or operable portions of semiconductor devices. The substrate will typically include one or more insulative layers of etchable material. By way of example, the insulator layer  14  may be constructed of silicon dioxide. The conductive layer  11  is typically constructed of a metal, such as aluminum, tungsten or copper, and may also be fabricated of a non-metallic conductive material, such as polysilicon. Also, the contact  13  and the interconnect  12  are constructed preferably of a metal such as aluminum or tungsten, or a non-metallic conductive material such as polysilicon. 
     The procedure for construction of the wafer begins in FIG. 1 with the deposition of the material of the insulator layer  14  upon the electrically conductive layer  11 . A typical depth of the layer  14  is approximately 1.5 microns. To facilitate the description, the interface between the layer  14  and the conductive layer  11  is referred to as the bottom surface of the layer  14 , the opposite surface being designated the top surface. Following the deposition of the material of layer  14 , preferably the top surface of the layer  14  is then planarized to remove any undulations. Planarization may be conducted in conventional fashion by use of photoresist-RIE-etchback or chemical mechanical polishing (CMP) of the top surface of layer  14 . During the planarization step, the thickness of the layer  14  is typically reduced to approximately 1.3 microns. This is followed by deposition of a layer of photoresist  15  on the top of the top surface of layer  14 . Using a mask and well-established photolithography steps, an opening  16  is created in the photoresist  15 , the opening extending through the layer of photoresist  15  down to the top surface of the layer  14 . 
     Referring now to FIG. 2, an oxide etch, for example, is applied to create via  17 . Via  17  extends from the top surface of insulator layer  14  to the bottom surface of layer  14  and exposes a portion of the surface  18  of the electrically conductive layer  11 . The layer of photoresist  15  is then removed, resulting in the structure shown in FIG.  2 . 
     The procedure continues, as shown in FIG. 3, by forming a protective layer  19  within via  17 . The protective layer  19  covers the exposed surface  18  of the electrically conductive layer  11  in via  17  during subsequent trench etch and processing. The protective layer  19  may be any material which will not develop out during the subsequent photoprocessing steps and, preferably, is comprised of an organic anti-reflective coating (ARC). Layer  19  may also comprise a negative photoresist, or any other material that is not photoreactive. With vias having a relatively small geometry, the protective layer  19  preferably fills the via. Large, vias may be partially filled, as illustrated in FIG.  3 . The protective layer  19  will typically also form a coating  20  on the sidewalls of via  17 , and a coating  21  on the top surface of insulator layer  14 . It is preferred that the thickness of coatings  20  and  21  be less than the depth  22  of the protective layer  19 . The protective layer  19  may be deposited by spinning onto the wafer, or by any other means suitable for applying a photoresist material. This procedure results in the structure shown in FIG.  3 . 
     Referring now to FIG. 4, a second photoresist layer  24  is then applied to the coating  21  of protective layer  19  by masking and use of a developer. Photoresist layer  24  is preferably a positive photoresist. If negataive resist is used to form the pattern then positive resist can be used for the protective film. The insulator layer  14  is then partially etched by reactive ion etching (RIE) or other suitable means to form a horizontal trough  24  at the location of the via  17 . The etchant should etch the oxide or the material of insulator layer  14  selectively with respect to the anti-reflective coating or other material of the protective layer  19 . Accordingly, the etchant used to create the trough  24  does not completely remove the protective layer  19  from the bottom of via  17 . This selective etch produces the structure shown in FIG.  5 . 
     As shown in FIG. 5, oversizing of the trough  24  in the direction transverse to the via  17  allows for some misalignment among the masks of the via and trench photolithography processes so that, even if the trough mask is not centered along an axis of via  17 , an adequate opening can still be created. With respect to misalignment of the mask in the longitudinal direction of the trough  24 , the trough  24  extends for a sufficient distance beyond the via  17  to insure an adequate area of intersection of the via  17  with the trough  24 . The via can also be fabricated to an oversized width in the transverse direction of the via  17  to allow for some misalignment among the masks so that even if the trough is not over-sized, and not centered along an axis of the via, an adequate opening can still be created. Alternatively, the via and trough can both be fabricated without any oversizing. 
     The procedure continues with a stripping off of photoresist layer  24  and an etching of the protective layer  19  at the bottom of via  17  and coating  21 , as shown in FIG.  5 . If an organic anti-reflective coating is used as the protective layer  19 , removal is preferably accomplished in situ by use of an oxide plasma etch. Alternatively, any post ash treatment or wet cleanse removal process can be used where suitable for the various types of protective layers that may be used. In FIG. 6, the photoresist layer  24  has been stripped and the residual protective layer  19  has been removed from the bottom of via  17 . 
     The via  17  and trough  24  are next filled with an electrically-conductive material, preferably a metal such as that employed in the construction of the conductive layer  11 . For example, in the event that the conductive layer  11  is constructed of aluminum, then the via  17  and the trough  24  are filled with aluminum by physical or chemical vapor deposition, or by electroplating if copper. The metal in the trough  24  is then planarized down to the top surface of the insulator layer  14 . This produces the structure of the portion of the wafer shown in FIG.  7 . The portion of the metal  10  deposited within the via  17  has become the stud of a contact  13  to the underlying metal  11 ; the portion of the metal deposited in the trough  24  has become the interconnect  12 . 
     The above process steps can be repeated in succession a plurality of times in order to fabricate multiple levels of via contacts and trough interconnects to form multi-level ULSI circuits. 
     The above description and accompanying drawings are only illustrative of preferred embodiments which can achieve and provide the objects, features and advantages of the present invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. The invention is only limited by the spirit and scope of the following claims.