Abstract:
A process and apparatus directed to forming a terraced film stack of a semiconductor device, for example, a DRAM memory device, is disclosed. The present invention addresses etch undercut resulting from materials of different etch selectivity used in the film stack, which if not addressed can cause device failure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of integrated circuits and, in particular, to a method and process of forming a terraced film stack in an integrated circuit, such as dynamic random access memories (DRAMs). 
     BACKGROUND OF THE INVENTION 
     As integrated circuits continue to scale to still smaller feature sizes, shrinking device geometry and differing material properties pose challenges for feature processing at 90 nm and below. One problem is that of etch undercut that occurs when etching a film stack consisting of several different materials.  FIG. 22A  illustrates a stack of materials to be etched using a photoresist  400 . In this example, the stack consists of a metal layer  402 , such as titanium, an insulating layer  404 , such as tetraethylorthosilicate (TEOS) or other oxide, and other film layers, such as a polysilicon layer  406 . The photoresist layer  400  is patterned on top, and all of the layers below are etched. Etch selectivity, which describes the etching rate of one material relative to the etching rate of another material, is poor between the Ti metal layer and the TEOS insulating layer. Accordingly, while polysilicon layer  406  is being cleaned, the TEOS insulating layer  404  is unintentionally etched as well, as illustrated in  FIG. 22B . That is, as the polysilicon in polysilicon layer  406  underneath the TEOS insulating layer  404  is being etched vertically, the TEOS insulating layer  404  is etched laterally. Additional undercutting may further result from a subsequent cleaning prior to a deposition as depicted by  FIG. 22C , resulting in an undercut trench  408 . Such an undercut trench becomes difficult to reliably fill using conventional techniques without creating voids in the fill. These voids can be fatal to device performance. 
     SUMMARY OF THE INVENTION 
     It is against the above background that the present invention provides a method and apparatus directed to forming a terraced film stack in a semiconductor device, for example, a DRAM device, which provides a number of advancements and advantages over the prior art. 
     In one embodiment, a method of forming a memory device is disclosed. The method comprises providing a substrate assembly having underlying material layers, and providing an insulating layer over the underlying material layers. The method further includes providing a first metal layer on the insulating layer, providing a photoresist with a first pattern, and etching the insulating layer and the first metal layer through the first pattern to expose at least one of the underlying material layers, the etching defining in the insulating layer a first cavity having a first width. The method also includes etching the photoresist to provide a second pattern, etching the first metal layer through the second pattern to define a second cavity over the first cavity, the second cavity having a second width larger than the first width, removing the photoresist, and depositing a second metal layer over the substrate to fill the first and second cavities. 
     In another embodiment, a memory device having a terraced film stack is disclosed, which comprises a substrate assembly having underlying material layers. An insulating layer is provided over the underlying material layers. The insulating layer has a first cavity having a first width. A metal layer is provided on the insulating layer. The metal layer has a second cavity over the first cavity. The second cavity has a second width larger than the first width, and a material layer is provided over the substrate to fill the first and second cavities. 
     These and other features and advantages of the invention will be more fully understood from the following description of various embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of the embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is a cross-sectional view of the early stages of fabrication of a semiconductor device in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 1 . 
         FIG. 3  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 2 . 
         FIG. 4  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 3 . 
         FIG. 5  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 4 . 
         FIG. 6  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 5 . 
         FIG. 7  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 6 . 
         FIG. 8  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 7 . 
         FIG. 9  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 8 . 
         FIG. 10  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 9 . 
         FIG. 11  shows the semiconductor device of  FIG. 1  at a processing step according to an alternate embodiment of the present invention. 
         FIG. 12  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 11  according to an alternate embodiment of the present invention. 
         FIG. 13  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 12  according to an alternate embodiment of the present invention. 
         FIG. 14  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 13  according to an alternate embodiment of the present invention. 
         FIG. 15  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 14  according to an alternate embodiment of the present invention. 
         FIG. 16  shows the semiconductor device of  FIG. 1  at a processing step subsequent to that shown in  FIG. 15  according to an alternate embodiment of the present invention. 
         FIG. 17  shows another embodiment of a semiconductor device at a processing step according to another alternative embodiment of the present invention. 
         FIG. 18  shows the semiconductor device of  FIG. 17  at a processing step subsequent to that shown in  FIG. 17  according to an alternate embodiment of the present invention. 
         FIG. 19  shows the semiconductor device of  FIG. 17  at a processing step subsequent to that shown in  FIG. 18  according to an alternate embodiment of the present invention. 
         FIG. 20  shows the semiconductor device of  FIG. 17  at a processing step subsequent to that shown in  FIG. 19  according to an alternate embodiment of the present invention. 
         FIG. 21  shows the semiconductor device of  FIG. 17  at a processing step subsequent to that shown in  FIG. 20  according to an alternate embodiment of the present invention. 
         FIGS. 22A ,  22 B, and  22 C depict a conventional etching process resulting in an undercut trench. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that various structural, logical, and electrical changes may be made without departing from the spirit or scope of the invention. Additionally, well-known structures, processes, and materials associated with microelectronic device fabrication have not been shown in detail in order to avoid unnecessarily obscuring the description of the embodiments of the invention. 
     Furthermore, skilled artisans appreciate that elements in the figure are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of the various embodiments of the present invention. 
     The term “substrate” used in the following description may include any semiconductor-based structure that has an exposed substrate surface. Structure should be understood to include silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. When reference is made to a substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation. 
     The present invention relates to forming, during a buried bit line connection process flow, low resistance contacts to a substrate in the peripheral circuit logic area and to poly plugs in the memory cell array area formed as part of a memory device, such as a DRAM memory device. The present invention will be described as set forth in an exemplary embodiment illustrated below. Other embodiments may be used and structural or logical changes may be made without departing from the spirit or the scope of the present invention. 
     In accordance with the present invention, a method is provided for forming low resistance contacts for both N and P doped active regions in a peripheral logic circuitry area, which is typically formed outside of and around a memory cell array area. Referring now to the drawings, where like elements are designated by like reference numerals,  FIGS. 1 through 16  illustrate exemplary embodiments of the fabrication steps and resulting structures in accordance with the present invention. 
     Referring to  FIG. 1 , a first embodiment of a semiconductor device is illustrated wherein on a substrate  100 , a memory cell array indicated generally by reference numeral  102  and a peripheral circuitry area, indicated generally by reference numeral  104  are shown during an early stage of fabrication. The peripheral circuitry area  104  is typically either an N-channel transistor area or a P-channel transistor area. The memory cell array  102  includes gate stacks  106 ,  108 ,  110 ,  112 , where in one embodiment, gate stacks  108  and  110  in the memory cell array comprise electrically isolated word lines  114 ,  116 . Active areas are provided about the gate stacks  106 ,  108 ,  110 ,  112 , such as the doped active areas  120 ,  122 ,  124  that form Field Effect Transistors (FETs) provided between field isolation areas  118 ,  126 . 
     Each of the gate stacks  106 ,  108 ,  110 ,  112  includes a layer of oxide  128 , such as silicon dioxide in contact with the substrate, a layer of polysilicon  129  provided on the oxide, a conductive gate layer  130  provided on the poly, an insulating cap layer  132 , and insulating sidewalls  134 . Provided between the gate stacks  106 ,  108 ,  110 ,  112  are polysilicon (poly) plugs  136 ,  138 ,  140 . The polysilicon (poly) plugs  136 ,  140  shown in  FIG. 1  will connect with subsequently formed memory cell capacitors and poly plug  138  will connect with a subsequently formed bit line. Accordingly, gate stacks  108 ,  110  are part of access transistors  142 ,  144  for respective memory cells. Additionally, gate stacks  106 ,  112  formed part of other memory cells in a different cross-sectional plane from that illustrated, which are used for self-aligned fabrication processes, and field oxide regions  118 ,  126  are used to isolating the memory cells in the memory cell array  102 . 
     A doped well  146  may be provided in the substrate  100  and associated with a respective memory cell array  102  and peripheral circuitry area  104 . For the N-channel transistors, the doped well  146  is a p-well, while for the P-channel transistors the doped well is a n-well, as is well known in the art. 
     As further shown in  FIG. 1 , planarized first insulating layer  148 , formed of, for example, borophosphosilicate glass (BPSG) or silicon dioxide has been formed over the gate stacks and active areas. The first insulating layer  148  is then planarized by chemical mechanical polishing (CMP) or other suitable means. A second insulating layer  150 , formed of, for example, tetraethylorthosilicate (TEOS) or other oxide, is formed over the first insulating layer  148 . The second insulating layer  150  is deposited with a thickness, for low resistance contacts of current integration size and levels, in a range of about 5 Angstroms to about 10,000 Angstroms. Of course, one skilled in the art will be able to easily vary the relevant dimensions to fit the particular application. If desired, the second insulating layer  150  may also by planarized by chemical mechanical polishing (CMP) or other suitable means; however, this step may be skipped as the first insulating layer  148  is planar. The substrate assembly shown by  FIG. 1  serves as the starting foundation for the invention which is discussed hereafter. 
     The process of the present invention begins by applying a photoresist mask  152  to the second insulating layer  150 . Opening  154  in the mask defines an etch location of a peripheral contact to other wordlines and actives areas. As shown in  FIG. 2 , a first portion of the first and second insulating layers is removed by etching to expose, for example, an active area  156  which is N+ doped for N-channel transistors, and P+ doped for P-channel transistors. It is also possible to dope the active area  156  after the etching operation instead of doping such areas prior to etching. The contact opening  158  is thus provided, as shown in  FIG. 2 . 
     As shown by the structure illustrated in  FIG. 3 , after contact opening  158  is formed, such as by reactive ion etching (RIE), the photoresist mask  152  is removed and a low resistance metal film layer  160  is deposited by CVD over the second insulating layer  150 . The metal film layer  160  is titanium which will cover the contact opening  158 , and form titanium silicide (TiSi x ) in the peripheral circuitry area  104  in a subsequent heating cycle when the layers are annealed at temperatures above 650° C. The metal film layer  160  is deposited with a thickness in a range of about 1 Angstrom to about 5,000 Angstroms. As the second insulating layer  150  is intact over the memory cell array area  102 , no CVD Ti comes into contact with poly plug  138 , which will connect with a subsequently formed bit connection. 
     In another embodiment, TiSi x  can be provided in the contact opening  158  by reacting chemically vapor deposited Ti with Si from the substrate  100  or with Si simultaneously added from the vapor phase. For example, the titanium silicide areas in the contact opening  158  may be formed by depositing Ti from the precursor TiCl 4 , with the Si coming from the substrate  100  or from added gas-phase SiH 4  or SiH 2 Cl 2 . 
     After Ti deposition, a second photoresist mask  162  is provided over the Ti film layer  160  to a thickness standard in the art, and patterned to provide an opening  164  located over the memory cell array area  102 , and in particular poly plug  138  as illustrated by  FIG. 4 . As shown by the structure illustrated in  FIG. 5 , bit connection opening  166  is formed by anisotropically etching through the first and second insulating layers  150 ,  160 , thereby opening the bit connections in the memory cell array area  102 . It is to be appreciated that the etching process to form the bit connection openings in the memory cell array area  102  can be one or more process steps (in-situ or ex-situ). 
     In the first part of the bit connection opening formation process, the Ti metal film layer  160  is anisotropically etched using a reactive halogen containing plasma etch process, such as chlorine, fluorine, and the like, which is very selective and stops at the first insulating layer  150 . In the second part of the bit connection opening formation process, the first insulating layer  150  is then anisotropically etched using a reactive halogen containing plasma etch process to remove the portion of the first insulating layer  150  over the bit connections, thereby exposing the bit connection poly plugs, such as for example, poly plug  138 . 
     At this point in the bit connection opening formation process, the steps illustrated by  FIGS. 6 and 7  and explained hereafter, are performed in order to provided a terrace film stack. In order to address metal undercut during a pre-clean process, such as with an aqueous or non-aqueous mixture of HF and/or NH 4 F, to a subsequent metal deposition step, a second etch is conducted to the bit connection opening  166 . To conduct the second etch, the second photoresist mask provides a second pattern having an opening  168  that is wider than the bit connection opening  166  as illustrated by  FIG. 6 . In one embodiment, the photoresist layer  162  is isotropically etched using an oxygen containing plasma etch process to expose precise portions of the Ti film layer  160  around the connection opening  166 . Lastly, the exposed portions of the Ti film layer  160  are then anisotropically etched by a reactive halogen containing plasma etch process as is illustrated by  FIG. 7 . 
     As shown by  FIG. 8 , the second photoresist mask  162  is stripped and then the metal deposition pre-clean step is performed. It is to be appreciated that the pre-clean process further widens the connection opening  166 , as indicated by the dotted lines, but not as wide as opening  168 . In the pre-clean process, the TEOS insulating layer  150  etches faster than the CVD Ti film layer  160 , and without the widening process providing opening  168 , the Ti film layer  160  would most likely get undercut. The trimming of the photoresist layer  152  and subsequent additional Ti etch will result in a terraced film stack as illustrated in the subsequent process flow which prevents void formation in the bit connection. 
     Referring now to  FIG. 9 , after the formation and cleaning of the contact opening, a low-resistivity metal mode titanium/tungsten nitride/tungsten (MMTI/WN/W) terraced film stack is provided. First, a metal mode (metallic) titanium film layer  170  is deposited, using a physical vapor deposition (PVD) process, over the memory cell array and peripheral circuitry areas  102  and  104 , respectively, which fills into the openings  154 ,  166 ,  168  ( FIG. 8 ). It is to be appreciated that the metal mode titanium film layer  170  does not form suicides or ultra thin silicides, thus providing good contact to the poly plug  138  without voiding. The metal mode titanium film layer  170  is deposited with a thickness in a range of about 1 Angstrom to about 5000 Angstroms. 
     Next the WN/W layer  172  is deposited using either a PVD or CVD process, which completely fills the peripheral contact opening  154 . The WN/W film layer  172  is deposited with a thickness in a range of about 5 Angstroms to about 5000 Angstroms. Finally, a nitride capping layer  174  is deposited and planarized to have a thickness in a range of about 100 Angstroms to about 10,000 Angstroms. 
     As shown in  FIG. 10 , a directional etching process or other suitable process is used to etch through a photoresist mask (not shown) to remove portions of layers  160 ,  170 ,  172 ,  174  in areas not desired and in order to form low resistance contacts  176 ,  178 . The contacts  176 ,  178  may be of any suitable size and shape so as to provide a low resistance vertical path to the active areas  122 ,  146 . The contacts, such as contact  176 , in the peripheral circuitry area  104  are preferably of a smaller area than the contacts, such as contact  178 , in the memory cell array area  102 . 
     An alternate embodiment is described with reference to  FIGS. 11–16 . Like numerals from the first described embodiment are utilized where appropriate, with differences being indicated by 200 series numerals or with different numerals.  FIG. 11 , shows a processing step conducted similar to the processing steps shown in  FIG. 1 , except that the first photoresist mask  152  is patterned to provide the contact opening  164  in the memory cell array area  102 , and not the peripheral circuitry area  104  as in  FIG. 1 . A directional etching process or other suitable process occurs to etch through the first insulating layer  150  as indicated by the dotted lines in  FIG. 11 , thus exposing poly plug  138 . 
     Referring to  FIG. 12 , the photoresist mask layer  152  is then removed after the etching process, and the metal mode titanium layer  170  is deposited over the memory cell array and the peripheral circuitry areas  102  and  104 , respectively. The metal mode deposition is then followed by a deposition of a tungsten nitride layer  200 . Accordingly, the metal mode titanium layer  170  is formed over the exposed outer surfaces of poly plug  138 . Alternatively, layer  170  may comprise titanium, titanium nitride, tungsten, cobalt, molybdenum or tantalum, but any suitable metal may be used. Additionally, each layer  170 ,  200  may be planarized by, for example, by CMP after deposition. 
     As shown in  FIG. 13 , the second photoresist layer  162  has been deposited over the substrate to fill opening  164  above the poly plug  138 . The photoresist layer  162  is then patterned to form the etching opening  154  for the subsequently formed peripheral contact. 
     As shown in  FIG. 14 , a directional etching or other suitable etch process is performed to etch through layers  148 ,  150 ,  170 , and  200  to form the contact opening  154  so as to expose a contact area in the substrate  100 . It is to be appreciated that the metal mode titanium layer  170  and tungsten nitride layer  200  are used as a hard mask if needed, such that only the first and second insulating layers  148 ,  150  are etched after etching portions of layers  170 ,  200  with the directional etching process. The contact opening  154  in one embodiment is of a smaller diameter than the opening  164  above the poly plug  138 . 
     After formation of the peripheral contact opening  154 , the process steps for forming the terraced film stack as explained previously above in reference to  FIGS. 6 and 7  is conducted. As explained above, the second photoresist layer  162  is used again to provide an opening that is wider than the contact opening  154 . In one embodiment, the second photoresist layer  162  is isotropically etched using an oxygen containing plasma etch process to expose precise portions of the tungsten nitride layer  200  around the connection opening  154 . Lastly, the exposed portions of the tungsten nitride layer  200  and the underling metal mode titanium layer  170  are then anisotropically etched by a reactive halogen containing plasma etch process to widen opening  154  to the dashed line  155 . 
     Next, the second photoresist layer  162  is striped away, and the titanium layer  160  is deposited by CVD as shown by  FIG. 15 . As mentioned previously above, the CVD Ti layer  160  provides a low resistance periphery contact, which due to the process flow illustrated in  FIGS. 11–14 , does not coming into contact with the poly plug  138  in the memory cell array area  102 , thus preventing voiding. An adhesion/barrier layer  202  formed from a suitable material such as titanium nitride is then deposited by CVD or other suitable deposition process. This deposition is then followed by a conductive layer  204  formed from a suitable conductive material such as tungsten or other metal to fill the contact opening  154  as illustrated by  FIG. 16 . The nitride capping layer  174  is then deposited and layers  174 ,  204 ,  202 ,  160 ,  200 ,  170  are etched and patterned so as to form contacts  206 ,  208  having a top portion situated on the second insulating layer  150  as also shown by  FIG. 16 . The contacts  206 ,  208  may be of any suitable size and shape so as to provide a low resistance vertical path to the active areas of the memory cell array and peripheral circuitry areas  102  and  104 , respectively. 
     In accordance with the present invention the contacts are formed after the formation of the capacitors. In particular, the process of forming the contacts begins after the completion of all high temperature processing steps utilized in wafer fabrication and after any other temperature changes that affect the metal layers provided in the contact formation process. In one embodiment, the process begins after the heat cycles used for cell poly activation and capacitor formation. The contacts may be formed prior to forming upper cell plate contacts to the capacitor of the memory device but subsequent to high temperature processing treatment for the capacitor. Furthermore, the present invention is not limited to the illustrated layers. Any suitable number and/or arrangement of conductive and insulating layers may be used without departing from the spirit of the invention. 
     For example, referring to  FIG. 17 , a second embodiment of a semiconductor device is illustrated, wherein like numerals from the first described embodiment are utilized where appropriate, with differences being indicated by 300 series numerals or with different numerals. On a substrate  100 , a memory cell array indicated generally by reference numeral  102  is shown during an early stage of fabrication. The memory cell array  102  includes gate stacks  106 ,  108 ,  110 . Active areas are provided about the gate stacks  106 ,  108 ,  110 , such as the doped active areas  120 ,  122 , that form Field Effect Transistors (FETs) provided between field isolation areas  118 ,  126 . 
     Each of the gate stacks  106 ,  108 ,  110 , includes a layer of oxide  128 , such as silicon dioxide in contact with the substrate, a layer of polysilicon  129  provided on the oxide, a conductive gate layer  130  provided on the poly, an insulating cap layer  132 , and insulating sidewalls  134 . Provided between the gate stacks  106 ,  108 ,  110 , are polysilicon (poly) plugs  136 ,  138 . Additionally, a trench capacitor, generally indicated by symbol  300 , is provide below the gate stacks, and in particular, centrally below gate stack  108 . 
     As further shown in  FIG. 17 , a first insulating layer  148 , formed of, for example, borophosphosilicate glass (BPSG) or silicon dioxide surrounds the gate stacks  106 ,  108 ,  110  and remaining active areas. The first insulating layer  148  and insulating cap layer  132  is planarized, such as by chemical mechanical polishing (CMP) or other suitable means. 
     A second insulating layer  150 , formed of, for example, tetraethylorthosilicate (TEOS) or other oxide, is formed over the first insulating layer  148  and insulating cap layer  132 . The second insulating layer  150  is deposited with a thickness, for low resistance contacts of current integration size and levels, in a range of about 5 Angstroms to about 10,000 Angstroms. Of course, one skilled in the art will be able to easily vary the relevant dimensions to fit the particular application. If desired, the second insulating layer  150  may also by planarized by chemical mechanical polishing (CMP) or other suitable means; however, this step may be skipped as the first insulating layer  148  is planar. A low resistance metal film layer  160  is deposited by CVD over the second insulating layer  150 . In one embodiment, the metal film layer  160  is titanium or other suitable metal or metal based film. The metal film layer  160  is deposited with a thickness in a range of about 1 Angstrom to about 5,000 Angstroms. 
     Next, the process of the present invention begins by applying a photoresist mask  152  to the metal film layer  160 . The openings in the mask defines etch locations, and as shown in  FIG. 18 , portions of the second insulating layer  150  and metal film layer  160  are removed by etching to expose, for example, portions of the insulating cap layer  132  and the first insulating layer  148 . 
     A second etch is then conducted to the metal film layer  160 . To conduct the second etch, the photoresist mask  152  provides a second pattern having openings wider than the previous pattern openings illustrated by  FIG. 17 . In one embodiment, the photoresist layer  152  is isotropically etched using an oxygen containing plasma etch process to expose precise portions of the metal film layer  160 . Lastly, the exposed portions of the metal film layer  160  are then anisotropically etched by a reactive halogen containing plasma etch process as is illustrated by the dotted lines in  FIG. 19 . 
     As shown by  FIG. 20 , the photoresist mask  152  is stripped and then a metal deposition pre-clean step is performed. In one embodiment, it is to be appreciated that the pre-clean process further etches the insulating layer  150 , as indicated by the dotted lines. In this embodiment, a TEOS insulating layer  150  etches faster than the CVD Ti film layer  160 , and without the trimming process discussed above, the Ti film layer  160  is often undercut, which in a subsequent material deposition step, would result in void formation. The subsequent material deposition of a material  302  is illustrated by  FIG. 21 . The material  302  in one embodiment is a metal or material containing metal. It is to be appreciated that the trimming of the photoresist layer  152  and subsequent additional metal film layer etch results in a terraced film stack which prevents void formation. 
     The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the present invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.