Abstract:
A tungsten stud, stacked via process, has been developed, featuring smooth planar topographies at all metal levels. The desirable topography is obtained by allowing the tungsten stud to reside at the same level, or slightly above the level, of the top surface of the via hole insulator. This is achieved via an insulator etch back procedure, performed after metal stud formation.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a process for manufacturing a semiconductor device, and more specifically to a method of forming metallization levels with smooth topographies on the semiconductor substrate. 
     (2) Description of the Prior Art 
     The trend in the semiconductor industry has been to continually increase device density, while maintaining or reducing the cost of semiconductor devices. The ability to significantly reduce dimensions, of critical device features, has enabled the semiconductor industry to fabricate more circuits on a specific size silicon chip, thus increasing device density. Cost reductions have also been realized by the ability of the industry to obtain smaller, and thus a greater amount of chips, for a specific size silicon wafer. Thus the trend to device micro-minaturazation has been, and is continuing to be, a strong objective of the semiconductor industry. Many semiconductor fabrication disciplines have been a major contributor in the realization of the micro-miniaturazation objective. For example more sophisticated exposure cameras, as well as more sensitive photoresist materials, have allowed the photolithographic discipline to routinely produce sub-micron images in photoresist. Anisotropic, reactive ion etching, (RIE), processes have in turn allowed the transfer of these sub-micron images, in photoresist, to be successfully transferred to underlying materials used in the fabrication of advanced silicon devices. Other semiconductor fabrication disciplines, such as low pressure chemical vapor deposition, (LPCVD), as well as ion implantation, have also been major contributors to the goal of micro-miniaturazation. 
     In addition to specific fabrication disciplines, aiding the advancement to smaller devices, several key processes have also been major contributors. For example, the ability to use sidewall spacers, by first depositing a material on a specific shape, and then anisotropically removing this material, everywhere except on the sides of the specific shape, has allowed the semiconductor industry to conserve valuable silicon area. Another technique employed to conserve silicon real estate, thus increasing device density, has been the stacked via approach. The stacked via approach is where basically all the vias or contacts, between interconnecting metallizations, or between a metallization and an underlying silicon device element, are created to reside on the same vertical axis. To successfully use the stacked via approach it is essential to create via hole metal fills, as planar, or as smooth as possible. For example if an irregularity, in terms of a metal seam exists in a lower via hole metal fill, this irregularity will be transferred to upper via hole metal fills, usually in a more exaggerated form then had existed for the lower via hole metal fills. This phenomena can result in metal discontinuities, leading to yield losses, or a lack of a sufficient amount of metal in the via hole, leading to possible electromigration failures due to increased current densities. 
     The attainment of smooth or planar metallizations has been addressed by Deleonibus, et al, in U.S. Pat. No. 4,592,802, and by Aoyama, et al, in U.S. Pat. No. 4,520,041, however these disclosures do not address the practical situation in which an overetch of the metal fill has to be performed, to clear the unwanted material from specific areas. The overetch will result in the metal fill residing well below the top surface if the via hole insulator, making it extremely difficult for subsequent via hole formation and metallizations to proceed in a planar manner. This invention will describe a process, which allows metal overetch, however uses a unique process that will allow subsequent via hole creation and metallization processes to still result in planar togographies. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a process in which stacked vias can be fabricated, exhibiting planar via hole, metal fills. 
     It is another object of this invention to fill a via hole, in an insulator, with a metal and selectively etch back the metal to insure removal of the metal from all areas except in the via hole. 
     It is still another object of this invention to selectively remove some insulator material, that the via hole was formed in to allow the surface of the metal fill, in the via hole to be at the same level as the top surface of the insulator material. 
     It is still yet another object of this invention, to selectively remove an amount of insulator material, that the via hole was formed in, to allow the surface of the metal fill, in the via, to be higher then the top surface of the insulator material. 
     In accordance with the present invention a process is described for fabricating a stacked via process, using tungsten studs. An insulator layer is deposited on a semiconductor surface, which is comprised of exposed oxide, as well as exposed silicon regions. A via hole is formed in the insulator layer, exposing an underlying silicon region. A tungsten layer is deposited in the open via hole, as well as on the insulator surface, outside the via hole. A selective, anisotropic etch is used to remove all the tungsten from the surface of the insulator layer, followed by an overetch, to insure complete removal of the unwanted tungsten. The metal overetch results in the surface of the tungsten, in the via hole, being below the top surface of the insulator, therefore another selective etch is used, in this case to remove insulator material, to result in the tungsten fill being planar with the top surface of the via hole, or in some cases higher then the top surface of the via hole. An interconnecting metallization is then deposited, resulting in a planar layer on the underlying tungsten via hole fill, then patterned, followed by deposition of another insulator layer. The next via hole is then created, followed by creation of either another tungsten fill process, or another interconnecting metallization. If the tungsten via hole fill is used for this upper level via hole, identical processing, consisting of selective tungsten overetch, followed by selective insulator removal, is employed to result in the desired planarity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other advantages of this invention are best described in the preferred embodiment with reference to the attached drawing that include; 
     FIGS. 1-3, showing prior art for stacked via structures. 
     FIG. 4, which schematically, in cross-sectional representation, shows an early stage of fabricating stacked via structures exhibiting planar surfaces. 
     FIGS. 5a-5c, which schematically illustrate one embodiment of the planar stacked via structure. 
     FIGS. 6a-6c, which schematically illustrate another optic, n used with the first embodiment of the planar stacked via structure. 
     FIG. 7, which schematically illustrates a second embodiment of the planar stacked via structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of forming a planar stacked via structure will now be covered in detail. This structure can be used as part of metal oxide semiconductor field effect transistors, (MOSFET) devices that are currently being manufactured in industry, therefore only the specific areas unique to understanding this invention will be described in detail. 
     FIG. 1 schematically shows the prior art, used to achieve stacked via structures. A single crystalline silicon, P type substrate 1, having a &lt;100&gt; orientation is used. An oxide layer 2, is grown and patterned, using conventional photolithographic and dry etching techniques, to produce an opening, 3, in oxide layer 2. After photoresist removal and careful wet cleans, an N+ region, 4, is formed in the P type silicon via conventional ion implantation or conventional diffusion processes. A silicon oxide layer 5, is deposited using either atmospheric pressure chemical vapor deposition, (APCVD), or plasma enhanced chemical vapor deposition, (PFCVD), techniques, at a temperature between about 350° to 600° C., to a thickness between about 5000 to 8000 Angstroms. Standard photoresist and RIE processing, using CHF 3  and argon, are used to produce a first via hole 6. The process continues with the deposition of a metal, in this case tungsten, 7, via use of LFCVD, at a temperature between about 350° to 500° C. to a thickness between about 5000 to 9000 Angstroms. It is important that the thickness of tungsten be great enough to completely fill first via hole, 6. The choice of tungsten is based on the ability of this metal to withstand high current densities, in the contact hole, without the risk of electromigration failures. In addition the low solubility of tungsten in silicon, makes the risk of penetration failures, commonly seen with aluminum metallizations, remote. 
     The critical step next performed is the removal of unwanted tungsten from the surface of oxide layer, 5. This step is performed using a selective RIE process, which basically does not etch the underlying oxide layer, 5, at the conclusion of the tungsten etch. However to insure that all the tungsten is removed from all parts of the wafer, since the uniformity of the deposited metal is not 100%, a significant tungsten overetch is needed. This overetch results in a depression of the tungsten filled via, or tungsten stud, 7, in the first via hole 6. This is illustrated schematically in FIG. 2. Next a first interconnecting metallization, such as aluminum-copper, 8, or aluminum-copper-silicons deposited using r.f. sputtering is performed to a thickness between about 3000 to 5000 Angstroms. It however can be seen that the depressed tungsten stud, 7, led to a lack of complete planarity, resulting in an irregularity or metal seam, 9, in the first level interconnect metallization. FIG. 3 shows that the creation of the stacked via process, when applied to an underlying interconnecting metallization that exhibits this scenario, results in the transfer and enhancement of this irregularity to upper level layers. For example an interlevel dielectric layer, 10, is deposited, followed by conventional photolithographic and dry etching procedures, used to create a second via hole, 11. It can be seen in the stacked via approach the irregularity 9, is exposed. After photoresist removal, and careful wet cleans, a second level metallization, 12, has difficulty in depositing uniformally, due to the irregularity, 9, which was initially spawned by the depressed tungsten stud, 7. Therefore it is imperative that a stacked via structure start with a planar fill for the first via hole, so that irregularities are not transferred and enhanced with upper level via hole and metallization patterning. 
     The process, designed to overcome the problem of seams or irregularities in various levels of metal studs or interconnects, will now be described. Referring now to FIG. 4, again a blanket, selective RIE-process is used to remove the unwanted tungsten, 7, from the surface of silicon oxide layer, and an additional RIE overetch, is performed to assure complete removal of any residual metal that may remain due to uniformity variations experienced with the metal deposition, creating the case in which the surface of the tungsten, 7, in the first via hole, 6, is below the surface of silicon oxide layer 5. As was seen with the prior art in FIGS. 1-3, severe irregularities in subsequent metallizations can now occur. FIGS. 5a-5c, will describe a first embodiment, designed to overcome the problem described with the prior art. Another selective RIE process, in this case designed to selectively remove silicon oxide layer 5, is applied to the structure shown in Fig. 4. The etchback is performed to remove a thickness of the silicon oxide layer, 5, between about 1000 to 2000 Angstroms. FIG. 5a, illustrates the case in which the controlled etchback resulted in the previously recessed tungsten stud, 7, in via hole, 6, now being planar in relation to the surface of silicon oxide layer, 5. The attainment of a planar structure, at this stage of processing, will enable subsequent processing to yield non-seamed stacked via structures. 
     Continuing with the structure shown in FIG. 5a, a first interconnecting metallization, 8, such as aluminum-copper, or aluminum-copper-silicon, is deposited, via r.f. sputtering, to a thickness between about 3000 to 5000 Angstroms. After the use of standard photolithographic and RIE procedures, using a chlorine based etchant, the first interconnect metallization structure, 8, is obtained, and shown schematically in FIG. 5b. It is important to note that the first interconnect metallization structure is smooth, not indicating irregularity, 9, observed with prior art in Fig. 2. After photoresist removal, via oxygen plasma ashing, followed by careful wet chemical cleans, an interlevel dielectric layer of silicon oxide, 10, is deposited using PECVD processing, at a temperature between about 300° to 500° C., to a thickness between about 1000 to 3000 Angstroms. Planarazation techniques, using chemical mechanical polishing, are used, followed by photolithographic and RIE processing, to create second via hole, 11. The RIE was performed using a CHF 3  --CF 4  -argon chemistry. This is seen in FIG. 5c. Finally a second interconnecting metallization, 12, such as aluminum-copper, or aluminum-copper-silicon, deposited via the use of r.f. sputtering, is performed to a thickness between about 3000 to 4000 Angstroms. Photolithographic and RIE processing, again using a chlorine based chemistry to etch the metal, produce second interconnect metallization structure, 12, exhibiting a planar contact to underlying first interconnect metallization, 8. The absence of irregularities, observed with prior art structures, is directly attributed to the initial planar tungsten stud, 7, obtained using the insulator etchback process. 
     Referring back to the insulator etchback process, described in FIG. 5a, occasionally thickness variations in oxide layer, 5, may exist across a wafer. Therefore the resulting height of the tungsten stud structure, 7, in relation to the top surface of oxide layer, 5, may vary. The process is thus designed to account for these possible uniformity variations via a mild intentional overetch of oxide layer, 5, and thus in some regions of the wafer the tungsten stud, 7, may be higher then the top surf ace of oxide layer, 5. This is schematically illustrated in FIG. 6a. However the planar surface of tungsten stud 7, still allows the first level interconnect metallization, 8, to be formed without irregularities, as shown schematically in FIG. 6b. Formation of subsequent second via hole, 11, and second interconnect metallization, 12 are also, produced without severe topography problems, as shown in FIG. 6c. Again the smooth topography, obtained via the etchback of oxide layer, 5, did not allow the creation of metal seams and irregularities to be transferred to upper level structures. 
     A second embodiment of this invention is the use of a second tungsten stud structure connecting first and second interconnect metallizations. This is shown schematically in FIG. 7. After deposition of interlevel dielectric layer, 10, and formation of second via hole, using identical processing conditions as previously described, for this embodiment, shown in FIG. 7, another tungsten deposition is performed, rather than the aluminum interconnect layer previously used in the first embodiment. The unwanted tungsten is again removed via blanket, selective RIE process, again using conditions previously described for this procedure, and again resulting in a recessed tungsten stud structure, 13. However the key feature of this invention a blanket selective insulator RIE etchback, in this example applied to interlevel dielectric layer, 10, results in lowering the top surface of interlevel dielectric layer, 10, to a level in which tungsten stud 13, is now planar with the top surface of interlevel dielectric layer, 10. A second interconnect metallization structure, 12, is again created, using identical procedures used and described for the first embodiment. Again it can be seen in FIG. 7, that the resulting structures exhibit planarity, again attributable to the planarization steps initially employed via the blanket, selective insulator etchback process. 
     This invention for an improved tungsten stud process for stacked via applications, although described as a simple contact to an N type diffusion in a semiconductor substrate, can be applied to more complex complimentary metal oxide semiconductor, (CMOS), as well as to BiCMOS structures. 
     While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.