Abstract:
Methods for improving post etch in via or trench formation in semiconductor devices. A preferred embodiment comprises forming a re-capping layer over a dielectric film following an initial etch to form a feature in the dielectric film, followed by additional etch and etch back processing steps. The re-capping method provides protection for underlying films and prevents film damage post etch. Uniform feature profiles are maintained and critical dimension uniformity is obtained by use of the methods of the invention. The time dependent dielectric breakdown performance is increased.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/112,584, entitled “Post Etch Dielectric Film Re-Capping Layer,” filed on Nov. 7, 2008, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to semiconductor processing and methods for improving post etching characteristics of dielectric films for vias or for trench etching. More particularly, in some embodiments, the invention relates to a method for providing a post etching re-capping layer over a dielectric film to improve critical dimension (CD) and time-dependent-dielectric-breakdown (TDDB) characteristics of semiconductor devices produced by the process. 
     BACKGROUND 
     Generally, semiconductor wafers are produced using photolithography, deposition, etching, damascene and polish steps to form circuitry and conductive layers surrounded by dielectric materials to produce a plurality of integrated circuit dies on a semiconductor wafer. 
     In conventional semiconductor processing, typically, copper or aluminum conductor layers are formed and patterned as “top level metal” connectors within layers lying above the semiconductor substrate. The substrate, which for this discussion includes semiconductor over insulator or other epitaxial or grown layers, typically has transistors or other devices formed with doped regions that provide p-n or n-p junctions. The transistors are often planar MOS devices and in many processes, complementary MOS devices are formed by providing a p type substrate, N type MOS devices are formed in the substrate, N-wells are formed in the substrate and P type MOS devices are formed in the wells. These devices are frequently paired to form CMOS inverters, which as is known in the art, may be coupled to form logic gates, registers, memories, processors and other functions needed to form an advanced integrated circuit. 
     To complete the integrated circuit, electrical connections are formed using dielectrics, including without limitation high K and low K dielectric materials, as well as oxides, nitrides, and other dielectrics known to those skilled in the art, and metallization layers separated by the insulating layers. The metal layers are patterned as conductive lines. Vias are used to vertically couple these conductors. The conductors may be formed of aluminum and its alloys, copper and its alloys, and other conductive materials. If aluminum is used for conductors, a deposit, pattern, and etch process is used to form the conductors. If copper is used, a single or dual damascene process using a film deposition, photolithography, etch and chemical mechanical process (CMP) to remove excess material over the filled trenches may be used. Presently, copper or an alloy of copper is often used with single and dual damascene processes. 
     The etching process used in the conventional back end of the line (BEOL) metallization and patterning steps will damage film quality. This impacts, among other objective reliability criteria, the TDDB. Conventional approaches to improve the film quality after etch include photoresist (PR) trimming, CD shrinkage and others. These process improvements usually involve polymerizing using etch techniques and therefore are subject to etching profile damage. Critical dimension uniformity (CDU) will also be impacted negatively by these steps. For improving process TDDB, CD shrinkage from, or with PR trimming are usually used. However the capabilities of the photolithography process remain a bottleneck for advanced semiconductor process technology. 
       FIG. 1  depicts, in a cross sectional view, an intermediate stage of a conventional BEOL copper semiconductor process. In  FIG. 1 , a metal layer  101  is shown that was previously formed by, for example, typical copper metallization steps such as electro-deposition and CMP over a semiconductor substrate (not shown). An etch stop layer (ESL)  103  is shown overlying the metal layer  101 . This ESL layer will be used to stop etch processes above the metal layer  101  in subsequent process steps. Overlying the ESL layer  103  is a dielectric layer  105 . Presently, so-called low-K dielectric materials may be used. Low-K dielectrics have a dielectric constant lower than that of silicon dioxide (about 3.5). Recently, extreme low-K or ELK materials are also being used; these have a dielectric constant below 2.0. Alternative dielectric materials include oxides, nitrides, and even high-K dielectrics, but low-K materials are preferred presently for insulation between the metal layers. 
     Overlying the dielectric layer  105  is a layer of dielectric anti-reflective coating (DARC)  107 . DARC is used to prevent unwanted reflections from interfering with the exposure of photoresist in subsequent processes. Overlying the DARC is a bottom anti reflective coating layer (BARC)  109 . BARC layer  109  is also used to prevent unwanted reflections from interfering with subsequent photoresist processes. 
     Overlying the BARC layer  109  is a layer of photoresist (PR)  111  that has already been patterned. PR patterning is accomplished by exposure and development to form a pattern for etching. The etch process is selective so that areas protected by the PR are not etched, and areas exposed to the etchant are etched, with the ESL stopping the etching. 
     In this simple cross section, three via positions  102 ,  104  and  106  are shown, similar process steps may be used to form trenches, for example, as well as vias. The vias will, when finally completed, be filled with a conductive material and form connections between metal layer  101  and the next metal layer formed above layer  101  in a multiple level metallization scheme as is typically used. For example if copper layer  101  was “metal  1 ”, then the next metal layer formed above the structure of  FIG. 1  would be “metal  2 ”. If instead, layer  101  were “metal  5 ” then the next overlying layer of metal would be “metal  6 ”. Modern semiconductor processes may have as few as one metal layer and possibly many more metal layers. The vias are used to electrically couple the conductors on different layers by forming vertical connections through the dielectric layers. 
       FIG. 2  depicts the structure of  FIG. 1  following additional processing steps. In  FIG. 2  the vias (alternatively, trenches)  102 ,  104  and  106  are shown etched through the DARC layer  107  and the dielectric layer  105  to the ESL layer  103 . Etching is well known in the art and typical processing steps may be used such as RIE and plasma etch processes. Wet and dry etch may be used. 
       FIG. 3  depicts the structure of  FIG. 2  following still more processing steps. In  FIG. 3 , sacrificial plug material is used to fill the vias  102 ,  104  and  106 . This via fill material will be removed later and is provided to protect the sidewalls and bottom of the low-K via hole structure. An etch back step is performed to remove some of the via fill material at the top of the vias adjacent DARC layer  107 . In this manner, the DARC layer may be thinned or etched to reduce the thickness of the DARC layer. 
       FIG. 4  depicts in two views the structure of  FIG. 3  following additional processing steps. In  FIG. 4 , the upper portion depicts a plan view of the vias  102 ,  104  and  106  and the conductor trenches  117  being formed for the metal trenches Mx. The metal layer Mx could be any metal layer overlying a previous metal layer, (for example metal layer  7  would be M 7 ) and will typically be formed using dual damascene or single damascene processes, with copper deposited in the vias and the trenches overlying the vias, and then a copper metal polishing (CMP) step to remove excess material from the conductor regions to complete both the vias and the overlying conductors. 
     In the lower portion of  FIG. 4  the cross section taken along the dashed line A-A′ through the via portions  102 ,  104  and  106  is shown. A second BARC layer  110  is now provided overlying the structure and adjacent the DARC layer  107 . A second layer of PR  112  is provided and patterned to set up the removal of the via filler  113  by another etch step to complete the vias. 
       FIG. 5  depicts the completed vias  102 ,  104  and  106  in the structure of  FIG. 4  after additional processing steps. The second PR layer  112 , and the second BARC layer  110  are removed following the etch process to remove material  113  from the vias. As can be seen, the tops of the vias  102 ,  104  and  106  are rounded and sloped. The shape is due to the etch processes which damage the DARC film  107  and the top portions of the dielectric layer  105  within the via regions  102 ,  104  and  106 . This etch damage causes non uniform spacing, lower minimum spacing between the vias, between vias and lines, and line to line spacing. 
       FIG. 6  depicts in a close up cross section an example via  115  produced using the conventional processes described above. As can be seen, the sides of the dielectric layer  105  and the DARC layer  107  forming the via  115  are sloped to open wider at the top of the via. In the conventional process, the top critical dimension and the etch stop layer will be damaged by the use of the etch processes. Top opening, or bowing profile vias, or even ESL punch through may occur. This can cause “bird beak” effects, for example. 
       FIGS. 7A and 7B  depict actual test structures formed in a conventional semiconductor process using the process steps of  FIGS. 1-6 . In  FIG. 7A , a pair of vias is shown in a SEM photograph looking down on via structures from above. The via shapes can be seen to be elongated near the center. In  FIG. 7B , a SEM of the overlying conductors formed on the vias of  FIG. 7A  is shown in a SEM photo. The minimum spacing between lines formed over these vias is shown to be 21 nanometers, less than the desired minimum spacing. This minimum spacing design violation is due to the irregular shaped opening at the top of the vias. The design parameters are not being met in this example, as the etch process is causing the spacing to change. 
     One disadvantage of the prior art is that the known approaches to improving the TDDB and via shape post etch problems also have disadvantages. The PR trimming and CD shrinkage of deposition mode in etching processes used in the prior approaches tend to induce additional profile damage. CD uniformity control will then be worse than if these techniques were not used. 
     A second disadvantage of the prior art is that the various approaches add complexity to the semiconductor processing and thus tend to increase cost and process time. In spite of the additional steps, the photolithography processes tend to limit the advance of semiconductor processing. A continuing need thus exists for semiconductor process methods to improve the TDDB reliability, maintain CD uniformity, and maintain via to via, via to line, and line to line spacing parameters in the semiconductor devices produced, with low cost and without the disadvantages of the known solutions used in the prior art. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by illustrative embodiments of the present invention which provide methods for re-capping an ARC layer to improve the etch process for vias or trenches, to enhance the TDDB parameter for the devices, to maintain spacing uniformity between vias and between vias and lines, and between lines, and to improve the CD uniformity. 
     In accordance with an exemplary embodiment of the present invention, a method for improving the post etch profile of a via or trench through dielectric films is provided, wherein following a first etch process to form the feature, a re-capping deposition of additional film is performed at the remaining top ARC layer. Subsequent etching steps then do not damage the films, the profiles of the etched features are maintained, critical dimension uniformity is achieved, and minimum spacing between features such as vias, vias to line, and line to line, is increased. The time dependent dielectric breakdown (TDDB) parameter is increased. 
     In accordance with another exemplary embodiment of the present invention, a method for forming a via through a dielectric overlying an etch stop layer comprises depositing a first layer of antireflective coating over the dielectric, depositing a second antireflective coating, depositing a photoresist, patterning the photoresist to define the vias, etching to form the vias using the etch stop layer to stop the etching and removing the photoresist and the second antireflective coating. A re-capping step is performed, depositing additional film material on the remaining first antireflective coating. The vias are then filled with a sacrificial material, an etch back step is performed, another layer of antireflective coating is deposited, another layer of photoresist is deposited, the additional layer of photoresist is patterned to remove the via filler material, another etch process is performed and the finished vias have a reduced thickness layer of the first antireflective coating at the top of the via. 
     Embodiments of the present invention improve CD uniformity, TDDB performance, and spacing. Damage to films in the dielectric layers and the etch stop layer is reduced, the vias achieved maintain a more uniform profile, the methods protect against interface damage or depletion, post etch striation and “bird beak” effects are reduced, and when an extreme low K dielectric is used, the re-capping step provides good adhesion to this layer. 
     A further advantage of certain embodiments of the present invention is that the formation of trenches in dielectric films may be improved by recapping the dielectric films following a first etch process to define the feature, then the additional etchback and etch steps will not damage the films defining the trench and the uniformity of the trenches formed will be increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  is a cross sectional view of an intermediate stage semiconductor device where vias will be formed; 
         FIG. 2  is another cross sectional view of the device of  FIG. 1  following additional processing steps; 
         FIG. 3  is a another cross sectional view of the device of  FIG. 2  following additional processing steps; 
         FIG. 4  depicts in a plan view and in another cross sectional view the device of  FIG. 2  following additional processing steps; 
         FIG. 5  is a cross sectional view of the structure of  FIG. 4  including vias produced in a dielectric layer using conventional process steps; 
         FIG. 6  is a close up cross sectional view of one of the vias of  FIG. 4 ; 
         FIGS. 7A and 7B  depict test structures produced using conventional process steps,  FIG. 7A  is a plan view of vias, and  FIG. 7B  is a plan view of conductors overlying the vias; 
         FIG. 8  is a cross sectional view of a semiconductor structure at an intermediate stage depicting the added re-capping layer provided by an exemplary method embodiment of the present invention; 
         FIG. 9  is a cross sectional view of the structure of  FIG. 8  following additional processing steps; 
         FIG. 10  is a close up cross sectional view of a via of  FIG. 9  produced using an exemplary method embodiment of the present invention; 
         FIGS. 11A and 11B  depict test structures produced using an exemplary method embodiment of the present invention,  FIG. 11A  is a plan view of vias, and  FIG. 11B  is a plan view of conductors overlying the vias; and 
         FIG. 12  depicts a table illustrating comparisons of measured feature distances for a standard process device and a device produced using an exemplary method of the present invention. 
     
    
    
     The figures are illustrative, and are not to scale, and some features may be omitted for clarity. Like reference numerals for like elements are used. 
     DETAILED DESCRIPTION 
     The making and using of the exemplary embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to illustrative embodiments in a specific context, namely via formation between metal layers in a copper semiconductor process. The invention may also be applied, however, to other feature formations where etching would otherwise damage films, for example, in forming trenches using etch steps. The methods of the present invention are not limited to a particular process or feature size. 
     In  FIG. 8 , a structure formed using an exemplary method is shown in cross section. This figure shows the result of an additional method step performed between the via etch step of  FIG. 2  in the conventional process, and the via filling step of  FIG. 3 . The novel additional step in one exemplary embodiment is to perform a re-capping on the DARC film layer  107  by forming an additional layer  120 . The layer  120  may comprise, in one embodiment, the same material used for the DARC layer. In other embodiments, a film of a differing composition may be used. The thicknesses of the additional dielectric re-capping layer  120  may be, for a current process example, between 75-200 Angstroms, and may range from 30-300 Angstroms, depending on the process in which the re-capping layer  120  is used. 
     A variety of materials may be used for the re-capping layer. The material chosen should in various embodiments should have good adhesion to the low-K or ELK dielectric layer, and be compatible with the photoresist used. Nitrogen-free ARC or NF ARC is one possible material that may be selected for layer  120  although the re-capping method is not limited to these materials. Exemplary ARC layers that might be used for re-capping are materials having SiwCxOyHz, where w, x, y, and z are integers indicating a compound formula. For example, SiC, SiO2, SiCO, or SiCOH may be used. In some applications, a nitrogen containing dielectric such as SiN or SiON may be used as well. 
     In measuring and qualitatively analyzing semiconductor processes, an inspection after etch procedure is typically used and measurements taken on features. The via after etch inspection (AEI) critical dimension (CD) shrinkage in an example process where an example embodiment of the invention was applied was observed to be from 2-10 nanometers, depending on the dielectric film thicknesses and the deposition methods used. 
       FIG. 9  depicts, in a cross sectional view, the structure obtained for the re-capped DARC via structure of  FIG. 8  after the remaining via or trench process steps, such as described above with respect to  FIGS. 4-6  are performed. As can be seen from  FIG. 9 , the sidewalls of the vias  115  are now vertical (no slope or widening at the top) and the remaining layer  107  of DARC material has sharp sidewalls, not rounded. The use of the re-capping step of the preferred method has protected the ARC and ELK films and has reduced, or eliminated, post etch damage as seen in the prior art. 
       FIG. 10  depicts in a close up cross sectional view the sidewall profile of a via  115  obtained with the recapping layer  120  applied to DARC layer  107 . The vertical profile obtained using the dielectric capping layer provides physical improvement in the process as well as reliability improvement over the conventional processes. By maintaining the vertical slope, the feature dimensions (critical dimensions) are maintained at the designed sizes. Via spacing uniformity will therefore be maintained, as each via opening will maintain its original shape and size throughout the post etch processes. 
       FIGS. 11A and 11B  provide two views of an actual test structure produced using an exemplary method embodiment of the present invention in the semiconductor process as for  FIGS. 7A and 7B , the process now being modified by the use of the re-capping method. In  FIG. 11A , two adjacent vias are shown in a SEM photo. Shown from above, the regular shapes can be seen. In  FIG. 11B , the minimum spacing observed between features on the test structure is shown. Here, the conductors overlying the vias in  FIG. 11A  are shown in a SEM photo. The minimum spacing obtained is now 40 nanometers, a 90% increase compared to the SEM of  FIG. 7B  which showed a 20 nanometer spacing. This improvement is obtained using the method embodiments of the invention. 
       FIG. 12  depicts in a table form some comparison data obtained from test devices produced using the standard process flow without the use of the embodiments of the invention, and for devices implemented using the re-capping method embodiments for the dielectric layer used to protect and enhance the via structures, where a 100 Angstrom thickness recapping film was used. The following abbreviations are used to label rows in  FIG. 12 : AEI is “after etch inspection”, API is “after polish inspection”, PCM is an area of the circuit that is the “process control monitor”, PCM mean is a measurement of the average, or mean, critical dimension “CD” in the PCM part of an integrated circuit, SRAM is an area of the integrated circuit that is “Static Random Access Memory”; SRAM mean (VIA-VIA) is average or mean spacing measured via-to-via in the SRAM area, SRAM mean space is the average or mean distance of spacing between via-to-via; SRAM-RY is a circuit area implementing a particular SRAM example layout RY, SRAM RY mean spacing is an average or mean spacing measure in the circuit area containing SRAM-RY, and SRAM-RY min. space is the minimum spacing on via to trench in the SRAM-RY portion of the integrated circuits. In  FIG. 12 , the column labeled “STD” is for devices fabricated without the re-capping layer. The column labeled “Re-cap” is for devices implemented when one of the re-capping embodiments was used. In an after etch inspection (AEI) of certain areas in these ICs, use of the embodiments caused the SRAM mean via-to-via spacing to increase, and the SRAM mean space to increase, while the PCM mean spacing decreased. 
     Also as shown in  FIG. 12 , in the portion labeled API, the SRAM-RY mean spacing (the spacing between lines labeled in the SRAM-RY array) increased by 6 nanometers, and the SRAM-RY minimum spacing increased from 29.0 to 40.9 nanometers. 
     The re-capping methods of the exemplary illustrative embodiments described above may also be advantageously used for trench formation. These methods and the structures produced are additional alternative exemplary embodiments of the invention. Again, a re-capping film is deposited on the DARC dielectric to prevent post etch damage and CD nonuniformity in subsequent etch steps for forming the trench. The use of the exemplary embodiments in trench formation will improve the CMP polish window for the process. 
     Advantages of the use of the various embodiment methods are that CD uniformity is maintained. The re-capping of the invention maintains via and trench feature dimensions throughout the post etch processes. As a result, AEI CD shrinkage is reduced and CD striation is improved. Extreme low-K film damage protection is provided. Spacing between vias, from via to line, and from line to line is maintained without the disadvantages of the prior art. The protection of the ELK film results in the TDDB parameter for the process being improved, the breakdown voltage VBD for the vias is improved, and the resistance Rc is comparable to the conventional processes. 
     The advantages of the use of the exemplary methods may be seen in several tests. For example, CD uniformity will increase. AEI CD shrinkage will improve. When extreme low-K dielectrics (ELKs) are used, ELK damage protection will increase. Wafer acceptance test (WAT) leakage window measurements will improve, and reliability analysis TDDB will improve. 
     The use of the exemplary embodiments may also provide additional features over conventional semiconductor processes. Post etch deposition of the re-capping layer may reduce or avoid film damage caused by subsequent exposure to etch plasma processes. The interfaces may be protected from damage or depletion by subsequent processing. The post etch striation observed and so called “bird beak” effects may be improved. Adhesion characteristics to ELK materials may be improved by the additional re-capping layer 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the features and functions discussed above can be implemented in software, hardware, or firmware, or a combination thereof. As another example, it will be readily understood by those skilled in the art that the re-capping material may be varied, as it could be the same material for the dielectric film or another film material, while remaining within the scope of the present invention. 
     Moreover, the scope of the present application is not intended to be limited to the particular illustrative embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.