Patent Publication Number: US-2022221797-A1

Title: Freeze-less Methods for Self-Aligned Double Patterning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application 63/135,217, filed on Jan. 8, 2021, which application is hereby incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to microfabrication, and, in particular embodiments, to microfabrication of integrated circuits using self-aligned double patterning. 
     BACKGROUND 
     In material processing methodologies (such as photolithography), creating patterned layers typically involves the application of a thin layer of radiation-sensitive material, such as photoresist, to an upper surface of a substrate. This radiation-sensitive material is transformed into a patterned mask that can be used to etch or transfer a pattern into an underlying layer on a substrate. Patterning of the radiation-sensitive material generally involves exposure by a radiation source through a reticle (and associated optics) onto the radiation-sensitive material using, for example, a photolithographic exposure system. 
     This exposure creates a latent pattern within the radiation-sensitive material which can then be developed. Developing refers to dissolving and removing a portion of the radiation-sensitive material to yield a relief pattern (topographic pattern). The portion of material removed can be either irradiated regions or non-irradiated regions of the radiation-sensitive material depending on a photoresist tone and/or type of developing solvent used. The relief pattern can then function as a mask layer defining a pattern. 
     Preparation and development of various films used for patterning can include thermal treatment (e.g. baking). For example, a newly applied film can receive a post-application bake (PAB) to evaporate solvents and/or to increase structural rigidity or etch resistance. Also, a post-exposure bake (PEB) can be executed to set a given pattern to prevent further dissolving. Fabrication tools for coating and developing substrates typically include one or more baking modules. Some photolithography processes include coating a substrate with a thin film of bottom anti-reflective coating (BARC), followed by coating with a resist, and then exposing the substrate to a pattern of light as a process step for creating microchips. A relief pattern created can then be used as a mask or template for additional processing such as transferring the pattern into an underlying layer. 
     The minimum resolution attainable with a single lithographic exposure is limited, amongst other things, by the wavelength of light used (the so-called diffraction limit). Techniques such as immersion lithography can be utilized to lower the diffraction limit. Multiple patterning processes such as Self-Aligned Double Patterning (SADP) are increasingly being used for scaling semiconductor features below photolithographic limits. Multiple patterning processes can double pitch (for each additional patterning) and thus help to achieve feature sizes that are otherwise unattainable. 
     However, multiple patterning processes are frequently costly and complex. Additionally, multiple patterning process flows can be incompatible with high volume manufacturing. Further, many multiple patterning techniques require additional process steps such as etching, deposition, development, and treatments which also increase complexity and reduce throughput. Therefore multiple pattern processes that reduce cost, reduce complexity, and/or increase compatibility are desirable. 
     SUMMARY 
     In accordance with an embodiment of the invention, a method of patterning a substrate includes depositing an overcoat in openings of a relief pattern supported by a substrate. The relief pattern includes a solubility-shifting agent and a first deprotectable monomer sensitive to the solubility-shifting agent. The overcoat includes a second deprotectable monomer sensitive to the solubility-shifting agent. The relief pattern has a first solubility threshold relative to a predetermined developer while the overcoat has a second solubility threshold relative to the predetermined developer that is lower than the first solubility threshold. The method also includes activating the solubility-shifting agent to at least reach the second solubility threshold of the overcoat without reaching the first solubility threshold of the relief pattern, diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the overcoat to form soluble regions in the overcoat, and developing the substrate with the predetermined developer to remove the soluble regions of the overcoat. The soluble regions are soluble in the predetermined developer while the relief pattern remains insoluble in the predetermined developer. 
     In accordance with another embodiment of the invention, a method of patterning a substrate includes depositing an overcoat in openings of a relief pattern supported by a substrate. The relief pattern includes a solubility-shifting agent and a first deprotectable monomer having a first activation energy. The overcoat includes a second deprotectable monomer having a second activation energy. The first activation energy is higher than the second activation energy. The method also includes deprotecting the second deprotectable monomer without deprotecting the first deprotectable monomer to form soluble regions in the overcoat by activating the solubility-shifting agent and diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the overcoat, and developing the substrate with the predetermined developer to remove the soluble regions of the overcoat. The soluble regions are soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer. 
     In accordance with still another embodiment of the invention, a method of patterning a substrate includes forming a relief pattern on a substrate from a layer of photoresist by exposing the photoresist to actinic radiation including a first wavelength to activate a first photoacid generator. The photoresist includes the first photoacid generator and a solubility-shifting agent. The method also includes depositing a deprotectable resin in openings of the relief pattern, activating the solubility-shifting agent, diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the deprotectable resin to form soluble regions in the deprotectable resin by deprotecting the deprotectable resin, and developing the substrate with the predetermined developer to remove the soluble regions of the deprotectable resin. The soluble regions are soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer. 
    
    
     
       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 drawings, in which: 
         FIGS. 1A-1F  illustrate a conventional antispacer process flow including a freeze step. The conventional antispacer process occurs through the dissolution of an acid-sensitive overcoat via acid diffusion from a photoresist mandrel; 
         FIGS. 2A-2E  illustrate an example process flow for patterning a substrate to form antispacer features that avoids additional dissolution-inhibiting steps by activating a solubility-shifting agent to at least reach a solubility threshold of an overcoat without reaching a solubility threshold of a relief pattern in accordance with an embodiment of the invention; 
         FIGS. 3A-3D  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility-shifting agent is utilized as a thermal acid generator when solubilizing an overcoat material in accordance with an embodiment of the invention; 
         FIGS. 4A-4C  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility shifting agent is utilized as a photoacid generator when forming a relief pattern and as a thermal acid generator when solubilizing an overcoat material in accordance with an embodiment of the invention; 
         FIGS. 5A and 5B  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility shifting agent is utilized as a photoacid generator when solubilizing an overcoat material in accordance with an embodiment of the invention; 
         FIGS. 6A and 6B  illustrate an example process flow for patterning a substrate to form antispacer features where a photodestroyable quencher is utilized as a solubility shifting agent when solubilizing an overcoat material in accordance with an embodiment of the invention; 
         FIG. 7  illustrates two qualitative graphs where the left graph illustrates an example relationship between resist thickness and exposure dose and where the right graph illustrates an example relationship between solubility and deprotection in accordance with an embodiment of the invention; 
         FIG. 8  illustrates four qualitative graphs of potential scenarios in which the dissolution contrast and sensitivity of the photoresist and overcoat are considered in accordance with embodiments of the invention; 
         FIG. 9  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention; 
         FIG. 10  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention; and 
         FIG. 11  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. The edges of features drawn in the figures do not necessarily indicate the termination of the extent of the feature. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the various embodiments described herein are applicable in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use various embodiments, and should not be construed in a limited scope. 
     The order of discussion of the different steps as described herein has been presented for the sake of clarity. In general, these steps can be performed in any suitable order. Additionally, although each of the features, techniques, configurations, etc. described herein may be discussed in different places of this disclosure, it is intended that each of the concepts can be executed independently of each other or in combination with each other. Accordingly, the present invention can be embodied and viewed in many different ways. 
     The ever continuous scaling of microelectronics requires improved patterning resolution. One approach is spacer technology to define a sub-resolution line feature via atomic layer deposition (ALD). A challenge of spacer techniques, however, is that if the opposite tone feature is desired, spacer techniques can be complex due to involving over-coating another material, chemical mechanical planarization (CMP), and reactive ion etch (RIE) to exhume the spacer material leaving a narrow trench, which can be costly. 
     Antispacer technology is a self-aligned technology that uses the diffusion length of a reactive species to define a critical dimension (CD), creating a narrow trench. To form a narrow slot (in contrast to, e.g., a narrow trench), the reactive species may be controlled spatially via exposure through a mask. Reactive species may be controlled uniformly across a wafer by decomposition of a thermal acid generator via a bake process. Antispacer techniques may enable access to narrow slot contact features at dimensions beyond the reach of advanced lithographic capabilities. 
     Some conventional antispacer process flows are complex and highly inefficient due to the addition of several process steps. For example, one conventional antispacer process includes an additional overcoat step of patterned structures, an additional bake to diffuse a reactive species into the patterned structures from the overcoat, and an additional overcoat removal step. Another overcoat step is then followed by a development step to form the antispacer structures. 
     Other conventional antispacer processes diffuse a reactive species from patterned structures into an overcoat, but require a “freeze” step (i.e. a treatment that neutralizes a solubility-shifting potential of a layer having an acid generator). The freeze step is necessary because the reactive species is included in the patterned structures and therefore undesirably solubilizes the patterned structures themselves when the future antispacer regions of the overcoat are solubilized. For example, the use of a thermal freeze process can inhibit a de-protected photoresist mandrel from solubilizing in an aqueous developer that is subsequently used to remove a de-protected (or de-crosslinked or otherwise changed in solubility) overcoat to reveal antispacer features. 
     Yet freeze processing is imperfect resulting in reduced uniformity and increased CD. Furthermore, the additional freeze step reduces throughput. The incorporation of thermal freeze functionality into a photoresist is a significant challenge due to the stringent requirements for high volume manufacturing and the reactive environment of the polymer resin. As a result, antispacer processes requiring fewer process steps that are compatible with high volume manufacturing flows are desired. 
     The techniques described herein include patterning a substrate without the need for a freeze step. That is, the described techniques are “freeze-less” methods of patterning a substrate capable of forming sub-resolution antispacer features. In various embodiments, a method of patterning a substrate includes forming an overcoat over a relief pattern on a substrate. The relief pattern includes a solubility-shifting agent that is activated and diffused into the overcoat to form soluble regions in the overcoat while maintaining insolubility of the relief pattern. The soluble regions of the overcoat are then subsequently removed by developing the substrate. 
     The solubility-shifting agent may advantageously enable formation of sub-resolution antispacer features without reaching the solubility threshold (e.g., a de-protection threshold) of the relief pattern relative to a predetermined developer (e.g. an aqueous developer). The solubility-shifting agent may be an acid generator. For example, the solubility-shifting agent may be a photoacid generator (PAG), a thermal acid generator (TAG), a photodestroyable quencher (PDQ), or other suitable solubility-shifting agent. 
     The techniques described herein may advantageously pair the innate dissolution contrast of a selected overcoat material and a selected relief pattern material (e.g. a photoresist) such that the solubility of the overcoat may be altered to form the desired antispacer structures while the relief pattern remains entirely or sufficiently insoluble in the developing solution (e.g. insoluble relative to the solubility of the overcoat in the predetermined developer). An advantage afforded by this process is to provide a means to circumvent any need for additional steps (e.g. a freeze step) to prevent the dissolution of the relief pattern. 
     Various embodiments described herein include controlling photoresist contrast and photosensitivity relative to an acid-sensitive overcoat to enable efficient and facile formation of antispacer features compatible with high volume manufacturing process flows. Embodiments herein will be described in detail relative to a conventional antispacer flow that includes a freeze step. Additionally, the photosensitivity and dissolution parameters which may be leveraged for successful development will be described. 
       FIGS. 1A-1F  illustrate a conventional antispacer process flow including a freeze step. The conventional antispacer process occurs through the dissolution of an acid-sensitive overcoat via acid diffusion from a photoresist mandrel. The conventional antispacer process flow disadvantageously results in the deprotection of the photoresist to an extent that it becomes solubilizes alongside the reacted overcoat. To prevent dissolution of the deprotected photoresist during development, an additional component or functionality must be included in the photoresist formulation to prevent dissolution along with an additional processing step. 
     Referring to  FIG. 1A , a conventional antispacer process flow  100  includes forming a structured photoresist pattern  101  from a layer of photoresist on a substrate no using photolithography. The structured photoresist pattern  101  includes photoresist mandrels  102  separated by patterned openings  105 . An additional acid source  103  is included in the photoresist (separate from the PAG used to form the photoresist mandrels  102 ). A process-compatible crosslinker  104  must also be included in the photoresist. 
     Referring now to  FIG. 1B , the substrate  110  with the photoresist mandrels  102  is then coated with an acid-sensitive resin  106  cast in a solvent that will not intermix with the underlying photoresist. As illustrated in  FIG. 1C , the acid source  103  within the photoresist is then activated  108  (by irradiation or baking) and the substrate  110  is baked to diffused  107  the acid into the surrounding acid-sensitive resin  106  causing solubility-changing reactions within the acid-sensitive resin  106 . 
     The diffusion depth  109  into the acid-sensitive resin  106  defines the CD of the antispacer features (i.e. antispacer thickness). This is illustrated in  FIG. 1D  by soluble resin regions in which are now soluble in an aqueous developer. However, the activation and diffusion of the acid source  103  also has the undesirable effect of solubilizing the photoresist mandrels  102  resulting in soluble photoresist mandrels  112 . That is, in the conventional antispacer process flow  100 , the photoresist becomes deprotected beyond a development (i.e. solubility) threshold of the photoresist in the aqueous developer. 
     The solubility shift of the photoresist would prevent the formation of antispacer features if the substrate  110  were developed at this stage because the soluble photoresist mandrels  112  would be removed in the developer. To address this solubility shift in the photoresist, a thermal freeze process  113  is performed to initiate a chemical reaction of the process-compatible crosslinker  104  in the photoresist that form crosslinking bonds  114  inhibiting solubility of the photoresist in the aqueous developer to form insoluble crosslinked photoresist mandrels  115 . 
     Such a thermal freeze process, however, will result in additional acid diffusion. This additional acid diffusion must be accounted for in the available process window to achieve the desired antispacer CD. Furthermore, the development of an innate functionality in the resin or of an additive that will inhibit dissolution requires the material be thermally stable, non-reactive to strong acids, exhibit minimal absorption to the irradiating wavelength and not interfere with the patterning capabilities of the photoresist. This makes the thermal freeze process  113  more prone to defects and not a preferred patterning technique. 
     Referring now to  FIG. 1F , once the acid has deprotected the acid-sensitive resin  106  to a desired depth/thickness (diffusion depth  109 ) and with the photoresist mandrels inhibited from dissolution, the substrate  110  is then developed in the aqueous developer to remove the soluble resin regions  111  and form conventional antispacers  116  between the insoluble crosslinked photoresist mandrels  115  and the remaining insoluble resin structures  117 . Yet the additional thermal freeze process  113  to form the crosslinking bonds  114  also have the undesirable effect of producing enlarged and irregular antispacer widths ( 118 ,  119 ) relative to the diffusion depth  109 . 
     The following described embodiments advantageously avoid the need for a dissolution-inhibiting step (e.g. a thermal freeze step) by instead selecting a combination of mandrel material, overcoat material, and solubility-shifting agent sufficient to achieve dissolution contrast. For example, an overcoat material may be advantageously selected such that it will achieve a sharp change in solubility at a level of deprotection well below that required of a photoresist used to form mandrels of a relief pattern thereby leaving the photoresist predominantly protected and insoluble in a predetermined developer. 
     Embodiments provided below describe various methods of patterning a substrate, and in particular, methods of forming antispacer features without additional dissolution-inhibiting steps. The following description describes the embodiments.  FIGS. 2A-2E  are used to describe an embodiment process flow for patterning a substrate to form antispacer features. Four more embodiment process flows are described using  FIGS. 3A-3D, 4A-4C, 5A-5B, and 6A-6B . Solubility shifting is discussed using qualitative graphs in  FIG. 7  while four potential scenarios of embodiment process flows are described using  FIG. 8 . Three embodiment methods are described using  FIGS. 9-11 . 
       FIGS. 2A-2E  illustrates an example process flow for patterning a substrate to form antispacer features that avoids additional dissolution-inhibiting steps by activating a solubility-shifting agent to at least reach a solubility threshold of an overcoat without reaching a solubility threshold of a relief pattern in accordance with an embodiment of the invention. 
     Referring to  FIG. 2A , a process flow  200  begins with a relief pattern  220  on a substrate  210 . The relief pattern  220  may be formed on the substrate  210  as part of the process flow  200  or be formed as part of a separate process and used as a starting point for the process flow  200 . For example, the example process flow  200  may include forming the relief pattern  220  using a lithographic or other suitable patterning process (e.g. from a layer of photoresist using a photolithographic process). In one embodiment, the relief pattern  220  is formed on the substrate  210  from a layer of photoresist using an immersion lithography process. As a specific example, the relief pattern  220  may be formed using a 193 nm immersion lithography process. 
     It should be noted that here and in the following a convention has been adopted for brevity and clarity wherein elements adhering to the pattern [x10] may be related implementations of a substrate in various embodiments. For example, the substrate  210  may be similar to the substrate  110  except as otherwise stated. An analogous convention has also been adopted for other elements as made clear by the use of similar terms in conjunction with the aforementioned three-digit numbering system. 
     The relief pattern  220  includes structures  222  on the substrate  210  separated by openings  226 . The structures  222  and openings  226  of the relief pattern  220  may be arranged in any desired pattern such as a uniform pattern of mandrels or an irregular pattern included various shapes, dimensions, and spacing. The material of the relief pattern  220  includes a solubility-shifting agent  224 . The solubility-shifting agent  224  in the structures  222  has not been activated at this stage of the process flow  200 . Consequently, the structures  222  are insoluble relative to a predetermined developer. 
     The predetermined developer may be any suitable developer. In various embodiments, the predetermined developer is an aqueous developer and comprises tetramethylammonium hydroxide (TMAH) in some embodiments. 
     In order to facilitate a solubility shift in the overcoat  230 , diffusion of a small molecule reactive species generated by the application of energy from the solubility-shifting agent  224  may be used. The solubility-shifting agent  224  may be an acid generator such as a PAG and/or a TAG, and may also be another compound acting as an acid generator such as a PDQ. However, any suitable compound capable of imparting a solubility shift at a prescribed point in the process flow  200  may be used as the solubility-shifting agent  224 . Several specific implementations of the solubility-shifting agent  224  are described below in other embodiments. 
     Referring now to  FIG. 2B , an overcoat  230  is deposited on the substrate  210  to at least partially fill the openings  226  between the structures  222 . In various embodiments, the overcoat  230  fills the openings  226 . An optional overburden  232  (overcoat material residing above top surfaces of the structures  222 ) may result in some embodiments. 
     Now referring to  FIG. 2C , the solubility-shifting agent  224  is activated by applying activation energy E to the relief pattern  220  to generate acid  242  which is diffuses  244  a predetermined distance  246  from the structures  222  into the overcoat  230 . The predetermined distance  246  (i.e. the diffusion depth) may be controlled by any combination of diffusion variables to achieve a desired depth of diffusion which is related to the thickness of the solubilized overcoat  230 . For example, possible variables that can be modified include, but are not limited to, acid molecular weight, acid concentration, bake temperature, bake time, base concentration and polymer composition. 
     The desired CD can be tuned through molecular weight modification of the reactive species generated from the solubility-shifting agent  224 , molecular structure of the reactive species, as well as the bake temperature and bake time. Additionally, the CD can be controlled by the composition of the overcoat  230  the reactive species is diffusing into. The polarity of components within the overcoat  230  may affect acid diffusivity and the activation energy of the reactive species-sensitive component of the overcoat  230  is an additional means to control processing. 
     The activation energy E may take any suitable form. For example, the activation energy E may be supplied in the form of actinic radiation, thermal energy, or a combination of the two. Light (e.g. electromagnetic radiation) and heat (e.g. convective, conductive, or radiative thermal energy) generate acid from PAGs or TAGs respectively. If a PAG is to be activated, some regions or the entire substrate  210  (including all supported layers) may be irradiated at a corresponding wavelength that activates the PAG. A TAG may be activated by baking the substrate  210  to a temperature sufficient to decompose the TAG. However, the specific mechanism for activating the solubility-shifting agent  224  depends on the selected solubility-shifting agent and may also include other mechanisms. The activation and diffusion may be performed sequentially or simultaneously. 
     The result of the activation and diffusion of the solubility-shifting agent  224  are soluble regions  250  adjacent to the structures  222  of the relief pattern  220  which remain insoluble relative to the predetermined developer. The soluble regions  250  have a predetermined width  247  that is proportional to the predetermined distance  246 . Since, the diffusion into the overcoat  230  causes the solubility shift, the predetermined width  247  of the soluble regions  250  may be equal to the predetermined distance  246  (as illustrated). However, in some cases the solubility-shifting agent  224  (e.g. generated acid) may diffuse into regions of the overcoat  230  (farthest from the structures  222 ) without reaching the solubility threshold of the overcoat  230 . Therefore, the predetermined width  247  of the soluble regions  250  is less than or equal to the predetermined distance  246  in practice. 
     Referring now to  FIG. 2E , the substrate  210  supporting the relief pattern  220  and the overcoat  230  with the soluble regions  250  is developed using the predetermined developer to form antispacer features  260 . Specifically, the result of the development process are antispacer features  260  defined by the structures  222  of the original relief pattern  220  and remaining overcoat structures  262  that were not solubilized during the activation and diffusion of the solubility-shifting agent  224 . 
     Notably and advantageously, after the activation and diffusion, no additional steps are required to prevent the structures  222  from dissolving in during development with the predetermined developer. Particularly, with the relief pattern material, the overcoat material, and the solubility-shifting agent already selected to prevent a solubility-shift in this scheme, the soluble regions  250  of the overcoat  230  can be removed without removing the structures  222 . 
     Additionally, in contrast to the conventional antispacer process flow  100 , the resulting antispacer features  260  may have an antispacer width  248  that is advantageously substantially equal to the predetermined width  247 . Further, preventing the solubilizing of the structures  222  and resultant avoidance of crosslinking (or other dissolution-inhibiting process steps) may advantageously result in uniform antispacer features  260 . Consequently, a smaller CD may be achievable in addition to the efficiency, compatibility, and simplicity that are afforded from the process flow  200 . 
     The type of materials selected for the structures  222  of the relief pattern  220  (e.g. photoresist), the overcoat  230  (e.g. a resin), and the solubility-shifting agent  224  (a PAG, TAG, PDQ, etc.) may affect the process flow  200 . For example, selections can be made which can maximize the selectivity between the deprotection rates of the relief pattern material compared to the overcoat material. In one embodiment, the overcoat  230  comprises a developable bottom anti-reflective coating (dBARC). The dBARC may advantageously de-crosslink in the presence of a low acid concentration (relative to an acid concentration necessary to deprotect the relief pattern material, for example). 
     The activation energy of a deprotectable monomer within the relief pattern  220  may be high relative to the overcoat  230  to allow for higher deprotection kinetics within the overcoat  230  relative to the structures  222 . In some embodiments, the relief pattern  220  comprises a high activation energy leaving group such as methyl adamantyl methacrylate (MAMA), isoadamantyl methacrylate (IAM), or tert-Butyl acrylate (TBA). 
     The activation energy of a deprotectable monomer included in the overcoat material may affect the choice of relief pattern material. For example, if the overcoat  230  comprises a deprotectable monomer with a low activation energy such as a monomer with an acetal functionality such as i-butoxyethyl methacrylate (BEMA) or a low activation ester functionality such as tert-Butyl cyclopentyl methacrylate (TBCPMA), then the activation energy of the relief pattern material may be lower than adamantyl methacrylate, such as that found with the ethyl cyclopropyl methacrylate (ECPMA) leaving group. 
     Remaining monomers in the relief pattern may be used for line formation and etch pattern transfer specifications. Conventional monomers used in standard lithographic patterning can be selected for this purpose. In some embodiments, the overcoat  230  is transparent over a range of wavelengths (e.g. so that actinic radiation within the range can pass through the overcoat). However, this is not a requirement of the overcoat  230 . 
       FIGS. 3A-3D  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility-shifting agent is utilized as a TAG when solubilizing an overcoat material in accordance with an embodiment of the invention. The process flow of  FIGS. 3A-3D  may be a specific implementation of other process flows or various stages of other process flows described herein such as the process flow of  FIGS. 2A-2E , for example. Similarly labeled elements may be as previously described. 
     Referring to  FIGS. 3A-3D , a process flow  300  optionally includes forming a relief pattern  320  on a substrate  310 . To form the relief pattern  320 , a layer of photoresist  327  supported by the substrate  310  is exposed to actinic radiation  321 . The exposure activates a PAG  325  included along with a TAG  324  (a solubility-shifting agent) in the layer of photoresist  327 . The PAG  325  is sensitive to a wavelength λ that is included in the spectrum of the actinic radiation  321 . The layer of photoresist  327  is exposed to the actinic radiation  321  through a photomask  323  (or obscured by opaque structures formed on the layer of photoresist  327 ) to form a latent pattern in the layer of photoresist  327 . 
     A relief pattern  320  including structures  322  separated by openings  326  is formed by developing the substrate  310  to remove the latent pattern ( FIG. 3B ). Alternatively, the process flow  300  may begin with the relief pattern  320  already formed. The structures  322  include both the TAG  324  which was not activated by the actinic radiation  321  and the PAG  325  which was shielded from the actinic radiation  321  by the photomask  323 . As a result, the structures  322  are insoluble relative to a predetermined developer at this stage of the process flow  300 . 
     As illustrated in  FIG. 3C , the TAG  324  is activated to generate an acid  342  that is diffused  344  a predetermined distance  346  by applying heat  340  having a predetermined temperature T to the substrate  310  including an overcoat  330  formed over the relief pattern  320 . Notably, the PAG  325  is still present in the structures  322 , but is not activated by the applied heat  340 . The diffusion process may also be promoted partially or entirely using a separate application of heat (e.g. a bake that uses different parameters, such as duration and/or temperature, than heat  340  applied to activate the TAG  324 ). 
     Antispacer features  360  with antispacer width  348  are then formed by developing the substrate  310  supporting the relief pattern  320  and the overcoat  330  using the predetermined developer ( FIG. 3D ). Specifically, the result of the development process are antispacer features  260  defined by the structures  322  of the original relief pattern  320  and remaining overcoat structures  362  that were not solubilized during the activation and diffusion of the TAG  324 . The PAG  325  was not activated during the application of heat  340  and the activated TAG  324  is insufficient to solubilize the photoresist of the structures  322 , but sufficient to solubilize regions of the overcoat  330  to which an appropriate amount of acid is diffused. For example, the solubility threshold of the photoresist of the relief pattern  320  may be sufficiently higher than the solubility threshold of the overcoat  230 . Additionally or alternatively, the acid  342  may be a weak acid that is not capable of significant deprotection of the photoresist. 
     The use of the TAG  324  as a secondary source of acid may advantageously be significant variable for tuning selectivity because the strength of the acid generated may be selected to allow for reaction with the overcoat  330  while minimally reacting with the photoresist of the relief pattern  320  at a given process temperature T. 
     As described above, both a PAG and TAG may be included in the formulation of a bottom layer of photoresist. In this specific example, the PAG is intended to deprotect the photoresist upon initial light exposure and subsequent bake and develop steps to form the initial relief pattern on the substrate. The TAG is subsequently activated thermally at a temperature higher than that of the relief pattern post-exposure bake (PEB). 
       FIGS. 4A-4C  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility shifting agent is utilized as a PAG when forming a relief pattern and as a TAG when solubilizing an overcoat material in accordance with an embodiment of the invention. The process flow of  FIGS. 4A-4C  may be a specific implementation of other process flows or various stages of other process flows described herein such as the process flow of  FIGS. 2A-2E , for example. Similarly labeled elements may be as previously described. 
     Referring to  FIGS. 4A-4C , a process flow  400  is similar to the process flow  300  of  FIGS. 3A-3D  except that an additional TAG is not required because a PAG  425  included in a layer of photoresist  427  that is used to generate a relief pattern  420  on a substrate  410  also acts as a TAG  424  at higher temperatures. 
     The exposure of actinic radiation  421  (including wavelength λ) through a photomask  423  activates the PAG  425  to form a latent pattern in the layer of photoresist  427 . As before, the relief pattern  420  including structures  422  separated by openings  426  is formed by developing the substrate  410  to remove the latent pattern ( FIG. 4B ). The structures  422  include the PAG  425  (TAG  424 ) which was not activated by the actinic radiation  421  due to being shielded from the actinic radiation  421  by the photomask  423 . As a result, the structures  422  are insoluble relative to a predetermined developer at this stage of the process flow  400 . 
     As illustrated in  FIG. 4C , the TAG  424  is activated to generate an acid  442  that is diffused  444  a predetermined distance  446  by applying heat  440  having a predetermined temperature T to the substrate  410  including an overcoat  430  formed over the relief pattern  420 . Antispacer features are then formed by developing the substrate  410  supporting the relief pattern  420  and the overcoat  430  using the predetermined developer. 
     In this specific example, the PAG  425  functions to deprotect the photoresist upon initial light exposure and subsequent bake and develop steps to form the initial relief pattern on the substrate, but then subsequently functions as a TAG (the TAG  424 ) at a temperature higher than that of the relief PEB. 
       FIGS. 5A and 5B  illustrate an example process flow for patterning a substrate to form antispacer features where a solubility shifting agent is utilized as a PAG when solubilizing an overcoat material in accordance with an embodiment of the invention. The process flow of  FIGS. 5A and 5B  may be a specific implementation of other process flows or various stages of other process flows described herein such as the process flow of  FIGS. 2A-2E , for example. Similarly labeled elements may be as previously described. 
     The previous two process flows describe a solubility-shifting agent that functions as a TAG to form soluble regions in an overcoat. Alternatively, another PAG activated at a different wavelength than a PAG used to form the initial relief pattern may be used. For example, the polymer in the overcoat layer may have a deprotectable monomer that is activated with a weaker acid than a deprotectable monomer in the photoresist of the relief pattern. This composition may advantageously facilitate little or no change in the CD of the initial structures of the relief pattern while enabling facile solubilizing of the overcoat layer. Examples of deprotectable monomers (solubility-switching groups) that react at lower temperatures or with weaker acids compared to the initial relief pattern are low activation energy ester leaving groups, such as those mentioned above, and acetal leaving groups. 
     Referring to  FIG. 5A , a process flow  500  includes a layer of photoresist  527  disposed on a substrate  510 . Both a first PAG  525  and a second PAG  524  are included in the layer of photoresist  527  which is exposed to actinic radiation  521  including a first wavelength λ 1  through a photomask  523 . As illustrated in  FIG. 5B , an overcoat  530  is then exposed to actinic radiation  540  including a second wavelength λ 2  (different from the first wavelength λ 1 ) which activates the second PAG  524  but does not activate the first PAG  525  remaining in the structures  522  of the relief pattern. The overcoat  530  is transparent to the second wavelength λ 2 . Heat  541  (e.g. energy supplied for diffusion) may also be applied to diffuse  544  acid  542  generated from the second PAG  524  into the overcoat  530 . The heat  541  may be applied before, during and/or after applying the actinic radiation  540  including the second wavelength λ 2 . 
       FIGS. 6A and 6B  illustrate an example process flow for patterning a substrate to form antispacer features where a PDQ is utilized as a solubility shifting agent when solubilizing an overcoat material in accordance with an embodiment of the invention. The process flow of  FIGS. 6A and 6B  may be a specific implementation of other process flows or various stages of other process flows described herein such as the process flow of  FIGS. 2A-2E , for example. Similarly labeled elements may be as previously described. 
     Referring to  FIGS. 6A and 6B , a process flow  600  is similar to the process flow  300  of  FIGS. 3A-3D  except that the additional TAG included in a layer of photoresist  627  used to generate a relief pattern  620  on a substrate  610  is also a PDQ  624  that is employed as a quencher during formation of the relief pattern  620 . As before, the layer of photoresist  627  including a PAG  625  and the PDQ  624  is exposed to actinic radiation  621  including a wavelength λthrough a photomask  623 . As illustrated in  FIG. 6B , heat  640  is applied to an overcoat  630  which activates and diffuses  644  the PDQ  624  to generate an acid  642  strong enough to deprotect the overcoat  630 . 
     In order to facilitate activation of the PDQ  624  without activation of the PAG  625  (e.g. still in the relief pattern  620 ), differentiation between conditions sufficient to activate the PDQ  624  and the PAG  625  may be used. For example, the PDQ  624  may generate a weak acid while the PAG  625  may generate a strong acid. If the stronger PAG  625  is activated the desired selectivity may not be achieved. The differentiation between the activation conditions for the PDQ  624  and the PAG  625  is a wavelength difference in one embodiment and is a thermal difference in another embodiment. 
     In various embodiments, the PDQ  624  is a camphorsulfonic acid PDQ. In the presence of a superacid in the relief pattern, the camphorsulfonic acid salt acts as a quencher due to pKa differences with the superacid. Diffusing camphorsulfonic acid into the overcoat  630  (e.g. containing acetal functionality) may facilitate reaction with the polymer to form a soluble material in the regions it diffuses into. Camphorsulfonic acid may also deprotect low activation energy esters such as TBCPMA at slightly elevated temperatures. 
       FIG. 7  illustrates two qualitative graphs where the left graph illustrates an example relationship between resist thickness and exposure dose and where the right graph illustrates an example relationship between solubility and deprotection in accordance with an embodiment of the invention. 
     Chemically-amplified photoresists (CAR) undergo a solubility shift upon exposure to a specific wavelength of light via acid-catalyzed deprotection of the polymer resin by acid generated from PAG decomposition. A metric of a high performance photoresist is a large dissolution contrast which is often represented by a plot of photoresist thickness vs. exposure dose, also known as a contrast curve. In a positive photoresist, when achieving a specific threshold of polymer de-protection any excess de-protection dramatically and non-linearly increases the solubility of the film in aqueous developer as illustrated in  FIG. 7 . 
     The techniques described herein function by selecting or formulating the overcoat and photoresist such that the overcoat will exceed its deprotection threshold (solubility threshold) within a process window before the photoresist becomes deprotected. 
       FIG. 8  illustrates four qualitative graphs of potential scenarios in which the dissolution contrast and sensitivity of the photoresist and overcoat are considered in accordance with embodiments of the invention. 
     Graph (A) illustrates an ideal scenario in which the overcoat exhibits high dissolution contrast with a deprotection threshold (solubility threshold) well below that of the photoresist such that complete dissolution of the overcoat can be attained before the solubility of the photoresist is affected. A photoresist with higher dissolution contrast will provide a wider process margin. 
     Graph (B) is a similar scenario to graph (A); however the critical deprotection thresholds of the overcoat and photoresist begin to overlap preventing the reproducible dissolution of the de-protected overcoat with retention of the photoresist mandrel. Variables, including but not limited to, acid strength, developer concentration, polymer composition, bake temperature and bake time may be adjusted to increase the process window between the two deprotectable films so that the ideal scenario of graph (A) may be approached or achieved. 
     Graph (C) illustrates a scenario in which the photoresist is more sensitive than the overcoat resulting in complete dissolution of the photoresist prior to critical de-protection of the overcoat. This system would require a freeze step and functionality within the photoresist to prevent dissolution of the de-protected resin. 
     Graph (D) is an example of a system in which the overcoat exhibits low dissolution contrast and high sensitivity relative to the photoresist. This system may advantageously achieve reproducible sub-resolution antispacer features if two process windows are considered. In one process window, the dissolution rate of the deprotected overcoat relative to the protected overcoat is sufficient to clear the antispacer region while retaining the surrounding overcoat. In a second process window, the degree of acid deprotection required to solubilize the overcoat results in minimal dissolution of the photoresist for which the structures of the relief pattern will be retained after the antispacer is solubilized. 
     Various compositions can be selected for use with processes herein. Overcoat compositions may be polymer resins, and the resins may be made up of multiple monomer types. A majority of the monomers within the overcoat may be similar in structure to that of the photoresist so that both films have similar etch rates. 
     The aspect of the overcoat that defines a solubility contrast relative to the photoresist is the composition of the acid-sensitive monomer. To maximize the selectivity, the ratio of the activation energy between the resist and overcoat sufficient to undergo the solubility-changing reaction should be high. Accordingly, the photoresist may have a higher activation energy deprotectable monomer such as MAMA or TBA. The overcoat may then have a lower activation energy monomer such as an acetal, ECPMA, or another low activation energy ester functionality (e.g. even lower than ECPMA). 
     As can be appreciated by those of skill in the art, other chemical combinations can be selected for use herein. The overcoat material may have a small dissolution rate R min  in TMAH that allows the development of the soluble region in the overcoat. To generate unexposed film thickness loss in the overcoat, a monomer with inherent TMAH solubility may be used. An example of this is the inclusion of one of the following monomers as examples: dihexafluoro alcohol (DiHFA), methyl methacrylate (MAA), and phenol. 
     As mentioned above, to simplify the formulation of the relief pattern material, it may be advantageous to incorporate a very low activation energy deprotectable monomer in the overcoat layer such as an acetal or a low activation energy ester. This may advantageously allow weak acid components used in the formation of the relief pattern to diffuse into the overcoat and induce a switch in solubility in the regions activated by acid diffusion. Many PAGs also act as TAGs at higher temperatures as they reach their thermal decomposition. Therefore, in some formulations, no additional TAG is required in the relief pattern material. 
       FIG. 9  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention. The method of  FIG. 9  may be combined with other methods and performed using the systems and apparatuses as described herein. For example, the method of  FIG. 9  may be combined with any of the embodiments of  FIGS. 2A-8 . Although shown in a logical order, the arrangement and numbering of the steps of  FIG. 9  are not intended to be limiting. The method steps of  FIG. 9  may be performed in any suitable order or concurrently with one another as may be apparent to a person of skill in the art. 
     Referring to  FIG. 9 , step  901  of a method  900  of patterning a substrate includes depositing an overcoat in openings of a relief pattern supported by a substrate. The relief pattern may include a solubility-shifting agent and a first deprotectable monomer sensitive to the solubility-shifting agent. The overcoat may include a second deprotectable monomer sensitive to the solubility-shifting agent. The relief pattern has a first solubility threshold relative to a predetermined developer while the overcoat has a second solubility threshold relative to the predetermined developer. The second solubility threshold is lower than the first solubility threshold. 
     Step  902  includes activating the solubility-shifting agent to at least reach the second solubility threshold of the overcoat without reaching the first solubility threshold of the relief pattern. The solubility-shifting agent is diffused a predetermined distance from structures of the relief pattern into the overcoat to form soluble regions in the overcoat in step  903 . The soluble regions are soluble in the predetermined developer while the relief pattern remains insoluble in the predetermined developer. Steps  902  and  903  may be performed simultaneously, separately, or partially overlapping. The substrate is developed with the predetermined developer to remove the soluble regions of the overcoat in step  904 . 
       FIG. 10  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention. The method of  FIG. 10  may be combined with other methods and performed using the systems and apparatuses as described herein. For example, the method of  FIG. 10  may be combined with any of the embodiments of  FIGS. 2A-8 . Additionally, the method of  FIG. 10  may be combined with the method of  FIG. 9 , for example. Although shown in a logical order, the arrangement and numbering of the steps of  FIG. 10  are not intended to be limiting. The method steps of  FIG. 10  may be performed in any suitable order or concurrently with one another as may be apparent to a person of skill in the art. 
     Referring to  FIG. 10 , step  1001  of a method  1000  of pattering a substrate includes depositing an overcoat in openings of a relief pattern supported by a substrate. The relief pattern includes a solubility-shifting agent and a first deprotectable monomer having a first activation energy while the overcoat includes a second deprotectable monomer having a second activation energy. The first activation energy is higher than the second activation energy. 
     In step  1002 , the second deprotectable monomer is deprotected without deprotecting the first deprotectable monomer to form soluble regions in the overcoat by activating the solubility-shifting agent and diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the overcoat. The soluble regions are soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer. The substrate is then developed with the predetermined developer to remove the soluble regions of the overcoat in step  1003 . 
       FIG. 11  illustrates an example method of patterning a substrate in accordance with an embodiment of the invention. The method of  FIG. 11  may be combined with other methods and performed using the systems and apparatuses as described herein. For example, the method of  FIG. 11  may be combined with any of the embodiments of  FIGS. 2A-8 . Additionally, the method of  FIG. 11  may be combined with any of the methods of  FIGS. 9 and 10 , as examples. Although shown in a logical order, the arrangement and numbering of the steps of  FIG. 11  are not intended to be limiting. The method steps of  FIG. 11  may be performed in any suitable order or concurrently with one another as may be apparent to a person of skill in the art. 
     Referring to  FIG. 11 , step  1101  of a method  1100  of patterning a substrate includes forming a relief pattern on a substrate from a layer of photoresist by exposing the photoresist to actinic radiation including a first wavelength to activate a first photoacid generator. The photoresist includes the first photoacid generator and a solubility-shifting agent. A deprotectable resin is deposited in openings of the relief pattern in step  1102 . 
     In step  1103 , the solubility-shifting agent is activated. Step  1104  includes diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the deprotectable resin to form soluble regions in the deprotectable resin by deprotecting the deprotectable resin. The soluble regions are soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer. Steps  1103  and  1104  may be performed simultaneously, separately, or partially overlapping. The substrate is then developed with the predetermined developer to remove the soluble regions of the overcoat in step  1105 . 
     In the preceding description, specific details have been set forth, such as a particular geometry of a processing system and descriptions of various components and processes used therein. It should be understood, however, that techniques herein may be practiced in other embodiments that depart from these specific details, and that such details are for purposes of explanation and not limitation. Embodiments disclosed herein have been described with reference to the accompanying drawings. Similarly, for purposes of explanation, specific numbers, materials, and configurations have been set forth in order to provide a thorough understanding. 
     Various techniques have been described as multiple discrete operations to assist in understanding the various embodiments. The order of description should not be construed as to imply that these operations are necessarily order dependent. Indeed, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments. 
     “Substrate” or “target substrate” as used herein generically refers to an object being processed in accordance with the invention. The substrate may include any material portion or structure of a device, particularly a semiconductor or other electronics device, and may, for example, be a base substrate structure, such as a semiconductor wafer, reticle, or a layer on or overlying a base substrate structure such as a thin film. Thus, substrate is not limited to any particular base structure, underlying layer or overlying layer, patterned or un-patterned, but rather, is contemplated to include any such layer or base structure, and any combination of layers and/or base structures. The description may reference particular types of substrates, but this is for illustrative purposes only. 
     Example embodiments of the invention are summarized here. Other embodiments can also be understood from the entirety of the specification as well as the claims filed herein. 
     Example 1. A method of patterning a substrate, the method including: depositing an overcoat in openings of a relief pattern supported by a substrate, the relief pattern including a solubility-shifting agent and a first deprotectable monomer sensitive to the solubility-shifting agent, the overcoat including a second deprotectable monomer sensitive to the solubility-shifting agent, where the relief pattern includes a first solubility threshold relative to a predetermined developer and the overcoat includes a second solubility threshold relative to the predetermined developer lower than the first solubility threshold; activating the solubility-shifting agent to at least reach the second solubility threshold of the overcoat without reaching the first solubility threshold of the relief pattern; diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the overcoat to form soluble regions in the overcoat, the soluble regions being soluble in the predetermined developer while the relief pattern remains insoluble in the predetermined developer; and developing the substrate with the predetermined developer to remove the soluble regions of the overcoat. 
     Example 2. The method of example 1, where the solubility-shifting agent is a thermal acid generator, and where activating the solubility-shifting agent includes applying heat to the substrate to activate the solubility-shifting agent. 
     Example 3. The method of example 2, further including: forming the relief pattern on the substrate, the relief pattern further including a photoacid generator different from the solubility-shifting agent, where forming the relief pattern includes forming the relief pattern from a layer of photoresist by exposing the layer of photoresist to actinic radiation to activate the photoacid generator. 
     Example 4. The method of example 2, further including: forming the relief pattern on the substrate, the solubility-shifting agent also being a photoacid generator, where forming the relief pattern includes forming the relief pattern from a layer of photoresist by exposing the layer of photoresist to actinic radiation to activate the solubility-shifting agent. 
     Example 5. The method of example 1, where the solubility-shifting agent is a first photoacid generator activated at a first wavelength, the relief pattern further includes a second photoacid generator activated at a second wavelength different from the first wavelength, and activating the solubility-shifting agent includes exposing the first photoacid generator to actinic radiation including the first wavelength to activate the first photoacid generator. 
     Example 6. The method of one of examples 1 and 2, where the solubility-shifting agent includes a photodestroyable quencher. 
     Example 7. The method of example 6, where the photodestroyable quencher is camphorsulfonic acid. 
     Example 8. The method of one of examples 1 to 7, where the overcoat includes a developable bottom anti-reflective coating. 
     Example 9. The method of one of examples 1 to 8, where the first deprotectable monomer includes methyl-adamantyl methacrylate, isoadamantyl methacrylate, or tert-Butyl acrylate. 
     Example 10. The method of one of examples 1 to 9, where the second deprotectable monomer includes acetal or ester functionality. 
     Example 11. The method of one of examples 1 to 10, where the predetermined developer includes tetramethylammonium hydroxide. 
     Example 12. A method of patterning a substrate, the method including: depositing an overcoat in openings of a relief pattern supported by a substrate, the relief pattern including a solubility-shifting agent and a first deprotectable monomer having a first activation energy, and the overcoat including a second deprotectable monomer having a second activation energy, the first activation energy being higher than the second activation energy; deprotecting the second deprotectable monomer without deprotecting the first deprotectable monomer to form soluble regions in the overcoat by activating the solubility-shifting agent and diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the overcoat, the soluble regions being soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer; and developing the substrate with the predetermined developer to remove the soluble regions of the overcoat. 
     Example 13. The method of example 12, where the solubility-shifting agent is a thermal acid generator, and where activating the solubility-shifting agent includes applying heat to the substrate to activate the solubility-shifting agent. 
     Example 14. The method of example 12, where the solubility-shifting agent is a photoacid generator, and where activating the solubility-shifting agent includes exposing the photoacid generator to actinic radiation to activate the photoacid generator. 
     Example 15. The method of one of examples 12 and 13, where the solubility-shifting agent includes a photodestroyable quencher. 
     Example 16. A method of patterning a substrate, the method including: forming a relief pattern on a substrate from a layer of photoresist by exposing the photoresist to actinic radiation including a first wavelength to activate a first photoacid generator, the photoresist including the first photoacid generator and a solubility-shifting agent; depositing a deprotectable resin in openings of the relief pattern; activating the solubility-shifting agent; diffusing the solubility-shifting agent a predetermined distance from structures of the relief pattern into the deprotectable resin to form soluble regions in the deprotectable resin by deprotecting the deprotectable resin, the soluble regions being soluble in a predetermined developer while the relief pattern remains insoluble in the predetermined developer; and developing the substrate with the predetermined developer to remove the soluble regions of the deprotectable resin. 
     Example 17. The method of example 16, where the solubility-shifting agent is a thermal acid generator, and where activating the solubility-shifting agent includes applying heat to the substrate to activate the solubility-shifting agent. 
     Example 18. The method of example 16, where the first photoacid generator is activated at the first wavelength, the solubility-shifting agent is a second photoacid generator activated at a second wavelength, and activating the solubility-shifting agent includes exposing the second photoacid generator to actinic radiation including the second wavelength to activate the second photoacid generator. 
     Example 19. The method of one of examples 16 and 17, where the solubility-shifting agent includes a photodestroyable quencher. 
     Example 20. The method of one of examples 16 to 19, where the photoresist includes a higher contrast than the deprotectable resin relative to the predetermined developer. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.