Patent Publication Number: US-11656550-B2

Title: Controlling semiconductor film thickness

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/073,047, filed on Sep. 1, 2020, which is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to semiconductor fabrication, and, in certain embodiments, to controlling semiconductor film thickness. 
     BACKGROUND 
     Constructing electrical circuits involves depositing numerous layered materials across various features or structures, as well as patterning, etch, and fill processes. As design innovation for next generation transistors moves to smaller dimensions and vertical architectures, desire for technology that precisely controls film thickness within a die and across a wafer increases. Etch processes can be timed to remove a portion of a film without an endpoint; however, such processes have poor locational control and high variability. 
     SUMMARY 
     In certain embodiments, a method for processing a semiconductor substrate includes receiving a substrate having microfabricated structures defining recesses and depositing a resin film on the substrate. The resin film fills the recesses, covers the microfabricated structures, and is initially resistant to development by a solvent. The method includes depositing a first overcoat film on the substrate. The first overcoat film contains a first agent-generating ingredient that generates, in response to actinic radiation, a first solubility-changing agent. The method includes exposing the first overcoat film to first sufficient actinic radiation to generate the first solubility-changing agent within the first overcoat film. The method includes diffusing the first solubility-changing agent a first predetermined depth into the resin film causing a first portion of the resin film to become soluble to the first solvent, and developing the first overcoat film and the first portion of the resin film using the first solvent. The method includes depositing a second overcoat film on the substrate. The second overcoat film contains the first agent-generating ingredient that generates, in response to actinic radiation, the first solubility-changing agent. The method includes exposing the second overcoat film to second sufficient actinic radiation to generate the first solubility-changing agent within the second overcoat film. The method includes diffusing the first solubility-changing agent a second predetermined depth into the resin film causing a second portion of the resin film to become soluble to the first solvent, and developing the second overcoat film and the second portion of the resin film using the first solvent resulting in the resin film being recessed respective first combined depths in the recesses. 
     In certain embodiments, a method for processing a semiconductor substrate includes receiving a substrate having microfabricated structures defining recesses and depositing a resin film on the substrate. The resin film fills the recesses, covers the microfabricated structures, and is initially resistant to development by a first solvent. The method includes depositing a first overcoat film on the substrate. The first overcoat film contains a first agent-generating ingredient that generates, in response to actinic radiation, a first solubility-changing agent. The method includes exposing the first overcoat film to sufficient actinic radiation to generate the first solubility-changing agent within the first overcoat film. The method includes diffusing the first solubility-changing agent a first predetermined depth into the resin film causing a first portion of the resin film to become soluble to the first solvent, and developing the first portion of the resin film using the first solvent. The method includes depositing a second overcoat film on the substrate. The second overcoat film contains a second agent-generating ingredient that generates, in response to heating of the substrate, a second solubility-changing agent. The method includes baking the substrate sufficiently to generate the second solubility-changing agent within the second overcoat film and diffuse the second solubility-changing agent a second predetermined depth into the resin film causing a second portion of the resin film to become soluble to the first solvent. The method includes developing the second portion of the resin film using the first solvent resulting in the resin film being recessed respective combined depths in the recesses. 
     In certain embodiments, a method for processing a semiconductor substrate includes depositing a resin film on a substrate that has microfabricated structures defining recesses. The resin film fills the recesses and covers the microfabricated structures. The method includes performing, using a photoacid generator (PAG)-based process, a localized removal of the resin film to remove the resin film to respective first depths in the recesses, at least two depths of the respective first depths being different depths. The method includes repeatedly performing, using a thermal acid generator (TAG)-based process and until a predetermined condition is met, a uniform removal of a remaining portion of the resin film to remove a substantially uniform depth of the resin film in the recesses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS.  1 A- 1 J  illustrate cross-sectional and plan views of an example semiconductor substrate during an example process for processing the substrate; 
         FIGS.  2 A- 2 I  illustrate cross-sectional and plan views of an example substrate during an example process for processing the substrate; 
         FIG.  3    illustrates example effects of varying depths of diffusion of a solubility-changing agent into a fill material; 
         FIGS.  4 A- 4 H  illustrate cross-sectional views of example substrate portions having pre-patterned features during example process for processing the substrate portions; 
         FIGS.  5 A- 5 C  illustrate cross-sectional views of example substrate portions having pre-patterned features during portions of example process for processing the substrate portions; 
         FIG.  6    illustrates an example method for processing a semiconductor substrate; 
         FIG.  7    illustrates an example method for processing a semiconductor substrate; 
         FIG.  8    illustrates an example method for processing a semiconductor substrate; 
         FIGS.  9 A- 9 C  illustrate example PAGs and TAGs that may be used in overcoat films; 
         FIGS.  10 A- 10 B  illustrate example modification of the solubility of overcoat films and/or a fill material; 
         FIG.  11    illustrates examples of stacked transistor architectures that may benefit from precise film height control to selectively grow n-type and p-type silicon-germanium (SiGe); and 
         FIGS.  12 A- 12 B  illustrate that a step in a self-aligned block (SAB) process flow may benefit from a partial recess of a specific film, such as a spin-on carbon. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Throughout the deposition, patterning, and removal processes associated with forming a semiconductor device, it may be desirable to control the height of a deposited film for various reasons. For example, it may be desirable to remove a portion of a deposited film (e.g., in a trench) to achieve a certain height of that deposited film within the trench. Conventional removal processes, such as timed wet or dry etch processes, for removing portions of a deposited layer are often difficult to control and suffer other problems, such as planarization problems. These problems become even more prevalent as feature sizes continue to shrink or vary across the surface of a semiconductor wafer being processed. 
     Embodiments of this disclosure provide techniques to control film thickness for a semiconductor substrate. The substrate may have pre-patterned features that include, for example, structures defining recesses. The film being controlled may be a fill material, such as a polymer resin, deposited over the pre-patterned features, filling the recesses and covering the structures. Across a semiconductor wafer that includes the substrate, precisely and repeatably reducing the fill material to particular target heights (thicknesses) within recesses may be desired, and those target heights might vary from among recesses. Certain embodiments accomplish this film thickness control without using an etch stop layer or other timed etch process used with conventional etch techniques in which control of film height is desired. 
     The fill material may be initially resistant to removal (e.g., development) by a solvent (e.g., developer) to be used in removing a portion of the fill material. Certain embodiments use a cyclic process that includes depositing an overcoat film containing an agent-generator that, in response to a stimulus, generates an agent in the overcoat film. The agent is then diffused into the fill material to a predetermined depth, causing a portion of the fill material to become de-protected (removable/developable) relative to the solvent. The overcoat film and the de-protected portion of the fill material are then removed using the solvent. This process can be repeated until the fill material in the recesses reaches one or more corresponding target heights. 
     Certain embodiments use a PAG-based process to reduce at least a portion of a height of a resin film in recesses of a substrate. For example, the agent-generator in the overcoat film may be a photo-activated agent generator (e.g., a PAG) that is activated in response to actinic radiation. This PAG-based process may be repeated a suitable number of times until target films heights (e.g., within recesses) are achieved. 
     Certain embodiments use a TAG-based process to reduce at least a portion of a height of a resin film in recesses of a substrate. For example, the agent-generator in the overcoat film may be a thermally-activated agent generator (e.g., a TAG) that is activated in response to heat. This TAG-based process may be repeated a suitable number of times until target films heights (e.g., within recesses) are achieved. 
     Certain embodiments combine one or more iterations of the PAG-based process to establish height variations in the resin film with one or more iterations of the TAG-based process to uniformly further reduce the film height thickness until the target film heights (e.g., within recesses) are achieved. 
     That is, embodiments provide modulation of film thickness and profile by location across a wafer by generation and diffusion of acid from an overcoat into an acid-de-protectable resin, followed by development. Depth of the acid de-protection into the resin film may be defined by the amount of acid produced in, and diffused from, the overlying overcoat. Locational height control may be achieved using photoacid and/or thermal-acid generator containing overcoats. Embodiments can be used with backside overlay control techniques as well as location-based critical dimension optimizer platforms for front side treatment. 
     Certain embodiments also provide improved planarity. For example, certain conventional etch techniques introduce or exacerbate planarization problems, particularly as pitches between structures of a substrate, or the widths of those structures, vary. Certain embodiments of this disclosure are able to control removal of a fill material to target heights with little to no effects introduced by the varying topography of a substrate. 
       FIGS.  1 A- 1 J  illustrate cross-sectional and plan views of an example semiconductor substrate  100  during an example process  102  for processing substrate  100 , according to certain embodiments. Process  102  includes stages  104   a - 104   j , though process  102  may include more or fewer stages if appropriate. Substrate  100  may be part of a larger semiconductor device, such as part of a larger semiconductor wafer. In certain embodiments, process  102  includes repeatedly performing a PAG-based process to remove a fill material from a recess of substrate  100  until the fill material is a predetermined height within the recess. 
     As shown in  FIG.  1 A  at stage  104   a , substrate  100  includes base portion  106  and microfabricated structures  108  formed on base portion  106 . Structures  108  define recesses  110 . This disclosure contemplates structures  108  being patterned into any suitable features. For example, although this disclosure primarily describes “recesses,” other suitable features might be formed in or on a semiconductor substrate, including (whether or not considered “recesses”) lines, holes, trenches, vias, and/or other suitable structures, using embodiments of this disclosure. Structures  108  and recesses  110  may be formed using conventional lithography processes and/or other suitable deposition and etch processes. Base portion  106  and structures  108  may include the same or different materials (or combinations of materials), as appropriate. 
     Substrate  100  generically refers to a workpiece being processed in accordance with embodiments of this disclosure. Substrate  100  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  100  is not limited to any particular base structure, underlying layer or overlying layer, patterned or un-patterned, but rather, may include any such layer or base structure, and any combination of layers and/or base structures. Substrate  100  may be a bulk substrate such as a bulk silicon substrate, a silicon on insulator substrate, or various other semiconductor substrates. 
     Structures  108  have respective top surfaces  112 , and recess  110  has bottom surface  113 . In certain embodiments, structures  108  and recesses  110  differ in height relative to each other. For example, in certain embodiments, recesses have a height  114  (in a z-direction from a bottom of base portion  106  to bottom surface  113  of recess  110 ), and structures  108  have a second height  116  (from a bottom of base portion  106  to top surfaces  112  of structures  108  in the z-direction). In certain embodiments, the height difference of structures  108  and recess  110  relative to each other may be between 10 nm and 100 nm (e.g., greater than 50 nm). In other embodiments, the height difference may be greater than 5 microns, in case of a deep opening/trench for example. Structures  108  are separated by a gap (e.g., defined by recess  110 ), which may have any suitable width  118  for a given application. 
     As shown in  FIG.  1 B  at stage  104   b , a fill material  120  has been deposited on substrate  100 . Fill material  120  may be deposited in any suitable manner. For example, fill material  120  may be deposited using spin-on deposition (or spin-coating), spray-coating, roll-coating, chemical vapor deposition (CVD), or any other suitable deposition technique. Fill material  120  fills recess  110  and covers structures  108 . In subsequent photolithography steps, it may be desirable to recess, via photolithographic development techniques, fill material  120  into recess  110 , such that fill material  120  has a particular height within recess  110 . 
     In certain embodiments, fill material  120  is a resin film, such as a polymer resin. Fill material  120  may have a photo-de-protectable property and, as deposited, may be resistant to being dissolved by a given solvent (which also may be referred to as a developer). As will be described in later stages, after exposure to a particular acid, however, the fill material  120  can experience a solubility change after which the fill material  120  (or portions thereof) is no longer protected from the solvent and will dissolve in the solvent. For example, in certain embodiments, fill material  120  is an acid de-protectable polymer, and a portion of the polymer will react with a certain species (e.g., an acid) to decompose in order to shift solubility of fill material  120  such that fill material  120  will dissolve or otherwise wash away if de-protected in a particular way. As particular examples, fill material  120  may be a copolymer or terpolymer composed of multiple types of monomers with at least one of the monomers able to decompose in the presence of a strong acid to make a more polar group like a carboxylic acid terminal group, so that fill material  120  will be more soluble in an aqueous medium. As a particular example, fill material  120  may include multiple monomer types including an acid sensitive monomer such as tert-butyl acrylate or methyl adamantyl methacrylate. 
     In certain embodiments, fill material  120  includes a photosensitive material such as a positive, negative, or a hybrid toned photoresist. In one example, fill material  120  includes phenol formaldehyde resin or a diazo-naphthoquinone based resin. In certain embodiments, fill material  120  may include a chemically amplified resist. In other embodiments, fill material  120  may include a non-chemically amplified resist material, such as PolyMethyl MethAcrylate (PMMA) or Hydrogene SilsesQuioxance (HSQ). 
     It may be desirable to remove a portion of fill material  120 , including within recess  110 , such that fill material has a predetermined target height  121  within recess  110 . In this example, target height  121  is shown to be measured from bottom surface  113  of recess  110 ; however, the target height of fill material  120  within recess  110  may be measured from any suitable location, such as the bottom of base portion  106 . Target height  121  also may be considered a target thickness of fill material  120 . Fill material  120  is initially resistant to development by one or more solvents that will be used in a subsequent process to remove portions of fill material  120 . 
     As shown in  FIG.  1 C  at stage  104   c , an overcoat film  122  has been deposited on substrate  100 . Overcoat film  122  may be deposited in any suitable manner, including spin-on deposition (or spin-coating), spray-coating, roll-coating, CVD, or any other suitable deposition technique. Overcoat film  122  contains a photo-activated agent generator that generates, in response to actinic radiation, a solubility-changing agent for changing the solubility of another material (e.g., the material of overcoat film  122  and/or fill material  120 ) to be soluble in one or more solvents to be used in a subsequent removal process. In certain embodiments, the photo-activated agent generator is a PAG and the solubility-changing agent is acid. 
     Aside the photo-activated agent generator, overcoat film  122  might or might not include the same material as or a similar material to fill material  120 . In certain embodiments, in addition to the photo-activated agent generator, overcoat film  122  may include a polymer resin that has a solubility in a solvent (to be used subsequently to remove a de-protected portion of fill material  120  in recess  110 ) that is similar to the solubility of the de-protected fill material  120  in the solvent, so that the de-protected portion of fill material  120  and overcoat film  122  can be removed in one step. In certain embodiments, the photo-activated agent generator of overcoat film  122  is pre-formulated in the material (e.g., the resin) of overcoat film  122 . 
     As shown in  FIG.  1 D  at stage  104   d , overcoat film  122  is exposed to actinic radiation  124  for a suitable time period. In particular, overcoat film  122  is exposed to sufficient actinic radiation  124  to cause the photo-activated agent generator (e.g., the PAG) in overcoat film  122  to generate a solubility-changing agent  126  (e.g., acid) within overcoat film  122  such that overcoat film  122  now includes solubility-changing agent  126 . Solubility-changing agent  126  causes overcoat film  122  to become solubilized, such that overcoat film  122  is now soluble in one or more solvents to be used in a subsequent removal process. 
     Actinic radiation  124  may include light at a suitable wavelength and having other suitable characteristics to activate the photo-activated agent generator (e.g., the PAG) in overcoat film  122 , causing the photo-activated agent generator in overcoat film  122  to generate solubility-changing agent  126  (e.g., acid) within overcoat film  122 . Characteristics of actinic radiation  124  that may affect whether the photo-activated agent generator in overcoat film  122  is activated to generate solubility-changing agent  126  within overcoat film  122  (and in what quantities) include content of overcoat film  122 , the type of photo-activated agent generator, the wavelength of actinic radiation  124 , the time period for which overcoat film  122  is exposed to actinic radiation  124 , and other suitable factors. 
     A predetermined photo-activated agent generator (e.g., PAG) may be sensitive to either a predetermined wavelength or to a predetermined range of wavelengths, permitting the use of various exposure sources. As just one example, the wavelength of actinic radiation  124  may be in a range of about 170 nm to about 405 nm, and exposure time may be about 10 seconds to about a minute (for a wafer of which the portion shown in  FIGS.  1 A- 1 J  is a part). The polymer of fill material overcoat film  122  can be transparent or near-transparent to the predetermined wavelength. 
     It should be understood, however, that the values and actinic radiation sources are provided as examples only. In certain embodiments, as described below with reference to  FIGS.  4 B and  4 F , substrate  100  is part of a larger substrate, and actinic radiation  124  is part of a larger pattern of actinic radiation that is directed to an overcoat film (of which overcoat film  122  is a part) on the larger substrate. Exposure to actinic radiation (e.g., light) can be executed with a scanner using a mask-based exposure, or via a direct-write exposure step, or a flood exposure, as just a few examples. A physical lithographic exposure stepper or scanner can be used as well. In another example, a comparably simple scanning laser system could be used that can vary the exposure energy spatially across a surface of a wafer. The particular wavelength of and exposure time to actinic radiation  124  suitable for a given implementation may be affected by the tool used, including the intensity of the laser. 
     As shown in  FIG.  1 E  at stage  104   e , to modify at least a portion of fill material  120  to be soluble in a solvent to be used in a later removal process, solubility-changing agent  126  has diffused into fill material  120 , causing a portion (de-protected portion  120   a ) of fill material  120  to become soluble to the solvent to be used in a later removal process. De-protected portion  120   a  is generally shown as the portion of fill material  120  into which solubility-changing agent  126  has diffused. Diffusion of solubility-changing agent  126  into fill material  120  results in solubility-changing reactions within fill material  120  to the depth at which solubility-changing agent  126  (e.g., to the predetermined depth) diffuses into fill material  120 , resulting in de-protected portion  120   a . De-protected portion  120   a  of fill material  120  then becomes soluble to one or more particular solvents, which also may be referred to as developers. The de-protection reaction resulting from the diffusion of solubility changing agent  126  into a portion of fill material  120  (creating de-protected portion  120   a ) could be a de-crosslinking reaction within the portion of fill material  120 . A similar reaction may occur within overcoat film  122  to cause overcoat film  122  to become solubilized. 
     Solubility-changing agent  126  may be diffused into fill material  120  using any suitable process. In certain embodiments, a thermal process (e.g., heat  127 ) is used to diffuse solubility-changing agent  126  into at least a portion of fill material  120 . For example, to apply heat  12 , substrate  100  may be baked, and the heat associated with baking substrate  100  causes solubility-changing agent  126  to diffuse into at least a portion of fill material  120 . Substrate  100  may be baked via a substrate plate in a suitable tool, via ambient heat in a substrate processing chamber of a suitable tool, a combination of these, or in any other suitable manner. 
     In certain embodiments, solubility-changing agent  126  is diffused a predetermined depth into fill material  120  to modify the solubility of fill material  120  to the predetermined depth. The predetermined depth might or might not be sufficient to reach target height  121  for fill material  120  in recess  110 . In the illustrated example, the predetermined depth of stage  104   e  is insufficient to recess fill material  120  to target height  121  of fill material  120  in recess  110 . 
     The depth to which solubility-changing agent  126  is diffused into fill material  120  may be affected by and/or controlled using a variety of factors, including the content of overcoat film  122  (including the type of the photo-activated agent generator in overcoat film  122 , other ingredients of overcoat film  122 , and concentration of the photo-activated agent generator in overcoat film  122 ), characteristics of actinic radiation  124  (e.g., used at stage  104   d , or later stages), content of fill material  120 , width  118  in relation to a difference between height  116  and  114  (which may be referred to as the aspect ratio of recess  110  and may affect the ability of actinic radiation  124  to activate the photo-activated agent generator to generate solubility-changing agent  126 , particularly as fill material  120  has been recessed into recess  110  at later stages), exposure dose of actinic radiation  124 , heating (e.g., bake) time and temperature, and any of a variety of other factors. 
     As shown in FIGURE F at stage  104   f , overcoat film  122  and de-protected portion  120   a  of fill material  120  have been removed. In certain embodiments, overcoat film  122  and de-protected portion  120   a  of fill material  120  are developed using a solvent  128 , causing overcoat film  122  and de-protected portion  120   a  of fill material  120  to be removed from substrate  100 . 
     This disclosure contemplates solvent  128  including any suitable substance for removing overcoat film  122  and de-protected portion  120   a  of fill material  120 . As just one example, solvent  128  may include an aqueous solution of tetramethyl ammonium hydroxide that is capable of solubilizing an acid-deprotected resin (e.g., de-protected portion  120   a  of fill material  120 ). Solvent  128  also may be referred to as a developer. 
     Removal of overcoat film  122  and de-protected portion  120   a  of fill material  120  causes a change in height of fill material  120  in recess  110  commensurate with exposure dose (e.g., the depth of diffusion of solubility-changing agent  126  into fill material  120 , or the depth of de-protected portion  120   a  of fill material  120 ). 
     This process of depositing overcoat film  122  (stage  104   c ), exposure to actinic radiation  124  (stage  104   d ), diffusion via baking for a time period (stage  104   e ), and development of a de-protected portion of fill material  120  (stage  1040  may be repeated until a cumulative depth of fill material  120  de-protection and development (removal) reaches target height  121  such that remaining fill material  120  in recess  110  is approximately at target height  121 . For example,  FIGS.  1 G- 1 J  illustrate a second iteration of this cyclic process, which in this example is sufficient to achieve target height  121  of fill material  120  in recess  110 . 
     In particular,  FIG.  1 G  illustrates stage  104   g  in which overcoat film  122  has again been deposited on substrate  100 . Overcoat film  122  again contains a photo-activated agent generator (e.g., a PAG) that generates, in response to actinic radiation, a solubility-changing agent (e.g., acid) for changing the solubility of the material of overcoat film  122  and/or fill material  120  to be soluble in one or more solvents to be used in a subsequent removal process. 
       FIG.  1 H  illustrates stage  104   h  in which overcoat film  122  is exposed to actinic radiation  124 , causing the photo-activated agent generator in overcoat film  122  to generate solubility-changing agent  126  within overcoat film  122  such that overcoat film  122  now includes solubility-changing agent  126  and causing overcoat film  122  to become solubilized (soluble in one or more solvents to be used in a subsequent removal process). 
       FIG.  1 I  illustrates stage  104   i  in which solubility-changing agent  126  has diffused into fill material  120 , causing a further portion (de-protected portion  120   b ) of fill material  120  to become soluble to a solvent (e.g., solvent  128 ). De-protected portion  120   b  is generally shown as the portion of fill material  120  into which solubility-changing agent  126  has diffused. As described above, solubility-changing agent  126  may be diffused into fill material  120  using a thermal process (e.g., baking of substrate  100 ). In certain embodiments, solubility-changing agent  126  is diffused a predetermined depth into fill material  120  to modify the solubility of fill material  120  to the predetermined depth. In this example, the predetermined depth is sufficient to de-protect fill material  120  to target height  121 . 
       FIG.  1 J  illustrates stage  104   j  in which overcoat film  122  and de-protected portion  120   b  of fill material  120  have been removed. In certain embodiments, overcoat film  122  and de-protected portion  120   a  of fill material  120  are developed using solvent  128 , causing overcoat film  122  and de-protected portion  120   a  of fill material  120  to be removed from substrate  100 . In this example, removal of overcoat film  122  and de-protected portion  120   b  of fill material  120  causes a change in height of fill material  120  in recess  110  such that the remaining fill material  120  in recess  110  is substantially at target height  121 . 
     Although in the illustrated example, two iterations of the cyclic process are sufficient to achieve target height  121  of fill material  120  in recess  110 , this disclosure contemplates any suitable number of iterations being sufficient to reach target height  121  for a given application. For example, more than two iterations may be appropriate to remove sufficient fill material  120  to reach target height  121  of fill material in recess  110 . In another example, a single iteration (e.g., of stages  104   b - 104   f ) may be adequate to remove sufficient fill material  120  to reach target height  121  of fill material in recess  110 . Furthermore, the predetermined depth of diffusion of solubility-changing agent  126  into fill material  120  and subsequent removal of a de-protected portion of fill material  120  may be the same from one iteration to the next (and potentially across all iterations) or may vary from one iteration to the next (and potentially across all iterations), according to particular needs. 
     Subsequent processing may then be performed on semiconductor substrate  100 . For example, process  102  may be integrated into a process for forming a semiconductor device using a variety of deposition and etch processes. 
       FIGS.  2 A- 2 I  illustrate cross-sectional and plan views of substrate  100  during an example process  202  for processing substrate  100 , according to certain embodiments. In particular, process  202  includes one or more iterations of using a PAG-based process (e.g., process  102 ) for locational de-protection of portions of fill material  120  and one or more subsequent iterations of a TAG-based process for de-protection of portions of fill material  120 . 
       FIGS.  2 A- 2 F  generally correspond to  FIGS.  1 A- 1 F , and details described above with respect to  FIGS.  1 A- 1 F  that are not repeated are incorporated by reference. In general,  FIGS.  2 A- 2 F  illustrate an iteration of receiving substrate  100  (stage  204   a ); depositing fill material  120  over substrate  100  (fill material  120  filling recess  110  and covering structures  108 , fill material  120  being initially resistant to development by solvent  128 ) (stage  204   b ); depositing overcoat film  122  (containing a photo-activated agent generator (e.g., PAG) that generates, in response to actinic radiation  124 , solubility-changing agent  126  (e.g., acid)) on substrate  100  (stage  204   c ); exposing overcoat film  122  to actinic radiation  124  to generate solubility-changing agent  126  within overcoat film  122  (stage  204   d ); diffusing (e.g., by exposing substrate  100  to heat) solubility-changing agent  126  a predetermined depth into fill material  120 , causing a portion (e.g., de-protected portion  120   a ) of fill material  120  to become soluble to solvent  128 ) (stage  204   e ); and developing overcoat film  122  and de-protected portion  120   a  of fill material  120  using solvent  128 , causing overcoat film  122  and de-protected portion  120   a  of fill material  120  to be removed from substrate  100  (stage  204   f ). That is,  FIGS.  2 A- 2 F  illustrate an iteration of a PAG-based process for removing a portion of fill material  120  in recess  110 . 
       FIGS.  2 G- 2 I  illustrate a TAG-based process, which may be performed one or more times, for removing additional portions of fill material  120  in recess  110  until target height  121  is reached. As shown in  FIG.  2 G  at stage  204   g , an overcoat film  222  has been deposited on substrate  100 . Overcoat film  222  may be deposited in any suitable manner, including spin-on deposition (or spin-coating), spray-coating, roll-coating, CVD, or any other suitable deposition technique. Overcoat film  222  contains a thermally-activated agent generator that generates, in response to heat, a solubility-changing agent for changing the solubility of another material (e.g., the material of overcoat film  222  and/or fill material  120 ) to be soluble in one or more solvents to be used in a subsequent removal process. In certain embodiments, the thermally-activated agent generator is a TAG and the solubility-changing agent is acid. 
     Aside from the thermally-activated agent generator, overcoat film  222  might or might not include the same material as or similar material to fill material  120 . In certain embodiments, in addition to the thermally-activated agent generator, overcoat film  222  may include a polymer resin that has a solubility in a solvent (to be used subsequently to remove a de-protected portion of fill material  120 ) that is similar to the solubility in the solvent of the de-protected fill material  120 , so that the de-protected portion of fill material  120  and overcoat film  222  can be removed in one step. In certain embodiments, the thermally-activated agent generator of overcoat film  222  is pre-formulated in the resin of overcoat film  222 . 
     As shown in  FIG.  2 H  at stage  204   h , overcoat film  222  is exposed to heat  127  for a suitable time period. In particular, overcoat film  222  is exposed to sufficient heat  12  to cause the thermally-activated agent generator (e.g., the TAG) in overcoat film  222  to generate a solubility-changing agent  226  (e.g., acid) within overcoat film  222  such that overcoat film  222  now includes solubility-changing agent  226 . Solubility-changing agent  226  causes overcoat film  222  to become solubilized, such that overcoat film  222  is now soluble in one or more solvents to be used in a subsequent removal process. In certain embodiments, a thermal process (e.g., heat  127 ) is used to activate the thermally-activated agent generator within overcoat film  222 . For example, to apply heat  12 , substrate  100  may be baked, and the heat associated with baking substrate  100  causes the thermally-activated agent generator to generate solubility-changing agent  226  within overcoat film  222 . Substrate  100  may be baked via a substrate plate in a suitable tool, via ambient heat in a substrate processing chamber of a suitable tool, a combination of these or in any other suitable manner. 
     Continuing with stage  204   h  in  FIG.  2 H , in addition to causing the thermally-activated agent generator in overcoat film  222  to generate solubility-changing agent  226  within overcoat film  222 , the thermal process applied to (e.g., heating of) substrate  100  also causes solubility-changing agent  226  to diffuse a predetermined depth into fill material  120 . Diffusion of solubility-changing agent  226  into fill material  120  modifies at least a portion (de-protected portion  220   a ) of fill material  120  to be soluble in a solvent to be used in a later removal process. De-protected portion  220   a  is generally shown as the portion of fill material  120  into which solubility-changing agent  226  has diffused. Diffusion of solubility-changing agent  226  into fill material  120  results in solubility-changing reactions within fill material  120  to the depth at which solubility-changing agent  226  (e.g., to the predetermined depth) diffuses into fill material  120 , resulting in de-protected portion  220   a . De-protected portion  220   a  of fill material  120  then becomes soluble to one or more particular solvents, which also may be referred to as developers. The de-protection reaction resulting from the diffusion of solubility changing agent  226  into a portion of fill material  120  (creating de-protected portion  220   a ) could be a de-crosslinking reaction within the portion of fill material  120 . A similar reaction may occur within overcoat film  222  to cause overcoat film  222  to become solubilized. 
     In certain embodiments, solubility-changing agent  226  is diffused a predetermined depth into fill material  120  to modify the solubility of fill material  120  to the predetermined depth. The predetermined depth might or might not be sufficient to reach target height  121  for fill material  120  in recess  110 . In the illustrated example, the predetermined depth of stage  204   h  is sufficient to recess fill material  120  to target height  121  of fill material  120  in recess  110 . In embodiments in which the predetermined depth of stage  204   h  is insufficient to recess film material  120  to target height  121  of fill material  120  in recess  110 , one or more additional iterations of stages  204   g - 204   i  may be performed. 
     The depth to which solubility-changing agent  226  is diffused into fill material  120  may be affected by and/or controlled using a variety of factors, including the content of overcoat film  222  (including the type of the thermally-activated agent generator in overcoat film  222 , other ingredients of overcoat film  222 , and concentration of the thermally-activated agent generator in overcoat film  222 ), the temperature of heat  127 , the length of time for which substrate  100  is exposed to heat  127  (e.g., the time period of the bake), content of fill material  120 , and any of a variety of other factors. 
     In certain embodiments, as described below with reference to  FIG.  5 A- 5 C , substrate  100  is part of a larger substrate, and heat  127  is applied across multiple (and potentially all) portions of the larger substrate. Exposure to heat  12  may cause a substantially uniform amount of solubility-changing agent  226  to be generated within overcoat film  222 . Furthermore, exposure to heat  127  may cause a substantially uniform depth of diffusion of solubility-changing agent  226  into fill material  120 . 
     As shown in  FIG.  2 I  at stage  204   i , overcoat film  222  and de-protected portion  220   a  of fill material  120  have been removed. In certain embodiments, overcoat film  222  and de-protected portion  220   a  of fill material  120  are developed using a solvent  228 , causing overcoat film  222  and de-protected portion  220   a  of fill material  120  to be removed from substrate  100 . 
     This disclosure contemplates solvent  228  including any suitable substance for removing overcoat film  222  and de-protected portion  220   a  of fill material  120 . As just one example, solvent  228  may include an aqueous solution of tetramethyl ammonium hydroxide that is capable of solubilizing an acid-deprotected resin (e.g., de-protected portion  220   a  of fill material  120 ). In certain embodiments, if a resin (e.g., fill material  120 ) is designed to interact with a solubility changing agent other than an acid generator, then it may be possible to use an organic solvent as solvent  228 . Solvent  228  might or might not be the same as solvent  128 . Solvent  228  also may be referred to as a developer. 
     Removal of overcoat film  222  and de-protected portion  220   a  of fill material  120  causes a change in height of fill material  120  in recess  110  commensurate with exposure dose (e.g., the depth of diffusion of solubility-changing agent  226  into fill material  120 , or the depth of de-protected portion  220   a  of fill material  120 ). In this example, removal of overcoat film  122  and de-protected portion  120   b  causes a change in height of fill material  120  in recess  110  such that the remaining fill material  120  in recess  110  is substantially at target height  121 . 
     This process of depositing overcoat film  222  (stage  204   g ), heating substrate  100  (stage  204   h ), and subsequent development of de-protected portion  120   a  of fill material  120  (stage  204   i ) is repeated until a cumulative depth of fill material  120  de-protection and development reaches target height  121 . For example,  FIGS.  2 G- 2 I  illustrate a first iteration of this cyclic TAG-based process, which in this example is sufficient to achieve target height  121  of fill material  120  in recess  110 . In other examples, additional iterations TAG-based may be used to remove sufficient fill material  120  to reach target height  121  of fill material in recess  110 . 
     Although in the illustrated example of  FIGS.  2 A- 2 I  a single iteration of using the photo-activated solubility-changing agent-generating ingredient (the PAG-based process) is illustrated and described, this disclosure contemplates process  202  including multiple iterations of using the photo-activated solubility-changing agent-generating ingredient prior to one or more iterations using the thermally-activated agent generator (the TAG-based process) to achieve target height  121  of fill material  120  in recess  110 , according to particular needs. Furthermore, whether considering the PAG-based process or the TAG-based process, the predetermined depth of diffusion of solubility-changing agent  126 / 226  into fill material  120  and subsequent removal of a de-protected portion of fill material  120  may be the same from one iteration to the next (and potentially across all iterations) or may vary from one iteration to the next (and potentially across all iterations), according to particular needs. 
     Subsequent processing may then be performed on semiconductor substrate  100 . For example, process  202  may be integrated into a process for forming a semiconductor device using a variety of deposition and etch processes. 
       FIG.  3    illustrates example effects of varying depths of diffusion of a solubility-changing agent  126 / 226  into fill material  120 , according to certain embodiments. In general,  FIG.  3    illustrates that, according to certain embodiments, as the depth of diffusions of solubility-changing agent  126 / 226  (e.g., acid) into fill material  120  increases, an increasing amount of fill material  120  is removed during a subsequent development process, which decreases the height of post-development fill material  120  in recess  110 . Portions of fill material  120  into which solubility-changing agent  126 / 226  (e.g., acid) diffused become soluble to solvent  128 / 228 , which allows solvent  128 / 228  to remove those portions of fill material  120  when fill material  120  is developed using solvent  128 / 228 . The example embodiments control film height by location through acid diffusion into an acid solubility-changeable resin layer in which a greater degree of acid diffusion results in a larger change in film thickness per overcoat cycle. Thus, by controlling the depth of diffusion of solubility-changing agent  126 / 226  into fill material  120 , the amount of fill material  120  removed (e.g., de-protected portion  120   a / 120   b / 220   a ) in a subsequent removal process can be controlled. Factors potentially affecting the depth of diffusions are described above. 
       FIGS.  4 A- 4 H  illustrate cross-sectional views of example substrate portions  400   a - 400   d  having pre-patterned features during example process  102  (described above with reference to  FIGS.  1 A- 1 J ) for processing substrate portions  400   a - 400   d , according to certain embodiments. For ease of reference, substrate portions  400   a - 400   d  may be referred to collectively as substrate  400 . Substrate portions  400   a - 400   d  may be part of a same substrate  400 , or may be part of different substrates  400 . Substrate  400  may be part of a larger semiconductor device, such as part of a larger semiconductor wafer. Furthermore, substrate portions  400   a - 400   d  may be part of a same semiconductor wafer or one or more different semiconductor wafers. In certain embodiments, process  102  includes repeatedly performing a PAG-based process to remove a fill material from a recess  110  of substrate  400  until the fill material is a predetermined height within the recess  110 . To the extent not repeated, details related to substrate  100  and process  102  described with reference to  FIGS.  1 A- 1 J  (or elsewhere) are incorporated by reference. 
     As shown in  FIG.  4 A , in addition to base portion  106 , substrate  400  includes multiple structures  108  that define multiple recesses  110 . Although structures  108  are shown as generally having the same shapes, heights, and pitches, structures  108  may have any suitable shapes, heights, and/or pitches, including varying shapes, heights, and/or pitches. Additionally, although recesses  110  are shown as generally having the same shapes and depths, recesses  110  may have any suitable shapes and/or depths, including varying shapes and/or depths. This disclosure contemplates the structures  108  being patterned into any suitable features. 
     As shown in  FIG.  4 A  (corresponding to stage  104   c ), fill material  120  has been deposited on substrate  400 , with fill material  120  filling recesses  110  and covering structures  108 , and overcoat film  122  has been deposited on substrate  400 . In subsequent photolithography steps, it may be desirable to recess, via photolithographic development techniques, fill material  120  into recesses  110 , such that fill material  120  has a particular height within recesses  110 . A target height  121  for recessing fill material  120  within recesses  110  is indicated for each recess  110 . In this example, a different target height  121  is desired for each recess  110 , with little or no recessing of fill material  120  being desired for the right-most recess  110  in  FIG.  4 A . This disclosure contemplates a same target height  121  being desired for two or more (and potentially all) recesses  110 , however. 
     As described above, overcoat film  122  contains a photo-activated agent generator (e.g., a PAG) that generates, in response to actinic radiation  124 , a solubility-changing agent  126  (e.g., acid) for changing the solubility of overcoat film  122  and/or fill material  120  to be soluble in one or more solvents (e.g., solvent  128 ) to be used in a subsequent removal process. 
     As shown in  FIG.  4 B  (corresponding to stage  104   d ), overcoat film  122  is exposed to sufficient actinic radiation  124  for a sufficient time period to cause, where desired, the photo-activated agent generator (e.g., the PAG) in overcoat film  122  to generate solubility-changing agent  126  (e.g., acid) within overcoat film  122  such that overcoat film  122  now includes solubility-changing agent  126 . In the example of  FIG.  4 B , actinic radiation  124  is a pattern of actinic radiation that is directed to overcoat film  122 . 
     As described above, characteristics of actinic radiation  124  affect the amount of the photo-activated agent generator in overcoat film  122  that is activated. That is actinic radiation  124  having certain characteristics causes greater amounts of the photo-activated agent generator in overcoat film  122  to be activated, resulting in greater amounts of solubility-changing agent  126  being generated in those areas of overcoat film  122 . Actinic radiation  124  having certain other characteristics causes lesser amounts of the photo-activated agent generator in overcoat film  122  to be activated, resulting in less solubility-changing agent  126  being generated in those areas of overcoat film  122 . The amount of solubility-changing agent  126  in a particular area of overcoat film  122  affects how much solubility-changing agent  126  will be available for diffusion into fill material  120  in a subsequent heating step. 
     Thus, a pattern of actinic radiation  124  can be tailored to activate greater amounts of the photo-activated agent generator in overcoat film  122  overlying areas of fill material  120  where greater depth of diffusion of solubility-changing agent  126  and ultimately removal of fill material is desired and to activate lesser amounts of the photo-activated agent generator in overcoat film  122  overlying areas of fill material  120  where lesser depth of diffusion of solubility-changing agent  126  and ultimately removal of fill material is desired. Although use of a pattern of actinic radiation  124  to vary the depth of diffusion of solubility-changing agent  126  and ultimately remove of fill material  120  is described, the pattern of actinic radiation  124  could be designed to cause the photo-activated agent generator in overcoat film  122  to generate solubility-changing agent  126  in substantially equal amounts in one or more portions of overcoat film  122 , such as when the target height  121  for fill material  120  in recesses  110  underlying those one or more portions of overcoat film  122  is substantially the equal. 
     In the example illustrated in  FIG.  4 B , the pattern of actinic radiation  124  is designed to cause the photo-activated agent generator in overcoat film  122  to generate a decreasing amount of solubility-changing agent  126  from overcoat film  122  over substrate portion  400   a  (the left side of  FIG.  4 A ) to overcoat film  122  over substrate portion  400   d  (the right side of  FIG.  4 A ), with little to no solubility-changing agent  126  being generated over recess  110  of substrate portion  400   d  (as no actinic radiation  124  is applied over substrate portion  400   d ). In certain embodiments, the ability to control activation of the photo-activated agent generator in overcoat film  122  and subsequent diffusion (e.g., by adjusting the exposure dose of actinic radiation  124 ) may be affected by a resolution limit of an exposure tool. 
     As shown in  FIG.  4 C  (corresponding to stage  104   e ), to modify at least a portion of fill material  120  to be soluble in solvent  128 , solubility-changing agent  126  has diffused into fill material  120 , causing a portion (de-protected portion  420   a ) of fill material  120  to become soluble to solvent  128 . De-protected portion  420   a  is generally shown as the portions of fill material  120  into which solubility-changing agent  126  has diffused. In certain embodiments, solubility-changing agent  126  is caused to diffuse into at least a portion of fill material  120  (creating de-protected portion  420   a ) using a thermal process (e.g., application of heat  12  for a suitable time period). In the example shown in  FIG.  4 C , solubility-changing agent  126  is diffused into recesses  110  at varying predetermined depths. Additionally, the predetermined depths in this example are insufficient to recess fill material  120  to target heights  121  of fill material  120  in recesses  110 . 
     As illustrated in  FIG.  4 D  (corresponding to stage  104   f ), overcoat film  122  and de-protected portion  420   a  of fill material  120  have been removed. In certain embodiments, overcoat film  122  and de-protected portion  420   a  of fill material  120  are developed using solvent  128 , causing overcoat film  122  and de-protected portion  420   a  of fill material  120  to be removed from substrate  400 . Removal of overcoat film  122  and de-protected portion  420   a  of fill material  120  causes changes in height of fill material  120  in recesses  110  commensurate with exposure dose (e.g., the depth of diffusion of solubility-changing agent  126  into fill material  120 , or the depth of de-protected portions  420   a  of fill material  120 ). 
     This process of depositing overcoat film  122 , exposure to actinic radiation  124 , diffusion via baking for a length of time, and subsequent development of de-protected portion  120   a  of fill material  120  is repeated until a cumulative depth of fill material  120  de-protection and development in each recess  110  reaches corresponding target heights  121 . For example,  FIGS.  4 E- 4 H  illustrate a second iteration of this cyclic process, which in this example is sufficient to achieve target heights  121  of fill material  120  in recesses  110 . Additional or fewer iterations may be appropriate to remove sufficient fill material  120  to reach target heights  121  of fill material in recesses  110  in particular implementations. Furthermore, the predetermined depth of diffusion of solubility-changing agent  126  into fill material  120  and subsequent removal of a de-protected portion of fill material  120  may be the same from one iteration to the next (and potentially across all iterations) or may vary from one iteration to the next (and potentially across all iterations), according to particular needs. 
     In particular, as illustrated in  FIG.  4 E  (corresponding to stage  104   g ), overcoat film  122  has again been deposited on substrate  400 . Overcoat film  122  again contains a photo-activated agent generator (e.g., a PAG) that generates, in response to actinic radiation  124 , a solubility-changing agent  126  (e.g., acid) for changing the solubility of the material of overcoat film  122  and/or fill material  120  to be soluble in solvents  128 . 
     As illustrated in  FIG.  4 F  (corresponding to stage  104   h ), overcoat film  122  is exposed to a pattern of actinic radiation  124 , causing the photo-activated agent generator in overcoat film  122  to generate solubility-changing agent  126  within overcoat film  122  such that overcoat film  122  now includes solubility-changing agent  126  and causing overcoat film  122  to become solubilized (soluble in solvent  128 ). It should be understood that the pattern of actinic radiation  124  used in  FIG.  4 F  might or might not be the same as the pattern of actinic radiation  124  used in  FIG.  4 B , depending on the desired predetermined depth of diffusion of solubility-changing agent  126  into fill material  120  in a subsequent processing step. 
     As illustrated in  FIG.  4 G  (corresponding to stage  104   i ), solubility-changing agent  126  has diffused into fill material  120 , causing a further portion (de-protected portion  420   b ) of fill material  120  to become soluble to solvent  128 . De-protected portion  420   b  is generally shown as the portion of fill material  120  into which solubility-changing agent  126  has diffused. As described above, solubility-changing agent  126  may be diffused into fill material  120  using a thermal process (e.g., baking of substrate  400 ). In certain embodiments, solubility-changing agent  126  is diffused a predetermined depth into fill material  120  to modify the solubility of fill material  120  to the predetermined depth, and in this example, the predetermined depth is sufficient to de-protect fill material  120  to target heights  121  in recesses  110 . 
     As illustrated in  FIG.  4 H  (corresponding to stage  104   j ), overcoat film  122  and de-protected portion  420   b  of fill material  120  have been removed. In certain embodiments, overcoat film  122  and de-protected portion  420   a  of fill material  120  are developed using solvent  128 , causing overcoat film  122  and de-protected portion  420   a  of fill material  120  to be removed from substrate  400 . In this example, removal of overcoat film  122  and de-protected portion  120   b  of fill material  120  causes changes in height of fill material  120  in recesses  110  such that the remaining fill material  120  in recesses  110  is substantially at target heights  121 . 
     Subsequent processing may then be performed on semiconductor substrate  400 . For example, process  102  may be integrated into a process for forming a semiconductor device using a variety of deposition and etch processes. 
     Process  102  may provide one or more technical advantages. For example, removing fill material  120  by creating de-protected portions of fill material  120  using a solubility-changing agent  126  that is generated from a photo-activated agent in an overcoat film  122  may provide a precise way to change the height of fill material  120 . As another example, the ability to direct a pattern of actinic radiation  124  toward overcoat film  122  may allow fill material  120  to be removed at differing precise depths within one or more of recesses  110 , and ultimately allow different target heights  121  to be reached. 
       FIGS.  5 A- 5 C  illustrate cross-sectional views of example substrate portions  400   a - 400   d  having pre-patterned features during portions of example process  202  (described above with reference to  FIGS.  2 A- 1 I ) for processing substrate portions  400   a - 400   d , according to certain embodiments. In certain embodiments, process  202  includes one or more iterations of performing a PAG-based process to establish potentially varying heights (by removing fill material  120  to varying depths in recesses  110 ) of fill material  120  on various substrate portions  400   a - 400   d  and one or more subsequent iterations of performing a TAG-based process to potentially uniformly remove portions of fill material  120  on various substrate portions  400   a - 400   d . To the extent not repeated, details related to substrate  100 , process  202 , and substrate portions  400   a - 400   d /substrate  400  described with reference to  FIGS.  2 A- 2 I  and/or  FIGS.  4 A- 4 H  (or elsewhere) are incorporated by reference. 
     Rather than beginning at stage  202   a  of process  202 ,  FIG.  5 A  begins at a step analogous to stage  204   g  of  FIG.  2 G . That is,  FIG.  5 A  illustrates substrate  400  following at least one iteration of a PAG-based process to remove a portion of fill material  120  to varying predetermined depths within recesses  110 , setting different relative heights of remaining fill material  120  in recesses  110 . For example, just prior to the state of substrate  400  illustrated in  FIG.  5 A , substrate  400  may be in a state corresponding to  FIG.  4 D .  FIGS.  5 A- 5 C  illustrate a TAG-based process, which may be performed one or more times, for removing additional portions of fill material  120  in recess  110  until target height  121  is reached. 
     As shown in  FIG.  5 A  (corresponding to stage  204   g ), an overcoat film  222  has been deposited on substrate  400 . Overcoat film  222  contains a thermally-activated agent generator (e.g., a TAG) that generates, in response to heat, solubility-changing agent  226  (e.g., acid) for changing the solubility of another material (e.g., the material of overcoat film  222  and/or fill material  120 ) to be soluble in solvent  228  to be used in a subsequent removal process. 
     As shown in  FIG.  5 B  (corresponding to stage  204   h ), overcoat film  222  is exposed to sufficient heat  127  for a suitable time period to cause the thermally-activated agent generator (e.g., the TAG) in overcoat film  222  to generate solubility-changing agent  226  (e.g., acid) within overcoat film  222  such that overcoat film  222  now includes solubility-changing agent  226 . The thermal process applied to (e.g., heating of) substrate  400  also causes solubility-changing agent  226  to diffuse a predetermined depth into fill material  120 . Diffusion of solubility-changing agent  226  into fill material  120  modifies at least a portion (de-protected portion  520   a ) of fill material  120  to be soluble in solvent  228 . De-protected portion  520   a  is generally shown as the portion of fill material  120  into which solubility-changing agent  226  has diffused. 
     In certain embodiments, solubility-changing agent  226  is diffused a predetermined depth into fill material  120  to modify the solubility of fill material  120  to the predetermined depth. The predetermined depth might or might not be sufficient to reach target heights  121  for fill material  120  in recesses  110 . In the illustrated example, the predetermined depth is sufficient to recess fill material  120  to target heights  121  of fill material  120  in recesses  110 . In embodiments in which the predetermined depth is insufficient to recess film material  120  to target heights  121  in recesses  110 , one or more additional iterations of the process illustrated in  FIGS.  5 A- 5 C  may be performed. 
     In certain embodiments, heat  127  is applied across substrate  400 , and exposure to heat  12  may cause a substantially uniform amount of solubility-changing agent  226  to be generated within overcoat film  222 . Furthermore, exposure to heat  127  may cause a substantially uniform depth of diffusion of solubility-changing agent  226  into fill material  120 . 
     As shown in  FIG.  5 C  (corresponding to stage  204   i ), overcoat film  222  and de-protected portion  420   a  of fill material  120  have been removed. In certain embodiments, overcoat film  222  and de-protected portion  420   a  of fill material  120  are developed using solvent  228 , causing overcoat film  222  and de-protected portion  420   a  of fill material  120  to be removed from substrate  400 . In this example, removal of overcoat film  122  and de-protected portion  120   b  of fill material  120  causes a change in height of fill material  120  in recess  110  such that the remaining fill material  120  in recesses  110  is substantially at target heights  121 . 
     This process of depositing overcoat film  222 , heating of substrate  400 , and subsequent development of de-protected portion  520   a  of fill material  120  is repeated until a cumulative depth of fill material  120  de-protection and development in recesses  110  reaches corresponding target heights  121 . For example,  FIGS.  5 A- 5 C  illustrate a first iteration of this cyclic process, which in this example is sufficient to achieve target heights  121  of fill material  120  in recesses  110 . Additional or fewer iterations may be appropriate to remove sufficient fill material  120  to reach target heights  121  of fill material in recesses  110  in particular implementations. Furthermore, the predetermined depth of diffusion of solubility-changing agent  226  into fill material  120  and subsequent removal of a de-protected portion of fill material  120  may be the same from one iteration to the next (potentially across all iterations) or may vary from one iteration to the next (potentially across all iterations), according to particular needs. 
     Subsequent processing may then be performed on semiconductor substrate  400 . For example, process  202  may be integrated into a process for forming a semiconductor device using a variety of deposition and etch processes. 
     Process  202  may provide one or more technical advantages, which may be in addition to advantages described above with reference to process  102 . In certain embodiments, recess  110  has a high aspect ratio (e.g., the difference between height  116  and height  114  is significantly larger than width  118 ), which can impede a path for the wavelength of light (the actinic radiation  124 ) suitable for activating the photo-activated agent generator in overcoat film  122  to reach the photo-activated acid generator in overcoat film  122  for activation. In general, a PAG-based process may begin encountering difficulties in activating the PAG in overcoat film  122  when the lateral dimension of the feature (e.g., recess  110 ) is much less than the wavelength of the impinging radiation. The greater the aspect ratio of the feature (e.g., recess  110 ) and thereby the depth in which the photons of actinic radiation  124  are to interact with overcoat film  122 , the lower the efficiency of photon interaction within the feature at a given dimension less than the incoming wavelength of actinic radiation  124 . As just one example, width  118  of the gap between structures  108  could be about 20 nm and a depth of recess  110  could be about five times that or more. A thermally-activated agent generator in overcoat film  222 , which is activated by heat  127  rather than actinic radiation  124 , does not rely on a particular wavelength of light and generates a substantially uniform amount of solubility-changing agent  226  in response to sufficient heat. 
     In process  202 , one or more iterations of a PAG-based process may be performed to establish relative differences in target heights  121  in recesses  110  based on locations of recesses  110 , while one or more subsequent iterations of the TAG-based process may be performed to substantially uniformly continue recessing fill material  120  in recesses  110  until target heights  121  are reached, while maintaining the relative differences in target heights  121  established using the one or more iterations of the PAG-based process. Furthermore, the TAG-based process is particularly efficient, as the thermal process used to activate the thermally-activated agent generator to generate solubility-changing agent  226  also causes solubility-changing agent  226  to diffuse the predetermined depth into fill material  120  without using a separate step (and potentially a separate tool) for exposure to actinic radiation  124 . 
       FIG.  6    illustrates an example method for processing a semiconductor substrate, according to certain embodiments. In general, the method described with reference to  FIG.  6    corresponds to process  102  described above with reference to  FIGS.  1 A- 1 J and  4 A -H. 
     At step  600 , a substrate  100 / 400  having microfabricated structures  108  defining recesses  110  is received. At step  602 , fill material  120  is deposited on substrate  100 / 400 , filling recesses  110  and covering microfabricated structures  108 . Fill material  120  may be a resin, and is initially resistant to development by a solvent  128 . At step  604 , overcoat film  122  is deposited on substrate  100 / 400 . Overcoat film  122  contains a photo-activated agent generator (e.g., a PAG) that generates, in response to actinic radiation, a solubility-changing agent  126  (e.g., acid). 
     At step  606 , overcoat film  122  is exposed to sufficient actinic radiation  124  to cause the photo-activated agent generator in overcoat film  122  to generate solubility-changing agent  126  within overcoat film  122 . Actinic radiation  124  may be a pattern of actinic radiation  124  directed at substrate  100 / 400  and designed to achieve variation in predetermined depths of removal (and ultimately remaining heights of) fill material  120  in recesses  110 . At step  608 , solubility-changing agent  126  is diffused a predetermined depth into fill material  120 , causing de-protected portion  120   a / 420   a  of fill material  120  to become soluble to solvent  128 . This may include multiple different predetermined depths across substrate  100 / 400 . In certain embodiments, substrate  100 / 400  is baked (or otherwise heated) to cause solubility-changing agent  126  to diffuse the predetermined depth into fill material  120 . At step  610 , overcoat film  122  and de-protected portion  120   a / 420   a  of fill material  120  is developed using solvent  128 . 
     At step  612 , a determination is made regarding whether a predetermined condition is met. In general, the determination made at step  612  relates to whether target heights  121  of fill material  120  in recesses  110  have been achieved. For example, the predetermined condition may include determining whether a predetermined number of cycles of steps  604 - 610  have been performed, the predetermined number of cycles having been predetermined to be sufficient to achieve target heights  121  of fill material  120  in recesses  110 . As another example, the predetermined condition may include a real-time analysis of substrate  100 / 400  to determine whether target heights  121  of fill material  120  in recesses  110  have been achieved. 
     If a determination is made at step  612  that the predetermined condition is not met, then the method returns to step  604  to perform another cycle of steps  604 - 610 . If a determination is made at step  612  that the predetermined condition has been met, then the method proceeds to step  614 , with target heights  121  of fill material  120  in recesses  110  having been achieved. At step  614 , subsequent semiconductor fabrication processes may be performed. 
       FIG.  7    illustrates an example method for processing a semiconductor substrate, according to certain embodiments. In general, the method described with reference to  FIG.  7    corresponds to process  202  described above with reference to  FIGS.  2 A- 2 I and  5 A- 5 C . 
     Steps  700 - 710  generally correspond to steps  600 - 610  of the method described with respect to  FIG.  6   ; thus, the details of steps  600 - 610  are incorporated by reference and not repeated. At step  712 , a determination is made regarding whether a predetermined condition is met. For example, the predetermined condition may include determining whether a predetermined number of cycles of steps  704 - 710  have been performed. In certain embodiments, the predetermined condition is whether a single cycle of steps  704 - 710  (e.g., the PAG-based process) has been performed; however, this disclosure contemplates multiple cycles of steps  704 - 710  (e.g., the PAG-based process) being performed prior to advancing to step  714 . 
     If a determination is made at step  712  that the predetermined condition is not met, then the method returns to step  704  to perform another cycle of steps  704 - 710  (e.g., the PAG-based process). If a determination is made at step  712  that the predetermined condition has been met, then the method proceeds to step  714 . At step  714 , overcoat film  222  is deposited on substrate  100 / 400 . Overcoat film  222  contains a thermally-activated agent generator (e.g., a TAG) that generates, in response to heat, a solubility-changing agent  226  (e.g., acid). At step  716 , substrate  100 / 400  is baked sufficiently to generate solubility-changing agent  226  within overcoat film  222 , and to diffuse solubility-changing agent  226  a predetermined depth into fill material  120 , causing a portion (e.g., de-protected portion  220   a / 520   a ) of fill material  120  to become soluble to solvent  228 . At step  718 , overcoat film  122  and de-protected portion  220   a / 520   a  of fill material  120  is developed using solvent  228 . 
     At step  720 , a determination is made regarding whether a predetermined condition is met. In general, the determination made at step  720  relates to whether target heights  121  of fill material  120  in recesses  110  have been achieved, and may be analogous to the predetermined condition described above at step  612  of  FIG.  6   . If a determination is made at step  720  that the predetermined condition is not met, then the method returns to step  714  to perform another cycle of steps  714 - 718 . If a determination is made at step  720  that the predetermined condition has been met, then the method proceeds to step  722 , with target heights  121  of fill material  120  in recesses  110  having been achieved. At step  722 , subsequent semiconductor fabrication processes may be performed. 
       FIG.  8    illustrates an example method for processing a semiconductor substrate, according to certain embodiments. At step  800 , fill material  120  is deposited on substrate  100 / 400 , filling recesses  110  and covering microfabricated structures  108  of substrate  100 / 400 . At step  802 , using a PAG-based process, a localized removal of fill material  120  is performed to remove fill material  120  to respective first depths in recesses  110 . At step  804 , a determination is made regarding whether a predetermined condition is met. For example, the predetermined condition may include determining whether a predetermined number of cycles of step  802  have been performed. In certain embodiments, the predetermined condition is whether a single cycle of step  802  has been performed; however, this disclosure contemplates multiple cycles of step  802  being performed prior to advancing to step  806 . If a determination is made at step  804  that the predetermined condition is not met, then the method returns to step  802  to perform another cycle of step  802 . If a determination is made at step  804  that the predetermined condition has been met, then the method proceeds to step  806 . 
     At step  806 , using a TAG-based process, a uniform etch of a remaining portion of fill material  120  is performed to remove a substantially uniform depth of fill material  120  in recesses  110 . At step  808 , a determination is made regarding whether a predetermined condition is met. In general, the determination made at step  808  relates to whether target heights  121  of fill material  120  in recesses  110  have been achieved, and may be analogous to the predetermined condition described above at step  612  and  720  of  FIGS.  6  and  7   , respectively. If a determination is made at step  808  that the predetermined condition is not met, then the method returns to step  806  to perform another cycle of step  806 . If a determination is made at step  808  that the predetermined condition has been met, then the method proceeds to step  810 , with target heights  121  of fill material  120  in recesses  110  having been achieved. At step  810 , subsequent semiconductor fabrication processes may be performed. 
       FIGS.  9 A- 9 C  illustrate example PAGs and TAGs that may be used in overcoat films  122 / 222 , according to certain embodiments.  FIG.  9 A  illustrates example ionic PAGs, including triphenylsulfonium triflate and Bis(4-tert-butylphenyl)iodonium triflate, that may be used as the photo-activated agent generator of overcoat film  122 .  FIG.  9 A  also illustrates example non-ionic PAGs, including N-Hydroxynaphthalimide triflate and N-Hydroxy-5-norbornene-2,3-dicarboximide perfluoro-1-butanesulfonate, that may be used as the photo-activated agent generator of overcoat film  122 . In general, whether ionic or non-ionic, PAGs may decompose upon exposure to a specific wavelength (or range of wavelengths) of light, generating a strong acid.  FIG.  9 B  illustrates example polymer-bound PAGs that may be used as the photo-activated agent generator of overcoat film  122 .  FIG.  9 C  illustrates example TAGs that may be used as the thermally-activated agent generator of overcoat film  222 . These TAGs may decompose at elevated temperatures, generating a strong acid. In certain embodiments, TAGs may include sulfonate esters, onium salts or halogen-containing compounds to name just a few examples. 
       FIGS.  10 A- 10 B  illustrate example modification of the solubility of overcoat films  122 / 222  and/or fill material  120 . In particular,  FIGS.  10 A- 10 B  illustrate polymer-solubility changing interactions with strong acid.  FIG.  10 A  illustrates a tert-butoxycarbonyl (t-BOC) de-protection chemistry, which may be used in certain photoresists. The material t-BOC may be one of several monomers that make up the polymer of fill material  120  and/or overcoat films  122 / 222 . In this example, the protected polymer is hydrophobic (t-butyl group), and the de-protected polymer is hydroxide, carboxylic acid.  FIG.  10 B  illustrates a vinyl ether de-crosslinking, which may be used in certain developable bottom anti-reflective coatings (dBARCs). In certain embodiments, interaction with strong acid causes a de-crosslinking reaction to occur, making the reacted portion of the film (e.g., fill material  120  and/or overcoat films  122 / 222 ) more soluble in a given developer (e.g., solvent  128 / 228 ). 
     It should be understood that the example chemistries and systems described above with reference to  FIGS.  9 A- 9 C and  10 A- 10 B  are provided as examples only, and that this disclosure contemplates using any suitable chemistries and systems. 
     Although this disclosure has been described in the context of a particular microfabrication process (recessing a fill material  120  to target heights  121  within one or more recesses  110  in a substrate  100 / 400 ), this disclosure may be used with any suitable microfabrication process. For example, this disclosure contemplates using techniques described herein to control the height of any film or other structure/feature of a semiconductor device, whether or not such film or other structure/feature is wholly or partially in a recess. 
     A specific example application of embodiments herein is the construction of three-dimensional transistor architectures in which n-type field effect transistors (NFETs) and p-type FETs (PFETs) are stacked on top of one another. This can include a vertical stack of lateral gate-all-around (GAA) transistors. Epitaxial silicon-germanium (SiGe) growth doped with electron-rich (n-type) species can occur at both upper and lower layers of uncovered silicon. The upper silicon layer, however, can be designed to have electron-deficient (p-type) SiGe. Therefore, after the n-type SiGe is grown, a corresponding feature is filled to a depth that will cover the lower silicon level while leaving the upper silicon level exposed (uncovered) for subsequent silicon etch and regrowth of p-type SiGe. Use of film height control embodiments herein may provide improved control and/or across-wafer uniformity of film height.  FIG.  11    illustrates examples of stacked transistor architectures that may benefit from precise film height control to selectively grow both n-type and p-type SiGe. 
     The process of SAB is a method to pattern dense features at advanced processing nodes. A step in the SAB process flow may benefit from a partial recess of a specific film, such as a spin-on carbon film as illustrated in  FIGS.  12 A- 12 B . If this film is over or under etched relative to the surrounding spacers by even a small margin, the final pattern in the process flow might not be transferred correctly, resulting in a failure. Techniques herein provide a highly planar surface across potentially the entirety of a wafer, which may improve the control and reproducibility of the SAB process. 
     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. 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. 
     While this disclosure 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 disclosure, 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.