Patent Publication Number: US-7910270-B2

Title: Reticle constructions

Description:
This patent resulted from a divisional of U.S. patent application Ser. No. 12/476,085, which was filed on Jun. 1, 2009 now U.S. Pat. No. 7,754,399, and which is hereby incorporated herein by reference; which resulted from a divisional of U.S. patent application Ser. No. 12/196,144, which was filed on Aug. 21, 2008, which is now U.S. Pat. No. 7,556,897, and which is hereby incorporated herein by reference; which resulted from a continuation of U.S. patent application Ser. No. 11/453,425, which was filed on Jun. 14, 2006, is now U.S. Pat. No. 7,432,025, and which is hereby incorporated herein by reference; which resulted from a continuation of U.S. patent application Ser. No. 10/915,936, which was filed on Aug. 10, 2004, is now U.S. Pat. No. 7,442,472, and which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention pertains to methods of forming reticles, and also pertains to reticle constructions. 
     BACKGROUND OF THE INVENTION 
     Radiation patterning tools are utilized during semiconductor processing to pattern radiation (such as, for example, ultraviolet light). The patterned radiation is projected against a radiation-imageable material (such as, for example, photoresist) and utilized to create a pattern in the radiation-imageable material. The utilization of patterned radiation for forming a desired pattern in a radiation-imageable material is typically referred to as photolithography. The radiation-patterning tools can be referred to as photomasks or reticles. The term “photomask” is traditionally understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the term “reticle” is utilized to generally refer to any radiation-patterning tool, regardless of whether the tool is utilized to pattern an entirety of a substrate or only a portion of the substrate. 
     An exemplary method of utilizing a reticle to pattern radiation is described with reference to  FIG. 1 . A reticle construction  10  is shown provided above a semiconductor substrate  12 . The substrate  12  has a radiation-imageable material  14  thereover. Radiation  16  is passed through reticle construction  10 . The radiation is patterned by construction  10  to form a desired radiation pattern which is directed toward radiation-imageable material  14  and ultimately is utilized to form a desired image within the radiation-imageable material. The desired image can include a pattern for forming semiconductor circuit elements, such as, for example, a pattern which can be transferred to one or more materials underlying the radiation-imageable material  14  to form patterned electrically conductive circuit elements (for instance, source/drain regions, wordlines, bitlines, capacitor electrodes, etc.) and/or patterned electrically insulative circuit elements (for instance, gate dielectric, capacitor dielectric, etc.). 
     The reticle construction  10  comprises a base  18 , projecting features  20 , and windows  22  between the projecting features. The projecting features can comprise phase-shifting material (such as, for example, silicon nitride, silicon oxynitride, molybdenum silicide and/or Mo w Si x N y O z , where w, x, y and z are numbers greater than zero), and/or opaque material (such as, for example, chromium). The projecting features  20  and the windows  22  together create the pattern in the radiation passing through reticle construction. 
     Only a fragment of the reticle construction  10  is shown in  FIG. 1 , and such fragment is part of a so-called main-field portion of the reticle. The main-field portion is a part of the reticle having windows therein for patterning radiation to ultimately form circuit elements associated with a semiconductor assembly. The reticle will typically also have a boundary portion extending around the main-field portion. The boundary portion has the primary function of blocking the light, but can have some patterned regions therein corresponding to non-circuit elements (i.e., patterned regions which do not form circuit elements associated with a semiconductor assembly). The patterned regions can be utilized for, among other things, calibration and mask alignment. 
       FIG. 2  shows a view from the bottom of the reticle construction  10 , and diagrammatically illustrates the full construction to show that the reticle comprises a main-field region  30  containing the projecting features  20  and windows  22 , and comprises a boundary region  32  surrounding the main-field region. The boundary region  32  will typically be covered by an opaque material (such as, for example, chromium) so that the boundary region blocks light from passing therethrough. 
     A continuing goal of semiconductor fabrication is to increase the density of structures formed across a semiconductor substrate (i.e., to increase the level of integration), which spawns a continuing goal to improve fabrication of the reticles utilized for patterning semiconductor substrates. Accordingly, it is desired to develop improved reticle constructions, and improved methods for forming reticle constructions. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention encompasses a method of forming a reticle. A reticle substrate is provided. The reticle substrate has a relatively transparent base and a relatively opaque material over the base. The substrate comprises a main-field region where windows utilized for patterning circuit elements of semiconductor constructions are to be formed, and a boundary region where windows utilized for patterning circuit elements of semiconductor constructions will not be formed. The main-field region has a lateral periphery, and the boundary region surrounds an entirety of the lateral periphery of the main-field region. A thickness of a majority of the relatively opaque material of the main-field region is reduced relative to a thickness of the majority of the relatively opaque material of the boundary region. 
     In one aspect, the invention encompasses a method of forming a reticle. A reticle substrate is provided which has a relatively transparent base, a phase-shifting material over the base, and a relatively opaque material over the phase-shifting material. The substrate comprises a defined main-field area having a lateral periphery, and a defined boundary area surrounding an entirety of the lateral periphery of the main-field area. The relatively opaque material within the main-field and boundary areas is defined to be first and second portions of the relatively opaque material, respectively. A first mask is provided which covers a region of the second portion of the relatively opaque material and leaves a region of the first portion exposed. The exposed relatively opaque material is thinned while the remainder of the relatively opaque material is protected with the mask. The utilization of the first mask during the thinning of the relatively opaque material can be referred to as first level processing, and areas of the boundary region containing non-primary patterns can also be exposed and thinned during the first level processing. The mask is removed, and thereafter a second mask is formed and patterned over the main-field area. The second mask can also be formed and patterned over the boundary area. The pattern from the second mask is transferred into the main-field area to pattern the phase-shifting material. 
     In one aspect, the invention encompasses an intermediate construction for fabrication of a reticle. The construction includes a relatively transparent base, and a relatively opaque material over the base. The construction is divided between a main-field region where windows utilized for patterning circuit elements of semiconductor constructions are to be formed, and a boundary region where windows utilized for patterning circuit elements of semiconductor constructions will not be formed. The main-field region has a lateral periphery, and the boundary region surrounds an entirety of the lateral periphery of the main-field region. A majority of the relatively opaque material of the main-field region has a reduced thickness relative to a majority of the relatively opaque material of the boundary region. 
     In one aspect, the invention encompasses a reticle construction. The construction includes a relatively transparent base, a phase-shifting material over the base, and a relatively opaque material over the phase-shifting material. The construction is divided amongst a main-field region where windows utilized for patterning semiconductor constructions extend through the phase-shifting material, and a boundary region which lacks windows utilized for patterning semiconductor constructions. The main-field region has a lateral periphery, and the boundary region surrounds an entirety of the lateral periphery of the main-field region. The majority of the boundary region has the relatively opaque material, and a minority of the main-field region has the relatively opaque material. The relatively opaque material of the main-field region is associated with a relatively opaque blocker, and is thinner than the relatively-opaque material of the boundary region. The invention can also include aspects in which at least some of the blockers have thickness of the relatively-opaque material which are about the same as the thickness of the relatively-opaque material of the majority of the boundary region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
         FIG. 1  is a diagrammatic, cross-sectional view of a reticle construction and a semiconductor construction, with the reticle construction shown being utilized to pattern radiation directed toward the semiconductor construction in accordance with a prior art process. 
         FIG. 2  is a diagrammatic view of a surface of a prior art reticle construction. 
         FIG. 3  is a diagrammatic, cross-sectional view of a reticle construction shown at a preliminary processing stage of an exemplary aspect of the present invention. 
         FIG. 4  is a diagrammatic view of a surface of the reticle construction shown at the processing stage of  FIG. 3 , with the cross-section of  FIG. 3  being along the line  3 - 3  of  FIG. 4 . 
         FIG. 5  is a diagrammatic, cross-sectional view of the  FIG. 3  construction shown at a processing stage subsequent to that of  FIG. 3 . 
         FIG. 6  is a view of a surface of the reticle construction at the processing stage of  FIG. 5 , with the cross-section of  FIG. 5  extending along the line  5 - 5  of  FIG. 6 . 
         FIG. 7  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 5 . 
         FIG. 8  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 7 . 
         FIG. 9  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 8 . 
         FIG. 10  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 9 . 
         FIG. 11  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 10 . 
         FIG. 12  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 11 . 
         FIG. 13  is a view of a surface of a reticle construction shown at the processing stage of  FIG. 12 , with the cross-section of  FIG. 12  being along the line  12 - 12  of  FIG. 13 . 
         FIG. 14  is a diagrammatic, fragmentary view of a surface of a reticle construction illustrating an exemplary pattern which can be formed within the main-field of a reticle. 
         FIG. 15  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 3  in accordance with a second exemplary aspect of the present invention. 
         FIG. 16  is a view of the  FIG. 3  cross-section shown at processing stage subsequent to that of  FIG. 15 . 
         FIG. 17  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 16 . 
         FIG. 18  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 17 . 
         FIG. 19  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 18 . 
         FIG. 20  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 11  in accordance with a third aspect of the invention. 
         FIG. 21  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 20 . 
         FIG. 22  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 18  in accordance with a fourth aspect of the invention. 
         FIG. 23  is a view of the  FIG. 3  cross-section shown at a processing stage subsequent to that of  FIG. 22 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     One aspect of the present invention is a recognition that it can be advantageous to utilize a different thickness of a relatively opaque material (such as, for example, chromium) over a main-field region of a reticle than over a boundary region of the reticle. Specifically, it is recognized that it is generally easier to form tight-tolerance patterns through thin materials than through thicker materials, and it is recognized that the tight-tolerance patterns will generally be formed within the main-field region of a reticle construction rather than within the boundary region. It is also recognized that it can be advantageous to leave a thick portion of relatively opaque material over the boundary region of a reticle construction in that such may do a better job of blocking stray light than would a thin portion of the relatively opaque material. 
     An exemplary method of forming a reticle construction in accordance with an aspect of the present invention is described with reference to  FIGS. 3-13 . 
     Referring initially to  FIG. 3 , a reticle substrate  50  is illustrated at a preliminary processing stage. The substrate  50  comprises a relatively transparent base  52 , a phase-shifting material  54  over the base, and a relatively opaque material  56  over the phase-shifting material. The term “relatively” is utilized throughout this document to indicate that a material has a particular quantitative property relative to another. For instance, the term “relatively opaque” is utilized to indicate that a material is more opaque than another material, with such other material being referred to as a “relatively transparent” material. 
     The relatively transparent material  52  will typically comprise, consist essentially of, or consist of quartz. The relatively opaque material  56  will typically comprise, consist essentially of, or consist of chromium. The phase-shifting material  54  will typically comprise, consist essentially of, or consist of one or more of silicon nitride, silicon oxynitride, molybdenum silicide and Mo w Si x N y O z , where w, x, y and z are numbers greater than zero. 
     The substrate of  FIG. 3  is shown to comprise a main-field region  58  and a boundary region  60  around the main-field region. A dashed border  59  is provided to demarcate a boundary between the main-field region and the boundary region. 
     Material  56  comprises an upper surface  57 , and  FIG. 4  shows a view of substrate  50  along such upper surface (i.e., shows a top view of the  FIG. 3  substrate). The  FIG. 4  view shows main-field region  58  having a lateral periphery defined by demarcation line  59 , and shows boundary region  60  entirely surrounding the lateral periphery of the main-field region. Although the shown main-field region comprises a rectangular lateral periphery, it is to be understood that the lateral periphery of the main-field region can have any suitable shape. 
     A series of marks  62  are provided within boundary region  60  to illustrate the exemplary locations where alignment marks can ultimately be formed. Such alignment marks can be utilized for aligning various masks utilized during the fabrication of the reticle, as well as, or alternatively for aligning the reticle during utilization of the reticle to pattern light during semiconductor fabrication. 
     It is noted that the demarcation  59  between the main-field and boundary regions of the substrate is provided for illustrative purposes, and frequently the border between the main-field and boundary regions is not a well-defined line. Regardless, persons of ordinary skill in the art will recognize that there is a main-field region of a reticle which can be defined as a region where openings will ultimately be formed for generating a circuit element pattern within a radiation-imageable material during a semiconductor fabrication process, and that such main-field region will be spaced from the edges of the reticle by a region which is not utilized to generate circuit element patterns within the radiation-imageable material. Persons of ordinary skill in the art will also recognize that the spacing between the main-field region and the edge of the reticle is a boundary region, and that such boundary region will typically extend entirely around the main-field region as is diagrammatically illustrated in  FIGS. 3 and 4 . 
     Referring next to  FIGS. 5 and 6 , a patterned mask  64  is formed over the upper surface  57  of relatively opaque material  56 . Mask  64  can comprise, consist essentially of, or consist of, for example, photoresist, and can be formed into the shown pattern utilizing photolithographic processing. 
     Relatively opaque material  56  can be considered to comprise a first portion within main-field  58 , and a second portion within boundary region  60 . Patterned mask  64  can cover a majority of the second portion of the relatively opaque material  56  while leaving a majority of the first portion of such relatively opaque material uncovered. In the shown aspect, the mask covers an entirety of the portion of the relatively opaque material  56  within boundary region  60 , and leaves an entirety of the relatively opaque material  56  within main-field region  58  uncovered (i.e., exposed). The locations  62  of the alignment markings are shown in  FIG. 6  to illustrate an optional aspect in which the patterned mask  64  covers all of the material  56  within the boundary region  60  except for locations  62  where alignment marks are to be formed. 
     Referring next to  FIG. 7 , a pattern is transferred from mask  64  to the underlying material  56 . Such reduces a thickness of the relatively opaque material of the main-field region  58  relative to a thickness of the relatively opaque material of the boundary region  60 . If the locations of the alignment marks  62  ( FIG. 6 ) are exposed during such etch, the locations of the alignment marks within boundary region  60  will also be reduced in thickness. 
     Although an entirety of the material  56  within main-field region  58  is shown being reduced in thickness, it is to be understood that the invention encompasses other aspects in which some of the material of main-field region  58  is protected by the patterned mask (discussed in more detail below with reference to  FIGS. 15 and 16 ), and accordingly wherein only some of the material within the main-field region is reduced in thickness during the processing of  FIG. 7 . Regardless, there will typically be a majority of the relatively opaque material  56  within the main-field region which is reduced in thickness relative to a majority of the relatively opaque material  56  of the boundary region. 
     Referring to  FIG. 8 , the patterned mask  64  ( FIG. 7 ) is removed, and masking material  66  is formed over the relatively opaque material  56 . Masking material  66  can comprise, consist essentially of, or consist of, for example, photoresist. 
     Referring to  FIG. 9 , a pattern is formed within masking material  66 , and specifically openings  68  are formed to extend through the material  66  within the main-field region  58 . The patterning of material  66  can be accomplished by, for example, photolithographic processing. The patterned material  66  forms a patterned mask over material  56 . The patterned material  64  (discussed above with reference to  FIGS. 5 and 6 ), and the patterned material  66  can be referred to as first and second patterned masks, respectively, to distinguish the patterned masks from one another. 
     Referring next to  FIG. 10 , openings  68  are extended into relatively opaque material  56  with a suitable etch, and accordingly a pattern from the patterned mask of material  66  is transferred into the material  56  of the main-field region. 
     The thinning of material  56  of the main-field region can provide numerous advantages for the patterning of the material. For instance, the etch through thinned material  56  can require less time than would an etch through the original thickness of material  56 . Further, etches through thin materials can typically be conducted with fewer complications and with tighter control of opening dimensions than can etches through thicker materials. Additionally, the etch through the thinned portions of material  56  can be conducted with thinner masking material  66  than can an etch through thicker portions of material  56  in many cases. This is because the etch utilized for material  56  is seldom 100% selective for material  56  relative to material  66 . Accordingly, some of the masking material  66  is removed during the etching of material  56 . The amount of masking material removed increases with the duration of the etch, which in turn increases with the thickness of material  56 . Accordingly, the thinned material  56  can be etched with a thinner mask  66  than could a thicker material  56 . The thinner mask can frequently be patterned with more stringent pattern control than can a thicker mask. For instance, if photolithographic processing is utilized to pattern a photoresist mask, the patterning can typically be conducted with more stringent control of the final pattern when the masking material  66  is thin than when the masking material  66  is thick. 
     Referring to  FIG. 11 , masking material  66  ( FIG. 10 ) is removed, and openings  68  are extended through phase-shifting material  54 . In some aspects, openings  68  can be extended into material  54  prior to removal of mask  66  ( FIG. 10 ). In other aspects, mask  66  can first be removed, and subsequently material  56  can be utilized as a hard mask during an etch to extend the openings  68  into phase-shifting material  54 . Extension of the openings into material  54  can be considered a transfer of the original pattern formed within masking material  66  ( FIG. 9 ) into main-field region  58  to pattern the phase-shifting material  54 . The construction of  FIG. 11  can be referred to as an intermediate construction for fabrication of a reticle. 
     In the shown aspect of the invention, the openings are extended to the upper surface of the relatively transparent base  52 . It is to be understood, however, that the invention includes other aspects (not shown) in which at least some of the openings are extended only partially into phase-shifting material  54  so that the openings do not reach the surface of base  52 , as well as aspects in which at least some of the openings are extended all the way through phase-shifting material  54  and partially into base  52 . 
     Referring next to  FIG. 12 , material  56  is subjected to an etch which removes the thinned portions of the material while leaving the thickened portions of the material. The shown etch has removed material  56  from main-field region  58 , while leaving material  56  over boundary region  60 . 
       FIG. 13  shows a top view of the construction of  FIG. 12 , and shows remaining phase-shifting material  54  patterned within main-field region  58  to leave windows  68  within the main-field region. Such windows can subsequently be utilized during a lithographic process, such as, for example, a process analogous to that described above with reference to prior art  FIG. 1 . 
     The features shown in  FIG. 13  are for diagrammatic purposes only, and it is to be understood that the relative scale of the features to the size of the reticle is much different than that which would typically be utilized. Specifically, there would typically be orders of magnitude more individual features formed within the main-field regions of a reticle than are shown in  FIG. 13 . 
     The alignment locations  62  are shown within boundary region  60  of the  FIG. 13  structure, and alignment marks can be present in such alignment locations at the processing stage of  FIG. 13 . The alignment marks can have been formed, for example, during the patterning conducted with the first masking material  64  ( FIG. 7 ), and/or during the patterning conducted with the second masking material  66  ( FIG. 10 ), and/or during other process steps which are not shown. If alignment marks are formed during the patterning of both the first and second masking materials, the locations of the alignment marks formed during the patterning of the second masking material can be at the same location as the locations of the alignment marks formed during the patterning of the first masking material or can be at different locations. 
     The invention can be utilized for applications in which all of the relatively opaque material  56  is removed from the main-field region. In other aspects, the invention can be applied to fabrication processes in which some of the relatively opaque material is to be left within the main-field region. 
     An exemplary application in which it is desired to leave some of the relatively opaque material within the main-field region is described with reference to  FIG. 14 . Specifically,  FIG. 14  shows a fragmentary top view of a reticle construction  100  comprising a base  102  and a pair of circuit patterns  104  and  106  over the base. The circuit patterns  104  and  106  can, for example correspond to patterns formed in phase-shifting material. The circuit patterns  104  and  106  comprise relatively thin and tightly packed components  108  and  110 , respectively; and relatively wide (i.e., less tightly packed) features  112  and  114  respectively. Relatively opaque regions  120  and  122  are provided over the wider regions  112  and  114  to block stray-light effects and other undesired effects that may otherwise occur. In applications in which the relatively opaque material contains chromium, the components  120  and  122  can be referred to as chrome blockers. It is common for a plurality of chrome blockers to be provided within the main-field region of a reticle. 
       FIGS. 15-21  illustrate exemplary methodologies by which relatively opaque blockers (for example, chrome blockers) can be provided within the main-field region of a reticle while utilizing aspects of the present invention. Similar numbering will be utilized in referring to  FIGS. 15-21  as was used above in referring to  FIGS. 3-13 , where appropriate. 
     Referring initially to  FIG. 15 , such shows a reticle construction  150  comprising the base  52 , phase-shifting material  54 , and relatively opaque material  56  discussed previously. The reticle is divided amongst a defined main-field region  58  and the defined boundary region  60 , with a border between such regions being diagrammatically illustrated with the dashed line  59 . 
     A patterned mask of material  152  is formed over material  56 . Material  152  can comprise the same masking material as described previously for material  64  of  FIG. 5 . The difference between the construction  150  of  FIG. 15  and the construction  50  of  FIG. 5  is that there is a block of the masking material over main-field region  58  of the construction  150 , whereas the entirety of the main-field is uncovered in the construction  50  of  FIG. 5 . 
     Referring to  FIG. 16 , a pattern from the patterned masking material  152  is transferred to the material  56  with an appropriate etch. Such thins material  56  within the main-field region but leaves thickened projections of material  56  where the material  56  is covered by masking material  152 . One of such projections (labeled  154  in  FIG. 16 ) is shown within the main-field region. Although only one projection is shown in the main-field region, it is to be understood that there would typically be more than one of such projections in the main-field region. 
     Referring next to  FIG. 17 , masking material  152  ( FIG. 16 ) is removed, and subsequently a patterned masking material  156  is formed over relatively opaque material  56 . The masking material  156  can be identical to the material  66  discussed above with reference to  FIG. 8 . Patterned masking material  156  has a plurality of openings  158  extending therethrough. 
     Referring next to  FIG. 18 , the openings  158  are extended through materials  56  and  54  in processing analogous to that discussed above regarding  FIGS. 10 and 11 , and masking material  156  is removed. 
     Referring next to  FIG. 19 , thinned regions of material  56  are removed in processing analogous to that discussed above with reference to  FIG. 12 . Such leaves a segment of material  56  extending around the boundary region  60 , and also leaves a segment of material  56  within the main-field region  58  as projection  154 . The projection  154  of relatively opaque material  56  can be utilized as a blocker, such as, for example, one of the blockers  120  and  122  discussed above with reference to  FIG. 14 . 
     The processing discussed above with reference to  FIGS. 15-19  illustrates one exemplary method for forming a blocker in accordance with an aspect of the present invention.  FIGS. 20 and 21  illustrate another exemplary method for forming a blocker. 
     Referring initially to  FIG. 20 , such shows a reticle construction  200  at a processing stage subsequent to  FIG. 11 . A patterned mask of material  202  has been formed over a thinned portion of relatively opaque material  56  in main-field region  58 . Material  202  can comprise any suitable masking material, and in particular aspects will comprise, consist essentially of, or consist of photoresist. 
     Referring to  FIG. 21 , construction  200  is illustrated after it has been subjected to processing analogous to that discussed above with reference to  FIG. 12  for removal of thinned material  56 , and after subsequent removal of masking material  202 . The masking material  202  has protected a portion of the thinned material  56  during the etching of the remainder of the thinned material  56 . The protected portion of material  56  forms a projection  204  within the main-field region  58 . Such projection can be a blocker analogous to the blockers  120  and  122  discussed above with reference to  FIG. 14 . 
       FIGS. 22 and 23  illustrate another exemplary method for removing the thinned material  56  in the main-field region and forming a blocker.  FIG. 22  shows construction  150  of  FIG. 18  at a processing stage subsequent to  FIG. 18 . Specifically a resist material  302  has been patterned to form a protective mask over some portions of material  56  while leaving other portions exposed. The material  302  can be patterned with, for example, an e-beam or laser mask pattern generation tool. The material  302  can be considered a third level, and the alignment of the third level to the underlying levels can be accomplished utilizing alignment marks built during either or both of the first and second level processes which formed the patterned material  56  of  FIG. 18 . 
       FIG. 23  shows construction  150  after exposed regions of the thinned opaque material  56  are removed, and after subsequent removal of masking material  302 . The patterned material  56  of  FIG. 23  has a relatively-opaque blocker over the main-field region which is approximately the same thickness as the majority of material  56  over the boundary region. 
     It is to be understood that processing analogous to that of  FIGS. 22 and 23  can also be utilized in applications in which blockers are not formed over the main-field region. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.