Abstract:
Methods are provided for fabricating a semiconductor device. One method comprises providing a first pattern having a first polygon, the first polygon having a first tonality and having a first side and a second side, the first side adjacent to a second polygon having a second tonality, and the second side adjacent to a third polygon having the second tonality, and forming a second pattern by reversing the tonality of the first pattern. The method further comprises forming a third pattern from the second pattern by converting the second polygon from the first tonality to the second tonality forming a fourth pattern from the second pattern by converting the third polygon from the first tonality to the second tonality forming a fifth pattern by reversing the tonality of the third pattern, and forming a sixth pattern by reversing the tonality of the fourth pattern.

Description:
PRIORITY CLAIM 
     This is a divisional of U.S. application Ser. No. 12/480,232, filed Jun. 8, 2009. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to methods for fabricating semiconductor devices, and more particularly relates to methods for fabricating semiconductor devices using a double patterning lithography technique and to photomasks for such a technique 
     BACKGROUND 
     There is a continuing trend within the microelectronics industry to incorporate more circuitry having greater complexity on a single integrated circuit (IC) chip. Maintaining this trend generally entails shrinking the size of individual devices within the circuit by reducing the critical dimensions (CDs) of device elements along with the pitch, or the CD of such an element added to the spacing between elements. Microlithography tooling and processing techniques play an important role in resolving the features necessary to fabricate devices and accordingly, are continually under development to meet industry milestones relating to the CD and pitch characteristic of each new technology generation. 
     High numerical aperture (NA) 193 nanometer (nm) optical projection stepper/scanner systems in combination with advanced photoresist processes now are capable of routinely resolving complex patterns that include isolated and dense resist features having CDs and pitches, respectively, well below the exposure wavelength. However, to meet the requirements of device design rules which continue to push the resolution limits of existing processes and tooling, other more specialized techniques have been developed to further enhance resolution. These include double patterning techniques (DPT) in which device patterns having potentially optically unresolvable features are decomposed into two or more complementary, and more easily resolvable patterns, each containing features with larger CDs and/or a relaxed pitch. One such DPT is referred to as litho/etch/litho/etch (LELE), and involves two separate lithographic exposures each followed by an etch process. These exposures are performed using different photomask reticles, each designed to image a portion of the total pattern. However, this scheme includes an additional etch step between lithography steps which increases fabrication cost and adds complexity to process logistics as wafers are transported between lithography and etch areas of a typical fab line. Other DPT processing options include spacer lithography processing often used, for example, in the fabrication of FinFet devices. However, these processes typically introduce several additional steps into a fabrication sequence adding yet further cost and complexity to the overall device fabrication process. 
     DPT processes which utilize a single etch step known as litho/freeze/litho/etch (LFLE) have also been developed. In LFLE, a first pattern is imaged into a first layer of photoresist, and the resist layer then is “frozen” rendering it unaffected by a second, subsequent photoresist process. A second pattern, complementary to the first pattern, then is formed into the second resist layer. However, the resolution conventionally attainable, for example, for isolated and dense trenches using existing LFLE processes is still limited to that of the lithography process. 
     Accordingly, it is desirable to provide methods for fabricating semiconductor devices using DPT processes which provide improved resolution over existing DPT processes. Further, it is also desirable to provide methods for designing photomask patterns for such DPT processes. Furthermore, it is also desirable to provide photomasks for such DPT processes. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     SUMMARY 
     Methods are provided for fabricating a semiconductor device. In accordance with an exemplary embodiment of the invention, one method comprises the steps of providing a first pattern design having a first polygon, the first polygon having a first tonality and having a first side and a second side, the first side adjacent to a second polygon having a second tonality, and the second side adjacent to a third polygon having the second tonality. The method also comprises forming a second pattern design by reversing the tonality of the first pattern design, and wherein the first polygon is converted from the first tonality to the second tonality, and wherein the second and third polygons are both converted to the first tonality. The method further comprises forming a third pattern design from the second pattern design by converting the second polygon from the first tonality to the second tonality forming a fourth pattern design from the second pattern design by converting the third polygon from the first tonality to the second tonality forming a fifth pattern design by reversing the tonality of the third pattern design, and forming a sixth pattern design by reversing the tonality of the fourth pattern design. 
     A method is provided for fabricating a semiconductor device in accordance with another exemplary embodiment of the invention. The method comprises providing a semiconductor substrate forming a first photoresist layer overlying the semiconductor substrate, and forming a first opening in the first photoresist layer, the first opening having a first side surface and a second side surface substantially parallel each other. The method also comprises freezing the first photoresist layer, forming a second photoresist layer on the first photoresist layer and in the first opening, and forming a second opening in the second photoresist layer, the second opening having a third side surface substantially parallel to the first and second side surfaces and positioned overlying the substrate between the first and second side surfaces, and a fourth side surface substantially parallel to the third side surface and positioned overlying the first photoresist layer. 
     A photomask set suitable for a double patterning lithography technique is provided. The set comprises a first photomask comprising a first surface, a first substantially opaque layer on the first surface, and a first substantially transparent polygon etched into the first substantially opaque layer, the first substantially transparent polygon having a first side and a second side substantially parallel to each other. The set also comprises a second photomask comprising a second surface, a second substantially opaque layer on the second surface, and a second substantially transparent polygon etched into the second substantially opaque layer, the second substantially transparent polygon having a third side and a fourth side substantially parallel to the first and second sides, and wherein the first and second substantially transparent polygons overlap when the first and second photomasks are aligned to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIGS. 1-7  schematically illustrate methods for fabricating semiconductor devices using a double patterning technique, in accordance with an exemplary embodiment of the present invention; 
         FIGS. 8-9  schematically illustrate methods for fabricating semiconductor devices using a double patterning technique, in accordance with another exemplary embodiment of the present invention; 
         FIGS. 10-14  schematically illustrate, in cross-section, methods for fabricating semiconductor devices using a double patterning technique, in accordance with a further exemplary embodiment; and 
         FIGS. 15-16  schematically illustrate, in cross-section, methods for fabricating semiconductor devices using a double patterning technique, in accordance with yet another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Manufacturing of modern semiconductor devices requires high resolution DPT lithography processing methods. In accordance with one embodiment, one such method includes fabricating semiconductor devices using a DPT process which provides improved resolution over existing DPT processes, and designing pattern layouts for such a DPT process. This method involves decomposing a master pattern layout for a semiconductor device layer into two separate and complementary sub patterns for use in a DPT process. The layout of the master pattern contains isolated and/or dense features having CDs and pitches, respectively, that are difficult or impossible to resolve with an existing lithography process and tool set, while each of the sub patterns contains only resolvable isolated and/or dense features. The two sub patterns are sequentially imaged into separate layers of photoresist such that the resulting photoresist mask is open in intersecting or common regions between the two sub patterns. Accordingly, the previously unresolvable isolated and/or dense photoresist mask features are formed by the separate patterning of a combination of two sub patterns having larger, more resolvable features. Further, photomasks suitable for use with these methods are also provided. 
       FIGS. 1-7  illustrate schematically, methods for fabricating semiconductor devices using a master pattern layout for a semiconductor device layer which has been decomposed into a pair of sub patterns, in accordance with exemplary embodiments of the invention. The sub patterns will be used in conjunction with a DPT, described more fully below, to fabricate a photoresist mask on a semiconductor substrate that will be used in the fabrication of a semiconductor device. The photoresist mask has isolated and dense features having improved resolution and smaller pitch than otherwise attainable by directly patterning the master layout using a single lithography step. While the decomposition of a master pattern layout having a particular geometry and configuration of features is illustrated, it will be appreciated that the methods depicted in  FIGS. 1-7  can be used to decompose other master pattern layouts having features of other geometries in any number of different configurations. 
     Referring to  FIG. 1 , in accordance with an exemplary embodiment, these methods begin by providing an original master pattern layout  100 . Master pattern layout  100  represents a two-dimensional design layout containing features pertaining to a particular layer of a semiconductor device, and is not limited with respect to dimension or to the number or shape of features contained therein. While pattern layout  100  may contain any number of features as required for implementing the device, for the sake of simplicity and clarity, this invention will be described in the context of a T-shaped feature  104  surrounded by a field  108 . In this example, feature  104  is composed of three individual rectangular polygons  110 ,  112 , and  114 . However, those of skill in the art will appreciate that feature  104  may include any number of overlapping or non-overlapping polygon shapes including rectangles, trapezoids, and the like, that when composed, form the desired feature. Further, no distinction is made regarding the manner by which a designed feature is formed, such as whether the feature is drawn directly, or whether the feature is formed indirectly as a central region bounded by a plurality of other drawn features. 
     The design contained within pattern layout  100  will be ultimately transferred onto a photomask as either substantially opaque or substantially transparent features. Accordingly, the polygons used to compose features and field regions may be designated in accordance with the resultant photomask tonality. For example, design features that result in a substantially transparent mask region may be designated as “clear” or “brightfield” tone features, and are illustrated without cross-hatching in  FIGS. 1-7 . Conversely, design features that result in a substantially opaque mask region may be designated as “dark” or “darkfield” tone features, and are illustrated with cross-hatching in  FIGS. 1-7 . Accordingly, polygons  110 ,  112 , and  114  are designated as clear or brightfield features while surrounding regions of field  108  are designated as dark or darkfield. The invention addresses the situation in which any or all of polygons  110 ,  112 , and  114  has a CD that is below the resolution limit of an existing photoresist process and exposure toolset with acceptable processing latitude. For example, any or all of sides  109 ,  111 , and  113  of polygons  110 ,  112 , and  114 , respectively, has a dimension (as represented by double-headed arrows  117 ,  118 , and  119 , respectively) below the threshold CD resolution limit. Accordingly, as will be described more fully below, master pattern layout  100  will be decomposed and divided into two separate sub patterns to be used in conjunction with a double patterning technique as a means of resolving such features. 
     Next, the tonality of master pattern layout  100  is globally reversed to form inverse master pattern  120 , as illustrated in  FIG. 2 . That is, regions formerly brightfield in master pattern layout  100  are transformed into darkfield, and those originally darkfield are converted to brightfield. For example, polygons  110 ,  112 , and  114  now appear as darkfield features on a brightfield background of field  108 . 
     Inverse master pattern  120  then is converted into two separate complementary inverse sub patterns  124  and  126 , as illustrated in  FIGS. 3-4 , respectively. Referring to  FIG. 3 , inverse sub pattern  124  contains each of the elements of inverse master pattern  120 , and also includes polygons  128  and  140  added as darkfield polygons to supplement the CD of polygons  110 ,  112 , and  114 . That is, a polygon  128  is added adjacent to a side  132  of polygon  112 , and adjacent to a side  136  of polygon  114 , and a polygon  140  is added adjacent to a side  144  of polygon  110 . As used herein, polygons described as adjacent each other, share a common side. Further, polygons sharing a common side are substantially parallel to each other along this side. Referring to  FIG. 4 , inverse sub pattern  126  also includes each of the elements of inverse master pattern  120 , and also includes additional darkfield polygons  148  and  152  to supplement the CD of polygons  110 ,  112 , and  114 . That is, polygon  148  is added adjacent a side  156  of polygon  112 , and is thus substantially parallel to polygon  112  along side  156 . Polygon  152  is added adjacent sides  160  and  164  of polygons  110  and  114 , respectively, and is thereby substantially parallel to these sides. 
     Next, the tonality of inverse sub patterns  124  ( FIG. 3) and 126  ( FIG. 4 ) are reversed to form normal sub patterns  170  and  174 , respectively, as illustrated in  FIGS. 5-6 . Referring to  FIG. 5 , normal sub pattern  170  includes polygons  110 ,  112 ,  114 ,  128 , and  140  converted to brightfield features, while field  108  is converted to a darkfield region. When represented on a photomask, the combination of brightfield polygons  110  and  140  will form a clear substantially rectangular feature having a CD (as represented by double-headed arrow  178 ) which is resolvable with an acceptable amount of processing latitude. Similarly, polygon  114  in combination with polygon  128 , and polygon  112  in combination with polygon  128  have CDs (as represented by double-headed arrows  182  and  186 , respectively) each acceptably resolvable. 
     Referring to  FIG. 6 , normal sub pattern  174  includes polygons  110 ,  112 ,  114 ,  148 , and  152  converted to brightfield features, while field  108  is converted to a darkfield region. When these polygons are fabricated onto a photomask, the combination of brightfield polygons  110  and  152  will form a clear feature having a CD (as represented by double-headed arrow  190 ) which is resolvable with acceptable processing latitude. Similarly, polygon  114  in combination with polygon  152 , and polygon  112  in combination with polygon  148  (as represented by double-headed arrows  194  and  198 , respectively) form clear features having CDs that are also acceptably resolvable. When aligned to and overlaid on each other, the intersection between normal sub patterns  170  and  174  comprises polygons  110 ,  112 , and  114 , as polygons common to both sub patterns. The remaining polygons  128 ,  140 ,  144 , and  152  are contained within only one of the sub patterns. Accordingly, the intersecting polygon set, illustrated in  FIG. 7  as double cross-hatched regions, comprises the same polygons having the same tonality as contained within master pattern layout  100 . 
       FIGS. 8-9  schematically illustrate, in cross-section, photomasks  200  and  204  fabricated using normal sub patterns  170  and  174 , respectively, in accordance with an exemplary embodiment. Photomasks  200  and  204  form an exemplary set of photomasks configured for use with a DPT lithography process, and comprise photomask blanks  205  and  206 , respectively, each made from a material substantially transparent to radiation of the exposing source wavelength such as, for example, quartz. Photomasks  200  and  204  contain normal sub patterns  170  and  174 , respectively, reproduced in any magnification such as, for example, 1×, 4×, 5×, and the like, suited for the optical lithography system to be used. Photomask  200  has opaque and clear features representative of normal sub pattern  170 , and includes polygons  110  and  140  merged together as a single clear feature  208  having sides  209  and  211 , and having opaque regions  212  representative of field  108  ( FIG. 5 ) on either side thereof. In one embodiment, sides  209  and  211  are substantially parallel. Similarly, photomask  204  includes polygons  110  and  152  ( FIG. 6 ) merged together as a clear feature  216  between opaque regions  220 , and having sides  217  and  219 . In another embodiment, sides  217  and  219  are substantially parallel. In a further embodiment, when photomasks  200  and  204  are in alignment with each other, sides  209  and  211  on photomask  200  are substantially parallel to sides  217  and  219  on photomask  204 . Opaque regions  212  and  220  comprise a layer of a material such as, for example, chromium or chromium oxide, that is substantially opaque to radiation of the wavelength to be used for exposure. 
       FIGS. 10-14  schematically illustrate, in cross-section, methods for fabricating a semiconductor device  400  including the steps of patterning a substrate  224  in and upon which the semiconductor device is fabricated, using photomasks  200  and  204 , in accordance with another exemplary embodiment. Referring to  FIG. 10 , substrate  224  may be any of the types commonly used in the fabrication of semiconductor devices such as silicon, germanium, a III-V material such as gallium arsenide, or another semiconductor material. Substrate  224  may be a bulk wafer or may be of a layered configuration such as, for example, a semiconductor-on-insulator (SOI) configuration comprising a thin layer of monocrystalline semiconductor material on an insulating layer supported by a semiconductor carrier wafer. Substrate  224  may optionally include without limitation one or more layers of additional materials having a surface  228 . These layers may also include any of the materials commonly used in semiconductor device fabrication such as, for example, semiconductor materials, dielectrics, conductive metal layers, and the like, and may include an organic or inorganic anti-reflective coating (ARC) or layer as a means of enhancing subsequent photoresist processing. While for ease of description the method described herein is illustrated by its application to the formation of semiconductor devices having isolated trenches, those of skill in the art will appreciate that the method is applicable to the fabrication of a myriad of semiconductor devices. 
     A photoresist layer  232  is applied overlying surface  228 . Photoresist layer  232  is an organic photoresist or “resist” layer sensitized to exposure from radiation of a particular wavelength or range of wavelengths. Such exposure wavelengths include, but are not limited to, 365 nanometer (nm), 248 nm, 193 nm, 157 nm, 126 nm, or 13.4 nm. Photoresist layer  232  may be either a positive acting or “positive tone” resist designed to be removed by a developer in regions exposed to actinic radiation, or a negative acting or “negative tone” resist designed to be removed by a developer in unexposed regions. Because this invention is directed to the formation of features on a semiconductor device of either positive or negative tone, the photoresist tonality is selected so as to complement the mask design and result in the formation of features having the desired tone in the resist film. Accordingly, to fabricate a trench using the exemplary clear features  208  and  216  of photomasks  200  and  204 , respectively, a positive tone resist is selected and herein described. However, those of skill in the art will appreciate that these methods may be applied with equal result in the formation of a trench in a photoresist layer using a negative tone resist in conjunction with photomasks having a reversed tonality. Layout patterns for such photomasks can be prepared using methods previously described and illustrated in  FIGS. 1-7  by globally reversing the tone of the pattern at each step. Photoresist layer  232  is applied using a suitable technique, such as, most commonly, by using a spin-coating and post application bake (PAB) process sequence. 
     Next, photomask  200  is used to selectively expose layer  232 , as illustrated in  FIG. 11 . Photomask  200  is used in conjunction with an optical exposure system, such as, for example, a step-and-scan lithography system having a lens assembly configured to project and focus normal sub pattern  170  onto layer  232 . Exposing radiation (as represented by arrows  236 ) passes through clear feature  208 , and is substantially blocked from reaching photoresist layer  232  by opaque regions  212 . Following exposure, layer  232  is developed to form an opening  240  having side surfaces  242  and  246 . In one embodiment, side surfaces  242  and  246  are substantially parallel to each other. Resist development typically includes immersion of photoresist layer  232  in a suitable developing solution and, depending upon the type of resist used, also commonly includes a post exposure bake (PEB) step performed prior to developer immersion. 
     Photoresist layer  232  is stabilized or “frozen” (as represented by dash lines  244 ) using a suitable freezing process, as illustrated in  FIG. 12 . The term “freezing” as used herein in the context of a photoresist, means that the resist layer has been chemically and structurally stabilized for use in conjunction with a subsequent photoresist patterning process. Accordingly, a frozen resist layer substantially maintains pattern fidelity for use as an etch mask when subjected to the coating, baking, exposure, and development steps associated with the second resist layer. The freezing of layer  232  may be performed using a suitable process including but not limited to exposure to heat and/or ultraviolet (UV) radiation (as represented by lines  243 ), ion bombardment, electron bombardment, and the like, in a manner that cross-links polymers within the resist. Alternatively, freezing may be done through a chemical treatment which stabilizes the resist to the above processes. Next, a photoresist layer  248  is applied as a blanket coating overlying photoresist layer  232  and within opening  240 . Layer  248  may be applied as previously described with reference to layer  232 , and illustrated with respect to  FIG. 11 . 
     Photoresist layer  248  is selectively exposed to radiation (as represented by arrows  252 ) through photomask  204 , as illustrated in  FIG. 13 . Exposing radiation passes through a clear feature  216  to expose layer  248 , but is blocked from reaching layer  248  by opaque features  220 . Layer  248  optionally is subjected to a post-exposure bake, and is developed following exposure as previously described with reference to layer  232 , and illustrated with respect to  FIG. 11 . The exposure and developing processes result in an opening  256  in photoresist layer  248  having a side surface  260  positioned overlying substrate surface  228  and a side surface  264  positioned overlying photoresist layer  232 . In one embodiment, side surfaces  260  and  264  are substantially parallel each other and to side surfaces  242  and  246 . Because photoresist layer  232  was frozen as described above, side surface  242  remains substantially unaltered during the patterning of photoresist layer  248  and, in conjunction with side surface  260 , form an opening  268  which provides access to surface  228 . Accordingly, because opening  268  constitutes the intersection of openings  240  and  256 , it has a CD (as represented by double headed arrows  272 ) less than that of either opening, and substantially identical to the original desired opening as represented by polygon  110  illustrated in  FIG. 1 . A trench  280  then may be etched into surface  228  through opening  268  using the composite of patterned layers  232  and  248  as an etch mask via a suitable wet or dry etch process, as illustrated in  FIG. 14 . 
       FIGS. 15-16  schematically illustrate, in cross-section, methods for forming a photoresist mask overlying surface  228  comprising a dense trench pattern, in accordance with another exemplary embodiment. This method begins with steps that are illustrated in  FIGS. 10-12 , and previously described. Following a blanket application, photoresist layer  248  is patterned using a photomask  284 , as illustrated in  FIG. 15 . Photomask  284  contains a dense line/space pattern which includes substantially opaque features  288  adjacent substantially transparent features  292  and  296 . Layer  248  is exposed (as represented by arrows  300 ) through photomask  284 , and is subsequently developed as previously described to form openings  304  and  308 . Opening  304  has a side surface  312  positioned overlying layer  232  and a side surface  316  positioned overlying substrate surface  228 . In one embodiment, side surfaces  312  and  316  are substantially parallel. In another embodiment, side surfaces  312  and  316  are substantially parallel to each other and to side surfaces  242  and  246 . Opening  308  has a side surface  320  positioned overlying layer  232  and a side surface  324  positioned overlying substrate surface  228 . As described above, side surface  242  is frozen and as such is substantially unaffected by the processing of photoresist layer  248  and, in conjunction with side surface  316 , forms a photoresist mask opening  328  that exposes surface  228 . Similarly, a photoresist mask opening  332  that exposes surface  228  is bounded by side surface  246  of photoresist layer  232  in conjunction with side surface  324  of photoresist layer  248 . Photoresist mask openings  328  and  332  are separated by resist line  336  formed overlying surface  228  comprising photoresist layer  248 . Accordingly, because each of photoresist mask openings  328  and  332  is formed by the intersection of two larger resist openings, the resulting pitch (as represented by double headed arrows  340 ) of photoresist mask openings  328  and  332  is less than the pitch (measured at 1×, and as represented by double headed arrows  344 ) of features used to form them in photomask  284 . Next, substrate  224  may be suitably etched through photoresist mask openings  328  and  332  to form trenches  348  and  350 , respectively, as illustrated in  FIG. 16 . 
     Accordingly, methods have been provided for fabricating a semiconductor device including the steps of forming a photoresist mask pattern having enhanced resolution and pitch of isolated and dense features, respectively. These methods include a process for decomposing a single master pattern layout that contains potentially unresolvable isolated or dense features into a pair of sub patterns containing only larger, more resolvable features. The sub patterns are imaged onto two separate photomasks and used with an LFLE-type DPT process to form the final photoresist mask. In addition to isolated features, these methods may be used to enhance the resolution of features having other shapes composed of isolated segments such as L-shaped, T-shaped, or U-shaped designs, and the like. Methodologies related to both pattern generation and photoresist processing are compatible with either a positive or negative tone photoresist process by adjusting the tone of pattern layouts and photomasks appropriately. Accordingly, these methods may be used to further extend the resolution limits of the lithography tool set and photoresist processes with which they are used. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.