Abstract:
The present invention provides an attenuated phase shift mask (“APSM”) that, in each embodiment, includes completely transmissive regions sized and shaped to define desired semiconductor device features, slightly attenuated regions at the edges of the completely transmissive regions corresponding to isolated device features, highly attenuated regions at the edges of completely transmissive regions corresponding to closely spaced or nested device features, and completely opaque areas where it is desirable to block transmission of all radiation through the APSM. The present invention further provides methods for fabricating the APSMs according to the present invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/629,641, filed Jul. 29, 2003, now U.S. Pat. No. 6,908,715, issued Jun. 21, 2005, which is a continuation of application Ser. No. 09/809,720, filed Mar. 15, 2001, now U.S. Pat. No. 6,599,666, issued Jul. 29, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to photolithography techniques used in semiconductor device manufacturing processes. Specifically, the present invention relates to a multi-layer, attenuated phase-shifting mask or reticle that reduces problems associated with side lobe printing in areas including closely spaced or nested features, while maximizing resolution and depth-of-focus performance for isolated features of a semiconductor device.  
         [0004]     2. State of the Art  
         [0005]     Photolithography processes are essential to the fabrication of state of the art semiconductor dice. Such processes are used to define various semiconductor die features included in semiconductor dice and generally include exposing regions of a resist layer to patterned radiation corresponding to the semiconductor die circuit feature to be defined in a substrate underlying the layer of resist. After exposure, the resist layer is developed to selectively reveal areas of the substrate that will be etched to define the various device features while selectively protecting those areas of the substrate which are not to be exposed to the etching process. In order to properly form a radiation pattern over a resist layer, the radiation is generally passed through a reticle or mask which projects the semiconductor die feature pattern to be formed in the resist layer.  
         [0006]     Various types of photolithographic masks are known in the art. For example, known masks often include a transparent plate covered with regions of a radiation blocking material, such as chromium, which define the semiconductor die feature pattern projected by the mask. Such masks are called binary masks since radiation is completely blocked by the radiation blocking material and fully transmitted through the transparent plate in areas not covered by the radiation blocking material. However, binary masks cause significant fabrication problems, particularly where semiconductor die dimensions shrink below 1 μm.  
         [0007]     As the pattern features of a binary mask are defined by boundaries between opaque, radiation blocking material and material which is completely radiation transmissive, radiation passing through a binary mask at the edge of a pattern feature will be diffracted beyond the intended image boundary and into the intended dark regions. Such diffracted radiation prevents formation of a precise image at the feature edge, resulting in semiconductor die features which deviate in shape or size from the intended design. Because the intensity of the diffracted radiation drops off quickly over a fraction of a micron, diffraction effects are not particularly problematic where semiconductor dice have dimensions on the order of 1 μm. However, as feature dimensions of state of the art semiconductor dice shrink well below 0.5 μm, the diffraction effects of binary masks become terribly problematic.  
         [0008]     Another type of mask known in the art is an attenuated phase shift mask (APSM). APSMs were developed to address the diffraction problems produced by binary masks and are distinguished from binary masks in that, instead of completely blocking the passage of radiation, the less transmissive regions of the mask are actually partially transmissive. Importantly, the attenuated radiation passing through the partially transmissive regions of an APSM generally lacks the energy to substantially affect a resist layer exposed by the mask. Moreover, the partially transmissive regions of APSMs are designed to shift the passing radiation 180° relative to the radiation passing through the completely transmissive regions and, as a consequence, the radiation passing through the partially transmissive regions destructively interferes with radiation diffracting out from the edges of the completely transmissive regions. Thus, the phase shift greatly reduces the detrimental effects of diffraction at the feature edges, thereby increasing the resolution with which sub-micron features may be patterned on a resist layer.  
         [0009]     A conventional APSM  4  is illustrated in drawing  FIG. 1 . As can be seen, the APSM  4  includes a transparent substrate  6  coated with a partially transmissive material  7  (to ease description, drawing  FIG. 1  provides a greatly simplified APSM). The partially transmissive material  7  has been patterned to form a completely transmissive region  8  and two attenuated regions  10   a ,  10   b . The attenuated regions  10   a ,  10   b  of a typical APSM  4  are typically designed to allow the passage of between about 4% (low transmission) and 20% (high transmission) of the incident radiation  12 . The partially transmissive material  7  forming the attenuated regions  10   a ,  10   b  is formed to a thickness that shifts the incident radiation  12  one hundred eighty degrees (180°) out of phase.  
         [0010]     Also provided in drawing  FIG. 1  is a graph  16  illustrating the electromagnetic intensity (plotted on the vertical axis) of the radiation passing through the APSM  4  relative to the position (plotted on the horizontal axis) on the surface of the exposed resist. As shown, the intensity curve  18  includes a first component  20  located primarily between the edges  22   a ,  22   b  formed between the attenuated regions  10   a ,  10   b  and the completely transmissive region  8  of the APSM  4 . The first component  20  of the intensity curve  18  corresponds to the electromagnetic intensity of the radiation passing through the completely transmissive region  8  of the APSM  4  illustrated in drawing  FIG. 1 . As can be seen in the graph  16 , the electromagnetic intensity of the radiation falls to zero at points  24   a ,  24   b , which are near the edges  22   a ,  22   b . Points  24   a ,  24   b  correspond to the locations where the magnitudes of the in phase radiation passing through the completely transmissive region  8  and the out of phase radiation passing through the attenuated regions  10   a ,  10   b  are equal. Beyond points  24   a ,  24   b  and moving away from the edges  22   a ,  22   b , the electromagnetic intensity of the transmitted radiation grows again to a steady value as indicated by the second curve components  26   a ,  26   b . The second curve components  26   a ,  26   b  represent the electromagnetic intensity of the radiation passing through the attenuated regions  10   a ,  10   b  of the APSM  4 .  
         [0011]     The electromagnetic intensity represented by the second curve components  26   a ,  26   b  is also known as “ringing effects,” and one significant disadvantage of APSMs is that such ringing effects become much more severe as feature density of an APSM increases. As device features designed into an APSM are spaced closer and closer together, the ringing effects of adjacent device features begin to overlap, and as the ringing effects overlap, the electromagnetic intensity of such ringing effects becomes additive. These increased ringing effects are known as “additive side lobes,” “additive ringing effects,” or “proximity effects.” In contrast to isolated ringing effects produced by isolated device features, the electromagnetic intensity of additive side lobes created by closely spaced (i.e., ≦0.5 μm) or nested device features often becomes sufficiently intense to cause printing of the resist layer, which is commonly termed “side lobe printing.” 
         [0012]     Illustrated in drawing  FIG. 2  is the additive ringing effects associated with conventional APSMs having closely spaced feature formations. As illustrated in drawing  FIG. 2 , a second APSM  30  includes a transparent substrate  32  coated with a partially transmissive phase-shifting film  34  (again, for ease of description, drawing  FIG. 2  provides a greatly simplified APSM). The partially transmissive phase-shifting film  34  has been patterned to form four attenuating regions  36   a - 36   d  and three completely transmissive regions  38   a - 38   c , which are closely spaced. Radiation  39  incident on the APSM  30  passes through the completely transmissive regions  38   a - 38   c  and the attenuated regions  36   a - 36   d  and impinges upon the surface of the resist layer to be patterned (not illustrated in drawing  FIG. 2 ).  
         [0013]     Included in drawing FIG. 2  is a graph  40  illustrating the electromagnetic intensity of the radiation incident upon the surface of the resist layer to be patterned. The graph  40  includes an intensity curve  42  made up of various components, with the first components  43   a - 43   c  illustrating the electromagnetic intensity of the radiation passing through the completely transmissive regions  38   a - 38   c  of the APSM  30 , the second components  44   a ,  44   b  illustrating the electromagnetic intensity of the ringing effects produced by the radiation passing through the isolated attenuated regions  36   a ,  36   d , and the third components  46   a ,  46   b  illustrating the electromagnetic intensity of the additive side lobes produced by the dense feature arrangement formed by the closely spaced attenuated regions  36   b ,  36   c . As can be seen in drawing  FIG. 2 , the magnitude of the second components  44   a ,  44   b  (represented by line “I 1 ”), which illustrate the intensity of the ringing effects produced by isolated attenuated regions  36   a ,  36   d , is significantly lower than that of the third components  46   a ,  46   b  (represented by line “I 2 ”), which illustrate the electromagnetic intensity of the additive side lobes.  
         [0014]     Provided in drawing  FIG. 3  is a cross-sectional view of a partially fabricated structure  50  after exposure through the APSM  30  illustrated in drawing  FIG. 2 . The partially fabricated structure  50  includes a semiconductor substrate  52  and a developed resist layer  54 . The developed resist layer  54  exhibits a set of depressions  56   a ,  56   b  resulting from the relatively high electromagnetic intensity of the additive side lobes caused by the dense feature arrangement of the APSM  30 . As device feature density increases, so will the intensity of the additive side lobes and the extent to which the resist layer is patterned due to exposure to additive ringing effects. Thus, depressions in the resist layer due to additive ringing effects may, in some situations, degrade the resist layer to such an extent that entire semiconductor dice become unusable due to damage incurred during a subsequent etch process.  
         [0015]     As is well known in the art, the ringing intensity is inversely related to the attenuation of the partially transmissive material used in APSMs. Increasing the attenuation of the partially transmissive material will, therefore, decrease any resultant ringing effects, while decreasing the attenuation will increase any resultant ringing effects. Thus, the intensity of additive side lobes produced by closely formed features in an APSM may be decreased simply by increasing the attenuation of the partially transmissive regions included therein.  
         [0016]     However, increasing the attenuation of the partially transmissive areas of APSMs also has significant drawbacks. For example, increasing the attenuation of the partially transmissive areas decreases print performance as well as the resolution and depth-of-focus achievable by the APSM. Reduction of depth-of-focus and resolution characteristics of an APSM are particularly problematic in the fabrication of state of the art semiconductor devices, which requires that an APSM accurately project images corresponding to sub-0.5μm device features while focusing such images through relatively thick layers of resist. In addition, even with the most precise fabrication equipment, sub-micron deviations from the optimum focus position of the APSM relative to the resist layer to be patterned will occur, and decreasing the depth-of-focus of an APSM increases the probability that fabrication defects may result from such slight deviations. Therefore, increasing the attenuation of the partially transmissive materials included in state of the art APSMs requires a careful compromise between control of additive ringing effects and maximization of resolution and depth-of-focus performance.  
         [0017]     Furthermore, state of the art semiconductor dice often include closely spaced or nested features as well as features which are isolated. It would, therefore, be an improvement in the art to provide an APSM that includes highly attenuated regions (i.e., attenuating regions allowing about 4% to about 10% transmittance of incident radiation) where necessary to control additive ringing but also includes slightly attenuated regions (i.e., attenuating regions allowing about 12% to about 20% transmittance of incident radiation) where isolated device features are to be formed. Such an APSM would enable control of additive ringing effects where needed without compromising resolution and depth-of-focus performance where additive ringing effects are of little or no concern.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018]     The present invention addresses the foregoing needs by providing an APSM that, in each embodiment, includes completely transmissive regions sized and shaped to define desired device features, slightly attenuated regions at the edges of completely transmissive regions corresponding to isolated device features, highly attenuated regions at the edges of completely transmissive regions corresponding to closely spaced or nested device features, and completely opaque areas where it is desirable to block transmission of all radiation through the APSM. The present invention further provides methods for fabricating the APSMs according to the present invention.  
         [0019]     In one embodiment, the method of fabricating an APSM according to the present invention includes providing a transparent substrate. The transparent substrate is coated with a first attenuating layer that shifts the phase of transmitted radiation by 180° and is only slightly attenuated. The first attenuating layer is coated with a second attenuating layer that does not shift the phase of passing radiation, but further attenuates the intensity of any radiation passing therethrough. An opaque layer is then formed over the second attenuating layer. Using this intermediate structure, a desired APSM according to the present invention may be formed.  
         [0020]     To form a desired APSM, the opaque layer is coated with a resist. The resist is then patterned to create a first patterned resist defining the semiconductor die feature pattern to be projected by the finished APSM. The opaque layer is then etched.  
         [0021]     After the opaque layer is etched, the first patterned resist may be left in place and the second and first attenuating layers may be etched using the first patterned resist as a template. The first patterned resist is then stripped, leaving a first intermediate mask structure including completely transmissive regions corresponding to the pattern to be projected by the mask and completely opaque regions where the opaque layer and first and second attenuating layers remain intact. Alternatively, the first patterned resist may be removed after etching the opaque layer, and the opaque layer alone may be used as the template for etching the first and second attenuating layers.  
         [0022]     A second layer of resist is deposited over the first intermediate mask structure. The second layer of resist is patterned to define a second patterned resist, which exposes areas of the intermediate mask structure wherein slightly attenuated regions will be formed. The opaque layer and the second attenuating layer in the areas exposed by the second patterned resist are then etched, revealing slightly attenuating regions formed from portions of the first attenuating layer. The second patterned resist is then stripped, and the resulting second intermediate mask structure includes completely transmissive regions sized and shaped in accordance with the device pattern to be projected by the mask, slightly attenuated regions, which shift passing radiation one hundred eighty degrees (180°), and opaque regions where the opaque layer, as well as the first and second attenuating layers, remain intact. Preferably, the slightly attenuated regions are provided at the edges of each of the completely transmissive regions corresponding to isolated device features, thereby maximizing image resolution and depth-of-focus performance where it is not necessary to increase attenuation to combat the negative effects produced by additive side lobes.  
         [0023]     A third resist is deposited over the second intermediate mask structure. The third resist is then patterned to form a third patterned resist, which exposes areas of the second intermediate mask structure wherein highly attenuated regions will be formed. The areas exposed by the third patterned resist are then etched to remove only the opaque layer, thereby defining regions were incident radiation is phase shifted one hundred eighty degrees (180°) and highly attenuated as it passes through both the first and second attenuating layers. Preferably, such highly attenuated regions are formed at the edges of completely transmissive regions corresponding to closely spaced or nested device features, thereby increasing the resolution of such semiconductor die features projected by the finished mask, while minimizing or eliminating any defects from additive ringing effects.  
         [0024]     The third patterned resist is then stripped leaving a completed mask according to the present invention. The completed mask, therefore, includes completely transmissive regions corresponding to the pattern to be projected by the mask, slightly attenuated regions, which phase shift passing radiation one hundred eighty degrees (180°), highly attenuated regions, which also shift passing radiation 180°, and opaque regions where the opaque layer and first and second attenuating layers remain intact.  
         [0025]     As can be appreciated, the method of the present invention is highly adaptable and can be used to fabricate APSMs having any desired feature pattern. Moreover, the steps of the method can be modified in several aspects while still obtaining a desired APSM. For example, etch stop techniques can be incorporated into the method of the present invention to eliminate one or more etching steps. However, in each of its embodiments, the method of the present invention advantageously produces APSMs including completely transmissive regions, slightly attenuated regions, and highly attenuated regions, and the size, shape, and position of these various regions can be modified or adjusted to produce any desirable semiconductor die feature pattern.  
         [0026]     Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0027]     The figures presented in conjunction with this description are not actual views of any particular portion of a device or component, but are merely representations employed to more clearly and fully depict the present invention.  
         [0028]      FIG. 1  provides a schematic illustration of a prior art APSM as well as a graph depicting the electromagnetic intensity of radiation projected through the prior art APSM;  
         [0029]      FIG. 2  provides a schematic illustration of a second prior art APSM as well as a graph depicting the electromagnetic intensity of radiation projected through the second prior art APSM;  
         [0030]      FIG. 3  schematically illustrates a cross-section of a partially fabricated semiconductor device structure after exposure through the second prior art APSM illustrated in  FIG. 2 ;  
         [0031]      FIG. 4  schematically illustrates a cross-section of a first intermediate mask structure formed in the first embodiment of the method of the present invention;  
         [0032]      FIG. 5  schematically illustrates a first patterned resist formed over the first intermediate mask structure of  FIG. 4 ;  
         [0033]      FIG. 6  provides a schematic illustration of a second intermediate mask structure formed in the first embodiment of the method of the present invention;  
         [0034]      FIG. 7  schematically illustrates the intermediate mask structure depicted in  FIG. 6 , after a second patterned resist for creating slightly attenuated regions is formed thereover;  
         [0035]      FIG. 8  provides a schematic illustration of a third intermediate mask structure formed in the first embodiment of the method of the present invention;  
         [0036]      FIG. 9  and  FIG. 10  schematically illustrate the third intermediate mask structure depicted in  FIG. 8 , after a third patterned resist for creating highly attenuated regions is formed thereover and the resulting structure is subjected to a selective etch process;  
         [0037]      FIG. 11  schematically illustrates a first embodiment of the APSM of the present invention;  
         [0038]      FIG. 12  schematically illustrates a cross-section of a first intermediate mask structure formed in the second embodiment of the method of the present invention;  
         [0039]      FIG. 13  schematically illustrates a first patterned resist formed over the first intermediate mask structure of  FIG. 12 ;  
         [0040]      FIG. 14  provides a schematic illustration of a second intermediate mask structure formed in the second embodiment of the method of the present invention;  
         [0041]      FIG. 15  schematically illustrates the intermediate mask structure depicted in  FIG. 14 , after a second patterned resist for creating slightly attenuated regions is formed thereover;  
         [0042]      FIG. 16  provides a schematic illustration of a third intermediate mask structure formed in the second embodiment of the method of the present invention;  
         [0043]      FIG. 17  and  FIG. 18  schematically illustrate the third intermediate mask structure depicted in  FIG. 16 , after a third patterned resist for creating highly attenuated regions is formed thereover and the resulting structure is subjected to a selective etch process; and  
         [0044]      FIG. 19  schematically illustrates a second embodiment of the APSM of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]     A first embodiment of the method of forming an APSM according to the present invention is schematically illustrated in drawing  FIG. 4  through drawing  FIG. 11 . In each of these figures, the structures representing the various intermediate APSM structures, as well as the complete APSM, are greatly simplified for ease of description.  
         [0046]     As is illustrated in drawing  FIG. 4 , the method of the present invention includes providing a transparent substrate  60 , such as quartz, fused silica, or other type glass substrates, etc. The transparent substrate  60  is then coated with a first attenuating layer  62 , such as a layer of chromium oxynitride (CrO x N y ) or chromium fluoride (CrF x ). The first attenuating layer  62  is preferably highly transmissive (i.e., allows about 12%-20% transmission) and shifts the phase of any passing radiation by 180°. The first attenuating layer  62  is then coated with a second attenuating layer  64 , such as a layer of molybdenum silicide oxynitride (MoSiO x N y ). The second attenuating layer  64  is formed such that the second attenuating layer  64  does not shift the phase of passing radiation, but simply further attenuates the intensity of any passing radiation. Preferably, the total attenuation of radiation passing through the first attenuating layer  62  and the second attenuating layer  64  is about 90% to about 96%, allowing about 4% to about 10% transmission. An opaque layer  66 , for example, a layer of chromium, is then formed over the second attenuating layer  64 , resulting in a first intermediate mask structure  70  that may be used to form a desired mask according to the first embodiment of the APSM of the present invention.  
         [0047]     As is illustrated in drawing  FIG. 5 , to form a desired APSM according to the first embodiment of the APSM of the present invention using the first intermediate mask structure  70 , a first patterned resist  72  is formed over the opaque layer  66 . The first patterned resist  72  is formed by first coating the opaque layer  66  with any suitable resist and patterning the resist by known methods to define a desired feature pattern to be projected by the completed mask. After formation of the first patterned resist  72 , the opaque layer  66  is etched to reveal areas of the second attenuating layer  64 . Any suitable etch process may be used to etch the opaque layer. For example, where the opaque layer includes chromium, a Cl 2 /O 2  plasma etch process or a suitable wet etch process may be used to etch the opaque layer  66 .  
         [0048]     After the opaque layer  66  is etched, the first patterned resist  72  may be left in place. The second attenuating layer  64  and the first attenuating layer  62  may then be etched using the first patterned resist as a template, revealing the underlying transparent substrate and forming various completely transmissive regions  74   a - 74   f  (shown in drawing  FIG. 6 ), which correspond in size, shape, and location to the device pattern to be projected by the completed APSM. The second attenuating layer  64  and first attenuating layer  62  are etched using any suitable method. However, where the second attenuating layer  64  includes MoSiO x N y , an SF 6  or CF 4  based plasma etch process is preferably used, and where the first attenuating layer  62  includes CrFx, the first attenuating layer  62  is preferably etched in a Cl 2 /O 2  plasma. Moreover, where it is used as a template for the formation of the completely transmissive regions  74   a - 74   f , the first patterned resist  72  is stripped after the completely transmissive regions  74   a - 74   f  are formed, leaving a second intermediate mask structure  76 , which is illustrated in drawing  FIG. 6 , including completely transmissive regions  74   a - 74   f.    
         [0049]     Once the second intermediate mask structure  76  is formed, slightly attenuated regions are formed where desired. Slightly attenuated regions are created using a second patterned resist  78  formed over the second intermediate mask structure  76 , as shown in drawing  FIG. 7 . The second patterned resist  78  is formed by first coating the first intermediate mask structure  76  with any suitable resist and patterning the resist by known methods to create exposed areas  77   a - 77   b  on the second intermediate mask structure  76 , wherein slightly attenuated regions are to be created.  
         [0050]     As is shown in drawing  FIG. 8 , which illustrates a third intermediate mask structure  82 , the slightly attenuated regions  80   a - 80   d  are created by etching the opaque layer  66  and the second attenuating layer  64  in the exposed areas  77   a - 77   b  created by the second patterned resist  78 . Again, the opaque layer  66  and the second attenuating layer  64  can each be etched by known etch processes, such as those already discussed. As can also be seen in drawing  FIG. 8 , the slightly attenuated regions  80   a - 80   d  are preferably formed at the edges of isolated completely transmissive regions  74   a ,  74   f . Because the slightly attenuated regions  80   a - 80   d  are formed using portions of the first attenuating layer  62 , which shifts transmitted radiation 180°, radiation transmitted through the slightly attenuated regions  80   a - 80   d  destructively interferes with radiation diffracting out from the edges of the isolated completely transmissive regions  74   a ,  74   f , thereby greatly increasing the resolution with which the isolated completely transmissive regions  74   a ,  74   f  define desired device features. Moreover, because slightly attenuated regions  80   a - 80   d  allow transmission of about 12% to about 20% of the incident radiation, the slightly attenuated regions  80   a - 80   d  serve to maximize depth-of-focus performance.  
         [0051]     As shown in drawing  FIG. 9 , to create highly attenuated regions where desired, a third patterned resist  84  is formed over the third intermediate mask structure  82 . The third patterned resist  84  is created by first coating the third intermediate mask structure  82  with a suitable resist. The resist is then patterned by known methods to expose an area  85  of the third intermediate mask structure  82  wherein highly attenuated regions are to be created.  
         [0052]     Highly attenuated regions  86   a - 86   e  are then formed by selectively etching the opaque layer  66  in the exposed area  85  (see drawing  FIG. 10 ). The opaque layer  66  can be etched using any suitable etch process, such as the processes already discussed herein. After formation of the highly attenuated regions, the third patterned resist  84  is stripped, leaving a complete APSM  88  according to the first embodiment of the APSM of the present invention (shown in drawing  FIG. 11 ). It is easily appreciated from reference to drawing  FIG. 11  that the highly attenuated regions  86   a - 86   e  are preferably formed at the edges of closely spaced transmissive regions  74   b - 74   e  which are closely spaced. Because of the one hundred eighty degree (180°) phase shift provided by the first attenuating layer  62  and the high total attenuation provided by the highly attenuated regions  86   a - 86   e , the highly attenuated regions  86   a - 86   e  formed at the edges of closely spaced completely transmissive regions  74   b - 74   e  greatly increase the resolution with which the isolated completely transmissive regions  74   a ,  74   f  define desired device features, while minimizing or eliminating any fabrication defects that may otherwise occur due to additive ringing effects.  
         [0053]     As can be appreciated by reference to drawing  FIG. 11 , even after formation of completely transmissive regions  74   a - 74   f , slightly attenuated regions  80   a - 80   d , and highly attenuated regions  86   a - 86   e , portions of the opaque layer  66  remain, forming opaque regions  90   a - 90   d . Opaque regions  90   a - 90   d  may be maintained on the finished APSM to prevent exposure to even attenuated radiation where attenuated radiation is not needed to increase image resolution. The first embodiment of the method of the present invention, therefore, provides an APSM having completely transmissive regions, highly attenuated regions, slightly attenuated regions, and opaque regions, which work in concert to maximize image resolution and depth-of-focus for isolated features, while minimizing or eliminating any defects caused by additive ringing effects in areas of high feature density and preventing any defects caused by transmission of attenuated radiation where attenuated radiation is not needed to enhance resolution and depth-of-focus.  
         [0054]     In a second embodiment, described in conjunction with drawing  FIG. 12  through drawing  FIG. 19 , the method of the present invention involves the use of etch stop technology. As illustrated in drawing  FIG. 12 , the method of the second embodiment also involves providing a transparent substrate  60 , which may also be, for example, a quartz, fused silica, or other glass substrate. A first attenuating layer  100  comprising CrF x  is deposited over the transparent substrate, followed by the formation of an etch stop layer  102  over the first attenuating layer  100 . The etch stop layer  102  may be formed of any suitable etch stop material that will allow for an etch selectivity between it and the etching chemistry utilized to etch the material adjacent to it (i.e., second attenuating layer  104  shown in  FIG. 12 ). For example, the first etch stop layer  102  may be formed of silicon dioxide (SiO 2 ).  
         [0055]     The first attenuating layer  100  is only slightly attenuating, allowing about 12% to about 20% transmission. Moreover, the first attenuating layer  100  may be formed such that the first attenuating layer  100  induces a one hundred eighty degree (180°) phase shift in radiation passing through the first attenuating layer  100 . Alternatively, the first etch stop layer  102  may be formed to induce a one hundred eighty degrees (180°) phase shift, while the first attenuating layer  100  serves only to attenuate passing radiation, or the first attenuating layer  100  and first etch stop layer  102  may be formed such that radiation must pass through both layers  100 ,  102  to be shifted one hundred eighty degrees (180°) out of phase. Where the first attenuating layer  100  is formed such that the first attenuating layer  100  both attenuates passing radiation and shifts the passing radiation one hundred eighty degree (180°) out of phase, the first etch stop layer  102  is formed to allow passage of radiation without inducing any further phase shifts.  
         [0056]     As shown in drawing  FIG. 12 , a second attenuating layer  104  is formed over the etch stop layer  102 . The second attenuating layer  104  is also preferably formed of CrF x  and further attenuates passing radiation. The second attenuating layer  104  is preferably formed such that radiation passing through both the first attenuating layer  100  and the second attenuating layer  104  is highly attenuated (i.e., the combined attenuation of the first attenuating layer  100  and the second attenuating layer  104  is about 90% to about 96%, resulting in about 4% to about 10% transmittance). However, the second attenuating layer  104  does not induce any phase shift in radiation passing therethrough. Once the second attenuating layer  104  is formed, an opaque layer  106  is provided over the second attenuating layer  104 , resulting in a first intermediate mask structure  108 . As is true in the first embodiment of the method of the present invention, the opaque layer  106  may be formed of any suitable material known in the art and by any suitable method, such as a deposited chromium layer.  
         [0057]     Using the first intermediate mask structure  108 , an APSM according to the second embodiment of the APSM of the present invention may be fabricated. Forming an APSM according to the second embodiment using the first intermediate mask structure  108  involves formation of a first patterned resist  110  over the opaque layer  106  of the first intermediate mask structure  108 , as is shown in drawing  FIG. 13 . The first patterned resist  110  is created by first coating the opaque layer  106  with any suitable resist and patterning the resist by known methods to define the desired feature pattern to be projected by the completed APSM. After formation of the first patterned resist  110 , the opaque layer  106  and the second attenuating layer  104  are etched in a single step using a Cl 2 /O 2  plasma etch process, which will stop at the etch stop layer  102 . With the first patterned resist  110  still in place, the etch stop layer  102  is etched using a fluorine-based plasma etch process, and the first attenuating layer  100  is then etched using a second Cl 2 /O 2  plasma etch process.  
         [0058]     After etching the first attenuating layer  100 , the first patterned resist  110  is stripped, leaving a second intermediate mask structure  112 , as illustrated in drawing  FIG. 14 . The second intermediate mask structure  112  includes completely transmissive regions  114   a - 114   f  which correspond in size, shape and location to the device pattern to be projected by the mask.  
         [0059]     As can be seen in drawing  FIG. 15 , a second patterned resist  116  is then formed over the second intermediate mask structure  112 , in order to form slightly attenuated regions where desired. The second patterned resist  116  is formed by first coating the second intermediate mask structure  112  with any suitable resist and patterning the resist by known methods to create exposed areas  118   a ,  118   b  of the second intermediate mask structure  112  wherein slightly attenuated regions are to be formed.  
         [0060]     As can be appreciated by reference to drawing  FIG. 16 , which illustrates a third intermediate mask structure  120 , the slightly attenuated regions  122   a - 122   d  are then created by etching the opaque layer  106  and the second attenuating layer  104  in a single step using a Cl 2 /O 2  plasma etch process, which stops at the exposed portions  124   a - 124   d  of the etch stop layer  102 , thereby reducing the number of etch steps necessary to form the slightly attenuated regions  122   a - 122   d  relative to the first embodiment of the method of the present invention.  
         [0061]     Further illustrated in drawing  FIG. 16  is that the slightly attenuated regions  122   a - 122   d , are preferably formed only at the edges of isolated completely transmissive regions  114   a ,  114   f . Because the slightly attenuated regions  122   a - 122   d  are formed of portions of the first attenuating layer  100  as well as portions of the etch stop layer  102 , the slightly attenuating regions  122   a - 122   d  shift transmitted radiation one hundred eighty degrees (180°), and the radiation transmitted through the slightly attenuated regions  122   a - 122   d  destructively interferes with radiation diffracting out from the edges of the isolated completely transmissive regions  114   a ,  114   f , thereby greatly increasing the resolution with which the isolated completely transmissive regions  114   a ,  114   f  define desired device features. Moreover, because slightly attenuated regions  122   a - 122   d  allow transmission of about 12% to about 20% of the incident radiation, the slightly attenuated regions  122   a - 122   d  serve to maximize depth-of-focus performance.  
         [0062]     As illustrated in drawing  FIG. 17 , a third patterned resist  128  is formed over the third intermediate mask structure  120  to create highly attenuated regions where desired. The third patterned resist  128  is created by first coating the third intermediate mask structure  120  with any suitable resist. The resist is then patterned by known methods to expose an area  130  of the third intermediate mask structure  120  wherein highly attenuated regions are to be created.  
         [0063]     Highly attenuated regions  132   a - 132   e  are then formed by selectively etching the opaque layer  106  in the exposed area  130  (see drawing  FIG. 18 ). Though the opaque layer  106  can be etched using any suitable etch process, where chromium is used as the opaque layer  106 , for example, a Cl 2 /O 2  plasma etch process or a suitable wet etch process may be used. After formation of the highly attenuated regions  132   a - 132   e , the third patterned resist  128  is stripped, leaving a complete APSM  134  according to the second embodiment of the APSM of the present invention (shown in drawing  FIG. 19 ).  
         [0064]     As was true in the first embodiment of the APSM of the present invention, the highly attenuated regions  132   a - 132   e  included in the second embodiment of the APSM  134  of the present invention are preferably formed at the edges of the closely spaced completely transmissive regions  114   b - 114   e . The one hundred eighty degree (180°) phase shift provided by the first attenuating layer  100  and/or the etch stop layer  102  and the high total attenuation provided by the combined attenuations of the first attenuating layer  100  and the second attenuating layer  104 , enhance the resolution of the images projected by the closely spaced completely transmissive regions  114   b - 114   e , while minimizing or eliminating any fabrication defects that may otherwise occur due to additive side lobes produced by the closely spaced completely transmissive regions  114   b - 114   e.    
         [0065]     Reference to drawing  FIG. 19  highlights that, even after formation of completely transmissive regions  114   a - 114   f , slightly attenuated regions  122   a - 122   d , and highly attenuated regions  132   a - 132   e , portions of the opaque layer  106  remain, forming opaque regions  140   a - 140   d . Again, opaque regions  140   a - 140   d  may be maintained on the finished APSM to prevent exposure to even attenuated radiation where attenuated radiation is not needed to increase image resolution. The second embodiment of the method of the present invention, therefore, also provides APSMs having completely transmissive regions, highly attenuated regions, slightly attenuated regions, and opaque regions, which work in concert to maximize image resolution and depth-of-focus for isolated features, while minimizing or eliminating any defects caused by additive ringing effects in areas of high feature density and preventing any defects caused by transmission of attenuated radiation where attenuated radiation is not needed to enhance resolution and depth-of-focus.  
         [0066]     Though the method and APSM of the present invention have been described and illustrated herein with reference to two different embodiments, such descriptions and illustrations do not limit the scope of the present invention. The method of the present invention and design of an APSM according to the present invention are highly adaptable. For example, the method disclosed herein can be used to fabricate APSMs having any desired feature pattern. Moreover, the steps of the method and composition of the APSMs can be modified in several aspects while still obtaining an APSM according to the present invention. For instance, the method of the present invention may utilize etching processes different from those discussed herein. Additionally, materials different than those described herein, such as, different substrate materials, different attenuating materials, different light blocking materials, or different etch stop materials, may be used in conjunction with the method of the present invention to fabricate APSMs according to the present invention having a different material composition than the APSMs according to the first and second embodiments. Therefore, the present invention is not to be defined or limited by the illustrative and descriptive examples provided herein, but, instead, the scope of the present invention is defined by the appended claims.