Abstract:
An alternating phase shift reticle for a capacitor layout scheme for a memory device and a method for its fabrication is disclosed. The alternating phase shift mask has regions of 0 and 180 degree phase shifts arranged in a way such that all sides of each region corresponding to a given phase shift value are bounded by areas corresponding to an opposite phase shift value. The reticle can be used to produce densely packed capacitor features, in which the variance between the actual exposure pattern and the desired exposure pattern is reduced.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a photolithography reticle for use in manufacturing semiconductor devices, and more particularly to a reticle and method of making it, which can be used to sharpen the light used to expose a masking material and thereby improve the definition of fabricated features. 
     BACKGROUND OF THE INVENTION 
     Photolithography is commonly used in semiconductor device fabrication to pattern various photomasks used in processing a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and template such as a mask or reticle. During the exposure process, radiant energy, such as ultraviolet light, is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, forming thereafter a resist mask on the wafer. The resist mask can be used further to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes. 
     An integral component of the photolithographic process is the reticle. The reticle includes the pattern for passing, blocking, or phase shifting light to expose the photoresist which is used to form features (e.g., transistor or capacitor structures) at a particular layer of a semiconductor device. The reticle is typically a quartz plate coated with a patterned light blocking material such as, for example, chromium. This type of mask is typically called a binary mask because light is completely blocked by the light blocking material and fully transmitted through the unblocked quartz portions. 
     Binary masks pose various problems when fabricating dense circuits. Light passing through the edge of a pattern within the mask (e.g., the boundary between a light blocking region and a transparent region) is oftentimes diffracted. This means that instead of producing a very sharp image of the edge on the resist layer, some lower intensity light diffracts beyond the intended edge boundary and into the regions expected to remain dark. Hence, the resultant feature shapes and sizes deviate somewhat from the intended IC design. Because integrated circuit manufacturers have continued to reduce the geometric size of the IC features, this diffraction can produce wafers having dies with incomplete or erroneous circuit patterns. 
     Attenuated phase shift masks (PSMs) have been used to overcome the diffraction effects and to improve the resolution and depth of focus of images projected onto a target (i.e., the photoresist). Attenuated PSMs utilize partially transmissive regions in addition to the light blocking and light transmissive regions used in binary masks. The partially transmissive regions typically pass (i.e., do not block) about three to eight percent of the light they receive. Moreover, the partially transmissive regions are designed so that the light they do pass is shifted by 180 degrees in comparison to the light passing through the transparent (e.g., transmissive) regions. Thus, some of the light spreading outside of the transparent region defined by the PSM pattern edge destructively interferes with light passing from the partially transmissive regions. This way, the detrimental effects caused by diffraction can be controlled. 
     As it is known in the art, reticle layouts are generated for each photoresist layer that must be patterned. Each reticle layout includes a pattern for blocking/passing light which is designed to produce, through the exposed photoresist, corresponding circuit features. One such representative reticle is shown in FIG.  1 . This reticle is a 0-180 degree attenuated phase shift reticle used to produce patterned areas for etching an insulated layer, e.g. a BPSG layer, to produce wells for fabricating container capacitors. Portions of an original reticle layout have been modified by OPC (optical proximity correction) techniques to generate a modified layout so that if exposure were directed through a reticle having such a modified layout, the photoresist would be exposed in a pattern which includes features which more closely approximates the corresponding desired feature in a circuit layout. The modified layout may be generated using any known algorithms or by other techniques, for instance, using trial and error through experience with particular layouts. 
     An attenuated phase shift mask which has different regions of differing phase shift values may be made in a variety of ways. For example, the 0-180 degree phase shift mask, illustrated in FIG. 1, can be made by taking a substrate of a transparent material, such as quartz, having a thickness such that incident light passing through the layer is of the same phase as the light entering the layer (0 degree phase shift), and etching into the side of the quartz layer where light exits to a depth which will shift the phase of incident exposure by 180 degrees (relative to the 0 degree regions, i.e., the regions of the layer which are not etched) to produce the 180 degree phase shift regions. A chrome layer is also applied on the light entering side to block these portions of the quartz substrate where incident light should not pass through the substrate and a layer of partially transmissive material is also applied on the quartz layer at the light entry side, over the etched regions, to form the partially transmissive 180 degree phase shift regions. 
     This way, transmissive or transparent regions  12  of open quartz, the partially transmissive regions  14 , and the blocked regions combine to form a light pattern in a photoresist layer. The transparent regions  12  pass the light without a phase shift. The partially transmissive regions  14  pass only about six percent of the light they receive with a 180 degree phase shift. The material used to form the partially transmissive regions  14  is any suitable opaque material, for example, molybdenum silicide (Mo—Si) or chromium fluoride. A preferred material for use in making the transparent regions  12  of reticle  10  is quartz. However, any other suitable light transmissive material such as soda-lime glass, borosilicate glass, or other similar natural or synthetic materials can be used also. Light blocking regions are typically formed with a chrome layer on the quartz substrate. 
     Although the FIG. 1 attenuated phase shift reticle is adequate for many applications, as semiconductors sizes continue to decrease, the light pattern produced by it becomes an increasing problem. FIG. 2 is the aerial image response with a critical dimension contour (CD) of the printing image of a capacitor design formed with the attenuated phase shift reticle layout of FIG.  1 . FIG. 2 depicts different regions which correspond to different light intensities which are produced by the FIG. 1 reticle. The contour of the desired printing image is delineated by line  21 . When exposed in a positive photoresist, the area encapsulated within line  21  of FIG. 2, that is the area including zones  23 ,  25 , and  27 , is removed and an etch opening is formed in a photoresist. Unfortunately, the light intensity contours of adjacent areas in FIG. 2, such as regions  23  and  25  for example, are not sharply defined since at sub-micron levels, light is diffracted and affected by proximity effects. Accordingly, there is a blurring of light, or stated otherwise, a light transition region across the boundaries of the defined intensity regions  23 ,  25 , and  27 . 
     Proximity effects occur primarily when very closely spaced circuit pattern features are lithographically transferred to a resist layer on a wafer. The light waves of the closely spaced circuit features interact, thereby distorting the final transferred pattern features. Accordingly, features that are in close proximity to other features tend to be more significantly distorted than features which are relatively isolated from other features. 
     As a consequence of the unsharp profiles in light intensity from one region to the next, the edges of the developed photoresist pattern tend to be less well defined in these areas than in other areas of the masked pattern. In small, dense integrated circuits, such as VLSI, these blurred images can cause printing of features which may significantly degrade a circuit&#39;s performance, since the correspondence between the actual circuit design and the final transferred circuit pattern on the photoresist layer is decreased. Further, unsharp profiles can result in a loss of wafer surface area, which correspondingly reduces the total area available for deposited conductors and accordingly results in undesirable increase in contact resistance. 
     Accordingly, there is a need for a simplified phase shift reticle which can be used to precisely fabricate small circuit features, for example, closely spaced wells for container capacitors used in a memory circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides an alternating phase shift mask for a capacitor layout scheme for a memory device integrated circuit. The alternating phase shift mask has regions of 0 and 180 degree phase shifts arranged in a way such that all sides of each region corresponding to a given phase shift value are bounded by areas corresponding to an opposite phase shift value. 
     The present invention also provides a method for producing an alternating phase shift reticle having regions of 0 and 180 degree phase shifts arranged in a way such that all sides of each region corresponding to a given phase shift value are bounded by areas corresponding to an opposite phase shift value. The reticle can be used to produce densely packed capacitor features, in which the variance between the actual exposure pattern and the desired exposure pattern is reduced. The alternating phase shift reticle of the present invention counteracts the diffraction and proximal effects, while improving both the resolution and depth of focus of the transmitted light. 
     Additional advantages and features of the present invention will become more readily apparent from the following detailed description of the invention, which is provided in connection with accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an aerial image of a conventional attenuated phase shift reticle for fabricating capacitor container wells. 
     FIG. 2 illustrates the aerial light intensity image contours produced by the reticle layout of FIG.  1 . 
     FIG. 3 illustrates an alternating phase shift mask constructed in accordance with the invention. 
     FIG. 4 illustrates the aerial light intensity image contours produced by the reticle layout of FIG.  3 . 
     FIG. 5 illustrates a reticle undergoing an intermediate stage of processing according to the present invention. 
     FIG. 6 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  5 . 
     FIG. 7 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  6 . 
     FIG. 8 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  7 . 
     FIG. 9 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  8 . 
     FIG. 10 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  9 . 
     FIG. 11 illustrates a reticle undergoing an intermediate stage of processing according to the present invention at a point subsequent to that shown in FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
     The terms “wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in or on the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide. 
     Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 3 illustrates a portion of an alternating phase shift reticle according to a preferred embodiment of the present invention, which is adapted to form a layout or design for container capacitors in a memory device, e.g. a DRAM, at a particular level of fabrication. 
     Referring to FIG. 3, the phase shift reticle  50  includes a transparent substrate  52  (FIGS. 5-8) made of quartz or glass. The transparent substrate  52  is partitioned into a plurality of triangular regions, spaced apart in a predetermined ordered, and arranged in a matrix array in a way to define 0-degree phase shift regions  54  and 180-degree phase shift regions  56 . Each of the 0-degree phase shift regions is bounded on all sides by 180-degree phase shift regions. Similarly, each of the 180-degree phase shift regions is bounded on all sides by 0-degree phase shift regions. 
     For example, as illustrated in FIG. 3, 0-degree phase shift area ABC is completely bounded by three 180-degree phase shift areas, identified as CAD, ABF, and CBE, respectively. In turn, 180-degree phase shift area CDA is completely bounded by three 0-degree phase shift areas, which are identified as ABC, CDG, and ADH, respectively. 
     FIG. 4 illustrates the aerial light intensity image response of the alternating phase shift reticle of the present invention, shown in FIG.  3 . The contour of the printing image is delineated by line  31 . Similar to the aerial image response of the prior art, which was shown in FIG. 2, regions  33 ,  35 , and  37  of FIG. 4 define the areas that will subsequently be removed to form a well or hole for a capacitor layout. 
     A comparison between the light intensity contours of FIG.  2  and those of FIG. 4 reveals an increased and sharper contrast between different zone areas corresponding to different light intensities, such as for example zones  25  and  35 . The edges and corners of the pattern of FIG. 4 are also more sharply defined than those of the pattern corresponding to FIG.  2 . Thus, the rounding effects that characterized the FIG. 1 reticle are less pronounced when using the reticle of the present invention. Accordingly, the reticle of the invention provides an improved correspondence between the original circuit design and the pattern transferred to the photoresist, affords a more reliable print of smaller and more densely packed IC features, and provides a wider latitude for reticle misalignment. 
     The present invention provides an alternating phase shift mask with 0-degree phase shift regions bounded all around by 180-degree phase shift regions, and with 180-degree phase shift regions bounded all around by 0-degree phase shift regions. Although the present invention has been described with reference to triangular alternating phase shift regions, it is to be understood that modifications can be made to the invention and equivalents substituted for described and illustrated structures without departing from the spirit and scope of the invention. For example, the alternating phase shift regions may have a rectangular or other suitable shape, as long as the sides of all 0-degree phase shift areas are bounded by 180-degree areas and all sides of 180-degree phase shift areas are bounded by 0-degree phase shift areas. 
     The method for fabricating a reticle according to the present invention will now be described with reference to FIGS. 5-11. Reference is first made to FIG. 5. A material layer  54  is deposited over a reticle transparent substrate  52 , the latter of which may be formed of silica glass, fused quartz glass, borosilicate glass or another material transparent to various types of radiation commonly used in semiconductor lithographic operations, by any conventional method. Material layer  54  may be either a partially light transmissive layer or an opaque layer, depending on other features which are to be created using the reticle, a portion of which is illustrated in FIGS. 5-11. For a partially light transmissive layer, layer  54  may be an attenuating material such as a molybdenum suicide. For an opaque layer, layer  54  may use materials such as chrome, aluminum, iron oxide, gold, or tungsten, to name just a few. Since chrome is most widely used, and for simplification of the method steps, reference will be made hereinafter to layer  54  as being chrome layer  54 . However, it should be understood that those skilled in the art will recognize that a light transmissive material could also form layer  54 . 
     Layer  54  of FIG. 5 may be deposited onto transparent substrate  52  by conventional processes such as sputtering, chemical vapor deposition (CVD) or electron beam deposition (EBD). 
     Next, as shown in FIG. 6, a pattern layer  55  is then deposited over chrome layer  54 . Pattern layer  55  may be made of any material used to transfer a pattern to a subsequent layer and will depend upon the radiation characteristics of the equipment used in subsequent steps. For example, where an electron beam direct write system is used, pattern layer  55  will be formed of an electron beam sensitive photoresist. Alternatively, where an optical system is used to generate radiation of a particular wavelength, pattern layer  55  will be a conventional photoresist material. 
     Reference is now made to FIG.  7 . After preparing reticle substrate  52  with chrome layer  54  and pattern layer  55 , pattern layer  55  is exposed to radiation by a scanning electron beam or light, such as from a laser. Radiation emerging from a radiation source is imaged onto pattern layer  55 . The imaging process results in the transfer of 180-degree phase shift region pattern to pattern layer  55 . This way, the 180-degree region pattern is exposed. This step in the fabrication process, at which 180-degree regions are patterned, is referred to in the art as the “first write.” 
     The pattern layer  55  is written with an electron beam direct write system and the 180-degree region pattern is developed to arrive at the structure illustrated in FIG.  7 . As mentioned before, while the transfer of pattern will typically use an electron beam direct write system, it is also possible to perform pattern transfer using an optical imaging process using radiation having a wavelength ranging from the deep-UV to about 200 nanometers to optical wavelengths up to about 440 nanometers. 
     Reference is now made to FIG.  8 . After preparing reticle substrate  52  with chrome layer  54  and pattern layer  55 , the pattern layer  55  is then developed to obtain areas  68  (future 180-degree regions) on reticle substrate  52  and chrome layer  54 . As shown in FIG. 8, areas  66  (future 0-degree regions) have portions of pattern layer  55  on them, while areas  68  (future 180-degrees regions) have only chrome on them, from chrome layer  54 . 
     Next, the structure of FIG. 8 is placed in a high density plasma etcher and etched into the quartz to the desired depth to obtain 180-degree phase shift regions  88 , as shown in FIG.  9 . Areas  66 , containing the remaining pattern layer  55 , are written and developed during a process step generally known as the “second write.” At the end of the second write, remaining resist layer of areas  66  is removed and the structure of FIG. 10 is obtained. Next, remaining portions  76  (FIG. 10) of chrome layer  54  are etched off, producing 0-degree phase shift regions  86  and 180-degree phase shift regions  88  on reticle substrate  52 , as illustrated in FIG.  11 . Each of the 0-degree phase shift regions  86  is bounded by 180-degree phase shift regions  88 , and each of the 180-degree phase shift regions  88  is bounded by 0-degree phase shift regions  86 . 
     The reticle of FIG. 11 can now be used to produce finely spaced features, such as capacitor wells, in an insulating layer of a semiconductor device. 
     Although exemplary embodiments of the present invention have been described, it is not intended that the present invention be limited to the illustrated embodiments. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.