Abstract:
A photoresist mask used in the fabrication of an integrated circuit is described. This mask can include a first portion having a phase characteristic; a second portion being located proximate the first portion and having the same phase characteristic as the first portion; and a segment disposed between the first portion and the second portion to prevent phase conflict between the first portion and the second portion.

Description:
FIELD OF THE INVENTION  
         [0001]    The present specification relates generally to the field of integrated circuits and to methods of manufacturing integrated circuits. More particularly, the present specification relates to a dark field trench in an alternating phase shift mask to avoid phase conflict.  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to put millions of devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC.  
           [0003]    One limitation to the smallness of IC critical dimensions is conventional lithography. In general, projection lithography refers to processes for pattern transfer between various media. According to conventional projection lithography, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film or coating, the photoresist. An exposing source of radiation (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern.  
           [0004]    Exposure of the coating through a photomask or reticle causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.  
           [0005]    Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation.  
           [0006]    Conventional projection lithographic processes are limited in their ability to print small features, such as, contacts, trenches, polysilicon lines or gate structures. As such, the critical dimensions of IC device features, and, thus, IC devices, are limited in how small they can be.  
           [0007]    The ability to reduce the size of structures, such as, shorter IC gate lengths depends, in part, on the wavelength of light used to expose the photoresist. In conventional fabrication processes, optical devices expose the photoresist using light having a wavelength of 248 nm (nanometers), but conventional processes have also used the 193 nm wavelength. Further, next generation lithographic technologies may progress toward a radiation having a wavelength of 157 nm and even shorter wavelengths, such as those used in EUV lithography (e.g., 13 nm).  
           [0008]    Phase-shifting mask technology has been used to improve the resolution and depth of focus of the photolithographic process. Phase-shifting mask technology refers to a photolithographic mask which selectively alters the phase of the light passing through certain areas of the mask in order to take advantage of destructive interference to improve resolution and depth of focus. For example, in a simple case, each aperture in the phase-shifting mask transmits light 180 degrees out of phase from light passing through adjacent apertures. This 180 degree phase difference causes any light overlapping from two adjacent apertures to interfere destructively, thereby reducing any exposure in the center “dark” comprising an opaque material, such as chrome.  
           [0009]    An exemplary phase-shifting mask  10  is illustrated in FIG. 1. Phase-shifting mask  10  includes a transparent layer  12  and an opaque layer  14 . Opaque layer  14  provides a printed circuit pattern to selectively block the transmission of light from transparent layer  12  to a layer of resist on a semiconductor wafer. Transparent layer  12  includes trenches  16  which are etched a predetermined depth into transparent layer  12 . The light transmitted through transparent layer  12  at trenches  16  is phase-shifted 180 degrees from the transmission of light through other portions of phase-shifting mask, such as portions  18 . As the light travels between phase-shifting mask  10  and the resist layer of a semiconductor wafer below (not shown), the light scattered from phase-shifting mask  10  at trenches  16  interferes constructively with the light transmitted through phase-shifting mask  10  at portions  18 , to provide improved resolution and depth of focus.  
           [0010]    As mentioned, various different wavelengths of light are used in different photolithographic processes. The optimal wavelength of light is based on many factors, such as the composition of the resist, the desired critical dimension (CD) of the integrated circuit, etc. Often, the optimal wavelength of light must be determined by performing a lithography test with photolithographic equipment having different wavelengths. When a phase-shifting mask technique is utilized, two different phase-shifting masks must be fabricated, each mask having trenches  16  suitable for phase-shifting light of the desired wavelength. The fabrication of phase-shifting masks is costly. Further, comparison of the effect of the two different wavelengths printing processes is difficult and requires complex software processing to provide a suitable display.  
           [0011]    One difficulty in using phase-shifting mask technologies is phase conflict. Phase conflict arises when two separate areas on a phase-shifting mask have the same phase shift characteristic and are so close in proximity that there is a bridging between the two areas. Bridging, or the effective photo-connection of two separate areas in the mask, results in a less than accurate mask. As such, phase-shifting masks are designed to avoid proximity of areas where the light will have the same phase going through both areas. This design constraint can limit the size and complexity of the phase-shifting mask, and, thus, the pattern on the IC.  
           [0012]    Thus, there is a need for an improved phase-shifting mask. Further, there is a need for avoiding phase conflict issues in phase shift masks. Further still, there is a need for a dark field trench in an alternating phase shift mask having a high transmittance area to avoid phase conflict.  
         SUMMARY OF THE INVENTION  
         [0013]    An exemplary embodiment relates to a photoresist mask used in the fabrication of an integrated circuit. This mask can include a first portion having a phase characteristic, a second portion being located proximate the first portion and having the same phase characteristic as the first portion, and a segment disposed between the first portion and the second portion to prevent phase conflict between the first portion and the second portion.  
           [0014]    Another exemplary embodiment relates to a photoresist mask configured for use in an integrated circuit fabrication process. This mask can be made by a method including depositing a phase shift material over an opaque layer, and selectively removing the phase shift material except at a location between two phase shift mask portions having the same phase characteristic.  
           [0015]    Another exemplary embodiment relates to a phase shifting mask. This phase shifting mask can include a first section with an alternating phase shift characteristic, a second section which is proximate to the first section and has the same alternating phase shift characteristic as the first section, and a third section with a high transmittance attenuating phase shift characteristic being formed at the location of the potential phase conflict section. A potential phase conflict section is located between the first section and the second section.  
           [0016]    Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The exemplary embodiments will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and:  
         [0018]    [0018]FIG. 1 is an exemplary conventional phase-shifting mask;  
         [0019]    [0019]FIG. 2 is a top view of an exemplary phase-shifting mask illustrating phase conflict;  
         [0020]    [0020]FIG. 3 is a top view of an exemplary phase-shifting mask in accordance with an exemplary embodiment;  
         [0021]    [0021]FIG. 4 is a top view of a phase-shifting mask with an area of phase conflict;  
         [0022]    [0022]FIG. 5 is a cross-sectional view of the phase-shifting mask illustrated in FIG. 4 about the line  4 - 4 ;  
         [0023]    [0023]FIG. 6 is a cross-sectional view of the phase-shifting mask illustrated in FIG. 4 about the line  5 - 5 ;  
         [0024]    [0024]FIG. 7 is a cross-sectional view of the phase-shifting mask of FIG. 3 about the line  4 - 4 , illustrating a phase shifting material deposition step in an exemplary method of making the phase-shifting mask;  
         [0025]    [0025]FIG. 8 is a cross-sectional view of the phase-shifting mask of FIG. 3 about the line  4 - 4 , illustrating an etching step in an exemplary method of making a phase-shifting mask;  
         [0026]    [0026]FIG. 9 is a cross-sectional view of the phase-shifting mask of FIG. 3 about line  5 - 5 , illustrating a phase shifting material deposition step in an exemplary method of making a phase shifting mask;  
         [0027]    [0027]FIG. 10 is a cross-sectional view of the phase-shifting mask of FIG. 3 about line  5 - 5 , illustrating an etching step in an exemplary method of making a phase-shifting mask;  
         [0028]    [0028]FIG. 11 is a cross-sectional view of the phase-shifting mask of FIG. 3 about line  4 - 4 , illustrating a 180 degree phase area formation step;  
         [0029]    [0029]FIG. 12 is a cross-sectional view of the phase-shifting mask of FIG. 3 about line  5 - 5 , illustrating a 180 degree phase area formation step;  
         [0030]    [0030]FIG. 13 is a top view of an exemplary phase-shifting mask having a 180 degree phase area; and  
         [0031]    [0031]FIG. 14 is a cross-sectional view of the phase-shifting mask of FIG. 13 about line  6 - 6  after an etching step. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0032]    Referring to FIG. 2, a top view of a conventional phase shifting mask  20  illustrates a phase shift area  22 , a phase shift area  24 , and a phase shift area  26 . Phase shift area  22 , phase shift area  24 , and phase shift area  26  are separated by portions of a chrome layer  28 . As described with reference to FIGS.  4 - 5  below, phase shift area  22 , phase shift area  24 , and phase shift area  26  are defined by removed portions of chrome layer  28 , exposing portions of a quartz layer.  
         [0033]    In an exemplary embodiment, phase shift area  22  and phase shift area  24  have a phase shift characteristic of phase 0° and phase shift area  26  has a phase shift characteristic of phase 180°. Phase shift area  22  and phase shift area  24  are separated only by a small area  23  of chrome in chrome layer  28 . In an exemplary embodiment, small area  23  is a distance of 0.16 μm separating phase shift area  22  and phase shift area  24 . The separation distance used is dependent on the design rule employed for a specific integrated circuit design. This small distance results in a potential phase conflict where bridging may occur with respect to the light waves passing through phase shift area  22  and phase shift area  24 . As discussed above, phase conflict results in less accurate lithographic operations using phase shifting mask  20 .  
         [0034]    [0034]FIG. 3 illustrates a top view of a phase shifting mask  30 . Phase shifting mask  30  can include a phase shift area  32 , a phase shift area  34 , a phase shift area  36 , and a phase attenuating segment  38 . Phase shift area  32 , phase shift area  34 , and phase shift area  36  are defined by removed portions of a chrome layer  39 . Layer  39  can be chrome oxide or other absorbing opaque material. Removed portions of chrome layer  39  expose portions of a quartz layer described further with reference to FIGS.  5 - 12  below.  
         [0035]    In an exemplary embodiment, phase shift area  32  and phase shift area  34  have a phase shift characteristic of phase 0° and phase shift area  36  has a phase shift characteristic of phase 180°. Phase attenuating segment  38  is located on top of chrome layer  39  between phase shift area  32  and phase shift area  34  in order to prevent phase conflict. Phase attenuating segment  38  can be a high transmittance attenuating material. In an exemplary embodiment, phase attenuating segment  38  is made of a molybdenum silicon (MoSi) material.  
         [0036]    Advantageously, phase attenuating segment  38  attenuates a portion of the light waves in order to prevent phase conflict between phase shift area  32  and phase shift area  34 . In an exemplary embodiment, phase attenuating segment  38  attenuates 20-40% of transmitted light. Phase conflict can occur between any closely located phase shifting areas that have the same phase shift characteristic. Phase attenuating segment  38  helps to prevent any bridging in the light waves passing through phase shifting mask  30 . Thus, the photo margin in this area of potential conflict is improved. The area between phase shift area  32  and phase shift area  34  where phase attenuating segment  38  is located can be called a dark field trench layer.  
         [0037]    [0037]FIG. 4 illustrates a top view of a phase-shifting mask  40 . Phase-shifting mask  40  can include a phase shift area  42 , a phase shift area  44 , and a phase shift area  46 . Phase shift area  42 , phase shift area  44 , and phase shift area  46  are defined by removed portions of a chrome layer  49 . Layer  49  can be chrome oxide or any of a variety of absorbing opaque materials.  
         [0038]    Phase-shifting mask  40  differs from phase-shifting mask  30  described with reference to FIG. 3 in that phase shift area  42  and phase shift area  44  have merged into one area because there is a phase conflict in area  38  described with reference to FIG. 3. It is desirable to avoid phase conflict from causing this merging.  
         [0039]    [0039]FIG. 5 illustrates a cross-sectional view of a portion  50  of phase-shifting mask  40  described with reference to FIG. 4. Portion  50  includes a quartz layer  52  and a chrome layer  54 . FIG. 6 illustrates a cross-sectional view of a portion  60  of phase-shifting mask  40  described with reference to FIG. 4. Portion  60  is shown in a cross-sectional view about line  5 - 5  in FIG. 4. Portion  60  includes a quartz layer  62  and a chrome layer  64 .  
         [0040]    [0040]FIG. 7 illustrates an exemplary step in a method of making phase-shifting mask  30  described with reference to FIG. 3. In an exemplary embodiment, a layer of molybdenum silicon (MoSi) or any other phase shifting material is deposited over chrome layer  54  and quartz layer  52  of portion  50  described with reference to FIG. 5. Phase shifting material layer  72  is coated with an e-beam resist or a photoresist and patterned to form a photoresist feature  74 . A variety of machines may be employed to provide a coating of e-beam resist, such as, ETCT&#39;s MEBES-4500 or MEBES-X, Toshiba EBM-3500, and JEOL JBX-9000MV. Alternatively, photoresist may be deposited utilizing a machine, such as, an optical machine such as ETCT&#39;s ALTA-3700. In an exemplary embodiment, phase shifting material layer  72  is etched using photoresist feature  74  as a pattern and the resist layer is stripped, forming a phase shifting material feature  82  illustrated in FIG. 8.  
         [0041]    [0041]FIG. 9 illustrates an exemplary step and a method of making phase-shifting mask  30  described with reference to FIG. 3. FIG. 9 illustrates portion  60  described with reference to FIG. 6 as a cross-sectional view about line  5 - 5  in FIG. 3. In an exemplary embodiment, a phase shifting material layer  92  is deposited over chrome layer  64  and quartz layer  62 . A resist feature  94  is formed over phase shifting material layer  92  to pattern phase shifting material  92 . Any of a variety of techniques may be utilized to pattern phase shifting material  92 . FIG. 10 illustrates portion  60  after a patterning step is performed to form a phase shifting material feature  1002 . FIGS. 9 and 10 illustrate the same steps as shown in FIGS. 7 and 8. Phase shifting material feature  82  described with reference to FIG. 8 and phase shifting material feature  1002  described with reference to FIG. 10 correspond to phase attenuating segment  38  described with reference to FIG. 3.  
         [0042]    [0042]FIG. 11 illustrates an exemplary step in formation of a 180 degree phase area in a phase-shifting mask. In an exemplary embodiment, an e-beam resist or photoresist layer  1112  is deposited and patterned selectively in order to expose a portion  1114 . FIG. 12 also illustrates deposition of photoresist layer  1112 . Exposed portion  1114  is then subjected to an etching or removal process to form a trench in the quartz layer. Such a trench results in a phase shifting.  
         [0043]    [0043]FIG. 13 illustrates a phase-shifting mask  1300 . Phase-shifting mask  1300  is similar to phase-shifting mask  30  described with reference to FIG. 3 with the exception that all of phase-shifting mask  1300  except portion  1302  is covered by photoresist layer  1112  described with reference to FIGS. 11 and 12. FIG. 14 illustrates an etching step performed in which portion  1302  of phase-shifting mask  1300  described with reference to FIG. 13 is etched to form a phase 180 degree area.  
         [0044]    While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. The invention is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations that nevertheless fall within the scope and spirit of the appended claims.