Patent Publication Number: US-2007111109-A1

Title: Photolithography scattering bar structure and method

Description:
BACKGROUND  
      Photolithography is a process used in semiconductor integrated circuit device fabrication to produce device structures on semiconductor or other substrates. Distortions of device structures are becoming evident in view of the shrinking of the dimensions of the device structures as compared to the radiation wavelengths used during photolithography. One source of distortion is due to light scattered or otherwise effected by adjacent structures. Distortion in size and shape of the projected image exhibited by this phenomenon is called proximity effect.  
      In optical proximity correction (OPC), a resolution enhancement technique using scattering bars has been introduced to counter proximity effects and to reduce distortion. Scattering bars are sub-resolution assist features (SRAF) that are placed on a mask (e.g., reticle or photo-mask) adjacent to isolated features and/or semi-isolated features. Isolated and semi-isolated design features, such as metal lines, trenches, or gate polysilicon, are generally exposed and/or printed on the substrate at a feature size significantly different from the same design feature surround by other nearby features. This phenomenon is known as an isolated/dense proximity effect. The use of scattering bars enables these isolated and/or semi-isolated design features to form more like dense features. In this manner, the usable resolution of an imaging system may be extended without decreasing the radiation wavelength or increasing a numerical aperture of the imaging tool, although such processes can be used for additional benefit.  
      Conventional scattering bars are narrow lines placed adjacent to existing design features. The scattering bars are parallel with the isolated feature, often with scattering bars placed on either side of an isolated feature. These types of scattering bars are commonly called edge scattering bars. Where there are semi-isolated features, for example two parallel lines spaced apart from one another, a center scattering bar is typically placed in parallel with and between the semi-isolated features. However, when the semi-isolated features are beyond a certain distance apart, the center scattering bar become too far spaced from the semi-isolated features and the benefit of using scattering bars significantly diminishes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.  
       FIG. 1  is a block diagram of a photolithography system that can benefit from one or more embodiments of the present invention.  
       FIG. 2  is a simplified graphical representation of an embodiment of a mask with vertical scattering bars;  
       FIG. 3  is a simplified graphical representation of an embodiment of a mask with center scattering bars;  
       FIG. 4  is a simplified graphical representation of an embodiment of a mask with edge scattering bars;  
       FIG. 5  is a simplified graphical representation of another embodiment of a mask with scattering bars;  
       FIG. 6  is a simplified graphical representation of yet another embodiment of a mask with scattering bars; and  
       FIG. 7  is a simplified flowchart of an embodiment of a method of adding and arranging scattering bars to a mask. 
    
    
     DETAILED DESCRIPTION  
      It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.  
      Sub-wavelength photolithography has presented new challenges to producing or printing features such as metal lines, trenches, polysilicon structures, and so-forth onto a substrate. These challenges include image distortion in the form of line-end shortenings, corner roundings, isolated/dense proximity effects, and adverse impacts on the depth of focus (DOF). Resolution enhancement technologies (RET) have been devised to extend the usable resolution of an imaging system without decreasing the wavelength of the light or increasing the numerical aperture of the imaging tool. RET includes phase-shifting masks, off-axis illumination (OAI), and optical proximity correction (OPC). The present disclosure provides new and unique scattering bars to make isolated and semi-isolated features of a mask print more like features in a dense area of the mask. The term scattering bars refer to both scattering bars and anti-scattering bars. The disclosure herein introduces scattering bars that are placed perpendicularly to existing isolated and semi-isolated features on a mask. The isolated and semi-isolated features are also referred to herein as “non-dense” features.  
      Referring to  FIG. 1 , the reference numeral  6  refers, in general, to a photolithography system that can benefit from one or more embodiments of the present invention. The photolithography system  6  includes a light source  7  for projecting a radiation  8  onto a substrate  9  through a mask  10 . Although not shown, various lenses can also be provided, as well as other light manipulating and/or transmitting devices. In furtherance of the present embodiment, the substrate  9  is a semiconductor wafer for receiving an integrated circuit pattern from the mask  10 . The patterns from the mask  10  will appear on a layer of the substrate  9 , thereby creating an integrated circuit device, or chip, when combined with other layers. The mask  10  includes a plurality of design features, some of which are located in densely populated areas of the mask, others of which are located in areas that are not as dense.  
      Referring now to  FIG. 2 , the mask  10  of  FIG. 1 , herein referred to as mask  10   a , includes a design feature  12  such as an isolated or semi-isolated, electrically connected metal line of an integrated circuit pattern. The design feature  12  is spaced far apart from other design features on one or both sides of the feature. The mask  10  also includes a first group of scattering bars  13  disposed proximate to and substantially perpendicular to the feature  12 . The scattering bars  13  can be either transparent or opaque, and for the sake of further example can be non-conducting (e.g., dummy) metal lines. The scattering bars  13  have a predefined width and pitch selected to enhance imaging of the feature  12 . For example, the scattering bars  13  may be extended as close to the design feature  12  as necessary for optimized imaging effect of the feature  12  during a lithography pattering process, while maintaining a predefined critical distance to the feature. Although the scattering bars  13  are illustrated as linear lines in  FIG. 1 , in alternative embodiments, the scattering bars  14  may be broken lines or other shapes. The scattering bars  13  may be disposed at various regions proximate the feature  12  and may be disposed in various groups, each having an individual width, pitch, and/or length.  
      In the present embodiment, the mask  10   a  includes a second plurality of scattering bars  14  disposed proximate the feature  12 . The scattering bars  14  are disposed substantially parallel with the feature  12 . The scattering bars  14  may be combined with the scattering bars  13  in various ways such as those examples illustrated in FIGS.  2  to  5 . For example, the scattering bars  14  may be disposed in one region and the scattering bars  13  may be disposed in another region of the mask  10   a . A procedure to place various perpendicular and/or parallel scattering bars may be rule-based with a set of predefined rules or model-based with various options including width, pitch, and/or other parameters for optimizing an imaging effect. Since vertical scattering bars (or assist features) are employed, scattering bars are capable of be disposed effectively such as with increased scattering bar area. In alternative embodiments, combination of perpendicular, parallel, and tilted scattering bars may be used, as desired.  
      Referring now to  FIG. 3 , in another embodiment, the mask  10  of  FIG. 1 , herein referred to as mask  10   b,  includes semi-isolated features  15 ,  16 ,  17  and two groups of center scattering bars  18 ,  19 . In the present example, semi-isolated features  15 - 17  are spaced apart and arranged generally parallel with one another. In conventional OPC, a single center scattering bar (CSB) would be centered and placed in parallel between adjacent semi-isolated features  15  and  16 , and another CSB would be placed between adjacent semi-isolated features  16  and  17 . However, this does not improve the DOF because the distance between the scattering bars and the existing design features is too large. Instead, a plurality of new scattering bars are formed perpendicular to the existing features  15 - 17 . As shown in  FIG. 2 , a first series of parallel scattering bars  18  is formed and placed inbetween and perpendicular to semi-isolated features  15  and  16 . A second series of parallel scattering bars  19  is placed inbetween and perpendicular to semi-isolated features  16  and  17 . The series of parallel scattering bars  18 ,  19  formed perpendicularly to the existing non-dense design features  15 - 17  creates a region of dense features to mitigate or eliminate proximity effects and improve the DOF.  
      Referring now to  FIG. 4 , in another embodiment, the mask  10  of  FIG. 1 , herein referred to as mask  10   c , includes an isolated feature  22  surrounded on both sides by edge scattering bars  24 ,  26 . In conventional OPC, two parallel edge scattering bars (ESBs) would be placed in parallel on each side of isolated feature  22 . According to the method described herein, a first series of parallel scattering bars  24  are placed adjacent and perpendicular to isolated feature  22  on one side, and a second series of parallel scattering bars  26  are placed adjacent and perpendicular to isolated feature  22  on the other side.  
      Referring now to  FIG. 5 , in another embodiment, the mask  10  of  FIG. 1 , herein referred to as mask  10   d , includes semi-isolated or non-dense design features  32 ,  34 ,  36 . In the present embodiment, non-dense features  32 - 36  may be spaced apart and arranged generally parallel with one another, although such an arrangement is not required. In this embodiment, scattering bars oriented parallel with the semi-isolated features as well as scattering bars oriented perpendicularly with the semi-isolated features are added to the mask design. A first series of parallel scattering bars  41  are formed and placed between and perpendicular to non-dense features  32  and  34 . A second series of parallel scattering bars  44  are placed in parallel between non-dense features  32  and  34 . On the other side of non-dense feature  34  are a series of parallel scattering bars  42  formed and placed perpendicularly to semi-isolated features  32  and  34 . On the same side of semi-isolated feature  34 , a second series of parallel scattering bars  46  are placed in parallel between non-dense features  34  and  36 . This example illustrates an embodiment in which parallel and perpendicular scattering bars may both be employed in OPC. These parallel and perpendicular scattering bars create a region of dense features that mitigates or eliminates proximity effects and improves the DOF.  
      Referring now to  FIG. 6 , in another embodiment, the mask  10  of  FIG. 1 , herein referred to as mask  10   e , includes semi-isolated or non-dense features  52 ,  53 ,  54 . Semi-isolated features  52 - 54  may be spaced apart and arranged generally parallel with one another. In this embodiment, scattering bars oriented parallel with the semi-isolated features as well as scattering bars oriented perpendicularly with the semi-isolated features are added to the mask design. A first series of parallel scattering bars  56  are formed and placed between and perpendicular to semi-isolated features  52  and  53 . A second series of parallel scattering bars  58  are placed in parallel with and between semi-isolated features  53  and  54 . This example illustrates an embodiment in which parallel and perpendicular scattering bars may both be employed in OPC. These parallel and perpendicular scattering bars create a region of dense features that mitigates or eliminates proximity effects and improves the DOF.  
       FIG. 7  is a simplified flowchart  60  of an embodiment of a method for adding scattering bars. The method  60  may be incorporated with one or more other OPC methods in processing an overall design for the mask  10  ( FIG. 1 ). In step  62 , a non-dense design feature is identified. This includes isolated and semi-isolated feature features already existing in the mask design. One way to identify a non-dense design feature is to have a minimum size for a scattering bar and to determine if such a scattering bar can be positioned between two design features while maintaining design rule requirements.  
      At step  64 , a plurality of scattering bars are formed perpendicular to the non-dense design feature on a first side. In some embodiments, a plurality of parallel scattering bars may be formed parallel with the non-dense design feature on the same side and combined with the perpendicular scattering bars. Various combination may be implemented using a rule-based method, model-based method, or other proper methods for optimized imaging of the non-dense design feature during a lithography patterning process. In another embodiment, the above described scattering bars may be disposed between two non-dense design features.  
      At step  66 , a second plurality of scattering bars may be formed on the second side of the non-dense design feature and placed perpendicularly with respect to the design feature. A set of parallel scattering bars may be collectively disposed on the second side of the non-dense design feature. This process may be repeated for each identified isolated and semi-isolated feature to increase the design density around these isolated and semi-isolated feature features. After all non-dense design features have been processed, the process ends in step  68 .  
      The method  60  only serves as an example as to how the perpendicular scattering bars are incorporated into a mask pattern, and it is understood that other methods may be used. For example, a region having non-dense design features may be identified and various perpendicular scattering bars and optional parallel scattering bars are disposed such that the imaging of the non-dense design features in the selected region is enhanced and optimized. The identified region may have a dimension to include at least portion of a non-dense design feature, one non-dense design feature, or a plurality of non-dense design features. For example, the identified region may include a round area having a predefined radius.  
      Using the scattering bars as described above, the lithography DOF is increased without introducing additional semiconductor fabrication steps. These additional assist features increase the DOF and resolution for isolated and semi-isolated feature features, and also reduce a mask error enhancement factor (MEEF). The manufacture of such assist features is also relatively easy, because the assist features are easily programmable with existing design-rule check (DRC) tools. It has been shown that the lithography process window increases approximately 20% when the perpendicular scattering bars are compared with the conventional parallel scattering bars. Depending on the application, the optimal width of the scattering bars, the optimal spacing between the scattering bars, and the optimal spacing between the scattering bar and the existing isolated or semi-isolated feature may be determined on a case-by-case basis.  
      Thus, the present disclosure provide many embodiments of masks, methods for making masks, photolithography systems, and devices produced by such systems.  
      In one embodiment, a photolithography mask includes a design feature located in an isolated or semi-isolated region of the mask and a plurality of parallel linear assist features disposed substantially perpendicular to the design feature. In some embodiments, the plurality of parallel linear assist features include a first series of parallel assist features disposed on a first side of the design feature and perpendicularly thereto, and a second series of parallel assist features disposed on a second side of the design feature and perpendicularly thereto.  
      In one embodiment, a method of forming a mask includes forming a first non-dense feature on the mask and forming a plurality of parallel assist features disposed substantially perpendicular to the at least one non-dense design feature.  
      In one embodiment, a device, such as a semiconductor device, includes at least one linear non-dense feature on a first layer of the semiconductor device and a plurality of parallel linear assist features on the first layer of the semiconductor device, disposed substantially perpendicular to the at least one linear non-dense feature.  
      Although embodiments of the present disclosure have been described in detail, those skilled in the art should understand that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure. For example, an assist feature can be created as a part of a previous design feature. More specifically, an assist feature can be a protrusion from a nearby design feature, arranged and positioned proximate to another non-dense design feature as in one of the embodiments listed above. Accordingly, all such changes, substitutions and alterations are intended to be included within the scope of the present disclosure as defined in the following claims.