Patent Publication Number: US-7718348-B2

Title: Photolithography process and photomask structure implemented in a photolithography process

Description:
REFERENCE TO CO-PENDING APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/894,924, filed on Jul. 20, 2004 which is now U.S. Pat. No. 7,435,512. 
    
    
     RELATED APPLICATION 
     This divisional patent application claims the priority benefit of Taiwan Patent Application No. 092135864, filed on Dec. 17, 2003. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a semiconductor process, and more particularly to a photolithography process and a photomask structure that can improve the dimensional transfer of pattern images from a photomask onto a substrate in the photolithography process. 
     2. Description of the Related Art 
     In the manufacture of a semiconductor device, a photolithography process is commonly conducted to transfer or image specific patterns predefined on a photomask to a semiconductor wafer or substrate. For this purpose, a photosensitive material (also called photoresist) is initially formed on the substrate. The photoresist layer can be either a positive photoresist or negative photoresist layer depending on whether an assigned region is to be removed after development. Then, the photoresist layer is exposed through the photomask to specific light radiation, so that the pattern defined on the photomask is imaged on the photoresist layer. The radiation used during exposure can include deep ultraviolet (DUV), X rays, electronic beams, ion beams or the like. After the exposure is completed, the substrate is processed to develop the transferred pattern and thereby form a patterned photoresist layer through which the substrate is etched to form the desired circuit pattern. 
     Since the circuit patterns become increasingly smaller, a photolithography process of higher precision is required. By reducing the wavelength of the exposure light in the photolithography process, small active devices and transistors can be realized by establishing small critical dimension. The critical dimension of the circuit pattern is defined as the smallest pattern line width. 
     A photomask conventionally includes one or more circuit patterns. The photomask can be of positive or negative type depending on whether the pattern images formed on the photomask are opaque or not. The negative photomask is usually preferred because light scattering in the negative photomask is in a smaller amount and particles falling in opaque regions are less likely to be developed. When light strikes fine particles, the particles absorb light energy. A part of the light energy is absorbed and becomes an internal energy, while the other part of the light energy emerges out under the form of light scattering. Light scattering is one issue to be solved in optical exposure. During the exposure, the particles contained in the photomask scatter light so that the critical dimension of the photoresist pattern formed on the substrate is biased with from the dimension set in the photomask pattern. 
       FIG. 1  is a schematic view of a conventional photomask. A conventional photomask  100  can include a first pattern  110  and a second pattern  120  of different sizes and an opening area  130 . The opening area  130  is located between the first pattern  110  and the second pattern  120  and surrounds the second pattern  120 . In  FIG. 1 , the first pattern  110  and the second pattern area  120  exemplary include line-shaped patterns and have similar line widths  112 ,  122  and inter-line pitches  114 ,  124 , but the first pattern  110  occupies an area larger than the second pattern  120 . In other words, the first pattern  110  has higher pattern density than that of the second pattern  120 , i.e. the light transmission rate is higher through the first pattern  110  than through the second pattern  120 . 
     During the exposure, the photomask  100  is placed on a semiconductor substrate having a photoresist layer thereon, and a light radiation is projected through the photomask  100  onto the substrate. The opening area  130  may cause light scattering around the second pattern area  120  of the photomask  100 , which may result in the accumulation of light energy in the underlying photoresist layer. 
       FIG. 2  is a cross-sectional view of a pattern image obtained by the conventional photolithography process. After development, first and second photoresist pattern images  210 ,  220  are formed on the semiconductor substrate  10  corresponding to the first and second pattern  110 ,  120  of the photomask  100  as illustrated in  FIG. 1 . As shown in  FIG. 2 , the photoresist pattern image  220  formed on the substrate  10  has dimensions biased from those of the corresponding second pattern  120  set in the photomask  100 . Reference numeral  222 ′ indicates the target line width as set in the photomask, while reference numeral  222  indicates the actual line width obtained after development. As shown, the line width  222  of the photoresist pattern image  220  is smaller than the line width  212  of the first photoresist pattern image  210 . Since the first and second patterns  110 ,  120  are dimensionally configured with a same line width in the photomask  100 , the pattern image of the second pattern  120  thus has been dimensionally biased in the photolithography process. 
     Conventionally, the pattern of the photomask having a smaller pattern density is configured beforehand to compensate the critical dimension biases occurring when pattern images of different densities are formed. However, this preliminary compensation becomes difficult to achieve as the critical dimensions of the semiconductor devices are increasingly reduced. 
     Therefore, a need presently exists for a photolithography process that can prevent the dimensional biases of pattern images formed in a photolithography process. 
     SUMMARY OF THE INVENTION 
     The application describes a photolithography process using a photomask that can prevent critical dimension biases of pattern images of different pattern densities. A circuit pattern thereby can be formed without the need of a conventional compensation process. 
     In one embodiment, the photolithography process includes the following steps. A photoresist layer is formed over a substrate. A photomask is aligned over the substrate, wherein the photomask includes a first pattern, a second pattern and a dummy pattern around the second pattern. The substrate is exposed to a light radiation through the photomask. The photoresist layer then is developed to form images of the first and second patterns in the photoresist layer. The dummy pattern is configured to allow light transmission in a substantially small amount so that the dummy pattern is not imaged while the first and second patterns are transferred on the photoresist layer during exposure. 
     In one embodiment, the dummy pattern is dimensionally configured to include a pattern density substantially smaller than a pattern density of the first and second patterns. In some embodiments, a line width and an inter-line pitch of the dummy pattern are substantially small so that the dummy pattern is not imaged while the first and second patterns are transferred on the photoresist layer during exposure. In a variant embodiment, the dummy pattern is placed at a distance from the second pattern to prevent image bias of the second pattern. 
     The foregoing is a summary and shall not be construed to limit the scope of the claims. The operations and structures disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the invention, as defined solely by the claims, are described in the non-limiting detailed description set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a conventional photomask; 
         FIG. 2  is a cross-sectional view of pattern images obtained by a conventional photolithography process; 
         FIG. 3  is a planar view of a photomask according to one embodiment of the invention; 
         FIG. 4  is a planar view of a photomask according to another embodiment of the invention; 
         FIG. 5  is a planar view of a photomask according to another variant embodiment of the invention; 
         FIG. 6  is a cross-sectional view of a substrate provided with photoresist pattern images obtained by a photolithography process using a photomask according to an embodiment of the invention; and 
         FIG. 7  is a flowchart of a photolithography process implemented according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT(S) 
     In a photolithography process, a photomask carries patterns of different sizes and shapes to be transferred on a photoresist layer laid on a substrate. Due to optical scattering occurring in the exposure step, the critical size of the pattern images defined in the photoresist layer might experience undesirable biases. 
       FIG. 3  is a schematic view of a photomask that can alleviate the foregoing problems in a photolithography process according to an embodiment of the invention. The photomask  300  includes at least a first pattern  310 , a second pattern  320 , and a dummy pattern  340  around the second pattern  320 . It is understood that the pattern shown in  FIG. 3  is a simplified representation used for purpose of illustration and practically can be embodied in many forms and shapes. For the purpose of clarification, “line width” herein refers to either a transverse dimension of a photoresist pattern image or the corresponding pattern dimension defined in the photomask. “Inter-line pitch” means the distance between two pattern lines. 
     In the illustrated embodiment, the first and second patterns  310 ,  320  include linear patterns, and the second pattern  320  occupies an area smaller than the first pattern  310 . The linear first pattern  310  includes lines respectively having a line width and an inter-line pitch that are respectively equal to the line width and inter-line pitch of the linear pattern  320 . In other words, the first and second patterns  310 ,  320  have different pattern densities that result in differential light transmission rates in their respective areas, i.e. the light transmission rate is higher through the first pattern  310  than through the second pattern  320 . 
     The photomask  300  can be made of adequate materials such as quartz. According to the design demand, the photomask  300  can be made of a transparent or opaque base material. In an embodiment where the photomask  300  is made of a transparent base material, the first and second patterns  310 ,  320  are opaque. Conversely, if the photomask  300  is made of an opaque base material, the first, second patterns  310 ,  320  are transparent. The first and second patterns  310 ,  320  can exemplary include circuit patterns implemented to form an integrated circuit device. 
     As shown in  FIG. 3 , a dummy pattern  340  is formed surrounding the second pattern  320  of lower light transmission rate. The dummy pattern  340  occupies an area larger than the second pattern  320 . The dummy pattern  340  can be formed in the shape of straight lines, holes, or the like. In the illustrated embodiment, the dummy pattern  340  includes a plurality of opaque straight lines spaced apart from one another and parallel to the linear second pattern  320 . 
     The dummy pattern  340  is configured to allow light transmission without being transferred during exposure of the photolithography process. In an embodiment, the inter-line pitch and the line width of the linear dummy pattern  340  can be set smaller than those of the second pattern  320 . The pattern density of the dummy pattern  340  has a light transmission rate sufficiently low to prevent its transferring during exposure. The dummy pattern  340  is spaced apart from the second pattern at a sufficient distance to prevent a biased image of the second pattern. 
       FIG. 4  illustrates a photomask pattern according to a variant embodiment of the invention. The photomask  300  at least includes a first pattern  310 , a second pattern  320  and a dummy pattern  350  around the second pattern  320 . The first pattern  310  and the second pattern  320  include lines having similar line width and inter-line pitch. The dummy pattern  350  surrounds the second pattern  320  and occupies an area larger than that of the second pattern  320 . The dummy pattern  350  includes a plurality of spaced opaque straight lines, but it is understood that diverse shapes of the dummy pattern  350  may be adequate. In the illustrated example, the direction of the linear dummy pattern  350  is approximately orthogonal to that of the linear second pattern  320 . The dummy pattern  350  is dimensionally configured to allow light transmission without being imaged during exposure. To this end, the inter-line pitch and the line width of the linear dummy pattern  340  are set smaller than those of the second pattern  320 . 
       FIG. 5  illustrates another variant embodiment of the invention. A photomask  400  is constructed from a base substrate of opaque material and includes a first pattern  410 , a second pattern  420  and a dummy pattern  440 . The patterns  410 ,  420 ,  440  are formed by gap structures such as slits or holes allowing light transmission. The first pattern  410  occupies an area larger than the second pattern  420 , but has the same line width and the same inter-line pitch as the second pattern  420 , i.e. the first and second patterns  410 ,  420  have different pattern densities that result in a light transmission rate higher through the first pattern  410  than through the second pattern  420 . The dummy pattern  440  surrounds the second pattern  420  and occupies an area larger than that of the second pattern  420 . The dummy pattern  440  can include spaced lines. In this embodiment, the direction of the linear dummy pattern  440  is orthogonal to that of the second pattern  420 . The line width and the inter-line pitch of the dummy pattern  440  are smaller than those of the second pattern  420  so that the dummy pattern  440  is not transferred during exposure. 
       FIG. 7  is a flowchart of a photolithography process implemented according to an embodiment of the invention. Initially, a photoresist layer is formed over a surface of a substrate ( 702 ). The photoresist layer can be either a negative or positive photoresist layer formed by spin-coating. A photomask then is aligned over the substrate ( 704 ). The photomask includes first and second patterns of different pattern densities to be transferred on the photoresist layer of the substrate, and a dummy pattern surrounding the second pattern of lower pattern density. The configuration of the dummy pattern can be as described in the previous embodiments. An exposure then is performed by irradiating a light beam through the photomask ( 706 ). The photoresist layer then is developed to form a photoresist pattern ( 708 ). 
       FIG. 6  is a cross-sectional view of photoresist pattern images obtained by a photolithography process using a photomask according to an embodiment of the invention. The substrate  600  includes a first pattern image  610  and a second pattern image  620  both of which are developed into a photoresist layer. By using a photomask configured with a dummy pattern according to the invention, the pattern line width  622  of the pattern image  620  and the pattern line width  612 ,  622  of the first and second pattern image  610 ,  620  are similar to the original pattern dimensions set in the photomask. Feature biases between patterns of different densities, i.e. of different light transmission rates, can be thereby advantageously overcome. 
     As described above, the dummy pattern set in the photomask is configured to allow reduced light transmission at an adjacent area surrounding the second pattern so that the dummy pattern is not transferred during exposure. This reduced light transmission reduces light scattering at the area around the second pattern, which promotes optical accumulation at the areas of the first and second pattern of the photomask. The developed pattern images on the photoresist layer thus can have critical dimensions consistent with the preset dimensions of the photomask patterns, and no compensation process thus is needed. As a result, the manufacture time and manufacture cost can be reduced. 
     Realizations in accordance with the present invention have been described in the context of particular embodiments. These embodiments are meant to be illustrative and not limiting. Many variations, modifications, additions, and improvements are possible. Accordingly, plural instances may be provided for components described herein as a single instance. Additionally, structures and functionality presented as discrete components in the exemplary configurations may be implemented as a combined structure or component. These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined in the claims that follow.