Patent Publication Number: US-2005123845-A1

Title: Method of adjusting deviation of critical dimension of patterns

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a photolithography process, and more particularly, to a method of adjusting deviation in a critical dimension (CD) of patterns formed by a photolithography process.  
      A claim of priority is made to Korean Patent Applications No. 10-2004-0056426 and No. 10-2004-0001099, filed respectively on Jul. 20, 2004 and Jan. 8, 2004. The disclosures of these Korean Patent Applications are incorporated herein by reference in their entirety.  
      2. Description of the Related Art  
      As integration density in semiconductor devices increases, a CD of patterns formed in the semiconductor devices decreases accordingly. Where the CD of a pattern is smaller than the wavelength of light from an exposure source, an optical proximity effect occurs due to diffraction. The optical proximity effect refers to distortion of the patterns caused by a combination of factors, including the difference in local pattern densities, adjacent patterns on a photomask, and deviation of the CD due to exposure limits. “Deviation of CD” for the patterns refers to a deviation between a desired CD and an actual CD. Since distortion of the patterns is typically associated with deviation of CD, the metric, deviation of CD, is taken to generally imply pattern distortion in a broad sense.  
      A conventional method of adjusting the deviation of CD utilizes an optical proximity correction (OPC) technique. The OPC technique uses a revised photomask to adjust deviation of CD. In other words, where deviation of CD occurs, a conventional photomask is revised to have new patterns that take into account the deviation of CD. Thus, local deviation of CD, e.g., CD distortion in a central or outer portion of a pattern, is effectively mitigated.  
      The OPC technique has at least two shortcomings. First, the OPC technique is not readily applicable to deviation of CD caused by the density of adjacent patterns or the position of patterns. Second, since the OPC technique requires revision and reproduction of a photomask, it is generally neither cost nor time effective.  
      Many semiconductor manufacturing processes include a process for simultaneously forming a plurality of identical patterns such as gate lines, bit lines, and metal interconnection lines. Where such a process is used to form patterns on a semiconductor substrate and deviation of CD occurs, the uniformity of the patterns is usually compromised. For example, in one case, the CD of an outer pattern in a plurality of patterns (hereinafter, the “outer pattern CD”) has a desired size, but the CD of a central pattern in the plurality of patterns (hereinafter, the “central pattern CD”) is smaller than the outer pattern CD. In other words, even where no deviation occurs in the outer pattern CD, deviation may occur in the central pattern CD. In another case, although the central pattern CD has a desired size, the outer pattern CD is larger than the central pattern CD. In yet another case, the central pattern CD is smaller than a desired size, and the outer pattern CD is larger than the desired size. In yet another case, the central pattern CD is larger than the outer pattern CD.  
      In order to address the deviation of CD problems described, a revised photomask is typically produced and used as described above. As previously mentioned, revising and reproducing a photomask is neither cost effective nor time effective. It often happens that as many as three or more revisions of a photomask are required.  
      Another method of adjusting deviation of CD involves forming gratings on a rear surface of a photomask.  FIGS. 1A and 1B  illustrate a conventional method of adjusting deviation of CD using gratings.  FIG. 1A  shows a case where no gratings are formed and  FIG. 1B  shows a case where gratings are formed on the rear surface of a photomask. In  FIGS. 1A and 1B , illustration (a) denotes a relative intensity of incident light, (b) denotes a relative intensity of light that has passed through the photomask, and (c) denotes the relative distribution of an outer pattern CD and a central pattern CD.  
      Referring to  FIG. 1A , incident light is projected with a uniform intensity onto the entire surface of a photomask  10 , as shown in  FIG. 1A (a) and the incident light is transmitted with a uniform intensity through a quartz substrate  11  of photomask  10 , as shown in  FIG. 1A (b). However, the CD of patterns formed on a semiconductor substrate using photomask  10  is rather non-uniform, as shown in  FIG. 1A (c). In  FIG. 1A (c), a central pattern CD (CD 1 ) is larger than an outer pattern CD (CD 2 ). Assuming a target CD is CD 1 , a deviation of CD is therefore defined as ΔCD=CD 2 −CD 1 .  
      Referring to  FIG. 1B , incident light is projected with a uniform intensity onto the entire surface of a photomask  20 , as shown in  FIG. 1B (a) and the incident light is transmitted with a non-uniform intensity through a quartz substrate  21  of photomask  20 , as shown in  FIG. 1B (b). While incident light transmitted through a central portion of quartz substrate  21  has a relatively low intensity, incident light transmitted through outer portions of quartz substrate  21  has a relatively high intensity. The non-uniform intensity of incident light transmitted through quartz substrate  21  is caused by gratings  23  formed on a rear surface of photomask  20 . Referring to  FIG. 1B (b), gratings  23  are formed more densely in the central portion of photomask  20  than in the outer portions thereof. By controlling the intensity of incident light using gratings  23 , the CD of patterns formed on a semiconductor substrate through photomask  20  can be adjusted to be uniform, as shown in  FIG. 1B (c).  
      Unfortunately, the formation of gratings  23  on photomask  20  deteriorates the resolution of the patterns by lowering the contrast of pattern images and reducing the corresponding normalized image log slope (NILS).  FIG. 2A  is a graph showing the contrast of pattern images as a function of grating density for photomask  20 .  FIG. 2B  is a graph showing NILS as a function of grating density for photomask  20 . The results shown in  FIGS. 2A and 2B  were obtained using an 8% attenuated phase shift mask having a 0.7 numerical aperture (NA), annular-type apertures, and 150-nm-line-and-space patterns. Referring to  FIGS. 2A and 2B , as the density of gratings  23  on photomask  20  increases, the contrast of pattern images and NILS decreases.  
      Additionally, the formation of gratings  23  on photomask  20  may damage the front surface of photomask  20 . Furthermore it is generally difficult to precisely match grating patterns according to a given deviation of CD. Moreover, although the foregoing method successfully adjusts global deviation of CD according to positions on the semiconductor substrate, it fails to adjust local deviation of CD.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method of adjusting the deviation of CD for patterns formed by a photolithography process. The deviation of CD is adjusted by forming a recess, an undercut, and/or an isotropic groove in a transparent substrate of a photomask with size smaller than the wavelength of incident light used in the photolithography process. Where a recess and an undercut are formed, the deviation of CD is typically adjusted by a larger amount than where a recess and an isotropic groove are formed. Accordingly, a method of adjusting deviation of CD by forming the recess and the undercut is preferably used to increase or decrease a general pattern CD across an entire substrate, while a method of adjusting deviation of CD by forming the recess and the isotropic groove is preferably used to increase or decrease a fine pattern CD in a selected portion of the substrate.  
      The present invention prevents degradation of the contrast of pattern images and reduction of normalized image log slope. The present invention also prevents the photomask from being damaged when the deviation of CD is adjusted. Furthermore, where different CDs are applicable for various patterns formed on a substrate, the present invention provides a method for adjusting the deviation of CD across the entire substrate by performing an etch mask forming process only once.  
      According to one aspect of the present invention, a method of adjusting deviation of CD for patterns formed on a device substrate by a photolithography process using an exposure source of wavelength λ is provided. The method comprises providing a photomask comprising a transparent substrate and a light-blocking pattern formed on the transparent substrate. The method further comprises performing the photolithography process using the photomask and etching a CD deviation region in the transparent substrate to a depth smaller than wavelength λ, wherein the CD deviation region corresponds to a region in the device substrate where CD deviation otherwise occurs as a result of the photolithography process.  
      According to another aspect of the present invention, a method of adjusting a CD for patterns formed on a device substrate by a photolithography process using an exposure source of wavelength λ is provided. The method comprises providing a photomask comprising a transparent substrate and a light-blocking pattern formed on the transparent substrate, and forming a material pattern on the device substrate from a material layer is using a photolithography process and an etching process using the photomask. The method further comprises measuring a CD for the material pattern, defining a positive CD deviation region and a negative CD deviation region in the transparent substrate by calculating a deviation of CD for the material pattern, wherein the deviation of the CD for the material pattern is calculated by comparing the measured CD of the material pattern to a target CD. The method further comprises forming a recess in the positive CD deviation region, and forming an undercut in the negative CD deviation region.  
      A depth of the recess and a width of the undercut are preferably determined by experimental data obtained under experimental conditions similar to the processing conditions. The recess is preferably formed by performing an anisotropic etching process using the light-blocking pattern as an etch mask. The undercut is preferably formed by performing a chemical dry etching process or a wet etching process using the light-blocking pattern as an etch mask.  
      According to still another aspect of the present invention, a method of adjusting a CD for patterns formed on a device substrate by a photolithography process using an exposure source of wavelength λ is provided. The method comprises providing a photomask comprising a transparent substrate and a light-blocking pattern formed on the transparent substrate, and forming a material pattern on the device substrate from a material layer using the photolithography process and an etching process using the photomask. The method further comprises measuring a CD for the material pattern, defining a positive CD deviation region and a negative CD deviation region in the transparent substrate by calculating a deviation of CD for the material pattern, wherein calculating the deviation of CD for the material pattern comprises comparing the measured CD with a target CD for the material pattern, forming an isotropic groove having a predetermined depth in the positive CD deviation region, and forming a recess having a predetermined depth in the negative CD deviation region.  
      According to still another aspect of the present invention, a method of adjusting deviation of a CD for patterns formed on a device substrate using a photomask is provided. The method comprises providing the photomask, wherein the photomask comprises a transparent substrate and defining a first positive CD deviation region, a second positive CD deviation region, and a third positive CD deviation region in the photomask, wherein the first, second, and third positive CD deviation regions correspond to respective patterns deviating from a first CD, a second CD, and a third CD. The method further comprises forming a recess having a predetermined depth in the transparent substrate in each of the first through third CD deviation regions, and forming a second recess and/or an isotropic groove inside the recess.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings illustrate several selected embodiments of the present invention, and are incorporated in and constitute a part of this specification. In the drawings:  
       FIGS. 1A and 1B  illustrate a conventional method of adjusting deviation of a CD of patterns using gratings.  
       FIG. 2A  is a graph showing contrast as a function of grating density, for pattern images formed by a photomask in  FIG. 1B ;  
       FIG. 2B  is a graph showing NILS as a function of grating density, for pattern images formed by the photomask in  FIG. 1B ;  
       FIG. 3A  is a cross-sectional view of a photomask used in a method of adjusting deviation of CD according to one embodiment of the present invention;  
       FIG. 3B  is a cross-sectional view of a photomask used in a method of adjusting deviation of CD according to another embodiment the present invention;  
       FIG. 4A  is a cross-sectional view of a photomask having a recess;  
       FIG. 4B  is a cross-sectional view of a photomask having an undercut;  
       FIG. 5A  is a graph showing optical intensity for light transmitted through the photomask in  FIG. 4A  as a function of a distance from the center of the photomask;  
       FIG. 5B  is a graph showing a CD of patterns formed using the photomask in  FIG. 4A  as a function of depth of the recess in the photomask, measured where threshold optical intensity is set to 0.2 based on the graph shown in  FIG. 5A ;  
       FIG. 6A  is a graph showing optical intensity for light transmitted through the photomask in  FIG. 4B  as a function of a distance from a center of the photomask;  
       FIG. 6B  is a graph showing CD for patterns formed using the photomask in  FIG. 4B  as a function of a width of the undercut in the photomask, measured where threshold optical intensity is set to 0.2 based on the graph shown in  FIG. 6A ;  
       FIG. 7A  is a cross-sectional view of a photomask having a recess;  
       FIG. 7B  is a cross-sectional view of a photomask having an isotropic groove;  
       FIG. 8A  is a graph showing CD for patterns formed using the photomask in  FIG. 7A  as a function of the width of the recess;  
       FIG. 8B  is a graph showing CD for patterns formed using the photomask in  FIG. 7B  as a function of an opening size of the isotropic groove;  
       FIG. 9A  is a cross-sectional view of a photomask having a first recess and a second recess;  
       FIG. 9B  is a cross-sectional view of a photomask having a recess and an isotropic groove;  
       FIG. 10A  is a graph showing CD for patterns formed using the photomask in  FIG. 9A  as a function of the width of the second recess;  
       FIG. 10B  is a graph showing CD for patterns formed using the photomask in  FIG. 9B  as a function of the opening size of the isotropic groove;  
       FIG. 11  is a flowchart illustrating a method of adjusting deviation of CD for patterns according to one embodiment of the present invention;  
       FIGS. 12A through 12C  are cross-sectional views illustrating a method of adjusting deviation of CD for patterns formed by a photomask having a positive CD deviation region;  
       FIGS. 13A through 13C  are cross-sectional views illustrating a method of adjusting deviation of CD for patterns formed using a photomask having a negative CD deviation region; and,  
       FIGS. 14A and 14B  are cross-sectional views illustrating a method of adjusting deviation of CD for patterns formed using a photomask having a plurality of different-sized CD deviation regions.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which several exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers is exaggerated for clarity. Also, like reference numerals refer to like elements throughout the drawings and the written description.  
      According to the present invention, a deviation of CD for patterns is adjusted by forming a recess and/or an undercut in a transparent substrate in a photomask. The recess and/or the undercut are generally formed by anisotropic dry etching and/or isotropic etching on a front surface of the photomask, i.e., a surface of the photomask on which a light-blocking pattern is formed. The recess and/or undercut adjust the deviation of CD by varying the intensity of incident light transmitted through the photomask. Typically the recess and/or the undercut have a smaller depth or width than the wavelength of the incident light.  
       FIG. 3A  is a cross-sectional view of a photomask  30  used in a method of adjusting the deviation of a CD according to the present invention. In photomask  30 , a recess  33  is formed in a light-transmitting region of a transparent substrate  31 . Recess  33  is preferably formed by performing anisotropic dry etching using a photoresist pattern (not shown) and/or a light-blocking pattern  32 .  
      Referring to  FIG. 3A , recess  33  is formed in transparent substrate  31  of photomask  30  with a predetermined width w 1  and depth d 1 . Deviation of CD for patterns formed using photomask  30  varies according to width w 1  and depth d 1 . A relationship between the deviation of CD, and width w 1  and depth d 1  will be described in some additional detail later.  
      Width w 1  is preferably less than or equal to a distance “wp” across a gap in light-blocking pattern  32 . Depth d 1  is preferably smaller than a wavelength of incident light received by photomask  30 . This preferred feature of the present invention prevents the phase of light transmitted through photomask  30  from being inverted.  
       FIG. 3B  is a cross-sectional view of a photomask  40  adapted for use within a method of adjusting deviation of a CD for patterns according to the present invention. In photomask  40 , an undercut  43  is formed in a transparent substrate  41 . Undercut  43  is preferably formed by isotropic wet etching or isotropic dry etching using a photoresist pattern (not shown) and/or a light-blocking pattern  42 . As a result of isotropic etching, undercut  43  is formed in both a light-blocking region and a light-transmitting region of transparent substrate  41 . Although there is a correlation between a horizontal etch rate and a vertical etch rate for transparent substrate  41 , the horizontal etch rate is typically higher than the vertical etch rate.  
      Referring to  FIG. 3B , a portion of undercut  43 , which is formed in the light-blocking region of transparent substrate  41 , has a predetermined width w 2 , an opening size w 2 ′, and a depth d 2 . Deviation of CD for patterns formed using photomask  40  varies according to width w 2 , opening size w 2 ′, and depth d 2 . The relationship between deviation of CD, width w 2 , opening size w 2 ′, and depth d 2  will be described in some additional detail later.  
      Although an undercut typically refers to a feature occurring underneath something else, undercut  43  should be interpreted to comprise both an etched portion formed in the light blocking region (e.g., a region under light-blocking pattern  42 ) and an etched portion formed in the light-transmitting region. Width w 2  refers to a width of the etched portion of undercut  43  formed in the light blocking region. Opening size w 2 ′ refers to a width of the an etched portion of undercut  43  formed in the light-transmitting region. Opening size w 2 ′ of undercut  43  is typically less than or equal to a distance “wp” across a gap in light-blocking pattern  42 . The term “undercut” is generally used herein where the opening size is approximately equal to the distance across a gap in the light-blocking pattern, while the term “isotropic groove” is used where the opening size is smaller than the distance across a gap in the light-blocking pattern.  
       FIGS. 4A and 4B  illustrate photomasks used in a first experimental example elucidating the present invention.  FIG. 4A  is a cross-sectional view of a photomask  130  having a recess  133  and  FIG. 4B  is a cross-sectional view of a photomask  140  having an undercut  143 .  
      Referring to  FIG. 4A , recess  133  is formed across an entire light transmitting region of photomask  130 . Recess  133  has a width w 3  equal to a distance “wp” across a gap in a light-blocking pattern  132 , and a depth d 3  in a transparent substrate  131 .  
      Referring to  FIG. 4B , undercut  143  is formed across an entire light transmitting region of photomask  140 . Undercut  143  has an opening size w 4 ′ equal to a distance “wp” across a gap in a light-blocking pattern  142 , a width w 4 , and a depth d 4  in a transparent substrate  141 .  
       FIGS. 4A and 4B  can be viewed as special examples of photomasks  30  and  40  shown in  FIGS. 3A and 3B , respectively.  
       FIG. 5A  is a graph showing optical intensity of light transmitted through photomask  130  as a function of depth d 3 . Optical intensity was measured for values of depth d 3  smaller than a wavelength λ of incident light. Specifically, optical intensity was measured for values in a range of 0 to 240 nm, at intervals of 40 nm. These measurements involved incident light having a wavelength of 248 nm. Specifically, the experimental observations involved a 248 nm KrF light source. Referring to  FIG. 5A , where depth d 3  is smaller than wavelength λ, increasing depth d 3  decreases the greatest optical intensity of light transmitted through photomask  130 .  
       FIG. 5B  is a graph showing variation in CD of patterns measured where threshold optical intensity is set to 0.2 on the basis of the graph shown in  FIG. 5A . Where the threshold optical intensity is set to values other than 0.2, values for CD of patterns change but relative differences between CDs of patterns show the same general trend as seen in  FIG. 5B . Referring to  FIG. 5B , CD of patterns tends to decrease as depth d 3  increases.  
      Therefore, CD of patterns is reduced by forming recess  133  to have width w 3  equal to distance “wp” and then increasing depth d 3  of recess  133 . Therefore, the amount of an adjustment to the CD of patterns varies by controlling depth d 3  of recess  133 . In the first experimental example, increasing depth d 3  by 10 nm adjusts the CD of patterns by about 3 nm. Accordingly, the method of adjusting the CD of patterns by forming recess  133  in the entire light-transmitting region of transparent substrate  131  can be applied to a positive CD deviation region in a photomask, particularly when the CD of patterns is adjusted by a relatively large value, as will be seen even more clearly in a second experimental example.  
       FIG. 6A  is a graph showing optical intensity as a function of width w 4  of undercut  143  shown in  FIG. 4B . Width w 4  is preferably smaller than a wavelength λ of incident light.  FIG. 6A  shows optical intensity for light transmitted through photomask  140  for different values of w 4  in undercut  143 . Optical intensity is shown in  FIG. 6A  for values of width w 4  from 0 nm to 200 nm, shown in intervals of 50 nm.  
      Referring to  FIG. 6A , as width w 4  of undercut  143  increases, the maximum optical intensity of light passing through photomask  140  tends to increase as well. Meanwhile, where width w 4  of undercut  143  was 0 nm, the maximum optical intensity is lower than where a binary mask (BM) is used. This is because where width w 4  of undercut  143  is 0 nm, only a recess having a predetermined depth was formed in the light-transmitting region of photomask  140 .  
       FIG. 6B  is a graph showing variation of CD of patterns measured where the optical intensity is set to 0.2 on the basis of the graph shown in  FIG. 6A . Referring to  FIG. 6B , as width w 4  of undercut  143  increases, the CD of patterns increases monotonically.  
      Therefore, a CD of patterns is increased by forming undercut  143  under light-blocking pattern  142 . Moreover, as width w 4  of undercut  143  increases, the CD of patterns is adjusted by a larger amount. In this first experimental example, where width w 4  of undercut  143  is increased by 10 nm, the CD of patterns is adjusted by about 5 nm. Accordingly, the method of adjusting a CD of patterns by forming undercut  143  in transparent substrate  141  can be applied to a negative CD deviation region of a photomask, particularly where the CD of patterns is adjusted by a relatively large value, as will be seen more clearly in a second experimental example.  
      In summary, the optical intensity of light transmitted through photomasks  130  and  140  varies according to depth d 3  of recess  133  formed in photomask  130  and width w 4  of undercut  143  formed in photomask  140 . By controlling depth d 3  and width w 4 , a CD of a pattern corresponding to recess  133  or undercut  143  is readily adjusted. The CD of the pattern is adjusted by forming recess  133  in photomask  130  to an appropriate depth d 3  and forming undercut  143  in photomask  140  to an appropriate width w 4 . Typically, an etch mask forming process is performed two or more times to form recess  130  or undercut  143  to different depths d 3  and widths w 4  respectively, according to positions of the light-transmitting region. This is because depth d 3  of recess  130  and width w 4  of undercut  143  each depend on the process time. Nevertheless, the method of adjusting CD of patterns illustrated in the first experimental example is useful in adjusting a CD of patterns by a large value and adjusting a CD of patterns for an entire photomask.  
       FIGS. 7A and 7B  illustrate photomasks used in a second experimental example elucidating the present invention.  FIG. 7A  is a cross-sectional view of a photomask  230  having a recess  233  and  FIG. 7B  is a cross-sectional view of a photomask  240  having an isotropic groove  243 .  
      Referring to  FIG. 7A , recess  233  is formed in a light transmitting region of a transparent substrate  231 . Recess  233  has a width w 5  and a depth d 5 . A light-blocking pattern  232  is formed over a light blocking region of transparent substrate  231  and a distance “wp” spans a gap in light-blocking pattern  232 . Photomask  230  differs from photomask  130  shown in  FIG. 4A  and used in the first experimental example in that width w 5  in recess  233  is smaller than “wp”.  
      Referring to  FIG. 7B , isotropic groove  243  is formed in a light transmitting region in a transparent substrate  241 . Isotropic groove  243  has an opening size w 6 ′, a width w 6 , and a depth d 6 . A light-blocking pattern  242  is formed over a light blocking region of transparent substrate  241  and a distance “wp” spans a gap in light-blocking pattern  242 . Photomask  240  shown in  FIG. 7B  differs from photomask  140  shown in  FIG. 4B  and used in the first experimental example in that opening size w 6 ′ in isotropic groove  243  is smaller than distance “wp”.  
      In the second experimental example, depth d 5  of recess  233  in photomask  230  in  FIG. 7A  is maintained constant while width w 5  is varied. Also in the second experimental example, depth d 6  and width w 6  of isotropic groove  243  in photomask  240  shown in  FIG. 7B  are maintained constant while opening size w 6 ′ is varied.  
      Photomasks  230  and  240  shown in  FIGS. 7A and 7B  can be viewed as special examples of photomasks  30  and  40  shown in  FIGS. 3A and 3B , respectively.  
       FIG. 8A  is a graph showing CD of patterns as a function of width w 5  of recess  233  shown in  FIG. 7A .  FIG. 8B  is a graph showing CD of patterns as a function of opening size w 6 ′ of isotropic groove  243  shown in  FIG. 7B .  
      Experiments were performed using photomasks  230  and  240 , each having 600 nm 1:3 line-and-space patterns, an ArF light source, a lens having a 0.85 numerical aperture (NA), and 0.55/0.85-annular apertures. The graphs shown in  FIGS. 8A and 8B  are obtained through a process similar to that described in the first experimental example with reference to  FIGS. 5B and 6B .  
      Referring to  FIG. 8A , where recess  233  is formed with a depth d 5  of 28.8 nm (i.e., 30° of ArF wavelength) and a width w 5  smaller distance “wp”, the CD of patterns was larger than where recess  233  is not formed (i.e., where width w 5  is 0 nm). Also, as width w 5  of recess  233  increases, the CD of patterns increases at first and then begins to decrease after it reaches a certain value.  
      Therefore, the CD of patterns is readily increased by varying the width w 5  of recess  233 . In this experimental example, as width w 5  of recess  233  increases by 10 nm, the CD of patterns increases by about 0.1 nm. Accordingly, the method of adjusting a CD of patterns by forming recess  233  is readily applied to a negative CD deviation region, particularly when a relatively fine CD adjustment is required, as shown in the first experimental example.  
      Referring to  FIG. 8B , where isotropic groove  243  is formed with a width w 6  of 28.68 nm (i.e., 30° of ArF wavelength) and an opening size w 6 ′ smaller than distance “wp”, the CD of patterns is smaller than where isotropic groove  243  is not formed (i.e., where opening size w 6 ′ is 0 nm). However, following a pattern similar to that of the graph shown in  FIG. 8A , as opening size w 6 ′ increases, the CD of patterns increases. Once opening size reaches a certain value, the CD of patterns will eventually begin to decrease.  
      Therefore, the CD of patterns is readily reduced by varying opening size w 6 ′ of isotropic groove  243 . In this experimental example, where opening size w 6 ′ is between 30 and 90 nm, increasing opening size w 6 ′ of isotropic groove  243  by 10 nm increases the CD of patterns by about 0.7 nm. Accordingly, the method of adjusting a CD of patterns by forming isotropic groove  243  is readily applied to a positive CD deviation region, particularly where relatively fine CD adjustment is required, as shown in the first experimental example.  
      Consequently, the optical intensity of incident light transmitted through photomasks  230  and  240  varies with width w 5  of recess  233  and opening size w 6 ′ of isotropic groove  243 . Thus, by controlling width w 5  of recess  233  formed in photomask  230  and opening size w 6 ′ of isotropic groove  243  formed in photomask  240 , a CD of patterns corresponding to recess  233  and isotropic groove  243  are controlled. As a result, the CD of patterns is readily adjusted by forming recess  233  in photomask  230  to an appropriate width w 5  and forming isotropic groove  243  in photomask  240  to an appropriate opening size w 6 ′.  
      Width w 5  of recess  233  and opening size w 6 ′ of isotropic groove  243  are finely controlled by controlling the size of an etched mask pattern used to form photomasks  230  and  240 , respectively. Depth d 5  of recess  233  and depth d 6  of isotropic groove  243 , each of which is a function of process time, are formed with constant values throughout the exposed light-transmitting region due to an etch mask. Therefore, as seen in the second experimental example, a CD of patterns is readily adjusted by appropriately forming photomasks  230  and  240  by performing an etch mask process only once.  
       FIGS. 9A and 9B  illustrate photomasks used in a third experimental example elucidating the present invention.  
      Referring to  FIG. 9A , a first recess having a depth R is formed in a light transmitting region of a transparent substrate  331  of a photomask  330 . A second recess  333  is formed in a portion of the light transmitting region. A light-blocking pattern  332  is formed over a light blocking region of transparent substrate  331  and a gap spanning a distance “wp” is formed in light-blocking pattern  332 . Second recess  333  is formed with a depth d 7  and a width w 7 . In the third experimental example, depth d 7  and depth R are maintained constant while width w 7  is varied.  
      Referring to  FIG. 9B , a recess is formed to a depth R in a light-transmitting region of a transparent substrate  341  of a photomask  340 . An isotropic groove  343  is formed in a portion of the light transmitting region. A light-blocking pattern  342  is formed over a light blocking region of transparent substrate  341  and a gap spanning a distance “wp” is formed in light-blocking pattern  342 . Isotropic groove  343  is formed with a width w 8 , an opening size w 8 ′, and a depth d 8 . In the third experimental example, depth d 8 , depth R, and width w 8  are maintained constant while opening size w 8 ′ is varied.  
       FIG. 10A  is a graph showing CD as a function of width w 7  of second recess  333  shown in  FIG. 9A .  FIG. 10B  is a graph showing CD of patterns as a function of opening size w 8 ′ of isotropic groove  343  shown in  FIG. 9B . Experiments were performed using photomasks  330  and  340 , each having 600 nm 1:3 line-and-space patterns, an ArF light source, a lens having a 0.85 NA, and 0.55/0.85-annular apertures. The graphs shown in  FIGS. 10A and 10B  are obtained through the process described in the first experimental example with reference to  FIGS. 5B and 6B .  
      The graphs shown in  FIGS. 10A and 10B  are similar to the graphs shown in  FIGS. 8A and 8B , respectively. However, each of photomasks  330  and  340  used to obtain the graphs shown in  FIGS. 10A and 10B  are initially recessed to a predetermined depth R. Accordingly, where the same threshold optical intensity is applied and a recess having the same width and depth is formed, the CD of patterns formed using photomask  230  shown in  FIG. 7A  is generally larger than the CD of patterns formed by photomask  330  shown in  FIG. 9A . Similarly, where the same threshold optical intensity is applied and an isotropic groove having the same width, opening size, and depth is formed, the CD of patterns formed using photomask  240  shown in  FIG. 7B  is generally larger than the CD of patterns formed using photomask  340  shown in  FIG. 9B .  
      The third experimental example combines certain aspects of the first and second experimental examples. Specifically, the third experimental example illustrates what happens to a CD of patterns where a depth of a recess or isotropic groove is offset and a width thereof is varied. Accordingly, the third experimental example can be appropriately applied where global CD adjustment is required across the entire photomask and fine CD adjustment is required in a portion of the photomask.  
      A method of adjusting deviation of a CD of patterns will now be described with reference to  FIG. 11 .  
       FIG. 11  is a flowchart illustrating a method of adjusting a deviation of a CD of patterns using the first experimental example according to an embodiment of the present invention.  
      Referring to  FIG. 11 , a photomask is prepared in an operation S 11 . The photomask is a binary mask (BM) including a light-blocking pattern and a transparent substrate. The light-blocking pattern is formed on a front surface of the transparent substrate. A light-blocking region and a light-transmitting region are defined by the light-blocking pattern on the transparent substrate. The light-blocking pattern is formed to a predetermined size according to a target CD of patterns. For example, where the target CD of patterns for a 4× photomask is 150 nm, the light-blocking pattern has a size of 600 nm.  
      Next, a material pattern is formed on a device substrate by performing an exposure process and a developing process using the photomask in an operation S 12 . An anisotropic dry etching process is additionally performed where necessary to form the material pattern. The exposure process is performed using a light source emitting light having a wavelength λ. In the present invention, any type of light source can be used. For example, a 248 nm KrF light source or a 196 nm ArF light source is typically employed. Also, the material pattern can be formed of any kind of material, for example, photoresist, an insulating material such as silicon oxide, a conductive material such as aluminum and tungsten, or a material such as chrome for forming a light-blocking pattern of a photomask.  
      Thereafter, the CD of the material pattern is measured in an operation S 13 . The CD of the material pattern is typically measured using an aerial image measurement system (AIMS) or a scanning electronic microscope (SEM). These apparatuses enable the measurement of a distribution of CD according to positions on the device substrate, as well as the maximum and minimum CD.  
      Thereafter, the CD measured in operation S 13  is compared with the target CD in an operation S 14 . In some instances the measured CD differs from the target CD because of photolithography limit due to a reduction of design rules and an optical proximity effect (OPE). In other words, the measured CD is sometimes larger than the target CD, which is referred to as a positive deviation of CD. Alternatively, the measured CD is sometimes smaller than the target CD, which is referred to as a negative deviation of CD. In some cases, no deviation of CD occurs. In some instances, the positive deviation of CD or the negative deviation of CD occurs by a constant value throughout the entire substrate. Alternatively, in other cases a deviation of CD of patterns differs according to positions on a substrate. In yet other cases, positive deviation of CD and negative deviation of CD even occur on a single substrate simultaneously.  
      Following operation S 14  a positive CD deviation region is defined on the photomask corresponding to a portion of the device substrate where positive deviation of CD occurs, and a negative CD deviation region is defined on the photomask corresponding to a portion of the device substrate where negative deviation of CD occurs. In a region of the photomask corresponding to a portion of the device substrate where the measured CD is equal to the target CD, no adjustment of the CD of patterns is required.  
      In an operation S 15 , a process of adjusting a deviation of CD is performed based on the result of the comparison operation S 14 . To adjust the deviation of the CD, an etch process for forming a recess or an undercut in the photomask is performed as described in the first experimental example. Alternatively, an isotropic groove or a recess is formed in the photomask as described in the second experimental example. Otherwise, a light-transmitting region is recessed to a predetermined depth, and then an isotropic groove or a recess may be formed as described in the third experimental example.  
      For example, a recess or an isotropic groove may be formed in the positive CD deviation region of the photomask. An undercut or a recess may be formed in the negative CD deviation region of the photomask. Where both a positive CD deviation region and a negative CD deviation region are defined in a single photomask, the recess or the isotropic groove is typically formed in the positive CD deviation region and the undercut or the recess is typically formed in the negative CD deviation region. In this case, the recess, the isotropic groove, and the undercut are not required to be formed in a specific order.  
      The above-described adjustment process will now be described in further detail.  
       FIGS. 12A through 12C  illustrate a process for adjusting a CD of patterns corresponding to a positive CD deviation region of a photomask.  
       FIG. 12A  is a cross-sectional view of a photomask where a positive CD deviation region is defined.  FIGS. 12B and 12C  are cross-sectional views illustrating a method of adjusting a CD of patterns formed using the photomask shown in  FIG. 12A .  
      Referring to  FIG. 12A , a photomask comprises a transparent substrate  51  and light-blocking patterns  52  ( 52   a,    52   b,  and  52   c ). An unadjusted region and a positive CD deviation region are defined within the photomask. Light-blocking patterns  52   a,    52   b,  and  52   c  shown in  FIG. 12A  are illustrated by way of example.  
      Referring to  FIG. 12B , a photoresist pattern  55  is formed on light-blocking patterns  52   a  and  52   c  to expose a light-transmitting region in the positive CD deviation region. Photoresist pattern  55  also covers the entire unadjusted region. In some instances, photoresist pattern  55  is selectively formed on light-blocking pattern  52   b  as well. An anisotropic dry etching process is performed to form a recess having a vertical profile. Where the anisotropic dry etching process is performed, photoresist pattern  55  and light-blocking pattern  52   b,  which is exposed in the positive CD deviation region, are used as an etch mask. As a result, a recess  53  having a predetermined depth d 9  is formed in a light-transmitting region of a transparent substrate  51   a  in the positive CD deviation region of the photomask. Depth d 9  of recess  53  varies according to CD deviation and is preferably smaller than a wavelength λ of incident light. As described above, where the depth of recess  53  is smaller than wavelength λ, the CD of a pattern can be reduced. For example, where an ArF light source is utilized, depth d 9  of recess  53  is 240 nm or less. Once recess  53  is formed, photoresist pattern  55  is removed. Thus, a photomask used to form patterns with an adjusted CD is obtained.  
      Where the CD of patterns formed by the photomask is adjusted by different values according to different positions, an etching process is typically performed twice or more. For example, suppose that a first region of the photomask requires a first recess having a first depth, a second region requires a second recess having a second depth, and the second depth is larger than the first depth. In this case, a first photoresist pattern is formed to expose both the first region and the second region. By using the first photoresist pattern as a photomask, the first and second regions of the photomask are etched to the first depth, thereby forming the first recess. Then, the first photoresist pattern is removed, and a second photoresist pattern is formed to expose the second region. The second region of the photomask is then etched to the second depth using the second photoresist pattern as an etch mask, thereby forming the second recess. Then, the second photoresist pattern is removed. Thus, the first recess having the first depth is formed in the first region of the photomask, and the second recess having the second depth is formed in the second region of the photomask.  
      Referring to  FIG. 12C , a photoresist pattern  55   a  is formed on transparent substrate  51   a  to expose only a portion of the light-transmitting region in the positive CD deviation region. Photoresist pattern  55   a  is formed to an appropriate size in consideration of an opening size w 10 ′ of an isotropic groove  54  to be formed during a subsequent process. Photoresist pattern  55   a  is formed to cover the entire unadjusted region. In some instances, photoresist pattern  55   a  is selectively partially or wholly formed on light-blocking patterns  52   a,    52   b,  and  52   c  as well. An isotropic dry or wet etching process is performed to form isotropic groove  54 . The etching process is performed using photoresist pattern  55   a  and light-blocking pattern  52 , which is exposed on the positive CD deviation region, as an etch mask. As a result, the isotropic groove  54  having predetermined depth d 10 , width w 10 , and opening size w 10 ′ is formed in a transparent substrate  51   b  of the positive CD deviation region. Photoresist pattern  55   a  is then removed. Thus, a photomask used to form patterns with an adjusted CD is obtained.  
      Where the CD of patterns formed by the photomask is adjusted by a different value according to different positions, a photoresist pattern is typically formed such that a size “A” of the light-transmitting region exposed by the photoresist pattern is different according to the positions of the photomask. For example, suppose that a first region of the photomask requires a first isotropic groove having a first opening size, a second region of the photomask requires a second isotropic groove having a second opening size, and the second opening size is larger than the first opening size. In this case, the photoresist pattern is typically formed such that size “A” of the light-transmitting region exposed by the photoresist pattern is larger in the second region than in the first region. Then, an isotropic etching process is performed using the photoresist pattern as an etch mask, and the photoresist pattern is removed. Thus, the first isotropic groove having the first opening size is formed in the first region, and the second isotropic groove having the second opening size, which is larger than the first opening size, is formed in the second region.  
      A method of etching a photomask where a negative CD deviation region is defined will now be described.  
       FIGS. 13A through 13C  illustrate a process of adjusting a CD in a negative CD deviation region.  
       FIG. 13A  is a cross-sectional view of a photomask where a negative CD deviation region is defined.  FIGS. 13B and 13C  are cross-sectional views illustrating a method of adjusting a CD of patterns of the photomask shown in  FIG. 13A .  
      Referring to  FIG. 13A , a photomask comprises a transparent substrate  151  and light-blocking patterns  152  ( 152   a,    152   b,  and  152   c ). An unadjusted region and a negative CD deviation region are defined within the photomask. Light-blocking patterns  152   a,    152   b,  and  152   c  are shown in  FIG. 13A  by way of example.  
      Referring to  FIG. 13B , a photoresist pattern  155  is formed on light-blocking patterns  152   a  and  152   c  in order to expose the entire light-transmitting region of the negative CD deviation region. Photoresist pattern  155  also covers the entire unadjusted region. In some instances, photoresist pattern  155  is also selectively formed on light-blocking pattern  152   b.    
      An isotropic etching process is performed to form an undercut  153 . When the isotropic etching process is performed, photoresist pattern  155  and light-blocking pattern  152   b,  which is exposed on the negative CD deviation region, are used as an etch mask. As a result, undercut  153 , which has a predetermined width w 11  is formed under the light-transmitting region of a transparent substrate  151   a  and light-blocking patterns  152  in the negative CD deviation region. Width w 11  of undercut  153  varies with a deviation of CD and is preferably smaller than a wavelength λ of incident light and smaller than ½ a width of each light-blocking pattern  152 . Thereafter, photoresist pattern  155  is removed. Thus, a photomask used to form patterns with an adjusted CD is obtained.  
      Where the CD of the photomask is adjusted by a different value according to different positions, an etching process is typically performed twice or more. For example, suppose that a first region of the photomask requires a first undercut having a first width, a second region thereof requires a second undercut having a second width, and the second width is larger than the first width. In this case, a first photoresist pattern is formed to expose both the first region and the second region. By using the first photoresist pattern as a photomask, the first and second regions of the photomask are isotropically etched, thereby forming the first undercut having the first width. Then, the first photoresist pattern is removed, and a second photoresist pattern is formed to expose the second region. Thereafter, by using the second photoresist pattern as an etch mask, the second region of the photomask is isotropically etched, thereby forming the second undercut having the second width. The second photoresist pattern is then removed. Thus, the first undercut having the first width is formed in the first region of the photomask, and the second undercut having the second width is formed in the second region of the photomask.  
      Referring to  FIG. 13C , a photoresist pattern  155   a  is formed on transparent substrate  151   a  to expose only a portion of the light-transmitting region in the negative CD deviation region. Photoresist pattern  155   a  is formed to an appropriate size in according to a width w 11  of a recess  154  to be formed during a subsequent process. Photoresist pattern  155   a  is formed to cover the entire unadjusted region. In some instances, photoresist  155   a  is also partially or wholly formed on the light-blocking patterns  152   a,    152   b,  and  152   c.    
      An anisotropic dry etching process is performed to form recess  154 . The etching process is performed using photoresist pattern  155   a  and light-blocking pattern  152 , which is exposed in the negative CD deviation region, as an etch mask. As a result, recess  154 , which has a predetermined depth d 12  and width w 12  is formed in a transparent substrate  151   b  of the negative CD deviation region. Photoresist pattern  155  is then removed. Thus, a photomask used to form patterns with an adjusted CD is obtained.  
      Where the CD of the photomask is adjusted by a different value according to different positions, a photoresist pattern is typically formed such that width w 12  of the light-transmitting region exposed by the photoresist pattern is different according to positions within the photomask. For example, suppose that a first region of the photomask requires a first recess having a first width, a second region of the photomask requires a second recess having a second width, and the second width is larger than the first width. In this case, the photoresist pattern is formed such that width w 12  of the light-transmitting region exposed by the photoresist pattern is larger in the second region than in the first region. Then, an anisotropic dry etching process is performed using the photoresist pattern as an etch mask, and the photoresist pattern is removed. Thus, the first recess having the first width is formed in the first region, and the second recess having the second width, which is larger than the first width, is formed in the second region.  
      The present invention is used not only to adjust the CDs of individual patterns formed on a device substrate but also to improve the uniformity of patterns by adjusting general deviation of CD of patterns. To improve the uniformity of patterns, the entire device substrate is generally divided into respective regions and CD of patterns is adjusted in the respective regions. The above-described first through third experimental examples can be applied in the same manner.  
      Hereinafter, a detailed method of improving the uniformity of patterns will be described with reference to  FIGS. 14A and 14B .  
       FIGS. 14A and 14B  illustrate a method of adjusting deviation of a CD of patterns in a photomask where a plurality of different-sized CD deviation regions are defined.  FIG. 14A  is a cross-sectional view of a photomask before a deviation of CD of patterns is adjusted, and  FIG. 14B  is a graph showing CD of patterns for respective regions.  
      Referring to  FIG. 14A , light-blocking patterns  420  ( 421 ,  422 ,  423 ,  424 ,  425 , and  426 ) having the same size are wholly or partially formed on a transparent substrate  410  of a photomask  400 . Photomask  400  is divided into regions I through VI to facilitate explanation. Light-blocking patterns  420  are typically line type patterns. Where light-blocking patterns  420  are line type patterns, photomask  400  is generally a photomask used to form bit lines or metal interconnection lines.  
       FIG. 14B  shows relative CD of patterns with respect to positions of the patterns on a substrate when a photolithography process is performed using photomask  400 . Referring to  FIG. 14B , the CD of a pattern is larger in portions of a device substrate corresponding to outer portions of photomask  400  than in portions of the device substrate corresponding to central portions of the photomask  400 . More specifically, the CD of patterns formed on the portions of the device substrate corresponding to regions I and VI of the photomask  400  is CD 3 , and the CD of the patterns formed on the portions of the device substrate corresponding to regions III and IV is CD 5 .  
      In addition to the example in  FIG. 14B , there are cases where the CD of patterns is less in portions of the device substrate corresponding to outer portions of a photomask than in portions of the device substrate corresponding to central portions of the photomask. Alternatively, the CD of patterns may have the form of a sine wave. In these and other cases, adjustment of the CD of patterns is accomplished using the method of the present invention.  
      Describing a first case, a target CD is CD 3 , regions II through V are defined as negative CD deviation regions. In the case of  FIGS. 14A and 14B , the CD of patterns corresponding to regions II through V of photomask  400  are adjusted to CD 3  by etching the regions II through V using the methods described with respect to  FIGS. 13B  or  13 C.  
      Using the method described with respect to  FIG. 13B  an undercut having a first width is formed in each of regions II and V, and an undercut having a second width is formed in each of regions III and IV. Here, the second width is larger than the first width. The first and second widths vary according to several parameters, including, for example, the wavelength of incident light, the type of aperture, the amount of CD deviation, the type and size of a light-blocking pattern, and the distance between adjacent light-blocking patterns. The first and second widths are generally determined using experiments involving respective process conditions. As stated above, to form undercuts having different widths in respective regions of a photomask, an etch mask forming process should be performed several times.  
      Using the method described with respect to  FIG. 13B , a recess having a first width is formed in each of regions II and V, and a recess having a second width larger than the first width is formed in each of regions III and IV. The first and second widths vary according to several parameters, including, for example, the depth of a recess, the wavelength of incident light, the type of aperture, the amount of CD deviation, the type and size of a light-blocking pattern, and the distance between adjacent light-blocking patterns. The first and second widths are typically determined using experiments involving respective process conditions. As stated above, by controlling the width of a light-transmitting region exposed by an etch mask, recess having different widths are formed in respective regions of a photomask using a one-time etch mask forming process.  
      Describing a second case, a target CD is CD 5 , regions I, II, V, and VI of photomask  400  are defined as negative CD deviation regions. In this case, the CD of patterns corresponding to regions I, II, V, and VI of photomask  400  is adjusted to CD 3  by etching regions II through V using the methods described in relation to  FIG. 12B and 12C .  
      Using the method described with respect to  FIG. 12B , a recess having a first depth is formed in each of regions II and V of transparent substrate  410 , and a recess having a second depth larger than the first depth is formed in each of regions I and VI thereof. The first and second depths vary according to several parameters, including, for example, the wavelength of incident light, the type of aperture, the amount of CD deviation, the type and size of a light-blocking pattern, and the distance between adjacent light-blocking patterns. The first and second depths are generally determined using experiments involving respective process conditions. As stated above, to form recesses having different depths in respective regions of a photomask, an etch mask forming process is typically performed several times.  
      Using the method described with respect to  FIG. 12C , an isotropic groove having a first opening size is formed in each of regions II and V of transparent substrate  410 , and an isotropic groove having a second opening size is formed in each of regions I and VI thereof. The first and second opening sizes vary according to several parameters, including, for example, the depth and width of the isotropic groove, the wavelength of incident light, the type of aperture, the amount of a CD deviation, the type and size of a light-blocking pattern, and the distance between adjacent light-blocking patterns. The first and second opening sizes can be determined using experiments involving respective process conditions. As stated above, by controlling the width of a light-transmitting region exposed by an etch mask, isotropic grooves having different opening sizes can be formed in respective regions of a photomask using a one-time etch mask forming process.  
      Describing a third case, a target CD is CD 4 , while regions I and VI of the photomask are defined as positive CD deviation regions, regions III and IV thereof are defined as negative CD deviation regions. In this case, the CD of patterns corresponding to regions I and VI of transparent substrate  410  are adjusted by etching regions I and VI using the methods described with respect to  FIGS. 12B and 12C . The CD of patterns corresponding to regions III and IV of transparent  410  are typically adjusted by etching the regions III and IV using the methods described with respect to  FIGS. 13B and 13C . A detailed description of the aforementioned methods will be omitted to avoid repetition.  
      Describing a fourth case, a target CD is CD 6  and the entire region of photomask  400  is defined as a positive CD deviation region. The amount of deviation of CD of patterns is smallest in regions III and IV of transparent substrate  410  and greatest in regions I and VI thereof. In this case, as described in the foregoing third experimental example, a recessed recess or a recessed isotropic groove are formed by etching photomask  400 . Specifically, in a first adjustment operation, a recess having a predetermined depth is formed in the entire light-transmitting region of transparent substrate  410  as described in a method with respect to  FIG. 12B , thereby reducing the CD of patterns. Thereafter, in a second adjustment operation, a recess, an undercut, a recess, or an isotropic groove are formed in respective regions of the transparent substrate  410 , thereby adjusting the CDs of patterns corresponding to the respective regions. In the first adjustment operation, the recess is formed to an arbitrary depth.  
      For example, in the first adjustment operation, a recess having a predetermined depth L 1  may be formed in the entire light-transmitting region of photomask  400  such that the CD of patterns corresponding to regions I and VI becomes the target CD, i.e., CD 6 . As a result, in the second adjustment operation, the CD of patterns is adjusted by etching photomask  400  in the same manner as when the target CD is CD 3 .  
      Alternatively, in the first adjustment operation, a recess having a predetermined depth L 2  is formed in the light-transmitting region of photomask  400  such that the CD of patterns corresponding to regions II and V becomes the target CD, i.e., CD 6 . In this case, L 2  is smaller than L 1 . As a result, in the second adjustment operation, the CD of patterns is adjusted by etching photomask  400  in the same manner as when the target CD is CD 4 .  
      Alternatively, a recess having a predetermined depth L 1  is formed in the entire light-transmitting region of photomask  400  such that the CD of patterns corresponding to regions III and IV becomes the target CD, i.e., CD 6 . In this case, L 3  is smaller than L 2 . As a result, in the second adjustment operation, the CD of patterns is adjusted by etching photomask  400  in the same manner as when the target CD is CD 5 .  
      According to the present invention, the CDs of patterns are adjusted by forming a recess, an undercut, and/or an isotropic groove in a transparent substrate of a photomask to a size smaller than a wavelength of incident light. Where a recess and an undercut are formed, a deviation of CD of patterns is typically adjusted by a larger amount than where a recess and an isotropic groove are formed. Accordingly, a method of adjusting deviation of CD of patterns by forming the recess and the undercut is typically used to increase or decrease a general CD of patterns in the entire substrate, while a method of adjusting deviation of CD of patterns by forming the recess and the isotropic groove is typically used to increase or decrease a fine pattern CD in a portion of the substrate.  
      In comparison with a conventional method of adjusting deviation of CD of patterns involving the formation of gratings on a rear surface of a photomask, the present invention prevents degradation of the contrast of pattern images and reduction of normalized image log slope (NILS). Also, the photomask is prevented from damage resulting from the formation of gratings.  
      Above all, where different amounts of deviation of CD of patterns occur throughout the entire substrate, the present invention provides a method for adjusting the deviation of CD of patterns throughout the entire substrate by performing an etch mask forming process only once. Thus, cost and time taken to adjust the pattern CDs are minimized.  
      The preferred embodiments disclosed in the drawings and the corresponding written description are teaching examples. Those of ordinary skill in the art will understand that various changes in form and details may be made to the exemplary embodiments without departing from the scope of the present invention which is defined by the following claims.