Abstract:
Embodiments relate to a method for fabricating a mask used in a photolithography process. According to embodiments, a Design dimensions of DI model considering a line width of a photoresist after Photo Engraving Process (PEP) are corrected to be used. Among the design dimensions, a design dimension for a distance between patterns forming a mask and design dimension for each region included in each of the patterns may be differently corrected.

Description:
[0001]     The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2006-0073478 (filed on Aug. 3, 2006), which is hereby incorporated by reference in its entirety.  
       BACKGROUND  
       [0002]     Photolithography related processes may be significant in a semiconductor fabrication process. This may be because a photolithography process is an important aspect regarding how highly integrated a device may be and the degree of precision provided in a fabrication process.  
         [0003]     As devices get more highly integrated, a pattern size should necessarily be reduced. In order to resolve such a small pattern, a fabrication process may need to be delicate.  
         [0004]     Resolution Enhancement Technology (RET), a technology for resolution improvement and fidelity, has been developed because photolithography equipment may not be able to keep up with a pattern becoming smaller. Among this technology, Optical Proximity Correction (OPC) may be in use and OPC based on a simulation model may be in great use.  
         [0005]     Among OPC related technologies, an OPC based on a model may be used in the foundry business. This may be because the OPC can be easily applied to various client databases or layouts unlike an OPC based on rules.  
         [0006]      FIG. 1  is a diagram illustrating a related art mask pattern.  
         [0007]     Critical Dimensions (CDs) of the related art mask pattern are to consider a CD of an after-exposure photoresist rather than a target CD of a layer intended for etching with a predetermined photoresist as an etching mask.  
         [0008]     Accordingly, a structure such as a device isolation film which may have a great influence on a degree of integration of a semiconductor device has a problem that after etch, a trench may not be formed with a CD of a desired size.  
         [0009]     In other words, there is a problem caused by considering a CD of an after-exposure photoresist without targeting a CD after a final etch process in designing a mask pattern.  
       SUMMARY  
       [0010]     Embodiments relate to a method for designing a region, which is a target of a mask pattern, considering an after-etch CD and forming a device isolation film using a mask.  
         [0011]     Embodiments relate to a method for fabricating a mask, and more particularly, to a method for fabricating a mask allowing a device isolation film to have more accurate Critical Dimension (CD) and a method for forming a device isolation film using the mask.  
         [0012]     According to embodiments, there may be provided a method for fabricating a mask used in a photolithography process, wherein design dimensions of DI model considering a line width of a photoresist after Photo Engraving Process (PEP) may be corrected to be used, and wherein, among the design dimensions, a design dimension for a distance between patterns forming a mask and design dimension for each region included in each of the patterns may be differently corrected.  
         [0013]     According to embodiments, a method for forming a device isolation film may include forming sequentially an oxide film, a nitride film, and a Tetra Ethyl Ortho Silicate (TEOS) film on a silicon substrate, forming a photoresist having a predetermined pattern on the TEOS film, etching part of the TEOS film, the nitride film, the oxide film, and the silicon substrate using the photoresist as an etching mask, and after etching of the part of the silicon substrate, performing an etching process for forming a trench in the silicon substrate with the photoresist as an etching mask. 
     
    
     DRAWINGS  
       [0014]      FIG. 1  is a diagram illustrating a related art mask pattern.  
         [0015]     FIGS.  2  to  5  are process cross-sectional diagrams illustrating a method for forming a device isolation film according to embodiments.  
         [0016]      FIGS. 6 and 7  are graphs of distribution of CD versus pitch of mask pattern.  
         [0017]      FIG. 8  illustrates a design of a mask fabricated according to embodiments.  
         [0018]      FIG. 9  illustrates a pattern of a mask fabricated according to embodiments.  
         [0019]      FIGS. 10 and 11  are Scanning Electron Microscope (SEM) photographs of an active region after a device isolation film may be formed using a mask fabricated in accordance with embodiments. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Referring to  FIG. 2 , oxide film  2 , nitride film  3 , and Tetra Ethyl Ortho Silicate (TEOS) film  4  may be sequentially deposited on silicon substrate  1 .  
         [0021]     Photoresist  5  may be formed to have a predetermined pattern on TEOS film  4 . In embodiments, after Photo Engraving Process (PEP), photoresist  5  may have a predetermined pattern as shown in  FIG. 2 .  
         [0022]     Referring to  FIG. 3 , TEOS film  4 , nitride film  3 , and oxide film  2  may be sequentially etched using photoresist  5  as an etching mask. In embodiments, even a predetermined portion of silicon substrate  1  can be etched together.  
         [0023]     In embodiments, silicon substrate  1  may be etched to a predetermined depth in a secondary etching process for forming a device isolation film.  
         [0024]     In other words, the device isolation film may be formed using photoresist  5  as an etching mask in first and second etching processes. In the first etching process, oxide film  2 , nitride film  3 , and TEOS film  4  formed on silicon substrate  1  may be etched and an upper part of silicon substrate  1  may be partially etched. In the second etching process, an insulation material may be buried within silicon substrate  1  and may be etched to form an active region.  
         [0025]     Referring to  FIG. 5 , photoresist  5  may be removed. Trench  6 , which may be provided within silicon substrate  1 , may be filled with an insulation material.  
         [0026]     The following description is based on experiments implemented to fabricate a pattern of a mask considering an after-etch CD.  
         [0027]      FIGS. 6 and 7  are graphs of distribution of CD versus pitch of mask pattern. In  FIG. 6 , the horizontal axis represents a pitch (nm) of a mask pattern, and the vertical axis represents a CD (nm) of an after-PEP photoresist. In  FIG. 7 , the horizontal axis represents a pitch (nm) of a mask pattern, and the vertical axis represents a difference (nm) between a CD of an after-PEP photoresist and a CD of an after-etch layer.  
         [0028]      FIG. 6  is a graph of a distribution of a CD of an after-PEP photoresist shown as a pitch of a mask pattern (a distance between patterns) varies.  
         [0029]     Referring to  FIG. 6 , it may be appreciated that when a mask has a size of about 120 nm to 170 nm, a CD of an after-PEP photoresist, which may vary depending on a pattern having a pitch of a diversity of sizes, may vary as the pitch increases. In embodiments, in photolithography process, a difference in an actual etched portion may occur depending on a distance between patterns that light cannot pass through by the proximity effect of light.  
         [0030]     Referring to  FIG. 6 , it may be appreciated that a CD may decrease and may again increase at a semi-dense portion (e.g., a portion with a pitch of about 200 nm to 600 nm). A CD may be almost the same as that of an Iso-line (a portion with a pitch of about 1500 nm or more) when a pitch is within a range of about 700 nm or more.  
         [0031]      FIG. 7  is a graph illustrating a difference between a CD of an after-PEP photoresist and a CD of an after-etch layer depending on a mask pattern having a pitch of a diversity of sizes.  
         [0032]      FIG. 7  illustrates a difference between a CD of a photoresist and a CD of an after-etch layer as a pitch of a mask pattern varies.  FIG. 7  illustrates that the mask pattern may be etched with more accuracy as the CD difference (a so-called CD bias) between the photoresist and the after-etch layer is smaller. In  FIG. 7 , Y-axis denotes the CD difference between the photoresist and the after-etch layer.  
         [0033]     Referring to  FIG. 7 , a variation of bias depending on a size of a mask may not be so great. In other words, masks having a diversity of sizes such as mask  120  to mask  170  may have almost the same CD bias distribution.  
         [0034]     In embodiments, a dense line (a portion with a pitch of about 600 nm or less) may have a bias of about 10 nm. In embodiments, an Iso-line (a portion with a pitch of about 1500 nm or more) may have a bias of about 42 nm.  
         [0035]     Therefore, an OPC model simply modeled using only data on a CD of an after-PEP photoresist without considering these biases may not be able to satisfy a process target in photolithography process. As a result, it may cause inconsistency with a target in an etching process for forming a pattern in a wafer. This may make it very difficult to perform accurate OPC.  
         [0036]      FIG. 8  illustrates a design of a mask fabricated according to embodiments.  
         [0037]     Experimental data for fabricating, by the mask design of  FIG. 8 , a mask considering a target CD after etching will be provided below.  
         [0038]      FIG. 8  illustrates a variety of formats of patterns. The respective patterns may be formed to be at a predetermined interval. The shown mask pattern shows PSC mark  50  and an SRAM.  
         [0039]     The SRAM may be comprised of first pattern  10 , second pattern  20 , third pattern  30 , and fourth pattern  40  each formed to be at a predetermined distance. Respective patterns  10 ,  20 ,  30 , and  40  may be of a shape of a series of regions which light may not pass through.  
         [0040]     In embodiments, first pattern  10  and second pattern  20  may be of an H shape. Third pattern  30  and fourth pattern  40  may be of a rectangular shape.  
         [0041]     Reference numeral  6  denotes a distance between first pattern  10  and second pattern  20 . Reference numeral  4  denotes a distance between second pattern  20  and fourth pattern  40 . Reference numeral  1  denotes a distance between third pattern  30  and fourth pattern  40 .  
         [0042]     Reference numeral  5  denotes a thickness of any one of regions of second pattern  20  which light may not pass through. Reference numerals  2  and  3  denote regions of fourth pattern  40  which light may not pass through.  
         [0043]     In other words, the mask patterns shown in  FIG. 8  can be classified into the first to fourth patterns. The respective patterns may be formed to be at a predetermined distance. Here, a distance between the patterns and a thickness of a region constituting each pattern can be designed in a desirable size.  
         [0044]     Referring to experimental data below, a description of a design for the mask pattern may be apparent. However, Tables 1 to 3 below show many pieces of experiment data and various target CDs for respective regions constituting the mask pattern. Thus, each Table will be divided and described.  
                                   TABLE 1                                   Division   Target (nm)   DI model (nm)   FI model (nm)                           A   105   132   133           B   105   125   127           C   147   137   133           1   151   130   131           2   115   146   142           3   166   194   187           4   345   303   317           5   115   154   138           6   152   126   128                      
 
         [0045]     Table 1 shows a target CD of an after-etch layer, data on a design by general DI model according to the related art art, and data on a design by FI model according to embodiments, based on the fabricating the mask pattern of  FIG. 8 .  
         [0046]     Embodiments use general DI model targeting a CD of an after-PEP photoresist, and suggests FI model to design a CD of an after-etch layer with more accuracy.  
         [0047]     In embodiments, a design for a gap between patterns and a design for a region constituting each pattern may be applied differently.  
         [0048]     Referring to  FIG. 8 , distance  6  between first pattern  10  and second pattern  20 , distance  4  between second pattern  20  and fourth pattern  40 , and distance  1  between third pattern  30  and fourth pattern  40  may be designed to be greater than a target CD by a predetermined length. For example, when a target CD of distance  6  between first pattern  10  and second pattern  20  is equal to 152 nm, a mask may be designed to have a size of 128 nm greater by a predetermined value than a size of 126 nm of DI model considering a CD of an after-PEP photoresist.  
         [0049]     The distance between the patterns may be designed greater than a DI model value considering a CD of an after-PEP photoresist. In such a method, distances  1 ,  4 , and  6  between the patterns may be designed.  
         [0050]     A region constituting each pattern may be designed smaller than a DI model value considering a CD of an after-PEP photoresist. Describing a design for a partial region  5  of second pattern  20  for example, when a target CD is equal to 115 nm, region  5  may be designed to have a size of 154 nm in DI model but may be designed to have a size of 138 nm smaller by a predetermined value than the 154 nm in FI model in accordance with embodiments.  
         [0051]     As described above, FI model considering a CD of an after-etch layer may use a value of DI model, and may have a value greater or smaller within 10% of a design dimension of the DI model.  
         [0052]     An after-PEP CD by DI model and FI model will be described with reference to Table 2. An after-etch CD will be described with reference to Table 3. However, experiments were performed with different target CDs.  
                                   TABLE 2                                   Division   Target (nm)   DI model (nm)   FI model (nm)                           A   105   106   108           B   115   117   117           C   134   133   134           1   141   142   139           2   125   124   128           3   176   171   171           4   335   328   341           5   125   125   127           6   142   135   127                      
 
         [0053]     Table 2 shows data on a target CD of an after-PEP photoresist pattern, data on a CD of a photoresist observed from general DI model, and data on a CD of a photoresist observed from FI model according to embodiments.  
         [0054]     FI model is OPC considering a CD of an after-etch layer. Therefore, it may be appreciated that a CD of an after-PEP photoresist pattern may have a great difference with an actual target value as shown in Table 2.  
         [0055]     DI model is OPC considering a CD of an after-PEP photoresist. Therefore, an actual CD of a photoresist may be closer to a target CD.  
         [0056]     For example, in embodiments when a target is equal to 142 nm, a CD may be equal to 135 nm in DI model but may be equal to 127 nm in FI model.  
         [0057]     However, much importance to an operation of a semiconductor device actually is that how much accurately an active region may be fabricated using a device isolation film formed after etching. Therefore, it may be appreciated that embodiments may be more effective in Table 3 below.  
                                   TABLE 3                                   Division   Target (nm)   DI model (nm)   FI model (nm)                           A   145   143   146           B   135   134   135           C   116   115   116           1   117   119   116           2   150   147   152           3   196   205   200           4   315   298   313           5   145   159   154           6   101   105   100                      
 
         [0058]     Table 3 shows data on a target CD, data on a CD of an after-etch layer etched using a mask designed by DI model for the target CD, and data on a CD of an after-etch layer etched using a mask designed by FI model for the target CD.  
         [0059]     As a result, it may be appreciated that a CD of layer actually observed in FI model for a target CD may be more accurate. For example, in embodiments, for distance  6 , when a target CD is equal to 101 nm, a CD may be equal to 105 nm in DI model but may be equal to 100 nm in FI model.  
         [0060]     It may be desirable that a mask pattern be fabricated such that a distance between respective mask patterns is greater than that of DI model by a predetermined dimension and a region constituting each pattern is smaller than that of DI model by a predetermined dimension.  
         [0061]      FIG. 9  illustrates a pattern of a mask fabricated in accordance with embodiments.  FIGS. 10 and 11  are Scanning Electron Microscope (SEM) photographs of an active region after a device isolation film may be formed using a mask fabricated in accordance with embodiments.  
         [0062]     The mask of  FIG. 9  is a result of using a DI model value considering a CD of an after-PEP photoresist and applying a design for a distance between patterns of a mask and a design for a region constituting a pattern differently, according to embodiments.  
         [0063]     In  FIGS. 10 and 11 , an active region may be formed with more accuracy where a device isolation film is formed using a mask in accordance with embodiments.  
         [0064]     A method for fabricating a mask and a method for forming a device isolation film using the mask may be advantageous in the improvement of device integration because a target CD may be realized with more accuracy.  
         [0065]     It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments. Thus, it is intended that embodiments cover modifications and variations thereof within the scope of the appended claims. It is also understood that when a layer is referred to as being “on” or “over” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present.