Abstract:
A lithography method for improving contrast includes the following steps: To provide a light source. To provide a first plate including at least one opening rotates according to at least one angular velocity. To provide a mask having patterns on it. To provide a second plate including at least one block corresponding to the opening rotates according to the same angular velocity as the first plate. The method also includes a step to perform an exposure process such that zero order light diffracted by the mask is hindered by the block.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a lithography method, and more particularly, to a lithography method utilizing a designed coherent plate in conjunction with a matching diffraction plate to form patterns having a superior contrast in a photoresist layer.  
         [0003]     2. Description of the Prior Art  
         [0004]     In integrated circuit manufacturing processes, a lithographic process has become a mandatory technique. In a lithographic process, a designed pattern, such as a circuit pattern, a doping pattern, a contact hole pattern, or a trench pattern, is created on one or several photo masks, then the pattern on the photo mask is transferred by light exposure, with a stepper or a scanner, into a photoresist layer on a semiconductor wafer. Only by using a lithographic process can a wafer producer precisely and clearly transfer a complicated circuit pattern onto a semiconductor wafer.  
         [0005]     It is an important issue for solving resolution of the lithographic process due to the reducing device sizes of the semiconductor industry. Theoretically, using short wavelengths of light to expose a photoresist layer will improve the resolution right away. Short wavelengths of light are desirable as the shorter the wavelength, the higher the possible resolution of the pattern. This method, though it seems simple, is not feasible. First, light sources for providing short wavelengths of light are not accessible. Secondly, the damage of equipment is very considerable when short wavelengths of light is used to expose a photoresist layer, leading to a shorted equipment lifetime. The cost is thus raised, which makes products not competitive. Due to the conflicts between theory and practice used in manufacturing, the manufacturers are all devoted to various researches so as to overcome this problem.  
         [0006]     Please refer to  FIG. 1 ,  FIG. 1  is a schematic diagram illustrating a lithography method according to the prior art. As shown in  FIG. 1 , light beams originating from a light source  12  pass through a coherent plane  14  first, then evenly illuminate a mask  16  having patterns  18  on it. Diffraction effects thus occur because the patterns  18  on the mask  16  hinder incident light. The coherent plane  14  is usually a lens. However, after light passing through the lens, the original function of space variables g(x,y,z) is transformed to a function of angular spatial frequencies G(f x ,f y ,f z ) by a Fourier transformation (G(f x ,f y ,f z )=F{g(x,y,z)}.  
         [0007]     Please refer to  FIG. 2 ,  FIG. 2  is a schematic diagram illustrating the types of light functions before and after a Fourier transformation. In order to facilitate illustration, the zero order light and the ±first order light are both shown in  FIG. 2 . However, the ±first order light is not separated out until the incident light is diffracted by the patterns  18 . As shown in  FIG. 2 , the types of these two functions are different from each other although they both represent light intensity. Later, the even incident light diffracted by the patterns  18  is separated into diffraction light of different orders.  
         [0008]     Please refer back to  FIG. 1 , the diffraction light of different orders is thereafter incident upon a diffraction plane  22  of projection lens  24  to allow the projection lens  24  to collect the diffraction light of different orders and to focus them on a wafer  26 . The diffraction plane  22  is usually a lens. After light passing through the lens, the transformed function of angular spatial frequencies G(f x ,f y ,f z ) is transformed back to another function of space variables g′(x,y,z) by another Fourier transformation (g′(x,y,z)=F{G(f x ,f y ,f z )}, and the type of g′(x,y,z) is the same as that of g(x,y,z). Similarly, the types of these two functions are different from each other although they both represent light intensity.  
         [0009]     Please refer to  FIG. 1  and  FIG. 3 ,  FIG. 3  is a schematic diagram illustrating light of different orders collected by a numeric aperture  28 . As shown in  FIG. 3 , the zero order light and part of the ±first order light are collected by the numeric aperture (NA)  28  of the projection lens  24  after this Fourier transformation, and are focused to the wafer  26 . However, the smaller the critical dimension (CD) is, the larger the diffraction angle of the incident light is with the same exposure light source. That means, when the critical dimension of the patterns  18  is very small, the diffraction angle is large to cause a large period of the zero order light (ΔP, as shown in  FIG. 2 ). Please refer to  FIG. 4 ,  FIG. 4  is an image intensity versus position curve acquired by performing the prior art lithography method. As shown in  FIG. 4 , the resulted curve is formed by adding up the intensity of the zero order light, partial of the +first order light, and partial of the −first order light. It is worth noting that the resulted curve has an I min  not equal to zero due to the existence of the zero order light.  
         [0010]     Since the contrast of an image is defined as C=(I max −I min )/(I max +I min ), the smaller the I min  is, the higher the contrast is. Once the I min  is high, the image contrast is poor, leading to unsatisfied resolution. Actually, the zero order light, becoming a constant in a Fourier transform series, does not carry any pattern signals. Rather, it represents the background intensity (I min ). That means, in order to obtain an increased contrast and a satisfied resolution, the zero order light needs to be eliminated.  
         [0011]     Therefore, it is very important to develop a lithography method to eliminate the zero order light so as to effectively improve the contrast and resolution of the patterns. This method is able to be applied to small-sized patterns, and should not damage equipment when using the current equipment. In addition, this method should not add any difficulty and complexity to routine processing, and should be implanted to the production line very easily without causing extra labor cost.  
       SUMMARY OF INVENTION  
       [0012]     It is therefore an objective of the claimed invention to provide a lithography method utilizing a designed coherent plate in conjunction with a matching diffraction plate to resolve the above-mentioned problem.  
         [0013]     According to the claimed invention, a lithography method for improving contrast comprising eliminating zero order light by utilizing a first plate in conjunction with a matching second plate is provided. The method comprises the following steps: To provide a light source. To provide a first plate comprising at least one opening rotates according to at least one angular velocity. To provide a mask having patterns on it. To provide a second plate comprising at least one block corresponding to the opening rotates according to the same angular velocity as the first plate. The method also comprises a step to perform an exposure process such that the zero order light diffracted by the mask is hindered by the block.  
         [0014]     The present invention method for improving the contrast of patterns utilizes a designed coherent plane in conjunction with a matching diffraction plane. The background intensity (I min ) is therefore zero by effectively eliminating the zero order light, which becomes a constant in a Fourier transform series and does not carry any pattern signals. The contrast of patterns is thus increased to improve the resolution of patterns. In summary, the present invention method can be applied to small-sized patterns, and does not damage equipment when using the current equipment. In addition, the present invention method does not add any difficulty and complexity to routine processing, and can be implanted to the production line very easily without causing extra labor cost.  
         [0015]     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the multiple figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram illustrating a lithography method according to the prior art.  
         [0017]      FIG. 2  is a schematic diagram illustrating the types of light functions before and after a Fourier transformation.  
         [0018]      FIG. 3  is a schematic diagram illustrating light of different orders collected by a numeric aperture.  
         [0019]      FIG. 4  is an image intensity versus position curve acquired by performing the prior art lithography method.  
         [0020]      FIG. 5  is a schematic diagram illustrating a lithography method according to the present invention.  
         [0021]      FIG. 6  is a schematic diagram illustrating a coherent plane according to a first preferred embodiment of the present invention.  
         [0022]      FIG. 7  is a schematic diagram illustrating the working principle of the present invention method.  
         [0023]      FIG. 8  is a schematic diagram illustrating light of different orders collected by a numeric aperture.  
         [0024]      FIG. 9  is an image intensity versus position curve acquired by performing the present invention lithography method.  
         [0025]      FIG. 10  is a schematic diagram illustrating a coherent plane according to a second preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Please refer to  FIG. 5  and  FIG. 6 ,  FIG. 5  is a schematic diagram illustrating a lithography method according to the present invention,  FIG. 6  is a schematic diagram illustrating a coherent plane  104  according to a first preferred embodiment of the present invention. As shown in  FIG. 5 , light beams originating from a light source  102  pass through a coherent plane  104  first. As shown in  FIG. 6 , a plurality of concentric ring-shaped regions  105  are included in the coherent plane  104 , and the plurality of concentric ring-shaped regions  105  take a center of the coherent plane  104  as center points. Each of the ring-shaped regions  105  comprises at least one opening  107  in a slit shape. The opening  107  in each of the ring-shaped regions  105  is interlaced with each opening  107  in each other ring-shaped region  105 . The coherent plane  104  rotates according to at least one angular velocity.  
         [0027]     Actually, the coherent plane  104  may have different designs, not limited in the design shown in  FIG. 6 . The coherent plane  104  may comprise only one ring-shaped region  105  taking a center of the coherent plane  104  as a center point, and the ring-shaped region  105  comprises at least one opening  107  in a slit shape. No matter how, light beams originating from the light source  102  pass through the openings  107  to form small partial coherent illumination. The coherent plane  104  may be a lens stacked with a baffle plate or other apparatus.  
         [0028]     However, after light passing through the coherent plane, the original functions of space variables g 1 (x,y,z), g 2 (x,y,z), g 3  (x,y,z), etc. are transformed to functions of angular spatial frequencies G 1 (f x ,f y ,f z ), G 2 (f x ,f y ,f z ), G 3 (f x ,f y ,f z ), etc., respectively, by Fourier transformations (G 1 (f x ,f y ,f z )=F{g 1 (x,y,z), G 2 (f x ,f y ,f z )=F{g 2 (x,y,z), etc.}. Please refer to  FIG. 7 ,  FIG. 7  is a schematic diagram illustrating the working principle of the present invention method. As shown in  FIG. 7 , the types of light functions before and after passing the coherent plane  104  are different from each other, by taking one of the functions as an example, although they both represent light intensity. In order to facilitate illustration, the zero order light and the ±first order light are both shown in  FIG. 7  at the beginning. However, the ±first order light is not separated out until the incident light is diffracted by the patterns  108  (as shown in  FIG. 5 ).  
         [0029]     Please refer back to  FIG. 5 , light beams passing through the coherent plane  104  then evenly illuminate a mask  106  having patterns  108  on it. Diffraction effects thus occur because the patterns  108  on the mask  106  hinder incident light. Later, the even incident light diffracted by the patterns  108  is separated into diffraction light of different orders. The diffraction light of different orders is thereafter incident upon a diffraction plane  112  of projection lens  114  to allow the projection lens  114  to collect the diffraction light of different orders and to focus them on a wafer  116 .  
         [0030]     A plurality of blocks  118  which are corresponding to the openings  107  are included in the diffraction plane, and the diffraction plane  112  rotates according to the same angular velocity as the coherent plane  104 . Since the site and dimensions of each of the blocks  118  are decided through sophisticated calculation by a computer, the unwanted light can be hindered by the blocks  118 . In the present invention method, each of the blocks  118  hinders the zero order light passing through the corresponding opening  107 , as shown in  FIG. 7 . The diffraction plane  112  may be a lens stacked with a baffle plate or other apparatus. Actually, each of the blocks may be regarded as a filter in this optical system. Furthermore, any design with which light beams passing through the coherent plane can evenly illuminate the mask, and the first order light diffracted by the mask can be eliminated effectively is within the scope of the present invention method.  
         [0031]     Later, the transformed functions of angular spatial frequencies G 1 (f x ,f y ,f z ), G 2 (f x ,f y ,f z ), G 3 (f x ,f y ,f z ), etc. are transformed back to functions of space variables g 1 ′(x,y,z), g 1 ′(x,y,z), g 1 ′(x,y,z), etc., respectively, by Fourier transformations (g 1 ′(x,y,z)=F{G 1 (f x ,f y ,f z ), g 2 ′(x,y,z)=F{G 2 (f x ,f y ,f z ), etc.} after light passing through the diffraction plane  112 . The type of g 1 (x,y,z) is the same as that of g 1 ′(x,y,z). Similarly, the types of the functions before and after passing through the diffraction plane  112  are different from each other although they both represent light intensity. Since each of the blocks  118  hinders the zero order light passing through the corresponding opening  107  as mentioned previously, some of the light disappears.  
         [0032]     Please refer to  FIG. 8 ,  FIG. 8  is a schematic diagram illustrating light of different orders collected by a numeric aperture  122 . As shown in  FIG. 8 , the zero order light is eliminated. Therefore, part of the +first order light and the −first order light are collected by the numeric aperture  122  of the projection lens  114  and are focused to the wafer  116 . Please refer to  FIG. 9 ,  FIG. 9  is an image intensity versus position curve acquired by performing the present invention lithography method. As shown in  FIG. 9 , the resulted curve is formed by adding up the intensity of partial of the +first order light and partial of the −first order light. It is worth noting that the resulted curve has an I min  equal to zero due to the eliminating of the zero order light. Actually, the zero order light, becoming a constant in a Fourier transform series, does not carry any pattern signals. Therefore, no pattern signal are lost when the background intensity (I min ) is zero. As a result, patterns (not shown) having a superior contrast are formed in a photoresist layer (not shown) on the wafer  116 .  
         [0033]     Since the contrast of a image is defined as C=(I max −I min )/(I max +I min ), the smaller the I min  is, the higher the contrast is. When the I min  is equal to zero, a superior image contrast is resulted in, leading to a satisfied resolution.  
         [0034]     Please refer to  FIG. 10 ,  FIG. 10  is a schematic diagram illustrating a coherent plane  204  according to a second preferred embodiment of the present invention. The only difference between the first preferred embodiment and the second preferred embodiment is the shape of the opening. As shown in  FIG. 10 , a plurality of concentric ring-shaped regions  205  are included in the coherent plane  204 , and the plurality of concentric ring-shaped regions  205  take a center of the coherent plane  204  as center points. Each of the ring-shaped regions  205  comprises at least one opening  207  in a circular shape. The opening  207  in each of the ring-shaped regions  205  is interlaced with each opening  207  in each other ring-shaped region  205 . Therefore, light beams originating from the light source (not shown) pass through the openings  207  to form small partial coherent illumination. Actually, different pupil functions (P) are involved in the calculation when the openings  107 ,  207  are in different shapes. Since the working principle in other portions of the second preferred embodiment is the same as that of the first preferred embodiment, it is not mentioned redundantly.  
         [0035]     It is worth noting that the center point of the coherent plane is not light transmitting. In Fourier transformation, the maximum value occurs at the origin (x=0, y=0). The center point thus becomes a very bright spot. Under the circumstances, the center point is designed as not light transmitting to avoid uneven illumination and unwanted light revealing. In addition, the light source may comprise an on-axis illumination light source, such as a circular illumination, or an off-axis illumination light source, such as an annular illumination, a dipole illumination, a tripole illumination, or a quadruple illumination. Although different illumination methods will provide different illumination patterns, the same working principle is employed. No matter what kind of illumination method is utilized, the diffraction plane in conjunction with the designed coherent plane can be found out through sophisticate calculation.  
         [0036]     The present invention lithography method, used for improving contrast of patterns, utilizes a designed coherent plane in conjunction with a matching diffraction plane. Therefore, the zero order light is eliminated to result in an I min  equal to zero, leading to a superior image contrast. When applying the present invention method to a practical production line, the resolution of patterns is improved. The equipment is not damaged. Furthermore, the processing complexity and labor cost are not increased.  
         [0037]     In contrast to the prior art method, the present invention method utilizes a designed coherent plane in conjunction with a matching diffraction plane. By effectively eliminating the zero order light, which becomes a constant in a Fourier transform series and does not carry any pattern signals, the background intensity (I min ) is zero. The contrast of patterns is thus increased to improve the resolution of patterns. In summary, the present invention method is able to be applied to small-sized patterns, and does not damage equipment when using the current equipment. In addition, the present invention method does not add any difficulty and complexity to routine processing, and can be implanted to the production line very easily without causing extra labor cost.  
         [0038]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.