Abstract:
A phase-shifting mask suited for equal line/space, small pitched, dense line pattern is disclosed. The phase-shifting mask includes a transparent substrate, a partially shielded mesa line pattern of first phase formed on the substrate, and a clear recessed line pattern of second phase etched into the substrate and is disposed right next to the partially shielded mesa line pattern. The partially shielded mesa line pattern has a plurality of alternating 5%-10% transmittance light-shielding regions and clear regions of the first phase. The partially shielded mesa line pattern and the clear recessed line pattern have the same line width. The light that passes through the clear regions of the first phase and the light that passes through the clear recessed line pattern of second phase have a phase difference of 180 degree.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a phase-shifting mask (PSM), and more particularly, to a chromeless PSM for equal line/space dense line patterns and lithographic method of employing such chromeless PSM.  
         [0003]     2. Description of the Prior Art  
         [0004]     Lithography processing, which is an essential technology when manufacturing conventional integrated circuits, is used for defining geometries, features, lines, or shapes onto a die or wafer. In the integrated circuit making processes, lithography plays an important role in limiting feature size. By using lithography, a circuit pattern can be precisely transferred onto a die or wafer. Typically, to implement the lithography, a designed pattern such as a circuit layout pattern or an ion doping layout pattern in accordance with a predetermined design rule is created on one or several mask in advance. The pattern on the mask is then transferred by light exposure, with a stepper and scanner, onto the wafer.  
         [0005]     It is critical in this field to solve resolution of the lithographic process as the device sizes of the semiconductor industry continue to shrink to the deep sub-micron scale. There are primarily two methods in the prior art for improving resolution. One method involves using short wavelengths of light to expose a photoresist layer on the semiconductor wafer. Short wavelengths of light are desirable as the shorter the wavelength, the higher the possible resolution of the pattern. Another method involves the use of a phase-shifting mask (PSM) to improve the resolution of the pattern transferred to the semiconductor wafer.  
         [0006]     Please refer to  FIG. 1 , which is a structural diagram of a prior art alternating phase-shifting mask  10 . As shown in  FIG. 1 , a fully opaque material such as chrome is used in a non-transparent region  12  of the alternating phase-shifting mask  10 , and the non-transparent region  12  is flanked by transparent regions  14 ,  16 . Both of the transparent regions  14 ,  16  are made of quartz. The thickness of the transparent region  14  is less than that of the transparent region  16 . Therefore, light that passes through the transparent region  14  has a  180 -degree phase shift relative to light that passes through the thicker transparent region  16 , which results in destructive interference and image contrast. Consequently, during the lithographic process, a dark unexposed region falls on an area of a photoresist layer and is located below the non-transparent region  12  of the alternating phase-shifting mask  10 .  
         [0007]     However, the alternating phase-shifting mask (alt-PSM)  10  has to perform a double-exposure/two-mask lithography process involving a trim mask to complete pattern transferring. The first mask is a phase-shifting mask and the second mask is a single-phase trim mask. The phase-shifting mask primarily defines regions requiring phase shifting. The single-phase trim mask primarily defines regions not requiring phase shifting. However, this optical proximity correction (OPC) technique suffers from transmission imbalance occurred in phase shifted and non-phase-shifted regions and other flaws caused by alt-PSM.  
         [0008]     Therefore, a chromeless phase-shifting mask is developed. Please refer to  FIG. 2 , which is a structural diagram of a prior art chromeless phase-shifting mask  20 . As shown in  FIG. 2 , the chromeless phase-shifting mask comprises a transparent region  22  made of quartz, and the transparent region  22  is flanked by two transparent regions  24 ,  26  also made of quartz. The transparent region  22  is thicker than both the transparent regions  24 ,  26 , which causes a 180 degree phase-shifting in light passing through the transparent regions  24 ,  26 .  
         [0009]     In other words, the transparent regions  24 ,  26  are phase-shifting regions, and the transparent region  22  is a non-phase-shifting region. Because of this 180-degree phase difference, there is destructive interference at the phase boundaries of the phase-shifting regions  24 ,  26  and the non-phase-shifting region  22 . Consequently, during the lithographic process, a dark unexposed region falls on an area of a photoresist layer and is located below the non-phase-shifting region  22  of the chromeless phase-shifting mask  20 .  
         [0010]     However, with the increase of packing density of devices such as dynamic random access memory (DRAM) devices, a pitch between adjacent micro features of the device such as word line pitch shrinks dramatically. Please refer to  FIG. 3 .  FIG. 3  is a plan view of a portion of word lines  32  overlying a semiconductor wafer  30 . As shown in  FIG. 3 , pitch P of the word lines  32  is equal to the combination of line width L and the spacing S between two adjacent word lines  32  (P=L+S). When the line width L is less than or equal to 100 nm, and the pitch P is substantially equal to the twice of the line width L of the device and forms a dense pattern, light of 0 degree phase-shifting and light of 180 degrees phase-shifting cancel out. Therefore, the prior art chromeless phase-shifting mask fails to transfer the dense pattern.  
       SUMMARY OF INVENTION  
       [0011]     It is therefore an object of the claimed invention to provide a chromeless phase-shifting mask for solving the above-mentioned problems.  
         [0012]     According to the claimed invention, a chromeless PSM for forming equal line/space, small pitched, dense line pattern is disclosed. The chromeless PSM includes a transparent substrate, a partially shielded mesa line pattern of first phase formed on the substrate, and a clear recessed line pattern of second phase etched into the substrate and is disposed right next to the partially shielded mesa line pattern. The partially shielded mesa line pattern has a plurality of alternating  5 %- 10 % transmittance light-shielding regions and clear regions of the first phase. The partially shielded mesa line pattern and the clear recessed line pattern have the same line width. The light that passes through the clear regions of the first phase and the light that passes through the clear recessed line pattern of second phase have a phase difference of 180 degree.  
         [0013]     From one aspect of this invention, a chromeless PSM comprises a transparent substrate; a plurality of first phase-shifting line patterns having a first substrate thickness of first phase disposed on the transparent substrate along a first direction, wherein each of the first phase-shifting line patterns is disposed thereon clear regions of the first phase and 5%-10% light transmittable block areas. A plurality of 100% light transmittable second phase-shifting line patterns, in parallel with the first phase-shifting line patterns, has a second substrate thickness of second phase. The first phase-shifting line patterns and second phase-shifting line patterns have the same line width and are alternately disposed on the transparent substrate.  
         [0014]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0016]      FIG. 1  is a structural diagram of a prior art alternating phase mask;  
         [0017]      FIG. 2  is a structural diagram of a prior art chromeless phase-shifting mask;  
         [0018]      FIG. 3  is a plan view of a portion of word lines overlying a semiconductor wafer;  
         [0019]      FIG. 4  is a plan view of a portion of the layout of a chromeless PSM in accordance with one preferred embodiment of this invention;  
         [0020]      FIG. 5  is a schematic, cross-sectional view of the chromeless PSM taken along line I-I of  FIG. 4 ;  
         [0021]      FIG. 6  is a schematic, cross-sectional view of the chromeless PSM taken along line II-II of  FIG. 4 ;  
         [0022]      FIG. 7  is a plan view of a portion of the resultant equal line/space dense line pattern transferred from the chromeless PSM of this invention to a photoresist film coated on a wafer;  
         [0023]      FIG. 8  is a schematic diagram illustrating the CD uniformity of the equal line/space dense line patterns  202   a  and  202   b  of  FIG. 7  according to the first preferred embodiment of this invention;  
         [0024]      FIG. 9  is a plan view of a portion of the layout of a chromeless PSM in accordance with second preferred embodiment of this invention;  
         [0025]      FIG. 10  is a schematic, cross-sectional view of the chromeless PSM taken along line I-I of  FIG. 9 ;  
         [0026]      FIG. 11  is a schematic, cross-sectional view of the chromeless PSM taken along line II-II of  FIG. 9 ;  
         [0027]      FIG. 12  is a plan view of a portion of the resultant equal line/space dense line pattern transferred from the chromeless PSM of  FIG. 9  to a photoresist film in accordance with the second embodiment of this invention; and  
         [0028]      FIG. 13  is a schematic diagram illustrating the CD uniformity of the equal line/space dense line patterns  202   a  and  202   b  of  FIG. 12  according to the second preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]     In describing the preferred embodiment of the present invention, reference will be made herein to  FIGS. 4-13  of the drawings, wherein like numerals designate like components, areas or regions. Features of the invention are not drawn to scale in the drawings.  
         [0030]     The present invention pertains to an improved chromeless phase-shifting mask (PSM), which is capable of solving equal line/space, small pitched, dense line patterns such as word lines or gate conductors of trench-capacitor dynamic random access memory (DRAM) devices having critical line width that is less than or equal to 100 nanometers. The critical dimension (CD) uniformity of the resultant equal line/space dense line pattern transferred from the chromeless PSM of this invention to a photoresist film coated on a wafer is also enhanced.  
         [0031]     Please refer to  FIGS. 4-7 , wherein  FIG. 4  is a plan view of a portion of the layout of a chromeless PSM in accordance with one preferred embodiment of this invention;  FIG. 5  is a schematic, cross-sectional view of the chromeless PSM taken along line I-I of  FIG. 4 ;  FIG. 6  is a schematic, cross-sectional view of the chromeless PSM taken along line II-II of  FIG. 4 ;  FIG. 7  is a plan view of a portion of the resultant equal line/space dense line pattern transferred from the chromeless PSM of this invention to a photoresist film coated on a wafer.  
         [0032]     As shown in  FIG. 4 , the chromeless PSM in accordance with one preferred embodiment of this invention comprises a transparent quartz substrate  100 , a plurality of first phase-shifting line patterns  102   a - 102   f  arranged in parallel with each other along the reference Y-axis, and a plurality of second phase-shifting line patterns  104   a - 104   e  arranged in parallel with each other along the reference Y-axis. According to the preferred embodiment, the line width of each of the first phase-shifting line patterns  102   a - 102   f  and the line width of each of the second phase-shifting line patterns  104   a - 104   e  are the same.  
         [0033]     The aforesaid first phase-shifting line patterns  102   a - 102   f  and second phase-shifting line patterns  104   a - 104   e  are alternately formed on the quartz substrate  100 . By way of example, the second phase-shifting line pattern  104   a  is disposed between the first phase-shifting line pattern  102   a  and the first phase-shifting line pattern  102   b , the second phase-shifting line pattern  104   b  is disposed between the first phase-shifting line pattern  102   b  and the first phase-shifting line pattern  102   c , and so forth. Besides, along each of the first phase-shifting line patterns  102   a - 102   f , a plurality of block areas  106   a - 106   f  are provided. The block areas are disposed spaced apart from each other along each of first phase-shifting line patterns  102   a - 102   f . As shown in  FIG. 4 , for example, a 100%-light transmission first phase-shifting area  108   a  is disposed between two adjacent block areas  106   a  along the first phase-shifting line pattern  102   a . The block areas are equal in size and have a light transmission rate of about 5%-10%, i.e., merely 5%-10% of the incident light can pass through each of the block areas.  
         [0034]     According to the first preferred embodiment, each of the block areas  106   a - 106   f  is masked by a phase shifter such as MoSi, MoSiO, MoSiON or the like.  
         [0035]     Therefore, the second phase-shifting line patterns  104   a - 104   e  of the chromeless PSM of this invention are 100% light transmittable. Each of the first phase-shifting line patterns  102   a - 102   f  of the chromeless PSM encompasses alternating 100% light transmittable clear areas and 5%-10% light transmittable block areas. The phase-shifting mask of this invention is partially shielded along the mesa line pattern  102   a - 102   f  of first phase. According to the first preferred embodiment, the length of one side of each of the rectangular 5%-10% light transmittable block areas  106   a - 106   f  along the reference Y-axis ranges approximately from λ/4 to 3λ/4 (λ: wavelength of the exposure light source of the stepper and scanner, in nanometer). The length of one side of each of the rectangular 100% light transmittable clear areas  108   a - 108   f  along the reference Y-axis ranges approximately from λ/4 to 3λ/4.  
         [0036]     As shown in  FIG. 5  and  FIG. 6 , the thickness of the quartz substrate  100  underneath each of the first phase-shifting line patterns  102   a - 102   f  is denoted as t 1 , and the thickness of the quartz substrate  100  underneath each of the second phase-shifting line patterns (also referred to as “clear recessed line patterns”)  104   a - 104   e  is denoted as t 2 , wherein t 1 is greater than t 2 (t 1 &gt;t 2 ), such that light passing through the quartz substrate  100  having different thicknesses produces image contrast. The phase difference between the phase of light passed through the first phase-shifting line patterns  102   a - 102   f  and the phase of light passed through the second phase-shifting line patterns  104   a - 104   e  is 180 degree. Preferably, the phase of light passed through the first phase-shifting line patterns  102   a - 102   f  is 0-degree, while the phase of light passed through the second phase-shifting line patterns  104   a - 104   e  is 180-degree (π).  
         [0037]     According to the first preferred embodiment, the plurality of spaced apart 5%-10% light transmittable block areas  106   a - 106   f , which are disposed on each of the first phase-shifting line patterns  102   a - 102   f , are aligned with the reference X-axis. By providing such unique layout of the chromeless PSM, resultant dense line patterns  202   a - 202   f  transferred from the chromeless PSM of this invention to a photoresist film coated on a wafer is depicted in  FIG. 7 . However, the critical dimension (CD) uniformity of the resultant dense line pattern (in equal line/space fashion) is still not satisfactory.  
         [0038]     Please refer to  FIG. 8 .  FIG. 8  is a schematic diagram illustrating the CD uniformity of the equal line/space dense line patterns  202   a  and  202   b  of  FIG. 7  according to the first preferred embodiment of this invention. As can be seen in this figure, the variation of the CD of the line pattern  202   a  or  202   b  is high, and leads to wavelike line profiles.  
         [0039]     Please refer to  FIGS. 9-12 , wherein  FIG. 9  is a plan view of a portion of the layout of a chromeless PSM in accordance with second preferred embodiment of this invention;  FIG. 10  is a schematic, cross-sectional view of the chromeless PSM taken along line I-I of  FIG. 9 ;  FIG. 11  is a schematic, cross-sectional view of the chromeless PSM taken along line II-II of  FIG. 9 ;  FIG. 12  is a plan view of a portion of the resultant equal line/space dense line pattern transferred from the chromeless PSM of  FIG. 9  to a photoresist film in accordance with the second embodiment of this invention.  
         [0040]     As shown in  FIG. 9 , the chromeless PSM in accordance with the second preferred embodiment of this invention comprises a transparent quartz substrate  100 , a plurality of first phase-shifting line patterns  102   a - 102   f  arranged in parallel with each other along the reference Y-axis, and a plurality of second phase-shifting line patterns  104   a - 104   e  arranged in parallel with each other along the reference Y-axis. According to the second preferred embodiment, the line width of each of the first phase-shifting line patterns  102   a - 102   f  and the line width of each of the second phase-shifting line patterns  104   a - 104   e  are the same.  
         [0041]     Likewise, the aforesaid first phase-shifting line patterns  102   a - 102   f  and second phase-shifting line patterns  104   a - 104   e  are alternately formed on the quartz substrate  100 . By way of example, the second phase-shifting line pattern  104   a  is disposed between the first phase-shifting line pattern  102   a  and the first phase-shifting line pattern  102   b , the second phase-shifting line pattern  104   b  is disposed between the first phase-shifting line pattern  102   b  and the first phase-shifting line pattern  102   c , and so forth. Besides, along each of the first phase-shifting line patterns  102   a - 102   f , a plurality of block areas  106   a - 106   f  are provided. The block areas are disposed equally spaced apart from each other along each of first phase-shifting line patterns  102   a - 102   f . As shown in  FIG. 9 , for example, a 100%-light transmission first phase-shifting area  108   a  is disposed between two adjacent block areas  106   a  along the first phase-shifting line pattern  102   a . These block areas are equal in size and have a light transmission rate of about 5%-10%, i.e., merely 5%-10% of the incident light can pass through each of the block areas.  
         [0042]     Therefore, the second phase-shifting line patterns  104   a - 104   e  of the chromeless PSM of this invention are 100% light transmittable. Each of the first phase-shifting line patterns  102   a - 102   f  of the chromeless PSM encompasses alternating 100% light transmittable clear areas and 5%-10% light transmittable block areas. According to the second preferred embodiment, the length of one side of each of the rectangular 5%-10% light transmittable block areas  106   a - 106   f  along the reference Y-axis ranges approximately from λ/4 to 3λ/4 (λ: wavelength of the exposure light source of the stepper and scanner in nanometer). The length of one side of each of the rectangular 100% light transmittable clear areas  108   a - 108   f  along the reference Y-axis ranges approximately from λ/4 to 3λ/4. Each of the block areas  106   a - 106   f  may be formed from phase shifter such as MoSi, MoSiO, MoSiON or the like.  
         [0043]     As shown in  FIG. 10  and  FIG. 11 , the thickness of the quartz substrate  100  underneath each of the first phase-shifting line patterns  102   a - 102   f  is denoted as t 1 , and the thickness of the quartz substrate  100  underneath each of the second phase-shifting line patterns  104   a - 104   e  is denoted as t 2 , wherein t 1 is greater than t 2 (t 1 &gt;t 2 ), such that light passing through the quartz substrate  100  having different thicknesses produces image contrast. The phase difference between the phase of light passed through the first phase-shifting line patterns  102   a - 102   f  and the phase of light passed through the second phase-shifting line patterns  104   a - 104   e  is 180 degree. Preferably, the phase of light passed through the first phase-shifting line patterns  102   a - 102   f  is 0-degree, while the phase of light passed through the second phase-shifting line patterns  104   a - 104   e  is 180-degree (π).  
         [0044]     According to the second preferred embodiment, the 5%-10% light transmittable block areas on two adjacent first phase-shifting line patterns are not aligned with the reference X-axis. For example, as best seen in  FIG. 9 , the spaced apart 5%-10% light transmittable block areas  106   a ,  106   c , and  106   e , which are disposed on each of the first phase-shifting line patterns  102   a ,  102   c , and  102   e , respectively, are aligned with the reference X-axis, while the spaced apart 5%-i  0 % light transmittable block areas  106   b ,  106   d , and  106   f , which are disposed on each of the first phase-shifting line patterns  102   b ,  102   d , and  102   f , respectively, are aligned with the reference X-axis. By providing such improved layout of the chromeless PSM, resultant dense line patterns  302   a - 302   f  transferred from the chromeless PSM of this invention to a photoresist film coated on a wafer is depicted in  FIG. 12 . The CD uniformity of the resultant dense line pattern (in equal line/space fashion) is enhanced.  
         [0045]      FIG. 13  is a schematic diagram illustrating the CD uniformity of the equal line/space dense line patterns  202   a  and  202   b  of  FIG. 12  according to the second preferred embodiment of this invention. As can be seen in this figure, the variation of the CD of the line pattern  302   a  or  302   b  is reduced.  
         [0046]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.