Patent Publication Number: US-7709166-B2

Title: Measuring the effect of flare on line width

Description:
RELATED PATENT DOCUMENTS 
     This patent document is a continuation under 35 U.S.C. §120 of U.S. patent application Ser. No. 10/566,804 filed on Jan. 30, 2006, now U.S. Pat. No. 7,556,900 which is a 35 U.S.C. §371 national stage entry of International Application No. PCT/IB2004/02444 filed on Jul. 31, 2004 which claims priority benefit under 35 U.S.C. §119 of U.S. Application No. 60/492,130 filed on Aug. 1, 2003 to which priority is also claimed here. 
    
    
     The invention relates to semiconductor process. More particularly the invention relates to measuring the degree of flare distortion of printed features. 
     The electronics industry continues to rely upon advances in semiconductor technology to realized higher-function devices in more compact areas. For many applications, realizing higher-functioning devices requires integrating a large number of electronic devices into a single silicon wafer. As the number of electronic devices per given area of the silicon wafer increases, the manufacturing process becomes more difficult. 
     A large variety of semiconductor devices has been manufactured having various applications in numerous disciplines. Such silicon-based semiconductor devices often include metal-oxide-semiconductor (MOS) transistors, such as p-channel MOS (PMOS), n-channel MOS (NMOS) and complementary MOS (CMOS) transistors, bipolar transistors, BiCMOS transistors. 
     Each of these semiconductor devices generally includes a semiconductor substrate on which a number of active devices are formed. The particular structure of a given active device can vary between device types. For example, in MOS transistors, a active device generally includes source and drain regions and a gate electrode that modulates current between the source and drain regions. 
     One important step in the manufacturing of such devices is the formation of devices, or portions thereof, using photolithography and etching processes. In photolithography, a wafer substrate is coated with a light-sensitive material called photo-resist. Next, the wafer is exposed to light; the light striking the wafer is passed through a mask plate. This mask plate defines the desired features to be printed on the substrate. After exposure, the resist-coated wafer substrate is developed. The desired features as defined on the mask are retained on the photo resist-coated substrate. Unexposed areas of resist are washed away with a developer. The wafer having the desired features defined is subjected to etching. Depending upon the production process, the etching may either be a wet etch, in which liquid chemicals are used to remove wafer material or a dry etch, in which wafer material is subjected to a radio frequency (RF) induced plasma. 
     Often desired features have particular regions in which the final printed and etched regions have to be accurately reproduced over time. These are referred to as critical dimensions (CDs). As device geometry approaches the sub-micron realm, wafer fabrication becomes more reliant on maintaining consistent CDs over normal process variations. The active device dimensions as designed and replicated on the photo mask and those actually rendered on the wafer substrate have to be repeatable and controllable. In many situations, the process attempts to maintain the final CDs equal to the masking CDs. However, imperfections in the process or changes in technology (that may be realized in a given fabrication process, if the process were “tweaked”) often necessitate the rendering of final CDs that deviate from the masking CDs. 
     U.S. Pat. No. 5,757,507 of Ausschnitt et al. relates generally to manufacturing processes requiring lithography and, more particularly, to monitoring of bias and overlay error in lithographic and etch processes used in microelectronics manufacturing which is particularly useful more monitoring pattern features with dimensions on the order of less than 0.5 micron. 
     U.S. Pat. No. 5,962,173 of Leroux et al. relates generally to the field of fabricating integrated circuits and more particularly to maintaining accuracy in the fabrication of such circuits having extremely narrow line elements such as gate lines. 
     U.S. Pat. No. 5,902,703 of Leroux et al. relates generally to the field of fabricating integrated circuits and more particularly to maintaining accuracy in the fabrication of such circuits having relatively narrow line elements such as gate lines. The invention is also directed to the verification of stepper lens fabrication quality. 
     U.S. Pat. No. 5,976,741 of Ziger et al. relates generally to methods of determining illumination exposure dosages and other processing parameters in the field of fabricating integrated circuits. More particularly, the invention concerns methods of processing semiconductor wafers in step and repeat systems. 
     U.S. Pat. No. 6,301,008 B1 of Ziger et al. relates to semiconductor devices and their manufacture, and more particularly, to arrangements and processes for developing relatively narrow line widths of elements such as gate lines, while maintaining accuracy in their fabrication. 
     U.S. Patent Application US 2002/0182516 A1 of Bowes relates generally to metrology of semiconductor manufacturing processes. More particularly, the present invention is a needle comb reticle pattern for simultaneously making critical dimension (CD) measurements of device features and registration measurements of mask overlays relative to semiconductor wafers during processing of semiconductor wafers. This reference and those previously cited are herein incorporated by reference in their entirety. 
     Lens aberrations limit the quality of reproducing a mask pattern into a photo resist. A significant type of lens aberrations is flare in which stray light can distort printed features. Flare can vary widely from stepper to stepper even from lenses of the same stepper family. In the manufacture of semiconductors, it is important to be able to assess the effect of flare due to various exposure tools. In general, flare degrades resist profiles by exposing regions that otherwise should remain underexposed. There is a need for quantifying the effects of flare so that the user may take action to minimize these effects so that product yield is increased and costs may be lowered. 
     In an example embodiment, in a photo lithography process on a photo resist coated substrate, there is a method for determining the effect of flare on line shortening. The method comprises printing a dark-field mask at a first die position on the substrate and in a first exposure. The dark-field mask includes a flare pattern corresponding to one corner of the dark-field mask, a correction box opening, and a focus box pattern on the substrate. In a second exposure, the method prints a clear-field mask including another flare pattern corresponding to an opposite corner of the clear-field mask. At a second die position on the substrate, a composite mask pattern based on features of the dark-field mask and the light field mask is printed. The printed patterns are developed and measurements are obtained from the printed patterns. As a function of the measurements, the effect of flare is determined. A feature of this embodiment further comprises, measuring the dimensions of the flare box pattern of features printed with the dark-field mask and features printed with the clear-field mask, measuring the dimensions of the correction box features printed during the first exposure and features printed during the second exposure, and measuring the dimensions of the focus box pattern printed at the second die position. 
     In another example embodiment, in a photo-lithography process on a photo resist coated substrate, there is a method for determining the effect of flare on line shortening. The method comprises, printing a first mask including a flare pattern corresponding to one corner of the first mask, at a first die position on the substrate and in a first exposure; in a second exposure, a second mask including another flare pattern corresponding to an opposite corner of the second mask, is printed. At a second die position on the substrate, a composite mask pattern based on features of the first mask and the second mask is printed. The printed patterns are developed and measurements are obtained from the printed patterns. As a function of the measurements, the effect of flare is determined. 
     In yet another embodiment, there is a system within a stepper apparatus, in a photo lithography process on a photo resist coated substrate, for determining the effect of flare on line shortening. The system comprises means for printing, at a first die position on the substrate and in a first exposure, a first mask that includes a flare pattern corresponding to one corner of the first mask, and in a second exposure, means for printing a second mask that includes another flare pattern corresponding to an opposite corner of the second mask. At a second die position on the substrate, there are means for printing a composite mask pattern based on features of the first mask and second mask. There are means for developing the printed patterns and obtaining measurements from the printed patterns. As a function of the measurements, there are means for determining the effect of flare. 
     In yet another embodiment, there is a mask set for use in a wafer stepper. The mask set comprises a first mask having features of predetermined dimensions laid out in a dark-field and a second mask having features of predetermined dimensions laid out in a clear-field. Included in the first mask, the features are comprised of a first portion of a flare pattern, a first portion of a box-in-a-box correction pattern, and a first portion of a focus box pattern. Included in the second mask, the features are comprised of a second portion of the flare pattern, a second portion of the box-in-a-box correction pattern, the second portion of the box-in-a-box correction pattern alignable to the first portion of the box-in-a-box correction pattern, and a second portion of the focus box pattern, the second portion of the focus box pattern alignable to the first portion of the focus box pattern. An additional feature of this embodiment, is that a the mask set further comprises a third mask having features of predetermined dimensions, wherein the features are defined by the combination of the features of the first mask and the features of the second mask. 
     The above summaries of the present invention are not intended to represent each disclosed embodiment, or every aspect, of the present invention. Other aspects and example embodiments are provided in the figures and the detailed description that follows. 
    
    
     
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  depicts a composite grating box used for calculating flare according to an embodiment of the present invention; 
         FIG. 2A  is a dark-field mask used in the printing of the composite grating box of  FIG. 1 ; 
         FIG. 2B  illustrates a light-field mask used in combination with the dark-field mask of  FIG. 2A  to print the composite grating box of  FIG. 1   
         FIG. 2C  depicts the combination of the masks of  FIGS. 2A and 2B  according to an embodiment of the present invention; 
         FIG. 3  depicts the scattering of light passing through a mask feature and the stepper lens; 
         FIG. 4  depicts a dark-field focus box pattern 
         FIG. 5A  depicts a dark-field mask incorporating the focus-box pattern of  FIG. 4 ; 
         FIG. 5B  depicts a light field mask used in conjunction with the mask depicted in  FIG. 5A ; 
         FIG. 5C  illustrates the combination of masks shown in  FIGS. 5A and 5B ; 
         FIG. 6  is a flowchart of an example process according to the present invention; and 
         FIG. 7  is a flowchart of another example process according to the present invention. 
     
    
    
     The present invention has been found to be useful for quantifying the effects of flare in reproducing a mask pattern onto a photo resist coated substrate. 
     Refer to  FIG. 1 . In an example embodiment according to the present invention, a modified box-in-a-box structure  100  is constructed using two masks, a sparse (clear-field) mask that is sensitive to flare and a dense (dark-field) mask that is far less sensitive to flare. To enhance the effect of flare, the sides of the box-in-a-box are composed of grating structures. The left and bottom elements  110  are exposed using a dark-field mask. The top and right elements  120  are exposed from a clear-field mask. 
     In an example embodiment, the user may assume that the masking can be done such that there is negligible overlay error for exposing two masks to attain the elements  110  and  120 . The apparent misalignment is a relative measure of the effect of flare line length shortening. It is expected that the elements  120  exposed with the clear-field mask are somewhat shorter than those elements  110  exposed with the dark-field mask. In performing optical length measurements, df x  is the field optical length measurement of the part of the box structure exposed with the dark field mask; lf x  is the corresponding optical length measurement of the part of the box exposed with the light field mask. Likewise, the corresponding optical length measurements in the y-direction are df y  and lf y . 
     There are two errors with calculating flare using the structure of  FIG. 1 . First, there is always some misalignment when interchanging two different masks to attain elements  110  and  120 . This may be corrected by constructing a correction box, a box-in-a-box pattern from the two masks. Refer to  FIGS. 2A-2B . The dark-field mask  200  is composed of grating structures  210  and an alignment box  230 . The clear-field mask  205  is composed of grating structure  220  and another alignment box  240 . In using these masks on a photo resist coated substrate, the user may print the dark-field pattern of  FIG. 2A . The clear-field pattern of  FIG. 2B  is aligned to the print of  FIG. 2A  with the two alignment boxes  230  and  240 . The resulting pattern  215  of  FIG. 2C  produces the flare pattern of grating structures  210  and  220  and a box-in-a-box alignment patter  250  (boxes  230  and  240 ). The measurement of interest is the width of each side. Differences in the left and right or top and bottom are the sum of the misalignment between the two masks and any flare differences. We can subtract the effect of mask overlay with the standard box-in-box. 
     A second potential error that may be encountered is due to long range flare effects in which scattered light exposes those areas masked off. Refer to  FIG. 3 . In the arrangement  300 , light  310  is projection through a mask  320  having a defined designed feature  330 . Aberrations in lens  340  scatter the light  310   b . This scattering may cause the printed features to be larger than those designed. In an extreme, example, the scattered light would evenly expose areas under the masked regions. One of the assumptions being made is that regions under the chrome are not exposed. The effect of long-range flare is worse with the light field mask. Refer to  FIG. 5 . For example, the procedure is to expose  FIG. 5A  and then  FIG. 5B . We assume that the left and bottom regions which are masked off area (the thick “L” of the  515  structure), remain completely unexposed. 
     A correction scheme according to an embodiment of the present invention may be used to quantify this error. Refer to  FIG. 4 . 1). One or more stepper fields are exposed with a focus box pattern  400  using a dark-field mask. The focus box pattern  400  is composed of solid regions  410 ,  420  and lines and spaces  430 ,  440  in the x and y-direction. 2). The same pattern is exposed in a region that has been masked off in a prior clear-field exposure. 3). The correction for prior exposure due to flare is the difference in line shortening between the focus box structure in the single exposed field and the doubly exposed field. 
     According to another embodiment of the present invention, the focus box pattern of  FIG. 4  may be incorporated into the flare measurement structure of  FIGS. 2A-2C . Refer to  FIGS. 5A-5C . A dark-field mask  500  is composed of part of a flare pattern  510 , an alignment box  520  and a focus box pattern  530 . A corresponding clear-field mask  505  is composed of another alignment box  525  (which aligns to alignment box  520 ), the region defined by focus box pattern  530  is blocked out by region  535 . The printed composite structure,  FIG. 5C  is the combination of dark field mask  500  and clear-field mask  505 . Patterns  510  and  515  make the flare pattern (“A”). The correction box (“B”)  535 , and focus box (“C”) pattern  530  are the combination of the dark field mask  500  and light-field mask  505 . 
     Refer to  FIG. 6 . In an example embodiment, in a wafer-stepper apparatus, upon a resist-coated substrate, the user may print the dark-field mask pattern  610  of  FIG. 5A . Overlaid upon the exposed  FIG. 5A  pattern, the user may print the clear-field mask pattern  620  of  FIG. 5B . The resist coated-coated substrate has undergone a double-exposure for the  FIG. 5A  and  FIG. 5B  masks. Having printed  FIG. 5A  and  FIG. 5B  mask patterns as a double-exposure, the user steps to another location on the substrate and prints the composite mask pattern  630  of  FIG. 5C . The printed pattern is developed  640 . The user obtains measurements from the printed pattern  650 . After obtaining the measurements, the user may determine the effect of flare  660  on line shortening. 
     Refer to  FIG. 7 . In another example embodiment, on a substrate, the user may print the combination mask pattern of  FIGS. 5A and 5B , as depicted in  FIG. 5C  in a single exposure. The user prints the combination pattern  710  which includes the flare pattern ( 510 ,  515 ), correction box ( 535 ), and focus box ( 530 ). The printed pattern is developed  720 . The effect of long-range flare on line shortening  730  is determined. 
     For more information on the measuring of process induced line-shortening, refer to provisional U.S. Patent Application No. 60/468,892 titled, “Overlay Box Structure for Measuring Process-Induced Line Shortening Effect,” of Yuji Yamaguchi and Pierre Leroux, the application being assigned to Koninklijke Philips Electronics N.V. of The Netherlands, the application is incorporated by reference in its entirety. 
     Further information on the measuring of lines and spaces may be found in provisional U.S. Patent Application No. 60/468,893 titled, “A Method and Lithographic Structure for Measuring Lengths of Lines and Spaces,” of Yuji Yamaguchi, the application being assigned to Koninklijke Philips Electronics N.V. of The Netherlands, the application is incorporated by reference in its entirety. 
     In taking measurements upon the composite structure of  FIG. 5C  printed on a wafer substrate, a general calculation for the effect of flare on line shortening of horizontal features is denoted by, “R” the measured registration reported by the alignment-measuring patterns, as depicted in  FIG. 5C . For example, in the structures A, B, C, the general expression of misalignment is represented by:
 
 R   x =right leg−left leg)/2  (1)
 
 R   y =(top leg−bottom leg)/2  (2)
 
     In an example embodiment according to the present invention, the user may determine the degree of misalignment in the “B” structure  535  between the overlay  FIG. 5A  and  FIG. 5B  in the x-direction where the misalignment is the difference in width between the structure&#39;s left leg (cb 1   x ) and right leg (cb 2   x ) and in the y-direction where the misalignment is the difference between the bottom leg (cb 1   y ) and the top leg (cb 2   y ). 
     Likewise, the user may determine the degree of misalignment in the “A” structure between the light-field features  515  and dark-field features  510 . The apparent misalignment in the x-direction, between the light/dark field features is the difference in width between the structure&#39;s left leg (df x ) and right leg (lf x ) and the apparent misalignment in the y-direction between the light/dark field features is the difference in width between the structure&#39;s top leg (lf y ) and bottom leg (df y ). 
     The user may compare the line shortening between the “C” structure  530  in the single versus double exposed fields where line shortening in the x-direction, is the difference in width between the left leg (fb 1   x ) and right leg (fb 2   x ) or in the y-direction, the top leg (fb 2   y ) and bottom leg (fb 1   y ). The intermediate results of the calculations determined from the three structures, is used to calculate the flare.
 
 ls   x   Flare   =r   x   lf-df   −r   x   1st-2nd mask   −r   x   Multiple-Single Exposure   (3)
 
 ls   y   Flare   =r   y   lf-df   −r   y   1st-2nd mask   −r   y   Multiple-Single Eyposure   (4)
 
     where r x   lf-df , r x   1st-2nd mask  and r x   Multiple-Single Exposure  are the measured misalignments in the x direction for the light field/dark field structures (A), the standard box in box (B) that measures the alignment between the two exposures and the measured difference between the focus box structure (C) exposed twice versus a single time. 
     where r x   lf-df , r x   1st-2nd mask  and r x   Multiple-single  Exposure are the measured misalignments in the x direction for the light field/dark field structures (A), the standard box in box (B) that measures the alignment between the two exposures and the measured difference between the focus box structure (C) exposed twice versus a single time. 
     In a particular example process, the techniques according to the present invention may be applied. The alignment tool reports r x  and r y  which are the differences in left leg and right leg or top leg and bottom leg optical width measurements. By overlapping  FIG. 5A  and  FIG. 5B , the image of  FIG. 5C  is rendered. Also, one may assume that the pattern shown in  5 C is exposed on another stepper field. Table 1 outlines the example values obtained. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example Data to Determine Flare 
               
            
           
           
               
               
               
            
               
                   
                 rx 
                 ry 
               
            
           
           
               
               
               
               
               
            
               
                 Structure 
                 Overlapped 5A/5B 
                 5C 
                 Overlapped 5A/5B 
                 5C 
               
               
                   
               
               
                 A 
                 0.013 
                   
                 0.013 
                   
               
               
                 B 
                 0.005 
                   
                 0.005 
               
               
                 C 
                 0.152 
                 0.150 
                 0.183 
                 0.181 
               
               
                   
               
               
                 Flare in x direction 
               
               
                 1s x   flare  = 0.013 − 0.005 − (0.152 − 0.150) 
               
               
                 Flare in y direction 
               
               
                 1s y   flare  = 0.013 − 0.005 − (0.183 − 0.181) 
               
            
           
         
       
     
     While the present invention has been described with reference to several particular example embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention, which is set forth in the following claims.