Abstract:
A method of measuring the overlay accuracy of a multi-exposure process is provided. The characteristic of this invention is utilizing a scanning electron microscope for monitoring the overlay accuracy real-time during the multi-exposure processes in stead of the conventional optical measurement method.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a method of measuring the overlay accuracy, and more particularly to measure the overlay accuracy of the multi-exposure process.  
         [0003]     2. Description of the Prior Art  
       SUMMARY OF THE INVENTION  
       [0004]     In the semiconductor fabrication process, the wafer is sawed along scribe lines into a plurality of chips. Overlay marks are arranged on the scribe lines at the four corners of the edge of each chip to measure whether the test pattern of the mask is precisely transferred to the photoresist layer and aligned with the previous layer of the wafer after a photolithography process. By the above test exposure process, the parameters of the formal exposure process will be adjusted on the basis of the overlay information from the overlay marks.  
         [0005]     A conventional method for measuring the overlay accuracy is utilizing an Overlay apparatus to scan the overlay marks on the scribe lines of each chip for acquiring the overlay information.  FIG. 1A  shows a vertical view of a conventional structure of an overlay mark  100 , and  FIG. 1B  illustrates a cross-sectional structure of the overlay mark  100  taken along a cutting line  1 B- 1 B′ of  FIG. 1A .  
         [0006]     First, referring to  FIG. 1A  and  FIG. 1B , four outer recesses  102  are formed on a previous layer  106  above a substrate layer  108 . The outer recesses  102  of the overlay mark  100  are respectively formed into a first rectangle, and each outer recess  102  is a side of the first rectangle and the adjacent sides are not connected. The hollow structure of the outer recesses  102  can be the result from an etching process on the previous layer  106  or the result of filling to a trench (not shown) of the substrate layer  108  by the previous layer  106 . The outer recess  102  is used as a reference mark for a following test exposure process to measure whether a photoresist pattern is precisely aligned with it from a mask.  
         [0007]     Next, referring to  FIG. 1A , four inner photoresist patterns  104  are transferred and formed from a test mark (not shown) to the previous layer  106  by a photolithography process (comprises photoresist coating, exposure and development processes). The inner patterns  104  of the overlay mark  100  are also respectively formed into a second rectangle and are enclosed by the first rectangle. Each inner photoresist pattern  104  is a side of the second rectangle and the adjacent sides are not connected. Four outer recesses  102  and four inner photoresist patterns  104  could be divided into a vertical mark and a horizontal mark.  FIG. 1B  just shows a cross-sectional structure of the vertical mark of the overlay mark  100  taken along the cutting line  1 B- 1 B′ of  FIG. 1A . A vertical centerline (not shown) of the opposite inner photoresist patterns  104  is set to match another vertical centerline (not shown) of the opposite outer recesses  102  when the mask is initially aligned with the previous layer  106 . And the alignment of horizontal mark of the overlay mark  100  are also be set as the same way of the vertical mark as above.  
         [0008]     After the inner photoresist patterns  104  were formed, an Overlay apparatus (not shown) is used to detect the overlay mark  100  with a optical scanning, along with the vertical direction for the horizontal mark and the horizontal direction for the vertical mark of the overlay mark  100 .  FIG. 1C  shows the return signal waveform from the vertical mark of the overlay mark  100  as shown in  FIG. 1B . The peak signals of the outer recesses  102  in  FIG. 1B  are read first and denoted as  102 ′ and  102 ′ in  FIG. 1C , and the peak signals of the inner photoresist patterns  104  are then read and denoted as  104 ′ and  104 ′. Next, the mean value of the peak signals  102 ′ and  102 ′ is obtained and expressed it by a dotted midline  110 , and the mean value of the peak signals  104 ′ and  104 ′ is also obtained and expressed it by another dotted midline  112  in  FIG. 1C . The related position and shift distance of the midlines  110  and  112  will be calculated as a horizontal error from the overlay mark  100 . And the vertical error of the overlay mark  100  will also be calculated as the same way by the scanning to the horizontal mark. Finally, an overlay error composed of the vertical error and the horizontal error of the overlay mark  100  is obtained.  
         [0009]     Four overlay errors, collected from the overlay marks  100  on four corners of the chip will help to judge whether a scale error, a rotation error, or a translation error is occur during this test exposure process. And the parameters of the following formal exposure process will be adjusted when this test exposure process is not reaching the required accuracy.  
         [0010]     The above conventional method for measuring the overlay error is utilizing an Overlay apparatus to scan the overlay marks  100  on the scribe lines of each chip for acquiring the overlay information. But the optical detecting resolution is depended on the wavelength of the light source in the Overlay apparatus. Hence, the scale of the overlay mark  100  must reach a recognizable size to cooperate with the specific Overlay apparatus.  
         [0011]     The conventional method for monitoring the alignment accuracy of the photolithography process is utilizing an Overlay apparatus to scan the overlay marks  100  at a specific time. The conventional method cannot provide a real-time monitoring of alignment accuracy; hence, the time spent in this alignment step will increase and influence the whole semiconductor fabrication process.  
         [0012]     The conventional method is used for the single exposure process to measure whether the photoresist patterns  104  transferred from a mask are precisely aligned with the recesses  102  in the scribe lines of the chip. Hence, in a multi-exposure process, the conventional method cannot be used to measure whether the latter photoresist patterns transferred from a second mask are precisely aligned with the former photoresist patterns transferred from a first mask. 
     
    
     BEIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  illustrates a vertical view of a conventional structure of an overlay mark.  
         [0014]      FIG. 1B  illustrates a cross-sectional structure of the overlay mark.  
         [0015]      FIG. 1C  illustrates a return signal waveform from a vertical mark of the overlay mark from the conventional detecting process.  
         [0016]      FIG. 2A  illustrates a vertical view of a first overlay check pattern on a first mask according to a first preferred embodiment of the present invention.  
         [0017]      FIG. 2B  illustrates a vertical view of a second overlay check pattern on a second mask.  
         [0018]      FIG. 2C  illustrates a vertical view of a first trench and a second trench on a photoresist layer.  
         [0019]      FIG. 2D  illustrates a cross-sectional structure of a first vertical trench and a second vertical trench on the photoresist layer.  
         [0020]      FIG. 3A  illustrates a vertical view of a first overlay check pattern on a first mask according to a second referred embodiment of the present invention.  
         [0021]      FIG. 3B  illustrates a vertical view of a second check pattern on a second mask.  
         [0022]      FIG. 4A  illustrates a vertical view of a first overlay check pattern on a first mask according to a third preferred embodiment of the present invention.  
         [0023]      FIG. 4B  illustrates a vertical view of a second overlay check pattern on a second mask.  
         [0024]      FIG. 4C  illustrates a vertical view of a first trench and two second trenches on a photoresist layer. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     The present invention has been made in view of the above problems of the conventional method for measuring the overlay accuracy by utilizing an Overlay apparatus. The present invention is providing a method for measuring the overlay tolerance during a multi-exposure process. According to the present invention, a scanning electron microscope (SEM), providing a high detecting resolution is utilized for monitoring the overlay accuracy instead of the overlay apparatus of the conventional method.  
         [0026]     The scanning electron microscope could be used to observe the surface structure of a specimen. A high-energy electron beam, emitted from an electron gun of the scanning electron microscope, was incident to the entire surface of the specimen, causing secondary signals (i.e. secondary electrons) to be ejected from the surface of the specimen and then collected and counted by an electronic detector. Finally, an image of the surface structure of the specimen could be obtained and monitored. Because electrons have a much smaller wavelength than light, they can resolve smaller structures than light can. Hence, in the following statement of a preferred embodiment of the present invention, a SEM is used to monitor the overlay accuracy by scanning the trenches transferred by different masks during a multi-exposure process.  
         [0027]     A first preferred embodiment of the present invention is providing a method to measure the overlay accuracy of a multi-exposure process. First, four of the first overlay check patterns  202  are formed at four corners of a first mask  200 , and a vertical view of a first overlay check pattern  202  is shown in  FIG. 2A . Referring to  FIG. 2A , a first vertical-trench pattern and a first horizontal-trench pattern are provided on each first overlay check pattern  202 . Then, a first photolithography process is executed (comprises photoresist coating, exposure and development processes), transferring four first overlay check patterns  202  to a photoresist layer  220  with the positive photoresist process, and four first trenches are formed on a photoresist layer  220 . Each first trench is composed of a first vertical trench  224  and a first horizontal trench  230  shown in  FIG. 2C . In this first preferred embodiment of the present invention, the first mask  200  is provided with four first overlay check patterns  202 . But the number of the first overlay check pattern  202  is not restricted to four according the present invention, and the position and the arrangement of the first overlay check pattern  202  is not limited to the four corners of a first mask  200 .  
         [0028]     Next, four second overlay check patterns  212  are formed at four corners of a second mask  210  (corresponding to the four corners of the first mask), and a vertical view of a second overlay check pattern  212  is shown in  FIG. 2B . Referring to  FIG. 2B , two second vertical-trench patterns and two second horizontal-trench patterns are provided on each second overlay check pattern  212 , and between the two second vertical-trench patterns, the corresponding first vertical-trench pattern is aligned as the midline, and between the two second horizontal-trench patterns, the corresponding first horizontal-trench pattern is aligned as the midline. Afterward, a second photolithography process is executed (comprises exposure and development process), transferring four second overlay check patterns  212  to the photoresist layer  220  with the positive photoresist process, and four second trenches are formed on the photoresist layer  220 . Each second trench is composed of two second vertical trenches  226             228  and two second horizontal trenches  232             234  shown in  FIG. 2C .  
         [0029]     A vertical view of a first trench formed on the photoresist layer  220  by the first photolithography process and a second trench formed on the photoresist later  220  by the second photolithography process are shown in  FIG. 2C . The arrangement of the first vertical trench  224  and the two second vertical trenches  226             228  could be passed through by a horizontal scan line  22 H, and the horizontal scan line  22 H is perpendicular to the first vertical trench  224 . The arrangement of first horizontal trench  230  and the two second horizontal trenches  232             234  could also be passed through by a vertical scan line  22 V, and the vertical scan line  22 V is perpendicular to the first horizontal trench  230 . Hence, taking the horizontal scan line  22 H as the section-line for the photoresist layer  220 , the relative positions between the first vertical trench  224  and the two second vertical trenches  226             228  could be observed in  FIG. 2D . Referring to  FIG. 2D , first, the photoresist layer  220  is formed above a previous layer  222  by the first photolithography process. Then the first vertical trench  224  is formed on the photoresist layer  222 . Finally, the two second trenches  226             228  are formed on two sides of the first vertical trench  224  and have individual intervals a and b to the first vertical trench  224 .  
         [0030]     In this first preferred embodiment according to the present invention, in order to measure the overlay accuracy for the second mask  210  to the first mask  200 , the positions of the second overlay check patterns  212  arranged on the second mask  210  must correspond with the positions of the first overlay check patterns  202  arranged on the first mask  200 . After the first trench transferred from the first overlay check pattern  202  of the first mask  200  is formed on the photoresist layer  220 , the second trench transferred from the second overlay check pattern  212  will be formed on the same photoresist layer  220  on the principle of making the intervals from the first vertical trench  224  to the adjacent second vertical trenches  226  and  228  to be equal. And the intervals from the first horizontal trench  230  to the adjacent second horizontal trenches  232  and  234  are also arranged to be equal when the second mask  210  has no overlay error with the first mask  200 . According to the above arrangement, the overlay accuracy between the second mask  210  and the first mask  200  in a multi-exposure process could be detected by measuring whether the intervals between the first trench and the second trenches are equal or not.  
         [0031]     Afterward, a scanning electron microscope (not shown) is used to detect the intervals from the first trench to the adjacent second trenches to obtain an overlay error between the second mask  210  and the first mask  200 . The intervals from the first vertical trench  224  to the adjacent second vertical trenches  226  and  228  are measured, scanned along the horizontal scan line  22 H to get a horizontal overlay error of the overlay error. And the intervals from the first horizontal trench  230  to the adjacent second horizontal trenches  232  and  234  are measured and are scanned along the horizontal scan line  22 V to get a vertical overlay error by the scanning electron microscope. Referring to  FIG. 2D  and taking the first vertical trench  224  and the second vertical trenches  226             228  as an example, the shift-direction of the horizontal overlay error depends on the magnitude of the intervals a and b. If the interval a is greater than the interval b, it means that the second vertical trenches  226             228  transferred from the second mask  210  has a shift-movement toward the leaf from the first vertical trench  224  transferred from the first mask  200 . And if the interval a is smaller than the interval b, the direction of the horizontal overlay error could be observed toward the right. Then the magnitude of the horizontal overlay error could be calculated as half the difference between the intervals a and b. The vertical overlay error, determined by the intervals between the first horizontal trench  230  and the adjacent second trenches  232             234  is also measured by the same way as the horizontal overlay error in the above description.  
         [0032]     In this first preferred embodiment according to the present invention, an overlay error composed of a horizontal overlay error and a vertical overlay error could be calculated by scanning the positions of the first trench and the second trench formed at one corner of the photoresist layer  220 . And the whole overlay result between the first mask  200  and the second mask  210  could be obtained by collecting four overlay errors arranged at four corners of the photoresist layer  220 .  
         [0033]     In this first preferred embodiment of the present invention, the amount and the positions of the first trench and the second trench are dependent on the first overlay check pattern  202  of the first mask  200  and the second overlay check pattern  212  of the second mask  210 . But the positions of first trench and the second trench are not limited to arrange on the scribe lines of each chip, other positions in the principle of not influence the original layout during the multi-exposure process also could be arranged to form the first trench and the second trench. In addition, the first mask  200  and the second mask  210  are not limited to be used on the adjacent exposure process during a multi-exposure process, the overlay accuracy between any two masks could be calculated by arranging a first overlay check pattern and a second overlay check pattern, as described in the above description of the first preferred embodiment according to the present invention.  
         [0034]     In this first preferred embodiment of the present invention, the forming position of the second vertical-trench patterns are parallel but not overlap with the forming position of the first vertical-trench pattern, and the forming position of the second horizontal-trench patterns are parallel but not overlap with the forming position of the first horizontal-trench pattern. But according to the present invention, the forms of the first overlay check pattern  202  of the first mask  200  and the second overlay check pattern  212  of the second mask  210  are not limited as the above first preferred embodiment of the present invention. In a second preferred embodiment of the present invention, a vertical view of a first overlay check pattern  302  of a first mask  300  and a second overlay check pattern  312  of a second mask  310  could be shown in  FIG. 3A  and  FIG. 3B . Referring to  FIG. 3A  and  FIG. 3B , two first vertical-trench pattern and a first horizontal-trench pattern are provided on the first overlay check pattern  302 , and a second vertical-trench pattern and two second horizontal-trench pattern are provided on the second overlay check pattern  312 . The second vertical-trench pattern is aligned with the midline position between the two first vertical-trench patterns, and between the two second horizontal-trench patterns, the corresponding first horizontal-trench pattern is aligned as the midline. In this second preferred embodiment of the present invention, the pattern design of the first overlay check pattern  302  and the second overlay check pattern  312  is different with the pattern design of the first overlay check pattern  202  and the second overlay check pattern  212  of the first preferred embodiment as the above description, but the detecting method and result could be the same as the first embodiment. Hence, other pattern design such as arranging two first vertical-trench patterns and two horizontal-trench patterns on the first overlay check pattern, and arranging a second vertical-trench pattern and a second vertical-trench pattern on the second overlay check pattern are also could be implemented according to the present invention.  
         [0035]     In a third preferred embodiment of the present invention, the arrangement of the first overlay check pattern  402  of a first mask  400  and the second overlay check pattern  412  of a second mask  410  could be shown in  FIG. 4A  and  FIG. 4B . It is observed that the first vertical-trench pattern and the first horizontal-trench pattern are connected to form an L-shape pattern of the first overlay check pattern  402 . And the second vertical-trench patterns and the second horizontal-trench patterns are also connected to form two L-shape patterns of the second overlay check pattern  412 . Hence, in a photoresist layer  420 , the first trench  422  transferred from the first mask  400  and the second trenches  424             426  transferred from the second mask  410  could be passed through by a horizontal scan line  44 H and a vertical scan line  44 V, and the horizontal scan line  44 H and the vertical scan line  44 V are respectively perpendicular to the first trench  422 . Next, a scanning electron microscope (not shown) is used to detect a vertical overlay error and a horizontal error between the first trench  422  and the second trenches  424             426  along the horizontal scan line  44 H and the vertical scan line  44 V. Finally, the overlay error between the first mask  400  and the second mask  410  could be obtained according to this preferred embodiment.  
         [0036]     As above descriptions of the first, second and third preferred embodiments according to the present invention, the present invention is utilizing a scanning electron microscope to detect the relative positions between the first trench and the second trench transferred from different masks during a multi-exposure process. In addition, with the high detecting resolution of the scanning electron microscope, the size of the trenches formed on the photoresist layer could be reduced and will not be limited to being formed on the scribe lines of a chip of the conventional method.  
         [0037]     Skilled workers will further recognize that many changes may be made in the details of the above-described embodiment of this invention without departing from the underlying principles thereof. Accordingly, it will be appreciated that this invention is also applicable to color synchronization applications other than those found in multimedia projectors. The scope of the present invention should, therefore, be determined only by the following claims.