Abstract:
An optical monitor includes a body having a first plurality of parallel, substantially opaque, spaced apart lines thereon, and the second plurality of parallel, substantially opaque, spaced apart lines thereon, with a relatively small angle between the first and second pluralities of lines. A an image of the lines of the first plurality thereof is provided on the semiconductor body, upon relative movement of the monitor toward and away from the semiconductor body, the line images move relative to the semiconductor body. The images of the lines of the second plurality thereof provided on the semiconductor body move in a different manner upon relative movement if the monitor toward and away from the semiconductor body: The moiré fringe formed on the semiconductor body from images of the first and second plurality of lines during such movement is analyzed in order to achieve proper focus of the image on the semiconductor body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to optical apparatus in semiconductor technology, and more particularly, to a test monitor for use in focusing an image on a semiconductor wafer. 
     2. Discussion of the Related Art 
     Typically, an optical system  30  (FIG. 1) used for patterning photoresist  32  on a semiconductor wafer  34  comprises a light source  36 , a mask or reticle  38  having opaque lines  40  and transparent portions  42 , and a lens  44 , the light from the light source  36  passing through the transparent portions  42  of the mask/reticle  38  through the lens  44  and to the photoresist  32 , with light being blocked from reaching the lens  44  (and photoresist  32 ) by the opaque lines  40  of the  38  mask/reticle. 
     As is well known, there is a need to position the wafer  34  at a proper distance with respect to the lens  44  so that images that fall on the photoresist  32  of the wafer  34  will be in the plane of best focus. 
     Typically, prior to actual fabrication of semiconductor devices, a test focus monitor in the form of for example a reticle is used as part of the overall system to achieve proper focus of the image on the wafer. An example of such a monitor is shown and described in the paper entitled “New Phase Shift Gratings For Measuring Aberrations”, by Hiroshi Nomura, published by SPIE, dated Feb. 27, 2001, which is herein incorporated by reference. FIGS. 2-4 herein show a monitor  50  configured as shown in FIGS. 3 and 5 of that paper. The monitor  50  is made up of a quartz base  52  which is transparent to light, and which has a plurality of parallel, opaque, spaced apart lines  54  on the base  52 , the lines  54  having a first set of adjacent ends  55 , and a second, opposite set of adjacent ends  56 . The area between each adjacent pair of lines  54  is transparent to light and is made up of regions  58  which pass light therethrough without changing the phase thereof, and regions  60  which pass light therethrough which shift the phase of such light by 90° (the phase shifting caused by recesses  62  in the base  52 —FIGS. 3 and 4 and the above cited paper). Each of the lines  54  has a region  58  and a region  60  which are aligned along and on one side thereof, and a region  58  and a region  60  which are aligned along and on the opposite side thereof. Each of the lines  54  has a region  58  on one side thereof opposite a region  60  on the other side hereof, these regions  58 ,  60  running from end  55  of that line to adjacent to the middle thereof, and furthermore, each of the lines  54  has a region  60  on the one side thereof opposite a region  58  on the other side thereof, these regions  60 ,  58  running from end  56  to adjacent the middle thereof. 
     FIGS. 3 and 4 are views similar to that shown in FIG. 1, but incorporating the monitor  50  of FIG. 2 as a part of the system  30 . FIG. 3 includes a sectional view of the monitor  50  taken along the line  3 — 3  of FIG. 2, showing a cross-section of the upper area  50 A of the monitor  50 , wile FIG. 4 includes a sectional view of the monitor  50  taken along the line  4 — 4  of FIG. 2, showing a cross-section of the lower area  50 B of the monitor  50 . As will be seen, with reference to the upper area  50 A of the monitor  50  (FIG.  3 ), moving the wafer  34  and lens  44  relatively together and apart causes the shadows  64 A,  64 B,  64 C formed on the photoresist  32  of the wafer  34  (formed by the opaque lines  54 ) to shift (downward as the wafer  34  and lens  44  are moved relatively further apart). Meanwhile, with reference to the lower area  50 B of the monitor  50  (FIG.  4 ), moving the wafer  34  and lens  44  relatively together and apart causes the shadows  64 D,  64 E,  54 F formed on the photoresist  32  of the wafer  34  to shift (upward as the wafer  34  and lens  44  are moved relatively further apart). The dotted lines  66  in FIGS. 3 and 4 indicate the traverse of the shadows  64 A,  64 B,  64 C,  64 D,  64 E,  64 F as the wafer  34  is so moved relatively toward and away from the lens  44 . 
     These paths are plotted in FIG. 5, and the intersections thereof indicate the best focus of the image on the wafer  34 . 
     FIG. 6 includes FIGS. 6A-6F which are views taken along the lines  6 A— 6 A,  6 B— 6 B,  6 C— 6 C,  6 D— 6 D,  6 E— 6 E, and  6 F— 6 F of FIGS. 3 and 4. With the wafer  34  and lens  44  closest together as shown in FIGS. 3 and 4, FIGS. 6A and 6D show the simultaneous positions of the shadows  64 A- 64 F on the photoresist  32  determined by the respective areas  50 A,  50 B of the monitor  50  With the wafer  34  and lens  44  so positioned relative to each other, the photoresist  32  is exposed to light from the light source  36  and is then developed to determine photoresist lines, which corresponds to the positions of the shadows  64 A- 64 F. As will be seen, the lines of FIGS. 6A and 6D are misaligned As the wafer  34  and lens  36  are moved relatively further apart to an intermediate position as shown in FIGS. 3 in  4 , FIGS. 6B and 6E show the simultaneous positions of the shadows  64 A- 64 F on the photoresist  32  determined by the respective areas  50 A,  50 B of the monitor  50 . Again, the photoresist  32  is exposed to light from the light source  36  and is then developed to determine photoresist lines that correspond to the positions of the shadows  64 A- 64 F. As will be seen, the lines of FIGS. 6B and 6E are substantially in alignment. Then, as the wafer  34  and lens  44  are moved relatively further apart, i.e., to their most far apart positions as shown in FIGS. 3 and 4, FIGS. 6C and 6F show the simultaneous is positions of the shadows  64 A- 64 F on the photoresist  32  determined by their respective areas  50 A,  50 B of the monitor  50 . Again, with the wafer  34  and lens  44  so positioned relative to each other, the photoresist  32  is exposed to a light from the light source  36  and is then developed to determine photoresist lines, which correspond to the positions of the shadows  64 A- 64 F. As will be seen, the lines of FIGS. 6E and 6F are misaligned. 
     It will be seen that changing the distance between the lens  44  and wafer  34  causes the shadows  64  A- 64 C to move further in and out of alignment with the shadows  64 D- 64 F. The process of moving the lens  44  and wafer  34  relatively closer together and further apart, along with the corresponding exposure and development of the photoresist  32  accompanying each adjustment is repeated until the lines formed in the photoresist  32  are substantially straight. This is illustrated in FIG. 6 of the above cited paper. 
     While such an approach is useful, only a relatively coarse reading of focus is achievable. For example, with reference to FIG. 6 of the above cited paper, only a small shift in the pattern from top to bottom is shown over a range of 400 nm of relative movement between the wafer  34  and lens  44 . With device dimensions continually being reduced, there is a need to achieve a proper reading of focus within a much smaller range of lens-wafer relative movement, for example, 200 nm or less. 
     Therefore, what is needed is an apparatus which is capable of providing proper focus of an image on a wafer through a very small range of relative movement between a lens and a wafer. 
     SUMMARY OF THE INVENTION 
     In the present invention, an optical tool is provided, made up of a tool body having a first plurality of parallel, substantially opaque, spaced apart lines thereon, and a second plurality of parallel, substantially opaque, spaced apart lines thereon with a relatively small angle between the first and second pluralities of lines. As an image of the lines of the first plurality thereof is provided on a semiconductor body, such line images move relative to the semiconductor body as the semiconductor body is moved relatively toward and away from the optical tool. Furthermore, as an image of the lines of the second plurality thereof is provided on the semiconductor body, such line images move relative to the semiconductor body as the semiconductor body is moved relatively toward and away from the optical tool, but in a manner different from the movement of the image of the lines of the first plurality thereof. The moiré fringe formed on the semiconductor body from images of the first and second plurality of lines is analyzed in order to achieve proper focus of an image on the semiconductor body. 
     The present invention is better understood upon consideration of the detailed description below, in junction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as said preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an elevational view of a typical optical system used for patterning photoresist of a semiconductor wafer; 
     FIG. 2 is a plan view of a test focus monitor of the prior art; 
     FIG. 3 is an elevational view of an optical system illustrating use of the test focus monitor of FIG. 2; 
     FIG. 4 is an elevational view similar to that shown in FIG. 3 again illustrating use of the test focus monitor of FIG. 2; 
     FIG. 5 is an elevational view showing the paths of the shadows falling on the wafer as the optical system of FIGS. 3 and 4 is used; 
     FIG. 6 includes FIGS. 6A-6F further illustrating use of the system of FIGS. 3 and 4; 
     FIG. 7 is a plan view illustrating portions of the monitor of the present invention; 
     FIG. 8 illustrates moiré fringe patterns formed by use of monitor portions of FIG. 7; 
     FIG. 9 is a view similar to that shown in FIG. 3, illustrating operation of the upper area of a monitor portion shown in FIG. 7; 
     FIG. 10 is a view similar to that shown in FIG. 9, illustrating operation of the lower area of a monitor portion shown in FIG. 7; 
     FIG. 11 is a view similar to that shown in FIG. 10, illustrating operation of the lower area of another monitor portion shown in FIG. 7; 
     FIG. 12 is a view similar to that shown in FIG. 10, illustrating operation of the upper area of said another monitor portion of FIG. 7; 
     FIGS. 13-15 illustrate moiré fringe patterns formed by use of the monitor portions of FIG. 7; 
     FIG. 16-20 are photographs of results in using the present invention; and 
     FIG. 21 is a graphical representation of moiré fringe shift plotted against programmed defocus. 
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to a specific embodiment of the present invention which illustrates the best mode presently contemplated by the inventors for practicing the invention. 
     FIG. 7 shows portions  100 ,  102 ,  104 ,  106  of a monitor  108  of the present invention. The monitor  108  includes portion  100  that is like that shown in FIG.  2 . That is, a quartz base  110  which is transparent to light has a plurality of parallel, opaque, spaced apart lines  112  on the base  110 , the lines  112  having a first set of adjacent ends  114 , and a second, opposite set of adjacent end s  116 . The area between each adjacent pair of lines  112  is transparent to light and is made up of regions  118  which pass light therethrough without change in phase thereof, and regions  120  which pass light therethrough which shift the phase of such light by 90° (the phase shifting caused by recesses  121  in the base  110 ). Each of the lines  112  has a region  118  and a region  120  which are aligned along and on one side thereof, and a region  118  and a region  120  which are aligned along and on the opposite side thereof. Each of the lines  112  has a region  118  on one side  1 hereof opposite a region  120  on the other side thereof, these regions  118 ,  120  running from the  114  end of that line  112  to adjacent the middle thereof. Furthermore, each of the lines  114  has a region  120  on the one side thereof opposite a region  118  on the other side thereof, these regions  118 ,  120  running from the end  116  to adjacent the middle thereof, all as shown and described with regard to FIG. 2 above. 
     Spaced a distance Y from the monitor portion  100  is the monitor portion  102 . The monitor portion  102 , similar to the monitor portion  100 , is made up of a quartz base  110  which is transparent to light, and which has a plurality of parallel, opaque, spaced apart lines  122  on the base  110 , having a first set of adjacent ends  124 , and a second opposite set of adjacent ends  126 . Again, the area between each adjacent pair of lines  122  is transparent to light and is made up of regions  128  that pass light therethrough without changing the phase thereof, and regions  130  that pass light therethrough which and shift the phase of such light by 90° (the phase shifting caused by recesses  132  in the base  110 ). Each of the lines  122  has a region  128  and a region  130  which are aligned along and on one side thereof, and a region  128  and a region  130  which are aligned along and or the opposite side thereof. Each of the lines  122  has a region  128  on one side thereof opposite a region  130  on the other side thereof, these regions  128 ,  130  running from end  124  of the the  122  to adjacent the middle thereof, and furthermore each of the lines  122  has a region  130  the one side thereof opposite the region  128  on the other side thereof, these regions  128 ,  130  running from end  126  of the line  122  to adjacent the middle thereof. 
     However, important differences exist between the monitor portion  100  and monitor portion  102 . Initially, with regard to the phase shifting and non-phase shifting regions between adjacent pairs of lines, the positions of the phase shifting and non-phase shifting regions of the monitor portion  102  are reversed as compared to the corresponding areas of the monitor portion  100 . Furthermore, the lines  122  of the monitor portion  102  are at a small angle relative to the lines  112  of the monitor portion  100 . The importance of these features will be discussed further on. 
     Also shown in FIG. 7 are other portions  104 .  106  of the monitor  108 . The monitor portion  104  is similar to monitor portion  100 , but includes no regions that shift the phase of the light passing through the transparent portions between the lines  134 . The monitor portion  106  is similar to monitor portion  102 , but again including no regions that shift the phase of light passing through the transparent portions between the lines  136 . The monitor portion  104  and monitor portion  106  are spaced apart a distance Y, as are the monitor portion  100  and monitor portion  102 . The lines  134  of the monitor portion  104  are parallel to the lines  112  of the monitor portion  100 , and the lines  136  of the monitor portion  106  are parallel to the lines  122  of the monitor portion  102 . That is to say, the angle between the lines  112 ,  122  is the same as the angle between the lines  134 .  136 . 
     Initially, the monitor portion  104  is used in the apparatus  30  of FIG.  1 . This results in shadows being formed on the photoresist  32  of the wafer  34  which correspond to the lines  134 . The monitor  108  is then moved a distance Y as shown in FIG. 7, so that the shadow portions formed by the monitor portion  106  overlie the lines formed in the photoresist  32 . The photoresist  32  is then developed resulting in the pattern of lines  138  shown in FIG.  8 . Because of the angle between the lines  134  and the lines  136 , moiré fringes  140 ,  142  are revealed as shown in FIG.  8 . With the distance Y being held constant for this operation, it will be understood that the distance A between the revealed moiré fringes  140 ,  142  will remain constant whenever this operation is repeated. Furthermore, with no phase shift regions in the monitor portions  104 ,  106 , the distance between the moiré fringes  140 , 142  formed on the photoresist  32  will be substantially the same over a range of distances between the lens  44  and wafer  34 . Thus, the image as shown in FIG. 8 can be used as a reference, as will be described further on. 
     FIG. 9 and 10 are similar to FIGS. 3 and 4, but with the monitor portions  100 ,  102  of FIG. 7 as part of the system  30 . FIG. 9 includes a sectional view of the monitor portion  100  taken along the line  9 — 9  of FIG. 7, showing a cross-section of the upper area  100 A of the monitor port  100 , while FIG. 10 shows a sectional view of the monitor portion  100  taken along the line  10 — 10  of FIG. 7, showing a cross-section of the lower area  100 B of the monitor portion  100 . Similar to the above description with regard to FIG. 3 and 4, with reference to the upper area  100 A of the monitor portion  100  (FIG.  9 ), moving the wafer  34  and lens  44  relatively together and apart causes the shadows  144 A,  144 B,  144 C formed on the photoresist  32  of the wafer  34  (formed by the opaque lines  112 ) to shift (downward as the wafer  34  and lens  44  are moved relatively further apart). Meanwhile, with reference to the lower area  100 B of the monitor portion  100  (FIG.  10 ), moving the wafer  34  and lens  44  relatively together and apart causes the shadows  144 D,  144 E,  144 F formed on the photoresist  32  of the wafer  34  to shift (upward as the wafer  34  and lens  44  are moved relatively further apart). The dotted areas  146  in FIGS. 9 and 10 indicate the traverse of the shadows  144 A-F as the wafer  34  is so moved relatively toward and away from the lens  44 . 
     FIGS. 11 and 12 are also similar to FIGS. 3 and 4, but with the monitor portion  102  as part of the system. FIG. 11 includes a sectional view of the monitor portion  102  taken along the line  11 — 11  of FIG. 7, showing a cross-section of the upper area  102 A of the monitor portion  102 , while FIG. 12 shows a sectional view of the monitor portion  102  taken along the line  12 — 12  of FIG. 7, showing a cross-section of the lower area  102 B of the monitor portion  102 . In this embodiment, with the phase shifting regions and non phase shifting regions reversed in position as compared to the monitor portion  100  as described above, the movement of the formed shadows  150 A- 150 F will be the opposite of that as described with regard to the monitor portion  100 . That is, with reference to the upper area  102 A of the monitor portion  102  (FIG.  11 ), moving the wafer  34  and lens  44  relatively together and apart causes the shadows  150 A-C formed on the photoresist  32  of the wafer  34  (formed by the opaque lines  122 ) to shift (upward as the wafer  34  and lens  44  are moved relatively further apart). Meanwhile, with reference to the lower area  102 B of the monitor portion  102  (FIG.  12 ), moving the wafer  34  and lens  44  relatively together and apart causes shadows  150 D-F formed of the photoresist  32  of the wafer  34  to shift (downward as the wafer  34  and lens  44  are moved relatively further apart). 
     With the wafer  34  in for example the position shown at X relative to the lens  44  (FIGS.  9  and  10 ), and the monitor portion  100  positioned to receive light from the light source  36 , the photoresist  32  is exposed to light from the light source  36  and is then developed to determine a latent image of lines  152  in the photoresist (FIG. 13) which correspond to the vertical lines  112  of FIG.  7 . It will be understood that through the above analysis, the photoresist line portions  152 A formed by the upper area  100 A may not be aligned with the photoresist line portions  152 B formed by the lower area  100 B, similar to the situation shown and described above with regard to FIG.  6 . 
     Then, the monitor  108  is moved a distance Y to position the monitor portion  102  to receive light from the light source  36  (the lens  44  and wafer  34  remaining distance X apart), and the photoresist  32  is again exposed to light from the light source  36  and then developed to determine photoresist lines  154  which correspond to the angled lines  122  of FIG.  7 . Again, it will be understood that through the above analysis, the line portions  154 A formed by the upper area  102 A may not be aligned with the line portions  154 B formed by the lower area  102 B. 
     With the lines  154  being at a small angle relative to the lines  152 , moiré fringes  156 ,  158  are formed certain distance B apart. It will be seen that as the lens  44  and wafer  34  are moved relatively further apart, the line portions  154 A will shift leftward, as will the line portions  152 B. At the same time, the line portions  154 B will shift rightward, as will be line portions  152 A. With the distance between the lens  44  and wafer  34  so increasing, the lateral shift of the line portions  154 A relative to the line portions  152 A causes the moiré fringe  156  of FIG. 13 to move upward away from the center of the pattern. At the same time, the lateral shift of the line portions  152 B relative to the line portions  154 B causes the moir é fringe  158  of FIG. 13 to move downward away from the center of the pattern. The result of this movement is shown in FIG.  14 . Thus, moving the wafer  34  and lens  44  relatively closer together from their most far-apart position as described increases the distance between the moiré fringes  156 , 158 , to a distance C. Further such relative movement of the lens  44  toward the wafer  34  causes the distance between the moiré fringes  156 , 158  to be further increased (Distance D, FIG.  15 ). 
     After the formation of the moiré fringe pattern of FIG. 8, which can be used as a baseline pattern, a number of exposures and developments of the photoresist  32  are undertaken as described above for various distances between the wafer  34  and lens  44 . Best focus is achieved when the distance between the moiré fringes  154 ,  156  formed by this series of operations closely matches the distance A between the moiré fringes of the baseline pattern of FIG.  8 . 
     Because of the relatively small angle between the lines  112  and lines  122 , i.e., 30 degrees or less, and preferably 10 degrees or less, the moiré fringes  156 ,  158  move at a much greater rate than the lateral movement of lines as described above with regard to the previous system (FIG.  6 ). Each moiré fringe  156 ,  158  is actually moved a distance which is a multiple of the distance moved by the shadows of FIG. 6 of the previous system, the multiple being dependent on the angle between the lines  112  and lines  122 , with a smaller angle providing a greater multiple. This provides a much more sensitive reading of focus. 
     FIG. 15-19 are photographs illustrating the practice of the present invention. The focus is run through a range of 400 nm for angles of one set of lines  112  relative to the other set of lines  122  of 3°, 5° and 7°. It will be seen that the shift in moir é fringe as described above is readily observed in these photographs, and indeed is readily observed within a range of 200 nm or less of relative movement of the lens  44  and wafer  34 . 
     FIG. 20 illustrates the sensitivity of the present apparatus, wherein programmed wafer-lens distance is plotted against measured moiré fringe shift, i.e., distance between the moir é fringes as the lens  44  and wafer  34  are moved relatively closer together and further apart. It will readily be observed that the system is quite sensitive well within a range of movement of 200 nm or less. 
     The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings. 
     The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.