Abstract:
A mask useable to direct an electron beam having a cone side and a grade side. The cone side includes a plurality of troughs that extend partially into the mask but do not extend to the grade side. Apertures are formed in the bottoms of the troughs that extend through to the grade side. The apertures are separated by both actual and virtual tie bars. In a preferred embodiment, some of the virtual tie bars extend from the grade side, through the trough to the cone side. The occurrence of these cone side virtual tie bars varies in concentration across the extent of the mask such that the concentration is heavy in areas where clipping is desired, and light in areas where clipping is not desired. In another preferred embodiment, the size, shape and location of the cone side virtual tie bars vary across the extent of the mask to accommodate for the angle of the electron beam travel and the necessity for localized clipping.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 60/334,212 filed Nov. 30, 2001, and U.S. Provisional Patent Application No. 60/411258 filed on Sep. 16, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Cathode ray tubes (hereinafter “CRTs”), such as those used in televisions, oscilloscopes, and computer screens, operate by shooting electron beams from an electron gun toward a screen coated with a layer of phosphor or, in the case of color screens, three different color-emitting phosphors. Between the screen and the gun, lies a metal mask used to direct the electron beam, thereby providing a control mechanism for guiding the beam to strike a desired place on the screen. In order to allow the electron beam to pass through the mask and strike the screen, the mask is constructed and arranged with hundreds of troughs, usually arranged in vertical columns. The troughs are substantially open on the screen (or “cone”) side of the mask and extend partially through the thickness of the mask but stop short of the gun (or “grade”) side of the mask. At the deepest portion of the troughs are defined hundreds of thousands of apertures that extend through to the grade side of the mask. These apertures allow the electron beams to pass through the mask and strike the screen.  
           [0003]    By thinking of the apertures as tiny, yet three-dimensional, tunnels one quickly realizes that those beams directed toward the edges of the screen are likely to strike the walls of the apertures due to the angular difference between the direction of travel of the electron beam and the direction the aperture extends. The troughs are formed as an attempt to shorten the length of the tunnels, by making the opening on the cone side larger than the opening on the grade side.  
           [0004]    Additionally, the length of the troughs are typically interrupted by a plurality of bridge like supports known as “tie bars”. The tie bars add rigidity and strength to the mask and provide beam control in a vertical direction. For purposes of this application, tie bars are distinguished by being either grade side or cone side tie bars and by being either actual or virtual tie bars. Reference is made to FIGS. 1 a  and  1   b  for explanation.  
           [0005]    [0005]FIGS. 1 a  shows a mask  1  with a cone side  2  and a grade side  3 . A plurality of troughs  4  are formed in the cone side  2  and are interrupted by actual tie bars  5 , which span the troughs  4 , and virtual tie bars  6 , which extend partially across but do not span the troughs  4 . Virtual tie bars are especially prevalent in semi-tension masks. At the bottom of the troughs  4 , the mask  1  defines a plurality of apertures  7 , through which the electron beam (not shown) may pass.  
           [0006]    [0006]FIG. 1 b  shows the grade side  3  of mask  1 . It can be seen that the apertures  7  extend through the grade side  3  and are separated by actual tie bars  8  and virtual tie bars  9 . As stated above, a distinction is drawn between actual tie bars  5  on the cone side  2  and actual tie bars  8  on the grade side  3  as well as between virtual tie bars  7  on the cone side  2  and virtual tie bars  9  on the grade side  3 .  
           [0007]    Again, tie bars generally function to provide vertical control of the electron beam, and, especially in the case of actual tie bars, add rigidity and strength to the mask. However, the tie bars on the grade side of the mask are frequently sufficient to accomplish both of these functions. The tie bars on the cone side of the mask add rigidity but often interfere with the electron beam.  
           [0008]    There is a need for a mask that minimizes, selectively eliminates, or completely eliminates electron beam interference without adversely compromising the structural integrity of the mask.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The grade tie bar only (“GTO”) concept of the present invention is premised on the idea of reducing or even eliminating the number of tie bars that are present on the screen side of the aperture mask. The result is a mask where the tie bars are primarily or solely on the grade side of the mask.  
           [0010]    In a first embodiment of the GTO concept, actual tie bars only are present on the grade side of the mask and no actual (nor any virtual) tie bars are present on the cone side of the mask. Such a design increases the electron beam transmission through the apertures, especially at the edges of the tube. This design also allows for the use of a thicker aperture mask material for both standard formed mask and semi-tension masks than currently used, while maintaining equivalent or improved brightness, thereby improving microphonic and magnetic performance in the tube.  
           [0011]    In a second embodiment of the GTO concept, the grade side of the mask is comprised of both actual tie bars and virtual tie bars and no actual nor virtual tie bars are present on the cone side of the mask. This design enhances the advantages in beam transmission that are known to be achieved through the use of virtual tie bars. For example, the design makes it easier to form desirable shapes of virtual tie bars, particularly along the minor axis and in the diagonal corners. Furthermore, this design leads to improved uniformity of both virtual tie bar shape and beam clearance through the mask, especially at the outer periphery. This results in improved overall visual uniformity of the tube. Finally, the design leads to the formation of improved slot shape, thus improving the squareness of the electron beam as it strikes the tube surface.  
           [0012]    In a third embodiment of the GTO concept, the grade side of the mask is comprised of both actual tie bars and virtual tie bars. In those areas of the cone side that align with actual tie bars on the grade side, there exist corresponding cone side actual tie bars. In those areas of the cone side that align with virtual tie bars on the grade side, however, there are no cone side tie bars (neither actual nor virtual).  
           [0013]    The mask of this third embodiment similarly offers advantages in beam transmission performance and uniformity but may have greater strength than the mask of the second embodiment. The greater strength may result from the presence of a cone side actual tie bar in the area that aligns with a grade side actual tie bar that, in turn, leads to the presence of greater material in this region. The greater amount of material results in the mask having greater capability to support a load or tension that may be applied to the mask, and reduces the tendency for damage due to handling.  
           [0014]    In a fourth embodiment of the GTO concept, the grade side of the mask again is comprised of both actual tie bars and virtual tie bars. In those areas of the cone side that align with actual tie bars on the grade side, there are no cone side tie bars (neither actual nor virtual). In this respect, this fourth embodiment is similar to the previously described second embodiment. However, the areas of the cone side that align with virtual tie bars on the grade side are treated differently from those of the second embodiment.  
           [0015]    We first discuss the center region of the mask. In those center areas of the cone side that align with virtual tie bars on the grade side, there are no cone side tie bars (neither actual nor virtual).  
           [0016]    We next discuss the extreme region along the x-axis of the mask and the diagonal corners of the mask. In those extreme x-axis and diagonal corner areas of the cone side that align with virtual tie bars on the grade side, there exist corresponding virtual tie bars.  
           [0017]    We next discuss the y-axis areas of the mask. In those y-axis areas of the cone side that align with the virtual tie bars on the grade side, there are no cone side tie bars (neither actual nor virtual).  
           [0018]    Finally, there exist transition zones on the cone side of the mask between those areas that do not have cone side tie bars (neither virtual nor actual) and those areas that have virtual tie bars. More specifically, there exist transition zones on the cone side of the mask from (1) the center of the mask (no tie bars) to the extreme x-axis and diagonal corners of the mask (virtual tie bars) and (2) from the end of the y-axis (no tie bars) to the diagonal corners (virtual tie bars). These transition zones may be based on x and y coordinates as well as the angle of electron beam transmission through the mask.  
           [0019]    This fourth embodiment may be referred to as a graded grade tie bar only (GGTO) design due to the “graded” nature of the mask that results from the use of the transition zones. This design leads to the formation of improved and more precise shapes of the virtual tie bar in those areas where there are no aligning cone side virtual tie bars, namely, in the center of the mask and along the y-axis. The beam clearance through the virtual tie bars in these areas is also improved.  
           [0020]    The design of the fourth embodiment also provides a way to “clip” the electron beam at locations where it is desirable to do so, namely, along the x-axis and along the diagonal axis of the mask. In other words, the presence of cone side virtual tie bars at the extreme x-axis area and at the diagonal corners of the mask, and the presence of transition zones there between, serves as a mechanism to “clip” the beam in these locations thus reducing or eliminating any electron beam passage through the virtual tie bar slot. This design improves the virtual tie bar shape and resultant electron beam shape at the center section of the mask and along the y-axis, while maintaining the capability to “clip” the electron beam passage through the virtual tie bars in the vicinity of the major axis and diagonal corners of the mask.  
           [0021]    In a fifth embodiment, the design and advantages are very similar to the fourth embodiment except that in areas of the grade side that have actual tie bars, there are corresponding actual tie bars on the cone side. As explained in embodiment three, the cone side actual tie bars may improve the overall strength of the mask and reduce damage during handling.  
           [0022]    By way of example only, included herewith as FIGS. 8 and 9 are photographs of the beam shape as it passes through the slot at beam angle. These figures demonstrate a prior art, non-GTO design (FIG. 8) and a GTO design in accordance with the invention, in this case the third embodiment of the invention (FIG. 9). On the left side of each of these FIGS. are a series of photographs of various locations (as determined by x-y coordinates) on a mask as viewed from the cone side of the mask. On the right side of each of these Exhibits are a series of photographs showing the result of an electron beam passing through the aperture shown in the corresponding left column picture.  
           [0023]    As can be seen in the left photographs of FIG. 8, the prior art mask is comprised of aligning virtual tie bars both on the grade and the cone side. As can be seen in the left photographs of FIG. 9, the mask of this embodiment of the present invention is comprised of virtual tie bars only on the grade side (no virtual or actual tie bars on the cone side that line up with the grade side virtual tie bars).  
           [0024]    The advantages of the present invention are readily apparent by comparing the right photographs in FIG. 8 with the right photographs of FIG. 9. The right photographs of FIG. 8 show that in the area of the virtual tie bars there is an inconsistent transmission of the electron beam. In fact, in some cases the beam transmission in the area of the virtual tie bar has been completely obstructed (e.g., the photograph of the mask at X=250, beam angle=45 degrees). In contrast, the right photographs of FIG. 9 show a very consistent transmission of the electron beam in the area of the virtual tie bars. Moreover, the transmission is shown to be very crisp and square as compared to the photographs of FIG. 8.  
           [0025]    [0025]FIG. 10 depicts a series of photographs showing cross-sectional views through slots on an aperture mask in accordance with one embodiment of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 a  is a perspective view of the cone side of a mask of the prior art;  
         [0027]    [0027]FIG. 1 b  is a perspective view of the grade side of the mask of FIG. 1 a;    
         [0028]    [0028]FIG. 2 a  is a perspective view of one of the five basic types of tie bar configurations of the present invention;  
         [0029]    [0029]FIG. 2 b  is a perspective view of one of the five basic types of tie bar configurations of the present invention;  
         [0030]    [0030]FIG. 2 c  is a perspective view of one of the five basic types of tie bar configurations of the present invention;  
         [0031]    [0031]FIG. 2 d  is a perspective view of one of the five basic types of tie bar configurations of the present invention;  
         [0032]    [0032]FIG. 2 e  is a perspective view of one of the five basic types of tie bar configurations of the present invention;  
         [0033]    [0033]FIG. 3 is a map of a preferred embodiment of the mask of the present invention;  
         [0034]    [0034]FIG. 4 is a map of another preferred embodiment of the mask of the present invention;  
         [0035]    [0035]FIG. 5 is a map of another preferred embodiment of the mask of the present invention;  
         [0036]    [0036]FIG. 6 is a map of another preferred embodiment of the mask of the present invention;  
         [0037]    [0037]FIG. 7 is a map of another preferred embodiment of the mask of the present invention;  
         [0038]    [0038]FIGS. 8 a  and  8   b  are photographs of an electron beam as it passes through a non-GTO mask of the prior art at beam angle;  
         [0039]    [0039]FIGS. 9 a  and  9   b  are photographs of the electron beam as it passes through a GTO mask of the present invention at beam angle;  
         [0040]    [0040]FIG. 10 is a series of photographs showing cross-sectional views through slots on an aperture mask in accordance with one embodiment of the present invention; and,  
         [0041]    [0041]FIG. 11 is a map of another preferred embodiment of the mask of the present invention.  
         [0042]    [0042]FIG. 12 is a series of photographs showing the transitioning shape and size of the virtual tie bars of the preferred embodiment mapped in FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    Referring now to FIG. 2 a - e , there are shown five sections ( 1 ), ( 2 ), ( 3 ), ( 4 ) and ( 5 ), of mask having various tie bar configurations. Explanation will first be made pertaining to the construction of each of these configurations. Later, the masks of the present invention will be discussed referring to these five sections. This convention will allow maps to be drawn representative of masks having one or more of these configurations in various areas of the mask. All configurations ( 1 )-( 5 ) show a section of a mask  10  having a cone side  12  and a grade side  14 . A trough  16  is formed in the cone side  12 , but does not extend through to the grade side  14 . At the bottom of the trough  16 , two or more apertures  18  are formed that extend through to the grade side  14 .  
         [0044]    Referring now to configuration ( 1 ), it can be seen that a pair of apertures  18  are separated by a solid, actual tie bar  20  formed in the grade side  14  of the mask  10 . For purposes of clarity, such actual tie bars  20  shall hereinafter be referred to as grade side actual tie bars  20 . In configuration ( 1 ), there is no tie bar (neither virtual nor actual) formed in the cone side  12  that corresponds to the grade side actual tie bar  20 .  
         [0045]    Configuration ( 2 ) is somewhat similar to configuration ( 1 ) except that a pair of apertures  18  are separated and defined by a virtual tie bar  22  formed in the grade side  14  (hereinafter “grade side virtual tie bar  22 ”). The grade side virtual tie bar  22  differs from the grade side actual tie bar  20  of configuration ( 1 ) due to a gap  24  formed in the grade side virtual tie bar  22 . In configuration ( 2 ), there is no tie bar (neither virtual nor actual) formed in the cone side  12  that corresponds to the grade side virtual tie bar  22 .  
         [0046]    Configuration ( 3 ), like configuration ( 1 ), has a pair of apertures  18  that are separated by a grade side actual tie bar  20 . However, configuration ( 3 ) also has a cone side actual tie bar  26  that corresponds to the grade side actual tie bar  20 . The cone side actual tie bar  26  is relatively flush with the surface of the cone side  12  of the mask  10  and spans the entire trough  16  formed in the cone side  12 . The cone side actual tie bar  26  is said to “correspond” to the grade side actual tie bar  20  because it is relatively close to having the same location in terms of position on the mask  10 , and differs in location only based on cone side  12  or grade side  14 . It will be seen, however, that the cone side actual tie bar  26  may be slightly offset from the grade side actual tie bar  20 , vertically, horizontally, or both, and still be considered to be “corresponding” to the grade side actual tie bar  20 . The slight offset is useful to enhance beam clearance in some portions of the mask  10 .  
         [0047]    Configuration ( 4 ) is similar to configuration ( 3 ) in that configuration ( 4 ) also has a grade side actual tie bar  20  and a corresponding tie bar on the cone side  12 . However, configuration ( 4 ) has a cone side virtual tie bar  28  that corresponds to the grade side actual tie bar  20 . The cone side virtual tie bar  28  is distinguished by a gap  30  that extends from the surface of the cone side  12  to the inner most limit  32  of the grade side actual tie bar  20 .  
         [0048]    Configuration ( 5 ) is similar to configuration ( 4 ) in that configuration ( 5 ) has a cone side virtual tie bar  28 . However, the cone side virtual tie bar  28  of configuration ( 5 ) corresponds to a grade side virtual tie bar  22  rather than a grade side actual tie bar  20 . The gap  30  of the cone side virtual tie bar  28  is shown as being aligned with the gap  24  of the grade side virtual tie bar  22 , but it will be seen that in some preferred embodiments, the gap  30  of the cone side virtual tie bar  28  may be offset horizontally, vertically, or both from the gap  24  of the grade side virtual tie bar  22 .  
         [0049]    Having explained the five tie bar configurations ( 1 )-( 5 ) found in the various embodiments of the present invention, it is now possible to discuss mask design. Referring now to FIG. 3, there is shown a first embodiment of the GTO concept. It can be seen that the mask  10  includes, by way of convention, an x-axis and a y-axis. A plurality of apertures are found relatively uniformly throughout the mask  10 . It is understood that the apertures are designated by a ( 1 ) signifying they are of configuration ( 1 ) of FIG. 2 a . It is further understood that though FIG. 3 shows only a limited number of columns having a limited number of apertures each, in actuality there are hundreds of columns each having hundreds of apertures. Thus, FIG. 3 represents a mask  10  defining hundreds of thousands of apertures having configuration ( 1 ) of FIG. 2 a.    
         [0050]    [0050]FIG. 4 shows another embodiment of the mask  10  of the present invention. The mask  10  of FIG. 4 is made up of a combination of tie bar configurations ( 1 ) and ( 2 ) of FIGS. 2 a  and  2   b . Preferably, the combination is arranged such that a grade side actual tie bar  20  of configuration ( 1 ) alternates with one or more grade side virtual tie bars  22  of configuration ( 2 ), as shown. More preferably, there are between five and twenty five grade side virtual tie bars  22  or configuration ( 2 ) between each grade side actual tie bar  20  of configuration ( 1 ). There are no cone side actual tie bars  26  or cone side virtual tie bars  28  in this embodiment.  
         [0051]    [0051]FIG. 5 shows a third embodiment of the mask  10 , which is similar to the second embodiment shown in FIG. 4. A uniformly distributed combination pattern of two different configurations is provided. However, in FIG. 5, those areas that have grade side actual tie bars  20  also have cone side actual tie bars  26 . Thus, configuration ( 3 ) has replaced configuration ( 1 ) in the embodiment of FIG. 5. Thus, the mask  10  of FIG. 5 has more strength and rigidity than the mask  10  mapped in FIG. 4.  
         [0052]    [0052]FIG. 6 shows a fourth preferred embodiment of mask  10 . The numbers signifying the configuration from FIG. 2 are drawn smaller in order to more accurately map the various zones of the mask  10 . Inference is not being made that the apertures are arranged more densely or that the mask is larger than those shown in FIGS.  3 - 5 . The pattern of the mask  10  includes a center zone  34 , a pair of zones  36  located at the outer edges of the x-axis, a pair of zones  38  located along the y-axis, and four diagonal zones  40  located in the corners of the mask.  
         [0053]    The center zone  34  and the zones  38  located near the y-axis include configurations ( 1 ) and ( 2 ) from FIG. 2. Thus, on the grade side of the mask  10  in these areas, there are grade side actual tie bars  20  and grade side virtual tie bars  22  that separate the various apertures  18 . On the cone side of the mask  10  in these areas, there are no cone side tie bars, just a plurality of troughs  16 .  
         [0054]    The zones  36  along the x-axis and the diagonal zones  40  include configurations ( 1 ) and ( 5 ) from FIG. 2. On the grade side of the mask  10  in these areas, there are grade side actual tie bars  20  and grade side virtual tie bars  22 . On the cone side of the mask  10 , there are no cone side tie bars that correspond to the grade side actual tie bars  20 . However, there are cone side virtual tie bars  28  that correspond to the grade side virtual tie bars  22 .  
         [0055]    Noticeably, this GGTO embodiment includes transition zones  42  between the aforementioned zones  34 ,  36 ,  38 , and  40 . The transition zones  42  are made up of all three of the different configurations ( 1 ), ( 2 ) and ( 5 ) found on the mask  10 . However, the concentration of configurations ( 2 ) and ( 5 ) change depending on location. For example, though the transitions zones  42  are shown as being empty for purposes of clarity, the areas in zones  42  that are near zones  34  and  38  have a high concentration of configuration ( 2 ) and a smaller concentration of configuration ( 5 ). These concentrations gradually reverse in zones  42  near zones  36  and  40  such that the concentration of configuration ( 5 ) is high and the concentration of configuration ( 2 ) is small. Configuration ( 1 ) remains relatively constant through the entire mask  10  as the grade side actual tie bars  20  of configuration ( 1 ) are necessary to provide structural integrity to the mask  10 .  
         [0056]    [0056]FIG. 7 shows an embodiment of mask  10  that is essentially the same as that shown in FIG. 6 with one difference: those areas that have grade side actual tie bars  20  also have cone side actual tie bars  26 . The cone side actual tie bars  26  may improve the overall strength of the mask and reduce damage during handling. In terms of the convention of FIG. 2, configuration ( 1 ) has been replaced with configuration ( 3 ).  
         [0057]    Thus, the center zone  34  and the zones  38  located near the y-axis include configurations ( 2 ) and ( 3 ) from FIG. 2. On the grade side of the mask  10  in these areas, there are grade side actual tie bars  20  and grade side virtual tie bars  22  that separate the various apertures  18 . On the cone side of the mask  10  in these areas, there are no cone side tie bars that correspond to the grade side virtual tie bars  22 , but there are cone side actual tie bars  26  that correspond to the grade side actual tie bars  20 . The zones  26  along the x-axis and the diagonal zones  40  include configurations ( 3 ) and ( 5 ) from FIG. 2. On the grade side of the mask  10  in these areas, there are grade side actual tie bars  20  and grade side virtual tie bars  22 . On the cone side of the mask  10 , there are cone side actual tie bars  26  that correspond to the grade side actual tie bars  20 , and cone side virtual tie bars  28  that correspond to the grade side virtual tie bars  22 .  
         [0058]    There are also transition zones  42  between the aforementioned zones  34 ,  36 ,  38 , and  40 . The transition zones  42  are made up of all three of the different configurations ( 2 ), ( 3 ) and ( 5 ) found on the mask  10 . The concentration of configurations ( 2 ) and ( 5 ) change depending on location in the same manner as the embodiment shown in FIG. 6. Again, the transitions zones  42  are shown as being empty for purposes of clarity, and the areas in zones  42  that are near zones  34  and  38  have a high concentration of configuration ( 2 ) and a smaller concentration of configuration ( 5 ). These concentrations gradually reverse in zones  42  near zones  36  and  40  such that the concentration of configuration ( 5 ) is high and the concentration of configuration ( 2 ) is small. Configuration ( 3 ) remains relatively constant through the entire mask  10  as the grade side actual tie bars  20 , reinforced with the cone side actual tie bars  26  of configuration ( 3 ) are necessary to provide structural integrity to the mask  10 .  
         [0059]    [0059]FIG. 11 shows another preferred embodiment exhibiting the GGTO concept of the present invention. Rather than the occurrence of screen side virtual tie bars transitioning from sparse to densely populated as the distance from the center of the mask increases, in this embodiment, the size and shape of the cone side virtual tie bars  28  transition from essentially non-existent in the center of the mask to relatively prominent toward the extreme x-axis areas and the diagonal corners. Notably, the offsets between the grade side virtual tie bars  22  and the cone side virtual tie bars  28  increase as the outer periphery of the mask  10  is reached. A different convention is used in FIG. 11 to describe the mask  10  than that of FIGS. 3 through 7 because it is important to show the changing shape and offset of the screen side virtual tie bars  28 .  
         [0060]    Specifically, FIG. 11 is drawn from the perspective of the electron gun to show the purpose of the offsets between the grade side virtual tie bars  22  and the cone side virtual tie bars  28 . Thus, the gun side  14  of the mask is shown while features on the screen side  12  are shown in phantom lines. The various aperture shapes are detailed in enlarged views (a) through (k), which are generally representative of the other apertures in the areas of the mask  10  from which they project. The enlarged views (a) through (k) all show a fully shaped aperture  18  surrounded by grade side virtual tie bars  22  and two partial views of adjacent apertures  18 . Details on the cone side  12  are shown in phantom where hidden and the features that show through the apertures  18  are shown in regular lines.  
         [0061]    Enlarged view (a) is representative of the apertures  18  found in the center of the mask  10 . The grade side virtual tie bars  22  are formed opposite a cone side trough  16  but there are no corresponding cone side virtual tie bars. Notably, the individual arms of the grade side virtual tie bars  22  are approximately the same size.  
         [0062]    Moving outward from the center vertically, views (b) and (c) are representative of the apertures  18  found generally along the y-axis of the mask  10 . Because the cone side trough  16  runs vertically, electron beams that bend from the center in a vertical direction do not require clipping. Therefore, it is optimal to maintain an absence of cone side virtual tie bars along the y-axis. Thus, the shape of the apertures  18  do not vary along the y-axis.  
         [0063]    However, moving outward from the center horizontally, the electron beam begins to encounter interference with the side walls of the virtual tie bars. Thus, looking first at views (d) and (e), it can be seen that arms of the grade side virtual tie bars  22  are beginning to grow longer on the outer side of the aperture  18  and shorter on the side of the aperture toward the center of the mask  10 . Additionally, clipping control is provided by way of cone side virtual tie bars  28 . Notably, the cone side virtual tie bars  28  are still relatively small and are more prominent on the outside of the aperture  18  than toward the center of the mask  10 . Also, the trough  16  and the cone side virtual tie bars  28  are becoming slightly offset from the grade side features toward the outside of the mask  10 .  
         [0064]    Transition in this manner continues and becomes most prevalent at the outer edges of the mask  10 . Looking now at views (f) and (g), it can be seen that the outer arms of the grade side virtual tie bars  22  are significantly larger than the opposing inner arms. Also, the corresponding cone side virtual tie bars  28  are larger and more offset toward the outside of the mask than those of views (d) and (e) due to the increased angle of beam travel. Because the features of views (f) and (g) are still located relatively close to the x-axis, there is no vertical offset between the grade side features and the cone side features.  
         [0065]    Transition away from the x-axis toward the four corners of the mask  10  is marked by vertical offset between the grade side features and the cone side features, as shown in views (h), (i), (j), and (k). The size and shape of the virtual tie bars  22  and  28  is approximately the same as in views (f) and (g), however the cone side virtual tie bars  28  are offset (shown exaggerated in the views for clarity) toward the outside of the mask  10  in both the horizontal and the vertical directions, thereby allowing for the angle of travel of the electron beam.  
         [0066]    [0066]FIG. 12 is a series of photographs (a) through (e) taken from various points on a mask  10  of the embodiment of FIG. 11 from the cone side  12  of the mask  10 . Each photograph includes a grade side actual tie bar  20 , a corresponding cone side actual tie bar  26 , a grade side virtual tie bar  22  defining a gap  24 , three apertures  18  and a trough  16 .  
         [0067]    The series begins with photograph (a), taken from the center of the mask  10 . With the exception of the cone side actual tie bar  26 , the trough  16  is uninterrupted with cone side virtual tie bars. Notably, the arms of the grade side virtual tie bar are of equal size such that the gap  24  is centered within the aperture  18 .  
         [0068]    The series progresses outwardly from the center of the mask  10  with photograph (b). A cone side virtual tie bar  28  begins to appear on the outer side of the aperture  18 . Also, the outer arm of the grade side virtual tie bar  22  is just slightly longer than the opposite arm on the inner side of the aperture  18  toward the center of the mask  10 .  
         [0069]    Next, photograph (c) was taken at a point along the x-axis of the mask  10  approximately midway between the center of the mask and the outer right edge as viewed from the cone side  12 . The outer arm of the cone side virtual tie bar  28  is becoming more prominent while an inner arm of the cone side virtual tie bar  28  is just beginning to appear. Similarly, the outer arm of the grade side virtual tie bar  22  is growing while the inner arm is shrinking as a lateral offset begins to appear.  
         [0070]    Photograph (d) was taken at a point even closer to the right edge of the mask  10  on the cone side  12 . Both arms of the cone side virtual tie bar  28  are now readily apparent, with the outer arm being significantly larger than the inner arm. Likewise, the inner arm of the grade side virtual tie bar  22  is much shorter than the outer arm of the grade side virtual tie bar  22 . This accentuates a noticeable lateral offset with the gap  24  well left of center of the aperture  18 .  
         [0071]    Photograph (e) was taken at the right edge of the mask  10  on the cone side  12 . Prominent are the cone side virtual tie bar  28  and the lateral offset between the various features on the cone side  12  and the features on the grade side  14 . The well formed outer arm of the cone side virtual tie bar  28  provides significant clipping to angled electron beam that would otherwise distort the features of the apertures  18 .