Patent Publication Number: US-2006008709-A1

Title: Mask and method for determining mask pattern line length

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2004-204053, filed on Jul. 12, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      This invention relates to a mask and a method for determining the line length of a mask pattern and, more particularly, to a mask used for forming a circuit pattern in a semiconductor device and a method for determining the line length of a mask pattern formed on such a mask.  
      (2) Description of the Related Art  
      In recent years semiconductor devices have become minute and their integration levels have become high. Accordingly, there have been severer demands for accuracy in the line length of mask patterns formed on photo masks (masks) used for fabricating them.  
      Usually items for guaranteeing the line length of a mask pattern include within-mask uniformity, linearity, a proximity effect, and X-Y difference. In addition to them, in recent years great importance has been attached to the amount of a shift of the position of an edge of a mask pattern from a designed value in masks for advanced devices. The reason for this is that in device fabrication a circuit pattern portion formed by transferring an edge portion of a line pattern onto a wafer often overlaps with a hole pattern for connecting the circuit pattern portion to another layer with the circuit pattern portion as a wiring edge. For example, if a device is fabricated with a mask on which the amount of a shift of the position of an edge of a line pattern is large, it may be impossible to ensure a sufficient contact area between the line pattern and a hole pattern. In this case, the contact will be bad.  
      At present the integration levels of advanced devices become higher and the number of their functions increases. This leaves no extra space in writing lines and contact holes at design time. Therefore, mask patterns must be formed with great accuracy on masks used for forming circuit patterns in devices. Meanwhile, when mask patterns are formed on masks, the positions of the edges of the mask patterns may shift from their designed values. Therefore, to fabricate high-quality devices, the line length of mask patterns formed on masks must be determined in advance with great accuracy and must be guaranteed.  
      Conventionally, scanning electron microscopes (SEMs) have widely been used for determining the line length of mask patterns. When the line length of mask patterns is determined with SEMs, the upper limit of line length which can be determined on SEM screens is about 3 μm at magnification (50,000, for example) necessary for ordinary accuracy guarantees.  
       FIG. 11  shows an example of a mask pattern.  
      It is assumed that a device includes a layer on which a circuit pattern having a shape shown in  FIG. 11  is formed and that the longer line length (about 2.5 to 3.0 μm) of a mask pattern  100  used for forming the circuit pattern must be guaranteed. In such a case, the entire mask pattern  100  is displayed on a SEM screen and line length L from the left end to the right end will be determined. In certain circumstances, the magnification of the SEM is lowered, the entire mask pattern  100  is displayed on the SEM screen, and its line length is determined.  
      The above-mentioned use is a simple example and SEMs are used for, for example, determining the line length of circuit patterns after exposure. For example, a method for forming a pattern with long line length for detecting a shift in position used for determining a shift in position with an optical measuring apparatus and a pattern with short line length for detecting a shift in position used for determining a shift in position with a SEM is proposed (see, for example, Japanese Unexamined Patent Publication No. Hei 11-297588).  
     SUMMARY OF THE INVENTION  
      A mask having a mask pattern is provided by the present invention. This mask has auxiliary patterns near the mask pattern used for determining the line length of the mask pattern.  
      In addition, a method for determining the line length of a mask pattern is provided by the present invention. In this method for determining the line length of a mask pattern, the line length of the mask pattern is found by determining the position of the mask pattern in respect to auxiliary patterns located near the mask pattern.  
      The above and other features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view showing the structure of a main portion of a mask.  
       FIG. 2  is a view for describing a method for determining the line length of a mask pattern.  
       FIG. 3  is a plan view of a main portion of an evaluation mask used for doing simulations of light intensity.  
       FIG. 4  shows results obtained by doing simulations of light intensity for a binary mask on which S=0.1 μm.  
       FIG. 5  shows results obtained by doing simulations of light intensity for a binary mask on which S=0.2 μm.  
       FIG. 6  shows results obtained by doing simulations of light intensity for a binary mask on which S=0.3 μm.  
       FIG. 7  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.1 μm.  
       FIG. 8  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.2 μm.  
       FIG. 9  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.3 μm.  
       FIG. 10  is a plan view showing the structure of a main portion of a mask on which an isolated pattern is formed.  
       FIG. 11  shows an example of a mask pattern.  
       FIG. 12  shows another example of a mask pattern. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As described in the “Description of the Related Art,” in the conventional method for determining the line length of a mask pattern with a SEM, an error in the line length of a mask pattern determined may be caused by problems, such as the repeatability of difference in line length in a screen, a change with the passage of time in line length, and a change with the passage of time in difference in line length between magnifications.  
      The problem of the repeatability of difference in line length in a screen is as follows. For example, there is a difference between a value obtained by determining the line length of a mask pattern displayed in the center of a SEM screen and a value obtained by determining the line length of the mask pattern displayed at the edge of the SEM screen, and this difference is not repeatable. When the whole of one mask pattern is displayed on a screen, both ends of the mask pattern may be at the edges of the screen, depending on the magnification of the SEM. Therefore, a value obtained by determining the distance between both ends of the mask pattern may differ from the actual value.  
      The problem of a change with the passage of time in line length is as follows. A value obtained by determining the line length of the same mask pattern varies each time determination is made (from determination date to determination date, for example). The larger the size of a mask pattern becomes, the larger the amount of a change with the passage of time in line length becomes. This is based on the principle of SEM calibration.  
      The problem of a change with the passage of time in difference in line length between magnifications is as follows. A value obtained by determining the line length of the same mask pattern varies when the magnification of the SEM is changed. In addition, difference in line length between magnifications varies each time determination is made (from determination date to determination date, for example). Therefore, when the magnification of the SEM is lowered to a value smaller than an ordinary value so that the entire mask pattern will be displayed on the screen, a value obtained by determining the line length of the mask pattern at this magnification may differ from a value obtained by determining the line length of the mask pattern at the ordinary magnification. In this state of things it is impossible to compare the line length of this mask pattern with that of another mask pattern.  
      In the conventional method for determining the line length of a mask pattern with a SEM, these factors often exert an influence simultaneously. Moreover, it is very difficult to separate or specify these factors. Therefore, even if the mask pattern is formed to design, a determination error may occur because of the SEM, being an apparatus for determining line length. As a result, it is possible that the line length of the mask pattern obtained will not be correct.  
       FIG. 12  shows another example of a mask pattern.  
      Mask patterns formed on masks include an isolated pattern  200  around which other mask patterns do not exist. Essentially, the position of the edge of the isolated pattern  200  must also be guaranteed.  
      Even if the whole of a mask pattern cannot be displayed on a SEM screen at predetermined magnification, it is possible in some cases to grasp the position of the edge of the mask pattern in respect to a second mask pattern around the edge by displaying the edge and the second mask pattern at the magnification.  
      With the isolated pattern  200 , however, a second mask pattern which can be used as a standard does not exist around it. Accordingly, the position of the edge of the isolated pattern  200  cannot be grasped. If the magnification of the SEM is lowered to display the isolated pattern  200  and another mask pattern, the above-mentioned problem of a difference in line length between magnifications or the like will occur. That is to say, under the present conditions, it is virtually impossible to guarantee accuracy in determining the line length of the isolated pattern  200 .  
      The present invention was made to solve such a problem. An object of the present invention is to provide a mask on which the line length of a mask pattern can be determined with great accuracy and a method for determining the line length of such a mask pattern.  
      Embodiments of the present invention will now be described in detail with reference to the drawings.  
       FIG. 1  is a plan view showing the structure of a main portion of a mask.  
      With a mask  10  shown in  FIG. 1 , a light shielding film of chromium (Cr) or molybdenum silicide (MoSi) is formed on a glass substrate made from, for example, silica. That is to say, light transmitting portions which transmit light at the time of transferring onto a wafer and light shielding portions which shield light at the time of transferring onto a wafer are formed on the mask  10 . A pattern  11  to be determined the line length of which is to be determined and minute auxiliary patterns  12  which are formed near the edges of the pattern  11  to be determined and which are used for determining the line length of the pattern  11  to be determined are the light transmitting portions of the mask  10 .  
      The pattern  11  to be determined corresponds to a circuit pattern transferred onto a wafer in the fabrication of a device.  
      The auxiliary patterns  12  are formed so that their shape, line length, and positions will not exert an influence upon functions which the circuit pattern formed on the wafer by transferring the pattern  11  to be determined should originally have.  
      Accordingly, it is preferable that the auxiliary patterns  12  the line length of which does not exert an influence upon the line length of the circuit pattern formed by transferring the pattern  11  to be determined should be formed at appropriate positions. Moreover, it is preferable that the auxiliary patterns  12  should not be transferred onto the wafer. Therefore, when the mask  10  is designed, the line length of the auxiliary patterns  12  and the distance between the auxiliary patterns  12  and the pattern  11  to be determined must adequately be considered. The line length of the auxiliary patterns  12  and the distance between the pattern  11  to be determined and the auxiliary patterns  12  will be described later in detail.  
      The pattern  11  to be determined and the auxiliary patterns  12  are formed in this way. As a result, after the mask  10  is fabricated, the line length of the pattern  11  to be determined can be obtained with great accuracy by determining the distance between the pattern  11  to be determined and the auxiliary patterns  12  with a SEM. A method for determining the line length of the mask pattern formed on the mask  10  by using a SEM will now be described.  
       FIG. 2  is a view for describing a method for determining the line length of the mask pattern.  
      To determine the line length of the pattern  11  to be determined, a distance W 1  between an edge (edge opposite the auxiliary pattern  12 )  11   a  of the pattern  11  to be determined and a left-hand edge (far edge from the pattern  11  to be determined)  12   a  of the auxiliary pattern  12  is determined first.  
      Then a distance W 2  between the edge  11   a  of the pattern  11  to be determined and a right-hand edge (near edge from the pattern  11  to be determined)  12   b  of the auxiliary pattern  12  is determined in the same way.  
      A distance W (=(W 1 +W 2 )/2) between the edge  11   a  of the pattern  11  to be determined and the position of the center of gravity (position of the midpoint between the edges  12   a  and  12   b ) of the auxiliary pattern  12  is calculated.  
      After the distance W is calculated in this way, the difference (=W−W′) between the distance W and a designed value W′ of the distance between the edge  11   a  of the pattern  11  to be determined and the position of the center of gravity of the auxiliary pattern  12  is calculated. This difference corresponds to the amount of a shift in the position of the edge  11   a  of the pattern  11  to be determined.  
      The amount of a shift in the position of the edge  11   a  of the pattern  11  to be determined can be calculated by using one of the distances W 1  and W 2  as the distance between the pattern  11  to be determined and the auxiliary pattern  12 . As described above, however, by calculating the distance W on the basis of the position of the center of gravity of the auxiliary pattern  12 , the influence of an error in the line length of the auxiliary pattern  12  itself can be eliminated.  
      In  FIG. 2 , only one edge portion of the pattern  11  to be determined is shown. However, the same process that is described above is performed on the other edge portion of the pattern  11  to be determined shown in  FIG. 1 . That is to say, by finding the position of the other edge of the pattern  11  to be determined in respect to the auxiliary pattern  12  opposite it, the amount of a shift in the position of the other edge of the pattern  11  to be determined is calculated. Then the line length of the pattern  11  to be determined is found from the amount of shifts of the positions of both edges of the pattern  11  to be determined.  
      By locating the auxiliary patterns  12  near both edges of the pattern  11  to be determined in this way, shifts of the positions of both edges of the pattern  11  to be determined can be quantified with great accuracy and the line length of the pattern  11  to be determined can be found with accuracy.  
      Moreover, by locating the auxiliary patterns  12 , the amount of shifts of the positions of both edges of the pattern  11  to be determined can be determined with the auxiliary patterns  12  and both edge portions of the pattern  11  to be determined displayed near the center of the screen. This significantly reduces the amount of a determination error caused by a difference in line length in a screen.  
      The auxiliary patterns  12  are minute, so the amount of a change with the passage of time in line length which occurs as a result of using a SEM can be reduced. Therefore, the amount of a determination error caused by a change with the passage of time in line length can be reduced significantly.  
      In addition, even if the line length of the pattern  11  to be determined is about 2.5 to 3.0 μm, that is to say, even if the pattern  11  to be determined is comparatively large, the line length of the pattern  11  to be determined can be determined at the same magnification that is used for another mask pattern with a line length of about 1 μm with both edge portions of the pattern  11  to be determined displayed near the center of the screen. This eliminates determination errors caused by a difference in line length between magnifications and a change of it with the passage of time.  
      The line length of the auxiliary patterns  12  and the distance between the pattern  11  to be determined and the auxiliary patterns  12  that must be considered at the time of designing the mask  10  will now be described. In this case, simulations of light intensity are done with a mask pattern shown in  FIG. 3  as an example.  
       FIG. 3  is a plan view of a main portion of an evaluation mask used for doing simulations of light intensity.  
      On an evaluation mask  20  shown in  FIG. 3 , a pattern  21  to be determined is formed and auxiliary patterns  22  are formed near both edges of the pattern  21  to be determined. The size of the pattern  21  to be determined is 0.6 μm×2.5 μm. The line length (S) of the auxiliary patterns  22  and the distance (D) between the edge of the pattern  21  to be determined and the edge of the auxiliary pattern  22  that do not exert an influence upon the line length of a circuit pattern formed by transferring the pattern  21  to be determined are found by doing simulations of light intensity. Simulation conditions are shown in Table 1.  
                               TABLE 1                                              CONDITIONS FOR LOCATING           EXPOSURE       AUXILIARY PATTERNS                                         No.   MASK TYPE   WAVELENGTH   NA   σ   S (μm)   D (μm)               1   BINARY   ArF   0.70   0.80   0.1, 0.2, 0.3   0.2, 0.4, 0.6,                               1.2, 2.4       2   HALF TONE   ArF   0.70   0.85/   0.1, 0.2, 0.3   0.2, 0.4, 0.6,                       0.425       1.2, 2.4                  
 
      Masks used are of two types. One is a binary mask (No. 1) on which Cr is used for forming a light shielding film and the other is a half tone phase shift mask (half tone mask) (No. 2) on which MoSi is used as a shifter. For both mask types, an exposure wavelength is 193 nm obtained by using an ArF exima laser as a light source. The transmittance of the half tone mask is about 6%. The numerical aperture (NA) of a lens is 0.70. The ratio (σ) of the NA of the lens to the NA of the light source is 0.80 for the binary mask and 0.85/0.425 for the half tone mask.  
      For both mask types, simulations are done with the line length (S) as 0.1 μm, 0.2 μm, or 0.3 μm and the distance (D) as 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm. The values of the line length (S) and the distance (D) shown in Table 1 are those determined on the evaluation mask  20 .  
      For both mask types, mask patterns are reduced at a magnification of ¼ and are transferred onto wafers when these simulations of light intensity are done.  
       FIGS. 4 through 6  show results obtained by doing simulations of light intensity for binary masks.  FIG. 4  shows results obtained by doing simulations of light intensity for a binary mask on which S=0.1 μm.  FIG. 5  shows results obtained by doing simulation of light intensity for a binary mask on which S=0.2 μm.  FIG. 6  shows results obtained by doing simulations of light intensity for a binary mask on which S=0.3 μm.  
      In each of  FIGS. 4 through 6 , a horizontal axis indicates the amount (in nm) of a defocus of an exposure apparatus and a vertical axis indicates the difference (in μm) between the line length of a circuit pattern formed on a wafer and the line length of a circuit pattern formed on a wafer without locating the auxiliary patterns  22 .  FIG. 4  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.1 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm.  FIG. 5  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.2 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm.  FIG. 6  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.3 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm.  
      As can be seen from the results of the light intensity simulations shown in  FIGS. 4 through 6 , conditions for locating the auxiliary patterns  22  which make the difference between the line length of the circuit patterns formed on the wafers zero do not exist. Accordingly, it is assumed that the allowable value of a total shift in the line length of a circuit pattern caused by locating the auxiliary patterns  22  is 1 nm or less. If a total shift in the line length of the circuit pattern is not greater than this allowable value, it is safe to consider that the auxiliary patterns  22  do not exert an influence upon the line length of the circuit pattern formed by transferring.  
      As can be seen from  FIG. 4 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.1 μm and the distance (D) is 0.6 μm, 1.2 μm, or 2.4 μm. As can be seen from  FIG. 5 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.2 μm and the distance (D) is 1.2 μm or 2.4 μm. As can be seen from  FIG. 6 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.3 μm and the distance (D) is 1.2 μm or 2.4 μm. Therefore, with the binary mask, conditions for locating the auxiliary patterns  22  are line length (S)≦0.3 μm and distance (D)≧1.2 μm.  
      If conditions for locating the auxiliary patterns  22  are considered from the viewpoint of the fabrication of a mask, the value of the line length (S) should be as great as possible so that a margin of pattern resolution can be ensured in the fabrication of the mask. The value of the distance (D) should be as small as possible so that the auxiliary patterns  22  will not be very far from the pattern  21  to be determined and so that the amount of a determination error which occurs as a result of using a SEM can be minimized. Therefore, with the binary mask, line length (S)=0.3 μm and distance (D)=1.2 μm can be obtained as conditions for properly locating the auxiliary patterns  22 .  
       FIGS. 7 through 9  show results obtained by doing simulations of light intensity for half tone masks.  FIG. 7  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.1 μm.  FIG. 8  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.2 μm.  FIG. 9  shows results obtained by doing simulations of light intensity for a half tone mask on which S=0.3 μm.  
      In each of  FIGS. 7 through 9 , a horizontal axis indicates the amount (in μm) of a defocus of the exposure apparatus and a vertical axis indicates the difference (in μm) between the line length of a circuit pattern formed on a wafer and the line length of a circuit pattern formed on a wafer without locating the auxiliary patterns  22 .  FIG. 7  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.1 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm. FIG.  8  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.2 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm.  FIG. 9  shows the relationship between the amount of a defocus of the exposure apparatus and the difference between the line length of the circuit patterns which is obtained when the line length (S) is 0.3 μm and the distance (D) is 0.2 μm, 0.4 μm, 0.6 μm, 1.2 μm, or 2.4 μm.  
      As can be seen from the results of the light intensity simulations shown in  FIGS. 7 through 9 , conditions for locating the auxiliary patterns  22  which make the difference between the line length of the circuit patterns formed on the wafers zero do not exist. Accordingly, it is assumed that the allowable value of a total shift in the line length of a circuit pattern caused by locating the auxiliary patterns  22  is 1 nm or less. This is the same with the above-mentioned binary mask.  
      As can be seen from  FIG. 7 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.1 μm and the distance (D) is 1.2 μm or 2.4 μm. As can be seen from  FIG. 8 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.2 μm and the distance (D) is 1.2 μm or 2.4 μm. As can be seen from  FIG. 9 , a total shift in the line length of the circuit pattern is smaller than or equal to 1 nm when the line length (S) is 0.3 μm and the distance (D) is 2.4 μm.  
      Line length (S)=0.3 μm and distance (D)=2.4 μm are obtained as conditions for locating the auxiliary patterns  22  in a way that is the same with the binary mask. From the view point of determining line length, however, there is no advantage to locating the auxiliary patterns  22  at a place 2.4 μm distant from the pattern  21  to be determined the size of which is 0.6 μm×2.5 μm. This does not differ greatly from conventional methods. Therefore, with the half tone mask, line length (S)=0.2 μm and distance (D)=1.2 μm can be considered as conditions for properly locating the auxiliary patterns  22 .  
      As stated above, conditions for properly locating the auxiliary patterns  22  in respect to the pattern  21  to be determined vary depending on which of the binary mask or the half tone mask is used. Basically, if the line length (S) is increased to stably fabricate a mask, then the distance (D) must also be increased to prevent a shift in the line length of a circuit pattern caused by light which passes through the auxiliary patterns  22 . When the auxiliary patterns  22  are formed, this must be taken into consideration. As a result, it is necessary to set the line length (S) and the distance (D) for each pattern  21  to be determined according to the type of a mask used.  
      In the above examples, the results of the light intensity simulations obtained when a reduction magnification of ¼ is used at exposure time are shown. If this reduction magnification is changed, the conditions of the line length (S) and the distance (D) for the auxiliary patterns  22  must be reset.  
      By forming a mask on the basis of the above-mentioned simulations results of light intensity, the amount of a shift in the position of the edge of a pattern to be determined can be determined by using auxiliary patterns and the line length of the mask pattern can be determined with great accuracy.  
      Conventionally, it has been virtually impossible to determine the amount of a shift in the position of the edge of an isolated pattern. However, by forming the above-mentioned auxiliary patterns, the amount of a shift in the position of the edge of such an isolated pattern can also be determined.  
       FIG. 10  is a plan view showing the structure of a main portion of a mask on which an isolated pattern is formed.  
      On a mask  30  shown in  FIG. 10 , a pattern  31  to be determined corresponding to a circuit pattern formed on a wafer by transferring is formed and an auxiliary pattern  32  is formed near the edge of the pattern  31  to be determined.  
      By forming the auxiliary pattern  32  near the edge of the isolated pattern  31  to be determined in this way, the amount of a shift in the position of the edge of the pattern  31  to be determined can be determined with great accuracy at proper magnification even if there is no mask pattern around the pattern  31  to be determined.  
      As has been described in the foregoing, in the present invention, by locating auxiliary patterns near a pattern to be determined used for determining the line length thereof, the amount of a shift in the position of the edge of the pattern to be determined can be determined with great accuracy and the line length of the mask pattern can be found with great accuracy. Accordingly, this method is effective in guaranteeing the line length of a pattern. In addition, the amount of a shift in the position of the edge of a pattern can be controlled strictly, so the number of fabricated devices in which the contact is bad can be reduced. Therefore, high-quality devices can be fabricated.  
      In the present invention, the auxiliary patterns are located near the mask pattern the line length of which is to be determined, the position of the mask pattern in respect to the auxiliary patterns is determined, and the line length of the mask pattern is determined. As a result, the amount of a shift in the position of the mask pattern from a designed value can be determined with accuracy and the line length of the mask pattern can be determined with great accuracy. This enables the fabrication of high-quality devices.  
      The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.