Patent Publication Number: US-2006000816-A1

Title: System for and method of zoom processing

Description:
FIELD OF THE INVENTION  
      The present invention relates to laser processing systems, and more specifically, to an improved system for and method of processing tapered or not-tapered features of almost any geometrical shape by using laser processing systems.  
     BACKGROUND OF THE INVENTION  
      There is an ever-increasing demand for smaller electronic devices in today&#39;s high-tech marketplace. As a result, new and innovative fabrication techniques that are well-suited to small devices have become a focal point of many manufacturers. Manufacturers have turned to laser processing as a means of fabrication (e.g., for blowing fuses, via and hole drilling, ablation patterning, or resistor trimming). However, most laser processing systems are costly and inefficient. For example, single-feature laser processing systems process one feature (e.g., pattern, hole, or via) through ablative, additive, or transformational means at a time, and are, therefore, incapable of efficiently operating in large-volume manufacturing environments.  
      Exemplary products made with laser processing systems include inkjet nozzles, system LSI chips, printed circuit boards, etc. The market for replacement inkjet cartridges and inkjet nozzles is in the tens of millions of dollars ($USD) per year. With a market size of this magnitude, companies that create incremental cost savings in manufacturing can potentially realize millions of dollars of additional profit.  
      The hole shapes required in inkjet nozzles are generally conical and symmetrical. However, other shapes (e.g., pyramids, straight cylinders) can be imagined that may be useful in a variety of applications. Inkjet nozzles contain rows of holes (for example, 4 rows with 38 beams in each row) that are shaped in order to best project ink when they are used in an inkjet printer. These holes are drilled as specified with a tapered, conical shape, in which the input end of the hole is wider than the exit hole. The shape and measurements of the hole (input diameter, exit diameter, and taper) are critical to the product quality and the operation of the end application. For example, the taper of a drilled hole affects the fluid dynamics of ink in an inkjet printer nozzle. What is needed is a way to improve control of the resulting feature shape during laser processing. It should be noted that the definition of hole in the more general context can refer to the additive creation of shaped features to a workpiece or non-ablative transformation of the material properties (such as refractive index, transmissivity, etc.) of the workpiece.  
      Many laser-processing manufacturers have sought to reduce the cost of manufacturing by increasing yield. Increasing yield often requires higher optical power from the laser in order to reduce the processing time for each feature, thus increasing yield. This increase in optical power often has the negative effect of lowering the quality in the fabricated devices because of overexposure and thermal effects. Therefore, there exists a need to reduce cost by increasing manufacturing yield without sacrificing manufacturing quality. Likewise, there exists a need to increase manufacturing quality without sacrificing manufacturing yield.  
      In U.S. Pat. No. 6,627,844, entitled “Method of laser milling,” a method of milling is described whereby a single or parallel processing laser system is used to process a wide variety of complex shapes on a workpiece. The &#39;844 patent describes a way to provide control of the processing beam(s) that allow(s) for almost any feature shape to be processed. However, the &#39;844 patent requires the use of a scanning mirror, such as a galvanometer or PZT mirror, to direct the spot of the beam on the workpiece. The scanning mirrors used in the &#39;844 patent are expensive to purchase and require frequent maintenance. Additionally, the milling algorithms described in the &#39;844 patent take too much time to complete, which further decreases yield and increases the final cost of the finished product. What is needed is a less expensive way to manufacture workpieces that have a wide variety of specified shapes.  
      It is an object of this invention to provide a way to improve control of the resulting feature shape during laser processing.  
      It is another object of this invention to provide a way to reduce cost by increasing manufacturing yield without sacrificing manufacturing quality.  
      It is yet another object of this invention to provide a way to increase manufacturing quality without sacrificing manufacturing yield.  
      It is yet another object of this invention to provide a less expensive way to manufacture workpieces that have a wide variety of specified shapes.  
     SUMMARY OF THE INVENTION  
      A laser processing system for precision manufacturing is operated by adjusting a scan lens within the system to create a wide variety of features on a workpiece. The zoom scan lens is adjusted continuously within the system to alter radius of an annulus of the processing beam(s), resulting in change of feature size on the final workpiece. The zooming of the scan lens is performed in combination with adjustments to the laser power and dwell time in order to maintain optimum power-per-unit area for high-quality laser processing. The invention is well-suited for drilling tapered, conical holes, such as those found in inkjet nozzles, but may be applicable for processing tapered or non-tapered features of almost any geometrical shape. Such processes include additive, ablative, or material transformation methods.  
      Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
       FIG. 1  illustrates a laser processing system;  
       FIG. 2  illustrates an improved method of operating a laser processing system and maintaining optimum power-per-unit area for high-quality processing;  
       FIG. 3A  shows an exemplary perspective view of a workpiece with conical workpiece features;  
       FIG. 3B  shows an exemplary side view of a workpiece with conical workpiece features;  
       FIG. 4A  shows an exemplary perspective view of a workpiece with pyramidal workpiece features;  
       FIG. 4B  shows an exemplary side view of a workpiece with pyramidal workpiece features;  
       FIG. 5A  shows an exemplary perspective view of a workpiece with arbitrary workpiece features;  
       FIG. 5B  shows an exemplary side view of a workpiece with arbitrary workpiece features. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
      The present invention includes a laser processing system for precision manufacturing and a method of using the system. More specifically, the invention includes a system for and method of laser zoom-processing to create a wide variety of features and feature shapes on the workpiece.  
      The present invention allows for faster processing of features and feature shapes on the workpiece by utilizing a larger beam diameter (relative to the overall feature size) and zooming in (processing a smaller and smaller area of the workpiece as it zooms) as the workpiece is processed to create the specified feature shape. This is a different processing method than is described in the &#39;844 patent above, in which a smaller beam diameter (relative to the overall feature size) is utilized and directed by a scanning mirror (such as a PZT scan mirror, or a galvanometer), according to a milling algorithm, to process the workpiece in a stepwise fashion.  
      For illustration purposes, this invention will be described in the context of a parallel laser processing system. Parallel laser processing systems process more than one feature at once and often employ a beam splitter to divide the optical power into a plurality of sub-beams, which process the workpiece in parallel. The present invention also applies to single-feature processing; those familiar with the technical details of laser processing systems will be able to modify the invention to accommodate single-feature processing after reading the description below.  
       FIG. 1  illustrates a laser processing system  100 , including the following elements: a laser  110 , a computer  112 , a beam  115 , a first mirror  120 , a shutter  125 , an attenuator  130 , a second mirror  135 , a beam expander  140 , a spinning half-wave plate  155 , a DOE  165 , a plurality of sub-beams  170 , a zoom scan lens  175 , a workpiece  180 , and a workpiece holder  185 , arranged as shown.  
      Laser  110  provides sufficient pulse energy to ablate material in workpiece  180 . In one example, laser  110  is a picosecond (ps) laser (bandwidth less than 0.1 nanometer (nm)) that consists of an oscillator and a regenerative amplifier, for which the oscillator output power equals 35 milliwatts (mW), the pulse width is approximately 15 ps, the regenerative amplifier output power is 1 Watt (W) at 1 kilohertz (kHz), the energy per pulse is 1 millijoule (mJ), the power stability is 1.7% over 12 hours, and the pointing stability is approximately 1%.  
      Beam  115  is the pulse energy emitted by laser  110 .  
      First mirror  120  and second mirror  135  are conventional mirrors used to direct or steer beam  115  along a specified path. It should be noted that the actual number of mirrors used to steer beam  115  may vary, depending the specific layout of the optical path of the drilling system.  
      Shutter  125  is a conventional mechanical shutter, such as those made by Vincent Associates (e.g., model # LS6ZMZ). The purpose of shutter  125  is to allow beam  115  to illuminate workpiece  180  when shutter  125  is in the open state and to prevent beam  115  from illuminating workpiece  180  when shutter  125  is in the closed state.  
      Computer  112  is one example of a controller that can be used in accordance with the present invention. Other types of controllers according tot he present invention are purely mechanical controls and/or an electromechanical control system. Preferably, the controller is computer  112 , such as a personal computer, which can include conventional input devices (e.g., keyboard, mouse); output devices (e.g., monitor, printer, disk, etc); communication components (e.g., network card, serial ports); an operating system (e.g., Microsoft Windows, Linux); and software to convert product specifications into instructions for elements within laser processing system  100 . As shown in  FIG. 1 , computer  112  has communication links to shutter  125 , attenuator  130 , zoom scan lens  175 , and workpiece holder  185 . Computer  112  coordinates the movements of one or more of these elements when processing complex features (such as shaped and tapered holes) in workpiece  180 . In this example, computer  112  contains software applications capable of converting product, laser, and material specifications into processing algorithms required by laser processing system  100  in order for it to produce products that meet specifications. Computer  112  has access to lookup tables that contain historical data from various combinations of lasers, workpiece materials, and processing methods. It should be noted that the use of a computer is not necessarily required. In the case of a fixed line manufacturing system, an electromechanical system including elements such as gears, switches, etc. can be utilized to control the various essential elements of the laser processing system.  
      Attenuator  130  is a filter that continuously controls the energy outside laser  110 . Attenuator  130 , as shown in  FIG. 1 , includes a half-wave plate, such as those manufactured by CVI Laser (e.g., model # QWPO-1053-06-2-R10), followed by a polarizer, such as one manufactured by CVI (e.g., model # CPAS-10.0-670-1064).  
      Beam expander  140  is used in the present invention to match the spot size of beam  115  to the pupil size of zoom scan lens  175 . The specifications of beam expander  140  are selected in coordination with the specifications of beam size of laser  110  and zoom scan lens  175 . The laser beam size from beam expander  140  should be the same size or slightly smaller than the pupil size of zoom scan lens  175 . One example of a beam expander is a pair of negative and positive lenses, with a focal length of −24.9 millimeters (mm) for the negative lens, and 143.2 mm for the positive lens.  
      Spinning half-wave plate  155  changes the polarization of beam  115  to increase the smoothness of the features in workpiece  180 . In one example in which laser processing system  100  is drilling tapered holes in workpiece  180 , such a change in polarization decreases rippling on the walls of the hole. In one embodiment, spinning half-wave plate  155  is a half-wave plate, such as those made by CVI Laser (e.g., model # QWPO-1053-06-2-R10), that spins at 600 revolutions per minute (RPM) and is driven by an electric motor.  
      DOE  165  is a compound diffractive optical element (DOE) that performs the functions of: (1) shaping beam  115  to create an annulus that will produce the specified feature shape on workpiece  180  and (2) splitting beam  115  to provide for parallel processing of workpiece  180 . In another example, DOE  165  may be two DOEs that perform these two functions. In yet another example, DOE  165  may simply act as a beam shaper for creating one specified feature shape at a time. It should be noted that the word “annulus” in this description can not only mean a “circular shape with an inner and outer radius” but also an arbitrary shape whose perimeter thickness is defined by the focused laser beam or sub-beams and a size that can generically be described as a “radius” even though the shape is not circularly symmetric.  
      The pattern of sub-beams  170  output by DOE  165  is pre-determined by the product specifications. In one example, DOE  165  splits beam  115  into  152  beams in a pattern of  4  rows with  38  beams in each row.  
      Zoom scan lens  175  is a zooming scan lens that is able to adjust the annulus of sub-beams  170  at the point of contact with workpiece  180 . Zoom scan lens  175  determines the spot size of sub-beams  170  upon workpiece  180 . Zoom scan lens  175  is controlled by computer  112 , which, when system  100  is in operation, adjusts the spot size of sub-beams as they impact workpiece  180  in order to create a wide variety of tapered features on workpiece  180 . The combined size of sub-beams  170  as they enter zoom scan lens  175  must be less than or equal to the pupil size of zoom scan lens  175 . Telecentricity is required to keep the incident angle between sub-beams  170  and workpiece  180  perpendicular, which is necessary to parallel process features in workpiece  180 . In alternate embodiments for which the axes of the holes do not need to be parallel to each other, a non-telecentric scan lens can be used.  
      Workpiece  180  is the target of laser processing system  100 . In one example, workpiece  180  is a stainless steel inkjet nozzle foil; however, the present invention may be generalized to a variety of workpiece materials, such as polymers, semiconductor metals, or ceramics. In alternate embodiments, laser processing system  100  can process features of a wide variety of shapes and tapers in workpiece  180 .  
      Workpiece holder  185  is used in a laser drilling system to support workpiece  180  during laser drilling. Workpiece holder  185  is made of a hard, durable, stiff, and heat-resistant material (e.g., steel, aluminum, machinable ceramic, and the like). Workpiece holder  185  is generally attached to the stage in a laser drilling system with nuts and bolts or other similar attachment means. In one example, workpiece holder  185  is attached to a fixed stage. In other examples, workpiece holder  185  is attached to a stage that is moveable on a single axis such as an x-axis that alters position of the workpiece surface respective of the beam in an xy plane, a z-axis that alters length of the beam path by moving the xy plane in a z-direction orthogonal to the xy plane, a theta-axis that rotates the workpiece in the xy plane orthogonal to the beam path. In some embodiments, the beam path is orthogonal to the xy plane; in other embodiments the beam path is not orthogonal to the xy plane. Another single axis direction available in some embodiments is a phi-axis that rotates the xy plane about the x-axis, thereby controlling an angle of incidence between the xy plane and the beam path. Yet further embodiments have the workpiece holder  185  attached to a stage that is moveable on more than one axis, such as an xy stage, an xz stage, an x-theta stage, an x-phi stage, an xyz stage, an xyz-theta stage, an xyz-phi stage, or an xyz-theta-phi stage.  
      Movement on the z axis can occur based on an ablation or modification rate of workpiece material to control depth of ablation or modification. Zoom scan lens  175  can also be used to control depth of ablation based on the ablation rate of the material. The z-axis movement can be coordinated with the control of the zoom scan lens  175  to extend the range of this depth. Also, machining can be provided on a free-form basis in workpieces that are not flat. This capability is provided to some extent with zoom lens control, but adding the z-axis movement can extend the range of variation in the z-direction.  
      Movement of workpiece holder  185  can be coordinated with control of zoom scan lens  175 , attenuator  130 , and shutter  125  to accomplish an unprecedented range of shape control. For example, attenuator  130  can be used to control laser energy and shutter  125  can be used to control dwell time. Together, these two components can be controlled to keep the energy per unit area on the workpiece constant; which makes the amount of material being ablated or modified dependent on the area impinged by the shaped beam. Accordingly, zoom scan lens  175  and workpiece holder  185  can control depth of ablation or modification on the z axis based on the amount of area being impinged by the beam according to the known shape of the beam and the known shape of the workpiece. Also, it is possible to cause workpiece holder  185 , and zoom scan lens  175 , attenuator  130 , and shutter  125  to be controlled as a function of a z axis input, a DOE selection input, a laser selection input, and a workpiece selection input, thereby greatly simplifying operation to achieve the wide variety of shapes.  
      In operation, laser  110  emits beam  115  along the optical path identified in  FIG. 1  above. Beam  115  propagates along the optical path, where it is incident upon first mirror  120 . First mirror  120  redirects beam  115  along the optical path, where it is incident upon shutter  125 . To begin laser processing, computer  112  sends a signal to shutter  125  to open and illuminate workpiece  180 . Beam  115  exits shutter  125  and propagates along the optical path to attenuator  130 . Attenuator  130  filters the energy of laser  110  in order to precisely control ablation parameters. Beam  115  exits attenuator  130  and propagates along the optical path, where it is incident upon second mirror  135 . Second mirror  135  redirects beam  115  along the optical path, where it is incident upon beam expander  140 .  
      Beam expander  140  increases the size of beam  115 . Beam  115  exits beam expander  140  and propagates along the optical path, where it is incident upon spinning half-wave plate  155 . Spinning half-wave plate  155  changes the polarization of beam  115 . Upon exiting spinning half-wave plate  155 , beam  115  propagates along the optical path, where it is incident upon DOE  165 .  
      DOE  165  performs two functions:  1 ) shaping beam  115  to create the annulus of light required to create specified features on workpiece  180 ; and  2 ) splitting beam  115  into a plurality of sub-beams  170 , which allows parallel processing of workpiece  180 . Sub-beams  170  exit DOE  165  and propagate along the optical path, where they are incident upon zoom scan lens  175 . Zoom scan lens  175  determines the spot size of sub-beams  170  upon workpiece  180 . As determined by laser processing algorithms, computer  112  sends signals to adjust the annulus of sub-beams  170  at the point of contact with workpiece  180 . Sub-beams  170  exit zoom scan lens  175  and propagate along the optical path, where they are incident upon workpiece  180 . Sub-beams  170  ablate workpiece  180 , which is held in position by workpiece holder  185 .  
       FIG. 2  shows an improved method  200  of operating a laser processing system and maintaining optimum power-per-unit area for high-quality processing.  
      All workpiece materials (e.g., polymers, metal foils, SiO2 substrates) have an optimum power-per-unit area for high-quality processing. Adjustments made by computer  112  include speed of zoom scan lens  175  and amount of laser power allowed to propagate through attenuator  130 .  
      Method  200  includes the steps of:  
      Step  210 : Obtaining specifications for final product  
      In this step, specifications for final product are analyzed and converted to a digital format. Specification details include feature shape and size, quality, materials, manufacturing cost, and the like. This specification is available to computer  112 . In one example, the specification is stored on a disk within computer  112 . In another example, computer  112  accesses the specification via a communication means, such as a network or the Internet. In one example, the specification is stored in a computer-aided design (CAD) file. In another example, the specification is stored in a database table similar to that shown in Table 1 below.  
               TABLE 1                          Sample of specification data                                 Feature   Melt   Pattern of                                         Material_name   shape?   Absorption?   Temp?   Size?   # of Features?   features?                                                 SteelFoil1   Cone   1.88 × 10 5  cm −1     1535° C.   20 μm   500   Regular               @ 1000 nm               Grid       AlFoil2   Polygon1   1.21 × 10 6  cm −1      660° C.   40 μm   200   Linear               @ 1035 nm       PolymerFilm1   Cylinder   2.08 × 10 6  cm −1      110° C.   80 μm   2000   Random               @ 632 nm       . . .       . . .   . . .                  
 
      Method  200  proceeds to step  220 .  
      Step  220 : Selecting combination of optical power and material  
      In this step, computer  112  determines the best combination of optical power and product material to meet product specifications from step  210 . Examples of possible lasers include CW, nanosecond, picosecond, femtosecond, and others. Software operating on computer  112  reviews historical results that are stored in a database (not shown). Software operating on computer  112  selects the best combination of laser and processing method, based on historical data that shows results obtained when workpiece material selected in step  210  is used. In one example, computer  112  accesses a database (not shown) with product specification and results data for the available lasers and processing methods.  
               TABLE 2                          Sample of laser characteristics data accessed by computer 112                                 Wave-   Pulse   Repetition                                     Laser_name   length   Energy   Pulse_width   Spot_size   Rate               Picosecond1   1053 nm    1 mJ   20 ps   10 μm   1 kHz       CW    248 nm   n/a   n/a   10 μm   continuous       Picosecond2   1064 nm   10 mJ   40 ps   10 μm   2 kHz       . . .       . . .   . . .   . . .                  
 
      Method  200  proceeds to step  230 .  
      Step  230 : Developing algorithm for laser processing to specification In this step, an algorithm is developed that combines the characteristics of the laser and materials to meet the product specification. This algorithm is used by computer  112  to direct how sub-beams  170  ablate workpiece  180 . The algorithm is used by computer  112  to control shutter  125 , attenuator  130 , and zoom scan lens  175  and to produce the specified shape in workpiece  180 .  
               TABLE 3                          Sample of laser processing data accessed by computer 112                                 Laser_Processing   Hole Shape   Pattern   Multi-step   Other?               Zoom Processing -   Cone   A1   MS1   . . .       Algorithm-ZP1       Zoom Processing -   Polygon1   A2   MS2   . . .       Algorithm-ZP2       Zoom Processing -   Cylinder   A3   MS3   . . .       Algorithm-ZP3       . . .                  
 
      Method  200  proceeds to step  240 .  
      Step  240 : Starting laser processing system  
      In this step, laser processing system  100  starts. Computer  112  sends a signal to shutter  125  to open. Processing of workpiece  180  begins. Method  200  proceeds to step  250 .  
      Step  250 : Adjusting zoom scan lens  
      In this step, zoom scan lens  175  is adjusted by computer  112  to set the radius of the annulus of sub-beams  170 .  
       FIG. 3A  shows an exemplary perspective view of workpiece  180  and further includes a circular feature perimeter  310  and a plurality of sub-beam spot size annuli  320 .  
       FIG. 3B  shows an exemplary side view of workpiece  180  and further includes circular feature perimeter  310  and the plurality of sub-beam spot size annuli  320 .  
      The annuli of sub-beams  170  can initially be set to match the size of feature perimeter  310 . In one example in which inkjet nozzle holes are being manufactured, zoom scan lens  175  is set to create sub-beam spot size annulus  320 A (at its widest, at the beginning) and, as material in workpiece  180  is ablated, the annulus radius is decreased to sub-beam spot size annulus  320 B, which decreases the radius of the hole created in workpiece  180  and eventually creates a conical hole, as shown in  FIG. 3 . Computer  112  continuously makes adjustments to zoom scan lens  175 , based on the specifications of laser  110 , workpiece  180 , and the specifications determined in steps  210 ,  220 , and  230  above. These continuous adjustments to zoom scan lens  175  result in smooth workpiece features, as shown in  FIGS. 3A , and  3 B. Method  200  proceeds to step  260 . In another example, the location for starting the processing can be at any arbitrary point inside the perimeter of the annuli of the sub-beams. Additionally, processing can be bi-directional. For example, processing can be started at the center of the feature, moved out to the maximum of the perimeter and then swept back to remove another layer of material. For thick or hard materials it may be necessary to remove the material layer-by-layer in a similar way to that described in U.S. Pat. No. 6,627,844. However, in this case it is not necessary to move the beam but merely zoom in and out as each layer is removed.  
      Method  200  can be used to create features of almost any shape. Examples of possible feature shapes, not intended to be a complete list, are depicted in  FIGS. 3A, 3B ,  4 A,  4 B,  5 A, and  5 B.  
       FIG. 4A  shows an exemplary perspective view of workpiece  180  and further includes a pyramidal feature perimeter  410  and a plurality of sub-beam spot size annuli  420 .  
       FIG. 4B  shows an exemplary side view of workpiece  180  and further includes pyramidal feature perimeter  410  and the plurality of sub-beam spot size annuli  420 .  
       FIG. 5A  shows an exemplary perspective view of workpiece  180  and further includes an arbitrary feature perimeter  510  and a plurality of sub-beam spot size annuli  520 .  
       FIG. 5B  shows an exemplary side view of workpiece  180  and further includes arbitrary feature perimeter  510  and the plurality of sub-beam spot size annuli  520 .  
      Step  260 : Adjusting dwell time and laser power  
      In this step, adjustments are performed to the dwell time and laser power simultaneously to counteract the effect of step  250  (which increases the energy per unit area), in order to maintain optimum laser power-per-unit area for high quality. By maintaining the optimum amount of power-per-unit area on workpiece  180 , method  200  produces workpieces with improved quality features.  
      Within step  260 , zoom scan lens  175  is adjusted by computer  112  such that the dwell time of sub-beams  170  incident upon workpiece  180  is adjusted to meet product specifications determined in steps  210 ,  220 , and  230  above. Dwell time refers to the amount of time that sub-beams  170  are incident upon workpiece  180  (also known as the amount of time that sub-beams  170  dwell on the surface of workpiece  180 ). In one example in which laser processing system  100  is used to drill shaped holes, dwell time correlates to the amount of material abated from workpiece  180 .  
      Also within this step, attenuator  130  is adjusted by computer  112  in order to adjust the power of beam  115  (and subsequently sub-beams  170 ) to meet product specifications that are determined in steps  210 ,  220 , and  230  above. Computer  112  keeps the energy per unit area constant by attenuating laser power with attenuator  130 .  
      Method  200  proceeds to step  270 .  
      Step  270 : Is workpiece processing complete? 
      In this decision step, computer  112  makes a determination if the processing algorithm is complete. If the workpiece processing is complete, method  200  continues on to step  280 . If not, method  200  returns to step  240 .  
      Step  280 : Ending laser processing  
      In this step, computer  112  sends a signal to shutter  125  to close and laser processing ends. After this step, method  200  ends.  
      The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.