Patent Abstract:
Disclosed herein is a laser processing apparatus including a beam swinging unit provided between a pulsed laser oscillator and a focusing unit for swinging the optical path of a pulsed laser beam oscillated from the pulsed laser oscillator and then introducing the pulsed laser beam to the focusing unit. The beam swinging unit includes a polygon scanner provided on the upstream side of the focusing unit for scanning the pulsed laser beam oscillated from the pulsed laser oscillator and introducing the pulsed laser beam scanned to the focusing unit and an acoustooptic deflecting unit provided on the upstream side of the polygon scanner and on the downstream side of the pulsed laser oscillator for deflecting the optical path of the pulsed laser beam oscillated from the pulsed laser oscillator and introducing the pulsed laser beam deflected to the polygon scanner.

Full Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a laser processing apparatus for performing laser processing to a workpiece such as a semiconductor wafer held on workpiece holding means. 
     Description of the Related Art 
     In a semiconductor device fabrication process, a plurality of crossing division lines (streets) are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby define a plurality of separate regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the division lines to thereby divide the regions where the devices are formed from each other, thus obtaining a plurality of individual semiconductor chips. 
     In recent years, a semiconductor wafer intended to improve the processing performance of semiconductor chips (devices) such as ICs and LSIs has been put into practical use. This semiconductor wafer is composed of a substrate such as a silicon substrate and a functional layer formed on the front side of the substrate, wherein the functional layer is composed of a low-permittivity insulator film (low-k film) and a functional film formed on the low-k film, the functional film forming a plurality of circuits. Thus, the semiconductor devices are formed from the functional layer. The low-k film is formed from an inorganic film of SiOF, BSG (SiOB), etc. or an organic film such as a polymer film of polyimide, parylene, etc. 
     Division of such a semiconductor wafer along the division lines is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding the semiconductor wafer as a workpiece, cutting means for cutting the semiconductor wafer held on the chuck table, and moving means for relatively moving the chuck table and the cutting means. The cutting means includes a rotating spindle adapted to be rotated at high speeds and a cutting blade mounted on the rotating spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on one side surface of the base along the outer circumference thereof. The annular cutting edge is an electroformed diamond blade formed by bonding diamond abrasive grains having a grain size of about 3 μm, for example. 
     However, it is difficult to cut the low-k film mentioned above by using the cutting blade. That is, the low-k film is very brittle like mica. Accordingly, when the semiconductor wafer having the low-k film is cut along the division lines by using the cutting blade, there arises a problem such that the low-k film may be separated and this separation may reach the devices to cause fatal damage to the devices. 
     To solve this problem, Japanese Patent Laid-open No. 2005-64231 discloses a wafer dividing method including the steps of applying a laser beam along both sides of each division line on a semiconductor wafer to form two laser processed grooves along each division line, thereby dividing a stacked layer (functional layer) including a stack of low-k films, and next positioning a cutting blade between the two laser processed grooves along each division line to relatively move the cutting blade and the semiconductor wafer, thereby cutting the semiconductor wafer along each division line. 
     SUMMARY OF THE INVENTION 
     However, when the laser beam is applied along each division line to remove the stacked layer including the stack of low-k films by ablation, thereby forming the laser processed grooves along each division line, there arises a problem such that fusion debris may scatter from the stacked layer to enter the laser processed grooves. Accordingly, in order to form a laser processed groove having a sufficient width, the laser beam must be applied along each division line plural times, causing a reduction in productivity. 
     Further, also in a technique of dividing a wafer along division lines by applying a laser beam having an absorption wavelength to the wafer along each division line to form a division groove along each division line by ablation, thereby obtaining a plurality of individual device chips, there arises a problem such that fusion debris may enter the division groove. Accordingly, in order to form a desired division groove necessary for division of the wafer, the laser beam must be applied along each division line plural times, causing a reduction in productivity. 
     It is therefore an object of the present invention to provide a laser processing apparatus which can efficiently perform ablation. 
     In accordance with an aspect of the present invention, there is provided a laser processing apparatus including a chuck table for holding a workpiece; a laser beam applying unit for laser-processing the workpiece held on the chuck table; and a moving mechanism for relatively moving the chuck table and the laser beam applying unit; the laser beam applying unit including a pulsed laser oscillator for oscillating a pulsed laser beam, focusing means for focusing the pulsed laser beam oscillated from the pulsed laser oscillator and applying the pulsed laser beam focused to the workpiece held on the chuck table, and a beam swinging unit provided between the pulsed laser oscillator and the focusing means for swinging the optical path of the pulsed laser beam oscillated from the pulsed laser oscillator and then introducing the pulsed laser beam to the focusing means; the beam swinging unit including a polygon scanner provided on the upstream side of the focusing means for scanning the pulsed laser beam oscillated from the pulsed laser oscillator and introducing the pulsed laser beam scanned to the focusing means, and an acoustooptic deflecting unit provided on the upstream side of the polygon scanner and on the downstream side of the pulsed laser oscillator for deflecting the optical path of the pulsed laser beam oscillated from the pulsed laser oscillator and introducing the pulsed laser beam deflected to the polygon scanner, whereby the optical path of the pulsed laser beam is swung by the combination of the deflection of the optical path by the acoustooptic deflecting unit and the deflection of the optical path by the polygon scanner and the pulsed laser beam thus swinging is applied to the workpiece held on the chuck table. 
     Preferably, the acoustooptic deflecting unit includes first acoustooptic deflecting means for deflecting the optical path of the pulsed laser beam in an X direction and second acoustooptic deflecting means for deflecting the optical path of the pulsed laser beam in a Y direction perpendicular to the X direction. 
     According to the laser processing apparatus of the present invention, the optical path of the pulsed laser beam is swung by the combination of the deflection of the optical path by the acoustooptic deflecting unit and the deflection of the optical path by the polygon scanner, so that the pulsed laser beam thus swinging is applied to the workpiece held on the chuck table. Accordingly, ablation by the pulsed laser beam is performed overlappingly, so that fusion debris can be prevented from entering the laser processed groove, thereby efficiently forming the laser processed groove in a low-k film, substrate, etc. 
     Further, in the case of swinging the optical path of the pulsed laser beam in the Y direction by operating the acoustooptic deflecting unit and also swinging the optical path of the pulsed laser beam in the X direction by operating the polygon scanner, a laser processed groove having a desired width can be formed in a low-k film, substrate, etc. 
     Further, in the case of swinging the optical path of the pulsed laser beam in the X direction by operating the polygon scanner and also swinging the optical path of the pulsed laser beam in the X direction by operating the acoustooptic deflecting unit, it is possible to form a low-density area where the spacing between the pulses is large and a high-density area where the spacing between the pulses is small, that is, the pulses are concentratedly applied. For example, by concentratedly applying the pulsed laser beam at the same position, a hole can be formed. 
     The manner of application of the pulsed laser beam can be adjusted by changing the rotational speed of the polygon scanner. However, it is difficult to instantaneously adjust the manner of application of the pulsed laser beam due to the effect of inertial force. In contrast thereto, in the laser processing apparatus according to the present invention, the manner of application of the pulsed laser beam can be instantaneously adjusted by operating the acoustooptic deflecting unit to deflect the optical path of the pulsed laser beam without changing the rotational speed of the polygon scanner. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a laser processing apparatus according to a preferred embodiment of the present invention; 
         FIG. 2  is a block diagram showing the configuration of laser beam applying means included in the laser processing apparatus shown in  FIG. 1 ; 
         FIG. 3  is a plan view of an essential part of the laser beam applying means shown in  FIG. 2 ; 
         FIG. 4  is a plan view for illustrating the condition of pulses applied to a workpiece in the case that a pulsed laser beam oscillated from a pulsed laser oscillator of the laser beam applying means is swung in a Y direction; and 
         FIGS. 5A and 5B  are plan views for illustrating the condition of pulses applied to a workpiece in the case that the pulsed laser beam oscillated from the pulsed laser oscillator is swung in an X direction. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the laser processing apparatus according to the present invention will now be described in detail with reference to the attached drawings.  FIG. 1  is a perspective view of a laser processing apparatus  1  according to this preferred embodiment. The laser processing apparatus  1  shown in  FIG. 1  includes a stationary base  2 , a chuck table mechanism  3  for holding a workpiece, the chuck table mechanism  3  being provided on the stationary base  2  so as to be movable in a feeding direction (X direction) shown by an arrow X, and a laser beam applying unit  4  provided on the stationary base  2 , the laser beam applying unit  4  having laser beam applying means to be hereinafter described in detail. 
     The chuck table mechanism  3  includes a pair of guide rails  31  provided on the stationary base  2  so as to extend parallel to each other in the X direction, a first slide block  32  provided on the guide rails  31  so as to be movable in the X direction, a second slide block  33  provided on the first slide block  32  so as to be movable in an indexing direction (Y direction) shown by an arrow Y perpendicular to the X direction, a cover table  35  supported by a cylindrical member  34  standing on the second slide block  33 , and a chuck table  36  as workpiece holding means. The chuck table  36  has a vacuum chuck  361  formed of a porous material. A workpiece such as a disk-shaped semiconductor wafer is adapted to be held under suction on the upper surface of the vacuum chuck  361  as a holding surface by operating suction means (not shown). The chuck table  36  is rotatable by a pulse motor (not shown) provided in the cylindrical member  34 . The chuck table  36  is provided with clamps  362  for fixing an annular frame supporting a semiconductor wafer as the workpiece through a protective tape. 
     The lower surface of the first slide block  32  is formed with a pair of guided grooves  321  for slidably engaging the pair of guide rails  31  mentioned above. A pair of guide rails  322  are provided on the upper surface of the first slide block  32  so as to extend parallel to each other in the Y direction. Accordingly, the first slide block  32  is movable in the X direction along the guide rails  31  by the slidable engagement of the guided grooves  321  with the guide rails  31 . The chuck table mechanism  3  further includes X moving means  37  for moving the first slide block  32  in the X direction along the guide rails  31 . The X moving means  37  includes an externally threaded rod  371  extending parallel to the guide rails  31  so as to be interposed therebetween and a pulse motor  372  as a drive source for rotationally driving the externally threaded rod  371 . The externally threaded rod  371  is rotatably supported at one end thereof to a bearing block  373  fixed to the stationary base  2  and is connected at the other end to the output shaft of the pulse motor  372  so as to receive the torque thereof. The externally threaded rod  371  is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of the first slide block  32  at a central portion thereof. Accordingly, the first slide block  32  is moved in the X direction along the guide rails  31  by operating the pulse motor  372  to normally or reversely rotate the externally threaded rod  371 . 
     The lower surface of the second slide block  33  is formed with a pair of guided grooves  331  for slidably engaging the pair of guide rails  322  provided on the upper surface of the first slide block  32  as mentioned above. Accordingly, the second slide block  33  is movable in the Y direction along the guide rails  322  by the slidable engagement of the guided grooves  331  with the guide rails  322 . The chuck table mechanism  3  further includes Y moving means  38  for moving the second slide block  33  in the Y direction along the guide rails  322 . The Y moving means  38  includes an externally threaded rod  381  extending parallel to the guide rails  322  so as to be interposed therebetween and a pulse motor  382  as a drive source for rotationally driving the externally threaded rod  381 . The externally threaded rod  381  is rotatably supported at one end thereof to a bearing block  383  fixed to the upper surface of the first slide block  32  and is connected at the other end to the output shaft of the pulse motor  382  so as to receive the torque thereof. The externally threaded rod  381  is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of the second slide block  33  at a central portion thereof. Accordingly, the second slide block  33  is moved in the Y direction along the guide rails  322  by operating the pulse motor  382  to normally or reversely rotate the externally threaded rod  381 . 
     The laser beam applying unit  4  includes a support member  41  provided on the stationary base  2 , a casing  42  supported by the support member  41  so as to extend in a substantially horizontal direction, laser beam applying means  5  provided on the casing  42 , and imaging means  6  provided on the casing  42  at a front end portion thereof for detecting a subject area to be laser-processed. The imaging means  6  includes illuminating means for illuminating the workpiece, an optical system for capturing an area illuminated by the illuminating means, and an imaging device (CCD) for imaging the area captured by the optical system. 
     The laser beam applying means  5  will now be described with reference to  FIGS. 2 and 3 . The laser beam applying means  5  includes a pulsed laser oscillator  51 , power adjusting means  52  for adjusting the power of a pulsed laser beam oscillated from the pulsed laser oscillator  51 , focusing means  53  for focusing the pulsed laser beam adjusted in power by the power adjusting means  52  and applying this pulsed laser beam to the workpiece held on the chuck table  36 , and beam swinging means  54  provided between the power adjusting means  52  and the focusing means  53  for swinging the optical path of the pulsed laser beam oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52  and then introducing this pulsed laser beam to the focusing means  53 . The pulsed laser oscillator  51  oscillates a pulsed laser beam LB having a wavelength of 355 nm, for example. The focusing means  53  includes a telecentric fθ lens  531  for focusing the pulsed laser beam oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52 . The pulsed laser oscillator  51  and the power adjusting means  52  are controlled by control means  7 . 
     The beam swinging means  54  is composed of a pair of first acoustooptic deflecting means  55  and second acoustooptic deflecting means  56  for deflecting the optical path of the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52 , direction changing means  57  for changing the traveling direction of the pulsed laser beam LB whose optical path has been deflected by the first acoustooptic deflecting means  55  and the second acoustooptic deflecting means  56 , and a polygon scanner  58  for scanning the pulsed laser beam LB whose traveling direction has been changed by the direction changing means  57  and then introducing this pulsed laser beam LB to the focusing means  53 . 
     The first acoustooptic deflecting means  55  includes a first acoustooptic device  551  for deflecting the optical path of the pulsed laser beam LB in the X direction in cooperation with the polygon scanner  58 , a first RF (radio frequency) oscillator  552  for generating an RF signal to be applied to the first acoustooptic device  551 , a first RF amplifier  553  for amplifying the power of the RF signal generated by the first RF oscillator  552  and applying the amplified RF signal to the first acoustooptic device  551 , and first deflection angle adjusting means  554  for adjusting the frequency of the RF signal to be generated by the first RF oscillator  552 . The first acoustooptic device  551  can adjust the angle of deflection of the optical path of the pulsed laser beam LB according to the frequency of the RF signal applied. The first deflection angle adjusting means  554  is controlled by the control means  7 . 
     The second acoustooptic deflecting means  56  includes a second acoustooptic device  561  for deflecting the optical path of the pulsed laser beam LB in the Y direction perpendicular to the X direction, a second RF oscillator  562  for generating an RF signal to be applied to the second acoustooptic device  561 , a second RF amplifier  563  for amplifying the power of the RF signal generated by the second RF oscillator  562  and applying the amplified RF signal to the second acoustooptic device  561 , and second deflection angle adjusting means  564  for adjusting the frequency of the RF signal to be generated by the second RF oscillator  562 . The second acoustooptic device  561  can adjust the angle of deflection of the optical path of the pulsed laser beam LB according to the frequency of the RF signal applied. The second deflection angle adjusting means  564  is controlled by the control means  7 . 
     The laser beam applying means  5  further includes laser beam absorbing means  59  for absorbing the pulsed laser beam LB deflected by the first acoustooptic device  551  as shown by a broken line in  FIG. 2  in the case that an RF signal having a predetermined frequency is not applied to the first acoustooptic device  551 . 
     The direction changing means  57  is composed of a first direction changing mirror  571  and a second direction changing mirror  572 . The first and second direction changing mirrors  571  and  572  function to change the traveling direction of the pulsed laser beam LB whose optical path has been deflected by the first acoustooptic deflecting means  55  and the second acoustooptic deflecting means  56 , thereby introducing the pulsed laser beam LB to the polygon scanner  58 . The polygon scanner  58  is composed of a polygon mirror  581  and a scan motor  582  for rotating the polygon mirror  581  in the direction shown by an arrow A in  FIG. 2  to thereby scan the pulsed laser beam LB in the X direction. In this preferred embodiment, the polygon mirror  581  has a regular octagonal outer circumference forming eight reflection surfaces  581   a . The scan motor  582  of the polygon scanner  58  is controlled by the control means  7 . 
     In the laser processing apparatus  1  described above, the pulsed laser beam LB is applied by the laser beam applying means  5  in the following manner. For example, in the case that the rotational speed of the polygon mirror  581  constituting the polygon scanner  58  is 500 revolutions per second, the moving time of each reflection surface  581   a  of the polygon mirror  581  is 1/4000 second because the polygon mirror  581  has the eight reflection surfaces  581   a . On the other hand, in the case that the repetition frequency of the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  is 40 kHz, the number of pulses of the pulsed laser beam LB to be applied to each reflection surface  581   a  of the polygon mirror  581  is 10. 
     As shown in  FIG. 2 , the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52  is introduced to the beam swinging means  54 . At this time, a voltage of 10 V, for example, is applied with a predetermined cycle (e.g., 1/4000 second) to the first acoustooptic device  551  by the first deflection angle adjusting means  554  of the first acoustooptic deflecting means  55  controlled by the control means  7 . As a result, the pulsed laser beam LB introduced to the beam swinging means  54  is guided through the first and second direction changing mirrors  571  and  572  of the direction changing means  57  to the polygon mirror  581  of the polygon scanner  58  as shown by a solid line in  FIG. 2 . The polygon mirror  581  is rotated at a predetermined speed (e.g., 500 revolutions per second) in the direction of the arrow A, so that 10 pulses (LB- 1  to LB- 10 ) of the pulsed laser beam LB are introduced from each reflection surface  581   a  of the polygon mirror  581  to the telecentric fθ lens  531  of the focusing means  53  so as to be arranged in the X direction. 
     On the other hand, a voltage of 5 V to 15 V, for example, is applied with a predetermined cycle (e.g., 1/4000 second) to the second acoustooptic device  561  by the second deflection angle adjusting means  564  of the second acoustooptic deflecting means  56  controlled by the control means  7 . As a result, the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52  is deflected in the Y direction in the range of 10 pulses (LB- 1  to LB- 10 ) and then guided through the direction changing means  57  to the polygon mirror  581  of the polygon scanner  58  as shown in  FIG. 3 . In this manner, the optical path of the pulsed laser beam LB is swung in the Y direction by the second acoustooptic device  561  by varying the voltage to be applied to the second acoustooptic device  561  by the second deflection angle adjusting means  564  controlled by the control means  7 . 
     As described above, a predetermined voltage (e.g., 10 V) is applied to the first acoustooptic device  551  by the first deflection angle adjusting means  554  of the first acoustooptic deflecting means  55 , and a voltage in a predetermined range (e.g., 5 V to 15 V) is applied to the second acoustooptic device  561  by the second deflection angle adjusting means  564  of the second acoustooptic deflecting means  56 . As a result, the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52  is deflected by the first acoustooptic deflecting means  55  and then applied through the direction changing means  57 , the polygon mirror  581 , and the telecentric fθ lens  531  to the workpiece held on the chuck table  36  in such a manner that 10 pulses (LB- 1  to LB- 10 ) of the pulsed laser beam LB are arranged in the X direction and deviated from each other in the Y direction in the range of 50 μm, for example, as shown in  FIG. 4 . In this case, a laser processed groove having a width of 50 μm can be formed. 
     There will now be described another case such that a voltage in a predetermined range (e.g., 5 V to 15 V or 15 V to 5 V) is applied to the first acoustooptic device  551  by the first deflection angle adjusting means  554  of the first acoustooptic deflecting means  55 . In the case that a voltage of 5 V to 15 V is applied to the first acoustooptic device  551  with a predetermined cycle (e.g., 1/4000 second), the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  and adjusted in power by the power adjusting means  52  is deflected from a single dot and dash line toward a double dot and dash line shown in  FIG. 2 . The pulsed laser beam LB thus deflected from the single dot and dash line toward the double dot and dash line is introduced through the direction changing means  57  to the polygon mirror  581 . Since the polygon mirror  581  is rotated at a predetermined speed (e.g., 500 revolutions per second) in the direction of the arrow A, the pulsed laser beam LB is introduced to the polygon mirror  581  in such a manner as to be deflected in the same direction as the rotational direction of the polygon mirror  581 . As a result, 10 pulses (LB- 1  to LB- 10 ) of the pulsed laser beam LB applied through the telecentric fe lens  531  are arranged in the X direction in the condition where the spacing between any adjacent ones of the pulses is large as shown in  FIG. 5A . 
     Conversely, in the case that a voltage of 15 V to 5 V is applied to the first acoustooptic device  551  with a predetermined cycle (e.g., 1/4000 second), the pulsed laser beam LB oscillated from the pulsed laser oscillator  51  are adjusted in power by the power adjusting means  52  is deflected from the double dot and dash line toward the single dot and dash line shown in  FIG. 2 . The pulsed laser beam LB thus deflected from the double dot and dash line toward the single dot and dash line is introduced through the direction changing means  57  to the polygon mirror  581 . Since the polygon mirror  581  is rotated at a predetermined speed (e.g., 500 revolutions per second) in the direction of the arrow A, the pulsed laser beam LB is introduced to the polygon mirror  581  in such a manner as to be deflected in the direction opposite to the rotational direction of the polygon mirror  581 . As a result, 10 pulses (LB- 1  to LB- 10 ) of the pulsed laser beam LB applied through the telecentric fθ lens  531  are arranged in the X direction in the condition where the spacing between any adjacent ones of the pulses is small as shown in  FIG. 5B . 
     In the laser beam applying means  5  according to this preferred embodiment described above, the optical path of the pulsed laser beam is swung by the combination of the deflection of the optical path by the first acoustooptic deflecting means  55  and the second acoustooptic deflecting means  56  and the deflection of the optical path by the polygon scanner  58 , so that the pulsed laser beam thus swinging is applied to the workpiece held on the chuck table  36 . Accordingly, ablation by the pulsed laser beam is performed overlappingly in the X direction, so that fusion debris can be prevented from entering the laser processed groove, thereby efficiently forming the laser processed groove in a low-k film, substrate, etc. 
     Further, in the case of swinging the optical path of the pulsed laser beam in the Y direction by operating the second acoustooptic deflecting means  56  and also swinging the optical path of the pulsed laser beam in the X direction by operating the polygon scanner  58 , a laser processed groove having a desired width can be formed in a low-k film, substrate, etc. 
     Further, in the case of swinging the optical path of the pulsed laser beam in the X direction by operating the polygon scanner  58  and also swinging the optical path of the pulsed laser beam in the X direction by operating the first acoustooptic deflecting means  55 , it is possible to form a low-density area where the spacing between the pulses is large and a high-density area where the spacing between the pulses is small, that is, the pulses are concentratedly applied. For example, by concentratedly applying the pulsed laser beam at the same position, a hole can be formed. 
     The manner of application of the pulsed laser beam can be adjusted by changing the rotational speed of the polygon mirror  581  constituting the polygon scanner  58 . However, it is difficult to instantaneously adjust the manner of application of the pulsed laser beam due to the effect of inertial force. In contrast thereto, in the laser beam applying means  5  according to this preferred embodiment, the manner of application of the pulsed laser beam can be instantaneously adjusted by operating the first acoustooptic deflecting means  55  and the second acoustooptic deflecting means  56  to deflect the optical path of the pulsed laser beam without changing the rotational speed of the polygon mirror  581 . 
     The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.

Technology Classification (CPC): 1