Patent Document

RELATED APPLICATIONS  
       [0001]     This is a continuation of U.S. patent application Ser. No. 10/891,719, filed Jul. 14, 2004, now U.S. Pat. No. 6,891,601, which claims benefit of U.S. Provisional Application No. 60/488,141, filed Jul. 17, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to the field of semiconductor processing devices and, more particularly, to a system for dynamically aligning a wafer in Z-, tip-, tilt-, and yaw- (theta) axes relative to a wafer processing device.  
       BACKGROUND INFORMATION  
       [0003]     There are various prior Z Tip Tilt (“ZTT”) devices for adjusting the height and parallelism of a semiconductor wafer in a semiconductor processing machine. ZTT devices typically control positioning of Z-axis displacement, rotation about an X-axis, and rotation about a Y-axis while the semiconductor wafer is moving in the X-Y directions under a semiconductor processing machine, such as an optical inspection system. The ZTT device dynamically compensates for non-flatness of the wafer and should be stiff to provide high bandwidth positioning.  
         [0004]     Typical ZTT devices are mounted on an X-Y positioning stage and should be sufficiently lightweight and compact to maintain the dynamic performance of the X-Y stage. The ZTT positioning device should also be accurate within a few nanometers, be geometrically stable, and have a sensitive and repeatable driving system. Moreover, ZTT devices should prevent contact between the wafer and the processing system, should not generate particles that could contaminate the wafer, and should be sufficiently reliable to maintain wafer processing throughput.  
         [0005]     A conventional approach for providing ZTT positioning integrates two or more separate technologies or products, such as mechanically splitting the Z-axis (vertical) positioning and the tip and tilt positioning, an approach which typically results in very large, high profile, high-mass mechanisms. When splitting the Z-axis and tip/tilt positioning, the most common approach maintains a fixed wafer Z-axis position and, instead, moves the wafer inspection/processing elements. This approach complicates the design of the inspection/processing elements (typically a multi-element optical assembly) and increases the risk of particulate contamination because the vertical translation stage is typically located directly above the wafer. Also, because the moving mass of the Z-axis translation stage (and the elements it carries) is greater than that of a wafer chuck, the resulting dynamic performance is inadequate for many high-throughput applications.  
         [0006]     Another conventional approach also mounts the tip and tilt positioners above the wafer. A problem with this approach is maintaining co-location of the inspection/processing system focal point and the tip and tilt positioner axes to prevent X-Y translation of the inspection/processing point as the tip and tilt angles are changed. Of course, mass, complexity, and contamination risk remain problems with this over-wafer configuration.  
         [0007]     Several conventional approaches exist for providing tip and tilt positioning beneath the wafer chuck, such as on the X-Y stage carriage. For example, stacking two goniometric cradle stages with coincident rotational axes provides tip and tilt rotation about a common point located at the wafer surface. This approach provides relatively large tip and tilt positioning angles but is problematic because it employs mechanical bearings and drive screws, has a high profile, and cannot directly measure the tip and tilt angles. Alternatively, this cradle approach may be further coupled to a Z-axis stage that is also located on the X-Y stage carriage. The most common conventional Z-axis stages for mounting to an X-Y stage employ either a horizontal wedge driven by a mechanical actuator or linear motor, a single drive screw with a vertical guide way, or three or four small vertical drive screws that turn synchronously to provide Z-axis movement. All these approaches are overly tall and massive to achieve suitable dynamic performance in high throughput applications.  
         [0008]     Another conventional tip and tilt positioner approach employs flexure mechanisms driven by mechanical or piezo-electric actuators connected to a support plate that rests on a pivot point defining the center of tip and tilt rotation. In this approach, two identical flexures spaced apart by 90 degrees and at a same radius from the pivot point, provide rotation about one axis and translation along another axis. The combination of rotation and translation creates the tip and tilt positioning. However, the flexures must be compliant through the rotational axis while providing stiffness for the mechanical structure. This tradeoff limits either rotational range or stiffness.  
         [0009]     Another conventional flexure approach employs a single stage that provides tip, tilt, and a small amount (less than 1 mm) of Z movement, by simultaneous actuation of two opposing flexures. This approach employs four flexures, a support plate, but no centered pivot point. The four flexures are spaced apart 90 degrees around the circumference of the support plate. Tip and tilt movement is provided by actuating two opposing flexures in opposite directions. Z-axis movement is provided by actuating all four flexures in the same direction. This approach also suffers from limited range or a lack of mechanical stiffness.  
         [0010]     In addition to ZTT positioning, many wafer processing applications also require rotational angle (theta) positioning about the Z-axis. Theta positioning typically includes static “fine theta” adjustments for aligning a wafer when it is loaded on a chuck and “dynamic theta” adjustments for maintaining alignment during movements of the X-Y axis positioner. The fine and dynamic theta positioners are typically mounted on the X-Y positioning stage. The fine theta positioner should be close to the wafer to avoid X-Y errors, whereas the dynamic theta positioner should be mounted at a lower position to compensate for parasitic rotations of the wafer.  
         [0011]     As with the ZTT positioners, the fine and dynamic theta positioners should be lightweight, compact, and stiff to provide suitable dynamic performances; accurate to within a few nanometers; stable, sensitive, and repeatable; should not generate wafer contaminating particles; and be sufficiently reliable to maintain machine throughput.  
         [0012]     A common conventional theta positioner employs a mechanical rotary stage mounted to the X-Y positioning carriage. Such a rotary stage includes a rotating carriage supported by a worm-gear driven radial bearing set. Alternatively, a direct-drive torque motor may drive the stage. However, the mass, height, and inherent mechanical properties of the bearing stage compromise the X-Y stage performance. Moreover, achieving a desired zero-dither performance for the theta stage requires adding a brake or locking mechanism to the stage, which further increases the mass and complexity of the positioner.  
         [0013]     A solution for providing suitable theta positioning performance employs a simple two-plate air bearing structure in which a flat reference plate is mounted to the X-Y stage carriage. An upper plate having pressure and vacuum orifices is installed above the reference plate forming an air bearing gap between the two plates. The upper plate is tangentially driven by a linear actuator on one end and is supported by a rigid flexure mechanism on the opposite end to form a pivot point for the theta adjustment. After adjustment, the air bearing pressure supply is blocked, allowing the remaining vacuum to adhere, and thereby lock, the upper and lower plates together. However, because the stage is locked, it cannot provide the dynamic theta adjustments required by some applications. Moreover, the travel range of this approach is limited by the rigid flexure mechanism and by a lateral shift that occurs between the actuator contact point. Another disadvantage of this approach is that the center of rotation is offset from the X-Y carriage center, making it necessary to compensate in X-Y for the theta offset angle.  
         [0014]     A solution for providing both very fine theta adjustment within about one degree and high-bandwidth response employs differential positioning of two parallel stages connected by a single perpendicular stage. This approach, referred to as an H-bridge configuration, employs flexures at each end of the single perpendicular stage to allow a small amount of individual mechanical movement between two connected parallel stages. This movement creates an offset angle of the single stage with respect to the parallel axes and, in turn, the desired theta offset functionality. While this solution adds little hardware to the X-Y system to provide theta functionality, it still has a limited travel range and provides no way to lock the theta position. High-bandwidth theta adjustments are possible with the H-bridge configuration, but because flexures are needed to accommodate the differential movement of the parallel stages, the dynamic response of the X-Y stage is reduced by the flexure compliance.  
       SUMMARY OF THE INVENTION  
       [0015]     An object of the invention is, therefore, to provide a wafer positioning stage that provides Z-axis, tip, and tilt positioning in a single mechanism that is integrated with the X-Y carriage without compromising the dynamic performance of the X-Y stage or related system elements.  
         [0016]     An advantage of the invention is that it also provides fine and dynamic theta positioning with fine adjustment capability, moderate travel range, high-bandwidth response, zero angular dither at any desired position, negligible influence on X-Y stage throughput, and angular rotation through the X-Y carriage rotational center.  
         [0017]     A ZTT positioner of this invention employs a flexible disk that allows Z-axis displacement and tolerates tip and tilt rotations. The disk has minimum mass, stiffness in the X and Y directions, and high damping to avoid vibration. A driving system employs three non-contacting voice coil motors each having a spring to compensate for the moving mass. Position feedback is provided by non-contacting linear encoders coupled to each voice coil motor. The motors and encoders are mutually angularly spaced apart 120 degrees around the circumference of the disk to provide high sensitivity and accuracy.  
         [0018]     The ZTT flexible disk includes multiple laminated plates. The upper plate is formed from a very stiff, low mass, ceramic material. The interface to the X-Y stage depends on the application, but could include a theta stage for angular alignment, a lift pin mechanism, and a wafer chuck. The ZTT positioner further includes adjustable hard limits to prevent contact between the wafer and the processing system.  
         [0019]     Fine and dynamic theta positioners of this invention together provide fine adjustment capability, moderate travel range, high-bandwidth mechanical response, zero angular dither at the desired position, negligible influence on the X-Y stage throughput, and angular rotation through the center of the X-Y stage. The theta positioner is preferably integrated with the ZTT positioner.  
         [0020]     The fine theta positioner employs an air bearing rotary stage with a centered pivot point to allow rotation through a few degrees. The air bearing rides on air pressure that is preloaded with a vacuum. After fine theta alignment, the pressure is shut off, thereby vacuum clamping the fine theta mechanism to a reference surface. The clamping provides a very stiff mechanism having minimum size and mass. The air bearing employs three air pads with an integrated interface for mounting the wafer chuck. The fine theta driving system employs a non-contacting voice coil motor. The angular feedback is provided by a non-contacting, high resolution angular encoder. During clamping, the motor and encoder are in a closed-loop configuration to ensure accurate angular positioning.  
         [0021]     The dynamic theta positioner employs a flexible pivot driven by three piezo actuators spaced apart  120  degrees about a pivot point. The flexible parts are oriented to focus rotation about the pivot point, thereby avoiding parasitic X-Y displacements during angular rotation. The three flexible mechanisms each have a small size, but are mounted at a large radial distance from the pivot point to provide high ZXY stiffness even when loaded with several kilograms. The dynamic theta positioner is substantially frictionless and is clean and reliable.  
         [0022]     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a pictorial side elevation view of a preferred embodiment of a ZTT-theta positioner of this invention.  
         [0024]      FIG. 2  is an exploded isometric pictorial view of the ZTT-theta positioner of  FIG. 1  showing major ZTT positioner components.  
         [0025]      FIG. 3  is an isometric pictorial view of a flexible disk component of the ZTT-theta positioner shown in  FIG. 2 .  
         [0026]      FIG. 4  is an exploded isometric pictorial view of flexible disk, ceramic plate, voice coil motor, and linear encoder scale components of the ZTT-positioner.  
         [0027]      FIGS. 5A and 5B  are respective enlarged isometric and side sectional views depicting in greater detail adjustable hard Z limit components of the ZTT voice coil motor components shown in  FIG. 4 .  
         [0028]      FIG. 6  is an exploded isometric pictorial view of fine theta positioning components shown in  FIG. 1 .  
         [0029]      FIG. 7  is an exploded isometric pictorial view of dynamic theta positioning components of this invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]      FIGS. 1 and 2  show respective side elevation and exploded views of a preferred embodiment of a ZTT-Theta positioner  10 , which is assembled between the top of an X-Y stage  12  and the bottom of a wafer mounting chuck  14 . X-Y stage  12  moves in X- and Y-axis directions relative to a flat surface  16 , such as a granite slab. ZTT-Theta positioner  10  is mounted to an upper surface  18  of X-Y stage  12  and acts to accurately move chuck  14  in the Z-axis direction, tip (roll) chuck  14  about the X-axis, tilt (pitch) chuck  14  about the Y-axis, and rotate (yaw) chuck  14  about the Z-axis. Accordingly, chuck  14  undergoes six-axes of controlled movement in the X, Y, Z, roll, pitch, and yaw directions.  
         [0031]     ZTT-Theta positioner  10  is a low-profile assembly occupying only about 35 mm of the total 115 mm height of X-Y stage  12 , positioner  10 , and chuck  14 . X-Y stage  12  is electrically connected to a controller (not shown) by a flexible cable  20 . The low-profile reduces angular torque by keeping the mass as low as possible and limiting the amount of rotational inertia.  
         [0032]      FIG. 3  shows a flexible disk  22  that acts as a guideway for ZTT-Theta positioner  10  by allowing Z-axis displacements and tip and tilt rotations while having high stiffness in the X-axis, Y-axis, and theta directions. Flexible disk  22  is preferably divided into three fixed sectors  24  that includes openings  25  and static mounting points  26  and three movable sectors  28  that have movable mounting points  30 . Mounting points  26  and  30  are preferably all at the same radial distance from the rotational center of flexible disk  22 . Openings  25  are preferably triangular with rounded corners to reduce stiffness of flexible disk  22  and equalize flexure of sectors  24  and  28 . Equalizing the flexure provides a same amount of displacement relative to the radius at the points of attachment. Static mounting points  26  are also formed in mounting interface members  31 , which are in the form of annular segments ( FIG. 4 ) and are fitted on the top and bottom sides of static mounting regions  26  of fixed sectors  24  to secure them either directly or indirectly to upper surface  18  of X-Y stage  12 . Slits  32  extending radially from the center to points near the periphery of flexible disk  22  intersect the shorter side boundaries of mounting interface members  31 . Slits  32  define boundary lines between and permit relative movement in three axes (Z, roll, and pitch directions) of adjacent fixed sectors  24  and movable sectors  28 . Mounting interface members  33  in the form of annular segments ( FIG. 4 ) radially extending from a common hub include apertures  34  that are axially aligned with movable mounting points  30 . Fasteners (not shown) extending through apertures  24  and movable mounting points  30  secure movable sectors  26  to an upper plate  35  ( FIGS. 2 and 4 ) that supports chuck  14 .  
         [0033]     Upper plate  35  is preferably formed from silicon carbide (SiC) ceramic material to provide low mass, high stiffness, and low thermal expansion. Sectors  24  and  28  of flexible disk  22  are optimized in size and position to provide a high stiffness in the X, Y, and theta directions. Flexible disk  22  is preferably a multilayered structure that is composed of several thin steel disk elements bonded together with double-sided tape to provide a high damping factor to avoid vibration and improve the ZTT movement bandwidth. A motive force necessary to provide suitable Z-axis displacements is substantially lower than the force required with a single thick disk. Although flexible disk  22  has a relatively low displacement range, it is very reliable because there is no stress in the steel and the double-sided tape bonds large surfaces. Moreover, flexible disk  22  is very clean and operates without lubrication.  
         [0034]      FIG. 4  shows a ZTT driving system that movably couples upper plate  35  to X-Y stage  12 . The driving system employs multiple extensible mechanisms, preferably three voice coil motors including motor magnets  36  that are fixedly mounted on X-Y stage  12  and motor coils  38  that are attached to upper plate  35 . There is no contact between motor magnets  36  and motor coils  38 , resulting in a direct drive arrangement between the moving upper plate  35  and X-Y stage  12 . As indicated in  FIG. 4 , motor magnets  36  and motor coils  38  pass through triangular shaped cutout regions  40  in flexible disk  22  and make no contact with it. Reliability is improved by making the gap between motor magnets  36  and motor coils  38  sufficiently large to avoid contact when the tip and tilt angles are at a maximum. The moving mass is compensated for by coil springs  42  that are located around and pass through cutout regions  42  in flexible disk  22  to motor coils  38  provide force against X-Y stage  12 . Because flexible disk  22  does not generate significant force over its five mm travel range, coil springs  42  efficiently reduce the electrical current required by motor coils  38 , thereby reducing temperature rise and increasing the thermal stability of the ZTT mounting interfaces.  
         [0035]     ZTT-Theta positioner  10  provides a ±2 mm Z-axis travel range with 70 nm repeatability and a 5 μm step and settle time of 40 msec. ZTT-Theta positioner  10  also provides ±0.5 mdegree tip and tilt rotational ranges with 2 μradian repeatability. Alternatively, extensible mechanisms including short stroke linear motors (of which voice coil motors are of one type) and piezoelectric mechanisms may be employed.  
         [0036]     ZTT position sensing is provided by three linear optical encoders, each of which includes an optical sensor head  44  and a linear scale  46  ( FIG. 5A ), optical sensor heads  44  are mounted adjacent to motor magnets  36  on X-Y stage  12 , and linear scales  46  are mounted adjacent to coil springs  42  on upper plate  35 . Linear scales  46  have a large range of angular tolerance that allows accurate Z displacement measurements when the tip and tilt angles are maximum. The three Z positions are sufficiently well known to provide a Z-axis translation resolution of 20 nm.  
         [0037]     The three motor coils  38  and linear scales  46  are mounted in height-reducing recesses formed in upper plate  35 . Metallic inserts in the recesses provide high stiffness mounting surfaces. The mounting surfaces are also very accurate and flat because of the ceramic material processes forming upper plate  35 . Upper plate  35  further includes the necessary interface for mounting an air bearing rotary stage and a lift pin mechanism for wafer leveling on chuck  14  and optional theta alignment mechanisms that are described with reference to  FIGS. 6-8 . Upper plate  35  also includes optional interfaces for mounting any ancillary components necessary for particular wafer processes. Finally, reference mirrors may be mounted on upper plate  35  to accommodate interferometer-based X-Y positioning measurements.  
         [0038]      FIGS. 5A and 5B  show respective isometric and side sectional views of a Z-axis movement adjustable hard limit mechanism  50  of this invention. ZTT-Theta positioner  10  preferably employs three hard limit mechanisms  50 , each of which is integrated with the supports for motor magnets  36  and motor coils  38 . A static fork  52  is mounted on a motor coil support  54 , and a moving fork  56  is mounted on a motor magnet support  58 , which is attached to upper surface  18  of X-Y stage  12 . The guideway of moving fork  56  is a pneumatic jack  60  that includes a clamping capability. A spring  62  preloads moving fork  56  to provide Z-axis downward displacement bias. Motor magnets  36  and motor coils  38  provide the driving motive force for adjusting the position of moving fork  56 .  
         [0039]     During the adjustment process, moving fork  56  is unclamped and driven upward by pneumatic jack  60  against the urging of spring  62 . Meanwhile, the ZTT controller moves motor coils  38  and thereby static forks  52  to the commanded upper hard limit, at which position moving forks  56  are clamped by pneumatic jack  60 . The upper hard limit positioning is very precise because it employs ZTT motor coils  38  and its associated linear scale  46 . Accordingly, Z-axis displacement can be very close to the upper hard limit. Hard limit mechanisms  50  are equally spaced about the periphery of ZTT-Theta positioner  10 , the diameter of which is sufficiently close to the wafer diameter to avoid Z-axis offsets in the presence of tip and tilt angles.  
         [0040]     ZTT-Theta positioner  10  optionally includes fine and dynamic theta positioner mechanisms.  
         [0041]      FIG. 6  shows a fine theta positioner  70  of this invention that includes a flexible disk  72  formed as a multilayered structure that is composed of several thin steel disks are bonded together with double-sided tape to provide a high damping factor. Flexible disk  72  provides stiffness in the X, Y, and theta directions and efficient decoupling in the Z-axis direction.  
         [0042]     Flexible disk  72  includes three arms  73  mutually angularly spaced apart by 120 degrees. The end of each arm includes an air pad  74 , which expels from its periphery pressurized air to form an air bearing region for frictionless movement of air pad  74  across a reference surface  76  embedded in upper plate  35 . Within reference surface  76  is a vacuum port  77  that provides an offsetting vacuum pressure bias that is slightly less than the air pressure creating the air bearing. When the air pressure is interrupted, the vacuum pressure dominates and clamps air pad  74  to reference surface  76 , thereby locking in the currently selected fine theta positioning angle. The air bearings also contribute to improved reliability, and the high damping factor of flexible disk  72  avoids vibrations, reduces parasitic forces on air pads  74 , and improves fine theta positioning bandwidth.  
         [0043]     A ball bearing  78  fitted into a pedestal mounted on upper plate  35  rides in a centered pivot point  80  fitted in a hub  81  of flexible disk  72  that defines the center of theta rotation. The three air pads  74  are affixed and thereby linked to ball bearing  78  by flexible disk  72 . Alternatively, air pads  74  may include ports for both the pressurized air and vacuum pressure, or some combination of permanent-magnets, electro-magnets, and springs may provide suitable attracting and/or repulsing forces.  
         [0044]     The fine theta driving system employs a voice coil motor that includes a motor coil  82  that is attached outboard of one of air pads  74  and a motor magnet  84  that is attached to X-Y stage  12 . The maximum radial position of motor coil  82  provides sufficient torque for the small, low mass voice coil motor. The voice coil motor provides non-contacting, direct drive between air pads  74  and X-Y stage  12 . Reliability is increased by making the gap between motor coil  82  and motor magnet  84  sufficiently large to avoid contact when the fine theta angle is maximized.  
         [0045]     Fine theta position feedback is provided by a rotary encoder that includes an optical sensor  86  that is mounted on X-Y stage  12  and an encoder scale  88  that is mounted outboard of one of air pads  74 . The rotary encoder provides direct angular information of the fine theta angle. Encoder scale  88  employs a Renishaw encoder supporting less than five pradians of resolution across ±3 degrees of rotation.  
         [0046]     Fine theta positioner  70  includes an angular clamping capability. During angular alignment, the air bearing is pressurized and there is, therefore, no friction to impede a sensitive, accurate angular displacement. When the target angular position is reached, the air pressure is cut off, allowing the vacuum to clamp air pads  74  to reference surfaces  76 . The high preload of the vacuum ensures a stiff and stable theta angle relative to X-Y stage  12 . During clamping, the controller servo loop is closed to ensure an accurate target alignment angle. After clamping, the servo loop is opened to eliminate current flow through motor coil  82 , thereby eliminating heat generation to ensure thermal stability.  
         [0047]     Each of air pads  74  further includes a chuck mounting interface  90  composed of a cone and a ball that decouple theta stresses from chuck  14 . Chuck  14  is rigidly affixed to air pads  74  by screws.  
         [0048]      FIG. 7  shows a dynamic theta positioner  100  of this invention that includes three static bases  102  mounted at 120-degree intervals on a reference surface, such as X-Y stage  12 . Dynamic theta positioner  100  further includes three movable bases  104  that are interconnected to static bases  102  by flexures  106 . Flexures  106  provide decoupling between static and movable bases  102  and  104 , which are linked together by piezo actuators  108 . Movable bases  104  are coupled to static points  26  of flexible disk  22  ( FIG. 3 ). When optionally installed, dynamic theta positioner  100  renders unnecessary and therefore replaces the flexible disk mounting interface  31  shown in  FIG. 4 .  
         [0049]     The orientation of the decoupling between static and movable bases  102  and  104  is directed radially toward a centered pivot point  110  that provides accurate theta rotation without X-Y parasitic displacement. Flexures  106  are optimized to ensure high stiffness in the X-Y directions. The spacings between adjacent ones of the three pairs of interconnected static bases  102  and movable bases  104  are sufficiently large to provide high stiffness in the Z-axis, tip, and tilt directions. The spacings also provide a free area  112  for integrating other functions, such as ZTT, fine theta, and an optional lift pin mechanism  114  for assisting wafer loading on chuck  14 . Lift pin mechanism  114  integrates with fine theta positioner  70  ( FIG. 6 ) and includes three tubular vacuum supply lines  116  that extend upwardly through holes  118  in chuck  14  ( FIG. 2 ). Lift pin mechanism  114  provides vacuum supply lines  116  with about 6 mm of travel for vacuum gripping and moving wafers to and from chuck  14 . All other integrated functions are mounted above dynamic theta positioner  100  to ensure proper dynamic theta alignment.  
         [0050]     Piezo actuators  108  are preloaded for displacement in forward and reverse direction without hysteresis. Piezo actuators  108  include integrated position sensors to provide accurate displacements without drift and hysteresis.  
         [0051]     Angular position feedback may be provided by the optical system that measures wafer alignment during XY displacement, or be provided by an interferometer having reference mirrors mounted close to chuck  14  or the wafer. In either alternative, dynamic theta positioner  100  provides dynamic rotation of the wafer to within 0.5 μradian across an angular travel of ±10 μradians.  
         [0052]     Referring again to  FIG. 2 , shown are the spatial relationships among various components of the ZTT positioner, fine theta positioner  70 , and dynamic theta positioner  100  of this invention.  
         [0053]     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments.  
         [0054]     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of this invention should, therefore, be determined only by the following claims.

Technology Category: g