U.S. patent application number 08/687643 was filed with the patent office on 2001-11-22 for indexer for magazine shelves of a magazine and wafer-shaped objects contained therein.
This patent application is currently assigned to Jenoptick Aktiengesellschaft. Invention is credited to SCHLEHAHN, VOLKER, SCHULTZ, KLAUS.
Application Number | 20010043850 08/687643 |
Document ID | / |
Family ID | 7773280 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010043850 |
Kind Code |
A1 |
SCHLEHAHN, VOLKER ; et
al. |
November 22, 2001 |
INDEXER FOR MAGAZINE SHELVES OF A MAGAZINE AND WAFER-SHAPED OBJECTS
CONTAINED THEREIN
Abstract
An indexer for magazine shelves of a magazine and wafer-shaped
objects contained therein has the object of ensuring accurate
access in any desired and predeterminable magazine plane, also for
magazines which are provided with an opening on only one removing
and charging side, by means of an all-purpose indexing, wherein it
is possible to differentiate between various standardized magazine
and wafer formats The magazine shelves and the wafer-shaped objects
are detected by an optoelectronic sensor arrangement, at least a
portion of which is designed as a distance measuring system The
indexer is applicable in the manufacture of integrated circuits, in
particular, for handling wafer-shaped objects in the form of
semiconductors and masks
Inventors: |
SCHLEHAHN, VOLKER; (JENA,
DE) ; SCHULTZ, KLAUS; (JENA, DE) |
Correspondence
Address: |
MARK F. HARRINGTON
PERMAN & GREEN, LLP
425 POST ROAD
FAIRFIELD
CT
06430
US
|
Assignee: |
Jenoptick
Aktiengesellschaft
|
Family ID: |
7773280 |
Appl. No.: |
08/687643 |
Filed: |
July 26, 1996 |
Current U.S.
Class: |
414/416.01 ;
414/937 |
Current CPC
Class: |
H01L 21/67778 20130101;
H01L 21/67265 20130101; Y10S 414/113 20130101 |
Class at
Publication: |
414/416.01 ;
414/937 |
International
Class: |
B65B 021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 1995 |
DE |
195 35 871.6 |
Claims
What is claimed is:
1. In an indexer for magazine shelves of a magazine and
wafer-shaped objects contained therein, such as for semiconductor
wafers and masks, the magazine and a first handling plane for
removing and charging being adjustable vertically relative to one
another for the processing of such wafer-shaped objects, having an
optoelectronic sensor arrangement for detecting the objects and
magazine shelves relative to a reference plane which is in a fixed
relationship to the first handling plane, the improvement
comprising at least a part of the optoelectronic sensor arrangement
being designed as a distance measuring system.
2. The indexer according to claim 1, wherein the sensor arrangement
is designed completely as a distance measuring system and contains
a transmitter and a receiver arranged at an open side of said
magazine so that a measurement ray bundle proceeding from said
transmitter with its center ray lying in the reference plane
successively the wafer-shaped objects and shelf-forming projections
by vertical adjustment of the magazine relative to the reference
plane, said objects and projections being distinguishable from one
another because of the different distances relative to the
transmitter
3. The indexer according to claim 1, wherein said optoelectronic
sensor arrangement comprises a distance measuring system containing
a transmitter and a receiver arranged at an open side of the
magazine in the reference plane and an additional receiver which is
arranged at the opposite open side of the magazine, wherein the
distance measuring system serves to detect shelf-forming
projections and the additional receiver serves to detect the
presence of objects.
4. The indexer according to claim 3, wherein means direct a
measurement ray bundle proceeding from the transmitter on said
objects and projections so as to be inclined in the reference plane
relative to the vertical incident radiation.
5. The indexer according to claim 1, wherein said optoelectronic
sensor arrangement comprises a distance measuring system containing
a transmitter and a receiver arranged at an open side of the
magazine, an additional transmitter on the same side, and an
additional receiver arranged at the opposite open side of the
magazine, wherein the distance measuring system serves to detect
shelf-forming projections and the additional receiver serves to
detect the presence of objects.
6. The indexer according to claim 5, wherein means direct a
measurement ray bundle proceeding from said additional transmitter
is directed on the objects so as to be inclined in the reference
plane relative to the vertical incident radiation
7. The indexer according to claim 4, wherein said additional
receiver is provided for detecting the measurement ray bundle
passing unaltered in a straight line through the magazine when no
objects are present
8. The indexer according to claim 6, wherein said additional
receiver is provided for detecting the measurement ray bundle
passing unaltered in a straight line through the magazine when no
objects are present
9. The indexer according to claim 4, wherein the additional
receiver is provided for detecting the measurement ray bundle which
is offset by a plane-plate effect of the object which is
transparent frontwise from one edge to the other.
10. The indexer according to claim 6, wherein the additional
receiver is provided for detecting the measurement ray bundle which
is offset by a plane-plate effect of the object which is
transparent frontwise from one edge to the other
11. The indexer according to claim 1, wherein the transmitter and
the receiver of the distance measuring system are combined in a
constructional unit.
12. The indexer according to claim 1, wherein a returning device is
provided in a second handling plane parallel to the first handling
plane for positioning wafer-shaped objects projecting from the
magazine, the actuation of the returning device being contingent
upon the value of the sensor signal of the distance measuring
system in the case of wafer-shaped objects which are opaque
frontwise from one edge to the other.
Description
BACKGROUND OF THE INVENTION
[0001] a) Field of the Invention
[0002] The invention is directed to an indexer for magazine shelves
of a magazine and wafer-shaped objects contained therein, in
particular semiconductor wafers and templates or masks, the
magazine and a first handling plane for removing and charging being
adjustable vertically relative to one another for the processing of
such wafer-shaped objects, with an optoelectronic sensor
arrangement for detecting the objects and magazine shelves relative
to a reference plane which is in a fixed relationship to the first
handling plane Such technical solutions are applicable in the
manufacture of integrated circuits, in particular for handling
tasks, and are known, e.g., from DE 43 06 957 C1.
[0003] b) Description of the Related Art
[0004] In the manufacture of integrated circuits, wafer-shaped
objects such as semiconductor wafers and masks must be transported
between different processing stages to individual processing
machines. In increasing measure, such transporting takes place in
standardized transport containers, referred to as standard
mechanical interface boxes (SMIF boxes), the magazine within whose
shelves the wafer-shaped objects are located being fastened in a
suitable manner to the base of these transport containers. For the
purpose of charging the processing machines, the magazines are
unloaded from the transport containers by suitable devices and the
wafer-shaped objects are removed by a removing and charging
mechanism. After processing, the wafer-shaped objects are returned
to the shelves of the magazine and the magazine is returned to the
transport container.
[0005] A disadvantage in the possible use of impact pressure
sensors or reflex couplers which act on the rear side of the
wafer-shaped objects to detect the latter consists in that the
magazine must be handled in a determined sequence. In so doing, the
objects may not be removed from the magazine in an arbitrary
manner. Rather the charging magazine must be emptied from the
bottom up and the dispensing magazine must be filled from the top
down because of the required sensor arrangement and the removing
and charging of semiconductor wafers associated therewith.
Consequently, the allocation of the object to a determined level is
not adhered to. Such technical solutions cannot be applied for the
conventional removal of random samples for inspection purposes or
for use in the above-mentioned transport containers.
[0006] It is known from U.S. Pat. No. 4,895,486 to determine the
presence of wafer-shaped objects in a carrier (magazine) and their
position relative to a reference plane in the carrier by means of a
monitoring device in that a first signal indicating the presence of
such an object is combined with a position signal for the object.
The first signal is obtained by an optoelectronic sensor which
monitors the space in which the objects can be found. The second
signal is formed via a position encoder coupled with a drive for
moving the carrier up and down To determine the reference plane and
possible resting place of the objects, the space in the carrier is
divided vertically into segments In addition to a segment serving
as a reference plane and segments without wafer-shaped objects,
window segments in which objects may be present are defined. An
indexing of the carrier is effected in that, after the reference
plane in the carrier is detected by measuring techniques, the
locations of the window segments are determined and stored by
computer based on construction data of the respective carrier being
used.
[0007] Although the quantity of objects and the locations in which
they are deposited relative to a reference plane with in the
carrier can be determined by means of the described solution, the
carrier or the removing and charging mechanism must be positioned
in the grid dimension of the shelves of the carrier in order to
remove the objects from the carrier. If divergent carrier
geometries and tolerances are not allowed for in so doing, errors
cannot be ruled out. Problems arise in particular when an empty
carrier is to be charged optionally.
[0008] Further, it is possible to monitor the correct position of
the object in the carrier by means of an additional optoelectronic
sensor when the carrier is being transported upward in the vertical
direction. This is done in order to prevent damage to objects
protruding from the carrier when returning the latter to the
transport containers. If it is detected that an object is
protruding from the carrier due to defective or incorrect operation
of the handling system, the transport of the magazine is halted and
manual intervention on the part of the operator is required to
eliminate the error. While the detection of protruding or
projecting objects has practical importance, the required manual
intervention causes unnecessary delays in continued processing by
interfering with the clean room conditions and, in some cases, the
climatic conditions of the processing machines within the machine
enclosure. This can result in failure of the machine.
[0009] The solution described in DE 43 06 957 C1 meets all of the
demands mentioned above in that the position of the wafer-shaped
objects as well as the position of the magazine shelves relative to
a reference plane are detected by means of a bundle of measurement
rays emitted by an optoelectronic sensor formed of a transmitter
and receiver The reference plane is in a fixed constructional
relationship with a handling plane for removing and charging. The
magazine is adjustable vertically in a measurable manner relative
to a handling plane for removing and charging by means of a
magazine receiving device via a magazine elevator.
[0010] A disadvantage consists in that magazines must be restricted
to those in which two opposite sides are freely accessible. This
indexing arrangement is not applicable in magazines which are only
open at the side serving for removing and charging. Another
disadvantage consists in that it is not possible to differentiate
directly between different magazine sizes or formats.
OBJECT AND SUMMARY OF THE INVENTION
[0011] Therefore, the problem arises of ensuring accurate access in
any desired and predeterminable magazine plane, also for magazines
which are provided with an opening on only one removing and
charging side, by means of an all-purpose indexing, wherein it is
possible to differentiate between various standardized magazine and
wafer formats. A primary object of the present invention is to
overcome this stated problem.
[0012] This object is met, according to the invention, by an
indexer for magazine shelves of a magazine and wafer-shaped objects
contained therein, in particular semiconductor wafers and masks,
the magazine and a first handling plane for removing and charging
being adjustable vertically relative to one another for the
processing of such wafer-shaped objects, with an optoelectronic
sensor arrangement for detecting the objects and magazine shelves
relative to a reference plane which is in a fixed relationship to
the first handling plane, in that at least a part of the
optoelectronic sensor arrangement is designed as a distance
measuring systme.
[0013] If the wafer-shaped objects are opaque frontwise from one
edge to the other, the entire sensor arrangement can be designed as
a distance measuring system which detects radiation scattered at
the edges of the objects and magazine shelves.
[0014] In this case, the distance measuring system contains a
transmitter and a receiver arranged at an open side of the magazine
so that a bundle of measurement rays proceeding from the
transmitter with its center ray lying in the reference plane
detects successively the wafer-shaped objects and shelf-forming
projections for the objects by means of the vertical adjustment of
the magazine relative to the reference plane, the objects and the
projections being distinguishable from one another because of the
different distances relative to the transmitter
[0015] As a result of the vertical adjustment, an image of the
magazine shelves and of the wafer-shaped objects contained therein
is generated by an amplitude modulation of the output signal. This
is brought about by the change in the distance between the
transmitter and the reflecting item in the reference plane vertical
to the direction of movement The received signal, whose value
depends on the distance between the transmitter and the point of
incidence, is converted into an analog signal by means of an
electronic amplifier
[0016] The sensor system, whose bundle of measurement rays lies in
a horizontal plane, determines the vertical position and, by
determining the coordinates in the horizontal plane, ascertains
whether the item in question is a magazine shelf for a wafer-shaped
object or the object itself.
[0017] In the case of wafer-shaped objects which are transparent
frontwise from one edge to the other, the light scattered in the
backward direction can only be measured under certain
conditions.
[0018] Therefore, the optoelectronic sensor arrangement
advantageously comprises a distance measuring system, which is
arranged at an open side of the magazine in the reference plane and
contains a transmitter and a receiver, and an additional receiver
which is arranged at the opposite open side of the magazine. A
bundle of measurement rays proceeding from the transmitter is
directed on the objects and projections so as to be inclined in the
reference plane relative to the vertical incident radiation.
[0019] Whereas the distance measuring system serves to detect
shelf-forming projections, a parallel-plate effect brought about by
the presence of an object can be utilized for detecting the object
owing to the arrangement of an additional receiver. The additional
receiver can be placed either at the location struck by the bundle
of measurement rays if no object is located in the magazine shelf
or at the location struck by the measurement ray bundle which is
offset owing to the presence of an object.
[0020] A sensor arrangement formed of a distance measurement system
which contains a transmitter and a receiver arranged at an open
side of the magazine, an additional transmitter on the same side,
and an additional receiver arranged at the opposite open side of
the magazine is also suitable for objects which are transparent
frontwise from one edge to the other.
[0021] In this sensor arrangement, a bundle of measurement rays
proceeding from the additional transmitter is directed only on the
object so as to be inclined in the reference plane relative to the
vertical incident radiation. The distance measuring system serves
to detect shelf-forming projections. The additional receiver is
used in the manner described above
[0022] The receiver and transmitter of the distance measuring
system are advantageously combined in a structural unit.
[0023] By means of the technical solution according to the
invention, the actually occurring ratios in a magazine to be
indexed are determined in that the magazine shelves and the
wafer-shaped objects contained therein are detected with measuring
techniques. Accordingly, the objects may be removed and restored
optionally so at to enable any type of re-sorting or rearrangement
between magazines with different shelf spacing as well as with
respect to a reference plane within the magazine. Empty magazines
can also be filled as desired. The technical solution also allows
the use of magazines or magazine-like containers which are closed
on all sides but that side having the charging and removing
opening.
[0024] Given a suitable design of the magazine, which is often
ensured at the present time owing to the global standard, different
magazine formats can be determined by means of the value of the
analog signal alone. The position of the wafer-shaped object can
also be valuated in every plane parallel to the reference plane
using the value of the analog signal determined by the distance and
this valuation can be used to distinguish between wafer sizes or to
monitor the correctly positioned depositing of the wafer-shaped
object following a handling process or prior to the start of the
handling process.
[0025] If depositing is not effected in the proper location, a
returning device is advantageously provided in a second handling
plane parallel to the first handling plane for positioning
wafer-shaped objects protruding from the magazine. The actuation of
the wafer returning device is contingent upon the value of the
sensor signal of the distance measuring system in the case of
wafer-shaped objects which are opaque frontwise from one edge to
the other. In the case of transparency, another sensor system can
be used for detecting objects protruding from the magazine.
Triggered by the sensor signal, the wafer returning device guides
the object back into the magazine shelf without external
intervention. Further transport of the magazine in the transport
containers is ensured and damage to protruding objects is prevented
without manual intervention.
[0026] The invention will be explained more fully in the following
with reference to the schematic drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] In the drawings:
[0028] FIG. 1 shows part of a handling device partially in
section;
[0029] FIG. 2 shows a sensor arrangement for a wafer magazine;
[0030] FIG. 3 shows a first sensor arrangement for a mask
magazine;
[0031] FIG. 4 shows a second sensor arrangement for a 5"-mask
magazine in a section through a magazine in the plane of a magazine
shelf with a mask below shelf-forming projections;
[0032] FIG. 5 shows a third sensor arrangement for a 6"-mask
magazine in a section through a magazine in the plane of a magazine
shelf with a mask below shelf-forming projections;
[0033] FIG. 6 shows a wall section from a magazine for objects of a
first, smaller format;
[0034] FIG. 7 shows a wall section from a magazine for objects of a
second, larger format;
[0035] FIG. 8 shows the wall portions in FIGS. 4a and 4b projected
one on top of the other together with associated optoelectronic
scan images; and
[0036] FIG. 9 shows a top view of the magazine of a wafer returning
device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] A device for handling wafer-shaped objects, only part of
which is shown in FIG. 1, contains in its interior a magazine seat
2 which can be raised and lowered in the z direction (vertically to
the supporting surface of the handling device) via a spindle drive
1. The spindle 1 is driven by a stepper motor 3 which is outfitted
with an angle measuring system 4 so that the distance traversed
when raising or lowering can be determined via the pitch of the
spindle.
[0038] Together with a control computer 25, not shown, the stepper
motor 3 along with its control electronics and the angle measuring
system 4 form a position regulator of a magazine elevator, the
spindle 1 and magazine seat 2 also making up a part of the
latter.
[0039] A removing and charging device 5 has a handling arm 6
working in a handling plane H-H and is fastened to a shared frame 9
as are the magazine elevator and an optoelectronic sensor
containing a transmitter 7 (visible in FIG. 2) and a receiver 8 in
a housing. A bundle 10 of measurement rays proceeding from the
transmitter 7 extends with its a center ray in a reference plane
E-E for indexing magazine shelves located in a magazine 11 and
wafer-shaped objects 12 contained therein. The distance between
plane E-E and the handling plane H-H is selectable within the
operating range of the spindle drive 1.
[0040] In FIG. 2, the transmitter 7 and the receiver 8 are arranged
adjacent to one another. The measurement ray bundle 10 is directed
on shelf-forming projections 13 at walls 14 and 15 and on the front
edge of inserted semiconductor wafers 16 as wafer-shaped objects.
By means of diffuse reflection of the ray bundle 10 either at one
of the projections 13 or at the front edge of an inserted
semiconductor wafer 16, analog signals corresponding to the
distances are generated. The values of the analog signals differ
from one another such that the projections 13 can be clearly
distinguished from semiconductor wafers 16. The oblique irradiation
of the magazine 11 shown in the drawing was chosen in order to
create favorable conditions for diffuse reflection.
[0041] In the mask magazine shown in FIG. 3, a measurement ray
bundle 17 proceeding from a transmitter 18 is directed parallel to
walls 19, 20 onto shelf-forming projections 21 and, if required,
simultaneously on masks 22 located in the shelves. The masks lie on
supports 23 of the shelves 21 and are prevented from slipping by
means of lateral stops 24.
[0042] Owing to the different distance between the transmitter 18
and projection 21 on the one hand and the transmitter 18 and mask
22 on the other hand, analog signals of different values are also
obtained at a receiver 25 in a mask magazine. The projection 21 and
the mask 22 can be distinguished on the basis of the value of the
analog signal.
[0043] If the objects to be detected are transparent frontwise from
one edge to the other (masks with transparent lateral faces), it is
advisable, in accordance with FIGS. 4 and 5 to direct the
measurement ray bundle 17 and 29, respectively, on the mask 22 and
the webs 21 in the reference plane so as to be inclined at an acute
angle relative to the vertical incident radiation.
[0044] The construction according to FIG. 4 provides another
receiver 26 at the opposite open side of the magazine which
measures in a spatially-sensitive manner. Due to a plane-plate
effect, the measurement ray bundle 17 exits so as to be offset
relative to its entrance into the mask 22, depending on the index
of refraction, when a mask 22 is located in a magazine shelf.
[0045] In the more favorable variant in technical aspects regarding
the arrangement, the additional receiver 26 is arranged at the
location struck by the measurement ray bundle when there is no mask
22 located in the magazine shelf. If a mask 22 is located in the
magazine shelf, the receiver 26 does not receive any signal
Conversely, the additional receiver can naturally also be provided
at the location of incidence of the offset measurement ray
bundle.
[0046] In FIG. 5, an additional transmitter 27 and an additional
receiver 28 are provided with a measurement ray bundle 29 which is
inclined relative to the vertical incident radiation. The manner of
operation corresponds in an analogous manner to that of the
arrangement shown in FIG. 4. In both cases, the distance measuring
system serves to detect the shelf-forming projections 21 and the
additional receiver 26 and 28, respectively, serves to detect the
mask 22.
[0047] The distances between the magazine shelves and the mask
magazines are subject only to slight tolerances due to their
process of manufacture. This fact along with the fact that
standardized magazines are frequently employed in a device in
semiconductor manufacturing offers the possibility of determining
the position of the magazine shelves indirectly and accordingly
also distinguishing the magazine format for an empty magazine.
[0048] FIGS. 6 to 8 serve to illustrate the procedure for
determining the different magazine format. FIG. 6 shows a
5"-magazine and FIG. 7 shows a 6"-magazine. The reference numbers
correspond to those of FIG. 3. In FIG. 8, the two magazines are
projected on top of one another corresponding to their positions in
a processing device.
[0049] With a stationary sensor arrangement corresponding to FIG.
3, the measurement ray bundle 17 scans the webs 30 in the 5"-mask
magazine shown in solid lines due to the vertical movement of the
projections 21 and scans the supports 23 in the 6"-mask magazine
shown in dashed lines. The location of support in the 5"-mask
magazine can be determined based on the fixed geometric
relationship between the support and web. Typical signal waveforms
occurring during scanning are designated by 31 for the 5"-mask
magazine and by 32 for the 6"-mask magazine.
[0050] Scanning is effected by the measurement ray bundle 17 as a
result of the vertical movement of the magazine, wherein the analog
value of the sensor signal and the associated value of the vertical
position which is determined via the position regulator of the
magazine elevator are stored. Threshold values sw1 and sw2 can be
set for the analog signal in order to reduce data.
[0051] The magazine shelves and masks can be exactly positioned
with respect to the handling plane H-H by means of a selectable
correction value corresponding to the distance between the web 30
and the support 23 and the linking of the stored vertical position
values with the threshold values.
[0052] As will be clear from FIG. 9 with reference to a
semiconductor magazine, the use of a distance measuring system is
also suitable for testing the correctness of the position of
wafer-shaped objects which are opaque frontwise from one edge to
the other by means of the value of the analog signal determined by
the distance in every plane parallel to the reference plane.
[0053] If the wafer-shaped objects have not been deposited
correctly, a returning device 33 in a second handling plane
parallel to the first handling plane E-E serves to push the
protruding semiconductor wafers 16 back into the magazine.
Accordingly, transport of the magazine 11 can proceed without
interference and without manual intervention.
[0054] The returning device 33 is formed of a linkage mechanism or
lever mechanism 34 which is driven by an electric motor and
executes a swiveling movement of 90.degree. during a revolution of
its drive (not shown). The reversal point of the lever of the
mechanism 22 is so arranged that it pushes the semiconductor wafer
16 into the magazine 11 and then returns to an initial position
which is monitored by a limit switch.
[0055] When the object is transparent from one edge to the other,
another sensor shown in dashed lines can be used, this sensor
having a transmitter 35 and a receiver 36, and its sensor signal
determines the actuation of the returning device 33. A returning
device can be used in an analogous manner in wafer magazines.
[0056] The stepper motor 3, angle measuring system 4, transmitters
7, 18, 27 and 35, receivers 8, 25, 26 and 36, another sensor 37
which is rigidly connected with the frame 9, the lever mechanism
33, and the limit switch are connected with a control computer for
carrying out the invention. Analog-to-digital converters are
connected between the sensors and the control computer.
[0057] After the magazine seat 2 is positioned relative to the
additional sensor 37, which is at a known distance relative to the
handling plane H-H and reference plane E-E, and after the counter
of the angle measuring system is reset to zero, the magazine 11 is
automatically removed by its base from a dust-proof transport
container, not shown, and is taken over by the magazine seat 2 for
indexing the magazine shelves and the wafer-shaped objects 12, 22
located therein. The magazine seat 2 is then lowered, i.e., moved
in the negative z direction, until exceeding a threshold value sw2
of threshold values sw1 and sw2 which are advantageously determined
for the purpose of data reduction. The distance of the base of the
magazine 11 from the reference plane E-E is thus detected.
[0058] As the magazine 11 moves through the measurement plane E-E
in the direction of its shelves, which are located one above the
other, an amplitude-modulated sensor output signal is obtained at
the receivers 8, 25, 26 and 28 as a function of the path, this
sensor output signal representing the imaging of the magazine
shelves and objects 12, 22 or the image is generated from this
sensor output signal by taking into account the fixed geometrical
relationships. By means of the control computer, the sensor output
signal, after being converted from analog to digital, is combined
with the measurement signal of the angle measuring system 4 and
stored
[0059] In order to remove an object 12, 22 from the magazine 11 or
wafer magazine or to place it in an empty magazine shelf, a value
for the distance between the measurement plane E-E and the first
handling plane H-H is added to the determined counter reading and
the magazine 11 or wafer magazine is moved into the corresponding z
position by means of the magazine lift
[0060] While the foregoing description and drawings represent the
preferred embodiments of the present invention, it will be obvious
to those skilled in the art that various changes and modifications
may be made therein without departing from the true spirit and
scope of the present invention
* * * * *