U.S. patent application number 14/549024 was filed with the patent office on 2015-03-19 for display device and method for manufacturing display device.
The applicant listed for this patent is Sony Corporation. Invention is credited to Toshiaki Arai.
Application Number | 20150076481 14/549024 |
Document ID | / |
Family ID | 37693900 |
Filed Date | 2015-03-19 |
United States Patent
Application |
20150076481 |
Kind Code |
A1 |
Arai; Toshiaki |
March 19, 2015 |
DISPLAY DEVICE AND METHOD FOR MANUFACTURING DISPLAY DEVICE
Abstract
A display device and method for manufacturing same are provided.
The display device including a plurality of unit pixels disposed in
the matrix on a substrate, each of the unit pixels has a thin film
transistor at a place other than the center of the pixel, and unit
pixels in a first row and unit pixels in a second row adjacent to
the first row are arranged so that they are symmetric with respect
to a first virtual plane orthogonal to a main surface of the
substrate.
Inventors: |
Arai; Toshiaki; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sony Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
37693900 |
Appl. No.: |
14/549024 |
Filed: |
November 20, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11458873 |
Jul 20, 2006 |
8928638 |
|
|
14549024 |
|
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Current U.S.
Class: |
257/40 ;
438/23 |
Current CPC
Class: |
G09G 5/10 20130101; H01L
27/1296 20130101; H01L 27/3262 20130101; H01L 27/1274 20130101;
G09G 2300/0426 20130101; G09G 3/30 20130101; H01L 27/1285 20130101;
H01L 21/02686 20130101; H01L 2227/323 20130101 |
Class at
Publication: |
257/40 ;
438/23 |
International
Class: |
H01L 27/32 20060101
H01L027/32; H01L 21/02 20060101 H01L021/02; H01L 27/12 20060101
H01L027/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2005 |
JP |
2005-217819 |
Claims
1. A display device comprising a plurality of unit pixels provided
with an organic EL element and a plurality of thin film transistors
and disposed in the two-dimensional matrix on a substrate, wherein
a first unit pixel and a second unit pixel in two rows adjacent to
each other are disposed in arrangement forms symmetric with respect
to the boundary between the rows, in each of the two rows, the
individual unit pixels are disposed in the same arrangement form in
the row direction, and the thin film transistors of the unit pixels
in the two rows have channel regions formed from thin film
semiconductor layers crystallized by irradiation with pulse lasers
having a major axis width over the unit pixels in the two rows.
2. A method for manufacturing a display device including a
plurality of unit pixels provided with an organic EL element and a
plurality of thin film transistors and disposed in the
two-dimensional matrix on a substrate, the method comprising the
steps of: forming a first unit pixel and a second unit pixel in two
rows adjacent to each other into arrangement forms symmetric with
respect to the boundary between the rows, forming the individual
unit pixels into the same arrangement form in the row direction in
each of the two rows, and forming channel regions from thin film
semiconductor layers crystallized by irradiation with pulse lasers
having a major axis width over the unit pixels in the two rows in
the thin film transistors of the unit pixels in the two rows.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 11/458,873 filed Jul. 20, 2006, which
application claims priority to Japanese Patent Application JP
2005-217819 filed in the Japanese Patent Office on Jul. 27, 2005,
the entire contents of which is incorporated herein by
reference.
BACKGROUND
[0002] The present invention relates to a display device provided
with a plurality of unit pixels driven by thin film transistors and
a method for manufacturing the display device.
[0003] In recent years, flat panel display type display devices
based on an active matrix system provided with driving elements
including film transistors (TFTs) have been noted. Most of all, a
display device taking the advantage of organic electro luminescence
(EL) has been developed toward commercialization.
[0004] Here, the principle of light emission of a unit pixel of an
organic EL display device based on the active matrix system will be
described with reference to an equivalent circuit of the unit pixel
shown in FIG. 7. A signal line is represented by Y1, a current
supply line is represented by Y2, and scanning lines are
represented by X1 and X2.
[0005] A voltage is always supplied to the current supply line Y2.
When a scanning pulse is applied to the scanning line X1 and a
predetermined signal is supplied to the signal line Y1, a second
transistor Tr2 is turned on, and a predetermined signal is written
into a storage capacitor C. A first TFT transistor Tr1 is turned on
based on the written signal, a current in accordance with the
amount of signal is supplied through the current supply line Y2 to
a light emitting portion EL and, thereby, the light emitting
portion EL emits light.
[0006] A plurality of the unit pixels are arranged in the matrix,
so that a display device is configured.
[0007] The configuration of each thin film transistor will be
described below. FIG. 8 shows a bottom gate type known thin film
transistor 101 having a channel region made of p-Si.
[0008] In the thin film transistor 101, a gate electrode 103 is
disposed on a molybdenum thin film on a substrate 102. a gate
insulating layer 104 made of SiN and SiO.sub.2 and a crystalline Si
layer 105 are disposed over the gate electrode 103 and the
periphery thereof.
[0009] The crystalline Si layer 105 is provided by irradiating an
amorphous silicon (.alpha.-Si) layer with a laser like an excimer
laser.
[0010] In the case where crystallization is effected with the
excimer laser or the like, as shown in FIG. 9A, pulsed laser light
203 shaped to have substantially the same width as the width of a
unit region 202 including a thin film transistor formation portion
is stepped sequentially in a direction perpendicular to a major
axis direction (width direction) of the laser light 203, and is
continuously applied to amorphous silicon. In FIG. 9A, reference
numeral 204 denotes a laser irradiation region.
[0011] However, the pulsed laser light 203 has energy variations of
about 5% on a pulse basis. Furthermore, the laser light 203 also
has an energy distribution in a minor axis direction (movement
direction a). Therefore, as shown in FIG. 9B, an intense energy
point (indicated by a broken line in the drawing) may occur at an
energy irradiation end.
[0012] In the case where laser light exhibiting nonuniformity due
to variations among pulses or in a pulse, as described above, is
applied, in order to leveling the irradiation energy in the unit
region 202 to be irradiated, for example, a step movement between
infinitesimal sections in the minor axis direction and laser
irradiation are repeated. Furthermore, 90% to 95% of regions of the
individual pulses are allowed to overlap one another and, thereby,
the difference in cumulative irradiation energy in a unit region
202 is reduced.
[0013] However, the crystallinity of individual unit pixels may not
be satisfactorily leveled by such a technique, and luminance
variations are recognized during the operation of a display device
121 (refer to FIG. 10).
[0014] For the purpose of leveling characteristic variations among
TFT elements in a current mirror or a differential circuit,
Japanese Unexamined Patent Application Publication No. 10-197896,
for example, proposes that TFT elements in the circuit are arranged
to become symmetric with respect to a point, the TFT elements
located at the positions symmetric with respect to the point are
connected in series, individual characteristic variations which
inevitably occur within a certain range are leveled and, as a
result, the characteristics of nearby two pairs of TFT series
circuits are leveled.
[0015] However, according to the technique disclosed in Japanese
Unexamined Patent Application Publication No. 10-197896, since
there is a need for connecting the thin film transistors in series,
the configuration of the driving element becomes complicated, and
the yield is decreased. In addition, when the laser is applied with
the step movement, as described above, the gate wiring is also be
irradiated with the laser light. For example, in the case where the
gate wiring is made of a material, such as aluminum, having low
heat resistance, the heat due to the laser irradiation is conducted
to the gate wiring and, therefore, abnormal crystal growth, e.g.,
hillock or whisker, is allowed to proceed, and short-circuiting
results. Therefore, there is a limit of material to be used as the
gate wiring.
SUMMARY
[0016] The present invention addresses the above identified, and
other problems. It is desirable to provide a display device, in
which characteristic variations of TFT are reduced and the yield
reduction is suppressed, and a method for manufacturing the
dispersion device.
[0017] A display device according to an embodiment of the present
invention includes a plurality of unit pixels provided with thin
film transistors and disposed in the matrix on a substrate, wherein
unit pixels in a first row and unit pixels in a second row adjacent
to the first row are arranged so that they are symmetric with
respect to a first virtual plane orthogonal to a main surface of
the above-described substrate.
[0018] A method for manufacturing a display device including a
plurality of unit pixels provided with thin film transistors and
disposed in the matrix on a substrate, according to an embodiment
of the present invention, includes the steps of arranging unit
pixels in a first row and unit pixels in a second row adjacent to
the first row so that they are symmetric with respect to a first
virtual plane orthogonal to a main surface of the above-described
substrate and crystallizing silicon constituting at least a part of
the above-described thin film transistors by irradiating
simultaneously the unit pixels in the above-described first row and
the unit pixels in the above-described second row with laser
light.
[0019] In the display device according to an embodiment of the
present invention, a plurality of unit pixels provided with thin
film transistors are disposed in the matrix on the substrate, and
the unit pixels in the first row and the unit pixels in the second
row adjacent to the first row are arranged so that they are
symmetric with respect to the first virtual plane orthogonal to the
main surface of the above-described substrate. Therefore, silicon
in the channel regions of unit pixels in the first row and the
second row may be substantially equally crystallized. Consequently,
driving elements including highly uniform thin film transistors may
be formed.
[0020] In the method for manufacturing a display device according
to an embodiment of the present invention, the unit pixels in the
first row and the unit pixels in the second row adjacent to the
first row are irradiated with the laser light having a uniform
energy distribution. Consequently, the unit pixels in the first row
and the unit pixels in the second row may be irradiated with the
laser light having a uniform energy distribution. Furthermore, the
unit pixels in adjacent two rows are subjected to simultaneous
laser annealing, so that the process tact may be reduced by almost
a half
[0021] In the method for manufacturing a display device according
to an embodiment of the present invention, even in the case where
components, e.g., wirings, composed of a material, e.g., aluminum
or silver, having low heat resistance or a material, e.g., copper,
having the diffusibility are disposed in advance, it is possible to
irradiate a channel region simply with a laser region having
uniform intensity, while the wiring portion is avoided from being
irradiated.
[0022] Additional features and advantages are described herein, and
will be apparent from, the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1 is a schematic diagram showing the configuration of
an example of a display device according to an embodiment of the
present invention.
[0024] Each of FIGS. 2A and 2B is a schematic diagram for
explaining an example of a method for manufacturing a display
device according to an embodiment of the present invention.
[0025] FIG. 3 is a schematic diagram for explaining a method for
manufacturing a display device according to an embodiment of the
present invention.
[0026] FIG. 4 is a schematic diagram for explaining another example
of a method for manufacturing a display device according to an
embodiment of the present invention.
[0027] Each of FIGS. 5A to 5C is a step diagram for explaining an
example of a method for manufacturing a display device according to
an embodiment of the present invention.
[0028] Each of FIGS. 6A to 6C is a schematic sectional diagram in
an example of a method for manufacturing a display device according
to an embodiment of the present invention.
[0029] FIG. 7 is an equivalent circuit diagram for explaining
driving of a unit pixel constituting a display device.
[0030] FIG. 8 is a schematic sectional diagram of a thin film
transistor constituting a display device in the past.
[0031] FIG. 9A is a schematic diagram for explaining an example of
laser annealing, and FIG. 9B is a schematic diagram showing an
example of energy intensity of the laser light.
[0032] FIG. 10 is a schematic diagram showing the configuration of
a display device in the past.
DETAILED DESCRIPTION
[0033] Embodiments according to the present invention will be
described below with reference to drawings.
Display Device
[0034] A display device according to an embodiment of the present
invention will be described.
[0035] In a display device 21 according to the present embodiment,
as shown in FIG. 1, a plurality of unit pixels 22 provided with
thin film transistors are disposed in the matrix on a substrate,
and the arrangements in a first unit pixel 22a and a second unit
pixel 22b adjacent to each other are symmetric with respect to a
first virtual plane 23 orthogonal to a main surface (that is, the
drawing) of the above-described substrate.
[0036] In the present embodiment, among a plurality of thin film
transistors constituting driving elements in the first unit pixel
22a and the second unit pixel 22b, for example, thin film
transistors 1 serving as switching transistors are arranged to
become symmetric with respect to the first virtual plane 23.
[0037] In the display device 21 according to the present
embodiment, the laser irradiation to crystallize semiconductor
layers of the thin film transistors may be conducted efficiently.
That is, the unit pixels in two rows sandwiching the first virtual
plane 23 may be crystallized by one operation.
[0038] When a 12-inch diagonal organic EL display was produced on
the basis of a display device configuration according to the
present embodiment, a tact time with respect to the laser
irradiation was able to be reduced by almost a half as compared
with that in the past, and it was also ascertained that highly
uniform luminance was exhibited.
Method for Manufacturing Display Device
[0039] A method for manufacturing a display device according to an
embodiment of the present invention will be described.
[0040] The method for manufacturing a dispersion device according
to the present embodiment is a method for manufacturing a display
device in which, as shown in FIG. 2A, a plurality of unit regions
26 corresponding to unit pixels 22 including a thin film transistor
1 and other plural thin film transistors (not shown in the drawing)
are disposed in the matrix, and a first unit region 26a and a
second unit region 26b adjacent to each other are arranged to
become symmetric with respect to a first virtual plane 23
orthogonal to a main surface of the above-described substrate.
[0041] Here, the pulsed laser light L used for step irradiation has
energy variations of about 5% on a pulse basis. In the present
embodiment, the first unit region 26a and the second unit region
26b are simultaneously subjected to the step irradiation of the
laser light L with an excimer laser. Therefore, the process tact
may be reduced by almost a half.
[0042] In the present embodiment, the step irradiation of the laser
light L is applied while pulsed laser light from an excimer laser
is shaped into a line and the width in a minor axis direction is
adjusted to become slightly smaller than two pixel lengths. In
order to level the influence of energy variations among pulses, it
is preferable that the irradiation is about 8 to 20 pulses per
place, that is, per unit region.
[0043] In the case where the energy distribution in the minor axis
direction is not always stable, it is preferable that the number of
pixels included in the minor axis direction is specified to be 1 to
2 pixels from the viewpoint of reduction of variations among
pixels. In addition, if necessary, at least a part, for example, a
half, of the laser irradiation is conducted while a substrate
constituting ultimately the display device 21 is rotated by 180
degrees and, thereby, the characteristics of the thin film
transistors disposed at symmetric positions in the adjacent pixels
may be further leveled.
[0044] In the case where a gate wiring 27, for example, is composed
of a material having the low heat resistance or a material having
the diffusibility, deformation, denaturation, or diffusion of the
metal may occur due to a thermal damage resulting from the laser
irradiation.
[0045] In order to address this, for example, in adjacent portions
25 adjacent to a first virtual plane 23, as shown in FIG. 2B, it is
preferable that thin film transistors composed of a highly
heat-resistant material are simply irradiated with the laser light
and a low-resistance electrically conductive material electrically
connected to thin film transistors composed of a low-resistance
metallic material is not irradiated with the laser light. In this
case, it is more preferable that gate electrodes and the like in
the adjacent portions 25 are composed of a metal having the heat
resistance higher than the heat resistance of the gate wiring
27.
[0046] The laser light to be applied may has the above-described
energy distribution in a minor axis direction of the laser light,
and an intense energy point may occur at a laser irradiation end.
If the intense energy point is contained in a unit region,
variations occur in the amount of energy applied and, thereby,
individual differences occur with respect to characteristics in
individual unit regions. However, in the present embodiment, it is
possible to set the irradiation region in such a way as to
selectively exclude the intense energy point from the unit region.
Therefore, occurrences of individual differences and luminance
variations of the display device 21 based thereon may be
avoided.
[0047] That is, in the method for manufacturing a display device
according to the present embodiment, the arrangement shape in the
unit region 26 may be selected in such a way that a channel region
of the transistor is not irradiated with a nonuniform portion of
the laser energy. For a common excimer laser, it is preferable that
about 8 to 20 pulses per place are applied in order to level the
influence of energy variations among pulses. However, a slight
movement within the range not allowing the channel region to
include the intense energy portion at the irradiation end may be
repeated in order to reduce the influence of the energy
distribution in a pulse.
[0048] FIG. 3 is a schematic diagram showing an example in which
scanning irradiation of the laser light is conducted with a solid
laser. In general, since the output of the solid laser is lower
than the output of the excimer laser, in many cases, the width on
the order of a few hundred micrometers in a major axis direction is
simply ensured to supply energy required for crystallizing a thin
film transistor, for example. Therefore, for the laser irradiation,
as shown in FIG. 3, it is preferable that scanning in the minor
axis direction is conducted on a row-by-row basis, sliding in the
major axis direction is repeated after each scanning is finished
and, thereby, crystallization is conducted on a line-by-line
basis.
[0049] This is because few solid lasers are capable of outputting
high energy required for crystallization at wavelengths suitable
for being absorbed by amorphous silicon serving as a precursor to
constitute the channel region of the thin film transistor. In
particular, for a middle or large display, it is difficult to
ensure one pixel length as the major axis width.
[0050] One pixel may also be annealed by a plurality of scanning
irradiation. In that case as well, it is desirable that the
above-described nonuniform irradiation end of the laser energy is
not applied to the channel portion of the thin film transistors 1
required to have uniform transistor characteristics.
[0051] In particular, when it is difficult to ensure adequately
large laser width (major axis width) relative to the width of two
adjacent unit regions 26, in contrast to the above description, it
is preferable that in the arrangement shapes in the adjacent two
unit regions 26a and 26b, thin film transistors, in which channels
are required to be crystallized, are collectively arranged in the
adjacent portions 25 adjacent to the first virtual plane 23, as
shown in FIG. 4. In this manner, every two pixels in ultimately
produced display device 21 are crystallized by one operation and,
thereby, the tact time may be reduced by almost a half. In this
case, even when the energy distribution in the width direction
orthogonal to the scanning direction is not always constant, if the
distribution is bilaterally symmetric, the element characteristics
of the two regions 26a and 26b are kept constant. Therefore, the
flexibility in usable lasers is increased.
[0052] A method for manufacturing a thin film transistor 1
constituting each unit pixel of an ultimately produced display
device will be described below with reference to FIG. 5. In the
present embodiment, an example in which the thin film transistor 1
is formed simply in a scan line portion connected to a switching
transistor and required to have low resistance will be described,
although the present invention is not limited thereto.
[0053] As shown in FIG. 5A, a substrate 2 having the insulating
property is prepared. A molybdenum thin film is formed on the
surface thereof by, for example, sputtering, and a gate electrode 3
is formed thereon by, for example, photolithography and etching.
Although not shown in the drawing, a low-resistance wiring is
electrically connected to this gate electrode 3, and the laser
irradiation described below is conducted in such a way as to avoid
this low-resistance wiring.
[0054] As shown in FIG. 5B, for example, a crystalline Si layer 5
is formed on this gate electrode 3 with a gate insulating layer 4
made of SiN and SiO.sub.2 therebetween. The crystalline Si layer 5
is crystallized by conducting laser irradiation after amorphous Si
is formed. For the crystallization method by using the laser, a
laser may be directly applied at wavelengths suitable for being
absorbed by amorphous silicon. Alternatively, a technique may be
adopted, in which a photo-thermal conversion layer is formed
thereon, laser irradiation is conducted at wavelengths suitable for
being absorbed by the photo-thermal conversion layer so as to
crystallize amorphous silicon by the heat generated from the
photo-thermal conversion layer and, thereafter, the photo-thermal
conversion layer is removed.
[0055] The laser irradiation is conducted by scan irradiation of
laser light L having a major axis width, that is, a width in a
direction orthogonal to a scanning direction, of a little less than
300 .mu.m, which is slightly narrower than the size of the unit
pixel, in a laser minor axis direction. At this time, it is
preferable to avoid the above-described gate wiring 27 composed of
a lamination structure of aluminum from being irradiated with a
laser, in order that aluminum constituting the gate wiring is not
damaged by the heat due to the laser irradiation.
[0056] Since the gate electrode 3 is composed of a material having
a thermal conductivity higher than those of the Si layer 5 and the
gate insulating layer 4, when the laser light L is moved with the
scanning irradiation and comes near the arrangement portion of the
gate electrode 3, the heat due to the irradiation begins to diffuse
from the Si layer to the gate electrode 3, and the heat, which
contributes to crystallization, may become nonuniform in the Si
layer 5. Therefore, particularly in the case where an improvement
of the uniformity in the crystallization is required, for example,
a wider gate electrode 3 is disposed and, thereby, a heat transfer
component having thermal conductivity higher than that of the gate
insulating layer is disposed on the side upstream to the ultimately
produced channel region with respect to the laser light scanning
Consequently, the gate electrode 3 may be thermally saturated and
become in a preannealed state before the laser light L reaches a
portion corresponding to the channel region, and the uniformity of
the crystallinity of the channel region may be improved.
[0057] An amorphous Si layer 6 is formed following the laser light
L scanning irradiation. Subsequently, an etching stop layer 7 is
made of, for example, SiN, in the portion ultimately corresponding
to the channel region on the resulting amorphous Si layer 6. In the
Si layer 5 and the amorphous Si layer 6, a channel region 15 is
formed immediately below the etching stop layer 7, and a source
region 12 and a drain region 13 are formed on two sides
thereof.
[0058] Thereafter, an n+amorphous Si layer 8 is formed over the
resulting etching stop layer 7 and an exposed portion of the
peripheral amorphous Si layer 6.
[0059] As shown in FIG. 5C, first to third metal layers 9 to 11 are
formed over the n+amorphous Si layer 8 and the etching stop layer
7, and the first to third metal layers 9 to 11 on the etching stop
layer 7 and the n+amorphous Si layer 8 of the channel portion are
etched. In this manner, a source electrode 16 and a drain electrode
17 are made of the first to third metal layers 9 to 11 on the
source region 12 and the drain region 13. Furthermore, a SiN layer
14 serving as a passivation film is formed all over the surface, so
that a bottom gate type transistor based on the inverted staggered
type configuration is produced, for example.
[0060] Preferably, low-resistance wirings, e.g., the gate wiring
27, are formed simultaneously with the formation of this thin film
transistor 1.
[0061] Specifically, as shown in FIG. 6A, low-resistance
electrically conductive films composed of a Mo layer 18 made of
molybdenum and an Al layer 19 made of aluminum are formed by
lamination in advance at the position where the gate wiring 27 is
formed ultimately, and the gate electrode 3 made of the Mo thin
film, which is common to the formation of the above-described thin
film transistor 1, and the gate insulating layer 4 are formed into
predetermined shapes thereon. As shown in FIG. 6B, on the thus
formed gate wiring 27, a first metal layer 9, a second metal layer
10, a third metal layer 11, and a SiN layer 14 are formed by
lamination.
[0062] The low-resistance electrically conductive films, that is,
the Mo layer 18 and the Al layer 19, electrically connected to the
gate electrode 3 may be formed to the end of the gate wiring 27.
However, an example, in which these layers are terminated while not
reaching the end portion and the wiring up to the end portion is
formed by the gate electrode 3 alone, is shown in FIG. 6C. The gate
electrode 3 is connected to the first metal layer 9, the second
metal layer 10, and the third metal layer 11, independent of one
another, through a contact hole disposed in the gate insulating
layer 4 in advance. Consequently, a power source (not shown in the
drawing) and the gate wiring 27 are allowed to become electrically
continuous.
[0063] As described above, it becomes possible to appropriately
crystallize the thin film transistor 1 while a reduction in the
yield of the wiring is suppressed by conducting laser irradiation
in such a way as to avoid the gate wiring 27 including the
low-resistance electrically conductive, that is, the low-resistance
electrically conductive films composed of the Mo layer 18 and the
Al layer 19 in the present embodiment.
[0064] Up to this point, the display device and the method for
manufacturing the display device according to the embodiments of
the present invention have been described. However, the materials
to be used, the amount thereof, and the numerical conditions of the
treatment time, the size, and the like included in the
above-described embodiments are no more than preferable
exemplifications. The sizes, shapes, and arrangement relationships
in each drawing used for the explanations are also
exemplifications. That is, the present invention is not limited to
the above-described embodiments.
[0065] For example, titanium, tungsten, tantalum, chromium,
vanadium, nickel, and alloys thereof may be used in place of the
above-described molybdenum.
[0066] It should be understood by those skilled in the art that
various modifications, combinations, sub-combinations and
alterations may occur depending on design requirements and other
factors insofar as they are within the scope of the appended claims
or the equivalents thereof.
[0067] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *