Title: Apparatus for growing stoichiometric lithium niobate and lithium tantalate single crystals and method of growing the same
Abstract: A method for growing stoichiometric lithium niobate and lithium tantalate single crystals is provided. A crystal growing apparatus that includes a long crucible with a separation member therein is used. A solid feed material is quenched from a molten state, solidified in batches or sintered before charged in the long crucible to obtain substantially stoichiometric solids. The separation member divides the long crucible into a melting zone and a feeding zone located under the melting zone, and it could effectively prevent bubble formation in the growing crystal. The stoichiometry of the axial and radial composition can be well controlled, and the control of the diameter of the crystal body is easily achieved as well.
Patent Number: 6,926,771 Issued on 08/09/2005 to Lan
| Inventors:
|
Lan; Chung-Wen (Taipei Hsien, TW)
|
| Assignee:
|
National Taiwan University (Taipei, TW)
|
| Appl. No.:
|
064880 |
| Filed:
|
August 27, 2002 |
Foreign Application Priority Data
| Feb 27, 2002[TW] | 91103520 A |
| Current U.S. Class: |
117/13; 117/948 |
| Intern'l Class: |
C30B 015/10; C30B 029//30 |
| Field of Search: |
117/13,206,208,214,948
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Norton; Nadine G.
Assistant Examiner: Anderson; Matthew
Attorney, Agent or Firm: Jiang Chyun IP Office
Claims
1. A method for growing stoichiometric lithium niobate and lithium tantalate
single crystals, comprising:
charging a solid feed material in a feeding zone of a long crucible, wherein
the feeding zone is in a lower portion of the long crucible;
placing a separation member above the feeding zone of the long crucible;
charging a zone stuff in a melting zone of the long crucible, wherein the melting
zone of the long crucible is located in an upper portion of the long crucible;
melting the zone stuff while keeping the solid feed material in the feeding zone
as solid;
placing a crystal seed into the melting zone of the long crucible;
pull-growing a crystal body after the crystal seed is melted; and
pushing the long crucible upward as the crystal body is grown.
2. The method of claim 1, wherein the solid feed material contains stoichiometric
lithium niobate.
3. The method of claim 1, wherein the zone stuff contains 58-60% Li
2O/(Li
2O+Nb
2O
5).
4. The method of claim 1, wherein the solid feed material contains stoichiometric
lithium tantalate.
5. The method of claim 1, wherein the zone stuff contains 58-60% of Li
2O/(Li
2O+Ta
2O
5).
6. The method of claim 1, further comprising doping the solid feed material in
the feeding zone with a dopant which has a first concentration.
7. The method of claim 6, further comprising doping the zone stuff in the melting
zone with a dopant which has a second concentration, wherein the ratio of the first
concentration with respect to the second concentration is K, a segregation constant
for the dopant.
8. The method of claim 6, wherein the dopant is selected from magnesium oxide,
zinc oxide, manganese, cerium, terbium, and iron.
9. The method of claim 1, wherein a pulling rate of the crystal body is proportional
to a pushing rate of the long crucible.
10. The method of claim 1, wherein a ratio of a pulling rate of the crystal body
with respect to a pushing rate of the long crucible is approximately equal to a
ratio of an inner cross sectional area of the long crucible with respect to a cross
sectional area of the crystal body, depending on a sintering density of the feeding material.
11. The method of claim 1, further comprising:
slowly cooling down a chamber to room temperature after the crystal body is grown
a predetermined length, and
removing the crystal body.
12. A method for growing stoichiometric lithium niobate and lithium tantalate
single crystals, comprising:
charging a solid feed material in a long crucible;
charging a zone stuff above the solid feed material in the long crucible;
melting the zone stuff while keeping the underlying solid feed material in a
solid phase;
placing a crystal seed in the zone stuff of the long crucible;
pull-growing a crystal body after the crystal seed is melted; and
pushing the long crucible upward as the crystal body is grown.
13. The method of claim 12, wherein the solid feed material contains stoichiometric
lithium niobate.
14. The method of claim 12, wherein the zone stuff contains 58-60% of Li
2O/(Li
2O+Nb
2O
5).
15. The method of claim 12, wherein the solid feed material contains stoichiometric
lithium tantalate.
16. The method of claim 12, wherein the zone stuff contains 58-60% of Li
2O/(Li
2O+Ta
2O
5).
17. The method of claim 12, further comprising doping the solid feed material
in a feeding zone with a dopant which has a first concentration.
18. The method of claim 17, further comprising doping the zone stuff in the melting
zone with a dopant which has a second concentration, wherein a ratio of the first
concentration with respect to the second concentration is K, a segregation constant
for the dopant.
19. The method of claim 17, wherein the dopant is selected from magnesium oxide,
zinc oxide, manganese, cerium, terbium, and iron.
20. The method of claim 12, wherein a pulling rate of the crystal body is proportional
to a pushing rate of the long crucible.
21. The method of claim 12, wherein a ratio of a pulling rate of the crystal
body with respect to a pushing rate of the long crucible is approximately equal
to a ratio of an inner cross sectional area of the long crucible with respect to
a cross sectional area of the crystal body, depending on a sintering density of
the feeding material.
22. The method of claim 12, further comprising:
slowly cooling a chamber to room temperature after the crystal body is grown
a predetermined length, and
removing the crystal body.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Taiwan application serial no.
91103520, filed Feb. 27, 2002, the full disclosure of which is incorporated herein
by reference.
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a method of growing single crystals. More specifically,
the present invention relates to a method of growing substantially stoichiometric
lithium niobate (LN) and lithium tantalate (LT) single crystals.
2. Description of the Related Art
LN and LT crystals have been widely used as a photoelectric material, especially
for photo-transmission, audio and visual appliance, double-frequency laser, and
optical storage medium. A conventional method of growing LN and LT crystals, called
as Czochralski method (Cz method), is well known in the art and as described hereafter.
FIG. 1 and FIG. 2 respectively show phase diagrams of a congruent composition
with lithium and niobium or lithium and tantalium. Here, a composition with a stoichiometric/non-stoichiometric
amount of reactants is also called a stoichiometric/non-stoichiometric composition.
That is, in the congruent composition, the mole ratio of lithium to niobium or
lithium to tantalium is not about 50%. Therefore, a crystal body obtained by the
conventional Cz method usually has a non-stoichiometric composition. However, stoichiometric
LN and LT crystals are preferable because of their superior photoelectric properties.
A method of continuously growing a stoichiometric LN and LT is thus needed.
Recently, a double-crucible method has been proposed by National Institute
for Research in Inorganic Materials (NIRIM), Japan. FIG. 3 is a schematic, cross-sectional
view of an apparatus used in a conventional double-crucible method.
In FIG. 3, a melt with a lithium-rich composition for growth of crystals is placed
in two separate crucibles. A conventional crystal growing apparatus 300
includes a chamber 302, an external crucible 304, an inner crucible
306, a heater 308, and a crystal pulling system 310. The crystal
growing apparatus 300 is provided with a powder feeding system 312
for automatically charging the raw material having the stiochiometric composition.
The external crucible 304 is arranged inside the chamber 302. The
inner crucible 306 is arranged in the external crucible 304 and has
a small opening 314 on the sidewall of the inner crucible 306 near
a bottom thereof. The heater 308 surrounds the chamber 302, including
a bottom and a sidewall thereof. The powder feeding system 312 is arranged
outside the heater 308 and is provided with a tube 318 extending
into the chamber 302. An outlet of the tube 318 is located between
the external crucible 304 and the inner crucible 306. The crystal
pulling system 310 is located above the inner crucible 306.
When the crystal growing process is performed, the solid raw material with about
58% Li is melted and entered into the inner crucible 306 and the external
crucible 304 inside the chamber 302. Then, a crystal seed 320
is placed in the crystal pulling system 310 and dipped in the melt 322
within the inner crucible 306. The crystal seed 320 is pulled up
at a constant speed while being rotated to grow a crystal body 324. As the
crystal body 324 is gradually grown, the powder material 326 is added
into the melt 328 within the external crucible 304 at a rate that
can compensate the consumed material consumption. Meanwhile, the powder material
326 added in the melt 328 is continuously melted by the heater 308.
The melt 328 flows into the inner crucible 306 through the opening
314 of the inner crucible 306. Thereby, the composition of the crystal
body 324 can be constant.
The double-crucible method grows stoichiometric lithium niobate (LiNbO
3,
LN) and lithium tantalate (LiTaO
3, LT) single crystals by keeping the
composition of the melt 422 in the inner crucible 306 at point A
of the phase diagram of FIG. 1 and FIG. 2. Theoretically, the stoichiometric
crystals can be obtained as long as the charged amount of powder stuff 326
is precisely controlled to grow the crystal body 324. Automatically feeding
powder stuff is requisite for the double-crucible method to grow the stoichiometric
crystals. However, it needs high level technology and high production cost. Further,
in consideration of thermal aspect, there is a significant difference between the
melting points of the melt 322 and 328 respectively in the inner
crucible 306 and the external crucible 304, which causes the control
of the heater 308 difficult. Therefore, the double-crucible method is difficult
to be commercialized at a large scale.
SUMMARY OF INVENTION
It is an object of the present invention to provide a method of growing stoichiometric
lithium niobate and lithium tantalate single crystals and an apparatus therefor.
With the method of the invention, it is easier to obtain the stoichiometric single crystals.
It is another object of the present invention to provide a method of growing
stoichiometric
lithium niobate and lithium tantalate single crystals and an apparatus therefor,
in which the uniformity of the composition of the single crystal can be improved.
It is still another object of the present invention to provide a method of growing
stoichiometric lithium niobate and lithium tantalate single crystals and an apparatus
therefor, in which a diameter of the crystal can be easily controlled.
It is still another object of the present invention to provide a method of growing
stoichiometric lithium niobate and lithium tantalate single crystals and an apparatus
therefor, in which power consumption is reduced and thus the production cost is decreased.
In order to achieve the above and other objectives of the invention, a crystal
growing apparatus of growing stoichiometric lithium niobate and lithium tantalate
single crystals is provided. The crystal growing apparatus includes a chamber,
a long crucible, a heating system, a separation member, a pushing/rotating system
and a crystal pulling system. The long crucible is provided with a separation member
a melting zone and a feeding to define a feeding zone and a melting zone. The provision
of the separation member further prevents bubbles generated when the solid feed
material is melted from being included in the crystal body. The solid feed material
can be quenched from a molten state, solidified in batches or sintered to obtain
stoichiometric solids before charged into the feeding zone of the long crucible.
With the design of the melting zone in the apparatus of the present invention,
the axial composition and radial composition can be controlled well. Thereby, substantially
stoichiometric lithium niobate and lithium tantalate single crystals can be obtained.
Furthermore, a method of growing stoichiometric lithium niobate and
lithium tantalate single crystals using a crystal growing apparatus is also provided.
The crystal growing apparatus includes a chamber, a long crucible, a heating system,
a pushing/rotating system and a crystal pulling system. In the method of the claimed
invention, a solid feed material charged into a lower portion of the long crucible
is kept in a solid state. A zone stuff is charged into the long crucible above
the solid feed material. The solid feed material can be quenched from a molten
state, solidified in batches or sintered before charged into the long crucible
in order to grow substantially stoichiometric lithium niobate and lithium tantalate
single crystals.
In another aspect of the invention, a method of growing stoichiometric lithium
niobate and lithium tantalate single crystals by using at least a long crucible
and an external heater is provided. A solid feed material is charged into the long
crucible. A separation member is placed into the long crucible. A solid feed material
is placed into the long crucible on the separation member, and then gradually melted
by the external heater. The long crucible is pushed upward as a crystal body is
grown. The zone stuff is separated from the solid feed material by the separation
member to further control an admix rate of solid feed material/zone stuff. The
provision of the separation member further prevents bubbles generated when the
solid feed material is melted from being included in a crystal body. The ratio
of the pulling rate of the crystal body and the pushing rate of the long crucible
is in proportion to the ratio of the sinner cross section area of the long crucible
and the cross section area of the crystal body in order to obtain the crystal body
with uniform composition. Preferably, the ratio of the pulling rate of the crystal
body to the pushing rate of the long crucible is substantially equal to the ratio
of the inner cross section area of the long crucible to the cross section area
of the crystal body.
In still another aspect of the invention, an apparatus for growing stoichiometric
lithium niobate and lithium tantalate single crystals is provided. The apparatus
includes a chamber, a long crucible, a separation member or an inner crucible,
a heating system, a pushing/rotating system, and a crystal pulling system. The
long crucible is arranged inside the chamber. The separation member, such as an
insulative plate, a shallow crucible, or a crucible with perforated wall, is arranged
in the long crucible to divide the long crucible into a melting zone and a feeding
zone. The heating system surrounds a sidewall of the chamber, corresponding to
locations of the melting zone and the feeding zone. Furthermore, the crystal pulling
system is located above the chamber to pull up a crystal seed during crystal growth.
The pushing/rotating system is located under the long crucible to rotate and push
the long crucible up.
In still another aspect of the invention, an apparatus for growing stoichiometric
lithium niobate and lithium tantalate single crystals is provided. The apparatus
includes a chamber, a long crucible, a heating system, a pushing/rotating system,
and a crystal pulling system. The long crucible is arranged inside the chamber.
The heating system surrounds a sidewall of the chamber for melting solids therein.
Furthermore, the crystal pulling system is located above the chamber for holding
a crystal seed and pulling up the crystal seed during crystal growth. The pushing/rotating
system is located under the long crucible to rotate and push the long crucible up.
In view of foregoing, with the use of the crystal growing apparatus of the present
invention, in which the long crucible is provided with or not provided with a separation
member, it is much easier to grow a crystal body with a controlled composition.
Furthermore, in the case of the crystal growing apparatus provided with
the separation member, such as an insulative plate, a shallow crucible or a shallow
crucible with a perforated wall is provided, the zone stuff in the melting zone
is separated from the solid feed material in the feeding zone. The provision of
the separation member further prevents bubbles generated when the solid feed material
is melted from being included in the crystal body. If the separation member is
a crucible with a shallow perforated wall, it is preferable to fix the separation
member by three external Pt/Rh rods that are secured at an upper part of the long
crucible. It is easy to take the separation member out of the long crucible after
the crystal growth.
The solid feed material can be quenched from a molten state, solidified in batches
or sintered before be charged the long crucible. Therefore, the composition of
the grown crystal is more uniform.
In the apparatus of the present invention, the melting zone of the long crucible
is located above the feeding zone of the long crucible. The solid feed material
and the zone stuff are separately prepared and sequentially charged into the long
crucible. Thereby, the stoichiometry of the axial and radial composition can be
well controlled, and the control of the diameter of the crystal body is easily
achieved as well.
Furthermore, the heat required for the present invention is applied
around the sidewall of the external crucible, instead of the whole apparatus. Therefore,
the energy can be saved and the production cost can be thus reduced.
BRIEF DESCRIPTION OF DRAWINGS
It is to be understood that both the foregoing general description and the following
detailed description are exemplary, and are intended to provide further explanation
of the invention as claimed.
The accompanying drawings are included to provide a further understanding of
the invention, and are incorporated in and constitute a part of this specification.
The drawings illustrate embodiments of the invention and, together with the description,
serve to explain the principle of the invention. In the drawings,
FIG. 1 is a phase diagram of LiNbO
3;
FIG. 2 is a phase diagram of LiTaO
3;
FIG. 3 is a schematic, cross-sectional view of an apparatus used in a conventional
double-crucible method;
FIG. 4 is a schematic, cross-sectional view of an apparatus for growing stoichiometric
lithium niobate and lithium tantalate single crystals according to one preferred
embodiment of the present invention;
FIG. 5 is a schematic, cross-sectional view of an apparatus for growing stoichiometric
lithium niobate and lithium tantalate single crystals according to another preferred
embodiment of the present invention, wherein an example of a separation member
is shown; and
FIG. 6 is a flow chart showing a method of growing stoichiometric lithium niobate
and lithium tantalate single crystals according to one preferred embodiment of
the present invention.
DETAILED DESCRIPTION
Reference will now be made in detail to the present preferred embodiments
of the invention, examples of which are illustrated in the accompanying drawings.
Whenever possible, the same reference numbers are used in the drawings and the
description to refer to the same or like parts.
In the invention, a zone melting Czochralski method (hereafter, zone melting
Cz
method) is used. Stoichiometric lithium niobate (LN) and lithium tantalate (LT)
single crystals are obtained by the zone melting Cz method that uses a crystal
growing apparatus provided with a long crucible. The long crucible can be preferably
provided with a separation member therein.
FIG. 4 is an apparatus for growing stoichiometric lithium niobate and lithium
tantalate single crystals according to one preferred embodiment of the present
invention. The crystal growing apparatus
400 includes a chamber
402,
a long crucible
404, a separation member
406, a heating system
408,
a pushing/rotating system
410 and a crystal pulling system
412. The
long crucible
404 is arranged inside the chamber
402. The heating
system
408 preferably surrounds sidewall of the chamber
402. The
long crucible
404 particularly has a high wall. Preferably, the height of
the wall of the long crucible
404 is larger than its diameter. The separation
member
406 is arranged inside the long crucible
404 to define a melting
zone
414 in an upper portion of the long crucible
404 and form a
feeding zone
416 in a lower portion of the long crucible
404. The
provision of the separation member
406 further prevents bubbles generated
when the solid feed material is melted from being included in the crystal body.
The separation member
406 can be an insulative plate, a shallow crucible,
or a crucible with a perforated wall, for example. The separation member
406
can be made of, for example, platinum or iridium. The heating system
408
is arranged outside the chamber
402, corresponding to locations of the feeding
zone
416 and the melting zone
414. The heating system
408
includes a preheater
418 for preheating the solid feed material in the feeding
zone
416, and a post-heater
420 for gradually reducing a temperature
of the grown crystal in the melting zone
414. The crystal pulling system
412 is arranged above the melting zone
414 for holding the
crystal seed
422 and pulling up the crystal seed
422 during crystal
growth. The pushing/rotating system
410 is located under the long crucible
404 to rotate and push up the long crucible
404 during crystal growth.
During crystal growth, the feeding zone
416 in the long crucible
404
is charged with a solid feed material. The solid feed material can be quenched
from a molten state, solidified in batches or sintered before charged into the
feeding zone
416 to obtain stoichiometric solids. Alternatively, a solid
feed material prepared previously is charged into the long crucible. The separation
member
406 is placed on the feeding zone
416 of the long crucible
404. Then, a zone stuff having composition at point A of FIG. 1 is placed
into the long crucible
404.
The separation member
406 used here can be an insulative plate, a shallow
crucible (as shown in FIG.
4), or a crucible with a perforated wall (as
shown in FIG.
5). The separation member
406 is provided and arranged
over the feeding zone
416, then an admix rate of zone stuff/solid feed material
can be further controlled. If the separation member is a crucible with a shallow
perforated wall, it is preferable to fix the separation member by three external
Pt/Rh rods that are secured at an upper part of the long crucible. It is easy to
take the separation member out of the long crucible after the crystal growth.
The separation member
406 is optionally provided. If the separation member
406 is not provided, a lower portion of the long crucible
404, where
the solid feed material is located, is also called feeding zone
416. Similarly,
an upper portion of the long crucible
404, where the zone stuff is located,
is also called melting zone
414, as illustrated in the above embodiment
shown in FIG.
4. The solid feed material in the feeding zone
416
is kept dense so that the solid feed material would not be affected by the temperature
of the melting zone
414, and the formation of bubbles is minimized. This
purpose can be achieved by densification or solidification of the solid feed material
in the feeding zone
416. The condition of densification includes longer
sintering time or the use of solid feed material with fine particles. The solidification
is performed step-by-step so as to reduce the solidification rate or quenching
from a molten state.
A crystal seed
422 is placed in the crystal pulling system
412
and
dipped into the zone stuff of the melting zone
414. The seed
422
is pulled up and rotated at a constant speed to grow a crystal body
424,
under the condition that the pulling speed Uc of the seed
424 (cross sectional
area, Ac) and the upward pushing rate (Uf) of the long crucible
404 (inner
cross sectional area, Af) satisfy the relationship ρcUc×Ac=ρfUf×Af;
pc and pf are the density of the crystal and the feed, respectively. As the crystal
body
424 is gradually grown, the pushing/rotating system
410 pushes
the long crucible
404 upward to melt the solid feed material near the melting
zone
414 at a proper temperature. A solid-liquid phase
426 can be
thereby kept at a substantially the same level. The proper temperature recited
above can be the point A of FIG.
1 and FIG. 2, for example. The crystal
body has a well-controlled diameter and composition after a self-stabilization
stage of heat transfer. The method of the present invention is particularly more
convenient than that of the prior art. The crystal body obtained by the present
invention has axial and radial compositions with improved uniformity. More particularly,
the stoichiometric ratio of Lithium and Niobium, or Lithium and tantalum is about
1, which means the optical properties of LN and LT are superior to those obtained
non-stoichiometrically. If a doping system is further used, the solid feed material
in the feeding zone
416 is doped with a dopant which has a concentration
C
0. The zone stuff in the melting zone
414 has a dopant concentration
C
0/K (wherein K is a segregation constant). The dopant can be magnesium
oxide, zinc oxide, manganese, cerium, terbium, or iron.
EXAMPLE
The following examples illustrate the present invention in more detail with reference
to FIG. 6.
FIG. 6 is a flow chart of a method of growing stoichiometric lithium niobate
and lithium tantalate single crystals according to one preferred embodiment of
the present invention.
With reference to FIG. 6, the solid feed material is charged in the long crucible
(step 500). The solid feed material has subtantially stoichiometric composition
of LN or LT. Preferably, the solid feed material includes at the molar ratio of
1:1. The feed material is either solidified in batches, quenching from a molten
state, or sintered to obtain stoichiometric solids before charged in the long crucible.
Alternatively, the specific powder material which is previously prepared is charged
in the long crucible.
The separation member 406 is placed in the long crucible (step 502).
A zone stuff is prepared (step 504). The zone stuff includes 58%-60% Li
2O/(Li
2O+Nb
2O
5)
or 58-60% Li
2O/(Li
2O+Ta
2O
5). The zone
stuff is charged into the long crucible (step 506). The zone stuff is melted
(step 508). A molten phase of the solid feed material that near the melting
zone is also separated from a non-molten phase of the solid feed material by the
separation member. The non-molten phase of the solid feed material in the feeding
zone 416 is kept dense. Therefore, the step 502 is preferable because
it helps not only to prevent bubbles generated when the solid feed material is
melted from being included in the crystal body, but also to well control the admix
rate of the solid feed material in the feeding zone and the zone stuff in the melting zone.
The crystal seed 422 is dipped in the zone stuff in the melting zone by
means of a crystal pulling system (step 510). After the crystal seed is
melted, the crystal pulling system 412 pulls the crystal seed up while rotating
the crystal seed to grow a crystal body (step 512). The zone stuff is consumed
as the crystal is grown, and replenished with the underlying feed material that
has turn into melt near the melting zone 414. Therefore, the long crucible
404 is pushed upward by means of, for example, a pushing/rotating system
as the crystal body is grown (step 514).
Further, the pushing rate of the long crucible 404 is in relation
with the pulling rate of the crystal body. That is, the ratio of the pulling rate
of the crystal body and the pushing rate of the long crucible, depending on the
density of the feed, is about equal to the ratio of the inner cross sectional area
of the long crucible and the cross sectional area of the crystal body. For example,
when the crystal body to be grown has a diameter that is half of the diameter of
the long crucible, the pushing rate of the long crucible is one fourth of the pulling
rate of the crystal body. In a step 516, once the crystal body with a desired
length is obtained, the crystal body is pulled out of the liquid level of the zone
stuff melt at a higher pulling speed, and then is cooled down slowly. The crystal
body is removed after cooling to room temperature.
The present invention has the following advantages over the prior art:
1. With the apparatus of the present invention having the long crucible with
or
without the separation member, it is much easier to grow a crystal body with a
controlled composition.
2. The solid feed material can be solidified in batches, quenched from a molten
state, or sintered before charged into the long crucible. The zone stuff is consumed
as the crystal is grown, and replenished with the underlying feed material that
has turn into melt near the melting zone.
3. By defining the melting zone in the long crucible, the stoichiometry and the
dopant concentration of the axial and radial composition can be well controlled.
Furthermore, the control of the diameter of the crystal body is easily achieved.
4. The required heat is applied around the sidewall of the external crucible,
instead of the whole apparatus. Therefore, energy consumption and thus fabrication
cost are reduced.
5. The arrangement of the separation member in the long crucible further enables
the control the admix rate of the zone stuff in the melting zone and the solid
feed material in the feeding zone. It also prevents bubbles generated by melting
the solid feed material from being included in the crystal body.
It will be apparent to those skilled in the art that various modifications and
variations can be made to the structure of the present invention without departing
from the scope or spirit of the invention. In view of the forgoing, it is intended
that the present invention cover modifications and variations of this invention
provided they fall within the scope of the following claims and their equivalents.
*
