U.S. patent application number 11/105000 was filed with the patent office on 2005-08-25 for back-biased face target sputtering based memory.
Invention is credited to Nagashima, Makoto.
Application Number | 20050183945 11/105000 |
Document ID | / |
Family ID | 34274230 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050183945 |
Kind Code |
A1 |
Nagashima, Makoto |
August 25, 2005 |
Back-biased face target sputtering based memory
Abstract
Systems and methods are disclosed for forming memory arrays on a
substrate in an air-tight chamber in which an inert gas is
admittable and exhaustible; a pair of target plates placed at
opposite ends of said air-tight chamber respectively so as to face
each other and form a plasma region therebetween; a pair of magnets
respectively disposed adjacent to said target plates such that
magnet poles of different polarities face each other across said
plasma region thereby to establish a magnetic field of said plasma
region between said target plates; a substrate holder disposed
adjacent to said plasma region, said substrate holder adapted to
hold a substrate on which an alloyed thin film is to be deposited;
and a back-bias power supply coupled to the substrate holder.
Inventors: |
Nagashima, Makoto; (Tokyo,
JP) |
Correspondence
Address: |
TRAN & ASSOCIATES
6768 MEADOW VISTA CT.
SAN JOSE
CA
95135
US
|
Family ID: |
34274230 |
Appl. No.: |
11/105000 |
Filed: |
April 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11105000 |
Apr 13, 2005 |
|
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10662862 |
Sep 15, 2003 |
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Current U.S.
Class: |
204/192.12 ;
204/298.16; 204/298.18 |
Current CPC
Class: |
H01J 37/3402 20130101;
C23C 14/352 20130101 |
Class at
Publication: |
204/192.12 ;
204/298.18; 204/298.16 |
International
Class: |
C23C 014/32 |
Claims
What is claimed is:
1. A facing targets sputtering device for semiconductor
fabrication, comprising: an air-tight chamber in which an inert gas
is admittable and exhaustible; a pair of target plates placed at
opposite ends of said air-tight chamber respectively so as to face
each other and form a plasma region therebetween; a pair of magnets
respectively disposed adjacent to said target plates such that
magnet poles of different polarities face each other across said
plasma region thereby to establish a magnetic field of said plasma
region between said target plates; a substrate holder disposed
adjacent to said plasma region, said substrate holder adapted to
hold a substrate on which an alloyed thin film is to be deposited;
a back-bias power supply coupled to the substrate holder; wherein
the substrate includes one or more memory arrays formed
thereon.
2. A facing targets sputtering device according to claim 1, wherein
the back-bias power supply is a DC or an AC electric power
source.
3. A facing targets sputtering device according to claim 1, further
comprising a first target power supply coupled to one of the target
plates.
4. A facing targets sputtering device according to claim 3, wherein
the first target power supply is a DC or an AC electric power
source.
5. A facing targets sputtering device according to claim 1, further
comprising a second target power supply coupled to the remaining
target plate.
6. A facing targets sputtering device according to claim 1, wherein
the first and second target power supplies comprises DC and AC
electric power sources.
7. A facing targets sputtering device according to claim 1, further
comprising a robot arm to move the wafer.
8. A facing targets sputtering device according to claim 1, further
comprising a magnetron coupled to the chamber.
9. A facing targets sputtering device according to claim 1, further
comprising a chuck heater mounted above the wafer.
10. The apparatus of claim 1, wherein the FTS further comprises
first and second targets mounted in parallel.
11. The apparatus of claim 10, further comprising magnets
positioned between the first and second targets.
12. The apparatus of claim 10, further comprising a power supply
coupled to the magnets and the targets.
13. The apparatus of claim 10, wherein the substrates are
positioned perpendicularly to the planes of the targets.
14. The apparatus of claim 13, further comprising a substrate
holder to secure the substrate.
15. The apparatus of claim 1, wherein the semiconductor layer is a
CMOS layer.
16. A method for sputtering a thin film onto a substrate,
comprising: providing at least one target and a substrate having a
film-forming surface portion and a back portion; creating a
magnetic field so that the film-forming surface portion is placed
in the magnetic field with the magnetic field induced normal to the
substrate surface portion back-biasing the back portion of the
substrate; and sputtering material onto the film-forming surface
portion, wherein the thin forming surface portion comprises a
memory cell.
17. A method as in claim 16 including providing a pair of said
targets opposed to each other where the substrate is disposed
between the targets.
18. A method as in claim 16, further comprising swinging the wafer
using a pendulum.
19. A method as in claim 16, further comprising supporting a chuck
from underneath rather than side-way.
20. A method as in claim 16, further comprising providing a
plurality of sources to deposit materials onto the substrate.
Description
[0001] This application is a continuation in part (CIP) application
of Ser. No. 10/662,862, the content of which is incorporated by
reference.
BACKGROUND
[0002] The present invention relates to systems and methods for
fabricating semiconductor devices at low temperature.
[0003] Electronic systems have become a ubiquitous fixture in
modern society. These electronic systems range from simple,
hand-held calculators to more complex systems including computers,
personal digital assistants (PDAs), embedded controllers and
complex satellite imaging and communications systems. As noted in
U.S. Pat. No. 6,862,211, many electronic systems include a
microprocessor that performs one or more functions based on data
provided to the microprocessor. This data is typically stored in a
memory device of the electronic system such as a common dynamic
random access memory (DRAM) device. A DRAM typically includes an
array of memory cells that store data as binary values, e.g., 1's
and 0's. In a conventional DRAM, the data is stored by controlling
the charge on capacitors in each cell of the DRAM. Data in the
array is "randomly accessible" since a processor can retrieve data
from any location in memory by providing the appropriate address to
the memory device. One problem with conventional DRAM is that the
device is "volatile." This means that when power is turned off to
the system using the DRAM, the data in the memory device is
lost.
[0004] In a parallel trend, various semiconductor fabrication steps
need to be done at low temperature. For instance, when applying a
ferroelectric thin film to a highly integrated device, conventional
processes do not provide a ferroelectric thin film which
sufficiently fulfills various conditions, such as denseness and
evenness on the thin film surface required for fine processing and
formation of film at a relatively low temperature.
[0005] U.S. Pat. No. 5,000,834 discloses a vacuum deposition
technique known as face target sputtering to form thin films on
magnetic recording heads at low temperature. The sputtering method
is widely used for forming a thin film on a substrate made of PMMA
because of intimacy between the substrate and the thin film formed
therethrough. The amorphous thin film of rare earth--transition
metal alloy formed through the sputtering method is applied to an
erasable magneto-optical recording medium. The sputtering method is
performed as follows: Positive ions of an inert gas such as Argon
(Ar) first created by a glow discharge are accelerated toward a
cathode or target, and then they impinge upon the target. As a
result of ionic bombardment, neutral atoms and ions are removed
from the target surface into a vacuum chamber due to the exchange
of momentum therebetween. The liberated or sputtered atoms and ions
are consequently deposited on a preselected substrate disposed in
the vacuum chamber.
[0006] U.S. Pat. No. 6,156,172 discloses a plasma generating unit
and a compact configuration of the combination of plasma space and
substrate holders for a facing target type sputtering apparatus
which includes: an arrangement for defining box-type plasma units
supplied therein with sputtering gas mounted on outside wall-plates
of a closed vacuum vessel; at least a pair of targets arranged to
be spaced apart from and face one another within the box-type
plasma unit, with each of the targets having a sputtering surface
thereof; a framework for holding five planes of the targets or a
pair of facing targets and three plate-like members providing the
box-type plasma unit so as to define a predetermined space apart
from the pair of facing targets and the plate-like members, which
framework is capable of being removably mounted on the outside
walls of the vacuum vessel with vacuum seals; a holder for the
target having conduits for a coolant; an electric power source for
the targets to cause sputtering from the surfaces of the targets;
permanent magnets arranged around each of the pair of targets for
generating at least a perpendicular magnetic field extending in a
direction perpendicular to the sputtering surfaces of the facing
targets; devices for containing the permanent magnets with target
holders, removably mounted on the framework; and a substrate holder
at a position adjacent the outlet space of the sputtering plasma
unit in the vacuum vessel. The unified configuration composed of a
cooling device for cooling both the backside plane of the targets
and a container of magnets in connection with the framework
improves the compactness of sputtering apparatus.
SUMMARY
[0007] In one aspect, a facing targets sputtering device for
semiconductor fabrication includes an air-tight chamber in which an
inert gas is admittable and exhaustible; a pair of target plates
placed at opposite ends of said air-tight chamber respectively so
as to face each other and form a plasma region therebetween; a pair
of magnets respectively disposed adjacent to said target plates
such that magnet poles of different polarities face each other
across said plasma region thereby to establish a magnetic field of
said plasma region between said target plates; a substrate holder
disposed adjacent to said plasma region, said substrate holder
adapted to hold a substrate on which an alloyed thin film is to be
deposited; and a back-bias power supply coupled to the substrate
holder.
[0008] In another aspect, a method for sputtering a thin film onto
a substrate includes providing at least one target and a substrate
having a film-forming surface portion and a back portion; creating
a magnetic field so that the film-forming surface portion is placed
in the magnetic field with the magnetic field induced normal to the
substrate surface portion; back-biasing the back portion of the
substrate; and sputtering material onto the film-forming surface
portion.
[0009] Advantages of the invention may include one or more of the
following. The substrate temperature in forming a thin film is
approximately that of room temperature, and the process requires a
short time. Since the thin film is formed at a very low temperature
during substantially the whole process, the process can be applied
to a highly integrated device to deposit an additional layer with a
plurality of elements without damaging other elements previously
deposited using conventional deposition.
BRIEF DESCRIPTION OF THE FIGURES
[0010] In order that the manner in which the above-recited and
other advantages and features of the invention are obtained, a more
particular description of the invention briefly described above
will be rendered by reference to specific embodiments thereof,
which are illustrated, in the appended drawings. Understanding that
these drawings depict only typical embodiments of the invention and
are not therefore to be considered to be limiting of its scope, the
invention will be described and explained with additional
specificity and detail through the use of the accompanying drawings
in which:
[0011] FIG. 1 shows one embodiment of an apparatus for fabricating
semiconductor.
[0012] FIG. 2 is an exemplary electron distribution chart.
[0013] FIG. 3 shows one embodiment of an FTS unit.
[0014] FIG. 4A shows one embodiment of a second apparatus for
fabricating semiconductor.
[0015] FIG. 4B shows one embodiment of a second apparatus for
fabricating semiconductor.
[0016] FIG. 5 shows an SEM image of a cross sectional view of an
exemplary device fabricated with the system of FIG. 1.
[0017] FIG. 6 is an enlarged view of one portion of the SEM image
of FIG. 5.
DESCRIPTION
[0018] Referring now to the drawings in greater detail, there is
illustrated therein structure diagrams for a semiconductor
processing system and logic flow diagrams for processes a system
will utilize to deposit a memory device at low temperature, as will
be more readily understood from a study of the diagrams.
[0019] FIG. 1 shows one embodiment of an apparatus for fabricating
semiconductor. An embodiment reactor 10 is schematically
illustrated in FIG. 1. The reactor 10 includes a metal chamber 14
that is electrically grounded. A wafer or substrate 22 to be
sputter coated is supported on a pedestal electrode 24 in
opposition to the target 16. An electrical bias source 26 is
connected to the pedestal electrode 24. Preferably, the bias source
26 is an RF bias source coupled to the pedestal electrode 24
through an isolation capacitor. Such bias source produces a
negative DC self-bias VB on the pedestal electrode 24 on the order
of tens of volts. A working gas such as argon is supplied from a
gas source 28 through a mass flow controller 30 and thence through
a gas inlet 32 into the chamber. A vacuum pump system 34 pumps the
chamber through a pumping port 36.
[0020] An FTS unit is positioned to face the wafer 22 and has a
plurality of magnets 102, 104, 106, and 108. A first target 110 is
positioned between magnets 102 and 104, while a second target 120
is positioned between magnets 106 and 108. The first and second
targets 110 and 120 define an electron confining region 130. A
power supply 140 is connected to the magnets 102-108 and targets
110-120 so that positive charges are attracted to the second target
120. During operation, particles are sputtered onto a substrate 22
which, in one embodiment where the targets 110 and 120 are
laterally positioned, is vertically positioned relative to the
lateral targets 110 and 120. The substrate 22 is arranged to be
perpendicular to the planes of the targets 110 and 120. A substrate
holder 24 supports the substrate 22.
[0021] The targets 110 and 120 are positioned in the reactor 10 in
such a manner that two rectangular shape cathode targets face each
other so as to define the plasma confining region 130 therebetween.
Magnetic fields are then generated to cover vertically the outside
of the space between facing target planes by the arrangement of
magnets installed in touch with the backside planes of facing
targets 110 and 120. The facing targets 110 and 120 are used as a
cathode, and the shield plates are used as an anode, and the
cathode/anode are connected to output terminals of the direct
current (DC) power supply 140. The vacuum vessel and the shield
plates are also connected to the anode.
[0022] Under pressure, sputtering plasma is formed in the space 130
between the facing targets 110 and 120 while power from the power
source is applied. Since magnetic fields are generated around the
peripheral area extending in a direction perpendicular to the
surfaces of facing targets 110 and 120, highly energized electrons
sputtered from surfaces of the facing targets 110 and 120 are
confined in the space between facing targets 110 and 120 to cause
increased ionized gases by collision in the space 130. The
ionization rate of the sputtering gases corresponds to the
deposition rate of thin films on the substrate 22, then, high rate
deposition is realized due to the confinement of electrons in the
space 130 between the facing targets. The substrate 22 is arranged
so as to be isolated from the plasma space between the facing
targets 110 and 120.
[0023] Film deposition on the substrate 22 is processed at a low
temperature range due to a very small number of impingement of
plasma from the plasma space and small amount of thermal radiation
from the target planes. A typical facing target type of sputtering
method has superior properties of depositing ferromagnetic
materials at high rate deposition and low substrate temperature in
comparison with a magnetron sputtering method. When sufficient
target voltage VT is applied, plasma is excited from the argon. The
chamber enclosure is grounded. The RF power supply 26 to the chuck
or pedestal 24 causes an effective DC `back-bias` between the wafer
and the chamber. This bias is negative, so it repels the
low-velocity electrons.
[0024] FIG. 2 illustrates an exemplary electron distribution for
the apparatus of FIG. 1. The electron distribution follows a
standard Maxwellian curve. Low energy electrons have two
characteristics: they are numerous and they tend to have
non-elastic collisions with the deposited atoms, resulting in
amorphization during deposition. High-energy electrons come through
the back-biased shield, but they effectively "bounce" off the atoms
without significant energy transfer--these electrons do not affect
the way bonds are formed. This is especially true because high
energy electrons spend very little time in the vicinity of the
atoms, while the low energy electrons spend more time next to the
atoms and can interfere with bond formation.
[0025] The presence of the large positively biased shield affects
the plasma, particularly those close to the pedestal electrode 24.
As a result, the DC self-bias developed on the pedestal 24,
particularly by an RF bias source, may be more positive than for
the conventional large grounded shield, that is, less negative
since the DC self-bias is negative in typical applications. It is
believed that the change in DC self-bias arises from the fact that
the positively biased shield drains electrons from the plasma,
thereby causing the plasma and hence the pedestal electrode to
become more positive.
[0026] FIG. 3 shows another embodiment of an FTS system. In this
embodiment, a wafer 200 is positioned in a chamber 210. The wafer
200 is moved into the chamber 210 using a robot arm 220. The robot
arm 220 places the wafer 200 on a wafer chuck 230. The wafer chuck
230 is moved by a chuck motor 240. One or more chuck heaters 250
heats the wafer 200 during processing.
[0027] Additionally, the wafer 200 is positioned between the heater
250 and a magnetron 260. The magnetron 260 serves as highly
efficient sources of microwave energy. In one embodiment, microwave
magnetrons employ a constant magnetic field to produce a rotating
electron space charge. The space charge interacts with a plurality
of microwave resonant cavities to generate microwave radiation. One
electrical node 270 is provided to a back-bias generator such as
the generator 26 of FIG. 1.
[0028] In the system of FIG. 3, two target plates are respectively
connected and disposed onto two target holders which are fixed to
both inner ends of the chamber 210 so as to make the target plates
face each other. A pair of permanent magnets are accommodated in
the target holders so as to create a magnetic field therebetween
substantially perpendicular to the surface of the target plates.
The wafer 200 is disposed closely to the magnetic field (which will
define a plasma region) so as to preferably face it. The electrons
emitted from the both target plates by applying the voltage are
confined between the target plates because of the magnetic field to
promote the ionization of the inert gas so as to form a plasma
region. The positive ions of the inert gas existing in the plasma
region are accelerated toward the target plates. The bombardment of
the target plates by the accelerated particles of the inert gas and
ions thereof causes atoms of the material forming the plates to be
emitted. The wafer 200 on which the thin film is to be disposed is
placed around the plasma region, so that the bombardment of these
high energy particles and ions against the thin film plane is
avoided because of effective confinement of the plasma region by
the magnetic field. The back-bias RF power supply causes an
effective DC `back-bias` between the wafer 200 and the chamber 210.
This bias is negative, so it repels the low-velocity electrons.
[0029] FIG. 4A shows one embodiment of a second apparatus for
fabricating semiconductor. In the system of FIG. 4A, multiple 1-D
deposition sources are stacked in the deposition chamber. The
stacking of the sources reduces the amount of wafer travel, while
significantly increasing deposition uniformity. A wafer 300 is
inserted into a chamber 410 using a robot arm 420 moving through a
transfer chamber 430. The wafer 300 is positioned onto a rotary
chuck 440 with chuck heater(s) 450 positioned above the wafer. A
linear motor 460 moves the chuck through a plurality of deposition
chambers 470.
[0030] The system of FIG. 4A provides a plurality of one
dimensional sputter deposition chambers. Each chamber can deposit a
line of material. By moving the wafer 300 with the linear motor
460, 2-d coverage is obtained.
[0031] Turning now to FIG. 4B, a second embodiment of a fabrication
apparatus is shown. In this embodiment, a chuck 500 is positioned
inside a chamber. The chuck 500 supports a wafer 502. The chamber
has vacuum bellows 510. The chuck 500 is driven by a wafer rotator
520 which rotates the wafer 502 and the chuck 500 in a
pendulum-like manner. The chuck 500 is also powered by a linear
motor 530 to provide up/down motion. A plurality of sources 540-544
perform deposition of materials on the wafer 502.
[0032] The system of FIG. 4B gets linear motion of the wafer 502
past the three sources for uniform deposition. This is done through
a chuck supported from underneath rather than from the side. A
jointed pendulum supports the wafer and keeps the wafer at a
constant vertical distance from the target as the pendulum swings.
The system swings the wafer using a pendulum. The system is more
stable than a system with a lateral linear arm since the chuck 500
is heavy and supports the weight of the wafer, a heater, and RF
back-bias circuitry and would require a very thick support arm
otherwise the arm would wobble. Also, the linear arm would need to
extend away from the source, resulting in large equipment. In this
implementation, the arm sits below the chuck, resulting in a
smaller piece of equipment and also the arm does not have to
support much weight.
[0033] In one embodiment, a process for obtain 2D deposition
coverage is as follows:
[0034] Receive desired 2D pattern from user
[0035] Move chuck into a selected deposition chamber;
[0036] Actuate linear motor and rotary chuck to in accordance with
the 2D pattern
[0037] Move current wafer to next deposition chamber
[0038] Get next wafer into the current chamber and repeat
process.
[0039] FIG. 5 shows an SEM image of an exemplary device fabricated
with the system of FIG. 1, while FIG. 6 is an enlarged view of one
portion of the SEM image of FIG. 5. The device of FIG. 5 was
fabricated at a low temperature (below 400.degree. C.). At the
bottom of FIG. 5 is an oxide layer (20 nm thick). Above the oxide
layer is a metal layer, in this case a titanium layer (24 nm
thick). Above this layer is an interface layer, in this case a
platinum (Pt) interface face layer (about 5 nm). Finally, a
crystallite PCMO layer (79 nm thick) is formed at the top. Grains
in this layer can be seen extending from the bottom toward the top
with a slightly angled tilt. FIG. 6 shows a zoomed view showing the
Ti metal layer, the Pt interface layer and the PCMO grain in more
details.
[0040] Although one back-biased power supply is mentioned, a
plurality of back-bias power supplies can be used. These power
supplies can be controllable independently from each other. The
electric energies supplied can be independently controlled.
Therefore, the components of the thin film to be formed are easily
controlled in every sputtering batch process. In addition, the
composition of the thin film can be changed in the direction of the
thickness of the film by using the Facing Targets Sputtering
device.
[0041] One or more electronic devices can be formed on the wafer.
The device can be non-volatile memory such as magneto-resistive
random access memory (MRAM). Unlike conventional DRAM, which uses
electrical cells (e.g., capacitors) to store data, MRAM uses
magnetic cells. Because magnetic memory cells maintain their state
even when power is removed, MRAM possesses a distinct advantage
over electrical cells.
[0042] In one embodiment, the MRAMs formed using the above FTS has
two small magnetic layers separated by a thin insulating layer
typically make up each memory cell, forming a tiny magnetic
"sandwich." Each magnetic layer behaves like a tiny bar magnet,
with a north pole and south pole, called a magnetic "moment." The
moments of the two magnetic layers can be aligned either parallel
(north poles pointing in the same direction) or antiparallel (north
poles pointing in opposite directions) to each other. These two
states correspond to the binary states--the 1's and 0's--of the
memory. The memory writing process aligns the magnetic moments,
while the memory reading process detects the alignment. Data is
read from a memory cell by determining the orientation of the
magnetic moments in the two layers of magnetic material in the
cell. Passing a small electric current directly through the memory
cell accomplishes this: when the moments are parallel, the
resistance of the memory cell is smaller than when the moments are
not parallel. Even though there is an insulating layer between the
magnetic layers, the insulating layer is so thin that electrons can
"tunnel" through the insulating layer from one magnetic layer to
the other.
[0043] To write to an MRAM cell, currents pass through wires close
to (but not connected to) the magnetic cell. Because any current
through a wire generates a magnetic field, this field can change
the direction of the magnetic moment of the magnetic material in
the magnetic cell. The arrangement of the wires and cells is called
a cross-point architecture: the magnetic junctions are set up along
the intersection points of a grid. Word lines run in parallel on
one side of the magnetic cells. Bit lines runs on a side of the
magnetic cells opposite the word lines. The bit lines are
perpendicular to the set of word lines below. Like coordinates on a
map, choosing one particular word line and one particular bit line
uniquely specifies one of the memory cells. To write to a
particular cell (bit), a current is passed through the word line
and bit line that intersect at that particular cell. Only the cell
at the crosspoint of the word line and the bit line sees the
magnetic fields from both currents and changes state.
[0044] In one exemplary memory cell array shown in FIG. 7, word
lines for selecting rows and bit lines for selecting columns are
arranged to intersect at right angles. Memory cells are formed at
intersections, and a peripheral driver circuit for selectively
allowing information to be written into or read from the memory
cells and an amplifier circuit which for reading the information
are also formed. The peripheral circuit section includes a word
line driver circuit and bit line driver circuit and a signal
detecting circuit such as a sense amplifier, for example.
[0045] It is to be understood that various terms employed in the
description herein are interchangeable. Accordingly, the above
description of the invention is illustrative and not limiting.
Further modifications will be apparent to one of ordinary skill in
the art in light of this disclosure.
[0046] The invention has been described in terms of specific
examples which are illustrative only and are not to be construed as
limiting. The invention may be implemented in digital electronic
circuitry or in computer hardware, firmware, software, or in
combinations of them.
[0047] Apparatus of the invention for controlling the fabrication
equipment may be implemented in a computer program product tangibly
embodied in a machine-readable storage device for execution by a
computer processor; and method steps of the invention may be
performed by a computer processor executing a program to perform
functions of the invention by operating on input data and
generating output. Suitable processors include, by way of example,
both general and special purpose microprocessors. Storage devices
suitable for tangibly embodying computer program instructions
include all forms of non-volatile memory including, but not limited
to: semiconductor memory devices such as EPROM, EEPROM, and flash
devices; magnetic disks (fixed, floppy, and removable); other
magnetic media such as tape; optical media such as CD-ROM disks;
and magneto-optic devices. Any of the foregoing may be supplemented
by, or incorporated in, specially-designed application-specific
integrated circuits (ASICs) or suitably programmed field
programmable gate arrays (FPGAs).
[0048] While the preferred forms of the invention have been shown
in the drawings and described herein, the invention should not be
construed as limited to the specific forms shown and described
since variations of the preferred forms will be apparent to those
skilled in the art. Thus the scope of the invention is defined by
the following claims and their equivalents.
* * * * *