Note: Descriptions are shown in the official language in which they were submitted.
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~IGH-OXYGEN-CONTENT SILICON MONOCRYSTAL SUBSTRATE FOR
SEMICONDUCTOR DEVICES AND PRODUCTION MET~OD THEREFOR
BACKGROUND OF T~E lNV~. 1 lON
- Field of the Invention
The present invention relates generally to a
silicon monocrystal substrate capable of absorbing
significant metal contamination. In addition, the
invention relates to a method for producing a silicon
crystal substrate with a significant oxygen
concentration. In particular, the invention relates to
a method and device for producing a
high-oxygen-concentration silicon substrate by way of
crystal growth.
Description of the Background Art
Silicon substrates are widely used for
producing various semiconductor devices. In such
semiconductor devices, it is generally preferable to
minimize leak current. It is known that the leak
current can be lowered by an effect called intrinsic
gette~ing (I.G.). The I.G. effect can be achieved as a
result of defects formed in the internal structure of
the silicon substrate.
As is well known, a silicon substrate is
derived from a silicon crystal body prepared by growing
a silicon monocrystal from molten polycrystalline
silicon by the Czochralski method (hereafter referred to
as the ''CZ method''), for instance. In the CZ method,
the monocrystal silicon body is drawn slowly out of a
bath of molten polycrystal silicon. Silicon substrates
are obtained by sectioning or ''wafering'' the finished
silicon monocrystal body.
The finished silicon crystal body contains a
great amount of oxygen. The oxygen in the silicon
crystal body generates defects or crystal dislocations
such as dislocation loops, stacking faults and so forth
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due to oxygen segregation during heat treatment of the
silicon substrate. The defects in the finished
semiconductor device degrade its rated characteristics
in particular, lower its break-down voltage, and
increase its leak current. As a result, the production
yield of semiconductor devices is significantly lowered.
On the other hand, it has been found that the
defects in semiconductor devices may serve to absorb
metal contaminants by the so-called intrinsic gettering
or I.G. effect. For instance, in semiconductor devices
in which the surface of the silicon substrate is the
major active region, such as in insulated-gate
field-effect transistors (MOS-FET's) or integrated
circuits employing MOS-FET's, defects in the silicon
substrate outside of the major active region exhibit the
I.G. effect, absorbing metal contaminants from the
active regions. This helps reduce the leakage current
of the semiconductor devices.
However, there are difficulties in achieving a
consistent I.G. effect in mass production. For
instance, in cases where the silicon crystal body is
grown by the conventional CZ method, the concentration
of defects in the crystal body tends to be substantially
different at the top, corresponding to the beginning of
growth, than at the bottom, corresponding to the end of
growth, due to thermal hysteresis. Furthermore,
although a high oxygen concentration is preferable to
enhance the I.G. effect, when the oxygen concentration
is excessively high, defects tend to form even at the
surfaces of the semiconductor devices, resulting in
deterioration of the characteristics of the
semiconductor devices as set forth above. In addition,
in some semiconductor production processes, attention
must be paid to precise control of the oxygen
concentration or special I.G. treatments must be
performed in view of the heat-treatment conditions
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required for production of some semiconductor devices.
Thus, it is a continuing problem in the art of
efficiently manufacturing silicon substrates for
semiconductor devices to obtain a substantially high
concentration o oxygen, sufficient to enhance the I.G.
effect to lower leak current without generating an
adverse effect for defects in the finished semiconductor
device, especially after heat treatment.
SUMMARY OF T~E lNV~;17l ION
Therefore, it is a general object of the
invention to provide a silicon substrate and production
method therefor, which can resolve the problems set
forth above.
Another object of the invention is to provide
a silicon substrate containing a relatively high
concentration of oxygen without deterioration of its
characteristics due to oxygen segregation, dislocation
loops, stacking faults and so forth.
A further object of the present invention is
to provide a method for producing silicon substrates as
a starting material for production of semiconductor
devices, which allows high yield without causing
deterioration of the characteristics of the finished
products.
In order to accomplish the above-mentioned and
other objects, a method for producing silicon substrates
includes growing the silicon crystal body at a speed
higher than is conventionally used. It has been found
that the growth rate of the silicon crystal body exerts
a significant influence on generation of defects in the
silicon crystal body. Furthermore, according to the
present invention, the oxygen concentration in the
silicon crystal body or the silicon substrate is
significantly higher than in the conventional silicon
crystal bodies or substrates. Accelerating the growth
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of the silicon crystal body significantly suppresses
separation of the oxygen in the crystal body. This
reduces the number of defects or dislocations formed in
the crystal body during heat treatment during production
of the semiconductor devices.
In the preferred process, according to the
present invention, the growth rate of the silicon
crystal body is greater than or equal to 1.2 mm/min.
Furthermore, the preferred oxygen concentration in the
grown silicon crystal body is greater than or equal to
1.8 x 1018cm~3.
According to the invention, a silicon
substrate cont~i n i ng oxygen in concentrati~ns greater
than or equal to 18 x 1018cm~3 can achieve a leak current
less than or equal to 1 x 10-1.
Acco~ding to one aspect of the invention, a
method of producing a silicon substrate with enhanced
oxygen concentration for semiconductor devices,
comprises the steps of:
growing a silicon monocrystal from a silicon
melt at a rate of growth greater or equal to 1.2 mm/min.
with applying more heat to the surface of said silicon
melt than to the remainder thereof; and forming the
silicon substrate by cutting the silicon monocrystal.
The preferred growth rate of the silicon
monocrystal is greater than or equal to 1.2 mm/min. On
the other hand, the preferred oxygen concentration in
the silicon substrate is greater than or equal to 1.8 x
1018cm~3. Further preferably, the growth rate of the
silicon monocrystal is preferably in the range of
approximately 1.5 mm/min. to 2.1 mm/min.
In the preferred embodiment, the silicon
monocrystal growth step comprises the steps of: placing
the silicon in a crucible; heating the silicon so as to
maintain the silicon in a fIuid state by applying more
heat to the surface of said fluid state silicon than to
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the remainder thereof; and gradually drawing the silicon
monocrystal out of the silicon melt in the crucible.
In the step of heating the silicon, the heat
applied thereto is sufficient to prevent the surface of
the silicon from solidifying. Nore preferably, in the
step of heating the silicon, more heat is applied to the
surface of the silicon than to the remainder of the
silicon melt.
In an alternative embodiment, the method
further comprises the step of applying a magnetic field
to the silicon. Furthermore, the preferred method may
further comprise the step of driving the crucible to
. rotate. The rotation speed of the cruci,ble can be
controlled so as to adjust the oxygen concentration in
~5 the silicon substrate.
According to another aspect of the invention,
an apparatus for growing a silicon monocrystal with
enhanced concentration of oxygen as a source for silicon
substrates for semiconductor devices, comprises a
crucible for receiving a silicon; a heater means for
heating the silicon in a fluid state, said heater means
applies more heat to the surface of the silicon melt
than to the remainder thereof; and a drawing means for
drawing the silicon monocrystal from the silicon melt in
the crucible at a rate so as to prevent loss of oxygen
from the substrate during subsequent heat treatment in
a process of fabricating the semiconductor devices.
Preferably, the drawing rate of the silicon monocrystal
is greater than or equal to 1.2 mm/min. In addition,
the preferred oxygen concentration in the silicon
substrate is greater than or equal to 1.8 x 10l8cm~3. The
heating means applies sufficient heat to prevent the
surface of the silicon melt from solidifying. The
heating means thus applies more heat to the surface of
the silicon melt than to the i~emainder of the silicon
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melt.
The apparatus further comprises means for
applying a magnetic field to the silicon melt. In
addition, the apparatus may further comprise means for
driving the cr~cible to rotate. The crucible driving
means drives the crucible at a variable speed allowing
adjustment of the oxygen concentration in the silicon
substrate.
According to a further aspect of the
invention, a semiconductor device is produced from a
silicon substrate having an oxygen concentration greater
than or equal to 1.8 x 10 cm and having a leakage
current value less than l x 10 A.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more
fully from the detailed description given herebelow and
from the accompanying drawings of the preferred
embodiment of the invention, which, however, should not
be taken to limit the invention to the specific
embodiment but are for explanation and understanding
only.
In the drawings:
Fig. l is a cross-section through a silicon
crystal growing apparatus implementing the preferred
embodiment of the silicon crystal production method
according to the invention;
Fig. 2 is a perspective view of part of the
heating element of Fig. l;
Fig. 3 is a three-dimensional graph of the
observed relationships among the crystal growth rate,
the oxygen concentration and stacking fault density;
Fig. 4 is a graph of heat treatment time
versus oxygen concentration;
Figs. 5 and 6 show the results of measurements
of leak current of a number of sample diodes obtained by
the silicon substrate production method according to the
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invention and by a conventionally known process
respectively; and
Fig. 7 is a cross-section through a modified
embodiment of the silicon crystal growing apparatus
implementing the preferred embodiment of the silicon
crystal production process according to the invention.
DESCRIPTION OF T~E PREFERRED EMBODIMENT
Referring now to the drawings, Fig. 1 shows a
silicon monocrystal growing apparatus implementing the
preferred embodiment of a silicon substrate producing
method according to the present invention. As will be
seen from Fig. 1, the preferred embodiment of the
silicon substrate producing method includes a process
for growing a silicon monocrystal body as a starting
material for silicon substrates. According to the
preferred process, the silicon monocrystal is grown by
the CZ method.
In the monocrystal growing apparatus of the
present invention, silicon 3 in a quartz crucible 2
disposed within a graphite crucible 1 is melted. A
graphite heat generator 4 and a heat-insulating material
9 surround the crucible 1. Additional plural cooling
jackets 10a, 10b and 10c surround the insulating
material 9. The cooling jacket 10b has a window 12 for
allowing observation of the drawn monocrystal 6. An
exhaust pipe 13 is provided in the floor of the cooling
jacket 10b for exhausting inert gas serving as an
atmosphere introduced into the jackets 10a, 10b and 10c
from above. A shaft 8 fixed to the lower surface of the
crucible 1 passes loosely through an aperture 10d in the
floor of the cooling jacket 10a and is used to rotate
and lift or lower the crucible 1. The lower edge of the
heat generator 4 is fixed to a ring plate 14, which in
turn is fixed to a pair of shafts 15 passing loosely
through two apertures 10e and 10f in the floor of the
cooling jacket 10a. The shafts 15 are used to lift or
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lower the heat generator 4. A molybdenum cylindrical
heat shield 16 with an inner diameter slightly larger
than an outer diameter of the monocrystal 6 is disposed
above the liquid silicon 3 and around the monocrystal 6.
Within the heat shield 16, a seed crystal 5 is held by a
chuck 7 attached to the lower end of a draw shaft 17 so
that a cylindrical monocrystal 6 may be grown from the
seed crystal 5.
In the CZ method, the maximum monocrystal
growth rate Vmax can be expressed as follows, assuming
that the solid-liquid interface between the monocrystal
6 and the liquid 3 is flat and no radial temperature
gradient exists in the monocrystal 6:
_ k dT
V - . (--)
where k denotes the thermal conductivity of the
monocrystal 6, h denotes the heat of solidification, ~
denotes the density, and dT/dX denotes the temperature
gradient in the solid phase of the monocrystal 6 at the
solid-liquid interface. Specifically, X denotes
distance along the longitudinal axis of the monocrystal
6. In the above expression, since k, h, and e are
inherent properties of the material, it is thus
necessary to increase the temperature gradient dT/dX in
order to increase or obtain a large maximum monocrystal
growth rate V . In the above-mentioned CZ method,
however, since the monocrystal 6 is heated by radiation
from the surface of the liquid 3, the inner wall of the
crucible 2, and the heat generator 4, the value of the
temperature gradient dT/dX is inevitably limited, so
that the growth rate has always been relatively small in
practice.
As will be appreciated from the above
discussion, the growth rate of the silicon monocrystal
can be accelerated by reducing the heat applied to the
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molten silicon 3 by the heat generator 4 and thus
lowering the temperature of the molten silicon.
Although this has a direct proportional effect toward
lowering the thermal gradient, by the Stefan-Boltzmann
law, the heat r~diated toward the monocrystal is reduced
to a much greater extent, so that the net effect is an
increase in the temperature gradient dT/dX. However,
reducing the heat generated by the heat generator 4 in
order to obtain a higher growth rate means that the
surface of the molten silicon will tend to solidify
since the surface of the molten silicon is cooled by
exposure to the gaseous furnace atmosphere. This limits
the extent to which the temperature of the molten
silicon 3 can be lowered.
The heat generator 4 of the preferred silicon
monocrystal growing apparatus is designed to apply
enough heat to the surface of the molten silicon 3 to
maintain the silicon in the liquid state. In
particular, the heat generator 4 of the preferred
construction is designed to apply more heat to the
surface of the molten silicon than to the remaining body
of molten silicon so as to allow the temperature of the
molten silicon 3 to be minimized.
Fig. 2 shows the structure of the heat
generator 4. The heat generator 4 is made of a
conductive material such as graphite, and is generally
in the form of a cylindrical sleeve with a tapered
portion 4a at its upper end. The heat generator 4 is
formed with alternating upper grooves 4b and lower
grooves 4c, each extending parallel to the vertical axis
of the heat generator 4. This construction provides the
cylindrical shell with a serpentine configuration
suitable for use as an electrical heating element. In
addition, the upper ends of the lower grooves 4c are
angularly bifurcated to form two short grooves 4d and 4e
extending at an angle of 45 with respect to the groove
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4c. Current passes through each section defined by
adjoining upper and lower grooves 4b and 4c and
generates heat due ~o o'~ loss.
In order to grow the monocrystal 6 with the
seed crystal 5 from the melted silicon material by means
of the monocrystal growing apparatus constructed as
described, the two crucibles 1 and 2 are rotated in a
clockwise direction by the shaft 8 and the grown
monocrystal 6 is rotated by the shaft 17 in a
counterclockwise direction, for instance, or vice versa.
At the same time, the draw shaft 17 is lifted gradually
by means of a driving mechanism (not shown) to draw the
monocrystal 6 out of the melt. Additionally, the two
crucibles 1 and 2 are both raised gradually so that the
surface of the liquid 3 can be kept at a predetermined
position with respect to the heat generator 4.
The apparatus described above has the
following advantages: the upper end 4a of the heat
generator 4 is tapered and, in addition, the bifurcated
grooves 4d and 4e are formed at the upper ends of the
lower grooves 4c, and the cross-sectional area of the
tapered portion 4a is smaller than the rest of the heat
generator 4. In particular, the cross-sectional area
near the bifurcated grooves 4d and 4e is quite small.
Therefore, as current passes through the heat generator
4, the tapered portion 4a of the heat generator 4 is
heated to a higher temperature than other portions of
the heat generator 4. As a result, the difference in
temperature between the melt 3a located vertically
opposite the tapered portion 4a and at the inner wall of
the crucible 2, and the maximum value within the body of
the melt 3 is small.
Furthermore, since the tapered portion 4a
increases the total electrical resistance of the heat
generator 4 relative to conventional models, the
temperature of the heat generator 4 will be higher,
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assuming equal amounts of current. Therefore, in this
embodiment, the current through the heat generator 4 can
be smaller than that in conventional heaters of similar
design.
As explained, it is necessary to increase the
temperature gradient (dT/dX) within the solid-phase
monocrystal 6 at the solid-liquid interface in order to
increase the maximum growing speed Vmax. Accordingly,
it would be preferable to reduce the heat output of the
heat generator 4, because the monocrystal is heated by
radiation from the heat generator 4.
In the apparatus according to the present
invention, even if the heat output of the heat generator
4 is reduced in order to increase the temperature
gradient (dT/dX), since the above-mentioned maximum
difference in temperature between the surface 3a and the
body of the melt 3 is small, it is possible to prevent
the surface of the melt 3 from solidifying at the inner
wall of the crucible 2. As a result, it is possible to
markedly increase the growth rate by as much as
0.2 mm/min, for instance, over-conventional systems.
Additionally, it is possible to grow the monocrystal 6
continuously, thus increasing productivity and
decreasing the cost of monocrystal manufacture.
The preferred embodiment of the method for
producing or manufacture the silicon substrate according
to the present invention employs the apparatus set forth
above. It has been found in the present invention that
the crystal growth rate exerts a great influence upon
the generation of crystal defects, especially stacking
faults. Therefore, in the present invention, the
crystal growth rate is set to a value higher than 1.2
mm/min in order to obtain a silicon crystal body with an
oxygen conlcentration of more than 1.8 x lO 8 cm 3.
Silicon substrates are thus manufactured by wafering
this silicon monocrystal body. Setting the silicon
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monocrystal growth rate higher that in the conventional
systems prevents segregation of oxygen in succeeding
heat treatments and so prevents the accompanying loss of
grown silicon monocrystal quality. Therefore, it is
possible to increase the oxygen concentration. In the
present invention, oxygen concentrations of 1.8 x 10
cm or more, can be achieved and thereby it is possible
to obtain an enhanced I.G. effect.
The following discussion is directed to the
finished silicon substrate produced by the preferred
method accroding to the invention utilizing the
apparatus of Figs. 1 and 2.
A silicon monocrystal body was drawn and grown
by the CZ method. A wafer was cut from the monocrystal
body. The surfaces of the wafer were mirror-polished,
and then twice subjected to heat treatment at a
temperature of 1100C for 2 hours within a dry oxygen
atmosphere. Thereafter, the wafer was etched to a depth
of 13 ~m by the so-called dry etching method to reveal
faults. In order to perform this test, various samples
were obtained by varying the growth rate of the silicon
monocrystal body in the CZ process. Also various
samples were obtained at various oxygen concentrations.
The density of stacking faults in these samples was
measured. The results of these measurements are shown
in Fig. 3.
The results shown in Fig. 3 indicate that
essentially no stacking faults are formed if the silicon
monocrystal growth rate is greater than or equal to 1.2
mm/min. Furthermore, it was also confirmed that no
stacking faults are generated during treatment of the
silicon wafer or silicon substrate, including surface
polishing.
In addition, the changes in oxygen
concentration due to heat treatment at 750 C were
measured. Fig. 4 shows the results of these
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measurements in the form of a curve relating oxygen
concentration to heat treatment time. In the drawing,
the curves 21 to 23 represent the relationship between
oxygen concentration and heat treatment time at a
crystal growth rate of greater than 1.2 mm/min. The
initial oxygen concentrations for curves 21 to 26 were
1.644 x 10 cm , 1.667 x 1ol8 cm 3, 1.709 x 1ol8 cm 3,
1,866 x 10 cm 3, 2.019 x 1018 cm 3, 2.019 x 1018 cm 3,
and 1.737 x 10 cm , respectively. Although the
oxygen concentration eventually drops as oxygen is
driven out of the silicon substrate or silicon
monocrystal body by the heat treatment, it is clear that
in the case of the high initial oxygen concentrations
due to the present invention, represented by the curves
24 to 26, the change is small even after a relatively
long heat treatment and measurable oxygen loss occurs
only after a very long time.
As will be appreciated from Figs. 3 and 4, it
is clear that high-speed crystal growth results in fewer
faults.
In the next test, diodes were prepared by
forming a n -P junction on silicon substrates obtained
by the present invention and the conventional method,
and the p-n junction leak current was measured for each
diode. In this case, a p-type region was formed on the
n-type silicon substrate, and n regions having an area
of 2.4 x 10 1 cm piece/cm were formed. The
measurement was performed by applying a testing voltage
of + 5V to the n region. The results of tests on
silicon substrates formed from a silicon monocrystal
body grown by the CZ method at a crystal growth rate
greater than or equal to 1.2 mm/min and which have an
oxygen concentration of 2.0 x 10 cm , are shown in
Fig. 5. On the other hand, Fig. 6 shows the result of
the tests performed on silicon substrates formed from a
silicon monocrystal body grown by the conventional
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silicon monocrystal growing method at a crystal growth
rate of 0.6 to 0.9 mm/min. In Figs. 5 and 6, the
abscissa is the measured leak current and the ordinate
is the number of samples exhibiting the indicated leak
current. As c~n be understood by a comparison of Figs.
5 and 6, in the case of silicon substrates manufactured
by the present invention, the leak current is reliably
decreased to 10 A or less. This may be due to the
pronounced I.G. effect produced by the high oxygen
concentration.
It should be appreciated that the preferred
method according to the present invention can provide a
high-oxygen-concentration silicon monocrystal body. It
is also possible to accurately select the oxygen
concentration from a wide range by applying a crystal
growth method in which a magnetic field is applied to a
silicon melt in a quartz crucible and the crucible is
rotated as necessary. An example of this crystal growth
method employing a magnetic field will be explained with
reference to Fig. 7.
In the drawing, the entire apparatus is
designated generally by the reference numeral 31. A
quartz crucible 32 retains molten silicon from which a
crystal is grown. The crucible 32 is rotated about its
central axis at an adjustable rotational speed. A
heating means 34 surrounds the crucible 32. The heating
means 34 may be a cylindrical electric heater 35 similar
to the heater 4 of the previous embodiment. A
cylindrical heat insulating body, or a jacket 36 cooled
by water, as necessary, is provided outwardly of the
heating means. A direct current magnetic field
generating means 37 made up of a permanent magnet or an
electromagnet is located outwardly of the jacket 36. A
silicon monocrystal seed is designated by the reference
numeral 38 while a drawing chuck is shown at reference
numeral 39. The drawing chuck 39 raises the silicon
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monocrystal seed 38 while rotating the seed about the
rotational axis of the crucible.
The electrical power supply to the heating
means 34 is dc current with 4% or less ripple or a 1 kHz
or higher alternating or pulsating current. This type
of current has been proven adequate to prevent
unnecessary resonance between the heating means 34 and
the magnetic field.
The monocrystalline silicon seed 38 is drawn
away from the molten silicon surface at a predetermined
speed so as to induce growth of a silicon monocrystal
40. In this case, varying the rotational speed of the
crucible 32 in particular also changes the oxygen
concentration in the finished crystal 40. This is due
to the following reason. The molten silicon in the
crucible has an effective viscosity enhanced by
application of a magnetic field. Since the silicon is
rotated relative to the crucible rotation, frictional
contact between the molten silicon 3 and the inner walls
of the crucible 32 results. Therefore, oxygen in the
walls of the crucible 32, specifically of the quartz, is
dissolved in the molten silicon 33. The oxygen
concentration in the growing crystal 40 thus increases
because the dissolved oxygen increases with increasing
frictional contact, that is, with increasing rotational
speed of the crucible relative to the molten silicon 33.
Moreover, it has been confirmed that a higher oxygen
concentration in the crystal can be achieved, if the
rotational speed of the crucible is sufficiently high,
when a magnetic field is applied than when no magnetic
field is applied.
As described above, since it is possible to
maintain a high oxygen concentration, the present
invention has many advantages. For instance, the
effects of thermal hysteresis as the crystal body is
being pulled can be essentially eliminated. Since the
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oxygen concentration is high, an extremely high I.G.
effect can be obtained in heat treatment. In addition,
crystal faults in the substrate surface can be
suppressed. Because of these advantages, in
semiconductor elements formed on the silicon substrate,
many significant advantages are achieved, such as the
lowering of the leak current, the improvement of
breakdown voltage, increased uniformity of
characteristics, the improvement of product yield, and
so forth.
While the present invention has been disclosed
in terms of the preferred embodiment in order to
facilitate a better understanding of the invention, it
should be appreciated that the invention can be embodied
in various ways without departing from the principle of
the invention. Therefore, the invention should be
understood to include all possible embodiments and
modifications to the shown embodiments which can be
embodied without departing from the principle of the
invention set out in the appended claims.
