U.S. patent application number 13/550789 was filed with the patent office on 2013-04-11 for method for forming metal film with twins.
The applicant listed for this patent is Tsung-Cheng CHAN, Yu-Lun CHUEH, Chien-Neng LIAO. Invention is credited to Tsung-Cheng CHAN, Yu-Lun CHUEH, Chien-Neng LIAO.
Application Number | 20130089674 13/550789 |
Document ID | / |
Family ID | 48042258 |
Filed Date | 2013-04-11 |
United States Patent
Application |
20130089674 |
Kind Code |
A1 |
CHUEH; Yu-Lun ; et
al. |
April 11, 2013 |
METHOD FOR FORMING METAL FILM WITH TWINS
Abstract
A method for forming a metal film with twins is disclosed. The
method includes: (a) forming a metal film over a substrate, the
metal film being made of a material having one of a face-centered
cubic crystal structure and a hexagonal close-packed crystal
structure; and (b) ion bombarding the metal film at a film
temperature lower than -20.degree. C. in a vacuum chamber and with
an ion-bombarding energy sufficient to cause plastic deformation of
the metal film to generate deformation twins in the metal film.
Inventors: |
CHUEH; Yu-Lun; (Hsinchu
City, TW) ; CHAN; Tsung-Cheng; (Hsinchu City, TW)
; LIAO; Chien-Neng; (Hsinchu City, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CHUEH; Yu-Lun
CHAN; Tsung-Cheng
LIAO; Chien-Neng |
Hsinchu City
Hsinchu City
Hsinchu City |
|
TW
TW
TW |
|
|
Family ID: |
48042258 |
Appl. No.: |
13/550789 |
Filed: |
July 17, 2012 |
Current U.S.
Class: |
427/533 |
Current CPC
Class: |
C25D 5/10 20130101; C25D
5/48 20130101; C25D 17/001 20130101; H01L 2924/0002 20130101; H01L
23/53228 20130101; H01L 21/76838 20130101; H01L 2924/00 20130101;
C25D 7/12 20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
427/533 |
International
Class: |
B05D 5/00 20060101
B05D005/00; B05D 3/06 20060101 B05D003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 11, 2011 |
TW |
100136726 |
Claims
1. A method for forming a metal film with twins, comprising: (a)
forming a metal film over a substrate, the metal film being made of
a material having one of a face-centered cubic crystal structure
and a hexagonal close-packed crystal structure; and (b) ion
bombarding the metal film at a film temperature lower than
-20.degree. C. in a vacuum chamber and with an ion-bombarding
energy sufficient to cause plastic deformation of the metal film to
generate deformation twins in the metal film.
2. The method of claim 1, wherein the metal film in step (a) is
made of a material selected from the group consisting of copper,
nickel, silver and gold.
3. The method of claim 2, wherein the metal film in step (a) is
made of copper.
4. The method of claim 3, wherein the film temperature ranges from
-25.degree. C. to -125.degree. C.
5. The method of claim 3, wherein the ion-bombarding energy ranges
from 4.0 KeV to 5.0 KeV.
6. The method of claim 3, wherein the deformation twins formed in
step (b) have an average twin spacing that ranges from 8.3 nm to
45.6 nm.
7. The method of claim 1, wherein the film temperature is
controlled using liquid nitrogen.
8. The method of claim 7, wherein the liquid nitrogen is introduced
to the vacuum chamber through a metal tube, the metal tube being in
contact with the metal film and having a thermal conductivity
greater than 270 W/mK.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority of Taiwanese application
No. 100136726, filed on Oct. 11, 2011.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method for forming a metal film,
and more particularly to a method for forming a metal film with
twins.
[0004] 2. Description of the Related Art
[0005] It is well known to persons having ordinary skill in the art
of metallurgy that, after a metal material is applied with a shear
stress through a cold working procedure, line defects in the metal
material, such as dislocations, may be moved by virtue of slip
system in the metal material itself. The moved dislocations may
become entangled with one another. The application of cold working
also increases planar defects in the metal material, such as twins.
The tangled dislocations and the planar defects enhance mechanical
strength of the metal material, such as hardness. However, the
abovementioned cold working procedure is limited for application
only to bulks, and is not suitable for a thin metal film.
[0006] In recent years, to enhance the strength of a metal film in
order to expand the applicability of the metal film, growth twins
have attracted much attention in the field of thin film processing.
It has been recognized that the smaller the twin spacing of growth
twins, the greater the mechanical strength will be. The theory and
mechanism behind this result are similar to those of grain size
strengthening. That is, fineness of grains facilitates enhancement
of the mechanical strength of the metal material.
[0007] In an article entitled "High-strength sputter-deposited
copper foils with preferred orientation of nanoscale growth twins"
in APPLIED PHYSICS LETTERS 88, 173116 (2006), X. Zang et al
disclosed magnetron sputtering copper at a deposition rate of 0.5
nm/s to 2.0 nm/s to produce a 20 .mu.m copper foil on a Si (100)
substrate with a native oxide layer. No heating or cooling was
applied to the Si substrate during the formation of the copper
foil.
[0008] Analytic results of the copper foil using transmission
electron microscope (TEM) indicate that the copper foil
sputter-deposited at a rate of 1.8 nm/s has columnar grains with an
average size of 43 nm. The TEM results also show an extremely high
density of planar defects within the columnar grains. Also,
analytic results of the copper foil using high resolution
transmission electron microscope (HRTEM) indicate that the planar
defects are growth twins with {111} interfaces. The spacing between
two adjacent twins is approximately 5 nm, and the planar defects of
{111} twin interfaces are stacked along the growth direction of the
copper foil. This confirms that the planar defects are stacking
faults (SF) within the copper foil. In other words, the twins in
the copper foil resulted from the stacking faults of the
close-packed plane of the copper foil.
[0009] In addition, five uniaxial tensile tests were performed on
the copper foils . The results indicate that the average elastic
modulus, the average tensile strength, and the average yield
strength of the copper foils are approximately 110 GPa, 1.2 GPa and
1.1 GPa, respectively. Further, the hardness of the copper foil, as
measured by a nanoindenter, was 3.5 GPa.
[0010] In an article entitled "Microstructural stability during
cyclic loading of multilayer copper/copper samples with nanoscale
twinning" in Scripta Materialia (2009) 1073-1077, C. J. Shute et al
disclosed sputtering of copper using magnetron sputtering
deposition technique to produce a 178 .mu.m copper multilayer film
on a Si (100) substrate. The thickness of the copper multilayer
film is sufficient to allow the copper multilayer film to be
removed from the Si (100) substrate.
[0011] The copper multilayer film has a mirror-like surface that is
in contact with the Si (100) substrate, and a dull surface that is
opposite to the mirror-like surface. A Vickers hardness measurement
was conducted on the copper multilayer film. The result indicates
that the hardness of the mirror-like surface and the dull surface
are 1.1 GPa and 1.9 GPa, respectively. In addition, an analysis of
the copper multilayer film was performed by means of a focus ion
beam (FIB) microscope. The results indicate that the structure of
the copper multilayer film changes from a non-columnar
microstructure to a nanotwinned columnar microstructure from the
mirror-like surface to the dull surface.
[0012] From the above, it is found that, when a metal film with
twins are formed using magnetron sputtering deposition technique, a
predetermined deposition thickness is required to convert the
microstructure of the metal film into a twin structure. For a
copper wire that is widely used in a semiconductor device, the
thickness of the copper wire is required to be in the range from
about 300 nm to about 400 nm. Therefore, the methods for forming a
metal film with twins disclosed by C. J. Shute et al and C. Zang et
al are not suitable for the nano-scale copper wire that is used in
the semiconductor field.
SUMMARY OF THE INVENTION
[0013] Therefore, the object of the present invention is to provide
a method for forming a metal film with twins, that is suitable for
use in the semiconductor field.
[0014] According to this invention, a method for forming a metal
film with twins comprises:
[0015] (a) forming a metal film over a substrate, the metal film
being made of a material having one of a face-centered cubic
crystal structure and a hexagonal close-packed crystal structure;
and
[0016] (b) ion bombarding the metal film at a film temperature
lower than -20.degree. C. in a vacuum chamber and with an
ion-bombarding energy sufficient to cause plastic deformation of
the metal film to generate deformation twins in the metal film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Other features and advantages of the present invention will
become apparent in the following detailed description of the
preferred embodiment of the invention, with reference to the
accompanying drawings, in which:
[0018] FIG. 1 shows consecutive steps of the preferred embodiment
of a method for forming a metal film with twins according to the
present invention;
[0019] FIG. 2 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in
Comparative example 1 (CE1);
[0020] FIG. 3 is a diagram illustrating an average twin spacing
distribution of the first copper layer in CE1;
[0021] FIG. 4 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in
Comparative example 2 (CE2);
[0022] FIG. 5 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in Example 1
(E1) of the preferred embodiment of a method for forming a metal
film with twins according to the present invention;
[0023] FIG. 6 is a diagram illustrating an average twin spacing
distribution of the first copper layer in E1;
[0024] FIG. 7 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in Example 2
(E2) of a method for forming a metal film with twins according to
the present invention;
[0025] FIG. 8 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in Example 3
(E3) of a method for forming a metal film with twins according to
the present invention;
[0026] FIG. 9 is a diagram illustrating an average twin spacing
distribution of the first copper layer in E3 of the present
invention; and
[0027] FIG. 10 is a transmission electron microscope image
illustrating a microstructure of a first copper layer in Example 4
(E4) of a method for forming a metal film with twins according to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0028] Referring to FIG. 1, the preferred embodiment of a method
for forming a metal film with twins according to the present
invention comprises:
[0029] (a) forming a metal film 3 over a substrate 2, the metal
film 3 being made of a material having one of a face-centered cubic
crystal structure and a hexagonal close-packed crystal structure;
and
[0030] (b) ion bombarding the metal film 3 at a film temperature
lower than -20.degree. C. in a vacuum chamber 4 and with an
ion-bombarding energy sufficient to cause plastic deformation of
the metal film 3 to generate deformation twins in the metal film
3.
[0031] The ion-bombarding energy is sufficient to provide the metal
film 3 with a strain energy that is higher than a stacking fault
energy of the metal film 3 such that the metal film 3 undergoes
plastic deformation to generate deformation twins.
[0032] The ion bombarding of this embodiment is achieved by
bombarding the metal film 3 with the ion beams generated by two ion
guns 41 that are disposed respectively in the vacuum chamber 4,
thereby providing stress to the metal film 3. In addition, since
the face-centered cubic crystal structure and the hexagonal
close-packed crystal structure have both close-packed planes, once
an error of the stacking sequence (i.e., the stacking fault) of the
close-packed planes in the face-centered cubic crystal structure
and the hexagonal close-packed crystal structure occurs, a twin
structure may be easily generated. Therefore, in the method of the
present invention, by virtue of provision of an ion-bombarding
energy that is sufficient to provide the metal film 3 with a strain
energy higher than the stacking fault energy of the metal film 3,
the metal film 3 can undergo plastic deformation. Since the strain
energy is higher than the stacking fault energy, the plastic
deformed metal film 3 that undergoes such plastic deformation
releases the overall energy of its material system by virtue of the
formation of the stacking faults in order to reduce the overall
energy of the material system itself, thereby forming deformation
twins in the metal film 3. It is noted that the overall energy of
the material system of the metal film 3 undergoing plastic
deformation may also be released by virtue of dislocation slip and
dislocation climb. However, if the overall energy of the material
system of the metal film 3 undergoing plastic deformation is
released by virtue of the dislocation slip and dislocation climb,
the probability of the generation of stacking faults would be
correspondingly reduced. Further, it is noted that, when the metal
film 3 undergoing plastic deformation is at a low temperature, the
dislocation slip and dislocation climb can be effectively
suppressed. Therefore, to prevent generation of an elevated
temperature of the metal film 3 when the ion bombarding is
performed, which results in an increase in the probability of
dislocation slip and dislocation climb that reduce the probability
of stacking faults, the film temperature (i.e., the temperature of
the metal film 3) in the step (b) of the method of the present
invention is controlled at a temperature lower than -20.degree.
C.
[0033] Preferably, the metal film in the step (a) is made of a
material selected from the group consisting of copper, nickel,
silver and gold. In an example of the present invention, the metal
film in the step (a) is made of copper, and the deformation twins
formed in the step (b) have an average twin spacing that ranges
from 8.3 nm to 45.6 nm.
[0034] Moreover, to ensure the deformation twins can be distributed
uniformly in the metal film 3 and the average spacing of the
deformation twins can be reduced, in the case where the metal film
3 is made of copper, preferably, the film temperature in the step
(b) ranges from -25.degree. C. to -125.degree. C. and the
ion-bombarding energy in the step (b) ranges from 4.0 KeV to 5.0
KeV. However, since different metals have different stacking fault
energies, the ion-bombarding energy in step (b) should be dependent
on the material of the metal film 3, and thus should not be limited
in the range of 4.0 KeV to 5.0 KeV. For example, since the stacking
fault energy of aluminum is higher than that of copper, when the
metal film 3 is made of aluminum, the ion-bombarding energy in the
step (b) may be higher than 5.0 KeV to ensure the deformation twins
of the metal film 3 can be distributed uniformly.
[0035] Preferably, the film temperature is controlled using liquid
nitrogen.
[0036] Preferably, the liquid nitrogen is contained in a Dewar
chamber 5 and is introduced to the vacuum chamber 4 through a metal
tube 6. The metal tube 6 is cooled as the liquid nitrogen passes
therethrough and the cooled metal tube 6 is arranged so that the
metal film 3 and the substrate 2 are placed thereon by virtue of a
metal clamp 61 disposed on the metal tube 6. Thus, the metal film 3
is in contact with and is fixed on the metal tube 6, thereby
cooling the metal film 3 to the desired film temperature.
Alternatively, the metal film 3 may also be cooled to the film
temperature by virtue of allowing the liquid nitrogen to flow to
the metal film 3 through a plurality of through holes 62 formed in
a metal wall of the metal tube 6. Preferably, the metal tube 6 and
the metal clamp 61 are made from a metal material have a thermal
conductivity greater than 270 W/mK. More preferably, the metal tube
6 and the metal clamp 61 have a thermal conductivity greater than
400 W/mK, which may be, for example, made of pure copper having a
thermal conductivity of about 401 W/mK.
[0037] It is noted that, when performing in step (b) of the method
of the present invention, the temperature at the Dewar chamber 5
and the temperature at the metal film 3 (i.e., film temperature)
are separately measured, and the temperature of the Dewar chamber 5
is lower than that of the metal film 3 by about 50.degree. C.
[0038] Further, it is noted that, when the ion bombarding is
performed, the metal atoms on the surface of the metal film 3 will
be sputtered out, thereby resulting in a decrease in the thickness
of the metal film 3 as the time for ion bombarding is increased.
Therefore, preferably, the metal film 3 in the step (a) has a first
predetermined thickness that is large enough to leave a second
predetermined thickness after the metal film 3 in the step (a) is
subjected to the ion bombarding in the step (b).
[0039] It should be noted that the first predetermined thickness of
the metal film 3 before the step (b) is performed and the second
predetermined thickness of the metal film 3 after the step (b) is
performed are determined based on the subsequent applications
thereof. For example, for forming a copper wire, a copper seed
layer may be previously formed on a device surface. Further, a
copper layer with a 1 .mu.m thickness (i.e., the first
predetermined thickness) is formed on the copper seed layer by
electrical plating. Next, the copper layer is ion bombarded to
generate deformation twins in the copper layer, followed by
chemical mechanical polishing (CMP) the copper layer. Finally, the
polished copper layer is patterned to produce a copper wire. The
thickness of the copper wire is required to be about 300 nm to
about 400 nm. Therefore, when the method of the present invention
is applied to a copper wire, the first predetermined thickness
should be greater than 400 nm. However, the first predetermined
thickness and the second predetermined thickness are not the
technical features of the present invention and may vary depending
on the subsequent application.
[0040] Also, it is noted that, to prevent the film temperature of
the metal film 3 from increasing too fast as the ion bombarding is
performed, which results in an increase in the probability of
dislocation slip and dislocation climb in the metal film 3, the
sputtering rate is controlled to decrease as the film temperature
in the step (b) increases, and the ion bombarding is performed
intermittently.
COMPARATIVE EXAMPLE 1 (CE1)
[0041] A method for forming a metal film with twins was performed
as follows.
[0042] First, a first copper layer with about 8 .mu.m thickness and
a second copper layer with about 20 .mu.m thickness were deposited
respectively on a first silicon oxide substrate and a second
silicon oxide substrate by electrical plating. Next, the copper
layers were placed in a vacuum chamber and then liquid nitrogen
contained in a Dewar chamber was introduced to the vacuum chamber
through a Cu tube with about 400 W/mK thermal conductivity. The Cu
tube was connected to the Dewar chamber and the copper layers were
disposed on the Cu tube. At this time, the copper layers were ion
bombarded using an ion-bombarding energy of 5.0 KeV. In Comparative
example 1 (CE1), the temperature of the Dewar chamber and the
temperature of the copper layers (i.e., the film temperature) were
25.degree. C. and 75.degree. C., respectively. The sputtering rate
of the ion bombarding was about 50 nm/min.
[0043] The first and second copper layers were subsequently used as
the samples for analysis using transmission electron microscopy
(TEM) and for hardness testing. To perform the transmission
electron microscopy analysis for observing the twins structure, the
first copper layer on the first silicon oxide substrate was
previously removed from the first silicon oxide substrate before
the ion bombarding was performed. The first copper layer was simply
placed in the vacuum chamber and was then ion bombarded to reduce
the thickness thereof to become transparent. To perform the
hardness test by virtue of a nanoindenter and avoid the substrate
effect resulting from the hardness test by the nanoindenter, the
second copper layer was placed in the vacuum chamber together with
the second silicon oxide substrate when the ion bombarding was
performed, and the residual thickness of the second copper layer is
10000 nm after the ion bombarding was performed.
COMPARATIVE EXAMPLE 2 (CE2)
[0044] The conditions for performing the ion-bombarding procedure
in Comparative example 2 (CE2) were generally the same as those in
Comparative example 1 (CE1). Comparative example 2 (CE2) and
comparative example 1 (CE1) are different in that, in CE2, the
temperature of the Dewar chamber was -25.degree. C., the
temperatures of the first copper layer and the second copper layer
(i.e., the film temperatures) were 25.degree. C.; and the
sputtering rate of the ion bombarding was about 60 nm/min.
EXAMPLE 1 (E1)
[0045] The conditions for performing the ion-bombarding procedure
in Example 1 (E1) of a method for forming a metal film with twins
according to the present invention were generally the same as those
in Comparative example 1 (CE1). Example 1 (E1) and Comparative
example 1 (CE1) are different in that, in E1, the temperature of
the Dewar chamber was -75.degree. C.; the film temperature for each
of the first and second copper layers was -25.degree. C.; and the
sputtering rate of the ion bombarding was about 85 nm/min.
EXAMPLE 2 (E2)
[0046] The conditions for performing the ion-bombarding procedure
in Example 2 (E2) were generally the same as those in Comparative
example 1 (CE1). Example 2 (E2) and Comparative example 1 (CE1) are
different in that, in E2, the temperature of the Dewar chamber was
-125.degree. C.; the film temperature of each of the first and
second copper layers was -75.degree. C.; and the sputtering rate of
the ion bombarding was about 90 nm/min.
EXAMPLE 3 (E3)
[0047] The conditions for performing the ion-bombarding procedure
in Example 3 (E3) were generally the same as those in Comparative
example 1 (CE1). Example 3 (E3) and Comparative example 1 (CE1) are
different in that, in E3, the temperature of the Dewar chamber was
-175.degree. C.; the film temperature of each of the first and
second copper layers was -125.degree. C.; and the sputtering rate
of the ion bombarding was about 100 nm/min.
EXAMPLE 4 (E4)
[0048] The conditions for performing the ion-bombarding procedure
in Example 4 (E4) were generally the same as those in Example 3
(E3). Example 4 (E4) and Example 3
[0049] (E3) are different in that, in E4, the sputtering rate of
the ion-bombarding energy was 4 KeV.
[0050] <Analysis>
[0051] It can be seen from the transmission electron microscopy
image shown in FIG. 2 that the twin spacing of the deformation
twins in the first copper layer of CE1 at a film temperature of
75.degree. C. and with an ion-bombarding energy of 5 KeV is about
45 to 90 nm, and the deformation twins in CE1 are not distributed
uniformly.
[0052] Statistical measurement of the twin spacing of the first
copper layer in CE1 was performed by selecting randomly 200 copper
grains from the transmission electron microscopy image, and
calculating the average twin spacing of the 200 copper grains. It
can be seen from the twin spacing distribution diagram of FIG. 3
that the deformation twins in the first copper layer of CE1 have an
average twin spacing of 54.4 nm, and the standard deviation is
.+-.26.2 nm.
[0053] It can be seen from the transmission electron microscopy
image shown in FIG. 4 that the twin spacing of the deformation
twins in the first copper layer of CE2 at a film temperature of
25.degree. C. and with an ion-bombarding energy of 5 KeV is about
45 nm, and the deformation twins are not distributed uniformly.
[0054] It can be seen from the transmission electron microscopy
image shown in FIG. 5 that the twin spacing of the deformation
twins in the first copper layer of E1 at a film temperature of
-25.degree. C. and with an ion-bombarding energy of 5 KeV is about
35 nm.
[0055] It can be seen from the twin spacing distribution diagram of
FIG. 6 that the deformation twins in the first copper layer of E1
have an average twin spacing of 33.4 nm, and the standard deviation
is .+-.12.2 nm.
[0056] It can be seen from the transmission electron microscopy
image shown in FIG. 7 that the twin spacing of the deformation
twins in the first copper layer of E2 at a film temperature of
-75.degree. C. and with an ion-bombarding energy of 5 KeV is about
30 nm.
[0057] It can be seen from the transmission electron microscopy
image shown in FIG. 8 that the twin spacing of the deformation
twins in the first copper layer of E3 at a film temperature of
-125.degree. C. and with an ion-bombarding energy of 5 KeV is about
25 nm.
[0058] It can be seen from the twin spacing distribution diagram of
FIG. 9 that the deformation twins in the first copper layer of E3
have an average twin spacing of 17.5 nm, and the standard deviation
is .+-.9.2 nm.
[0059] It can be seen from the transmission electron microscopy
image shown in FIG. 10 that the twin spacing of the deformation
twins in the first copper layer of E4 at a film temperature of
-125.degree. C. and with an ion-bombarding energy of 4 KeV is about
35 nm.
[0060] <Hardness Testing>
[0061] Hardness for each of the second copper layers in CE1, CE2,
and E1 to E4 was measured using a nanoindenter. Hardness testing
was performed by pressing a probe of the nanoindenter against a
surface of the copper layer at a loading rate of 1 mN/sec to make
an indentation, and, when the depth of the indentation reaches 100
nm, retracting the probe at an unloading rate of 1 mN/sec. The
results, the process parameters, and the twin spacing are
collectively shown in the following Table 1.
TABLE-US-00001 TABLE 1 Average Film Ion-bombarding twin Example
temperature energy spacing Hardness No. (.degree. C.) (KeV) (nm)
(GPa) CE1 75 5 54.4 .+-. 26.6 2.0 CE2 25 5 -- -- E1 -25 5 33.4 .+-.
12.2 2.8 E2 -75 5 -- -- E3 -125 5 17.5 .+-. 9.2 3.6 E4 -125 4 --
3.4
[0062] It can be seen from Table 1 that when the film temperature
decreases, the average twin spacing of the deformation twins of the
first copper layer decreases while the hardness of the second
copper layer is increased. For a copper wire used in the
semiconductor field, such high hardness is sufficient to prevent
undesirable electromigration phenomenon, thereby enhancing the
reliability of a semiconductor device.
[0063] By virtue of ion bombarding and adjusting the film
temperature to control the twin spacing of the copper layers, a
copper film with twins having a thickness smaller than 500 nm can
be obtained. Therefore, the method for forming a metal film with
twins is more suitable for use in the semiconductor field as
compared to the methods disclosed by C. J. Shute et al and C. Zang
et al.
[0064] While the present invention has been described in connection
with what is considered the most practical and preferred
embodiment, it is understood that this invention is not limited to
the disclosed embodiment but is intended to cover various
arrangements included within the spirit and scope of the broadest
interpretations and equivalent arrangements.
* * * * *