U.S. patent application number 15/494867 was filed with the patent office on 2017-11-02 for wafer processing method.
The applicant listed for this patent is DISCO CORPORATION. Invention is credited to Masaru Nakamura.
Application Number | 20170316977 15/494867 |
Document ID | / |
Family ID | 60157120 |
Filed Date | 2017-11-02 |
United States Patent
Application |
20170316977 |
Kind Code |
A1 |
Nakamura; Masaru |
November 2, 2017 |
WAFER PROCESSING METHOD
Abstract
A wafer is transferred to a holding surface of a chuck table by
using a transfer unit having a suction pad. The front side of the
wafer is held under suction through a protective tape on the
holding surface, and the suction pad is removed from the back side
of the wafer. A modified layer is formed on the back side of the
wafer along division lines. The wafer is transferred by mounting
the wafer held by the suction pad on the holding surface and
sandwiching the wafer between the suction pad and the holding
surface of the chuck table. A suction force is applied to the
holding surface of the chuck table to thereby hold the front side
of the wafer through the protective tape on the holding surface of
the chuck table under suction, and the suction pad is then removed
from the back side of the wafer.
Inventors: |
Nakamura; Masaru; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DISCO CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
60157120 |
Appl. No.: |
15/494867 |
Filed: |
April 24, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/67092 20130101;
H01L 21/6838 20130101; H01L 2221/68327 20130101; H01L 21/78
20130101; H01L 21/6836 20130101; H01L 2221/6834 20130101 |
International
Class: |
H01L 21/78 20060101
H01L021/78; H01L 21/683 20060101 H01L021/683; H01L 21/268 20060101
H01L021/268; H01L 21/683 20060101 H01L021/683 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2016 |
JP |
2016-089088 |
Claims
1. A wafer processing method for dividing a wafer into a plurality
of individual device chips along a plurality of crossing division
lines formed on a front side of said wafer, the front side of said
wafer being partitioned by said division lines to define a
plurality of separate regions where a plurality of devices are
formed, said individual device chips corresponding to said
respective devices, said wafer processing method comprising: a
protective tape attaching step of attaching a protective tape to
the front side of said wafer; a holding step of holding said
protective tape attached to the front side of said wafer on a
holding surface of a first chuck table under suction; a grinding
step of grinding the back side of said wafer held on said first
chuck table under suction, thereby thinning said wafer; an
unloading step of holding a back side of said wafer held on said
first chuck table by using a suction pad of transfer means after
performing said grinding step, and next moving said suction pad to
thereby unload said wafer from said first chuck table; a transfer
step of transferring said wafer to a holding surface of a second
chuck table by operating said transfer means after performing said
unloading step, next holding said protective tape attached to the
front side of said wafer on the holding surface of said second
chuck table under suction, and next removing said suction pad from
the back side of said wafer; and a modified layer forming step of
applying a laser beam having a transmission wavelength to said
wafer to the back side of said wafer along each division line in
the condition where the focal point of said laser beam is set
inside said wafer after performing said transfer step, thereby
forming a modified layer inside said wafer along each division
line; said transfer step including a mounting step of mounting said
wafer held by said suction pad to the holding surface of said
second chuck table, a sandwiching step of removing a suction force
applied to said suction pad after performing said mounting step,
and then sandwiching said wafer between said suction pad and the
holding surface of said second chuck table, and a suction holding
step of applying a suction force to the holding surface of said
second chuck table after performing said sandwiching step to
thereby hold said protective tape attached to the front side of
said wafer on the holding surface of said second chuck table under
suction, and next removing said suction pad from the back side of
said wafer.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a wafer processing method
for dividing a wafer into individual device chips by applying a
laser beam to the wafer in the condition where the focal point of
the laser beam is set inside the wafer to thereby form a modified
layer inside the wafer.
Description of the Related Art
[0002] A plurality of devices such as integrated circuits (ICs) and
large scale integrations (LSIs) are formed on the front side of a
wafer so as to be separated by a plurality of crossing division
lines. The wafer thus having the plural devices is divided along
the division lines into individual device chips by using a dicing
apparatus or a laser processing apparatus, for example. The device
chips divided from the wafer are used in electrical equipment such
as mobile phones and personal computers.
[0003] Further, there has been proposed a technique including the
steps of attaching a protective tape to the front side of a wafer,
next grinding the back side of the wafer to thereby thin the wafer,
and next applying a laser beam having a transmission wavelength to
the wafer to the back side of the wafer along each division line in
the condition where the focal point of the laser beam is set inside
the wafer to thereby form a modified layer as a division start
point inside the wafer along each division line (see Japanese
Patent No. 3408805, for example).
[0004] According to the technique disclosed in Japanese Patent No.
3408805, the width of each division line can be reduced to thereby
increase the number of device chips that can be manufactured from
one wafer as compared with the case of using a conventional dicing
apparatus to form a division start point along each division
line.
SUMMARY OF THE INVENTION
[0005] The present inventor has found that in performing laser
processing for forming a modified layer inside a wafer, there is a
case that the wafer may be broken at a position different from each
division line where the modified layer is to be formed (as shown by
a solid line C in FIG. 9). Such a breaking phenomenon causes a
problem such that the devices formed on the front side of the wafer
may be partially damaged to remarkably reduce the production
efficiency of each device chip.
[0006] Under these circumstances, the present inventor has closely
studied the cause of the above breaking phenomenon to obtain the
following findings. After performing a grinding step of grinding
the back side of a wafer in a grinding apparatus, the wafer is
unloaded from a chuck table included in the grinding apparatus and
then transferred to a chuck table included in a laser processing
apparatus by using transfer means having a suction pad. In the
laser processing apparatus, a laser beam is applied to the wafer
held on the chuck table to form a modified layer inside the wafer.
In transferring the wafer from the chuck table of the grinding
apparatus to the chuck table of the laser processing apparatus, the
wafer is held by the suction pad under suction. At this time, a
nonuniform internal stress is generated inside the wafer by a
suction force applied to the suction pad. When this internal stress
is not sufficiently relieved and the wafer is then held on the
chuck table of the laser processing apparatus under suction, the
internal stress due to the suction holding by the suction pad is
partially left inside the wafer. In the next modified layer forming
step, the laser beam is applied to the wafer having such a residual
internal stress. Due to the application of the laser beam, the
residual internal stress causes the breaking phenomenon at an
unintentional position.
[0007] It is therefore an object of the present invention to
provide a wafer processing method which can prevent the breaking
phenomenon at an unintentional position in a wafer in performing
laser processing for forming a modified layer inside the wafer.
[0008] In accordance with an aspect of the present invention, there
is provided a wafer processing method for dividing a wafer into a
plurality of individual device chips along a plurality of crossing
division lines formed on the front side of the wafer, the front
side of the wafer being partitioned by the division lines to define
a plurality of separate regions where a plurality of devices are
formed, the individual device chips corresponding to the respective
devices, the wafer processing method including a protective tape
attaching step of attaching a protective tape to the front side of
the wafer; a holding step of holding the protective tape attached
to the front side of the wafer on a holding surface of a first
chuck table under suction; a grinding step of grinding the back
side of the wafer held on the first chuck table under suction,
thereby thinning the wafer; an unloading step of holding the back
side of the wafer held on the first chuck table by using a suction
pad of transfer means after performing the grinding step, and next
moving the suction pad to thereby unload the wafer from the first
chuck table; a transfer step of transferring the wafer to a holding
surface of a second chuck table by operating the transfer means
after performing the unloading step, next holding the protective
tape attached to the front side of the wafer on the holding surface
of the second chuck table under suction, and next removing the
suction pad from the back side of the wafer; and a modified layer
forming step of applying a laser beam having a transmission
wavelength to the wafer to the back side of the wafer along each
division line in the condition where the focal point of the laser
beam is set inside the wafer after performing the transfer step,
thereby forming a modified layer inside the wafer along each
division line; the transfer step including a mounting step of
mounting the wafer held by the suction pad to the holding surface
of the second chuck table; a sandwiching step of removing a suction
force applied to the suction pad after performing the mounting
step, and then sandwiching the wafer between the suction pad and
the holding surface of the second chuck table; and a suction
holding step of applying a suction force to the holding surface of
the second chuck table after performing the sandwiching step to
thereby hold the protective tape attached to the front side of the
wafer on the holding surface of the second chuck table under
suction, and next removing the suction pad from the back side of
the wafer.
[0009] According to the present invention, in transferring the
wafer from the suction pad to the second chuck table, the suction
force applied to the suction pad holding the wafer under suction is
removed to thereby relieve an internal stress in the wafer.
Accordingly, the internal stress due to the suction holding by the
suction pad is not left in the wafer and the wafer is next held on
the second chuck table under suction. As a result, even when the
laser beam is applied to the back side of the wafer held on the
second chuck table in the modified layer forming step, there is no
possibility that the devices may be partially damaged.
[0010] The above and other objects, features and advantages of the
present invention and the manner of realizing them will become more
apparent, and the invention itself will best be understood from a
study of the following description and an appended claim with
reference to the attached drawings showing a preferred embodiment
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a perspective view showing a protective tape
attaching step of attaching a protective tape to a silicon
wafer;
[0012] FIGS. 2A and 2B are perspective views showing a holding step
of holding the silicon wafer on a first chuck table of a grinding
apparatus;
[0013] FIG. 3 is a perspective view showing a grinding step of
grinding the back side of the silicon wafer held on the first chuck
table;
[0014] FIG. 4 is a perspective view schematically showing an
unloading step of unloading the silicon wafer from the first chuck
table;
[0015] FIGS. 5A to 5C are side views for specifically illustrating
the unloading step shown in FIG. 4;
[0016] FIG. 6 is a perspective view schematically showing a
transfer step of transferring the silicon wafer from the first
chuck table to a second chuck table of a laser processing
apparatus;
[0017] FIGS. 7A to 7D are side views for specifically illustrating
the transfer step shown in FIG. 6; and
[0018] FIG. 8A is a perspective view showing a modified layer
forming step of forming a modified layer inside the silicon wafer
held on the second chuck table;
[0019] FIG. 8B is an enlarged sectional view of the silicon wafer
shown in FIG. 8A; and
[0020] FIG. 9 is a perspective view showing a prior art related to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] A specific preferred embodiment of the wafer processing
method according to the present invention will now be described in
detail with reference to the attached drawings. Referring to FIG.
1, there is shown a silicon wafer 10 as a workpiece. The silicon
wafer 10 has a front side 10a and a back side 10b. A protective
tape 20 is attached to the front side 10a of the silicon wafer 10,
so as to protect the front side 10a (protective tape attaching
step). A plurality of crossing division lines 14 are formed on the
front side 10a of the silicon wafer 10 to thereby define a
plurality of separate regions where a plurality of devices 12 such
as ICs are formed. The silicon wafer 10 originally has a thickness
of 755 .mu.m, for example, before grinding. The protective tape 20
is composed of a base sheet and an adhesive layer formed on one
side of the base sheet. The base sheet has a thickness of 100
.mu.m, and the adhesive layer has a thickness of approximately 5
.mu.m. The base sheet is formed of polyvinyl chloride (PVC), and
the adhesive layer is formed of acrylic resin.
[0022] After performing the protective tape attaching step, a
holding step is performed as shown in FIGS. 2A and 2B. Referring to
FIG. 2A, there is shown a grinding apparatus 4 (the whole
configuration thereof being not shown). The grinding apparatus 4
includes a first chuck table 30 having a holding surface 32 for
holding the silicon wafer 10. As shown in FIG. 2A, the silicon
wafer 10 with the protective tape 20 attached to the front side 10a
is placed on the holding surface 32 of the first chuck table 30 in
the condition where the protective tape 20 is in contact with the
holding surface 32 and the back side 10b as a work surface is
oriented upward (see FIG. 2B). The first chuck table 30 is
rotatable by a rotational drive mechanism (not shown). The holding
surface 32 is formed of a porous material allowing air
communication, and it is connected to suction means (not shown). In
the condition shown in FIG. 2B, the suction means is operated to
firmly hold the silicon wafer 10 through the protective tape 20 on
the holding surface 32 of the first chuck table 30 under suction
without displacement (holding step).
[0023] After performing the holding step, a grinding step is
performed by using the grinding apparatus 4 as shown in FIG. 3.
Referring to FIG. 3, the grinding apparatus 4 includes grinding
means 40 for grinding the back side 10b of the silicon wafer 10
held on the first chuck table 30 to thereby reduce the thickness of
the silicon wafer 10. The grinding means 40 includes a spindle 42
adapted to be rotated by a rotational drive mechanism (not shown),
a mounter 44 fixed to the lower end of the spindle 42, and a
grinding wheel 46 mounted on the lower surface of the mounter 44.
The grinding wheel 46 is composed of a base and a plurality of
abrasive members 48 fixed to the lower surface of the base so as to
be arranged annularly along the outer circumference of the
base.
[0024] In the condition where the silicon wafer 10 is held through
the protective tape 20 on the first chuck table 30 under suction,
the first chuck table 30 is rotated at 300 rpm, for example, in the
direction shown by an arrow 30a in FIG. 3, and the spindle 42 is
also rotated at 3400 rpm, for example, in the direction shown by an
arrow 42a in FIG. 3. Thereafter, the grinding means 40 is lowered
to bring the abrasive members 48 of the grinding wheel 46 into
contact with the back side 10b of the silicon wafer 10. Further,
the grinding means 40 is fed downward by a predetermined amount at
a feed speed of 1 .mu.m/second, for example, in the direction
perpendicular to the holding surface 32 of the first chuck table
30. At this time, the thickness of the silicon wafer 10 may be
measured by a contact type measuring gauge (not shown) during the
grinding operation. For example, the back side 10b of the silicon
wafer 10 is ground until the thickness of the silicon wafer 10
becomes a predetermined thickness of 60 .mu.m, for example. Thus,
the grinding step is finished.
[0025] After performing the grinding step, an unloading step is
performed as shown in FIG. 4 in such a manner that the silicon
wafer 10 with the protective tape 20 is unloaded from the first
chuck table 30. The unloading step will now be described in more
detail with reference to FIGS. 5A to 5C. As shown in FIGS. 5A to
5C, the unloading step is performed by using transfer means 50 (the
whole configuration thereof being not shown). The transfer means 50
includes a transfer arm 52 and a suction pad 54 provided at the
front end of the transfer arm 52 so as to be oriented downward. The
transfer arm 52 is movable horizontally and vertically by a moving
mechanism (not shown). The suction pad 54 is a disk-shaped member
having substantially the same size as that of the first chuck table
30. The suction pad 54 has a lower surface functioning as a suction
holding surface 56. The suction holding surface 56 is formed of a
porous material allowing air communication, and it is connected
through the transfer arm 52 to suction means (not shown).
[0026] In performing the unloading step, the transfer arm 52 is
first moved by the moving mechanism to position the suction pad 54
directly above the silicon wafer 10 held on the first chuck table
30 as shown in FIG. 5A. Thereafter, the moving mechanism is
operated to lower the transfer arm 52 until the suction holding
surface 56 of the suction pad 54 comes into abutment against the
back side 10b of the silicon wafer 10 held on the first chuck table
30. At this time, the distance between the suction holding surface
56 of the suction pad 54 and the back side 10b of the silicon wafer
10 is measured by a proximity sensor (not shown). Thus, the silicon
wafer 10 is sandwiched between the first chuck table 30 and the
suction pad 54 as shown in FIG. 5B.
[0027] Thereafter, the suction means connected to the suction pad
54 is operated to hold the back side 10b of the silicon wafer 10
under suction. Accordingly, the silicon wafer 10 is held under
suction by both the holding surface 32 of the first chuck table 30
and the suction holding surface 56 of the suction pad 54.
Thereafter, a suction force applied to the first chuck table 30 is
removed, so that the silicon wafer 10 is held under suction only by
the suction pad 54. Thereafter, the transfer arm 52 is lifted to
thereby move the silicon wafer 10 away from the first chuck table
30 as shown in FIG. 5C. Thusly, the unloading step is finished.
[0028] After performing the unloading step, a transfer step is
performed as shown in FIG. 6 in such a manner that the silicon
wafer 10 with the protective tape 20 is transferred to a second
chuck table 60 included in a laser processing apparatus 6 (the
whole configuration thereof being not shown) for forming a modified
layer inside the silicon wafer 10. In the transfer step, the
silicon wafer 10 is held on the second chuck table 60 in the
condition where the protective tape 20 is in contact with the upper
surface of the second chuck table 60. Thereafter, the suction pad
54 is moved away from the back side 10b of the silicon wafer 10. As
similar to the first chuck table 30, the upper surface of the
second chuck table 60 functions as a holding surface 62 for holding
the silicon wafer 10 under suction. The holding surface 62 is
formed of a porous material allowing air communication, and it is
connected to suction means (not shown). The laser processing
apparatus 6 is located adjacent to the grinding apparatus 4, and
the transfer means 50 is so arranged as to transfer the silicon
wafer 10 from the grinding apparatus 4 to the laser processing
apparatus 6. The transfer step will now be described in more detail
with reference to FIGS. 7A to 7D.
[0029] As shown in FIG. 7A, the moving mechanism for moving the
transfer arm 52 is operated to move the transfer arm 52 and
position the suction pad 54 holding the silicon wafer 10 directly
above the second chuck table 60 of the laser processing apparatus 6
for forming a modified layer inside the silicon wafer 10.
Thereafter, the moving mechanism is operated to lower the transfer
arm 52 until the protective tape 20 attached to the front side 10a
of the silicon wafer 10 held by the suction pad 54 comes into
abutment against the holding surface 62 of the second chuck table
60. At this time, the distance between the protective tape 20 of
the silicon wafer 10 and the holding surface 62 of the second chuck
table 60 is measured by a proximity sensor (not shown). Thus, the
silicon wafer 10 is mounted on the holding surface 62 of the second
chuck table 60 in the condition where the silicon wafer 10 is held
by the suction pad 54 as shown in FIG. 7B (mounting step).
[0030] After performing the mounting step, a suction force applied
to the suction pad 54 is removed. At this time, a suction force has
not yet been applied to the holding surface 62 of the second chuck
table 60. The removal of the suction force applied to the suction
pad 54 may be effected by physically blocking a suction passage
connected to the suction pad 54 or by stopping a suction pump
included in the suction means connected to the suction pad 54. Any
other methods for removing the suction force applied from the
suction pad 54 to the silicon wafer 10 may be adopted. Accordingly,
the silicon wafer 10 is physically sandwiched between the suction
pad 54 and the second chuck table 60 without receiving a suction
force from the suction pad 54 and the second chuck table 60 as
shown in FIG. 7C (sandwiching step).
[0031] After performing the sandwiching step, a suction force is
applied to the holding surface 62 of the second chuck table 60,
thereby holding the silicon wafer 10 through the protective tape 20
on the holding surface 62 under suction. Thereafter, the transfer
arm 52 is lifted to move the suction pad 54 away from the back side
10b of the silicon wafer 10 as shown in FIG. 7D. Thus, the silicon
wafer 10 is held by only the second chuck table 60 under suction
(suction holding step). In this manner, the mounting step, the
sandwiching step, and the suction holding step are sequentially
performed to thereby finish the transfer step.
[0032] After performing the transfer step, a modified layer forming
step is performed as shown in FIGS. 8A and 8B in such a manner that
a laser beam is applied from laser beam applying means 70 included
in the laser processing apparatus 6 to the silicon wafer 10 along
each division line 14 to thereby form a modified layer 10c as a
division start point inside the silicon wafer 10 along each
division line 14. More specifically, an alignment step is first
performed to align a laser beam applying position with a
predetermined one of the division lines 14 by using imaging means
(not shown). Thereafter, the laser beam applying means 70 of the
laser processing apparatus 6 is operated to oscillate a laser beam
having a transmission wavelength to the silicon wafer 10 from a
laser oscillator (not shown) included in the laser beam applying
means 70. The laser beam oscillated is applied through focusing
means 72 included in the laser beam applying means 70 to the back
side 10b of the silicon wafer 10 held through the protective tape
20 on the second chuck table 60 in the condition where the focal
point of the laser beam is set inside the silicon wafer 10. At the
same time, the second chuck table 60 is moved in the X direction
shown by an arrow X in FIGS. 8A and 8B at a predetermined feed
speed. Accordingly, the modified layer 10c is formed inside the
silicon wafer 10 along the predetermined division line 14. Although
not shown, the laser processing apparatus 6 further includes X
moving means for moving the second chuck table 60 in the X
direction, Y moving means for moving the second chuck table 60 in
the Y direction perpendicular to the X direction, and rotating
means for rotating the second chuck table 60. By controlling the
laser beam applying means 70, the X moving means, the Y moving
means, and the rotating means, the laser processing operation
mentioned above is repeated along all of the other division lines
14 to thereby form a plurality of similar modified layers 10c
inside the silicon wafer 10 along all of the other division lines
14. Thereafter, an external force may be applied to the silicon
wafer 10 by using any external force applying means known in the
art to thereby divide the silicon wafer 10 into individual device
chips corresponding to the respective devices 12.
[0033] For example, the modified layer forming step using the laser
beam applying means 70 is performed under the following processing
conditions. As described above, the silicon wafer 10 originally has
a thickness of 755 .mu.m, and the back side 10b of the silicon
wafer 10 is ground by the grinding step mentioned above to reduce
the thickness of the silicon wafer 10 to a thickness of 60 .mu.m.
Accordingly, the thickness of the silicon wafer 10 in the modified
layer forming step is 60 .mu.m.
[0034] Wavelength: 1342 nm
[0035] Average power: 0.18 W
[0036] Repetition frequency: 80 kHz
[0037] Spot diameter: 1 .mu.m
[0038] Work feed speed: 180 mm/second
[0039] Focal position: 32 .mu.m from the front side 10a (28 .mu.m
from the back side 10b).
[0040] With the above configuration of the present invention, the
following particular effects can be exhibited. In the transfer step
of transferring the silicon wafer 10 to the second chuck table 60
of the laser processing apparatus 6, the silicon wafer 10 held by
the suction pad 54 is brought into abutment against the holding
surface 62 of the second chuck table 60. Before applying a suction
force to the second chuck table 60, the suction force applied to
the suction pad 54 is removed. In this condition, no suction force
is applied from the suction pad 54 and the second chuck table 60 to
the silicon wafer 10, and the silicon wafer 10 is physically
sandwiched between the suction pad 54 and the second chuck table
60. Accordingly, it is possible to once completely relieve an
internal stress generated in the silicon wafer 10 due to the
suction holding by the suction pad 54. Further, since the silicon
wafer 10 is physically sandwiched between the suction pad 54 and
the second chuck table 60, the silicon wafer 10 can be thereafter
held on the second chuck table 60 under suction without
displacement. As a result, even when the silicon wafer 10 is
subjected to laser processing in the modified layer forming step,
there is no possibility of unintentional breaking due to residual
internal stress in the silicon wafer 10. Accordingly, it is
possible to prevent the problem that the devices 12 formed on the
silicon wafer 10 may be partially damaged to cause a reduction in
production efficiency.
[0041] In the above preferred embodiment, the unloading step and
the transfer step are performed by using the same suction pad 54 of
the transfer means 50 to transfer the silicon wafer 10 from the
first chuck table 30 of the grinding apparatus 4 to the second
chuck table 60 of the laser processing apparatus 6. However, this
configuration is merely illustrative and any other modifications
may be made. For example, the silicon wafer 10 may be once
transferred from the first chuck table 30 of the grinding apparatus
4 to a cleaning table as another table by using the suction pad 54
in the unloading step. After cleaning the silicon wafer 10 on the
cleaning table after performing the grinding step, the silicon
wafer 10 may be next transferred from this cleaning table to the
second chuck table 60 of the laser processing apparatus 6 by using
another suction pad in the transfer step. In this case, the
transfer step shown in FIGS. 7A to 7D may be similarly performed in
transferring the silicon wafer 10 from the cleaning table to the
second chuck table 60.
[0042] Further, while the silicon wafer 10 is used as a workpiece
in this preferred embodiment, the workpiece usable in the present
invention may include any wafer such that a modified layer may be
formed inside the wafer along each division line after grinding the
wafer. Examples of such a wafer include sapphire, silicon carbide
(SiC), lithium tantalate (LT), and lithium niobate (LN) wafers.
[0043] The present invention is not limited to the details of the
above described preferred embodiment. The scope of the invention is
defined by the appended claim and all changes and modifications as
fall within the equivalence of the scope of the claim are therefore
to be embraced by the invention.
* * * * *