Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVE~TION
FieLd of the Invention
This invention relates to semiconductor image sensors
having charge-coup~ed devices (CCD) operating as electro-
optic transducers and charge-transfer devices and as a self-
scanning matrix structure.
The Prior Art
There are existing image sensors constructed on the sur-
face of a semiconductor divided into elements arranged in hori-
zontal rows and vertical columns. Each element includes a
qemiconductive photo-sensor and a charge-transfer device con-
nected thereto to connect all of the photo-sensor elements of
a vertical column to a vertical shift register. One end of
each of the vertical shift registers is connected to the hori-
zontal shift register located on the same semico~nductor sub-
strate.
The vertical shift registers are operated in unison to
transfer charge carriers from position to position along the
vertical shift registers 90 that they eventually reach the
hor~zontal shift register. The carriers reach the horizontal
shift register at the equivalent of one horizontal line of the
video image at a time, and they are transferred along the hori-
zontal shift register at a rate equivalent to the horizontal
I scanning speed of a television system. At the output of the
I horizontal shift re$ister, the carriers are applied to a utili-
~ zation circuit.
;i The conventional elements of an image sensor according to
the prior art include an insulating layer on the semiconductor
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subctrate and a transparent electrode on the insulating layer.
One part of each element is arranged so that the transparent
electrode is spac2d from the substrate onLy by the insulating
layer so that photons that reach the transparent electrode
can produce minority charge carriers in the region of the sub-
strate directly behind that part of the transparent electrode.
The elemental area also includes one electrode of the vertical
shift register to which that element is connected. Part of
the electrode is separated from the semiconductor substrate
by the insulating layer, and between that part and the part
of the transparent electrode that is also separated from the
substrate only by the insulating layer is a tranfer electrode.
The transfer electrode is also separated from the semiconductor
substrate by the insulating layer, and a part of the shift
register electrode overlaps the transfer electrade but is in-
sulated therefrom by additional insulating material. The
transparent electrode also extends over the transfer electrode
and the shift register electrode but is separated from each
of them by more of the insulating material of the insulating
layer. An opaque layer overlies the transfer electrode and
the shift register electrode, leaving substantially only the
area to be illuminated uncoated.
The vertical shift registers of the prior art are operated
as two-phase devices, which are clocked at the horizontal repe-
tition rate. The transparent electrode has a fixed voltage
applied to it and the transfer electrode is pulsed at field
repetition rate so that during one field interval, charge car-
riers from those elements lying in odd horizontal lines will ~ -
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be transferred to their respective vertical shift registers,
and during the remaining alternate field intervals, those
elements lying in even horizontal lines will transfer their :~
carriers to their respective vertical shift registers.
The prior art structure requires three types of
electrodes, one to receive the proper potential to oPerate the
photc-sensors, another to receive the clock pulses, and a third
to ~r~nsfer charge carriers from the photo-sensor regions to
regi~ns controlled by the electrodes operated by the clock
pulses.
SUMMARY OF THE INVENTION
The present invention is a semiconductor image sensor
with sensor elements arranged in vertical columns and hori-
zontal rows and being capable of generally the same type of
operation as the prior art structures but without the necessity
of providing transfer electrodes and the voltage heretofore
required for such electrodes.
Further in accordance with the present invention,
carriers are transferred by an overflow operation to improve
resolution of the image sensor.
According to one aspect of this invention a transfer
gate-le~s photo-sensor element for a semiconductor image sensor,
said photo-sensor element comprises: a substrate of one con-
ductivity type; an insulating layer on said substrate; a first
... .
.` electrode on said insulating layer having first means for apply-
ing a potential to said first electrode, said first electrode
having a first electrode portion which is spaced from said sub-
- ~trate by a thickness of said insulating layer for defining, in
combination with the potential applied to said first electrode
30 by said first means, a first region in said substrate which is
... ,.; .
a potential well for minority charge carriers in said substrates
', a second electrode in said insulating layer having second me~ns
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for appl~ing a potential to said second electrode, said second
electrode having a second electrode portion adjacent one side
o~ said first electrode and an adjacent third electrode portion
remote from said first electrode, said second electrode portion
being spaced from said substrate by a thickness of s~id insulat- -
ing layer which is greater than the thickness of said insulating
layer which spaces said third electrode portion from said sub-
strate for defining, in combination with the potential applied
to said second electrode by said second means, a second region
in said substrate adjacent said first region which is a poten-
tial barrier for said minority charge carriers in said sub-
strate relative to said potential well in said first region,
and for defining with said third electrode portion in combin~ -
tion with the potential applied to said second electrode by
said second means a third region in said substrate adjacent said
second region and which is a potential well for said minority
charge carriers in said substrate relative to said potential
barrier in ~aid second region; and a third electrode in said
insulating layer and extending generally parallel to said
second electrode and electrically insulated from the latter
by said insulating layer, ~aid third electrode being connected
with said second means for receiving a potential therefrom,
said third electrode having a fourth electrode portion over-
r"
i lapping said third electrode portion and being spaced from said
substrate by a thickness of said insulating layer which is
larger than said thickness of the insulating layer which spaces
aaid third electrode portion from said substrate, at least one
of said second and third electrodes having a cutout portion co-
operating with an ad~acent portion of the other of ~aid second
.- 30 and third electrodes to define an opening above said first
region.
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According to another asDect of this invention a semi-
condu~or image sensor is composed of a plurality of charge-
- coup~-~ transfer gate-less photo-sensor elements each as recited
above and with said first electrode further including a con-
necting portion extending from said side of said first elec-
tro~e to which said second electrode portion of said second
electrode is adjacent over said second electrode for connection
to a first electrode portion of a first electrode of another
of said photo-sensor elements, said insulating layer spacing
said connecting portion of said first electrode from said
second electrode. .
According to a further aspect of this invention a
semiconductor image sensor is composed of a plurality of charge-
coupled transfer gate-less photo-sensor elements each as recited
, . above and with said first electrode further including a fifth
; electrode portion along a side thereof which is different from
said side of said first electrode adjacent to said second elec-
trode and which is spaced from said substrate by a thickness
of said insulating layer greater than the thickness of said
insulating layer which spaces said first electrode portion of
said first electrode from said substrate for defining, in com-
. bination with the potential applied to said first electrode by
~aid first means, a fourth region which i8 a potential barrier
for said minority charge carriers relative to said potential
~ well in said first reg~on of said substrate, said potential;1, barrier in said fourth region being lower than said potential ;.
barrier in said second region of said substrate to control the
number of said minority charge carriers which can be stored in
said potential well in 8aid first region of said substrate.
BRiEF DæSCXIPTION OF THE DRAWINGS
Fig. 1 is a schematic representation of a conventional .
semiconductor image 80n80r employing an interline transfer , .
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operation.
Fig. 2 is a cross-sectional view along the line 2-2
of a fragment of the sensor in Fig. 1.
Fig. 3 is a plan view of a fragment of a semiconductor
image sensor according to the present invention.
Figs. 4-6 are cross-sectional views of a fragment of
the
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device in Fig. 3 alsng the lines 4-4, 5-5, and 6-6, respective-
ly, in Fig. 3.
FigC 7A-7G are crocs-sectional views of a semiconductor
structure illuctrating a sUccecsion of steps in the production
of the device shown in Figs. 3-6.
Figs. 8A-~C are waveforms of voltage pulses applied to
operate the device in Figs. 3-6.
Fig. 9 is a plan view of another fragment of a semicon-
ductor image sen~or according to the invention.
Fig. 10 is a cross-sectional view of a fragment of the
tevice in Fig. 9 along the line 10-10.
DESCRIPTIO~ OF THE PREFERRED EMBODIME~TS
The semiconductor images sensor in Fig. L is constructed
on the major surface of a semiconductor substrate (not shown)
and incLudes a matrix of photo-sensor elements l arranged in
vertical columns and horizontal rows. All of the photo-sencor
eLements 1 in a given vertical column are connected to the same
: vertical shift register 2, and one end of each of the vertical
shift registers 2 is connected to a horizontal shift register
3, which is also located on the semiconductor substrate. Minor-
ity carriers generated in each of the photo-sensor elements l
in alternate rows are simultaneously transferred to the vertical
shift registers 2. The charges thus transferred are transported
sequentially along the vertical shift registers 2 to the hori-
zontal shift regicter 3 and are transferred from left to right
along the shift register 3 to an output terminal connected to
a utilization circuit. Then the minority carriers in the re-
maining alternate horizontal rows, which represent horizortal
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scanning lines of an interiac2d scanning system, are trans-
ferred to the vertical Chift registers 2 and along the verti-
cal shif~ registers to the horizontal shift regicter 3. The
rate of transfer of charge carriers along the shift register
3 correspondc to the scanning rate of a single horizontal line
in a television syctem, and the rate of transfer of carriers
along the vertical shift registers 2 corresponds to the verti-
cal scanning speed of a television system.
Fig 2 is a cross-sectional view of one of the elemen~s
of the device in Fig. 1. The photo-sensor l is constructed
on a major surface of a semiconductor substrate 10, which, in
this embodiment, is an N-type substrate. An insulating layer
11 is formed on one surface of the substrate and in the region .
of the photo-sensor 1, a transparent electrode 12 is formed
directly on the surface of the insulating layer 11 The verti-
cal shift register 2 to which this particular photo-sensor
element 1 is connec~ed includes a shifting electrode.l3, which
is fonmed on another part of the insulating layer 11. Still
another electrode L4, referred to as the transfer electrode,
is formed on the layer 11 bet~een the photo-sensor 1 and part
of the shifting electrode 13. Another part of the shifting
electrode 13 overlaps the transfer electrode 14 and is insu-
; lated therefrom by the same insulating material that makes up
the layer 11. The transparent electrode 12 also overlaps the
transfer electrode 14 and the shifting electrode 13, as w211,
but is insul~ted from both of them by more of the insulatinO
~` material of the type in the layer 11. The portion of the struc-
ture shown in Fig. 2 that includes the transfer electrode 14
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is identified as the transfer portion 15. Vertical edges of
the columns are defined by c'nannel stops 16 of ~7~ type con-
ductivity and by an opa~ue light shield layer 17.
The operation of the element n Fig. 2 and of the sensor
in Fig. l includes the application of a suitable fixed voltage
Vs to the transparent electrode 12. This voltage Vs may be,
for example, about -20 volts Each of the vertical shift regis-
ters 2 in Fig. 1 includes a separate electrode 13 of the type
shown in Fig. 2 for each of the elements 1. Alternate shifting
electrodes 13 in the various vertical shift registers 2 have
clock pulses ~ 1 at, for example, 0 volts appLied to them,
and the remaining alternate shift electrodes 13 have clock
pulses ~ 2 at, for example, -20 volts applied to them. At the
end of each horizontal line interval the pulses shift, and
those electrodes to which the 0 volts was applied receive
the -20 volts, and vice versa. A voltage Vt of, for example,
-20 volts is applied to the transfer electrodes 14 for a
fixed interval that corresponds to one field interval.
When light is incident on the photo-sensors 1 in Fig. 1,
a pattern of charges is produced In the single element in
Fig. 2, the incident light on the exposed part of the trans-
psrent electrode 12 having a suitable voltage Vs applied to it
cause9 minority carriers (holes~ to be generated and to be
stored at the immediately adjacent part of the semiconductor
substrate 10 in a region thereof defined as having a potential
well (with respect to holes). When the minority carriers are
accumulated, the potential of the well rises according to charges
on the holes stored in that well. As long as the transfer
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electrode hac a voltage Vt = O applied to it, the potential
of the transferring portion 15 is high, and stored carriers
in the photo-sensor 1 are not transferred to the shift egis-
ter 2. On the other hand when the voltage Vt = -20 volts is
applied to the trancfer electrode 14, the potential of that
portion of the semiconductor substrate 10 under the electrode
14 is lowered according to the dotted line 21, and minority
carriers in the photo-sensor 1 are transferred to the shift
register 2.
The vertical shift registers 2 in Fig. 1 operate ~ccord-
ing to a two-phase principle so that alternate electrodes 13
must not have charge carriers transferred to them by their re-
spective photo-sensors 1. This permits the charges that are
transferred to the remaining alternate electrodes 13 from their
respective photo elements 1 to be transerred along the verti-
cal shift registers 2 to the shift register 3. After trans-
ferring the charge carriers, the voltage Vt on the transfer
electrodes 14 returns to Vt = O, and the carriers that have
been transferred to the vertical shift registers 2 can be trans-
ferred along these shift registers to the horizontal shift
register 3.
One of the disadvantages of the electrode configuration
in Fig. 2 is that three kinds of electrodes are required. In
addition, the voltage of the transfer electrode 14 is influenced
by the voltage of the shifting electrode 13 if the transfer
electrode 14 is made of high resistivity poly-silicon.
The embodiment of the invention in Figs. 3-6 will now
be described. The semiconductor image sencor according to
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this invention has photo-C~ncor elements arranged in a matrix
22 with a plurality of vertical CCD shift regicters 23 separating
the eleme-nts 22 into vertical columns A transfer portion 24
is located between each of the elements 22 and its vertical
shift register 23, and overfLowing portions 25 defined by po-
tential barriers in the substrate 21 also extend vertically
aLong the elements Z2 to separate the columns of photo-sensor
elements 22 and adjacent shift register elements 23. This em-
bodiment of the invention also includes a horizontal CCD shift
register (not shown) at the corresponding end of each of the
vertical shift registers 23.
Each photo-sensor 22 includes a portion of a transparent
electrode 26 applied to the exposed surface of an insulating
layer 27 on the surface of the semiconductor substrate 21.
Shifting electrodes 28a and 28b are also applie~ to the insulating -
layer 27 but are separated from different portions of the sur-
face of the substrate 21 by different thi^knesses of the layer
27 to constitute vertical transfer devices 29. Clock pulses
01 and 02 are apPlied to alternate sets of shift-
ing electrodes 28a and 28b. As shown particularly in Fig. 5,
the shifting electrodes 28a are applied over thin portions of
the insulating ~yer 27. As shown particularly in Fig. 6, the
shifting electrodec 28b are applied over thicker portions of
the insulating layer 27. Each of the shifting electrodes 28a
and 28b has an edge portion with a greater thickness of the
layer 27 between it and the substrate 21 than the central por-
tion of that shi~ting electrode. As shown in Fig. 4, the
application of clock pulsec voltages of Yl = -15V and !~2-
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adjacent Ce~fi~6lo 5 8= OV to / electrodes 28a and 28b forms a stairli~e potential
well 30 within t~ subctrate 21,
The tran~ferring portion 24 incl~des an extended portion
of the first shifting electrod2 28a separated from the sub-
strate 21 by a relativeiy thick portion of the insulating layer
27 located between the photo-sencor 22 and the shift register
23 for that photo-sencor. As shown in Fig, 5, the potential
33a of the transfer portion 24 is higher than the potential
33b due to the shift regicter electrode 28a, In this embodi-
ment, the thickness tl of the insulating layer 27 under the
transfer portion 24 of the electrode 28a is larger than the
thickness t2 of that portion of the layer 27 under the central
part of the electrode 28a. The edge of this shifting electrdde
28a away from transfer portion 24 also has a relatively thick
portion of the insulating layer 27 between it and the su'~;trate
2l to form a channel stopping region 30 in which the potential
i8 relatively high to prevent the leakage of carriers from one
vertical column to the next. A P+ region 31 extending vertically
adjacent the region 30 also helps to isolate the vertical columns.
A voltage V9l is applied to the transparent electrode 26
to ~orm a deep potential well 33c, as shown in Fig, 5, when
carriers are to be stored. A voltage Vs2 is applied to produce
a shallow potential 33c~ which is higher than the potential 33b
of the shift register 23, when the shifting electrode 28 has
the lower (negative) level of the clock pulse applied to
it. As a result, carriers stored in the photo-sensor 22 are
transferred to the shift register 23, The potential 33c' of
this part of the cemiconductor substrate 21 is lower than the
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potential 33b' of the shlLt register 23 when the higher level
of the clock pulse is ap?Lied to the elec,rode 28a of the shift
register, The potential 33c in the potential well under the
exposed part of the trans?arent electrode 26 ic, of course,
raised to some extent acco.ding to the quantity of stored charges
35.
The overflow portion 2~ includes the opposite conductivity
region 31 that extends vertically along the elements 22 on the
substrate 21 and also includes a portion of the transparent
electrode 26 on a thick section of the insulating layer 27 be-
tween the photo-sensor 22 and the region 31. The overflow por-
tion 32 has a potential 33d that is higher than the potential
33c of the photo-sensor 22 when the voltage Vsl is applied to
the transparent electrode 26, The potential 33d is lower than
the potential 33a of the tranferring portion 24 ~hen either
the higher or the lower (more negative) level of the clock
pulse is applied to the electrode 28a, Thus, when strong light
i9 incident on the photo-censor ~2 so as to cause excessive
voltage build-up due to the storage of an excessive number of
carriers 35, the extra carriers 35a overflow to the region 31,
to which a reverse bias i9 applied. Otherwise the surplus
carriers, if they raised the potential 33c above the level of
the ~arrier 33a, would flow into the shift register 23. The
difference between the ~arriers 33a and 33d is indicated by
a vertical measurement d, If surplus carriers flowed into
~;
the shift register 23, they would reduce the resolution of the
image sensor.
In the embodiment ~ho-~ in Figs. 3-6, the thic~ness t3
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of the insulating layer 27 in the overflow portion 32 is
greater than the thic~ness t4 in the portion of the photo-
sensor 22 that operat2C in ctoring minority carriers. If
the thickness tl is equal to the thickness t3 and if the
thickness t2 is equal to the thickness t4, the voltage
applied to the transparent electrode 26 is more negative than
the lower (negative) leveL of the cLock pulse (~ lSVolts)
applied to the shifting electrode 28 .
The transparent electrode 26 covers all of the insulating
layer 27, but a light shield and conductive layer 34 covers
most of the layer 26 except ~or the portion devoted to the
photo-sensor 22.
Fig. 7 illustrates the method of manufacturing an image
sen90r of the type just described in connection with Figs. 3-6.
A silicon dioxide layer 27a having a thickness ~f about .4 microns
i9 formed on a major surface of the N type silicon substrate
21. The layer 27a is selectively etched, and P+ type regions
31 are diffused through the resulting windows 36 (Fig. 7A).
Thereafter the layer 27a is again selectively etched to
form a silicon dioxide layer 27b having a thickness of .1 micron
(Fig. 7~).
A first electrode 28a of doped poly-silicon is deposited
on the portion 27b of the layer 27a and adjacent portions of
the layer 27a. The electrode 28a also overLaps the adjacent
channel stopping region 31 (Fig. 7C).
An additional silicon dioxide layer 27c having a thic~-
:`~
: ness of about .3 micron ic formed over the entire exposed
~,
surface (Fig. 7D). A cecond electrode 28b OL doped poly-silicon
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(shown in Fig. 6) is ~electLvely deposited on the layer 27c
to be vertically in line t~ith the electrode 28a.
- The layers 27a and 27c are selectively etched to form a
thin silicon dioxide layer 27d having a thicknesc of about
.1 micron in a region that will later become the photo-sensor
22 (Fig. 7E).
A transparent layer 26 of tin oxide or thin, doped poly-
silicon is deposited over the entire exposed surface of the
device (Fig. 7F).
A light shield layer 34 of aluminum is deposited on por-
tions of the layer 26 except those that are to be used as the
photo-sensor 22 (Fig. 7G).
The voltage pulses used in the operation of the device
in Figs. 3-6 are shown in Figs. 8A-8C. Fig. 8A represents
the voltage Vs applied to the transparent electrode 26. This
voltage has a value of -20 volts corresponding to Vsl during
the interval Tl corresponding to the visible part of a tele-
vision field. The voltag2 Vs has a value of -10 volts corres-
ponding to V92 during an interval of T2 that corresponds to
the vertical blanking interval. Figs. 8B and 8C show clock
pulses 01 and 02~ which are square wave pulses having a duty
cycle twice t~e horiæontal line scanning interval of the tele-
vision system with the polarity of 02 opposite to that of 01
Both of the pulses 01 and 02 have a positive value of Ovolts
and a negative value of -15 volts, and they are applied to
alternate shifting electrodes of the vertical shift register
to be operated as a two-phase CCD
When the voltage Vsl of Fig. 8A is applied to the trans-
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parent el~ctrode 26, minor-ty carriers are generated in the
photo-sensors 22 according to the strength of the ligh~ in-
cident thereon and are stored at the potential well 33c, which
is shown particularly in Fig. 5. When the voltage Vs2 is ap-
plied and a cLock pulse vaLue of -15 volts is also applied to
the shifting electrode 28a, the potential 33c' of the photo-
sensor is made higher than the potential 33a of the transferring
portion 24, and carrierC are transferred to the shift register
23. On the other hand, if the 0 volt value of the clock pulse
is applied to the shift register 23,the potential 33cr of the
photo-sensor 22 is lower than the potential 33a~ of the trans-
fer portion 24, so that carriers are not transferred. Since
the clock pulses 01 and 02 are applied in opposite polarity
to adjacent sets of electrodec 28a and 28b? carriers are trans-
ferred from alternate photo-sensors 22 to their respective
shift registers 23, and carriers are transferred at 2Tl inter-
vals in each cell.
Carriers simultaneously transferred from each photo-sensor
22 to the corresponding vertical shift register 23 are sequent-
; ially transferred vertically by the cloc~ pulses 01 and 02.
Carriers from, for exampLe, odd numbered fields are thus in-
serted into the horizontal CCD shift register at one line in-
. tervals and are transferred to an output circuit during each
line interval. After trans~2rring the carriers, which is
equivalent to scanning the i~age optically produced on the
semiconductor image sensor, in one field interval, the voltage
Vs2 is applied to the trancparc~nt electrode 26, and carrierC
that correspond to even numbered fields are simultaneously
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trancferred to the vertical Chift registers and are se4uent-
ially read out by the horizo~tal shift register in the follow-
ing field interval.
Figs. 9 and 10 show an improved embodiment of a hori-
zontal three-phace CCD shift register 37 according to the
invention, This Chift register has a first set of electrodes
44 made up of electrodes 44a, 44b, 44c,... and a second set
of electrodes made up of electrodes 45a, 45b, 45c,......... These
electrodes are formed on the surface of the semiconductor
according to the method of manufacture of vertical shift regis- ;
ters, Each pair of corresponding first and second electrodes
44 and 45 is connected to each other to constitute shifting
electrodes 46. The length ,~ of the three shifting electrodes
46 corresponds to the entire width of each basic element of
the semiconductor image sensor,including a photo'-sensor 40,
a transferring portion 39, a vertical shift register 38, and
a channel stopper 41. Three phase clock pulses 01~ 02 and 03
are applied to sets of three shifting electrodes 46. Each
shifting electrode 46 to which the lowest level pulse is
applied has a stairlike potential well 47 so that carriers are
collected at the well under the first electrode 44. Conse-
quently the effective length of the shifting electrode 46 is
equal to the length Ll of the first electrode 44 rather than
to the length L of the shifting electrode 46. This raises t~e
transfer speed and enables higher frequency driving clock
pulses to be employed. While this invention has been described
in terms of specific embodiments, it will be apparent to those,
skilled in the art that modifications may be made therein within
the true scope of the invention as defined by the following
claims.
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