Note: Descriptions are shown in the official language in which they were submitted.
P~IN 77l17
]3KS~IW
25.~.75
1055159
"Charge transfer device".
.,
TIie invention relntes to a charge transfer
device comprising a semiconductor body having a surface-
adjoining semiconductor layer in which means are present
to locally introduce into the semiconductor layer infor-
mation in the form o.f mobile charge carriers and means
to read out said information elsewhere in the layer,
and an electrode system for capacitively generating .
electr~c fields in the semiconductor layer is present
on the surface by means of which fields the charge
can be transported to the read-out means through the
semiconductor layer in a direction parallel to the se-
miconductor layer, which electrode system comprises a
~ series of electrodes which are insulated from the sur-
face of the body an insulating layer and which are as-
sociated alternately with a first conductor layer~ here-
ina~ter termed lowermost.conductor layer9 and with a
second conductor layer, hereinaf-ter termed uppermost
conductor layer, each electrode of the uppermost con-
ductor layer extending to above an adjacent electrode
20 of the lowermost conductor layer and being separated
herefrom by an intermediate insulating oxide layer
which has been obtaincd by partly oxidi7ing the elec-
trodes as~ociated wi1;h the lowermost conductor layer.
~harge transfer de~ices form a generally
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known class of semiconductor devices which provideim-
portallt and wide appli.cation possibilities, for exam-
ple in the fi.eld of delay lines, f`ilters and picture
sensors. Known constructions Or charge transfer de-
vices are inter a.l.ia the bucket brigade device or B.B.D.
and the charge coupled device, also referred to as ~.C.D.
In these devices the introduced information in the form
of charge packets is each time.transported from a ~torage
site below an electrode to a subsequent storage site
below an adjacent electrode, more or less in a step-
wise manner, by applying suitable clock voltages to the
electrodes.
In most of the practical embodiments the semi-
conductor body consists of silicon. In those cases in
15 which the device forms a charge-coupled device with
charge transport along the surface, the semiconductor
. ~ , .
layer in which the charge.transport takes place may
occupy the whole semiconduct-or body. In charge-coupled
devices with bulk transport, however, the semiconductor
.layer will usually occupy only a thin, relatively high-
ohmic sub-layer of the body of a conductivity type
which, via a p-n junction on the side present opposite
to the surface, changes into a second sub-layer of the
body of the second conductivity type.
In practical embodiments~ the electrode.s of
- thc lowermost conductor layer (metalli~ation layer)
which are provided first during manufacturing the de-
.
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vice are usually formed by silicon layers in a poly-
crystalline form which are deposited on the insulating
layer. By oxidation of the silicon, the eloctrodes of
the lowermost conductor layer may be ~urrounded by a
silicon oxide layer Or the desired thicknoss which in-
sulates the silicon electrodes electrically from the
electrodes of the uppermost conductor layer to be pro-
vided in a subsequent process step. These electrodes
may be, for example, of aluminium.
The use of electrode systems in the form of
multilayer metallization in charge transfer devices
presents many advantages over, for example, the use
of a monolayer metallization, both technologically and
as regards the electric functioning of the devices, and
is therefore generally known. For example, the inter-
electrode space can be made vexy small, which is an
important advantage in charge-coupled devices because
said inter-electrodes spaces are often responsible for
the formation of potential wells or potentiaI hills
between successive charge storage sites and can hence
adversely influ~nce the transport efficiency and/or
the transport rate of the device. . . . . . . . . .
The oxide layer which separates the electrodes
of the lowermost and uppermost conductor layer from each
other cannot be made arbitrarily thin~ as will be ob-
vious, but will have to be at least so thick that
breakdown between the electrodes is prevented at
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the clock voltagcs to be ~pplied to tho electrodes. The
oxidation treat.nent ~hich is carried out to obtain said
oxide ~ layer can hence also oftel1 influence the si-
licon oxide layer on the surface of the body which in-
sulates the electrodes from the body. In particular,
for example, the semiconduotol body may further oxi-
di~e, notably between the silicon electrodes~ so that
the silicon oxide layer between the silicon electrodes
can become thicker than below said electrodes.
It has already been suggested, in order to
avoid said drawback, to provide on the surface of the
semiconductor body an oxide layer and thereon a silicon
nitride layer. A dielectric in the form of such a double
layer has the advantage that it does not change or at
least hardly changes during the oxidation of the sili-
con electrodes provided (on the nitride layer), in par-
ticular as a result of the action of the nitride layer
masking the semiconductor body against oxidation.
However, a number of drawbacks are associated
with the use as an insulating layer ~ such a dielectric
consisting of a double layer. For example, it may be
necessary in behalf of thee~hing of contact holes in
the nitride layer at the area of the input and/or the
output of the device, to provide on the nitride layer
25a an extra auxiliary masking layer, for example, of si-
licon oxide. Said auxiliary masking layer should be
removed again in a further etching tr~atment.
-- 5
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,, : , ,, " , "
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~urtllermore, as is known, it is often desired
to subject the device to a so-called anneal treatment
in a suitable medium during or at the end of the manu-
facturing process, so as to reduce the number of surface
states at the interface betwecrl the semiconductor body
and the insulating layer on the surface of the semicon-
ductor body. It has beèn fo-md that such a treatment
in the presence of a nitride layer is often less effec-
tive than is desired, in particular when the device oc-
cupies a surface area which is comparatively large for
a semiconductor device. A presumable cause hereof re-
sides in the comparatively very large density of the
silicon nitride material as a result of which it is
substantially impenetrable even for~ for example,
hydrogen molecules, so that only lateral diffusion
of gas molecules through the oxide layer Or the crystal
is possible.
One of the objects of the present invention
is therefore to provide a charge transfer device of
the kind described in the preamble in which the oxi-
dation treatment to oxidize the electrode of the lower-
most conductor layer does not or at least substantially
not vary the insulating layer between the electrodes
and the surface of the semiconductor body, and in which
the said difficulties can be avoided at least for the
greater part.
: The invention is based inter alia on the re-
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1055159
cognition of the fact that by locally rcmoving the si-
licon nitricle ]ayor, after the said oxidation treatment,
the operation of tlle device to be manufactured need not
or at least need hardly be influenced, ~qhile a structure
S is obtained which is very simple to m~lufacture techno-
logically.
Therefore, a chàrge transfer device of the
kind described in the preamble is characterized accord-
ing to the invention in that the insulating layer which
insulates the electrode from the surface of the semicon.-
ductor body comprises two sub-layers of different ma-
terials, namely a first sub-layer which adjoins the
surface of the body and which extends both below the
electrodes of the lowermos.t conductor layer and below
1~ the electrodes of the uppermost conduc-tive layer, and
a second sub-layer ~qhich is provided on the first sub-
layer and which shows apertures present below the elec-
- trodes associated -.~ith the uppermost conductor layer
and which is of a material which masks the underlying
semiconductor material against oXidation and which can
be etched selectively relative to the material of the
first sub-layer.
During the manufacture of a device according
to the invention, after the oxidation of the electrodes
of the lowermost conductor layer, the second sub-layer
(the nitride layer) may be subjected to an e-tching
treatment without an extra pho-to-masking step. The
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electrode~s of the lowerlllost conductor layer and the
oxide layer growll on said el~ctrodes mask the under-
lying nitride against said etching treatment so that
the nitrido layer will be removed only between said
eloctrodes. Moreover, the nitride layer may at the
same time~ be removed at the area of the input and~or
the output of the device to be manufactured, so that
it is not necessary in a later production stage to
provide contact holes in the nitride layer, which con-
siderably simplifies the manufacture of the device.
Further advantages of the device will become
apparent from the following description of the Figures.
A preferred embodiment of a charge transfer
device according to the invention is characterized in
that the electrodes of the uppermost conductor layer
are provided on the first sub-iayer of the insulating
layer through the apertures in the second sub-layer.
DUQ to the local removal of the ~nitride
layer, the insulating layer below the electrodes of
the uppermost conductor layer may be thinner than
beiow the electrodes of the lowermost conductor layer
in certain embodiments of a charge transfer device ac-
cording to the invention. In many cases, however, this
need not be annoying since the thickness of the second
sub-layer (nitride layer) is generally small as compar-
ed with the first sub-iayer (oxide layer) of the in-
sulating layer.
s
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However, the sa~d diffcrence can be compensated
for in a simple manner by subjecting the device to an
oxidation treatment after the local removal of the nitride
layer (second sub-layer). ~t the area of the apertures
in the second sub-layer~ the body may further oxidize
so that in said places t~le insulating layer can become
thicker, whilQ as a result of the action of the second
sub-layer masking against oxidation, the semiconductor
body does not further oxidize below the electrodes of
the lowermost conductor layer. A further preferred em-
bodiment of a charge transfer device according to the
invention is therefore characterized in that the first
sub-layer of the insulating layer is formed by an oxide
layer which, at the area of the apertures in the second
sub-layer below the electrodes of the uppermost conduc-
tor layer, has a larger thickness than below the elec-
trodes of the lowermost.conductor layer.
In most of the cases the oxide layer to oe
formed below the apertures in the second sub-layer is
very thin - in practical embodiments not more than a
few hundreds of Angstrom units -, in particular with
respect to the oxide layer which is provided on the
electrodes ~f the lowermost conductor layer. The
extra oxidation step which is necessary to grow said
thin oxide layer may therefore be of a short duration
or may take place at a comparat~vely low temperature
and, if desired, be combined with one or more rurther
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25.8 .75
10551S9
temperaturc treatments, for e~ample, gettering.
The dielectric constants of thc two sub~layers
will gellerally be different. In most o:f the cascs the
dielectric constal1t of the second sub-:layer maskin~
a~a:inst ox:idation will be largor than that of the oxide
lay~r. A .further preferred embodirllorlt of a charge trans-
fcr device according to the invention in which said dif-
ference has been compensated for at least partly, is
characterized in that the thickness of the insulating
layer at the area of the apertures in the second sub-
layer below the electrodes of the uppermost conductor
layer is substantially equal to and preferably smaller
than the overall thickness of the insulatinglayer below
the electrodes of the lowermost conductor layer.
- Although other materials may also be used,
the semiconductor body preferably consists of silicon
;~S~\o~ ~
and the second sub-layer of the in~ ati~ layer mask-
ing the body against oxidation consists of silicon
nitride. The electrodes of the lowermost conductor
layer which should be of an oxidizable material prefer-
ably consist of silicon which usually is deposited in
a polycrystalline form on the second sub-layer and is
doped with a suitable impurity to reduce the resist-
ance.
The invention present.s important advantages
in any type of charge transfer devices. However, a
preferred ernbodiment of a charge trarlsfer device ac-
.- 10
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cording to.the inventioll which, as experiments have
demollstrated, h~s particularly good electric properties
is characterized in that the device belongs to the type
o~ charge-coupled devices in whicll the charge transport
takes pl.ace at least mainly via the interior of the se-
miconductor body, means being present to insulate the
semiconductor layer fro!n the surroundings, the semi-
conductor layer having a thickness and a doping con-
centration at which a depletion zone can be obtained
19 throughout the thickness of the semiconductor layer
by means of an electric field while avoiding break-
down.
The invention relates in addition to a method
of manufacturing a charge transfer device comprising a
semiconductor body having an electrode system which is
provided on a surface for capacitively generating elec-
tric fields in the body'by means of which electric
charge can be transported through the body, which
electrode system comprises a series of electrodes
which are insulated from the surface of the body by
an insulating -layer nnd which alternately belong to '
a first conductor layer, hereinafter referred to as
lowermost conductor layer, and to a second conductor
layer, hereinafter referred to as uppermost conduc-tor
layer, each electrode o~ the uppermost conductor layer
extending to above an adjacent electrode of the lower-
most conductor layer and '~eing separ~ted herefrom by'
- 11 -
.
~ . . ' " , : , , . ; ~, .
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1055159
al~ intermediate insulatin~ layer. Accordillg to the in-
ventioll, sucll a method is characterized in that thc i.n-~
sulating layer which insu]ates tlle el~ctrodcs from the
surface Or tlle semicondllctor body is provided in thc
forlll of a double layer comprising a first sub-].ayer ad-
JOinillg the sur:~ace o~ the. body nnd a second sub-layer
provided thereon and consisting of a material differ-
ing f`rom the first sublayer and maslcing the semiconduc-
tor body against oxidation, and that, after providing
.10 the electrodes belonging to the lowermost conductor
layer, said electrodes are subjected to an oxidation
treatment to obtain the said intermediate insulating
layer, the second sub-layer masking the underlying
. material of the semicond.uctor body against oxidation
.15 during said oxidation treatment, and that after the
oxidation treatment the second sub-layer is subjected
to an etching treatment as a result of whlch the second
sub-layer is removed.locally, the electrodes of the
lowermost conductor layer ~th the oxide layer formed
~hereon serving as an etching mask, and that after
said etching treatment the electrodes of the uppermost
conductor layer are provided which are sepa~ated from
the surface of the semicondllctor body at least mainly
only by the first sub-layer of the insulating layer.
By u.sing such a meth~d the drawbacks already described
above can be avoided at least for the greater part.
An important pre~erred embodiment of a method
~ 12
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according to the invention is charactorized in that af-
ter the oxidation treatlllellt ~ the electrodes of the
low~rmost conductor layer to obtain the said inter-
mediate insulating layer, the semicorlductor body is
subjected to a gettering treatllleIlt in behalf Or wllich
thc semiconductor body is covered, at lcast at its
major sllrfaces, with an impurity-doped gettering oxide
layer which is separated from the said surface by ,a
screening layer which is of the same material as the
said second sub-layer and which is provided prior to
the said etching treatment above the clec*rodes of the
lowermost conductor layer and the second sub-layer and
which is removed again entirely during the etching
treatment in which the second sub-layer of the in-
sulating layer is locally,removed.
The invention will now be described in greater
detail with reference ta a few embo'diments and the ac-
companying diagrammatic dra~ing, in which
Fig. 1 is a sectional view of a part of a
charge transfer device according to the invention;
Figs. 2-6 are sectional views of the device
shown in Fig. 1 during a number of stages of the manu-
facture thereof;
Fig. 7 is a sectional view of a part of a -,
25 ' further charge transfer device according to the in~
vention;
Figs. 8 a~d 9 are sectional views of the
, ~
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1055159
device shown in Fig. ~ during various stages of manu-
facture.
It is to be noted that the Figures are dia-
gral~llatic only and are not dra~n to scale for reasons
of clarity.
Fig. 1 is a cross-sectional view parallel to
the charge transport direction of a part of a charge
transf`er device according to the invention.
The device comprises a semiconductor body
of silicon having an _-type semiconductor layer 3
which adjoins the surface 2 and which, in case the
charge transport takes place along the surface 2,
may extend throughout the body 1, but which in the
present case in which the charge is transported
through the bulk of the body is formed only by a
~"~ surface layer of the body which, via a ~-n ~unction
4, changes into a ~-type part or substrate 5.
~ The semiconductor layer 3 comprises an elec-
`~ tric input having the contaot 6 and the contact 7
which is of the same conductivity as and has a higher
doping than the layer 3. It will be obvious that,
` besides via the electric input contact 6j 7, the
information can also be i~troduced into the semi- --
,; ,
conductor layer differently, for example, by absorp-
tion of electromagnetic radiation. The semiconductor
layer 3 furthermore has means to read out said infor-
mation elsewhere in the layer 3, which means are denoted
' . , : .
~, ' ,
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1055159
dia~ramIllatically by the output contact 8 wl1ich i9 con-
tacted to the layer 3 via the higllly dopod n-type con-
tact zone g.
Present on the surface 2 is an electrode sys-
te?ll for capacitively genorating electric fields in the
layor 3 by means of ~Y}liC}l the charge can bo transport-
ed tllrollgh the semiconductor layer 3 in a direction pa-
rallel to the layer from the input 6, 7 to the output
8~ 9. The device may be operated as a two-phase (four-
10 phase) or as a three-phase charge transfer device. De-
pendent hereon, the electrodes 10, 11 belonging to the
electrode system may be connected together with two or
three clock lines not further shown in Fig. 1 to apply
cloc~ voltages. The electrode system comprises a series
15 of electrodes 10, 11 which are insulated from thé sur-
face 2 of the body 1 by an insulating layer 12, 13.
The electrodes 10, 11 belong alternately to afirst
conductor layer, hereinafter termed lowermost conduc-
tor layer, and to a second conductor or metallization
20 layer, hereinafter termed uppermost conductor layer,~
the electrodes of the lowermost conductor layer being
referenced 10 and the electrodes of the uppermost con-
ductor layer being referenced 11. The electrodes 11
of the uppermost conductor layer each extend to abo~e
25 the adjacent electrodes 10 of the lowerrnost conductor
layer, as is shown in F~. 1, and are separated here-
from by an intermediate oxlde layer 1ll obtained by
- 15
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.. . . . . . . . . .
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1055159
o.Yi~lizillg the electrodes ~0 partly. By USi31g such an
electrode systel.l, the inrluellce of the finite distance
betl~oen thc electrocles mutually on, for example, the
transport efriciency O:r t:he device can be consid~rably
reduced.
Acoorcling to the invelltioll, the insulating
layer 12~ 13 which insulates the electrodes 10, 11
f`rom the surface 2 of the body 1 comprises two sub-
layers of different materials which are referenced 12
and 13, respectively. The first sub-layer 12 adjoins
the surf`ace 2 of the body 1 and extends both below
the electrodes 10 of the lowermo~ conductor layer and
below the electrodos 11 of tho uppermost conductor
layer. Said sub-layer is formed by a silicon oxide
layer ~rhich in the present embodiment has been ob-
tained by conversion of semiconductor material of
the semiconductor body 1 by oxidation. A second sub-
layer 13 which, unlike the sub-layer 12, does not ex-
tend below all the electrodes is present on the oxide
layer 12 and has apertures 15 (see also Figs. 5 and 6)
below the electrodes 11 of the uppermost conductor
layer. The sub-layer 13 is of a material which masks
the under]ying semiconductor material of the body
against oxidation and which can selectively be etched
with respect to the silicon oxide of the f~st sub-
layer. Although, of course, other materials may also
bo considered for this purpose, si1icon nitride is a
16
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material whicll can advantageously be used for the second
Slll~-l~y(3:1:` 13.
~ccord:ing to the invention, one of t~e im-
portant advantagcs of a device as is sho~rn in Fig. 1
i9 that, as ~i]l becollle apparent hereinafter, the manu-
facture thereof is simple in spite of the prosence of
the silicon nitride layer.
As is furthermore shown in ~ig. 1, the elec-
trodes ll of the uppermost conductor layer are directly
10 provided on the first sub-layer 12 of the insulating
làyer12, 13 through the apertures in the second sub-
layer 13.
The electrodes 10 of the lowermost conduc-
tor layer which consist o~ an oxidizable material are
15 formed by layers of silicon ~rhich is deposited in a
polycrystalline form on the second sub-layer 13 of
silicon nitride. The electrodes 11 of the uppermost
conductor layer are of aluminium but may, of course,
also consist of other suitable materials, for example,
20 silicon.
As already noted, the device shown in Fig. 1
belongs to the type of charge-coupled device in which
the charge transport takes place at least mainly via
the interior of the semiconductor body. For that pur-
25 pose, means are present to insulate the semiconductor
layer 3 - at least during operation - from its sur-
roundings. These means :Lnclude inter alia the ~-n
,
- 17
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.
r}IN 771I7
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~junct.ion 4 on the lower ~ de of the layor 3 which, dur-
ing operatioIl~ can be b:iased in the reversl3 direction,
an~l the p-type isolation ~.olle 16 which, viewecl OII the
surface 2, surrouIlds t;he layer 3 0ntirely. ThC isolation
ZOllQ 1 l~ hich may extend throughout the thic.l;ness of thc
layer 3 in this case extQIlcls only over a part Or said
thickIless. 13y applying a sufficiently low voltage to
the isolation zone 16, the island insulation may be
completed . by means of an e:Lectric field which extends
below the isolation zone 16 in the body 1.
The thickness and the doping concentration
in the semiconductor layer 3 are chosen to be so small
that a depietion zone can be obtained throughout the
thiclcness of the semiconductor layer by means of an
electric field while avoiding breakdown. Potential
minima may then be formed in the depleted semicon-
ductor layex 3 so that; majority charge carriers can
be stored and transported at a finite distance from
the surface 2. Such a thin high-ohmic layer may be
formed, as in the embodiment described, by an epi-
taxial layer which is grown on the substrate 5,
but may also be obtained, for example, by redoping a
comparatively thin surface par t of the substrate 4 by
means of, for example, ion implantation.
Tho manufacture of the device shown in
Fig. 1 will now be described in greater detail also
wi.th reference to Figs. 2 to 6. Starting material is
.
Pl-IN 77~7
25.8.7~
10~5~59
the ~-typc sil:icon substr,atc 5 whic]1 ~las a resistivil;y
wllicll prererably exceecls 10 ollm.cm .~l~ has a thic]cness'
of app~o~iIllately 250/um. Thc other dilIlerlsiolls are not
criticaI. and arc assumed to be sufriciently 1.argc for
the devi.co to be nlanuf`actured. An n-type epitaxial
layer 17, thickness, for example, 5/um doping concen-
tration approxiluately 6.1014 atoIns/cm3~ is grown on
the substrate 5 by means of an epitaxy process.
.The ~-type isolati.on zone 16, the n-type
contact ~ones 7 and 9 and possible further zones of
further circuit elements may be provided in the epi-
taxial layer 17 in the usual manner and by means of
known diffusion and/or ion implantation techniques,
after which the surface 2 is provided with the oxide
layer .12 which is obtained by thermal oxidation at the
surface of the semiconductor body~ The thickness of the
silicon oxide layer is approximately 800 A.
For the protection of the oxide layer 1Z
against inter alia further oxidation treatments, the
silicon nitride layer 13 is deposited in a thickness '~-
of approxi~ately 350 ~ on the oxide layer 12 by means
of deposition from the gaseous phase. Fig. 2 shows the
device in this stage of' the manufacture.
The electrodes 10 of the lowermost conductor
layer are then provided on the nitride layer 13 (see
Fig. 3) by depositing a polycrysta],line silicon layer
which :i~ removed a~aln locally by means of etching so
,. _ 19
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1055159
that the e1,oct;rodes 10 al~d possi~ly furtIler connections
are obtained OII the nitride layer 13. The thickness of
the electrocle~s 10 is, for exnlllp.Le, approxilllately O.G/um.
The material of the eloctrodes 10 may further comprisc
a suitable iIllpurity~ for example, boron or phosphorus,
in a su.fficiently high concentration to roduce the re-
sistivity.
By heating at approximately 1000 C in an
oxidizing medium, the silicon electrodes 10 may then
be oxidized to obtain the silicon oxide layers 14 (see
Fig. 4-) w]lich will insulate the electrodes 10 from the
electrodes 11 of the uppermost conductor layer to be
pro~ided afterwards. The thickness of the oxide layer
14 is ohosen to be at least solarge that at the c3.ock
voltages to be applied to the electrodes 10, 11, break-
,~ dowI~ between the electrodes is prevented. A specific
value for this thickness is approximately 0.3/um.
It is to be noted that during said oxidation
treatment the oxide layer 12 on the surface 2 of the
semiconductor body does substantially not vary, in par-
ticular as regards the thickness, as a result of the
presence of the nitride layer 13 masking against oxi-
dation.
After the oxidation treatment the silicon
nitri.de layer 13 is swbjected to a selec-tive etching
treattlIent in an aqueous phosphoric acid solution at
; approximately 180C, in w'hich the sili.corl oxi.de is
. ~ ~0 ~
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25.~.75
lOS5~59
not nttacked or is at lea~t substalltially not attacked
and ill which t}le nitridQ layer is removed abovc 1;hc
zones 7, 9 and 16 to be contacted and botween tho elec-
trodes 10. ~s a result of said etching treatment -
t~hich may be carried 011t withollt; the usual photom~sk-
ing techn:iques - apertures 15 are formed in the nitride
layer bet-~een the electrodes 10, see Fig. 5.
In the resulting structure contact windo-s
may tllen be provided in the insulating layer 12 in be-
half` of the contacting of the isolation zones 16 and
the provision of the input and output contacts at the
area of thc contact zones 7 and 9. As a rcsult of`-the
local removal of the nitride layer 13 it is necessary
only to provide the contact windows 18 in -the oxide ~-
layer 12 so that problems associated with the provision
o~ contact windows in a nitride layer can be avoided.
As is usual, an etching mask consisting of a layer of
photolacquer can simply be provided on the oxide layers
12 and 14, af`ter which the windows can be etched in the
oxide layer 12 in a s~itable etching bath simultaneously
with contact windows (not shown~ in the oxide layers
14, after which the layer of photolacquer may be re-
moved again. Fig. 6 shows the device in this stage of
its manufacture.
The contacts 6 and 8 of the input and the
OUtpllt, respectively, of` the device and the contacts
19 of the isolation zones 16 may be provided simul-
~1 --
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.
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t;an~ously with the e:Lectr~odes 11 of the uppermost con-
ductor .layer by dopositing a layer of aluminium in ~}lich
tlle electrodes 11 and the necessary condllctor tracks can
be obtail1ecl in tlle~ usual manner l)y etching.
At`ter etchi.l1g, the contacts 7, 9 and 19 may
l)e fllrther al.loyed by lleat:illg the devlcQ to, for exam-
ple, approximately 4500C in a medium to whic}l has been
added, for example, H2 so as to reduce surPace states
at the interface between the surface 2 of the semicon-
ductor body 1 and the oxide layer 12. It is to be not- .
ed that such an afterfiring treatment generally proves
to be particularly effective, in comparison with struc-
tures in which thc nitride layer 13 extends through
the surface 2 and is not provided with the local aper-
ture 15, which may also be considered as an irnportant
advantage of the device.
The oxide lay~r 12 in the charge transfcr
dc-vice shown in Fig. 1 shows a uniform thickness. As
a result of this the overall thickness of the dielec-
20 tric below the electr-Jdes 10 of the lo~-Termost conductor
layer is slightly larger as a result of the presence of
the nitride layer 13 than below the electrodes 13 of
the uppermost conductor layer. In many cases this dif-
fercnce is no objection, the more so since the thicX-
ness o~ the nitride layer 13 is small as compared
with the underlying oxide .1.ayex 12. IIowever, the i.n-
vention pres~nts the furtller advantage that this dif-
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lO551S9
ference in a device according to the i.nvention can be
compensated. ~or in a very simp.1.e manner as will now
be described in de~ta.il with rcferellce to the ensuing
cmbodiment. Thls embod:iment relates to a charge-
coupled device ~hich is substantially identical to that
of the preceding emboclimcnt and, as will be apparent
from Fig. 7, is therefore referred to by the same
re~erence numerals as ~ar as corresponding identical
components are concerned.
The electrodes 10, 11 are insulated from the
semiconductor body 1 by an intermediate insulating layer
which again comprises two sub-layers of different ma-
terials. The lowermost sub-layer 22 which is formed
by a silicon oxide layer obtained by oxidation at the
1.5 surfac.e of the body again extends .throughout the semi-
conductor layer 3. The second sub-layer 23 of silicon
nitride again shows apertures 2~ below the electrodes
11 of the uppermost conductor layer. The electrodes 11
are provided on the oxide layer 22 via said apertures.
~nlike the oxide layer 12 in the preceding
embodiment, the oxide layer 22 does not have a uni-
~orm thickness but at the-area of the apertures 24in
the nitride layer 23 below the electrodes 11 of the
. uppermost conductor layer it shows a larger thickness
than below the electrodes 10 of the lowermost conduc-
tor layor. The efI'ective thickness of the insulating
layer 22 below the elect:ro-les 11 may h.ence be e~lual
,
. - X3
~' 7 j
~5.~.7
1055159
c-r at least be substanti<llly equal to the erfective
-thlcklless o~ thc insulating 1nycr 22, 23 below the
electro~-'es 10, so tha-t the charge storage capacity
below the electrodos 10, i 1 per unit of surfacc C~l
~e substallt:ially the same at least witll the same
voltage.
Because the die:Lectric constant of the ni-
tride layer 23 masl{illg against oxidation is generally
larger than that of th0 oxide layer 22, the thicknoss
Or the oxide layer 22 below the electrodes 11 is chosen
1;o be smaller than the overall thickness of the oxide
layer 22 and the nitride layer 23 below the electrodes
10 of the lowermost conductor layer.
The manufacture of the charge transfer device
sho~Yn in Fig. 7 is also particularly simple and in par-
ticular with respect to the device described in the pre-
ceding embodiment it requires no extra critical and/or
laborious photomasking steps. Starting material for the
manufacture may be a structure as is sho-rn in Fig. 5
of the preceding embodiment in which instead of the
layers 12 and 13 the layers 22 and 23 are provided
on the body in a thickness of approximately 800 A
and 350 A, respectively.
After providing the oxide layers ll~ by oxi-
dat:Lon of the polycrystalline silicon electrodes 10 -
in which the semiconductor body is masked against oxi-
dation by the sil:icon nitl~ide layer 23 - an additional
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25.8.75
1055159
protective layer 25 is provided tl~roughollt the surface
of the~ device. Sa.id laycr in the prcscnt embodiment al-
so consists of si.licon nitride, see ~ig, 8. A phospho~1s-
doped oxide layer 26 ~Yhich forms a getter.ing layer i9
then provided on the lo~er side of the body 1. At the
same time a similar phospllorus-doped oxi.de layer 27 ,is
deposited on the upper side and is separated from the
oxide layer 14 by the intermediate additi.onal silicon
nitride layer 25.
It is to be noted that it is generally known
andusllal in semiconductor technology during the manu-
facture of a semiconductor device to screen the upper
side of the device where usually the active elements
are present before providing the gettering layer 26
by depositing on said side a silicon oxide layer from
the gaseous phase. The phosphor oxide layer 26 may
then be provide'd on the lower side, di~.fusion of phos-
phorus on the upper side of the device being prevented
by means of the provided silicon oxide screening layer.
In a subsequent process step the screeningr layer should
usually be removed again entirely or at least partlyO
In particular because the screening layer generally
shows some spreading in thickness, the passiva,jting
layer present on the surface of the body and usually
also consisting of silicon oxide may also be attackec1
duri~g the etching away. The possi,bility even exi.sts
that the pa~sivating layer i.s etched locally throu~,rh-
, - ~5
Pf~N 77
~ 8.7
1055159
01lt it:s th:Lckll~ss so that apertures are forlrlod in the
pa~s:ivatillg layer :La wllich shortcirc~ t mcly occur.
IIow~ver, this dr~wbac.~ can b~ avo.ided like in the pre-
sel1t elllbocli.l1lellt; by provi.ding a screcnil1g layer 25
WhiCll Cclll ~e otched selectively relati.ve to silicon
oxide on th~ uppcr side of the device before provid-
.ing the gottering layer 26. .In the presQnt embodimQnt
th~ screening lay~r 25 WhiC]l protccts the polycrystal-
- l:ine silicon electrodes 10 from the pho~sphor oxide
layer 27 consists of silicon nitride which can be
etched selectively relative to the silicon oxide layer
14 above the polycrystalline silicon electrodes 10.
lIowever, i.nstead of silicon nitride, of course other
mate~rials, for example, alumillium oxide or double layers
i5 of, for example, silicon nitride and silicon oxide which
is deposited on the nitride, may also be used. In ad-
dition said method may also be used advantageously
during the manufacture of charge transfer devices
other than those described here.
The oxide layer ~ on the upper side is then
removed again, for example, by means of` etching, in
which the oxide layer 26 on the lower side of the body
can be masked by a layer of photolacquer provided on
the whole lower side. After removing the oxide layer
27 the additional si.licon nitride lay~r 25 is rcmoved
by etching in phosphp~ic acid at a temperature of ap-
proximat;ely 180C. In this etching treatmQnl the si.li-
- 2G _
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PllY 77~l/
1055159
con oxide presellt :Ls not or at least hardly attackcd.
At. the samo time the silicon nitride layer 23 in so
far as it is n~ maskcd by the c.lectrodes 10 and the
as~ociatcd oxide laycrs 14 is removed so that thc oxide
layc?r 22 above the input and output ZOIIOS 7 and 9 and
abo~re the isol.ation zone 16 is exposed and ap-3rtures
2l~ are ~orllled in the nitride layer 23 between the elec-
trodes 10.
A so-called "getter-drive-in" step or gctter
aft;erfiring step is then carried out in which heavy
metal atoms presumably present in the body 1 dif~use in
an accelerated manner in the direction of the body
layer 26. This "getter-drive-in" step is carried out
at a temperature of approximately 1000C in a medium
whicll is oxidizing at least ~or a comparatively short
tinle. Below the apertures 24 in the nitride layer and
above the zones, 7, 9 and,16 where the semiconductor
- body 1 is no longer masked agai.nst oxidation by the
nitride layer 23, the oxide layer 22 locally increases
in thickness during the getterillg treatrnent as a -result
of oxidation at the surface 2. The oxldation is continued
until the oxide layer 22 below the apertures 24 (see
Fi6r. 9) is approximately 200 A thicker than below the
electrodes 10. The overall thickness Qf the insulating
layer 22 at the area o:~ the apHrtures 24 therl i.s ap-
proximately 1000 A si?.icon oxi.de, whi]e the insulat-
in~r layer below the electrodes 10 consis-ts of approxi-
, - ~7
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25.~,7~
lOSSlS9
nlately 800 ~ silicon oxide ancl of` 350 ,~ silicon nitricle.
After the combilled gettering treatmellt and
oxidation treatmerlt, contc~ct wil1(lows may be provided
in thc ox:i~le layer 22 a.t the area of tho zones lG, 7
and 9 and in thc oxide laycrs 1l~, whicll requirQs no
extra laboriolls steps since the nitl~ide .1.ayer 23 i.s
locally removed entirely. Just as in the preceding
embodiment, the electrodes 11 of` alumi.nium and the
contacts 6, 8 and 16 may thell be provided in the
1 10 usual manner.
It will be obvious that the invention is
no-t restricted to the embodiments described but that
many variations are possible to those skilled in the
art without departing from the scope of this inventi.on.
For example, instead of` a hoJnogeneously doped
semiconductor layer 3, a semiconductor lnyer may advan-
tageously be used which has a relatively highly doped
thin surf`ace layer and an underlying and adjoining
relatively low doped th:ick region. Such a highly dop-
ed thin surf`ace lay~r is shown in Fig. 1 by the broken
lines 28.
The oxidation treatment which is carried out
simultaneously with the gettering treatrnent so as to
locally increase the thickness of` the oxide layer 22
in the device according to the second embodiment may
al~o be carr.i.cd out during other temperature treat-
ments or in a separate process step,
,' ' ' ,: ' ~ ,
N 77ll7
25.~.75
1055159
Materials othcr thall those mcntionod here may
also be used advantagcously. For cxample, the electrodos
10 of tho lowerlnost conductor layer may consi..st of a
suitable metal, I`or example, alumil1:ium or tantalum,
bcsides o~ polycrystalline silicon.
Instead of in charge-coupled devices with
bulk transport, the imrention may also be used in
other types of charge transfer devices, for example,
in charge-coupled devices having charge transport
along the surface of the semiconductor body or in
bucket brigade devices.
Furthermore, in the embodiments described
the aluminium electrodes 11 may eacll be connected
conductively to an adjacent silicon electrode 10,
15 either externally, or via contact windows in the
oxide layers 15.
_ 2~ -
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