Note: Descriptions are shown in the official language in which they were submitted.
20092~2
-1- R '~
BILAYER OXIDE FILM AND PROCES~; FOR PRODUCING SA~F
. _ .
This invention relates to a process for producing films
made up of two different metal oxides in which a first metal
oxide predominates at one side of the film and a second metal
oxide predominates at the other side of the film (such films
being referred to hereinafter as bilayer oxide films). The
invention also relates to bilayer oxide films produced by the
method and to devices incorporating such films.
Metal oxides have a variety of useful physical and
chemical properties, such as inertness, stability, abrasion
resistance, resistance to high temperatures, etc. This makes
metal oxides particularly useful as coatings for other
materials or in other applications in which they are required
to be in the form of supported or unsupported thin films. We
have also found it particularly useful for certain applica-
tions to provide bilayer oxide films which make use of the
different properties of two different metal oxides.
Metal oxide films are often produced by vapou~ deposition
techniques but these methods are not always entirely suitable.
For example, these methods involve high temperatures due to
the high melting point (for evaporation) and high heat of
condensation (for sputtering) of these oxide materials.
Consequently heat sensitive substrates can be damaged or
destroyed. Vapour deposition techniques also usually involve
the use of high vacuums which may cause out-gassing from
certain materials, such as paper, making the methods unsuited
for the coating of such substrate materials. A further
disadvantage is that it is expensive using the known
techniques to produce bilayer films, i.e. those in which a
first metal oxide predominates at one side of the film and a
second metal oxide predominates at the other side of the film.
It is accordingly an object of the present invention to
provide a process for producing bilayer metal oxide films
which overcomes these disadvantages and makes it possible
to produce films of this type as coatings on a variety of
substrates or as unsupported films.
~Q~92~
One possible way of producing bilayer oxide films is by
anodizing a structure comprising a layer of a metal which can
be anodized to consumption (e.g. aluminum) deposited on a
metal which at least forms a barrier anodic film (e.g. a
valve metal). ~owever, oxide films formed in this way
normally adhere tenaciously to the substrate metal and cannot
therefore be transferred to other substrates. This type of
procedure is mentioned in Russian Patent No. 817 099 to
P. P. Rhanzhin et al published on ~arch 30, 1981. In this
reference, an aluminum foil is deposited (by rolling or
vacuum spraying) onto a substrate made of a valve metal
(e.g. titanium) and anodization is carried out until all the
aluminum and some of the underlying valve metal is consumed.
According to this reference, the anodic layer can easily be
separated from the substrate ~by a slight mechanical action~
with the separation taking place at the interface between the
aluminum oxide layer and the valve metal oxide layer. It
would therefore seem that a bilayer oxide film is not produced
when the film is separated from the substrate. Moreover,
attempts to duplicate this procedure have resulted in the
formation of anodized films that cannot easily be removed
from the suLstrate.
A similar procedure is disclosed in an article by W.~.
Baun entitled "Anodization of evaporated aluminum on Ti-
6 wt ~ Al - 4 wt~ V", Journal of Materials Science, 15 (1980)
2749 - 2753. 8aun tried to adapt the well-known technology
for porous anodizing Al, used to prepare the surface for
adhesive bonding, to Ti by evaporating a layer of Al onto a
Ti substrate and then treating it according to the standard
procedures known for Al. The anodic film thus obtained did
not have the porous structure associated with similarly
prepared films on Al alloys. Rather than a bilayer film,
a non-porous mixed Al-Ti oxide structure was formed.
Furthermore, the article states that the duplex oxide forms a
strong bond to the underlying metal and presumably couId not
be easily detached.
Despite these negative indications of the prior art, we
2009212
bave found that anodization techniques can be used to form
bilayer films a~d that the films can be made readily and
uniformly detachable from the metal substrates and are thus
transferable.
According to one aspect of the invention there is provided
a process for producing a bilayer oxide film comprising a
layer containing aluminum oxide and a layer containing an
oxide of a valve metal, sai~ process comprising: providing a
substrate comprising said valve metal or an anodizable alloy
of the valve metal, at least at an exposed surface thereof;
forming a coating on said surface of a material selected from
aluminum and anodizable a~uminum alloys; anodizing said
coated substrate for a sufficient time and at a sufficient
voltage to consume said coating and some of said valve metal
of said substrate to form a bilayer oxide film, said anodiz-
ation being carried out in the presence of an adhesion-
reducing agent capable of reducing the strength of attachment
of the bilayer film to the remaining metal; and detaching
said bilayer film from said remaining metal.
According to another aspect of the invention there is
provided a bilayer oxide film comprising: a layer of aluminum
oxide and a layer of an oxide of a valve metal, said layers
being integral said film having been produced by a process
which comprises: providing a substrate comprising said valve
metal or an anodizable alloy of the valve metal, at least at
an exposed surface thereof; forming a coating on said surface
of a materizl selected from aluminum and anodizable aluminum
alloys; anodizing said coated substrate for a sufficient time
and at a sufficient voltage to consume said coating and some
of said valve metal of said substrate to form a bilayer oxide
film, said anodization being carried out in the presence of
an adhesion-reducing agent capable of reducing the strength
of attachment of the bilayer film to the remaining metal; and
detaching said bilayer film from said remaining metal.
The invention also relates to a polymer film having a
coating of a bilayer oxide film and to magnetic recording
media incorpcrating a bilayer oxide film.
2~0921~
-- 4 --
The ~bilay~r~ oxide films produced by the process of the
present invention may consist of two homogenous layers with a
sharp interface between them, or two homogenous layers
separated by a region in which the two different oxides
are mixed or mingled together, or a layer at one surfaçe
consisting predominantly of one oxide and a layer at the
other surface consisting predominantly of the other oxide,
with a gradual change of ratio of the oxides fron one surface
to the other. The actual structure obtained depends on the
choice of the valve metal and on the conditions employed for
the anodization step. However, it should be understood that
the term ~bilayer" as used throughout this specification is
intended to encompass all these types of structure.
The valve metals are a group of metals including Ta, Nb,
Zr, ~f and Ti which, when subject to anodization, form a
substantially non-porous barrier anodic film having a maximum
thickness dependent upon~the voltage employed for the anodiz-
ation step. These metals are not anodized to consumption
unless they are present in extremely thin layers that can be
consumed before the maximum anodic film thickness is reached.
In the present invention, the valve metal layers have such a
thickness that some of the valve metal remains unconsumed
after the a~odization procedure.
The preferred valve metals are tantalum, niobium and
titanium, and tantalum is most preferred because of the
ability of its oxide to resist attack by most chemicals.
It will be appreciated from the above that, as well as
using aluminum and the valve metals themselves, it is also
possible to use anodizable alloys of these metals in the
process of the present invention. For the sake of simplicity
in the following description, however, reference is generally
made only to the metals themselves.
The invention is described in more detail below with
reference to the accompanying drawings, in which:
Fig. 1 is a series of cross-sections showing the steps in
a preferred process according to the present invention;
Fig. 2 is an enlarged cross-sectional view of the product
of step (c) as shown in Fig. l;
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Fig. 3 is an enlarged cross-sectional view of the product
of step (e) of Fig. l;
Fig. 4 is a cross-sectional view similar to Fig. 2 but
showing a metal deposited in the pores;
Fig. 5 is a cross-sectional view similar to ~ig. 3, but
showing a metal deposited into the pores;
Fig. 6 is a s~hematic representation of apparatus
suitable for forming a continuous web of plastic material
having an attached bilayer anodic film;
Fig. 7 is a cross-sectional view of a magnetic recording
media produced by a process of the invention; and
Figs. 8, 9 and 10 are cross-sections of additional
structures comprising two bilayer oxide films and/or two
polymer sheets in combination;
Figs. 11 to 13 are a series of photomicrographs showing
actual structures of intermediates and products produced in
Examples 1 to 3.
It should be noted that in the various figures of
drawings (apart from Figs. 11 to 13) no attempt has been made
to show the relative thicknesses of various layers to scale.
The present invention is based, at least in part, on the
finding that bilayer oxide films can be formed by anodization
and that certain materials, notably fluorides, are capable of
making the bilayer oxide film easily detachable at the inter-
face between the layer of valve metal oxide and the
unconsumed valve metal when such materials are present during
the anodization step. These materials are referred to herein
as "adhesion-reducing agentsn.
When the adhesion-reducing agent is a fluoride, it may be
ifl the form of simple salts, e.g. NaF or RF, or in the form
of complex salts, or fluorine-containing compounds or in
acids such as hydrofluoric acid, fluoroboric acid, etc.
While the Baun article referred to above refers to an initial
treatment of the valve metal with a fluoride-containing
etchant, and some experiments were carried out in
fluoride-containing etchants, there is no mention of this
producing a detachable film and, to the contrary, it is
stated that the duplex oxide forms a strong bond.
~ O ~ 9 ~ ~ 2
The adhesion-reducing agent may be added to the electro-
lyte prior to or during the anodization step or may be coated
onto the metal surface prior to the start of the anodization
procedure. It is alternatively possible to carry out a part
of the anodization step without the adhesion reducing agent
and to carry out another part of the anodization procedure in
the presence of the adhesion-reducing agent.
The amount of the adhesion-reducing agent required in any
particular case can be determined by simple trial and experi-
mentation. When the agent is a fluoride, the amount can be as
low as about 0.003% by volume of the electrolyte, but is more
usually at least about 0. 05~ by volume of the electrolyte.
Prior to the anodization step, the structure to be anodized
is prepared by providing an aluminum coating on a valve metal
substrate. The valve metal may be in the form of a self-
supporting body, sheet, foil or plate, but is preferably
itself in the form of a thin layer supported on a further
co-anodizable substrate (e.g. an aluminum foil, sheet or
plate). This reduces costs because valve metals tend to be
expensive. The valve metals can be coated on the substrates
by any suitable techniques, but vapour deposition techniques
such as physical vapour deposition (PVD) or chemical vapour
deposition (CVD) are particularly preferred because the
characteristics of the resulting valve metal layer make
subsequent separation of the bilayer film highly reliable over
large areas. Sputtering and vacuum evaporation are the most
preferred techniques. The thickness of the valve metal layer
should be large enough that not all of the metal is consumed
during the anodization step, and thicknesses of at least 20 nm
are normally suitable.
As noted above, in the region where the anodized aluminum
and anodized valve metal meet following the anodization step,
there may be a sharp division between the aluminum oxide layer
and the valve metal oxide layer, or there may be a region
where the oxides mix or merge. In any event, separation of
the film at this interface is not desired since this would
leave most or all of the underlying barrier layer on the
substrate metal during detachment of the film. We have found
2~0~2~2
that this undesired separation does not normally take place
in the process of the present invention and can be avoided
entirely by choosing an appropriate coating method for the
aluminum which results in intimate contact between the
aluminum layer and the valve metal layer. While a variety of
coating methods are suitable, vacuum sputtering and vacuum
evaporation of the aluminum are preferred. In particuiar,
sputtering is relatively quick and inexpensive and can give
good contro' over the aluminum coating thickness.
The aluminum oxide layer of the bilayer film may be either
non-porous or porous depending on the electrolyte used for the
anodi2ation step. When the electrolyte contains certain acids,
e.g. phosphoric acid, sulfuric acid or oxalic acid, partial
dissolution of the oxide film takes place as the anodization
proceeds and open pores extend inwardly from the oxide surface
towards the substrate metal. 20wever, when the anodization
proceeds into the valve metal, a non-porous barrier layer is
formed, even when such acids are present in the electrolyte.
The film resulting from the porous anodization is particularly
preferred in the present invention because it co~prises pores
which are open at one surface of the film but are closed at the
other surface, and this has particular advantages for reasons
that will be apparent later. ~oreover, the non-porous barrier
layer of valve metal oxide tends to be dense and durable so
that it provides an impermeable protective surface on one side
of the film. Because of the preferred status of these pore-
containing films, the following description is largely
concerned with them, but it should be kept in mind that films
having a non-porous aluminum oxide layer can also be formed by
the same techniques simply by using an electrolyte that does
not contain a pore-forming acid.
The thickness of the aluminum coating to be applied to
the substrate valve metal depends on the desired thickness of
the anodic film. Normally, the aluminum can be applied to a
thickness in the range of 10 nm to tens of microns (e.g. 50~)
which is then converted to an aluminum oxide layer generally
30 to 40~ thicker during the anodization step. When the
2009212
resulting anodic film is very thin (less than 1~00 nm), the
film may appear to be coloured prior to its detachment from
the substrate valve metal as a result of light interference
and absorption effects, and such colour-generating structures
are the subject of our co-pending U.S. patent application
Serial No. 306,766 filed on February 3, 1989. ~oweve~, the
J bilayer oxide films resulting f;om the detachment of these
extremely thin films from the substrate metals may be useful
for the purposes mentioned herein and consequently are
included within the scope of the present invention.
The anodization step can be carried out in the conven-
tional manner suitable for the anodization of aluminum or
aluminum alloys. For example, a voltage in the range of
3-200 volts but more conveniently in the range of 5-25 volts
may be employed at ambient temperature ln a suitable electro-
lyte containing sulfuric, phosphoric acid or oxalic acid if a
pore-containing film is desired. The time required for the
anodization depends on the thickness of the required duplex
film, and the anodization normally proceeds at a rate of 0.1
to 1 micron per minute.
It was mentioned above that the thickness of the valve
metal oxide layer of the anodic films depends on the voltage
used for the anodization procedure and thicker films are
produced at higher voltages. Thus, either high voltages can
be used for the anodization cf aluminum to produce thick films
of valve metal oxides or these can be formed by initially
anodizing the Al at a lower convenient voltage e.g. 5-25 volts
and then raising the voltage once the aluminum has been
consumed to a value within the range of about 25 to 200
volts. The choice of voltage and technique depends to some
extent on what pore size is required in the Al oxide since
pore diameter is proportional to anodizing voltage to the
extent of about 10~/v. Using either technique, valve metal
oxide layers of up to 350 nm in thickness can be obtained.
The bilayer oxide films can be separated from the under-
lying substrate metal and transferred to another substrate
by any convenient method but the most effective way is by
adhering a thin flexible sheet of the new substrate onto the
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anodized film while it is still attached to the substrate
valve metal and then gradually peeling the sheet and the
attached bilayer oxide film from the substrate metal. If an
adhesive is used for the attachment of the flexible sheet to a
pore-containing film, the adhesive may enter the pores in the
bilayer film. If this is not desired, a heat sealable
material (e.g. a polyester or polypropylene) may be used as
the new substrate since this allows the attachment of a sheet
without the use of an adhesive. When an adhesive is to be
employed, it is preferable to use a W -curable adhesive (e.g.
NORLAND OPTICAL ADHESIVE) because such adhesives tend to be
very transparent when cured and because the curing step
(exposure to ultraviolet light) is quick and effective.
The film may subsequently be removed from the new
substrate, if desired, and either transferred to a further
substrate (e.g. if it is desired to have the open pore surface
of a pore-containing film exposed) or used in unsupported
form. When an unsupported film is desired, the substrate to
which it was originally transferred should be removed by some
method that does not apply destructive physical force to the
thin film, e.g. the substrate can be oxidized (e.g. burned
off) if it is organic or it could be dissolved if it is
soluble (e.g. polyvinyl alcohol) in a suitable solvent.
While it is preferable to apply the bilayer oxide film to
a plastic substrate, the film can in fact be applied to a sub-
strate of ~irtually any kind either with or without the use of
an adhesive as the substrate permits. Examples of substrates
other than plastic include textiles, wood and paper.
Instead of making the new substrate to which the anodic
film is initially applied thin and flexible in order to
facilitate peeling of the film from the metal, the metal
substrate itself can be made thin and flexible so that this
can be peeled away from the anodic film when the latter is
attached to a less flexible material. This has the advantage
that the anodic film remains flat and securely supported
during the 2etachment from the metal and is thus less suscep-
tible to cracking or breaking. The metal substrate can be
made sufficiently flexible either by using a thin foil of the
-- 10 --
12
valve metal or a thin foil of a less expensive metal (e.g.
Al) carrying a thin layer of the valve metal.
For some applications it may be necessary to deposit a
material into the pores of a pore-containing anodic film.
For example, when the film is to be used as a magnetic
coating, a magnetic metal such as Fe,Ni, or Co may be
deposited into the pores. The deposition of such materials
into the pores can conveniently be carried out prior to the
removal of the duplex film from the substrate valve metal,
for example either by electrodeposition or by electroless
deposition using standard techniques known in the art of
anodizing aluminium. When electrodeposition is employed, the
metal tends to be deposited at the bottom regions of the
pores (adjacent to the valve metal oxide layer). On the
other hand, electroless deposition tends to deposit the metal
evenly over the internal pore surfaces. The method
appropriate for the intended application can thus be selected.
When the bilayer oxide film is applied to a substrate,
the result is a substrate having an oxide coating on one
side. If desired, more complicated structures can be
produced by building upon the basic structure. For example,
the anodization procedure can be repeated and a second
bilayer film can be adhered to the first or to the uncoated
side of the substrate. Alternatively, two coated substrates
can be produced and adhered together. Such structures may be
desired to increase oxygen or moisture retardation of the
substrate or for specialized applications.
The invention, and particularly its preferred embodiments
relating to pore-containing anodic films, are described in
more detail in the following with reference to the
accompanying drawings.
Fig. 1 shows the steps of a preferred process according
to the present invention. In step (a) a substrate 10, e.g.
an aluminum foil, is coated with a thin layer 11 of a valve
metal, e.g. tantalum. This is preferably carried out by
vacuum sputtering, but could alternatively be carried out by
vacuum evaporation, plasma spraying etc.
2~09212
In step (b), the resulting structure is coated with a
thin layer 12 of aluminum or an anodizable aluminum alloy,
again by employing any suitable coating technique, e.g. one
of those mentioned above.
In step (c), the aluminum layer is anodized to consumption
and a surface region of the valve metal layer is also anodized
until the current drops to zero. As a result, an anodized
bilayer 13 is formed on the valve metal layer 11.
In step (d), the structure is coated with a flexible sheet
14 which is adhered to the bilayer 13.
In step (e) the flexible sheet 14 is used to detach the
bilayer 13 rom the valve metal layer 11.
The product of step (c) of Fig. 1 is shown on an enlarged
scale and in greater detail in Fig. 2. The bilayer 13 is an
anodic film having an upper part 13A comprising A1203 and
containing pores 15, and a lower part 13B which is substan-
tially non-porous and contains the oxide of the valve metal.
The dotted line X-Y represents the position of the aluminum/
valve metal interface before the anodization step and it
can be seen that the anodization has proceeded for a short
distance into the valve metal layer 11.
When the anodization is carried out in the presence of
fluoride ions or other adhesion-reducing agents, the film 13
is quite easily detachable from the underlying substrate
at the interface between layers 11 and 13. This detachment
can be carried out as in step (e) of Fig. 1 and the result-
ing structure is shown on an enlarged scale in Fig. 3. The
structure consists of the bilayer 13 attached to the flexible
film 14 on the open-pore side of the bilayer. The exposed
opposite surface of the bilayer is imperforate and contains
(or consists entirely of) the oxide of the valve metal.
If desired, a metal or other depositable material may be
deposited in the pores 15 of the bilayer film. For example,
if the product of step (c) of Fig. 1 is subjected to
electrodeposition of a metal prior to step (d), a metal
deposit 16 is formed at the very bottoms of the pores 15, as
shown in Fig. 4. Following step (e), the detached film has
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- 12 -
... .
the structure as shown in Pig. 5 where the metal deposits 16
in the porous bilayer film 13 are protected by the imperforate
part 133 of the bilayer on one side, and by the overlying
flexible strip 14 on the other side.
If desired, the detached bilayer film 13 can be
transferred to a different support and the flexible strip 14
removed, thus allowing free access of any fluid to the pores
15 and any deposits therein.
Fig. 6 shows a simplified apparatus for the continuous
production of plastic sheet having a bilayer anodic film as
a surface coating on one side. A continuous foil 50 of
aluminum having a surface coating of tantalum followed by
aluminum on one side (the underside) is fed fr~m a payoff
roll 51 through an electrolyte bath 52. During the passage
of the foil through the bath, porous anodization takes place
in the presence of fluoride which is dissolved in the
electrolyte. On emergence from the bath, the foil 50 has a
surface anodic film 53 on one side which consists of a porous
aluminum oxide layer and an underlying layer of non-porous
tantalum oxide. After passing through rinsing and drying
stations, 54,55 respectively, the foil is wound around a
heated metal drum 56 of large diameter (approximately 2
feet). A heat sealable plastic sheet 57 is fed from pay off
roll 58 into a nip formed between drum 56 and a counter chill
roll 59. In the nip, the plastic sheet is pressed against
the heated porous surface of the anodic film and is heat
sealed to the film. Upon leaving the nip the anodic film is
detached from the metal foil substrate and forms a coating
for the plastic sheet. The stripped metal foil 60 is wound
up on take up roll 61 and the coated plastic sheet 62 is
wound up on take up roll 63.
The anodic films produced by the present invention, at
least in its preferred forms, are particularly suitable for
use in two specific applications, namely the production of
air- and moisture-impermeable packaging films and the
production of magnetic recording media. These specific
applications are described in more detail below.
20~9212
- 13 -
Resistance to oxygen and moisture penetration in plastic
packaging is usually improved by metalLizing the plastic with
a thick, opaque layer of aluminum. There has been much
interest and effort devoted to realizing a visually trans-
parent barrier film that would have consumer appeal. The
avoidance of metal layers would also allow for microwave
compatibility.
Recently, a number of processes have been announced in
which transparent films are produced by vapour depositing
silicon oxide onto a plastic film (see for example Paper,
Film and Foil Converter, June 1988 pp. 102-104). This
approach, however, has a number of disadvantages, including
the need for a polymeric top-coat to plug cracks and defects
in the deposited silicon dioxide film, the limited fold
endurance and crease resistance, the somewhat yellow colour of
the film composition having the best barrier properties, etc.
Accordingly, there is a need for a transparent barrier
film having good adhesion to plastic, flexibility and dur-
ability. These features are present in the bilayer oxide film
of the present invention, at least in its preferred forms.
When a pore-containing film is produced, the porous anodic
side provides an excellent surface for heat-seal laminating
to the plastic substrate. This is evidenced by the
well-established technology for adhesive bonding of aluminum
based on a phosphoric acid porous anodized film as an
intermediate bonding layer (see J.D.Minford Adhesive Age 17
(1974) 24, the disclosure of which is incorporated herein by
reference). The heat-seal polymer in such a process
infiltrates the porous structure to some extent leading to a
composite polymer/oxide structure which is stronger and more
flexible than an oxide film directly deposited onto a plastic
substrate.
The valve metal oxide layer of the bilayer film, and
especially tantalum oxide, is dense and amorphous as well as
highly chemically resistant offering good barrier properties
and durability. Moreover, anodic tantalum oxide has been
found to be surprisingly ductile relative to other oxides
which are normally brittle (see S.F. 3ubar and D.A. Vermilyea,
- 14 -
2009212
J. Electrochem. Soc. 113, (1966) 892 and ibid 1l4 (1967) 882,
the disclosures of which are incorporated herein by
reference). Thus the bilayer films have good flexibility.
An example of a preferred plastic film having an attached
bilayer anodic film would have the structure as shown in Fig.
3 with the bilayer 13 comprising a porous aluminum oxide
layer 13A and a dense tantalum oxide layer 13B, the poroùs
surface being heat sealed to a conventional packaging film
14. As can be seen, the plastic of the film partially
infiltrates the porous Al oxide layer.
The other area of particular applicability relates to the
production of magnetic recording devices.
Conventional magnetic media for recording information
storage consist typically of fine magnetic particles, such as
iron oxide, dispersed in a polymeric binder media which is
spin coated as a thin film onto a rigid disk or applied to a
flexible web for tape or floppies. More recently the use of
continuous thin magnetic films vacuum deposited onto a disk
or flexible web has been developed.
A third type of magnetic media for rigid disks consists
of an Al platter anodized to provide a porous anodic film
over the surface in which magnetic particles such as cobalt
are electrodeposited into the pores (see S. Kawai, R. Ueda,
J. Electrochem. Soc. 122 (1975) 32.). Recently this type of
media has been extended to flexible substrates (see N. Tsuya,
T. Tokushima, M. Shiraki, Y. ~mehara, IEEE Trans. Mag. vol.
24 (1988), 1790) in a process where plastic film sùch as
polyester is vacuum deposited by evaporation with Al to a
thickness of several microns then anodized and electro-
deposited as for rigid media.
A disadvantage of this approach is that using a thin film
initially and particularly after an appreciable portion of Al
is consumed in the anodizing, the residual Al cannot support
high currents and so the anodizing can only be accomplished at
low rates. Additionally, the appreciable resistance of the
web at its centre relative to the edges, leads to non-uniform
anodizing across the width of the web. Moreover, the heat
9-2 1 ~
generated due to the resistance of the Al film can have a
deleterous effect on the porous anodic film, which is sen-
sitive to temperature, and hence on the magnetic properties.
All of these difficulties would be removed by having a
much thicker conductive layer of Al than is actually needed
for the anodic film thickness, but this would seriously limit
the rate of the vacuum deposition stage.
Our present invention can be used to form the electro-
deposited anodic film on a foil of Al sputtered with an Al/Ta
film, and apply the anodization process to then transfer this
film to a plastic substrate. The anodizing can be done at
very high speed and with excellent uniformity due to the
thick conductive foil.
The Ta oxide in the bilayer anodic film plays a definite
role as a protective overlayer of precisely controlled
thickness. In ccn~ional processing such a protective layer
(typically SiO2) is sputtered as an additional process step.
An example of a recording medium of this type would have
the structure shown in Fig. 7 in which the bilayer film 13
comprises a porous aluminum oxide layer 13A and a dense
tantalum oxide layer 13B acting as a hard protective covering
of precise thickness. The pores 15 contain an electro- -
deposited magnetic material 16 such as cobalt. The film is
heat sealed to a plastic substrate 14 such as a conventional
tape or disk of the type used for recording media. The
structure is very similar to Fig. 5, except that the pores 15
are almost completely filled with the magnetic deposits 16.
Structures having more than one bilayer oxide film can be
produced by building upon the basic structure of Fig. 3.
Examples of such structures are shown in Figs. 8, 9 and 10.
Fig. 8 shows a structure in which a polymer sheet 14 is
provided with two bilayer oxide films 13 and 13' on one
surface. A structure of this type can be formed by first
forming the structure of Fig. 3, except that, in this case, a
layer of adhesive 17 was used to adhere the sheet 14 to the
bilayer film 13 pcior to the cemoval of the film from the
valve metal, and this layer remains in the resulting
packaging structure as shown. After removal of the bilayer
- 16 -
200~212
film 13 from the underlying valve metal, the valve metal is
again coated wi~h aluminum or an anodizable aluminum alloy
and anodized to form a second bilayer film 13'. A further
layer of adhesive 17' is coated on the resulting bilayer film
and the previously coated sheet 14 is then attached to the
bilayer film 13' via the adhesive layer 17' and the film 13'
is peeled from the valve metal. The resulting structure is
then as shown in Eig. 8.
The structure shown in Fig. 9 has two layers of polymer
sheet 14 and 14' joined together via two bilayer films 13 and
13' and three adhesive layers 17, 17' and 17 "'. This
structure is formed by first producing two structures of the
type shown in Fig. 3 (again with an adhesive layer between
the polymer sheet and the bilayer film), and then adhering
the two structures together with the bilayer films facing
each other via a further adhesive layer 17~.
The structure of Fig. 10 hs two layers of polymer sheet
14 and 14' attached via a single bilayer film 13 and two
adhesive layers 17 and 17'. This structure is formed by
first forming a structure of the type shown in Fig. 3 (with
an adhesive layer 17) and then attaching a further layer of
polymer sheet to the bilayer film 13 via a further layer of
adhesive 17'.
Clearly further structures could be formed by similar
techniques.
The following Examples illustrate the invention.
Example 1
Tantalum metal was sputtered onto an aluminum foil to a
thickness of 5000~. Aluminum metal was then sputtered onto
the tantalum coated foil to a thickness of 7000~.
The aluminum was anodized to consumption at 20V in
a 120g/1 solution of phosphoric acid maintained at 30C. The
electrolyte was doped with hydrofluoric acid at the level of
0.1~ by volume and the anodization was continued into the
tantalum layer until the current decayed to a
low level. This produced a tantalum oxide barrie~ layer
approximately 340A in thickness, corresponding to 20V of
2~092~2
anodizing, between the tantalum and the porous aluminum oxide
layer. A flexible plastic strip was heat sealed
to the structure and the anodic bilayer was detached from the
remaining tantalum by pulling one edge of the plastic strip
and peeling away the bilayer.
Fig. 11, comprising five cross-sectional transmission
electron micrographs (a), (b), (c), (d), and (e), all at a
magnification of 28,000X, illustrates the actual structures
produced in accordance with this Example. Micrograph (a)
shows the as-sputtered Al on Ta structure deposited onto Al
foil. Micrograph (b) shows the as-anodized sputtered film
with the porous anodic film (13) formed on the tantalum (11)
after the anodization step. Micrograph (c) shows the
as-anodized film with the oxide layer in the process of
separating from the tantalum underlayer. Micrograph (d)
shows the porous anodic film following its separation from
the tantalum. The non-porous Ta2O5 barrier layer,
approximately 340 ~thick, is visible on one side of the
film. Micrograph (e) shows the tantalum layer (11) remaining
after separation of the porous film.
Example 2
Tantalum was sputtered on to aluminum foil to a thickness
of 1500~. Aluminum was then sputtered on to the Ta coated
foil to a thickness of 4000~. The coated foil was then
anodized in 1.2M phosphoric acid at 30 and 15V until the top
Al layer was consumed and converted to oxide.
The foil sample was then transferred to a bath of 0.4M
phosphoric acid, doped with .005% by volume of hydrofluoric
acid and anodized at 21C and 90V for three minutes.
The anodized foil was then heat-sealed to polyethylene
laminated polyester film at a temperature of 150~. The
aluminum foil was then peeled away from the polymer film
transferring the anodic bilayer to it.
~ igure 12 shows the resulting structures. Photomicro-
graph 12(a) at 80,000X magnification shows a large diameter
porous Al anodic layer on a thick 1500 ~ non-porous Ta oxide
- 18 -
2~0~212
layer prior ta separation of the bilayer from the aluminum
foil. An intermediate region of intermixed Al and Ta oxide
is visible for this relatively thick bilayer film.
Photomicrograph 12(b), also at 80,000X shows the bilayer
anodic film transferred to the polymer substrate.
Example 3
Aluminum was sputtered onto Tantalum coated aluminum foil
as in Example 2. The coated foil was then anodized in 1.2~
phosphoric acid, doped with O.OOS~ by volume of hydrofluoric
acid, at 30C and 15V until the top Al layer was consumed and
the underlying Ta layer partially anodized to a thickness of
25~ . The sample was then transferred to a standard Ni
ANOLOK* electrodeposition bath. Electrodeposition of
magnetic Ni particles into the pores of the anodic film was
carried out using alternating current at 15V rms and 60Hz for
20 sec. The Ni loaded bilayer oxide was then transferred to
a plastic substrate via heat-seal laminating as in Example 2.
Figure 13 shows the resulting structures. Photomicro-
graph 13(a) at 46,000X magnification shows the Ni loaded
bilayer oxide prior to separation from the Al substrate.
Photomicrograph 13(b), also at 46,000X, shows the same
structure transferred to the polymer substrate ~ith a
relatively thin non-porous Ta oxide layer at the outer
surface above the Ni deposits.
.
* Registered Trade Mark of
Alcan Aluminum Limited.
