Note: Descriptions are shown in the official language in which they were submitted.
PHD 84.100 1 24-6-1985
A process for the production of bismuth-substituted ferrimagnetic garnet
films.
The invention relates to a process for the production of
bismuth-substituted ferrimagnetic garnet films on a substrate by HF
cathode sputtering using a target containing in the form of oxides the
elements involved in the build-up of the film.
A particular problem in the deposition of bismuth-
substituted iron-garnet films using HF cathode sputtering is the
considerably higher sputtering output of bis~uth in an oxide compound as
compared with the other constituents of the layer. In the performance of
the conventional HF plasma sputtering with the parameters generally
used, an impoverishment of the bismuth content in the films for~ed on
the substrate electrode thus occurs, owing to back-sputtering processes
and thermal processes, e.g. as a consequence of bombardment with
energetic secondary electrons. The problem of bismuth impoverishment is
solved in a previously developed process firstly by incorporating an
excess of bismuth oxide in the target and secondly by making fine
adjustment of the bismuth content of the growing film possible by means
of an HF bias voltage applied to the substrate electrode (referred to as
~substrate biasU).
The production of thin ferrimagnetic garnet films has
acquired increasing importance in the last few years because of the
possibilities of using them in the magnetic or magneto-optical field,
e.g. as displays, for optical printers, in bubble-domain devices, in
gyro lasers and for parts used in optical communication, e.g. active
elements in storage devices which are addressable by ~eans of
electromagnetic radiation.
In the production of garnet films deposition by means of
HF cathode sputtering (NF plasoa sputtering of RF diode sputtering) has
become more interesting for industrial purposes because it is still the
~eans by which the difficulty of achieving homogeneous deposition of
dielectric coatings over even fairly large areas can most readily be
overcome.
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The traditional process of depositing single-crystal
garnet films by liquid-phrase epitaxy (LPE) on single-crystal
substrates with a reasonably suitable lat~ice constant is costly
and has as ye~ not developed to the point where large-series
production can be undertaken with reproducible success.
Difficulties occur with, for example, the slngle-crystal
substrates which, depending on the dimension of the single-crystal
from which they are cut, cannot be produced in any desired size.
Compared with the liquid-phase epitaxy process, HF
plasma sputtering offers a number of advantages for the productlon
of garnet layers: no noble metals are needed for the equipment
used; the great technological effort involved in ~he production of
single-crystal substrates can be dispensed with; and a continuous
production process is possibleO The HF plasma process further
offers the advantage of greater versatility since a wide range of
substrate/film combinations is possible and larger-dimension
layers can be deposited.
Using the previously developed process it is possible to
deposit reproducibly films oi a desired stoichiometric composition
~0 whose intrinsic~ optical, magneto-optical, magnetic and
crystallographic properties are definitely on a par with those of
films grown by liquid-phase epitaxy. It has, however, been found
that iilms deposited by this process may contain defects in the
~orm of particles measuring 10 ym, which are undesirable when the
aim is to grow films of the greakest possible perfection.
The invention sets out to improve the processes re~erred
to in the p~eamble in such a way that bismuth-substituted
20104 7895
ferrimagnetic garnet films which are practlcally defect-free can
be produced by HF cathode sputtering.
This aim is achieved by the invention through the use of
a target which includes bismuth in ~he form of an oxide compound
with a cer~ain stoichiometry, bismuth and at least one element
participating in the film build up, being incorporated at fixed
lattice sites, which element has a sput~eriny rate which ls
distinctly lower than the sputtering rate of bismuth, and in which
the target electrode used is a magnetron into which the HF power
required for the sputteriny proceæs is fed.
~ he invention is based on the knowledge that the defects
found in HF cathode sputtered films are attributable to embedded
particles originating from the taryet. Because of the very high
transfer loss of bismuth the conventional targets contain excess
free
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PHD ~4.100 3 24-6-19a5
Bi2o3. The output of Bi3~ in the spu~ter process being hiyh in
comparison with the other constituent materials of the target, the
~-. Iosc:s
~` surface of the target ~aeses drastically large amounts of Bi2o3 and
splits. ~aterials with a lower sputtering rate, e.g. Gd3Fe5012,
are left behind, protruding from the target. The high nega~ive
electrical potential of the target s surface during the HF cathode
sputtering then causes repulsion of the similarly charged, protruding
particles and their incorporation in the growing film. It can also be
presumed that the B ~o3 matrix of the known targets in the vicinity
of the surface disintegrates as a result of crystallographic junctions
being crossed and loosens the link with the o~her phase cons~ituents.
It has bean found that defects of the kind described
above in the deposited film can be almost entirely avoided if care is
taken to ensure that bismuth is present in the target not in the form of
Bi2o3 but, for example, in the form of a binary oxide compound or a
ternary oxide phase. Because of the molecular binding of Bi3+ to other
elements with a lower vaporisation output, e.g. Fe3+ and/or Gd3~,
the effective vaporisation output of Bi3+ is reduced.
If the quantity of bismuth present in the target does not
exceed one bi-atom/formula unit, perfect films, i.e. films not
incorporating foreign particles, can be achieved with a conventional H~
cathode sputtering apparatus in a diode set-up, subject to the condition
that Bi3+ in the target is molecularly bound to one or more cations
with a lower sputtering output. If higher bismuth contents have to be
achieved in the layers deposited and/or if, in order to achieve
spontaneous crystallisation in the growing film, it is necessary to
operate with higher temperatures at the substrate, as is possible only
in oxygen-free plasma because of corrosion of the apparatus, the plasma
must be kept a~ay from the growing film in accordance with another
finding on which the invention is based, in order to eliminate to a
large extent back-sputtering losses of Bi3+; this can be achieved by
using a magnetron cathode.
In a further, ~ore advanced embodiment of the invention
the target electrode used is therefore a magnetron into which the HF
power required for the sputter process is fed. Associated herewith is
the advantage that targets can be used whose composition is relatively
close to the stoichiometric composition of the film to be deposited, and
PHD 84.100 4 24-6-1985
in which the bismuth contenk is thus only slighter higher than that
corresponding to the stoichiometric composition of the film to be
deposited. A magnetron ensures that the plasma concentrates in the
vicinity of the target and hence that ~he ion bombardment of the growing
film is drastically reduced, so that Bi3+, once present in the
deposited film, is not back-sputtered.
Furthexmore, the use of a magnetron is associated with
the advantage that the distance between the target and khe substrate can
be considerably more than doubled as compared with the known process, in
which a conventional HF cathode sputtering installation is used in a
diode set-up because the sputtering rate of the target material as a
whole achievable with a magnetron is considerably higher.
In a further embodiment of the invention a distance of
bet~een 40 and 100 mm between the target electrode and the substrate
electrode is therefore selected. As already explained in more detail
above, a particular problem in the deposition of bismuth-substituted
ferrimagnetic garnet using HF cathode sputtering is that the bismuth in
an oxide compound has a considerably higher sputter output than the
other constituents of the layer. Not only is this effect, naturally,
perceptible during the sputtering of the target but it also operates if
the growing film is subjected to ion bombardment from the plasma, when
back-sputtering of the Bi3+ already present in the layer occurs, so
that a ~i3~ impoverishment of the film takes place. When a magnetron
electrode is used, the distance between the target electrode and the
substrate electrode can, because of the high sputtering power of the
magnetron, be chosen such that the said back-sputter effects scarcely
occur.
In a further embodiment of the invention the substrate i5
heated to a temperature in the range from 300 to 650C, and preferably
to a temperature of approx. 550C. This range is of extreme importance
precisely for glass substrates. Under certain conditions optimum films
are not obtained below 300C, and under certain conditions 650C is
likewise too high for many kinds oE glass. Precisely at a process
temperature of approx. 550C a possible second phase with an extremely
fine distribution occurs so that, accordingly, a deviation from the
formula composition of the target with a particular stoichiometry is
uncritical. This is accompanied by the advantage that losses of Bi3+,
PHD 84.100 5 24-6-1985
caused, for example, by condensation phenomena, are reduced as a
consequence of an already commencing formation of crystallites in the
radiologically amorphous film. This also means that if work is condu~ted
at temperatures as low as, e.g 550C, spontaneously crystallised
films can be achieved even without re-tempering.
In a further embodiment of the invention, ~he film is
deposited in an inert-gas atmosphere at a pressure of 1.1 to 10 x 10 3
mbar. It has been found that the proportion o~ bismuth present~ in the
: ` ~. ' .r '`
` ` film can be appreciably increased if deposition taXes place in a pure
inert-qas atmosphere instead of an inert-gas atmosphere containing
oxygen, bettex results being obtained if krypton is chosen.
In a further embodiment of the invention films with a
general composition of (Gd, Bi)3(Fe, Ga, Al)5012 are deposited.
Good results were obtained when the target used had one of khe following
compositions (% by weight):
Gd203 25.7 Gd203 17.2
BiFeO3 48.7 si2 3Gdo.7Fe5o12 71.1
Fe203 17.6 or Fe23 3.1
20 Al203 3.4 Al23 3-4
Ga203 5.2 Ga203 5.2
Bismuth-substituted ixon-garnet films produced in
accordance ~ith the process according to the invention have the
advantage that their optical and magneto-optical properties and also the
temperature-dependence of the saturation magnetisation more or less
agrees with the values of films of correspondins comp~sition obtained by
means of liquid-phase epitaxy, but also that, unlike films produced bY
means of liquid-phase epitaxy, the films in question show great
homogeneity over their entire film thickness.
A further advantage of the process according to the
invention is that it is possible to ~ork with an oxygen-free inert-gas
plasma, so that essentially higher substrate temperatures than with the
already proposed process, which employs an oxygen atmosphere, are
possible. A higher substrate temperature promotes a spontaneous
crystallisation of the deposited film.
P~D 84.100 6 24-6-1985
ExamPles
As an exa~ple of implementation of the invention we now
describe the production of bismuth-substituted gadolinium-iron garnet
films, in which part of the iron is replaced by aluminium and
gallium. For the HF cathode sputtering commercially available cathode-
sputtering installations powered by HF voltage, with a magnetic target
electrode (magnetron), can be used, a measurement device for measurement
of the r.m.s. HF voltage both at the target electrode and al50 on the
substrate electrode being provided as an ancillary device. The magnetron
electrode is built in horizontally into a conventional vacuum
container. The HF power required for the sputtering process is fed into
the target electrode via impedance-matching elements, the adjustment of
the HF voltage at the target electrode being e~fected by adjustment of
the capacitances of the impedance-matching elements. The HF voltage at
the target electrode is kept constant to within ~ 1% by computer
control. The energy source used for the sputtering device is a normal HF
generator which is operated with an output power of 160 W. The operating
frequency is 13.56 MHz. The targets (cathode-sputtering sources) used
are bodies produced by hot-pressing from oxides of the elements involved
in the film structure, with a diameter of 75 mm, a thickness of 4 mm and
a porosity of ~ ~~ with the following compositions tfigures in ~0 by
weight):
25 Gd203 25.7 Gd203 17.2
BiFeO3 48.1 Bi2 3GdO~7Feso12 71.1
Fe203 17.6 Fe203 3.1
Al23 3-4 Al23 3 4
Ga203 5.2 Ga203 5.2
The ceramic process for production of the target from the
oxides stated is conducted in such a manner that no free Bi2o3 is
back-sputtered.
The target body is fastened to a carrier plate with an
effective heat-conducting adhesive ~epoxy resin filled with silver
powder) and this composite structure is then fastened to the target
electrode.
PHD 84.100 7 29-~-1985
The target body also be produced by cold-pressing of ~che
oxides of the elements involved in the film structure and subsequent
sintering.
In order to dissipa~e unwanted heat water-cooled target
electrodes can effectively be used.
The substrate electrode used was an electrode, suitable
for high frequencies, with a diameter of 150 mm, heated to a temperature
between 355 and 550 C. The substra~es used were Ca/~g/Zr-substituted
gadolinium-gallium-garnet single-crystal slices with (111) orientation
or plates of glass with a high transformation temperature and a thermal
expansion coefficient suitable for the film material to be deposited.
The choice of the substrate depends on the purpose for which the
deposited film is used. If large-area films are required, e.g. for
displays, glass substrates are particularly suitable.
The sputtering equipment is then evacuated to a pressure
of approx. 10-15 mbar with a vacuum pump, and then an inert gas,
preferably krypton, is introduced at a pressure of 5 x 10-3 mbar. The
inter-electrode distance is 50 - 80 mm. The deposition rate is approx.
0.5 to 2 /um/h when a hot-pressed target body is used.
The HF voltage measured at the HF lead-in at the rear of
the target electrode is C 200 Vrms; the substrate voltage is floating.
Before the deposition of the films (typical fil~
thicknesses are from 1 to 2 lum) the target and substrate are each
etched by plasma sputtering in order to ensure a balanced distribution
of the phase constituents at the surface. The preliminary-etching time
is reduced in further experiments on the same target to approx. 0.5 to 1
h, until the equilibrium potential at the target has stabilised.
After deposition in the cathode-sputtering installation
the layers are largely amorphous to X-rays. A conversion into
crystalline bismuth-substituted iron-qarnet films is obtained by heating
in an oxygen atmosphere at a temperature of approx. 730C if single-
crystal qarnet substrates are used and at a temperature of approx.
780C if giass substrates are used.
The particular advantages of the process in question are
that, despite the fact that the sputtered production of Bi3+ in the
deposited film is appreciably higher than the other constituents of the
target, high bismuth contents can be reproducibly achieved, the
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.,
PHD 84~100 8 Z4-6-1985
deposited films sho~ great homogeneity throughout the layer thickness
and are practically free from defects in the form of fairly large
particles torn from the target as a consequence of splitting caused by
the greatly differing sputtering rates of the other ions to be removed
from the target such as Fe3+, Gd3+, Ga3~ and Al3~.
How good the behaviour of the process is with regard to
the transfer of the elements involved from the target to the growing
film is shown by the following table (in which the layer compositions
indicated are not stoichiometric).
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PHD 84.100 9 24-6-1985
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PHD 84.100 10 24-6-1985
Experiments have shown that a change of the argon
pressure (3.5-~5-~7x10-3 mbar) has scarcely any ef2ect on the
quantity of bismuth deposited in the film. A pure argon atmosphere is
found to be advantageous in respect of the quantity of bismuth deposited
in the film whereas the presence of oxygen in the inert-gas atmosphere
ff~ C! *
~ adversely cf~ccts the transfer of bismuth from the target into the film.
