Method of manufacturing perovskite lead scandium tantalate

A method is described of manufacturing perovskite lead scandium tantalate comprising the pre-reaction of scandium and tantalum oxides at temperatures between 1000.degree. C. and 1400.degree. C. to form scandium tantalate. The scandium tantalate is then reacted with lead oxide to form the desired perovskite phase lead scandium tantalate. In one embodiment there is described a method for the deposition of perovskite lead scandium tantalate films from metal organic precusors. The availability of metal organic precursors allows the deposition of thin films directly from solution or by MOCVD techniques. One particular material, lead scandium tantalate Pb(Sc.sub.0.5 Ta.sub.0.5)O.sub.3, is described. The principal features of the invention are the deposition of scandium and tantalum components from solution or by MOCVD onto the required substrate. These are then prereacted to yield scandium tantalate. A lead containing film or vapor is then reacted with the scandium tantalate to form the perovskite phase lead scandium tantalate.

The present invention relates to a method of manufacturing perovskite lead 
scandium tantalate (PST) and more particularly, but not exclusively, to a 
method of manufacturing PST thin films. 
The fabrication of ceramic derived thermal detectors often requires the 
production of very thin slices of material. This is traditionally achieved 
by difficult and time consuming lappina dn polishing techniques. In order 
to develop alternate routes to thin film ceramic, methods exploiting the 
recently developed emtal organic precursors of these ceramics have been 
extensively utilised. These techniques include the deposition of thin 
films from metal organic solutions and by metal organic chemical vapour 
deposition (MOCVD). Both these methods allow the controlled deposition of 
ceramic films with thicknesses less than 0.1 .mu.m and upwards. 
These techniques have been successfully developed for material such as lead 
titanate but extension to lead scandium tantalate (PST) has been prevented 
by the inability to deposit the PST films with the desired 100% perovskite 
rystal structure. By depositing the lead, scandium and tantalum from a 
single metal organic solution only small amounts of the perovskite phase 
were obtained after firing. 
An objective of the present invention is to provide an improved method of 
manufacturing perovskite lead scandium tantalate. 
According to the present invention there is provided a method of 
manufacturing perovskite lead scandium tantalate comprising heating 
scandium oxide and tantalum oxide to form scandium tantalate and heating 
the scandium tantalate in the presence of lead. 
In one embodiment a film of perovskite lead scandium tantalate is formed by 
first depositing layers of scandium oxide and tantalum oxide or a mixed 
oxide layer of scandium oxide and tantalum oxide, heating the layers or 
the mixed layer to form a layer of scandium tantalate and heating the 
scandium tantalate layer in the presence of lead. 
Preferably the scandium tantalate is heated in the presence of lead oxide. 
In one embodiment, perovskite PbSc.sub.1/2 Ta.sub.1/2 O.sub.3 (PST) films 
are manufactured by first depositing layers of Sc.sub.2 O.sub.3. Ta.sub.2 
O.sub.5 or a mixed oxide layer of Sc.sub.2 O.sub.3 and Ta.sub.2 O.sub.5, 
heating to form ScTaO.sub.4 and then heating the ScTaO.sub.4 layer in the 
presence of lead. In one embodiment the layers of scandium and tantalum 
oxides or the mixed oxide layer of scandium and tantalum oxides are heated 
to temperatures above 1000.degree. C., and preferably to temperaturse 
between 1000.degree. C. and 1400.degree. C. 
A mixed layer of Sc.sub.2 O.sub.3 and Ta.sub.2 O.sub.5 may be deposited 
from solution, starting with scandium (III) acetylacetonate (Scacac).sub.3 
and tantalum ethoxide Ta(OEt).sub.5 as precursors in methoxyethanol 
solution, consisting of taking the Sc(acac).sub.3 into solution at for 
example 120.degree. C. cooling to 90.degree. C. adding Ta(OEt).sub.5, 
complexing at 120.degree. C., cooling and spinning the resulting solution 
onto the selected substrate and drying the film at 150.degree. C. Thick 
layers of mxied scandium and tantalum oxides are formed by repeating the 
process. 
Alternatively mixed layers of scandium and tantalum oxides are formed by 
using metal organic chemical vapour deposition (MOCVD), consisting of 
passing volatile metal organic compounds of scandium and tantalum over a 
heated substrate at reduced pressure where they decompose to give the 
oxides. The metal organic compounds are advantageously scandium 
(FOD).sub.3 as described later and tantalum ethoxide respectively. 
Typically the perovskite PST layers may be formed by reacting ScTaO.sub.4 
layers in the presence of lead oxide at temperature for example of between 
850.degree. C. and 1300.degree. C. The lead oxide may in one embodiment be 
in the form of a vapour produced by lead zirconate or a mixture of lead 
zirconate and lead oxide. 
Alternatively the lead oxide is first deposited as a layer onto the surface 
of the ScTaO.sub.4 layer; for example by spinning onto the ScTaO.sub.4 a 
dehydrated solution of lead acetate in methoxyethanol, followed by a 
drying stage and firing. 
In a further embodiment the lead oxide is deposited from a vapour of a 
metal organic precursor such as lead tertiary butoxide and the atmosphere 
is a low pressure mixture of argon and oxygen. Alternatively the metal 
organic precursor is lead (FOD).sub.2. 
In other embodiments the lead oxide is deposited by evaporation or is 
deposited by sputtering or is deposited by settling of particles from a 
suspension of PbO in a fluid, or is deposited by dipping, or is deposited 
by spraying. 
Details of the deposition methods referred to above are well known in the 
art and will therefore not be described herein.

The first method to be described hereinafter is the deposition of the oxide 
layers from solution. 
After taking into account such factors as reactivity, solubility, organic 
content, decomposition temperature, cost and availability, lead (II) 
acetate Pb(OAc).sub.2.3H.sub.2 O, Sc(III) acetyl acetonate Sc(acac).sub.3 
and tantalum (V) ethoxide Ta(OEt).sub.5 have been identified as suitable 
precursors for the solution route. The deposition for PST involves a 
two-step process (i) the deposition and preraction of Sc and Ta to give 
ScTaO.sub.4 and (ii) the reaction of ScTaO.sub.4 with lead to form PST. 
The precursor solution for depositing ScTaO.sub.4 is prepared by reacting 
Sc(acac).sub.3 and Ta(OEt).sub.5 in methoxyethanol solution as shown in 
the scheme below: 
##STR1## 
A thin film of the above is fabricated by spin coating at 2000 rpm on a 
standard photoresist spinner using a polished and thoroughly cleaned 
substrate such as single crystal sapphire. A film of about 0.6 .mu.m is 
built up by successive depositions with intermediate drying at 150.degree. 
C/10 mins. X-ray diffraction analysis of such a film after annealing at 
1400.degree. C./2 hrs. should show it to be 100% ScTaO.sub.4, as shown by 
the X-ray diffraction curve given in FIG. 1. These conditions are not 
exclusive and other etmperature time annealing combinations are also 
successful. Reaction with lead is subsequently accomplished by 
successively depositing lead containing layers onto the ScTaO.sub.4 using 
a solution of dehydrated Pb(OAc).sub.2 in methoxyethanol by spinning, 
dipping or spraying. The resultant film is then annealed at 900.degree. 
C./3 hrs. in a double crucible arrangement (FIG. 2) with lead zirconate 
(PZ) spacer pwder. x-ray diffraction analysis has shown the film to be a 
single phase sharp perovskite like that as shown in FIG. 3. Alternatively, 
i has also been shown that PST can be produced by thermally annealing 
ScTaO.sub.4 in a high partial pressure of lead oxide thereby obviating the 
second lead deposition. A high partial pressure of lead oxide can be 
produced by annealing the layers in an apparatus such as in FIG. 2 where 
the powder is PbZrO.sub.3 or a mixture of PbZrO.sub.3 and PbO. Placing 
more PbO in this powder will increase the partial pressure of the lead 
oxide in the atmosphere over the layer. This process involving a 
prereaction stage is novel for the deposition of thin films and bulk 
ceramic PST. 
Alternative metal organic precursors which can be used in the solution 
deposition route are: 
(i) Sc(OR).sub.3 where R is an alkyl gruop such as 
R=Me, CH.sub.2 CH.sub.3, CH(CH.sub.3).sub.2, C(CH.sub.3).sub.3 
Sc(DPM).sub.3, DPM=2,3,6,6-Tetramethyl-3,5-heptanedione 
Sc(FOD).sub.3. FOD=Fluorinated octanedione 
The composition of Sc(FOD).sub.3 is scandium 
1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione. 
(ii) Ta(OR).sub.5 where R is an alkyl group such as 
R=CH.sub.3, CH.sub.2 CH.sub.3, CH(CH.sub.3).sub.2, Me(CH.sub.2).sub.2, 
Me(CH.sub.2).sub.3, C(CH.sub.3).sub.3 
(iii) Pb(OR).sub.2 where R is an alkyl group such as 
R=CH.sub.3, CH.sub.2 CH.sub.3.Me(CH.sub.2).sub.3, CH(CH.sub.3).sub.2 
This thin film deposition process is not restricted to the sol-gel method 
as it has also been shown that ScTaO.sub.4 can be deposited using MOCVD 
(metallogranic chemical vapor deposition) and volatile scandium and 
tantalum precursors. The resultang films can be reacted with lead in a 
lead rich environment at 1200.degree. C. to give perovskite PST. 
The metal oxides of scandium and tantalum can be deposited onto a heated 
substrate in an apparatus as described above. The volatile scandium and 
tantalum precursors are placed in two of the stainless steel bubblers and 
a carrier gas (which can be argon or a mixture of argon and oxygen) is 
passed through each bubbler at a known rate. Each bubbler is heated to a a 
temperature determined to give a known vapour pressure of each precursor. 
(Provision is also made for the inclusion of bubblers containing other 
metal organic precursors which may be required, such as lead, titanium or 
niobium). Volatile precursors which have been selected as being 
particularly suitable for the growth of the scandium and tantalum oxides 
being Sc(FOD).sub.3 having a composition scandium 
1,1,1,2,2,3,3-Heptafluoro-7,7-dimethyl-4,6-oxtanedione. 
For scandium 
EQU Sc(OR.sub.1).sub.3 
where R.sub.1 is an alkyl or substituted alkyl or 
##STR2## 
where R' and R" may be individually selected from alkyl, aryl, alkoxy or 
fluorinated alkyl. Preferably R' is tertiary butyl and R" 
heptafluoro-propyl. R.sub.2 is preferably hydrogen but may alternatively 
be halogen or low alkyl. 
For tantalum 
EQU Ta(OR).sub.5-n (Y).sub.n n=0-3 
where R is alkyl or substituted alkyl and Y is a substituted pentanedione. 
Preferably R is ethyl and n=0. 
The conditions which are suitable for depositing the scandium and tantalum 
oxides are shown in FIG. 5. It can be seen that the tantalum oxide can be 
grown much more rapidly than scandium oxide. A mixed scandium/tantalum 
oxide layer can be grown under similar conditions to those stipulated in 
FIG. 5. 
If this is fired at 1000.degree. C. for 16 hours then a ScTaO.sub.4 phase 
is formed as shown in the X-ray diffraction trace in FIG. 5. Higher 
temperature firing, up to 1400.degree. C. is permissible. This firing 
could be carried out in the MOCVD reactor. The ScTaO.sub.4 layer formed by 
the MOCVD route can be reacted to form perovskite PST in one of the 
following ways: 
(a) annealing in a PbO-rich atmosphere 
(b) depositing a lead acetate layer onto the surface as described above and 
annealing the layer in the apparatus described in FIG. 2. 
(c) growing a PbO layer onto the surface of the ScTaO.sub.4 using the MOCVD 
technique and annealing the composite layer in the apparatus described in 
FIG. 2. The PbO layer can be grown in the same apparatus shown in FIG. 4 
as is used for the ScTaO.sub.4 growth. Lead tertiary butoxide has been 
identified as a suitable volatile precursor for the PbO growth. 
Alternative volatile lead oxide precursors which could be used are 
Pb(R).sub.2 where R is alkyl or alkoxy (OR.sub.1), where R.sub.1 is 
preferably tertiary butoxide but may be alkyl or substituted alkyl or R 
may be a substituted diketonate. 
##STR3## 
Where R' and R" may be individually selected from alkyl-, aryl, alkoxy or 
fluorinated alkyl. R.sub.2 is preferably hydrogen but may alternatively be 
halogen or low alkyl. 
The conditions for the growth of lead oxide from lead t-butoxide are given 
in FIG. 5. It can be seen that lead oxide is deposited in a mixed 
argon/oxygen atmosphere at low pressure. If the atmosphere is pure argon 
then metallic lead is deposited. This is a feasible variation on the 
process, as metallic lead can be oxidized during the final annaling by 
conducting said annealing in an oxygen containing atmosphere. 
In feasible modifications to the processes described above, the lead oxide 
layer could be deposited by MOCVD onto a ScTaO.sub.4 layer grown via the 
solution route. Alternatively, the lead containing layer can be deposited 
onto a ScTaO.sub.4 layer formed via either solution or MOCVD routes by any 
one of the following methods: 
(a) evaporation of PbO 
(b) evaporation of Pb followed by thermal oxidation during the final anneal 
(c) sputtering of PbO 
(d) sputtering of Pb followed by thermal oxidation during the final anneal 
(e) allowing a suspension of PbO particles formed by grinding PbO in a 
fluid to settle onto the ScTaO.sub.4 surface. 
Whereas the above described describes various embodiments of the present 
invention it will be appreciated that other embodiments exist which fall 
within the scope of the present invention. For example various photolytic 
processes may be employed wherein ultra-violet light is used to irradiate 
precursors in order to promote their decomposition. In another example 
plasma enhancement may be employed.