A METHOD OF MANUFACTURING PEROVSKITE LEAD SCANDIUM TANTALATE

Abstract: A method is described of manufacturing perovskite lead scandium tantalate comprising the pre-reaction of scandium and tantalum oxides at temperatures between 1000 DEG C and 1400 DEG 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(Sc0.5Ta0.5)O3, 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 vapour is then reacted with the scandium tantalate to form the perovskite phase lead scandium tantalate.

Description: A METHOD OF MANUFACTURING PEROVSKITE 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 lapping and polishing techniques. In order to develop alternate routes to thin film ceramic, methods exploiting the recently developed metal 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 μm 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 crystal 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 PbScι/ 2 Taι/2θ3 (PST) films are manufactured by first depositing layers of SC 2 O3, Ta2θs or a mixed oxide layer of SC 2 O 3 and Ta 2 θs, heating to form ScTaθ 4 and then heating the ScTaθ4 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°C, and preferably to temperatures between 1000°C and 1400°C. A mixed layer of SC 2 O3 and Ta2θs may be deposited from solution, starting with scandium (III) acetylacetonate Sc(acac) 3 and tantalum ethoxide Ta(0Et)s as precursors in methoxyethanol solution, consisting of taking the Sc(acac)3 into solution at for example 120°C, cooling to 90°C, adding Ta(OEt)5, complexing at 120°C, cooling and spinning the resulting solution onto the selected substrate and drying the film at 150°C. Thick layers of mixed scandium and tantalum oxides are formed by repeating this 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)3 as described later and tantalum ethoxide respectively. Typically the perovskite PST layers may be formed by reacting ScTaθ 4 layers in the presence of lead oxide at temperatures for example of between 850°C and 1300°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 ScTaθ 4 layer; for example by spinning onto the ScTaθ 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)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 present invention will be described further, by way of examples, with reference to the accompanying drawings in which :- Figure 1 is an x-ray diffraction curve for ScTaθ 4 employing a SOL-GEL method of preparation according to an embodiment of the present invention; Figure 2 illustrates a double crucible arrangement utilised for annealing a sample in accordance with an embodiment of the present invention; Figure 3 is an x-ray diffraction curve of the annealed sample of Figure 2; Figure 4 is an x-ray diffraction curve for ScTaθ 4 employing a MOCVD method of preparation and Figure 5 is a table of the deposition conditions for PST. 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)2 .3H_2θ, Sc(III) acetyl acetonate Sc(acac)3 and tantalum (V) ethoxide Ta(0Et)s have been identified as suitable precursors for the solution route. The deposition for PST involves a two-step process (i) the deposition and prereaction of Sc and Ta to give ScTaθ 4 and (ii) the reaction of ScTaθ4 with lead to form PST. The precursor solution for depositing ScTaU 4 is prepared by reacting Sc(acac)3 and Ta(0Et)s in methoxyethanol solution as shown in the scheme below: CH3OCH 2 CH2OH add Sc(acac)s ~τ— > > cool to 90°C -— > ScTa(OR) x complex Heat to 120°C Ta(0Et) 5 Heat to 120°C, reflux and cool 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 O.όμm is built up by successive depositions with intermediate drying at 150°C/10 mins. X-ray diffraction analysis of such a film after annealing at 1400°C/2 hrs. should show it to be 100% ScTaθ 4 , as shown by the X-ray diffraction curve given in Figure 1. These conditions are not exclusive and other temperature time annealing combinations are also successful. Reaction with lead is subsequently accomplished by successively depositing lead containing layers onto the ScTaθ4 using a solution of dehydrated Pb(OAc > 2 in methoxyethanol by spinning, dipping or spraying. The resultant film is then annealed at 900°C/3 hrs. in a double crucible arrangement (Figure 2) with lead zirconate (PZ) spacer powder. X- ray diffraction analysis has shown the film to be a single phase sharp perovskite like that as shown in Figure 3. Alternatively, it has also been shown that PST can be produced by thermally annealing ScTaθ 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 Figure 2 where the powder is PbZrθ 3 or a mixture of PbZrθ 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)3 where R is an alkyl group such as R = Me, CH 2 CH 3 , CH(CH 3 )2, C(CH 3 ) 3 Sc(DPM)3, DPM = 2, 2, 6, 6 - Tetramethyl -3, 5 - heptanedione Sc(FOD)3, FOD = Fluorinated octanedione The composition of Sc(FOD)3 is scandium 1, 1, 1, 2, 2, 3, 3 - heptafluoro -7, 7 - dimethyl - 4, 6 - octanedione. ii) Ta(OR)s where R is an alkyl group such as R = CH3, CH2CH3, CH(CH 3 ) 2 , Me(CH 2 ) 2 , Me(CH 2 )3, C(CH 3 )3 iiϊ) Pb(OR) 2 where R is an alkyl group such as R = CH3, CH2CH3, Me(CH 2 )3, CH(CH 3 )2 This thin film deposition process is not restricted to the sol -gel method as it has also been shown that ScTaθ4 can be deposited using MOCVD (metallorganic chemical vapour deposition) and volatile scandium and tantalum precursors. The resultant films can be reacted with lead in a lead rich environment at 1200°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 ar a known rate. Each bubbler is heated to 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) 3 having a composition scandium 1, 1, 1, 2, 2, 3, 3 - Heptafluoro, - 1, 1 - dimethyl - 4, 6 - octanedione. For scandium - Sc(OR ι)3 where Ri is an alkyl or substituted alkyl where R ' and R" may be individually selected from alkyl, aryl, alkoxy or fluorinated alkyl. Preferably R' is tertiary butyl and R" heptafluoro-propyl. R2 is preferably hydrogen but may alternatively be halogen or low alkyl. For tantalum Ta(OR)5- n (Y) n n = 0-3 where R is alkyl or substituted alkyl and Y is a substituted pentane- dione. Preferably R is ethyl and n = 0. The conditions which are suitable for depositing the scandium and tantalum oxides are shown in Figure 5. It can be seen that the δ 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 Figure 5. If this is fired at 1000°C for 16 hours then a ScTaθ 4 phase is formed as shown in the X-ray diffraction trace in Figure 5. Higher temperature firing, up to 1400°C is permissible. This firing could be carried out in the MOCVD reactor. The ScTaθ 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 Figure 2. c) growing a PbO layer onto the surface of the ScTaθ 4 using the MOCVD technique and annealing the composite layer in the apparatus described in Figure 2. The PbO layer can be grown in the same apparatus shown in Figure 4 as is used for the ScTaθ 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) 2 where R is alkyl or alkoxy (ORχ), where R \ is preferably tertiary butoxide but may be alkyl or substituted alkyl or R may be a substituted diketonate. Where R' and R" may be individually selected from alkyl-, aryl. alkoxy or fluorinated alkyl. R 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 Figure 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 oxidised during the final annealing 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 ScTaθ 4 layer grown via the solution route. Alternatively, the lead containing layer can be deposited onto a ScTaθ4 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 ScTaθ4 surface. Whereas the above description 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.

Claims: CLAIMS:- 1. 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. 2. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 1 wherein 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. 3. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 1 or claim 2 wherein the scandium tantalate is heated in the presence of lead oxide. 4. A method of manufacturing perovskite PbScι/2Taι/2θ3 (PST) thin films by first depositing layers of SC2O3, Ta2θs or a mixed oxide layer of SC2O3 and Ta2θs, heating to form ScTaθ4 and then heating the ScTaθ 4 layer in the presence of lead oxide. 5. A method of manufacturing perovskite lead scandium tantalate films as claimed in claim 2 or claim 4 wherein the mixed oxide layer of scandium oxide and tantalum oxide is deposited from solution. 6. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 5 wherein the solution comprises starting with scandium (III) acetylacetonate Sc(acac)3 and tantalum ethoxide Ta(0Et) 5 as precursors in methoxyethanol solution, the method comprising taking the Sc(acac)3 into solution adding Ta(0Et)s, complexing, cooling and spinning the resulting solution onto a selected substrate and drying the resulting film. 7. A method of manufacturing perovskite lead scandium tantalate films as claimed in any one of claims 2, 4, 5 and 6 wherein a plurality of layers of scandium oxide and tantalum oxide or a plurality of mixed oxide layers are formed by repeating the method of deposition a plurality of times. 8. A method of manufacturing perovskite lead scandium tantalate as claimed in any one of claims 1 to 4 wherein layers of scandium oxide and tantalum oxides or mixed layers of scandium and tantalum oxides are formed by using chemical vapour deposition, comprising passing compounds of scandium and tantalum over a substrate where they decompose to produce the oxides. 9. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 8 wherein the layers of scandium oxide and tantalum oxides or mixed layers of scandium and tantalum oxides are formed by using metal organic chemcial vapour deposition, comprising passing the metal organic compounds of scandium and tantalum over a heated substrate where they decompose to produce the oxides. 10. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 9 wherein the metal organic scandium compound is scandium (FOD)3 as described in the specification. 1 1 . A method of manufacturing perovskite lead scandium tantalate as claimed in claim 9 or 10 wherein the tantalum oxide precursor is a tantalum alkoxide. 12. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 9 or 10 wherein the tantalum oxide precursor is tantalum ethoxide. 13. A method of manufacturing perovskite lead scandium tantalate films as claimed in any one of claims 1 to 12 wherein the films are formed by reacting scandium tantalate in the presence of lead oxide at temperatures between 850°C and 1300°C. 14. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 3 or claim 13 wherein the lead oxide is in the • form of a vapour produced by lead zirconate or a mixture of lead zirconate and lead oxide. 15. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 3 or claim 13 wherein the lead oxide is first deposited as a layer onto the surface of the scandium tantalate layer. 16. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 15 wherein the lead oxide is deposited by spinning onto the scandium tantalate a solution of lead acetate in methoxyethanol, followed by a drying stage and heating. 17. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 15 wherein the lead oxide is deposited from a vapour of a metal organic precursor. 1 8. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 17 wherein the metal organic precursor is lead tertiary butoxide and the atmosphere is a mixture of argon and oxygen. 19. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 18 wherein the atmosphere of argon and oxygen is maintained at a low pressure. 20. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 17 wherein the metal organic precursor is lead (FOD)2 as described in the specification. 21. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 15 wherein the lead oxide is deposited by evaporation. 22. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 15 wherein the lead oxide is deposited by sputtering. 23. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 15 wherein the lead oxide is deposited by settling of particles from a suspension of lead oxide in a fluid. 24. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 16 wherein the lead oxide is deposited by dipping. 25. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 16 wherein the lead oxide is deposited by spraying. 26. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 1 or claim 2 wherein lead is evaporated onto the scandium tantalate and thermal oxidation of the lead occurs during subsequent heating. 27. A method of manufacturing perovskite lead scandium tantalate as claimed in claim 1 or 2 wherein lead is sputtered onto the scandium tantalate and thermal oxidation of the lead occurs during subsequent heating. 28. A method of manufacturing perovskite lead scandium tantalate as claimed in anyone of claim 1 to 27 wherein the scandium and tantalum oxides are heated to between 1000°C and 1400°C to form the scandium tantalate. 29. A method of manufacturing perovskite lead scandium tantalate substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: CONSIDINE LAURENCE, GOOSEY MARTIN TREVOR, PATEL ANIL, WHATMORE ROGER WILLIAM, CONSIDINE, LAURENCE, GOOSEY, MARTIN, TREVOR, PATEL, ANIL, WHATMORE, ROGER, WILLIAM
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1989
Main classification: C04B35
IPC codes: C01G35, C04B35, C23C16