A METHOD FOR THE PRODUCTION OF THIN FILM PEROVSKITE PHASE LEAD SCANDIUM TANTALATE

Abstract: This invention is directed to a method for producing thin film perovskite phase lead scandium tantalate, which method includes the following steps: introducing oxygen and organometallic lead scandium and tantalum gaseous reagents into a mixing chamber and forming a gaseous admixture thereof; and, admitting this gaseous admixture into a reactor vessel and exposing the same to a heated substrate in the presence of water vapour to form a film upon the surface of the substrate.

Description: A METHOD FOR THE PRODUCTION OF THIN FILM PEROVSKITE PHASE LEAD SCANDIUM TANTALATE The present invention concerns inprovements in or relating to methods for the production of thin film perovskite phase lead scandium tantalate and in particular metal organic chemical vapour deposition (MOCVD) methods for producing thin film for piezoelectric device applications. 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 micron and upwards. These techniques have been successfully developed for materials such as lead titanate but extension to lead scandium tantalate (PST) has been prevented by the inability to deposit the PST films with a requisite body centre cubic (perovskite) crystal structure. Hitherto co-deposition of lead scandium and tantalum oxides from metal organic precursors has yielded films in which the resultant lead scandium tantalate exhibits either only the face centre cubic phase ( pyrochlore ) or at best body centre cubic phase ( perovskite ) containing a high percentage of the undesired f.c.c. phase. In our co-pending United Kingdom Patent Application No. 8809955.1 a solution to the aforesaid problem has been disclosed. The method disclosed therein, however, comprises four stages: a first stage where scandium oxide and tantalum oxide are co-deposited to form a thin film; a second stage where this film is annealed; a third stage where lead oxide is then deposited onto this annealed film; and, a final and fourth stage wherein the resultant bilayer is reannealed to yield the lead scandium tantalate compound , which compound exhibits the perovskite phase. Although this method provides a solution to the problem it is none the less time consuming and therefore not wholly satisfactory. The present invention is intended to provide a more direct route to the solution of this same problem. It has now been found possible to co-deposit all three metallic oxides from vapour phase organometallic reagents and to form from this a film that can be annealed in a single step to form perovskite PST. Under appropriate conditions the oxides will deposit as the perovskite phase without annealing stage. In accordance with the present invention there is provided a method for producing thin film perovskite phase lead scandium tantalate, which method includes the following steps: introducing oxygen and organometallic lead scandium and tantalum gaseous reagents into a mixing chamber and forming a gaseous admixture thereof; and. admitting this gaseous admixture into a reactor vessel and exposing the same to a heated substrate in the presence of hydrolising agent to form a film upon the surface of the substrate. In appropriate circumstances, wherein the gaseous reagents are admixed in stoichiometric proportions it is possible thus to form a perovskite phase lead scandium tantalate film directly. In other circumstances, such perovskite phase can be formed by following the aforesaid steps with an anneal of the substrate. It is believed that hydrolosis occurs during the complex pyrolytic reaction that ensues at the substrate - gas interface. It follows that it is possible that other hydrolising agents, for example volatile (i.e. lower) alcohols, could be used in place of the water vapour. The same metal organic reagents may be utilised as described in our co-pending United Kingdom Patent Application GB 8809955.1. Thus, for lead oxide, any volatile lead compound could potentially be used, preferably a lead alkoxide such as lead tertiary butoxide or a β- diketonate: where R? is preferably hydrogen but may alternatively be halogen or a lower alkyl. R' and R" may be individually selected from alkyl, aryl, alkoxy or fluorenated alkyl. Thus for the scandium oxide, scandium β-diketonates may be used: where R 2 . R'. and R" have the same significance as above. Thus for the tantalum oxide, tantalum alkoxides such as methoxide or ethoxide may be used or alternatively an alkoxvtantalum β- diketonate mav be suitable: where R-i is a lower alkyl and R2, R' and R" have the same significance as above. The presently preferred choice of precursors is : ( 1 ) lead bis (2,2-dimethyl-6,6,7,7.8,8,8-heptafluoro octan-3.5- dionate ); (2) scandium tris (2.2-dimethyl-6,6,7,7,8,8,8-heptafluoro octan- 3,5-dionate); and, (3 ) tantalum pentaethoxide . Precursors (1) and (2) are referred to as lead Fod and scandium Fod , respectively. In the drawings accompanying this specification: Figure 1 is a schematic drawing of apparatus for conducting MOCVD deposition in accordance with the present invention; and. Figures 2 and 3 are x-ray diffraction curves for perovskite phase lead scandium tantalate films produced in accordance with this invention and with and without annealing respectively. Embodiments of this invention will now be described and reference will be made to the Figures. The description that follows is given by way of example only. The apparatus shown in Figure 1 includes: a set of six mass flow controllers 1. 3. 5, 7, 9 and 11: a set of four heated stainless steel bubblers 13, 15. 17 and 19; a mixing chamber 21 ; and a reactor vessel 23. A substrate 25 is supported within the vessel 23 upon a heater 27. Exhaust gases are removed from the reactor vessel 23 by a pumping arrangement 29 consisting of a cold trap 31 , a throttle valve 33. a fore-line trap 35. a Roots pump 37 and a rotary pump 39. The reactor vessel 23 is provided with a UV/visible window 41. and a capacitance manometer 43. Inert gas, e.g. argon, is admitted into the mixing chamber 21 directly via one of the mass flow controllers, controller 1. and. indirectlv via three of the stainless steel bubblers. bubblers 13, 15 and 17 and three of the mass flow controllers, controllers 3, 5 and 7 respectively. Each of the three stainless steel bubblers 13, 15 and 17 contain the organometallic lead, scandium and tantalum precursors. Vapours of these are entrained and also admitted into the mixing chamber 21. These are admixed with oxygen which is admitted directly into the mixing chamber 21 via one of the remaining mass flow controllers, controller 11. The admixture of gases and vapours is admitted into the reactor vessel 23 at an inlet 45 close to a second inlet 47. Water vapour is admitted at this second inlet 47 from the remaining heated stainless steel bubbler, bubbler 19 (which contains water). Optionally, inert gas may be passed through the remaining mass flow controller, controller 9 and the bubbler 19. Deposition of lead scandium tantalate film is achieved as follows: The precursors (1), (2) and (3) are separately heated to temperatures sufficient to allow their transport by a bubbled gas flow under vacuum. Temperatures found suitable for each of the above precursors are approximately : (1) 80°C, (2) 90°C, (3) 55°C, with a gas flow through each of 10, 10. and 4 sscm respectively. The vapours are then passed into the mixing chamber 21 to be mixed with argon (4 sscm) and oxygen (23 sscm). The mixed gases and vapours are then transported to the reactor vessel 23 containing the heated substrate 25. which may be for example of sapphire, magnesium oxide or aluminium nitride, and heated to approximately 650°C, where pyrolysis of the organo-metallic vapours occurs resulting in an oxide film of lead, scandium, and tantalum. Provided that water vapour is introduced into the gas stream e.g. at or near the inlet 45 where the organo-metallic vapours are introduced into the reactor vessel, then the deposited film, if lead oxide rich, can subsequently be annealed to give perovskite lead scandium tantalate. This result is surprising since a similar oxides film deposited by solgel techniques will not give exclusively this phase on annealing. Furthermore, if the proportions of the precursors are carefully controlled to give a relative lead content close to stoichiometric composition then the film deposited is pure perovskite phase and requires no annealing. Example 1 Experimental parameters are given in the following table: TABLE 1 Using the conditions in this table, a lead scandium and tantalum containing film with element ratios of 3.9:1:1.3 for Pb:Sc:Ta is formed as an amorphous layer. Subsequent annealing at 900°C for 10 hrs surrounded by but not contacted by lead zirconate gives a lead scandium tantalate film with perovskite structure (figure 2). Example 2 An alternative set of experimental parameters is given in the next table: TABLE 2 Using the conditions in Table 2 a film containing lead scandium and tantalum in the ratio 2.96 : 2.31 : 1 is formed which on X-ray analysis (figure 3) proves to be lead scandium tantalate with a perovskite structure. 1 0 Example 3 Alternatively an alcohol may be used as the additional oxygen source in place of water. Using the above conditions lead, scandium and tantalum oxides were deposited as an amorphous film which when annealed as in exl gave lead scandium tantalate in the perovskite phase.

Claims: CLAIMS 1. A method for producing thin film perovskite phase lead scandium tantalate, which method includes the following steps: introducing oxygen and organometallic lead scandium and tantalum gaseous reagents into a mixing chamber and forming a gaseous admixture thereof; and, admitting this gaseous admixture into a reactor vessel and exposing the same to a heated substrate in the presence of hydrolising agent to form a film upon the surface of the substrate. 2. A method as claimed in claim 1, in which the hydrolising agent is water vapours. 3. A method as claimed in claim 1, in which the hydrolising agent is a volatile alcohol. 4. A method as claimed in any one of the preceding claims, in which said gaseous reagents are lead alkoxide such as lead tertiary butoxide or a β-diketonate, scandium oxide such as scandium β- diketonates and tantalum alkoxide such as methoxide, ethoxide or alkoxvtantalum β-diketonate. 5. A method as claimed in any one of the preceding claims, in which said gaseous reagents are lead bis (2,2-dimethyl-6,6,7,7,8,8,8- heptafluoro octan-3,5-dionate), scandium tris (2,2-dimethyl- 6,6,7,7,8,8,8-heptafluoro octan-3,5-dionate), and, tantalum pentaethoxide. 6. A method as claimed in any one of the preceding claims, in which said gaseous reagents are mixed in said chambers in the presence of an inert gas. 7. A method as claimed in any one of the preceding claims, in which said substrate is made of sapphire, magnesium oxide or aluminium nitride. 8. A method as claimed in any one of the preceding claims, in which the substrate is heated to 650°C. 9. A method as claimed in any one of the preceding claims, in which the gaseous reagents are admixed in stoichiometric proportions. 10. A method as claimed in any one of claims 1 to 8, further including the step of annealing said substrate. 11. A method as claimed in any one of the preceding claims, in which said gaseous reagents are heated to temperatures sufficient to allow their transport to the mixing chamber by a bubble gas flow under vacuum. 12. A method as claimed in claim 11, in which said lead precursor is heated to 80°C, said scandium precursor is heated to 90°C and said tantalate precursor is heated to 55°C.

Inventors: HASDELL NEIL BERNARD, TRUNDLE CLIVE, HASDELL, NEIL, BERNARD, TRUNDLE, CLIVE
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1989
Main classification: C23C16
IPC codes: C01G35, C23C16, H01L41