PREPARATION OF THIN FILM CERAMICS BY SOL GEL PROCESSING

Abstract: A method of making a mixed sol to produce a titanate-based or zirconate-based ceramic. A metal salt such as lead acetate is dissolved in a stabilising agent such as glacial acetic acid, used in a concentration such that subsequent addition of titanium and zirconium alkoxide sols is effected without precipitation. PZT ceramic films made from the mixed sols have: dielectric constant 150-4000; dielectric loss factor 0.001-0.08; saturation polarisation 0.8 mu C/cm<2>.

Description: PREPARATION OF THIN FILM CERAMICS BY SOL GEL PROCESSING This invention concerns a method of making a mixed sol of a composition to be gelled and heated to produce a titanate-based or a zirconate-based ceramic. The invention is of particular interest in the preparation of thin films of the ceramic. Specific compositions comprise electronic ceramics such as PZT (lead zirconate titanate), which have application in infra red detectors, surface acoustic wave devices, and particularly in electrooptic switching devices. G. Yi et al (J. Appl. Phys. 64 (5), 1 September 1988, 2717 - 2723) described a method of making a sol for this purpose. The method involved dissolving lead acetate in acetic acid (using 1 ml of glacial acetic acid for each 2g of lead acetate trihydrate), adding zirconium propoxide and then adding titanium isopropoxide. The solution was agitated by mixing in an ultrasonic cleaner until all the condensed solid had dissolved. Ethylene glycol was then added in order to prevent cracking and to improve the surface smoothness of the film. In our hands, this method gave rise to several problems: - Our attempts to achieve dissolution of the condensed solid that results when titanium alkoxides are added, were unsuccessful. In fact ultrasonic agitation was found to aggravate the problem. Alkoxide condensates are inherently unstable, moisture sensitive, and hence precipitate irreversibly out of solution. Ultrasonification is therefore unlikely to effect redissolution of a precipitated alkoxide condensate. - Ethylene glycol addition resulted in further formation of condensed solid, in this instance associated with the presence of unreacted alkoxy groups. In common with all alkoxide derived feedstock materials for the preparation of ceramics, the resulting sols were unstable on storage. This invention results from our attempts to overcome the problems associated with the aforesaid method. In one aspect the invention provides a method of making a mixed sol of a composition to be gelled and heated to produce a titanate-based or a zirconate- basdd ceramic, by forming a solution of at least one metal salt in a stabilising agent, adding to the solution a titanium alkoxide sol and/or a zirconium alkoxide sol to form a mixed sol precursor and adding water to form the mixed sol, characterised in that the amount of the stabilising agent used is sufficiently great to prevent turbidity when the alkoxide sol is added to the solution. Titanate-based and zirconate-based ceramics are a known class of materials, including particularly the PZT ceramics which have a perovskite structure. The formula of the ceramic is not a feature of the invention. The amounts of various metal solutions and sols can be chosen by known means to form a mixed sol of composition appropriate to produce the ceramic. In particular, dopant solutions and sols can be used in conventional manner. When the ceramic contains Zr and Ti , the zirconium alkoxide sol is preferably added to the solution, prior to addition of the titanium alkoxide sol . The prevention of condensation reaction resulting in turbidity can be achieved by the use of a variety of stabilising agents. These include monocarboxyl ic acids, generally C1 to C4 alkanoic acids such as acetic acid; dicarboxylic acids; acid anhydrides; glycols; chelating agents e.g. ethyl acetoacetate; and mixtures. In certain instances the addition of a separate solvent may be required for complete stability and iscibility of chemical feed constituents. The extent of the reaction between the stabilising agent and the titanium and zirconium alkoxides is uncertain, and it is convenient to define the concentration of the stabilising agent in terms of the lead (or other) salt. We prefer to use at least 5 moles e.g. 6.6 moles of stabilising agent (such as acetic acid) per mole of lead (or other) salt. The at least one metal salt generally includes Pb which may be partly or indeed completely replaced by one or more of Ca, Sr, Ba, La, Mg , Na, K, Pb, Li, Cu, Au , Ag and Cd. The system may also include Al , Si, Na, K, Sc, V, Ga, Ge , As, Rb, Y, Nb, Mo, Tc, Ru, Rh, Pd, Ag , Cd, In, Sn , Sb, Te , Cs, La, Hj , Ta, W, Pt , Au, Hg, Tl, Bi, or a rare earth. The metal salts should be soluble in the chosen stabilising agent; for example, lead acetate is preferably used with acetic acid. The mixed sol may contain a dopant which may be a salt solution as indicated and/or an alkoxide sol of one or more of the above metals which sol may be added before or after the titanium and/or zirconium alkoxide sols. The amount of the stabilising agent used needs to be sufficiently great to prevent turbidity, resulting from condensation, when the titanium or zirconium alkoxide sol is added to the solution. With acetic acid it is believed that some of the alkoxy groups in the partly condensed alkoxide sol are replaced by acetate ligands. The resulting alkoxy acetate condensate is then found to be stable to precipitation. (This work is published in the Journal of the American Ceramic Society 64_ [5] 821-826 (1989) in an article by R. E. Riman, R. R. Landham, H. K. Bowen entitled "Synthesis of uniform titanium and 1:1 stontium-titanium carboxyhydrosols by controlled hydrolysis of alkoxymetal carboxylate precursors). Both titanium and zirconium sols comprise complexes of one metal atom with four alkoxide groups. Replacement of the first alkoxide group by carboxylate (such as acetate) i,s very easy, replacement of the second quite easy, of the third quite difficult, and of the fourth alkoxide group very difficult. ("Chemical Properties of metal alkoxides", chapter 1, p.202, in "Metal Alkoxides" editors D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Academic Press, 1978). rBy way of example, at least 0.6 ml, e.g. about 1 ml to about 3 ml of glacial acetic acid is preferably used per gram of lead acetate trihydrate. The use of unnecessarily large quantities of acetic acid or other stabilising agent should be avoided, because it increases the amount of organic material that needs to be burnt out of the ceramic on firing. To the dehydrated solution is added first the required amount of a zirconium alkoxide sol, followed by the required amount of a titanium alkoxide sol. The nature of the alkoxide is not critical; typically alkoxides derived from C1 to C4 alkanols are used. The sols are added to the solution slowly with stirring to avoid localised precipitation, and preferably at temperatures not in excess of 80°C, e.g. in the range of 20-60°C. The reagents should be as pure as possible, and steps should preferably be taken to eliminate Na, K and Li cations and 0 anions. The resulting mixed sol preferably contains at least 5 moles e.g. at least 6.6 moles of acetate stabiliser per mole of lead salt and constitutes another aspect of this invention. The resulting sol precursor is diluted with water to form the desired sol. Preferably the water is added in a mixture with an alcohol, again to avoid possible localised precipitation. Enough of this diluent is used to provide a stable sol of convenient application viscosity. Unreacted alkoxy groups are hydrolysed by the water, resulting in further controlled condensation. Then a glycol, such as ethylene glycol, surfactants compatible with the sol or drying control chemical agents (DCCA's) [See D. R. Ulrich; Am. Ceram. Soc. Bull., 64- [11] 1444-1448 (1985)] e.g. formamide and glycerol may be added to the sol in a concentration to improve its film-forming properties. Because unreacted alkoxy groups are no longer present, chemical reactions between alkoxy groups and the film forming additives do not take place, for example, the glycol addition does not result in formation of a condensed solid. The sol can be formed into a film on a non-porous surface by any convenient application technique such as flow or roller coating, brushing, spraying or particularly spin coating. Film thickness can be controlled by appropriate control of viscosity, feedstock dilution in alcohol and coating conditions. Thin films less than 0.5 microns thick (before firing) may be prepared by a single application, thicker films can be prepared by multiple applications with appropriate thermal treatment between applications. Suitable non-porous surfaces include glass, platinum and titanium. The films are first dried and then heated, e.g. to 300 to 700 C, to burn out the organic species, this annealing treatment reduces the film thickness by up to 70% . To avoid cracking, the films are preferably heated at a rate of 1 20°/min e.g. 5 - 15°/min After annealing, the amorphous films are further heat treated to develop the desired crystalline phase, e.g tetragonal PZT. The appropriate thermal treatment in the temperature range 400 to 1150°C imparts the required electrical characteristics in the film. In another aspect the invention provides a PZT film of composition Pb Zr Q 0-1 Q Ti Q 0-1 Q 0 3 preferably Pb Zr Q 4 _ Q 6 Ti Q 6 _ Q 4 0 3 with appropriate dopants where desired having the following electrical characteristics : Dielectric constant 150 - 4000 Dielectric loss factor 0.001 - 0.08 Saturation polarisation 0 - 80 preferably 2 - 40 μC/cm . The film may have a polarisation electric field hysteresis loop showing high remanent polarisation, high saturation polarisation with the difference between the saturation and the remanent polarisation being small and preferably not greater than half the value of the saturation polarisation. Dielectric loss factor should be measured on a high quality electrode comprising a silicon wafer with successive sputtered coatings of titanium and platinum beneath the PZT film. The thin film can either behave as a ferroelectric and be used for switching, capacitor or for electromechanical coupling applications, or alternatively, the thin film can be non-ferroelectric and be used as a passive dielectric layer. The invention gives rise to the following advantages: a) Currently used techniques for the preparation of thin films on a range of substrates such as silicon or quartz, are sputtering, MOCVD, chemical vapour deposition and plasma vapour deposition. The problems associated with these techniques are high capital costs, difficult processing controls, use of expensive precursors, low through-put and size limitations. The use of sol-gel processing solves most of these problems in that the equipment needed is inexpensive, processing control is easy leading to a homogeneous product, precursors are inexpensive and no problems arise regarding through¬ put or size. b) Existing sol gel techniques for the preparation of thin films suffer from the problems that the feeds are generally irreproducible and have a short shelf-life and that the properties of the films produced from the feeds vary considerably and are for that reason unacceptable. Sols made according to the present invention have reproducible compositions and long shelf-lives, e.g. two months or more. Reference is directed to the accompanying drawing which is a flow diagram for preparation of PZT feed. Example 1 A mixed sol, of a composition to be gelled and heated to produce a PZT ceramic material, was prepared as shown in the flow diagram of Figure 1. Crack free films of this mixed sol were deposited on glass, platinum and titanium by standard spin coating techniques. The deposited films were annealed under the following conditions: Heat at 1°C per minute to 600°C; hold for 2 hours at 600°C; cool to room temperature at 1°C per minute. This anneal was effective to burn out organics. After annealing, the amorphous films were further heat treated to develop the desired crystalline phase, tetragonal PZT. The appropriate thermal treatment in the range 650 - 1150°C imparted the required electrical characteristics in the film. The electrical properties of a typical PZT film (Pb Zr Q 52 τi o 48 °3^ for e d by the above method were as follows: Dielectric constant (epsilon) - 250 Loss Factor (tan delta) - 0.022 2 Saturation Polarisation (P ) - 7μC/cm Switching time - 54 ns with a polarisation-electric field hysteresis loop that is symmetrical about the polarisation-electric field origin showing a difference between the saturation and remanent polarisation of less than half the value of the saturation polarisation. ' These results are consistently better than any previously reported for spin-coated sol-gel derived films. For example, in the aforementioned paper, Yi et al reported that their ceramic had a loss factor of 0.1 - 0.7. These higher values preclude the use of such films in many intended applications, for which films according to the present invention are suitable. Example 2 A mixed sol of a composition to be gelled and heated to produce a PZT ceramic material was prepared as follows. 30g of lead acetate trihydrate was dissolved In 36 mis of glacial acetic acid at 80°C. The solution was then cooled to room temperature. 17.76-g of zirconium n-propoxide (70% solution in n- propanol) was added slowly to the solution and with stirring. 11.61g of titanium iso-propoxide was then added ' to the resultant mixture with stirring in order to form the * PZT sol. 15 mis of water was then added to the sol with stirring, and finally 4.3g of ethylene glycol was added to complete the sol formation. Prior to spin coating, the sol was diluted to achieve the desired rheology for spin coating onto a substrate. To dilute the sol, 9g of sol was added to 6g of a n- proptmol/water mixture, the composition of the n- propanol/water mixture being 82.5 wt% n-propanol, 17.5 wt% water. The diluted sol was then spin coated at 1000 rpm for 12 sees onto a platinum coated substrate to form a thin film. The deposited films were annealed under the following conditions:- Heat at 10°/min to 400°C, hold for 1 minute then cool to room temperature. This anneal was effective to burn out organics. After annealing, the amorphous films were further heat treated to develop the desired perovskite crystalline phase. The appropriate thermal treatment in the range 400 - 1150°C imparted the required electrical characteristics in the film. The properties of a typical PZT film (Pb Zr Q 48 Ti Q 52 0.,) formed by the above method were as follows:- Dielectric constant (epsiloπ) - 870 at 5 volts Film thickness - 230θ8 Loss factor (tan delta) - 0.025 2 Saturation Polarisation at 5V, P - 30 μC/cm with a polarisation - electric field hysteresis loop that is symmetrical about the polarisation - electric field origin showing a difference between the saturation and remanent polarisation of less than half the value of the saturation polarisation. Example 3 A mixed sol to be gelled and heated to produce a PZT ceramic of composition Pb Zr Q 40 Ti Q 6 Q 0 3 was prepared by following the method of Example 2 with the quantities of alkoxides adjusted accordingly. In this example the amounts of reagents used was as follows:- 36 mis glacial acetic acid 30g lead acetate trihydrate 14.79g zirconium n-propoxide (70% solution in n-propanol ) 13.47g titanium i so-propoxide 15 mis water 4.3g ethylene glycol The sol produced was diluted as described in Example 2 prior to spin coating on a platinum coated substrate at 1000 rpm for 12 seconds. The deposited films were then annealed and further heat treated under the same conditions described in Example 2. The properties of a typical PZT film of composition Pb Zr Q . Ti Q g 0 3 formed by the above method were as follows:- Dielectric constant (epsilon) - 660 at 5 volts Film thickness - 230θ8 Loss factor (tan delta) - 0.02 Saturation Polarisation at 5V, P - 30 μC/cm with a polarisation - electric field hysteresis loop that is symmetrical about the polarisation - electric 5 field origin showing a difference between the saturation and remanent polarisation of less than half the value of the saturation polarisation. Example 4 A mixed sol to be gelled and heated to o produce a PZT ceramic of composition Pb 0.97 La 0.33 (Zr 0.3 Ti 0.7 } 0.9925 °3 was re ared b y following the method of Example 2 with the quantities of alkoxides adjusted accordingly, and with a lanthanum salt being dissolved in the glacial acetic acid after 5 the lead acetate trihydrate addition but before the addition of the alkoxides. In this example the amounts of reagents used were as follows:- 36 is glacial acetic acid 29.1g lead acetate trihydrate 0 1.03g lanthanum nitrate hexahydrate 11.01g zirconium n-propoxide (70% solution in n-propanol ) 15.60g titanium iso-propoxide 15 mis water 5 4.3g ethylene glycol The sol produced was diluted as described in Example 2 prior to spin coating on a platinum coated substrate at 1000 rpm for 12 seconds. The deposited films were then annealed and further heat treated under the same conditions described in Example 2. The properties of a typical PZT film of composition Pb Q>97 La Q>33 (Zr Q>3 Q 7 ) Q 9925 0 3 formed by the above methods were as follows:- Dielectric constant (epsilon) - 780 at 5 volts Film thickness - 23008 Loss factor (tan delta) - 0.02 2 Saturation Polarisation at 5V, P - 20 μC/cm with a polarisation - electric field hysteresis loop that is symmetrical about the polarisation - electric field origin showing a difference between the saturation and remanent polarisation of less than half the value of the saturation polarisation. Example 5 A mixed sol to be gelled and heated to produce a PZT ceramic of composition Pb 0.985 (Zr 0.3 Ti 0.7 ) 0.97 Nb 0.03 °3 as P re P ared b y following the method of Example 2 with the quantities of zirconium and titanium alkoxides adjusted accordingly, and with a niobium alkoxide dissolved in n-propanol being added to the sol after the titanium iso propoxide addition but before the addition of water and ethylene glycol. In this example the amounts of reagents used were as follows:- 36 mis glacial acetic acid 29.53g lead acetate trihydrate 10.76g zirconium n-propoxide (70% solution in n-propanol ) 15. 5g titanium iso-propoxide 0.75g niobium ethoxide dissolved in 5 mis n- propanol 15 is water 4.3g ethylene glycol The sol produced was diluted as described in Example 2 prior to spin coating on a platinum coated substrate at 1000 rpm for 12 seconds. The deposited films were then annealed and further heat treated under the same conditions described in Example 2. The properties of a typical PZT film of composition Pb Q _ 985 (Zr Q>3 Ti 0>7 ) 0>97 Nb Q>03 0 3 formed by the above methods were as follows:- Dielectric constant (epsilon) - 450 at 5 volts Film thickness - 230θ8 Loss factor (tan delta) - 0.02 Saturation Polarisation at 5V, P - 20 μC/cm with a polarisation - electric field hysteresis loop that is symmetrical about the polarisation - electric field origin showing a difference between the saturation and remanent polarisation of less than half the value of. the saturation polarisation. Example 6, A mixed sol to be gelled and heated to produce a lead zirconate (PZ) ceramic of composition PbZr0 3 was prepared by following the method of Example 2 with the appropriate quantity of zirconium alkoxide added and with no titanium alkoxide being added at all. In this example the amounts of reagents used was as fol lows :- 3 * 6 mis glacial acetic acid 30g lead acetate trihydrate 36.97g zirconium n-propoxide (70% solution in n-propanol ) 15 mis water 4.3g ethylene glycol The sol produced was diluted as described in Example 2 prior to spin coating on a platinum coated substrate at 1000 rpm for 12 seconds. The deposited films were then annealed and further heat treated under the same conditions described in Example 2 to give a lead zirconate thin film. Example 7 A mixed sol to be gelled and heated to produce a lead titanate (PT) ceramic of composition PbTi0 3 was prepared by following the method of Example 2 with the appropriate quantity of titanium alkoxide added and with no zirconium alkoxide being added at all. In this example the amounts of reagents used was as follows:- 36 mis glacial acetic acid 30g lead acetate trihydrate 22.46g titanium iso-propoxide 15 mis water 4.3g ethylene glycol The sol produced was diluted as described in Example 2 prior to spin coating on a platinum coated substrate at 1000 rpm for 12 seconds. The deposited films were then annealed and further heat treated under the same conditions described in Example 2 to give a lead titanate thin film.

Claims: CLAIMS 1. A method of making a mixed sol of a composition to be gelled and heated to produce a titanate-based or a zirconate-based ceramic, by forming a solution of at least one metal salt in a stabilising agent, adding to the solution a titanium alkoxide sol and/or a zirconium alkoxide sol to form a mixed sol precursor and adding water to form the mixed sol, characterised in that the amount of the stabilising agent used is sufficiently great to prevent turbidity when the alkoxide sol is added to the solution. 2. A method as claimed in Claim 1, wherein the stabilising agent is acetic acid. 3. A method as claimed in Claim 2, wherein the metal salt is lead acetate. 4. A method as claimed in Claim 3, wherein at least 0.6 ml of glacial acetic acid is used per gram of lead acetate trihydrate. 5. A method as claimed in any one of Claims 1 to 4, wherein the zirconium alkoxide sol is added to the solution prior to addition of the titanium alkoxide sol . 6. - A method as claimed in any one of Claims 1 to 5, wherein water mixed with an alcohol is added to the sol precursor. 7. A method as claimed in any one of Claims 1 to 6, wherein an alkoxide sol of at least one metal other than titanium and zirconium is also added to form the mixed sol. 8. A method as claimed in Claim 7 wherein, after hydrolysis of metal-alkoxy groups in the sol precursor, a glycol is added to the mixed sol in a concentration to improve its film-forming properties. 9. A method as claimed in any one of Claims 1 to 8, wherein the mixed sol i s formed into a film on a non-porous surface and the film is heated to produce - the desired titanate-based ceramic. 10. A PZT film of composition Pb Zr Q Q , Q Ti Q Q , Q 0 3 with appropriate dopants where desired having the following electrical characteristics : 0 Dielectric constant 150 - 4000 Dielectric loss factor 0.001 - 0.08 2 Saturation polarisation 0 - 80 μC/cm 11. A mixed sol of a composition to be gelled and heated to produce a PZT ceramic of composition 5 Pb Zr Q 0 _ 1 Q Ti Q 0 _. Q 0 3 with appropriate dopants where required, comprising at least 5 moles of acetate stabiliser per mole of lead salt. 0 5 0 5

Inventors: GROVES PAUL, LANDHAM ROWENA ROSHANTHI, STEWARD NIGEL I, TEAT SIMON J, WARRENDER THERESA E, GROVES, PAUL, LANDHAM, ROWENA, ROSHANTHI, STEWARD, NIGEL, I., TEAT, SIMON, J., WARRENDER, THERESA, E.
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1990
Main classification: C01G25
IPC codes: C01G23, C01G25, C04B35, H01L41