WO1990013149A1

WO1990013149A1 - SOL GEL PROCESS FOR PREPARING Pb(Zr,Ti)O3 THIN FILMS

Info

Publication number: WO1990013149A1
Application number: PCT/CA1990/000123
Authority: WO
Inventors: Michael Sayer; Guanghua Yi
Original assignee: Queen's University At Kingston
Priority date: 1989-04-27
Filing date: 1990-04-18
Publication date: 1990-11-01

Abstract

A method for preparing crack free transparent ferroelectric polycrystalline Pb(Zr,Ti)O3 thin films (PZT or PLZT) and other complex oxide thin films by a sol-gel process and the product thereof is described. A stock solution containing the organic precursors to the metal oxides together with a chelating agent may be spin coated onto a flat substrate or dip coated or painted onto a curved substrate or even metallic wire, and subsequently dried and fired. The addition of firing additives to the solution prevents cracking of the film. Alternatively an intermediate layer of Al2O3 or other oxide may be deposited on the substrate before deposition of the PZT or other oxide materials.

Description

SOL GEL PROCESS FOR PREPARING Pb(Zr,Ti)0₃ THIN FILMS

Field of Invention

This invention relates to the production of thin-film piezoelectric materials. More particularly this invention relates to a sol-gel process for producing thin films of ferroelectric or piezoelectric materials such as Pb(Zr,Ti)0₃, known as PZT, or lanthanum doped PZT, known as PLZT, or PZT doped with elements such as niobium and the product thereof. It also relates to a sol gel process for producing crack free thin films of other complex oxides such as zirconia, calcium apatite, and barium titanate, among others.

Background of Invention The ferroelectric effect is the property of certain crystalline dielectric materials to retain polarization following their polarization. The piezoelectric effect is the property of certain crystalline dielectric materials, when polarized, to generate an electric field or potential of one polarity when a compressive force is applied and the reverse polarity when a tensile force is applied. Conversely, a piezoelectric material will tend to compress if an electric field of one polarity is applied and to expand if an electric field of the opposite polarity is applied.

Thin film materials which are transparent and show good optically non linear properties are in great demand for integrated optics among other uses for such thin f ilm materials. Undoped PZT is an opaque ferroelectric cerami and therefore its nonlinear optical effects, if any, canno be measured. However, the addition of lanthanum to form La doped PZT, or PLZT, causes the material to becom transparent and to show large electro-optic effects. I addition to electro-optic properties, PZT has a large pyro electric response, large electro-mechanical coupling coeffi cient high dielectric constant and a large spontaneou polarization, all of which are useful properties for incor poration-into infra red detectors, surface acoustic wav devices, ferroelectric nonvolatile semiconductor memories, and devices requiring transtarent high permittivity layer such as electroluminescent displays.

Several methods for producing PZT, or PLZT (lanthanu doped) thin films have been developed over the past decade, primarily using techniques such as flash and electron bea evaporation, rf sputtering, ion beam deposition an epitaxial growth by rf sputtering. However such technique are generally difficult to control, are relatively tim consuming and require expensive apparatus. Chemical sol ge processing of PZT of PLZT thin films has also bee suggested. Sol gel processing offers significant advantage over vacuum deposition techniques such as easier compositio control and film homogeneity, easier fabrication of larg area thin films, low cost and short processing cycles. However, sol gel processing is not without its difficulties, particularly with respect to fabricating relatively thic crack free PZT thin films. The thickness of PZT or PLZ thin f ilm strongly influences f ilm properties such as dielectric constant, remanent polarization and coercive field.

While the present invention will be described with particular reference to the production of PZT and PLZT thin films, the sol-gel processing method is equally applicable to other complex oxide thin films. As used herein the term

"complex oxide" includes:

Alumina • Zirconia

Calcium titanate

Calcium apatite

Lithium niobate

Barium titanate

Tantalum zirconate

Magnesium tantalate

Yttrium Barium Copper Oxides

Yttrium Oxide

Barium Calcium Thalium Copper Oxide

Object of Invention

It is, therefore, an object of the present invention to overcome the deficiencies of the prior art and provide an improved sol gel processing method for making thin film piezoelectric materials which are substantially crack free, and which have improved physical properties.

Another object of the invention is to provide PZT thin films having a piezoelectric coupling coefficient of at least 1%. It is yet another object of the invention to provide crack free complex oxide thin films by an improved sol gel process. 4 Statement of Invention

By one aspect of this invention there is provided a method for preparing a composite of a substrate havin deposited thereon a crack-free ferroelectri polycrystalline lead zirconate titanate thin film, comprising

(a) dissolving lead acetate in acetic acid, adding thereto zirconium propoxide followed by titanium isopropoxide in relative proportions to produce a selected ratio of metals, and agitating

10 until all solids are in solution;

(b) adding a firing agent selected from glycerol, ethylene glycol, tetra ethylene glycol, polyethylene glycol and mixtures thereof, to said solution;

15

(c) adjusting viscosity of said solution;

(d) coating said substrate with said solution so as to form a wet film thereon;

(e) firing said coated substrate at a

20 temperature in the range 300°C-550°C so as to pyrolyse said film;

(f) annealing said pyrolysed film at a temperature up to about 600 °C for sufficient time to produce a perovskite structure in said film;

25 and

(g) electrically poling said film so as to produce piezoelectric properties therein. By another aspect of this invention there is provided a method for preparing a composite of a substrate having deposited thereon a crack free ferroelectric polycrystalline lead zirconate titanate thin film, comprising:

(a) depositing an oxide film on said substrate;

(b) dissolving lead acetate in acetic acid, adding thereto zirconium propoxide followed by titanium isopropoxide in relative proportions to produce a selected ratio of metals, and agitating until all solids are in solution;

(c) adding a firing agent selected from glycerol, ethylene glycol, tetra ethylene glycol, polyethylene glycol and mixtures thereof to said solution;

(d) adjusting viscosity of said solution;

(e) coating said oxide coated substrate with said solution so as to form a wet film thereon;

(fj firing said coated substrate at a temperature in the range 300°C-550°C so as to pyrolize said film.

(g) annealing said pyrolysed film at a temperature up to about 600°C for sufficient time to produce a perovskite structure in said film; and

(h) electrically poling said film so as to produce piezoelectric properties therein. 6 By yet another aspect of this invention there i provided a composite comprising a substrate having deposite thereon a substantially crack-free ferroelectric poly crystalline lead zirconate titanate thin film having piezoelectric coupling coefficient of at least 1%. By further aspect of this invention there is provided a metho for preparing a composite comprising a substrate havin deposited thereon a crack free complex oxide thin film, comprising:

(a) preparing a solution containing precursors for said complex oxides;

(b) adding to said solution a firing agent selected from glycerol, ethylene glycol, tetra ethylene glycol, polyethylene glycol and mixtures thereof in relative proportions and amounts so as to provide a solution having a selected viscosity; _ c) coating said substrate with said solution so as to form a wet film thereon; and

(d) firing said coated substrate at a temperature in the range 300°C-550°C so as to pyrolyse said film.

Brief Description of Drawings

Fig. 1 is a block flow diagram illustrating sol gel processing of PZT films. Detailed Description of Preferred Embodiments

Sol gel processing has been described previously (see, for example, Sol-Gel Processing of PbTi0₃, PbZr0₃, PZT and PLZT Thin Films, Budd et al Brit. Cer. Proc. 3_6 (1985) pp 107-121) and depends upon the fact that the components of a complex solution can remain mixed during firing if a gel, a form of polymerised liquid, can be formed. The standard procedure normally includes the steps of (a) forming a stock solution (b) coating the solution onto a substrate either by spin or dip coating (c) firing the wet film or coating to an inorganic form, (d) annealing at high temperature to obtain the required structure and (e) polarizing the film to induce piezoelectric activity. It has now been discovered that the basic process can be improved in ways which are vital to industrial application and which permit the use of a wide range of compositions. Specifically a new stable precursor solution using a chelating agent has been developed and an additive which controls and improves the firing cycle for film fabrication and which improves the surface smoothness of the final film and substantially eliminates macro cracking of the final surface has also been developed. Figure 1 illustrates a preferred process for making the improved PZT and PZLT films of the present invention. The choice of the precursor compounds and the solvents therefor is important to success. The compounds should have high metal content and high solubility in the selected solvent. They should thermally decompose without evaporating and be chemically compatible with each other. Preferred compounds having the aforesaid properties include: Lead acetate, Zirconium propoxide and Titanium propoxide. The solven must have an appropriate boiling point and suitabl viscosity and surface tension properties. Preferre solvents include water and propanol. The chelating agent, which is required to prevent hydrolysis of the properties i preferably glacial acetic acid although other acids can b used. In order to substantially eliminate macroscopi cracking and to improve the surface smoothness of the film, a firing additive selected from glycerol, ethylene glycol,

10 tetra ethylene glycol, polyethylene glycol and mixture thereof is preferably incorporated.

*_

Preparation of Solutions for PZT films

Lead acetate is dissolved in heated acetic acid in the

¹⁵ proportion of 2 g lead acetate to 1 ml acetic acid an heated to 105°C to remove water. The dehydrated solutio should be cooled below 80°C before sequentially adding th required quantities of zirconium propoxide and titaniu isopiropoxide. It is essential that the zirconium propoxid

2⁰ is '-stdded first because it reacts with the acetic acid t fortα a non-hydrolysable solution which in turn protects th titanium isopropoxide from hydrolysis and condensation whe it is added. (inversely, if titanium isopropoxide is adde fiist it reacts with the acetic acid to form mono o

²⁵ diacetylates and condensation occurs with the formation o polytitanyl aσetylates. The zirconium-titanium bearin mixture is agitated, preferably in an ultra sonic cleanin

tank, until all of the solids present have dissolved. The firing additive which controls the viscosity and the decomposition temperature, is selected from glycerol, ethylene glycol, tetra ethylene glycol, polyethylene glycol or mixtures thereof, and can then be added to the stock solution. Ethylene glycol or glycerol is preferably added in the proportion of 1 ml to 10 g of lead acetate in the solution depending on the thickness of film required. The viscosity and surface tension of the solution can then be adjusted by adding water, propanol or mixtures thereof. Thinner films result from lower viscosity solutions and propanol reduces surface tension thereby improving wettability of the substrate.

Particularly suitable solutions for preparing single layer films 1 m thick of (a) PZT having a composition near the morphotropic phase boundary (Zr/Ti = 53.5/46.5) and (b) alumina are shown below in Table 1.

Table 1

Film Chemical Quantity

(a) PZT Lead Acetate Pb(CH-,C0-_>)-,3H O 12 . . 00 g Acetic Acid CH₃COOH 6 ml Zirconium propoxide Zr(C₃H2θ)₄ 5. . 54 g Titanium isopropoxide Tif(CH-,j cHO]_/ 2 , . 18 g Ethylene Glycol 0HCH.,CH₅0H 1 , .2 ml Distilled Water H₂0 6 ml

(b) Al₇0τ Aluminum isopropoxide Al[ (CH-,) ^CHO], 4 = g Nitric Acid HNO, ^~ 5 ml

Ethylene Glycol OHCH_pCH OH 4 ml Propanol CH₃CH₂CH₂OH 4 ml The final solution, which is stable in air, may be filtered through a membrane filter (pore size 0.22 μm) and stored in a sealed container.

Substrate Surface Preparations

PZT may be deposited on numerous substrates such as silica wafers, mica, fused quartz, borosilicate glass, Corning 7509 glass coated with stannic oxide based trans¬ parent conducting coating, and stainless steel, gold or platinum plates. Crack free surface coatings are most easily applied to conducting glass substrates but in the case of fused quartz it is preferable to first apply a special surface treatment as described in more detail below. Substrates must be thoroughly cleaned before depositing the coating, preferably by boiling in water with a detergent, rinsing in distilled water followed by ultra sound to remove organic contaminants and vapour degreasing in methanol. After cleaning, the precursor films, such as those described in Table 1, can be deposited on the substrate by spin coating, dip coating or by painting. The spin coating technique is preferred for flat substrate surfaces as the thickness of the film may be controlled by the speed of rotation of the spinner and the concentration and viscosity of the solution. A spinner such as a "Headway Research Incorporated photoresist spinner" operated at 8500 rpm for 20 seconds is particularly suitable. More complex shapes cannot readily be prepared by spin coating and recourse may be had to dip coating or painting. In this case it is usually necessary to dilute the above stock solution with propanol or a mixture of propanol and water. The film thickness can be controlled by the amount of propanol and water or the ratio of propanol to water and by controlling the speed of pulling the dip coated article.

Firing

Heretofore films made by sol gel processing have usually suffered macro cracking during firing. Such cracking can be substantially reduced or eliminated by the use of firing additives to the stock solution or, in certain cases, by deposition of an intermediate buffer layer.

After an organo-metallic film has been deposited and dried it must be pyrolysed to remove the organics and induce the solid state reaction which produces the PZT. An increase in the density of the film and sufficient time for a solid state diffusion reaction to take place are both required. The normal approach to avoid cracking is to pyrolyse the film rapidly by transferring the substrate to a surface which is well above the pyrolysing temperature. According to the present invention, the pyrolysis may be better controlled if the time or temperature range over which the film is in a viscous liquid state is extended. This may be accomplished by adding a relatively small amount of a firing additive selected from ethylene glycol, glycerol and tetra ethylene glycol to the stock solution in order to raise the viscous state temperature. The resulting films are crack-free, very transparent and have improved surface smoothness. Glycerol and tetra ethylene glycol are preferred additives. When using glycerol the wet film may be fired by raising the temperature gradually which is particularly advantageous especially when coating metallic substrates such as stainless steel or alumel.

It is known that in normal ceramics, firing cracks can be controlled if the material is maintained under a compressive load. In a preferred, and alternate, embodiment of the present invention, this technique may be applied and is particularly useful on substrates such as silicon and borosilicate glass. A thin intermediate or buffer layer may be deposited onto the substrate and fired on prior to the deposition of PZT. The thickness of the buffer layer depends on the substrate and film composition but 100 A is usually sufficient. For applications requiring a transparent insulating layer Alumina (A1₂0₃) has good properties and is easy to fabricate by sol gel processing. A typical mixture for this purpose is shown in Table 1 (b) , above. For transparent, conducting coatings indium tin oxide or stannic oxide conducting coatings may be used. Other conducting coatings include Al, Au, Cr, Ni and Co which may be produced by known vacuum deposition or chemical techniques.

The firing schedule which pyrolyses the organo-metallic compounds to an inorganic film is key to the preparation of crack free films having the desired crystal structure, grain size, transparency and surface roughness. The solvents evaporate with a consequent large change in volume and the generation of internal stress at temperatures between room temperature and about 250°C. As the temperature increases the lead acetate dehydrates, melts and decomposes. At this stage, the dried film becomes "wet" again and the organic compounds begin to decompose. The internal stress is relaxed and the volume change of the film continues. At the end of this stage, the organic film changes to a fine mixture of oxides of lead, titanium, zirconium and free carbon. At still higher temperature, the free carbon oxidizes and the mixture of oxides transforms to a transparent amorphous PZT film.

It is believed that the high boiling point and latent heat of vaporization of the ethylene glycol raises the solution evaporation temperature in the first phase towards the "melt" temperature in the second phase. This retains atom mobility and significantly reduces the tendency to crack. Raising the firing temperature also reduces this tendency. Films can be fired either in an oven or by placing the substrate on a hot plate. In the latter case, the rapid rise in temperature and the fact that the substrate temperature is higher than the film temperature lead to fewer cracks in the fired film. As the wet film pyrolyses and solidifies on the hot expanded substrate, the tendency on cooling is to create compressive or lower tensile stress. There are, however, some limitations on the speed and temperature of firing. In the case of thick films, the firing temperature should be limited to 350-C- 550°C as the films become milky and less transparent at higher firing temperatures.

Annealing

As fired films are basically amorphous and need annealing at temperatures up to about 600°C for up to six hours in order to cause the amorphous structure to change to a perovskite structure.

Electrical Post Treatment After annealing the crystal structure of the film is generally that of randomly oriented crystallites, although some evidence of at least some orientation exists. In order to induce piezoelectric properties, however, it is necessar to electrically pole the films by application of an electric field of between 2 and 3 KV/mm when the films are held at temperature of about 175°C. Films produced by the aforesai processing steps have been shown to have properties superior to those obtainable by other processing methods. For example, the best piezoelectric coupling coefficient heretofore achieved, in zinc oxide, is of the order of 0.7%. In contrast films according to the present invention have a piezoelectric coupling coefficient of at least 1.0%.

PZT thin films have numerous applications including use in electro-optic displays and electroluminescent devices. They are used in electronic applications such as ferro¬ electric gate insulators or capacitors in non-volatile memories. A third field of use includes high frequency ultrasonic transducers for non-destructive evaluation and medical and biological purposes, while a fourth field includes fibre optic sensors and devices.

While this invention has been described with particular reference to PZT and PLZT, it will be appreciated that the processing techniques described are equally applicable to the deposition of other complex oxide crack-free thin films such as those listed hereinabove. It has been found that the thickness of the crack-free film and the smoothness of the film can -be controlled specifically by the composition and viscosity of the firing additive. Modification of the polyethylene glycol mixture is therefore important. It has also been found advantageous to process and fire under vacuum in order to extract the water uniformly and ^•cause uniform decarbonization.

Claims

We Claim

1. A method for preparing a composite of a substrat having deposited thereon a crack-free ferroelectric . polycrystalline lead zirconate titanate thin film, i which a substrate is coated with a solution containin precursors of said thin film, fired and electricall poled, and characterized by:

(a) dissolving lead acetate in acetic acid, adding thereto zirconium propoxide followed by

10 titanium isopropoxide in relative proportions to produce a selected ratio of metals, and agitating until all solids are in solution;

(b) adding a firing agent selected from glycerol, ethylene glycol, tetra ethylene glycol,

₁-. polyethylene glycol and mixtures thereof to said solution;

(c) adjusting viscosity of said solution;

(d) firing said coated substrate at a temper gU _%,„, ature in the range 300 -C-550 - C so as to pyrolyse

■ 20 said film; and

^'■ ( e) annealing said pyrolysed f ilm at a temperature up to about 600 °C for sufficient time to produce a perovskite structure in said film;

25 2. A method as claimed in claim 1 characterized in that said substrate is selected from silicon, silicon coated with metals such as aluminum, platinum, titanium nitride, mica, fused quartz, glass , alumina, stainless steel , alumel , gold and platinum.

3. A method as claimed in claim 2 characterized in that said coating step is selected from spin coati . ng, di.p coati. ng and painting.

4. A method as claimed in claim 1 characterized in that the viscosity of said solution is adjusted by addition of a di luent selected from water , propanol and mixtures thereof .

5. A method for preparing a composite of a substrate having deposited thereon a crack free ferro¬ electric polycrystalline lead zirconate titanate thin film, characterized by:

( a ) depos iting an oxide f i lm on sa id substrate ;

(b) dissolving lead acetate in acetic acid, adding thereto zirconium propoxide followed by titanium isopropoxide in relative proportions to produce a selected ratio of metals , and agitating until all solids are in solution;

( c) adding a f iring agent selected from glycerol , ethylene glycol , tetra ethyl glycol , polyethylene glycol and mixtures thereof to said solution;

(d) adjusting viscosity of said solution; (e) coating said oxide coated substrate with said solution so as to form a wet film thereon;

( f ) f iring said coated substrate at a temperature in the range 300 °C-550 °C so as to pyrolize said film;

( g) annealing said pyrolysed f ilm at a temperature up to about 600°C for sufficient time to produce a perovskite structure in said film; and

(h) electrically poling said film so as to produce piezoelectric properties therein.

6. A method as claimed in Claim 5 characterized in that said substrate is coated with an organic oxide or nitrate precursor solution , which is dried and fired at a temperature in the range 400- 600 °C so as to deposit said oxide film on said substrate .

7. A method as claimed in Claim 5 characterized in that said oxide f ilm is depos ited by vacuum deposition.

8. A method as claimed in Claim 5 characterized in that said oxide film is selected from alumina, indium tin oxide and stannic oxide.

9. A method as claimed in Claim 6 wherein said oxide film is selected from alumina, indium tin oxide and stannic oxide .

10. A method as claimed in Claim 8 characterized in that said oxide film is selected from alumina , indium tin oxide and stannic oxide.

11 . A compos ite compr i s ing a substrate having deposited thereon a ferroelectric polycrystalline lead zirconate titanate thin film characterized in that said thin film is substantially crack-free and has a piezoelectric coupling coefficient of at least 1% .

12 . A composite as claimed in Claim 11 characterized in that said substrate is selected from, silicon , mica , fused quartz , alumina , glass , stainless steel, alumel, gold and platinum.

13. A method for preparing a composite comprising a substrate having deposited thereon a crack free complex oxide thin film, characterized by:

(a) preparing a solution containing precursors for said complex oxides;

(b) adding to said solution a firing agent selected from glycerol, ethylene glycol, tetra ethylene glycol, polyethylene glycol and mixtures thereof in relative proportions and amounts so as to provide a solution having a selected viscosity;

(c) coating said substrate with said solution so as to form a wet film thereon; and (d) firing said coated substrate at a temperature in the range 300°C-550°C so as to pyrolyse said film.

14. A method as claimed in claim 13 characterized in that said complex oxides are selected from:

Alumina Zirconia

Calcium titanate Calcium apatite Lithium niobate Barium titanate Tantalum zirconate Magnesium tantalate Yttrium Barium Copper Oxides

Yttrium Oxide . Barium Calcium Thalium Copper Oxide.

15. A method as claimed in claim 14 characterized in that said firing is effected in vaccuo.