Process for forming thin films of functional ceramics

Details Process for forming thin films of functional ceramics Process for forming thin films of functional ceramics

1998-09-28

2001-02-20

Talbot, Brian K. (Department: 1762)

Coating processes

Electrical product produced

Metallic compound coating

C427S226000, C427S419300

Reexamination Certificate

active

06190728

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for forming a thin film of functional ceramic, such as a thin film composed of a composite oxide of a plurality of metals, particularly to a ferroelectric thin film, for example, PZT, PLZT, etc.
DESCRIPTION OF THE PRIOR ART
Perovskite-type Pb(Zr
x
Ti
1−x
)O
3
ceramics and films which are composite oxides have high ferroelectric property and show excellent piezoelectric and pyroelectric properties, and have been used in many applications such as sensor elements. Recently, not only PZT which is a composite oxide consisting of lead, zirconium and titanium, but also PLZT which is a composite oxide composed of lead, lanthanum, zirconium and titanium have attracted attention for their high ferroelectric properties.
These functional ceramics are generally used in a form of thin films when applied to sensors. They are prepared by forming a thin film consisting of lead, zirconium, and titanium (or their oxides) through sputtering process, chemical vapor deposition (CVD) process, etc. and forming crystals under high temperature condition. In this occasion, heat-resistive substrates such as silicon wafers, alumina or the like have been conventionally employed. However, they are too expensive to be used as substrates, and possibility of using more economical materials have been seeked. However, less expensive materials such as aluminum, glass, etc. have low melting points and cannot be used as substrates of the composite oxide thin films.
Moreover, electrodes made of platinum or the like provided on the substrates for applying the thin films to sensors are expensive, and results in high cost of the whole substrate in respect of materials and workability. At the same time, the above process which is treated at high temperature requires an expensive furnace for high temperature operation, and is disadvantagous also in respect of installation cost.
Recently so-called sol-gel process is one of the most promising methods because it offers precise control of a composition at a molecular level as well as the lower processing temperature.
The inventors of this invention have already demonstrated that it was possible to prepare a ferroelectric PZT thin film at comparatively low temperature, employing as a seeding layer a thin film of lead and titanium (or their compounds) formed by the sol-gel process, and stacking on the seeding layer a layer of lead, zirconium and titanium (or their compounds)which is formed by the sol-gel process and contains relatively large amount of zirconium (This process is hereinafter referred to as “single-seeding process”).
However, annealing at 600° C. or more is still required to form a fully functional thin film even by this single-seeding process, which therefore cannot be said as a satisfactorily low temperature.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for forming thin films of functional ceramics overcoming the above drawbacks of the conventional art, in other words, a process for forming thin films of functional ceramics without a need of annealing at high temperature.
In order to solve the above problem, there is provided a process for forming a functional ceramic thin film having a crystal of a composite oxide consisting of two or more metal elements and oxygen, which comprises steps of alternately stacking seeding layers having the same crystalline structure as the composite oxide and formable at a temperature lower than the crystallization temperature of the composite oxide, and layers containing a larger amount of a specified metal element than said seeding layers, and then annealing the resultant layers to form an integral body.
DETAILED DESCRIPTION OF THE INVENTION
In this invention it is preferable that content of said specified metal element in the seeding layer is zero, because a desired composite oxide thin layer can be thus obtained at much lower temperature.
This invention is most suitable for the formation of a thin film of a composite oxide of lead, zirconium and titanium having perovskite-type cristal structure, or a thin film of a composite oxide of lead, lanthanum, zirconium and titanium having similarly perovskite-type cristal structure. In these instances, the specified metal element comprises zirconium and lanthanum.
In this invention, the seeding layer has the same crystal structure as the crystal structure of the composite oxide in the object functional ceramics, but its crystallization temperature is lower than that of the composite oxide. Into the crystal structure of the seeding layer is supplied the specified metal element from the layer having more said specified metal element than the seeding layer. Then the crystal grows making this seeding layer as a seed of the crystal, and a generally uniform crystal structure is formed. A favorable composite oxide thin layer can be obtained even at low temperature by thus alternately stacking these layers. (A process of alternately stacking the two layers in this way is hereinafter referred to as “multi-seeding process”.)
Through the multi-seeding process, a composite oxide thin film having an excellent ferroelectric property can be obtained at such low temperature as would be impossible with the conventional single-seeding process in which one each of the seeding layer and the layer having more the specified metal element than the seeding layer are stacked, and then annealed to give a functional ceramic thin film having the composite oxide crystal.
In this invention, the seeding layer has preferably a thickness of 10 nm to 40 nm. A uniform and favorable seeding layer is hardly obtainable when the thickness is below 10 nm. When the thickness is over 40 nm, it is difficult to obtain a uniform crystal unless treated at high temperature. On the other hand, the layer containing more the specified metal element has preferably a thickness of 20 nm to 80 nm. Outside the range of thickness above mentioned, it would be difficult to obtain a uniform functional ceramic.
The layers are preferably prepared by chemical solution deposit (CSD) process because this process facilitates a precise control of the thickness of the layers. The CSD process includes a dipping process in which a substrate is dipped into an alcohol solution of alkoxide of desired elements such as lead, titanium, lanthanum, zirconium, and slowly withdrawn while forming a film on the surface, and a spinning process in which the alkoxide alcohol solution is dropped onto the rotating substrate to form the film thereon. The thickness of the film can be controlled by viscosity of the solution, withdrawal speed (dipping process), rotation speed (spinning process), concentration of the solution, etc. The alkoxide of the film thus prepared is hydrolized and an oxide is then formed. The stacked body can be prepared by repeating the abovementioned film formation.
In this invention, the required annealing temperature in oxygen containing atmosphere is at lowest 450° C. At this temperature it is possible to obtain the composite oxide thin film having sufficient dielectric property. Annealing at much higher temperature can be conducted, but the range of 450-600° C. would be appropriate to achieve the object of this invention. If the material for the composite oxide contains a lead compound, the annealing must be conducted in the temperature range where a large evaporation is avoided.
The annealing is generally conducted in air. It is sufficient if the processing atmosphere contains oxygen to such extent as the composite oxide is formed.
The substrate to be used should be heat-resistive to 450° C. or more. A glass substrate is preferably employed in view of its low cost. If a transparent electrode such as ITO is provided on the substrate, the obtained composite oxide thin film can be used as a sensor.
A platinum electrode generally used when transparency is not required is very expensive and needs further processes to be used as an electrode, and thus raises the cost for the substrate. However, according to this invention, an aluminum substrate can

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