Dielectric Materials

This invention relates to dielectric materials and in particular, but not exclusively, high dielectric constant dielectric materials for capacitors and other devices such as sensors.

Dielectric materials are used in various electrical components for various purposes. In the case of capacitors dielectric materials may be employed as the dielectric between metallic electrodes, or in the case of sensors, dielectric materials may be employed to provide substrates on which sensing films are arranged. Conventionally the dielectric materials may be powders which are mixed with binder and solvents, formed to a required shape and then sintered in order to reduce porosity.

The dielectric material most commonly employed is barium titanate which has a dielectric constant of the order of 1500. Very much higher dielectric constant values are, however, provided by bulk dielectric materials such as PbFe.sub.2/3 W.sub.1/3 O.sub.3, PbFe.sub.1/2 Nb.sub.1/2 O.sub.3 and PbZn.sub.1/3 Nb.sub.2/3 O.sub.3. With suitable choice of materials it has already been demonstrated that multilayer ceramic capacitors can be produced in which the ceramic dielectric constant is 20,000 or more. These known multilayer ceramic capacitors were, however, manufactured using the conventional process of mixing powders with binders and solvents and forming them to a required shape, by, for example, pressing. After screen printing of electrodes thereon and the assembly of stacks of electrode-carrying ceramic bodies, the stacks must be fired in order to sinter the ceramic to reduce porosity.

It is an object of the present invention to provide a method of manufacturing a layer of dielectric material of the formula Pb(X,Y)O.sub.3 in a simple, inexpensive and reliable manner such that the layer has good electrical properties.

According to one aspect of the present invention there is provided a high dielectric constant dielectric material of the general formula Pb(X,Y)O.sub.3 produced in thin film form by chemical vapour deposition.

According to another aspect of the present invention there is provided a method of manufacturing thin films of high dielectric constant materials of the general formula Pb(X,Y)O.sub.3 by chemical vapour deposition, comprising directing vapours of the constituent elements onto a heated substrate together with an oxidising gas.

In this specification, high dielectric constant values are to be understood to mean values generally greater than 5,000.

The sole FIGURE shows a schematic arrangement of an apparatus for producing thin fim dielectrics.

The present invention is based on the recognition that such high dielectric constant materials with the general formula Pb(X,Y)O.sub.3, where X and Y include Mg, Zn, Fe, Ni, Co, Nb, Ta, W, Sn, Ti and Zr, lend themselves particularly well to preparation by chemical vapour deposition so that very thin films thereof can be readily prepared, with an attendant reduction in size of capacitor elements, for example. This is because all of ceramic material constituents have volatile compounds, for example Pb(C.sub.2 H.sub.5).sub.4, NbCl.sub.5, WF.sub.6, zinc acetylacetonate, which can be easily vaporised and oxidised in suitable reactors.

Hence the present invention provides a method of manufacturing thin films by directing vapours of reactants containing lead and at least two additional metals and an oxidising gas onto a heated substrate, the films having a dielectric constant generally greater than 5,000.

Thus the preparation of single high dielectric constant dielectric materials or a mixture of several high dielectric constant dielectric materials may be achieved at a heated surface by directing on to it suitable vapours of the elements required together with an oxidising gas such as oxygen, water vapour etc.

In this way a range of materials can be prepared in thin film form for use in capacitor or sensor elements, for example. Whilst the general formula of the dielectric materials quoted above was Pb(X,Y)O.sub.3, the materials may contain PbO and two or more oxides of the elements Mg, Zn, Fe, Ni, Co, Nb, Ta, W, Sn, Ti, Zr, for example. The thickness of the films may be of the order of one micron, for example in the range 0.03 micron to several microns.

The apparatus illustrated somewhat schematically in the accompanying drawing has been employed to produce oxide films on suitable substrates by the hydrolysis of vapours of PbBr.sub.2, FeCl.sub.3 and NbCl.sub.5 at temperatures in the range 500.degree. to 800.degree. C.

The FeCl.sub.3 and NbCl.sub.5, both in solid form, were contained in vessels 1 and 2 and heated (by means not shown) to create vapours to be transported by a flow of nitrogen to a reaction vessel 3 disposed in a furnace 4 and containing a substratre 5 on which an oxide film was to be formed. The temperatures used were 180.degree. to 290.degree. C. for FeCl.sub.3 and 150.degree. to 240.degree. C. for NbCl.sub.5. The PbBr.sub.2 was prepared in situ by passing bromine vapour entrained in nitrogen from a vessel 6 containing liquid bromine, kept at room temperature, and over molten lead 12 contained in a graphite boat 7 in the vessel 3. The vessel 3 was heated to various temperatures in the range 500.degree. to 800.degree. C. during different experimental runs. Water vapour (oxidising gas) in the vessel 3 was produced by passing nitrogen through water in a vessel 8 which was kept at room temperature.

By selecting suitable nitrogen flow rates, oxide films 1 to 5 micrometer thick on substrate areas of approximately one square cm have been achieved in 20 to 100 minutes, with dielectric constant values varying from 160 to 128000. Typical nitrogen flow rates through the reagent vessels were 150 to 325 ml/min for the FeCl.sub.3 in vessel 1, 80 to 250 ml/min for the NbCl.sub.5 in vessel 2, 0 to 30 ml/min for the bromine in vessel 6 and 600 to 1200 ml/min for the water in vessel 8. In the case of 0 ml/min nitrogen flow through vessel 6, the stop cocks of vessel 6 were open and significant diffusion of bromine into the gas stream occurred. The nitrogen flow rates were indicated on flow meter tubes 9. The effluent gas from vessel 3 was bubbled through water in vessel 10. The connecting tubes between vessels 1, 2 and 3, and vessel 3 where it extends out of furnace 4 were heated in order to prevent condensation of the vapours on the walls thereof. The heating was achieved by means of electrical heating tape (not shown) wrapped around the tubes and the end of vessel 3 and also vessels 1 and 2, in order to provide the heating thereof. The vessels 1 and 2 and the furnace 4 each had a respective thermocouple 11 and temperature regulation means (not shown).