Composite Metal Polymer Films

This invention relates to composite metal polymer films and to the production of such films by evaporation techniques.

It has been proposed in U.K. patent specification No. 1,072,049 to produce by vapor deposition composite films comprising metal and poly (p-xylylene) for use as thin film resistors in electric circuitry. In this prior specification considerable stress has been placed upon the production of an atomic or nearly atomic dispersion of metal in a polymer matrix leading in general to composite films having a positive temperature coefficient of resistance. Furthermore, the pressures at which the deposition process is carried out in specification No. 1,072,049 are determined by the chemistry of the polymer-forming component and are usually relatively high, e.g., about 1 micron.

It has now been found possible to produce composite metal polymer films having unusual and very valuable properties in which the metalliferous particles are dispersed in a polymeric matrix in the form of three dimensional dispersions of island structures, such products usually having a negative temperature coefficient of resistance. For example, with certain copper-polymer films surface and bulk resistivity values change by up to seven orders of magnitude over a temperature range of only 200.degree. C. This high temperature sensitivity makes the material useful as a temperature sensor or temperature controller element.

Composite metal polymer films in accordance with the present invention are produced by so controlling the relative rates of deposition of metal and polymer particles that insufficient metal is deposited to form a continuous film, the metal particles being laid down instead in the form of islands with intervening zones of polymeric material thus resulting in a product which is initially electrically open circuited or having very high resistivity, e.g., greater than 10.sup.12 ohms per square.

To achieve stable electric properties it is necessary to subject the three-dimensional dispersion of metalliferous material in the polymeric matrix to a subsequent heat treatment. This heat treatment will for convenience be described in this specification as "annealing." The effect of annealing is time/temperature dependent and actual values of resistivity can be pre-determined by terminating annealing when the required value is reached. Annealing may be carried out at temperatures up to about 150.degree. C or even higher but in most cases it is unnecessary to anneal at temperatures higher than about 200.degree. C because little or no advantage is gained thereby. The principal effect of annealing is oxidative in the case of copper and aggregation in the case of silver. With copper, annealing may follow closely upon deposition whereas with silver a delay period is desirable before annealing is commenced, about two days being a recommended delay period.

To produce the composite films of the present invention techniques broadly similar to that described in our prior U.S. Pat. No. 1,298,453 may be used. However, for the purposes of our prior patent irradiation of the polymeric material during deposition is essential whereas in relation to the present invention an irradiation treatment either during or subsequent to the deposition process is merely optional. Irradiation generally toughens the polymer film and limits the degree of aggregation of metal particles and for this reason is usually undesirable although there may be circumstances in which its use is indicated.

The polymeric material and the metal are evaporated and deposited from different sources and strict control over the rate of evaporation and deposition is required. Particular importance is attached to the use of a constant rate of evaporation/deposition of the metallic material.

A variety of polymeric materials may be used. Polyethylene is eminently suitable as a polymeric material but valuable films can also be made with polycarbonates, polyvinylidine chloride and other polymers including polyethylene terephthalate. Examples of metals which may be used are gold, aluminum, chromium, tellurium, silver and copper.

Whilst the nature of the metalliferous particles in the film of the present invention is not at present fully known with certainty, it is believed that the metalliferous material is partly in the elementary metallic state and partly (in some cases almost entirely) in the form of a metal oxide, the relative proportion depending to some extent on the metal itself and the conditions used to prepare the product, particularly in the annealing step. It is probable that the metalliferous particles consist of a metallic core surrounded by a layer of metal oxide. The properties of the resultant film are unusual in that they are typical neither of the metal nor of the metal oxide. Thus in the case of silver and copper the proportion of oxide is significant whilst with aluminum and chromium the oxidation occurring during annealing proceeds probably further than would be desired for the best results unless protective non-oxidizing atmospheres are used. When gold is used as the metal very little oxide is present.

When composite films are produced according to the invention, and annealing is carried out at the higher temperatures, e.g., 200.degree.-220.degree. C, it is found in many cases that the amount of polymer remaining in the final product is quite small. The composite nature of the film in such cases is therefore of transient importance, the function of the polymeric matrix being primarily to prevent coalescence of the metalliferous islands during deposition and to allow aggregation or oxidation of discrete island structures as separate entities. In applying the process of the present invention to metals which form oxides with semiconducting properties, therefore, e.g., copper, tin, iron, a new technique for producing oxide films becomes possible at temperatures much lower than is used at present. For example prior methods of producing copper oxide semiconductors require oxidation temperatures of the order of 1200.degree. C; with the method described above comparable results are achieved at about 200.degree. C. The invention therefore also comprises a process for the production of a metal oxide film in which metalliferous particles and polymer particles are co-deposited on a substrate at such relative rates that metal particles are deposited as island structures separated by intervening zones of polymeric material, and the resulting composite film is subjected to an oxidative treatment in which substantial oxidation of the separate metal islands is achieved before loss of the polymer by oxidation and/or evaporation.

The invention is illustrated in the accompanying drawings of which,

FIG. 1 is a diagrammatic representation of the apparatus used and,

FIG. 2 illustrates composite films deposited therein.

FIGS. 3 to 9 are described later.

Referring to FIG. 1, the apparatus includes a vacuum chamber (not shown) in which the apparatus illustrated is enclosed. The apparatus comprises an electrically heated glass substrate 1 protected by a removable shutter (not shown) and disposed horizontally about 20 cm. above a polymer source 2 and a metal source 3. The polymer source 2 has the same form as that described with reference to the drawing in U.K. Pat. No. 1,298,453 and is activated from a power supply 4. The metal source 3 comprises molybdenum strip or boat containing metal granules and is electrically heated through an automatic evaporation rate controller 5 energized by a power supply 6.

The polymer and metal sources are shielded from each other and from other parts of the apparatus by means of shields 8 which provide for the emission of divergent beams of particles towards the substrate 1. The evaporation of polymer is controlled by a polymer monitor 9 and monitor supply 10, evaporation of metal being controlled by a metal monitor 11 through the evaporation rate controller 5. The configuration of the shields 8 is such that the beams of polymer and metal particles overlap where they impinge on the substrate 1.

An ultraviolet discharge lamp (not shown) mounted inside the chamber is used to radiate and toughen the polymer film if desired.

Electrical connections (not shown) passing through the vacuum chamber wall permit electrical measurements to be made on the deposited films during and subsequent to deposition.

In operating the apparatus gold conducting lands 12 (see FIG. 2) are first deposited on the substrate by known evaporation techniques using appropriate masks. The vacuum system is then brought to atmospheric pressure and electrical connections are made to the lands using conducting colloidal silver paste. The polymer and metal sources 2 and 3 are then charged with material and the system is re-evacuated down to pressures of about 10.sup.-5 torr using a pumping system that includes a liquid-nitrogen-cooled trap. The substrate temperature is maintained at 30.degree. C during deposition.

The sources 2 and 3 are brought to operating temperature. When using polyethylene as the polymer an operating temperature of 380.degree. C is typical. The conditions are adjusted until a deposition rate of about 17 A to about 30 A min.sup.-1 is achieved. Similarly the metal deposition rate is adjusted to the desired level, e.g., with a range of 40-85 A min.sup.-1 in the case of silver and within a range of 30-70 A min.sup.-1 in the case of copper. When the appropriate level is achieved it is maintained at a constant value.

The protective shutter is then removed from the substrate and co-deposition allowed to proceed through suitable defining masks to establish the pattern of films 13 shown in FIG. 2.

At the end of co-deposition a second set of gold lands 14 is evaporated over the metal polymer film in an appropriate pattern.

After a suitable delay period annealing of the composite metal polymer film is achieved by raising the substrate temperature to values up to 200.degree. C. Annealing is carried out either at reduced pressure of 10.sup.-1 torr or atmospheric pressure.

Typical conditions for polyethylene-silver and polyethylene-copper composite films are as follows:

Experimental Results

I. Polyethylene-Silver

Immediately following deposition electrical resistivity was found to be unstable with time, and an annealing process was required before reproducibility was obtained. The annealing was carried out after a delay period following deposition of the film. In this way ageing effects were eliminated. The change in specific resistivity with annealing time is illustrated in FIG. 3 after various delay periods, viz, A, 15 hrs., B, 24 hrs., C, 2 days, D, above 2 days.

FIGS. 4 and 5 show electron micrographs of the structure of films respectively before and after annealing at 90.degree. C. It can be observed that island growth has occurred during annealing, islands having diameter within the range 150-1000 A being seen. Annealing was carried out both at a reduced pressure (10.sup.-2 torr) and in the atmosphere, in each case leading to similar island growth behavior.

At the completion of annealing the variation of resistivity with film temperature is shown in FIG. 6.

The resulting films usually possess a resistivity greater than about 10.sup.8 ohm per square and have negative temperature coefficient of resistance. In contrast where the metal evaporation rate is raised to such high level that the island structures of metal are in contact the products have resistivity less than 10.sup.4 ohm per square and have positive temperature coefficient of resistance.

Once annealed, films exhibited no detectable changes in resistivity due to ageing effects.

All electrical measurements were made using gold contacts. Current-voltage relationships were in all cases found to be ohmic.

II. Polyethylene-Copper

Composite films of polyethylene and copper were formed in a similar manner to those of polyethylene and silver. The main difference in production lies in the annealing process which may be carried out at temperatures of up to 200.degree. C, although it is usually done at 140.degree. C, and is always carried out in atmosphere. The effect of this treatment was to convert islands of copper to Cu.sub.2 O. Island size was smaller than those obtained with silver, island diameters were found to be about 110 A, with little variation in size throughout the film. FIGS. 7 and 8 show electron micrographs of films respectively before and after annealing at 150.degree. C. In the case of copper it was possible to perform electrical measurements both parallel and perpendicular to the surface of the film, in each case the value of specific resistivity of the composite material was the same.

After annealing the variation of resistivity with film temperature was found to exhibit the characteristics shown in FIG. 9. FIG. 9 refers to surface resistivity measurements of two films, annealed at 375.degree. K (graph A) and at 493.degree. K (graph B) respectively. It is observed that a plot of log resistivity versus 10/T1/4 results in a straight line, indicative of a hopping mechanism of electrical conduction.

Gold electrodes were used for all measurements. As with silver composite films, ohmic characteristics were always exhibited. Dielectric breakdown was caused when the applied field was in excess of about 10.sup.6 Vm .sup.- 1.

No variation of resistivity with film age has been detected.

Films of a given resistance value have been produced simply by altering the aspect ratio in the case of high resistance values (surface conduction) or electrode size for low resistance values (bulk conduction).