GB1597403A - Temperature-measuring resistors for resistance thermometers and methods of manufacturing same - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TEMPERATURE-MEASUR1NG RESISTORS FOR RESISTANCE THERMOMETERS AND METHODS OF MANUFACTURING SAME (71) We, W. C. HERAEUS GMBH, a German Body Corporate, of 12-14 Heraeusstrasse, D-6450 HANAU, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to methods of producing temperature-measuring resistors for resistance thermometers of the kind having a thin film of platinum in a specific geometrical pattern, e.g. in a meander shape, on an electrically insulating substrate, of alumina for example. Hereinafter such a resistor will be referred to as "of the kind described".

It is an object of the invention to provide a simple method of manufacturing thin-film platinum resistance thermometers which also guarantees that the electrical values (specific resistance and temperature co-efficient) are those required in order to be able to keep the resistance to a specific value and so that the resistor track is sufficiently long while being of small thickness and width. If for example a method known from thin-film techniques such as photolithography is used to produce the pattern and if attempts are then made to etch the platinum chemically, it is found that the resistance to etching of the very pure platinum of the resistor is so great that it cannot be dissolved by any known etchants without at the same time the mask which is required with this method being destroyed.

Nor can the object be achieved by electrochemical etching using pulsed currents of specific forms, since the platinum has to be connected as the cathode and the hydrogen which forms lifts the thin film of platinum forming the resistor from the substrate.

The ion-etching method also leads to difficulties because the surface of the resistor platinum, which has to be preserved, has to be covered with a substance which provides protection from ion sputtering.

However, this substance is able in turn to react with the platinum under the influence of fast ions, as a result of which there is an irreversible change in the electrical characteristics of the thin film of platinum.

Producing the pattern by treatment with a laser beam can be ruled out for application to the subject of the invention due to the comparatively high manufacturing costs.

The invention consists in a method of manufacturing a temperature-measuring resistor of the kind described, which method comprises the following steps: a) a thin film of a metal from the iron family or of the metal copper, siver, gold or palladium is deposited on the substrate, b) a mask is formed from a plastics resist on the thin film which has been deposited, c) the mask is dried at 100 to 1600C and the film not covered by the mask is etched away and the mask is then washed off, d) a thin film of platinum is applied to the substrate and on the unetched thin metal film, the substrate being at a temperature of 400 to 700 C, by condensation from the gaseous phase, and e) the product of the last step is treated with aqua regia so that portions of the platinum film lying on and forming an alloy with the unetched metal film, and said metal film are etched away and a platinum film in a pattern constituting the resistor track is left.

The drying temperature in step (c) may be 1200C.

In order that the invention can be more clearly understood, reference will now be made to the accompanying schematic drawings which illustrate one particular embodiment of resistor produced by the method and in which: Fig. 1 is a plan view of a temperaturemeasuring resistor made according to the invention, Fig. 2 is a cross-section on line A-B of Fig. 1, and Fig 3 shows the sequence of the individual method steps by, in each case, a crosssection through the product on line A-A as in Fig. 2.

Referring now to the drawings, a substrate 1, which may for example consist of alumina or a similar suitable ceramic material of high purity such as is described in German Offenlegungsschrift 25 27 739 for example, has applied thereto firstly a thin film 2 of nickel or of another metal of the iron family, to a thickness of between approximately 0.5 and 2 micrometers and preferably between 1 and 1.5 micrometers.

This constitutes method step (a).

On the thin film 2 of the said metal is then formed a mask 3 from a known plastics resist suitable for the purpose. This constitutes method step (tor).

Once the mask has dried, the exposed metal of the thin film 2 is etched away chemically. This constitutes method step (c).

A solution of ferric chloride (feel) has proved suitable for use as an etchant. After this, a thin film 4 of platinum is applied to the substrate so treated, which is heated, by condensation from the gaseous phase.

This constitutes method step (d).

The basic product obtained after step (d) is then treated with aqua regia, thus leaving a pattern 5 of platinum (the resistor track), which gives the end product shown in Fig. 2. This constitutes method step (e).

At the edge the resistor track has electrically conductive connections to outer electrodes 6.

The invention enables the solubility of platinum to be markedly increased by alloying with an added amount of the said metals.

It is of great advantage that in the case of the invention the platinum alloyed with the said metal, such as nickel, is dissolved by the final treatment with aqua regia and only the pure platinum of the resistor remains, unaffected.

Modifications may of course be made to the method described without departing from the scope of the invention as defined by the appended claims. Thus the nickel content of the platinum alloy is not of vital importance. It may for example be at least 20% by weight.

As well as the said metals of the iron family (iron, cobalt and nickel) copper, silver, gold or palladium may be considered.

The resist used for the mask must of course be light-sensitive but otherwise there are no restrictions on its composition.

Other etching solutions other than ferric chloride may also be used in step (c) if desired (in place of one mentioned?. Ion- etching is also possible with the method according to the invention. This method is however more expensive than the chemical etching described in the example.

WHAT WE CLAIM IS: 1. Method of manufacturing a temperature-measuring resistor of the kind desribed, which method comprises the following steps: a) a thin film of a metal from the iron family or of the metal copper, silver, gold or palladium is deposited on the substrate, b) a mask is formed from a plastics resist on the thin film which has been deposited, c) the mask is dried at 100 to 1600 and the thin film not covered by the mask is etched away and the mask is then washed off, d) a thin film of platinum is applied to the substrate and on the unetched thin metal film, the substrate being at a temperature of 400 to 700"C, by condensation from the gaseous phase, and e) the product of the last step is treated with aqua regia so that portions of the platinum film lying on and forming an alloy with the unetched metal film, and said metal film are etched away and a platinum film in a pattern constituting the resistor track is left.

2. A method according to claim 1, wherein the drying temperature of step (c) is 1200C.

3. A temperature-measuring resistor whenever produced by a method according

Claims (1)

1. to claim 1.

   4. A temperature-measuring resistor substantially as hereinbefore described with reference to the accompanying drawings.