US 3563862 A
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.HIGH PRECISION ANODIZING 0F THIN FILMS Filed Nov. 4, 1968 United States Patent Int. Cl. B23p 1702,- C23b 5/48 US. Cl. 204-15 4 Claims ABSTRACT OF THE DISCLOSURE To obtain high precision control of the anodization of thin metal films by measurement of the resistance of the said layer in the course of the operation, the measurement is performed on an electrolyte free surface.
Anodization is carried through a movable arm carrying a pad filled with electrolyte the movement of which is synchronized with the A-C oxidising current. The film surface is dried when the pad is removed.
BACKGROUND OF THE INVENTION The present invention relates to improvements in processes for the anodization of thin-film devices and more particularly to the control of this operation. It is well known to produce devices from thin metal films deposited upon insulating substrates. These devices alford such advantages from the view-point of overall dimensions and reliability that their industrial development has been very rapid in recent years. Such devices consist mainly in metal deposits produced by evaporation in vacuo, by cathode sputtering or by any other method known per se. These films, which are very thin, are relatively sensitive to weathering agents, and a protective oxide layer is employed to passivate the surface. This protection is often afforded through anodization. A simple method of controlling the degree of oxidation consists in measuring the resistance of the layer in the course of the operation. When the operation is automatic, the measurement permits to limit its duration. In the case of the manufacture of thin-film resistors, the component consists of a patterned thin metal deposit on a non-conductive support. A coarse adjustment of the value of the resistance is effected by means of the parameters defining the deposit: geometrical shape, thickness, etc. The precise adjustment of the value of the resistance is effected by anodizing the resistive layer.
The present invention is aimed at increasing the precision of the anodization control by increasing the precision of the dynamic resistance measurement during oxidation. It is mainly characterised in that it makes it possible to eliminate the measurement error due to the presence of electrolyte on the surface of the film the resistance of which is being controlled.
PRIOR ART In processes hitherto described, such as in US. Pats. 3,341,444, filed on Sept. 1, 1964 and 3,341,445 filed on Sept. 1, 1964, the resistance measurement is effected by connecting to a measuring bridge the two terminals of the resistor which is immersed in the electrolyte bath serving for the oxidation. In the process described in the aforesaid patent and its addition, an alternating voltage is applied to the oxidising bath and the measurement takes place during the half-cycles of the current which do not produce the oxidation. During measurement, the resistance of the metal film is connected in parallel with the resistance consisting of the electrolyte layer which con- 3,563,862 Patented Feb. 16, 1971 sists of that fraction of the bath situated between the two terminals of the resistor. The measured resistance value is therefore the resultant of these two parallel-connected resistances.
The present invention has for its object to eliminate the influence of the electrolyte during the measurement, and it is essentially characterised in that the layer to be anodized is only in contact with the electrolyte during the time when it acts as an anode, the said layer being electrolyte-free during the measuring time.
DETAILED DESCRIPTION The invention will be readily understood with reference to the following description and to the figure accompanying it, which are given by way of illustration of the invention and have no limiting character, and which concern the manufacture of thin-film resistors, it being understood that this application does not limit the scope of application of the invention.
There is shown at 1 the resistor to be adjusted, which consists of a metal deposit of given pattern on an insulating substrate 1. The terminal contacts of the metal deposit are connected, generally by soldering, to two leads .2, 2'. These two leads are connected through a switch (not shown) to the resistance measuring circuit (not shown), for example a measuring bridge of a type known per se. The switch makes it possible, to interconnect the lead 2 to a first output terminal of an alternatingcurrent supply source. The switch is so designed as to connect the resistor to the said terminal during the halfcycles in which the latter is positive in relation to the second terminal of the same supply. A pad 3 filled with electrolyte is mounted on the core 4 of an electromagnet 5 which is supplied with alternating current synchronized with the supply source serving for the oxidation. Pad 3 is connected to the second supply terminal. The winding of the electromagnet is so chosen that the pad is in contact with the layer to be anodized during the half-cycles in which the oxidation occurs. It is removed from the resistor as shown in the figure during the half-cycle in which no oxidation is performed and during which the measurement of the resistance takes place. It is to be understood that the alternating displacement of the core may be controlled at any frequency provided that the oxidation current and the movement of the core are synchronous.
The mechanical arrangement of the various parts of the circuit is schematically illustrated, it being understood that these elements may be incorporated into a complex assembly operating simultaneously on a large number of resistors 1, it being possible for the measurement to be made multi-successively on the various resistors under treated through an appropriate switching device.
In a particular embodiment of the invention, the pad 3 consists of felt, plastic foam, eider chamois skin or any other porous material which does not leave any fluff and which is filled with 0.1% aqueous orthophosphoric acid solution, the refilling with electrolyte being effected by injection of liquid controlled by an electromagnetic valve every 3 minutes. The alternating-current supply source is a source whose frequency is between 15 and c./s. The frequency is chosen as a function of the range of resistance values to be obtained.
The result of the measurement may be used for automating the operation. The resistance value to be reached is stored in a memory, and the measured value may be compared with the desired value, there being derived from this comparison an error signal which serves to control the anodic oxidising circuit either on an on-off basis or progressively, for example by control of the anodic oxidation voltage.
By applying the invention, on an automatic machine, resistance values to within 0.1% were obtained.
It is to be understood that the same result would be obtained by displacing, the thin-film circuit, such as the resistor ,1 described in the foregoing instead of the electrolyte filled pad. Depending on the resistance range, it may be necessary to eliminate completely the electrolyte film remaining on the surface of the layer in the course of the oxidation. When the electrolyte is not very liquid, withdrawal of the pad may be insufficient. The drying may be etfected, for example, by means of a jet ;of dry hot air blown onto the surface during the withdrawal of the buffer. When the thin film device is displaced, its own movement may be utilised to remove any electrolyte remaining on the surface through mechanical force.
1. A process for controlling the resistance of a thin metallic film during anodization thereof comprising the following steps:
oxidizing the thin metallic film by means of an arm the end of which carries a pad filled with electrolyte,
mechanically interrupting the oxidation circuit by distancing said pad and said thin film surface,
drying the film surface when the pad is no longer contacting the surface,
measuring intermittently the resistance value through fixed contacts on the film when said pad and thin film are distanced.
References Cited UNITED STATES PATENTS 3,148,129 9/1964 Basseches et al. 204-228 3,341,444 9/1967 Chapelle 204228 3,341,445 9/1967 Gerhard 204228 FOREIGN PATENTS 1,060,712 3/1967 Great Britain 204-15 OTHER REFERENCES Western Electric Technical Digest No. 4 October 1966 pp. 1 and 2.
JOHN H. MACK, Primary Examiner -T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 204224