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Methods of trimming film resistors
| Details |
Inventors: Oberholzer, David J.;
Assignee: Western Electric Co., Inc. (New York, NY)
Primary Examiner: Albritton; C. L.
Assistant Examiner:
Attorney, Agent or Firm: Schellin; W. O.
A bar-type film resistor (40) is adjusted to a high degree of precision by longitudinally offset plunge cuts (41, 42 and 43) extending from alternately opposite edges (29, 49) into the resistive layer (13), by a trim cut (46) which extends from the most recently made plunge cut (43) longitudinally through the resistor (40), and by a shave cut (47) which is coextensive with the trim cut and preferably overlapping therewith. The trim cut (46) and any shave cut (47) are located at a predetermined distance from a reference edge (29). The most recently completed plunge cut extends from its starting edge (29) into the resistive layer (13) to distances greater than those at which the trim cut and the shave cut are to be located. |
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DETAILED DESCRIPTION General Considerations Referring now to FIG.
1, there is depicted a typical bar resistor 12 of a desired width W and a length L which is formed as a layer 13 of material of a desired resistivity on an insulating substrate 14.
Conductive terminals 16 bound two opposite edges 18 and 19 of the resistor 12.
The spacing between the edges 18 and 19 defines the length L of the resistor 12.
In a typical example where the bar resistor 12 is a thin film resistor, the resistive layer may be tantalum nitride which is deposited to a typical thickness in the range of 400 Angstrom units.
Such a thickness may yield a theoretical resistivity of, for example, 300 ohms per square.
In the example of the thin film resistor 12, the conductive terminals may be formed of gold.
The resistor may, for example, have a length L of 48 mils and a width W of 12 mils, 1 mil being one one-thousandth of an inch or 25.
4.
times.
10.
sup.
-6 meters.
The nominal resistance of the untrimmed resistor 12 amounts, consequently to 4 squares of 12 mils multiplied by the nominal sheet resistance per square, or 1200 ohms.
However, since the sheet resistance of the resistive film may vary, for example, between 200 and 400 ohms per square, the untrimmed resistance of the bar resistor 12 is likely to be anywhere between 800 and 1600 ohms.
The ultimately desired resistance of a resistor for which the untrimmed bar resistor 12 is therefore desirably in the range of 2000 ohms or greater, the increase in resistance being achieved by plunge cuts 21, where the last of such plunge cuts 21 is typically terminated with a right-angle longitudinal fine trim cut 22.
Such longitudinal fine trim cut 22 is started according to a prior art technique as a continuation of the current plunge cut 21 when the resistor has reached a predetermined percentage value of its desired, final value.
While such known trimming technique yields, in most instances, resistor values with reasonable tolerances, it does occur that the final values of resistors show, on occasion, undesirable variations from the desired value and its specified tolerance
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Fault Detection 
