Assertion (A) : The value of thin film resistor can be changed even after fabrication. Reason (R) : This is done by means of trimming. Select your answer using the codes given below.
Both (A) and (R) are true and (R) is correct explanation of (A).
Examine the assertion. A thin-film resistor is a strip of resistive material — nichrome, tantalum nitride or cermet — deposited a few hundred nanometres thick on a substrate. Its value is
\(R=R_s\dfrac{L}{W}\)
where Rs is the sheet resistance in ohms per square and L/W is the number of squares. Because the film is exposed on the surface, its geometry can still be altered after the wafer is made. The assertion is true.
Examine the reason. The process for doing this is called trimming. Material is removed from the film — classically by abrasive or laser cutting — which narrows the effective width or lengthens the current path, so the resistance can only be adjusted upward, and only in the increasing direction. The reason is true.
Does the reason explain the assertion? Yes, completely and directly. The assertion states that the value can be changed after fabrication; the reason names the exact mechanism by which that is done. There is nothing else to the claim — trimming is the answer to "how". So (R) is the correct explanation of (A), which is code 1.
How trimming is actually carried out.
| Method | Action |
|---|---|
| Laser trimming | A focused laser vaporises an L-shaped or plunge cut into the film; resistance is monitored live and the beam stops at the target value. |
| Abrasive trimming | A fine air-driven abrasive jet erodes the film — cheaper, coarser. |
| Anodisation | Electrochemical oxidation of a tantalum film thins the conducting layer. |
Because the value is measured in a feedback loop while the cut proceeds, tolerances of 0.01 % are achievable — far better than the ±10 % typical of an untrimmed deposited film, and far better than any diffused resistor can offer.
The contrast with monolithic resistors. A diffused resistor is buried under an oxide layer, so its value is fixed the moment the diffusion is done and cannot be altered. This ability to trim is a principal reason thin-film networks are chosen for precision analogue work such as data converters and instrumentation amplifiers, where matched, accurate ratios matter.
Hence, both statements are true and (R) is the correct explanation of (A).
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