What is a Thin Film Resistor?

M.J. Casey

A thin film resistor is a resistor, a common electronic component, that is made by vacuum deposition methods to place a resistive material onto a substrate. It is differentiated from thick film resistors not by material or function but by means of manufacture. That rather technical definition is better understood in parts.

Thin film resistors may include compounds of bismuth.
Thin film resistors may include compounds of bismuth.

A resistor refers to an element in the architecture of electronics that hinders the flow of electricity. An increase in the resistance value will require an increase in the applied voltage to maintain a constant electrical current. Mathematically, it is the R value in Ohm’s Law, which relates current (I) and voltage (V) — shown by the formula V=I/R — or in another version, expresses the relation of power (P) to voltage — as in the formula P=V2/R. By enabling changes to voltage or current, resistance is part of the “language” of electronics that allows mathematical expressions to be evaluated electronically.

A resistor is an electronic component that can lower a circuit’s voltage and its flow of electrical current.
A resistor is an electronic component that can lower a circuit’s voltage and its flow of electrical current.

There are two principal methods of vacuum deposition used in the manufacture of a thin film resistor. The material to be used as the resistor, called the resistive material, is vaporized by the application of electrical heat and then condensed on a surface. Secondly, in “sputtering,” the ions from a gaseous plasma impact and energize the resistive material molecules. These molecules are ejected away from the material and onto a substrate.

The first method can be visualized as spray paint, a direct application of material. The second can be thought of as the splash from a wheel going through a mud puddle, an indirect application of material. In either case, the layer of resistive material is so thin that it is only a few atoms or molecules deep. By conducting the process in a vacuum, an even layer is achieved, and contaminants or unwanted chemical reactions are avoided.

The materials from which the resistors are made include compounds of tantalum, bismuth and ruthenium, as well as chromium, nickel and lead. There are thousands of possible compounds, including newer organic mixtures. A thin film resistor is more expensive than other types but also has tighter tolerances. These resistors are typically used in more demanding applications such as high-frequency communications and computing.

Like their thick film cousins, thin film resistors can be trimmed to increase the precision of their rating. Resistors are trimmed by slightly over-depositing the required material. Then computer controlled lasers etch away at the material until the desired resistance value is achieved. The refinement of laser trimming an already accurate thin film resistor is an indication of the very tight tolerances required by today's small, fast, powerful and lower-heat-producing electronic devices.

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