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Thin-film resistors, though more expensive than thick-film, are the darling of the microwave resistor industry, because they have better electrical properties.
Thin-film resistor fabrication
The thin-film resistors start with a "hard" substrate such as silicon, GaAs or alumina. An extremely thin layer (hundreds of Angstroms) of resistor material is deposited over the entire substrate surface, using a sputtering process. Usually a conductor layer is deposited on top of the resistor layer. Using a photo-lithographic process the substrate is patterned and the two layers are etched away independently, so you have a part with both a conductor and resistor pattern.
Thin-film resistor materials are usually tantalum nitride (Tan) or nichrome. A limited range of sheet resistance is possible, from perhaps 5 ohms/square to 250 ohms per square. Thick film technology provides a far greater range.
Click here to learn more about tantalum nitride resistors and the skin effect.
Temperature coefficient of resistance (TCR)
Thin-film chip resistors usually are not offered, because thick-film is cheaper. However, in applications such as where temperature coefficient is important, thin-film chip resistors are sometimes used. Temperature coefficients lower than 25 PPM/C are possible.
According to Paul, typical TCR for nichrome resistor material is 0-50 ppm. TaN resistor material is typically -100 ppm. Thanks!
By the way, nichrome is a portmanteau from the words nickel and chromium.
Temperature stabilization
The sheet resistance of films such as tantalum nitride is known to increase with time as the surface layer oxidizes (the oxide is even less of a conductor than the original material). In order to minimize this effect, resistors are stabilized by baking them it temperatures as high as 400 degrees C for up to one hour. The resistance value can increase as much as 40% during the stabilization, so the original geometry must take this into account and the resistors must start out substantially below their target value. This is very effective in preventing further change in resistance over time. What is happening when resistors are stabilized? The top surface is oxidizing, which increases its resistance.