here to go to our main page on resistors
here to go to our page on resistor math
here to go to our page on thick-film resistors
Thin-film resistors, though more
expensive than thick-film, are the darling of the microwave resistor
industry, because they have better electrical properties.
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
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.
here to learn more about tantalum nitride resistors and the
Temperature coefficient of resistance
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.
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.