Resistor Fabrication

Understanding thin film resistor fabrication

A basic understanding of thin film resistor fabrication can assist you in your circuit design. Thin film resistors are fabricated by sputtering a high-resistance film of Tantalum- Nitride (TaN) or NiChrome (NiCr) under the conductor layer and selectively etching the resistor elements. The following table shows how typical characteristics of Tantalum-Nitride (TaN) and Nichrome (NiCr) films can be used to obtain desired performance.

Film Type Sheet Resistivity (ohm/square) TCR (ppm/°C) Passivated Tolerances (no trim) Laser Trimmed Tolerances
TaN 10–150 -100 ± 50 ± 10% ± 0.5–10%
NiCr 50–225 0 ± 50 ± 10% ± 0.5–10%

Design Rules

Once you have selected a resistor film type and desired resistivity, the elements are designed under the following rules:

R = p(L/W)
R = total resistance of the elements (ohms)
P = sheet resistivity (ohms/square)
L = resistor length (distance between the conductors)
W = resistor width (width of the resistor element)

This formula demonstrates that the aspect ratio of L/W becomes a simple means of designing for the total resistance of the element. Examples of resistor elements, using a 50 ohm/square film, are demonstrated here:

L = .5W
L/W = .5
R = 25 ohmsL = W
L/W = 1.0
R = 50 ohmsL = 2W
L/W = 2.0
R = 100 ohms