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 |
10–200 |
-100 ± 50 |
± 10% |
± 0.5–10% |
| NiCr |
50–225 |
0 ± 50 |
± 10% |
± 0.5–10% |
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 ohms
L = W
L/W = 1.0
R = 50 ohms
L = 2W
L/W = 2.0
R = 100 ohms
|
|
