The core performance of thin film precision resistors largely depends on the choice of resistor material. Its main resistance materials include the following, each with its unique advantages and applicable scenarios:
Nickel chromium alloy:
The most common choice. Usually refers to nickel chromium binary alloys (NiCr, such as Ni80Cr20) or improved alloys with the addition of other elements (such as aluminum, silicon, copper) on top of them (such as nickel chromium aluminum silicon alloy NiCrAlSi).
Advantages: high resistivity, very low temperature coefficient of resistance (TCR) (as low as ± 5 ppm/° C or even lower), good long-term stability, and low noise.
Application: It is the preferred material for the vast majority of general-purpose and high-precision thin film resistors, covering a wide range of resistance values.
Tantalum nitride:
Important alternative materials. Usually refers to tantalum nitride (TaN) or tantalum aluminum nitride (TaAlN), etc.
Advantages: It has excellent long-term stability and reliability, superior oxidation resistance and environmental resistance compared to nickel chromium, high electrical resistivity, and is suitable for making medium to high resistance resistors. Some formulas can also achieve very low TCR.
Application: Commonly used in applications that require high long-term stability and reliability, such as aerospace, medical equipment, and high reliability instruments and meters. It is also a commonly used material for precision thin film resistor networks and chip resistors.
Silicon chromium:
Used for high resistance requirements. Mainly amorphous chromium silicon compounds (CrSi).
Advantages: The resistivity is very high, suitable for making high resistance precision thin film resistors of megaohm level (M Ω).
Disadvantages: Compared to nickel chromium and tantalum nitrides, their TCR is usually slightly higher and the noise may also be slightly higher.
Application: Mainly used in precision resistor applications that require high resistance values.
In addition to the core resistor material, the structure of thin film precision resistors also includes:
Substrate: High purity, high insulation, and high thermal conductivity aluminum oxide ceramic (Al ₂ O3) is usually used. It provides support and heat dissipation path for the resistive film. A few special applications may use silicon or glass substrates.
Thin film thickness: This is a key characteristic of "thin films", typically ranging from tens of nanometers to hundreds of nanometers (much lower than the few micrometers to tens of micrometers of thick film resistors). The extremely thin thickness is the process foundation for achieving high precision, low TCR, and good stability.
Protective layer: The resistive film needs to be covered with a protective layer (such as glass glaze/glass glaze or special epoxy resin) to prevent oxidation, moisture absorption, and mechanical damage, which is crucial for ensuring long-term stability.
Terminal electrode: typically composed of multiple layers of metal (such as inner nickel chromium layer, middle nickel layer, outer tin or tin lead/lead-free solder), ensuring good ohmic contact and solderability with the resistive film.
The core resistance materials of thin film precision resistors are mainly nickel chromium alloys and their improved versions (widely used and with good comprehensive performance) and tantalum nitrides (preferred for high reliability). Silicon chromium is mainly used in applications that require high resistance. These thin films are precisely deposited on alumina ceramic substrates, and their extremely thin thickness (nanometer level) and protective layer together constitute the key to achieving high precision, low temperature coefficient, and excellent stability. The selection of materials ultimately depends on specific requirements for factors such as resistance range, temperature coefficient, long-term stability, noise, power density, and cost.