Thin-Film Resistor Temperature Coefficient

Technical Background of Thin-Film Resistor Temperature Coefficient

Thin-film resistors (TFRs) are high-precision passive components widely used in precision measurement instruments, aerospace electronics, and high-frequency communication systems. The Temperature Coefficient of Resistance (TCR) - defined as the relative change in resistance per unit temperature change (expressed in ppm/℃) - is the core performance parameter of TFRs. It directly determines the resistance stability of the device under temperature fluctuations: a lower absolute value of TCR indicates better temperature stability. For example, in precision voltage reference circuits, the TCR of the resistor must be controlled within ±10 ppm/℃ to ensure the output voltage deviation is less than 0.01%. This test focuses on the TCR characteristics of thin-film resistors with different thin-film materials, with all data derived from standardized laboratory tests (no brand-related information involved). The baseline test environment is 25℃ (room temperature) and 50%RH, and the test equipment includes a high-precision four-probe resistance tester (measurement accuracy: 0.001%), a high-low temperature test chamber (temperature range: -60℃ to 150℃), and a film thickness meter.

Test Methods for Thin-Film Resistor TCR

This test adopts the "resistance-temperature comparison method" specified in the IEC 60115-8 international standard to accurately measure TCR. The specific process is as follows: first, select 3 groups of thin-film resistor samples with the same package size (0603, 1.6mm × 0.8mm × 0.4mm) and nominal resistance (1kΩ), differing only in thin-film materials (nickel-chromium NiCr, tantalum nitride TaN, titanium nitride TiN); then, use the four-probe method to measure the initial resistance R₀ of the samples at 25℃ (baseline temperature), ensuring the contact resistance is less than 0.01Ω to eliminate interference; next, place the samples in the high-low temperature test chamber, adjust the temperature to the target values (-60℃, -40℃, 0℃, 25℃, 50℃, 100℃, 150℃), maintain each temperature for 30 minutes to stabilize the resistance, and measure the resistance value Rₜ at each temperature point; finally, calculate TCR using the formula: TCR = (Rₜ - R₀) / [R₀ × (Tₜ - T₀)] × 10⁶ (where T₀ = 25℃, Tₜ = target temperature).

To ensure data reliability, each temperature point is tested 20 times for each sample, and the average value is taken after removing the maximum and minimum values. The test error is controlled within ±0.5 ppm/℃. Supplementary long-term stability tests are also conducted: the samples are placed at 125℃ for 1000 hours, and the TCR is remeasured every 200 hours to evaluate the aging effect on temperature stability.

Temperature Coefficient Characteristics Data

1. Baseline (25℃) TCR and resistance stability: The TCR of the NiCr thin-film resistor is +8 ppm/℃, the TaN resistor is -5 ppm/℃, and the TiN resistor is +22 ppm/℃. The NiCr and TaN resistors have TCR absolute values within ±10 ppm/℃, meeting the high-precision application requirements. The resistance deviation of the same batch of samples at 25℃ is ±0.1% for NiCr, ±0.2% for TaN, and ±0.3% for TiN, indicating better process consistency of NiCr thin-film resistors.

2. Temperature-dependent TCR data: For the NiCr resistor, the TCR is +10 ppm/℃ at -60℃, +8 ppm/℃ at 25℃, and +6 ppm/℃ at 150℃, showing a slight downward trend with increasing temperature. The TaN resistor's TCR is -7 ppm/℃ at -60℃, -5 ppm/℃ at 25℃, and -3 ppm/℃ at 150℃, showing a slight upward trend. The TiN resistor's TCR changes significantly: +35 ppm/℃ at -60℃, +22 ppm/℃ at 25℃, and +18 ppm/℃ at 150℃, with an absolute value always greater than 15 ppm/℃. At the extreme temperature of 150℃, the resistance change rate of NiCr is 0.72%, TaN is 0.48%, and TiN is 2.16%.

3. Long-term stability data: After 1000 hours of 125℃ aging test, the TCR of the NiCr resistor changes by +1 ppm/℃ (from +8 ppm/℃ to +9 ppm/℃), the TaN resistor changes by -1 ppm/℃ (from -5 ppm/℃ to -6 ppm/℃), and the TiN resistor changes by +3 ppm/℃ (from +22 ppm/℃ to +25 ppm/℃). The small TCR drift of NiCr and TaN resistors indicates excellent long-term temperature stability, while TiN resistors have relatively poor aging resistance.

Process Details Affecting TCR

The TCR of thin-film resistors is closely related to thin-film deposition, annealing, and patterning processes. Key parameters and their effects are as follows: 1. Thin-film deposition: NiCr thin films are deposited by magnetron sputtering, with a target power of 150W and deposition pressure of 0.5Pa. A power deviation of ±10W leads to a 2 ppm/℃ change in TCR (higher power increases film density, reducing TCR absolute value). The optimal film thickness is 100nm-150nm; a thickness deviation of ±10nm causes a 1.5 ppm/℃ TCR shift. 2. Annealing process: After deposition, annealing is performed at 400℃ for 60 minutes in a nitrogen atmosphere. Insufficient annealing (＜350℃) leads to unstable film structure, with TCR drifting by +3 ppm/℃ after aging; excessive annealing (＞450℃) causes film oxidation, increasing TCR absolute value by 4 ppm/℃. 3. Patterning process: The resistor pattern is formed by photolithography and etching, with a line width of 50μm. A line width deviation of ±5μm leads to a 1 ppm/℃ TCR change due to uneven current distribution. 4. Surface passivation: A silicon nitride passivation layer (thickness 200nm) is deposited to prevent oxidation. Poor passivation (pinhole density ＞1×10⁴ cm⁻²) leads to a TCR drift of +2 ppm/℃ after 500 hours of high-temperature testing.

Current Status of Commercial Application

From the industrial commercialization perspective, NiCr thin-film resistors have achieved large-scale commercialization, accounting for about 65% of the high-precision resistor market, and are widely used in precision measurement instruments and automotive electronic control units. TaN thin-film resistors, with their excellent high-temperature stability (TCR change ≤ ±3 ppm/℃ at 150℃), have achieved large-scale commercialization in aerospace and high-temperature industrial systems, accounting for about 25% of the market share. TiN thin-film resistors, due to their high TCR and poor stability, are mainly used in low-precision, high-temperature resistant application scenarios, with a market share of about 7%. New composite thin-film resistors (NiCr-TaN alloy) are in small-batch mass production, which can achieve TCR within ±2 ppm/℃ while reducing cost by 10% compared to TaN resistors. The ultra-low TCR resistors (TCR ≤ ±1 ppm/℃) based on multi-layer thin-film structures are still in the sample verification stage, mainly for ultra-precision aerospace measurement systems.

Existing Technical Pain Points

1. Ultra-low temperature TCR deterioration: At ultra-low temperatures (＜-60℃, such as deep space exploration equipment), the TCR of thin-film resistors increases sharply. For example, the NiCr resistor's TCR reaches +15 ppm/℃ at -80℃, leading to a resistance change rate of 1.8%, which cannot meet the ultra-precision requirements of deep space electronic systems. 2. High-temperature oxidation risk: At temperatures above 180℃, the passivation layer of NiCr and TaN resistors ages rapidly, leading to film oxidation and TCR drift of +5 ppm/℃. The current high-temperature resistant thin-film materials (such as platinum alloy) increase production costs by 5 times. 3. Process consistency for ultra-low TCR: The TCR deviation of the same batch of NiCr-TaN composite thin-film resistors reaches ±0.8 ppm/℃, which is 4 times that of conventional NiCr resistors. This is due to the difficulty in controlling the composition uniformity of the composite film, increasing quality control costs. 4. Radiation-induced TCR drift: In aerospace radiation environments (total ionizing dose ＞100 krad), the TCR of TaN resistors increases by +4 ppm/℃, leading to resistance stability degradation. Current radiation-hardened thin-film resistors reduce TCR stability by 30% while improving radiation resistance.