Evaluating Temperature Coefficient Wander in Thin Film Platinum Resistance Elements
Evaluating temperature coefficient wander in thin film platinum RTDs requires tracking substrate strain, glass passivation diffusion, and multi-point calibration.
Residual Lattice Strain
Crystalline Tension
Internal mechanical stresses remaining in a crystalline material after processing or deformation alter the spacing of the atomic planes. This condition, known as residual lattice strain, occurs when manufacturing steps like grinding or rapid cooling leave localized forces trapped within the atomic structure. It affects the electronic properties of semiconductor materials.
Physical Distortion
Displacement of atoms from their equilibrium positions in the lattice alters the bandgap energy and carrier mobility of the material. In sensors, residual lattice strain changes the electrical resistivity through the piezoresistive effect. This alteration causes the sensor output to deviate from its designed nominal value.
Metrological Consequence
Stability of the sensor calibration is compromised when these internal stresses relax over time or during thermal cycles. The residual lattice strain slowly dissipates, causing a continuous, non-linear drift in the zero-point voltage of the transducer. This drift complicates long-term measurements because it cannot be predicted by standard linear models, requiring frequent checks against a reference.
High-precision instruments must therefore be fabricated using techniques that minimize the accumulation of these microscopic stresses.
Relief Process
Thermal annealing at high temperatures is used to reorganize the crystalline structure and release the trapped forces. Cooling the material slowly after annealing prevents new strains from developing. This treatment stabilizes the lattice structure for long-term use.