Meaning
Accelerated aging procedures designed to induce and measure the formation of voids in metallic interconnects under sustained high temperatures provide a benchmark for wafer level reliability. A stress migration bake involves placing semiconductor samples in an oven for hundreds or thousands of hours without applying an electrical current. The test isolates the effects of mechanical stress from those of electromigration.
Thermal Stressor
High heat acts as the catalyst for the movement of atoms within the metal structures. During a stress migration bake, the temperature is typically held between one hundred and fifty and two hundred degrees Celsius. This range is high enough to promote atomic diffusion but low enough to avoid melting the materials.
Void Morphology
Microscopic gaps form in the metal lines as atoms migrate toward regions of lower stress. After the stress migration bake is complete, the samples are inspected using scanning electron microscopy to locate these defects. The size and frequency of the voids indicate how well the metallization process resists mechanical degradation.
Validation Metric
Results from the test are compared against industry standards to determine if the batch meets the requirements for commercial use. A successful stress migration bake shows no significant change in the electrical resistance of the test structures. If the resistance increases beyond a set threshold, the fabrication process must be adjusted to reduce internal stress.
