Abstract
As the trend for electronic devices toward smaller size and higher performance continues to grow, line width and line spacing become smaller. For three-dimensional integrated circuit packages with through-silicon vias, the diameter for the through-silicon via is about 5 to 10 µm and the line width is about 10 to 20 µm or even less. For the reduced line width, the current density will rise to 106 A/cm2 in the metal trace. When the current density in the aluminum trace is higher than 105 A/cm2, voids are observed at the cathode and hillocks are observed at the anode. The voids lead to open circuit, whereas the hillocks lead to short circuit. For three-dimensional integrated circuit package structure, stress is accumulated during chip-to-chip stacking. Therefore, a study of the relationship between electromigration and applied mechanical stress is very important. The residual stress of the aluminum thin film was measured and controlled within ± 50 MPa at an experiment temperature of 200 °C through annealing. A nanoindenter was used to measure the Young’s modulus of the sputtered aluminum thin film. After the process temperature of the aluminum thin film was obtained, the residual stress was simulated using the finite element method. The simulation result matches the measurement result obtained by Stoney’s equation. When the body force of the wafer was considered in the simulation, the simulation result shows that the scan length is an important parameter for stress measurement. A 30° scan direction or four-pin design for symmetric support of the warpage measurement system is recommended. Silicon substrate is widely used for the external stress applied on the samples. The applied stress on the aluminum thin film is usually less than 60 MPa due to the strength of the silicon substrate. Glass and silicon substrates with different orientations were tested in the present research. Using a two-step cutting method to reduce front-side chipping and die saw before the grinding method to reduce backside chipping, 275 MPa of average external stress could be applied on the aluminum thin film. The unpassivated aluminum Blech strip with stress control through annealing was designed and fabricated. External mechanical stress was applied using a four-point bend equipment. The present study found that the critical product was reduced from 1,294 to 1,281 A/cm when 120 MPa mechanical stress was applied, and increased to 1,315 A/cm when -120 MPa mechanical stress was applied. Tensile stress increases electromigration, and compressive stress delays electromigration.