Applications - Under-Bump Metallization (UBM)
As the foundation of wafer bumping processing, the Under-Bump Metallization (UBM) stack must provide a strong, stable low resistance electrical connection to the IC’s pads and the surrounding passivation layer. Low stress films are critical. Materials selection and deposition process governs the initial tensile or compressive stresses within the layers. This generally requires multiple layers of different metals, such as an adhesion layer, a diffusion barrier layer, as well as solders layer(s).

A typical UBM sequence started with cleaning after IC pad opening. An adhesion layer is then sputtered onto the IC wafer followed by second metal sputtering. A litho step is used to delineate the pad contact areas for electroplating of barrier layer and solder metals. Subsequently, the resist strip and metal etch are used to form pad isolation.