Adhesion poses a significant challenge in thin-film technology, where poor adhesion is often the standard, and addressing this issue requires special attention. Certain materials exhibit poor adhesion due to their chemical nature; for instance, noble metals, being non-reactive, do not readily form bonds across interfaces. Additionally, adhesion is influenced by surface cleanliness, and residues or contaminants from previous steps can lead to suboptimal adhesion.
Deposition process variables also play a role in adhesion. In sputtering, energetic ions and atoms can displace contamination and weakly bonded film atoms. However, in evaporation, the arriving atoms may lack sufficient energy to displace weakly bonded atoms.
To mitigate adhesion challenges, adhesion layers are introduced as additional films. Adhesion layers aim to improve adhesion without significantly affecting the device’s structure or operation. The selection of adhesion layer materials is based on their ability to form bonds. Titanium and chromium are commonly used materials for adhesion layers, and other oxide-forming metals like aluminum also exhibit good adhesion. The thickness of the adhesion layer typically ranges around 10 nm, serving primarily as a surface treatment. Adhesion layers are deposited immediately after each other with the structural film in the same vacuum chamber. This sequential deposition ensures a clean surface for the adhesion layer, addressing a key factor in poor adhesion.
Typical pairs of adhesion layer/noble metal include Ti/Pt and Cr/Au, and vice versa. Adhesion layers are also beneficial for near-noble refractory metals such as tungsten.
The tape pull test is a common method for evaluating adhesion. In this test, adhesive tape (often standard office tape) is applied to the thin film and then pulled off. If the film peels off with the tape, it indicates a failure in adhesion. More advanced tests involve quantifiable pull force measurements to assess adhesion performance accurately.