Hybrid Systems-in-Foil (HySiF) – Low Stress CFP Process for Biomedical Application

Flexible Hybrid Systems-in-Foil (HySiF) allow the combination of large-area thin-film electronics with ultrathin, high performance silicon chips. Monitoring and determining the long-term reliability, and robustness, especially for biomedical application, is crucial. Biocompatible materials such as polyimides and noble metals are required, in order to prevent from side effects and to reduce the risk of rejection reactions. Based on the previously published data from our group, the scope of this work is to present the ongoing improvements of the process using a low stress polyamic acid based polyimide. The advantage over other organic materials is its low thermal expansion coefficient (CTE), which is close to that of silicon, resulting in reduced stress in the system, particularly, during the manufacturing process. However, unlike benzocylobutene based polymers, polyimide PI2611 shows poor planarizing behavior, leading to more inhomogeneous substrate layers and thus, affecting process steps, such as, photolithography, silicon chip embedding, metal etching, via opening and, direct laser writing, especially in terms of accuracy. To address those issues, we have investigated the homogeneity of multiple polymer layers using both spectroscopic reflectometry and surface profilometry. Based on a fabricated HySiF demonstrator, the crucial process steps are discussed and the reliability of the system is verified by thorough measurements under biological conditions.