Low temperature cure of BCB and the influence on the mechanical stress

Abstract

Thin film polymers play an essential role in system integration. The curing temperature and the mechanical stress of the polymers are crucial for 3D-Integration. Low temperature cure is beneficial because the risks of damaging parts of integrated components like MEMS/MOEMS and memory chips or interconnections based on low temperature solders are reduced. A high mechanical stress in the polymer layer on top of a Si wafer can result in a large wafer/chip bow. This is a very important aspect if the wafers are thinned to less than 150 μm for the application of Si interposer or other 3D-integration techniques. In contrast to most of the PIs and PBOs BCB can be cured below 250°C. In this paper low temperature curing profiles similar to Epoxy cure have been performed. The degree of cure was measured by FT-IR-spectroscopy to guarantee the stability of the films for further processing. An 80% degree of polymerization was set as a minimum value. The curing mechanism was controlled by DSC. Based on this result a new BCB curing model can be postulated which helps to predict the degree of cure for different curing profiles. In contrast to the literature published the last 15 years this model is based on a 2-step polymerization mechanism which is much more accurate for the polymerization above 80%. Curing BCB at lower temperature helps to reduce the mechanical stress in the film. The stress was measured by wafer bow for different curing profiles. Tensile tests using 13 μm free-standing BCB-films which have been cured using different curing profiles were performed to measure the elongation to break, tensile strength and the Young's Modulus. Additionally, there was a viscoelastic stress relaxation process in the BCB layer explored, which until today has never been described in literature. This relaxation is an important aspect of the simulation of BCB-based buildups. This paper is therefore a comprehensive study of the mechanical properties of BCB in relation to the curing process with a focus on a temperature range below 200°C.