Integration of benzocyclobutene polymers and silicon micromachined structures using anisotropic wet etching

Integration of thick, low-k dielectric benzocyclobutene (BCB) film with deep etched structures in silicon allows the fabrication of microelectromechanical systems (MEMS) devices with low parasitic loss. A fabrication process is developed for integration of 1-μm-thick BCB low-k dielectric film and 200-μm-deep anisotropically etched grooves in silicon with potassium hydroxide (KOH). In order to protect the low-k film during the highly corrosive, long, high-temperature KOH etching process, gold (Au) is used as an etch mask. Chromium (Cr) is used to improve the adhesion of Au to the underlying BCB layer. Metal-BCB adhesion is the key parameter in this masking design. Partial cure of BCB at 210 °C for 40 min with appropriate surface treatment (adhesion promoter) prior to metallization and full cure at 250 °C for 1 h after metallization, together with Cr/Au sputtering at 200 °C, improves the adhesion dramatically. The adhesion strength of metal films to BCB was experimentally verified in a qualitative manner. V grooves were etched into silicon in 20 wt%, 80 °C KOH solution for 8 h in the presence of 1 μm BCB film. BCB was protected and kept intact with an Au mask layer during the etch process. In order to understand the mechanism of the adhesion improvement, the interface between BCB and the Cr/Au layer was studied using secondary ion mass spectroscopy and Auger electron spectroscopy. Adhesion improvement which is mainly due to cure management and use of adhesion promoter is associated with (1) the diffusion of silicon and carbon from the polymer structure into the Cr layer, and (2) the chemical interaction of BCB and Cr at the interface mainly in the form of the oxidation of Cr. The integration of BCB and the KOH etching process which was obtained by improving the adhesion of metal etch mask to the BCB film, together with the study of the interfaces, allow us to use thick low dielectric constant BCB film for fabrication of MEMS devices with very low parasitic loss. © 2004 American Vacuum Society. [DOI: 10.1116/1.1787519]