Reactive ion etching for bulk structuring of polyimide

This paper presents a study on bulk structuring of polyimide by reactive ion etching (RIE). The need for deep structuring of polyimide is exemplified by the design of a MEMS thermal flow sensor. The results, however, are broadly applicable and in no way limited to a specific kind of sensor or actuator. Etch rate, etch surface roughness, and etch mask undercutting are examined and their dependence on etch power (100–300 W), etch pressure (13.3–80 Pa), etch gas flow (20–150 sccm), and etch gas mixture (O2 and SF6) is analysed. While increasing pressure and etch power in the range investigated yield an approximately linear increase in etch rate, a distinct maximum of etch rate at specific values of etch gas flow and etch gas composition is detected. The highest overall etch rate obtained is approximately 1.5 μm/min at 120 sccm total flow, 300 W etch power, 80 Pa (600 mTorr) etch pressure and an etch gas composition of 97.5 sccm O2 to 22.5 sccm SF6. The corresponding etch surface roughness Ra is approximately 1 μm. Mask undercutting, given as etch rate parallel to the surface divided by etch rate normal to it, is approximately 0.4. The results obtained prove that conventional RIE equipment can successfully be employed to conduct bulk structuring of polyimide.