Fast and robust hydrogen sensors based on discontinuous palladium films on polyimide, fabricated on a wafer scale
Fast hydrogen sensors based on discontinuous palladium (Pd) ﬁlms on supporting polyimide layers, fabricated by a cost-efﬁcient and full-wafer compatible process, are presented. The ﬁlms, deposited by electron-beam evaporation with a nominal thickness of 1.5 nm, consist of isolated Pd islands that are separated by nanoscopic gaps. On hydrogenation, the volume expansion of Pd brings initially separated islands into contact which leads to the creation of new electrical pathways through the ﬁlm. The supporting polyimide layer provides both sufﬁcient elasticity for the Pd nanoclusters to expand on hydrogenation and a sufﬁciently high surface energy for good adhesion of both ﬁlm and contacting electrodes. The novel order of the fabrication processes involves a dicing step prior to the Pd deposition and stencil lithography for the patterning of microelectrodes. This allows us to preserve the as-deposited ﬁlm properties. The devices work at room temperature, show response times of a few seconds and have a low power consumption of some tens of nW.