Design, fabrication, performance and reliability of Pt- and RuO₂-coated microrelays tested in ultra-high purity gas environments

Abstract

We have fabricated, tested and analyzed the reliability of Pt- and RuO2-coated ohmic microrelays in ultra-high purity gas environments. RuO2-coated relays could survive 3 × 108 contact cycles without electrical degradation, while Pt-coated devices degraded after 105 cycles. Thermally actuated microrelays were fabricated using a process that employed a polysilicon surface-micromachined substructure. After releasing the devices, just a few blanket metal depositions were required to create the different coatings. This method allowed direct comparisons between different coating materials, and was enabled by a self-aligned shadow mask that provides electrical isolation between different traces. Testing was performed in a clean environment achieved through in situ ultra-high vacuum bakeouts, chamber cooling to <5 × 10−9 Torr and chamber refill with ultra-high purity gases. The RuO2 coatings were formed by two avenues—reactive sputtering and thermal oxidation. No significant difference in contact resistance or reliability was detected for these two deposition methods. For all coatings, post-test analysis by scanning electron microscopy and Auger electron spectroscopy indicated no difference in carbon concentration on real contact versus non-contacting areas, implying that carbon did not play a role in limiting the switches' reliability. The Pt-coated switch reliability limit was attributed to surface wear rather than to the growth of a contaminating film. For the RuO2 switches, trace resistance was reduced by ten times using an Al underlayer, so that the total device resistance was compatible with commercial device requirements. Because RuO2 is expected to be resistant to hydrocarbon contamination, this work shows that the RuO2 coating provides a promising path toward achieving ultra-high reliability ohmic microswitches.