Selective All-Wet Metallization of Silicon for Hysteretic Power Microswitches

Abstract

Metallization of semiconductors has been very important for improving the performance of today's integrated circuits (IC) and microsystems. Electrochemical metallization methods, among other metallization schemes, have received increased attention due to certain unique advantages they offer – such as low film stress, ease of fabrication and low-cost. In this category, electroless deposition (ED) methods have also seen an increase in the number of applications, including in biomedical devices, micro-electro-mechanical-systems (MEMS), fuel cells, ultralarge scale integration (ULSI) and molecular nanodevices. Electroless deposition offers high quality coatings that are selective and conformal without the need for external electrical connections.

In this research, the selective and conformal nature of ED is tapped to fabricate high-aspect ratio (exceeding 200:1), extremely-compliant (spring constant of 0.02 N/m), free-standing mechanical structures metalized with a single-mask, self-aligning process flow. Specifically, the unique features of ED allows fabrication of very long (up to 1 mm in length), slender (5 µm x 5 µm in cross-section) metalized micro-cantilever beams with zero-curvature and no fundamental limit except for molecular diffusion. These micro-machined structures metalized with such a simple process sequence can help achieve reduced cost and high yield in the realization of a wide variety of electro-mechanical transducers, including capacitive sensors, accelerometers, RF elements, and microswitches/relays. Particularly, the proposed fabrication flow has been applied for a single-mask, self-aligning fabrication of electrostatic microswitches/relays, whose fabrication flow has not changed much since their first examples have emerged nearly two decades ago, as the first prototype of the integration of ED with the microswitch/relay fabrication. The fabricated microswitches have been actuated both quasi-statically and in the transient mode, and the electroless Au-to-Au contact from the source to the drain has been characterized. It has been found that the current-voltage (I_ds-V_ds) curves show ohmic characteristics with modest contact resistance of 5 kΩ. Moreover, the fabricated devices display over-damped behavior with a switching time of around 80 µs and squeeze-film damping of 0.0023 N-s/m². Lifetime/reliability experiments have shown promising results (exceeding 2000 stable hot cycling with a current load of 1 mA); however more improvement is necessary for an eventual marketable product. What is more, intentional stress-imbalance is induced on the fabricated micro-structures to determine thin-film stresses for the electroless Cu (tensile stress of 45 MPa for 100 nm thick film) and Au/Cu (tensile stress of 56 MPa for 100 nm thick film) for the material characterization purposes.