Amorphous Metal NEMS

A new approach

In this work, thin-film amorphous WNx is investigated for the formation of micro/nano-electro-mechanical (M/NEM) switches with sub-1 V pull-in voltage. M/NEM switches provide an alternative switching device to the complementary metal oxide semiconductor (CMOS) transistors. Typically these switches have both zero leakage current due to the existence of an air gap in off-state, and high on-state current (Ion) due to direct metallic contact. M/NEM switches generally are fabricated using polycrystalline-silicon (poly-Si) as the structural layer integrated with one or more layers of metallic material. A mechanical switch comprises electrodes with a small air gap separating the conductive electrodes in the off state.

For a normally-off switch design, electrostatic force causes the conductive electrodes to come into contact when the electric field induced by an applied control (gate) voltage is higher than a critical value. The critical value of applied voltage is called the 'pull-in voltage' (Vpi) and is analogous to the threshold voltage (Vt) of a CMOS transistor. Vpi is dependent on the switch geometry, and should be minimized to minimize the energy required to operate the switch. Reducing the air-gap decreases the pull-in voltage, however, achieving an ultra-small air gap and compliant structure is challenging from a fabrication perspective.

“We have developed a process flow for the fabrication of amorphous metal based nano-electro-mechanical switches with ultra-low power consumption and sub-1 V switching voltage.”

To achieve this scaling, a low-stress, homogeneous structural material is required. A monocrystalline layer could be used as the structural material, such as expensive silicon-on-insulator substrates. The other choice of a homogeneous material is an amorphous layer. An amorphous metallic material that is electrically conductive and which has high hardness and high Young's modulus would be attractive as both a structural material and an electrode material. Due to high adatom mobility of metal to form crystalline grains during thin-film deposition, an amorphous pure metal layer is difficult to achieve. Amorphous tungsten nitride (WNx) has been reported in the past to be electrically conductive.

So where to from here then?

M/NEM switches offer an exciting alternative to silicon-based switches for specific applications such as ultra-low power electronics and harsh environment electronics. The sharp switching curve obtained for mechanical switching can be used to reduce the driving voltages of circuits thus resulting in substantial energy savings. In the future, further research can be done to effectively integrate amorphous metal based M/NEM switches with state-of-the-art silicon electronics to obtain robust, low-power electronics.

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