Paper MN+IJ+TR-ThA9
Improvement in Mechanical Contact Reliability with ALD TiO₂ Coating

Thursday, November 12, 2009, 4:40 pm, Room B3

Introduction: In order to overcome energy limits of CMOS, micro-electro-mechanical relays are now being investigated. High endurance is required for relay-based ICs to be viable, and has been a challenge due to stiction and wear. In this work, we demonstrate that a mechanical contact can be made to be very reliable if the surfaces of the conductive electrodes are coated with an ultra-thin layer of titanium dioxide (TiO₂) by atomic layer deposition (ALD).

Device structure: A 3-terminal (3-T) relay design was used: an electrically conductive mechanical beam (source) is actuated electrostatically by applying a voltage to an underlying electrode (gate) separated from the beam by an air gap. If the applied bias is above a threshold voltage (V_TH), the tip of the beam is deflected to bring it into contact with a fixed electrode (drain).

Device fabrication: First, tungsten gate and source electrodes were formed on top of a thermally oxidized Si wafer using sputter deposition. Then, a sacrificial low-temperature oxide (LTO) layer was deposited and patterned. The top W electrode was then sputter deposited and etched. A heavily doped polycrystalline silicon-germanium (poly-SiGe) structural layer was then deposited and patterned. The top W electrode is attached to the bottom of the poly-SiGe beam. The beam was then released in HF vapor. Immediately afterwards, the relay was coated with ALD TiO₂ at 275^oC using titanium tetrachloride (TiCl₄) as the precursor material. One ALD cycle consists of one pulse of TiCl₄ followed by Ti oxidation, and deposits ~0.25A of TiO₂. TiO₂ – W is a moderate potential barrier for electron in the ON state.

Results: W contacts were coated with either 3, 6 or 12 cycles of ALD TiO₂. 3 cycles coated contacts have poor stability and degrade with time. This is attributed to tungsten native oxide growth. Devices were characterized by applying an actuation bias V_GS=12V and source-drain bias V_DS=50mV. The estimated force in the contact region is 9μN. Measurements are done after 100 ON/OFF switching cycles, to stabilize the contact resistance. Linear I_DS-V_DS characteristics have been measured for both 6 and 12 cycles of ALD TiO₂. Reported contact resistances are 85.2kΩ and 1.47MΩ, for a contact area of 15μm². No stiction or contact degradation is observed. If properly biased, 6 and 12 ALD TiO₂ cycles have an excellent yield and a good reliability (max. number of switching cycles tested thus far = 500).

Conclusion: We have found that coating of tungsten with ALD TiO₂ is an efficient way to reduce contacts ageing, stiction, and W oxidation. A contact resistance of 85.2kΩ has been measured and suggests the use of W-W contacts for relay-based ICs.