VO₂-Based MEMS Mirrors

Abstract

This paper reports the integration of vanadium dioxide (VO 2 ) thin films in a microelectromechanical systems (MEMS) mirror device, where the actuation is mainly due to the solid-solid phase transition of VO 2 . The fabrication process described in this paper provides the details that will enable the integration of VO 2 thin films at any step during the fabrication of rather complex MEMS devices. The present VO 2 -based MEMS mirror device is operated electro-thermally through integrated resistive heaters, and its behavior is characterized across the phase transition of VO 2 , which occurs at a temperature of ∼68°C and spans about 10°C. The maximum vertical displacement of the mirror platform is 75 µm and it occurs for an input voltage of 1.1 V. This translates to an average power consumption of 6.5 mW per mirror actuator and a total power consumption of 26.1 mW for the entire device. The studies included in this paper are key for future device improvements and further development of MEMS mirror actuation technology, which could include the use of the hysteresis of VO 2 for programming tilting angles in MEMS mirrors.[ 2016-0016]Index Terms-Microelectromechanical systems (MEMS)-mirrors, vanadium dioxide, phase-change materials.

Modeling of MEMS Mirrors Actuated by Phase-Change Mechanism

“…The added stress can move the transition temperature of the VO${}_2$, which has been observed previously in VO 2 thin films [42,43]. Even more relevant to the present case, this effect was also observed in VO 2 -based MEMS mirrors [10], where it was found that individual leg actuation and piston-like actuation required different actuation voltages—note that, during individual actuation, the remaining mirror legs add a stress that is not present during piston-like movement. The pseudo-creep is not included in the modeling of the device, in order to focus on the fundamental thermal and mechanical dynamics in the general case, and, as verified in later experiments, the presented model (ignoring the creep effect) shows adequate capability in predicting the mirror dynamics.…”

“…The VO 2 -based MEMS mirror presented here follows the same fabrication process as in [10], but different metal layers are used to increased the yield per wafer—this is discussed in more detail in Section 2.2. Device fabrication starts with a two–inch double–sided p-type <111> polished Si wafer as the substrate with a thickness of $300 \mathrm{sans-serif\mu }\mathrm{normalm}$.…”

“…Although higher VO 2 orientations are expected from crystalline substrates (i.e., quartz, sapphire), their processing represents major fabrication processing hurdles. The VO 2 is deposited by pulse laser deposition (PLD) and patterned with dry etching using reactive ion etch (RIE), following the procedure shown in [10]. …”

“…In our previous work [10], we used Ti/Pt for the metallization. The rationale for the combination of these metals was to have a high-temperature metal in case it was necessary to change the order of the deposition of materials in the fabrication process flow that would require the deposition of VO 2 after the metallization.…”

“…A new method of actuation for MEMS mirror was presented in [10], where the conventional ET actuation mechanism (i.e., using two materials with different TEC) was replaced with a smart material, vanadium dioxide (VO 2 ), which goes through a phase transition that can be induced by a gradient of temperature. VO 2 has a reversible solid-to-solid phase transition that comes with drastic changes in the mechanical [11], electrical [12], and optical properties [13] of the material.…”