A micromachined thermo-optical light modulator based on semiconductor-to-metal phase transition

In this research, a micromachined thermo-optical light modulator was realized based on the semiconductor-to-metal phase transition of vanadium dioxide (VO2) thin film. VO2 undergoes a reversible phase transition at approximately 68°C, which is accompanied with drastic changes in its electrical and optical properties. The sharp electrical resistivity change can be as great as five orders. Optically, VO2 film will switch from a transparent semiconductor phase to a reflective metal phase upon the phase transition. The light modulator in this research exploits this phase transition related optical switching by using surface micromachined low-thermal-mass pixels to achieve good thermal isolations, which ensures that each pixel can be individually switched without cross talking. In operation, the pixel temperature was controlled by integrated resistor on each pixel or spatially addressed thermal radiation sources.; Active VO2 thin film was synthesized by thermal oxidation of e-beam evaporated vanadium metal film. The oxidized film exhibits a phase transition at ∼65°C with a hysteresis of about 15°C. A transmittance switching from ∼90% to ∼30% in the near infrared and a reflectance switching from ∼50% to ∼15% in the visible have been achieved. The surface microstructure was studied and correlated to its optical properties. A study on the hysteresis loop reveals that the VO2 can be repetitively switched between the "on" and "off" states.; The micromachined thermal isolation pixel was a bridge-like silicon dioxide platform suspended with narrow supporting legs. The pixel design was optimized with both thermal and optical simulations. The VO2 light modulator was fabricated by surface micromachining based on dry processing. Silicon dioxide was deposited on a polyimide sacrificial layer by PECVD and patterned to form the structural pixel. Vanadium film was e-beam evaporated and patterned with lift-off process. It was thermally oxidized into VO2 at 390°C. The thermal isolation pixel was anchored on substrate by aluminum pedestals. Finally, the structure was released in an oxygen plasma barrel asher. The VO2 array was experimentally tested and its light switching and modulation ability were demonstrated. Further study shows that the surface micromachining process has no degrading effect on the optical property of VO2 thin film.