Thermal Bubble Microfluidic Gate Based on SOI Wafer

This paper presents a simple process using a silicon-on-isolator (SOI) wafer to fabricate a silicon-based vertical microheater to generate thermal bubbles for the applications in microfluidic systems. The fabrication process consists of only two photolithography masks that provide an effective way of manufacturing a resistive bulk microheater and a high-aspect-ratio microchannel. The electrothermal property of the proposed microheater has been characterized and verified by finite-element analysis and experiment. According to the design concept and experimental results, the largest temperature occurred at the smallest neck section due to the nonuniform property of the resistance along the length of the arch-type microheater, and thus, the vapor bubble was generated and attached to the vertical sidewall of the microheater. For a typical microheater design, bubble nucleation could be generated under a applied voltage of 5 V, and the bubble could obstruct the entire 100 mum width of the microchannel when the applied voltage reaches 8 V. A switching test has showed that the silicon-based microheater has a good thermal resistance behavior for long-term reliability and that the modulation of output flow rate is easily handled by the sizes of the thermal bubbles. Moreover, the bubble can be formed with a steady growth, even when the maximum fluid velocity is larger than 920 mum/s in a microchannel with a rectangular cross section 100 mum wide and 50 mum high. These results reveal that the microfluid gate presented here is well designed and that bubble sizes are stable and controllable.