Thermally actuated MEMS seal for vacuum applications

Abstract

This thesis presents the design, fabrication and testing of a new, leak-free, permanently sealable MEMS valve for use in vacuum applications. This device is different from existing MEMS valves in that it is leak-free in the closed state and has a relatively high flow rate in the open state. This device relies on the surface tension of a molten seal material to establish a permanent seal over its initially-open port upon heating. The sealable port is a through via located in the center of an isolated silicon island supported on a thermally-insulating silicon nitride membrane in the center of a die. The through via is surrounded by a moderately high aspect ratio ring of indium solder. To seal the solder over the through via, the island and solder are heated by passing a current through a resistive heater on the back side of the device. Upon thermal actuation, the hollow cylinder of solder reflows into a toroid due to surface tension. For sufficiently high solder aspect ratios, the inner edges of the toroid meet in the center, thereby plugging the via. The heater is then turned off, solidifying the solder and forming a permanent seal. Individual subsystems of the device were first analytically modeled using structural, thermal, electrical and geometrical models to optimize the device features. The sealing and thermal isolation subsystems were then separately fabricated and tested to verify the analytical models and key fabrication processes. The individual subsystems were then combined into the final device. Tests on the final device indicate an open state flow rate of 60 to 400 standard cm³ per minute (sccm), a closed state leak rate not detectable above that of the test jig used (10-⁴ sccm), and an open-to-closed flow rate ratio of greater than 10⁵ to 10⁶.