Packaging of the MIT Microengine

Abstract

The MIT Microengine Project is an on-going effort to build a MEMS based gas turbine engine and turbo generator. This thesis covers the development and testing of the packaging needed for the microengine and related devices. The packaging of the microengine includes the fluidic, electrical, and sensor connections that interface the engine with the supplies and instruments necessary for operation. Making connections to the device is difficult because each connection must be gastight, able to bond to a device made entirely of silicon, and capable of withstanding temperatures from 25°C to 900°C. The method chosen for connection was glass sealing, which has been used extensively for metal-to-glass seals in electronics packaging but is a novel approach for making metal-to-silicon connections. The three strength-limiting factors found in metal-to-glass-to-silicon seals were the existence of voids in the glass, the lack of wetting of the glass on the silicon, and the residual stress in the silicon due to thermal expansion mismatch with the glass. Sessile drop tests were conducted to determine which process parameters affect these strength-limiting factors. Voids were found to depend on the type of glass used, specifically the manufacturing process of the preforms, and wetting was found to depend on both the atmosphere used in the furnace while making the bond and the type of glass being used. Test samples were made using both the baseline process and the improved process recommended by the results of the sessile drop tests. Mechanical testing of the samples confirmed that using the improved process increased the strength and reliability of the joints. Macro connections are the interface between the micro tubes and wires exiting a device and the macro supplies and instruments in the lab. Several different options were explored for making these connections, three of which were tried on different devices. A fixed plate method is recommended for making macro connections due to its support of the tubes, protection of the device, and ease of assembly. Inlet, exhaust, and electrical connections were also considered and several different options for each are presented. The results of this thesis are an improved process for making metal-to-glass-to-silicon seals, a roadmap for packaging future devices, and recommendations for interfacing the microengine with the micro aerial vehicle. Future work in packaging should include a more detailed study of wetting, the manufacturing process of preforms, and the possibility of wafer level packaging.