Damage and Failure Mechanisms in High Pressure Silicon-Glass-Metal Microfluidic Connections
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Damage and Failure Mechanisms in High Pressure Silicon-Glass-Metal Microfluidic Connections Dong-Jin Shim1, Hong-Wei Sun1, Srikar T. Vengallatore2, and S. Mark Spearing1 1
Department of Aeronautics and Astronautics, Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA 02139, USA 2 Department of Mechanical Engineering, McGill University 817 Sherbrooke Street, W. Montreal, Quebec, Canada H3A 2K6 ABSTRACT The characteristics and mechanisms of damage and failure in microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass seals have been investigated. These joints are representative of seals for the MIT microrocket which is a silicon-based MEMS device. A key concern in such joints is the occurrence of cracks in silicon and glass due to residual stresses caused by a large thermal excursion during processing and the dissimilar coefficients of thermal expansion of the constituent materials. Joints with two types of glass compositions and joint configurations were fabricated, tested, and inspected. Axial tension tests were performed to investigate load carrying capability and the effect of thermally-induced cracks. Finite element models were used to obtain residual stresses due to the fabrication, and the location of the cracks from the experiments were found to coincide with the locations of the maximum principal stresses. The current work shows that the certain types of thermally-induced cracks are more detrimental to joint strength than others and a good bond between the Kovar tube and the silicon sidewall can help increase joint strength via shear load transfer. INTRODUCTION The application of microsystems in power generation and propulsion necessitates the development microfluidic connection technologies capable of operating at high temperatures and pressures. These connections are required to transport fuels, oxidizers, and coolants from external sources to the MEMS device. In the current work, the application of interest is the MIT microrocket [1,2] whose temperature can reach in the excess of 700 K and pressures up to approximately 15 MPa. To meet requirements of the microrocket, the glass sealing approach is used to connect the microfluidic channels in the Si device to Kovar metal tubes. Glass sealing techniques are commonly used in the microelectronics packaging to hermetically seal and provide electrical isolation, e.g., in optical and microwave packages [3]. The challenge in applying and adapting the glass sealing technique for packaging of the microrocket is that the three materials being bonded, i.e., silicon, Kovar, and glass, have different coefficients of thermal expansion (CTE). The high processing temperature required to fabricate the joint gives rise to concern of thermally-induced cracks in the glass and silicon, which are brittle materials. Such thermallyinduced cracks are a key concern because they may cause leaks, reduce the load carrying capability of the joint and lead to premature failure. The current investigation builds upon t
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