Fabrication, Assembly, and Testing of Metal-based Microchannel Heat Exchangers

Microsystem technologies are believed to be an important part of the technological foundation upon which the world can continue to improve the standard of living in the twenty-first century. Microelectromechanical systems (MEMS) have been studied intensely for over two decades. Si-based MEMS, fabricated by surface and bulk micromachining techniques derived from IC processing technology, dominate the current MEMS market. Although metal-based microdevices and microsystems have not received as much attention in comparison, early examples of metal-based microdevices exist in the literature, including microchannel heat exchangers (MHEs) and magnetically driven microactuators. As these early examples show, the physical properties of metals afford metal-based microdevices functionalities not achievable with Si-based counterparts, be it increased thermal conductivities in the case of MHEs or magnetism in the case of magnetic actuators. Metal-based high-aspect-ratio microscale structures (HARMS) are basic building blocks for metallic microdevices. One important factor responsible for the present paucity of metal-based microdevices and microsystems is the current lack of suitable fabrication techniques for metal-based HARMS. In order for metalbased microdevices to be competitive in the market place, fabrication of metal-based HARMS should be simple and fast, and assembly of metallic HARMS into metal-based microdevices should be simple, fast, and reliable. The lack of such fabrication and assembly techniques for metal-based microdevices, coupled with the ready availability of technology for fabrication and assembly of Si-based MEMS, is largely responsible for the absence of metalbased microdevices and microsystems on the current MEMS market. Molding replication offers an important alternative toward efficient fabrication of metal-based HARMS. The process of replication involves the use of primary HARMS, produced by a combination of lithography, etching, deposition, and other techniques, as a mold insert to create the negative of the insert pattern in metals by direct compression molding. One important example of molding replication is contained within the LiGA protocol for HARMS fabrication, combining deep X-ray/UV lithography (Lithographie) on polymeric resists, metal electrodeposition (Galvanoformung) into developed resist recesses, and molding replication (Abformung) of secondary HARMS. Primary HARMS made by lithography and electrodeposition (LiG) are expensive due to the high equipment and process costs of lithography and the low speed of electrodeposition. The importance of the molding replication step is that secondary HARMS can be produced in different engineering materials without repeating the lithography/developing/deposition steps, and thus at low cost and high throughput. Since 2003, successful HARMS replication by direct compression molding has been demonstrated in Pb and Zn, Al, Cu, Ni and Ni-Ti. Two critical elements were needed for successful molding replication of reactive metals such as Cu and Al. The first one is to control the near-surface chemical/mechanical interactions between the mold insert and the molded metal. This was achieved via conformal deposition of suitable ceramic coatings, such as a-Si:N and Ti-C:H, over HARMS mold inserts. The second one is to improve the mechanical properties of the mold insert bulk at elevated molding temperatures. This was achieved by fabricating mold inserts out of refractory metals and alloys with μEDM. Using these surface-engineered, refractory, mold inserts, microchannels with complex geometries have been replicated successfully in metals such as Cu and Al by compression molding. In addition to fabricating metallic microchannels by molding replication, Al-based microchannels were successfully bonded to other Al microchannels and flat plates by eutectic bonding with vapor-phase co-deposited Al-Ge thin film intermediate layers. Multilayered microchannel arrays were successfully assembled to form metal-based MHE prototypes. Measured tensile bond strengths exceeded 75MPa and reaching as high as 165MPa.