Evolution of Microchannel Flow Passages--Thermohydraulic Performance and Fabrication Technology

Abstract

This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may be influenced by rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research in microchannel flow passages has paralleled the advancements made in fabrication technology. The earliest microchannels were built in silicon wafers by anisotropic wet chemical etching and sawing. While these methods have been exploited successfully, they impose a number of significant restrictions on channel geometry. A variety of advanced micromachining techniques have been developed since this early work. The current state of fabrication technology is reviewed, taxonomically organized, and found to offer many new possibilities for building microchannels. In particular anisotropic dry etching and other high aspect ratio techniques have removed many of the process-induced constraints on microchannel design. Other technologies such as surface micromachining, microstamping, hybridization, and system-on-chip integration will enable increasingly complex, highly functional heat transfer devices for the foreseeable future. It is also found that the formation of flow passages with hydraulic diameters below the microchannel regime will be readily possible with current fabrication techniques.