IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 18 (2008) 115022 (13pp) doi:10.1088/0960-1317/18/11/115022 Air cooling of a microelectronic chip with diverging metal microchannels monolithically processed using a single mask Youngcheol Joo 1 , Hsin-Chih Tim Yeh 2 , Kiet Dieu 2 and Chang-Jin Kim 2 1 Department of Mechanical Engineering, Soonchunhyang University, Asan-si, Chungnam 336-745, Korea 2 Mechanical and Aerospace Engineering Department, UCLA, Los Angeles, CA 90095, USA E-mail: ychjoo@sch.ac.kr Received 13 May 2008, in final form 26 August 2008 Published 7 October 2008 Online at stacks.iop.org/JMM/18/115022 Abstract A single-mask process is used to fabricate metal microchannels of 5–10 μm in width, 10–40 μm in height and millimeters in length, monolithically (i.e., no bonding) on the chip front side. Taking advantage of the small size and the diverging cross-section allowed for these microchannels, we explore the air cooling of a microelectronic chip, addressing the limitations of the liquid cooling with the well-known silicon bulk-etched microchannels. Upon the air flow, a distributed array of temperature sensors integrated on a heater chip reads a temperature drop (e.g., from 90 ◦ C to 25–27 ◦ C) and confirms an effective cooling. A thermal analysis further predicts a heat removal capacity of more than 35 W cm −2 by optimized microchannels with a pressure drop of 30 psi (207 kPa). The compatibility with IC fabrication and the use of air as the coolant makes the chip packaging and the system implementation of the reported approach simpler and economical for microelectronic chip cooling. (Some figures in this article are in colour only in the electronic version) 1. Introduction 1.1. Motivation With the increase in device density on integrated circuit (IC) chips, heat removal has been an important consideration. Among various cooling schemes proposed to date, heat sinks with microchannels are among the most promising [1]. The first was made of silicon with a channel cross-section on the order of 50 μm in width and 300 μm in depth, reporting a heat removal flux as high as 790 W cm −2 by flowing a cooling liquid through the channels [2]. However, this heat sink suffered from the cost of implementation such as bonding of the heat sink to the chip, modification of the chip packaging procedure and the complexity of the electronic system hosting coolant recirculation, as well as the cost of the silicon microchannel heat sink. To address these problems, we develop a microchannel fabrication method that is of low cost not only for the formation of the channels but also for the chip packaging. Using only a standard ultraviolet (UV) light source and one photomask minimizes the cost of the channel fabrication. By building metal microchannels of a hydraulic diameter of only 10–30 μm and allowing a diverging cross- section, we verify that even air can be explored as a cooling medium, which would not call for the complex electronic system that a liquid cooling would. Over the past 20 years, the construction of miniaturized channels has become increasingly important not only for IC chip cooling but more so in many other applications. A number of micro analysis systems such as gas chromatography, liquid chromatography, free-flow fractionation and electrophoresis have been developed by micromachining technology, requiring such microchannels for reactant delivery and biochemical reaction chambers. Other applications are found in liquid flow sensors, pressure and drag force flow sensor, densitometer, micropumps, micro thermal actuator and in passive liquid flow 0960-1317/08/115022+13$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK