Heat Transfer at Small Dimensions MICROSCALE THERMAL ENGINEERING OF ELECTRONIC SYSTEMS

The electronics industry is encountering thermal challenges and opportunities with lengthscales comparable to or much less than one micrometer. Examples include nanoscale phonon hotspots in transistors and the increasing temperature rise in onchip interconnects. Millimeter-scale hotspots on microprocessors, resulting from varying rates of power consumption, are being addressed using two-phase microchannel heat sinks. Nanoscale thermal data storage technology has received much attention recently. This paper provides an overview of these topics with a focus on related research at Stanford University. INTRODUCTION Thermal management has been an important concern for the electronics industry for several decades, and it has grown more critical in the past few years owing to increases in the total heat generation rate, and the heat generation rate per unit area, of modern micrporcessors. This trend has been accompanied by an extension of the dimensions of many challenging problems down to deep sub-micrometer lengthscales. In the past, the main thermal management activities were in air routing and system level management and the design of metal heat sinks, heat pipes, and vapor chambers. While these activities remain important, a new set of challenges and opportunities at the micro and nanoscale are growing crucial. These are occurring primarily, or close to, the semiconductor chip, as summarized in these four examples: Nanoscale Transistor Hotspots: As silicon transistor channel lengths decrease below 100 nm, the spatial volume and number of lattice vibrational states gaining energy from electron scattering are also decreasing. This results in extreme nonequilibirum phonon populations within the device, which impedes conduction cooling and reduces both the source injection and drain series resistances. The impact on device design will be dramatic for transistors below the 50 nm node. Interconnect Heating: Another on-chip thermal problem is the temperature rise in the metal interconnect structures on chips. The trends to higher current interconnect densities, extreme interconnect aspect ratios, and more metal layers are raising the metal temperature rise to tens of degrees Celsius. This problem is aggravated by trends to novel organic and porous dielectrics, which impeded thermal conduction. Novel Thermal Microdevices for Information Storage and Diagnostics: Advancements in micro and nanomachining have enabled new realms of imaging and data storage technologies, which are finding impact for the IC industry. This section of the paper presents two novel thermal microstructures including cantilever beams for data storage and microlenses for precise scanning. Two-Phase Microchannel Heat Sinks and Chip-Level Hotspots: Highly nonuniform rate of heat generation on a semiconductor chip increases the peak chip temperature for a