Single-crystal silicon is being increasingly employed in a variety of new commercial products not because of its well-established electronic properties, but rather because of its excellent mechanical properties. In addition, recent trends in the engineering literature indicate a growing interest in the use of silicon as a mechanical material with the ultimate goal of developing a broad range of inexpensive, batch-fabricated, high-performance sensors and transducers which are easily interfaced with the rapidly proliferating microprocessor. This review describes the advantages of employing silicon as a mechanical material, the relevant mechanical characteristics of silicon, and the processing techniques which are specific to micromechanical structures. Finally, the potentials of this new technology are illustrated by numerous detailed examples from the literature. It is clear that silicon will continue to be aggressively exploited in a wide variety of mechanical applications complementary to its traditional role as an electronic material. Furthermore, these multidisciplinary uses of silicon will significantly alter the way we think about all types of miniature mechanical devices and components.

Silicon as a mechanical material

We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

https://microfluidics.stanford.edu/Publications/Micropumps_Cooling/Laser%20Review%20of%20Micropumps%20in%20JMM.pdf

A review of micropumps

This paper presents the current state of understanding of the factors that limit the continued scaling of Si complementary metal-oxide-semiconductor (CMOS) technology and provides an analysis of the ways in which application-related considerations enter into the determination of these limits. The physical origins of these limits are primarily in the tunneling currents, which leak through the various barriers in a MOS field-effect transistor (MOSFET) when it becomes very small, and in the thermally generated subthreshold currents. The dependence of these leakages on MOSFET geometry and structure is discussed along with design criteria for minimizing short-channel effects and other issues related to scaling. Scaling limits due to these leakage currents arise from application constraints related to power consumption and circuit functionality. We describe how these constraints work out for some of the most important application classes: dynamic random access memory (DRAM), static random access memory (SRAM), low-power portable devices, and moderate and high-performance CMOS logic. As a summary, we provide a table of our estimates of the scaling limits for various applications and device types. The end result is that there is no single end point for scaling, but that instead there are many end points, each optimally adapted to its particular applications.

/pdf/device-scaling-limits-of-si-mosfets-and-their-application-299ftwsxth.pdf

Device scaling limits of Si MOSFETs and their application dependencies

A Three Dimensional Structure (3DS) Memory (100) allows for physical separation of the memory circuits (103) and the control logic circuit (101) onto different layers (103) such that each layer may be separately optimized. One control logic circuit (101) suffices for several memory circuits (103), reducing cost. Fabrication of 3DS memory (100) involves thinning of the memory circuit (103) to less than 50 microns in thickness and bonding the circuit to a circuit stack while still in wafer substrate form. Fine-grain high density inter-layer vertical bus connections (105) are used. The 3DS memory (100) manufacturing method enables several performance and physical size efficiencies, and is implemented with established semiconductor processing techniques.

Three dimensional structure memory

The problem of achieving compact, high-performance forced liquid cooling of planar integrated circuits has been investigated. The convective heat-transfer coefficient h between the substrate and the coolant was found to be the primary impediment to achieving low thermal resistance. For laminar flow in confined channels, h scales inversely with channel width, making microscopic channels desirable. The coolant viscosity determines the minimum practical channel width. The use of high-aspect ratio channels to increase surface area will, to an extent, further reduce thermal resistance. Based on these considerations, a new, very compact, water-cooled integral heat sink for silicon integrated circuits has been designed and tested. At a power density of 790 W/cm2, a maximum substrate temperature rise of 71°C above the input water temperature was measured, in good agreement with theory. By allowing such high power densities, the heat sink may greatly enhance the feasibility of ultrahigh-speed VLSI circuits.

High-performance heat sinking for VLSI

Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Heat Transfer in Microchannels—2012 Status and Research Needs

: The design of high-speed integrated circuits and systems is often constrained by thermal considerations. As late as 1981 it was authoritatively predicted that the maximum achievable power flux for liquid-cooled, densely-packed integrated circuits (ICs) would be about 20 W/sq cm. Convective heat-transfer theory indicates that well over 1000 W/sq cm can be compactly removed from ICs at normal operating temperatures, provided microscopic (e.g., 50-microns wide) extended-surface structures are used. The difficulty of constructing high-conductance, low-stress thermal interfaces between ICs and heat sinks suggests the use of an integral heat sink. Accordingly, IC microfabrication techniques were employed to design, fabricate, and test novel, ultracompact water-cooled, laminar-flow, optimized plate-fin and pin-fin heat sinks directly within standard-thickness silicon substrates. Worst-case thermal resistances as low as 0.083 deg C/W were measured from 1-sq cm thin-film resistors (e.g., a 108 deg C temperature rise at 1309 W), in good agreement with predictions. Further increases in heat transfer are achievable. The use of integral liquid-cooled heat sinks in multichip systems presents potential yield, reliability, cost and packaging problems. Attachment of unmodified ICs to micro-heat sinks seems a more attractive approach. A novel die-attachment technique has been developed which avoids the problems of conventional attachments. In this technique, a liquid partially fills an array of micron-wide repentant capillaries in the heat sink substrate, so that surface tension holds the polished back of an IC in intimate thermal contact with the heat sink. The bond is void-free, virtually stress-free, long-lived, and allows repeated detachment and replacement of ICs without damaging the heat sink substrate. The repentant grooves were fabricated by a novel process using electroless plating of nickel onto vertical silicon microgrooves.

/pdf/heat-transfer-microstructures-for-integrated-circuits-48f4gastz4.pdf

Heat-transfer microstructures for integrated circuits

A property of an insurance policy may be determined, at least in part, upon characteristics of a vehicle autonomous drive mode selection system. The characteristics may pertain to any capability, configuration, and/or operating state of the autonomous drive mode selection system (and/or vehicle). For example, a property of the insurance policy may be based upon whether the autonomous drive mode selection system is enabled or disabled. In some embodiments, the property of the insurance policy may be based upon which of one or more autonomous driving modes is selected via an autonomous drive mode selection system. The property of the insurance policy may be dynamic, and may be updated in response to changes to the autonomous drive mode selection system.

Systems and methods for insurance based on monitored characteristics of an autonomous drive mode selection system

Two bodies such as an integrated circuit and a heat sink are maintained in a low stress and high thermal conductance abutment by pressure from capillary attraction. A surface of one of the bodies is provided with grooves preferably having reentrant surfaces. A surface of the other body is brought into abutment with the grooved surface with a liquid therebetween which partially fills the grooves. The grooves act as reservoirs for excess liquid so that a minimum thickness liquid interface is achieved, and the grooves enable trapped gas to escape out the open ends thereby preventing voids between the two bodies. Importantly, the groove geometry enforces a well-defined attractive force between the two surfaces due to the liquid surface tension.

David B. Tuckerman

Papers

High-performance heat sinking for VLSI

Heat Transfer in Microchannels—2012 Status and Research Needs

Heat-transfer microstructures for integrated circuits

Systems and methods for insurance based on monitored characteristics of an autonomous drive mode selection system

Heat sink and method of attaching heat sink to a semiconductor integrated circuit and the like