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Demand of cooling solution for microelectronics devices

As integrated circuits become faster and more densely packed with transistors, the power density increases and the heat generated as a by-product becomes more severe. There is a growing demand for heat transfer expertise to aid with the design of micro fabricated transistors, sensors, and actuators for the integrated circuits and MEMS industries. Heat generation and conduction influence the reliability of semiconductor devices and interconnects.

The fundamental need to improve cooling efficiency spans much of developing microelectronics technology. There is constant pressure on military electronics system designers to reduce system volume while increasing electronics complexity and power density. Compound semiconductor phased array radar modules are trending to higher power, smaller module size, and larger arrays. Additional needs for cooling optoelectronics, laser diodes, high speed processors and communication equipment are increasingly demanding. Heat transfer is also important for multilevel interconnects in fast logic circuits, for which the trend to larger numbers of level increases the peak temperature rise induced by Joule heating. Despite the migration from high power bipolar to low power CMOS logic, the lowering of device operating voltage, and the use of sleep mode in circuit design, the power density of micro electronics systems continues to increase: higher frequency clocks have made CMOS as power hungry as bipolar; low voltage has led to high current, high loss power conversion and transmission; and micro sensors and their heat-producing analog circuitry are proliferating.

Heat buildup is becoming one of the major limitations to creating tomorrow?s more compact, complex microelectronics devices. The increasing integration of electronic system is leading to challenges for the thermal design of packaging, development, and assessment of high performance heat sinking solution.


Cooling solutions and trends

Traditionally microelectronics chip substrates are made of fiberglass-epoxy-type materials laden with copper interconnects that help conduct heat into the substrate and away from the micro chip. Other designs include supplementary heat spreaders of solid copper or other conductive materials within the substrate. In the recent past, highly conductive diamond and diamond-coated substrates have been developed, but their use is drastically limited by their high cost.

To solve the heat-buildup problem in specialty microelelctronic devices and some laptop computers, microchip manufacturers have added small tubular heat pipes to the surfaces of microchips that help carry some heat away form the hottest circuitry and transfer it to other structural elements, such as a printed circuit board's external surfaces. But the surface connections where tubular heat pipes meet substrate typically conduct heat very poorly and therefore aren't effective heat removes.

Microjet technology (in which a vibrating diaphragm causes an alternating suction and blowing of air through an orifice, which can be directed onto a chip or package to be cooled) can be used as an alternative solution to the currently used CPU fans. These devices have first been fabricated using either electromagnetic or piezoelectric drivers. The electromagnetic device has been the most reasonable solution due to the high sound pressure levels created by the piezoelectric devices. The electromagnetic mircojet cooling device consists of a circular flexible membrane under tension.

Currently microelectronics systems use bulk conductive substrates to spread heat from hot components to channel and to dissipate thermal energy to heat sinks. When applications involve higher heat fluxes, a pumped coolant approach is generally used.


Why Micro Heat Pipe ?

Designers need a thermal management tool kit with many options to solve the many heat problems constraining advances in microelectronics technology today. Conventional methods of cooling are not an ideal way to overcome the heat problem. A simple solution would be using micro heat pipe as integrated part in the silicon substrate of the processors. Currently, silicon/working fluid micro heat pipe are being fabricated and tested to verify the operation of the micro heat pipe as a thermal heat spreader.

Micro heat pipe, two-phase convection devices, yield by far the highest cooling rates per unit volume in electronic systems, as evidence by the effectiveness and large commercial impact of heat pipes in portable systems. Other two-phase devices that provide even higher cooling rates include microchannels, microjets, and vapor compression refrigerators. However, these devices are rarely found in mass-produced electronic systems because they require high-pressure pumping.

The micro heat pipes built directly within the substrate material carry heat away from the chip much more efficiently than conduction through solid copper or transport through larger heat pipes on the substrates surface and thus allow for greater heat reduction and improved temperature control. The closer the coolant is to the circuit, the more heat can be managed. As the coolant directly underneath the operating micro chip heats up and evaporates, it moves through the galleries to cooler areas, where it re-condenses, distributing its heat throughout the substrate. Capillary pressure on the re-condensed liquid then pull the coolant back to hotter regions where evaporation has occurred, similar to the way a candle wick draws molten wax upward to the flame.

The amount of capillary pressure inside the tubes is a function of the size of the passages. As the structures get smaller, more pumping action is created within the substrate. This cyclic evaporation and condensation effect distributes heat throughout the substrate much more efficiently than simple conduction through a solid. A coolant and micro pipe geometry can be selected that best transfers heat given each device's design and operating temperature range.