Dual Action Improves Micropump Performance

One of the approaches people have been examining for heat removal in upcoming ICs is the placement of fluid-filled microchannels on the die. Researchers in Suresh Garimella's group at the Cooling Technologies Research Center at Purdue University (West Lafayette, Ind.) recently announced the development of a key component of that approach — MEMS micropumps to move fluid through those microchannels with improved efficiency.

According to the 2005 International Technology Roadmap for Semiconductors (ITRS), the projected power consumption of systems-on-a-chip (SoCs) will increase by a factor of 10 over the next 15 years, reaching several hundred watts per die. Heat sinks with fins will not be sufficient.

A wide variety of heat removal approaches are currently under investigation, from thermoelectric coolers to metal-filled vias in three-dimensional ICs. The use of fluid-filled microchannels is compatible with all of these approaches. Microchannels can be made using CMOS-compatible processes, as can most micropumps, including the one developed by Garimella's group.

The use of micropumps on the die itself is attractive for a few reasons. Having pumps at the site to be cooled gives the greatest control over the local flow rates, and therefore the greatest control of heat removal. Also, having many micropumps throughout the flow loop on the die can reduce the pressure requirements for external pumps. Reducing the pressure of external pumps makes the cooling system more reliable; this means that smaller external pumps are needed. Minimizing the size of an external pump, or even eliminating it, is the most important benefit.

Many micropump schemes have been explored in recent years. The technology that Garimella's group developed uses two pumping mechanisms simultaneously: valveless diaphragm pumping with nozzle diffusers and electrohydrodynamic (EHD) pumping. Their findings indicate that the flow rate created by the combination is greater than the sum of the flow rates using each mechanism individually.

A valveless diaphragm pump with nozzle diffusers is actually quite inefficient (Figure ). Fluid exits through both nozzles during compression and enters through both during expansion. The pump depends on the geometry of the nozzles to end up with a net flow in the desired direction. Its advantage is that it has only one moving part, the diaphragm, which is piezoelectrically actuated in this case. Using another mechanism to force the flow in the right direction reduces backflow through both nozzles, which significantly improves the diaphragm pump's efficiency.

The valveless diaphragm pumps depend on the nozzle geometry to produce a net flow, while the electrohydrodynamic (EHD) pumps create ions in the fluid and move them along in a "stepper motor" fashion.

EHD pumping has been investigated for many applications. If there is an electrical conductivity gradient normal to the flow direction, then ions can be formed in the fluid with the introduction of electric fields. Since conductivity is usually temperature-dependent, a temperature gradient in the fluid will ensure a conductivity gradient. These fields are imposed by applying voltages on a series of electrodes. By varying the voltages on the electrodes, the ions can be made to move in the same way that a stepper motor works, and the ions drag the fluid with viscous forces.

The concern about using EHD is that it adds energy into the fluid to make it move, and the goal is to remove heat. Fortunately, this effect is less pronounced at smaller geometries. Typically, microchannels are no wider than 100 µm and no deeper than 150 µm. Calculations with representative parameters have shown that the heat removed using the combined pump is nearly 10,000× the energy added by using EHD.

One of the keys to making EHD work is to have ions remain ionized long enough to travel the distance between two electrodes. Ion lifetime and viscosity are key parameters of the fluid. In its proof-of-concept studies, the group used water "doped" with potassium chloride. Two recent graduates from the group, Vishal Singhal and Dan Schlitz, cofounded Thorrn Micro Technologies (Redwood City, Calif.), which promotes the use of ion-driven air. The choice of fluid seems to be a wide-open issue.

The upcoming heat removal requirements for ICs are daunting. Innovations like microchannels and micropumps, along with power-efficient designs, provide some hope that those requirements will be met.

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