Ph.D. Oral Exam Announcement

Evelyn Wang enwang at
Wed Jan 11 10:26:20 PST 2006

Ph.D. Oral Examination, Stanford University

Characterization of Microfabricated Two-Phase Heat Sinks for IC Cooling

Evelyn N. Wang 
Advisor: Thomas W. Kenny and Kenneth E. Goodson
Department of Mechanical Engineering

Time:  4:15 P.M. (refreshments will be served at 4:00 P.M.)
Date:  Tuesday, January 17th, 2006
Location:  Building 530, Room 127

The increasing heat generation rates in integrated circuit (IC) chips pose
severe thermal management challenges for the semiconductor industry.  The
cooling capacity of conventional heat sinks will soon reach their limit and
novel methods for heat dissipation from ICs need to be developed.  Two-phase
microchannels and microjets have received attention because they promise
compact and efficient cooling solutions. 

This talk focuses on microchannel cooling technologies during incipient
boiling, where bubbles nucleate, grow, and depart from nucleation sites on
channel walls.  Understanding bubble dynamics and determining departure
criteria are critical in these devices such that local dry-out and
subsequently poor cooling in regions can be avoided. Silicon microchannels
with hydraulic diameters less than 400 um were fabricated with heaters and
sensors.  When heating power was applied, bubbles formed due to
heterogeneous nucleation and grew from the channel side-walls.  In order to
understand bubble dynamics, micron-resolution particle image velocimetry
(uPIV) was used to obtain two-dimensional liquid velocity fields surrounding
the nucleating bubbles.  However, the limited information from the data
requires the development of a hybrid method to reconstruct the
three-dimensional geometry and associated three-dimensional velocity field.
The combination of experiments and numerical simulations with this
methodology yields important information such as bubble geometry, growth
rates, contact angles, and forces that contribute towards the understanding
of the physical mechanisms behind growth and departure.  
Some recent efforts on two-phase microjet impingement cooling will also be
discussed.  In this project, microjet test structures were designed,
fabricated, and characterized using a custom heater device.  Heat removal of
over 90 W/cm^2 using a 4-microjet array was demonstrated, which suggests
microjet impingement is a promising cooling solution.  Insights into
microjet hydrodynamics were also obtained with flow visualizations.

These current studies show promise towards developing optimized MEMS
two-phase heat sinks for future IC chip cooling.  The heat sinks are
intended for the eventual integration into a closed-loop electroosmotically
pumped cooling system.

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