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Technology
That's All Wet:
Microspray Cooling
by David Brown and Marlys
Amundson
Like firefighters battling a blaze, researchers in the UCLA Henry
Samueli School of Engineering and Applied Science are spraying water
onto the hottest areas of semiconductor chips to cool them down.
This new technology may improve the efficiency of communications
platforms aboard unmanned aircraft and the performance of the electronic
drives for electric car and train motors. It may also prove to be
an enabling technology for gallium nitride on sapphire chips, which
have poor thermal conductivity.
Spray cooling allows transistors to be driven harder and produce
more power. Spray cooling also enables chips to survive in harsh
environments that would otherwise cause them to fail.
Under the leadership of Elliott Brown, professor of electrical engineering
and Vijay K. Dhir, dean of the School, UCLA researchers found that
liquid spray- cooling could improve the performance of transistors
as much as 34 percent.
“We were able to achieve significantly greater power than can be
obtained from the same chips using conventional (solid-state conductive)
cooling alone,” notes Brown, who calls it “a promising approach
to improving thermal management at elevated power dissipation.”
Brown and Dhir discovered that compared to other fluidic methods
such as liquid immersion or forced-convective (fan) cooling, spray
cooling improves the transfer of heat away from the chip by combining
the effects of convection with vaporization.
Although applying the concept to electronics is not new - there
are commercially available products that spray-cool the entire package
of components, including the circuit boards - the UCLA team is the
first to employ micro-spraying of water directly on the semiconductor
chip surface, which concentrates the spray to the hottest areas
on a chip. Targeting the hottest areas on the chip not only improves
the heat removal capability but also minimizes the amount of liquid
required, making it more efficient from a system standpoint.
Electrons and holes speeding through transistors create heat, a
negligible effect in small components such as cell phones. But when
the circuitry must generate large amounts of power - to drive motors
or operate radar equipment, for example - the temperatures of transistors
in these circuits can exceed 100 degrees Celsius. Above temperatures
of 150 degrees Celsius, chips break down faster and become unreliable.
At about 200 degrees Celsius, they cease to function.
In addition to increasing their power, maintaining chip temperatures
below 150 degrees Celsius allows power-amplifier chips to operate
in harsh environments such as those aboard unmanned aerial vehicles
(UAVs).
The cooling system Brown and Dhir have designed is small, lightweight,
and consumes only a small amount of power, a necessity in the cramped
confines of a UAV, where space and weight come at a high premium.
A key to their spray technology is a micronozzle array tailored
to match the size of the semiconductor chip. The individual nozzles
- only 35 microns in size - are bulk micromachined in a silicon
substrate, and are tailored to the chip design. In other words,
the nozzles are only located where devices on the chip generate
heat.
Because silicon is chemically robust with respect to acids and other
harsh chemicals and inexpensive to mass-produce, it is an ideal
material for the nozzle array. Reactive-ion etching, the same process
used in the fabrication of the transistors themselves, is used to
create the nozzles, producing very smooth sidewalls with less tendency
to trap contaminants and become clogged than small holes in metals
would have.
In the first set of experiments, Dhir and Brown applied spray cooling
to two types of chips: Insulated Gate Bipolar Transistors (IGBTs)
used to drive electric motors in trains, electric cars and elevators,
and LD-MOSFET transistors used in 500-MHz-to-2 GHz radio frequency
power amplifiers in radar transmitters and communications base stations.
Results for the IGBTs were impressive with an order-of-magnitude
improvement in heat removal. The same technique on 500-MHz LD-MOSFETs
boosted the RF output power by as much as 34 percent. Spray cooling
dissipated about 20 watts of heat in the 60-watt RF power amplifier
demonstration. They believe that given the level of heat removal
obtained in these tests, they will be able to cool power amplifiers
operating up to 100 W output and up to at least two GHz as they
refine the technology
More recently they have been applying the spray cooling technique
to gallium nitride on sapphire chips provided by Rockwell Scientific.
If it proves successful for gallium nitride devices, their technique
would serve as an enabling technology since sapphire's poor thermal
conductivity prevents high power levels from being obtained before
the device burns up.
"The voltage levels in gallium nitride transistors are a factor
of about five higher than in silicon," notes Brown, "which produces
greater RF output power but also creates more heat. The other possible
substrate for GaN transistors is silicon carbide which has far better
thermal conductivity than sapphire, but is also far more expensive."
A critical aspect of their work involved calculating the proper
water flow rates, requiring Dhir's expertise in phase change heat
transfer. They have found that for the device temperature range
of about 100 to 200 degrees Celsius, water is the best high-density
liquid coolant. Heat is disbursed by both thermal convection and
evaporation in about equal parts, allowing water to accommodate
a higher level of critical heat flux than alternative inert liquid
coolants can do.
"Similar to spraying your face with an atomizer on a hot, dry day,
atomizing the water increases the surface area, disbursing every
cubic centimeter into a zillion droplets. And each of those droplets
removes heat as it evaporates," Brown comments.
To prevent the water from affecting the electronics, they coated
the top surface of silicon die with Parylene-C, a truly conformal
polymer coating with excellent dielectric properties. The polymer
covers the sidewalls, trenches, and other exposed surfaces on the
chip.
Austin Cotler, a junior development engineer in electrical engineering,
and Matteo Fabbri, a doctoral student in mechanical and aerospace
engineering, are among those involved in this research project at
UCLA.
Direct spray cooling tests on LD-MOSFET and gallium nitride chips
were carried out at UCLA, while the tests on the IGBT chips were
performed at Rockwell Scientific.
For additional information on the research of Professor Elliott
Brown, please see http://www.ee.ucla.edu/faculty/bios/erbrown.htm.
For more information on Professor Vijay Dhir’s work, please see
http://www.mae.ucla.edu/academics/faculty/index.htm.
Photo: Todd
Cheney, UCLA Photography |
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