4.3. Microchannel model
Heat transfer and fluid flows in the phase separated microchannel array were modeled using the finite element method. Due to
significant computational requirements involved in solving 3D
flow and conjugate heat transfer problems, we only modeled a single period of the channels (one liquid channel and the two halves
of the adjacent vapor channels). The computational domain
(cross-sectional and top views) for the simulations is shown in
Fig. 3. We note that, by not allowing heat spreading in the transverse direction, this unit-cell model may significantly underpredict
the real-world performance for cooling semiconductor chips of finite widths.
The micro-perforations were not directly modeled in the microchannel model. The evaporation of the liquid in the perforations is
accounted for using an effective evaporation heat transfer coefficient. Corresponding mass sink/source terms were included at
the respective channel wall surfaces to represent mass loss/gain
due to evaporation.
The SiC chip (semiconductor device) is mounted on top of the
cooling device. The chip is 0.5 mm in length and is located
0.5 mm from the edge of the channels in the simulation domain.
An infinitely wide (in the transverse direction) heat source of
length 100lm (in the streamwise direction) is applied on the chip
surface at a constant heat flux of 1000 W/cm
2
.
As mentioned earlier, all channel dimensions are fixed except
the widths of the liquid and vapor channels, denoted by Wl and
Wv, which were varied parametrically. Most simulations were
performed for two different values ofhevap (=10 kW/m2K and 20 kW/m2K). In a few select caseshevap was varied in the range
5–30 kW/m2K to examine its impact. The values obtained from
the numerical simulation of the micro-perforations is corrected
to account for both the accommodation coefficient and the sidewall porosity. Note that for vaporization from a liquid–vapor
interface, we use the accommodation coefficient as the ratio of
the net mass flux at the interface to the mass flux at an interface
with no recondensation. It is always less than unity. The resultant
overall effective heat transfer coefficient is then used in the thermofluid model. This provides a lower bound for the effective heat
transfer coefficient. The change in the saturation temperature due
to the pressure drop along the microchannels is negligible and was
hence not accounted for.
No slip conditions were applied at all walls. Zero temperature
gradients (negligible conductive heat flux) were specified at the inlets and outlets of the channels. All the other surfaces were assumed to be thermally insulated. A uniform pressure boundary
condition was specified at the liquid inlet and the vapor outlet
viernes, 8 de noviembre de 2013
jueves, 7 de noviembre de 2013
------------------------------------------------------------------------
whereris the surface tension, Dc is the cavity mouth diameter
(modified to account for contact angle) [19], and qv andhfg are
the vapor density and latent heat of evaporation of the fluid, respectively. Fluids with low surface tension (well wetting fluids like FC-72 used in the present work) make it harder for cavities to trap gas.
Additionally, smaller cavities require larger wall superheats to
nucleate. Furthermore, even if bubbles do nucleate and grow, the
vapor would be vented out to the vapor channel through the pores
rather than growing in the liquid channel against the pressure. In
the present study, for FC-72 at atmospheric pressure and a contact
angle of 10 , the predicted wall superheat required for nucleation is
about 62 C (wall temperature’120 C). For the wall temperatures
expected in this study, nucleation is not expected to occur [20-21].
High-aspect-ratio microchannel geometry will allow us to
achieve significant enhancement in the effective heat transfer coefficient (per fin base area) over that of micro-gap flows between
non-finnednon-porous flat surfaces[21]. The thermal energy is
conducted along the perforated solid walls and then transferred
to the evaporating liquids in the micro-pores in our microchannel
device, which helps minimize the negative impact of the low thermal conductivity of a dielectric fluid.
For realistic thermofluid modeling of the chip/cooling device,
accurate thermodynamic and transport properties of FC-72 are
necessary. The properties of liquid FC-72, such as density (q), thermal conductivity (k), specific heat capacity at constant pressure
(Cp), viscosity (l), and surface tension (r) are readily available
[22]. However, many thermophysical properties of the vapor of
FC-72 are not available in the literature.
We develop models to predict some of the missing vapor properties required for the thermofluid modeling, focusing on near
atmospheric pressures. A four-parameter equation of state (EOS)
[23,24] is used to predict qandcpof saturated and superheated
FC-72 vapor. This EOS evaluates the pure component parameters
using the critical temperature (Tc), critical pressure (Pc) and the
accentric factor (x). The EOS is given as
whereris the surface tension, Dc is the cavity mouth diameter
(modified to account for contact angle) [19], and qv andhfg are
the vapor density and latent heat of evaporation of the fluid, respectively. Fluids with low surface tension (well wetting fluids like FC-72 used in the present work) make it harder for cavities to trap gas.
Additionally, smaller cavities require larger wall superheats to
nucleate. Furthermore, even if bubbles do nucleate and grow, the
vapor would be vented out to the vapor channel through the pores
rather than growing in the liquid channel against the pressure. In
the present study, for FC-72 at atmospheric pressure and a contact
angle of 10 , the predicted wall superheat required for nucleation is
about 62 C (wall temperature’120 C). For the wall temperatures
expected in this study, nucleation is not expected to occur [20-21].
High-aspect-ratio microchannel geometry will allow us to
achieve significant enhancement in the effective heat transfer coefficient (per fin base area) over that of micro-gap flows between
non-finnednon-porous flat surfaces[21]. The thermal energy is
conducted along the perforated solid walls and then transferred
to the evaporating liquids in the micro-pores in our microchannel
device, which helps minimize the negative impact of the low thermal conductivity of a dielectric fluid.
For realistic thermofluid modeling of the chip/cooling device,
accurate thermodynamic and transport properties of FC-72 are
necessary. The properties of liquid FC-72, such as density (q), thermal conductivity (k), specific heat capacity at constant pressure
(Cp), viscosity (l), and surface tension (r) are readily available
[22]. However, many thermophysical properties of the vapor of
FC-72 are not available in the literature.
We develop models to predict some of the missing vapor properties required for the thermofluid modeling, focusing on near
atmospheric pressures. A four-parameter equation of state (EOS)
[23,24] is used to predict qandcpof saturated and superheated
FC-72 vapor. This EOS evaluates the pure component parameters
using the critical temperature (Tc), critical pressure (Pc) and the
accentric factor (x). The EOS is given as
martes, 5 de noviembre de 2013
microchannel cooling device
We propose and analyze a novel two-phase microchannel cooling device that incorporates perforated
side walls for potential use as an embedded thermal management solution for high heat flux semiconductor devices. A dense array of perforated side walls separate alternating liquid and vapor microchannels,
allowing the vapor generated through evaporation of liquid supplied through micro-perforations to flow
only in the dedicated vapor channels. By separating the liquid and vapor flows, these ‘‘perspiring’’ side
walls enable us to circumvent flow instabilities and other challenges associated with conventional
two-phase microchannel cooling while at the same time effectively take advantage of the large extended
surface areas available in high-aspect-ratio microchannels. One implementation of our design is parametrically analyzed using finite element modeling, demonstrating the potential of our proposed device for
handling high heat flux electronic and optoelectronic semiconductor devices.
2013 Elsevier Ltd. All rights reserved.
1. Introduction
Continued miniaturization and increasing power ratings of electronic and optoelectronic devices have resulted in the need for active embedded cooling techniques with high heat flux removal
capability. Flow boiling in microchannels has received a lot of
attention for its promise in handling ultra-high heat fluxes
[1–12]. Practical applications of flow boiling in microchannels for
semiconductor device cooling, however, remain challenging due
to issues related to flow instabilities, complexity of two-phase flow
regimes, and excessive pressure drops associated with inlet constrictions. Ability to initiate and localize bubble nucleation in a
controlled manner may help partially mitigate some of these challenges but such ability remains elusive for most dielectric fluids
with very high wettability.
An alternative approach involves separating the liquid and vapor phases as an intrinsic part of the device design to help alleviate
some of the difficulties. An early attempt by David et al.[13]placed
a hydrophobic ‘‘breather’’ membrane on conventional microchannels to partially separate water vapor from liquid water and thereby mitigate flow instabilities. In this study, flow boiling in
microchannels with a 65lm thick porous PTFE membrane (with
220 nm pores) separating the liquid channels from the vapor channels was investigated. The device had 19 channels machined out of
copper with each channel being 19 mm long. The liquid microchannels were 130lm wide and 134lm deep, while the mating
vapor channel were 125lm wide and 132lm deep. Experiments
were performed with water as the test fluid. A thermofluid model
for the device (vented and non-vented) was also developed to predict the performance. The experiments showed that with vapor
venting, the quality of the two-phase flow in the microchannels
was lower. Experimental results showed that compared to a nonventing device, the venting device had 60% reduction in the pressure drop. Model predictions matched well with the experimental
results, except at higher mass flow rates (>600 kg/m
2
s) where the
model overpredicts the pressure drop. This was attributed to the
formation of churn-annular flow pattern observed at high mass
fluxes. Additionally, no two-phase flow instabilities were observed
in these experiments.
In these experiments, heat fluxes up to 70 W/cm
2
were used.
For low mass flux, the heat transfer coefficients measured for the
vented device was lower than that for the non-vented device. On
the other hand at higher mass fluxes, the heat transfer coefficients
for the vented device were approximately the same or slightly
higher than those measured for the non-vented device. Though
the vapor quality in the microchannels were lower in the case of
the vented device, the reduction in the fluid saturation temperature (due to reduction in the pressure drop, up to 4.4 C) and
changes in the flow regime resulted in the heat transfer rates being
similar to that obtained for the non-vented device. Due to lower
vapor quality, stratified flow was typically obtained which generally has lower heat transfer rates compared to annular or churnannular flows. The model predictions matched the experimental
results quite well, except at the lowest mass flux where they were
overpredicted. Though the experimental results obtained using the
0017-9310/$ - see front matter 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.022
⇑Corresponding author. Tel.: +1 310 825 9617.
E-mail address:gwarrier@ucla.edu(G.R. Warrier).
International Journal of Heat and Mass Transfer 68 (2014) 174–183
Contents lists available atScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
vented-device did not show much improvement in the heat transfer rates compared to the non-vented device, the results did show a
clear reduction in the pressure drop characteristics. The authors
hypothesized that the much higher heat transfer rates could be
realized with small liquid channel and very large vapor channels
separated by a porous membrane. Since evaporation does not take
place within (or on the surface of) the membrane, this approach relies on passive transport of vapor across the channels and then
across the membrane, which can be inefficient and incomplete.
The lack of comparable ‘‘hydrophobic’’ membranes, furthermore,
makes this approach difficult to apply for well wetting dielectric
fluids.
In another recent study, Narayanan et al.[14]proposed using a
‘‘nano-porous patch’’, where a thin porous membrane is used to
separate the vapor from a thin evaporating liquid layer of a controlled thickness. A gas jet was used to assist the removal of the vapor generated. Both modeling and experiments were conducted.
Modeling results showed that the heat transfer rates increase with
decrease in liquid film thickness and/or membrane thickness. Increase in the air flow rate also increases the heat transfer rate. With
liquid and membrane thicknesses of 1lm, heat fluxes of 700 W/
cm
2
were possible with FC-72 as the test liquid. Results for water
were consistently lower by factors of 2.2–3.
To validate the model results, experiments were conducted
with water as the test fluid. In these experiments, liquid film
thicknesses of 200 and 400lm were tested with a 60lm thick
membrane with 20 nm pores (porosity = 50%). Since the location
of the liquid–vapor interface is a key input parameter in the
model, and because it could not be measured in the experiments, two extreme cases were considered – one where the liquid completely fills the membrane (i.e., evaporation occurs at
the membrane outlet) and the other where the membrane is
completely filled with vapor (i.e., evaporation occurs at the
membrane inlet). Experiments show that compared to singlephase air jet cooling, evaporative cooling enhance heat transfer
by a factor of 2. For surface temperature above 70 C, the experimental results were in agreement with model predictions. At
lower wall temperature, the models underpredict the experimental results. This discrepancy was attributed to leakage through
the membrane.
The results from[14]show that as high heat flux removal can be
achieved with evaporative cooling. Moreover, with the porous
membrane completely filled with liquid, the resistance to mass
transfer is minimized resulting in the highest heat transfer rates.
The ‘‘horizontal’’ porous patch cooling device implemented in
[14], however, suffered from important limitations. The liquid
layer defined by the porous membrane has to be kept very thin
( 1lm) because the thermal energy needs to be first conducted
across this liquid layer. Such a thin liquid layer, however, presents
significant hydraulic resistance to the flow parallel to the membrane, leading to an excessive pressure drop. The horizontal configuration of the cooling device also limits the heat transfer surface
area to only that of the exposed heat source.
In this article, we propose a novel two-phase microchannel
cooling device with perforated side walls. This device helps circumvent the limitations of the previous cooling device by decoupling the main heat conduction paths (along the microchannel
walls) from the liquid supply paths (along dedicated ‘‘wide’’ liquid
channels) and by taking advantage of large extended surfaces
available in high-aspect-ratio microchannel walls for evaporation
heat transfer. We provide details of one practical implementation
of the device concept and thermofluid modeling results.
The cooling device proposed in this study involves a situation
very similar to that studied in[14], where the porous side wall is
completely filled with liquid and evaporation occurring at the vapor channel. Subsequently, mass transfer resistance is expected
to be negligible. The results from[13,14]show that the inclusion
of a porous membrane to separate the liquid and vapor flow paths
improves the heat transfer and pressure drop characteristics of
cooling devices that involve boiling or evaporation. An additional
benefit of separating the two phases is the mitigation of instabilities related to the pressure drop. Due to the differences in the
geometry and the operating conditions, it is not possible to compare the data obtained from these studies to the numerical simulation results obtained in this study. However, the modeling
methodology adopted and the performance of the devices in the
two studies discussed above shows that the concept of our high
heat flux cooling device and the thermofluid modeling approach
used are appropriate.
2. Cooling device concept
We propose a novel embedded evaporative cooling device
based on phase-separated microchannels with perforated (‘‘perspiring’’) side walls for the thermal management of ultra-high heat
flux devices. A schematic of this device is shown inFig. 1. In this
cooling device, each liquid channel is separated from the two adjacent vapor channels by side walls that contain arrays of micro perforations. Liquid flowing in the liquid channels is transported
through the perforations by capillary action and evaporate into
the vapor channels to efficiently remove heat. Conceptually, our
approach may loosely be viewed as a geometric adaptation of transpiration cooling systems investigated to tackle demanding thermal management challenges of hypersonic vehicles. Our unique
perforated channel walls of high aspect ratio offer dual benefits
by serving both as extended heat transfer surfaces (fins) and as micro-porous membranes for phase separation.
We select silicon as our channel wall material in part to take
advantage of its unique micromachinability, such as photo-electrochemical (PEC) etching and deep reactive ion etching (DRIE). This
also helps ensure that our ‘‘embedded’’ cooling solution is compatible with recent trends towards heterogeneous integration of highpower-density circuits built on SiC or diamond substrates with silicon-based CMOS circuits. Examples of the former may include
GaN-based microwave or RF power amplifiers and other power
switching/control devices, and high-performance microprocessors.
Major R&D efforts are currently under way to prepare GaN[15]devices on diamond substrates. The relatively small mismatch in
CTEs between Si and SiC and between Si and diamond helps reduce
thermal stress on the microchannels during thermal cycling.
The microfabrication processes considered for our cooling device can be scaled up to create multiple microchannel groups on
a single substrate to cover large areas. Alternatively, multiple cooling devices may be placed side to side to handle an array of chips
over a large area.
3. Coolant selection and its thermophysical properties
The peak junction temperature specified for wide-bandgap GaN
devices is typically over 200 C. Previous modeling and depth-resolved Raman measurements showed, however, that a large portion of the temperature drop occurs across the device buffer
layers and interfaces rather than the substrate [16]. With a target
top ‘‘substrate’’ temperature of 90 C, we choose as our working
fluid Fluorinert™ FC-72 with a boiling point of 56 C (at 1 atm)
and with material compatibility superior to hydrofluoroethers
(HFEs). Different Fluorinert fluids with higher boiling points (e.g.,
FC-77 at 97 C) may be used instead if higher condenser temperatures are desired.
Heterogeneous bubble nucleation generally occurs at microscopic cavities that entrap gas[17]and the wall superheat required
side walls for potential use as an embedded thermal management solution for high heat flux semiconductor devices. A dense array of perforated side walls separate alternating liquid and vapor microchannels,
allowing the vapor generated through evaporation of liquid supplied through micro-perforations to flow
only in the dedicated vapor channels. By separating the liquid and vapor flows, these ‘‘perspiring’’ side
walls enable us to circumvent flow instabilities and other challenges associated with conventional
two-phase microchannel cooling while at the same time effectively take advantage of the large extended
surface areas available in high-aspect-ratio microchannels. One implementation of our design is parametrically analyzed using finite element modeling, demonstrating the potential of our proposed device for
handling high heat flux electronic and optoelectronic semiconductor devices.
2013 Elsevier Ltd. All rights reserved.
1. Introduction
Continued miniaturization and increasing power ratings of electronic and optoelectronic devices have resulted in the need for active embedded cooling techniques with high heat flux removal
capability. Flow boiling in microchannels has received a lot of
attention for its promise in handling ultra-high heat fluxes
[1–12]. Practical applications of flow boiling in microchannels for
semiconductor device cooling, however, remain challenging due
to issues related to flow instabilities, complexity of two-phase flow
regimes, and excessive pressure drops associated with inlet constrictions. Ability to initiate and localize bubble nucleation in a
controlled manner may help partially mitigate some of these challenges but such ability remains elusive for most dielectric fluids
with very high wettability.
An alternative approach involves separating the liquid and vapor phases as an intrinsic part of the device design to help alleviate
some of the difficulties. An early attempt by David et al.[13]placed
a hydrophobic ‘‘breather’’ membrane on conventional microchannels to partially separate water vapor from liquid water and thereby mitigate flow instabilities. In this study, flow boiling in
microchannels with a 65lm thick porous PTFE membrane (with
220 nm pores) separating the liquid channels from the vapor channels was investigated. The device had 19 channels machined out of
copper with each channel being 19 mm long. The liquid microchannels were 130lm wide and 134lm deep, while the mating
vapor channel were 125lm wide and 132lm deep. Experiments
were performed with water as the test fluid. A thermofluid model
for the device (vented and non-vented) was also developed to predict the performance. The experiments showed that with vapor
venting, the quality of the two-phase flow in the microchannels
was lower. Experimental results showed that compared to a nonventing device, the venting device had 60% reduction in the pressure drop. Model predictions matched well with the experimental
results, except at higher mass flow rates (>600 kg/m
2
s) where the
model overpredicts the pressure drop. This was attributed to the
formation of churn-annular flow pattern observed at high mass
fluxes. Additionally, no two-phase flow instabilities were observed
in these experiments.
In these experiments, heat fluxes up to 70 W/cm
2
were used.
For low mass flux, the heat transfer coefficients measured for the
vented device was lower than that for the non-vented device. On
the other hand at higher mass fluxes, the heat transfer coefficients
for the vented device were approximately the same or slightly
higher than those measured for the non-vented device. Though
the vapor quality in the microchannels were lower in the case of
the vented device, the reduction in the fluid saturation temperature (due to reduction in the pressure drop, up to 4.4 C) and
changes in the flow regime resulted in the heat transfer rates being
similar to that obtained for the non-vented device. Due to lower
vapor quality, stratified flow was typically obtained which generally has lower heat transfer rates compared to annular or churnannular flows. The model predictions matched the experimental
results quite well, except at the lowest mass flux where they were
overpredicted. Though the experimental results obtained using the
0017-9310/$ - see front matter 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.022
⇑Corresponding author. Tel.: +1 310 825 9617.
E-mail address:gwarrier@ucla.edu(G.R. Warrier).
International Journal of Heat and Mass Transfer 68 (2014) 174–183
Contents lists available atScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
vented-device did not show much improvement in the heat transfer rates compared to the non-vented device, the results did show a
clear reduction in the pressure drop characteristics. The authors
hypothesized that the much higher heat transfer rates could be
realized with small liquid channel and very large vapor channels
separated by a porous membrane. Since evaporation does not take
place within (or on the surface of) the membrane, this approach relies on passive transport of vapor across the channels and then
across the membrane, which can be inefficient and incomplete.
The lack of comparable ‘‘hydrophobic’’ membranes, furthermore,
makes this approach difficult to apply for well wetting dielectric
fluids.
In another recent study, Narayanan et al.[14]proposed using a
‘‘nano-porous patch’’, where a thin porous membrane is used to
separate the vapor from a thin evaporating liquid layer of a controlled thickness. A gas jet was used to assist the removal of the vapor generated. Both modeling and experiments were conducted.
Modeling results showed that the heat transfer rates increase with
decrease in liquid film thickness and/or membrane thickness. Increase in the air flow rate also increases the heat transfer rate. With
liquid and membrane thicknesses of 1lm, heat fluxes of 700 W/
cm
2
were possible with FC-72 as the test liquid. Results for water
were consistently lower by factors of 2.2–3.
To validate the model results, experiments were conducted
with water as the test fluid. In these experiments, liquid film
thicknesses of 200 and 400lm were tested with a 60lm thick
membrane with 20 nm pores (porosity = 50%). Since the location
of the liquid–vapor interface is a key input parameter in the
model, and because it could not be measured in the experiments, two extreme cases were considered – one where the liquid completely fills the membrane (i.e., evaporation occurs at
the membrane outlet) and the other where the membrane is
completely filled with vapor (i.e., evaporation occurs at the
membrane inlet). Experiments show that compared to singlephase air jet cooling, evaporative cooling enhance heat transfer
by a factor of 2. For surface temperature above 70 C, the experimental results were in agreement with model predictions. At
lower wall temperature, the models underpredict the experimental results. This discrepancy was attributed to leakage through
the membrane.
The results from[14]show that as high heat flux removal can be
achieved with evaporative cooling. Moreover, with the porous
membrane completely filled with liquid, the resistance to mass
transfer is minimized resulting in the highest heat transfer rates.
The ‘‘horizontal’’ porous patch cooling device implemented in
[14], however, suffered from important limitations. The liquid
layer defined by the porous membrane has to be kept very thin
( 1lm) because the thermal energy needs to be first conducted
across this liquid layer. Such a thin liquid layer, however, presents
significant hydraulic resistance to the flow parallel to the membrane, leading to an excessive pressure drop. The horizontal configuration of the cooling device also limits the heat transfer surface
area to only that of the exposed heat source.
In this article, we propose a novel two-phase microchannel
cooling device with perforated side walls. This device helps circumvent the limitations of the previous cooling device by decoupling the main heat conduction paths (along the microchannel
walls) from the liquid supply paths (along dedicated ‘‘wide’’ liquid
channels) and by taking advantage of large extended surfaces
available in high-aspect-ratio microchannel walls for evaporation
heat transfer. We provide details of one practical implementation
of the device concept and thermofluid modeling results.
The cooling device proposed in this study involves a situation
very similar to that studied in[14], where the porous side wall is
completely filled with liquid and evaporation occurring at the vapor channel. Subsequently, mass transfer resistance is expected
to be negligible. The results from[13,14]show that the inclusion
of a porous membrane to separate the liquid and vapor flow paths
improves the heat transfer and pressure drop characteristics of
cooling devices that involve boiling or evaporation. An additional
benefit of separating the two phases is the mitigation of instabilities related to the pressure drop. Due to the differences in the
geometry and the operating conditions, it is not possible to compare the data obtained from these studies to the numerical simulation results obtained in this study. However, the modeling
methodology adopted and the performance of the devices in the
two studies discussed above shows that the concept of our high
heat flux cooling device and the thermofluid modeling approach
used are appropriate.
2. Cooling device concept
We propose a novel embedded evaporative cooling device
based on phase-separated microchannels with perforated (‘‘perspiring’’) side walls for the thermal management of ultra-high heat
flux devices. A schematic of this device is shown inFig. 1. In this
cooling device, each liquid channel is separated from the two adjacent vapor channels by side walls that contain arrays of micro perforations. Liquid flowing in the liquid channels is transported
through the perforations by capillary action and evaporate into
the vapor channels to efficiently remove heat. Conceptually, our
approach may loosely be viewed as a geometric adaptation of transpiration cooling systems investigated to tackle demanding thermal management challenges of hypersonic vehicles. Our unique
perforated channel walls of high aspect ratio offer dual benefits
by serving both as extended heat transfer surfaces (fins) and as micro-porous membranes for phase separation.
We select silicon as our channel wall material in part to take
advantage of its unique micromachinability, such as photo-electrochemical (PEC) etching and deep reactive ion etching (DRIE). This
also helps ensure that our ‘‘embedded’’ cooling solution is compatible with recent trends towards heterogeneous integration of highpower-density circuits built on SiC or diamond substrates with silicon-based CMOS circuits. Examples of the former may include
GaN-based microwave or RF power amplifiers and other power
switching/control devices, and high-performance microprocessors.
Major R&D efforts are currently under way to prepare GaN[15]devices on diamond substrates. The relatively small mismatch in
CTEs between Si and SiC and between Si and diamond helps reduce
thermal stress on the microchannels during thermal cycling.
The microfabrication processes considered for our cooling device can be scaled up to create multiple microchannel groups on
a single substrate to cover large areas. Alternatively, multiple cooling devices may be placed side to side to handle an array of chips
over a large area.
3. Coolant selection and its thermophysical properties
The peak junction temperature specified for wide-bandgap GaN
devices is typically over 200 C. Previous modeling and depth-resolved Raman measurements showed, however, that a large portion of the temperature drop occurs across the device buffer
layers and interfaces rather than the substrate [16]. With a target
top ‘‘substrate’’ temperature of 90 C, we choose as our working
fluid Fluorinert™ FC-72 with a boiling point of 56 C (at 1 atm)
and with material compatibility superior to hydrofluoroethers
(HFEs). Different Fluorinert fluids with higher boiling points (e.g.,
FC-77 at 97 C) may be used instead if higher condenser temperatures are desired.
Heterogeneous bubble nucleation generally occurs at microscopic cavities that entrap gas[17]and the wall superheat required
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