FIELD OF THE INVENTION
The present invention relates to micro-heat pipes for efficient chip/electronic cooling
systems and the like. More particularly, the present invention is directed to a micro-
channel based heat pipe for efficient and reliable cooling system for miniature size high
performance components used in integrated circuits in various applications such as in high
density semiconductor processors generating high heat fluxes and thus having high heat
dissipation demands to ensure safe and reliable performance of such components/device.
Importantly, the micro-heat pipe of the present invention is adapted to dissipate heat in
miniaturized pipes at reduced dry-out lengths. The micro-heat pipe of the present
invention is also simple to develop by following a fabrication process involving photo-
lithography. Advantageously the micro heat pipe is adapted such as to facilitate the
required capillary pumping for liquid flow without additional space requirement and with
less frictional resistance. The design parameters for fabrication of such micro-channel
based heat pipe involve the shape, size, surface profile of the heat pipe. Importantly, the
heat pipe of the invention specifically avoids the requirement of space consuming wicks
usually used in heat pipes such as those in laptops and most electronic devices. Also,
advantageously, the heat pipe of the invention is directed to improvement in the capillary
action compared to conventional heat pipes by achieving surface tension effect which
would be higher in wickless micro heat pipes of the invention for its higher surface area-to-
volume ratio or a lower form factor. Such wickless construction of heat pipes with reduced
form factor could favour more efficient transport of concentrated heat compared to
conventional wick based heat pipe. The micro-heat pipe according to the present invention
is thus directed to favour faster heat dissipation from micro-electronic components such as
chips or high density semiconductor processors, enabling further miniaturization of
electronic devices and ensuring reliable performance through effective cooling through
proper integration of the micro-heat pipe with said high performance electronic
components and is thus capable of wide industrial application as a thermal management
system for small form-factor requiring electronic devices such as in High density
semiconductor industry, hardware fabrication industry or in R & D set ups.

BACKGROUND ART
The existing art of cooling of delicate electronic components relies on cooling by rejecting
heat from transistors and high density semiconductors through heat pipes in ambience.
Small high performance components used in integrated circuits in various applications
such as in high density semiconductor processors cause high heat fluxes and have high
heat dissipation demands. A cooling system is considered to be efficient when it does not
increase heat dissipation at the expense of system reliability. The cooling systems are
always required to meet such crucial requirement. Moreover, an efficient and reliable
cooling system is of paramount importance in sensitive electronic circuits and devices
since high temperature induces mechanical, electrical and chemical changes, which impair
the performance reliability of the electronic system. This in turn imposes limitations on the
miniaturization of components and induces to maintain a high form factor. It is well
understood that a low form-factor is a critical design requirement in most electronic
devices.
It is well known that the existing techniques used for heat dissipation in laptops makes
use of a heat pipe with a wick in it. Heat pipes with wicks are used since they utilize the
latent heat of phase change with negligible scope for sensible heating. In the conventional
heat pipe, evaporator is attached to a heat source e.g. chip. The condenser is attached
with a heat sink e.g. fin to dissipate heat. The presence of wick material is needed for
liquid reserve and fluid transport. The heat enters the evaporator section and causes the
liquid in the wick to change its phase into vapor. Small temperature differences between
both ends of heat pipe cause a small pressure difference and allow the vapor to flow
toward the condenser area where the heat is released. The vapor is condensed to liquid
and flow back to evaporator due to capillary action. This technique is very effective since
the thermal conductivity along the length is high. However, the primary disadvantage is
the presence of the wick in the pipe as it leads to a high form factor, which reduce the
scope of miniaturization. A large volume to surface area ratio leads to inefficient control
over interfacia! transport. Moreover, the liquid encounters considerable frictional resistance
while flowing through the dense wicks and these types of wick based heat pipes have a
tendency of drying-out at even moderate heat fluxes.
A number of attempts have been made in the existing art to improve the efficiency of heat
transport mechanism for micro heat pipe design and applications.

US 6595270 entitled 'Using micro heat pipes as heat exchanger unit for notebook
applications' is directed to a system including a plurality of micro heat pipes MHPs) used to
transfer heat from a die in closed contact with the die through a die attach block to absorb
heat generated by the die and transport the same to another block at other end of the
pipes where the heat is released to ambience .The evaporation and the condensations
ends of said heat pipes are metallic while having an intermediate portion with wicks and a
fluid favoring transport of heat from the source (evaporation end) to sink (at condensation
end), said condensation end being subjected to air flow. Further, the invention involves
heat pipes that are circular sections of dimension upto 1 mm. Thus the prior patent clearly
indicates a macro scale fabrication and is no way related to the wick free micro heat pipe
device with low form factor and high heat dissipation efficiency which is the requirement
of the art.
US 4,675,783 is titled 'Heat pipe heat sink for Semiconductor devices' and is basically
directed to a heat pipe heat sink for cooling of semiconductor device characterized by
arranging heat pipes in zig-zag form toward the direction of air flow around the fin
sections in the heat sink. The semiconductor device is mounted on a block at the heat-in
end housing plurality of heat pipes and the arrangement involves a large number of
radiating fins crossing over the heat pipes and fitted to the protruding portions of heat
pipes at the heat-out section of heat pipes. The said prior art also does not target the
desired reduction in form factor with improved efficiency of heat dissipation.
US 20060113662A1 is on a micro heat pipe with wedge capillaries and teaches the
provision of a heat pipe comprising an elongated hollow housing having a condenser end
and an evaporator end, wherein a corrugated wick is disposed within the housing. The
wick comprises a plurality of wedge-shaped capillaries extending from the condenser end
to the evaporator end. A liquid is set in fluid communication with the corrugated wick. This
prior art document thus again suggest the requirement of wick in heat pipes which in turn
is known to affect the requirement of reduced form factor in micro heat pipes.
US 20050026015 is directed to micro heat pipe embedded bipolar plate for fuel cell stacks
and provides for a system and method for distributing heat in a fuel cell stack through
bipolar interconnection plates having one or more heat pipes disposed within the plate.
Thus the bipolar interconnection plate comprising heat pipe disposed embedded in the
planar support member for receiving and distributing heat in the fuel cell stack wherein the

heat pipe contains a working fluid that comprises liquid metal and the micro heat pipe do
not significantly increase the size or weight of the fuel cell stack and provides greater
temperature uniformity among the fuel cell units within the stack thus improving power
generation efficiency of the stack. This prior art thus clearly suggests the use of liquid
metal as working fluid and do not involve phase change. Further the disclosure makes no
specific mention about the micro scale dimension for the pipe/channel fabrication.
US 4921041 entitled 'Structure of a heat pipe' speaks about structure of a elongated heat
pipe in the form of a closed-loop in which the working fluid which is preferably a bi-phase
non-condensate type fluid circulates at high speed under its own vapour pressure, so as to
repeat vaporization and condensation and thus carrying out a heat transfer. Check
valve(s) selective disposed in said closed loop heat pipe that propels and amplifies forces
generated by the heat carrying fluid and its vapour to circulate in the stream direction.
This invention is thus directed to a loop type heat pipe that is neither having micro section
nor suggests benefit of lower form factor without use of any wick or valve in said micro
heat pipe.
Thus it is apparent from the above general state of the art that the presently available
heat pipes to cool heat generating devices and components including high density
semiconductor processor/chip and the like in electronic hardware devices donot specifically
direct to provide an efficient heat dissipation system by providing means for fabrication of
micro-heat pipes having optimized design parameters directed to achieve favored low
form-factor along with optimized cooling performance thereby making possible further
miniaturization of electronic components/devices while ensuring the system reliability in
the long run.
There has therefore been a persistent need in the related art to develop cooling systems
involving heat pipes adapted for efficient cooling of the high heat flux generating
components in electronic appliances adapted to favor the much required low form-factor
along with optimized cooling performance thereby making possible further miniaturization
of electronic components/devices while ensuring the system reliability in the long run.

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide a heat pipe /cooling system
involving micro heat pipe for electronic cooling which would on one hand meet the
required need for low form-factor and on the other hand capable of efficient cooling of high
heat flux generating semiconductor processors/chip or the like components in electronic
devices.
Another object of the present invention is directed to developing a micro-heat pipe based
cooling system without the use of wick in said pipe for fluid transporting as heat carrying
media.
A still further important object of the present invention is directed to optimize the design
parameters for fabrication of said wick free micro-heat pipe for electronic cooling to
achieve a lower volume to surface area ratio and thus a lower form factor, favoring further-
miniaturization of the micro-heat pipe based cooling system as well as electronic systems.
A still further object of the present invention directed to micro-heat pipe for effective
electronic cooling wherein a wick-free construction would favour heat to be transported
over a much longer pathway than the traditional heat pipes involving wicks.
A still further object of the present invention directed to micro-heat pipe for effective
electronic cooling wherein the heat pipe free of any wicks and thus with possible reduced
form factor would be capable of heat dissipation in miniaturized pipes at reduced dry-out
lengths.
A still further object of the present invention directed to micro-heat pipe without wicks for
effective electronic cooling involving a simple and effective fabrication process which
would favour achieving desired design parameters in the geometry of the micro channel
obtained employing simple photo-lithography.
A still further object of the present invention directed to micro-heat pipe for effective
electronic cooling involving enhanced capillary pumping action of the working fluid
compared to conventional heat pipes.

A still further object of the present invention directed to micro-heat pipe for effective
electronic cooling wherein high heat dissipation rate is ensured by reducing the form factor
by eliminating the need for a wick in the heat pipe and modulation of dynamic contact
angle at liquid/solid interface while heat is transferred.
A still further object of the present invention is directed to micro-heat pipe for effective
electronic cooling involving selective working fluid (coolant) based on properties
comprising i) completely wetting of micro channel and (ii) high latent heat of vaporization.
A still further object of the present invention directed to micro-heat pipe for effective
electronic cooling involving selective surface profile of micro channel adapted for
achieving desired slip effect or nanobuble formation mechanism at interface favouring
heat dissipation rate.
According to yet another object of the present invention the same is directed to micro-
heat pipe for effective electronic cooling and its simple and effective fabrication involving
an interfacial heat transport involving a selective combination of thin film evaporation ,slip
effect and hydrophobic interaction.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a micro heat
pipe with reduced form factor for reliable cooling in miniaturized scale particularly of chips
and integrated circuit devices and like miniaturized component electronic cooling
comprising :
micro channel based heat pipe adapted for thin film evaporation, slip effect and
hydrophobic interactions to dissipate heat.
Importantly, in the above micro heat pipe the dissipation rate for a given spatial constraint
is selectively achieved based on the shape and size of the heat pipe and the surface
profile/roughness as a controlling parameter for the hydrophobic interactions and nano
bubble mechanism.

In particular, the above micro heat pipe of the invention comprises evaporation section,
adiabatic section and condenser section with the evaporation section adapted to vaporize
the coolant liquid which then passes through the adiabatic to the condenser section and in
the condenser section the vapor releases its latent heat and gets liquefied with the liquid
flowing towards the evaporative section due to capillary pumping generated by the
change in the radius of curvature of the liquid meniscus to thereby achieve the uniform
temperature distribution.
Advantageously, the micro heat pipe of the invention comprises higher surface tension
effect due to higher surface area to volume ratio favouring enhanced capillary action.
Importantly, also the micro channel comprise rough channel walls made of a hydrophobic
material which favour roughness induced phase-transformation in narrow confinement
thereby favouring formation of nano-bubbles for transporting of liquid and concentrated
heat.
In accordance with a preferred aspect of the invention , the micro heat pipe is selectively
provided with design parameters based on end application/use selected from (a) shape
and size of the micro-channel heat pipe;(b) surface profile of the heat pipe; and (c ) the
selection of the working fluid / liquid (coolant) is based on i) completely wetting (ii) high
latent heat of vaporization preferably pentane, water and (d) material of the micro-
channel preferably silicon wafer.
In accordance with a preferred aspect of the invention , the micro heat pipe comprises of
a triangular cross-section wherein the apex angle of the triangle and other linear
specifications are selectively provided depending upon the given spatial constraints.
In the above disclosed micro heat pipe of a triangular cross-section the same comprises of
length in the range of few milimeters to centimeters preferably about 20 mm, width of the
order of microns and preferably about 120 μm with an apex angle of the triangle cross
section of the micro channel determined by the preferential slip planes of the substrate
materials such as preferably about 72° Substrates.

In accordance with another aspect of the present invention there is provided a method or
producing the micro heat pipe comprising fabrication of the micro channel by photo-
lithography after computer simulation and identifying desired design parameters.
In accordance with a preferred aspect in the above method the micro channel is obtained
of silicon wafers with surface selectively etched for defining desired surface roughness.
In accordance with a more preferred aspect of the invention the above method involving
photo-lithography comprises:
i) subjecting the silicon wafer to oxidation;
ii) covering the oxidized wafer with photoresist;
iii) exposing the wafer to UV light through a photomask ultraviolet radiation;
iv) dissolving the unexposed photresist in developer solution;
v) subjecting the unprotected oxide to selective etching and removing the rest of
the photoresist ; and
vi) finally doping the wafer.
The method of producing the micro heat pipe further comprises testing for performance
evaluation of the silicon substrates prior to assembling the micro heat pipe.
In accordance with yet further aspect the method of producing the micro heat pipe
comprises optimizing the design specifications of the heat pipe by selectively providing:
i) shape and size to minimize the shape/form factor and high surface to volume ratio for
the micro heat pipe;and
ii) selectively providing the surface profile preferably the grooved profile by specifying the
surface roughness preferably in the range of 4 nm to 25 nm (r.m.s. value) and surface
material chemistry using silicon wafers.
In accordance with a more preferred aspect of the invention the above method of producing
the micro heat pipe comprises said step of testing the silicon wafers for suitability as a heat
pipe comprising the steps of :
testing the axial temperature profiles on the microgrooved Silicon substrate bonded on a
thin aluminum plate ensuring no vapor loss from cell/set up as a function of position and

input power, with and without the working liquid (coolant), using thermocouples made of
Copper-Constantan, each individually calibrated with an accuracy of ±0.1°C.
In the above test method the thermocouples are routed through a channel selector to a
micro-voltmeter, which measures the emf developed very accurately and a stand and base
plate arrangement is provided to keep the substrate inclined at any tilt angle.
Moreover, for the purposes of testing the channels of the heat pipe of the invention, the
heat spreading capabilities of the grooved surface utilizing change of phase heat transfer is
noted to chart the propagation of the dry out point along the length of the channel, with
changes in inclination of the substrate.
Importantly also, to evaluate the heat spreading enhancement, temperature profiles are
plotted against the axial distance from the heater, more particularly, T - Tref is plotted,
where T^gf is the prevailing ambient temperature, the driving force for heat transfer
against each positions obtained and the effect of any small perturbations in ambient
temperature is minimized.
Moreover, to establish substantially higher cooling performance the grooved wafer profile
is selectively provided during fabrication of the heat pipes of the invention.
The details of the invention, its objects and advantages are explained hereunder in greater
detail in relation to non-limiting exemplary illustrations as per the following accompanying
figures:
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the schematic illustration of conventional Heat Pipe using wick.
Figure 2: is the schematic illustration of the basic structure of a preferred embodiment of
the micro-heat pipe according to the present invention having selective sectional
geometry to favor low form factor and ensuring desired flow of working fluid.

Figure 3: is the illustration of the process flow chart for the electronic cooling system using
micro-heat pipe of the invention.
Figure 4: is the schematic illustration of the sequential stages of Photo-lithographic etching
process for fabrication of silicon wafer based micro-channel of desired configuration for use
as micro-heat pipe.
Figure 5: is the schematic illustration of the working mechanism of fluid flow based heat
transport in Micro-Heat Pipe according to the present invention.
Figure 6: is the schematic illustration of the Testing Set-up for performance evaluation of
the lab scale experimentation for electronic cooling system using micro-heat pipe
according to the invention.
Figure 7: is the graphical plot of T-Tref against the distance from the heater to establish the
cooling performance/efficiency of the micro-heat pipe based cooling system test set up
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention thus relates to micro heat pipes with reduced form factor and free of
wicks adapted to maximize heat dissipation from electronic components including the
chip/semiconductor processor for efficient cooling and ensuring reliable part performance.
Advantageously, the heat pipe according to the present invention makes use of thin film
evaporation, slip effect and hydrophobic interactions to dissipate heat in miniaturized pipes
at reduced dry-out lengths. The need for a wick is completely eliminated in the heat pipe
of the present invention and the heat transfer process is controlled solely by dynamic
contact angle modulation at the fluid/pipe-wall interface. Thus the micro-channel/heat
pipes of the invention is adapted to having low form factor and thus favoring further
miniaturization of components/systems. The surface roughness in the micro heat pipe,
which is an outcome of the fabrication process using etching of silicon wafers by photo-
lithography, can be used as a design parameter based on hydrophobic interaction coupled
with nanobubble mechanism. Nanobubble formation in the solid-liquid interface reduces
the flow friction, augmenting the fluid-mediated dissipation of generated heat. Further, the
11

formation of nanobubble and consequent flow friction can be adjusted to favorable
magnitude by manipulating surface roughness parameters and surface chemistry of
fabricated micro-heat channel/pipes. In addition, micro grooves are provided on the heat
pipes to enhance the capillary pumping action and surfactants are added to augment the
spreading on the surface.
The constructional and design parameters for the micro-heat pipe according to the
invention and the working principle of the electronic cooling system making use of said
micro-heat pipes to achieve desired heat dissipation and cooling efficiency are described in
greater details with reference to the accompanying non limiting illustrative drawings.
Reference is first invited to accompanying Figure 1, which schematically illustrates the
construction and working of the conventional Heat Pipe using wick. As it can be seen, the
wicks present in such heat pipes carry/transport the condensed liquid at the cold end
towards the hot end for evaporation and thereby removing the heat by way of latent heat
of evaporation through liquid to vapour phase-change mechanism. The vapor moves inside
the heat pipe in the free space due to the pressure gradient between the evaporator (high
pressure) and condenser (low pressure) end. However, the liquid encounters considerable
frictional resistance while flowing through the dense wicks placed in longitudinal grooves
running parallel to axis of the heat pipe directed from condenser (heat-out) end to the
evaporator (heat-in) end and these types of wick based heat pipes have a tendency of
drying-out at moderate heat fluxes. Moreover, the incorporation of wicks renders these
heat pipes unsuitable for application in situations with space constraints (e.g. for cooling of
electronic circuitry) and is having relatively higher magnitude of volume to surface area
ratio resulting in higher form factor.
The presence of wick material is needed for liquid reserve and fluid transportation in
conventional heat pipes. The heat enters the evaporator section and causes the liquid in
the wick to change its phase into vapor. Small temperature differences between both ends
of heat pipe cause a small pressure difference and allow the vapor to flow toward the
condenser area where the heat is released. The vapor is condensed to liquid and flow back
to evaporator according to capillary action aided by wick embedded grooves. This
technique is very effective since the thermal conductivity along the length is high.
However, the primary disadvantage is the presence of the wick in the pipe as it leads to a
high form factor with reduced the scope of miniaturization. A large volume to surface area
ratio leads to inefficient control over interfacial transport. Moreover, the liquid encounters
12

considerable frictional resistance while flowing through the dense wicks and these types of
wicked heat pipes have a tendency of drying-out at even moderate heat fluxes. The use of
wickless, micro-grooved heat pipes are thus the only choice for such specialized electronic
cooling applications facilitating possibility of further miniaturization as attempted in the
present invention.
Reference is now invited to the accompanying Figure 2, that illustrates the basic structure
of a preferred embodiment of the micro heat pipe for electronic cooling achieved based on
optimization of design parameters, according to the present invention. The Figure 2,
clearly illustrates the advantageous features of wickless micro-heat pipe of the invention.
Because of the selective triangular section geometry according to the preferred
configuration wherein the corners provide the necessary capillary pumping for liquid flow
without additional space requirement and with less frictional resistance. Grooved surfaces
are fabricated to form desired configuration of the micro heat pipe, by photolithography
and etching of the silicon wafer, discussed at a later stage. While the heat pipe is
integrated to any heat generating component e.g. chip/semiconductor device, the heat
enters the evaporator section and causes the liquid inside the micro-channel to change its
phase into vapor.
Accompanying Figure 2 shows a triangular configuration of micro channel for use as wick
free micro heat pipe in an electronic cooling system. The typical dimensions of such micro
channel tested for optimal cooling performance are in the order of width:100-120μm,
length:20mm, apex angle of triangular cross section:72°. This angle and other linear
dimensions are design parameters dependent upon the spatial constraint. The V-shaped
microgrooves are etched on the silicon wafer using chemical machining method, and a
computer controlled mechanical surface profilometer is used to verify their apex angles
and dimensions, as stated.
Reference is next invited to accompanying Figure 3 that illustrates the flow process chart
for the development of a micro heat pipe based electronic cooling system ensuring its
reliable performance. It is apparent from the flow of events, that the first activity is the
design of micro heat pipe using CAD. The critical design parameters are first selected and
various alternatives are then tested for cooling performance virtually using computer
simulation. The various parameters are then short listed to limit the complexity of
fabrication procedure for the micro heat pipe. The lab scale fabrication of micro channel is
13

carried out using photolithography. The cooling performance is then tested for optimal
performance. In case of unsatisfactory/unreliable cooling performance with respect to
defined critical performance parameters, the critical design parameters are revalidated
through simulation for optimization of cooling performance/heat dissipation rate as objective
function.
The basic steps involved in the process of developing the micro-heat pipe based cooling
system are:
The critical design parameters under consideration for developing various design alternatives
to arrive at the optimal solution, comprise the following variables:
a. Shape and size of the micro-channel heat pipe.
b. Surface profile of micro-channel.
c. Choice of working fluid and material of micro-channel, depending on the demand of
performance and reliability of cooling system. Typically, the working fluid used is
pentane or water.
The present invention thus involves the following design standards for a optimal solution of
the electronic cooling system using micro-heat pipe:
-The objective function for the basic design directed to optimize the heat dissipation rate,
under given spatial constraints and channel surface material and roughness;
-Determine size and shape selectively to minimize the shape/form factor and high surface
to volume ratio for the micro heat pipe.
-process of fabrication of micro channel/heat pipe comprises etching of silicon wafers by
photolithography;
-characterizing the surface profile by specifying the surface roughness and surface
material chemistry;
-control on the selective use of frequency of the spectral light and the light exposure
during photo lithography used as determinant of roughness parameter, when typical
material used is silicon wafers, the order of roughness parameter maintained in the range
of 4 nm to 25 nm(r.m.s. value).
14

Reference is now invited to the accompanying Figure 4 which illustrates schematically the
different stages of the fabrication of the micro channel according to the invention for
effective electronic cooling system, using etching of silicon wafers by photolithography and
using selective frequency of and controlled exposure to incident light energy in said
process of etching. As clearly apparent from the accompanying figure showing sequential
stages of fabrication, silicon wafer is first oxidized to form a layer of silicondioxide at the
top layer, then it is covered with a photo resist material layer of preferred thickness. In
the next step the wafer is exposed to UV light through a photomask providing means for
directed/obstructed incidence of light energy on the photoresist surface. Thus a portion of
photoresist unexposed to UV light is dissolved in a developer solution. The unprotected
portion of the silicon dioxide layer then etched away using hydrofluoric acid, to form the
micro channel of selective size and shape. The remaining photoresist is then removed to
make the silicon dioxide wafer ready for use as micro channel for cooling system. For the
present cooling system, chemical machining method is successfully used to fabricate V-
shaped axial microgrooves of 100 micron width and 2 cm length with a spacing of 200
microns and an apex angle of 72 ° on a silicon substrate.
The constructional feature of micro channel that contributes towards thin film evaporation,
slip effects, and hydrophobic interactions is the small micro scale dimensions of the
microchannel/grooves and their surface roughness which is not really a constructional
feature but is a natural outcome of the fabrication processes; for micro-scale these are
critically important since the roughness elements may be of comparable dimensions with
the system length scales. Surface roughness is a common outcome of any manufacturing
process, which acts as a flow inhibitor in the traditional scale. However, in the micro scale
fabrication, it may promote the inception of nanobubbles adhering to the microchannel
walls, facilitated by an additional chemical treatment requirement making the
microchannel substrate to be hydrophobic.
The working mechanism of the micro heat pipe according to the present invention is
illustrated in accompanying Figure 5. The micro heat pipe utilized in the electronic cooling
system according to the present invention and as illustrated in Figure 5, is divided into
three sections. Heat is input through the evaporator section at one end of the pipe
vaporizing the coolant liquid. The vapor then passes adiabatically through the micro
section channel to the condenser portion at the other end. In the condenser section, the
vapor releases its latent heat of condensation to the surroundings and gets liquefied. The
15

liquid flows towards the evaporator section by capillary pumping, due to pressure gradient
generated by the change in the radius of the liquid meniscus.
The flow of liquid towards the hot spot is sustained by a pressure gradient present in the
liquid. The liquid meniscus at the corner near the hot region will be depressed more
(towards the corner) compared to that at the cold end. This difference in shape will give
rise to a reduced pressure at the hot end compared to the cold end and cause a pressure
jump at the liquid-vapor interface according to the Young-Laplace equation ( curvature
effect), and this difference in pressure will drive the liquid flow. Surface tension of working
fluid is thus an important characteristic property affecting fluid flow and heat transport, as
this shape dependent pressure jump is a function of the curvature and the surface tension
of the liquid. The liquid must also wet the solid i.e. the wall of the micro heat pipe in
contact with the fluid, completely to achieve proper spreading and efficient cooling. The
vapor is condensed to liquid and flow back to evaporator according to capillary action. This
capillary action is enhanced compared to conventional heat pipe since surface tension
effect is higher in micro heat pipe for higher surface area-to-volume ratio. This causes
more efficient transport of concentrated heat compared to conventional heat pipe.
The heat dissipation and thus the cooling technique employed in the electronic cooling
system according to the invention using micro heat pipe comprises, thin film evaporation,
slip effect and efficient transport by advantageously utilizing hydrophobic interactions
which are further discussed hereunder.
Thin Film Evaporation: relating to latent heat loss by evaporation of thin liquid layer at
interface at heat-in end.
Slip Effect: relating to heat dissipation due to relative slippage of liquid layer during fluid
transport.
Hydrophobic Interaction: relating to rough micro channel substrates made of hydrophobic
materials that induces formation of tiny/nano bubbles adhering to the walls in tiny channels.
This incipient vapor layer acts as an effective smoothening blanket, by disallowing the liquid
on the top of it to be directly exposed to the rough surface asperities. In such cases, the
liquid is not likely to feel the presence of the wall directly and may smoothly sail over the
intervening vapor layer shield. Thus, instead of 'sticking' to a rough channel surface, the
16

liquid may effectively 'slip' on the same. This results in a consequent augmentation in heat
transfer and dissipation rates. This effect cannot be favourably exploited in traditional heat
pipes, since larger-sized confinements cannot trigger nanobubble formation.
The above advantages in the heat pipe of the invention could be possible only because of
the complex thermo-physical interactions between narrow confinements with rough
hydrophobic surfaces. The consequent flow physics automatically promotes slip effects on
the walls, which is an effect that favour efficient heat transport in the micro-domain.
Further, the slip effect augments the local heat transfer rate and leads to a thin film
evaporation. These advantages cannot be achieved with traditional heat pipes because of
larger dimensions of the cross section of the flow pathways. The present invention exploits
favorable flow physics through miniaturization, which makes it technically superior to
achieve transport rates much greater than the traditional ones. Additionally, the wick-free
design of micro heat pipe allows the heat to be transported over a much longer pathway
than the traditional heat pipes aided by the flow physics as described above, so that heat
may be dissipated from intricate interiors of a device to the surroundings.
Reference is now invited to the accompanying Figure 6 that schematically illustrates the
Testing Set-up for performance evaluation of the lab scale experimentation for electronic
cooling system using micro-heat pipe according to the invention. To evaluate the cooling
performance and heat dissipation rate for any configuaration of micro heat pipe/channel of
given geometry/design parameters has been fabricated, temperature distribution has been
studied and heat flux through channel wall has been measured experimentally using the
set-up as illustrated in said Figure 6.
Utilizing the above testing set-up experiments were next carried out in a specially
designed cell to study the onset and propagation of dry out point for fluid flow and
evaporation on a micro grooved silicon surface. Chemical machining method was
successfully used to fabricate V-shaped axial microgrooves of 100 micron width and 2 cm
length with a spacing of 200 microns and an apex angle of 72 ° on a silicon substrate. The
substrate with pentane as the coolant liquid was isolated from the surroundings inside the
experimental chamber. Controlled heat was supplied to the top of the substrate and axial
temperature distribution was accurately measured as a function of heat input rate and
inclination of the substrate to the horizontal. The V-shaped microgrooves were etched on
the silicon wafer using chemical machining method, and a computer controlled mechanical
17

surface profilometer wais used to verify their apex angles and dimensions. The cell was
equipped with a standard heat source, providing stable measurable power to the hot end,
mimicking the hot spots on electronic chips. The axial temperature profiles on • the
microgrooved Silicon substrate are measured, with and without the working liquid
(coolant), using thermocouples, each individually calibrated with an accuracy of ±0.1°C.
The results so obtained are compared with the temperature profiles corresponding to a
non-grooved substrate.
The silicon substrate of size (0.8 cm x 2.8 cm), was bonded on a thin aluminum plate, so
that the stress due to the tightening of the bolts, was absorbed by the aluminum plate
rather than the brittle and already weakened (due to the etching of the grooves) silicon
substrate. Such an arrangement ensured that there was absolutely no vapor loss from the
experimental chamber. A resistance heater and small thermocouples used to measure
axial temperature as a function of position and input power, are attached to the back
surface of the substrate. The resistance heater was held in place by a teflon base that kept
the heater pressed against the substrate assembly. This minimized heat dissipation into
the surrounding from the heater, by transferring all the heat into the system. The
substrate was enclosed in an aluminum chamber to completely isolate it from the
environment. The top part of the cell has a glass window through which the evaporating
meniscus can be viewed. The substrate is held between two of the three aluminum plates
using 0-rings and symmetric screws along the periphery of the plate.
The bottom aluminum plate was provided with a rectangular opening, through which the
connections for the thermocouple and heater are made. A liquid entry port was made in
the middle aluminum plate, near the centre, which allows charging of the working fluid
(coolant), at the beginning of the wet runs. A specially fabricated stand and base plate
arrangement was used to keep the substrate inclined at any tilt angle. The constant
controlled power supply is obtained from an accurate stabilized DC regulated power supply
unit. The thermocouples for temperature measurement were fabricated using Copper-
Constantan and were each individually calibrated. The thermocouples were routed through
a channel selector to a micro-voltmeter, which measures the emf developed very
accurately. Experiments were conducted to evaluate the heat spreading capabilities of the
grooved surface utilizing change of phase heat transfer and to chart the propagation of the
dry out point along the length of the channel, with changes in inclination of the substrate.
18

To evaluate the heat spreading enhancement, temperature profiles were plotted against
the axial distance from the heater.
Reference is now invited to the accompanying Figure 7 which illustrates graphically the
cooling performance of the micro channel based electronic cooling system according to the
present invention through experimental set-up and test data as stated above. By plotting
T - Tref , where Tref was the prevailing ambient temperature, the driving force for heat
transfer against position, the effect of any small perturbations in ambient temperature is
minimised. The relative cooling effect, between 5-7 °C for the low heat inputs was studied
of the grooved wafer compared to the non-grooved wafer that established substantial
cooling achieved through use of the grooved wafer.
It is thus possible by way of the present invention to provide a micro-heat pipe/channel
based electronic cooling system adapted to cool high-density semiconductor processor or
chip in electronic/hardware devices so as to favor efficient heat transport by preferred low-
form-factor design configuration and fabrication of the micro channel from silicon wafer by
etching using photolithography. The micro-heat pipe/ channel based electronic cooling
system of the present invention would favor efficient cooling in a wick free channel
configuration on one hand and on the other hand enable rapid heat dissipation and
resultant efficient cooling performance in a low form-factor or high surface to volume
configuration of the micro-channel section, enabling heat to be transported over a much
longer pathway than the traditional heat pipes aided by the flow physics that facilitate
dissipation of heat from intricate interiors of a device to the surroundings. The micro heat
pipe based electronic cooling system as of the present invention further avoids tendency of
drying-out at even moderate heat fluxes, and is capable of meeting the need for reducing
the size of the thermal management system along with an increased efficacy in heat
dissipation. The invention thus solves the problem of thermal packaging and overheating
at miniaturized scale with heat pipes free of wicks and thus reduced form factor and
thereby ensuring wide industrial application in a number of electronic and hardware
devices involving heat generating miniature components such as the chip or high
performance semiconductor processor.

WE CLAIM:
1. A micro heat pipe with reduced form factor for reliable cooling in miniaturized scale
particularly of chips and integrated circuit devices and like miniaturized component
electronic cooling comprising :
micro channel based heat pipe adapted for thin film evaporation, slip effect and
hydrophobic interactions to dissipate heat.
2. A micro heat pipe as claimed in claim 1 wherein the dissipation rate for a given spatial
constraint is selectively achieved based on the shape and size of the heat pipe and the
surface profile/roughness as a controlling parameter for the hydrophobic interactions and
nano bubble mechanism.
3. A micro heat pipe as claimed in anyone of claims 1 or 2 comprising evaporation section,
adiabatic section and condenser section with the evaporation section adapted to vaporize
the coolant liquid which then passes through the adiabatic to the condenser section and in
the condenser section the vapor releases its latent heat and gets liquefied with the liquid
flowing towards the evaporative section due to capillary pumping generated by the
change in the radius of curvature of the liquid meniscus to thereby achieve the uniform
temperature distribution.
4. A micro heat pipe as claimed in anyone of claims 1 to 3 comprising higher surface
tension effect due to higher surface area to volume ratio favouring enhanced capillary
action.
5. A micro heat pipe as claimed in anyone of claims 1 to 4 wherein the micro channel
comprise rough channel walls made of a hydrophobic material which favour roughness
induced phase-transformation in narrow confinement thereby favouring formation of nano-
bubbles for transporting of liquid and concentrated heat.

6. A micro heat pipe as claimed in anyone of claims 1 to 5 with selectively provided design
parameters based on end application/use selected from (a) shape and size of the micro-
channel heat pipe;(b) surface profile of the heat pipe; and (c ) the selection of the working
fluid / liquid (coolant) is based on i) completely wetting (ii) high latent heat of vaporization
preferably pentane, water and (d) material of the micro-channel preferably silicon wafer.
7. A micro heat pipe as claimed in anyone of claims 1 to 6 comprising of a triangular cross-
section wherein the apex angle of the triangle and other linear specifications are
selectively provided depending upon the given spatial constraints.
8. A micro heat pipe as claimed in claim 7 of a triangular cross-section comprises of
length in the range of few millimeters to centimeters, width of the order of microns, with
an apex angle of the triangle cross section of the micro channel determined by the
preferential slip planes of the substrate materials, such as 72° for Silicon substrates.
9. A method of producing the micro heat pipe as claimed in anyone of claims 1 to 7
comprising fabrication of the micro channel by photo-lithography after computer
simulation and identifying desired design parameters.
10. A method as claimed in claim 9 wherein the micro channel is obtained of silicon wafers
with surface selectively etched for defining desired surface roughness.
11. A method as claimed in claim 10 wherein photo-lithography comprises:
i) subjecting the silicon wafer to oxidation;
ii) covering the oxidized wafer with photoresist;
ill) exposing the wafer to UV light through a photomask ultraviolet radiation;
iv) dissolving the unexposed photresist in developer solution;
v) subjecting the unprotected oxide to selective etching and removing the rest of
the photoresist ; and
vi) finally doping the wafer.

12. A method of producing the micro heat pipe as claimed in anyone of claims 9 to 11
comprising testing for performance evaluation of the silicon substrates prior to assembling
the micro heat pipe.
13. A method of producing the micro heat pipe as claimed in anyone of claims 9 to 12
comprising optimizing the design specifications of the heat pipe by selectively providing:
i) shape and size to minimize the shape/form factor and high surface to volume ratio for
the micro heat pipe;and
ii) selectively providing the surface profile preferably the grooved profile by specifying the
surface roughness preferably in the range of 4 nm to 25 nm (r.m.s. value) and surface
material chemistry using silicon wafers.
14. A method of producing the micro heat pipe as claimed in anyone of claims 9 to 13
comprising said step of testing the silicon wafers for suitability as a heat pipe comprising the
steps of :
testing the axial temperature profiles on the microgrooved Silicon substrate bonded on a
thin aluminum plate ensuring no vapor loss from cell/set up as a function of position and
input power, with and without the working liquid (coolant), using thermocouples made of
Copper-Constantan, each individually calibrated with an accuracy of ±0.1°C.
15. A method of producing the micro heat pipe as claimed in claim 14 wherein the
thermocouples are routed through a channel selector to a micro-voltmeter, which
measures the emf developed very accurately and a stand and base plate arrangement is
provided to keep the substrate inclined at any tilt angle.
16. A method of producing the micro heat pipe as claimed in anyone of claims 11 to 15
wherein the heat spreading capabilities of the grooved surface utilizing change of phase
heat transfer is noted to chart the propagation of the dry out point along the length of the
channel, with changes in inclination of the substrate.
17 A method of producing the micro heat pipe as claimed in anyone of claims 11 to 16
wherein to evaluate the heat spreading enhancement, temperature profiles are plotted

against the axial distance from the heater, more particularly, T - Tref is plotted, where
Tref is the prevailing ambient temperature, the driving force for heat transfer against
each positions obtained and the effect of any small perturbations in ambient temperature
is minimized.
18. A method of producing the micro heat pipe as claimed in anyone of claims 11 to 17
wherein to establish substantially higher cooling performance the grooved wafer profile is
selectively, provided.
19. A micro heat pipe with reduced form factor for reliable cooling in miniaturized scale
particularly of chips and integrated circuit devices and like miniaturized component
electronic cooling, its manner of manufacture and testing substantially as hereindescribed
and illustrated with reference to the accompanying figures.

Micro-heat pipes for efficient cooling of miniaturized electronic components and the like,
involving thin film evaporation, slip effect and hydrophobic interactions to dissipate heat. A
micro-channel based heat pipe for efficient and reliable cooling system for miniature size
high performance components including high density semiconductor processors, at reduced dry-out lengths. Importantly, the micro heat pipes avoids wicks, leading to low form factor. It is simple to develop by fabricating with photo-lithography process, and facilitates capillary pumping for liquid flow at less frictional resistance, due to higher surface tension effect for its higher surface area-to-volume ratio. The micro-heat pipes are adapted for wide industrial application such as in High density semiconductor industry, hardware fabrication industry or in R & D set ups.