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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to active solid state devices combined with a
housing and, more particularly, to fluid cooling for such devices.
2. Description of Prior Art
Boiling: Background
Boiling is one of the heat transfer methods which uses liquids, and it
requires the least equipment and expense. In boiling, pockets of vapor at
a hot surface to be cooled generate bubbles repeatedly, which, in
escaping, agitate the fluid very close to the hot surface. In most forced
liquid convection systems it is difficult to cause mixing of hot and
cooler liquids on a fine scale next to the hot surface. Cool fluid is
brought down to the hot surface, and when it is heated, the fluid is
forced upwardly. In rising, the bubbles expand while fluid on the hot
surface is evaporated. Initially the process occurs at the hot surface
with the evaporation of a thin film of fluid beneath a bubble known as a
microlayer. Only a fraction of the heat transferred from the surface is in
the form of latent heat within bubbles. The bulk of the heat is carried by
liquid convection currents. See G. Leppert, "Boiling," Advances in Heat
Transfer, Vol. 1, p. 185 (1964).
Performance is usually displayed in a boiling curve such as FIG. 1 which
relates the power flux G (watts/meter.sup.2) to the temperature difference
T.sub.s -T.sub.sat. T.sub.s is the surface temperature, and T.sub.sat is
the saturation temperature of the fluid at the operating pressure
(approximately the temperature of the bulk fluid). The improvement in
performance which accompanies the onset of boiling is evident. Importantly
enough, the system is inherently unstable as shown by the fact that a heat
flux slightly greater than the critical flux G.sub.c causes the surface
temperature to increase suddenly. Since a new operating temperature for a
G greater than G.sub.c is often unacceptably large (beyond the curve of
FIG. 1), point G.sub.c is often referred to as "burn-out." Such behavior
distinguishes liquid systems from those which rely upon conduction or gas
convection (natural or forced). For solid conductors or gases, performance
is enhanced as the temperature difference between the hot surface and the
ambient medium increases. The systems are always stable. The problem
within liquids is associated with bubble formation. As more heat is
generated at the surface, bubbles are generated more rapidly and closer
together. The escaping bubble streams interfere with each other.
Eventually, a point is reached at which the bubbles merge to form a vapor
film which holds the liquid in suspension above the surface to be cooled.
Reliability is another limiting feature of boiling systems. To be
effective, boiling must occur reproducibly when the heat flux or
temperature reaches a certain level. Often this does not occur. The cause
is a deficiency of stable nucleation sites (points where bubbles tend to
form) on the surface. Since heat transfer within the boiling mode is much
more effective than in the natural convection mode, the temperature
decreases dramatically once boiling begins. This lagging, hysteretic
behavior is particularly unsuitable for systems in which the power level
varies. The system may operate at significantly higher temperatures at low
power levels than at high ones. The above problems can be alleviated
considerably by the use of artificial nucleation surfaces to be described.
Enhancement of Boiling by Surface Treatment:
Effect of Surface Cavities on Boiling
Since the effectiveness of boiling is closely connected with bubble
generation, considerable effort has been made by several workers to
understand the process. While a bubble 10 in FIG. 2 adheres to a surface
to be cooled, some evaporation occurs through a thin annular film of fluid
beneath the bubble known as a microlayer 8. See H. H. Jawurek,
"Simultaneous Determination of Microlayer Geometry and Bubble Growth in
Nucleate Boiling," Int. J. Heat Mass Trans. 12, 843 (1969); R. Mesler, "A
Mechanism Supported by Extensive Experimental Evidence to Explain High
Heat Fluxes Observed During Nucleate Boiling," AICHE J., 12, 246 (1976);
Cooper et al, "The Microlayer in Nucleate Pool Boiling," Int. J. Heat Mass
Trans. 12, 895 (1969); Voutsinos et al, "Laser Interferometric
Investigation of the Micro-Layer Evaporation Phenomenon," J. Heat Trans.
97, 88 (1975); Foltz et al, "The Measurement of Surface Temperatures with
Platinum Films During Nucleate Boiling of Water," AICHE J. 16, 44 (1970).
As evaporation occurs, a dry zone 7 of surface 9 is formed within the
center of the bubble 10. The heat transfer coefficient of the layer 8 is
extremely large. Performance can be improved if microlayer evaporation is
extended over larger dimensions or longer times.
Study of the reliable initiation of bubbles with little superheat (T.sub.s
-T.sub.sat) is a more productive line of inquiry. If a bubble is suspended
within a liquid, a pressure difference exists between the gas and the
liquid.
The excess in pressure is absorbed by the surface tension of the interface.
If S is the interfacial surface tension and r is the bubble radius, the
difference between the pressure of the vapor in the bubble, p.sub.v, and
the pressure of the liquid (immediately outside the bubble), p.sub.l is:
p.sub.v -p.sub.l =2S/r 1
If the bubble is in equilibrium, the vapor pocket neither grows nor
contracts. In that case, the pressures within the vapor and liquid phases
are related by the Clapeyron relation (Cole, "Boiling Nucleation,"
Advances in Heat Transfer 10, 84, 95 (1974)):
p.sub.v -p.sub.l .perspectiveto..rho..sub.v L(T.sub.be
-T.sub.sat)/T.sub.sat 2
T.sub.sat is the temperature associated with a liquid pool of infinite
extent in equilibrium with its vapor and subjected to a liquid pressure
p.sub.l. T.sub.be is the average equilibrium temperature of the vapor in
the bubble and liquid consistent with that pressure difference (i.e., no
growth, no shrinkage). L is the latent heat of the liquid, and .rho..sub.v
is the vapor density. If the two equations are combined, the variation of
equilibrium bubble temperature with bubble radius is obtained:
T.sub.be =T.sub.sat (1+2S/.rho..sub.v Lr) 3
If the actual bubble temperature is higher than T.sub.be, a bubble of
radius r grows. If the temperature is lower, the bubble contracts. Eq. 3
establishes the minimum temperature necessary to produce active boiling
from a bubble of radius r. Clearly, the larger the initial size of the
bubble, the lower the temperature needed to instigate bubble growth and
boiling.
If the heated surface is perfectly smooth, as shown in FIG. 2, initiation
of a bubble 10 requires an undesirably large amount of superheat and is as
likely to occur within the bulk of the fluid 11 as upon the surface. This
reflects the large vapor pressure of a gas 12 confined to such small
dimensions. For a surface 14 in FIG. 3A which is not smooth, the situation
is dramatically different. If stable vapor pockets or cavities 15 exist in
or on the surface, sites for bubble initiation are always available. The
larger the pocket or cavity 15, the smaller the amount of superheat
necessary to initiate bubble growth. Reliability sets a limit on their
size. If the sites are too large, filling by liquid renders them
inoperable. Several researchers have concluded that there is an ideal
geometry for an active nucleating site--a reservoir cavity (Hsu, "On the
size Range of Active Nucleation Cavities on a Heating Surface," J. Heat
Trans., 84, 207 (1962); Cole, op cit.). The fluid in contact with the pore
wall pulls the liquid-vapor interface upward with a force inversely
proportional to the radius of curvature of the interface as in FIG. 3A. If
the pore is just a cylindrical cavity 36, (FIG. 3B), once the gas-fluid
interface 37 enters, the radius of curvature r is fixed. If the pressure
applied by the liquid 33 above is too large, interface 37 descends to the
bottom of wall 38 and the cavity 36 is filled with liquid. In the case of
a reservoir cavity 15 in FIG. 3A, when the interface 17 reaches the lower
lip of the pore 16, interface 17 can pivot around a corner 20. In the
process the magnitude of the radius of curvature r decreases before
increasing again. In this region interface 17 is stable, since, as the
liquid 13 attempts to descend farther into cavity 15, the surface tension
produces an increasing force acting to retard the flow. In operation in
FIG. 3A cavity 15 in surface 14 of a body to be cooled is initially filled
with vapor 19. As the liquid 13 forms more vapor 19, liquid 13 is driven
up through pore 16, and a bubble (not shown) escapes, releasing pressure
in cavity 15 so interface 17 can project again well within pore 16 to
place liquid 13 in contact with walls of pore 16 to produce more vapor 19.
This process is far more efficient than that of FIG. 2 because formation
of vapor 19 is facilitated by presence of a large liquid vapor interface
17.
As long as the meniscus (interface 17) can support the liquid 13 above it,
the cavity 15 stays dry. The site remains active because a macroscopic
liquid-vapor interface 17 is always available at which evaporation can
occur. We suggest that a rough approximation is sufficient, which is that
boiling can be said to occur when the bubble radius predicted by Eq. 3
equals the cavity radius. With that assumption, if the liquid 13 is Freon
113 fluorocarbon, a superheat of 30.degree. C. requires a 0.4 micro-meter
cavity; a superheat of 10.degree. C. requires a 1 micro-meter cavity; and
a superheat of 1.degree. C. requires an 11 micro-meter cavity.
Alternative Approaches to Surface Conditioning
In the development of a continuously active surface, several approaches are
available. The first is to design a cavity shape which can be formed in a
surface which is applicable to the particular type of cooling liquid and
suface 14 material of the body to be coated and to make such cavities in
the surface 14 serve as nucleation sites in FIG. 3A. The effectiveness of
the technique depends partially upon correct placement of the cavities 15.
If all is done properly, there is evidence that heat transfer can be
improved considerably. (Heled et al, "Pool Boiling from Large Arrays of
Artificial Nucleation Sites," Int. J. Heat Mass Trans. 8, 1261 (1965)).
A similar technique is to coat a surface with an array of spots. Then a
cooling fluid is selected so as to wet the surface, but not the spots. The
approach is not to trap vapor but to provide sites at which vapor growth
can occur more easily. Again the spacing and size are critical parameters.
In addition, surface properties such as contact angle must remain fixed.
Another method uses the observation that a roughened surface exhibits
better boiling characteristics than the same surface when smooth.
(Bergles, "Recent Developments in Convective Heat Transfer Augmentation,"
App. Mech. Rev., 26, 675 (1973)). The roughening process apparently
creates a spectrum of sites of varying size and activity. At low heat
fluxes only the most active sites operate. With increasing flux levels
other sites are activated. A process is needed to create active sites
preferentially. Surface abrasion with various types of sandpaper has been
successful in shifting the boiling curve to lower superheats. (Corty et
al, "Surface Variables in Nucleate Boiling," Chem Eng. Prog. Sym. Series
51, 1 (1955)). Coating with a thin layer of low thermal conductivity and
heat capacity, a layer of coarse particles, or a layer of porous material
all act to increase the critical heat flux G.sub.c. (Butler, et al
"Improved Pool Boiling Heat Transfer to Helium from Treated Surfaces and
its Application to Superconducting Magnets," Int. J. Heat Mass Trans. 13,
105 (1970). Virtually anything which disrupts the uniformity of the
surface enhances its heat transfer capabilities. The objective is the
creation of more than enough active sites for any occasion. The system
selects the appropriate number of sites for proper operation.
The techniques discussed up to this point are passive. Other active methods
are known to increase heat transfer. One is film evaporation.
Unfortunately, most methods are incompatible with integrated circuits. A
frequent suggestion is the application of electric fields in which bubbles
are pulled away from the surface electrically, and the onset of film
boiling is delayed. However, noticeable effects occur only at extremely
high field strenghts in which there is a large probability that the fluid
will experience an electrical breakdown.
Surface Treatment--Silicon Chips
Production of Porous Surfaces
Since the subject matter involved herein concerns cooling integrated
circuits, treatments must be compatible with silicon device processing.
Because of the improvements cited in the literature, interest centers upon
porous surfaces for promotion of nucleation of cooling fluids. Performance
of a particular technique is compared with that of an etched silicon
surface and an etched and polished one. The former corresponds to the
backside of an integrated circuit chip, and the latter to the front side
on which the device processing is performed. Specific methods of
preparation are:
Sandblasting: An abrasive in the form of 27 micro-meter aluminum oxide
powder is carried within a gas jet in an abrasive unit. Only a few seconds
of impingement are needed to roughen the surface. Cooling is enhanced by
this process.
Dendritic Coatings: A dendritic tungsten layer can be grown by chemical
vapor deposition (J. J. Cuomo, "Low to High-Temperature Capillary," IBM
Technical Disclosure Bulletin, 18, 1239, (Sept. 1975), and Cuomo et al, "A
New Concept for Solar Energy Thermal Conversion," Appl. Phys. Lett. 26,
557 (1975). The surface is covered by an array of blades approximately 10
micro-meters in height. Chemical vapor deposition requires use of high
temperatures, which renders the process inapplicable to treatment of
substrates which can be harmed by exposure to high temperatures. The
structure has very little porosity and is open so there is very little
capability to trap gas. Since gas can escape, efficient boiling is
prevented. Bakelaar in "Substrate Mounted Heat Pipe for Chip Cooling," IBM
Technical Disclosure Bulletin 14, 2690 (Feb. 1972) describes coating the
surfaces of circuit chips solder bonded to a support with a dendritic-type
of wicking material, cooled by a dielectric fluid.
Porous Silicon for Cooling: A silicon wafer for a semiconductor device is
etched electrolytically when used as an electrode within an electrolytic
cell by anodizing in HF (Gregor et al, "Porous Silicon Interface to
Enhance Heat Transfer from Silicon Substrate to Liquid," IBM Technical
Disclosure Bulletin 19, No. 3, 1120 (Aug. 1976). Variation of the current
density and duration of application alters the layer thickness. The size
and spacing of the pores depend upon the type of silicon doping (p or n)
and the electrical conductivity. In the n-type material used, openings a
few micro-meters in diameter taper inward and develop cross-channels.
However, this involves use of a wet chemical process which can be
hazardous when manufacturing solid-state devices because the chemicals may
be exposed to surfaces which should be protected from such wet chemicals.
Porous Films in General: A porous structure not intended for cooling,
composed of copper, is produced by sputtering within a 240 micro-meter
argon atmosphere at low substrate temperatures. Thornton et al, "Tubular
Hollow Cathode Sputtering onto Substrates of Complex Shape," J. Vac. Sci.
Technol, 12, No. 1, 93 (1975). Porous aluminum was obtained with
evaporation techniques in argon pressure and it was shown that the
pressure can be used to change the porosity (Silvestri, "Forming Porous
Aluminum," IBM Technical Disclosure Bulletin, Vol. 19, No. 9, p. 3622 Feb.
1977). Miersch and Sachar in "Enhancement of Boiling Heat Transfer by a
Submerged Capillary Structure," IBM Technical Disclosure Bulletin, 18 p.
3843 (Apr. 1976) described formation of a surface composed of a porous
material, with several layers, the topmost of which has jets evenly spaced
apart. The pore size and spacing are important factors. Production of
cavities in a surface adapted for heat transfer is achieved in U.S. Pat.
No. 4,050,507 to R. C. Chu et al, which shows drilling holes in the back
of a wafer with a high energy beam such as a laser to provide nucleate
boiling. This subjects the device to an extremely large amount of heat
which would destroy many thin film devices.
Channels or Islands: Pearson in "Integrated Circuit Chip Cooling," IBM
Technical Disclosure Bulletin, 19, 460-1 (July 1976) describes use of a
porous silicon film made porous to remove areas where it is intended to
form channels or spaces around islands. All porous material is removed.
Macroscopic Porous Structure: U.S. Pat. No.3,739,235 of Kessler for a
"Transcalent Semiconductor Device," describes coating of a chip with
metallic layers with many solder-plated copper particles which are bonded
together to form a porous capillary structure of a very coarse nature. A
fluid such as water saturates the porous structure.
In general, the prior art has taught the desirability of providing porous
structures of some kind which should include partially enclosed cavities.
However, the techniques used have been deficient in that they did not
supply sufficient cooling capacity per unit time and/or they required such
high temperatures of fabrication as to risk damage to the sensitive
structures. Treatment by scratching a substrate with sandpaper or
sandblasting are inadequate because (1) there is a risk of dirt and dust
being left on the opposite side of the chip (2) the cooling efficiency of
such treatments is relatively poor, largely because no porosity is
provided by such superficial treatment. Reactive ion etching to produce
holes and laser drilled holes provide no pores communicating between
adjacent cavities which is believed by applicants to be essential for
proper efficiency of wicking of the cooling film. Dendritic tungsten
coatings may provide wicking, but they do not provide cavities for
trapping gas, and they require chemical vapor deposition at unacceptably
high temperatures for many devices. Wet chemical etcing techniques for
producing grooves and the like involve the risk of damage to the chip,
operator safety exposure, and chemical disposal problems.
SUMMARY OF THE INVENTION
In accordance with this invention, a thin film solid state device is formed
on a substrate. The reverse side of the substrate is coated with a highly
porous thin film structure deposited in an evacuated chamber in the
presence of a substantial atmosphere of a gas. The thin film structure
includes microscopic capillary structures and cavities with transverse
interconnection between cavities. A cooling liquid is retained in contact
with the porous film.
In another aspect of this invention, the porous film is deposited by a
technique of vacuum deposition selected from evaporation and sputtering at
substantial pressures of a gas nonreactive with the material being
deposited.
In accordance with this invention, a substrate for carrying a thin film
solid state device upon a first surface is coated with a vacuum deposited
highly porous thin film structure deposited at a relatively low
temperature and a relatively high pressure upon the opposite surface of
the substrate from the first surface. The structure comprises a film with
a plurality of microscopic cavities and capillary structures with a
substantial number of transverse interconnections between capillaries.
Preferably, the cooling system includes a substrate having a vacuum
deposited highly porous thin film structure upon a cooling surface. A
chamber is formed by the substrate and an enclosure for holding a cooling
fluid in contact with the porous structure. The porous structure comprises
a porous thin film deposited in a vacuum chamber in the presence of a gas
nonreactive with the components of the structure.
It is preferred that the thin film structure comprise a thin film of
aluminum which is vacuum deposited at a relatively low temperature in a
relatively high pressure of an inert gas. The pressure should be greater
than about 0.5 millitorr. It is desirable that the thickness of the film
structure be at least as great as the film pore size, providing for
substantial trapping of gas in microscopic pores. Further in accordance
with the invention, the porous thin film structure comprises aluminum
deposited in an argon pressure between about 0.5 millitorr and 100
millitorr.
It is preferred that the porous thin film structure comprise aluminum
deposited by sputtering in an argon atmosphere at a pressure of between
about 0.5 millitorr and 100 millitorr. It is alternatively preferred that
the porous thin film structure comprise aluminum deposited by evaporation
in an argon atmosphere at a pressure of between about 0.5 millitorr and
100 millitorr.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a logarithmic graph of power flux G in units such as watts/sq.
in. vs. temperature difference between the temperature T.sub.s of the hot
surface to be cooled and the temperature T.sub.sat which is the saturation
temperature of the cooling fluid employed at the operating pressure.
FIG. 2 shows a bubble formed on a flat, hot surface under which remains an
annular layer of liquid referred to as a microlayer.
FIG. 3A shows a sectional view of a fragment of a hot surface in which a
cavity is formed through which a liquid meniscus extends through a pore
into a pocket of vapor.
FIG. 3B shows a sectional view similar to that of FIG. 3A for a cylindrical
cavity.
FIG. 4A shows a photograph of a vertical cross-section of an aluminum film
sputtered under a high pressure of argon.
FIG. 4B shows a photographic perspective view of the upper surface of the
film of FIG. 4A.
FIG. 4C shows an enlarged view of a fragment of the photographic view of
FIG. 4B.
FIG. 5A shows a photograph of a vertical cross-section of an aluminum film
evaporated under a high pressure of argon.
FIG. 5B shows a photograph of a perspective view of the upper surface of
the film of FIG. 5A.
FIG. 6 shows a test apparatus for measuring the effectiveness of a cooling
film on a substrate.
FIGS. 7A and 7B show curves of heat injection into a coolant versus surface
temperature of the surface to be cooled.
FIG. 8 shows a curve of heat injection into a coolant from three aluminum
films sputtered in three different pressures of argon, as a function of
surface temperature of the substrate coated with the aluminum surface.
FIG. 9 shows a miniature heat pipe adapted to operate to cool a cooling
surface with a fluid coolant contained in the heat pipe.
FIG. 10 shows the range of size of structures produced by evaporating and
sputtering aluminum films as measured from photographs such as FIGS. 4B,
4C, and 5B.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As can be seen from the above discussion, covering a heated surface with a
liquid which boils at a desired operating temperature is desirable.
Preferably, the surface should not be smooth unless an acceptable
temperature is significantly higher than the temperature at which boiling
initiates. Use of a smooth surface can result in overheating and
destruction of an integrated circuit formed on a chip when a circuit is
turned on or when the amount of power dissipated changes very
significantly. Furthermore, once boiling commences, heat flux G introduced
to the fluid must be kept below G.sub.c, the critical value, so film
boiling will not occur, when bubbles are generated so rapidly that they
coalesce across the entire hot surface before leaving the surface. Such
boiling vapor films separate the liquid from the surface so that the vapor
film forms an insulating layer leading to rapid increase of the
temperature of the hot surface to unacceptably high temperatures.
Artificial nucleation sites have been provided by numerous techniques to
improve the rate of formation of bubbles and to overcome such problems as
film boiling.
FIG. 4A shows a photograph of an SEM image of a vertical section of a new
thin film of porous aluminum about 15.mu. meters thick made by sputtering
aluminum at a pressure of 20 millitorr of argon onto a chip. The section
is through a cleaved portion of the film and is shown magnified 4000 times
at an angle of 90 degrees. FIG. 4B shows the same sputtered aluminum film
in a perspective view from above at an angle of 45 degrees, magnified 3000
times. FIG. 4C is a similar view to FIG. 4B magnified 12,000 times. FIG.
4A shows surprising structures with substantial transverse
interconnection, substantial cavities capable of trapping vapor and the
like, with openings provided at the top for escape of vapor and entrance
of liquid during reciprocation of the liquid interface in active boiling.
It can be seen by reference to FIG. 8 that this film cools 15 watts stably
at 60.degree. C. for a 15.mu. meter thick film.
The porosity consists of vertical pores with many interconnecting channels
making the films useful as both a source for initiating nucleate boiling
and as a wick when employed as a heat pipe material. In this case,
capillary action of the porous Al draws fluid from the periphery onto the
heated area where evaporation occurs. The mechanical attachment of a
nonintegrable wick structure has been cited in the literature as a severe
limitation in the implementation of heat pipes mounted directly on chips.
By making the wicking material (i.e., porous aluminum) an integral part of
the structure, this difficulty is avoided.
The closely spaced vertical columns of metal (aluminum in this case) can be
varied in height, width and spatial density as a function of the
deposition rate and the pressure of the gas (argon in this case). By
adjustment of the above parameters, myriads of cavities of the sort shown
in FIG. 4A can be formed. This type of structure greatly increases the
value of the critical heat G.sub.c.
FIGS. 5A and 5B show SEM photographs of an evaporated thin film structure
about 4 micro-meters thick. FIG. 5A shows a vertical section. The film is
generally similar in appearance to FIG. 4A for a thin film of aluminum
deposited upon a substrate by means of evaporation of the aluminum in a
vacuum chamber containing an atmosphere of 6 millitorr of argon gas. FIG.
5B shows a 45.degree. SEM photograph of the upper surface of the
evaporated film. The effectiveness of this film as a cooling material is
illustrated in FIG. 7B, curve 7, where it has an excellent characteristic,
cooling at 10 watts stably at 60.degree. C.
FIG. 6 shows apparatus useful for measuring the effectiveness of cooling of
a given thin film layer applied to an n-type silicon wafer 60 which can be
3.2 cm. in diameter with 10 ohm-cm. resistivity. A heating strip 61 is
evaporated onto the bottom side of the wafer 60, where an integrated
circuit would have been deposited if the wafer were used as a part of an
integrated circuit system. Strip 61 is composed of chromium (500 A) and
platinum (2000 A). The metal film 61 which acts as a heater is originally
0.32 cm. by 1.9 cm. However, once the electrical terminals are attached to
the ends, the thin film length is reduced to 1.3 cm. A chromel-alumel
thermocouple 62 monitors the temperature at the center of the strip 61.
The tip is coated with an epoxy to render it electrically insulating and
is then mechanically attached to the heater. The wafer is clamped to a
water-cooled copper ring 63, and the chamber 64 is filled with Freon 113
fluorcarbon coolant 65. The current and the voltage are continuously
monitored. Ring 63 is cooled by cooling coil 66. A cover 67 is provided
with an opening through which vapor from coolant 65 can leave chamber 64.
When the polished side of a wafer is exposed to Freon 113 fluorocarbon,
boiling begins when strip 61 reaches 100.degree. C. When the critical flux
G.sub.c is reached, 4 watts are dissipated into the fluid directly over
the strip 61. When a wafer coated with porous aluminum is used, boiling
begins when strip 61 reaches 60.degree. C. In this case, the critical flux
G.sub.c corresponds to a power dissipation of 9 watts.
Two separate tests are conducted for each surface: one with the cell dry
and the other with the cell filled with fluid 65. To calculate the power
actually carried away by the fluid 65 at a certain film temperature, the
input power to a dry cell which results in that temperature is subtracted
from the input power to the wet cell. Until the surface begins to boil,
the curves which relate power to temperature are almost coincident. The
implication is that in the absence of boiling almost all of the heat flows
through the wafer 60 to wall 63. The little heat which is injected into
the liquid is the result of natural convection. The strength of convection
is largest directly over the heater. When boiling does occur, the surface
pattern of the bubbles conforms to the outline of the heater. Heat flows
either into liquid 65 directly above heater 61 or through the wafer 60 to
wall 63. The heat flux associated with a surface temperature is calculated
by dividing the above difference by the area of the heater, 0.39
cm..sup.2.
Variation of Heat Transfer with Surface Preparation
A summary of the results of some experiments on surface roughening is
provided in FIGS. 7A and 7B, which show the measured heat injection into
Freon 113 flurocarbon as a function of surface temperature for several
silicon surface treatments: (curve 1) polished and etched Si; (curve 2)
polished Si; (curve 3) porous n.sup.- Si (4 micro-meters thick); (curve 4)
porous n.sup.- Si (100 micro-meters thick) in FIG. 7A; and (curve 5)
dendritic tungsten; (curve 6) sandblasting (27 micro-meter abrasive);
(curve 7) porous aluminum deposited at a pressure of 6 millitorr as stated
in connection with FIGS. 5A and 5B and 4 micro-meters thick shown in FIG.
7B.
All methods lower the surface temperature required to initiate boiling of
Freon 113 flurocarbon (Tbp=47.3.degree. C.) and raise the critical heat
flux beyond that of polished silicon. The data indicate that the power
injected into the fluid remains relatively constant at sufficiently large
temperatures. However, FIG. 1 predicts that the surface temperature
increases dramatically when the heat flux exceeds G.sub.c. The difference
results from the parallel thermal path through the silicon. Since the
thermal resistance in the film boiling mode is much larger than the value
for the conduction path through the wafer, heat in excess of Gc is
channeled through the silicon. The system remains at the upper limit of
the nucleate boiling range. For a sufficiently large amount of
dissipation, conduction can no longer cope with the excess power at a
temperature less than T.sub.c. Beyond this point the system passes into
film boiling. The value of G in the plateau region corresponds to G.sub.c
for the system.
Use of the data to indicate hysteresis should also be discussed.
Conventionally, a system is claimed to exhibit hysteresis when, in the
process of increasing and then decreasing the heat flux, the temperature
proceeds along different paths. All of the power is assumed to be injected
into the liquid. In the experiment, two parallel paths exist for the heat,
one into the liquid and the other through the wafer. At low power levels,
the thermal resistance of the path through the wafer is less than that of
the path into the fluid. At high power levels, the reverse is true. In the
experimental system, when hysteresis is displayed in the form of
considerable superheat followed by a sudden transition to boiling, not
only does the temperature suddenly decrease, but the flux of heat into the
fluid increases. The situation resembles a chip mounted on a substrate by
solder pad bonds. The solder pads provide parallel paths for the heat flow
other than through the back of the silicon chip.
Porous aluminum is the most attractive surface conditioner with respect to
lowering the temperature at which boiling begins, increasing G.sub.c, and
minimizing hysteretic behavior. The process is dry, can be performed at
low temperatures, and is delicate enough to be considered for chip
processing. The degree of improvement in boiling characteristics is
impressive. From FIGS. 7A and 7B, curve 1 and curve 7 show that the wafer
temperature at boiling initiation decreases from over 100.degree. C. to
less than 60.degree. C. The heat flux increases from 15 watts/cm..sup.2 to
32 watts/cm..sup.2. Hysteresis diminishes to less than 5.degree. C.
Deposition of the porous film can be at room temperature and is by
conventional processes (evaporation and sputtering). In the case where an
elevated temperature is allowed, the process can be performed at an
intermediate stage of the device fabrication with little effect upon
either the previous or subsequent stages.
Depositions of porous Al form mechanically sturdy films in intimate contact
with the Si substrate or chip.
Engineering capability: Experiments have shown that porosity can be varied
with proper choice of argon pressures and films can be grown over a wide
range of thicknesses. (Films up to 10.mu. micro-meters have been grown).
The film can be deposited in a batch fabrication step under clean
conditions and at low temperatures, both of which are critical in insuring
device reliability and reproducibility. The effects on device
characteristics of depositing aluminum are known to be harmless. Thus, a
large amount of retesting to determine the effects can be eliminated. The
data base already exists.
In heat transfer studies, identical silicon wafers processed with porous Al
allow approximately 3 times the power (18 watts) to be dissipated from the
same area (1/16 sq. in.) into Freon 113 fluorocarbon as from polished
silicon (conventional backside treatment). There is a significant decrease
in the amount of superheat (temperature of surface in excess of saturation
temperature of fluid at operating pressure) necessary to initiate boiling
and in extent of hysteresis (difference in temperature at a given power
level when boiling is and is not occurring) to a few degrees. In both
respects, porous Al compares favorably or significantly exceeds results of
other treatments (sandblasting, porous Si, tungsten).
Porous films were applied in a sputtering chamber as explained in
connection with Examples I-III below in a glow discharge sputtering system
of the type described in U.S. Pat. No. 3,616,450 of Clark entitled
"Sputtering Apparatus." The system had the trade name Sloan Sputtergun
Model S-310 manufactured by Sloan Technology Corporation, 535 East
Montecito Street, Santa Barbara, California 93103. It is described in the
Installation and Operating Instructions manual distributed by that company
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