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This invention is directed to a semiconductor device of the type requiring
cooling during its use. Such semiconductor devices include thyristors or
silicon-controlled rectifiers which operate in conductive states wherein
the current flow through the device produces a relatively large amount of
heat, which must be dissipated to prevent breakdown or destruction of the
device. Different types of cooling means have been used, one type of which
is a heat pipe structure to which this invention relates.
BACKGROUND OF THE INVENTION
A semiconductor device, such as a thyristor or silicon controlled
rectifier, may be cooled by a pair of heat pipe structures that are fixed
to the opposed major surfaces of the wafer, which forms the semiconductor
portion of the device. A semiconductor device of this type is described in
detail in U.S. Pat. No. 3,739,235 issued to Sebastian William Kessler, Jr.
on June 12, 1973. The semiconductor wafer normally has a conductive
cathode electrode coating on one major surface of the wafer and a
conductive anode electrode coating on the opposite major surface of the
wafer. Surrounding the cathode electrode coating on the same major surface
of the wafer and electrically insulated from it, there is normally an
annular conductive electrode coating forming the gate electrode of the
device. One heat pipe structure is bonded to the cathode electrode coating
while the second heat pipe structure is bonded to the anode electrode
coating on the opposite wafer surface. Heat developed in the wafer during
the operation of the device flows from the wafer into the two respective
heat pipe structures where it is dissipated.
For many applications of thyristors, the current rating of the device is
its surge current capability rather than its continuous current rating.
The reason for this is that many uses of these devices are for motors and
contactor, where a high short-circuit current occurs. The lower the
temperature of the semiconductor wafer of these devices during a surge of
current, the greater is the surge current capability of the device. It has
been found that a semiconductor device, such as a transcalent thyristor,
which is cooled with heat pipes has a higher continuous current rating per
unit area of the emitter than when the device is cooled by other means.
However, the surge capability of the device is only slightly better than
these similar devices that are cooled by other means.
An analysis of the operation of semiconductor transcalent thyristors shows
that the cooling of the semiconductor device by the heat pipe structures
does not begin until 5 to 6 milliseconds after the current starts to flow.
Any cooling during the first 5 to 6 milliseconds is due to the volume heat
capacity of the materials adjacent to the major surfaces of the
semiconductor wafer. In these semiconductor devices, the porous wick
structure of the heat pipes is in direct contact with the cathode and
anode electrode coatings respectively. This wick structure has 54% of the
density of solid copper metal and is filled with water. Although the water
has a large specific heat, it contributes little to the heat capacity of
the wick because its density and its thermal conductivity are small. The
poor thermal conductivity of the water limits the rate with which it is
able to absorb the heat. For these reasons then, the heat pipes bonded to
the semiconductor wafer lag in their heat dissipation after the current
flow has started during the initial current surge.
When current initially begins to flow through the device, during the first
1/2 cycle of a 60 Hz current, for example, the peak surge of current can
be one of 10,000 amperes at 5 volts, or with 50 kilowatts of power. If the
heat generated is not instantly absorbed or dissipated, the device can be
destroyed. This dissipation of heat must occur during the first 1/4 to 1/2
cycle of operation.
SUMMARY OF THE INVENTION
Therefore, in accordance with embodiments of the invention a semiconductor
device comprises a semiconductor wafer having at least one major surface
and a heat pipe including a hermetically closed envelope. The heat pipe
envelope comprises a metal wall portion substantially conforming with and
fixed to the major surface of the wafer. The wall portion of the envelope
has a coefficient of thermal expansion conforming closely with the
material of the wafer and is tightly bonded to the wafer surface with a
solid bond making a good thermal contact to the wafer surface and
providing good thermal conductivity between the wafer and heat pipe. This
particular heat pipe design increases the surge current capability of the
semiconductor device and permits it to operate at a lower temperature
during the first 1/2 cycle of operation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal view, partially in section, of a semiconductor
thyristor cooled by heat pipe structures, in accordance with an embodiment
of the invention.
FIg. 2 is an enlarged sectional view of a portion of the semiconductor
device of FIG. 1.
FIG. 3 is a graph schematically disclosing the maximum temperatures
calculated during operation of semiconductor devices of the type shown in
FIGS. 1 and 2.
An embodiment of the invention is shown in FIGS. 1 and 2, as a
semiconductor thyristor cooled by heat pipe structures bonded to the
thyristor. The semiconductor device may be, for example, of the type
described in detail in U.S. Pat. No. 3,739,235, referred to above. The
device comprises a semiconductor wafer 10 of silicon which is formed of
layers of doped semiconductor material providing an NPNP semiconductor
thyristor device. Shown in detail in FIG. 2, the layered wafer 10 includes
on one major surface a cathode electrode conductive coating 12 and on the
opposite major surface of the wafer a conductive anode electrode coating
14. Although not shown in the figures, the configuration of the wafer 10
is that of a substantially round disc.
On the one major surface of the wafer 10 and adjacent to the cathode
electrode coating 12 and insulatingly spaced therefrom is an annular
conductive coating 16 forming the gate electrode. A conductive metal lead
structure 18 is brazed to the gate electrode 16. Bonded directly to the
opposed major surfaces of the semiconductor 10 are two heat pipes
respectively, 20 and 22. The heat pipes include two hermetically sealed
envelopes comprising copper cylinders 24 and 26. One open end of the
cylinder 24 is hermetically closed by a metal wall portion consisting of a
round metal plate 28 metal-brazed around its periphery across and to the
open end of the cylinder 24. In a smiliar manner, the corresponding open
end of the cylinder 26 is hermetically closed by a second metal plate 30
metal-brazed around its periphery and across its open end. The opposite
ends of cylinders 24 and 26 are hermetically closed as by brazing by
respective plates 32 and 34, as indicated in FIG. 1. The cylinders 24 and
26 are metal-brazed directly to the respective electrode conductive
coatings 12 and 14.
The opposed major surface areas of the wafer 10 are flat. The conductive
electrode coatings 12 and 14 conform with these surfaces. Plates 28 and 30
form wall portions of the respective heat pipe envelopes and are formed
flat to conform with the flat electrode coatings 12 and 14. The plates 28
and 30 are bonded across their surfaces with a solid metal bonding to the
coatings 12 and 14, respectively. The bonding is a metal-brazing or
soldering of all of the conforming surface areas of plates 28 and 30 to
the respective electrode coatings 12 and 14. This solid bonding of the
heat pipe wall formed by plates 28 and 30 to the wafer provides good
thermal contact between these parts, which in turn provide good thermal
and electrical conductivity between the wafer 10 and heat pipes 20 and 30.
The inner surfaces of both of the heat pipes 20 and 22 are covered with a
capillary wick structure, which extends over and is bonded tightly to the
inner surfaces of the closed cylinders 24 and 26, respectively. As shown,
a capillary wick liner 36 is bonded to the inner surface of the cylinder
24 and across the inner surface of the end plate 28. In a similar manner,
a capillary wick liner 38 extends over and is bonded to the inner surface
of cylinder 26 and continuously over the respective end plate 30.
As is well known and as described in the above cited U.S. Pat. No.
3,739,235, the wick liners 36 and 38 consist primarily of porous copper
layers, in which an amount of a working fluid, such as water, is used. In
the operation of the device, heat generated by current flow through the
semiconductor wafer 10 is conducted away from the wafer by the two heat
pipes 20 and 22. Water within the capillary liners 36 and 38 is evaporated
from the portions of the heat pipe liners adjacent to the major surfaces
of the semiconductor wafer 10. The water vapor formed from the evaporation
condenses on the cooler walls of the two heat pipes, which are removed
from the semiconductor wafer. The condensed water flows back through the
capillary wick liners 36 and 38 toward the end portions of the heat pipes
which are attached to the opposed surfaces of the wafer 10. The water is
distributed over the plates 28 and 30 by triangularly shaped vanes 37
formed of the porous copper wick material and extending integrally from
the walls of cylinders 24 and 26 to the centers of the respective plates
28 and 30.
To further aid in the cooling of the heat pipe cylinders, a spiral thin
metal fin structure 48 is bonded to the external surface of cylinder 24
and a similar fin structure 52 is bonded to the external surface of
cylinder 26. These fins 48 and 52 provide a greater surface area for
air-cooling of the outer surfaces of the heat pipe cylinders 24 and 26,
respectively.
The cylinder 24 provides an electrical cathode lead from an external
circuit to the cathode electrode coating 12 and the heat pipe cylinder 26
provides the anode lead directly to the anode electrode coating 14. The
external circuit is connected to the cylinders 24 and 26 by a threaded
cathode bolt 44 fixed to the end plate 32 of the cylinder 24 and a
threaded anode bolt 46 connected to the end plate 34.
The two heat pipes 20 and 22 have hermetically closed envelopes formed
respectively by the cylinder 36 and the end plates 28 and 32 and the
cylinder 38 with its end plates 30 and 34. To further protect and shield
the exposed portions of the semiconductor wafer 10 there is provided
another envelope structure 53 extending between the two heat pipes. This
envelope structure includes a ceramic cylinder 54 coaxially positioned
around the heat pipe cylinder 26 and bonded to the wall of cylinder 26 by
a copper, flanged-disc 56 brazed at its outer periphery to one end of the
ceramic cylinder 54 and at its thinner periphery to the outer wall of
cylinder 26. As schematically shown in FIG. 1, the ring 56 is of a
corrugated construction to relieve the difference in thermal expansion of
the connected parts.
Envelope structure 53 also includes a weld sleeve 58 made of Kovar (Trade
Mark), for example, attached to the ceramic ring 54 by an annular ring 60
brazed to one end of the Kovar sleeve 58 and joined at its inner periphery
to the other end of the ceramic ring 54. The other end of the weld sleeve
58 is bonded to the outer surface of the heat pipe cylinder 24 by means of
the annular ring 62, which is brazed at its outer periphery to the other
end of the Kovar weld sleeve 58 and at its inner periphery to the outer
surface of cylinder 24. The lead 18 to the gate electrode coating 16
extends through an opening in the ring 62 and is attached to a lead
terminal 64, as indicated in FIG. 1 and is used to connect the gate
electrode into the external working circuit. The annular rings 56, 60 and
62 are of an irregular cross-sectional design to provide strain isolation
connections between the rings 54 and 58 and the cylinders 24 and 26.
As described above, the surge current capability of thyristor devices of
the type shown in FIGS. 1 and 2, in many applications depends upon their
capability of absorbing a high current surge without failure. The current
surge normally is that provided by the initial current within the first 5
to 6 milliseconds of operation. It is necessary that the semiconductor
devices be protected from a high temperature operation, at which the
device will fail. As set forth above the poor thermal conductivity of the
water used in the heat pipes and the lessened heat conductivity of the
porous copper of the wick do not provide sufficient cooling during the
initial portion of the current surge.
In accordance with the invention, it was found that the use of the metal
end plates 28 and 30 provides a considerable increase in the cooling of
the semiconductor device during the initial surge portion of the operating
current. It was found that the metal plates 28 and 30 bonded directly to
the respective electrode coatings 12 and 14 provide a structure which
initially absorbs and stores the heat formed in the semiconductor 10
during the surge portion of the current. The heat storing capability of
plates 28 and 30 effectively dissipates the heat from the semiconductor
100, so that the maximum temperature at which the semiconductor 10
operates during the initial surge current is lowered by 60.degree. to
70.degree.C. This, then, enables the semiconductor to operate at a higher
surge current and provides greater utility for the semiconductor device.
FIG. 3 discloses the maximum temperature conditions which silicon
semiconductor thyristors of the type described above, experienced within
the six milliseconds after the application of a peak power surge of 53.9
kilowatts of the first half cycle of a 60 Hz current. The tests were run
with different thicknesses of metal plates 28 and 30 and with metal plates
made of tungsten and copper in combiination with different types of porous
wick constructions. For example, curve 70 represents the maximum
temperatures observed under the peak power conditions by the thyristor
device when no plates 28 and 30 were used and when the plates were formed
of tungsten metal and were of different thicknesses for a series of tests.
During these tests the wick material 36 and 38 used was of a porous silver
metal. Curve 70 indicates that without the use of plates 28 and 30 and
with the silver wick liners, the initial maximum temperature the silicon
wafer reached was around 270.degree.C. By using tungsten plates of
different thicknesses, the maximum temperature reached during the first
six milliseconds of the initial surge conditions could be lowered to a
temperature of about 220.degree.C.
If the wick structure were formed of porous copper, results represented by
curve 72 were obtained by the use of tungsten plates 28 and 30 of varying
thicknesses. The copper wick alone held the temperature of the device to
around 260.degree.C. The use of tungsten plates of increasing thickness
lowered the initial maximum temperature of the device to about
220.degree.C. Curve 74 indicates the maximum temperatures measured when
copper plates were utilized for the end plates 28 and 30, in combination
with copper wick liners 36 and 38. With this arrangement, different
thicknesses of the copper plates 28 and 30 lowered the maximum temperature
under the given conditions to a minimum of around 200.degree.C.
The results shown in FIG. 3 indicate that the maximum temperatures
experienced by the semiconductor devices in the initial 6 milliseconds of
the first half cycle of the power surge were lower with the use of end
plates 28 and 30 bonded directly to the electrode coatings on the opposite
major surfaces of the semiconductor 10. As mentioned above, these plates
apparently are able to absorb and store the developed heat in the very
short time when the surge current conditions of the device are
experienced. This is the time before the cooling capabilities of the heat
pipes are effective to dissipate the heat from the wafer 10. The device
then is able to operate at higher surge currents than possible without the
plates 28 and 30. The results shown in FIG. 3 indicate, furthermore, that
during the operation of the device, the presence of the metal plates 28
and 30 prevents the temperature of the semiconductor device rising to a
value it would have without the plates.
FIG. 3 further shows that the use of end plates 28 and 30, which are
thicker than 1000 microns, do not improve the operating characteristics of
the semiconductor device and are not beneficial in further lowering the
operating temperatures, during the initial 6 milliseconds of the power
surge. However, there is a disadvantage in using end plates thicker than
1000 microns, as the temperature differential between the silicon wafer 10
and the evaporator portions of the heat pipes will increase during
continuous operation because the thermal conductivity of the metal plates
28 and 30 is not as great as the vapor space of the heat pipes. For
example, a tungsten end plate having a thickness of 625 microns will have
a temperature gradient of 2.45.degree.C. A thicker end plate would have
even a greater temperature gradient. Thus, it is seen that for the optimum
surge current capability of the device and for also the optimum continuous
current operation, there is an optimum thickness less than 1000
micrometers for the metal plate between the silicon wafer surface and the
evaporator structure of the heat pipe.
In the test results indicated in FIG. 3, tungsten and copper end plates
were used. However, copper is not the best thermal match for the silicon
of the wafer 10 and does not retain its bond to the surface of the silicon
as well as other metals, such as tungsten and molybdenum, which have
thermal expansion characteristics which more closely match that of the
silicon of wafer 10. Silver can also be used for the plates 28 and 30.
The plates 28 and 30 conduct the current from all portions of the
respective electrode coatings 12 and 14 to which they are conductively
brazed. The current conduction is uniformly from plates 28 and 30 to the
cylinders 36 and 38, acting as the respective cathode and anode leads of
the device. Thus, the plates 28 and 30 serve to prevent the occurrence of
"hot spots" in the conductive coatings 12 and 14, which would cause the
device to break down.
A preferred embodiment of the invention utilizes copper cylinders 24 and
26, which are machined to provide the desired wall thickness of around
0.038 cm. Molybdenum end plates 28 and 30 are brazed into the opposite
ends of cylinders 24 and 26 with a gold-nickel alloy composition. The
inner surfaces of cylinders 24 and 26 and those of end plates 28 and 30
are nickel plated. The outer surfaces of plates 28 and 30 are ground to
provide an accurate flatness of the surfaces, so as to form a good
matching fit to the respective flat opposed surfaces of wafer 10. These
matching surfaces of the wafer 10 and the plates 28 and 30 are solidly
metal-brazed or soldered together so that the electrode coatings 12 and 14
are completely bonded over their surfaces to the respective plates 28 and
30. The semiconductor wafer may be formed in the manner disclosed in the
above cited U.S. Pat. No. 3,739,235 with the opposed electrode coatings 12
and 14 respectively brazed to the ends of cylinders 24 and 26.
The wick liners 36 and 38, inside the cylinders 24 and 26 are formed
somewhat in the manner described in British Pat. No. 1,361,269 issued to
RCA Corporation. Such a method includes the use of a cylindrical mandrel
of less diameter than the inner diameter of cylinders 24 and 26. The
mandrel is of a cylindrical configuration and is coaxially mounted within
the heat pipe cylinder so that it is spaced from the walls of the cylinder
a distance equal to the desired thickness of the capillary wick liners.
The space between the mandrel and the inner surfaces of the cylinders 24
and 26 is filled with a fine copper powder, of which the copper particles
have been silver plated, in the manner described in the above mentioned
British patent. The fine powder fills the space between the mandrel and
the inner wall of the heat pipe cylinder. The end of the mandrel is
appropriately slotted and the powder will fill the slots to form the vane
37. The mandrel is normally of stainless steel having a chromic oxide
formed on its surface.
The cylinders with their mandrels filled with a powdered wick material are
then heated to a temperature at which the silver plated copper powder
fuses together to provide the porous wick liner constructions indicated at
36 and 38. The heat pipe cylinders then are stacked together with the
semiconductor wafer 10 in between. Using an appropriate solder material,
the cylinders 24 and 26 and their respective end plates 28 and 30 are
bonded by metal brazing directly to the electrode coatings 12 and 14. The
brazing of the heat pipes to the semiconductor surfaces may be done
separately or simultaneously during the heating of the pipes to form the
wick liners within the two cylinders 24 and 26. After the heat pipes have
been assembled to the wafer 10, the wafer envelope 53 consisting of the
ceramic cylinder 54 and the weld sleeve 58 may be assembled to the
cylinders 24 and 26. This assemblying of the envelope 53 to the ceramic
wafer 10 may also be done during the heating and brazing of the other
parts of the semiconductor device.
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