Note: Descriptions are shown in the official language in which they were submitted.
RD-11272
~3~
This invention relates to apparatus for cooling
semiconductor devices and more particulatly to apparatus
for providing fluid cooling to semiconductor devices in order
to achieve high power operation.
To achieve high power operation in semiconductor
devices, it is necessary to provide an efficient means
for removing heat. In semiconductor devices of the prior
art utilizing a silicon wafer, one side of the silicon
wafer has been attached to a tungsten disk by a silicon-
10 aluminum braze to provide support for the fragile silicon wafer.
Heat sinks, both air cooled and liquid cooled, have been
placed on either or both sides of this structure. To
obtain good thermal and electrical conductivity between
the elements of the device, large mechanical presses have
been necessary to squeeze the wafer-disk-heat sink structure
together with several thousand pounds force. Large clamps
are required to hold this structure together at this force
level, even during device operation. However, a
characteristic of such devices is that the interfaces
between the heat sinks and the silicon wafer are dry. Dry
interfaces are those interfaces where one surface is in
contact with another but not joined thereto; consequently
the thermal and electrical conductivities across these
interfaces are limited. Thus, air and liquid cooled
semiconductors devices of the prior art are limited in the
amount of power they can handle because of the dry interfaces
within the device. Additionally, differential thermal
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RD-11272
~ 1793
expansion and contraction between the elements of the device
with variation of device temperature may cause scrubbing
or even cracking of the silicon wafer.
The present invention concerns a fluid cooled
semiconductor device in which the strain caused by
differential thermal expansion and contraction has been
reduced. As used herein, "fluid" may signify either a gas or
a liquid. In the preferred embodiments of the invention,,
most interfaces between the heat sinks and silicon wafer-
are comprised of thermo-compression diffusion bonds. It should
be apparent to tlose skilled in the art that improved electronic
performance is to be gained by such stress reductions
within the semiconductor device and by eliminating dry
interfaces and their inherent limited thermal and
electrical conductivity.
It is one object of this invention to provide
a fluid cooled semiconductor device with increased
electrical and thermal conductance between the elements
thereof in order to achieve high power operation.
~ further object of this invention is to provide
a fluid cooled semiconductor device with reduced
stress between the elements thereof.
These and other objects of the invention will become
apparent to those skilled in the art upon consideration
of the following description of the invention.
Brief Summary of the Invention
The present invention is directed toward increasing
the electrical and thermal conductances of the interfaces
between the elements comprising a fluid cooled semicondurtor
device and reducing stress between these elements.
~ 793 R~-11272
.
In accordance with the invention, most of these interfaces
are replaced by highly thermally conductive and highly
electrically conductive thermo-compression diffusion bonds.
One preferred embodiment of the electronic semicon~lctor
S device of the invention includes a first metallic layer
situated atop,and joined to, a first electrode of a silicon
wafer having at least first and second
electrodes on opposite sides thereof. A second metallic layer-
is situated atop and joined to this first metallic layer.
A first structured copper strain buffer having first and
second opposed surfaces is located above this second metallic
layer and attached thereto. This structured copper strain
buffer includes a bundle of substantially parallel closely
packed strands of copper of substantially equal length.
One common end of these strands of copper is thermo-compression
diffusion bonded into a first metallic sheet so as to form
said first opposed surface. The remaining second opposed
surface of this first structured copper strain buffer is
thermo~compression diffusion bonded into the second
metallic layer. A first metallic heat sink is situated
atop the first metallic sheet and thermo~compression diffusion
bonded thereto to provide cooling of the semiconductor
device.
A third metallic layer is situated below the second
electrode of the silicon wafers and joined thereto. A
fourth metallic layer is located below and joi~ed to this
third metallic layer. A second structured copper strain
buffer having third and fourth opposed surfaces and being
essentially identical to the one discussed above is
situated below this fourth metallic layer and one common
~3-
1~1793 RD-11272
. ;.
end of its copper strands is thermo-compression diffusion
bonded into a second metalllc sheet so as to form the
fourth opposed surface. The remaining third opposed surface
of this second structured copper strain buffer is thermo-
compression diffusion bonded into the fourth metallic layer.
A second metallic heat sink similar to the first metallic
heat sink discussed above is thermo-compression
dLffusion bonded into the second metallic sheet.
Alternately, in accordance with the invention, electrical
contacts are operatively connected to the side of the wafer
including the second electrode, in lieu of attaching the
third and fourth metallic layers, second structured copper
strain buffer and second metallic heat sink thereto. In the
electronic semiconductor device thus formed, the silicon
wafer is provided with single-sided cool mg.
Because thermo-compression diffusion bonds are present
r~ ~
at the described interface locations, the above discussed
embodiment of the invention includes no dry interfaces.
Thus, thermal and electrical conduction across the
interfaces of the device of the invention are not degraded
by the limiting effects of dry interfaces. Heat is
efficiently conducted away from the silicon wafer and
electric current is efficiently provided to the silicon
wafer.
In accordance with another preferred embodiment of the
invention, a metallic support plate is situated between the
.
silicon wafer and the third metallic layer
of the first described embodiment. One side of the
support plate is joined to the silicon wafer by a metallic
braze. The opposite side of the -support plate
--4--
~ 3 RD-11272
is thermo-compression diffusion bonded to the second
structured copper strain buffer. In all other respects
this embodiment of the invention is essentially identical
to~the first described embodiment. Alternately, the
opposite side of the support plate is operatively con-
nected to electrical contacts, in lieu of attaching the
second structured copper strain buffer, the second metallic
heat sink and associated metallic layers thereto.
In a further preferred embodiment of the invention, the
silicon wafer may comprise a thyristor with an anode on one
side thereof and constituting the second electrode, and a
cathode and gate electrodes on the opposite side thereof and
constituting a split first electrode. In this embodiment
a means for connecting the gate to circuits external to the
device is provided. `
~The features of the inventLon believed to be novel
; are set forth with particularity in the appended claims.
The invention itself, however, both as to organization and
method of operation, together with further ob~ects and
advantages thereof, may best be understood by`reference to
the following description taken in conjunction with the
accompanying drawi~.gs
Descript~on of the Drawings
Figure 1 is a cross-sectional view of one embodiment `~
of a fluid cooled semiconductor device constructe-d in
accordance with the present invention.
Figure 2 is a top view of the fluid cooled semiconductor
device of Figure 1.
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~ . : . . .
- ~3~7~3 RD-11272
Figure 3 is a cross-sectional view of another embodiment
of the invention with thermo-compression diffusion bonds
at selected interfaces within the device and with a
fluid-cooled heat sink.
Figure 4 is a top view of the embodiment of Figure 3.
Figure 5 is a cross-sectional view of another embodiment
of the invention constructed with an air cooled heat sink.
Figure 6 is a top view of the embodiment of Figure 5.
Figure 7 is a side view of the diffusion bonding
press used to form the thermo-compression diffusion
bonds of the fluid cooled semiconductor device of the
- invention.
Figure 8 is a cross-sectional view of another
embodiment of the invention with thermo-compression
diffusion bonds at selected interfaces of the device.
Figure 9 is a cross-sectional view of a fluid
cooled thyristor device with gate connecting means and with
thermo-compression diffusion bonds at selected interfaces
within the device.
Figure 10 is a top view of the fluid cooled thyristor
device of Figure 9.
Figure 11 is a cross-sectional view of a fluid cooled
thyristor device with thermo-compression diffusion bonds at
selected interfaces within the device.
Figure 12 is a cross-sectional view of a hermetically
sealed fluid cooled semiconductor device made in accordance
with the invention.
Figure 13 is a side view of the embodiment of Figure 12.
Detailed DescriPtion of the Preferred Embodiment
_
Figure 1 illustrates one embodiment of ~he fluid
cooled semiconductor device of the invention, wherein
semiconductor device 10 is comprised of a silicon wafer
--6--
~11~\~r~
RD-11,272
~131793
12 with cathode 12a and anode 12b portions on opposlte
sides thereof. Silicon wafer 12 may include a standard
metallization applied to the external surfaces of each of
electrodes 12a and 12b to facilitate electrical connection
thereto.
A first metallic layer 13 is deposited in contact
with cathode 12a of silicon wafer 12 as shown in Figure 1.
A second metallic layer 15 is deposited in contact with
metallic layer 13. Metallic layer 13 may be comprised of
titanium, chromium or nickel, and metallic layer 15 may be
comprised of gold, silver or copper. Second metallic
layer 15 is situated in contact with one surface of
structured copper strain buffer 14, as shown in Figure 1.
Structured copper strain buffer 14 is comprised of a
bundle of substantially parallel closely-packed
copper strands 16 of substantially equal length. One
common end of copper strands 16 is thermo-compression diffusion
bonded into metallic layer 15. The other common end of
copper strands 16 is thermo-compression diffusion bonded
into metallic sheet 18. Metallic sheet 18 may be
comprised of highly electrically conductive metals
such as copper, gold or silver~ for example. Typically,
the copper strands 16 are each 10 mils in diameter,
although somewhat larger or smaller diameters may
be used. The thickness of structured copper strain
buffer 14 is typically selected to be within the
range of 0.1 cm to 1 cm, although strain buffers of greater
~.
- 1131~3 RD-]1272
or smaller thickness may be used. Best strain relieving results
are obtained when the natural oxide coating is present on the
copper strands 16. With such a non-sticking coating present on
the surface of copper strands 16, the individual strands are
relatively free to move and thereby relieve stress when
sandwiched between two surfaces, as is the case with the
embodiment of the invention shown in Figure 1.
Metallic sheet 18 of structured copper strain buffer 14
is thermo-compression diffusion bonded to fluid cooled
metallic heat sink 20. Heat sink 20 contains a chamber
22 wi~hin and has an input 20a to allow fluid to enter
chamber 22. Heat sink 20 is provided with an output 20b
to allow fluid passing through the chamber to exit
the heat sink. Liquid and/or gas coolants may be passed
through heat sink 20 to draw heat away from the heat sink.
This results in the cooling of silicon wafer 12. For ~xample~
guc~ ~lu~dq as ~ter and the refrigerant R113 manufactured by the
Dow Chemical Company may be used to achieve this cooling. The
cooling fluid may be in both liquid and vapor phases as it
passes through heat sLnk 20. The cooling fluld may be
pressurized to assure that it remains in liquid phase as it
passes through heat sink 20. Alternately, unpressurized
fluids may be passed through heat sink 20 ln order to achieve
cooling. The geometry of the heat sink may vary. One
version is shown in Figures 1 and 2, for example.
A third metallic layer 25 is deposited in contact
with anode 12b of silicon wafer 12. A fourth metallic
layer 27 is deposited in contact with metallic layer 25 as
shown in Figure 1. Metallic layer 25 may be comprised of
titanium, chromium or nickel and metallic layer 27 may be
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RD-11,272
~31793
comprised of gold, silver or copper. Fourth metallic
layer 27 is situated in contact with one surface of
structured copper strain buffer 24 as shown in
Figure 1. Structured copper strain buffer 24
(similar to structured copper strain buffer 14) is
situated with one common end of its copper strands 26
thermo-compression diffusion bonded into fourth metallic
layer 27. The remaining common end of copper strands
26 is thermo-compression diffusion bonded into a metallic
sheet 28. Metallic sheet 28 is comprised of a highly
electrically conductive material, for example, copper,
gold or silver.
A metallic heat sink 30, essentially similar
to heat sink 20, is thermo-compression diffusion bonded
to metallic foil 28 of structured copper strain buffer
24. Heat sink 30 has an input 30a for receiving cooling
fluid and an output 30b for exiting cooling fluid.
Heat sink 30 includes a chamber 32 through which cooling
fluid passes from input 30a to output 30b, resulting
in the removal of heat generated by silicon wafer 12.
Heat sinks 20 and 30 may be used to connect cathode 12a
and anode 12b, respectively, to circuitry external to
device 10.
The diffusion bonding press 40 illustrated in Figure 7
is used to form the thermo-compression diffusion bonds in the
semiconductor device of the invention, such as those between
structured copper strain buffer 14 and heat sink 20, and
between structured copper strain buffer 24 and heat sink 30.
g
,,i,~
~1~17~3 RD-11272
Diffusion bonding press 40 is comprised of an upper metallic
plate 42 oriented parallel to a lower metallic plate 44,
with a space provided therebetween. Metallic pressing
block 46 is positioned at the center of the side of upper
plate 42 facing lower plate 44. Metallic bolts 58 and 59
pass through respective holes in upper plate 42 and lower
plate 44 and are threaded into lower plate 44 to connect
the two plates together as illustrated in Figure 7.
Metallic bolts 58 and 59 are comprised of a steel
other than stainless steel, while upper plate 42, lower
plate 44 and metallic pressing block 46 are comprised
of stainless steel. To achieve the thermo-compression
diffusion bond between heat sink 20 and structued copper
strain buffer 14, it is necessary to position heat sink
20 in contact with structured copper strain buffer 14,
as shown in Figure 1, and situate the combined structure
of sink 20 and buffer 14 between metallic pressing block
46 and lower plate 44 of press 40. A conventional press
is used to squeeze upper plate 42 and lower plate 44
together and while such pressure is applied to these
plates, bolts 58 and 59 are tightened.
The thermo-compression diffusion bond between heat
sink 20 and structured copper strain buffer 14 is
actually formed when press 40 containing this sink-buffer
assembly is placed in an inert atmosphere and heated
-- 10 --
~ 793 RD-11272
at a temperature in the range of 300-400C for an interval of
approximately 15 minutes to 5 hours. When press 40 is so
heated, upper plate 42, lower plate 44 and metallic
pressing block 46 expand to a greater total extent than do
nonstainless steel metallic bolts 58 and 59. Thus,
~ force is exerted between pressing block 46 and lower plate
44 resulting in the squeezing of heat sink 20 and structured
copper strain buffer 14 together and the thermo-compression
diffusion bonding of each to the other. The sink-buffer
structure is then removed from diffusion bonding press 20.
The thermo-compression diffusion bond between heat sink 30
and structured copper strain buffer 24 is achieved in
the same manner as described above.
As a result of the thermo-compression diffusion bond
formed between sink 20 and buffer 14, and between
sink 30 and buffer 24, the cathode 12a of silicon
wafer 1~ (with metallic layers 13 and 15 thereon) is
situated in abutting contact with buffer 14,and the anode
12~ (with metal:Lic layers 25 and 27 thereon) is situated
in abutting contact with bufer 24 to form fluid cooled
semiconductor device 10 of Figure 1. To achieve even better
cooling of silicon wafer 12, the above-mentioned "abutting -
contacts" may be replaced by thermo-compression diffusion
bonds between the respective materials.
The term "element" as used herein designates any of the
various layers and components constituting the fluid
cooled semiconductor device to be described.
As mentioned above, many different geometrics of fluid-
cooled heat sinks may be used in practicing the invention.
A preferred heat sink geometry is shown in Figure 3
~31~93 RD-11272
which illustrates a cooled semiconductor device 31 that is
similar in structure and function to device 10 of Figure 1
except for the geometry of heat sinks 34 and 35. Heat
sink 34 comprises a metallic block including a cavity 34c
with an inlet 34a for allowing cooling fluid to enter cavity
34c and outlets 34b to allow the cooling fluid to
- exit cavity 34c. A central plate 36 includes an aperture
36a connected to inlet 34a and causes the cooling fluid
to be guided radially outward from the central
portion of cavity 34c. The cooling fluid thus
continuously flows across the bottom portion of cavity
34c in close proximity to the interface comprising the
thermo-compression diffusion bond of structured copper
strain buffer 14 to heat sink 34. Heat sink 35 is
similar to heat sink 34 and is thermo-compression diffusion
- bonded tQ structured copper strain buffer 24. Heat sink 3S
includes a metallic block having a cavity 35c, a fluid inlet 35a,
fluid outlets 35b, and central plate 3~ with aperture 37a.
As shown in Figure 4 and partially in Figure 3, device
31 is provided with electrical contact bars 38a and 38b to
facilitate connection of cathode 12a and anode 12b,
respectively, to external circuitry.
Figure 5 depicts an air-cooled semiconductor device 39
similar Ln structure and function to the devi~es shown in
Figures 1 and 3, except that air-cooled heat sinks 44 and
46 are used in place of heat sinks 20 and 30 9 respectively,
of Figure 1 and heat sinks 34 and 35, respectively, of
Figure 3. Air cooled heat sink 44 includes a block of metal
41 thermo-compression diffusion bonded to metallic sheet 18
3Q of structured copper strain buffer 14. Heat sink 44 further
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~13~793 RD-11272
includes a metallic fin structure 45 fastened to block 41
with a bolt 48. A plurality of fin-like heat dissipating
projections 51 are included in fin structure 45. These
projections are shown in top view in Figure 6.
Air cooled heat sink 46 includes a metallic block 43
ther~o_compressiondiffusion bonded to metallic sheet 28 of
structured copper strain buffer 24. Heat sink 46
- further includes fin structure 47 fastened to block 43 with
a bolt 49. Alternately, fastening bolts 48 and 49 are
not required if heat sinks 44 and 46 are each respectively
fabricated as a single metallic member. That is, the
fin structures and metallic blocks may be combined as one
to form heat sinks 44 and 46.
Metallic braids 55 and 57 are connected to heat sinks
44 and 46, respectively, to facilitate connection of device 39
to external electrical circuitry.
In the embodiment of Figure 5, silicon wafer 12 is
depicted as a thyristor, although it could be a diode,
transistor or other semiconductor device as well. Thyristor
12 includes a cathode 12a and anode 12b on opposite sides of
the silicon wafer. Thyristor 12 includes a gate 12c
situated on the cathode side of thyristor 12. Because
of the presence of gate 12c in the interior of device 39,
a means must be provided for connecting it to circuitry
external to device 39. For example, Figure 5 shows
a channel 21 disposed above gate 12c and passing through
strain buffer 14 and metallic layers 13-and 15. Block 41
includes a channel 41b with one end thereof in communication
with channel 21 and the other end in communication with
the external surface of block 41. An electrically conductive
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: . - : .: : - - . . . .
,
_ li3~793 RD-11272
wire 17 (gate lead) i8 situated in channels 41b and 21.
Gate lead 17 is connected to gate 12c and is surrounded
by.electrically insulative material 19 to prevent it from
contacting the metallic parts of device 39 and becoming
electrically short-circuited theret~.
Figure 8 shows another fluid cooled
semiconductor device 60 without dry interfaces between the
elements thereof. Semiconductor device 60 comprises
a silicon wafer 12 with a cathode 12a and an anode
12b on opposite sides thereof. A first metallic layer
62 is deposited in contact with cathode 12a of
silicon wafer 12. A second metalllc layer 64 is deposited
in contact with metallic layer 62 as shown in Figure 8. Metallic
layer 62 may be comprised of titanium, çhromium or nickel
and metallic layer 64 may be comprised of gold, silver
or copper. One common end of the copper strands 16 of
. structured copper strain buffer 14 ~ully described in
the discussion of the semiconductor device 10 of Figure 1)
is thermo-compression diffusion bonded into metallic
layer 64. The metallic sheet 18 of structured copper
strain buffer 14 is thermo-compression diffusion bonded
to fluid cooled heat sink 20 (also fully described above).
A support plate 66 is joined to the anode 12b of
silicon wafer 12 via metallic braze 68 (an aluminum braze,
for example) to provide structural integrity to the rather
fragile wafer 12. Support plate 66 is comprised of
tungsten, molybdenum or other metal with a thermal
coefficient of expansion similar to that of silicon.
A metallic layer 67 is deposited in contact with support
plate 66 and a metallic layer 69 is deposited in contact with
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~131~7~33 RD-11272
layer 67 as shown in Figure 8. Metallic layer 67 may be
comprised of such metals as titanium, chromium or nickel and
metallic layer 69 may be comprised of gold, silver or
copper. One common end of the copper strands 26 of structured
copper stain buffer 24 is thermo-compression dlffusion
bonded to metallic layer 69. Fluid cooled heat sink
30 is thermo-compression diffusion bonded to metallic foil
28 of structured copper strain buffer 24.
To form the thermo-compression diffusion bond to
such support plates (shown as 66 in Figure 8, for example) and
a structured copper strain buffer (24, for example), the
surface of support plate 66 is first cleaned to remove
any oxide coating therefrom. Sputter etching, for example,
may be used to accomplish such cleaning. Metallic layer 67
comprised of titanium, nickel or chromium, for example, is
then deposited on the cleaned surface of support plate 66.
Metallic layer 69 comprised of copper, gold or silver, for
example, is then deposited on metallic layer 66 to provide a
bbnding surface. Structured copper strain buffer 24 is
then positioned in substantial abutment with metallic layer
69. The assembly formed by support plate 66, metallic
layer 67 and 69 and structured copper strain buffer 24
(together with the remainder of the particular embodiment
of the semiconductor device of the invention) is si~uated in
press 40 of Figure 7 between pressing block 46 and lower plate
44. It is generally desirable to form simultaneously as many
of the thermo-compression diffusion bonds in the device of
the invention as possible (excluding the structured copper
strain buffers which are convenient~y preformed). Thus,
the entire device is situated in press 40 to form the desired
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~3~9~3 RD-11272
diffusion bonds between the elementæ thereof.
In the manner described with regard to Figure 7 above,
bolts 58 and 59 of preæs 40 are tightened. Preææ 40
with the aææembly to be bonded together contained therein
iæ placed in an inert atmoæphere. Preæs 40 iæ heated
at a temperature within the range of 300C-400C which
squeezes the assembly toge~her at high pressure, typically
-2Q,000 to 50,0~0 psi and causes t4e desired thermo-compression
diffusion bondæ to be formed. After an interval of about
15 mlnuteæ to 5 hour~, the completed semiconductor device is
removed from presæ 4Q.
The thermo-compression diffusion bonding method thus
described may also be used to bond other copper memberæ,
in addition to structured copper members, to a support
lS plate, uch aæ 66. For example, a copper tab for
electrical connection purposeæ may be bonded to a tungsten
or D lybdenum pla~e using this method ,
~317~3 RD-11272
Figure 9 shows a fluid cooled thyristor device 80 which
is similar to device 10 illustrated in Figure 1 except for the
following modifications. In device 80, semiconductor wafer 12
comprises a thyristor, with an anode 12b on one side thereof
and a cathode 12a and a gate 82 on the opposite side thereof.
In order to gain access to gate 82, it is necessary to provide
gate connecting means 83 to facilitate connection of the gate
to external circuitry. Thus, heat sink 84 and strain buffer 14
are provided with a hole 90 po~itioned above gate 82 such
that a lead wire 83 may be connected to gate 82. With lead
wire 83 in place and connected to gate 82, hole 90 is packed
with an electrically insulating material 92 to prevent gate
- 82 from being short-circuited to heat sink 84 and strain
buffer 14. Heat sink 84 is identical to the heat sink 30 of
Figure 1, except for aperture 90. Fluid inlet 84a an-d fluid
outlet 84b of Figure 6 correspond in structure and function
to inlet 20a and outlet 20b, respectively, of Figure 1, while
fluid chamber 86 corresponds to fluid chamber 22 of Figure 1.
Heat sink 84 is situated atop, and thermo-compression
diffusion bonded to, metallic sheet 16 of structured copper
strain buffer 14.
Figure 10 is a top view of fluid cooled thyristor devlce
80, showing the positioning of hole 90 and lead wire 83
as it passes through heat sink 84.
Figure 11 shows another embodiment of the fluid cooled
thyristor device of the invention. Fluid cooled th~ristor
device 100 is subst~antially similar to fluid cooled semi-
~131793
RD~ 72
conductor device 60 shown in Figure 8 with the following
exceptions. Silicon wafer 12 comprises a thyristor having
an anode 12b on one side thereof and a cathode 12a and a
gate 82 on the opposite side thereof. A heat sink 84 is
constructed in the same manner as the heat sink illustrated in
Figure 9 and includes a hole 104 extending therethrough to
provide a means for connecting a lead wire 102 to gate 82
located below heat sink 84. Hole 104 further extends through
structured copper strain buffer 14 and metallic layers 62 and
64 and is situated above gate 82. Lead wire 102 passes from
the area external to semiconductor device 100 through hole 104
and is connected to gate 82. Lead wire 102 is surrounded with
an electrically insulating material 106 to prevent wire 102
from short-circuiting to heat Rink 84, structured copper
strain buffer 14 or metallic layers 62 and 64. Thus, gate 82
of thyristor 12 may be connected to circuitry external to
semiconductor device 100.
~gure 12 shows another fluid cooled semiconductor
device 110 which avoids the use of dry interfaces
between the elements thereof. Semiconductor device 110 is a
preferred embodiment and is similar to device 31 of
Figure 3 with the following exceptions. Silicon
wafer 12 is a thyristor with a cathode 12a and an anode 12b
on opposite sides thereof. A gate 12c is centrally
situated on the cathode side of wafer 12. To connect
gate 12c to circuitry external to device 110, a channel 21-is
disposed above gate 12c and passes through metallic layers 13
and 15 and structured copper strain buffer 14. A channel
141 is situated in the portion of heat sink 34 below central
plate 36. One end of channel 141 is situated in the portion
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: 11317'~3 RD-11272
of heat sink 34 below central plate 36. One end of
channel 141 is in communication with channel 21 and the
other end of channel 141 is in communication with the
exterior of device llO. A portion of an electrically
conductive wire 117 (gate lead) is sîtuated in channels
21 and 141 and connected to gate 12c. As shown in Figure
- 12, this portion of gate lead 117 is surrounded by
electrically insulative material 119 to prevent lead 117
from short circuiting to other components of device 110.
The remaining portion of gate lead 117 passes through a
hole in housing 111 to the exterior of device llO. Gate
. lead lL7 is sealed to this hole in housing 111.
Device 110 includes hermetically sealed housing 111 to
protect the components of device llO situated between
heat sinks 34 and 35 from external contaminants as well
as internal vapor conden~ation. In the embodiment shown in
`. Figure 12, housing 111 exhlbits a ring-like shape with
upper and lower surfaces. The geometry of housing 111
may vary to accommodate the particular size and shape of
silicon wafer 12 and associated components used. Housing
111 is made of electrically insulative material.such as
a ceramic. The upper and lower surfaces of housing.lll,
respectively, include a metallic flange llla and lllb,
Flange llLaaligns with a metallic flange 121a extending
from the lower portion of heat sink 34 as shown in Figure 12.
Flange lllb aligns with a metallic flange 121b
extending from the upper portion of heat sink 35. The
space enclosed by housing 111 is filled with a gas such
as dry air, nitrogen or other gases to prevent the
condensation of water vapor on the internal components
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.
~ 7~3 RD-11272
of device llO. ~'langes l~la and lllb are respectively
hermetically sealed to flanges l21a and lZil~ to prevent
the above-mentioned gas from escaping from device 110
and to prevent contaminants from reaching the internal
components of device 110. Housing 111 is provided with
circular ribs 113 to increase the effective voltage
insulating capability of housing 111.
~~gure ]8 is a side view of semiconductor
device 110.
It should be understood that many variations of the par-
ticular semiconductor devices disclosed above are possible and
are within the scope of the invention. Referring again to
Figure 12, for example, the invention may be practiced
with or without hermetically sealed housing 111 although
superior results are achieved when housing 111 ~s used.
Silicon wafer 12 may comprise virtually any semiconductor
device such as diodes, transistors and thyristors, for
example, with means provided for connecting each electrode
of the wafer to external electrical circuitry.
Either side of silicon wafer 12 may be attached
to a metallic support plate having a thermal coefficient
of expansion similar to that of silicon, such as tungsten
and molybdenum, for example, to increase the structural
integrity of the relatively fragile wafer 12. This wafer-plate
assembly may be substituted for wafer 12. The
remainder of the semiconductor device of the invention
is constructed in the same manner as disclosed above.
Further embodiments of the invention are achieved
in that strain buffers 14 and 24 of Figures 1, 3, 5, 8, 9,
11 and 13 may have a metallic sheet thermo-compression
diffusion bonded to each side thereof, instead of,
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~131~93 RD-11272
as depicted in the drawings with metallic sheets respectively
bonded to one side thereof. In such further embodiments, it
is important that the thickness of the metallic sheets
closest to silicon wafer 12 be rela~ively thin, typically
1-3 mils thick. Somewha~ smaller thicknesses of such metallic
sheets may be used as well on larger thicknesses, providing
the thickness is not so large as to significantly reduce
the stress relieving capabilities of structured copper
strain buffers 14 and 24. The thickness of the metallic
sheets 18 and 28 (that is, the sheets furthest from
~ silicon wafer 12) is typically in the range o 1/2 mil to
; 20 mils, which is a thickness commensurate to that possessed
by metallic foils, and conveniently lies in the range of 1-3
mils. Alternatively, thicknesses of metallic sheets 18
and 28 somewhat smaller than 1/2 mil may be used, provlding
the thickness is sufficient to provide structural integrity
to structured copper disks 16 and 26, respectively, joined
thereto. As another alternative, the thickness of metallic
sheets 18 and 28 may be larger than 20 mils, even so large
as to form a block of metal, provided the thickness is not so
large as to substantially increase the thermal resistance
of strain buffers 14 and 24.
The foregoing describes a fluid cooled semiconductor
device employing structured copper strain buffers to connect
each side of a silicon wafer to respective fluid
cooled heat sinking means. Thermo-compression diffusion bonds
are used at the above described interfaces of the device,
thereby eliminatlng dry interfaces. Electrical conductance to
the electrodes of the device is thereby significantly increased
while thermal conductance and capability to remove heat from
-21-
~13~793 RD-11272
the device are also increased.
While only certain preferred features of the invention
have been shown by way of illustration, many modifications and
changes will occur to those skilled in the art. I. is,~l~ere-
fore, ~o be understood that the appended claims are intendedto cover all such modifications and changes as fall within the
true spirit of the invention.
