Low thermal resistance, low stress semiconductor package

A semiconductor device having especially low thermal impedance between a semiconductor element and a heatsink is described wherein the semiconductor element is directly disposed upon a thin, low thermal impedance current-spreading layer on an electrically isolated layer. An electrode connected to the current-spreading layer surrounds the semiconductor element so that it introduces no additional thermal impedance between the semiconductor element and the heatsink. Relatively inexpensive electrode materials may be used in accordance with the invention without increasing thermal impedance.

This invention relates in general to semiconductor packages and more 
particularly to isolated semiconductor packages wherein especially low 
thermal impedance and long life, under conditions of repeated temperature 
cycling, are required. 
An important function of a semiconductor device package is to provide 
effective thermal conduction between the semiconductor element for which 
the package is provided and an external heatsink to which the device is 
attached during operation. While a number of considerations determine the 
efficiency of heat transfer between the semiconductor element and the 
ultimate heatsink this invention is primarily concerned with those factors 
which are attributable to the structure of the semiconductor device 
itself, including the semiconductor element, the electrical insulating 
layer which isolates the semiconductor element from the device heatsink, 
and the device heatsink itself which is adapted to be connected to an 
external heatsink by conventional mechanical means. A typical 
semiconductor device in accordance with the prior art includes a number of 
different materials as well as a plurality of interfaces thermally and/or 
electrically coupling the materials for electrical connection to the 
semiconductor element and for thermal connection between the semiconductor 
element and the external heat-sink. The intimate assembly of these varied 
materials can lead to thermal stresses at the interfaces between the 
several elements as the device is subjected to heating and cooling during 
normal operation. It is common-place during the operation of such devices 
that the temperature thereof alternately increases and decreases as a 
result of the device being turned on and off. Differential expansion of 
package elements made of materials having different coefficients of 
thermal expansion inevitably stresses the interfaces therebetween and is a 
major cause of thermal fatigue in the package, and ultimately in the 
premature failure of the device. Where it is desired to package several 
semiconductor elements in a single hybrid module, the problem is 
aggravated inasmuch as additional parts which are required to be provided 
in the assembly in order to interconnect two or more devices, add thermal 
resistance in the path between the semiconductor element and the heatsink 
as well as potentially increasing the number of interfaces between 
materials having different coefficients of thermal expansion, which 
interfaces are the potential sites of failure due to thermal fatique. 
Because of its excellent electrical and thermal conductivity, copper is a 
desirable material for interconnecting multiple semiconductor elements 
within a single semiconductor module. In order to provide a surface which 
is readily soldered, copper parts are oftentimes nickel plates, or in the 
alternative, plated with a combination of nickel and silver. These plated 
copper elements may be soldered both to a metallized ceramic insulating 
member and to the electrode or stress relief plate of a semiconductor 
element or subassembly. Ceramic insulating members such as beryllium oxide 
and aluminum oxide are readilyy metallized as, for example, with 
molymanganese, or in the alternative, with copper according to the direct 
bond process described for example in U.S. Pat. No. 3,994,430 to Cusano, 
et al., of common assignee herewith, the contents of which are herein 
incorporated by reference. Typically, solder has been utilized as a 
joining medium. The melting point of lead-tin solder, which is widely 
used, depends upon the relative proportions of the two ingredients. For 
example, 37% lead-63% tin solder has a melting temperature of 181.degree. 
C., while 50--50 lead-tin solder has a melting point of 216.degree. C. 
Where materials with substantially different co-efficients of thermal 
expansion are joined with a solder layer, substantial stresses are 
produced in the solder layer due to both the bonding itself as well as 
when thermal cycling occurs. Lead-tin solders of the types described have 
a relatively low melting point and therefore exhibit relatively low 
residual stress (stress due to bonding dissimilar materials). They are, 
however, particularly subject to fatigue as the frequency of occurrence of 
thermal cycling stresses increases. For example, silicon devices are 
oftentimes bonded to a piece of refractory metal of low thermal 
coefficient of expansion close to that of silicon. Tungsten, molybdenum, 
and the like, are oftentimes employed. The thermal coefficient of 
expansion for molybdenum is 4.9.times.10.sup.-6 in./in. .degree.C. The 
thermal coefficient of expansion for copper is 17.0.times.10.sup.-6 
in./in. .degree.C. The stresses developed at the refractory metal-copper 
interface are extremely high, and further, are dependent on the 
temperature at which the bond is formed, higher bonding temperatures 
resulting in higher residual stress. Accordingly, low temperature solders 
are preferred in order to maintain the stress level at the interface at an 
acceptable level. As has been mentioned, however, such low temperature 
soft solders have a limited fatigue life due in part to the absence of 
annealing of such layers, non-complete plastic deformation, and the like. 
A partial solution to the problem of limited fatigue life has been to 
employ hard solders such as gold-tin eutectic solders having melting 
points in the range of 280.degree. C. Such hard solders are better able to 
withstand the stresses caused by differential thermal coefficient of 
expansion in dissimilar materials than the low temperature soft solders, 
but the high mechanical strength of the solder material introduces further 
problems which may be equally as detrimental as short thermal fatigue 
life. For example, when a high expansion coefficient material is directly 
bonded to the surface of a semiconductor element using hard solder, 
fracture at the soldered interface of the brittle semiconductor element is 
likely to occur. Such hard solder materials are also of very high cost. 
As has been mentioned, thermal stresses developed at interfaces of two 
dissimilar materials are directly dependent on the bonding temperature. In 
small devices, i.e., in devices having a small physical size, the use of 
higher temperature bonding materials (such as solder of 280.degree. C. 
melting) may represent an acceptable solution. In large devices, the 
stresses developed in the solder during bonding are oftentimes sufficient 
to physically deform one or the other of the bonded materials resulting in 
bowing or the like, as is commonly found. Such bowing leads to lack of 
flatness of parts of the semiconductor package especially the device 
heatsink which may result in thermal runaway problems inasmuch as the 
device may not be well coupled to an external heatsink. In an extreme 
case, the deformation of the package elements may be sufficient to crack 
to otherwise damage the semiconductor element resulting in device failure. 
Accordingly, in high current, large area devices, compression packaging 
systems have been employed. The various elements of the device are placed 
in intimate but non-bonded contact and are urged together by internal or 
external spring forces. Such devices are widely employed in the high power 
semiconductor industry. Such compression devices suffer from several 
disadvantages, however. They are somewhat more complex and of 
substantially higher cost, requiring not only an external heatsink, but 
additionally, a clamp or the like, for providing the required compressive 
forces for proper operation. Such systems are of higher cost and greater 
complexity than devices where such additional structural elements are 
eliminated. Further, the efficiency of heat transfer from the device to an 
external heatsink where dry interfaces are employed is substantially less 
than a device wherein bonded or other wet interfaces are properly 
utilized. 
Accordingly, it is an object of this invention to provide a semiconductor 
device having, in combination, lower thermal impedance between a 
semiconductor element and the device heatsink and lower thermal stresses 
during operation due to differential thermal expansion of the elements of 
the package. 
It is another object of this invention to provide a multi-element 
semiconductor package wherein two or more semiconductor elements are 
connected by low electrical resistance members which are disposed so as 
not to increase the thermal impedance between the semiconductor elements 
and the device heatsink. 
It is another object of this invention to provide such an improved 
semiconductor device wherein high temperature soft solders such as those 
consisting mainly of lead, or hard solders, may be employed so as to 
provide high reliability and long lifetime even in the presence of 
repeated temperature cycling. 
It is still another object of this invention to provide such a 
semiconductor device at a low cost and an easy-to manufacture form. 
Briefly stated and in accordance with one aspect of this invention, a 
semiconductor device is provided having heatsink adapted to be bonded to 
an external heatsink or heat exchanger. A layer of electrically insulating 
material is disposed on the device heatsink in low thermal impedance 
relation thereto. A relatively thin, current-spreading layer of highly 
annealed, electrically conductive material is bonded to the 
current-spreading layer. A relatively thicker, higher conductivity layer 
surrounds the electrical element and contacts the thin layer for providing 
low impedance electrical connection to the surface of the semiconductor 
element in contact with the thin, current-spreading layer. The layer 
provides electrical connection to the semiconductor element without 
introducing either the thermal impedance or the differential thermal 
coefficient of expansion of a thicker, completely nonannealed layer. 
Essentially, the thin layer functions solely as a current-spreading layer 
being substantially transparent from a thermal viewpoint.

FIG. 1 is a section view of a prior art device including a semiconductor 
element 10 mounted to a thermal stress relief plate 12 which element and 
stress relief plate are bonded to electrode 14 by solder layer 16. 
Electrode 14 includes an interconnecting portion 18 which may conveniently 
provide electrical connection to another semiconductor element within the 
same package. Electrode 14 is further bonded to insulating layer 20 which 
is bonded to internal heatsink 22. The bonding of insulating layer 20 to 
heatsink 22 as well as the bonding of the electrode 18 to insulating layer 
20 is accomplished by metallizing the insulating material as for example 
with an Mo-Mn system as is well known to those skilled in the art and then 
soldering the thus metallized insulating material to the heatsink and to 
the electrode. Conventionally, both the electrode and the heatsink are 
selected to have low thermal and electrical resistance and in many cases 
may be made of copper. Thus interfaces 24 and 26 between insulating layer 
20 and electrode 18 and heatsink 22, respectively, are oftentimes 
multi-element interfaces and are illustrated herein as single layers 
solely for the purpose of simplifying the drawing. High current carrying 
capacity terminal 28 is also bonded to electrode 14 by brazing or the like 
and provides electrical connection to the lower terminal of the device. 
Connection to the upper contact of semiconductor device 10 is conveniently 
made by flexible lead 30, washer 32, and bonding layer 34. Washer 32 may 
conveniently be a molybdenum washer which is soldered to the top surface 
of semiconductor element 10. Likewise, lead 30 may be soldered to washer 
32 at an interface which is not shown in detail herein. 
Heat, generated within semiconductor element 10, must flow through stress 
relief plate 12, interface 16, electrode 14, interface 24, insulating 
member 20, interface 26, heatsink 22, and ultimately through the interface 
between heatsink 22 and an external heatsink prior to being dissipated 
into the air. The presence of a large number of heretofore essential 
elements and the interfaces between them increases the thermal impedance 
of a prior art device. Further, the presence of a plurality of dissimilar 
materials between semiconductor device 10 and the external heatsink, which 
materials have different coefficients of thermal expansion, results in the 
creation of substantial mechanical stresses, especially at the interfaces 
as the device heats and cools during normal operation. 
A semiconductor device in accordance with one embodiment of the instant 
invention is illustrated at FIG. 2. It will be understood by those skilled 
in the art that while this invention is especially useful in and provides 
superior results for semiconductor devices, including two or more 
semiconductor elements in a single package, the application thereof is not 
so limited and single element semiconductor devices are equally within the 
ambit of the invention. Accordingly, FIG. 2 illustrates but a single 
semiconductor element in a form which is a portion of a two-element 
semiconductor device but which with only very slight modifications as will 
be hereinbelow described provides an exemplary, single element 
semiconductor device. 
FIG. 3 is an exploded view of the structure of FIG. 2, wherein the 
individual elements may be more easily seen. Like reference numerals in 
FIGS. 2 and 3 denote like elements. A base member 40 is provided which 
base member is characterized by high thermal conductivity, and which, due 
to equalization of stresses on both sides of the electrically insulating 
layer may be thinner than has been heretofore possible without suffering 
from undesirable distortion due to stresses formed on bonding of the 
device. Typically, base 40 is made of copper which has excellent thermal 
conductivity and is relatively easy to fabricate in a desired shape. While 
copper is a presently preferred material for base member 40, this 
invention allows the use of other materials as, for example, steel or 
combinations of two or more materials, such as copper and steel, which 
materials are of lower cost. The strength and coefficient of thermal 
expansion of steel both allow and permit a relatively thinner base member 
to be utilized; for example, in accordance with this invention a steel 
base member having a thickness between one-third and one-sixth that of a 
copper base member is advantageously employed. 
Due to the relatively small thickness which may be employed in accordance 
with this invention, base 40 may easily be stamped from a larger sheet of 
material and thus provided in an extremely low cost manner as compared to 
prior art methods wherein the thickness of material required eliminated 
punching or stamping as a method for forming the base. Base 40 acts as a 
heatsink for the device and also to transfer heat to an external heatsink 
to which it is mounted. Insulating layer 42 is provided with bonding layer 
44 on a lower surface thereof which layer is selected to be compatible 
with and bondable to base 40 by soldering or the like. In accordance with 
a presently preferred embodiment of this invention, layer 44 is a direct 
bonded, copper layer; insulating layer 42 is a ceramic layer of, for 
example, alumina or beryllia, or a like material having a high thermal 
conductivity and which bonds directly to layer 44 as described for example 
in U.S. Pat. No. 3,994,430 to Cusano, et al., the contents of which are 
incorporated by reference herein. Alternatively, bonding layer 44 may be a 
metallized layer such as is formed by the moly-manganese metallization 
process well known to those skilled in the art. Or, as yet another 
alternative, any layer of material which forms a low thermal impedance 
bond to base 40 may be employed. The upper surface of insulating layer 42 
is provided with current-spreading layer 46 which is a low electrical 
resistance layer which performs dual functions of conducting current from 
semiconductor subassembly 50 to electrode 48 as will be described and of 
providing thermal connection between semiconductor element 50 and 
insulating layer 42. Preferably, current-spreading layer 46 is also a 
direct bonded copper layer similar to and formed simultaneously with layer 
44 in order to minimize the stress on insulating layer 42. In accordance 
with the preferred embodiment of this invention, layers 44 and 46 are each 
about 10 mils thick and exhibit the completely annealed characteristics of 
direct bond copper as described in the Cusano, et al. patent. In 
accordance with a preferred embodiment of this invention, insulating 
member 42 is bonded to current-spreading layer 46 and bonding layer 44; in 
accordance the direct bond process and copper layers 44 and 46 are nickel 
plated and solder coated prior to being bonded to base 40. It is preferred 
that base 40 also be nickel plated in order to enhance soldering thereto. 
In order to increase the voltage handling capability of a device in 
accordance with this invention, current-spreading layer 46 and bonding 
layer 44 are made slightly smaller than insulating layer 42 in order to 
provide longer creep and strike paths between the two layers and between 
current spreading layer 46 and base 40. Further the corners of current 
spreading layer 46 are provided with a radius as indicated which further 
increases the creep and strike distances as well as eliminates thermal 
stress concentration points developed at the corners of direct bonded 
copper to ceramic interfaces. Electrode 48 is preferably a low electrical 
resistance electrode which will support a high current in accordance with 
the current capability of the device. It is not necessary, however, that 
electrode 48 have a particularly low thermal impedance inasmuch as it 
functions solely as a current-carrying electrode and is not in the thermal 
path between semiconductor subassembly 50 and heatsinkable base 40. 
Preferably, electrode 48 is fabricated of the same material and has 
similar thermal expansion characteristics as base 40 so that the stresses 
on the opposite sides of insulating member 42 are equalized. In accordance 
with an alternative embodiment of this invention, electrode 48 may be 
fabricated of a material which exhibits good electrical conductivity but 
which may have relatively poor thermal conductivity as compared with 
copper. Aluminum has been found to be an advantageously employed material 
for electrode 48 inasmuch as its electrical conductivity is relatively 
high and its cost is low compared to that of copper. Electrode 48 is 
provided with a connecting portion 52 which will be seen hereinbelow to 
provide electrical connection between multiple elements in a single device 
package in accordance with this invention. Electrode 48 is further 
provided with opening 54 into which semiconductor subassembly 50 is 
disposed in direct contact with current spreading layer 46. Preferably, 
opening 54 is provided with a beveled region 54 in order to increase the 
high voltage capability of the device by increasing the distance between 
semiconductor element 58 and electrode 48. Semiconductor subassembly 50 
includes semiconductor element 58 and stress relief member 60 which are 
preferably bonded together prior to the assembly of this device. The use 
of such semiconductor subassemblies for higher current devices is well 
known to those skilled in the art, and tungsten and molybdenum are 
commonly employed as stress relief plates when silicon semiconductor 
elements are utilized inasmuch as there is good thermal coefficient of 
expansion compatibility therebetween. Preferably, contact is made to the 
upper surface of semiconductor element 58 by thermally compatible washer 
62 which is preferably of molybdenum or tungsten and which may be suitably 
plated as for example with nickel-silver alloy and which is soldered to 
the upper surface of element 58. Where desired, a solder preform 64 may be 
used. Electrical lead 66 is bonded to upper stress relief washer 62 in a 
conventional manner by brazing at a relatively high temperature of about 
700.degree. C. prior to bonding washer 62 to semiconductor element 58. 
The assembly of base 40; insulating member 42, to which current spreading 
layer 46 and bonding layer 44 have been previously applied, electrode 48, 
and the previously assembled subassembly which includes (semiconductor 
subassembly 50) and stress relief washer 62, lead 66 and (semiconductor 
subassembly 50) and the elements therebetween, is preferably accomplished 
in a single high temperature soldering step wherein a solder comprising 
about 95.2 percent lead, 0.05 percent tin, and 2.5 percent silver is used. 
This assembly operation may conveniently be carried out in a tunnel oven 
or the like. Solder interfaces are thereby provided between bonding layer 
44 and base 40, between electrode 48 and current-spreading layer 46, 
between semiconductor subassembly 50 and current spreading layer 46, and 
between semiconductor subassembly 50 and washer 62. 
A number of advantages of the instant invention may be readily observed by 
a comparison now between the structure of the prior art illustrated at 
FIG. 1 and the instant invention illustrated at FIG. 2. It will be seen 
that electrode 48 does not contribute to the thermal impedance between 
semiconductor subassembly 50 and base 40 while the electrode 18 in 
accordance with the prior art is thermally interposed between 
semiconductor subassembly 12 and base 22. The thermal impedance of a 
device in accordance with the priot art is increased not only by the 
presence of electrode 18 in the thermal path between the semiconductor 
device and the ultimate heatsink, but also by the presence of an 
additional interface 16 between the semiconductor element and the 
electrode. Neither the electrode nor the interface contribute to the 
thermal impedance of a device in accordance with the instant invention. 
Further, semiconductor subassembly 12 is bonded, in the prior art to 
electrode 18 which is, as is the case of the instant invention, a copper 
electrode having thermal characteristics which are very poor match to 
those of silicon and of molybdenum which are used for the semiconductor 
element and backup plate, respectively. Non-annealed copper is required 
for electrode 18 inasmuch as substantial mechanical strength is a 
requirement for processing handleability. The instant invention offers a 
substantial improvement inasmuch as non-annealed material is disposed 
between semiconductor subassembly 50 and relatively thin and highly 
annealed, current-spreading layer 46. Accordingly, the thermal stresses 
generated during temperature cycling of a device in accordance with the 
instant invention are significantly lower than has been heretofore 
possible, resulting in substantially improved reliability of the device, 
and equally important, allowing the use of high temperature soldering 
operations and higher temperature solders in assembly of the device, which 
greatly improves the thermal fatigue capabilities of the device. 
FIG. 4 is an outline drawing of a two-element semiconductor device in 
accordance with this invention. A plastic or otherwise insulating housing 
70 is bonded to base 40 and electrical terminals 72, 74, and 76 provide 
high current connection to the device while gate terminals 78 and 80 
provide low current control connection. 
The structure of a device according to FIG. 4 may be seen in the cross 
section view of FIG. 5 which also illustrates the interconnection of two 
semiconductor elements in a single package. 
Like elements in FIGS. 2, 3, 4, and 5 are designated with like reference 
numerals. High current carrying capacity terminal posts 28 and 84 are 
connected to the electrodes 48 and 86. Screw-type terminals 72, 74, and 76 
are connected to terminal posts 28 and 84 and to lead 66 by low 
temperature soldering for high mechanical strength. Gate leads 78 and 80 
are likewise connected to gate electrodes 88 and 90 of semiconductor 
elements 58 and 92, respectively. A number of unique advantages and 
further details of the package illustrated at FIGS. 4 and 5 may be further 
appreciated by reference to co-pending U.S. Patent Application, Ser. No. 
963,807, of common assignee herewith. 
While the invention has been illustrated and described in accordance with a 
presently preferred embodiment thereof, various changes and modifications 
may occur to those skilled in the art which do not depart from the true 
spirit and scope of the invention. For example, although it is presently 
preferred to provide a relatively thick, direct bond copper 
current-spreading electrode in accordance with this invention, it is also 
appreciated that a relatively thinner current-spreading layer may be 
employed, fabricated for example according to the moly-manganese 
metallization process. Specifically, the thickness of metallization 
achievable directly according to the moly-manganese process is on the 
order of one or two mils, while direct copper bond electrodes may be 
provided having the thickness of ten mils or greater in a single step. 
Where the thinner current-spreading layers are used, it is preferred that 
electrical connection be made not only between electrode 48 and 
current-spreading layer 46, but also directly between electrode 48 and 
stress relief electrode 60 as shown in FIG. 6. This connection may readily 
be achieved by the introduction of solder ring 96 into peripheral space 94 
which surrounds stress relief electrode 60. Where the spacing between 
electrode 48 and stress relief electrode 60 is sufficiently narrow, on the 
order of 5-10 mils or less, solder will be introduced into peripheral 
space 94 by capillary action during the assembly of the elements of this 
invention as hereinabove described. 
In order to enhance this capillary action, it is preferred to nickel or 
nickel-silver plate thermal stress relief member 60 on the back and at 
least half way up the edges thereof. A comparison of the thermal impedance 
of a device fabricated in accordance with the prior art and a device 
fabricated in accordance with the instant invention as illustrated at FIG. 
2 has demonstrated that the thermal impedance of a prior art device has 
thermal impedance measured between the junction and base 22 of between 
about 0.22.degree. and 0.28.degree. C. per watt while the thermal 
impedance of a device in accordance with this invention is about 
0.15.degree. C. per watt. Further, it is estimated that thermal fatigue 
life is at least an order of magnitude better than that of prior art 
devices, all these improvements being achieved in a device which has a 
lower cost than has been heretofore possible. 
These and other modifications and changes are intended to be within the 
scope of the appended claims.