Hermetically sealed semiconductor ceramic package

A ceramic hermetic package is provided having conductive material bonded into apertures in the ceramic housing that connect with a die mounted on a thin substrate through wire and pad connections. The substrate has a heat sink affixed to the bottom and electrical connections are formed on the opposite face of the package from the heat sink so that the encased semiconductor may be mounted on circuit boards in either a vertical position or in a position where the heat sink is horizontal but facing upward.

BACKGROUND OF THE INVENTION 
This invention relates generally to an electronic or semiconductor device 
package structure and, more particularly, a package for a semiconductor 
device as a hermetically sealed unit. 
Semiconductor devices employed in high current type applications, commonly 
referred to as power devices, generate a substantial amount of heat in 
operation of the device. In the past it has been very common to have a 
package known as a TO-3 package, which essentially had a large heat sink 
area, the die being encapsulated by a metal cap, while the leads were 
brought out through metal-to-glass seals. If demand calls for a package 
which must be electrically isolated, a ceramic substrate has to be placed 
in the package and, if the package was a power package and had to 
dissipate heat, the ceramic substrate would generally be beryllium oxide, 
and in order to help to dissipate the heat, that is spread it, an 
efficient means must be provided in contact with the substrate. Whatever 
this means may be, it must not stress the substrate by differences in 
thermal expansion coefficients. The commercial acceptability, therefore, 
of such devices is based upon their ability to dissipate the generated 
heat at a high rate as excessive heat retention can destroy the 
semiconductor die within the device. 
Miniaturized packages, whether they be a high power device or a low power 
device, still have to dissipate heat. Problems arise because there is a 
linear coefficient of expansion of the semiconductor die of approximately 
3.5.times.10.sup.-6, and this has to be closely matched in some way to the 
material upon which the die is mounted or there will be undue stresses 
developed which can cause failure of the die or create die attachment 
reliability problems. 
Another consideration is fabrication of the unit as it is desirable to 
simultaneously perform soldering operations. 
Another problem with prior art semiconductor packages has been the power 
loss problem and inductance in the leads as the operating frequency is 
increased. This is due in part to the use of poor conductors and 
conductors of a small size that lead away from the die. The present 
invention accordingly has the object to eliminate these problems for radio 
frequency operation by utilizing rectangular copper clad conductors which 
are useful to one 1 GHz. 
It is, therefore, a primary object of this invention to obviate some of the 
foregoing problems inherent in prior art packages. 
It is a further object of the invention to provide a structure for a 
package of a semiconductor device which closely matches the thermal 
coefficients of expansion while providing good heat dissipation and 
electrical insulation. 
It is another object of the invention to provide a semiconductor package 
having terminals which permit it to be mounted directly on a circuit board 
with the heat sink dissipating area mounted on top of the package spaced 
from the board in what is commonly termed "surface mount". 
It is still another object of the invention to provide a semiconductor 
package where the package may be mounted on a circuit board in a vertical 
direction where the heat sink rises vertically from the board in surface 
or through board fashion. 
Another object of the invention is to provide a package wherein the 
connections can be preformed in one step and the cover may be attached in 
a subsequent step without destroying the previous connection joints. 
SUMMARY OF THE INVENTION 
The objects noted above are accomplished by providing a hermetically sealed 
ceramic base package that has a thin thermally conductive insulating 
ceramic layer on which the die is mounted and away from which leads are 
directed, first in the plane of the die and then at right angles thereto 
through a ceramic side wall. The package with ceramic substrate and side 
walls supports and electrically insulates the die, its leads, and provides 
a housing which may be easily hermetically sealed. 
According to this invention, a semiconductor die is packaged on an 
insulating substrate. A continuous ceramic wall rises from the insulating 
substrate, forming a part of an enclosure about the semiconductor die. A 
passageway is created at the juncture of the wall and the upper surface of 
the substrate, and an aperture or via passes down from the top surface of 
the wall to the substrate where a lead connecting plate is placed. The via 
is coated with a refractory metal and is filled with a copper conductor 
which is plated or clad with silver, and then a connector pad is placed 
over the copper conductor and electrically conductive material is placed 
in the recess to contact the copper conductor. When the entire structure 
is brazed or fired, a hermetic seal of the electrical leads is created, 
the silver coated copper conductor bonding to the ceramic metalized 
aperture or via. The semiconductor die can then be connected to the 
electrically conductive material in the lead aperture or recess, and the 
semiconductor device is soldered to its lead connecting plate, preferably 
by using a high temperature solder. The entire package may then be 
subjected after testing to a bake-out process that removes moisture, and 
then a cover may be placed over the wall which is soldered onto the top 
surface of the wall with a low temperature solder to form an 
environmentally impermeable seal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to FIGS. 1 and 2, the details of the semiconductor package 
are readily seen. The package generally designated 10 has a cavity 12 in 
which a semiconductor die 13 may be disposed. Referring particularly to 
FIG. 2, it will be seen that there is a bottom ceramic layer or substrate 
14 which may be made of alumina (Al.sub.2 O.sub.3), and on the upper 
surface of layer 14 a metallized surface 16 is suitably placed thereon, 
onto which, by solder a die 13 may be secured. Preferably, from a 
production standpoint, as will become apparent, the die 13 is secured with 
a 95-5 solder onto the metallized layer 16. 
A continuous ceramic wall 20 is formed with three layers of ceramic 
designated 21, (see FIG. 2). The lower of the three layers is formed with 
a recess 22 while the other two layers have only an aperture cut therein. 
The arrangement is such that when the three layers are superimposed, the 
aperture 24 will be in alignment as will the semi-circular portion of the 
cutout 22. The aperture 24 is preferably metallized with a refractory 
metal such as tungsten or molybdenum which will withstand high heat upon 
firing. Located within the aperture is preferably a copper or other 
superior conductor 26 that has been clad or plated with silver. In the 
recess 22 is a lead connecting plate 28 which abuts the copper conductor 
26, and lying over the lead connecting plate 28 is a layer of aluminum 
clad copper with an under side solder face 30 onto which the electrical 
conductor or wire bond from the die will be affixed. Terminal pads 32 are 
secured as by brazing to the electrical conductive material 26. 
Further included in the semiconductor package is a heat dissipating member 
36 which will be secured onto the ceramic substrate 14 by brazing. The 
heat dissipating member 36 will preferably comprise a material of 
relatively low coefficient of thermal expansion substantially 
corresponding to the coefficient of expansion of the semiconductor device 
and the ceramic substrate 14. This arrangement provides a reliable 
mounting for the device and a suitable high thermal conductivity property 
for rapidly withdrawing heat from the semiconductor. Preferably the heat 
dissipating member may comprise any of the various conventional composite 
metal materials having a desired low coefficient of thermal expansion and 
a desired high thermal conductivity. 
Referring to FIG. 5, an alternate configuration for a MOSFET type device is 
illustrated, and in this particular instance a common drain has been 
provided with the large metallized pattern as seen at 40. Additionally, 
the terminal pads 32' have been configured for edge mounting by having a 
portion thereof as at 33 lying at right angles and over the edge of the 
ceramic package. In addition, the electrical conductive material 26' has 
been configured as a rectangle in order to provide a low impedance to high 
frequencies. 
A still further configuration of a semiconductor package is seen in FIG. 6 
where the wall 20' is circular. 
Each of the embodiments will have a lid 38 (see FIG. 3) which will have a 
metallized peripheral surface as at 39 that will lie on the metallized 
surface of the top surface of the wall 20 which has been illustrated in 
FIG. 1 by suitable shading. The lid will lie on the wall in such a way as 
to leave the terminal pads free. Accordingly, when the device is fully 
assembled, it is heated in a suitable atmosphere to eliminate all of the 
moisture, and then the lid is placed thereon and may be secured with a low 
melting point solder such as a PbSn 60-40 solder or gold eutectic solder 
applied to the metallized areas which will melt at a sufficiently low 
temperature that will not damage the completed package. 
The package may be readily configured by first creating a plurality of 
layers by cutting the same from alumina green tape. The layers may then be 
suitably screen printed with metallization in the areas which have been 
described in connection with the package above as, for example., the 
mounting for the semiconductor, the apertures or via and the underside or 
bottom surface of the ceramic substrate 14. The layers of alumina green 
tape are fixtured together and fired in a 3000.degree. F. kiln. 
Electrically conductive material 26 is inserted into the apertures. A lead 
connecting plate 28 is laid on the metallized ceramic substrate so as to 
interconnect with the electrically conductive material 26. A terminal pad 
32 is placed on top of the wall, and the entire structure is fired for 
brazing to a temperature of approximately 1500.degree. F. Since the 
conductive material 26 is coated with silver, when the entire structure is 
fired, and the temperature exceeds the combining temperature of 
1435.degree. F., there will be formed a silver, copper eutectic at the 
same time a bond is made to the lead connecting plate 28. Accordingly, in 
one firing operation the copper conductive pin 26, silver-clad connecting 
plate 28 and terminal pad 32 are integrated in the ceramic package. After 
firing all of the metallized surfaces, may then be plated with nickel 
which then permits easy soldering of the remaining materials onto them. To 
complete the structure, the die 13 is placed on the metalized surface 16 
and suitable wire bond pads 30 are placed on the lead plates 28 and, a low 
temperature solder operation is performed to solder the die and the wire 
bond pads 30 for example, with PbSn 95-5 solder. Subsequently, suitable 
wire connector bonds are placed between the die and the pads 30. A cover 
38 that consists of a layer of fired alumina, which has been metallized on 
its periphery and then coated with a layer of presolder 39, is then placed 
over the open well, if you will, at which time hermetic encapsulation can 
take place at a low temperature. 
The end result is a ceramic package which has side walls and a bottom wall, 
which is an insulating substrate, onto which a suitable heat sink plate 36 
of many configurations may be placed. By the proper selection of thin 
alumina layer, performances potentially in excess of a dissipation of a 
1/2.degree. C. per watt can be achieved.