Semiconductor device of the type sealed in glass having a silver-copper bonding layer between slugs and connection conductors

The invention relates to a semiconductor device of the type sealed in glass, comprising a semiconductor body having a pn-junction between opposing faces which are connected to slugs of a transition metal, said slugs being connected to copper-containing connection conductors by a bonding layer, the bonding layer comprising, in addition to copper and silver, more than 1 wt. % germanium.

BACKGROUND OF THE INVENTION 
The invention relates to a semiconductor device of the type sealed in 
glass, comprising a semiconductor body having a pn-junction between 
opposing faces which are connected to slugs of a transition metal, said 
slugs being connected to copper-containing connection conductors by means 
of a silver- and copper-containing bonding layer, at least the 
semiconductor body and a part of the slugs being covered with the glass. 
Such a device, which is also referred to as glass-bead diode, is 
particularly suitable for applications in which a very good passivation, a 
hermetic sealing of the semiconductor body, mechanical strength or 
resistance against high temperatures are important factors. The glass 
passivates the semiconductor body and forms a barrier against moisture. 
Apart from the semiconductor body, the glass also covers the part of the 
slugs adjacent to said semiconductor body. In such a device, the slugs of 
a transition metal, the glass and the semiconductor body have coefficients 
of thermal expansion which are of the same order of magnitude to preclude 
defects caused by thermal expansion. The connection conductors serve to 
enable the semiconductor device to be electrically connected, for example, 
to a printed circuit board. Such a device is made by soldering the 
semiconductor body to the slugs and the slugs to the connection conductors 
at a relatively high temperature by means of a so-called "brazing 
process", that is, a soldering process at a relatively high temperature 
(above approximately 450.degree. C.). Subsequently, the glass is provided 
at a temperature which is lower than that of the soldering process. 
A device of the type mentioned in the opening paragraph is known from the 
abstract in the English language of Japanese Patent Application 
JP-A-55-39617 in which a silver-copper solder is used to connect 
molybdenum slugs to copper connection conductors in the form of connection 
wires. 
Said known device has the drawback that the tensile strength of the 
connection between the slugs and the connection wires is insufficient. 
SUMMARY OF THE INVENTION 
It is an object of the invention, inter alia, to obviate said disadvantage. 
To this end, the device in accordance with the invention is characterized 
in that, in addition to copper and silver, the bonding layer comprises 
more than 1 wt. % germanium. 
This results in a sufficiently great tensile-strength of the bonding layer. 
In practice it is found, for example in the case of copper conductors, 
that pulling the connection conductors causes them to break, whereas the 
bonding layer remains intact. 
Preferably, the device in accordance with the invention is characterized in 
that the bonding layer comprises less than 5 wt. % germanium. If the 
germanium content exceeds 5 wt. %, the bonding layer is relatively hard 
and brittle, so that in practice said bonding layer is relatively 
difficult to apply. 
An additional advantage is obtained if the bonding layer comprises between 
20 and 30 wt. % copper. In practice, such a bonding layer has a relatively 
low melting point in the range between approximately 780.degree. and 
800.degree. C. 
If the slugs and the connection conductors are soldered together at an 
elevated temperature for a relatively long period of time, and if use is 
made of connection conductors comprising almost exclusively copper, 
problems regarding the strength of the bonding layer may occur in the 
manufacturing process. Preferably, the connection conductors comprise, in 
addition to copper, a metal from the group formed by iron, cobalt and 
nickel. It has been found that in the case of connection conductors 
comprising iron, cobalt or nickel, problems regarding the strength of the 
bonding layer do not occur, even if the soldering process is carried out 
at an elevated temperature for a long period of time. It is assumed that 
in the case of connection conductors comprising almost exclusively copper, 
germanium of the bonding layer diffuses into the copper connection 
conductors during soldering at an elevated temperature, so that the 
quantity of germanium in the bonding layer decreases to a level which, in 
the case of long soldering times at an elevated temperature, yields a 
bonding layer whose strength is insufficient. It is assumed that the iron, 
cobalt or nickel in the connection conductors largely precludes diffusion 
of germanium into the connection conductor, so that the quantity of 
germanium in the bonding layer remains large enough to yield a strong 
bonding layer. The semiconductor device preferably has connection 
conductors which comprise a core containing iron and, around said core, a 
sheath containing copper. Such a connection conductor has proved to be 
very satisfactory in practice. 
Preferably, the bonding layer also comprises less than 1 wt. % of a metal 
from the group formed by iron, nickel or cobalt. Such a metal causes the 
adhesion of the bonding layer to the transition metal to improve. It is a 
so-called "nucleation element", which ensures that sufficient germanium is 
present at the interface between transition metal and bonding layer. 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The FIGURE shows a glass-bead-diode semiconductor device of the type sealed 
in glass, comprising a semiconductor body 1 having a pn-junction 2 between 
opposing faces 3, 4, which are connected to slugs 5 of a transition metal, 
said slugs 5 being connected to copper-containing connection conductors 8 
by means of a silver- and copper-containing bonding layer 10, at least the 
semiconductor body 1 and a part of the slugs 5 being covered with the 
glass 12. The slugs are composed of metals such as tungsten or molybdenum. 
Alternatively, use is made of combinations of metals, for example, from 
the group formed by iron, nickel and copper, which combinations are also 
known by names such as dumet, fernico or invar. In practice, use is 
preferably made of molybdenum slugs 5. Said slugs 5 are connected to the 
semiconductor body 1 by means of a bonding layer 14. 
Such a device is manufactured by providing a titanium layer, silver layer 
and aluminium layer in thicknesses of 0.2 .mu.m, 3.65 .mu.m and 2.4 .mu.m, 
respectively, on the semiconductor body, for example, by means of vapor 
deposition. After the soldering operation, the silver and aluminium layers 
form a bonding layer 14 of Ag89A111, the numbers representing percentages 
by weight. Subsequently, parts of the glass-bead diode, such as the 
connection wires 8, the slugs 5 and the semiconductor body 1, are 
positioned relative to each other as shown in the FIGURE. At the location 
of the bonding layer 10, plates are provided which are made from the 
material of said bonding layer 10. Such plates are manufactured by means 
of known standard techniques, for example, by melting the metals of the 
bonding layer 10 in the desired ratio and by means of techniques such as 
rolling and blanking. After the parts have been positioned, they are 
brought to a temperature of approximately 830.degree. C. The semiconductor 
body 1 is then soldered to the slugs 5 by means of the bonding layer 14, 
and said slugs 5 are soldered to the connection wires 8 by means of the 
bonding layer 10, so that a soldered structure is formed. For the 
manufacture of glass-bead diodes, preferably, temperatures below 
approximately 830.degree. C. and soldering times below 20 minutes are 
observed, because at higher temperatures and longer soldering times 
material of the bonding layers 10, 14 diffuses away and the properties of 
the bonding layers undergo a change. After the soldering process, the 
soldered structure is cleaned by means of so-called lye etching in an 
alkaline solution of NaOH (40 g/l) to allow the glass 12 to adhere well to 
the structure. After said cleaning step, a glass suspension, for example, 
of lead-borate glass or zinc-borate glass, also referred to as Schott 
glass or Ferro glass, is provided on the structure thus formed. The glass 
suspension is sintered at a lower temperature, in this example a 
temperature ranging between 700.degree. and 730.degree. C., than the 
temperature used during soldering. The glass 12 covers the semiconductor 
body 1 and at least a part of the slugs 5 (see FIGURE). 
In practice, the diameter of the slugs 5 and the bonding layer 10 is 
approximately 50% larger than the diameter of the connection wires. For 
example, use is made of a connection wire having a diameter of 1.322 and a 
section at the location of the bonding layer of 2.0 mm. Materials which 
are customarily used to solder copper-containing wires to slugs of a 
transition metal are AgCu28 having a melting temperature of 780.degree. C. 
and AgCu26.6Pd5 having a melting temperature of 820.degree. C., the 
numbers representing percentages by weight. If, for example, AgCu28 is 
used as the bonding layer 10, pulling the connection wires causes the 
connection between the slugs 5 and the connection wires to be broken in 
the silver-copper bonding layer 10 in all cases, in spite of the fact that 
the bonding layer 10 has a larger diameter than the connection wires. In 
the case of AgCu26.6Pd5 breakage in the bonding layer 10 occurs in 
approximately ten percent of the cases. 
In accordance with the invention, the bonding layer 10 comprises, in 
addition to copper and silver, more than 1 wt. % germanium. This results 
in a much greater tensile strength of the bonding layer 10. The bonding 
layer 10 preferably comprises between 20 and 30 wt. % copper. Experiments 
in which use is made of a bonding layer 10 of AgCu28 having a germanium 
content of 2, 3 and 5 wt. %, and molybdenum slugs 5 and copper connection 
wires 8 show that pulling the connection wires causes them to break, 
whereas the bonding layer 10 remains intact. The soldering temperature at 
which the bonding layer 10 is provided is not very critical. For example, 
such a bonding layer 10 can be provided at a soldering temperature in the 
range between, for example, 795.degree. and 900.degree. C., without the 
properties of the bonding layer being adversely affected. Moreover, the 
slugs 5 and the connection wires are moistened very satisfactorily by the 
bonding layer 10 in the manufacturing process. 
The bonding layer 10 preferably comprises less than 5 wt. % germanium. If 
the germanium content exceeds 5 wt. %, the bonding layer 10 has a great 
tensile strength but the plates consisting of the material of the bonding 
layer, which are used for soldering, are very difficult to manufacture. 
If the slugs 5 and the connection wires are soldered together at an 
elevated temperature for a relatively long period of time, and if 
connection conductors 8, which comprise almost exclusively copper, are 
used, problems regarding the strength of the bonding layer 10 may occur in 
the manufacturing process. Preferably, the connection conductors 8 
comprise, in addition to copper, iron, cobalt or nickel. It has been found 
that in the case of connection conductors 8 which comprise iron, cobalt or 
nickel, problems regarding the strength of the bonding layer 10 do not 
occur, even if the soldering process is carried out at an elevated 
temperature for a long period of time. Preferably, use is made of a 
connection wire 8 having a core of iron, for example Low Carbon (LC) or 
Interstitional Free (IF) iron, which is provided with a copper sheath. A 
connection wire 8 having, for example, an iron core of 0.8 mm and a copper 
sheath having a thickness of 0.26 mm proves to be very satisfactory. Such 
a connection wire is also known by the name Fecuma wire with a 70% IACS 
(International Accepted Conductivity Standard). 
Preferably, the bonding layer 10 also comprises less than 1 wt. % of a 
metal from the group formed by iron, nickel or cobalt. Such a metal allows 
a better adhesion of the bonding layer 10 to the transition metal to be 
achieved. Said metal is a so-called "nucleation element" which ensures 
that sufficient germanium is present at the interface between the 
transition metal and the bonding layer 10. For example, AgCu282Fe0.2, 
AgCu28GeCo0.3, AgCu28Ge2Ni0.1 can be used as the material for the bonding 
layer, the numbers representing percentages by weight. Bonding layers 
comprising iron or cobalt are better moisturizers of slugs of transition 
metals than bonding layers comprising nickel, whereas nickel or 
cobalt-containing bonding layers have a greater strength than 
iron-containing bonding layers. Preferably, use is made of a bonding layer 
of AgCu28Ge2Co0.3, because this layer combines satisfactory moistening 
properties with a great strength. 
The invention is not limited to the above-described exemplary embodiments. 
For example, in the exemplary embodiments the semiconductor body 1 is a 
diode provided with one pn-junction 2 between opposing faces 3, 4. It is 
alternatively possible, however, that the semiconductor body 1 comprises a 
number of series-arranged pn-junctions 2. The semiconductor body 1 shown 
in the FIGURE then comprises a number of interconnected semiconductor 
bodies which each include a pn-junction 2. The semiconductor device can 
then suitably be used to rectify relatively high voltages. In the 
foregoing, a specific technique for the manufacture of the bonding layer 
10 is mentioned. However, this is not to be understood to mean that the 
device in accordance with the invention can only be manufactured by means 
of such a technique. For example, instead of the plates used in the 
example, layers of the desired composition can be provided on the slugs or 
on the connection conductors by means of electrodeposition, "chemical 
vapor deposition" (CVD) or vapor-deposition techniques. The bonding layer 
can alternatively be provided by means of a paste. The glass 12 can also 
be provided in a manner which differs from the one described herein, for 
example by providing a glass tube around the semiconductor body and the 
slugs and melting said glass tube. In the example, connection wires are 
used as connection conductors. Said connection conductors may very well 
have a different shape, for example a cylindrical or rectangular box shape 
to enable, for example, surface mounting in accordance with a so-called 
"surface mounting" technique (SMD).