Package for semiconductor devices with first and second metal layers on the substrate of said package

A package for sealing an MOS type semiconductor element having a die-attach area comprising a first metal layer composed of gold and a second metal layer composed of a metal having a good bondability to aluminum metal, which is electrically connected to the first metal layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a package for sealing a semiconductor 
element. More particularly, the invention relates to an improvement in a 
die-attach area of a package for sealing on MOS type semiconductor 
element. 
2. Description of the Prior Art 
A metal oxide semiconductor (MOS) is very poor in the voltage resistance 
and current resistance because the thickness of an insulating film for 
insulating and protecting MOS integrated circuits is very small. 
Accordingly, if static charges are intruded from the outside in the 
interior of a device including such semiconductor element while the device 
is being used or transported, breakdown is caused in the insulating film 
by these static charges, resulting in deterioration of inherent properties 
of the element or complete breakage of the element, and no normal 
operation can be expected anymore. 
As means for protecting the semiconductor element from breakdown by static 
charges, there has heretofore been adopted a method in which a common 
grounding electrode is formed on the element and this common electrode is 
connected to a grounding terminal. 
A conventional package for sealing an MOS type semiconductor element 
comprises a metal layer 2 composed of gold, which is disposed in the inner 
bottom portion (the die-attach area) of a substrate 1 composed of a heat 
resistant insulating substance as shown in FIG. 1. An MOS type 
semiconductor element 3 is placed on the metal layer 2 and gold silicon 
eutectic reaction is caused between silicon constituting the element 3 and 
gold of the metal layer 2 to bond the element 3 tightly to the substrate 
1. Respective electrodes of the element 3 are connected to terminals of a 
lead frame 4 through aluminum fine wires 5. A common grounding electrode 
of the element 3 is connected to the metal layer 2 through aluminum fine 
wire 5' for ensuring common grounding, and the metal layer 2 is 
electrically connected to a grounding terminal of the lead frame 4 through 
an aluminum fine wire through this connection is not specifically 
illustrated in the drawing. 
After mounting of the semiconductor element the wire bonding, a cap member 
6 is bonded on the substrate 1 by a sealing member 7 such as glass or 
resin, whereby the interior of the package is sealed completely 
air-tightly. 
In case of this conventional package, by the operation of heating the 
sealing member for attaining air-tight sealing, purple plague is caused in 
the bonding area between the aluminum fine wire 5' for common grounding 
and the metal layer 2, and therefore, breakage of the aluminum fine wire 
or insufficient contact takes place in this bonding area. As a result, the 
semiconductor device fails to make a normal operation. This is a fatal 
defect involved in the conventional semiconductor device. 
As means for eliminating this defect, there has been proposed a method in 
which a dummy chip of silicon having an aluminum electrode formed on the 
top face thereof according to a known technique such as vacuum deposition 
or spattering is separately mounted on the metal layer 2, the aluminum 
electrode of this dummy chip is connected to the common grounding 
electrode of the semiconductor element 3 through an aluminum fine wire and 
the aluminum electrode is further connected to the grounding terminal of 
the lead frame through an aluminum fine wire. 
According to this method, the aluminum fine wire for common grounding of 
the semiconductor element 3 is not allowed to have direct contact with 
gold of the metal layer 2 but is allowed to fall in contact with the 
aluminum electrode of the dummy chip, and therefore, purple plague is not 
caused and complete common grounding is possible. 
In this method, however, the dummy chip of silicon, which is inherently 
unnecessary, should be mounted and the troublesome step of fixing this 
small dummy chip should inevitably be conducted. Further, at the wire 
bonding step, in order to use a wire bonder, it is necessary to arrange 
the dummy chip very precisely at a predetermined position. Therefore, this 
method is not satisfactory in the operation facility and reliability. 
As another means for eliminating the above-mentioned defect, there may be 
considered a method in which a gold fine wire is used instead of the 
aluminum fine wire for common grounding of the semiconductor element. 
However, when an ultrasonic wire bonder performing bonding by utilizing 
frictional heat is used for wire bonding, since gold is soft and slip is 
caused on the gold fine wire, heat necessary for fusion bonding cannot be 
obtained and complete bonding cannot be expected. Further, since the gold 
fine wire is very expensive, there is brought about a disadvantage that 
the manufacturing cost of the semiconductor device is increased. 
It is therefore a primary object of the present invention to provide a 
package for sealing a metal oxide semiconductor element having a very high 
reliability, in which occurrence of purple plague by the heating operation 
at the step of forming air-tight sealing can be completely prevented. 
Another object of the present invention is to provide a package for sealing 
metal oxide semiconductor element, which can be mass-produced at a low 
cost with good operation facility. 
SUMMARY OF THE INVENTION 
The package for sealing an MOS type semiconductor element according to the 
present invention has a die-attach area comprising a first metal layer 
composed of gold and a second metal layer composed of a metal having a 
good bondability to aluminum metal, which is electrically connected to the 
first metal layer. The MOS type semiconductor element is fixed to the 
first metal layer by gold silicon eutectic reaction and a common grounding 
electrode of the semiconductor element is electrically connected to a 
grounding terminal through the second metal layer by an aluminum fine wire 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will now be described in 
detail by reference to FIGS. 2 and 3, where members same as those of the 
conventional package are indicated by the same reference numerals as used 
in FIG. 1. 
The package for sealing an MOS type semiconductor element according to the 
present invention comprises a first metal layer 2' composed of gold 
disposed in the inner bottom portion (the die-attach area) of a substrate 
1 composed of a heat-resistant insulating substance, for example, a 
ceramic, and a second metal layer 2" composed of a metal having a good 
bondability to aluminum metal, such as silver (Ag), a mixture of silver 
(Ag) and platinum (Pt), a mixture of silver (Ag) and palladium (Pd), or a 
mixture of silver (Ag), palladium (Pd) and platinum (Pt), which is also 
disposed in the die-attach area of the substrate 1. In this package, both 
the metal layers 2' and 2" are electrically connected to each other. 
These metal layers 2' and 2" are formed, for example, by coating a gold 
paste and a paste of an Ag-Pd mixture on predetermined areas of the 
die-attach area of the substrate 1 according to a known coating technique, 
such as silk screening, and drying the coated pastes at a predetermined 
temperature. Both the metal layers 2' and 2" are partially overlapped or 
bonded to each other (See an area A in FIG. 3). These metal layers 2' and 
2" may be bonded to each other throughout the end edges thereof or at 
several points on the end edges thereof. 
As in the conventional package, an MOS type semiconductor element 3 is 
mounted on the first metal layer 2' composed of gold, and by causing 
eutectic reaction silicon constituting the element 3 and gold of the first 
metal layer 2', the semiconductor element 3 is tightly bonded and fixed to 
the substrate 1. Respective electrodes of the element 3 are connected to 
terminals of a lead frame 4 composed of Kover through aluminum fine wires 
5. 
In the present invention, the common grounding electrode of the element 3 
is connected to the second metal layer 2" on the substrate 1 through an 
aluminum fine wire 5' by means of, for example, an ultrasonic wire bonder. 
Further, according to similar procedures, the second metal layer 2" is 
connected to a grounding terminal of the lead frame 4 through an aluminum 
fine wire, though this connection is not specifically shown in the 
drawing. 
After completion of connection of the respective electrodes of the element 
3 to respective terminals of the lead frame 4 and common grounding of the 
element 3, a cap member 6 composed of a ceramic is fixed onto the 
substrate through the lead frame 4 by a sealing 7 composed of, for 
example, low-melting-point glass, whereby a semiconductor device having 
the interior thereof sealed completely air-tightly is formed. 
As will be apparent from the foregoing illustration, in the novel package 
for sealing an MOS type semiconductor element according to the present 
invention, since a metal layer composed of gold is arranged on the area 
where the semiconductor element 3 is mounted, the advantage of the 
conventional technique that the semiconductor element can be tightly 
bonded to the substrate by eutectic reaction is not lost at all, and since 
a metal layer composed of Ag, an Ag-Pt mixure, an Ag-Pd mixture or an 
Ag-Pd-Pt mixture is disposed on the area to which an aluminum fine wire 
for common grounding of the semiconductor element is connected, good 
bondability to aluminum is attained and occurrence of purple plague by the 
heating operation at the sealing step can be prevented. As a result, a 
semiconductor device having a very reliability can be obtained. 
Further, in the present invention, a dummy chip need not be formed or 
expensive gold fine wire need not be used, and therefore, a package for 
sealing an MOS type semiconductor element which is excellent in 
adaptability to the mass production can be prepared at a low cost with 
good operation facility. 
As will be apparent from the foregoing illustration, in the package for 
sealing an MOS semiconductor element according to the present invention, 
the structure of the die-attach area on which the semiconductor element is 
mounted is important and critical, and the present invention is not 
limited by other structure features of the package. 
While the invention has been shown and described with reference to a 
preferred embodiment thereof, it will be obvious to those skilled in the 
art that various changes and modifications may be made therein without 
departing from the scope of the invention as defined by the following 
claims.