Semiconductor device

Details Semiconductor device Semiconductor device

1999-09-14

2001-02-20

Clark, Sheila V. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

C257S771000, C257S765000

Reexamination Certificate

active

06191485

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and in particular to metal layers, such as electrodes and wires, which are formed on the semiconductor devices.
BACKGROUND OF THE INVENTION
In recent years, electronic equipment in which semiconductor devices are mounted has been used in a wider range of applications, or under severe conditions or environments, such as vibrations, temperature cycles, stresses, and various atmospheres, which require high reliability of the electronic equipment. To keep up with the expansion of the use of electronic equipment, it is extremely important to provide semiconductor devices having sufficiently high reliability. In particular, it has been strongly desired to improve the reliability of connecting portions at which the semiconductor devices and external components or devices are electrically joined to each other. Known methods for electrically connecting the semiconductor devices with the external components include wire bonding, soldering, and pressure welding, for example.
Among the above-indicated connecting methods, soldering is used in many applications since it enables a large number of connecting portions to be joined together at a time at a relatively low temperature. FIG.
5
(
a
) is a cross-sectional view showing an electrode portion of a semiconductor device prior to soldering.
An electrode consisting of an alloy layer (hereinafter referred to as “Al layer”)
2
having a thickness of 0.5 &mgr;m and containing aluminum as a major component, a nickel (Ni) layer
3
having a thickness of 0.5 &mgr;m, and a gold (Au) layer
4
having a thickness of 0.2 &mgr;m is formed on a semiconductor substrate
1
made of silicon (Si), by electron beam deposition or other method. The Ni layer
3
is used for soldering, and the Au layer
4
is provided for preventing oxidation of the electrode surface and improving wettability.
FIG.
5
(
b
) is an enlarged cross-sectional view showing the electrode portion after soldering. A terminal
6
is joined to the Ni layer
3
with a lead-tin solder
5
interposed therebetween. After actually using the semiconductor device thus obtained by soldering, it was found that part of its electrode suffered from considerable reduction in the joining strength.
FIG.
5
(
c
) is an enlarged cross-sectional view showing the electrode portion in which the considerable reduction in the joining strength was observed.
In the electrode portion of FIG.
5
(
c
), an aluminum-nickel (Al—Ni) intermetallic compound
7
developed or grew from the interface between the Al layer
2
and the Ni layer
3
, down to the interface between the Al layer
2
and the Si substrate
1
, and clearances
8
were formed between the Al—Ni intermetallic compound
7
and the Si substrate
1
. Observing the electrode portion of FIG.
5
(
b
) more carefully, it was also found that an Al—Ni intermetallic compound
7
was formed between the Al layer
2
and the Ni layer
3
.
Since the electrode is subjected to high temperatures in the range of 300 to 500° C. during lamination of the layers and soldering, and also under conditions of the use thereof, the Al—Ni intermetallic compound
7
grows from the interface between the Al layer
2
and the Ni layer
3
, and the clearances
8
are formed probably because of volume changes (volume decrease) due to phase changes from the Al layer
2
and Ni layer
3
to the Al—Ni intermetallic compound
7
. As a result, the joining strength between the terminal
6
and the Si substrate
1
is rapidly reduced.
In addition to the high temperature as described above, electrodes of semiconductor devices are generally exposed to other thermal stresses caused by, for example, heating of the Si substrate when Al layer or Ni layer is formed thereon, heat treatment for removing defects in the Si substrate through electron beam irradiation, heat treatment for removing water vapor adhering to the surface, abnormal high temperatures due to temperature variations within the device during the above heat treatments, abnormal high temperatures due to temperature variations among lots during the heat treatment processes, and heat generated it local portions of the semiconductor device due to concentration of current during the use of the device.
SUMMARY OF THE INVENTION
It is therefore an objet of the present invention to provide a semiconductor device having a laminated metal layer in which a metal layer containing Al as a major component and a metal layer containing Ni as a major component are laminated in contact with each other, which device assures high reliability for a long period of time.
To accomplish the above object, the present invention provides a semiconductor device comprising: a semiconductor substrate; and a laminated metal layer formed in the semiconductor substrate, the laminated metal layer comprising a first metal layer containing aluminum as a major component and a second metal layer containing nickel as a major component, which arc laminated in contact with each other, wherein the ratio (t
Al
/t
Ni
) of the thickness (t
A1
) of the first metal layer containing aluminum as a major component to the thickness (t
Ni
) of the second metal layer containing nickel as a major component is at least 5.
When the electrode of the semiconductor device in the form of the laminated metal layer is exposed to high temperature, an Al—Ni intermetallic compound grows at a boundary or interface between the metal layer whose main component is Al and the metal layer whose main component is Ni, which ale laminated on each other to provide the electrode. The Al—Ni intermetallic compound is mainly NiAl
3
. If the ratio (t
Al
/t
Ni
) of the thicknesses of these metal layers is controlled to 5 or larger, the Al—Ni intermetallic compound does not reach the interface between the substrate as a base and the Al layer. Here, problems due to the formation of the Al—Ni intermetallic compound arise where the metal layers have crystal lattices of Al and Ni as main lattices, for example, where the metal layers contain at least 50% of Al atoms and Ni atoms, respectively. Although an Al—Ni intermetallic compound may be formed even with the smaller contents, such an intermetallic compound has a small influence on the resulting device.
In one form of the invention, a solder is deposited on the second metal layer containing nickel as a major component.
If the metal layer whose main component is Ni is soldered to an external component, tile metal layers of Ni and Al are subjected to heat treatment, and an Al—Ni intermetallic compound is likely to be Honed. In this case, too, the problem of reduced strength can be avoided by controlling the thickness ratio of these layers to 5 or larger as described above.


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patent: 3781596 (1973-12-01), Galli et al.
patent: 3990094 (1976-11-01), Konantz et al.
patent: 4358784 (1982-11-01), Wislocky et al.
patent: 4372809 (1983-02-01), Grewal et al.
patent: 4378410 (1983-03-01), Bailey
patent: 5227332 (1993-07-01), Morris
patent: 5408120 (1995-04-01), Manabe et al.
patent: 6011313 (2000-01-01), Shangguan et al.
patent: 4-85972 (1992-03-01), None
patent: 4-72764 (1992-03-01), None
patent: 6252091 (1994-09-01), None

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