Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Flip chip
Reexamination Certificate
2002-02-27
2003-08-05
Whitehead, Jr., Carl (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Flip chip
C257S737000, C257S690000, C438S613000, C438S108000
Reexamination Certificate
active
06603207
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrode structure for a semiconductor device mounted on a circuit board and a method for forming the electrode structure, and a mounted body including the semiconductor device, and more particularly to an electrode structure for a semiconductor device mounted on a circuit board in the face-down state and a method for forming the electrode structure, and a mounted body including the semiconductor device.
BACKGROUND OF THE INVENTION
Conventionally, a semiconductor device is often mounted on a circuit board by soldering. In recent years, however, the package for the semiconductor device has been small-sized and the number of connecting terminals has been increased. Consequently, the space between the connecting terminals has been reduced. Thus, it has become gradually harder to apply the soldering technique according to the prior art.
A method for directly attaching the semiconductor device onto the circuit board to reduce the mounting area so that the circuit board can be used efficiently has been proposed. In particular, the following mounting method is considered effective because the mechanical strength obtained after connecting the semiconductor device to the circuit board is great and direct connection can be performed (for example, “Microelectronics Packaging Handbook; Nikkei BP published on Mar. 27, 1991”). According to the above-mentioned method, a semiconductor device having an electrode structure is manufactured in which a deposited film such as an adherence metal or a diffusion preventive metal is formed on an aluminum electrode in advance when connecting the semiconductor device to the circuit board and a bump electrode made of a solder is formed on the deposited film by a plating method, and the semiconductor device is provided on the circuit board in the face-down state and heated at a high temperature so that the solder (bump electrode) and the terminal electrode of the circuit board are fused.
According to this method for forming the bump electrode, the electrode structure is complicated so that a lot of steps are necessary. For this reason, a method for easily forming the bump electrode on the aluminum electrode of the semiconductor device by using the wire bonding technique has been proposed (for example, Japanese Unexamined Patent Application No. 49-52973).
An example of the electrode structure for a semiconductor device and a method for forming the electrode structure and the mounted body including the semiconductor device according to the prior art will be described below with reference to the drawings.
FIG. 3
is a sectional view showing the electrode structure for the semiconductor device according to the prior art.
FIG. 4
is a sectional view showing the mounted body including the semiconductor device according to the prior art.
As shown in
FIG. 3
, an aluminum electrode
11
is formed on an IC substrate
10
. A passivation film
13
is formed on the IC substrate
10
so as to cover the peripheral portion-of the aluminum electrode
11
. A bump electrode
12
is formed on the aluminum electrode
11
by a wire bonding method.
As shown in
FIG. 4
, the semiconductor device having the bump electrode
12
is aligned in the face-down state in such a manner that the bump electrode
12
closely comes in contact with a terminal electrode
15
of a circuit board
14
, and is provided on the circuit board
14
. The bump electrode
12
and the terminal electrode
15
are fused together by thermo-compression bonding or ultrasonic welding. Thus, a mounted body including the semiconductor device is obtained.
However, the semiconductor device having the above-mentioned electrode structure and the mounted body including the semiconductor device have the following problems.
More specifically, the aluminum electrode is exposed around the bump electrode. For this reason, if water exists in the exposed aluminum electrode portion, there is a possibility that aluminum flows out in a comparatively short time so that the semiconductor device is not electrically conducted to the circuit board. In order to enhance the reliability of a junction, it has been proposed to fill a gap between the semiconductor device and the circuit board with an insulating resin. In this case, however, there is a possibility that impurity ions such as chlorine contained in the insulating resin promote the corrosion of the aluminum electrode so that the semiconductor device is not electrically conducted to the circuit board. In the case where an organic board such as a glass epoxy board is used for light weight and low cost, the corrosion of the aluminum electrode becomes more serious because the organic board is generally permeable to water and a lot of impurity ions are present in the organic board.
SUMMARY OF THE INVENTION
In order to solve the above-mentioned problems in the prior art, it is an object of the present invention to provide an electrode structure for a semiconductor device and a method for forming the electrode structure and a mounted body including the semiconductor device in which the semiconductor device can be easily connected to a circuit board reliably.
To accomplish the above-mentioned object, the present invention provides an electrode structure for a semiconductor device which is suitable for mounting on a circuit board in the face-down state, comprising a bump electrode formed on an aluminum electrode of the semiconductor device; and an aluminum oxide film sufficient for the prevention of corrosion of the aluminum electrode, which is formed on a surface of the aluminum electrode exposed around the bump electrode. According to the electrode structure for the semiconductor device, the aluminum electrode is not corroded due to water and impurity ions when mounting the semiconductor device on the circuit board.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the aluminum oxide film for the prevention of corrosion should be obtained by further oxidizing a natural oxide film on the surface of the aluminum electrode.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 5 to 20% of the thickness of the aluminum electrode. According to the preferred example, a practical electrode structure for the semiconductor device can be obtained. In other words, the thickness of the aluminum oxide film which is smaller than 5% of the thickness of the aluminum electrode is not practical because it is very hard to form the aluminum oxide film densely. Furthermore, the thickness of the aluminum oxide film which is greater than 20% of the thickness of the aluminum electrode is not practical because the resistance value of the aluminum electrode portion is increased too much.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the thickness of the aluminum oxide film should be 0.05 to 0.2 &mgr;m. According to the preferred example, a practical electrode structure for the semiconductor device can be obtained. In general, the thickness of the aluminum electrode is about 1 &mgr;m. Consequently, the aluminum oxide film having a thickness of 5 to 20% of the thickness of the aluminum electrode can be obtained.
In the electrode structure for a semiconductor device according to the present invention, it is preferable that the bump electrode should be made of Au. According to the preferred example, the surface of the bump electrode is not oxidized at the step of forming an oxide film on the aluminum electrode of the semiconductor device. Accordingly, the bump electrode made of Au is convenient for the following bonding step. The reason is that if an oxide film is formed between the bonding layer and the surface of the bump electrode, a good electrical conduction cannot be obtained.
The present invention provides a method for forming an electrode structure for a semiconductor device which is mounted on a circuit board i
Berezny Nema
Jr. Carl Whitehead
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