JPS5923112B2 - Electrode formation method for semiconductor devices - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming an electrode of a semiconductor device having a multilayer electrode including a gold layer.

One of the methods for forming a multilayer electrode including a gold layer (hereinafter referred to as a gold multilayer electrode) is a plating method as shown below.

First, a barrier metal, gold, is successively deposited on the silicon wafer in which contact holes have been opened by vapor deposition or sputtering.

Then, using photolithography, a mask for selective gold plating is created by leaving photoresist in areas that do not require metal wiring. After that, gold and nickel were plated sequentially, and then the photoresist used as a mask for selective plating was removed, and the gold layer and barrier metal layer under the photoresist were removed by chemical etching or other methods using the nickel layer as a mask. Remove them in sequence. Thereafter, the nickel layer used as an etching mask for the gold and barrier metal layers is removed to complete the gold multilayer electrode.
On the other hand, in order to improve the reliability of the semiconductor device and the yield during assembly, the entire surface of the chip except for the bonded portion is sometimes covered with a silicon oxide film as a protective film.

However, in a semiconductor device having a conventional gold multilayer electrode as described above, since the uppermost layer of the electrode is made of gold, the adhesion to the silicon oxide film is weak, and there is a risk that the protective film may peel off at that portion.

As a method to improve this, it is known to interpose a metal layer with strong adhesive strength between the gold and the protective film. The nickel layer described above can be used.

That is, the upper layer of nickel, which serves as an etching mask for the metal and barrier metal layer, is left as is, a protective film is applied, and then photolithography is used to protect the part to be bonded (hereinafter referred to as a bonding pad). Opening the membrane and subsequently etching the underlying nickel layer exposes the gold at the bonding pads, completing the electrode. However, in such conventional methods, since the mutual diffusion coefficient between gold and nickel is large, especially when using the Shigaku reaction using the vapor phase growth method as a protective film formation method, even if the growth temperature is about 500°C, Alloying of gold and nickel may occur, and the nickel on the bonding pad may not be completely etched, leaving a large amount of nickel on the bonding pad, making it impossible to bond to the pad during the semiconductor device assembly process. There was a problem that the chip could become defective.

The present invention was made for the purpose of eliminating the above-mentioned drawbacks.
Before forming the protective film, that is, before heat treatment, the nickel on the bonding pad is selectively etched away to prevent alloying of gold and nickel during the formation of the protective film, and to protect the bonding pad with the remaining nickel layer. This is intended to strengthen the adhesive force with the membrane.

An example in which the present invention is applied to the manufacture of bipolar transistors will be described below.

1 to 6 are longitudinal cross-sectional views of a semiconductor device in the main steps of forming electrodes of a bipolar transistor according to the method of the present invention.

First, as shown in FIG. 1, a silicon wafer 1, which is a semiconductor substrate, on which a base region 2 and an emitter region 3 have already been formed and is covered with a silicon oxide film 4 is prepared.
A contact hole 6 for the base region 2 and a contact hole 5 for the emitter region 3 are opened in this silicon oxide film 4.

Next, silicon oxide film 4 and contact holes 5 and 6 are formed.
A barrier metal layer 7, such as a titanium or tungsten alloy layer, and a gold layer 8 are sequentially deposited on the exposed silicon wafer 1 by sputtering or the like. The gold layer 8 is deposited for the purpose of maintaining good coverage of plating to be performed later, and its thickness may be as thin as 1000 Å or less. Further, a photoresist film is deposited on the gold layer 8, and a photoresist mask 9 with a desired pattern is formed using photolithography. Next, as shown in Figure 2,
Using the photoresist as a mask, a gold layer 10 is formed by selective plating, and subsequently a nickel layer 11 is also formed by selective plating. Note that the gold and nickel plating liquid may be any ordinary one, and the lower the current density, the better the results. Next, as shown in FIG. 3, the photoresist mask 9 is removed.

Next, as shown in FIG. 4, the gold layer 8 and the titanium-tungsten alloy layer 7 are selectively etched away using the nickel layer 11 as a mask.

At this time, as the etching liquid for the gold layer 8 and the titanium-tungsten alloy layer 7, for example, an iodine-potassium iodide solution and a hydrogen peroxide solution may be used, respectively. Next, as shown in FIG. 5, the nickel layer 11 of the bonding pad portion 12 is selectively etched away using photolithography to partially expose the gold layer 10. As the nickel etching solution at this time, dilute nitric acid is preferably used.
Finally, as shown in FIG. 6, a protective film 13 made of a silicon oxide film is deposited on the entire surface of the wafer, and the protective film on the bonding pad portion 12 is selectively etched away using photolithography.

An embodiment of the present invention has been described above, and as is clear from the above description, according to the present invention, the nickel layer 11 on the bonding pad portion 12 is partially removed before forming the protective film. When the protective film is formed by heating, the nickel layer 11 no longer exists on the bonding pad portion 12, and therefore, alloying of gold and nickel does not occur in the same portion, so that the bonding strength in that portion is sufficiently increased. It can be made larger and the other parts of the gold layer 1
Since the nickel layer 11 is formed on the layer 0, the adhesive strength with the protective film 13 is extremely good.

In the above embodiments, the present invention was applied to the formation of electrodes of bipolar transistors. However, the present invention improves the adhesion between the protective film and the electrodes, and increases the bonding strength at the bonding pad. Since a specific portion of the nickel layer 11 is removed in advance as described above in order to improve the electrode structure, it can be widely applied to many other semiconductor devices that adopt the same electrode structure.

Furthermore, although a titanium-tungsten alloy is used as an example of the barrier metal, other metals or alloys may be used as long as they meet the purpose of the barrier metal.
Further, the above-mentioned nickel layer is provided to increase the adhesive strength with the gold layer and the protective film, and may be replaced with another gold layer having similar characteristics.

[Brief explanation of the drawing]

1 to 6 are longitudinal cross-sectional views of a semiconductor device in the main steps of forming electrodes of a bipolar transistor according to the method of the present invention. 1...Silicon wafer, 2...Base diffusion region, 3...Emitter diffusion region, 4...
... Silicon oxide film, 5 ... Emitter contact hole, 6 ... Base contact hole, 7 ... Barrier metal layer, 8 ... Gold layer,
9...Photoresist mask, 10...
・Metsuki gold layer, 11...Metsuki nickel layer, 1
2...Bonding Patsudo Music 13...
··Protective film.

Claims (1)

[Claims] 1. A step of depositing an insulating film on a semiconductor substrate in which a functional region is formed, and forming a required contact hole in the insulating film to reach the semiconductor substrate, the semiconductor substrate being exposed on the insulating film and in the contact hole. A process of completely depositing a barrier metal layer, which is a metal through which gold does not pass, on the barrier metal layer, with or without a single layer or multiple deposited metal layers. A step of forming nickel on the metal layer by a plating method using a mask having the shape of A method for forming an electrode for a semiconductor device, comprising the steps of etching to expose gold and forming a protective film on the metal layer.