JPS592177B2 - Method for manufacturing high power semiconductor devices - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-power semiconductor conductor, and particularly to a metallization layer on the entire back surface of the chip and a metallization method.

In recent years, as semiconductor devices have become more powerful, chips have become larger, and it has become necessary to braze the entire back surface of the L chip to the case.

For this reason, conventionally, in order to perform uniform brazing, the first
As shown in the figure, after an aluminum thin film J2 was attached to the entire back surface of the silicon semiconductor chip 1, a gold thin film 3 was attached to the surface thereof, and this gold thin film 3 was brazed to the case. In this conventional method, after the chip metallization process,
Although the surface is not oxidized during various etching and heat treatment processes, the use of gold has the disadvantage of high costs.

In addition, when gold is directly attached to silicon, the diffusion rate of gold ions into silicon is much faster than that of other metal ions, so the withstand voltage value may decrease during long-term use of semiconductor devices. This leads to deterioration of characteristics such as an increase in leakage current, resulting in poor reliability. Conventionally, a structure in which a nickel layer is formed on a thin aluminum film formed on the entire back surface of a semiconductor chip without using gold, and this nickel layer is brazed to the case has also been used.

A nickel surface originally has a very good elongation of the solder material, making it a suitable metal for a soldering surface, but it has the disadvantage that the surface is easily oxidized. Therefore, if the nickel layer is formed after the aluminum layer is formed, the surface will undergo surface oxidation during the subsequent stabilization process of the semiconductor chip surface. Therefore, when using nickel as a brazing surface, the nickel layer is not formed following the formation of the aluminum layer, but after the process such as surface treatment of the semiconductor chip is completed and immediately before the chip is brazed and fixed to the case. Formation of a nickel layer had to be carried out. However, although the formation of the aluminum layer and the nickel layer are the same process as the metal vapor deposition, the fact that these cannot be performed consecutively complicates the manufacturing process, not only requires manufacturing effort but also makes it uneconomical. be.

For this reason, conventionally, nickel was not used as the brazing surface, but gold was used as described above.

In view of the above points, the present invention uses nickel as the brazing surface for the gold layer D, and performs the formation of the nickel layer subsequent to the formation of the aluminum layer, and in addition, brazes the nickel surface to the case with the nickel surface as a clean surface free from oxidation. We will provide you with ways to do so, and
The purpose of this invention is to solve the above-mentioned conventional drawbacks. According to the present invention, after forming an aluminum layer on the entire back surface of a semiconductor chip, a nickel layer is subsequently formed, and in order to avoid oxidation of the surface of the nickel layer, after forming the nickel layer, an aluminum layer is further formed as a protective film on top of the nickel layer. After forming a layer of aluminum and carrying out various surface treatments such as stabilization treatment on the surface of the semiconductor chip with this triple metallized layer of aluminum, only the outermost aluminum layer is formed just before soldering the semiconductor chip to the case. is removed to expose the nickel layer, which is then brazed to the case.

As a result, the nickel layer can be brazed onto the case while keeping the surface clean. Hereinafter, the present invention will be described in detail with reference to illustrative embodiments.

FIG. 2 shows cross-sectional views at each step to explain the metallization method according to the present invention.

Referring to FIG. 2), an aluminum layer 2 is formed by a conventional method on the back surface of the silicon chip 1 on which necessary circuit elements have been formed.

Subsequently, this is heat treated at around 400 DEG C. to form an aluminum-silicon alloy layer along the contact area between the silicon chip 1 and the aluminum layer 2. This alloy layer is shown in Figure 2) after 4
is shown. Next, a nickel layer 5 is formed on the surface of the aluminum layer 2 (FIG. 2).

This nickel layer is later used as a soldering surface to the case. Subsequently, an aluminum layer 6 is further formed on the surface of the nickel layer 5 (FIG. 2).

This aluminum layer 6 serves to protect the nickel surface during the subsequent chip surface stabilization process. That is, after forming the metallized layer into a three-layer structure of aluminum and aluminum, various treatments normally performed after the metallization step, such as passivation, wax coating, chip cutting, and wax removal, are performed. The presence of the aluminum layer 6 protects the nickel layer 5 from acids, resist stripping solutions, alkaline solutions, etc. used in these treatments. Of course, since the aluminum layer 6 is subject to oxidation and etching, it is necessary to make it sufficiently thick. Usually 2 to 3 μm is sufficient. After the various treatments performed after the metallization step are completed and immediately before the chip is soldered to the case with FlC, the outermost aluminum layer 6 is removed to expose the nickel layer 5 (FIG. 2).

The aluminum layer 6 is removed by etching with diluted hydrofluoric acid. Since nickel is not etched at all with dilute hydrofluoric acid, etching automatically stops when the aluminum is completely removed, exposing a clean nickel surface. Then, apply this exposed clean nickel surface to the solder material 7.
(Fig. 2)). In this brazing process, the surface of the nickel layer 5 is ensured as a clean surface, so the solder material spreads well, and therefore there is almost no generation of bubbles, making it possible to perform brazing with low thermal resistance. According to the present invention, since gold is not used in the metallized layer for brazing the semiconductor chip to the case, it is inexpensive and also has the disadvantages of using gold.
Special considerations to prevent gold from adhering to the chip and deterioration of properties due to gold adhesion are eliminated. Moreover, while using nickel instead of gold, the nickel deposition can be carried out sequentially with the aluminum deposition, yet oxidation of the nickel brazed surfaces is prevented and complete brazing to the case is achieved. Therefore, it is possible to completely braze the chip to the case while satisfying the requirements of ease of manufacture, simplicity, and low cost. Since oxidation of the soldering surface is reliably prevented, complete brazing with low thermal resistance of the case can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of a conventional metallized semiconductor chip, and Fig. 2 (1) to 2) are cross-sectional views sequentially showing each step of metallization and brazing to a case according to the present invention. be. 1... Silicon chip, 2... Aluminum layer, 4...
Aluminum-silicon alloy layer, 5...Nickel layer, 6...
- Aluminum layer, 7... Brazing material, 8... Case.

Claims (1)

[Claims] 1. In a method of manufacturing a high power semiconductor device by metallizing the entire back surface of a semiconductor chip of a high power semiconductor element, subjecting the chip to necessary processing, and then brazing the metallized layer to a case, the entire back surface of the chip is metallized. First, an aluminum thin film is deposited and partially alloyed with chip silicon, and then a nickel thin film is deposited on the aluminum thin film, and then an aluminum thin film is deposited as a surface protection film on the nickel thin film, and the aluminum-nickel-aluminum A three-layer metallized layer is formed in succession, and just before brazing and fixing the chip to the case, only the outermost aluminum thin film is removed to expose the nickel layer, and the nickel layer is brazed to the case. A method of manufacturing a high power semiconductor device, characterized in that: