JPH01307219A - Electrode forming method for semiconductor device - Google Patents

Abstract

PURPOSE:To retard dry corrosion (thermal fatigue phenomenon) by vacuum- evaporating alloy containing gold and germanium on a heated silicon substrate, inducing eutectic reaction, and vacuum-evaporating a single substance of nickel or an alloy containing nickel. CONSTITUTION:When alloy containing gold and germanium is attached on a silicon substrate, as a first metal film, by vacuum evaporating, eutectic reaction or combination reaction is induced at the same time with vacuum evaporation, by heating the silicon substrate at a temperature of 340-400 deg.C, thereby strengthening the bonding between the silicon substrate and the first metal film. Since the metal film is vaporized and activated in vacuum, eutectic and combination are obtained at a temperature considerably lower than the eutectic temperature and the combination temperature. Then, continuously, a second metal film is vacuum-evaporated, and soldering with a lead frame is facilitated. As the second metal film, therefore, metals such as nickel and silver which are easy to be bonded to solder material are preferable. Thereby, the thermal fatigue life is lengthened, and the working process is cut down.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for forming electrodes of semiconductor devices, particularly power semiconductor devices that handle relatively large amounts of power.

BACKGROUND OF THE INVENTION Conventionally, as a method for forming electrodes of power semiconductor devices, taking a power transistor as an example, the emitter and base electrodes are formed using an aluminum evaporated film, and the collector electrode is formed using an Au-3i alloy or nickel vanadium chromium. Transition metals such as these were widely used.

Points to keep in mind when forming electrodes for power semiconductor devices are:
During use of a semiconductor device, it is important to prevent dry corrosion of electrode materials that often occurs, increasing thermal resistance and causing disconnection defects. This phenomenon is generally called the "thermal fatigue phenomenon".

In particular, in resin-sealed semiconductor devices, dry corrosion is more likely to occur than in hermetic semiconductor devices, resulting in poor reliability. In order to prevent such dry corrosion, improvements have been made to electrode structures, electrode materials, sealing materials, bonding processes, and the like.

Problems to be Solved by the Invention The present invention provides a method for forming electrodes of a highly reliable power semiconductor device by delaying dry corrosion (thermal fatigue phenomenon).

Conventionally, when forming an electrode on a silicon substrate, there are many methods in which a metal film is deposited on the silicon substrate by vacuum evaporation, and then heat treatment is performed to form eutectic or combine the metal film. In this case, it takes time to perform heat treatment after vacuum deposition, and the surface may be oxidized or scratches may occur due to handling, resulting in the disadvantage that uniform adhesion cannot be obtained. As a result, oxidation progressed further starting from oxidized areas and scratches, promoting dry corrosion.

Means for Solving the Problems In order to solve these problems, in the present invention, when the first metal film is deposited on the silicon substrate by vacuum evaporation, the temperature of the silicon substrate is raised from 340°C to 400°C. This is intended to cause a eutectic reaction or a combination reaction simultaneously with vacuum evaporation, thereby strengthening the bond between the silicon substrate and the first metal film. Since the metal film is vaporized in a vacuum, it is activated and can undergo eutectic and combination at a temperature considerably lower than the eutectic and combination temperatures shown in the phase diagram. Subsequently, a second metal film is vacuum deposited to facilitate soldering to the lead frame.

Therefore, a metal that easily adheres to the solder material, such as nickel or silver, is suitable for the second metal film.

According to the present invention, alloying of the first metal film and the silicon substrate is achieved at low temperatures, and highly stable contact electrode formation can be realized.

Embodiments Embodiment 1 The present invention will be explained in detail using a power transistor as an example.

All diffusion processes have already been completed and aluminum films have been formed on the emitter and base electrodes.The silicon substrate is heated to 340°C in a vacuum evaporation tank to form an Au-Ge alloy (Ge).
(content 15%) was deposited to a thickness of 1.2 μm. The eutectic temperature of silicon and gold is 370° C., but it can be understood from the state of the film after vapor deposition that even at 340° C., a eutectic reaction occurs between the silicon substrate and gold during vapor deposition. Thereafter, as a second metal film, chromium/chromium-nickel alloy (Ni content 35%)/nickel was deposited with a thickness of 0.05 μm and 0.1 μm, respectively.
um and 0.3 um were continuously deposited.

Example 2 The same silicon substrate as in Example 1 was heated to 400°C, and Au
-Ge-8b alloy (Ge content 15%.

sb content 1.5%) was deposited to a thickness of 2.0 μm. Then, as a second metal film, chromium/chromium-nickel alloy (Ni content 35%) was deposited with a thickness of 0.05 μm and a thickness of 0.05 μm, respectively.
Continuous deposition of 1 μm and 0.3 μm was performed.

The substrate on which the electrodes had been formed was divided into chips and adhered to a TO220 type lead frame using Pb/Sn. After wire bonding and resin sealing, a dry corrosion test using heat resistance was conducted. The conditions for thermal fatigue are: semiconductor chip 1:5W (1=5A, V
=IA) was applied intermittently for 5 minutes on/off. (The temperature difference at this time was 105°C. 130°C when on, 25°C when off.) Figure 1 shows the method of the present invention (Examples 1 and 2) and the conventional method (the first metal film was The results of a thermal fatigue test are shown below for a sample obtained by vacuum evaporating at 400° C., heating to 400° C. for eutectic formation, and then depositing a second metal film.

The results were shown as changes in thermal resistance and number of on/off cycles.

In the conventional method, the thermal resistance increases after about 20,000 cycles,
In contrast to the observed deterioration, the method of the present invention causes no deterioration of thermal resistance at all. It is clear that the thermal fatigue life is long.

As described in detail, according to the present invention, compared to conventional methods, it is possible not only to obtain a highly reliable semiconductor device, but also to reduce the number of work steps (heat treatment is no longer required), which improves manufacturing efficiency. There are many advantages.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram of thermal fatigue test results showing the effectiveness of the method of the present invention. 1...Example characteristic curve, 2...Conventional example' characteristic curve.

Claims (1)

[Claims] A step of first vacuum-depositing an alloy containing at least gold and germanium as a first metal film onto a silicon substrate heated from 340° C. to 400° C., and causing a eutectic reaction between silicon and gold during the deposition; 1. A method for forming an electrode for a semiconductor device, comprising the step of: then depositing at least nickel alone or an alloy containing nickel as a second metal film.