JP2009099655A - Lead-free soldering method for wide gap semiconductor chips - Google Patents

Abstract

A lead-free soldering method for a wide-gap semiconductor chip, and a lead-free soldering method capable of improving the bonding state when soldering using lead-free solder is performed in an electrode structure of a wide-gap semiconductor chip A wide gap semiconductor chip is provided.
A protective layer for preventing oxidation of the outermost electrode surface during lead-free soldering is formed on an electrode of a wide gap semiconductor chip, and soldering is performed on the protective layer with lead-free solder. Lead-free soldering method for wide gap semiconductor chips.
[Selection] Figure 1

Description

  The present invention relates to a method of soldering electrodes of a wide gap semiconductor chip such as SiC with lead-free solder.

Conventionally, for example, as illustrated in FIG. 1A and FIG. 2A, in a wide gap semiconductor chip, an Ag oxide film (“Ag layer” in the figure) is formed on the outermost surface of the chip electrode. When lead-based solder is used, the wettability thereof is better than that of lead-free solder, so that no major problem has occurred in the bonding state between the electrodes.
However, lead-free solders such as tin-silver-copper (Sn-Ag-Cu) and gold-tin (Au-Sn) have poor wettability compared to lead-based solder, and are therefore greatly affected by the state of the electrode surface. For this reason, even if soldering is performed under the same conditions as lead-based solder, an unjoined portion as illustrated in FIG. 2B may be produced, and a joint state equivalent to lead-based solder is created. It is extremely difficult.
As a technique for overcoming the problems in the case of using this lead-free solder, for example, the following can be considered.
1. The oxide layer on the outermost surface of the electrode is removed with chemicals (so-called wet etching).
2. It is removed by a physical method such as dry etching.

However, in one method, for example, it can be removed by using a solution of zinc and hydrochloric acid. However, this has a problem that the surface electrode of the chip is violated. Sex is reduced.
As the second method, for example, there is a plasma etching method or the like. However, since the temperature of the chip itself increases, there is a concern about damage to the metal used for the electrode.
If both 1 and 2 are not soldered immediately after removal, an oxide layer is formed again on the outermost surface, which makes it difficult to set the timing for soldering.

  The present invention has been made in view of the above circumstances, and in the electrode structure of a wide gap semiconductor chip, the bonding state when soldering using lead-free solder is made favorable, It is an object of the present invention to provide a lead-free soldering method for a wide gap semiconductor chip and a wide gap semiconductor chip by lead-free soldering without causing the same problems.

In order to solve the above problems, the present invention forms a protective layer on the electrode of a wide gap semiconductor chip to prevent oxidation of the outermost surface of the electrode during lead-free soldering, and the lead-free solder is formed on the protective layer. Provided is a lead-free soldering method for a wide gap semiconductor chip, characterized by performing soldering.
In order to solve the above problems, the present invention provides a protective layer for preventing oxidation of the outermost surface of the electrode during lead-free soldering on the electrode of the wide gap semiconductor chip, and the lead-free coating is provided on the protective layer. Provided is a wide gap semiconductor chip by lead-free soldering, which is characterized by being soldered with solder.
According to the present invention having the features as described above, in the electrode structure of the wide gap semiconductor chip, by providing one or more protective layers, the oxide layer itself is not formed on the outermost electrode surface during lead-free soldering. Therefore, it is possible to realize lead-free soldering having a good joined state in which an unjoined portion between the electrode outermost surface and lead-free solder as illustrated in FIG. 2B does not occur.
As the protective layer, a material that does not need to be removed before soldering and that is compatible with lead-free solder or that is absorbed by lead-free solder during soldering is selected. Examples of materials considered as candidates include gold and preflux. Gold film formation can be performed by vacuum evaporation or plating.

[First embodiment]
FIG. 2B shows an embodiment of the present invention, and a single gold layer is provided to cover the outermost surface of the electrode as the protective layer as described above.
Here, Sn is applied to each of the wide gap semiconductor chip that does not cover the outermost surface of the electrode with the gold thin film layer (thickness of 100 nm or more) in FIG. 2A and the gold thin film layer that is covered with the gold thin film layer in FIG. When soldering using a lead-free solder of -Ag-Cu was performed, an image obtained by an X-ray fluoroscopic apparatus as shown in FIGS. 3A and 3B was obtained. As a result of calculation from this image data, it was found that when the same lead-free solder was used and soldered under the same conditions, so-called voids, which are unjoined portions, decreased from 35% to 15%.
When there are many voids, the shear strength becomes low and the heat conduction becomes bad, especially in power devices, but a fatal problem arises.However, according to the present invention, a very good bonding state can be obtained, and the shear strength and heat conduction are reduced. Improvements can be made.
The soldering conditions are preferably soldering in an environment where oxygen is eliminated as much as possible, such as heating in an inert gas such as nitrogen or heating in a vacuum.

(A) and (b) are schematic diagrams of a conventional example and an embodiment of the present invention, respectively. (A) and (b) are electron micrographs of a conventional example and an embodiment of the present invention, respectively. (A) and (b) are the X-ray fluoroscopic image figure of a prior art example and one Example of this invention, respectively.

Claims (8)

  A wide-gap semiconductor, characterized in that a protective layer that prevents oxidation of the outermost electrode surface during lead-free soldering is formed on the electrode of the wide-gap semiconductor chip, and soldering is performed on the protective layer with lead-free solder Chip lead-free soldering method.   2. The lead-free soldering method for a wide gap semiconductor chip according to claim 1, wherein the protective layer does not need to be removed before soldering and is made of a material that is compatible with lead-free solder or is absorbed by lead-free solder. .   3. The lead-free soldering method for a wide gap semiconductor chip according to claim 1, wherein the protective layer is made of gold.   4. The lead-free soldering method for a wide gap semiconductor chip according to claim 1, wherein one or more protective layers are formed.   A protective layer for preventing oxidation of the outermost electrode surface during lead-free soldering is provided on the electrode of the wide gap semiconductor chip, and soldering is performed on the protective layer with lead-free solder. Wide gap semiconductor chip by lead-free soldering.   7. The wide gap semiconductor chip by lead-free soldering according to claim 6, wherein the protective layer does not need to be removed before soldering and is made of a material that is compatible with lead-free solder or is absorbed by lead-free solder.   7. The lead-free soldering method for a wide gap semiconductor chip according to claim 5, wherein the protective layer is made of gold.   The wide gap semiconductor chip by lead-free soldering according to any one of claims 5 to 7, wherein at least one protective layer is provided.