JP2020043183A - Semiconductor device - Google Patents

Abstract

To provide a vertical semiconductor device in which the amount of current that can be flowed through a wire bonded to the surface of an electrode layer can be increased.SOLUTION: A semiconductor device includes: a semiconductor layer 1 where forward current flows from the front surface 1a to the back surface 1b; and an electrode layer (surface electrode layer 2) formed on the front surface 1a of the semiconductor layer 1. The electrode layer includes a first conductive layer 21, a second conductive layer 22 and a third conductive layer 23 laminated in order on the front surface 1a of the semiconductor layer 1. The first conductive layer 21 is constituted from a material containing aluminum as a main component, and the second conductive layer 22 is constituted from a material having a lower resistivity than any of the materials constituting the first conductive layer 21 and the third conductive layer 23, respectively.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a semiconductor device such as a diode chip, and more particularly to a vertical semiconductor device in which a forward current flows from a front surface to a back surface.

  2. Description of the Related Art In recent years, in many electric devices, those requiring a large current have been increased in accordance with high performance. For this reason, a power supply circuit capable of supplying a large current to an electric device is required. Then, in order to realize such a power supply circuit, it is required to develop various electronic components such as semiconductor devices constituting the power supply circuit that can cope with a large current.

  In a high-output power supply circuit, many large-capacity capacitors are often incorporated, and a large surge current is generated. For this reason, it is desired that various electronic components such as a semiconductor device constituting a power supply circuit have a large surge current withstand capability (showing how much surge current can be tolerated by the amount of current). In this sense, there is a demand for the development of electronic components that can handle large currents.

  A semiconductor device (such as a rectifier diode chip) constituting a power supply circuit includes a vertical semiconductor device including a semiconductor layer in which a forward current flows from the front surface to the back surface, and an electrode layer formed on the surface of the semiconductor layer. May be used. Further, when such a semiconductor device is mounted on a circuit board, a wire for energizing the semiconductor device may be bonded to the surface of the electrode layer (wire bonding). Conventionally, a wire made of aluminum is used for the current-carrying wire, and the entire electrode layer is made of aluminum in accordance with the material of the wire so that the wire can be joined to the electrode layer. Many (see, for example, Patent Document 1).

  Also, in such a vertical semiconductor device, it is required to develop a device capable of handling a large current under the above-mentioned background.

JP 2018-14490A

  When the wire is bonded to the surface of the electrode layer as described above, the wire is locally bonded to the surface of the electrode layer. Also, current tends to flow through low resistance paths within the semiconductor device. For this reason, the current flowing into the electrode layer from the wire flows along the shortest path from the bonding point of the wire to the electrode layer on the back surface side of the semiconductor layer without substantially spreading around the path. That is, it was easy for the current to concentrate at the joints of the wires. Therefore, when an attempt is made to flow a large current through the semiconductor device, there has been a problem that the junction is heated abnormally due to the concentration of the current at the junction, and the junction is melted and damaged due to this. In order to solve such a problem, the use of a means for closely connecting a plurality of wires on the surface of the electrode layer can avoid the concentration of current, but such means is not very practical.

  Accordingly, an object of the present invention is to increase the amount of current that can flow through a wire bonded to the surface of an electrode layer in a vertical semiconductor device.

  A semiconductor device according to the present invention includes a semiconductor layer through which a forward current flows from a front surface to a back surface, and an electrode layer formed on the front surface of the semiconductor layer. The electrode layer includes a first conductive layer, a second conductive layer, and a third conductive layer sequentially stacked on the surface of the semiconductor layer. The first conductive layer is made of a material containing aluminum as a main component, and the second conductive layer is made of a material having a lower resistivity than any of the materials forming the first and third conductive layers. Have been.

  According to the semiconductor device, since the first conductive layer formed on the surface of the semiconductor layer is formed of a material containing aluminum as a main component, a compound having high resistivity is formed at the interface with the semiconductor layer. As a result, contact resistance between the first conductive layer and the semiconductor layer is less likely to occur.

  Further, since the second conductive layer having lower resistivity than any of these layers is interposed between the first conductive layer and the third conductive layer, the second conductive layer is bonded to the surface of the electrode layer (the surface of the third conductive layer). The current that has flowed into the electrode layer from the drawn wire is likely to flow laterally along the second conductive layer. Therefore, the current (forward current) flowing in the semiconductor device can be provided with a lateral spread, and as a result, the current can be prevented from being concentrated on the joint portion of the wire.

  According to the present invention, it is possible to increase the amount of current that can flow through the wire bonded to the surface of the electrode layer.

1 is a longitudinal sectional view conceptually showing a rectifier diode chip according to an embodiment. FIG. 3 is a conceptual diagram illustrating a flow of a current in a surface electrode layer. It is an observation image of a longitudinal section of a surface electrode layer without a barrier metal layer. It is an observation image of the surface of the third conductive layer in which copper is diffused. 4 is an observation image of a vertical cross section of a surface electrode layer on which a barrier metal layer is formed.

  An embodiment in which the present invention is applied to a rectifier diode chip will be specifically described. FIG. 1 is a longitudinal sectional view conceptually showing a rectifier diode chip. As shown in FIG. 1, the rectifier diode chip includes a semiconductor layer 1, a front electrode layer 2, and a back electrode layer 3.

  The semiconductor layer 1 includes an n-type semiconductor layer 11 and a p-type semiconductor layer 12 formed on the n-type semiconductor layer 11. The front surface 1a of the semiconductor layer 1 is constituted by the p-type semiconductor layer 12, and the back surface 1b of the semiconductor layer 1 is constituted by the n-type semiconductor layer 11. Therefore, a forward current flows through the semiconductor layer 1 from the front surface 1a to the back surface 1b in the vertical direction. That is, the rectifier diode chip of the present embodiment is a vertical semiconductor device.

  The surface electrode layer 2 is an electrode layer formed on the surface 1a of the semiconductor layer 1, and functions as an anode in the present embodiment. The surface electrode layer 2 is connected to another electronic component or terminal (not shown) via the wire W by bonding the wire W to the surface 2a (wire bonding; see FIG. 2). The back electrode layer 3 is an electrode layer formed on the back surface 1b of the semiconductor layer 1, and functions as a cathode in the present embodiment. The back surface electrode layer 3 is connected to wiring provided on a circuit board (not shown) by soldering or the like.

  As shown in FIG. 1, the surface electrode layer 2 includes a first conductive layer 21, a second conductive layer 22, and a third conductive layer 23 sequentially stacked on the surface 1a of the semiconductor layer 1. Furthermore, the surface electrode layer 2 is provided between the first barrier metal layer 24A interposed between the first conductive layer 21 and the second conductive layer 22, and between the second conductive layer 22 and the third conductive layer 23. And a second barrier metal layer 24B.

  These layers are formed using a thin film forming technique such as vacuum evaporation or sputtering. Specifically, after the semiconductor layer 1 is formed, a thin film made of a material (such as aluminum) for forming the first conductive layer 21 is formed on the surface 1a of the semiconductor layer 1 by vacuum deposition or sputtering. It is formed using a technique. After that, using the same thin film forming technique, thin films to be the first barrier metal layer 24A, the second conductive layer 22, the second barrier metal layer 24B, and the third conductive layer 23 are sequentially stacked. After these thin films are formed (laminated), the thin film is subjected to an etching process to form the surface electrode layer 2.

  The first conductive layer 21 and the third conductive layer 23 are made of a material containing aluminum as a main component. Specifically, the first conductive layer 21 and the third conductive layer 23 are made of a conductive material such as pure aluminum or an aluminum alloy containing an additive such as silicon. Note that the materials forming the first conductive layer 21 and the third conductive layer 23 may be the same or different from each other.

  Since the first conductive layer 21 formed on the surface 1a of the semiconductor layer 1 is made of a material containing aluminum as a main component, the contact resistance between the semiconductor layer 1 and the first conductive layer 21 is increased. Is less likely to occur. This is because a p-type semiconductor having a high impurity concentration has good compatibility with aluminum, and even when heat treatment is performed, it is difficult to form a compound having high resistivity at the interface between them, and even if a compound (silicide) is formed, This is because the resistivity is low. Further, since the third conductive layer 23 forming the surface 2a of the surface electrode layer 2 is made of a material containing aluminum as a main component, a current-carrying current bonding to the surface 2a of the surface electrode layer 2 is performed similarly to the related art. As the wire W (see FIG. 2), a wire containing aluminum as a main component can be used.

  The second conductive layer 22 is formed of a material having a lower resistivity than any of the materials forming the first conductive layer 21 and the third conductive layer 23. Specifically, the material forming the second conductive layer 22 contains at least one of copper, silver, and gold as a main component. These materials have a low resistivity and are preferable as the material forming the second conductive layer 22. Among them, copper is particularly preferable because it has low resistivity and is inexpensive as compared with silver and gold.

  According to such a configuration of the surface electrode layer 2, the flow of current in the surface electrode layer 2 can be improved. FIG. 2 is a conceptual diagram showing the flow of current in the surface electrode layer 2. As shown in FIG. 2, since the second conductive layer 22 having a lower resistivity than any of these layers is interposed between the first conductive layer 21 and the third conductive layer 23, the surface electrode layer 2 The current flowing into the surface electrode layer 2 from the wire W (see FIG. 2) bonded to the surface 2a (the surface of the third conductive layer 23) is likely to flow in the lateral direction along the second conductive layer 22. Therefore, the current (forward current) flowing through the rectifier diode chip can have a lateral spread, and as a result, the current can be prevented from being concentrated on the joint portion of the wire W. Therefore, in the rectifier diode chip of the present embodiment, which is a vertical semiconductor device, the amount of current that can flow through the wire W bonded to the surface 2a of the front electrode layer 2 can be increased.

  In addition, by configuring a part of the surface electrode layer 2 (that is, the second conductive layer 22) with a material having a low resistivity, the resistance of the surface electrode layer 2 can be reduced. Therefore, energy loss in the surface electrode layer 2 can be reduced.

  Furthermore, copper, silver, gold, and the like, which can be selected as the main components of the material forming the second conductive layer 22, have a high thermal conductivity, so that heat is easily radiated from the surface electrode layer 2. Therefore, the heat dissipation of the rectifier diode chip can be improved.

  On the other hand, the material forming the second conductive layer 22 is often a material that is easily diffused. Therefore, the material of the second conductive layer 22 diffuses into the first conductive layer 21 and the third conductive layer 23, and the diffused material becomes an alloy with the material forming the first conductive layer 21 and the third conductive layer 23. There is a possibility that the strength of the first conductive layer 21 and the third conductive layer 23 may be reduced due to the formation. Further, the material diffused from the second conductive layer 22 diffuses into the semiconductor layer 1 through the first conductive layer 21, and the diffused material forms a defect in the semiconductor layer 1, and the characteristic of the semiconductor layer 1 ( Forward voltage and recovery characteristics).

  For example, copper, which can be selected as a main component of the material forming the second conductive layer 22, is easily diffused and easily forms an alloy with aluminum. And the alloy of copper and aluminum has low strength and is brittle. Therefore, when copper is selected as the main component of the material forming the second conductive layer 22, the copper is used as the material forming the first conductive layer 21 and the third conductive layer 23 (aluminum that is the main component). By forming an alloy with the first conductive layer 21, the strength of the first conductive layer 21 and the third conductive layer 23 may be reduced.

  FIG. 3 is an observation image of a longitudinal section of the surface electrode layer 2 having no barrier metal layer. The observation image in FIG. 3 shows that copper, which is the main component of the second conductive layer 22, diffuses into the first conductive layer 21 and the third conductive layer 23 so that it is difficult to specify the boundary between the conductive layers. ing. FIG. 4 is an observation image of the surface of the third conductive layer 23 in which copper is diffused. The observation image in FIG. 4 shows that many cracks are generated on the surface of the third conductive layer 23, which indicates that the strength of the third conductive layer 23 is reduced by the diffusion of copper. As described above, copper forms an alloy with aluminum, which is a main component of the first conductive layer 21 and the third conductive layer 23, so that the strength of those conductive layers is reduced.

  Therefore, in the present embodiment, the first barrier metal layer 24A is formed between the first conductive layer 21 and the second conductive layer 22, and the second barrier metal layer 24A is formed between the second conductive layer 22 and the third conductive layer 23. A metal layer 24B is formed (see FIG. 1). The first barrier metal layer 24A and the second barrier metal layer 24B are made of a material (a so-called barrier metal) that prevents a material such as copper from diffusing (that is, passing through). Specifically, the material constituting each of the first barrier metal layer 24A and the second barrier metal layer 24B contains at least one of titanium, tungsten, molybdenum, and tantalum as a main component. Among them, titanium is particularly preferable because it is less expensive than other materials. Note that the materials constituting the first barrier metal layer 24A and the second barrier metal layer 24B may be the same or different from each other.

  FIG. 5 is an observation image of a longitudinal section of the surface electrode layer 2 on which the barrier metal layer is formed. According to the observation image in FIG. 5, since the boundaries between the respective layers can be specified, the diffusion of copper as the main component of the second conductive layer 22 is prevented by the first barrier metal layer 24A and the second barrier metal layer 24B. You can see that there is.

  As described above, the first barrier metal layer 24A is formed so that the material (main component) of the second conductive layer 22 is diffused into the first conductive layer 21, and furthermore, the semiconductor layer 1A passes through the first conductive layer 21. Prevent from spreading inside. As a result, a decrease in the strength of the first conductive layer 21 and a change in the characteristics of the semiconductor layer 1 are prevented. Further, the second barrier metal layer 24 </ b> B prevents the material (main component) of the second conductive layer 22 from diffusing into the third conductive layer 23. As a result, a decrease in the strength of the third conductive layer 23 is prevented.

  Here, titanium, tungsten, molybdenum, tantalum, or the like that can be selected as a main component of the material constituting each of the first barrier metal layer 24A and the second barrier metal layer 24B is a material having low conductivity. Therefore, the first barrier metal layer 24A and the second barrier metal layer 24B are preferably formed to be thinner than other layers in the surface electrode layer 2. As an example, the film thickness of the first barrier metal layer 24A and the second barrier metal layer 24B is preferably 0.3 μm or less.

  Note that the thickness of the barrier metal layer also depends on the smoothness of the surface 1a of the semiconductor layer 1. For example, when the surface 1a has irregularities of about 1 to 2 μm, the thickness of the barrier metal layer is preferably about 0.3 μm. On the other hand, when the smoothness of the surface 1a is increased by mirror polishing, the thickness of the barrier metal layer can be further reduced according to the smoothness.

  In the rectifier diode chip, when the material forming the second conductive layer 22 hardly diffuses, or when the diffusion does not adversely affect other layers, the surface electrode layer 2 is formed of the first barrier metal. It is not necessary to include both or one of the layer 24A and the second barrier metal layer 24B.

  Further, in the rectifier diode chip, the current-carrying wire W bonded to the surface 2a of the surface electrode layer 2 is a wire mainly containing at least one of aluminum, copper, and gold having low resistivity. May be used. In this case, the third conductive layer 23 forming the surface 2a of the surface electrode layer 2 is preferably made of the same material as the wire W in order to facilitate bonding with the wire W. Further, the configuration of the surface electrode layer 2 of the rectifier diode chip can be applied to a surface electrode layer provided in various vertical semiconductor devices not limited to the rectifier diode chip.

  The description of the above embodiment is illustrative in all aspects and should not be construed as limiting. The scope of the present invention is defined by the terms of the claims, rather than the embodiments described above. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the claims.

REFERENCE SIGNS LIST 1 semiconductor layer 1a front surface 1b back surface 2 front electrode layer 2a front surface 3 back electrode layer W wire 11 n-type semiconductor layer 12 p-type semiconductor layer 21 first conductive layer 22 second conductive layer 23 third conductive layer 24A first barrier metal layer 24B Second barrier metal layer

Claims (6)

A semiconductor layer in which a forward current flows from the front surface to the back surface,
An electrode layer formed on the surface of the semiconductor layer;
With
The electrode layer includes a first conductive layer, a second conductive layer, and a third conductive layer sequentially stacked on the surface of the semiconductor layer,
The first conductive layer is made of a material containing aluminum as a main component,
The semiconductor device, wherein the second conductive layer is made of a material having a lower resistivity than any of the materials forming the first conductive layer and the third conductive layer.   The semiconductor device according to claim 1, wherein a material forming the second conductive layer includes at least one of copper, silver, and gold as a main component. The electrode layer,
A first barrier metal layer interposed between the first conductive layer and the second conductive layer;
A second barrier metal layer interposed between the second conductive layer and the third conductive layer;
The semiconductor device according to claim 1, further comprising:   4. The semiconductor device according to claim 3, wherein a material constituting each of the first barrier metal layer and the second barrier metal layer contains at least one of titanium, tungsten, molybdenum, and tantalum as a main component.   The semiconductor device according to claim 1, wherein a wire is bonded to the third conductive layer, and the electrode layer is connected to another electronic component or a terminal via the wire.   The semiconductor device according to claim 5, wherein the wire includes at least one of aluminum, copper, and gold as a main component.

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