Semiconductor device having protection film with recess

A semiconductor device may include a semiconductor substrate, a first metal film covering a surface of the semiconductor substrate; a protection film covering a peripheral portion of a surface of the first metal film; and a second metal film covering a range extending across a center portion of the surface of the first metal film and a surface of the protection film, wherein a recess may be provided in the surface of the protection film, and a part of the second metal film may be in contact with an inner surface of the recess.

TECHNICAL FIELD

A technique disclosed herein relates to a semiconductor device.

BACKGROUND

A semiconductor device of Japanese Patent Application Publication No. 2010-272711 includes a semiconductor substrate, a first metal film (emitter electrode film) covering a surface of the semiconductor substrate, a protection film (passivation film) covering a peripheral portion of a surface of the first metal film, and a second metal film (additional electrode) covering a range extending across a center portion of the surface of the first metal film and a surface of the protection film.

SUMMARY

In the semiconductor device of Japanese Patent Application Publication No. 2010-272711, a boundary between the second metal film and the protection film exists on the first metal film. When the semiconductor device generates heat, each of the first metal film, the second metal film, and the protection film thermally expands. Since the second metal film has a liner expansion coefficient different from that of the protection film, the first metal film under the second metal film has an expansion rate different from that of the first metal film under the protection film. Due to this, a high stress is applied to the first metal film under the boundary between the second metal film and the protection film, and thus a crack is likely to occur in the first metal film at that position.

Meanwhile, in general, a stress is applied to a metal film on a semiconductor substrate toward a center of the metal film. Therefore, in Japanese Patent Application Publication No. 2010-272711 as well, a stress is applied to the first metal film and the second metal film toward respective centers of the metal films. That is, the stress is applied to the first metal film and the second metal film in a direction separating those metal films from the protection film. When a crack occurs in the first metal film as described above, the force acting in the direction along which the first and second metal films are separated from the protection film concentrates on an interface between the second metal film and the protection film. Thereby, there may be a case where the second metal film is detached from the protection film, and the first and second metal films slide toward their centers. As a result, an abnormality occurs in an electrical property of the semiconductor device.

Therefore, a semiconductor device in which a first metal film and a second metal film are less likely to slide is disclosed herein.

The semiconductor device disclosed herein may comprise a semiconductor substrate; a first metal film covering a surface of the semiconductor substrate; a protection film covering a peripheral portion of a surface of the first metal film; and a second metal film covering a range extending across a center portion of the surface of the first metal film and a surface of the protection film. A recess may be provided in the surface of the protection film, and a part of the second metal film may be in contact with an inner surface of the recess.

In the above-described semiconductor device, a part of the second metal film is arranged in the recess provided in the surface of the protection film. According to this configuration, even when a force acts on the second metal film in a direction separating the second metal film from the protection film, the second metal film is held by the protection film due to the part of the second metal film arranged in the recess. Therefore, the second metal film is less likely to be detached from the protection film. Due to this, the second metal film, and the first metal film being in contact with the second metal film are less likely to slide.

DETAILED DESCRIPTION

As shown inFIG. 1, a semiconductor device10according to an embodiment comprises a semiconductor element20, a heatsink block80, a front surface side radiator plate81, a rear surface side radiator plate82, and sealing resin83. The semiconductor element20includes a semiconductor substrate, a front surface electrode, and a rear surface electrode. It should be noted that the front surface electrode and the rear surface electrode are not shown inFIG. 1. The heatsink block80is fixed to the front surface electrode of the semiconductor element20via a solder layer91. The front surface side radiator plate81is fixed to a front surface of the heatsink block80via a solder layer92. The rear surface side radiator plate82is fixed to the rear surface electrode of the semiconductor element20via a solder layer93. The sealing resin83covers a lower surface of the front surface side radiator plate81, the heatsink block80, the semiconductor element20, and an upper surface of the rear surface side radiator plate82.

As the semiconductor element20, for example, an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a diode, or the like can be used. The semiconductor element20generates heat while operating. As shown inFIG. 2, the semiconductor element20includes a semiconductor substrate24, a front surface electrode23, a protection film26, and a rear surface electrode28.

The semiconductor substrate24has a plate-like shape, and is constituted of silicon (Si), silicon carbide (SiC), or the like, for example. A liner expansion coefficient of the semiconductor substrate24is 3 to 5 ppm.

The front surface electrode23and the protection film26are provided above the semiconductor substrate24. The front surface electrode23includes first metal films21and a second metal film22.

FIG. 3is a top view of the semiconductor element20. It should be noted that the second metal film22is not shown inFIG. 3. Further, inFIG. 3, a range covered by the protection film26is hatched. As shown inFIG. 3, two first metal films21are provided on a front surface of the semiconductor substrate24. Each of the first metal films21partially covers the front surface of the semiconductor substrate24. The first metal films21have electrical conductivity, and are constituted of aluminum alloy (AlSi), for example. A liner expansion coefficient of the first metal films21is approximately 23 ppm.

The protection film26is constituted of resin, and has an insulation property. The protection film26is constituted of polyimide, for example. A liner expansion coefficient of the protection film26is approximately 35 ppm. As shown inFIGS. 2 and 3, the protection film26covers a portion of the front surface of the semiconductor substrate24which is not covered by the first metal films21, and a peripheral portion of a front surface of each first metal film21. The protection film26includes openings52above the respective first metal films21. A center portion of the front surface of each first metal film21is arranged within a corresponding one of the openings52. Therefore, the center portion of the front surface of each first metal film21is not covered by the protection film26. A recess51is provided in a surface of the protection film26. The recess51extends in a ring shape so as to surround the two openings52(i.e., the center portions of the respective first metal films21).

As shown inFIG. 2, the second metal film22covers a range extending across the center portion of the front surface of each first metal film21and the front surface of the protection film26. Therefore, the second metal film22covers side surfaces of the openings52of the protection film26. Due to this, on each first metal film21, a boundary54between the second metal film22and the protection film26exists. The second metal. film22has electrical conductivity; and is constituted of nickel (Ni), for example. A liner expansion coefficient of the second metal film22is approximately 13 ppm. The second metal film22covers an entirety of the front surface of the protection film26in a range where the recess51is provided. A part of the second metal film22is arranged in the recess51. The recess51is filled with the second metal film22over its entire region in a direction along which the recess51extends. The second metal film22arranged in the recess51is in contact with an inner surface of the recess51(i.e., its bottom surface and side surface). An entirety of the front surface of the second metal film22is covered by the solder layer91.

When the semiconductor device10generates heat, each of the first metal films21, the second metal film22, and the protection film26thermally expands. The liner expansion coefficient of the protection film26is greater than the liner expansion coefficient of the second metal film22. Therefore, an expansion rate of the first metal films21under the protection film26is greater than an expansion rate of the first metal films21under the second metal film22. Due to this, a high stress is applied to the first metal films21under the boundary54between the second metal film22and the protection film26, and a crack is likely to occur in the first metal films21at these positions.

Meanwhile, a stress is constantly applied to the first metal films21and the second metal film22toward respective centers of the metal films. The first metal films21and the second metal film22are deposited on the semiconductor substrate24by sputtering or vapor deposition in a high-temperature of around 300 degrees Celsius. When being deposited, stress is hardly applied to the first metal films21and the second metal film22. During cooling after the deposition, the first metal films21, the second metal film22, and the semiconductor substrate24shrink. At this occasion, since the liner expansion coefficient of the semiconductor substrate24is smaller than the liner expansion coefficients of the first metal films21and the second metal film22, the first metal films21and the second metal film22do not sufficiently shrink, and stress is generated toward the center of each metal film. Since an operating temperature of the semiconductor device10is equal to or lower than the above-mentioned deposition temperature, stress is constantly applied to the first metal films21and the second metal film22toward the respective centers thereof. That is, stress is applied to the first metal films21and the second metal film22in a direction separating these metal films from the protection film26. The higher the temperature of the semiconductor device10is, the smaller the stress applied to the first metal films21and the second metal film22becomes, and the lower the temperature of the semiconductor device10is, the greater the stress becomes. Depending on usage environment and conduction state of the semiconductor device10, the temperature of the semiconductor device10changes, and metal fatigue that leads to a crack accumulates in the first metal films21.

Further, the entirety of the front surface of the second metal film22is covered by the solder layer91. The solder layer91is formed in a high-temperature melting state on the front surface of the second metal film22, and then shrinks while cooling down and solidifying. Therefore, the solder layer91also causes the stress toward the centers of the first metal films21and the second metal film22(i.e., the stress acting in the direction separating these metal films from the protection film26) to be generated in the first metal films21and the second metal film22.

When a crack occurs in the first metal film(s)21under the boundary54as described above, the force acting in the direction separating the first metal films21and the second metal film22from the protection film26concentrates on an interface between the second metal film22and the protection film26. However, in the semiconductor device10of the present embodiment, a part of the second metal film22is arranged in the recess51provided in the front surface of the protection film26. According to such a configuration, even when the force acts on the second metal film22in the direction separating the second metal film22from the protection film26, the second metal film22is held by the protection film26due to the part of the second metal film22arranged in the recess51. Therefore, the second metal film22is less likely to be detached from the protection film26and is less likely to slide therefrom. Due to this, the first metal films21and the second metal film22can be suppressed from sliding, and high reliability of the semiconductor device10can be realized.

An example of a manufacturing method of the semiconductor device10will be described hereinbelow. It should be noted that this manufacturing method has a characterizing feature in forming the recess51, thus a process related to forming the recess51will be described below but explanations of the other processes will be omitted.

Firstly, as shown inFIG. 4, the first metal films21and the protection film26are deposited on the semiconductor substrate24. The protection film26is deposited to cover the front surface of the semiconductor substrate24in a range not covered by the first metal films21, and the first metal films21. Next, as shown inFIG. 5, a resist100is formed. An opening101is provided in the resist100located above a range where the recess51of the protection film26is to be formed, and an opening102is provided in the resist100located above a range where each of the openings52is to be formed. A width of each opening102is wider than a width of the opening101. Each opening102has a substantially square shape, and the opening101extends in a ring shape so as to surround the openings102. Next, as shown inFIG. 6, the protection film26inside the openings101and102is removed by isotropic etching. Inside the openings102, the first metal films21are exposed. Since the width of the opening101is narrower than the width of each opening102, an etching rate is lower inside the opening101than inside the openings102. Here, the etching is stopped before the first metal films21are exposed inside the opening101. Thereby, the recess51is formed inside the opening101. Thereafter, the resist100is removed. Next, as shown inFIG. 7, the second metal film22is deposited to cover the recess51. Due to this, the configuration in which a part of the second metal film22is arranged in the recess51can be obtained.

In the above-described embodiment, the second metal film22is constituted of nickel. However, the second metal film22may be constituted of another metal which can be joined by soldering. For example, the second metal film22may be constituted of gold, silver, copper, tin, or the like. Further, in a case where the second metal film22is connected to an external device by means other than solder joint, the second metal film22may be constituted of a metal which cannot be joined by soldering.

Some of technical elements disclosed herein will be listed below. It should be noted that the respective technical elements are independent of one another, and are useful solely or in combinations.

In a configuration example disclosed herein, a recess extends in a ring shape so as to surround a center portion of a first metal film.

According to such a configuration, a second metal film can be suppressed from being detached from a protection film over its entire peripheral portion.

In a configuration example disclosed herein, a solder layer covering a front surface of the second metal film is further provided.

According to such a configuration, a higher stress is applied to the second metal film by the solder layer. Therefore, by providing the recess in the surface of the protection film, the second metal film is more effectively suppressed from being detached.

While specific examples of the present invention have been described above in detail, these examples are merely illustrative and place no limitation on the scope of the patent claims. The technology described in the patent claims also encompasses various changes and modifications to the specific examples described above. The technical elements explained in the present description or drawings provide technical utility either independently or through various combinations. The present invention is not limited to the combinations described at the time the claims are filed. Further, the purpose of the examples illustrated by the present description or drawings is to satisfy multiple objectives simultaneously, and satisfying any one of those objectives gives technical utility to the present invention.