JP2013235948A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce contact resistance between an electrode pad and a conductor in a pressure-contact semiconductor device.SOLUTION: A semiconductor device 1 comprises a conductor 4 brought into pressure contact with an electrode pad 3 on a semiconductor element 2 and harder than the electrode pad 3. A concave-convex part 5 is formed on an end surface of the conductor 4 bonded to the electrode pad 3. A salient 53 having a spiry longitudinal surface is formed at an edge part of a salient 51 in the concave-convex part 5. The end surface of the conductor 4 having the concave-convex part 5 is preferably covered with a conductive material softer than the electrode pad 3 and the conductor 4 and having low electric resistance.

Description

  The present invention relates to a pressure contact structure in a semiconductor device, and more particularly to a pressure contact structure between an electrode pad on a semiconductor element and a conductor.

  As a typical insulated power semiconductor module, there is an IGBT (Insulated Gate Bipolar Transistor) module used in a power converter such as an inverter. In addition, standards for “insulated power semiconductor modules” or “Isolated power semiconductor devices” typified by this IGBT module are established in JEC-2407-2007 and IEC60747-15, respectively.

  The structure of a general insulated power semiconductor module disclosed in Non-Patent Document 1 will be described. In the insulated power semiconductor module 60 shown in FIG. 4A, the semiconductor element 61 such as an IGBT or a diode which is the switching element shown in FIG. 4B is connected to a DBC (Direct Bond) via its lower electrode layer. Copper) Soldered onto the copper circuit foil 63 of the substrate 62. The DBC substrate 62 is obtained by directly bonding a copper circuit foil 63 to both surfaces of an insulating plate 64 made of ceramics or the like. The DBC substrate 62 is connected to a copper base 65 for heat dissipation via a solder portion 66.

  The upper electrode layer of the semiconductor element 61 is bonded with an aluminum wire 67 with ultrasonic waves and is electrically connected to, for example, another copper circuit foil 63 on the DBC substrate 62. A copper terminal 68 for connecting electricity from the copper circuit foil 63 of the DBC substrate 62 to the outside is connected to the copper circuit foil 63 by soldering. Further, this is surrounded by a plastic case 69 and filled with silicone gel or the like for electrical insulation. Here, generally, the solder joint between the semiconductor element 61 and the DBC substrate 62 has a high melting point and is joined by two reflows to the solder joint between the DBC substrate 62 and the copper base 65.

  In recent years, the operating temperature of semiconductor elements has been increased, and the operating temperature is 175 to 200 ° C., which is close to the melting point of general-purpose solder materials. For this reason, metal-based high-temperature solder (Bi, Zn, Au), compound-based high-temperature solder (Sn—Cu), low-temperature sintered metal (Ag nanopaste), and the like have been proposed as alternative materials. Further, SiC, which is a next-generation semiconductor element, has been reported to operate at 250 to 300 ° C.

  On the other hand, a flat pressure contact structure package 70 illustrated in FIG. 5A is known as a semiconductor module that does not employ solder connection (Non-Patent Documents 1, 2, etc.). As shown in FIG. 5B, the upper electrode layer of the semiconductor element 71 in the flat pressure contact structure package 70 is provided on the Mo plate 73 in contact with the contact terminals 72. A guide 74 for positioning the semiconductor element 71 and the contact terminal 72 is provided at the end of the semiconductor element 71.

  The flat pressure contact structure package 70 can cool the semiconductor element 71 from both sides and can be electrically and thermally connected to the outside without using solder. For this reason, both sides of the flat pressure contact structure package 70 are generally pressed by heat sinks to cool both surfaces of the package 70 and the heat sink is used as a conductive member.

  The pressure contact must be electrically insulated between the upper and lower heat sinks of the flat pressure contact structure package 70, and the pressure contact is performed by a leaf spring, but the design pressure contact force is equal to the electrode post of the flat pressure contact structure package 70. It is necessary to make it take. If the pressure contact is poor, the semiconductor element 71 is destroyed. In addition, skill is required for constructing the circuit, for example, because the heat sink and the pressure spring contact the leaf spring to prevent downsizing.

  For this reason, the flat type pressure contact structure package 70 is applied to a limited apparatus, and the above-described insulating power semiconductor module which is easy to use has been widely used instead.

  In order to improve the reliability to temperature cycle, power cycle, etc., there is a problem caused by the difference in thermal expansion of each member (semiconductor, metal, ceramics, etc.) constituting the semiconductor module. That is, between the DBC substrate and the copper base, and between the DBC substrate and the copper terminal, a shear stress acts on the solder between the copper and ceramics due to the difference in thermal expansion coefficient. There is a risk of peeling. Furthermore, cracks may also occur in the solder between the semiconductor element and the DBC substrate. Depending on the conditions, even at the connection portion of the aluminum wire on the semiconductor element, stress is generated due to the difference in thermal expansion between the aluminum and the semiconductor element, and the aluminum wire is fatigued.

  In recent years, since the power density increases year by year and the bonding temperature inside the semiconductor element increases, the shear stress at the solder joint and the stress applied to the aluminum wire have increased. On the other hand, it is necessary to prevent the influence of thermal expansion from becoming apparent during the period until the design life of the semiconductor module is reached. With the advent of wide-band cap semiconductor elements that can be used at high temperatures such as SiC and GaN, further reduction of the effect of thermal expansion is required.

The Institute of Electrical Engineers, High Performance and High Functionality Power Device / Power IC Research Special Edition, “Power Device / Power IC Handbook”, Corona, 1996. 6.7, p.289, p.336 Akihiro Mori, Yasukazu Seki, “Recent Advances in Large Capacity IGBTs”, The Institute of Electrical Engineers of Japan, Vol. 118, 1998, p276

Japanese Patent Laid-Open No. 5-13409

  In order to achieve high reliability, environmental friendliness, and convenience at the same time, there is a need to realize an easy-to-use insulated power semiconductor module that does not use solder bonding or wire bonding like pressure welding. Further, semiconductor modules that make use of the performance of semiconductor elements that can be used at high temperatures, such as SiC and GaN, are also required to have improved reliability such as temperature cycle and power cycle.

  However, in the pressure contact structure, the conductor is in direct contact with the electrode pad on the semiconductor chip. However, as shown in FIG. 6A, the electrode pad 81 and the conductor 82 There is a contact resistance due to the interface state. As this cause, as shown in FIG. 6 (b), surface contamination, adsorbed material, surface oxide film, altered layer accompanying processing, void layer 83 according to surface roughness, and the like can be mentioned. On these contact surfaces, the contact portion slides due to the pressure contact force, and a part of the film is broken and pressed, and when a low resistance member comes into contact, it becomes conductive, but it is significantly larger than the original resistivity of the electrode material. Become.

  Accordingly, the semiconductor device according to claim 1 includes a conductor harder than the pad pressed against the electrode pad on the semiconductor element, and an uneven portion is formed on an end surface of the conductor joined to the pad. A convex part having a spire-like longitudinal section is formed at the edge of the part. According to the present invention, the bonding area between the electrode pad and the conductor is increased, and the contact resistance between the pad and the conductor is reduced. In particular, when the conductor is pressed against the electrode pad, the contact resistance between the pad and the conductor can be reduced with a smaller pressing force. Moreover, it becomes easy to suppress the resistance distribution between the electrode pad and the conductor with respect to the variation in the pressure contact force.

  A semiconductor device according to a second aspect is characterized in that, in the semiconductor device according to the first aspect, an end face of the conductor having the concavo-convex portion is covered with a conductive material that is softer and lower in electrical resistance than the pad and the conductor. According to the present invention, the contact resistance between the electrode pad and the conductor is further reduced.

  According to a third aspect of the present invention, in the semiconductor device of the second aspect, the film thickness of the coating is larger than the surface roughness of the end face. According to the present invention, since the gap between the fine irregularities due to the surface roughness of the end face of the conductor in contact with the electrode pad is filled with the conductive material, the contact resistance between the pad and the conductor is further reduced.

  The semiconductor device according to claim 4 is the semiconductor device according to any one of claims 1 to 3, wherein a distance from a lower end of the convex portion to an apex of the convex portion having a spire shape in the longitudinal section is equal to or less than a thickness of the pad. It is characterized by. According to the present invention, when the electrode pad and the conductor are joined, the uneven portion of the conductor falls within the thickness range of the pad.

  A semiconductor device according to a fifth aspect is the semiconductor device according to any one of the first to fourth aspects, wherein the concavo-convex part having the convex part having a spire shape in the longitudinal section is formed by ion milling. According to the present invention, a convex portion having a spire shape in the longitudinal section can be efficiently formed at the edge of the convex portion in the concave and convex portion.

  Therefore, according to the above invention, the contact resistance between the electrode pad and the conductor in the pressure contact type semiconductor device can be reduced.

The longitudinal cross-sectional view which showed the joining state of the conductor and electrode pad of embodiment of this invention. (A) is a longitudinal cross-sectional view of the semiconductor device in embodiment of this invention, (b) is a perspective view of the device. Explanatory drawing of the process (a)-(d) which forms the convex part of a longitudinal cross-section spire in the edge of a convex part. (A) is a perspective view of a solder connection type semiconductor module, (b) is a longitudinal cross-sectional view around an electrode pad on a semiconductor element in the module. (A) is a perspective view of a press-contact type semiconductor module, (b) is a longitudinal cross-sectional view of the module. (A) is the longitudinal cross-sectional view explaining the contact state of the conventional conductor and electrode pad, (b) is an expanded longitudinal cross-sectional view of the same state.

  A semiconductor device according to an embodiment of the present invention will be described below with reference to the drawings. The present invention is not limited to this embodiment, and can be implemented with various modifications within the scope of the claims.

  The semiconductor device 1 of the present embodiment shown in FIG. 1 includes a conductor 4 that is harder than the pad 3 that presses against the electrode pad 3 on the semiconductor element 2. A plurality of concave and convex portions 5 including convex portions 51 and concave portions 52 are formed on the end face of the conductor 4 to be joined to the electrode pad 3. Further, a convex portion 53 having a spire-like longitudinal section is formed at the edge of the convex portion 51.

  Examples of the conductor 4 include the conductors 41 to 43 provided in the pressure contact type semiconductor module 20 illustrated in FIG. The conductors 41 to 43 are conductors for electrically connecting the electrode pads 21 to 23 on the semiconductor element 2 to the extraction electrodes 24 to 26, respectively. In particular, the conductor 43 is a conductor connected to the electrode pad 23 functioning as a gate pad on the semiconductor element 2 and is formed in a needle shape.

  In the power semiconductor chip, an Al alloy containing several percent of Si is used as a material for the electrode pad 3 after being formed into a thickness of several microns to about 10 microns. On the other hand, in the pressure contact type power semiconductor chip, the conductor 4 applied as a contact electrode reduces the generation of interface stress due to the difference in thermal expansion with the semiconductor chip when the temperature changes, and the pressure contact force is completely transmitted to the semiconductor chip. For this reason, a material mainly composed of a low thermal expansion, high hardness metal such as Mo alloy or W alloy is used. In addition, the needle-like conductor 4 having a tip diameter of 1 mm or less, the main component of which is the low thermal expansion and high hardness metal, is applied to a gate electrode or the like that does not flow a large current.

  The laminated body composed of the semiconductor element 2, the conductors 41 to 43, and the lead electrodes 24 to 26 is housed in the ceramic case 27 as shown in FIG. A cooling member 29a is disposed on the extraction electrodes 24 and 26 via an insulating member 28a, and a cooling member 29b is disposed on the extraction electrode 25 via an insulating member 28b.

  The concave / convex portion 5 having the convex portion 53 is formed on the end face of the conductor 4 by an ion milling method. Specifically, the uneven portion 5 made of the material of the conductor 4 is formed on the end face of the conductor 4 made of a material (for example, Mo alloy, W alloy) harder than the material (for example, Al alloy) constituting the electrode pad 3. It is formed by an ion milling method. For example, the ion milling method disclosed in Patent Document 1 (paragraph [0003]) is applied to this method. The ion milling method is a method of removing a target object by physically causing plasma of an inert gas such as Ar to collide with a workpiece. When the distance H1 from the lower end of the convex portion 51 to the apex of the convex portion 53 having a spire-like vertical section is set to be equal to or less than the thickness H2 of the electrode pad 3, the conductor 4 and the electrode pad 3 are joined when the conductor 4 and the electrode pad 3 are joined. The uneven portion 5 can be accommodated within the range of the thickness H <b> 2 of the electrode pad 3.

  Specific steps (a) to (d) of the ion milling method will be described with reference to FIG.

  Step (a): A metal layer 501 made of the same material as the conductor 4 on the end face of the conductor 4 made of a material (for example, Mo alloy, W alloy, etc.) harder than the material (for example, Al alloy) constituting the electrode pad 3 Is formed. The end face of the conductor 4 is flat, and the surface roughness Ra is processed in advance to, for example, 0.5 μm or less.

  Step (b): A resist layer 502 made of a photosensitive resin is directly formed on the formed metal layer 501, and this resist layer 502 is patterned. Specifically, a photosensitive resin is applied to the thickness of the electrode pad 3 or less, and a circular shape having a diameter of several microns to several tens of microns, for example, according to the shape (for example, circular shape) of the end face of the conductor 4 by a known photolithography technique. The resist layer 502 of the pattern is evenly arranged.

  Step (c): The metal layer 501 is patterned by etching by an ion milling method using the patterned resist layer 502 as a mask. In this step, the component of the layer 501 scattered from the metal layer 501 collides with and adheres to the side wall of the resist layer 502.

  Step (d): Then, the patterned metal layer 501 is formed on the end face of the conductor 4 by removing the resist layer 502 by wet etching using an organic solvent or dry etching using ozone gas. At the edge of the metal layer 501, a burr layer 503 having a spire-like longitudinal section made of the components of the same layer 501 scattered in the step (c) remains. The metal layer 501 and the burr layer 503 become the convex part 51 and the convex part 53 of the concave-convex part 5 shown in FIG.

  By patterning the plurality of metal layers 501 having the burr layer 503 at the edge by the above-described steps (a) to (d), at the end face of the conductor 4 joined to the electrode pad 3 on the semiconductor element 2. A plurality of concave and convex portions 5 including the convex portions 51 and 53 and the concave portion 52 are efficiently formed.

  Moreover, in order to eliminate the space | gap resulting from the surface roughness of the end surface of the conductor 4, when the end surface of the conductor 4 is coated with a conductive material after the step (d), the contact resistance between the electrode pad 3 and the conductor 4 is further increased. To reduce. For example, when the electrode pad 3 is made of an Al alloy and the conductor 4 is made of an Mo alloy or a W alloy, an Ag alloy, for example, is formed by a vacuum deposition method as a conductive material that is softer and lower in electrical resistance than the electrode pad 3 and the conductor 4. The end face of the conductor 4 is coated with a predetermined film thickness (for example, about 0.5 μm). The film formation may be performed by other methods such as a plating method and a sputter deposition method. When the film thickness of the coating is made thicker than the surface roughness of the end face of the conductor 4 in contact with the electrode pad 3, the gap between the fine irregularities due to the surface roughness of the end face is filled with the conductive material. The contact resistance between the pad 3 and the conductor 4 can be further reduced.

  When the conductor 4 manufactured in the steps (a) to (d) is in pressure contact with the electrode pad 3 on the semiconductor element 2, the uneven portion 5 of the conductor 4 becomes an oxide film or an adsorption coating on the electrode pad 3 as shown in FIG. Etc. and dig into the inside of the pad 3. By forming the concavo-convex portion 5 on the end face of the conductor 4 in pressure contact with the electrode pad 3, the bonding area between the electrode pad 3 and the conductor 4 is expanded. As a result, the contact resistance between the conductor 4 and the electrode pad 3 is reduced as compared with a conductor that does not have the uneven portion 5.

  In particular, the convex portion 53 is formed at the edge of the convex portion 51 in the concave and convex portion 5, so that the bonding area between the electrode pad 3 and the conductor 4 is further expanded, and the contact resistance between the electrode pad 3 and the conductor 4 is reduced. Further reduction. When the conductor 4 is pressed against the electrode pad 3, the contact resistance between the electrode pad 3 and the conductor 4 can be reduced with a smaller pressing force. Moreover, it becomes easy to suppress the resistance distribution between the electrode pad 3 and the conductor 4 with respect to the variation in the pressure contact force.

  Furthermore, the end surface of the conductor 4 having the concavo-convex portion 5 is covered with a conductive material that is softer and lower in electrical resistance than the electrode pad 3 and the conductor 4, thereby further reducing the contact resistance between the electrode pad 3 and the conductor 4.

  In addition, since the film thickness of the coating is set to be thicker than the surface roughness of the end surface, the gap between the fine irregularities due to the surface roughness of the end surface of the conductor 4 in contact with the electrode pad 3 is caused by the conductive material. As a result, the contact resistance between the pad 3 and the conductor 4 can be further reduced.

  Furthermore, when the distance H1 from the lower end of the convex portion 51 to the apex of the convex portion 53 is set to be equal to or less than the thickness of the electrode pad 3, the concave and convex portion 5 of the conductor 4 when the electrode pad 3 and the conductor 4 are joined. Falls within the thickness range of the electrode pad 3.

  Moreover, the uneven | corrugated | grooved part 5 which has the convex part 53 can form the convex part 53 efficiently in the edge part of the convex part 51 in the uneven | corrugated | grooved part 5 by the ion milling method.

DESCRIPTION OF SYMBOLS 1 ... Semiconductor device 2 ... Semiconductor element 3 ... Electrode pad 4 ... Conductor 5 ... Uneven part, 51, 53 ... Convex part, 52 ... Concave part

Claims (5)

Provided with a harder conductor than the pad pressed against the electrode pad on the semiconductor element,
A semiconductor device, wherein an uneven portion is formed on an end face of a conductor to be joined to the pad, and a convex portion having a spire shape in the longitudinal section is formed at an edge of the convex portion in the uneven portion. 2. The semiconductor device according to claim 1, wherein an end face of the conductor having the concavo-convex portion is covered with a conductive material that is softer and lower in electrical resistance than the pad and the conductor. The semiconductor device according to claim 2, wherein a film thickness of the coating is thicker than a surface roughness of the end face. The semiconductor device according to any one of claims 1 to 3, wherein a distance from a lower end of the convex portion to a vertex of the convex portion having a spire shape in the longitudinal section is equal to or less than a thickness of the pad. 5. The semiconductor device according to claim 1, wherein the concavo-convex portion having the convex portion having a spire shape in the longitudinal section is formed by ion milling.

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