Copper bonding ribbon, method for manufacturing copper bonding ribbon, and semiconductor device

Abstract

The purpose of the present invention is to provide a copper bonding ribbon capable of securing bonding strength without damaging an object to be bonded, extending the life of the cutter, reducing electric conductivity, and suppressing oxidation progress of the surface of the copper ribbon. This copper bonding ribbon comprises a copper alloy having a copper purity of 99.99 mass% or more. The copper alloy contains 5-40 ppm by mass of silver, 0-10 ppm by mass of phosphorus, 0-8 ppm by mass of iron, silicon, arsenic, and antimony, respectively, with respect to the total amount. The total content of phosphorus, iron, silicon, arsenic, and antimony is 0-30 ppm by mass. In a cross section perpendicular to the longitudinal direction, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundary is 30% to less than 60%, (1) the arithmetic average roughness Ra in the longitudinal direction of the plane is 30-150 nm, (2) the arithmetic average roughness Ra in the direction perpendicular to the longitudinal direction of the plane is 30-150 nm, and the value obtained by dividing (1) the arithmetic average roughness Ra in the longitudinal direction by (2) the arithmetic average roughness Ra in the direction perpendicular to the longitudinal direction is 0.75-1.15.

Description

Copper bonding ribbon, copper bonding ribbon manufacturing method, and semiconductor device

[0001] The present invention relates to a copper bonding ribbon (hereinafter also referred to as "copper ribbon"), and in particular to a copper ribbon suitable for applications involving large currents, such as power semiconductors and connecting the bus bars of lithium-ion batteries in electric vehicles to the electrodes of the lithium-ion batteries, a method for manufacturing the same, and a semiconductor device using the copper ribbon.

[0002] Generally, "semiconductor" is a general term for CPUs (central processing units) and memories, whose main roles are "calculation" and "storage." Semiconductors are used in consumer devices such as personal computers, smartphones, and televisions. On the other hand, power semiconductors are responsible for driving motors, charging batteries, and supplying power to operate microcomputers and LSIs (large-scale integrated circuits). Power semiconductors are mainly used to change voltage and frequency, and for power conversion (converting direct current to alternating current or alternating current to direct current). Power semiconductors are also called power semiconductors, power devices, power elements, and power semiconductor elements.

[0003] "Inverters" found in energy-saving home appliances such as air conditioners, refrigerators, and washing machines are a familiar example of the use of power semiconductors. Inverters control the rotation speed of a motor by converting frequency. By freely changing the rotation speed of a motor, inverters reduce unnecessary motor movement and contribute to energy savings. On the other hand, air conditioners without inverters adjust the room temperature by repeatedly turning the motor on and off, which can result in problems such as a lack of temperature stability and high power consumption. These functions of an inverter are achieved by "switching" the current on and off precisely using power semiconductors (power transistors).

[0004] In addition to energy-saving home appliances, power semiconductors are also widely used in the transportation sector, for example, in electric vehicles and hybrid vehicles. In the transportation sector, power semiconductors such as IGBTs (insulated gate bipolar transistors) are used to convert and control power. An IGBT consists of an IGBT and bonding wires that connect the IGBT chips (power chips) to each other and to connect the IGBT chips to external electrodes. Because a large current flows through the bonding wire for power semiconductors, a relatively thick aluminum wire with a diameter of 40 μm or more and 700 μm or less is used.

[0005] As described above, while general semiconductors are primarily used for low-power computing, power semiconductors are primarily used for high-power control, and thus can be considered completely different. The bonding wire used in general semiconductors only carries a small current. Therefore, thin bonding wires made of Au, Ag, or Cu with a wire diameter of 30 μm or less are often used for general semiconductors. The bonding method involves melting one end of the wire to form a molten ball, applying heat, ultrasound, and pressure to the wire to form a first bond to a semiconductor chip electrode, etc., and then a second bond to an external substrate or electrode, etc. In contrast, the bonding wire used in power semiconductors requires a large current to be passed through it, so as mentioned above, thick aluminum wires with a wire diameter of 40 μm or more, which have relatively low electrical resistance and are inexpensive, are used. The bonding method for power semiconductors does not create a molten ball, but instead applies ultrasound and pressure to the wire at room temperature to form a first bond to an electrode, etc., and then a second bond to an external substrate or electrode, etc.

[0006] Furthermore, unlike general semiconductors, power semiconductors require repeated on-off switching of large currents, as described above, requiring bonding wires that can withstand the thermal loads caused by heat generation and cooling. Furthermore, because they are used in applications requiring extremely high safety, such as automobiles and trains, they require high levels of durability and reliability. For these reasons, the required characteristics and properties of bonding wires for general semiconductors and those for power semiconductors are completely different.

[0007] The 21st Conference of the Parties to the United Nations Framework Convention on Climate Change (COP21), which aimed to reduce greenhouse gas emissions as a measure to combat global warming and climate change, adopted the Paris Agreement, which calls for greenhouse gas emissions reductions from 2020 onward. Following this, the global movement toward a decarbonized society has accelerated in recent years. The automotive industry plays a major role in this movement. European countries have announced a policy to completely ban the sale of internal combustion engine vehicles (gasoline and diesel vehicles) by 2035 and limit sales to electric vehicles (including plug-in hybrid vehicles). The United States has also set a goal of making electric vehicles 50% of its new car market by 2030. This trend is also seen in emerging countries such as China and India, and Japan also plans to switch all new car sales to electric vehicles (including hybrid vehicles) by 2035.

[0008] As the shift to electric vehicles accelerates, there are issues that hinder their widespread adoption: their short driving range (the distance they can travel from a fully charged state until the charge runs out) and the lack of charging facilities (locations).

[0009] In particular, improving the performance of power semiconductors is essential to extending the driving range. Electric vehicles are made possible by converting the direct current (DC) of the battery, which is the energy source, into alternating current using an inverter, which is a power semiconductor, to drive the motor.

[0010] Until now, power semiconductors have been made of silicon (Si), but in recent years, power semiconductors made of silicon carbide (SiC) have been released. Conventional Si power semiconductors generate energy losses when current is applied and when switching is performed on and off. All of this electrical energy loss is released as heat (heat generation) from the inverter. A heat dissipation mechanism is required to suppress this heat generation, which ultimately reduces the living space of electric vehicles, increases vehicle weight, and shortens their driving range.

[0011] The application of SiC power semiconductors to electric vehicles can reduce energy loss both during current flow and during on / off switching operations. This allows for the miniaturization of passive elements such as transformers, which benefit from increased switching frequencies (higher frequencies), and makes it possible to reduce the size and weight of power semiconductors, such as inverters. Furthermore, the simplification of the heat dissipation mechanism makes it possible to make the entire electric vehicle more compact (lighter), and the highly efficient operation of the inverter can extend the driving range per charge.

[0012] In addition, the widespread use of battery charging equipment is essential for the widespread use of electric vehicles, but at the same time, there is a growing need for rapid charging. As a result, there is a demand for bonding wires that can withstand the large currents that come with rapid charging.

[0013] Amid this trend toward miniaturization and support for large currents, aluminum is currently the primary material used for most bonding wires in power semiconductors. In addition to power semiconductors, aluminum bonding wire is also used as the wire connecting the busbars and electrodes of lithium-ion batteries in mobile lithium-ion batteries in electric vehicles. Because aluminum has high thermal and electrical resistance, it generates a lot of heat when current is applied, and its heat dissipation effect is poor. Therefore, the use of aluminum wire can be considered to be counter to the trend toward miniaturization of power semiconductors. Furthermore, it has been reported that the heat generated by the passage of large currents at the joints of weak aluminum wire causes grain coarsening, resulting in the destruction of the joints.

[0014] Therefore, copper wire has attracted attention as an alternative material to aluminum wire. Compared to aluminum wire, copper wire has lower thermal and electrical resistance, which helps prevent heat generation, and its high recrystallization temperature means it can withstand large currents.

[0015] Furthermore, in recent years, expectations have been rising for copper ribbons, which have a larger bonding area than copper wires. Because both copper wires and copper ribbons are joined in a loop shape between the first and second junctions, the bending process places stress on the copper wires and ribbons during the joining process. In particular, with hard materials like copper, a large wire diameter (thickness) can lead to cracks during bending. Therefore, when replacing copper wires with copper ribbons to increase the bonding area, it is common to make the copper ribbon thicker and wider, so that the copper ribbon can withstand bending. A larger bonding area increases the bonding strength and allows for greater current flow. However, issues unique to copper and the ribbon shape arise.

[0016] As mentioned above, copper wire generates less heat than aluminum wire and can withstand high power. However, copper is much harder and more resistant to crushing than aluminum, so a large load is required when joining copper wire. This is not a problem when joining objects such as circuit boards, which are strong and unlikely to break even under load. However, when joining delicate objects such as IC chips, the hardness of copper combined with the large load increases the likelihood of damaging the IC chip. Furthermore, copper ribbon has a larger cross-sectional area than copper wire, so the cutting area is also larger. This, combined with the hardness of copper and the large cutting area, creates a new problem: the lifespan of the cutter used to cut the ribbon after the second joining is shortened.

[0017] If the cutter life is shortened, the cutter will need to be replaced more frequently, which will reduce productivity and increase the cost of purchasing cutters, resulting in higher manufacturing costs.In addition to the cost issue, using a dull cutter increases the likelihood of burrs or remaining material remaining on the cut surface after cutting.

[0018] Patent Document 1 describes a copper rolled sheet in which the ratio of the length of special grain boundaries to the length of all crystal grain boundaries is 60% or more. The invention described in Patent Document 1 aims to improve fatigue properties, and does not suggest anything about cutter life or bonding strength. Furthermore, since this is a wiring material for electronic components such as terminals, connectors, and lead frames, or interconnectors that connect electrodes in solar cell panels, the preferred range of the length of the special grain boundaries, i.e., the coincidence grain boundaries, is different from that of wiring materials used in power semiconductors and lithium-ion batteries.

[0019] Patent Document 2 describes an invention of a copper alloy strip for LED lead frames, and the invention described in Patent Document 2 specifies the average roughness in the rolling direction of the strip surface in order to improve the reflectivity of the Ag-plated reflective film. However, the invention described in Patent Document 2 is not intended for use as a wiring material for power semiconductors or lithium-ion batteries, and there is no suggestion regarding cutter life or bonding strength.

[0020] The invention described in D3 relates to a ribbon made of an aluminum alloy with a concave end surface, and aims to obtain a high-strength aluminum alloy having a ribbon shape comparable to that of copper-based and iron-based materials. D3 does not suggest anything about the cutter life or bonding strength of copper ribbon materials.

[0021] JP 2012-062498 JP 2014-189852 WO2018 / 181329

[0022] As described above, the problem to be solved by the present invention is to provide a copper bonding ribbon (hereinafter also referred to as "copper ribbon") that can ensure bonding strength without damaging fragile bonding objects such as IC chips, and a copper ribbon that contributes to extending the life of cutters. In addition to the above, the present invention also aims to provide a copper bonding ribbon that can suppress surface oxidation over time without reducing electrical conductivity.

[0023] Additionally, the present invention aims to provide a method for manufacturing the copper ribbon and a power semiconductor device using the copper ribbon.

[0024] As a result of extensive research, the inventors have invented a copper bonding ribbon that can simultaneously solve multiple problems, by using the copper bonding ribbon of the following embodiment, and the copper bonding ribbon and semiconductor device manufactured by the manufacturing method of the embodiment, which ensures bonding strength without damaging fragile bonding objects such as IC chips, contributes to extending the life of the cutter that cuts the copper bonding ribbon after bonding, does not reduce electrical conductivity, and can suppress oxidation of the copper bonding ribbon surface over time.

[0025] The copper ribbon of the embodiment is a copper bonding ribbon made of a copper alloy having a copper purity of 99.99% by mass or more, wherein the copper alloy contains, relative to the total amount, 5 ppm by mass or more and 40 ppm by mass or less of silver (Ag), 0 ppm by mass or more and 10 ppm by mass or less of phosphorus (P), 0 ppm by mass or more and 8 ppm by mass or less of iron (Fe), silicon (Si), arsenic (As), and antimony (Sb), respectively, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less, (1) in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, and (2) the arithmetic average roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) The arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and (1) the arithmetic mean roughness (Ra) in the longitudinal direction divided by (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less.

[0026] In addition, it is preferable that the end face of the copper bonding ribbon of the embodiment has a curvature, and it is preferable that the value obtained by dividing the radius of curvature by the thickness of the copper bonding ribbon is 0.50 or more and 0.90 or less.

[0027] In addition, it is preferable that the width of the copper ribbon of the embodiment is 0.3 mm or more and 5.0 mm or less, and the thickness is 0.05 mm or more and 0.5 mm or less.

[0028] Further, a method for producing a copper ribbon according to an embodiment is a method for producing a copper bonding ribbon, the method comprising the steps of: preparing a copper wire made of a copper alloy having a copper purity of 99.99% by mass or more, containing 5 ppm by mass or more and 40 ppm by mass or less of silver (Ag), 0 ppm by mass or more and 10 ppm by mass or less of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) each at 0 ppm by mass or more and 8 ppm by mass or less, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less; drawing the copper wire to obtain a copper wire; rolling the copper wire to form a copper ribbon; and performing a final heat treatment on the copper ribbon by electrical heating. The copper bonding ribbon is characterized in that, in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, (1) the arithmetic mean roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and the value obtained by dividing (1) the arithmetic mean roughness (Ra) in the longitudinal direction by (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less.

[0029] A power semiconductor device comprising at least one substrate, at least one semiconductor element on the substrate, an electrode on a surface of the semiconductor element, a semiconductor element-on-substrate on the semiconductor element, at least one circuit pattern on the substrate, and copper bonding ribbons connecting one or more selected from the group consisting of: the electrodes on the surface of the semiconductor element to each other, the electrodes on the surface of the semiconductor element to an external electrode, the electrodes on the surface of the semiconductor element to one of the circuit patterns, the electrodes on the surface of the semiconductor element to a terminal, two adjacent circuit patterns among the circuit patterns, one of the circuit patterns to a terminal, the substrate to the substrate, and the semiconductor element-on-substrate to the circuit pattern, The copper bonding ribbon is made of a copper alloy having a copper purity of 99.99% by mass or more, the copper alloy containing, relative to the total amount, 5 ppm by mass or more and 40 ppm by mass or less of silver (Ag), 0 ppm by mass or more and 10 ppm by mass or less of phosphorus (P), 0 ppm by mass or more and 8 ppm by mass or less of iron (Fe), silicon (Si), arsenic (As), and antimony (Sb), respectively, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less, (1) in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, (2) the arithmetic average roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) The arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and the value obtained by dividing (1) the arithmetic mean roughness (Ra) in the longitudinal direction by (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less. In this specification, the symbol "to" represents a numerical range from the value on the left of the symbol to the value on the right of the symbol.

[0030] The copper bonding ribbon of the present invention can simultaneously solve several problems: it does not damage fragile objects to be bonded, such as IC chips, ensures bonding strength, contributes to extending the life of the cutter that cuts the ribbon after bonding, does not reduce electrical conductivity, and can suppress oxidation of the copper ribbon surface over time.

[0031] 1 is a photograph showing a portion of a cross section perpendicular to the longitudinal direction of a copper bonding ribbon according to an embodiment; FIG. 2 is an oblique overhead photograph of a copper bonding ribbon according to an embodiment, and a photograph showing an enlarged schematic view of a joint; FIG. 3 is a cross-sectional view showing a schematic configuration of a semiconductor device according to an embodiment; FIG. 4 is a cross-sectional view showing a schematic configuration of a semiconductor device according to another embodiment; FIG. 5 is a cross-sectional view showing a schematic configuration of a semiconductor device according to an embodiment, the semiconductor device having a ribbon connecting circuit patterns; FIG. 6 is a graph showing the change over time in the CuO oxide film ratio after storage tests of Example 6, Example 13, and Comparative Example 14; FIG. 7 is a diagram showing a schematic view of a portion of a lithium ion battery module; FIG. 8 is a diagram showing a schematic view of a portion of a lithium ion battery module according to another embodiment; FIG. 9 is a diagram showing a schematic view of an entire lithium ion battery module; FIG. 10 is a diagram showing the proportion of coincidence grain boundary lengths measured by EBSD on a cross section perpendicular to the longitudinal direction of a copper bonding ribbon according to an embodiment;

[0032] A copper bonding ribbon according to an embodiment of the present invention will now be described. Figures 1 and 2 are photographs showing the copper bonding ribbon according to this embodiment. Figure 1 is a photograph showing a portion of a cross section of the copper bonding ribbon in a direction perpendicular to the longitudinal direction. Figure 2 is a diagonal overhead photograph of the copper bonding ribbon. The copper ribbon according to this embodiment is made of a copper alloy having a copper purity of 99.99% by mass or more, and contains 5 to 40 ppm by mass of silver (Ag) and 0 to 10 ppm by mass of phosphorus (P) relative to the total amount. The copper ribbon according to this embodiment further contains one or more elements selected from the group consisting of iron (Fe), silicon (Si), arsenic (As), and antimony (Sb), each at 0 to 8 ppm by mass, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) is 0 to 30 ppm by mass. In the copper ribbon of the embodiment, in a cross section perpendicular to its longitudinal direction, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%. Also, on the surface of the copper ribbon of the embodiment, (1) the arithmetic mean roughness (Ra) in the longitudinal direction is 30 nm or more and 150 nm or less, (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 30 nm or more and 150 nm or less, and the value obtained by dividing (1) the arithmetic mean roughness (Ra) in the longitudinal direction by (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less. In this specification, unless otherwise specified, the arithmetic mean roughness (Ra) is simply referred to as "average roughness".

[0033] The coincidence boundaries are low-energy grain boundaries, and exist from Σ1 to Σ49 in a face-centered cubic crystal. The length of the coincidence boundary in this embodiment is the value of the Σ3 coincidence boundary among the above-mentioned coincidence boundaries.

[0034] The shape of copper ribbons depends primarily on the manufacturing method. There are two main methods for manufacturing copper ribbons. One involves producing a plate-shaped ingot in a melting and casting process, rolling the ingot to the desired thickness and stretching it longitudinally, and finally cutting the end faces perpendicularly (slitting). The other involves producing a cylindrical ingot in a melting and casting process, subjecting the ingot to continuous casting, then repeatedly drawing the ingot to produce a wire of a certain thin diameter, and finally rolling the wire to stretch it longitudinally to the desired copper ribbon thickness. These methods produce thin, long, flat copper ribbons. When the copper ribbon is laid flat, the top surface (or the opposite surface) is the copper ribbon plane, the direction parallel to the copper ribbon plane and perpendicular to the copper ribbon longitudinal direction is the copper ribbon width, and the areas near both ends of the width are the copper ribbon end faces. The end faces of copper ribbons manufactured using the slitting method are perpendicular to the copper ribbon plane. Copper ribbons produced by the latter method, which involves rolling after wire drawing, do not have their end faces cut off at the end, so their end face shapes are arcs or elliptical arcs (hereinafter collectively referred to as "arcs"). This area with curvature is called the end face. In rare cases, copper ribbons have their end faces rounded by passing them through a die after slitting, and these copper ribbons are also included in the category of copper ribbons with curvature.

[0035] The following describes in detail the process of trial and error that led to the invention, as well as the configuration and manufacturing method of the copper ribbon of this embodiment.

[0036] (Extending cutter life) If the load on the cutter is large when cutting the copper ribbon, the cutter will lose its sharpness early and will need to be replaced more frequently. Not only will the cost of the cutter increase, but the more frequently the cutter needs to be replaced, the more times the bonding machine will have to be stopped, which will reduce the productivity of the bonding process.

[0037] The cutter used in this embodiment is, for example, a front cutter 01-25028 manufactured by Kulicke & Soffa, size item number 13 (or size item number 33). The dimensions of the cutter are a total length of approximately 20 mm, a width of approximately 3 mm, and a thickness of approximately 1 mm, and the length of the cutting edge portion is approximately 2.5 mm, the length of the portion other than the cutting edge (attachment portion) is approximately 17.5 mm, the width of the cutting edge tip is approximately 2.2 mm, and the thickness is approximately 0.05 mm.

[0038] First, the inventors carefully investigated numerous prototypes made under various manufacturing conditions to find a way to cut copper ribbon without putting strain on the cutter. As a result, they discovered that there is a correlation between the metal structure of the cross section perpendicular to the longitudinal direction, i.e., the rolling direction of the copper ribbon, i.e., the ratio of coincidence grain boundaries (special grain boundaries) to the total grain boundary length, and the strain on the cutter. Note that hereinafter, the cross section perpendicular to the longitudinal direction of the copper ribbon will also be referred to as the "cross section."

[0039] A coincidence boundary is a grain boundary structure in which, despite being located at a grain boundary position, it is at a crystal lattice position, and therefore has a regular atomic arrangement with very few disturbances or gaps compared to a normal grain boundary. In other words, the grain boundary is strong because there are few gaps in the grain boundary. However, because it is strong, a load is placed on the cutter when cutting. Therefore, the inventors thought that if the proportion of the coincidence boundary length could be controlled to be small, the life of the cutter could be extended.

[0040] The length of the coincidence grain boundary can be measured by electron backscattered diffraction (EBSD). The inventors have conducted research into the relationship between the ratio of the length of the coincidence grain boundary to the total length of the grain boundaries and the life of the cutter. As a result, they have discovered that the life of the cutter is not shortened if the ratio of the length of the coincidence grain boundary to the total length of the grain boundaries is less than 60%.

[0041] However, if the proportion of the CBL length is too small, there is a high risk of the oxidation resistance of the copper ribbon decreasing. Oxidation resistance is also related to the elements contained in the product of the present invention, as described below. The inventors conducted storage tests on several copper ribbons with different proportions of the CBL length to investigate the correlation between the proportion of the CBL length and oxidation resistance. As a result, they discovered that if the proportion of the CBL length is 30% or more, oxidation resistance is achieved, i.e., the proportion of the copper (II) oxide (CuO) film on the copper ribbon surface after 6 months of storage testing is 30% or less. From this, it was found that the proportion of the CBL length is suitable in the range of 30% to less than 60% of the total length of the crystal grain boundaries. Furthermore, the proportion of the CBL length is preferably 33% to less than 55%, and more preferably 35% to less than 50%.

[0042] As described above, by setting the proportion of coincidence grain boundary lengths to less than 60%, an improvement in cutter life of about 10% was achieved. Hoping to extend the cutter life even further, the inventors conducted extensive research, focusing on the shape of the copper ribbon. As a result, they discovered that there is a correlation between the end face shape of the copper ribbon and cutter life.

[0043] As mentioned above, the end face shape of a copper ribbon depends on the manufacturing method of the copper ribbon. The end face of a copper ribbon manufactured by the slitting method is perpendicular to the plane of the copper ribbon. Copper ribbons manufactured by the method of rolling after wire drawing do not have their end faces cut off at the end, so the end face shape of the copper ribbon is arc-shaped.

[0044] The inventors conducted repeated experiments to determine whether a copper ribbon with a straight or arc-shaped end face was more effective in extending the cutter's lifespan, and found that a copper ribbon with an arc-shaped end face was more effective in extending the cutter's lifespan.

[0045] Furthermore, the inventors focused on the radius of curvature of the arc of the end face. Having noticed that the radius of curvature varies depending on the thickness of the copper ribbon, the inventors compared the value obtained by dividing the radius of curvature of the end face of the copper ribbon by the thickness of the copper ribbon with the cutter life, and evaluated the correlation. As a result, they found that a copper ribbon with a value obtained by dividing the radius of curvature by the thickness of the copper ribbon of 0.50 or more and 0.90 or less is most effective in extending the cutter life.

[0046] The reason for the correlation between the end face shape of the copper ribbon and the cutter lifespan is speculated as follows. When cutting a copper ribbon, it is rare for the edge of the cutter's cutting edge to contact the copper ribbon plane perfectly parallel; instead, there is a slight difference in angle between the edge of the cutter's cutting edge and the copper ribbon plane. In other words, the blade often contacts the copper ribbon plane at a slight angle. This increases the likelihood that the cutter blade will hit the end face of the copper ribbon (especially the boundary between the rolled plane and the end face). In this case, if the end face is perpendicular to the rolled surface (copper ribbon plane), the initial contact of the cutter will hit a sharp part (right-angle part), causing considerable damage to the cutter. On the other hand, if the end face is arc-shaped, the initial contact will be softer than at a right angle, which is speculated to ultimately lead to a longer cutter lifespan.

[0047] The inventors produced numerous prototypes by varying the crushing rate (processing rate) during roll rolling of the drawn wire, and among copper ribbons of different dimensions, they found a curvature radius that is effective in improving the cutter life. They found that copper ribbons with a curvature radius of the end face of the copper ribbon divided by the thickness of the copper ribbon of 0.90 or less are effective in improving the cutter life, while a curvature radius of more than 0.90 is ineffective. Furthermore, a curvature radius of less than 0.50 does not affect the cutter life, but cracks may occur on the end face of the copper ribbon or the copper ribbon itself may warp. Based on these findings, they found that the optimal range for the curvature radius of the end face of the copper ribbon divided by the thickness of the copper ribbon is 0.50 or more and 0.90 or less. Furthermore, the curvature radius of the end face of the copper ribbon divided by the thickness of the copper ribbon is preferably 0.58 or more and 0.82 or less. A curvature radius of 0.66 or more and 0.74 or less is even more preferable.

[0048] The radius of curvature of a copper ribbon can be measured as follows. After the copper ribbon is manufactured, the copper ribbon is wound onto a spool, for example, to a length of 100 m, and the radius of curvature is measured approximately at the leading end, trailing end, and intermediate point. Specifically, it is preferable to measure the radius of curvature at both ends of the copper ribbon width direction at these three locations, obtaining a total of six measured values, and then calculate the average value. Samples are taken by cutting out portions of the copper ribbon at three locations: the leading end, the trailing end, and intermediate point. The taken samples are embedded in resin and polished to expose a cross section perpendicular to the longitudinal direction. The radius of curvature of the copper ribbon end face can be automatically calculated by specifying three points on the exposed cross section: the inflection point between the copper ribbon plane (rolled surface) and the end face, the outermost point on the end face, and the inflection point between the plane (rolled surface) of the copper ribbon and the plane (rolled surface) on the opposite side (back surface) of the copper ribbon plane (rolled surface) and the end face.

[0049] (Chip Damage) Next, we will explain how to reduce damage during bonding when the object to be bonded with the copper ribbon is a delicate object, such as an IC chip, that is vulnerable to external loads and easily damaged. Copper is much harder than the gold, silver, and aluminum that have traditionally been used for bonding wire, so the basic solution is to soften the copper ribbon, and the challenge is how to obtain sufficient bonding strength with as small a load as possible.

[0050] The inventors of the present invention thought that if the bonding area could be increased in addition to softening the copper ribbon, it might be possible to increase the bonding strength even with a small load. Previous attempts have been made to roughen the copper ribbon surface to increase the bonding area, but most of these have involved roughening only in one direction, such as perpendicular to the longitudinal direction, or by chemically roughening the surface using chemicals.

[0051] For example, if scratches (patterns) are created only in the longitudinal direction of the copper ribbon plane, as if the unevenness of the rolling rolls is transferred to the copper ribbon surface, unevenness will be formed in the direction perpendicular to the longitudinal direction, but unevenness is unlikely to be formed in the longitudinal direction. This unevenness will have a sharp sawtooth shape, and the distance between the peaks (convex) and valleys (concave) will likely be short. As a result, when joining the copper ribbon, the peaks are unlikely to be crushed, and the contact between the peaks and the joining partners, i.e., point contact, will occur, which reduces the joining area and makes it difficult to expect an increase in joining strength.

[0052] The method of chemically dissolving and roughening a material with a chemical is costly due to the use of the chemical, and furthermore, there is a possibility that the chemical may contaminate the material.

[0053] The inventors thought that by forming a rectangular surface with equal unevenness in both the longitudinal direction and the direction perpendicular to the longitudinal direction without using chemicals and with wide spacing between peaks and valleys, it would be possible to obtain high bonding strength with a small load without damaging even delicate objects to be joined.The inventors thought that a rectangular surface would have more cushioning properties than a sawtooth surface and would also be able to reduce the load on the objects to be joined.

[0054] Based on these considerations, the inventors used various manufacturing methods to create prototypes with different copper ribbon surface roughnesses and conducted extensive research to determine what surface roughness would provide high bonding strength with a small load. As a result, they discovered that the range in which high bonding strength can be achieved with a small load is when (1) the average roughness in the longitudinal direction of the copper ribbon plane and (2) the average roughness in the direction perpendicular to the longitudinal direction are both 30 nm to 150 nm, and the value obtained by dividing (1) the average roughness in the longitudinal direction by (2) the average roughness in the direction perpendicular to the longitudinal direction is 0.75 to 1.15. When the values ​​of (1) the average roughness in the longitudinal direction and (2) the average roughness in the direction perpendicular to the longitudinal direction are less than 30 nm or exceed 150 nm, the bonding strength is weakened. When the value obtained by dividing (1) the average roughness in the longitudinal direction by (2) the average roughness in the direction perpendicular to the longitudinal direction is less than 0.75 or exceeds 1.15, the possibility of damaging the objects to be bonded increases. Furthermore, (1) the average roughness in the longitudinal direction and (2) the average roughness in the direction perpendicular to the longitudinal direction are both preferably 55 nm or more and 130 nm or less, more preferably 60 nm or more and 120 nm or less. Furthermore, the value obtained by dividing (1) the average roughness in the longitudinal direction by (2) the average roughness in the direction perpendicular to the longitudinal direction is preferably 0.80 or more, even more preferably 0.85 or more.

[0055] (Issues of oxide generation and electrical conductivity) Next, the composition of the copper ribbon of this embodiment will be described. As mentioned above, the copper ribbon has a larger bonding area (volume) than the copper wire. For example, when comparing a copper wire for power devices having a wire diameter of 200 μm with a copper ribbon having a thickness of 200 μm, the same diameter as the copper wire, and a width of 1250 μm, the volume of the copper ribbon is about 8 times larger than that of the copper wire, and the bonding area is also similarly larger, given the same length. As such, the copper ribbon has a larger bonding area than the copper wire, and therefore a higher bonding strength. Furthermore, because the copper ribbon has a larger bonding area than the copper wire, fewer bonds are required than the copper wire to pass the same current, and the number of bonding operations is reduced, which greatly contributes to productivity.

[0056] However, copper surfaces are relatively susceptible to oxidation. When joining copper wires, pressure is applied from the radial direction of the wire by a tool at the joining point, causing the wire to plastically deform while being joined (crimped). Therefore, even if the wire surface is oxidized, sufficient joint strength can be achieved because the unoxidized copper inside is exposed during plastic deformation.

[0057] In contrast, copper ribbons have a flat shape, so they do not undergo significant plastic deformation during bonding, which differs from copper wires, which are flattened during bonding. Therefore, the amount of unoxidized copper exposed inside the ribbon during plastic deformation is significantly less than when wire bonding is performed. Therefore, oxidation of the copper ribbon surface has a greater impact on bonding strength than oxidation of the copper wire surface, posing a more serious problem. Furthermore, unlike ball bonding, copper ribbons are bonded at room temperature, so surface oxidation of copper ribbons has an even greater negative impact on bonding strength.

[0058] Furthermore, the copper ribbon surface becomes hardened considerably as oxide formation progresses, which may adversely affect the life of the cutter blade. Also, if the copper ribbon is to be bonded to a delicate object such as an IC chip, the copper ribbon with oxide formation on its surface is more likely to damage the IC chip due to its hardness.

[0059] As mentioned above, copper ribbon has a larger bonding area (volume) than copper wire, allowing for a larger current to flow at one time. However, if the electrical resistance of the copper ribbon itself is high, the loss due to resistance increases. Therefore, in order to reduce the loss due to resistance, the inventors wanted to ensure that the copper ribbon (purity 99.99% by mass) of the embodiment has an electrical conductivity of at least 99%, with the electrical conductivity of ultra-high purity copper (purity 99.999% by mass), which has an even lower impurity content, being 100%.

[0060] As mentioned above, the widespread adoption of electric vehicles is expected as a measure to combat global warming (reducing carbon dioxide emissions). However, the driving distance (cruising range) of electric vehicles on a full charge is shorter than that of gasoline-powered vehicles, which is a problem in the widespread adoption of electric vehicles. Reducing the size (weight) of vehicles is an effective way to extend the driving range of electric vehicles. However, if the energy loss during switching (on / off) in power semiconductors is large, the heat emitted also increases, requiring a large cooling space to cool the power semiconductors. This cooling space hinders the miniaturization of electric vehicles. High electrical resistance increases energy loss, so reducing the electrical resistance of power semiconductors is an important key to the widespread adoption of electric vehicles.

[0061] Here, copper wires can be bonded even when the bonding space within a semiconductor device is small, because their small cross-sectional area gives them sufficient conformability and flexibility. Therefore, even if copper wires have a high electrical resistance, the loss due to electrical resistance can be compensated for by increasing the number of copper wire bonds.

[0062] In contrast, copper ribbons generally have a larger cross-sectional area than copper wires, making them less adaptable to the tool during bonding than copper wires. In other words, copper ribbons have poor adaptability and flexibility, making them difficult to apply to bonding that involves sharp bends or twists, other than unidirectional, linear bonding. Therefore, it is difficult to increase the number of bonds with copper ribbons, and high electrical resistance directly leads to energy loss. Therefore, high electrical conductivity of the copper ribbon itself is a very important requirement.

[0063] In order to simultaneously solve the problems of oxide formation on the copper ribbon surface and electrical conductivity, the inventors investigated the composition of the copper alloy that constitutes the copper ribbon. The inventors sought trace elements that effectively suppress the progression of oxide formation in the copper base material and suppress an increase in electrical resistance. As a result of extensive experiments using a variety of elements added to the copper base material, they discovered that maintaining a copper purity of 99.99% by mass or higher, adding silver (Ag) as a trace element, and controlling the contents of phosphorus (P), iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) are effective in suppressing copper oxide formation and an increase in electrical resistance.

[0064] As a result of conducting many experiments, the inventors have discovered that, from the viewpoint of suppressing the generation of surface oxides and maintaining electrical conductivity, it is preferable that the content of silver (Ag) is 5 ppm by mass or more and 40 ppm by mass or less, that phosphorus (P) is 0 ppm by mass or more and 10 ppm by mass or less, that the content of iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) is 0 ppm by mass or more and 8 ppm by mass or less, and that the total of phosphorus (P), iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less, relative to the entire copper ribbon.

[0065] It was discovered that when the silver (Ag) content is 5 mass ppm or more and 40 mass ppm or less, and at the same time, phosphorus (P) is 0 mass ppm or more and 10 mass ppm or less, iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) are each 0 mass ppm or more and 8 mass ppm or less, and the total of the above trace elements excluding silver (Ag) is 0 mass ppm or more and 30 mass ppm or less, the effect of delaying the generation of copper oxide is easily obtained, and the electrical resistance of the entire copper ribbon does not increase too much.

[0066] In particular, in the manufacturing process of the copper ribbon of this embodiment, when the copper ribbon is exposed to high temperatures during heat treatment, the rate of copper oxide generation increases, although this depends on the heat treatment atmosphere. In this case, by controlling the composition of the copper ribbon of this embodiment and by controlling the aforementioned proportion of the coincidence grain boundary length to 30% or more, the rate of copper oxide generation on the copper ribbon surface during heat treatment can be slowed.

[0067] The susceptibility of the copper ribbon surface to oxidation can be evaluated by measuring the copper oxide (II) (CuO) oxide film formed on the copper ribbon surface after a storage test. In the storage test, the copper ribbon sample is placed in a container or the like without sealing, and left at room temperature of 35°C and humidity of 75% RH. Under these conditions, the sample is left for a maximum of 10 months, during which the CuO oxide film formed on the copper ribbon surface is measured at the start of storage (1 hour), and then every other month, for example, after 1 month, 2 months, and 3 months, up to 10 months. In the process of copper oxidation, copper oxide (I) (CuO) is first formed on the copper ribbon surface. ２ O) is generated, and as oxidation progresses, the proportion of CuO increases. Therefore, by calculating the proportion of CuO oxide film, it is possible to evaluate the ease of oxidation generation on the copper ribbon surface. The progress of oxidation can be expressed by the following formula: "Proportion of CuO oxide film = ((CuO oxide film) / (CuO oxide film + Cu ２ In this embodiment, the ratio of the CuO oxide film after 6 months is used as the evaluation standard, and if the ratio of the copper (II) oxide (CuO) oxide film is 30% or less, it is rated as "B", meaning that it is above the minimum level. If the ratio of the copper (II) oxide (CuO) oxide film is 25% or less, it is rated as "A", meaning that it is good. If the ratio of the copper (II) oxide (CuO) oxide film is 20% or less, it is rated as "S", meaning that it is better. The analysis method will be described later.

[0068] The International Annealed Copper Standard (IACS) was used to evaluate electrical conductivity. Samples with an IACS decrease rate of less than 0.5% compared to the IACS (100%) of 99.999% pure copper at room temperature (20°C) were rated "S," meaning excellent. Samples with an IACS decrease rate of 0.5% or more but less than 0.8% compared to the IACS (100%) of 99.999% pure copper at room temperature (20°C) were rated "A," meaning good. Samples with an IACS decrease rate of 0.8% or more but less than 1.0% compared to the IACS (100%) of 99.999% pure copper at room temperature (20°C) were rated "B," meaning that they met or exceeded the minimum practical level. Samples with an IACS drop of 1.0% or more compared to the IACS (100%) of 99.999% pure copper at room temperature (20°C) were rated "F" in the sense that they were likely to cause practical problems. The measurement method for IACS will be described in detail later.

[0069] The dimensions of the copper ribbon in this embodiment are, for example, preferably a width of 0.3 mm to 5.0 mm and a thickness of 0.05 mm to 0.5 mm, and more preferably a width of 1.0 mm to 3.0 mm and a thickness of 0.1 mm to 1.0 mm. The width of the copper ribbon is the maximum distance from the vertex of the end face (the point located outward) to the vertex of the other opposite end face, and the thickness of the copper ribbon is the maximum distance between the plane of the copper ribbon and the surface on the opposite side.

[0070] (Method for manufacturing copper ribbon) Next, an example of a method for manufacturing a copper ribbon according to an embodiment will be described. Note that the method for manufacturing a copper ribbon is not limited to the manufacturing method described below. It is also desirable to adjust the conditions appropriately in consideration of the weight of the copper ribbon to be manufactured and the processing capacity of the heat treatment furnace. As described above, the copper ribbon according to this embodiment employs a process in which a copper wire having a relatively thin wire diameter is manufactured, and then the copper wire is rolled to form a copper ribbon.

[0071] First, molten copper is prepared by dissolving silver in high-purity copper of 99.99% by mass or higher according to the composition of the resulting copper ribbon. A heating furnace, such as an arc heating furnace, a high-frequency heating furnace, a resistance heating furnace, or a continuous casting furnace, is used for melting. While atmospheric melting is acceptable, it is preferable to melt the molten copper in the heating furnace while maintaining it in a vacuum or an inert gas atmosphere such as argon or nitrogen to prevent air from being mixed in. The molten material is then solidified by continuous casting from the heating furnace to a predetermined wire diameter to produce a strand. Alternatively, the molten copper may be cast into a mold to produce a cylindrical ingot, which is then extruded to a predetermined wire diameter using an extruder to produce a strand. Alternatively, a rough wire may be produced by a DIP forming method or an SCR method and used as a strand.

[0072] The wire obtained in the above process is drawn into an intermediate wire with a wire diameter of 900 μm. Next, the intermediate wire is subjected to intermediate heat treatment by heating at 400°C to 600°C for 60 to 420 minutes. The intermediate heat treatment may be performed using a "combustion furnace" using oil or gas as a heat source, or an "electric furnace" using electrical energy as a heat source. The intermediate heat treatment may be performed using a "batch method" or a "continuous method (tubular furnace method)." Alternatively, an "electric method" in which the wire is heated by directly passing an electric current through it may be used. The heat treatment atmosphere is preferably an inert gas atmosphere to prevent oxidation of the wire.

[0073] Next, the wire after the intermediate heat treatment is drawn to the target final wire diameter. The final wire diameter can be determined based on the target width and thickness of the copper ribbon and the radius of curvature of the end face. For example, to produce a copper ribbon with a width of 1.25 mm, a thickness of 0.2 mm, and a radius of curvature of 180 μm, it is preferable to draw the copper wire to a wire diameter of 700 μm. In the wire drawing process, the wire can be passed through multiple carbide dies or diamond dies in sequence to gradually reduce the wire diameter. The wire reduction rate (also referred to as the area reduction rate or processing rate) before and after one die drawing process is preferably 5% or more and 30% or less.

[0074] The copper wire drawn to the final wire diameter is rolled to the target copper ribbon dimensions (width, thickness, radius of curvature) using a two-stage roll mill. Depending on the dimensions of the copper ribbon, it is preferable to roll it several times using two-stage rolls. When processing from a wire into a ribbon shape, the cross-sectional area reduction rate is preferably 3% or more and 15% or less. When processing from a ribbon shape into a ribbon shape, the cross-sectional area reduction rate (rolling reduction rate) per operation is preferably 10% or more and 60% or less.

[0075] As a finishing touch, the ribbon processed to the final dimensions is subjected to a heat treatment (final heat treatment). The final heat treatment conditions are the most important step for manufacturing the copper ribbon of the embodiment. After various heat treatment experiments and trial and error, the inventors have determined that the following method is the most preferable for manufacturing the copper ribbon of the embodiment. A resistance heating method was adopted for the final heat treatment. The resistance heating conditions are a voltage value of 18 V to 24 V, a distance between the electrode terminals of 900 mm to 1300 mm, a ribbon running speed of 70 m / min to 150 m / min, and the second electrode is immersed in cooling water (the ribbon moves into the cooling water while being heated). Pure water is used as the cooling liquid, and the temperature of the pure water is preferably 30°C to 60°C, and the dissolved oxygen concentration in the pure water is preferably 7 mg / L or less. To prevent oxidation of the copper ribbon, the area between the electrode terminals through which the copper ribbon passes is preferably covered in a cylindrical shape, and nitrogen gas is preferably flowed at a flow rate of, for example, 30 L / min to 50 L / min. Although the electrical heating method has been described here, other heat treatment methods may be adopted to achieve the surface roughness and the proportion of corresponding grain boundary length (also referred to as the corresponding grain boundary density) of the copper ribbon in the above-mentioned embodiment.

[0076] Until now, the roughness of ribbon surfaces has often depended on the unevenness of the roller surfaces used during rolling. In other words, many manufacturing methods have been used to control surface roughness by creating unevenness on the rollers according to the target roughness and then transferring that unevenness to the material being rolled. However, these conventional manufacturing methods can result in large differences in the average roughness in the longitudinal direction and the direction perpendicular to the longitudinal direction. As mentioned above, this results in a microscopic sawtooth-shaped surface, which makes sharp contact with the objects to be joined, and can potentially damage the objects to be joined, such as delicate IC chips.

[0077] The method for controlling the surface roughness of a copper ribbon according to this embodiment is distinct from previous methods. In other words, the method for controlling the surface roughness of a copper ribbon according to this embodiment does not control the roughness of the copper ribbon surface by physical or mechanical means, but rather controls the unevenness of the copper ribbon surface by promoting the growth of crystal grains inside the copper ribbon through optimal heat treatment, and utilizing the fact that the grown crystal grains emerge on the ribbon surface. Unlike unevenness formed physically or mechanically, unevenness formed by the natural growth of crystal grains due to the influence of the heat treatment temperature is thought to have stable surface properties and improve adhesion.

[0078] In the copper ribbon manufacturing method of this embodiment, it is possible to reduce the bias of the copper ribbon surface roughness in a predetermined direction. That is, the same level of roughness is achieved in both (1) the longitudinal direction and (2) the direction perpendicular to the longitudinal direction. Ideally, the closer the value obtained by dividing the average roughness in the longitudinal direction by the average roughness in the direction perpendicular to the longitudinal direction is to 1, the better. This is because, when viewed microscopically, the irregularities have a bowl-shaped shape, which increases the bonding area relative to the objects to be bonded. This not only increases the bonding strength, but also reduces the load during bonding and makes it less likely to damage the objects to be bonded.

[0079] The inventors have discovered that in order to achieve the surface shape of the copper ribbon described above, the conditions of the final heat treatment described above are important, and that heat treatment using an electric current heating method is particularly suitable.

[0080] (Semiconductor Device) Next, the configuration of a semiconductor device 100 using the copper ribbon of the embodiment will be described with reference to FIG.

[0081] As shown in FIG. 3, the semiconductor device 100 includes a semiconductor element 1, a metal film 2, a ribbon 3, a circuit pattern 41, a metal pattern 42, an insulating member 43, a heat dissipation member 5, a bonding material 6, a case 7, terminals 8, and a sealing material 9.

[0082] In this embodiment, the semiconductor element 1 is, for example, a power semiconductor used as a semiconductor for power supply. Examples of the semiconductor element 1 include a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT).

[0083] The semiconductor element 1 is formed by laminating, in this order, an electrode 11, a substrate portion 13, and a back electrode 12. The electrode 11 is, for example, an aluminum (Al)-silicon (Si) electrode, and the substrate portion 13 is, for example, a silicon (Si) substrate, a silicon carbide (SiC) substrate, a gallium nitride (GaN) substrate, or the like.

[0084] The metal film 2 is provided on the surface of the electrode 11 opposite the substrate portion 13 so as to cover the surface of the electrode 11. The metal film 2 is a nickel (Ni) film, copper (Cu) film, titanium (Ti) film, tungsten (W) film, or the like, and is a film formed by electroplating, electroless plating, vapor deposition, sputtering, or the like. The nickel (Ni) film includes a nickel (Ni) electroless plating film, and specific examples thereof include an electroless nickel (Ni)-phosphorus (P) plating film and an electroless nickel (Ni)-boron (B) plating film. Other preferred embodiments of the metal film 2 will be described later.

[0085] The ribbon 3 is made of the copper ribbon of the above-described embodiment, and has the same configuration and characteristics as described above. The ribbon 3 is bonded to the surface of the metal film 2.

[0086] Next, we will explain other configurations of semiconductor device 100. Within semiconductor device 100, a semiconductor circuit is formed by semiconductor element 1, ribbon 3, terminals 8, circuit pattern 41, and metal pattern 42. Within semiconductor device 100, ribbon 3 is bent, and at this bent portion, ribbon 3 is bonded to semiconductor element 1, terminal 8, circuit pattern 41, etc.

[0087] In the semiconductor device 100, a bonding material 6, a metal pattern 42, an insulating member 43, a circuit pattern 41, a bonding material 6, and a semiconductor element 1 are layered in this order on the surface of the heat dissipation member 5. The bonding material 6 is made of solder, silver (Ag), or the like, and bonds the heat dissipation member 5 to the metal pattern 42, and the circuit pattern 41 to the back electrode 12 of the semiconductor element 1. The insulating member 43 is an insulating substrate, or the like.

[0088] The case 7 is a ring-shaped housing having an internal space, and is provided so as to surround the outer periphery of the heat dissipation member 5. The internal space of the case 7 accommodates the semiconductor element 1, the metal film 2, the ribbon 3, the circuit pattern 41, the metal pattern 42, the insulating member 43, the bonding material 6, and the sealing material 9 described above.

[0089] The terminals 8 function as connection terminals for connecting to external devices. The terminals 8 are provided on the upper surface of the case 7, and are arranged so that one end protrudes into the internal space of the case 7 and the other end protrudes outside the case 7. The internal space of the case 7 is filled with a sealing material 9 that encloses the semiconductor element 1, metal film 2, ribbon 3, circuit pattern 41, metal pattern 42, insulating member 43, and bonding material 6. The sealing material 9 is a gel-like sealing resin, a hardened mold resin, or the like.

[0090] The semiconductor device 100 shown in FIG. 3 may have electrodes 11 on multiple semiconductor elements 1, multiple circuit patterns 41, and multiple terminals 8. The semiconductor device 100 preferably has an electrode-circuit pattern bonding structure including one electrode 11 of the multiple semiconductor elements 1, one circuit pattern 41 of the multiple circuit patterns 41, and a wire 3 connecting the electrode 11 to the circuit pattern 41. The semiconductor device 100 also preferably has an electrode-terminal connection structure including one terminal 8 of the multiple terminals 8, one electrode 11 of the multiple electrodes 11, and a wire 3 connecting the terminal 8 to the electrode 11. The semiconductor device 100 also preferably has a circuit pattern-terminal connection structure including one terminal 8 of the multiple terminals 8, one circuit pattern 41 of the multiple circuit patterns 41, and a wire 3 connecting the terminal 8 to the circuit pattern 41. The semiconductor device 100 preferably includes one or more electrode-circuit pattern bonding structures, electrode-terminal connection structures, and circuit pattern-terminal connection structures, and preferably includes two or more of each. That is, the copper wire of the embodiment can be used to connect the electrode 11 and the circuit pattern 41 , to connect the terminal 8 and the electrode 11 , or to connect the terminal 8 and the circuit pattern 41 .

[0091] The semiconductor device 100 may further have a substrate (semiconductor element upper substrate) on the semiconductor element 1, and in this case, the semiconductor device 100 preferably has a bonding structure including the semiconductor element upper substrate, a circuit pattern 41, and wires 3 connecting the semiconductor element upper substrate and the circuit pattern 41. The same applies to semiconductor devices 101 and 103 described below.

[0092] FIG. 4 shows a semiconductor device 101 having a lead frame as another embodiment of the semiconductor device. In FIG. 4, components having the same functions as those of the semiconductor device 100 shown in FIG. 3 are designated by the same reference numerals, and detailed description thereof will be omitted. The semiconductor device 101 shown in FIG. 4 includes a lead frame LF in addition to a semiconductor element 1, a metal film 2, a ribbon 3, an insulating member 43, a bonding material 6, and a sealing material 9. Because the semiconductor device 101 shown in FIG. 4 includes the lead frame LF, it does not include a case 7, but a case 7 may be included. The lead frame LF is bonded to the surface of the insulating member 43 and has the same function as the circuit pattern 41 of the semiconductor device 100 shown in FIG. 3. Although the lead frame LF and the insulating member 43 are bonded to each other in FIG. 4, a metal plate (not shown) may be disposed between the lead frame LF and the insulating member 43.

[0093] The sealing material 9 is provided so as to enclose the semiconductor element 1, the metal film 2, the ribbon 3, the insulating member 43, the bonding material 6, and the lead frame LF. However, the ends of the lead frame LF protrude outside the sealing material 9, and the lead frame LF constitutes an electrical circuit for the semiconductor element 1 and the ribbon 3, and the protruding ends function as terminals 8 for connecting the semiconductor device 101 to external equipment.

[0094] FIG. 5 schematically illustrates a semiconductor device 103 having a ribbon 3 connecting circuit patterns 41 to each other as another embodiment of a semiconductor device. The semiconductor device 103 differs from the semiconductor device 100 shown in FIG. 3 in that it has another circuit pattern 41 adjacent to the circuit pattern 41 of the semiconductor device 100 shown in FIG. 3 , and that adjacent circuit patterns 41 are connected by a ribbon 3; however, the other configurations are the same. The semiconductor device 103 shown in FIG. 5 may have multiple circuit patterns 41 and includes a bonding structure including two adjacent circuit patterns among the multiple circuit patterns 41 and a ribbon 3 connecting the two adjacent circuit patterns. The semiconductor device 103 includes one or more such bonding structures, and preferably includes two or more. That is, the copper ribbon of the embodiment can be used to connect circuit patterns 41 to each other.

[0095] Next, a method for manufacturing the semiconductor devices 100, 101, and 103 shown in FIGS. 3, 4, and 5 will be described. First, the components constituting the semiconductor devices 100, 101, and 103 are prepared, stacked, and bonded together according to the configuration described above. Then, an end of the ribbon 3 is bonded to the surface of the metal film 2 by ultrasonic bonding or the like. Then, the other end of the ribbon 3 is wedge-bonded (second bonding) to an external electrode (terminal 8 in FIGS. 3 and 4 or LF in FIG. 4). The ribbon 3 is the copper ribbon of the above-described embodiment. Then, a sealing resin is injected into the semiconductor devices 100 and 103 and cured to form the sealing material 9. In the case of the semiconductor device 101, a lead frame carrying the semiconductor element 1 and the like is placed in a mold, and a sealing resin is injected into the mold and then cured to form the sealing material 9.

[0096] In a semiconductor device incorporating the copper ribbon of the above-described embodiment, there is no chip damage at the ribbon joint and the joint strength is maintained, so the joint 31 can be stably maintained for a long period of time. Furthermore, the extended life of the cutter reduces cutter consumption costs and cutter replacement frequency, resulting in excellent production efficiency. As a result, by using the copper ribbon of the embodiment, semiconductor devices with high production efficiency and long-term reliability can be manufactured.

[0097] Examples will be described below, but the present invention is not limited to the following examples.

[0098] The copper ribbons of the examples were produced by the following method and conditions. Copper with a purity of 99.999% by mass or higher and silver as an additive element were melted together. A continuous casting furnace (heating furnace) was used for melting. To prevent contamination by oxygen and other elements from the atmosphere, the molten copper in the heating furnace was evacuated and then melted in a nitrogen atmosphere. The melted material was solidified by continuous casting from the heating furnace to a predetermined diameter to produce a wire.

[0099] The wire obtained above was drawn to a diameter of 0.9 mm, and then subjected to intermediate heat treatment at 500°C for 120 minutes. The drawing was performed using multiple carbide or diamond dies to gradually reduce the wire diameter. The area reduction rate before and after each drawing was in the range of 5 to 30%. Taking into account the target dimensions of the copper ribbon (width 1.25 mm, thickness 0.2 mm, end face curvature radius 180 μm), the final wire diameter was set to 580 μm.

[0100] The copper wire drawn to the final wire diameter was set in a rolling mill and rolled to the target copper ribbon dimensions using two-stage rolls. The cross-sectional area reduction rate when processing from wire to ribbon shape was 3% to 15%. Furthermore, the cross-sectional area reduction rate per roll when rolling from ribbon shape to ribbon shape was 10% to 60%. The copper ribbon of the example was rolled twice using two-stage roll rolling, with the first cross-sectional area reduction rate being approximately 9% (processed from a wire diameter of 700 μm (0.7 mm) to a ribbon width of 1.0 mm x ribbon thickness of 0.35 mm) and the second cross-sectional area reduction rate being 28.6% (ribbon width of 1.25 mm x ribbon thickness of 0.20 mm).

[0101] The copper ribbon processed to the final dimensions was subjected to a final heat treatment in an electric heating furnace. The conditions for the electric heating were a voltage of 18 V to 24 V, a distance between the electrode terminals of 900 mm to 1300 mm, a copper ribbon running speed of 70 m / min to 150 m / min, and the second electrode was immersed in cooling water. Thus, the copper ribbon was immersed in the cooling water in a heated state. Pure water was used as the coolant, and the electric heating was performed at a temperature of 30°C to 60°C and a dissolved oxygen concentration of 7 mg / L or less. The area around the electrodes through which the copper ribbon passed was maintained in a nitrogen atmosphere. The conditions were varied within the above ranges to obtain samples of Examples 1 to 90 shown in Tables 1, 2, and 3. After the final heat treatment, the copper ribbon was rewound onto multiple spools in approximately 100-m increments.

[0102] The copper purity and some of the contained elements of the copper ribbons of the comparative examples are shown in Table 4. The copper ribbons of the comparative examples were produced under the same conditions as those of the examples, except that some or all of the production conditions, such as the final diameter of the copper wire, the area reduction rate in the rolling process, and the final heat treatment conditions (electrical heating conditions), were outside the range of the conditions of the examples. As a result, copper ribbons of comparative examples 1 to 33 were obtained.

[0103] However, in Examples 37, 46, Comparative Examples 15, and 25, the end faces were cut (slit) after the heat treatment to obtain ribbons with straight end faces, and then the obtained ribbons were rewound onto spools.

[0104] Next, we will explain the measurement methods for each characteristic of the copper ribbon in the examples and comparative examples. (Measurement of the Correspondence Grain Boundary Length Ratio) The correspondence grain boundary length ratio (correspondence grain boundary density) of the cross section perpendicular to the longitudinal direction of the copper ribbon was measured as follows. The manufactured copper ribbon was cut into several centimeters in length to prepare multiple samples. Taking great care to avoid deformation of the sample, it was attached straight and flat to an Ag-plated metal frame (metal plate). The sample and metal plate were then placed in a cylindrical mold with the metal plate at the bottom of the cylinder. An embedding resin was poured into the mold, and a curing agent was then added to harden the resin. The cylindrical resin containing the hardened sample was then roughly polished with a polisher to expose a cross section perpendicular to the longitudinal direction of the copper ribbon. The cut surface was then polished to a final finish, followed by ion milling to remove residual strain on the polished surface and obtain a smooth surface. The ion milling device was fine-tuned so that the cut surface of the copper ribbon was perpendicular to the longitudinal direction.

[0105] In this embodiment, the coincidence grain boundary density was measured and calculated by electron backscattered diffraction (EBSD). Specifically, the ribbon sample was attached to the sample stage of a field emission scanning electron microscope (FE-SEM) equipped with an EBSD measurement device (OIM Data Collection manufactured by EDAX / TSL) so that the cross section of the ribbon sample (i.e., the polished surface of the sample) was parallel to the sample stage. The following settings were used: acceleration voltage 15 kV, tilt angle 70 degrees, measurement point interval 1.5 μm, magnification 60 times; a boundary where the orientation difference between adjacent measurement points exceeds 5° is regarded as a grain boundary; and if two or more pixels are connected with an orientation difference of 5° or less, it is recognized as one crystal grain.

[0106] The total length (μm) of the coincidence grain boundaries was divided by the total length (μm) of the crystal grain boundaries to calculate the coincidence grain boundary density (%). The Σ3 value in EBSD was used for the coincidence grain boundaries. The measurement points were selected from the periphery of the leading end (the longitudinal leading end), the periphery of the trailing end (the end opposite the longitudinal leading end), and the intermediate portion of an approximately 100 m copper ribbon. The coincidence grain boundary density was measured in cross sections perpendicular to the longitudinal direction at these three points, and the average values ​​are listed in Table 1.

[0107] (Arithmetic mean roughness (Ra) of copper ribbon plane) The average roughness in the longitudinal direction and the average roughness in the direction perpendicular to the longitudinal direction of the copper ribbon plane were measured using a surface roughness meter (small micro-profile measuring instrument, model ET200, manufactured by Kosaka Laboratory). The main measurement conditions were a cutoff of 0.25 mm, a probe movement speed of 0.05 mm / sec, and a probe load of 100 μN. As described above, the leading edge and trailing edge of an approximately 100 m copper ribbon, as well as an intermediate portion between them, were selected, and the average roughness was measured at these three locations. The average roughness in the direction perpendicular to the longitudinal direction was measured by moving the probe along a straight line from a position 10% inward relative to the copper ribbon width from the end of the copper ribbon to a position 10% inward from the opposite end of the copper ribbon. For example, for a copper ribbon with a width of 1.25 mm, the average roughness was measured by moving the probe 1.0 mm in the direction perpendicular to the longitudinal direction. Next, for the average roughness in the longitudinal direction, a measuring needle was moved along three straight lines parallel to the rolling direction, passing through three points that divide the ribbon width into four equal parts (i.e., the center of the ribbon, and points located 25% of the ribbon width from both ends of the ribbon), the same distance (1.0 mm) as that measured in the direction perpendicular to the longitudinal direction, to measure the average roughness. As with the measurement of the average roughness in the direction perpendicular to the longitudinal direction, the average values ​​of nine measured values ​​were used: three points around the leading edge of the copper ribbon, three points around the trailing edge of the copper ribbon, and three points in the intermediate portion between the leading edge and the trailing edge.

[0108] ((1) Average roughness in the longitudinal direction of the copper ribbon plane / (2) Average roughness in the direction perpendicular to the longitudinal direction) Using the (1) average roughness in the longitudinal direction and (2) average roughness in the direction perpendicular to the longitudinal direction of the copper ribbon plane measured above, the value obtained by dividing the (1) average roughness in the longitudinal direction by the (2) average roughness in the direction perpendicular to the longitudinal direction, i.e., the value of (1) average roughness in the longitudinal direction / (2) average roughness in the direction perpendicular to the longitudinal direction, was calculated and is shown in Tables 1, 2 and 3.

[0109] (Curvature Radius of Copper Ribbon End Face) The curvature radius of the copper ribbon was measured approximately at the front end, rear end, and midway point of a 100 m copper ribbon wound in the same manner as described above. Specifically, the curvature radius of both ends in the copper ribbon width direction at these three locations was measured, obtaining a total of six measured values, and the average value was calculated. The collected sample was embedded in resin and polished to expose a cross section perpendicular to the longitudinal direction. An image analyzer (WinROOF, manufactured by Mitani Shoji Co., Ltd.) was used to automatically calculate the curvature radius of the copper ribbon end face by specifying three points on the exposed cross section: the inflection point between the copper ribbon plane (rolled surface) and the end face, the outermost point on the end face, and the inflection point between the plane (rolled surface) and the end face on the opposite side (back side) of the copper ribbon plane (rolled surface).

[0110] Although the methods for measuring the properties of the copper ribbon in the examples have been described above, the length of the copper ribbon sample used to measure the properties of the copper ribbon can be adjusted as appropriate. For example, if the length of the copper ribbon is less than approximately 100 m, the properties of the copper ribbon may be measured by measuring the properties at approximately the front end, rear end, and intermediate points between the front end and rear end along the length of the copper ribbon in the longitudinal direction.

[0111] Furthermore, when performing EBSD measurements, it is preferable to measure at a magnification that allows the entire cross section of the ribbon sample to fit in the EBSD image. The copper ribbon in the examples has dimensions of 1.25 mm wide and 0.2 mm thick. Therefore, a magnification of 60x was used so that the entire cross section fits in a single image. However, when measuring copper ribbons larger than those in the examples using EBSD, it is preferable to use a magnification smaller than 60x so that the entire cross section fits in a single image. For example, if the copper ribbon has dimensions of 2.0 mm wide and 0.3 mm thick, a magnification of 45x will allow the entire cross section to fit in a single image. If the entire ribbon cross section cannot fit in a single image even after adjusting the measurement magnification, the entire cross section is measured by dividing it into multiple parts, and then the images of the divided cross sections are joined to create a single image with minimal overlapping areas, and each property is calculated using this image. If the overlapping areas of the measurement images are not large, the results obtained by the measurement can be determined to be reliable.

[0112] Next, the evaluation method for each sample in the examples and comparative examples will be described. (Cutter Life Evaluation Method) Using an ORTHODYNE 3600Plus bonder, continuous ribbon bonding was performed on a copper plate measuring 50 mm long x 50 mm wide x 1 mm thick, with a load of 2300 gf, a power (ultrasonic output setting) of 220, and a hold time of 150 milliseconds. For each sample, bonding was performed with a combination of the first and second bondings, n = 1 (1 set). In this case, if the bonder stopped after 5,000 or fewer consecutive bondings due to a cutting error such as a copper ribbon not being cut, it was evaluated as "F," meaning failure. If the bonder stopped after more than 5,000 or fewer consecutive bondings but less than 7,000, it was evaluated as "B," meaning no practical problem. If the bonder stopped after more than 7,000 or fewer consecutive bondings but less than 10,000, it was evaluated as "A," meaning good. When the bonder stopped after bonding more than 10,000 consecutive pairs, it was evaluated as excellent, with an "S" rating. These results are shown in Tables 1, 2, 3 and 4.

[0113] (Bonding Strength Evaluation Method) Using an ORTHODYNE 3600Plus bonder, a semiconductor package having a semiconductor element (chip electrode) and a lead frame was bonded with a holding load of 2300 gf, a power (ultrasonic output setting) of 220, and a holding time of 150 milliseconds. Ten pairs of bonding were performed, each pair consisting of a semiconductor element (first bond) and a lead frame (second bond). The center of the bonding loop was then cut, and a peel test was performed on 20 samples (n = 20) at the first and second bonded portions using a STELLAR 4000 from Nordson Advanced Technologies Co., Ltd. The peel test was performed using a 5 kg load cell at a tensile speed of 300 μm / s. If none of the 20 samples had a copper ribbon peeled from the bonded portion (zero peels), the sample was rated "S," indicating excellent results. When the number of copper ribbons that did not peel from the joint was 15 to 19, that is, when the number of copper ribbons that peeled from the joint was 1 to 5, it was rated as "A," meaning that it was good. When the number of copper ribbons that did not peel from the joint was 10 to 14, that is, when the number of copper ribbons that peeled from the joint was 6 to 10, it was rated as "B," because there was room for improvement but it was unlikely to cause a practical problem. Furthermore, when the number of copper ribbons that peeled from the joint was 11 or more, it was likely to cause a practical problem, so it was rated as "F," meaning that it failed. The evaluation results are shown in Tables 1, 2, 3, and 4.

[0114] (Chip Damage Evaluation Method) The chip damage evaluation was carried out under the same conditions as in the bonding evaluation method described above. 20 pairs of semiconductor elements (first bonding) and lead frames (second bonding) were bonded, and then the metal film on the semiconductor elements was dissolved with a chemical. The exposed chip electrodes were observed under a metallurgical microscope. If there were no cracks or scratches, the result was considered very good and was marked with an "S." If there were no cracks but shallow scratches (traces), the result was considered good and was marked with an "A." If there were scratches but no cracks, the result was considered acceptable and was marked with a "B." If there was even one crack, the result was considered poor and was marked with an "F." These results are shown in Tables 1, 2, 3, and 4.

[0115] (Overall Evaluation) The overall evaluation was determined as follows based on the five evaluations in Tables 1, 2, 3, and 4 above, and is shown in Tables 1, 2, 3, and 4. When there was at least one "S" among the five evaluations and the others were "S" or "A," the overall evaluation was marked with the symbol "E" (Excellent), meaning excellent. When there were three or more "B"s and no "F"s, the overall evaluation was marked with the symbol "P" (Passable), meaning a passing grade. When there was a combination of "A," "S," and "B" other than those mentioned above and no "F," the overall evaluation was marked with the symbol "G" (Good), meaning good. When a sample had even one "F" evaluation, the overall evaluation was marked with the symbol "F" (Fail), meaning failure. The specific combinations of each evaluation (regardless of order) are as follows: "E": SSSSS, SSSSA, SSSAA, SSAAA, SAAAA "G": SSSSB, SSSAB, SSSBB, SSAAB, SSABB, SAAAB, SAABB, AAAA, AAAAB, AAABB "P": SSBBB, SABBB, SBBBB, AABBB, ABBBB, BBBBB "F": When there is at least one F.

[0116]

[0117]

[0118]

[0119]

[0120] The copper purity and the concentrations (content ratios) of some of the contained elements in the examples and comparative examples are shown in Tables 1, 2, 3, and 4. Each concentration was calculated using inductively coupled plasma-mass spectrometry (ICP-MS) (8800ICP-MS manufactured by Agilent Technologies Inc.) owned by the applicant, Tanaka Electronics Co., Ltd.

[0121] (Evaluation of oxidation resistance) Next, the oxidation resistance of the samples of the examples and comparative examples was evaluated as follows. Each sample of the examples and comparative examples was left at room temperature of 35°C and humidity of 75% RH for 6 months. After that, the oxidation state of the copper ribbon surface was analyzed. In the process of copper oxidation, copper (I) oxide (Cu ２ O) is generated, and as oxidation progresses, the proportion of CuO increases. Therefore, by calculating the proportion of the CuO oxide film, the ease with which oxidation is generated (oxidation resistance) of the copper ribbon can be evaluated. The proportion of the CuO oxide film is calculated as follows: (CuO oxide film) / (CuO oxide film + Cu ２ The film thickness was calculated by converting the film thickness (O oxide film) into a percentage.

[0122] Fig. 6 is a graph showing the change over time in the proportion of CuO oxide film after storage tests in Example 6, Example 13, and Comparative Example 14. In Fig. 6, the open circles represent the results of Example 6, the closed circles represent the results of Example 13, and the open squares represent the results of Comparative Example 14. As shown in Fig. 6, it can be seen that the copper ribbons of the Examples containing 5 ppm by mass or more of silver have a lower rate of increase over time in the proportion of CuO oxide film than the copper ribbons of the Comparative Examples containing less than 5 ppm by mass of silver.

[0123] The oxide film measurement results were calculated as thickness values ​​calculated by sequential electrochemical reduction analysis (SERA). ２ The SERA analysis to determine the thickness of the copper ribbon was carried out using a QC-200 manufactured by ECI Technology, Inc., as follows: A copper ribbon was sandwiched between two O-rings, and a region of 2.1 cm in diameter was isolated with a gasket. A borate buffer solution was injected into the region and saturated with nitrogen. A current of 150 μA / cm was applied to the region. ２ A current density (I) of 1000 kJ / s is applied, and Cu appears between -0.30 V and -0.60 V. ２ The time (seconds) (t) required for the O reduction reaction and the CuO reduction reaction occurring between -0.60 V and -0.85 V was measured. ２ The thickness T (nm) of each of the CuO layers was calculated based on T = K I t using the constant K obtained from Faraday's law. The value of the constant K for CuO is 6.53 × 10 －５ (cm３ / A·sec), and Cu ２ The value of K for O is 2.45 x 10 －４ (cm ３ / A·sec).

[0124] The measurement points were the peripheral portion of the leading end and the peripheral portion of the trailing end of the copper ribbon, and the intermediate portion, i.e., three locations on the surface, and the average value was calculated. Samples with a CuO oxide film ratio of 20% or less were rated "S," meaning excellent. Samples with a CuO oxide film ratio of more than 20% but less than 25% were rated "A," meaning good. Samples with a CuO oxide film ratio of more than 25% but less than 30% were rated "B," meaning above the minimum practical level. Samples with a ratio of more than 30% were rated "F," meaning there was a high possibility of practical defects.

[0125] Here, we will explain the reason why a CuO oxide film ratio of 30% or less was judged to be acceptable. In a power module, an IGBT, MOSFET, or diode chip is mounted on a substrate in which a ceramic plate (silicon nitride, alumina, etc.) and a copper plate are bonded together using active metal brazing (AMB) or direct copper bonding (DCB), and a circuit is formed on the substrate using ribbon bonding.

[0126] For example, in a copper ribbon bonded to a copper plate of a DCB substrate, a temperature cycle test (e.g., from -65°C to 150°C) is performed on the finished product sealed with silicone resin. The thermal expansion and contraction of the silicone resin exerts stress on the ribbon bond, causing the ribbon to peel. The temperature conditions for this temperature cycle test are determined by global automotive standards. The inventors believed that the growth of an oxide film on the copper ribbon surface due to heat is one of the main causes of this peeling. They then found that if the proportion of CuO film on the copper ribbon surface is 30% or less after the copper ribbon has been left for six months at room temperature of 35°C and humidity of 75%, peeling at the ribbon bond during the temperature cycle test can be significantly reduced.

[0127] Furthermore, in recent years, the guaranteed operating temperature range for power semiconductors has widened, and there are cases where a guarantee of operation is required within a range of, for example, -65°C to 175°C. In these temperature cycle tests, an even higher load is applied to the copper ribbon compared to the test conditions described above, making the ribbon more likely to peel from the joint. The inventors discovered that if the ratio of the CuO oxide film on the ribbon surface can be reduced to 30% or less in the storage test described above, such high requirements can be met, ribbon peeling from the joint does not occur, and stable bonding strength can be obtained even under harsher conditions.

[0128] In addition to the problem of ribbon peeling due to the generation of a copper oxide film, the inventors have discovered that the expansion of the guaranteed temperature range may also cause the problem of a decrease in electrical conductivity at high temperatures. That is, electrical conductivity tends to decrease as the temperature increases. As mentioned above, the copper ribbon of the embodiment has a larger volume than copper wire, and therefore has the advantage of being able to pass a large current at a high voltage. However, if the electrical resistance increases and the electrical conductivity decreases due to an increase in temperature, the current value may decrease and the driving force may decrease.

[0129] Therefore, in the manufacture of power semiconductor devices, it is necessary to quantitatively predict the increase in electrical resistance of copper ribbons at higher temperatures. If the electrical resistance increases beyond this prediction, not only will the designed current value not be achieved, but the heat generated by the ribbon due to the increase in electrical resistance may also have a negative impact on surrounding components. For example, there is a risk of serious problems such as the encapsulation resin of the surrounding components melting due to the heat generated by the ribbon.

[0130] The inventors have repeatedly conducted extensive research and experiments into the rate of increase in electrical resistance of copper ribbons and the heat generated as a result of the expansion of the guaranteed temperature range, particularly in high temperature ranges. As a result, they discovered that if the electrical conductivity (IACS) at room temperature is 99% or higher (preferably 99.5% or higher), the increase in electrical resistance will be within the range (within about 1.7 times) expected due to a normal temperature increase in the manufacture of power semiconductor devices.

[0131] In addition, in order to suppress the increase in electrical resistance and keep the proportion of CuO oxide film on the copper ribbon surface at 30% or less, in addition to controlling the corresponding grain boundary density as described above, the inventors have repeatedly conducted experiments in which trace amounts of various elements are added to high-purity copper to evaluate the electrical resistance and the formation of CuO oxide film. As a result, it has been found that these two problems can be solved simultaneously by containing 5 mass ppm to 40 mass ppm of silver (Ag), 0 mass ppm to 10 mass ppm of phosphorus (P), 0 mass ppm to 8 mass ppm of iron (Fe), silicon (Si), arsenic (As), and antimony (Sb) each relative to the entire copper ribbon, and the total content of these elements other than silver (Ag) being 0 mass ppm to 30 mass ppm.

[0132] The electrical conductivity was measured at the same area using the IACS (International Annealed Copper Standard) and the average value was calculated. The IACS measured the electrical resistivity of each copper ribbon using a four-terminal method with a potential lead wire distance of 100 mm and a direct current of 200 mA. The electrical resistivity of annealed standard soft copper at room temperature (20°C) was 1.7241 × 10 －２ The electrical resistivity value of each copper ribbon is expressed as a ratio when the IACS in μΩm is set to 100%. Samples with an IACS decrease rate of less than 0.5% compared to the IACS of 99.999 mass% pure copper were rated "S", meaning excellent. Samples with an IACS decrease rate of 0.5% or more but less than 0.8% were rated "A", meaning good. In addition, when the IACS decrease rate is 0.8% or more but less than 1.0%, it is rated "B", meaning that it is at or above the minimum practical level, and when it is 1.0% or more, it is rated "F", meaning that there is a high possibility of practical problems occurring.

[0133] Here, the results of the examples and comparative examples will be considered. First, the effect of the coincidence grain boundary density will be considered. The copper ribbon of Example 25 (Table 1) has a coincidence grain boundary density of 59.7%, and the copper ribbon of Comparative Example 29 (Table 3) has a coincidence grain boundary density of 59.6%, and both received a cutter life rating of "B" (no practical problem). In contrast, the copper ribbon of Comparative Example 8 has a coincidence grain boundary density of 61.3%, and the copper ribbon of Comparative Example 13 has a coincidence grain boundary density of 63.4%, and both received a cutter life rating of "F" (fail: bonder stopped due to cutting error). From these results, it can be seen that a coincidence grain boundary density of 60% is the critical value, and that a value less than 60% is significantly superior in extending the cutter life.

[0134] In addition, a storage test was conducted on samples with particularly low CBC densities. As a result, the evaluation of the CuO oxide film on the sample surface after the storage test of copper ribbons with CBC densities of 30.1% (Example 14) and 30.3% (Example 68) was "B" (above the minimum practical level), and the evaluation of the CuO oxide film on copper ribbons with CBC densities of 28.1% (Comparative Example 4) and 27.5% (Comparative Example 10) was "F" (fail). From this, it can be seen that the oxidation resistance of copper ribbons is significantly superior to expectations when the CBC density is 30% or more, with the CBC density being the boundary at 30%.

[0135] Next, we will consider the influence of the radius of curvature of the end face. The copper ribbons of Examples 37, 46, Comparative Examples 15, and 25, which were slit so that the end faces were linear, were expected to have excellent cutter life because their CGB densities were low at around 30%, but the cutter life evaluation was a relatively low "B" (no practical problems). In contrast, the copper ribbons of Example 3 (CGB density 45.6%) and Example 33 (CGB density 46.7%), which have CGB densities in the 40% range, were expected to have a longer cutter life due to the CGB density compared to copper ribbons with CGB densities in the 30% range. However, because the end faces were arc-shaped, the cutter life was evaluated as "S" (excellent).

[0136] In Example 66, the corresponding grain boundary density was 36.0%, and the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.88, resulting in an evaluation of cutter life of "A" (good). In contrast, in Example 67, which has the same corresponding grain boundary density (35.0%), the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.91, resulting in an evaluation of cutter life of "B" (no practical problem). From these results, it can be seen that the evaluation of cutter life is even better when the value obtained by dividing the radius of curvature by the thickness of the copper ribbon is 0.90 or less, with the boundary being 0.90.

[0137] For example, in Example 4, the corresponding grain boundary density was 48.8%, and the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.51, resulting in an evaluation of cutter life of "A" (good). Similarly, in Example 49, the corresponding grain boundary density was 51.0%, and the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.52, resulting in an evaluation of cutter life of "A". In contrast, in Example 16, the corresponding grain boundary density was 49.5%, and the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.48, resulting in an evaluation of cutter life of "B" (no practical problem). Similarly, in Example 48, the corresponding grain boundary density was 50.5%, and the value obtained by dividing the radius of curvature by the thickness of the copper ribbon was 0.47, resulting in an evaluation of cutter life of "B". Incidentally, the cutter life evaluation "B" here was given because, although it was not a practical problem, slight warping of the ribbon was confirmed. These results show that the value obtained by dividing the radius of curvature by the thickness of the copper ribbon is 0.50, and that when the value is 0.50 or more, the cutter life evaluation is even better.

[0138] Next, the average roughness will be considered. For example, Example 87 had an average roughness of 32.3 nm in the longitudinal direction and 40.3 nm in the direction perpendicular to the longitudinal direction, and the bondability was evaluated as "B" (unlikely to cause practical problems). Similarly, Example 90 had an average roughness of 34.2 nm in the longitudinal direction and 31.1 nm in the direction perpendicular to the longitudinal direction, and the bondability was evaluated as "B". In contrast, Comparative Example 1 had an average roughness of 28.7 nm in the longitudinal direction and 33.4 nm in the direction perpendicular to the longitudinal direction, and the bondability was evaluated as "F" (fail). Comparative Example 3 had an average roughness of 31.2 nm in the longitudinal direction and 29.0 nm in the direction perpendicular to the longitudinal direction, and the bondability was evaluated as "F". From the above, it can be seen that the average roughness in the longitudinal direction and the average roughness in the direction perpendicular to the longitudinal direction have a critical value of 30.0 nm, and that when the average roughness is 30.0 nm or more, the bondability is remarkably excellent.

[0139] Furthermore, Example 26 had an average roughness in the longitudinal direction of 147.5 nm and an average roughness in the direction perpendicular to the longitudinal direction of 148.1 nm, and the bondability was evaluated as "B" (unlikely to cause practical problems). Example 10 had an average roughness in the longitudinal direction of 121.7 nm and an average roughness in the direction perpendicular to the longitudinal direction of 149.5 nm, and the bondability was evaluated as "B". In contrast, Comparative Example 5 had an average roughness in the longitudinal direction of 153.8 nm and an average roughness in the direction perpendicular to the longitudinal direction of 149.2 nm, and the bondability was evaluated as "F" (fail). Comparative Example 9 had an average roughness in the longitudinal direction of 140.0 nm and an average roughness in the direction perpendicular to the longitudinal direction of 155.0 nm, and the bondability was evaluated as "F". From the above, it can be seen that the average roughness in the longitudinal direction and the average roughness in the direction perpendicular to the longitudinal direction have a critical value of 150 nm, and that when the average roughness is 150 nm or less, the bondability is remarkably excellent.

[0140] Next, oxidation resistance and electrical conductivity will be examined. For example, the silver content of Example 2 was 5 ppm by mass, the CuO oxide film ratio was evaluated as "B" (above the minimum practical level), and the IACS (electrical conductivity) decrease rate was evaluated as "S" (excellent). In contrast, the silver content of Comparative Example 27 was 4 ppm by mass, the IACS decrease rate was evaluated as "S", but the CuO oxide film evaluation was "F" (high possibility of practical defects). Furthermore, the silver content of Example 51 was 39 ppm by mass, the CuO oxide film ratio was evaluated as "S" (excellent), and the IACS decrease rate was evaluated as "A" (good). In contrast, the silver content of Comparative Example 25 was 42 ppm by mass, the CuO oxide film ratio was evaluated as "A", but the IACS decrease rate was evaluated as "F". From the above, it can be seen that the silver content is preferably 5 ppm by mass or more and 40 ppm by mass or less.

[0141] Furthermore, the phosphorus content of Example 36 was 0 ppm by mass, the CuO oxide film ratio was evaluated as "B", and the IACS reduction rate was evaluated as "S". The phosphorus content of Example 39 was 10 ppm by mass, but the CuO oxide film ratio was evaluated as "B" and the IACS reduction rate was also evaluated as "B". In contrast, the phosphorus content of Comparative Example 23 was 12 ppm by mass, and the CuO oxide film was evaluated as "B", but the IACS reduction rate was evaluated as "F". This shows that the phosphorus content is preferably 0 ppm by mass or more and 10 ppm by mass or less.

[0142] Furthermore, the iron content of Example 4 was 0 ppm by mass, the CuO oxide film ratio was evaluated as "B", and the IACS reduction rate was evaluated as "S". The iron content of Example 9 was 8 ppm by mass, but the CuO oxide film ratio was evaluated as "B" and the IACS reduction rate was also evaluated as "B". On the other hand, the iron content of Comparative Example 17 was 10 ppm by mass, and the CuO oxide film evaluation was "A", but the IACS reduction rate was evaluated as "F". This shows that the iron content is preferably 0 ppm by mass or more and 8 ppm by mass or less.

[0143] Furthermore, the silicon content of Example 5 was 0 ppm by mass, the CuO oxide film ratio was evaluated as "B", and the IACS reduction rate was evaluated as "S". The silicon content of Example 11 was 8 ppm by mass, but the CuO oxide film ratio was evaluated as "B", and the IACS reduction rate was evaluated as "A". On the other hand, the silicon content of Comparative Example 21 was 10 ppm by mass, and the CuO oxide film evaluation was "B", but the IACS reduction rate was evaluated as "F". This shows that the silicon content is preferably 0 ppm by mass or more and 8 ppm by mass or less.

[0144] The arsenic content of Example 8 was 0 ppm by mass, the CuO oxide film ratio was evaluated as "A", and the IACS reduction rate was evaluated as "A". The arsenic content of Example 22 was 8 ppm by mass, but the CuO oxide film ratio was evaluated as "B" and the IACS reduction rate was also evaluated as "A". On the other hand, the arsenic content of Comparative Example 18 was 10 ppm by mass, and the CuO oxide film was evaluated as "B", but the IACS reduction rate was evaluated as "F". This shows that the arsenic content is preferably 0 ppm by mass or more and 8 ppm by mass or less.

[0145] Furthermore, the antimony content of Example 14 was 0 ppm by mass, the CuO oxide film ratio was evaluated as "B", and the IACS reduction rate was evaluated as "S". The antimony content of Example 57 was 8 ppm by mass, but the CuO oxide film ratio was evaluated as "A" and the IACS reduction rate was evaluated as "B". On the other hand, the antimony content of Comparative Example 7 was 10 ppm by mass, and the CuO oxide film was evaluated as "B", but the IACS reduction rate was evaluated as "F". This shows that the antimony content is preferably 0 ppm by mass or more and 8 ppm by mass or less.

[0146] In Example 15, the total content of phosphorus, iron, silicon, arsenic, and antimony was 0 ppm by mass, the CuO oxide film ratio was evaluated as "A," and the IACS reduction rate was evaluated as "S." In Example 38, the total content of phosphorus, iron, silicon, arsenic, and antimony was 30 ppm by mass, but the CuO oxide film ratio was evaluated as "A" and the IACS reduction rate was evaluated as "B." On the other hand, in Comparative Example 29, the total content of phosphorus, iron, silicon, arsenic, and antimony was 31 ppm by mass, the CuO oxide film was evaluated as "F," and the IACS reduction rate was also evaluated as "F." This shows that the total content of phosphorus, iron, silicon, arsenic, and antimony is preferably 0 ppm by mass or more and 30 ppm by mass or less.

[0147] Next, we will explain the application of the copper ribbon of the embodiment to the connection between the busbars and the electrodes of the lithium-ion batteries in the lithium-ion battery module of an electric vehicle. A lithium-ion battery is a unit made up of several dozen columnar lithium-ion batteries, and the gaps between adjacent lithium-ion batteries are filled with a gel-like substance to prevent contact between adjacent batteries and to absorb vibrations while the vehicle is in operation.

[0148] FIG. 7 is a schematic diagram showing a portion of a lithium-ion battery module. The lithium-ion battery module shown in FIG. 7 includes a lithium-ion battery 51, a cathode bus bar 52, an anode bus bar 53, and a copper ribbon 50 that electrically connects the lithium-ion battery 51 and the cathode bus bar 52. The lithium-ion battery module shown in FIG. 7 also includes another copper ribbon 50 that electrically connects the lithium-ion battery 51 and the anode bus bar 53. The copper ribbon 50 is the same as that of the above-described embodiment. The lithium-ion battery module shown in FIG. 7 has an anode and a cathode disposed on the top and bottom surfaces, respectively, of a cylindrical lithium-ion battery, and is typically mounted on a vehicle with the anode facing up and the cathode facing down.

[0149] FIG. 8 is a schematic diagram illustrating a portion of another type of lithium-ion battery module. The lithium-ion battery module shown in FIG. 8 includes a lithium-ion battery 51, a cathode bus bar 52, an anode bus bar 53, and a copper ribbon 50 that electrically connects the lithium-ion battery 51 to the cathode bus bar 52. The lithium-ion battery module shown in FIG. 8 also includes another copper ribbon 50 that electrically connects the lithium-ion battery 51 to the anode bus bar 53. The copper ribbon 50 can be the same as that of the above-described embodiment. The lithium-ion battery module shown in FIG. 8 has both an anode and a cathode disposed at the center and outer edge of the top surface of a cylindrical lithium-ion battery, respectively, and is typically mounted on a vehicle with the anode facing up.

[0150] Fig. 9 is a diagram showing a schematic diagram of the entire lithium-ion battery module. As shown in Fig. 9, the lithium-ion battery module contains a plurality of lithium-ion batteries 51, and is configured by filling the spaces between the plurality of lithium-ion batteries 51 with a gel-like material 54. Copper ribbons 50 are connected to the lithium-ion batteries 51.

[0151] In this lithium-ion battery module, the use of the copper ribbon with good bondability of the embodiment allows for maintaining good bond strength in the bonding between the busbar and the copper ribbon and the bonding between the electrodes of the lithium-ion battery and the copper ribbon (ribbon bonding). The reasons for this are, for example, as follows: Because ultrasonic waves are used in ribbon bonding and the lithium-ion battery is not fixed in place with a gel-like substance, the lithium-ion battery itself vibrates due to ultrasonic waves during ribbon bonding. Therefore, if the bonding strength is weak, the copper ribbon will peel off from the bonded portion. Furthermore, by using the copper ribbon with high bondability of the embodiment, peeling and breakage from the bonded portion can be avoided not only during bonding but also in the face of vibrations while the vehicle is running.

[0152] Furthermore, because automobiles are used in harsh environments, from deserts to tropical rainforests and cold regions, the bonding ribbons installed in automobiles are also susceptible to corrosion (oxidation). The copper ribbon of this embodiment has the function of suppressing the generation of CuO, and from the viewpoints of oxidation resistance and corrosion resistance, it is suitable for joining the bus bars of lithium-ion battery modules used in electric vehicles to the electrodes of lithium-ion batteries.

[0153] Furthermore, as shown in Figure 9, a lithium battery module is an assembly of many lithium-ion batteries, so the number of cuts to be made to the copper ribbon is also large. Therefore, the copper ribbon of this embodiment also has the effect of extending the cutter life, making it very suitable for lithium battery modules.

[0154] Furthermore, copper ribbons with low electrical resistance (high electrical conductivity) such as those in this embodiment are less likely to generate heat, and therefore can improve the safety of lithium-ion batteries, which are at risk of explosion due to high heat (e.g., 80°C or higher).

[0155] (Evaluation of Bondability Between Battery Electrodes and Bus Bars) Using the copper ribbons of Examples 1 to 90 and Comparative Examples 1 to 33, the bonding mode between a lithium-ion battery electrode and a bus bar was simulated. Two copper plates were used, and a first bonding was performed on the first plate, followed by a second bonding on the second plate, which was positioned approximately 20 mm lower than the first plate. Except for the difference in the positional relationship between the first and second bonding, the bondability and cutter life were evaluated under the same conditions as in the above-described Examples using the same ORTHODYNE 3600Plus bonder. As a result, the evaluation results were the same as those in Tables 1, 2, 3, and 4.

[0156] From the above, the copper ribbon of the embodiment can simultaneously solve the problems of good bonding, suppression of chip cracking, extending the life of the bonder cutter, suppression of CuO oxide film generation, and suppression of the rate of decrease in IACS by controlling the trace elements and their concentrations, average surface roughness, aspect ratio of the average surface roughness, and corresponding grain boundary density within predetermined ranges when bonding inside power semiconductor devices, etc.

[0157] Furthermore, the copper ribbon of the embodiment has excellent bonding properties, a low rate of CuO oxide film formation, high bonding strength, high electrical conductivity, and is resistant to heat generation, making it highly suitable not only for power semiconductor applications, but also for bonding bus bars and electrodes of lithium-ion batteries in lithium-ion battery modules for electric vehicles.

[0158] The copper ribbon of this embodiment can greatly contribute to the development of the automotive industry, power electronics industry, electric railways, power industry, etc., and ultimately contribute greatly to reducing greenhouse gas emissions and preventing global warming.

[0159] 100, 101... semiconductor device, 1... semiconductor element, 2... metal film, 3... ribbon, 41... circuit pattern, 42... metal pattern, 43... insulating member, 5... heat dissipation member, 6... bonding material, 7... case, 8... terminal, 9... sealing material, LF... lead frame, 50... copper ribbon, 51... lithium ion battery, 52... cathode side bus bar, 53... anode side bus bar, 54... gel-like substance 54

Claims

1. A copper bonding ribbon made of a copper alloy with a copper purity of 99.99% by mass or more, wherein the copper alloy contains, relative to the total amount, 5 ppm by mass or more and 40 ppm by mass or less of silver (Ag), 0 ppm by mass or more and 10 ppm by mass or less of phosphorus (P), 0 ppm by mass or more and 8 ppm by mass or less of iron (Fe), silicon (Si), arsenic (As) and antimony (Sb), respectively, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less, and in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, (1) the arithmetic average roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) A copper bonding ribbon characterized in that the arithmetic mean roughness (Ra) in a direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and the value obtained by dividing (1) the arithmetic mean roughness (Ra) in the longitudinal direction by (2) the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less.

2. The copper bonding ribbon of claim 1, wherein the end faces of said copper bonding ribbon have a curvature.

3. A copper bonding ribbon as described in claim 1 or claim 2, wherein the value obtained by dividing the radius of curvature of the end face of the copper bonding ribbon by the thickness of the copper bonding ribbon is 0.50 or more and 0.90 or less.

4. A copper bonding ribbon as described in claim 1 or claim 2, wherein the width of the copper bonding ribbon is 0.3 mm or more and 5.0 mm or less, and the thickness is 0.05 mm or more and 0.5 mm or less.

5. A method for producing a copper bonding ribbon, comprising the steps of: preparing a copper wire made of a copper alloy having a copper purity of 99.99% by mass or more, containing 5 ppm by mass or more and 40 ppm by mass or less of silver (Ag), 0 ppm by mass or more and 10 ppm by mass or less of phosphorus (P), 0 ppm by mass or more and 8 ppm by mass or less of iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) relative to the total amount of the copper alloy; and a total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) of 0 ppm by mass or more and 30 ppm by mass or less; drawing the copper wire to obtain a copper wire; rolling the copper wire to form a copper ribbon; and performing a final heat treatment on the copper ribbon by electrical current heating; A manufacturing method in which, in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, the ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, (1) the arithmetic mean roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) the arithmetic mean roughness (Ra) in a direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and a value obtained by dividing the arithmetic mean roughness (Ra) in the longitudinal direction by the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less.

6. A power semiconductor device comprising at least one substrate, at least one semiconductor element on the substrate, an electrode on a surface of the semiconductor element, a substrate on the semiconductor element, at least one circuit pattern on the substrate, and a copper bonding ribbon connecting one or more of the electrodes on the surface of the semiconductor element, the electrodes on the surface of the semiconductor element and an external electrode, the electrodes on the surface of the semiconductor element and one of the circuit patterns, the electrodes on the surface of the semiconductor element and a terminal, two adjacent circuit patterns among the circuit patterns, one of the circuit patterns and a terminal, the substrate and a substrate, and the substrate on the semiconductor element and the circuit pattern, wherein the copper bonding ribbon is made of a copper alloy having a copper purity of 99.99% by mass or more, The copper alloy contains, relative to the total amount, silver (Ag) in an amount of 5 ppm by mass or more and 40 ppm by mass or less, phosphorus (P) in an amount of 0 ppm by mass or more and 10 ppm by mass or less, and iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) in an amount of 0 ppm by mass or more and 8 ppm by mass or less, respectively, and the total content of phosphorus (P), iron (Fe), silicon (Si), arsenic (As) and antimony (Sb) is 0 ppm by mass or more and 30 ppm by mass or less, and in a cross section perpendicular to the longitudinal direction of the copper bonding ribbon, a ratio of the corresponding grain boundary length to the total length of the crystal grain boundaries is 30% or more and less than 60%, (1) the arithmetic average roughness (Ra) in the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, (2) The arithmetic mean roughness (Ra) in a direction perpendicular to the longitudinal direction of the copper bonding ribbon plane is 30 nm or more and 150 nm or less, and the value obtained by dividing the arithmetic mean roughness (Ra) in the longitudinal direction by the arithmetic mean roughness (Ra) in the direction perpendicular to the longitudinal direction is 0.75 or more and 1.15 or less.

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