JP2012084788A - STRAIGHT ANGLE SILVER (Ag) CLAD COPPER RIBBON FOR HIGH-TEMPERATURE SEMICONDUCTOR ELEMENT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silver clad copper ribbon for a high-temperature semiconductor element capable of improving the bonding reliability with a pad electrode interface and suppressing peel-off of a clad layer at a clad interface.SOLUTION: In a silver clad copper ribbon composed of a high-purity silver clad layer and a high-purity copper core material tape, a heat treatment is performed at a temperature of 450-750°C after clad processing to resolve a processing structure of the clad layer. Thereby, development of the processing structure composed of a high-distortion fine crystal structure in the clad layer, which is formed at a pad interface at bonding, is suppressed to reduce a hardness difference with the other regions of the clad layer and prevent clack generation. A fine granular crystal structure is formed under a high-temperature use environment to suppress dispersion of Al and the like from a pad layer and suppress generation of an intermetallic compound at the pad interface and clad interface. As a result, the bonding strength and the reliability can be improved.

Description

    The present invention relates to a rectangular silver clad copper ribbon, particularly a high-temperature power semiconductor element, for joining a semiconductor element and a substrate-side lead frame part in an electronic component and a semiconductor package by ultrasonic bonding at a plurality of locations and connecting them in a loop shape. The present invention relates to a flat silver clad copper ribbon for connecting the substrate and the lead frame portion on the substrate side.

As bonding pads mounted on semiconductor elements such as silicon (Si), silicon carbide (SiC), diamond (C), gallium nitride (GaN), and sapphire, aluminum (Al) or nickel (Ni) has been mainly used so far. , Pads made of copper (Cu), palladium (Pd), gold (Au), and alloys thereof are used.
In addition, the substrate side lead frame has an aluminum (Al) alloy, iron (Fe) alloy, copper (Cu) alloy, noble metal plating such as gold (Au) or silver (Ag), copper (Cu), nickel (Ni) Lead frames made of copper (Cu) alloy or iron (Fe) alloy plated with aluminum or vacuum deposition of aluminum (Al), or ceramic lead frames equipped with leads made of these metals or alloys are mainly used. ing. To connect the pad electrode and the lead frame by ultrasonic bonding, a flat aluminum ribbon has been mainly used so far. The ultrasonic bonding method for a flat rectangular ribbon is a method in which a cemented carbide tool is pressed onto an aluminum ribbon and bonded by the pressing load and the energy of ultrasonic vibration. The effect of applying ultrasonic waves is to increase the bonding area to promote the deformation of the aluminum ribbon, and to destroy and remove the oxide film of about 1 nanometer (nm) naturally formed on the aluminum ribbon. A new surface in which metal atoms such as Al) are exposed on the lower surface and plastic flow is generated at the pad interface between the opposing bonding pad electrode such as aluminum (Al) or nickel (Ni) and the aluminum ribbon, and the pad interface is in close contact with each other. In this case, both the pad electrode and the aluminum ribbon are bonded interatomically while gradually increasing.

As described above, the material of the pad electrode of the semiconductor element and the lead frame on the substrate side connected via the flat rectangular aluminum ribbon are different. For this reason, even by ultrasonic bonding that does not involve a metallurgical melting process, these different types of intermetallic compounds are generated at these bonding interfaces, so that it has not always been possible to achieve strong and reliable bonding.
As a solution to these problems, a clad ribbon in which a copper (Cu) alloy such as Kovar is deposited on one side of aluminum (Al) has been proposed (Japanese Patent Publication No. 60-22827, Patent Document 1 described later). This is because the electrode on one semiconductor element to be bonded is an aluminum pad and the other is a dissimilar metal such as a lead frame, so the bonding wire is bonded to a metal with ultrasonic bonding characteristics suitable for these dissimilar metals As a clad laminate, the corresponding metal surface of the wire is to be bonded to the aluminum pad and the Kovar lead.
However, this clad ribbon is ultrasonically bonded to the pad electrode with the aluminum (Al) layer on the aluminum clad side (first bond) and then ultrasonically bonded to the lead frame side (second bond) by using Kovar or the like. In order to match the Kovar side of the clad ribbon with the lead tip made of a hard metal material, it was necessary to twist and rotate the clad ribbon after the first bond, or to reverse the direction and make the second bond .
In addition, since the copper (Cu) alloy is hard and aluminum (Al) is soft as the material constituting the clad ribbon, if the clad ribbon is bent or twisted during loop formation, the copper (Cu) alloy and aluminum (Al) are hardened. Due to the difference, peeling occurred from the clad interface of the clad ribbon. Heat treatment was also considered to strengthen this clad interface, but a brittle intermetallic compound layer of copper (Cu) and aluminum (Al) was formed at the interface between copper (Cu) and aluminum (Al). As a result, the adhesion strength of the clad interface is lowered, and as a result, the clad interface tends to peel off.

In order to prevent the formation of intermetallic compounds due to the mutual diffusion of copper (Cu) and aluminum (Al) at the coating layer interface and bonding interface, between the copper (Cu) alloy and aluminum (Al) As an intermediate layer, a diffusion prevention layer such as nickel (Ni) or titanium (Ti), tungsten (W), chromium (Cr) is also provided.
However, the formation of these intermediate layers has the effect of suppressing peeling of the clad interface at the time of loop formation, but at the junction between the pad electrode to which the ultrasonic wave is applied and the flat clad ribbon, a copper (Cu) alloy Each layer, nickel (Ni) intermediate layer, and aluminum (Al) layer has its own plastic flow, so it is difficult to suppress the peeling of the pad interface. On the contrary, the bonding strength between the pad electrode and the cladding ribbon It became clear that the variation of was large.

In addition, power semiconductor devices used so far may reach a temperature of 150 ° C., but these flat rectangular bonding ribbons applied to the power semiconductor devices are also miniaturized, densified and mounted. As the capacity of components has increased, the current must be increased and the current density increased. On the other hand, the pad electrode on the semiconductor element can be gold (Au), nickel (Ni), aluminum (Al) or an alloy thereof. For example, since it is an Al—Cu alloy or an Al—Si alloy, high bonding reliability in ultrasonic bonding to these pads is required. For such applications, one side of the above-described aluminum (Al) is made of Kovar or the like. A clad ribbon coated with a copper (Cu) alloy is insufficient.
Japanese Patent Application Laid-Open No. 2007-324603 (Patent Document 2 described later) was proposed in response to such a request, and aluminum (Al) or silver (with high conductivity copper (Cu) as a core material) was proposed. Ag) or the like is clad to achieve both high conductivity and ultrasonic bondability to the pad.
According to the present invention, it is suggested that the copper (Cu) layer on the inner side of the wire may be clad with outer silver (Ag) (Patent Document 2, [0007] paragraph, [0032] paragraph) described later, In this way, it is said that highly reliable ultrasonic bonding can be achieved. According to it, a large current capacity is achieved by the inner copper (u), whereas a small thickness silver (Ag) cladding layer, practically a thickness of 1 to 200 nm, preferably about 20 to 25 nm coating layer Therefore, it is said that excellent ultrasonic bonding to the pad can be achieved (paragraph [0017] in Patent Document 2 described later).

  Such a large-capacity clad ribbon is used in a relatively high-temperature environment such as a relatively high-temperature semiconductor requiring a heat-resistant temperature of 100 to 150 ° C., particularly an air conditioner, a solar power generation system, a hybrid vehicle, and an electric vehicle. The operating condition of the semiconductor element is higher than that of a normal semiconductor element. For example, a clad ribbon used for a power semiconductor in a relatively high temperature environment used for in-vehicle use needs to withstand a junction temperature of usually 100 to 150 ° C. at the maximum. Under such a relatively high temperature environment, internal oxidation of the clad ribbon material is cited as an issue, and improvement in oxidation resistance of the clad ribbon material is required, such as covering the clad ribbon surface with a stable coating.

Japanese Patent Publication No. 60-22827 JP 2007-324603 A

In the ultrasonic bonding using the clad ribbon for power semiconductor elements exposed to a relatively high temperature as described above, a loop of the clad ribbon is formed after the first bond to the pad electrode, and the second bond is made to the lead frame. In some cases, further third and fourth multiple bonds were made, and the clad ribbon was cut with a cutter after the final bond.
However, with clad ribbons in which aluminum (Al) or silver (Ag) is clad on a copper (Cu) core material so far, it has been difficult to simultaneously satisfy the bonding strength at the clad interface and the connection reliability at the pad interface. It was.
For example, increasing the thickness of the silver (Ag) clad layer that is in direct contact with the pad electrode and reducing the load applied to the pad electrode during bonding by a relatively soft clad layer to prevent chip cracking during the first bond, When a loop is formed and bent and joined, the degree of deformation differs depending on the difference in hardness between copper (Cu) and silver (Ag) with increased thickness. The clad interface of the silver (Ag) clad layer peels off.

Conversely, if the thickness of the silver (Ag) clad layer is reduced, the silver (Ag) clad layer between the pad electrode and the pad electrode does not work as a so-called cushion layer. The copper (Cu) core layer is deformed by the ultrasonic energy, and the interface on the copper (Cu) side is directly exposed on the pad electrode. For this reason, the pressing load and ultrasonic energy at the time of bonding are directly transmitted to the pad via the exposed copper (Cu) interface, and chip damage occurs in the semiconductor element on the pad electrode side.
Moreover, as the capacity of relatively high-temperature power semiconductor elements increases, the operating temperature of power semiconductors used in photovoltaic power generation systems, hybrid vehicles, electric vehicles, etc. becomes higher, and such silver (Ag) cladding When it becomes a high-temperature power semiconductor that uses a copper ribbon at a high temperature of 150 ° C. or higher, aluminum (Al), gold (Au), or nickel (Ni) of the pad electrode diffuses in the silver (Ag) cladding layer, and copper (Cu) Al / Cu-based intermetallic compounds, Au / Cu-based brittle compounds, Ni / Cu-based intermetallic compounds, etc. are formed at the cladding interface at the cladding interface where the core material and the silver (Ag) cladding layer are joined. . Since these compounds proliferate and grow at high temperatures, cracks occurred from the clad interface, and the bonding strength of the clad ribbon at the clad interface was not sufficient. In particular, the closer the operating temperature is to a high temperature up to 300 ° C., the faster the diffusion rate of the pad metal that diffuses in the silver (Ag) cladding layer, resulting in insufficient bonding strength at the cladding interface.
In addition, as a result of an increase in the number of elements diffused from the pad electrode such as aluminum (Al) existing in the silver (Ag) cladding layer, an Al / Ag-based fragile compound phase is formed in the silver (Ag) cladding layer, This resulted in a lack of long-term joint reliability at the pad interface.

  Therefore, in order to deal with these problems, the silver-bonded copper ribbon that is second-bonded by drawing a loop from the first bond draws a loop from the first bond and is second-bonded even in a high-temperature operating environment of 150 ° C. to 300 ° C. During ultrasonic bonding, the bonding strength of the clad interface between the copper (Cu) core material and the silver (Ag) clad layer is improved while preventing the peeling of the silver (Ag) clad layer as before, and the pad It is an object of the present invention to ensure the bonding reliability at the pad interface where the aluminum (Al), gold (Au) or nickel (Ni) layer of the electrode and the silver (Ag) clad layer on the ribbon side are first bonded. .

As means for solving the above-mentioned problems, the present inventors have bonded at the pad interface when the second-bonded silver-clad copper ribbon is left in a high-temperature operating environment of 150 ° C. to 300 ° C. by drawing a loop from the first bond. Attention was focused on changing the fine crystal structure of the silver (Ag) clad layer formed along with this to a recrystallized structure having a relatively small grain size.
When the state of ultrasonic bonding of the silver clad copper ribbon to the pad electrode with the first bond is observed in detail, the pad bonding interface of the silver clad copper ribbon is made of copper (Cu) deformed by an ultrasonic tool along with ultrasonic vibration. A mechanical compressive force is applied to the pad electrode through the silver (Ag) cladding layer, and a fine crystal structure having a dense transition network is formed at the pad interface of the silver (Ag) cladding layer. The present inventors previously heat-treated the silver-clad copper ribbon to change it to a structure in which the high strain of the processed structure at the pad interface of the silver (Ag) clad layer has been eliminated, so that the high-temperature operating environment of 150 ° C. to 300 ° C. However, the present inventors succeeded in forming a stable granular crystal structure on the pad interface of the silver (Ag) clad layer that is not affected by the clad processing of the silver clad copper ribbon.

In other words, the silver-clad copper ribbon after the silver-clad processing has a strained texture because the clad layer undergoes plastic deformation as the clad is processed. Conventionally, since the degree of processing was not so large, it was used for bonding as it was, but when bonding with such a bonding ribbon, plastic deformation was performed by an ultrasonic tool near the bonding interface with the pad of the cladding layer as described above. Therefore, the processing structure accompanying the cladding processing is further improved in the processing degree, and a fine crystal structure having a dense transition network as described above is formed.
This crystal structure region has a hardness higher than that of the surrounding crystal structure, and is rapidly and highly recrystallized by being placed in a high temperature use environment for a long time. End up.
Therefore, if such a fine crystal structure formed at the time of bonding to the pad exists in the vicinity of the pad interface of the silver (Ag) clad layer, the high strain region and the recrystallized region near the clad interface Since there is a large difference in hardness due to such a change in the internal structure, cracks occur in the silver (Ag) cladding layer where the change in the internal structure is significant, and the bonding reliability at the pad interface of the silver-clad copper ribbon Decreases.
The present inventors annealed the bonding ribbon after the clad processing, thereby relaxing the degree of processing of the high strain region formed at the time of bonding to the pad to suppress the hardness and recrystallizing in a high temperature use environment. When the high strain region at the pad bond interface in the silver (Ag) cladding layer is changed to a granular recrystallized structure, the hardness difference due to the change in the internal structure is suppressed. It has been found that the occurrence of cracks at this interface can be prevented. This granular recrystallized structure has a relatively small grain size but is formed faster and larger as the temperature becomes higher in the use environment, and is relatively large as it is during annealing in the silver (Ag) clad layer immediately above it. The difference in hardness between the two structures is reduced before crystal grains and cracks are generated. Further, by changing from a fine crystal structure, which is a processed structure, to a granular recrystallized structure, aluminum (Al), gold (Au), nickel (Ni) of a pad electrode that diffuses and enters into a silver (Ag) cladding layer. , And the formation of Al / Cu intermetallic compounds at the cladding interface is hindered.
Furthermore, the relatively large silver (Ag) crystal grains in the silver (Ag) cladding layer other than the vicinity of the pad bond interface also have high strain formed by the first bond under a high temperature operating environment of 150 ° C. to 300 ° C. The strain is removed from the crystal grains, the interdiffusion between copper (Cu) and silver (Ag) of the core material is increased, and the bonding strength of the clad interface of the silver clad copper ribbon is increased.
However, when the purity of silver (Ag) exceeds 99.999 mass%, the recrystallized structure region in the silver (Ag) cladding layer undergoes secondary recrystallization and becomes coarse, and the silver (A Ag) The crystal grains in the high strain region in the cladding layer also grow coarsely and become larger than the secondary recrystallized structure. Such coarse crystal grains are not preferable because they have low strength, are likely to crack due to thermal stress applied in the vicinity of the pad interface, and the bonding reliability at the pad interface decreases during high temperature operation.

The characteristics of the silver clad copper ribbon of the present invention used for a power semiconductor used in an environment of 150 to 300 ° C. are as follows. That is, the present invention relates to a rectangular silver clad comprising a silver (Ag) clad layer and a copper (Cu) core material tape for connecting a pad of a semiconductor element and a substrate in a loop shape by ultrasonic bonding at multiple locations. In copper ribbon,
The silver-clad copper ribbon is heat-treated at 450 ° C. to 750 ° C. after the clad, and the heat-treated silver (Ag) clad layer has a purity of 99% by mass to 99% having a Vickers hardness of 30 to 80 Hv. .99% by mass of silver (Ag),
The heat-treated copper (Cu) core tape is composed of copper (Cu) having a Vickers hardness of 50 to 80 Hv and a purity of 99.9% by mass to a purity of 99.9999% by mass.

The thickness of the silver (Ag) clad layer is appropriately determined depending on the pressure applied by the ultrasonic tool during ultrasonic bonding, the ultrasonic energy (amplitude of specific frequency × time), and the heating temperature of the pad electrode. A range of 5 μm to 200 μm is preferred for forming the tissue. When the thickness is within the preferred range in Patent Document 2, a granular crystal structure cannot be formed, and a metal such as aluminum (Al) diffuses from the pad electrode to the cladding interface in a high-temperature semiconductor. Can not prevent. In addition, when the thickness of the silver (Ag) cladding layer is less than 5 μm, all silver (Ag) in the silver (Ag) cladding layer is changed to a fragile Ag—Al compound under a high temperature operating environment of 150 to 300 ° C. In addition, copper (Cu) diffused in the silver (Ag) cladding layer also forms Al / Cu intermetallic compounds, etc., and cracks occur at the cladding interface between the silver (Ag) cladding layer and the copper (Cu) core material. Will occur.
On the other hand, the thickness of the silver (Ag) cladding layer with respect to the thickness of the copper (Cu) core material is preferably in the range of 1/10 to 2/3.

Here, “cladding” is a well-known technique in which a silver (Ag) thin tape layer is rolled over the entire surface of a copper (Cu) core material tape by applying pressure and heat, and overlay bonded. Since the obtained silver clad copper ribbon is rolled, it is thought that the processing strain remains. This is removed by heat treatment after cladding.
For this reason, the silver (Ag) clad layer preferably has as high a purity as possible, and silver (Ag) having a purity of 99.9% by mass is more preferable than silver (Ag) having a purity of 99% by mass. However, if the purity becomes too high, as described above, the crystal structure tends to be coarsened, and therefore silver (Ag) with a purity of 99.99 mass% is preferable to silver (Ag) with a purity of 99.999 mass%. Similarly, copper (Cu) having a purity of 99.99 mass% to a purity of 99.999 mass% is preferable.
However, the purity of silver (Ag) and copper (Cu) and the kind of trace additives can be selected as appropriate according to the purpose of the high-temperature semiconductor to be used. With silver-clad copper ribbons, work hardening during ultrasonic bonding is possible. Therefore, the hardness of the silver (Ag) clad layer is set to 30 to 80 Hv, and the hardness of the copper (Cu) core tape is set to 50 to 80 Hv. Note that the use of higher-purity copper (Cu), such as copper (Cu) with a purity of 99.99% or more and further copper (Cu) with a purity of 99.995% or more, is useful for loop formation and bonding. From the viewpoint of reducing work hardening at the time. Due to such high purity, even if a steep loop is drawn at the time of loop formation, it becomes difficult to peel off from the clad interface between the copper (Cu) core material tape and the silver (Ag) clad layer. Further, at the time of the first bonding, the work hardening of the silver clad copper ribbon due to mechanical compression of the cemented carbide tool hardly occurs, and there is an effect of preventing chip damage of the pad electrode.

  Under high temperature environment, mutual diffusion occurs at the clad interface between silver (Ag) and copper (Cu) in the silver clad copper ribbon, and the bonding strength is strengthened, but the metal of the pad electrode is in the silver (Ag) clad layer. Al and Cu-based intermetallic compounds and the like are formed by diffusing to form cracks at the cladding interface. Therefore, it is effective to form a diffusion prevention layer between the clad interface between silver (Ag) and copper (Cu), and the diffusion prevention layer is known nickel (Ni), zinc (Zn) or titanium (Ti), In addition to tungsten (W) and chromium (Cr), gold, palladium, platinum, and other platinum group metals that are completely dissolved with Cu can be wet-plated, clad, or vacuum deposited. This diffusion prevention layer is extremely thin with respect to the total film thickness of the silver (Ag) clad layer and the copper (Cu) core tape, and is only a film thickness on the order of several percent at the maximum. The effect of hardness can be ignored.

The silver (Ag) clad layer in the present invention has a fine grain boundary by changing a high strain fine crystal structure in the vicinity of the pad interface formed at the time of the first bonding into a granular crystal structure under a high temperature operating environment. There is an effect of delaying the progress of aluminum (Al) or the like which diffuses through the metal. Therefore, there is an effect of delaying the formation of an Al / Cu intermetallic compound or the like at the cladding interface.
On the other hand, since the silver clad copper ribbon uses a high-purity metal, the interdiffusion between silver (Ag) and copper (Cu) at the clad interface is promoted. As a result, the clad interface under a high temperature operating environment of 150 ° C. to 300 ° C. This has the effect of increasing the bonding strength.
Moreover, the hardness difference resulting from the difference in crystal structure in the silver (Ag) clad layer is obtained by removing the strain from the crystal grains having high strain formed by the first bond and forming a granular crystal structure. The difference in hardness in the silver (Ag) cladding layer is reduced. For this reason, even if left in a high temperature operating environment of 150 ° C. to 300 ° C., cracks do not enter from different locations of the crystal structure near the pad interface in the silver (Ag) cladding layer, and the pad electrode and silver (Ag) ) There is an effect of improving the bonding reliability at the pad bonding interface with the cladding layer.

FIGS. 1A to 1C are cross-sectional structure photographs (100 times) showing the relationship between the annealing temperature after cladding and the metal structure of the Ag / Al junction interface after the reliability test. No annealing, (B) The annealing treatment of the present invention was performed. Similar results were obtained when the annealing temperature ranged from 450 to 760 ° C. (C) The annealing temperature was higher than the range of the present invention. FIG. 2 shows a cross-sectional structure of the pad joint after a thermal cycle test at 175 ° C./−50° C. (3 minutes at each temperature for a total of 1000 cycles). (A) The magnification is 5 times and (B) the magnification is 10 times, and no cracks are observed at the pad interface. FIG. 3 shows a cross-sectional structure after a thermal cycle test at 175 ° C./−50° C. (3 minutes at each temperature for a total of 1000 cycles) after ultrasonic bonding of a non-annealed comparative bonding ribbon after cladding. (A) The magnification is 5 times, (B) the magnification is 10 times, and cracks are generated at the pad interface. FIG. 4 is a structure micrograph showing the details of the annealing temperature after cladding and the Ag / Al junction interface after the reliability test. FIG. 5 is a diagram showing a state in which a pad of a semiconductor element and a lead frame are connected by ultrasonic bonding using a conventional clad bonding ribbon.

  In the bonding ribbon of the present invention, the purity of the copper (Cu) core material tape is preferably 99.9% or more. This is to reduce the work hardening during loop deformation as much as possible, increase the bonding speed, and increase the number of connections per unit time. The purity and type of the copper (Cu) core tape is appropriately determined depending on the semiconductor to be used, the lead frame, etc. However, in order to avoid work hardening of the copper (Cu) core tape and mixing of impurities during bonding, the purity is 99.99. 99 mass% or more, More preferably, purity 99.995 mass%-purity 99.999 mass% is desirable. The Vickers hardness of the copper (Cu) core tape is preferably in the range of 50 Hv, which is the hardness of copper (Cu) having a purity of 99.9999% by mass or more, and 80 Hv that avoids work hardening during ultrasonic bonding.

  The purity of the silver (Ag) clad layer is preferably 99.999% by mass or less. This is because a granular structure is formed on the pad interface in the silver (Ag) cladding layer under a high temperature operating environment of 150 ° C. to 300 ° C. after ultrasonic bonding, and the aluminum (Al) metal or the like of the pad electrode is silver (Ag). This is for avoiding diffusion in the cladding layer and ensuring the bonding reliability. The range of 30 Hv, which is the hardness of silver (Ag) with a purity of 99.999% by mass or more of the silver (Ag) clad layer, to 80 Hv that prevents chip cracking of the semiconductor element is preferable.

  The hardness of the copper (Cu) core tape of the present invention is more preferably 1.5 times or less of the hardness of the silver (Ag) cladding layer. This is because even if a steeper loop is drawn, it is difficult to peel off from the Cu / Ag interface. Furthermore, it is for suppressing the excessive deformation | transformation and peeling of the silver (Ag) clad layer in the junction part of ultrasonic bonding, and ensuring stable joining reliability and avoiding chip damage.

  Further, the thickness of the silver (Ag) clad layer is preferably 200 μm or less from the viewpoint of peeling resistance with the copper (Cu) core material tape at the time of loop formation. Further, when the film thickness of the silver (Ag) cladding layer is too thin, less than 5 μm, a sufficient granular crystal structure cannot be formed under a high temperature operating environment of 150 ° C. to 300 ° C., and the metal of the pad electrode and silver While (Ag) forms a brittle crystal grain structure, it cannot prevent a metal such as aluminum (Al) from diffusing from the pad electrode to the cladding interface under a high-temperature operating environment. More preferably, the region is 20 to 80 μm, and the region of 20 to 50 μm is most excellent from the economical viewpoint.

Examples of the present invention will be described below.
[Example 1]
[Production of copper (Cu) core tape]
A copper (Cu) wire rod having a diameter of 700 μm and a purity of 99.99% by mass was rolled to produce a copper (Cu) core tape having a width of 1.9 mm and a thickness of 0.16 mm. Next, when the rolled tape was fully annealed in a hydrogen mixed nitrogen atmosphere at 750 ° C., the core material tape had a Vickers hardness of 70 Hv to 55 Hv. The properties of this core tape are shown in Table 1.
In addition, when a copper (Cu) tape having a purity of 99.9999 mass% rolled in the same manner was fully annealed, the Vickers hardness was reduced to 55 to 50 Hv.

[Preparation of silver (Ag) clad layer]
A silver (Ag) tape material having a purity of 99.9% by mass was rolled to produce a silver (Ag) clad layer tape having a width of 1.9 mm and a thickness of 0.06 mm. Next, when the clad layer tape was fully annealed, the clad layer had a Vickers hardness of 60 Hv to 45 Hv. Table 1 shows the characteristics of this cladding layer.
Similarly, when a silver (Ag) tape having a purity of 99.9999% by mass was fully annealed, the Vickers hardness decreased to 35 Hv.

[Production of silver clad copper ribbon]
Hydrogen mixed nitrogen gas is flowed into the upper and lower two-roll type thermocompression bonding apparatus, the silver (Ag) clad layer and the copper (Cu) core tape are joined, and then cold-rolled, width 2.0 mm, thickness 0.2 mm Silver clad copper ribbon. Next, these silver clad copper ribbons were washed and then fully annealed in a hydrogen mixed nitrogen atmosphere at 700 ° C. to obtain a silver clad copper ribbon (1).

[Observation of silver clad copper ribbon]
This silver-clad copper ribbon (1) is etched with dilute nitric acid, ion milled at 5 kV × 30 minutes (angle 45 °, eccentricity 3 mm), and then the longitudinal section is examined with a metal microscope (Olympus model PMG3). Observation was performed at 5 times, 10 times, and 100 times, respectively.
From the results of line analysis, it was found that no interdiffusion occurred at all at the 700 ° C. full annealing at the cladding interface of silver (Ag) and copper (Cu).

[Ultrasonic bonding test]
Next, this silver-clad copper ribbon (1) was ultrasonically bonded (first bond) on an aluminum (Al) plate (thickness 2 mm) having a purity of 99.99% by mass, and then subjected to Ni plating of 3 μm and having a purity of 99. Ultrasonic bonding (second bonding) was performed on a 95% by mass copper (Cu) substrate (thickness 2 mm). The apparatus is a fully automatic ribbon bonder 3600R type manufactured by Orthodyne Electronics Co., at a frequency of 80 kHz, and the load and ultrasonic load conditions are such that the collapse width is 1.01 to 1.02 times. All samples were bonded under the same conditions. The loop length of the bonding ribbon was 50 mm, the loop height was 30 mm, and the conditions were set such that the sliding resistance received from the ribbon, the path, and the tool was greater than the normal conditions. Then, when ultrasonic bonding was performed with n = 50 in each sample, the number of cuttings of the ribbon generated during bonding was examined, and none was cut. Moreover, the joint strength was measured from the side of the joint using the DAGE Universal Bond Tester PC5000 type from the side of the ribbon, and the determination results are shown in Table 1.

[Observation of internal structure before reliability test]
The cross section in the longitudinal direction of the silver-clad copper ribbon (1) immediately after the first bonding was etched by ion milling, and the clad interface and pad interface were observed with a metal microscope and subjected to line analysis. From a cross-sectional photograph of a metallographic microscope, it was found that a fine crystal structure with high strain was formed at the pad interface of the silver (Ag) cladding layer, and a large crystal grain structure was formed at the cladding interface. Moreover, it was found from the results of line analysis that interdiffusion has not yet occurred at the cladding interface of silver (Ag) and copper (Cu).

〔Reliability test〕
Subsequently, the reliability test of this silver clad copper ribbon (1) was conducted by a thermal cycle test at 175 ° C./−50° C. (3 minutes at each temperature for a total of 1000 cycles). The equipment used is Hitachi Heat Shock Tester: ES-60LMS [Liquid Example Cycle Tester].
In the high temperature reliability test, the rate of decrease in the shear strength after the reliability test with respect to the shear strength immediately after the first bond was measured and is also shown in Table 1. In addition, the determination is based on the strength ratio after the reliability test, and the strength ratio after the reliability test of 0.9 or more is represented by a double circle (◎), and is 0.7 or more and less than 0.9. A thing was described with a single circle (O), and a thing less than 0.7 was described with a cross (x) mark.
As is clear from Table 1, the high temperature bonding reliability of the silver clad copper ribbon (1) of the present invention is excellent. This is because the fine crystal structure of the high strain at the pad interface of the silver (Ag) clad layer is a fine crystal. This is due to the change to the grain structure (lower diagram in FIG. 4).

[Observation after reliability test]
After etching the longitudinal section of the silver clad copper ribbon (1) after the reliability test by ion milling, the clad interface and the pad interface were observed with a metal microscope and subjected to line analysis. From the cross-sectional photographs of the metallurgical microscope (FIGS. 1-B, 2 and 4), the fine crystal structure of high strain changes to a fine grain structure at the pad interface of the silver (Ag) cladding layer. It can be seen that a larger crystal grain structure is formed at the interface. In addition, from the results of line analysis, mutual diffusion occurs at the silver (Ag) and copper (Cu) clad interface, and aluminum (Al) diffuses too much near the pad interface in the silver (Ag) clad layer. I found out.
In addition, the silver clad copper ribbon after the same clad in FIG. 4 was subjected to heat treatment (lower stage) and non-heat treated (upper stage) by bonding with an aluminum pad, and after each thermal cycle test. The more detailed organization status is shown.
According to FIG. 4, when heat treatment is not performed, the crystal structure in the vicinity of the Ag bonding interface is coarse and grows to a coarse crystal, and cracks and voids are formed at the Ag layer bonding interface after the thermal cycle test. It can be seen (the upper right figure in FIG. 4). On the other hand, in the case of the heat treatment of the present invention, the crystal structure in the vicinity of the Ag joint interface after bonding is changed to a fine granular crystal structure (lower left and right diagram in FIG. 4). A fine granular crystal structure is maintained.
[Example 2]

[Production of copper (Cu) core tape]
In the same manner as in Example 1, cold-working was performed by depositing nickel (Ni) with a purity of 99.9% by mass on a copper (Cu) wire having a diameter of 500 μm with a purity of 99.999% by magnetron sputtering. Thus, a copper (Cu) core tape having a width of 1.7 mm and a thickness of 0.46 mm was produced. Next, when full annealing was performed in the same manner as in Example 1, the Vickers hardness of the core material tape was changed from 70 Hv to 55 Hv.

[Preparation of silver (Ag) clad layer]
In the same manner as in Example 1, a silver (Ag) wire having a diameter of 600 μm and a purity of 99.999% by mass was rolled to produce a silver (Ag) clad layer having a width of 2.0 mm and a thickness of 130 μm. When full annealing was performed in a hydrogen mixed nitrogen atmosphere, the silver (Ag) cladding layer had a Vickers hardness of 40 Hv to 35 Hv. Table 1 shows the configurations of the core tape and the clad layer.

[Production of silver clad copper ribbon]
In the same manner as in Example 1, a silver clad copper ribbon was obtained and fully annealed at 740 ° C. to obtain a silver clad copper ribbon (2).

[Observation of silver clad copper ribbon]
This silver clad copper ribbon (2) was observed in the same manner as in Example 1 with a cross section in the longitudinal direction with a metal microscope, and the clad interface was subjected to line analysis.
From a cross-sectional photograph of a metallographic microscope, it was confirmed that a fine crystal structure with high strain was formed at the pad interface of the silver (Ag) clad layer, and a large crystal grain structure was formed at the clad interface. Moreover, it was found from the results of line analysis that interdiffusion has not yet occurred at the cladding interface of silver (Ag) and copper (Cu).

[Ultrasonic bonding test]
Next, this silver clad copper ribbon (2) was tested in the same manner as in Example 1, and the determination results are shown in Table 1. No ribbon was cut during n = 50 bonding.

[Observation of internal structure before reliability test]
In the same manner as in Example 1, it was observed with a metal microscope and subjected to line analysis. From the cross-sectional photograph of the metallographic microscope, as in Example 1, a fine crystal structure with high strain is formed at the pad interface of the silver (Ag) cladding layer, and a large crystal grain structure is formed at the cladding interface. confirmed.

〔Reliability test〕
Next, in the same manner as in Example 1, this silver-clad copper ribbon (2) was subjected to a thermal cycle test at 175 ° C./−50° C. (3 minutes at each temperature for a total of 1000 cycles). The equipment used is Hitachi Heat Shock Test Equipment: ES-60LMS [Liquid Cooling Cycle Tester].
As is clear from Table 1, the high-temperature bonding reliability of the silver-clad copper ribbon (1) of the present invention is excellent as in Example 1, and this is a high strain at the pad interface of the silver (Ag) clad layer. This is because the fine crystal structure is changed to a fine crystal grain structure.

[Observation after reliability test]
The cross section in the longitudinal direction of the silver clad copper ribbon (2) after the reliability test was etched by ion milling, and then the clad interface and the pad interface were observed with a metal microscope and subjected to line analysis. From the observation by a cross-sectional photograph of a metallographic microscope, although the fine crystal structure of high strain is changed to a fine grain structure at the pad interface of the silver (Ag) clad layer, the silver clad copper of Example 1 is changed at the clad interface. It was observed that the crystal grain structure did not grow as the larger grain structure of the ribbon (1). Also, from the results of the line analysis, the silver (Ag) and copper (Cu) clad interface has some degree of interdiffusion unlike the first example, and is performed near the pad interface in the silver (Ag) clad layer. As in Example 1, it was found that the diffusion of aluminum (Al) was not so advanced.

[Torsion test]
Next, with respect to the silver clad copper ribbons (1) and (2), one end of a sample of about 1 m is fixed, and the other end is rotated 15 times to the right at 1 revolution per minute, and then rotated 15 times to the left to reverse the silver ( The peeling of the cladding interface with Ag) and copper (Cu) or nickel (Ni) was observed.
No peeling of the boundary surface occurred in any of the silver-clad copper ribbons (1) and (2). From this, it can be seen that the present invention is excellent in the peeling prevention characteristic of the silver (Ag) clad layer.
Further, regarding the heat treatment conditions, Table 1 shows the results of Example 3 in which the silver-clad copper ribbon of Example 1 was annealed by heating in a current-carrying electric furnace and Example 4 in which the annealing temperature was 400 ° C. When the annealing temperature is 400 ° C., the core material and the cladding layer are high in hardness, and the reliability test results are slightly inferior.

Comparative example

[Production of copper (Cu) core tape]
A core tape having a Vickers hardness of 50 Hv was prepared in the same manner as in Example 1.

[Preparation of silver (Ag) clad layer]
A silver (Ag) clad layer having a Vickers hardness of 60 Hv was prepared in the same manner as in Example 1.

[Production of silver clad copper ribbon]
The silver (Ag) clad layer and the copper (Cu) core material tape were joined in the same manner as in Example 1, and except for the full annealing process in a hydrogen mixed nitrogen atmosphere at 450 ° C. to 750 ° C. Similarly, it was set as the silver clad copper ribbon: Comparative Example 1.

[Observation of silver clad copper ribbon]
After etching the silver clad copper ribbon of Comparative Example 1 by ion milling, the cross section in the longitudinal direction was observed with a metallographic microscope, and line analysis of silver (Ag) and copper (Cu) was performed.
As a result of the line analysis, it was found that no interdiffusion occurred at all in the clad interface of silver (Ag) and copper (Cu) as in Example 1.

[Ultrasonic bonding test]
Next, the first and second bonds of the silver-clad copper ribbon of Comparative Example 1 were made in the same manner as in Example 1. Then, when ultrasonic bonding was performed with n = 50 in each sample, the number of cuttings of the ribbon generated during bonding was examined, and none was cut. Moreover, the joint strength was measured from the side of the joint using the DAGE Universal Bond Tester PC5000 type from the side of the ribbon, and the determination results are shown in Table 1.

[Observation of internal structure before reliability test]
About the silver clad copper ribbon of the comparative example 1 immediately after the 1st bonding, it observed with the metal microscope similarly to Example 1, and analyzed the line. From a cross-sectional photograph of a metallurgical microscope, as in Example 1, a fine crystal structure with high strain is formed at the pad interface of the silver (Ag) cladding layer, and a large crystal grain structure is formed at the cladding interface. I understood. Moreover, it was found from the results of line analysis that interdiffusion has not yet occurred at the cladding interface of silver (Ag) and copper (Cu).

〔Reliability test〕
Subsequently, the silver clad copper ribbon of Comparative Example 1 was subjected to a reliability test in the same manner as in Example 1. In the high temperature reliability test, the rate of decrease in the shear strength after reliability with respect to the shear strength immediately after the first bond was measured, and is also shown in Table 1. In addition, the determination is based on the strength ratio after the reliability test, and the strength ratio after the reliability test of 0.9 or more is represented by a double circle (◎), and is 0.7 or more and less than 0.9. A thing was described with a single circle (O), and a thing less than 0.7 was described with a cross (x) mark.
As is apparent from Table 1, the high-temperature bonding reliability of the silver-clad copper ribbon of Comparative Example 1 is inferior, and this is because, as described later, a high-strain fine crystal structure is present at the pad interface of the silver (Ag) clad layer. This is due to the change to a fragile grain structure composed of a compound of silver (Ag) and aluminum (Al).

[Observation after reliability test]
The cross section in the longitudinal direction of the silver-clad copper ribbon of Comparative Example 1 after the reliability test was etched by ion milling, and the clad interface and pad interface were observed with a metal microscope and subjected to line analysis. From the cross-sectional photographs of the metallurgical microscope (FIG. 1-A, FIG. 3 and FIG. 4 top view), the fine crystal structure of high strain changed to a brittle crystal grain structure at the pad interface of the silver (Ag) clad layer. It can be seen that a larger crystal grain structure is formed at the interface. In addition, from the results of line analysis, diffusion of aluminum (Al) occurs at the clad interface between silver (Ag) and copper (Cu), and an intermetallic compound of aluminum (Al) and copper (Cu) is formed. It was found that a compound of silver (Ag) and aluminum (Al) was formed by diffusion of aluminum (Al) in the vicinity of the pad interface in the silver (Ag) cladding layer.
Furthermore, as Comparative Example 2, the hardness of the core material and the clad layer and the reliability test results are shown in Table 1 for the annealing temperature of the silver clad copper ribbon of Example 1 exceeding 770 ° C. of the present invention. . From the cross-sectional photograph of the metal microscope, it can be seen that the vicinity of the pad interface of the silver (Ag) clad layer has a coarse crystal structure and cracks are generated at the bonding interface (FIG. 1-C).
When the annealing temperature exceeds the range of the present invention, the hardness of the core material and the clad layer is lowered, but the reliability test result is poor. This is because recrystallization progresses remarkably as the annealing temperature rises, resulting in a coarse crystal structure, which indicates that it cannot be used as a silver-clad copper ribbon outside the annealing temperature range of the present invention. .

DESCRIPTION OF SYMBOLS 1 Bonding ribbon 2 Silver (Ag) clad layer 3 Copper core material 4 Aluminum pad 5 Lead frame

Claims (5)

In a rectangular silver clad copper ribbon composed of a silver (Ag) clad layer and a copper (Cu) core material tape for connecting a semiconductor element pad and a substrate in a loop shape by ultrasonic bonding at multiple locations,
The silver-clad copper ribbon is heat-treated at 450 ° C. to 750 ° C. after the clad, and the heat-treated silver (Ag) clad layer has a purity of 99% by mass to 99% having a Vickers hardness of 30 to 80 Hv. The heat-treated copper (Cu) core tape is composed of 999 mass% silver (Ag) and has a Vickers hardness of 50 to 80 Hv and a purity of 99.9 mass% to 99.9999 mass% copper (Cu A flat silver (Ag) clad copper ribbon for high-temperature semiconductor elements.   The rectangular silver clad copper ribbon for high-temperature semiconductor elements according to claim 1, wherein the silver (Ag) clad layer has a thickness of 5 µm or more and 200 µm or less.   The rectangular silver clad copper ribbon for high-temperature semiconductor elements according to claim 1, wherein the purity of the copper (Cu) core material tape is 99.99 mass% to 99.999 mass%.   The rectangular silver clad copper ribbon for a high-temperature semiconductor device according to claim 1, wherein the purity of the silver (Ag) clad layer is 99.9 mass% to 99.99 mass%.   The flat silver-clad copper ribbon for a high-temperature semiconductor element according to claim 1, wherein the high-temperature semiconductor element is used in an operating environment of 150C to 300C.