Al bonding wire

Abstract

Provided is an Al bonding wire capable of achieving sufficient bonding reliability at the bonding portion of the wire under high-temperature operating conditions of a semiconductor device using the Al bonding wire. The Al bonding wire is characterized in that it is composed of Al or an Al alloy, has an average grain size of 0.01 to 50 μm in a core material cross section perpendicular to the wire axis, and, when crystal orientation is measured in the core material cross section perpendicular to the wire axis, has an orientation ratio of 30 to 90% of the <111> orientation with an angular difference of 15° or less relative to the wire length direction.

Description

Technical Field

[0001] The present invention relates to Al bonding wire. Background Art

[0002] In semiconductor devices, electrodes formed on semiconductor elements are connected to electrodes on lead frames or substrates by bonding wires. As the material used for bonding wires, gold (Au) or copper (Cu) is used in integrated circuit semiconductor devices such as super LSI, while aluminum (Al) is mainly used in power semiconductor devices. For example, Patent Document 1 shows that a bonding wire is used in a power semiconductor module. An example of aluminum bonding wire (hereinafter referred to as "Al bonding wire") is shown. In a power semiconductor device using Al bonding wire, wedge bonding is used as a bonding method for both connection to electrodes on a semiconductor element and connection to electrodes on a lead frame or substrate.

[0003] Power semiconductor devices using Al bonding wire are often used in high-power equipment such as air conditioners and solar power generation systems, as well as in automotive semiconductor devices. In these semiconductor devices, the bonding portion of the Al bonding wire is exposed to high temperatures of 100-150°C. When using Al bonding wire composed solely of high-purity Al, the wire tends to soften in such temperatures, making it difficult to use in high-temperature environments.

[0004] Patent Document 2 discloses an Al wire containing 0.02 to 1% by weight of Fe. In Al wires that do not contain Fe, the high temperatures experienced during semiconductor use will cause recrystallization directly above the wire bonding interface, forming small crystal grains that can cause cracks. To address this issue, the inclusion of 0.02% or more of Fe can increase the recrystallization temperature. Annealing after wire drawing can reduce the wire grain size before bonding to 50 μm or more. Larger grain sizes prevent recrystallization even at the high temperatures experienced during semiconductor use, preventing cracks.

[0005] Patent Document 3 discloses a bonding wire comprising Al-0.1 to 1 wt% X, where X is at least one metal selected from Cu, Fe, Mn, Mg, Co, Li, Pd, Ag, and Hf, and having a wire thickness (diameter) of 50 to 500 μm, as a bonding wire that suppresses the growth of cracks in the connection even when high currents are repeatedly passed through it, thereby achieving a highly reliable connection.

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-314038

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 8-8288

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 2008-311383 Summary of the Invention

[0011] [Technical problem to be solved by the invention]

[0012] In semiconductor devices using either pure Al bonding wire or Al bonding wire using Al alloys as described in Patent Documents 2 and 3, sufficient bonding reliability of the bonding portion of the bonding wire may not be achieved under high-temperature conditions during operation of the semiconductor device.

[0013] An object of the present invention is to provide an Al bonding wire capable of achieving sufficient bonding reliability of a bonding portion of the bonding wire under a high-temperature operating state of a semiconductor device using the Al bonding wire.

[0014] [Technical means for solving technical problems]

[0015] That is, the gist of the present invention is as follows.

[0016] [1] An Al bonding wire characterized in that it is composed of Al or an Al alloy, has an average grain size of 0.01 to 50 μm in a cross section of a core material perpendicular to the wire axis, and, in a result of measuring crystal orientation in the cross section of the core material perpendicular to the wire axis, has an orientation ratio of 30 to 90% of the crystal orientation <111> having an angular difference of 15° or less with respect to the wire length direction.

[0017] [2] The Al bonding wire according to [1], wherein the wire has a hardness of 20 to 40 in Hv.

[0018] [3] The Al bonding wire according to [1] or [2], wherein the wire diameter is 50 to 600 μm.

[0019] [Effects of the Invention]

[0020] According to the present invention, the bonding reliability of the bonding portion of the bonding wire can be sufficiently ensured in a high-temperature state in which a semiconductor device using the Al bonding wire operates. DETAILED DESCRIPTION

[0021] It has been clarified that even in semiconductor devices using Al bonding wire composed of pure Al or an Al alloy such as those described in Patent Documents 2 and 3, prolonged operation at high temperatures can lead to a decrease in the bonding strength of the wire's joints, effectively preventing sufficient bonding reliability. Observation of a cross-section of a bonding wire in a semiconductor device after prolonged high-temperature operation reveals that recrystallization caused by the high-temperature environment increases the crystal grain size and reduces the <111> orientation ratio (described later). This reduces the wire strength compared to the initial stage, leading to delamination at the bonding interface and reduced bonding reliability.

[0022] In response to this, the present invention sets the average grain size of the core material cross section perpendicular to the wire axis in Al bonding wire, whether pure Al or an Al alloy, to 0.01 to 50 μm. Measurement of the crystal orientation of the core material cross section perpendicular to the wire axis reveals that the ratio of the crystal orientation <111> with an angle difference of 15° or less relative to the wire length direction (hereinafter referred to as the "crystal <111> orientation ratio") is 30 to 90%. This ensures the reliability of the bonding portion even when the semiconductor device is used continuously for a long period of time in a high-temperature environment, even after prolonged high-temperature operation. This is described in detail below.

[0023] The following describes a test for evaluating the reliability of a joint after prolonged exposure to high temperatures.

[0024] The Al bonding wire used consisted of an Al alloy containing 0.5% Fe by mass and pure Al. The wire diameter after drawing was 200 μm. Heat treatment was performed during the drawing process, with or without the need for it. When heat treatment was performed, the cooling conditions were set to either slow cooling or rapid cooling. The wire after drawing was subjected to a tempering heat treatment to adjust the Vickers hardness to Hv40 or less. By varying the heat treatment conditions during and after drawing, the wire's grain size and <111> orientation ratio were varied.

[0025] In the semiconductor device, the first bonding portion between the semiconductor chip and the bonding wire and the second bonding portion between the external terminal and the bonding wire are both wedge bonds.

[0026] High-temperature, long-term exposure is tested through power cycling tests. This test involves repeatedly heating and cooling a semiconductor device with an Al bonding wire. Heating takes two seconds until the bonding portion of the wire reaches 140°C, followed by cooling down to 30°C over five seconds. This heating and cooling cycle is repeated 200,000 times.

[0027] After the above-mentioned long-term high-temperature exposure, the shear strength of the first joint was measured to evaluate the joint reliability. The results showed that when the wire grain size was 0.01 to 50 μm and the <111> orientation ratio was 30 to 90% (the conditions of the present invention), both the Al alloy and pure Al exhibited a joint shear strength of at least 90% of the initial strength, indicating sufficient joint reliability. In contrast, when the conditions of the present invention were exceeded, the joint shear strength was less than 50% of the initial strength, indicating insufficient joint reliability.

[0028] Average grain diameter of wire

[0029] In the present invention, the average grain diameter in a cross section of the core material perpendicular to the wire axis (a cross section perpendicular to the wire axis; a cross section perpendicular to the wire length direction (C cross section)) is 0.01 to 50 μm. The average grain diameter is measured using a measurement method such as EBSD (Electron Back Scatter Diffraction Patterns) to determine the area of ​​each grain. The average diameter of the grains, calculated as a circle, is used.

[0030] If the average grain diameter is 0.01 μm or more, the recrystallization is appropriately carried out by the tempering heat treatment during wire drawing, the wire will soften, and the occurrence of chip breakage during bonding, the reduction of the bondability of the bonding part, and the reduction of reliability during long-term use at high temperature can be prevented. On the other hand, if the average grain diameter exceeds 50 μm, the wire will be excessively recrystallized, resulting in a reduction in reliability during long-term use at high temperature. By performing tempering heat treatment during wire drawing, the average grain diameter in the C section of the wire can be set to 0.01 to 50 μm. The average grain diameter is preferably 0.05 μm or more, more preferably 0.1 μm or more. The average grain diameter is also preferably 20 μm or less, more preferably 10 μm or less.

[0031] 《Crystalline <111> Orientation Ratio of Wires》

[0032] In the present invention, when measuring crystal orientations in a cross section of the core perpendicular to the wire axis, the ratio of crystal orientations <111> with an angle difference of 15° or less relative to the wire length direction (the crystalline <111> orientation ratio) in the cross section of the core perpendicular to the wire axis is 30 to 90%. Here, the cross section of the core perpendicular to the wire axis refers to a cross section perpendicular to the wire length direction (C-cross section). EBSD can be used to measure the crystalline <111> orientation ratio. Using a cross section of the core perpendicular to the wire axis (a cross section perpendicular to the wire axis; a cross section perpendicular to the wire length direction) as the inspection surface, the crystalline <111> orientation ratio can be calculated using analysis software included with the device. This orientation ratio calculation method calculates the area ratio of the <111> orientation as the crystalline <111> orientation ratio, taking only the area of ​​crystal orientations determined based on reliability within the measurement region as the total area. In calculating the orientation ratio, areas where crystal orientation measurement is impossible or, even if measurement is possible, areas where the orientation analysis reliability is low are excluded from the calculation.

[0033] If the crystal <111> orientation ratio is 90% or less, appropriate recrystallization based on the tempering heat treatment during wire drawing will soften the wire, preventing chip breakage during bonding, reduced bondability of the bonded portion, and reduced reliability during long-term use at high temperatures. On the other hand, if the crystal <111> orientation ratio is less than 30%, it means that the wire has been excessively recrystallized, reducing the reliability of the bonded portion and the reliability during long-term use at high temperatures. By heat treating the wire during wire drawing and rapidly cooling it after the heat treatment, in conjunction with the tempering heat treatment after wire drawing, the crystal <111> orientation ratio in the cross section perpendicular to the length of the wire can be set to 30-90%. The crystal <111> orientation ratio is preferably 60% or more, more preferably 70% or more. The crystal <111> orientation ratio is also preferably 85% or less, more preferably 80% or less.

[0034] Vickers Hardness of Wires

[0035] In the present invention, preferably, in the cross section (C cross section) perpendicular to the length direction of the wire of the bonding wire, the Vickers hardness is in the range of Hv20 to 40. By setting it to Hv40 or less, chip breakage will not occur during bonding, good bonding is achieved, and wire arcs can be easily formed for wiring of semiconductor devices. On the other hand, when the Vickers hardness is reduced to less than Hv20, it means that the recrystallization of the wire has been excessively carried out, and it is difficult to obtain sufficient strength even if precipitates are formed by aging heat treatment, and the reliability of the bonding portion may be reduced. Therefore, the lower limit of the Vickers hardness is preferably set to Hv20. As mentioned above, heat treatment is performed during the wire manufacturing process, and tempering heat treatment is performed during the wire drawing process, thereby enabling the Vickers hardness of the wire to be set to the range of Hv20 to 40.

[0036] Wire diameter

[0037] In the present invention, the preferred bonding wire diameter is 50 to 600 μm. Because high currents flow through power devices, wires of 50 μm or greater are generally used. However, wires of 600 μm or greater become difficult to handle or are not compatible with wire bonding machines, so wires of 600 μm or less are used.

[0038] Wire components

[0039] The Al bonding wire of the present invention is applicable to both pure Al and Al alloys. Al alloys may include elements such as Fe and Si. Examples include Al-Fe alloys and Al-Si alloys. The Al content is preferably 95% by mass or greater, 96% by mass or greater, 97% by mass or greater, 98% by mass or greater, or 98.5% by mass or greater. Preferred examples of Al alloys include Al-0.5% by mass Fe alloys and Al-1% by mass Si alloys.

[0040] Manufacturing method of bonding wire

[0041] The bonding wire of the present invention is manufactured by obtaining an Al alloy containing predetermined components and performing heat treatment during the wire drawing process in addition to rolling and wire drawing processes of a conventional method.

[0042] During the wire drawing process, heat treatment and subsequent rapid cooling treatment are carried out. The heat treatment can be carried out at the stage when the wire diameter is about 1 mm. The heat treatment conditions in the wire drawing are preferably set to 600-640°C and 2-3 hours. The rapid cooling treatment after the heat treatment is rapid cooling in water. If heat treatment is not carried out, the crystallization <111> orientation ratio will deviate from the upper limit after the following tempering heat treatment. In addition, even if heat treatment is carried out and the cooling conditions are set to slow cooling, or the heat treatment temperature is too high, the crystallization <111> orientation ratio will deviate from the lower limit after the following tempering heat treatment.

[0043] Tempering heat treatment is performed during or after the wire drawing process, or both. Increasing the tempering heat treatment temperature and duration increases the average grain size, reduces the <111> orientation ratio, and lowers the Vickers hardness. Tempering heat treatment conditions can be selected to achieve the desired average grain size, <111> orientation ratio, and Vickers hardness within a heat treatment temperature range of 250-350°C and a heat treatment time range of 5-15 seconds.

[0044] [Example]

[0045] Example 1

[0046] An Al alloy containing 0.5% by mass of Fe was prepared, the alloy was made into an ingot, the ingot was groove-rolled, and wire drawing was performed. Heat treatment was performed at the stage where the wire diameter was 800 μm. Thereafter, the final wire diameter was set to the wire diameter shown in Table 1 for die wire drawing, and tempering heat treatment was performed after the wire drawing was completed. The heat treatment conditions during the wire drawing process were set to the standard conditions at 620°C, 3 hours, and rapid cooling (water cooling), a part was not heat treated (Comparative Example 3), and the cooling conditions were set to slow cooling (air cooling) (Comparative Example 4). In addition, the tempering heat treatment conditions after wire drawing were set to the standard conditions in the range of 270±10°C and 10 seconds, a part was set to a temperature lower than the standard (Comparative Example 1), and a temperature higher than the standard (Comparative Example 2). In Examples 1 to 9 of the present invention, the temperature of the tempering heat treatment was varied within the standard conditions.

[0047] Using this wire, the average grain size, the orientation ratio of the crystal orientation <111> with an angle difference of 15° or less relative to the wire length direction (crystallographic <111> orientation ratio), and Vickers hardness were measured in a core material cross section perpendicular to the wire axis (C cross section).

[0048] The average grain size was measured by obtaining the area of ​​each grain using the EBSD method, converting the area of ​​each grain into the area of ​​a circle, and taking the average of the diameters.

[0049] The crystal <111> orientation ratio is measured by performing EBSD on a cross section of the core material perpendicular to the wire axis (a cross section perpendicular to the length of the bonding wire). The crystal <111> orientation ratio is then calculated using the aforementioned steps using the analysis software included with the device.

[0050] The Vickers hardness was measured using a micro Vickers hardness tester, and the hardness was measured at the center position in the radial direction of the C-section.

[0051] In the semiconductor device, the semiconductor chip electrodes are made of Al-Cu, the external terminals are made of Ag, the first bonding portion between the semiconductor chip electrodes and the bonding wires, and the second bonding portion between the external terminals and the bonding wires are both wedge bonds.

[0052] The bonding quality of the bonding wire in the semiconductor device was evaluated based on whether there was any initial bonding failure (non-bonding) at the first bonding portion (before prolonged exposure to high temperatures). Bonded wires were marked as ◯, while those that were not bonded were marked as ×, and the results were recorded in the "Bondability" column of Table 1.

[0053] Semiconductor device chip cracking was evaluated by dissolving the metal on the pad surface with acid and observing the area beneath the pad for chip cracks using a microscope. No cracks were rated as ◯, while cracks were rated as ×, and the results were recorded in the "Chip Crack" column in Table 1.

[0054] High-temperature, long-term exposure is tested through power cycling tests. This test involves repeatedly heating and cooling a semiconductor device with an Al bonding wire. Heating is performed for 2 seconds until the temperature of the bonding wire joint reaches 140°C, followed by cooling for 5 seconds until the temperature of the joint reaches 30°C. This heating and cooling cycle is repeated 200,000 times.

[0055] After the high-temperature, extended period, the shear strength of the first joint was measured to evaluate the reliability of the joint. Shear strength measurements were compared with the initial joint shear strength. A score of 95% or greater of the initial joint strength was designated as ◎, 90% to 95% as ○, 70% to 90% as △, and less than 70% as ×. These scores are recorded in the "Reliability Test" column of Table 1. × indicates failure.

[0056] [Table 1]

[0057]

[0058] The results are shown in Table 1. Numerical values ​​outside the range of the present invention are underlined.

[0059] In Examples No. 1 to 9 of the present invention, the average grain diameter and crystal <111> orientation ratio of the wires are within the range of the present invention, the wire hardness is good, and the bonding, chip cracking, and reliability tests are all good. As mentioned above, in Examples 1 to 9 of the present invention, the temperature of the tempering heat treatment is varied under standard conditions. For wires with the same diameter, the higher the tempering heat treatment temperature under standard conditions, the larger the crystal grain size and the smaller the crystal <111> orientation ratio. In particular, in Examples No. 1, 4, and 7 of the present invention, the crystal grain size and crystal orientation are within the most preferred ranges, and the results of the reliability test are particularly good (◎).

[0060] Comparative Examples No. 1 to 4 are comparative examples.

[0061] Comparative Example No. 1 had a tempering heat treatment condition lower than the standard condition, a low average grain size outside the range, a high <111> orientation ratio outside the range, and a hardness higher than the preferred condition. Bondability, chip cracking, and reliability tests were all poor.

[0062] Comparative Example No. 2 had a tempering heat treatment condition higher than the standard condition, an average grain size exceeding the upper limit, a low <111> orientation ratio outside the range, and a hardness lower than the preferred condition, resulting in a failed reliability test.

[0063] Comparative Example No. 3 did not undergo heat treatment during wire drawing. Although the average grain size was within the range of the present invention, the <111> orientation ratio of the crystal was high and outside the range, and the hardness exceeded the preferred condition, resulting in a poor reliability test.

[0064] Comparative Example No. 4 was subjected to slow cooling during the heat treatment during wire drawing. Although the average grain size was within the range of the present invention, the <111> orientation ratio of the crystal was low and outside the range, resulting in a poor reliability test.

[0065] Example 2

[0066] Al bonding wires having a wire diameter of 200 μm and having the wire composition shown in Table 2 were produced. The production conditions and evaluation items were the same as those in Example 1. Table 2 shows the results.

[0067] [Table 2]

[0068]

[0069] It is clear from Table 2 that any wire composition can obtain the wire structure of the present invention and achieve good wire quality.

Claims

1. An Al bonding wire, characterized in that: The wire is composed of Al or an Al alloy, with an Al content of 95% by mass or more, and has an average grain diameter of 0.01 to 50 μm in a cross section perpendicular to the wire axis, wherein the average grain diameter is obtained by measuring the area of ​​each grain using EBSD, and the average diameter of each grain is calculated as the average of the diameters when the area of ​​each grain is considered as a circle. The results of crystal orientation measurements on a cross section perpendicular to the wire axis showed that the ratio of the crystal orientation <111>, with an angle difference of 15° or less relative to the wire length direction, was 30% to 90%. The measurement of the ratio of the crystal orientation <111> was performed using EBSD, with a core cross section perpendicular to the wire axis as the inspection surface, and the analysis software included with the device calculated the ratio of the crystal orientation <111>. The wire diameter is 50 to 600 μm.

2. The Al bonding wire according to claim 1, wherein The average grain diameter is greater than 0.05 μm.

3. The Al bonding wire according to claim 1, wherein The average grain diameter is greater than 0.1 μm.

4. The Al bonding wire according to claim 1, wherein The average grain diameter is 20 μm or less.

5. The Al bonding wire according to claim 1, wherein The average grain diameter is 10 μm or less.

6. The Al bonding wire according to claim 1, wherein The orientation ratio of the crystal orientation <111> is 60% or more.

7. The Al bonding wire according to claim 1, wherein The orientation ratio of the crystal orientation <111> is 70% or more.

8. The Al bonding wire according to claim 1, wherein The orientation ratio of the crystal orientation <111> is 85% or less.

9. The Al bonding wire according to claim 1, wherein The orientation ratio of the crystal orientation <111> is 80% or less.

10. The Al bonding wire according to claim 1, wherein The hardness of the conductor is 20 to 40 in Hv. The Vickers hardness was measured using a micro Vickers hardness tester, and the hardness was measured as the hardness at the center position in the radial direction of a cross section perpendicular to the longitudinal direction of the wire.

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