Bonding wire for semiconductor device

Abstract

The present invention addresses the problem of maintaining and improving productivity of a bonding process by improving a capillary tip's resistance to build-up while ensuring bonding reliability of a primary junction (also referred to as a 1st junction, an FAB junction, or a ball junction) and of a secondary junction (also referred to as a 2nd junction or a wedge junction), and the problem of providing such an Ag bonding wire.　The bonding wire that solves the aforementioned problem is this Ag bonding wire made of an Ag alloy containing: In at 0.05-3.0 mass%; one or more of Pd, Pt, and Au at 0.010-5.0 mass%; one or more of Sc, Ti, and Mn at 0.0005-0.050 mass%; and one or more of P, Zn, Bi, and Cu at 0-0.030 mass%.

Description

Bonding wire for semiconductor devices

[0001] The present invention relates to a bonding wire for a semiconductor device, which is used to connect electrodes on a semiconductor element to wiring on a circuit wiring board, such as external leads.

[0002] As a bonding wire for semiconductor devices (hereinafter referred to as bonding wire or simply wire) for joining electrodes on a semiconductor element to external leads, a thin wire with a wire diameter of about 15 to 50 μm is mainly used. Bonding wire is required to have excellent ball formability, ball bondability, wedge bondability, loop formability, etc., and Au (gold) bonding wire has been mainly used to comprehensively satisfy these requirements. However, since Au is expensive, a low-cost wire material to replace Au is required, and Ag (silver) bonding wire (hereinafter referred to as Ag bonding wire) has been proposed as one such material.

[0003] For example, Patent Document 1 proposes an Ag bonding wire in which the Pd and Pt contents and the P, Cr, Zr, and Mo contents are optimized to ensure bonding reliability in high-temperature environments even when using molded resins with a high S (sulfur) content and to suppress chip damage during ball bonding.

[0004] Patent Documents 2 and 3 propose Ag-Pd-Pt ternary Ag bonding wires for use with ultra-high frequency signals (several GHz band).

[0005] Patent Document 4 proposes an Ag bonding wire containing one or more of In, Ga, and Cd in order to satisfy bonding reliability, spring performance, and chip damage performance.

[0006] Patent Document 5 proposes an Ag bonding wire containing one or more of Be, B, P, Ca, Y, La, and Ce, and one or more of In, Ga, and Cd, in order to achieve the bonding strength of the ball bonding portion required for high-density mounting while ensuring bonding reliability and to improve the leaning characteristics of the upright part of the ball.

[0007] International Publication No. 2021 / 205931 Japanese Patent Application Laid-Open No. 2014-201797 Japanese Patent Application Laid-Open No. 2014-222725 Japanese Patent No. 5839763 International Publication No. 2017 / 187653

[0008] In response to the trend toward higher integration of semiconductor devices and the need for higher density packaging, bonding wires are required to have not only improved bonding reliability but also loop shape controllability. In particular, with Ag bonding wires, it has been found that during the process of melting the wire tip to form a ball (free air ball: FAB) for ball bonding to a chip electrode, some elements contained in the Ag wire are concentrated and precipitated on the FAB surface. When such elemental precipitation occurs on the FAB surface, the precipitates, although in small amounts, adhere to the capillary, gradually accumulating and forming a build-up at the capillary tip. It has been found that this build-up at the capillary tip induces wire non-adhesion and wire dislodgment, thereby reducing the productivity of the bonding process.

[0009] The present invention aims to provide an Ag bonding wire that maintains and improves the productivity of the bonding process by improving the build-up resistance of the capillary tip while ensuring the bonding reliability of the primary bonding (also called 1st bonding, FAB bonding, or ball bonding) and the secondary bonding (also called 2nd bonding or wedge bonding).

[0010] The present inventors have made extensive development efforts to achieve the above object and have obtained the following findings.

[0011] (A) Analysis of the built-up formed at the tip of the capillary revealed that it was an element contained in the Ag wire. It is believed that the elements (especially In) contained to improve the characteristics of the bonding wire locally concentrated on the surface of the FAB during the FAB formation process, and that this concentrated element adhered to the capillary tip when the FAB was pressed against the capillary tip during ball bonding (primary bonding). It was found that such deposits accumulated at the capillary tip after repeated bonding tens of thousands or hundreds of thousands of times, forming a built-up.

[0012] (a) In the process of improving Ag bonding wire by adding various elements to it, it was found that Sc, Ti, and Mn have the effect of suppressing the formation of build-up (especially build-up of In-based compounds) at the capillary tip. It was also confirmed that these elements do not deteriorate bonding wire properties (primary bond reliability, secondary bond (wedge bond) reliability, FAB shape controllability, and loop shape controllability).

[0013] The present invention was made based on these findings, and its gist is as follows. [1] A bonding wire for a semiconductor device, comprising: 0.05% to 3.0% by mass of In; 0.010% to 5.0% by mass in total of one or more of Pd, Pt, and Au; 0.0005% to 0.050% by mass in total of one or more of Sc, Ti, and Mn; 0 to 0.030% by mass in total of one or more of P, Zn, Bi, and Cu, with the balance consisting of Ag and inevitable impurities. [2] The bonding wire for a semiconductor device according to [1] above, comprising 0.0001% to 0.030% by mass in total of one or more of P, Zn, Bi, and Cu. [3] The bonding wire for a semiconductor device according to [1] or [2], wherein in a cross section perpendicular to the wire axis of the bonding wire, the ratio of the total length of the twin grain boundaries to the total length of the crystal grain boundaries is 40% or more.

[0014] The Ag bonding wire according to the present invention can maintain and improve productivity in the bonding process by suppressing the generation of built-up at the tip of the capillary while ensuring the bonding reliability of the primary bonding and secondary bonding.

[0015] The following description is based on one embodiment of the present invention (hereinafter simply referred to as the present invention). Unless otherwise specified, "%" for components indicates mass % in the wire. When no lower limit is specified or when the lower limit is 0%, this also includes the case where the component is not contained (0%). As mentioned above, in this specification, semiconductor device bonding wire may be referred to as bonding wire or simply as wire.

[0016] [In: 0.05% to 3.0%] In has the effect of improving the reliability of the primary bonding (ball (FAB) bonding) of Ag bonding wire. For example, when a HAST test (high temperature and high humidity test) was conducted in an atmosphere at a temperature of 130°C and a humidity of 85%, a wire containing and not containing In was evaluated. It was confirmed that the time until the shear strength (shear strength) of the ball bonded portion reached half of its initial strength was more than twice as long for the wire containing In compared to the wire not containing In. Therefore, the In content is set to 0.05% or more. The lower limit of the In content can be 0.10%, 0.15%, 0.20%, 0.30%, 0.40%, or 0.50%.

[0017] On the other hand, if the In content is too high, stress concentration occurs during ball bonding during the bonding process, making chip damage more likely. Furthermore, In is likely to concentrate on the FAB surface, which is thought to easily form In-based precipitate buildup at the tip of the capillary. Therefore, the In content is set to 3.0% or less. The upper limit of the In content can be 2.7%, 2.5%, 2.3%, 2.1%, or 1.9%.

[0018] [One or more of Pd, Pt, and Au: 0.010% to 5.0%] Pd, Pt, and Au have reduced sulfur (sulfur) adsorption compared to Ag (excellent sulfurization resistance). Therefore, sulfurization of the Ag bonding wire surface can be suppressed, and the floor life (usage life of the bonding wire) can be extended. Furthermore, these elements have a strong bond with In, and when contained in combination, they are effective in suppressing deterioration over time through a synergistic effect. Therefore, the total content of one or more of Pd, Pt, and Au is set to 0.010% or more. The lower limit of the total content of these elements can be 0.030%, 0.050%, 0.070%, 0.100%, 0.200%, 0.300%, 0.400%, or 0.500%.

[0019] On the other hand, if the content of Pd, Pt, and Au is too high, the resistivity increases, resulting in poor electrical and thermal conductivity, making it unsuitable for practical use. Therefore, the total content of one or more of Pd, Pt, and Au is set to 5.0% or less. The upper limit of the content can be 4.5%, 4.0%, 3.5%, 3.0%, or 2.5%.

[0020] [One or more of Sc, Ti, and Mn: 0.0005% to 0.050%] It has been confirmed that the inclusion of Sc, Ti, and Mn suppresses the formation of built-up at the tip of a capillary (particularly the build-up of In-based precipitates). While the mechanism is unclear, the inclusion of Sc, Ti, and Mn is thought to suppress the concentration of In on the FAB surface. Therefore, it is thought that the adhesion of In-based precipitates to the tip of the capillary is suppressed when the tip of the capillary is pressed against the FAB during the bonding process. Therefore, the total content of one or more of Sc, Ti, and Mn is set to 0.0005% or more. The lower limit of the content can be 0.0010%, 0.0020%, 0.0040%, 0.0070%, or 0.010%.

[0021] On the other hand, if the content of Sc, Ti, and Mn is high, the strength of the wire itself increases, and for example, the breaking strength during secondary joining (wedge joining) increases, making spring failure more likely to occur. Therefore, the total content of one or more of Sc, Ti, and Mn is set to 0.050% or less. The upper limit of the content can be 0.045%, 0.040%, 0.035%, or 0.030%.

[0022] [One or more of P, Zn, Bi, and Cu: 0% to 0.030%] P, Zn, Bi, and Cu do not necessarily have to be contained (they may be 0%). However, the inclusion of P, Zn, Bi, and Cu can improve the sphericity of the FAB shape. This is presumably because it affects the fluidity and wettability of the melt when the FAB melts and becomes spherical, improving the sphericity.

[0023] On the other hand, if the content of P, Zn, Bi, and Cu is high, the shape of the FAB deteriorates and the primary bondability (ball bondability) deteriorates. Therefore, it is preferable that the total content of one or more of P, Zn, Bi, and Cu is 0.030% or less. The upper limit of the content can be 0.026%, 0.022%, 0.018%, or 0.015%.

[0024] To ensure the effects of containing P, Zn, Bi, and Cu, the total content of one or more of P, Zn, Bi, and Cu should be 0.0001% or more. The lower limit of the content can be 0.0005%, 0.0010%, 0.0020%, 0.0030%, 0.0040%, or 0.0050%.

[0025] The remainder of the above components is Ag and impurities. Here, the term "impurities" refers to components that are mixed in due to various factors in the manufacturing process, including raw materials such as silver ingots, when industrially manufacturing Ag bonding wire, and are acceptable within a range that does not adversely affect the present invention.

[0026] [Ratio of total length of twin grain boundaries to total length of crystal grain boundaries in a cross section perpendicular to the wire axis: 40% or more] Twin grain boundaries generally have lower interfacial energy and are more stable than high-angle grain boundaries, making them less likely to migrate even at high temperatures. Therefore, even when solidifying from the molten state during primary bonding (ball bonding), the wire's crystal grains near the ball are more likely to be maintained. This reduces the formation of heat-affected zones due to FAB, thereby minimizing wire strength loss. This ensures the strength of the ball neck (upright portion) and improves leaning characteristics and loop shape controllability. Furthermore, twin grain boundaries, like normal high-angle grain boundaries, can block dislocation movement, and cross-slip occurs at their interfaces. Therefore, materials with many twin grain boundaries have superior ductility while maintaining the same strength as normal materials. Therefore, complex looping can be formed as intended. In other words, a high proportion of twin grain boundaries is effective for loop shape controllability for complex looping, such as low and high loops, which are required for high-density semiconductor packaging, and for leaning characteristics to avoid contact with adjacent wires. Therefore, in the Ag bonding wire of the present invention, the ratio of the total length of the twin grain boundaries (total length of the twin grain boundaries) to the total length of the crystal grain boundaries (total length of the crystal grain boundaries) in a cross section perpendicular to the central axis of the wire (central axis of the bonding wire) is preferably 40% or more. The ratio of the length of the twin grain boundaries to the length of the crystal grain boundaries is more preferably 50% or more, or 60% or more.

[0027] A cross section perpendicular to the central axis of the wire can be exposed by ion beam processing. The total length of the twin grain boundary and the total length of the crystal grain boundary can be determined by measuring the entire cross section of the wire using electron backscattered diffraction (EBSD). The EBSD method can identify the crystal grain boundary by determining the crystal orientation difference between adjacent measurement points. The crystal grain boundary is defined as a high-angle grain boundary when the orientation difference between adjacent crystal grains is 15 degrees or more, and the region surrounded by the high-angle grain boundary is defined as a single crystal grain. Among them, those in a specific positional relationship (a positional relationship rotated 60 degrees around the <111> axis) are twin grain boundaries, and are calculated together with the total length of the crystal grain boundary using dedicated analysis software (for example, OIM analysis manufactured by TSL Solutions, etc.). Then, the total length of the crystal grain boundaries in the entire cross section perpendicular to the central axis of the wire is taken as the total length of the crystal grain boundaries, and the total length of the twin grain boundaries is taken as the total length of the twin grain boundaries, and the ratio of the total length of the twin grain boundaries to the total length of the crystal grain boundaries is calculated.

[0028] [Method for Manufacturing Bonding Wire] An example of a method for manufacturing a bonding wire according to the present invention will be described. The method for manufacturing a bonding wire according to the present invention is not limited to the manufacturing method described below, and other manufacturing methods are also included in the bonding wire according to the present invention as long as they produce an Ag bonding wire that satisfies the requirements defined in the present invention.

[0029] Silver (Ag) with a purity of 4N to 5N (99.99% to 99.999%) is melted, and a predetermined amount of a predetermined element such as In is added, followed by continuous casting to obtain a cylindrical ingot. Next, this ingot is continuously drawn using a die. During this process, intermediate heat treatment at 350°C to 550°C and wire drawing are repeated until the wire reaches the final diameter. The wire drawing can be performed using a continuous wire drawing device equipped with multiple diamond-coated dies.

[0030] It is recommended to perform heat treatment (intermediate heat treatment) twice during the wiredrawing process. For example, by setting the area reduction rate at each stage of the wiredrawing process (one wiredrawing process) to 18% to 30% and performing intermediate annealing after one to three stages of wiredrawing, the grain boundary length of twins (annealing twins) in the wire can be increased.

[0031] The area reduction rate per stage is set to 18% or more, preferably 20% or more, and 30% or less, preferably 28% or less.

[0032] The intermediate annealing temperature is preferably 350°C to 550°C. Preferably, the first intermediate heat treatment temperature is 450°C to 550°C, and the second intermediate heat treatment temperature is 350°C to 450°C. It is also effective to set the next (second) heat treatment temperature lower than the previous (first) intermediate heat treatment temperature. This allows the length of the twin grain boundary to be increased.

[0033] Furthermore, after the final wire drawing, a final heat treatment may be performed. The temperature conditions for the final heat treatment may be set so that the breaking elongation of the wire falls within a predetermined range. The final heat treatment may be performed at a temperature in the range of 200 to 700°C for 5 seconds or less, preferably 3 seconds or less, or 2 seconds or less. The atmosphere for the heat treatment may be, for example, an inert gas such as nitrogen gas or Ar gas, or forming gas (5% H ２ -N ２ It is preferable to use a non-oxidizing atmosphere such as

[0034] The following examples are described. The raw material Ag used had a purity of 99.99% or higher, with the remainder consisting of inevitable impurities. The added elements, such as In, also had a purity of 99.9% or higher, with the remainder consisting of inevitable impurities.

[0035] Ag bonding wires were manufactured having the component compositions shown in Table 1. To obtain the Ag alloys used for each bonding wire, predetermined amounts of raw materials such as Ag and In were loaded into a cylindrical carbon crucible, and the crucibles were heated in a vacuum or under N 2 atmosphere using a high-frequency furnace. ２ The mixture was melted by heating to 1080°C to 1600°C in an inert atmosphere such as Ar gas, and then continuously cast to obtain Ag alloy ingots with diameters of 4 to 6 mm.

[0036] The Ag alloy ingot was then drawn continuously using a die to produce wires with diameters of 200 μm to 600 μm. This process is called the primary wiredrawing process.

[0037] Thereafter, wire was repeatedly drawn to obtain a wire with a final diameter of φ15 μm to φ25 μm. A commercially available lubricant was used for the wire drawing, and the wire feed speed during drawing was 100 m / min to 600 m / min. This process is called the secondary wiredrawing process. Two intermediate heat treatments were performed between the secondary wiredrawing processes (first: 450°C to 550°C, second: 350°C to 450°C). For the intermediate heat treatments, the wire was continuously passed through an Ar gas atmosphere. The wire feed speed during the intermediate heat treatments was 20 m / min to 120 m / min.

[0038] The grain size and the proportion of twin grains were adjusted by adjusting the area reduction rate and wire feed speed in the secondary wiredrawing process, the wire diameter during intermediate heat treatment, and the intermediate heat treatment temperature, and as a result, the ratio of the total length of twin grain boundaries to the total length of crystal grain boundaries was adjusted.A higher area reduction rate, a faster wire feed speed, a thinner wire diameter during intermediate heat treatment, and a higher temperature tend to increase the proportion of twin grain boundaries.

[0039] After the secondary wiredrawing, a final heat treatment was carried out so that the wire's breaking elongation would be approximately 9 to 25%. The wire feed rate during the final heat treatment was 50 m / min to 150 m / min, the final heat treatment temperature was 300°C to 550°C, and the heat treatment time was 0.2 to 1.0 seconds. In this way, the wire samples listed in Table 1 were prepared.

[0040] [Evaluation of Primary Bonding Reliability] Samples for evaluating the primary bonding reliability were manufactured as follows. Each wire sample was ball-bonded to an electrode formed of a 1.0 μm thick aluminum (Al) film on a silicon substrate on a general lead frame using a commercially available wire bonder, and then sealed with a commercially available epoxy resin (semiconductor sealant). ２ +5% H ２ The gas was supplied at a flow rate of 0.4 to 0.6 L / min, and the ball diameter was adjusted to be 1.5 to 1.6 times the wire diameter.

[0041] The bonding reliability was evaluated by a high temperature, high humidity test (HAST). For the evaluation, the bonding reliability evaluation sample was exposed to a high temperature, high humidity environment at a temperature of 130°C and a relative humidity of 85% using an unsaturated pressure cooker tester, and a bias of 5 V was applied. The bonding life of the ball bond was measured by conducting a shear test on the ball bond every 48 hours, and the time until the shear strength value became half (1 / 2) of the initial shear strength. The shear test was performed after removing the resin by acid treatment to expose the ball bond.

[0042] The shear tester used for the HAST evaluation was a DAGE tester. The shear strength value was calculated by arithmetically averaging the measurements at 10 randomly selected ball joints. A joint life of 288 hours or more was rated as "excellent," 144 hours or more but less than 288 hours as "good," 96 hours or more but less than 144 hours as "passable," and a joint life of less than 96 hours as "fail." The evaluation results are shown in Table 1.

[0043] [Secondary Bond Reliability Evaluation (Wedge Bond Pull Strength)] Samples for evaluating secondary bond reliability were manufactured as follows. Each wire sample was wedge bonded onto an Ag-plated Cu alloy lead frame using a commercially available wire bonder. A total of 320 wedge bonds were manufactured at 80 locations in four directions (four directions clockwise: 0°, 90°, 180°, and 270°, with an arbitrary direction as viewed from above the lead frame as the reference direction (0°)). The evaluation samples were manufactured using wires immediately after manufacturing, wires stored in a clean room at room temperature for one week, and wires stored in a clean room at room temperature for two weeks. A pull test of the wedge bonds was performed on each sample.

[0044] The pull strength value was calculated by randomly selecting 15 pieces in each direction and calculating the arithmetic mean of the measurements for a total of 60 pieces in the four directions. A pull strength of 4 gf or more was rated as "excellent," a pull strength of 3 gf or more but less than 4 gf was rated as "good," and a pull strength of less than 3 gf was rated as "fail." The evaluation results are shown in Table 1.

[0045] [Productivity Evaluation: Continuous Bonding Evaluation, Observation of Capillary Tip] Continuous bonding was evaluated by using each wire sample to bond 200,000 times continuously onto an Ag-plated Cu alloy frame using a commercially available wire bonder. Continuous bonding was evaluated based on the number of times the device stopped during the 200,000 bonding runs. A device with zero device stoppages was rated "excellent," one or two device stoppages was rated "good," and a device with three or more device stoppages was rated "fail." The evaluation results are shown in Table 1.

[0046] Next, after 200,000 consecutive bondings, the face surface of the capillary tip was observed with an SEM, and the degree of built-up adhesion was visually evaluated. On the entire face surface of the capillary tip, if no built-up (deposit) product was visible or only a few scattered build-up products were evaluated as "excellent," if the product was present on less than 1 / 4 of the face surface, it was evaluated as "good," and if the product was present covering 1 / 4 or more of the face surface, it was evaluated as "fail." The evaluation results are shown in the "Capillary Observation" column of Table 1.

[0047] [FAB Shape Evaluation] Regarding the FAB shape, balls (FABs) for ball bonding were formed using each wire sample with a commercially available wire bonder, and the FAB shapes were observed in this state using an SEM. A total of 50 FABs were formed and evaluated. Spherical FABs were rated as good, and eccentric or irregularly shaped FABs were rated as bad. FABs with zero defects out of the 50 were rated as "excellent," those with one to two defects as "good," and those with three or more defects as "fail." The evaluation results are shown in Table 1.

[0048] [Evaluation of Loop Shape Controllability] The loop shape controllability was evaluated in terms of low loop shape controllability and leaning characteristics. Low loop shape controllability was evaluated by bonding 100 wire samples of each wire to an evaluation lead frame with a loop length of 1 mm and a loop height of 60 μm using a commercially available wire bonder. Next, the presence or absence of neck damage at the ball bond was evaluated using a scanning electron microscope (SEM). Neck damage was judged to be present if cracks occurred in the neck portion of the ball bond or if the neck portion was deformed and the wire became thinner. If three or more of the 100 wires had neck damage, they were rated as "fail," two were "pass," one was "good," and zero was "excellent." Good and excellent were considered pass marks.

[0049] The leaning characteristics were evaluated using 100 wire samples bonded to a lead frame for evaluation using a commercially available wire bonder with a loop length of 5 mm and a loop height of 0.5 mm. The evaluation method involved observing the upright portion of the wire from the horizontal direction of the chip and evaluating the maximum distance (leaning distance) between the upright portion of the wire and a perpendicular line passing through the center of the ball bond. If the leaning distance was smaller than the wire diameter, the leaning was considered good, but if it was larger, the upright portion was tilted and the leaning was considered poor.

[0050] Regarding loop shape controllability, out of 100 evaluations of low loop shape controllability and leaning interval evaluation, those with zero neck damage and zero leaning defects were rated as "excellent," those with one neck damage or one to three leaning defects were rated as "good," those with two neck damage or four to five leaning defects were rated as "passable," and those with three or more neck damage or six or more leaning defects were rated as "fail." The evaluation results are shown in Table 1.

[0051] As can be seen from Table 1, the Ag bonding wire according to the present invention has good primary bonding reliability and secondary bonding reliability, suppresses built-up at the capillary tip, and has excellent continuous bonding properties. This confirms that the Ag bonding wire according to the present invention does not impede the productivity of the bonding process. Furthermore, it was confirmed that the Ag bonding wire according to the present invention has excellent FAB shape and loop shape controllability. In other words, it was confirmed that the Ag bonding wire according to the present invention can maintain and improve productivity while maintaining bonding reliability.

[0052]

[0053] The present invention can be used in the semiconductor industry.

Claims

1. A bonding wire for semiconductor device comprising: 0.05% by mass to 3.0% by mass of In; a total of 0.010% by mass to 5.0% by mass of one or more of Pd, Pt, and Au; a total of 0.0005% by mass to 0.050% by mass of one or more of Sc, Ti, and Mn; a total of 0% by mass to 0.030% by mass of one or more of P, Zn, Bi, and Cu; and the remainder being Ag and unavoidable impurities.

2. A bonding wire for a semiconductor device as described in claim 1, containing one or more of P, Zn, Bi, and Cu in a total amount of 0.0001 mass % to 0.030 mass %.

3. A bonding wire for a semiconductor device as described in claim 1 or 2, wherein in a cross section perpendicular to the wire axis of the bonding wire, the ratio of the total length of twin grain boundaries to the total length of crystal grain boundaries is 40% or more.

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