JP2011044729A5 - - Google Patents

Description

Bonding wires for semiconductor devices

  The present invention relates to a bonding wire for a semiconductor device used for connecting between an electrode on a semiconductor element and an external terminal.

  Currently, thin wires (bonding wires) with a wire diameter of about 20 to 50 μm are used as bonding wires for bonding between electrodes on semiconductor elements and external terminals that are wiring of circuit wiring boards (lead frames, substrates, tapes, etc.). It is mainly used. Bonding wires are generally joined by ultrasonic thermocompression bonding, and a general-purpose bonding apparatus, a capillary jig used for connection through the bonding wire, or the like is used. After the wire tip of the bonding wire is heated and melted by arc heat input to form a ball by surface tension, this ball part is crimped and bonded onto the electrode of the semiconductor element heated within a range of 150 to 300 ° C. Then, the bonding wire is directly bonded to the external lead side by ultrasonic pressure bonding.

  In recent years, the structure, materials, connection technology, etc. of semiconductor packaging have been diversified rapidly. For example, in addition to the QFP (Quad Flat Packaging) that uses the current lead frame, the packaging structure uses a substrate, polyimide tape, etc. New forms such as BGA (Ball Grid Array) and CSP (Chip Scale Packaging) have been put into practical use, and bonding wires with improved loop characteristics, bonding properties, mass production usability, etc. have been demanded.

  Narrow pitches are being made to reduce the spacing between adjacent bonding wires. As a request for a bonding wire corresponding to this, thinning, high strength, loop control, improvement in bonding property, etc. are required. The loop shape has become complicated due to the high density of semiconductor packaging. As the loop shape classification, the loop height and the wire length (span) of the bonding wire are used as indices. In the latest semiconductors, there are an increasing number of cases in which conflicting loop formations such as a high loop and a low loop, a short span and a long span are mixedly mounted in one package. To achieve this with a single type of bonding wire, strict material design of the bonding wire is required.

  Conventionally, gold of high purity 4N type (purity> 99.99 mass%) has been mainly used as a material for the bonding wire. In order to improve characteristics such as high strength and high bonding, a small amount of alloy elements are adjusted. Recently, a gold alloy wire having a purity of 2N (purity> 99%) with an additive element concentration increased to 1% or less has been put into practical use for the purpose of improving the reliability of the joint. By adjusting the type and concentration of the alloy element added to gold, it is possible to increase the strength and control the reliability. On the other hand, the alloying may cause adverse effects such as a decrease in bondability and an increase in electrical resistance, and it is difficult to comprehensively satisfy various characteristics required for bonding wires.

  Further, since gold is expensive, other types of metals having a low material cost are desired, and a bonding wire made of copper having a low material cost and excellent electrical conductivity has been developed. However, copper bonding wires have problems in that the bonding strength is reduced due to the oxidation of the wire surface, and that the wire surface is easily corroded when sealed with a resin. These are also the reasons why the practical application of copper bonding wires has not progressed.

  All the bonding wires put to practical use so far have a single-layer structure. Even if the material changed, such as gold or copper, the alloy element was uniformly contained inside, and the wire cross section of the bonding wire was a single-layer structure. In some cases, a thin natural oxide film, an organic film for surface protection, or the like is formed on the wire surface of the bonding wire, but these are also limited to the extremely thin region (up to several atomic layers level) on the outermost surface.

  In order to respond to various needs required for bonding wires, a bonding wire having a multilayer structure in which another metal is coated on the wire surface has been proposed.

  As a method for preventing surface oxidation of a copper bonding wire, Patent Document 1 proposes a bonding wire in which copper is coated with a noble metal such as gold, silver, platinum, palladium, nickel, cobalt, chromium, titanium, or a corrosion-resistant metal. . In addition, from the viewpoints of ball formability, prevention of deterioration of the plating solution, and the like, Patent Document 2 describes a core material mainly composed of copper, a dissimilar metal layer made of a metal other than copper formed on the core material, and There has been proposed a bonding wire formed on the dissimilar metal layer and having a coating layer structure made of an oxidation-resistant metal having a melting point higher than that of copper. Patent Document 3 has a core material containing copper as a main component, and an outer skin layer containing copper and a metal different from one or both of the core material and its component or composition on the core material. A bonding wire that is a thin film having a thickness of 0.001 to 0.02 μm has been proposed.

  Also, many multi-layer structures have been proposed for gold bonding wires. For example, Patent Document 4 proposes a bonding wire in which the outer peripheral surface of a core wire made of high-purity Au or Au alloy is covered with a coating material made of high-purity Pd or Pd alloy. Patent Document 5 proposes a bonding wire in which the outer peripheral surface of a core wire made of high-purity Au or Au alloy is covered with a coating material made of high-purity Pt or Pt alloy. Patent Document 6 proposes a bonding wire in which the outer peripheral surface of a core wire made of high-purity Au or Au alloy is coated with a coating material made of high-purity Ag or Ag alloy.

  As the wire characteristics of bonding wires used in mass production, the loop control in the bonding process is stable, the bondability is improved, the deformation of the bonding wire is suppressed in the resin sealing process, and the long-term reliability of the connection part It is desired that high-density mounting such as state-of-the-art narrow pitch and three-dimensional wiring can be achieved by satisfying comprehensive characteristics such as performance.

  In connection with ball bonding, it is important to form a ball with good sphericity at the time of ball formation and to obtain sufficient bonding strength at the bonding portion between the ball portion and the electrode. Further, in order to cope with a decrease in the bonding temperature, a thinning of the bonding wire, and the like, a bonding strength, a tensile strength, and the like at a portion where the bonding wire is wedge-connected to the wiring portion on the circuit wiring board are also required.

  The surface property of the wire often affects the use performance, and for example, the occurrence of scratches or scrapes becomes a problem in mass production use. There is a risk of electrical short-circuiting with an adjacent bonding wire due to scraping, and scratches cause damage to bonding wire quality and reliability, such as bonding wire manufacturing yield and wire deformation during resin sealing. In addition, by pursuing the stability of loop shape control and improving the bonding property at a low temperature, it is impossible to put it to practical use unless it is able to adapt to the strict requirement of managing the defect occurrence rate in the semiconductor manufacturing process on the order of ppm.

  Although such a bonding wire having a multilayer structure for semiconductors is expected to be put into practical use, it has not been put into practical use so far. While surface modification and high added value are expected due to the multi-layer structure, bonding wire productivity and quality, yield in the bonding process, performance stability, and long-term reliability when using semiconductors are comprehensive. It must be satisfied.

JP-A 62-97360 JP 2004-64033 A JP 2007-12776 Japanese Unexamined Patent Publication No. 4-79236 Japanese Patent Laid-Open No. 4-79240 JP-A-4-79242

  In conventional bonding wires with a single layer structure (hereinafter referred to as single layer wires), the addition of alloying elements is effective to improve the tensile strength, joint strength, reliability, etc. There are concerns about the limits. A bonding wire having a multilayer structure (hereinafter referred to as a multi-layer wire) is expected to further improve characteristics and increase added value as compared to a single-layer wire. For example, in order to prevent the surface of a copper bonding wire from being oxidized, it is possible to coat the surface of the wire with a noble metal or an oxidation-resistant metal as a multi-layered wire that provides high functionality. Even a gold bonding wire is expected to have an effect of reducing the resin flow by coating the wire surface with a high-strength metal or alloy.

  However, the present inventors have evaluated the semiconductor packaging in consideration of needs for higher density, smaller size, thinner thickness, etc., and as a result, there are many practical problems that will be described later in multilayer wires. It has been found.

  For multi-layer wires, scratches, scrapes, etc. on the wire surface in the state used to connect the final wire product or semiconductor element due to wire drawing in the wire manufacturing process and complicated loop control in the wire bonding process This is a problem. For example, a flaw on the surface of the wire may cause a sub-micron groove, and in the example of shaving failure, scraping of canna scraps occurs in the wire longitudinal direction of the multilayer wire, and the shaving length is several hundred. It may be up to μm. Due to surface scratches and scraping, the loop shape becomes unstable, the multi-layered wire is damaged and its strength is reduced, and when scraped scrapes come into contact with the adjacent multi-layered wire, a short circuit failure occurs, causing practical trouble. Cause.

  The frequency of defects related to surface scratches and scraping increases as the wire diameter decreases, which is disadvantageous for narrow-pitch connections. In addition, loop control is complicated because high loops and low loops are mixed. As it rises, it becomes difficult to adapt to 3D connections. The frequency of surface flaws tends to increase when forming a low loop. If these defects are not reduced, there is a concern that the practical range of the multilayer wire is limited.

  In addition to such direct defects, there are concerns about indirect defects due to the occurrence of scratches and scraping on the wire surface, or a decrease in yield. For example, even if scratches and scrapes once generated in the process of manufacturing a multi-layer wire cannot be detected in the final product, the thickness of the skin layer becomes uneven or internal cracks remain, resulting in a loop shape. May be unstable. In addition, there are cases where scratches, scraping, etc. occur on the back side of the loop, which is difficult to detect by loop appearance inspection with an optical microscope in the mass production process. Even if it is difficult to directly recognize the causal relationship with scratches and shavings, these defects adversely affect the production yield.

  Surface scratches, scraping frequency, and phenomena vary depending on the skin layer material, but no sufficient countermeasures have been taken so far. The frequency of occurrence of multi-layer wires is often higher than that of single-layer wires. This is because multi-layer wires impose stresses and strains on the skin layer in the process of forming loops, and the wire manufacturing process. Differences in conditions are considered to be involved.

  When a loop is formed with a multi-layer wire, the linearity of the loop may be reduced, and problems such as a fall, sagging, or bending of the multi-layer wire may occur. As the linearity of the loop is lowered, there is a problem that the manufacturing yield is lowered.

Typical problems of the ball joint portion of the multilayer wire are a petal phenomenon and a misalignment phenomenon. The petal phenomenon is a phenomenon in which the vicinity of the outer periphery of the ball joint part is deformed in a petal shape and deviates from roundness, and when joining on a small electrode, the ball protrudes or the joint strength is reduced. Cause failure. The misalignment phenomenon is a phenomenon in which the ball portion formed at the tip of the wire is formed asymmetrically with respect to the wire axis and becomes, for example, a golf club shape. When the misalignment ball is joined in a narrow pitch connection, There is a problem of causing short circuit contact. The frequency of occurrence of the petal phenomenon and the misalignment phenomenon in these multi-layer wires tends to increase more than that of single-layer wires, and this is one factor that causes a reduction in productivity. Therefore, it is necessary to tighten the management standards of the wire bonding process.

In the multilayer copper wire, the effect of delaying the oxidation can be expected as compared with the single layer copper wire, but the effect varies greatly depending on the composition, structure, thickness, etc. in the skin layer or near the wire surface. It is important to optimize the structure of the multilayer copper wire. In order to ensure workability equivalent to that of gold wire, it is necessary to ensure that, for example, wedge bondability, loop shape, and the like do not deteriorate even after storage in the atmosphere for about two months. This requires an improvement in the life of several tens of times compared with the shelf life of the single-layer copper wire, and considerably strict conditions are required for materials mainly composed of copper.

  The present invention solves the above-mentioned problems of the prior art, and in addition to the conventional basic performance, it aims to improve the performance of the wire surface such as scratches and scrapes, stabilization of the loop shape, and good ball formation. Another object of the present invention is to provide a bonding wire for a semiconductor device.

  As a result of studying a bonding wire having a multi-layer structure in order to solve the problems such as surface scratches and scraping of the wire, the present inventors have been able to control the structure of the skin layer in a specific skin layer. I found it effective.

  This invention is made | formed based on the said knowledge, and makes it the summary of the following structures.

A bonding wire for a semiconductor device according to claim 1 of the present invention is for a semiconductor device having a core material made of a conductive metal and a skin layer mainly composed of a metal different from the core material on the core material. A bonding wire, wherein the metal of the skin layer is a face-centered cubic crystal, the thickness of the skin layer is in the range of 0.005 to 0.2 μm, and the crystal in the wire longitudinal direction on the crystal plane of the surface of the skin layer of orientation <hkl>, the wire angle difference with respect to the longitudinal direction Ri der orientation ratio is more than 50% of the <111> containing up to within 15 °, between the core and the skin layer, the It has an intermediate metal layer made of a component different from the main component constituting the skin layer and the core material .

A bonding wire for a semiconductor device according to a second aspect of the present invention is the bonding wire for a semiconductor device according to the first aspect, wherein an angle difference with respect to the longitudinal direction of the wire is included in the crystal orientation <hkl> in the longitudinal direction of the wire on the crystal plane of the cross section of the core material. The total of the orientation ratios of <111> and <100> including up to 15 ° is 30% or more.

The semiconductor device bonding wire according to claim 3 of the present invention, in claim 1 or 2, the crystal orientation of the wire longitudinal direction of the surface of the skin layer is the area of the crystal grains is <111>, the wire surface It is characterized by being 30% or more as a percentage of the total area.

A bonding wire for a semiconductor device according to a fourth aspect of the present invention is characterized in that, in any one of the first to third aspects, the main component constituting the skin layer is any one of Pd, Pt, Ru, and Ag. .

A bonding wire for a semiconductor device according to a fifth aspect of the present invention is the bonding wire for a semiconductor device according to the first to fourth aspects, wherein a diffusion layer having a concentration gradient of main components of the outer skin layer and the core material is provided between the outer skin layer and the core material. It is characterized by having.

The bonding wire for a semiconductor device according to a sixth aspect of the present invention is the bonding wire for a semiconductor device according to the fourth or fifth aspect , wherein the main component constituting the core material is Cu and a total amount of one or more of B, Pd, Bi, P, and Zr is 5 It is characterized by containing in the range of ~ 300ppm.

A bonding wire for a semiconductor device according to a seventh aspect of the present invention is the bonding wire for a semiconductor device according to the fourth or fifth aspect , wherein the main component constituting the core material is Cu and Pd is contained in a range of 5 to 10,000 ppm to constitute the skin layer. The main component is Pd or Ag.

A bonding wire for a semiconductor device according to an eighth aspect of the present invention is the bonding wire for a semiconductor device according to the fourth or fifth aspect , wherein the main component constituting the core material is Au, and the total amount of at least one of Be, Ca, Ni, Pd, and Pt is 5 It is characterized by containing in the range of ~ 8000ppm.

  With the bonding wire for a semiconductor device according to the present invention, the surface properties can be improved by suppressing scratches and scraping on the surface of the wire. In addition, the linearity of the loop and the stability of the loop height can be improved. Moreover, stabilization of the joining shape of the bonding wire for semiconductor devices can be promoted. As a result, it is possible to provide a high-performance bonding wire for a semiconductor device that can be applied to the latest semiconductor mounting technologies such as thinning, narrowing, long span, and three-dimensional mounting.

EBSP measurement result of multi-layered bonding wire (wire diameter 25μm) (colored region oriented in <111> direction in the longitudinal direction of wire. Crystal grain boundaries are shown as lines)

  A bonding wire for a semiconductor device (hereinafter referred to as a bonding wire) is composed of a core material made of a conductive metal and a skin layer mainly composed of a face-centered cubic metal different from the core material on the core material. As a result of studying, the inclusion of a conductive metal in the vicinity of the surface of the bonding wire can be expected to improve wedge bondability, but on the other hand, wire drawing in the wire manufacturing process and complicated loops in the wire bonding process It has been found that the occurrence of scratches, scraping, etc. on the wire surface in control and the like becomes a problem and the stability of the loop shape is not sufficient.

  Therefore, as a result of examining multi-layered bonding wires that can respond to new needs such as narrow pitch connection, strict loop control of 3D connection, improvement of yield in wire drawing processing of thin wires, etc. It has been found that it is effective to control the tissue of the epidermis layer. In particular, by controlling the specific crystal orientation by paying attention to the relationship between the surface texture of multi-layered wire and the performance of wire bonding, which has been hardly known until now, workability, bondability, and loop control are controlled. It was confirmed for the first time that comprehensive improvement of sex and so on was possible. Furthermore, it has been found that the control of the combination of the structure of the skin layer and the core material is effective.

That is, a bonding wire having a core material made of a conductive metal and a skin layer having a face-centered cubic metal as a main component different from the core material on the core material, the surface of the skin layer It is necessary that the bonding wire be characterized in that 50% or more of the crystal orientation <hkl> in the wire longitudinal direction on the crystal plane is <111>. If it is this bonding wire, the high effect which suppresses the generation | occurrence | production of the flaw of a wire surface in the wire drawing process in a wire manufacturing process, the complicated loop control in a wire bonding process, etc. and a chip | tip will be acquired.

  If the component of the skin layer is a face-centered cubic metal, there is no yield drop during processing, good workability, and easy adaptation to complicated processing such as wire drawing and loop control, bending, etc. .

Among the crystal orientation <hkl> in the wire longitudinal direction of the bonding wire, 50% or more is <111>, which makes it difficult to achieve both the surface hardness, workability, bending resistance, etc. of the skin layer. Comprehensive improvement is possible, and as a result, scratches and scrapes on the wire surface can be suppressed. The <111> orientation of the face-centered cubic metal is the densest direction, and the more the <111> orientation gathers in the skin layer , the more the surface mechanical properties tend to improve.For example, the hardness increases and elastic deformation occurs. Strong, highly resistant to plastic deformation and toughness is enhanced. If the proportion of the <111> orientation in the crystal orientation is 50% or more, a sufficient effect of suppressing flaws and scraping on the wire surface can be obtained. Preferably, if the ratio of the <111> orientation is 60% or more, the effect of suppressing the cutting is enhanced, and even a long span having a wire length of 5 mm or more can be cut and scratches can be reduced. More preferably, if it is 70% or more, the effect of suppressing scratches is further enhanced. For example, even in a low loop with a loop height of 65 μm or less, scratches and scraping can be suppressed and stable loop formation becomes possible.

In the multilayer wire, the skin layer and the core material are composed of different components. Therefore, it is relatively easy to separate and control the tissue of the skin layer covering the wire surface. The effect of improving the characteristics by controlling the surface texture is also high. In this respect, it differs from conventional single layer wire tissue control. A single-layer wire can manage the texture and crystal orientation of the entire wire, but it is difficult to control the structure separately from the inside of the wire only in the vicinity of the surface. Therefore, the structure control of the skin layer of the multilayer wire requires a unique concept of the multilayer wire, and the management of the texture and crystal orientation in the wire cross section of the single layer wire cannot be applied.

If the total proportion of <111> and <100> in the crystal orientation <hkl> in the wire longitudinal direction in the crystal plane of the surface of the skin layer is 60% or more, the variation in loop height is reduced, The loop can be controlled stably at high speed. In a normal wire connection process, a complicated operation is performed such that a bonding wire passing through a hole in a capillary is drawn out or pulled back. This is an operation in which the bonding wire enters and exits at a considerably high speed on the order of several tens of milliseconds. Although the specific effects and relationships of the <111> and <100> orientations are not clear, it is thought that the loop height is stabilized by lowering the sliding resistance between the bonding wire and the capillary. In other words, to stabilize such slidability and loop height, it is effective to keep crystal orientations other than <111> and <100> low. If the total proportion of <111> and <100> in the skin layer is 60% or more, a high effect of stabilizing the loop height can be obtained with a general span with a wire length of 3 mm or less. Preferably, if it is 80% or more, a high effect of stabilizing the loop height can be obtained even with a long span having a wire length of 5 mm or more. In addition, as an effect of increasing the ratio of <111> and <100>, the thickness of the skin layer can be made uniform by suppressing the non-uniformity of the film thickness in the post-deposition processing and heat treatment processes. Is also advantageous.

  If the total ratio of <111> and <100> in the crystal orientation <hkl> in the wire longitudinal direction in the cross-section of the core material is 15% or more, abnormal deformation in which the ball joint is greatly deviated from the perfect circle The defect which generate | occur | produces can be suppressed. This abnormal deformation is one of the most problematic forms of ball bonding, such as causing a direct electrical short with adjacent electrodes. As a standard for determining the abnormal deformation, an elliptical shape in which the ratio of the major axis size to the minor axis size of the ball joint portion is 1.3 times or more is determined to be defective. Even if it occurs suddenly at a low rate, wire bonding productivity is hindered. Preferably, if the sum of the proportions of <111> and <100> is 30% or more, small petal failure in which the vicinity of the outer periphery of the ball joint is unevenly deformed is reduced, and the ball joint is perfectly round. It can be stabilized close to. If the roundness is good, it is advantageous for reduction of the bonding area, manufacturing management of the bonding process becomes easy, or productivity of narrow pitch bonding can be improved. The solidification structure of the ball part largely reflects the structure of the core material, and it was confirmed that it was effective to increase the proportion of <111> and <100> in the crystal orientation <hkl> of the core material. . Regarding the control of the crystal orientation of the core material, it was confirmed that a single layer wire is not effective enough, but a multi-layer wire can provide a high effect. Although this factor has not been fully elucidated, the effect of the structure of the core material on the structure of the ball part is due to the stepwise melting of the multilayer wire first with the skin layer and then with the core material. Is presumed to be large. This effect has been confirmed to be higher in the case of a normal ball size. For example, when joining normal size balls with a ratio of initial ball diameter / wire diameter of 1.9 to 2.2, it is possible to improve roundness by reducing shape defects such as anisotropy and petal shape at the ball joint. . As a result of investigating the deformation behavior of the ball portion by compressive deformation and application of ultrasonic waves, it was confirmed that the ball bond shape had a small correlation with the structure of the skin layer, but rather the structure of the core material was dominant. Here, if the total proportion of <111> and <100> in the core material is less than 15%, the frequency of occurrence of abnormal deformation at the time of ball bonding is high, and if less than 30%, When the balls are joined, the frequency of petal-like or elliptical deformation increases, which may be defective. The influence of the wire structure on the ball deformation is more noticeable in the multilayer wire and is often different from the influence of the structure of the single-layer wire. Preferably, if the total proportion of <111> and <100> in the core material is 50% or more, the bonded shape of the small-diameter ball can be stabilized. For example, when joining small diameter balls whose initial ball diameter / wire diameter ratio is in the range of 1.5 to 1.7, by improving the roundness of the ball joint, it is also good for narrow pitch joining with electrode spacing of 40 μm or less A ball-joint shape can be obtained. There is no particular upper limit to the total proportion of <111> and <100> in the core material, but if it is 85% or less, there is an advantage that control during production is relatively easy.

A synergistic effect can be expected by combining the structure of the core material and the structure of the skin layer described above, and it becomes possible to simultaneously improve the control of the loop shape and the stabilization of the ball deformation. That is, 50% or more of the crystal orientation <hkl> in the wire longitudinal direction in the crystal plane of the surface of the skin layer is <111>, and the crystal orientation <hkl> in the wire longitudinal direction in the cross section of the core material It is desirable that the bonding wire has a multilayer structure in which the proportion of <111> and <100> is 40% or more. This is a typical example of three-dimensional mounting, which is a multilayer chip connection in which a plurality of chips are stacked, BGA, and CSP, which are recently used in multi-stages where the loop height of adjacent bonding wires is greatly different in the range of 60 to 500 μm. The overall characteristics of the bonding wire can be improved in connection (Multi-Tier Bonding) or the like.

  In the description so far, the actions and effects exerted by the proportion of a specific orientation are described with reference to the measurable crystal orientation. If thinning progresses to adapt to narrow pitches in the future, the effect of the surface will increase, and the effect of crystal orientation will be more accurately understood based on the surface of the bonding wire. Is possible.

Specifically, it is a multi-layer bonding wire in which 50% or more of the crystal orientation <hkl> in the longitudinal direction of the wire on the surface of the skin layer is <111>, and further on the surface of the skin layer. It is desirable that the area of the crystal grains whose crystal orientation in the longitudinal direction of the wire is <111> is 30% or more as a ratio with respect to the total area of the wire surface. As a result, the effect of stabilizing the loop shape is enhanced, and in particular, even with the bonding wire thinned to a diameter of 22 μm or less, the loop characteristics are stabilized and effective in reducing scratches and scraping. For thin bonding wires with a wire diameter of 22 μm or less, the area of crystal orientation that is difficult to measure increases due to increased wire strain, etc. There is a tendency that the area where the loop characteristics cannot be accurately grasped increases. Therefore, if the area of the <111> crystal grains on the surface of the skin layer is set to an appropriate ratio (appropriate area ratio) with respect to the total area of the wire surface (area ratio), good characteristics can be obtained even with thin wires. The reason why the area ratio is 30% or more is that when the area ratio is less than 30% even if 50% or more of the crystal orientations are <111>, a narrow pitch connection using a bonding wire having a wire diameter of 22 μm or less Then, scratches, scrapes, etc. may not be suppressed. Preferably, when the area ratio is 40% or more, even when a thin wire having a diameter of 18 μm or less is connected, scratches and scraping during loop formation can be reduced. More preferably, if the area ratio is 50% or more, the effect of suppressing flaws and scraping with fine wires having a diameter of 18 μm or less is further enhanced, which is advantageous for narrow pitch connection with a pitch of 40 μm or less.

A bonding wire with a multi-layer structure in which 50% or more of the crystal orientation <hkl> in the longitudinal direction of the wire on the surface of the skin layer is <111>, and the average size of the crystal grains on the surface of the skin layer If the bonding wire has a multilayer structure, the ratio of the average size in the longitudinal direction of the wire to the average size in the circumferential direction (aspect ratio of the crystal grain size) is 3 or more, a loop-formed bonding wire The linearity of can be improved. When forming a loop by exiting or returning from the hole at the tip of the capillary, the bonding wire may fall down due to friction with the inner wall of the capillary, curl failure due to bending, bonding failure due to sagging, etc. As a result, the yield decreases. In order to suppress such defects and improve the loop linearity, it has been found effective to increase the aspect ratio of the crystal grain size on the surface of the skin layer. When the aspect ratio is increased, crystal grains long in the wire longitudinal direction form a fibrous structure, which is advantageous in reducing distortion and deformation variation remaining on the bonding wire during loop formation. If the aspect ratio is 3 or more, a sufficient effect of improving loop linearity can be obtained. Preferably, when the aspect ratio is 5 or more, good loop linearity can be obtained even with a long span having a diameter of 25 μm or less and a wire length of 5 mm or more. More preferably, if the aspect ratio is 10 or more, the effect of improving the loop linearity is enhanced even with an ultra-long span having a wire length of 7 mm or more.

The face-centered cubic metal that is the main component of the skin layer is a metal that is different from the conductive metal that is the main component of the core material. It is also desirable that the metal be effective. Specifically, Pd, Pt, Ru, Rh, Ag are candidates, and if more importance is attached to practicality, cost performance, etc., it is more desirable to be any one metal of Pd, Pt, Ru, Ag. . The main component here is an element having a concentration of 50 mol% or more. Pd has advantages such as sufficient adhesion to the sealing resin, sufficient bonding to the electrode, and easy quality control. Pt is relatively easy to stabilize the ball shape. Ru is easy to form a hard and dense film, and the material cost is relatively low. Rh is oxidation resistance such performance is good, because the material cost is expensive, considering the future thinning or the like is expected. Since Ag is soft, it is relatively easy to suppress flaws due to wire drawing of the wire on which the skin layer is formed, and the material cost is also low, so it is useful for cost-sensitive semiconductors.

That is, the skin layer is preferably a pure metal whose main component is any one of Pd, Pt, and Ru, or an alloy whose main component is the conductive metal. A pure metal has the advantage that oxidation resistance and bondability are easily improved, and an alloy has the advantage of suppressing wire deformation during resin sealing by increasing the tensile strength and elastic modulus. Here, the pure metal corresponds to a part of the skin layer including a layer having a concentration of 99 mol% or more, or an average concentration of the skin layer excluding the diffusion layer being 80 mol% or more. The alloy contains 50 mol% or more of at least one metal selected from Pd, Pt, and Ru.

Conductive metal constituting the core material, Cu, Au, Ag is a candidate, if practicality, Cu, it is desirable to mainly containing any one of Au. Cu has low material cost, high electrical conductivity, and relatively good operability such as easy formation of a good ball by spraying a shielding gas during ball formation. Au has strong oxidation resistance, does not require a shielding gas or the like at the time of ball formation, has a good deformation at the time of bonding, and has an advantage that it is easy to ensure the bonding property. Ag is excellent in electrical conductivity, but it is somewhat difficult to draw, and it is necessary to optimize the manufacturing technique. On the other hand, Cu and Au are also advantageous in that they have been used as single-layer bonding wire materials.

If the core material is an alloy containing a conductive metal as a main component, it may be advantageous for thinning the wire due to an increase in wire strength or improving the bonding reliability. In the case of Cu alloy, by containing one or more of B, Pd, Bi, P , and Zr in the range of 5 to 300 ppm, the span of up to about 5 mm can be achieved due to the increase in tensile strength and elastic modulus of the bonding wire. The effect of improving the linearity in the long span can be obtained. In order to enhance the additive action, it was confirmed that a Cu single layer wire is not sufficient, but a higher effect can be obtained by applying it to a multilayer wire whose main component is Cu. That is, the core material is a Cu alloy containing B, Pd, Bi, P , Zr in the range of 5 to 300 ppm, and the skin layer is composed mainly of any one of Pd, Pt, Ru, Of the crystal orientation <hkl> in the longitudinal direction of the wire in the surface crystal plane, the proportion of <111> is 50% or more, thereby further enhancing the effect of improving the long span linearity. This is presumably because the linearity is improved by the synergistic effect of the skin layer in which the crystal orientation is controlled and the core material containing the alloy element.

Of the crystal orientation <hkl> in the wire longitudinal direction in the crystal plane of the surface of the skin layer, the proportion of <111> is 50% or more, the main component constituting the skin layer is Pd or Ag, and the core material is If the main component is Cu and the bonding wire has a multi-layer structure containing Pd in the range of 5 to 10,000 ppm in the core material, it suppresses scratches and scraping, stabilizes the shape and height of the loop, or ball It becomes easy to comprehensively satisfy the stabilization of the crimping shape of the joint. In the heat treatment process in wire manufacturing, when the Pd in the core material and Pd and Ag in the skin layer diffuse to each other near the interface between the core material and the skin layer, the change in Pd concentration is made uniform and gentle. Due to the synergistic action, it is possible to obtain an action of reducing peeling / scraping near the upper surface of the loop and a high action of reducing variation in shape such as the fall of the loop and bending. This concentration change is effective not only for the entire wire but also for the neck portion that is affected by the heat of ball melting, and is therefore effective for stabilizing the loop height. Also, with the combination of Cu core material and Pd skin layer, when the ball melts, the mixing of Cu, Pd, and Ag may become uneven, resulting in ball shape irregularities. By containing, the effect of rounding the shape of the ball joint is enhanced. Here, regarding the Pd concentration contained in the core material, the above effect is confirmed if it is 5 ppm or more, and the improvement effect is more remarkable if it is preferably 200 ppm or more. With respect to the upper limit of the Pd concentration, chip damage due to ball hardening can be suppressed if it is 10000 ppm or less, and if it is 8000 ppm or less, the effect of suppressing chip damage is further enhanced, which is advantageous for narrow pitch connection. .

In the case of an Au alloy, if one or more of Be, Ca, Ni, Pd, and Pt are contained in the range of 5 to 8000 ppm, the same effect is obtained, and it is easy to ensure good linearity. It becomes. That is, the core material is an Au alloy containing one or more of Be, Ca, Ni, Pd, and Pt in a range of 5 to 8000 ppm, and the skin layer is mainly composed of any one of Pd, Pt, and Ru, It is desirable that the proportion of <111> in the crystal orientation <hkl> in the longitudinal direction of the wire in the crystal plane on the surface of the skin layer is 50% or more.

  In the structure of the bonding wire having a multilayer structure, the above-mentioned skin is provided by having an intermediate metal layer made of a component different from the main component constituting the skin layer and the core material between the skin layer and the core material. It is more advantageous to control the orientation of the crystal orientation of the layer. The formation of the skin layer may be affected by the crystal orientation of the base, and it is relatively easier to control the crystal orientation of the intermediate metal layer formed on the core material than to control the crystal orientation of the core material. Because there is. Specifically, the same face-centered cubic metal as that of the skin layer is preferable as the intermediate metal layer. In particular, it is more preferable that the lattice constant of the metal of the skin layer is close to the lattice constant of the metal of the intermediate metal layer.

  That is, a multi-layered bonding wire characterized by having an intermediate metal layer made of a component different from the main component constituting the skin layer and the core material between the skin layer and the core material is desirable. . As an effect of adding the intermediate metal layer, the peel strength, which is one of the indicators of the joint strength of the wedge joint, can be increased by improving the adhesion between the skin layer and the core material. Here, the peel strength can be measured by a simple method of measuring the pull strength in the vicinity of the wedge joint. Accordingly, the peel strength can be increased by inserting the intermediate metal. Here, the component of the intermediate metal layer should be selected in combination with the component of the skin layer and the core material, and is preferably a metal component as described above, in particular, Au, Pd, Pt is more preferable. More preferably, when the combination of the main component of the skin layer / core material is Pd / Cu, if the main component of the intermediate metal layer is Au, it is advantageous for controlling the crystal orientation of the skin layer, and the skin layer / Adhesion at each interface of the intermediate metal layer / core material is also relatively good. In addition, when the combination of the main component of the skin layer / core material is Pd / Au, if the main component of the intermediate metal layer is Pt, it is advantageous for the control of crystal orientation, the composition of the skin layer, and the uniformity of the film thickness. is there.

  If the thickness of the skin layer is in the range of 0.005 to 0.2 μm, it is advantageous for the control of the crystal orientation of the skin layer as described above, and it is easy to satisfy the required characteristics such as bondability and loop control comprehensively. Become. If the thickness is 0.005 μm or more, a sufficient effect of the skin layer with controlled crystal orientation can be obtained. If the thickness exceeds 0.2 μm, hardening due to alloying of the ball part becomes significant and cracks occur in the chip during bonding. Etc. may cause problems.

  Preferably, when the thickness of the skin layer is in the range of 0.01 to 0.15 μm, a desired loop shape can be stably formed without reducing the speed even with complicated loop control. More preferably, if it is in the range of 0.020 to 0.1 μm, it is easy to obtain a stable film quality, such as improving the processing efficiency of the film forming process while maintaining the use performance of the bonding wire.

  If the thickness of the intermediate metal layer is in the range of 0.005 to 0.2 μm, it becomes easy to control the crystal orientation of the skin layer, improve the adhesion at the interface with the core material, and support complex loop control. it can. Preferably, when the thickness is in the range of 0.01 to 0.1 μm, it becomes easy to ensure uniformity of film thickness and reproducibility.

  Here, the boundary between the skin layer and the core material is a portion where the total detected concentration of the conductive metal constituting the skin layer is 50 mol%. Therefore, the skin layer referred to in the present invention is the surface from the site where the total detected concentration of the conductive metal constituting the skin layer is 50 mol%, that is, the total detected concentration of the conductive metal constituting the skin layer is It is a site of 50 mol% or more.

Crystal orientation in the present invention, it is preferable that the angle difference in the crystal orientation with respect to the wire longitudinal direction of the bonding wire can contain more than 15 °. Normally, even if attention is paid to a crystal orientation in a certain direction, individual crystals have a certain degree of angle difference, and a slight angle difference is also caused by an experimental method such as sample preparation or a crystal orientation measurement method. Here, if the range of the angle difference is within 15 °, it has the characteristics of each crystal orientation, and the degree of influence on the various characteristics of the bonding wire can be effectively utilized.

  Regarding the texture of the surface of a fine line having a diameter of about 25 μm, little is known so far, and there are few reports on the texture of the outermost surface of a multilayer wire of fine lines. In order to accurately measure a texture in a metal wire having a relatively soft and thin wire diameter such as a bonding wire, an advanced measurement technique is required.

  The texture measurement method is advantageous for narrowing down the measurement area or obtaining information only on the outermost surface, so the recently developed Electron Back Scattering Pattern (hereinafter referred to as EBSP) method. Can be used. By measuring the texture by the EBSP method, the texture of the surface or the cross section can be measured with high accuracy and sufficient reproducibility even with a fine wire such as a bonding wire. With this measurement method, the crystal orientation of submicron fine crystal grains, the distribution of crystal orientation on the wire surface, and the like can be measured with high accuracy and good reproducibility regarding the microstructure of the bonding wire.

In the EBSP method, it is usually difficult to measure the crystal orientation with high accuracy when the unevenness and curved surface of the sample are large. However, if measurement conditions are optimized, highly accurate measurement and analysis are possible. Specifically, the bonding wire is fixed linearly on a plane, and a flat portion near the center of the bonding wire is measured by the EBSP method. With respect to the measurement region, if the size in the circumferential direction is 50% or less of the wire diameter with the center in the longitudinal direction of the wire as the axis, and the size in the longitudinal direction of the wire is 100 μm or less, measurement efficiency can be improved in addition to accuracy. Preferably, if the size in the circumferential direction is 40% or less of the wire diameter and the size in the longitudinal direction of the wire is 40 μm or less, the measurement efficiency can be further improved by shortening the measurement time.

  In order to perform high-accuracy measurement using the EBSP method, the area that can be measured at one time is limited. Therefore, it is desirable to perform measurement at three or more locations and obtain average information in consideration of variations. It is preferable to select the measurement location so that different regions can be observed in the circumferential direction so that the measurement locations are not close to each other.

For example, in the measurement of a bonding wire with a wire diameter of 25 μm, a bonding wire fixed on a flat plate so as to change the wire direction as much as possible is used, and the size is 8 μm in the circumferential direction around the wire axis and 30 μm in the wire longitudinal direction. It is possible to obtain average information on the crystal orientation of the wire surface by measuring three locations at a distance of 1 mm or more in a single measurement area. However, the selection of the measurement region and location is not limited to this, and it is desirable to optimize the measurement device and the wire state.

Also, when measuring the crystal orientation of the core material, wire longitudinal vertical section or, both measurements parallel cross section of the wire near the center in parallel to the wire longitudinal direction is also possible. Preferably, the polishing surface required by the vertical cross section can be obtained more easily. When the cross section is produced by mechanical polishing, it is desirable to remove the skin layer by etching in order to reduce residual distortion of the polished surface.

In the analysis of measurement results by the EBSP method, by using the analysis software installed in the device, the area ratio occupied by the crystal grain area in each orientation relative to the measurement area of the wire surface described above, or within the measurement area The ratio of each crystal orientation can be calculated by using the total area of the crystal grains or regions that can identify the crystal orientation as a population. Here, the minimum unit for calculating the area of the crystal orientation may be a crystal grain or a part of a minute region in the crystal grain. Regarding the size of the crystal grains, the average size in the wire longitudinal direction and the circumferential direction can be calculated.

  In manufacturing the bonding wire of the present invention, a process for forming a skin layer on the surface of the core material and a processing / heat treatment process for controlling the structure of the skin layer, the diffusion layer, the core material and the like are required.

  Methods for forming the skin layer on the surface of the core include plating, vapor deposition, and melting. In the plating method, electrolytic plating and electroless plating can be used properly. In electrolytic plating, the plating rate is fast and the adhesion to the substrate is good. Electrolytic plating may be performed once, but it can be divided into thin plating called flash plating and main plating that grows the film after that, and the film quality is more stable by performing these steps separately. It is advantageous to make. Solutions used for electroless plating are classified into substitutional type and reduction type. If the film is thin, substitutional plating alone is sufficient, but when forming a thick film, the reduction type is used after substitutional plating. It is effective to apply plating step by step. The electroless method is simple and easy to use, but requires more time than the electrolysis method.

  In the vapor deposition method, physical adsorption such as sputtering, ion plating, and vacuum deposition, and chemical adsorption such as plasma CVD can be used. All of them are dry-type, and cleaning after film formation is unnecessary, and there is no concern about surface contamination during cleaning.

  Regarding the stage of plating or vapor deposition, either the method of forming a conductive metal film with a target wire diameter or the method of forming a film on a thick core material and then drawing multiple times to the target wire diameter Is also effective. In the former film formation with the final diameter, manufacturing, quality control and the like are simple, and the combination of the latter film formation and wire drawing is advantageous in improving the adhesion between the film and the core material. As a specific example of each forming method, a method of forming a film while continuously sweeping a wire in an electrolytic plating solution on a thin wire having a target wire diameter, or immersing a thick wire in an electrolytic or electroless plating bath Then, after the film is formed, a method of drawing the wire to reach the final diameter is possible.

  Here, in the final plating method in which the skin layer is formed with the final wire diameter described above, only the heat treatment step is performed after the film formation. Moreover, in the large-diameter plating method for forming a film on a large-diameter core material, it is necessary to combine a processing step up to a target wire diameter and a heat treatment step.

  In the processing step after forming the skin layer, roll rolling, swaging, die drawing and the like are selected and used properly. Controlling the processing structure, dislocations, defects at the grain boundaries, etc. by the processing speed, pressing rate or die area reduction rate, etc. also affects the structure of the skin layer, adhesion, and the like.

  The crystallographic orientation of the texture on the surface and inside of the skin layer cannot be controlled simply by forming, processing and heating the wire. Even if the processing strain relief annealing at the final wire diameter used in normal wire manufacturing is applied as it is, loop control becomes unstable due to a decrease in adhesion between the skin layer and the core material, or the skin layer in the longitudinal direction of the wire It is difficult to control the homogeneity, the distribution of the skin layer, the diffusion layer, etc. in the wire cross section. Therefore, by comprehensively combining the conditions for forming the skin layer, processing conditions such as the area reduction rate in the wire drawing process, speed, the timing of the heat treatment process, optimization of the temperature, speed, time, etc. It becomes possible to control the texture stably.

In the wire rolling and wire drawing process, a processed texture is formed, and in the heat treatment process, recovery and recrystallization proceed to form a recrystallized texture, and these textures are interrelated and finally the skin. The texture and crystal orientation of the layer are determined. In order to orient the crystal orientation of the skin layer to <111>, it is more effective to use a processed texture. The orientation ratio to <111> can be increased by optimizing the processing conditions of the wire drawing after the film formation. Although the orientation rate to <111> by the wire drawing process varies depending on the wire conditions such as the composition before processing, in order to make the <111> orientation rate of the above-described skin layer 50% or more, for example, the processing rate is set to It is effective to raise it to 80% or more. Preferably, the effect of increasing the <111> orientation ratio in the entire bonding wire can be enhanced by setting the processing rate to 95% or more.

  In the heat treatment step, it is effective to perform the heat treatment once or a plurality of times. The heat treatment process is classified into annealing immediately after film formation, annealing in the middle of processing, and finish annealing at the final diameter, and it is important to select and use them properly. The final skin layer, the diffusion behavior at the interface between the skin layer and the core material, and the like vary depending on the processing stage at which the heat treatment is performed. In one example, intermediate annealing is performed during the processing after plating treatment, and further, the wire is drawn, and the final annealing is performed at the final diameter, so that the surface layer / It is advantageous to improve adhesion by forming a diffusion layer at the interface of the core material.

As a heat treatment method, heat treatment is performed while continuously sweeping the wire, and the temperature in the furnace, which is a general heat treatment, is not constant, but a temperature gradient is provided in the furnace, which is a feature of the present invention. It becomes easy to mass-produce bonding wires having a skin layer and a core material. Specific examples include a method of introducing a temperature gradient locally and a method of changing the temperature in the furnace. In order to suppress the surface oxidation of the bonding wire, it is also effective to heat while flowing an inert gas such as N ₂ or Ar into the furnace.

  In the melting method, either the skin layer or the core material is melted and cast, and it is excellent in productivity by drawing after connecting the skin layer and the core material with a large diameter of about 10 to 100 mm. Compared to plating and vapor deposition methods, the alloy component design of the skin layer is easy, and there are advantages such as easy improvement of properties such as strength and bondability. In a specific process, a melted conductive metal is cast around a core wire prepared in advance to form a skin layer, and a hollow cylinder of a conductive metal prepared in advance is used. It is divided into a method of forming a core wire by casting. Preferably, it is easier to stably form a concentration gradient or the like of the main component of the core material in the skin layer by casting the core material inside the latter hollow cylinder. Here, if a small amount of copper is contained in the skin layer prepared in advance, the copper concentration on the surface of the skin layer can be easily controlled. Also, in the melting method, it is possible to omit the heat treatment work for diffusing Cu into the skin layer, but further improvement in characteristics can be expected by applying heat treatment to adjust the Cu distribution in the skin layer. .

  Further, when such a molten metal is used, at least one of the core wire and the skin layer can be manufactured by continuous casting. By this continuous casting method, the process is simplified as compared with the above casting method, and the wire diameter can be reduced to improve the productivity.

When bonding using a multilayer copper wire whose main component is copper, a shielding gas is required when forming the ball, and an N ₂ mixed gas containing H ₂ in the range of 1 to 10% Or pure N ₂ gas. For conventional single-layer copper wires, a mixed gas typified by 5% H ₂ + N ₂ has been recommended. On the other hand, with a multilayer copper wire, good bondability can be obtained even if inexpensive pure N ₂ gas is used, so that the running cost can be reduced as compared with the standard gas of 5% H ₂ + N ₂ gas. The purity of N ₂ gas is desirably 99.95% or more. That is, it is desirable to use a bonding method in which an arc discharge is generated while spraying N ₂ gas having a purity of 99.95% or more on or around the wire tip to form a ball portion, and the ball portion is joined.

Moreover, adhesiveness can be improved by forming a diffusion layer between the skin layer and the core material. The diffusion layer is a region formed by mutual diffusion of the main components of the core material and the skin layer, and has a concentration gradient of the main components. By forming a diffusion layer, the adhesion between the core material and the skin layer can be improved to suppress the peeling of the skin layer during loop control and bonding, and by having a concentration gradient, the conductive metal can be used as the skin layer. The wire deformation in the control at the time of the loop subjected to the complicated plastic deformation can be stabilized as compared with the case where the concentration is uniform throughout. In addition to the effect of suppressing scratches and scraping by increasing the <111> orientation of the surface of the skin layer to 50% or more, the effect can be further improved if there is a diffusion layer having a concentration gradient. confirmed. The concentration gradient in the diffusion layer is desirably such that the degree of concentration change in the depth direction is 10 mol% or more per 1 μm. Preferably, if it is 5 mol% or more per 0.1 μm, a high effect of mutual use can be expected without impairing the different physical properties of the skin layer and the core material. The thickness of the diffusion layer is preferably in the range of 0.002 to 0.2 μm. This is because the effect is small if the thickness of the diffusion layer is less than 0.002 μm, and it is difficult to identify by analysis, and if it exceeds 0.2 μm, the structure of the skin layer is affected. This is because it is difficult to form the film stably. In order to control this diffusion layer, it is effective to use heat treatment. As described above, by controlling the degree of diffusion by combining heat treatment and processing, a desired diffusion layer can be uniformly formed in the circumferential direction of the wire or in the longitudinal direction of the wire .

  For concentration analysis of the skin layer, the core material, etc., a method of analyzing the surface of the bonding wire while digging in the depth direction by sputtering or the like, or a method of performing line analysis or point analysis on the wire cross section is effective. The former is effective when the skin layer is thin, but if it is thick, it takes too much measurement time. Analysis of the latter cross section is effective when the skin layer is thick, and it is advantageous that the concentration distribution over the entire cross section, reproducibility confirmation in several places, etc. are relatively easy. When the skin layer is thin, the accuracy decreases. It is also possible to measure by increasing the thickness of the diffusion layer by obliquely polishing the bonding wire. In the cross section, line analysis is relatively simple. However, if you want to improve the accuracy of the analysis, it is also effective to narrow the analysis interval of the line analysis or perform point analysis focusing on the area to be observed near the interface. . The analyzers used for concentration analysis should use electron microanalysis (EPMA), energy dispersive X-ray analysis (EDX), Auger spectroscopic analysis (AES), transmission electron microscope (TEM), etc. Can do. In particular, the AES method is effective for concentration analysis of the thinnest region because of its high spatial resolution. Further, for the investigation of the average composition, etc., it is possible to dissolve in acid or the like stepwise from the surface portion, and obtain the composition of the dissolution site from the concentration contained in the solution. In the present invention, it is not necessary that the concentration values obtained by all the analysis methods satisfy the specified range of the present invention, and if the concentration value obtained by one analysis method satisfies the specified range of the present invention, the effect can be obtained. It is

  Examples will be described below.

  Cu, Au, Ag used as the core material of the bonding wire is a high purity material with a purity of about 99.99% by mass or more, and it is made of Au, Pt, Pd, Ru, Rh used for the skin layer or intermediate metal layer. Raw materials with a purity of 99.9% by mass or more were prepared.

  In order to use a wire thinned to a certain wire diameter as a core material and form a different metal layer on the surface of the wire, an electrolytic plating method, an electroless plating method, a vapor deposition method, a melting method, or the like was performed and heat treatment was performed. A method of forming the skin layer with the final wire diameter and a method of forming the skin layer with a certain wire diameter and then further thinning to the final wire diameter by wire drawing were utilized. As the electrolytic plating solution and the electroless plating solution, a plating solution commercially available for semiconductor applications was used, and the sputtering method was used for vapor deposition. Prepare a wire with a diameter of about 15 to 1500 μm in advance, coat the wire surface by vapor deposition, plating, etc., draw it to 15 to 50 μm of the final diameter, finally remove the processing strain and elongate value 5 to 15% The heat treatment was applied so as to be in the range. If necessary, after wire drawing to a wire diameter of 25 to 200 μm, diffusion heat treatment was performed, and then wire drawing was further performed. The area reduction rate of the wire drawing dies was prepared in the range of 5 to 15% per die, and the introduction of processing strain on the wire surface was adjusted by the combination of these dies. The drawing speed was optimized between 20 and 500 m / min.

  When using the melting method, a method of casting a molten metal around a core wire prepared in advance and a method of casting a molten metal in the central portion of a hollow cylinder prepared in advance were employed. Thereafter, forging, roll rolling, die drawing, and the like, and heat treatment were performed to produce a wire.

Regarding the heat treatment of the wire of the example of the present invention, the wire was heated while continuously sweeping. A method of introducing a temperature gradient locally, a method of changing the temperature in the furnace, and the like were used. For example, a heat treatment furnace modified so that the furnace temperature can be controlled by dividing into three parts was used. In an example of the temperature distribution, the distribution of high temperature → intermediate temperature → low temperature or intermediate temperature → high temperature → low temperature was obtained from the wire insertion port toward the outlet, and the heating length was also controlled. Along with the temperature distribution, the wire sweep speed was also optimized. In the atmosphere of heat treatment, an inert gas such as N ₂ or Ar was also used for the purpose of suppressing oxidation. The gas flow rate was adjusted in the range of 0.0002 to 0.004 m ³ / min and used for temperature control in the furnace. As the timing of heat treatment, when the skin layer is formed after heat treatment is performed on the wire after wire drawing, the heat treatment is performed once or twice or more before, during or immediately after the skin layer is formed. And so on.

  The processing level by rolling and wire drawing after the skin layer is formed can be organized by the cumulative processing rate calculated by the area ratio between the wire at the time of film formation and the final wire diameter. When the processing rate (%) is less than 30%, it is expressed as R1, R2 when 30% or more and less than 70%, R3 when 70% or more and less than 95%, and R4 when 95% or more.

In order to control the surface texture of the skin layer, it is necessary to optimize material factors such as material, composition, and thickness, and process factors such as film formation conditions and processing / heat treatment conditions. In the embodiment, as a measure for increasing the <111> ratio in the longitudinal direction of the wire on the surface of the skin layer, it is effective to increase the processing rate, reduce the initial film thickness, lower the heat treatment, and the like. As an example, if the processing rate is R2 to R4, it is relatively easy to increase the <111> ratio. In one comparative example, in order to reduce the <111> ratio, it was effective to reduce the processing rate or to perform the heat treatment at a high temperature or for a long time.

Regarding the observation of the structure of the wire surface, the crystal orientation was measured by the EBSP method in a certain region of the surface in the skin layer of the bonding wire. In the preparation of the measurement sample, 3 to 5 bonding wires were fixed on the flat plate so as to change the wire direction as much as possible. The observation area was a square area including the wire axis, and the size was 5-10 μm in the circumferential direction and 10-50 μm in the wire longitudinal direction as one measurement area. The measurement locations were 3 to 10 locations, which were selected at a distance of 0.5 mm or more. The measurement point interval was 0.01 to 0.2 μm.

  In observing the structure of the core material, the crystal orientation was measured by the EBSP method using a sample obtained by polishing the cross section of the bonding wire and reducing the surface processing strain by chemical etching. The cross section was measured mainly with a cross section perpendicular to the longitudinal direction of the wire, but measurement was also performed on a cross section parallel to the longitudinal direction of the wire and passing through the central axis as necessary while examining the sample state, reproducibility, and the like.

Dedicated software (such as OSL analysis manufactured by TSL) was used for analysis of EBSP measurement data. The crystal orientation in the measurement area was analyzed, and the ratio of crystal grains such as <111> and <100> orientation was obtained. The orientation was determined with reference to the wire longitudinal direction of the bonding wire, and included up to 15 ° angle difference of each crystal orientation. Regarding the calculation method of the ratio of crystal grains, the ratio of each orientation that calculates the total area of the measurement area as a population (hereinafter referred to as area ratio) and the crystal orientation that can be identified based on the reliability within the measurement area Two kinds of ratios (hereinafter referred to as azimuth ratios) for calculating only the area as a population were obtained. In the latter process of obtaining the orientation ratio, the calculation was performed by excluding the part where the crystal orientation could not be measured, or the part where the reliability of orientation analysis was low even though the crystal orientation could be measured. Here, the reliability may include parameters prepared in the analysis software.For example, using several parameters such as Confidential Index (CI value) and Image Quality (IQ value), the sample state, analysis purpose, etc. It is desirable to select judgment criteria according to the above.

Depth analysis by AES was used to measure the film thickness on the wire surface, and surface analysis and line analysis by AES, EPMA, etc. were performed to observe element distribution such as concentration of grain boundaries. In depth analysis by AES, measurement was performed in the depth direction while sputtering with Ar ions, and the unit of depth was displayed in terms of SiO ₂ . The conductive metal concentration in the bonding wire was measured by ICP analysis, ICP mass spectrometry or the like.

For connecting the bonding wires, a commercially available automatic wire bonder was used to perform ball / wedge bonding. A ball was produced at the tip of the wire by arc discharge, it was joined to the electrode film on the silicon substrate, and the other end of the wire was wedge joined to the lead terminal. The shielding gas used to suppress oxidation during ball formation was mainly pure N ₂ gas. The gas flow rate was adjusted in the range of 0.001 to 0.01 m ³ / min.

  As a bonding partner, an Al alloy film (Al-1% Si-0.5% Cu film, Al-0.5% Cu film) having a thickness of 1 μm, which is a material of an electrode film on a silicon substrate, was used. On the other hand, a lead frame whose surface was Ag-plated (thickness: 2 to 4 μm) was used as a partner for wedge bonding. It has been confirmed that the same effect as that of the lead frame can be obtained with respect to the bonding property to the Au / Ni / Cu electrode on the BGA substrate by using some wire samples.

  In the evaluation of scratches, scrapes, etc. on the wire surface, the appearance of the bonded loop was examined. It can be evaluated including effects before loop formation, such as scratches and scrapes generated in the wire manufacturing process. The wire diameter is 25 μm. There were two types of wire lengths: a general span of 2 mm and a long span of 5 mm. A trapezoidal loop with a target height of 100 to 250 μm was formed, and 1000 bonding wires were observed with a projector. Scratch observation was centered on the outside of the loop, and scraping observation was centered on the neck near the ball joint, which was frequently generated, and scratches with a size of 10 μm or more were counted. In addition, as a low loop evaluation, a low loop having a target height of about 65 μm was formed, and the occurrence of scratches and scraping was observed in the same manner. In general, the longer the wire length or the lower the loop height, the higher the degree of rubbing of the wire surface, so that the evaluation becomes more severe. If there are more than 4 scraps and scratches are noticeable, it is judged that there is a problem and is indicated by a cross, and the range is 1 to 3 scrapes, but there are many scratches and there is a concern about the impact on capillary clogging etc. Is marked with △ because it is judged that improvement is necessary, and when there are no major scratches in the range of 1 to 3 shavings, the wire surface is relatively good, When no shaving occurred and scratches were not conspicuous, it was judged to be stable and good and represented by ◎. Scratches and shavings were judged to be affected somewhat by the observer's personal judgment, so two or more observers were evaluated and ranked according to average information.

  Two types of wire diameters of 22 μm and 18 μm were used in the evaluation of scratches and scrapes on the wire surface of fine wires. A trapezoidal loop with a wire length of 2 mm and a target height of 70-200 μm was formed, and 1000 bonding wires were observed with a projector. The same criteria for scratches and scrapes as described above were adopted.

  In order to evaluate the linearity of the bonded loop, bonding was performed with three types of wire span (span): normal span of 2 mm, long span of 5 mm, and ultra-long span of 7 mm. The wire diameter is 25 μm. Thirty bonding wires were observed from above with a projector, and the deviation of the part where the bonding wires were farthest from the straight line connecting the joints between the ball side and the wedge side was measured as the amount of bending. If the average amount of bending is less than one wire diameter, it is judged to be good. Is not marked as a problem.

Regarding the loop shape stability in the bonding process, 30 trapezoidal loops were connected with a long span of 5 mm and a loop height of 200 to 250 μm, and evaluated from the standard deviation of the height. The wire diameter is 25 μm. An optical microscope was used for height measurement, and the position was measured at two locations near the top of the loop and at the center of the loop. If the standard deviation of the loop height was 1/2 or more of the wire diameter, it was judged that the variation was large, and if it was less than 1/2, it was judged that the variation was small and good. Judgment based on that standard, if the variation is small at all three locations, the loop shape is judged to be stable, indicated by ◎, and if there is only one location with large variation, Since it was good, it was indicated by a mark ◯, in the case of two places, a △ mark, and in the case of large variations in all three places, a mark X.

  In determining the bonded shape of the press-bonded ball portion, 200 bonded balls were observed, and the roundness of the shape, abnormal deformation failure, dimensional accuracy, etc. were evaluated. The wire diameter is 20 μm. Two types were evaluated: a case of forming a normal size ball having an initial ball diameter / wire diameter ratio of 1.9 to 2.2 and a case of forming a small diameter ball having a ratio of 1.5 to 1.7. If there are 5 or more defective ball shapes such as anisotropy and petals that deviate from a perfect circle, it is judged as defective, and if there are 2 to 4 defective ball shapes that deviate from a perfect circle, they are classified into two. If one or more abnormal deformations have occurred, improvement in mass production is desirable. If no abnormal deformation has occurred, it can be used. Since it is good, it is indicated by a circle.

  For the evaluation of peel joint strength, a pull test of a wedge joint was used. The wire diameter is 25 μm and the span is 2 mm. In this method, the hook hooked on the loop was moved upward at a position closer to the wedge joint than 3/4 of the wire length, and the breaking strength of the bonding wire was measured. Since the pull strength depends on the wire diameter, loop shape, bonding conditions, etc. of the bonding wire, the relative ratio (Rp) of pull strength / wire tensile strength was used instead of the absolute value. If the Rp is 20% or more, the wedge bondability is good, and it is judged that there is no problem if it is marked ◎, 15% or more and less than 20%. △ mark, and if it is 10% or more, there is a problem in the mass production process.

  In order to evaluate the adhesion between the outer skin layer and the core material in the loop formation, the loop was observed from above with an optical microscope to examine the occurrence of peeling of the outer skin layer. A normal loop having a wire diameter of 25 μm and a span of 3 mm was used, and 400 loops were observed. Compared with the number of peels, if it is zero, it is judged to be good, and if it is 1 to 4, there is no problem in normal use but improvement may be required, so Δ mark, 5 or more If there is a problem in the mass production process, it is indicated by a cross.

  In the depth analysis of AES spectroscopic analysis, when a diffusion layer having a concentration gradient between the skin layer and the core material is confirmed, and the thickness of the diffusion layer is in the range of 0.002 to 0.2 μm, The “Diffusion layer” column is indicated by a circle.

  In the evaluation of the damage to the chip, the ball part was bonded onto the electrode film, and then the electrode film was removed by etching, and the damage to the insulating film or the silicon chip was observed by SEM. 400 electrodes were observed. If no damage is found, mark it as “◯”, if there are 2 or less cracks, it is judged as a satisfactory level, and if there are 3 or more cracks, it is judged as a level of concern and marked as “X”.

  Tables 1 and 2 show examples of bonding wires according to the present invention and comparative examples.

The bonding wire according to the first aspect are examples 25-30, the bonding wire according to the second aspect embodiment 25 to 30, the bonding wire according to the third aspect Examples 25-30, 4th billing Bonding wires according to the fifth embodiment are examples 25 to 30 , bonding wires according to the fifth claim are embodiments 25 to 30 , bonding wires according to the sixth claim are embodiments 27, and bonding wires according to the eighth claim are performed. This corresponds to Example 30 . Comparative Examples 1 to 6 show results when the first claim is not satisfied.

FIG. 1 shows an example of the EBSP measurement result on the surface of the bonding wire of Reference Example 4. The region where the crystal orientation in the longitudinal direction of the wire is within an angle difference of 15 ° from the <111> orientation was colored, and the crystal grain boundary where the angle difference was 15 ° or more was displayed as a line. The <111> area ratio in FIG. 1 was 88%.

  A part of the evaluation results will be described for representative examples of each claim.

The bonding wires having a multilayer structure of Examples 25 to 30 have a ratio (<111> orientation ratio) of <111> in the crystal orientation in the longitudinal direction of the wire on the surface of the skin layer according to the present invention is 50% or more. As a result, it was confirmed that scratches and scrapes on the wire surface were reduced. On the other hand, in Comparative Example 6 relating to the bonding wire having a multilayer structure in which the <111> orientation ratio on the surface of the skin layer is less than 50%, a number of scratches were confirmed even in the case of normal loop formation. As a preferable example, in Examples 26 , 27 and 29 in which the <111> orientation ratio of the skin layer is 60% or more, scratches and scraping can be reduced even in a long span, and the <111> orientation ratio of the skin layer is further reduced. In Examples 26 and 29 , which are 70% or more, it was confirmed that scratches and chipping defects were suppressed even under severe conditions of low loop.

The bonding wires of the multilayer structure of Examples 25 to 30 have a azimuth ratio of 60% or more of the total of <111> and <100> on the surface of the skin layer according to the present invention. It was confirmed that the variation in loop height under normal loop conditions was suppressed and stable. Preferably, in Examples 26 , 29 , and 30 in which the orientation ratio is 80% or more, the loop height can be stabilized even with a long span of 5 mm span.

The bonding wires having a multilayer structure of Examples 25 to 30 have a normal ball size because the total orientation ratio of <111> and <100> in the cross section of the core material according to the present invention is 30% or more. It was confirmed that the shape of the ball joint could be reduced and the shape could be stabilized. Preferably, in Examples 26 , 29 , and 30 in which the azimuth ratio is 50% or more, it was confirmed that the roundness of the ball joint portion was improved even with a small-diameter ball having severe bonding conditions.

The bonding wires having a multilayer structure of Examples 26 to 30 are usually provided with an aspect ratio of 3 or more in the wire longitudinal direction / circumferential direction of the average size of crystal grains on the surface of the skin layer according to the present invention. It was confirmed that the linearity of the loop was good in the condition of 3 mm span. Preferably, in Examples 26 , 27 , 29 , and 30 in which the aspect ratio is 5 or more, it was confirmed that linearity can be improved even with a long span of 5 mm, which is a severe joining condition. More preferably, in Examples 26 and 29 in which the aspect ratio is 10 or more, it was confirmed that the linearity can be improved even with an extremely long span of 7 mm, which is a severe loop condition.

The bonding wires having a multilayer structure of Examples 25 to 30 are based on the present invention, in which the area of crystal grains having a crystal orientation of <111> in the longitudinal direction of the wire on the surface of the skin layer is a ratio (<111> area) to the wire surface. When the ratio was 30% or more, it was confirmed that the wire surface was a fine wire having a diameter of 22 μm, and the flaws and scraping of the wire surface were reduced. Preferably, it was confirmed that Examples 26, 29 , and 30 having a <111> area ratio of 40% or more can suppress scratches and scrapes even with a fine 18 μm extra fine wire. More preferably, in Example 29 in which the <111> area ratio is 50% or more, it was confirmed that the effect of suppressing scratches and scraping was further enhanced by an 18 μm fine wire.

The bonding wires having a multilayer structure of Examples 25 to 30 have the above-mentioned <111> orientation ratio of 50% or more according to the present invention, and have an intermediate metal layer between the skin layer and the core material. Thus, it was confirmed that the peel strength at the wedge joint can be increased.

The bonding wires having a multilayer structure of Examples 25 to 30 have an orientation ratio of 15% or more of the total orientation of <111> and <100> in the cross section of the core material according to the present invention. Thus, it was confirmed that the abnormal deformation of the ball joint portion can be reduced and the shape can be stabilized.

The bonding wires having the multilayer structure of Examples 25 to 30 were excellent in reducing chip damage because the thickness of the outer skin layer according to the present invention was in the range of 0.005 to 0.2 μm.

The bonding wires having a multilayer structure of Examples 25 to 30 have a diffusion layer having a concentration gradient between the skin layer and the core material according to the present invention, so that there is no peeling above the loop and the adhesion of the skin layer is good. It was confirmed that.

The bonding wire having a multilayer structure of Example 27 is Cu, the main component constituting the core material according to the present invention, and one or more of B, Pd, P, and Zr (in this case, P) is 5 to 300 ppm. It was confirmed that the linearity of the loop having a span of about 5 mm was improved by inclusion in the range. Similarly, in Example 30 , the main component constituting the core material according to the present invention is Au, and contains one or more of Be, Ca, Ni, and Pd (in this case, Ca) in the range of 5 to 8000 ppm. As a result, it was confirmed that the linearity was improved. Here, with respect to the action of improving the linearity of the loop having a span of about 5 mm, it is also effective that the aspect ratio in the wire longitudinal direction / circumferential direction is 5 or more, and the effect by addition of the above alloy components It can be difficult to identify. On the other hand, it was confirmed from Table 1 that although the aspect ratio is less than 5, the linearity of about 5 mm span can be improved by containing the above alloy components.

The bonding wire having a multilayer structure shown in Table 1 is Cu, the main component constituting the core material according to the present invention, containing Pd in a range of 5 to 10,000 ppm, and the main component constituting the skin layer is Pd or It was confirmed that a high effect of reducing peeling / scraping near the upper surface of the loop can be obtained by using Ag. Preferably, in the bonding wire for a semiconductor device , since the Pd concentration is 200 ppm or more, the above effect is more remarkable. Moreover, since the Pd content in the bonding wire for semiconductor devices is in the range of 5 to 8000 ppm, it was confirmed that chip damage was suppressed.

Claims (8)

A bonding wire for a semiconductor device having a core material made of a conductive metal and a skin layer mainly composed of a metal different from the core material on the core material,
The metal of the skin layer is face-centered cubic, and the thickness of the skin layer is in the range of 0.005 to 0.2 μm,
Of the crystal orientation <hkl> in the wire longitudinal direction in the crystal plane of the surface of the skin layer, the orientation ratio of <111> including an angle difference of up to 15 ° with respect to the wire longitudinal direction is 50% or more,
A bonding wire for a semiconductor device, comprising an intermediate metal layer made of a component different from a main component constituting the skin layer and the core material between the skin layer and the core material. Of the crystal orientation <hkl> in the wire longitudinal direction on the crystal plane of the cross-section of the core material, the sum of the orientation ratios of <111> and <100> including an angle difference up to 15 ° with respect to the wire longitudinal direction The bonding wire for a semiconductor device according to claim 1, wherein the bonding wire is 30% or more. Area of crystal grains the crystal orientation of the wire longitudinal direction is the <111> in the surface of the skin layer, as claimed in claim 1 or 2, wherein the as a percentage of the total area of the wire surface is 30% or more Bonding wire for semiconductor devices. 4. The bonding wire for a semiconductor device according to claim 1, wherein a main component constituting the skin layer is any one of Pd, Pt, Ru, and Ag. 5. Between the core and the skin layer, the semiconductor device according to claim 1, characterized in that it comprises a diffusion layer having a concentration gradient of the main component of the skin layer and the core material Bonding wire. The main component constituting the core material is Cu and contains one or more of B, Pd, Bi, P, and Zr in a total amount range of 5 to 300 ppm. The bonding wire for semiconductor devices as described in 2. 6. The main component constituting the core material is Cu, containing Pd in a range of 5 to 10,000 ppm, and the main component constituting the skin layer is Pd or Ag. 2. A bonding wire for a semiconductor device according to item 1. The main component constituting the core material is Au, and contains one or more of Be, Ca, Ni, Pd, and Pt in a total amount of 5 to 8000 ppm. The bonding wire for semiconductor devices as described in 2.