JP2009140953A - Bonding wire for semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bonding wire principally comprising copper that enhances bondability in addition to conventional basic performance. <P>SOLUTION: A bonding wire for semiconductor device has a core material principally comprising copper, and an outer layer provided on the core material and comprising an oxidation resistant metal having a thickness of 20-150 nm, wherein the 0.2% yield strength is 0.07-0.14 mN/μm<SP>2</SP>, the maximum yield strength is 0.20-0.28 mN/μm<SP>2</SP>, the elongation ε per unit cross-section is between ε1 and ε2, and the ε1 and ε2 can be expressed as follows; ε1=(-0.001×R+0.055) and ε2=(-0.001×R+0.068), assuming R is the diameter of the wire. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a bonding wire for a semiconductor device used for connecting an electrode on a semiconductor element and wiring of a circuit wiring board (lead frame, substrate, tape, etc.).

  Currently, fine wires having a wire diameter of about 20 to 50 μm are mainly used as bonding wires for semiconductor devices (hereinafter referred to as bonding wires) for bonding between electrodes on semiconductor elements and external terminals. 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. A semiconductor element heated at a temperature of 150 to 300 ° C. after the tip of the wire is heated and melted by arc heat input to form a free air ball (hereinafter also simply referred to as “ball” or “FAB”) by surface tension. The ball portion is bonded to the electrode by pressure bonding, and then the bonding wire is directly bonded to the external lead by ultrasonic pressure bonding.

  In recent years, semiconductor packaging structures, materials, connection technologies, etc. have been diversified rapidly. For example, packaging structures use substrates, polyimide tapes, etc. in addition to QFP (Quad Flat Packaging) using the current lead frame. New forms such as BGA (Ball Grid Array) and CSP (Chip Scale Packaging) have been put into practical use, and there is a demand for bonding wires with improved loop characteristics, bonding properties, mass productivity, and the like. Even in such bonding wire connection technology, in addition to the current mainstream ball / wedge bonding, wedge / wedge bonding suitable for narrow pitches directly bonds the bonding wire at two locations, improving the bondability of fine wires Is required.

  The materials used as bonding partners of the bonding wires are also diversified, and copper suitable for finer wiring has been put to practical use in addition to the conventional Al alloy as the wiring and electrode material on the silicon substrate. In addition, Ag plating, Pd plating, etc. are applied on the lead frame, and copper wiring is applied on the resin substrate, tape, etc., and noble metal elements such as gold and alloys thereof are formed thereon. In many cases, a film is applied. It is required to improve the bonding property of the bonding wire and the reliability of the bonded portion according to such various bonding partners.

  Conventionally, gold of high purity 4N type (purity> 99.99 mass%) has been mainly used as a material for the bonding wire. However, since gold is expensive, a bonding wire of another kind of metal having a low material cost is desired.

  In the demand from the wire bonding technology, it is important to form a ball having good sphericity when forming the ball and obtain a sufficient bonding strength at the bonding portion between the ball portion and the electrode. In addition, in order to cope with lower bonding temperature, thinner bonding wires, etc., it is possible to perform continuous bonding without causing peeling or the like when the bonding wire is connected to the lead terminal or the wiring board with a wedge. Bonding strength is required.

  In the resin sealing process in which high-viscosity thermosetting epoxy resin is injected at a high speed, the bonding wire is deformed and comes into contact with the adjacent wire, and further, the pitch, lengthening, and thinning are progressing. Therefore, it is required to suppress the wire deformation at the time of resin sealing as much as possible. Although the deformation can be controlled to some extent by increasing the wire strength, it is difficult to put it to practical use unless the problems such as difficulty in loop control and a decrease in strength during bonding are not solved.

  Furthermore, long-term reliability when a semiconductor element to which a bonding wire is connected and mounted is actually used is also important. In particular, semiconductor elements mounted on automobiles are required to have high reliability in harsh environments such as high temperatures, high humidity, and thermal cycles in order to ensure strict safety. Even in such an unprecedented harsh environment, high reliability must be maintained without deteriorating at the joint where the bonding wire is connected.

  As the wire characteristics satisfying the above requirements, loop control in the bonding process is easy, and the bonding property to the electrode part and the lead part is improved, and excessive wire deformation in the resin sealing process after bonding is suppressed. In addition, it is desired to satisfy comprehensive characteristics such as long-term reliability of the connection portion and stability of the joint portion in a harsh environment.

  Copper is used as a raw material in order to improve the material cost, excellent electrical conductivity, ball bonding (hereinafter also referred to as “1st bonding”), wedge bonding (hereinafter also referred to as “2nd bonding”), and the like. Bonding wires have been developed, and Patent Document 1 is disclosed. However, a problem with copper bonding wires is that the bonding strength decreases due to oxidation of the wire surface, and that the wire surface is easily corroded when sealed with resin. In addition, since the hardness of the ball portion of the copper bonding wire is higher than that of Au, when the ball is deformed and bonded on the pad electrode, there is a problem that the chip is damaged such as a crack. Regarding wedge bonding of copper bonding wires, there is a concern that the bonding margin is narrower than that of Au and mass productivity is reduced. These are the reasons why the practical application of copper bonding wires does not progress.

As a method for preventing the surface oxidation of a copper bonding wire, Patent Document 2 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. . Further, from the viewpoints of ball formability, prevention of deterioration of the plating solution, and the like, Patent Document 3 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 4 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. Further, from the viewpoint of improving the 2nd bondability, Patent Document 5 has a core material mainly composed of copper and a coating layer formed on the core material, and has a 0.2% proof stress of 0.115 mN / Bonding wires having a size of μm ² or more and 0.165 mN / μm ² or less have been proposed. Similarly, Patent Document 6 includes a core material mainly composed of copper and a coating layer formed on the core material, the tip of the bonding wire is suspended so as to contact a horizontal plane, and 15 cm above the tip. A bonding wire is proposed in which the radius of curvature of an arc formed by cutting the wire and dropping the bonding wire onto the horizontal plane is 35 mm or more.

  Although such copper bonding wires for semiconductors have great practical expectations, they have not been put to practical use. The productivity and quality of wire manufacturing, yield in the bonding process, performance stability, and long-term reliability when using semiconductors must be comprehensively satisfied.

As the wire characteristics used in mass production, the loop control in the bonding process is stable, the bondability is also improved, the wire deformation in the resin sealing process is suppressed, the long-term reliability of the joint, etc. By satisfying the overall characteristics, it is desired to be able to cope with high-density mounting such as state-of-the-art narrow pitch connection and multilayer chip connection.
JP-A-61-99645 JP-A-62-97360 JP 2004-64033 A JP 2007-12776 A JP 2005-123540 A JP 2005-123511 A

  Conventional copper-based bonding wires having a single-layer structure (which is an uncoated copper-based bonding wire, and a thin natural oxide film layer having a thickness of 1 to 2 nm may be formed on the surface of the wire. As a practical problem of single-layer copper wire), it is easy to oxidize the wire surface and to reduce the bonding strength. Therefore, as a means for preventing the surface oxidation of the copper bonding wire, it is possible to coat the wire surface with a noble metal or an oxidation resistant metal.

  The present inventors evaluated in consideration of needs such as high density, miniaturization, and thinning of semiconductor mounting. As a result, a conventional multilayer copper wire having a structure in which the surface of a copper bonding wire is covered with a metal different from copper ( The uncoated copper wire is referred to as a single-layer copper wire, whereas the copper wire coated with a single coating layer is referred to as a multilayer copper wire (hereinafter referred to as a conventional multilayer copper wire). It turns out that many practical problems remain.

  Conventionally, when a ball is formed at the tip of a multilayer copper wire, a flat ball deviated from a true sphere is formed, an unmelted wire remains in the ball, or bubbles are generated. If such an abnormal ball portion is bonded onto the electrode, it may cause problems such as a decrease in bonding strength and chip damage.

  In practice, when a ball is formed from a conventional multilayer copper wire, it is more likely to cause a defective shape of the ball joint and a decrease in bonding strength than when a single layer copper wire or a current mainstream gold bonding wire is used. It becomes a problem. Specific examples of defects include the formation of flat balls that deviate from the true sphere, misalignment in which the ball is tilted with respect to the wire, wire that remains unmelted inside the ball, ) May be a problem. When such an abnormal ball part is joined on the electrode, the joint part protrudes from the electrode surface due to eccentric deformation in which the ball is displaced from the center of the wire, elliptical deformation, petal deformation, etc. This may cause problems such as a decrease in bonding strength, chip damage, and problems in production management. Such poor initial bonding may induce the above-described deterioration in long-term reliability.

  As a technique for solving the problems related to ball bonding of conventional multilayer copper wires, Patent Document 3 discloses that the thickness of the outer skin layer is 0.001 to 0.02 μm. The outer skin layer here also includes a concentration gradient region, and it is also described that the concentration of the metal M constituting the outer skin layer is 10 mol% or more at the boundary between the outer skin layer and the core material. In the evaluation of the present inventors, it has been observed that by reducing the thickness of the outer skin layer in this way, the above-mentioned problem of the ball joint portion is partially improved. It has been confirmed that the frequency of flat balls increases as the thickness of the outer skin layer decreases, when the device is used in a new environment for applications such as devices. It has also been confirmed that the problem of insufficient improvement in wire wedge bonding occurs due to thinning.

Conventionally, as a means for improving the bondability in 2nd bonding of a multilayer copper wire, Patent Document 5 proposes a bonding wire having a 0.2% proof stress of 0.115 mN / μm ² or more and 0.165 mN / μm ² or less. ing. In Patent Document 6, the radius of curvature of an arc formed by hanging the tip of the bonding wire so as to contact the horizontal plane, cutting 15 cm above the tip and dropping the bonding wire onto the horizontal plane is 35 mm. The bonding wire which is the above is proposed. However, in the bonding wires according to Patent Documents 5 and 6, there is a concern that sufficient bonding strength cannot be obtained in the 2nd bonding, and that the electrode is damaged in the first bonding, and the reliability is lowered. was there.

  An object of the present invention is to solve the above-described problems of the prior art and to provide a bonding wire mainly composed of copper that can further improve the bondability in addition to the conventional basic performance.

In order to achieve the above object, an invention according to claim 1 is composed of a core material mainly composed of copper and an oxidation-resistant metal having a thickness of 20 nm or more and 150 nm or less provided on the core material. a bonding wire having an outer layer, 0.2% proof stress 0.07mN / μm ² or more 0.14mN / μm ² or less, the maximum yield strength is 0.20mN / μm ² or more 0.28mN / μm ² or less, and The elongation value per unit cross-sectional area (% / μm ² ) is ε1 or more and ε2 or less, and ε1 and ε2 are ε1 = (− 0.001 × R + 0.055), where R is the wire diameter of the wire. , Ε2 = (− 0.001 × R + 0.068).

In the invention according to claim 2, the number of particles in the C cross section is N / μm ² or less, the average crystal grain size in the C cross section is G μm or more, and the N and G are N = (− 0 .01 × R + 0.7) G = (0.02 × R + 0.8).

  The invention according to claim 3 is characterized in that the oxidation-resistant metal contains as a main component at least one element selected from Pd, Pt, and Rh.

  In the invention according to claim 4, the core material contains one or more elements selected from P, B, Bi, Sn, Ag, and Mg, and the concentration of the element in the entire wire is a total. It is the range of 0.0001 mol% or more and 0.03 mol% or less.

  According to the first aspect of the present invention, by setting the thickness of the outer layer to a predetermined thickness, it is possible to improve the pressure ball shape and bonding strength and reliability evaluation in FAB formation and ball bonding. In addition, by having predetermined mechanical characteristics, it is possible to reduce damage to the electrode and neck in ball bonding, improve the shape and bonding strength in wedge bonding, and improve the bonding margin. It can improve mass production stability such as bonding / loop performance stability and long-term reliability when using semiconductors.

  Moreover, according to the invention of Claim 2, the bonding wire which has the said predetermined | prescribed mechanical characteristic can be obtained.

  In addition, according to the third aspect of the invention, the ball shape can be stabilized and the 2nd bondability can be improved.

  Moreover, according to the invention of Claim 4, the roundness at the time of a ball deformation | transformation and loop controllability can be improved.

1. Embodiments Hereinafter, preferred embodiments of the present invention will be described. As a result of investigating a bonding wire composed of a core material mainly composed of copper and an outer layer containing an oxidation-resistant metal, wedge bonding properties can be achieved by including an oxidation-resistant metal near the surface of the wire. However, it has been found that unstable formation of the ball and breakage of the wedge joint in the reflow process become new problems. Therefore, as a result of examining copper-based bonding wires that can respond to new mounting needs such as narrow pitch small ball bonding and further improve mass productivity, it has an outer layer and a specific thickness range Found that it was effective. Furthermore, it has been found that control of the composition and structure of the outer layer and the core material is effective.

  In general, the surface of copper wire is easier to oxidize and is harder than gold wire, so in 1st bonding, shape abnormality, insufficient bonding strength, damage to semiconductor elements (chip damage), and minute damage to the neck There is a problem that damage (neck damage) occurs. Further, in the 2nd junction, there are problems that the junction margin is narrow and daily changes with time are severe, and that the bonding strength is easily affected by the bonding direction and the mass production stability of the 2nd junction is lacking. Here, the junction margin means the stability of the 2nd junction, and the controllability of the loop, that is, the variation in the loop height and the suppression of the neck damage.

On the other hand, the bonding wire according to the present invention has a core material mainly composed of copper, and an outer layer containing an oxidation-resistant metal having a different component and composition from the core material provided on the core material. and 0.2% yield strength 0.07mN / μm ² or more 0.14mN / μm ² or less, the maximum yield strength is 0.20mN / μm ² or more 0.28mN / μm ² or less, and an elongation value per unit cross-sectional area (% / Μm ² ) is ε1 or more and ε2 or less, and ε1 and ε2 are ε1 = (− 0.001 × R + 0.055), ε2 = (− 0.001 × R + 0.068) is desirable. For convenience of explanation, in the present specification, the mechanical property of the bonding wire according to the present invention is referred to as “soft mechanical property”. Here, the 0.2% yield strength is evaluated as the yield strength of a material that does not have a clear yield point, and is a stress value when a permanent strain of 0.2% is generated. The maximum proof stress refers to the breaking strength of the bonding wire. Further, the elongation value means the maximum elongation value of the wire at the time of breaking. The unit of ε1 and ε2 is% / μm ² .

  Since this bonding wire has the above-mentioned soft mechanical characteristics, damage to the pad as an electrode in the first bonding (hereinafter referred to as “pad damage”) and damage to the neck portion (hereinafter referred to as “neck damage”). The shape and bonding strength in the 2nd bonding can be improved. In addition, since the bonding wire has soft mechanical characteristics, the bonding margin is improved, so that mass production stability in the bonding process can be improved. In general, in a 2nd joint, when a shape defect occurs due to a mesh or the like, the joint strength is lowered. In addition, when a bonding failure such as peeling occurs, the automatic wire bonder stops.

When the 0.2% proof stress exceeds 0.14 mN / μm ² , the bonding wire becomes hard, so that chip damage and neck damage increase, and the shape in 2nd bonding deteriorates and bonding strength decreases. As a result, productivity in the bonding process is lowered.

On the other hand, if the 0.2% proof stress is less than 0.07 mN / μm ² , the maximum proof strength of the bonding wire will be reduced, so the bonding strength at the 2nd junction will be reduced and the long-term reliability when using semiconductors will be reduced. When the resin is sealed, the bonding wire is deformed to cause a short circuit defect.

When the maximum proof stress exceeds 0.28 mN / μm ² , the bonding wire becomes hard, so that chip damage and neck damage are increased, and the shape in 2nd bonding is deteriorated, bonding strength is lowered and bonding is performed. Productivity in the process will deteriorate.

When the maximum proof stress is less than 0.20 mN / μm ² , the maximum proof strength of the bonding wire is lowered. Therefore, the bonding strength at the 2nd junction is lowered, and the long-term reliability when using the semiconductor is lowered. The bonding wire is deformed to cause a short circuit defect.

  Furthermore, when the elongation value per unit cross-sectional area exceeds the above ε2, the maximum proof stress of the bonding wire is reduced, so that the bonding strength in the 2nd junction is reduced, and the long-term reliability when using the semiconductor is reduced. Since the straightness of the shape and the bonding wire is deteriorated, the bonding wire is deformed at the time of resin sealing, and a short circuit defect occurs.

  On the other hand, when the elongation value per unit cross-sectional area is less than ε1, the bonding wire becomes hard, so that chip damage and neck damage increase, the shape in 2nd bonding deteriorates, bonding strength decreases, As a result, productivity in the bonding process is lowered.

Further, the number of particles in the C cross section is N / μm ² or less, the average crystal grain size in the C cross section is G μm or more, and the N and G are N = (− 0.01 × R + 0.7), By setting G = (0.02 × R + 0.8), a bonding wire having soft mechanical properties can be obtained. Therefore, with this bonding wire, as described above, pad damage, shape and bonding strength in 2nd bonding, and mass production stability in the bonding process can be improved. The C cross section is a cross section perpendicular to the drawing direction of the bonding wire.

  Here, the average crystal grain size means that the grain boundary recognition angle is divided into 15 ° or more, which is a large-angle grain boundary, twins are also regarded as grains, and a crystal grain size distribution image is drawn. The distribution is calculated. The particle size (equivalent circle diameter) distribution and basic statistics were calculated by using spreadsheet software based on the data of each crystal grain obtained by Backn Electron Scatter Pattern (hereinafter referred to as “EBSP”) processing software.

  The hardness of the bonding wire according to the present invention is such that the crystal structure of the C cross section is made of a mixture of fine grains and coarse grains by appropriate types and addition amounts of dopants, heat treatment, and optimization of the processing process. From the grain structure, it is possible to optimize and control the medium-sized sized structure. By controlling the grain size of the crystal structure to a sized structure in this way, it is possible to obtain a uniform deformation rather than a local deformation such as a grain boundary sliding deformation at a coarse grain boundary. It is possible to obtain a copper wire having softening mechanical properties in which 2% yield strength, maximum yield strength, and elongation value per unit cross-sectional area are within a certain range.

  In this way, the bonding wire according to the present invention increased the average particle size by reducing the fine particles and decreased the hardness to soften the core material. Thereby, chip damage and neck damage at the 1st junction can be reduced.

  By reducing this chip damage, the bonding wire according to the present invention can be applied to a chip using a low-K material corresponding to high density and high speed of a semiconductor, and even to a high performance LSI. The range of application of copper wire can be expanded.

  Moreover, since the core material was softened by making the crystal structure of C cross section into a sized structure, the bonding wire according to the present invention can improve deformation performance. As a result, the neck damage of the bonding wire according to the present invention is reduced, so that a low loop corresponding to thinning of semiconductor mounting and multi-layer stacking is possible.

  In addition, since the bonding wire according to the present invention has softened core material, ductility is improved and wear and dirt at the capillary tip can be reduced, so that the shape of the 2nd joint is stabilized. As a result, the bonding wire according to the present invention reduces the frequency of capillary replacement, and further, the surface of the bonding wire is less likely to be oxidized and stable in the atmosphere. Since the frequency of replacement can also be reduced, the productivity of the bonding process can be improved and the manufacturing cost can be easily reduced.

When the number of particles in the C cross section exceeds N particles / μm ² , the fine particles increase and the bonding wire becomes hard, so chip damage and neck damage increase, and the shape in the 2nd bonding deteriorates and bonding The strength will decrease.

  In addition, when the average crystal grain size of the C cross section is less than G μm, fine grains increase and the bonding wire becomes hard, so chip damage and neck damage increase, and the shape in the 2nd junction deteriorates or is bonded. The strength will decrease.

  Further, it is desirable that the outer layer has a thickness of 20 nm to 150 nm. With this bonding wire, it is possible to improve reliability evaluation in FAB formation, pressure ball shape and bonding strength in 1st bonding, and PCT (Pressure Cooker Test).

  In the bonding wire according to the present invention, by having the outer layer, the influence of the surface oxide is eliminated, and the bonding property with Au, Ag, Pd, Cu, etc., which are the main bonding partners of the 2nd bonding portion, is stabilized. The bonding margin can be widened, daily changes with time can be suppressed, and the bonding strength can be prevented from being affected by the bonding direction.

  Further, in the bonding wire according to the present invention, by setting the thickness of the outer layer to 20 nm or more, the wettability of the wire surface is stabilized, so that the ball eccentricity can be reduced. On the other hand, when the outer layer thickness is set to 150 nm or less, the uniformity of the ball portion is improved, so that an abnormal shape such as a shrinkage nest can be reduced. In this way, the bonding wire according to the present invention can stabilize the ball formability and the bonding strength.

Incidentally, in the conventional copper wire, N ₂ + 5% gas mixed with H ₂ gas was required at a flow rate of about 1.0 to 2.0 L / min during ball formation. Further, conventionally, there has been a problem that the ball forming property is lowered with a bonding wire simply provided with an outer layer.

On the other hand, in the bonding wire according to the present invention, the ball shape can be stabilized by having the thickness of the outer layer not less than 20 nm and not more than 150 nm, and by having oxidation resistance, H ₂ gas can be omitted, and 4N or more. It becomes possible to produce stably with N ₂ gas. In addition, since the flow rate can be halved to about 0.5-1.0 L / min, the manufacturing process can be omitted together with the omission of H ₂ gas, and the basic unit of shielding gas can be reduced, resulting in significant production. Running costs can be reduced.

  If the outer layer thickness is less than 20 nm, the ball will be eccentrically deformed. Due to the eccentric deformation of the ball, a load and an ultrasonic wave are poorly transmitted, and the bonding strength in the first bonding is lowered. Further, when the outer layer thickness is less than 20 nm, the effect of suppressing the oxidation of the ball surface is small, and as a result, the 1st bondability is lowered. On the other hand, if the thickness of the outer layer exceeds 150 nm, a shrinkage nest is generated during solidification, causing a so-called peach-shaped deformation in which the ball has a peach-like shape. Due to the deformation of the ball, the ground contact is poor, the load and the ultrasonic wave are poorly transmitted, and the bonding strength in the first bonding is lowered. Due to such poor bonding in the 1st bonding, it becomes difficult to improve the reliability evaluation. Further, when the outer layer thickness is less than 20 nm, the effect of suppressing oxidation is small, so that a joint failure occurs even in the wedge joint, and it is difficult to improve the reliability evaluation in the 2nd joint.

  The oxidation-resistant metal that is the main component of the outer layer is a metal other than copper, and is preferably a metal that is effective in improving the bondability of the bonding wire and is effective in preventing copper oxidation. For example, Pd, Pt, Rh, etc. are mentioned. Pd and Pt are relatively easy to stabilize the ball shape. Pd has a relatively low material cost and good adhesion to copper, so that the utility value as an outer layer is further increased.

  The main component of the core material is copper, and the characteristics are improved by adding an alloying element and the components and composition in the copper alloy. The core containing copper as a main component contains one or more additive elements selected from P, B, Bi, Sn, Ag, and Mg, and the concentration of the additive elements in the entire bonding wire is 0 in total. By being in the range of 0.0001 mol% or more and 0.03 mol% or less, effects such as an increase in breaking elongation in the peel test of the wedge bonding are increased. In particular, in the peel test for wedge bonding, the improvement effect is high. As an effect of these alloy elements, by controlling the processing of the core material and the formation of texture by recrystallization in wire manufacturing and wedge bonding, it effectively works to increase the breaking elongation of the bonding wire in the vicinity of the wedge bond. It is thought that. In addition, when the oxidation-resistant metal constituting the outer layer is Pd, Pt, and Rh, the added element in the core material synergizes with the oxidation-resistant metal due to ball melting, so that a perfect circle when the ball is deformed This has the effect of further improving the performance. With regard to such an additive effect, it is found that the effect is promoted when the outer layer and the additive element are used in combination, compared to the case where the additive is added to a conventional copper-based bonding wire in which the outer layer is not formed. It was done. If the concentration of the additive element is less than 0.0001 mol%, the above improvement effect may be reduced. If it exceeds 0.03 mol%, a wrinkle-shaped depression may be generated on the ball surface and the ball shape may become unstable.

  In manufacturing the bonding wire of the present invention, a step of forming an outer layer on the surface of the core material, and a processing / heat treatment step for controlling the structure of the outer layer, the core material, and the like are required. First, in order to control the composition and thickness of the outer layer and the core material, it is first important to manage the thickness and composition at the initial stage of forming the outer layer in the step of forming the outer layer.

  In addition, it is effective to perform the heat treatment once or a plurality of times for the above-described method for obtaining the soft mechanical properties. For example, a method of heat treatment at a temperature higher than the recrystallization temperature before forming the outer layer, a method of heat treatment at a temperature higher than the recrystallization temperature immediately after forming the outer layer, or a temperature higher than the recrystallization temperature during the wire drawing after the outer layer is formed. It can also be produced by appropriately combining any one of the method of heat-treating and finish annealing at the final wire diameter, or two or more methods. It is also effective to change the recrystallization progress of the core material by controlling the heat treatment temperature. By adopting the above-described method, a bonding wire having desired soft mechanical characteristics can be manufactured.

  A plating method is used for forming the outer layer on the surface of the copper core. As the plating method, either electrolytic plating or electroless plating can be used. Electrolytic plating called strike plating or flash plating has a high plating speed and good adhesion to the substrate. 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, reduction type plating is used after substitutional plating. It is effective to apply stepwise. The electroless method is simple and easy to use, but requires more time than the electrolysis method.

  In the present specification, a method of manufacturing a bonding wire will be described in which a film is formed on a thick core material by plating, and then a wire is drawn a plurality of times to a target wire diameter. Thus, the adhesion between the film and the core material can be improved by combining film formation and wire drawing. As a specific example, a technique of drawing a thick copper wire in an electrolytic or electroless plating bath to form a film and then drawing the wire to reach the final diameter is possible.

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

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 an outer 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 temperature gradient method, a positive temperature gradient near the furnace inlet (temperature rises with respect to the wire sweep direction), a stable temperature range, and a negative temperature gradient near the furnace outlet (temperature falls with respect to the wire sweep direction) It is effective to give a temperature gradient in a plurality of regions. This improves adhesion without causing separation between the outer layer and the core material in the vicinity of the furnace inlet, promotes diffusion of copper and oxidation-resistant metal in a stable temperature region, and forms a desired concentration gradient, Further, by suppressing excessive oxidation of copper on the surface in the vicinity of the furnace outlet, it is possible to improve the bondability and loop controllability of the obtained bonding wire. In order to obtain such an effect, it is desirable to provide a temperature gradient at the entrance / exit of 10 ° C./cm or more.

  In the method of changing the temperature, it is also effective to create a temperature distribution by dividing the furnace into a plurality of regions and performing different temperature control in each region. For example, the inside of the furnace is divided into three or more locations, temperature control is performed independently, and both ends of the furnace are set to a temperature lower than that of the central portion, so that the same improvement effect as in the case of the temperature gradient can be obtained. Further, in order to suppress the surface oxidation of the bonding wire, the bonding strength of the wedge joint can be increased by setting the outlet side of the furnace to a low temperature at which the oxidation rate of copper is low.

2. Examples Hereinafter, examples of the present invention will be described. As examples, bonding wires according to claims 1 to 4 were prepared for three types of wire diameters (φ20 μm, φ33 μm, and φ50 μm), and the average crystal grain size G and the number N of particles were measured for the prepared bonding wires. Moreover, in order to confirm the effect of the said Example, the comparative example was produced. The production method and analysis method of the examples are as follows.

  As the raw material of the bonding wire, the copper used for the core material is a high-purity material having a purity of about 99.99 mass% or more, and the Pd, Pt, and Rh materials of the outer layer are materials having a purity of 99.99 mass% or more. Prepared.

  To form a copper-based bonding wire that has been thinned to a certain wire diameter and to form a different oxidation-resistant metal layer on the wire surface, after forming an outer layer by electrolytic plating and electroless plating with a certain wire diameter Furthermore, a method of thinning to the final wire diameter by wire drawing was used. As the plating solution, a plating solution commercially available for semiconductor applications was used.

Prepare a bonding wire with a diameter of about 6 mm to 100 μm in advance, coat the outer surface of the wire with a plating method, repeat the heat treatment and cold wire drawing twice or more, and draw the wire to the final diameter. It was. Finally remove work strain, by heat treatment, 0.2% proof stress 0.07mN / μm ² or more 0.14mN / μm ² or less, the maximum yield strength is 0.20mN / μm ² or more 0.28mN / μm ² And the elongation value per unit cross-sectional area (% / μm ² ) is ε1 or more and ε2 or less, and ε1 and ε2 are ε1 = (− 0.001 × R + 0) where R is the wire diameter of the wire. 0.055) and ε2 = (− 0.001 × R + 0.068), and the bonding wire according to the example was formed. The values of ε1 and ε2 are as shown in Table 1.

A scanning electron microscope (SEM) was used for measuring the film thickness of the C cross section on the wire surface, and depth analysis using a fluorescent X-ray analyzer or Auger spectroscopy (AES) was also used. In the 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 film thickness of Pd was set to a value where the Pd concentration was 50 mol%. Therefore, the outer layer referred to in the present invention is a portion from the surface where the total Pd detection concentration constituting the outer layer is 50 mol%, that is, the portion where the total Pd detection concentration is 50 mol% or more. The analysis of the oxidation resistant metal in the bonding wire was measured by ICP analysis, ICP mass spectrometry, or the like.

Further, for the produced bonding wire, the number of particles in the C cross section and the average crystal grain size of the C cross section were measured, and the number of bonding wire particles according to the example was N / μm ² or less, and the average crystal grain size was It was confirmed that it was G μm or more. N and G are N = (− 0.01 × R + 0.7) and G = (0.02 × R + 0.8). The values of N and G are as shown in Table 2. An EBSP (Electron Backscatter Pattern) method was used to measure the average crystal grain size and the number of particles of the bonding wire.

  As measurement examples, FIG. 1 shows a crystal grain size distribution image of φ20 μm, and FIG. 2 shows a crystal grain size distribution image of φ50 μm. Moreover, the result of having calculated the data of each crystal grain obtained by EBSP processing software with spreadsheet software is shown in FIG. 3, FIG. 4 (φ20 μm), and FIG. 4 (φ50 μm).

  Tables 3 to 5 show the breakdown of the bonding wires produced as described above. Table 3 shows data of bonding wires having a wire diameter of φ20 μm, Table 4 shows data of bonding wires having a wire diameter of φ33 μm, and Table 5 shows data of bonding wires having a wire diameter of φ50 μm.

For connecting the bonding wires, a commercially available automatic wire bonder was used to perform ball / wedge bonding. A free air 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 standard 5 vol% H ₂ + N ₂ gas and pure N ₂ gas were used as the shielding gas for suppressing oxidation during ball formation. The gas flow rate was adjusted in the range of 0.5 to 0.7 L / min.

  As a bonding partner, an Al alloy film (Al-0.5 mass% 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: 1 to 4 μm) or a resin substrate having an electrode structure of Au plating / Ni plating / Cu was used as a partner for wedge bonding.

  In the evaluation of the free air ball shape, the ratio of the ball diameter / wire diameter was in the range of 1.7 to 2.0. Fifty balls before joining were observed with an SEM to evaluate whether the ball shape, eccentricity (center misalignment), and surface irregularities were large.

  Regarding the free air ball shape, if there are 5 or more abnormally shaped balls, it is defective and needs to be improved. If the FAB is waving, the number of occurrences of △ marks and abnormally shaped balls is one or less. If it was less than one and a beautiful perfect circle was drawn, it was marked with “◎” and indicated in the “shape” column of FAB formation.

  In the determination of eccentricity (center misalignment), if there are 5 or more misalignments, it is judged as defective, and if it is 2 or 4, it is desirable to improve if necessary, so Δ mark, 1 misalignment or less In addition, it can be judged that there is no serious problem in practical use, and is marked in the “Eccentricity” column of FAB formation when it is not generated at all.

  As for the surface shape (uneven shrinkage nest), there are 5 or more uneven shrinkage nests, and if the unevenness is large, x mark is 4 or less, but if the unevenness is large, improvement is necessary. However, it was judged that there were no irregularities due to small irregularities, and a circle mark was given, and when no irregularities were generated, a circle mark was given, and it was written in the “surface shape (unevenness shrinkage nest)” column of FAB formation.

  In the determination of the pressure-bonded ball shape, 500 bonded balls were observed to evaluate the roundness of the shape, dimensional accuracy, and the like. The ratio of the ball diameter / wire diameter was in the range of 1.7 to 2.2. If there are 5 or more defective ball shapes such as anisotropy and petal shape deviating from a perfect circle, it is determined as defective, and if there are 2 to 4 defective ball shapes, improvement is desirable as necessary. If the number of Δ marks or defective balls is 1 or less, the result is good. If no defective balls are generated, the mark is marked in the “Press-bonded ball shape” column of 1st bonding.

  In the determination of the bonding strength, 20 shear strength measurement tests are performed on the 1st side where the ball diameter / wire diameter ratio is in the range of 1.7 to 2.2 and the ball height is in the range of 8 to 16 um. If the Min value of the shear strength is less than 10 gf, the 1st joint strength is unstable. Therefore, if the mark is 10 gf or more and less than 15 gf, it can be improved by changing the joining conditions. Therefore, Δ mark, 15 gf or more and less than 20 gf. If it exists, it is judged that there is no problem in practical use, and if it is in the range of 20 gf or more, it is good and is marked in the “Joint strength” column of 1st joint because it is good.

  In addition, after bonding, the wire was etched and peeled to evaluate the presence or absence of pad damage. The load, the ultrasonic time and the ultrasonic force under the wire bonding conditions were unified and evaluated. Check 500 points. If there are 5 or more damages, mark X. If damages are 2-4, improvement is desirable depending on customer conditions. If there was no damage at all, it was marked with “◎” in the “pad damage” column of the 1st joint.

  Further, the neck portion of the bonded wire was observed with a scanning electron microscope (SEM) to evaluate neck damage. As a part that is easily damaged at the neck, the outside of the neck in the direction opposite to the wedge bonding was carefully observed. As for the form of damage, micro cracks, large cracks, wrinkled irregularities, etc. were investigated. When the loop height is lowered, when the narrow diameter is narrowed, the occurrence rate of neck damage usually increases, so that the evaluation becomes stricter. Therefore, evaluation was carried out as a trapezoidal loop with a wire diameter of 20um, a loop height of 100um during bonding, and a loop length of 2mm. Check 100 wires, if there are 8 or more damage, × mark, 5-7 marks △, 2-4 marks ○, 0-1 marks ◎ This is shown in the “neck damage” column of the 1st joint. The neck damage was evaluated only for the bonding wire having a wire diameter of φ20 μm. In this bonding wire having a diameter of φ20 μm, FIG. 7 shows a photograph of the neck portion according to the comparative example and the example in the first bonding in the case of bonding with a bonding height of 10 μm and a bonding length of 2 mm. Thus, although the neck damage was contained in the neck part in the comparative example, it was confirmed that the neck damage was not entered in the examples.

  For determination of 2nd bonding, observe the bonded 2nd side at 500 locations, check the occurrence of turning and perform a pull test of 20 2nd bonding portions, and if the Min value of the pull strength is less than 3 gf, Since the 2nd bonding strength is insufficient, it can be improved by changing the bonding conditions if it is in the range of x mark and 3 gf or more but less than 4 gf. In the range of 6 gf or more, it is good, and it is marked in the “shape” column of the 2nd junction because it is good.

  In the determination of mass productivity evaluation, 40 balls were observed initially at 100,000 wires and 300,000 wires, and a sample was prepared under the initially set conditions, and the ball diameter, ball height, 1st shear strength, 2nd pull strength, Stitch shape comparison was performed and evaluated. If the shape changes or strength decreases with 3 or more items, there is a problem with the continuous bonding property. Therefore, if the cross mark is 2 items or more to less than 4 items, it is necessary to improve the bonding conditions, etc. If it is less than 1 item, improvement is desirable depending on the conditions. Therefore, if there is no change in the shape or decrease in strength, it is indicated in each column of mass productivity evaluation.

  In PCT, heating was performed for 200 hours in a high-temperature and high-humidity environment at 121 ° C., 2 atm and humidity of 100%. Thereafter, the electrical characteristics of 40 wires were evaluated. When the ratio of the wire whose electrical resistance has increased to 3 times or more of the initial value is 30% or more, it is X because of the bonding failure, and the ratio of the wire whose electrical resistance has increased 3 times or more is 5% or more and less than 30%. In the case of the range, since it can be used for an IC whose reliability requirement is not strict, the proportion of the wire whose electrical resistance has increased 3 times or more is less than 5% and the rate of increase of 1.5 times or more is 10 If the percentage is less than 30%, there is no practical problem. Therefore, it is good if the ratio of the wire that has risen to 1.5 times or more is less than 10%. In the "" column.

  Tables 6 to 8 show the evaluation results of the bonding wires according to the present invention and comparative examples. Table 6 shows data of bonding wires having a wire diameter of φ20 μm, Table 7 shows data of bonding wires having a wire diameter of φ33 μm, and Table 8 shows data of bonding wires having a wire diameter of φ50 μm. As for the bonding wires of Examples 1 to 83, it was confirmed that the reliability evaluation could be improved by FAB formation, pressure bonding ball shape and bonding strength in 1st bonding, and PCT when the thickness of the outer layer was 20 nm or more and 150 nm or less. It was done. On the other hand, in Comparative Examples 1, 5, 10, 14, 18, 23 in which the thickness of the outer layer is less than 20 nm, and Comparative Examples 15, 17, 19, 22, 25 in which the thickness of the outer layer exceeds 150 nm, the PCT It was confirmed that the reliability evaluation was low.

Bonding wires of Examples 1-83, the 0.2% proof stress is 0.07mN / μm ² or more 0.14mN / μm ² or less, the maximum yield strength is 0.20mN / μm ² or more 0.28mN / μm ² or less, and The elongation value per unit cross-sectional area (% / μm ² ) is ε1 or more and ε2 or less, and ε1 and ε2 are ε1 = (− 0.001 × R + 0.055), where R is the wire diameter of the wire. , Ε2 = (− 0.001 × R + 0.068) (Table 1), it was confirmed that pad damage, shape and bonding strength in 2nd bonding, and mass productivity evaluation can be improved. On the other hand, Comparative Examples 2-4, 6-9, 11-14, 16, 17, 20, 21, 21, 24, 25, and 0.2% proof stress and maximum proof strength, where the elongation per unit cross-sectional area is out of the above range. However, in Comparative Examples 15 and 22, which are out of the above range, it was confirmed that the pad damage, the shape at the 2nd junction, and the mass productivity evaluation were not improved.

In the bonding wires of Examples 1 to 83, the number of particles in the C cross section is N / μm ² or less, the average crystal grain size in the C cross section is G μm or more, and the N and G are N = (− 0 .01 × R + 0.7) G = (0.02 × R + 0.8) makes it possible to obtain a bonding wire having soft mechanical properties. Therefore, with this bonding wire, as described above, pad damage, shape and bonding strength in 2nd bonding, and mass productivity evaluation can be improved. On the other hand, in Comparative Examples 3, 4, 7-9, 11-14, 16, 17, 20, 21, 21, 24, 25 in which the average crystal grain size and the number of particles are out of the above ranges, the pad damage and the shape in the 2nd junction It was confirmed that the mass productivity evaluation was not improved.

It is an EBSP measurement result in a bonding wire having a diameter of 20 μm. It is an EBSP measurement result in a bonding wire having a diameter of 50 μm. It is a graph which shows the result of having calculated the data of each crystal grain obtained by the EBSP processing software in the bonding wire of φ20 μm by the spreadsheet software. (A) Comparative example, (B) Example It is a table | surface which shows the result of having calculated the data of each crystal grain obtained by the EBSP processing software in the bonding wire of φ20 μm by the spreadsheet software. It is a graph which shows the result of having calculated the data of each crystal grain obtained by the EBSP processing software in the bonding wire of φ 50 μm by the spreadsheet software. (A) Comparative example, (B) Example It is a table | surface which shows the result of having calculated the data of each crystal grain obtained by the EBSP processing software in the bonding wire of φ50 μm by the spreadsheet software. It is the SEM photograph which image | photographed the mode of the neck part after bonding in the bonding wire of (phi) 20micrometer.

Claims (4)

A core mainly composed of copper;
A bonding wire for a semiconductor device having an outer layer made of an oxidation-resistant metal having a thickness of 20 nm or more and 150 nm or less provided on the core;
0.2% proof stress 0.07mN / μm ² or more 0.14mN / μm ² or less, the maximum yield strength is 0.20mN / μm ² or more 0.28mN / μm ² or less, and an elongation value per unit cross-sectional area (% / Μm ² ) is not less than ε1 and not more than ε2, and the ε1 and the ε2 are R when the wire diameter of the wire is R,
ε1 = (− 0.001 × R + 0.055)
ε2 = (− 0.001 × R + 0.068)
A bonding wire for a semiconductor device, characterized in that The number of particles in the C cross section is N / μm ² or less, the average crystal grain size in the C cross section is G μm or more, and the N and G are:
N = (− 0.01 × R + 0.7)
G = (0.02 × R + 0.8)
The bonding wire for a semiconductor device according to claim 1, wherein:   2. The bonding wire for a semiconductor device according to claim 1, wherein the oxidation-resistant metal contains, as a main component, one or more elements selected from Pd, Pt, and Rh.   The core material contains one or more elements selected from P, B, Bi, Sn, Ag, and Mg, and the total concentration of the elements in the entire wire is 0.0001 mol% or more and 0.03 mol% or less. The bonding wire for a semiconductor device according to claim 1, wherein the bonding wire is in a range of